GB2160117A

GB2160117A - Apparatus for the distillation of fresh water from sea water

Info

Publication number: GB2160117A
Application number: GB08432110A
Authority: GB
Inventors: Gamal El Din Nasser
Original assignee: DVT DEUTSCH VERFAHRENSTECH
Current assignee: DVT DEUTSCH VERFAHRENSTECH
Priority date: 1982-05-24
Filing date: 1984-12-19
Publication date: 1985-12-18
Also published as: GB2160116B; GB8432110D0; GB2160116A; GB8311155D0; GB2120720B; GB2160115B; GB2160115A; GB2120768A; GB2120950B; GB2120950A; GB2120768B; GB8432108D0; GB2160117B; GB8432109D0; GB2120720A; GB8311153D0; GB8311154D0

Abstract

Process and apparatus for the distillation of fresh water from sea water by a combined film, vertical tube evaporation (VTE) and multiple stage flash (MSF) evaporation in a multiple stage process, in which after passing through a first VTE process stage and prior to entering the following VTE process stage, the salt water is distributed to form a seal between the VTE process stages. The fresh water distillate from a VTE process stage undergoes flash evaporation in at least one MSF process stage for each VTE process stage. The fresh water distillate and brine residue in each case pass together through all the following VTE and MSF process stages in such a way that the brine residue can be removed from the final VTE process stage and the fresh water distillate from the final MSF process stage. The steam used for heating the sea water is successively condensed and passed through all the evaporation stages, starting with the first VTE process stage. Inert gases produced during the stagewise evaporation are removed by suction. <IMAGE>

Description

1

SPECIFICATION

Process and apparatus for the distillation of fresh water from sea water Technical field and background art

The invention relates to a process and to an appa ratus for distilling fresh water from sea water by film evaporation in a multiple stage vertical tube evaporation (VTE) process by which the sea water 75 to be evaporated is passed through tubular heat exchangers heated by primary vapour or steam.

It has hitherto been proposed, for example, in German Patent No. 23 34 481 to derive fresh water from sea water by passing the sea water (heated 80 by means of a preheater constructed as a succes sive stage heater) successively through the individ ual pressure stages of a falling film evaporator heated in the first stage with primary steam only.

Partial evaporation of the sea water occurs in the 85 individual stages, whilst the unevaporated sea water flows in to the next stage, which is now heated with mixed steam. After passing through the final stage, the sea water has been separated into brine and distillate (conveniently called fresh 90 or clean water). The preheater and failing film evaporator are subdivided into pressure and tem perature stages, in accordance with the required reduction in the temperature at which the sea water boils in the successive stages. In the prior proposals the evaporator and preheater are in the form of tubular heat exchangers arranged as verti cal columns within a support structure. These tu bular heat exchangers comprise densely packed bundles of tubes carried between longitudinally 100 opposed end flanges to have a relatively high me chanical strength, particularly in the longitudinal direction. These tube bundles are mounted with their bottom flanges on supporting grates to ex tend vertically and provide the individual evapora- 105 tion stages as a column within a container. Such a column (which generally has fewer than 15 verti cally disposed stages) will usually have an ade quate mechanical strength, so that there is no need to provide a special supporting frame for the con- 110 tainer. The aforementioned tubular exchangers have an economic length of approximately 7 m, and fifteen stages are usually provided for the column with an overall temperature differential of ap- proximately + 12WC being available for the stage- 115 by-stage evaporation. However, even with only 15 stages the overall height of the column is usually more than 100 m. The efficiency of the multieffect evaporation process is dependent on the number of evaporation stages which can be obtained within the temperature differential between the first and final stage of the evaporator; there is consequently a desire to provide a process and apparatus for desalinating sea water by which the number of stages in the evaporator column can be 125 significantly increased to provide efficient and economic desalination with a relatively uncomplicated arrangement of stages and sealing between those stages.

GB 2 160 117 A 1 Statements of invention and advantages

According to the present invention there is provided a process for the distillation of fresh water from sea water by film evaporation within a multi- ple stage vertical tube evaporation WTE) process in combination with flash evaporation within a multiple stage flash evaporation (MSF) process which comprises passing heated sea water through a column of successive VTE process stages; condensing steam derived from the individual WE process stages and passing the fresh water condensate through successive MSF process stages which are associated at least one with each stage of the said WE process stages and by which the condensate is subjected to flash evaporation and applying the steam resulting from the flash evaporation to heat the respectively associated stage of the WE process, and wherein prior to entering a VTE process stage the salt water/brine is distributed over that stage to form a seal between adjacent WE process stages. Preferably primary steam is introduced into the first stage of the WE process for heating that stage and subsequent stages of the WE process are heated with steam which results from flash evaporation of fresh water derived from the condensation of said primary steam and from the steam components of the preceding WE process stage or stages.

Further according to the present invention there is provided apparatus for carrying out the process as specified in the immediately preceding paragraph which comprises a sea water preheater column and a film evaporator column for the VTE process, each of said preheater and evaporator columns comprising heat exchanger plates having a spaced array of reinforcing beads formed therein, said beads being uniformly disposed and aligned longitudinally and transversely of the plates in grid- like manner; said plates being disposed in pairs with the plates in each pair positioned adjacent and in mirror image to each other so that the beads form between the pair of plates tubular ducts extending transversely of the plates and the beads on the exterior of a pair of plates form with an adjacent heat exchanger plate of a further pair of such plates slot-like ducts extending longitudinally of the plates or vice versa; said tubular ducts and slot-like ducts providing conduits in one case for the through flow of water and in the other case for the through flow of steam, and wherein the plates in the film evaporator column form vertically disposed stages for the VTE process, the plates in said stages carrying salt water distributing means by which the brine is distributed over the respec- tive stages of the VTE process to form seals between those stages. Preferably the heat exchanger plates for the water preheater column are arranged so that water flows upwardly through the slot-like ducts and steam flows through the tubular ducts and the heat exchanger plates for the film evaporator column are arranged so that water flows downwardly through the tubular ducts and steam flows through the slot-like ducts.

