THERMAL MEMBRANE DISTILLATION SYSTEM

Abstract: Thermal membrane distillation system whereby a product liquid layer within space (3) is separated from a brine liquid layer within space (2) by a microporous, hydrophobic membrane (1) and the product liquid layer is also separated from a coolant liquid layer within space (5) by a barrier (4). The product liquid is formed by condensing vapor from a brine which has passed through the hydrophobic membrane (1) as a result of a temperature gradient across it. The heat of condensation is transferred to a coolant liquid through a fluid impervious barrier (4) whereby a counter flow relationship is established between the brine and coolant. As a preferred embodiment, the membrane distillation system can be conformed into a spiral configuration.

Description: Desc iption THERMAL MEMBRANE DISTILLATION SYSTEM Technical Field This invention relates to distillation purifi- cation systems and more particularly to distillation systems using hydrophobic membranes to desalinate water. Background Art The distillation process has long been recognized as a means for purifying a chemical substance from a mixture of chemical substances. Early methods involved boiling the mixture and condensing out the substance to be purified. However, it has more recently been recognized that the ability of certain membranes to selectively pass the vapor phase of water while retarding the transport of the liquid phase will eliminate the need for violent boiling and use less energy to achieve distillation. Evaporation of water requires: (1) that the water to be extracted or purified has a lower evaporation temperature than the remaining species in the mixture and (2) a coordinated heat input and removal process to extract water by evaporation and condensation. In this manner, large amounts of energy are transported by means of latent heat, although the temperature gradient between evaporation and condensation required to drive the process is usually very small. The evaporation temperature of a substance in a mixture is higher than that of the pure substance at the same ambient pressure. This temperature difference is called "evaporation temperature elevation." A minimum temperature difference between that of the evaporating mixture and condensing or "pure" liquid ( T ) must be maintained in order to proceed with mass transport. At a value smaller than T MTKΓ ' mass transport can actually be reversed even if the condensing surface temperature is lower than the mixture solution temperature. Evaporation temperature is also a function of partial vapor pressure. At temperatures below normal boiling point, evaporation can be continued by lowering the ambient pressure in the chamber. A multiple-stage flash evaporator is designed to recognize these factors, and make the distillation process energy efficient. The cooling necessary for condensation is done by a liquid, such as the feed water brine, which absorbs the latent heat of condensation from chamber to chamber through a non- permeable heat transfer surface in the form of a tube or radiator. The chamber is selected so that each chamber has its own equilibrium chamber pressure. Chamber pressure is selected and controlled so that the coolant always has a temperature lower than the evaporating solution plus T M IN* In a counter current configuration, the latent heat of condensation becomes a regenerative heat source for the incoming brine, before any additional heat input is required. The brine solution is then further heated to eventually become the evaporating solution. The temperature of the evaporating solution open to the chamber continues to drop from chamber to chamber as pure vapor is evaporated. The number of stages or chambers of the evaporator is selected in order to balance out or optimize the pressure/temperature profile and flow rates in addition to the operating and capital costs. Generally, the larger the number of stages, the lower the energy costs will be. The temperature difference of the condensing surface and evaporating liquid becomes smaller with increasing number of stages, hence one trades off energy costs for capital costs. The number of stages is bound by ■ ~ the TM,, τ I.,, . and TM„ τ IN. τ is a function of the mixture concentration. One means of increasing the efficiency of such a system is to decrease the vapor gap between the evaporating and condensing surfaces for it is known that, the performance ratio is inversely proportional to the distance between these surfaces. However, reduction of this distance increases the potential for contamination of the distillate by the distilland. Objects It is thus an object of the present invention to disclose a means of improving the efficiency of a flash evaporator system while minimizing capital and operating costs. It is yet another object of the present invention to teach the use of hydrophobic porous membranes across which the distillate vapor can pass, thus greatly decreasing the vapor gap of the system. It is still another object of the present invention to optimize a thermal membrane distillation system's design parameters by taking advantage of the properties of hydrophobic porous membranes