U.S. patent application number 17/034748 was filed with the patent office on 2021-01-21 for characteristics of tunable adsorbents for rate selective separation of nitrogen from methane.
The applicant listed for this patent is Philip A. Barrett, Steven J. Pontonio, Neil A. Stephenson, Nicholas R. Stuckert. Invention is credited to Philip A. Barrett, Steven J. Pontonio, Neil A. Stephenson, Nicholas R. Stuckert.
Application Number | 20210016218 17/034748 |
Document ID | / |
Family ID | 1000005167461 |
Filed Date | 2021-01-21 |
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United States Patent
Application |
20210016218 |
Kind Code |
A1 |
Stuckert; Nicholas R. ; et
al. |
January 21, 2021 |
CHARACTERISTICS OF TUNABLE ADSORBENTS FOR RATE SELECTIVE SEPARATION
OF NITROGEN FROM METHANE
Abstract
The present invention generally relates to a process that
utilizes tunable zeolite adsorbents in order to reduce the bed size
for nitrogen removal from a methane (or a larger molecule)
containing stream. The adsorbents are characterized by the rate of
adsorption of nitrogen and methane and the result is a bed size
that is up to an order of magnitude smaller with these
characteristics (in which the rate selectivity is generally 30)
than the corresponding bed size for the original tunable zeolite
adsorbent that has a rate selectivity of >100x.
Inventors: |
Stuckert; Nicholas R.;
(Grand Island, NY) ; Pontonio; Steven J.; (Eden,
NY) ; Stephenson; Neil A.; (E. Amherst, NY) ;
Barrett; Philip A.; (Tonawanda, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Stuckert; Nicholas R.
Pontonio; Steven J.
Stephenson; Neil A.
Barrett; Philip A. |
Grand Island
Eden
E. Amherst
Tonawanda |
NY
NY
NY
NY |
US
US
US
US |
|
|
Family ID: |
1000005167461 |
Appl. No.: |
17/034748 |
Filed: |
September 28, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/US2019/024581 |
Mar 28, 2019 |
|
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17034748 |
|
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62649798 |
Mar 29, 2018 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 53/047 20130101;
B01J 20/186 20130101; B01D 2253/108 20130101; B01D 2257/102
20130101; B01D 2256/245 20130101 |
International
Class: |
B01D 53/047 20060101
B01D053/047; B01J 20/18 20060101 B01J020/18 |
Claims
1. A pressure swing adsorption process for kinetic separation of
N.sub.2 from a feed stream comprising at least methane and N.sub.2,
said process comprising feeding the feed stream to an adsorbent bed
comprising adsorbent having: a rate of adsorption of at least 0.036
mmol/g/min for N.sub.2 as determined by the Hiden method and a rate
of adsorption of methane that is 1/6.sup.th to 1/10000.sup.th the
adsorbent's adsorption rate for N.sub.2 as determined by the Hiden
method and recovering a product stream containing said at least
methane gas with a reduced level of N.sub.2.
2. The process of claim 1 wherein the adsorbent has an adsorption
rate of at least 0.143 mmol/g/min for N.sub.2 impurity as
determined by the Hiden method and an adsorption rate for methane
that is 1/10.sup.th to 1/1000.sup.th of the adsorption rate for
N.sub.2 as determined by the Hiden method.
3. The process of claim 1 wherein said adsorbent comprises zeolite
A, X, Y, chabazite, mordenite, faujasite, clinoptilolite, ZSM-5, L,
Beta, or combinations thereof.
4. The process of claim 3 wherein said adsorbent is a zeolite
exchanged with at least one cation selected from Li, Na, K, Mg, Ca,
Sr, Ba, Cu, Ag, Zn, NH4+ and combinations or mixtures thereof
5. The process of claim 1 wherein said adsorbent is zeolite A.
6. The process according to claim 2 where the feed stream that may
contain additional gas species such as ethane, propane, butane and
hydrocarbons with more than 4 carbon atoms and may include
adsorbents to remove said hydrocarbons.
7. A process according to claim 2 where the feed stream that may
contain additional gas species such as water, carbon dioxide or
sulfur species and may include adsorbents to remove said
species.
8. An adsorbent for the kinetic separation of N2 impurity from a
feed stream comprising at least methane and nitrogen gas, said
process comprising feeding the feed stream to an adsorbent bed
comprising an adsorbent having: a rate of adsorption of at least
0.036 mmol/g/min for N.sub.2 as determined by the Hiden method, and
a rate of adsorption for methane that is 1/6.sup.th or less than
the adsorbent's adsorption rate for N.sub.2 as determined by the
Hiden method.
9. The adsorbent of claim 8 wherein the adsorbent has an adsorption
rate of at least 0.143 mmol/g/min for said N.sub.2 as determined by
the TGA method and an adsorption rate for the methane that is
1/10.sup.th or less of the adsorbent's adsorption rate for N.sub.2
as determined by the Hiden method.