Still further according to the present invention there is provided sea water desalination apparatus 2 comprising a pressure-tight container, a water preheater column of heat exchanger plates and a vertically disposed falling film evaporator column of heat exchanger plates, said columns being housed within the container and wherein opposed side walls of the container carry a vertically spaced array of supporting frames on which the heat exchanger plates of the falling film evaporator column are mounted as slide-in units to provide a predetermined number of stages to the failing film evaporator column substantially over the height of the container; said preheater column extending substantially over the height of the container adjacent to the falling film evaporator column and there being provided sealing means, deflector and baffle means between the side walls of the container and the falling film evaporator and preheater columns and between the preheater column and the evaporator column to provide discrete precle- termined pressure stages for the evaporator column which pressure stages correspond to the number of slide-in units and communicate with pressure stages of the preheater column.

By use of banks of the aforementioned heat ex- changer plates with the reinforcing beads arranged to provide the tubular ducts and the slot-like ducts it is possible to assemble columns within a sealed container which columns, particularly for the falling tube evaporator have far more than the previ- ously mentioned 15 stages (there usually being provided more than 50 stages), each of which can be made readily accessible and located within the sealed container and sealed to form the respective stages for the VTE process.

When desalinating sea water by use of a multistage evaporation process it is unavoidable that gas residues, particularly inert gas, will collect in the individual stages of a falling film evaporator. Preferably these inert gases are removed by suc- tion during the stagewise evaporation of the salt water. With the aforementioned prior proposal of forming evaporator columns with bundles of tubular heat exchangers there is no precise condensation end of a given stage and consequently there is not the possibility of removing inert gases from such an end (this is because of the spacial arrangements of the individual tubes in the bundles). By having the tubular ducts of the heat exchanger plates in the apparatus of the present invention ar- ranged so that water flows downwardly through those ducts as a previously mentioned preference the slot-like ducts for each pair of plates for the film evaporator column may be interconnected by a zone in the respective geometrical centres of those plates through which zone non-condensable gas components can be removed from the bottom of the pairs of heat exchanger plates in each stage.

With the previously proposed heat exchangers comprising a bundle of tubes it is difficult to dis- tribute the sea water which is to be evaporated uniformly over the inner surfaces of the walls of the individual tubes in each stage. Furthermore, if the sea water is uniformly distributed at the beginning of each stage of the bundle of tubes it is gen- erally regarded as impractical to maintain a liquid GB 2 160 117 A 2 film over tube lengths of approximately 7 m. The lack of liquid film within the bundle of tubes decreases the efficiency of vaporisation and by the present invention this problem can be alleviated by the use of the beads forming the tubular ducts be- tween the opposed plates where an effective film can be maintained within the relatively short tubu lar duct parts formed by the respective opposed sets of beads.

In a multiple stage flash evaporation (MSF) proc ess preheated sea water which is to be evaporated flows through numerous flash chambers the lower part of each of which is constructed as a weir and flow passage. The upper part of each flash cham- ber houses preheater tubes through which passes the sea water prior to entering the MSF process and on which preheater tubes steam is condensed. The fresh water distillate and salt water/brine condensate formed during the MSF processes can be removed from the final flash chamber as discussed in Ullman "Enzyklop5die der Chemie", 3rd Edition, Volume 18, page 465. The efficiency of a sea water distillation process is dependent upon the number of evaporation stages which can be obtained within the temperature differential between the first and final stages available for the evaporation of the sea water. MSF process plants have hitherto been proposed having a horizontally orientated cellular construction and these have been found to permit a larger number of stages than any VTE process plants and up to 36 stages have been achieved with such a horizontal construction. However, in achieving flash evaporation a mere 60% of the total cost is involved in the actual evaporation of the sea water and the horizontally orientated plants have not proved so successful compared with the so-called failing film evaporators for the VTE process which have a significantly higher evaporation efficiency. In addition, plants operating on the basis of the MSF process require significantly more space than do vertically orientated plants for failing film evaporators according to the VTE processes. A feature of the present invention is the utilisation of advantages derived from both the MSF and VTE processes for distilling fresh water from sea water to improve the fresh water yield per unit energy which is consumed in carrying out the processes and to achieve this advantage with apparatus which is readily assembled to provide the required stages for the preheater and evaporator columns within a pressure tight con tainer and where relatively simple sealing arrange ments can be provided between the respective stages.

In one form of the process of the present inven tion for the distillation of fresh water from sea water by film evaporation in the multiple stage VTE process, after passing through the first VTE process stage and in each case before entering the following VTE process stage, the sea water is passed over a weir and undergoes flash evaporation in at least one MSF process stage. The fresh water distillate and the brine are passed through a roll the following VTE process and MSF process stages in such a way that the brine can be re- 3 GB 2 160 117 A 3 moved from the final VTE process stage and the fresh water distillate from the final MSF process stage. The steam for heating the sea water is suc cessively developed in all the evaporation stages starting with the first VTE process stage and ini tially is mixed with primary steam which is intro duced into the first VTE process stage.

The apparatus of the present invention is prefer ably arranged so that when using banks of heat exchanger plates for the preheater and for the fall- 75 ing film evaporator, opposing side walls of the sealed container carry supporting frames having bearing surfaces on which the banks of plates for the evaporator column are received as slide-in units. Banks of the plates can also be located in the 80 sealed container to extend over the entire height of the container to form the preheater. Deflectors will usually be provided beteen the side walls of the container and between the columns to form the re quired number of pressure stages (corresponding 85 to the number of slide- in units of the evaporator column); these stages in the evaporator column are sealed from each other and communicate, in each case, in pressure tight manner with corre sponcling pressure stages of the preheater.

