EP2104547A2

EP2104547A2 - Purification of a h2/co mixture with heater-skin temperature control

Info

Publication number: EP2104547A2
Application number: EP07871905A
Authority: EP
Inventors: Christian Monereau; Guillaume Rodrigues; Simon Jallais
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2006-12-18
Filing date: 2007-12-11
Publication date: 2009-09-30
Also published as: US20100031819A1; FR2909898B1; WO2008087312A3; CN101563145A; US8221526B2; WO2008087312A2; FR2909898A1; CN101563145B

Abstract

The invention relates to a method for purifying or separating a supply gas flow containing at least one impurity, in which: a) said supply gas flow is contacted with a first adsorbent for the adsorption-removal of at least one said impurity; b) recovering said purified or separated gas; c) heating a regeneration gas containing at least hydrogen (H2) and carbon monoxide (CO) using a heater having a skin temperature (T1) of between 150°C and 200°C during the gas heating phase; and d) periodically regenerating the adsorbent of step a) with the regeneration gas heated during step c) at a regeneration temperature (T2) such that: T2 = T1 - ΔT with 5°C < ΔT < 50°C.

Description

Purification of a H ₂ / CO mixture with control of the skin temperature of the heater

The invention relates to a process for the adsorption purification of a gaseous mixture rich in hydrogen and carbon monoxide, commonly known as H2 / CO mixture or synthesis gas, before its cryogenic treatment in order to produce a rich fraction. CO, and / or one or more mixtures of H2 / CO of determined content, such as for example a mixture of 50% mole H2 / 50% mole CO. and usually a hydrogen-rich fraction.

The synthesis gas type mixtures can be obtained in several ways, in particular: by steam or CO2 reforming, by partial oxidation, by mixed processes, such as the ATR (Auto Thermal Reforming) process which is a combination of steam reforming and partial oxidation, from gases, such as methane or ethane, by gasification of coal, or recovered as waste gas downstream of acetylene production units. In addition to hydrogen and carbon monoxide as the main compounds, many impurities such as carbon dioxide, water or methanol are often part of the synthesis gas.

Among the purification processes, the TSA type method (Swing Adsorption Temperature) is a cyclic process in which each of the adsorbers alternates adsorption stages during which the impurities are retained in the adsorbent and regeneration steps during which a heating phase is used in particular to extract the impurities from the adsorbent. The typical operating cycle of this type of unit is described in WO-A-03/049839.

The units of the TSA-type purification processes are generally dimensioned so as to obtain a synthesis gas of cryogenic quality, ie such that, during the cooling of said synthesis gas in the cold box, the possible deposits of impurities are sufficiently weak to ensure satisfactory operation of said box cold for several years, therefore without clogging or thermal deterioration of the exchange line and without risk to the safety of equipment.

This results in a residual CO2 content generally of the order of 0.1 ppm maximum and even lower levels in the ppb for other impurities.

In order to limit the interventions on these purification units, they are also dimensioned with enough initial margins to ensure correct operation for several years without having to replace the adsorbents.

Despite all the precautions, it appeared that the life of these units was significantly shorter than originally planned.

In normal operation, a CO ₂ analyzer ensures the purity of the gas produced. It makes it possible to modify the cycle, for example to shorten the adsorption phase, if detecting premature CO2 breakthroughs linked to a degradation of the performance of the purification unit as mentioned above. The fact remains that despite these precautions, we note after a few years of operation, a degradation of the separation performance of the cold box ensuring the cryogenic separation of the synthesis gas.

This lack of performance is attributed to a degradation of heat exchange by solid deposition on the plates of the exchangers. A shutdown of the unit and its warming (defrosting) solves the problem but of course a significant cost if it is not a stop programmed in advance. Given the margins on the exchangers taken during the design of the cold boxes, these effects are only felt after a relatively long operating period, greater than the year, more generally of the order of 2 or 3 years. This does not make it possible to know if the entrainment of traces of impurities by the purified synthesis gas, a priori of water and CO2, to the cold box occurs after more than one year of service, after several months or only a few weeks of operation.

