WO2019010547A1

WO2019010547A1 - Thermal separation process and apparatus

Info

Publication number: WO2019010547A1
Application number: PCT/AU2018/050994
Authority: WO
Inventors: Burkhard Sebastian SEIFERT
Original assignee: Seifert Burkhard Sebastian
Priority date: 2017-07-14
Filing date: 2018-09-13
Publication date: 2019-01-17
Also published as: AU2018299217A1; DE102018002568A1

Abstract

A thermal separation process for the separation of liquids, the method including: humidifying air in an evaporator by evaporation of pure water from a heated raw water; condensing pure water out of the humidified air in a condenser, wherein heat of condensation is used to preheat the raw water in the condenser; and further heating the pre-heated raw water with an external heater to form the heated raw water; wherein an adjustable active wall separating the evaporator and the condenser allows control of air flow between the evaporator and the condenser.

Description

THERMAL SEPARATION PROCESS AND APPARATUS

Priority Claim

[0001 ] This patent application claims convention priority from German patent application 10 2017 006 726.0 filed on 14 July 2017 and also from German patent application 10 2018 002 568.4 filed on 28 March 2018, the entire contents of both of which are to be taken as incorporated herein by this reference.

Technical Field of Invention

[0002] This invention relates to processes and apparatus for the thermal separation of liquids, including for example the production of potable, desalinated or otherwise usable water from a raw water such as a saline or brackish water, or conversely for the production of high salinity concentrates (such as brine) from such raw waters, generally utilising distillation techniques such as humidification- dehumidification methods (referred to hereafter as "HD-methods").

Background of the Invention

[0003] For a large proportion of society, the supply of clean water is an increasingly pressing task. In many cases, the treatment of raw waters (such as sea water or brackish water or other saline waters that, without treatment, would be unusable) has been a solution. In addition to mechanical (such as reverse osmosis) and electrical (such as electrodialysis) treatment processes, thermal processes are also known.

[0004] In relation to known thermal separation processes, water to be desalinated is heated with solar energy in flat reservoirs, with evaporated pure water condensing on an inclined transparent reservoir cover and flowing to a purified water container, with the heat of condensation being released at the reservoir cover. An example of this technology, referred to simply as a "solar still" is described in the book of A.B. Meinel and M.P. Meinel: Applied Solar Energy, Addison Wesley Publishing Co., pages 554-554, where a production rate of 3.29 kg purified water per square meter of reception area per day is mentioned. [0005] In the case of modern thermal separation processes, the heat of condensation of the purified water is often utilised multiple times through stepwise decreased pressure (through multi-stage flash evaporation or multi-effect distillation), or the vapor is compressed in such a way that the condensation and re-evaporation can occur without pressure grading. These processes for multiple use of the supplied energy are mainly used in large scale plants, requiring substantial capital and energy costs. As a desalination process, this technology is therefore particularly prevalent in arid countries where there are low prices for fossil fuels and high water purification requirements.

[0006] Known desalination techniques that can be operated at ambient and low temperatures utilize humidification-dehumidification methods. HD-methods have been known for many decades and an overview of developments in this area is presented in B. Seifert et al.: "About the History of Humidification-Dehumidifica^^ Desalination Systems", October 2013, Conference Paper: International Desalination Association World Congress on Desalination and Water Reuse At: Tianjin, China. However, despite the technical advantages of HD-methods, they have typically not been adopted in large scale operations, primarily for economic reasons.

[0007] Typical setups for HD-methods generally consist of an evaporator (being a humidifier), a condenser (being a dehumidifier) and an external heater. Raw water to be treated (for desalination, for example) is preheated in the condenser and further heated in the external heater so that it is hot when it enters the evaporator, subsequently cooling in the evaporator by the evaporation of pure water contained in the raw water. The evaporated pure water is absorbed by the air stream in the evaporator. The components of the raw water which do not evaporate are either discharged from the process or are partly returned to the process, so that a more concentrated raw water is achieved. Subsequently, desalinated water is obtained from the humidified air in the condenser by condensation on its cooling surfaces.

[0008] The movement of the air from an evaporator to a condenser, through which evaporated pure water is transported to the condenser, can occur by free convection, due simply to differences in density caused by temperature and concentration differences, or by forced convection caused by fans. [0009] The increase of the temperature of a raw water flow in a condenser can be seen as a good approximation for the efficiency of the process. In relation to Equations (1 ) and (2) below, the enthalpy increase ΔΗ ₂ of a raw water flow in a condenser is the product of the temperature increase (t₂ - t-i) of the raw water in the condenser, the specific heat capacity c_p,_w of the raw water and the raw water mass flow L. This increase in enthalpy ΔΗ12 of the raw water flow also results approximately from the condensation enthalpy used, i.e. the product from the condensate mass flow M (fresh water mass flow) and the specific enthalpy of condensation Ah_v (= specific enthalpy of evaporation).

[0010] Also, the enthalpy increase ΔΗ23 of the raw water flow L by an external heat supply Q_ext can be calculated as the product of the temperature increase (t₃ - 1₂) in the external heater with the specific heat capacity c_p wof the raw water and the raw water mass flow L. The water mass flow L is usually adjusted proportional to the external heat input to maintain a constant temperature t₃ of raw water entering the humidifier. Therefore, the ratio of the mentioned temperature differences is a measure of the multiple use of the condensation energy. It is referred to as the effectiveness or performance ratio PRHD, but is in essence a measure of efficiency:

PRHD = ΔΗ₁₂ / Qext = (t₂ - ti ) / (t₃ (1 )

[001 1 ] The efficiency PRHD therefore is a good approximation to the ratio of the actually obtained condensate quantity (the purified water quantity) M to the purified water quantity Q_ext / Ah which can be generated alone with an externally supplied heat Qext, (namely, without using condensation heat):

PRHD « M / (Qex, / Ah_v) (2)

[0012] Heat and mass transfer in the humid air takes place in the evaporator between raw water, which has previously been heated, and upwards flowing air. Pure water evaporates from the raw water stream, which cools down.

[0013] In the condenser, vapor absorbed in the evaporator condenses from the humid air on condensation surfaces (for example on condenser tubes), and the condensation enthalpy released, as mentioned above, is transmitted to the raw water stream to be heated, ensuring its preheating. [0014] The coupled heat- and mass-transfers in the stream of humid air in the evaporator and in the condenser requires driving gradients of temperature and concentration between the state in the main flow of the air and the state of the air at the surface of the water. The air adjoining the surface of the water is approximately in the saturation state corresponding to the local temperature of the water surface. The temperature and concentration gradient which is decisive for the heat and mass transfer is thus limited by the saturation condition of the humid air. The saturation condition for a certain temperature at the surface is given by the maximum possible vapor content of the air at this temperature, i.e. by the saturation vapor pressure. The saturation condition is only dependent on the temperature and is therefore referred to as a saturation line.

