FIELD OF THE INVENTION

The invention relates to a membrane distillation system comprising a heat transfer means, at least one membrane distillation module and a condenser, which distillation modules are pressure -wise coupled in series, such that each subsequent module operates at a lower pressure than a preceding module, wherein said at least one membrane distillation modules comprises:

A liquid channel with a fluid entry and a fluid exit and a hydrophobic, vapor permeable membrane through which evaporated liquid may pass; and

- a vapor space adjacent to the liquid channel and separated therefrom through the hydrophobic, vapor permeable membrane, through which evaporated liquid may pass into the vapor space, which vapor space further comprises a condensation wall and a distillate exit for a distillate generated by means of condensation of the evaporated liquid.

The invention further relates to a membrane distillation process, wherein such a membrane distillation system is used for water purification, for air conditioning and/or for other liquid purification applications, such as the removal of water from ethanol.

BACKGROUND OF THE INVENTION

The use of membrane distillation for desalination of water has already been investigated in the 1980s. EP0094543A2 discloses a membrane distillation system as defined hereabove, and discloses a series of graphs in its Figures 11 to 18, in which the volume of distillate is shown as a function of temperature and underpressure. While the primary embodiment shows a single distillation module, it is clear from f.i. Fig. 2 that a plurality of membrane distillation modules may be put in series.

Further elaborations of said basic principles are known from WO-A 2005/089914, WO-A

2007/054311 and WO-A 2010/0127818. WO-A-2005/089914 discloses in its Fig. 2 an embodiment of a series coupling of several distillation modules. The first of said distillation modules mere generates vapour, which is used as a heat source in the second distillation module for the needed evaporation from the feed liquid, and then converted into distillate. The second distillation module again generates new vapour that is used as a heat source in the third distillation module and thus condensated into distillate. The vapour resulting from the third distillation module is condensated in a separate condenser module, particularly a cooler in the form of a heat exchanger. This type of membrane distillation with a sequence of distillation module, that are operated at an increasing underpressure (thus from high to low), is also known as vacuum membrane distillation. In the embodiment of WO-A-2005/089914, a plurality of liquid channels and vapor spaces is put in parallel within one distillation module, so as to increase the effective surface area. This is moreover an effective design for heat exchange; a single vapour channel is present between two liquid channels, thus minimizing heat losses to the environment. If the liquid channel is designed to be narrow, the feed liquid can easily fill the liquid channel, leading to maximation of the effective surface area.

An effective implementation providing such narrow liquid channels hereof is disclosed in WO-A 2010/0127818, in the form of a weldable and stackable frame, in which the required channels within one distillation module are defined. If desired, fluid and/or vapour connections between subsequent distillation modules may be implemented into the stack of frames. Each frame may be provided with foils chosen from a.o. hydrophobic membranes and condensation walls. Hence, a liquid channel is defined with a frame provided with a condensation wall foil on one side, and a hydrophobic membrane foil on the opposed side. As a result hereof, the system may have a membrane distillation module and a condenser that are integrated into one physical system. Under pressure channels are moreover implemented herein for ensuring the setting of the intended underpressure through the complete system.

WO-A 2007/054311 discloses furthermore an arrangement for controlling the underpressure in the series of distillation modules, and more particularly such that the full system runs at an underpressure. Thereto, an underpressure pipe system is shown extending to the vapour spaces of the membrane distillation modules, a condenser module as well as the preceding heat transfer means with a separate heat generator. Moreover, several underpressure control means in the form of U-tubes (typically siphons) are present, i.e. in the feed line, in the system's distillate exit and in the brine exit, , which further comprises a brine pump.

As shown most clearly in Fig. 5 of WO2007/054311 , the cooling liquid is driven with a pump through a cooling channel of a separate condenser. This cooling channel is separated through a condensation wall from an adjacent vapour channel, in which distillate is generated. While the feed and the distillate are held at an underpressure, the cooling channel is at atmospheric pressure. The known implementation has the disadvantage that the condenser needs to be a separate entity made in another technology than the preceding distillation modules. Foils that are typically used as membranes, or as condensation walls, may be damaged or torn, if a pressure difference over the foil exceeds a maximum value. The combination of a vapour channel at deep underpressure and a cooling channel at atmospheric pressure is highly sensitive to such damaging, particularly because the pressure difference is maintained over a long period and because variations in the pressure in the vapour channel may occur, for instance during start up, but also otherwise unexpectedly, such as due to power interruptions (and therewith changes in the pressure), malfunctioning of vacuum pumps, ageing of the system, or for any other reason. It would be however desirable to integrate the condenser with the preceding distillation modules, for instance on the basis of the technology shown in WO2010/127818. Integration of modules into an assembly leads to shorter connections between the channels in adjacent modules, i.e. the connection of the liquid channels, and the connection of vapour space and vapour channel. As a result, underpressures may be defined and controlled better, which significantly increases the yield of the system.

One possible solution is that the cooling channel is made part of a closed system of cooling liquid. The cooling liquid runs then in a cycle between the condenser and a separate heat exchanger. In this configuration, the cooling liquid in the cycle can be held at a predefined underpressure and be driven with a pump. Such a solution however results in additional components and complexity.

Moreover, a closed system of liquid requires regularly inspection, so as to see whether the level of liquid is sufficient. This is particularly true when the membrane distillation system is used outside in a hot climate. SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an improved system and an improved membrane distillation process, such that the risk of damage to the condenser module is lowered considerably without an increase in complexity and cost of the system.

According to a first aspect of the invention, a membrane distillation process is provided, wherein distillate is generated from a feed by means of evaporation to vapour in a first liquid channel, which vapour is removed through a membrane and subsequently condensed in a vapour channel, wherein heat transfer occurs from the vapour channel to any subsequent liquid channel, and wherein the evaporation of feed in the first liquid channel occurs upon heat transfer from a heating channel in which a vapour flow condenses. In accordance with the invention, a - second - underpressure is applied to an outlet of the cooling channel, so that the cooling liquid is sucked through the condenser module by means of a pressure difference between an inlet and the outlet of the cooling channel of the condenser module.

According to a further aspect of the invention, a membrane distillation system is provided in which the said membrane distillation process can be run, so as to convert a feed into distillate and non- evaporated, concentrated feed, under application of an underpressure. The distillate is generated in at least one membrane distillation modules and a condenser module, which comprises a vapour channel for receiving said vapour, a condensation wall, and a cooling channel through which, in use, a cooling liquid flows. The cooling channel constitutes part of an open cooling system, in which cooling liquid flows in use from an inlet to an outlet under suction by means of a pressure difference, wherein the outlet is held in use at a - second - underpressure and wherein a pumping means is present for pumping cooling liquid at the outlet from the second underpressure to a higher pressure.

