Ind. Eng. Chem. Res. 1992,31, 2040-2042
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RESEARCH NOTES Separation of Close-Boiling Substituted Phenols by Anhydrous Calcium Hydroxide and Recovery of Phenols from Calcium Phenoxides by Carbonation Separation of close-boiling phenolic mixtures of p-cresol/ 2,6-xylenol and o-methoxyphenol/alkylphenols has been successfully accomplished by "dissociation extraction" using solid anhydrous calcium hydroxide. Separation factors having very high values were realized. The phenols were recovered from their calcium salts by carbonation of filtered calcium salts suspended in a nonpolar organic medium.
Introduction Close-boilingmixtures of xylenols, ethylphenols, cresols, etc. are encountered in tar acids obtained from low-temperature carbonizationof coal or wood. Such mixtures are also obtained in petroleum refining and also in some coal-gasification processes. The toxic, corrosive, and chemically reactive nature of phenols is a potential source of problems in direct utilization of the resulting coal liquids as synthetic fuels. Phenols could well be a source of value-added products. As the boiling points are very clwe (201-207 "C), separation of these phenols from the corresponding distillate fraction is difficult by conventional methods of separation such as distillation. In such cases, the strategy of selective reactions may prove to be useful. Dissociation extraction is one of such methods which exploits the differences between dissociation constants and distribution coefficients of the components of acidic/basic mixtures and has been widely employed for various industrially important mixtures (Anwar et al., 1974, 1977; Wadekar and Sharma, 1981; Jagirdar and Sharma, 1981; Gaikar and Sharma, 1984). The method based on dissociation extractive crystallization has also provided excellent results in the separation of p-cresol/2,6-xylenol and o-methoxyphenol/alkylphenols (Gaikar and Sharma,1987; Gaikar et al., 1989). Extractive separation with hydrotropes was also successfullytried for the separation of 2,6-xylenol/p-cresol (Gaikar and Sharma, 1986). The removal of p-cresol impurity from technical grade 2,6-xylenol containing 345% p-cresol has been successfully accomplished by the selective catalytic reaction of p-cresol with formaldehyde (Ciernik and Spousta, 1986; Ciernik et al., 1988)and selective solubilizationof p-cresol in the micellar or microemulsion media (Mahapatra and Sharma, 1987). Very recently, Chaudhuri et al. (1990) have reported the removal of p-cresol impurity from technical grade 2,6-xylenol containing 2% p-cresol by the selective O-alkylation of p-cresol with isobutylene in the presence of acid catalysts like cation-exchange resin Amberlyst 15. The major cost factor deciding the economics of dissociation extraction or dissociation extractive crystallization as a separation process is the cost of the chemicals, i.e., the cost of the neutralizing agent and the cost of the extracting agent required to recover the extracted material. The aqueous phase used in the liquid-liquid dissociation extraction becomes a wastewater stream which may need further processing to meet statutory requirements. The aim of this work was to develop a process where a cheap neutralizing agent could be used to separate close-boiling
phenolic substances and where recovery can be done in a facile manner. Earlier, Schlosberg and Scouten (1988) suggested recovery of phenolic materials, as a mixture, with calcium hydroxide. In the present study, solid anhydrous calcium hydroxide was used as a neutralizing agent for the separation of close-boilingphenolic mixtures. The recovery of phenols from hydroxycalcium phenoxides has been successfully accomplished by thermal decomposition at 550+50 "C (Schlosberg and Scouten, 1988). Very recently, Scouten and Dougherty (1990) have reported the regeneration of calcium phenoxide by steam stripping. In this investigation the recovery of phenols from their calcium salts was successfully accomplished by carbonation. This mild recovery process is likely to be more cost-effective in practice.
