U.S. patent application number 10/636726 was filed with the patent office on 2004-02-12 for membrane process for separating sulfur compounds from fcc light naphtha.
Invention is credited to Chuba, Michael R., Minhas, Bhupender S., Saxton, Robert J..
Application Number | 20040026321 10/636726 |
Document ID | / |
Family ID | 26695106 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040026321 |
Kind Code |
A1 |
Minhas, Bhupender S. ; et
al. |
February 12, 2004 |
Membrane process for separating sulfur compounds from FCC light
naphtha
Abstract
A process for the separation of sulfur compounds from a
hydrocarbon mixture using a membrane is provided. Preferred
hydrocarbon mixtures are oil refining fractions such as light
cracked naphtha. Membranes are composed of either ionic or
non-ionic materials and preferentially permeate sulfur compounds
over other hydrocarbons. A single or multi-stage membrane system
separates the hydrocarbon mixture into a sulfur-rich fraction and a
sulfur-lean fraction. The sulfur-lean fraction may be used in fuel
mixtures and the sulfur-rich fraction may be further treated for
sulfur reduction.
Inventors: |
Minhas, Bhupender S.; (Bel
Air, MD) ; Chuba, Michael R.; (Wrightstown, NJ)
; Saxton, Robert J.; (Pleasanton, CA) |
Correspondence
Address: |
EXXONMOBIL RESEARCH AND ENGINEERING COMPANY
P.O. BOX 900
1545 ROUTE 22 EAST
ANNANDALE
NJ
08801-0900
US
|
Family ID: |
26695106 |
Appl. No.: |
10/636726 |
Filed: |
August 7, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10636726 |
Aug 7, 2003 |
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10021800 |
Dec 12, 2001 |
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6649061 |
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60258583 |
Dec 28, 2000 |
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Current U.S.
Class: |
210/640 ;
210/651; 585/818 |
Current CPC
Class: |
C10G 2400/02 20130101;
B01J 20/28033 20130101; B01J 47/12 20130101; B01D 61/246 20130101;
B01D 61/362 20130101 |
Class at
Publication: |
210/640 ;
210/651; 585/818 |
International
Class: |
B01D 061/36; B01D
061/00; C07C 007/144 |
Claims
What is claimed is:
1. A process for separating sulfur compounds from a hydrocarbon
mixture containing at least one sulfur compound and hydrocarbons
comprising the steps of: (a) contacting said hydrocarbon mixture
with a first compartment of a membrane module, said membrane module
further comprising a second compartment and a hydrophilic membrane
separating said first compartment and said second compartment; (b)
selectively permeating said sulfur compounds of said hydrocarbon
mixture through said membrane such that a sulfur-rich fraction
collects in said second compartment and a sulfur-lean fraction is
retained in said first compartment; and (c) retrieving said
sulfur-rich fraction from said second compartment and said
sulfur-lean fraction from said first compartment.
2. The process of claim 1 further comprising the steps of: (d)
contacting said sulfur-rich fraction of step (c) with a first
compartment of a further membrane module, said further membrane
module comprising a second compartment and a hydrophilic membrane
separating said first compartment and said second compartment; (e)
selectively permeating sulfur compounds of said sulfur-rich
fraction of step (d) through said membrane such that a further
sulfur-rich fraction accumulates in said second compartment and a
further sulfur-lean fraction is retained in said first compartment;
and (f) retrieving said further sulfur-rich fraction and said
further sulfur-lean fraction; (g) repeating steps (d), (e) and (f)
using said sulfur-rich fraction until a final sulfur-rich fraction
of desired sulfur content is obtained; and (h) retrieving said
final sulfur-lean fraction.
3. The process of claim 1 wherein said hydrocarbon mixture is
obtained from an oil refining process.
4. The process of claim 1 wherein said hydrocarbon mixture is a
light cracked naphtha.
5. The process of claim 1 wherein said sulfur compound is thiophene
or a derivative of thiophene.
6. The process of claim 1 wherein said hydrophilic membrane is an
ionic membrane.
7. The process of claim 6 wherein said membrane is selected from
the polymers of perfluorosulfonic acid and derivatives thereof.
8. The process of claim 1 wherein said hydrophilic membrane is a
water-soluble membrane.
9. The process of claim 1 wherein said hydrophilic membrane is a
non-ionic membrane.
