U.S. patent application number 10/616053 was filed with the patent office on 2004-06-03 for use of membranes to separate organic liquids having different polarities.
Invention is credited to Altman, Lawrence J., Buchanan, J. Scott, Diehl, John W..
Application Number | 20040104170 10/616053 |
Document ID | / |
Family ID | 25399040 |
Filed Date | 2004-06-03 |
United States Patent
Application |
20040104170 |
Kind Code |
A1 |
Buchanan, J. Scott ; et
al. |
June 3, 2004 |
Use of membranes to separate organic liquids having different
polarities
Abstract
A method for separating at least one lower polarity fluid from a
mixture of fluids having varying polarity, comprising contacting at
least one low polarity or non-polar polymeric membrane with the
mixture of fluids under conditions such that the at least one lower
polarity fluid selectively permeates through the membrane, wherein
the membrane is one which has a ratio of heteroatoms chemically
bonded to the carbon atoms in the membrane to the number of carbon
atoms of less than about 0.2, preferably less than about 0.05.
Inventors: |
Buchanan, J. Scott;
(Lumbertville, NJ) ; Altman, Lawrence J.;
(Sarasota, FL) ; Diehl, John W.; (Paulsboro,
NJ) |
Correspondence
Address: |
Exxon Mobil Research and Enigineering Company
P.O. Box 900
Annandale
NY
08801-0900
US
|
Family ID: |
25399040 |
Appl. No.: |
10/616053 |
Filed: |
July 9, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10616053 |
Jul 9, 2003 |
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09891908 |
Jun 26, 2001 |
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6620958 |
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Current U.S.
Class: |
210/649 ;
210/650; 210/652; 210/653 |
Current CPC
Class: |
B01D 61/362 20130101;
C07C 29/76 20130101; Y02P 20/582 20151101; C07C 29/76 20130101;
C07C 68/08 20130101; B01D 61/246 20130101; C07C 68/08 20130101;
C07C 31/202 20130101; C07C 69/96 20130101; B01D 71/24 20130101;
C07B 63/00 20130101 |
Class at
Publication: |
210/649 ;
210/650; 210/652; 210/653 |
International
Class: |
B01D 061/00 |
Claims
What is claimed is:
1. A method for separating at least one lower polarity fluid from a
mixture of fluids having varying polarity, comprising: contacting
at least one low polarity or non-polar polymeric membrane with said
mixture comprising fluids of varying polarity under conditions such
that said at least one lower polarity fluid selectively permeates
through said membrane.
2. The method of claim 1, further comprising the step of eluting
said at least one lower polarity fluid which has permeated through
said membrane.
3. The method of claim 1, wherein said mixture comprising fluids of
varying polarity comprises dimethyl carbonate, ethylene glycol and
methanol, and said lower polarity fluid comprises dimethyl
carbonate.
4. The method of claim 1, wherein said membrane is an integral part
of a chemical reactor.
5. The method of claim 3, wherein said mixture comprising fluid of
varying polarity are formed via the reaction of ethylene carbonate
and methanol.
6. The method of claim 1, wherein two or more low polarity or
non-polar polymeric membranes are contacted by said mixture in
series, wherein the permeated liquid from one membrane contacts the
next adjacent membrane.
7. The method of claim 6, wherein said membranes have different
flux rates and different selectivity relative to the selectively
permeable fluid or fluids which contact each respective
membrane.
8. The method of claim 1, wherein said membrane further comprises a
porous support layer.
9. The method of claim 1, wherein said membrane is a composite
membrane comprising a plurality of polymeric layers.
10. The method of claim 1, wherein said lower polarity fluid
comprises hydrogen.
11. The method of claim 1, wherein said membrane is a synthetic or
naturally occurring latex membrane, wherein said synthetic latex
membrane is selected from the group consisting of: polyisoprene,
styrene-butadiene copolymers, neoprene and mixtures thereof.
12. The method of claim 1, wherein said membrane is one which has a
ratio of heteroatoms chemically bonded to the carbon atoms in said
membrane to the number of carbon atoms of less than about 0.2.
13. The method of claim 12, wherein said ratio is less than about
0.05.
14. A process for producing a dialkyl carbonate which comprises the
following steps: (a) reacting an alkanol with an alkylene
carbonate, thereby forming a product mixture comprising said
dialkyl carbonate, said alkanol and said alkylene carbonate; and
(b) separating at least a portion of said dialkyl carbonate from
said product mixture by contacting at least one low polarity or
non-polar polymeric membrane with said product mixture under
conditions which produce a permeate comprising said dialkyl
carbonate in a concentration higher than in said product mixture
from step (a).
15. The process of claim 14, wherein said product mixture further
comprises an alkylene glycol.
16. The process of claim 14, wherein said dialkyl carbonate is
dimethyl carbonate.
17. The process of claim 14, wherein said membrane is one which has
a ratio of heteroatoms chemically bonded to the carbon atoms in
said membrane to the number of carbon atoms of less than about
0.2.
18. The method of claim 17, wherein said ratio is less than about
0.05.
