Duplex adsorption process

Abstract: A duplex thermal swing adsorption process is carried out in one or more stages , each having a normal bed portion and an inverted bed portion that enables the less selectively adsorbable light component of a fluid mixture to be displaced from the normal bed portion and the more selectively adsorbable heavy component to be displaced from the inverted bed portion.

Description: Background of the Invention Field of the Invention The invention relates to fluid separation using a thermal swing adsorption process. More particularly, it relates to the use of said process to enhance the recovery of two purified product fractions. Description of the Prior Art Thermal swing adsorption (TSA) processes and systems are well known in the art (e.g. US-A-3 928 004) for achieving desirable separation and purification of a feed gas stream containing a more selectively adsorbable component and a less selectively adsorbable component. The more selectively adsorbable component is adsorbed as the feed gas is passed, at a lower adsorption temperature, over an adsorbent bed capable of selectively adsorbing said more selectively adsorbable component. Upon subsequent increase of the bed temperature to a higher desorption temperature level, the more selectively adsorbable component is desorbed from the adsorbent. TSA processing is commonly carried out in systems containing more than one adsorbent bed, with each bed undergoing a processing sequence, on a cyclic basis, comprising (a) lower adsorption temperature feed - adsorption of the more selectively adsorbable component - discharge of less selectively adsorbable component, (b), higher temperature desorption and removal of the less selectively adsorbable component, typically from the feed end of the bed, and (c) cooling of the bed to said lower adsorption temperature. TSA processing is suited for fluid separation operations in a variety of industrial applications, particularly in relatively small sized operations for which the use of cryogenic air separation plants may not be economically feasible. TSA processing is also well suited to the drying of fluids. In such drying applications, the moist fluid is passed to the feed end of an adsorbent bed that preferentially adsorbs the water as the more selectively adsorbable component thereof. As water is removed from the adsorbent at the feed end of the bed, the bed becomes loaded with water and loses its adsorptive capacity with respect to additional quantities of moist fluid passed therethrough. The following fluid, depleted of moisture, encounters a zone of relatively dry adsorbent and emerges from the discharge end of the bed as a dry feed fluid product. As such drying operations continue, an adsorption front of the more selectively adsorbed component, or mass transfer zone, moves through the bed from the feed end in the direction of the opposite, discharge end thereof, until the adsorption front reaches the vicinity of the discharge end and nearly all of the bed is water loaded. Before further drying can be achieved, the bed must be regenerated, i.e. the more selectively adsorbed water must be desorbed and removed from the bed. In TSA processing for drying, the selectively adsorbed water can be removed from the bed by heating the bed from its lower adsorption temperature to a higher desorption temperature, and by flowing a dry purge fluid through the bed from the discharge to the feed end thereof. The adsorption front of more selectively adsorbed water is thus driven back to the feed end of the bed. In a properly designed system, the concentration of the water impurity in the fluid will increase when the temperature is increased from its lower adsorption level. The amount of purge gas then required for desorption and removal of the more selectively adsorbed water is less than the amount of fluid dried during adsorption. A portion of the dry fluid product is typically used as the purge fluid, with the remaining dry fluid being removed from the system as the ultimate product stream. Such conventional processing is suitable for air drying application because the moist feed air is freely available, and a high degree of recovery of the air is thus unnecessary. That is, the loss of air in the waste stream after purging is not of major importance. As those skilled in the art will appreciate, however, such a process may not be desirable or even satisfactory for the purification of a valuable fluid that must be recovered without appreciable loss. There remains a need in the art for improved TSA processing that is relatively simple, that uses only a few adsorbent beds and that can economically be employed to purify or separate a feed fluid stream with high recovery of the desired product fluid or fluids. It is an object of the invention to provide an improved TSA process for separating a multicomponent feed fluid stream into two purified streams, without appreciable loss of the desired purified product fluids. It is another object of the invention to provide an improved TSA process for efficiently separating a binary fluid stream into two pure fluid fractions with a high recovery of both components of said fluid stream. It is another object of the invention to provide a TSA process for the removal of a more selectively adsorbable impurity from a less selectively adsorbable fluid so as to produce a purified fluid stream with a high recovery of the desired product fluid. It is a further object