PTQ Q3 2024 Issue

Sweet gas

Vent

Regenerated solution

Chemical addition

Oxidiser

Absorber

Recovered solution

Sour gas

Rich solution

Slurry

Sulphur lter

Air

Sulphur cake

Figure 1 Block flow diagram of a liquid redox system

associated with the spent material. The solid material can plug downstream equipment and cause further operating issues, possibly plugging equipment to the point that it cannot be used. Additional developments have been made in this industry to minimise these issues, but they have not been completely removed. Caustic treatment for H₂S removal from gas streams is very effective and operates the same as discussed previ- ously in the liquids treating section. The main limitation of using caustic, however, is that as the mass flow rate of H₂S increases, the cost can become prohibitive. For gas streams with higher levels of H₂S than are eco - nomical for scavengers to be cost-effective, liquid redox processes are an excellent option. These are typically more cost-effective from 1 TPD of sulphur removed up to approximately 20 TPD of sulphur removed. Liquid redox can process any kind of vapour stream, from natural gas to syngas to contaminated air. As a result, the technology has been applied in a wide variety of applications to achieve 99.9%+ H₂S removal in a single stage. A simplified block flow diagram of this type of system is shown in Figure 1 . In the absorber, H₂S contacts the liquid redox solution and is converted to elemental sulphur and water. The solution is then routed to the oxidiser, where the catalyst is regenerated using air. The regenerated solution is pumped to the absorber. A slip stream solution from the oxidiser is routed to a sulphur filter to produce sulphur cake. Depending on the type of sulphur filter chosen, a settler may be employed to concentrate the solution into a slurry with higher solids content. This can improve the operation of the sulphur filter. The liquid redox process offers several potential advantages: • H₂S removal efficiencies of 99.9%+ in a single stage. • Effective over a wide range of H₂S concentrations. • Ability to run smoothly despite large fluctuations in feed gas rate and gas composition. • Near ambient temperature operation (low energy and inherently safe). • Capable of operating at pressures from atmospheric to greater than 1,000 psi. • Production of sulphur cake or molten sulphur, depending on quantities and resulting economics. The keys to high operability of liquid redox systems include:

• Strategies to keep solids moving and prevent plugging. • Full understanding of salt formation chemistry and disposition. • Highly reliable sulphur filter operation with retention of solution chemicals while also ridding the system of unwanted salts. Biological processes are another solution that can be used in a similar space to liquid redox. These systems use an alkaline solution to remove H₂S from a feed gas stream and then convert the sulphide to elemental sulphur using bacte- ria. The main advantages of this type of system include not having to deal with solid sulphur issues and no requirement to purchase the required catalyst and chemicals needed in the redox systems. Disadvantages include maintaining a bacteria culture tuned to the feedstock and upsets that may cause downtime if the bacteria are destroyed. Other options are readily available depending on a cli- ent’s inlet contaminants, flow rates, and locations, including combustion or other solutions. Each solution has its own set of benefits or challenges. Conclusion The world has become more cognisant of the damage result- ing from sulphur emissions, which has prompted the push to remove and treat contaminants that impact the environment. As sulphur is a primary culprit, sulphur removal and treat- ment are key requirements for ensuring contaminants are removed from liquid and gas streams and the environment is protected from SO2 emissions and acid rain. There are many solutions available, and each should be considered carefully based on operating and capital costs as well as actual capa- bilities and final specifications that can be achieved.

References 1 Pyburn et al., 1978. 2 Nielsen et al., 1997. 3 Mokhatab et al., 2015.

Cyndie Fredrick is Chief Executive Officer at Merichem Technologies, with more than 25 years of experience in the downstream and mid- stream oil and gas sectors. She has a background in technology licensing, EPC, midstream, downstream, and owner engineering and operations. She holds a bachelor’s degree in chemical engineering from the University of Arkansas, and an MBA from Rice University.

PTQ Q3 2024