Patent Publication Number: US-2017361277-A1

Title: Vacuumed gap membrane distillation (vagmed) module, multi-stage vagmed systems, and vagmed processes

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is the National Stage of International Application No. PCT/162015/002518, filed 17 Dec. 2015, which claims the benefit of and priority to U.S. Provisional Patent Application No. 62/095,136 entitled “VACUUMED GAP MEMBRANE DISTILLATION (VAGMED) MODULE, MULTI-STAGE VAGMED SYSTEMS, AND VAGMED PROCESSES”, filed on 22 Dec. 2014, all of which are expressly incorporated by reference as if fully set forth herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to membrane-based separation systems and processes, in particular membrane-based distillation systems and processes for desalination. 
     BACKGROUND 
     Membrane distillation (MD) is a thermally driven membrane-based separation process, considered as one of the technologies that are emerging as alternative desalination processes. MD utilizes a hydrophobic, micro-porous membrane as a contactor to achieve separation by liquid-vapor equilibrium. Pre-heated feed solution is brought into contact with the membrane which allows only the water vapor to go through the membrane pores so that it condenses on the other side of the membrane. This vapor is driven across the membrane by the difference in the partial vapor pressure maintained at the two sides of the membrane created by the difference of temperatures (feed/coolant). 
     Conventional desalination technologies such as multi-stage flash distillation (MSF) and reverse osmosis (RO) are not only highly energy intensive processes but they require huge investment cost and large footprint (including extensive pretreatment required for the RO process); whereas MD operates at ambient pressure and lower temperatures (40-90° C.) so that any low grade heat source (solar, waste heat and low-enthalpy geothermal) can be sufficient for its operation. Moreover the scalability, low-cost polymeric materials for the installation, and the very high salt rejection reaching 99.95% (theoretically 100%) regardless of the feed concentration, makes MD as an attractive alternative desalination process. 
     The major configurations that have been employed in MD process are direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), vacuum membrane distillation (VMD) and sweeping gas membrane distillation (SGMD). In all configurations, hot feed solution is in direct contact with the membrane. In DCMD, both hot and cold streams are in direct contact with the membrane. In AGMD, a stagnant air gap is maintained between the membrane and a condensation surface (the coolant flows in the external side of the condensation surface). Distilled water could also be filled in the air gap, known as liquid gap MD configuration. In VMD and SGMD, vacuum and a cold inert gas are passed through the permeate side, respectively, so that the vapor coming across the membrane from the feed is condensed outside the membrane module. 
     Accordingly, there is a need to address the aforementioned deficiencies and inadequacies. 
     SUMMARY 
     The present disclosure provides novel vacuumed gap membrane distillation (VAGMED) modules, multi-stage VAGMED systems, and VAGMED processes for different applications, such as water desalination, thermal and RO brines treatment, water reclamation and water reuse, and a membrane-based module for use in such systems. Current technologies such as MSF and RO are very high energy intensive and require high investment cost, large foot print, including extensive pretreatment for RO, and they are not environmentally friendly (emission of CO 2  and high chemicals consumption). 
     VAGMED operates at low temperature (40-90° C.) so that it is suitable for renewable energy, such as solar, low-enthalpy geothermal and any kind of low-grade waste heat such as from cooling towers, nuclear power stations. It operates at atmospheric pressure, which further reduces the operational cost. Also, the salinity of the feed water does not have much of an effect on VAGMED systems allowing operation at very high recoveries. 
     The novel design employed in the fabrication of the present VAGMED module ensures enhanced water production during the process. In addition, present device has advantages of being modular, having a lower footprint, being compact, having higher thermal efficiency through its more efficient heat recovery system, low investment cost (low-cost polymeric materials for the membrane and module fabrication), and having wide applications, such as water desalination, brines treatment, reclamation and reuse treatment units/plants. 
     In an embodiment, among others, a membrane distillation module is provided, comprising: a) a condenser including a condensation surface; b) a first passageway having an inlet for receiving a first feed stream and an outlet through which the first stream can pass out of the first passageway, the first passageway configured to bring the first feed stream into thermal communication with the condensation surface; c) an evaporator including a permeable evaporation surface allowing condensable gas to pass there through; d) a second passageway having an inlet for receiving a second feed stream and an outlet through which the second feed stream can pass out of the second passageway, the second passageway configured to bring the second feed stream into communication with the permeable evaporation surface; e) an enclosure providing a vacuum compartment within which the condenser, the evaporator and the first and second passageways of the module are contained; and f) a vacuum system coupled to the vacuum compartment of the enclosure. The vacuum system can be configured to control the pressure within the vacuum compartment of the module by adjusting the amount of vacuum applied to the vacuum compartment of the module relative to the saturation pressure of the second feed stream in the module and to remove uncondensed gas from the vacuum compartment. 
     In an embodiment among others, a multi-stage system is provided including a plurality of membrane distillation modules, each of the plurality of membrane distillation modules comprising the aforementioned membrane distillation module. The plurality of the membrane distillation modules can be coupled in series such that the outlet of the first passageway of one of the plurality of modules is coupled to the inlet of the first passageway of another of the plurality of modules and the inlet of the second passageway of the one of the plurality of modules is coupled to the outlet of the second passageway of the another of the plurality of modules, or vice versa. The system can include means for passing the first feed stream out of the outlet of the first passageway of the first module to the inlet of the first passageway of the another module; means for passing the first feed stream out of the outlet of the first passageway of the another module to the inlet of the second passageway of the another module wherein the first feed stream becomes the second feed stream to the another module; means for passing the second feed stream out of the outlet of the second passageway of the another module to the inlet of the second passageway of the first module, or vice versa. The vacuum system can be configured to control the pressure within the vacuum compartment of each module by adjusting the amount of vacuum applied to the vacuum compartment of each module relative to the saturation pressure of the second feed stream in each module and to remove non-condensable gas and uncondensed condensable gas (if any) from the vacuum compartment. Means for collecting condensate from each module can also be provided. In one or more aspects the second feed stream can incur a reduction in temperature due to evaporation of condensable gas from the second feed stream within the evaporator of each module of the plurality of membrane distillation modules. The number of modules of the plurality of membrane distillation modules can be determined based on including a module of the plurality of the membrane distillation modules for every 2-3° C. or more reduction in the temperature of the second feed stream in each of the modules. The first stream exiting the outlet of the first passageway of one of the modules can be heated to form the second feed stream and then delivered to the inlet of the second passageway of another one of the plurality of modules, or the second steam exiting the outlet of the second passageway of the another one of the plurality of modules is cooled to form the first stream and then delivered to the inlet of the first passageway of the one of the modules. 
