U.S. patent application number 10/307597 was filed with the patent office on 2003-07-03 for light hydrocarbon separation using 8-member ring zeolites.
Invention is credited to Olson, David H..
Application Number | 20030121415 10/307597 |
Document ID | / |
Family ID | 27118106 |
Filed Date | 2003-07-03 |
United States Patent
Application |
20030121415 |
Kind Code |
A1 |
Olson, David H. |
July 3, 2003 |
Light hydrocarbon separation using 8-member ring zeolites
Abstract
A method of selectively adsorbing propylene in mixtures of
propylene/propane and propylene/olefins through the use of zeolites
having structures with a maximum of 8-member rings of tetraheda
controlling the diffusion rate. Suitable zeolite adsorbents are
those having the CHA and ITE structure types. Other 8-member ring
zeolites, including aluminosilicates, with a Si:Al molar ratio of
at least about 200 and having substantially no free acid are also
suitable adsorbents.
Inventors: |
Olson, David H.;
(Pennington, NJ) |
Correspondence
Address: |
DANN DORFMAN HERRELL & SKILLMAN
SUITE 720
1601 MARKET STREET
PHILADELPHIA
PA
19103-2307
US
|
Family ID: |
27118106 |
Appl. No.: |
10/307597 |
Filed: |
December 2, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10307597 |
Dec 2, 2002 |
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09844314 |
Apr 27, 2001 |
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6488741 |
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09844314 |
Apr 27, 2001 |
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09768943 |
Jan 23, 2001 |
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Current U.S.
Class: |
95/144 |
Current CPC
Class: |
B01D 2253/108 20130101;
B01D 2256/24 20130101; B01D 53/02 20130101; B01D 2253/1085
20130101 |
Class at
Publication: |
95/144 |
International
Class: |
B01D 053/02 |
Claims
What is claimed:
1. A process for kinetic separation of a light hydrocarbon mixture
comprising at least two components, by preferentially adsorbing one
of said two components on a zeolite adsorbent, said zeolite phase
containing 8-member rings of tetrahedra as the pore opening
controlling hydrocarbon diffusion, the other of said two components
being non-preferentially adsorbed, said process comprising the
steps of: (a) contacting said light hydrocarbon mixture with a
zeolite adsorbent, characterized by having a diffusion rate which
is at least 50 times greater for the preferentially adsorbed
component as compared to said non-preferentially adsorbed
component; and (b) recovering said preferentially adsorbed
component.
2. A process as claimed in claim 1, wherein said hydrocarbon
mixture includes an olefin as said preferentially adsorbed
component and a paraffin as said non-preferentially adsorbed
component.
3. A process as claimed in claim 2, wherein said olefin is
propylene and said paraffin is propane.
4. A process as claimed in claim 3, wherein said zeolite has a
diffusion rate which is at least 200 times greater for propylene
than for propane.
5. A process as claimed in claim 1, wherein said zeolite has a
silica-to-alumina ratio greater than about 200.
6. A process as claimed in claim 1, wherein said zeolite has a
silica-to-alumina ratio greater than about 500.
7. A process as claimed in claim 1, wherein said zeolite has a
silica-to-alumina ratio greater than about 1000.
8. A process as claimed in claim 1, wherein said zeolite has a
silica-to-alumina ratio greater than about 2000.
9. A process as claimed in claim 1, wherein said zeolite has alkali
metal cations as the ions balancing the framework charge.
10. A process as claimed in claim 1, wherein said zeolite has a
propylene adsorption capacity greater than 40 mg/g.
11. A process as claimed in claim 1, wherein said zeolite is of CHA
structure type.
12. A process as claimed in claim 11, wherein said zeolite has a
silica-to-alumina ratio greater than about 200 and contains alkali
metal ions as the cations balancing the framework charge.
13. A process as claimed in claim 11, wherein said zeolite has a
silica-to-alumina ratio greater than about 500 and contains alkali
metal ions as the cations balancing the framework charge.
14. A process as claimed in claim 11, wherein said zeolite has a
silica-to-alumina ratio greater than about 1000 and contains alkali
metal ions as the cations balancing the framework charge.
15. A process as claimed in claim 11, wherein said zeolite has a
silica-to-alumina ratio greater than about 2000 and contains alkali
metal ions as the cations balancing the framework charge.
16. A process as claimed in claim 11, wherein said CHA structure
type 8-member rings zeolite is selected from the group consisting
of Si-CHA, high silica chabazite, AlPO.sub.4-(CHA), CaAPO-44,
CaAPO-47, GaPO.sub.4-34, LZ-218, Linde D, Linde R, MeAPO-47,
MeAPSO-47, Phi, SAPO-34, SAPO-47, Si-CHA, SSZ-13, Wilhendersonite,
ZK-14 and ZYT-6 and mixtures thereof.
17. A process as claimed in claim 1, wherein the zeolite has the
ITE structure type.
18. A process as claimed in claim 17, wherein the zeolite has the
ITE structure type, a silica-to-alumina ratio greater than about
200 and contains alkali metal ions as the cations balancing the
framework charge.
19. A process as claimed in claim 17, wherein the zeolite has the
ITE structure type, a silica-to-alumina ratio greater than about
500 and contains alkali metal ions as the cations balancing the
framework charge.
