Pressure swing adsorption method with additional elution

Abstract

A pressure swing adsorption process for producing a gas stream enriched in a compound X from a feed gas stream is provided. The process includes at least 2 adsorbers, with each adsorber being subjected to a pressure cycle having a high pressure and a low pressure. The process includes adsorption at the high pressure with production of the gas stream enriched in compound X; then depressurization; then elution at the low pressure; and finally repressurization to the high pressure. The elution gas is made up fractions of gas stream resulting from the depressurization of an adsorber and the gas stream enriched in compound X. The pressure cycle has a phase time corresponding to the duration of a pressure cycle divided by the number of adsorbers and the fraction of the gas stream enriched in compound X is determined as a function of the phase time.

Claims

1. A pressure swing adsorption process for producing a gas stream enriched in a compound X from a feed gas stream, the process comprising at least 2 adsorbers, each adsorber being subjected to a pressure cycle having a high pressure and a low pressure, the process comprising the following successive steps: a) adsorption at the high pressure with production of the gas stream enriched in compound X; b) depressurization to the low pressure; c) elution at the low pressure by means of an elution gas; d) repressurization to the high pressure wherein: the elution gas comprises a fraction of a gas stream resulting from the depressurization of an adsorber and a fraction of the gas stream enriched in compound X, the fraction of the gas stream enriched in compound X is adjustable via an adjustment means, and the pressure cycle has a phase time corresponding to the duration of a pressure cycle divided by the number of adsorbers and the fraction of the gas stream enriched in compound X is determined as a function of the phase time.

2. The process claim 1, wherein the fraction of the gas stream enriched in compound X is adjusted as a function of at least one of the operating conditions of the pressure swing adsorption process.

3. The process of claim 2, wherein the operating conditions include the feed flow rate of the pressure swing adsorption process, the low pressure, the high pressure, the temperature of the feed stream, the composition of the gaseous feed stream and the required concentration of compound X in the enriched gas stream.

4. The process claim 1, wherein the adjustment means comprise an automatic valve.

5. The process of claim 4, wherein the automatic valve also performs another function in the pressure swing adsorption process cycle.

6. The process of claim 1, wherein the pressure cycle comprises from 1 to 6 equalizations.

7. The process of claim 1, wherein the depressurization step b) comprises several sub-steps and the fraction of the gas stream resulting from the depressurization is withdrawn at the same time as or at least partially with another sub-step.

8. The process of claim 1, wherein the fraction of the gas stream enriched in compound X is withdrawn throughout the duration of a phase time.

9. The process of claim 1, wherein the compound X is hydrogen or CO2.

10. A pressure swing adsorption process for producing a gas stream enriched in a compound X from a feed gas stream, the process comprising at least 2 adsorbers, each adsorber being subjected to a pressure cycle having a high pressure and a low pressure and comprising the following successive steps: a) adsorption at the high pressure with production of the gas stream enriched in compound X; b) depressurization to the low pressure; c) elution at the low pressure by means of an elution gas; d) repressurization to the high pressure wherein: the elution gas comprises a fraction of a gas stream resulting from the depressurization of an adsorber and a fraction of the gas stream enriched in compound X, the fraction of the gas stream enriched in compound X is adjustable via an adjustment means, and the pressure cycle has a phase time corresponding to the duration of the pressure cycle divided by the number of adsorbers and the fraction of the gas stream enriched in compound X is selected so that the phase time corresponds to the nominal phase time 5%, the nominal phase time corresponding to the phase time of the PSA under design conditions.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawing, in which like elements are given the same or analogous reference numbers and wherein:

(2) FIG. 1 illustrates the difference between the standard regulation and the regulation according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

(3) The design point of the PSA is point A of the graph. For the nominal flow rate of feed gas (100), the PSA unit has been optimized with a minimum phase time Tin order to have the minimum volume of adsorbentand for the required extraction efficiency A.

(4) It is not necessary here to go back into detail of the determination of the optimal phase time which may be based on the minimal duration required for a step (owing to the adsorption kinetics, pressure drops, etc.) or on a series of steps that have to keep within a phase time.

(5) The elution gas thus originates on the one hand from a co-current depressurization step and on the other hand from the production. The conventional regulation will make it possible to maintain the nominal efficiency A by increasing the phase time in inverse proportion to the reduction of the feed flow rate. Thus at 80% of the nominal flow rate, the phase time will correspond to 1.25 T, at 50% to 2 T, at 33% to 3 T.

(6) The new regulation according to the invention corresponds to the thick line from the same FIG. 1.

(7) The overdesign of the PSA is taken advantage of relative to the design point brought about by the reduction in flow rate in order to use a cycle of lower productivity and of higher efficiency. In order to do this, the amount of elution gas originating from the production is reduced. The curve AB of the graph is moved over at constant phase time T with an efficiency that increases until reaching B for a flow rate of 70% of the nominal flow rate. In the example given, below 70% of the flow rate, a conventional regulation is returned to, that is to say that the phase time will be increased, but this will make it possible to maintain the maximum efficiency B and not the lower design efficiency A.

(8) According to the design conditions, the point B may correspond to the case where the amount of elution gas originating from the production has been canceled or else to the case where the total flow rate of elution gas becomes too low to be able to ensure the purity desired for the production. Below that, the performance drops and offers no advantage.

(9) It is advisable here to make a certain number of observations.

