IL28707A - Heat and/or mass transfer - Google Patents

Description

PATENTS AND DESIGNS ORDINANCE SPECIFICATION Improvements in heat and/or mass transfer ABRAHAM KOGAN, an Israel citizen, of 35a Rehov Trspeldor, Haifa do hereby declare the nature of this invention and in what manner the same is- to he performed, to he particularly described and ascertained in and by the following statement:- This invention relates t© the transfer of heat and/or mass. More particularly it is concerned with heat and/or mass transfer hereinafter referred t© as "transfer process of the kind specified", involving vaporisation and condensation in the su stantial absence of a solffd heat transfer condensation surface, such as a metallic condenser, wherein the transfer is improved In efficiency. It is further concerned with a transfer process of the kind specified* comprising tw© liquid streams having different solute concentrations, the two streams being at appropriate temperatures, such that the vapor pressure above a first stream is higher than the vapor pressure above a second stream, the streams being positioned so that vapor emanating from the first stream condenses and or dissolves in the second stream whereby the solute concentration in the first stream increases and the solute concentration in the second stream is decreased. The second stream may be substantially devoid of solute at the outset, he streams may interaet in a eeries of sequential stages and flow eo-currently or counter-currently in a continuous manner.

Thus, if the first stream is brine and the second stream is converted water the brine stream will become progressively cooled and concentrated whilst the converted water stream will become progressively heated. Such a transfer process can form the basis of operation of a multiple flash evaporator-condenser especially for use in the desalination of saline water.

The efficiency of a process of the kind specified is critically important in desalination processes because of the lew cost requirement for so-called sweet water. The It has been found in practice that with the condensation of the vapour on the surface of the cooler liquid a thin warm localized barrier layer of non-condensible gases is formed ©n the surface and this layer tends to insulate the bulk of the cooler liquid below from the vapours above thereby obstructing the vapour flow and reducing the rate of condensation* furthermore, it has been found that the directional flow ©f the vapours t© the upper surface of the condensing liquid hinders the diffusional removal of non-condensible gases from the condensing liquid surface and it has been found that in the course of a short time, a relatively high concentration of such gases is built up close to the surface ©f the condensing liquid which inhibits the condensation of the vapours thereon.

In an actual experiaent in which two streams of water with initial temperatures of 31.0°C and 26.S°©» respectively, were flown eoantercurrently in adjacent channels through an evacuated enclosure, it has been found that even when using water deaerated to contai less than 0.2 p.p.m. Oxygen, traces of non-condensible gases accumulated in the vicinity of the free surface ©f the cooler water stream, inhibit3¾)strongly vapour condensation. The measured concentration of non-condensible gases near the cold water surface was about 40 times higher than the concentration measured at a distance of 10 cm above this surface. Half an hour a$ter start of the experiment the concentration of non-condensible gases at the cold water free surface attained the value of SO 000 p. .m. When this thin concentrated layer Israel Institute of Technology TAB Rep. No. 60).

It is an objec of the present invention therefore to provide a new and improved heat and/or mass transfer process of the kind specified in which the above referred to desiderata are satisfied and the disadvantages wholly or largely overcome.

According to the present invention there is provided a process for transferring heat and/or mass from one stream of liquid of higher vapor pressure to another stream .of liquid of lower vapor pressure comprising the steps of flowing the liquid of lower vapor pressure along a channel which includes an apertured partition between the two streams, the liquid of lower vapor pressure flowing across the apertured partition, flowing the stream of liquid of higher vapor pressure along a channel which is separated from the apertured partition by a vapor transfer region and evolving a vapor from the liquid of higher vapor pressure to build up in the vapor transfer region beneath the apertured partition a vapor pressure higher than the pressure above the liquid of lower vapor pressure to prevent flow of the liquid of lower vapor pressure through the apertured partition while introducing the evolved vapor into the liquid of lower vapor pressure through the apertured partition.

-Preferably said second stream is arranged to flow in a supercritical channel flow. By "supercritical channel flow1* is meant channel flow under such conditions whereby small disturbances which may arise in any portion of the flow are not transmitted upstream of that portion.

