GB2360004A

GB2360004A - Composite membrane comprising natural rubber latex and hydrophilic colloid layers

Info

Publication number: GB2360004A
Application number: GB0003238A
Authority: GB
Inventors: Virender Nath Malhotra; Shantha Walpalage
Original assignee: EAST LONDON, University of
Current assignee: EAST LONDON, University of
Priority date: 2000-02-11
Filing date: 2000-02-11
Publication date: 2001-09-12
Anticipated expiration: 2020-02-11
Also published as: GB2360004B; GB0003238D0

Abstract

A composite membrane comprising a first layer formed from a hydrophilic colloid, optionally blended with a natural rubber latex, and a second layer formed from a natural rubber latex in the absence of hydrophilic colloid. A method of removing water from a mixture of ethanol and water using the membrane in a pervaporation process is also claimed.

Description

1 2360004 PERVAPORATION SEPARATION OF ETHANOL/WATER MIXTURES This

invention relates to the pervaporation separation of mixtures of ethanol and water to 5 remove predominantly water therefrom.

Pervaporation is a well-known technique for the separation or concentration of one liquid component present in a mixture thereof with at least one other miscible liquid. In this technique the mixture is placed in contact with a liquid impervious membrane and the vapour phase permeate (which will be richer in one of the components than those present in the mixture) is condensed and collected on the other side of the membrane, where a vacuum or inert gas flow would be provided.

In recent years, pervaporation has been tried in the separation (by which we include component concentration) of various mixtures including azeotropic liquid mixtures. In Polyme International, IQ (1993), 123-128, Huang and Rhim describe attempts to use the technique with ethanol-water mixtures. The article lists various polymer membranes which have been used, and specifically describes the use of certain poly(vinyl alcohol) (PVA) membranes modified with maleic acid.

In general, the membranes previously used or suggested for pervaporation of ethanol-water mixtures have not been entirely satisfactory. In many, the selectivity is too low to be of much practical use. In others, the flux rate (i.e. the throughput) is too low to be very useful.

The structure of some membranes appears to change in use so that they can only be used for a short time before the overall performance becomes unacceptable. Others are not useful at high temperatures.

We have now found that the separation of ethanol-water mixtures by pervaporation can be 33 0 improved by utilising a membrane containing a hydrophiEc colloid. More particularly, we have now found membranes of excellent quality, durability and utility for the separation of ethanol-water mixtures which can be made of vulcanised natural rubber latex blended with suitable hydrophilic organic compound(s).

2 Broadly, the invention provides a method of removing water from a mixture of ethanol and water, comprising subjecting the mixture to pervaporation using a water-selective vulcanised natural rubber latex membrane containing at least one suitable hydrophilic colloid as the blending agent. The ethanol/water mixture may contain any concentration of ethanol. typically the mixture would contain from ') to 99.0 wt% ethanot.

According to one aspect of the invention there is provided a method of removing water from a mixture of ethanol and water, comprising subjecting the mixture to pervaporation using a water-selective composite membrane in which comprises a first layer formed from a hydrophilic colloid and a second layer formed from a natural rubber latex and no hydrophilic colloid.

According to another aspect of the invention there is provided a composite membrane comprising: (a) a first layer comprising either (1) a blend of a natural rubber latex and a hydrophilic colloid, or (2) a hydrophilic colloid and not including any natural rubber latex; and (b) a second layer comprising a natural rubber latex and not including any hydrophilic colloid.

The first layer may be formed from one or more hydrophilic colloids alone or may be formed from a blend of one or more hydrophilic colloids and a natural rubber latex. Both the first and second layers may, of course, contain other additives conventionally used with natural rubber latices.

We have found that it is possible to make membranes from predominantly compounded natural rubber latex mixture containing one or more hydrophilic colloid blending agents for pervaporation of ethanol-water mixtures, which membranes give excellent selectivity, have a high flux rate, and do not deteriorate quickly in use even at high temperatures. A prevulcanised rubber latex mixture may also be used before the addition of the blending agents and the formation of a membrane followed by drying at a suitable temperature. Alternatively, the thin film formed from the mixture containing un-vulcanised latex and its normal compounding ingredients may be vulcanised at the end before using the membrane in pervaporation.