The present invention removes the need for separate pumps and reduces the vulnerability of the foil-based condensation wall. Rather operating the cooling system at constant pressure, the inventors have understood that the cooling liquid should be flowing under the driving force of suction, e.g. towards an underpressure that is applied to the outlet of the cooling channel. In this manner, the cooling liquid flows in an open system.

Use of an open system for the cooling liquid has important advantages, in that the components, but also the cooling liquid may be integrated in the rest of the system, i.e. be reused. The cooling liquid may for instance be reused, at least partially, as a feed source. It may alternatively or additionally be used for diluting the non-evaporated and concentrated feed flowing from the last membrane distillation module - the latter also known as brine.

For sake of clarity, the cooling liquid that has passed the cooling channel of the condenser module will be referred to as a warmed-up cooling liquid. Generally, the cooling liquid will indeed be warmed up in order to absorb the heat from the condensation wall. However, there may be conditions, such as during start up that there is no or no significant warming up of the cooling liquid. The term 'warmed-up cooling liquid' is nevertheless used in general.

The underpressure applied to the cooling channel could be applied directly from a vacuum pump, which could even be an independent vacuum pump. However, it appears beneficial that a single vacuum pump defines setting the underpressure throughout the system. It appears even more beneficial that the pressure in the cooling channel is set indirectly. This allows that any pressure variation in the system is also transmitted to the cooling channel, so as to harmonize the system. For sake of clarity, it is added that the term 'vacuum pump' is herein used to refer to a pump that is suitable to provide a relatively low pressure, such as for instance 100 mbar or lower.

In one preferred embodiment, any non-evaporated and concentrated feed, i.e. brine, is transferred to a low-pressure brine collector vessel. Furthermore, at least part of the cooling liquid is transferred into the brine collector vessel after passing the cooling channel of the condenser module. The combination of the brine with the cooling liquid results in a less concentrated brine that is reduced in temperature and typically has a higher vapour pressure. As a result, the brine may be pumped away more easily, and particularly with a lower risk of cavitation and the negative effects thereof on pump performance and lifetime. In one specific embodiment, use can be made of continuous or semi-continuous pumping, and more specifically using a frequency-controlled pump. This prevents or at least significantly reduces the number of times that the pump need to be started, which is a most sensitive moment for the generation of cavitation. Moreover, no separate vessel for cooling water is needed, nor a separate pump coupled to such a separate cooling water vessel for pumping the cooling water from the low pressure to higher pressure, such as atmospheric pressure. Particularly in this embodiment, the underpressure may thus be communicated to the brine collector vessel, particularly from the vapour channel of the condenser module. The

communication of this low pressure along the path of the cooling liquid will then result in the pressure difference over the cooling channel of the condenser module, which provides a driving force for the cooling liquid. This coupling to a brine collector vessel has turned out to be a suitable manner of limiting the pressure difference over the suitably foil-based condensation wall of the condenser module.

If the cooling liquid is coupled to the brine collector vessel, suitably a protection device is present for preventing flow of liquid from the brine collector vessel into the vapour channel of the condenser during a current interruption. Such a protection device is for instance implemented as a closing valve at the exit of the cooling channel, or as a closing valve of the said first connection. The flow into the vapour channel would be detrimental for the distillate quality. Moreover, the provision of typically salt brine into a vapour channel for distillate may well result in a contamination that is difficult to remove. The flow into the vapour channel may result through the pressure communication means between condenser module and brine collector vessel, if an overflow of the brine collector vessel occurs.

In an alternative embodiment, the warmed-up cooling liquid does not flow into a brine collector vessel. Separate pumping means are present. This embodiment is suitable for large systems requiring a large flow of cooling liquid. Small systems are for instance systems with a distillate output of up to 0.1 m ³ /hour or 0.2 m ³ /hour or even up to 0.5 m ³ /hour. Larger system are for instance systems of at least 0.3 m ³ /hour, suitably at least 0.5 m ³ /hour and preferably at least 1.0 m ³ /hour. Small systems are intended for individual households or groups of households. Larger systems are intended for industrial use and/or for facilities for a complete community, such as a village, institute, hotel, city, farm.

In one specific implementation, the pumping means for the warmed-up cooling liquid comprise a first pump for pumping from the underpressure to an overpressure, and a second pump of a jet pump type. The benefit hereof is that the jet pump may be suitably used as the system's vacuum pump, i.e. it may operate as a cryogenic pump.

The use of jet pumping has furthermore been found very beneficial, in that liquid with a temperature higher than 20 °C, particularly with a temperature of more than 25°C up to 40°C or even 50°C may be used as cooling liquid. This was a rather unexpected finding, with a major benefit, since cold water is not available in many tropical locations in need for clean water. Even seawater in tropical locations is typically higher than 25 °C. The jet pump may additionally be used for pumping the brine to atmospheric pressure. It will be understood that two or more jet pumps may be coupled in parallel to each other and in series with the first pump, one as the system's vacuum pump and the other as the brine pump. A further advantage of using the jet pump as the brine pump is that no intermediate brine collector vessel is needed: the brine may directly be transported, as a continuous flow, to the jet pump.

Most suitably, an arrangement is made wherein a first and a second jet pump are combined, both of which are fed with cooling liquid. Moreover, brine is added to the first jet pump, while the system's underpressure is taken from the second jet pump. Herein, a pressure communication line is coupled between said second jet pump and at least one module, suitably the condenser module. In an even further implementation, a bypass line is provided around said jet pump, more particularly between the first pump and an outlet for the cooling liquid, such as a tank or open water, such as sea, canal, river or lake. The bypass is suitably provided with a valve that is controllable, but such valve may be left out after system optimization and be replaced by any means defining a flow through said bypass

Suitably, part of the warmed-up cooling liquid is used as a source for feed. Thereto, in one embodiment, the open system of cooling liquid comprise a network with a first connection to waste, for instance the brine collector vessel, and a second connection to a feed inlet of the system. Regulating means are present for setting a flow ratio between the first connection and the second connection. The regulating means are for instance embodied as a valve or restriction, and are more preferably arranged in the first connection, i.e. between a split point and the brine collector vessel. Since the pressure difference extends to the brine collector vessel, the regulating means may be embodied quite simply.

