U.S. patent application number 13/063254 was filed with the patent office on 2011-07-07 for process for enhanced propylene yield from cracked hydrocarbon feedstocks and reduced benzene in resulting naphtha fractions.
This patent application is currently assigned to Patent Department. Invention is credited to Wu-Cheng Cheng, Lloyd S. White, Richard F. Wormsbecher.
Application Number | 20110163002 13/063254 |
Document ID | / |
Family ID | 42005398 |
Filed Date | 2011-07-07 |
United States Patent
Application |
20110163002 |
Kind Code |
A1 |
White; Lloyd S. ; et
al. |
July 7, 2011 |
PROCESS FOR ENHANCED PROPYLENE YIELD FROM CRACKED HYDROCARBON
FEEDSTOCKS AND REDUCED BENZENE IN RESULTING NAPHTHA FRACTIONS
Abstract
A process in which a catalytic cracking unit is operated to
crack a hydrocarbon feedstock in a manner to enhance light olefin
yields. The accompanying benzene-containing naphtha product stream
is further processed through a benzene selective membrane to
provide a low content benzene stream. Refiners frequently operate
their cracking units to optimize light olefin yields, e.g.
propylene, in response to needs in the petrochemical industry, and
it has been discovered that units operated in this manner
frequently produce naphtha containing increased amounts of benzene.
The method of this invention therefore allows one to operate the
unit when it is desired to optimize light olefin yields, yet at the
same time produce a naphtha yield having a low benzene content. The
invention is particularly useful when the cracking unit utilizes
pentasil zeolites at increased concentrations to enhance light
olefins yield.
Inventors: |
White; Lloyd S.; (Mountain
View, CA) ; Wormsbecher; Richard F.; (Dayton, MD)
; Cheng; Wu-Cheng; (Ellicott City, MD) |
Assignee: |
Patent Department
DES PLAINES
IL
|
Family ID: |
42005398 |
Appl. No.: |
13/063254 |
Filed: |
September 11, 2009 |
PCT Filed: |
September 11, 2009 |
PCT NO: |
PCT/US09/05095 |
371 Date: |
March 10, 2011 |
Current U.S.
Class: |
208/95 |
Current CPC
Class: |
B01D 71/54 20130101;
B01D 71/64 20130101; B01D 71/70 20130101; B01J 29/40 20130101; B01D
61/362 20130101; C10G 2400/02 20130101; C10G 2300/301 20130101;
C10G 2300/807 20130101; C10G 2400/20 20130101 |
Class at
Publication: |
208/95 |
International
Class: |
C10G 61/04 20060101
C10G061/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 15, 2008 |
US |
61192074 |
Claims
1. A catalytic cracking process comprising: (a) introducing a
hydrocarbon feedstock into a reaction zone of a catalytic cracking
unit which feedstock is characterized as having: an initial boiling
point from about 120.degree. C. with end points up to about
850.degree. C.; (b) catalytically cracking said feedstock in said
reaction zone employing a cracking catalyst, temperature,
catalyst-to-oil ratio, pressure, steam dilution and space velocity
such that an olefin yield from the unit is enhanced to produce a
product comprising olefin, naphtha and benzene, wherein the product
comprises about 6 to about 20% propylene based on the weight of the
hydrocarbon feedstock introduced in step (a) above; (c)
fractionating the product into at least a light olefin-containing
fraction, and a benzene-containing naphtha fraction; (d) recovering
the light olefin-containing fraction; (e) contacting the
benzene-containing naphtha fraction with a membrane having a
sufficient flux and selectivity to separate a benzene-enriched
permeate fraction and a benzene-deficient naphtha retentate
fraction, said benzene-enriched permeate fraction being enriched in
benzene compared to the retentate; (f) recovering the
benzene-deficient naphtha retentate fraction; and (g) routing the
benzene-enriched permeate fraction for further processing.
2. A catalytic cracking process according to claim 1, wherein the
cracking catalyst in step (b) comprises a pentasil zeolite crystal
content in the range of 2 to about 20% by weight of the
catalyst.
3. A catalytic cracking process according to claim 1, wherein the
cracking catalyst in step (b) comprises a catalyst additive
comprising ZSM5 in the range of 10 to about 80% by weight of the
catalyst additive.
4. A catalytic cracking process according to claim 1, wherein the
benzene-containing naphtha fraction in step (c) comprises 0.6 to
about 3% by weight benzene.
5. A catalytic cracking process according to claim 1, wherein the
benzene-containing naphtha fraction in step (c) is light cat
naphtha having a boiling point in the range of 50.degree. to
105.degree. C., and comprises 1.2 to about 6% by weight
benzene.
6. A catalytic cracking process according to claim 2, wherein the
benzene-containing naphtha fraction in step (c) is light cat
naphtha having a boiling point in the range of 50.degree. to
105.degree. C., and comprises 1.2 to about 6% by weight
benzene.
7. A catalytic cracking process according to claim 1, wherein the
process is a deep catalytic cracking process, and the conditions in
the reaction zone of step (a) are as follows: TABLE-US-00004
Temperature, .degree. C. 505-575 Cat./Oil 9 to 15 Pressure,
atmospheres 0.7 to 1.5 Steam Dilution, wt % of feed 10 to 30 WHSV,
hr.sup.-1 0.2-20 Pentasil crystal content, 10-75 % by weight
catalyst
8. A catalytic cracking process according to claim 1, wherein the
membrane in step (e) comprises a member selected from the group
consisting of polyimide, polyurea-urethane, polysiloxane and
combinations thereof.
9. A catalytic cracking process according to claim 2, wherein the
membrane in step (e) comprises a member selected from the group
consisting of polyimide, polyurea-urethane, polysiloxane and
combinations thereof.
10. A catalytic cracking process according to claim 5, wherein the
membrane in step (e) comprises a member selected from the group
consisting of polyimide, polyurea-urethane, polysiloxane and
combinations thereof.
11. A catalytic cracking process according to claim 1, where in the
benzene-containing naphtha fraction is contacted with the membrane
under pervaporation conditions.
12. A catalytic cracking process according to claim 1, wherein the
benzene-deficient naphtha retentate comprises less than 1% by
weight to about 100 ppm benzene.
13. A catalytic cracking process according to claim 1, wherein the
benzene-deficient naphtha retentate comprises less than 0.6% by
weight to about 100 ppm benzene.
14. A catalytic cracking process according to claim 1, wherein the
benzene-enriched permeate comprises 1 to about 10% by weight
benzene.
15. A catalytic cracking process according to claim 1 further
comprising fractionating the benzene-containing naphtha fraction
from step (c) to remove C.sub.5 fractions prior to contacting the
benzene-containing naphtha fraction with a membrane in accordance
with step (e).
