Direct oxidation sulphur recovery process for very lean acid gas applications

Due to higher demand in desulphurisation and new environmental regulations, Companies need to find robust and efficient technologies while considering the best approach to minimise CAPEX and OPEX.

Thibaut Heim, Benoit Mares & Vincent Simonneau

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Article Summary

With more stringent regulations in terms of sulphur emissions to the atmosphere, some Companies have no choice but investing in new sulphur removal units. Some industries need to treat diluted acid gas with H2S concentrations below 15% vol. containing various impurities with, in some instances, huge hydraulic capacities. Some technologies can offer suitable solutions in terms of performance but investment and operating costs can be very high. 

The SmartSulfTM technology used in a Direct Oxidation scheme can offer a proven solution for industries dealing with low H2S concentration (gasification, biogas, carbon black, natural gas…). The technology can be used in new facilities or, thanks to its low footprint, used in existing facilities for retrofitting. The core of the process is the implementation of a cooling system inside the catalytic reactor. Details and principle of operation of such cooling system are presented. The process configuration can also propose a sulphur subdewpoint mode where sulphur is directly adsorbed on the catalyst leading to higher recoveries. Based on a smart switching system, the process offers a simple and fully automatic operation allowing continuous adsorption and regeneration cycles.

Several configurations are available for the SmartSulf in direct oxidation mode based mainly on the acid gas H2S concentration and on the required performance.

This paper focus on the SmartSulf process used in direct oxidation mode and gives feedbacks from the existing facilities where the technology is already in operation for many years.

Sulphur recovery from diluted acid gas is a challenge. Historically, liquid redox processes have been widely used for sweetening of such gases. Even if liquid redox processes are capable of meeting typical treated gas specification, these facilities often suffer from very high operating cost, low availability and a low quality sulphur product, which usually must be disposed rather than sold as product. H2S treatment through standard Claus unit is not possible as lean H2S gases below 15% vol. cannot allow flame stability. An enrichment section and/or gas reheating methods can be implemented but leads to a lot of capital cost, and makes operation more expensive and less reliable. The SmartSulf process used in Direct Oxidation mode offers a proven solution for the treatment of lean gases. The main feature of this process is its simplicity, ease of operation and relatively low capital and operating cost.

SmartSulf description

The core of the SmartSulf process is to implement a cooling system into a catalytic bed so as to limit an increase of temperature caused by the exothermicity of the Claus reaction.

The SmartSulf process can be used downstream a thermal Claus section for treating high H2S acid gas content (Claus reaction occurring at high temperature thanks to H2S rich gas burning) or used directly to treat an H2S lean gas (H2S gas being too lean for combustion). 

In the direct oxidation mode the feed gas containing H2S is mixed with a stoichiometric amount of air (O2) and reactions occur on the catalyst as per Eq. I. Part of the H2S is oxidised to elemental sulphur and side reactions form some SO2 which continue to react as per Claus reaction (Eq. II). Claus reaction being exothermic in the catalytic stage, a decrease in temperature favours the Claus equilibrium towards a higher sulphur conversion.

Eq. I: Direct oxidation of H2S    
H2S + 3/2 O2 →  SO2 + H2O + heat of reaction
H2S + 1/2 O2  → → 1/x Sx + H2O + heat of reaction

Eq. II: Claus reaction    
2 H2S + SO2 → → 3/x Sx + 2 H2O + heat of reaction
where x = 2, 4, 5, 6, 8 sulphur molecules of different sizes, according to temperature

In SmartSulf process, the reactor contains an internal heat exchanger which removes the heat of reaction. This results in a number of advantages:
• The chemical equilibrium is shifted in the direction of increased sulphur formation.
• The sulphur recovery rate rises substantially.
• Operating the reactor becomes a lot easier because the outlet temperature of the reactor remains almost constant, independent of fluctuations in feed gas volume and composition.
Basically two reactors configurations are possible with such process:
• Either the outlet gas temperature is set above the sulphur dewpoint temperature.
• Either the outlet gas temperature is set below the sulphur dewpoint temperature. In this configuration at least two reactors need to be installed operating in a switching mode. Alternately, one reactor is in adsorption mode (COLD mode), the other one is in regeneration mode (HOT mode) to desorb the sulphur settled on the catalyst. Operating mode is changed through special 4-ways valves sequence.

The reactors configuration selection is based on the performance required by the Customer.

Distinctive design of SmartSulf technology
Use of thermoplates for internal cooling of catalytic reactors
Internally cooled catalytic reactors have been successfully used in many applications. They are applied primarily for selective reactions, where a rigorous temperature control is required, or in reactions where the chemical equilibrium is strongly temperature dependent.
The catalytic reactors incorporated in SmartSulf use thermoplates as heat exchangers. The basic element of a thermoplate heat exchanger is the thermoplate itself, as shown in Figure 2.

A thermoplate consists of two metal sheets welded together along their edges and point-welded across their surfaces. This is accomplished with precise fabrication machinery that facilitates the manufacturing of large area exchangers at low cost. The plates are expanded by injecting high-pressure liquid between the metal sheets, which opens channels for the cooling medium. The expansion generates the typical cushion shape, and the point and seam welds of the thermoplates are gap free. Multiple thermoplates are combined to form a heat exchanger package, which is then inserted in a shell to complete the heat exchanger.

For application in a reactor, the catalyst is poured into the spacing between thermoplates. Vertical plane walls are formed by the thermoplates and allow easy filling of the catalyst particles.

Several thousand such thermoplate heat exchangers have been built and installed worldwide. They are in service in even the most severe applications, such as condensation of phosgene, which is not only highly toxic, but also very corrosive when in contact with water. Single heat exchangers with several thousand square meters of exchanger surface have been installed and operated. The thermoplate heat exchanger is considered a proven technology. These exchangers are compact and light weight, have low pressure drop, and provide high heat exchange coefficients; they are ideal for sulphur recovery reactors.

The outer and inner fluid channels are completely separated from each other by seam welds. As in contrast to other plate heat exchangers, there is no contact between adjacent thermoplates; each thermoplate is self-contained and no forces are transferred to the next plate. The catalyst particles are insulated and do not experience mechanical stress.

The distance between plates, pitch of the point welds, dimensions and number of thermoplates can vary widely. Therefore, thermoplate reactors can be optimally tailored to each sulphur recovery application.

4-ways valves for switching operation (subdewpoint configuration)
4-ways valves are the results of several years of development and are specifically dedicated to the SmartSulf operation.
The main characteristics of the valves are the following:
• Only one actuator actuates the two valves minimising the failure rate.
• These valves are specially designed for sulphur service meaning that the entire body, stem and disc are fully steam jacketed. No sulphur accumulation is possible in the valve itself as this is a no pocket design.
• Mechanical design of the valve takes into account thermal gradient through the valve during normal operation and after switch-over where the valve will handle cold gas and hot gas alternately. 
• The sealing of the valve has been thoroughly studied so as to avoid any gas leakage from one pass to the other with a special sealing.

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