IL39234A - Distillation apparatus - Google Patents

Description

on"? yaum ipn 'apnna o**? ^ IMPROVEMENTS1 RELATING TO DISTILLATION APPARATUS This invention relate:?, to distillation of lic ids and more par icularly to distillation apparatus v.'hich is used for trans erring the vapour of a solution through a . : membrane without trans erring a liquid- from the solution through the sam membrane. ■ Distillation of certain solutions., such as salt, water, is very efficiently carried out by the use of vapour-permt.able membranes wherein a solution to be distilled is heated with the vapour therefrom nrj.grating through the vapour-permeable, membrane without allowing the passage of liquid therethrough. Such a membrane is shown in Israeli Patent' Application No. £6866..in which polyvinyl chloride, cellulose nitrate, cellulose acetate, cellulose, triacetate, nylon or polytetrafluore hylene are described as being useful as materials for such membranes. In another proposal polyvinylldene fluoride is set forth as being a suitable material for such vapour permeable membranes. These, proposals also teach the use of elongated channels formed by corrugations on ■ the surface of a vapour-impermeable film, in conjunction with adjacent vapour permeable membranes to provide flow spaces on both sides of. the film. The corrugated design was 'primarily • ■ intended to improve flow through a multiple stage still, i.e., a still comprising- a series of vapour-permeable membranes and vapour-impermeable films 'in face-to-face relationship.

In other words, the disposition of two ver thin sheets such as are appropriate in such a still construction have a tendency to adhere to one another possibly inhibiting flow between these two faces. The addition of the elongated depression or corrugations in that structure had the effect of allowing relatively unrestricted flow from end-to-end across a given sheet.

It is a problem with the desalination of sea -water in apparatus involving vapourisation across a. membrane, that there is a tendency for the development of encrustations on the distilland side of the membrane where the saline water flows. These encrustations are formed by various chemical components of saline water that come out of solution ; and adhere to the membrane. With the corrugated film-.. arrangement described above, encrustations may still develops, and if the membrane has a flat surface which contacts the crests of the .corrugations, when an encrustation grows to block completely the channel in which it is situated further flow of distilland through that channel is prevented. , In this way the efficiency of of the apparatus is impaired since the rate/flow of distilland and the area of membrane over which evaporation can occur may be reduced in relatively large steps.

In order to overcome this and to maintain the effective area of the membrane as high as possible despite blockages which may occur in some channels, we propose to provide the face of the membrane bounding the distilland channels formed by the corrugations of an adjacent film with a eeries of parallel _ ridges and depressions extending substantially perpendicularl to the direction of the film corrugations.

Thus, according to this invention, a distillation stage for use in a rautiple stage distillation apparatus comprises a micro-porous membrane which is impermeable to liquid and permeable to ' the vapour thereof and which has one face formed with a number . o 'parallel ridges and depressions, and a pair of liquid and vapour impermeable corrugated films disposed one on each side and in contact with the faces of the membrane so that the corrugations form channels betwee the films and the membrane, the films and the membrane being arranged so. that the corrugations extend substantially at right angles with respect to the ridges and depressions. ■ ' Preferably the channels between the ridged surface of the membrane and the adjacent film are arranged to receive distilland liquid, preferably so that the liquid is caused to flow in the direction of the film corrugations. Since the corrugations and ridges run perpendicularly with respect to each other, the contact between the membrane and the film occurs only at the points of intersection between the ridges and the crests of the corrugations. Thus, there is always an alternative path for liquid to flow round any obstruction that may present itself, such as in the form of an encrustation, in. the depressions or corrugations.

For example, if a corrugation becomes blocked at any point along its length, the path of liquid flow' in that corrugatio deviates at the obstruction into the next free corrugation via an adjacent depression. Once past the obstruction the liquid v/ill tend to spill back into the first corrugation via a further depression.

