IL31106A - Process and installation for the distillation of a saline solution - Google Patents

Description

PROCESS O SALINE COMMISSARIAT The present invention is directed of distillation of a saline solution which j y applicable to the desalination of sea water as well as to an installation for carrying out said process any like The invention is more especially concerned with processes in which the saline solution has previously been heated in the liquid state is subjected to a series of Successive expansions at progressively decreasing the vapor produced at each stage being condensed as fresh Distillation is thus carried out within a series of expansion chambers at progressively decreasing the vapor produced being condensed in contact with a surface which is provided within each chamber and maintained at a colder Said condensation surface is usually formed by tubes through which the saline feed solution is circulated prior to This saline solution passes through tubes from one expansion stage to the next in counterflow to the solution which vaporizes within the chambers In these conventional processes present a major problem of scale In point of progressively as the temperature of the saline feed solution rises as a result of heat transfer with the vapor which deposits of various salts and in particular calcium form inside the In order to limit scale it is usually necessary to subject the saline solution to preliminary softening treatments which increase the cost of the distillation process to a considerable proposed in for fluid being a liquid substantially insoluble in the The fluid is a or a This proposal eliminates the high temperature heat transfer surfaces of heat exchanger tubes and vessel walls upon which the scale formin deposit forms in the usual process of heating the feed contradistinction with these known methods this invention especially relates to a flash evaporator system of the type in which the water to be evaporated is initially heated to a predetermined top temperature and is then directed through a plurality of flash chambers for flash evaporation at successively lower temperatures and The present invention makes it possible to vent the disadvantages referred to by eliminating the of saline solution within the tubes of those stages in which deposits appear without any detriment to optimum thermal this result being achieved by virtue of an auxiliary fluid circulation the invention makes it possible to retain the usual advantages of tion processes which entail successive expansions and especially to secure freedom from any danger of contamination of the fresh water which is With this the invention proposes a process for the distillation of a saline solution which is applicable primarily to sea water and comprises heating said solution in the liquid state followed by vaporization by successive expansions at progressively decreasing pressures and condensing the vapor which is produced at each expansion stage as fresh this condensation being carried out in the stages by with the saline solution prior to said process being essentially characterized in that it comprises circulating within a loop a fluid which is immiscible with water and which is then successively preheats the saline tion by being contacted directly with said solution and causes condensation of the vapor produced within the pressure The invention is also directed to an installation for the distillation of a saline solution as primarily applicable to the distillation of sea wherein said installation comprises a series of expansion chambers each containing cooling tubes for causing the condensation of the vapor which is and means for circulating the saline solution successively through said tubes of the chambers at successively decreasing said tion being essentially characterized in that it further comprises a circulation loop for an auxiliary fluid which is immiscible with water and means for circulating said auxiliary fluid between successively a heat said heater and the cooling tubes of the pressure expansion in the application of the invention to the desalination of sea the stages are those in which the temperature is lower than a value of the order of so that the scale formation caused by the saline solution within the condenser tubes accordingly remains very In those stages in which the ture is higher than and in which the sea water could produce substantial deposits of condensation is carried out by circulating the auxiliary fluid within the condenser with the result that any problem of scale formation is the process and installation in accordance with the invention can profitably utilize the advantages of a process entailing direct contacting with an auxiliary fluid which is immiscible with water efficiency of heat transfer and absence of scale while remaining free from any of the disadvantages normally attendant upon this type of transfer there is never any direct contact within the expansion chambers between the auxiliary fluid and the vapor or condensed fresh with the result that the fresh water is never liable to be contaminated or to acquire an unpleasant taste advantages of the process and heat exchanger in accordance with the invention may become more readily one particular embodiment which is chosen by way of example will be described It will naturally be understood that this description is not intended to imply any limitation so far as the scope of the invention is The single figure of the accompanying drawings illustrates a particular form of construction of the lation for the distillation of a saline solution as plated by the said installation being employed in the particular case described for the desalination of sea water in order to produce drinkable The installation of 1 is essentially made up of a heat exchanger 3 for heating sea water and of a series of expansion chambers Inside these the sea water which is heated in the liquid state within the heat exchanger 3 is subjected to a series of successive expansion the different chambers being maintained at progressively decreasing At each expansion stage 1 n a fraction of the sea water is with the result that the tion which is finally discharged at the outlet of the last chamber is a brine which is more highly enriched in solved salts than the incoming sea The vapor which is produced at each stage condenses in contact with cooling tubes which are disposed in the top portion of each of the chambers Condensation results in a supply of fresh water which is collected in a tray The fresh water produced at each stage is mixed with the fresh water derived from the preceding stages the direction as the sea water and is finally collected at the outlet of the last chamber 2 The expansion stages are divided into two groups the stages 1 to and the stages to The limit between the two groups is fixed at a stage at which the temperature of condensation of fresh water is of the order of In the the cooling which is necessary for the condensation of the vapor produced by expansion is carried out by the incoming sea water which flows within the tubes from the chamber to the chamber is to say countercurrent to the hot sea water which vaporizes within the chambers before being directed towards the heat exchanger In the the tubes carry a not of sea but of an auxiliary fluid which passes from one stage to the next from the of the chamber to the tubes of the chamber and current to the sea water which The auxiliary fluid employed is a liquid which is immiscible with In the particular case herein said fluid is an oil which has a higher density than Said auxiliary fluid flows through a circulation loop which comprises the cooling tubes of the stages followed by a heat source 8 in which the auxiliary fluid delivered from stage 1 is heated by a process with any heating and finally the heat exchanger At the outlet of said heat exchanger the auxiliary fluid turns to the cooling tubes of the chamber Within the sea water and gives up heat to this In the particular case the heat exchanger consists of a dispersion column in which the auxiliary fluid and the sea water are circulated in current the sea water in the upward direction and the auxiliary fluid in the downward The sea water is the dispersed This concept is of special interest since the persed phase is water for known reasons of thermal balance within the must circulate in a smaller tity than the other the heat transfer process takes place under the best conditions of output since the liquid which is heated sea rises within the column whilst the liquid which cools passes downwards within the In convection currents which are generated within the column as a result of the temperature differences between the base and the top of the column are accordingly in the direction of flow and in ho way affect the advantages of countercurrent The use of an oil of higher density than water as auxiliary fluid in the installation herein described is not attended by the same disadvantages as in conventional lations although oils of this type usually have a high degree of In in the expansion the oil flows only through the relatively hot The viscosity of the oil is then distinctly lower than at a colder ture and this is beneficial from the point of view of pumping In those stages in which the temperature is higher than about the auxiliary liquid serves to inhibit from the presence of sea the cooling tubes of these stages can be formed of a material which is less noble than if they had been intended to carry a tion of sea an outer coating on mild steel being therefore sufficient for the This advantage pensates for the disadvantage of a larger surface which is made necessary by the physical properties of the auxiliary fluid employed which are less ble than those of On the other since the incoming sea water is circulated within the cooling tubes of the that is to say at a temperature which is below approximately scale formation within these tubes is any circulation of auxiliary fluid within these tubes is to be avoided by reason of the fact in the case of the oils which are usually available and which have a greater density than the auxiliary fluid would have viscosity at this in the process according to the the use of an oil which is contacted directly with water in order to heat this latter does not carry any of the advantages which are often cited as reasons for rejecting this In point of this oil is contacted with the sea water prior to evaporation but is not again contacted with the fresh water which is From a health and sanitation the auxiliary fluid can therefore not contaminate the fresh water in impart an pleasant taste to the since the auxiliary fluid employed is usually a costly an economic bonus arises from the fact that no loss of fluid is insufficientOCRQuality

