U.S. patent application number 17/042627 was filed with the patent office on 2021-03-18 for process for handling variable flow rates and compositions in pressure swing adsorption systems.
The applicant listed for this patent is PRAXAIR TECHNOLOGY, INC.. Invention is credited to Cynthia A. Hoover, Yang Luo, Michael St. James, Nicholas R. Stuckert.
Application Number | 20210077942 17/042627 |
Document ID | / |
Family ID | 1000005288752 |
Filed Date | 2021-03-18 |
View All Diagrams
United States Patent
Application |
20210077942 |
Kind Code |
A1 |
Stuckert; Nicholas R. ; et
al. |
March 18, 2021 |
PROCESS FOR HANDLING VARIABLE FLOW RATES AND COMPOSITIONS IN
PRESSURE SWING ADSORPTION SYSTEMS
Abstract
The present invention generally relates to a process for
responding to feed flow variations by changing the process cycle
and thereby increasing the productivity and capacity of the system
significantly over constant process systems. This increases the
flexibility a PSA system for customers that do not require a
constant or uniform product flow rate and/or for processes and
applications that experience feed streams that vary in flow,
temperature, and/or composition.
Inventors: |
Stuckert; Nicholas R.;
(Grand Island, NY) ; St. James; Michael; (Calgary,
CA) ; Hoover; Cynthia A.; (Grand Island, NY) ;
Luo; Yang; (Amherst, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PRAXAIR TECHNOLOGY, INC. |
Danbury |
CT |
US |
|
|
Family ID: |
1000005288752 |
Appl. No.: |
17/042627 |
Filed: |
March 29, 2020 |
PCT Filed: |
March 29, 2020 |
PCT NO: |
PCT/US2019/024848 |
371 Date: |
September 28, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62649798 |
Mar 29, 2018 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 2259/40011
20130101; B01D 2253/108 20130101; B01D 53/047 20130101; B01D
2256/245 20130101; B01D 53/0423 20130101; B01D 2256/16 20130101;
B01D 2257/102 20130101; B01D 2257/104 20130101; B01D 2256/10
20130101; B01D 2259/404 20130101; B01D 2257/504 20130101; B01D
2259/4062 20130101 |
International
Class: |
B01D 53/047 20060101
B01D053/047; B01D 53/04 20060101 B01D053/04 |
Claims
1. A method for maximizing product production under variable feed
conditions in a PSA system adapted for separating a pressurized
feed supply gas containing at least one more readily adsorbable
component from at least one less readily adsorbable product gas
component to produce a stream of product gas enriched with said
less readily adsorbable component and a stream of offgas that is
enriched in said more readily adsorbable component, wherein said
PSA system comprises a feed gas step, product gas make step, a
product pressurization step, a high pressure equalization step,
product make step that overlaps with feeding the bed, at least one
equalization up step and one equalization down step, and a blow
down step to depressurize the bed, wherein when the Required
Processing Power of said PSA system is greater than 1, the PSA
process cycle is modified by making at least one of the following
cycle changes: a. Substitute at least one feed step for an
equalization step provided that the cycle retains at least one
equalization step pair; or b. Substitute at least one blow down
step or purge step pair for an equalization step pair provided that
the cycle retains at least one equalization step pair; or c.
Substitute at least one feed step for at least one blow down step
or purge step pair provided that the cycle retains at least one
blow down step or purge step pair; d. Substitute an overlap feed
and product pressurization step for a product pressurization step;
or e. Substitute a purge step pair for at least one blowdown
step.
2. The method of claim 1 wherein when the Required Processing Power
of said PSA system is greater than 1, the PSA process cycle is
modified by: a. Substituting at least one feed step for an
equalization step provided that the cycle retains at least one
equalization step pair; and b. Substituting at least one blow down
step or purge step pair for an equalization step pair provided that
the cycle retains at least one equalization step pair.
3. The method of claim 1 wherein when the Required Processing Power
of said PSA system is less than 1, the PSA process cycle is
modified by making at least one of the following cycle changes: a.
Substitute an equalization step pair for a feed step provided that
the cycle retains at least one feed step; or b. Substitute an
equalization step for a blow down step or a purge step pair
provided that the cycle retains at least one blow down step or
purge step pair; or c. Substitute at least one blow down step or
purge step pair for at least one feed step provided that the cycle
retains at least one feed step; or d. Substitute a product
pressurization step for an overlap feed and product pressurization
step; or e. Substitute at least one blowdown step for a purge step
pair.
4. The method of claim 1 wherein when the Required Processing Power
of said PSA system is less than 1, the PSA process cycle is
modified by: a. Substituting an equalization step pair for a feed
step provided that the cycle retains at least one feed step; and b.
Substituting an equalization step for a blow down step or a purge
step pair provided that the cycle retains at least one blow down
step or purge step pair.
5. The method of claim 3 wherein for Required Processing Power of
less than 1, there is a deadband of up to 0.2 within no change to
the process cycle is implemented.
6. The method of claim 3 wherein the Required Processing Power is
less than 0.8.
7. The method of claim 1 wherein the product gas is methane and the
more readily adsorbable component is N.sub.2 and/or CO.sub.2.
8. The method of claim 1 wherein the product gas is helium and the
more readily adsorbable component is N.sub.2 and/or CO.sub.2 and/or
methane and/or other hydrocarbons.
