IL31044A - Apparatus for increasing the concentration of a less voltatile liquid fraction in a mixture of liquids - Google Patents

Description

APPARATUS FOR INCREASING THE CONCENTRATION OF A LESS VOLATILE LIQUID FRACTION IN A MIXTURE OF LIQUIDS nan-wra ηιποπ ρ^ππ na*nn mairft ]ρηπ This invention relates to an aApparatus for increasing the concentration of a less volatile liquid fraction in a mixture of the liquid and a more volatile liquid, and particularly for the concentration of heavy water in a mixture including ordinary water. The apparatus comprises a multiplicity of microporous membranes substantially impermeable to the liquids of the mixture and permeable to the vapors of the liquids, sandwiched together to form a column between means located at the ends of the column for creating a temperature gradient across the column. The mixture is introduced into a medial section of the column and circulated toward the hotter end thereof as thin films between adjacent membranes to cause flow of the xnMnex less volatile (higher boiling point) fraction toward the hotter more end of the column and flow of the ±ssx volatile (lower boiling point) fraction toward the colder end resulting in stripping or depletion of the lower boiling point fraction in the colder portion of the column and concentration or enrichment in the hotter portion of the column.

Existing systems for isotope concentration and particularly for the concentration of deuterium require a very large capital investment represented by the cost of elaborate and extensive equipment as well as a relatively large quantity of the isotope, e.g., heavy water, in the form of "hold-up" in the system which frequently amounts to the major part of one year's production.

An object of the present invention is to provide a system for concentrating a liquid fraction Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

The apparatus embodying the invention includes, as a basic component thereof, a multiplicity of micro-porous patent and operates according to essentially the same basic principles as the distillation above apparatus described in the Hsodsgaanac application. The micro-porous membrane is a thin, porous polymeric film having a high percentage of voids in the form of microscopic passages extending completely through the membrane. The pores or passages contain substantially only gases and are of a size such that they will readily pass gases including the vapor of a particular liquid such as water, but will not pass the liquid in the absence of a substantial hydrostatic pressure differential. This impermeability of a membrane to the liquid is a function of pore size, surface tension of the liquid, and contact angle which, as described in the aforementioned application, are determined according to well-known physical principles governing the flow of liquids by capillary action, to enable the employment of a membrane having the largest possible pores consistent with I the impermeability to the liquid. The porous membrane functions as a liquid-impermeable., vapor-permeable barrier between a body of a liquid mixtjure such as saline water or heavy and ordinary water, and another body of liquid including a higher proportion (up to 100%) of the more volatile liquid fraction. The liquids are circulated as thin films in direct contact with opposite sides of the membrane to vaporize the liquid, and heat is transferred from the liquid on the opposite side of the membrane causing the vapors to pass through the membrane and be condensed on the cooler side thereof.

A single microporous membrane with layers of liquid in contact with opposite surfaces and means for transferring heat to and from the layers of liquid constitute a still which lends itself advantageously to multiple staging in which a plurality of the porous membranes are arranged in face-to-face relation. Heat is transferred from the liquid on the hot side of each membrane to the liquid on the cold side of the membrane of the next succeeding stage so as to form a multiple-effect still comprising a multiplicity of distillation stages, each including a microporous membrane, wherein heat is the force which drives the vapors across the barrier between adjacent layers of liquid. The distillation or fractionating apparatus thus constituted is characterized by efficiencies which compare favorably with those of more complex and expensive existing structures because heat transfer is optimized by circulating the liquids as relatively thin films, and liquid circulation is optimized by supporting the thin films of liquids only externally, that is , at their two major outer surfaces. Because a large number of membranes may be assembled to form a multiple-effect still which is quite small and compact, the apparatus is operated at or near ambient pressure and the operating temperatures do not, as a rule, exceed the boiling point of the liquid, e.g. , water, the distillation or fractionating apparatus itself as well as the supporting equipment are substantially less complex and expensive than existing apparatus.