[0015] The enthalpy of the air in a cross-section of the evaporator, or of the condenser, can be represented by an operating line as a function of the temperature of the raw water stream in the cross-section under consideration (corresponding to the energy balance of the evaporator or the condenser). When the state of the air approaches the saturation state, the driving gradients disappear. Then, the transfer resistances required for heat and mass transfer for evaporation or for condensation would also have to disappear, with correspondingly large transfer areas and transition coefficients. As a result, in practice, the ratio of internally recovered energy to the externally supplied energy, which is possible by the counter-current principle, is limited to small values, so that the efficiency PRHD is also limited to small values under such conditions.

[0016] A known measure for realizing a higher efficiency PRHD is a multi-stage arrangement wherein the operating lines of individual stages are adapted to the course of the saturation line so that the distance between the operating lines and the saturation line over the entire height of the evaporator and the condenser is sufficiently large, i.e. there is a sufficiently high driving gradient for the heat and mass transfer. The construction of such a desalination plant, having several stages with a fan at each stage for the appropriate distribution and circulation of humid air, was realized in a large pilot plant described in the aforementioned book by A.B. Meinel and M. P. Meinel: Applied Solar Energy on pages 556-557. However, the described pilot plant had the disadvantage that the necessary effort was too great, and no successor plants were built. [0017] For smaller plants, test facilities have been developed and tested in which an evaporator and a condenser are arranged next to each other with no partition therebetween so that a crossflow from the evaporator into the condenser forms. However, in this case, the entire circulation of humid air is caused by free convection (i.e. by differences in density due to temperature and concentration differences). The use of free convection requires a specific setup of evaporator and condenser where there are low flow resistances, which either limits the heat and mass transfer possible, or which requires very large transfer areas in the evaporators and condensers, often rendering them uneconomic.

[0018] The invention is intended to present a process and apparatus which enable a thermal separation process with high efficiency PRHD and with low costs per generated quantity of purified liquid. Advantages can then be realised in small and cost effective, yet still efficient, plants for independent, decentralized purified liquid supply, such as for the supply of desalinated water.

[0019] Before turning to a summary of the present invention, it must be appreciated that the above description of the prior art has been provided merely as background to explain the context of the invention. It is not to be taken as an admission that any of the material referred to was published or known, or was a part of the common general knowledge in the relevant art.

Summary of the Invention

[0020] In its broadest form, the invention resides in the introduction of an adjustable "active wall" to a thermal separation system based on the abovementioned HD-methods, preferably used in conjunction with an optimization control system, for separating evaporator from condenser and allowing the control of air flow between evaporator and condenser.

[0021 ] The present invention provides a thermal separation process for the separation of liquids, the method including:

a. humidifying air in an evaporator by evaporation of pure liquid from a heated raw liquid;

b. condensing pure liquid out of the humidified air in a condenser, wherein heat of condensation is used to preheat the raw liquid in the condenser; and c. further heating the pre-heated raw liquid with an external heater to form the heated raw liquid;

wherein an adjustable active wall separating the evaporator and the condenser allows control of air flow between the evaporator and the condenser.

[0022] The present invention also provides thermal separation apparatus for the separation of liquids, the apparatus including:

a. an evaporator for humidifying air by evaporation of pure liquid from a heated raw liquid;

b. a condenser for condensing pure liquid out of the humidified air, wherein heat of condensation is used to preheat the raw liquid in the condenser;

c. an external heater for heating the pre-heated raw liquid to form the heated raw liquid; and

d. an adjustable active wall separating the evaporator and the condenser, the adjustable wall capable of controlling air flow between the evaporator and the condenser.

[0023] In a preferred form, the active wall is a barrier arranged between the evaporator and the condenser, thereby separating the evaporator from the condenser and, when not in use (namely, when the active wall is, for example, closed), essentially sealing the evaporator from the condenser. However, the active wall is adjustable in that it preferably includes controllable ventilators able to be configured between open and closed positions to increase or decrease the size and/or direction of one or more cross-flow openings in the active wall, and thus control the flow rate of air passing through the ventilators between condenser and evaporator.

[0024] The active wall is preferably further adjustable by way of one or more controllable blowers able to increase or decrease the velocity and amount of air passing through the ventilators, the blowers in one preferred form being formed integrally with the active wall, although in other forms being physically separated from the active wall. Indeed, it is envisaged that in a preferred form of active wall, a ventilator may actually incorporate a blower, such as a fan, so that the preferred ventilating and blowing functions of the adjustable active wall are both provided by the same, single element. [0025] The combined effect of being able to control such ventilators and blowers is the control of the air flow between evaporator and condenser, ideally continuously through all operating phases of the apparatus and ideally in response to changes in the external heat supply, for reasons that will be explained further below.

[0026] In one form, the active wall is an integral unit and is provided with control elements for controlling blower power and ventilator configuration, including the adjustment of some cross-flow openings in preference to other cross-flow openings. In this respect, it may be desirable to have some cross-flow openings (such as those towards the bottom of an active wall) open or closed at certain stages of operation, so as to control the direction and location of air flow within the condenser and evaporator.

[0027] In another form, the apparatus may be provided by way of multiple modules, for example modules which are stacked on top of each other, each module having condenser, evaporator and active wall as generally outlined above, with an individual active wall for each module. The use of an active wall enables multiple arrangements of condenser and evaporator, as optimum air flow conditions can be achieved with adjustable ventilators and cross-flow openings, virtually independently of their location. As an example, two condensers can be located on opposite sides adjacent to one evaporator or vice versa. Also, an evaporator can be connected to more than one condenser with active walls. It is also possible to connect one condenser with more than one evaporator with active walls.

[0028] In relation to operating temperatures for the processes and apparatus of the present invention, heat sources at low temperatures (in particular below 100°C) can be readily used. The process of the present invention can thus be used in connection with other processes, ideally which release heat at low temperatures. Large plants can thus be advantageously operated, as mostly the corresponding external heat source is available at low cost. The process of the present invention can of course be adjusted to the temperature range of the external heat source. For example, the process can be operated with a heat source of 80°C, by using appropriate temperatures ti to t₃ for the dimensioning. [0029] The low operating temperature and the operation at ambient pressure enables the use of cost effective and corrosion resistant materials for the apparatus components and sheets for the apparatus casing. In particular, plastic such as polypropylene is suitable as a construction material.

[0030] The present invention is of course suitable not only for the recovery of purified water from seawater, brackish water or contaminated waters, but also for concentrating liquids, for example, such as for liquids requiring concentration through the evaporation of a water fraction. In this respect it will be appreciated that the main product may not be the condensate from a condenser, but the concentrate, which might be referred to as "brine" or "concentrate", from an evaporator, as it may be required for example in metallurgical processes or in a sugar refining process as molasses for the food industry.

