Ind. Eng. Chem. Res. 2007, 46, 7721-7728
7721
Catalytic H2S Conversion and SO2 Production over Iron Oxide and Iron Oxide/γ-Al2O3 in Liquid Sulfur Michael A. Shields,* Norman I. Dowling, and Peter D. Clark Alberta Sulphur Research Ltd., Department of Chemistry, UniVersity of Calgary, #6-3535 Research Road N.W., Calgary, Alberta, Canada T2L 2K8
A stirred-glass autoclave containing liquid sulfur and solid iron oxide catalyst was used to study low-tonnage sulfur recovery from H2S-containing gas streams. The objectives were to test the feasibility of using both liquid sulfur as a reaction medium and iron oxide as a direct oxidation catalyst for prolonged H2S conversion. Using a 1.60% H2S and 0.80% O2 (balance N2) feed gas, fresh iron oxide acted primarily as a scavenger for bulk H2S removal from the inlet gas stream. Following the scavenging phase, the steady-state iron oxide/ sulfide was able to maintain low catalytic activity (30% conversion). The steady-state catalyst did, however, have a strong ability to generate significant amounts of SO2 in the presence of inlet feed O2. Data showed that this SO2 production resulted from the oxidation of the liquid sulfur over the steady-state iron oxide/ sulfide. The rate of SO2 formation was shown to be directly proportional to the concentration of O2 in the inlet feed gas. Although H2S conversions over steady-state iron oxide/sulfide ended up being lower than expected, the ability to strictly control the amount of SO2 generated from the system was advantageous. By incorporating γ-Al2O3 into a liquid sulfur reactor containing steady-state iron oxide/sulfide, the dual-catalyst system achieved 97% conversion of the H2S to elemental sulfur. 1. Introduction The removal of H2S from sour gas streams is an important step in bringing these streams to pipeline and market specifications. Since H2S is toxic, unpleasantly odorous, and corrosive, capture and conversion of the H2S is very important. Industrially, the most common approach to H2S removal is through conversion of the H2S to elemental sulfur. Sulfur in its elemental state is an important feedstock in fertilizer and chemical manufacturing and can be stored in block form when produced in excess to demand. Industrially, large-tonnage H2S gas processing (10-2 000 ton/ day) incorporates a Claus plant for conversion to elemental sulfur. Discovered in the 19th century and developed throughout the previous century, the Claus process continues to be the method of choice for large-scale H2S conversion.1,2 In its infancy, the chemistry consisted of a catalytically promoted reaction between H2S and O2, thus producing elemental sulfur and H2O (eq 1).
H2S + 1/2O2 f 1/8S8 + H2O
(1)
To improve the process, this single-step partial oxidation of H2S was split into two separate steps.2 Coined the Modified Claus Process, it consists of a first step in which one-third of the incoming H2S is combusted with a stoichiometrically controlled amount of O2 at 1200 °C, generating SO2 and sulfur and leaving some H2S unreacted (eq 2).
H2S + 3/2O2 f SO2 + H2O
(2)
The SO2 subsequently reacts in the furnace with the remaining two-thirds of the H2S from the original gas stream, thus producing elemental sulfur. Overall, 60-70% of the initial H2S is converted to elemental sulfur prior to leaving the furnace (eq * To whom correspondence should be addressed. E-mail:
[email protected].
3). This sulfur vapor (S2) is then subsequently condensed to liquid (S8).
2H2S + SO2 h 3/2S2 + 2H2O
(3)
The remaining gas-phase sulfur species (2:1 H2S/SO2) are sent through a series of sequentially lower temperature catalytic converters (320-200 °C). Within each converter, elemental sulfur is produced and then condensed (eq 4).
2H2S + SO2 h 3/8S8 + 2H2O
(4)
Using three catalytic converters, maximum achievable conversion to elemental sulfur is on the order of 98% due to thermodynamic limitations. The effluent gas from the converter train contains small amounts of sulfur-containing compounds (i.e., H2S, SO2, COS, CS2, and Svap) that are recovered in tail gas processing units, resulting in a total sulfur recovery > 99%. Many different approaches have been developed for tail gas treatment processes (i.e., H2S conversion from low H2S content gas streams). These include sub-dew point processes (e.g., Sulfreen),3 catalytic-oxidation processes (e.g., Superclaus),4 and a variety of scavenging technologies and incineration methods. Although different in design and methodology, all aim for complete H2S removal from the acid gas stream. Designed primarily for Claus tail gas cleanup, some of these processes also have the potential to be incorporated as a stand-alone system for low-tonnage sulfur recovery (