10. The process of claim 10 wherein said membrane comprises
polyvinylpyrrolidone.
11. The process of claim 10 wherein said membrane comprises
cellulose triacetate.
12. The process of claim 1 which is performed under pervaporation
conditions.
13. The process of claim 1 which is performed under perstraction
conditions.
14. The process of claim 2 wherein said membrane of said further
membrane module is an ionic membrane.
15. The process of claim 14 wherein said membrane is selected from
the polymers of perfluorosulfonic acid and derivatives thereof.
16. The process of claim 2 wherein said hydrophilic membrane is a
non-ionic membrane.
17. The process of claim 2 wherein said hydrophilic membrane is a
water-soluble membrane.
18. The process of claim 16 wherein said membrane comprises
polyvinylpyrrolidone.
19. The process of claim 16 wherein said membrane comprises
cellulose triacetate.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is based on Provisional U.S. Application
60/258,583 filed Dec. 28, 2000.
BACKGROUND OF THE DISCLOSURE
[0002] 1. Field of the Invention
[0003] The present invention relates to a process for the
separation of sulfur compounds from hydrocarbon mixtures using a
membrane.
[0004] 2. Background of the Invention
[0005] Sulfur compounds are impurities in gasoline that compromise
vehicle emission controls by poisoning the catalytic converter. In
an effort to further decrease emissions, the U.S. government has
recently proposed a nationwide reduction of sulfur in gasoline from
current levels at 300-1000 ppm to an average of 30 ppm (Federal
Register, 64(92), May 13, 1999). Gasoline producers, both domestic
and foreign, selling fuel in the U.S. would be expected to comply
by the year 2004.
[0006] Presently, the conventional process for reducing sulfur
content in gasoline involves hydrotreating in which sulfur
compounds are converted to volatile hydrogen sulfide and other
organics. This energy intensive process, requiring elevated
temperature and pressure, is expensive for obtaining the proposed
lowered sulfur levels. Alternative processes with more efficient
sulfur-reducing technology are needed to maintain progress toward
cleaner burning fuels.
[0007] The use of membrane separation technology, in which select
compounds or types or compounds can be separated from an organic
mixture by permeation through a membrane, has been reasonably well
developed. Separation processes that incorporate membranes present
an attractive option for large-scale purification of petroleum
fractions because of their inherent simplicity, versatility, and
low energy consumption.
[0008] Typically, membrane separation processes rely on the
affinity of a specific compound or class of compounds for the
membrane. In this way, the components of a mixture with specific
affinity for the membrane will selectively sorb onto the membrane.
The sorbed compounds diffuse, or permeate, through the membrane and
are removed on the opposite side. Continual withdrawal of permeated
compounds from the membrane maintains the driving force for the
separation process. Removal of permeated compounds is usually
achieved by pervaporation or perstraction methods. Pervaporation
employs a vacuum on the permeate side of the membrane, removing the
permeated compounds in gaseous form, while perstraction employs a
liquid sweep stream, continually washing away permeate.
[0009] The chemical properties of the membrane dictate the type of
compound that has affinity for it. Some types of membranes are
composed of charged chemical groups and are, therefore, considered
ionic in character. An example of an ionic membrane is Nafion.RTM.
(available from DuPont, of Wilmington, Del.), which is a polymer of
perfluorosulfonic acid that has been used principally in the
dehydration of liquid organic mixtures as described in U.S. Pat.
No. 4,846,977. Only few examples exist for the use of Nafion.RTM.
in separating organic compounds. U.S. Pat. No. 4,798,764 describes
the separation of methanol from dimethyl carbonate or methyl
t-butyl ether. The use of Nafion.RTM. membranes for the separation
of mixtures of styrene and ethylbenzene has also been reported
(Cabasso, Ind. Eng Chem. Prod Res. Dev. 1983, 22, 313). U.S. Pat.
No. 5,498,823 reports the enhanced separation of unsaturated
organic compounds using silver ion-exchanged Nafion.RTM. membranes.
A related ionic membrane composed of sulfonated polysulfone has
been also used for the separation of aromatics and non-aromatics as
disclosed in U.S. Pat. No. 5,055,631. To date, the use of ionic
membranes, such as Nafion.RTM., for the separation of sulfur
compounds from liquid organic mixtures has not been reported.