10. The adsorbent of claim 8 which comprises zeolite A, X, Y,
chabazite, mordenite, faujasite, clinoptilolite, ZSM-5, L, Beta, or
combinations thereof
11. The adsorbent of claim 10 wherein said adsorbent is a zeolite
is exchanged with at least one cation selected from Li, Na, K, Mg,
Ca, Sr, Ba, Cu, Ag, Zn, NH4+ and combinations or mixtures
thereof.
12. The adsorbent of claim 10 wherein said adsorbent is zeolite A.
Description
RELATED APPLICATIONS
[0001] This is a continuation-in-part application of and claims
benefit of International Application No. PCT/US2019/024581, filed
on Mar. 28, 2019, which claimed the benefit of U.S. Provisional
Application Ser. No. 62/649,798, filed on Mar. 29, 2018, both of
which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention generally relates to adsorbent
characteristics used in a process to separate nitrogen from
methane.
BACKGROUND OF THE INVENTION
[0003] Since nitrogen adsorption from methane is a relatively
unexplored area, it is important to draw the background from
similar adsorption processes such as pressure swing adsorption
(PSA), vacuum swing adsorption (VSA) and vacuum pressure swing
(VPSA) which have been commercially utilized for bulk air
separation, as well as trace air contaminant removal, for many
decades. In PSA and VPSA processes, compressed air is pumped
through a fixed bed of an adsorbent exhibiting an adsorptive
preference for one of the main constituents, typically Na in bulk
air separation, CO.sub.2 and H.sub.2O in air prepurification, or CO
and CO.sub.2 in H.sub.2 purification, etc., whereby an effluent
product stream enriched in the lesser-adsorbed constituent is
obtained. Improvements in these processes remain important goals,
one principal means of which is the discovery and development of
better process cycles. Significant improvements have been achieved
in not only recovery of gas but also reductions in overall system
size. These improvements also continue to provide important
benefits even while the adsorbent being used in conjunction with
the system is constantly improved and replaced with better
alternatives.
[0004] A large majority of processes operate through the
equilibrium adsorption of the gas mixture and kinetic separations
have lately attracted considerable attention with the development
of functional microporous adsorbents and efficient modeling tools.
Still, relatively few steric separation processes have been
commercialized. Kinetically based separation involves differences
in the diffusion rates of different components of the gas mixture
and allows different molecular species to be separated regardless
of similar equilibrium adsorption parameters. Kinetic separations
utilize adsorbents like carbon molecular sieves since they exhibit
a distribution of pore sizes which allow the different gaseous
species to diffuse into the adsorbent at different rates while
avoiding exclusion of any component of the mixture. Kinetic
separations can be used for the separation of industrial gases, for
example, for the separation of nitrogen from air and argon from
other gases. In the case of the nitrogen/oxygen separation (for
example, oxygen and nitrogen differ in size by only 0.02 nm), the
separation is efficient since the rate of transport of oxygen into
the carbon sieve pore structure is markedly higher than that of
nitrogen. Hence, the kinetic separation works, even though the
equilibrium loading levels of oxygen and nitrogen are virtually
identical.
[0005] Kinetically based separation processes may be operated, as
noted in U.S. Patent Application Publication No. 2008/0282884, as
pressure swing adsorption (PSA), temperature swing adsorption
(TSA), partial pressure swing or displacement purge adsorption
(PPSA) or as hybrid processes comprised of components of several of
these processes. These swing adsorption processes can be conducted
with rapid cycles, in which case they are referred to as rapid
cycle thermal swing adsorption (RCTSA), rapid cycle pressure swing
adsorption (RCPSA), and rapid cycle partial pressure swing or
displacement purge adsorption (RCPPSA) technologies, with the term
"swing adsorption" taken to include all of these processes and
combinations of them.
[0006] The faster the beds perform the steps required to complete a
cycle, the smaller the beds can be when used to process a given
hourly feed gas flow. Several other approaches to reducing cycle
time in PSA processes have emerged which use rotary valve
technologies (U.S. Pat. Nos. 4,801,308; 4,816,121; 4,968,329;
5,082,473; 5,256,172; 6,051,050; 6,063,161; 6,406,523; 6,629,525;
6,651,658; and 6,691,702). A parallel channel (or parallel passage)
contactor with a structured adsorbent may be used to allow for
efficient mass transfer in these rapid cycle pressure swing
adsorption processes. Approaches to constructing parallel passage
contactors with structured adsorbents are known (U.S. Patent
Application Publication No. 2008/0282892). These demonstrate the
benefit of having high rates of adsorption of the contaminant in
equilibrium processes and provide the basis for why increasing
rates of adsorption helps to intensify the process.
