Abstract:
Multi Effect Distiller (MED) with vertical flat-plate, falling-film heat transfer mechanism. A multitude of alternatively arranged or “checkered”, rectangular shaped evaporator and condenser cells form one layer between two vertical flat plate walls. Multitude of layers—each comprised of alternating evaporator and condenser cells—form the block-shaped MED unit. The evaporator and condenser cells are against each other, sharing common vertical heat transfer walls. The simultaneous propagation of multi effect distillation occurs in two dimensions along the longitudinal vertical plane of the heat exchanger. One end of the distiller is heated, while the other end is cooled.

Description:
TECHNICAL FIELD 
       [0001]    The present application relates generally to thermal distillation and more particularly relates to multi effect distillers (MED) used for desalination of water. 
       BACKGROUND OF THE INVENTION 
       [0002]    Multi-effect Evaporative Distillation is a known technology in the field of large scale water desalination for removal of dissolved solids from undrinkable water sources such as seawater or brackish ground-water. This is a separation process with heat as driving force. 
         [0003]    There are two basic principles used in desalination: Membrane Separation and Distillation. Membrane systems use sophisticated semi-permeable porous materials and various driving forces (electrical potential, pressure or temperature gradient) to separate the water molecules from salt ions through the membrane, as the membrane blocks the passage of larger molecules. Distillation systems use heat and/or pressure gradient to vaporize the water and separate it from salt ions and a cooling source to condense the water vapor back to liquid. Desalination systems provide fresh water from seawater on coastal regions and from brackish groundwater in inland areas. Groundwater desalination is also referred as water reclamation. 
         [0004]    There are four known basic distillation based desalination processes: Falling Film Evaporation, Flash Vaporization, Vapor Compression and Humidification. The subject of the present application is one of the falling film evaporation based technologies that is considered the most efficient and it will be described in details in the following sections of this application. The other three are described briefly for background purposes: The Flash Vaporization uses differential pressure—driven by the temperature gradient between the heat source and the cooling source—to flash-out the vapor from the liquid water as it passes from a higher to a lower pressure stages. There is a multitude of flash-vessels (or stages) connected in series. These systems are referred as multi-stage flash or MSF. In vapor compression systems, mechanical or thermal jet compression is applied to the water vapor to drive the process and maintain the differential pressure between the evaporation and condensation. With humidification systems, another gas—typically air—is circulated as a working fluid to carry the water vapor from the point of evaporation to the point of condensation. The operation is based on the capacity of air to absorb water and to naturally circulate (rise) if heated. The humidification technologies closely mimic the water-cycle of nature. 
         [0005]    The currently known multi effect distillers (MED) are comprised of shell and tube evaporator-condensers units arranged in series, either horizontally or vertically. Each evaporator/condenser unit in the chain process is referred to as one effect. The water vapor from the evaporator of the upstream effect enters to the condenser of the downstream effect. The latent heat of the condensing water vapor is then used to evaporate the water in the next evaporator in the chain and so on. This cascading distillation process is referred to as propagation of evaporative effect from the heat source to the heat sink. Number of effects in a MED process is equal to the number of condenser-evaporator pairs. This number is an expression of the number of times the unit of input (heat) energy is utilized for distillation of water. Higher number of effects results in higher energy efficiency of the system. Typically the shell side of the heat exchanger is the evaporator, while the tube side is the condenser. The known MEDs are expensive because their geometry is complex, the materials required for construction are specialty-alloys and the required manufacturing process is labor-intensive. The horizontal tube-bundles of the condensers are sprayed with seawater such that the outer surfaces of the tubes are partially wetted. This results in relatively low heat-transfer efficiency. 
         [0006]    The energy efficiency of thermal desalination is often expressed as Gained Output Ratio (GOR) which is the ratio of the total latent heat of evaporation of the distillate to the input energy. Another often used, similar measure is the performance ratio (PR) which is the ratio of mass flow of distilled water to the mass flow of heating steam at saturated condition. 