[0010] In contrast to ionic membranes, non-ionic membranes are made
from those materials lacking charged chemical groups. Chemical
affinity for these membranes is usually governed by the hydrophilic
or hydrophobic nature of the membrane material. Hydrophilic
membranes have affinity for water or other polar compounds, and
those membranes with affinity for water are often water-soluble.
Hydrophilic membranes include both ionic and non-ionic membranes.
However, the non-ionic membranes generally contain polar chemical
groups such as hydroxyl, carboxyl, sulfonyl, carbonyl, or amine
groups. Examples of hydrophilic non-ionic membranes include
polyvinylalcohol (PVA), cellulose acetates, and polyvinylamine.
Hydrophobic membranes, on the other hand, have little affinity for
water or polar compounds and generally lack or contain a small
proportion of charged or polar chemical groups. Examples of
hydrophobic membranes include polyethylene and polystyrene.
[0011] A wide variety of non-ionic membranes have been used in
separation processes. U.S. Pat. Nos. 5,905,182, 5,019,666,
4,997,906, 4,944,880, 4,532,029, 4,802,987, 4,962,271, 5,288,712,
5,635,055, 3,556,991, 3,043,891, and 2,947,687 describe the
separation of aromatics from hydrocarbon mixtures using a wide
variety of non-ionic membrane materials. Non-ionic membranes have
also been used in the separation of aromatics containing
heteroatoms from hydrocarbon mixtures as disclosed in U.S. Pat.
Nos. 5,643,442 and 5,396,019. The aforementioned patents, which are
incorporated herein by reference, disclose membrane separation
processes directed to the separation of aromatics and non-aromatics
using hydrophobic membranes.
[0012] The proposed mandate for lowered sulfur levels in gasoline
has made it imperative to improve or replace existing methods for
desulfurization of petroleum fractions. A more cost-effective
method for reducing sulfur content in petroleum fractions is a
primary goal of the oil refining industry. Current membrane
separation technology shows potential for meeting future standards,
but has not yet been used specifically for this purpose.
SUMMARY OF THE INVENTION
[0013] This invention relates to a process for the separation of
sulfur compounds from hydrocarbon mixtures, preferably oil refining
fractions, using a membrane. The membrane may be composed of any
material, ionic or non-ionic, that preferentially permeates sulfur
compounds over hydrocarbons. The hydrocarbon mixture is split by
one or more membranes forming sulfur-rich and sulfur-lean
fractions. The sulfur-lean fraction may be incorporated into fuel
mixtures and the sulfur-rich fraction may undergo further treatment
for reduction of sulfur levels.
[0014] The present invention provides a process for separating
sulfur compounds from a hydrocarbon mixture containing at least one
sulfur compound and hydrocarbons comprising the steps of:
[0015] (a) contacting said hydrocarbon mixture with a first
compartment of a membrane module, said membrane module further
comprising a second compartment and a hydrophilic membrane
separating said first compartment and said second compartment;
[0016] (b) selectively permeating said sulfur compounds of said
hydrocarbon mixture through said membrane such that a sulfur-rich
fraction accumulates in said second compartment and a sulfur-lean
fraction is retained in said first compartment; and
[0017] (c) retrieving said sulfur-rich fraction from said second
compartment and said sulfur-lean fraction from said first
compartment.
[0018] The present invention also provides a process which further
comprises the steps of:
[0019] (d) contacting said sulfur-rich fraction of step (c) with a
first compartment of a further membrane module, said further
membrane module comprising a second compartment and a hydrophilic
membrane separating said first compartment and said second
compartment;
[0020] (e) selectively permeating sulfur compounds of said
sulfur-rich fraction of step (d) through said membrane such that a
further sulfur-rich fraction accumulates in said second compartment
and a further sulfur-lean fraction is retained in said first
compartment; and
[0021] (f) retrieving said further sulfur-rich fraction and said
further sulfur-lean fraction;
[0022] (g) repeating steps (d), (e) and (f) using said sulfur-rich
fraction until a final sulfur-rich fraction of desired sulfur
content is obtained; and
[0023] (h) retrieving said final sulfur-lean fraction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematic showing a process for the separation
of sulfur compounds from light cracked naphtha using a membrane
system.
[0025] FIG. 2 is a schematic showing a process for the separation
of sulfur compounds from light cracked naphtha using a multi-staged
membrane system under pervaporative conditions.
