Improving the Operation of Split-Flow Sulfur Recovery Plants with Membrane Technology
Authors: Vaidya Milind M. , Duval Sebastien , Hamad Feras , O'Connell John , Ghulam Shabbir , Al-Talib Ahmed , Bahamdan Ahmad A. , Al-Otaibi Faisal D. 
Published: November 2020

Sulfur recovery units are used in gas plants and refineries to convert hydrogen sulfide (H2S) to elemental sulfur. H2S occurs naturally in natural gas or is formed as a by-product in refinery and/or petrochemical processing systems (e.g. hydro-treatment). H2S is highly toxic, therefore requiring removal from the gas stream. Efficient processing of H2S and other sulfur-containing compounds is vital in reducing emissions to meet increasingly stringent environmental regulations. In a sulfur recovery unit, an acid gas feed stream containing H2S and a source of oxygen, such as air, are fed to a free-flame furnace, followed by multiple catalytic stages to convert most of the H2S into elemental sulfur.

This paper introduces a new approach for the operation of split-flow sulfur recovery plants. For lean acid gas (i.e. low hydrogen sulfide content gas), only a fraction of the acid gas is sent to the reaction furnace of the modified Claus sulfur recovery units. The remaining acid gas, or bypassed feed, is merged with the outlet of the reaction furnace stream, downstream of the waste heat recovery unit and upstream of the first sulfur condenser. One benefit of bypassing the reaction furnace is the reduction of the formation of COS and CS2 within the furnace due to the high concentration of CO2 and kinetic limitation. A shortcoming, however, is that a fraction of the heavy hydrocarbons, i.e. Benzene, Toluene and Xylene (BTX), are not destroyed in the furnace reactor as a result of the bypass. BTX can deposit and rapidly deactivate the catalytic beds. To mitigate this risk, an activated carbon bed is installed to trap these hydrocarbons upstream of the catalyst bed. The cyclical nature of the operation of the activated carbon bed (adsorption, regeneration) as well as the risk inherent to the replacement of the contaminated beds call for an alternative approach. In this paper, the improvements discussed relate to the use of membranes that separate BTX along with H2S. The H2S- and BTEX-rich stream is routed to the reaction furnace, unloading the activated carbon bed and thus improving the sulfur recovery plant operability. An associated benefit is the possibility to route a larger fraction of the feed H2S within the reaction furnace such that the risk of O2 breakthrough to the catalyst bed is reduced while operating at a large by-pass fraction.

A series of process simulations with different acid gas compositions and different membrane properties were evaluated to determine the impact of membrane selectivity on the acid gas composition sent to the reaction furnace.

The results show a significant reduction of BTX and other hydrocarbons sent to catalytic converters and an acid gas enrichment with significant increase in H2S content in the acid gas feeding the reaction furnace. The ability of the membrane to concentrate BTX along with H2S can lead to a new operating philosophy for lean acid gas sulfur recovery plants.