Abstract:
A membrane module comprising an outer casing having an interior region, a seal disposed within the outer case, thereby dividing the interior region into a first chamber and a second chamber, and a plurality of hollow fiber membranes extending through the first chamber and the second chamber, where at least a portion of the plurality of hollow fiber membranes have first segments located within the first chamber and second segments located within the second chamber, the first segments being configured to allow vapor transmission therethrough, and the second segments being configured to substantially prevent vapor transmission therethrough, and further configured to allow transmission of thermal energy therethrough.

Description:
BACKGROUND 
   The present invention relates to distillation systems for liquid treatment processes. In particular, the present invention relates to distillation systems containing hollow fiber membrane modules for separating distillate fluids from feed solutions in liquid treatment processes. 
   In recent years, membrane distillation has become increasingly popular in a variety of fluid-treatment applications. The membranes are typically hydrophobic and microporous to keep the feed solution separated from that of the distillate during operation. Hollow fiber membranes are typically employed in tube/shell configurations, where bundles of hollow fiber membranes are arranged along the longitudinal axes of the modules. For membrane distillation, each hollow fiber membrane in the bundle is typically a hydrophobic, microporous membrane having an exterior surface and an inner hollow tubular region. The exterior surfaces of the hollow fiber membranes face a shell side of the module, which is the portion of the module containing the feed solution. The inner hollow tubular regions define a tube side of the module, which provides a conduit for collecting the distillate fluids separated from the feed solution. 
   During operation, the feed solution is typically heated to form a temperature differential across the wall of the hollow fiber membranes. This temperature differential creates a vapor pressure differential between the tube side and the shell side of the module, which causes vapor transport through the hollow fiber membranes. The transmitted vapor then condenses within the hollow fiber membranes, thereby providing the desired distillate fluid. 
   During steady state operations, the mass transfer across the hollow fiber membranes can provide good product rates of distillate fluids for a variety of distillation systems. However, a common issue with hollow fiber membrane modules involves the thermal efficiencies of the modules. As the distillate fluid evaporates and transmits through the hollow fiber membranes, latent heat accompanies the transmitted vapor, passing from the shell side to the tube side of the module. Furthermore, the temperature differential between the shell side and the tube side of the module can result in conductive heat transfers across the wall of the hollow fiber membranes. Overall, a large amount of heat is transported through the membrane. If the heat is not recuperated, a high operating cost in terms of thermal energy would result. Thus there is an ongoing need for increased thermal efficiencies in distillation systems containing hollow fiber membrane modules. 
   SUMMARY 
   The present invention relates to a hollow fiber membrane module and a distillation system incorporating the hollow fiber membrane module. The hollow fiber membrane module includes an outer casing having an interior region divided into a first chamber and a second chamber, and a plurality of hollow fiber membranes extending through the first and second chambers. At least a portion of the hollow fiber membranes have first segments located within the first chamber and second segments located within the second chamber, where the first segments allow vapor transmission therethrough, and the second segments allow transmission of thermal energy therethrough, but substantially prevent vapor transmission therethrough. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of a distillation system that includes multiple hollow fiber membrane modules. 
       FIG. 2  is a schematic illustration of an interior of one of the hollow fiber membrane modules. 
       FIG. 3  is an expanded sectional view of an exemplary hollow fiber membrane in the hollow fiber membrane module. 
       FIG. 4  is a schematic illustration of an interior of an alternative hollow fiber membrane module. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic illustration of distillation system  10 , which includes membrane modules  12 , feed loop  14 , and distillate loop  16 , and is a suitable system for separating a desired distillate fluid from a feed solution. Examples of suitable feed solutions for use with distillation system  10  include solutions containing compounds that are separable by vapor pressure differentials across hydrophobic, microporous membranes, such as seawater, brackish water, and other aqueous brine solutions for water and wastewater treatment processes. As used herein, the term “solution” refers to a carrier liquid that contains one or more solids that are fully dissolved, partially dissolved, dispersed, emulsified, or otherwise suspended in the carrier liquid(s). For example, the feed solution may be an aqueous brine solution containing salt that is at least partially dissolved in a water carrier. In this example, distillation system  10  may be used to separate at least a portion of the water from the aqueous brine solution to provide the clean water as the distillate fluid. Distillation system  10  also desirably includes standard fluid processing equipment (not shown), such as process control units, fluid pumps, and filters. 