3 Whilst the method of the invention is useful for increasing the ethanol concentration in mixtures thereof with water, containing any concentration of ethanol, it is especially. useful fer treating azeotropic mixtures that contain 96.0 wt% ethanol. By the method of the invention, it is possible to obtain mixtures of increased ethanol content, and even ethanol that is substantially free of water.

The formation of thin films of vulcanised natural rubber latex is generally well known and is widely used, for example, in the manufacture of rubber gloves, condoms and other thin-walled rubber articles such as balloons. In general, the natural rubber latex is compounded with a vulcanising agent (normally sulphur although other compounds can be used instead of sulphur) and an accelerator, and then after forming the film, the mixture is heated to effect vulcanisation. Alternatively, the mixture can be heated to cause pre-vulcanisation, and then formed into a film, which can subsequently be further heated to complete the vulcanisation process. The process of forming films from the mixture(s) of latex and the blending ingredients is the same, but special attention must be paid to the level of mixing which must be such as to yield a uniform presence of the additive(s) in the mixture.

The thin films used as pervaporation membranes in accordance with the present invention can be made in the same way. We have found, however, that their properties and qualities for use 20 as water selective pervaporation membranes will vary in dependence on a number of factors.

One factor is the degree of vulcanisation (cross-1 inking). Unvulcanised natural 1 rubber latex films, whilst giving some selectivity as pervaporation membranes, are not used in accordance with the invention since they tend to be tacky and difficult to handle. Some vulcanisation is needed to avoid these problems and to prevent (or reduce) swelling of the membrane in use.

However, the extent of vulcanisation must not be so great as to seriously affect deleteriously the steady state flux or the selectivity of the membrane in use. Over- vulcanisation of natural latex rubber results in a relatively hard, non-elastic material, and this is of course to be avoided. The degree of vulcanisation should be sufficient to provide a strong elastic membrane. For example, films made from natural latex rubber containing 1. 5 parts by weight sulphur per hundred parts by weight rubber (1.5 phr), and heated for 12 minutes at 12WC gave good water-selectivity membranes. The films were of a wet thickness of about 100 micrometers. These films were considered to be fully vulcanised in the sense that all the 4 sulphur had reacted. They were flexible elastic films of good strength (having regard to their thickness), and perforation-free. As a general matter, the amount of sulphur to be included will usually be up to about 2.5 phr, preferably about 1.5 phr, but other amounts can be used.

For the purposes of the present invention where the amount of crosslinking is generally low, we prefer to use swelling index as a measure of the crosslinking. Swelling index (S.I.) measures the change in area of a rubber test-piece on immersion in toluene to equilibrium at 25'C. The lower the crosslink density, the higher the degree of swelling and truly uncrosslinked rubber will dissolve in toluene. To measure S.l., we stamp circular discs of 40mm diameter from a thin rubber sheet, mark the edge of the disc with non-soluble ink and immerse it in toluene in a petri dish. With thin discs, equilibrium is usually reached in 30 minutes. A microscope slide is then placed on the swollen disc to hold it flat and the petri dish placed over mill imetre-squared graph paper. The swollen diameter is measured in two orthogonal directions and averaged. Since area is proportional to the square of the diameter (d), S.I. is defined as d' swollen - d'initial. With discs of diameter 40mm, d 2 (initial) is 1600 d 2 initial mm 2. For the present invention, the S.I. of the vulcanised rubber membranes should preferably be from 2.7 to 6.4, more preferably between 3). 15 and 4.9.

Another factor which can have some influence on the selectivity and other properties of the membrane is the protein content of the natural rubber latex. As is well known, there is a small amount of protein present in natural latex rubber, largely in the water phase and on or around the surface of (but not within) the rubber particles. We have found that it is advantageous to increase the amount of protein in the latex such as by adding a protein. The amount of nitrogen- containing additive is not critical but will be no more than the amount in natural rubber latex with which it is blended and may usually be very much less.

