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Compact modular sulphur recovery solutions

Prosernat has been awarded a contract for the delivery of modular Sulphur Recovery Units (SRUs) for an Early Production Facility (EPF) project in Middle East.

Benoît Mares and Christian Streicher
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Article Summary
For this project the SmartSulf™ technology licensed has been selected among other options less favourable in terms of capex and opex. The unit capacity is of 2 parallel trains of 100 TS/d each. The specificity of the SmartSulf process technology which allows to achieve high sulphur yields without need for additional tail gas treatment, has been considered as a definite advantage for the project. The modular delivery by Prosernat allows to keep with the tight overall project schedule. The overall project is being developed on a BOT (Build-Operate-Transfer) model and the capacity of Prosernat to follow the BOT Contractor in terms of contractual scheme was also a key decisive factor.

A compact, low cost technology
Local environmental regulations required for this kind of SRU an overall sulphur recovery of at least 99.4 %. This level can be guaranteed with the SmartSulf process without need for installation of any downstream Tail Gas treatment Process. The concept of the SmartSulf process is based on the use of a 2 catalytic stages Claus unit, different from conventional Claus by the fact that the catalytic reactors are operated under isothermal conditions and the second catalytic stage is operated under subdewpoint conditions in order to achieve much higher sulphur yields than in a conventional Claus process.  This concept is described with Figure 1. The fundamental idea of the SmartSulf process is removing the heat produced by the exothermic Claus reaction directly in the catalyst bed rather than in a downstream heat exchanger. This controls the temperature throughout the catalyst bed within a narrow range. The heat exchanger applied is a thermoplate with large clearances. The space in between the plates is filled with catalyst so that the cooling heat exchanger is embedded in the catalyst.

The acid gas is fed in a thermal section, strictly similar to what is found in conventional Claus design. After sulphur condensation and reheating the tail gas from thermal stage is fed to the first catalytic stage. Both catalytic reactors are equipped with an internal heat exchanger in order to control the temperature in the reactors. The heat of the Claus catalytic reaction is being removed by circulating water counter-currently in the heat exchanger inside of the reactor. The first catalytic reactor is being operated at temperatures above sulphur subdewpoint. However by controlling the temperature at the outlet of the first reactor higher catalytic conversions are already being achieved than in a conventional first stage Claus reactor. After sulphur condensation the gas from the first reactor is then fed to the second catalytic reactor. This reactor is being operated at temperatures below sulphur subdewpoint, thus achieving much higher conversions. Because the temperature is kept controlled until the outlet of the reactor the conversions achieved are in fact even higher than those obtained in conventional subdewpoint tail gas treatment processes. For a typical acid gas in a refinery overall sulphur recoveries are above 99.5 %. Tail gas form the SmartSulf unit can therefore in most cases be directly sent to the incinerator without the need for any downstream tail gas treatment unit. This is being the case for the early production facilities considered in Middle East.

Once the second catalytic reactor is loaded with liquid sulphur, the couple of automatic 4 ways valves are being rotated so that the first reactor originally operated in “hot mode” becomes now operated in subdewpoint mode. The second catalytic reactor is now operated in “hot mode” and therefore regenerated by vaporising all the accumulated sulphur, which is then condensed in the external sulphur condenser.

The SmartSulf process is a real breakthrough in sulphur recovery technologies, allowing high sulphur recoveries without need for tail gas treatment unit. To summarise the key reasons for selecting Prosernat and its SmartSulf process for this EPF project have been:
• The capacity of the process to meet local environmental regulations ;
• The compactness of the process with a minimum amount of equipment items compared to other technologies ;
• The low cost of this technology, compared to alternative Claus + tail gas treatment options ;
• The capacity of Prosernat to supply the unit fully modularised within a tight timeframe ;
• The capacity of Prosernat to support its customer for such BOT project through financing and continuous operational support.

Construction of the SmartSulf unit
Each of both 100 TS/d train of the SmartSulf will be delivered modularised in the following form (see also Figure 2 3D model of the SRU):
• 1 module for feed gas pre-treatment (knock-out drum + air pre-heater)
• 3 modules for the whole catalytic section (including sulphur condenser and  4-ways valves)
• 1 package for the thermal  section with reaction furnace, burner and waste heat boilers with base plates and access platform supplied loose for site installation
• 1 incinerator conditioning skid (for tail gas and combustion air)
• 1 loose incinerator and burner

Specific equipments
The SmartSulf reactors for the project are 3.6 m diameter. Those reactors are proprietary items based on specific manufacturing knowledges especially for the assembly of internals (header, plates), mechanical design and testing of the reactors, welding procedures, thermal stress considerations, non destructive test.

Another specific item for the SmartSulf application is the 4-ways valves. The process is cyclic and designed so that after 24h, the two reactors switches. The second Reactor that contains the catalyst loaded with sulphur has to be regenerated. For that purpose, switching 4-way valves are actuated and modify the process gas circulation through the unit. The former second reactor becomes the first one and vice-versa. The switching procedure is completely automatic and takes only a few seconds.

The two 4-ways valves for the JPF case are 16 inches and are especially designed for sulphur environment applications. Full availability of the valves is achieved through tailor made design such as full steam jacket of the valve (body and disc), free draining body with no possible sulphur accumulation, sealing system to ensure Class IV tightness… Those valves are delivered by Prosernat as proprietary items.

As mentioned earlier the sequence is completely automatic. Prosernat develops the complete functional analysis and also the logic diagrams as required by the Client. For the project, the logic was implemented by the Client directly in its control system. The advantage is that the sequence is programmed using the same hardware and software as the ones used for the overall plant control and monitoring. It simplifies a lot in terms of maintenance.

Plot area
The plot area required for the SmartSulf unit is similar to the plot area required for the installation of a standard Claus unit with two catalytic stages. The two 100 TS/d trains of SmartSulf are easily installed in a 50 m x 70 m foot print. In fact an existing Claus unit can be retrofitted into a SmartSulf unit and has been already been done in past projects. 

Modularisation strategy and schedule
The modularisation strategy for this project was selected due to the location of the site and fast track project schedule.
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