[0031 ] Additionally, it will be appreciated that by the recirculation of brine emerging from an evaporator, into a raw water stream passing into a condenser, the concentration of a product to a desired level can be improved. Such concentrating of the brine can be carried out without the addition of fresh liquid in a batch operation, or brine may be continuously removed and fresh liquid added in a continuous operation. Also, with the use of the apparatus of the present invention, such brine concentration can be conducted at reasonable temperatures. The process of the present invention is therefore applicable for the concentration of liquids, for which other methods, such as reverse osmosis, may not be suitable. For example, some metallurgical processes require liquids of very high concentration which can be produced with the process of the present invention. Other applications include the concentration or even crystallisation of not only salts but also other dissolved substances such as sugar.

[0032] By definition, optimum operating conditions lead to a maximum purified water volume for a given apparatus and a given external heat supply. In accordance with Equation (2) above, this is approximately proportional to the product of the efficiency PRHD and the externally supplied heat Q_ext. If the heat Q_ext is generated by the use of solar radiation, a variable heat supply is to be expected so that the optimization should advantageously take place continuously. [0033] In a preferred form of the invention, an optimisation control system is adopted whereby the temperatures of the raw water at the inlet (t-i ) into the condenser and at its outlet (t₂) are measured continuously for the optimization, as is the temperature of the raw water after the external heater (t₃). In this way, the efficiency PRHD according to Equation (1 ) can be determined continuously and without reaction time.

[0034] Thus, the ongoing optimization of the operation of the apparatus of the invention can be carried out by maximizing the efficiency PRHD- In contrast to the usual optimization, in which the generated condensate water quantity is used as a control variable, the efficiency is determined more precisely and substantially more quickly using the above preferred technique, thus improving the optimization process. Indeed, the fast and continuous measurement and analysis of the efficiency PRHD allows continuous optimisation and control of blower speeds and ventilator configuration using a control system.

[0035] The main objective of the present invention is to increase the efficiency PRHD with new approaches and consequently to reduce the required energy input per kg of purified water produced, leading to better economics for the method. In this respect, some factors that are relevant to heat and mass transfer deliberations are the size of the transfer surfaces in the condenser and evaporator, the heat transfer coefficient a for the heat transfer and β for the mass transfer, and the temperature and concentration gradients, with the transport coefficients a and β being approximately proportional to the square root of the relative velocity at the exchange surfaces.

[0036] If, for example, for a given heat transfer surface and amount of condensate, the relative velocity is increased by 10-fold, the driving gradients (temperature and concentration) can be reduced to approximately one third. This can be approximated with a reduction of Δί₂3 to one third, which results according to equation (1 ) to an increase of PRHD by three fold. For a given condensate mass flow M, only one third of the external heat input Q_ext is required. If heated with solar collectors, then only one third of a collector surface would be required. In addition, the operating lines are optimised, as described above. If it is then also possible to increase the relative velocity for heat and mass transfer, this leads to a significant contribution to reduce investment cost for the production of water or for the concentration of process water.

[0037] Finally, the maximal theoretical value for PRHD depends on the salt content of the raw water. It is approximately 120 for a salt content of seawater at 0.035, which is far above the values of traditional HD-based desalination systems where the maximum PRHD values are typically below 4. It has been determined by the Applicant that the main reason for the low PRHD value for these known systems is the fact that the relevant relative velocity near the transfer surfaces for heat- and mass transfer is small. Therefore, during steady-state operation, only the same amount of humid air condenses in the dehumidifier which previously evaporated into the air flow in the humidifier. Increasing the air flow velocity is very limited, as the contact time for heat- and mass-transfer is reduced.

[0038] The effect of low heat- and mass-transfer coefficients due to the low relative velocity are, in the case of these traditional systems, compensated with a corresponding large driving force, (namely a high external heat input Q_ext.), leading to the low efficiency PRHD_. In contrast, the present invention enables the increase of the relative velocity of the humid air at the transfer surfaces in the evaporator and condenser, by the improved control of air flow between the evaporator and the condenser via the adjustable active wall and preferably also by generating secondary air flows, as described further below.

[0039] Before turning to a description of several preferred embodiments with reference to the accompanying drawings, reference will now be made to a further modification to the process and apparatus of the present invention, by way of the adoption of a secondary air flow to assist further with the control and increase of the relative velocity referred to above.

[0040] In a preferred form, a secondary air flow is generated within either or both of the condenser and the evaporator, ideally lateral to the main air flow and ideally at a significantly higher relative velocity in order to further improve heat and mass transfer.

[0041 ] Such a secondary air flow may be generated by the adoption of deformable or displaceable plates, or fixed or elastic wings, preferably arranged axially vertically, located within the internals of the condenser and/or evaporator, or by oscillating elements such as oscillating fans. Additionally, the internals of the evaporator and the condenser could also be equipped with perforated plates, to create an oscillating secondary flow through the holes of the plates.

[0042] Another possibility for the generation of a secondary air flow according to the present invention is the use of a rotating conveyor system, such as blower-wheel (rotors), which effect the relative velocity whereby the secondary air flow does not need to oscillate. In addition, or as a replacement to influence the relative velocity with a secondary air flow, the evaporator/condenser internals could be moved mechanically. It will thus be appreciated that various methods may be adopted to increase the relative velocity on the surfaces for heat and mass transfer, in combination or individually. For example, for the evaporator, for an advantageous combination of mechanical movement and the stimulation of a secondary air flow, internals such as round brushes can be rotated on a vertical axis.

[0043] Additionally, instead of the abovementioned secondary flow, it may also be advantageous to introduce a flow in the other direction to the flow of humidified air. As the mainly vertical direction of the velocity of the main air flow of the process of the invention is usually small, with a pulsating vertical secondary air flow it can be possible and advantageous to generate a periodic reverse vertical flow.

Brief Description of Figures

[0044] To illustrate the present invention, reference will now be made to several preferred embodiments that are illustrated in Figures 1 to 10.