[0026] FIG. 3 is a diagram of a spiral wound membrane module.
DETAILED DESCRIPTION OF THE INVENTION
[0027] As used herein, "hydrocarbon mixtures" means both synthetic
mixtures and oil refining fractions, each of which contain sulfur
compounds. Preferred hydrocarbon mixtures include FCC gasoline
mixtures and light cracked naphthas (LCN). Hydrocarbons in the
mixture encompass aliphatic, aromatic, saturated, and unsaturated
compounds composed essentially of carbon and hydrogen. Preferred
hydrocarbons are compounds that are commonly found in oil refining
fractions including, but not limited to, benzene, toluene,
napthenes, olefins and parrafins. The sulfur compounds in the
hydrocarbon mixtures may be in any concentration, but levels of
from about 1 ppm to about 10,000 ppm are preferred, and levels of
from about 10 ppm to about 4000 ppm are more preferred. Also, the
term "sulfur compounds" means inorganic or organic compounds
comprising at least one sulfur atom. Preferably, sulfur compounds
of the present invention are thiophenes and derivatives
thereof.
[0028] As used herein, "permeate" refers to the portion of the
hydrocarbon mixture that diffuses across a membrane, and
"retentate" refers to the portion of the hydrocarbon mixture that
does not pass through the membrane. Accordingly, the term "permeate
side" refers to that side of the membrane on which permeate
collects which is also the second compartment of the membrane
module. The term "retentate side" refers to that side of the
membrane which contacts the hydrocarbon mixture and also refers to
the first compartment of the membrane module. In addition, the term
"sulfur-rich" means having an increased content of sulfur relative
to the hydrocarbon mixture and "sulfur-lean" means having a
decreased content of sulfur relative to the hydrocarbon
mixture.
[0029] According to the present invention, the term "perstraction"
refers to a method for removing permeate from a membrane module
involving a liquid sweep stream. In perstraction, a liquid sweep
stream is passed through the second compartment of the membrane
module, or permeate side of the membrane, preferably countercurrent
to the direction of flow of the hydrocarbon mixture on the
retentate side. The permeate dissolves into the sweep stream and is
carried away by the flow, thereby preventing the accumulation of
preferentially permeated components such as sulfur compounds. The
sweep liquid preferably has affinity for, and is miscible with, the
permeated components. Methanol is a preferred sweep liquid for
membrane systems employing Nafion.RTM.-type membranes as it would
also serve as a transport agent for enhancing flux and selectivity
of the membrane.
[0030] As used herein, the term "pervaporation" refers to another
method of removing permeate from the membrane module. In this
method, the permeate is removed from the permeate side of the
membrane as a vapor. Thus, a vacuum or lowered pressure must be
maintained on the permeate side such that permeated components of
the mixture will vaporize upon transfer across the membrane.
Transfer of the permeable components across the membrane is
ultimately driven by the difference in vapor pressure between the
liquid hydrocarbon mixture on the retentate side of the membrane
and the partial pressure of the permeate vapor on the permeate side
of the membrane.
[0031] As used herein, "hydrophilic" means having an affinity for
water or polar compounds. Additionally, "ionic" means having acidic
or charged chemical groups and "non-ionic" means having neutral
chemical groups.
[0032] According to the present invention, "membrane system" is a
component of a process that preferentially separates sulfur
compounds from hydrocarbon mixtures. The membrane system is
single-staged comprising one membrane module, or multi-staged,
comprising more than one membrane module. "Membrane module" refers
to a membrane assembly comprising a membrane, feed and permeate
spacers, and support material, assembled such the membrane
separates a first compartment from a second compartment. The
membrane module may be formed in any workable configuration such as
flat sheet, hollow fibers, or spiral-wrapped.
[0033] As used herein, "transport agent" refers to an additive in
the hydrocarbon mixture for augmenting flux and selectivity of the
separating membrane. Transport agents include, but are not limited
to, alcohols, glycols, ethers or any other compounds that are
miscible with hydrocarbon mixtures, are sorbed by the ionic
membrane, and increase flux through the membrane. Preferred
transport agents are alcohols. A low boiling transport agent such
as methanol is a more preferred transport agent because of ease of
removal by distillation. The quantity of transport agent added to
the hydrocarbon mixture is preferably about 1% to about 20% by
weight. Addition of about 10% by weight of methanol is more
preferred. The transport agent also may comprise the sweep stream
in perstraction processes.