16. A method of producing light olefins and low benzene-containing
naphtha in a catalytic cracking unit from a hydrocarbon feedstock,
the method comprising: (a) selecting catalyst, temperature,
catalyst-to-oil ratio, pressure, steam dilution and/or space
velocity to enhance olefin yield in the catalytic cracking unit;
(b) contacting the feedstock with cracking catalyst in the unit,
thereby producing a product comprising olefin, naphtha and benzene;
(c) fractionating the product into at least a light
olefin-containing fraction, and a benzene-containing naphtha
fraction; (d) recovering the light olefin-containing fraction; (e)
contacting the benzene-containing naphtha fraction with a membrane
having a sufficient flux and selectivity to separate a
benzene-enriched permeate fraction and a benzene deficient naphtha
retentate fraction, said benzene-enriched permeate fraction being
enriched in benzene compared to the retentate; (f) recovering the
benzene deficient naphtha retentate fraction; and (g) routing the
benzene-enriched permeate fraction for further processing.
17. A method according to claim 16, wherein the benzene-containing
naphtha fraction in step (c) has a boiling point in the range of
50.degree. C. to about 220.degree. C., and the fraction comprises
0.6 to about 3% by weight benzene.
18. A method according to claim 16, wherein the benzene-containing
naphtha fraction in step (c) has a boiling point in the range of
50.degree. C. to about 105.degree. C., and the fraction comprises
1.2 to about 6% by weight benzene.
19. A method according to claim 16, wherein the product from step
(b) comprises 6 to about 20% by weight propylene based on the
weight of the feedstock in step (a).
20. A method according to claim 16, wherein the membrane in step
(e) comprises a member selected from the group consisting of
polyimide, polyurea-urethane, polysiloxane and combinations
thereof.
21. A method according to claim 16 wherein the benzene-containing
naphtha fraction is contacted with the membrane under pervaporation
conditions.
22. A method according to claim 16, wherein the benzene-deficient
naphtha retentate comprises less than 1% by weight to about 100 ppm
benzene.
23. A method according to claim 16, wherein the benzene-deficient
naphtha retentate comprises less than 0.6% by weight to about 100
ppm benzene.
24. A method according to claim 18, wherein the benzene-deficient
naphtha retentate comprises less than 1% by weight to about 100 ppm
benzene.
25. A method according to claim 19, wherein the benzene-deficient
naphtha retentate comprises less than 1% by weight to about 100 ppm
benzene.
26. A method according to claim 16, wherein the feedstock is
contacted with a catalyst comprising ZSM5.
27. A method according to claim 16, wherein the feedstock in step
(b) is contacted with a catalyst comprising 2 to about 20% by
weight pentasil crystal.
28. A method according to claim 19, wherein the feedstock in step
(a) is contacted with a catalyst comprising 2 to about 20% by
weight pentasil crystal.
29. A method according to claim 27, wherein the pentasil is
ZSM-5.
30. A method according to claim 28, wherein the pentasil is
ZSM-5.
31. A method according to claim 16, wherein the cracking catalyst
in step (b) comprises a catalyst additive comprising ZSM5 in the
range of 10 to about 80% by weight of the catalyst additive.
32. A method according to claim 19, wherein the cracking catalyst
in step (b) comprises a catalyst additive comprising ZSM5 in the
range of 10 to about 80% by weight of the catalyst additive.
Description
PRIORITY CLAIM OF EARLIER NATIONAL APPLICATIONS
[0001] This application is a national stage entry under 35 U.S.C.
.sctn.371 of International Application No. PCT/US2009/005095 filed
Sep. 11, 2009 which claims benefit of U.S. Provisional Application
No. 61/192,074 filed Sep. 15, 2008.
BACKGROUND OF THE INVENTION
[0002] The invention relates to catalytic cracking processes
conducted to produce light olefins. The invention further relates
to methods for reducing benzene in naphtha fractions produced in
such processes.
[0003] Benzene is a known carcinogen that arises in the production
of gasoline. Regulations in the European Union, US, and other
locations require less than 1% benzene in gasoline. Gasoline is
produced at a refinery by blending component streams, including
butane, isopentane, alkylate, isomerate, straight run naphtha,
hydrocrackate, catalytic naphtha, steam cracked naphtha, coker
naphtha, pyrolysis gasoline, catalytic reformate, vacuum gas oil,
and oxygenates. The naphthas formed from catalytic cracking, e.g.,
fluidized catalytic cracking (FCC), can be further fractionated
into light cat naphtha, intermediate cat naphtha, and heavy cat
naphtha.
[0004] The product distribution from current FCC processes
comprises a number of constituents in addition to gasoline naphtha.
While gasoline is of primary interest to most refiners, light
olefins and LPG are also found in the FCC product, and are
increasingly becoming of interest to refiners as those products
become more valuable. The light olefins produced can be used for a
number of purposes, e.g., they are upgraded via sulfuric or HF
alkylation to high quality alkylate. LPG is used for cooking and/or
heating purposes. Accordingly, operators of FCC units can vary the
content of their products depending upon the markets they are
serving and the value associated with each of the components found
in an FCC product.
[0005] Propylene is a particular light olefin in high demand. It is
used in many of the world's largest and fastest growing synthetic
materials and thermoplastics. Refiners are relying more and more on
their FCC units to meet the increased demand for propylene, thus
shifting the focus of the traditional FCC unit away from
transportation fuels and more toward petrochemical feedstock
production as operators seek opportunities to maximize margins.
Indeed, the FCC unit provides one third of the world's propylene,
and being able to increase propylene output from the unit is of
value when propylene prices are high.
[0006] If a refinery cannot expand its existing FCC unit, the
unit's operators have rather limited options for increasing light
olefin production. Reported options include: (a) using additive
ZSM-5 catalyst and/or additive in the FCC unit; and (b) increasing
the severity of the conditions, e.g., temperature, in the unit,
e.g., production of cracked gas from gas oil over pentasil
zeolites, e.g., ZSM-5.
[0007] It has been noted, however, that processes such as the above
typically produce a product that, when fractionated to the gasoline
naphtha streams, have higher concentrations of benzenes compared to
units run at conditions to maximize gasoline yields. Naphtha
fraction from a FCC unit operated to enhance light olefin yields
can contain more than 2% benzene. The source of increased benzene
is not readily recognized by refiners.
[0008] Accordingly, there can be reluctance to rely on FCC units
for substantially meeting olefin needs, or reluctance to maximize
the use of the pentasil additive catalysts. The reluctance is
further reinforced given that when refiners use pentasil additives
to enhance olefin yields, gasoline yields are often sacrificed. In
other words, the refiner is facing the additional issue that the
yield of a valuable product is being reduced in addition to the
fact the process will require processing the product to remove the
increased amount of benzene.
[0009] Polymeric membranes have been reported to separate
aromatics.
[0010] U.S. Pat. No. 2,930,754 (Stuckey), U.S. Pat. No. 2,958,656
(Stuckey), U.S. Pat. No. 3,370,102 (Carpenter et al.), U.S. Pat.