as related to heat and mass transfer, particularly in a counter flow configuration. These and further objects of the present invention will be more fully appreciated when considering the following disclosure and appended drawings wherein: FIG. 1 is a cross-sectional representation of one embodiment of a thermal membrane distillation system of the present invention; PIG. 2 depicts a cross-sectional depiction of a "stacked" thermal membrane distillation system of the present invention; FIG. 3 illustrates a spiral wound distillation system corresponding to the linear system of FIG. 1; and FIG. 4 illustrates a spiral wound thermal membrane distillation system, again, of a stacked configuration. Summary of the Invention The present invention deals with a thermal membrane distillation system whereby a product liquid layer is separated from a brine liquid layer by a microporous, hydrophobic membrane and the product liquid layer is also separated from a coolant liquid layer by a barrier. The product liquid is formed by condensing vapor from a brine which has passed through the hydrophobic membrane as a result of a temperature gradient across it. The heat of condensation is transferred to a coolant liquid through a fluid impervious barrier whereby a counter flow relationship is established between the brine and coolant. As a preferred embodiment, the membrane distillation system can be conformed into a spiral configuration. Detailed Description of the Invention Turning to FIG. 1, a counter flow thermal membrane distillation system is shown whereby the hot distilland or brine is shown flowing from side 7 to side 6 within space 2 being bounded on its top surface by microporous hydrophobic membrane 1. Above the membrane is located distillate or fresh water, which is produced by vapor migration through the membrane and condensation within space 3 due primarily to low temperature impervious barrier 4. The temperature of barrier 4 is maintained by coolant within space 5 which travels from surface 6 to surface 7, counter currently with the distilland within area 2. The structure depicted above incorporating a hydrophobic microporous membrane represents a significant advantage over conventional multiple-staged flash evaporator systems. In a multiple staging condition liquids contact both sides of the membrane sealing off individual pores to form microporous chambers. In such systems, a vapor gap is established between the distilland and distillate whereby heat is employed to warm the distilland providing for evaporation across the vapor gap and condensation at a surface remote from the body of distilland liquid. Because only a vapor diffusion gap is employed by prior art multi-staged flash evaporator systems, a vigorous heating of the distalland can cause impurities to travel the vapor gap and contaminate the distillate. Although this can be significantly prevented by increasing the physical dimension of the vapor gap, any increase in such dimension greatly reduces distillation efficiency. By providing a microporous hydrophobic membrane, the distilland and distillate surfaces can be separated by the mere thickness of the membrane itself, while virtually eliminating contamination across the membrane. Another distinction between the thermal membrane distillation system of the present invention and prior art multi-staged flash evaporators is that the prior art devices are generally composed of multiple discrete stages where the present system is of a continuous design. As a result, it is necessary to design the counter-flow system so that the hydrophobic membrane is capable of transferring substantially pure water vapor from the distilland throughout the entire linear length of the system where local temperature and pressure conditions vary throughout. In designing such a system, it is recognized that the high temperature end (side 7 of FIG. 1) characteristically encloses distilland and product liquid of relatively high vapor pressures. Due to the non-compartmentalized structure, there is a tendency for the migration of vapor from high temperature end 7 to Ipw temperature end 6 through the maze of inter¬ connecting channels in the microporous membrane. It has thus been recognized that the thermal membrane distillation system of the present invention will function as long as the temperature gradient along the outer surface of the membrane is minimal. That is, (T-.-T 7 )/L is minimal where T.. is the temperature at which the hot distilland enters the system (side 7) and T_ is the temperature at which it exits (side 6). L is the linear length of the membrane. In other words, the temperature gradient across the membrane should be very large in comparison to that along the membrane. Ideally, the flow parameters are chosen so that the vapor mass transfer of the pure water across the membrane is at least five times greater than the vapor mass transfer due to the temperature gradient along the surface of the membrane. In order to eliminate the need for the substantial linear length needed to derive a volume of distillate for commercial application, it is proposed that the present invention be configured in the form of the spiral such as that shown