Reference is now made to the drawing wherein: Figure 1 is a somewhat schematic sectional view of fractionating apparatus embodying the invention; Fig. 2 is an enlarged fragmentary sectional view of the apparatus; and Fig. 3 is a fragmentary perspective view, partially in section showing an alternative embodiment of the appeiratus .

The basic component of the fractionating apparatus of the invention is a microporous membrane, designated 10, preferably formed of a polymeric material which, in membrane form, is non-wettable or at least poorly wettable by the liquid, e.g., water, for which the apparatus is designed. Desirable characteristics of the membrane are minimum thickness consistent with the requisite structural strength; high resistance to the passage of liquids; low resistance to the passage of gases; a high proportion of voids; high resistance to conductive heat flow; inertness with respect to the liquid; low absorptivity of the liquid; physical strength and integrity at elevated temperatures in the presence of a liquid; and uniformity with respect to the physical properties, especially thickness, pore size and distribution and thermal conductivity. As an example, a membrane found to be useful in apparatus constructed according to the invention for increasing the concentration of heavy water in a mixture of heavy water and ordinary water, is formed of poly-1, 1-vinylidene fluoride having a thickness of approximately .005 inch, including approximately 75 percent voids and characterized by an ooze point for water of 20 psi and air flow rate of the order of one liter per minute per square centimeter. Membranes of this material and type have a surface which, on a microscopic scale, is quite rough, and have been found to exhibit contact angles in excess of 90°, so that effectively, they are non-wettable by water. !E^xxsxxiaiffit-dLx&ad-^d^ linasies x-i xx i¾«cix&<¾y: - ^ The basic unit of the apparatus of the invention comprises a multiplicity of membranes 10 arranged in face-to-face stacked relation to form a column generally designated 12, in which the membranes are disposed in generally parallel relation and spaced from one another by shallow passages having depths of the order of the thickness of the membranes, e.g., .003-.005 inch, to provide channels through which a mixture such as ordinary water and heavy water may be circulated as thin films supported at their outer boundaries by and in direct contact with the microporous membranes .

As previously noted, this arrangement of membranes for circulating the liquid as thin films is advantageous because of the improved heat transfer thus obtained and the fact that there is nothing between the membranes to interfere with the circulation of the liquid mixture.

The membranes may take any convenient form including circular or rectangular, are generally coextensive in area with one another, and are sealed together at their margins by gaskets to prevent the escape of liquid from between the membranes at the margins thereof. The gaskets may be formed of a polymeric or elastomeric material having good adherent properties with respect to the membrane and preferably being thermosetting although compositions such as epoxy resin and silicon rubber, adapted to be applied in a liquid state or a solid thermoplastic state, may be Qf preferred because /their ease of formation and for reasons which will appear hereinafter. By way of example, gasket materials found to be useful in apparatus of this type because of their adherent properties and the ease with which gaskets can be formed include adhesive compounds of nitrile and urethane elastomers sold by B. F. Goodrich Company under the designations, respectively, A-178-B and A-1247-B.

Means for transferring heat to the column and transferring heat from the column are provided at opposite ends of the column for creating a temperature gradient across the column. In the form shown, these means comprise blocks 16 and 18 at opposite ends of the column providing chambers 20 and 22 through which, respectively, a heated fluid such as steam, and a coolant fluid such as water, may be circulated. The specific construction of the means for transferring heat to and from the column is unimportant as long as the means are constructed and arranged so as to . transfer heat to and from the .end layers or films of the liquid being circulated through the column and provide for a fairly . constant temperature gradient.

