WO2008151913A1

WO2008151913A1 - Psa process suitable for recovering specific compounds

Info

Publication number: WO2008151913A1
Application number: PCT/EP2008/056323
Authority: WO
Inventors: Martine Schneider; Guillaume De Souza; Bruno Alban
Original assignee: L'air Liquide Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 2007-06-14
Filing date: 2008-05-22
Publication date: 2008-12-18
Also published as: FR2917304B1; FR2917304A1

Abstract

Process for separating a gas mixture by adsorption, said gas mixture comprising at least four compounds belonging to at least four respective groups of increasing adsorptivities, in which N adsorbers (where N ≥ 2) are used which follow, in a staggered fashion, a pressure cycle, the depressurization phase of which starts: either with a step during which all of the streams leaving the adsorbers in cocurrent depressurization mode and rich in compounds of the second group are sent to the adsorbers undergoing the elution step; or with a step during which the stream leaving the adsorbers undergoing cocurrent depressurization and rich in compounds of the second group leaves the system consisting of the N adsorbers; so as at least partly to separate the compounds of the second group; and includes a partial balancing of the pressures so as to produce, during the production phase, a stream of gas enriched with compounds of the first and third groups but depleted in compounds of the second group.

Description

PSA process suitable for recovering specific compounds

The present invention relates to a process for separating a gas mixture by adsorption, said mixture comprising at least a first compound, a second compound, a third compound and a fourth compound belonging to at least a first group, a second group, a third group and a fourth group respectively, said groups having increasing adsorptivities .

The invention relates more particularly to adsorption treatment units of the PSA (pressure swing adsorption) type. Such a unit implements a process in which at least one adsorber that follows a cycle, in which an adsorption phase substantially at a high pressure of the cycle, a depressuπzation phase down to the low pressure of the cycle and a repressurization phase back up to the high pressure of the cycle come after one another, is used.

The depressurization phase down to the low pressure of the cycle generally comprises at least the following various steps : a first cocurrent depressurization step during which the stream coming from the adsorber is sent to another adsorber in the repressurization phase - there is therefore complete pressure balance; a second cocurrent depressurization step during which the output stream coming from the adsorber is directed to another adsorber in the elution step as stream for eluting the adsorbent material of another adsorber (purge & elution) ; and a third depressurization step (or blow down) during which the stream coming from the adsorber leaves the unit.

For example, document EP-A-I 023 934 is known which teaches an improved hydrogen-recovery process employing these various depressurization steps. Following this, it should be pointed out that the depressuπzation phase invariably starts with a first cocurrent depressuπzation step by pressure balancing with an adsorber undergoing repressuπzation .

This type of process makes it possible for example to recover hydrogen from a given feed gas with a concern for energy efficiency of the process associated with optimizing the pressure balancing.

However, these cycles are not particularly relevant when certain species have to be at least partly separated from the main stream.

This is because, considering a gas mixture comprising at least a first compound, a second compound, a third compound and a fourth compound belonging to at least a first group, a second group, a third group and a fourth group respectively, said groups having increasing adsorptivities, the processes according to the prior art do not allow effective separation of the compounds of the third group so as to recover a stream of purified gas enriched with compounds of the first and third groups but depleted in compounds of the second group.

Starting from this, one problem that arises is how to provide a process for improving the separation of a gas mixture comprising at least four groups of compounds of increasing adsorptivities, so as to recover a stream of gas enriched with compounds of the first and third groups.

One solution of the invention is therefore a process for separating a gas mixture by adsorption, said mixture comprising at least a first compound, a second compound, a third compound and a fourth compound belonging to at least a first group, a second group, a third group and a fourth group respectively, said groups having increasing adsorptivities, in which N adsorbers (where N > 2) are used, each adsorber following, in a staggered fashion, a pressure cycle in which a production phase at the high pressure (HP) of the cycle, a depressuπzation phase down to the low-pressure (LP) of the cycle and a repressurization phase back up to the high pressure (HP) of the cycle come after one another, characterized in that at least the depressurization phase starts : either with a cocurrent depressurization step during which all of the stream or streams leaving the adsorber or adsorbers in cocurrent depressurization mode and rich in compounds of the second group is/are sent to the adsorber or adsorbers undergoing the elution step, so as at least partly to separate the compounds of the second group; or with a cocurrent depressurization step during which the stream leaving the adsorber or adsorbers undergoing cocurrent depressurization and rich in compounds of the second group leaves the system consisting of the N adsorbers so as at least partly to separate the compounds of the second group; and includes a partial balancing of the pressures between at least one adsorber undergoing cocurrent depressurization and at least one adsorber in the repressurization step, so as to produce, during the production phase, a stream of gas enriched with compounds of the first and third groups but depleted in compounds of the second group.

