FR2899890A1 - Pressure swing adsorption process to produce hydrogen-rich gaseous fraction from feed gas containing hydrogen, comprises introducing feed gas through first adsorber at high pressure cycle to eliminate adsorbent in first adsorber - Google Patents

Abstract

The pressure swing adsorption process to produce hydrogen-rich gaseous fraction from a feed gas containing hydrogen, comprises introducing the feed gas through a first adsorber at a high pressure cycle to eliminate adsorbent in the first adsorber to produce the hydrogen-rich gaseous fraction, depressurizing the first adsorber to recover the hydrogen-rich gaseous fraction, eluting the first adsorber by an elution gas rich in hydrogen, and repressurizing the first adsorber with the generated gaseous fraction obtained from the depressurizing step. The pressure swing adsorption process to produce hydrogen-rich gaseous fraction from a feed gas containing hydrogen, comprises introducing the feed gas through a first adsorber at a high pressure cycle to eliminate adsorbent in the first adsorber to produce the hydrogen-rich gaseous fraction, depressurizing the first adsorber to recover the hydrogen-rich gaseous fraction, eluting the first adsorber by an elution gas rich in hydrogen, repressurizing the first adsorber with the generated gaseous fraction obtained from the depressurizing step and by feed gas and/or gas rich in hydrogen, and eliminating impurities from an adsorbent material. The depressurization is carried out against the first adsorber until a low pressure cycle, which is lower than the high pressure cycle. The elution gas rich in hydrogen is formed by the pressure swing adsorption process in which secondly formed hydrogen rich gas has an elution ratio of less than 0.5. The elution of the first adsorber is carried out through the fraction of the gas rich in hydrogen from the external gas source by vehicle channelization of a purge gas from (petro) chemical or oil unit, such as oil refinery. The hydrogen content present in the elution gas rich in hydrogen, which is obtained from the pressure swing adsorption process is less than the hydrogen content present in the feed gas introduction step. Forming a part of the elution gas is deduced to obtain a constant production of hydrogen. The step of eluting the first adsorber is based on the gaseous fraction enriched in hydrogen obtained from a permeation membrane unit. The pressure swing adsorption process has 4-16 adsorbers. The produced hydrogen-rich gaseous fraction contains 98-99.5 mol.% purity.

Description

The invention relates to a PSA H2 process with an elution step in the cycle

  production / purification of the gas mixture to be treated, the elution being carried out with a hydrogen-rich stream derived at least in part from a source external to the PSA process itself.

  Gas separation or purification processes by adsorption are widely used in industry. Thus, the production of high purity hydrogen (> 98 mol%) is usually carried out by PSA (Pressure Swing Adsorption) process which is a gas separation / purification process by adsorption of one or more constituents of the gas on a or more adsorbents subjected to cycles of production with implementation of pressure variations. PSA H2 is commonly referred to as such a process. Generally speaking, in a unit employing such a process PSA H2, during a first stage of the so-called adsorption stage cycle, a gaseous mixture to be treated, also called a feed gas, containing hydrogen and one or several other gaseous components pass through, at the high pressure of the production cycle, one or more layers of adsorbent materials which preferentially retain all or part of the other constituents of the mixture while allowing the hydrogen to pass through them. This adsorption step will be designated step A below. The compounds adsorbed during step A are then extracted from the adsorbents, during a so-called regeneration stage step or even desorption stage, by operating a reduction of the gas pressure exerted on the adsorbent materials until the pressure is reached. cycle, also known as regeneration pressure or desorption pressure. The adsorbents are usually placed in adsorption vessels called adsorbers in which the circulation of the pressurized gases may be in particular axial or radial. In particular, the adsorbent beds comprise particles of activated alumina type adsorbents, silica gel, active charcoal and zeolite which make it possible to adsorb various constituents of the hydrogen-based feed gas, said adsorbents being distributed in layers. successive in each adsorber. A PSA H2 unit usually has several adsorbers, typically from 3 to 16 adsorbers. The very principle of a PSA H2 process being to cyclically chain such adsorption and regeneration / desorption steps, it is then immediately understood that each adsorber is successively subjected to the various stages of the cycle.