An example of a distillation stage in accordance with the present invention is described in the following description of a distillation apparatus, referring to the k the accompanying drawings, in v/hich w Figure 1 is a perspective view of the apparatus with parts broken away to illustrate some internal detail; Figure 2 is a perspective view- of a section of the stack of membranes and films contained in the apparatus shown in Figure 1, the components being cut away in a step fashion to illustrate constructional detail; Figure 5 is a partially exploded perspective view of the stack section shown in Figure 2; Figure K is a section through the apparatus, on a distorted scale taken on the lines : in Figure 1; and Figure 5 is a section through the apparatus on a distorted scale taken on the line 5j5 in Figure 1.

There is illustrated a multiple stage distillation apparatus, or still, embodying the invention, being shown in Figure 1 as generally designated 10. The still includes as basic components thereof, a multiple stage distillation unit 12 composed of a multiplicity of pprous membranes and barrier films secured together in stacked relation and enclosed in a ■ housing designated 14. The membranes and films are sheet-like elements all .substantially rectangular and having the same dimensions "and are arranged with their edges in alignment so that the unit 12 takes the form of a relatively thin parallel-epipedon.

In more detail, the unit component 12 includes a multiplicity of microporous, vapour permeable membranes l6 and liquid and vapour impermeable barrier films l8. The me anes and barrier films are arranged in face-to-face relation in alternating order to form two rectangular stacks which are joined together in a manner to be described to form the unit 12. Each membrane 16 is preferably formed with a smooth or plane surface and its other surface having a corrugated appearance being formed with alternating parallel depressions and raised sections having a generally sinusoidal configuration. Each membrane is formed with a high proportion, e.g. in the order of 80 , of microscopic through passages or pores for conducting the vapour of a liquid, such as water, while preventing the passage" of liquid by capillary action. Although polymeric materials useful for the membranes and methods of manufacture are disclosed in the aforementioned patent, particular mention should be made of polyvinylidene fluoride as a preferred membrane material and the so-called " solvent-non- solvent" casting process as the preferred method of forming the membrane.

- Although the construction and composition of barrier films 18 is described in the aforementioned patent, it should be noted that the preferred polymeric material for the barrier films is a polycarbonate such as sold by General Electric Co. under the trademark "Lexan" . Films l8 are corrugated so that when interposed between adjacent porous membranes 16, as shown in Figures and 5, the corrugations will contact the surfaced of the porous membranes forming channels for the flow of liquids in contact with the membranes. As' indicated in the drawings, the corrugated barrier films l8 cooperate with porous membranes ΐβ to form a multiplicity of distilland. channels 20 through which a feed or distilland- liquid such as salt water is circulated; and a multiplicity of distillate channels 22 in which vapour of the distilland liquid is condensed to form distillate liquid.

The essentia], components of a distillation stage of the still comprise a microporous membrane l6 through which the vapour . of the distilland is transferred and means such as a pair of barrier films l8 cooperating with the membrane to form distillate and distilland channels on opposite sides of the membrane. For the purpose of clarity of illustration, the thickness of the components has been exaggerated and the still is shovm' in the drawings as including only four distillation stages. However, it should be understood that in actual practice, such a still would normally ■ comprise a very large number of distillation stages, for example, as many as l8o. The actual number of stages, however, will depend upon the. temperature differential between the hottest and coldest stages and the temperature differential between succeeding stages. In the multiple stage still, (see Figures and 5) heat is transferred to the distilland liquid in the distilland channel (or channels) 20 of the first or hottest stage (or stages) to vaporize the distilland liquid. Heat is transferred from the distillate liquid of the last or coldest stage (or stages) of the still to condense the vapour transferred through 'the adjacent membrane to form distillate in a distillate channel 22. in each stage, energy is transferred as flux, i.e. vapour, through the porous membrane and then transferred by conduction through the adjacent' barrier film from the distillate to the distilland liquid of the next succeeding stage. In the preferred form shown in the drawings, the still comprises two series of distillation stages with the first or hottest stages being located innermost and successive cooler stages arranged outwardly therefrom. Thus, distillation unit 12 comprises two inner porous membranes 16 and, alternating outwardly therefrom, barrier films 18 and additional . porous membranes, four membranes and four barrier films being illustrated in the drawings.