20. A process as claimed in claim 17, wherein the zeolite has the
ITE structure type, a silica-to-alumina ratio greater than about
1000 and contains alkali metal ions as the cations balancing the
framework charge.
21. A process as claimed in claim 17, wherein the zeolite has the
ITE structure type, a silica-to-alumina ratio greater than about
2000 and contains alkali metal ions as the cations balancing the
framework charge.
22. A process as claimed in claim 17, wherein said ITE structure
type is ITQ-3.
23. A process as claimed in claim 17, wherein the zeolite has the
SAPO-34 structure type, and contains alkali metal ions as the
cations balancing the framework charge.
24. A process for kinetic separation of a light hydrocarbon mixture
comprising at least two components, by preferentially adsorbing one
of said two components on a zeolite adsorbent, said zeolite phase
containing 8-member rings of tetrahedra as the pore opening
controlling hydrocarbon diffusion, the other of said two components
being non-preferentially adsorbed, said process comprising the
steps of: (a) contacting said light hydrocarbon mixture with a
zeolite adsorbent, characterized by having a diffusion rate which
is at least 50 times greater for the preferentially adsorbed
component as compared to said non-preferentially adsorbed
component; and (b) recovering said non-preferentially adsorbed
component.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This Application is a continuation-in-part of my co-pending
application Ser. No. 09/768,943, filed on Jan. 23, 2001, the
disclosure of which is incorporated herein by specific
reference.
FIELD OF THE INVENTION
[0002] The invention relates generally to a process for the
adsorptive separation of low molecular weight hydrocarbons. In a
preferred embodiment, the invention relates to the kinetic
separation of propylene from other hydrocarbons, in particular,
from propane. The process employs at least one substantially
acid-free zeolite having an 8-member ring channel system. Preferred
zeolite structure types are CHA and ITE.
RELATED ART
[0003] It has long been known that certain porous substances such
as zeolites which have certain selective adsorption characteristics
are useful in separating a hydrocarbon mixture into its component
parts. Similarly, it is well known in the separation art that
certain crystalline zeolites can be used to separate certain
hydrocarbons from feed mixtures.
[0004] The separation of propylene-propane mixtures is one of the
most important operations in the petrochemical industry. Such
mixtures usually result from the thermal or catalytic cracking of
hydrocarbons, and the majority of them represent coproducts with
ethylene. Their separation is of great economic consequence, since
the separated propylene has many uses, one of the most important
being as monomer feedstock for polypropylene elastomer production.
For most end uses the propylene must have a high purity. The
propane fraction can be recycled to the cracking step or used
separately, e.g., as liquefied petroleum gas (LPG) for heating.
[0005] The conventional method for separating the propylene-propane
mixture is fractional distillation. The relative volatility for the
mixture is in the range of 1.09-1.15 (Laurance and Swift, 1972),
depending on composition and pressure of operation. A large number
of contacting stages are required (over 100), and the associated
high reflux ratio requires a large input of energy. The U.S.
Department of Energy has reported that the propylene/propane
separation is the most energy-intensive single distillation
practiced commercially (Wiley, 1992).
[0006] A significant amount of the light olefins produced during
the refining of crude oil is used as refinery fuel. Cost-effective
separation technologies will be required that can withstand the
harsh operating environment of a petroleum refinery. In addition to
traditional refinery and olefin plant applications, paraffin
dehydrogenation units for the production of olefins from natural
gas liquids (NGL) have recently been brought on stream. For these
small units, typically less than 5,000 bbl/day of olefin
production, novel separation technologies may be more competitive
with more traditional unit operations.
[0007] Recent federal regulations, such as the Clean Air Act,
mandate that hydrocarbon emissions from refineries and chemical
plants be reduced to low levels. To reduce the economic penalty of
environmental compliance, low-cost hydrocarbon separation
technologies are required. For facilities located in nonattainment
areas, the need to reduce hydrocarbon losses is especially
critical. Facilities can no longer afford to dispose of waste
hydrocarbon streams in their flare systems. Purge streams from
polyolefin reactors and vents from polymer storage facilities,
which were once flared, must be redirected to recovery systems. For
streams containing a mixture of paraffinic and olefinic material,
economic and process considerations may dictate that the olefin be
recovered and recycled.
[0008] Propylene is obtained by recovery from petroleum feed stocks
which include mixtures of olefins and paraffins. Although propylene
can be separated without difficulty from certain olefins and
paraffins by fractional distillation, that separation technique is
less useful for olefins and paraffins having similar boiling points
and similar volatilities, in particular propylene and propane. For
this reason, the separation of propylene in highly purified form
from a mixture containing propylene and propane is difficult and,
as noted above, consumes a large amount of energy.
SUMMARY OF THE INVENTION
[0009] It is an object of this invention to provide a zeolite
adsorbent material that will separate propylene from
propylene/propane and propylene/olefin mixtures in a cost efficient
manner. To that end, the zeolites used in practicing this invention
are relatively inexpensive. The zeolites also have a high
adsorption capacity for propylene and a high degree of selectivity
for propylene over propane and other olefins. These zeolites are,
therefore, suitable for large scale commercial operations.
[0010] It is another object of this invention to provide a more
environmentally acceptable process for obtaining highly purified
propylene from a propylene/propane mixture.
