GB2304296A

GB2304296A - Distillation process

Info

Publication number: GB2304296A
Application number: GB9517076A
Authority: GB
Inventors: Henri Gerhard Willem Pierson
Original assignee: D and C Ltd
Current assignee: D and C Ltd
Priority date: 1995-08-21
Filing date: 1995-08-21
Publication date: 1997-03-19
Also published as: ES2121541B1; GB9517076D0; ES2121541A1; JPH09103763A

Abstract

A process for the extraction of a purified solvent, in particular water, from a solution, in particular sea water, involves adding the solution to a bed of heated particles. The solvent is driven off and condensed elsewhere. As shown, a bed 9 is heated and agitated whilst sea water is sprayed on to it via nozzles 15. The bed is composed of particles, especially particles of the solute. In preparing fresh water from the sea, the preferred particles are of sea salt. The sea water may be heated by heat-exchange with the outgoing distillate, and excess salt is harvested via port 4. Preferably the bed is at a temperature of around 80 to 90 degrees C. In other embodiments, the bed may be in a rocking drum, or may be fluidised by a gas.

Description

PROCESS The present invention relates to a process for the extraction of purified solvent from a solution, and especially for the extraction of water from aqueous solutions. The invention finds particular, but not exclusive, use in relation to the desalination of water, particularly sea water, to provide water that is fit for drinking.

Desalination of water, especially sea water, is a topic of growing importance. The relentless development of tourism in coastal areas has resulted in an increased need for potable water. There is also an increasing need to provide a better quality water supply to the growing populations of various developing countries. In this regard, there is a desire for increased amounts of water for both drinking and agricultural purposes.

Present processes for the desalination of sea water aim to provide essentially separate salt and water residues, or at least a salt residue and a considerably more dilute salt solution.

Various processes are available for achieving the above aim. Current desalination techniques may involve evaporation of the seawater (perhaps by multiple-effect distillation, multi-stage flash evaporation or forcedcirculation vapor-compression, for example). Alternative techniques involve freezing the seawater followed by harvesting the thus formed ice crystals, and also electrodialysis. The desired objective of such processes is generally to lower the saline content of the water to be suitable for drinking purposes. This level is generally taken to be about 500ppm salines or less.

There are various problems associated with the present desalination processes. As regards the thermal evaporation processes, the disadvantages include the high capital cost of the thermal evaporator apparatus and also the relatively high energy cost in producing the necessary thermal energy to effect the evaporation. In addition, the classical evaporators are typically constructed from expensive materials which are often extremely difficult, if not impossible, to repair on site (such as in a developing country). Furthermore, problems often arise with fouling of the evaporator by the saline residues or their products of thermal decomposition.

In addition, when it is desired to purify solvents other than water, it is often necessary to employ an evaporation/distillation process, which have the same disadvantages as discussed above. According there is a desire to provide alternative purification systems.

The present invention seeks to alleviate at least some of the aforementioned problems.

In one respect, the present invention involves supplying the solution to be purified to a hot bed, to vaporise the solvent component of the solution while retaining solid components on the bed, and subsequently condensing the vapour, which may be done in a conventional manner, to provide a purified liquid. Often, the solution to be purified will be an aqueous solution.

Accordingly, in one broad aspect the present invention relates to a process for the desalination of an aqueous salt solution, the process comprising supplying the aqueous salt solution to a bed of salt which is at a temperature sufficient to vaporise water from the solution, collecting the water vapour produced and condensing it remotely from the bed.

In cases where the material in the salt bed essentially corresponds to that in the aqueous solution, the present invention effectively allows for the salt in the aqueous solution to be harvested and usefully recycled as a heat-transfer medium in a subsequent desalination process.

In preferred embodiments the present process relates to the desalination of an aqueous solution which is sea water.

It will be appreciated that the desalination process is not necessarily limited to the removal of sodium chloride from seawater, since the seawater will typically contain a host of dissolved salts. Generally, seawater is expected to contain appreciable amounts of calcium bicarbonate, calcium sulphate, magnesium sulphate and magnesium chloride in addition to sodium chloride.