The distillate and distilland channels are formed and separated from one another to confine and conduct the flow of the liquids within the channels by damming or blocking the channels in selected regions. The channels are dammed or blocked by sealing elements located within the channels between adjacent membranes and barrier films and bonded to the facing surfaces of the membrane and films. The distilland channels are located on the inner or hot side of each microporous membrane 16 and the distillate channel are located on the opposite sides of the membranes from the distilland channels. The channels are blocked in such a way that the flow of the distilland and distillate liquids is in parallel directions from end-to-end of the unit .12 and in the form -shown, sealing elements are bonded to the opposite lateral margins of adjacent membranes and barrier films to prevent the admissio or escape of liquids from the distilland and distillate channels at the sides thereof. The distillate channels 22 are blocked at opposite ends by sealing elements 26 and 28 secured between and to the end marginal surfaces of adjacent membranes and barrier films between the cold side of each membrane and the hot side of the adjacent barrier film. The distilland channels 20 are blocked at one end, termed the exit end, by sealing elements JO also bonded to the end marginal surfaces of adjacent membranes and barrier films. The opposite ends of the distilland channels remain open at the edges of the membranes to permit the introduction of distilland liquid into the distilland channels.

Means are provided for withdrawing liquid from any particular channel at or near an end thereof at which the channel is blocked. These means include outlets or conduits provided by holes formed in alignment through the stack of membranes, barrier films and sealing elements. Where a hole is formed in a sealing element within a particular 'channel, there is no communication between the channel and the hole because the channel is blocked in regions surrounding the hole. However, where the channel remains unblocked in regions surrounding the hole, there will be communication between' the hole and the channel. In this way, each set of aligned holes through stacked membranes, films, and sealing elements forms a conduit through the stack communicating with selected channels.

The distillation unit. 12 comprises two sets of' membranes and barrier films secured together in face-to-face stacked relation by sealing elements bonded to the membranes and barrier ' films. '.

Each of these sets, one of which is shown in exploded form in · Figure * is bounded on one face by a microporous membrane 16, on its opposite face by a barrier film l8 and includes a multiplicity (one of each are shown) of barrier films and membranes arranged in alternating order between the outer membrane and barrier film. A pair of these sets are arranged with membranes . l6 in spaced face-to-face relation separated by a corrugated spacer sheet 32. The spacer 32 is formed of a liquid and vapour impermeable polymeric sheet material such as a polycarbonate and is corrugated so as to cooperate with the adjacent membranes 16 to form -heating channels y\ through which a heating liquid is · circulated for transferring heat to the still to help establish thermal gradients from the inside of the still outwardly towards the exterior thereof. The thickness of the sheet material comprising the spacer 2 may be substantially greater than the thickness of the sheet material comprising the barrier films l8 . and the corrugations in the spacer 2 are substantially deeper, e.g. have a greater amplitude than the corrugations in the barrier films 18, so that the flow capacity of the channels 3^ s substantially greater than the flow capacities of the channels 20 and 22 defined by the corrugations in the barrier films. The corrugations. . of the spacer 2 are parallel with the corrugations of the barrier films, and the spacer 3 is secured to the adjacent membranes l6, and the channels 3 blocked at their lateral edges, by seating elements 36 · By virtue of this construction, a heating liquid can be introduced into the channels at one end edge of the · . stack and withdrawn from the channels at the opposite end edge thereof after giving up heat to the liquids within the distillate and distllland channels during its passage through the channels 34. In the preferred example of the invention, the heating liquid is the feed or distllland liquid, e.g. salt water, so that a portion of the heating liquid circulated through the channels 34 is transferred as vapour through the inicroporous membranes l6 defining the outer sides of the heating channels.