No. 4,115,465 (Elfert et al.), U.S. Pat. No. 4,944,880 (Ho et al.),
U.S. Pat. No. 5,028,685 (Ho et al.), U.S. Pat. No. 5,063,186
(Schucker), and U.S. Pat. No. 5,635,055 (Sweet et al.) all relate
to membranes for aromatic/non-aromatic separations, but none
address benzene removal from hydrocarbon streams.
[0011] U.S. Pat. No. 6,180,008 (White) and U.S. Pat. No. 6,187,987
(Chin et al.) refers to polyimide membranes and processes using
hyperfiltration to recover aromatic solvents. Benzene removal from
hydrocarbon streams, however, is not addressed.
[0012] U.S. Pat. No. 5,914,435 (Streicher and Prevost) describe a
process where a sidestream from a distillation column enriched in
benzene is treated with a membrane permeation zone in order to
reduce the benzene content of the treated hydrocarbon stream. The
membrane is selective for benzene, and at least part of the
retentate low in benzene is divided into two streams and recycled
to two different levels in the distillation column. It is believed
that the distillation column is a naphtha pre-fractionating column
designed to separate C.sub.5 to C.sub.10 hydrocarbons, wherein the
those hydrocarbons having a boiling point in the range of
150.degree. to 200.degree. C. are collected at the bottom of the
column and hydrocarbons having a boiling point of about 50.degree.
C. are collected off the top of the column.
[0013] A publication titled "Reduce Your Tier 2 Gasoline Compliance
Costs with Grace Davison S-Brane.TM. Technology" and presented at
the Spring 2002 NPRA meeting (AM-02-21) by J. Balko describes
reducing sulfur content in gasoline by employing S-Brane.RTM.
membranes. See also U.S. Pat. No. 6,896,796 (White, Wormsbecher,
and Lesemann). Balko generally mentions retentate aromatics level
(particularly benzene) is substantially reduced by the process
using the S-Brane membrane, but there is no mention of doing so in
connection with a gasoline stream relating to olefin production,
and sulfur reduction was the primary purpose of using the S-Brane
membrane. Indeed, Balko does not provide supporting data on benzene
removal.
[0014] The following references also describe using membranes to
remove sulfur from hydrocarbon feeds. Except for the '761 to Balko,
these references do not mention benzene removal. U.S. Pat. No.
6,649,061 (Minhas et al.); U.S. Pat. No. 7,048,846 (White et al.);
and U.S. Pat. No. 7,267,761 (Balko).
[0015] Jonquieres, R. Clement, P. Lochon, J. Neel, M. Dresch, and
B. Chretien; "Industrial state-of-the-art of pervaporation and
vapour permeation in the western countries"; J. MEMBRANE SCI. 206
(2002) 87-117, states that "the petrochemical industry is now
considering these new separation processes as very good candidates
to take up the coming world-wide challenge of aromatics removal
from gasoline that remains one of the current great issues of
public health", but no additional references or information is
given. See also, A. Jonquieres, R. Clement, and P. Lochon;
"Permeability of block copolymers to vapors and liquids"; PROG.
POLYM. SCI. 27 (2002) 1803-1877.
[0016] U.S. Pat. No. 6,232,518 (Ou) refers to using cyclodextrins
for removal of benzene from hydrocarbon streams.
[0017] J. Garci Villaluenga and A. Tabe-Mohammadi; "A review of the
separation of benzene/cyclohexane mixtures by pervaporation
processes"; J. MEMBRANE SCI. 169 (2000) 159-174 reviews existing
technologies for recovery of benzene. Removal of benzene to low
levels, however, is not addressed.
[0018] New regulations are calling for lower levels of benzene in
gasoline. Since the FCC unit produces blending components for
gasoline, keeping the benzene levels low is critical to a refiner.
It would therefore be desirable to have a process that allows for
increased propylene production while simultaneously lowering the
benzene levels in FCC gasoline produced during that production. As
evidenced from above discussion, a practical solution to this
dilemma has not been disclosed or suggested.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is schematic illustration showing one embodiment of
the invention wherein light cat naphtha is treated with a membrane
to reduce the amount of benzene in the stream.
[0020] FIG. 2 is a graph illustrating the invention's performance
in reducing benzene concentrations with respect to membranes M1 and
M2 described in Example 3.
SUMMARY OF INVENTION
[0021] We have found that a membrane process can be used to address
the undesirable buildup of benzene when cracking feedstocks to
light olefins, e.g., propylene, using pentasil zeolite catalysts or
other conditions known to enhance light olefin yields, thereby
allowing for process conditions that improves light olefin yield
from an FCC unit, while at the same time providing a more
environmentally friendly naphtha stream that can be blended into
the gasoline pool. Light olefins are intended herein to mean
ethylene, propylene and butylenes, i.e., olefins containing two to
four carbons (C.sub.2 to C.sub.4).
[0022] The invention comprises introducing a hydrocarbon feedstock
into a reaction zone of a catalytic cracking unit, which feedstock
is characterized as having an initial boiling point from about
120.degree. C. with end points up to about 850.degree. C. The
feedstock is cracked in the reaction zone of the unit by contacting
the feedstock with a cracking catalyst under conditions of
temperature, catalyst-to-oil ratio, pressure, steam dilution and
space velocity such that a light olefin yield from the unit is
enhanced compared to that of the same unit operating at more
typical conditions, e.g., those listed below.
TABLE-US-00001 Temperature, .degree. C. 500-535 Cat./Oil 5 to 10
Pressure, atmospheres 1 to 4 Steam Dilution, wt % of feed 1 to 5
WHSV, hr.sup.-1 125-200 Pentasil zeolite crystal 0-1 content, wt %
catalyst
[0023] The unit thereby produces a product comprising light olefin,
naphtha and benzene, wherein the product comprises about 6 to about
20% propylene based on the weight of the hydrocarbon feedstock
introduced in the cracking step above. The product is then
fractionated into at least an light olefin-containing fraction, and
a benzene-containing naphtha fraction. Such streams can include
full range cat naphtha (boiling point in the range of 50.degree. C.
to about 220.degree. C., and 0.6 to about 3% by weight benzene) or
light cat naphtha (boiling point in the range of 50.degree. C. to
about 105.degree. C., and 1.2 to about 6% by weight benzene).
[0024] The light olefin-containing fraction is recovered, and the
benzene-containing naphtha fraction is contacted with a membrane.
The membrane selected for use is benzene selective, and therefore
should have a sufficient flux and selectivity to separate from the
naphtha a benzene-enriched permeate fraction and a benzene
deficient naphtha retentate fraction, said benzene-enriched
permeate fraction being enriched in benzene compared to the
retentate. One then recovers the benzene deficient naphtha
retentate fraction, and routes the benzene-enriched permeate
fraction for further processing.
[0025] In operating such a process, one has at its disposal a
method of producing light olefins and low benzene-containing
naphtha in a catalytic cracking unit when the unit's conditions are
selected to enhance light olefin yield in the catalytic cracking
unit, e.g., conditions such as type and composition of catalyst,
temperature, catalyst-to-oil ratio, pressure, steam dilution and/or
space velocity.