This degradation has been reported to be due to chemical reactions between the adsorbent and the adsorbate and / or reactions between synthesis gas components promoted by the adsorbent. The reactivity of the H2 / CO mixtures is indeed well known at high temperature, but document US-A-5,897,686 teaches that several reactions occur during the repressurization phase of the purification, which is a substep of the regeneration. It mentions in particular the two reactions: - methanation: CO + 3H ₂ → CH ₄ + H ₂ O the Boudouard reaction: 2 CO ^~~ ^ C + CO ₂

According to this document, the problem encountered is related to the formation of water in the adsorbent and the recommended solution is to add at the adsorber head a bed of sieve 3A which does not adsorb CO avoids the formation in -situ of the said water. This document recommends a regeneration temperature of between 100 ^° C. and 400 ^° C., which classically corresponds to a skin temperature of the heater of the order of 150 ^° C. to 450 ^° C.

Some chemical reactions may also be catalyzed by secondary component deposits on the adsorbent surface. Deposits of metals such as iron, nickel, copper, facilitate the reactions mentioned above. For some of them, their origin is due to the decomposition of metal-carbonyls formed upstream of the purification.

Progressive stocking of the adsorbents by traces of impurities that are not or poorly regenerable is also a plausible hypothesis, given the very high number of secondary reaction products that can be produced in synthetic reactors, from coal or natural gas used as raw material. or be involved in upstream prepurification processes, such as methanol washing or amine washing.

The document WO-A-2006/034765 describes a process for purifying a gas stream rich in carbon monoxide and hydrogen, in which the gas stream is brought into contact with an adsorption layer containing a silica gel. and the adsorption layer is regenerated with a gas whose temperature is between 70 ^° C. and 150 ^° C., which normally corresponds to a skin temperature of the heater of the order of 150 ^° C. to 200/250 ^° C.

The skin temperature of the heater is defined as the temperature at which the regeneration gas is subjected by passing through the heater, that is to say at the temperature of the exchange surface in contact with the gas. It is known, moreover, that for a given thermal power (Q) expressed for example in Kcal / h, the exchange surface to be installed (S) is inversely proportional to the temperature difference ΔT between the skin temperature Ti of the heating surface and the temperature of the regeneration gas T 2 From this, it is easily understood that to reduce the necessary exchange surface, and therefore the investment, it is necessary to use a skin temperature Ti as high as possible .

From there, it is common to use in the state of the art a skin temperature T ₁ , such that Ti = T2 + ΔT with ΔT> 50 ⁰ C and preferably ΔT of the order of 100 ⁰ C.

In a refinery, a chemical or petrochemical plant, to heat a fluid at a temperature of 170 ⁰ C, it is conventional to use vapors 250/270 ⁰ C or more.

According to the teaching of document WO-A-2006/034765, the claimed process makes it possible to limit the formation of formic acid and to extend the life of the adsorbents of said purification.

However, several chemical reactions occur during heating, which is a substep of regeneration.

Despite all these poisoning hypotheses, the main reason for introducing impurities into the cold box has not yet been clearly identified. From there, one of the problems is to provide a cryogenic quality synthesis gas without having to intervene prematurely on the purification units and / or on the cold box by proposing an effective process for purifying a mixture H2 / CO containing at least one impurity, so as to avoid or minimize parasitic reactions.

The solution of the invention is then a process for purifying or separating a feed gas stream containing at least one impurity, wherein: a) said feed gas stream is brought into contact with a first adsorbent to remove by adsorption at least said impurity, b) recovering said purified or separated gas, c) a regeneration gas containing at least hydrogen (H ₂ ) and carbon monoxide (CO) is heated by means of a heater whose skin temperature (Ti) is between 150 ^° C. and 200 ^° C. C during the heating phase of the gas, and d) periodically regenerating the adsorbent of step a) with the regeneration gas heated in step c) at a regeneration temperature (T 2) such that:

T ₂ = Ti - AT with 5 ° C <ΔT <50 ⁰ C

Depending on the case, the method according to the invention may have the following characteristics: the regeneration temperature (T ₂ ) is such that the temperature difference (ΔT) is between 5 and 40 ^° C., preferably between 5 and 40 ^° C. 25 ⁰ C; the skin temperature (Ti) of the heater is equal to or less than 190 ^° C. and in that the regeneration temperature (T ₂ ) at the inlet of the adsorber is equal to or greater than 150 ^° C. the skin temperature Ti is equal to or lower than 185 ° C, preferably equal to or lower than 175 ° C, and in that the regeneration temperature T ₂ at the inlet of the adsorber is equal to or greater than 135 ° C, preferably equal to or greater than