[0011] It is yet another object of the invention to carry out a
separation of other light hydrocarbons using the kinetic based
process of this invention in an energy-efficient manner. It is a
further objective of the invention to provide a separation process
that is highly efficient in having high diffusion rates for
propylene relative to propane.
[0012] The invention can be summarized as encompassing certain
zeolite adsorbents, methods for preparation of such zeolites, and
their use in methods of separating a mixture containing at least
two hydrocarbons.
[0013] The zeolites of the invention comprise or consist of
zeolites having a channel structure that has 8-member rings of
tetrahedra defining diffusion in the channels. The zeolites are
also substantially acid-free. Zeolites containing as constituents
both aluminum and silicon must have an Si:Al molar ratio of at
least about 200. Low silicon zeolites such as aluminophosphate-type
zeolites are also suitable, provided they satisfy the other
requirements stated herein. Examples of suitable
aluminophosphate-type zeolites and zeolites containing as
constituents aluminum and silicon are set forth below.
[0014] The zeolites of the invention are also characterized as
having a propylene/propane diffusion ratio of greater than about 50
(at 80.degree. C. and 600 torr hydrocarbon pressure) and having a
propylene absorption capacity greater than 40 mg/g at 80.degree. C.
and 600 torr propylene pressure. Suitable zeolites are those having
an 8-member ring channel structure and a molar ratio of silica to
aluminum of at least about 200:1 and may have at least one low
activity metal cation to minimize the acid activity of the zeolite
so that it does not result in significant polymerization of the
olefins. Preferred zeolites are those having CHA and ITE-type
structures.
[0015] The invention also provides a process for separating
hydrocarbons from a hydrocarbon mixture and, in particular,
propylene from a propane/propylene mixture using the
above-described zeolite adsorbent. The process comprises the steps
of contacting the hydrocarbon mixture with at least one of the
zeolites to effect separation of propylene. In a preferred
embodiment, the zeolite adsorbent separates propylene from
propylene/propane and propylene/propane/ethylene mixtures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 shows the amounts of propylene adsorbed in milligrams
per gram of zeolite Si-CHA over time and under conditions of 600
torr propylene and 80.degree. C.
[0017] FIG. 2 shows the amounts of propylene adsorbed in milligrams
per gram of zeolite Si-CHA over time and under conditions of 600
torr propylene and 30.degree. C.
[0018] FIG. 3 shows adsorption isotherms for propylene adsorbed, in
milligrams per gram of zeolite, on Si-CHA at 30.degree.,
45.degree., 60.degree., 80.degree. and 100.degree. C.
[0019] FIG. 4 shows the amount of propane adsorbed, in milligrams
per gram of zeolite, on Si-CHA zeolites over time and under
conditions of 80.degree. C. and 600 torr propane.
[0020] FIG. 5 shows the temperature dependence of diffusion rates
for propylene and propane in a zeolite Si-CHA.
[0021] FIG. 6 shows the amount of propylene adsorbed in milligrams
per gram of zeolite ITQ-3 over time and under conditions of 600
torr propylene and 80.degree. C.
[0022] FIG. 7 shows the amount of propylene adsorbed in milligrams
per gram of zeolite ITQ-3 over time and under conditions of 600
torr propylene and 30.degree. C.
[0023] FIG. 8 shows adsorption isotherms for propylene adsorbed, in
milligrams propylene per gram of zeolite, on ITQ-3 zeolite at
pressures of about 0 to 700 torr and temperatures of 40.degree. C.,
60.degree. C., 80.degree. C., 100.degree. C., 120.degree. C. and
140.degree. C.
[0024] FIG. 9 shows the rates of adsorption of propylene and
propane in milligrams propylene/propane per gram of zeolite ITQ-3
structure type over time and under conditions of 80.degree. C. and
650 torr hydrocarbon.
[0025] FIG. 10 shows the amount of propylene adsorbed in milligrams
propylene per gram of zeolite ZSM-58/DD3R over time and under
conditions of 600 torr propylene and 80.degree. C.
[0026] FIG. 11 shows the amount of propylene adsorbed in milligrams
propylene per gram of zeolite ZSM-58/DD3R over time and under
conditions of 600 tort propylene and 30.degree. C.
[0027] FIG. 12 shows the amount of propane adsorbed in milligrams
propane per gram of zeolite ZSM-58/DD3R overtime and under
conditions of 600 torr propane and 80.degree. C.
[0028] FIG. 13 shows the amount of propane adsorbed in milligrams
propane per gram of zeolite ZSM-58/DD3R over time and under
conditions of 600 torr propane and 30.degree. C.
[0029] FIG. 14 shows the amount of propylene adsorbed in milligrams
propylene per gram 10-member ring zeolite HZSM-5 over time and
under conditions of 600 torr propylene and 30.degree. C.
[0030] FIG. 15 shows the amount of propane adsorbed in milligrams
propane per gram 10-member ring zeolite HZSM-5 over time and under
conditions of 600 torr propane and 30.degree. C.
DETAILED DESCRIPTION OF THE INVENTION
[0031] Zeolites are commonly classified as large-pore, medium-pore
and small-pore and in most cases the channel system of such
zeolites arc accessed via 12-member rings (or larger), 10-member
rings, or 8-member rings (or smaller), respectively.
