Membrane process for separation of fluid mixtures

A process for the separation of liquid component mixtures by combined pervaporation-vapor permeation with a feed flow in the form of a vapor-liquid mixture being employed and the feed flow being passed from bottom to top as a vapor-liquid mixture over a vertically arranged membrane, so that a permanent thorough mixing of vapor and liquid results, and no stationary liquid film is able to form on the surface of the membrane.

The present invention relates to a process for separation of fluid mixtures 
by means of pervaporation and vapor permeation. 
Processes for the separation of fluid mixtures by means of pervaporation 
and vapor permeation are known to those skilled in the art. The fluid 
mixture to be separated, the feed flow, is brought into contact with the 
first side, the feed side, of a non-porous (pore-free) membrane, which has 
a preferred permeability for at least one of the components of the fluid 
mixture. 
This component is transported through the membrane if a lower partial 
pressure of said component is provided by appropriate devices on the 
second side of the membrane, the permeate side, than on the feed side. 
Generally, the reduction of partial pressure on the permeate side occurs 
by applying a vacuum, with the permeate being obtained in a vaporous form 
and being sucked off or condensed, but other possibilities of maintaining 
a partial pressure gradient are known to those skilled in the art. Thus 
the (permeating) component, preferably transported through the membrane, 
is enriched on the permeate side and depleted in the feed flow. Membranes 
that allow water to preferably permeate, with organic components being 
retained, are known to those skilled in the art, as well as are such that 
allow organic components to permeate preferably, but retain water, as are 
such that allow certain organic classes of substances to permeate, 
preferably with others being retained. Consequently, organic solutions can 
be dehydrated by means of pervaporation and vapor permeation, organic 
components can also be removed from water and gas flows and organic 
mixtures can be separated as well. 
It is further known, that the mass transfer through the membrane (flux) 
increases as the temperature increases; pervaporation and vapor permeation 
are thus performed at the highest temperatures possible, with a limitation 
given generally by the thermal stability of the membranes. Considering 
pervaporation, the feed flow is passed over the membrane in liquid phase, 
the temperature and the pressure thereof may be freely chosen, provided 
that the pressure is higher than the vapor pressure of the feed mixture. 
Thus there is a higher flexibility in the choice of operating conditions 
for pervaporation. Vaporization of the permeate requires the supply of the 
adequate heat of vaporization, which is withdrawn from the sensible heat 
of the liquid feed flow. Thus the temperature thereof is reduced as is the 
flux through the membrane. In pervaporation, the total surface of the 
membrane, being necessary for the respective task of separation, is known 
in the art to be divided into a number of subunits or steps. A heat 
exchanger is connected between two respective steps, the amount of heat, 
withdrawn from the feed flow by vaporization of the permeate, is 
reintroduced into the feed flow before it enters the next step or subunit. 
A disadvantage of this procedure is that a number of heat exchangers and 
steps are arranged through which the feed flows in series, requiring 
higher costs for components and piping. Additionally, only a part of the 
membrane area is operated at the optimal temperature, the remaining part 
always at a lower temperature. Advantageous though, is that only the 
amount of heat actually needed for vaporization of the permeate has to be 
supplied. 
In vapor permeation the total feed is vaporized in a preceding vaporizer 
and passed over the membrane in the vapor phase. No additional heat has to 
be withdrawn from the system for the penetration of the permeate, the 
change of temperature caused by the Joule-Thompson effect disregarded. The 
total membrane area is operated at a constant temperature and thus at 
maximal flux, no division into steps is necessary. This leads to an 
improved utilization of the membrane area and a simplification of the 
arrangement. Disadvantageous, however, in vapor permeation is the need of 
the total feed flow to be vaporized first and condensed again after, thus 
increasing distinctively the energy consumption as compared to 
pervaporation because of the restricted heat recovery in the condensation. 
