AU658544B1 - Pressure swing adsorption process for purifying a high pressure feed gas mixture with respect to its less strongly adsorbed component - Google Patents

Description

iu 4 Regulation 3.2

AUSTRALIA

Patents Act 1990 SOM:!'W~ nl'PE::- SnE T p I C2E I OJS[: 1-: APPLICANT: AIR PRODUCTS AND CHEMICAL~S INC NUM4BER: FILING DATE: Invention Title: PRESSURE SWING ADSORPTION PROCESS FOR PURIFYING A HIGH PRESSURE FEED GAS MIXTURE WITH RESPECT TO ITS LESS STRONGLY ADSORBED COMPONENT The following statement is a full des(c-,ription of this invention, including the best method of perfr-mJ.!ng it known to me: I la PRESSURE SWING ADSORPTION PROCESS FOR PURIFYING A HIGH PRESSURE FEED GAS MIXTURE WITH RESPECT TO ITS LESS STRONGLY ADSORBED COMPONENT TECHNICAL FIELD The present invention relates to a pressure swing adsorption process for purifying a high pressure (greater than 200 psig) feed gas mixture with respect to its less strongly adsorbed component. An important application of the present invention is the purification of a high pressure natural gas feed stream with respect to its methane/C 2 hydrocarbon component wherein said component is produced at high purity and high recovery.

•meg PrsueBACKGROUND OF THE INVENTION Pressure swing adsorption (PSA) purification cycles wherein a high 10 pressure feed gas mixture is purified with respect to its less strongly adsorbed component are taught in the art. The less strongly adsorbed component in such a process can include one or more species and generally constitutes at least 75% of the feed mixture on a volume basis. The remaining more strongly adsorbed component in such a process can also include one or more species and is generally either discarded as waste or, where natural gas is the feed, burned for its fuel value. At a minimum, these cycles consist of the following three steps: passing the feed gas mixture throug!h an adsorption bed containing an adsorbent selective for the adsorption of the more strongly 20 adsorbed component to produce an adsorption bed saturated with the more strongly adsorbed component and a product stream enriched in the less strongly adsorbed component; depressurizing the adsorption bed to ambient pressure to produce a waste stream enriched in the more strongly adsorbed component; repressurizing the adsorption zone to the pressure level of the feed gas mixture prior to starting a new cycle.

To' improve the purity of the less strongly adsorbed component produced in step product stream, the PSA art further teaches the use of a purge step whereby the adsorption bed is purged with a stream consisting primarily of the less strongly adsorbed component immediately after the depressurization step. Such low pressure purgilig increases the purity of the product stream produced in step because it purges the bed of any of the more strongly adsorbed component which may remain in the bed

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rd 1_ -2after the depressurization step and which can therefore contaminate the product effluent in the subsequent adsorption step. One trade-off associated with low pressure purging is that it typically requires another bed be added to the multi-bed system in order to maintain continuous product withdrawal.

To reduce power requirements in PSA cycles, the PSA art further teaches the use of one or more pressure equalization transfers, during each of which, a portion of the depressurization effluent from one bed in a multi-bed system is transferred to another bed as partial repressurization ooeo i0 gas, thereby equalizing the pressures of the two beds involved in each pressure equalization transfer. In this way, the pressure energy of the feed stream can be at least partially recovered. In the case of high pressure feed PSA cycles, the high feed pressure will generally justify the use of multiple pressure equalization transfers. One trade-off associated 15 with pressure equalization .is that the adsorption capacity of the bed is "reduced in the subsequent adsorption step. This is because depressurization effluent, which contains a significant amount of the more strongly adsorbed component, tends to be adsorbed by the bed and thus uses up some of the adsorption capacity of the bed. Another trade-off associated with pressure equalization is that each pressure equalization transfer typically requires another bed ne added to the multi-bed system in order to maintain continuous product withdrawal.

An example of a PSA cycle for purifying a high pressure feed gas mixture with respect to its less strongly adsorbed component which utilizes both low pressure purging and pressure equalization is US Patent 3,986,849 by Fuderer et al. Fuderer specifically utilizes three pressure equalization transfers to partially recover the pressure energy of his high pressure feed gas mixture.

