In the western USA and Canada, there are many natural gas wells that are sour which are treated by the Claus Reaction to remove H2S. The first step involves oxidation to a sufficient fraction of the H2S to SO2

2 H2S(g) + 3 O2(g)Û 2 SO2(g) + 2 H2O(g)

for a subsequent reaction that is performed catalytically. Other components of the natural gas also are oxidized to give CO, CO2 and water. After this oxidation, the stream is cooled and passed to a Claus reactor. A typical gas composition in mole % fed to a catalytic Claus reactor is :

N2 2%

CH4 74%

C2H6 8%

H2S 4%

H2O 4%

SO2 4%

CO2 2%

CO 2%

The Claus reaction is given below and is performed on a g-Al2O3 catalyst.

H2S(g) + 1/2 SO2(g)Û H2O(g) + 3/2 S(g and/or s)

There are two forms of sulphur which can result, gas or solid, depending upon conditions. If solid sulphur results, due to supersaturation, it will precipitate in the pores of the catalyst and decrease the number of catalytically active sites available, thus deactivating the catalyst. The catalyst is porous g-Al2O3 beads with a diameter of 0.2 cm which has a bulk density 1.4 gm/cc, a particle density of 2.1 gm/cc and a specific surface area of 100 m2/gm (the density of g-Al2O3 is 3.6 gm/cc). The forward rate on a fresh catalyst is pseudo-first order with respect to H2S with a rate constant given by :

k1 = 8.9 x 1016 exp[(-40,000 cal/mole)/RT)] .

a) For a 10 m tubular reactor what conversion of the feed is expected? The feed flow rate is 40Kg/hr at a temperature of 150oC and a pressure of 200KPa. Assume that the reactor is well insulated and consists of a packed catalyst bed with an inside diameter of 30 cm. Also assume that the catalyst is not deactivated by the solid sulphur for this calculation.

c) Run the same calculations from a and b above accounting for deactiviation of surface sites as the sulfur is condensed in the pores of the catalyst. Pore condensation follows the Modified BET isotherm and is limited to n layers adsorbed. The adsorbed amount, G, is given by:

where Gm is the monolayer coverage, c = 0.1, n = 10 and x is the ratio of partial pressure of sulfur in the gas phase to its equilibrium partial pressure.

S(s-rhombic) MP =112.8˚C ∆Hfusion= 1.7175 KJ/mole

density = 2.07gm/cc BP = 444.6˚C ∆Hvaporization = 2,200 cal/mole

Cp˚(cal/deg/mole) = a +bT+cT2+dT3 (T˚K)