Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MULTI-STAGE PERVAPORATION PROCESS RUN AT
PROGRESSIVELY HIGHER VACUUM, HIGHER TEMPERATURE OR
BOTH AT EACH SUCCESSIVE RETENTATE STAGE
Brief Description of the Invention
The present invention is directed to a
multi-stage pervaporation process wherein each succes-
sive stage, which may contain one membrane unit or
multiple membrane units run either in series or in
parallel is run at a progressively stronger vacuum, a
progressively higher temperature or both than the
preceding stage. This process allows a multi component
mixture to be membrane separated into multiple frac-
tions of different molecular type and of different
boiling point. In the process, the Eeed is initially
heated to an appropriate temperature and fed to a
membrane pervaporation stage operated at low (i.e.,
weak) vacuum in order to produce a permeate and a
retentate. The retentate, either as is or with any
additionally needed heating to replace the lost heat of
evaporation from the first stage (i.e. at isothermal
conditions) or alternatively heated to a higher temper-
ature than the initial temperature, is then fed to a
second pervaporation stage operated at a higher vacuum
than stage 1 to yield a second permeate and a second
retentateO Alternatively if the retentate from each
stage prior to being sent to the next succeeding stage
is heated to a temperature higher than that of the
preceding stage, the vacuum employed in the succeeding
stage can be the same as that of the preceding stage,
that is, the process can be run at isobaric conditions.
Preferably both the temperature and the pressure are
increased in each succeeding stage.
Any number of stages (n) can be employed with
the retentate from stage n-1 being the feed to stage n
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run at a higher vacuum, a higher temperature or both
than that used at stage n-l. The lighter, lower
boiling components are thus recovered as permeate in
the initial stages, and the heavier components are
recovered in the later stages. The maximum operating
temperature and vacuum are set with due regard paid to
the nature of the heaviest component to be recovered as
well as the mechanical limits of the membranes used in
the pervaporation stages.
sackqround of the Invention
The separation of components from mixtures
using membranes has become an accepted form of separa-
tions technology in the literature. Processes such as
reverse osmosis, dialysis, ultrafiltration, pervapora-
tion, perstraction, forward osmosis, etc. have been
described. The use of multiple membrane stages in
membrane separation processes has also been described.
Thus, in ~.S. Patent 2,923,749, a process for
separating organic compounds is described - a process
especially useful for separating aromatics from non-
-aromatics. In the process, the ~eed is sent to a
membrane separation zone preferably operated under a
vacuum (pervaporation) to yield a permeate and a
retentate. A number of permeation zones may be used.
When the concentration of the co~ponents thersin is
suitable, permeate and retentate may be recycled to the
various stages.
U.S. Patent 2,947,687 describes hydrocarbon
separation by the use of a permeation membrane operated
under perstraction or pervaporation conditions~ By
using a sweep gas or by maintaining the permeate zone
at sub atmospheric pressure, permeate is removed from
the permeation zone. Retentate from one separation
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stage can be used as the feed to subsequent membrane
separation zones. In the Example each permeation stage
is maintained at the same vacuum level - about 100 mm
Hg abs. Each successive stage is employed to remove
additional quantities of the same component. See also
U.S. Patent 3,043.891.
U.S. Patent 2,985,588 describes a membrane
permeation separation technique which provides trouble
free start-up and shut down procedures. In the process
the permeation temperature is maintained between the
softening point transition temperature and not higher
than 20C above the first order transition temperature
displayed by the plastic membrane; the mixture of
molecules in the feed zone is maintained in the liquid
state; the mixture of molecules in the permeate zone is
maintained in the vapor state; the absolute pressure in
the permeate zone is maintained at less than one-half
te.g., one-tenth~ of the vapor pressure normally
exerted by the mixture in the permeate zone. In the
Example supporting the description of the figures a
hydrocarbon mixture containing 50% benzene is sent to a
membrane unit at about 90C. The retentate containing
about 23% benzene (the remainder being other hydrocar-
bons) is sent to a second membrane unit at about 110C.
This higher temperature can be employed because of the
lower concentration of benzene present in the feed to
the second membrane unit. The retentate from this unit
containing about 7.5% benzene (the remainder being
other hydrocarbons) is sent to a third membrane unit
after being heated to about 120~C. This higher temper-
ature is permissible because of the lower concentration
of benzene present in the feed to thi third membrane
unit.
In this patent temperature staging is used to
separate benzene from a mixture of benzene with other
2~58S~i
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hydrocarbons. The patent does not show that tempera-
ture staging can be used to fractionate a multi-compo-
nent feed mixture into numerous fractions of different
molecular types and different boiling points, the
earlier stages containing a majority of the lighter
simpler, lower boiling components and the later stages
containing the heavier more complex higher boiling
components.
Brief Description of the Fi~ure
Figure 1 is a schematic of an embodiment of
the present invention containing multiple membrane
pervaporation stages.
Figure 2 is a schematic of an embodiment of
the present invention functioning as a multi-staged
membrane distillation unit.
