The modern contact plant is fundamentally the same as that of 10 years ..... through a pipe that extends down the center of the air inlet Hue. The mol...
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., A Staff-IndustrgCollaborative Report.
heir readings on the electrical conductivity of the solutions. A rotameter and control valve for the dilution water to the 98% acid pump tank is located beside the 98% acid recorder. The dilution water valve is operated automatically through the recording instrument in some units.





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with pneumatic liquid level gages. A pressure switch, connect,ed in parallel with the gages, causes an alarm to sound whenever the acid level becomes too lo~v,and a signal light on the control panel indicates the particular tovier that needs attention. STORAGE A S D FlLLING



Vol. 40, No. 8


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I(e1ation of Unit Capacity to Cost of hrid


Teniperat,ures at any of 20 control points in the system can bct quiclrlp read from a temperature indicator that gives readings from chi,oniel-alumel thermocouples. X record of all converter tc:mperatures is maintained by a multiple-point recorder. Liquid lcvel gages are installed in the control room, so that the Icvel of all process acid tanks can bc. seen at a glance. These gages determine the acid levels by slowly bubbling air into the bottom of the tanks and measuring thr pneumatic pressure required to overcome the hydrostatic head. Manometers t o register gas pressures at inlet and outlet of the blon-er are also provided. Boiler feedn-ater aiid steam pressure gagos and a steam drum water level indicator provide index t o the operat ion of the wa,ste heat boiler system. .Automatic controls eliminate the need for. eveii m e operator iii inany emergeneics. One such device stops the sulfur pump and air blower in the event of low Tvatcr 1 1 in the steam drum. -1float in the drum operates a Mercoid switch in the power circuit. Another lfercoid switch will stop the sulfur pump if there is an interruption in the b l o ~ c rpon-er supply. Tho acid distributor palls at the top of the drying aiid absorbing towers arc equipped

The product, oleum may be piped to other I\fonsant,o departments from the vertical steel storage tanks by means of horizontal acid pumps located at grade beside the tanks. These same pumps are used to deliver the product acid to tank car loading racks, where as many as 18 cars may be loaded siinultaneously for shipment outside the plant. On occasions when 66" BB. (93.2%) acid is desired, product 98y0 acid is bled off from the outlet of the 98yc drying acid cooler and delivered by gravity to a dilution boot made of cast iron pipe fitted nit,h a Duriron liner, TT-here water is added t o produce the desired acid strength. Acid strength is checked in a hydrometer boot supplied by a continuous streani of t,he diluted acid. The dilution boot discharges by- gravity to a cast. iron cooler similar to that used for the 98y0 process acid, in order to remove the heat of dilution, and then flom to one of the storage tanks. Demand for sulfuric acid in carboys and drums requires thr periodic filling of such containers from the storage tanks. TREND TOWARD L4RGER Uh-fTS

The trend in contact sulfuric acid manufacturc in recent, ycari; has been to large single units. Sulfur-burning plants are now being built in single units-i.e., no equipment in parallcl--h-ith capacities up to 500 tons (lOOVc basis) of acid daily. Thcsc. larger units result in a greatly decreased capital cost on a pci. ton basis. Labor requiremeiits for the larger units are usually no more than for the smallest size of unit. Total maintenance cost per year on a large unit represents only a small percentage incrcnsv over that for I-he smaller unit. Thus, cost of manufacture dccreases as the size of the unit increases. The world's largest contact sulfuric acid uiiits are beliovod to be those desigiied by ?ironsanto in 1942 for the East Tennessee Ordnance T o r k s . This plant,, now owned aiid operated by the Tennessee Copper Company, produces over 650 tons (100% basis) of new acid daily from by-product sulfur dioxide gas received from nearby ore roasters. Although its wet gas purification sj-stcm is in a single line, RIonsanto rates this plant as two 300-ton units, because it contains tn.0 converters in parallel.

Contact Sulfuric Acid Plant (100-Ton Capacity) Designed by hlonsanto Chemical Company for Swift and Company, Agricola, Fla.

August 1948



about 30 cubic feet per minute of gas are purged from the system continuously.