[0026] Being able to remove benzene in this fashion allows a
refiner to maximize its light olefin yield from a FCC product
stream. When the unit is utilizing pentasil zeolite for its olefin
production, the stream can contain propylene in the range of 6 to
about 20% by weight propylene based on the weight of the feedstock
to the FCC unit.
[0027] Accordingly, aromatic selective membranes that
preferentially remove benzene from gasoline feedstocks, provide for
simultaneous increased yield of light olefins, e.g., propylene,
from an FCC unit while still producing a large fraction of gasoline
with less than 1% benzene levels, and preferably less than 0.6%.
Pervaporation with an aromatic selective membrane is a preferred
process for removing benzene.
[0028] Catalysts and process conditions can moreover be adjusted in
an FCC unit to increase the yield of C.sub.3 and C.sub.4 light
olefins to greater than 20 weight % from a lights stripping tower.
Even though such a process leaves a naphtha fraction that can
contain more than 1% benzene, the membrane step of the invention
splits the gasoline fraction into a major fraction, the retentate,
with less than 1% benzene, and a minor fraction, the permeate,
containing greater than 1% benzene. The major retentate fraction
can be directly used in the gasoline pool, while the minor permeate
fraction is sent for further processing in the refinery.
DETAILED DESCRIPTION OF THE INVENTION
Catalytic Cracking Processes
[0029] The catalytic cracking process of this invention is
preferably a FCC process. Catalysts used in FCC processes are in
particle form, usually have an average particle size in the range
of 20 to 200 microns, and circulate between a cracking reactor and
a catalyst regenerator. In the reactor, hydrocarbon feed contacts
hot, regenerated catalyst that vaporizes and cracks the feed. A FCC
unit can be operated under a range of conditions, wherein the
reaction temperatures range from about 400.degree. to 700.degree.
C. with regeneration occurring at temperatures of from about
500.degree. to 900.degree. C. The particular conditions depend on
the petroleum feedstock being treated, the product streams desired
and other conditions well known to refiners. For example, lighter
feedstock can be cracked at lower temperatures. The catalyst (i.e.,
inventory) is circulated through the unit in a continuous manner
between catalytic cracking reaction and regeneration while
maintaining the equilibrium catalyst in the reactor. The invention
can be employed in a FCC unit under conventional cracking
conditions. Typical conditions found in a FCC unit are listed
below.
TABLE-US-00002 Temperature, .degree. C. 500-535 Cat./Oil 5 to 10
Pressure, atmospheres 1 to 4 Steam Dilution, wt % of feed 1 to 5
WHSV, hr.sup.-1 125-200
[0030] Certain embodiments of the invention will utilize conditions
that are somewhat more severe. These more severe processes include
those known as Deep Catalytic Cracking (DCC), Catalytic Pyrolysis
Process (CPP), and Ultra Catalytic Cracking (UCC). Illustrative
conditions for the more severe processes are listed in the table
below.
TABLE-US-00003 DCC CPP UCC Temperature, .degree. C. 505-575 560-650
550-570 Cat./Oil 9 to 15 15 to 25 18 to 22 Pressure, atmospheres
0.7 to 1.5 0.8 1 to 4 Steam Dilution, wt % of feed 10 to 30 30 to
50 20 to 35 WHSV* 0.2 to 20 -- 50 to 80 *weight hourly space
velocity (hr.sup.-1)
[0031] Those of ordinary skill in the art are familiar as to when
such processes can be used with the invention. When the invention
is used with such processes, certain modifications to the invention
may be required, e.g., activity and attrition may require
alteration of the catalyst, in order to optimize the catalyst
composition's effectiveness in those processes. Such modifications
are known to those skilled in the art. For example, when using
increased amounts of pentasil zeolites such as ZSM5, the FCC unit
can be operated under conventional FCC conditions listed in the
first column of the table above to enhance light olefin yields.
[0032] The cracking reaction deposits carbonaceous hydrocarbons or
coke on the catalyst, thereby deactivating it. The cracked products
are separated from the coked catalyst. The coked catalyst is
stripped of volatiles, usually with steam, in a catalyst stripper
and then regenerated. The catalyst regenerator burns coke from the
catalyst with oxygen containing gas, usually air, to restore
catalyst activity and heat catalyst to, e.g., 500.degree. C. to
900.degree. C., usually 600.degree. C. to 750.degree. C. The hot
regenerated catalyst recycles to the cracking reactor to crack more
fresh feed. Flue gas from the regenerator may be treated to remove
particulates or convert CO, and then discharged into the
atmosphere. The FCC process, and its development, is described in
the Fluid Catalytic Cracking Report, Amos A. Avidan, Michael
Edwards and Hartley Owen, in the Jan. 8, 1990 edition of the OIL
& GAS JOURNAL.
[0033] A variety of hydrocarbon feedstocks can be cracked in the
FCC unit to produce light olefins and gasoline. Typical feedstocks
include in whole or in part, a gas oil (e.g., light, medium, or
heavy gas oil) having an initial boiling point above about
120.degree. C. (250.degree. F.), a 50% point of at least about
315.degree. C. (600.degree. F.), and an end point up to about
850.degree. C. (1562.degree. F.). The feedstock may also include
deep cut gas oil, vacuum gas oil, coker gas oil, thermal oil,
residual oil, cycle stock, whole top crude, tar sand oil, shale
oil, synthetic fuel, heavy hydrocarbon fractions derived from the
destructive hydrogenation of coal, tar, pitches, asphalts,
hydrotreated feedstocks derived from any of the foregoing, and the
like. As will be recognized, the distillation of higher boiling
petroleum fractions above about 400.degree. C. must be carried out
under vacuum in order to avoid thermal cracking. The boiling
temperatures utilized herein are expressed in terms of convenience
of the boiling point corrected to atmospheric pressure. Even high
metal content resids or deeper cut gas oils having an end point of
up to about 850.degree. C. can be cracked.
[0034] Y-type zeolites are typically used in FCC processes to
produce gasoline. These zeolites include zeolite Y (U.S. Pat. No.
3,130,007); ultrastable Y zeolite (USY) (U.S. Pat. No. 3,449,070);
rare earth exchanged Y (REY) (U.S. Pat. No. 4,415,438); rare earth
exchanged USY (REUSY); dealuminated Y (DeAlY) (U.S. Pat. No.
3,442,792; U.S. Pat. No. 4,331,694); and ultrahydrophobic Y (UHPY)
(U.S. Pat. No. 4,401,556). These zeolites are large-pore molecular
sieves having pore sizes greater than about 7 Angstroms. In current
commercial practice most cracking catalysts contain these
zeolites.
[0035] Metal cation exchanged zeolites, e.g., MgUSY, ZnUSY and
MnUSY zeolites, can also be employed and are formed by using
exchange solutions containing the metal salts of Mg, Zn or Mn or
mixtures thereof in the same manner with respect to the formation
of REUSY except that a salt of magnesium, zinc or manganese is used
in lieu of the rare-earth metal salt used to form REUSY. The
content and manufacture of these catalysts are well known in the
art.
