The present invention relates to distillation systems for liquid treatment processes. In particular, the present invention relates to distillation systems containing hollow fiber contactor 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. Both hollow fiber and flat sheet membranes have been used in laboratory studies. For membrane distillation, 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. Each hollow fiber membrane in the bundle is typically a hydrophobic, microporous membrane having an exterior surface and an inner hollow tubular region. In some membrane distillation systems, 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 membrane distillation, the feed solution is typically preheated to form a temperature differential across the hollow fiber membranes. This temperature differential creates a vapor pressure differential between the shell side and the tube side of the fibers in the module, which causes a portion of the feed solution to evaporate near the pore entrance on the feed solution side and the vapor to transmit through the hollow fiber membranes. The transmitted vapor then condenses at a gas/liquid interface near the pore entrance of the membranes on the distillate side, thereby providing the desired distillate. Because the vaporization of a liquid is involved in the separation process, a large amount of thermal energy is transferred from the feed solution to the distillate. Being able to recoup the thermal energy significantly affects the thermal efficiency of the membrane distillation processes. Thus, there is an ongoing need for the recuperation of thermal energy in membrane distillation processes.
Embodiments of the present invention include a contactor module with an outer casing having an interior region, pluralities of hollow fiber membranes and hollow tubes, and a distillate collection chamber. The hollow fiber membranes extend through the interior region and are configured to allow vapor transmission. The hollow tubes extend through the interior region and are configured to substantially prevent vapor transmission and to allow transmission of thermal energy. Distillate is collected in the distillate collection chamber, through which the hollow fiber membranes and hollow tubes extend.
One exemplary embodiment of the present invention includes a distillation apparatus having second feed inlet and outlet manifolds and a contactor module. The contactor module includes an outer casing having an interior region, four seals within the outer casing dividing the interior region into three chambers, pluralities of hollow fiber membranes and hollow tubes, and a distillate outlet. The hollow fiber membranes extend from a first feed inlet chamber to a first feed outlet chamber and are configured to allow vapor transmission. The hollow tubes extend from the second feed inlet manifold to the second feed outlet manifold and are configured to substantially prevent vapor transmission and to allow transmission of thermal energy.
Further exemplary embodiments include a system for separating distillate from a feed solution having first and second feed loops for providing the feed solution, at least one contactor module having three chambers, pluralities of hollow fiber membranes and hollow tubes, and a distillate outlet. The hollow fiber membranes extend from a first feed inlet chamber to a first feed outlet chamber, are in fluid communication with the first feed loop, and are configured to allow vapor transmission. The hollow tubes extend from a second feed inlet manifold to a second feed outlet manifold, are in fluid communication with the second feed loop, and are configured to substantially prevent vapor transmission and to allow transmission of thermal energy.
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 includes standard fluid processing equipment (not shown), such as process control units, fluid pumps, and filters.
Contactor module 12 is a hollow fiber contactor module that separates the distillate fluid from the feed solution (e.g., water from an aqueous brine solution) via vapor pressure differentials. As discussed below, contactor module 12 is divided into a shell side (not shown in
Contactor modules 12 in distillation systems 10 according to the present invention can operate in liquid gap or air gap modes. In both operational modes, the first tube side and the shell side are separated by a hydrophobic membrane and the first tube side contains the feed solution. In liquid gap mode, liquid distillate is allowed to contact both the membrane and a cooling/condensing surface. This differs from the operation of a module using air gap mode. In air gap mode, distillate contact with both the membrane and the condensing surface is avoided. In some cases, such contact can impair the function of contactor module 12.
In liquid gap mode, the shell side of contactor module 12 contains a liquid (distillate). The shell side also contains an element that has a temperature lower than the first tube side. According to the present invention, this element is the exterior surface of the second tube side. The second tube side contains feed solution that is at a lower temperature than the feed solution in the first tube side. The second tube side cools the air or distillate in the shell side, and thus creates a vapor pressure differential between the first tube side and the cooler shell side. This vapor pressure differential causes transport of part of the feed solution from the first tube side to the shell side. The part of the feed solution transported across the hydrophobic membrane may then condense in the shell side on the exterior surfaces of the first and second tube sides and then join with the distillate and be collected. In air gap mode, the shell side of contactor module 12 contains a gas (air).