The expression "stream of gas enriched with compounds of the first and third groups" is understood to mean a stream of gas whose content of compounds of the first and third groups is higher than the content of compounds of the first and third groups of the gas mixture to be separated.

Likewise, the expression "stream of gas depleted in compounds of the second group" is understood to mean a stream of gas whose content of compounds of the second group is lower than the content of compounds of the second group of the gas mixture to be separated. Moreover, each pressure of the pressure cycle does not exceed HP and is not less than LP, where LP < HP.

Depending on the case, the process according to the invention may have one or more of the following features: at least one stream of gas at the low pressure (LP) of the cycle, preferably at a pressure between 1 and 2 bar, which is enriched with compounds of the second group, is recovered during the depressurization phase; - a countercurrent depressurization step, during which the stream leaving the adsorber or adsorbers undergoing countercurrent depressurization leaves the system consisting of the N adsorbers, is carried out during the depressurization phase; - the first group includes hydrogen and/or the third group includes carbon monoxide; the compounds of the second group are chosen from nitrogen, argon and rare gases (rare gases are understood to mean helium, neon, krypton, xenon and radon) ; - the system consisting of N adsorbers is a PSA unit; the high pressure of the cycle is between 15 and 40 bar, preferably between 20 and 30 bar; said N adsorbers comprise adsorbents having a selectivity, with respect to compounds of the second group, of not less than 2, preferably not less than 4, for compounds of the third group, and a selectivity, with respect to compounds of the second group, of not less than 1, preferably between 1 and 2, for compounds of the fourth group; and the gas mixture comes from a steam or oxygen reforming process, from a partial oxidation process, from a coal, residue or biomass gasification process or from hybrid processes, or is a product or an effluent of a chemical reaction, preferably a product of the Fisher-Tropsch reaction or from the generation of methanol, oxo-alcohols or DME from syngas. The term "hybrid process" is understood to mean a combination of steam reforming and partial oxidation. The compounds of the fourth group are all the compounds that are present in the stream to be treated and that do not form part of the other three groups. These may comprise alkanes (methane, ethane, etc.), CO₂, etc.

The four groups of compounds involved within the context of the present invention may be defined in the following manner: first group: compounds of very low adsorptivity, that is to say those that are adsorbed less than all those of the other groups; second group: compounds that have a low adsorptivity, i.e. those that are more adsorbable than those of the first group but less adsorbable than those of the third and fourth groups; - third group: adsorbable compounds, i.e. those that are more adsorbable than those of the first and second groups but less adsorbable than those of the fourth group; and fourth group: highly adsorbable compounds, i.e. those which are adsorbed more than the compounds of the other groups.

Thus, in the invention, the object is to improve the recovery of the compounds of the first and third groups in a stream produced at a pressure close to the feed pressure.

To do this, one solution of the invention is to start the depressuπzation phase that follows the high-pressure adsorption phase with a step different from the usually proposed pressure balancing.

The invention is applicable to all PSA processes, independently of the adsorbents used, of the number of adsorbers and of the operating conditions: pressure, temperature, composition and inflow rate.

The following notations will be used in the rest of the text:

B (standing for "balancing") : partial pressure balancing between at least one adsorber at the start of cocurrrent depressurization and at least one adsorber undergoing the repressuπzation step;

PP (standing for "provide purge") : a step during which all of the stream or streams leaving the adsorber or adsorbers undergoing cocurrent depressurization is/are sent to the adsorber or adsorbers undergoing the elution step;

DU (standing for "dump up") : a cocurrent depressurization substep during which the stream leaving the adsorber or adsorbers undergoing cocurrent depressurization leaves the system consisting of the N adsorbers;

BD (standing for "blow down") : a countercurrent depressurization substep during which the stream leaving the adsorber or adsorbers undergoing countercurrent depressurization leaves the system consisting of the N adsorbers; and elution: a substep during which an adsorber is countercurrently flushed at the low pressure of the cycle with a stream coming from another adsorber undergoing the PP substep and producing an effluent at low pressure.

As mentioned above, the depressurization phase starts: either with a cocurrent depressurization step during which all of the stream or streams leaving the adsorber or adsorbers in cocurrent depressurization mode and rich in compounds of the second group is/are sent to the adsorber or adsorbers undergoing the elution step, so as at least partly to separate the compounds of the second group; or with a cocurrent depressurization step during which the stream leaving the adsorber or adsorbers undergoing cocurrent depressurization and rich in compounds of the second group leaves the system consisting of the N adsorbers so as at least partly to separate the compounds of the second group .

This alternative solves the stated problem well.

