Process for the separation of an organic liquid mixture

A process for the separation of a two component liquid mixture is described in which the liquid mixture is passed along one side of a membrane and an extracting agent is passed on the other side of the membrane, the process being characterized by the use of a membrane impermeable to at least one component of the liquid mixture and permeable to the extracting agent.

BACKGROUND OF THE INVENTION 
The invention relates to an improved process for the separation of a liquid 
mixture of two components by contacting the mixture with an extracting 
agent in which one component is soluble and the other component is 
insoluble or substantially insoluble, subsequently separating the second 
component from the extracting agent which contains the dissolved component 
and finally separating the dissolved component from the extracting agent. 
The process called extraction is well known, particularly for the 
separation of a mixture of two liquids. The insoluble component is called 
the raffinate, and the dissolved component is designated the extract. 
However, mixtures of a liquid and a solid material may also be separated 
in this way. The extracting agent is sometimes referred to as solvent, in 
particular when a single solid and a single liquid component are involved. 
Naturally, the mixture may comprise more than two materials, but, 
according to the present process, it is divided into two components. 
One drawback of extraction processes is the necessity of expenditure of 
large amounts of energy in the separation of the second component from the 
extracting agent. For example, if distillation is employed for the 
separation, large quantities of heat must be used. It is the object of the 
invention to reduce the cost of this separation. 
SUMMARY OF THE INVENTION 
Accordingly, the invention relates to a process for the separation of a 
liquid mixture of first and second components (A and B) by contacting the 
mixture with an extracting agent in which the second component (B) is 
soluble and the first component (A) is insoluble or substantially 
insoluble, subsequently separating the first component (A) from the 
extracting agent containing the dissolved second component (B), and 
finally separating the second component (B) from the extracting agent, the 
process being characterized in that a stream of the mixture of the first 
and second components (A and B) is passed along one side of a membrane, 
while a previously formed stream of the extracting agent containing the 
dissolved second component (B) is passed along the other side of the 
membrane, with the membrane being substantially permeable to the 
extracting agent but non-permeable to the first component (A), after which 
the mixture of the first and second components (A) and (B), which has 
taken up the extracting agent at least partly, is passed to a separator in 
which the first component (A) is separated from the extracting agent 
containing the dissolved second component (B), thus forming the stream 
that is passed along the other side of the membrane. For simplicity 
hereinafter, the first and second components will be designated as A and 
B, respectively, and, as indicated, the components may comprise mixtures 
themselves. 
In itself, the use of membranes is known, for instance in dialysis 
processes. However, in the known membrane processes, some component or 
impurity or other migrates through the membrane, whereas in the present 
invention, the solvent or extracting agent migrates through the membrane. 
In the case of a membrane along both sides of which a flow can be 
maintained, a liquid to be purified may be present on one side and a sweep 
liquid on the other side. In addition, the known membrane processes have 
been put to use almost exclusively in inorganic chemistry, for instance in 
the desalination of seawater, but the invention is particularly, though 
not exclusively, suitable for use in organic processes. The invention is 
particularly suited for extraction processes used in the petroleum 
industry for the separation of hydrocarbon mixtures. 
The invention, therefore, relates particularly to a process for the 
extraction of a hydrocarbon mixture by contacting it with an extracting 
agent, as described herein. 
Examples of such hydrocarbon extractions are given in the following Table. 