[0007] In the case of kinetic-controlled PSA processes, the
adsorption and desorption are more typically caused by cyclic
pressure variation, whereas in the case of TSA, PPSA and hybrid
processes, adsorption and desorption may be caused by cyclic
variations in temperature, partial pressure, or combinations of
pressure, temperature and partial pressure, respectively. In the
exemplary case of PSA, kinetic- controlled selectivity may be
determined primarily by micropore mass transfer resistance (e.g.,
diffusion within adsorbent particles or crystals) and/or by surface
resistance (e.g., narrowed micropore entrances). For successful
operation of the process, a relatively and usefully large working
uptake (e.g., the amount adsorbed and desorbed during each cycle)
of the first component and a relatively small working uptake of the
second component may preferably be achieved. Hence, the kinetic-
controlled PSA process requires operation at a suitable cyclic
frequency, balancing the avoidance of excessively high cycle
frequency where the first component cannot achieve a useful working
uptake with excessively low frequency where both components
approach equilibrium adsorption values.
[0008] Some established kinetic-controlled PSA processes use carbon
molecular sieve adsorbents, e.g., for air separation with oxygen
comprising the first more- adsorbed component and nitrogen the
second less adsorbed component. Another example of
kinetic-controlled PSA is the separation of nitrogen as the first
component from methane as the second component. Those may be
performed over carbon molecular sieve adsorbents or more recently
employing a hybrid kinetic/equilibrium PSA separation (principally
kinetically based but requiring thermal regeneration periodically
due to partial equilibrium adsorption of methane on the adsorbent
material) over titanosilicate based adsorbents such as ETS-4 (U.S.
Pat. Nos. 6,197,092 and 6,315,817). Thermal regeneration is
described as the method of passing heated gas across the adsorbent
bed in order to cause desorption of the methane. In order to
minimize the time required for thermal regeneration, slow rates of
methane uptake are chosen, which also correspond to the primary
benefit of the ETS-4 which is disclosed as high rate selectivity,
exceeding 100.times. the nitrogen uptake rate over methane as the
primary benefit of these adsorbents. The relatively slow rate of
uptake for nitrogen compared to an equilibrium process is seen as
unavoidable for rate selective processes, in order to maintain high
recovery. As a result, the bed sizes to process the gas are
relatively large compared to equilibrium processes.
[0009] Another patent utilizing molecular sieves for the removal of
nitrogen from natural gas is U.S. Pat. No. 4,964,889 which
discloses the use of a clinoptilolites zeolite containing magnesium
cations for the removal of nitrogen. The authors again teach the
primary benefit of the zeolites is high rate selectivity, exceeding
100.times. the nitrogen uptake rate over methane as the primary
benefit of these adsorbents. Again, the slow rate of uptake of
nitrogen is seen as necessary and unavoidable, in order to have
high recovery of methane and also to prevent methane poisoning.
Again, as a result the bed sizes to process the gas are relatively
large compared to equilibrium processes.
SUMMARY OF THE INVENTION
[0010] The present invention generally relates to adsorbent
characteristics used in a process to separate nitrogen from
methane. More specifically, the present invention relates to a
process that utilizes tunable zeolite adsorbents in order to reduce
the bed size for nitrogen removal from a methane (or a larger
molecule) containing stream. The adsorbents are characterized by
the rate of adsorption of nitrogen and methane and the result is a
bed size that is up to an order of magnitude smaller with these
characteristics (in which the rate selectivity is generally 30)
than the corresponding bed size for the original tunable zeolite
adsorbent that has a rate selectivity of >100.times..
DETASILED DESCRIPTION OF THE FIGURES
[0011] FIG. 1. Shows the rate selectivity dependence of modified 4A
on changing uptake rate of nitrogen.
[0012] FIG. 2. Outlines the TGA method sequence to measure the
rates of adsorption of nitrogen and methane.
[0013] FIG. 3. Shows an example of a TGA plot that is obtained
following the method outlined in FIG. 2.
[0014] FIG. 4. Shows an expansion of the same plot in FIG. 3 to
illustrate the features observed during gas switching.
[0015] FIG. 5. A breakthrough experiment diagram showing the
characterization of tunable zeolite 4A and clinoptilolite TSM-140
as compared to the minimum characteristics described herein and the
ideal characteristics for this invention.
[0016] FIG. 6. A diagram showing a typical application of this
system to a natural gas well head feed stream, post hydraulic
fracturing.
DETAILED DESCRIPTION OF THE INVENTION
[0017] Separating nitrogen and methane has historically presented a
challenge. While carbon-based adsorbents are readily available to
adsorb methane from nitrogen, this leaves the methane at ambient
pressure while the nitrogen is produced near the feed pressure.
Typically, the methane is required at the feed pressure and the
nitrogen at ambient pressure. It is then preferred to adsorb the
nitrogen. The present invention generally relates to adsorbent
characteristics used in a process to separate nitrogen from
methane.
[0018] The adsorbents of the invention are characterized by the
rate of adsorption of nitrogen and methane. These material
characteristics are used in a pressure swing adsorption (PSA)
process, in order to adsorb the nitrogen and allow the methane to
pass through the adsorption bed at or around the feed pressure.
Demonstration of effect and benefit is shown in the examples of
modeling, bench characterization and pilot testing.