[0034] As used herein, "Nafion.RTM.-type membrane" refers to a
polymer of perfluorosulfonic acid or a derivative thereof.
Derivatives include, but are not limited to, Nafion.RTM.-type
membranes having undergone ion-exchange or reaction with organic
bases. "Nafion," according to L. Gardner's Chemical Synonyms and
Tradenames, 9.sup.th ed., 1989, is defined as a perfluorosulphonic
acid membrane (DuPont).
[0035] The hydrocarbon mixtures treated by the present invention
encompass both synthetic mixtures and authentic oil refining
fractions, each of which contain sulfur compounds. Preferable
hydrocarbon mixtures include FCC gasoline mixtures and light
cracked naphthas (LCN). The sulfur compounds in the hydrocarbon
mixtures may be in any concentration, but levels of from about 1
ppm to about 10,000 ppm are preferred, and levels of from about 10
ppm to about 4000 ppm are more preferred. Sulfur compounds include
organic and inorganic compounds. Preferred sulfur compounds are
organic compounds. More preferred sulfur compounds are thiophenes
and derivatives thereof. Hydrocarbons in the mixture include, but
are not limited to, aliphatic, aromatic, saturated and unsaturated
compounds composed essentially of carbon and hydrogen. Preferred
hydrocarbons are compounds that are commonly found in oil refining
fractions including, but not limited to, benzene, toluene,
naphthenes, olefins and paraffins.
[0036] A transport agent may be optionally added to the hydrocarbon
mixture to augment flux and selectivity of the separating membrane.
Preferred transport agents include, but are not limited to,
alcohols, glycols, ethers or any other compounds that are miscible
with hydrocarbon mixtures and enhance flux through a membrane. More
preferred transport agents are alcohols. A low boiling transport
agent such as methanol is even more preferred. The quantity of
transport agent added to the hydrocarbon mixture is preferably
about 1% to about 20% by weight. Addition of about 10% by weight of
methanol is more preferred.
[0037] According to the present invention, membrane separation of
sulfur compounds from hydrocarbon mixtures involves the selective
permeation, or diffusion, of sulfur compounds through a membrane.
Generally, select diffusion of components of a mixture is
controlled by the affinity of the components for the membrane.
Components having greater affinity for the membrane permeate more
rapidly. Thus, in the present invention, membranes which have
affinity for, or preferentially permeate, sulfur compounds are
preferred. Membranes can be of any suitable composition, and
incorporate either or both inorganic and organic materials.
Membranes may also possess either ionic or non-ionic properties.
Ionic membranes generally contain charged chemical groups including
salts and acids, while non-ionic membranes contain neutral chemical
groups.
[0038] Preferred ionic membranes according to the present invention
include Nafion.RTM.-type acidic membranes, such as Nafion.RTM. 117,
that have been optionally treated by ion-exchange reactions or with
bases. Nafion.RTM. belongs to a class of solid superacids that
generally exhibit acid strength greater than 100% sulfuric acid.
Nafion.RTM. is strongly hydrophilic. Nafion.RTM. is preferred for
selectively permeating sulfur compounds which are generally more
polar than other components of petroleum fractions and other
hydrocarbon mixtures. Ion-exchanged Nafion.RTM. membranes, in which
the acidic protons are replaced by other cations, are also within
the scope of this invention. Examples of suitable cations include,
but are not limited to, inorganic ions such as silver, copper,
sodium, and organic ions such as tetraalkylammoniums and
tetraalkylphosphoniums. In another aspect of the present invention,
the Nafion.RTM.-type membranes may be treated with organic bases
including, but not limited to, triethanolamine and pyridine,
thereby forming organic salts. Nafion.RTM.-type membrane
modification by reaction with organic bases results in increased
selectivity for sulfur compounds over saturates and olefins.
[0039] Ionic membranes generally perform best in the presence of a
transport agent. For example, when a Nafion.RTM.-type membrane is
contacted with a transport agent, it swells from sorbtion of the
transport agent, changing the microstructure of the polymer such
that flux through the membrane is enhanced. Transport agents
preferably include alcohols, glycols, ethers or any other compounds
that are miscible with hydrocarbon mixtures, are sorbed by the
ionic membrane, and increase flux through the membrane.