For the purposes of illustration, operation in liquid gap mode will be described in detail. Differences between liquid gap mode and air gap mode, where applicable, are provided below. During operation, the heated feed solution flows from first feed loop 14 to the first tube side of contactor module 12. While the feed solution flows through the first tube side of contactor module 12, part of the feed solution evaporates and the resulting vapor is transported through the first tube walls into the distillate section and a slightly concentrated feed solution remains as residual. The vapor condenses on the shell side of first tube walls in the distillate section 18 of contactor module 12. The residual concentrated feed solution exits the first tube side of contactor module 12 and enters the second feed loop 16 where it is cooled. The cooled residual concentrated feed solution flows from second feed loop 16 to the second tube side of contactor module 12. While the cooled residual concentrated feed solution flows through the second tube side of contactor module 12, heat transfers from the vapor and condensate in the distillate section 18 of contactor module 12 to the second tube side. The cooled residual concentrated feed solution exits the second tube side of contactor module 12 and reenters the first feed loop 14.
While distillation system 10 is shown with a single contactor module (i.e., contactor module 12), distillation system 10 may alternatively include additional numbers of contactor modules 12. Examples of suitable numbers of contactor modules 12 for use in distillation system 10 range from one to one hundred; particularly suitable numbers of contactor modules 12 for use in distillation system 10 range from one to twenty five; and even more particularly suitable numbers of contactor modules 12 for use in distillation system 10 range from one to ten. The contactor modules 12 can be arranged in serial configurations, parallel configurations, and combinations thereof.
First feed loop 14 is a fluid pathway for the feed solution, and includes feed source line 20, feed conveyance lines 22a-22b, feed inlet line 24, feed outlet line 26, feed reservoir tank 28, fluid pump 30, and heat exchanger 32. Feed source line 20 is a valve-controlled fluid conduit for transferring the feed solution from feed reservoir tank 28 to first feed loop 14. During operation of distillation system 10, portions of the feed solution are separated within contactor module 12 to provide the distillate in distillate section 18. As such, feed source line 20 provides additional feed solution to mix with the residual concentrated feed solution and balance the flow of feed solution in first feed loop 14.
Feed conveyance lines 22a-22b are fluid conduits for first feed loop 14, and provide a clockwise flow path in the embodiment shown in
Feed reservoir tank 28 is a container that supplies feed solution into feed conveyance line 22a by way of feed source line 20. Feed reservoir tank 28 provides a reservoir of feed solution for the operation. Additional feed solution may be needed to balance the volume of condensate produced or maintain pressure within first feed loop 14. When additional feed solution is needed, feed solution from feed reservoir tank 28 is added to first feed loop 14. Fluid pump 30 is a pump that interconnects feed conveyance lines 22a and 22b, and is configured to deliver the feed solution within feed loop 14 to the first tube side of contactor module 12.
As discussed below, the feed solution is maintained in a liquid phase to prevent contaminants of the feed solution from entering the shell side and mixing with the distillate in distillate section 18. This would otherwise reduce the purity of the distillate, and could potentially require discarding the distillate product. The pressure of the feed solution entering contactor module 12 is also less than a liquid penetration pressure of the first tube side membranes (first tubes) of contactor module 12. The liquid penetration pressure is the pressure at which liquid from the feed solution penetrates through the pores of the first tube side membranes (first tubes) of contactor module 12 (i.e., from the tube side to the shell side), and is a function of several factors, such as the average pore radius of the membranes and the average liquid contact angle of the membranes (i.e., hydrophobicity of the membranes). Liquid penetration of the feed solution from the first tube side into the shell side of contactor module 12 may result in the contamination of the distillate, thereby reducing the purity of the distillate. Resistance to the penetration of liquid into the membrane pores allows the shell side to be pressurized to a pressure below the liquid penetration pressure.
Heat exchanger 32 is a heat-providing heat exchanger that interconnects feed conveyance line 22c and feed inlet line 24, thereby increasing the temperature of the feed solution passing from feed conveyance line 22c to feed inlet line 24. Accordingly, heat exchanger 32 heats the feed solution to an elevated temperature before it enters the first tube side membranes of contactor module 12. As discussed below, heating the feed solution allows the feed solution to enter contactor module 12 at an elevated temperature to increase the separation rate within contactor module 12.