TABLE A 
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Name of Process 
Mixture Component A 
Component B 
Extracting Agent 
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deasphalting 
vacuum residue 
asphaltic 
deasphalted 
propane, butane 
bitumen oil 
furfural extraction 
(deasphalted) 
furfural furfural 
furfural 
residual oil, 
raffinate 
extract (lub 
spindle oil, oil) 
heavy cycle oil 
phenol extraction 
(deasphalted) 
phenol phenol phenol 
residual oil, 
raffinate 
extract 
spindle oil, 
heavy cycle oil 
SO.sub.2 extraction 
(deasphalted) 
SO.sub.2 raffinate 
SO.sub.2 extract 
liquid SO.sub.2 
residual oil 
spindle oil 
heavy cycle oil 
edeleanu extraction 
(unrefined) 
(refined) 
kerex liquid SO.sub.2 
kerosine fraction 
kerosine (aromatics) 
edeleanu extraction 
(unrefined) 
SO.sub.2 raffinate 
superbenzex 
liquid SO.sub.2 
gasoline fraction (aromatics) 
sulfolane extraction 
platformate 
sulfolane 
aromatic 
sulfolane 
raffinate 
extract 
glycol extraction 
platformate 
glycol aromatic 
diethylene 
raffinate 
extract 
glycol 
dewaxing waxy lubricating oil 
paraffinic wax 
dewaxed oil 
methylethyl 
ketone, propane 
10. 
urea dewaxing 
heavy gas oil, 
n-paraffin 
dewaxed 
dichloromethane 
fuel oil wax/urea adduct 
oil 
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Processes 9 and 10 of Table A relate to liquid-solid separations, the other 
processes are liquid-liquid extractions. Asphaltic bitumen (process 1) may 
at times be very tough, yet from a physical point of view it is a liquid. 
As indicated, the invention has the advantage that the extracting agent, 
which previously had to be separated from component B in a cumbersome and 
costly manner (for instance through distillation, crystallization or 
stripping) and subsequently reintroduced into the mixture to be separated, 
is now transferred in a simple way from component B, which has already 
been separated, to the mixture that has yet to be treated. Separation and 
reintroduction can be carried out at the same temperature and pressure, 
which means an important saving in energy cost. 
Depending on the conditions used, a larger or a smaller portion of the 
total quantity of extracting agent will be transferred through the 
membrane. Therefore, there will be certain cases in which one more final 
separation will be carried out, but this separation will always be carried 
out on a smaller scale and will therefore be less expensive than a process 
which is not preceded by the removal of extracting agent through a 
membrane. 
If the separator in which component A is separated from the stream of 
extracting agent and component B is not quite perfect, viz., if, upon 
leaving the separator, component A is still mixed with a (small) quantity 
of extracting agent, then a final separation of extracting agent and 
component A should still be carried out. In the processes used up till now 
that separation was usually carried out by distillation, crystallization 
or stripping, but also in these processes a membrane along both sides of 
which a flow can be maintained may be used, if desired, and thus the 
extracting agent may be transferred at least partly to the mixture to be 
separated. In this case the membrane to be used should be substantially 
impermeable to component B. However, in view of the relatively small 
quantities of extracting agent which in actual practice are generally 
yielded by component A, the saving achieved will be less here. 
The membrane is substantially permeable to the extracting agent and 
impermeable to component A. In one embodiment of the invention, the 
membrane is also substantially permeable to component B, which is 
advantageous when the concentration of component B is higher in the 
mixture of components A and B than in the stream containing the extracting 
agent. In that case the existing concentration gradient allows component B 
to diffuse from one side of the membrane to the other, which is just what 
is aimed at, viz. separation of components A and B. But if the 
concentration of component B in the stream containing the extracting agent 
is higher and the membrane is permeable to component B, care should be 
taken that this concentration is reduced, for instance by the addition of 
extracting agent. The latter, in other words, means a higher "solvent 
ratio". 
It is known that in membrane processes of the type according to the 
invention, concentration polarization may arise. As a consequence, in the 
present process, where the extracting agent diffuses from the "other side" 
to the "one side" of the membrane, a layer almost exclusively consisting 
of component B may form along the "other side" of the membrane and, 
similarly, a layer almost exclusively consisting of extracting agent may 
form along the "one side" of the membrane. Naturally, this local reversal 
of the concentration gradient has a restraining influence on the diffusion 
of the extracting agent. In order to prevent such concentration 
polarization, the streams are preferably pumped rapidly along both sides 
of the membrane and, in order not to use too large a membrane surface, the 
streams are recirculated. Thus, as homogeneous a mixture as possible will 
constantly be present both on one side and on the other side of the 
membrane. Pumping rates will preferably be so high that a molecule which 
can diffuse through the membrane will pass along the membrane some 10 to 
30 times before diffusing through it. 
