Chapter Vlll SEPARATON OF ORGANC LQUD MXTURES BY PERVAPORATON USNG NATURAL RUBBER MEMBRANES Results of this chapter have been communicated for publication in J. Appl. Polym. Sci a fractionation pmcess which uses a dense polymeric 1 membrane as a separation barrier between the liquid feed and permeant vapour. t has been the object of numerous experimental and theoretical investigations over the past several years [$]. The method is usually employed for the separation of azeohopes, [6] isomers [7] and for the removal or recovery of trace substances [8]. Recently the interest rose on the application of the pervaporation process for the separation of organic mixtures in industrial processes [9,1]. Many interesting studies haw been reported on the use of ebstomeric membranes for penmporation process. For example. Lamer et al [ll used a
silicon membrane to extract aroma compounds from dilute aqueous solutions. A resistance-in-series model was used by them to describe the flux of organic compounds through the membrane. Duval et al. [12] indgated the separation properties of dense polymeric membranes towards a mixture of toluene and acetone. They found that the separation properties of the membranes were improved by introducing various active carbons and zeolites into a thin polymeric film to form a heterogeneous membrane. Nijhuis et al. [31 used a wide range of homogeneous ehmeric membranes for the removal of volatile organics from water. They related permeation and sorption data obtained for the elastomers to the chemical and physical nature of the elastomers through the solubility parameter and the glass transition temperature respectively. They attributed the diierences in the permeabiity of the organic component through the membrane to structural parameters in the membrane material such as degree of unsaturation and presence of steric side groups. Dih et al [14] studied the transport of ethanovwater mixture through ethylene-vinyl acetate (EVA) membranes. They observed that for a membrane with 37% vinyl acetate content, ethanol permeability increased with ethanol activity in the membrane whik the water permeabiity decreased with water activity. he et al [15] examined the permeation of steroids fhrough poly(ether urethane) and EVA. This chapter deals with the pervaporation separation of diierent organic liquid mixtures using M membranes. The effects of feed composition, vulcanizing agents and cure time of the membranes and the molecular size of the permeate on the separation process have been examined.
V.l. Results and discussion V1.l.l. Separation of aliphatic hybrocarbondacetone mixtures Vl1.1..a. Sw& chamctemcs The swelling behaviour of the membranes was assessed by immersing them separately in mixtures of n-hexane and acetone of different compositions at 28 C for 72 h. After reaching equilibrium, the membranes were taken out from the mixtures, the surface wiped with a filter paper and weighed immediately in an elecbonic balance that measured reproducibly within.1 g. The swelling ratio (S,) was determined as where Wd and W, are the weight of dry and swollen membranes, respectively. The plots of swelling ratio versus feed composition for the membranes vulcanized by the four systems, viz., CV, EV, DCP and mixed are given in Figure V.1. t is seen that the swelling ratio increases with increase in the concentration of n-hexane in the feed. The same result is obtained with n-heptanelacetone and n-odane/acetone mixtures. This clearly indicates the preferential affinity of the membranes towards aliphatic hydrocarbons than acetone. Of the four system, the swelling ratio increases in the order DCP < W < mixed < CV.
2.5 - * DCP * EV * MXED CV 2 51.5 - a Z - 1 3 () 1.5 2 3 4 5 6 7 8 WT. % OF n-hexane N THE MXTURE n~ur~ VUL1. V&Wn feed. of swelling rat& with the we@ht per cent of n-heme " the
The effect of feed composition on the pervaporation performance for the DCP membrane is given in Figure V.2. t is seen that the permeation rate through the membrane increases with increase in the concentration of n-hexane in the feed. This is due to the strong interaction between NR and n-hexane owing to the closer solubii parameter values. (The solubility parameter values of NR, n-hexane and acetone are 16.2, 14.9 and 2.3 (~/m~)', respectively). This interaction effectively increases the frequency and amplitude of rubber chain motions thereby allowing the permeate molecules to pass through the membrane easily. t is also shown in Figure W11.2 that the weight per cent of n- hexane in the permeate increases as its concentration in the feed increases. Figure V11.3 has the Mliation of flux (permeation rate) as a function of weight per cent of n-hexane in the feed for the four types of membranes. t is seen that the membrane vulcanbd by DCP, with highest degree of crosslinking, exhibits the lowest flux while that cured by CV shows the highest. These observation pamuel the decrease in the values of swelling ratio which increase in the order DCP < EV < mixed < CV. The diierences in the permeation rate through the membranes can be attributed to the diierences in the nature and diibution of crossb in the rubber network The flexible network of the CV membrane, having polysulphidic linkages, allows the passage of permeate molecules relatively easy. The DCP membrane has stable C-C linkages between rubber chains, as pointed out earlier, which effectively reduces the flexibility of the network and thus reduces the permeation rate.