Likewise, in the context of the salt bed, the term salt is not limited to what is customarily known as table salt, sodium chloride. A host of alternative salts may be effective in the present process. Generally, a hygroscopic salt will be selected, such as an alkali or alkaline earth metal chloride, for example. Without being bound by theory, it is believed that hygroscopic salts, which are capable of absorbing water will act, when hot, in a similar fashion to a very thin film evaporator.

Thus, the hot salt bed of hygroscopic salt particles may provide a relatively large surface area to enable efficient evaporation. Preferred hygroscopic salts thus include sodium chloride and magnesium chloride.

The hot bed most preferably contains a major proportion of salt in the form of sodium chloride.

In the present process the collected water vapour is condensed remotely from the salt bed, leaving the salts and other mineral components from the solution behind in the salt bed. In cases where the process is applied to sea water from relatively unpolluted areas, the salt residue could be harvested and utilised as natural table salt. For this reason it is particularly preferable to employ a bed of principally the same salt as that intended to be harvested from the seawater. In this regard a sodium chloride salt bed is preferred. Alternatively, depending upon the nature of pollutants or contaminants in the sea water, the salt residue could be harvested as industrial salt, for example.

The present process may utilise salt beds at various temperatures provided the temperature is sufficient to vaporise water from the aqueous solution at a reasonable rate. The precise temperature required will vary in dependence upon the desired rate of reaction and various other process conditions.

In embodiments of the present invention, in cases where the process is conducted at essentially atmospheric pressure, the aqueous solution is typically supplied to a salt bed which is at a temperature of at least about 60"C.

Of course, water will evaporate from an aqueous solution to a greater or lesser extent at any temperature above freezing point. However, in terms of economic viability of the present process, it is probable that the salt bed will be raised to an average temperature of at least about 600C. The appropriate upper limit for the temperature of the salt bed will also generally be dictated by economic factors.

To illustrate the above, in an agitated salt bed a given quantity of water may evaporate in about ten minutes at about 70"C, in about five minutes at about 100" C and in about 1.5 minutes at about 1200C. Whilst, on the face of things the higher temperature salt bed is advantageous in effecting more rapid evaporation, it is often expensive to generate heat at a temperature in excess of 100 C. In contrast, temperatures of about 70" may be provided by what might otherwise be considered as "waste heat", such as from power stations, or even by way of solar heating.

As a result, in most cases, the use of a salt bed at a temperature of between about 700C and about 1000C may be advantageous, and temperatures of from about 75"C to about 95"C, especially about 80"C to about 900C particularly preferable.

In some embodiments it is preferred that the salt bed comprise essentially anhydrous or dry salt, that is salt with a very low water content. In other embodiments, the bed may comprise a salt melt. As noted above, the use of dry salt at relatively low temperatures (600C or above) may be preferred over the use of the an extremely hot salt melt.

Preferred embodiments of the present process involve agitation of the salt bed. This is desirable to ensure the free flow of the granular salt and also to avoid the formation of balls of salt. The agitation may be effected by use of an agitator or by providing a tumbling or rotating action in the salt bed. In cases where a dry salt bed is employed as opposed to a salt melt, vaporisation of the water component of the solution may optionally be assisted by supplying hot air to fluidise the dry salt bed.

In another aspect, the present invention provides apparatus for the desalination of an aqueous solution, the apparatus comprising a housing having a reservoir, a port for supply and removal of salt from the reservoir, an inlet port for supply of solution generally in the form of a spray to the reservoir and an outlet port for removal of water vapour from the housing, means for heating salt in the reservoir to a temperature sufficient to effect vaporisation of solution entrant the housing, and a condensation station for condensing vapour released from the outlet port in the housing.

As noted above in relation to the discussion of the present process, the apparatus may be employed with sodium chloride and various other salts.

Typically, the reservoir is located in a lower region of the apparatus, suitably below an inlet for the supply of aqueous solution (such as seawater) and also in turn below an outlet port for removal of water vapour.

Suitably the reservoir has an internal surface for contacting the salt bed which is made of a material which has some resistance to corrosion by the salt(s).

