CA1136575A - Sea - or brackish water desalination apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:
The adoption of vertical preheaters instead of the horizontal ones in an installation for desalinating brackish or sea water affords outstanding advantages in that not only a considerable savings of costly construction materials such as cupronickel alloys is achieved, but the manipulation for assembling and disassembling the tube bundles becomes much simpler. In addition, a considerable savings in the electric power useup can be obtained.

Description

113~57S

The present invention relates to an improvement of the apparatus for the desalination of both sea water and brackish water as disclosed in the U.S. patent N 3,961,658 issued June 8, 1976 to Pagani.
The apparatus according to the above cited patent operates accordlng to a multiple stage method, for the detailed description of which reference is invited to the specification of the parent patent, said method comprising the use of one or more vertical cylindri~al columns which are partitioned into a number of stages, each stage consisting of two liquid-film vertical evaporators having a cross-sectional outline in the form of a cricular segment and having, between the two bodies a preheater for the salt water which is ar-ranged horizontally.
A stage formed in the manner recalled above is suit-able for installations which have a from average to high output, whereas for installations of small output, an apparatus such as the one disclosed in U.S. patent N 4,062,734 issued on December 13, 1977 to Pagani, may be employed.
The latter Patent provides an embodiment of the tube bundle of each stage as an entity having the tube plate with a circular cross-section and the preheater, which is horizontal, inserted into a through-tube which diametrically passes through the brackish-water tub.
In installations producing high output, it has now been found that the vertical arrangement for the preheaters as well as the evaporators involves surprising advantages over those provided by U.S. patent N 3,961.658 issued June 8, 1976, inventor Giorgio Pagani~both from the point of view of the invested capital and the overall electricity usage.
According to the present invention there is provided an improved multiple effect distillation apparatus for the -1- ~
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-` 1136S75 desalination of sea water wherein a column of superposed cylindrical sections are provided, each section having the following elements: at least one falling film evaporator disposed in each said section other than the lowermost section, each said evaporator having a bundle of vertical tubes; a collection tub within each said section, with the tub in the uppermost section being connected at its upper end to a brackish water feed tube with the remaining tubs in the succeeding lower sections being connected at their upper ends to said vertical tubes of the film evaporator, disposed in the section which is immediately superjacent, and wherein the bottom of the tub in the lowermost section is connected to a brackish water discharge tube, and with the bottoms of the tubs of each remaining section being connected to the upper end of the vertical tubes of the associated film evaporator of that section; a restriction means placed on the bottom of each tub, said restriction means for permitting the flow of brackish water towards the underlying evaporators and dissapating the positive pressure difference existing between the tub and the evaporator; openings formed in the upper portion of each tub in each section other than the uppermost section, said openings to allow vapor to flow from each said tub about the associated evaporator of that section;
at leastone siphon tube for withdrawing the condensate collected at the bottom of each section and for reintroducing said condensate at an intermediate point of the succeeding subjacent section; means for transferring any inext gases from each section to a vacuum system for collecting said inert gases;
means for introducing sea water to the tub of the uppermost section and means for introduction of steam to the exterior surface of said film evaporator of said uppermost section;

means for removing condensate from the lowermost section wherein said improvement comprises; preheating means, said preheating means including a plurality of longitudinally -extending, cylindrical ducts disposed in a substantially vertical orientation, with one of said ducts being located within each section of said apparatus and oriented in end to end, colinear relationship, all of said ducts being in com-munication to define a flow path, such that brackish water may be passed through said ducts prior to its introduction to the uppermost section of said apparatus whereby the vapors in each section functions to elevate the temperature of the brackish water within the ducts of the preheating means to facilitate its vaporization within said apparatus.
Embodiments of the present invention will now be briefly described, this disclosure being intended as an explanation rather than a limitation of the invention.
Objects and advantages of the subject invention will become apparent in light of the specification taken in conjunction with the accompanying drawings in which:
Figure 1 is a cross-sectional view of one chamber of a desalination apparatus taken along the line B-B in Figure 2 and illustrates the preheating means of the subject invention.
Figure 2 is a cross-sectional view of a desalination apparatus taken along the line A-A in Figure 1 and illustrating the location of the preheating means of the subject invention.
With reference to FIGURE 1, which shows a longitu-dinal cross-sectional view of a column trunk a single desalina-tion stage is described.
The single chamber illustrated in Figures 1 and 2 is part of a desalination apparatus which is similar to the desalination apparatus described in U.S. Patent No. 3,961,658, with the differences therebetween being discussed in detaii 1~3657S

