WO1998025679A1

WO1998025679A1 - Method for desalinating salt-containing water, single-effect or multiple-effect distillation apparatus and modular element suitable for a single-effect or multiple-effect distillation apparatus

Info

Publication number: WO1998025679A1
Application number: PCT/NL1997/000676
Authority: WO
Inventors: Pieter Robert Bom
Original assignee: Pieter Robert Bom
Priority date: 1996-12-09
Filing date: 1997-12-09
Publication date: 1998-06-18
Also published as: NL1004733C2; AU5345698A

Abstract

The invention relates to a method of desalinating salt-containing water by a single-effect or multiple-effect distillation process. The raw water (5) to be treated is successively fed to each effect in order to be at least partly evaporated by film evaporation in each effect at an essentially constant pressure level in each effect. Each effect comprises at least one channel system (12) having channels which have flow from top to bottom, which channels are heated and through which the raw water is passed in such a way that there is formed along the channel walls a film of raw water which at least partly evaporates during the passage through the channel system (12). The evaporation takes place stage-by-stage in each effect in an above-mentioned channel system for each stage and, between two successive stages, vapour formed in the first of said two successive stages is removed from the effect. The invention furthermore relates to an apparatus which is suitable for performing the method according to the invention. For this purpose, said apparatus is provided between two successive stages with removal means (13) for the interim removal of vapour from the effect between two successive stages. In this connection, each effect is accommodated in a process space which is sealed off essentially in a pressure-tight manner with respect to the environment. The multiple-effect distillation apparatus may at the same time be built up in a modular manner from one layer of modular elements (10) per stage.

Description

Method for desalinating salt-containing water, single-effect or multiple- effect distillation apparatus and modular element suitable for a single- effect or multiple-effect distillation apparatus

The present invention relates to a method for desalinating water, in particular sea water, by a single-effect or multiple-effect distillation process in which raw water to be treated is fed to the one effect or successively to each effect in order to be at least partly evaporated by film evaporation in the one effect or each effect at a pressure level which is essentially constant for each effect, and in which the one effect or each effect comprises at least one channel system having channels which extend in an inclined or vertical manner and have flow from the top downwards, which are heated and through which the raw water is passed in such a way that there is formed along the channel walls a film of raw water which at least partly evaporates during the passage through the channel system.

Such a method is disclosed in the article entitled "Seawater

Desalination Plant for Southern California, Part 1, by David W. Dean &

Design Team in Desalination & Water Re-use, Issue 5/1. May 1995- The latter describes an apparatus and a method for desalinating water in which the method is performed in an apparatus comprising a number of 30 effects placed one above the other, and in which each effect comprises a bundle of tubes. Preheated sea water is distributed at the top of the tower above the first bundle of tubes in order to flow through the latter forming a thin film of water on the inside of the tubes. Said tubes are heated by means of hot water vapour which then condenses in the process.

At the end of the first bundle of tubes, the unevaporated water appears to be collected in a collecting space together with the vapour formed.

The unevaporated water is then passed on to the second bundle of tubes and the water vapour collected is passed over the other side (the outside) of the second bundle of tubes in order to heat said bundle of tubes. When passed onto the second bundle of tubes, the raw water originating from the first bundle is first passed through one or more nozzles in order to meter the raw water in accordance with the pressure difference between the two effects, the raw water remaining at boiling point (or being at boiling point) spontaneously undergoing a drop in temperature dependent on the pressure difference with the production of a corresponding amount of vapour. This process is repeated for each effect until the entire tower of 30 effects has been traversed. The tubes have a diameter of 2 inches (5-08 cm) and are 10 feet (3.048 m) long. A disadvantage of such a method and apparatus as disclosed in the abovementioned article is that a portion of the film evaporates while flowing through the tubes and that the vapour thus produced forms, as it were, a plug in each tube, which plug increases in size on its way downwards and, as a result of the relatively great length of the tube (10 feet) , requires a relatively large pressure difference in order to reach the outflow opening of the tube at the required velocity, which pressure difference forms a pressure loss which reduces the pressure difference available for heat transfer and, consequently, the temperature difference, which has a disadvantageous effect on the good operation of the distillation process.

The object of the present invention is to provide an improved method and apparatus for desalinating water, which method and apparatus, inter alia, overcomes the abovementioned disadvantage.

