There have been many studies on the kinetics of the water-gas shift reaction, but the results are at a poor level. Many experimental techniques and data evaluation methods are used in P'7, but quite often the limitations of experimental methods are not sufficiently emphasized. These, to name but a few, may be due to the non-experimental modification technique, insufficient care in observing errors inherent in this technique, and variations in the performance of the actual catalytic materials used.
RATE EQUATION
SURFACE PHENOMENON CONTROLLING
The kinetic and statistical derivations of the Langmuir isotherm are based on the following assumptions: -. A). If the Band R adsorptions are fast compared to the first step, they can be treated as adsorption equilibria and the first. The rate of the reverse reaction is then the collision of S with adsorbed R. The concentrations of the adsorbed particles are represented by cB and cR", the concentration of free active species, centers by cl and the total concentration of active centers by L. The chemisorption can then be written as a chemical reaction between a gas particle and an active center. 1. 2.8) forms the rate-determining step.
2.12) The elimination of the un)<:.nown surface con.eentrations gives the rate
PORE DIFFUSION RESISTANCE.CONTROLS
The kinetics of the displacement reaction was investigated by BOHLBRO (11) at atmospheric pressure over a commercial iron oxide - chromium oxide catalyst in the temperature range from 3300 to 5000 °C. In the early works of POPOVA (84) and BELONOGOVA and POPOVA (9) simple. degree equation of the form:. Integration of the equation gave:. moles of converted CO)/(moles of converted CO in' equilibrium).
Z. S REVERSE SHIFT REACTION
WORK ON OTHER OXIDE CATALYSTS
The most active mixture was ZnO:Cr203:O. It was then said to be very active even at l7SoC. The reaction was found to be of first order with respect to CO and the calculated activation energy was lS,K, called gm. The rate was thought to be controlled by the reaction rate of CO with the oxygen layer on the catalyst surface. -2 mm.) had shown that the size of the grains had no influence on the conversion rate and implied that the reaction was not inhibited by the diffusion processes.
INTEGRAL REACTOR
By assuming that the reaction plug flow velocities in differential reactors are found in a forward fashion, as the integration of the design equation becomes quite simple. 1), we obtain the expression that allows: "US to find the reaction rates in integral reactors. Equation (4.4) shows that the reaction rate "at any value of xA is simply the slope of the curve; so at a number of xA value s, the slope s of this curve (i.e. the rate s of the reaction) as well as the corresponding concentrations of the reaction CA can be obtained.
A TYPICAL PROGRAMME FOR KINETIC EXPERIMENTS
Integral analysis provides a straightforward, fast procedure for testing some of the impler rate expressions. This function can be considered as a graphical form of the velocity equation and can be used for preliminary calculations. A ,fit to the integral-reaet As the purpose of the investigation. was not only the establishment of the rate equation but aha information about .the catalyst, the change in mechanism when promoter s are added etc., the. investigation can be advantageously carried out using the initial-rate method. Determination of the activation energy can provide information about the influence of additives and of different treatment methods. using a statistical design. method helps to greatly reduce the bulk of the work and produces data of reasonable accuracy. water-gas shiR reacts the.more generalized form of exponential type rate expression can be. r " differential reaction rate. k = rate constant. p = partial pressure of the relevant component. Experimental conditions are chosen to test the validity of the above equation (4.7) and also to evaluate the parameters A, E/R. These three pipes had short right angles bent at the axis of the main pipe and turned along the direction of flow. The cylindrical shell was built with 1/16 inch thick S.S. plate, and the base was of 1/8 inch thick S. In the center of the base a. pipes were connected to lead the water to the pump. The water inlet pipe was in the center of the top and was 10 inches long and 3/4 inch a.d. Two thermocouple wells (approximately 2.5 mm in diameter), placed axially and along the wall, were suspended from a -i-inch S; S. disk that closed off the top of the reactor. The bottom of the reactor led to a 3/4 O. pipe connection via a pair of S. Each pair of flanges was secured with 8 2BA nuts and bolts. An overflow pipe was installed from the side wall near the top. The recirculation pipe for cooling water was kept from the center of the bottom. The connecting wires up to the condenser were 3/4 inchO.D. S. S. stir s and as after. the capacitor was made of copper. The hot connections (except the two in the reactor) were placed in the center of the flow lines. Boiler performance. was checked by measuring the condensate for corresponding calibrated water flow. The gas-steam mixture was brought to the reaction temperature by a 1 K,W heater around the. side arm of the reactor. reactor had two 400 W~Electrothermal cable heaters that were controlled separately. The soap bubble column was made. glass