US20210077942A1

US20210077942A1 - Process for handling variable flow rates and compositions in pressure swing adsorption systems

Info

Publication number: US20210077942A1
Application number: US17/042,627
Authority: US
Inventors: Nicholas R. Stuckert; Michael St. James; Cynthia A. Hoover; Yang Luo
Original assignee: Praxair Technology Inc
Current assignee: Praxair Technology Inc
Priority date: 2018-03-29
Filing date: 2020-03-29
Publication date: 2021-03-18
Also published as: CA3095357C; CN112020389A; CA3095132C; WO2019191421A1; WO2022066365A1; MY194862A; CN112020389B; US11471820B2; WO2019191594A1; MX2023003551A; RU2756180C1; CA3103291C; CA3193693A1; CA3103291A1; MX2020010139A; MY194670A; RU2020133778A3; CA3095357A1; US20210016218A1; CN111918711A

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.

Description

RELATED APPLICATIONS

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

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

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.
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.
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.
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.
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

DETAILED DESCRIPTION OF THE FIGURES

FIG. 1 shows the process for a 4121 cycle from the view of a single bed.

FIG. 2 shows the process for a 4131 cycle from the view of a single bed.

FIG. 3 shows the process for a 4122 cycle from the view of a single bed.

FIG. 4 shows the process for a 4221 cycle from the view of a single bed.

FIG. 5 shows the pressure trace for the 4122, 4131 and 4221 cycles.

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.

FIG. 7 shows an example of how to switch from a 4131 cycle to a 4122 cycle and the reverse.

FIG. 8 shows an example of how to switch from a 4122 cycle to a 4221 cycle and the reverse.

The legend for FIGS. 1-4 and 6-7 is:

- F—feed step and make product if at pressure
- EQD1—first equalization down
- EQD2—second equalization down
- EQD3—third equalization down
- X—Idle step
- BD—bed blow down/vent
- EQU3—third equalization up
- EQU2—second equalization up
- EQU1/F—first equalization up, overlap feed
- PP—product pressurization
- PP/F—product pressurization, overlap feed

DETAILED DESCRIPTION OF THE INVENTION

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.
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.
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.
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.
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.
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.
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.
$W_{q} (P, T) = X_{q} * \frac{{(K_{q} * P_{q})}^{\frac{1}{n_{q}}}}{1 + {Σ_{i = 0}^{m} (K_{i} * P_{i})}^{\frac{1}{n_{i}}}}$ $K_{i} = e^{- (A 1_{i} + \frac{A 2_{i}}{T})}$ $n_{i} = A 3_{i} + \frac{A 4_{i}}{T}$
q is the component being evaluated
T—temperature in Kelvin of the gas and adsorbent
P—pressure in Pascal of the gas
P_q—partial pressure in Pascal of the gas q
W_q—amount of component q adsorbed
A1, A2, A3, A4, X—fitting parameters, subscripts denote which gas the parameters correspond to
m is the number of components in the feed stream
$Required Processing Power = \frac{F_{n} * (W_{q} (P_{f}, T_{f}) - W_{q} (P_{v}, T_{v}))}{F * (W_{q} (P_{f^{n}}, T_{f^{n}}) - W_{q} (P_{v n}, T_{v n}))}$
Where:
q is the component being evaluated
W_q—amount of component q adsorbed as defined by a multicomponent temperature dependent isotherm, preferably the LRC isotherm
Pf—original feed pressure
Pfn—new feed pressure
Pv—original vent pressure
Pvn—new vent pressure
Tf—original feed temperature
Tfn—new feed temperature
Tv—original vent temperature
Tvn—new vent temperature
F—original feed flow rate
F_n—new feed flow rate
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.
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.
A component that is more readily adsorbable means that it can have:

- 1) a higher isosteric heat of adsorption than the less readily adsorbable component
- 2) a higher rate of adsorption that the less readily adsorbable component
- 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.

