May-2024

Optimise sulphur recovery plant emissions during unit upset conditions

Knowledge, understanding, and awareness training are essential to maintain process instrumentation for optimal information gathering.

Jochen Geiger, Michael Gaura and Anantha Kukkuvada
AMETEK Process Instruments

Article Summary

Working conditions for refineries are changing, and such changes might be challenging. In several geographical areas, the historical supplier of crude oil has changed, resulting in new crude oil compositions. The method of supply may also have changed from pipelines, with stable composition, towards ocean vessel supply, with different compositions.

Regardless, final product quality from refineries needs to be stable, and quantities need to be ever increasing. At the same time, environmental impacts are being more strictly regulated, increasing monitoring and reporting requirements. Emissions always need to be reduced. In summary, more flexibility from refiners is required.

All of this puts more load and attention on sulphur recovery units (SRUs), which are expected to operate continuously and in an optimised manner. As discussed in several papers, such SRUs require some special attention with regard to safety and operational monitoring.

On its own, elemental sulphur is not an issue. It is one of the most common elements (by mass) on earth and is essential to life. Sulphur is present in living organisms, including humans, and can be used in fertiliser production. It even has medicinal applications. However, sulphur can also be harmful to both humans and the environment. Hydrogen sulphide (H₂S) concentrations of just a few parts per million (ppm) in ambient air can lead to illness and death at sustained levels greater than 100 ppm. Sulphur dioxide (SO₂) is also toxic to humans at very low concentrations but has received significant attention based on its negative impacts on the environment. Specifically, it can destroy vegetation and wildlife and contribute to the production of acid rain.

The basic chemistry of sulphur recovery has been known for more than 90 years. In today’s world, the capacity of modern SRUs can range from tens to thousands of tons of sulphur production per day. The most significant improvements have been made to the overall recovery efficiency.

Although it was sufficient to operate at 80-85% in the 1970s and between 95% and 99% in the 1990s, the current requirement is to operate at recovery efficiency rates of 99.9+%.
Considering that we rely on the same chemistry as 90 years ago, except for tail gas clean-up units, such efficiency improvements only became possible using reliable process instrumentation (see Figure 1).

The challenges are:
• Understanding of the environmental impact of each single instrument.
• Knowing the potential improvement of using the best instrument combination.
• Keeping the instruments in operational conditions.
• Mitigating upset conditions by understanding ‘unexpected’ instrument behaviours.

The following discussion will investigate each sample point and review from a control aspect and the potential environmental impact. When considering AT1 feed gas to the Claus reaction furnace, also known as the feed gas analyser (refer to AT1 in Figure 1), the bespoke feed gas (in refineries) can consist of two different source streams. The first is the acid gas stream, and the second is the sour water stripper (SWS) gas.
While acid gas should be more stable, the SWS is known to be a potential source of upsets. Against this backdrop, we should not fall into the belief that no conditions can cause upset conditions for SRUs originating from acid gas.

Refinery operating conditions
Refineries need to operate their SRUs with both acid and SWS gas. This might not be the case for every single unit (train), but as a general operating condition, it can be taken as given. One of the problems of SWS gas comes from ammonia (NH₃) as part of the SWS stream composition, requiring operation of the Claus reaction furnace at a higher temperature (<1,200°C) in order react and destroy the ammonia before entering the heat exchanger.

The key aspect of any feed gas measurement and control is to mitigate upset conditions. Such upset conditions can be caused by rapid changes in hydrocarbons in the feed gas. We can observe such changes in both streams, acid and SWS gas. Hydrocarbons need a significantly higher amount of oxygen (O₂) than H₂S to react in the Claus furnace, as shown in Table 1.

Depending on SRU design, the sudden occurrence (or sudden disappearance) of hydrocarbons can cause operational issues. The potential impact can even affect bypass recovery processing steps with potentially drastic consequences. In the case where only the tail gas clean-up unit gets bypassed, the overall sulphur dioxide (SO₂) emissions could potentially increase by a factor of 100. Normal SO₂ concentrations after final incineration, in combination with an amine-based tail gas treatment unit (TGTU), are 200-400 ppmv SO₂, which will jump to 10,000/20,000 ppmv in cases of TGTU bypass.

Even when not approaching such dramatic process conditions, it can be said that a closed feed gas control loop can reduce emissions by 10-15% of the overall emission rate. Flaring events can be reduced by 65-95%.1 Considering that sources of hydrocarbon upsets are caused upstream from the SRU, another new challenge will be caused by frequent changes in crude composition, with most refineries not designed for such changes in crude composition.

A reliable understanding of the feed gas composition (H₂S, NH₃, THC, CO₂) will make process control more effective. Fast responses to compositional changes will allow a fast reaction on the acid gas-to-air ratio, resulting in an overall emissions reduction.

The AT2 sulphur pit is a safety measurement where H₂S must be maintained below the explosive limit (lower explosive limit [LEL] 3.25 vol%). SO₂ should be measured to provide an early warning of sulphur fire.