[0036] The amount of Y-type zeolite in the catalyst composition for
use in the invention should be sufficient to produce molecules in
the gasoline naphtha range. In general, zeolite Y will be present
in amounts ranging from 1 to 99% by weight of the catalyst.
Catalysts comprising about 12 to about 60% by weight Y-type zeolite
are more typical, with specific amounts depending on amount of
activity desired. The amount of Y-type zeolite typically is such
that the total amount of Y-type zeolite and the pentasil described
below comprises at least about 35% by weight of the total catalyst
composition.
[0037] As indicated above, adding pentasil zeolites to the catalyst
inventory is one method of operating a FCC unit to enhance light
olefin yield in accordance with this invention. These catalysts are
well known and are commonly called additive catalysts. The pentasil
zeolites suitable for this invention include those zeolite
structures having a five-membered ring. In preferred embodiments,
the catalyst composition of this invention comprises one or more
pentasils having an X-ray diffraction pattern of ZSM-5 or ZSM-I1.
Suitable pentasils include those described in U.S. Pat. No.
5,380,690, the contents of which are incorporated by reference.
Commercially available synthetic shape selective zeolites are also
suitable.
[0038] The preferred pentasil zeolites generally have a Constraint
Index of 1-12. Details of the Constraint Index test are provided in
J. CATALYSIS, 67, 218-222 (1981) and in U.S. Pat. No. 4,711,710,
both of which are incorporated herein by reference. Such pentasils
are exemplified by intermediate pore zeolites, e.g., those zeolites
having pore sizes of from about 4 to about 7 Angstroms. ZSM-5 (U.S.
Pat. No. 3,702,886 and U.S. Pat. No. Re. 29,948) and ZSM-I1 (U.S.
Pat. No. 3,709,979) are preferred. Methods for preparing these
synthetic pentasils are well known in the art. The preferred
embodiments of pentasil have relatively low silica-to-alumina
ratios, e.g., less than 100:1, preferably less than 50:1. A
preferred embodiment of this invention has a silica-to-alumina
ratio less than 30:1. The pentasil may also be exchanged with metal
cations. Suitable metals include those metal dopants described in
U.S. Pat. No. 6,969,692 B2, the contents of which are incorporated
by reference. Briefly these metals can be alkaline earth metals,
transition metals, rare earth metals, phosphorus, boron, noble
metals and combinations thereof. Catalysts comprising ZSM-5
pentasils are commercially available from W.R. Grace &
Co.-Conn, and sold as Olefins Ultra.RTM., Olefins Extra.RTM., and
OlefinsMax.RTM. brand catalysts. Olefins Ultra.RTM. HZ additive is
particularly suitable for use in this invention.
[0039] Use of these additive catalysts in combination with the
macroporous zeolite Y catalysts to enhance light olefins yield is
known, and can be used in this invention in accordance with
techniques and under conditions known in the art. For example,
refiners can add pentasil containing catalysts as additive
catalysts to their FCC units, with 10-80 wt %, typically 12 to 35
wt %, and more typically 25 to 50 wt. %, pentasil zeolite in an
amorphous support. In this instance, the pentasil is added as
particles that are separate from the particles containing the
conventional large pore zeolite catalysts. These additives are
manufactured to have physical properties that allow them to
circulate between the reaction zone and regeneration zone with the
large pore zeolite cracking catalyst. Using pentasil in a separate
additive allows a refiner to retain the ability to use the myriad
types of commercially available large pore zeolite cracking
catalyst available today, and allows a refiner to switch between
focusing production on gasoline and production of light
olefins.
[0040] Regardless of the pentasil zeolite content in the additive
particle, the amount of pentasil zeolite crystal in the total
inventory of catalyst should be in a quantity sufficient to enhance
olefin yields of the FCC unit compared to when such catalyst are
not present, or only low concentrations (e.g., 1% by weight or
less) are present, e.g., to enhance octane. A "pentasil zeolite
crystal" is meant to refer to crystalline pentasil zeolite in the
neat form. The selected pentasil zeolite crystal content,
preferably ZSM-5, will preferably be in the range of 2 to about 20%
by weight of the catalyst to be used with this invention. The
amount of the pentasil crystal can be calculated from a catalyst
particle containing support and/or matrix utilizing x-ray
diffraction techniques known to those skilled in the art.
[0041] High pentasil zeolite content catalyst particles will be
preferable in this invention in order to obtain the crystal amount
in the aforementioned range. Utilizing the high content particles
should avoid dilution of the gasoline cracking activity of the
zeolite Y. Such high pentasil zeolite content catalysts are known
and described in U.S. Pat. No. 6,916,757, the contents of which are
incorporated by reference.
[0042] These catalyst additives can be used under a wide range of
conditions in the FCC unit, with light olefins yields depending on
the conditions used. Temperature severity, i.e., higher
temperatures, in combination with use of pentasil zeolites such as
ZSM5 will typically result in enhanced light olefins yields, but
frequently also mean lowered gasoline yields with increasing
benzene content.
[0043] A particularly suitable pentasil zeolite catalyst for use in
a FCC unit to enhance olefin yields from a FCC unit is described in
WO 2006/050487 A1, and US 2008/0093263 A1, the contents of which
are incorporated by reference. Briefly, this catalyst is formulated
with a Y-type zeolite to contain pentasil zeolite in a range of
about 10% to about 50%, and a pentasil zeolite to Y-type zeolite
ratio of at least 0.25. The ratio of pentasil zeolite to Y type
zeolite for this catalyst should in general be no more than about
3.0. Typical embodiments of the invention comprise about 10% to
about 30% by weight pentasil zeolite, and more typically 10 to
about 20% by weight. The amount of pentasil zeolite in this
catalyst is generally such that the amount of pentasil zeolite and
Y-type zeolite described above is at least 35% by weight of the
total catalyst composition.
[0044] Another suitable pentasil zeolite catalyst for use in this
invention is one containing at least 1% by weight iron oxide based
on the weight of the particles containing the pentasil zeolite.
Such catalysts are described in WO 2007/005075 A1, the contents of
which are incorporated by reference. These iron oxide-containing
pentasil zeolites typically comprise 1 to 10% by weight iron oxide,
and the iron in that amount is outside the framework of the
pentasil framework, e.g., iron present in the pentasil particles'
matrix, as opposed to that present in the pentasil's silica alumina
framework. WO 2007/005075 A1 describes methods of preparing these
type of pentasil catalysts, and how such catalysts can be
incorporated into the catalyst inventory of a FCC unit. These
catalysts are particularly designed to enhance the olefin yield of
the FCC unit, and this invention would have particular utility with
a FCC unit whose catalyst inventory contains such catalysts.
[0045] The invention is also suitable for use with an FCC unit
whose catalyst inventory comprises an iron-based catalyst such as
that described in US 2006/0011513 A1, wherein the pentasil catalyst
comprises a metal phosphate binder, especially those catalysts that
comprise an iron phosphate binder.
