000
~ n d Eng. . Chem. Process ~ e s Dev. . 1905, 24,886-887
Two-Step Separation Process: Alternative to Distillation Separation of close-boillng and/or exploshre mixtures which are often encountered in practlce has been discussed by a new two-step single-stage process. The distinct feature highlighted in this communication Is the expbitatbn of the differences in solubilities of the isomers of organic adds/bases in various organic sohrents. As an iHwtratkm, a mixture of 0 - and p-nitrophenols has been successfullyseparated Into pure isomers, with recovery of more than 95% of starting total phenols in a single stage by adopting this new strategy. Introduction Separation of isomeric/nonisomeric close-binding mixtures is often an industrial problem. Jagirdar and Sharma (1981a,b) have reported the separation of some such mixtures by adopting various modified gas-liquid, liquidliquid dissociation extraction processes. Wadekar and Sharma (1981) have published a state-of-the art review on the conventional dissociation extraction which gives salient details on various aspects of this process. Recently, Jagirdar and Lawson (1984) have reported the separation of mixture of nitrophenols by modified solid/liquid dissociation extraction using aqueous sodium hydroxide. However, this solid/liquid extraction does not appear to be very economic as it demands constant consumption of neutralizing reagents. In this communication, an attempt has been made to exploit a new two-step strategy to separate various difficult mixtures of organic acids/ bases. In this process various physicochemical properties such as solubilities in organic solvent in the first step and dissociation constants and distribution coefficients in the second step have been successfully exploited. It is interesting to note that due to the ortho effect (because of hydrogen bonding), there is a substantial difference in the solubilities of the ortho and para isomers in various organic solvents. For instance, o-nitrophenol (0-NP) is much more soluble than p-nitrophenol (p-NP) in solvents such as benzene, chloroform, carbon tetrachloride, etc. (see Table I). In the present study, separation of mixtures of 0-NP and p-NP has been carried out by this strategy. The similar approach can be extended for separation of various other mixtures. Recently, Jagirdar (1984) has also reported the separation of 0- and p-nitroanilines using a similar approach. Experimental Section The various chemicals used in the experimental study were of high purity. The synthetic mixtures of 0-NP and p-NP of different compositions were prepared. Experimental procedure employed for the first step was similar to that deacribed by Jagirdar (1984). The organic solvent phase (extract I) was quantitatively collected and was subjected to conventional diesociation extraction in the second step. Experimental details of this step can be found elsewhere (Jagirdar and Sharma, 1981a). The nitrophenols in the mixture were quantitatively analyzed by HPLC in a p-Porasil column (3.9 mm X 30 cm) with a spectrophotometer (Perkin-Elmer LC55 with a flow-through cell) at a wavelength of 284 nm. The solvent used in the analysis was toluene with a flow rate of 2 mL/min. All the experiments were conducted under ambient conditions and at 20 f 1 "C.
Theory The knowledge of various physicochemical properties will be very useful in the design of this process without involving excessive experimental work. In the first step, since it is the differential solubility that is exploited, it is easy to calculate the minimum volume of the organic solvent required for the complete dissolution of the more soluble compound, say, o-nitrophenol in the present study. Details on the various theoretical aspecta and predictions
Table 1. Physicochemical Properties of Nitrophenols o-nitro- p-nitrophenol property phenol 44-45 113-114 melting point, "C subl. boiling point, O C 214.5 7.14 7.22 pk,," in aq solutions at 25 "C solubility,bscg/100 g water of solvent benzene chloroform carbon tetrachloride
0.208"
107.3816.6" 1.28l' 99.6816.6' 2.9914" 40.4216.6' 0.0514"
distribution coeffd benzene 210 between the org. solvent and water at 20 "C chloroform 223 carbon tetrachloride 116 'Serjeant and Dempsey (1979). (1927). dKorenmanet al (1976).
1.32"
* Vaubel (1895).
