CA1268428A - Gas separation - Google Patents
Gas separationInfo
- Publication number
- CA1268428A CA1268428A CA000493160A CA493160A CA1268428A CA 1268428 A CA1268428 A CA 1268428A CA 000493160 A CA000493160 A CA 000493160A CA 493160 A CA493160 A CA 493160A CA 1268428 A CA1268428 A CA 1268428A
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- CA
- Canada
- Prior art keywords
- gas
- bed
- pressure swing
- swing adsorption
- psa
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
- Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
- Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
Abstract
Abstract Gas separation A raw ammonia synthesis gas, preferably containing an excess of nitrogen, is converted to synthesis ready ammonia synthesis gas by two pressure swing adsorption (PSA) stages operated in succession. If the raw gas contains carbon monoxide this is chemically removed, eg by methanation between the PSA stages. In the first PSA stage, CO2 is removed and waste gas from the second PSA stage is preferably fed back to the first PSA stage for sweeping, purging or repressurising. In a preferred system the first PSA cycle includes a sweep with CO2-rich gas prior to depressurisation and purge with second PSA waste gas. In the second PSA stage inert medium boiling gases, and any unwanted nitrogen are removed as the waste gas.
Description
~2~42~3 Gas separation This invention relates to gas separation and in particular to producing synthesis-ready ammonia synthesis gas from a raw synthesis gas.
Such a raw synthesis gas is generally prepared, using a hydrocarbon feedstock, for example natural gas or naphtha, by the sequence of primary and secondary steam reforming or partial oxidation (with air, or oxygen-enriched air being employed in the secondary reforming or partial oxidation step) to give a gas stream containing hydrogen;
nitrogen and argon (from the air); carbon oxides; steam (as an e~cess of that required for reforming or as produced in the partial oxidation); and, generally, a small amount of methane. This gas mixture is then subjected to the shift reaction wherein carbon monoxide reacts with steam to produce hydrogen, thereby increasing the hydrogen content of the gas, and carbon dioxide. The shifted gas is then cooled, eg to a temperature below 50C to condense the steam present as liquid water which is then separated. The resultant gas is the raw synthesis gas.
For use in ammonia synthesis the synthesis-ready gas should contain hydrogen and nitrogen, should be essentially free of gases such as steam, carbon dioxide, and carbon monoxide, which deactivate the ammonia synthesis catalyst, and desirably is essentially free of inerts such as methane and argon in order to minimise any purge from the ammonia synthesis loop.
The ammonia synthesis reaction involves the reaction of 1 mole of nitrogen with 3 moles of hydrogen and so, in order to minimise any purge from the synthesis loop, the synthesis-ready ammonia synthesis gas desirably has a ; H2/N2 molar ratio near to 3, for example in the range 2.5 to 3.1. However it is often advantageous to conduct the aforementioned secondary reforming or partial oxidation steps :
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Such a raw synthesis gas is generally prepared, using a hydrocarbon feedstock, for example natural gas or naphtha, by the sequence of primary and secondary steam reforming or partial oxidation (with air, or oxygen-enriched air being employed in the secondary reforming or partial oxidation step) to give a gas stream containing hydrogen;
nitrogen and argon (from the air); carbon oxides; steam (as an e~cess of that required for reforming or as produced in the partial oxidation); and, generally, a small amount of methane. This gas mixture is then subjected to the shift reaction wherein carbon monoxide reacts with steam to produce hydrogen, thereby increasing the hydrogen content of the gas, and carbon dioxide. The shifted gas is then cooled, eg to a temperature below 50C to condense the steam present as liquid water which is then separated. The resultant gas is the raw synthesis gas.
For use in ammonia synthesis the synthesis-ready gas should contain hydrogen and nitrogen, should be essentially free of gases such as steam, carbon dioxide, and carbon monoxide, which deactivate the ammonia synthesis catalyst, and desirably is essentially free of inerts such as methane and argon in order to minimise any purge from the ammonia synthesis loop.
The ammonia synthesis reaction involves the reaction of 1 mole of nitrogen with 3 moles of hydrogen and so, in order to minimise any purge from the synthesis loop, the synthesis-ready ammonia synthesis gas desirably has a ; H2/N2 molar ratio near to 3, for example in the range 2.5 to 3.1. However it is often advantageous to conduct the aforementioned secondary reforming or partial oxidation steps :
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2 B 33273 with such an amount of air, or oxygen-enriched air that in the raw synthesis gas there is an e~cess of nitrogen over that required in the synthesis-ready ammonia synthesis gas.
The raw synthesis gas thus usually contains hydrogen, carbon monoxide, carbon dioxide, methane, argon, and nitrogen, the latter often being in an excess of that required for ammonia synthesis.
Carbon monoxide, nitrogen, and the inerts such as methane and argon, have boiling points, at atmospheric pressure, in the range -100 to -200C: such gases are hereinafter termed medium boiling gases and are hereinafter referred to as ~B.
It is therefore necessary, in order to convert the raw synthesis gas into synthesis-ready ammonia synthesis gas, to separate from that raw synthesis gas carbon oxides and, ln order to minimise the purge, if any~ from the ammonia synthesis loop, it is desirable to separate from the raw synthesis gas any excess of nitrogen and other MB gases.
It has been proposed in GB-A-2126573 to carry out such a separation by a pressure swing adsorption (PSA) process but the percentage recovery of hydrogen is low (72.4%) and the PSA product gas has to be methanated and dried before it can be contacted with an iron ammonia synthesis catalyst. Other proposals, su h as in GB-A-2103199 or European Chemical News 1978, 20 October, 39 - 47, have involved feeding a N2-free raw gas to the PSA system and adding N2 in the course of the PSA cycle or thereafter; such proposals are unsatisfactory in requiring an external supply of substantially pure nitrogen. It appears that recovery of the C02 from such proposed processes is inefficient owing to the low pressure and/or purity of the CO2-containing waste stream from the PSA system, and consequently they do not provide an attractive route to an integrated ammonia/urea process or to manufacture of solid or liquid C02.
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The raw synthesis gas thus usually contains hydrogen, carbon monoxide, carbon dioxide, methane, argon, and nitrogen, the latter often being in an excess of that required for ammonia synthesis.
Carbon monoxide, nitrogen, and the inerts such as methane and argon, have boiling points, at atmospheric pressure, in the range -100 to -200C: such gases are hereinafter termed medium boiling gases and are hereinafter referred to as ~B.
It is therefore necessary, in order to convert the raw synthesis gas into synthesis-ready ammonia synthesis gas, to separate from that raw synthesis gas carbon oxides and, ln order to minimise the purge, if any~ from the ammonia synthesis loop, it is desirable to separate from the raw synthesis gas any excess of nitrogen and other MB gases.
It has been proposed in GB-A-2126573 to carry out such a separation by a pressure swing adsorption (PSA) process but the percentage recovery of hydrogen is low (72.4%) and the PSA product gas has to be methanated and dried before it can be contacted with an iron ammonia synthesis catalyst. Other proposals, su h as in GB-A-2103199 or European Chemical News 1978, 20 October, 39 - 47, have involved feeding a N2-free raw gas to the PSA system and adding N2 in the course of the PSA cycle or thereafter; such proposals are unsatisfactory in requiring an external supply of substantially pure nitrogen. It appears that recovery of the C02 from such proposed processes is inefficient owing to the low pressure and/or purity of the CO2-containing waste stream from the PSA system, and consequently they do not provide an attractive route to an integrated ammonia/urea process or to manufacture of solid or liquid C02.
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3 B 33273 In the present invention these disadvantages are overcome by the use of two PSA stages.
According tc the invention there is provided a process for the production of synthesis-ready ammonia synchesis gas from a raw gas containing hydrogen, carbon dioxide and medium boiling gas including nitrogen comprising removing carbon dioxide and unwanted medium boiling gas by pressure swing adsorption characterised by (a) subjecting the raw gas to a first PSA system effective to remove carbon dioxide while leaving hydrogen and medium boiling gas largely unadsorbed; and (b) subjecting the unadsorbed gas to a second PSA
system effective to separate therefrom a waste gas stream containing said unwanted medium boiling gas, including the excess, if any, of nitrogen; and wherein, if the unadsorbed gas leaving the first PSA system contains carbon monoxide, subjecting that unadsorbed gas to chemical removal of carbon monoxide, preferably by catalytic methanation, prior to subjecting said unadsorbed gas to said second PSA system.
~ach PSA system can be broadly of the type wherein there is a plurality of adsorbent beds and each bed takes part, successively, in steps including adsorption, pressure equalisation, depressurisation to exterior~ and repressurisation, with an optional purge step between depressurisation and repressurisation.
In the ensuing description of the preferred PSA
systems, the terms "inlet" and "outlet" refer to the direction of flow of gas during the adsorption step, and the terms "co-current" and "couater-current"~mean towards such outlet and inlet respectively.
In the first PSA system the pressure of the raw gas entering a bed undergoing adsorption duty is preferably in the range 25 to 50, especiaily 30 to 40, bar abs. The temperature in the fiFst~PSA~syste= is preferablY higher than ~ ;
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According tc the invention there is provided a process for the production of synthesis-ready ammonia synchesis gas from a raw gas containing hydrogen, carbon dioxide and medium boiling gas including nitrogen comprising removing carbon dioxide and unwanted medium boiling gas by pressure swing adsorption characterised by (a) subjecting the raw gas to a first PSA system effective to remove carbon dioxide while leaving hydrogen and medium boiling gas largely unadsorbed; and (b) subjecting the unadsorbed gas to a second PSA
system effective to separate therefrom a waste gas stream containing said unwanted medium boiling gas, including the excess, if any, of nitrogen; and wherein, if the unadsorbed gas leaving the first PSA system contains carbon monoxide, subjecting that unadsorbed gas to chemical removal of carbon monoxide, preferably by catalytic methanation, prior to subjecting said unadsorbed gas to said second PSA system.
~ach PSA system can be broadly of the type wherein there is a plurality of adsorbent beds and each bed takes part, successively, in steps including adsorption, pressure equalisation, depressurisation to exterior~ and repressurisation, with an optional purge step between depressurisation and repressurisation.
In the ensuing description of the preferred PSA
systems, the terms "inlet" and "outlet" refer to the direction of flow of gas during the adsorption step, and the terms "co-current" and "couater-current"~mean towards such outlet and inlet respectively.
In the first PSA system the pressure of the raw gas entering a bed undergoing adsorption duty is preferably in the range 25 to 50, especiaily 30 to 40, bar abs. The temperature in the fiFst~PSA~syste= is preferablY higher than ~ ;
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4 B 33273 in conventional PSA systems in order to increase the purity and/or pressure of the C02-rich stream evolved in the final depressurisation to exterior or in the purge step. Suitable temperatures are up to 200C, especially in the range 60 -150C. Such a PSA process is among those described in our European application and US application claiming priority from UK application 8426393. However, conventional temperatures of up to about 50C can also be used satisfactorily, especially if, as described hereinafter, waste gas from the second PSA system is fed to the first PSA
system.
The adsorption step in the first PSA system is preferably terminated sufficiently before the C02-front has reached the bed outlet in order to ensure that the C02 content of the unadsorbed gas, integrated over the adsorption step, is at the required level, typically under 0.5 and preferably under 0.2% V/v. The C02 content of the gas tends to rise towards the end of the adsorption step as its adsorption tail approaches the bed outlet. The ~2/MB molar ratio of the unadsorbed gas has a maximum value at an intermediate part of the adsorption step, because at the beginning MB gas adsorbed in preceding equalisation steps and any purge step, if those steps are counter-current, becomes desorbed; and because at the end the MB adsorption front has reached or passed the bed outlet.
~ fter the adsorption has proceeded to the desired extent in a bed, the pressure in that bed is reduced by one or more pressure equalisation steps in which gas from the outlet of the bed that has finished its adsorption duty is released, co-curren~ly out therefrom and is fed, preferably counter-currently, into a recipient bed that has been depressurised and may have been purged and may have been partly repressurised.
There may be one or more presure equalisation .. , ~ .