     In an embodiment, among others, a method of membrane distillation is provided. The method can comprise the steps of: a) providing a module, the module including a condenser and an evaporator, the condenser of the module including a condensation surface, the evaporator of the module including a permeable evaporation surface allowing condensable gas to pass there through, a first passageway having an inlet for receiving a first feed stream and an outlet through which the first feed stream can pass out of the first passageway, the first passageway configured to bring the first feed stream into thermal communication with the condensation surface, and a second passageway having an inlet for receiving a second feed stream and an outlet through which the second feed stream can pass out of the second passageway, the module including an enclosure providing a vacuum compartment within which the condenser, the evaporator and the first and second passageways of the module are contained, wherein the vacuum compartment of the module is coupled to a vacuum system; b) providing a first feed stream to the inlet of the first passageway of the condenser of the module; c) cooling the condensation surface of the condenser of the module with the first feed stream; d) passing the first feed stream out of the outlet of the first passageway of the module; e) passing a second feed stream to the inlet of the second passageway of the module; f) evaporating condensable gas from the second feed stream and passing the condensable gas formed through the evaporation surface of the evaporator of the module; g) condensing the condensable gas on the condensation surface of the condenser within the module; h) passing the second feed stream out of the second passageway of the module; and i) using the vacuum system to control the pressure within the vacuum compartment of the module by adjusting the amount of vacuum applied to the vacuum compartment of the module relative to the saturation pressure of the second feed stream in the second passageway of the module. 
     In one or more aspects of the method, a plurality of membrane distillation modules can be provided. Each of the plurality of membrane distillation modules can comprise the aforesaid module. The plurality of modules can be coupled in series such that the outlet of the first passageway of one of the plurality of modules is coupled to the inlet of the first passageway of another of the plurality of modules and the inlet of the second passageway of the one of the plurality of modules is coupled to the outlet of the second passageway of the another of the plurality of modules, or vice versa. The method can include passing the first feed stream out of the outlet of the first passageway of the first module to the inlet of the first passageway of the another module; cooling the condensation surface of the condenser of the another module with the first feed stream; passing the first feed stream out of the first passageway of the another module; passing the second feed stream to the inlet of the second passageway of the another module, or vice versa. The method can include evaporating condensable gas from the second feed stream and passing the condensable gas formed in the another module through the evaporation surface of the evaporator of the another module; condensing the condensable gas on the condensation surface of the condenser within the another module, and passing the second feed stream out of the second passageway of the another module to the inlet of the second passageway of the first module. The second feed stream can incur a reduction in temperature due to evaporation of condensable gas from the second feed stream within the evaporator of each said module of the plurality of membrane distillation modules. The number of modules of the plurality of membrane distillation modules can be determined based on including a module of the plurality of membrane distillation modules for every 2-3° C. or more reduction in the temperature of the second feed stream in each of the membrane distillation modules. The first feed stream, after exiting the outlet of the first passageway of one of the modules, can be heated to form the second feed stream and then delivered to the inlet of the second passageway of another one of the plurality of modules, or the second steam, after exiting the outlet of the second passageway of the another one of the plurality of modules, is cooled to form the first stream and then delivered to the inlet of the first passageway of the one of the modules. 
     In any one or more aspects of the various embodiments, the first feed stream can be selected from the group consisting of seawater, brine solution, industrial waste water, produced water, brackish water and non-potable water and the condensable gas is water vapor. The first feed stream or the second stream, or both, can be de-gasified to remove non-condensable gas from the first feed stream or the second feed stream, or both, prior to being delivered to the module. The first feed stream can include a salt, a mixture of a salt and an organic contaminant or a mixture of a salt and an inorganic contaminant. The first feed stream can be a cold feed stream relative to temperature of the second feed stream, or the first feed stream can be cooled to have a colder temperature relative to the temperature of the second feed stream prior to being delivered to the inlet of the hollow body of the condenser of the module. The first feed stream, after exiting the first passageway of the module, can be heated to form the second feed stream and then delivered to the inlet of the second passageway of the module. The second feed stream can be a hot feed stream relative to the temperature of the first feed stream, or the second feed stream can be heated to have a hotter temperature relative to the temperature of the first feed stream prior to being delivered to the inlet of the second passageway of the module. The second feed stream after exiting the module can be cooled to form the first feed stream and then delivered to the inlet of the first passageway of the module. The second feed stream can include a salt, a mixture of a salt and an organic contaminant or a mixture of a salt and an inorganic contaminant. The permeable evaporation surface of the module can be selected from the group consisting of micro-porous hydrophobic membranes, nanocomposite membranes, surface modified membranes, dual layer composite hydrophobic/hydrophilic membranes, and modified ceramic membranes. The vacuum system can be used to control the pressure within the vacuum compartment of the module to be about 1% to about 5% below the saturation pressure of the second feed stream passed to the inlet of the hollow body of the evaporator of the module. 
     In any one or more aspects of the various embodiments, the condensation surface and the permeable evaporation surface can be configured in an opposed, spaced apart relationship forming an air gap there between within which condensable gas can be received. The condensation surface, the permeable evaporation surface, or both can be configured as a flat sheet. The condensation surface, the permeable evaporation surface, or both can be configured as a sheet having a non-flat configuration. Thus, for example, one of the condensation surface(s) or the permeable evaporation surface(s) can be a flat sheet while the other has a non-flat configuration. The non-flat configuration can be a sheet having a zigzag, sinusoidal, etc. or a hollow tube configuration. The condensation surface(s) and the permeable evaporation surface(s) can be hollow/hollow, flat/hollow, hollow/fat, flat/flat etc. The ratio of condensation surface area to permeable evaporation surface area can be 1:1, or more than or less than 1:1. The flow of the first and second streams can be from the inlet and out through the outlet of the first and second passage ways or in reverse flow. 
     Other devices, systems, processes, features, and advantages of the present disclosure for vacuumed gap membrane distillation (VAGMED) will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
       Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIGS. 1A and 1B  depict a schematic diagram of an exemplary membrane-based separation module for use in a multi-stage vacuumed gap membrane distillation VAGMED system of the present disclosure, A) as a flat sheet and B) as a hollow fiber. Evaporation and condensation are done inside the same module. 
         FIG. 2  depicts staging the MD process to maintain heated feed at the water-vapor saturation curve. 
         FIGS. 3A-3C  depict various process flow diagrams of the present disclosure for: A) a flat sheet VAGMED process using the module of  FIG. 1A ; B) a hollow fiber once through VAGMED module, wherein (the broken line represents a simple brine recycle VAGMED mode; and C) a hollow fiber complete brine recycle VAGMED process, the systems of  FIGS. 3B and 3C  using the module of  FIG. 1B . 
         FIG. 4A  depicts one way of a number of possible ways of fabricating a hollow fiber MD module of  FIG. 1B , and  FIG. 4B  depicts a picture of a prototype unit of a hollow fiber MD module. 
         FIGS. 5A-5D  depict various embodiments of a module of  FIG. 1B . 
         FIGS. 6A-6E  depict various additional embodiments of exemplary mixed evaporation and condensation hollow fibers module of the present disclosure.  FIGS. 6F-H  depict pictures of a prototype of a hollow fiber unit for use in a module. 
         FIG. 7  depicts an embodiment of a mixed flat sheet evaporation and condensation hollow fibers module. 
         FIG. 8  depicts simulated results of an VAGMED process including a staging flow diagram. 
         FIG. 9  depicts an example of a VAGMED reversal process design having multi-stage modules of treating hot feed sources as opposed to cold feed sources. 
     