[0019] In order to intensify the process and reduce the bed sizes
of the adsorption system, an adsorbent having an increased rate of
adsorption of nitrogen was developed. An examination of the role of
the uptake rate of nitrogen, the selectivity that was
correspondingly displayed in the material compared to the uptake
rate of methane, and the final product purity desired, demonstrated
that the long-held wisdom of higher selectivity was incorrect. What
is best is a high uptake rate of the contaminant (nitrogen in this
case), and even a moderate selectivity of around 6x is sufficient
to provide similar performance to state of the art materials that
have a selectivity generally around 100.times..
[0020] Previous adsorbent applications describe characteristics of
adsorbents that are favorable or required for a separation or to
improve a separation. Previous patents have described materials
that are favorable for kinetic-controlled purification of gases or
processes that are favorable for kinetic-controlled separations. In
U.S. Pat. No. 6,315,817 as an example, a specification for which
variant of ETS-4 or the characteristics required is missing, and
the majority of ETS-4 products that can be made to fit the
description do not work. Specifically, barium exchanged ETS-4 is
commercially available for this separation; however the moisture
content is noted to be critical to performance. No characteristics
exist for rate selective adsorption of nitrogen from natural gas
(methane). The only characterization is that the benefit of the
material is the high rate selectivity (up to and exceeding
100.times.) of the rate of uptake of nitrogen over methane. The
result is very large bed sizes that have come to define the entire
area of rate selective adsorbent processes.
[0021] This present invention defines tunable adsorbent
characteristics and a process that allows one to reduce the bed
sizes for nitrogen removal from a methane (or a larger molecule)
containing stream. The result is a bed size that is up to an order
of magnitude smaller with these characteristics (in which the rate
selectivity is generally 30) than the corresponding bed size for
the original tunable zeolite adsorbent that has a rate selectivity
of >100.times..
[0022] In one embodiment adsorbents having crystalline inorganic
frameworks can be utilized in accordance with the present
invention. Crystalline inorganic adsorbents are defined as any
microporous aluminosilicate having a regular arrangement of atoms
in a space lattice. Zeolites are a preferred crystalline inorganic
framework. Zeolites are porous crystalline aluminosilicates which
comprise assemblies of SiO.sub.4 and AlO.sub.4 tetrahedra joined
together through sharing of oxygen atoms. The general
stoichiometric unit cell formula for a zeolite framework is:
M.sub.x/m(AlO.sub.2)x(SiO.sub.2)zH.sub.2O
where M is the cation with a valence of m, z is the number of water
molecules in each unit cell, and x and y are integers such that y/x
is greater than or equal to 1. The ratio of oxygen atoms to
combined aluminum and silicon atoms is equal to 2. Therefore, each
aluminum atom introduces a negative charge of one (-1) on the
zeolite framework which is balanced by that of a cation. To
activate the zeolite the water molecules are completely or
substantially removed by raising the temperature or pulling vacuum.
This results in a framework with the remaining atoms intact
producing cavities connected by channels or pores. The channel size
is determined by the number of atoms which form the apertures
leading to the cavities as well as cation type and position.
Changing the position and type of the cation allows one to change
and fine tune channel size and the properties of the zeolite,
including its selectivity. For instance, the sodium form of Zeolite
A has a pore size of .about.4.ANG. and is called a 4A molecular
sieve. If at least 40% of the sodium ions are exchanged with a
larger potassium ion, the pore size is reduced to .about.3.ANG.. If
these are exchanged with >70% calcium, one calcium ion replaces
two sodium ions and the pore opening is increased to .about.5.ANG..
The ability to adjust pores to precisely determine uniform openings
allows for molecules smaller than its pore diameter to be adsorbed
while excluding larger molecules. The Si/Al ratio can also be
varied to modify the framework structure and provide selectivity
required for a given separation. This is why zeolites, known as
molecular sieves, are very effective in separating on the basis of
size.
[0023] Some non-limiting examples of zeolites that can be employed
in the context of the invention include zeolite A, X, Y, chabazite,
mordenite, faujasite, clinoptilolite ZSM-5, L, Beta, or
combinations thereof. The above zeolites can be exchanged with
cations including Li, Na, K, Mg, Ca, Sr, Ba, Cu, Ag, Zn, NH4+ and
mixtures thereof. In one embodiment zeolite 4A is a preferred
adsorbent.
[0024] The adsorbents are characterized by the rate of adsorption
of nitrogen and methane. These material characteristics are used in
a pressure swing adsorption (PSA) process, in order to adsorb the
nitrogen and allow the methane to pass through the adsorption bed
at or around the feed pressure. Demonstration of effect and benefit
is shown in the examples of modeling, bench characterization and
pilot testing. Previous disclosures on cycles do not define the
rate characteristics required for the cycle to work. This is an
important consideration for rate selective materials as the
majority of materials fail to deliver sufficient separation under
any process conditions even while providing a substantial
difference in adsorption rates. When rate selectivities are
disclosed for materials for this separation, they are generally
100.times. or higher.