Preferred embodiments of the present process comprise a steel housing coating at least in part with a relatively non-corrodible material such as rubber or PVDF. The housing will typically be constructed so as to allow reasonable access to the reservoir to enable it to be relined, if necessary, once the lining has degraded to a certain extent. Any such coating may, if desired, be provided only at regions of the housing which are likely to contact the hot salt bed. In some cases a coating for guarding against corrosion may inhibit the heating means for raising the temperature of the salt bed. Thus, this will generally be taken into account when selecting appropriate material/extent for the coating.

In preferred embodiments, the inlet port for supply of aqueous solution to the reservoir is effective for delivering the solution in the form of a spray. A suitable inlet port comprises a manifold which includes two and preferably more spaced spray nozzles. Suitably, any such spray nozzles are arranged to supply the aqueous solution over a major proportion of the reservoir.

In use of the apparatus, the rate of supply of aqueous solution to the reservoir may be selected so as to effect vaporisation of the aqueous component at a reasonable rate and particularly under conditions which are not too violent or vigorous. In this respect, the size and number of nozzles for supply of aqueous solution may be selected so as to enable the reaction to proceed at a desired rate.

In some embodiments, the housing of the present apparatus may also accommodate a heating means for heating salts in the reservoir. Suitably, the heating means are operable to raise the temperature of the salt bed to at least 70"C for the reasons discussed above. In some embodiments the heating means are operable to raise the temperature of the salt bed at least to the melting point of salt. Example heating means may be direct fired, indirectly fired or optionally solar assisted.

In some embodiments the housing of the present apparatus may accommodate a condensation station, located remotely from the reservoir. Alternatively, an independent housing may be provided to accommodate the condensation station.

In the present apparatus the housing is constructed to permit both the supply and removal of salt to the salt bed. In this respect, a single port may be provided for both the supply and removal of salt or alternatively independent inlet and outlet ports may be provided.

In preferred embodiments means are provided for agitating the salt bed. This is to inhibit the undesirable formation of balls or lumps of salt and also to encourage free flow of granular material. This may take the form of an agitator or stirrer. Alternatively, the salt bed may be carried on a moving belt which is agitated as it progresses. As a further alternative, the salt bed could take the form of a fluidised bed, agitated by introduced hot air.

The present apparatus may be constructed so as to be useful in both batch and continuous processes.

Embodiments of the present invention have various advantages. In particular, the thermal desalination process can be employed at relatively low temperatures of about 700C to 900C. Also, as a result of it operating at lower temperatures the apparatus may be constructed from materials which are cheaper than those employed in classical evaporator apparatus. As noted above, example materials which may be suitable for the present apparatus include rubber lined mild steel and synthetic materials such as PVDF. The present process and apparatus also avoid the inherent fouling problems of traditional apparatus. This is because it is not necessary to separate out any salt residues from the present apparatus in order to enable it to function efficiently. Rather in the present case, any salt residues are simply added to the salt bed to be included in the heat transfer medium.

Partly as a consequence of the reduced fouling maintenance costs are considerably reduced as compared to prior apparatus.

In addition to the above, since the process operates at relatively low temperatures, lower grade fuels may be employed as compared to those required in the current apparatus and processes. Also, as the present apparatus is relatively simple in construction and the process is operable at relatively low temperatures, it is suitable for use even in relatively primitive conditions in developing countries.

In addition, the present invention provides apparatus employable for the purification of solutions containing a solvent other than water. That aspect of the invention concerns a housing having a reservoir, a port for supply and removal of crystalline material to the reservoir, an inlet port for supply of solution to the reservoir and an outlet for removal of solvent vapour from the housing means for heating the crystalline material in the reservoir to a temperature sufficient to effect vaporisation of solution entrant the housing and a condensation station for condensing vapour released from the outlet port in the housing. Suitably the crystalline material is capable of adsorbing the solvent in a generally similar fashion to the manner in which hygroscopic materials adsorb water.

Embodiments of the present invention will now be described further, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a sectional view of an example apparatus according to a first embodiment, and Fig. 2A is a sectional view of apparatus according to a second embodiment and Fig 2B an end view.

Turning firstly to Fig. 1, this illustrates apparatus 1 which includes a housing 2 having a reservoir 8. The housing 2 has a port 4 for supply and removal of salt to the reservoir 8. The housing 2 is typically constructed of steel and provided with a PVDF lining at least in the region of reservoir 8 which is to contact salt. Salt in the reservoir 8 provides a salt bed 9. The salt bed may be a mixture of various compounds, but typically contains a major proportion of sodium chloride.