The process of the present invention utilises the VTE and MSF processes to provide a single evapo ration process and by this makes it possible, not only to utilize the evaporation energy supplied in an optimum manner, but also to provide an ex tremely simple construction of the evaporator and preheater columns (particularly where the columns are formed by the stamped or pressed flat heat ex changer plates) which permits the efficient use of a predetermined large number of stages for the available overall temperature and pressure gra dient for the process in a vertically constructed pressure container. The required MSF process stages can be constructed as curved deflectors or baffles, which simultaneously serve as boundaries 105 for the pressure chambers having different pres sure levels. The vertically arranged preheater which is housed in the pressure-tight container preferably has its heating surfaces directly exposed to the steam chambers of the individual VTE and 110 MSF process stages.

In the first VTE process stage, the sea water will likely be heated to approximately 130'C in the preheater and be evaporated by means of live steam which is introduced into that stage at 130'C. The live steam condensate obtained is supplied to the distillation process via the first MSF process stage and as distillate passes through all the MSF process stages to be passed out of the pressure con- tainer as fresh water at a temperature of approximately 28.9'C. The sea water (which may conveniently be termed brine) flowing through the first VTE process stage dams up at the distributing means to form pools and a weir by which the brine is distributed for the following VTE process stage. Preferably the pairs of heat exchanger plates of the VTE process stages for the falling film evaporator column have channels in their upwardly directed side edges through which channels the salt water is supplied to the tubular ducts formed between the plates and the distributing means comprises rods which are accommodated in the channels. These rods form restrictors to salt water flow to the tubular ducts so that the pools of salt water de- velop in the channels to form the seals. By means of an overflow and the aforementioned restrictors part of the brine may be passed directly into the following MSF process stage with a lower pressure than in the preceding stage, whereby flash evaporation takes place. The main part of the brine flows through the next VTE process stage. As a result of the pressure reduction and heat transfer through the condensing distillate steam of the preceding process stage, part of the brine again evaporates in this VTE process stage. As a result of a phase change, distillation vapours separated from the brine pass into the mixed steam or vapour flow whilst, after leaving this VTE process stage. the brine collects at the distributing means and weir of the following VTE process stage, whereupon the process is repeated with modified saturated steam temperatures and pressures until all the VTE process stages have been traversed. The distillation steam is passed through the deflectors which pref- erably simultaneously act as centrifugal drip separators to the condensation surfaces of the following process stage, where the distillation steam is condensed on the heat exchanger surfaces of the, in each case, associated pressure stage of the preheater. The distillate condensate obtained is also collected by deflectors and is supplied via restrictors to the next lower pressure level of the column and in each case part of the distillate water is evaporated.

In the upper process stages of the evaporator and preheater columns the steam component may be directly mixed and condensed with the steam component of the VTE process stage in the manner above described. In the lower process stages of the columns, the distillate water is flashed preferably in a plurality of flash stages for each stage of the VTE process. The steam produced in these latter MSF process stages, preferably, is condensed only on the exposed heat exchange surfaces of the associated stages of the preheater.

Preferably the process comprises flash evaporating brine residue derived from the low pressure side of a VTE process stage in a plurality of MSF process stages which are associated with a com- mon VTE process stage. In a preferred embodiment the process comprises distilling the sea water in a 55 stage VTE process and a 91 stage MSF process and in which 43 MSF process stages are associated one with each of the first 43 VTE process stages and 48 MSF process stages are associated four with each of the final 12 VTE process stages. The process advantageously comprises progressively preheating the sea water prior to its distillation in stages which correspond in number to, and are associated one with each of, the MSF process stages, the sea water being preheated in the final stage by primary steam supplied to the first VTE process stage to which the sea water is to be subsequently subjected and the sea water in the other preheating stages being heated with steam 4 GB 2 160 117 A 4 derived from the distillate of the VTE and MSF process stages. Within this process it is preferred that the inert gases which are obtained in each process stage are removed at the end regions of each condensation process of the VTE and MSF process stages. Preferably the distillate, brine, steam temperature and steam pressure are maintained substantially constant in the respectively associated stages of the VTE process and the MSF process and in associated stages for preheating the sea water.

The heat exchanger plates forming the water preheater column may have transversely extending areas which are responsive to pressure of the sea water flowing upwardly through the slot-like ducts, these areas responding by moving into abutment with opposing areas to separate the tubular ducts through which steam flows into vertically disposed sections for predetermined stages of the preheater column.

Preferably the heat exchanger plates of the film evaporator are arranged so that the aforementioned tubular ducts are vertically aligned and form circumferential surfaces for the film evaporation of salt water supplied thereto and the transversely disposed beads form successively arranged crossconnections between said circumferential surfaces to distribute the salt water and regulate the steam pressure. The number of heat exchanger plates in the VTE process stages of the film evaporator column may vary from stage to stage so that the number of heat exchanger plates is reduced and the geometrical spacing between the stages is increased whereby the through- flow area for the steam in the respective stages is progressively increased from the sea water inlet stage to the finalstage of the evaporator column. The water preheater column may have a heating surface presented within the tubular ducts through which steam is in- tended to flow and which heating surface is determined for a predetermined pressure and temperature stage of the preheater column by the number of tubular ducts formed within the respective stages of that column.

Drawings One embodiment of apparatus for, and a process of, sea water desalination in accordance with the present invention will now be described, by way of example only, with reference to the accompanying 115 illustrative drawings, in which:- Figure 1 is a flowchart showing the sea water desalination apparatus generally; Figure 2 is a perspective view of part of an un- treated water preheater column incorporated in the 120 apparatus; Figure 3 is a perspective view of part of a failing film evaporator column incorporated in the apparatus; Figure 4 is a diagrammatic view of the columns shown in Figures 2 and 3 with, in each case, the first and last stages of those columns being shown; Figure 5 is a cutaway view showing part of the preheater column of Figure 2 and part of the falling 130 film evaporator column of Figure 3 with sealing arrangements for those columns and in which the evaporator column part has two vertical tube evaporator process stages and one or more multiple stage flash evaporation stages; Figure 6 is an exploded perspective view of a failing film evaporator unit included in the apparatus of Figure 3; Figure 6A is a section through part of the unit in Figure 6 to show tubular ducts and slot-like ducts formed between the plates, and Figure 7 is a process diagram for the desalination of the sea water.