[0025] Surprisingly, it has been found that lowering rate
selectivity of the adsorbent allows one to reduce the bed size
required to process a specific feed stream thereby lowering cost
performance. It has also been noted that a higher rate selectivity
generally corresponds to a lower uptake rate of nitrogen. When
attempting to shrink the apparent pore size of an adsorbent, the
decreasing rate of the larger molecule occurs much faster than the
smaller molecule. However, the decreasing rate of uptake of
nitrogen decreases the productivity of the adsorbent during a fixed
period of time. Thus, it is important to find a balance between the
two instead. One might think that increasing the rate of uptake of
the larger molecule would cause the adsorbent to saturate with the
larger molecule. In typical rate selective separations however,
steady state adsorption is achieved relatively quickly in a matter
of minutes to hours to a few days (N.sub.2PSA, or carbon molecular
sieves for example). In a scenario where it is desired to decrease
the rate to the point that the larger molecule did not reach steady
state during operation, the rate of uptake of nitrogen would have
to be slowed substantially such that the adsorbent productivity
would suffer even more and would not be practically useful. Another
alternative has been to employ periodic thermal regeneration of the
adsorbent, however even in this case the productivity of the
adsorbent suffers significantly. The essentially finding here is
that when two overlapping distributions of gas sizes are present,
in order to find the optimal rate, one must take into account the
rate of adsorption of the contaminant, rather than seek only higher
selectivity.
[0026] In accordance with the present invention it was discovered
that contaminant rate selective adsorbent must have a rate of
contaminant uptake at least 6.times. greater than the product, and
ideally >20.times. product, as characterized by the rate
measurement from a gravimetric pressure microbalance such as a
Hiden IGA unit i.e., it is not required to have a rate selectivity
of greater than 100.times. the product as taught by the
state-of-the-art in the field.
[0027] According to the invention, the adsorbent is characterized
by the following characteristics: [0028] 1. The rate of uptake of
nitrogen is greater than 0.1 wt %/min (0.036 mmol/g/min), in
another embodiment greater than 0.4wt % min determined by the
gravimetric method going from vacuum to 1 Bar pressure of >99.9%
Nitrogen (less than 0.1 ppm H.sub.2O) at 35.degree. C. for nitrogen
measurements and from vacuum to 1 Bar pressure of >99.9% Methane
(less than 0.1 ppm H.sub.2O) at 35.degree. C. for methane
measurements, these targets may vary up to 50% for differing
pressures and desired product compositions, and these numbers are
based on a feed gas stream of 600 psig. [0029] 2. The rate of
methane uptake, as characterized by a pressure microbalance
gravimetric system such the Hiden IGA, must be less than 1/6.sup.th
the uptake rate of nitrogen at 1 atm >99.9% CH4, in another
embodiment less than 1/10.sup.th at 1 atm >99.9% CH.sub.4. The
rate of methane uptake should obviously not be zero and should be
greater than 1/10000.sup.th, in another embodiment greater than
1/1000.sup.th the uptake rate of nitrogen at 1 atm >99.9%
CH.sub.4. In one embodiment the rate of methane uptake, as
characterized by the Hiden unit, is less than 1/6.sup.th but
greater than 1/10000.sup.th the uptake rate of nitrogen at 1 atm
>99.9% CH.sub.4, in another embodiment less than 1/10.sup.th but
greater than 1/1000.sup.th at 1 atm >99.9% CH.sub.4.
[0030] In additional embodiments the adsorbent is characterized by
the following: [0031] 3. A heat of adsorption as determined by
isotherm measurements fit with the LRC method show that a heat of
adsorption (i.e. -A2).gtoreq.10 kcal/mol and .ltoreq.25 kcal/mol.
[0032] 4. A heat of adsorption of methane (or a larger molecule) is
.ltoreq.200% that of nitrogen, in another embodiment from about 50%
to about 200% of that of nitrogen, and in another embodiment from
about 50% to .ltoreq.125% of that of nitrogen. [0033] 5. A total
adsorption capacity as determined by the Hiden gravimetric
measurements that are allowed up to one week to equilibrate, which
is preferably greater than 0.4 mmol/g N.sub.2. Adsorption capacity
of nitrogen of .gtoreq.0.2 wt %, in another embodiment .gtoreq.0.7
wt % for a fresh sample activated at 350 to 400.degree. C. for 8
hours under vacuum and measured in a 1 Bar of >99.9% nitrogen at
35.degree. C.
[0034] While these characteristics are primarily described for the
separation of nitrogen from methane (natural gas), it should be
noted that they will apply to other kinetic based separations as
well. The process may also include other adsorbents to remove a
range of contaminants that are present in the feed stream including
hydrocarbons that contain more than 4 carbon atoms, moisture,
carbon dioxide, sulfur containing species or other species that may
reduce the working capacity of the adsorbent described herein.
These adsorbents could comprise activated carbon, silica, alumina,
zeolites, titanosilicates, iron based, amine containing adsorbents
or mixtures thereof. Typically, silica and alumina adsorbents are
used for initial water removal, followed by zeolites. Typically,
titanosilicates, zeolites, activated carbon, amine containing, or
iron-based adsorbents are used for sulfur removal. Typically,
zeolites, titanosilicates, activated carbon, silica or amine
containing adsorbents are used for carbon dioxide removal.