For a fuller discussion of the concept of supercritical channel flow reference can be made to Engineering Hydraulics, By virtue of the fact that the vapour makes contact with the second liquid stream throughout the "bulk thereof the effective area of contact is greatly increased. Furthermore, the very penetration of the vapour bubbles into the second liquid stream results in the rapid and continuous renewal of the contact surface between the vapour and the second liquid stream, !his rapid renewal of the surface contact is actually enhanced by the shear forces in the boundary layer near the interspace between the bubbles and the surrounding liquid.

Furthermore, since the vapours rise through the second liquid stream from below there is no longer any question of the non-condensible gases accumulating -in steadily increasing concentration near the vapour contact surface of the second liquid stream and being prevented from being removed by downwardly flowing vapours. Instead non-condensible gases move, under their own buoyancy, to the upper surface of the second stream where they are continuously and naturally drained off.

According to the present invention there is also provided apparatus for use in the heat and/or mass transfer process as defined above and comprising a multistage housing accommodating both streams, a flow channel for one of the streams which includes an apertured partition dividing each stage of the housing into upper and lower sections and carrying the stream of liquid of lower vapor pressure through a heat and/o mass transferring stage of the housing, a flow channel for the stream of liquid of higher vapor pressure separated from the apertured partition by a vapor transfer region and carrying the stream of liquid of higher vapor pressure through the respective region and the apertured partition into the stream of liquid of lower vapor pressure, the pressure differential between the vapor pressures of the two streams causing the vapor evolved from the liquid of higher vapor pressure to flow into the stream of liquid of lower vapor pressure while preventing the flow of the stream of liquid of lower vapor pressure through the apertured partition, means separating the vapor transfer regions into a plurality of vapor transfer regions of progressively decreasing vapor pressures in the downstream direction of flow of the stream of liquid of higher vapor pressure, and a restricted passage between the stages for the flow of the stream of liquid of higher vapor pressure from a stage in which the vapor transfer region is of higher vapor pressure to a stage in which it is at lower vapor pressure, said flow sealing the restricted passage.

With such apparatus the pressure difference which exists between the lower evaporator chamber and the upper condenser chamber prevents the second feed stream from penetratin through the apertures in the partition, and, provided the second feed stream is sufficiently shallow, this pressure difference should be large enough to overcome the hydrostatic head of liquid in the condenser chamber and vapour from the first feed stream will then pass from the evaporator chamber through the apertures into the bulk of the second liquid stream wherein it condenses by direct contact with the second liquid stream.

In this way the first liquid stream is cooled as a result of vaporisation whilst the second liquid stream is heated as a result of condensation; heat is transferred. Furthermore, mass is transferred from the first to the second liquid stream. The a aratus serves therefore for the transfer of both heat Preferably, each enclosure is sub-divided by substantiall vertically disposed partitions into a plurality of stages whilst successive enclosures are connected, one with the other, so that the two liquid streams can flew, preferabl j countercurrently from enclosure to enclosure. Co-current flow is possible a well.

In accordance with a preferred embodime t flow of the first stream takes place under gravity whilst the second stream flowsj at least partially, against gravity under a driving force which arises from the variation of vapour pressure from stage to stage in the manner described in my co-pending Israel Application Ho. 23554 to which reference should be made.

Preferably, both a reams should be comparatively shallow and this again ©an be achieved by increasing the incline of the streams.

To obtain high efficiency of vapour flow from the evaporator chamber to the condenser chamber through the apertured partition it is necessary to reduce ae far as possible the pressure losses caused to the vapour stream in passing through th© apertures. Ac@ording to the present invention such conditions are ensured by the particular shape of the apertures. Any solid protuberances on the upper surface ®f the apertured plate, such as risers or slotted caps, are avoided. In preferred embodiments of the present invention the apertures have a tapering, conical or bell shape, with a large cross section on the evaporator chamber side and with a small cross section on the condeiser chamber side. With such shapes the restriction of the effective cross section of the vapour stream upon approaching the entry of an aperture is In a preferred embodiment of the present invention the axes of the apertures are directed at an angle to the p&ane of the partition and in the same general direction as the direction of flow of the second liquid stream thereon, l!he vapour passing through each aperture is thus substantially directed into the general flow direction ©f the second liquid stream. She obstruction caused to the flow of the seeond liquid stream by the entry of the vapour streams from each aperture into the bulk of liquid is thus minimized. Moreover, the momentum of the vapour stream is not lost completely, it being transferred t© a large extent to the second liquid stream.