Detailed description of drawings

As is shown by the flowchart of Figure 1, cold sea water is supplied for preheating by means of a pump 10 to a vertical untreated water preheater column VW. The preheater VW has a plurality of vertically disposed stages VSt, to VSt, the pressure within which increases progressively and within which the untreated water is heated with condensing steam; the untreated water is then subjected to evaporation in a vertical falling film evaporator col- umn FV. The evaporator FV has a plurality of vertically disposed stages, FSt, to FSt, the pressure within which reduces progressively and this column is also heated with condensing steam. For this latter purpose primary steam is supplied by means of pipe 11 to the first state FSt, which forms the first vertical tube evaporation (VTE) stage of the process and which simultaneously serves for heating the uppermost stage VSt, of the preheater, these two stages being at the highest pressure in the respective columns. The preheated sea water passes by way of an overflow 12 from the preheater column VW into the first stage FSti of the falling film evaporator FV and from there into the adjacent underlying stage FSt, and so on until it has passed through all the stages FSt, to FSt.. The steam formed in the first stage FSti and the steam component resulting from the flashing of the condensate from the first stage condenses in the region indicated at 14 and the resultant water flows in to the second stage FSt2 of the falling film evaporator where it partly vapourises in the lower pressure region and the resultant steam is used for heating that stage and the respectively associated stage (VSt, to VStJ of the preheater (as is indicated by the connections 15, 16 and 17). In a similar manner, the following associated stages of the falling film evaporator and of the untreated water preheater are combined, so that the process is repeated in each of the respective and progressive stages. The steam remaining in the final stage FSt, passes through a condenser 21 and the resulting water together with the clean water condensate is removed by means of a pump 23. After passing through all the process stages the brine which is collected beneath the final stage FSt, at 25 is removed through a conduit 24. Each region 14 forms a stage of the Multiple Stage Flash (MSF) evaporation process.

The falling film evaporator column FV is formed by a plurality of identical and pairwise-joined heat GB 2 160 117 A 5 exchanger plates 30. As shown in Figures 3, 6 and 6A each heat exchanger plate 30 is a generally cor rugated-like sheet shaped by stamping or pressing to have, in both the longitudinal and transverse di rections, a uniformly arranged array of reinforcing 70 beads 32, 33 which are aligned in grid-like manner.

The beads 32 can be provided with grooves 32' to be of corrugated form. The grooves 32' serve to in crease the surface area of the tubular ducts of which the respective beads 32 are to form part.

The heat exchanger plates are arranged longitudi nally and in pairs with the plates in each pair being the mirror image of each other with reference to their respective beads 32. The plates 30 in each pair have their opposed and transversely extending 80 even side edges 40 abutting each other and these abutting side edges 40 are welded together. As two of the aforementioned plates are positioned adjacent to each other in m i rror-sym metrical man ner the beads 33 will form slot-like ducts 35 be tween the adjacent heat exchanger plates 30. The opposed beads 32 of the adjacent heat exchanger plates of two of said pairs, that is between adja cent plates 30 of the two adjacent pairs of heat ex- changer plates, consequently define tubular ducts 90 34 which adjacent plates are seam welded together as shown in Figure 6A. By this technique (welded edges 40 of two plates forming a pair and at right angle to said edges seam welded edges W' of two adjacent plates of two pairs) a bank of the plates 95 can be assembled for each film evaporator stage which form in the vertical direction a plural ity of juxtaposed tubular ducts 34 and in the longi tudinal direction a plurality of juxtaposed slot-like ducts 35 (so that a cross flow of the heat exchang- 100 ing media can be provided through these ducts). In addition the plates 30 in each pair have the mar ginal edge regions 40' of their opposed longitudi nally extending side edges dog-legged in shape to provide a longitudinally extending channel 42 be105 tween those plate pairs. The channels 42 formed by the plate pairs extend longitudinally over the width of each falling film evaporator stage to com municate with the tubular ducts 34. In each chan nel 42 is placed a distributor rod 41 (which is of corresponding length to the channel). The rods 41 serve to distribute and provide a uniform flow of brine between the individual tubular ducts 34 of the individual stages and to regularise the differ ences in the salt concentration of the untreated water. In each case, the channels 42 in a bank of plates 30 open upwardly to be directed towards the bank of plates 30 in the overlying stage of the failing film evaporator (see Figure 3) so that brine will fall progressively into the channels 42 of the 120 vertically disposed array of banks to form pools in the respective channels and to be distributed by the rods 41 into the ducts 34. The banks of heat exchanger plates are held in position by means of opposed side walls 205, 206 of a sealed container F 125 within which the two columns M and FV are lo cated. Thus in the column FV1 the transverse and upwardly directed tubular ducts 34 (formed from individual duct sections in accordance with the length of the identical beads 32) form evaporation 130 surfaces for the film evaporation of the sea water which is to be fed into those ducts (as indicated by the arrows 34' in Figure 6), whilst steam is supplied through the longitudinally extending slot-like ducts 35 (as indicated by the arrows 35'). The array of beads 33 of the heat exchanger plates form cross connections extending over the width of the plates for the sea water flowing through the tubular ducts 34. The water/steam mixture leaving an individual section of a tubularduct 34, (that is from between opposed beads 32) as a result of the above mentioned cross-connection formed by the beads 33 is immediately distributed uniformly over all the following (down stream) beads 32; consequently the steam pressure within the individual tubular ducts 34 as well as differences in the salt concentration can be distributed uniformly before the water/steam mixture flows into the subsequent individual sections (that is the beads 32) of the ducts 34.

