Patent Publication Number: US-2023150838-A1

Title: Solar powered desalination with direct heat transfer

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the production of potable water by the desalination of saline water. In general, the potable water shortage is becoming more and more acute. Conflicts over potable water are already taking place with increasing frequency. There are continuous efforts to convert more sea or saline water into potable water. 
     The two common desalination technologies of importance are SWRO (Sea Water Reverse Osmosis) and thermal distillation. 
     In SWRO, sea water is pumped at high pressure through a water permeable membrane. Water, the product, permeates through the membrane. Soluble salt content in the sea water is retained by the membrane, and the amount of salt in the membrane increases over time. The salt must be discarded when its concentration reaches a high level, typically between 20-50% of the feed water. The entire process is very energy intensive. 
     Thermal distillation is another process widely employed to produce potable water. The process, as the name implies, consists of boiling sea water, and condensing the vapor (steam) to produce potable water. When water is boiled off, soluble salts are left behind in the remaining seawater. The solution becomes more and more concentrated with salt, which then has to be discarded to prevent damage to the equipment. Typically, around 20% of the feed is used, whereas the balance of around 80% has to be discarded. The process is very energy intensive. 
     These two commercially significant processes require enormous amounts of energy of either fossil fuels or fossil fuel derived energy. In addition, waste disposal is a significant drawback. Both processes produce very large amounts of waste, i.e., highly saline water that must be discarded. Treating this waste is difficult and expensive. Ocean disposal causes damage to the marine environment. 
     In a conventional heating process used in distillation, the heat is transferred between two media which are separated by a barrier between them. The barrier, usually a metal or a material such as graphite, enables the heat transfer but prevents the mixing of fluids from opposite sides of the barrier. The present invention eliminates the need for such barrier. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for desalination of water, using a distillation process which is powered by solar energy. In the present invention, a solid material is heated by solar means, and the solid material heats the saline liquid by direct contact. Thus, the present invention eliminates the use of any barrier between the heated solid material and the liquid to be heated. 
     The hot solid material is introduced at the top of a chamber or tower and sinks to the bottom, transferring heat to the saline liquid which is pumped upward in the chamber. The solid material is preferably an impervious material such as silicon carbide, quartz, graphite, ceramics etc. 
     The saline water is heated and evaporates. The water vapor is then guided through pipes and heat exchangers to a barometric condenser. The vacuum created by the barometric condenser not only condenses the vapor, thereby producing water as a product, but also creates the condition in the equipment for the efficient removal and recovery of remaining salts in the saline water. 
     After having been used to heat the water, the solid material is extracted, renovated, and recycled to continue the process. 
     The retrograde salts, calcium carbonate and calcium sulfate, precipitate due to a high saline temperature, and are removed by a desludging unit. The halides NaCl and MgCl 2  undergo crystallization and finally precipitate in two separate chambers where they can be removed as solidified salts. Therefore, the process is a ZLD (Zero Liquid Discharge) process as there is virtually no liquid discharge at all. 
     The present invention therefore comprises an efficient and economical process which combines barrier free solar heating and vacuum cooling, while eliminating or substantially mitigating the disadvantages discussed above. 
     The present invention therefore has the primary object of providing a desalination process which is powered by solar energy. 
     The invention has the further object of providing a desalination process which uses thermal distillation, and in which saline water is heated by direct contact with a hot solid material. 
     The invention has the further object of providing an efficient process for desalination, while also producing commercially useful residues from the saline water being treated. 
     The invention has the further object of producing potable water, and other products of commercial use, while generating a minimum of waste, and while minimizing the overall costs of maintenance and energy. 
     The invention has the further object of providing an apparatus which performs the above-described functions. 
     The reader skilled in the art will recognize other objects and advantages of the invention, from a reading of the following brief description of the drawings, the detailed description of the invention, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    provides a schematic diagram of an apparatus made according to the present invention. 
         FIG.  2    provides a schematic diagram of the circulation devices, and associated components, used in the present invention, the circulation devices being identified as CD1 and CD2 in  FIG.  1   . 