The housing 2 is also provided with an inlet port 10 for supply of sea water (or other solution) to the reservoir 8. The inlet 10 comprises a manifold 14 which includes a plurality of spaced nozzles 15. The manifold 14 is capable of supplying sea water in the form of a spray over a major proportion of the salt bed 9. A stirrer 18 is also provided for agitating the salt bed 9 to avoid lump formation.

Heating means for raising the temperature of the salt bath 9 are illustrated schematically at 40. Various conventional forms of heating means may be employed, as discussed above. In the interests of economic efficiency it is desirable to employ heat exchange to utilise "waste" heat generated from an industrial process.

In the upper region of the housing 2 there is an outlet for removal of water vapour 20, which communicates with a condensation station, illustrated schematically at 30. Various forms of conventional apparatus may be employed to condense the water vapour and they need not be described in detail for the present purposes.

The apparatus of Fig. 1 may be employed in a batch process, for periodic harvesting of aqueous components and salt residues. In use, salt is provided in reservoir 8 to produce a salt bed 9, and the temperature of the bed is raised to a desired level (generally between about 70"C to 100"C) by heating means 40. The bed 9 is agitated by stirrer 18.

Sea water is subsequently supplied to the hot salt bed via inlet 10, manifold 14 and nozzles 15. As the sea water contacts the salt bed 9, the hygroscopic hot salt particles absorb the aqueous component of the seawater in what is thought to be essentially the same manner as a thin film evaporator. The aqueous component vaporises and rises to an upper region of the housing 2 to be released via outlet 20. The water vapour is subsequently channelled to the condensation station 30 to be condensed in a conventional fashion.

The salt and other mineral residues present in the sea water are retained in the salt bed 9. Periodically, salt is removed from the salt bed 9 via outlet 4 to ensure that the level of the salt bed 9 does not go beyond a desired maximum.

Throughout the process of supply of sea water to the salt bed, the hot bed is preferably agitated by an agitator 18, to avoid lump formation in the bed.

An alternative embodiment of the present invention is shown in Fig. 2. In this figure the apparatus illustrated is suitable for use in a continuous rather than a batch process. In this case, the apparatus includes a housing 102 which defines a reservoir 108 for a hot salt bed 109.

Again, heating means 140 are provided to raise the temperature of the salt bed.

In this case, the housing 102 is in the form of a drum 103 which rocks from side to side on rollers 107 to agitate salt in the salt bed 109. The agitation also ensures that excess salt is released from outlet 104 in the housing 102. The drum 103 is typically made of steel with a rubber or PVDF lining in the region of the kiln which is to contact concentrated salt in the bed.

In the drum 103 a manifold 114 extends along a major proportion of the length of the drum. As before, the manifold 114 includes a plurality of spaced nozzles 115.

At a first end of the housing 102 there is provided an inlet 110 for supply of sea water to the manifold 114. At the opposite end of the housing there is provided an outlet 120 for water vapour and which communicates with a condensation station (such as in the previous embodiment).

The illustrated apparatus may be employed by operating the heating means to raise the temperature of the salt bed to a desired level. The drum 103 is rocked on rollers 107 to agitate salt in the salt bed. Sea water is supplied to the salt bed via inlet 110 and nozzles 115.

As the sea water contacts the hot salt bed, the aqueous component of the sea water vaporises and is released from the housing 102 via outlet 120. The hot walls of the drum 103 will also assist in driving off moisture from the salt bed. The dry salt will then be contacted by fresh seawater, in a cycle. The water vapour produced is then channelled to a condensation station, remote from the salt bed 109. The salt and other residues from the sea water are retained in the salt bed and, as and when the level of salt in the bed exceeds a desired amount, salt is released from outlet 104.

It will be appreciated that various modifications may be made to the illustrated embodiments. In particular, although the above discussion concentrated on a process for the desalination of sea water by employing a salt bed, of course the process is applicable to the purification of solvents generally by use of hot beds comprising inert crystalline materials which may be other than salts.