Further, it is known to those skilled in the art, that the vapor has to be 
passed over the membrane at saturation conditions. Any superheating of the 
vapor leads to a strong reduction of activity and thus of the driving 
force for the transport through the membrane. Owing to the concentration 
change of the feed along the membrane, the saturation conditions (boiling 
temperature and vapor pressure) change so that saturated vapor conditions 
can only be maintained by means of costly compression, heating, or 
cooling. This can cause a total loss of the cost advantage that vapor 
permeation first gains over pervaporation. There has been no lack of 
attempts to unite the advantages of both methods while avoiding their 
drawbacks. So it has been suggested in U.S. Pat No. 5,151,190 to reheat 
the retentate of membrane step and to recirculate part of it to the feed 
flow, thus increasing by means of this recirculation the supplied amount 
of heat and thereby the average temperature of the feed flow between entry 
and outlet of a membrane step. This procedure doesn't really succeed, 
however, if a certain final concentration of the retentate has to be 
achieved, because then a high number of steps with interconnected heat 
exchangers is required and the redilution causes a lower degree of 
efficiency of the membrane area than can be achieved in a pervaporation 
process. 
In U.S. 4,405,409 it has been suggested to use the waste heat of a 
preceding distillation column as a heat source for pervaporation. The 
exhaust vapor of the distillation column heats conventional heat 
exchangers between the single membrane steps. It has been suggested for 
the dehydration of ethanol to feed back into the first membrane step, 
connected after predistillation, a portion of the retentate of this 
membrane step in order to increase the operation temperature of this first 
step. An essential disadvantage of this procedure is that the operation 
temperature of pervaporation is linked to that of the distillation column 
and can not be freely chosen. 
In DE 34 10 155 it has also been suggested to use the exhaust vapor of a 
preceding distillation column as an energy source for pervaporation. Here 
the heat exchangers heated by the exhaust vapor are integrated into the 
membrane steps. A feed compartment is proposed, one wall of which is 
formed by the feed side of the membrane, and in which the feed flow is 
heated by another wall, serving as a heat exchange area. A device that is 
appropriate for this process is described in EP 118 760, with an 
electrical heating being disclosed. A similar suggestion is found in U.S. 
Pat. No, 3,608, 610, with the permeate side of the membrane being heated. 
As on one hand the integration of a heat exchange area causes considerable 
problems in sealing, and on the other hand an electrical heating cannot be 
performed with inflammable media because of safety reasons, none of those 
devices has been realized to date in practice. In EP 294 827 it has been 
suggested to film a liquid film of 0.1 to 10 mm in thickness from the feed 
flow on the feed side of the membrane and to heat it by means of contact 
with vaporous feed flow. The vaporous and the liquid portion of the feed 
mixture can be directed as cocurrent, cross, or counter current flow. It 
is an essential feature of EP 294 827 that the vaporous form of the/bed 
flow is totally condensed after leaving a membrane step and supplied as a 
liquid to the same step in order to form the indispensable liquid film on 
the membrane. It turned out that there is no advantage of the EP 294 827 
teaching as compared to pervaporation or vapor permeation alone. The 
membrane is only in contact with a completely formed liquid film, which on 
the other side is in contact with a vapor phase. Indeed, substance can 
condense from the vapor phase into the liquid film, but it has to be 
transported through it. The same applies to the heat that is brought into 
the liquid film from the vapor. The completely formed liquid film is an 
additional resistance to mass transfer as well as to heat transport. If 
more than one membrane step is required for achieving a specified final 
concentration, the retentate of the first step has to be vaporized again, 
directed through the next step and condensed again, in order to form said 
liquid film, causing a distinct additional consumption of energy thereby. 