Some membrane materials are sensitive to thermal degradation. Thus, the elevated temperature of the feed solution entering contactor module 12 should be balanced to provide a high vapor pressure driving force for distillation and limit the thermal degradation of the first tube side membranes (first tubes) of contactor module 12 (not shown in
Second feed loop 16 is a fluid pathway for the residual concentrated feed solution, and includes residual concentrated feed solution outlet line 36, heat exchanger 38, residual concentrated feed solution conveyance line 40, residual concentrated feed solution inlet line 44 and discharge line 34. Second feed loop 16 also optionally includes residual concentrated feed solution circulation pump 42. In some embodiments, fluid pump 30 of first feed loop 14 may provide enough pressure to drive the system without the need for an additional pump in second feed loop 16.
Second feed loop 16 provides a flow of cool fluid (residual concentrated feed solution) to the second tube side of contactor module 12 to assist the condensation process. Heat exchanger 38 is a cooling heat exchanger that interconnects residual concentrated feed solution conveyance line 40 and residual concentrated feed solution inlet line 44, thereby lowering the temperature of the residual concentrated feed solution flowing through second feed loop 16. Suitable lowered temperatures for the residual concentrated feed solution may vary depending on the composition of the residual concentrated feed solution. For aqueous residual concentrated feed solutions, examples of suitable lowered temperatures range from about 5° C. to about 75° C., with particularly suitable lowered temperatures ranging from about 20° C. to about 55° C.
Residual concentrated feed solution conveyance line 40 is a fluid conduit for second feed loop 16, and provides for a counter-clockwise flow path for the residual concentrated feed solution in the embodiment shown in
During operation, the feed solution flows through first feed loop 14, and is respectively pumped by fluid pump 30 and heated by heat exchanger 32 to the elevated temperature. While flowing through first feed loop 14, the feed solution flows into the first tube side of contactor module 12 via feed inlet line 24. Also during operation, a supply of the residual concentrated feed solution flows through second feed loop 16, and is cooled by heat exchanger 38 to a reduced temperature. While flowing through second feed loop 16, the cooled residual concentrated feed solution flows into the second tube side of contactor module 12 via residual concentrated feed solution inlet line 44.
The cooled residual concentrated feed solution and the heated feed solution flowing into contactor module 12 create a temperature differential between the first tube side and the shell side of contactor module 12. The cooled residual concentrated feed solution on the second tube side cools the distillate on the shell side to provide a temperature differential between the shell side and the first tube side. For example, a feed solution entering the first tube side of contactor module 12 at about 80° C. and a residual concentrated feed solution entering the second tube side of contactor module 12 at about 40° C. provide a temperature differential of about 40° C. between the first and second tube sides. As the cooled residual concentrated feed solution on the second tube side cools the distillate on the shell side, the temperature differential between the first tube side and the shell side increases. This correspondingly increases the separation rate of the distillate from the feed solution, where the separated distillate transfers from the first tube side to the shell side of contactor module 12. The separated distillate then exits contactor module 12 via distillate outlet line 46, and is subsequently collected in recovery tank 48 as the distillate product (e.g., clean water in a water treatment process). Recovery tank 48 is a container for receiving the distillate obtained from contactor module 12.
Distillate outlet line 46 may contain a flow transducer 47 to monitor the flow through distillate outlet line 46 and into recovery tank 48. The information collected from flow transducer 47 may be used to regulate the addition of additional feed solution to first feed loop 14 via feed source line 20. This allows distillation system 10 to operate in a steady state and at optimal efficiency. Additional feed solution is also added to first feed loop 14 via feed source line 20 to account for any residual concentrated feed solution removed from second feed loop 16 through discharge line 34. If discharge line 34 is closed, no residual concentrated feed solution is removed and all of the residual concentrated feed solution is returned to contactor module 12 and first feed loop 14. In exemplary embodiments of distillation system 10, discharge line 34 removes between about 1% and about 67% of the residual concentrated feed solution. The rate of removal is dependent upon the distillation application, the salt concentration of the feed solution and the overall water recovery desired.