The untreated water preheater VW, partly shown in Figure 2 is constructed from heat exchanger plates 30 combined to form plate pairs in a similar manner to that described above for the evaporator W. However, unlike the failing film evaporator, the heat exchanger plate pairs in the preheater VW are oriented in such a way that the tubular ducts 34 are positioned horizontally and the slot-like ducts 35 vertically.

As can be gathered from Figure 4, the untreated water preheater column VW extends substantially to the height of the sealed container F and the tubular ducts 34 are subdivided into vertically spaced sections which are sealed from each other by transversely extending unstamped areas 37 (see Figure 2) within the individual heat exchanger plates 30. These spaced sections correspond in number to the desired number of stages. The aforementioned subdivision and sealing between sections of the tubular ducts 34 in the preheater column is effected by arranging for unstamped areas 37 of each of the heat exchanger plates in each adjacent part of such plates to be pressed against each other in response to pressure of un- treated water in the slot-like ducts 35. The total number of horizontally disposed tubular ducts 34 of the preheater column VW is consequently subdivided into a plurality of groups each of which cornprises several tubular ducts.

The geometrical arrangement of the unstamped areas 37 is, in each case, determined in accordance with the pressure and temperature stages of the column selected for the multieffect evaporation process as a function of the temperature range for the whole preheater column. In the present example 91 stages are provided the respective pressures within which progressively decrease from the top to the bottom of the columns. Starting from the inlet stage VSt. and extending to the outlet stage VSt, the vertical distance between two unstamped areas 37 becomes progressively smaller from stage to stage (and consequently so does the number of tubular ducts 34 in each stage).

The slot width of the slot-like ducts 35 is determined by the spacing with which the beads 33 are 6 GB 2 160 117 A 6 stamped or pressed. In the stamping operation is not carried out at a particular point in the plates, for example over the area 39 (Figure 2), then a by pass will be formed between the slot-like ducts.

Cold sea water is fed into the bottom of the ver- 70 tically disposed slot-like ducts 35 of the heat ex changer plates of the first, bottom stage VSt. of the untreated water preheater column VW and passes through the column up to the final top stage VSt,. Simultaneously heating steam is fed, at 75 each stage, between the heat exchanger plates (at right angles to the sea water flow) through the tu bular ducts 34. Only in the top stage VSt, and FSt, of the preheater and evaporator columns is hot pri mary steam fed into the sealed container; all of the 80 other stages in the two columns are heated by sec ondary steam, (that is steam which is generated within the various stages of the failing film evapo rator FV as a result of vaporisation of distilled water derived from the preceding stage or stages). 85 The lowest steam temperature and pressure is in the bottom stage VSt. and FSt_ Figures 3 and 4 show the stagewise arrangement of the banks of heat exchanger plates of the failing film evaporator column. Although the vertically ar- 90 ranged untreated water preheater column VW ex tends over the entire height of the sealed container (as an assembly subdivided into stages by the un stamped areas 37) the failing film evaporator col umn FV is subdividied into individual banks or sections corresponding to the number of stages of the column. These banks for the evaporator col umn FV are arranged on horizontal planes E, to En with varying vertical spacings between those planes. The number of planes E, to E. corresponds 100 to the number of stages in the preheater. One plane is therefore associated with each of the pre determined pressure and temperature stages St, to St, for the multieffect evaporation process and comprises a stage of the preheater VW, a stage of 105 the falling film evaporator FV and at least one MSF evaporator (see Figures 1 and 7).

For the purpose of locating the banks of plates for the evaporator column FV in their respective planes, a frame construction 301 is carried by the opposed side walls 205 and 206 of the sealed con tainer (see Figure 4). The side walls 205, 206 are fixed but the container also has opposed side walls 207 and 208 (Figure 5) which are movable to per mit access to the columns (preferably in the man ner disclosed in our co-pending Patent Application No. (Ref. FJWIGDG/PB3201-)). The frames 301 are located in the appropriate number of planes E, to E. with the required vertical spacing and corre spond to the selected number of stages; on the frames in each of the planes is arranged a bank of the failing film evaporator plates shown in Figure 3. Deflectors or baffles LB are provided for each stage of the evaporator which is used for flash evaporation to ensure that, in each such stage, steam and clean water is directed and transferred as appropriate. Also vacuum/suction systems are provided (shown diagrammatically in Figure 7 with restricto rs/va Ives 405) for controlling the pressure differentials between the stages in the columns and for removing non-condensable inert gas as produced in the condensation zone in each stage of the failing film evaporator FV. This latter condensation zone is formed by the unstamped areas 39 in the geometrical centres (see Figures 3 and 7) of the heat exchanger plates 30 which, in the plates of the falling film evaporator FV are vertically disposed. The areas 39 connect all the slotlike ducts 35 of each pair of plates and the inert gas components can therefore be discharged from the bottom slots 35 in the banks of the heat exchanger plates of each stage of the failing film evaporator FV.

The rods 41 in the channels 42 act as restrictors which allow pools of brine to form in those channels during the flow of the brine down the successive stages of the evaporator column FV. This has the effect of distributing the brine over the tubular ducts 34 which communicate with the respective channels 42. Furthermore the pools or weirs which are effectively formed by the rods 41 to the flow of brine serve as seals to maintain and regulate the steam pressure within the individual tubular ducts 34.

In a practical example (Figure 7) of the combined multieffect evaporation process for sea water desalination by use of the apparatus as above described, the untreated water preheater VW and the falling film evaporator FV have 55 vertical tube evaporation (VTE) process stages and 91 multiple storage flash (MSF) stages associated therewith. In each case the individual VTE process stages have associated therewith the deflectors LB which extend over the entire width of each stage between the opposed side walls 205 and 206. The upper (first) 43 VTE stages are associated, one each, with 43 IVISF stages while the lower (final) 12 VTE stages are associated, four each with 48 MSF stages (see Figure 7).