[0040] Non-ionic membranes are also suitable for the present
invention. Preferred membranes are composed of hydrophilic
materials including, but not limited to, cellulose triacetate (CTA)
and polyvinylpyrrolidone (PVP). Hydrophilic properties generally
enhance the selective membrane permeation of sulfur compounds in
hydrocarbon mixtures. Furthermore, no transport agent is generally
required to observe reasonable levels of flux and selectivity when
using non-ionic membranes. In fact, the PVP and CTA membranes show
a surprising, but desirable, simultaneous increase in flux and
selectivity upon increasing temperature of feed. This result is in
contrast to what has been observed for hydrophobic membranes, such
as polyimides, under similar conditions which usually show a
decrease in selectivity and an increase in flux with increasing
temperature.
[0041] Other types of membranes include inorganic membranes
comprising ceramics, inorganic oxides, metal foils, or carbon.
[0042] The present invention encompasses a process for the
separation of sulfur compounds from hydrocarbon mixtures. According
to the process, a hydrocarbon mixture is divided into a sulfur-rich
fraction, i.e., sulfur-rich permeate, and a sulfur-lean fraction,
i.e., sulfur-lean retentate, using a membrane system. The
sulfur-rich fraction, or sulfur-rich permeate, corresponds to the
portion of the hydrocarbon mixture that diffused through the
membrane. The sulfur-lean fraction, or sulfur-lean retentate,
corresponds to the portion of the hydrocarbon mixture that did not
pass through the membrane. The hydrocarbon mixture treated by the
process is preferably light cracked naphtha (LCN); however, any oil
refining fraction or organic mixture contaminated with sulfur
compounds is suitable. The sulfur compounds in the hydrocarbon
mixtures may be in any concentration, but levels of from about 1
ppm to about 10,000 ppm are preferred, and levels of from about 10
ppm to about 4000 ppm are more preferred.
[0043] Incorporated into the membrane separation process is a
membrane system which separates sulfur compounds from hydrocarbon
mixtures. The membrane system can be single-staged comprising one
membrane module, or multi-staged comprising more than one membrane
module. Each module has at least two compartments, a first
compartment and a second compartment, separated by a membrane
assembly, the assembly preferably comprising a membrane, feed
spacers, and support material. The first compartment receives the
hydrocarbon mixture in liquid form while the second compartment
collects the portion of the hydrocarbon mixture that has permeated
through the membrane. The permeate is removed from the second
compartment to maintain a chemical gradient that drives the
transfer of sulfur compounds across the membrane.
[0044] Removal of the permeate is accomplished by either
perstraction or pervaporation. In perstraction, a liquid sweep
stream is passed through the second compartment of the membrane
module, preferably countercurrent to the direction of flow of the
hydrocarbon mixture in the first compartment. The permeate
dissolves into the sweep stream and is carried away by the flow,
thereby preventing the accumulation of preferentially permeated
components such as sulfur compounds. The sweep liquid preferably
has affinity for, and is miscible with, the permeated components.
Methanol is a preferred sweep liquid for membrane systems employing
Nafion.RTM.-type membranes as it would also serve as a transport
agent for enhancing flux and selectivity of the membrane.
[0045] Under pervaporative conditions, the permeate is removed from
the second compartment as a vapor. Thus a vacuum or lowered
pressure must be maintained in the second compartment such that
permeate will vaporize upon transfer across the membrane. The
driving force for transport across the membrane is the difference
in vapor pressure between the liquid hydrocarbon mixture and the
permeate partial pressure. Vaporized permeate can be subsequently
condensed with a chiller. The vapor is cooled and condensed to a
liquid and may be optionally heated prior to delivery to subsequent
membrane modules. A detailed discussion of perstraction and
pervaporation can be found in Membrane Handbook, W. S. Ho and K. K.
Sirkar, Eds., Chapman and Hall, 1992, herein incorporated by
reference.