This means, first, that part of the mixture of components A and B which has 
taken up the extracting agent at least partly, is preferably recycled and 
reintroduced into the stream of the mixture of components A and B 
(upstream of the membrane) and is thus passed along the first or "one" 
side of the membrane again. An additional advantage is that the mixture of 
components A and B is prediluted prior to being passed along the membrane. 
Particularly in the case of highly viscous liquids, such as residual oils, 
this is an advantage since this improves their pumpability and/or offers 
the opportunity of lowering the temperature. 
Secondly, this means that part of the stream of the extracting agent 
containing the dissolved component B, which is passed along the other side 
of the membrane, is preferably recycled and reintroduced into the stream 
of the extracting agent containing the dissolved component B, and is thus 
passed along the other side of the membrane again. 
Although in a number of cases a single membrane will be sufficient, there 
are many cases in which, in view of the imperfections of the commercially 
available units, the use of more than one membrane will be preferred, 
either for achieving a higher capacity (parallel arrangement), or deeper 
extraction (series arrangement). In the latter embodiment, a number of 
membranes are arranged in series, the stream of components A and B which 
has been passed along the first or "one side" of any of the membranes 
subsequently being passed along the first or "one side" of the following 
membrane, if present, and, similarly, the stream of the extracting agent 
containing the dissolved component B, which has been passed along the 
"other side" of any of the membranes subsequently being passed along the 
"other side" of the preceding membrane, if present. Obviously, if no 
following or preceding membrane is available, the stream is further 
treated as described hereinbefore on the subject of a single membrane. In 
other words, then the stream of components A and B is supplied to the 
separator and the other stream, now consisting almost entirely of pure 
component B, may optionally be subjected to a final purification treatment 
(e.g., stripping). 
When more than one membrane is used, the number of membranes will be 
dependent on the type of process, the supply rates per unit time and the 
size of the membranes, but preferably the number will be 2 to 20, in 
particular 4 to 10. 
The membrane material may be chosen from the materials known for the 
purpose, such as polyethylene, polypropylene, cellulose acetate, butyl 
rubber, methyl rubber, silicon rubber, polystyrene, 
polytetrafluoro-ethylene and other polymeric materials. The material 
should be insoluble both in components A and B and in the extracting 
agent, and it should also be totally or practically impermeable to 
component A and, on the other hand, permeable to the extracting agent. 
Fouling of the membrane by solid or viscous elements of the mixture of 
components A and B need not be feared, for probably owing to the membrane 
being continuously "rinsed" by diffusing extracting agent, the membrane 
remains clean in actual practice. 
The configuration in which the membrane material is used may be one of the 
membrane units along both sides of which a flow can be maintained, which 
in themselves are known, such as the flat sheet or the tubular membrane 
unit. However, such configurations are not very economical of space and 
therefore do not achieve a high packing density (m.sup.2 membrane/m.sup.3 
apparatus). 
Preference is given to the use of the spirally wound membrane which has 
been described in the applicant's copending U.S. application, Ser. No. 
471,104, now abandoned, entitled Spiral Wound Membrane, filed even date 
herewith, which application is incorporated herein by reference. The 
latter membrane combines the advantages, such as pressure resistance, low 
initial expense and high packing density, of the well-known spirally wound 
membranes along one side of which a flow can be maintained with the 
possibility of maintaining a flow on both sides.

DETAILED DESCRIPTION OF THE INVENTION 
In order to illustrate the invention more fully, reference is made to the 
accompanying schematic drawing. FIG. 1 represents a general flow diagram 
of the process according to the invention, and FIGS. 2 and 3 are 
elaborations thereof representing flow diagrams of a solvent dewaxing unit 
and a deasphalting unit, respectively. 