Thus, another aspect of the present invention concerns a process for the purification of a solution, the process comprising supplying the solution to a bed of crystalline material which is at a temperature sufficient to vaporise solvent from the solution, collecting the vapour produced and condensing it remotely from the bed.

In this aspect, various crystalline materials may be suitable in addition to, or as an alternative to, a salt.

Claims

1. A process for the extraction of a purified solvent from a solution comprising the steps of supplying the solution to be purified to a hot bed of crystalline material at a temperature sufficient to vaporise the solvent component of the solution while retaining solid components on the bed, collecting the vapour produced, and condensing the vapour remotely from the bed.

2. A process for the desalination of an aqueous solution comprising supplying the aqueous salt solution to a bed of salt which is at a temperature sufficient to vaporise water from the solution, collecting the water vapour produced and condensing it remotely from the bed.

3. A process as claimed in claim 2 in which the aqueous solution is sea water.

4. A process as claimed in claim 2 or claim 3 in which the salt of the bed is an hygroscopic salt such as an alkali or alkaline earth metal chloride.

5. A process as claimed in any one of claims 2 to 4 in which the salt of the bed is sodium chloride.

6. A process as claimed in any one of claims 2 to 5 and carried out at atmospheric pressure, the temperature of the salt bed being at least about 60 C.

7. A process as claimed in claim 6 in which the temperature of the salt bed is between about 70 C and lOO C, preferably between about 75 C and 95 C, and more preferably between about 80 C and 90'C.

8. A process as claimed in any one of claims 2 to 7 in which the salt of the bed comprises essentially anhydrous or dry salt.

9. A process as claimed in any one of claims 2 to 7 in which the salt of the bed is a salt melt.

10. A process as claimed in any one of claims 1 to 9 in which the salt bed is agitated.

11. A process as claimed in claim 10 in which agitation of the salt bed is effected by use of an agitator or by providing a tumbling or rotating action in the salt bed.

12. A process as claimed in claim 10 together with claim 8 in which hot air is supplied to the salt bed to fluidise the bed thereby to assist in vaporisation of the water component of the solution.

13. Apparatus for the extraction of purified solvent from a solution comprising a housing having a reservoir, a port for supply and removal of a bed of crystalline material to and from the reservoir, an inlet port for supply of solution, preferably in the form of a spray, to the reservoir and an outlet port for removal of vapour from the housing, means for heating crystalline material in the reservoir to a temperature sufficient to effect vaporisation of solution in the housing, and a condensation station for condensing vapour released through the outlet port from the housing.

14. Apparatus as claimed in claim 13 for the desalination of an aqueous solution in which the crystalline material is salt and the reservoir is located in a lower region of the apparatus below the inlet port for the supply of aqueous solution and below the outlet port for the removal of water vapour.

15. Apparatus as claimed in claim 14 in which the reservoir has an internal surface for contacting the salt bed and made of a material having some resistance to corrosion by the salt.

16. Apparatus as claimed in claim 15 in which the housing is of steel coated at least in part with a relatively noncorrodible material such as rubber of PVDF.

17. Apparatus as claimed in any one of claims 14 to 16 in which the inlet port comprises a manifold including at least two spaced spray nozzles arranged to supply the solution over a major proportion of the reservoir.

18. Apparatus as claimed in any one of claims 14 to 17 in which the heating means are operable to raise the temperature of the salt bed to at least 70 C.

19. Apparatus as claimed in any one of claims 14 to 18 in which the condensation station is within the housing at a location remote from the reservoir.

20. Apparatus as claimed in any one of claims 14 to 18 in which the condensation station is in a further housing independent of the housing containing the reservoir.

21. Apparatus as claimed in any one of claims 13 to 20 and including means for agitating the bed of material.

22. Apparatus as claimed in claim 21 in which the agitating means comprise an agitator or a stirrer.

23. Apparatus as claimed in any one of claims 13 to 20 in which the bed material is carried on a moving belt which is agitated as it progresses.

24. Apparatus as claimed in any one of claims 13 to 20 in which the bed material is fluidised by the introduction of hot air.

25. A method of extracting a purified solvent from a solution substantially as described with reference to the accompanying drawings.

26. Apparatus for the extraction of purified solvent from a solution substantially as described with reference to and as illustrated by the accompanying drawings.