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B 33~73 steps. Thus, as in ~he relevant part of the 4~bed system depicted in flgure 2 of USP 3430418, on which the system described hereinafter is based, ~here may be a single equalisation step. Alternatively, as in the relevant parts of the modified 4-bed system depicted in figure 2 of USP
3564816 or of the 5-bed system depicted in figure 3 of USP
3430418, there may be two equalisation steps. Alternatively, as in the relevant parts of the syRtems using 6 or more beds, for example the 8-bed and 10-bed systems depicted in USP
3986849, there may be three equalisation steps. Although the pressure equalisations are illustrated by those references, as described below, the PSA cycles that may be used in the process of the invention differ from the disclosed cycles in other respects.
After the pressure equalisation step or steps, the bed is subjected to depressurisation. The depressurisation can be conducted in a variety of ways. For example, the depressurisation may be effected in one step, to de orb C02, and to release the relatively small amount of MB and H2 remaining in the bed, to the exterior. Alternatively the depressurisation may be effected in a plurality of steps:
thus the depressurisation may be conducted, preferably co-currently, in one or more steps, (hereinafter termed the penultimate depressurisation) to an intermediate pressure level releasing to the exterior an MB-rich/C02-lean gas and then in a final step, preferably counter-currently, to release to the exterior an ~B-lean/C02-rich gas.
Alternatively, in a multiple step depressurisation, in the penultimate depressurisation, the released gas may be used, as described below, to purge another bed: in this mode o~
operation the gas released to the exterior as the purge from said another bed is enriched in C02 by the C02 purged from said another bed: as described above, in the final depressurisationj the bed is depressorised, preferably ~, .:
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~ 34~8 counter-currently.
The ter~inal pressure of co-current penultimate depressurisation with product release is preferably at least 2 bar abs and typically up to 10 bar abs. The gas released in this step is leaner in CO2 the higher the pressure. The terminal pressure of final, countercurrent, depressurisation is conveniently in the range 1 to 3 bar abs. However to ensure more complete C02 desorption the final depressurisation may be to a ter~linal sub-atmospheric pressure. The C02-rich stream released in this final depressurisation step is richer in C02 the lower the terminal pressure of the penultimate depressurisation. However the lower the latter pressure, the less will be the proportion of the C02 in the raw gas that is recovered in the final depressurisation C02 product stream.
After final depressurisation the bed is optionally purged. ~or example, as described above, the bed can be purged, preferably counter-currently, with a through-current of gas fed counter-currently out of a bed undergoing penultimate depressurisation. Alternatively the bed can be purged, preferably counter-currently, with a through-current of ~B-rich/C02-free gas from a depressurisation in the second PSA system. As a further alternative the bed can be purged, preferably counter-currently, with a through-current of gas of low or zero C02 content from an exterior source. If two or more such purges are used, they are preferably effected in the above order.
Ater final depressurisation, and after purge, if used, the bed is repressurised by gas released from a bed undergoing pressure equalisation as described above and by a feed of unadsorbed gas from the outlet line of a bed, or beds, on adsorption duty. The latter repressurisation may be operated during, as well as after, the repressurisation resulting from a bed undergoing pressure equalisation, in order to avoid excessive fluctuations in the flow rate of , : ~
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unadsorbed gas leaving the bed, or beds> undsrgoing adsorptio~ duty. Alternatively or addieionally, the final repressurisation may be by gas recycled from the second PSA
system: this gas may be waste g8S or product gas fro~ the second PSA syste~.
If the unadsorbed, essentially C02 free> gas from the first PSA system contains C0 as an ~ gas> this unadsorbed gas is then sub~ected to chemical removal o~ C0.
This chemical removal is preferably a catalytic methanation process to convert all the C0 present to methane. This methanation process may also convert most> or all, of any C02 present ln the unadsorbed gas from the first PSA systPm. The methanation catalyst can be of the well-tried supported nickel and/or cobalt type> containing for example 5 _ 70% W/w of such metal (calculated as monoxide) on a refractory comprislng alumina> spinel> cement or aluminosilicate. If desired> however> a supported ruthenium catalyst can bs used.
The temperature need not be controlled to prevent methanation of C2 and thus is conveniently in the range 250 - 400C at the catalyst outlet. Whereas methanation produces by-product water vapour> most of this water vapour can conveniently be removed by coolin~ to condense the water vapour as llquid water wbich is readily separated. Any residual water vapour can be removed by the second PSA system. In some cases the second PSA system can remove all of the water vapour produced by the methanation step. Generally no separate adsorptive water vapour removal step is necessary.
The second PSA system may be of the type described in European application 85301022, which w;ll Dublish as Eæ'A-157480 and which corresponds to USSN 703531, operated und~r conditions to adjust the H2/MB molar ratio to that required in the s~nthesis-ready amm~nia synthesis gas.
As in the first PSA sy.stem, each bed successively , 342~3 undergoes adsorption, pressure equalisation, depressurisation (preferably in two stages, co current followed by counter-current), and repressurisation with an optional purge step, preferably using co-current depressurisation gas from an other bed, between the depressurisation and repressurisation steps.
In order to limit the magnitude of fluctuations in the composition and flow rate of unadsorbed gas in the adsorption step of each PSA system, each &ystem preferably includes enough beds to permit at least two beds to be used on adsorption duty simultaneously, but out of step with one another. Likewise there are preferably sufficient beds to permit beds to be on simultaneous, but out of step, duties in those other steps, ie depressurisation and purge (if any), where gas is released from the PSA system.
The waste-gas from the second PSA will be essentially C02 free, particularly where there is a methanation step in which any C02, as well as C0, is methanated, between the first and second PSA systems. This waste-gas will also be MB-rich/~2-lean. As mentioned above, this waste gas from the second PSA system can be used in the first PSA system. It is therefore possible that the second PSA waste gas can be taken at a higher pressure than is disclosed in the aforesaid European application 85301022. A
waste gas pressure of at least 2 bar abs. and up to one quarter of the second PSA system gas inlet pressure is preferred. This waste gas may required compression and/or heating to the first PSA system operating conditions but, even so, such use of the second PSA system waste gas may be advantageous.
In one mode of operation waste gas from the second PSA system is fed into a bed of the first PSA system after depressurisation to exterior as part of the repressurisation of that bed. This has the advantage that it returns to the : ' : : .. '-.
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:, 8~2i~3 process any a2 in the waste gas of the second PSA system.
Part of the waste gas from the second PSA system may be vented in order to prevent accumulation of MB gas in the unadsorbed gas from the first PSA system.
In another mode of operation, a bed of the first PSA system, that has finished its~ adsorption duty but has not been depressurised to exterior, is swept by a through current, preferably co-currently, of waste gas from the second PSA system, directly, or indirectly, into a recipient bed of the flrst PSA system after that recipient bed has been depressurised (and purged, if used) and before that recipient bed has finished its adsorption duty. The first PSA system bed may be s~ept before or after its pressure equalisation step. The effect of this sweep is to displace the unadsorbed gas, which contains H2 and MB and has a H2/MB ratio greater than that of the waste gas from the second PSA system, that is still present in that bed of the first PSA system into the recipient bed: in this way the H2 of the unadsorbed gas in the bed being swept is not lost during subsequent depressurisation to exterior or pur~e (if used).
~ The waste gas from the second PSA system can be a gas from co-current depressurisation, counter-current depressurisation or purge or more than one of these. Since by the invention a2 is retained in the process, that gas need not be taken at minimal pressure in order to minimise H2 loss. However, at the preferred inlet and equalisation pressures of the first PSA system, it is usually necessary to compress the waste gas from the second PSA system before feeding it to the first PSA system. ~ ;
If desired the gas used to sweep the bed of the first PSA system can include one or more other streams of suitable composition, namely C02-lean or C02-free, possibly containing MB gases tbat will be subsequently purged, and advantageously containing H2. Such other streams include ;
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intermediate pressure waste gas from the first PSA, or ammonia synthesis purge gas.
There are three preferred ways of effecting a first PSA system sweep with waste gas from the second PSA system:
two of these preferred ways involve sweeping after the pressure equalisation step of the first PSA system and are termed ~intermediate pressure sweep while the third way, termed high pressure sweep , involves sweaping before pressure equalisation.
In the first intermediate pressure sweep mode of operation, the waste gas from the second PSA system i9 fed into the inlet of a bed of the first PSA system that has just undergone pressure equalisation and is still connected to the recipient bed undergoing repressurisation. By this means gas still in the bed after equalisation is swept into the recipient bed and replaced by second PSA waste gas. The extent to which gas is replaced depends on how much waste gas is available from the second PSA system and on the pressure to which (if at all) it is necessary or convenient to compress it. After this step both the post-equalisation bed and the recipient bed are at a pressure between feed and equalisation; the recipient bed contains unadsorbed product gas and the post-equalisation bed contains second PSA waste gas and may contain some unadsorbed product gas. The post-equalisation bed can be equalised with a fully regenerated, ie depressurised and optionally purged possibly partly repressurised, bed if one is available~ or can be depressurised to waste, possibly stagewise at 2 or more pressure levels.
One further advantage of the use, as described above, of the waste gas from the second PSA system, or unadsorbed product gas swept out thereby, for repressurisation is that less, possibly none, of the gas from the unadsorbed product line of the first PSA system need to :;
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~;2 6842~3 be diverted into repressurisation, and hence the flow of unadsorbed product gas from the first PSA system is subject to less fluctuation.
In the second intermediate pressure sweep mode of operation, the waste gas from the second PSA system is fed into a bed that has undergone pressure equalisation, has been disconnected from the recipient bed, and has been con~ected to the inlet of a compresser, the outlet of which feeds into the raw gas inlet line of the first PSA system. By this means gas still in the bed after equalisation is swept into the PSA raw gas inlet and replaced by second PSA waste gas.
The extent to which gas is replaced depends on how much gas is available, but less gas is needed than in the first intermediate pressure sweep mode because it is used only at equalisation pressure, not at pressures up to feed pressure.
Since the swept out gas passes into the raw gas inlet, a destination for it is continuously available. After this step, the post-equalisatlon bed contains second PSA waste gas and may contain unadsorbed product gas if the quantity of second PSA waste gas was insufficient to sweep out all the unadsorbed product gas. If more tha~ this sufficient second PSA waste gas is available, it can be passed into the compressor and raw gas feed inlet, and thereby ~ in the second PSA waste gas will be retained in the process. After the sweep, the post-equalisation bed can be equalised with a fully regenerated, possibly partly repressurised, bed if one is available, or can be depressurised to~waste, possibly stagewise at 2 or more pressure levels.
In the high pressure sweep mode of operation, the waste gas from the second PSA system is fed into a bed that has completed its adsorption step, but has not been equalised, and has been connected to the raw gas inlet line of the first PSA system, possibly by way of a booster compressor recovering the pressure-drop through the bed.
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Thereby unadsorbed product gas stlll in the bed is swept into the raw gas inlet line and recovered.
Since the bed is at feed pressure, the quantity of waste gas from the second PSA system required is almost double ehat for the above described second mode of intermediate pressure sweep in which almost half the : unadsorbed product gas in the bed is recovered by equalisation. After high pressure sweep the bed can be equalised with a fully regenerated, possibly partly repressurised, bed; such a bed is available if a system of 4 or more beds is used. As an altecnative, the swept bed can be depressurised to waste, possib:Ly stagewise at 2 or more pressure levels; in such a system there is an energy penalty in tha. gas is let down from the highest pressure in the system after having been compressed, but for some users the penalty may be mieigated if there is a use for waste gas at relatlvely high pressure or if there is reason for having only 2 or 3 beds instead of the 4 or more required when equalisation is practised.
In the aforementioned `'sweeping" modes of operation, the waste gas from the second PSA system is preferably passed through the bed being swept co-currently in order to minimise deso~ption of C02. This is more critical for the first mode of intermediate pressure sweep since any desorbed C02 would be adsorbed at the outlet of ehe recipient bed (assuming flow into lts outlet as in equalisation) and thus would contaminate the unadsorbed gas stream fed to the second PSA system in the next adsorp~ion step using that bed.
Alternatively in this mode of intermediate pressure sweep, the swept out gas is received co-currently, and this is preferred.
In the second intermediate, and in the high, pressure sweep modes of operation, any desorption of C02 is less important since this desorbed C02 will be returned to - : :. : :.; :.: .: ~ : -- :. . .: . :. .-~
:' . . ` ' ~;8~28 the raw synthesis gas inlet line.
