INDUSTRIAL A N D ENGI.VEERING CHEMISTRY
966
Vol. 19. No. 9
Elimination and Recovery of Phenols from CokePlant Ammonia Liquors' By Robert hi. Crawford COhlMONWBALTH BUILDING,PITTSBURGH, P A .
I
N PREVIOUS articles2 the writer has described a successful method for eliminating phenols from ammonia still wastes by the continuous, countercurrent extraction of the raw liquor with benzene-or, more properly, motor fuel-in a process wherein the phenols removed from the liquor are recovered as a salable by-product in the form of crude tar acids. The purpose of this article is to describe a further substantial improvement in the process wherein a less costly and more efficient solvent, readily available a t any coke plant, can be utilized and additional salable crude tar acids can be incidentally recovered. Motor fuel is so easily available a t any coke plant having light-oil and benzene equipment, that its use as a solvent for extracting the ammonia liquor is logical. Owing, however, to the relatively high vapor pressure of such a solvent a t the temperature of the operation and also to its solubility in the extracted liquor, the item of "solvent loss'' constitutes one of the major costs of the operation. The "solvent loss" is determined by inventory and is the quantity of new solvent added to the operation from time to time. It is really an apparent solvent loss and is greater than the actual solvent loss, which cannot be accurately determined because that portion of solvent leaving the system in solution in the extracted ammonia liquor passes through the ammonia stills with the liquor, is vaporized, and passes into the main gas stream where its vapors augment the normal light-oil content of the gas, and is recovered along with the normal light oil in the benzene scrubbers. Therefore, this recovery of solvent is charged as a consumption, although i t is an apparent, and not an actual consumption of solvent. Phenol Extraction Efficiency of Coal-Tar Light Oils
A motor fuel solvent is also unsatisfactory for other reasons and in the endeavor to seek means for improving the operation an investigation was made of the use of neutral coaltar light oils for this purpose. Representative crude ammonia liquor was procured from a coke plant and light oils were obtained by distilling representative coke-plant tars. These light oils contained tar acids in varying amount, and in order to provide neutral light oils for the tests, they were separately extracted with 10 per cent caustic soda solution until the tar-acid content was below 1 per cent. A typical light oil as used for the tests had the following general properties : specific gravity a t 30" C., 0.953; limpid point, 40" C., tar acids, 0.6 per cent. Distillation data for this oil are as follows: =
c.
Below 170 Below 190 Below 210
70
b y uol. 18
41 61
c. Below 235 Below 300 Residue and loss
%
b y wol.
73 83 17
Measured quantities of the ammonia liquor were extracted with measured quantities of motor fuel and the neutral lightoil solvents, and the distribution coefficient (partition ratio) of the phenols was determined. I n every case the light-oil solvents exhibited much higher phenol extraction efficiency and lower solubility in the ex-
' Receired May 2 4 , 1927 2 THISJOUX\AL, 18, 313 (1926), 19, 168 (1027), Blast Furnace Steel Plant, 14 (1926).
tracted liquor than a motor-fuel solvent; with a solvent-toliquor ratio of 1:1, which is common in practice, the phenol coefficient with motor fuel was 73 per cent, whereas with the light-oil solvents i t averaged 93 per cent; and whereas the solubility of motor fuel in the extracted liquor was 5 per cent, the light-oil solvents were soluble only to the extent of about 1 per cent. With the light-oil solvents, no difficulty was experienced by the formation of emulsions so long as clean light oils were distilled from the tars. Even with repeated use on fresh ammonia liquor-removing the phenols from the oil with caustic soda before each extraction-no emulsions xere encountered and separations were clean. The reddish color in the ammonia liquor, which appears to be due to dispersed free carbon, or tarry matter, in solution, was substantially removed by the solvents. This feature is important, because a motor-fuel solvent does not dissolve free carbon, or tarry matter, so readily, and in practice, if the cycling motor fuel is not removed periodically for cleaning, emulsions form which appear to be due to dispersed free carbon segregated in the motor fuel, and separations are incomplete. With the light-oil solvents the free carbon seems to be taken into solution readily and not in the dispersed condition. Production and Use of Solvent
The most logical source of a light-oil solvent for the purpose is from the tar normally produced a t the coke plant itself. To obtain the solvent, the raw tar would be passed continuously through a simple steam-heated, multi-pass dehydrator for distilling off a light-oil fraction having a specific gravity of about 0.95 at 30" c. Dehydration of the tar is effected in this distillation, and a simple decanter would separate the water and oil condensate; the water, or, more properly, weak ammonia liquor, sent to the liquor storage, and the light oil pumped to the phenol extraction plant. The light oil thus collected contains tar acids which must be removed before the oil is suitable for a solvent. This is easily accomplished by introducing the oil into the phenol extraction system a t the first caustic extractor along with the normal cycling solvent, where the phenols are removed to augment the normal phenol recovery from the extraction of the ammonia liquor. The acid-free, or neutral, light oil leaving the caustic extractors then enters the cycling system as the solvent for extracting the liquor. Since new solvent is being added to the extraction system continuously from the tar dehydrator, a quantity of cycling extraction solvent must be removed. A suitable quantity would, therefore, be bled from the extraction system and mixed continuously with the distilled tar leaving the tar still, and thus prevent depletion of the normal coke-plant tar yield, except for water and acids removed from, and mechanical losses of, the light oil. It is obvious that the tar produced by such a system would be substantially dehydrated and would be preferred by the tar distiller to ordinary grades of raw tar. The accompanying flow sheet illustrates this process. The feature of a continuous supply of fresh solvent to, and the continuous removal of used solvent from, the ammonia-liquor extraction plant is an important advantage because the solvent can be repleiiished automatically and
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September, 1927
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
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968
INDUSTRIAL A N D ENGINEERING CHEMISTRY
there is an ideal outlet for impurities which tend to accumulate in such cyclic systems. Assuming the sales value of coke-oven tar to be 5 cents per gallon, the writer estimates the cost of the light-oil solvent to be less than 8 cents per gallon. This cost includes operating and fixed charges and assumes the small loss of tar volume a t the price of the tar with no credit taken for the ammonia liquor recovered from the tar.