[0045] Figure 1 is a schematic representation of apparatus in accordance with a first embodiment of the present invention;

[0046] Figure 2 is a psychrometric chart in which the specific enthalpy of humid air is illustrated as a function of the temperature of raw water showing operating lines for the evaporator and condenser in a conventional single-stage apparatus;

[0047] Figure 3 is a psychrometric chart similar to the chart in Figure 2, but illustrating operating lines for the embodiment illustrated in Figure 1 ; [0048] Figure 4 is a schematic representation of an adjustable active wall for use with the embodiment illustrated in Figure 1 ;

[0049] Figure 5 is a schematic representation of an optimised control scheme for use with the embodiment of Figure 1 ;

[0050] Figure 6 is a schematic representation of apparatus in accordance with a second embodiment of the present invention, representing a two-stage version of the apparatus;

[0051 ] Figure 7 shows the geometric design for the determination of the increasing salinity of the brine for a multi-stage embodiment, as shown in the embodiment of Figure 6 for two stages;

[0052] Figure 8 is a schematic representation of apparatus in accordance with a third embodiment of the present invention, similar to the two-stage version of Figure 6 but with emphasis on maximising brine concentration and recovery including crystallisation;

[0053] Figure 9 is a simplified front view of an evaporator in cross-section, suitable for use in the apparatus of the present invention, equipped having with deformable plates for the generation of an oscillating secondary air flow; and

[0054] Figure 10 is a top view of an evaporator and a condenser suitable for use in the apparatus of the present invention, both being equipped with deformable plates for the generation of an oscillating secondary air flow.

Detailed Description of Preferred Embodiments

[0055] Figure 1 illustrates an example of apparatus in accordance with the present invention, for the separation of liquids, the apparatus in this embodiment being a desalination apparatus having an evaporator 1 , a condenser 2, and an adjustable active wall 3 located between the evaporator 1 and the condenser 2 and separating the evaporator 1 from the condenser 2. An external heater 4 serves to further heat the raw water stream that has been preheated in the condenser 2, the raw water stream having been conveyed through the raw water pump 5 through the condenser, and then through the external heater 4 to the raw water distributor 6 in the evaporator 1 . Brine discharged from the evaporator 1 via the brine outlet 7 is collected in the brine container 8 or is discharged via the overflow 9. The raw water pump 5 can be supplied with the aid of the mixer 48 with a mixture of fresh raw water from the raw water suction container 10 and with brine from the brine container 8.

[0056] In this embodiment, the external heat source for the external heater 4 is solar radiation, which is captured with a solar receiver 1 1 and is delivered to the external heater 4 through a secondary circuit with a circulation pump 12 to a heat carrier (for example, water which has been treated with anti-corrosion agent). This advantageously avoids the use of expensive material in the solar receiver 1 1 .

[0057] Humid air circulates in the closed circuit between the evaporator 1 and the condenser 2 in the desalination apparatus through the adjustable active wall 3. In this embodiment, the circulation of humid air is controlled by the active wall 3 via a blower 13 in the form of a fan, in conjunction with an adjustable ventilator 14 having a plurality of adjustable cross-flow openings. Pure water evaporated in the evaporator 1 from the heated raw water is condensed in the condenser 2 as fresh water from the humidified air, whereby it emits heat to the colder raw water which flows through the pipes 15 for heat transfer in the condenser (the pipes 15 being, for example, tube bundles).

[0058] The evaporator 1 is preferably a column with internal elements 16 (for example, packing elements per se or with a column packing) in order to achieve a suitably large liquid surface, which is advantageous for the evaporation from the downflowing heated raw water stream. This raw-water stream is supplied to these internal elements with the aid of a distributor 6 at the top of the evaporator.

[0059] In the condenser 2, the vapor absorbed by the circulating air is condensed as fresh water, so that it runs off and can be collected as a product in the fresh water container 17. The condenser 2 preferably consists of a plurality of layers of pipes arranged approximately parallel, which are connected at the ends via connecting pipes. In this case, the tubes of each layer can be passed through in parallel, while the superimposed layers are flowed through one another from bottom to top. By the parallel and series connection of many small pipes, large condensation surfaces can be created. However, other capacitor designs are also suitable, for example plate heat exchangers.

[0060] An improvement in the heat and mass transfer in the condenser can be achieved by recycling (not shown) a part of the condensate obtained from the collecting tank 17 for the fresh water into the condenser 2 with a pump (not shown) so that the condenser surfaces can be intensively purged. The preferred thermal insulation for the components operated at higher temperatures, in particular the evaporator 1 and the condenser 2, are also not shown in Figure 1 .

[0061 ] In this embodiment, the active wall 3 is arranged as a barrier in the form of an insert between the evaporator 1 and the condenser 2, which while providing for a physical connection between the evaporator 1 and the condenser 2 allows there to be fluid communication between the evaporator 1 and the condenser 2 via the cross-flow openings and their adjustability.

[0062] As mentioned above, the active wall 3 includes a single blower 13, with its motor mounted outside the condenser/evaporator arrangement. In a further preferred embodiment, however, an evaporator and a condenser may be mounted as modules upon the active wall 3, thus being connected to one another via the active wall 3. In this form, the housings of the evaporator 1 and the condenser 2 might then consist of heat-insulated hoods, which are easy to assemble and easy to remove for the maintenance of the fittings.

[0063] The psychrometric chart of Figure 2 shows the specific enthalpy (ordinate values) of humid air in imaginary horizontal cross sections of an evaporator or of a condenser as a function of temperature (abscissa values) of a raw water flow in the concerned cross section due to the energy balance. A conventional arrangement with a single-stage HD-system and a constant mass flow G of the proportion of dry air and a constant raw water mass flow L in the circuit is assumed for a constant external heat input C

[0064] The slope of the operating lines 19 and 20 is approximately proportional to the ratio of the raw water mass flow L to the air mass flow G. Since this ratio is constant in the case of Figure 2, straight operating lines result. The saturation line 21 of the humid air saturated with water vapor runs between the two operating lines. The specific enthalpy of the humid air is, as usual, related to the proportion of dry air. The enthalpy of the saturated humid air (saturation line) is only dependent on the temperature (abscissa value).

[0065] The enthalpy of the humid air (ordinate value) remains approximately unchanged during the overflow of the humid air from an evaporator into the condenser at the upper end of the two columns, but the associated raw water temperature (abscissa value) is the temperature t₃ of the raw water entering the evaporator after passing the external heater, while at the top of the condenser the raw water reaches the final temperature t₂. At the lower end of the evaporator, the concentrated raw water (brine) exits with a temperature t₄ which is higher than the temperature t-ι of the raw water flowing into the condenser. The brine is enriched with salt by the evaporation of pure water. Of course, in the illustrated embodiment of the apparatus of the present invention in Figure 1 , the stream of the humid air is conveyed by the blower 13 from the condenser 2 into the evaporator 1 at the lower end of the active wall 3.