The heat transfer means of the present invention could be provided with a heat generator, such as known from WO2007/054311A1. Herein a separate circuit is provided with which steam is generated that is condensed in the heating channel and then recirculated. The feed then enters the assembly of the heat transfer module, the membrane distillation modules and the condenser module in the first membrane distillation modules. It is not excluded, when using such a separate circuit that the heat exchanging fluid is a fluid different from water.

Alternatively, heat is added to the feed. Feed then enters the heat transfer means, and produces sufficient steam in the heating channel so as to enable heat transfer to the adjacent liquid channel of first distillation module. The architecture of the present invention already supports this embodiment, in that the cooling liquid is both warmed up and decreased in pressure. It therewith can easily be pretreated for use as a feed. More preferably, feed is pretreated such that a two-phase feed is obtained. This has turned out to increase the system' s efficiency significantly. Moreover, the distillate from this heating channel in the first distillation module may be added to the distillate output.

Preferably, the heat transfer means are embodied as a heat transfer module. Such a heat transfer module may be integrated with the one or more distillation modules into an assembly. The heat transfer module more suitably operates as a distillation module, in that steam is separated from liquid feed and transferred to a vapour channel adjacent to the liquid channel of the first distillation module. A condensation wall is present between said vapour channel and said liquid channel for optimum heat transfer.

Alternatives to the use of a heat transfer module are by no means excluded. Steam could be injected directly into the vapour channel adjacent to the liquid channel, i.e. as supplied via a steam inlet. Furthermore, the heat transfer means could be embodied as a unit combined with a heat exchanger and to be located adjacent - either above, below or laterally adjacent- to one or more of the distillation modules. Preferably such unit is located substantially below one or more distillation modules, such that steam resulting therefrom is easily coupled into the said vapour channel.

Rather than, or in addition to, taking the flow through the above mentioned second connection as the feed source, the liquid in the brine collector vessel could be used as a feed source. The advantage of using the liquid from the brine collector vessel is its lower pressure and higher temperature. However, the concentration of salts may be somewhat higher.

Further alternatives for generating heat to the heat transfer module are not excluded. A

combination of both mentioned embodiments is not excluded either, for instance in that the cooling liquid is used as a feed source, and is optionally pretreated, but that additional steam is injected into the feed prior to and/or upon entry of the heat transfer module.

The membrane distillation system of the invention most suitably comprises a physically integrated modular assembly of the membrane distillation modules and the condenser module, and optionally any heat transfer module. More preferably, use is made of a single frame that defines the various channels. The functions or condensation wall and membrane are therein defined by means of application of dedicated foils. Such a modular frame defining various channels, including a liquid channel, a pressure connection, an inert gas channel, a distillate channel is for instance known from WO- A 2010/127818, which is herein included by reference.

Rather than a single physically integrated modular assembly, a plurality of such assemblies may be present. This is particularly feasible in the context of the invention, as several voluminous supplementary components such as vacuum pumps, recirculation lines of a steam riser circuit, siphons are no longer necessary. Instead, a plurality (for instance two or three or four) physically integrated modular assemblies may be stacked on top of each other. It will be understood that further components such as a pretreatment module and means for setting the underpressure may be common to a plurality of such assemblies.

The means for applying an underpressure to the condenser in the present invention suitably comprise a vacuum pump as well as any connections between such vacuum pump and the condenser module. The connections may extend to both the vapour channel and the cooling channel in the condenser module. Alternatives are envisagable, such as a connection to a pipe to or from the cooling channel rather than the cooling channel itself; a single connection to the condenser module with an appropriate pressure equilibration connection between vapour channel and cooling channel.

Preferably, the underpressure in the brine collector vessel is derived from the underpressure in one of the modules of the system, more preferably in vapour channel of the condenser module, by means of pressure communication means.

In a further embodiment, a protection device is present for preventing overflow of the brine collector vessel in case of current interruption. An overflow of the brine collector vessel would result in flow of brine into the vapour channel through the pressure communication means. Such entrance of brine in the vapour channel inevitably leads to contamination of the distillate, and of a contamination of the vapour channel that is difficult to remove. A first embodiment of the protection device is an electric normally closed valve (V2). Another embodiment of the protection device is is a hydrophobic means installed in the pressure communication means (i.e. pressure line) between brine vessel and condenser. Examples of such hydrophobic means are for instance filters, membranes, but also coatings. Suitable results were obtained with a filter with a pore size of 0.1- 0.4 microns turned out suitable.

Rather than a single distillate output, more than one output could be present, allowing different portions of the distillate to be collected separately. In a single distillate output, all distillate exits in the vapour channels are typically coupled to one distillate channel leading to a single distillate collector. In a system with a plurality of distillate output, more than one of such distillate channel and such distillate collector is present. Most suitably, a vapour channel has a single distillate exit to one of the distillate channels.

The system of the invention is suitably a small scale or medium scale system, and is for instance used for household applications for drinking water and the like. For household applications, a volume of water in drink water quality of 100 liter per day (1-10 liter per hour) may be sufficient.

For larger applications, a throughput of 70 liters per hour is feasible, which may well be increased up to for instance 100-150 liter per hour.

The system suitably comprises rather conventional supplementary elements, such as vessels for storage, pumps for throughout of brine and distillate and filters and/or sieves for removal of particulates, biological organisms and the like.

Most suitably, the generated distillate is stored at reduced pressure, particularly a pressure lower than atmospheric pressure. The advantage of storage at reduced pressure is that risk of

contamination of the distillate with bacteria and/or other microbiological contamination is reduced. Furthermore, such a system is preferably provided with reservoirs at different temperatures, so as to provide hot water for f.i. cooking, cold water for f.i. drinking and medium temperature water for f.i. washing, cleaning and the like. Such reservoirs with different temperatures are not merely beneficial for a user, but also, particularly in a hot and sunny environment, it tends to cost time to cool the distillate to a desired temperature. Hence, means and flows may be designed so as to ensure that water of a desired temperature is generated in a sufficient quantity. Thermostats for setting and controlling the desired temperature in a reservoir may be present.

According to a further aspect of the invention, that is most suitably combined with the preceding embodiments, the system comprises a brine collector vessel into which, in use, non-evaporated and concentrated feed from the liquid channel of a membrane distillation module flow. The brine pump is a frequency controlled pump for continuous pumping of the liquid out of the brine collector vessel, when the pump is in a pumping mode.