Description
[0001] The present invention relates to the separation of organic
liquids. More specifically it relates to a process for separating
organic liquids based upon their polarity utilizing a low polarity
or non-polar membrane.
BACKGROUND OF THE INVENTION
[0002] It is well known to separate mixtures of liquids by various
techniques including adsorption or distillation. These conventional
processes, however, generally have high capital costs. For example,
separating liquids by distillation requires expensive distillation
towers, heaters, heat exchangers, as well as a substantial amount
of auxiliary equipment, such as, pumps, collection vessels, vacuum
generating equipment, etc. Distillation operations also generally
have high operating costs associated with heating, cooling and
material transfer.
[0003] Additionally, the specific properties of the materials being
separated may warrant equipment or processing beyond that required
for simple distillation to complete the separation. For example,
when the mixture to be separated forms an azeotrope, the separation
may require a series of steps (e.g., use of two or more towers) or
by the addition of other materials to the separation system.
[0004] Adsorption systems also encounter comparable problems to
those associated with distillation.
[0005] Thus, it would be advantageous to be able to separate
mixtures of materials which are difficult or expensive to separate
by distillation or adsorption systems.
[0006] The use of membrane technology to separate mixtures which
are difficult to separate by distillation or adsorption are known
in the art and include the use of porous and non-porous membranes.
Non-porous membranes are used to separate mixtures of miscible
liquids by exploiting the differences in the rate of transport
through the membrane by means of a solution and diffusion
mechanism. Methods have been proposed which utilize membranes to
separate mixtures of organic substances or water/organic substance
mixtures through pervaporation, vapor permeation or perstraction.
Although each of these techniques rely upon a solution and
diffusion mechanism for transport through the membrane, the
operating parameters are quite different.
[0007] In the case of pervaporation, the liquid to be subjected to
separation is fed on one side of a membrane, while the pressure is
decreased or a carrier gas is passed on the other side of the
membrane to permeate the material to be separated in the form of a
gas through the membrane. Vapor permeation differs in that a vapor
of a mixture is fed on the one side of the membrane and the
material permeated through the membrane is recovered by cooling and
condensing the permeated vapor. Perstraction differs from
pervaporation in that the material to be separated is permeated
through the membrane as a liquid and the carrier stream is also a
liquid.
[0008] Examples of methods employing such membrane separations
include separation of organic substance/water mixtures using a
polymeric membrane having active anionic groups, separation of
ethanol/water mixtures using a cellulose acetate membrane or a
polyvinyl alcohol membrane, separation of organic substance/water
mixtures or organic substance mixtures using a poly acrylonitrile
copolymer membrane and separation of organic substance mixtures
using a cross-linked polyvinyl alcohol membrane.
[0009] U.S. Pat. No. 4,798,674 to Pasternak et al. describes a
method for concentrating a charge solution containing a
C.sub.1-C.sub.3 alcohol and an organic oxygenate selected from
organic ethers, aldehydes, ketones and esters through pervaporation
using a membrane of cross-linked polyvinyl alcohol and a high
molecular weight ion exchange resin in membrane form. The alcohol
permeates the membrane at a higher rate than the oxygenate, thus
concentrating the oxygenate.
[0010] The present inventors have unexpectedly discovered that a
liquid of reduced polarity relative to a mixture of liquids having
varying polarity can be selectively separated from the mixture
using a low polarity or non-polar, non-porous membrane.
SUMMARY OF THE INVENTION
[0011] The present invention is a method for separating at least
one lower polarity fluid from a mixture of fluids having varying
polarity.
[0012] A method for separating at least one lower polarity fluid
from a mixture of fluids having varying polarity, comprising
contacting at least one low polarity or non-polar polymeric
membrane with the mixture comprising fluids of varying polarity
under conditions such that the at least one lower polarity fluid
selectively permeates through the membrane, wherein the membrane is
one which has a ratio of heteroatoms chemically bonded to the
carbon atoms in the membrane to the number of carbon atoms of less
than about 0.2, preferably less than about 0.05. Preferably, the
method includes the step of eluting the at least one lower polarity
fluid which has permeated through the membrane. The eluting step
includes passing a solvent fluid over the side of the membrane
opposite to the side which is contacted with the mixture under
conditions such that the lower polarity fluid is carried away from
the membrane. The mixture typically comprises fluids of varying
polarity, e.g., dimethyl carbonate, ethylene glycol, and methanol,
and wherein the lower polarity fluid comprises dimethyl carbonate.
It is also desirable to use the method of the present invention
when hydrogen is the lower polarity fluid.
[0013] The present invention also includes the use of such a
membrane integral to a chemical reactor, wherein the mixture
comprising the fluid of varying polarity is formed via the reaction
of ethylene carbonate and methanol.
[0014] Optionally, two or more low polarity or non-polar polymeric
membranes are contacted by the mixture in series, wherein the
permeated liquid from one membrane contacts the next adjacent
membrane and so forth. The membranes preferably have different flux
rates and different selectivities relative to the selectively
permeable fluid or fluids which contact each respective membrane.
The membrane may further comprise a porous support layer and
typically is a composite membrane comprising a plurality of
polymeric layers.