In order to obtain an overall high rate of passage ©f vapour through the apertured partition it is important t© ensure stable plate operation in the sense that all the apertures in the plate should be active in passing vapour through them. Moreover, it is desirable that the vapour flow through the apertured partition should be uniformly distributed between all the apertures, so that each aperture should let through it a vapour flow of equal rate. his stability and uniformity of vapour flow through the apertured partition is obtained in accordance with the invention by positioning the apertured partition between the evaporator chamber and condenser chamber at an angle of inclination to the horizontal sufficient to ensure supercritical channel flow of the second water stream. Under such flow conditions the disturbances introduced into the second liquid stream by the penetration of a vapour stream through any aperture in the apertured plate are net transmitted upstream, and do not, therefore, influence flow conditions near apertures situated upstream. 28707 2 S!hus there is no substantial variation of total resistance to vapour flow from one aperture to another and under such conditions ©table and uniform vapour flow is obtained.

Equipment for use in accordance with the present invention will now be described by way of example and with reference to the accompanying drawings in which t Pig, 1 is a schematic side elevation of a heat and/or mass transfer equipment in accordance with the present invention} Fig. 2 is a schematic side elevation of one (the fourth) enclosure of the equipment shown in Fig. 1 on an enlarged scale j Figs. 5, 4 and 5 are longitudinal sectional views of differing forme of transverse partitions for use in equipment shown in Figs. 1 and 2; Fig, 6 is a schematic plan view of a still further form of transverse partitions for use i equipment shown in Figs. 1 and 2 \ and Fig. 7 is a schematic sectional view of a modified form of enclosure in accordance with the present invention.

Referring to the drawings, the equipment comprises a plurality (six) of enclosures 1-6 preferably of trapezoidal shape, each enclosure being, as shown clearly in Fig. 2 of the drawings, divided by means of a plurality of vertical partitions 7 into successive stages (l/l, 1/2 ... /n) (6/1, 6/2 ... 6/n)» each stage being divided by means of an upper transverse inclined perforated partition 8 into an upper condenser chamber a and a lower evaporator chamber b. Communication between successive evaporator chambers a is via an aperture 9 formed in the lower region of the separation vertical partitions 7 whilst communication between successive condenser chambers a is ia the ap 10 between the u er ed e of the vertical located in the lower region of each evaporator chamber b ana extending transversely th reaoross is a perforated, transversely directed plate 12 which is inclined with respect to the lower surface of the enclosure.

In operation a saline water stream 13 (constituti g the '' i s 11 feed stream) is passed through a direct contact heat exchanger 14 wherein it absorbs heat from a converted water stream 15 (constituting the "second" feed stream) and from thence passes through an external heater 16 (for xample, a steam heater) to enter the evaporator chamber a of the first stage l/l of the first enclosure 1. The saline water fills the lower region of the evaporator chamber a penetrating, in a fountain-like manner^ through the apertures in the trans* verse plate 12 thereby effectively mixing the saline water stream 13 in the evaporator chamber a. By virtue of the inclination of the transverse plate 12 the aaline water stream flowing along the pi&te 12 is relatively shallow thus facili-tating effective vapour disengagement. Means (not shown) are provided for continuously evacuating non-eendensible gases from the condenser chambers* As a result of the difference of temperatures prevailing in - successive stages and the consequent difference in vapour pressures prevailing in successive evaporator, chambers the saline liquid stream passes from stage to stage through the apertures 9 in the Vertical partitions 7 and against the influence of gravity in the. manner described in my e©- STo. 23534 pending patent application/to which reference can be mad*. 3?he converted water stream 15 which ha© been cooled in the direct contact heat exchanger 14 is fed into the eondenser chamber of the final condenser stage 6/n of for the production of a relatively shallow l quid layer which reduees the hydrostatic pressure of this liquid stream.

In order to ensure that the rate of downward flow ®f the cool converted water stream 15 under gravity is equal to the rate ©f upward flow of the warm saline water stre&ffl 13 under the vapour pressure driving force, and that there is n© undue accumulation ©f either stream as a relatively thick layer, and ea ing in aind the influence of friction, the enclosures are given a trapezoidal shape. A® a result the rates of flew of the two streams 15 and 15 are rendered substantially ef al and, additionally, the appropriate inclination ©f the transverse partitions 8 and plates 12 required t© achieve relatively shallow strearos can be provided for.