The Present Invention
The present invention is a process for
separating feed streams containing multiple components.
The process employs a multi-membrane staged pervapora-
tion scheme wherein each stage may contain one membrane
unit or multiple membrane units run either in series or
in parallel and wherein each membrane stage in series
is run at a progressively stronger vacuum, higher
temperature or both. The process is especially useful
for separating components from wide boiling range
mixtures.
In the process of the present invention, a
multi-component feed is fed to a first membrane stage.
The feed is heated to a temperature appropriate for the
lightest, lowest boiling component of the feed. The
first membrane pervaporation stage is run at a low
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vacuum. A first permeate of light, lower boiling
material is recovered, and a retentate is produced.
This retentate is fed to a second membrane pervapora-
tion stage which i5 run at a higher temperature, a
stronger vacuum or both than the preceding stage. In
this second stage a second permeate is recovered which
is heavier and higher boiling than the permeate of the
preceding stage. A second retentate is also produced
which is fed to a third membrane pervaporation stage
which is run at a still higher temperature, a higher
vacuum or both than the preceding secsnd stage. In
each of these retentate cascade steps, when run at
successively higher vacuum, the retentate can be fed to
the next succeeding stage as it comes from the preced-
ing stage or it can be at least subjected to interstage
reheat to make up for heat lost due to evaporation of
the permeate in the preceding stages such conditions
being called isothermal conditions. Alternatively, the
retentate can be fed to the next succeeding stage at a
temperature higher than the preceding stage but the
succeeding stage is run at the same vacuum pressure as
the preceding stage. Optimally, the amount of heat
supplied in each heating/reheating is enough to evapo-
rate the desired permeate for that particular stage and
the heating is, therefore, to a temperature higher than
in the preceding zone while each stage is run at a
successively greater vacuum than the preceding stage.
The maximum amount of heat which can be employed is
determined by the mechanical maximum operating tempera-
ture of the membrane at the vacuum employed at the
particular stage running at this temperature.
The process of the preeant invention may
employ as many membrane pervaporation stages as there
are recoverable components or fractions in the initial
feed. The only requirement i8 that the temperature,
vacuum or both used in stage n be greater than the
vacuum in the preceding stage n-1. Different me~branes
can be employed in each stage in order to obtain the
optimum performance.
Because each stage is run using only as much
vacuum, heat or both as necessary to pervaporate a
particular component or narrow range of components, the
condensers associated with each stage can be scaled and
powered accordingly. Thus, in the initial stages run
at minimal heat and low vacuum, the permeate condensers
can use cooling water. ~he later stages, run at higher
temperatures, successively higher vacuums or both can
still use cheap cooling water condensers because the
heavier permeated components are more easily condensed
than the lighter ones evaporated in the earlier stages.
This scheme, wherein only cheap cooling water condens-
ers are needed is much more economical than current
schemes which use single pervaporation units to perform
broad cut separations of feeds and therefore require
expensive refrigerated condensers to condense the
lighter components.
The process of the present invention is
useful for separating complex multi component feeds,
especially those having a wide boiling range. It has
special utility in those instances where different
components of a stream possess approximately the same
boiling point so traditional distillation cannot be
used. Thus, for example, in fractionating catalytic
naphthas containing mixtures o~ aromatics and non-aro-
matics, the present process can be used to recover
number of fractions of aromatics as permeates boiling
at different temperatures. Non-aromatics boiling at
substantially the same temperature as the aromatics are
not permeated and remain in the retentate ~ractions.
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Because the pervaporation process of the
present invention can be used to separate feeds on the
basis of both molecular type and boiling point, the
present pervaporation process combines the benefit of
both distillation and membrane separation in a single
process. One process can be used in place of two,
thereby simplifying material handlings and energy
requirements.
In practicing the process of the present
invention the different pervaporation units may employ
the same or different membranes depending on the
separation desired at each stage.
In separating aromatics from non aromatics
various membranes can be employed. U.S. Patent
2,923,749, 2,947,687, 3,043,891 and 2,985,588 suggest
the use of various cellulose esters, cellulose ethers,
and mixed cellulose esters and ethers and irradiated
polyethylene as useable membranes. The membranes can
be used 2S such or they can have their permeability
increased by using various diluents or feed additives
such as oxygenated hydrocarbons, halogenated hydrocar-
bons, ~ulfur containing organic compounds, nitrogen
containing organic compounds etc. U.S. Patent
4,115,465 teaches polyurethane membranes for use in the
pervaporative separation of aromatics from various non
aromatic compounds.
Recently polyurea-urethane membranes have
been described as useful for aromatic-non-aromatic
separations under pervaporation conditions. The
membranes are described in copending application USSN
108,821 and USSN 108,822, both assigned to the same
assignee as the present invention.
a
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USSN 108,822 teaches a particular polyurea-
urethane membrane for aromatic-non aromatic separation.