[0015] The present invention also includes a process for producing
a dialkyl carbonate which comprises the following steps: (a)
reacting an alkanol with an alkylene carbonate, thereby forming a
product mixture comprising the dialkyl carbonate, the alkanol, the
alkylene carbonate and, optionally, an alkylene glycol; and (b)
separating at least a portion of the dialkyl carbonate from the
product mixture by contacting at least one low polarity or
non-polar polymeric membrane with the product mixture under
conditions which produce a permeate comprising the dialkyl
carbonate, preferably dimethyl carbonate.
[0016] Additional objects, advantages and novel features of the
invention will be set forth in part in the description and examples
which follow, and in part will become apparent to those skilled in
the art upon examination of the following, or may be learned by
practice of the invention. The objects and advantages of the
invention may be realized and attained by means of the
instrumentalities and combinations particularly pointed out in the
appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] As used herein and in the claims, the term "lower polarity"
when referring to a fluid (e.g., liquid and/or gas) means that the
fluid with lower polarity is of relatively lower polarity as
compared to at least one other fluid of higher polarity in a
mixture of fluids. For example, assume a fluid mixture contained
fluid 1, fluid 2, fluid 3, and fluid 4 and that each successively
listed fluid was of higher polarity than the preceding listed
fluids (i.e., as to polarity: fluid 1<fluid 2<fluid
3<fluid 4). Then each of fluids 1 through 3 could qualify as a
fluid of lower polarity, because at least one fluid 4 in the
mixture of four fluids is of a higher polarity than each of fluids
1 through 3.
[0018] The present invention is a method for separating a mixture
of organic fluids (e.g., liquids and/or gases) based upon their
relative polarity. More specifically, it is a method for
selectively separating a liquid or liquids of relatively lower
polarity from a mixture of liquids having varying polarity using a
low polarity or non-polar, non-porous polymeric membrane.
[0019] The process of the present invention is accordingly
suitable, for example, for the following separation tasks, i.e.,
the removal of dialkyl carbonates from alcohols, the removal of
dialkyl carbonates from diols, the removal of dialkyl carbonates
from alcohol/water mixtures or diol/water mixtures, the removal of
alkyl pyridines from pyridine, and the removal of esters from
reaction mixtures containing acids and alcohols as starting
materials. The process of the present invention may also be
applicable for separating hydrogen from gaseous mixtures, for
example, from syngas containing hydrogen, carbon monoxide, carbon
dioxide and methane.
[0020] Membranes that are useful according to the present invention
include those membranes made from polymeric materials which have
low polarity or non-polar. The membranes are preferably non-porous
polymeric membranes (i.e., non-porous in the sense of not
permitting macroscopic sized particles to pass therethrough). The
polymeric membranes may be synthetic membranes or they may be made
from naturally occurring polymeric materials, for example,
naturally occurring latex.
[0021] The suitability of a given low polarity or non-polar
membrane for use in the present invention may be determined by a
competitive diffusion test, in which a mixture of a polar species
and a non-polar species is allowed to diffuse through the membrane.
One such test is described below in Example 1. A relative
diffusivity (i.e., diffusivity of non-polar species divided by
diffusivity of polar species) greater than 1 is required, and
greater than 3 is preferred.
[0022] A membrane useful in the present invention is a low polarity
or non-polar latex-based membrane formed from natural latex found
in the Hevea brasilensis tree. Natural Hevea latex has been
described as a cytoplasmic system containing rubber and nonrubber
particles dispersed in an aqueous serum phase. Generally, Hevea
natural rubber contains about 93 to 95 wt % Cis-1-4-polyisoprene.
The nonrubber portion consists of moisture (0.30-1.0 wt %), acetone
extract (1.5-4.5 wt %), protein (20.-3.0 wt %) and ash (0.2-0.5 wt
%).
[0023] The double bonds in such natural rubber undergo the usual
chemical reactions, such as, addition, substitution and
epoxidation. Thus, the natural rubber can be treated or modified to
change its physical properties. For example, natural rubber can be
chlorinated to improve its resistance to chemical attack, reacted
with peracids to provide an epoxidized natural rubber which has
increased oil resistance and decreased air permeation, or
vulcanized (or crosslinked) to improve toughness over a greater
range of temperature. The membrane can also be a synthetic low
polarity or non-polar latex membrane.
[0024] The membrane can also be a synthetic low polarity or
non-polar polymeric based membrane, for example, polyisoprene,
styrene-butadiene copolymer, or neoprene. The synthetic low
polarity or non-polar polymeric membrane may also be composed of a
mixture of two or more polymers. The molecular structure of the
polymeric membrane will determine its relative polarity. Generally,
most unsubstituted aliphatic hydrocarbon or silicone polymers
and/or elastomers will have a relatively low polarity or non-polar
and will be suitable for use in the present invention, provided
that they otherwise exhibit the required physical
characteristics.
[0025] For carbon-based non-polar polymer membranes, it is
preferred that the ratio of heteroatoms, such as oxygen, nitrogen
or chlorine, chemically bonded to the carbon atoms in the membrane
polymer to the number of carbon atoms be less than 0.2 heteroatoms
per carbon atom, and more preferably less than 0.05. Thus, some
common condensation polymers, such as nylon 6,6 (polyamide) and
polyethylene terephthalate fall outside this preferred range of
heteroatom content.