150 ⁰ C; the regeneration gas further contains methane (CH ₄ ) and / or nitrogen (N ₂ ); the regeneration heater is a steam heater, the pressure of said vapor being less than 15 bars effective, preferably of the order of 8 to 12 bars; by the term "effective" it is emphasized that this pressure has been measured by taking the earth's atmospheric pressure as zero, the steam used in the regeneration heater is obtained by expansion of a higher pressure steam, - the regeneration heater is a electric heater equipped with a skin temperature control means, the feed gas contains at least hydrogen (H ₂ ) and carbon monoxide (CO), the hydrogen content of the feed gas is between about 30 and 75 mol% and that the carbon monoxide content is between about

25 and 60 mol%; at least one impurity of the feed gas stream is carbon dioxide

CO ₂ ; at least one impurity of the feed gas stream belongs to the group formed by water and alcohols, in particular methanol; the first adsorbent of step a) contains activated alumina and / or silica gel and / or activated charcoal; the activated alumina and / or the silica gel and / or the activated carbon are arranged in successive layers in any order or intimately mixed within at least one adsorption bed; - Activated carbon is an activated carbon treated to adsorb more specifically secondary impurities selected from nitrogen oxides, sulfur compounds, amines and their decomposition products; said feed gas stream is contacted with a second adsorbent comprising zeolite; the zeolite is chosen from zeolites of type X, LSX, 4A or 5A; the partial pressure of CO in the regeneration gas heated in step c) is less than 2 bars absolute, preferably less than 1 bar absolute, more preferably equal to or less than 0.5 bar absolute; the feed gas is obtained by steam reforming, by partial oxidation, by gasification of coal or residues, or by a mixed process; by mixed process is meant a combination of steam reforming and partial oxidation; the feed gas undergoes pretreatment, such as washing with amines or methanol, before being purified.

The invention will now be described in more detail with reference to the appended FIG.

Figure 1 depicts a purification unit for carrying out the method according to the invention. The synthesis gas resulting from an amine wash 10 is directed via the valve 21 - valve 22 closed - to the adsorber 11 consisting of a bed of activated alumina 110 followed by a bed of zeolite 111 in which are respectively retained the water and CO2 contained in the synthesis gas and the gas thus purified is sent to the cold box 60 via the valve 31-valve 32 closed and then is introduced into the cryogenic exchanger principal 70.

The regeneration gas 30, a hydrogen-rich fraction containing CO and / or CH4, is heated during the heating phase through the steam heater 80 by means of high or medium pressure steam 50. The inlet temperature in the adsorber 12 is regulated by means of a temperature probe 13 and a by-pass circuit of the exchanger controlled by the valve 44. At the end of the heating step, the valve 43 is closed and the Cold regeneration gas is directed to the adsorber 12 starting to cool the screen bed 121 freed of previously adsorbed CO2 while pushing the residual heat front through the activated alumina bed 120. A temperature probe 14 on the circuit regeneration gas evacuation 40 makes it possible to control the smooth running of the heating and cooling steps.

The details of the control of the unit as well as the transient stages and the corresponding circuits allowing a stabilized operation of the units upstream and downstream of the purification unit are not described here for the sake of simplification.

More generally, concerning the main constituents of synthesis gases that are conventionally treated in units of this type, the hydrogen content remains approximately in the range 30 to 75 mol% and the carbon monoxide in the range 25 to 75 mol%. at 60 mol%.

With regard to the main impurities, the carbon dioxide (CO2) can vary between approximately 5 and 500 molar ppm. If the decarbonation is carried out by washing with amines, the synthesis gas is moreover normally saturated with water. In the case of cryogenic washing with alcohols, it is generally possible to find between 20 and 500 molar ppm of residual CO2. Of the alcohols, methanol is the most common impurity.

The pressure of the synthesis gas is generally between 10 and 70 bar, many units however operating between 15 and 50 bar.

The temperature of the feed gas is in the range 5 to 50 ^° C., more generally between 15 and 40 ^° C. in the case of an amine wash and in the range -70 to -20 ^° C. after a washing with alcohols , usually methanol. In the latter case, it is also possible to heat the synthesis gas and to purify at room temperature. The choice is not dictated by purification alone but by global thermal balances around the entire washing, purification, cold box.

