FR3120546A3 - Method for treating a gas by pressure swing adsorption with low elution ratio - Google Patents

Abstract

The invention relates to a process for treating a gas by means of pressure swing adsorption by means of a unit comprising at least four cylinders, the unit treating a feed gas at a pressure below 14 bara, the process implements an elution pressure greater than 0.8 bara, with an elution ratio P/F strictly less than 1. Figure of abstract: FIG. 1

Description

Process for treating a gas by pressure swing adsorption with low elution ratio

The present invention relates to a process for treating a gas by pressure swing adsorption (PSA, for "Pressure Swing Adsorption") in particular to produce an enriched gaseous stream from a gaseous supply stream, and more particularly the gas treatment units by pressure modulation adsorption (PSA unit) implementing at least 4 adsorbers (or cylinders).

The invention finds a particularly advantageous, but not exclusive, application with PSAs H2, O2 or CO2.

In general, a gas phase adsorption process makes it possible to separate one or more molecules from a gas mixture containing them, by exploiting the difference in affinity of one or more adsorbents for the different constituent molecules of the mixture. The affinity of an adsorbent for a molecule depends on the one hand on the structure and the composition of the adsorbent and on the other hand on the properties of the molecule, in particular its size, its electronic structure and its multipolar moments. An adsorbent can be for example a zeolite, an activated carbon, an optionally doped activated alumina, a silica gel, a carbonaceous molecular sieve, a metallo-organic structure, an oxide or hydroxide of alkali or alkaline-earth metals, or a structure porous preferably containing a substance capable of reversibly reacting with molecules, substance such as amines, physical solvents, metal complexing agents, metal oxides or hydroxides for example.

The most classic adsorbent materials are in the form of particles (beads, sticks, crushed, etc.) but also exist in structured form such as monoliths, wheels, contactors with parallel passages, fabrics, fibers, etc.

We can distinguish 3 main families of adsorption process: lost charge processes, temperature modulation processes called TSA (Temperature Swing Adsorption) and finally PSA (Pressure Swing Adsorption) processes.

In lost charge processes, a new charge is placed when the one in use is saturated with impurities or more generally when it can no longer play its protective role sufficiently.

In TSA-type processes, the adsorbent at the end of its use is regenerated in situ, i.e. the arrested impurities are evacuated so that the said adsorbent recovers most of its adsorption capacity and can restart a purification cycle, the essential regeneration effect being due to a rise in temperature.

Finally, in PSA-type processes, the adsorbent at the end of the production phase is regenerated by the desorption of the impurities obtained by means of a drop in their partial pressure. This pressure drop can be obtained by lowering the total pressure and/or by flushing with a gas that is free or contains few impurities.

Pressure swing adsorption processes are used both to remove traces of impurities, for example with a content of less than one percent in the feed gas, and to separate mixtures containing tens of percent of different gases. In the first case, we generally speak of purification (for example gas drying) and separation in the second case (for example production of oxygen or nitrogen from atmospheric air).

In the context of the present invention, the term PSA designates any gas purification or separation process implementing a cyclic variation of the pressure seen by the adsorbent between a high pressure, called adsorption pressure, and a low pressure, called regeneration pressure. Thus, this generic name of PSA is used interchangeably to designate the following cyclic processes, to which it is also common to give more specific names depending on the pressure levels involved or the time required for an adsorber to return to its initial point (cycle time) :

• VSA processes in which the adsorption takes place substantially at atmospheric pressure, preferably between 0.95 and 1.25 bar abs and the desorption pressure is below atmospheric pressure, typically from 50 to 400 mbar abs;
• MPSA or VPSA processes in which the adsorption takes place at a high pressure above atmospheric pressure, typically between 1.5 and 6 bar abs, and the desorption at a low pressure below atmospheric pressure, generally between 200 and 600 mbar abs;
• actual PSA processes in which the high pressure is substantially greater than atmospheric pressure, typically between 3 and 50 bar abs and the low pressure substantially equal to or greater than atmospheric pressure, generally between 1 and 9 bar abs;
• RPSA (Rapid PSA) processes for which the duration of the pressure cycle is typically less than one minute;
• URPSA (Ultra Rapid PSA) processes for which the duration of the pressure cycle is of the order of a few seconds maximum.

It should be noted that these various designations are not standardized and the limits are subject to variation.

It is recalled that unless otherwise stated, the use of the term PSA here covers all these variants.

An adsorber will therefore begin a period of adsorption until it is loaded with the constituent(s) to be stopped at high pressure, then will be regenerated by depressurization and extraction of the adsorbed compounds before being restored to start again. a new period of adsorption. The adsorber then carried out a pressure cycle and the very principle of the PSA process is to chain these cycles one after the other; it is therefore a cyclical process. The time it takes for an adsorber to return to its initial state is called cycle time. In principle, each adsorber follows the same cycle with a time shift called phase time or more simply phase. We therefore have the relationship: phase time = cycle time / number of adsorbers. We see that the number of phases is equal to the number of adsorbers.