       SUMMARY OF THE INVENTION 
       [0007]    The present application thus describes one embodiment of the invention that may take the form of a two dimensional Multi Effect Distiller (MED) with flat-plate, falling-film, heat transfer mechanism. The invention may be used for distillation of water or any other liquid solution that contains dissolved solute mater. It may be used as concentrator of a solution and for separation of the solvent liquid from the solution. The evaporator and condenser surfaces may be vertically oriented heat transfer planes. The space “sandwiched” between two heat transfer planes may form a layer of evaporator and condenser cells. The rectangular shaped evaporator and condenser cells may be alternatively “checkered” against each other, sharing common heat transfer walls: One evaporator cell may share a common heat transfer wall with one adjacent condenser cell. The evaporator and condenser cells may form a checkered pattern in one layer. Multitude of layers may also form a block shaped desalinator apparatus. A multitude of evaporator and condenser cells may be arranged in an alternating three-dimensional matrix configuration. The position of each cell in the MED block can be defined by 3 numbers for its position in the respective rows, columns and layers of the MED matrix. The simultaneous propagation of multi effect distillation occurs in two dimensions along the longitudinal vertical plane of the heat exchanger. The cells may be filled with water vapor (or vapor of other liquid) such that a portion or all of the cells may be operating below atmospheric pressure. A set of condensing cells on one end of the desalinator may serve as heating cells while a set of evaporating cells may serve as cooling cells. The condensed water vapor collected from the condenser cells may be drained from the unit as desalinated water. 
         [0008]    The present application further describes the two dimensional propagation of multi effect distillation process of the invention. The distillation process propagates in two directions simultaneously in the vertical plane of the desalinator device. This vertical plane lies along the length of the desalinator, connecting the hot end with the cold end. The evaporator and condenser cells—in the same plane—form a checkered-pattern layer. The desalination apparatus consists of multitude of layers. The two dimensional propagation of distillation effect in one of these layers is described as follows: In a vertical direction the propagation is from top to bottom: for example from an evaporator cell through a condenser cell below to an evaporator cell below and so on. In a horizontal direction, from the hot to the cold end: for example from a condenser cell through a horizontally adjacent evaporator cell to an adjacent condenser cell and so on. 
         [0009]    The present application further describes the flow of saline water (as an example of a solution) in the invention. The saline water flows from top to bottom by gravitation as falling film through the evaporator cells. The saline water enters into a top evaporator cell and flows down on the walls as thin liquid film to the bottom of the cell. Some of the water evaporates (as the cell walls are heated from the adjacent condenser cells) therefore the saline water at the bottom is more concentrated than at the top. The concentrated saline water flows from the bottom of the evaporator cell to the top of the two evaporator cells located below in the adjacent layers on each side of the top evaporator cell through collection-transfer troughs (or gutters). The concentrated saline water enters to the top of the evaporator cell from the adjacent layers from both sides. The concentrated saline water is then mixed with saline feedwater in a mixing bulkhead at the top of the evaporator cell. The feedwater is supplied through distribution nozzles. The saline mixture then is channeled through narrow slots that provide an even flow-distribution of the thin falling film to the walls of the evaporator cells. The flow pattern then repeats itself: The concentrated saline water flows down on the walls and across to the adjacent layers to the top of the two evaporator cells located below on each side of the top evaporator cell . . . and so on. 
         [0010]    The present application further describes the flow of brine in the invention. Once the saline concentrate reaches the lowest evaporator cells then the concentrate—or brine is collected at the bottom of the distiller desalinator apparatus. This bottom collection pan is filled with brine. It is closed off from the condenser cells above it and is opened to all lower level evaporator cells. The evaporator cells are only hydraulically connected with each other through the brine pan such that vapor cannot escape, due to sealing effect of the liquid in the pan. Generally speaking the pressure is the same in all of those brine-pan cells that are at the same distance from the hot end of the apparatus (or they are in the same stage of evaporation effects). The liquid flow is largely unrestricted through large openings of the brine-pan across adjacent layers in the same evaporation effect (as these pan cells are all under isobar conditions). The brine flows horizontally in the general direction from the hot end to the cold end of the apparatus. The driving force of the flow is the pressure gradient between consecutive evaporation effects. The liquid flow is restricted in this direction by orifices as the brine cascades from one effect to the next. The collected brine leaves the apparatus at the cold end. 