9. The method of claim 1 wherein the product gas is hydrogen and
the more readily adsorbable component is N.sub.2 and/or CO.sub.2
and/or methane and/or other hydrocarbons.
10. The method of claim 1 wherein the system has at least 4
adsorbent beds but less than 25.
11. The method of claim 1 wherein the product gas is N.sub.2 and
the more readily adsorbable component is O.sub.2.
12. A method of claim 1 wherein an intermediate processing cycle is
created to facilitate the transition for option c.
13. The method of claim 1 wherein each adsorption bed contains
zeolitic material.
14. The method of claim 1 wherein each adsorption bed contains
adsorbent materials used in H.sub.2 PSA, the product gas is H.sub.2
and the more readily adsorbable component is selected from one or
more of CO, CO.sub.2, CH.sub.4, N.sub.2, Ar, and hydrocarbon.
15. The method of claim 1 wherein said adsorbent is selected from
at least one of activated carbon, Zeolite, 5A, CaX, LiX.
16. The method of claim 1 wherein PSA system comprises a 4131
design cycle, and wherein when the Required Processing Power for
said system is greater than 1, the design cycle is modified to a
4122 cycle according to the following cycle chart:
17. The method of claim 1 wherein the PSA system comprises a 4122
design cycle, and wherein when the Required Processing Power for
said system is greater than 1, the design cycle is changed to a
4221 cycle according to the following cycle chart:
18. A method of claim 3 wherein the PSA system comprises a 4221
design cycle, wherein when the Required Processing Power for said
system is less than or equal to 1, the design cycle is modified to
a 4122 cycle according to the following cycle chart provided that
the Required Processing Power for the 4122 cycle is also less than
or equal to 1:
19. The method of claim 18 wherein the cycle is not modified until
the Required Processing Power for the 4122 cycle is less than about
0.95 or less than about 0.9, or less than 0.8.
20. The method of claim 3 wherein the PSA system comprises a 4122
design cycle, wherein when the Required Processing Power for said
system is less than or equal to 1, the design cycle is modified to
a 4131 cycle according to the following cycle chart provided that
the Required Processing Power for the 4131 cycle is less than or
equal to 1:
21. The method of claim 20 wherein the cycle is not modified until
the Required Processing Power for the 4122 design cycle is lower
than about 0.95 or lower than about 0.9, or lower than 0.8.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 62/649,798, filed on Mar. 29, 2018, which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a process to respond to
feed flow variations by changing the process cycle and thereby
increasing the productivity and capacity of the system
significantly over constant process systems.
BACKGROUND OF THE INVENTION
[0003] Typically pressure swing adsorption (PSA) systems have an
optimal design condition that is the peak performance achievable
for the system. Under steady conditions, this design is acceptable
most of the time. Occasionally variances occur and processes to
handle the variances range from restricting the flow, to reducing
the number of beds (effective physical size) of the system. Other
methodologies have been suggested to address the issue of variable
feed flow, composition, and temperature for PSA processes.
Traditionally these methodologies are targeted toward bringing the
feed stream within optimal operating parameters for the system.
[0004] U.S. Pat. No. 5,258,056. describes a turndown methodology to
produce substantially less product in response to declining
customer product demand. This is done by reducing the number of
beds online and by taking substantially less feed flow.
[0005] U.S. Pat. No. 7,641,716 describes a throttling methodology
to maintain a constant feed. This consists of valves located before
the system to keep the flow rate at the optimal rate to achieve
peak performance for the system.
[0006] U.S. Pat. No. 6,030,435 describes regulating the feed flow
temperature in order to keep the temperature of the system at the
optimal temperature for peak performance of the PSA process.
[0007] All these methodologies involve changing the feed stream
rather than changing the process. The present invention offers a
different approach for regulating pressure swing adsorption (PSA)
systems by changing the process cycle and thereby increasing the
productivity and capacity of the system significantly over constant
process systems.
SUMMARY OF THE INVENTION
[0008] The present invention generally relates to a process for
responding to feed flow variations by changing the process cycle
and thereby increasing the productivity and capacity of the system
significantly over constant process systems. This increases the
flexibility a PSA system for customers that do not require a
constant or uniform product flow rate and/or for processes and
applications that experience feed streams that vary in flow,
temperature, and/or composition.
DETAILED DESCRIPTION OF THE FIGURES
[0009] FIG. 1 shows the process for a 4121 cycle from the view of a
single bed.
[0010] FIG. 2 shows the process for a 4131 cycle from the view of a
single bed.
[0011] FIG. 3 shows the process for a 4122 cycle from the view of a
single bed.
[0012] FIG. 4 shows the process for a 4221 cycle from the view of a
single bed.
[0013] FIG. 5 shows the pressure trace for the 4122, 4131 and 4221
cycles.
[0014] FIGS. 6, 6A, 6B, and 6C show the cycle chart for a 12 bed 24
step process and corresponding cycles that could be used as part of
this invention.
[0015] FIG. 7 shows an example of how to switch from a 4131 cycle
to a 4122 cycle and the reverse.
[0016] FIG. 8 shows an example of how to switch from a 4122 cycle
to a 4221 cycle and the reverse.
