Gas 2024 Issue

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 fur - nace, as shown in Table 1 . Depending on SRU design, the sudden occurrence (or sudden disappearance) of hydrocarbons can cause opera- tional issues. The potential impact can even affect bypass recovery processing steps with potentially drastic conse- quences. 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 con- ditions, 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 fre- quent 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 effec - tive. 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 explo- sive limit [LEL] 3.25 vol%). SO₂ should be measured to pro - vide an early warning of sulphur fire. A smouldering sulphur fire is not uncommon; exposed iron pipe reacts with sulphur from pyrites, a slow but exo- thermic reaction that can result in an ignition source. For example, with H₂S as the fuel source and pyrophoric fire as the ignition source, waiting only for oxygen will make the fire triangulate. At the AT3 tail gas analyser, also called the air demand analyser, the entire process control of the modified Claus process is guided by this measurement. The leading com- ponents to be measured here are H₂S and SO₂. Their components can be of interest but do not require process control. Taking a step back, what does the process control

Ratio of O₂ needed per mole HC compared to moles of H2S

Compound moles O₂ per mole HC

Ratio of O₂ needed per mole HC compared to mole of H₂S

Methane

4 7

Ethane

3.5

Propane Butane Pentane Hexane

10 13 16 19

6.5

9.5

Table 1

The challenges are: • Understanding of the environmental impact of each sin- gle instrument. • Knowing the potential improvement of using the best instrument combination. • Keeping the instruments in operational conditions. • Mitigating upset conditions by understanding ‘unex- pected’ 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 composi - tion, 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

Tail gas

Claus beds & condensers

Burner

Reaction furnace

Acid gas

To tail gas clean-up unit or incinerator

AT- 301

FT- 201

Main air

Trim air

FV-101

FV-102

Sulphur

FT- 101

FT- 102

Combustion air

Figure 2 Feedback plus ratio control

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