Vol. 19,
XO.
9
Coke and gas plants which do not have light-oil recovery equipment can now utilize the ammonia-liquor extraction process for the elimination and recovery of phenols from still wastes. This has not heretofore been feasible because such plants could not afford to purchase supplies of motor-fuel solvent as the process would operate a t prohibitive costs. Applications have been made for patents covering the improved process.
Solvent Balance’ By Bruce K. Brown and Charles Bogin COMMERCIAL SOLVENTS CORPORATION, TERRE HAUTE,IND.
The proper balancing of solvents and non-solvents in OR several decades ranges which represent norlacquer formulation is discussed. Empirically deternitrocellulose solvents mal room conditions. mined rates of evaporation for individual solvents proand diluents have been A direct comparison of the vide more accurate indices of evaporating tendencies classified according to boiling vapor pressures of solvents at than are afforded by boiling point or vapor pressure point, those boiling below the room temperature is not a determination. boiling point of water being true index of their comparaThe amount of non-solvent tolerated by a nitrodesignated as low boilers and tive volatility. Vapor prescellulose solution varies with the concentration, solthe others as high boilers. sure determinations are based vent, and non-solvent. Dilution ratios are deterDespite its popularity, this on the number of molecules mined by titrating a solution of nitrocellulose with nonclassification is extremely inleaving the liquid state rather solvent until precipitation of some of the nitrocellulose accurate and the erroneous than on the weight of the occurs. The technic of the determination is described idea that the boiling point of molecules. Two solvents of and a number of dilution ratios for common solvents a solvent is an index of its identical vapor pressure will are given. behavior in a lacquer has not evaporate at the same rate tended to obstruct scientific a t a given temperature, unlacquer formulation. less their molecules are of the same weight and their latent The term “boiling point” has no scientific significance heats of vaporization are identical. The solvent having except that it describes the temperature at which the vapor the greatest molecular weight will evaporate most rapidly. pressure of the liquid under consideration is equal to one While it is possible to determine the vapor pressures of lacquer atmosphere of pressure. The ratio of the volatility of two solvents at room temperature, and then to correct these liquids at room temperature cannot be predicted by a com- figures for molecular weight and latent heat, so that the data parison of their boiling points. For example, there is a dif- for various solvents can be directly compared, such a proference of only 2 degrees in the boiling points of toluene and cedure is too complicated for everyday use-particularly since isobutyl alcohol, yet a t room temperature the former evap- new solvents whose physical properties are not well known orates almost three times as rapidly as the latter. While are being added monthly to the range of selection for lacquer xylene and normal amyl acetate boil at practically the same manufacture. temperature, xylene evaporates 125 per cent as rapidly as As yet, no better means of determining the volatility of the ester. Similarly, butanol boils at a temperature about solvents has been developed than the employment of purely 22 degrees below xylene, yet it evaporates only about 80 per empirical evaporation tests of the type reported by Davidson2 and Gardner.3 cent as fast. A factor representing rate of evaporation, whether it is Recognition of the disparity between the boiling point relations of various solvents and their volatility a t room tem- derived by physico-chemical calculation or by empirical test, perature has led to a study of vapor pressure curves. The is but an approximation of the behavior of the solvent in vapor pressures of a number of lacquer solvents a t varying question in so far as practical lacquers are concerned. Every temperatures hare been plotted as curves, from which data mixture of solvents and diluents which might be employed attempts have been made to compare the volatility of the as the liquid portion of a lacquer may produce binary, ternary, solvents. I n a number of cases the curves for various sol- or quaternary minimum vapor pressure mixtures of such vents will be found to cross during their ascent. At these complexity as to defy any prediction based merely on knowlpoints the solvents have the same vapor pressure, though edge of the vapor pressures of the unit liquids. For example, they may differ conversely above and below that critical point. so simple a mixture as 30 per cent benzene and 70 per cent’ ethyl alcohol evaporates (at room temperature) about 25 Importance of Evaporation Rate per cent more rapidly than the rate that might be derived The use of vapor pressure curves may be conceded to be from the formula for the evaporation of “perfect mixtures.” a vastly more accurate method of solvent evaluation than a I n general, substances which form constant-boiling mixtures study of boiling points, but in the opinion of the writers such also form mixtures of minimum vapor pressure at room temcurves will assist the formulator of lacquers only in EO far perature, but the proportions of ingredients present in the as they improve his knowledge of the principles of evaporation. mixture volatilized a t room temperature may differ from It may fairly be said that the only sectors of vapor pressure the proportions present a t the boiling point. curves that are of practicaI interest are those temperature Water blush, gum blush, cotton blush, improper drying 1 Presented as a part of the Symposium on Lacquers, Surfacers, and time, poor flow, orange peel, and a variety of other lacquer Thinners before the Section of Paint and Varnish Chemistry a t the 73rd
F
Meeting of rhe American Chemical Society, Richmond, Va., April 11 to 16, 1927.
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THISJOURNAL, 18,669 (1926). P a n t M f ~ s . ’Assocn. U . S., Tech. Car. 218 (1924)