The distillation unit 12 is enclosed within a housing 14 providing for circulation of the various liquids including the feed liquid utilized both for heating and the distilland, and a cooling liquid. In the form shown in Figures 1, 4 and 5 the housing 14 comprises first and second complementary sections 38 and 40, each having a generally rectangular main wall 42 and dependent side walls 44 each formed with a flange 46. The housing sections 38 and 4o are secured to one another at the flanges 4β to form a ■ shallow- enclosure having length and width-dimensions exceeding the length and width of the unit 12 and a depth dimension, measured between the main walls 42, approximately equal to the depth of the unit 12. To facilitate fabrication and assembly, housing sections 38 and 4θ are preferably identical in size, shape and conformation. Thus, only one se of tools is required and selective assembly is made unnecessary.

The corners of the housing are chamfered to form dependent corner walls designated 48 disposed at 5° angles with respect to the dependent side walls 44 and including projecting sections 50 for receiving the corners of the unit 12 located within the enclosure provided by the housing 14.

A sealant adapted to adhere to the housing sections and to the edges of the distillation unit 12 is introduced into the projecting sections 50 to form a seal between the corners of the unit 12 and the corner walls 48 of the housing to form four chambers between the side walls of the housing and the edges of the unit 12, these chambers being designated first, second, third and fourth and numbered 56, , 8, and 54 respectively. The second chamber 5 is located at the edge of the unit 12 at which , the distilland channels 22 are unblocked or open, and the fourth chamber 54 is located at the opposite edge of the unit 12 where all of the channels, except the heating channels J are blocked. Thus, feed or distilland liquid introduced into the second chamber 52 will enter and flow through the distilland channels 20 and the heating channels toward the opposite edge of the unit.

The portion of the heated distilland liquid which passes through the heating channels \ and is cooled during its passage, is collected in the fourth chamber from which it is conducted from the still. · The first and third chambers 6 and 8 are located at the other two sides of the still; that is, at the sides thereof at which all of the distilland, distillate, and heating channels are blocked, and communicate with coolant channels designated 60 through which a coolant liquid may be circulated in contact . . with the outer barrier films 18 of the unit 12. The main walls k2 of the housing are formed with corrugations similar to those in the. spacer, sheet 32, but disposed at right angles to the latter, which cooperate with the outer barrier films 18 to form the coolant channels 60. The coolant channels are blocked at the edges of the unit adjacent the second and fourth chambers 52 and 54 by sealing elements 62 bonded to the facing surfaces of the outer barrier films 18 and the main walls k2 to prevent the exchange of liquid between the cooling channels and the second and fourth chambers. By virtue of this construction, a coolant liquid introduced into the first chamber 6 will be conducted through the channels 60 against the outer surfaces of the unit 12 to the third chamber 58 from which the coolant liquid may be withdrawn, thus establishing thermal gradients across the sections of the unit 12 between the heating channels 3 and the coolant channels 60.

The structure of the still is especially designed to facilitate fabrication and assembly of the components as well as to optimize liquid transfer, heat exchange, and free flow of liquids particularly insofar as these factors effect efficiency. With regard to fabrication and assembly of the still 10, all of the components thereof are preformed or fabricated as sheets and films so that assembly is essentially a positioning and stacking operation followed by the application of heat and pressure to the assembly to bond the elements to one another to form an integral unit 12 which is then sealed within the housing.

The membranes 16, barrier films- 18, and sealing elements are constructed and positioned to provide for the necessary channels and conduits for the various liquids and to reduce resistance to flow of the liquids.

In the operation of the still, the heated distilland liquid, e.g. saline water, is caused to flow through the distilland channels between adjacent membranes and barrier films in a direction parallel to the direction of the corrugations of the barrier films. The distillate or- product is formed by condensation of vapour which passes through the membranes into the distillate channels, and, in the preferred form of still shown, is withdrawn from the end of the still at which the distilland liquid is introduced so that the flow of distillate is counter to the flow of distilland. The distilland liquid is introduced into the distilland channels at one end of the unit 12 from the second chamber 52 and flows through the distilland channels to conduits 88 formed by aligned holes lso designated 88 in the membranes 16, the barrier films 18, and the sealing elements 26 and 62, as well as additional sealing elements 0 located within heating channels 34 and '' bonded to the opposite surfaces of the spacer sheet 32 and the adjacent membranes 16. The sealing elements 90 are spaced from one another to permit flow of the heating liquid from the heating channels into the fourth chamber 5J while blocking the channels in regions surrounding the holes 88 thereby preventing flow of the heating liquid into the holes 88.