Unusually Small Monsanto-Designed U n i t for 10 Tons Daily under ConstrucGon i n Cuba

An over-all efficiency of over 99.57, is reported. Production rate has exceeded 200 tons per day and operation has been steady and reasonably trouble-free in recent years. Laboratory and plant tests have shown that increased corrosion rates in the Trail unit are due to the high sulfur dioxide content rather than the high oxygen content. Corrosion problems, as might be suspected, have been most critical in the converters and heat exchangers. However, aluminum-coated surfaces are exceedingly passive to the gas mixtures used as long as the coating remains unbroken. Although this installation has proved the technical practicability of the process, thc immediate outlook for the use of oxygen in acid manufacture does not appear attractive. However, if future developments in industrial oxygen production result in a substantial decrease in price the picture may change radically. ACKNOWLEDGMENT

Sulfur in bags at extreme l e f t


The promise of large scale commercial oxygen production has caused some interest in the study of its possible use for sulfuric acid manufacture. The use of pure oxygen in place of atmospheric air in the contact process would greatly decrease the size of the equipment involved for a given capacity, almost entirely eliminate waste gases to the atmosphere, and increase over-all efficiency. However, somewhat different and more expensive materials ?f construction would be required because of the higher temperatures and gas concentrations that would be encountered in the sulfur burner and in the converter. This would partially offset the advantage of smaller vessels. There could be no saving in labor requirements, and maintenance costs probably would be of the same order, if not higher. Thus the price of oxygen would have to be offset by an increase in yield above that obtained in present design, by a saving in capital cost, or both. A single industrial installation for the use of pure oxygen in a contact acid plant has been made at the Consolidated Mining and Smelting Company works in Trail, British Columbia (8). Slthough this installation was made under exceptional circumstances which alter the economy of the operation, it affords valuable technical data regarding the feasibility of the process. At the beginning of the war the Trail plant found it necessary to double its sulfuric acid production. Pure sulfur dioxide was available from a n existing plant which acidulated ammonium bisulfitk solution. Oxygen (99% pure) was available as a byproduct of the electrolytic hydrogen production in the ammonia synthesis operation. Under these conditions it was practical to convert a n existing contact plant producing 35 tons per day to a cyclic process designed to handle a mixture of 25% sulfur dioxide, 30% oxygen, and 45% nitrogen. The plant contains two converter vessels in parallel, each 9 feet in diameter. The shells are of welded construction, bricklined at the bottom, and insulated on the outside. Three layers of vanadium pentoxide catalyst are used with intermediate cooling provided after each layer by tubes circulating cool air. The gas enters the converters a t 402” C. and leave: at 528” C. Temperatures in the three catalyst layers are 682 , 642”, and 638 C., respectively. The gas passes through each converter at a rate of 3000 cubic feet per minute, and after passing through the absorbing towers is recycled to the blowers with the addition of sulfur dioxide and oxygen. Because of the small amount of nitrogen impurity that enters with the sulfur dioxide and oxygen,

The authors wish to express their. appreciation lor. the valued assistance of T. R. Harney of Monsanto Chemical Company in the preparation of this article. LITERATURE CITED

(1) Chem. Eng., 55, No. 2, 104-5, 128 (1948). (2) Fairlie, A. M., “Sulfuric Acid Manufacture,” New York, Heinhold Publishing Corp., 1936. (3) Fasullo, 0. T., Chemicallnds., 60, 838, 840, 842 (1947). (4) Federal Specifications, 0-A-11;Joint Army-Navy Specificatiorrs,

A-179. Furnas, C. C., “Rogers Manual of Industrial Chemistt.y.” 6th ed., p. 256, New York, D. Van Nostrand Co., 1942. (6) Lee, J. A., Chem. Eng., 55, No. 4, 119-21 (1948).


(7) Parsons, H. O., Chem. Eng. News, 23, 1334-5 (1945). (8) Snowball, A. F., Can. Chem. ProcessInd., 32, 1110-14 (1947). (9) U. S.Dept. Commerce, Statistica2 Abstracts (1939). (10) White. Alonso, 111, Chem. Eng. News, 23, 1154 (1945). RECEIVED June 19, 1948.

Loading Tank Cars with Acid