In the method of the invention, an increase in operating temperature increases the flux rate but decreases pennselectivity. It is, therefore, a matter of compromise to select an acceptable flux rate whilst maintaining adequate permselectivity. Generally, the flux rate increases as the pressure on the exit side of the membrane is reduced. The effect of this on pennselectivity is relatively small. The flux rate increases as the membrane thickness is reduced but the effect may not be pronounced within the usual, range of operation. Permselectivity decreases as thickness is reduced.

The nature of the natural rubber latex is not critical, and any of the well known natural rubberlatices can be used. Thus, in principle, any commercially available stabilised natural rubber latex is suitable. However, we have found that high-ammonia (HA) latex is excellent for the purpose.

In general, the thickness of the vulcanised natural latex rubber membranes for use in the present invention will be from about 25 to 225 micrometres, most preferably around 75 to 100 micrometres. As will be understood, whilst thinner membranes can give a higher flux rate, they are generally weaker and so have a shorter life. The thicker membranes, being stronger, can have a longer life but may give a significantly lower flux rate. The thicknesses referred to are wet thicknesses, i.e. the thickness of the wet latex film as cast and before drying. Dry thickness is generally a half to two-thirds of the wet thickness.

The invention will be further described with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of laboratory equipment for carrying out the method of the invention; and FIG. 2 is a vertical sectional view of one example of a pervaporation cell.

Referring to the drawings, Fig. 1 shows a chiller unit A the top of which is connected by line T to the top of a pervaporation cell H, and the bottom of the unit is connected by line U to the bottom of the cell H. In line U is a cold liquid circulating pump B. Lines T and U are interconnected by line V which connects T and U to a hot water bath D equipped with a stirrer and thermostatic controller. Line V includes a hot water circulating pump E. Isolating valves C are provided in lines T, U and V where indicated.

A high pressure oxygen-free nitrogen cylinder L, provided with a pressure regulator K, is connected to cell H, with a pressure gauge G adjacent the cell. Cell H is provided with a drain valve 1.

6 Line W connects cell H via metal-metal coupling J to two liquid nitrogen traps M each containing liquid nitrogen N in a flask 0. Vacuum pump P evacuates line W and. a vacuum regulating valve Q is provided, downstream of which is vacustat gauge R and a vacuum gauge S.

The cell of Fig. 2 is an example of the type of cell H shown in Fig. 1. The cell comprises a hollow body 20 with caps 21, 22 closing each end. The body contains a glandless stainless steel expandable bellows 5 connected via a passageway 6 in cap 21 to an oxygen-free nitrogen supply line 2 (with an associated pressure gauge 1). At the bottom of body 20 is the membrane 6 under test, held at its periphery to body 20 by 0-rings 10 and 15 in cap 22. A porous stainless steel support 8 for the membrane is provided. Cap 22 includes a port 1 for conducting permeate vapour out of the cell (and, in Ej&J, to the cold traps M). Around body 20 and caps 21, 22 is a jacket 23 with connections 14 and 4 for receiving and exiting a heat medium to maintain the desired temperature.

The body 20 is connected to ducts 11 and 12 which are, respectively, inlet and outlet ducts for continuous supply of, or for filling with, the water/ethanol mix to be treated.

in the operation of the cell, membrane 6 is mounted in position and the body 20 is filled with ethanol/water mixture using ducts 11 and 12. Heat medium is supplied to the jacket 23 and nitrogen to the bellow 5. Referring to Fig. 1, heat medium is stored in A and conducted in lines T and U, with optional passage as derived through water bath D. the nitrogen gas is supplied from L. A vacuum is applied by pump P to draw pervaporate out of outlet 13 of the cell through line W to the cold traps M where it is condensed. No further detailed description will be given since the method of pervaporation is well known.

In order that the invention may be more fully understood, the following examples are given by way of illustration only.

Example 1 - Membrane (Thin film) preparation a) Mixtures were made up of high ammonia natural rubber latex (HA latex), sulphur and an accelerator, the sulphur contents being 0.2.or 0.5 or 1.0 or 1.5 7 phr. The mixtures contained (in parts per hundred dry rubber by weight) antioxidant (1 phr), zinc oxide (1.5 phr), accelerator (1.0 phr), the natural rubber latex containing 60% rubber solids. Herein after this mixture will be called "compounded HA rubber latex". The contents of a mixture were mixed with a high shear mixer without trapping air. In the absence of this precaution, an imperfect membrane containing pin holes and/or thin patches may be formed. This will give rise to abnormally high rate of flux and very low selectivity of water in the permeate i.e. pervaporate mixture.