[0017] The legend for FIGS. 1-4 and 6-7 is: [0018] F--feed step and
make product if at pressure [0019] EQD1--first equalization down
[0020] EQD2--second equalization down [0021] EQD3--third
equalization down [0022] X--Idle step [0023] BD--bed blow down/vent
[0024] EQU3--third equalization up [0025] EQU2--second equalization
up [0026] EQU1/F--first equalization up, overlap feed [0027]
PP--product pressurization [0028] PP/F--product pressurization,
overlap feed
DETAILED DESCRIPTION OF THE INVENTION
[0029] The present invention relates to a control method to respond
to feed flow variations by changing adoption a new process cycle
and thereby increasing the productivity and capacity of the system
significantly over constant process systems.
There are two specific cases presented as to why this is necessary
and the benefits that it imparts. This first is control of low
kinetic difference systems. In these systems adsorbent rate
selectivity is typically low (less than 100). As a consequence, the
timing for adsorption during a cycle has a very narrow window that
is sufficient to adsorb the contaminant, but not substantially
adsorb the product. This is an issue because the state of the art
all teaches that process cycles can be altered with cycle timing in
order to respond to changing feed conditions (flow, pressure,
temperature, composition, etc.). If the timing of a process cycle
for one of these low selectivity kinetic processes (LSKPs) is
increased, the adsorption of the product increases and the recovery
does not increase as is taught in the prior art. Additionally if
the adsorption time is shortened, the amount of contaminant
adsorbed decreases and the amount of feed stream that can be
processed while maintaining product purity decreases. Since almost
all feed streams have variations, controlling LSKPs becomes
critical to having a viable commercial system.
[0030] In order to control LSKPs different cycles are used to
handle different flow conditions. The cycles are typically chosen
to have the best performance over a feed flow regime and are used
to handle the feed flow variations. The design point would be the
cycle that is chosen to best suit the application based on highest
recovery and lowest capital (which is synonymous with highest feed
flow potential). In the state of the art, these considerations
would be accounted for, the optimal cycle would be chosen, and that
cycle would be used for the life of the system. Here it is
demonstrated that the optimal cycle can be changed to accommodate
expected or unexpected variations in the feed stream, leading to a
more flexible system and ability to design a system for multiple
feed stream conditions.
[0031] Another problem that is extremely similar is for typical PSA
processes. Typically, PSA processes attempt to control the feed
stream and adjust it to fit the optimal design or reduce the number
of beds online to meet a reduced flow (which also changes the
process cycle). These cycles are usually deemed turndown modes and
an excellent example would be H2PSA systems. When the systems are
originally designed, the maximum flow rate and the target recovery
are used to design a system to meet those objectives. This makes
sense at the time because the feed stream ahead of the H2PSA is
well controlled by other processes. However, if the plant wished to
expand capacity, a new H2PSA system would need to be built or
hardware modifications are needed for the new cycles to accommodate
the additional flow as the old one cannot handle the flow according
to state of the art process cycles. The present invention takes a
different approach in that the cycle/process is modified in order
to fit the feed stream variability. Specifically, a lower recovery
cycle can be chosen to increase the total production of the system
and utilize the increased feed stream capacity, without the
requirement to deploy additional capital. This has substantial
benefits for customers that are able to take an unregulated flow of
product or are looking to increase the flow of the product.
Additionally, by being able to increase processing capability by
slightly lowering recovery, the system can capitalize on
opportunities where flow requests exceed design conditions. These
can happen during specific instances when a customer's primary
supplier of hydrogen goes down and the secondary producer wishes to
meet the increase in demand on their system. When flow is lower
than design conditions, adding back or even increasing equalization
steps allows for higher system recovery by increasing the void
recovery, and thereby reduction operating costs. This methodology
adds considerable economic benefit over current designs by
processing up to 60% or more flow than the design condition and
increasing production by as much as 25% or more over
state-of-the-art process cycles.
[0032] These two applications of the method for process control by
varying process cycles can be summarized as applying to systems
that are poly bed, in one embodiment 4 or more, in another
embodiment from 4 beds up to 25 beds, with at least one
equalization header and preferably two or more equalization
headers. The design basis cycle is the cycle which is used to
typically run the system at the design condition (feed flow and
feed composition specification). This poly bed system is then
enabled by the process methodology to respond to variable feed
temperatures, flow rates, and compositions beyond the typical
conditions the system was designed for or could be designed for
using the state or the art teachings. The trade-off is increased
processing capacity for reduced recovery, which is substantially
different than prior art methodologies wherein the goal is not to
regulate the flow, but to adjust to the flow. The design cycle
typically has at least 1 equalization step and in another
embodiment 2 or more. FIG. 2 is a 4-1-3-1 cycle and is
representative of the state-of-the-art design cycles for a 4 bed
process. It has one bed on feed, one product make step, 3
equalization steps, two idle steps, and an overlapping feed and
product pressurization step. The design cycle has at least one feed
step, has at least one product make step, has at least a blow down
step and may or may not have a purge step. All cycles should have
at least one feed step, at least one product make step and at least
a blow down step. Almost all cycles will have at least one
equalization step pair and it would be rare if any of the proposed
cycles do not have at least one equalization step pair.
[0033] An example of a cycle that could be switched to from a
4-1-3-1 design cycle of FIG. 2, is the 4-1-2-2 cycle shown in FIG.