If sweeping by second PSA waste gas is continued until such waste gas has passed into a bed undergoing repressurisation or into the raw gas feed, there will be an accumulation of MB gases in the combination of the two PSA
sys~ems. The same will occur if second PSA waste gas is used directly in repressurisation.
If the adsorbent is correctly choqen and operated, the increased MB partial pressure may result in increased adsorption, so that such accumulation is limited. Otherwise it may be desirable to maintain a purge of ~B-rich gas at a suitable point in the system.
Especially in the second intermediate pressure, and in the high pressure, sweep modes, wherein the waste gas from the second PSA system is accumulated in the first PSA system under raised pressure, a subsequent depressurisation to purge another bed can be carried out at two or more pressure levels, so as to produce first a waste gas rich in fuel values but lean in C02 and finally a waste gas rich in C02.
It will be appreciated that the waste gas from the second PSA may be used both for sweeping, prior to depressurisation, and also for repressurisation as described above.
In an another alternative, a bed in the first PSA
system that has completed is adsorption duty is swept with a C02-rich gas and the swept-out gas is returned to the raw synthesis gas inlet line. After depressurising the bed to recover C02, the depressurised bed is purged with waste gas from the second PSA system.
The C02-rich gas used for this C02-sweeping is conveniently derived from the C02-rich gas recovered from the depressurisation of another bed. If desired, the required C02-rich gas can be supplied by an autonomous circulation system. More conveniently, the requirement of the recipient ~ ~
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~61~428 bed is provided from the storage capacity of pipework and any reservoirs in the C02 collecting system or, in a suitably designed system, from a bed undergoing desorption of C02-rich gas in the same time period. The C02-rich gas may need to be compressed, depending on the pressure of the gas to be swept out. Often the C02-rich gas will be compressed in the course of use further downstream in a process sequence making for example solid or liquid C02 or urea; thus the gas used in sweeping can conveniently be takea from such a downstream source.
The bed subjected to C02-sweeping has preferably been at least once downwardly pressure-equalised with another bed. In such equalisations, carried out with co-current flow from the bed, the C02 front stays in the bed and thus the gas passed into the recipient bed or beds is H2 + MB gas low in C2 and, since it enters the recipient bed(s) counter-currently, drives back the C02 front in such bed(s). By sweeping after equalisation the pressure of the C02-rich gas need not be so high as would be necessary for sweeping before equalisation.
C02-sweeping is continued preferably co-currently and until the C02 front has moved towards the bed outlet but ramains within the bed. A small C02 content in the swept-out gas is not harmful if that gas is fed to the raw synthesis gas inlet tlme of the first PSA system and then a balance may be struck between the advantage of more complete sweeping and the disadvantage of adding more C02 to the inlet gas.
The effect of the C02-sweeping step is to expel H2 and M3 gases from the void space in the bed and also to desorb H2 and ~B gases. Consequently the gas desorbed in the subsequent depressurisation consists almost entirely of C02 and is very suitable for further processing.
For a C02-sweepino step the C02-rich gas needs to be compressed, for example to 6 to 12 bar abs, but such - ': . ~ ,. . .: -, :
:
~26~2~3 pressures would com~only be needed for the further processing already mentioned. For C02 recovery the depressurisation, which is preferably counter-current, is for example from 6 -12 bar abs and can go down, if desired, to less than 1 bar ; 5 abs., f or example to 0.1 bar abs., depending on the extent to which C02 is required.
.~fter the C02-sweep and the depressurisation with C02-recovery, the bed is purged, preferably counter-currently, with waste gas from the second PSA system. The effect of this purge is to desorb still more C02 since the waste gas from the second PSA system is substantially C02-free and thus subjects the bed to a still lower C02 pressure than would be attained in normal depressurisation to below atmospheric pressure. Since the purge gas is not obtained from another bed in the first PSA system, its supply does not depend on a step occurring in another bed of the first PSA
system; therefore purging need not be rigidly synchronised with steps occurring in other beds.
The purge outlet gas consists mainly of MB gases, a s~all percentage of H2 and a content of C02 depending on the extent to which C02 was recovered in the depressurisation to C2 recovery step. If it is desired to keep down the C02 content Gf the purge outlet gas but very high C02 recovery is not required, the void space gas present after the depressurisation can itself be swept out wholly or partly to waste, by means of the second PSA waste gas to be used as purge gas.
The purging pressure, whatever the C02 content intended in the outlet purge gas, can be superatmospheric, and then the outlet purge gas can be used as a source of power, by heating it and expanding it (preferably with combustion) in a gas turbine; the gas turbine exhaust can supply at least part of the heat required before expansion.
To obtain the required purging pressure the second : - ~ :, ;
''' ., . : :
.
B~ 8 PSA waste gas can if necessary be compressed. However the waste gas may be taken from the second PSA system at superatmospheric pressure, for example in the range 2 - 10 bar abs. and the inlet and outlet ~2/N2 ratios of the second PSA system chosen accordingly.
By the use of a C02-sweep, and purge with waste gas from the second PSA system, a large extent of C02 desorption is possible. It is therefore possible to use adsorber beds in the first PSA system of unconventionally small volume in proportion to the flow rate of gas to be purified.
Alternatively or additionally, aclsorption step times can be unconventionally long, for example 4 - 8 minutes, and thus time is available durlng each adsortion step for several short steps, of which equalisations, sweeps and depressurisations are examples.
Since the C02-sweep step returns H2 to the process, the first PSA system does not rely on multiple pressure equalisations to decrease ~2 loss and therefore need not include a large number of beds. It appears that the most convenient number will be 4 through 6 or possibly 8 if it is desired to have overlapping operation of successive adsorbers.
Whereas it was indicated above that the first PSA
system could advantageously be operated "hot" with inlet temperatures above 40C in order to maximise the C02 content of the stream of recovered C02, where a C02-sweep and second PSA waste gas purge are employed, such higher temperature operation can be avoided.
The adsorbent charged to the PSA beds can be any of those considered suitable for PSA, including silica gels, active carbons and zeolites. The adsorbent for at least the beds of the first PSA system preferably includes a zeolite, since zeolites are capable of adsorbing C02 much more preferentially to the MB gases than the other adsorbents.
- , :
Such specificity is less at the high temperatures that may be used in the first PSA system than at conventional PSA
temperatures, but is fully adequate. When using a high temperature first PSA system, with a zeolite adsorbent, the effect is to produce a C02 adsorption isotherm that resembles the ambient temperature isotherms for active carbon and silica gel. A suitable zeolite is of the A type, for example calcium A (Zeolite A i9 defined in D W Breck's "Zeolite Molecular Sieves" and is available from several manufacturers under different trade names). The following table sets out values of the adsorption constants K, where K _ kg. mols of adsorbed gas per kg of adsorbent pressure in bar abs.
for C02 and N2 and of the ratio KC02/KN2 for a zeolite and an active carbon. The values at 30, 70 and 100C are quoted from page 565 of "Gas Purification" by A L Kohl and S C
Riesenfeld, 3rd ed. 1979 (Gulf Publications, Houston, Texas, USA) and those at 150C are obtained by Clausius Clapeyron-~ype calculation.
Table _ . I
; K, kg. mol kg 1 bar 1 at TC
T = 30T = 70 T = 100 T = 150 Zeolite N2 26 x 10 5 11.9 x 10 5 7.4 x 10-5 3.9 x 10 5 C2 0.1610.041 0.0179 0.0058 ratio 619344 241 148 . . _ Carbon N2 41.5 x 10 5 23.8 x 10 5 16.9 x 10 5 10.7 x 10 5 C2 4.46 x 10-3 1.45 x 10-3 0 73 x 10-3 0.29 x 10-3 ratio 10.7 6.1 4.33 2.71 . . _ Whereas, in the absence of a purge by second PSA
.
. .
:' ~ . . : :
2~3 system is preferably mainly carbon, when such a purge is used that adsorbent is preferably zeolite.
The present invention is of particular use in combinations wherein (a) the C02-containing gas released in the first PSA
system is fed to a C02 purification system such as an adsorptive or liquid-absorption or distillation C02 recovery system; for this purpose the product C02-containing gas can, if desired, be compressed; and/or tb) with or without the above-mentioned C2 concentration, the C02-containing product gas is fed to urea synthesis; and/or (c) unadsorbed gas from the second PSA system is fed to ammonia synthesis, the latter advantageously being the source of ammonia for urea synthesis where, as mentioned above, the C02-containing product gas is used for urea synthesis.
Usually the product gasesj ie C02-product gas released in the first PSA system, and the synthesis ready ammonia synthesis gas produced as the unadsorbed gas stream in the second PSA system have to be compressed before further use.
Since the efficiency of compressors tends to vary : as the molecular weight of the gas being compressed varles it is advantageous, particularly in the case of the second PSA
system to have sufficient beds that at least two beds are on adsorption duty at any one time, but out of phase with one another, in order to minimise fluctuations in the composition of the gas fed to the compressor.
The chemical step preceding the first PSA system is preferably a catalytic shift step preferably decreasing the C0 content to at most 1, preferably und~r 0.5, % V/v on a dry basis. Cooling and liquid water sep~aration are effected before the first PSA system, but the residual water vapour can be removed in the PSA system, without a separate drying A ~
, :; '.; ~: '~:: ,, : ', ;
:: .
~842~3 , operation.
Upstream of shift and water removal the preferred stage is hydrocarbon air-steam reforming ln which catalytic primary steam reforming is effected in indirect heat exchange with catalytic air reforming and the product raw gas has an (H2 + CO)/(~B - CO) molar ratio iu the range 1.25 to 2.5, especially 1.5 to 2.1, a C02 content in the range 10 to 25 V~v, and at least 90% V/v of the ~ is N~.
Preferred embodiments of the invention are shown in the accompanying drawings in which Figure 1 is a block diagram of the overall flow sheet, Figure 2 is the flow sheet of the second PSA
system, and Figures 3 - 10 are flow sheets of the first PSA
system showing various alternative embodiments making use of the waste gas from the sacond PSA
unit as follows:
for final repressurisation (Figure 3);
as a counter-curren~ purge (Figure 4);
as a co-current sweep then counter-current purge (Figure 5);
as a co-current sweep then intermediate co-current repressurisation (Figure 6);
as a co-current intermediate pressure sweep with recycle (Figure 7);
as a co-current high pressure sweep with recycle (Figure 8);
as a counter~current purge in a cycle using a sweep with C02-rich gas to recycle (Figure 9); and as in Figure 9 but for a 6 bed PSA unit (Figure 10) .
In the flow sheets the dotted horizontal lines represent divisions between successive time intervals of the PSA cycle.
: ,..
:
... .
.: :, : - ~ :
: ~ . - :- ~,.
-: ., ~: ,.
4~8 With the 4-bed units of Figures 2 to 9 there are four such intervals labelled Tl to T4 while in the 6-bed unit of Figure 10 there are 6 time intervals Tl to T~.
The connections to the beds are positioned, in the flow sheets, so that gas entering the left hand side (LHS), and/or leaving the right hand side (R~S), of a bed is flowing co-currently. Conversely gas entering the R~S, and/or leaving the L~S, of a bed is flow:Lng counter-currently~
In the flow sheets and ensuing description the following abbreviations are employed:
AD - adsorption duty (co-current) El = intermediate equalisation (co-current - may be omitted).
E2 = final equalisation (co-current - may be the only equalisation as shown in Figures 2 -8) Dl = intermediate depressurisation (co-current -omitted in Figures 5, 9 and 10) D2 - final depressurisation (counter-current) PU = purge (counter-current) SW - sweep (co-current) Rl = initial repressurisation (counter-current) R2 = intermediate repressurisation (counter-current - omitted in Figures 2 - 5, 7 and 8) : 25 R3 = final repressurisation (co-current ) IG = inlet raw synthesis gas W = waste gas from the second PSA system SR = synthesis ready ammonia synthesis ~as U = unadsorbed gas from the first PSA system ~ = unadsorbed gas from the first PSA system after methanation and water removal.