Films of varying thicknesses were formed from the mixture given in (a) by applying a small quantity on to a glass plate and wiping with an applicator to spread the mixture to an even thickness. The wet thicknesses were 25, 50, 75 and 100 micrometres. After drying at room temperature for ten minutes, the films were placed in an oven, for crosslinking, at temperatures which varied from 60'C to 12 O'C for curing times which varied from 10 to 2 5 minutes. Next, a thin layer of a homogeneous mixture of the compounded HA rubber latex and a blending ingredient consisting of one hydrophilic colloid or a mixture of two such colloids was applied on the above film. The composite membrane was vulcanised again in the oven i.e. further cured at a temperature and time as mentioned above. The composite film so formed was carefully lifted from the glass plate and mounted on to a filter paper, Whatman Grade 6 catalogue number 1006240, for subsequent use in the pervaporation experiments. The thickness of the top blended layer was varied in the range of 25 to 100 microns (wet). The weight percent of one or more blending ingredient(s) was varied in the range of 1.0 to 20 percent on dry rubber content basis that is, the dry rubber content in the top layer of the composite membrane was varied from 99 to 80 weight percent respectively. The blending ingredient(s) was selected from a group of hydrophilic colloids with a specific range of molecular mass. For the present work, three types of hydrophilic colloids namely, alginic acid (sodium salt), carboxy methyl cellulose (sodium salt)[CMC1 and methyl cellulose[MC] were used. Each colloid was selected from either "low" or "medium" or "high" molecular mass category, which is defined and is available commercially from companies such as " Sigma-Aldrich, (1999) UK". This company assigns each b) 8 molecular mass a range of values and defines the corresponding viscosity in a solution of a given strength, in percent by weight, in water.

Hydrophilic colloid High Medium Molecular Low molecular mass molecular mass and viscosity & value of viscosity mass & value of viscosity Sodium salt of 600,000 to About 250,000 and 4 to Less than 90,000 and carboxy methyl 900,000 8 poise @ 250 C in 2 1.5 to 3 poise@250C cellulose also labelled 15 to 30 poise wt% aqueous solution. in 1.0 wt% aqueous CMC here. @ 250 C solution.

in 1.0 wt% aqueous solution.

Methylcellulose About 63,000 About 33,000 About 17,000 and 25 also labelled MC here. and 15 Centi-poise @ 250 C Poise @ 250 C in 2 wt.% aqueous in2% soln by wt. aqueous soln Sodium salt of the 120,000 to 80,000 to 120,000 & 12,000 to 80,000 & alginic acid. 190,000 & 35 poise @ 250C in 2 2.5 poise @ 250C in 2 poise @ wt% aqueous soIn. wt% aqueous soIn.

25'C in 2 wt.% aqueous soln.

C) Another type of composite membrane was also formed where the base layer was prepared from the compounded HA rubber latex and a coat of the top layer was formed from either CMC, or MC, or sodium salt of the alginic acid.

9 d) Membranes were also made, for comparison only, from the compounded HA rubber latex mixture alone. The composition of this mixture has been givenin (a) above.

Example 2 - Pervaporation a) The apparatus shown schematically in Fig. 1 of the accompanying drawincys was used C> for all the experimental work. The pervaporation cell used in this work has been shown in Fig.2.

b) Each of the membranes employed was tested for any inherent defects (e.g. pinholes) and then (if found satisfactory) used for pervaporation of an ethanol-water mixture.

Pervaporation flux measurements were conducted at a constant temperature within the range of 5'C to WC. Before the start of an experiment, a selected temperature was obtained by continuously circulating either the heating or cooling liquid through the outer jacket of the pervaporation cell for the entire duration of the experimental work.

An Edwards vacuum pump was used to evacuate the cold traps and the vacuum line, The vacuum line was connected to the downstream compartment and the cold traps which were immersed in liquid nitrogen simultaneously. A vacustat was used to measure the vacuum in the line. In a few experiments, a nitrogen pressure of 0.3 to 0.4 mPa gauge was applied indirectly to the feed solution via the stainless steel bellows.