This is because, in the first case, the fact of starting the regeneration phase with a PP step makes it possible, at the end of the adsorption step when the fronts of the compounds of the frrst group have passed, to redirect the stream rich in compounds of the second group into a purge stream and therefore to partially separate it. The position of substep B necessary for recycling the compounds of the third group will depend on the application and on the composition of the incoming stream.

In the second case, the fact of starting the regeneration phase with a DU step allows the compounds of the second group to be partially separated by making it leave the system consisting of N adsorbers directly with a cocurrent depressuπzation step. As previously, the position of substep

E necessary for recycling the compounds of the third group will depend on the application and on the composition of the incoming stream.

This alternative is not a priori obvious since the pressure for the balancing streams is lost. Specifically, since the balancing steps B is delayed in the succession of depressuπzation steps, it is natural for these balancing streams to be at a pressure strictly lower than the high pressure of the cycle. However, this pressure reduction consequently results in an energy loss. Thus, the present invention is properly applied to processes in which the first object is to enrich the main stream produced with compounds of the first and third groups relative to the feed gas, while depleting the latter of compounds of the second group, this being the case when the feed gas mixture comprises at least four compounds divided into four groups of increasing adsorptivities .

Moreover, the process according to the invention makes it possible to recover, by combining the low-pressure streams (those coming from the elution and/or BD steps) , a stream of gas enriched with compounds of the second group so as to at least partly recycle the compounds of the second group. In fact, all the streams may or may not be combined/mixed dependrng on the requirements. Thus, one and the same apparatus (same cycle, etc.) may have different arrangements of the low-pressure streams. If for example the first and third compounds are compounds of no useful value, there is no need to separate them. In contrast, if the fourth compound is instead a fuel (hydrocarbons, such as methane, ethane, etc.) and the second compound is nitrogen, it is preferable to separate them into a hydrocarbon-rich stream (and therefore coming from the steps at the end of regeneration, since these molecules are more "fixed") and into another stream rich in nitrogen (?? with streams coming from the start of regeneration) .

The invention will now be described in greater detail by means of two examples given by way of illustration, namely a comparative example and an example according to the invention .

Comparative example

Consider a stream of gas composed of 20% of each of the gases in guestion, namely H₂, N₂, CO, CH₄ and CO₂.

The objective is to optimally separate the nitrogen, a weakly adsorbable compound, from the hydrogen, the least adsorbable compound, and from the CO, which may be recycled with the hydrogen .

These compounds may be put into four groups according to the invention : group 1: H₂; group 2 : N₂; group 3: CO; - group 4: CH₄ and CO₂. A bed composed mainly (90%) of a silicalite preceded by an alumina guard bed as adsorbent for the PSA process is employed.

In many actual applications, minor compounds (water, heavier hydrocarbons, etc.) are added to the abovementioned major ones, the guard bed being used to stop these minor compounds that would be prejudicial to the proper operation of the "core" bed composed of a silicalite, since it is the latter that will efficiently separate the compounds of the above four groups.

The entire cycle of the PSA process is characterized by: twelve cylindrical bottles that operate in parallel, i.e. together (with a volume of 13 m³ and a height/diameter ratio of 4), three of which are used at any moment for the production of the high-pressure stream; a high pressure of 25 bar and a low pressure close to atmospheric pressure; and - feed and adsorber wall temperatures of 30⁰C.

Cycle 1 (Figure 1) implemented is a cycle optimized for the separation in question, while maintaining the conventional balancing at the start of depressurization. The cycle allows the production of a high-pressure stream in which it is desired to collect H2 and CO and eight low-pressure streams in which substantial nitrogen recycling is desired.

The results below derive from software used to simulate all the cyclic adsorption processes. This software is comparable to the commercially available ADSIM™ software and is based on solving the matter and energy balances iteratively.

This software requires the following to be known: the physical properties of the adsorbent (size, bed density, porosity, etc.), the adsorption and co-adsorption properties at equilibrium for an adsorbate/adsorbent pair based in general on experimental isotherms, and the adsorption rate based on the interpretation of experimental break-through curves .

Table 1 expresses the percent yield for each of the gases in the high-pressure stream of the cycle and for the sum of the low-pressure streams of the cycle.

Table 1: yields of the various compounds in the high-pressure stream and low-pressure streams obtained during a conventional cycle

Example according to the invention

Consider, as previously, a stream of gas composed of 20% of each of the gases in question, namely H₂, N₂, CO, CH₄ and CO₂.

These compounds may be put into four groups according to the invention : - group 1 : H₂; group 2: N₂; group 3: CO; group 4: CH₄ and CO₂.

