624 THE COLLIERY GUARDIAN. March 30, 1917. the use of the benzol group of explosives, particularly trinitrotoluol and tetranitro aniline, is by no means confined to military purposes, and their consumption for commercial blasting operations will doubtless increase to a very large extent, because the war has given such a convincing demonstration of their superior qualities. The demand for military explo- sives is likely to continue on a considerably greater scale than before the war, on account of the increased amount of military training that is being required in every country, and on account of the general recogni- tion of the wisdom of accumulating and storing large reserve supplies of the principal explosives. It is one of the peculiar merits of trinitrotoluol that it can be stored indefinitely without risk of deterioration, and the importance of conserving this valuable asset in national defence cannot be over-estimated. The vast development of American chemical indus- tries ensures a market for large quantities of benzol products to be used for synthetic purposes. Benzene, itself, is an indispensable source of nitrobenzene and aniline with its long series of valuable derivatives. The manufacture of synthetic phenol, which is far out- stripping that of carbolic acid made directly from coal tar, may be considered as a very important outlet for pure benzene, on account of the great increase in the production of the various condensation products of phenol and formaldehyde. Operation of Benzol Recovery Plant. The benzol recovery process dealt with is the Koppers process, which is typical of modern practice, and is the one almost exclusively used in the benzol plants now under construction in America. The wash oil generally used to absorb the benzol is a petroleum product known as straw oil, of which at least 90 per cent, should distil between 250 and 350 ash er Steam Supply Water Supply i—B-----1 Naphtalene Separating I Sump | Gas Supply Gas Return Add Drain Tankyt>12- £ 3 5 £ degs. Cent. A good absorbent oil has a specific gravity of less than 0-88 at 15 degs. Cent., and is readily fluid at 4 degs. Cent. It contains no naphthalene or pitch, and exerts a good solvent action on benzol. In best practice the amount of benzol absorbed (technically the “ enrichment ”) is kept between 2 and 3 per cent, of the absorbing oil. Too high enrichment is likely to lead to loss of benzol, and too low enrichment may involve needless consumption of absorbing oil. In European practice, heavy tar oils are used almost exclu- sively as absorbing and steam media, and such materials may find increasing application in America. Such tar oils should contain less than 7 per cent, naphthalene, and 90 per cent, of the material should distil between 200 and 300 degs. Cent. The drawing shows a flow sheet of a benzol plant with the principal features of operation. Previous to treatment with the absorbing oil, the gas should be cooled to a suitable temperature, which may vary from plant to plant, and depends principally upon such factors as its "moisture and naphthalene content, temperature of the wash oil, and per cent, enrichment desired. This cooling is accomplished by means of the cooler A, which is preferably of the direct contact type. The water not only acts as a cooling medium, but mechanically washes a large portion of the naphthalene out of the gas, and carries it into the separating sump B. The cool gas then passes into the benzol washers C. These are tall scrubbing towers of the hurdle type, effecting a very intimate and prolonged contact between gas and oil. The debenzolised gas passes out of the last washer through the return main to its point of consumption. The fresh wash oil is pumped from the circulating tank D, over the scrubbers in an opposite direction to the flow of the gas, maintaining the counter-current principle that should be adopted in nearly all scrubbing operations, and bringing the fresh washing medium into the scrubbing system at a point where the gas contains the least light oil vapours. The distribution of the wash oil over the tops of the scrubbers is a very important matter, and should be done as uniformly as possible. The enriched wash oil accumulates in tanks usually located underneath the scrubbers, and is pumped from these to the benzol recovery plant, to be heated for the purpose of releasing its benzol consti- tuents. Part of this heating is accomplished by the utilisa- tion of the heat in the hot debenzolised wash oil leaving the still. Two heat economisers are used. The cold oil first enters the heat exchanger E, where