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Nov-2013

Sulphur recovery from lean 
sour gas streams

Gas streams are often treated with amine or membrane systems to meet specifications before the gas can be sent to a pipeline or used in a fuel gas system. The amine and membrane systems often produce an acid gas stream that needs further treatment, if there is any hydrogen sulphide (H2S) present, to meet sulphur emission norms.

Matt Thundyil, David Seeger, Ramiro G Vazquez and Sameer Pallavkar
GTC Technology
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Article Summary
In cases where the sour gas contains a high carbon dioxide (CO2) concentration, or a low H2S concentration relative to CO2, this sour gas stream is often called a “lean acid gas” with H2S content typically 0.1-10%. There are few economically attractive options for treating a lean acid gas to meet permit requirements and produce saleable sulphur. An alternative proven approach is presented in this paper where the lean sour gas stream is treated catalytically to produce elemental sulphur, through the “direct oxidation to sulphur” of the H2S. This approach has applicability in treating conventional natural gas, shale gas and associated gas streams.

Introduction
Over the past 20 years, environmental specifications for sulphur emissions have tightened, resulting in an increasing need for cost-effective desulphurisation technology. As Figure 1 demonstrates, there are two dominant approaches for desulphurisation: (1) At low total sulphur levels (<0.1 ton/day), scavengers are preferred; and (2) at high sulphur levels (>20 tons/day), amine units in combination with Claus plants are preferred, producing high quality sulphur. In the intermediate range, 0.1-20 tons/day, there are few options that produce high quality sulphur in a cost-effective manner. The recent excitement related to tight oil and gas production, has also highlighted the need for cost-effective sulphur removal technology.

Over the past decade, scientists at TDA Research developed a catalytic route to the direct oxidation of H2S that can be used to produce “Claus quality” elemental sulphur from acid gas streams lean in H2S.1-3 GTC Technology US, LLC, has acquired the exclusive licensing rights to this technology and markets it under the name GT-DOS. This technology is particularly relevant in the gas treating space, where gas producers have to manage lean sour gas streams in the context of associated gas production, conventional natural gas production associated with souring of fields, and conventional gas production in high CO2 fields.

GT-DOS Technology: Process Description
GT-DOS Technology uses a patented mixed-metal oxide catalyst to achieve the direct oxidation to sulphur of H2S.1-2 (Figure 2 shows a block flow diagram for GT-DOS Technology.)

The GT-DOS process can be used to treat sour natural gas streams, or amine acid-gas, or membrane acid-gas streams. In the GT-DOS process, the air and the acid gas are preheated before they are mixed. The mixed gases are heated and enter the GT-DOS reactor where the temperature is maintained above the sulphur dewpoint. Then the mixed gas-air stream is directed to the GT-DOS reactor where the H2S and O2 react, under slightly sub-stoichiometric conditions, according to the below chemical equation.

2 H2S + O2 → 2 H2O + 2 S

The catalyst selectivity is extremely high, and not equilibrium limited. Typically, in a single pass, 90% of the H2S is converted to elemental sulphur (S). Any CO2 and hydrocarbons pass through essentially unreacted. The oxidation of H2S to S is exothermic; so the temperature of the gas will rise. The process is designed to limit the temperature increase, to mitigate SO2 formation at higher temperatures. The gas exiting the GT-DOS reactor is then directed to a sulphur condenser where the temperature is reduced and sulphur condenses from the gas phase to a molten sulphur phase. The heat of condensation can be integrated into the energy scheme of the process. Also, in the sulphur condenser, the molten sulphur is separated from the gas, and the sulphur is directed from the condenser to sulphur storage vessels or a sulphur pit, or formed into solid sulphur.
Combining the GT-DOS process with a tail gas clean up technology (such as scavenging, or hydrogenation, amine, recycle) allows it to deliver 99.9%+ removal at a moderate and acceptable operating and capital cost.

Case study
The direct oxidation catalyst technology patented by TDA has been licensed to two end-customers. This case study presented details the application of the GT-DOS direct oxidation catalyst in the direct treatment of associated gas.  The inlet gas stream composition was:

Operating results
Figure 3 illustrates the modular direct oxidation unit using the TDA-patented direct oxidation catalyst, and installed at a gas plant location in the US.

The gas composition shown in Table 2 indicates the performance of process and the catalyst. The results indicate that over 88% of the H2S was removed. The hydrocarbon compositions, within the error of measurement, suggest that the hydrocarbons passed through the reactor unreacted.
The H2S conversion (to sulphur) over time was measured. This is illustrated in Figure 4. The stability of conversion can be seen.

The direct treatment of associated gas, at pressure, for the removal of sulphur is among one of the most aggressive environments that a catalyst can be subjected to, due to the long list of paraffins, olefins, aromatics, naphthenes and other compounds that can be present. The stable conversion efficiency demonstrated illustrates the robustness of the catalyst.

By comparison, the treatment of amine or membrane acid gas streams is relatively easier, because most of the constituents of the raw gas will not make their way into the acid gas streams, and variations in the raw gas composition is modulated by the amine or membrane units. Therefore, the use of GT-DOS to treat amine acid gas or membrane acid gas streams is also attractive.
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