Typically, silica gel or activated carbon are used for hydrocarbon
removal.
[0035] In the event that one of these adsorbents fails to remove
the species, thermal regeneration may be performed to remove that
species from the adsorbent described and still fall within the
realm of this invention which is to eliminate thermal regeneration
from being used to remove the product gas of the invention. Methods
to characterize the adsorbent are given below.
Pilot Description
[0036] 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.
[0037] The conceptual process flow diagram is presented in FIG.
6.
[0038] 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 17
kscfd 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.
LRC Description
[0039] Adsorbents were characterized using the loading ratio
correlation (LRC) method as described herein and based on the
article "Multicomponent Adsorption Equilibria on Molecular Sieves"
by Yon and Turnock, published as part of the AIChE Symposium
Series, 117, Vol. 67, in 1971 in Adsorption Technology. Isotherm
measurements were performed by using an IGA balance as described
below, for temperatures of 20.degree. C., 35.degree. C. and
50.degree. C.
Hiden IGA Description (Equilibrium and Rate)
[0040] Rate and equilibrium characterization of samples were
performed using a Hiden IGA pressure microbalance (Model#HAS022650)
which measures single component gas uptake and was used to examine
the adsorption of N.sub.2 and CH.sub.4. The samples were loaded and
gas adsorptions were measured as instructed in the IGA Systems User
Manual #HA-085-060. Each sample was loaded and activated in situ
under vacuum with a temperature ramp of 0.7 C/min to between 350
and 400.degree. C. and held for 12 hours. It was then cooled to the
adsorption test temperature at a rate of 1.degree. C./min. The
amount of gas adsorbed by the adsorbent is measured in micrograms
at a fixed temperature controlled by a constant temperature bath.
The pressures points are taken from 0.1 bar to 10 Bar allowing up
to 7 days to reach equilibrium. Equilibrium and leak check
verification is done by a desorption isotherm that matches the
adsorption isotherm. A buoyancy correction was determined using
helium and this was used to adjust the microgram weight for
buoyancy effects using the molecular weight of the gas being
measured. The buoyancy corrected microgram weight was used to
calculate uptake using standard methods and using the activated
sample weight. For rate measurements, the test gas (N.sub.2 or
CH.sub.4) was introduced at 1 Bar then the sample was held at
pressure recording the weight as a function of time. System
dynamics require approximately 2 minutes to stabilize. Weight data
after 2 minutes were corrected for buoyancy and converted to
uptakes in weight % or mmol/g and the uptake versus time data were
fit to a first order process to obtain rates. Each material was
tested first for N.sub.2, prior to being reactivated before
repeating the test using CH.sub.4.
Breakthrough Description
[0041] A breakthrough test system was created to test the adsorbent
samples using a 12'' long 1'' pipe filled with adsorbent. A
breakthrough test was run by first saturating the bed with a flow
of 300 sccm at 400 psig of 99% methane (where methane is
>99.99%) and 1% helium (where helium is >99.99%) gas for 2
hours, then a flow of 300 sccm of a 49.75/49.75/0.5 mixture of
N.sub.2 (where nitrogen is >99.99%)/CH.sub.4/He was introduced
as a feed gas to the adsorbent bed and the outlet gas was measured
using a gas chromatography mass spectrometer. The breakthrough was
recorded as a nitrogen breakthrough example. After 30 minutes this
flow was switched to 300 sccm of 99% nitrogen and 1% helium and
held for 2 hours. Then the flow was switched back to the 300 sccm
of 49.75/49.75/0.5 mixture of N.sub.2/CH.sub.4/He and this was
recorded as the methane breakthrough. These breakthrough curves
were then used with gPROMS software provided by Process Systems
Enterprise, Inc. (PSE) to automatically perform parameter
estimation of a model that was created as a replica of the system.
The libraries supplied with the adsorption aspect of Process
Builder from PSE are sufficient to replicate these results. A
detailed description and instructions on how to perform these
simulations is provided by PSE.
Modeling Description
[0042] 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.
TGA Rate Measurements
[0043] A TGA method was developed to assess comparative nitrogen
rates that involves both an in-situ activation step followed by
adsorption tests using oxygen and nitrogen at 25.degree. C. The
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 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. Rate measurements for selectivity determinations
relied exclusively on a Hiden pressure microbalance rather than the
TGA method.
EXAMPLE 1. MODELING RESULTS FOR HIGHER SELECTIVITY RATIOS
[0044] This example demonstrates that once the rate selectivity is
above 30, the ratio of uptake rates (N.sub.2/CH.sub.4) as measured
via Hiden microbalance, does not significantly impact performance
until a ratio of uptake rates is greater than 1,000,000 which has
not been achieved in an economically viable offering. At a ratio of
1, the system works against the desired separation to instead
produce a purified product of nitrogen. At ratios above 5, the
adsorption of methane becomes too low on a normal cycle and the
product of purified methane begins to emerge. At ratios above 35
the adsorption of methane fails to negatively impact the
performance of the system with proper process cycles and the system
performs at peak performance for the majority of selectivity ratios
studied. The exception is that above a selectivity of 1,000,000,
then the adsorbent does not reasonably saturate with methane during
the expected lifetime of the system (>5 years) and thereby
increases the working capacity of nitrogen almost 100% vs CSS
conditions. Selectivity ratios of almost 1,000,000 have never been
reported in literature and are currently .about.10,000 times higher
than the state of the art.