it is heated by benzol vapours and steam from the still H, thence it is conducted to a second heat exchanger F, where it receives additional heating by means of hot deben- zolised wash oil leaving the still H. It is then still further heated to the maximum temperature desired by means of live steam in a superheater G, from which it passes into still H; this still is composed of a series of superimposed sections or chambers, as in common distillation practice. The heated oil flows down through these sections, while steam is blown directly into the lowest section, and travels in a direction oppo- site to that of the oil. The mixture of benzol and water vapour is partially rectified in the upper portion of the still, and then enters the heat exchanger E, as mentioned before, where it is condensed and the partial preheating of the enriched wash oil is effected. The remaining vapours are completely condensed, and the total condensate cooled in a water cooled condenser I. The light oil is separated from the water in the con- densate by means of the separator J. The debenzolised wash oil, after leaving the still, passes through the heat exchanger F, where it gives up a part of its heat to the enriched wash oil, as stated Soda Mixing Tank Soda Storage Tank M AylYEtorage ft V Tank, T Acid Boiler Agitator Pure Cooler Receiving ranks Crude Products Storage Tank Crude Products Storage Tank Crude Products Storage Tank Crude Cooler ^Receiving lanks Meter Tanks Washed Products Crude Products Storage Tank Storage Tan's Pure Products Storage Tank 3 Pure Products Storage Tank Pure Products | Storage Tank J Pure Produrts Loading Pump Loading "xPump JLoading —kPump Loading Pump Pure Produces Storage Tank □I Flow Sheet of Benzol Plant. above. Then it is finally cooled in the water cooler K. The cool oil is then delivered to circulating tank D, thus completing the cycle. The improvements that have been made in recent practice have to do principally with effecting as great economy as possible of heat in the cycle of gas treat- ment, distillation, and cooling through which the wash oil passes. In the Koppers system it is calculated that the devices for heat economy reduce the steam con- sumption in distilling the enriched wash oil by more than 80 per cent.; 1,000 gals, enriched oil 77 to 284 degs. Fahr.; (1,000x7-3x0-537x207)=811,440 British thermal units = 840 lb. of steam at full pressure, or, per 1,000 gals, purified benzols, 55 x 840 = 46,200 lb. of steam which, at 2c. per 100 lb., costs 9*24 dols.; of this steam, heat exchanger F cooling the hot debenzolised oil from 275 to 170 degs. Fahr, in preheating the enriched oil, saves per 1,000 gals, enriched oil (973 gals, debenzolised oil), 937X 7-3X0-54x105 = 402,675 British thermal units = 417 lb. of steam, or 49-8 per cent, of the total; heat exchanger E heating the cold, enriched oil by the benzol vapours and steam from the still, adds a recovery of about 256,000 British thermal units per 1,000 gals, wash oil, equivalent to 2651b. of steam,, or 31-5 per cent, of the total; in the two heat exchangers, therefore, the total saving is 417+265 = 682 lb. of steam per 1,000 gals, of enriched wash oil circulated, or 37,5101b. of steam for every- 1,000 gals, of purified benzols produced, and the cost of steam for this pur- pose is reduced from 9-24 to 1-72 dols. per 1,000 gals, of product. Further economy lies in the saving of cooling water circulation which would be required to cool the debenzolised wash oil. Th& light oil is accumulated in a drain tank H, shown in the lower part of the figure, and portions are taken for distillation in still L. This still is usually known as the crude still, the first distillation of the wash oil being made for the purpose of effecting an approxi- mate separation of several fractions of different boiling points preliminary to washing and final rectification. This and subsequent distillations are made intermit- tently in stills of large capacity (6,000 to 12,000 gals.), which give better fractionations than are possible in smaller apparatus. The continuous type of stills has not been found satisfactory in this work. The crude still does not require the elaborate dephlegmator that is necessary on the final rectifying stills. The heating is accomplished by means of internal steam coils and a direct steam spray. The benzol and toluol are prin- cipally distilled off by indirect heat, using the steam coils, and the higher boiling constituents xylol, solvent naphtha, etc., are then distilled over by introducing steam directly into the still. After the benzol, toluol, xylol, and