This cycle generally includes periods among:

• Production or Adsorption during which the feed gas is introduced through one end of the adsorber, the most adsorbable compounds are preferentially adsorbed and the gas enriched in the least adsorbable compounds (product gas) is extracted through the second end . Adsorption can take place at rising pressure, at substantially constant pressure, or even at slightly falling pressure;
• Depressurization during which the adsorber, which is no longer supplied with feed gas, is evacuated by at least one of its ends of some of the compounds contained in the adsorbent and the free volumes. By taking as a reference the direction of circulation of the fluid during the adsorption period, it is possible to define co-current, counter-current or simultaneously co-current and counter-current depressurizations;
• Elution or Purge, during which a gas enriched in the less adsorbable constituents (purge gas) is circulated through the adsorbent bed to help desorb the more adsorbable compounds. The Purge is generally done against the tide;
• Repressurization during which the adsorber is at least partially repressurized before resuming a period of Adsorption. The repressurization can be done counter-current and/or co-current;
• Dead time during which the adsorber remains in the same state. These dead times can be an integral part of the cycle, allowing the synchronization of steps between adsorbers or be part of a step that ended before the allotted time. The valves can be closed or left as they are, depending on the characteristics of the cycle.

In PSA cycles, it is common practice to use one or more feed-elution step(s) consisting of co-current depressurizing a bottle in order to supply the gas used to elute one or more other bottles from the PSA .

It is known that the amount of elution gas sampled during the feed-elution steps impacts the performance of the process. It is conventional to use the elution ratio P/F which is the ratio, in real cubic meters, that is to say under the respective operating conditions of pressure and temperature, of the quantity of elution gas used P on the amount of feed gas F entering the adsorber during the cycle. Many documents state that the P/F elution ratio must be greater than 1 to achieve substantially pure product gas.

We can cite for example "Pressure Swing Adsorption" by Ruthven, Farooq and Knaebel (chapter 6.1.4) which indicates a theoretical minimum of P/F equal to 1 and a recommended minimum value of 1.15 in the case of a PSA air dryer.

We can also cite “Adsorption Technology and Design” by Crittenden and Thomas (chapter 5.7) which recommends a P/F ratio between 1 and 2 for PSAs in general.

P/F ratios are often used in the literature with different definitions.

For example, document US5248321 calls P/F the ratio of the molar rate of elution to that of supply. The P/F estimated according to our definition remains slightly lower than 1, but it is a VSA process with regeneration under high vacuum (0.03 bara).

US8901359 describes a prior art ethanol drying process with a typical P/F of 0.07 and explains that the recommended value is usually close to 1, but the P/F ratio is time limited. available for the elution step in a 2 or 3 bottle PSA cycle.

The document US8901359 consists in increasing the P/F ratio to a value close to 1 by increasing the time allocated to the elution in order to improve the performance of the drying process.

It follows that, in the case of a PSA with at least 4 bottles where the time allocated to elution can be greater, a P/F of around 1 would give better performance.

In certain cases of separation to be carried out, compressing the gas to be separated to a high pressure represents a prohibitive energy cost (elimination of CO2 in a blast furnace gas). It is then preferable to design a PSA operating at medium pressure. In this case, the constraint of maintaining a P/F ratio greater than 1 induces a limitation in performance. Indeed, the real volume corresponding to the same quantity (mass or molar) of supply gas is inversely proportional to the pressure, which leads automatically when a lower operating pressure is used, at the same ratio P/ F and at constant elution pressure, to increase the elution rate proportionally. This additional flow of elution gas corresponds to a loss of recoverable product which leaves with the residual.

In the case of a blast furnace gas from which we want to remove CO2, we studied the performance of a PSA with 8 cylinders and 2 balances at different pressures and P/F.

It shows that, contrary to expectations, for an operating pressure lower than 14 bara, the P/F ratio giving the optimal performance is lower than 1.

It is also observed that increasing the operating pressure above 14 bara leads to a lower optimal yield of the PSA.

The present invention aims to effectively remedy these drawbacks by proposing a gas treatment unit by pressure swing adsorption by means of a gas treatment unit by pressure swing adsorption comprising at least four adsorbers, the unit treating a feed gas at a pressure lower than 14 bara, the method implementing an elution pressure higher than 0.8 bara, with an elution ratio P/F strictly lower than 1, said ratio representing the quantity of used elution gas P to the quantity of feed gas F entering the adsorber during a pressure cycle, said quantity of used elution gas P and said quantity of feed gas F being each expressed in meters real cubes, that is to say under respective operating conditions of pressure and temperature.

The invention will be better understood on reading the following description and on examining the accompanying figures. These figures are given only by way of illustration but in no way limit the invention.

The is a plot of unit yield versus P/F elution ratio for different operating pressures.