CR = C02-rich gas separated in first PSA system ~IR = C02-lean/MB-rich gas separated in first PSA :
system :~ . .. ,. ., . .. :,.
~2~
P ~ product gas (CR plus MR where these are not taken off separately).
In the process of these embodiments a stream of raw ammonia synthesis gas, IG, from the process sequence of natural gas/steam primary reforming, air secondary reforming, heat exchange between the primary and secondary reforming, catalytic shift, cooling, and water separation, is fed, via line 10 to the inlet gas manifold 12 of a firs~ PSA unit 14, optionally via a heat exchanger 16 where a hot inlet gas, IG, is required. PSA unit 14 has a plurality of beds, eg 4, 6, 8 or 10, of adsorbent and programmeld valve actuators providing an uninterrupted succession of regenerated adsorbent beds and steps of pressure equalisation, depressurisation, purge, and repressurisation with an optional sweep step after adsorption but before depressurisation.
The raw synthesis gas, IG, contains H2, N2, C0, Ar, C~4 and C02 and the PSA unit 14 is effective to give a stream, U, of unadsorbed gas, which is essentially free from C02, leaving PSA unit 14 via manifold 18. PSA unit 14 also gives one or two gas streams containing the C02 removed from the synthesis gas IG. As shown in Figure 1, a C02-lean/~B-rich stream MR (which will contain a little H2) and a C02-rich/~-lean stream CR (which may contain a small amount of H2), leave the PSA unit 14 via manifolds 20 and 22 respectively. Alternatively, as will be described there may be a single product gas stream P containing the C02 and some ~B and a little ~2 leaving PSA unit 14: for convenience of description its manifold is also designated 22.
Where separate MR and CR streams are produced, the MR stream is taken from manifold 20 and may be used as a fuel for a gas turbine (not shown) driving, for example, one or more compressors. The CR stream or P stream, is taken from manifold 22 of PSA unit 14 to C02 recovery steps (not shown).
-, ,,- ~ , - .' :: '~ ~ ' ' ' ' ' ~ , - , ' . ~ .: : ~ ., ~;8~2~3 If the temperature at which PSA unit 14 is operated is above the critical temperature of C02 and high enough to limit adsorption of MB gases, the C02 content of the unadsorbed gas stream, ie stream U, is low, for example 0.5%
V/v or less, but not much of the MB and H2 of the raw synthesis gas are separated in the PSA unit 14.
The unadsorbed gas stream, ie stream U, is fed from manifold 18 to a heat e~changer 24 wherein it is heated and passed to a methanator 26 in which it encounters a supported nickel catalyst. The temperature of the methanation inlet gas is controlled at such a level that both C0 and C02 are metha~ated. The resulting methanated gas is cooled in heat exchanger 24 as the source of heat for heating the gas fed to methanator 26, cooled further in heat exchanger l6 as the source of heat for the raw gas, IG, entering PSA unit 14 where the latter is of the hot type, and finally cooled in cooler/separator 28 wherein liquid water is separated and removed via line 30. From cooler/separator 28, the methanated gas from which water has been separated, ie gas stream M, is fed to the inlet manifold 32 of a second PSA
unit 34. In PSA unit 34, which is operated atJ for e~ample, 30C, the methanated gas ~ is separated to give a waste gas W
containing CH4, N2, and Ar, as its main components, and the :
synthesis ready ammonia synthesis gas SR which has an H2:N2 ratlo within the range 2.5 to 3.1. The waste gas W will contain a little H2. The W and SR gases leave PSA unit 34 via manifolds 36 and 38 respectively.
PSA unit 34 is of the same general type as that of unit 14: a flo~ chart for a 4-bed PSA unit 34 is shown in Figure 2.
During the first time interval Tl, bed A is used for adsorption duty. In this duty the inlet of bed A i3 connected to the inlet manifold 32 for the methanated gas M
and its outlet is connected to the outlet manifold 38 ;
: ., ,,'.: : , .--:
" : :: ' :
supplying the synthesis ready ammonia synthesis gas SR.
In the second time interval, wherein bed D is used for the adsorption duty, bed A is first subjected to a pressure equalisation step E2 wherein the inlet to bed A is closed and its outlet is connected to the outlet of bed B
which has just been purged in step PU. When the pressures in beds A and B are equal, the outlet of bed A is disconnected from that of bed B and is connected to the outlet of bed C
whose inlet is connected to the ~aste gas manifold 36. Bed A
is thus subjected to a co-current depressurisation Dl wherein the gas in bed A passes co-currently out of bed A to bed C
through which it passes, counter-currently, as a purge PU.
In the third time interval T3, wherein bed B is : used for adsorption duty, the outlet of bed A is closed and its inlet ls connected to the waste gas manifold 36: the bad is thus subjected to a counter-current depressurisation D2.
The outlet of bed A is then connected to the outlet of bed D, which has just undergone the equalisation step E2, so that the gas released during the co-current depressurisation step Dl of bed D flows counter-currently through bed A as a purge, ie step PU.
In the fourth time interval T4, wherein bed C is used for adsorption duty, the inlet of bed A is closed and its outlet is connected to the outle of bed B (which has just finished its adsorption duty AD) so that the gas released from bed B during its equalisation step E2 effects counter-current repressurisation ~1 of bed A. Finally the outlet of bed A is closed and its inlet connected to the SR gas outlet manifold 38 (in Figure 2, since this manifold 38 is being supplied, in this time interval, with gas from bed C, the connection to bed A is shown from the SR outlet time fro~ bed C), to effect co-current repressurisation, step R3.
Bed A is thus ready to recommence adsorption duty.
As shown in Figure 2 the other beds go through the same cycle .,, ' :: :, : : : . -,: : :.:
:
~. ~'' ,', , ~%6~3~28 but out of phase with one another.
Some or all of ehe waste gas W from PSA unit 34 ~ay be used as a fuel in a furnace or gas turbine, possibly in admixture with the MR (or P) gas stream from PSA unit 14.
~owever, it i5 preferrred that at least part of the waste gas W from PSA unit 34 is fed back to PSA unit 14 via line 40 in Figure 1. Unless PSA unit 34 is operated under conditions giving a waste gas W at a suffic:Lently hlgh pressure for its intended use in PSA unit 14, it will generally be necessary to compress waste gas W in compressor 42 before it is supplied to the waste gas inlet manifold 44 of PSA unit 14.
Various uses of the waste gas W from PSA unit 34 in the PSA unit 14 are shown in the flow sheets of Figures 3 to 10.
The cycle of the PSA unit 14 is similar to that described above for PSA unit 34. However, as shown in Figures 3 - 5, 9, and 10, the gas released from the bed in the final, counter-current, depressurisation step D2, and the purge step PU may bs collected separately to give, respectively a C02-rich gas stream CR and a MB-rich gas stream ~R. Such separate collection of CR and MR can also be adopted, if desired, in the embodiments of Figures 6 - 8.
Alternatively, in the arrangements of Figures 3 - 5, the final depressurisation and purge gases can be collected as a single stream of product gas, P, as show~ in the embodiments of Figures 6 - 8.
In the embodiment of Figure 3, the waste gas W from PSA unit 34 is used for the co-current repressurisation step R3 instead of using gas from the manifold 18 supplied with gas from a bed undergoing adsorption duty AD. This has the advantage that H2 otherwise lost in the waste gas W from PSA
unit 34 is returned to the system.
Alternatively, in an embodiment not illustrated, this final rapressurisation, or at least the initial part ,:
' .~ . " : ' "' . "'' ' .
thereof, may be affected with part of the SR gas leaving PSA
unit 34 instead of with the waste gas: this has the advantages that the amount of MB returned to YSA unit 14 in this repressurisation step R3 is minimised and also that less, if any, compression of this returned gas is required.
In the embodiment of Ei~ure 4 the waste gas W from PSA unit 34 is used as the purge gas: thus the gas leaving the bed undergoing the co-current depressurisation stap Dl in PSA unit 14 is taken directly to the ~R gas outlet manifold 20 while the waste gas W from PSA unit 34 is fed to the outlet of the bed of PSA unit 14 undergoing the purge step PU. This has the advantage that the MR gas has a lower C02 content than in the embodiment of Figure 3. Consequently in this embodiment generally the CR and ~R gases will be collected separately, as shown, rather than combined to give a single product gas P.
In the embodiment of Figure 5, the co-current depressurisation step Dl is replaced by a sweep step SW
wherein the waste gas W from PSA unit 34 is fed to the inlet of a bed that has undergone the equalisation step E2. The outlet of the bed undergoing the sweep step SW is connected, .
as in the embodiment of Figure 3, to the outlet of a bed undergoing the counter-current purge PU. This has the advantage that the CR gas produced in the counter-current depressurisation step D2 following the sweep step SW will contain virtually no hydrogen.
In the embodiments of Figures 6 and 7 a co-current sweep step SW using the waste gas W from PSA unit 34 is interposed between the equalisation step E2 and the co-current depressurisation step Dl. In the Figure 6 embodimentthe gas swept from the bed is used for an intermediate, co-current, repressurisation step R2. This has the advantage that H2 in the waste gas W is retu~ned to the system.
However it will be seen that the pressure at which the waste ,. ~ , - , ~:
, :
4~
gas has to be supplied increases, as in the embodiment of Figure 3 as repressurisation proceeds. Thus during the -course of the sweep the pressure has to increase from that remaining after equalisation towards the adsorption pressure.
In the embodiment of ~igure 7 the gas swept from the bed is returned, via a compressor 46, to the inlet gas manifold 12.
This has the advantage of returnLng the ~2 as in the Figure 6 embodiment with the further advantage that, unlike the Figure 3 and Figure 6 embodiments, the compressor 42 in the waste gas W inlet line 40 is not subject to a fluctuating load.
In the embodiment of F:Lgure 8 the cycle is similar to that of Figure 7 except that thé sweep step SW is before, instead of after, the equalisation step E2. This is more advantageous than the system of Figure 7 where the waste gas W from PSA unit 34 is at a higher pressure.
In the embodiment of Figure 9 the waste gas W from PSA unit 34 is used as purge gas, as in the Figure 4 embodiment, but also a co-current sweep step SW is employed in place of the co-current depressurisation with the swept gas being returned to the inlet gas manifold 12 via compressor 46 as in the Figure 7 embodiment. In this case the gas used for the sweep step SW is C02-rich gas CR taken, via compressor 48, from the manifold 50 supplied with gas from the counter-current depressurisation step D2. Although in this embodiment there is no bed undergoing step D2 at the time of the sweep step SW, a sufficient reservoir of C~ gas wil~ probably be availabIe in the pipework and/or a small reservoir vessel can be provided. Alternatively the CR gas used for the sweep can be gas from external processing of the CR gas exported from PSA unit 14. By means of this C02-rich gas sweep, the CR gas produced in the subsequent counter-curren~ depressurisation will be particularly pure.
In this embodiment two equalisation steps E2 and E1 are shown: before the final equallsation E2 in which the : ~ :
-:.. ,: ;: :
.
~2~342~
released gas effects initial counter-current repressurisation Rl of a bed that has been purged, an intermediate equalisation step E1 is employed for intermediate counter-current repressurisation R2 of another bed that has already been subjected to the initial repressurisation Rl by gas released from a final equalisation step E2. Such multiple equalisations may also be adopted where appropriate in the embodiments of Figures 3 to 8.
In the embodiment of Figure 9 each time interval is typically about 6 minutes. Referring to bed A, at the end of interval Tl, the C02 adsorption front is well short of the bed outlet. In intervcal T2 the two equalisation steps are first effected with the result that bed B has been repressurised to over half the adsorption pressure and bed C
to over one quarter of ths adsorption pressure. Beds B and C
have had such C02 adsorption fronts as remained moved back towards the bed inlets. Up to three quarters of the ~2 left in bed A at the end of the adsorption step, ie in the void space and as adsorbed gas, has been returned to the process.
In the sweep step SW in interval T2 the gas is allowed to flow until most of the H2 and ~B gas has been swept out of the bed, but is stopped before significant C02 breaks through. The gas in bed A is now almost exclusively C02. In the counter-current depressurisation step D2 also in interval T2, the major proportion of the gas is exported as CR gas and only a minor proportion is used for the sweep. It will be appreciated that the CR export can be taken from the mani~old 50 before compressor 48 if desired: however since the exported CR gas will normally have to be compressed in its subsequent processing, it is often more convenient to effect the compression of all the gas released in the counter-current depressurisation step D2 and to export the CR
gas, as shown, from manifold 22 after compressor 48. After the counter-current depressurisation step D2, the purge step , ~, .