More particularly, the membrane distillation system according to this aspect of the invention comprises at least one membrane distillation module and a condenser module, in which, in use, a feed is converted into distillate and brine, which distillate is generated in the at least one membrane distillation modules and the condenser modules, and which brine flows at least from a last membrane distillation module that precedes the condenser module into a brine collector vessel, wherein a brine pump is present for pumping liquid from the brine collector vessel to a higher pressure,

Wherein the brine pump is a frequency controlled pump for continuous pumping of the liquid out of the brine collector vessel, when the pump is in a pumping mode.

According to again a further aspect of the invention, a method is provided comprising the steps of guiding non-evaporated feed from the last module to a brine collector vessel, and pumping the liquid out of the brine collector vessel occurs continuously, when the pump is in a pumping mode. Frequency-controlled pumping allows continuous or semi -continuous pumping.

More particularly, this method comprises the steps of:

- generating distillate from a feed by means of evaporation to vapour in at least one membrane distillation module and a condenser module to which vapour of a preceding, last membrane distillation module is transferred,

- removing non-evaporated, concentrated feed as brine at least from the last membrane distillation module to a brine collector vessel, and

- pumping the liquid from the brine collector vessel to a higher pressure,

wherein the pumping the liquid out of the brine collector vessel occurs continuously, when the pump is in a pumping mode.

The use of frequency-controlled pumping significantly reduces the effects of cavitation, which most strongly occur upon switching on a pump. As known per se, for instance from the field of marine engineering, cavitation is due to the phenomenon that vapour bubbles are formed and grow locally due to acceleration in the water inlet. The thermodynamically instable bubbles collapse after a short time, causing shock waves to occur. This cavitation reduces pump performance significantly and it reduces pump lifetime dramatically. Cavitation was found, in experiments, leading to this aspect of the invention, to make the brine pump that operates on a low pressure vulnerable.

The frequency control is preferably coupled to a sensor detecting a volume in the brine vessel (which may be a tank or a tube). Upon a detection of a volume reduction, the frequency of pumping is reduced, and vice versa. It will be understood that rather than detecting volume, use may be made of a system detecting a flow rate into the brine vessel. Sensors for volume detection, typically level detection, are known per se and suitably provide an electrical signal to a controller. Examples include mechanical-electrical sensors, optical sensors, capacitive sensors, magnetic sensors.

Frequency-controlled pumping is moreover useful in the context of the present system, because a variation in the brine flow is expected: the brine flow during steady-state production is likely to be significantly less than during start up. Typically, the reduction may be in the order of 30-70%. This aspect of the present invention is particularly suitable for systems with a relatively small distillate output of less than 1 m ³ per hour, such as 0.1, 0.3 or 0.5 m ³ per hour. Such systems are deemed suitable for a single household, or a couple of households. Use of drinking water of a single average household in the Netherlands was, in 2008, nearly 0.4 m ³ drinking water per day. Such a capacity may be easily generated with a system having an output of 0.1 m ³ per hour.

The pumping in accordance with the invention is more preferably carried out in one of at least three modes; in a first mode, the pump is off. In a second mode, the pump operation is frequency controlled. In a third mode, the pump operates at maximum frequency.

For specifying the pumping mode, and an appropriate pumping frequency in the second mode, suitably the level or the inflow of the brine collector vessel is sensed with one or more sensors.

Suitable sensors are level sensors, and alternatively flow sensors. Most suitably, use is made of sensors for sensing a minimum level and an upper limiting level. The upper limiting level need not be the absolute maximum, but a limiting value above which another operation, for instance at maximum flow speed, is obtained.

Rather than relying on sensors only, the control may be operated via an algorithm in a controller, wherein a flow into the brine collector vessel is estimated on the basis of input parameters, such as the feed flow and optionally flow of cooling liquid.

The use of a brine collector vessel moreover has the advantage of buffering, and, in case of combining cooling liquid and brine, to obtain adequate mixing. Particularly, in one suitable embodiment, the brine originates from the last distillation module, whereas the pressure in the brine collector vessel is taken from the condenser. The brine flow will thus be at a higher pressure than the inflow of warmed-up cooling liquid. Experiments have shown that artefacts occur, in this embodiment, when a brine collector vessel is left out.

In a most suitable embodiment, the brine flow is injected into the brine collector vessel at a position lower than the inflow of warmed-up cooling liquid, and more preferably below a usual upper level of the liquid in the brine collector vessel. Due to a tendency of warm liquid to rise, as since the brine is warmer than the cooling liquid, a tendency to countercurrent flow between the brine and the cooling liquid inside the brine collector vessel may occur. Such flow behaviour is very good for mixing of the two flows. Moreover, the brine has a tendency to evaporate, particularly in view of the lower pressure in the brine collector vessel. By injecting it below the cooling liquid, evaporation of brine is suppressed. Too much evaporation of brine might cause problems in the vacuum lines of the system.

According to another aspect of the invention, the invention relates to a vacuum membrane distillation process, wherein use is made of cooling liquid of elevated temperature, without giving rise to process instability and/or inherent process termination. According to this method, use is made of a pumping configuration that is feasible to pump away cooling liquid and moreover to apply vacuum (i.e. underpressure of sufficient extent, for instance less than 100 mbar) to the used membrane distillation system.. The elevated temperature of the cooling liquid is more suitably a temperature of at least 25 °C. Preferably, the method involves pumping the said cooling liquid from an underpressure to an relative to atmospheric pressure, which underpressure results in flow of cooling liquid through a cooling channel, and thereafter accelerating said cooling liquid. The acceleration is preferably achieved in means that operate on the basis of the outlet flow and pressure of pumping means used in the pumping step. Preferably, this outlet flow and pressure is such that is constitutes a pressure relative to atmospheric pressure. The accelerating step suitably is made to result in cooling of the cooling liquid that has warmed up during use. The accerelating step furthermore is arranged to apply the system's vacuum. More particularly, a jet pump is used in the acceleration step. This jetpump is advantageously driven on the basis of the outlet pressure and flow from the said pumping means.

More particularly this aspect of the invention relates to a membrane distillation process comprising the steps of:

generating distillate from a feed by means of evaporation to vapour in at least one membrane distillation module and a condenser module to which vapour of a preceding, last membrane distillation module is transferred, said condenser module being provided with a flow of cooling liquid;

- removing non-evaporated, concentrated feed as brine at least from the last membrane distillation module; pumping said cooling liquid by means of a series arrangement of a first pump and a jet pump.

Preferably, this process is a vacuum membrane distillation process, wherein an underpressure is applied to any one module, and suitably the condenser module, and spread over the modules by means of pressure communication. More preferably, this underpressure is applied via a pressure line ending up at the jet pump. Even more preferably, use is made of a plurality of membrane distillation modules.