[0032] The zeolites of this invention have 8-member rings of
tetrahedra as the largest ring size, thereby limiting access to the
pore system by larger hydrocarbons. A large number of zeolites have
8-member rings of tetrahedra as the largest ring size. These
zeolites are referred to as 8-member ring or small pore
zeolites.
[0033] The Structure Commission of the International Zeolite
Association has assigned three letter structure type codes to all
zeolites of known structure. Frequently, different names have been
assigned to materials having the same structure type, such
materials being referred to as isotypes. This may occur in cases
where the materials have a composition different from the material
which was the basis for the original assignment of the structure
type code. As an example, the structure type code CHA is derived
from the zeolite mineral chabazite. The CHA structure type has the
following isotypes: AlPO.sub.4-(CHA), CaAPO-44, CaAPO-47,
GaPO.sub.4-34, LZ-218, Linde D, Linde R, MeAPO-47, MeAPSO-47, Phi,
SAPO-34, SAPO-47, Si-CHA, SSZ-13, Wilhendersonite, ZK-14 and ZYT-6,
in addition to the material chabazite. A description of these
structures, their type codes, and a listing of their isotypes are
given in Atlas of Zeolite Structure Types by W. M. Meier, D. H.
Olson and Ch. Baerlocher, Elsevier, 1996, which is incorporated by
reference herein.
[0034] A partial listing of such 8-member ring structures, taken
from Atlas of Zeolite Structure Types, is given below:
1 8-Member Ring Structures ABW Li-A (Barrer and White) AEI
AIPO.sub.4-18 AFX SAPO-56 APC AIPO.sub.4-C ATN MAPO-39 ATT
AIPO.sub.4-12-TAMU ATV AIPO.sub.4-25 AWW AIPO.sub.4-22 CHA
Chabazite, SSZ-13 DDR Deca-dodecasil 3R EAB TMA-E ERI Erionite JBW
NaJ (Barrer and White) KFI ZK-5 LEV Levyne LTA Linde Type A MER
Merlinoite PAU Paulingite PHI Phillilpsite RHO Rho RTE RUB-3 RTH
RUB-13 ZON ZAPO-M1 ITE ITE-3
[0035] Zeolites of the ITE structure type include ITQ-3. Zeolites
of the DDR structure type include DD3R, ZSM-58 and Sigma 1. The
zeolite structure types that are the most preferred adsorbents are
zeolites of the CHA (Chabazite), ITE and DDR (Deca-Dodecasil-R)
structure types and preferably CHA and ITE types. Descriptions of
zeolites of CHA and DDR structure types including identifications
of structure and chemical compositions are set forth in W. M.
Meier, et al., referenced hereabove. A description of ITQ-3
structure type zeolite is set forth in Published International
Application WO/98/54091 to M. Camblor et al., which is incorporated
herein by reference. A description of the CHA type zeolite, SSZ-13,
is set forth in U.S. Pat. No. 4,544,538 to Zones, which is
incorporated herein by reference.
[0036] Other preferred adsorbents are zeolites having 8-member
rings that have a high Si:Al mole ratio and are substantially acid
free. High Si:Al ratio is defined as a ratio of at least about 200,
more preferably at least 300 and most preferably at least 500. For
efficient, long-term operation of the adsorption process, it is
critical that the adsorbent have either essentially no acidity or
acid activity that does not polymerize olefins. Acidic sites in the
adsorbent will result in polymerization of the desired olefins that
would require thermal activation to remove. Accordingly, the term
"substantially acid-free" is used herein to encompass zeolites that
do not polymerize olefins in a manner that would prevent long-term
operation of the adsorption process. In addition to the cost of the
thermal treatment there is a substantial time requirement, as well,
both of which would significantly reduce operating efficiency.
Equilibrium should be reached in 60 minutes, more preferably in 30
minutes, and even more preferably in less than 15 minutes.
[0037] Control of acidity can be done in several ways. One method
of reducing acidity is to subject the zeolite to ion exchange with
low activity metal cations. Preferred cations for this purpose are
alkali metal cations, such as sodium, potassium or cesium. The
larger cations have the advantage of weaker interactions with
olefins and hence lower temperature and/or faster desorption of the
desired olefin. The preferred method of avoiding acidity and
ensuring the desired Si:Al ratio is to synthesize the zeolite
adsorbent with the lowest possible aluminum content. Practical
considerations may preclude reaching the low levels achievable at
the laboratory scale. Tetramethyl- and tetraethyl-orthosilicate
will produce the lowest alumina content zeolites; materials having
SiO.sub.2/Al.sub.2O.sub.3 ratios above about 50,000 should be
achievable. Using fumed silica as the silica source should result
in zeolite products having SiO.sub.2/Al.sub.2O.sub.3 ratios above
about 20,000. Colloidal silica sol, available in bulk commercial
scale quantities, will yield zeolites having
SiO.sub.2/Al.sub.2O.sub.3 ratios of about 3,000. Sodium waterglass,
also available in bulk quantities, is a widely used silica source
and can produce zeolites having SiO.sub.2/Al.sub.2O.sub.3 ratios of
about 600 (see G. H. Kuehl, in Verified Synthesis of Zeolitic
Materials, H. Robson, editor, Microporous and Mesoporous Materials,
Vol. 22 (1998)).