The membranes comprising the column, with the exception of the end membrane closest the cold end, are provided with openings or holes 24 to permit the mass o circulation or flaw of the liquid towards the hot end of the column of membranes. The hole patterns in adjacent membranes are staggered with respect to one another so that each hole in a given membrane is uniformly spaced from its nearest neighbors in the adjacent membranes to provide for a circulation of the liquid between adjacent membranes in a direction generally parallel with the membranes and normal to the direction of the temperature gradient across the membranes. In the example of the apparatus given useful for concentrating heavy water, the holes in the membranes may be of the order of one-sixteenth inch in diameter and spaced from one another to provide for substantial circulation and uniform distribution of liquid between adjacent membranes.

The operation of the apparatus depends upon the difference in boiling points of the liquid fractions making up the mixture to be enriched in one of the fractions, which o in the case of heavy water, for example, is 1.42 C. Heat is transferred to the mixture of liquids on one side of a membrane, tending to vaporize the liquid, primarily^ the fraction of the liquid having the lower boiling point. As a result, the mixture on the hot side of a membrane will be depleted by a portion of the lower boiling point fraction^ and conversely will contain a greater proportion of the higher boiling point fraction. This depletion-concentration effect takes place at each membrane of the column so that the net effect is bulk flow of the liquid mixture toward either or both ends of the column and flow of the vapor of the lower boiling oint fraction toward the cold end of the In the preferred form of the apparatus shown in the drawings, feed liquid, e.g., a mixture of and H20, is introduced into the column between membranes in the region of the medial portion of the column, and the liquid mixture is circulated toward the hot end of the column and withdrawn therefrom adjacent block 16. Thus, bulk flow of liquid from the medial portion of the column is toward the hot end of the column while mass flow of the vapors of the lower boiling point fraction, e.g. , ordinary water, is toward the cool end of the column or block 18.

Vapor transferred through the imperforate, end membrane closest block 18 is condensed and withdrawn as liquid from the column adjacent the cooling block. The rate of vapor transfer across the membranes at constant heat energy flux is an increasing function of temperature so that the quantity of liquid transferred by evaporation to a layer of liquid is greater than the quantity of liquid transferred by evaporation from the layer of liquid. Since the membrane closest the cold to liquid end of the column is imperforate/ liquid flow is toward the hot end of the column. The membranes located toward the cool end of the column function to strip heavy water from the bulk mixture, and membranes located between the point of introduction of feed liquid and the hot end of the. column function to concentrate the heavy water in the mixture of heavy water and ordinary water. The product consisting of water enriched with heavy water is withdrawn from the hot end of the column and the residue or mixture depleted by heavy water, is withdrawn from the column at the cool end thereof. The proportion of stripping stages to concentrating stages in a column may be 1:1 or may be varied as desired.

The feed, product and residue (depleted product) liquids are introduced into and withdrawn from the column through channels provided in gaskets .-4 between adjacent membranes. The supporting equipment for a column includes the necessary conduits connected by means, such as conventional manifolds, to the channels through the gaskets, together with means, such as pumps 26, for circulating the liquids through the column. A typical column designed for the concentration of heavy water may be operated between ambient temperature and the boiling point of water so that the auxiliary equipment should include means for circulating a coolant through chamber 22 and block 18 and supplying a heated fluid such as steam to chamber 20 of block 16. Blocks 16 and 18 themselves may be secured to one another and thereby function to hold the membranes in spaced relation in the column which may require no other means for retaining its integral condition inasmuch as the hydrostatic pressure of the liquids within the column is close to ambient pressure and may exceed ambient pressure only by the amount necessary to effect circulation of the liquid through the column.