Similar conduits 92 are provided at the opposite end of the still for withdrawing distillate liquid from the · · distillate channels 22. The conduits 92 are constituted by the holes, also designated 92, formed in the membranes, the barrier films, the sealing elements 62 and additional sealing elements 9^, similar to the elements 90, provided within the distilland channels 20 and heating channels 3 «. The elements 4 function to block the channels 20 and 3^ in the regions surrounding the holes 92 to prevent the flow of feed liquid into the conduits 92 through which the distillate is withdrawn from the still. The conduits 88 and 92 are offset from one another so that each of the conduits 88 is aligned with a space between a pair of conduits 92 and vice versa, for reasons which will appear hereinafter. It will be noted that the sealing elements 28 and 30, which block the ends, respectively, of the distillate and distilland channels are formed with sinusoidally shaped inner edges located so that the conduits 88 and 9 open into the distilland and distillate channels respectively. Because of the spacing of the conduits there is required to be some flow of the distillate and distilland liquids transverse to the direction of the corrugations in the barrier lms 18 jn order for the liquids to flow into the conduits, and the sinusoidal edge configuration of the elements 28 and 30 tends to guide the flow of the liquids towards the conduits 88 and 92 as required.

As previously mentioned, the surface of each porous membrane 16 defining one side o each distilland channel is formed with alternating parallel ridges and grooves or corrugations, extending perpendicularly to the corrugations of the barrier films 18. This construction is provided to. reduce the possibility of, and loss of efficiency due to, blockages within the distilland channels which occur, for example, through scaling, and to facilitate the flow of the distilland transverse to the barrier film corrugations in the regions of the conduits 88. The crossed corrugations or grooves in the facing surfaces which bound each distilland channel 20 perform several functions tending to achieve the desired aims. First, the crossed corrugations tend to increase turbulence within the distilland liquid which in turn tends to prevent scale accumulation and adherence thereof to the membrane and barrier film surfaces. Should any scale buildup occur within a channel formed by a groove in a barrier film, the grooves in the membrane and adjacent grooves in the film will provide alternative paths or channels around the obstruction, thereby eliminating regions of stagnant liquid in which the concentration of the scale producing agents might tend to build-up and produce additional scale. The turbulent flow and alternative flow paths provided by the crossed grooves also tend to prevent particulate matter from becoming lodged in the distilland channels and, in the event that a particle do become lodged in a distilland channel, alternative paths are again formed around the particle to prevent the occurrence of stagnant regions resulting in scale build-up „ Furthermore the grooves in the porous membranes provide channels for the transverse flow of the distilland liquid in the regions of conduits 88. This is particularly important in these regions since it is there that the temperature and the concentration of dissolved solids in the distilland are greatest, thus making conditions more conductive to scale formation.

The sealing elements are preformed of an adhesive material adapted to bond to adjacent membranes, barrier films, the main walls of the housing and the spacer sheet 32. The sealing elements may be preformed to the desired shape and thickness from an organic polymeric material in a thermoplastic state which is caused to bond by the application of heat and pressure.

The distillation unit 12 is then assembled by alternately positioning and stacking the membranes, and barrier films, and inserting the sheet 32 and the sealing elements as appropriat and then subjecting the stack of the assembled components to heat and pressure to activate and cure the adhesive sealing elements to bind the membranes, films, and spacer sheet to one another to form the various channels. The conduits 88 and 92 are then formed by the simple expedient of drilling through the unit in the appropriate places, a conventional paper drill being suitable for this purpose.