in FIG. 3. More speci¬ fically, heated distilland enters the system within channel 14 at the center of the spiral and winds its way radially outward within channel 13. In a counter- flow relationship, coolant enters channel 11 at the radial extremity of the spiral and proceeds to travel inwardly as shown in the figure. The resultant fresh distillate then collects within channel 12 and is withdrawn from the spiral in a manner similar to that depicted in FIG. 1. As in FIG. 1, the distillate and coolant are separated by impervious barrier 15 while the hot brine and pure product are in turn separated by microporous membrane 16. Besides providing for a structure which is more manageable in overall dimension by using a spiral configuration, this aspect of the present invention results in a reduction in the need for insulation for the spiral tends to be self- insulating in nature. Yet a further embodiment of the present invention is that shown in FIG. 2 whereby a composite structure is illustrated providing for an improved distillation scheme. In this embodiment, a center channel 23 is provided for housing the hot brine which is bounded on each side by hydrophobic microporous membranes 22 and 24. As in the previously described embodiment, a counter current flow arrangement is established between the hot brine within channel 23 and the cooling liquid in channels 27 and 28 which are separated from the distillate or condensate products within channels 21 and 25 by impervious partitions 20 and 26, respectively. Further, by forming symmetrical pairs, an adiabatic system is achieved. As shown in FIG. 2, it is the intent of the present invention to provide a hydrophobic membrane separating a distilland within space 23, such as salt water, and a distillate located within spaces 21 and 25, such as fresh water. It is the nature of the hydrophobic membrane to possess pores across the body thereof. Due to the properties of the non-wetting hydrophobic material at low pressures, liquid is prevented from entering the pores of the hydrophobic membrane. Initially, neither the distilland nor distillate penetrates within the gaseous region of the pores. In fact, two convex-shaped liquid-vapor surfaces are found to exist at each end of the various pores. In distillation apparatus such as that disclosed herein, typically after several days, the vapor barrier across the pores of the hydrophobic material is destroyed due to water-logging, and hence the distillation process through the membrane ceases. The distilland liquid-vapor interface migrates in a direction toward the distillate side until eventually it intersects the liquid-vapor interface on the distillate side of the membrane. This destroys the gaseous barrier needed for the distillation process to occur. It has been determined that this liquid migration can be substantially eliminated by providing a hydrophilic layer which is essentially non-porous onto the hydrophobic microporous membrane adjacent to the distilland side of said membrane. Ideally, for water desalinization purposes, the maximum pore diameter of the hydrophobic membrane is intended to be less than approximately 0.5 microns. Fluoro-substituted vinyl polymers which are suitably hydrophobic are ideal materials for the microporous hydrophobic membranes. Polytetrafluoro-ethylene is the most hydrophobic polymeric material known. Pol inyli ene fluoride, although somewhat less hydrophobic, also performs well. In one aspect of the present invention, a plurality of composite microporous membranes can be arranged in a multi-staged cell configuration. The main advantage of multiple—staged cell configurations is the conservation of heat energy. In a multiple stage cell, multiple composite membrane system, the heat rejected from one stage is used to provide the temperature gradient across another stage. The optimum number of stages is reached when the operating OMPI temperature of any additional stages is so low that the distillate production rate is uneconomically small. Applying this principle to the cell of FIG. 2, it is noted that the hot brine, in traveling in the direction of the arrow from the bottom to the top of the page loses temperature which is in part picked up by the distilland during evaporation through the microporous membranes. As a consequence, the cooling fluid traveling in the direction of the arrows from the top to the bottom of the page experiences a temperature rise. When the temperature difference between the hot brine in area 23 and the cooling fluid in areas 27 and - 28 gets sufficiently small, distillation slows to the point where it is no longer economically feasible to provide capital equipment for further multi-stage units. The membrane multi-stage evaporator of the present invention can be made more energy efficient by using, as cooling fluid, cold brine which is gradually heated while passing counter current to the hot brine used as the distilland. Thus, the "cooling fluid" is partially heated before being used, itself, as hot brine solution within chamber 23 of FIG. 2. Another incidental utility in practicing the claimed invention is the removal of low boiling temperature trace substances from the distilland. In