Ind. Eng. Chem. Process Des. Dev. 1980, 19, 231-237
23 1
The Reaction between Hydrogen Sulfide and Spherical Pellets of Zinc Oxide James B. Gibson, 111, and Douglas P. Harrison* Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803
The reaction between H,S and ZnO pellets was studied in a microbalance reactor between 375 and 800 O C . Rapid and essentially complete reaction was observed in the temperature range of 600-700 O C . Near 800 O C , slow decomposition of ZnO with subsequent zinc vaporization led to a vapor phase reaction with nonporous ZnS depositing on the pellet exterior and preventing further reaction. At temperatures below 600 O C , the reaction stopped well before total ZnO conversion was obtained. Experimental time-conversion results were compared to predicted values obtained by applying the grain model. All grain model parameters were determined using either independent measurements or literature correlations. Good agreement between experiment and prediction was obtained in the 600-700 OC temperature range where pore diffusion provided the predominant resistance. However, the model as originally formulated totally failed to predict the reaction “die-off” observed at lower temperatures. I t is believed that grain diffusion resistance is the most important cause of the low temperature deviation.
Introduction Zinc oxide is known to react with hydrogen sulfide over a wide range of temperature and pressure. The reaction to form zinc sulfide is currently used in the desulfurization of hydrocarbon gases for the synthesis of ammonia (Phillipson, 1970) and has been suggested for sulfur removal in synthetic natural gas processes (Bresler and Ireland, 1972). In spite of the current and potential interest, little is known of the properties of this reaction. Kinetic data previously published (Westmoreland et al., 1977) were based upon limited studies of the initial reaction rate between hydrogen sulfide and zinc oxide powders. The reaction was found to be first order with respect to hydrogen sulfide, and Arrhenius constants were reported. This paper examines the properties of the reaction in greater detail. Spherical pellets of zinc oxide were reacted under conditions where the global reaction rate was influenced by mass transfer and diffusion as well as the intrinsic surface chemistry. The resulting experimental data were analyzed using a reaction model which accounts for the simultaneous effects of the multiple resistances. All reaction parameters were evaluated independently, using either literature correlations or previously published experimental data. This approach avoided the procedure, used frequently in other studies, of evaluating model parameters numerically to provide the best “fit” to experimental data. While this latter method is almost certain to yield good agreement with experiment, it actually contributes little to the understanding of the system and is often costly in terms of computer time. Reaction Model The grain model (Szekely et al., 1976) was used to analyze experimental data. While somewhat more complex than the familiar unreacted core model, it is more satisfying in that physical properties of the solid reactant are included in the model equations. The grain model is well described in the literature and only a brief development is offered here. The zinc oxide-hydrogen sulfide reaction may be represented by the general stoichiometry gas and solid product (1) aAA(gas) + asS(solid) The stoichiometric coefficients of the reacting species, aA
-
0196-4305/80/1119-0231$01 .OO/O
and as, are considered to be negative numbers in the species continuity equations which follow. The mathematics is based upon the following assumptions: (1)the pellet retains its initial size and spherical shape throughout the reaction; (2) the reaction system is isothermal; (3) the intrinsic reaction rate is first order with respect to gas concentration, and (4)the pseudo-steady-state approximation is valid. The solid reactant is visualized as being composed of a large number of highly dense, spherical grains, each of which reacts individually according to the unreacted core model. In the overall pellet, however, the reaction occurs in a zone rather than a t a sharply defined boundary. The model is shown schematically in Figure 1. Reactant gas undergoes mass transfer from the bulk gas stream to the pellet surface. From the surface the gas diffuses between the grains, then through a solid product layer associated with each grain until reaction occurs at the unreacted core present in each grain. Thus, four resistances are included. When all resistances are important there is a gradual transition from largely solid product at the outside of the pellet to largely solid reactant near the center. The sharply defined reaction surface associated with the unreacted core model is a limiting case reached when diffusion between the grains is the controlling resistance. The reactant gas continuity equation, applied to the entire volume of the spherical pellet, is
with boundary conditions (3)
ddrC.1
r=O
@ 1980 American Chemical Society
=o
(4)
232 Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 2, 1980 L O W CONVERSION
HIGH
CONVERSION
reactant concentration in grain PS'
=MS reactant concentration in pellet CS,'
grain radius I
1
I
I
!
