Note: Descriptions are shown in the official language in which they were submitted.
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214PUS05006
PRESSURE SWING ADSORPTION PROCESS FOR PURIFYING A HIGH PRESSURE FEED :
GAS MIXTURE WITH RESPECT TO ITS LESS STRONGLY ADSORBE~ CO~lPONENT
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
5 feed stream with respect to its methane/C2 hydrocarbon component wherein -
said component is produced at high purity and high recovery. ~ ;
BACKGROUND OF THE INVENTION
Pressure swing adsorption (PSA) purification oycles wherein a high ~;
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:
~ a) passing the feed gas mixture through an adsorption bed
containing an adsorbent selective for the adsorption of the more strongly
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; . .
(b) depressurizing the adsorption bed to ambient pressure to
; piroduce a waste stream enriched in the more strongly adsorbed component;
(c) repressurizing the adsorption zone to the pressur-e 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 (a)'s 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 purging increases the
purity of the product stream produced in step (a) because it pulges the bed
of any of the more strongly adsorbed component wllich ma~. rema~n in ~he bed
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after 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 ``m
10 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 `
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 be 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 -
depressuri-ation; the larger the pressure swing, the more work thPra ,s
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available 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 purifyin~ a
high pressure feed gas mixture with respect to its less strongly adsorbed
component is advantageous in increasing both (1) recovery of the less
strongly adsorbed component and (2) feed capacity of the adsorption bed
beyond the associated power penalty.
An important application of the present invention is the purification
of a high pressure natural gas feed stream with respect to its methane/C
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
the United States has an abundance of natural gas, it contains impurities
such as water, sulfur-containing compounds, light hydrocarbons (ie C3
hydrocarbons; note that C2 hydrocarbons are generally not considered an
impurity), heavy hydrocarbons (ie C4+ hydrocarbons) and carbon dioxide
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-tKus will normally require heating of the adsorption bed vis-
a-vis mere depressurization of the-adsorption bed. The removal of the
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 stepsl 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 depressllrization step is perforn,ed to a sub-ambient pressure level. - -
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BRIEF DESCRIPTION OF THE DRAWINGS ~;
Figure 1 is schematic diagram depicting one embodiment of the present - ~
invention which utilizes six adsorption beds and three pressure ;-
equalization transfers.
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DETAILED DESCRIPTION OF THE INVENTION , `
The process of the present invention is best illustrated with
reference to a specific embodiment thereof such as Figure 1's embodiment ~ `
which utilizes six adsorption beds and three pressure equalization
transfers. Figure 1's process configuration consists of vacuum pump V1,
valves 1 through 31 and six adsorption beds B1 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
may also be used. Examples of such adsorbents are zeolites, aluminas,
activated carbons and silica gels.
The present invention's cycle of steps (a) through (f) (as defined in -;
Claim 4's embodiment which specifies three pressure equalization transfers)
are performed on each of Figure 1's six adsorption beds in a phased `
sequence as summarized in Table 1. In addition to summarizing Figure 1's
adsorption bed step sequence for a complete cycle, Table 1 also summarizes
Figure 1'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 (a) through (f) so that the relative times for each step can be -
clearly indicated. It should be recognized that Figure 1's embodiment and
the operation sequence of Table 1 is only an example. Other embodiments
can be easily designed by one skilled in the art. ~-
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TABLE 1
Time Interval
0 Z 2-4 4-6 6-8 8-10 10-12 12-14 14-16 16-18 18-Z0 20-22 22-
8ed
Operat10ntl)
B1 (a, ~a) (b)(i) (b)(ii) (b)(iii) (c) (c) (d) (8)(i) (e)(ii) (e)(iii)
B2 (e)~iii) 'f' (a) (a) (b)(i) (b)(ii) (b)(iii) tc) (c) (d) (e)(i) (e)
~3 (e)(l) (e)(ii) (e)(iii) 'f' (a) (a) (b)(i) (b)tii) (b)(iii) (c) (c)
84 (c) (d) (e)(i) (e)(ii) (e)(iii) 'f' (a) (a) (b)(i) (b)(ii) (b)(iii) ( ~:
B5 (b)(iii) (c) (c) (d) (e)(i) (e)(ii) (e)(iii) (f) (a) (a) (b)(i) (b)
B6 (b)(i) (b)(ii) (b)(iii) (c) (c) (d) (e)(i) (e)(ii) (e)(ili) (f) (a)
Va1~e
PosltlOn(2)
o o c c c c c c c c c
2 c c o o c c c c c o o
3 c c c c o o c c o c c
4 c c c c c c o o c c c
c c o o c c c c c c c -
6 o c c c o o c c c c c : .
7 o c c c c c o o c c o ,
c c c c c c c c o o c
9 c c c c o o c c c c c ;,
c o o c c c o~ o c c c ,'
11 o c c c c c c c o o c ~:
12 c c c c c c c c c c o
13 c c c c c c o o c c c ,:~
14 c c c o o c c c o o c ~
c c o c c c c c c c o .