   Membrane modules  12  are hollow fiber membrane modules that separate the distillate fluid from the feed solution (e.g., water from an aqueous brine solution). Examples of particularly suitable modules for membrane modules  12  include direct contact membrane distillation (DCMD) modules. As discussed below, membrane modules  12  are each divided into a shell side (not shown in  FIG. 1 ) and a tube side (not shown in  FIG. 1 ), where the shell side is in fluid communication with feed loop  14 , and the tube side is in fluid communication with distillate loop  16 . During operation, the feed solution travels from feed loop  14  to the shell sides of membrane modules  12 . While the feed solution flows through the shell sides of membrane modules  12 , the feed solution is separated into the desired distillate fluid and a residual feed solution. The desired distillate fluid transfers to the tube sides of membrane modules  12 , and enters distillate loop  16 . The residual feed solution remains on the shell sides of membrane modules  12  and reenters feed loop  14 . 
   While distillation system  10  is shown with three membrane modules  12  operating in a parallel arrangement, distillation system  10  may alternatively include additional or fewer numbers of membrane modules  12 . Examples of suitable numbers of membrane modules  12  for use in distillation system  10  range from one to one hundred; particularly suitable numbers of membrane modules  12  for use in distillation system  10  range from one to twenty five; and even more particularly suitable numbers of membrane modules  12  for use in distillation system  10  range from one to ten. 
   Feed loop  14  is a fluid pathway for the feed solution, and includes feed source line  18 , feed conveyance lines  20   a - 20   d , feed inlet lines  22 , feed outline lines  24 , discharge line  28 , feed reservoir tank  30 , and heat exchanger  26 . Feed source line  18  is a valve-controlled fluid conduit for transferring the feed solution from a feed solution source (not shown) to feed loop  14 . When distillation system  10  reaches a steady state operation, portions of the feed solution are separated within membrane modules  12  to provide the desired distillate fluid in distillate loop  16 . As such, feed line  18  desirably provides the feed solution to balance the steady state flow in feed loop  14 . 
   Feed conveyance lines  20   a - 20   d  are fluid conduits for feed loop  14 , and provide a counter-clockwise flow path in the embodiment shown in  FIG. 1 . Feed inlet lines  22  are fluid conduits that interconnect feed conveyance line  20   d  and membrane modules  12 , thereby allowing the feed solution to flow into the shell sides of membrane modules  12 . Feed outlet lines  24  are fluid conduits that interconnect feed loop line  20   a  to membrane modules  12 . This allows the distilled feed solution to exit the shell sides of membrane modules  12 , and reenter feed loop  14 . 
   Discharge line  28  is a valve-controlled fluid conduit interconnecting feed conveyance lines  20   a  and  20   b , which allows a portion of the feed solution to bleed out of feed loop  14 . This is beneficial to control the concentration of the residual feed solution during a steady state operation. Feed reservoir tank  30  is a container fed by feed conveyance line  20   b , which provides a reservoir of the feed solution during operation. Feed reservoir tank  30  is also connected to feed conveyance line  20   c  for supplying the feed solution to heat exchanger  26 . Heat exchanger  26  is a heat-providing heat exchanger that interconnects feed conveyance lines  20   c  and  20   d . Accordingly, heat exchanger  26  increases the temperature of the feed solution passing from feed conveyance line  20   c  to feed conveyance line  20   d . As discussed below, the increased temperature of the feed solution assists in creating vapor pressure differentials within membrane modules  12 , thereby allowing membrane modules  12  to transport the vapor from the feed solution side to the distillate side. 
   Distillate loop  16  is a fluid pathway for the distillate fluid, and includes heat exchanger  32 , distillate conveyance lines  34   a - 34   d , distillate reservoir tank  36 , distillate inlet lines  38 , distillate outlet lines  40 , recovery line  42 , and storage tank  44 . In addition to the distillate fluid obtained from the separation processes within membrane modules  12 , distillate loop  16  also desirably contains a flow of cool distillate fluid to further assist the separation processes. Heat exchanger  32  is a cooling heat exchanger that interconnects distillate conveyance lines  34   a  and  34   b , thereby lowering the temperature of the distillate fluid flowing through distillate conveyance line  34   a  to distillate conveyance line  34   b.    
