Tricresyl Phosphate as Solvent for Phenol Recovery from Gas-Plant Effluents C. H. S.TUPHOLME, 32. Russell Hill, Purley, Surrey, England
S
OME six years ago the British Institution of GasEngineers appointed a subcommittee to investigate the methods which might be adopted for minimizing the production of gas liquor effluents or for rendering such effluents suitable for discharge into city sewage systems or rivers. The most important effluent from gas plants is the spent liquor from the distillation of ammoniacal liquor, but, in those cases where the recovery of ammonia has ceased, the ammoniacal liquor itself is regarded as an effluent. The recovery of ammonium sulfate now being an unprofitable task, all British gas plants are faced to a greater or less degree with this problem of effluent disposal. During the past year this subcommittee has concentrated on two main aspects of the problem: the prevention of higher tar acids from occurring in the ammoniacal liquor, and the recovery of phenols from ammoniacal liquor. COMPARISON OF CRUDEBENZEKE AND TRICRESYL
tent of the liquors treated is 3 to 5 grams per liter, and the percentage purification is 92 to 95. The liquor is washed with 8 to 10 per cent of its volume of solvent, and the amount of the latter which is lost is stated to be 0.08 gram per liter of liquor, which, with tricresyl phosphate a t 1 s. 6 d. per pound, represents 1 s. 2 d. per 1000 gallons of liquor. The operating temperature is 20-30' C. (68-86" F,), chosen to compare with phenol washing. To obtain a basis of comparison between benzene and tricresyl phosphate as phenol solvents, a test was first carried out by the British committee referred to with crude benzene. Hot liquor was run into the washer to a depth of 6 inches above the bottom of the gage glass. The remainder of the washer was filled with crude benzene from horizontal retort gas, 4.16 imperial gallons being required. Ammoniacal liquor a t 61" C. (141.8" F.) and a t 11.5 gallons per hour was then passed through the washer for 3.5 hours, samples of the effluent being taken periodically after the liquor originally a t the bottom of the washer had been displaced. The results are given in Table I. TABLEI. WASHING OF AMMONIACAL LIQUORWITH CRUDE BENZENE
PHOSPHATE AS SOLVENTS It is unfortunately the case that the complete elimination of higher tar acids from ammoniacal liquor, which is accomplished by the separation of the tar from the gas while above the dewpoint, is accompanied by some increase in PERCENTAQE HIQHER PERCENTAQE TIME the phenol content. Phenols, however, are not so objecTAR REMOVED IN REMOVED IN AFTER SAMPLE START PHENOL WASHER ACIDB WABHER tionable in liquor as are the higher tar acids, since they do Parts Os/ not form colored compounds on atmospheric oxidation, and Hours Grams/l00 ec. % 100,000 % fed t 0 they are more easily removed from the liquor. Various Liquor 0.367 .. 59 .. washer 0.158 47 57 20 1 methods have been proposed for accomplishing this removal, Effluent 0.155 22 46 58 Effluent 2 and the one which has met with most favor involves the Effluent 3 0.163 55.5 51 13 0.180 51 .. Effluent 4 .. washing of the liquor with benzene and the recovery of the Effluent 0.201 45 55 5 6 0.224 39 6 .. .. phenols from the benzene either by distillation or by wash- Effluent 32 0.249 Effluent 7 . . .. ing with caustic soda solution. Several plants of this type Effluent 8 28.5 0.262 .. 21 0.290 9 .. have been erected in Germany and in the United States; Effluent 19 0.297 Effluent 10 only one so far is operating in Great Britain, although others The initial removal of phenol (57 per cent) was less than are contemplated. Investigation has shown that for efficient purification the washer should be of considerable height and that obtained with a countercurrent flow of solvent (65 per the liquor should be heated. It was concluded that the proc- cent a t the same liquor rate and 9 gallons per hour of benzene), ess could not normally be profitable unless some economical the reason being that the two lowest sieves were out of action. means of recovering the benzene dissolved in the liquor were The amount of phenol removed remained almost constant for employed. In the one British plant, active carbon is used for 30 minutes, after whickit fell until only 19 per cent was exthis purpose. tracted after 3.5 hours. The removal of the higher tar acids A solvent which is attracting much attention, however, fell