         FIG.  3    provides a schematic diagram of the solar heaters used in a preferred embodiment of the present invention, the solar heaters being identified schematically as SH1 and SH2 in  FIG.  1   . 
         FIG.  4    provides a schematic diagram of various parts of the apparatus of the present invention, showing a chamber into which heated solid material is introduced at its top, and saline liquid is introduced at its bottom, and showing the renovation, reheating, and recycling of solid material used to heat incoming saline water. 
         FIG.  5 A  provides an elevational view of a chamber or tower, made according to the present invention, and used for providing direct contact between heated solid material and a liquid to be desalinated. 
         FIG.  5 B  provides a top view of the chamber of  FIG.  5 A . 
         FIG.  6    provides a schematic, elevational view of an alternative embodiment in which the chamber includes baffle plates instead of a helical structure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In brief, the process of the present invention is as follows. First, solid material (preferably having the form of a plurality of small solid balls) is heated by solar means, outside a vessel or chamber. The heated balls are introduced into the vessel, at or near its top or upper region, while saline liquid is pumped upward within the vessel. The balls fall by gravity through the vessel, while continuously contacting the liquid. The balls heat the liquid so as to cause the liquid to vaporize, thus separating the liquid from the salts carried by the liquid. In this way, the process produces potable water and one or more solid residues which can be commercially useful. The solid balls are recovered from the bottom or lower region of the vessel, and are renovated and then reheated and reintroduced into the vessel without interrupting the process. 
     The term “renovated”, as used in this specification, means the removal of scale deposits which form on the surfaces of the solid material, as a consequence of the contact between the hot solid material and the saline liquid. 
     Scales form when soluble salts extracted from saline liquid become deposited on warm surfaces. Such scale formation interferes with heat transfer surfaces, thus reducing the efficiency of heat transfer. 
     It is expensive and difficult, and time consuming, to remove the scales directly. In the prior art, it has been necessary to shut down the process to remove the scales. In the present invention, scale formed on the surfaces of the solid balls is removed by abrasion created when the balls impact each other, and when they impact the surfaces on which they travel. 
     In the present invention, the hot surface is provided by the heated solid balls, which are continuously moving through the liquid, and which are continuously removed, renovated, reheated, and re-used. By keeping the solid material in a continuous state of motion, and by renovating the solid material by abrasion, the present invention avoids the problem of scale formation, and the process can be operated continuously, without disruption. 
     The detailed structure and operation of the apparatus of the present invention will first be described with reference to the schematic diagram of  FIG.  1   . Then, further details of the major components will be described with respect to the other figures. 
     In this specification, the terms “solid material” and “solid balls” will be used interchangeably, it being understood that a preferred form of solid material is that of solid balls. However, the invention is not limited to use with solids in the form of balls or spheres. 
     As shown in  FIG.  1   , cool and filtered raw saline water enters storage tank ST 1 , through a conduit represented by arrow  101 . The liquid is kept at a sufficient volume to allow a preferably constant flow of the saline which will enter the desalination process through pipe P 1 . 
     The saline water leaves the storage tank ST 1  via pipe P 1 , and flows towards heat exchanger HE 1 . In this heat exchanger, the saline liquid is preheated by heat exchange with circulating hot potable water. 
     The now preheated saline water then flows through pipe P 2 , and is further heated in heat exchanger HE 2 , which receives heat from vapor or steam created in chamber C 1 , as will be explained below. The water passing through heat exchanger HE 2  then flows through pipe P 3 , into the raw saline storage tank ST 2 . 
     From the storage tank ST 2 , a now warm saline flows further through pipe P 4 , into pipe P 5 , and then upwards into chamber C 1 . The chamber is preferably insulated. It is in this chamber that the saline liquid is placed in continuous contact with heated solid material, so that heat is transferred from the solid material to the liquid. The solid material preferably has the form of small balls or spheres, indicated symbolically by reference numeral  130 . The solid material flows countercurrently relative to the inflow of the saline liquid. That is, in chamber C 1 , the liquid flows upwardly, and the solid material flows downwardly. 