   Distillate conveyance lines  34   a - 34   d  are fluid conduits for distillate loop  16 , and provide a clockwise flow path for the distillate fluid in the embodiment shown in  FIG. 1 . Distillate reservoir tank  36  is a container fed by distillate conveyance line  34   b , which provides a reservoir of the cool distillate solution during operation. Distillate reservoir tank  36  is also connected to distillate conveyance line  34   c  for supplying the distillate fluid to distillate inlet lines  38 , thereby allowing the distillate fluid to flow into membrane modules  12 . Distillate outlet lines  40  are fluid conduits that interconnect membrane modules  12  and distillate conveyance line  34   d . This allows the distillate fluid to exit membrane modules  12 , and reenter distillate loop  16 . 
   Recovery line  42  is a valve-controlled fluid conduit connected to distillate conveyance line  34   d  for transferring a portion of the distillate fluid from distillate loop  16  to storage tank  44 . Storage tank  44  is a container for receiving the distillate fluid obtained from membrane modules  12 . As discussed above, when distillation system  10  reaches a steady state operation, a portion of the feed solution is separated at membrane modules  12  to provide the distillate fluid in distillate loop  16 . As such, recovery line  42  and storage tank  44  allow the obtained amounts of distillate fluid to be removed from distillate loop  16  and stored as a recovered product (e.g., clean water from a brine solution). This allows distillate loop  16  to substantially maintain a steady state flow. 
   During a steady state operation, the feed solution flows in a counter-clockwise direction around feed loop  14 , and is heated in part by heat exchanger  26  to an elevated temperature. Suitable elevated temperatures may vary depending on the composition of the feed solution. For aqueous feed solutions, examples of suitable elevated temperatures range from about 50° C. to less than 100° C., with particularly suitable elevated temperatures ranging from about 70° C. to about 90° C. While flowing through feed loop  14 , a portion of the heated feed solution flows into the shell sides of membrane modules  12  via feed inlet lines  22 , and the remaining portion of the heated feed solution continues to flow through feed conveyance line  20   d . In an alternative embodiment, feed loop  14  may require all of the heated feed solution to flow into the shell sides of membrane modules  12  via feed inlet lines  22 . In this embodiment, heat exchanger  26  only interconnects feed conveyance line  20   c  and feed inlet lines  22 . 
   Also during the steady state operation, a supply of the distillate fluid flows in a clockwise direction around distillate loop  16 , and is cooled in part by heat exchanger  32  to a lowered temperature. Suitable lowered temperatures for the distillate fluid may vary depending on the composition of the distillate fluid. For aqueous distillate fluids, examples of suitable lowered temperatures range from about 5° C. to less than 50° C., with particularly suitable lowered temperatures ranging from about 20° C. to less than 40° C. While flowing through distillate loop  16 , the cooled distillate fluid flows into the tube sides of membrane modules  12  via distillate inlet lines  38 . 
   The cool distillate fluid and the heated feed solution flowing into membrane modules  12  create temperature differentials between the shell and tube sides of membrane modules  12 . The temperature differentials correspondingly create vapor pressure differentials, higher vapor pressure on the shell side versus lower vapor pressure on the tube side. The membranes inside the modules  12  allow only vapor to be transported across the membrane wall into the bore side of the tube. The distillate fluid may then be collected in recovery tank  44  as the desired distillate product (e.g., clean water in a water treatment process). 
   While flowing through the shell sides of membrane modules  12 , the feed solution also loses thermal energy due to latent and conductive heat transfers. In a standard membrane module, the transferred heat will be carried away from the modules. However, as discussed below, membrane modules  12  also function as heat exchangers to recoup at least a portion of the thermal energy inside the modules. This increases the thermal efficiency of distillation system  10 , thereby reducing the amount of energy required and the size of heat exchangers  26  and  32 . As a result, distillation system  10  may operate with lower operation costs. Additionally, while membrane modules  12  are shown in  FIG. 1  in use with distillation system  10 , membrane modules  12  may alternatively be used with a variety of systems for separating a distillate fluid from a feed solution, and for reducing thermal energy requirements in the given systems. 
     FIG. 2  is a schematic illustration of an interior of membrane module  12 , which corresponds to one of the membrane modules  12  shown in  FIG. 1 . As shown in  FIG. 2 , membrane module  12  includes outer casing  46 , potting resin walls  48 ,  50 , and  52 , connection conduit  54 , hollow fiber membranes  56 , and baffles  58  and  60 . Outer casing  46  is a rigid structure extending along a longitudinal axis (referred to as longitudinal axis  62 ), and defines interior region  64  of membrane module  12 . Outer casing  46  also desirably includes couplings (not shown) for connecting membrane module  12  to feed inlet line  22 , feed outlet line  24 , distillate inlet line  38 , and distillate outlet line  40 . 
