of Cellulose and Related Materials,” Pergamon Press, New York, 1963, pp 197-234. Stephens, E. R., Burleson, F. R., Cardiff, E. A., J . Air Poll. Contr. Ass. 15(3), 87-9 (1965). Stephens, R., Burleson, F., Holtzclaw, K., J . Air Poll. Contr. ASS.19(4), 261-4 (1969). Taylor, 0. C., 1967, unpublished. Taylor, 0. C., J . Air Poll. Contr. Ass. 19(5), 347-51 (1969). Taylor, 0. C., Eaton, F. M., Plant Physiol. 41(1), 132-5 (1966).
Thomson, W. W., Dugger, W. M., Jr., Palmer, R . L., Botan. GOZ.126, 66-72 (1965). Todd, G. W., Garber, M. J., Botan. Gaz. 120, 75-80 (1958). Receiced for reoiew June 17, 1970. Accepted Nocember 9, 1970. This incestigation was supported in part by research grant AP 00213 and contract no. PH86-68-71 ,from the National Air Pollution Control Administration OJ the U S . Public Health Sercice.
Removal of Sulfur Dioxide from Stack Gases by a Modified Claus Process Robert T. Struck, Metro D. Kulik, and Everett Gorin Research Division, Consolidation Coal Co., Library, Pa. 15129
The results of a bench-scale study on the potential application of a low-temperature Claus process to the treatment of dilute SO2-containingstack gases are presented here. Optimum results are obtained at a temperature of about 100°C. Under these conditions, the sulfur formed quantitatively condenses on the catalyst and the product gas contains less than 50 ppm of sulfur compounds. Data are given for a two-stage regeneration process which eliminates catalyst poisoning caused by sulfate formation. Of the sulfur recovered, 93 % is elemental sulfur, the rest is ammonium sulfate. These results were obtained with a synthetic flue gas having the same composition as a power-plant stack gas, with the exception that NO, was absent. Preliminary results showed that the presence of NO, accelerates poisoning of the catalyst, but that the above twostage regeneration procedure is still effective for reactivating the catalyst. H
R
esearch and development is being conducted by government and industry to develop processes for removing SO, from stack gases. The removal of SO? from power-plant stack gases is, of course, receiving the major attention, but removal of SO, and H?S-SO> mixtures from other process tail gases is also important. The latter, for example, includes off gases from roasting of sulfide ores as well as tail gases from sulfuric acid manufacturing plants and from conventional Claus plants. A method for accomplishing this objective has been proposed based on the use of a “modified”-type Claus process. The process operates by injecting H2Sinto the gas to provide an HyS,’SO,mol ratio of 2, as required for the catalytic reaction, 2 H,S
+ SO? = X3 S, + 2 H?O
(1)
I t differs from the normal Claus process in that the proc:ss is carried out at a much lower temperature and the sulfur product is largely condensed on the catalyst. The conventional Claus process operates at a higher temperature which is always above the dew point of sulfur vapor. The conversions that can be achieved at the higher tempera626
Environmental Science & Technolog)
ture are limited by the thermodynamic equilibria, as has been previously discussed by Gamson and Elkins (1953). Generally, the conversion is below 95 %, even when more than one catalytic stage is used. One main potential advantage of a Claus-type process is that the amount of reductant required to produce elementary sulfur from SO? is reduced to very nearly the minimum of 2 mol/mol of sulfur as dictated by the overall process,
Other regenerable SOuremoval processes now under development require a minimum of 3, and as many as 4 mol of reductant, expressed as hydrogen, per mole of sulfur recovered (Bienstock et al., 1965; Oldenkamp and Margolin, 1969). The first attempt to apply the present type of process to gas purification was made to coke oven gas by Audas (1951). In this case, the process was applied in reverse-Le., SO? was added to the H?S-containing gas, and the modified low-temperature Claus process was conducted with condensation of sulfur on the alumina catalyst with subsequent regeneration. Application of the concept to flue gas treating was proposed by Kerr (1968). In both the Audas and Kerr processes, the sulfur-fouled catalyst