    
    
     DETAILED DESCRIPTION 
     Described below are various embodiments of the present devices, systems and methods for vacuumed gap membrane distillation. Although particular embodiments are described, those embodiments are mere exemplary implementations of the system and method. One skilled in the art will recognize other embodiments are possible. All such embodiments are intended to fall within the scope of this disclosure. Moreover, all references cited herein are intended to be and are hereby incorporated by reference into this disclosure as if fully set forth herein. While the disclosure will now be described in reference to the above drawings, there is no intent to limit it to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications and equivalents included within the spirit and scope of the disclosure. 
     The present disclosure provides novel modules, systems and methods for vacuumed gap membrane distillation (VAGMED). In various aspects a module is provided for use in a vacuumed gap membrane distillation system. In various other aspects multi-stage vacuumed gap membrane distillation systems employing a plurality of the modules are provided comprising a plurality of the modules. In various other aspects vacuumed gap membrane distillation processes are provided. 
     The present VAGMED modules and processes can incorporate a thermal membrane-based process. The present systems and processes can include one or more of the membrane-based modules. The present systems and processes can be multi-stage systems and processes that include one or more of the membrane-based modules that, for example can be installed in series. Each module can represent a single stage, or one stage of a plurality of stages or effects. 
     In any one or more aspects the module can include:
         1. A condensation surface. The condensation surface can be a thin polymeric or metallic plate or tubes (polymeric may be preferred to avoid corrosion of metal tubes). The condensation surface can be part of a heat exchanger that can be cooled by a cold feed stream. The cold feed stream can come from a feed source, or from a previous module, to recover the latent heat of condensation from vapor generated by the evaporation surface.   2. An evaporation surface. The evaporation surface can be a porous membrane. For example it can be a micro-porous, hydrophobic membrane that can be in a form of flat sheet or a hollow fiber. The membrane can be made of polymeric materials such as polypropylene, polyvinylidine fluoride, polytetrafluoroethylene, polyethylene, polyazoles etc. The membrane can receive a relatively hot feed stream as compared to the cold feed stream. The relatively hot feed stream can be from a heated stream source or from a previous module, for example in a counter-current flow regime with the cold feed stream that passes through the condensation surface. The relatively hot feed stream loses some of its mass and temperature through evaporation before it reaches the next module.   3. A vacuum compartment. The vacuum compartment or enclosure can be an airtight enclosure. The enclosure can be made of PVC or any other material that isolates the condensation and evaporation surfaces from the environment. The vacuum compartment can be connected to the next or a previous module or both via ports. In one or more aspects the vacuum compartment can have inlet and outlet ports for condensate transfer and inlet and outlet ports for connection to a vacuum system and applying vacuum. In various aspects the applied vacuum can be used to remove uncondensed gases present in the gap between the evaporators and condensers in the vacuum compartment. Raw feed water is de-aerated before it enters the system.       

