Multi-Objective Optimization to Predict Minimum Temperature for Efficient BTEX Destruction to Minimize Fuel Gas Consumption in Sulfur Recovery Units

Abstract

Benzene, Toluene, Ethylbenzene and Xylene (BTEX) present in feed gases to Sulfur Recovery Units (SRU) cause frequent catalyst deactivation. BTEX can be oxidized at the recommended temperatures above 1050°C. High temperatures are achieved through feed preheating and co-firing acid gas with fuel gas. However, temperatures above 1050°C is not required when BTEX concentration is low. A multi-objective optimization approach is deployed to minimize feed preheating temperature and fuel gas co-firing, while maintaining high BTEX destruction. A well validated model for Claus furnace from previous studies was used for furnace simulations. Claus furnace was modelled using Chemkin Pro, while catalytic section (including condensers, re-heaters and incinerator) was modelled using Aspen Hysys (Sulsim). MATLAB was used as a platform to link Chemkin Pro with Aspen Hysys. Optimization was performed in MATLAB using genetic algorithm. The objectives of optimization were to 1) Maximize sulfur recovery, 2) Minimize fuel gas consumption to furnace, 3) Minimize air and acid gas preheating temperature. As a constraint, total BTEX at waste heat boiler outlet (WHB) was maintained below 1ppm. The optimization range for fuel gas flow rate was from 29 to 2034 nm3/hr, air temperature from 180 to 360°C and for acid gas temperature, 180 to 230°C was considered. The feed properties and physical dimensions of SRU were obtained from an industrial SRU plant. Results show that furnace temperature of 1028°C needs to be maintained for maintaining BTEX destruction for the given feed condition examined. Thus, fuel gas co-firing can be reduced from base case value of 1773 nm3/hr to 29 nm3/hr, while air preheating temperature can also reduce from 325°C to 223°C. This can assist in reducing operational costs in sulfur recovery units considerably. The present work predicts the ideal conditions for BTEX destruction in SRUs based on inlet feed conditions. This approach can be used to seek favorable means of optimizing Sulfur recovery, decreasing fuel gas consumption in sulfur recovery units to reduce operating cost.