  There are currently many PSA H2 cycles, on the one hand, depending on whether one wishes to favor the investment or the recovery of hydrogen and, on the other hand, according to the production capacity of the PSA H2 unit which can range from a few tens of m3 / h to several hundred thousand m3 / h. As a result, in practice, most of the PSA H2 processes used industrially comprise a series of additional elementary steps making it possible to carry out a complete pressure cycle with the desired efficiency and / or corresponding to the PSA H2 unit considered. Thus, as the case may be, a PSA H2 cycle to which each adsorber is subjected may also include: one or more balancing steps (Eq) of pressure making it possible to recover a portion of the hydrogen contained in the adsorbent and the dead volumes of each adsorber at the end of adsorption (ie an adsorber entering the depressurization phase), and to valorise its pressure by recompressing another adsorber previously regenerated and being at a lower pressure, - a production step (PP) of gas, Elution during which a complementary fraction of hydrogen-rich gas is extracted from the adsorber in question and is used to facilitate the removal of the adsorbed compounds in an adsorber at low pressure. - A final decompression step (BD) that allows to evacuate some of the most adsorbable compounds by simple pressure drop. an elution step (P) during which the adsorber at low pressure is swept by the gas produced during step (PP). one or more balancing steps (E'q) of pressure from the gas produced during the above-mentioned step (Eq) to transfer gas under pressure from an adsorber at the end of adsorption to another adsorber at the beginning of the repressurization phase. a final repressurization step (Rep) to raise the balancing pressure up to the adsorption pressure and thus be able to restart a production cycle. Many improvements have already been made to industrial H2 PSA processes and units in terms of ease of operation, reliability and performance. Thus, in terms of performance, namely improvement of yield or decrease of investment, different approaches have been carried out, in particular: -adjunction of equipment for regenerating the adsorbent at lower pressure, such as a pump to vacuum or a gas ejector, or to perform some recycling of a portion of the gas, such as the addition of a compressor to recycle a portion of the waste gas. - Choice of adsorbents more effective, better adapted to each application, implemented in the form of successive beds or multiple layers. - more efficient pressure cycles by adding new substeps in the cycle, by different arrangement of steps, by shortening the cycle time ...

  With regard to the cycles of the H2 PSAs, the main improvement came from the use of a plurality of adsorbers making it possible not only to implement a greater number of pressure balances within the same cycle and thus to to improve the efficiency of the adsorption unit, but also to make special arrangements of the stages of the cycle and thus arrive at finally shorter cycles in duration than the cycles used previously, which makes it possible to limit the increase adsorbent needs and therefore the investment. Thus, US Pat. No. 3,986,849 teaches a cycle employing at least 7 adsorbers of which at least 2 adsorbers are concomitantly in the production phase, and with the production of at least 3 balances between adsorbers. Equilibrations (Eq) are carried out at co-current, ie in the same direction of circulation of the gas as that of its introduction into the adsorber considered during the adsorption step. This makes it possible to recover gaseous fractions rich in hydrogen which are introduced in counter-current into the adsorbers in repressurization (E'q). Similarly, the fraction of hydrogen-rich gas used for elution (PP) is also co-currently extracted. The final depressurization (BD) is in turn against the current because it is, in this step, to extract some of the impurities still contained in the adsorber. Elution is against the current. The cycle of Figure 2 of the above document is shown in the following Table 1. Table 1 Al T Al T A2 T A2 T A3 T A3 T Eq1 T Eq2 T Eq3 T PP T PP PP BD .LP (pp) 1 P (pp) 1 'P (pp) 1' E'q3 .L E ' q2 .L E'q1. ~ + R (f) TR (f) TT: co-current circulation; Countercurrent circulation It can be seen that the gas used for elution (PP) is conventionally taken from the adsorber after the last, ie the third, equilibration (Eq3) and before the final depressurization (BD) against current. We also see that in order to have a constant production rate, the repressurization by the charge gas R (f) is done simultaneously with the first equilibration (E'q1). Thereafter, unless otherwise indicated, the direction of flow of gas through the adsorber is as indicated above and the arrows will be omitted. It will be remembered that the final repressurization with a fraction of the production R (p) is done against the current R (p), I '. Furthermore, it is noted that cocurrent balancing is a step in which all or at least a large majority of the gas is withdrawn cocurrently, although it may be possible to extract a small amount simultaneously. against the current, that is to say a quantity not representing more than 20% by volume of the amount extracted co-current. In addition, the cycle corresponding to Figure 15 of this document has also been shown in Table 2 below. This is a variant of the cycle of Table 1 in which the elution gas is taken simultaneously with pressure balancing Eq2 and Eq3. Table 2 Al Al A2 A2 Eq1 Eq2 + PP Eq3 + PP BD P (pp) P (pp) E'q3 E'q2 E'q 1 + R (f) R (f) Finally, the cycle corresponding to Figure 9 of this document comprises an elution gas production step (PP) between the second and third balancing. Table 3 Al A1 A2 A2 Eq1 Eq2 PP PP Eq3 BD P (pp) P (pp) E'q3 E'q2 E'q 1 + R (f) R (f) Other documents introduce variations around the steps balancing or generation of elution and elution gases. Thus, US-A-6,210,466 teaches that the elution gas of PSA H2 may be provided by several adsorbers simultaneously and / or supply several adsorbers in the elution phase. It is also possible to have simultaneously the last pressure equalization and the supply of elution gas. US-A-6,379,431 discloses 3 or 4 equilibrium PSA H2 cycles based on the same principles. Alone, the arrangement detail of the substeps differs from one cycle to another.

