Abstract:
A thermal desalination system adapted to produce distilled water from feed water which may be sea water, based on a forward feed evaporator made up of a main water feed line, a vapor feed line, a distilled water main line and any number of effect groups. The effect groups include any number of effects which in turn include vapor inlets, water inflow lines, concentrate outflow lines that drain into a common concentrate drainage line, a vapor and water outlet in fluid communication with the distilled water main line, and heat transfer means that condense part of an inlet vapor to produce the distilled water.

Description:
FIELD OF THE INVENTION 
     This invention relates to distillation installations and methods, in particular a method using multi-effect evaporators. 
     BACKGROUND OF THE INVENTION 
     Distillation of water is a process in which various soluble materials such as salt, contaminants etc. are eliminated from water containing these materials, leaving clean, usually drinkable water. One known method for achieving such distillation relies on water evaporation, much like salt and scale being accumulated on the bottom of an electric kettle after water has evaporated. In this process during evaporation of the water, soluble materials that are not volatile remain in a solid state residue, usually in the form of salt and scale, and are disposed of. The vapor can then be condensed back into a state of liquid, resulting in contaminant free water. 
     U.S. Pat. No. 3,868,308 to the applicant discloses a multiple effect evaporator system, comprising a housing and a plurality of effects connected in a series to one another, each effect having a plurality of bundles of tubes. The system is built such that high temperature steam is introduced into the tubes of the first effect, while non-distilled water is sprayed against the outside of the tubes, causing the vapor in the tube to condense while evaporating a part of the non-distilled water. The remainder of steam from the tubes of the first effect, along with the evaporated water outside the tubes enters the tubes of the adjacent downstream effect, while the remainder of the non-distilled water which has not evaporated, is accumulated at the bottom of the effect housing in the form of a concentrate and is moved to be sprayed against the outside of the tubes of the adjacent upstream effect and so on and so forth. 
     Once water has completed its passage through all the effects of the evaporator system, the process yields distilled water on the downstream end of the evaporator and a warm concentrate in the form of a highly concentrated water solution of soluble materials on the upstream end. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a multi-effect evaporator adapted for distillation of water, comprising a plurality of effects connected in a series manner and arranged into groups including a most upstream group and subsequent downstream groups, each group having a most upstream effect and a most downstream effect and having a common parallel water feed inlet adapted to supply all effects in the group with feed water; the evaporator further including a main feed water line in fluid communication with the most upstream group; an array of heaters disposed along the line and adapted for heating the feed water before its entry into the effects of the most upstream group; each effect comprising heat transfer means adapted to receive an inlet vapor and produce from the feed water a first outlet vapor, leaving the remainder of the feed water as a concentrate, and to condense a part of the inlet vapor to produce distilled water, leaving the remainder of the first inlet vapor as a second outlet vapor; each effect comprising means for forwarding the first outlet vapor into an adjacent downstream effect, where it will constitute the first inlet vapor, and means for forwarding the second outlet vapor into one of the heaters for heating the feed water thereby; each group further comprising a pump adapted to extract the concentrate from the effects of the group and pump it into the common parallel water feed of an adjacent downstream group; and means for collecting the distilled water. 
     In one embodiment of the invention, the heat transfer means in each effect comprises a plurality of tubes with inter-tube spaces therebetween, the tubes being adapted for receiving the inlet vapor, and contacting the feed water to cause a heat transfer therebetween, resulting in the vaporization of a part of the feed water in the inter-tube spaces, to produce the first outlet vapor, leaving the remainder of the feed water as a concentrate, and resulting in the condensation of a part of the inlet vapor in the tubes, to produce the distilled water, leaving the remainder of the inlet vapor as the second outlet vapor. The tubes are further adapted for channeling the condensed water and the second outlet vapor from one side of the effect to the other. 