[0026] The membranes according to the current invention may also be
composed of two or more polymeric layers to form a composite
membrane. Preferable, the composite membrane will have a first
outer side composed of a low polarity or non-polar polymeric
material to provide for the less polar fluid(s) to be separated
from the mixture within the first outer side of the membrane.
Preferably, the composite membrane will have an inner layer or
layers between the first outer side and second outer side of the
composite membrane. The inner layers should be of a suitable
material to allow diffusion of at least one of the non polar fluids
to be separated from the starting mixture of fluids. Optionally,
the inner layer may be chosen such that it will allow for diffusion
of less than all of the non polar fluids that diffuse into the
first outer side of the membrane. For example, the inner layer or
layers may be of sufficient number and/or thickness to allow for
preferential diffusion of a molecule of smaller diameter from a
mixture of fluids with equal polarity. The inner layer or layers
are preferably made of a polymeric material. The second outer layer
should be chosen to allow for diffusion of the low polarity or
non-polar fluid(s) out of the membrane into a second fluid or
mixture of fluids. The second outer layer is preferably made of
polymeric material.
[0027] The physical characteristics required for a given membrane
will depend upon the chemical composition, temperature and pressure
of both the permeate and raffinate phases in contact with the
membrane. Essentially the membrane must maintain its integrity
while providing the required separation performance for the
substance being separated.
[0028] The separation performance for a membrane in accordance with
the invention is governed by solution-diffusion processes.
Typically, a first fluid mixture (e.g., a gaseous and/or liquid
mixture) of materials having varying polarity is contacted with a
first face of a suitable low polarity or non-polar membrane and a
second solvent fluid is contacted with the second face of the
membrane. The membrane is characterized by permitting: (a) sorption
in the first face of at least one component of the fluid mixture,
e.g., the least polar material in the mixture; (b) diffusion of the
one component across the thickness of the membrane; and (c)
desorption of the one component from the second face into the
solvent fluid. A chemical potential gradient or concentration
gradient for the one component is then established across the
membrane, the potential or concentration in the first fluid mixture
being greater than in the second solvent fluid side.
[0029] The overall rate of migration of the one component from the
first fluid mixture to the second fluid is dependent upon, inter
alia, the following: (1) extent and rate of sorption of that one
component in the first face of the membrane; (2) rate of diffusion
of the one component through the membrane; and (3) extent and rate
of desorption of the one component out of the second face into the
solvent fluid.
[0030] If either extent or rate of sorption is low then the overall
migration rate of the one component will be low regardless of the
diffusion rate of the component in the membrane or desorption rate
into the solvent fluid. The extent and/or rate of sorption of the
one component may be low, for example, because its concentration in
the first fluid mixture is low, leading to low rate of transfer of
the one component to the first face. Also, the one component may
appreciably swell or plasticize the first face and in doing so
permit sorption of other components from the first fluid mixture
into the first face. Such swelling may be restrained by
incorporating crosslinks in the membrane, by blending (i.e.,
alloying) the material of the membrane with substances (e.g.,
polymers) which are not swollen by the one component, by adding
substances which reduce the affinity of the membrane for the one
component, and by inducing the formation of micro crystals in the
latex material of the membrane. Further, the characteristic
dimensions of the interstices in the surface region of the membrane
in the steady-state under operating conditions should generally be
of a size which discourages sorption of unwanted components.
[0031] Generally the rate of diffusion of the one component
increases as its diameter decreases. However, among components
having substantially the same diameter, those of greater length
(i.e., greater aspect ratio) will generally diffuse less rapidly.
Components having great lengths (e.g., polymers) may not diffuse at
all even though they have small diameters in the extended (e.g.,
solvated or diffused) form and appreciable sorption (so-called
"snake-cage" effect). The interstices in the material of the
membrane must be appreciably larger than the characteristic
dimension of the diffusing component (e.g., the one component). In
some instances the overall permeability can be quite high, even
though the extent or sorption is low, owing to exceptionally large
diffusion coefficients. In some processes (e.g., pervaporation) the
rate of desorption of sorbed components into the solvent fluid in
contact with the second face of the membrane can be so high and/or
the chemical potential of the desorbed components in the solvent
fluid so low that the second face is essentially free of sorbed
components. In such a case, the overall rate of migration of the
one component may be almost entirely determined by the slow
diffusion in the second face. Small molecules and/or molecules
having high affinity for the material of the membrane will then be
favored.
[0032] If either the extent or rate of desorption is low, then the
overall migration rate of the one component will be low regardless
of the diffusion rate of the component across the membrane or
sorption rate from the first fluid mixture into the first face. The
extent of desorption may be low because the chemical potential of
the one component in the solvent liquid is not sufficiently low
compared to the chemical potential in the second face of the
membrane. This may, for example, be due to lack of sufficient
diffusion and convection to remove the desorbed one component from
the vicinity of the second face. On the other hand, the chemical
potential of the one component in the second solvent fluid may be
so low that there is essentially no sorbed component(s) in the
second face resulting in very low diffusion rates in the face.