         [0011]    The present application further describes the flow of water vapor and the flow of distilled water. The water evaporation happens in the evaporator cells as they are heated by the condenser cells located in the adjacent layers through shared heat transfer surfaces. The vapor flow from the evaporators is split into two streams. Both streams flow freely within the same layer from the evaporator cell to two of the adjacent condenser cells. Horizontally the vapor flows to the colder condenser cell toward the cold end of the desalinator through vapor passage openings in the wall dividing the evaporator cell from the condenser cell. Vertically the vapor may flow downward to the condenser cell underneath through a vapor passage louver. Therefore most every condenser cell is supplied with water vapor from two directions: a horizontal inflow from the hot-end and an upward vertical inflow. The water vapor is condensed on the walls of the condenser cell that are cooled by the evaporator cells located in the adjacent layers. The distilled, condensed water flows down on the walls of the cell and collects in the bottom of the cell. The distilled water is drained from the condenser cells through drainage tubes to the exterior of the desalinator. 
         [0012]    These and other features of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows a general arrangement of the Multi-Effect Evaporative Desalinator with falling film evaporator and condenser cells arranged in a matrix configuration. It also shows the two-dimensional (vertical and horizontal) propagation of the multi effect evaporative distillation process. 
           [0014]      FIG. 2  shows the vertical cross sections of the evaporator and condenser cells. It depicts the flow of saline water and water vapor through the propagation of evaporative distillation effects. 
           [0015]      FIG. 3  depicts the horizontal cross section of the brine pan or brine collection bulkhead. It shows the flow path of concentrated brine in the lowest section of desalinator. 
           [0016]      FIG. 4  is the detailed drawing of the junction of the evaporator and condenser cells. It depicts the mixing of concentrated saline water—collected from the evaporator cell above in the adjacent layer—with saline feedwater. It also shows the fresh water flash box and the distilled water drainage piping. 
           [0017]      FIG. 5  depicts the flow diagram of the desalinator apparatus in an Open-Loop Sea-(or Saline) Water Cooling application. 
           [0018]      FIG. 6  depicts the flow diagram of the desalinator apparatus in a Closed, Sea-(or Saline) Water-Loop Cooling application. This configuration is one of the dry-cooling or air cooled applications. 
           [0019]      FIG. 7  depicts the flow diagram of the desalinator apparatus in a Closed, Distilled Water-Loop Cooling application. This configuration is also one of the dry-cooling applications. 
           [0020]      FIG. 8  is the detailed drawing of the vertical cross section of the “cold” or condenser end of the desalinator in a closed distilled water loop cooling configuration. The final condenser cells are direct contact condensers. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Referring now to the drawings, in which like numerals indicate like elements throughout the several views,  FIG. 1  shows an isometric view of one embodiment of the Multi Effect Distiller  107 . Water desalination process is used as an example to describe the operation of the system. The heat input takes place in the heating cells  101 , on one end of the distiller, while the cooling takes place in the cooling cells  104 , at the opposite end. The heating cells  101  are layered alternating with evaporator cells  103 , forming a sandwiched structure while the cooling cells are similarly layered alternatively with condenser cells  104 . In the mid section of the distiller there are falling film evaporator- 103  and condenser cells  104  arranged in a checkered or matrix configuration. The water vapor generated in the evaporator cell  103  leaves and enters into the condenser cell vertically below and into the adjacent condenser cell horizontally forward toward the cooling cells  104 . This is a two-dimensional (vertical and horizontal) propagation of the multi effect evaporative distillation process  106 . 