[0046] As mentioned above, light olefins yields in a FCC unit are
also enhanced utilizing more severe operations such as DCC, UCC,
and CPP listed in the table above. These methods employ the same
catalysts typically used in FCC, but in amounts and ratios tailored
to the particular conditions selected. A range of conditions for
each of these operations is provided in the table above. These
processes and catalysts used therein are known in the art. See
Chapin et al., "Deep Catalytic Cracking, Maximize Olefin
Production", presented at 1994 NPRA Annual Meeting, San Antonio,
Tex., Mar. 20-22, 1994 (DCC); Meng et al., "Production of Light
Olefins by Catalytic Pyrolysis of Heavy Oil", PETROLEUM SCIENCE AND
TECHNOLOGY Vol. 24, pages 413-422, 2006 (CPP); and U.S. Pat. No.
5,846,402 (UCC).
[0047] It is also envisioned that the invention can utilize units
conducting a process known and licensed by Kellogg Brown and Root
as the Superflex process. In such an embodiment, the Superflex
process is by design operated to enhance light olefin yields when
directly processing light naphtha feeds, and the product from the
unit would then otherwise be processed in accordance with the
teachings herein.
[0048] The hydrocarbon effluent or product from the FCC unit varies
and depends not only on the feedstock, but also the conditions in
the unit. A hydrocarbon stream processed under typical FCC
conditions, as well as those processed in accordance with the
invention will result in product having specifications illustrated
in the examples below.
[0049] The product from the FCC unit is then routed to a
fractionation column for further processing according to this
invention. When the FCC unit is operated to enhance the light
olefin yield in the FCC product, the light olefins will comprise 6
to about 20% propylene based on the weight of the feed to the FCC
unit. Table 2 in the aforementioned NPRA presentation regarding DCC
processes for maximizing propylene yields is illustrative of the
FCC unit product specifications, e.g., about an 8% propylene yield
from an FCC unit operated to maximize C.sub.3 olefins. FCC product,
e.g., about 7% propylene yield, obtained under more conventional
conditions are illustrated in Table 3 of "Reformulated Gasoline:
The Role of Current and Future FCC Catalysts", Young et al.
presented at the 1991 NPRA Annual Meeting, Mar. 17-19, 1991, San
Antonio, Tex.
Fractionation
[0050] The FCC product is separated into various fractions
depending on the final product targets for the refinery. For the
purposes of this invention, the fractionation column, e.g., also
known as the FCC Main Column, separates the FCC product into at
least a light olefin-containing fraction and a naphtha fraction.
The light olefin-containing fraction generally comprises C.sub.4 or
lower saturated and/or unsaturated fractions. The column or
fractionator can be those known in the art. See FLUID CATALYTIC
CRACKING HANDBOOK-DESIGN, OPERATION, AND TROUBLESHOOTING OF FCC
FACILITY, Sadeghbiegi, pp. 18-21 (1995) Conditions for running
these columns vary depending on the number of fractions desired. A
refiner running a FCC unit will typically fractionate the FCC
product into a light olefin fraction, gasoline naphtha fraction,
light cycle oil (LCO) fraction, and heavy cycle or bottoms
fraction.
[0051] Light olefin fractions comprising C.sub.4 or lower saturated
and/or unsaturated fractions are typically distilled off as "wet
gas". Wet gas fractions are generally considered those fractions
having a boiling point of 50.degree. C. or lower. These fractions
are typically recovered in a compressor apparatus and
processed/distilled into the individual light olefins flashed from
the column.
[0052] The gasoline naphtha fraction generally comprises C.sub.5 to
C.sub.12 hydrocarbons. For purposes of this invention, the terms
"naphtha" and "gasoline naphtha" are used interchangeably herein to
indicate hydrocarbon streams found in refinery operations that have
a boiling range between about 50.degree. C. and about 220.degree.
C. The naphtha fractions contain various amounts of olefinic,
aromatic, and non-aromatic, e.g., aliphatic, hydrocarbon compounds
and are primarily differentiated by the following boiling ranges.
Light naphthas have a boiling point ranging from 50.degree. C. to
about 105.degree. C. Intermediate (mid) naphtha has a boiling point
ranging from 105.degree. C. to about 160.degree. C. Heavy cat
naphtha has a boiling point ranging from about 160.degree. C. to
about 220.degree. C.
[0053] Benzene has a boiling point of 80.degree. C. and significant
portions of the benzene content in the FCC product will distill
with the naphtha fraction. The naphtha fraction of product from a
FCC unit operating to produce enhanced olefin yields can comprise
0.6 to about 3% by weight benzene, and frequently above 1%
benzene.
[0054] If the FCC product is separated into the light, intermediate
and heavy cat naphtha fractions, significant amounts of benzene
will flash with the light cat naphtha, i.e., the 50 to 105.degree.
C. fraction. A light cat naphtha from a FCC unit operating to
produce enhanced olefin yields can comprise 1.2 to about 6% by
weight benzene, and frequently above 2% benzene. Such benzene
concentrations can be found in light cat naphtha produced in units
in which pentasil zeolite, e.g., ZSM5, crystal comprises 2 to 20%
by weight of the catalyst inventory.
[0055] Without being held to a particular theory, it is believed
that when processes of higher severity are utilized to produce more
light olefins in a FCC unit, dealkylation of aromatic side chain,
cyclization, and dehydrogenation reactions occur to create a
greater concentration of benzene. Processes relying on pentasil
zeolites to produce light olefins, on the other hand, cracks
molecules to olefin molecules that are removed from the naphtha
fraction, thereby resulting in a stream with higher benzene
concentrations. In other words, the pentasil catalyst removes
molecules from the naphtha stream that would otherwise dilute the
presence of benzene.
[0056] The benzene-containing naphtha fraction, e.g., full range or
light, is collected from the column and then routed to a membrane
for further processing according to the invention. See FIG. 1. The
remaining fractions coming off the column, e.g., LCO and HCO,
represent, respectively, hydrocarbon fractions in the
C.sub.12-C.sub.22 range and the C.sub.22 and higher range. It is
envisioned that these fractions will not typically require further
processing according to this invention. These latter streams are
frequently collected and routed for separate processing, or, e.g.,
recycle through the FCC unit.
Membrane Separation
[0057] Membranes useful in the present invention are those
membranes having a sufficient flux and selectivity to permeate at
least benzene in the presence of hydrocarbons containing multiple
aromatic-compounds, e.g., benzene-containing gasoline naphtha. Any
aromatic selective membrane can be used. Favorable membranes are
chosen on the basis of high productivity which is the combination
of high flux and good aromatics selectivity, and on the ability to
withstand hot operating temperatures typically in the range of
80.degree. to 120.degree. C. Benzene is highly permeable across
aromatic selective membranes. When you compare the transport rate
of benzene across a benzene selective membrane against transport
rates for compounds of C.sub.5 and above in the feedstock, the
relative rate of benzene removal is the fastest, thereby making
this invention rather efficient, and at the same time preserving
the valuable gasoline fractions in the retentate. Moreover, the
membrane systems can be designed to handle large-scale feed
streams, e.g., on the order of 5405 m.sup.3 per day (34,000
barrels/day), thereby offering potential economies of scale to the
refiner utilizing this invention.