  Moreover, US-A-6,454,838 claims PSA H2 cycle adsorber 6 and 4 balancing, the cycle schematically in Table 4 following. Table 4 AAAA Eq1 Eq2 I Eq3 PP PP BD + Eq4 BD P (pp) P (pp) E'q4 E'q3 I E'q2 II E'q 1 R (f / p) R (f / p) R ( f / p) As can be seen, the elution gas (PP) is extracted from the adsorber between equilibration Eq3 and a sub-step during which there is simultaneously cocurrent depressurization to perform a fourth equilibration and to countercurrent to start the final decompression. In order to be feasible without the use of intermediate gas storage capacity, this cycle comprises several idle times I. US-A-6,565,628 teaches cycles with 12, 14 or 16 adsorbers having 4 or 5 equilibriums, possibly including one sub-step with simultaneous supply of elution gas and start of final decompression against the current. The described cycle also includes short balancing times. In addition, EP-A-1,023,934 discloses a PSA H2 cycle in which the elution gas is temporarily stored in a capacity to be used with a time shift. From these documents, two main lessons emerge, namely that: a multiplication of the pressure balances makes it possible to reach the maximum yield that can be expected from the H2 PSA, taking into account its operating conditions, and a plurality of adsorbers with implementation of a plurality of substeps during a phase time, allows for the shortest possible cycles.

  However, the fact remains that the increase in yield unfortunately generates a significant additional cost at the industrial level. In particular, the multiplication of balancing steps intended to increase the yield negatively impacts the overall investment of the PSA process.

  Indeed, as already said, the pressure balancing steps between adsorbers having completed the adsorption phase and regenerated adsorbers are intended to recover the hydrogen that is in the adsorber at the end of the production step . However, when, for this purpose, the adsorber is depressurized by the production side, ie co-currently, the impurities contained inside said adsorber also migrate towards the outlet because of the decrease in pressure. total and gas flow from the entrance to the exit. So, the more balancing we do, the more we reduce the pressure, the more we recover hydrogen but the more we also advance the front of impurities. In addition, the elution gas supply step, which is also a countercurrent decompression step, thus continues this advancing effect of the impurities in the adsorber. However, since we do not want to pollute the adsorbers in regeneration and recompression, it is therefore necessary to increase the amount of adsorbent required and therefore to increase the volume of the adsorbers.

  Table 5 below shows the relationship between the number of equilibrations, the volume of adsorbent needed and the yield. It should be noted that in Table 5, an integral balancing number (1, 2 or 3 ...) corresponds to complete balancing between the adsorbers. In case of incomplete balancing or simultaneous steps, fractional or partial balances can be obtained. So, later, when we talk about cycles with 2 or 3 balances for example, it will be cycles to perform 2 or 3 balances that they are complete or partial. The values below correspond to that of PSA H2 units treating a feed gas containing hydrogen and other constituents, such as CO, CH4, N2, CO2, H2O from reforming with natural gas vapor. The operating pressures are the conventional pressures for this type of unit, namely an adsorption pressure of the order of 25 bar abs and a regeneration pressure of the order of 1.35 bar abs. TABLE 5 Amount Adsorbing volume Yield d (relative) balances in H2 (%) 0.7 100 81 1.5 128 84.8 2.0 146 86.5 2.5 167 87.7 3.0 188 88.7 3.3 201 89 We then note that for an 8% gain in efficiency, it is necessary to double the quantity of adsorbents, in constant phase time.

  Knowing that, to operate the most efficient cycles, it takes from 6 to 8 adsorbers minimum, it is understood that the increase in yield is associated with a significant increase in investment, with in particular a larger volume of adsorbent and an increase in the dimensions of the adsorbers, and therefore has a negative impact on the overall cost of the process.

  This is further confirmed by the publication: Parametric Study of a Pressure Swing Adsorption Process, AIChE 1999, Annual Meeting, Dallas, concerning the production of high purity hydrogen from a feed gas formed from a binary mixture methane-hydrogen. The pressure cycles used are similar to those described above. It is indicated that going from 1 to 3 balances can save 2.5% on the yield but at the expense of a 40% increase in the volume of adsorbent. In addition, EP-A-529513 states that the trend for H2 PSA units is to multiply the countercurrent balances to increase the hydrogen extraction efficiency. He indicates that this leads in particular to increase the necessary volume of adsorbent in order to avoid that the impurities do not pierce production side.

  The impact of the quantity of elution gas, taken during the PP steps, on the performance of the process is also known. For this purpose, it is conventional to use the elution ratio P / F which is the ratio (in actual cubic meters), ie under the operating conditions of pressure and temperature, of the quantity of gas of elution used at the amount of feed gas entering the adsorber during the cycle. Figure 2 of the aforementioned AIChE article shows that the optimum, whatever the cycle, is obtained for a ratio of 1.1 to 1.2. Indeed, when the ratio is less than 1, performance, especially productivity and yield, collapse. Hence, the PSA H2 units and processes that are currently in industrial operation are characterized by a P / F elution ratio, the amount of elution gas over the amount of feed gas under the actual conditions, greater than one. note that, for a PSA H2 unit, the temperature of the elution gas is very close to the temperature of the feed gas or the feed gas, and that the elution ratio (P / F) can be defined as being the ratio of the quantity of elution gas to the amount of filler gas (expressed in moles), multiplied by the ratio of the adsorption pressure to the regeneration pressure or more generally, if one of the pressures by example is not constant, as envisaged in one of the examples below, as the ratio in actual m3 of the amount of elution gas to the amount of feed gas, the gases being assumed to be at the same temperature . Subsequently, in the context of the present invention and in particular at the level of the claims, we adopt this last definition. It will be noted that even if, for whatever reason, the elution gas was at a temperature substantially different from that of the feed gas, the heat capacity of the adsorbent very much exceeds the heat capacity corresponding to the quantities of gas introduced. would warm or cool the gas in question after a few centimeters. To illustrate by example this definition, we consider the case of a PSA H2 unit, for example 6 adsorbers, processing 10,000 Nm3 / h of feed gas at a pressure of 29.8 bar absolute during a phase of phase 135 seconds and having only one adsorber in production. The quantity of feed gas entering during the production phase in this adsorber is then: 10000 x (135/3600) = 375 Nm3. Elution of the adsorber is via a gas flow rate of 940 Nm3 / h, for 90 seconds and at a pressure of 1.5 bar absolute.