This object is achieved in a method of the type according to the invention specified at the outset in that, in the one effect or each effect, the evaporation takes place stage-by-stage in an abovementioned channel system for each stage and in that, between two successive stages, vapour formed in the first of said two successive stages (or in stages prior thereto) is removed from the effect. The interim collection of the vapour and the interim removal of the vapour from the effect can be achieved in many ways. Thus, it is conceivable, for example, to provide one channel system which extends in a practically uninterrupted manner for each effect and to connect to each channel drainage means at intervals (viewed in the vertical direction) with which the vapour present at the connection can be drained from the respective channel. The evaporation in this system takes place, as it were, stage-by-stage by always removing the vapour formed after each stage and forming in a subsequent stage further vapour in a manner similar to that in the previous stage, the evaporation process again taking place in each stage essentially at the same or constant pressure level.

Viewed in terms of design, the method according to the invention can be achieved in an advantageous way if a collecting space is situated between the channel systems of two subsequent stages and if vapour essentially formed in the first of said two successive stages (or in stages prior thereto) is removed from the effect via said collecting space situated between two successive stages, preferably to a subsequent effect, if present, and if the residual raw water from said collecting space situated between two successive stages is passed on to the channel system of the second of the two successive stages. The pressure level prevailing in and at the residual raw water collected in the collecting space will essentially undergo no change while being passed on to the channel system of the subsequent stage. In such a design involving a collecting space, the channels systems are interrupted at certain vertical levels in order to debouch into such a collecting space so that it is possible to make do with one vapour discharge for each collecting space, which is an appreciably simpler design than the separate connection of vapour removal means to each channel of the channel system.

In order to remove entrained particles in the form of essentially unevaporated raw water from the vapour formed, it is advantageous according to the invention if the vapour removed from an abovementioned collecting space is passed through a droplet trap.

In order to counteract vapour plug formation and to minimize the pressure drop needed for the conveyance through the channel system, it is advantageous according to the invention if the vertical height of the at least one channel system is less than 1 metre, preferably between 20 and 60 cm. Channel systems involving channels whose vertical height is 50 cm with channels preferably arranged in an essentially completely vertical manner appear to yield particularly good results . At the same time, the diameters of the tubes may be in a range of less than 5 cm, for example 2 to 4 cm, but a range of approximately 1 to cm is also readily conceivable .

It may be pointed out in relation to the channel system that it can be formed in many ways. It is conceivable to form such a channel system by placing corrugated plates against one another, channels then being enclosed between the corrugated plates. But according to an advantageous embodiment, the at least one channel system comprises a bundle of parallel tubes arranged with mutual spacings such as, for example, so-called "fluted tubes". On the inside of such tubes, the film of raw water can be formed for the purpose of evaporating it, while the tubes have a hot medium, such as vapour formed from an earlier effect, flowing round the outside.

According to an advantageous embodiment, the vapour removed from the collecting spaces associated with one effect is brought together and distributed over the stages of the subsequent effect and used there to heat the channel system associated with said subsequent effect. First combining the vapour removed from the collecting spaces associated with one effect achieves the result that the pressure in the effect from which the vapour originates is kept uniform over said effect and that the heat content of the vapour fed to the subsequent effect is more or less an average of that of the vapour removed from the respective collecting spaces of the previous effect, which heat content will in practice differ to some extent between the collecting spaces associated with one effect. The present invention furthermore relates to a single-effect or multiple-effect distillation apparatus comprising for each effect a process space essentially sealed in a pressuretight manner with respect to the environment having an inlet for raw water debouching at the top, a drain for unevaporated residual raw water debouching at the bottom, at least one drain for vapour formed and preferably at least one inlet for heating vapour preferably originating from a previous effect and utilized as evaporating heat, at least one channel system of channels extending in a vertical or inclined manner being provided per effect, characterized in that each process space is subdivided into a number of successive stages having an abovementioned channel system for each stage and in that removal means are provided between the channel systems of two successive stages for removing vapour from the effect, essentially formed in the first of two successive stages (or in stages prior thereto) . Essentially the same statements apply to the advantages of such a single-effect or multiple-effect distillation apparatus as to the advantages of the method according to the invention.