[0033] The overall rate of migration and selectivity will be
affected by the specific composition of the membrane and the
physical characteristics (e.g., polarity) of the polymer employed.
For example, the proportions of elastomer, fillers, softeners and
vulcanizing agents present in the compounded latex rubber can
affect the selectivity and rate of migration. The molecular weight
and viscosity of the polymer or elastomer, and the thickness of the
membrane, can also affect the rate of migration.
[0034] Preferably, the addition of fillers and softeners will be
avoided or minimized, so that the membrane will contain none or
only small amounts of such additives or impurities, because of the
negative influence on the permeability of the membrane. The
membrane will generally be vulcanized (or cross-linked) by heating
during the preparation process of the membrane or, optionally, by
the addition of a small amount of a vulcanizing or cross-linking
agent during the preparation process.
[0035] The low polarity or non-polar latex membrane of the current
invention can be prepared in the form of a film by any process
known in the art, such as, for example, casting or coating an
aqueous dispersion or emulsion followed by drying. Such an aqueous
dispersion will generally contain about 5 to 10 wt % elastomer,
e.g., polyisoprene, and a crosslinking or vulcanizing agent, e.g.,
sulfur or sulfur species.
[0036] A membrane useful in the present invention will preferably
include a non-porous layer of a suitable low polarity or non-polar
polymer having a thickness of about 0.1 to 15 mils, preferably 0.5
to 5 mils. Preferably, the non-porous layer will be incorporated
into a composite structure which contains a carrier layer, having a
high degree of porosity and mechanical strength. The carrier layer
can comprise a layer of any suitable material, such as, a fibrous
or non-fibrous, woven or non-woven cloth or mesh, a wire or metal
mesh, or glass fibers. The carrier layer can be any porous,
flexible, material which is compatible with the chemical system
being contacted and which provides sufficient mechanical properties
under the specific operating conditions.
[0037] The membrane can be of any configuration which prevents the
flow of liquid from one side of the membrane to the other by any
means other than through the membrane itself. Typical
configurations include any configuration known in the art, such as,
flat sheets or films, tubes or hollow fibers. Although the use of a
single membrane is typical, the use of a series of membranes having
different rates of permeation and selectivity is also contemplated.
Generally, when such a series of membranes are employed, the
mixture of liquids having varying polarity will be contacted
successively with the membranes so that the permeated liquid from
one membrane is contacted with the next membrane in succession.
Typically, the membranes will be arranged so that they are
contacted in order of decreasing rate of permeation and increasing
selectivity.
[0038] The process of the present invention is particularly useful
for separating organic liquids having varying polarities that are
difficult or costly to separate by other methods, such as,
distillation. For example, mixtures of liquids, such as, dimethyl
carbonate and methanol, are difficult to separate by distillation
because an azeotrope is formed. However, since dimethyl carbonate
is less polar than methanol and will selectively permeate through
the low polarity or non-polar membranes of the present invention at
a faster rate than the methanol, it can be selectively separated
from the mixture.
[0039] The present process can be carried out under pervaporation
conditions, in which the mixture of fluids (e.g., liquids or gases)
having varying polarity is contacted with one side of the low
polarity or non-polar, non-porous membrane. The less polar fluid to
be separated from the mixture absorbs into the membrane and
diffuses therethrough, as discussed above. The permeate side of the
membrane is maintained at a pressure which is lower than the vapor
pressure of the permeate. Preferably, the permeate side of the
membrane is maintained at a low pressure below about 10 mm Hg. The
permeate which passes through the membrane and exits as a vapor may
be recovered by condensing at low temperature or alternatively may
be swept away by use of a moving stream of gas. Examples of
separations under pervaporation conditions that are contemplated
include separating methane from a mixture of methane and water
vapor, and CO (or possibly CO.sub.2) from syngas.
[0040] The present process can also be carried out under
perstraction conditions, in which the mixture of liquids having
varying polarity is contacted with one side of the low polarity or
non-polar, non-porous membrane. The less polar liquid to be
separated from the mixture absorbs into the membrane and diffuses
therethrough. The permeate which passes through the membrane is
swept away with a liquid solvent stream. The solvent can generally
include any substance in which the permeated substance being
separated will dissolve into or readily mix with. Preferably, a
solvent will be chosen which can easily be separated from the
desired permeated liquid. Typical solvents can include, for
example, methanol, heptane, pentane, hexane, cyclohexane, or any
other non-reactive, low boiling organic solvent.
[0041] The process conditions of the present invention will vary
depending on the composition of the mixture to be separated and the
required performance criteria of the specific membrane, since the
temperature can effect the diffusion rate through the membrane and,
thus, may effect the overall rate and selectivity. For example,
dimethyl carbonate produced by the transesterification reaction of
ethylene carbonate with methanol can be separated from the reactor
effluent stream by a process according to the present invention by
contacting the mixture (e.g., reaction effluent) on a first side of
the membrane at temperatures up to about 260.degree. C. Although
the pressure is not critical, since the rate of permeation is
controlled by a solution/diffusion mechanism, contacting mixtures
having pressures up to about 5000 psia are contemplated, with
pressure differentials across the membrane up to 600 psi being
contemplated. The permeate side of the membrane will preferably be
maintained under a vacuum when operating under pervaporation
conditions.