Materials and Experimental Section The phenolic compounds, p-cresol, o-methoxyphenol, o-ethylphenol, and 2,6-xylenol were of Fluka/AR grade; different solvents, such as n-heptane and toluene, were of AR grade. Calcium hydroxide was obtained from LobaChemie Indoaustranal Co. and was more than 96% pure. Carbon dioxide was obtained from Bombay Carbon Dioxide Gas Company. The experiments were conducted in a 0.05-m4.d. mechanically agitated glass reactor. A known weight of dry calcium hydroxide powder in stoichiometric deficiency was added to the solution of a synthetic mixture of phenols in an organic solvent, and powdered calcium hydroxide was suspended in the reaction medium by mechanical agitation. The reaction slurry was stirred for 3-5 h at room temperature (30 "C) and filtered to separate calcium salt. The analysis of the filtrate was done by gas-liquid chromatography (GLC). Carbonation. The resulting solid (Ca-phenates) from the filtration unit was suspended in n-heptane or toluene by mechanical agitation. Carbon dioxide gas was passed into the slurry through a sparger for 3-4 h at room temperature (30 "C). The flow rate of carbon dioxide was kept fixed at 1.5 cm3/s. The carbonation experiments were carried out at atmospheric pressure. The slurry was then filtered to separate calcium carbonate and the filtrate was analyzed on GLC. Results and Discussion Separation of 2,6-Xylenol/p-Cresol. Table I gives the values of separation factors for 2,6-xylenol/p-cresol,
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Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 2041 Table I. Separation of 2,6-Xylenol (A)/p-Cresol (B) concentration (mol/L) initial final extractant solvent B A B A sep factor Ca(OH)2 toluene 1.22 0.82 0.33 0.80 105 1.07 0.42 0.19 0.41 453 1.18 0.17 0.16 0.17 118 n-heptane 1.10 0.43 0.21 0.43 455 Table 11. Separation of o-Methoxyphenol (B)/Alkylphenols (A) (Extracting Agent, Ca(OH),; Solvent, Toluene/n -heptane) initial concn final concn (mol/L) (mol/L) system B A B A sep factor 46 o-methoxyphenol (B)/ 0.652 0.760 0.078 0.660 82 p-cresol (A) 0.791 0.872 0.031 0.670 91 0.790 0.620 0.026 0.470 76 0.750 0.460 0.008 0.211 a o-methoxyphenyl (B)/ 0.967 2.292 0.007 2.284 a 2,6-xylenol (A) 0.977 1.034 0.005 1.029 0.650 a 0.967 0.655 b 0.399 a 0.967 0.409 b o-methoxyphenyl (B)/ 0.970 0.960 0.009 0.812 649 o-ethylphenol (A) 0.997 0.590 0.011 0.580 c 0.900 1.800 0.013 1.600 547 0.970 1.250 0.013 1.091 602 Very high, approaching infinity. sis. e Very high, =MOO.
* Not detected in GLC analy-
where anhydrous calcium hydroxide was used to form a solid product with p-cresol preferentially. Extremely high values of separation factors of the order of 1W450 were obtained. These results are attractive since the highest separation factor in liquid-liquid dissociation extraction was 30 (Gaikar and Sharma, 1985). Separation of o-Methosyphenol/Alkylphenols. Table I1 gives the separation factors for various systems, which are of considerable industrial importance, when calcium hydroxide was used as neutralizing agent for substituted phenols. Toluene and n-heptane were used as the organic medium. (i) o-Methoxyphenollp -Cresol. The values of the separation factor (46-91) are markedly higher than those obtained by liquid-liquid dissociation extraction using n-octanol ( H . 4 ) as a solvent (Gaikar and Sharma, 1985) and by dissociation extractive crystallization using piperazine as extracting agent (16-52) (Gaikar et al., 1989). (ii) o-Methoxyphenol/2,6-Xylenol.It is seen from Table I1 that calcium hydroxide is effective in isolating almoet pure o-methoxyphenol from the mixture in the form of solid calcium salt. The values of the separation factor approach infinity. In the liquid-liquid dissociation extraction, the value of the separation factor was 42 using n-octanol as the best solvent (Gaikar and Sharma, 1985). Gaikar et al. (1989) have also observed high values of the separation factor (5-109) for this system using piperazine as neutralizing agent, and the separation factor decreased as the concentration of 0-methoxyphenol decreased in the initial organic mixture. (iii) o-Methoxyphenollo-Ethylphenol.Extremely high values of separation factors of the order of 550-5000 were obtained. The highest value of separation factor achieved in liquid-liquid dissociation extraction was 23.7 (Gaikar and Sharma,1985), and in dissociation extractive crystallization it was 56 (Gaikar et al., 1989). This method, therefore, seems to be an elegant method of separation which gives 85-90% o-methoxyphenol in a single stage
with 99% extraction of o-methoxyphenol starting from a typical mixture containing 50 w t % o-methoxyphenol. Effect of Solvents. Nonpolar solvents, such as nheptane and toluene, were found to be useful. Polar solvents, like diisopropyl ether, were also used. It was observed that phenols do not react with solid calcium hydroxide in the presence of diisopropyl ether. This may be due to the intermolecular hydrogen bonding between ether and phenols. Recovery of Phenols from their Calcium Salts. Phenolic substances were recovered from their calcium salts by carbonation in toluene medium, and the quantitative amount of phenols was obtained by this method.