3 which replaces an equalization set of steps with four additional
blowdown steps. Another example is the 4-2-2-1 cycle of FIG. 4
which instead replaces an equalization set of steps for four
additional feed steps. This methodology applies to both equilibrium
selective processes and kinetic selective processes, however, may
be utilized more frequently and favorably with kinetic selective
processes, particularly LSKPs.
[0034] The effect of the cycle changes (4-1-2-2 and 4-2-2-1 vs
4-1-3-1) on the pressure trace is shown in FIG. 5, demonstrating
the same system is capable of running all three cycles. The method
to switch between cycles is similar to that outlined by Baksh et
al. in EP2663382B1 and WO2012096812A1. Specifically, if a different
cycle is desired in order to adjust to changing feed
concentrations, then the changes should occur when the next step in
the cycle is most similar to the next step in the cycle of the
cycle being switched to. This is shown in FIG. 7 and the shaded
cells show steps that should not proceed changing to the next
cycles. The non-shaded cells show the step that could proceed
changing to the next cycle and an arrow is shown indicating which
step in the next cycle should be selected. It can also occur that
no next steps when switching from one cycle to another are
equivalent, in that case, an intermediate cycle can be run for a
short period of time, where there is no product taken from the
system and, the EQU1 steps or EQU1/F steps are replaced with PP
steps and the EQD1, EQD2 and X steps between these are replaced
with F steps. This is demonstrated in FIG. 8.
[0035] Since there are significant feed variations to accommodate
when selecting changes to the existing cycles, a methodology was
developed to correlate flow, pressure, temperature and composition
variations in terms of a single number. The reasoning behind the
generation of the single number is the adsorption isotherms being
used in the process and the effect of the feed flow variations on
them. Essentially the working capacity of the bed can be inferred
by the use of the LRC isotherm but is equally applicable to other
multicomponent isotherms that account for temperature effects as
well. A logarithmic extrapolation between the inlet and outlet
conditions that the bed experiences at the top of pressurization
and the bottom of pressurization can be used to generate the
starting points. Assuming that the end composition is always best
represented by the product purity at the top pressure and bottom
pressure, and that the feed is best represented by the feed inlet
at the top pressure and bottom pressure, we can then solve for the
working capacity of the bed at all conditions.
W q ( P , T ) = X q * ( K q * P q ) 1 n q 1 + .SIGMA. i = 0 m ( K i
* P i ) 1 n i ##EQU00001## K i = e - ( A 1 i + A 2 i T )
##EQU00001.2## n i = A 3 i + A 4 i T ##EQU00001.3##
[0036] q is the component being evaluated
[0037] T--temperature in Kelvin of the gas and adsorbent
[0038] P--pressure in Pascal of the gas
[0039] P.sub.q--partial pressure in Pascal of the gas q
[0040] W.sub.q--amount of component q adsorbed
[0041] A1, A2, A3, A4, X--fitting parameters, subscripts denote
which gas the parameters correspond to
[0042] m is the number of components in the feed stream
Required Processing Power = F n * ( W q ( P f , T f ) - W q ( P v ,
T v ) ) F * ( W q ( P f n , T f n ) - W q ( P v n , T v n ) )
##EQU00002##
[0043] Where:
[0044] q is the component being evaluated
[0045] W.sub.q--amount of component q adsorbed as defined by a
multicomponent temperature dependent isotherm, preferably the LRC
isotherm
[0046] Pf--original feed pressure
[0047] Pfn--new feed pressure
[0048] Pv--original vent pressure
[0049] Pvn--new vent pressure
[0050] Tf--original feed temperature
[0051] Tfn--new feed temperature
[0052] Tv--original vent temperature
[0053] Tvn--new vent temperature
[0054] F--original feed flow rate
[0055] F.sub.n--new feed flow rate
[0056] If the Required Processing Power is above 1 that means that
more intensified cycles are required (meaning less equalizations
and more time feeding and evacuating the beds). If this Required
Processing Power is below 1, that means there's more time available
for higher recovery by increasing adsorption feed time or the
number of beds for instance. By definition a RPP of 1 will
correspond to the maximum processing power of a cycle under
conditions that produce the most product at the desired purity.
[0057] For processes that contain a vacuum step, it is almost
always most beneficial to have the vacuum equipment fully utilized
as taught by U.S. Pat. No. 5,702,504 to Schaub et al. There then
exists a minimum number of vacuum steps that is taught here which
is that at least one bed is undergoing vacuum at substantially all
times of the cycle (momentary isolation from valve switching could
occur). Additionally, vacuum is best performed on one bed at a time
and the teaching here is that the maximum number of steps for a
vacuum containing PSA process is the same as the minimum which is
one bed on vacuum at substantially all times.
[0058] A component that is more readily adsorbable means that it
can have: [0059] 1) a higher isosteric heat of adsorption than the
less readily adsorbable component [0060] 2) a higher rate of
adsorption that the less readily adsorbable component [0061] 3)
both a higher isosteric heat of adsorption and a higher rate of
adsorption than the less readily adsorbable such that during the
design cycle basis, the more readily adsorbable component is lower
in concentration in the product stream than in the feed stream.
[0062] A more rigorous method for calculating these effects and the
optimal process cycle for a set of feed conditions is the modeling
detailed in the modeling description.