WT. % OF n-hexane N THE FEED Rgwe V11.2. Penmpomtion performance of DCP membmne with n-hexanelacetone
WT. % OF n-hexane N THE FEED Figure V111.3. Effect of vulcanizing system on the permeation rate of n-hexanel acetone mixtures of different compositions.
n order to see the influence of molecular weight of the predominantly permeating species on the separation process, mixtures of n-heptanelacetone and n-octane/acetone were also used as feed in the permeation cell. F~gure V11.4 shows the penmporation of 515 mixtures of n-hexane, n-heptane, and n-octane, a! belonging to the same homologous series, with acetone through the four types of membranes. t is seen that the flux decreases as the molecular weight of the preferentially permeating species increases. Thii is in quantitative agreement with our expectation that the low molecular weight species diises more easily through a compatible medium. Figure V.5 represents the decrease in the weight per cent of the three aliphatic hydrocarbons in the permeate with increase in their molecular weight. Another significant parameter controlling the separation process was found to be the cure time of the membranes. The increase in vulcanization time increases the number of crosslinks between rubber chains thereby, enhancing the rigidity of the macromolecular network This in turn reduces the flux but enhances the separation efficiency as shown in Figures V1.6 and V.7. Figure V.8 shows the effect of membranes thickness on the permeation behaviour of 515 n-hexanelacetone mixture through the DCP membrane. t is seen that the separation efficiency is independent of the thickness of the membrane but the flux decreases with increase in the thickness of the membrane.
- ffl u c 4- a ffl 3 5.- c * DCP * EV * MXED * CV MOLECULAR VVT. OF ALPHATC HYDROCARBON Figure W11.4. Effect of mdecukr weight of aliphatic hydmrbons on the permeation rate through different membranes.
W i2 7 - * D CP 8 E V *MXED * CV 2 a - z - 65 z m u a u >- a F: 6 a i a L 8 5 55 8 9 1 11 12 MOLECULAR Wr. OF ALPHATC HYDROCARBON Flgure V111.5. Variation of weight per cent of n-hexane in the feed with molecular weight of aliphatic hydrocarbons.
CURE TME ( min ) Figure V111.6. EHed of cure time on the permeation rate through NR membranes
CURE TME ( min ) Figure V111.7. Effect of cure time on the sepamtion efficiency of NR membranes
7 9 11 13 15 THCKNESS (,urn ) Rgure V111.8. Effect of membrane thickws on the pempomtion performance of NR membmne vulcanized by DCP.
V111.1.2. Separation of chlorinated hy&ocarbon/acetone mixtures The mixtures of CH2C12, CHCb and CC, with acetone, of different concentrations, were used for thii work. The swelling behaviour of the membranes in CC, acetone mixtures of different concentrations is presented in Figure V.9. The swelling ratio has been found to increase with increase in the concentration of CC, in the feed. The same trend was obtained with mixtures of CH2Cldacetone as well as CHCuacetone. Thii clearly indicates the higher affinity of the membranes towards chlorinated hydrocarbons. Figure V.1 shows the effect of feed composition on the permeation rate of CH2Cldacetone, CHClJacetone and CCWacetone mixtures through the membrane vulcanized by DCP. t is seen that the flux increases as the concentration of chlorinated hydrocarbons in the feed increases. Another interesting observation is that the flux is higher for CCuacetone mixture than the other two mixtures of a given composition. This is definitely due to the differences in the polarity of the chlorinated hydrocarbons. The dipole moment values of CH2C12, CHCl, and CC, are 1.6, 1.1 and D, respectively. Figure V.11 represents the weight per cent of chlorinated hydrocarbon in the permeate as a function of that in the feed for pervaporation through DCP membrane. t is seen that the amount of chlorinated hydrocarbons in the permeate increases as their concentration in the feed increases. This clearly shows that the membrane exhibits preferential permeation towards chlorinated hydrocarbons.
f DCP *EV +MXED +CV / WT. % OF CC, N THE MXTURE Figure V11.9. Variation of swelling mtio with weight per cent of CQ in the feed.
WT. % OF CHLORNATED HYDROCARBON N THE FEED Figure V.1. Effect of feed wmposition on the permeation mte of chlorinated hydmcahnlacetone mixbms.
w $ r" T W a 1 W 8- - z - z m u 4: 2 6- t W 2 - z a: 4- LL 8 s * CC14*CHC13* CH2C2 2 2 3 4 5 6 7 8 WT. % OF CHLORNATED HYDROCARBON N THE FEED R m V.11. Weight per cent of chlorinated hyd-n the feed. m the pemat. Vs h t h
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