[0038] As noted above, the zeolites or molecular sieves can also be
of the aluminophosphate-type. Aluminophosphate-type zeolites having
a channel structure that has 8-member rings of tetrahedra defining
diffusion in the channels includes those having AEI, AET, AFT, AFX,
ACL, APD, ATN, ATT, ATV, AWW, and ZON structure types. These
zeolites must also be substantially acid free, have a
propylene/propane diffusion ratio of greater than about 50 (at
80.degree. C. and 600 torr hydrocarbon pressure) and a propylene
adsorption capacity greater than 40 mg/g at 80.degree. C. and 600
torr pressure propylene.
[0039] The discovery that substantially acid-free zeolites having
8-member rings and Si:Al mole ratios of at least about 200 or
8-member ring aluminophosphate-based zeolites make good adsorbents
for separation of propylene from hydrocarbon mixture is of
considerable practical importance. While pure zeolites of the CHA
and ITE structure types are operable, specific zeolites of this
type may currently be very expensive to prepare in small
quantities, let alone the large amounts of zeolite that would be
required for industrial application. The ability to select
well-known and relatively inexpensive 8-member zeolites which can
then be made suitable for use by routine treatment steps is an
essential prerequisite for the commercial application of an
industrial process for separation of propylene by zeolite
adsorbents.
[0040] The zeolites of the present invention are characterized by
their unexpectedly high diffusional distinction between propylene
and propane. This distinction can be quantified in terms of the
ratio of the diffusion coefficients for these two hydrocarbons,
e.g., the ratio of D.sub.propylene/D.sub.propane. The effectiveness
of an adsorbent for separation increases with the magnitude of this
ratio, which is referred to herein as R.sub.D. It is well accepted
in the art that an adsorbent having an R.sub.D greater than about
50 would be an effective separation adsorbent for a two-component
system.
[0041] Where the R.sub.D value for propane/propylene approaches
unity, there is no kinetic based preferential adsorption of one
component over the other. As the value of R.sub.D becomes less or
greater than unity, there is a preferential selectivity by the
adsorbent for one of the two components. When comparing a R.sub.D
value of propylene relative to propane, a selectivity larger than
unity indicates preferential adsorption of propylene within the
adsorbent, while a R.sub.D value less than unity would indicate
that propane is preferentially adsorbed by the adsorbent.
[0042] The zeolites of the present invention have a R.sub.D ratio
(propylene/propane diffusion ratio) of greater than fifty (50),
preferably greater than one hundred (100), more preferably greater
than two hundred (200) and most preferably greater than five
hundred (500).
[0043] Another important adsorbent property of a zeolite is its
adsorption capacity, for example as measured by the weight of
hydrocarbon adsorbed per unit weight of adsorbent. A zeolite's
capacity to adsorb propylene is defined as the number of milligrams
of propylene adsorbed per gram of zeolite at equilibrium.
Equilibrium condition is defined as no change in amount of
propylene adsorbed by the zeolite. In principle, the higher the
adsorption capacity the less the adsorbent that will be required to
separate a given quantity of hydrocarbon, e.g., propylene, for an
adsorbent having a given R.sub.D. The zeolites of this invention
are also characterized by their high adsorption capacity.
Specifically, the zeolite of the present invention has a propylene
adsorption capacity of greater than 40 milligrams (mg) propylene
per gram (gm) zeolite measured at 80.degree. and a propylene
pressure of 600 torr. The zeolite's propylene adsorption capacity
at that temperature and pressure is preferably at least 60 mg/g and
more preferably 100 mg/g.
[0044] A high R.sub.D value is required to achieve effective
kinetic separation, and a moderate to high adsorption capacity is
required to make the separation process practical.
[0045] In the description that follows, unless otherwise stated,
adsorption capacities will be the values determined when the feed
is at 600 torr pressure and the adsorption chamber is at 80.degree.
C. Several different modes of making these measurements are
feasible.
[0046] Another method of controlling a zeolite's behavior as an
adsorbent for use in practicing this invention is by controlling
the size of the zeolite crystal. Adjusting this size, typically
achieved during the zeolite synthesis step, will allow the
attainment of practical cycle times, as in a pressure swing
adsorption process (PSA process). The adsorption rate vanes as the
inverse square of the radius of the crystal. Thus a change in the
average crystal size by a factor of 10 will produce a factor of 100
difference in the adsorption and desorption step times.
Accordingly, careful attention must be given to the control of the
size of the zeolite crystals employed. It is preferable that the
size of the zeolite crystal is no larger than 1.0 microns and more
preferably no larger than 0.1. microns.
[0047] The novel separation process of the instant invention
involves contacting a mixture containing propylene and one or more
hydrocarbons, in particular, propane, that exists either as a gas,
liquid or mixed phase with at least one of the class of zeolites
described above for a period of time to selectively adsorb
propylene within the internal pore structure of the zeolite. The
components of the hydrocarbon mixtures that are not adsorbed are
thus carried off. The propylene is thereafter recovered from the
internal pore structure of the zeolite by conventional desorbing
techniques such as stripping with another gas, pressure change,
temperature change or a combination of these methods. A single or
multistage pressure swing adsorption process (PSA process) is
typical of the type of configuration in which this invention may be
practiced. Similarly, a single or multistage membrane-based process
may be employed. The processes of this invention can also be
conducted in flow type (continuous) systems, e.g., a continuous
chromatographic type operation. In such a flow type system, a
hydrocarbon mixture is passed through a bed containing at least one
of the class of zeolites described above. Because of the high
selectivity of the zeolites used in practicing this invention, the
desired propylene is adsorbed or retained in the bed, while the
remaining hydrocarbons are removed. By this process, a highly
purified propylene product can be obtained.