A single-effect or multiple-effect distillation apparatus which can be achieved in a technically relatively simple and cheap manner is obtained according to the invention if the channels of each channel system debouch at their bottom into a collecting space for residual raw water, vapour formed and, preferably, any non-condensable gases (NCG) and/or uncondensed heating vapour, and if each collecting space situated between two stages is provided with a discharge for vapour to be collected therein and discharge/distribution means separate therefrom for distributing residual water over the subsequent channel system, if present, the discharge/distribution means being designed in such a way that the constancy of the pressure prevailing during operation in a process space is essentially undisturbed. Process space is understood in this connection as meaning the entire space or the entire section through which the raw water fed in at the beginning of the section is passed during the passage through the entire effect, it at least partly evaporating in the meantime, including the spaces in which heating by means of heating vapour, the passing of NCG and/or uncondensed vapour and the spaces through which the vapour formed from the raw water is discharged, including the droplet traps, takes place.

In order to obtain a uniform flow through the channels of the channel system of each subsequent stage, it is advantageous according to the invention if the stages are disposed one above the other and if the inlet mouths of the channel system of each subsequent stage open into the collecting space of the respective preceding stage and, for example, at some distance from the base of the collecting space in such a way that the inlet mouths each form an overflow brim. In this arrangement, the inlet mouths will generally be provided with inlet nozzles known per se for promoting the film formation on the inside walls of the channels .

In order to remove any residual liquid, in particular fine particles of raw water, from the vapour discharged at each stage, it is advantageous according to the invention if the vapour discharges are provided with a droplet trap, such as a demister.

According to the invention, it has been found that channel systems can advantageously be used which have a vertical height of less than 1 metre, preferably 20 to 60 cm. Such channel systems can be made up of a bundle of parallel tubes, the diameters of the tubes being in the range < 5 cm, for example 1 to 4 cm. In this connection, vertical height is understood as meaning the height measured in the vertical direction.

If a channel system extends essentially vertically, said vertical height will be equal to the length of the channel itself. As a consequence of the method and apparatus according to the invention, it becomes possible, in particular, to build up the process space of each effect from a number of essentially identical modular elements placed one on top of the other, the number of layers of modular elements preferably being equal to the number of stages in said process space. The invention therefore also relates to a single-effect or multiple-effect distillation apparatus in which the process space of each effect is made up of essentially identical modular elements and in which preferably the same modular ^"elements can be used for the various effects. In an advantageous manner from a design point of view, the or each modular element in this arrangement comprises a frame having a base plate and a top plate between which a channel system, preferably a bundle of parallel tubes arranged with mutual spacings, extends, and wherein the inlets of the channel system on the top side of the top plate are open and the outlets thereof on the bottom side of the base plate are open, and wherein the modular element on the bottom side and top side are provided with mutually complementarily formed circumferential edge parts which are designed in such a way that there forms between the channel systems of two modular elements stacked one on top of the other a collecting space which is bounded by the complementarily formed circumferential edge parts, the base plate of the uppermost modular element and the base plate of the lowermost modular element. The base plate and top plate of each modular element will, in this case, be mutually joined by means of walls provided around the channel system in such a way that a space is formed which is formed by said walls around the channel system and to which a heated or hot medium can be fed to heat the channel system extending through said space.

The frame of the modular element according to the invention can advantageously be manufactured from plastic, preferably as a moulded, in particular, injection-moulded product and the channel system can advantageously be manufactured from a metal having good heat conduction properties . Such a modular element can be manufactured relatively simply and cheaply, can be used in a cost-effective manner for constructing or assembling a process space needed for one effect and, if necessary, modular elements can, while the installation is shut down, be removed for inspection, cleaning, repair and other maintenance, or replacement.

The (condensation) space formed between the top plate and base plate is preferably in gas communication with the collecting space to be formed underneath the base plate. This is necessary, in particular, if, to heat the channel system, use is made of vapour which originates from a preceding effect and which is also referred to here as heating vapour. Any NCGs, such as carbonic acid gas, in said heating vapour, could remain behind in the condensation space and could accumulate, as a result of which the condensation of water vapour would be seriously hampered. Said gas communication ensures a continuous removal (through passage) of such NCGs, together with residual water vapour, as a result of which the condensation of water vapour can continue to take place in the condensation space. The NCGs ending up in said collecting space are passed to the first subsequent condensation space, to which space the same applies as described above.

The modular element will advantageously be provided in this case with a moisture trap, such as a droplet trap.