The cold traps were changed approximately once every hour in order to collect the condensed permeate vapour. The inlet and outlet ports of the cold trap were immediately sealed with rubber bellows to eliminate the loss of condensed permeate due to evaporation during the thawing process. The cold traps were weighed to determine the amount of permeate (flux) and the liquid transferred into the small sample bottle and placed in a refrigerator for chemical analysis by gas liquid column chromatography (GLC). However, in most cases the permeant was analysed immediately by a refractometer and the results were compared with the data obtained S from GLC analysis. The pervaporation experiment was carried out for a steady state period of approximately seven hours every day. In some cases, the test was continued the following day or for the rest of the following week. Hence, the pervaporation flux measurement was carried out for a maximum of 120 hours. Pervaporation experiments were also conducted with membranes that had already been used. The results (i.e. separation, flux) were found to be the same as with the freshly made membranes.

The feed was also analysed by gas chromatography or by refractive index measurement. The concentration of ethanol in the mixtures of ethanol and water was varied between pure water to pure ethanol. However, a large number of experiments were conducted within the range of 5 to 96 percent by weight of ethanol. A vast proportion of the experimental work that is 70 to 80 percent was carried out with the azeotropic concentration of ethanol.

c) Results The results are set out in the following Tables 1 to 11. In these Tables, the membrane thicknesses are of the wet membranes. Drying reduces the thickness of a wet membrane by about 50 percent.

TABLE 1

Steady state values of flux rate of the permeate (i.e. pervaporate) and its composition at 20 0 C of various water ethanol mixtures through 100 micrometre thick fully crosslinked (cured @1200 C for 12 minutes) compounded rubber latex membranes with 1.5 phr sulphur, made from compounded HA rubber latex are given below.

Wt % EtOH in Flux rate Wt % of ROH feed (g/M2 h) in permeate 0 10.38 0 15.58 9.0 25.97 22.9 25.97 53.4 25.97 73.5 96 31.6 81.0 51.94 100 As can be seen, these membranes are ethanol selective up to 50% ethanol feed, but at 96% are water selective. The change occurs between at 60 to 70 wt% of ethanol in the feed.

TABLE2

Steady state values of flux rate of the permeate (i.e. pervaporate) and its composition at 20 0 C for various water ethanol mixtures through 100 micrometre thick (wet) fully crosslinked (cured @1200 C for 12 minutes) composite membranes (rubber latex with 1.5 phr sulphur), made from one base layer of the compounded HA rubber latex and the top layer of the blended ingredients with the compounded HA rubber latex, are given below. The top 50 micrometre thick (wet) layer contained blend ingredients of 7.5 wt.% of high molecular mass CMC on dry rubber basis.

Wt % EtOH in Flux rate Wt % of EtOH feed (g/M 2 h) in permeate 0 41.55 0 36.36 3. 7 36.36 7.5 36.36 22.50 36.36 35.50 96 34.28 40.80 15.58 100 TABLE 3

Steady state values of flux rate of the permeate (i.e. pervaporate) and its composition at 20 0 C for various water ethanol mixtures through 100 micrometre thick (wet) fully crosslinked (cured @1200 C for 12 minutes) composite membranes (rubber latex with 1.5 phr sulphur), made from one base layer of the compounded HA rubber latex and the top layer of the blended 12 ingredients with the compounded HA rubber latex, are given below. The top 50 micrometre thick (wet) layer contained blend ingredients of 7.5 wt.% of high molecular mass MC on dry rubber basis.

Wt % EtOH in Flux rate Wt % of EtOH feed (g/M 2 h) in permeate 0 25.97 0 20.97 4.2 20.77 10 20.77 -3 1 2M7 41 96 20.77 49 15.58 100 TABLE4

Steady state values of flux rate of the permeate (i.e. pervaporate) and its composition at 20 0 C for various water ethanol mixtures through 100 micrometre thick (wet) fully crosslinked (cured @1200 C for 12 minutes) composite membranes (rubber latex with 1.5 phr sulphur), made from one base layer of the compounded HA rubber latex and the top layer of the blended ingredients with the compounded HA rubber latex, are given below. The top 50 micrometre thick (wet) layer contained blend ingredients of 7.5 wt.% of high molecular mass Alginic acid (sodium salt) (also called sodium alginate) on dry rubber basis.