solvent naphtha have distilled off, a certain amount of wash oil con- taining naphthalene remains in the still tank. The presence of wash oil in the light oil is due not only to mechanical trapping of the heavy oil during the distil- lation, but also to the actual distillation of some of its original constituents by agency of the direct steam used. The products recovered from 1,000 gals, of light oil vary according to the kind of coal coked, the regu- lation of the ovens and the method of operation of the light oil plant. In one plant that may be taken as fairly typical of a well-operated system, the yields average about as follow :—From 1,000 gals, crude light oil: 680 gals, crude benzol, 140 gals, crude toluol, 50 gals, crude xylol, 55 gals, solvent naphtha, 75 gals, wash oil residue with naphthalene. The wash oil remaining in the still is drained into cooling pan M, where it is cooled fin the air to crystal- lise out the naphthalene. The wash oil is drained away from the latter into tank N, and then is returned to the main circulating tank D. In large plants where the amount of naph- thalene is great, a centrifugal dryer is employed for the purpose of separating the small amount of oil remaining in the napthalene, and also for re-claiming the naphthalene from the separating sump at the foot of the gas cooler. The crude naphthalene so obtained can be sold as such, or may be put into the tar in the coke plant. The products obtained from the crude still will satisfy many commercial pur- poses in normal times. Moreover, at present the demands of chemical manu- facturers for benzol and toluol of a high degree of purity have made it advisable to accomplish the complete process of purification at the coke plant to serve this important part of the trade. For this purification, the crude benzols are first washed with sulphuric acid, and then with caustic soda and water. This operation is accomplished in agitator O, which is a large lead-lined vessel with an efficient mechanical mixing device for bringing the acid and benzol into inti- mate contact. The acid is commercial concentrated sulphuric acid (66 degs. Be). The quantity used is accurately measured from the meter tank p3. The caustic soda solution is prepared in the tank Q, and measured in the meter tank p2. The acid has the effect of reacting with and to a large extent polymerising most of the impurities which consist of various olefines and substances of similar char- acter, together with certain phenoloid bodies. This results in the forma- tion of resinous substances of very high boiling point, part of which are insoluble in the benzols, and settle out with the acid in the bottom of the agitator, while part go into solution, giving the benzol a dark brown or a reddish colour. The acid sludge is drawn off and treated, as will be described later. The caustic soda neutralises any traces of acid which may remain in the agitator and affect the removal of some of the phenoloid bodies. After the soda wash the benzol is a lighter brown colour, but always requires distillation. The washed benzol is delivered from the agitator to the still R. This still is generally of the same capacity as the crude still, but is provided with a very efficient dephlegmator. Some- times purified products of less exact boiling points are desired. In this case, the washed benzols are dis- tilled rapidly, simply to separate the benzol from the resinous materials in solution. The products so pro- duced are termed “ purified,” e.p., 90 per cent, purified benzol, 50 per cent, pure benzol, etc., the nomenclature being based on the percentage in test distilling under 100 degs. Cent. The distillation for the preparation of pure products is conducted more slowly, the con- densate being collected in receivers S, and tested care- fully before placing in the final storage tanks. Great improvement has been made in recent prac- tice in the distillation of pure benzol and toluol. In a well-designed plant the cuts are remarkably clean, and the percentage of intermediate fractions small. At one benzol plant pure benzene is regularly being pro- duced of a grade such that less than 5 per cent, distils below 80-1 degs. Cent., and 95 per cent, distils within 0-3 degs. Cent. Pure toluene also is being made, of which less than 5 per cent, distils below 110 degs. Cent, and 95 per cent, within 0*5 deg. Cent. This extra- ordinary degree of purity is of great advantage to manufacturers of explosives and synthetic chemicals requiring the use of pure benzols. The ease with which these distillations can be effected and the sharp-