hereinafter. A single chamber of the multi-chambered desalination apparatus comp~ises the following parts:
at least one vextical film evaporator 1, having a plurality of tubes lA (only one tube is illustrated).
At least one brackish water collecting tub 2 is provided in each section having its bottom end connected to the top of the evaporators 1. The tubs 2 in the lowermost cylindrical chamber of the apparatus (not shown) are connected to a brackish water removal means such as a discharge tube.
The tubs 2 in the uppermost section of the apparatus (not shown) are connected to a brackish water feeding tube.
A lamination system is provided in each tub 2 which essentially divides the tub into an upper section and a lower tub section 4. The lower second tub section 4 is connected to the top tube plate 8 of the underlying film evaporator 1.
The lamination system may consist of a plate with a narrowed orifice as shown in U.S. Patent No. 3,961,658. The lamination system 7 is intended to dissipate the pressure difference existing between the tub 2 and the associated evaporator.
Openings 3 formed through the top portion of the sidewalls of each tub 2 with the exception of that of the uppermost section or chamber (not shown).
One or more siphon tubes 9 for drawing the condensates collected at the bottom of each section to introduce the condensates into intermediate points of the subsequent section again.
Connection pipes 12 between a stage and the next one to transfer the inert gases which are possibly present to the vacuum pump.
The final condenser (not shown) placed beneath the last stage, is of the horizontal tube bundle type with fixed tube plates placed on diametrically opposite portions of the 1136~75 column, and condenses all the steam which is produced in the last stage.
In accordance with the subject invention, a new and improved preheating means is provided for preheating brackish water prior to its introduction to the tubs 2 in the uppermost section of the apparatus. By preheating the brackish water, less energy is needed to vaporize the water in the uppermost section. The new and improved preheating means consists of a plurality of cylindrical ducts 5 disposed in a substantially vertical orientation, with one duct being located in each section of the apparatus. The ducts 5 are in communication with each other and define a flow path for the brackish water. The bottom of each duct 5 rests against the plate 10 separating the sections of the apparatus. The top of each duct 5 is connected to a thermal expansion joint 6 which, in turn, is connected to the upper plate 10 of each respective chamber. The thermal expansion joint 6 is provided to dampen the effects of varying temperatures in each section of the apparatus, which cause the ducts 5 to expand at dif-ferent rates.
As illustrated in Figure 2, each of the tubs 2 hasa configuration approximating a section of a circle. The tubs 2 are oriented such that they occupy approximately 3/4ths of the interior of the chamber. Each duct 5 is cylindrical in configuration and occupies the remaining sector of the chamber. By this arrangement, efficient preheating of the brackish water is facilitated.
In use, sea or brackish ~ater is pumped through the new and improved preheating means, such that it is preheated by the vapors in the chambers prior to its introduction into the tubs 2 of the uppermost section of the apparatus.

Conveniently, as heat is transferred from the steam to the ~, 1~3~575 brackish water in the ducts, the temperature of the water vapor is reduced below the boiling point such that condensed fresh water is produced. Fresh steam, which is also introduced into the uppermost section of the apparatus, futhers heats the brackish water as it passes from the uppermost tub through the associated evaporators 1 and into the tubs 2 of the section immediately subjacent. The heating of the brackish water causes its partial evaporation, with the vapor passing into the interior of the cylindrical chamber through the openings 3 provided in the tubs 2. The openings 3 may be provided with demisters to prevent the espace of non-vaporized brackish water.
The non-vaporized brackish water passes through the lamination 7 into the lower tub 4 and thereafter into the subjacent set of evaporators 1. The brackish water in the evaporators 1 is heated by the steam within the chamber. As the steam heats the brackish water to its boiling point, the steam itself is condensed into pure water.
The operation within each chamber is identical, with the interior pressures decreasing from the uppermost chamber to the lowest. The pure water condensate formed in the bottom of each section is siphoned to the sectlon imme-diately below through siphon tubes 9. In the lowermost section, the remaining brackish water in the tubs 2 is removed via a discharge tube. As noted above, any inert gases which were produced are removed by a vacuum pump. Any steam remalning in the lowermost chamber is condensed by a horizontal condenser (not shown3 such that all the pure water may be removed from the apparatus.
The vertical preheaters of the subject invention have many advantages over the horizontal preheaters used heretofor in the prior art. These advantages which are more fully described hereinafter, include the reduction of the . ~,~

