New Strategies for Separations Through Reactions
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Sddhand, Vol. 10, Parts 1 & 2, April 1987, pp. 163-183. © Printed in India. New strategies for separations through reactions V G GAIKAR and M M SHARMA* Department of'Chemical Techno[ogy, University of Bombay, Matunga, Bombay 400 019, India Abstract. Separations through reactions can provide reliable and economically viable alternatives to established methods of separation, particularly for close boiling substances. New strategies in 'Dissociation Extraction' and "Dissociation Extractive Crystallization' for separation of close boiling acidic/basic mixtures have been highlighted. Separations with aqueous solutions of hydrotrope and aqueous micellar solutions have been brought out. Separations by membranes with facilitated transport is potentially attractive. Keywords. Separations through reactions; dissociation extraction; reactive crystallization; dissociation extractive crystallization; reactive distillation; separations with hydrotropes; micelles in separations: membrane separations; hydrometallurgical separations; separations with supercritical fluids. I. Introduction In chemical process industries separations of a variety of mixtures are frequently encountered and the cost of separation may dominate the capital investment and operational expenses. Quite often physical methods of separation, such as distillation, crystallization, solvent extraction followed by distillation and adsorp- tion, are used, which exploit the differences in physical properties like boiling points, solubility, melting points etc. However, for systems having close boiling points or which are thermally unstable, these methods are either not applicable or not economically viable. In such cases the strategy of selective reactions may prove to be attractive. There is, therefore, a clear incentive to probe newer methods of separations through reactions to achieve better selectivity and higher throughput. An ideal situation would be where separation and the desired reaction are conducted simultaneously. Separations through reactions have played a significant role in chemical industry from the early stages. The alkylation-dealkylation with isobutylene, with scpara- tion of alkylated products, has been successfully exploited for the separation of m-cresol~p-cresol (Stevens 1943). A similar strategy has been applied for separation of m-xylenc/p-xylene mixtures by reacting the mixtures with acetaldehy- de where p-xylene hardly reacts when m-xylene is present. Recently it has been claimed that m-xylene/p-xylene can be separated by fractional distillation in the presence of an organometallic compound where the relative acidities of the 163 164 V G Gaikar and M M Sharma different xylenes in exchanging hydrogen with metallic atoms is exploited (ANVAR 1975; Terill et al 1985; Cleary & Doherty 1985). The method of sulphonation-desulphonation also finds application in the separations of xylenes, dichlorobenzenes etc. It is known that/3-picoline does not condense with benzaldehyde whereas a- and y-isomers yield corresponding stilbazoles; this method was used by Schwarz (1891) for the isolation and was proposed as a convenient way of separating 7 and /3-picolines. Another strategy exploited the greater susceptibility of 7-picoline and 2,6-1utidine to oxidation as compared to /3-picoline (Coulson & Jones 1946). The separation of methyl ethers of m-/p-cresols has been carried out very recently by selective oxidation with MnO2 where p-isomer reacted selectively to form substituted benzaldehyde (Millington et al 1986). It is also possible to chlorinate m-cresol selectively, in the presence of a CuCI/CuCI2/HCI system. The resulting mixture of 4-chloro-3-methyl phenol with 2-chloro derivative and p-cresol can be easily separated (Sharma 1985). The removal of sulphur compounds like H2S, RSH and COS from CO2 is another large scale application of separations through reactions. The fast kinetics of H2S and RSH with alkaline solutions and the manipulations of the Operating conditions can lead tQ highly selective absorption of H2S and RSH from their mixtures with CO2. The use of hindered amines is a recent development in this field (Sarfori & Savage 1983); this was originally suggested by Sharma (1964). C4 olefins, found in a variety of streams in petrochemical plants and petroleum refineries, can be made free from vinyl acetylene by selective hydrogenation of vinyl acetylene to butadiene. Absorption in 50-60% H2SO4 has been widely employed, in the past, for selective absorption of isobutylene from mixtures with butenes. A recent state-of-art review by Sharma (1985) brings out different aspects of separations through reactions. This paper will be concerned with aspects which have not been discussed by Sharma. The separation of close boiling, isomeric/nonisomeric, acidic/basic/neutral, substances provides the most attractive situation for exploitation of reactions. Dissociation extraction is an approach of significant industrial importance for the separations of acidic/basic close boiling mixtures. 