Jun-2017

Improving sulphur plant performance

Strict attention to utilities is critical to ensure safe and reliable sulphur recovery unit operation

ELMO NASATO
Nasato Consulting

Article Summary

Current trends in the characteristics of crude oil supply, petroleum product demand, and tightening environmental regulations require continuous change in the worldwide refining and gas producing industry. Refiners and gas producers are confronted with more stringent environmental, safety and regulatory requirements that must be met while maintaining very high on-line factors in a safe and highly efficient operation.

More hydrotreating and increased processing severity are required for removing sulphur and nitrogen compounds from fuels to meet current and future environmental regulations. The increase in production of hydrogen sulphide (H2S) and ammonia (NH3) in combination with more stringent sulphur recovery levels, on-line factors, and safety requirements has placed new demands on the processing capability of refinery sulphur recovery units (SRU). These developments have led to an increased understanding of root cause failure adversely affecting SRU performance and on-line time.

In theory, the SRU is a relatively simple process operating unit, but the details in design and operation of the SRU, the upstream amine acid gas unit (AGU) and sour water stripper (SWS) acid gas unit are critical in maintaining very high on-line factors. In comparison with the process side of the SRU, the utility side of the SRU is frequently neglected in both the details of the conceptual design and in normal day-to-day operation. However, the utility side frequently provides harsh reminders of its importance and the need for keen attention in order to  ensure reliable, safe and high on-line operation of the SRU complex.

This article will describe some of the key design and operating parameters that are to be considered in the SRU, AGU and SWS in order to improve performance and on-line reliability of the SRU.

Reliability considerations
Acid gas flaring is prohibited by law in most regions, except for very short periods of time during an emergency situation, due to the toxic sulphur dioxide (SO2) that is generated. Redundant sulphur plants may be required and the refinery may need to curtail or limit throughput in its sulphur producing units until the sulphur plant can be repaired. This can lead to lost profits and, in some cases, fines by environmental agencies. Proven techniques exist in both the design and operation of the sulphur recovery block that will improve its reliability. These techniques have been shown to improve the on-stream factor dramatically and provide for up to the industry standard of five years between scheduled turnarounds.

As environmental regulations become more stringent, optimisation of the sulphur complex continues to be a design challenge as the industry continues to approach near 100% sulphur recovery levels. However, the challenge for operating companies is to take action to control excess emissions and maintenance costs. The goal for most operators is to eliminate unplanned shutdowns of the sulphur recovery operations. This goal has the benefit of achieving environmental compliance but also improves overall safety and economic viability through increased equipment reliability. The best practice for a SRU operation is to keep the unit hot; this implies starting the unit and desirably only shutting down for scheduled maintenance turnarounds. It is thermal cycling of the SRU complex that causes the most damage and places personnel and the environment at the greatest risk.

A typical refinery sulphur recovery operation, comprising of a two stage Claus unit and reduction-absorption type tail gas treating unit (TGTU) is shown in Figure 1.

Although the process is simple, problems do develop. Unexpected upsets and shutdowns of sulphur recovery operations cause excess SO2 emissions and increase plant maintenance costs. Examples of resulting cost increases include: direct replacement costs for damaged equipment, labour costs for repairs, and increased costs for maintaining greater inventories of spare catalyst and critical parts.

Based on operating experience, shutdowns in the sulphur complex can be linked directly to the SRU, TGTU but more importantly the AGU and upstream SWS unit. The four typical, high priority causes of unit failure (causes that fall in both the high probability and high consequence categories) can be categorised in general terms as follows:
• Fouling problems
• Instrumentation problems
• Utility problems
• Human or procedural errors.

It is worth noting that every refinery and gas plant is unique and consequently operating reliability issues are site specific. However, in general terms the above list of four problem items is common to most locations, regardless of location, operating philosophy and/or corporate culture.

Design impact of SRU and TGTU
In theory, the SRU is a relatively simple process operating unit, but the details in design of the SRU are critical in maintaining very high on-line factors. Some of the key design features of SRUs with higher reliability include:
• Generously sized knock-out drums are provided in the acid gas feed piping to remove free liquids, which helps to minimise process upsets and equipment problems.
• To achieve high sulphur recovery efficiency and reliability in the SRU, the flow measurement and control loops must be accurately tuned and utilise v-notch control valves, size permitting. The air control loop as a minimum should use a combination of feed forward and feedback whereas feed forward control is based on measured feed flow rates and independent feedback control from a tail gas air demand analyser measuring H2S and SO2.
• The acid gas burner must be a high efficiency burner capable of operating at high turndown and ensuring complete contaminant destruction.
• The reaction furnace must include a refractory system that is properly designed and installed for start-up, shutdown and normal operation. The refractory material must be compatible with a H2S, SO2 reducing environment in order to avoid thermal and mechanical degradation. The refractory design should include a properly designed external weather shield to control the furnace metal skin temperature.
• The waste heat boiler should be designed with thin tube sheet, proper strength weld tube/tube sheet connections for maximum tube sheet reliability including an engineered ceramic ferrule tube sheet protection system. Preferred generated steam pressure is 450 psig or 600 psig.
• Two or three catalytic reaction stages follow the thermal stage. Indirect steam reheaters using steam as the heating medium to control the reactor inlet temperatures.