[0066] The existence of some distance of the operating line 19 of an evaporator or the operating line 20 of a condenser from the saturation line 21 is necessary for the heat and mass transfer in the evaporator and in the condenser to produce the required temperature and concentration gradients between the saturated air at the water surface and the bulk flow of the humid air. An approximation of the operating line to the saturation line would require that the exchange surface areas become very large or the transfer resistance becomes very small. This would mean an infinitely growing effort so that a strong approach of the operating line to the saturation line must be avoided. With this in mind, Figure 2 shows the case in which the operating lines approach the saturation line, which thus represents approximately a limiting case. In this case, the operating line 20 of the condenser approaches the saturation line 21 at their ends, while the operating line 19 of the evaporator is tangential to the saturation line. However, in the case shown, the efficiency PRHD is less than 3, as can be seen from the relation of the significant temperature differences (t₂-ti) / (t₃-t₂), which can be seen as the ratio of the distance 22 to the distance 23.

[0067] In this respect, and as mentioned above, PRHD can be regarded as a good approximation of the ratio of the actual purified water flow to the purified water flow that would be achieved by simple evaporation of water (i.e. without recovering the condensation energy). It is assumed here that the mass flow L of the water in the condenser and external heater is constant. The ratio PRHD = (t2-ti) / (t3-t₂) can then be formed from differences of abscissa values. Accordingly, Figure 2 is limited by the curvature of the saturation line and by the condition that the operating line should not approach the saturation line too closely.

[0068] According to the present invention, high efficiencies can be achieved for apparatus that is in accordance with the present invention, when the course of the operating lines is adapted to the course of the saturation line by a combination of forced convection and controlled cross flow between evaporator and condenser. With this in mind, Figure 3 shows by way of example how the combination of these measures can be used to provide an adaptation of the inclinations of the operating lines according to the course of the saturation line with a sufficient distance over the entire humidification and condensation process.

[0069] In Figure 3, the efficiency PRHD is approximately 7, more than twice that of the conventional case of Figure 2, and it is submitted that this value is not to be seen as an upper limit. Theoretically, the maximum achievable value of PRHD in seawater desalination is higher than 100, because the temperature difference t₃-t₂ is limited only by the boiling point elevation of the sea water (approx. 0.5 K). A very high value of PRHD, however, will require a very high apparatus complexity so that there is an optimal configuration. For example this can be PRHD = 12.

[0070] The curved operating lines 19 and 20 of Figure 3 result from the influence of the active wall according to the present invention. The shown efficiency of PRHD = 7 means that seven times the purified water is obtained compared to a construction without recovery of the condensation heat. Because of the energy balance, the inclination of the operating line at a location is proportional to the ratio of the raw water mass flow L to the mass flow G of the air, the inclination can be adapted to the inclination of the saturation line over the entire process by means of the cross flow between the evaporator and the condenser. The mass flow rate G of the air is correspondingly adapted by means of the cross flow which means adjustment of the parameters of the active wall. In this respect, the necessary distance between the operating lines and the saturation line is mainly dependent on the size of the transfer areas and the intensity of the heat and mass transfer in the evaporator and the condenser.

[0071 ] As such, it can be seen from Figures 2 and 3 that for a high efficiency PRHD, the temperature difference (t₂-t-i ) must be large compared to the temperature difference (t₃-t₂). This is achieved according to the invention by an advantageous course of the operating lines, which in turn is achieved primarily by means of a preferred cross flow between evaporator and condenser, whereby the ratio of raw water mass flow L to air mass flow G is adapted. In addition, a high heat and mass transfer from the raw water in the evaporator and from the humidified air in the condenser is preferred.

[0072] In this respect, a high heat and mass transfer can be caused in the evaporator by a large supply of heated raw water, which trickles down the evaporator internals with large transfer surfaces, and by adapted flow of the humid air, which absorbs the evaporated water. In the condenser, the approach to the saturation line can be achieved by a wide range of condensation surfaces, e.g. through thin tubes, which are passed through with the raw water to be heated, and through adapted flow velocities. The flows are adjustable by controlling the pump 5 for raw water flow rate and by control of the blower 13 and the ventilator 14 with its cross-flow openings. The active wall provides additional possibilities for improving the heat and mass transfer, which is pointed out below in the discussion of Figure 6.

[0073] In order for the cross-flow openings to provide the preferred cross-flow between the evaporator and the condenser, numerous possibilities are possible. Compared to free convection, the use of blowers in the active wall allows efficient flow to the evaporation and condensation transfer surfaces, resulting in improved heat and mass transfer. The cross-flow openings can be designed as nozzles which effect the penetration of an air jet from the active wall, primarily transversely into the internals of evaporator and condenser. Alternatively, multiple fans can be installed in the active wall itself, which could be alternated with cross-flow openings in the active wall.

[0074] Cross-flow openings can be formed as lamellas, the elements of which may be adjustable, therefore influencing the air flow over the entire height of the evaporator and the condenser. Special constructions of a ventilator 14 could thus be used for the preferred influencing of the flow of the humid air. An example with lamellas is presented in Figure 4.

[0075] The manufacture of the ventilators in plastic makes it possible to make a variety of shapes for the cross-flow openings, e.g. circular or rectangular, but also nozzle-shaped openings.

[0076] The additional adoption of continuous or irregular movement of a ventilator in an active wall could be used to further increase the heat and mass transfer in a condenser and an evaporator by superimposing an oscillation on the air flow. For example, the lamellae 34 in Figure 4 could be adapted to oscillate at an angle with respect to a basic position so that the flow at the transfer surfaces in the condenser and in the evaporator constantly changes and stationary films or large drops with reduced heat and material transfer are avoided.

[0077] In the flow through cross-flow openings, care should also be taken that no raw water or no brine from the evaporator enters the condenser. In one embodiment, this can be achieved by avoiding the formation of fine drops in the evaporator. For example, in the distributor for the application of the raw water to the internals of the evaporator, spraying is preferably avoided and replaced by the draining large drops from the distributor. Also the internals of the evaporator can provide for a drain without distribution into fine drops.

[0078] Further in relation to cross-flow openings provided in an active wall, it is preferred that the active wall also acts as a demister to avoid transfer of salt water droplets from raw water in the evaporator reaching the condenser. This can be achieved, inter alia, by the shaping of the surface, by surface characteristic and by the position of the ventilator blades of the active wall. For example, the active wall ventilator blade can face in upwards in a condenser or downwards in an evaporator. In addition, the ventilator elements can be equipped with an extension such as guiding blades facing downwards into the evaporator as shown in Figure 4 for the guiding blades second from above. The surface characteristic of both extension and ventilator elements can be coated with a hydrophobic characteristic. A possibility is also to equip the guiding blade with flat brushes to avoid the entrainment of salt droplets into the condenser. [0079] A particularly advantageous solution for the forced circulation of the humid air is of course provided by blowers such as fans. In preferred forms, fans can be provided integrally with an active wall and their axes can be arranged in the plane of the active wall, so that the drive motor of a fan can be arranged in a simple manner outside the apparatus housing. However, axial and radial fans are also suitable, the drives of which may be protected against the humid air inside the apparatus. In Figure 4, a blower in the form of a fan 13 is positioned towards the lower end of the active wall, but it will be appreciated that such a blower may alternatively be positioned towards the upper end of the active wall, or towards the centre of the active wall, or may be closely adjacent the active wall or somewhat remote from the active wall (such as at a location in fluid communication with the evaporator).