         [0022]      FIG. 2  shows the flow of saline water  201 ,  205  and water vapor  202  (or other liquid solution and its vapor) through vertical cross sections of the evaporator and condenser cells. The saline feedwater  205  enters through horizontal feedwater pipes  209  that may run through the length of the MED device, parallel with the vertically oriented common sidewalls  211  of the evaporator and condenser cells. The feedwater  205  is mixed with the concentrated saline water  201  flowing from the sidewalls of the evaporator cells. The mixture is then distributed evenly by cap  212  along the top edge of the evaporator cell and flows down on the sidewalls  211  of evaporator cells below. The evaporator cells  203  are heated by the adjacent condenser cells  204  through the metal sidewalls  211 . The condenser cells are at slightly higher pressure and temperature than the adjacent evaporator cells. The vapor  202  from the evaporator cells flows to two directions: Downward through the separator cap  213  to the condenser cell below and horizontally and parallel with the sidewalls  211  through the separator end wall openings  206  to the adjacent condenser cell  204 . The walls of the condenser cells  204  are cooled by the adjacent evaporator cells through the metal sidewalls  211 . The water vapor condenses on the condenser walls and flows down as distilled water  210 . The distilled desalinated water is collected in the bottom of the condenser cells and passes through an orifice  214  to a flash-box  207  in the adjacent evaporator cell. The distilled water is then drained through drain pipe  208 . Further details of the operation are provided on  FIG. 4 . 
         [0023]      FIG. 3  shows the flow of concentrated liquid solution  301  in the lowest section of distiller. The concentrated solution (or liqueur)  301  cascades down through the evaporator cells to the concentrate pan  309  or collection bulkhead. In case of desalination of saline water, this concentrated liquid is referred to as brine. It flows on the walls of the evaporator cells  304  as falling film until it is collected in the bottom portion of the distiller. The collected brine  303  in the pan  309  is flowing horizontally from the heated end toward the cooled end, through subsequent orifices  310  that are sized to maintain the differential pressure between subsequent compartments of the pan. The brine is drained from the distiller at the cold end. Figure also shows the flow of distilled liquid  305 —water in case of desalinator. It is collected in the bottom of the condenser cells and flows through the orifices  311  to the flash box  307  located in the adjacent evaporator cell. From the flash box the distilled water is drained out of the distiller device through drain pipes  306 . 
         [0024]      FIG. 4  shows the details of one embodiment of the flow distribution and separation system. This is a cross sectional view of evaporator and condenser cells as it relates to the liquid flow distribution and separation of the falling film solution and distilled water. The concentrated liquid brine (liquor)  401  flows down the walls of the evaporator cells  404  and crosses through an opening  414  to the top portion of the evaporator cell bellow in the adjacent layer. The evaporated water (or solvent) vapors  402  flows into the condenser  405 . A separator cap  409  prevents the liquid brine from entering into the condenser. The water vapor  402  condenses to liquid distilled water  415  on the walls  412  and it flows down to and collects at the bottom of condenser cell  407  and drains out through the orifice  408 . The saline feedwater (or solution feed) is supplied through feedwater pipe  410  and nozzles  413 . The feedwater mixes with brine  401  flowing from the evaporator cell above forming a brine mixture  406 . The flow distribution of the brine to the walls  412  of the evaporator is accomplished by a distributor plate  417 . The flow is controlled by the vertical up and down movement of the plate that results in opening or closing of the gap  418 . The weight of the mixed brine pool  406  is countered and balanced by the force of spring mechanism  411 . The edges of the distributor plate  417  provide an even thickness of the falling film  401 . To prevent pane walls  412  from deflection or implosion—caused by the pressure differential between the evaporator cells  404  and adjacent condenser cells  405 —spacers  413  are installed to absorb the forces caused by differential pressure and maintain the cell-wall distances. 