A bed composed mainly (90%) of a silicalite preceded by an alumrna guard bed as adsorbent for the PSA process rs employed. The entire cycle of the PSA process is characterized by: twelve cylindrical bottles that operate in parallel, i.e. together (with a volume of 13 m³ and a height/diameter ratio of 4), three of which are used at any moment for the production of the high-pressure stream; a high pressure of 25 bar and a low pressure close to atmospheric pressure; and feed and adsorber wall temperatures of 30⁰C.

Cycle 2 is the cycle illustrating one of the examples of the innovation, namely a cocurrent depressuπzation substep, the output of which is directed into a purge stream. In the present case, the stream is separated into two purge streams. It may be seen that there are then three cocurrent balancing steps, then a PP substep and then a BD substep.

As in the case of the first cycle, a high-pressure stream rich in H2 and CO is separated from six low-pressure streams in which it is desired to increase the N₂ recycling.

Employing the same software as that used in the comparative example, the following results are achieved.

Table 2 expresses the percent yield for each of the gases in the high-pressure stream of the cycle and for the sum of the low-pressure streams of the cycle.

Table 2: yields of the various compounds in the high-pressure stream and the low-pressure streams obtained during a cycle having a PP at the start of the depressuπzation phase.

Thus, this example shows that, when a cocurrent depressuπzation substep, the output stream of which is directed into a purge stream, is included at the start of regeneration, then a 5% increase in CO yield in the high- pressure stream and a 7% increase in N₂ yield in the low- pressure streams are effectively achieved.

This type of separation process can be applied to various chemical processes in which the recovery of molecules is advantageous but in which the required level of purity is not critical. The cycles described will be particularly relevant when certain inert species have to be at least partly separated from the main stream.

Thus, with this process it is for example possible to capture CO2 and purge some of the inert species.

Claims

1. Process for separating a gas mixture by adsorption, said mixture comprising at least a first compound, a second compound, a third compound and a fourth compound belonging to at least a first group, a second group, a third group and a fourth group respectively, said groups having increasing adsorptivities, in which N adsorbers (where N > 2) are used, each adsorber following, in a staggered fashion, a pressure cycle in which a production phase at the high pressure (HP) of the cycle, a depressuπzation phase down to the low- pressure (LP) of the cycle and a repressurization phase back up to the high pressure (HP) of the cycle come after one another, characterized in that at least the depressurization phase starts: either with a cocurrent depressurization step during which all of the stream or streams leaving the adsorber or adsorbers in cocurrent depressurization mode and rich in compounds of the second group is/are sent to the adsorber or adsorbers undergoing the elution step, so as at least partly to separate the compounds of the second group; or with a cocurrent depressurization step during which the stream leaving the adsorber or adsorbers undergoing cocurrent depressurization and rich in compounds of the second group leaves the system consisting of the N adsorbers so as at least partly to separate the compounds of the second group; and includes a partial balancing of the pressures between at least one adsorber undergoing cocurrent depressurization and at least one adsorber in the repressurization step, so as to produce, during the production phase, a stream of gas enriched with compounds of the first and third groups but depleted in compounds of the second group.

2. Process according to Claim 1, characterized in that at least one stream of gas at the low pressure (LP) of the cycle, preferably at a pressure between 1 and 2 bar, which is enriched with compounds of the second group, is recovered during the depressuπzation phase.

3. Process according to either of the preceding claims, characterized in that a countercurrent depressurization step, during which the stream leaving the adsorber or adsorbers undergoing countercurrent depressurization leaves the system consisting of the N adsorbers, is carried out during the depressurization phase.

4. Process according to one of the preceding claims, characterized in that the first group includes hydrogen and/or the third group includes carbon monoxide.

5. Process according to one of the preceding claims, characterized in that the compounds of the second group are chosen from nitrogen, argon and rare gases.

6. Process according to one of the preceding claims, characterized in that the system consisting of N adsorbers is a PSA unit.

7. Process according to one of the preceding claims, characterized in that the high pressure of the cycle is between 15 and 40 bar, preferably between 20 and 30 bar.

8. Process according to one of the preceding claims, characterized m that said N adsorbers comprise adsorbents having a selectivity, with respect to compounds of the second group, of not less than 2, preferably not less than 4, for compounds of the third group, and a selectivity, with respect to compounds of the second group, of not less than 1, preferably between 1 and 2, for compounds of the fourth group .

9. Process according to one of the preceding claims, characterized in that the gas mixture comes from a steam or oxygen reforming process, from a partial oxidation process, from a coal, residue or biomass gasification process or from hybrid processes, or is a product or an effluent of a chemical reaction, preferably a product of the Fisher-Tropsch reaction or from the generation of methanol, oxo-alcohols or DME from syngas.