[0080] Adaptation of the air flow in an evaporator to conditions that provide for high efficiency can also be assisted by an internal subdivision by means of intermediate distributors in an evaporator so that humid air and heated raw water mass flow distribute as uniformly as possible over the cross-section of the evaporator.

[0081 ] A known measure for improving the performance of desalination plants is the recycling of part of the brine which leaves the evaporator and which still has a temperature which is above the temperature of the raw water to be heated when it enters the condenser. This return of brine to the raw water stream to be heated is particularly advantageous when the plant is started after the nocturnal shutdown in the case of solar energy use if hot brine has been stored in isolated containers the day before. The use in this manner of the energy from the brine is also possible in apparatus according to the present invention.

[0082] The cooling of the apparatus in the absence of external heating, in particular at night, is disadvantageous for water separation and can be reduced by means of measures according to the invention. In addition to the efficient isolation of the apparatus, any one or more of the following may be adopted:

a) the supply of hot brine in the vessel 8 of Figure 1 , which also allows night-time operation, during which time the external heating is replaced;

b) the adjustment of the circulation of the humid air between evaporator and condenser by the active wall, adapted to the heat capacity of the brine tank;

c) the prevention of circulation in the secondary circuit of the external heater; and/or d) the use of a heat-insulating cover for the solar receiver at night.

[0083] A particular advantage of the invention, compared to a subdivision of desalination apparatus to several separate stages, is that the active wall of the present invention allows continuous adaptation to changing operating conditions. This applies in particular to the adaptation to the externally supplied variable heat flow when using solar energy. By continuously adapting the air flow, the apparatus can be operated continuously under optimal conditions. This interaction is advantageously achieved by a preferred optimization control system. A further adaptation is the control of the raw water flow to be heated.

[0084] Figure 5 shows a simplified diagram for the optimized control of the blower 13 of the active wall 3 according to the invention, of the pump 5 for conveying the raw water flow L to be heated, and of the ventilator 14 and its cross-flow openings, all with the aid of a control device 40.

[0085] The heating power Q_ext externally supplied to the external heater 4 is generally to be regarded as a variable value of the control system, in particular in the case of solar energy use. The value to be continuously maximized by the control system is the efficiency PRHD- AS mentioned above, this optimization quantity is approximately proportional to the generated fresh water flow M, based on the external heating power Q_ext, according to the approximation equation (2), so that M is maximized at a given plant and heating power. The controllable variables which can be used in this case are, in particular, the power or the rotational speed of the fans and the pumps, the position of the actuating drives of the transverse flow openings and the mixing of the raw water with the brine from the container 8 with the mixer 48 which is controlled by the adjusting drive 49. When optimizing the adjustable variables, the possibilities of advanced control systems in the controller 40 can be used.

[0086] For optimizing the operation with the active wall, the temperature t| , t₂ and t₃ are measured at the measuring points 41 , 42 and 43. From this, PRHD can be determined according to equation (1 ) without delay time and with high precision. Limitations are obtained above all at the temperature t₃ of the heated raw water due to the resistance of the materials coming into contact with it. Exceeding the permissible temperature is primarily avoided by the adaptation of the raw water flow L, which can be measured with the mass flow measuring point 44.

[0087] The adjustment of the raw-water flow L takes place with known possibilities, e.g. by setting the motor speed of the pump motor 45. Likewise, the power of the blower 13 can be adjusted on its drive 46. For the adjustment of the transverse flow openings with the adjusting devices 47, there are a plurality of possibilities, one of which is illustrated in the explanation of Figure 4.

[0088] Figure 6 shows a simplified system configuration with two stages, whereby each stage mainly consists of evaporator 1 and condenser 2, and external heating 4 to continuously concentrate the feed raw water.to form a concentrated brine. In the description, the stages are distinguished using indices (i, i+1 ). An apparatus in accordance with the present invention can of course consist of a much larger number of stages. In each stage, the concentration of the brine is increased by withdrawal of a salt-free condensate.

[0089] The stage shown in Figure 6 on the right-hand side of the page is supplied from the brine pump 50 of the left stage. The brine can be cooled with a chiller 51 , to ensure a continuous operation and to increase the condensate mass flow. Each stage is preferably equipped with its own active wall 3.

[0090] With a mass balance for water and the salt content S, the following correlations are derived for stage i and the subsequent stage i+1

[0091 ] The brine-mass flow is described as L, the salt content as S and the produced condensate with the mass flow rate M (which contains no salt even at high brine concentrations).

[0092] As a good approximation for processes of this type, the condensate mass flow rate Mj which evaporated in stage i, can be determined with the simplified energy balance of the dehumidifier Μί * Δη_ν = l_i * Cp,w* At₁₂,i (6)

[0093] The left side of the equation (6) is the heat flux (with the specific heat of condensation Ah_v) released during condensation of the condensate mass flow Mj which will be used almost entirely to preheat the feed water, and respectively the brine mass flow Lj by the temperature increase At-i_2,i, shown on the right side of the equation. Equation (6) shows the correlation between the produced condensate mass flow Mj in the condenser and the temperature increase Δίι_2,ι of the brine in the condenser before it enters the external heater.

[0094] The relation between the salt contents of the brine Sj/S_i+i results from equation (3) above, namely:

S S_i+1 = L_i+1 / Li (7)

[0095] The increase of the salt concentration AS, according to equation (4) results from equation (6) and (7) by extraction of the condensate mass flow Mj , namely:

AS, = S_i+1 / (Ahv / Cp_iL * At₁₂,i) (8)

[0096] Figure 7 shows the enthalpy-concentration diagram (h-S-diagram) resulting from equation (8).

[0097] A similar enthalpy-concentration diagram is shown in the chapter titled "Eindampfen von Salzlosungen" on page 192 in F. Bosnjakovic: Technische Thermodynamik, II. Teil, 3. Auflage, Verlag Th. Steinkopff, Dresden und Leipzig (1960). The diagram IV/70 does not show a thermal separation process but does describe the vaporization of a salt solution.

[0098] The increase of the salt concentration AS, in stage i results from the reduction of the water content of the brine Lj due to the produced pure condensate mass flow Mj. The heat of condensation (= heat of vaporisation) Ah_v is indicated on the y-axis (S = 0). In Figure 7, these values are shortened due to space considerations. The enthalpy increase Ah, of the brine flow resulting from the preheating of the brine is also shown on the y-axis with the salt content Sj. [0099] Figure 7 shows the stepwise geometric determination of the salt content of successively arranged stages, whereby the condensate mass flow from each stage results from the realised temperature increase At-i₂,i of the brine in the condenser.