[0046] According to the present invention, the permeate is enriched
in sulfur and corresponds to the sulfur-rich fraction. The
retentate is depleted in sulfur and corresponds to the sulfur-lean
fraction. In a multi-stage membrane system, the permeate of the
initial membrane module may be optionally treated by another
membrane module, and the permeate of that module further treated by
another, proceeding indefinitely until a desired sulfur
concentration is obtained in the permeate. The sulfur-lean
retentate, exiting the membrane system preferably contains about 1
ppm to about 300 ppm sulfur, more preferably about 1 ppm to about
100 ppm, and most preferably about 1 ppm to about 50 ppm. The
sulfur-lean fraction ideally can be used directly in fuel
formulation. Permeate can be combined with other sulfur-containing
hydrocarbon mixtures, such as heavy cracked naphtha (HCN), for
conventional removal of sulfur compounds by hydrotreating. The
hydrotreated stream can be optionally combined with the sulfur-lean
fraction for further refining or fuel formulation.
[0047] Membrane modules are of reasonable size and shape, including
hollow fibers, stretched flat sheet, or preferably, spiral-wound
envelopes. In the spiral-wound configuration (FIG. 3), the open
sides of membrane envelopes are positioned and sealed over a
permeate receptacle such as perforated piping. The envelopes are
spirally wrapped around the receptacle to minimize volume. Feed
spacers, such as, for example, plastic netting or nylon mesh,
separate the membrane envelopes to allow penetration of the
hydrocarbon mixture between the wrapped layers. The interior of
each membrane envelope is fitted with a permeate spacer to channel
permeate toward the receptacle. The permeate spacer is composed of
a material that is flexible, porous, and inert such as polyester.
The membrane preferably is a composite comprising a stiff but
flexible porous backing which is directed toward the inside of the
envelope. Backing materials are preferably resistant to organic
mixtures and include polyester, ceramic, glass, paper, plastic, or
cloth. Cushions composed of a flexible, inert material may flank
either side of the permeate spacer inside the membrane envelope and
contribute to structural integrity of the membrane assembly under
applied pressure.
[0048] The membrane itself preferably possesses certain qualities
to function effectively in a process for separating sulfur
compounds from hydrocarbon mixtures. In addition to selectivity for
sulfur compounds, desirable membrane qualities include resistance
to operative conditions such as thermal stress, sustained pressure,
and prolonged contact with organic chemical mixtures. Membrane
thickness may vary from about 0.1 microns to about 200 microns, but
thinner membranes are preferred for maximum flux such as, for
example, membranes having a thickness of about 0.1 microns to about
50 microns, or more preferably, about 0.1 microns to about 1
micron.
[0049] Preferred non-ionic membranes of the present invention are
fabricated according to a proprietary method developed by Membrane
Technology and Research, Inc. (MTR) of Menlo Park, Calif. MTR
membrane designs are disclosed in U.S. Pat. Nos. 4,931,181;
4,963,165; 4,990,255; and 5,085,776, which are also incorporated
herein by reference. These membranes are composite membranes
prepared in a two-step process. The first step involves the
deposition of a microporous support layer, comprising polysulfones,
polyimides, or polyamides, onto a flexible porous backing made of
an inert material (i.e., polyester fabric, ceramic, glass, paper,
plastic, or cotton). The second step involves coating the
microporous layer with a dilute solution of polymer, resulting in a
thin, defect-free, selectively permeable layer which is responsible
for the selectivity of the membrane.
[0050] Typical process conditions according to the present
invention depend on several variables including membrane separation
method (i.e., pervaporation vs. perstraction) and feed composition.
Determination of appropriate pervaporative and perstractive
operating conditions is well within the capabilities of one skilled
in the art. Some typical operating parameters for perstractive
processes of the present invention include feed flow rates of from
about 30 to about 50 gpm, absolute membrane flux of from about 0.5
to about 150 kg.multidot.m.sup.-2.multidot.D.sup.-1, feed
temperature of from about 20.degree. C. to about 300.degree. C.,
and negligible pressure drop across the membrane. Additionally,
some typical operating parameters for pervaporative processes of
the present invention include feed flow rates of from about 30 to
about 50 gpm, absolute membrane flux of from about 0.5 to about 150
kg.multidot.m.sup.-2.multido- t.D.sup.-1, feed temperature of from
about 20.degree. C. to about 300.degree. C., and lowered pressure
on the permeate side measuring from about 1 to about 80 mmHg.