[0042] The process of the present invention may find particular use
when the mixture of liquids having varying polarity is an effluent
stream from a reactor wherein one of the components to be separated
is a product of the reaction. An example of such an effluent stream
is that obtained from the reaction of methanol and ethylene
carbonate, wherein the effluent stream may contain unreacted
methanol, unreacted ethylene carbonate, product dimethyl carbonate
and product ethylene glycol, and wherein the product to be
separated is dimethyl carbonate.
[0043] It is contemplated that the effluent stream from such a
reactor may have been subjected to preliminary separation, e.g.,
distillation, to yield, for example, an azeotrope of methanol and
dimethyl carbonate.
[0044] Thus, in one embodiment, the process of the present
invention will be incorporated into the purification steps of a
chemical synthesis, e.g., dialkyl carbonate production.
[0045] In another embodiment, the process of the present invention
can be incorporated into the reactor itself. This will be
particularly useful in connection with an equilibrium reaction
wherein a reaction product is selectively withdrawn from the
reaction mixture. By withdrawing the reaction product, the
equilibrium can be shifted to increase yield and selectivity and
possibly reduce the amount of reactants or recycle to the reactor.
An example of such a reaction is the transesterification reaction
between an alkanol and an alkylene carbonate which produces dialkyl
carbonate and alkylene glycol.
[0046] The reactants to the transesterification reaction (e.g.,
ethylene carbonate and methanol) are typically contacted in the
presence of a transesterification catalyst. The transesterification
catalyst can typically include any homogeneous or heterogeneous
catalyst known in the art which provides adequate reaction
kinetics.
[0047] Examples of such catalysts include: alkali metals or
alkaline earth metals, such as, lithium, sodium, potassium,
rubidium, cesium, magnesium, calcium, strontium, barium and the
like; basic compounds such as hydrides, hydroxides, alkoxides,
aryloxides and amides of alkali metals or alkaline earth metals and
the like; basic compounds, such as, carbonates and
hydrogencarbonates of alkali metals or alkaline earth metal, alkali
metal or alkaline earth metal salts of organic acids and the like;
tertiary amines, such as, triethylamine, tributylamine,
trihexylamine, benzyldiethylamine and the like; nitrogen-containing
heteroaromatic compounds, such as, N-alkylpyrrole, N-alkylindole,
oxazole, N-alkylimidazole, N-alkylpyrazole, oxadiazole, pyridine,
alkylpyridine, quinoline, alkylquinoline, isoquinoline,
alkylisoquinoline, acridine, alkylacridine, phenanthroline,
alkylphenanthroline, pyrimidine, alkylpyrimidine, triazine,
alkyltriazine and the like; cyclic amidines, such as,
diazabicycloundecene (DBU), diazabicyclononene (DBN) and the like;
thallium compounds, such as thallium oxide, thallium halides,
thallium hydroxide, thallium carbonate, thallium nitrate, thallium
sulfate, thallium salts of organic acids and the like; tin
compounds, such as, tributylmethoxytin, tributylethoxytin,
dibutyldimethoxytin, diethyldiethoxytin, dibutyldiethoxytin,
dibutyldiphenoxytin, diphenyldimethoxytin, dibutyltin acetate,
tributyltin chloride, tin 2-ethylhexanoate and the like; zinc
compounds, such as, dimethoxyzinc, diethoxyzinc, ethylenedioxyzinc,
dibutoxyzinc and the like; aluminum compounds, such as, aluminum
trimethoxide, aluminum triisopropoxide, aluminum tributoxide and
the like; titanium compounds, such as, tetramethoxytitanium,
tetraethoxytitanium, tetrabutoxytitanium,
dichlorodimethoxytitanium, tetraisopropoxytitanium, titanium
acetate, titanium acetylacetonate and the like; phosphorus
compounds, such as, trimethylphosphine, triethylphosphine,
tributylphosphine, triphenylphosphine, tributylmethylphosphonium
halides, trioctylbutylphosphonium halides,
triphenylmethylphosphonium halides and the like; zirconium
compounds, such as, zirconium halides, zirconocenes, zirconium
acetylacetonate, zirconium alkoxides, zirconium acetate and the
like; lead and lead-containing compounds, such as, lead oxides,
e.g., PbO, PbO.sub.2, Pb.sub.3O.sub.4 and the like; lead sulfides,
such as, PbS, Pb.sub.2S.sub.3, PbS.sub.2 and the like; lead
hydroxides, such as, Pb(OH).sub.2, Pb.sub.3O.sub.2(OH).sub.2,
Pb.sub.2[PbO.sub.2(OH).sub.2], Pb.sub.2O(OH).sub.2 and the like;
plumbites, such as, Na.sub.2PbO.sub.2, K.sub.2PbO.sub.2,
NaHPbO.sub.2, KHPbO.sub.2 and the like; plumbates, such as,
Na.sub.2PbO.sub.3, Na.sub.2H.sub.2PbO.sub.4,
K.sub.2PbO.sub.3,K.sub.2- [Pb(OH).sub.6], K.sub.4PbO4,