The flow rates of synthesis gas to be purified can range from a few hundred NmVh to several hundred thousand Nm / h. According to the invention, it has been demonstrated that the skin temperature of the heater plays an essential role in the introduction of impurities into the cold box.

Thus, it has been demonstrated by the inventors of the present invention that the skin temperature of the regeneration heater must be less than 200 ^° C., preferably less than or equal to 175 ° C. so as not to create traces of moisture in the regeneration heater. the regeneration gas, and secondly, the regeneration gas must have at the inlet adsorbers a sufficient temperature, greater than 130 ⁰ C, preferably greater than 140 ⁰ C, more preferably of the order of 150 ⁰ C .

Indeed, traces of moisture are likely to trap on the adsorbents and reduce their adsorption capacity. On the other hand, the use of relatively low temperature level to regenerate the adsorbents is not conducive to the desorption of impurities.

More precisely, if a steam temperature of 200 ^° C. or slightly less is to be adopted, taking into account the availability of the vapor and condensate networks, it will be necessary to regenerate at 150 ^° C. or more taking into account the probable presence of traces of humidity at the level of a few tens of ppb in the regeneration gas.

If we have a steam, less hot, of the order of 175 ° C for example, we can afford to regenerate at about 130 to 140 ⁰ C because the regeneration gas will be dry.

Given the heat losses, means for regulating the temperature of the regeneration gas, it can be seen that the only way to ensure a good regeneration of the purification unit, that is to say to regenerate with a level of sufficient temperature and with a dry or substantially dry gas is to use a small temperature difference between the skin temperature Ti and the regeneration temperature T ₂ .

It will be necessary to oversize the exchanger to limit the skin temperature Ti to about 175 ° C while having a temperature of the order of 140 to 150 ⁰ C.

It may also be advantageous to use improved insulation. In case of presence in the adsorbent of compounds that can decompose, polymerize or attack the structure of the adsorbent at high temperature, it is recommended to reach the final regeneration temperature in steps, for example one hour at 80 ^° C. at 120 ^° C. for one hour before performing the actual regeneration at 150 ^° C. or more. Alternatively, it is possible to increase the regeneration temperature using a ramp, for example 1 to 20 ^° C per minute.

A steam regeneration heater can be used in the context of the invention. In this case, a means of limiting the maximum temperature is to use a low-pressure steam, in particular a vapor at a pressure of less than or equal to 15 effective bars, preferably 8 to 12 bars.

However, the conventional steam networks found on this type of petrochemical units are at generally higher pressures, beyond 20 bar. This leads to not directly using the available pressure levels but to relax the steam up to 15 effective bars, preferably 8 to 12 bars from a higher pressure.

An electric heater can also be used. In this case the skin temperature of the heater is limited to a maximum temperature by means of a method defined during the design of the heater. It may be a control of the wall temperature of the heating element through one or more temperature probes for adjusting the electrical power. Other means are possible according to the technologies used, the regulation being able to be internal to the equipment (self-regulation) and in this case being part of the specification of the equipment, or external to the equipment, the temperature regulation in question. being treated in the central control system in the same way as the other regulations of the synthesis gas treatment unit.

Although temperature is the main factor in the occurrence of spurious reactions because of their kinetic limitation, a higher CO content may play a negative role. The adsorbent will preferably be chosen to regenerate at least during the heating phase, a gas stream containing less than 2 bar of CO partial pressure, preferably less than 1 bar and more preferably less than 0.5 bar. The regeneration flow rate, expressed as a% of the synthesis gas flow rate, varies greatly depending on the units. It can range from 5% to 50% and more depending on the separation process involved. This is essentially a function of the flow available to perform this regeneration: important if it is a question of a hydrogen fraction but very limited in the case of residuals or purges, on the other hand pressure and temperature conditions of the said purification with, for example, limited needs in the case of low temperature and high pressure of the synthesis gas .

The use of a heater at low temperature leads to preferentially use regeneration rates of at least 10% of the flow rate of the synthesis gas. The regeneration temperature of the adsorber less than 150 ^° C. leads to the use of regenerable adsorbents at this temperature level. It will therefore be preferable to use water or alcohols, generally methanol, activated alumina and / or silica gel and / or activated charcoal known for their lower affinity with water or alcohols than zeolites. X or A. To stop the CO 2, it will be possible conventionally to use a zeolite or a doped activated alumina.