'''`' .
-. ...
~26~42~3 is effected to purge the remaining C02 from the bed: since the time remaining in interval T2 is unlikely to be sufficient to effect completion of the purge, ~he purge can continue, as shown, into interval T3.
In the embodiment of Figure 10, a 6 bed arrangement for effecting the cycle of Figure 9 is shown. There it is seen that a bed is undergoing counter-current depressurisation D2 while the sweep SW is taking place and so no reservoir for CR gas is required. Since in the Figure 10 embodiment each time interval only includes 1 or 2 steps, whereas in the Figure 9 embodiment: up to 5 steps were required in each time interval, the time intervals ln the Figure 10 embodiment can, if desired, be shorter than those of the Figure 9 embodiment.
In a further embodiment, not shown in the Figures, the arrangement of Figure 9 is modified by the omission of the equalisation steps El and E2 and the corresponding initial and intermediate repressurisation steps Rl and R2.
In this embodiment the final repressurisation R3 is preferably using some of the synthesis ready ammonia synthesis gas from the second PSA unit. This embodiment has the advantage of reducing the number of beds required in the first PSA system, in some cases to as few as two, at the expense of increased power requirement Pg in compressors 46 and 48.
- ,::. :, ,. .,.: . . - ::: : : ' ~, :
system.
The adsorption step in the first PSA system is preferably terminated sufficiently before the C02-front has reached the bed outlet in order to ensure that the C02 content of the unadsorbed gas, integrated over the adsorption step, is at the required level, typically under 0.5 and preferably under 0.2% V/v. The C02 content of the gas tends to rise towards the end of the adsorption step as its adsorption tail approaches the bed outlet. The ~2/MB molar ratio of the unadsorbed gas has a maximum value at an intermediate part of the adsorption step, because at the beginning MB gas adsorbed in preceding equalisation steps and any purge step, if those steps are counter-current, becomes desorbed; and because at the end the MB adsorption front has reached or passed the bed outlet.
~ fter the adsorption has proceeded to the desired extent in a bed, the pressure in that bed is reduced by one or more pressure equalisation steps in which gas from the outlet of the bed that has finished its adsorption duty is released, co-curren~ly out therefrom and is fed, preferably counter-currently, into a recipient bed that has been depressurised and may have been purged and may have been partly repressurised.
There may be one or more presure equalisation .. , ~ .
~ , ~2~
B 33~73 steps. Thus, as in ~he relevant part of the 4~bed system depicted in flgure 2 of USP 3430418, on which the system described hereinafter is based, ~here may be a single equalisation step. Alternatively, as in the relevant parts of the modified 4-bed system depicted in figure 2 of USP
3564816 or of the 5-bed system depicted in figure 3 of USP
3430418, there may be two equalisation steps. Alternatively, as in the relevant parts of the syRtems using 6 or more beds, for example the 8-bed and 10-bed systems depicted in USP
3986849, there may be three equalisation steps. Although the pressure equalisations are illustrated by those references, as described below, the PSA cycles that may be used in the process of the invention differ from the disclosed cycles in other respects.
After the pressure equalisation step or steps, the bed is subjected to depressurisation. The depressurisation can be conducted in a variety of ways. For example, the depressurisation may be effected in one step, to de orb C02, and to release the relatively small amount of MB and H2 remaining in the bed, to the exterior. Alternatively the depressurisation may be effected in a plurality of steps:
thus the depressurisation may be conducted, preferably co-currently, in one or more steps, (hereinafter termed the penultimate depressurisation) to an intermediate pressure level releasing to the exterior an MB-rich/C02-lean gas and then in a final step, preferably counter-currently, to release to the exterior an ~B-lean/C02-rich gas.
Alternatively, in a multiple step depressurisation, in the penultimate depressurisation, the released gas may be used, as described below, to purge another bed: in this mode o~
operation the gas released to the exterior as the purge from said another bed is enriched in C02 by the C02 purged from said another bed: as described above, in the final depressurisationj the bed is depressorised, preferably ~, .:
..
~ 34~8 counter-currently.
The ter~inal pressure of co-current penultimate depressurisation with product release is preferably at least 2 bar abs and typically up to 10 bar abs. The gas released in this step is leaner in CO2 the higher the pressure. The terminal pressure of final, countercurrent, depressurisation is conveniently in the range 1 to 3 bar abs. However to ensure more complete C02 desorption the final depressurisation may be to a ter~linal sub-atmospheric pressure. The C02-rich stream released in this final depressurisation step is richer in C02 the lower the terminal pressure of the penultimate depressurisation. However the lower the latter pressure, the less will be the proportion of the C02 in the raw gas that is recovered in the final depressurisation C02 product stream.
After final depressurisation the bed is optionally purged. ~or example, as described above, the bed can be purged, preferably counter-currently, with a through-current of gas fed counter-currently out of a bed undergoing penultimate depressurisation. Alternatively the bed can be purged, preferably counter-currently, with a through-current of ~B-rich/C02-free gas from a depressurisation in the second PSA system. As a further alternative the bed can be purged, preferably counter-currently, with a through-current of gas of low or zero C02 content from an exterior source. If two or more such purges are used, they are preferably effected in the above order.
Ater final depressurisation, and after purge, if used, the bed is repressurised by gas released from a bed undergoing pressure equalisation as described above and by a feed of unadsorbed gas from the outlet line of a bed, or beds, on adsorption duty. The latter repressurisation may be operated during, as well as after, the repressurisation resulting from a bed undergoing pressure equalisation, in order to avoid excessive fluctuations in the flow rate of , : ~
. . .
: . . , .: ' ''~: ''` ~ , :
: : ~' .,~ ',' ' `
,, ~ . ,.
unadsorbed gas leaving the bed, or beds> undsrgoing adsorptio~ duty. Alternatively or addieionally, the final repressurisation may be by gas recycled from the second PSA
system: this gas may be waste g8S or product gas fro~ the second PSA syste~.
If the unadsorbed, essentially C02 free> gas from the first PSA system contains C0 as an ~ gas> this unadsorbed gas is then sub~ected to chemical removal o~ C0.
This chemical removal is preferably a catalytic methanation process to convert all the C0 present to methane. This methanation process may also convert most> or all, of any C02 present ln the unadsorbed gas from the first PSA systPm. The methanation catalyst can be of the well-tried supported nickel and/or cobalt type> containing for example 5 _ 70% W/w of such metal (calculated as monoxide) on a refractory comprislng alumina> spinel> cement or aluminosilicate. If desired> however> a supported ruthenium catalyst can bs used.
The temperature need not be controlled to prevent methanation of C2 and thus is conveniently in the range 250 - 400C at the catalyst outlet. Whereas methanation produces by-product water vapour> most of this water vapour can conveniently be removed by coolin~ to condense the water vapour as llquid water wbich is readily separated. Any residual water vapour can be removed by the second PSA system. In some cases the second PSA system can remove all of the water vapour produced by the methanation step. Generally no separate adsorptive water vapour removal step is necessary.
The second PSA system may be of the type described in European application 85301022, which w;ll Dublish as Eæ'A-157480 and which corresponds to USSN 703531, operated und~r conditions to adjust the H2/MB molar ratio to that required in the s~nthesis-ready amm~nia synthesis gas.
As in the first PSA sy.stem, each bed successively , 342~3 undergoes adsorption, pressure equalisation, depressurisation (preferably in two stages, co current followed by counter-current), and repressurisation with an optional purge step, preferably using co-current depressurisation gas from an other bed, between the depressurisation and repressurisation steps.
In order to limit the magnitude of fluctuations in the composition and flow rate of unadsorbed gas in the adsorption step of each PSA system, each &ystem preferably includes enough beds to permit at least two beds to be used on adsorption duty simultaneously, but out of step with one another. Likewise there are preferably sufficient beds to permit beds to be on simultaneous, but out of step, duties in those other steps, ie depressurisation and purge (if any), where gas is released from the PSA system.
The waste-gas from the second PSA will be essentially C02 free, particularly where there is a methanation step in which any C02, as well as C0, is methanated, between the first and second PSA systems. This waste-gas will also be MB-rich/~2-lean. As mentioned above, this waste gas from the second PSA system can be used in the first PSA system. It is therefore possible that the second PSA waste gas can be taken at a higher pressure than is disclosed in the aforesaid European application 85301022. A
waste gas pressure of at least 2 bar abs. and up to one quarter of the second PSA system gas inlet pressure is preferred. This waste gas may required compression and/or heating to the first PSA system operating conditions but, even so, such use of the second PSA system waste gas may be advantageous.
In one mode of operation waste gas from the second PSA system is fed into a bed of the first PSA system after depressurisation to exterior as part of the repressurisation of that bed. This has the advantage that it returns to the : ' : : .. '-.
.: : - . -, : . , ,::: :: : ,, :
.
"
:, 8~2i~3 process any a2 in the waste gas of the second PSA system.
Part of the waste gas from the second PSA system may be vented in order to prevent accumulation of MB gas in the unadsorbed gas from the first PSA system.
In another mode of operation, a bed of the first PSA system, that has finished its~ adsorption duty but has not been depressurised to exterior, is swept by a through current, preferably co-currently, of waste gas from the second PSA system, directly, or indirectly, into a recipient bed of the flrst PSA system after that recipient bed has been depressurised (and purged, if used) and before that recipient bed has finished its adsorption duty. The first PSA system bed may be s~ept before or after its pressure equalisation step. The effect of this sweep is to displace the unadsorbed gas, which contains H2 and MB and has a H2/MB ratio greater than that of the waste gas from the second PSA system, that is still present in that bed of the first PSA system into the recipient bed: in this way the H2 of the unadsorbed gas in the bed being swept is not lost during subsequent depressurisation to exterior or pur~e (if used).
~ The waste gas from the second PSA system can be a gas from co-current depressurisation, counter-current depressurisation or purge or more than one of these. Since by the invention a2 is retained in the process, that gas need not be taken at minimal pressure in order to minimise H2 loss. However, at the preferred inlet and equalisation pressures of the first PSA system, it is usually necessary to compress the waste gas from the second PSA system before feeding it to the first PSA system. ~ ;
If desired the gas used to sweep the bed of the first PSA system can include one or more other streams of suitable composition, namely C02-lean or C02-free, possibly containing MB gases tbat will be subsequently purged, and advantageously containing H2. Such other streams include ;
. - , . : . .
intermediate pressure waste gas from the first PSA, or ammonia synthesis purge gas.
There are three preferred ways of effecting a first PSA system sweep with waste gas from the second PSA system:
two of these preferred ways involve sweeping after the pressure equalisation step of the first PSA system and are termed ~intermediate pressure sweep while the third way, termed high pressure sweep , involves sweaping before pressure equalisation.
In the first intermediate pressure sweep mode of operation, the waste gas from the second PSA system i9 fed into the inlet of a bed of the first PSA system that has just undergone pressure equalisation and is still connected to the recipient bed undergoing repressurisation. By this means gas still in the bed after equalisation is swept into the recipient bed and replaced by second PSA waste gas. The extent to which gas is replaced depends on how much waste gas is available from the second PSA system and on the pressure to which (if at all) it is necessary or convenient to compress it. After this step both the post-equalisation bed and the recipient bed are at a pressure between feed and equalisation; the recipient bed contains unadsorbed product gas and the post-equalisation bed contains second PSA waste gas and may contain some unadsorbed product gas. The post-equalisation bed can be equalised with a fully regenerated, ie depressurised and optionally purged possibly partly repressurised, bed if one is available~ or can be depressurised to waste, possibly stagewise at 2 or more pressure levels.
One further advantage of the use, as described above, of the waste gas from the second PSA system, or unadsorbed product gas swept out thereby, for repressurisation is that less, possibly none, of the gas from the unadsorbed product line of the first PSA system need to :;
.
-:: :: -: . ,: .. ,. ::: ;
:: :;.:."., :
~- : :.:: .: :.