In each of the first 43 VTE process stages of the falling film evaporator FV, the deflectors LB are provided by (see Figure 5) deflectors 306 and 310 on the side of the evaporator column remote from the untreated water preheater VW and by deflec- tors 304, 305 and 308 on the side of the evaporator column adjacent to the untreated water preheater. The deflectors 306 and 310 are cross-sectionally curved in the manner of an open ellipse to act as centrifugal drip separators. The deflectors 304 are angled and curved so that, together with the walls of the deflectors 305, they form U-shaped channel ducts 312 for the collection of condensed clean, fresh, water derived from the VTE process stage with which the deflectors are respectively associ- ated. This condensed fresh water can flow through an array of holes 313 in the bottoms of the respective ducts 312 and into successive underlying ducts 312 (in successive pressure reduced stages in the columns) where it is subjected to flash evaporation in accordance with the IVISF process so that the resultant steam is available for heating the stage in which it is flashed.

As shown in Figure 7 there are four deflectors 305 for each deflector 304 in the final 12 of the WE process stages (which correspond to stages FSt,,, 7 GB 2 160 117 A 7 to FSt,, in Figure 1) so that four channel ducts 312 and thereby four MSF process stages are associ ated with each of these VTE process stages.

The deflectors 304 and 308 carry sealing surfaces 315 for separating the stages of the falling film. 70 The banks of plates 30 for the evaporator column FV are constructed as slide-in units on to the frames 301 with the side wall 207 removed. In or der to ensure reliable sealing the sealing surfaces 315 are preferably formed by silicone seals. As will 75 also be seen from Figure 5, the angular deflectors 310 are fixed to the removable container wall 207 to be openable therewith and when the wall 207 is closed to seal the container the U-shaped end 316 of the deflector 310 abuts a wall 317 of the defiec- 80 tor 306; a resilient seal 315 is provided between the end 316 and the wall 317. The deflectors ex tend between the fixed side walls 205 and 206. If required the deflectors 304, 305, 306 and 308 can be secured to the fixed side walls to form ties be85 tween those walls (particularly in the part of the sealed container between the two columns FV and VW). The deflectors 305 having sealing regions 318 (again achieved by interposing seals 315) which engage on the end faces of the associated stages 90 VSt of the preheater VW, specifically at the posi tions where the unstamped areas 37 subdivide the preheater column M into the stages as previously discussed. Resilient seals 315 on sealing regions 319 of deflectors 309 serve to seal the stages of the 95 preheater column VW with respect to the remova ble side wall 208 of the container. The deflectors 309 are fixed to the side wall 208 to be removable therewith and permit access to the preheater col umn. From the aforegoing it will be appreciated 100 that the deflectors 304 to 310 together with their associated seals serve to partition the individual process stages of the columns in a pressure-tight manner. The steam introduced in the uppermost process stages and the steam which is subse105 quently produced within each stage of the VTE/ MSF evaporator column FV can consequently flow to the associated stage of the preheater column VW, where it serves to heat the untreated water.

In the aforegoing example the individual rein- 110 forcing beads 32 of each heat exchanger plate 30 have a length of 35 mm; the tubular ducts 34 of the untreated water preheater a length of 350 mm; the slot-like ducts 35 of the falling film evaporator a length of 2160 mm; the failing film evaporator is 115 assembled from banks of plates 30 constructed as slide-in units each of which unit has a thickness of 500 mm, and the column has a height of 34 m.

In operation of the desalination process all the VTE and MSF process stages (with the exception of 120 the final condenser 21) are housed in the sealed pressure container with the various pressure subchambers being formed by the banks of plates in failing film evaporator for the VTE process stages and the connected deflectors LB as discussed above. The preheater M is vertically incorporated into the container with the stages of its tubular heating ducts 34 directly receiving steam from the respectively associated sub-chambers of the VTE 65 and MSF process stages (see Figure 7).

In the first VTE process stage FSt, of the falling film evaporator column FV, the sea water (already heated in the preheater VW to 130'C) is evaporated at 130'C by means of primary steam admitted through conduit 11. The distillate water which results passes through all 91 IVISF process stages and leaves the container at a temperature of approximately 28.9'C.

During its flow through the successive stages down the evaporator column FV the brine is dammed back by the resistance or weir effect provided by the rods 41. The rods 41 thus act as pressure reducing and distribution points, to a given brine level in the channels 42. Because of the pressure reduction and heat transfer caused by the condensing distillate steam of the preceding stage, part of the brine evaporates in the failing film heat exchanger of this stage. After leaving this heat exchanger, the distillate steam and brine are separated. The remaining brine again collects over the following VTE process stage and the process begins again with modified saturated steam temperatures and pressures.

The distillate or product steam is passed by the deflectors 304 (which simultaneously act as centrifugal drip separators) to the condensation surfaces of the following process stage. The steam is condensed firstly in the falling film evaporator and secondly on the heat exchange surfaces of the preheater. The resulting freshwater condensate is collected by the deflectors 305 and in the ducts 312 (see Figure 5) from which it flows by way of the restrictor holes 313 in to the next lower pressure level, where once again part of the fresh water evaporates. In the upper 43 stages of the evaporator column FV this latter steam is directly mixed and condensed with the steam component of the particular VTE process stage in the manner previously described. In the lower 12 stages of the evaporator column FV the product water is not flash evaporated in one stage and in fact the pressure within each of these lower VTE process stages is subdivided into a plurality of flash stages (that is four flash stages). The steam produced in these IVISF process stages is only condensed in the tubular ducts 34 of the heat exchanger plates in the preheater VW.

The salt water collecting in the centrifugal separators is supplied to the following stage by way of the restrictors formed by the rods 41 as previously described.