total weight of the apparatus, elimination of individual manifolds for each preheater, and the reduction of the pump size used to feed the brackish water to the apparatus. Further, the above described vertical arrangement facilitates the maintenance and replacement of the ducts.
Two examples will now be reported which have the purpose of better illustrating the invention without limiting its scope in any manner.

The starting data are:
Salinity of the sea water 35,000 ppm (Parts per million) pH of sea water at 20C 8.1 Temperature of sea water 26C
Primary steam 3 atm and 135C
Production of fresh water 1,500 c.metres an hour Temperature of the fresh water 46C
Salinity of fresh water 15 ppm The embodiment of the desalination unit with vertical preheaters is especially interesting for such a high output as can be seen in the following table which reports a comparison between the main features of the desalination apparatus for the two alternatives, i.e. with horizontal and with vertical preheaters, the heating efficiency being the same for both.
Horizontal Vertical preheaters preheaters N. of stages 15 15 N of columns 2 2 Dia.lst column8,000 milli- 6.500 milli-metres metres Dia.2nd column 8,000 " 6,500 "

Height 1st column37,200 " 41,400 "

Horizontal Vertical preheaters preheaters Height 2nd column 40,800 milli- 45,000 milli-metres metres Specific electricity useup 2.34 kWh/cu. 1,86 kWh/cu.
metre metre In addition to the comparison made hereabove it must be added that the horizontal preheaters would require tube plates having a diameter of 2,450 mm each, with serious attendant problems as regards their construction and the sup-porting of the bundles in the interior of the stages, due to their weight proper, for the tight seal problems for the flanges -~ with regard toair seeping from the outside and, lastly, that which is the most important aspect, the virtual impossibility of manipulating bundles of such a size when slipping them into and out of the columns for upkeep and repair operations.
The fact that with this invention the overall length of the two columns is by 8,400 mm longer than in the case of the horizontal preheaters is not such as to give a deceptive impression inasmuch as the considerable reduction of the diameter, along with dispensing with the external manifolds for the preheaters, said manifolds being compulsorily made of a previous material (Cu/ni 90/10) and the connecting pipes also of a costly material has the result of a considerable and actual savings in the initial cost, a savings which is best seen in the subsequent example hereof.

The starting data are:
Salinity of the sea water35,000 ppm pH of sea water at 20C 8.1 Temperature of sea water 26C

Primary steam 3 atm and 135C

- 1131i575 Fresh water production 535 cu.metres an hour Fresh water temperature 46C
Salinity of fresh water 15 ppm The following table clearly shows that the lesser output of 535 cu.metres an hour, which can be regarded as a medium-to-high magnitude, makes the advantages of the invention less conspicuous than in the previous example but, this notwithstanding, the savings of materials is still significant as will be seen hereafter.
Horizontal Vertical preheaters preheaters N of stages 15 15 N! of columns 2 2 Dia. 1st column 4,700 mm 4,350 mm Dia. 2nd column 4,700 mm 4,350 mm Height 1st column 37,200 mm 39,800 mm Height 2nd column 40,800 mm 44,00 mm Specific electricity useup2.35 kWh/cu.1,86 kWh/cu.
metre metre In this case the comparison between the two approaches has been made closer and has permitted that the actual eco-nomical advantage so achieved might be more accurately evaluated.
The overall weight of the two evaporating columns is reduced by about 33 metric tons and, of these, 5.5 tons are of a costly material ~Cu/ni 90/10) and the remainder is carbon steel coated by a costly material.
It should be added that in the case of a unit with laterally withdrawable horizontal preheaters, an external scaffolding must be provided outside the columns, with work flats at every stage and hoists for removing and installing the preheaters.