TABLE-US-00001 Ratio of rates of uptake BSF at year 5
(N.sub.2/CH.sub.4) (lbs/MMscfd Feed) 1 -- 10 3600 35 1200 100 1200
1000000 1200 10000000 600 For 35% N.sub.2 in feed to 20% N.sub.2 in
product at a recovery of 80% at 35 C.
EXAMPLE 2. MODELING RESULTS DEMONSTRATIONG REDUCED SELECTIVITY, AND
HIGHER UPTAKE OF NITROGEN BENEFIT
[0045] The commercial performance of the tunable zeolite 4A was
modeled with a relative rate of 0.9 wt % N.sub.2/min characterized
material. The recovery is the total hydrocarbons recovered from the
4-bed system. The production is the relative production of the
system at different conditions. The purity is the methane
concentration of the product. The N2 rate is the rate of uptake of
nitrogen on the material relative to the 0.9 wt % N.sub.2/min
uptake rate material. The CH.sub.4 rate is the rate of uptake of
methane on the modeled material relative to the same material basis
which was 0.03wt % CH.sub.4/min. The feed concentration is 35%
N.sub.2, 65% Methane at 35.degree. C.
TABLE-US-00002 TABLE 1 the rate selectivity dependence of modified
4A (WO201715431164) for changing uptake rate of nitrogen. Recovery
Purity Production N.sub.2 Rate CH.sub.4 Rate 66% 90% 100% 100% 100%
60% 90% 90% 100% 200% 47% 90% 71% 100% 400% 52% 90% 99% 20% 100%
21% 90% 95% 4% 100% 74% 90% 160% 200% 200% -- 55% -- 800% 800% 53%
90% 90% 20% 20%
Table 1 show that doubling the uptake rate of methane decreases the
recovery of a fixed bed size system, but only by .about.10%. This
demonstrates that higher selectivity only has a marginal benefit in
this regime.
[0046] In the case where the rate of uptake of both nitrogen and
methane are doubled the theoretical performance is significantly
higher, however this assumes the rate selectivity ratio is
maintained, which unfortunately it is not. This demonstrates that
higher uptake rates of nitrogen are preferred, as described
herein.
[0047] A 400% increase in methane uptake begins to lower the
recovery substantially more, however the material is still viable
for the separation. This further demonstrates that higher
selectivity is not the most important consideration even at the
edges of the proposed characteristics described herein.
[0048] If an 800% increase to both rates is modeled, the material
is no longer able to maintain a product purity, and instead begins
to remove methane from nitrogen. This is an effect of accounting
for physical restrictions within the system related to valve open
speeds and gas flows across the adsorbent. If a system were
designed to mitigate these, the higher rates of adsorption could be
tolerated, and the system size could be reduced even more. However,
it is important to note here that typical adsorption systems have
physical limits that render them unable to utilize such high rate
materials and the practical design point for materials when
considering these factors is a slower uptake rate of nitrogen and
methane.
[0049] In the case of lowered rates of adsorption of nitrogen but
maintained or lowered rates of adsorption of methane we see a drop
in the process performance, but the performance is not affected by
the rate of methane uptake in this regime. This suggests that at a
certain point, slowing the uptake rate of methane no longer
benefits the process as shown in example 1. At a certain point, the
rate of uptake of nitrogen is sufficiently slow that the process
begins to perform very poorly at a fixed bed size and can only be
remedied with very costly increases to the bed size. This
demonstrates the improvement discovered here of increased rates of
nitrogen uptake that the expense of even larger increases to the
rate of methane uptake.
EXAMPLE 3. FASTER SYSTEM RESPONSE
[0050] Since one of the benefits of higher selectivity is the
increased time to reach cyclic steady state (CSS), it's important
to note that CSS is reached significantly faster with these new
material characteristics. A defining characteristic of
state-of-the-art materials is strong competing adsorption via
methane which results in a lowered working capacity/minimal working
capacity after adsorbent saturation. In a normal PSA cycle with
ETS-4, very large adsorbent beds must be utilized with very low
recovery systems to generate a moderate purity product. To counter
this, one can implement methods to desaturate the adsorbent and
balance the economics of large beds with low recovery (normal) or
high capital (with desaturation). One example of desaturation is
thermal regeneration. U.S. Pat. No. 6,444,012 to Dolan et al
describes a method to desaturate the adsorbent by heating the
product stream (largely methane) in order to force methane out of
the pores via a TSA process. This consumes methane, energy (for
heating) and requires replacement capital of the beds not
undergoing thermal regeneration in order to maintain continuous
operation. Additionally, this requires that the adsorption rate of
methane is very slow. A very slow adsorption rate of methane
usually is associated with a slow adsorption rate of nitrogen. CSS
loading capacity of methane on Tunable 4A was reached in
approximately 30 minutes via modeling and pilot experiments for the
highest performing adsorption rates tested. This has additional
benefits such as being able to respond rapidly to changing feed
conditions. Oil and gas wells typically have significant
fluctuations and variations. Responding to these is an additional
benefit of faster adsorption rate-based processes.
