NATHAN FEDERGREEN' and ARTHUR J. WEINBERGER Stamford Research Laboratories, American Cyanamid Co., Stamford, Conn.
Methylstyrene-A Case Study in Spent Acid Catalyst Treatment A simple and novel system solves another waste disposal problem
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Methylstyrene is the product and its voluminous wastes contain both acid and mercury
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A design for large scale plant is proposed
PETRocmMIc processes often cause waste stream treatment problems. This is particularly true of processes involving alkylation with acetylene where the catalyst is a mercury salt in sulfuric acid, or a nickel or copper salt in hydrochloric acid. The spent catalyst will require tar removal and if re-used, the heavy metal and acid will need regenerating. These treatments can be complicated by corrosion problems, and the need for special processing techniques and equipment can seriously affect over-all cost. Rigid health laws have made effluent treatment an integral part of modern chemical plants, and the feasibility of a process can depend upon the satisfactory solution of such problems. A new method of manufacturing mcthylstyrene is a typical example. The process is attractive because the resulting polymer has a softening point about 20" F. above that of polystyrene and even more above that of polymethylstyrene previously available. This results from the presence of 30y0 of ortho isomer not found in other commercial methylstyrene. In this process described by Dixon and Saunders (5). acetylene reacts with toluene using a mercuric sul-
fate-sulfuric acid catalyst and resulting ditolylethane is cracked to give methylstyrene and toluene. T h e amount of acid used is considerable but not unusual with waste acid amounting to approximately 1 . 2 pounds per pound of methylstyrene. (LVaste acid-product ratios of 1 to 5 have been reported in petroleum processing and manufacture of T N T , DDT, and acetaldehyde.) In this process, recovery of large amounts of sulfuric acid is complicated by the presence of mercury. The method described subsequently for treating Tvaste acid is simple, yet it represents a departure from methods previously reported.
Present address, Celanese Corp. of America, Newark, N. J. 1
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Previous Wasfe-Sulfuric Acid Treatment Techniques
Lt'astes containing sulfuric acid, aromatic sulfonic acids, and other substances in varying concentrations, are produced by petroleum refining operations in considerable quantities and are usually treated by hydrolysis, combustion, neutralization, or extraction (3, 70, 77). Sometimes the treatment must be followed by a concentration step. I n one hydrolysis method, for example, the spent acid is diluted to 60y0 acidity and digested a t 100' C. T h e organic hydrolysis product forms a separate layer
and is removed. Then the acid 1a)er is concentrated and bleached ( 7 ) . In the combustion process. waste acid is either burned and the recovered sulfur dioxide converted into sulfuric acid; or it is decomposed into a liquid hydrocarbon and sulfur dioxide by mixing with a hidrocarbon diluent and heating to 260" C. under pressure ( 4 ) . For extraction methods. numerous processes have been proposed for extracting sulfonic acids ( 9 , 12, 74, 75, 7 9 ) . Neutralization is usually too expensive but it has been used for waste acid from T N T manufacture ( I 7 , 76) ; however recovery of fertilizer-grade ammonium sulfate (8, 7 8 ) and sulfonates (2, 6, 72) has been reported. Processes requirinq concentration of dilute sulfLiric acid present a problem, and several methods have been proposed for removal of organic matter and carbon (73, 20). A s the concentration increases above 60 to 65%, remaining organic matter will accelerate decomposition of sulfuric acid to sulfur dioxide. Recovery of Mercury from Waste Sulfuric Acid
Two methods for recovering mercury from sulfuric acid waste were developed for use in hydrating acetylene to produce
acetaldehyde (7). In one method the spent catalyst is settled in two successive stages. The first sludge, high in mercury content, is oxidized electrolytically and returned to the process; the second, low in mercury content, is roasted to distill the mercury. T h e settled acid may be fortified with fresh acid and re-used, while excess is treated with sodium sulfide to remove residual mercury before discharge. I n the other method, the catalyst contains ferric sulfate which is reduced by oxidizing the mercurous ion or elemental mercury produced in the acetaldehyde catalysis. T h e spent catalyst is settled to remove sludge, oxidized with nitric acid to regenerate the ferric ions, and recycled to the process. T h e sludge is then roasted to recover any mercury present.
