Hydrogenation Reactor
The tail gas coming from the final sulfur condenser in a Claus-based SRU contains enough unconverted SO2 and other sulfur species to present the plant with a significant challenge meeting sulfur emissions regulations if discharged to the stack without further treatment. Conventional treatment involves tail gas hydrogenation (also called hydrodesulphurization) to convert the SX, COS, CS2, and SO2 content to H2S so it can be removed and recycled via a downstream amine treating unit. Hydrogenation is followed by a direct contact water quench step to cool the gas to a low enough temperature to make H2S removal by an amine feasible.
Catalysts
The hydrogenation catalyst is usually cobalt-molybdenum (or nickel-molybdenum) supported on γ-alumina. The industrial manufacture of hydrodesulphurization catalysts consists of four steps: carrier preparation, addition of active compounds, calcination, and sulphiding. The oxide catalyst is activated by pre-heating in an atmosphere of hydrogen and hydrogen sulphide. The catalyst must be presulfided before it can be used. A frequent question engineers often ask is how well presulfiding really works. SulphurPro’s Tail Gas Hydrogenation Reactor model now allows that question to be answered.
Important Factors
The hydrogenation reactor model in OGT | SulphurPro® results from having taken great pains to ensure that the model reflects as much as possible the detail of the catalysts themselves. Model accuracy is critically important because hydrogenation is such an important step in determining the final sulphur recovery and establishing sulphur emissions.
The OGT | SulphurPro® simulator provides a tremendous wealth of information on the performance of multi-layered beds of spherical, cylindrical and trilobe catalyst particles including 11 reactions in horizontally and vertically aligned vessels, and bed heat loss, and the effect of catalyst aging and poisoning.
The OGT | SulphurPro Hydrogenation Reactor uses a robust numerical solver that permits different layers of catalysts to be modelled in a kinetic rate-based manner inclusive of hydraulic calculations for accurate pressure drop predictions across the catalyst bed. Rigorous handling of vessel geometries incorporating measured cylinder and head volumes are included to permit accurate accommodation of the plant thermocouple locations, and catalyst volume variation with catalyst bed depth for both horizontal and vertical geometries.
Tail gas hydrogenation reactor calculations are extremely detailed and are on a rate basis (not equilibrium) which leads to exceptional reliability and predictive power. The composition of the gas feeding and leaving the TGTU accurately reflects reality with OGT | SulphurPro®.
A Detailed Hydrogenation Model Really Matters
A simple example will show how an accurate fundamentals-based hydrogenation reactor model makes a difference. The case is a refinery SRU processing SWS gas through the first zone of a two-zone reaction furnace with amine acid gas (AAG) split between the two zones. (This approach allows the first zone to run hotter promoting ammonia destruction.) This is a two-converter-bed setup using alumina catalyst without oxygen enrichment.
When the unit is run with combustion air set to maintain a 2:1 H2S/SO2 ratio (defined to be zero excess air) the normal SO2 level from the hydrogenation reactor is quite acceptable. However, with even 2.5% excess air the SO2 level in the hydrogenation reactor effluent is predicted to escalate to 400 ppmv. This can play havoc with the quench system and, if not mitigated, will cause a host of problems ranging from corrosion and plugging of the quench to the rapid loss of activity of the amine solvent in the TGTU. The solution is to cut back on combustion air, or add more hydrogen to the reactor feed. However, none of this may be immediately obvious to the operators without using OGT | SulphurPro® to simulate the system, and possibly OGT | ProBot™ to monitor the system on-line and give advance warning of impending trouble. These situations are very easily revealed by SulphurPro simulation.