of the invention to provide a TSA process for the removal of a less selectively adsorbable impurity from a more selectively adsorbable component so as to produce a purified fluid stream with a high recovery of the desired product component. It is a further object of the invention to provide an improved TSA process for the removal of traces of nitrogen from an impure argon stream so that a purified argon stream can be produced with only a negligible loss of argon product in the purification process. With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly fronted out in the appended claims. In conformity with the subject invention a duplex temperature swing adsorption process as defined in claim 1 is provided. In the following a duplex pressure swing adsorption (PSA) process according to the parent application (patent-number EP-B-0 483 670) of the present divisional application is described, with the help of which the principle concept of the process of the present invention can be illustrated. According to the present application a fluid mixture is separated by using the duplex adsorption process in thermal swing processing (TSA) embodiments. In such TSA operation a lower adsorption temperature will correspond to a higher adsorption pressure of a PSA process, and a higher desorption temperature will correspond to a lower desorption pressure of a PSA process. While the PSA processing operations include provisions for compressing and depressing the stages and the feed streams thereto, the TSA processing operations include corresponding provisions for heating and cooling the stages and the feed streams thereto. For such heating operations, indirect heating or heating by the use of tubes embedded in the stages can be employed. Heat exchangers can be employed for desired cooling. Brief Description of the Drawings Fig. 1 is a schematic drawing illustrating a two-bed duplex PSA system. Fig. 2 is a schematic drawing illustrating a four-bed duplex PSA system. It will be appreciated in the art that the normal PSA process is generally carried out in two or more beds in which each bed, on a cyclic basis, undergoes a pressurization - depressurization sequence in which the more selectively adsorbable, or heavy, component is adsorbed at a higher adsorption pressure and desorbed at lower desorption pressure. In such normal processing sequence in each bed, the feed gas is compressed to the higher adsorption pressure and pressed to the feed end of the bed, with the heavy component(s) being selectively adsorbed and with at least some of the light component(s) being removed from the discharge end thereof as purified light gas product. The remaining gas removed from the discharge end of the bed during the high pressure adsorption step is conveniently expanded and used to provide low pressure, countercurrent flow purge gas passed to the discharge end of another bed in the system. When the adsorptive capacity of the first bed is nearly exhausted, as by the adsorption front of the more selectively adsorbable component passing through the bed to the vicinity of the discharge end thereof, the flow of feed gas to the bed is terminated, and the depressurization phase of the cycle is commenced. At this point, the first bed is typically cocurrently depressurized by the release of gas from the discharge end of the bed. The gas thus released from the discharge end of the bed is preferably expanded into the discharge end of the second bed, or another bed in the system, for pressure equalization and/or provide purge gas purposes. The bed is then further depressurized to a lower desorption pressure level as by countercurrent depressurization and release of gas from the feed end of the bed, with such gas comprising the more selectively adsorbable component of the feed gas and being passed to waste or other use when the less selectively adsorbable component is the desired product gas. Following purge by the addition of the purge gas to the discharge end of the bed at its lower desorption pressure level, the bed is repressurized to the higher adsorption pressure level. Typically, the bed is initially repressurized from the lower desorption pressure to an intermediate pressure by the passage of gas from the discharge end of the second bed, or of another bed in a system undergoing depressurization, to the discharge end of said first bed for pressure equalization purposes. Feed gas is then passed to the feed end of the bed in order to increase the pressure thereof from the intermediate pressure to the upper adsorption pressure. The passage of feed gas to the feed end of the bed at the upper adsorption pressure is then continued, with the less readily adsorbable component being withdrawn therefrom as the cyclic operation is continued in said first bed. To operate effectively, essentially all of the adsorbed heavy component(s) must be removed from the feed end of the bed and discharged to waste. A certain minimum amount of light component is necessarily lost in the waste, depending upon the components involved, the adsorbent material used in the beds, the pressure ratios involved and the like, in order to desorb and purge the more selectively adsorbable heavy impurities from the bed. The recovery of light gas component is inherently limited by this circumstance, particularly by the pressure ratio between the upper adsorption pressure and the lower desorption pressure inherent in such PSA processing. The normal PSA process is