The conventional wisdom in purifying a high pressure feed gas mixture with respect to its less strongly adsorbed component is that the high feed pressure provides enough driving force or work such that depressurization to sub-ambient pressure (and its associated power penalty) is not necessary. (The amount of work that is available to effect a separation in a PSA cycle is a function of the size of the pressure swing during depressurization; the larger the pressure swing, the more work there is

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-3available to effect the separation.) For example, Fuderer depressurizes to ambient pressure only. The present invention has unexpectedly found, however, that depressurization to sub-ambient pressure in a PSA cycle which utilizes low pressure purging and pressure equalization for purifying a high pressure feed gas mixture with respect to its less strongly adsorbed component is advantageous in increasing both recovery of the less strongly adsorbed component and feed capacity of the adsorption bed beyond the associated power penalty.

An important application of the present invention is the purification m of a high pressure natural gas feed stream with respect to its methane/C 2 hydrocarbon component wherein said component is produced at high purity and high recovery. This application is important because, as the awareness of the benefits from clean air increases, there is a trend towards replacing petroleum fuels by liquid methane in the transportation industry. Although 15 the United States has an abundance of natural gas, it contains impurities such as water, sulfur-containing compounds, light hydrocarbons (ie C 3 hydrocarbons; note that C 2 hydrocarbons are generally not considered an impurity), heavy hydrocarbons (ie hydrocarbons) and carbon dioxide S which have to be removed prior to liquefaction to obtain the liquid methane fuel. The removal of the water, sulfur-containing compounds and heavy .hydrocarbons is best accomplished by thermal swing adsorption (TSA) since regeneration of an adsorbent which is saturated with such compounds is difficult and thus will normally require heating of the adsorption bed visa-vis mere depressurization of the adsorption bed. The removal of the 2E remaining carbon dioxide and light hydrocarbons is best accomplished by the PSA process of the present invention.

SUMMARY OF THE INVENTION The present invention is a pressure swing adsorption (PSA) process for purifying a high pressure (greater than 200 psig) feed gas mixture with respect to its less strongly adsorbed component. In addition to the basic adsorption, depressurization and repressurization steps, the process of the present invention utilizes a low pressure purge step and one or more pressure equalization transfers. A key to the present invention is that the depressurization step is performed to a sub-ambient pressure level.

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-4- BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is schematic di,,gram depicting one embodiment of the present invention which utilizes six adsorption beds and three pressure equalization transfers.

DETAILED DESCRIPTION OF THE INVENTION The process of the present invention is best illustrated with reference to a specific embodiment thereof such as Figure l's embodiment which utilizes six adsorption beds and three pressure equalization 10 transfers. Figure l's process configuration consists of vacuum pump VI, valves 1 through 31 and six adsorption beds BI through B6 each containing an adsorbent selective for the adsorption of the more adsorbable component.

In the case of a natural gas feed, any adsorbent(s) capable of selectively adsorbing natural gas impurities may be used.- Multi-layers of adsorbents 15 may also be used. Examples of such adsorbents are zeolites, aluminas, activated carbons and silica gels.

The present invention's cycle of steps through (as defined in Claim 4's embodiment which specifies three pressure equalization transfers) are performed on each of Figure i's six adsorption beds in a phased sequence as summarized in Table 1. In addition to summarizing Figure l's "adsorption bed step sequence for a complete cycle, Table 1 also summarizes Figure l's valve sequence for a complete cycle. Table 1 utilizes 12 time intervals and a total elapsed time of 24 time units to cover the cycle of steps through so that the relative times for each step can be clearly indicated. It should be recognized that Figure l's embodiment and the operation sequence of Tabl.e 1 is only an example. Other embodiments can be easily designed by one skilled in the art.

5 TABLE 1 Time Interval 0-2 2-4 4-6 6-8 8-10 10-12 12-14 14-16 16-18 18-20 20-22 22-21 Bed Opera tion (1) B1 B2 B3 B4 B6 Valve -ftton z) .4 7 8 16 17 1.9 21 22 23 24 26 27 28 29 31 (a) (iii) (i) (c) (iii) (i) 0

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0 0 (iii) (i) (c) (iii) (f) (i (d) (c) (a) through correspond to steps through of the present invention as defined in Claim 4's embodimen which specifies three pressure equalization transfers.

o open; c closed -e I r Y -6- By way of example, Table l's step sequence and valve sequence will be described as it relates to the operation of Figure l's adsorption bed B1.