     Schematic drawings of various aspects of a module of the present disclosure are presented in  FIGS. 1A and 1B .  FIG. 1A  is a cross-sectional schematic view of an embodiment, among many, of a membrane-based separation module for use in the present system and method. The module  10  is a flat sheet module design. The module  10  of  FIG. 1A  includes one or more condensers  12  and evaporators  17 . The one or more condensers  12  can be one or more heat exchangers and can include a passageway or conduit  13  allowing for a first feed stream  14 , for example a cooling fluid, to pass through the condenser  12 . The cooling fluid can be a cold feed stream  14 . The cold feed stream  14  can be seawater, brine solution, industrial waste water, produced water, brackish water and/or non-potable water to be reused. The condenser  12  can include a condensation surface  15 . The condensation surface may be made of a metal, though other materials that allow efficient thermal conductivity for the cooling of the condensation surface  15  by the cold feed stream  13 . In various aspects a non-corrosive material may be preferred for the condensation surface, such as polymeric materials that have good thermal conductivity. 
     The evaporator  17  also includes a passageway or conduit  18  for allowing a feed stream  19 , sometimes referred to as a second feed stream, to pass there through. The second feed stream can be a heated feed stream  19  or hot stream, e.g. natural geothermal spring or a discharge having high temperature which is enough to drive the process (direct multi-stage process) without a heat input. The heated feed stream can be a cold feed stream that has been heated. The evaporator  17  also includes an evaporation surface  21  on the exterior of the evaporator. The evaporation surface  21  can be designed or configured to allow condensable gas, for example water vapor, to pass out of the passageway  18  there through to the outside of the evaporator. In various aspects, the evaporation surface can be a porous membrane that allows water vapor to pass from the conduit  18  through the membrane and out of the evaporator  17 . Suitable porous membranes can include micro-porous, hydrophobic membranes, nanocomposite membranes, surface modified membranes, dual layer composite hydrophobic/hydrophilic membranes, modified ceramic membranes, and any other membrane, coated or not, that permits that passage of water vapor. Exemplary micro-porous, hydrophobic membranes include nanofibrous membranes fabricated using electrospinning methodology, hollow fiber and flat sheet membranes fabricated using phase inversion methodology, and membrane surface modification using chemical vapor deposition (CVD), chemical treatment and plasma treatment methodologies. 
     In the embodiment of  FIG. 1A  the condensation surfaces  15  and the evaporation surfaces  21  are generally flat sheets. As a non-limiting example, the ratio of evaporation to condensation surfaces areas can be adjusted to 1:1 ( FIGS. 1A and 3A ). But it can also have a higher condensation surface area to the evaporation one by using different condensation surface geometries, such as sinusoidal, zigzag, etc. The one or more condensers  12  can be positioned generally parallel or planar to the evaporators  17  and further spaced apart from evaporator  17  to provide one or more air gaps  23  between the condensation surface  15  of the one or more condensers  12  and the evaporation surface  21  of the evaporator  17 . The one or more gaps  23  are configured to allow condensate, such as water condensation or distillate, to form in the one or more gaps  23  as result of the condensate that has passed through the evaporation surface  21  coming into contact with the cooler condensation surface(s)  15  of the condensers  12  and condensing in the air gaps  23 . 
     The one or more condensers  12 , evaporator  17  and air gap(s)  23  can be contained within a compartment or housing  24 . Evaporation and condensation can then take place inside the housing  24  of the module. The evaporation can produce condensable gases, such as water vapor, from the heated feed stream  19 . The heated feed stream  19  may also include non-condensable gases (for example, N 2 , O 2 , CO 2  and the like). Both condensable and non-condensable gases can pass through the evaporation surface  21 . Vacuum can be applied to the housing  24  and thereby to the air gaps  23  to help promote transfer of the condensable and non-condensable gases through the evaporation surface  21 , condensation of the condensable gases (e.g., water vapor) in the air gaps  23  within housing  24  and withdrawal of non-condensable gases, including excess uncondensed condensable gases, such as water vapor, from the air gaps  23 . In various aspects, due to controlled vacuum applied to the housing the spacing of the air gaps  23  or width between the condenser(s)  12  and evaporator(s)  17  may not be important. 
     As depicted in  FIG. 1A , the module  10  includes two condensers  12   a ,  12   b  having condensation surfaces  15   a  and  15   b , respectively. A cold feed stream  14  enters the bottom of condenser  12   a  passing through passageway  13  of condenser  12   a  and exiting the top of the condenser to be cycled to enter the top condenser  12   b  passing there through and exiting the bottom of the second condenser  12   b . The heated feed stream  19  can enter the top of evaporator  17  passing there through and exiting the bottom of the evaporator. The module, thus, provides counter-flow of the heated feed stream  19  in relation to the cold feed stream  14 . One skilled in the art will recognize, however, that the various flows depicted in  FIG. 1A  can be reversed in any combination. 
     A non-limiting schematic drawing of various aspects of a multi-stage vacuumed gap membrane distillation system employing the module of  FIG. 1A  is depicted in  FIG. 3A . In  FIG. 3A  a plurality of heat exchangers or condensers  12   a ,  12   b  and  12   c  are provided within one or more housings  24 . Two evaporators,  17   a  and  17   b , are alternatively positioned between the condensers. One skilled in the art, will recognize, however, that additional condensers and evaporators can be provided in a similar alternating fashion. The system can further include a conduit  31  for providing a cold feed stream  14  to the system. The conduit  31  can include a de-gasifier or de-aerator  32  to remove air and other non-condensable gases out of the cold feed stream  14  to improve the efficiency of the system. A portion of the conduit  31  connects de-aerator  32  to a first one of the condensers  12   a  for providing the cold feed stream  14 , for example seawater, from the de-aerator  32  to the passageway  13   a  of condenser  12   a . Additional conduits  33  can be provided to transport the cold feed stream  14  from one condenser to the next condenser in a series of condensers. For example a conduit  33  can provide the cold feed stream  14  from condenser  12   a  to the passageway  12   b  of the next condenser, in this case condenser  12   b . Similarly a conduit  33  is provided for transporting the cold feed stream  14  exiting the second condenser  12   b  for entry into the passageway  13   c  of a third condenser  12   c  and so on. 
     The cold feed stream  14  exiting condenser  12   c  can then be provided by way of an additional conduit  34  to a heat exchanger  35  for heating the stream. Conduits  36  then deliver the heated stream  19  for passing through a first evaporator  17   a , exiting evaporator  17   a  and delivering the heated stream  19  to a second evaporator  17   b , passing out of evaporator  17   b  to a discharge  37 . Any type of heat source  39  can be applied to the heat exchanger  35  for heating the stream. 
     Since the system runs under constant, or almost, temperature difference between evaporation and condensation in all stages, the same value of the temperature difference can be provided by the external heat source (typically low increase of 3-10° C.). Suitable heat sources include solar thermal collectors, low-enthalpy geothermal energy, low-grade waste heat from industrial plants, low-grade steam from nuclear power plants, or waste heat from diesel engines. If raw feed water is hot enough to drive the process without the need for external heat source, the process can be reversed. We call it direct multi-stage vacuumed gap membrane distillation unit ( FIG. 8 ) where a cooling medium is required instead of a heat source (see VAGMED Reversal Section below). 
     As can be seen the system of  FIG. 3A  thus provides counter-flow of the cold feed stream  14  through the condensers  12  in relation to the flow of the heated feed stream  19  through the evaporators  17 . The cold feed stream  14  can enter the system at a relatively cold temperature. In various aspects the temperature of the cold feed stream  14  can be an ambient temperature of about 20° C. to about 30° C., or more or less. In various aspects the cold feed stream  14  can be seawater, or other liquid including a concentration of salt or any other composition, e.g. industrial wastewater). The cold feed stream  14  can be heated to incrementally higher temperatures as it passes through the series of condensers  12  ultimately to the heat exchanger  35 . For example, the cold feed stream  14  can be successively heated as it passes through the series of condensers  12  by the release of latent heat in each condenser due to the condensation of water vapor on the condensation surfaces  15 . 
     The heat exchanger serves to heat the feed stream to provide a heated feed stream  19  to be delivered to the evaporators  17  and to promote production of water vapor. In one or more aspects the heat exchanger may heat the feed stream to the top brine temperature (TBT) of the stream. The heated feed stream  19  enters each of the evaporators  17  in series where water vapor from the heated feed stream  19  passes through the evaporation surfaces  21  of each successive evaporator into the air gaps  23  where the water vapor condenses on the colder condensation surfaces  15  of the condensers  12 . Due to the loss of water vapor from the series of evaporators  17  the heated feed stream  19  can be successively cooled as it passes from heat exchanger  35  through the various modules or stages to brine discharge  37 . 
     The system can be provided with a plurality of conduits  43  to collect the condensate or distillate  25  from the air gaps and remove it from the system. One or more vacuum lines  45  can be connected to the air gaps  23  to place the air gaps under vacuum and to remove uncondensed or excess water vapor from the vacuum compartment(s). The distillate collected will be relatively salt-free, the water vapor leaving behind the salt or brine in the heated feed stream  19 . The concentrated brine can be removed from the final evaporator  17   b  by way of the brine discharge  37 . Additionally, if desired a recycle loop  47  can be provided to recycle brine solution back to the conduit  31  connecting the de-aerator  32  to a first condenser  12   a.    
     Depicted in  FIG. 1B  is another embodiment of a membrane-based separation module for use in the present systems and methods. The module  50  of  FIG. 1B  operates on a principle similar to that of the module  10  of  FIG. 1A . Module  50  includes a condenser  52  and an evaporator  57 . Each can be comprised of one or more sections or tubes. As depicted in  FIG. 1B  each includes three sections or tubes installed within the same module enclosure  53  under vacuum. Either or both the condenser  52  and the evaporator  57  can be comprised of more tubes or fewer tubes with different arrangements, such as depicted in  FIG. 5  and  FIG. 6  discussed in more detail below. Condenser  52  includes one or more condensation surfaces  55 . Evaporator  57  includes one or more evaporation surfaces or membranes  61 . The various sections or tubes of the condenser  52  and the evaporator  57  are configured or positioned to provide air gaps  23  there between. As with the embodiment of  FIG. 1A , the spacing of the air gaps  23  or width between the condenser  52  and the evaporator  57  may not be important with the application of controlled vacuum. 