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Figure 1. Methylstyrene Waste Acid Treatment
T h e spent catalyst from the ditolylethane synthesis is a black, moderately viscous liquid smelling strongly of sulfur dioxide and containing some tarry organic material. A typical analysis is: Sulfuric acid Toluene sulfonic acid Water Tars Mercury
56%
35% 7% 2% 1200 p.p.m.
At proposed plant capacity the treatment must handle per year 38,000,000 pounds of sulfuric acid, 9,300,000 pounds of toluene and other organics, and 62,000 pounds of mercury. Losses of mercury must be negligible. Thus, consideration of alternatives to simplify waste problems is worth while. Recycling of spent acid, decomposition by burning, and development of a substitute catalyst were investigated and found impractical. Recycling, even on a limited basis, was unsatisfactory, for success of the alkylation depends upon a high toluene yield a t low conversion. Using the spent catalyst mixed with fresh acid and mercuric sulfate reduced the yield sufficiently to affect cost a t as little as 30y0 recycle. Also, the waste acid was more highly contaminated with organics than that produced from fresh acid. A combustion process seemed at-
Table 1. Effect of Stripping Temperature on Toluene Recovery from Waste Acid Batch Temp.,
c.
160-170 175-180 195-200
Toluene Recovered,
wt. LTqof
Waste Acid Fed 13.0-13.8 12.0 8.5- 9.0
WUNn!€
Flow sheet of laboratory apparatus for stripping spent acid
tractive because the waste acid contained sufficient organic matter to supply practically all the heat required for decomposing it to sulfur dioxide. However, mercury in the sulfur dioxide presented several complications in materials of construction and plant operation and had to be removed. It 'is impractical to rely on separating it from the sulfur dioxide because of low concentration and the probability of its forming a n aerosol on burning. Physical separation, even supercentrifuging, of the waste acid removed only 70% of the mercury. Consequently, the combustion process was eliminated. Several other catalysts were tried, but those showing promise gave poor yields. laboratory Development
After reviewing the alternatives, it was decided to investigate hydrolysis of the waste acid, for recovery of toluene would pay a large part of the total treatment cost. However, conventional hydrolysis with steam followed by separation of recovered toluene as a second liquid phase would leave a dilute acid. If instead, hydrolysis and stripping were done by blowing superheated steam through the waste acid, a higher acid concentration could be maintained, with toluene removed in vapor form. Preliminary laboratory tests (Figure 1) showed that steam blowing a t liquid temperatures of 160' to 170' C. gave good toluene recovery and a series of runs was made a t varying liquid temperatures (Table I). Below 160' C. the toluene distillation rate was low. At 160' C., some 3 to 5y0 of coke, based on weight of acid feed, was formed. This percentage increased a t higher temperatures. Most of the mercury vaporized and could be recovered from the distillate by settling or filtration. T h e remainder was absorbed by the coke formed during strip-
ping, and after filtration, mercury in the stripped waste acid was less than 2 p.p.m., while that of the coke ranged from 800 to 11,000 p.p.m. (dry basis) and could be stripped readily by roasting (Table I I). Foaming and coking could be reduced by starting with a heel of previously stripped acid, then adding fresh waste acid continuously. Several attempts were made in the laboratory to discharge the stripped acid continuously. However, the coke formed large agglomerates at the surface of the liquid and could not be discharged properly. Pilot Plant Trials
Since laboratory work indicated that draining and cleaning the hydrolysis vessel might be a problem, this was checked on a pilot plant scale using a glass-lined kettle, the material of construction proposed for the large scale plant (Figure 2). A series of runs was made using a semicontinuous feeding system. T h e heel was charged, the steam flow started, fresh waste acid was added over a 11/?- to 2-hour period, and then steam stripping was continued for an additional 1 1 / 2 to 4 hours. Some foaming was encountered but it was controlled by holding the liquid volume to 10 gallons
Table It.
Temp.,
c.
Removal of Mercury from Dried Coke Residual Heating Time, Mercury, Min. P.P.M.
315 315 415 450 450 450 540
47 103 75 30 75 120 75
Coke before heating
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4500 170