In order, however, t© avoid the vertical dimension of the enclosures 1-6 becoming unduly large the number ©f s a es in each enclosure is limited and, in view of the fact that the converted water stream 15 emerges from each enclosure at a level below that in which it is introduced into the next enclosure, pumps 17 are provided between successive enclosures so as to raise the converted water stream 15 to the required level.

As the preheated sali&e water stream 13 passes through successive evaporator chambers, flash evaporation ©f the saline water stream 13 takes place, the disengaging vapour rising and penetrating into the converted water stream 15 which flows along the inclined transverse apert red partition 8 in the respective condensation chambers and condensation of the vapour emitted in a particular evaper&tien chamber takes place throughout the converted water stream 15 in the associated condensation ehaaber. the vapour pressure established in each evaporation chamber is sufficiently high t© overcome the hydrostati© head of essure presented by the overhead shallow layer of the converted water stream 15 and in consequence the stream 15 continues t© flow along the apert red partitions 8 under the influence of gravity but doe» not percolate downwards through the apertures into the evaporation chamber©.

S e heated saline water stream 13 becomes cooled and more concentrated as a result of the evaporation therefroia whilst the converted water stream 15 beeeaaeai correspondingly heated s a result of the condensation of the vapour therein and acquires mass i the form of the condensed vapour.

As a result of the passage of the two streams through the stages of all of the enclosures the converted water stream 13 emerges* heated, from the first stage l/l of the first enclosure 1 and is pumped by means of a pump 18 through the direet contact heat exchanger 14 in which it gives up much of it® heat to the incoming saline water .stream 13. A portion of the converted water stream 15 emerging fram the heat exchanger 14 is recycled into the system so as to constitute the converted wate stream whilst the remainder ®f the converted water stream is reaoved as product water. Similarly, the concentrated residual brine is removed from the final stage S/n of a the final enclosure 6 via a pump 19,/portion of the concentrated "brine is removed as blowdown whilst the remainder is mixed with incoming make-up saline water and is passed through the direet contact heat exchanger 14 where it acquire® heat from the converted water stream 13 and emerge® therefrom to be further heated by the heater 16 and to be fed into the first stage l/l Various considerations govern the choice of the else, the shape, and the distribution of the apertures in the transverse partitions 8 between the evaporator and condenser chambers. Thus, on the one hand, these apertures must have a large enough overall area to allow for the effective penetrations of the vapour into the body of the converted water stream flowing on the apert red partition, ©n the other hand, the diameter of each aperture must be limited so as to avoid downflow of the converted water stream through the apertures from the condensing chambers into the evaporating chamber.

The overall area of the apertures depends, on the one hand, on the density of distribution of the apertures and, on the other nd* on the size (diameter) of each aperture. Thus, whilst theoretically it would be desirable t© distribute the apertures as elosely as possible together, a limiting factor arises in view of the fact that when the apertures are too close together it is found that the bubbles of vapour which pass through these closely spaced apertures into the converted water stream tend to coaleece into a single large bubble which often* passes straight through the converted water stream without condensing.

In psaetice it has been found, however> that with water temperatures which range between 30°Q and 60°C this danger of coalescence of vapour bubbles, with the consequent non-condensation thereof, is very considerably reduced with apertures whose diameters are less than 1.5 mm and whi h are so distributed as to have their centres spaced from each other by distances which are not less than 15 mm.

Similarly, with apertures of such sise and distribution no flow of the converted water stream under its own hydrostatic pressure and against the vapour pressure developed in the evaporating chamber. On the contrary, effective penetration of the vapour through the apertures and into the converted water stream has been found to take place.

It has furthermore been found to be advantageous to give the aperture a tapering, conical or bell shape in the manner illustrated in Figs. 5, 4 and 5. With such a shape a greater resistance is experienced by the liquid to downward flow agalnat the vapour pressure prevailing in the evaporation chamber than is experienced by the vapour to upward flow.