is cycled through a thermal regeneration step, where sulfur is removed by distillation at about 500°C. Princeton Research has undertaken more recent work to develop this type of process (Chem. Eng. News, 1968) under the auspices of the National Air Pollution Control Administration. Little information is available, however, about the results of their work at this time. One aspect of this type of process is that the hydrogen sulfide content of the treated gas must be maintained at a very low level, and thus the efficiencyof conversion must be very high. Consolidation Coal Co. undertook evaluation of the modified Claus process in its laboratories because, economically, it seems to be potentially one of the most attractive processes for treating flue gas. The present paper deals with a study of the major features of the process and the limitations in its potential application to particular gases. The following subjects are covered: thermodynamic limitations of the process, experimental determination of the effects of the major flue gas components and process variables, and a study of methods of catalyst regeneration. A two-step re-
, Q,,REACTOR,34mm
NORMAL+
VOL_ % \VET FEED _
ID
HzS SO2
FURNACE
HzO
0.6
0.3 0.0 99. I
93.1
OTHER
GLASS WOOL
DRY FEED 4-
0.6 0.3 6.0
QUARTZ CHIPS, 0 5 9 t o 1.17mm CATALYST, I 17 to 2.36mm, 2 5 or 76 mm DEEP QUARTZ CHIPS
GLASS wo3L
SULFUR TPAP
SCRUBBER
Figure 1. Experimental apparatus 103 110
I20
130
140 150 160
TEMPERATURE
generation process is described which overcomes the problem of poisoning due to sulfate formation in cycling catalyst through the process.
Results and Discussion Thermodynamic Relationships. The equilibrium curve calculated for a typical flue gas to which 2 mol of H r S per mole of SO, have been added is shown in Figure 2. These cal-
180
190
Figure 2. Equilibrium concentration in tail gas and percent of sulfur formed which condenses vs. operating temperature
Experimental
The apparatus used is shown in Figure 1. A fixed bed of catalyst is supported on quartz chips in a heated tube, through which the simulated stack gas flows. The gases are preheated by passage through Pyrex wool and quartz chips. A central thermocouple well with adjustable couple position is used to measure the bed temperature. The controlled temperature is taken as the hottest spot in the bed. Water is added to the incoming gases by bubbling one of the gas streams through a water bath held a t the proper temperature. Under the conditions used here, elemental sulfur remains on the catalyst and the tail gases pass out through a soda lime trap before being metered. At regular intervals, part of the tail gas is diverted through a n iodine scrubber to analyze for H2S and SOn.Catalyst bed depths of 1 and 3 in. and a reactor pressure of 810 m m H g absolute were used. In the runs carried out with added NO,, a separate stream diluted with nitrogen was fed through a calibrated rotameter to the SOystream manifold. In the feed and product gas, N O z was determined colorimetrically by batch absorption in sulfanilic acid (Cholak and McNary, 1943). Total N O plus N O y was determined colorimetrically on a separate gas sample. In this method, the NO plus NOs was oxidized by aqueous H 2 0 2 to form nitric acid, which was subsequently reacted with phenol disulfonic acid (Cholak and McNary, 1943). Thermal regeneration of the catalyst is carried out by passing nitrogen a t 28-1 13 liters/hr over the catalyst as it is heated above the boiling point of sulfur to distill off sulfur. The exit gases pass through a sulfur trap (dotted line, Figure 1) and then through the iodine scrubber to analyze for HyS and SOz liberated during stripping. The analysis for H2S and SO2 is based on their reactions with iodine, as was previously described by Doumani et al. (1944). Three different aluminas were used as catalysts in these tests. Catalyst A is a commercial dessicant alumina which contains 1 . 6 z alkali and 2 . 0 z silica. The others are purer, more expensive aluminas with properties as shown in Table I.
10 '
, 'C
Table 1. Catalyst Analyses
Catalyst composition, Ppm Na Si Fe Ca AI Surface area, m*/g Pore volume, cc/g Bulk density, g/cc a
A
C
H
11 ,900' 9,300' 840
5