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113~;57S

By virtue of the approach afforded by this invention, both the evaporating bundles and the preheaters can be with-drawn vertically from the column heads so that it is sufficient to cater for a hoist of proper force above the column tops and this hoist must be capable of servicing both columns and of resting on a platform fastened to the column heads.
By so doing, the savings in the scaffoldings relative to the servicing outside the columns can be estimated to about 140 metric tons, and the savings relative to the erection can be added thereto.
The powerful pump to be used for charging the evapo-rators become cheaper since, if the rate of delivery required is the same, the hydraulic head to be overcome is reduced from 180 metres to 120 metres.
The foregoing has also an influence on the reduction of the erection time, a factor which is not to be overlooked on account of the present situation in the market.
In addition to the reduction of the weight of the materials, which means a resultant considerable reduction inthe initial costs, a savings is also achieved in the running costs of the installation on account of the reduced usage of electric power.
For a yearly output distributed over 330 working days and for a cost of electricity of 30 It.lire per kWh such a savings will be: (2.34 - 1.86) x 1,500 x 24 x 330 x 30 =
174,636,000 It. Lire yearly, for Example 1 and, (2.35 - 1,86) x 535 x 24 x 330 x 30 = 62,286,840 It. LIRE yearly for Example

2.
It is apparent from Example 2 as reported above that this invention can be used with a vantage, as outlined above, also to installations having a potential medium-high outputs and, very presumably, already from outputs of from 350 to 400 ~` 113~575 cu.metres an hour whereas it is virtually unreplaceable for high outputs such as from 800 to 1,000 cu.metres an hoùr and over: installations with so high potential outputs are becoming ever and ever more widespread.

Claims (2)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improved multiple effect distillation ap-paratus for the desalination of sea water wherein a column of superposed cylindrical sections are provided, each section having the following elements:
a. at least one falling film evaporator disposed in each said section other than the lowermost section, each said evaporator having a bundle of vertical tubes;
b. a collection tub within each said section, with the tub in the uppermost section being connected at its upper end to a brackish water feed tube with the remaining tubs in the succeeding lower sections being connected at their upper ends to said vertical tubes of the film evaporator, disposed in the section which is immediately superjacent, and wherein the bottom of the tub in the lowermost section is connected to a brackish water discharge tube, and with the bottoms of the tubs of each remaining section being connected to the upper end of the vertical tubes of the associated film evaporator of that section;
c. a restriction means placed on the bottom of each tub, said restriction means for permitting the flow of brackish water towards the underlying evaporators and dissapating the positive pressure difference existing between the tub and the evaporator;
d. openings formed in the upper portion of each tub in each section other than the uppermost section, said openings to allow vapor to flow from each said tub about the associated evaporator of that section;
e. at least one siphon tube for withdrawing the condensate collected at the bottom of each section and for reintroducing said condensate at an intermediate point of the succeeding subjacent section;
f. means for transferring any inert gases from each section to a vacuum system for collecting said inert gases;
g. means for introducing sea water to the tub of the uppermost section and means for introduction of steam to the exterior surface of said film evaporator of said uppermost section;
h. means for removing condensate from the lowermost section wherein said improvement comprises:
preheating means, said preheating means including a plurality of longitudinally extending, cylindrical ducts disposed in a substantially vertical orientation, with one of said ducts being located within each section of said apparatus and oriented in end to end, colinear relationship, all of said ducts being in communication to define a flow path, such that brackish water may be passed through said ducts prior to its introduction to the uppermost section of said apparatus whereby the vapors in each section functions to elevate the temperature of the brackish water within the ducts of the preheating means to facilitate its vaporization within said apparatus.

2. An apparatus as recited in claim 2, wherein said preheating means further includes a thermal expansion joint interposed between the connection between each duct, said thermal expansion joint for dampening the different expansion of each said duct as a result of the varying temper-ature of each section.