In a further implementation, the series arrangement comprises a first pump and a parallel arrangement of at least a first jet pump and a second jet pump. More preferably, the first jet pump is also used for pumping the brine, and then second jet pump is used for the said application of the system's underpressure. This implementation is particularly suitable for generation of potable water, wherein the brine is not valuable. Furthermore, the parallel arrangement may further include a bypass, particularly a bypass with a valve.

Herein, the first pump suitably sucks the cooling liquid through a cooling channel of the condenser module and pumps it up to a pressure about atmospheric pressure.

Specific embodiments discussed with respect to one aspect are also deemed applicable to another aspect of the invention, unless clearly contradictory. This further holds for the embodiments defined in the dependent claims BRIEF INTRODUCTION OF THE FIGURES

These and other aspects of the invention will be further elucidated with reference to theFigures, which are purely diagrammatical and not drawn to scale, and wherein:

Fig .1 shows diagrammatically a general architecture in accordance with the prior art as shown in WO2005/089914A1 ;

Fig. 2 shows a schematical view according to one embodiment of the invention;

Fig. 3 shows a view corresponding to that of Fig. 2, but indicating the pressure operation of the system;

Fig. 4 shows a more detailed schematical view specifying sensors and valves for one embodiment, Fig. 5 shows a more detailed schematical view specifying sensors and valves for an alternative embodiment;

Fig. 6 shows a more detailed schematical view for a third embodiment, and

Fig. 7-9 show schematic views similar to those of Fig. 4-6 for further embodiments comprising a jet pump.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS The Figures are not drawn to scale and are intended purely for illustrative purposes. Equal reference numerals in different figures refer to equal or corresponding parts.

Fig .1 shows diagrammatically a general architecture in accordance with the prior art as shown in WO2005/089914 Al. As shown herein, the membrane distillation system 100 comprises a heat transfer module 40, a first membrane distillation module 14, a second membrane distillation module 16 and a separate condenser 42 in the form of a heat exchanger, particularly so as to preheat the feed. As is indicated in the Fig. 4, 5 and 6, the said modules 40, 14, 16, 42 are suitably physically integrated in a modular assembly 10. Cross-connections for pressure, distillate and connections between vapour channel and vapour spaces as well as between the liquid channels may be implemented herein. This integration is deemed beneficial so as to keep heat within the unit and to reduce pressure leakages as much as possible.

Rather than a heat transfer module 40 suitable for integration into the modular system, any other heat transfer means could be used for the provision of heat in the form of condensable vapour, particularly steam. More particularly, the heat transfer means constitute a steam generator for generating steam. Source of the steam may be a separate source, for instance a circulating hot water circuit, but alternatively the feed itself. The heat transfer means is further also referred to as a vapor generator module.

Liquid, i.e. particularly an aqueous solution or a fluid mixture, enters the system via a liquid line 7. It is optionally preheated in a heat exchanger 34, but still at atmospheric pressure. Thereafter, it enters the vapor generator module 40 at the fluid entry 8. Here the fluid runs in a liquid channel 12 and evaporates under the influence of the available heat, and subsequently leaves this vapor generator module 40 at the fluid exit 9 as a concentrated fluid. The fluid is thereafter led via a liquid connection 19 to the first membrane distillation module 14, which it enters at the fluid entry 8, runs through in liquid channel 12 and leaves at the fluid exit 9 as a further concentrated fluid. In order to ensure that distillation occurs, the pressure in the first membrane distillation module 14 is lower than that in the vapor generator module 40. Subsequently, the fluid is led through a liquid connection 19 to the second membrane distillation module 16, which it enters at the fluid entry 8, passes through liquid channel 12 and leaves through the fluid exit 9 as an even further concentrated fluid. This fluid is also known as brine and removed via brine exit 39. The brine is in fact present at atmospheric pressure, rather than at an underpressure.

Vapor is generated by means of distillation of the fluid. The vapor enters a vapor space 23 that is separated from the liquid channel 12 by means of substantially fluid-impermeable membranes 20. The substantially fluid-impermeable membranes 20 are particularly hydrophobic membranes. Evidently, if the fluid is non-aqueous but rather an organic liquid, the fluid-impermeable membranes will be chosen to be impermeable for said organic fluid. The generated vapor is led from the vapor space 23 to the vapour chamber of a subsequent module - in casu the first distillation module 14 - via vapour connection 29 and arrives in the vapour channel 21. The vapour channel 21 is provided with at least one condensation wall 24. The vapour will condensate at this condensation wall 24 and be converted into distillate. Simultaneously, heat generated in the condensation process is transferred to the liquid channel 12, which is located adjacent to the condensation wall 24. In the present embodiment, the vapour channel 21 is bound by two condensation walls 24 on opposite sides. While being advantageous, this is not strictly necessary. Liquid evaporating from the liquid channel 12 in the first membrane distillation module 14 enters the vapor space 23 through said hydrophobic membranes 20, and flows through vapour connection 29 into the vapour channel 21 of the second membrane distillation module 16. Liquid evaporating from the liquid channel 12 in the last membrane distillation module - in this embodiment the second membrane distillation module 16 - enters the vapor space 23 through hydrophobic membranes 20, and flows through vapour connection 29 into the condenser unit 42. It is therein converted into liquid and thereafter brought to a desired pressure (typically a higher pressure) by means of pump 38 (typically a vacuum pump). Instead of being led away as a separate liquid stream, the condensate from the condenser 42 may be added to a distillate collector. Distillate is collected in a distillate collector 54, from which it is pumped to a higher pressure using a vacuum pump 36. The distillate collector 54 is coupled to the distillate exits 52 of the first and second membrane distillation modules 12, 14 via a (distillate) connection 53.

It is observed for sake of clarity, that the number of fluid channels 12 in parallel within one module 40, 14, 16 may be specified on the basis of the intended distillate volume. While the number of fluid channels 12 is the same in each module 40, 14, 16 of the present embodiment, this is not necessary. While the present embodiment shows a design wherein the vapour space 23 and the vapour channel 21 are mutually coupled through an external vapour connection 29, this is not necessary. Alternative embodiments are envisageable wherein the vapour space 23 and the vapour channel 21 are merged into a "vapour channel space". In such case, the vapour channel space is suitably bounded on one side by a membrane 20 and on the opposite side by a condensation wall 24.