Claims (7)

What we claim is :

1. A process for the distillation of a saline solution comprising heating said solution in the liquid state followed by vaporization by successive expansions at progressively decreasing pressures and condensing the vapor which is produced at each expansion stage as fresh water, said condensation being carried out in the lower-pressure stages by heat transfer with the saline solution prior to heating, and circulating within a loop an auxiliary fluid which is immiscible with water and which is heated and then successively preheats the saline solution by being contacted directly with said solution and causes condensation of the vapor produced within the higher-pressure stages.

2. A process in accordance with Claim 1 , wherein the limit between the lower-pressure stages and the higher-pressure stages corresponds to a water temperature of approximately 65°C.

3. A process in accordance with Claim 1 wherein the preheating of the saline solution is carried out by dispersion of said solution in the auxiliary fluid which is circulated in countercurrent flow.

4. A process according to claim 1 , 2 or 3, as applied to the desalting of sea water.

5. An installation for the distillation of a saline solution and for carrying out the process in accordance with Claim 1 , comprising a series of expansion chambers each containing cooling tubes for causing the condensation of the vapor '.which is produced, means for circulating the saline solution successively through said tubes of the lower-pressure chambers, through a heater, then through said chambers at successively decreasing pressures and a circulation loop for an auxiliary fluid which is immiscible with water, and means for circulating said auxiliary fluid between auxiliary fluid between successively a heat source, said saline-solution heater and the cooling tubes of the higher-pressure expansion chambers.

6. An installation in accordance with Claim 4, characterized in that said saline-solution heater is a direct-contact exchange eolumri which is preferably of the dispersion type.

7. A process in accordance with Claim 1 for the distillation of a saline solution, substantially as hereinbefore described. B. An installation for the application of the process in accordance with Claim 1 , substantially as hereinbefore described with reference to and as described in the accompanying drawings. For the Applicants B. 25B4-3