Feb-2015

LO-CAT: a flexible hydrogen sulphide removal process

The hydrocarbon engineering industry faces tighter restrictions for hydrogen sulphide (H2S) emissions and expanding requirements for sulphur recovery. Operators place a high value on developing the right strategy for H2S removal and sulphur recovery.

William Rouleau and John Watson
Merichem Company

Article Summary

When H2S exceeds certain levels, sulphur abatement systems are often required. These investments can be expensive and complex. Fortunately, the LO-CAT technology offers a well-developed and economic solution that has been in commercial use for almost 35 years. The LO-CAT technology provides operators features, like: 100% turndown in respect to H2S concentration, flow rate and sulphur loading; single stage removal efficiencies in excess of 99.9%; and an ability to process any type of gas stream. These features make the process attractive as a standalone solution for sulphur removal.

Background
Hydrogen sulphide is an extremely toxic, corrosive and odorous gas, causing safety and material of construction issues in its unaltered form. For many years, high levels of H2S in many raw natural gas streams have required processing to reduce the contained acid gases before transport and distribution of the fuel to market.

Increasing concentrations of H2S can have several detrimental effects: 1) onset of odour problems; 2) corrosion of gas production hardware; 3) increasing SOx emissions from flaring or other combustion processes; and 4) possible health consequences for workers. The odour threshold for H2S is extremely low (0.05 to 0.1 ppmv), and levels of H2S above 10 ppmv are considered toxic, exceeding the Threshold Limit Value (TLV). Moreover, levels of H2S above 1000 ppmv (0.1 V%) in a breathing zone can rapidly lead to unconsciousness and death. Sites with high H2S levels pay special attention to design approval worker health and safety controls throughout the plant.

As environmental considerations increase on designers and site operators to further reduce sulphur emissions, selecting the correct processing choice from several options involves an analysis of capital cost minimisation, operational flexibility, operating cost, and the total cost of ownership associated with new facilities.
 
Sulphur Recovery via the LO-CAT Technology
The LO-CAT technology was developed to provide an isothermal, low operating cost method for carrying out the modified Claus reaction. The technology has been adopted by a number of industries, including the Oil & Gas, Coal, Power, and Food & Beverage.

The first commercial installation of LO-CAT technology took place in 1980. Starting with oil and gas production (upstream and midstream), and oil refining (downstream), the basic process has been continually improved and modified over time to allow for expanded use into other markets and industrial segments. From petrochemicals to metals (coke oven gas and direct reduced iron off gas), water and waste water treatment (municipal and industrial) to carbon dioxide products (food and beverage), this simple, robust technology has found success in multiple applications. More recently, unconventional resources and alternative energy sources such as shale gas, stranded offshore gas, biogas, and gasification syngas have been added to the portfolio of applications that successfully utilise LO CAT technology for sulphur recovery.

LO-CAT technology contains a proprietary liquid redox catalyst that converts H2S to solid elemental sulphur by carrying out the direct oxidation of H2S as follows:

Direct Oxidation Reaction H2S + 1/2 O2 → H2O + S°
As embodied in LO-CAT technology, the direct oxidation reaction is divided into five sequential steps:

1 – Absorption of H2S H2S (Gas) + H2O (Liquid) H2S (Aq) + H2O (Aq)
2 – Ionisation of H2S H2S (Aq) H+ + HS–
3 – Sulphide Oxidation HS– + 2Fe+++ S° + 2Fe++ + H+
4 – Absorption of Oxygen 1/2 O2 (Gas) + H2O (Liquid) 1/2 O2 (Aq) + H2O (Aq)
5 – Iron Oxidation 1/2 O2 (Aq) + H2O + 2Fe++ → 2 OH– + 2Fe+++

Equations 1 and 2 represent the absorption of H2S into the aqueous, chelated iron solution and its subsequent ionisation, while equation 3 represents the oxidation of hydrosulphide ions to elemental sulphur and the accompanying reduction of the ferric (active) iron to the ferrous (inactive) state. Equations 4 and 5 represent the absorption of oxygen (from ambient air) into the aqueous solution followed by oxidation of the ferrous iron back to the ferric state.

Equations 3 and 5 are very rapid. Consequently, LO-CAT systems generally produce relatively small amounts of byproduct thiosulphate ions. However, equations 1 and 4 are relatively slow and are the rate controlling steps in a LO-CAT System. It is interesting to note that the chelating agents do not appear in the process chemistry, and in the overall chemical reaction, the iron effectively cancels out. One may wonder why chelated iron is required at all, if it doesn’t take part in the overall reaction. In the LO-CAT process, iron serves two purposes in the process chemistry. First, it serves as an electron donor and acceptor, or in other words, a reagent. Secondly, it serves as a catalyst in accelerating the overall reaction. Because of this dual purpose, the iron is often called a “catalytic reagent”. Although there are many metals which can perform these functions, iron (Fe) was chosen for LO-CAT technology primarily because it is inexpensive and safe to operate. The chelating agent(s) do not take part in the process chemistry. Their role is simply to hold the iron ions in solution. Neither ferrous (Fe++) nor ferric (Fe+++) ions are very soluble or very stable in aqueous solutions. Iron will ordinarily precipitate at low concentrations as either ferric hydroxide Fe(OH)3 or ferrous sulphide (FeS). The chelating agents are organic compounds that wrap around the iron in a claw-like fashion, preventing the iron ions from forming precipitates. LO-CAT technology uses a proprietary blend of chelating agents to hold the iron in solution over a wide operating range. LO-CAT has developed into a versatile processing scheme for treating gas streams containing any amount of H2S. Advantages of these systems include the ability to treat both aerobic and anaerobic gas streams, H2S removal efficiencies in excess of 99.9%, essentially 100% turndown on H2S concentration and/or gas flow, and the production of only innocuous products and byproducts.

Elemental sulphur recovered by the LO-CAT technology is profoundly different from sulphur produced by other processes. LO-CAT sulphur has a particle size that ranges from 8 to 45 microns, much smaller than sulphur generated by other means. And since this solid sulphur is formed in an aqueous solution, it harbours no trapped H2S vapours that can evolve during subsequent handling. Sulphur recovered by the LO-CAT technology also has a relatively soft texture that can lead to large internal and external surface areas that support microbial action in the soil¹. LO-CAT sulphur is removed from the process as a “filter cake” composed of approximately 65 wt% sulphur and 35 wt% diluted LO-CAT solution. The moisture content of the cake favours low dust production during use. Additionally, particle size, surface area and moisture content characteristics exhibited by the produced LO CAT sulphur cake support good reactivity, applicability and safety for sulphur used in agricultural applications².