Effect of Water on Sulfur Dioxide Reduction by Carbon Monoxide Vuranel C. Okay and W. Leigh Short* Department of Chemical Engineering, University of Massachusetts, Amhmt, Mass.01001
In a previous paper by Querido and Short the process feasibility was demonstrated for the reduction of SO2 by CO at concentrations and temperatures typical of a power plant stack gas. This paper considers the effect of water vapor on the reduction of sulfur dioxide. It is shown that the water-gas shift reaction does not proceed for temperatures between 820 and 92OoF,at CO ratios of 1 -35 to 1 3 1 , and space velocities of 29,300 to 38,700 h r l . Nevertheless, the SO2 reduction activity is adversely affected by the presence of water. This effect is reversible and, if the water is removed, the higher level of catalyst activity is restored.
I n the search for a way of reducing sulfur dioxide emission in stack gases, one of the techniques that has been considered is the catalytic reduction of this pollutant by carbon monoxide. The advantages of this process would be numerous, such as the possibility of simultaneous removal of nitric oxide, the ease of storage of the saleable by-product sulfur, and the possible availability of carbon monoxide in required concentrations as a flue gas constituent from furnaces operating near stoichiometric ratio of fuel to air. Querido and Short (1973) have investigated the catalytic reduction of sulfur dioxide in the 2000 ppm concentration level with carbon monoxide to elemental sulfur. They used a synthetic gas mixture containing nitrogen, carbon monoxide, carbon dioxide, and sulfur dioxide to simulate the stack gas in that study, where they were eventually able to remove over 95% of sulfur compounds from the effluent stream using a system of two reactors operating at different temperatures. As well as successfully satisfying their goal of 90% minimum removal of sulfur compounds, their results also increased the need for further investigations to determine the effect of the other flue-gas constituents such as water, fly-ash, and hydrocarbons, on the reduction of sulfur dioxide. The dry process utilizes three main reactions of sulfur compounds, which are
+ so2 1/2SZ + 2coz co + '/ZS2 cos + 2c02 2 c o s + so2 2CO
+
(1)
(2) (3) If the water-gas shift reaction does not proceed, the only effect of water will be to reduce the partial pressure of all the reactants, and possibly act as a catalyst poison. However, if the water-gas shift reaction proceeds by -+
+ 8/2Sz
CO
+ HzO
+
Hz
+ COz
(4)
hydrogen is produced, raising the possibility of a host of reactions between hydrogen, sulfur, sulfur dioxide, and carbonyl sulfide
+ SO2 H2S + 2Hz0 Hz + '/zS2 H2S 2H2S + SO2 '/2Sz + 2Hz0 CO + HzS H2 + COS 3H2
+
+
-+
+
(5) (6)
(7) (8)
HzO
+ COS
+
COa
+ H2S
(9)
It is known that copper catalysts can promote the water-gas shift reaction, but concentrations of CO and H 2 0 normally used are much higher than in this case. It is also true that sulfur is a poison for the water-gas shift reaction. The goal of this study was to examine the effects of water on the sulfur dioxide reduction system. Specifically, there were several questions to be answered, and these were the following. (a) How is the catalyst affected by the presence of water? (b) Will the introduction of water into the system cause the water-gas shift reaction to proceed? (c) If the water-gas shift reaction does go, how will this affect the removal of sulfur compounds? (d) Will the presence of water in the system cause an unfavorable shift in catalyst selectivity? Experimental Apparatus