Wt % EtOH in Flux rate Wt % of EtOH feed (g/M 2 h) in permeate 0 311. 16 0 25.97 25.97 6.5 25.97 20 25.97 19 96 20.77 21 13 1 100 5.19 1 TABLE5

Steady state values of flux rate of the permeate (i.e. pervaporate) and its composition at 20 0 C for various water ethanol mixtures through 100 micrometre thick (wet) fully crosslinked (cured @1200 C for 12 minutes) composite membranes (rubber latex with 1.5 phr sulphur), made from one base layer of the compounded HA rubber latex and the top layer of the blended ingredients with the compounded HA rubber latex, are given below. The top 50 micrometre thick (wet) layer contained blend ingredients of different wt.% of high or medium or low molecular mass of the CMC (sodium salt) on dry rubber basis.

Wt% of CMC on dry High molecular mass Medium molecular Low molecular mass rubber basis in the CMC mass CMC Cme top layer of the composite membrane Wt% of Wt% of Wt% of (balance wt % is dry Flux rate EOH in the Flux rat( EOH in Flux rate EOH in rubber). g1M 2 h permeate g/m 2 h permeate g/m 2 h permeate 1.25 15.58 71.2 25.97 76 25.97 78 3.3 25.97 48 25.97 64 25.97 72 7.5 34.28 40.8 25.97 63 25.97 68 36.36 39 25.97 60 25.97 65 TABLE6

Steady state values of flux rate of the permeate (i.e. pervaporate) and its composition at 20 0 C for various water ethanol mixtures through 100 micrometre thick (wet) fully crosslinked (cured @1200 C for 12 minutes) composite membranes (rubber latex with 1.5 phr sulphur), made from one base layer of the compounded HA rubber latex and the top layer of the blended ingredients with the compounded HA rubber latex, are given below. The top 50 micrometre thick (wet) layer contained blend ingredients of different wt.% of high or medium or low molecular mass of MC on the dry rubber basis.

14 Wt% of MC on dry rubber High molecular mass Low molecular mass 1 basis in the top layer (balance Flux rate Wt% of E011 Flux rate Wt% of EOH wt% is dry rubber) g/M2 h in permeate g1M 2 h in permeate 3. 3 15.58 57 25.97 69 7.5 20.77 49 25.97 68 25.97 48 25.97 67.5 TABLE7

Steady state values of flux rate of the permeate (i.e. pervaporate) and its composition at 20 0 C for 96 wt% of ethanol in the feed mixtures through 100 micrometre thick (wet) fully crosslinked (cured @1200 C for 12 minutes) composite membranes (rubber latex with 1.5 phr 5 sulphur), made from one base layer of the compounded HA rubber latex and a top layer of the blended ingredients with the compounded HA rubber latex, are given below. The top 50 micrometre thick (wet) layer contained blend ingredients of different wt. % of high or medium or low molecular mass Alginic (sodium salt) (called sodium alginate) on dry rubber basis.

Wt% of High molecular mass Medium molecular Low molecular mass alginic(sodium mass salt)on dry rubber basis in the top layer Wt% Of Wt% of Wt% of of the membrane Flux rate EOH in Flux rate EOH in Flux rate EOH in (balance wt% on dry g/M2 h permeate g/M2 h permeate g/M 2 h permeate rubber) 1.25 31.16 57 31.16 60 25.97 65 3.3 25.97 32 31.16 39 25.97 56 7.5 25.97 21 31.16 3 6 25.97 48 25.97 15 31.16 32 25.97 43 TABLE8

Steady state values of flux rate of the permeate (i.e. pervaporate) and its composition at 20 0 C, 50 'C and 75 'C for 96 wt% of ethanol in the feed mixtures through 100 micrometre thick (wet) fully crosslinked (cured @1200 C for 12 minutes) composite membranes (rubber latex with 1.5 phr sulphur), made from one base layer of the compounded HA rubber latex and a top layer of the blended ingredients with the compounded HA rubber latex, are given below. The top 50 micrometre thick (wet) layer contained blend ingredient at 7.5 wt% of different high molecular mass colloids namely, CMC (sodium salt) or MC or Alginic acid (sodium salt) on dry rubber basis.