  The amount of elution gas is then: 940 x (90/3600) = 23.5 Nm3. Given the respective pressures, we obtain a P / F ratio of: (23.5 / 375) x (29.8 / 1.5) = 1.245 This calculation corresponds to a conventional cycle where the final repressurization is done from the hydrogen produced and where therefore all the feed gas is introduced into the adsorber at the production pressure. In a variant of this cycle, a portion of the feed gas is used to perform the final repressurization, here from 21.4 bar abs to 29.8 bar abs. The amount of gas involved for this final repressurization is 90 Nm3. The average pressure in the adsorber during this repressurization step is (21.4 + 29.8) / 2 = 25.6 bar abs.

  We can then calculate the quantity (in real m3) of feed gas entering the adsorber, without temperature correction, ie: 375 - 90 = 285 Nm3 at 29.8 bar abs, equivalent to 9.564 m3 for the production phase and 90 Nm3 at 25.6 bar abs for repressurization, equivalent to 3.516 m3, ie overall 13.08 m3. The amount of elution gas unchanged compared to the previous case is then: 23.5 / 1.5 = 15.67 m3.

  Under these conditions, the P / F ratio is: 15.67 / 13.08 = 1.2 It can thus be seen that the quantity of feed gas (in real m3) can be evaluated by taking into account the two stages during which this gas is introduced into the adsorber. Similarly, if the elution was done in two steps, for example, the same should be done to determine the amount of feed gas (in actual m3) used. It is also easy to perform these calculations for more complex cycles with for example several adsorbers simultaneously in the production phase or several adsorbers simultaneously in the elution phase, knowing that it is considered that each adsorber follows exactly the same cycle.

  The determination of the elution ratio is particularly easy when the calculation of the parameters, such as sizing, performance, etc., is performed by simulation using appropriate standard software. The summation of the m3 relating to the elution step and that of introduction of the feed gas can be easily automated. Once the P / F ratio defined, as indicated above, perform these calculations does not pose any difficulty to a skilled person. In particular, it can use the mole as a unit of quantity of gas (instead of Nm3) and express the pressures in Pascal. In any case, the fact that the P / F ratio must be greater than 1 is not limited to the H2 PSAs. Thus, in the work Pressure Swing Adsorption of Ruthven, Farooq, Knaebel, this point is approached, for example in the paragraph Purge flow rate, Air drying, PSA Processes of Chapter 6, where it is explained that for a unit PSA dryer of air, a P / F ratio of 1.15 is recommended. A ratio of 1 in theory makes it possible to roll back the impurities of what they advanced in phase of adsorption, that is to say to roll back the front of impurities until where it was before adsorption. The determination of the elution ratio of an industrial or pilot unit can be made from the measurement of the flow rates of supply and elution of the gas. It can also be determined from PSA process simulation software in the manner of what was done in the article presented at AIChE. A ratio lower than 1 can also be demonstrated experimentally because by decreasing the quantity of elution gas beyond this point, it is no longer possible to maintain the PSA's performance, particularly the purity of the production because we are witnessing then to a progressive advance of the impurities in the adsorbent.