Specifically:
[0063] 1) For higher flow rates and/or increased contaminant
concentrations and/or higher temperatures and/or higher product
draw rates (as defined by the Required Processing Power being
greater than 1) compared to the design case (Required Processing
Power of 1 by definition): [0064] a. Substitute at least one feed
step for an equalization step pair (not necessarily at the same
step number) but keeping at least one equalization step pair [0065]
b. And/or substitute at least one blow down step or purge step pair
for an equalization step pair (not necessarily at the same step
number) but keeping at least one equalization step pair [0066] c.
And/or substitute at least one feed step for at least one blow down
step or purge step pair (not necessarily at the same step number)
but keeping at least one blow down step or purge step pair [0067]
d. And/or substitute an "overlap feed and product pressurization"
step for a product pressurization step (not necessarily at the same
step number) [0068] e. And/or substitute a purge step for a
blowdown step (not necessarily at the same step number). [0069] 2)
For lower flow rates and/or decreased contaminant concentrations
and/or lower temperatures and/or lower product draw rates (as
defined by the Required Processing Power being less than 1)
compared to design case (Required Processing Power of 1 by
definition): [0070] a. Substitute an equalization step pair for a
feed step (not necessarily at the same step number) but keeping at
least one feed step; in another embodiment keeping at least 3 feed
steps, [0071] b. And/or substitute an equalization step pair for a
blow down step or purge step pair (not necessarily at the same step
number) but keeping at least one blow down step or purge step pair;
in another embodiment keeping at least 3 blow down steps or purge
step pairs, [0072] c. And/or substitute a least one blow down step
or purge step pair for at least one feed step (not necessarily at
the same step number) but keeping at least one feed step, in
another embodiment at least 3 feed steps, [0073] d. And/or
substitute a product pressurization step for an "overlap feed and
product pressurization" step (not necessarily at the same step
number) [0074] e. And/or substitute a blowdown step for a purge
step (not necessarily at the same step number). An optimal method
for control would be to start with change proposed as option b. and
then to use the change proposed as option a. (on the basis of the
original cycle, option c. on the basis of starting from option b.).
In the case that RPP exceeds 1 for the original basis then switch
cycles and start using option b., in the case that the RPP exceeds
the RPP of 1 as calculated for option b., then start using option
a. (on the basis of the original cycle, option c. on the basis of
starting from option b.). When going down and starting from option
a. (on the basis of the original cycle), then when the RPP is lower
than or equal to 1 as calculated for option b. (on the basis of the
original cycle), start using option b. (on the basis of the
original cycle). When the RPP is lower than or equal to 1 for the
RPP as calculated for the design cycle, start using the design
cycle. It is noted here that using a deadband of up to 0.2 for the
RPP when going down (essentially not choosing the next cycle until
the RPP is as low as 0.8) can be used to control the switching and
maintain stability during unstable flow conditions. In the case of
a LSKP system, using a design basis of 4-1-3-1 would mean that
option b. would be a 4-1-2-2 cycle and option a. would be a 4-2-2-1
cycle. It should be noted that the RPP of the proposed cycle needs
to be 1 or lower.
[0075] In one embodiment the invention relates to a method for
maximizing product production under variable feed conditions in a
PSA system adapted for separating a pressurized feed supply gas
containing at least one more readily adsorbable component from at
least one less readily adsorbable product gas component to produce
a stream of product gas enriched with said less readily adsorbable
component and a stream of offgas that is enriched in said more
readily adsorbable component, wherein said PSA system comprises
feed gas, product gas make step, a product pressurization step, a
high pressure equalization step, product make step that overlaps
with feeding the bed, at least one equalization up step and one
equalization down step, and a blow down step to depressurize the
bed, wherein when the Required Processing Power of said PSA system
is greater than 1, the PSA process cycle is modified by making at
least one of the following cycle changes: [0076] a. Substitute at
least one feed step for an equalization step provided that the
cycle retains at least one equalization step pair; or [0077] b.
Substitute at least one blow down step or purge step pair for an
equalization step pair provided that the cycle retains at least one
equalization step pair; or [0078] c. Substitute at least one feed
step for at least one blow down step or purge step pair provided
that the cycle retains at least one blow down step or purge step
pair; or [0079] d. Substitute an overlap feed and product
pressurization step for a product pressurization step; or [0080] e.
Substitute a purge step pair for at least one blowdown step.
[0081] In another embodiment, when the Required Processing Power of
said PSA system is greater than 1, the PSA process cycle is
modified by: [0082] a. Substituting at least one feed step for an
equalization step provided that the cycle retains at least one
equalization step pair; and [0083] b. Substituting at least one
blow down step or purge step pair for an equalization step pair
provided that the cycle retains at least one equalization step
pair.
[0084] In another embodiment, when the Required Processing Power of
said PSA system is less than 1, the PSA process cycle is modified
by making at least one of the following cycle changes: [0085] a.
Substitute an equalization step pair for a feed step provided that
the cycle retains at least one feed step; or [0086] b. Substitute
an equalization step for a blow down step or a purge step pair
provided that the cycle retains at least one blow down step or
purge step pair; or [0087] c. Substitute at least one blow down
step or purge step pair for at least one feed step provided that
the cycle retains at least one feed step; or [0088] d. Substitute a
product pressurization step for an overlap feed and product
pressurization step; or [0089] e. Substitute at least one blowdown
step for a purge step pair.