burette held vertically from the bottom of which carrier gas was directed upwards. Small amounts of soap solution were added to the gas stream through a side arm, which produced soap bubbles that climbed up the column. Timing the bubble rise from the dec mark gave the flow rate. meter- was noted from Hg- glass s thermometer s. The atmospheric pressure from a Fortin's barometer and the reactor pressure from the pressure indicator were taken. The flow rate of the gas mixture was measured by timing. the wet gas meter pointer for. Two evacuated sampling tubes were used for taking samples from the sampling line provided. from just before the wet test meter. The device is turned on for about 2-3 hours before use.. the analysis method - is described in the previous section. Each experimental sample was analyzed twice, each movement in the chromatograph was checked using a standard gas. sample at the beginning and end of each experimental sample analysis. From the ratio of the sample and the standard heights of the peaks.. the- composition of the sample. was calculated. All the catalysts were reduced with a gas mixture with the composition of PCO = PH 0= PCO = PH = 0.10 atm., and at the mass flow rate to be used in the actual experiment. Although for the sake of uniformity the experimental data are expressed with five decimal places, it. The significance test showed that at the 99% confidence limit, the effects of pea PH 0'T were strong, with a weaker effect (95% confidence limit by pea had no interaction. At a 99% confidence limit the effects of Pea pH a and T were significant and pea effects were significant at 950/0. In this experiment, the root mean square error of replication was very small (0.00005) compared to values of the order of 0.0006 obtained in experiments for the other three iron oxide-based catalysts for the same temperature range: A significance test was performed for this experiment based on the latter value. In significance tests, some interactions were found to be a significant event at the 99% confidence limit, except for the SSV catalyst. 74 - are given in Table 6.1 The summary of the results of previous kinetic studies of iron oxide-based shift catalysts in Table 7.1 shows results that are, at first glance, quite contradictory. 1 differed from the rest of the investigators by simply assuming either a straightforward first- or second-order rate equation. Second, any comparison of the effect of molecular species concentration on reaction rate must be based on two different criteria. For all iron oxide-based catalysts, reverse reaction rates were less than 10% of the forward. Again results in the case of the SSV catal'Yllt are consistent with previous work by showing no significant retardation effect. It appears from the previous discussion that these apparently contradictory results obtained by various workers may be valid, any divergence being due to the different detailed composition of the catalyst. Unfortunately, even if the detailed compositions of the commercial catalysts used in the present study were known, it would be extremely difficult to account for the effects either qualitatively or quantitatively. This is partly due to small reaction rate values at low temperatures, where experimental errors are magnified, and partly due to the presence of interactions in some of the experimental data. Moreover, the -ve value of the coefficient, according to Hougen-Watson, automatically rejects the mechanism. The findings of the present work require confirmation by repeating the factorial experiments at higher partial pressures of the components ip,volvecl. Table Ip. 1 Ref: 'table 10.19 15 For mass balances, calibrated values of inlet composition and mass flow were taken as given below and were. The total flow rate after the reaction would be the dry gas flow rate. If we divide the difference between these two by 8, we get the effect of the factor, since this "difference" was actually s\\m the difference of 8 pairs. Method lS the same as shown above, except that the level of interaction as log Peo x log prto, is defined in *,ay. In a treatment combination if both Pca and PH a are at the highest level (t) or both at the lowest level (-), then the interat10n will be at the highest level. 0.000935 mean square - 0.000935/1 The experimental data for each factorial experiment were correlated by fitting a linear regression equation of the form of Equation 4.7, with .; using the method of least squares. From the experimental data given in Table 10.20 a number of k values were calculated using the above equation and log k vs liT was plotted according to the Arrhenius relationship k = A e-E/RT. G-66 Ox:idee OI Zincj Cappel' and chrome. the complete separation function of adsorbed molecules. rate constant for the forward reaction. rate constant for the backward reaction. preliminary partial safe for reacting components. temperature rise in an adiabatic reactor.PROGRAMME FOR THE PRESENT WORK
STATISTICAL DESIGN METHOD - FACTORIAL EXPERIMENTS For the study of the variation brought about by delib\9rate changes in
EXPERIMENTAL PROCEDURE
EXPERIMENTAL. ACCURACY
27470 ItT!
KINETICS OF THE REACTION
G02 log PH
CALCULATION OF R:E::ACTIONRATE "1'''
34; 0.000935 Slgnl£icanee..te st
141 - APPENDICES
Chrome1~A1umel Thermocouple Calibration Chart Cold Junction O?C