[0058] The membrane will typically have a benzene enrichment factor
of greater than 1.5, preferably greater than 2, even more
preferably from about 2 to about 20, most preferably from about 2.5
to 15. Preferably, the membranes have an asymmetric structure,
which may be defined as an entity composed of a dense ultra-thin
top "skin" layer over a thicker porous substructure of a same or
different material. Typically, the asymmetric membrane is supported
on a suitable porous backing or support material.
[0059] In one embodiment of the invention, the membrane is a
polyimide membrane prepared from a Matrimid.RTM. 5218 or a Lenzing
polyimide polymer as described in U.S. Pat. No. 6,180,008, the
contents of which are incorporated herein by reference.
[0060] In another embodiment of the invention, the membrane is one
having a siloxane-based polymer as part of the active separation
layer, e.g., coated onto a microporous or ultrafiltration support
layer. Examples of membrane structure incorporating polysiloxane
functionality are found in U.S. Pat. No. 4,781,733; U.S. Pat. No.
4,243,701; U.S. Pat. No. 4,230,463; U.S. Pat. No. 4,493,714; U.S.
Pat. No. 5,265,734; U.S. Pat. No. 5,286,280; and U.S. Pat. No.
5,733,663; said references being herein incorporated by
reference.
[0061] In still another embodiment of the invention, the membrane
is an aromatic polyurea/urethane membrane as disclosed in U.S. Pat.
No. 4,962,271, incorporated herein by reference. Such
polyurea/urethane membranes are characterized as possessing a urea
index of at least 20%, but less than 100%, an aromatic carbon
content of at least 15 mole %, a functional group density of at
least about 10 per 1000 grams of polymer, and a C.dbd.O/NH ratio of
less than about 8.
[0062] The polyimide, polyurea-urethane, and polysiloxane based
membranes described above are particularly useful when separating
benzene from gasoline (e.g., light cat naphtha) produced in a unit
whose catalysts comprise pentasil zeolite crystal, e.g., ZSM-5, in
a range of 2 to about 20% by weight. As mentioned earlier, light
cat naphthas produced from such catalysts can comprise 1.2 to 6.0
weight percent benzene.
[0063] The membranes can be used in any convenient form such as
sheets, tubes or hollow fibers. Sheets can be used to fabricate
spiral wound modules familiar to those skilled in the art.
Alternatively, sheets can be used to fabricate a flat stack
permeator comprising a multitude of membrane layers alternately
separated by feed-retentate spacers and permeate spacers. This
device is described in U.S. Pat. No. 5,104,532, herein incorporated
by reference.
[0064] Tubes can be used in the form of multi-leaf modules wherein
each tube is flattened and placed in parallel with other flattened
tubes. Internally each tube contains a spacer. Adjacent pairs of
flattened tubes are separated by layers of spacer material. The
flattened tubes with positioned spacer material are fitted into a
pressure resistant housing equipped with fluid entrance and exit
means. The ends of the tubes are clamped to create separate
interior and exterior zones relative to the tubes in the housing.
Apparatus of this type is described and claimed in U.S. Pat. No.
4,761,229, herein incorporated by reference.
[0065] Hollow fibers can be employed in bundled arrays potted at
either end to form tube sheets and fitted into a pressure vessel
thereby isolating the insides of the tubes from the outsides of the
tubes. Apparatus of this type are known in the art. A modification
of the standard design involves dividing the hollow fiber bundle
into separate zones by use of baffles, which redirect fluid flow on
the tube side of the bundle and prevent fluid channeling and
polarization on the tube side. This modification is disclosed in
U.S. Pat. No. 5,169,530, herein incorporated by reference.
[0066] Multiple separation elements, be they spirally wound, plate
and frame, tubular, or hollow fiber elements can be employed either
in series or in parallel. U.S. Pat. No. 5,238,563, herein
incorporated by reference, discloses a multiple-element housing
wherein the elements are grouped in parallel with a feed/retentate
zone defined by a space enclosed by two tube sheets arranged at the
same end of the element.
[0067] Selective membrane separation of the benzene-containing
gasoline, e.g., whether it is the full range gasoline or light cat
naphtha, into permeate and retentate is conducted under
pervaporation or perstraction conditions. Preferably, the process
is conducted under pervaporation conditions.
[0068] The pervaporation process typically relies on vacuum on the
permeate side to evaporate or otherwise remove the permeate from
the surface to the membrane and maintain the concentration gradient
driving force which drives the separation process. The feed is in
the liquid and/or gas state. When in the gas state the process can
be described as vapor permeation. The maximum temperature employed
in pervaporation will be that necessary to vaporize the components
in the feed which one desires to selectively permeate through the
membrane, while still being below the temperature at which the
membrane is physically damaged. Pervaporation can be performed at a
temperature of from about 25.degree. C. to 200.degree. C. and
higher, the maximum temperature being that temperature at which the
membrane is physically damaged. The feed pressure into a membrane
unit is usually in the range of 1 to 20 atmospheres and the unit
operated under vacuum in the range of 0.1 to 300 millimeter of
mercury. It is preferred that the pervaporation process be operated
as a single stage operation to reduce capital costs. Alternatively
to a vacuum, a sweep gas can be used on the permeate side to remove
the product. In this mode the permeate side would be at atmospheric
pressure.
[0069] In a perstraction process, the permeate molecules in the
feed diffuse into the membrane film, migrate through the film and
reemerge on the permeate side under the influence of a
concentration gradient. A sweep flow of liquid is used on the
permeate side of the membrane to maintain the concentration
gradient driving force. The perstraction process is described in
U.S. Pat. No. 4,962,271, herein incorporated by reference.
[0070] Whether pervaporation or perstraction, the selected membrane
process can easily be adjusted to various products of the FCC unit
by changing feed flow rates or operating temperatures. This
invention therefore provides additional process control variables
to help the refinery increase yields, and still have a process that
handles the benzene-containing naphtha streams.
[0071] Very significant reductions of benzene in the naphtha are
achievable by the selective membranes according to this invention.
Generally, the retentate, also referred to herein as the benzene
deficient retentate will have less than 1% benzene and as low as
100 ppm, depending on the benzene concentration in the naphtha,
membrane separation conditions, etc. The invention preferably
reduces gasoline benzene such that the retentate contains less than
0.6% by weight benzene. Generally, sufficient benzene reduction is
readily achievable in the retentate while substantially or
significantly maintaining the level of gasoline naphtha molecules
in the retentate. The retentate stream is then routed to the
refiner's gasoline pool where other refinery streams are combined
to form gasoline product.