~;2 6842~3 be diverted into repressurisation, and hence the flow of unadsorbed product gas from the first PSA system is subject to less fluctuation.
In the second intermediate pressure sweep mode of operation, the waste gas from the second PSA system is fed into a bed that has undergone pressure equalisation, has been disconnected from the recipient bed, and has been con~ected to the inlet of a compresser, the outlet of which feeds into the raw gas inlet line of the first PSA system. By this means gas still in the bed after equalisation is swept into the PSA raw gas inlet and replaced by second PSA waste gas.
The extent to which gas is replaced depends on how much gas is available, but less gas is needed than in the first intermediate pressure sweep mode because it is used only at equalisation pressure, not at pressures up to feed pressure.
Since the swept out gas passes into the raw gas inlet, a destination for it is continuously available. After this step, the post-equalisatlon bed contains second PSA waste gas and may contain unadsorbed product gas if the quantity of second PSA waste gas was insufficient to sweep out all the unadsorbed product gas. If more tha~ this sufficient second PSA waste gas is available, it can be passed into the compressor and raw gas feed inlet, and thereby ~ in the second PSA waste gas will be retained in the process. After the sweep, the post-equalisation bed can be equalised with a fully regenerated, possibly partly repressurised, bed if one is available, or can be depressurised to~waste, possibly stagewise at 2 or more pressure levels.
In the high pressure sweep mode of operation, the waste gas from the second PSA system is fed into a bed that has completed its adsorption step, but has not been equalised, and has been connected to the raw gas inlet line of the first PSA system, possibly by way of a booster compressor recovering the pressure-drop through the bed.
- ,~ ..,: ::: . : ::
"
, . ., : ,: ; ~ . ;, : '- , ' , :
.. : :: - -~8~
Thereby unadsorbed product gas stlll in the bed is swept into the raw gas inlet line and recovered.
Since the bed is at feed pressure, the quantity of waste gas from the second PSA system required is almost double ehat for the above described second mode of intermediate pressure sweep in which almost half the : unadsorbed product gas in the bed is recovered by equalisation. After high pressure sweep the bed can be equalised with a fully regenerated, possibly partly repressurised, bed; such a bed is available if a system of 4 or more beds is used. As an altecnative, the swept bed can be depressurised to waste, possib:Ly stagewise at 2 or more pressure levels; in such a system there is an energy penalty in tha. gas is let down from the highest pressure in the system after having been compressed, but for some users the penalty may be mieigated if there is a use for waste gas at relatlvely high pressure or if there is reason for having only 2 or 3 beds instead of the 4 or more required when equalisation is practised.
In the aforementioned `'sweeping" modes of operation, the waste gas from the second PSA system is preferably passed through the bed being swept co-currently in order to minimise deso~ption of C02. This is more critical for the first mode of intermediate pressure sweep since any desorbed C02 would be adsorbed at the outlet of ehe recipient bed (assuming flow into lts outlet as in equalisation) and thus would contaminate the unadsorbed gas stream fed to the second PSA system in the next adsorp~ion step using that bed.
Alternatively in this mode of intermediate pressure sweep, the swept out gas is received co-currently, and this is preferred.
In the second intermediate, and in the high, pressure sweep modes of operation, any desorption of C02 is less important since this desorbed C02 will be returned to - : :. : :.; :.: .: ~ : -- :. . .: . :. .-~
:' . . ` ' ~;8~28 the raw synthesis gas inlet line.
If sweeping by second PSA waste gas is continued until such waste gas has passed into a bed undergoing repressurisation or into the raw gas feed, there will be an accumulation of MB gases in the combination of the two PSA
sys~ems. The same will occur if second PSA waste gas is used directly in repressurisation.
If the adsorbent is correctly choqen and operated, the increased MB partial pressure may result in increased adsorption, so that such accumulation is limited. Otherwise it may be desirable to maintain a purge of ~B-rich gas at a suitable point in the system.
Especially in the second intermediate pressure, and in the high pressure, sweep modes, wherein the waste gas from the second PSA system is accumulated in the first PSA system under raised pressure, a subsequent depressurisation to purge another bed can be carried out at two or more pressure levels, so as to produce first a waste gas rich in fuel values but lean in C02 and finally a waste gas rich in C02.
It will be appreciated that the waste gas from the second PSA may be used both for sweeping, prior to depressurisation, and also for repressurisation as described above.
In an another alternative, a bed in the first PSA
system that has completed is adsorption duty is swept with a C02-rich gas and the swept-out gas is returned to the raw synthesis gas inlet line. After depressurising the bed to recover C02, the depressurised bed is purged with waste gas from the second PSA system.
The C02-rich gas used for this C02-sweeping is conveniently derived from the C02-rich gas recovered from the depressurisation of another bed. If desired, the required C02-rich gas can be supplied by an autonomous circulation system. More conveniently, the requirement of the recipient ~ ~
:
:
. - ::: .:: .,,:,..,.: . . : .: ::
:: . : . : : .- .
~61~428 bed is provided from the storage capacity of pipework and any reservoirs in the C02 collecting system or, in a suitably designed system, from a bed undergoing desorption of C02-rich gas in the same time period. The C02-rich gas may need to be compressed, depending on the pressure of the gas to be swept out. Often the C02-rich gas will be compressed in the course of use further downstream in a process sequence making for example solid or liquid C02 or urea; thus the gas used in sweeping can conveniently be takea from such a downstream source.
The bed subjected to C02-sweeping has preferably been at least once downwardly pressure-equalised with another bed. In such equalisations, carried out with co-current flow from the bed, the C02 front stays in the bed and thus the gas passed into the recipient bed or beds is H2 + MB gas low in C2 and, since it enters the recipient bed(s) counter-currently, drives back the C02 front in such bed(s). By sweeping after equalisation the pressure of the C02-rich gas need not be so high as would be necessary for sweeping before equalisation.
C02-sweeping is continued preferably co-currently and until the C02 front has moved towards the bed outlet but ramains within the bed. A small C02 content in the swept-out gas is not harmful if that gas is fed to the raw synthesis gas inlet tlme of the first PSA system and then a balance may be struck between the advantage of more complete sweeping and the disadvantage of adding more C02 to the inlet gas.
The effect of the C02-sweeping step is to expel H2 and M3 gases from the void space in the bed and also to desorb H2 and ~B gases. Consequently the gas desorbed in the subsequent depressurisation consists almost entirely of C02 and is very suitable for further processing.
For a C02-sweepino step the C02-rich gas needs to be compressed, for example to 6 to 12 bar abs, but such - ': . ~ ,. . .: -, :
:
~26~2~3 pressures would com~only be needed for the further processing already mentioned. For C02 recovery the depressurisation, which is preferably counter-current, is for example from 6 -12 bar abs and can go down, if desired, to less than 1 bar ; 5 abs., f or example to 0.1 bar abs., depending on the extent to which C02 is required.
.~fter the C02-sweep and the depressurisation with C02-recovery, the bed is purged, preferably counter-currently, with waste gas from the second PSA system. The effect of this purge is to desorb still more C02 since the waste gas from the second PSA system is substantially C02-free and thus subjects the bed to a still lower C02 pressure than would be attained in normal depressurisation to below atmospheric pressure. Since the purge gas is not obtained from another bed in the first PSA system, its supply does not depend on a step occurring in another bed of the first PSA
system; therefore purging need not be rigidly synchronised with steps occurring in other beds.
The purge outlet gas consists mainly of MB gases, a s~all percentage of H2 and a content of C02 depending on the extent to which C02 was recovered in the depressurisation to C2 recovery step. If it is desired to keep down the C02 content Gf the purge outlet gas but very high C02 recovery is not required, the void space gas present after the depressurisation can itself be swept out wholly or partly to waste, by means of the second PSA waste gas to be used as purge gas.
The purging pressure, whatever the C02 content intended in the outlet purge gas, can be superatmospheric, and then the outlet purge gas can be used as a source of power, by heating it and expanding it (preferably with combustion) in a gas turbine; the gas turbine exhaust can supply at least part of the heat required before expansion.
To obtain the required purging pressure the second : - ~ :, ;
''' ., . : :
.
B~ 8 PSA waste gas can if necessary be compressed. However the waste gas may be taken from the second PSA system at superatmospheric pressure, for example in the range 2 - 10 bar abs. and the inlet and outlet ~2/N2 ratios of the second PSA system chosen accordingly.
By the use of a C02-sweep, and purge with waste gas from the second PSA system, a large extent of C02 desorption is possible. It is therefore possible to use adsorber beds in the first PSA system of unconventionally small volume in proportion to the flow rate of gas to be purified.
Alternatively or additionally, aclsorption step times can be unconventionally long, for example 4 - 8 minutes, and thus time is available durlng each adsortion step for several short steps, of which equalisations, sweeps and depressurisations are examples.
Since the C02-sweep step returns H2 to the process, the first PSA system does not rely on multiple pressure equalisations to decrease ~2 loss and therefore need not include a large number of beds. It appears that the most convenient number will be 4 through 6 or possibly 8 if it is desired to have overlapping operation of successive adsorbers.
Whereas it was indicated above that the first PSA
system could advantageously be operated "hot" with inlet temperatures above 40C in order to maximise the C02 content of the stream of recovered C02, where a C02-sweep and second PSA waste gas purge are employed, such higher temperature operation can be avoided.
The adsorbent charged to the PSA beds can be any of those considered suitable for PSA, including silica gels, active carbons and zeolites. The adsorbent for at least the beds of the first PSA system preferably includes a zeolite, since zeolites are capable of adsorbing C02 much more preferentially to the MB gases than the other adsorbents.
- , :
Such specificity is less at the high temperatures that may be used in the first PSA system than at conventional PSA
temperatures, but is fully adequate. When using a high temperature first PSA system, with a zeolite adsorbent, the effect is to produce a C02 adsorption isotherm that resembles the ambient temperature isotherms for active carbon and silica gel. A suitable zeolite is of the A type, for example calcium A (Zeolite A i9 defined in D W Breck's "Zeolite Molecular Sieves" and is available from several manufacturers under different trade names). The following table sets out values of the adsorption constants K, where K _ kg. mols of adsorbed gas per kg of adsorbent pressure in bar abs.
for C02 and N2 and of the ratio KC02/KN2 for a zeolite and an active carbon. The values at 30, 70 and 100C are quoted from page 565 of "Gas Purification" by A L Kohl and S C
Riesenfeld, 3rd ed. 1979 (Gulf Publications, Houston, Texas, USA) and those at 150C are obtained by Clausius Clapeyron-~ype calculation.
Table _ . I
; K, kg. mol kg 1 bar 1 at TC
T = 30T = 70 T = 100 T = 150 Zeolite N2 26 x 10 5 11.9 x 10 5 7.4 x 10-5 3.9 x 10 5 C2 0.1610.041 0.0179 0.0058 ratio 619344 241 148 . . _ Carbon N2 41.5 x 10 5 23.8 x 10 5 16.9 x 10 5 10.7 x 10 5 C2 4.46 x 10-3 1.45 x 10-3 0 73 x 10-3 0.29 x 10-3 ratio 10.7 6.1 4.33 2.71 . . _ Whereas, in the absence of a purge by second PSA
.
. .
:' ~ . . : :
2~3 system is preferably mainly carbon, when such a purge is used that adsorbent is preferably zeolite.
The present invention is of particular use in combinations wherein (a) the C02-containing gas released in the first PSA
system is fed to a C02 purification system such as an adsorptive or liquid-absorption or distillation C02 recovery system; for this purpose the product C02-containing gas can, if desired, be compressed; and/or tb) with or without the above-mentioned C2 concentration, the C02-containing product gas is fed to urea synthesis; and/or (c) unadsorbed gas from the second PSA system is fed to ammonia synthesis, the latter advantageously being the source of ammonia for urea synthesis where, as mentioned above, the C02-containing product gas is used for urea synthesis.
Usually the product gasesj ie C02-product gas released in the first PSA system, and the synthesis ready ammonia synthesis gas produced as the unadsorbed gas stream in the second PSA system have to be compressed before further use.