I RADIAL POSITION
U
RADIAL POSITION
Figure 1. The grain model.
The solid reactant continuity equation, in terms of the unreacted core radius of a single grain, is
Literature Parameters. The final group of parameters was evaluated directly from the literature or calculated using general correlations from the literature. Kinetic studies of the ZnO-H2S reaction using powdered samples of reagent grade zinc oxide have been reported (Westmoreland et al., 1977). From initial measurements at conditions where mass transfer effects were unimportant, the reaction was found to be first order with respect to H2S concentration with an Arrhenius rate constant of
k,' = kOe-E/RT= 9.46 with the initial condition r,'(t=O= r,' (6) There are two fractional conversion terms of interest. The local extent of reaction, x , can be obtained directly from the solution to eq 5 x = l - ( $ )
(7)
and the overall extent of reaction 2, is obtained by integrating the local extent of reaction over the entire pellet volume.
The species continuity equations are coupled, thus requiring that they be solved numerically. Details of the numerical procedure used in this work are available (Gibson, 1977). Parameter Evaluation One of the primary objectives of this study was to utilize literature correlations and independent measurements to determine model parameters, thereby avoiding the practice of numerically determining values which provide the best "fit" to experimental data. The parameters were calculated as follows. Measured Parameters. The mass, Wo,and radius, r,, of each pellet were directly measured. Since the pellets did not form perfect spheres, the radius was calculated from the average of four measurements taken across random diameters. The specific surface area, A,, could not be measured for a single pellet because of instrument limitations. Therefore, the average specific surface area of a number of pellets was measured with this average used to represent each individual pellet. All pellets were subjected to a pretreatment step described later. Calculated Parameters. Several parameters were calculated using the directly measured properties and known values of density and molecular weight. particle porosity
exp(-7236/R?7
X
(13)
The units of k,' are cm4/mg-molmin. The rate constant of equation 13 was used directly in the grain model. The particle diffusivity was estimated using the random pore model (Wakao and Smith, 1962)
De, = DAc;
(14)
where DA was determined by combining molecular and Knudsen diffusion coefficients _1 - -1 1
+-
DA
DM
DK
The molecular diffusivity was evaluated using the Chapman-Enskog formula (Reid et al., 1977) and the Knudsen diffusivity was obtained from (Satterfield, 1970)
Mass transfer coefficients were predicted using modifications of the Froessling method (Hughmark, 1967) NSh
=K
+ BNRemNScn
(17)
The above equations are sufficient to evaluate all model parameters with the exception of the grain diffusion coefficient, Dd. In general solid state diffusion coefficients cannot be independently predicted. Consequently, values of D,' were initially chosen so that the grain diffusion resistance was negligible. This assumption produced good agreement between the model and experimental data over a portion of the temperature range studied. When the model deviated seriously from experimental data, it was found that choosing values of' DA'so that grain diffusion resistance became important produced considerably better agreement than arbitrary variation of any other single parameter. In summary, the grain model, using methods of parameter evaluation described, is totally predictive whenever grain diffusion resistance is negligible. Only when this resistance must be included does it become necessary to evaluate a single parameter, DA', by fitting the experimental results. Energy Effects The previous development is based upon isothermal reaction conditions. Since the reaction is exothermic, it
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 2, 1980
Table I. Chemical and Physical Properties of G-72C Zinc Oxide (Manufacturer's Specifications) chemical composition bulk density surface area pore volume particle size weight loss on ignition
85 wt % ZnO (minimum) 69 * 4 lb/ft3 16.6-26.0 m'/g 0.22 cm3/g @ 0.25-0.035 um 0.14 cm3/g @ 800 A 0.02 cm3/g @ 140 A t o 3 / , 6 in. diameter spheres