     The module  50  can be configured to provide an enclosure  53  about the condenser  52  and evaporator  57  and the air gaps sealing the condenser  52  evaporator  57  and air gaps  23  from the environment and allowing vacuum to be applied to the gaps or spaces  23  within the module  50 . Thus, evaporation and condensation take place within the enclosure  53  of the module. Module  50  can further include a cold feed stream inlet port  14   a  and a cold feed stream outlet port  14   b  for delivering a feed stream  14  to the condenser  52 , allowing the cold feed stream  14  to pass through the condenser  52  and ultimately to exit the module. This hollow fiber module design ( FIG. 1B ) can have a ratio of condensation to evaporation surfaces of 1:1 ( FIG. 1A ), but also can have a higher condensation surface to the evaporation one or vice versa by increasing the number of fibers (evaporation or condensation as required). In addition, a mixture of flat sheet and hollow fiber can be used for evaporation and condensation, respectively, and vice versa, with different evaporation to condensation surface area ratios. 
     Similarly, the module  50  can include an inlet port  19   a  for receiving a heated feed stream  19  for delivery to evaporator  57  and an outlet port  19   b  for removing the heated feed stream from the evaporator  57  and the module. The module  50  can include an inlet and outlet ports  25   a, b  for receiving condensate or distillate  25  from another module and for removing condensate from module  50 , respectively, as well as inlet and outlet ports  63   a, b  for the vacuum system. While one inlet and one outlet of each of the various ports are depicted, one skilled in the art will understand that more than one of each of the ports can be provided. 
     Module  50  operates in a similar manner to module  10  in that the heated feed stream  19  is heated to promote production of condensable gases (e.g., water vapor) which can along with non-condensable gases pass through evaporation surface(s)  61  into the air gaps  23  where the condensable gases can condense on the surface(s)  55  of condenser  52  within the enclosure  53 . The condensate  25  can be removed from module  50  by way of the condensate outlet port  25   b . Excess water vapor and other non-condensable gases that do not condense within module  50  can be removed through the vacuum outlet port  63   b  by way of a vacuum system  45 . 
     Depicted in  FIGS. 3B and 3C  are various embodiments of multi-stage applications of the module  50  of  FIG. 1B . Three modules  60   a, b, c  are depicted in series for receiving a cold feed stream, for example, seawater. As in  FIG. 3A  the cold feed stream  14  can first be passed through a de-gasifier or de-aerator  32  to remove air and other gases from the stream. The cold feed stream  14  is then delivered to the cold stream inlet port  14   a  of a first module  50   a . Each of the modules includes a heat exchanger or condenser  52  and an evaporator  57  such as described in relation to  FIG. 1B . While the system is depicted as including three modules  50   a, b, c  in series the system can include 1, 2, 3, or more modules. The cold feed stream  14  enters the condenser  52   a  of module  50   a  passing there through to be delivered to condenser  52   b  of module  50   b  and from there on to condenser  52   c  of module  50   c  where the cold feed stream  14  exits the last module and is delivered to heat exchanger  35 . The cold feed stream  14  can be successively heated as it passes through the series of condensers  52  by the release of latent heat in each condenser due to the condensation of water vapor on the condensation surfaces  55 . 
     The heat exchanger  35  can be similar to that discussed above in relation to  FIG. 3A . The heat exchanger  35  heats the stream to provide a heated feed stream  19  and to promote production of condensable gases (e.g., water vapor). The heated feed stream  19  is delivered to evaporator  57   c  of module  50   c , from thereon to evaporator  57   b  of module  50   b  and onto evaporator  57   a  of module  50   a . Water vapor exits evaporators  57   a, b, c  through their respective evaporation surfaces  61  into the respective air gaps  23  of each of the modules where the water vapor condenses on the condensation surfaces  55  of the respective condensers  52 . Due to the loss of condensable gases from the series of evaporators  57  the heated feed stream  19  can be successively cooled as it passes from heat exchanger  35  through the modules to brine discharge  37 . 
     Condensate from module  50   c  can be passed out of the condensate outlet port onto module  50   b . Condensate from module  50   b  can be passed onto the condensate inlet port of module  50   a . Ultimately condensate  25  from the first module  50   a  can be collected and delivered by way of a conduit  65  to a storage tank  67 . 
     As an example, the heated feed stream  19  can exit the evaporator  57   a  of module  50   a  as concentrated brine solution  37  and can be delivered to brine storage tank  69 . Ultimately the concentrated brine discharge  37  may be distributed further, as desired, by pump  71 . Optionally a portion of the brine discharge  37  may be cycled or recirculated by conduit  73  back to be incorporated with the cold feed stream  14  optionally passing through a heat exchanger  75  where it may be cooled by a cooling medium  77  and then delivered by conduit  79  to join the cold feed stream  14  for delivery to the condenser  52   a  of module  50   a.    
     A vacuum system  45  can be configured to include a vacuum outlet port in module  50   c  where vacuum is drawn and delivered to vacuum inlet port of module  50   b  which has a vacuum outlet port delivering vacuum to the vacuum inlet port of module  50   a . Ultimately excess, uncondensed gases, for example uncondensed condensable gases (e.g., water vapor) and non-condensable gases, can be collected out of module  60   a  by the vacuum system  45 . Some of the excess condensable gases may condense in the vacuum system which condensate may be delivered by conduit  68  to the condensate storage tank  67 . Thus, similar to the system of  FIG. 3A , the system of  FIG. 3B  incorporates a plurality of modules  50  and can provide counter-current flow of a cold feed stream  14  in relation to a heated feed stream  19  to promote production of condensable gases and condensation of the gases to collect condensate or distillate  25 . Where the condensable gases are water vapor the condensate can be water that is substantially free of salt or brine. 
       FIG. 3C  depicts a further version of the system of  FIG. 3B  employing a plurality of the modules  50  depicted in  FIG. 1B . As depicted in  FIG. 3C  the system can include any number of modules beyond the three illustrated. The system of  FIG. 3C  operates in a similar manner to that of  FIG. 3B , having however an additional loop. In an aspect where the feed stream  14  is a form of salt water, the additional loop can be a brine recycle loop. Instead of delivering the cold feed stream  14  exiting the condenser  52   a  of the first module  50   a  directly to the condenser  52   b  of module  50   b , the cold feed stream  14  can be diverted to a brine collection tank  69  (brine pool) and a portion of brine solution in the brine collection tank  69  can be delivered as the cold feed stream  14  to condenser  52   b  of module  50   b . A pump  81  (that may be a brine recycle pump) may be provided to assist in delivering brine solution as a cold feed stream  14  to the second module  50   b . In this depicted embodiment, module  50   b  can be the last module in the series of multi-stage membrane distillation modules and module  50   a  can be a brine recycle module that is part of the brine recycle loop. One or more modules can be staged in series with module  50   a  as part of the brine recycle loop. 
     The cold feed  14  of the VAGMED modules, systems and processes can be raw seawater, thermal or membrane desalination brines, produced water, wastewater, groundwater or surface water. Referring to the multi-stage systems, for example depicted in  FIGS. 3A-3C , the cold feed stream  14  can enter the first module as a coolant for the condensation surface(s)  15 ,  55  to recover the latent heat of condensation from the vapor that condenses on the condensation surface(s) of that module and then flows to the next module (before last) to recover more energy in a similar manner and so on. The number of modules can be determined through specific designs depending on the plant size and the operating parameters. The operating parameters can include cold feed stream inlet temperatures, heated feed stream temperatures, flow rates, module length, and hydrophobic membranes specification. 
     The cold feed stream  14  exits the first module where its temperature can be increased to reach the top brine temperature (TBT) using a heat source  39  for heat exchanger  35 . In this case the “first module” is the module in the series closest to heat exchanger  35 . In the embodiments depicted in  FIGS. 3B and 3C  the first module is module  50   c . The heat source  39  can be solar thermal panels, low-enthalpy geothermal energy, low-grade waste heat, low-grade steam or electrical heat supply. The feed stream can then pass through heat exchanger  35  before it enters an evaporator  17 ,  57  having an evaporation surface  21 ,  61 . The evaporation surface can be a channel made of a micro-porous hydrophobic membrane of the first module to receive the heated feed stream  19  from the heat exchanger  35  ( FIGS. 3A-3C ). The temperature difference created by the heating source as well as the vacuum applied to the module compartment or enclosures  23 ,  53  cause part of the heated feed steam  19  to evaporate forming water vapor. The driving force for the evaporation can be created by a pressure difference between the two sides of the membrane of the evaporation surface. The water vapor can pass through the membrane pores to the condensation surface  15 ,  55  of the condenser  12 ,  52 . Condensation can take place in the air gap  23  between the hydrophobic membrane surface and the condensation surface within the housing  24  or enclosure  53 . 
     The air gap can be a small air gap. In various aspects the air gap can be up about 200 mm. In other aspects the air gap can be as small as about 1 mm, 2 mm or 5 mm. In yet other aspects the air gap can be more than 5 mm up to about 200 mm, under the condition of efficient vacuum system (efficient non-condensable gases removal). 
     After losing some mass due to evaporation that also leads to a drop in its temperature, the heated feed steam  19  enters the evaporator of the next module where more water vapor can pass through its evaporation surface. The vacuum inside the next module enclosure can be adjusted to be slightly lower than the saturation pressure of the temperature of the heated feed stream  19  entering the next module to aid further in promotion and passing of water vapor through the evaporation surface. Setting or adjusting the pressure to be slightly lower than the saturation has the benefit in aiding the gases passing through the membrane of the evaporation surface  21 ,  61  to overcome membrane structure resistance. By slightly lower we mean 1-5% of the saturation pressure depending upon the temperature of the heated feed stream entering the next module. The heated feed stream  19  can continue in a similar manner until it exits the last module (for example, module  50   a  depicted in  FIGS. 3B and 3C ) as a concentrated feed stream where it is discharged as brine  35  (once-through VAGMED) or recycled back fully or partially to the process as feed to a condenser of an intermediate module (for example, as feed to condenser  52   b  of module  50   b , as depicted in  FIG. 3C ), or after its temperature is lowered by a cooling medium  77  (brine recycled VAGMED) in heat exchanger  75 , as depicted in  FIG. 3B . 
     The maximum production of distillate  25  from the VAGMED system (theoretically the product is distilled water quality as pure vapor is condensed only if no membrane pore wetting occurs) depends on the temperature difference between the heated feed stream  19  that enters the first module and the temperature of the brine stream  65  exiting the process, as well as the heated feed stream  19  flow rate that enters the evaporation section of the system according to the following equation: 
     