Inert gases obtained during each flash evaporation and condensation are sucked out of each stage at two points - the suction taking place at the end regions of the condensation process. The inert gases obtained in the VTE process part are removed by suction through a cavity 400 between the deflectors and the movable side wall 207 of the container along, in each case, the lowermost slot cross-section 410 of the falling film evaporator, starting from the centre 39 (condensation end) of each heat exchange surface. The inert gases obtained during the condensation process at the preheater are removed by connections 403 at the other movable side wall 208 of the sealed con- 8 GB 2 160 117 A 8 tainer by way of a cavity 404 between the preheater and the side wall at the condensation end regions. The inert gas suction capacity for the individual stages is controlled by throttle valves 5 405 in connecting lines 406.

The above described combination of VTE and MSF processes leads to numerous advantages, which together bring about a distillate or fresh water production such as has hitherto not been considered commercially feasible, a particular contribution to this being made by the use of the plate-type heat exchanger. For example it is apparent from the aforegoing discussion that as a result of the limited free---tubelength" of only 35 mm for the individual failing film evaporator tubular ducts (that is the stamped bead portions 32), a film can be maintained over the entire stage length which could not be achieved with conventional failing film tubular evaporators having tube lengths of up to 7 m.. Furthermore, the use of the preferred form of heat exchanger plates makes it relatively simple to subdivide the available temperature and pressure ranges into a random number of individual ranges and this has a very favourable influence on the efficiency of the columns.

Claims

1. A process for the distillation of fresh water from sea water by film evaporation within a multiple stage vertical tube evaporation WTE) process in combination with flash evaporation within a multiple stage flash evaporation (MSF) process which comprises passing heated sea water through a col- umn of successive VTE process stages; condensing steam derived from the individual VTE process stages and passing the fresh water condensate through successive MSF process stages which are associated at least one with each stage of the said WE process stages and by which the condensate is subjected to flash evaporation and applying the steam resulting from the flash evaporation to heat the respectively associated stage of the VTE process, and wherein prior to entering a VTE process stage the salt water/brine is distributed over that stage to form a seal between adjacent VTE process stages.

2. A process as claimed in claim 1 which cornprises introducing primary steam into the first stage of the VTE process for heating that stage and heating subsequent stages of the VTE process with steam which results from flash evaporation of fresh water derived from the condensation of said primary steam and from the steam components of the preceding VTE process stage or stages.

3. A process as claimed in either claim 1 or claim 2 which comprises removing by suction inert gases obtained, during the stagewise evaporation.

4. A process as claimed in any one of the pre- ceding claims which comprises flash evaporating fresh water condensate derived from the low pres sure side of a VTE process stage in a plurality of MSF process stages which are associated with a common VTE process stage.

5. A process as claimed in any one of the pre- 130 ceding claims which comprises distilling the sea water in a 55 stage VTE process and a 91 stage MSF process and in which 43 MSF process stages are associated one with each of the first 43 VTE process stages and 48 MSF process stages are associated four with each of the final 12 VTE process stages.

6. A process as claimed in any one of the preceding claims which comprises progressively pre- heating the sea water in stages prior to its distillation, the sea water being preheated in the final stage by primary steam supplied to the first VTE process stage to which the sea water is to be subsequently subjected and the sea water in the other preheating stages being heated with steam derived from the distillate of the VTE and MSF process stages.

7. A process as claimed in any one of the preceding claims which comprises removing inert gases obtained in each process stage at the end regions of each condensation process of the VTE and MSF process stages.

8. A process as claimed in any one of the preceding claims which comprises maintaining sub- stantially constant the distillate, brine, steam temperature and steam pressure in the associated stages of the VTE process and the MSF process and in associated stages for preheating the sea water.

9. A process as claimed in claim 1 and substan tially as herein described.

10. Apparatus as claimed in any one of the preceding claims and comprising a plurality of said film evaporators in a vertically spaced array and in separate pressure stages, said evaporators overlying each other as a column so that brine will pass downwardly from the evaporation passages of each evaporator (other than the lowermost evaporator) to collect in the channel or channels of the immediately underlying evaporator.

Printed in the UK for HMSO, D8818935,

11.85, 7102. Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.

c

10. Fresh water derived from sea water by the process as claimed in any one of the preceding claims.

11. Apparatus for carrying out the process as claimed in claim 1 which comprises a sea water preheater column and a film evaporator column for the VTE process, each of said preheater and evap orator columns comprising heat exchanger plates having a spaced array of reinforcing beads formed therein, said beads being uniformly disposed and aligned longitudinally and transversely of the plates in grid-like manner; said plates being disposed in pairs with the plates in each pair posi- tioned adjacent and in mirror image to each other so that the beads form between the pair of plates tubular ducts extending transversely of the plates and the beads on the exterior of a pair of plates form with an adjacent heat exchanger plate of a further pair of such plates slot-like ducts extending longitudinally of the plates or vice versa; said tubular ducts and slot-like ducts providing conduits in one case for the through flow of water and in the other case for the through flow of steam, and wherein the plates in the film evaporator column form vertically disposed stages for the VTE process, the plates in said stages carrying salt water distributing means by which the brine is distributed over the respective stages of the VTE process to form seals between those stages.

12. Apparatus as claimed in claim 11 in which the heat exchanger plates for the water preheater column are arranged so that water flows upwardly through the slot-like ducts and steam flows through the tubular ducts and the heat exchanger 9 GB 2 160 117 A 9 plates for the film evaporator column are arranged so that water flows downwardly through the tubular ducts and steam flows through the slot- like d u cts.

13. Apparatus as claimed in claim 12 in which the heat exchanger plates forming the water preheater column have transversely extending areas which are responsive to pressure of the sea water flowing upwardly through the slot-like ducts, said areas responding by moving into abutment with opposing areas to separate the tubular ducts through which steam flows into vertically disposed sections for predetermined stages of the preheater column.

14. Apparatus as claimed in either claim 12 or claim 13 in which the heat exchanger plates of the film evaporator are arranged so that the tubular ducts are vertically aligned and form circumferential surfaces for the film evaporation of salt water supplied thereto and the transversely disposed beads form successively arranged cross-connections between said circumferential surfaces to distribute the salt water and regulate the steam pressure.