A single column constructed and operated in accordance with the invention will be characterized by particular concentration and stripping factors, that is factors by which the concentration of the higher boiling constituent in the mixture is, respectively, increased (multiplied) and reduced (divided). In a typical situation in which the initial concentration of the higher boiling constituent is relatively small, a complete plant or system will comprise a plurality of columns coupled in a cascade relationship (as seen in Figure 1) in which the concentrated product of each column is circulated as the feed of the next succeeding column and the depleted feed of each column is recirculated as the feed of the preceding column. The total number of membranes in each column as well as the numbers of membranes in the concentrating and stripping stages are dependent upon the concentration and stripping factors to be attained; and the number and capacities of the columns coupled together to form a complete plant or system will be dependent upon the concentration and stripping factors and output of the individual columns and the ultimate concentration to be attained. Succeeding stages or columns after the first will have successively smaller capacities each equal to the output of the preceding column, although each column may have the same concentration and stripping factors as every other column. The number of columns and the ultimate concentration of the higher boiling fraction will depend upon economics and particularly, a trade-off between concentration factors, capital costs and quantity of product, as will appear in a subsequent example.

Various constructions are possible for spacing adjacent membranes apart from one another to provide channels between the membranes through which the liquid may be circulated substantially as thin films. One expedient is to corrugate alternate membranes so that the alternate membranes have a generally sinusoidal cross sectional configuration and each corrugated membrane functions as a spacer between it and the adjacent membranes. In this form of the apparatus, the flow of the liquid between adjacent membranes will tend to be linear and in a direction parallel with the corrugations.

In an alternative embodiment shown in Fig. 3, all the membranes are corrugated and are arranged with the corrugations in adjacent membranes extending at right angles to one another. This arrangement has the advantage of permitting circulation of the liquid between adjacent membranes in substantially any direction. A membrane according to the example given, i.e., 0.005 inch thick, may be formed with sinusoidal corrugations having an amplitude of the order of 0.010 inch and a wave length of the order of a 0.012 inch. This general configuration is shown and described in the copending tfcxxS. patent No. 26866, filed November 16, 1966 - in Israel application ¾fx;j¾¾(8d^^ , is particularly desirable because of the ease with which it can be achieved and its resistance to compression when incorporated in a column. Gasket materials in fluid form or capable of being liquefied or softened by heat are particularly useful in combination with corrugated membranes because of their ability to flow into and fill the corrugations.

A typical system designed for the concentration of heavy water in a mixture of ordinary water and operated with a temperature differential ranging from ambient to the boiling point of water, includes a column having a total of approximately 55 membranes, each 0.005 inch thick with films of water between adjacent membranes of the order of 0.003-0.005 inch deep so that the total "height" of the column (exclusive of the heat transfer means) will not exceed 0.5 inch. A column constructed according to this example will have a concentration factor and a stripping factor each of the order of four , that is , the heavy water concentration in the product is four times that in the feed while 75% of the heavy water in the feed is recovered. As previously noted, the number of cascaded columns in a complete system or plant and the concentration of the higher boiling point constituent in the product of such a plant will be dependent upon the economic factors which determine the cost of the ultimate product. When the initial concentration is very small as in the case of heavy water in feed water, a system such as described may be more economically desirable despite its relatively low concentration factor because of its very small capital cost and its equally small operating cost. More expensive systems having higher concentration factors become economically justified when the initial concentration of the higher boiling point constituent is relatively high.

It is this trade-off between concentration factor and plant cost which determines the number of columns in and the concentration of the product of the heavy water system of the example, and on this basis, it is contemplated that a complete heavy water concentrating plant or system might consist of six stages or columns of the type described each having a capacity one-quarter that of the preceding column. Such a plant would concentrate an ordinary mixture of water and heavy water to 50 molal % heavy water at which concentration, other and substantially more costly systems having higher concentration ratios might prove to be more economical. Advantages of such a plant include the fact that the total plant hold-up would be of the order of six to seven hours and any desired amount of total plant capacity may be obtained merely providing additional systems or plants with six columns each, coupled in parallel with advantage that the individual six column units could be shut down for repairs or replacement without interferring with the production of the remaining units and when unit production is small compared with total capacity, the shut down of one or a few units would have negligible effect on total production.

The most important advantages of a system of this type become apparent when it is compared with a typical prior art distillation plant which may comprise a tower having 44 stages and a height of 200 feet, heavy water recovery of 1.9% and a hold-up time of the order of 75 days. The vast difference between the complexity, size and cost of the apparatus of the invention and prior art system is substantial while the difference in complexity and cost of the supporting equipment for the two systems is correspondingly great.