  A problem to be solved is therefore to improve the known PSA H2 processes so as to reduce their cost, that is to say the corresponding global investment, without this being accompanied by a decrease in the efficiency of hydrogen extraction. In other words, one of the aims of the invention is to propose an improved method making it possible to increase the productivity of a process or unit PSA H2 compared to the most efficient current units in order to obtain a technically effective solution for to separate the hydrogen-based mixtures and which is economically acceptable. Another object of the invention is to be able to valorize impure hydrogen flows, that is to say with contents of H 2 down to 70 or 75 mol%, and / or at low or medium pressure, that is to say about 2 to 15 bars absolute, but also at higher pressure. A solution of the invention is then a PSA process for producing a gaseous fraction enriched in hydrogen from a hydrogen-containing feed gas and at least one gaseous component to be removed using several adsorbers each containing one or more adsorbents, each adsorber being subjected to a production cycle comprising the steps of: a) introducing the feed gas into at least a first adsorber and adsorbing, at the top pressure of the cycle, at least one component to be removed on at least one adsorbent contained in said first adsorber so as to produce a gaseous fraction enriched in hydrogen, b) stopping the introduction of the feed gas into said first adsorber and depressurizing cocurrently said first adsorber to recover at least one gaseous subfraction rich in hydrogen, c) backpressively depressurizing said first adsorber to the low cycle pressure, said low pressure the cycle being lower than said high pressure of said cycle, thereby desorbing at least a portion of the constituents adsorbed during step a), d) eluting said first adsorber with at least one hydrogen-rich elution gas, e) repressurizing the first adsorber with at least one sub-fraction generated during the co-current depressurization of a step b) and with the feed gas and / or a hydrogen-rich gas, characterized in that at the stage d), at least a portion of the hydrogen-rich elution gas is derived from a hydrogen-rich gas source external to the process. Depending on the case, the process of the invention may comprise one or more of the following characteristics: in step d), the hydrogen-rich elution gas is formed of at least a first hydrogen-rich gas derived from a source of hydrogen-rich gas outside the process and optionally a second gas rich in hydrogen from the PSA process. in step d), the second hydrogen-rich gas resulting from the PSA process is either at least a part of a hydrogen-rich gas subfraction resulting from a step b), or at least a part of a gaseous fraction enriched in hydrogen produced in step a), or both. in step d), the elution of the first adsorber with the second hydrogen-rich gas resulting from the PSA process corresponds to an elution ratio (P / F) of less than 1. the second hydrogen-rich gas derived from PSA method corresponds to an elution ratio (P / F) of less than 0.8, preferably an elution ratio (P / F) of less than 0.5. in step d), the flow of hydrogen-rich elution gas consists solely of gas from a source of hydrogen-rich gas outside the process and of hydrogen-enriched gas produced during step a) . in step d), all of the hydrogen-rich elution gas is derived from a source of hydrogen-rich gas external to the process. in step d), the elution of the first adsorber is carried out from a hydrogen-rich gas fraction derived from a source of gas external to the process chosen from a pipe or a network of hydrogen pipes, a hydrogen storage or a source of production of a hydrogen-rich gas, in particular said pipe or pipeline network carries at least one purge gas from a chemical, petrochemical or petroleum unit, such as a petroleum refinery. in step d), the hydrogen-rich elution gas from a source of gas outside the process has a hydrogen content lower than the hydrogen content of the gas produced in step a). - The fraction of hydrogen-rich gas from step a) and forming a portion of the elution gas of step d) is taken to obtain a constant hydrogen production rate. it comprises from 1 to 4 pressure balancing. in step d), elution of said first adsorber is carried out from a hydrogen-rich gas fraction derived from the permeate of a membrane permeation unit. in step e), a partial recompression of the adsorber or adsorbers regenerated in steps c) and d) is carried out by a gas external to the process. from 2 to 26 adsorbers, preferably from 4 to 16 adsorbers, and / or several adsorbers are simultaneously in phase a), in phase b), in phase c), in phase d) and / or in phase e). in step a), at least part of the impurities is removed on at least one adsorbent material selected from silica gels, activated aluminas, activated carbons, zeolites, and combinations or mixtures thereof, preferably or the active carbons represent more than 50% by volume of the volume of adsorbent material used. the gaseous fraction enriched in hydrogen produced in stage a), has a purity in hydrogen of between 95 and 99.9 mol%, preferably between 98 and 99.5 mol%. The invention will now be described in more detail with reference to the appended figures, in which: FIG. 1 represents a conventional PSA process cycle according to the prior art; FIG. 2 represents a PSA H2 process cycle according to the present invention; and Figures 3 and 4 show variations of the basic cycle of the present invention. The invention relates to a PSA H2 process using a hydrogen separation / purification unit comprising several adsorbers each containing one or more adsorbents for stopping at least part of the constituents other than the hydrogen contained in the feed gas. to be purified / separated, each of the adsorbers operating according to a production cycle comprising at least one adsorption step, at least one cocurrent depressurization step for partially repressurizing a regenerated adsorber - said balancing step - optionally a co-current withdrawal stage of a portion of the elution gas, a final countercurrent depressurization step, an elution step at the low cycle pressure, repressurization steps with at least the gas from the one or more depressurization steps and a portion of the gas from the production or the feed gas.

  The elution gas is formed of a hydrogen-rich fraction from a source of hydrogen external to the PSA process, which is optionally supplemented by a stream from the PSA process, in particular by a fraction of the hydrogen withdrawn at co-current as indicated above and / or a part of the hydrogen produced during the production step.