During the first and second time intervals (time units bed BI undergoes the adsorption step or step of the present invention. The high pressure feed gas mixture enters bed Bi via open valve 1 to produce an adsorption bed saturated with the more strongly adsorbed component and a product stream enriched in the less strongly adsorbed component which 'product stream exits the bed via open valve 25. During the second time interval (time units a portion of the product effluent from bed B1 is used to both repressurize bed 82 via open valve 26 and purge bed B4 :via open valves 16 and 28.

~During the third, fourth and fifth time intervals (time units 4-10), bed BI undergoes the initial depressurization step or step of the present invention which is effected in three successive pressure 15 equalization transfers. During the first pressure equalization transfer (step corresponding to time units withdrawn gas from bed B1 is transferred via open valves 2 and 10 to bed 83 which is currently undergoing step (e)(iii) thereby equalizing the pressures of beds 81 and 83. During the second pressure equalization transfer (step (b)(ii) corresponding to time units withdrawn gas from bed 81 is transferred S via open valves 2 and 14 to bed B4 currently undergoing step (e)(ii) ."*thereby equalizing the pressures of beds BI and 84. During the third pressure equalization transfer (step (b)(iii) corresponding to time units 8-10), withdrawn gas from bed BI is transferred via open valves 3 and 19 to bed B5 currently undergoing step thereby equalizing the pressures of beds 81 and During the sixth and seventh time interval (time units 10-14), bed B1 undergoes the further depressurization step or step of the present invention. During the sixth time interval (time units 10-12), bed B1 is depressurized to ambient pressure by withdrawing a gas stream therefrom via open valves 3 and 31. During the seventh time interval (time units 12-14), bed BI1 is depressurized to a sub-ambient pressure level by withdrawing a gas stream therefrom via open valve 4 and vacuum pump Vi. The effluent from the further depressurization step is enriched in the morve adsorbable ,I l^ II M -7 component and is generally either discarded as waste or, where a natural gas pipeline is the feed, compressed and returned to the pipeline.

During the eighth time interval (time units 14-16), bed Bl undergoes the purge step or step of the present invention. With vacuum pump VI still operating, bed B1 is purged via open valves 4 and W- with a portion of the product effluent from bed B4 which is currentl, Ad'rgaing the adsorption step. The effluent from the purge step is ',ly handled in the same fashion as the effluent from the further depressurization step.

During the ninth, tenth and eleventh time intervals (time units 10 16-22), bed Bl undergoes the initial repressurization step or step of the present invention which is also effected in three successive pressure equalization transfers. During the initial pressure equalization transfer (step corresponding to time units 16-18), withdrawn gas from bed 83 which is currently undergoing step (b)(iii) is transferred to bed B1 via 15 open valves 3 and 11 thereby equalizing the pressures of beds B1 and B3.

During the subsequent pressure equalization transfer (step (e)(ii) corresponding to time units 18-20), withdrawn gas from bed B4 which is currently undergoing step is transferred to bed B1 via open valves 2 and 14 thereby equalizing the pressures of beds 81 and B4. During the final pressure equalization transfer (step (e)(iii) corresponding to time units 20-22), withdrawn gas from bed B5 which is currently undergoing step is transferred to bed 81 via open valves 2 and 18 thereby equalizing the pressures of beds Bl and Finally, during the twelfth time interval (time units 22-24), bed Bl undergoes the further repressurization step or step of the present invention. Bed Bl is further repressurized via open valves 25 and 30 to the pressure level of the feed gas mixture with a portion of the product effluent from bed B6 which is currently undergoing the adsorption step.

After repressurization, bed Bl's cycle is complete and a new cycle can commence. Each adsorption bed undergoes a similar sequence of operation as is described for bed Bl as can be further detailed from Table 1.

Is~aw~Ao~MsUII--~~--- -L I -8- It should be noted that other variation to Figure l's embodiment are possible such as the following: performing the pressure equalization steps between the product ends of the beds instead of between the feed ends of the beds; product assisted pressure equalization whereby a portion of the product gas from a bed on step is used for pressure equalization with a bed on step (e)(iii) (this helps to reduce product flow fluctuations at the cost of product recovery); pressure equalization assisted purging whereby some or all of the vacuum purge gas for step is obtained from the product end of a bed undergoing step and adding additional beds to the system in order to accomodate a cycle which performs simultaneous feeding and/or simultaneous sub-ambient depressurization of two or more beds.

