TECHNOLOGY
Sulfuric acid plants face emission limits Many abatement methods available, but all cost money; strict controls might shut down older, smaller plants Control regulations for the emission of sulfur dioxide from tail gas stacks of sulfuric acid plants are now being discussed in several areas of the United States. A common acceptable value for emissions is 500 p.p.m. S 0 2 . Just how realistic this value is remains to be seen, but a report recently commissioned by the National Air Pollution Control Administration (C&EN, June 22, page 15) should aid the local authorities in their deliberations. The report was compiled by the consulting division of Chemical Construction Corp., which spent a year surveying the existing acid plants in the U.S. and the various means for controlling the emissions from them. Objectionable emissions from sulfuric acid plants all enter the atmosphere from the tail gas stack. They result either from the incomplete conversion of the sulfur source to acid or from the carryover of acid mist droplets formed or entrained in the tail gas. Typical emissions from plants now in use are shown in the accompanying table. Although the sulfur dioxide level in the tail gases is apparently the only thing undergoing scrutiny by the control groups, other potential pollutants occasionally are present. In addition to the acid mist itself, the older chamber plants also emit small quantities of nitrogen oxides. Usual. The usual commercial practice for operating acid plants is to use S 0 2 feed temperatures of 820° to 840° F. in three- or four-stage converters containing 160 to 180 liters of vanadium pentoxide catalyst per daily ton of acid product. Under these conditions the overall S 0 2 conversion is 95 to 98%. Below" 820° F. the reaction rate drops off sharply even if there may be a higher conversion,, This is the core of much of the economics of plant operation: the optimum compromise between reaction rate and conversion efficiency. Increasing the capacity of a plant is usually done by lowering the conversion efficiency through an increase in the inlet S 0 2 concentration. In the sulfur-burning plants this is achieved by burning more sulfur without a corresponding increase in the inlet air; that is, the 0 2 : S 0 2 ratio is dropped. The effect is that the S 0 2 emission level in the tail gas stack rises correspondingly. Any emission control system will, therefore, have to ac-
commodate a wide range of S 0 2 feed concentrations. Baffling. Sulfuric acid mist, though less a problem than S 0 2 , is still more baffling. The mist consists of droplets of sulfuric acid, usually over 90% acid, formed in the vapor phase from water vapor and sulfur trioxide. Once formed, the mist is very stable and is not readily separated or absorbed. Most of the mists observed contained droplets in the 1- to 5-micron range. In spite of the precautions taken, mist formation occurs in most plants. The most common cause is hydrocarbons in the sulfur source, which produce water vapor during combustion. Another cause is nitrogen oxides in the converted gases. These may result from fixation of atmospheric nitrogen in the high-temperature furnaces but more commonly from nitrogen compounds in the sulfur. According to Chemico's evaluation, the nitrogen problem can be eliminated by proper operation of the plant. The exact mechanism of mist formation is not known but one possibility is formation as the result of oxidation of residual S 0 2 in the stack itself by the nitrogen oxides. Assessment. In assessing the available means for S 0 2 and mist removal, Chemico's report notes that there are 65 possible methods for S 0 2 control available and several methods of mist suppression. No blanket recommendation can be made because of the great variety of plant configurations now in use. However, Chemico did list 15 methods of S 0 2 removal as being the most promising for the array of plants now in service or on the drawing
boards. Most of the promising methods selected by Chemico involve extended conversion of the stack gas and/or chemical absorption and neutralization. The one method receiving the most attention is the dual-absorption method. In the dual absorption method pollution is reduced by converting a greater portion of the S 0 2 in the converter feed to sulfuric acid. After partial conversion in two or three stages, the S 0 3 is removed in a primary absorption tower and the remainder of the gas, now with a very high 0 2 : S 0 2 ratio, is returned to the converter. After being reheated, the remaining gas passes through one or two stages with sufficient catalyst to ensure that conversion is at least 99.5% of the original S 0 2 in the feed. This high potential conversion is hardly affected by gas composition in the first part of the converter and permits smaller equipment, partially offsetting the cost of the intermediate absorber. The dual-absorption system can be applied equally well to sulfur-burning or wet-gas plants, according to Chemico. Money. Regardless of the type of emission controls used, they will cost money, and to achieve the 500-p.p.m. limits now in prospect, it seems sure that some of the older and smaller plants will find the costs too much to bear. Doubtless tail gas control and mist suppression through mechanical or electrostatic filtering will provide an incentive for newer and larger sulfuric acid plants in the future. This may mean a readjustment of the logistics of acid manufacture, since little acid is transported more than 200 miles in the present manufacturing scheme.
Tail gases from sulfuric acid plants add to U.S. air pollution Plant type
Lead chamber Pre-1960, 3-stage contact S-feed Post-1960, 4-stage contact S-feed Pre-1960, wet gas 3-stage Post-1960, wet gas 4-stage Source:
Acid capacity N u m b e r of (short t o n s / y e a r plants 1 0 0 % acid) (U.S.)
Tail gas emissions SO2 Acid mist (p.p.m.) (mg./s.c.f.)
40
11,000,000
1500-4000
2-20
—60 -20
7,500,000 4,000,000
2000-10,000 4000-7000
2-50 2-50
Chemical Construction Corp.
JUNE 29, 1970 C&EN 43