  These flows resulting from the PSA process and used as a possible additional elution gas at low pressure can be produced during cocurrent depressurization steps during specific steps or sub-stages (PP) as in the cycles of Tables 1, 3 and 4. 4. They can also be extracted simultaneously with a balancing step (Eqi + PP) as in the cycle of Table 2, or even with counter-current depressurization (PPT + BD.I). They can also be taken from the workflow. As mentioned previously, the fact that the elution made only with PSA gas corresponds to a P / F <1 ratio, means in practice that the PSA plant can not ensure production at the required purity without the need to use another hydrogen-rich outer stream to ensure elution consistent with the desired performance. In a variant, this hydrogen-rich fraction originating from a source outside the PSA also participates in the repressurization of an adsorber. The external source may include various streams from different units or from different points of the same unit. In a standard PSA process, the gas making it possible to carry out the elution generally comes, as has been seen previously, from the last cocurrent balancing. Although it is still a hydrogen-rich fraction, its content of impurities is substantially greater than that of production. However, it can be seen by measurements on industrial units, on pilot or by simulation, that the impurity content of said elution gas depends on the operating conditions and, in particular, adsorption and regeneration pressures, but that medium, it can contain about 10 to 50 times the impurity content of hydrogen production. This fact is known for hydrogen of very high purity (> 99.99%) but remains true for hydrogen of less purity, that is to say only 99.9% or even 99%. This means that if it is desired to produce a hydrogen fraction containing a maximum of 1 mol% of methane by PSA, it is possible to use available gaseous streams containing, for example, 5 to 10% methane as eluting gas. This possibility of eluting with an external gas makes it possible either to limit the cocurrent depressurizations (removal of the stage PP of producing the elution gas internally to the PSA) or to make an additional balancing instead. of said step PP. In the first case, the investment is mainly favored because it limits the advance of impurities in the adsorber, progression directly related to the final pressure obtained at the end of co-current depressurizations. In the second case, the extraction yield is essentially favored by adding a balancing without increasing the volume of adsorbent. It should be noted that the elution gas is used at low pressure. The invention will therefore in particular make it possible to upgrade hydrogen sources at a pressure lower than the pressure of the PSA process without additional compression means. It should be noted that the need to invest a compressor to treat such a gas in the PSA generally removes any interest in the recovery of these low or medium pressure streams in order to add them to the main load of the PSA. These low or medium pressure streams are generally sent to the plant's fuel-gas network. It is often the same for the PSA waste. Taking some of them to elute is not a waste. Indeed, their composition is compatible with the achievement of an elution to cross the bed of adsorbents and thus cause the desorption of impurities before injecting these same flows with added said impurities desorbed in a fuel-gas network. If such a suitably pure gas - containing 5 to 10% methane in our example - is available in excess of the amount required for conventional elution, it may be advantageous to over-elute, that is, to say to go beyond the usual elution ratios, beyond P / F = 1.3 for example. Indeed, in a conventional PSA, the elution rate is determined to optimize the performance, in particular the extraction efficiency. We have seen that this corresponds to using a ratio P / F of the order of 1.1 to 1.2. At the end of the elution, the adsorber still contains a large amount of adsorbed impurities. It must therefore be sized to contain at the end of the adsorption cycle both the amount of impurities introduced during said adsorption cycle and the residual amount mentioned above. If more elution rate is used than necessary, during this step more impurities will be removed. It is easy to see that this results in a decrease in the volume of the adsorbers and in this way leads to a reduction in the investment. Reducing the size of the adsorbers can also reduce the loss of hydrogen by minimizing the amount contained in the interstitial spaces and the pore volumes, proportional to the volume of adsorbent. Over-elution with an external gas will therefore generally lead to a decrease in investment and an increase in hydrogen production. If the quantity of elution gas outside the PSA is not sufficient, it may be advantageous to add a hydrogen flow from the PSA itself, from a cocurrent depressurization step or taken from the production. This additional levy will naturally be limited and optimized, generally in order to obtain maximum hydrogen production. In this case, the additional flow taken from the production may be mixed with the external elution gas or be a separate elution sub-step. In the latter case, the elution with the flow of hydrogen from the production will be preferentially after the outer elution because of its greater purity in H2.

  A particular case is that where a gas is available at a pressure lower than the high pressure of the PSA process - thus not directly usable in the PSA - but too low in hydrogen content to be used as an elution gas. With conventional PSA H2 units, the only solution, if you still want to use it, is to invest in a compression machine to raise this gas or a portion of this gas to the adsorption pressure which is particularly expensive However, the method of the invention proposes a solution resulting in a lower investment. A hydrogen membrane is installed on all or part of the impure hydrogen and the permeate is used as elution gas. Since the hydrogen membranes are very selective and we will be able to extract the permeate at low pressure, just above the elution pressure, that is to say between 1.5 and 2 bar abs for example a very enriched H2 fraction is obtained that can be used even for a final production of high purity hydrogen. It should further be noted that if the hydrogen outside the PSA process is sufficiently pure and at intermediate pressure between the low regeneration pressure and the adsorption pressure, this hydrogen can be used to at least partially repressurize the adsorbers. Such repressurization makes it possible to reduce the number of balances because the hydrogen previously extracted during a balancing operation is recovered outside. This makes it possible to increase the productivity of the PSA process and to treat a larger charge rate if necessary or to use lower adsorbent volumes. We can then use indeed a cycle with 2 or 3 balances instead of a cycle with 4 balances. To illustrate the invention, it is considered a PSA H2 unit implementing a conventional production cycle with 4 physical balances corresponding to about 3.7 effective balancing and with an elution step performed using gas removed during a step of co-current depressurization. The cycle used is a conventional cycle of the prior art, as given in Table 6 and the pressure diagram to which each adsorber is subjected is shown schematically in Figure 1. This PSA unit comprises 10 adsorbers operating concomitantly, called A to J on the abscissa of FIG. 1, each adsorber A to J being subjected to a determined stage of the cycle corresponding to a given pressure or a range of pressures, as indicated on the ordinate of FIG.