As seen on the , 4 curves are shown, each showing the evolution of the efficiency of a unit comprising 4 adsorbers. Each curve relates to a specific operating pressure (respectively 8, 11, 14 and 18 bara). The operating pressure is the unit supply gas pressure. For each of these curves, the elution pressure is greater than 0.8 bara.

It can be seen that an optimal yield is obtained for operating pressures below 14 bara and when the P/F elution ratio is strictly less than 1.

It can be seen that for each of the pressures, an optimal efficiency is obtained for a particular value of P/F. This bell-shaped curve (yield vs P/F ratio) is usual, meaning that to the left of the optimal value of P/F, the adsorbent is not regenerated sufficiently, while conversely, to the right, more gas is used. elution as necessary.

It can also be seen that depending on the pressure of the supply gas, the maximum value of efficiency also follows a bell curve with the best efficiency obtained for 11 bara.

Obtaining a maximum efficiency value for such a low pressure is relatively surprising because this effect is usually encountered when operating at much higher pressures, such that the adsorbent is close to being totally saturated, that is to say that it has then reached its theoretical limit filling rate. It should also be noted that operation at 8 bara, a pressure for which there is little loss in efficiency compared to the best possible solution, can be interesting since this leads to limiting the compression of the supply gas with corresponding gains in energy. and in investment.

It is also surprisingly observed that optimal yields are achieved, for operating pressures below 14 bara, when the elution ratio P/F is strictly less than 1, even for particularly low operating pressures, such as for example with an optimal P/F of the order of 0.75 to 8 bara.

The results of this study show more generally that in the search for an optimal solution, it is advisable not to be limited either to the teaching of old rules, often established in simpler cases than those encountered at present, nor to the lesson drawn from a particular case and generalized too hastily.

We have just seen here that units of the PSA type could, contrary to a very widespread idea, not only operate with a purge ratio of less than 1, but also that this could correspond to an optimum. It is indeed not easy to be able to justify a priori in a theoretical and quantitative way such a result.

It is however conceivable that when there are several constituents to be stopped, namely a first constituent and a second constituent, in order to produce a flow enriched in the least adsorbable constituent from a feed containing these three constituents (the the least adsorbable constituent and the first and second constituents), the use of an overall ratio of the P/F type as defined previously can only be approximate.

In fact, in the general case, a multi-bed PSA will be used in this type of process with a first layer of adsorbent dedicated essentially to retaining the first constituent and a second layer of adsorbent dedicated to stopping the second constituent. The feed gas flow through the adsorber and its composition varies from inlet to outlet and so does the elution gas.

This is particularly true if the first and second adsorbent layers are considered separately: the gas passing through the second adsorbent layer no longer contains the first constituent previously stopped in the first adsorbent layer whereas the purge which will elute this first layer of adsorbent will include, in addition to the initial quantity of elution gas injected through the second layer of adsorbent (in practice, a stream consisting essentially of the least adsorbable constituent), the desorbed fraction of the second constituent .

It is understood that the estimation of the real power of regeneration will be different from one layer of adsorbent to another and, therefore, is more complex than in the case of the simple stoppage of traces of an impurity in a gas. not very adsorbable for which a theoretical approach can effectively lead to the use of a purge factor slightly greater than 1.

We can make this type of reasoning, certainly qualitative and approximate, with other parameters influencing the performance of PSA type units for which there are also a prioris that can lead the designer of the unit on the wrong track. Thus, it should no longer be surprising for those skilled in the art to notice, for example, that in certain cases:

• adding balancing steps does not improve performance;
• increasing the supply pressure or the HP/LP ratio is not always favorable and that there may be an optimum as demonstrated by this study;
• lengthening certain steps to be at equilibrium (adsorption, elution, etc.) and shortening others to create a kinetic limitation (balancing, etc.) can be interesting;
• using a feed gas that is too pure leads to a drop in the extraction efficiency;
• lowering the yield does not increase the purity.

Applying this type of approach has the consequence, when looking for the solution, of widening the value ranges in which we want to vary a given parameter affecting the performance of a unit (for example exploring a P/F ratio ranging from 0.5 to 1.5 instead of 1.05 to 1.25) and to add parameters (for example, shorten a step by extending it with a dead time that will be varied).

This can lead to a multiplication of possible cases which would no longer be compatible with the computer resources available or with the time allocated to the project.

It is then necessary to couple the adsorption process simulator with an optimum search software in a multi-parameter system. With such a system, there is no need to determine all the possibilities to retain the best one, the software finding the path itself to arrive at the result much more quickly.

Claims (1)

A method of treating gas by pressure swing adsorption using a pressure swing adsorption gas treatment unit having at least four adsorbers, the unit treating a feed gas at a pressure below 14 bara, the method implementing an elution pressure greater than 0.8 bara, with an elution ratio P/F strictly less than 1, said ratio representing the quantity of elution gas used P over the quantity of gas of feed F entering the adsorber during a pressure cycle, said quantity of elution gas used P and said quantity of feed gas F being each expressed in real cubic meters, that is to say in respective operating conditions of pressure and temperature.