       
         
           
             
               
                 
                   
                     M 
                     D 
                   
                   = 
                   
                     
                       
                         M 
                         F 
                       
                        
                       
                         ( 
                         
                           
                             T 
                             F 
                           
                           - 
                           
                             T 
                             B 
                           
                         
                         ) 
                       
                     
                     
                       ( 
                       
                         
                           
                             h 
                             g 
                           
                           
                             C 
                             p 
                           
                         
                         - 
                         
                           T 
                           B 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where M D  and M F  are the mass flow rates of the distillate  25  and heated feed stream  19 , respectively, T F  and T B  are the temperatures of the heated feed stream  19  that enters the first module and exits the last module, respectively, h g  and C p  are the average enthalpy of the generated water vapor and the average specific heat of the feed, respectively. 
     As mentioned above, if raw feed water is hot enough to drive the process without the need for external heat source, the process could be reversed. We call it direct multi-stage vacuumed gap membrane distillation process and device ( FIG. 9 ) where a cooling medium is required instead of a heat source. 
     The Removal of Non-Condensed Gases and VAGMED Staging 
     Reducing the module enclosure absolute pressure by applying a vacuum can increase the VAGMED condensable gas flux (such as water vapor flux), and help sustain evaporation and formation of condensable gas (such as water vapor), due to removal of the mass transfer resistance caused by the non-condensable gases that preoccupied the module enclosure. However, the more we reduce the enclosure pressure below the saturation pressure of the heated feed stream temperature, the lower the cooling temperature needed for condensing the condensable gas or water vapor. Therefore, in various aspects, from a practical point of view a preferred housing or enclosure pressure can be slightly below the saturation pressure of the heated feed stream  19  temperature to assure the complete removal of non-condensed gases and to sustain enough driving force for the condensable gas to overcome the hydrophobic micro-porous membrane structure mass transfer resistance. The non-condensed gases can be non-condensable gases, but they may also include uncondensed condensable gases. The preferred pressure also can be high enough to allow the condensable gas to condense on the condensation surface  15 ,  55 . Therefore, in various aspects the pressure difference between the enclosure (vacuum compartment) pressure and the saturation pressure of the heated feed stream  19  can be between about 1% and about 5% of the saturation pressure depending upon the temperature of the heated feed stream entering the evaporator within the vacuum compartment of the module. In various aspects the temperatures of the condensation surfaces  15 ,  55  can be maintained slightly lower than the saturation temperature of the water vapor. For example, the temperature of the condensation surfaces  15 ,  55  can be about 3° C. to about 7° C. lower than the saturation temperature of the condensable gas. 
     As mentioned earlier, the heated feed stream  19  temperature decreases along the membrane channel of the first module due to the heat loss through evaporation. Such decrease in temperature creates a practical difficulty in maintaining the enclosure pressure at the saturation pressure of the feed temperature in a single module. A solution to this problem can be through a staging of the evaporation and condensation processes. When the heated feed stream  19  temperature decreases by 3° C., for example, in the first module due to evaporation, the system can be configured such that it flows into another module (next stage) where the pressure is lowered relative to the saturation pressure at that heat feed stream temperature and so on (for the next stages). Thus in various aspects, the amount of vacuum applied to the next module vacuum compartment is increased in order to affect the lower pressure. The vacuum system  45  can be configured to affect a different pressure within the vacuum compartment  24 ,  53  of each module in the multi-stage system. For example, the vacuum system  45  can be configured to effect a different pressure within the vacuum compartment  24 ,  53  of each successive module such that the first stage module, the module closest to heat exchanger  35 , has the highest pressure and each module downstream from the first stage module has an increasingly lower pressure within its respective vacuum compartment. 
       FIG. 2  shows that in order to maintain the pressure at the saturation pressure of the feed temperature, infinite stages are required. In  FIG. 2  the temperature of the heated feed stream is shown along the x-axis. The stages are numbered such that stage no.  1  is the stage or module closest to heat exchanger  35 , stage no.  2  is next module in the series of modules moving away from heat exchanger  35 .  FIG. 2  shows that each successive stage or module in the series of modules has a lower heated feed stream temperature and a correspondingly lower saturation pressure. The vacuum system  45  can be configured to adjust the amount of vacuum applied to each successive stage in order to lower the pressure (i.e., increase the amount of vacuum) within each successive vacuum compartment relative to the lower saturation pressure in each stage. As noted above, in various aspects the vacuum system can be configured to effect a pressure within each successive vacuum compartment that is about 1-5% below the successively lower saturation pressure in each stage. 
     However, since this is not practically possible, in one or more aspects we can use one module for every 2-3° C. reduction in heated feed stream  19  temperature (higher ΔT leads to lower number of stages but lower efficiency). In this way, the highest absolute pressure is applied at the first stage (closest to heat exchanger  35 ) while the lowest one is maintained at the last stage (farthest from heat exchanger  35 ). Since the enclosures of the modules are connected together, the vacuum system is preferably connected at the last modules (as depicted in  FIGS. 3B and 3C ) to minimize the loss of condensable gas or water vapor. To remove non-condensable gases a venting line can be installed in each stage. The vacuum in the other modules can be adjusted by pressure control devices according the partial pressure of the heated feed stream  19  entering each module. In addition, all raw cold feed water  14  can be de-aerated  32  to remove a large amount of non-condensable gases, such as O 2 , CO 2  and N 2 . One skilled in the art will recognize that the temperatures and flow rates of the cold feed stream  14  and the heated feed stream, the saturation pressure (which is related to the temperature of the heated feed stream), the pressure (amount of vacuum) within in the vacuum compartment, and the amount of evaporation and condensation surface within a stage all affect the design of the stages and the number of stages (modules) of the present multi-stage distillation system. 
     One possible way of fabricating a hollow fiber module MD compartment design is represented in  FIGS. 4A and 4B  (see  FIG. 1B ). Design specifications such as the gap width between evaporation and condensation and their respective surface areas, packing density, module length and diameter, and the number of stages can be set for each configuration and specific application through modeling/simulation software. The compartment design of FIGS.  4 A and  4 B can be fitted internally with a plurality of condensation and evaporation tubes, as depicted in  FIG. 1B . Alternative embodiments for the fitting of the condensation and evaporation tubes are depicted in  FIGS. 5A-5D . 
     Another hollow fiber module design is a mixed configuration of evaporation (MD membranes) and condensation (condensation tubes) inserted in one shell and tube configuration, as shown in  FIGS. 6A-6H . Different configurations are possible, e.g. condensation tubes can be installed in the inner part and the MD hollow fibers (evaporation) in the outer part ( FIGS. 6B and 6C ). They can also be installed in the other way (hollow fiber membranes in the inner part,  FIG. 6D ) or completely mixed in one single bundle ( FIG. 6E ). 
     In yet another embodiment, the one or more modules of the present disclosure can have an evaporator configured with a sheet, either flat or non-flat, and a condenser configured with the above described hollow fiber configuration, or vice versa.  FIG. 7  depicts one aspect of such an embodiment including two modules  750   a, b  connected in series. Each module includes an evaporator  757   a, b  comprised of a pair of opposed flat sheet membranes that provide a passage therebetween for feed inlet  719 A and feed outlet  719 B. The feed passage ways of the two modules can be coupled by a conduit  736 . Each module  750   a, b  also includes condensers  752   a, b  comprised of a bundle of fibers such as condenser  53  of  FIG. 1B . Coolant, as described herein, can be fed into the condensers  752   a  and  752   b  of modules  750   a  and  b , respectively. Conduits  733   a ,  733   b  and  733   c  can be provided to couple the coolant passing out of and into the respective condensers. In the embodiment of  FIG. 7 , permeate passes through the flat sheet membranes  757   a, b  condensing in the airgaps or spaces  723  to be collected at the respective outlets  725   a  and  725   b  of the modules. One skilled in the art will recognize that the configuration of  FIG. 7  can be reversed wherein the evaporators  757   a  and  b  can each be comprised of a bundle of hollow fibers as described herein and the condensers  752   a, b  can be comprised of a sheet membrane, flat or non-flat, to which coolant can be provided on one side of the sheet condenser to provide a condensing surface on the opposed side of the sheet condenser. Evaporation and condensation surfaces can be optimized for each case. 
     We have thus described a number of configurations wherein the condensation surface(s) and permeable evaporation surface(s) have a hollow tubular design or, as described earlier a flat design as in for example  FIG. 1A . One skilled in the art will recognize that the condensation/evaporation surfaces can be hollow/hollow, flat/hollow, hollow/flat, flat/flat, etc. 
     As an example, simulated results of an VAGMED unit are presented in  FIG. 8  and Table 1. Twenty modules or stages are simulated therein. The system, however, can have more or less stages. In this example, the temperature of the heated feed stream  19  from heat exchanger  35  entering stage  1  is 70° C. This temperature of the heated feed stream can be related to the top brine temperature (TBT) of the stream and can be higher or lower. For example it can be 75° C. or 80° C. or another temperature. The temperature of the cold feed stream entering the system (at stage  20 ), though its temperature can be higher or lower. Similarly the flow rates of the cold feed and heated feed streams can be higher or lower than simulated (see Table 1). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Simulated Results of an VAGMED Process (Calculated Data) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Feed 
                 Feed 
                 Feed 
                 Feed in 
                   