Surprisingly, it has been shown that the described disadvantages, known in 
the art, can be avoided and the respective advantages of pervaporation and 
vapor permeation can be unified. The present invention relates thus to a 
combination of the processes of vapor permeation and pervaporation for the 
separation of fluid mixtures and particularly to a combination of vapor 
pemeation and pervaporation for the removal of at least one undesired 
minor component from a feed flow as described in claim 1. Essential 
features of the present invention are that 
the membrane is arranged in such a manner, that the feed mixture forming 
the feed flow flows from the bottom to the top along the feed side of the 
membrane, with the membrane being arranged vertically, 
no stationary liquid film forms on the membrane, 
the feed mixture is passed over the membrane as a vapor-liquid mixture 
whereby the ratio between vapor to liquid phase can freely be chosen in a 
wide range, 
vapor and liquid are in a thermodynamical equilibrium caused by permanent 
mixing of tile two phases, thus the vapor is always at saturation 
conditions, 
by permanent mixing of vapor and liquid, any liquid film forming on the 
feed side of the membrane is constantly destroyed, thus minimizing the 
transition resistance on the feed side of the membrane, and no stationary 
liquid film on the membrane can be formed. Devices (modules), appropriate 
for the performance of the process according to the present invention are 
described e.g. in DE 4 225 060 or in EP 0 214 496; both devices allow for 
the vertical arrangement of the membrane. 
In a preferred embodiment the feed flow to be separated, the feed mixture, 
is preheated to the desired operating temperature, e.g. 95.degree. C., in 
liquid phase. Then it is directed to a vaporizer, which is heated by 
vapor, heat transfer liquid, or electrically, with the heating power being 
fixed by means of appropriate control devices. By the fixed heating power, 
the mass of the feed flow, and its evaporation enthalpy, the ratio of 
vapor to liquid at the outlet of the vaporizer is fixed and can be 
controlled. The pressure is regulated in such a manner that its value 
corresponds to the boiling temperature in the vaporizer. 
In a further preferred embodiment, the heating power of the vaporizer is 
fixed in such a manner, that the molar portion of vapor in the feed flow 
to the module containing the membrane corresponds to 90% to 150% of the 
molar permeate flow. 
In another preferred embodiment, the retentate received at the outlet of 
the module is directed in liquid form to another module being operated by 
a pervaporation process. 
In a further preferred embodiment, one part of the feed mixture is being 
split off after the heat exchanger and is directed as a liquid into the 
module through a first line. The second part of the feed mixture is 
vaporized completely in the vaporizer and is directed into the module 
through a second line so that vapor is directed into the module, which is 
filled with liquid. 
In a further preferred embodiment, one part of the feed flow is vaporized 
completely in the vaporizer and passed into the module as vapor. The 
second part of the feed mixture is split off after the heat exchanger, and 
directed into the module through a separate line and by an additional 
pump, so that a fine dispersion of liquid particles in the vapor phase is 
formed, 
In a further preferred embodiment, the feed flow vapor and liquid portions 
are passed through several membrane modules.

the following examples and comparative examples illustrate the process of 
the present invention as compared to vapor permeation and to pervaporation 
alone. 
Comparative example 1 
A plate module, as described e.g. in DE 4 225 060, is loaded with a 
commercially available pervaporation membrane (PERVAP.RTM.1000 of GFT), 
the membrane area being 1 m.sup.2. The membrane is arranged horizontally 
in the module. On the permeate side of the module a vacuum of 3 mbar is 
applied, the resulting permeate being condensed at 0.degree. C. A feed 
mixture of 93.8% by weight ethanol and 6.2% by weight water is heated to 
95.degree. C. and passed through the module, on the retentate side of the 
module a pressure of 2.2 bar absolute being maintained. With a feed flow 
of 5 kg/h a product of 4.78 kg/h having a water content of 2.12% by weight 
and a temperature of 71.degree. C. is obtained. 
Comparative example 2 
The arrangement according to example 1 is varied in such a manner, that the 
feed mixture, heated in advance to 95.degree. C. is directed to an 
electrically heated vaporizer and completely vaporized. The membrane is 
arranged vertically in the module, the vapor flow is directed from top to 
bottom through the module. On the relentate side of the module a pressure 
of 1.9 bar absolute is maintained, corresponding to a vapor temperature of 
94.degree. C. In a condenser the vaporous retentate is completely 
condensed. For an amount of feed of 5 kg/h the vaporizer needs a heating 
power of 1.35 kW, after the condensation 4.71 kg/h of product, having a 
water content of 0.63% by weight, is obtained. The utilization of the 
membrane area is clearly better than in comparative example 1, as is shown 
by the lower water concentration of the retentate. 