The membrane is made from a polyurea-urethane copolymer
characterized by possessing a urea index of at least
20% but less than 100%, an aromatic carbon content of
at least about 15 mole %, a functional group density of
at least about 10 per 1000 grams of polymer and a
C=0/NH ratio of less than 8Ø Such polyurea-urethane
membranes are preferably anisotropic in nature. The
production of such anisotropic membranes is taught in
USSN 108,821. The membranes are prepared from the
corresponding co-polymer by synthesizing the copolymer
in a good solvent containing less than 5 vol% non-
solvent, producing a film of this polymer solution on a
support with a maximum pore size less than 20 microns,
subjecting the film to conditions such that the solvent
vapor pressure-time factor is 1000 mm Hg min. or less
and quenching the membrane film in a non-solvent
yielding the anisotropic membrane. Pervaporation
processes preferably use membranes in the form of
hollow fibers. Hollow fibers of the above described
polyurea-urethane polymer can be made by using hollow
fibers of regenerated cellulose as the support on which
the polymer is deposited, subjecting the ~ilm to
conditions resulting in the proper solvent vapor
pressure-time factor, preferably immediately quenching
the fiber in a non~-solvent.
The invention will be better understood by
reference to the Figure 1 which presents a particular,
non limiting embodiment.
The multi component feed to be separated into
selected components is passed through heater (1) and
heated to a temperature Tl sufficient to provide the
necessary heat to evaporate the desired boiling range
permeate. The heated feed is passed via line (2) to
the first pervaporative membrane unit (3) where it is
subject to a low vacuum sufficient to draw off the
light, low boiling permeate. The vacuum is produced by
vacuum pump (A). The amount of vacuum applied to each
membrane unit is controlled by valves (V1 and V2 ...VN
shown in the figure). This permeate is passed through
line (4) to chiller (5) where it is condensed and sent
to permeate storage vessel (6). This permeate can be
sent via line (7) to further processing, treatment or
blending units (not shown). The retentate from mem-
brane unit 3 is passed via line (8) to the second
membrane unit (9). The retentate in line 8 can be sent
to the second membrane unit (9) directly or it can
first be passed through a heater (15) wherein heat 105t
in stage 1 membrane unit to evaporation is replaced or
the retentate is heated to a temperature T2 sufficient
to provide the heat of evaporation of the next light-
est, higher boiling component present in the retentate.
In stage 2 membrane unit (9) the retentate from stage l
is subjected to the same or a higher vacuum than that
in stage 1 membrane unit 3 producing a second permeate
passed via line (10) to cooler (11) for storing in
permeate storage vessel (12). This permeate can be
passed via line (13) to further processing, treatment
or blending units ~not shown).
The retentate from stage 2 membrane unit (9)
is passed via line (14) through a series of stages
until the highest desired boiling component is recov-
ered. The retentate from heater n is passed to stage n
membrane uni~ wherein it is subjected to the highest
vacuum used in the process to produce a final permeate
stream (16) passed through chiller (~7) and sent to
permeate vessel (18~. The retentate, containing the
non-permeating species of the feed is sent via line 19
for treatment, processing, blending etc. not shown.
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Figure 2 describes a multi-stage membrane
distillation unit in which molecules can be separated
by boiling point as well as molecular type. A multi-
component feed to be separated into selected components
is passed through heater (1) and heated to a tempera-
ture Tl sufficient to provide the necessary heat to
evaporate the desired boiling range permeate. The
heated feed is passed via line (2) to the first pervap-
orative membrane unit (3) in stage 1 housing membrane A
which selectively permeates type A molecules. This
permeate is passed through line (4) to chiller (5)
where it is condensed and sent to permeate storage
vessel (6). This permeate can be sent via line (7) to
further processing, treatment or blending units (not
shown). The retentate from membrane unit (3) is sent
via line (8) to the second membrane unit (9) in stage 1
which houses membrane B which selectively permeates
type B molecules. A heater (not shown) can be employed
prior to unit 9 if required. The permeate from unit 9
is passed through line (10) to chiller (11) where it is
condensed and sent to perm~ate storage vessel (13).
This permeate can be sent via line (14) to further
processing, treatment or blending units. ~ny number of
membrane units housing different membranes which are
selective to different molecular types can be employed.
All the membrane units in stage 1 are subjected to a
vacuum sufficient to draw off the light low boiling
permeate. The vacuum is produced by a vacuum generat-
ing system (i.e., vacuum pump, steam ejector).
The retentate from the stage 1 membrane units
is passed via line 15 through a series of stages until
the highest desired boiling component is recovered.
The retentate from heater 16 is passed to membrane unit
17 in stage n wherein it iB subjected to the highest
vacuum used in the process to produce a final permeate
stream (18) passed through chiller (19j and sent to
permeate vessel (21). This permeate can be sent via
line (22) to further processing, treatment or blending
units. All membrane units are subjected to a vacuum
produced by a vacuum pump, the vacuum being delivered
to the membrane units via lines 12 and 20 and regulated
by means of valve VI. The retentate~ containing the
non-permeating species of the feed to membrane unit 17
is sent via line 23 for further treatment or process-
ing.