     The solid material exits the chamber at the bottom, through pipe P 5 , and flows into a circulation device CD 1 , which is more fully illustrated in  FIG.  2   , and which will be described later. The circulation device recirculates the solid material through pipe P 6 . Also, some of the saline liquid is recycled through pipe P 8 . 
     Chamber C 2 , which will also be described later, is similar in construction to that of chamber C 1 . Solid material flows out of chamber C 2  through pipe P 19  and into circulation device CD 2 , also illustrated more fully in  FIG.  2   , and which will be described later. 
     Pipe P 5 , at the bottom of chamber C 1 , is similar to pipe P 19 , at the bottom of chamber C 2 . Similarly, pipe P 8  corresponds to pipe P 31 , and pipe P 6  corresponds to pipe P 20 . Also, pipe P 4  corresponds to pipe P 18 . 
     In the case of both chambers C 1  and C 2 , the solid material leaving the chambers, at the bottoms thereof, is renovated and reheated. The details of the renovation and reheating will be described later. For present purposes, it should be understood that the solid material is conveyed to solar heaters SH 1  and SH 2 . These solar heaters receive solar radiation, symbolized by arrows  120  and  121 . The heated material is conveyed, through pipes P 7  and P 30 , respectively, into temporary heated storage units TS 1  and TS 2 , before re-entering the upper regions of the respective chambers C 1  and C 2 . 
     The preferably continuous flow of warm saline into the chamber C 1  at its bottom will cause an overflow of saline at its top which then flows through pipe P 10  into sludge chamber SC. A minor amount of the saline leaves the sludge chamber at its bottom through pipe P 11 , and is filtered in the filtration tank FT 1  and circulated back into the sludge chamber by pipes P 12  and P 10 . The majority of the saline runs by overflow through pipe P 14  into the crystallization spray chamber CSC 1 . 
     Inside the sludge chamber SC, retrograde salts such as calcium sulfate and calcium carbonate will precipitate and accumulate. These materials accumulate in the sludge chamber, and are funnelled into pipe P 11 , then filtered inside filtration tank FT 1 , and then withdrawn from the process as a product having commercial value. 
     The remaining filtered saline is pumped back into the sludge chamber SC through pipe P 12 . The saline enters pipe P 10 , preferably at the top of the sludge chamber. The continuous overflow of saline in the sludge chamber SC flows into pipe P 14  and is released as a spray inside the crystallization spray chamber CSC 1 , in which the halide sodium chloride is crystallized, preferably under vacuum, and funneled into pipe P 16 , where it is filtered in filtration tank FT 2  and subsequently withdrawn from the process as another product having commercial value. 
     In the crystallization spray chamber CSC 1 , the majority of the liquid saline is vaporized, leaving the crystallization chamber CSC 1  by pipe P 15 . A small part of the liquid is drained into the filtration tank FT 2 , and then fed into pipe P 18  and finally pipe P 19 . 
     The continuous flow of saline into chamber C 2 , through pipes P 18  and P 19 , causes an overflow of saline guided by pipe P 21  into crystallization spray chamber CSC 2 , where it is sprayed downwardly, releasing its heat as steam or vapor upwardly into pipe P 22  and subsequently pipe P 26 . The remaining liquid flows through pipe P 23 , and is filtered to remove the halide magnesium chloride, in filtration tank FT 3 , and the residue is recovered as a product of commercial value. The remaining saline is pumped back through pipe P 24 , entering pipe P 18 , and then flows into pipe P 19 , so as to be able to return to chamber C 2 . 
     The saline which passes filtration tank FT 2  is pumped through pipe P 18  into pipe P 19 , which connects chamber C 2  with circulation device CD 2 . The saline flows upwardly, through pipe P 19 , in the opposite direction to the solid material, and the saline is heated by direct contact with the solar heated solid material flowing downwardly, as was described with respect to chamber C 1 . The solid material finally drops through pipe P 19  into the circulation device CD 2 , to be described in detail later, from which it is recirculated by pipe P 20  into solar heater system SH 2 , and finally introduced, via solid storage TS 2 , into the chamber C 2  again, from the upper region, thus continuously reheating the saline liquid inside the chamber. 