15 Computer simulations of Finure I's embodiment for the purification of a feed stream at-400 psig and 74 0 F containing 86% methane and 11% ethane as the less adsorbable component and 3% carbon dioxide as the more adsorbable component where the adsorbent is a NaX zeolite yielded a product stream containing 93.2% methane, 6.8% ethane and only 50 ppm CO 2 The methane recovery in the product stream was 95.5% while the methane plus ethane recovery in the product stream was 91.0%. The feed capacity was 15.1 milli-lbmoles feed per Ib adsorbent. Such purity, recovery and feed capacity numbers represent significant improvements over the traditional PSA purification cycles which do not utilize sub-ambient depressurization.

The present invention has been described with reference to a specific embodiment thereof. This embodiment should not be seen as a limitation of the scope of the present invention; the scope of such being ascertained by the following claims.

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Claims (4)

1. 2. The process of Claim 1 wherein the less strongly adsorbed component constitutes at least 75% of the feed gas mixture on a volume basis.
2. 3. The process of Claim 2 wherein: the high pressure feed gas mixture is obtained from a natural gas pipeline; the less strongly adsorbed component of the feed gas mixture comprises methane and C 2 hydrocarbons; the more strongly adsorbed component of the feed gas mixture comprises carbon dioxide and C 3 hydrocarbons; the adsorbent comprises one or more adsorbents selected from the group consisting of zeolites, aluminas, activated carbons and silica gels; and the depressurization effluent from step and the purge effluent from step is compressed and returned to the natural gas pipeline. S.. S *5*S
3. 4. The process of Claim 1 wherein transfers are performed between those ends receive the feed gas mixture in step The process of Claim 1 wherein transfers are performed between those ends discharge the product stream in step said pressure equalization of the involved beds which said pressure equalization of the involved beds which
4. 6. The process of Claim 1 wherein step depressurization step is effected in three pressure equalization transfers; during the first of which, sub-step withdrawn gas from said bed is transferred to another bed of said plurality of said beds currently undergoing step (e)(iii) thereby equalizing the pressures of the two beds involved in this first pressure equalization transfer, and during the second of which, sub-step withdrawn gas from said bed is transferred to another bed of said plurality of said beds currently 11 undergoing step thereby equalizing the pressures of the two beds involved in this second pressure equalization transfer, and during the third of which, sub-step withdrawn gas from said bed is transferred to another bed of said plurality of said beds currently undergoing step thereby equalizing the pressures of the two beds involved in this third pressure equalization transfer; and wherein step initial repressurization step is similarly effected in three pressure equalization transfers; during the initial of which, sub-step withdrawn gas from a 10 bed currently undergoing step (b)(iii) is transferred to said bed thereby equalizing the pressures of the two beds involved in this initial pressure equalization transfer, and during the subsequent of which, sub-step withdrawn gas from a bed currently undergoing step is transferred to said bed thereby 15 equalizing the pressures of the two beds involved in this subsequent pressure equalization transfer, and during the final of which, sub-step withdrawn gas from a bed currently undergoing step is transferred to said bed thereby equalizing the pressures of the two beds involved in this final pressure 20 equalization transfer. 9*t9 .m *DATED this THIRD day of OCTOBER 1994. AIR PRODUCTSAND CHEICAS AIR PRODUCTS AND CHEMICALS INC THOMSON PIZZEY I ABSTRACT A pressure swing adsorption (PSA) process is set forth for purifying a high pressure (greater than 200 psig) feed gas mixture with respect to its less strongly adsorbed component. In addition to the basic adsorption, depressurization and repressurization steps, the process of the present invention utilizes a low pressure purge step and one or more pressure .go equalization transfers. A key to the present invention is that the depressurization step is performed to a sub-ambient pressure level. An important application of the present invention is the purification of a high pressure natural gas feed stream with respect to its methane/C 2 hydrocarbon component wherein said methane/C 2 hydrocarbon component is produced at high purity and high recovery. C o A C° a