                 Coolant 
                 coolant 
                   
                   
                   
               
               
                 Stage 
                 Feed in 
                 out 
                 in 
                 out 
                 salinity 
                 Feed outlet 
                 in 
                 out 
                 product 
                 membrane 
                 pressure 
               
               
                 No. 
                 kg/hr 
                 kg/hr 
                 temp C. 
                 temp C. 
                 wt % 
                 salinity wt % 
                 temp C. 
                 temp C. 
                 kg/hr 
                 area [m 2 ] 
                 (pa) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 920.00 
                 916.91 
                 70.00 
                 66.00 
                 4.20 
                 4.21 
                 63.00 
                 65.00 
                 3.10 
                 0.41 
                 30412.38 
               
               
                 2 
                 916.91 
                 913.83 
                 66.00 
                 66.00 
                 4.21 
                 4.23 
                 61.00 
                 63.00 
                 3.09 
                 0.43 
                 27879.84 
               
               
                 3 
                 913.83 
                 910.75 
                 66.00 
                 63.99 
                 4.23 
                 4.24 
                 59.00 
                 61.00 
                 3.08 
                 0.45 
                 25521.00 
               
               
                 4 
                 910.75 
                 907.68 
                 63.99 
                 61.98 
                 4.24 
                 4.25 
                 56.99 
                 58.99 
                 3.07 
                 0.47 
                 23326.25 
               
               
                 5 
                 907.58 
                 904.62 
                 61.98 
                 59.95 
                 4.26 
                 4.27 
                 54.55 
                 55.98 
                 3.07 
                 0.50 
                 21265.67 
               
               
                 6 
                 904.62 
                 901.57 
                 59.95 
                 57.92 
                 4.27 
                 4.29 
                 52.95 
                 54.95 
                 3.05 
                 0.53 
                 19393.69 
               
               
                 7 
                 901.57 
                 888.53 
                 57.92 
                 55.89 
                 4.29 
                 4.30 
                 50.92 
                 52.92 
                 3.05 
                 0.56 
                 17641.79 
               
               
                 8 
                 898.53 
                 895.50 
                 55.89 
                 53.65 
                 4.30 
                 4.31 
                 48.89 
                 50.89 
                 3.04 
                 0.60 
                 16021.48 
               
               
                 9 
                 895.50 
                 892.48 
                 53.85 
                 51.79 
                 4.31 
                 4.33 
                 46.85 
                 48.85 
                 3.03 
                 0.64 
                 14525.21 
               
               
                 10 
                 892.48 
                 889.46 
                 51.79 
                 49.74 
                 4.33 
                 4.34 
                 44.79 
                 46.79 
                 3.02 
                 0.66 
                 13145.12 
               
               
                 11 
                 889.46 
                 886.45 
                 49.74 
                 47.67 
                 4.34 
                 4.36 
                 42.74 
                 44.74 
                 3.01 
                 0.73 
                 11874.47 
               
               
                 12 
                 886.45 
                 883.45 
                 47.67 
                 45.60 
                 4.36 
                 4.37 
                 40.67 
                 42.67 
                 3.00 
                 0.78 
                 10706.36 
               
               
                 13 
                 883.45 
                 880.46 
                 45.60 
                 43.52 
                 4.57 
                 4.39 
                 58.60 
                 40.60 
                 2.99 
                 0.84 
                 9635.50 
               
               
                 14 
                 880.46 
                 877.47 
                 43.52 
                 41.43 
                 4.39 
                 4.40 
                 55.52 
                 55.52 
                 2.99 
                 0.90 
                 8654.93 
               