The invention furthermore relates also to a modular element suitable for a single-effect or multiple-effect distillation apparatus according to the invention.

In relation to the present invention, one effect is understood as meaning a portion of a distillation process apparatus in which water is evaporated at an essentially constant pressure level, the evaporation temperature determined by said pressure level or the boiling point determined by said pressure level being construed as essentially constant. Said term effect is essentially also employed with the same definition in the prior art known per se. In this connection, reference is made, inter alia, to "The U.S.A.I.D. Desalination Manual", pages 3^_6 et seq., August I98O.

The term raw water is understood as meaning the water to be desalinated or distilled which is fed to an effect or stage thereof. Said raw water will only partly evaporate in each effect and, in particular, in each stage and the residual raw water is then fed, again being referred to as "raw water", to a subsequent stage or, for example, to the uppermost stage in the same or in a subsequent effect or another effect for further evaporation. The present invention will be explained in greater detail below by reference to a drawing containing a diagrammatically depicted exemplary embodiment. In the drawing:

Figure 1 shows a diagrammatic view, partly in section, of a multiple-effect distillation apparatus (with three effects shown) according to the invention;

Figure 2 shows a detail from one effect according to Figure 1, showing one stage of said effect in greater detail; and

Figure 3 shows a diagrammatic sectional view along line III-III in Figure 2 of a modular element for a multiple-effect distillation apparatus according to the invention.

As shown diagrammatically in section in Figure 1, the multiple- effect distillation apparatus according to the invention can be made up of a housing 1, for example a concrete housing, in which towers 2, 3. 4 etc. of modular elements 10 are sited next to one another, one tower being in principle provided for each effect, in which arrangement each tower may possibly be made up of a plurality of subtowers/stacks of modular elements. This means that, for example, in the case of tower 2, essentially one and the same pressure level prevails from the top downwards. As is usual in the prior art, said pressure level will generally be a reduced pressure with respect to atmospheric pressure. The housing 1, made, for example, of concrete, provides an enclosure in which closable openings are provided for the purpose of assembly, maintenance and the like and within which the said reduced pressure can be maintained .

Although the method for desalinating water and the apparatus intended for the purpose will in practice often be subdivided into approximately ten or more effects , a possible way of operating an installation comprising three effects will be described below by reference to Figure 1, in which three effects sited next to one another are shown. At 5. water to be desalinated is fed in to be fed up via pump

6 and line 7 to spraying or distribution installation 8, which sprays/distributes raw water to be treated into a tank 11 which is present at the top of the stack of modular elements 10 and which operates as a distribution element. Via said tank 11, the raw water to be treated is distributed over a channel system 12 in order to flow downwards with film formation over the walls and then to flow out into a collecting space 13. Said collecting space 13 is in turn bounded at its bottom by a tank 11 which distributes the raw water collected in collecting space 13 and still to be treated further over the subsequent channel system 12 through which the raw water to be treated is again fed over the inside walls of the channels with film formation in order to emerge in the subsequent collecting space 13. This feeding through of raw water to be treated is repeated until the residual raw water is finally collected at the bottom of tower 4 in a collecting space 9 and enters a collecting container 21 via a drain 20. The residual raw water collected in the latter can again be fed up from collecting container 21 via line 22 and pump 6 in order to be retreated in the same tower 4. The raw water to be treated can be fed through via valve 23 and line 5. or untreated raw water can be passed on to the subsequent effect or the subsequent tower

(or subsequent effect and subsequent towers, respectively). In the latter, essentially the same process of treating raw water as just described by reference to tower 4 takes place, which process will be described in more detail further below. The most important difference between the processes in the various towers is the pressure level at which the process takes place. During the passage through the channel systems 12, the raw water will flow as a film over the inside walls of the channels of the channel system 12 in order then to evaporate relatively readily if the channels of the channel system 12 are heated. If possible, this heating preferably takes place by means of the heat of condensation of water vapour originating from a neighbouring (upstream) effect, the (water) vapour used for such heating being referred to here as heating vapour. An external or other heat source then has to be used only in the case of the first effect, in the case of Figure 1, the tower on the extreme left, that is to say tower 2. The channel systems 12 always debouch into collecting spaces 13. with the result that residual raw water and vapour evaporated during the passage through upstream channel systems are collected in each case in the collecting spaces 13- Said vapour is removed via a drain 14 , upstream of which there is connected a demister 15, drained from the collecting space 13 in order then to be fed with pressure equalization into a vertical collecting channel 25. which extends essentially vertically between two adjacent towers of modular elements 10 together with vapour originating from other collecting spaces 13 of the same effect, and then to be fed to the subsequent effect in order to be able to flow therein (in a way still to be explained further) over the outside of the channel system 12, after which the NCG possibly present and/or uncondensed water vapour can be added via passages 16 to the of vapour collected in the collecting space

13.