Conclusions The separation of close-boiling phenolic substances can be succeesfully carried out by dissociation extraction using powdered anhydrous calcium hydroxide. Very high values of the separation factor were realized, and a single-stage operation may suffice in some cases for nearly complete separation. Carbonation of calcium hydroxyphenates is an attractive method for the recovery of phenols from their calcium Salts. The proposed method is superior to the dissociation extraction in liquid-liquid systems with aqueous sodium hydroxide, in terms of both separation factor and the ease and cost of regeneration. Acknowledgment
P.K.P. is thankful to the University Grants Commission, New Delhi, India, for an award of a Senior Research Felowship. Registry No. Ca(OH)*, 1305-62-0; 2,6-xylenol, 576-26-1; p cresol, 106-44-5;o-methoxyphenol, 90-051; o-ethylphenol, 90-00-6.
Literature Cited Anwar, M. M.; Cook, S. T. M.; Hanson, C.; Pratt, M. W.T. Separation of mixtures of 2,6-lutidine with 3- and 4-picolines by dissociation-extraction. Roc. Znt. Solvent Extr. Conf. 1974, 1, 895-912. Anwar, M. M.; Cook, S. T. M.; Hanson, C.; Pratt, M. W.T. Separation of 2,3- and 2,6-dichlorophenolsby dissociation-extraction with monoethanolamine. Roc. Znt. Solvent Extr. Conf. 1977,2, 671-676. Ciernik, J.; Spousta, E. Chemical refining of 2,6-xylenol. Czech. Pat. 230,842,1986; Chem. Abstr. 1986,105,152704. Ciemik, J.; et al. Method of refining 2,6-xylenol. Czech. Pat. 240,893, 1988; Chem. Abstr. 1988,109,210689. Chaudhuri, B.; Patwardhan, A. A.; Sharma, M. M. Alkylation of substituted phenols with olefis and separation of close boiling substances via alkylation/dealkylation. Znd. Eng. Chem. Res. 1990,29,1025-1031. Gaikar, V. G.; Sharma,M. M. Separation of substituted phenols and chlorophenols by dissociation-extraction: New strategies; New regenerative methods. J. Sep. Process Technol. 1984,5,53-68. Gaikar, V. G.; Sharma, M. M. Dissociation-extraction: Prediction of separation factor and eelection of solvent. Solvent Extr. Ion Exch. 1985,3 (5), 679-696. Gaikar, V. G.; Sharma, M. M. Extractive separations with hydrotropes. Solvent Extr. Zon Exch. 1986,4 (4), 839-843. Gaikar, V. G.; Sharma,M. M. Dissociation-extractivecrystallization. Znd. Eng. Chem. Res. 1987,26,1045-1048. Gaikar, V. G.; Mahapatra, A.; Sharma, M. M. Separation of cloee b o i i point mixtures (p-cresollrn-cresol,guaiacol/alkylphenols. 3-picoline/4-picoline, substituted anilines) through dissociation extractive cryetallization. Znd. Erg. Chem. Res. 1989,28,199-204. Jagirdar, G. C.; Sharma, M. M. Separation of cloae-boiling mixtures of heterocyclic amines and LTC tar acids by dissociation-extraction. J. Sep. Process Technol. 1981,2 (31, 37-41.
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Mahapatra, A.; Sharma, M. M. New strategies in separation of close-boiling (solid) mixtures: Extraction into micellar and microemulaion media. Solvent Ertr. Zon Erch. 1987,5(4), 781-788. Schlosberg, R. H.; Scouten, C. G. Organic Chemistry of calcium. Formation and pyrolysis of hydroxy calcium phenoxides. Energy Fuels 1988,2,582. Scouten, C. G.; Dougherty, H. W. Organic chemistry of calcium. 3. Steam stripping of metal phenoxides liberates phenol and regenerates the metal hydroxide. Ind. Eng. Chem. Res. 1990, 29, 1721-1725. Wadekar, V. V.; Sharma, M. M. Separation of close boiling substi-
tuted phenols by dissociation-extraction. J. Chem. Technol. BiotechnoL 1981, 31, 279.