Whilst in the embodiments shown in Figs. 3 and 4 the axes of the apertures are directed substantially normally to the plane of the partition, in the embodiment shown in Fig. 5 the axis of the apertures is directed at an angle to the plane of the partition and in the same general direction as the direction of flow of the converted water stream thereon. With such a construction, the vapours flow into the liquid in the direction of flow thereof so that the momentum of vapour flow is not completely lost and is rather partially transmitted to the converted water stream. Furthermore, the vapour bubbles emerging from such inclined apertures tend t© be elongated and Inclined to the horizontal and thereby present an extended contact surface to the converted water stream so as to facilitate condensation.

Fig. 6 is a plan view of a partition which ©an be of metal or of a suitable plastic material and may be suitably insulated against heat transfer, and which is apertured with tapering apertures whose axes are inclined with effectively formed by punching or, alternativel , the partition can be cast ready apertured.

It will also be realised that by virtu¾ Qf the provisions of apertures whose axes are inclined to the p¾ane of the partitions and are directed in the general direction of flow of the cooling li¾uid a considerably increased resistance is presented to the downflow fo the converted water stream into the evaporation ehaiaber seeing that, in order for suc dewn ow to take place, the converted water stream mu¾t change its direction of flew by an angle of almost 180°.

In a practical example, using only one enclosure divided into four stages, the converted water stream was Introduced into the system in the last stage at a temperature ©f 38.8°0 whilst the heated aaline water stream was introduced into the system in the first stage at a temperature ©f 44.7°C, the rate of flow ©f each of the streams being 15 liters per minute. It is found that the converted water stream increased in temperature from enclosure to enclosure and left the enclosure at the first stage at a teaperature of 41.2°G, i.e. it had undergone a heating of 2.4°G whilst the heated saline water stream had undergone a cooling of a similar magnitude.

In this experiment both the upper apertured partition and the lower perforated plate were inclined at o 2 to the horison/i3n-! the respective flow direction, $h© water flows were in supercritical channel flow condition. fhe flow of vapour through the apertured partition wae stable and uniform.

In this experiment the rate of transfer of two streams amounts to 32 -Ι*·^- · will be readily hr.m . C appreciated that such a transfer ia very considerable and leads to very effective transfer of heat and/or mass.

Summing up the advantages of the heat and/or mass transfer method and equipment im cco dance with the present invention it can be stated that? 1. The effective area of contact between the converted water stream and the condensing vapour is very great as compared with conventional systems. 2. he very fact that the vapour bubbles penetrate into the body of the converted water stream results in the rapid and continuous renewal of the surface of contact between the liquid and the vapour, this being enhanced by the shear forces present in the boundary layer near the interface between the vapour bubbles and the surrounding liquid. 3. he present system is conducive to a continuous ^rain-off of non-eondensible gaaee seeing that bubbles of these noh^condensible gases form at the top of each vapour bubble in its passage through the liquid the bubble of non-condensible gas ultimately detaching itself from the vapour bubbles and being carried by buoyancy towards the upper surface of the converted water stream. This is in contrast to the conventional systems wherein the condensation of the vapour at the free surface ©f the cool stream was accompanied by the formation of a concentrated layer of non-iondensible gases at the interphase thereby tending to inhibit the continued condensation of the vapour. 4. $he use of tapered conical or bell shaped apertures in the partition between evaporator chamber and condenser - ΐβ - y chamber, with their longitudinal direction inclined to the plane of the partition, ensures vapour flow from the evaporator to the condenser with very small pressure losses.

. An inclination of the perforated partition sufficient to produce supercritical channel flow in the condenser chamber ensures stable and uniform vapour penetration through the partition, so that vapour flows simultaneously through all the partitions at a substantially equal rate* furthermore, and as explained above, the system allows itself to be readily adapted t© the system described in iay co-pending application wherein the heated saline water stream is raised from stage to stage by the driving force which arises out of the difference in vapour pressures between the stages* In such an arrangement therefore the vapour pressure is used both to overcome the hydrostatic head of the cool stream liquid and to propel the i line water stream from stage to stage against the influence of gravity.

Similarly, and as described in my co-pending application, each enclosure can be furthermore divided by a plurality ©f horizontal partitions into a plurality of pairs of coa-partments th© liquids flowing countereurrently in each pair of compartments transferring between them heat and mass.