[0048] The exact temperature at which the novel processes of this
invention are conducted will depend upon the type of separation
method employed. The temperature, however, must be maintained below
that required for a chemical reaction to occur, e.g. below the
cracking temperature. The temperature should thus be maintained
below about 250.degree. C. Preferably, the processes of this
invention can be conducted in the temperature range between
-100.degree. C. and about 250.degree. C. and preferably between
-20.degree. C. and +100.degree. C.
[0049] In the above, materials are described that are useful for
the separation of propylene from propane. In this embodiment,
propylene is the gas phase component preferentially adsorbed. In
the subsequent desorption step, separation and recovery of high
purity propylene is achieved. It should be understood that for a
different pair of light hydrocarbons, the gas phase component not
preferentially adsorbed may be the desired hydrocarbon. In this
instance, the non-adsorbed or slowly adsorbed hydrocarbon is
separated in high purity and subsequently the adsorbed component is
desorbed before this process is repeated.
[0050] Thus, the detailed discussion concerning propylene/propane
separation illustrates the potential of 8-member ring zeolites for
kinetic based separation. These teachings may in turn be applied to
other light hydrocarbon pairs, or mixtures, and cases where either
the fast adsorbing or the slow adsorbing component is the desired
higher value product.
[0051] The following examples are presented for illustration
purposes only and are not to be taken as limiting the present
invention in any way.
EXAMPLES
Preparation of Zeolite Adsorbents Tested
[0052] The Si-CHA (a pure silica version of the zeolite chabazite)
and ITQ-3 structure type zeolite used in the following examples
were obtained from M. Chamblor, who is identified as a joint
inventor in Published International Application No. WO 98/54091. A
synthesis of Si-CHA is reported in M. J. Diaz-Cabras, P. A. Barrett
and M. A. Camblor, Chem. Commun., 1998, 1881. A process for
preparing ITQ-3 is disclosed in Published International Application
No. WO 98/54091. A synthesis of DD3-R is reported in U.S. Pat. No.
4,698,217. A synthesis of SSZ-13 is reported in U.S. Pat. No.
4,544,538.
Measurement of Adsorption Capacities and Rates
[0053] Adsorption measurements were made using a computer
controlled thermogravimetric balance consisting of a TA51
thermobalance and associated TA-2000/PC control system. This one
atmosphere gas flow through electrobalance system was controlled
via Macintosh based LabView control software, Kinetic Systems
interface, mass flow controllers and Eurotherm temperature
controller. Typically, adsorption capacities and rates were
measured at 600 torr hydrocarbon pressure and 30.degree. and/or
80.degree. C.
[0054] The relative diffusion parameters were obtained as follows.
The diffusion parameter D/r.sup.2, where D is the diffusion
coefficient and r is the crystal radius, were derived from the
sorption measurements with the assumption that the plane sheet
model describes the diffusion process. Thus, for a given sorbate
loading, Q, the value Q/Q.sub..infin., where Q.sub..infin. the
equilibrium sorbate loading, is mathematically related to
(Dt/r.sup.2).sup.1/2 where t is the time (sec.) required to reach
the sorbate loading Q. Graphical solutions for the plane sheet
model are given by J. Crank in The Mathematics of Diffusion, Oxford
University Press, Ely House, London, 1967. The ratio of the
diffusions coefficients, D, for propylene and propane is taken as
the ratio of the respective D/r.sup.2 values and given the name
R.sub.D.
Example 1
[0055] Si-CHA zeolite having an 8-member ring CHA type structure,
having substantially equidimensional crystals ranging in size from
1 to 12 microns and having a dry composition of 100% SiO.sub.2 was
employed. The Si-CHA zeolite was contacted with propylene at a
pressure of 600 torr at 80.degree. C. until equilibrium was
obtained. At equilibrium, this zeolite adsorbed 90 milligrams
propylene per gram of zeolite. The rate of adsorption, defined as
D/r.sup.2 (wherein D is diffusion and r is radius of zeolite
crystal), was measured over time and calculated as
1.5.times.10.sup.-3 sec.sup.-1. The amount of propylene adsorbed in
milligrams per gram of zeolite over time is shown in FIG. 1.
Example 2
[0056] The Si-CHA zeolite employed in EXAMPLE 1 was contacted with
propylene at 600 torr pressure at 30.degree. C. until an
equilibrium was obtained. At equilibrium, the zeolite adsorbed 120
milligrams propylene per gram of zeolite. The rate of adsorption,
D/r.sup.2, was 4.6.times.10.sup.-4 sec.sup.-1. The adsorption rate
for propylene in milligrams per gram of zeolite, is shown in FIG.
2.
Example 2A
[0057] Isotherms for the adsorption of propylene by the Si-CHA
zeolite at 30.degree. C., 45.degree. C., 60.degree. C., 80.degree.
C. and 100.degree. C., were measured. The results are shown in FIG.
3.
