NEWS EDITION HARRISON E. HOWE. Editor
Published by the American Chemical Society VOLUME 18
NOVEMBER 10, 1940
NUMBER 21
New Synthetic Phenol Plant
New Durez synthetic phenol plant showing one of the first- and second-stage production buildings
D
UREZ
PLASTICS
AND
CHEMICALS,
INC., North Tonawanda, N. Y., recently announced the opening of a new $2,000,000 synthetic phenol plant, capable of producing 15,000,000 pounds of phenol a year. The unit, which has been in continuous operation for four months and is said to be the largest of its kind in the world, employs the Raschig process, a catalytic vapor-phase system for the chlorination of benzene and the hydrolysis of the chlorobenzene produced. The process, invented by Prahl and Mathes of the Raschig organization, Ludwigshafen, Germany, produces a phenol purer than U. S. P. and does so with less than 0.1 per cent of by-product. Processes heretofore employed are reported as yielding from 2 to 5 pounds of by-product per pound of phenol. The plant, built under the supervision of the process inventors, has been in the construction and trial stages for the past two
years. It employs many innovations and much equipment, socially designed to overcome process difficulties, and is housed in several huge buildings. There is complete duplication of equipment at every stage in case an emergency renders one system unfit for use. In spite of the plant's great size, however, it is operated at full capacity by a force of only six men per shift, automatic controls and alarms having been utilized to the fullest extent. The conversion of benzene to phenol is earned out in two catalytic, vapor-phase stages. In the first operation benzene is chlorinated with hydrochloric acid and air in the presence of a catalyst, the reaction being exothermic and proceeding at about 230° C. The reaction is represented by equation (1) C2H4 + HC1 +1/2O2- C2H4Cl + H2O (1) The first pass of the benzene-hydro-
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chloric acid gas mixture through the catalysts converts only 10 per cent of the available benzene to chlorobenzene. Formed also are minute quantities of polychlorobenzene and oxidation products. Separation of the mixture is accomplished by continuous fractional condensation whereby the chlorinated compounds and some unreacted benzene pre collected in liquid form for subsequent recovery by continuous fractional distillation. The balance of the benzene is recovered in a final condenser and in a scrubber where the tail gases are absorbed by an organic liquid from which it is continuously distilled and condensed for reuse. In practice the first stage system operates by drawing the reactants in the vapor state, at slight negative pressure, through contact chambers, a fractional condensing tower, and a final condenser. As conversion takes place, the products are removed from the fractional condenser,
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which operates at a temperature slightly above the boiling point of benzene. The benzene vapor which pass the fractional condenser are recovered from the final condenser and tail gas recovery system. The second stage reaction proceeds at about 425° C., is endothermic, and is represented by equation (2) C6H5 + H2O = C4H5OH + HCI (2) In the second stage only 10 per CEnt of the monochlorobenzene is converted in one pass through the catalyst and, since there is water present, traces of side-reaction products are formed, the actual products of the second-stage reaction being C4H5OH, HCI, H2O, oxydiphenyl, and diphenylether. By heat interchange between the sensible heat of the products of reaction and the azeotropie va|>or mixture of the reactants, the products are first celled to about 200° C. and then further cooled in a fractional condenser by being required to vaporize a liquid mixture of the reactanK Ry further condensation, a 15 per cent water solution of hydrochloric acid, containing about 5 per eent of the phenol formed, is recovered. The vapors leaving the fractional condenser contain about 95 per cent of the phenol formed, together with all the azeotropie mixture of the reactants. The phenol is recovered by scrubbing with hot water under temperature conditions such that the azeotropie vapor mixture of the reactants remains in the vapor state while the phenol is dissolved. The vapor mixture is later heated to reaction temperature and repassed through the secondstage contact chambers. The products of the second-stage reaction are thus collected in the form of a recovered hydrochloric acid solution containing dissolved phenol and a phenol-
N E WS E D IT ION
water solution. The phenol is then recovered by a continuous countercurrent iKjnzenc extraction. Tho combined benzene-phenol solutions |KISS to a continuous iractionating still, the distillate being benzene vapor which is |Kissed to the first-stage reaction. The condensation product from this column is crude phenol (97 per cent) which is subsequently recovered in pure state by continuous vacuum distillation. The recovered hydrochloric acid solution, after phenol has been removed, passes to hydrochloric acid vaporizers and then to the first-stage reaction. The extracted water solution of phenol is reheat**!, «tripped of benzene, and then cycles back to the scrubber to absorb more phenol. The process flow sheet is shown in Figure 1 and starting at El, a steam-heated tubular evai>orator, benzene is evajwrated from a mixture of benzene and phenol into fractionating column Cl. The l>enzene va|K>r coming from the top of Cl passes through a tubular heater, 2, in the vapor superheater, and then into vapor mixer, 3, where it is mixed with hydrochloric acid boiling from 4, and air which hw* been passed through tubular heater 2A. The va|x>r mixture, at a temperature of about 205° C , passes into contact chamber 6' where chlorination takes place. The chlorinated products then pass into scrubbing tower 8. Here chlorobenzene is condensed, removed from the bottom, and held in storage for subsequent continuous fractional distillation. Above tower 8 is condenser 8 which condenses benzene vapor driven off in the tower. Part of this distillate is used at* reflux and part to extract phenol in the second stage. A suction fan, 5, is used to draw the reactants through the first-stage system and to discharge them into a scrubbing
Vol. 18, No. 21 tower in which benzene is absorbed by a hydrocarbon oil. In the second stage, starting at B, a mixture of pure chlorobenzene and water is passed into the top of scrubbing column 25, within which the products of the second-stage reaction are circulated by a turlniblower, 21, Temperature conditions in column 25 are such that an azeotropie mixture of chlorobenzene and water is formed to augment the vapors leaving the column. Excels water added at this jioint absorbs hydrochloric acid from the vajKirs, the recovered solution flows from the bottom and is subsequently vaporized into the first stage system at •{. The va|wrs leaving the top of column 25 consist of chlorobenzene, water, and phenol and |wss into the bottom of the phenol washer 26 where they arc refluxed with hot water to dissolve the phenol. The phenolwater solution leaving the bottom of column 26 Is jMissed through the benzene extractor 28. The phenol-benzene extract from the top of extractor 28 flows to cvajK)rator El to supply the benzene vapor from the first stage. The crude phenol from the evaporator is sent to storage for subsequent purification. The chlorobenzene-watcr vajwr leaving the top of column 26 is picked up by turboblower 21 and |iassed through tubular heat exchanger ?2. Leaving exchanger 22 the reactants lack about 100° C. in temperature so they are passed through tubular heater 23 in the vapor superheater and then directly into the second-stage contact chambers, 24. From chamber 24 the hot products exchange sensible heat with the incoming reactants in exchanger U% are partly cooled, and go directly to the bottom of the acid recovery tower, 25. For all practical purposes the process is regenerative with respect to hydrochloric acid as evidenced by the fact that the normal oj>e rating efficiency of the hydrochloric acid recovery cycle is about 97 per cent. This efficiency includes loss of hydrochloric acid in the form of higher chlorinated l>cnzencs and all mechanical losses in handling. Only 0.1 |>ound of byproducts is formed JHT pound of pure phenol produced, being small quantities of »K>lychlorobcnzenes, diphenyl compounds, and tarry matter. Another feature of the process is that the first-stage catalyst is truly selective towards benzene in that unsaturated compounds or toluene arc not chlorinated but arc oxidized to CO» and CO. Thus no trace of cresols exists in the phenol which is kept rigidly to a specified 40.70°