Example 1 
The proceeding is akin to comparative example 2, the membrane being 
vertically arranged in the module. A heating power of only 0.25 kW is 
supplied to the vaporizer. The feed mixture is directed from bottom to top 
through the module, at the outlet of the module a liquid, containing only 
very few bubbles of vapor, having a temperature of 95.degree. C. and a 
water content of 0.51% by weight is obtained. As compared to comparative 
example 2 the utilization of the membrane is even improved, as the low 
water content of the retentate shows, but less than 1/5 of the heat needed 
in comparative example 2 is required. 
Comparative Example 3 
The same arrangement as in comparative example 1 is used, except for the 
module being loaded with a commercial high flux membrane of also 1 m.sup.2 
membrane area. The feed mixture consists of 87% by weight propanol-2 and 
of 13% water. It is also heated to 95.degree. C. and directed into the 
module in liquid form, the pressure at the outlet of the module being 2 
bar absolute. With an amount of feed of 10 kg/h, a product of 93.6% by 
weight propanol-2 and 6.4% by weight water is obtained at a temperature of 
52.degree. C. 
Comparative example 4 
The proceeding is akin to comparative example 2, except that the high flux 
membrane and a propanol-2-water mixture of comparative example 3 is used. 
A heating power of 2.5 kW has to be supplied to the vaporizer to 
completely vaporize the feed mixture of 10 kg/h of 87% by weight 
propanol-2 and 13% water, which has been preheated to 95.degree. C. After 
condensation, a product of 8.58 kg with a water content of 0.6% by weight 
is obtained. As compared to tile comparative examples 1 and 2 clearly 
better utilization of the membrane is achieved, as can be seen by the 
lower water content of the retentate as compared to comparative example 3. 
Example 2 
The mixture of comparative example 4 is treated according to comparative 
example 2, with a heating power of 0.45 kW being supplied to the vaporizer 
to only partly vaporize the feed mixture, and the feed flow is directed 
from bottom to top over the vertically arranged membrane. The liquid 
retentate at the retentate side of the module consists of 99.7% by weight 
propanol-2 and of 0.3% by weight water at a temperature of 94.degree. C. 
Here as well, an improved utilization of the membrane as compared to 
comparative examples 3 and 4 is also achieved: the additional expense of 
heat is less than 1/5 of that being required in comparative example 4. 
Example 3 
The fluid retentate being obtained in example 2 is passed directly into a 
module with horizontally arranged membranes, containing an area of 1 
m.sup.2 of the same membrane as in comparative examples 3 and 4 and as in 
example 2. At the outlet of this second module a retentate is obtained, 
having a water content of 0.04% by weight and a temperature of 74.degree. 
C. 
Comparative example 5 
The product obtained in comparative example 3 was heated again to 
95.degree. C. and was passed again through a module with an area of 1 
m.sup.2 of the same membrane and arrangement in order to obtain the same 
final water content as in example 3. The water concentration of the 
retentate was still 1.7% by weight at the outlet of the second module, 
only after a third module a final water content of the retentate of 0.04% 
by weight, corresponding to that of example 3 was obtained. A heating 
power of a total 0.48 kW was required for the reheating between the first 
and the second and the second and the third module. As compared to example 
3 the same water concentration is obtained in the last retentate, and the 
heat being supplied is almost the same, but 3 m.sup.2 Of membrane area are 
required as compared to 2 m.sup.2 in example 3. 
It is evident to anyone skilled in the art that the examples being noted 
herein for dehydration processes may be employed equivalently for the 
separation of other mixtures like organic-organic mixtures.