Finally, whilst in this application the use of the system for the transfer of heat and/or mass between a saline liquid and a converted water stream has been particularly described with specific reference to the use of the system in desalination of saline water, it will be readily nders ood that the system is equally applicable to heat and/or mass transfer between any pair of liquid streams which are at In all the embodiments described above the enclosures were given a trapezoidal shape so as to ensure that the overall direction of flow of the cool converted water (under gravity) has a steeper inclination as compared with the overall direction of flow of the warm saline water stream (partially against gravity under the vapour pressure driving force), This divergence between the overall directions of flow of the two streams is provided for in order to take care of frictional losses of the streams. In consequence of this trapezoidal shape the number of stages in each enclosure is limited and pumps must be provided between enclosures so as to E&se, in each instance, the converted water stream from enclosure to enclosure.

The embodiment shown in Fig. 7 of the drawings is so constructed as to avoid or reduce the necessity for diverging overall directions of flow of the two streams. This is achieved by recovery of part of the momentum imparted by the vapour flow to the cool converted water stream and yet ensures equal rates of flow of the two streams.

As seen in Pig. 7 of the drawings an enclosure 21 is provided with upper and lower parallel defining walls. Extending transversely through the enclosure is a transverse partition 22 which divides the enclosure into a lower evaporation and an upper condensation section 23 and 24. The condensation section 24 is divided by means of vertical partitions 25 into successive stages 24/1 ... 24/n. Similarly the evaporation section 23 is divided by vertical partitions 26 into successive evaporation stages 23/1 ··♦ 23/n.

Each vertical partition 26 is formed integrally with a perforated transversely directed plate 27 whieh is inclined with respect to the lower surface of the enclosure. directed flange 28 which is spaced from the adjacent portion of the transverse partition 22.

Each transverse partition sectio between successive vertical partitions 26 is constituted by a substantially horizontal apertured portion 22a, a short rising portion 22b, and longer descending portion 22c.

In operation, the saline water stream 29 ("first" feed stream) passes through the evaporator stages in a similar manner to the stream 13 in the embodiments previously described, whilst the converted water stream 30 flows in a counter current direction through the condenser stages.

In the present case however the vapour which passes from an evaporator to a condenser stage through the apertured portion 22a transfers sufficient momentum to the converted water stream so as to cause It to "climb" the rising portion 22b.

The perforated portion 22a can be horizontally disposed (as shown in the Figure) or it can be given a slight upward slope. Alternatively the portion 22a can be rendered slightly convex so as to achieve an even flow of vapour through the apertures and thereby ensuring an effective control on the thickness of the liquid layer on this portion 22a.

This modification reduces or eliminates the divergence between the overall directions of flow of the first and second feed streams and in this way a multistage system can be constructed without the necessity to provide for pumping between stages or groups of stages.

Claims (15)

1. A process for transferring heat and/or mass from one stream of liquid of higher vapor pressure to another stream of liquid of lower vapor pressure comprising the steps of flowing the liquid of lower vapor pressure along a channel which includes an apertured partition between the two streams, the liquid of lower vapor pressure flowing across the apertured partition, flowing the stream of liquid of higher vapor pressure along a channel which is separated from the apertured partition by a vapor transfer region and evolving a vapor from the liquid of higher vapor pressure to build up i the vapor transfer region beneath the apertured partition a vapor pressure higher than the pressure above the liquid of lower vapor pressure to prevent flow of the liquid of lower vapor pressure through the apertured partition while introducing the evolved vapor into the liquid of lower vapor pressure through the apertured partition.

2. A process according to Claim 1 in which the flow channel for the liquid of lower vapor pressure is sloped downwardly to insure supercritical channel flow along the apertured partition.

3. A process according to Claim 1 or 2 in which the evolved vapor is directed into the liquid stream of lower vapor pressure in a direction having a component in the direction of flow of the stream of liquid of lower vapor pressure.

4. A process according to Claim 1, 2 or 3 in which the streams of liquid are caused to flow in countercurrent directions. - 20 - 287C#2

5. A process according to any one of the preceding claims in which the channel for the stream of liquid of lower vapor pressure includes an upwardly sloped portion downstream of the apertured partition and utilizing the upward flow of vapor through the apertured partition to direct the stream up said inclined portion.