In order to ensure that, in each tower, the essentially constant pressure level needed for said effect can be maintained, the process space through which the raw water fed into said towers is fed is sealed off in an essentially pressure-tight manner with respect to the adjacent effects. For this purpose, there are provided around the inlet side of an effect sealing means comprising, inter alia, for example horizontal sealing strips 30 and 32 and vertical sealing strips (not shown). Furthermore, the passages 16 (via which any NCG and residual vapour originating from the one effect can be fed through to another effect) will be designed in such a way that they do not disturb the pressure level prevailing in the respective effects. In addition, all the process spaces are sealed off, at least in a virtually completely pressure-tight manner with respect to the outside environment by the said housing made, for example, of concrete.

The distillation process described above corresponds in outline with distillation processes known from the prior art, with the important difference that it is not known from the prior art to remove, in the interim, vapour formed in one effect, as happens via collecting spaces 13, demisters 15 and drains 14 in the case of the exemplary embodiment according to the invention shown. Said interim removal of the vapour formed in one effect has the great advantage that it counteracts water- vapour plug formation in the channel systems, with the result that the pressure drop needed for maintaining the process, in particular for maintaining the conveyance of raw water and vapour can be lower. A further advantage is that, as a consequence of the interim removal of vapour formed from an effect, the flow cross sections of the channels of the channel systems can also be reduced. Said reduction in the flow cross sections of the channel systems can go so far that the pressure drop required is the same as that used in the prior art, but, on the other hand, this still achieves the advantage that, as a consequence of the reduction in the flow cross section of the channels, the relative evaporation surface available for evaporation can be appreciably increased with the same space being occupied by the entire system, which results in a much more compact system.

In relation to the relative pressure level of each effect, it should also be pointed out that, according to Figure 1, this decreases from left to right, that is to say the pressure level in tower 2 is higher than the pressure level in tower 3 and the pressure level in tower

3 is higher than the pressure level in tower 4.

Figure 2 shows in greater detail one modular element which shows, in cross section, one stage from one effect of a multiple-effect distillation process. Figure 2 also shows the lowermost portion of the preceding modular element and the uppermost portion of the subsequent modular element. The reference numerals used in Figure 1 are used here as well. In addition, it should be noted that the channel system 12 is made up of a bundle of parallel tubes 35 arranged with mutual spacings (see also the cross-sectional view of the modular element 10 in Figure 3)- The tubes 35 extend between a top plate 36 and a base plate 37. which both form part of the frame of the modular element 10. The modular elements in Figure 2 are bounded at their top and bottom by planes 50, shown in

Figure 2 as centre lines. As will be clear from Figures 2 and 3. each modular element comprises a frame, preferably manufactured as a plastic injection-moulded product, the frame comprising a tank 11 surrounded by a peripheral upright wall 4l and is bounded at its bottom by the so-called top plate 36, furthermore a condensation space 43 which is bounded by the top plate 36, the base plate 37, the wall 42 and the two long side walls of the module (which form extensions of wall 4l) and through which the tubes 35 extend, and a space 44 at the bottom of the base plate 37 essentially open only downwards. As will furthermore be clear from

Figures 2 and 3. the top edges 45 and bottom edges 46 of each modular element will be formed so as to match one another in such a way that, when two or more modular elements 10 are stacked one on top of the other, an essentially fluid-tight and pressure-tight joint is brought about or can be brought about between the modular elements stacked one on top of the other. In this connection, sealing strips can also additionally be applied to the edges in order to improve the sealing.

In Figure 2, arrows 52 and 51 furthermore show diagrammatically the conveyance of vapour or residual raw water developed in the tubes 35 and fed through them.

As is clear especially from Figure 2, the collecting space 13 situated between two stages of one effect is formed by the tank 11 of the subsequent modular element and the space 44 which is open downwards (a sort of tank open downwards) of the earlier modular element preceding the subsequent modular element.

