filtration feed of gaseous reagents to the chemical-reaction zone is called filtration ... oxidation (combustion) of hydrogen sulfide the following chemical reactions.
Combustion, Explosion, and Shock Waves, Vol. 32, No. 6, 1996
MOTION OF THE CONCENTRATION
FRONT
IN CATALYTIC OXIDATION OF HYDROGEN TO CONDENSED
SULFIDE
SULFUR IN A FIXED BED
V. M. K h a n a e v , G. G. K u v s h i n o v , and O. N . Kovalenko
UDC 66.011+661.217
An unsteady-state mathematical model for condensed sulfur formation by catalytic hydrogen sulfide oxidation in a fixed catalyst bed is proposed. Analytical solutions for the model are obtained. The steady-state front of hydrogen sulfide oxidation is shown to occur. The front velocity, the time of front formation, and the effect of the catalyst activity on front formation are estimated. The analytical solutions are compared with experimental data.
INTRODUCTION Direct oxidation of hydrogen sulfide by oxygen for sulfur production is a promising process in the technology of purification of gases with low impurity concentration, since the technique and equipment for its implementation are simple. In many cases, this process is an alternative to the Claus process and is unrivaled with excess of oxygen. The successful realization of this process has become possible with the invention of new active catalysts operating at room temperature. A high degree of sulfur trapping and selectivity are achieved when the process occurs at reduced temperatures so that the resulting sulfur is deposited (condensed) from the gas phase on a catalyst. Sulfur deposition leads to gradual deactivation of the catalyst. When the process is realized in a fixed catalyst bed, variation in the hydrogen sulfide concentration, sulfur condensation, and catalyst deactivation can proceed as a wave propagating along the bed. This problem is of practical significance, because the conditions of front formation and propagation determine to a great extent the thickness of the catalyst bed, the switching-over frequency of the reactor from a hydrogen sulfide oxidation regime to a catalyst-recovery regime, and some other parameters, which are important for practical application. The process of propagation of the gas-phase exothermic reaction zone in an inert porous medium under filtration feed of gaseous reagents to the chemical-reaction zone is called filtration combustion of gases [1]. Under conditions .of a granular catalyst bed, combustion proceeds on the grain surface. In this case, wave regimes can be observed. In many cases, they are associated with thermal-front propagation along the fixed catalyst bed [2]. However, in this process, isothermal wave regimes of oxidation, which are due to catalyst deactivation and are poorly studied, are observed. M A T H E M A T I C A L M O D E L A N D BASIC P H Y S I C A L H Y P O T H E S E S In the general case, under oxidation (combustion) of hydrogen sulfide the following chemical reactions c&n occur:
+ (1/2)O2
(1/n)S + H20,
(1)
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 6, pp. 47-51, November-December, 1996. Original article submitted August 24, 1995; revision submitted May 8, 1996. 0010-5082/96/3206-0637 $15.00 ~ 1997 Plenum Publishing Corporation 637
H2S + (3/2)02 -~ SO2 + H20,
(1/n)Sn + 02 --+ SO2,
2H2S + SO2 ~ ( 3 I n ) S , + H20, H2S + 202 ---* SO3 + H20,
(2)
(1/n)Sn + (3/2)02 ~ SO3,
SO2 + (1/2)O2 --~ SO3,
where n = 1-8. Reaction (1) with approximately first-order kinetics with respect to hydrogen sulfide [3] plays a dominant role at low temperatures under sulfur condensation. Experimental results obtained in [4] in studying the Claus reaction (2) show that in the absence of the pore-diffusion resistance the reaction rate is a linear function of the degree of packing of inner grain cavities with hquid sulfur. In this paper, this dependence is used for reaction (1). For a fixed catalyst bed, at low concentrations of hydrogen sulfide when heat release and variation in gas flow velocity as a result of the reaction can be ignored, we have the following mathematical model of hydrogen sulfide oxidation to elemental sulfur: c3c
k~ .S(1
0c
0~
k -Z
~-flS(1 - ~p)c,
k
subject to the initial and boundary conditions z=0:
c=cin,
t=0:
T=I,
c=0,
~=0.
(3)
Here c and c~n are the instant and inlet concentrations of hydrogen sulfide on conversion to sulfur (kilogram of sulfur/m3), k is the reaction rate constant (m/sec), L is the reactor length (m), u is the gas velocity (m/sec), z is the variable coordinate along the length of the reactor (m), S is the specific surface of the catalyst (m2/m3), /~ is the mass transfer coefficient (m/sec), ¢ is the porosity of the catalyst bed; ~ is the porosity of the catalyst grain, ~ is the degree of cavity packing of the catalyst grain, p is the density of liquid sulfur, and t is time. ANALYSIS AND ANALYTICAL
S O L U T I O N OF T H E M O D E L
To analyze the model, it is appropriate to change over to nondimensional variables. For this purpose, we introduce the notation A = k f l S L / ( k + fl)u and w = cin/p(1 - ¢)¢s and use the nondimensional variables r = t w u / L , x = t - c/ci,~, and ~ = z / L . The nondimensional system of differential equations then becomes Ox Ox W~T T + ~ = A(1 - ~)(1 - x),
O~ A(1 - ~v)(1 - x). O-~ =
(4)
Estimation shows that w