16 o o c c c c c c c c c ; ~
17 c c c c c c c c o o c - .::- :
18 c c c c c o o c c c o
1 9 o o c c o c c c c c c
c c o o c c c c c c c
21 c c c c c c c c c c o
22 - o o c c c c c o o c c .:
23 c c o o c c o c c c c
24 c c c c o o c c c c c
i 0~ 0 c ~ c c c c o c I c i~ c
26 c o o o c c c c c o c : :~
27 ~ c c c o o o c c c ~ c c .-~
28 c o c c c o o o c c c
29 c c c o c c c o o 0 c
c c c c c o c c c o o
31 c o c o c o c o c o c
(l) (a) through (f) correspond to steps (a) througl1 (f) of the present invention as deflnec ln Clalm ~s embodln~
which specifies three pressure equali~ation transfers. :1:': :2
(2) o = open; c = closed
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By way of example, Table 1's step sequence and valve sequence will be
described as it relates to the operation of Figure 1's adsorption bed B1. ,;-.:~:
During the first and second time intervals (time units 0-4), bed B1
undergoes the adsorption step or step (a) of the present invention. The ~`.
high pressure feed gas mixture enters bed B1 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 2-4), a portion of the product effluent from bed B1 is .
used to both (l) repressurize bed 82 via open valve 26 and (2) purge bed B4
via open valves 16 and 28.
During the third, fourth and fifth time intervals (time units 4-10), i~
bed B1 undergoes the initial depressurization step or step (b) of the
present invention which is effected in three successive pressure ~ . `
equalization transfers. During the first pressure equalization transfer
(step (b)~i) corresponding to time units 4-6), withdrawn gas from bed Bl is :. . .
transferred via open valves 2 and 10 to bed B3 which is currently ~ ::
undergoing step (e)(iii) thereby equalizing the pressures of beds Bl and
B3. During the second pressure equalization transfer (step (b)(ii) .
corresponding to time units 6-8), withdrawn gas from bed B1 is transferred
via open valves 2 and 14 to bed B4 currently undergoing step (e)(ii) .
thereby equalizing the pressures of beds B1 and B4. During the third
pressure equalization transfer (step (b)(iii) corresponding to time units . ~.
~ 8-10), withdrawn gas from bed B1 is transferred via open valves 3 and 19 to
bed BS currently undergoing step (e)(i) thereby equalizing the pressures of
beds Bl and B5. .;::
During the sixth and seventh time interval (time units~10-14), bed B1 `
undergoes the further depressurization step or step (c) 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 B1 is depressurized to a sub-ambient pressure level by withdrawing a
gas stream therefrom via open valve 4 and vacuum pump Vl. The effluent
from the further depressurization step is enriched in the more adsorbable ;
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component and is generally either discarded as waste or, where a natural
gas pipeline is the feed, compressed ancl returned to the pipeline.
During the eighth time interval (time units 14-16), bed Bl undergoes
the purge step or step (d) of the present invention. With vacuum pump V1
still operating, bed B1 is purged via open valves ~ and 25 with a portion
of the product effluent from bed B4 which is currently undergoing the
adsorption step. The effluent from the purge step is generally handled in
the same fashion as the effluent from the further depressurization step.
During the ninth, tenth and eleventh time intervals (ti~e units
16-22), bed B1 undergoes the initial repressurization step or step (e) of
the present invention which is also effected in three successive pressure
equalization transfers. During the initial pressure equalization transfer
(step (e)(i) corresponding to time units 16-18), ~ithdrawn gas from bed B3
which is currently undergoing step (b)(iii) is transferred to bed B1 via 'J'`'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 (b)(ii) is transferred to bed B1 via open valves
2 and 14 thereby equalizing the pressures of beds B1 and B4. During the
20 final pressure equalization transfer (step (e)(iii) corresponding to time ~;
units 20-22), withdrawn gas from bed B5 which is currently undergoing step
(b)(i) is transferred to bed B1 via open valves 2 and 18 thereby equalizing
the pressures of beds B1 and B5.
~ Finally, during the twelfth time interval (time units 22-24), bed Bl undergoes the further repressurization step or step (f) of the present
invention. Béd 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.
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It should be noted that other variations to Figure 1's embodiment are
possible such as the following:
(1) performing the pressure equalization steps between the product -~
ends of the beds instead of between the feed ends of the beds;
(2) product assisted pressure equalization whereby a portion of the
product gas from a bed on step (a) 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); ~ ;
(3) pressure equalization assisted purging whereby some or all of
the vacuum purge gas for step (d) is obtained from the product end of a bed
undergoing step (b)(ii); and ~ ~ ~
(4) 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. ~ `
Computer simulations of Figure 1's embodiment for the purification of
a feed stream at 400 psig and 74 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 C02. The ~
20 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 lb 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.
O:\RJI~\2145006 .ADL
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