Reactor and Flow System. The reactor and flow system is shown schematically in Figure 1. The reactor is made of 0.25-in. titanium pipe, 32 in. long, with the catalyst bed beginning approximately 12 in. from the bottom. Alumelchrome1 thermocouples are attached t o the outside wall of the reactor, starting 0.5 in. above the bottom of the bed and continuing for 5 in. a t 1-in. intervals. A Leeds & Northrup millivolt potentiometer was used to measure temperature. The reactor was heated in a Lindbergh Heavi-Duty, three-zone oven which made it possible t o control temperature in the 4-in. catalyst beds to within h5OF. The reactor was mounted vertically resulting in convection gradients between the reactor a i d furnace. Hence, the catalyst bed lengths were limited to 5 in. to prevent axial temperature variations. The gas components, with the exception of nitrogen, were fed from pressurized tanks through a series of rotameters where the gases were blended to the desired concentrations. Nitrogen was taken separately from a pressurized tank and then through a rotameter. This known flow rate stream of nitrogen was then passed through a humidifier and demister that were set inside a constant temperature water bath, to saturate the stream with water. It was then blended with the rest of the gas mixture passing through heated lines to prevent water from condensing. Upstream samples were drawn through a l/c-in. sampling line before the reactor inlet, and the concentrations were determined on a wet basis. The Ind. Eng. Cham. Process Des. Develop., Vol. 12, No. 3, 1973 291
Elactricol healing topes
t
I
--I
t - - - - 1 -
Fixed h d
demisler
tor effluent DODlYIlS
To caustic bubblar
,
,-WF
\"'
Figure 1 . Reactor flow system 0.8
1.0
1.2 GO
1.4 RATIO
1.6
1.8
Figure 3. Calculated thermodynamic equilibrium concentration of SO2 as a function of CO ratio REACTANT CONCENTRATIONS' 2000 ppm SO1 3.40% Ot
IWO
tp -WET
B d
R,,"$I
'-
/ -0 /, 'i,.l.2
z
$100 0 0
3
-
/
e--------
REACTANT CONCENTRATIONS WET SYSTEM, OW SYSTEM, xKlOPpmS4 2oo0Wmq 3.40 % O1 3.40% 0, 14.0% CO, 14.0% C4 12.0% %O
-
---
5 IO
10
I2
cd4RATlo I 6
1.8
2.0
Figure 2. Calculated thermodynamic equilibrium concentration of SOzas a function of CO ratio
exit gas stream from the reactor contained some sulfur vapor, and was therefore passed through a line traced with electrical heating tapes to an electrostatic precipitator where most of the sulfur was condensed and removed. The precipitator consisted of a 1.5-in. diameter vertical aluminum tube, 24 in. in length, in which a copper wire was suspended axially, insulated from the aluminum pipe by plexiglass plugs, and powered by tapping the high-voltage source of a television set. To prevent condensing of water in the lines from the electrostatic precipitator to the gas chromatograph, this line was also traced with electrical heating tapes, and kept above the dewpoint of the gas stream. Analytical Equipment. All analyses were done on two gas chromatographs, a Perkin-Elmer 900 and a Varian Aerograph Series 1800. The Perkin-Elmer 900 was used for the analysis of CS?,SOz, COS, HzSJand COZ and the Varian 1800 was used for the analysis of Hz, G O , and Nz. A 6-ft long Porapak T column of 0.25 in. diameter was operated a t 130°, 30 cc/min helium flow rate, 150' detector temperature, and 300 mA detector current t o separate COz, SO?, HzO, and CSz. A 6-ft long Porapak Q-S column of 0.25 in. diameter was operated a t 130", 30 cc/min helium flow rate, 150" detector temperature, and 300 mA Jetector current to 292 Ind.
Eng. Chem. Process Des. Develop., Vol. 12, No. 3, 1973
TEMPERANRE. ' F
Figure 4. Equilibrium concentrations of HzS and COS with and without water in the reaction system
separate COz, COS, and HzS. These columns were used in the Perkin-Elmer Model 900 gas chromatograph. A 6-ft long molecular sieve 5A column of '/*-in. diameter was operated a t 70°, 50 cc/min helium flow rate, 120' detector temperature, and 250 mA detector current to separate Hz,Nz, and CO. Excellent resolution of the SO2 peak was obtained on the Porapak T column in the presence of large amounts of water vapor. Since COS and H2S eluted simultaneously on this column, Porapak Q-S column was used for the resolution of these components after removing the moisture. Water concentrations were calculated from the flow rates of the nitrogen stream, the main stream, and the humidifier temperature. Thermodynamic Calculations
Thermodynamic equilibrium compositions were calculated for the sulfur dioxide reduction reactions with and without water in the reactor feed gases. The results of the dry case was reported by Querido and Short (1973). To circumvent the problem of choosing a reaction system, the equilibrium was calculated by the minimization of free energy technique.
k
0.0 B W F c T