[0072] The benzene enriched permeate is routed to the refinery's
BTX unit (benzene, toluene, and xylene) for collection and
appropriate processing, use and/or disposal. The permeate from this
invention will commonly have benzene (and toluene) in amounts
ranging from 1 to 10% by weight benzene. Depending on the refinery,
the high benzene enriched permeate can be routed to processes that
further concentrate the stream to a substantially pure benzene for
use in chemical operations, or the permeate can be further
processed to become a feedstock in reforming or alkylation
processes.
[0073] Increased efficiency of the invention may be attained by
fractionating the benzene-containing naphtha (full range or light)
in a depentanizer distillation column prior to the membrane unit
for separating benzene from the naphtha fraction. Depending on the
membrane selected, C.sub.5 olefins in the gasoline stream will
usually remain in the retentate, but these olefins can also
concentrate in the permeate. Treating the naphtha stream removes
the C.sub.5 olefins from the permeate, and therefore, concentrates
benzene in the feedstock being directed to the membrane. Overall
this should reduce the stage cut requirement. The C.sub.5's have
been cut out, and higher benzene concentrations are removed at a
faster absolute rate than when lower concentrations of benzene are
present. The costs of condensing permeate without C.sub.5's will
also be reduced. The C.sub.5 olefins recovered from the
depentanizer can then be further processed for another refinery
need, or returned to the retentate stream prior to processing to
the gasoline pool. Depentanizers columns are known in the art. The
column to be used in this embodiment of the invention can be run to
flash fractions at the column's top at 50.degree. C., and bottom
fractions at a temperature in the range of 150.degree. to
200.degree. C.
[0074] To further illustrate the present invention and the
advantages thereof, the following specific examples are given. The
examples are given for illustrative purposes only and are not meant
to be a limitation on the claims appended hereto. It should be
understood that the invention is not limited to the specific
details set forth in the examples.
[0075] All parts and percentages in the examples, as well as the
remainder of the specification, which refers to solid compositions
or concentrations, are by weight unless otherwise specified.
However, all parts and percentages in the examples as well as the
remainder of the specification referring to gas compositions are
molar or by volume unless otherwise specified.
[0076] Further, any range of numbers recited in the specification
or claims, such as that representing a particular set of
properties, units of measure, conditions, physical states or
percentages, is intended to literally incorporate expressly herein
by reference or otherwise, any number falling within such range,
including any subset of numbers within any range so recited.
Example 1
[0077] A benzene selective polyurea/urethane membrane (M1) was made
in a two-step coating process. A solution was made that consists of
1.05 to 1 mole ratio of toluene diisocyanate terminated
polyethylene adipate and 4,4'-methylenebis(2,6-diethylaniline) at
4.0% solids in dioxane. This is allowed to react overnight for 16
hours to generate a polyurea/urethane solution. A polyacrylonitrile
ultrafiltration substrate was dip coated with the resulting
solution. The coated substrate was then transferred to a ventilated
oven at 100.degree. C. to dry and cure. In a second step, a
separate solution of toluene diisocyanate terminated polyethylene
adipate and 4,4'-methylenebis(2,6-diethylaniline) was prepared, but
this time at 2.65% solids in dioxane. This was allowed to react
overnight for 16 hours to generate a polyurea/urethane solution.
This solution was then dip coated onto the earlier coated
substrate, which had been allowed to age 7 days. The newly coated
substrate was again transferred to a ventilated oven at 100.degree.
C. The finished membrane was dry and durable.
[0078] Properties for the polymeric separation layer of M1 was
calculated using the methodology of Feimer et al. (U.S. Pat. No.
4,879,044). Calculated was an aromatic index=17.56, a urea
index=50, the sum of (C.dbd.O+NH)/1000 g=12.64, and C.dbd.O/NH
value=5.34.
Example 2
[0079] A second benzene selective membrane (M2) was made in a
two-step coating process similar to that described in Example 1. A
solution was made of 1.05 to 1 mole ratio of toluene diisocyanate
terminated polyethylene adipate and
4,4'-methylenebis(2,6-diethylaniline) at 2.65% solids in dioxane.
This was allowed to react overnight for 16 hours to generate a
polyurea/urethane solution. The same polyacrylonitrile
ultrafiltration substrate mentioned above was dip coated and then
transferred to a ventilated oven at 100.degree. C. to dry and cure.
The coated substrate was coated a second time with the same coating
above, after the first coating aged for 7 days. The finished
membrane was dry and durable.
[0080] M2 was another polyurea/urethane membrane with more aromatic
content and higher functional group density. Calculated was an
aromatic index=29.63, a urea index=50, the sum of (C.dbd.O+NH)/1000
g=12.97, and C.dbd.O/NH value=2.33.
Example 3
[0081] An FCC feedstock, having 25.5 API gravity, 11.94 K-Factor,
0.68% Conradson Carbon, and 0.12 wt % Nitrogen was treated in a
Davison Circulating Riser at 543.degree. C. (1010.degree. F.)
reactor temperature, 172 kPa (25 psig) reactor pressure and
704.degree. C. (1300.degree. F.) regenerator temperature, using a
catalyst blend of 80% Ultima and 20% Olefin Ultra. Propylene yield
was 8.37 weight % on the fresh feed basis. The light C.sub.3 and
C.sub.4 compounds were separated from naphtha by a distillation
column, operating with a bottom temperature of 32.degree. C.
(90.degree. F.) and the top temperature of -11.degree. C.
(12.degree. F.). The recovered naphtha contained 1.1 wt %
benzene.
[0082] The benzene selective membranes M1 and M2 were used to
further treat this naphtha in a pervaporation system at consisting
of a feed reservoir, circulation pump, flow meters, a test cell
containing a membrane sample located inside an oven, permeate
collection vessels, and a vacuum pump. The permeate traps are
cooled in liquid nitrogen (-195.degree. C.). The feed stream was
the FCC naphtha stripped of C.sub.3 and C.sub.4 compounds. A
membrane trial was run for each membrane M1 and M2 in the
pervaporation unit at 120.degree. C. and full vacuum to reduce the
benzene levels in this naphtha. A pressure regulator allowed the
naphtha to be pressurized to 552 kPa (80 psi) and remain as liquid
phase while hot. The vacuum pump generates a vacuum of less than 10
torr on the permeate side of the membrane.
[0083] Each permeate sample was collected for 1-2 hours. Collecting
multiple permeate samples generates a large stage cut. The
retentate was continuously returned to the feed reservoir for
recycle over the membrane. The retentate samples were collected at
the start of each fraction of permeate collection along with one
final sample at the end of the run.
[0084] Concentrations of hydrocarbons (weight %) were determined
for both retentates and permeates using standard GC methods.
[0085] Results are shown on the graph (FIG. 2) where stage cut
refers to the fraction of feed removed as permeate. M1 produced
retentate with less than 0.6% benzene at 25% stage cut. M2, a more
selective membrane, generated less than 0.6% benzene by 21% stage
cut.
* * * * *