Ca.sub.2PbO.sub.4, CaPbO.sub.3 and the like; lead carbonates and
basic salts thereof, such as, PbCO.sub.3, PbCO.sub.3.Pb(OH).sub.2
and the like; alkoxylead compounds and aryloxylead compounds, such
as Pb(OCH.sub.3).sub.2, (CH.sub.3O)Pb(OPh), Pb(OPh).sub.2 and the
like; lead salts of organic acids, and carbonates and basic salts
thereof, such as, Pb(OCOCH.sub.3).sub.2, Pb(OCOCH.sub.3).sub.4,
Pb(OCOCH.sub.3).sub.2.PbO.3H.sub.2O, and the like; organolead
compounds, such as, Bu.sub.4Pb, Ph.sub.4Pb, Bu.sub.3PbCl,
Ph.sub.3PbBr, Ph.sub.3Pb (or Ph.sub.6Pb.sub.2), Bu.sub.3PbOH,
Ph.sub.2PbO and the like wherein Bu represents a butyl group and Ph
represents a phenyl group; lead alloys, such as, Pb--Na, Pb--Ca,
Pb--Ba, Pb--Sn, Pb--Sb and the like; lead minerals, such as galena,
zinc blende and the like; hydrates of these lead compounds;
ion-exchangers, such as, anion-exchange resins having teriary amino
groups, amide groups, or at least one type of ion-exchange group
selected from the group consisting of sulfonate, carboxylate and
phosphate groups; strongly basic solid anion-exchangers having
quarternary ammonium groups as ion-exchange groups and the like;
solid inorganic compounds, such as, silica, silica-alumina,
silica-magnesia, aluminosilicate, gallium silicate, various types
of zeolites, various types of metal-exchanged zeolites,
ammonium-exchanged zeolites; and mixtures thereof.
[0048] Preferred homogeneous transesterification catalysts include
alcoholates and alkali hydroxides and carbonates, such as, sodium
methylate and sodium hydroxide. Preferred heterogeneous
transesterification catalysts include anion exchange resins having
tertiary amine, quaternary ammonium, sulfonic acid or carboxylic
acid functional groups, solid support catalysts containing alkaline
earth metal halides, such as, those described in U.S. Pat. No.
5,498,743, which is incorporated herein by reference, or inorganic
solid support catalysts alone, such as, alumina, pseudoboehmite,
MgO and MgO/Al.sub.2O.sub.3 hydrotalcites, or containing ions,
metals, compound or complexes of at least one element of Groups 1,
2, 4-10, 12 and 13-17 (IUPAC classification, previously Groups 1A,
2A, 4B-8B, 2B and 3A-7A) of the Periodic Table.
[0049] The catalyst can be utilized as a solid, as a solubilized
solid, or in liquid form with the preferred form being that of a
solubilized solid. A solid catalyst, such as, an alkali metal
carbonate or alkali metal halide can be solubilized in one or more
of the alkylene carbonate stream, alkanol stream or another stream
that may be conveyed to the reacted vessel. A solid catalyst may
also be employed in a fixed bed or ebullated bed arrangement or may
be fluidized in a manner so as to enhance the transesterification
reaction.
[0050] Transesterification reaction conditions generally comprise a
reaction temperature ranging from about 32.degree. F. (0.degree.
C.) to about 500.degree. F. (260.degree. C.), preferably from about
70.degree. F. (21.degree. C.) to about 400.degree. F. (204.degree.
C.), and more preferably from about 100.degree. F. (38.degree. C.)
to about 300.degree. F. (149.degree. C.). Excessively high
temperatures can result in the decomposition of the dialkyl
carbonate into undesirable thermolysis products, such as, carbon
dioxide and possibly reduced yield or selectivity of the membrane.
Exceedingly low temperatures can result in reduced alkylene
carbonate and alkanol conversion. Suitable reaction pressures
generally range from about 0 psig to about 5000 psig, preferably
from about 50 psig to about 1000 psig, and more preferably from
about 50 psig to about 500 psig. Excessively low pressures can
result in vaporization of the alkanol resulting in carryover of the
alkanol with the dialkyl carbonate-containing product.
[0051] In such a reaction, the latex-based low polarity or
non-polar, non-porous membrane, preferably a polyisoprene membrane,
will be incorporated into a transesterification reactor used to
react ethylene carbonate and methanol to produce dimethyl carbonate
and ethylene glycol. By utilizing such a membrane in accordance
with the present invention, the dimethyl carbonate product from the
reaction zone can be selectively separated and withdrawn from the
reaction mixture as permeate.
[0052] The raffinate stream will generally contain ethylene glycol,
a small amount of dimethyl carbonate, unreacted methanol and
unreacted ethylene carbonate. It may also contain homogeneous
transesterification catalyst, if used. The raffinate stream
containing the unreacted ethylene carbonate can be recycled to the
transesterification reactor or possibly directed to a hydrolysis
reactor for converting unconverted ethylene carbonate to ethylene
glycol.