rapidly from 20 to 6 per cent a t the end of one hour, and for the recovery of phenols, is tricresyl phosphate, which has during the same period the sulfide extracted fell from 25 to advantages over benzene in its low volatility and solubility 2 per cent. Tricresyl phosphate has a higher specific gravity than in water, and in its great capacity for dissolving phenols. A summary of an investigation is given later. The tricresyl ammoniacal liquor; therefore, in order to arrange for the phosphate was contained in an experimental washer, and am- liquor to pass in drops through the solvent, the washer was moniacal liquor a t 60" C. (140" F.) was passed through the inverted. The liquor thus entered a t the bottom and passed solvent, samples of the outgoing liquor being taken from time out a t the top. Apart from this change the conditions of to time. The initial efficiency of the tricresyl phosphate was the test were identical with that carried out with benzene. much greater than that of benzene, and its total capacity for The temperature of the washing was 60" C. (140" F.). The phenol was eight times that of benzene. In addition, thirty results are given in Table I1 and are represented graphically, times the quantity of higher tar acids was removed. The together with those of the benzene run, in Figure 1. phenols recovered from the tricresyl phosphate were of better The amount of phenol extracted by the tricresyl phosphate quality than those obtained from benzene. remained almost constant a t 80 per cent for the first two This process has been patented by I. G. Farbenindustrie, hours, compared with an initial value of 57 per cent for and two plants are operating in Germany of 100,000 and 20,- benzene. This is presumably one of the effects of the large 000 gallons capacity per day, respectively. The phenol con- distribution coefficient for phenol between tricresyl phos303
IKDUSTRIAL AND EKGINEERING CHEMISTRY
304
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'phate and water. Although this is only a minor factor in the ~ I E T H OOF D SRECOVERY OF P H E P ~ O L ~ velocity of extraction, it becomes important in this particular ORDINARY DISTILLATION.By simple distillation, using a case. I n this initial period the higher tar acids were removed to the extent of 40 per cent, compared with 20 per fractionating column, 19 per cent by volume of the used cent in the test with benzene. A complete analysis of the solvent came over below 270" C. The residual tricresyl liquor showed that the different figure had been reduced by phosphate, however, still contained 23 per cent of the phenol an even greater amount (54 per cent) from 373 to 170. The present before distillation, so that the recovery was by no removal of sulfide was initially 23 per cent, almost exactly means complete. It was also found that slight decomposition the same as in the benzene run, but this fell to zero after 8 of the solvent had taken place, producing cresols. The recovered solvent was dark colored but retained unimpaired hours. its solvent power for phenols. It had, however, become TABLE11. I v A S H I N G O F AM&fOXI.kC.4L LIQUOR' WITH TRICRESYLmuch more viscous, part of this change being due to some PHOSPHATE change in the solvent itself, and part due to decomposition of PERCENT-HIQHER PERPERCENTsubstances extracted from the liquor. Evidently such a AQE RETAR CENTAQE AQE RES A M P L E TIME PHENOL MOVED ACIDS REMOVED SWLFIDEb MOVED method of recovery is not suitable because of the effect on the Grams/ Parts Os/ Grams tricresyl phosphate itself, Hours 100 cc. % 100,000 c/o per 100 c c . % 33 The recovered phenols were colorless, but darkened some1 0.072 so 0.144 0.5 39 23 2 0.066 0.162 1 81.5 31 42 16 what on keeping. They were not completely soluble in 3 2 0.076 0.175 79 .. 9 0.100 4 72 3 37 31 ... ,. sodium hydroxide, indicating that some neutral oil was 5 0.115 4 68 .. 6 44.5 present, but they contained no catechol. They did, how6 o:i& 18 5 7 0 : iis 53.5 8 .. ever, contain other substances which had an oxygen absorp0.198 45 .. 0 : i92 8 10 0 0.249 32 10 ... 14 tion, but their nature was not investigated. 0.267 27 11 50 16 7 VACUUMDISTILLATION.This is the most satisfactory 0.298 4 13 18 51 20 0.315 15 15 24 method of recovering the phenols, since a t 12 mm. pressure 0.328 17 12 28 0.348 32 19 7 the phenols can be almost completely removed by heating 0.341 8 .. ... 