     A final circulation of pure potable water takes place at the barometric condenser BC. This barometric condenser creates a vacuum and condenses the incoming steam/vapor from the desalination process to be collected as the end product. The circulating potable water is pumped from the water circulation tank CT via pipe P 28  to the heat exchanger HE 1  and the barometric condenser BC, and back into the water circulation tank CT. The excess of condensed steam/vapor is finally collected as potable water through pipe P 29 . 
     During the desalination process, saline water will thus be split into three components, namely liquid, steam and/or vapor, and salts. 
     Stated in more detail, the steam leaving chamber C 1 , through pipe P 9 , the vapor leaving the crystallization spray chamber CSC 1 , through pipe P 15 , and the steam or vapor from the crystallization spray chamber CSC 2 , flowing through pipe P 22 , are condensed in the barometric condenser BC, to become potable water. The potable water flows through pipe P 27 , and enters a circulatory flow into a circulating tank CT, flowing through pipe P 28  and passing through heat exchanger HE 1 , to release heat, and enters the barometric condenser again. The steady inflow of steam and vapor from pipe P 26  being condensed by the barometric condenser causes an accumulation and finally an overflow of potable water inside the circulation tank CT which is then collected and withdrawn from the process as a product having commercial value. The potable water is collected through pipe P 29 . 
     In summary, in chamber C 1 , the warm raw saline enters the chamber at the bottom, flowing upwards and heated by contact with the solid material, which counterflows downwards through the chamber. The steam will leave the chamber C 1  at the top, through pipe P 9 , passing through heat exchanger HE 2 , through pipe P 26 , and towards the barometric condenser BC. 
     A secondary flow of steam and/or vapor will exit the crystallization spray chamber CSC 1 , joining the steam flow from chamber C 1 , also towards the barometric condenser BC. 
     A third flow of steam/vapor exiting the crystallization spray chamber CSC 2 , will again join the previous two streams on their way to the barometric condenser BC. 
     The steam and/or vapor from each of the chamber C 1 , the crystallization spray chamber CSC 1 , and the crystallization spray chamber CSC 2 , are all accumulated in pipe P 26 . Inside the barometric condenser BC, the incoming steam/vapor is liquified and finally collected as warm, potable water. 
       FIG.  2    provides details of the fluid circuit which includes chambers C 1  and C 2 , and which renovates the solid material. The components other than the chambers C 1 /C 2  comprise the circulation devices identified by CD 1  and CD 2  in  FIG.  1   . 
     The solid material, typically having the form of solid balls  130 , falls out of the chambers C 1  and C 2 , and is conveyed to vibrating sieve  102 . The balls are transported in saline liquid, which is moved through the system by liquid circulation pump  107 . This means of transport has been found to be dust-free, efficient, and economical. 
     The solid balls fall onto a conical abrasive screen  103  within the vibrating sieve  102 , and are shaken, rolled about, and abraded by the abrasive screen. In this way, the scale material, which has formed and accumulated on the surfaces of the balls, is removed. 
     The solid balls are then sprayed with saline liquid, by sprayer  104 , so as to flush off the debris on the balls, before they roll down a chute (not shown) to begin the reheating process. The conveying saline, the flushing saline, the scale material, and the detritus all flow through the abrasive screen and collect at the bottom of the vibrating sieve, and then drain through the outlet pipe  105  into a settling tank  106  where the solids settle and are removed. The filtered saline returns to the process via pipe P 4  and pipe P 18  of  FIG.  1   . In  FIG.  2   , there is shown pump  118  which pumps the filtered saline back into the chamber. Pump  118  is therefore an illustration of a means for directing saline water upwards through the chamber. The vibrating sieve  102 , and associated components, is a means for recovering and renovating the solid material. 
     Wet abrading is a fast, economical, environmentally friendly process for renovating or reconditioning the solid balls. The scale material and detritus are removed in solid form. 
     The solid balls may also be renovated or reconditioned by chemical, ultrasonic, or other means. 
       FIG.  3    provides a schematic diagram showing details of the solar heating of the solid material.  FIG.  3    shows the preferred structure for either or both of solar heaters SH 1  and SH 2  of  FIG.  1   . 