[0053] The membrane separation process can be operated under
pervaporation or perstraction conditions. Under pervaporation
conditions, the permeate side of the membrane will typically be
maintained under a vacuum and a sweep stream of an inert gas, e.g.,
N.sub.2, can be used to sweep the vapor phase permeate stream,
which contains the dimethyl carbonate, away from the membrane.
Under perstraction conditions, a liquid solvent stream, e.g.,
heptane, will be employed to sweep the liquid phase permeate
stream, which contains the dimethyl carbonate, away from the
membrane.
[0054] In certain circumstances it may be beneficial to utilize a
solvent which will be used with the substance being separated. For
example, in the case of a permeate stream containing an alkyl
carbonate, which will be used as an oxygenate additive for
gasoline, a suitable gasoline blending component may be employed as
the sweep stream solvent, thus avoiding a separation step for the
alkyl carbonate and the solvent.
[0055] The examples set forth below are for illustration purposes
only. The scope of the present invention is not in any way limited
by the examples set forth below.
EXAMPLES
[0056] Static experiments were performed in which various mixtures
of organic substances were placed on one side of a low polarity or
non-polar latex membrane and a solvent was placed on the other side
of the membrane. The static system was maintained at about
70.degree. F. (21.degree. C.) and the composition on each side of
the membrane was measured by GC as a function of time.
Example 1
[0057] In this example, a mixture containing 3.72 grams of dimethyl
carbonate (DMC) and 3.25 grams of ethylene carbonate (EC) was
combined with 10 ml of methanol (MeOH). The combination was placed
inside a Trojan.RTM. brand non-lubricated latex condom, as
commercially available from Youngs Rubber Co., division of
Carter-Wallace, New York, N.Y. The condom was tied off and
suspended in a solvent of 200 ml of MeOH contained in a glass
beaker. The composition of the combination inside the condom (side
1) and of the solvent outside of the condom (side 2) was measured
by gas chromatography over a period of 24 hours. The results are
listed below in Table 1.
1TABLE 1 Results of Static Experiment For DMC and EC (in wt %) Time
(hours) DMC (side 1) EC (side 1) DMC (side 2) EC (side 2) 0 54 46
.25 51 49 100 0 .83 52.6 47.4 100 0 2.25 52 48 96.5 3.4 5.75 46.2
53.8 95 5 24 20 80 88.4 11.6
[0058] A review of Table 1 reveals that DMC diffuses through the
latex membrane faster than EC.
Example 2
[0059] In a static experiment similar to Example 1, 10 mls of a
reaction product from a zeolite catalyzed transterification
reaction, containing 9.3 wt % DMC, 50.1 wt % hydroxyethyl methyl
carbonate (HMC), 28.4 wt % EC and 12.2 wt % ethylene glycol (EG),
was placed inside the Trojan.RTM. brand latex condom (side 1) and
tied off. The condom was then suspended in 400 mls of MeOH (side
2). The composition of each side was measured by GC over a period
of 48 hours. On side 1 the composition was determined for DMC, HMC
and EC as wt % of the total mixture. The amount of EG as a function
of time was not recorded. On side 2 the composition was determined
for DMC, HMC and EC as wt % relative to each other. The results are
listed below in Table 2.
2TABLE 2 Results of static Experiment For DMC, HMC and EC (in wt %)
Time DMC HMC EC DMC HMC EC (hours) (side 1) (side 1) (side 1) (side
2) (side 2) (side 2) 0 9.3 50.1 28.4 1.3 86.5 5.4 8.1 3.75 7 53.8
27.1 77.6 11.1 11.3 21.5 2.6 55.3 29.2 68.5 13.6 17.9 28 1.2 57.8
28 62 18 20 48 57.9 18.1 23.8
[0060] A review of Table 2 reveals that DMC diffuses through the
latex membrane faster than HMC. Although the amount of EG was not
quantified in the table above, it was noticed that it primarily
remained on side 1 of the latex membrane demonstrating that DMC
diffuses through the latex membrane faster than EG.
Example 3
[0061] In a static experiment similar to Example 1, a mixture of
5.0 grams of DMC and 15 grams of MeOH was placed on side 1 of the
latex condom membrane and 200 mls of heptane was placed on side 2
of the latex condom membrane. The composition of each side was
measured by GC over a period of 20 hours. The results are listed
below in Table 3.
3TABLE 3 Results of Static Experiment For DMC and MeOH (in wt %)
Time (hours) DMC (side 1) M (side 1) DMC (side 2) M (side 2) 0 16.3
83.7 1.67 6.8 93.2 30.7 69.3 20 6.3 93.7 24.3 75.6
[0062] A review of Table 3 reveals that DMC diffuses through the
latex membrane faster than MeOH.
[0063] While we have shown and described several embodiments in
accordance with our invention, it is to be clearly understood that
the same are susceptible to numerous changes apparent to one
skilled in the art. Therefore, we do not wish to be limited to the
details shown and described but intend to show all changes and
modifications which come within the scope of the appended
claims.
* * * * *