'Dissociation extractive crystal- lization' and 'dissociation extractive distillation' are the newly emerging fields for separation of acidic/basic mixtures. Separation by hydrotropes is yet another strategy by which separation factors for some systems can be enhanced many-fold. Selective solubilization with micellar solutions can provide yet another strategy where reactions also can be imposed to increase the rate and the extent of solubilization. Membrane processes supplemented by reactions or facilitated transport can provide answers in some of the difficult and/or energy-intensive situations. 2. Dissociation extraction This two-phase technique of liquid-liquid extraction, applicable to acidic/basic mixtures, exploits the differences between the dissociation constants and distribu- tion coefficients of the components of the mixture. New strategies for separations through reactions 165 2.1 Liquid-liquid dissociation extraction A dissociation extraction step involves equilibrating the mixture dissolved in a suitable water-immiscible organic solvent with an aqueous phase containing a stoichiometric deficiency of the neutralising agent. The term 'stoichiometric deficiency' implies that the amount of neutralising agent is just sufficient to neutralise the stronger component of the mixture. The competition between the components of the mixture for the neutralising agent results in the enrichment of the aqueous phase by the stronger component, while the organic phase gets enriched with the weaker component. The following equation gives the value of separation factor for the separation of acids, HA and HB (HA is the weaker acid) and published data correlate well with this equation (Anwar et al 1974): DA KB { N(6+1)+ TI(1/D~)+(KA/KB)(6/DA)]} (1) a- DB KA N(6+ I)+ T[(1/DB)(K~/KA)+(a/DA)] " where D and K are distribution coefficients and dissociation constants, respective- ly, and N= [A-]+[B }, "~ 6 = [AH],,~g/[HB]o..g,? at equilibrium. I T = [HAL, g+ [HB]o~,J After the initial theoretical development by Anwar et al (1971, 1973, 1974, 1979), this potentially attractive method of separation has been extensively exploited by Sharma and co-workers (or a number of systems of industrial relevance. The separation of N-alkylanilines and chlorobenzoic acids (Laddha & Sharma 1978); the separation of chlorophenols, p-cresol (or m-cresol)/2,6-xyleno[, chlorosubsti- tuted cresols and xylenols (Wadekar & Sharma 1981b,c); N-substituted anilines, chloroanilines and nitroanilines (Jagirdar & Sharma 1981b) are a few of the systems which have been tried. Wadekar & Sharma (1981a) have given a state-of-art review of this process, covering the literature upto 1981. 2.2 Regenerative dissociation extraction Sharma and co-workers have developed new regenerative processes so that the extractant can be reused; the cost of acid and alkali constitutes the dominant factor in separation via dissociation extraction. Wadekar & Sharma (1981b) suggested a thermal regenerative method for the recovery of ~cak neutralising agents like ammonia or methylamine from aqueous extracts. This could eliminate the solvent extraction of aqueous extract by a secondary highly polar solvent as suggested by Anwar et al (1979). Gaikar & Sharma (1984b) have proposed the method of carbonation of the aqueous extract to recover phenolics as a separate phase from the aqueous phase containing alkanolamines. Alkanolamines can be recycled for dissociation extraction step after desorption of carbon dioxide under boiling conditions; even CO2 can be recycled. 166 V G Gaikar and M M Sharma 2.3 Gas-liquid-solid dissociation extraction Jagirdar & Sharma (1981b) have extended the liquid~liquid mode of dissociation extraction to the gas-liquid-solid mode of dissociation extraction. Very high values of separation factors, in the range of 4-40, were realised when anhydrous HC1 gas was used to separate substituted anilines, chqoroanilines and nitroanilines. Considerably high values of selectivity (~ 100%) were observed when Gaikar & Sharma (1984a) applied this strategy for the separation of cumidines. Table 1 shows the separation factors for gas-liquid-solid dissociation extraction as compared to conventional liquid-liquid dissociation extraction. The process was also found to be thermally regenerative; the hydrochlorides of the bases can be decomposed by heating and the liberated HCI gas can be recycled. One particular advantage in this process for cumidines is that p-cumidine hydrochloride can be directly phosgenated to make the corresponding isocyanate;