Fig. 2 and 3 show the general architecture in accordance with one embodiment of the present invention. Fig. 2 shows schematically the flows of liquid and vapour. Fig. 3 shows schematically the pressures in the system. For sake of clarity, details within the modules 40, 14, 16 are not shown, but are suitably corresponding to those shown in Figure 1. As shown in Fig 2, the system comprises a heat transfer module 40, distillation modules 14, 16 and a condenser 42. All these modules are suitably physically integrated into a single assembly. The drawn lines 7, 19, 39, 47, 49 indicate substantially liquid flows. The liquid feed 7 goes through the modules 40, 14, 16 and liquid connections 19. It is finally converted into brine 39. The cooling liquid 47 passes the condenser 42 and is obtained as warmed up cooling liquid 49. This warmed up cooling liquid 49 is suitably divided into a first stream 49a that is merged with the brine 39, and a second stream 49b for other purposes, for instance use as a feed 7.

The dotted line 52, 53 indicate distillate flows. In this example architecture, the distillate flow 52 from the first distillation module 14 is merged with the distillate flow 53 from the second distillation module 16 and the condenser to arrive at a distillate collector 54. However, as shown in Fig. 4-6, the distillate flow (pipe) 52 may be integrated in the modular assembly, such that merely a single distillate exit 53 is present.

Furthermore vapour lines 107, 29 are shown. In accordance with the present invention, the feed 7 is pretreated in a pretreatment module 134 to obtain a multi-phase feed, i.e. a two-phase feed comprising a feed vapour 107 and a liquid feed 7. The feed vapour 107 and the liquid feed 7 are indicated separately in this Figure 2 for sake of clarity, but may physically be provided in a single pipe.

The architecture shown in Fig. 3 corresponds to that of Fig.2. For sake of clarity, the distillate channels 52, 53 are omitted. Fig. 3 intends to represent the pressure balance in one embodiment according to the invention. Particularly, the system 100 operates at underpressure. The pressure is defined with a limited number of pumps 35, 38. The pressures within the system 10 will be set during use. The pressure build-up is controlled through a vacuum pump 38. This pump sets an underpressure F which is communicated through the system via lines 101-104. Line 101 communicates the pressure to a condenser channel of the condenser 42 (i.e. the vapour channel coupled to a preceding module). Line 102 indicates the clamping vacuum (Dutch: klemvacuum) of the system. This line 102 is a branch of the main line 101, wherein vacuum pressure is brought between the individual modules for clamping them together. Line 103 communicates the pressure from the condenser 42 to a brine collector vessel 37. A further pump 35 is coupled thereto as an output valve. A similar pump, not shown, will be coupled a distillate collector vessel. Line 104 communicates the pressure from the condenser 42 to other modules 40, 14, 16. This

communications do not imply that the underpressure is identical everywhere in the system 100. An actual pressure is obtained as a dynamic equilibrium on the basis of temperature, actual amount of vapour in dependence of flow rate and evaporation rate plus condensation rate.

The membrane distillation processes resulting in evaporation of feed 7, 19 result in a pressure difference between each module 40, 14, 16. The pressure E at the entrance of the second distillation module 16 is thus higher than the pressure F in the brine collector vessel 37. The pressure D at the entrance of the first distillation module 14 is again higher than the pressure E. The pressure C at the entrance of the heat transfer module 40 is again higher. For instance, the pressure C is 0.4 bar, pressure D is 0.3 bar, pressure E is 0.2 bar and pressure F is 0.1 bar. In accordance with the invention, the feed is pretreated in a pretreatment module, so as to obtain a two-phase feed 7, 107. During this pretreatment, the feed is suitably reduced in pressure from pressure B to pressure C. Pressure B is for instance 0.7 bar. This pressure B is also available at the warmed up cooling liquid 49, notwithstanding the communicated low pressure F. In fact, atmospheric pressure A may exist at the inlet 47 of cooling liquid. The cooling liquid is then driven through the condenser on the basis of the existing pressure difference, in which process the pressure is significantly lowered relative to the inlet pressure A.

In the shown implementation of Fig. 4, a throttle valve VI is present, which lowers the Pressure B to a predefined maximum, for instance between 650 and 900 mbar, suitably in the range of 750- 800 mbar. This maximum setting of the Pressure B reduces a risk of damage to any foils in the condenser, more particularly any polymer foils in the condenser that are used as a condensation wall. This damaging is a risk, since the unexpected pressure differences over the foils may arise in the course of starting up and/or in case of system interruptions or failures. It will be understood that the predefined maximum may depend on the foil type in use. Furthermore, if the condensation wall were made of steel, aluminium, or a heat-conducting ceramic, the provision of a predefined maximum is not deemed necessary.

Hence, the system of the shown embodiment of the present invention may be operated with a minimum number of pumps 35, 38. Surprisingly, the stability in the system can be properly controlled, and a high distillate output may be obtained. This high distillate output is deemed due to the combination of the underpressure in the system together with the provision of a two-phase feed 7, 107 that more effectively results in the creation of a vapour flow of sufficient magnitude through vapour connection 29 to the condensation wall in the first membrane distillation module 14.

Fig. 4 shows schematically a more detailed view of a system 100 according to one embodiment of the invention, with an emphasis on all valves and sensors. The shown embodiment relates to a system with a hot water vessel 201 and a cooling water vessel 202 that are external to the system. Such a situation is for instance foreseen for use in industrial environments, or in combination with cooling or heating facilities, such as solar panels and/or caves for water disposal.

According to the shown embodiment, the feed 7 is herein pretreated to lower its pressure by means of valve V4. Heating means may be added for bringing the feed 7 to the desired temperature. The feed then passes the membrane destination assembly 10, comprising the heat transfer module 40, the condenser 42 as well as a series of membrane distillation modules 14, 16, 114, 116. The number of membrane distillation modules 14, 16, 114, 116 is open for design and typically ranges from 1 to 8, preferably 3 to 6. The distillate is thereafter removed via distillate channel 53 into a distillate collector vessel 54. The brine is removed via brine channel 39 to brine collector vessel 37. The brine 39 is merged with warmed up cooling liquid 49 that has passed the condenser 42 and was fed into the system from cooling water vessel 202 via cooling liquid line 47. The brine collector vessel 37 and the distillate collector vessel 54 are each coupled to a pump 35, 55 for pressure increase and transport. The brine is thereafter recirculated back into cooling water vessel 202 via recirculation line 205. It will be understood that alternatives are envisageable.

For an appropriate operation, a plurality of valves is provided. Valve V0 is a manual valve added for inspection purposes, in case of any leakage of vacuum (so that the desired underpressure is not reached). With this valve V0, on may find out easily, whether the leakage occurs in the assembly 10 or is related to the vacuum pump 38.