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:

- 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):
  - 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
  - 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
  - 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
  - d. And/or substitute an “overlap feed and product pressurization” step for a product pressurization step (not necessarily at the same step number)
  - e. And/or substitute a purge step for a blowdown step (not necessarily at the same step number).
- 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):
  - 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,
  - 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,
  - 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,
  - d. And/or substitute a product pressurization step for an “overlap feed and product pressurization” step (not necessarily at the same step number)
  - 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.

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:

- 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; or
- 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.

In another embodiment, 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.

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:

- 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.

In another embodiment, 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.
  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.

In one embodiment the product gas is methane and the more readily adsorbable component is N₂and/or CO₂.
In another embodiment the product gas is helium and the more readily adsorbable component is N₂and/or CO₂and/or methane and/or other hydrocarbons.
In another embodiment the product gas is hydrogen and the more readily adsorbable component is N₂and/or CO₂and/or methane and/or other hydrocarbons.
In yet another embodiment the product gas is N₂and the more readily adsorbable component is O₂.
The adsorbent beds of the invention typically contain contains zeolitic material and other optional adsorbents depending on the separation desired.
In one embodiment the adsorption bed contains adsorbent materials used in H₂PSA, the product gas is H₂and the more readily adsorbable component is selected from one or more of CO, CO₂, CH₄, N₂, Ar, and hydrocarbon.
In another embodiment the adsorbent is selected from at least one of activated carbon, Zeolite, 5A, CaX, LiX.
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:
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:
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:
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

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.
The conceptual process flow diagram is presented in FIG. 6.
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 H₂S compatible in order to remove all condensed liquids and sulfur before entering the PSA portion of the system.
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 ˜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 8×16 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° C. The shallow tray calcination method used a General Signal Company Blue-M electric oven equipped with a dry air purge. ˜500 g. dry wt. of the 8×16 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° C., followed by a 6 hour dwell time. The temperature was then increased to 200° C. gradually over the course of a 6 hour period, and further increased to 300° C. over a 2 hour period and finally increased to 595° C. over a 3 hour period and held there for 1 hour before cooling to 450° 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 8×16 U.S. mesh range.
Characterization of the modified 4A samples calcined at 595° 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 ˜0.2 weight %/minute as measured using the TGA method disclosed herein. When the product beads in Example 1 were calcined up to 575° C., the nitrogen uptake rate as performed in the test was determined to be ˜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° C., the nitrogen uptake rate as performed in the test was determined to be ˜1.2 weight %/minute as measured using the TGA method disclosed herein.

TGA Description

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.
The TGA method involves both an in-situ activation step followed by adsorption tests using oxygen and nitrogen at 25° C. The sample activation was performed by heating the sample under nitrogen purge at 2° C. per minute to 150° C., maintaining isothermal for 60 minutes, heating at 5° C./minute to 350° C., holding at 350° C. for 120 minutes, then cooling to 25° C. The nitrogen equilibrium capacity at atmospheric pressure and 25° C. is reported as the weight gain on cooling under nitrogen relative to the minimum weight at 350° 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

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

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.
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.
The methodology for switching between cycles can be extrapolated from those proposed by Baksh et al. and described previously.

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% N₂in CH4 feed, 10% N₂in product, variable feed flow, feed pressure 410 psig, product pressure 405 psig
**32-36% N₂in pipeline sales natural gas feed, 18-21% N₂in 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.

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% N₂in the product) can be seen in the table with the other cycles.

Example 2. H2PSA

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.
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 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

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. H₂Recovery is the recovery of the product from the feed relative to the recovery from the 12-3-4 cycle at its HOFF.
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.
The methodology for switching between cycles can be extrapolated from those proposed by Baksh et al. as described previously.

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 N₂and/or CO₂.

8. The method of claim 1 wherein the product gas is helium and the more readily adsorbable component is N₂and/or CO₂and/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 N₂and/or CO₂and/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 N₂and the more readily adsorbable component is O₂.

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 H₂PSA, the product gas is H₂and the more readily adsorbable component is selected from one or more of CO, CO₂, CH₄, N₂, 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.