conventional distillation processes, low boiling temperature components tend to evaporate right along with the low boiling temperature distillate. Thus, the distillate becomes contaminated. It is common to have low temperature boiling contaminants in the ground waters, particularly in areas proximate urban areas and these potentially toxic chemicals cannot be removed by ordinary water distillation plants. However, in the membrane thermal distillation system of the present invention, the incoming liquid is heated by the condensing distillate causing the low boiling temperature contaminants to bubble off as the liquid reaches the distilland heater. At that point, the various contaminants can be vented from the system before the liquid becomes the distilland in contact with the membrane. It has also been observed that due to the fact that both distilland and distillate are hotest on the same end of the membrane evaporator unit, there is a tendency for migration of vapor from the high temperature end to the low temperature end through the maze of interconnecting channels of the porous hydrophobic membrane itself. It is desirable that the temperature gradient along the outer surface of the membrane be kept small so that the driving force of vapor across the membrane from the distilland to distillate sides is great compared to the driving force along the linear length of the membrane. The performance ratio of such a counter current system can be mathematically described as PR = n x l- T 2 T l- 4 where n is a factor which expe imentally accounts for heat and mass transfer loss due to the inefficiencies influenced by the choice of flow rate, type of membrane and heat transfer coefficient of the channel flow. T-. is the hot solution temperature. T is the discharge solution temperature. T. is the coolant temperature at exit. It was discovered that if the convection system's performance ratio (PR) is greater than 10, substantially pure vapor could be added to the brine liquid will tend to increase the performance ratio without needing additional heat transfer surface area. OMPI There are several additional ways to maximize the performance of the evaporator shown in FIG. 2. One method would be to configure the evaporator in a vertical mode, somewhat as shown by the figure's orientation itself. In this way, additional pressure can be imposed upon the upstream end of the hot brine in order to increase the solution boiling temperature which would consequently increase the efficiency of the system. Yet another means of increasing efficiency is to provide a structure as shown in FIG. 4. By providing a spirally wound counter current structure, a rather long counter current path could be established without needing an evaporator of inconvenient length. Further, the spiral structure tends to be somewhat self-insulating, again adding to the efficiency of the system. Turning to FIG. 4, hot brine enters the system at the geometric center of the spiral as shown as area 70. The brine progresses through the spiral between membrane pairs 74 and 75. Rejected brine exits the system at the radial extremity of the spiral following channel 73, while fresh water distillate similarly is withdrawn from areas 71 and 72 at the radial extremity. Coolant is caused to flow in a counter current fashion beginning at the radial extremity and progressing inwardly within areas 76, 77, 78, etc. operating in much the same manner as the counter current flow arrangement of FIG. 2. Example 1 A countercurrent heat exchanger was made as depicted in Fig. 1. The cell had the following dimensions: Channel width 5" Channel length 27" 2 Membrane area 0.94 ft. Membrane thickness 0.01" Film thickness 0.005" Product channel thickness 0.02" Cooling channel thickness 0.125" As used in this example, brine entered the cell at a temperature of 190°F and entered the heater at 180.9°F. After distillation, the brine was measured to be at 183. °F where it was mixed with incoming cooling brine resulting in an overall cooling liquid input temperature of 176.8°F. The circulation rate was " " maintained at a steady 600 lbs./day yielding product distillate at 3.1 lbs./day. When corrections were made with enough membrane to extend intake water temperature to ambient, the performance ratio was calculated at 11.7. Example 2 The cell of Example 1 was again used in the manner disclosed in practicing this invention. In doing so, four different cases were generated as follows: OMPI Counterflow Heat Exchαngor Module Performance C a s e Item Units I II III IV Intake Temperature °F 80 90 '80 90 Intake TDS PPM 35,000 43,000 35,000 43,000 Fresh brine input concentratio Fresh brine temperature Cooling liquid entrance temper Cooling liquid exit temperatur Distilland input temperature Distilland exit temperature Fresh brine input flow Distilland input flow Distilland exit flow Distillate product flow Recirculated brine flow Rejected (spent) brine flow PR (Theoretical) PR (Practical) Although the present invention was disclosed as dealing with the desalinization of brine, the thermal membrane distillation system can be used in any environment now occupied by prior distillation systems employed today.