(10) Moving over the curve AB is normally the optimum since advantage is completely taken of the overdesign which is converted into efficiency. It is of course possible to use an approach intermediate between the two regulations described by using only a fraction of the overdesign for the purpose of increasing the efficiency and of at the same time adjusting the purity of the production to the targeted purity via an increase in the phase time.

(11) The potential gain in efficiency will depend on the point chosen for the design.

(12) For an H.sub.2-PSA, it may be several percent in the case of a moderate nominal efficiencyfor example going from 86% to 88%or several tenths of a percent in the case of an initial efficiency that is already very high, for example from 89.1% to 89.5%. It may however be observed that in the second case, it is generally a unit that produces large flow rates of hydrogen, more than 100 000 Nm.sup.3/h, and that the supplementary production is counted in hundreds of Nm.sup.3/h.

(13) Conversely, when starting from a relatively low efficiency, it is possible to achieve very significant gains in efficiency (point C from FIG. 1).

(14) Point D is placed as a matter of interest. It is generally possible to treat more feed gas than the nominal flow rate at the expense of a loss of efficiency. The regulation according to the invention, by adjusting the amount of elution gas, may also make it possible to find a better solution than the simple reduction of the phase time.

(15) The feed gas flow rate, more particularly a reduction in the flow rate treated, has been selected as an operating parameter that makes it possible to provide a margin regarding the volume of adsorbent as the most direct example. It is understood that other parameters may have the same effect such as a low pressure below the nominal low pressure, which facilitates the regeneration. Likewise, it is possible to have a purer feed gas, a lower purity required for hydrogen, more favorable temperature or pressure conditions of the feed gas, etc. In these cases, the correction linked to the purity of the hydrogen will be at first generally a determining factor in the regulation since the variation of these operating parameters (pressures, composition, etc.) will not be able to be directly taken into account in the regulation. In practice, a drop in the regeneration pressure for example, at an unchanged feed flow rate, will result, in order to maintain the required purity, in treating slightly more flow rate, i.e. in increasing the phase time. This increase in phase time will then make it possible to reduce the elution by the production in order to return to the nominal phase time and by so doing increase the extraction efficiency.

(16) It will be noted that in order not to disturb the production, it could be advantageous to draw off the adjustable fraction of elution gas withdrawn from said production in a continuous manner and with a constant flow rate. This fraction must therefore be withdrawn over one (or more) complete phase times.

(17) In order to better explain the principle of the invention, the case of H.sub.2-PSAs has been taken, which PSA process has probably known the most developments in recent years due to its success with various hydrogen-consuming industries.

(18) This is understood to mean hydrogen having a purity of greater than 95 mol %, generally greater than 98 mol % and that may reach, when the consuming process requires it, purities of greater than 99.99%.

(19) But the invention in its very principle may apply, a priori, to all PSA cycles in the most general sense since there is a relationship between the amount of elution gas, the productivity and the efficiency.

(20) Thus, it is possible to envisage, for example, a PSA unit intended to strip the CO.sub.2 from an oxygen blast furnace gas in order to recycle it while producing CO.sub.2, at a purity sufficient to sequester it either directly, or after a supplementary treatment (coldbox, membrane, other adsorption unit, etc.).

(21) The cycle of the CO.sub.2-PSA may comprise supplementary steps such as a Rinse step which consists in circulating in co-current mode, in the adsorbent bed, a fraction of the CO.sub.2 production at the pressure at the end of the last equalization in order to drive the lightest constituents (CO, CH.sub.4, H.sub.2, N.sub.2) from the adsorbent inlet zone in order to subsequently recover, during the Blowdown (counter-current depressurization), an effluent highly enriched in CO.sub.2. It will be noted that the CO.sub.2 may also be extracted from the adsorber during the elution phase, or at least during part of this next step with the (optional) aid of a vacuum pump. It is thus possible to recover a gas containing more than 80 mol % of CO.sub.2.

(22) The gas recovered at the outlet during the Rinse step is used during a purge step P to push the CO.sub.2 back toward the inlet end of the adsorber. The gas recovered at the purge outlet is discharged to outside of the unit or recycled in the feed gas.

(23) More simply, it may be a question of stripping the CO.sub.2 from a conventional (air combustion) blast furnace gas in order to reinject the CO.sub.2-depleted gas into the blast furnace.

(24) Among other applications of PSAs capable of using the regulation according to the invention, mention may be made, non-limitingly, of the production or purification of helium, the production of oxygen, the production or stripping of nitrogen, the separation of hydrocarbons, gas drying, etc.

(25) The basic applications envisaged for PSAs according to the invention are preferably processing units of high flow rate, for example more than 100 000 Nm.sup.3/h, with conventional adsorbents and standard valves. The phaseswithin the meaning of the definition given abovewill have a duration preferably equal to 10 seconds and more, resulting in cycles having a duration of generally more than one minute and preferably of at least 2 minutes.

(26) Nevertheless, the cycles according to the invention are also very suitable for the new adsorbent structures in the process of being developed.

(27) The expression new structures is understood to mean structured adsorbents. The adsorbent is no longer in the form of millimetric particles (beads, rods, crushed materials, pellets, etc.) but has more complex geometries and sizes of several centimeters or even several tens of centimeters. They may be monoliths, parallel-passage contactors, a set of parallel fibers, adsorbent fabric, etc. The cycle times are then generally much shorter, of the order of several seconds or tens of seconds.