As is furthermore clear from Figures 2 and 3, each modular element 10 is furthermore also provided with a droplet trap, for example a demister 15, which is connected upstream of drain 14. According to the invention, the modular elements may, for example, have dimensions of 1.5 ω x 1 m x 0.75 m (1 x w x h). Modular elements having such dimensions can be manufactured relatively easily as separate products and be sited as separate elements in an apparatus according to the invention. Such modular elements can appreciably facilitate the construction of an apparatus according to the invention and the maintenance thereof. In relation to the operation of a complete apparatus, it may furthermore be remarked that, in general:

The vapour formed from the raw water in the last effect is fed to another destination within the apparatus, for example to a final condenser (not shown) which is cooled by external means and in which the said vapour condenses , or to a vapour compression apparatus in which the pressure of the vapour is increased for use as heating vapour in the first effect. - After condensation, the vapour formed from raw water is collected as a distillate by one of the methods usual for this type of system, treated further, collected and drained as product water. The facilities needed for this purpose are present in each effect (not shown) and can be assumed per se to be known from the prior art.

The drainage of product water (distillate) , the feeding of feed water to the distillation process and the drainage of sluice water (concentrated raw water), as normal for this type of apparatus, balance one another. For this purpose, in addition to facilities for the drainage of product water, facilities

(not shown) known from the prior art and usual per se are also present for feeding in feed water and for draining sluice water.

Many variants of the method and apparatus are conceivable which fall within the scope of the invention and the attached claims. Thus, it is conceivable that one effect comprises a plurality of towers of essentially identical modular elements, in which case the towers may have mutually the same orientation, shape and dimensions and in which case they are preferably sited next to and against one another in such a way that the vapour outlet openings of all the modular elements are situated in one vertical flat plane. The number of towers of modular elements within one effect will in that case be dependent on the designed flow rate of distilled water per unit time, which flow rate has been used as starting point in designing the system. Furthermore, the method and the apparatus according to the invention may also comprise one effect.

It should furthermore be pointed out that the various towers of modular elements, each tower belonging to one effect, are shown in Figure 1 as leaning against one another by means of buttressing projections 60, 61. The loading due to the pressure difference which exists over the tower(s) in one effect is, in each effect, directed towards the low- pressure side. The total horizontal loading on a tower in the low- pressure direction therefore increases as a result of accumulation. Where this total horizontal loading becomes too great, the towers can be buttressed against vertical columns (not shown) associated with the housing structure, for example its roof structure, instead of against one another.

In the case of the multiple-effect distillation apparatus according to the invention, it is possible also to allow the feeding of raw water to the effect and the drainage of raw water from the effect to take place at one or more intermediate levels , in which case use can be made for each intermediate level of collecting spaces 13 situated between two stages of an effect. As a result, a saving can be achieved in the pumping power needed for circulating raw water.

Circulation of raw water may take place from one effect to the same effect or to another effect.

Claims

1. Method for desalinating water, in particular sea water, by a single-effect or multiple-effect distillation process in which raw water to be treated is fed to the one effect or successively to each effect in order to be at least partly evaporated by film evaporation in the one effect or each effect at a pressure level which is essentially constant for each effect, and in which the one effect or each effect comprises at least one channel system having channels which extend in an inclined or vertical manner and have flow from the top downwards, which are heated and through which the raw water is passed in such a way that there is formed along the channel walls a film of raw water which at least partly evaporates during the passage through the channel system, characterized in that, in the one effect or each effect, the evaporation takes place stage-by-stage in an abovementioned channel system for each stage and in that, between two successive stages, vapour formed in the first of said two successive stages (or in stages prior thereto) is removed from the effect.

2. Method according to Claim 1, characterized in that a collecting space is situated between the channel systems of two subsequent stages and in that vapour formed in the first of said two successive stages (or in stages prior thereto) is removed from the effect via said collecting space situated between two successive stages, preferably to a subsequent effect, if present, and in that the residual raw water from said collecting space situated between two successive stages is passed on to the channel system of the second of the two successive stages.

3. Method according to Claim 1 or 2, wherein the vapour removed from an abovementioned collecting space is passed through a purification apparatus, such as a droplet trap.