FIG. 1 schematically indicates the way in which a mixture to be separated 
(A+B) is fed to a membrane unit (M), where extracting agent (E) joins the 
stream of the mixture to be separated. The resulting stream is then fed to 
a separator (S), where component A is separated off. The remainder, i.e., 
component B and the extracting agent, is recirculated to the membrane unit 
(M) in order to transfer the extracting agent to a fresh supply of mixture 
to be separated. If the membrane is permeable to component B and if the 
concentration of component B is higher in mixture (A+B) than in mixture 
(E+B), there will also be migration of component B along the membrane in 
the direction opposite to that of the extracting agent (dashed line B'). 
Disregarding many details, FIG. 2 illustrates a flow diagram of a unit for 
dewaxing with the aid of a solvent, which, according to the invention, 
includes a membrane unit. The feed, for instance a waxy furfural 
raffinate, is supplied through line (1) to a membrane unit (2), where 
solvent (extracting agent) joins the feed through membrane (3). As solvent 
a mixture of aromatics (benzene, toluene, etc.) and methyl ethyl ketone 
may be used. Through conduits (4) and (5) the feed and the solvent are 
transferred further via a heat exchanger (6) and a cooler (7), and then to 
a vacuum rotary drum filter (8). In cooler (7), the feed and the solvent 
are cooled down to a temperature of about -20.degree. C. in order to allow 
the paraffins present therein to crystallize. The paraffin crystals are 
washed on the drum of drum filter (8) using a thin stream of solvent from 
line (9), subsequently scraped off, and, together with a small quantity of 
solvent still present, carried off through conduit (10) to a 
wax-processing unit (11). In unit (11), what is called "slack wax" is 
separated via line (12) from the solvent, and the solvent is fed through 
conduit (13) to a recirculation line (14). The filtrate from the vacuum 
rotary drum filter (8), which consists of dewaxed oil and solvent, is 
passed through a conduit (15) via heat exchanger (6) and fed to membrane 
unit (2). Both in heat exchanger (6) and in membrane unit (2) the 
feedstock is pre-cooled to some extent by its indirect contact with the 
cold filtrate. After a considerable part of the solvent has diffused 
through the membrane (3), the dewaxed oil, together with the remainder of 
the solvent, is fed through conduit (16) to a unit (17) for the processing 
of dewaxed oil. In reality, unit (17) will generally comprise two flashing 
columns, one operated at low pressure and temperature, and one at higher 
pressure and temperature, followed by a steam stripper, in conjunction 
with distillation columns for the removal of water from the solvent and a 
number of pumps, burners and reflux pipes, which together have been 
illustrated as unit (17). The required heat is supplied by a heat 
exchanger (18). Dewaxed oil is in the end separated via line 19, and the 
solvent is then passed through conduits (20) and (14), via a cooler (21), 
and reintroduced into the system, that is to say, the feedstock present in 
line 5. 
FIG. 3 is an illustration of a flow diagram of a deasphalting unit. For the 
sake of simplicity, most of the ancillary equipment known in itself and 
not essential to the invention has been left out of this illustration as 
well. 
More particularly, the feedstock, usually a vacuum residue, is fed through 
conduit (31) to a mixing vessel (32), where the feed is prediluted with a 
stream of feed already diluted with extracting agent leaving a conduit 
(33). Mixing vessel (32) also acts as a buffer vessel so as to eliminate 
changes in the supply. From mixing vessel (32) there is a continuous 
discharge, through conduit (34), of a stream part of which is fed, through 
a conduit (101) to a membrane unit (100) and the remaining part through 
conduits (106) and (201) to a membrane unit (200). In these membrane 
units, a certain amount of extracting agent, represented as streams (102) 
and (202), respectively, diffuses from a stream of deasphalted oil and 
solvent, supplied via conduits (103) and (203), respectively, through the 
membrane into the stream of the prediluted feed which has been supplied 
through conduits (101) and (201), respectively, and is discharged through 
conduits (104) and (204), respectively. Conduit (104) is connected with 
conduit (33) which leads to the mixing vessel and conduit (204) 
recirculates part of the diluted feed to membrane unit (200) through a 
conduit (205) and conduit (201), while the rest of the diluted feed is fed 
to a next membrane unit through a conduit (206). The stream of deasphalted 
oil with extracting agent, which has released part of the extracting agent 
in membrane units (100) and (200), respectively, is discharged through 
conduits (107) and (207), respectively. Part of it is recirculated to the 
original membrane unit through conduits (108) and (103) and (208) and 
(203), respectively, while the remainder is discharged through conduits 
(109) and (209), respectively. Conduit (209) is connected with the 
preceding membrane unit by conduit (103), while conduit (109) is connected 
with a stripper unit (35), in which the last residues of extracting agent 
are removed via line 36 from the stream of deasphalted oil (37) with the 
aid of steam. 