     The parallel water feed inlet in each group of the effects may comprise dispersion means adapted to introduce the feed into each of the effects of the group so as to allow its contact with the tubes. The water feed may be introduced into the dispersion means in various forms, e.g. thin film form, and temperatures, allowing optimization of the heat transfer and overall desalination process. 
     The number of effects comprised in one group may vary according to the water feed rate, water temperature and additional factors. The group formation may hold up to 30 effects maintaining high efficiency factors. 
     The tube bundle may be of any material allowing reasonable heat transfer between the vapor inside the tube and water on the exterior of the tube, such as aluminum or other metals or metal alloys. In operation, the tubes may be positioned horizontally or tilted at an angle as to aid the condensed vapor within the tubes to flow down to the opposite end of the effect by force of gravity. The tube shape is not restricted to a circular cross section and may be of other shapes, for example oval. 
     Further modifications may be made to the evaporator such as galvanization of the condenser tubes, adding ion traps, using various alloys for the condenser tubes etc. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the invention and to see how it may be carried out in practice, one embodiment of the present invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of a multi-effect evaporator according to one embodiment of the present invention, showing only the first and last effects of each of the groups; 
         FIG. 2  is an enlarged view of group  20   c  with upstream and downstream effects of the adjacent groups  20   b ,  20   d  of the evaporator shown in  FIG. 1 ; and 
         FIG. 3  is a schematic view of a feed heater used in the multi-effect evaporator shown in  FIGS. 1 and 2 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIGS. 1 and 2  show a multi-effect evaporator system  10  together with its main water feed line  11  extending above and along the evaporator  10  from its downstream end  10   b  to its upstream end  10   a , and with its main vapor feed line  12  associated with the upstream end  10   a  of the evaporator. The evaporator  10  is adapted for the distillation of water entering it from the main water feed line  11  using vapor entering it from the main vapor feed line  12 . 
     The evaporator  10  comprises a housing  8  and four groups of effects designated as  20   a  to  20   d , disposed therein, group  20   a  being the upstream-most group and group  20   d  being the downstream-most group. The evaporator  10  further comprises a condenser  13  adapted to condense vapor remains from the distillation process, a distilled water main line  16  adapted to collect distilled water produced by the evaporator, a product pump  14  adapted to withdraw from the evaporator the distilled water, and a brine pump  15  adapted to withdraw from the evaporator brine left after the distillation. The condenser and both pumps are located at the downstream end of the system. 
     The water feed line  11  is provided with a plurality of heaters  24 , each having a heater vapor inlet  41  adapted to receive vapor from each group  20  and to heat thereby the water in the feed line  11 . Each heater  24  further comprises a first outlet  27  (shown in  FIG. 3  and in  FIG. 1  only for the most upstream and downstream heaters of the groups  20   a  and  20   d  respectively) used for removing non-condensable gasses (NCG) and remaining vapor, in fluid communication with an NCG and vapor removal line  17 , and a second outlet  44  for withdrawal of water therefrom. 
     Each group of effects  20   a  to  20   d  comprises a plurality of effects  30  and a plurality of inlet chambers  40  connected in series with the effects so that each effect  30  has on its upstream side an inlet chamber  40 . The effects are denoted E 1  to E 20 , the most upstream and downstream effects of each effects group  20  being designated as  30   h  and  30   t , respectively. Groups  20   a ,  20   b ,  20   c  and  20   d  hold within them effects E 1  to E 6 , E 7  to E 11 , E 12  to E 16 , and E 17  to E 20  respectively. 
     Each of the effects groups  20  also comprises a common parallel water inlet  25  divided into dispersion means  26 , each adapted to introduce feed-water in a thin film form into one of the effects  30 . Each group  20  further comprises a concentrate drainage line  23  and a concentrate pump  21  connected therewith, both adapted to withdraw concentrate from the group  20 , the pump  21  being adapted to introduce the concentrate into the water inlet  25  of the subsequent downstream group. 