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

Claims (1)

1. Appln.No. 31044/2 ' C L A I S t 1. Apparatus or increasing the concentration of one liquid in a mixture of two liquids of which one is more volatile than the other, the apparatus comprising a number of thin membranes assembled face to face with channels therebetween' to form a column and each perforated by a multiplicity' of microscopic, gas-filled, through passages capable of passing the vapours of the liquids while preventing the passage of the liquids, means for introducing the mixture between a pair of the membranes between the ends of the column and causing the mixture to flow as a thin film between successive pairs of membranes towards one end of the column, means for withdrawing the mixture from the column ear the one end, means for transferrin heat to the film near the one end of the column and means for transferring heat from the film near the other end of the column whereby a temperature difference can be produced across each membrane causing part of the more volatile liquid on the hotter side of each membrane t evaporate and to pass through the membrane- as a vapour, the mixture withdrawn from the column near the one end thus haying an increased concentration of the less volatile liquid*, 2. Apparatus according to claim 1, wherein each of the. membranes except that nearest the other end of the column is formed with a number of openings for permitting the "liquid mixture to flov through the column between successive pairs of membranes toward the one end of said column* Appln.No. 31044/¾^ ' 3. Apparatus according to claim 2, wherein the openings in each of the membranes are staggered with respect to the openings in adjacent membranes to cause the liquid mixture to flow as a thin film between adjacent membranes. 4. Apparatus according to claim 2 or claim 3, including means at the other end of the column fo withdrawing from the column liquid which is transferred as vapour through the membrane nearest the other end of the column and condensed on the cold side thereof and which is depleted in the less volatile liquid. 5. Apparatus according to claim 1, wherein at least alternate membranes are corrugated to provide channels for circulating the liquid mixture between the membranes. 6i Apparatus according to claim 5* wherein all the membranes are corrugated and the corrugations of each membrane extend at an angle with respect to the corrugations of adjacent membranes. 7i, Apparatus according to claim 1> including at least a second column similar to the first and means for introducing the mixture which is withdrawn from near the one end of the first column and which has an increased concentration of the less volatile liquid between a pair of membranes between the ends of the second column and causing the mixture to flow as a thin film between successive pairs of membranes toward the one end of the second column and means fo withdrawing the mixture having a further increased concentration of the less volatile liquid from the second column near the one end of the second column* Appln.No.31044/&^_„ 8. Apparatus according to claim 7, including means for withdrawing mixture which is depleted of the less volatile liquid from the second column near the other end thereof and introducing the depleted mixture into the first column* 9· Apparatus according to any one of the preceding the claims, wherein membranes are substantially impermeable to water in the absence of an appreciable hydrostatic pressure difference across the membranes* 10. Apparatus according to claim 9, wherein the membranes are non-vettable by water* 11. Apparatus according to claim 1, substantially as described with reference to Figures 1 and 2, or to Figures 1 and 2 when modified in accordance with Figure 3 o the accompanying drawings* 12. A method of using the apparatus according to any one of the preceding claims to increase the concentration of one liquid in a mixture of two liquids of which one is more volatile than the other, the method comprising introducing the mixture between a pair of the membranes between the ends of the column and causing the mixture to flow as a thin film between successive pairs of membranes towards one end of the column* transferring heat to the film near the one end of the column and transferring heat from the film near the other end of the column to produce a temperature difference across each membrane causing part of the more volatile liquid on the hotter side of each membrane to evaporate and to pass through the membrane as vapour, and withdrawing the mixture having an increased concentration of the less volatile liquid from the 6 J Appln.No. 31044/^ 13. A method according to claim 12, substantially as described with reference to the accompanying drawings. November Tel-Aviv, -»o¾eever 8, 1968