restricted, therefore, in the separation and recovery of the less selectively adsorbable component(s) to applications involving the separation of selectively low-value feed streams. A duplex PSA process and system is illustrated in Fig. 1 in which the feed gas passed to each bed of a two-bed system at an intermediate point therein. By contrast, the feed gas is passed to a feed end of each bed in the normal PSA and the inverted PSA processes and systems referred to above. In the Fig. 1 embodiment, feed gas in line 1 is compressed by means of compressor 2 and passed, alternately, through valve 3 to adsorbent bed 4 at an intermediate point 5 between the ends thereof, or through valve 6 to bed 7 at intermediate point 8 thereof. Light component removed from the upper end of bed 4 is passed through valve 9 to line 10 for recovery as light component product gas. Similarly, light component removed from the upper end of bed 7 is passed through valve 11 to said line 10 for recovery as said light product gas. A portion of the light component removed from the top of either bed can be passed to the top of the other bed by passage through line 12 containing valve 13. Heavy component removed from the bottom of bed 4 can be passed through line 14 containing valve 15 for passage to line 16 containing valve 17 for recovery as heavy component product gas. Similarly, heavy component removed from the bottom of bed 7 can be passed through line 18 containing valve 19 for passage to said product line 16. The heavy component from beds 4 and 7 can also be passed through valves 20 and 21, respectively, for passage to compressor 22 and optional storage vessel 23 prior to recovery as heavy component product or recycle to the system. Valves 24 and 25 are provided so that the systems can conveniently continue to run in an unloaded condition without unneeded compression of the feed gas and heavy component product streams. The system shown in the Fig. 2 drawing is essentially the same as the two-bed duplex system of Fig. 1, except that the beds are divided at the feed point, resulting in a four-bed system. Thus, the Fig. 2 embodiment employs separate pairs of beds 4 and 4a and 7 and 7a, respectively, with feed gas passing to lines 5 and 8 communicating, respectively, with each pair of beds at an intermediate point there between. It will be understood that the upper portion of the duplex systems, i.e. the upper portion of bed 4 in the Fig. 1 embodiment, and beds 4 and 7 in the Fig. 2 embodiment, function as does the light-product end of a system for the normal PSA process. Similarly, the lower portion of the duplex systems, i.e. the lower portion of bed 4 in the Fig. 1 embodiment, and of beds 4a and 7a in the Fig. 2 embodiment, function as does the heavy-product end in the inverted PSA process of Wilson. Described with reference to the Fig. 2 embodiment for illustration, the feed gas comprising a binary mixture of light and heavy, i.e. less and more selectively adsorbable, components is passed in line 1 to compressor 2 for compression to a high adsorption pressure. During one portion of the overall processing cycle, the compressed feed gas is passed through valve 3 to line 5 for passage to the bottom of bed 4. This feed gas, combined with the effluent from the upper end of bed 4a, flows upward through bed 4 wherein the heavy component is adsorbed, leaving the less selectively adsorbed light component to pass through the beds and emerge from bed 4. A portion of this purified light gas stream is passed through valve 9 and is withdrawn in line 10 as light product. The remaining purified light gas is expanded to low pressure through valve 13 and enters the upper, light product end of bed 7. This low-pressure gas flows backward through bed 7 where it acts to displace from the bed the heavy component previously adsorbed thereon in an earlier step of the cyclic process. The effluent thus removed from bed 7 through the bottom end thereof is passed through line 8 directly to the top of bed 7a where it continues to act to drive the heavy component from the bottom end of bed 7a. The effluent from bed 7a is passed through valve 21, is compressed in compressor 22 and enters storage container 23. A portion of this gas is withdrawn through line 16 and valve 17 for recovery as heavy product. Another portion of the gas from storage vessel 23 is passed through line 14 and valve 15 to the bottom end of bed 4a. This gas then flows forward through bed 4a displacing the light component therein. The effluent from bed 4a, which ideally can be near the composition of the feed stream, is combined with the feed gas being passed through line 1 and valve 3 to line 5 for passage into the lower end of bed 4. This step of the overall cyclic processing sequence must be terminated before the adsorption front of more selectively adsorbable component breaks through bed 4 and passes into the light product removed from the system and/or the portion thereof recycled through line 12 and valve 13 to the upper end of bed 7. During the next step of the PSA processing sequence, beds 4 and 4a are depressurized from the upper adsorption pressure, and beds 7 and 7a are repressurized from the lower desorption pressure to said upper adsorption pressure. In one desirable embodiment, a pressure equalization step is employed so that desirable pressure recovery can be achieved. For this purpose, the feed to the system and output flows from the system can be suspended during the period of