     Each effect  30  comprises at its top a main water inlet  31  adapted to receive water to be distilled from the dispersion means  26 ; a vapor inlet  32  located on the upstream side of the effect; and a concentrate collector  33  at the bottom of the effect in fluid communication with the concentrate drainage line  23  of the effect&#39;s group. The vapor inlet  32  of the most upstream effect  30   h  of the first group  20   a  is in fluid communication with the main vapor line  12 , while the vapor inlet  32  of each of the other effects of each group is adapted to receive vapor, via its preceding inlet chambers  40 , from the immediately preceding adjacent upstream effect  30 . 
     Within the effect  30  is located a tube bundle  35 , consisting of horizontally coextending condenser tubes  35   a  of oval or circular cross-sections, with a space  35   b  therebetween. The upstream ends of the tubes  35   a  constitute the vapor inlet  32  of the effect, and the downstream ends of the tubes  35   a  constitute a first, water and vapor outlet  34  of the effect, for vapor and distilled water exiting the tubes, while the space  35   b  between the tubes constitutes a second, vapor outlet  36  of the effect for vapor created from the feed water in the space  35   b  between the tubes  35   a . The tubes are slightly downwardly inclined from the upstream side of the effect to its downstream side to allow water flow therein using gravitational forces. The tubes are located under the main inlet  31  to allow feed water from the dispersion means  26  to be sprayed thereon, to cause heat transfer between the vapor flowing within the tubes and the sprayed water. The ends of the tubes  35  are supported by and arranged within vertical tube sheets  39 . 
     Each effect further comprises a concentrate outflow line  37  which connects the concentrate collector  33  of each effect  30  with the concentrate drainage line  23 . The concentrate pump  21  of each group  20  is connected to the downstream end of the drainage line  23 . 
     As mentioned above, the inlet chamber  40  is disposed between each pair of two adjacent effects  30 , one effect being on the upstream side of the chamber and the other being on the downstream side of the chamber. The chamber  40  is adapted to receive water and vapor from first and second outlets  34  and  36  of the effect located on the upstream side of the chamber. The chamber is designed to allow vapor from outlet  34  to flow into the vapor inlet  32  of the effect  30  located on the downstream side of the chamber. The chamber also has at its top a vapor outlet  43  via which the vapor from outlet  36  is diverted to the vapor inlet  41  of the corresponding heater  24 . The chamber  40  further comprises a droplet separator  38  located at the vapor outlets  34 ,  36  of each effect and a distilled water collector  42  located at the bottom of the chamber, adapted to receive the distilled water from the first outlet  34  of the effect on the upstream side of the chamber. Connected to each distilled water collector  42  is a distilled water outflow line  47  leading to the distilled water main line  16 . 
     It should be appreciated that although described here to be common parallel drainages for both concentrate and distilled water, the concentrate collectors  33  may be connected in series to one another, and the distilled water collectors  42  may also be connected to one another in a series manner. 
     The heaters  24  may each be associated with one group  20  or with one inlet chamber  40  of each group  20 , as shown in the drawings. 
     The main water feed line  11  and vapor feed line  12  are connected to the first effect  30   h  of the first group  20   a . The condenser  13  is in fluid communication with the water and vapor outlet  34  of the last effect  30   t  of the last group  20   d  via pipe-line  13   a  and the brine pump  15  is in fluid communication with the concentrate drainage line  23  of the last group  20   d  via pipe-line  15   a.    
     In operation, feed water is introduced from an external source into the feed line  11  on the downstream end of the system, at about 25° C., and passed along the feed line  11  through the heaters  24 . The heaters  24  gradually heat the feed water until it is introduced into the common parallel water inlet  25  and the dispersion means  26  of the first group  20   a . The heating of the water is such that it reaches the first effect  30   h  of the first group  20   a  at its highest temperature which may reach 82-85° C. At the same time, vapor at a temperature slightly above 85° C. is introduced into the vapor inlet  32  of the first effect, i.e. into the upstream ends of the tubes  35   a  of the first group  20   a.    