4. Method according to one of the preceding claims , wherein the vertical height of the at least one channel system is less than 1 metre, preferably between 20 and 60 cm.

5. Method according to one of the preceding claims, wherein the at least one channel system comprises a bundle of parallel tubes arranged with mutual spacings.

6. Method according to Claim 5. wherein the diameters of the tubes are in a range of less than approximately 5 cm, preferably in the range from 1 to 4 cm.

7. Method according to one of the preceding claims, wherein the vapour removed from the collecting spaces associated with one effect is brought together and distributed over the stages of the subsequent effect and used there to heat the channel system associated with said subsequent effect.

8. Single-effect or multiple-effect distillation apparatus comprising for each effect a process space essentially sealed in a pressuretight manner with respect to the environment having an inlet for raw water debouching at the top, a drain for unevaporated residual raw water debouching at the bottom, at least one drain for vapour formed and preferably at least one inlet for heating vapour preferably originating from a previous effect and utilized as evaporating heat, at least one channel system of channels extending in a vertical or inclined manner being provided per effect, characterized in that each process space is subdivided into a number of successive stages having an abovementioned channel system for each stage and in that removal means are provided between the channel systems of two successive stages for removing vapour from the effect, essentially formed in the first of two successive stages.

9- Single-effect or multiple-effect distillation apparatus according to Claim 8, characterized in that the channels of each channel system debouch at their bottom into a collecting space for residual raw water, vapour formed and, preferably, any incondensable gases and/or uncondensed heating vapour, and in that each collecting space situated between two stages is provided with a discharge for vapour to be collected therein and discharge/distribution means separate therefrom for distributing residual water, if present, over the subsequent channel system, the discharge/distribution means being designed in such a way that the constancy of the pressure prevailing during operation in a process space is essentially undisturbed.

10. Single-effect or multiple-effect distillation apparatus according to Claim 8 or 9, wherein the stages are disposed one above the other and wherein the inlet mouths of the channel system of each subsequent stage open into the collecting space of the respective preceding stage.

11. Single-effect or multiple-effect distillation apparatus according to one of Claims 8-10, wherein the vapour discharges are provided with a purification device, such as a droplet trap.

12. Single-effect or multiple-effect distillation apparatus according to one of Claims 8-11, wherein the vertical height of each channel system is less than 1 metre, preferably 20 to 60 cm.

13. Single-effect or multiple-effect distillation apparatus according to one of Claims 8-12, wherein each channel system comprises a bundle of parallel tubes arranged with mutual spacings.

14. Single-effect or multiple-effect distillation apparatus according to Claim 12, wherein the diameters of the tubes are in the range of less than approximately 5 cm, preferably in the range of approximately 2 to 4 cm.

15. Single-effect or multiple-effect distillation apparatus according to one of Claims 8-14, wherein the process space of each effect comprises a number of essentially identical towers, each tower being built up of preferably essentially identical modular elements placed one on top of the other and the number of modular elements stacked one on top of the other in a tower being equal to the number of stages in said process space.

16. Single-effect or multiple-effect distillation apparatus according to Claim 15, wherein the modular element in this arrangement comprises a frame having a base plate and a top plate between which a channel system, preferably a bundle of parallel tubes arranged with mutual spacings, extends, and wherein the inlets of the channel system on the top side of the top plate are open and the outlets thereof on the bottom side of the base plate are open, and wherein the modular element on the bottom side and top side are provided with mutually complementarily formed circumferential edge parts which are designed in such a way that there forms between the channel systems of two modular elements stacked one on top of the other a collecting space which is bounded by the complementarily formed circumferential edge parts, the base plate of the uppermost modular element and the base plate of the lowermost modular element.

17. Single-effect or multiple-effect distillation apparatus according to Claim 16, wherein the frame of the modular element is manufactured from a plastic, preferably as a moulded product and wherein the channel system is manufactured from a metal.

18. Single-effect or multiple-effect distillation apparatus according to one of Claims 16-17, wherein the (condensation) space formed between the top plate and base plate is preferably in gas communication with the collecting space to be formed underneath the base plate.

19. Single-effect or multiple-effect distillation apparatus according to one of Claims 16-18, wherein the modular element is provided with a purification device, such as a droplet trap.

20. Modular element suitable for a single-effect or multiple-effect distillation apparatus according to one of Claims 16-19 •