               
                 15 
                 877.47 
                 874.50 
                 41.46 
                 39.34 
                 4.40 
                 4.42 
                 34.43 
                 36.43 
                 2.97 
                 0.98 
                 7756.26 
               
               
                 16 
                 874.50 
                 871.54 
                 39.34 
                 37.24 
                 4.42 
                 4.43 
                 32.34 
                 34.34 
                 2.96 
                 1.05 
                 6940.15 
               
               
                 17 
                 871.54 
                 868.59 
                 37.24 
                 35.13 
                 4.43 
                 4.45 
                 30.24 
                 32.24 
                 2.95 
                 1.15 
                 6195.69 
               
               
                 18 
                 868.59 
                 865.64 
                 35.13 
                 33.01 
                 4.45 
                 4.46 
                 28.13 
                 30.13 
                 2.94 
                 1.25 
                 5518.88 
               
               
                 19 
                 865.64 
                 862.71 
                 33.01 
                 3.089 
                 4.46 
                 4.48 
                 26.01 
                 28.01 
                 2.93 
                 1.36 
                 4905.07 
               
               
                 20 
                 862.71 
                 859.79 
                 3.059 
                 28.76 
                 4.48 
                 4.49 
                 28.89 
                 28.89 
                 2.92 
                 1.49 
                 4349.71 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Total 
                 60.27 
                 15.79 
               
               
                   
               
            
           
         
       
     
     VAGMED Reversal 
     One or more further embodiments are provided when the feed source input to the system is a hot feed source instead of a cold feed source and a cooling system is provided in place of a heating system at the heat exchanger (e.g., heat exchanger  35 ). 
     In the VAGMED Reversal process a hot feed source  114  (e.g., thermal brines, power plant condensers, boilers blow-down, hot or geothermal springs, wastewater of incinerators) enters the evaporator  157   a  of the last module  150   a . As in the aforementioned systems the evaporator  157   a  includes a flow channel and an outer evaporation surface  161   a . The evaporators  157  can include or be made of a micro-porous hydrophobic membrane as above. The module  150   a  can also include a condenser  152   a  having a condensation surface  155   a , as above. The condensers  152  can be formed of condensation bundle tubes, as illustrated for example in  FIGS. 6A-6H . The temperature difference created by the temperature of the hot feed source  114  as well as the vacuum applied within the enclosure of module  150   a  causes part of the feed to evaporate (the driving force is created by the pressure difference between the two sides of the hydrophobic membrane) where the condensable gases (water vapor) pass through the membrane pores to reach the condensation surface  155   a  of condenser  152   a . Condensation takes place within the gap  123  between the hydrophobic membrane of the evaporation surface  161   a  and the condensation surface  155   a . After losing some mass due to evaporation that also causes a drop in its temperature, the feed  114  enters the evaporator of the next module where the vacuum inside the module enclosure is adjusted to be slightly lower than the saturation pressure of the feed temperature to generate further condensable gases (water vapor). The feed  114  continues in similar manner until it exits the evaporator  157   c  of the first module  150   c.    
     To ensure a constant temperature difference between the evaporation and condensation surfaces in all modules, its temperature is further cooled by 3-7° C. in a heat exchanger  135  by a cooling medium such as (ambient seawater, cold water from cooling tower or air-fan cooler). The cooled feed stream  119  exiting heat exchanger  135  then enters the channels of the condenser  157   c  of the first module  150   c  as a coolant to recover the latent heat of the vapor that condenses on the condensation surface  155   c  of that module. Then, it flows to the next module (before last) to recover more energy in similar manner and so on. The number of modules can be determined through specific designs depending on the plant size and the operating parameters, such as feed/coolant inlet temperatures, flow rates, module length, and hydrophobic membranes specs. The feed stream  119  exits the last module  150   a  as concentrated feed (for example concentrated brine) where it can be passed to storage tank  169  and if desired, as hot brine feed  214  for a next similar unit/process to be further treated in a similar manner, for example delivery to the inlet of evaporator  257   a  of module  250   a.    
     The number of units/processes in series depends on the temperature of the brine discharged from the last unit/process, which in its turn depends on the available hot feed stream (a sufficient ΔT is required to drive the process). The maximum distillate production from the VAGMED (theoretically the product is distilled water quality as pure vapor is condensed only if no membrane pore wetting occurs) depends on the temperature difference between the hot feed that enters the first module and the brine temperature that exits the process, and the feed flow rate that enters the evaporation section of each process according to the following equation: 
     
       
         
           
             
               M 
               D 
             
             = 
             
               
                 
                   
                     M 
                     
                       F 
                        
                       
                           
                       
                        
                       1 
                     
                   
                    
                   
                     ( 
                     
                       
                         T 
                         
                           F 
                            
                           
                               
                           
                            
                           1 
                         
                       
                       - 
                       
                         T 
                         
                           B 
                            
                           
                               
                           
                            
                           1 
                         
                       
                     
                     ) 
                   
                 
                 
                   ( 
                   
                     
                       
                         h 
                         g 
                       
                       
                         C 
                         p 
                       
                     
                     - 
                     
                       T 
                       
                         B 
                          
                         
                             
                         
                          
                         2 
                       
                     
                   
                   ) 
                 
               
               + 
               
                 
                   
                     M 
                     
                       F 
                        
                       
                           
                       
                        
                       2 
                     
                   
                    
                   
                     ( 
                     
                       
                         T 
                         
                           F 
                            
                           
                               
                           
                            
                           2 
                         
                       
                       - 
                       
                         T 
                         
                           B 
                            
                           
                               
                           
                            
                           2 
                         
                       
                     
                     ) 
                   
                 
                 
                   ( 
                   
                     
                       
                         h 
                         g 
                       
                       
                         C 
                         p 
                       
                     
                     - 
                     
                       T 
                       
                         B 
                          
                         
                             
                         
                          
                         2 
                       
                     
                   
                   ) 
                 
               
               + 
               … 
               + 
               
                 
                   
                     M 
                     Fn 
                   
                    
                   
                     ( 
                     
                       
                         T 
                         Fn 
                       
                       - 
                       
                         T 
                         Bn 
                       
                     
                     ) 
                   
                 
                 
                   ( 
                   
                     
                       
                         h 
                         g 
                       
                       
                         C 
                         p 
                       
                     
                     - 
                     
                       T 
                       Bn 
                     
                   
                   ) 
                 
               
             
           
         
       
     
     where M D  and M F  are the mass flow rates of the distillate and heat feed, respectively, T F  and T B  are the temperatures of the heated feed that enters the first module and exits the last module, respectively, h g  and C p  are the average enthalpy of the generated water vapor and the average specific heat of the feed, respectively. 
     In the VAGMED reversal process ( FIG. 8 ), the energy consumption is significantly lower, as cooling medium is required rather than the need for energy intensive heat source to achieve TBT as it is the case for conventional systems. Since the system runs under constant, or almost constant, temperature difference between evaporation and condensation in all stages, the same value of the temperature difference is provided by the external cooling medium (typically a low decrease of 3-10° C.). 
     It should be emphasized that the above-described embodiments are merely examples of possible implementations. Many variations and modifications may be made to the above-described embodiments without departing from the principles of the present disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.