[0090] In another embodiment, when the Required Processing Power of
said PSA system is less than 1, the PSA process cycle is modified
by: [0091] a. Substituting an equalization step pair for a feed
step provided that the cycle retains at least one feed step; and
[0092] b. Substituting an equalization step for a blow down step or
a purge step pair provided that the cycle retains at least one blow
down step or purge step pair. In situations where the Required
Processing Power is less than 1, there is a deadband of up to 0.2
within no change to the process cycle is implemented. Depending on
the situation, no change is made to the process cycle unless the
Required Processing Power is less than about 0.95, in another
embodiment less than about 0.9 and in yet another embodiment less
than or equal to 0.8.
[0093] In one embodiment the product gas is methane and the more
readily adsorbable component is N.sub.2 and/or CO.sub.2.
[0094] In another embodiment the product gas is helium and the more
readily adsorbable component is N.sub.2 and/or CO.sub.2 and/or
methane and/or other hydrocarbons.
[0095] In another embodiment the product gas is hydrogen and the
more readily adsorbable component is N.sub.2 and/or CO.sub.2 and/or
methane and/or other hydrocarbons.
[0096] In yet another embodiment the product gas is N.sub.2 and the
more readily adsorbable component is O.sub.2.
[0097] The adsorbent beds of the invention typically contain
contains zeolitic material and other optional adsorbents depending
on the separation desired.
[0098] In one embodiment the adsorption bed contains adsorbent
materials used in H.sub.2 PSA, the product gas is H.sub.2 and the
more readily adsorbable component is selected from one or more of
CO, CO.sub.2, CH.sub.4, N.sub.2, Ar, and hydrocarbon.
[0099] In another embodiment the adsorbent is selected from at
least one of activated carbon, Zeolite, 5A, CaX, LiX.
[0100] In one embodiment according to the invention where PSA
system comprises a 4131 design cycle, and the Required Processing
Power for said system is greater than 1, the design cycle is
modified to a 4122 cycle according to the following cycle
chart:
[0101] In another embodiment wherein the PSA system comprises a
4122 design cycle, when the Required Processing Power for said
system is greater than 1, the design cycle is changed to a 4221
cycle according to the following cycle chart:
[0102] In another embodiment where the PSA system comprises a 4221
design cycle, and the Required Processing Power for said system is
less than or equal to 1, the design cycle is modified to a 4122
cycle according to the following cycle chart provided that the
Required Processing Power for the 4122 cycle is also less than
1:
[0103] In another embodiment where the PSA system comprises a 4122
design cycle, when the Required Processing Power for said system is
less than or equal to 1, the design cycle is modified to a 4131
cycle according to the following cycle chart provided that the
Required Processing Power for the 4131 cycle is less than or equal
to 1:
Pilot Description
[0104] The pilot system is a pressure swing adsorption system that
operates by exploiting the difference in adsorption capacity of an
adsorbent for the gas of interest over a specific pressure range.
When the vessel containing the adsorbent is pressurized, the
adsorbent will selectively adsorb the contaminant from the gas
stream and thus remove it from the product stream that exits
through the other end of the vessel. When vessel is depressurized,
the contaminant will desorb, and the adsorbent will be ready to
process the feed stream again. This process is made into a
semi-continuous batch process by having 1 vessel or more than 1
vessel available to process the gas at the majority of all times.
With more than 1 vessel to process gas, additional options are
available to further increase efficiency by retaining pressurized
gas in dead volume spaces (piping or the heads of the vessels) and
the process then has the ability to generate a continuous stream of
product.
[0105] The conceptual process flow diagram is presented in FIG.
6.
[0106] The pilot system employs multiple PSA vessels to achieve the
desired nitrogen rejection and hydrocarbon recovery target. The
current pilot PSA design consists of 4-6 vessels with process steps
consisting of 1 bed on feed and 1 bed on blowdown at a time. There
are 2-3 equalization steps as well as product pressurization and
purge steps. The pilot system was designed to process up to 17kscfd
and capable of using 1 to 4 inch diameter beds. During the initial
construction of the pilot test system the bed size was selected to
be 1 inch due to the adsorbent performance and with considerations
of adsorbent manufacturing. The height was based on maximum
available height in the container. The remaining components of the
design were based on similar 6 bed PSA pilot plant already in
operation. Full range control valves were used for all valves. The
system was constructed entirely of stainless steel grade 316.
Additionally, a pretreatment system of 304 stainless steel was
designed and built as H.sub.2S compatible in order to remove all
condensed liquids and sulfur before entering the PSA portion of the
system.
[0107] The material used in the pilot testing was created as
follows: 23.00 lbs. of zeolite 4A powder supplied by Jianlong (as
4A-D) on a dry weight basis (29.50 lbs. wet weight) was placed in a
WAM MLH50 plow mixer. With the mixer agitating, 2.16 lbs of MR-2404
(a solventless silicone containing silicone resin from Dow Corning)
was pumped in at rate of 0.07 lb/min. After the MR-2404 addition
was completed, 9.2 lbs of water was added at a rate of 0.3 lb/min
under constant stirring in the plow mixer. At the end of the water
addition, plow mixing was continued for an additional 5 minutes.