Example 3
[0058] The Si-CHA zeolite employed in Example 1 was contacted with
propane at 600 torr pressure and at 80.degree. C. for about 600
minutes as shown in FIG. 4. The rate of adsorption, D/r.sup.2 was
9.4.times.10.sup.-7 sec. Accordingly, the relative adsorption rates
of propylene to propane for this zeolite at 80.degree. C., also
referred to as R.sub.D is 1,550.
Example 4
[0059] The Si-CHA zeolite employed in EXAMPLE 1 was contacted with
propane at 600 torr pressure at 30.degree. C. The rate of
adsorption was 1.0.times.10.sup.-8 sec.sup.-. The relative
propylene/propane diffusion rates for this zeolite at 30.degree. C.
is 43,600.
[0060] The results set forth in the previous examples were analyzed
to determine the temperature dependence of diffusion rates for
propane and propylene with zeolite Si-CHA. That result is set forth
in FIG. 5. The relative diffusion rates of propylene to propane in
Si-CHA structure type zeolite range from 1,550 to 43,600 over the
temperature range of 80.degree. to 30.degree. C. These diffusion
rates for propylene and propane in Si-CHA structure type zeolite at
the temperatures tested are set forth below in TABLE 1.
2TABLE 1 Diffusion Rates of C.sub.3 Hydrocarbons in Si-CHA
Hydrocarbon Temperature (.degree. C.) Diffusion Parameter D/r.sup.2
R.sub.D Propylene 30 4.6 .times. 10.sup.-4 Propane 30 1.0 .times.
10.sup.-8 43,600 Propylene 45 8.4 .times. 10.sup.-4 Propane 45 8.9
.times. 10.sup.-8 9,500 Propylene 60 1.1 .times. 10.sup.-3 Propane
60 2.7 .times. 10.sup.-7 3,900 Propylene 80 1.5 .times. 10.sup.-3
Propane 80 9.4 .times. 10.sup.-7 1,500
Example 5
[0061] An ITQ-3 zeolite material having an 8-member ring channel
system of the ITE structure type with elongated crystals of about
0.2 to 15 microns in length and the shortest dimension for
diffusion from about 0.1 to about 1.0 micron and having a dry
composition of 100% SiO.sub.2 was employed. This ITQ-3 zeolite was
contacted with propylene at a pressure of 600 torr at 80.degree. C.
until equilibrium was obtained. At equilibrium, this zeolite
adsorbed 46 milligrams propylene per gram zeolite. The rate of
adsorption, D/r.sup.2, was 3.8.times.10.sup.-3 sec..sup.-1. The
amount of propylene adsorbed in milligrams per gram zeolite over
time is shown in FIG. 6.
Example 6
[0062] The ITQ-3 zeolite employed in Example 5 was contacted with
propylene at 600 torr pressure at 30.degree. C. until equilibrium
was reached. At equilibrium, this zeolite adsorbed 63 milligrams
propylene per gram of zeolite. The rate of adsorption, D/r.sup.2
was 1.5.times.10.sup.-3 sec.sup.-1. The amount of propylene
adsorbed in milligrams propylene per gram of zeolite over time is
shown in FIG. 7.
Example 6A
[0063] Adsorption isotherms for the adsorption of propylene by the
ITQ-3 zeolite were carried out. Isotherms for 40.degree. C.,
60.degree. C., 80.degree. C., 100.degree. C., 120.degree. C. and
140.degree. C. are set forth in FIG. 8.
Example 7
[0064] The ITQ-3 zeolite employed in EXAMPLE 5 was contacted with
propane at 650 torr pressure at 80.degree. C. The rate of
adsorption, D/r.sup.2, was 7.4.times.10.sup.-6 sec.sup.-1. The
relative propylene/propane diffusion rates, RD, at 80.degree. C.
for this ITQ-3 zeolite was 510. These data are set forth in FIG.
9.
Example 8
[0065] The ITQ-3 structure employed in EXAMPLE 5 was contacted with
propane at 600 torr pressure at 30.degree. C. The rate of
adsorption, D/r.sup.2, was 2.2.times.10.sup.-6 sec.sup.-1. The
relative propylene/propane diffusion rates, RD, at 3.sup.0.degree.
C. for this ITQ-3 zeolite was 690.
[0066] The results from EXAMPLES 5, 6, 7 and 8 for ITQ-3 zeolite in
terms of propylene diffusion parameter, D/r.sup.2, and the ratio of
adsorption of propylene to propane adsorption rates, R.sub.D, are
set forth in TABLE 2. A comparison of the relative diffusion
parameters of ITQ-3 for light hydrocarbons is set forth in TABLE 3.
The RD results come within the limitations for the invention.
3TABLE 2 Diffusion Rates of C.sub.3 Hydrocarbons in ITQ-3
Hydrocarbon Temperature (.degree. C.) Diffusion Parameter D/R.sup.2
R.sub.D Propylene 30 1.5 .times. 10.sup.-3 Propane 30 2.2 .times.
10.sup.-6 690 Propylene 80 3.8 .times. 10.sup.-3 Propane 80 7.4
.times. 10.sup.-6 510
[0067]
4 TABLE 3 Hydrocarbon Diffusion Parameter D/r.sup.2 (sec-1,
80.degree. C.) Ethane 1.4 .times. 10.sup.-2 Ethylene 1.1 .times.
10.sup.-2 Propane 7.4 .times. 10.sup.-6 Propylene 3.8 .times.