15. Apparatus as claimed in any one of claims 11 to 14 in which the number of heat exchanger plates in the VTE process stages of the film evaporator column varies from stage to stage so that the number of heat exchanger plates is reduced and 0 the geometrical spacing between the stages is increased whereby the through-flow area for the steam in the respective stages is progressively increased from the sea water inlet stage to the final stage of the evaporator column.

16. Apparatus as claimed in any one of claims 11 to 15 in which the water preheater column has a heating surface presented within the tubular ducts through which steam is intended to flow and which heating surface is determined for a predeter- mined pressure and temperature stage of the preheater column by the number of tubular ducts formed within the respective stages of that column.

17. Apparatus as claimed in claim 12 or any one of claims 13 to 16 when appendant thereto in which the slot-like ducts of each pair of heat exchanger plates for the film evaporator column are interconnected by a zone located in their respective geometrical centres through which zone non-con- densable inert gas components can be removed from the bottom of the pairs of heat exchanger plates.

18. Apparatus as claimed in claim 12 or in any one of claims 13 to 17 when appendant thereto in which the pairs of heat exchanger plates of the VTE process stages for the failing film evaporator column have channels in their upwardly directed side edges through which channels salt water is supplied to the tubular ducts and wherein the dis- tributing means comprises rods which are accommodated in said channels, the rods forming restrictors to salt water flow to the tubular ducts so that pools of salt water develop in the channels to form said seals.

19. Apparatus as claimed in any one of claims 130 11 to 18 and having a heat exchanger for condens ing steam derived from the final stage of the film evaporator column to provide fresh water there from.

20. Sea water desalination apparatus compris ing a pressure-tight container, a water preheater column of heat exchanger plates and a vertically disposed failing film evaporator column of heat ex changer plates, said columns being housed within the container and wherein opposed side walls of the container carry a vertically spaced array of sup porting frames on which the heat exchanger plates of the failing film evaporator column are mounted as slide- in units to provide a predetermined num- ber of stages to the falling film evaporator column substantially over the height of the container; said preheater column extending substantially over the height of the container adjacent to the falling film evaporator column and there being provided seal- ing means, deflector and baffle means between the side walls of the container and the falling film evaporator and preheater columns and between the preheater column and the evaporator column to provide discrete predetermined pressure stages for the evaporator column which pressure stages correspond to the number of slide-in units and communicate with pressure stages of the preheater column.

21. Apparatus as claimed in claim 20 in which the pressure stages of the water preheater column are determined by a vertically spaced array of abutting areas between adjacent heat exchanger plates which form that column and through which abutting areas flow is prevented.

22. Apparatus as claimed in either claim 20 or claim 21 in which the opposed side walls of the container which carry the supporting frames are fixed relative to an external supporting structure for the container and a pair of opposed movable side walls are provided which are openable to per mit access to the container, and wherein the falling film evaporator column and water preheater col umn have their respective stages determined by deflector and baffle means and sealing means car- ried by the movable and fixed side walls.

23. Apparatus as claimed in claim 22 in which deflector and baffle means provided between a movable side wall of the container and a respectively adjacent face of the water preheater column or of the failing film evaporator column are secured to the inner face of that movable side wall.

24. Apparatus as claimed in any one of claims 20 to 23 in which baffle and deflector means is carried by fixed opposed side walls of the container and between the failing film evaporator and preheater columns to provide fresh water collecting ducts and to serve for multi stage flash evaporation.

25. Apparatus as claimed in any one of claims 20 to 24 in which at least one of the supporting frames, deflector means or baffle means provides a tie between opposed fixed walls of the container.

26. Apparatus as claimed in any one of claims 20 to 25 in which seals are provided between the deflector and baffle means and the respectively asGB 2 160 117 A sociated stages of the falling film evaporator col umn and of the preheater column.

27. Apparatus for the distillation of fresh water from sea water substantially as herein described with reference to the accompanying illustrative 70 drawings.

Amendments to the claims have been filed, and have the following effect:

(a) Claims all above have been deleted.

(b) New or textually amended claims have been filed as follows:- 1. Apparatus for the distillation of fresh water from sea water/brine having a falling film evapora tor between two stages which are intended to be maintained at different pressures and which evap orator comprises an array of heat exchanger plates which form between them downwardly extending evaporation passages through which the sea water/brine can flow downwardly to be subjected to evaporation, and condensation passages sealed from said evaporation passages and within which steam can flow to condense on said plates; and wherein said plates form, at their upper ends, a channel which communicates with said evapora tion passages and within which a pool of sea water/brine can develop to be distributed to the evaporation passages and to provide a seal be tween the two stages.

2. Apparatus as claimed in claim 1 in which a distributor is located in said channel to provide a restrictor to the flow of sea water/brine to the evaporation passages and to distribute the sea water/brine to those passages.

3. Apparatus as claimed in claim 2 in which the distributor comprises a rod located in the channel.

4. Apparatus as claimed in any one of the pre ceding claims in which the plates form an array of channels which communicate with respective evaporation passages formed between the respec tive plates.

5. Apparatus as claimed in claim 4 when ap pendant to claim 3 in which each channel has a distribution rod therein.

6. Apparatus as claimed in any one of the pre ceding claims in which the plates have a spaced array of reinforcing beads or corrugations formed thereon, said beads or corrugations being uni formly disposed and aligned in grid-like manner so that the beads or corrugations form between the plates tubular ducts for the evaporation passages and slot-like ducts for the condensation passages.

7. Apparatus as claimed in any one of the pre ceding claims in which the plates are formed by stamping or pressing.

8. Apparatus as claimed in any one of the pre ceding claims in which the condensation passages are formed between pairs of plates and the evapo ration passages are formed between two adjacent plates being one plate from each of two said pairs.

9. Apparatus as claimed in claim 8 in which the plates are identical and those plates in the pairs are disposed oppositely and in mirror image to each other.