  Table 6 Al Al Al A2 A2 A2 Eq1 I Eq2 Eq3 Eq3 Eq4 PP PP BD Com BD BD P (pp) P (pp) P (pp) P (pp) P (pp) E'q4 E'q3 E'q3 E (p) R (p) R (p) It is found that it is a cycle with 10 adsorbers, 2 of which in the production phase. The generation of elution gas (PP) takes place after the 4th equilibration on a complete phase time and feeds 2 other adsorbers in elution step (P). The duration of the first equilibration - Eq1 and E'q1 - is shortened to the maximum in order to limit the quantity of repressurization gas introduced concomitantly and to recover a maximum of hydrogen during balancing, this being made possible here by the introduction of a dead time, which is represented by I in Table 6, in the third line, the middle substep between the first and the second balancing. Applied to a feed gas containing a minimum of 65/70% mole of hydrogen with hydrocarbons as main impurities, mainly methane, possibly traces of nitrogen, resulting from any chemical or petrochemical process, this conventional cycle, using also usual adsorbents activated alumina type, silica gel, activated carbon and zeolite to purify the gas, can produce 99 99.9 or even 99.9% pure hydrogen with a yield of the order of 90%.

  In order to represent the present invention, this basic cycle has been modified as given in Table 7.

  Table 7 Al Al Al A2 A2 A2 Eq1 I Eq2 Eq3 Eq3 Eq4 BD BD BD P (ext) P (ext) P (ext) P (ext) P (ext) E'q4 E'q3 E'q3 E'q2 E The elimination of the phase corresponding to the production of the elution gas by co-current depressurization makes it possible to eliminate an adsorber. We now have a type 9.2.4 cycle, that is to say now comprising only 9 adsorbers with always 2 adsorbers in the production phase and 4 balances. This removal of an adsorber which represents a significant gain on the investment is added to the gain presented above which corresponded to a decrease in volume of the unit adsorber, due to the progress of the impurities. This cycle is shown in Figure 2 of this document.

  Note that it is possible to retain all or part of the conventional steps producing the internal elution gas in addition to the external elution gas, but this does not simplify the PSA pressure cycle. This may be necessary if the external elution source is used or only temporarily usable. But also, it may be interesting to keep internal elution reduced if the external source of elution is not sufficient or if the purity of said external source is limited. In this case, the amount of elution gas withdrawn in this way will correspond to an elution ratio P / F less than 1, preferably less than 0.5. A cycle of this type is shown in FIG. 3. It will be noted that such a PSA can only function properly thanks to external elution. The various substeps P (ext) of Table 7 and of FIG. 2 correspond to the elution with a flux outside the PSA. It can be seen that during the first 2 substeps, two adsorbers are eluted in parallel with the flow. outside while, during the 3rd and last substep, only the first adsorber is eluted. Depending on the availability of the external elution flux, it will be possible to take a constant flow rate and periodically an adsorber will thus see an elution rate double that which it receives during the other substeps or else it is the elution rate. to each adsorber which will be kept constant and periodically less elution gas will be taken from the external source. In general, it is possible to use an additional capacity to temporarily store all or part of the elution gas and thus give itself an additional freedom parameter as to the development of the cycle. As mentioned above, the elution with the external gas can be completed by elution from a hydrogen fraction taken from the production. The cycle of Table 8 and Figure 4 is then obtained. Table 8 Al (Al) Al A2 (A2) A2 Eq1 (Eq1) Eq2 Eq3 (Eq3) Eq4 BD (BD) BD P (ext) (P (ext)) P (ext) P (p) (P (p)) E'q4 E'q3 (E'q3) E'q2 E'q1 (E'q1) R (p)

  P (p) being the elution carried out by a flow of hydrogen taken from the production. It will be noted that the intermediate substep (middle) is here identical to the first substep and the cycle is limited to only 2 steps per phase.

  Hydrogen removal from production is continuous. In the first sub-step, the hydrogen removed is used to elute the adsorber which has already been eluted by the gas from the external source. In the second substep, the hydrogen is used to perform the final repressurization using QR (p) flow. In this way, the production of hydrogen can be made constant if it is arranged that the flow taken from the production Q (p) is itself constant. In the case where the additional elution rate taken from the production is lower than the rate required to perform the final repressurization, it will be easy to make the production constant by sending the difference (R (p ') = final repressurization flow less flow additional elution) in the adsorber performing the first equilibration.

  The last line of Table 9 would then become: E'q1 + R (p ') R (p) The composition of the elution gas from an external source must be compatible with the purity required for production. It has already been seen that a membrane could make it possible to use a gas that is a priori too impure, for example with a purity lower than 90 mol%. An example has already been mentioned in which the gas coming from an external source was at a pressure lower than the adsorption pressure of the PSA, but it goes without saying that it is possible advantageously to treat by a unit of permeation a gas available at higher pressure and recover the permeate at low pressure to elute with a gas highly enriched in hydrogen. In the same way, it will be possible to use other available methods to eliminate certain compounds, such as heavy hydrocarbons, CO, possibly H2O, CO2 ... within the limit where the overall balance (investments / additional production) remains interesting. Given the composition of hydrogen-rich purge gases that can be found in chemistry and petrochemistry, typically 70 to 90% hydrogen, the purity of PSA production will be in the range 95 to 99.9 mol%. more particularly between 98 and 99.5%, which is sufficient for a large number of applications. It will be appropriate in the choice of adsorbents, to take into account the impurities that may be in the external elution gas. The use of activated carbon, activated alumina or silica gel is preferentially retained with respect to the use of zeolites more sensitive to contaminants.