   Potting resin walls  48 ,  50 , and  52  are seals extending perpendicular to longitudinal axis  62 , and are formed from one or more sealant materials, such as acrylate and epoxy-based materials. Accordingly, potting resin walls  48 ,  50 , and  52  divide interior region  64  into distillate inlet chamber  66 , contactor chamber  68 , recuperator chamber  70 , and distillate outlet chamber  72 . Distillate inlet chamber  66  is the chamber disposed between outer casing  46  and potting resin wall  52 , and is open to distillate inlet line  38 . Contactor chamber  68  is the chamber disposed between potting resin walls  50  and  52 , within outer casing  46 , and is open to feed inlet line  22  and connection conduit  54 . Recuperator chamber  70  is the chamber disposed between potting resin walls  48  and  50 , within outer casing  46 , and is open to feed outlet line  24  and connection conduit  54 . Distillate outlet chamber  72  is the chamber disposed between outer casing  46  and potting resin wall  48 , and is open to distillate outlet line  40 . 
   As discussed below, contactor chamber  68  is the section of membrane module  12  where separation through distillation occurs, and recuperator chamber  70  is the section of membrane module  12  where a portion of the heat from the hot distillate flowing on one side of hollow fiber membranes  56  is transferred to the cooler feed residual feed solution on the other side of hollow fiber membranes  56 . Suitable dimensions for contactor chamber  68  and recuperator chamber  70  may vary depending on the desired heat and mass transfer properties. Examples of suitable lengths along longitudinal axis  62  for each of contactor chamber  68  and recuperator chamber  70  range from about 13 centimeters (about 5 inches) to about 76 centimeters (about 30 inches), with particularly suitable lengths ranging from about 25 centimeters (about 10 inches) to about 51 centimeters (about 20 inches), and with even more particularly suitable lengths ranging from about 30 centimeters (about 12 inches) to about 41 centimeters (about 16 inches). Examples of suitable diameters orthogonal to longitudinal axis  62  for each of contactor chamber  68  and recuperator chamber  70  range from about 3 centimeters (about 1 inch) to about 41 centimeters (about 16 inches), with particularly suitable diameters ranging from about 5 centimeters (about 2 inches) to about 30 centimeters (about 12 inches), and with even more particularly suitable diameters ranging from about 8 centimeters (about 3 inches) to about 13 centimeters (about 5 inches). 
   Connection conduit  54  is a shell-side conduit that allows the feed solution to travel from contactor chamber  68  to recuperator chamber  70  during operation. Hollow fiber membranes  56  are a plurality of tubular membranes that extend along longitudinal axis  62  between distillate inlet chamber  66  and distillate outlet chamber  72 . Hollow fiber membranes  56  are desirably bundled together within the volume of interior region  64  to increase the total membrane surface area. In one embodiment, hollow fiber membranes  56  substantially fill the open volumes of contactor chamber  68  and recuperator chamber  70 . This allows the feed solution to travel through contactor chamber  68  and recuperator chamber  70  in the interstitial voids between the membranes. Alternatively, one or more portions of contactor chamber  68  and recuperator chamber  70  may be unoccupied by hollow fiber membranes  56 , thereby providing larger flow paths for the feed solution. 
   Hollow fiber membranes  56  each include an inner hollow region (not shown in  FIG. 2 ) that extends through potting resin walls  48 ,  50 , and  52  and baffles  58  and  60  along longitudinal axis  62 . This provides a tube-side flow path for the distillate fluid between distillate inlet chamber  66  and distillate outlet chamber  72  (i.e., the distillate fluid may flow through the inner hollow regions of hollow fiber membranes  56 ). Conversely, the shell-side flow path of the feed solution extends between the exterior surfaces of hollow fiber membranes  56 , within contactor chamber  68  and recuperator chamber  70 , and through connection conduit  54 . 
