Catalytic Water-Gas Reaction MASS TRANSFER AND CATALYST ACTIVITY F. G . LAUPICHLER Postbox 357, The Hague, Holland
B
ECAUSE of the importance of catalysis in accelerating chemical reactions, much work has been devoted to studying the mechanism of this phenomenon. Very little has been done, however, on the more practical side of the question, and scarcely any information is available for predicting quantitatively the effect of variations in the working conditions on a specific catalytic gas reaction. A welldefined basis is lacking even for predicting the amount of catalyst required to obtain a certain degree of conversion or the conversion rate for a given amount of catalyst; both of these questions are important to the designer and manufacturer of chemical plants. The purpose of this paper is to offer some contribution to the questions raised. The method of attacking the problem is based on the following considerations: When a gas mixture is passing along a catalyst surface, two different processes determine the rate of conversion-namely, (1) the transport of the reacting gases to the contact surface and the retransport of the reaction products from the catalyst to the core, and (2) the specific catalytic effect of the contact material. The mass transfer between the catalyst and the core can be treated in a general way by means of a set of eauations awplicable to 'any specikc chemical r e a c t i o n . The conversion rate on the contact surface depends not only on the properties of the catalyst but also on the special chemical reaction to be investigated, since frequently t h e speed of conversion cannot be derived from the stoichiometric reaction scheme. This discussion will be restricted to a specific reaction- the catalytic water-gas reaction, one of the important indust r i a l c a t a l y t i c processes. In deriving the relations for the speed of conversion and its dependence on t h e properties of the catalyst, a method is em(film) ' ployed that may prove useful in other cases. FIGURE1
In choosing the catalytic water-gas reaction, the author intends not only to discuss a number of typical effects valid for many other catalytic gas reactions but also to present another contribution (4, 6) to the knowledge of this special process for producing hydrogen.
Theory When gases are flowing past a surface in turbulent motion, a so-called gas film is formed on the surface in which the gas moves in laminar motion. The formation of this film is unfavorable to the mass transfer of the reactants of a gas mixture from the interior of the gas mass (core) to the solid surface, since the molecules have to penetrate the laminar layer. This molecular transfer by means of diffusion can take place only when a concentration gradient is produced between the surface and the boundary line between film and core. For this reason the concentration of the reactants in a gas mixture passing along a catalyst surface is lower a t the contact surface than in the core, whereas the concentration of the reaction products at the contact surface is higher than in the core, as shown in Figure 1. Consequently the gas composition a t the contact surface is different from that of the gas mass; the greater the film thickriess and the more different the diffusities of the various components, the greater will be the difference in the gas composition.
Mass Transfer If in the general chemical reaction
+ azAz + di there is ug + u4 = al + ut, as for the water-gas conversion reacaiAi
&a
tion; then when the gas mixture being cataIyzed has passed a certain amount of catalyst having a total surface, F , the mass transfer across any plane within the fdm by pure diffusion is Wi = - D , , d c , f / d L
(1)
whereL = distance of plane within film, measured from boundary line between film and core (Figure l),cm. cif = concentration of components A, at this plane, D, = diffusion moles/cc. coefficients of A , at temperature prevailin at this plane, sq. cm./sec. moles o f gases or vapors transported by diffusion per sec. through an area of 1 sq. cm. of this plane
Wi
=
i
= 1, 2, 3, 4
The reaction temperature is taken as constant. Since in the steady state W ,is independent of L , the integrs578
MAY, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
tion of Equation 1 from L = 0 to L = 6 and from cu = e,’ leads to:
cij
D. -’6 (c, - Cit)
W