10.sup.-3 Hexane 5.0 .times. 10.sup.-7
Example 9
[0068] A ZSM-58 zeolite material having an 8-member ring channel
system and of the DDR type structure was contacted with propylene
at 600 torr pressure at 80.degree. C. until equilibrium was
obtained. At equilibrium, this zeolite adsorbed 34 milligrams
propylene per gram of zeolite. The rate of adsorption, D/r.sup.2,
was 2.6.times.10.sup.-4 sec.sup.-1. The amount of propylene
adsorbed in milligrams per gram of this zeolite over time is shown
in FIG. 10.
Example 9A
[0069] The ZSM-58 zeolite material of EXAMPLE 9 was contacted with
propylene at 30.degree. C. and 600 torr propylene pressure. The
rate of adsorption curve is set forth in FIG. 11 At equilibrium, 44
mg of propylene are adsorbed per gram of zeolite. The adsorption
rate, D/r.sup.2, was 1.2.times.10.sup.-4 sec..sup.-1.
Example 10
[0070] The ZSM-58/DD3R zeolite employed in EXAMPLE 9 was contacted
with propane at 600 torr pressure at 30.degree. C. and 80.degree.
C. For each experiment, two adsorption steps are observed, the fast
step being completed at about 15 min.sup.1/2 (for 30.degree. C.)
and -7 min 1 (for 80.degree. C.). The rates of adsorption for the
fast adsorption process are respectively 2.8.times.10.sup.-4
sec.sup.-1 at 80.degree. C. and 1.1.times.10.sup.-4 sec.sup.-1 at
30.degree. C. The rates of adsorption for the slow adsorption step
or process are respectively 1.8.times.10.sup.-7 sec.sup.-1 at
80.degree. C. and 9.6.times.10.sup.-9 sec.sup.-1 at 30.degree. C.
The amount of propane adsorbed in milligrams per gram of this
zeolite over time, and at 600 torr pressure as shown in FIG. 12 for
80.degree. C. and in FIG. 13 for 30 C. The results for the fast and
slow adsorption steps or processes at 30.degree. C. and 80.degree.
C. are also set forth in TABLE 4 herebelow.
[0071] A summary of results obtained in the above-referenced
examples for Si-CHA, ITQ-3 and ZSM-58/DD3R zeolites is set forth in
TABLE 4.
5TABLE 4 Propylene Adsorption Capacities and Relative Diffusion
Rates Adsorption Pore Capacity (mg/g).sup.a Relative D's R.sub.D
Zeolite System 80.degree. C. 30.degree. C. (C3 = /C3.degree.,
80.degree. C. Si-CHA 3-d 90 120.sup.b 1,500(43,600 @ 30.degree. C.)
ITQ-3 1-d.sup.c 46 63 510 ZSM-58/DD3R 2-d 30 42 1.0 @ 30.degree. C.
and 0.9 @ 80.degree. C. (fast step) 12,400 @ 30.degree. C. (slow
step) 1,410 @ 80.degree. C. (slow step) .sup.aHydrocarbon pressure
= 600 torr. P/P.degree. = 0.023 at 80.degree. C. and 0.063 at
30.degree. C. .sup.bSimulated propylene pore capacity is 117 mg/g.
.sup.cMultifile diffusion.
[0072] While the results for Si-CHA and ITQ-3 zeolites show
excellent separation parameters, the results for ZSM-58/DD3R
zeolites indicate inadequate separation parameters. In TABLE 4, for
zeolite ZSM-58/DD3R, the term "fast step" is used to describe the
result obtained for the fast adsorption process which dominates the
adsorption during the early times on stream, e.g. 7-15 min.sup.1/2.
For the fast step the R.sub.D value is 1.0, meaning that the
propylene and propane have essentially equal adsorption rates. The
slow adsorption step, clearly observable at longer adsorption
times, gives R.sub.D values of 12,400 and 1,410, at 30.degree. and
80.degree. C., respectively. In a separation process using this
adsorbent, the adsorption associated with the "fast step" would
dominate and this adsorbent would be ineffective in bringing about
propylene/propane separation. This ZSM-58/DD3R is thus an
unacceptable adsorbent for this process.
Example 11
[0073] HZSM-5 zeolite having a silica to alumina ratio of 3,300 was
contacted with propylene at 30.degree. C. and 600 torr propylene
pressure. At equilibrium 72 mg/g of propylene was adsorbed. The
D/r.sup.2 was 0.15 sec.sup.-1. These data are shown in FIG. 14.
Example 12
[0074] HZSM-5 zeolite having a silica to alumina ratio of 3,300 was
contacted with propane at 30.degree. C. and 600 torr propane
pressure. At equilibrium, 74 mg/g of propane was adsorbed. The
D/r.sup.2 was 0.10 sec.sup.-1. These data are shown in FIG. 15. The
R.sub.D ratio is 1.4, demonstrating that 10-ring zeolites are not
effective for the kinetic separation of propylene from propane.
[0075] While certain embodiments of the present invention have been
described and/or exemplified above, various other embodiments will
be apparent to those skilled in the art from the foregoing
disclosure. The present invention is, therefore, not limited to the
particular embodiments described and/or exemplified, but is capable
of considerable variation and modification without departure from
the scope of the appended claims.
* * * * *