     The water from the feed line is sprayed downwards from the main water inlet  25  using dispersion means  26  which spray it in thin film form, i.e. about 0.2-0.3 mm, onto the tubes  35   a  of each of the effects  30  of the first group  20   a . Upon the contact of the water film with the tubes  35   a , a heat transfer process takes place between this film and the vapor flowing within the tubes  35   a , resulting in partial condensation of the vapor in the tubes  35  and partial evaporation of the feed water in the space  35   b  between the tubes. 
     The vapor that has condensed in the tubes  35   a  constitutes the distilled water and flows by force of gravity down the tubes, which are inclined, into the inlet chamber  40  located at the downstream end of the tubes, where it drips down to the distilled water collector  42 . The distilled water flows down from the collector  42  of each inlet chamber  40  to the distilled water main line  16  via the outflow line  47 . The remainder of the vapor that has not condensed in the tubes  35   a , flows into the inlet chamber  40  and is sucked via the vapor outlet  43  into the vapor inlet  41  of the respective heater  24  that utilizes the hot vapor to heat the feed water in the main water feed line  11 . 
     The feed water that has turned into vapor in the space  35   b  between the tubes  35   a  of the effect is forwarded via the chamber  40  to the vapor inlet  32 , and the upstream ends of the tubes  35   a , of the next, immediately adjacent effect. The feed water that has not evaporated, namely the concentrate, drips down to the concentrate collector  33  at the bottom of the effect  30 , from which the concentrate flows down into the concentrate drainage line  23  of the first group  20   a.    
     The concentrate is then pumped from the concentrate drainage line  23  using a concentrate pump  21  via pipe-line  22  into the common parallel water inlet  25  of the second group  20   b , where the process is repeated in this and subsequent groups, with the only difference between the first group  20   a  and all the subsequent groups  20   b  to  20   d , that the first group  20   a  receives feed water from the main water feed line  11  while the other groups receive feed water from the concentrate pumps  21 . Thus, the feed water and the vapor both move downstream during the process, the evaporator system thus being a forward feed flow multi-effect evaporator. 
     The feed heater  24  shown in  FIG. 3  is adapted to receive feed water through the inlet  11   a  and hot vapor with NCG through the inlet  41  and allowing heat transfer between the two. This process yields heated feed water leaving the heater  24  through the outlet  11   b  and condensed vapor, constituting distilled water, leaving the heater  24  through outlet  44 . The vapor that has not condensed is removed through outlet  27  via NCG and vapor line  17 . The outlet  44  may be in fluid communication with the distilled water collector  42  of the inlet chamber  40  or with the distilled water main line  16 . 
     The above described process produces distilled water and obtains the most highly concentrated brine at its lowest temperature due to the forward feed flow, as opposed to backward feed flow where the brine is produced at the highest temperature. Low temperature brine allows preventing the danger of scale and corrosion of the installation. The brine is removed from the system using a brine pump  15  via pipe-line  15   a  connected to the last concentrate drainage  23   d . At the downstream end of the system the distilled water from the distilled water main line  16  is introduced into a condenser  13  via the pipe-line  13   a , where it utilized to condense the vapor coming from the last effect  30   t  of the last group  20   d . From the condenser  13 , the product pumps  14  pumps out the distilled water using the pipe-line  14   a.    
     The operation of the above described multi-effect evaporator  10  may be further modified and enhanced by various means such as ion traps for heavy metals, pre-de-aeration of the water in a titanium tube, galvanic insulation of tubes from tube plates by elastomeric grommets and incorporating sacrificial aluminum-magnesium anodes in the submerged section of each effect. Furthermore, various types of water may be used for the process, the most common of which is sea-water. In addition, various numbers of effects for each group and, and a various number of effects altogether may be used in order to yield different results. 
     Those skilled in the art to which this invention pertains will readily appreciate that numerous changes, variations and modifications can be made without departing from the scope of the invention mutatis mutandis.