In the presence of identified secondary impurities from the upstream processes, it will be possible to add or use a bed of specific activated carbon as the first adsorbent layer, mixed with another adsorbent or before the zeolite bed. Among these secondary impurities, one can find according to the upstream process, oxides of nitrogen, sulfur products, amines, their products of decomposition, traces of acids such HCN, of mercury.

Claims

claims

A process for purifying or separating a feed gas stream containing at least one impurity, wherein: a) said feed gas stream is contacted with a first adsorbent to adsorb adsorbate at least said impurity, b) recovering said purified or separated gas, c) heating a regeneration gas containing at least hydrogen (H ₂ ) and carbon monoxide (CO) by means of a heater whose skin temperature (T ₁ ) is between 150 ⁰ C and 200 ⁰ C during the heating phase of the gas, and d) periodically regenerates the adsorbent of step a) with the regeneration gas heated in step c) to a regeneration temperature (T ₂ ) such that:

T ₂ ≈ Ti - ΔT with 5 ° C <ΔT <50 ⁰ C

2. Method according to claim 1, characterized in that the regeneration temperature (T ₂ ) is such that the temperature difference (ΔT) is between 5 and 40 ⁰ C, preferably between 5 and 25 ⁰ C.

3. Method according to one of claims 1 or 2, characterized in that the skin temperature (T]) of the heater is equal to or less than 190 ^° C and in that the regeneration temperature (T ₂ ) to the inlet of the adsorber is equal to or greater than 150 ^° C.

4. Method according to one of claims 1 or 2, characterized in that the skin temperature Ti is equal to or less than 185 ° C, preferably equal to or less than 175 ° C, and in that the regeneration temperature T ₂ at the inlet of the adsorber is equal to or greater than 135 ^° C., preferably equal to or greater than 150 ^° C.

5. Method according to one of the preceding claims, characterized in that the regeneration gas further contains methane (CH ₄ ) and / or nitrogen (N ₂ ).

6. Method according to one of the preceding claims, characterized in that the regeneration heater is a steam heater, the pressure of said vapor being less than 15 effective bars, preferably of the order of 8 to 12 bar.

7. Method according to claim 6, characterized in that the steam used in the regeneration heater is obtained by expansion of a higher pressure steam.

8. Method according to one of claims 1 to 5, characterized in that the regeneration heater is an electric heater equipped with a skin temperature control means.

9. Method according to one of the preceding claims, characterized in that the feed gas contains at least hydrogen (H2) and carbon monoxide (CO).

10. Method according to one of the preceding claims, characterized in that the hydrogen content of the feed gas is between about 30 and 75 mol% and that the carbon monoxide content is between about 25 and 60 molar%.

11. Method according to one of the preceding claims, characterized in that at least one impurity of the feed gas stream is CO2 carbon dioxide.

12. Method according to one of the preceding claims, characterized in that at least one impurity of the feed gas stream belongs to the group formed by water and alcohols, in particular methanol.

13. Method according to one of the preceding claims, characterized in that the first adsorbent of step a) contains activated alumina and / or silica gel and / or activated carbon.

14. The method of claim 13, characterized in that the activated alumina and / or silica gel and / or activated carbon are arranged in successive layers in any order or intimately mixed within at least one bed of 'adsorption.

15. Method according to one of claims 13 or 14, characterized in that the active carbon is an activated carbon treated to adsorb more specifically secondary impurities selected from oxides of nitrogen, sulfur compounds, amines and their products. decomposition.

16. Process according to one of the preceding claims, characterized in that said feed gas stream is brought into contact with a second adsorbent comprising zeolite.

17. The method of claim 16, characterized in that the zeolite is selected from X type zeolites, LSX, 4A or 5 A.

18. Method according to one of the preceding claims, characterized in that the partial pressure of CO in the regeneration gas heated in step c) is less than 2 bar absolute, preferably less than 1 bar absolute, more preferably equal or less than 0.5 bar absolute.

19. Method according to one of the preceding claims, characterized in that the feed gas is obtained by steam reforming, by partial oxidation, by gasification of coal or residues, or by a mixed process.

20. Method according to one of the preceding claims, characterized in that the feed gas is pretreated, such as washing with amines or methanol, before being purified.