Valve VI is a protection valve. It is defined so as to set a maximum to a pressure difference over the condenser 42. Therewith, polymer foils in the condenser 42 acting as condensation walls between a vapour channel and a cooling channel are protected, so as to prevent tearing, aging and the like. Valve VI is for instance embodied as a restriction device.

Valve V2 is a further protection device for the event of any current interruption. The valve V2 prevents inflow of brine into the condenser via vacuum line 103, when for instance pump 35 does not work and hence an overflow of the brine collector vessel 37 occurs. In such case of current interruption, valve V2 will close automatically, therewith preventing any overflow of the brine collector vessel 37. This valve V2 is arranged in the line for the warmed up cooling liquid, since the cooling liquid flow tends to be larger than the brine flow. Evidently, a similar valve may be arranged in the brine line 39, if necessary for the prevention of any overflow. A further valve may be arranged so as to prevent, at least substantially, the backwards flow of warmed up cooling liquid after turning the apparatus off.

Valve V3 serves a similar function for the feed line 7.

Valve V4 is a device for setting the underpressure in the feed line 7 and therewith creating a two- phase feed 7 + 107. Suitably, this valve V4 is implemented as a throttle valve, but alternatives such as a tap or valve are not excluded. The setting of valve V4 is suitably controlled through a controller (not shown), on the basis of the sensor signals obtained. Alternatively, the setting may be carried manually, so as to ensuring sufficient heat. The setting does not need to be modified thereafter. A reset may however be done if the system suffers from scaling and/or fouling, affecting the total feed inflow, or when less heat is available.

The flow sensor F is in a highly preferred implementation arranged upwards from the means for providing an underpressure, such as valve V4. If the flow sensor were placed downwards from the valve V4, vapour bubbles tend to make the sensing more complex, or could result in an inappropriate sensing result. The latter is caused in that the creation of steam in the feed flow accelerates the mixture. Hence, a mass or volume measurement is no longer representative.

Valves V5 and V6 are used for preventing backwards flow due to the pressure difference, particularly after turning off the apparatus.

Fig. 5 shows a schematical view of another embodiment of the system 100 of the invention.

According to this system, the part 49a of the warmed up cooling liquid 49 is recirculated into the feed 7. A further part 49b of the warmed-up cooling liquid 49 is combined with the brine 39 to the brine collector vessel 37. In this embodiment, the warmed up cooling liquid 49 is pretreated by means of valve V4 and a heat exchanger 34 to become a two-phase feed 7+107. The heat exchanger 34 is provided with a separate inlet 191 and an outlet 192 for the heat flow.

This configuration suitably includes two further valves. Valve V7 is a protection device so as to prevent backwards flow of warmed-up cooling liquid 49a after turning off the system, and/or in case of any current interruption.

Valve V8 is a device with which the flow rate ratio between the flows 49a and 49b can be set. This device is suitably controlled by a system controller on the basis of the sensor measurements in the course of operation. Evidently, a manual control may be used alternatively, wherein corrections are likely to be made only subsequent to an operation run.

Fig. 4 and 5 moreover both indicate sensors in the system, more particuarly a flow sensor F, a temperature sensor T, a pressure sensor P, level sensors L and a salinity sensor S. These sensors allow investigation of appropriate product quality and control of system stability and operation. Fig. 5 furthermore shows post treatment means. Herein, the distilled water is treated to obtain potable water. A carbondioxide vessel 53 is present, from which carbondioxide may be inserted in the system using valves V9, V10. Furthermore, a marble filter 151 and a UV lamp 152 are provided so as to obtain water that meets all quality standards. It will be understood that the shown post treatment is merely an example and may be left out or replaced with alternative post treatments. It is furthermore indicated that additional filters and check valves may be present in the system, which are known per se and have been left out for sake of clarity.

It is observed that the above mentioned valves are suitable for use for protection and control of the system, and are claimed as separate features of the present invention.

In preliminary experiments with a system as shown in Fig 5, a cooling liquid input of 15 liter per minute was used at atmospheric pressure. After passing the condenser, 20% thereof was recirculated to become feed 7. This feed was heated to 70 °C and its pressure was lowered to approximately 0.3 bar. As a result, 0,8 liter per minute of distillate was obtained. The system pressure applied through the vacuum pump was 75 mbar. A further increase of the distillate output to 70 liter per hour (1,2 liter per minute) was obtained by means of optimization of the temperature and underpressure of the two-phase feed 7+107. Use was made both potable water and sea water for test purposes, which led to virtually identical results with respect to stability and product output.

A third embodiment is shown in Fig. 6. This embodiment is deemed beneficial for water treatment systems particularly suitable for the production of a larger distillate production, for instance a distillate production of at least 250 liter output per hour and more preferably at least 400 liter output per hour. In accordance with this embodiment, the cooling liquid 47 passes the cooling channel of the condenser 42 to become a warmed-up cooling liquid flow 49. Part 49a of this flow 49 is transmitted to heat exchanger 34, so as to become feed 7. Another part 49b of the warmed-up cooling liquid flow 49 is pumped away to an output by means of pump 45. A protection valve VI 1 is present so as to prevent flow in an opposite direction. The pump is suitably a pump suitable for a liquid medium.

The pressure defined on the cooling liquid flow part is 100-500 mbar, suitably 200-400 bar and preferably in the range of 250-350 mbar, for instance 300 mbar. This pressure on part 49b results, in combination with an appropriate setting for valve or restriction VI to a pressure of 600-900 mbar, suitably 700-800 mbar, for instance 750 mbar in the condenser.

No cooling liquid flows in this embodiment into the brine vessel 37. The low pressure applied by vacuum pump 38 to the condenser channel of the condenser 42 is also in this embodiment transmitted to the brine vessel 37. The resulting pressure difference between the last stage 116 and the brine vessel ensures that brine will flow into the brine vessel 37. The brine vessel 37 is herein dimensioned such that the frequency controlled brine pump 35 will operate appropriately, either continuously, or in intervals. In order to operate in intervals, a valve could be added at the bottom of the brine vessel 37. Alternatively, the pump 35 could work as a valve or a valve could be coupled thereto.