Claims: Claims 1. In a thermal membrane distillation system wherein a product liquid layer is separated from a distilland liquid layer by a microporous, hydrophobic membrane and the product liquid layer is also separated from ; a coolant liquid layer by a barrier, said product liquid layer being formed by condensing vapor from the distilland liquid layer which has passed through the hydrophobic membrane as a result of a temperature gradient across it and transferred heat to the coolant liquid layer and barrier, the improvement comprising means for establishing a counter-flow relationship between the distilland liquid layer and the coolant liquid layer. 2. A thermal membrane distillation system in accordance with claim 1 further comprising means for conforming the layers and membranes into a spiral configuration. 3. A thermal membrane distillation system comprising: a pair of microporous hydrophobic membranes defining a first passageway for an aqueous solution; a pair of barrier membranes external to the hydrophobic membranes and defining second and third passageways for product liquid, the aqueous solution being at a higher temperature than the product liquid; and means for establishing a counter-flow of a coolant relative to the flow of said product liquid beyond each of the barrier membranes. 4. A thermal membrane distillation system in accordance with claim 3 further comprising means for stacking said thermal membrane distillation system. 5. A thermal membrane distillation system in accordance with claim 3 further comprising means for conforming said hydrophobic membrane and said pair of barrier membranes into a spiral configuration. 6. A thermal membrane distillation system comprising: a pair of microporous hydrophobic membranes defining a first passageway for an aqueous solution; a pair of barrier membranes external to the hydrophobic membranes and defining second and third passageways for product liquid, the aqueous solution being at a higher temperature than the product liquid; means for establishing a counter-flow of a coolant relative to the flow of said product liquid beyond each of the barrier membranes; and means for stacking said membranes and passageways. 7. A thermal membrane distillation system comprising: a pair of microporous hydrophobic membranes defining a first passageway for an aqueous solution; a pair of barrier membranes external to the hydrophobic membranes and defining second and third passageways for product liquid, the aqueous solution being at a higher temperature than the product liquid; eans for establishing a counter-flow of a coolant relative to the flow of said product liquid beyond each of the barrier membranes; and means for conforming said hydrophobic membrane and said pair of barrier membranes into a spiral configuration. 8. In a thermal membrane distillation system wherein a product liquid layer is separated from a brine liquid layer by a microporous, hydrophobic membrane and the product liquid layer is also separated from a coolant liquid layer by a barrier, said product liquid layer being formed by condensing vapor from the brine liquid layer which has passed through the hydrophobic membrane as a result of a temperature gradient across it and transferred heat to the coolant liquid layer and barrier, the improvement comprising: means for establishing a counter-flow relationship between the brine liquid layer and the coolant liquid layer; and means for maintaining a smaller temperature gradient along the surface of the hydrophobic membrane than the temperature gradient established across the hydrophobic membrane. 9. A thermal membrane distillation system in accordance with claims 1, 3, 5 or 7 further comprising means for maintaining a smaller temperature gradient along the surface of the hydrophobic membrane than the temperature gradient established across the hydrophobic membrane. -£TJR£ OMPI - WIPO 10. A thermal membrane distillation system in accordance with claim 8 wherein the flow rates and membrane dimensions are chosen so that the heat transfer due to mass transport of the product liquid across the hydrophobic membrane is at least five times greater than the heat transfer due to the temperature gradient along the surface of the hydrophobic membrane. 11. The thermal membrane distillation system of claim 1 wherein the distilland liquid is brine. 12. A thermal membrane distillation system in accordance with claims 1, 4, 5, 8 or 9 wherein the membranes are oriented so that the liquids flow in a vertical direction, using the hydraulic static pressure to increase the boiling temperature without pressurizing the system. 13. A thermal membrane distillation system in accordance with claims 1, 3, 5, 7 or 8 wherein said microporous, hydrophobic membrane comprises a plurality of membrane components, at least one of said components comprising a hydrophilic membrane. 14. A thermal membrane distillation system in accordance with claims 1, 3, 5, 7 or 8 further comprising: adding pure vapor to the brine liquid layer if the convection cooling system performance ratio is greater than 10.

Inventors: CHENG DAH YU, CHENG, DAH, YU
Applicants: INT POWER TECH, INTERNATIONAL POWER TECHNOLOGY, INC.
Application year: 1984
Main classification: B01D13
IPC codes: B01D3, B01D61, C02F1