time in which pressure equalization between the beds is being achieved. Thus, valves 3, 9, 11, 15, 19, 20 and 21 are all closed, while valve 13 is left open until the pressures equalize to some extent by the passage of gas from the top of bed 4, initially at the upper adsorption pressure, to the top of bed 7, initially at the lower desorption pressure. Valve 13 is then closed, and valves 20 and 19 are opened so that beds 4 and 4a can be further depressurized by the passage of gas from the bottom of bed 4a, while beds 7 and 7a are further repressurized to the upper adsorption pressure by the flow of gas from storage vessel 23 to the bottom end of bed 7a. During the carrying out of such steps, valves 24 and 25 can be opened when no flow is required from compressors 2 and 22, so that these units can continue to run in an unloaded form. During the next portion of the processing sequence, the feed gas from compressor 2 is passed through line 8 and valve 6 and flows upward through bed 7 at the upper adsorption pressure. Some of the purified light product separated from the more selectively adsorbable heavy component in bed 7 is passed through valve 11 and recovered as light product. The remaining portion of the purified light product is expanded through valve 13 and is passed to the upper end of bed 4, wherein it flows downward through the bed to displace the previously adsorbed heavy product. The effluent from bed 4 is passed downward through line 5 and into the upper end of bed 4a, where it flows downward carrying additional quantities of the previously adsorbed heavy component. The effluent from bed 4a is passed through valve 20, is compressed in compressor 22 and is passed to storage vessel 23. Some gas from storage vessel 23 is withdrawn through line 16 and valve 17 as heavy product, while additional gas from said storage vessel 23 is passed through line 11 and valve 19 to serve as displacement gas passed to the bottom end of bed 7a. The effluent from bed 7a is combined with feed gas in line 8 and is passed into the bottom of bed 7. Such operation is terminated before the adsorption waves in beds 7a and 7 break through into the effluent removed from the upper ends of said beds. In the final portion of the overall processing sequence, a pressure equalization is carried out as in the second portion thereof, so as to recover pressure in the beds initially at the upper adsorption pressure and thereby to pressurize the beds initially at lower desorption pressure to a higher intermediate pressure. At the end of the pressure equalization, valve 13 is closed, and valves 15 and 21 are opened to allow beds 7 and 7a to further depressurize and beds 4 and 4a to be repressurized. Upon thus completing the processing sequence, the cycle is repeated with fresh quantities of feed gas being passed to the beds in the processing sequence outlined above. It will thus be seen that this practice enables less selectively adsorbable light component to be displaced and recovered at one end of the system, and for more selectively adsorbable heavy component to be displaced and recovered at the other end of the system. The duplex pressure swing adsorption process employs a system having at least one pressure swing adsorption stage containing adsorbent material capable of adsorbing a more selectively adsorbable heavy component from the feed gas mixture containing said component and a less selectively adsorbable light component. In various embodiments, the use of two or more stages will be preferred, depending upon the overall conditions and requirements pertaining to a given application. Upon such selective adsorption, adsorption fronts of said more selectively adsorbable heavy component will tend to form in the adsorbent stage. Each stage will have the normal bed portion and the inverted bed portion referred to above. Each stage will undergo, on a cyclic basis, the processing sequence described above and more generally summarized as follows: The more selectively adsorbable heavy component is passed at an upper adsorption pressure to the bottom end of the inverted portion of the stage to displace less selectively adsorbable light component therefrom. The effluent gas thus displaced from the upper end of the inverted bed portion is passed into the lower end of the normal bed portion of the stage at the upper adsorption pressure for passage forward therethrough. The less selectively adsorbable light component is discharged from the upper end of the normal bed portion of the stage. The stage is then depressurized from the upper adsorption pressure to a lower desorption pressure. Less selectively adsorbable light component is then passed to the upper end of the normal bed portion at the lower desorption pressure for passage therethrough in a backward direction, thus displacing previously adsorbed heavy component therefrom. The effluent gas thus displaced from the bottom end of the normal bed portion is passed into the upper end of the inverted bed portion at the lower desorption pressure to displace additional quantities of the heavy component therefrom at said lower desorption pressure. The more selectively adsorbable heavy component is discharged from the bottom end of the inverted bed portion at said lower desorption pressure. The stage is then pressurized from the lower desorption pressure to the upper adsorption pressure. It will be understood that the feed gas mixture to be separated can be introduced into the stage at an