[0051] Advantages of the present invention are numerous. The
separation of sulfur compounds from hydrocarbon mixtures such as
oil refining fractions allows the concentration of sulfur
contaminants such that a smaller total volume of liquid needs to be
processed by conventional hydrotreating. Additionally, selectivity
of the membrane for sulfur compounds over unsaturated hydrocarbons
results in a low olefin content in the sulfur-rich stream and
reduced octane loss and hydrogen consumption during the
hydrotreating process.
[0052] Those skilled in the art will appreciate that numerous
changes and modifications may be made to the preferred embodiments
of the present invention, and that such changes and modifications
may be made without departing from the spirit of the invention. It
is, therefore, intended that the appended claims cover all such
equivalent variations as fall within the true spirit and scope of
the present invention.
EXAMPLES
Example 1
[0053] Pervaporative Process for Reducing Sulfur Content in Light
Cracked Naphtha Using a Membrane System.
[0054] This process is outlined in FIG. 1. Light cracked naphtha
(LCN) originating from an FCC main column and containing 1990 ppm
of sulfur is fed at a rate of 35 gpm into a membrane system and
maintained at a pressure of 50 psig. The membrane system is
composed of one spiral-wrapped membrane module. The membrane
comprises a thin layer of polyvinylpyrrolidone (PVP), derived from
a 0.5% aqueous PVP solution, layered on porous
polyvinylidenefluoride. The permeate side of the membrane module is
held under vacuum at a pressure of 50 mm Hg. Sulfur-rich permeate
vapor exits the membrane system and is condensed with a chiller
operating at 80.degree. F. The liquid is combined with heavy
cracked naphtha (HCN) derived from the FCC main column, and the
mixture is processed by conventional hydrotreating. The sulfur-lean
retentate exiting the membrane system, containing about 120 ppm
sulfur, is combined with the hydrotreated fraction of similar
sulfur content. The combined streams are used directly in the
gasoline pool, diluted by two volume equivalents ultimately giving
a fuel product with 30 ppm sulfur content.
Example 2
[0055] Perstractive Process for Reducing Sulfur Content in Light
Cracked Naphtha Using a Membrane System.
[0056] Light cracked naphtha originating from an FCC main column
and containing 1700 ppm sulfur is fed at a rate of 35 gpm into a
membrane system at ambient pressure. The membrane system is
composed of one membrane module, and the membrane has a thickness
of about 50 microns and comprises Nafion.RTM. 117 supported on
woven polyester. A sweep stream composed of methanol is fed to the
permeate side of the membrane at a rate of 35 gpm at ambient
pressure. The sulfur-rich permeate, containing about 1 weight % of
sulfur, mixes with the methanol sweep stream. The resulting mixture
is fed to a distillation unit in which the methanol is removed from
the sulfur-rich permeate. The distilled sulfur-rich permeate is
combined with heavy cracked naphtha (HCN) from the FCC main column
and hydrotreated. Similarly, the sulfur-lean retentate, containing
about 150 ppm sulfur and 5 weight % methanol, exiting the membrane
system, is fed to a separate distillation unit in which the
methanol is removed. The resulting methanol-lean retentate fraction
is used in gasoline mixtures after combining with hydrotreated
HCN.
Example 3
[0057] Multi-Stage Pervaporative Process for Reducing Sulfur
Content in Light Cracked Naphtha (LCN) Using a Membrane System.
[0058] This process is outlined in FIG. 2. LCN originating from an
FCC main column containing 1880 ppm sulfur is fed to a membrane
system. The membrane system is composed of two membrane modules, a
first stage membrane module and a second stage membrane module.
Both modules are operated under pervaporative conditions and have a
membrane comprising cellulose triacetate (CTA) mounted on porous
polyvinylidenefluoride. LCN initially enters the first module on
the retentate side of the membrane and sulfur-rich permeate vapor,
containing about 0.5% by weight sulfur, is drawn out of the
permeate side. First stage permeate vapor is condensed with a
chiller and heated to a temperature of 120.degree. C. before
entering the second stage membrane module on the retentate side of
the membrane. As in the first stage module, the permeate from the
second stage module is drawn away as a vapor and condensed. Sulfur
content of the permeate from the second stage module is enriched to
0.93% by weight. The sulfur-rich permeate from the second stage
module is mixed with HCN (1 weight % sulfur) from the FCC main
column and hydrotreated. The hydrotreated mixture, containing about
150 ppm sulfur, is combined with retentate from both of the
membrane modules for a combined sulfur content of about 150
ppm.
* * * * *