36 21 to 160" C. During this process the decomposition of the 0.356 22 5 .. .. ... .. 38 a Composition of ammoniacal liquor: 0-10 hours, 0.361 gram/100 cc. tricresyl phosphate is negligible, and the increase in viscosity phenol 54 parts Os/lOO,OOO higher t a r acids 0.209 gram/100 cc. sulfide. very much smaller: The increase which was shown in the 10-24 )hours, 0.366 gram/l00 cc. phenol, 54'parts 01/100 000 higher ta; acids. 0.222 aram/100 cc. sulfide: 24-38 hours, 0.373 a r a m h 0 0 cc. Dhenol. test, however, was due almost entirely to the decomposition 54 parts Oz/rOO 000 higher t a r acids 0 230 gram/100 r5. sulfide b It was founh t h a t owing t o the kxposure of the liquor t o air during the of substances extracted from the liquor, and it would seem washing process, the sulfide was reduced by volatilization b y about 7 per that, after the tricresyl phosphate had been used for a few cent; this has been allowed for In the figures given for percentage removed washings, it would become too viscous for further use without Figure 1 shows that, while the extraction of phenols with purification by distillation or some other means. This effect benzene fell to 20 per cent in 3.25 hours, 19 hours were re- might not be shown if the liquor were free from higher tar quired by tricresyl phosphate. Up to this point the benzene acids. had washed 8.4 times its volume of liquor with an average OF PROPERTIES OF TRICRESYL PHOSefficiency of 38.5 per cent, whereas tricresyl phosphate had TABLE111. COMPARISON PHATE ASD BENZENE washed 51.5 times its volume with an efficiency of 49 per cent. PROPERTY COMPIRIBON The ratio of the total amount of phenols extracted up to this Very much less with tricresyl phospoint by the two solvents was 7.5, compared with the value Ratio of solvent t o liquor phate much less with tricresyl phos8.0 calculated from the distribution coefficients (34.8 for tri- Height of washer required a t e for same purification Tricresyl phosphate very much greater Initial cost of solvent cresyl phosphate and 4.35 for benzene). Der nallon. and considerably . nreater The difference between the two solvents was even more ger unit iant Tricresyl pzosphate very much less b u t marked in the case of the higher tar acids. When the Solvent loss in operation probab y not when reckoned as cost Tricresyl phosphate much less efficiency of extraction had fallen to 5 per cent, the tricresyl Cost of recovering phenols from solvent phosphate had removed thirty times as much of these sub- Character of recovered phenols Tricresyl phosphate m u r h better Character of recovered solvent Tricresyl phosphate deteriorates and stances as had benzene. would need occasional purification; Percentage of higher tar acids removed
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$20 10
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I 1 I-! I 1 I 19 el 24 27 30 hOL7S AFTER START O I TEST
9 12 6
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FIGURE 1. WASHING OF h M O N I 4 C A L LIQUORWITH CRUDE BENZENEAND WITH TRICRESYL PHOSPHATE
At the end of the run the phenol content of the tricresyl phosphate, calculated from the amount extracted from the liquor, was 11.9 grams per 100 cc. That determined by the extraction of the used solvent with water was 12.2 grams per 100 cc. These quantities were calculated as ethyl alcohol, but in so far as cresols were present, the actual weights would be considerably greater. Three methods of recovery of the phenols were tried: ordinary distillation, vacuum distillation. and extraction with caustic soda solution.
where benzene is used and distilled after every washing, i t retains its properties unimpaired Greater with t f i c r e y l , phosphate, b u t probably diminishmg with continued use of same solvent
EXTRACTIOS WITH CAUSTIC SODA. The phenols can be extracted from tricresyl phosphate by caustic soda solution, but the method is not satisfactory, as alkali is lost in an emulsion with the solvent and there may also be hydrolysis of the latter. Those properties of tricresyl phosphate and benzene which have importance in the recovery of phenol from ammoniacal liquor are compared in Table 111. ACKSOWLEDGMEKT The writer is indebted to R. E. Gibson, chairman of the General Research Committee, Charles F. Botley, chairman of the Liquor Effluents and Ammonia Subcommittee, and J. R. W. Alexander, secretary of the Institution of Gas Engineers, for the above details. RECEIVEDNovember 3, 1932.