     After the solid balls have been renovated, the balls are ready to be reheated, as represented by solar heaters SH 1  and SH 2  of  FIG.  1   . 
     As shown in  FIG.  3   , the solid balls are sent first to a flat plate heater  108 , which allows the balls to be heated by solar radiation  109 . While the flat plate heater is efficient, it is unable to achieve the high temperature necessary for efficient operation of the process. 
     The solid balls are therefore transported by a vertical screw conveyor  110  to a secondary heater  111 , which comprises a line focus Fresnel reflector heater, where the balls are heated to a very high temperature. The line focus Fresnel reflector heater includes a quartz tube  113  located above an assembly of mirrors  112  which reflect solar radiation and concentrate it on the quartz tube. The quartz tube is supported by rollers and a drive mechanism (not shown) which rotates the tube around its axis, as indicated by arrow  114 . 
     The secondary heater  111  is mounted at an incline. The inlet end  115  of the quartz tube  113  where the solid balls are fed is higher than the outlet end  116 . The outlet end  116  is similarly higher than the inlet of temporary storage devices TS 1  and TS 2  of  FIG.  1   . 
     Solar radiation heats the solid balls inside the quartz tube. Rotating the quartz tube results in even heating of the solid balls, and facilitates moving the solid balls down the incline from the inlet towards the outlet of the secondary heater, and on to the inlet of the temporary storage devices TS 1  and TS 2  of  FIG.  1   . 
     One or both of heaters  108  and  111  can be considered a heating means in the present invention. 
     The solid balls could also be heated by a parabolic trough, or a flat or curved Fresnel lens concentrator. The quartz tubing can also be non-transparent, metallic, non-metallic such as graphite, for indirect heating of the solid balls. 
       FIG.  4    provides details of the structure of the chambers C 1  and C 2 , and shows these chambers as part of a closed loop which includes the renovation and heating devices associated with the respective chambers. Note that a single element C 1 /C 2  is shown in  FIG.  4   , representing both of chambers C 1  and C 2 . 
     The chambers C 1  and C 2  contain a vertical helix  10 . As described with respect to  FIG.  1   , a temporary solid material storage tank TS stores the solid material before it is dispensed into the chamber. The solid material then rolls, by gravity, along chute  25 , into the upper region of the chamber, where it will continue to travel, by gravity, while releasing its heat into the already pre-warmed saline liquid inside the chamber. The chute  25  can be considered a means for conveying the heated solid material into the chamber. The storage tank TS of  FIG.  4    is intended to represent either of TS 1  or TS 2  of  FIG.  1   . 
     The inside of the chamber may be provided with suitable structures to prolong the contact time of the saline with the solid material. For example, as shown in  FIG.  4   , the helix  10  of the chamber comprises an internal plate having the form of a spiral, which is shown explicitly in  FIGS.  4  and  5 A . The solid material is forced to roll down the spiral plate, thereby prolonging contact with the saline. While rolling, the heat from the solid material is efficiently released by the turbulence induced by the movement of the solid material, due to the shape of the solids and their movement towards the bottom of the chamber. 
     In an alternative embodiment, shown in  FIG.  6   , instead of a spiral or helical structure, the inside of the chamber C 1  (or C 2 ) has baffle plates  301 , extending from either side of the chamber, and arranged to project alternately from one side and the other, so as to cause the balls  130  to fall along a generally zig-zag path. 
     The solid material leaves the chamber C 1 /C 2  by either of pipes P 5  or P 19 , into the circulation device CD 1 /CD 2 , shown in  FIG.  4    and previously described with respect to  FIG.  2   . The circulation device drains off the saline at the top of the device before the solid material enters the solar heaters. The flat plate solar heater  108 , the secondary solar heater  111 , and the vertical screw conveyor  110  are the same as in  FIG.  3   . 
     As shown in  FIG.  4   , the hot solid material flows, by gravity, into storage tank TS. The storage tank TS could optionally be heated also, and could comprise a tertiary solar heater. This tertiary solar heater is designated generally by reference numeral  140  in  FIG.  4   . The drawing symbolically illustrates a solar concentrator and rays which converge on the storage tank. However, the storage tank TS could be heated instead by non-solar means. 