Optionally, a distillate recirculation pump 50 or blower 50A may be in fluid communication with the distillate section 18.
Potting resin walls 54, 56, 58, and 60 are seals extending generally perpendicular to longitudinal axis 76, and are formed from one or more sealant materials, such as acrylate, polyurethane, and epoxy-based materials. Accordingly, potting resin walls 54, 56, 58, and 60 divide interior region 74 into first chamber 64, second (contactor) chamber 66, and third chamber 68. First manifold 62 is a chamber disposed between first manifold casing 52a and potting resin wall 54, and is open to feed outlet line 26 in the embodiment illustrated in
According to the embodiment illustrated in
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When present, optional distillate recirculation pump 50 is in fluid communication with distillate section 18. Distillate recirculation pump 50 may be in communication with second (contactor) chamber 66 via contactor inlet 73. Distillate recirculation pump 50 may provide means for circulating distillate within distillate section 18. A portion of distillate may be collected during recirculation. Recirculation may also increase mixing on the shell side of contactor module 12, which increases the vapor pressure differential between the first tube side and the shell side. Distillate recirculation pump 50 may be configured to direct a suitable fluid, i.e. collected or additional distillate, into second (contactor) chamber 66. The delivered suitable fluid drives the distillate which has condensed on the shell side of hollow fiber membranes 78 and hollow recuperators 80 towards the distillate outlet section 72 and distillate outlet line 46 for collection. By driving the distillate towards the distillate outlet section 72, mixing on the shell side may be increased thereby increasing the vapor pressure differential between the first tube side and the shell side and, thus, increasing the rate at which distillate is separated from the feed solution by hollow fiber membranes 78.
For ease of manufacture and assembly, one or more groups of hollow fiber membranes 78 may be attached to fabric layers at predetermined intervals. Hollow recuperators 80 may also be attached to separate fabric layers at predetermined intervals. The layers of hollow fiber membranes 78 and hollow recuperators 80 can be arranged side by side and wound in a spiral fashion within the interior region as illustrated in
As discussed below, feed solution heated to an elevated temperature in first feed loop 14 is pumped through hollow fiber membranes 78. Due to vapor pressure differentials between the first tube side and the shell side, distillate separates from the feed solution within hollow fiber membranes 78, the first tube side, and the distillate crosses the hollow fiber membranes 78 and into the interstitial volume of interior region 74, the shell side. Meanwhile, residual concentrated feed solution cooled in second feed loop 16 is pumped through hollow recuperators 80. Because of temperature differentials between the second tube side and the shell side, the cooled residual concentrated feed solution on the second tube side absorbs heat energy from the warmer distillate in the interstitial volume of interior region 74 across hollow recuperators 80, thereby cooling the distillate.
The operation of distillation system 10 will be described with reference to the embodiment of
During operation, the heated feed solution enters first chamber 64 via feed inlet line 24. The feed solution is then directed to hollow fiber membranes 78. The feed solution enters the hollow fiber membranes 78 on the first tube side and travels towards third chamber 68 and residual concentrated feed solution outlet line 36. At the same time, cooled residual concentrated feed solution enters second manifold 70 via residual concentrated feed solution inlet line 44, and flows through the inner hollow regions of hollow recuperators 80 on the second tube side toward first manifold 62 and feed outlet line 26. As the cooled residual concentrated feed solution flows through the inner hollows of hollow recuperators 80, the temperature differentials between the interior and exterior surfaces of hollow recuperators 80 allows for the transfer of thermal energy within second (contactor) chamber 66. Since, the cooled residual concentrated feed solution has a temperature lower than the distillate on the shell side of contactor module 12, the cooled residual concentrated feed solution absorbs thermal energy across the walls of hollow recuperators 80. The cooled residual concentrated feed solution on the second tube side is thereby warmed and the distillate on the shell side is cooled. The residual concentrated feed solution traveling through hollow recuperators 80 is warmed as it passes through contactor module 12. This residual concentrated feed solution exits contactor module 12 at first manifold 62 through feed outlet line 26 and returns to first feed loop 14 where it is heated and returned to contactor module 12 as heated feed solution. The passage of residual concentrated feed solution through hollow recuperators 80 allows the residual concentrated feed solution to be preheated before it reaches heat exchanger 32 of first feed loop 14. The preheating of residual concentrated feed solution provides for heat recuperation and a reduction in energy needed to heat the feed solution before it enters contactor module 12.