Operating 7.5 wt% of high 7.5 wt% high 7.5 wt% high 0C molecular mass CMC molecular mass MC molecular mass temperature in (sodium salt) Alginic acid(sodium salt) flux rate Wt% of flux rate Wt% of flux rate wt% of g/m' h E011 in g/m' h EOH in g/m'h E011 in permeate permeate permeate 36.36 40.8 20.77 49 25.97 21 103.8 71 77.91 76 72.72 60 233.7 82 207.7 85 166.2 80 TABLE9

Steady state values of permeate flux rate and composition of pervaporate at 20 'C for 96 wt% ethanol mixtures through 100 micrometre thick (wet) fully crosslinked (cured @1200 C for 12 minutes) composite rubber latex membranes with 1.5 phr sulphur, made from one base layer of the compounded HA rubber latex and a top layer of the blended ingredients with the compounded HA rubber latex. The top 50 micrometre thick (wet) layer contained blend ingredients of different wt.% of high molecular mass Carboxy methyl cellulose (sodium salt) or Alginic (sodium salt) (called sodium alginate) on dry rubber basis. The base layer of the compounded rubber membrane was in contact with the feed solution durinp, pervaporatio experiment(s).

Blend ingredient of Wt% of blend Flux rate Wt% of EOH in high molecular mass ingredient in mixture g/M2 h permeate in the compounded HA latex rubber CMC 3.3 31.16 68 CMC 7.5 31.16 63 16 Alginic acid (sodium 3.3 25.97 61 salt) Alginic acid (sodium 20 25.97 56 salt) TABLE 10

Steady state values of flux rate of the permeate (i.e. pervaporate) andits composition at 20 0 C for 96 wt% of ethanol in the feed mixtures through 100 micrometre thick (wet) fully crosslinked (cured @120 0 C for 12 minutes) composite membranes (rubber latex with 1.5 phr sulphur), made from one base layer of the compounded HA rubber latex and a top layer of the blended ingredients with the compounded HA rubber latex, are given below. The top 50 micrometre thick (wet) layer was prepared from the compounded HA rubber latex solution blended with a different combination(s), as given below., of the wt% of high molecular mass of CMC (sodium salt), MC and Alginic acid (sodium salt) on dry rubber basis.

Blend ingredients of Wt% of blend Flux rate Wt% of EOH in high molecular ingredients g/m 2 h permeate masses in the compounded HA rubber latex CMC and Alginic 2.5 wt% CMC and acid (sodium salt) 7.5 wt% Alginic acid 25.97 19.5 CMC and Alginic 7.5 wt% CMC and acid (sodium salt) 2.5 wt% Alginic acid 31,16 nj 7 CMC, Alginic acid 2.5 wt% CMC, (sodium salt) and 7.5 wt% Alginic acid 25.97 19 MC and 2.5 wt% MC TABLE 11

Steady state values of flux rate of the permeate (i.e. pervaporate) and its composition at 20 0 C for 96 wt% of ethanol in the feed mixtures through 100 micrometre thick (wet) fully 17 crosslinked (cured @1200 C for 12 minutes) membranes, first prepared from the compounded HA rubber latex solution and cured and next coated with a different 100% solution of high molecular mass CMC (sodium salt) or MC or Alginic acid (sodium salt), are given below.

Coating of high molecular Flux rate Wt% of EOH in permeate mass on compounded rubber g/m 2 h latex film CMC (sodium salt) 36.36 48 Alginic acid (sodium salt) 31.16 40 MC 31.16 56 wt% of CMC & 50 wt% 25.97 36 alginic acid 18

Claims

CLAIMS:

3.

1 A method of removing water from a mixture of ethanol and water, comprising subjecting the mixture to pervaporation using a water-selective composite membrane in which comprises a first layer formed from a hydrophilic colloid and a second layer formed from a natural rubber latex and no hydrophilic colloid.