[0100] An advantage of this embodiment of the invention is that the produced solvent during the concentration of the liquid can be re-used, which reduces the procurement costs and disposal cost. Also other applications for the use of the solvent are possible.

[0101 ] According to this embodiment of the invention, the brine passes each process stage and the concentration is increased stepwise. This process cannot be continued an unlimited number of times, as the concentration approaches the saturation concentration. In general, the saturation concentration increases with increasing temperature. With cooling, the saturation line can be crossed. According to the invention, this crossing, which leads to crystallisation, should not occur within the apparatus, as then crystals grow on surfaces and can block the flow. In this respect, crystallisation can be allowed to occur in the evaporator if the internals of the evaporator are appropriately configured, such as by using brushes or other elastic internals (see Figure 8).

[0102] The cooling of the brine released from the last stage leads to crystallisation if the solubility of the salt decreases with decreasing temperature and the solubility limit is exceeded. The generation of crystals is shown in Figure 7 on the left side of the diagram. Crossing the solidification line 53 by cooling can lead to production of crystals, which corresponds to the position M on the isothermal line of the crystalliser. The mass fraction of the crystals can be determined with the mixing rule for salt (on the right side of the x-axis, S = 1 ) and the saturated solutions (on the solidification line 53) at the temperature in the crystalliser. Additional production of crystals from saturated solution can be achieved with additional stages, by brine recirculation and with the common techniques used in traditional crystallisation techniques.

[0103] Figure 8 shows a stage that processes brine with a high concentration. Below the evaporator 1 , a crystalliser 55 is shown in which crystals are kept in rotation by a rotational flow while they are growing in a saturated solution. Figure 8 shows a configuration directly located below the evaporator 1 which is advantageous to capture crystals.

[0104] A circulation pump 58 and a chiller 59 are also shown, which ensure the circulation of the solution. The supersaturation is created by the withdrawal of solvent in the evaporator 1 and by cooling in the chiller 59. Multiple construction types are known which allow a continuous or batch-wise crystallisation and removal of crystals. In addition, crystallisation can be accomplished at temperatures which allow the ability to construct using plastics, with only little mechanical strain because of the operation at ambient pressure.

[0105] The crystals can be discharged from the brine and mechanically separated from the almost saturated solution. Figure 8 shows for this separation a band filter 56, with the recirculation of brine with a pump 57 to the thermal separation process for further concentration at a higher temperature. The crystals held back by the band filter 56 are fed into the crystal container 60.

[0106] In this embodiment, the majority of the removed water was extracted from solution by the multistage process with relatively low effort (in comparison to traditional vaporization processes). The external heat flow, provided for each stage by external heaters 4, is not shown in Figure 7. However, it leads to a small increase in enthalpy at constant salinity S, within the specific stage which processes the brine, because the majority of the heating of the brine results from the counter-current flow process. The temperature increase of the external heater is:

At₂3,i = At₁₂,i / PRHD,i (10)

[0107] Because of the counter current flow of the process, a heat flow which is minimised by the divisor P RHD is externally supplied to the brine.

[0108] Returning now to a more general description of aspects of the present invention, albeit with reference to Figure 9, heated raw water is distributed in the evaporator 1 via the distributor 6 onto the schematically drawn internals 16, to flow downwardly, distributed within the internals, to humidify upward flowing air with pure water. The heated raw water cools down and reaches the brine outlet 7 with a higher concentration. [0109] In this embodiment, the evaporator 1 includes additional plates 61 arranged to generate a secondary air flow, the plates 61 being radially deformable and being arranged in pairs adjacent either side of the internals of the evaporator 1 , so as to be capable of synchronised deformation. A crank mechanism 62 acts as a drive to deform the plates 61 , which acts on the plates via bars 63. Figure 9 shows the plates deformed to a maximum extent internally (broken line) and externally (solid line) of the neutral positions (not shown) of the plates 61 . The continuous deformation of the plates 61 generates an oscillating secondary air flow that overlays the main air flow generated by the active wall's ventilators and blowers, improving relative velocity further and thus improving heat and mass transfer.

[01 10] In this respect, for a plate with length I, a width b and a maximal deformation of z_max (maximum distance from the neutral position), the moved volume V for a total stroke of a plate is calculated as:

V = 2 ^* l ^* b ^* z_max ^* F_v (1 1 )

[01 1 1 ] In Equation (1 1 ), the volume factor F_v depends on the form of the deformation of the plate. For a plate with a uniform thickness, which is, as shown, symmetrically deformed, F_v = 0.63. If the plate is not deformed but moves backwards and forwards without deformation and so is operated like a piston, F_v = 1 .

[01 12] The average air flow velocity w of the secondary flow between the plates 61 for a stroke frequency f results from: w = V ^* f / (l ^* b) = 2 ^* z_max ^* F_v ^* f (12)

[01 13] An average velocity w of the secondary flow of the magnitude of 10 times of the velocity of the main flow of the humid air, for example with a deformation of z_max = 4 cm to a stroke frequency f of approximately 4 per second.

[01 14] Figure 10 outlines the top view of the evaporator 1 and the condenser 2, separated by the adjustable active wall 3, with the plates 61 arranged sideways on the respective internals 15 and 16 and are shown with a maximum deformation. Because of the synchronised movement of the plates 61 , the secondary air flow they generate is lateral to the main air flow and is at a significantly higher relative velocity which is relevant for the improved heat and mass transfer. In this respect, the energy consumption for the movement of the plates is small, and the periodic storage of kinetic energy of the stroke movement for the plates is for example possible with the drive of a flywheel.

[01 15] It is also possible for the displaced air to be compensated for at the back side of the plates. The compensation flow can, for example, take place in the apparatus where the brine or the feed water leave the evaporator or condenser internals. The changing volume at the back side of the plates 61 can also, as an example, be closed with gaiters. The plates can also be placed in the inside, so that parts of the internals are flown through in counter current.

[01 16] The variety of possible forms and arrangements of oscillating displacement and/or a selectable stroke frequency enable opportunities to optimise the process and apparatus of the present invention. Instead of the deformation of the plates, an advantageous secondary flow can be created, for example, by fixed or elastic wings, preferably arranged axially vertically, located within the internals of the condenser and/or evaporator, or by oscillating elements such as oscillating fans. Additionally, the internals of the evaporator and the condenser could also be equipped with perforated plates, to create an oscillating secondary flow through the holes of the plates.