intermediate point between the normal bed portion and the inverted bed portion thereof during at least one of the steps in which (a) the heavy component is passed to the bottom end of the inverted bed portion of the stage, (b) the stage is depressurized from the upper adsorption pressure to a lower desorption pressure, (c) the light component is passed to the upper end of the normal bed portion of the stage and (d) the stage is pressurized from the lower adsorption pressure to the upper adsorption pressure. Upon completion of the processing sequence, the processing steps are repeated in the stage with additional quantities of the feed gas mixture in a desired relationship with other stages in the overall system likewise undergoing said processing sequence. For certain embodiments, it will be preferred to introduce the feed gas mixture to the stage during the passing of said more selectively adsorbable heavy component to the bottom end of the inverted bed portion of the stage at the upper adsorption pressure. In other embodiments, however, it may be preferred to add the feed gas mixture during the step in which the less selectively adsorbable light component is being passed to the upper end of the normal bed portion at lower desorption pressure, or during the pressurization or depressurization steps, or during a combination of said steps, depending upon the requirements of a given application. While the duplex process has been described herein with reference to the lower or bottom end, and to the upper end, of the normal bed portion and to the inverted bed portion of each stage, said reference to upper and lower ends will be understood to be for convenience only, coinciding with the illustrated positioning of the bed portions in the drawings. The stages, the individual portions thereof, and the flows to and from said stages and portions can also be positioned, in other desired configurations in practical commercial applications. As illustrated in the Figs. 1 and 2 embodiments, each PSA stage and similarly the TSA according to the present invention can comprise a single bed of adsorbent material, or each stage can comprise individual adsorbent beds for the normal bed portion and the inverted bed portion of each stage. At least two PSA or TSA stages are commonly preferred, but in particular fluid separation processes having particular processing requirements, a one stage system may be desirable. In the practice of the invention, a portion of the less selectively adsorbable light component discharged from the upper end of the normal bed portion of a stage is commonly recovered from the system for use as a desired product or co-product or for disposal from the system as a waste stream of the fluid separation operation. Another portion of said light component is commonly used for passage to the upper end of one or more stages during the displacement step in which such light component is passed to the upper end of the normal bed portion of a stage at the higher desorption temperature of the TSA process. Similarly, a portion of the more selectively adsorbable heavy component discharged from the lower end of the inverted bed portion of a stage is commonly recovered as a desired product or co-product or for disposal from the system as a waste stream of the fluid separation operation. Another portion of said heavy component is commonly cooled and passed to the lower end of one or more stages during the displacement step in which the heavy component is passed to the lower end of the inverted bed portion thereof at the lower adsorption temperature of the TSA process. It will be understood that the portion of said light and heavy components discharged from one stage can be used as displacement fluid in other stages, or in the stage from which it was discharged, depending on the circumstances pertaining to the overall system employed in the practice of the invention. The process of the invention can be used to achieve a variety of commercially significant fluid separations. TSA processes of particular commercial interest include gas separations, such as the separation and recovery of light nitrogen from heavy carbon monoxide and/or carbon dioxide components, and liquid separations such as the separation and recovery of water as the heavy component from ethanol, as the light component, and the separation and recovery of normal hydrocarbons from mixtures thereof with iso-hydrocarbons. Those skilled in the art will appreciate that various changes and modifications can be made in the details of the invention as described herein without departing from the scope of the invention as recited in the appended claims, For example, the process can be carried out using any adsorbent material capable of selectively adsorbing a more selectively adsorbable heavy component from a feed gas or other fluid mixture containing said component and a less selectively adsorbable light component Equilibrium type adsorbent materials, such as zeolitic molecular sieves, e.g. conventional 13X and 5A, can be used, as can rate-selective adsorbent materials, such as activated carbon adsorbents and 4A molecular sieves. It will also be appreciated that the number of stages employed can be varied, and variations can be made in the processing sequence in accordance with known practices in the PSA and TSA art, particularly in the pressure or temperature change steps, to smooth out the overall operation and the flow of desired product therefrom.