A method according to claim 1, wherein the first layer is formed from a blend of a natural rubber latex and a hydrophilic colloid.

2.

4.

5.

6.

7.

A method according to claim 1, wherein the first layer is formed from only one hydrophilic colloid.

A method according to claim 1, wherein the first layer is formed from at least two hydrophilic colloids.

A method according to claim 1, 2 or 3, wherein the first and second layers include additives in addition to the hydrophilic colloid.

A method according to any preceding claim, wherein the second layer comprises a vulcanised natural rubber latex.

A method according to any one preceding claim, wherein the hydrophilic colloid has a high molecular mass.

A method according to any preceding claim, wherein the first layer contains from 1.0 to 20 wt%, preferably 2 to 10 wt%, hydrophilic colloid on a dry rubber basis.

8. A method according to any preceding claim, wherein the hydrophilic colloid has a viscosity from 25 centipoise to 200 poise, preferably from 15 to 150 poise, when in a solution of distilled water at 25T and at a concentration of 2.0 weight percent.

19 9. A method according to any preceding claim, wherein the hydrophilic colloid has a molecular mass from 12,000 to 2,400,000, preferably from 120,000 to 900, 000.

10. A method according to any preceding claim, wherein the mixture contains from 3 to 99.0 wt% ethanol.

A method according to any preceding claim, wherein the mixture is an azeotropic mixture of ethanol and water.

12. A method according to any preceding claim, wherein the natural rubber latex is a high ammonia (HA) latex.

13. A method according to any preceding claim, wherein the degree of vulcanisation of the natural rubber latex in the membrane is such that the swelling index is from 2.7 to 6.4.

14. A method according to any preceding claim, wherein the base membrane is cast as a wet rubber latex film and then dried and vulcanised, and wherein thethickness of the wet latex film is from 25 to 225 micrometers.

15. A method according to any preceding claim, wherein the top layer of the composite membrane is cast as a wet film and then dried and vulcanised, and wherein the thickness of the wet film is from 25 to 225 micrometers.

16. A method according to any preceding claims, wherein the overall thickness of the composite membrane cast as a wet film is from 50 to 450 micrometers.

17. A method according to any preceding claim, wherein before forming the top or base layer of the membrane the contents of each appropriate mixture are mixed with a high shear mixer such that no air or gas is trapped during mixing.

18. A method according to any preceding claim, wherein the hydrophilic colloid comprises a sodium salt of carboxymethylcellulose, methyl cellulose or a sodium salt of alginic acid.

19. A composite membrane comprising: (a) a first layer comprising either (1) a blend of a natural rubber latex and a hydrophilic colloid, or (2) a hydrophilic colloid and not including any natural rubber latex., and (b) a second layer comprising a natural rubber latex and not including any hydrophilic colloid.

20. A membrane according to claim 19, wherein the first layer is formed from only one hydrophilic colloid.

21. A membrane according to claim 19, wherein the first layer is formed from at least two hydrophilic colloids.

A membrane according to claim 19, 21 or 22, wherein the first and second layers include additives in addition to the hydrophilic colloid and the natural rubber latex.

23. A membrane according to any preceding claim, wherein the second layer comprises a vulcanised natural rubber latex.

24. A membrane according to any one of claims 19 to 23, wherein the hydrophilic colloid has a viscosity from 25 centipoise to 200 poise, preferably from 15 to 150 poise, when in a solution of distilled water at 25'C and at a concentration of 2.0 weight percent.

25. A membrane according to any one of claims 19 to 24, wherein the hydrophilic colloid has a molecular mass from 12,000 to 2,400,000, preferably from 120,000 to 900,000.

A membrane according to any one of claims 19 to 25, wherein the hydrophilic colloid comprises a sodium salt of carboxyrnethylcellulose, methyl cellulose or a sodium salt of alginic acid.

27. A method of removing water from a mixture of ethanol and water substantially as herein described with reference to and as shown in the accompanying drawings.

21 28. A composite membrane substantially as herein described with reference to and as shown in the accompanying drawings.

29. A method of removing water from a mixture of ethanol and water substantially as herein described with reference to the examples.

30. A composite membrane substantially as herein described with reference to the examples.