   The segments of hollow fiber membranes  56  at contactor chamber  68  are formed from one or more hydrophobic, microporous materials that are capable of separating the distillate fluid from the feed solution via vapor pressure differentials. In contrast, the segments of hollow fiber membranes  56  at recuperator chamber  70  are non-porous, thereby preventing mass transfer across the membranes, but allow conductive heat transfer to occur. As such, recuperator chamber  70  functions as a heat exchanger for increasing the thermal efficiency of membrane module  12  and distillation system  10  (shown in  FIG. 1 ). 
   Baffles  58  and  60  are shell-side flow barriers (e.g., acrylate and epoxy-based barriers) within contactor chamber  68  and recuperator chamber  70  that direct the flow of the feed solution in cross patterns relative to hollow fiber membranes  56 . As shown, baffles  58  have axially-centric openings, thereby directing the flow paths of the feed solution toward the axial centers of contactor chamber  68  and recuperator chamber  70 . Baffles  60 , however, have openings that are non-axially centric (e.g., adjacent outer casing  46 ), which directs the flow paths of the feed solution away from the axial centers of contactor chamber  68  and recuperator chamber  70 . Accordingly, the alternating arrangement of baffles  58  and  60  force the feed solution to flow across hollow fiber membranes  56  in a quasi-cross flow geometry and the feed solution passes axially down chamber  68 , thereby increasing the mass and heat transfers across hollow fiber membranes  56 . Suitable numbers of baffles  58  and  60  may vary depending on the dimensions of contactor chamber  68  and recuperator chamber  70 , and on the desired flow paths of the feed solution. Examples of suitable numbers of baffles (e.g., baffles  58  and  60 ) for each of contactor chamber  68  and recuperator chamber  70  range from one baffle to ten baffles, with particularly suitable numbers of baffles ranging from one baffle to five baffles. 
   During operation, the heated feed solution enters contactor chamber  68  via feed inlet line  22 . The feed solution is then directed around the openings of baffles  58  and  60  toward potting resin wall  52 , where baffles  58  and  60  cause the feed solution to move in a cross flow pattern relative to hollow fiber membranes  56 . While the feed solution flows through contactor chamber  68 , the cool distillate fluid enters distillate inlet chamber  66  via distillate inlet line  38 , and flows through the inner hollow regions of hollow fiber membranes  56  toward distillate outlet chamber  72 . As the heated feed solution flows around the exterior surfaces of hollow fiber membranes  56 , the temperature differentials between the exterior surfaces and the inner hollow regions of hollow fiber membranes  56  create vapor pressure differentials across hollow fiber membranes  56  within contactor chamber  68 . This causes a portion of the distillate fluid in the feed solution to evaporate and transmit through the micropores of hollow fiber membranes  56 . When the vapor passes into the inner hollow regions of hollow fiber membranes  56 , the cool supply of distillate fluid causes the vapor to condense within the inner hollow regions and mix with the supply of distillate fluid. The distillate fluid then flows toward distillate outlet chamber  72 , and into distillate outlet line  40 . 
   As discussed above, while the distillate fluid separates from the feed solution, latent and conductive heat also transfers from the feed solution to the distillate fluid flowing through the inner hollow regions of hollow fiber membranes  56 . While flowing through contactor chamber  68 , the feed solution gradually cools down due to the heat transfer across hollow fiber membranes  56 . Thus, within contactor chamber  68 , the feed solution bulk temperature drops to the lowest value just before entering connection conduit  54  and recuperator chamber  70 . In contrast, while flowing through the inner hollow regions of hollow fiber membranes  56  within contactor chamber  68 , the distillate fluid gradually heats up due the heat transfer across hollow fiber membranes  56 . This heat transfer within contactor chamber  68  increases the temperature of the distillate fluid and decreases the temperature of the feed solution such that the bulk temperature of the distillate fluid entering recuperator chamber  70  on the tube side of membrane module  12  is greater than the bulk temperature of the residual feed solution entering recuperator chamber  70  on the shell side of membrane module  12 . 
   Upon entering recuperator chamber  70  from connection conduit  54 , the residual feed solution flows through recuperator chamber  70  toward feed outlet line  24 . Baffles  58  and  60  accordingly cause the distilled feed solution to flow in a quasi-cross flow path relative to hollow fiber membranes  56 . As discussed above, however, the segments of hollow fiber membranes  56  within recuperator chamber  70  are non-porous. Thus, distillate fluid vapor does not penetrate across hollow fiber membranes  56  within recuperator chamber  70 . Nonetheless, conductive heat is capable of traveling across the segments of hollow fiber membranes  56  within recuperator chamber  70 . As a result, at least a portion the thermal energy acquired during the separation process within contactor chamber  68  is transferred from the distillate fluid (i.e., from the tube side of membrane module  12 ) back to the feed solution (i.e., to the shell side of membrane module  12 ). 