The plow mixed powder product labeled hereinafter "the formulation"
was transferred to a tilted rotating drum mixer having internal
working volume of .about.75 L and agitated therein at a speed of 24
rpm. Mixing of the formulation was continued while beads were
gradually formed which had a porosity, as measured using a
Micromeritics Autopore IV Hg porosimeter on the calcined product,
in the 30-35% range. The beads were subjected to a screening
operation to determine the yield and harvest those particles in the
8.times.16 U.S. mesh size range. The product beads were air dried
overnight prior to calcination using a shallow tray method at
temperatures up to 595.degree. C. The shallow tray calcination
method used a General Signal Company Blue-M electric oven equipped
with a dry air purge. .about.500 g. dry wt. of the 8.times.16 U.S.
mesh adsorbent was spread out in a stainless steel mesh tray to
provide a thin layer. A purge of 200 SCFH of dry air was fed to the
oven during calcination. The temperature was set to 90.degree. C.,
followed by a 6 hour dwell time. The temperature was then increased
to 200.degree. C. gradually over the course of a 6 hour period, and
further increased to 300.degree. C. over a 2 hour period and
finally increased to 595.degree. C. over a 3 hour period and held
there for 1 hour before cooling to 450.degree. C. after which the
adsorbent was removed, immediately bottled in a sealed bottle and
placed in a dry nitrogen purged drybox. The calcined beads were
rescreened to harvest those particles in the 8.times.16 U.S. mesh
range.
[0108] Characterization of the modified 4A samples calcined at
595.degree. C. was performed using a thermogravimetric method as
described earlier in "ANRU TGA Testing". The nitrogen uptake rate
as performed in the test was determined to be .about.0.2 weight
%/minute as measured using the TGA method disclosed herein. When
the product beads in Example 1 were calcined up to 575.degree. C.,
the nitrogen uptake rate as performed in the test was determined to
be .about.0.7 weight %/minute as measured using the TGA method
disclosed herein. Subsequently, when the product beads in Example 1
were calcined up to 555.degree. C., the nitrogen uptake rate as
performed in the test was determined to be .about.1.2 weight
%/minute as measured using the TGA method disclosed herein.
TGA Description
[0109] Routine characterization of modified 4A samples was
performed using a thermogravimetric method using a TA Instruments
Q500 system installed in a glove box to minimize the impact of air
leaks. Nitrogen and oxygen gases supplied to the instrument were
high purity. The balance purge gas and gas 1 was nitrogen and a gas
2 corresponds to oxygen. For all experiments, a balance purge of 5
cc/minute was used and the gas directly over the sample was set to
95 cc/minute (nitrogen or oxygen). A sampling frequency of 0.5
sec/point was used for all adsorption steps. Alumina pans were used
for all studies and the sample size after activation was in the
range 100 to 120 mg.
[0110] The TGA method involves both an in-situ activation step
followed by adsorption tests using oxygen and nitrogen at
25.degree. C. The sample activation was performed by heating the
sample under nitrogen purge at 2.degree. C. per minute to
150.degree. C., maintaining isothermal for 60 minutes, heating at
5.degree. C./minute to 350.degree. C., holding at 350.degree. C.
for 120 minutes, then cooling to 25.degree. C. The nitrogen
equilibrium capacity at atmospheric pressure and 25.degree. C. is
reported as the weight gain on cooling under nitrogen relative to
the minimum weight at 350.degree. C. (the activated sample weight).
An assessment of relative rate for different samples and
preparation is captured by switching from nitrogen to oxygen. A
transient weight gain is observed followed by a drop attributable
to oxygen uptake followed by nitrogen leaving. A corresponding
switch from oxygen back to nitrogen results in a transient weight
loss followed by a weight gain attributable to oxygen loss followed
by nitrogen pickup. Values reported as "nitrogen uptake rate"
correspond to the maximum slope observed in the nitrogen uptake
portion and is equivalent also to the peak in the derivative weight
with respect to time for the same step. Values are reported in
weight %/minute.
Modeling Description
[0111] The results from the breakthrough test and parameters
obtained from the modeling were used with the methodology described
by Mehrotra, et al. in Arithmetic Approach for Complex PSA Cycle
Scheduling, Adsorption, 2010, pp. 113-126, vol. 16, Springer
Science+Business Media which details the basis for modeling PSA
processes. These simulations were performed using Process Builder,
from PSE.
Example 1. LSKP
[0112] A LSKP could be designed to handle a feed flow stream from a
well head during flowback after hydraulic fracturing of the well.
The state-of-the-art design condition would be based on the maximum
amount of value delivered by recovering the most methane available.
This design would call for a 4-1-3-1 cycle that could handle 5
MMscfd at a 35% N2 feed content and a 20% N2 product content. For
flow rates above 5 MMscfd and 35% N2 feed content, the extra gas
would be passed to the vent. For flow rates below 5 MMscfd and or
35% N2 feed content, the product gas would contain less than 20% N2
but the product flow rate would be substantially the same.
[0113] Using the proposed methodology, for feed streams above 5
MMscfd and 35% N2 in the feed, a switch to a 4-1-2-2 cycle would
enable the system to process up to 7 MMscfd and up to 45% N2 in the
feed stream while producing up to 35% more product than the
equivalent feed stream with the 4-1-3-1 cycle. Additionally,
switching to a 4-2-2-1 cycle would allow processing up to 10 MMscfd
and up to 70% N2 in the feed stream while producing up to 45% more
product that the equivalent feed stream with the 4-1-3-1 cycle and
venting methodology taught in the state of the art. These values
are shown in table 1 as demonstrated by modeling and pilot results.