Claims (16)

claims   A PSA process for producing a gaseous fraction enriched with hydrogen from a feed gas containing hydrogen and at least one gaseous component to be removed using several adsorbers each containing one or more adsorbents, each adsorber being subjected to a production cycle comprising the steps of: a) introducing the feed gas into at least a first adsorber and adsorbing, at the top pressure of the cycle, at least one component to be removed on at least one adsorbent contained in said first adsorber; producing a gaseous fraction enriched in hydrogen, b) stopping the introduction of the feed gas into said first adsorber and depressurizing said first adsorber cocurrently to recover at least one hydrogen-rich gas sub-fraction, c) performing a depressurization countercurrently from said first adsorber to the low pressure of the cycle, said low cycle pressure being lower than said high pressure of said cycle, thereby desorbing at least a portion of the components adsorbed during step a), d) eluting said first adsorber with at least one hydrogen-rich elution gas, e) repressurizing the first adsorber with at least one minus one sub-fraction generated during the co-current depressurization of a step b) and with charge gas and / or a gas rich in hydrogen, characterized in that in step d), at least one part of the hydrogen-rich elution gas is derived from a hydrogen-rich gas source external to the process.   2. Method according to claim 1, characterized in that in step d), the hydrogen-rich elution gas is formed of at least a first gas rich in hydrogen from a source of hydrogen-rich gas. outside the process and optionally a second gas rich in hydrogen from the PSA process.   3. Method according to one of claims 1 or 2, characterized in that in step d), the second gas rich in hydrogen from the PSA process is: -at least part of a rich gaseous subfraction hydrogen from a step b), and / or - at least a portion of a gaseous fraction enriched in hydrogen produced in step a).   4. Method according to one of claims 1 to 3, characterized in that in step d), the elution of the first adsorber with the second gas rich in hydrogen from the PSA process corresponds to an elution ratio ( PIF) less than 1.   5. Method according to claim 4, characterized in that the second gas rich in hydrogen from the PSA process corresponds to an elution ratio (P / F) of less than 0.8, preferably an elution ratio (P / F) lower than 0.5. 10   6. Method according to one of claims 1 to 5, characterized in that in step d), the flow of hydrogen-rich elution gas is formed solely of gas from a source of hydrogen-rich gas. external to the process and hydrogen enriched gas produced in step a). 15   7. Method according to one of claims 1 to 5, characterized in that in step d), all of the hydrogen-rich elution gas is derived from a source of hydrogen-rich gas external to the process.   8. Method according to one of claims 1 to 7, characterized in that in step 20 d), the elution of the first adsorber is carried out from a hydrogen-rich gas fraction from a source of gas external to the process selected from a pipe or a network of hydrogen pipes, a hydrogen storage or a source of production of a hydrogen-rich gas, in particular said pipe or pipe network conveys at least one gas of hydrogen purge from a chemical, petrochemical or petroleum unit, such as an oil refinery.   9. Method according to one of claims 1 to 8, characterized in that in step d), the hydrogen-rich elution gas from a source of gas outside the process has a hydrogen content less than the hydrogen content of the gas produced in step a). Process according to one of Claims 1 to 6 or 8 to 9, characterized in that the hydrogen-rich gas fraction from step a) and forming part of the elution gas of step d) is taken to obtain a constant hydrogen production rate. 11. Method according to one of claims 1 to 10, characterized in that it comprises from 1 to 4 pressure equalizations. 30 35 12. Method according to one of claims 1 to 11, characterized in that in step d), elution of said first adsorber is performed from a hydrogen-rich gas fraction from the permeate of a membrane permeation unit. 13. Method according to one of claims 1 to 12, characterized in that in step e) is carried out a partial recompression of the adsorber regenerated or in steps c) and d) by a gas outside the process. 10 14. Method according to one of claims 1 to 13, characterized in that it implements from 2 to 26 adsorbers, preferably from 4 to 16 adsorbers, and / or more adsorbers are simultaneously in phase a), in phase b), in phase c), in phase d) and / or in phase e). 15. Process according to one of claims 1 to 14, characterized in that in step a), at least a portion of the impurities is removed on at least one adsorbent material selected from silica gels, activated aluminas, active carbons, zeolites, and their combinations or mixtures, preferably the active carbon or carbons represent more than 50% by volume of the volume of adsorbent material used. 20 16. A process according to one of claims 1 to 15, characterized in that the gaseous fraction enriched in hydrogen produced in step a) has a hydrogen purity of between 95 and 99.9 mol%, preferably between 98 and 99.5 mol%. . 25 30