Since the efficiency of compressors tends to vary : as the molecular weight of the gas being compressed varles it is advantageous, particularly in the case of the second PSA
system to have sufficient beds that at least two beds are on adsorption duty at any one time, but out of phase with one another, in order to minimise fluctuations in the composition of the gas fed to the compressor.
The chemical step preceding the first PSA system is preferably a catalytic shift step preferably decreasing the C0 content to at most 1, preferably und~r 0.5, % V/v on a dry basis. Cooling and liquid water sep~aration are effected before the first PSA system, but the residual water vapour can be removed in the PSA system, without a separate drying A ~
, :; '.; ~: '~:: ,, : ', ;
:: .
~842~3 , operation.
Upstream of shift and water removal the preferred stage is hydrocarbon air-steam reforming ln which catalytic primary steam reforming is effected in indirect heat exchange with catalytic air reforming and the product raw gas has an (H2 + CO)/(~B - CO) molar ratio iu the range 1.25 to 2.5, especially 1.5 to 2.1, a C02 content in the range 10 to 25 V~v, and at least 90% V/v of the ~ is N~.
Preferred embodiments of the invention are shown in the accompanying drawings in which Figure 1 is a block diagram of the overall flow sheet, Figure 2 is the flow sheet of the second PSA
system, and Figures 3 - 10 are flow sheets of the first PSA
system showing various alternative embodiments making use of the waste gas from the sacond PSA
unit as follows:
for final repressurisation (Figure 3);
as a counter-curren~ purge (Figure 4);
as a co-current sweep then counter-current purge (Figure 5);
as a co-current sweep then intermediate co-current repressurisation (Figure 6);
as a co-current intermediate pressure sweep with recycle (Figure 7);
as a co-current high pressure sweep with recycle (Figure 8);
as a counter~current purge in a cycle using a sweep with C02-rich gas to recycle (Figure 9); and as in Figure 9 but for a 6 bed PSA unit (Figure 10) .
In the flow sheets the dotted horizontal lines represent divisions between successive time intervals of the PSA cycle.
: ,..
:
... .
.: :, : - ~ :
: ~ . - :- ~,.
-: ., ~: ,.
4~8 With the 4-bed units of Figures 2 to 9 there are four such intervals labelled Tl to T4 while in the 6-bed unit of Figure 10 there are 6 time intervals Tl to T~.
The connections to the beds are positioned, in the flow sheets, so that gas entering the left hand side (LHS), and/or leaving the right hand side (R~S), of a bed is flowing co-currently. Conversely gas entering the R~S, and/or leaving the L~S, of a bed is flow:Lng counter-currently~
In the flow sheets and ensuing description the following abbreviations are employed:
AD - adsorption duty (co-current) El = intermediate equalisation (co-current - may be omitted).
E2 = final equalisation (co-current - may be the only equalisation as shown in Figures 2 -8) Dl = intermediate depressurisation (co-current -omitted in Figures 5, 9 and 10) D2 - final depressurisation (counter-current) PU = purge (counter-current) SW - sweep (co-current) Rl = initial repressurisation (counter-current) R2 = intermediate repressurisation (counter-current - omitted in Figures 2 - 5, 7 and 8) : 25 R3 = final repressurisation (co-current ) IG = inlet raw synthesis gas W = waste gas from the second PSA system SR = synthesis ready ammonia synthesis ~as U = unadsorbed gas from the first PSA system ~ = unadsorbed gas from the first PSA system after methanation and water removal.
CR = C02-rich gas separated in first PSA system ~IR = C02-lean/MB-rich gas separated in first PSA :
system :~ . .. ,. ., . .. :,.
~2~
P ~ product gas (CR plus MR where these are not taken off separately).
In the process of these embodiments a stream of raw ammonia synthesis gas, IG, from the process sequence of natural gas/steam primary reforming, air secondary reforming, heat exchange between the primary and secondary reforming, catalytic shift, cooling, and water separation, is fed, via line 10 to the inlet gas manifold 12 of a firs~ PSA unit 14, optionally via a heat exchanger 16 where a hot inlet gas, IG, is required. PSA unit 14 has a plurality of beds, eg 4, 6, 8 or 10, of adsorbent and programmeld valve actuators providing an uninterrupted succession of regenerated adsorbent beds and steps of pressure equalisation, depressurisation, purge, and repressurisation with an optional sweep step after adsorption but before depressurisation.
The raw synthesis gas, IG, contains H2, N2, C0, Ar, C~4 and C02 and the PSA unit 14 is effective to give a stream, U, of unadsorbed gas, which is essentially free from C02, leaving PSA unit 14 via manifold 18. PSA unit 14 also gives one or two gas streams containing the C02 removed from the synthesis gas IG. As shown in Figure 1, a C02-lean/~B-rich stream MR (which will contain a little H2) and a C02-rich/~-lean stream CR (which may contain a small amount of H2), leave the PSA unit 14 via manifolds 20 and 22 respectively. Alternatively, as will be described there may be a single product gas stream P containing the C02 and some ~B and a little ~2 leaving PSA unit 14: for convenience of description its manifold is also designated 22.
Where separate MR and CR streams are produced, the MR stream is taken from manifold 20 and may be used as a fuel for a gas turbine (not shown) driving, for example, one or more compressors. The CR stream or P stream, is taken from manifold 22 of PSA unit 14 to C02 recovery steps (not shown).
-, ,,- ~ , - .' :: '~ ~ ' ' ' ' ' ~ , - , ' . ~ .: : ~ ., ~;8~2~3 If the temperature at which PSA unit 14 is operated is above the critical temperature of C02 and high enough to limit adsorption of MB gases, the C02 content of the unadsorbed gas stream, ie stream U, is low, for example 0.5%
V/v or less, but not much of the MB and H2 of the raw synthesis gas are separated in the PSA unit 14.
The unadsorbed gas stream, ie stream U, is fed from manifold 18 to a heat e~changer 24 wherein it is heated and passed to a methanator 26 in which it encounters a supported nickel catalyst. The temperature of the methanation inlet gas is controlled at such a level that both C0 and C02 are metha~ated. The resulting methanated gas is cooled in heat exchanger 24 as the source of heat for heating the gas fed to methanator 26, cooled further in heat exchanger l6 as the source of heat for the raw gas, IG, entering PSA unit 14 where the latter is of the hot type, and finally cooled in cooler/separator 28 wherein liquid water is separated and removed via line 30. From cooler/separator 28, the methanated gas from which water has been separated, ie gas stream M, is fed to the inlet manifold 32 of a second PSA
unit 34. In PSA unit 34, which is operated atJ for e~ample, 30C, the methanated gas ~ is separated to give a waste gas W
containing CH4, N2, and Ar, as its main components, and the :
synthesis ready ammonia synthesis gas SR which has an H2:N2 ratlo within the range 2.5 to 3.1. The waste gas W will contain a little H2. The W and SR gases leave PSA unit 34 via manifolds 36 and 38 respectively.
PSA unit 34 is of the same general type as that of unit 14: a flo~ chart for a 4-bed PSA unit 34 is shown in Figure 2.
During the first time interval Tl, bed A is used for adsorption duty. In this duty the inlet of bed A i3 connected to the inlet manifold 32 for the methanated gas M
and its outlet is connected to the outlet manifold 38 ;
: ., ,,'.: : , .--:
" : :: ' :
supplying the synthesis ready ammonia synthesis gas SR.
In the second time interval, wherein bed D is used for the adsorption duty, bed A is first subjected to a pressure equalisation step E2 wherein the inlet to bed A is closed and its outlet is connected to the outlet of bed B
which has just been purged in step PU. When the pressures in beds A and B are equal, the outlet of bed A is disconnected from that of bed B and is connected to the outlet of bed C
whose inlet is connected to the ~aste gas manifold 36. Bed A
is thus subjected to a co-current depressurisation Dl wherein the gas in bed A passes co-currently out of bed A to bed C
through which it passes, counter-currently, as a purge PU.
In the third time interval T3, wherein bed B is : used for adsorption duty, the outlet of bed A is closed and its inlet ls connected to the waste gas manifold 36: the bad is thus subjected to a counter-current depressurisation D2.
The outlet of bed A is then connected to the outlet of bed D, which has just undergone the equalisation step E2, so that the gas released during the co-current depressurisation step Dl of bed D flows counter-currently through bed A as a purge, ie step PU.
In the fourth time interval T4, wherein bed C is used for adsorption duty, the inlet of bed A is closed and its outlet is connected to the outle of bed B (which has just finished its adsorption duty AD) so that the gas released from bed B during its equalisation step E2 effects counter-current repressurisation ~1 of bed A. Finally the outlet of bed A is closed and its inlet connected to the SR gas outlet manifold 38 (in Figure 2, since this manifold 38 is being supplied, in this time interval, with gas from bed C, the connection to bed A is shown from the SR outlet time fro~ bed C), to effect co-current repressurisation, step R3.
Bed A is thus ready to recommence adsorption duty.
As shown in Figure 2 the other beds go through the same cycle .,, ' :: :, : : : . -,: : :.:
:
~. ~'' ,', , ~%6~3~28 but out of phase with one another.
Some or all of ehe waste gas W from PSA unit 34 ~ay be used as a fuel in a furnace or gas turbine, possibly in admixture with the MR (or P) gas stream from PSA unit 14.
~owever, it i5 preferrred that at least part of the waste gas W from PSA unit 34 is fed back to PSA unit 14 via line 40 in Figure 1. Unless PSA unit 34 is operated under conditions giving a waste gas W at a suffic:Lently hlgh pressure for its intended use in PSA unit 14, it will generally be necessary to compress waste gas W in compressor 42 before it is supplied to the waste gas inlet manifold 44 of PSA unit 14.
Various uses of the waste gas W from PSA unit 34 in the PSA unit 14 are shown in the flow sheets of Figures 3 to 10.
The cycle of the PSA unit 14 is similar to that described above for PSA unit 34. However, as shown in Figures 3 - 5, 9, and 10, the gas released from the bed in the final, counter-current, depressurisation step D2, and the purge step PU may bs collected separately to give, respectively a C02-rich gas stream CR and a MB-rich gas stream ~R. Such separate collection of CR and MR can also be adopted, if desired, in the embodiments of Figures 6 - 8.
Alternatively, in the arrangements of Figures 3 - 5, the final depressurisation and purge gases can be collected as a single stream of product gas, P, as show~ in the embodiments of Figures 6 - 8.
In the embodiment of Figure 3, the waste gas W from PSA unit 34 is used for the co-current repressurisation step R3 instead of using gas from the manifold 18 supplied with gas from a bed undergoing adsorption duty AD. This has the advantage that H2 otherwise lost in the waste gas W from PSA
unit 34 is returned to the system.
Alternatively, in an embodiment not illustrated, this final rapressurisation, or at least the initial part ,:
' .~ . " : ' "' . "'' ' .
thereof, may be affected with part of the SR gas leaving PSA
unit 34 instead of with the waste gas: this has the advantages that the amount of MB returned to YSA unit 14 in this repressurisation step R3 is minimised and also that less, if any, compression of this returned gas is required.
In the embodiment of Ei~ure 4 the waste gas W from PSA unit 34 is used as the purge gas: thus the gas leaving the bed undergoing the co-current depressurisation stap Dl in PSA unit 14 is taken directly to the ~R gas outlet manifold 20 while the waste gas W from PSA unit 34 is fed to the outlet of the bed of PSA unit 14 undergoing the purge step PU. This has the advantage that the MR gas has a lower C02 content than in the embodiment of Figure 3. Consequently in this embodiment generally the CR and ~R gases will be collected separately, as shown, rather than combined to give a single product gas P.
In the embodiment of Figure 5, the co-current depressurisation step Dl is replaced by a sweep step SW
wherein the waste gas W from PSA unit 34 is fed to the inlet of a bed that has undergone the equalisation step E2. The outlet of the bed undergoing the sweep step SW is connected, .
as in the embodiment of Figure 3, to the outlet of a bed undergoing the counter-current purge PU. This has the advantage that the CR gas produced in the counter-current depressurisation step D2 following the sweep step SW will contain virtually no hydrogen.