   During the steady state operation, heat exchanger  26  (shown in  FIG. 1 ) desirably maintains the feed solution within feed loop  14  (shown in  FIG. 1 ) at the elevated temperature despite heat losses within membrane module  12 . Similarly, heat exchanger  32  (shown in  FIG. 1 ) desirably maintains the distillate fluid within distillate loop  16  (shown in  FIG. 1 ) at the lowered temperature despite heat acquired within membrane module  12 . Thus, the heat recovery within recuperator chamber  70  reduces the amount of energy required by heat exchanger  26  to heat the feed solution flowing through feed loop  14 , and by heat exchanger  32  to cool the distillate fluid flowing through distillate loop  16 . This increases the thermal efficiency of distillation system  10  (shown in  FIG. 1 ), thereby reducing operation costs required to produce the distillate fluid. 
     FIG. 3  is an expanded sectional view of membrane  74 , which is an example of a single membrane of hollow fiber membranes  56  (shown in  FIG. 2 ). As shown in  FIG. 3 , membrane  74  includes segments  76  and  78 , and inner hollow region  80 , where segment  76  is the portion of membrane  74  within contactor chamber  68  (shown in  FIG. 2 ), and segment  78  is the portion of membrane  74  within recuperator chamber  70  (shown in  FIG. 2 ). Segments  76  and  78  each include membrane wall  82 , which is formed from one or more hydrophobic materials and includes a plurality of micropores (not shown) that allow the transmission of gases and vapors, but restricts the flow of liquids and solids. This allows the distillate fluid to separate from the feed solution via vapor pressure differentials. 
   Examples of suitable wall thicknesses for membrane wall  82  range from about 50 micrometers to about 200 micrometers, with particularly suitable wall thicknesses ranging from about 100 micrometers to about 150 micrometers. Examples of suitable average micropore sizes for membrane wall  82  range from about 0.1 micrometers to about 1.0 micrometers, with particularly average suitable micropore sizes ranging from about 0.3 micrometers to about 0.7 micrometers. Examples of suitable materials for membrane wall  82  include polymeric materials, such as polypropylenes, polyethylenes, polysulfones, polyethersulfones, polyetheretherketones, polyimides, polyphenylene sulfides, polytetrafluoroethylenes, polyvinylidene difluorides, and combinations thereof. 
   As further shown, segment  78  also includes coating  84 , which desirably covers the entire exterior surface of membrane wall  82  at segment  78 . Coating  84  is a non-porous coating that blocks the transmission of gases and vapors, thereby preventing mass transfer across membrane  74  within recuperator chamber  70 . Suitable materials for coating  84  include any hydrophobic polymeric material that are stable to the heat and aqueous solutions. Examples of suitable materials for coating  84  include the suitable polymeric materials discussed above for membrane wall  82 , except that coating  84  does not include micropores. Coating  84  may be formed on membrane wall  82  with a variety of coating techniques, such as dip coating, wash coating, and spray coating. In alternative embodiments, the micropores of membrane wall  82  at segment  78  may be filled to block the transmission of gases and vapors, or membrane wall  82  at segment  78  may be formed without micropores. In these embodiments, membrane wall  82  is capable of blocking the transmission of gases and vapors, and coating  84  may be omitted. 
   As discussed above, while within contactor chamber  68 , the cool distillate fluid flows through inner hollow region  80  of membrane  74  and the heated feed solution flows adjacent to the exterior surface of membrane wall  82 . This creates a vapor pressure differential across membrane wall  82 , allowing distillate fluid vapor to penetrate through the micropores of membrane wall  82  into inner hollow region  80 . Correspondingly, the distillate fluid vapor carries latent heat from the feed solution to the distillate fluid within inner hollow region  80 . Additionally, thermal energy also conductively transfers across membrane wall  82  from the feed solution to the distillate fluid within inner hollow region  80 . 
   Within recuperator chamber  70 , coating  84  substantially prevents mass transfer across segment  78  of membrane  74 . Thus, distillate fluid vapor does not penetrate across segment  78 . However, segment  78  allows thermal energy to conductively transfer across membrane wall  82  and coating  84 , thereby recovering heat from the distillate fluid flowing through inner hollow region  76  to the feed solution flowing adjacent coating  84 . This increases the thermal efficiency of membrane module  12  (shown in  FIG. 2 ). 