Claims: A duplex thermal swing adsorption process for separating a fluid mixture in a system having at least one thermal swing adsorption stage containing adsorbent material capable of adsorbing a more selectively adsorbable component from a feed fluid mixture containing said component and a less selectively adsorbable light component, thereby forming an adsorption front of said more selectively adsorbable heavy component in said adsorbent, each stage having a normal bed portion and an inverted bed portion and undergoing, on a cyclic basis, a processing sequence comprising: (a) passing said more selectively adsorbable heavy component at a lower adsorption temperature to the bottom end of the inverted bed portion of said stage to displace less selectively adsorbable light component therefrom, the effluent fluid thus displaced from the upper end of said inverted bed portion being passed into the lower end of the normal bed portion of said stage at said lower adsorption temperature for passage forward therethrough; (b) discharging less selectively adsorbable light component from the upper end of the normal bed portion of said stage; (c) heating said stage from the lower adsorption temperature to an upper desorption temperature; (d) passing less selectively adsorbable light component to the upper end of the normal bed portion at said upper desorption temperature for passage therethrough in a backward direction, thus displacing previously adsorbed heavy component therefrom, the effluent thus displaced from the bottom end of the normal bed portion being passed into the upper end of said inverted bed portion at said upper desorption temperature to displace additional quantities of the heavy component therefrom at said upper desorption temperature; (e) discharging heavy component from the bottom end of said inverted bed portion at said upper desorption temperature; (f) cooling said stage from said upper desorption temperature to the lower adsorption temperature; (g) introducing said fluid mixture to said stage at an intermediate point between said normal bed portion and an inverted bed portion during at least one of said steps (b), (c), (d) and (f); and (h) repeating said steps (a) - (g) on a cyclic basis with additional quantities of said fluid mixture, whereby the less selectively adsorbable light component and the more selectively adsorbable heavy component are both recovered at enhanced purity and recovery levels. The process of claim 1 in which the feed gas or fluid mixture, respectively, is introduced to the stage at said intermediate point during step (a). The process of claim 1 in which the feed gas or fluid mixture, respectively, is introduced to the stage at said intermediate point during step (d). The process of claim 1 in which each stage comprises a single adsorbent bed. The process of claim 1 in which each stage comprises an individual adsorbent bed for the normal bed portion and for the inverted bed portion. The process of claim 1 in which said system comprises two thermal swing adsorption stages. The process of claim 1 in which a portion of the less selectively adsorbable light component discharged in step (b) is recovered from the system and another portion is used for passage to the upper end of one or more stages during step (d) therein, and a portion of the more selectively adsorbable heavy component discharged in step (e) is recovered from the system and another portion is used for passage to the bottom end of one or more stages during step (a) thereof. The process of claim 1 in which said fluid mixture comprises argon as the less selectively adsorbable light component and impurities as the more selectively adsorbable component. The process of claim 1 in which said fluid mixture comprises water as the less selectively adsorbable light component and impurities as the more selectively adsorbable heavy component. The process of claim 1 in which said fluid mixture comprises nitrogen as the less selective adsorbable light component and carbon oxide impurities as the more selectively adsorbable heavy component. The process of claim 1 in which said fluid mixture comprises water as the more selectively adsorbable heavy component and ethanol as the less selectively adsorbable light component. The process of claim 1 in which said fluid mixture comprises n-hydrocarbons as the less selectively adsorbable light component and iso-hydrocarbons as the more selectively adsorbable components.

Inventors: LEAVITT FREDERICK WELLS, LEAVITT, FREDERICK WELLS
Applicants: PRAXAIR TECHNOLOGY INC, PRAXAIR TECHNOLOGY, INC.
Application year: 1991
Main classification: B01D53
IPC codes: B01D53