As the feed solution flows through the inner hollows of hollow fiber membranes 78, the temperature differentials between the interior and exterior surfaces of hollow fiber membranes 78 (the first tube side and the shell side, respectively) create vapor pressure differentials across hollow fiber membranes 78 within second (contactor) chamber 66. The vapor pressure differentials allow vapor from the feed solution in the hollow fiber membranes 78 to be transported from the tube side of hollow fiber membranes 78 to the shell side and condense as distillate. Distillate separates from the feed solution and passes across the hollow fiber membranes 78 from the first tube side to the shell side.
As discussed above, the elevated temperature of the feed solution on the first tube side and the reduced temperature of the distillate on the shell side substantially increase the vapor pressure differentials across hollow fiber membranes 78, thereby increasing the separation rate of distillate from the feed solution. The vapor pressure differentials cause a portion of the distillate in the feed solution to evaporate and transmit through the micropores of hollow fiber membranes 78. When the vapor passes from the inner hollow regions of hollow fiber membranes 78 into the shell side of the interior region 74, the cooled supply of distillate causes the vapor to condense near the pore entrances on the shell side (i.e. exterior surfaces) of hollow fiber membranes 78 (liquid gap mode) or the shell side of hollow recuperators 80 (air gap mode) in interior region 74 and mix with the distillate. As discussed below, in some embodiments, the exterior of the hollow fiber membranes 78 are hydrophobic. The hydrophobic exterior of hollow fiber membranes 78 prevents distillate from reentering the membrane pores and rejoining the feed solution on the first tube side of hollow fiber membranes 78. Due to the system pressure and/or gravity, the distillate flows toward distillate outlet section 72 once it enters the shell side, and proceeds into distillate outlet line 46.
The feed solution generally remains in a liquid state while flowing through the first tube side of contactor module 12, within the interior surfaces of hollow fiber membranes 78. The hydrophobic surface of the hollow fiber membranes 78 (first tubes) keeps feed solution from entering the membrane pores while allowing vapor transport from the first tube side to the shell side of contactor module 12. Additionally, the pressure of the feed solution entering contactor module 12 is also less than the liquid penetration pressure of hollow fiber membranes 78. If the pressure of the feed solution exceeds the liquid penetration pressure of hollow fiber membranes 78, portions of the liquid feed solution may be forced through the micropores of hollow fiber membranes 78 into the shell side of contactor module 12. Accordingly, the temperature and pressure of the feed solution entering contactor module 12 are balanced to prevent transmission of the feed solution across hollow fiber membranes 78.
As discussed above, while within second (contactor) chamber 66, the distillate flows through the shell side of interior region 74 of contactor module 12 and the heated feed solution flows through inner hollow region 86 of membrane 82 adjacent to the surface of membrane wall 84. This creates a vapor pressure differential across membrane wall 84, allowing distillate vapor to evaporate and penetrate through the micropores of membrane wall 84 into the shell side of interior region 74. When the distillate vapor passes into the shell side of interior region 74, the cool supply of residual concentrated feed solution within hollow recuperators 80 (second tubes) causes the distillate vapor to condense within the shell side of interior region 74 on the exterior of hollow recuperators 80 (second tubes) and mix with the distillate.
As discussed above, the pressure of the feed solution entering contactor module 12 is also less than the liquid penetration pressure of membrane wall 84 of membrane 82. The liquid penetration pressure of membrane wall 84 is generally dependent on the composition of membrane wall 84, the wall thickness of membrane wall 84, and on the sizes of the micropores of membrane wall 84. Examples of suitable wall thicknesses for membrane wall 84 range from about 50 micrometers to about 500 micrometers, with particularly suitable wall thicknesses ranging from about 100 micrometers to about 250 micrometers. Examples of suitable average micropore sizes for membrane wall 84 range from about 0.1 micrometers to about 0.6 micrometers, with particularly suitable average micropore sizes ranging from about 0.2 micrometers to about 0.4 micrometers.
Although the present invention has been described with reference to exemplary 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.