Except for connections to the conduits 88 and 92 and to the chambers 52, 5^, 56, and 8, the remainder of the assembly process involves locating the unit 12 between the housing sections 38 and together with the sealing elements 62, and subjecting the assembly, i.e. the still 10, to heat and pressure to bond the sealing elements 62 in place between the main walls 42 of the housing and the outer faces of the unit 12. The flanges 6 of the two housing sections are secured to one another either by conventional methods such as the use of an adhesive or by welding. An adhesive sealant in fluid form is introduced into the projecting sections 0 to form a seal between the corner walls of the housing and the corners of the unit 12 to divide the housing into the chambers 52, ^, 56, and 58. Sealants suitable for this purpose include water insoluble cements or heat curable rubber solutions having their .. v a low enough viscosity to permit ifee introduction into the projecting sections 50, such as by way of a hollow needle or syringe.

•Means are provided for introducing liquids into and withdrawing liquids from the still 10, specifically for introducing heated and deaerated feed liquid (distilland) into the second chamber 52, withdrawing the cooled and partially depleted heating liquid from the fourth chamber *1, withdrawing the effluent liquid from the distilland channels 20 by way of the conduits 88, withdrawing distillate liquid from the distillate channels 22 by way of the conduits 92, introducing coolant liquid into the first chamber 56, and withdrawing the coolant liquid from the third chamber 58. These means involve drilling holes in the main wall 42 of the housing section 38 which open one into each of the chambers 52.» 4, 56 and 8, and further holes through the main wall 42 and the elements 62 which open into the conduits 88 and 92. In the example shown, nipples or short tubes 100, 102, 104, 106, 108, 110 are then attached to the main wall 42 around the holes. The nipples 100, 102, 104, IO6, are secured around the holes opening into the chambers 52, 54, 6, and 8 respectively The nipples 108 are secured around the holes 88 in the main wall 42 and the nipples 110 are secured around the holes 92.· The nipple 100 therefore becomes the inlet for the feed distilland liquid and since the flow rate through it must be greater than the flow through any other nipple, the internal diameter of the nipple 100 is larger than the diameter of the other nipples. The nipples 108 and 110 may have substant ally the same diameter in examples where the effluent flow rate approximates the product flow rate.

The nipples may be formed of the same material as the housing and may be attached to the main wall 42 in any conventional manne such as by welding or by a suitable adhesive. In use, the nipples are connected to external supply and return conduits of a distillation system of which the still 10 forms a part.

It 'will.be seen from the foregoing that the distillation apparatus described provides a structure which is relatively simple to fabricate and assemble to form a relatively complex liquid distribution system providing for desirable heat exchange and temperature gradients; minimum resistance to liquid flow; poor conditions for scale formation; and optimum conditions for preventing flow blockages. The still structure is designed to permit fabrication entirely of polymeric materials in sheet form and provide for quick and easy connection to the other components of an overall distillation system including means for circulating liquids to and from the still.

Claims (6)

1. A distillation stage comprising a microporous membrane which is impermeable to liquid and permeable to the vapour thereof and which has one face formed with a number of parallel ridges and depressions, and a pair of liquid and vapour imperme able corrugated films disposed one on each side and in contact with the faces of the membrane so that the corrugations form channels between the films and the membrane, the films and the membrane being arranged so that the corrugations extend substantially at right angles with respect to the ridges and depressions . - .

2. A distillation stage according to claim 1, in which the other surface of the membrane is substantially smooth.

3. ■ A distillation stage according to claim 1 or claim 2, in which the ridges and depressions of the membrane have a generally sinusoidal configuration.

4. H. A distillation stage according to any one of claims 1 to 3, including means for causing a distilland liquid to flow between the ridged surface of the membrane and the adjacent film in the direction of the corrugations.

5. A distillation stage according to claim 4, in which the channels between the other surface of the membrane and the adjacent film are sealed around the whole periphery of the stage by means of sealing elements bonded between the margins of the membrane and the film.

6. A distillation stage according to claim 1, substantially as described with reference to_the accompanying drawings. Tel-Aviv, dated 16.4.1972