A line 105 is added between the vacuum line 101 and the outlet pipe 109 from the brine vessel. This line has a double function. First, it ensures that the low pressure applied to the condenser channel of the condenser 42 is not only applied to a top side of the brine vessel, but also to the outlet pipe 109. This leads to better pressure equilibration, and therewith increases the stability of the overall system. Secondly, the brine vessel and also the condenser itself can generate vapour that is sucked out by the vacuum lines connected to the brine vessel and condenser. This vapour tends to condense in the vacuumlines, forming water droplets that can hinder the operation of the vacuum pump. Line 105 with valve V12 acts as a trap for these droplets, releasing them into the brinestream when the brine pump is pumping. With this installation only condensed vapour can be removed; any vapour in the vacuumlines that does not condense is taken out by the vacuum pump. Valve V12 will only open when there is a water column on top of it (condens) that is higher than the water column on top of the lower level sensor inside the brine vessel. So the condens-trapping system needs to collect a certain amount of condens before it can be pumped out of the system by the brine pump.

As indicated, the embodiment of Fig. 6 makes use of a frequency-controlled brine pump. One advantage hereof is an increase of the flow rate. This helps to use a continuous or semi- continuous pumping rather than a batch-wise pumping. For instance, for a system with an output of 0.5 m ³ per hour, the brine flow is around 20 liters per minute in the start-up phase. However, after start-up, at a steady-state production, the brine flow decreases significantly, for instance with 30- 70%. Such reduction would likely lead to a repeated on- and off-switching of the pump, while cavitation is particularly to occur at switching the pump on. Adding warmed-up cooling liquid therefore reduces differences in flow (at least relatively) through the brine pump between start-up phase and steady-state production. Moreover, it increases the flow anyhow. The frequency- controlled brine pump can thus more easily be operated continuously or semi-continuously.

It is observed that frequency pumping is shown both in Fig. 5 and in Fig. 6 and is suitably combined, as shown in Fig. 5, with the configuration wherein at least part of the warmed-up cooling liquid is transported, after passing the condenser module to the brine collector vessel, so as to thin the non-evaporated and concentrated feed. The addition of cooling liquid reduces temperature and concentration of the brine. As a result, the liquid in the brine collector vessel may be pumped more easily. Moreover, this reduces a risk of corrosion and of crystallization of salts in the course of pumping, or particularly, when some brine is left behind in the pump after pumping. Furthermore, in view of reduced temperature, the vapour pressure of the brine may be increased, therewith reducing the possibility of cavitation effects in the brine flow.

Fig. 7 shows a further embodiment comprising a jet pump 48. Jet pumps are fed with a water flow of predefined pressure and volume and are capable of providing an air pressure and volume displacement. Jet pumps are particularly attractive for larger volumes, since then an electrical vacuum pump is expensive and requires a significant energy consumption. The jet pumps, which are also but less appropriately referred to as venturi pumps, are for instance known from the product catalogue "Korting Liquid Jet Liquid Ejectors" from Korting Hannover AG, Badenstedler StraBe 56, D- 30453 Hannover.

Similar to the embodiment of Fig. 6, the open cooling system comprises a network for the warmed- up cooling liquid 49 with a first connection 49a for use as a feed, and with a second connection 49b for disposal via a separate pump 45. Though the pump 45 is shown to be a frequency controlled pump, this is not deemed necessary. The pump 45 typically is designed for pumping from an underpressure, for instance in the order of 200-500 mbar, to an overpressure, for instance in the order of 1.5-3 bar, more preferably 2-2.5 bar. This high pressure is then used as a driving force for the jet pump 48. The jet pump 48 is herein used as the system's vacuum pump. A vacuum line 101 extends from the jet pump 48 to the vapour channel of the condenser module 42 of the physically integrated modular assembly comprising a heat transfer module 40, distillation modules 14, 16, 114, 116 and further the condenser module 42. The underpressure in the vapour channel of the condenser module 42, typically in the order of 50-90 mbar, is then communicated to the brine collector vessel 37 via pressure communication means 103. For reasons of clarity, it is added that the valves VI, V8 are shown in this Figure as restrictions. Such implementation is advantageous in view of its robustness and simplicity, but not strictly necessary.

Fig. 8 shows a further embodiment comprising again a jet pump 48. Herein, the jet pump 48 not only acts as the system's vacuum pump so as to apply an underpressure to the vapour channel of the condenser module 42 via vacuum line 101. The jet pump 48 furthermore acts as a brine pump. The brine outlet pipe 39 thereto is coupled to the vacuum line 101 adjacent to the jet pump, so that the brine 39 is sucked into the jet pump 48 together with any air from the vacuum line 101. The jet pump is in this embodiment suitably designed so as to remove a liquid flow. Moreover, this embodiment has the advantage that the brine collector vessel may be left out.

Fig. 9 shows again a further embodiment with a jet pump. In this embodiment, a first jet pump 48 is present to act as the system's vacuum pump. A second jet pump 58 is present to act as the brine pump.

Experiments with the system as shown in Fig. 9 have shown that it operates with cooling liquid in a large variety of temperature ranges, from room temperature up to 38°C. The operation with cooling liquid at higher temperatures is not excluded. The system operation was found to be stable, with the cooling liquid of 38°C. Pressure in the condenser module and temperature of the steam of the feed became somewhat higher. The distillate output was approximately 10% lower than in operation with cooling liquid of 20°C. This reduction in distillate output is considered marginal. . It is believed that the jetpump is capable of condensing vapour that comes out of the vacuum system. When running at high cooling liquid temperatures, the condenser does not condense all steam entering from the last stage. The remaining non-condensed steam is then withdrawn via the pressue communication means and the vacuum line 101, on the basis of the low pressure exerted by the pump 45. When a conventional vacuum pump was used, of the type indicated in Fig. 4-6, the vacuum pump "choked" on this steam, causing a raise in vacuum pressure and a possible run-off of the complete system. However, when a jetpump was used, this raise in vacuum pressure was not found. This is deemed due to condensation of said steam, thus maintaining the vacuum levels in the complete system.

It is believed that the jet pump cools the coolinq liquid and may show an additional pumping effect due to condensation of evaporated cooling liquid in the jet pump.

This result is found to be surprising in comparison to the use of a conventional vacuum pump. Such a conventional vacuum pump does not work with cooling liquid with elevated temperature. If the cooling reduces in a system with a conventional vacuum pump, vapour (e.g. steam) will remain in the condenser module. This will be drawn into the vacuum system, i.e. any lines for pressure communication that are coupled to and operating by the vacuum pump. This vapour ends up in the vacuum pump, resulting in a reduction of pumping force, which brings about a pressure increase (loss of underpressure) in the system. This merely increases the issue, leading in the end to a cancellation of the pressure difference, and hence termination of distillate production. While all of the Figures 2-9 show embodiments wherein the feed is pretreated and used for the heat generation to the heat transfer module, this is not essential for the present invention, and alternatives or variations may be envisaged.