The number of membrane units may vary, but in the embodiment described here 
the number is 7. For convenience, units (300), (400) and (500) have been 
left out of the figure; only units (100), (200), (600), and (700) have 
been shown. Corresponding numbers indicate corresponding parts. For 
instance, conduit (601) corresponds with conduits (201) and (101). The 
part of the stream of feed with extracting agent which is discharged 
through conduit (706) is fed to what is called a rotating-disc-contactor 
(38), where this stream is split up into an asphaltic bitumen fraction 
which is discharged through a conduit (39) and a fraction of deasphalted 
oil with extracting agent which is run through a conduit (40) into conduit 
(703), with the object of removing the solvent in membrane unit (700), and 
subsequently in units (600), (500), etc. The asphaltic bitumen fraction is 
freed from remaining extracting agent, if any, in a stripper (41), so that 
in the end a quantity of asphaltic bitumen can be discharged through 
conduit (42) and a quantity of extracting agent through conduit (43). Both 
the streams of conduits (43) and (36) are recirculated through a conduit 
(44) to the rotating-disc-contactor (38) so as to enable all the 
extracting agent to be mixed with the feed. 
The generation of steam and the subsequent separation of steam and 
extracting agent (e.g., propane) in stripping unit (35) will require a 
certain amount of energy which will be greater in proportion as the 
fraction of extracting agent in the deasphalted oil is larger. According 
to the invention, this fraction is small, since as much extracting agent 
as possible is transferred to the fresh feed via membrane units (100), 
(200) and so on. When a special membrane material, such as polypropylene, 
is used, there will in addition be a considerable part of deasphalted oil 
that is transferred direct from streams (101), (201), etc. into streams 
(107), (207), etc. via membranes (100), (200), etc., which results in 
additional savings on the overall cost. 
Although the transfer of extracting agent illustrated in this figure is 
countercurrent, this transfer may also take place co-currently. 
EXAMPLE 
For further elucidation of the process according to the invention, the 
results will now be given of an experiment in a dewaxing unit as 
corresponding to that of FIG. 2. Using a polypropylenepolyethylene 
copolymer membrane of 1 .mu.m in thickness, in the spiralled configuration 
described in my copending application, almost 74% of the solvent (50% v 
methyl ethyl ketone, 50% v aromatics) present in stream (15) is 
transferred from there to stream (4). Table B gives a summary of the 
effect this has on the processing of the dewaxed oil. All the quantities 
given are expressed in tons per day (tpd). 
TABLE B 
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Stream Number 
Membrane 
Composition 15 3 16 19 20 
______________________________________ 
without dewaxed oil (tpd) 
299 -- 299 299 -- 
solvent (tpd) 
1714 -- 1714 -- 1714 
with dewaxed oil (tpd) 
299 -- 299 299 -- 
solvent (tpd) 
1714 1264 450 -- 450 
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It is clear, that according to the invention, stream (16) contains much 
less solvent than it does in the embodiment in which no membrane is used 
and, that, consequently, less solvent need be flashed and stripped in 
processing unit (17). Therefore, less heat is required for the removal of 
the solvent from the dewaxed oil (2080 ton cal/h instead of 5220 ton 
cal/h).