(174d) A New Kinetic Model for the Simulation of the Claus Process

The modified Claus process disposes of H₂S in acid gas streams from refinery amine units and sour water strippers, and converts it to elemental sulfur. The feed mixes with air or oxygen and is sent to a burner/waste heat boiler where combustion and other reactions occur transforming H₂S to SO₂ and sulfur. The outlet is sent to a series of catalytic fixed beds where most of the remaining H₂S reacts with SO₂ to produce elemental sulfur and water (Claus reaction). First bed operating conditions and catalyst are selected to favor the hydrolysis of CS₂ and COS formed as byproducts in the furnace. Sulfur production in the catalyst bed is favored at low temperatures with each bed operating at a slightly lower temperature. Sulfur is extracted by inter-stage condensation and separation. Tail gas out of the final bed can be sent to a tail gas unit to achieve overall sulfur recovery > 99%.

The focus of this work is the formulation and application of a new kinetic model for the Claus catalytic fixed bed reactors. The proposed reaction mechanism accounts for surface adsorption of intermediate species in line with experimental data on observed surface species concentrations. Experimental data on Claus reaction, hydrolysis of CS₂ and COS on alumina and titania catalysts at different temperatures and feed compositions were used to estimate kinetic and adsorption parameters for each type of catalyst using available gradient-based algorithms in the Optimization Tool^TM of the ProMax® simulation software.

The kinetic rate expressions and parameters were included in a model for a commercial sulfur recovery unit created using ProMax. Different cases were evaluated to study the effect of variations of feed composition, reactor temperatures and titania content of the first catalytic bed on sulfur recovery, with the goal of maximizing sulfur recovery.