         [0025]      FIG. 5  depicts the flow diagram of the MED matrix distiller apparatus configured for desalination in an open (or once through) cooling-loop application. This configuration is useful in coastal installations where supply of seawater is not limited. As all distillation based processes, the MED system requires significant cooling. If seawater is available for cooling, this open loop could be the most cost effective and energy efficient solution. The heating  501 , the cooling  502 , the evaporator  503  and condenser  504  cells are arranged in a checkered-matrix configuration. Heating is provided by the heat source  505  through a heating loop  506 . Seawater  508  pumped by the main supply pump  513  through the cooling cells  502 . Portion of the leaving preheated seawater  511  is used as a portion of the feedstock of the distillation process  514 . The balance of the leaving seawater  509  is returned to the sea. The products of the desalination are the distilled water stream  507  leaving the condenser cells  504  and the concentrated brine  510  pumped from the brine collection pan. Portion of the leaving brine stream  512  is recirculated by mixing it with the preheated feedwater  511 . This mixture  514  is the feedstock of the distillation process. 
         [0026]      FIG. 6  illustrates the flow diagram of the MED distiller in a closed cooling-loop, useful where supply of saline feedwater is limited and air cooling is required. This configuration is similar to the open loop cooling system shown on  FIG. 5  except that the seawater intake  608  is only a process makeup and it is equal to the feedstock flow  611 . The closed cooling loop consists of the air cooled heat rejection device  615  (that may be a fin-fan cooler, natural draft cooling tower or other cooling apparatus) and cooling flow  609  circulated by pump  617 . The sum of seawater flows  609  and  608  equaling  616  is pumped through the cooling cells  602 . 
         [0027]      FIG. 7  shows the flow diagram of a novel, direct-contact condenser configuration that uses distilled water in the closed cooling loop. The heating loop  705 ,  706  is similar to the previously discussed configurations. Saline feedwater  715  enters the system and is preheated in heat exchanger  713  recovering the waste heat from the leaving distilled water. The preheated feedwater is blended with the recirculated brine  712  and the mixed feedstock  711  is fed into the evaporator cells  703  of the MED. The distilled water  717  is mixed with the direct contact condenser cooling water  718  (also a distilled water quality) and the mix  707  is partially cooled after passing through the heat exchanger  713 . Portion of the distilled water  708  is further cooled by a heat rejection device  714  (that may be a fin-fan cooler, natural draft cooling tower or other cooling apparatus). The cooled distilled water  708  is used for direct contact condenser cooling. This flow diagram is for interpretation of and in conjunction with  FIG. 8 . 
         [0028]    Further details of the direct contact condenser cooling is shown on  FIG. 8 . This condenser configuration is significantly different compared to the indirect condenser. In the indirect case the last (cold) column consists of closed loop cooling cells sandwiched between condenser cells. The water in the cooling loop is a saline solution and it removes the heat through the vertical walls, from the condenser cells that collect the distilled water. The cooling and the condenser cells are not connected. In the direct contact condenser configuration—shown on FIG.  8 —all cells of the last (cold) column  814  are condensing cells connected only to the distilled water loop. The top, spray-cooled DC 1  condenser-cells  805  are simply connected with DC 2  condenser-cells  806  positioned below the  805  cells. The  805  and  806  cells form a common, double, condenser cells in series. Cold distilled water is pumped through the spray header pipe  812  and sprayed through nozzles  813  into the cell volume of  805 . The fine distilled water droplets fill the volume of both  806  and  805  cells, also creating a falling film on the vertical walls of the cells. The water vapor  802  enters into condenser cell  806  through louver openings from evaporator cells  803 . Condensation of the water vapor happens by direct contact on the surfaces of sprayed distilled water droplets. To prevent distilled water splashing into the evaporator cells, the openings  807  are covered with splash preventer louvers  808 . Distilled water  809  from the adjacent condenser cell  804  also flows into the flash-box  815 , through the flow restrictor orifice as previously described.