TABLE-US-00003 wt %/min N2 CSS Time Pilot CSS Time model uptake
(min) (min) 1.2 20 22.5 0.9 30 30 0.6 120 45 0.1 -- 270
EXAMPLE 4. LOW RATE SELECTIVITY, HIGH UPTAKE RATE ADSORBENTS
[0051] A material was made to demonstrate the proposed benefit of
higher uptake rates of nitrogen even at reduced overall rate
selectivity, demonstrating the benefit illustrated by the model in
example 1.
[0052] 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.
[0053] Characterization of the tunable 4A samples calcined at
595.degree. C. was performed using a thermogravimetric screening
method as described earlier in "TGA description". 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.
Breakthrough Data from Model and Lab Experiment
[0054] The breakthrough data demonstrates the achievement of the
required rate characteristics, and is shown in the FIG. 5 by the
minimum required rates of adsorption for nitrogen, and maximum
rates of adsorption of methane, and the ideal rates of adsorption
for both of these. It is clear in the FIG. 5 that the actual
adsorbent had rates of adsorption of both components in between
these two extremes. These two extremes also determine the
characteristics alternative materials need to meet in order to have
high performance in this process. Also shown in the FIG. 5 is
clinopotilolite (clino) TSM-140 which is commercialy available.
This state of the art material does not have the uptake rate of
nitrogen to meet the characteristics described here.
Pilot Data
[0055] The relative rate of uptake correlates to the TGA
measurement. The selectivity is the rate of uptake of nitrogen
divided by the rate of uptake of methane as determined by the
breakthrough test and model fitting. The pilot recovery is the
recovery observed in the pilot system for a feed concentration of
35% nitrogen, a flow rate of 120 scfh using four 1'' beds filled
5.5' tall. The product impurity was held at 20% nitrogen. The
results show that with increasing selectivity, the pilot recovery
falls significantly, due to the inability of the material to
process enough gas to overcome the losses from void spaces. This is
an unfortunate reality of high selectivity is that is typically
corresponds to reduced uptake rate of nitrogen. When this is
considered, the recovery rises substantially, up to a point that
the process is unable to take advantage of the higher rate of
uptake to due to low response time of the system and the fast
uptake of methane which ultimately both work to lower recovery. One
way that others have overcome this low recovery is to increase the
bed size, thereby increasing the amount of gas the material is able
to process and offsetting the void space losses. Since this
modification is no longer necessary, this recovery gain is
equivalent on a commercial scale to lower bed sizes.
TABLE-US-00004 TABLE 2 showing the impact of relative uptake rate
on selectivity and on final recovery in the pilot system. N.sub.2
rate of uptake Rate Selectivity Pilot wt %/min (N.sub.2/CH.sub.4)
Recovery 0.1 120 2% 0.6 70 6% 0.9 44 24% 1.2 30 20%
EXAMPLE 5. MODELING SENSITIVITY TO PRODUCT PURITY AND PRESSURE
[0056] Another study was conducted to determine the optimal
adsorbent for a product purity of 5% nitrogen compared to 20%
nitrogen and for a feed stream of 200 psig compared to 600 psig.
The results show that some variation in the preferred adsorption
uptake rate of nitrogen and the subsequent ratio of uptakes
compared to methane exists, but that this variation is typically
limited to +/-50% between applications. Additionally, while the
optimal target can vary up to 50%, the performance for a 50%
variation in uptake rates does not generally cause a larger
variation in process performance.
TABLE-US-00005 TABLE 3 Model N.sub.2 rate of Rate Projected uptake
Selectivity Pilot Feed Product wt %/min (N2/CH4) Recovery impurity
impurity 0.1 120 4% 35% 20% 0.6 70 8% 35% 20% 0.9 44 28% 35% 20%
1.2 30 22% 35% 20% 0.1 120 8% 10% 5% 0.6 70 34% 10% 5% 0.9 44 32%
10% 5% 1.2 30 28% 10% 5%
Table 3 shows that for different processing feed impurities and
desired product impurity levels, the optimal adsorption rate of
nitrogen can vary accordingly. In particular, as the impurity level
is reduced, the impact of selectivity becomes more important and
the importance of the uptake rate of nitrogen begins to fall. This
should be considered when selecting the optimal characteristics for
the process. It also leads to the prospective of multiple layers of
varying uptake rate tunable zeolite 4A adsorbents in order to best
accomplish a separation process when there is a potential large
variation in impurity levels through the process, as a method to
best reduce the overall bed size of the system.
* * * * *