     FIG. 4  is a schematic illustration of an interior of membrane module  112 , which is an alternative to membrane module  12  (shown in  FIG. 2 ), where the reference numerals of the corresponding components are increased by 100. As shown in  FIG. 4 , membrane module  112  includes outer casing  146 , potting resin walls  148 ,  150 , and  152 , connection conduit  154 , and hollow fiber membranes  156 , which function in the same manner as the corresponding components of membrane module  12 . Accordingly, outer casing  146  is a rigid structure extending along longitudinal axis  162 , and defines interior region  164  of membrane module  112 . Outer casing  146  also desirably includes couplings (not shown) for connecting membrane module  112  to feed inlet line  22 , feed outlet line  24 , distillate inlet line  38 , and distillate outlet line  40  of distillation system  10  (shown in  FIG. 1 ). While shown with a tubular geometry, outer casing  146  may exhibit alternative geometries, such as rectangular geometries. 
   Potting resin walls  148 ,  150 , and  152  are seals that divide interior region  164  into distillate inlet chamber  166 , contactor chamber  168 , recuperator chamber  170 , and distillate outlet chamber  172 , where the length-to-width ratio of interior region  164  differs from that of interior region  64  (shown in  FIG. 2 ). Accordingly, in the embodiment shown in  FIG. 4 , the flow paths of the feed solution through contactor chamber  168  and recuperator chamber  170  are perpendicular to lengths of hollow fiber membranes  156 . This creates a cross flow relative to hollow fiber membranes  156  to increase the mass and heat transfers across hollow fiber membranes  156 . Thus, in this embodiment, baffles corresponding to baffles  58  and  60  (shown in  FIG. 2 ) are omitted. 
   Connection conduit  154  is a shell-side conduit that allows the feed solution to travel from contactor chamber  168  to recuperator chamber  170  during operation. Hollow fiber membranes  156  are a plurality of tubular membranes that extend along longitudinal axis  162  between distillate inlet chamber  166  and distillate outlet chamber  172 , and function in the same manner as hollow fiber membranes  56  (shown in  FIG. 2 ). Accordingly, the segments of hollow fiber membranes  156  at contactor chamber  168  are formed from one or more hydrophobic, microporous materials that are capable of separating the distillate fluid from the feed solution. In contrast, the segments of hollow fiber membranes  156  at recuperator chamber  170  are non-porous, thereby preventing mass transfer across the membranes, but allow conductive heat transfer to occur. As such, recuperator chamber  170  functions as a heat exchanger in the same manner as recuperator chamber  70  (shown in  FIG. 2 ) for increasing the thermal efficiency of membrane module  112  and distillation system  10 . 
   During operation, the heated feed solution enters contactor chamber  168  via feed inlet line  22 . While the feed solution flows through contactor chamber  168  toward connection conduit  154 , the cool distillate fluid enters distillate inlet chamber  166  via distillate inlet line  38 , and flows through the inner hollow regions of hollow fiber membranes  156  toward distillate outlet chamber  172 . The vapor pressure differentials that are created across hollow fiber membranes  156  within contactor chamber  168  cause a portion of the distillate fluid in the feed solution to evaporate and transmit through the micropores of hollow fiber membranes  156 . As discussed above, latent and conductive heat also transfers from the feed solution to the distillate fluid flowing through the inner hollow regions of hollow fiber membranes  156 . 
   Upon entering recuperator chamber  170  from connection conduit  154 , the distilled feed solution flows through recuperator chamber  170  toward feed outlet line  24 . As discussed above, the segments of hollow fiber membranes  56  within recuperator chamber  170  are non-porous. Thus, distillate fluid vapor does not penetrate across hollow fiber membranes  156  within recuperator chamber  170 . Nonetheless, conductive heat is capable of traveling across the segments of hollow fiber membranes  156  within recuperator chamber  170 . As a result, at least a portion the thermal energy acquired during the separation process within contactor chamber  168  is transferred from the distillate fluid back to the distilled feed solution. This heat recovery within recuperator chamber  170  reduces the amount of energy required by heat exchangers  26  and  32  (shown in  FIG. 1 ), thereby increasing the thermal efficiency of distillation system  10 . 
   Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.