CATALYTIC COMBUSTION OF CARBON MONOXIDE ON COPPER OXIDE Efeect
of Water Vapor A.
E.
COHEN AND
KEN NOBE
Department of Engineering, Uniuersity of California, Los Angeles, Calif.
The effect of water vapor on the catalytic combustion of carbon monoxide on copper oxide has been investigated. The water vapor content in the carrier gas stream was varied from 0 to 3000 pap.m.with the total gas flow maintained constant at 400 liters per hour (NTP) at temperatures ranging from 70" to 130' C. The initial concentration of carbon monoxide was varied from 500 to 1500 p.p.m. The reaction was first order for water vapor concentrations of 800 p.p.m. and above and 0.4 order in the absence of water vapor. The combustion rate of carbon monoxide was strongly affected by small changes in water vapor concentrations when the total water vapor content was less than 800 p.p.m.
UMEROUS investigations
on the catalytic oxidation of carbon The literature on this subject has been reviewed by Katz ( 4 ) . Because of the growing danger of air pollution in urban communities, control of emission of hydrocarbons, nitrogen oxides, and carbon monoxide from auto exhausts has been receiving very serious consideration in recent years. In fact, the state of California recently has instituted requirements to control auto exhaust emission of hydrocarbons and carbon monoxide. At the University of California a t Los Angeles, investigations have been in progress on the catalytic combustion of hydrocarbons and carbon monoxide and the catalytic dissociation of nitrogen oxides on copper oxide catalysts. During an investigation of the catalytic oxidation of carbon monoxide, it was observed that variations in water vapor content in the feed stream had a considerable effect on the degree of oxidation. The literature indicates that it has been known for some time that water decreases the catalytic oxidation of carbon monoxide. Katz (4) indicated that metal oxide catalysts were readily poisoned by water vapor. Engelder and Miller (3) observed that water vapor poisoned a catalyst which was a mixture of CuO and TiOz. There have been numerous other investigations on the poisoning effect of water vapor on hopcalite. Recently, more quantitative but limited studies of the poisoning of catalysts by water vapor in the oxidation of carbon monoxide were made by Bank and Verdurmen ( 2 ) . They report that a t 500' C. and a reaction time of 15 minutes, the oxidation of carbon monoxide on quartz was reduced from 46% to 17% by the addition of 3125 p.p.m. of water vapor. O n increasing the reactant stream concentration of water vapor to 9375 p.p.m., a t the same temperature and reaction time, there was a further decrease in the conversion to 15%. Since quantitative studies of the effect of water vapor content on the catalytic oxidation of carbon monoxide have not been too extensive, it seemed appropriate to extend our studies in this direction. Experimental
The copper oxide catalyst was the one prepared by Koutsoukos ( 5 ) . A copper hydroxide precipitate was formed by adding a stoichiometric quantity of KOH to a copper nitrate solution. The precipitate was then dried in an oven and thermally decomposed to CuO. 214
I L E C PROCESS DESIGN A N D DEVELOPMENT
I
EXHAUST GAS
N monoxide have been reported.
THER COUP
I
AIR
INLET
Figure 1.
Equipment and apparatus
The experimental apparatus is shown in Figure 1. The reactor was a vertical borosilicate glass tube (21-mm. i.d.) filled with glass beads to a depth of 40 cm. and then with 20 grams of catalyst pellets. Four horizontal glass thermocouple wells (1.5-mm. 0.d.) extended into the catalyst section of the reactor. Iron-constantan thermocouples were placed in each well. Three of the wells extended to the center of the tube. The fourth extended just 2 mm. inside the tube and was placed opposite the uppermost well. The entire catalyst bed and approximately 35 cm. of the glass beads were covered with two heating tapes. The electrical current to the tapes was regulated by two separate Variacs to facilitate achieving isothermal conditions in the bed. The position of the thermocouple wells allowed the measurement of axial and radial temperature gradients in the bed. These gradients were minimized by adjusting the two Variacs. Air from the campus supply line served as the carrier gas and oxidant, and was metered by a calibrated rotameter. The air passed initially through a drying tube containing molecular sieve (Linde, 13X). The drying tube was 2 inches in i.d. by 12 inches in length and contained 1.2 pounds of the desiccant. The linear velocity of the air in the drying tube was approximately 0.2 foot per second. The inlet air was a t room temperature. Under these operating conditions, according to manufacturer's specifications, the desiccant had to be changed every 10 hours in order to obtain the desired quality of the effluent stream-less than 1.5 p.p.m. of water vapor and less than 50 p.p.m. of COz. T o ensure that the water content of the air remained less than 1.5 p.p.m., the desiccant was changed a t least every 8 hours. Air of the same quality was used to purge the desiccant a t 650' F. during regeneration. To investigate the effect of water vapor on the oxidation of
CO, a known portion of the air stream, after the molecular sieve trap, was bubbled by means of a sintered glass bubbler through a washer bottle filled with water. The air leaving the washer bottle was assumed to be saturated, so that the water vapor content in the air stream was known. The carbon monoxide (Matheson C.P. grade, 99.5% minimum purity) was introduced to the main air stream before the latter entered the reactor tube. A rotameter was used to indicate the CO flow rate. The gas mixture was sampled at the entrance and exit of the catalyst bed with the exit stream continuously analyzed until steady state was achieved. An infrared analyzer (MSA Model IR 300) connected to a Leeds & Northrop pen recorder was used to determine the concentration of CO. The infrared analyzer was calibrated to read in the range 0 to 2000 p.p.m. of C O and had an accuracy of 1 2 7 , of full scale. The unit was calibrated every morning using a special span gas (MSA span gas containing 1850 9.P.m. of CO). Dry air which passed through the molecular sieve served as the zero gas. The same catalyst bed was used for all the experimental runs. The catalyst was activated for 48 hours before use by passing hot air at 400' C. through the bed. During the C O oxidation study, the CuO catalyst was heated to 210' C. before each run and held there for 10 minutes. The experimental points in each run were then taken in order of decreasing temperature. The surface area of the catalyst was determined by the BET method. The sample was preheated in vacuum for 12 hours, then the nitrogen adsorption isotherm a t liquid nitrogen temperature was determined. The pore volume of the catalysts was measured by the water injection method. The catalyst was in the form of 2 X 2 mm. cylindrical pellets, of 13.2 sq. meters per gram of surface area, and 745-A. mean pore radius.
800 PPM
100-
z 00
-
z-8
-
-
0
60
I-
z w
0
2 40a 20
I
I
1
I20
140
160
I
€lo-
OLO
100
BED TEMPERATURE
Figure 2. oxidation
- C'
Effect of water vapor content on CO Initial CO concentration, p.p.m.
+o
00 12
1100
AA
1500
Results
Experimental data for the effect of water vapor on the oxidation of carbon monoxide are shown in Figure 2. With the carrier gas flow rate maintained constant a t 400 liters per hour, per cent oxidation was plotted against bed temperature. The initial concentration of the carbon monoxide was varied from 500 to 1500 p.p,m. The amount of water vapor in the inlet stream was varied from 0 (