In the embodiments of Figures 6 and 7 a co-current sweep step SW using the waste gas W from PSA unit 34 is interposed between the equalisation step E2 and the co-current depressurisation step Dl. In the Figure 6 embodimentthe gas swept from the bed is used for an intermediate, co-current, repressurisation step R2. This has the advantage that H2 in the waste gas W is retu~ned to the system.
However it will be seen that the pressure at which the waste ,. ~ , - , ~:
, :
4~
gas has to be supplied increases, as in the embodiment of Figure 3 as repressurisation proceeds. Thus during the -course of the sweep the pressure has to increase from that remaining after equalisation towards the adsorption pressure.
In the embodiment of ~igure 7 the gas swept from the bed is returned, via a compressor 46, to the inlet gas manifold 12.
This has the advantage of returnLng the ~2 as in the Figure 6 embodiment with the further advantage that, unlike the Figure 3 and Figure 6 embodiments, the compressor 42 in the waste gas W inlet line 40 is not subject to a fluctuating load.
In the embodiment of F:Lgure 8 the cycle is similar to that of Figure 7 except that thé sweep step SW is before, instead of after, the equalisation step E2. This is more advantageous than the system of Figure 7 where the waste gas W from PSA unit 34 is at a higher pressure.
In the embodiment of Figure 9 the waste gas W from PSA unit 34 is used as purge gas, as in the Figure 4 embodiment, but also a co-current sweep step SW is employed in place of the co-current depressurisation with the swept gas being returned to the inlet gas manifold 12 via compressor 46 as in the Figure 7 embodiment. In this case the gas used for the sweep step SW is C02-rich gas CR taken, via compressor 48, from the manifold 50 supplied with gas from the counter-current depressurisation step D2. Although in this embodiment there is no bed undergoing step D2 at the time of the sweep step SW, a sufficient reservoir of C~ gas wil~ probably be availabIe in the pipework and/or a small reservoir vessel can be provided. Alternatively the CR gas used for the sweep can be gas from external processing of the CR gas exported from PSA unit 14. By means of this C02-rich gas sweep, the CR gas produced in the subsequent counter-curren~ depressurisation will be particularly pure.
In this embodiment two equalisation steps E2 and E1 are shown: before the final equallsation E2 in which the : ~ :
-:.. ,: ;: :
.
~2~342~
released gas effects initial counter-current repressurisation Rl of a bed that has been purged, an intermediate equalisation step E1 is employed for intermediate counter-current repressurisation R2 of another bed that has already been subjected to the initial repressurisation Rl by gas released from a final equalisation step E2. Such multiple equalisations may also be adopted where appropriate in the embodiments of Figures 3 to 8.
In the embodiment of Figure 9 each time interval is typically about 6 minutes. Referring to bed A, at the end of interval Tl, the C02 adsorption front is well short of the bed outlet. In intervcal T2 the two equalisation steps are first effected with the result that bed B has been repressurised to over half the adsorption pressure and bed C
to over one quarter of ths adsorption pressure. Beds B and C
have had such C02 adsorption fronts as remained moved back towards the bed inlets. Up to three quarters of the ~2 left in bed A at the end of the adsorption step, ie in the void space and as adsorbed gas, has been returned to the process.
In the sweep step SW in interval T2 the gas is allowed to flow until most of the H2 and ~B gas has been swept out of the bed, but is stopped before significant C02 breaks through. The gas in bed A is now almost exclusively C02. In the counter-current depressurisation step D2 also in interval T2, the major proportion of the gas is exported as CR gas and only a minor proportion is used for the sweep. It will be appreciated that the CR export can be taken from the mani~old 50 before compressor 48 if desired: however since the exported CR gas will normally have to be compressed in its subsequent processing, it is often more convenient to effect the compression of all the gas released in the counter-current depressurisation step D2 and to export the CR
gas, as shown, from manifold 22 after compressor 48. After the counter-current depressurisation step D2, the purge step , ~, .
'''`' .
-. ...
~26~42~3 is effected to purge the remaining C02 from the bed: since the time remaining in interval T2 is unlikely to be sufficient to effect completion of the purge, ~he purge can continue, as shown, into interval T3.
In the embodiment of Figure 10, a 6 bed arrangement for effecting the cycle of Figure 9 is shown. There it is seen that a bed is undergoing counter-current depressurisation D2 while the sweep SW is taking place and so no reservoir for CR gas is required. Since in the Figure 10 embodiment each time interval only includes 1 or 2 steps, whereas in the Figure 9 embodiment: up to 5 steps were required in each time interval, the time intervals ln the Figure 10 embodiment can, if desired, be shorter than those of the Figure 9 embodiment.
In a further embodiment, not shown in the Figures, the arrangement of Figure 9 is modified by the omission of the equalisation steps El and E2 and the corresponding initial and intermediate repressurisation steps Rl and R2.
In this embodiment the final repressurisation R3 is preferably using some of the synthesis ready ammonia synthesis gas from the second PSA unit. This embodiment has the advantage of reducing the number of beds required in the first PSA system, in some cases to as few as two, at the expense of increased power requirement Pg in compressors 46 and 48.
- ,::. :, ,. .,.: . . - ::: : : ' ~, :
Claims (10)
1. A process for the production of ammonia synthesis gas from a shifted gas containing (a) hydrogen, (b) carbon dioxide, (c) nitrogen in an excess of that desired in the ammonia synthesis gas, (d) carbon monoxide, but in an amount of not more than 1% v/v , and, optionally, (e) methane and/or argon; said process comprising:
A) subjecting the shifted gas to a first pressure swing adsorption system having a plurality of adsorbent beds each of which undergoes a pressure swing adsorption cycle effective to separate carbon dioxide, thereby producing a first unadsorbed product gas having a decreased carbon dioxide content, but a hydrogen:nitrogen molar ratio below that of the desired ammonia synthesis gas;
B) subjecting the first unadsorbed product gas to catalytic methanation to convert the carbon monoxide, and any residual carbon dioxide, in the first unadsorbed product gas to methane, thereby producing a methanated gas; and C) removing nitrogen from the methanated gas so as to increase the hydrogen:nitrogen ratio to that of the desired ammonia synthesis gas by subjecting the methanated gas to a second pressure swing adsorption system having a plurality of adsorbent beds each of which undergoes a pressure swing adsorption cycle effective to separate the requisite amount of nitrogen from the methanated gas as a waste gas stream, thereby leaving as the unadsorbed gas the desired ammonia synthesis gas.
A) subjecting the shifted gas to a first pressure swing adsorption system having a plurality of adsorbent beds each of which undergoes a pressure swing adsorption cycle effective to separate carbon dioxide, thereby producing a first unadsorbed product gas having a decreased carbon dioxide content, but a hydrogen:nitrogen molar ratio below that of the desired ammonia synthesis gas;
B) subjecting the first unadsorbed product gas to catalytic methanation to convert the carbon monoxide, and any residual carbon dioxide, in the first unadsorbed product gas to methane, thereby producing a methanated gas; and C) removing nitrogen from the methanated gas so as to increase the hydrogen:nitrogen ratio to that of the desired ammonia synthesis gas by subjecting the methanated gas to a second pressure swing adsorption system having a plurality of adsorbent beds each of which undergoes a pressure swing adsorption cycle effective to separate the requisite amount of nitrogen from the methanated gas as a waste gas stream, thereby leaving as the unadsorbed gas the desired ammonia synthesis gas.
2. A process according to Claim 1 wherein, in the shifted gas, the ratio of the sum, of the molar proportion of hydrogen and the molar proportion of carbon monoxide to the sum of the molar proportions of nitrogen, carbon monoxide, methane, and argon, is in the range 1.25 to 2.5, the carbon dioxide content is in the range 10 to 25% v/v, and at least 90% v/v of the total of nitrogen, carbon monoxide, argon, and methane, is nitrogen, and in the second pressure swing adsorption system, the amount of nitrogen removed is such as to give an ammonia synthesis gas having a hydrogen to nitrogen molar ratio in the range 2.5 to 3.1.
3. A process according to Claim 1 or Claim 2 wherein the first pressure swing adsorption system cycle includes a counter-current depressurisation step followed by a purge step and the gases released in said counter-current depressurisation and purge steps are collected separately as, respectively, a carbon dioxide-rich gas stream and a carbon dioxide-lean gas stream.
4. A process according to Claim 1 wherein at least part of the waste gas stream from the second pressure swing adsorption system is fed back to the first pressure swing adsorption system and is used therein as the gas employed for a sweeping, purging, and/or repressurisation operation therein.
5. A process according to Claim 4 wherein the first pressure swing adsorption system cycle includes a co-current sweep step and waste gas from the second pressure swing adsorption system is used in the sweep step of the first pressure swing adsorption system as sweep gas to sweep residual gas from a bed that has finished its adsorption step, but has not been depressurised, into another bed.
6. A process according to Claim 5 wherein the sweeping of the first pressure swing adsorption system bed is effected co-currently after a pressure equalisation step and the residual gas is swept into a bed undergoing repressurisation.
7. A process according to Claim 5 wherein the first pressure swing adsorption system cycle includes a purge step and the residual gas swept from the bed by the sweep step is used as the purge gas in the purge step operated on another bed of the first pressure swing adsorption system.
8. A process according to Claim 4 wherein the first pressure swing adsorptiono system cycle includes a sweep step and waste gas from the second pressure swing adsorption system is used in the first pressure swing adsorption system as sweep gas to sweep residual gas from a bed that has finished its adsorption step, but has not been depressurised, and said residual gas swept from the bed is recycled by adding it to the shifted gas fed to the first pressure swing adsorption system.
9. A process according to Claim 8 wherein the sweep step is effected after a pressure equalisation step.
10. A process according to Claim 4 wherein the first pressure swing adsorption system cycle includes a sweep step and a purge step, and carbon dioxide-rich gas released in a counter-current depressurisation step of the forst pressure swing adsorption system cycle is used, after compression, as the sweep gas to sweep residual gas from the bed after a pressure equalisationb step, to a compressor from whence it is recycled by adding it to the shifted gas fed to the first pressure swing adsorption system, and waste gas from the second pressure swing adsorption system is used as the purge gas in the purge step of the first pressure swing adsorption system cycle.
Applications Claiming Priority (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8426665 | 1984-10-22 | ||
| GB848426665A GB8426665D0 (en) | 1984-10-22 | 1984-10-22 | Gas separation |
| GB848429317A GB8429317D0 (en) | 1984-11-20 | 1984-11-20 | Gas separation |
| GB8429317 | 1984-11-20 | ||
| GB8432487 | 1984-12-21 | ||
| GB848432487A GB8432487D0 (en) | 1983-12-22 | 1984-12-21 | Liquid detergent compositions gas separation |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1268428A true CA1268428A (en) | 1990-05-01 |
Family
ID=27262488
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000493160A Expired - Fee Related CA1268428A (en) | 1984-10-22 | 1985-10-17 | Gas separation |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1268428A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2992307A1 (en) * | 2012-06-25 | 2013-12-27 | Air Liquide | PROCESS AND INSTALLATION FOR THE COMBINED PRODUCTION OF AMMONIA SYNTHESIS GAS AND CARBON DIOXIDE |
| CN110643404B (en) * | 2019-09-23 | 2024-04-09 | 山东红枫环境能源有限公司 | Two-stage series pressure swing adsorption biogas purification system capable of improving methane recovery rate |
-
1985
- 1985-10-17 CA CA000493160A patent/CA1268428A/en not_active Expired - Fee Related
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2992307A1 (en) * | 2012-06-25 | 2013-12-27 | Air Liquide | PROCESS AND INSTALLATION FOR THE COMBINED PRODUCTION OF AMMONIA SYNTHESIS GAS AND CARBON DIOXIDE |
| WO2014001672A1 (en) * | 2012-06-25 | 2014-01-03 | L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude | Method and installation for the combined production of ammonia synthesis gas and carbon dioxide |
| US9206041B2 (en) | 2012-06-25 | 2015-12-08 | L'Air Liquide, Société Anonyme pour l'Etude et l'Exploitation des Procédés George Claude | Method and installation for the combined production of ammonia synthesis gas and carbon dioxide |
| CN110643404B (en) * | 2019-09-23 | 2024-04-09 | 山东红枫环境能源有限公司 | Two-stage series pressure swing adsorption biogas purification system capable of improving methane recovery rate |
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