     The bottom of the temporary storage tank TS has a feed controller or dispenser  117  which can adjust the amount of solid material released from the temporary storage tank TS. 
     The solid material then enters chamber C 1  or C 2  through an inlet chute  25  at the upper side. The solid material storage facilities, the storage tank, and the storage dispenser are designed to store flexible amounts of solid hot material of desired shapes and/or size, to maintain the process heat capacity which is needed in times of insufficient solar radiation, such as on cloudy days, and during night, to ensure overall continuous production. 
     The highest temperature in chamber C 1  or C 2  will be at the top surface of the saline liquid inside the chamber, due to the heat exchange while in contact with the hot solid material entering the chamber. Some saline flashes off as steam, which leaves the chamber C 1  through pipe P 9 , and passes through heat exchanger HE 2 , to heat further the fresh warm raw saline on its way to the storage tank ST 2 . 
     In  FIG.  4   , vapor leaves the chamber C 1 /C 2  through port  26 , located at the top. Hot liquid leaves the chamber through port  27 , located at the upper right-hand side. 
       FIGS.  5 A and  5 B  show the structure of either of the chambers C 1  or C 2 . Chamber  200  comprises a generally cylindrical housing, in which there is disposed a vertical helix  201  which defines the spiral path for the solid balls described above. The vertical helix surrounds, and may be mounted to, a post  202 .  FIG.  5 B  provides a top view of the chamber  200 . 
     All process equipment which has a higher or lower temperature than ambient is preferably insulated by suitable insulation materials which prevent heat or cooling losses during the process. The chambers and other equipment are preferably fabricated from thermo plastic and/or saline resistant material. 
     The solid material used in the present invention preferably is made of solid, round balls which are inert and thermally stable. It is preferred that the solid material have a density substantially greater than that of the saline liquid, to allow easy separation of the solid from the liquid. 
     Regardless of the shape of the solid material, it is preferable that the material be pourable. That is, the solid material preferably comprises a plurality of pieces each having a sufficiently small diameter that the material can be efficiently poured and/or moved through a pipe or conduit by a liquid. The use of a large number of relatively small pieces also has the advantage of maximizing the amount of surface contact between the solid and the saline liquid. 
     The solid material may be made of ceramic, such as aluminum oxide, graphite, silicon carbide, or quartz. The preferred diameter of the balls is about 5-25 mm. The balls are easily removed from the vessel, and can be easily renovated by causing rolling and/or vibratory motion. 
     The solid material could also be non-spherical, and could vary in size from the range described above. The solid could be an alloy, a eutectic liquid or solid, or a liquid such as mercury. The solid material could have a shape like that of a ball bearing. Also, the solid material could include combinations of materials having different compositions, and could include pieces of varying shape and/or size. All such combinations of compositions, shapes, and sizes are within the scope of the present invention. 
     The present invention also overcomes the problem of scale formation in heat exchangers used in the thermal evaporation of saline liquids. 
     The spiral upward flow of saline liquid minimizes the mixing between incoming and exiting liquid. 
     The conical bottom of the chamber allows easy collection, recovery, and transfer of the solid balls to the location where they can be reheated. 
     The final products of the desalination process include 1) potable water, collected at the circulation tank CT, 2) retrograde products calcium carbonate and calcium sulfate, collected at the filtration tank FT 1 , 3) the halide sodium chloride at filtration tank FT 2 , and 4) the halides magnesium and potassium salt at filtration tank FT 3 . 
     The invention can be modified in various ways. The number of chambers can be increased or decreased. Although a dual solar heater, as shown, is preferred, there could be as few as one solar heater, or there could be more than two such heaters. There also could be non-solar heat sources, in addition to the solar heaters. 
     In another embodiment, the storage tank for the hot solid material could be divided into sections, with extra solar heating being directed at one of such sections. 
     The modifications indicated above, and others which will be apparent to those skilled in the art, should be considered within the spirit and scope of the following claims.