6. A process according to any one of the preceding claims including the step of transferring heat from the liquid which has emerged from the stream of liquid of lower vapor pressure to the liquid which is to be introduced as the stream of liquid of higher vapor pressure.

7. A process according to any one of the preceding claims for transferring heat and/or mass from an underneath stream of liquid to an overhead stream of liquid in a multistage housing ♦ comprising the steps of flowing the overhead stream along a flow channel which includes an apertured partition between the two streams in each of the stages of the housing, the liquid of lower vapor pressure flowing across the apertured partition, flowing the underneath stream of liquid beneath said apertured partition and beneath a vapor region intermediate the apertured partition and the underneath stream, evolving a vapor from the underneath stream in one of the stages of the housing to build up a vapor pressure in the vapor region to prevent flow of the overhead stream through the apertures and to introduce the evolved vapor into the upper stream through the apertures, flowing the vapor upwardly in that stage through the apertured partition into the overhead stream, and flowing the underneath stream to the next stage wherein the region below the apertured partition is at lower vapor pressure to evolve additional vapor therein which is transferred upwardly from the vapor region that stage, the pressure differential of the vapor regions ©f adjacent stages flowing the underneath stream from stage to stage.

8. A process according to Claim 7 in which the pressure differential between stages causes the underneath stream to flow at least in part against gravity in flowing from a vapor region of higher vapor pressure to a vapor region of lower vapor pressure.

9. A process according to Claim 7 or 8 including the step of building up a pressure head intermediate stages in the flow of the upper stream to act as a seal between stages and to insure a flow of the upper stream through each stage.

10. A process for transferring heat and/or mas® from a stream of liquid of lower vapor pressure substantially as hereinbefore described by way of example and with reference to the accompanying drawings.

11. An apparatus fo use in carrying out the process according to Claim 1 for transferring heat and/or mass from a stream of liquid of higher vapor pressure to a stream of liquid of lower vapor pressure through a plurality of stages comprising a multistage housing accommodating both streams, a flow channel for one of the streams which includes an apertured partition dividing each stage of the housing into upper and lower sections and carrying the stream of liquid of lower vapor pressure through a heat and/or mass transferring stage of the housing, a flow channel for the stream of liquid of higher vapor pressure separated from the apertured partition by a vapor transfer region and carrying the stream of liquid of higher vapor pressure through the respective stage of the housing, the vapor evolved from the stream of liquid of higher vapor pressure passing throug - 22 - differential between the vapor pressures of the two streams causing the vapor evolved from the liquid of higher vapor pressure to flow into the stream of liquid of lower vapor pressure while preventing the flow of the stream of liquid of lower vapor pressure through the apertured partition, means separating the vapor transfer regions into a plurality of vapor transfer regions of progressively deereasing vapor pressures in the downstream direction of flow of the stream of liquid of higher vapor pressure, and a restricted passage between the stages for the flow of the stream of liquid of higher vapor pressure from a stage in which the vapor transfer region is of higher vapor pressure to a stage in which it is at lower vapor pressure, said flow sealing the restricted passage.

12. An apparatus according to Claim 11 in which the flow channel for the liquid of lower vapor pressure descends from * stage to stage to cause the liquid stream of lower vapor pressure to flow from stage to stage by gravity.

13. An apparatus according to Claim 11 or 12 in which the channel for the liquid of lower vapor pressure includes an upwardly sloped portion downstream of the apertured partition, the flow of vapor through the apertured partition lifting the stream along said upwardly sloped portion,

14. An apparatus according to any one of Claims 11, 12 or 13 in which at least part of the channel for the stream of liquid of higher vapor pressure is inclined and including means for introducing the stream into a stage below the higher end of the flow channel, the low pressure in the vapor region of the stage evolving the vapor and flowing the stream against gravity to the higher end of said flow channel.

15. An apparatus according to any one of Claims 11 to 14 and including means upstream of said apertured partition for building up a pressure head which causes the upstream liquid to flow across the apertured partition. 16„ Apparatus for transferring heat and/or mass from a stream of liquid of lower vapor pressure substantially as hereinbefore described by way of example and with reference to the accompanying drawings. For the Applicant DR. R3IMQLD COM AJtfD PARTNERS By: r IS:CB