[01 17] Additionally, instead of the abovementioned secondary flow, it may also be advantageous to introduce a flow in the other direction. As the mainly vertical direction of the velocity of the main air flow of the process of the invention is usually small, with a pulsating vertical secondary air flow it can be possible and advantageous to generate a periodic reverse vertical flow. The increase of the relative velocity can also be achieved with small stroke volumes and a corresponding high stroke frequency. This can be used to ensure that no disadvantageous back flow is created.

[01 18] Another possibility for the generation of a secondary air flow according to the present invention is the use of a rotating conveyor system, for example blower- wheel (rotors), which effect the relative velocity whereby the secondary flow does not need to oscillate. In addition, or as a replacement to influence the relative velocity with a secondary air flow, the evaporator/condenser internals could be moved mechanically. It will thus be appreciated that various methods may be adopted to increase the relative velocity on the surfaces for heat and mass transfer, in combination or individually. For example, for the evaporator, for an advantageous combination of mechanical movement and the stimulation of a secondary air flow, internals such as round brushes can be rotated on a vertical axis.

[01 19] By the adoption of the process and apparatus of the present invention, it will be appreciated that large plants can be built to concentrate brine or to produce solvent. The advantages of a modular configuration can be used. For example, large systems can be built with serial and parallel operation of modules with a height of 2m, width of 2m and a depth of 2m, and continuously operated with a heat source of approximately 100 degrees Celsius.

[0120] On the other hand, the process and apparatus of the present invention can be used for the generation of pure water for households, for example for a desalination system which operates with a solar heater of 100 kWh/day, an efficiency of 7, and which produces approximately 1 cubic meter of water per day.

[0121 ] Advantageously, the process and apparatus of the present invention also allow for operation at the pressure of the humid air, which approximately corresponds to atmospheric pressure at the installation location, resulting in mechanical stress on the apparatus housing being low.

[0122] Also, with the use of the active wall and, ideally, the optimization control method outlined above, a high efficiency PRHD can be achieved by the process and apparatus of the present invention, allowing the externally supplied heat Q_ext to produce a multiple of condensate (product), compared to a plant without recovery of condensation heat with the same externally supplied heat. For this purpose, the cross flow between evaporator and condenser produced by forced convection with the aid of the active wall significantly improve the conditions for heat and mass transfer, the evaporation of pure water in the evaporator and condensation in the condenser, in comparison with known techniques.

[0123] Finally, the low operating temperatures (usually below 100°C) make it possible to use plastics, for example polypropylene, which are advantageously suitable for the cost-effective production of the evaporator and the condenser and enable a cost-effective, long-lasting, easy-to-maintain apparatus.

Claims

The claims are:

1 . A thermal separation process for the separation of liquids, the method including: a. humidifying air in an evaporator by evaporation of pure liquids from a heated raw liquid;

wherein an adjustable active wall connecting the evaporator and the condenser allows control of air flow between the evaporator and the condenser.

2. A process according to claim 1 , wherein the active wall includes controllable ventilators able to be configured between open and closed positions to increase or decrease the size and/or direction of one or more cross-flow openings in the active wall, and thus alter the flow of air passing through the ventilators between condenser and evaporator.

3. A process according to claim 2, wherein the active wall includes one or more controllable blowers able to increase or decrease the velocity and/or amount of air passing through the ventilators between condenser and evaporator.

4. A process according to claim 3, wherein the active wall includes control elements for controlling blower power and ventilator configuration.

5. A process according to any one of claims 2 to 4, wherein cross-flow openings oscillate to continuously change the air flow in the condenser and evaporator.

6. A process according to any one of claims 1 to 5, wherein control of the air flow between the evaporator and the condenser, via the adjustable active wall, enables the increase of relative velocity of humidified air at transfer surfaces in the evaporator and condenser.

7. A process according to any one of claims 1 to 6, wherein the control of the air flow between evaporator and condenser occurs continuously.

8. A process according to any one of claims 1 to 7, wherein the control of the air flow between evaporator and condenser occurs in response to changes in heat supply to the external heater.

9. A process according to any one of claims 1 to 8, wherein optimisation-control is used to control the air flow between the condenser and the evaporator by measuring the temperature t-ι at the entry of raw water to the condenser, the temperature t₂ at the exit of the condenser and the temperature t₃ at the exit of the external heater, and subsequently adjusting the active wall to control the air flow to maximise the efficiency P RHD in accordance with the equation:

PRHD = (t2 - ti)/(t₃ - t₂).

10. A process according to any one claims 1 to 9, wherein a secondary air flow is generated within either or both of the condenser and the evaporator.

1 1. A process according to claim 10, wherein the secondary air flow is lateral to the humidified air flow.

12. Thermal separation apparatus for the separation of liquids, the apparatus including:

13. Apparatus according to claim 12, wherein the active wall includes controllable ventilators able to be configured between open and closed positions to increase or decrease the size and/or direction of one or more cross-flow openings in the active wall, and thus alter the flow rate of air passing through the ventilators between condenser and evaporator.

14. Apparatus according to claim 13, wherein the active wall includes one or more controllable blowers able to increase or decrease the velocity and/or amount of air passing through the ventilators between condenser and evaporator.

15. Apparatus according to claim 14, wherein the active wall includes control elements for controlling blower power and ventilator configuration.

16. Apparatus according to any one of claims 13 to 15, wherein cross-flow openings oscillate to continuously change the air flow in the condenser and evaporator.

17. Apparatus according to any one of claims 12 to 16, wherein control of the air flow between the evaporator and the condenser, via the adjustable active wall, enables the increase of relative velocity of humidified air at transfer surfaces in the evaporator and condenser.

18. Apparatus according to any one of claims 12 to 17, wherein the control of the air flow between evaporator and condenser occurs continuously.

19. Apparatus according to any one of claims 12 to 18, wherein the control of the air flow between evaporator and condenser occurs in response to changes in heat supply to the external heater.

20. Apparatus according to any one of claims 12 to 19, wherein an optimisation- control is used to control the air flow between the condenser and the evaporator by measuring the temperature t-ι at the entry of raw water to the condenser, the temperature t₂ at the exit of the condenser and the temperature t₃ at the exit of the external heater, and subsequently adjusting the active wall to control the air flow to maximise the efficiency in accordance with the equation:

PRHD = (t2 - ti)/(t₃ - t₂).

21. Apparatus according to any one of claims 12 to 20, wherein the active wall includes lamellae and/or flat brushes at the cross-flow openings, whose position adjusts the air flow according to operating conditions such as external heat input.

22. Apparatus according to claim 21 , wherein elastic brushes or elastic packing materials are used as internals in the evaporator.

23. Apparatus according to claim 21 or 22, wherein a crystalliser is located below the evaporator.

24. Apparatus according to any one claims 12 to 22 wherein a secondary air flow is generated within either or both of the condenser and the evaporator.

25. Apparatus according to claim 23, wherein the secondary air flow is lateral to the humidified air flow.