1.5
1.6 0.12 Desvergnes
for the separation fador for the dissociation extraction step can be found in the review paper by Wadekar and Sharma (1981).
Results and Discussion The experimental resulta for the separation of 0- and p-nitrophenols are reported in Table 11. It is interesting to note that the results of the separation are similar to those otherwise expected based on solubility data in the first step and the predicted separation factor in the second step. It is also interesting to note that when the solvent was changed from benzene to carbon tetrachloride, an extraordinarily high degree of separation was achieved. The ratio of solubilities of o- and p-NP is as high as 800 at about 15 "C in carbon tetrachloride, which indicates a very high degree of separation in the first step itself. However, in order to get rid of small quantitiea of dissolved para isomer in carbon tetrachloride along with almost all o-NP, a contact with small volume of alkali in the dissociation extraction step can purify the organic phase leaving almost pure ortho isomer. Thus, when carbon tetrachloride was employed as a solvent, 99+ % of the phenols were recovered as pure ortho and para isomers indicating extraordinarily high recovery and separation of these species. This fantastic degree of separation in both the step (very high separation factors have been found in the dissociation extraction step; see Table 11)with solvents such as carbon tetrachloride warrants attention as it is very easy to recover and recycle the solvent without significant losses. Conclusion and Scope The mixture of 0- and p-nitrophenols with a feed composition of 4040% has been separated into almost pure ortho and paraisomers in a single-stage operation by adopting two-step strategy. A number of other industrially useful mixtures such as nitrocresols, nitrobenzoic acids,
~ 1 ~ 6 - 4 3 ~ 5 / a 5 / 1 1 2 4 - o a a a $ 0 1 . 5 ~0/ 01985 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985
887
Recycle
4 - 7 Solvent
Feed
Step 1 Preferential
Pure Raffinate p- isomer hose ( 1 )
Leoching
the feed
Neutralizing
u e e
e u u
D issociat ion Extraction
salts
I
I
Pure 2-isomer
Figure 1. Scheme for separation; two-step separation process.
u u u 01
I
chloroanilines, etc., can be subjected to separation by using a suitable organic solvent and adopting a similar process. The principles described in this process can be extended to the use of mixed solvents as well as for the separation of multicomponent mixtures. In the selection of solvent, one has to consider, apart from solubility data, other properties such as boiling point, toxicity, solubility in water, etc. It is possible to employ two or more stages in the dissociation extraction step if needed. A typical flowsheet of this process is shown in Figure 1. Registry No. o-NP, 88-75-5; p-NP, 100-02-7; carbon tetrachloride, 56-23-5; benzene, 71-43-2;sodium hydroxide, 1310-73-2.
Literature Cited Desvergnes, L. Rev. Chlm. Ind. 1927, 36, 194, 224. Jaghdar, G. C. Chem. Ind. (London) 1964, 506-587. Jaglrdar, Q. C.; Lawson, F. J . Sep. Process. Techno/. 1984. in Press. Jagirdar, G. C.; Sharma, M. M. J . Sep. Rocess Techno/. 1981a, 2(3), 37-41. Jaglrdar. G. C.; Sham, M. M. J . Sep. procesS Technol. WBlb, 2(4), 7-12. . Korenman. Ya. I.; Kotelyanskaya, E. 6.; Nefedova, T. A. Zh. M /Khlm. 1976. 49(5), 1112-1124. Serjeant, E. P.; Dempsey, B. “Ionlsatlon Constants of Organic Aclds In Aqueous Solutions”,IUPAC Chemical Data Series No. 23, 1st ed.;Pergaman Press, 1979. Vaubel, J . Prakt. Chem. 1695, (2)52. 72. Wadekar, V. V.; Sharma, M. M. J . Sep. Process Techno/. 1961, 2(1). 1-15.
Department of Chemical Engineering Monash University Clayton, Victoria, Australia, 3168
G. C . Jagirdar
Received for review March 13, 1984 Revised manuscript received September 10,1984 Accepted September 26,1984