Pradip I(.Pahari, Man Mohan Sharma* Department of Chemical Technology University of Bombay Matunga, Bombay 400 019, India Received for review July 26, 1991 Revised manuscript received April 28, 1992 Accepted May 12, 1992
Modified Joback Group Contribution Method for Normal Boiling Point of Aliphatic Halogenated Compoundst For the screening of alternatives of chlorofluorocarbons (CFCs), one of the limitations encountered is the nonavailability of primary data including normal boiling point and critical temperature. The Joback method, which was proposed for the prediction of boiling point from molecular structure of an organic compound, when tested for an aliphatic halogenated compound, was inadequate in accuracy. It appears that halogens have to be treated differently from other functionalgroups because of halogem-halogen and halogen-hydrogen nonbonded interactions. A modified Joback method, with an improved capability, is proposed with a reestimate of the group contribution by F and also with an addition of some groups and corrections for perfluorination, partial fluorination (with or without other halogens), perhalogenation (with or without F), and partial halogenation (without F). Introduction Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are being extensively used as refrigerants, blowing agents, and propellants and as solvents. Only bromochlorofluorocarbons (BCFCs) are used as fire extinguishing agents. CFCs possess most of the desirable characteristics but for their damaging effect on the stratospheric ozone layer and for their part in global warming by green house effect. The ozone layer depletion has become an international issue. “Montreal Protocol” prescribes curtailing the production and phasing out the use of CFCs on a global scale. There is a likely convention on the compounds contributing to greenhouse effect. Obviously, alternatives to CFCs have to be identified and developed. This has attracted the attention of scientists from many disciplines. Thermodynamic Screening from Normal Boiling Point The earliest screening by Midgley (1937) concluded that the potential refrigerants should be made up of some combinations carbon, hydrogen, nitrogen, oxygen, sulfur, fluorine, chlorine, and bromine. Ultimately the choice was for CFCs. Subsequently, CFCs found their way into many other industrial sectors. Now scientists and technologists are faced with a dilemma of identifying compounds which, as long as they are within the system, perform satisfactorily, perhaps even better than the currently used CFCs, but, when they leak out, should be harmless to human health and benign to the environment. Therefore, any alternative has to have a low or preferably zero ozone depletion potential (ODP) and also with relatively low global warming potential ( G W ) . These constraints limit the number of alternatives. One can screen various compounds using the desired thermodynamic criteria and find suitable alternatives. For an exhaustive thermodynamicscreening, one has to consider compounds for which even fundamental thermodynamic data are not readily available. Recently McLinden and Didion (1988), using various constraints for screening, concluded that the potential refrigerants could consist of the elements carbon, fluorine, hydrogen, and oxygen.
HFC134a was proposed as a potential substitute for CFC12. According to many comparative analyses, including the one by Devotta and Gopichand (1992), HFC134a and HFC152a are considered to be the most potential substitutes for CFC12. Still the question of alternative does not appear to be over. There are also some unresolved issues like the global warming potential, toxicity, compatibility, energy efficiency, etc. The other possible compounds from the combinations of H, F, C, and 0 are fluorinated carboxylic acids, alcohols, aldehydes, ethers, and ketones. A recent, thermodynamic assessment of some fluorinated ethers and amines by Devotta et al. (1992) has indicated that tetrafluorodimethyl ether (CHF20CHF2)is another potential alternative to CFC12. In all the above screenings, the starting point for screening among a family of compounds is the normal boiling point. Knowing the boiling point, one could proceed in predicting the required thermodynamic property, from the molecular structure of the compound using well-established group contribution methods, corresponding state methods, and a few correlations. Devotta et al. (1992) have used this approach in their analysis. The limitation of such a screening is that the normal boiling point of a compound should be known. If there is some way of predicting the properties only from the molecular structure, this limitation can also be dispensed. Prediction of Normal Boiling Point by Joback Met hod There are a few methods proposed for the estimation of normal boiling point. These have been reviewed by Lyman et al. (1982) and Reid et al. (1988). These methods require, besides molecular structure, some extra parameters, like critical temperature, molar refraction, ionization potential, etc. It is very unlikely that these parameters will be known for a compound when ita boiling point is not known. Although some of these parameters can be estimated through group contribution methods, the propagation of error through such a route would be fairly high, and some of these methods are not comprehensive enough
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