Additionally, shown in table 1 is that just choosing a 4-2-2-1
cycle or a 4-1-2-2 cycle as the design basis for the system, has
substantially lower recovery for the point at which the most value
can be generated by the system. Thus, while the 4-1-3-1 cycle is
still the best choice for the design basis for the system, it is
not the only cycle that should be employed during the operation of
the system.
[0114] The methodology for switching between cycles can be
extrapolated from those proposed by Baksh et al. and described
previously.
TABLE-US-00001 TABLE 1 Performance of various cycles Model Pilot
Feed Beds Feed Eqs BD Production* Production** 1.00 4 1 3 1 1.00
1.00 1.05 4 1 3 1 1.03 1.04 1.00 4 1 2 2 0.86 0.85 1.33 4 1 2 2
1.32 1.30 1.00 4 2 2 1 0.68 0.69 1.55 4 2 2 1 1.44 1.44 *35%
N.sub.2 in CH4 feed, 10% N.sub.2 in product, variable feed flow,
feed pressure 410 psig, product pressure 405 psig **32-36% N.sub.2
in pipeline sales natural gas feed, 18-21% N.sub.2 in product,
variable feed flow, feed pressure 380-405 psig, product pressure
375-400 psig Beds represents the total number of adsorbents beds in
the cycle. Feed represents the number of beds in the feed step at
one time. Eqs represents the number of equalization steps in the
cycle. BD represents the number of beds on blow down in the cycle.
Model recovery represents the simulated recovery for the cycle with
a consistent feed flow. Pilot recovery represents the recovery
demonstrated in the pilot test system. Model production represents
the simulated production relative to the base case for the cycle
with a consistent feed flow. Pilot production represents the
production demonstrated in the pilot test system relative to the
base case.
[0115] Table 1 shows the demonstration of the three different cycle
examples (4133, 4122, 4221). The feed of 105% for the 4-1-3 cycle
represents the maximum possible product production of the cycle
with any feed flow, but not the highest recovery. The process is
restricted because it is unable to make higher product production
at the desired purity. The ability to handle higher feed flow rates
while maintaining a constant product purity (20% N.sub.2 in the
product) can be seen in the table with the other cycles.
Example 2. H2PSA
[0116] As noted earlier, hydrogen PSA (H2PSA) systems can also
benefit substantially from the adoption of new cycles to increase
the product flow of the system, beyond the original design basis,
or design basis taught in the state of the art. In this instance a
12-3-4 cycle was chosen as the design for comparison. In the event
that the feed flow to the system is increased, the 12-3-4 cycle
cannot handle the flow and still meet the purity target required.
Initially the cycle time can be reduced for the cycle until the
system limitations are met or exceeded (cycle time, bed
fluidization etc.). Once this occurs, the full limit of the system
is reached using state of the art methodology.
[0117] Using the proposed methodology, table 2 was constructed
showing the effect of modifications to the process cycle. These
effects are a demonstration of the selection process, but other
factors should be considered when switching to a different cycle,
such as frequency of the cycle changes and the effect on the
production, as well as cycle compatibility based on the teachings
of Baksh et al.
TABLE-US-00002 TABLE 2 Relative Product Product H2 Produced
Produced Recovery at at at Cycle LOFF HOFF LOFF HOFF HOFF 12-3-4 0%
100% 0% 100% 1.00 12-3-3 100% 105% 100% 104% 0.99 12-3-3 105% 107%
104% 105% 0.98 pge 12-4-4 107% 108% 105% 106% 0.98 Fpp 12-4-4 108%
109% 106% 107% 0.98 FPPe 12-5-3 109% 112% 107% 109% 0.97 12-5-3
112% 114% 109% 110% 0.96 FPPe
[0118] Table 2 shows increasing feed processing capability and
increasing produced product at reduced overall recovery. Highest
product potential is the maximum production that could be obtained
by the cycle at the required product purity (99.999% H2) as
additional feed gas would need to be vented. These values are given
as a general approximation and should be seen as a demonstration of
the overall trend, rather than exact feed flows a different cycle
is used for. A copy of the model used is provided with PSE process
builder software. Lowest Feed Flow Optimal (LOFF) is the lowest
feed flow point at which this cycle has the highest product
recovery among all the cycles tested. Highest Feed Flow Optimal
(HOFF) is the highest feed flow point at which this cycle has the
highest product recovery among all the cycles tested or is no
longer able to produce more product at purity beyond this flow
rate. H.sub.2 Recovery is the recovery of the product from the feed
relative to the recovery from the 12-3-4 cycle at its HOFF.
[0119] Conventional PSA system handles variable feed composition
and flow by adjusting cycle time without changing the cycle and
cycle steps. Within one cycle, cycle step and sequence, such as
adsorption feed, equalization, purge, provide purge, blow down are
fixed. Control valves are sized accordingly. Therefore, system
processing range is limited for the feed and contaminant
composition. With the proposed new control method, allowing and
adopting new cycles to address wider feed flow and composition
provides additional operational freedom compares to conventional
PSA system.
[0120] The methodology for switching between cycles can be
extrapolated from those proposed by Baksh et al. as described
previously.
* * * * *