Nature at its finest: part one
Gas desulfurisation is particularly exacting for oil and gas projects with small to medium sulfur quantities. Through its THIOPAQ O&G technology, Paqell BV, a joint venture between Paques BV and Shell Global Solutions International BV, offers an attractive solution in this specific application area. The THIOPAQ O&G process distinguishes itself by integrating gas desulfurisation and sulfur recovery in a single unit. This provides the advantageous situation where the sour feed gas arrives straight from the source (at either low or high pressure), or indirectly downstream from an amine unit.
The THIOPAQ process, driven by the biotechnological conversion of bisulfide (HS-) to elemental sulfur (S8), was initially developed for sweetening H2S-containing biogas streams from anaerobic wastewater treatment units.1 Paques BV commercialised this process from 1991 onwards; today, over 200 THIOPAQ installations are operational worldwide. The THIOPAQ O&G process is a spin-off of this technology, tuned for the oil and gas industry. This article discusses the most recent breakthroughs relating to this technology.
Advanced bio-desulfurisation
The removal of H2S from natural and synthetic hydrocarbon gas streams traditionally takes place through the application of physicochemical processes, such as an amine process followed by a Claus unit, with the optional inclusion of a tail gas treatment unit. These processes typically operate at high temperatures and are costly, particularly in small scale applications, such as sulfur loads of less than 50 tpd. By contrast, the THIOPAQ O&G technology is characterised by its low costs for installation, operation and maintenance, as well as its reliability and simplicity.
The technology combines H2S removal with sulfur recovery in one process. What makes it unique is that it applies a living biocatalyst to oxidise HS- to elemental sulfur. This biocatalyst, a mixed population of sulfide oxidising bacteria, is fast growing and highly resistant to varying process conditions. The used bacteria are naturally occurring organisms, not genetically modified. In contrast with physicochemical processes, the microbiological conversions in the process proceed at ambient conditions. In addition, the process does not require addition of chelating agents, nor does it produce hazardous waste streams.
The overall process consists of three integrated, consecutive process sections: absorption, regeneration and sulfur recovery (Figure 1). In the absorption section, feed gas (not restricted to a minimum or maximum H2S level) is contacted counter currently with a mild alkaline carbonate/bicarbonate buffer solution at the desired pressure (up to 80 bara), through which H2S is absorbed with limited CO2 co-absorption. The ‘rich’ solution, containing the chemically bound HS-, is directed to the bioreactor, in the case of a unit with a pressure >4 bara, via a flash vessel. The treated gas from the absorber is routed through a treated gas knockout vessel before further processing. For high pressure gas, treated gas specifications of <4 ppmv H2S are reached. For low pressure gas, <25 ppmv of H2S is the guaranteed level.
Figure 1. General flow scheme of the THIOPAQ O&G technology.
In the regeneration section (the bioreactor), the ‘rich’ solution is mixed with air. Consequently, most of the HS- is oxidised into S8, while part of it (typically around 5%) is oxidised to sulfate (SO42-). The sulfur produced in the bioreactor remains in the solution as suspended solids. The vent air from the bioreactor typically contains <1 ppmv H2S and is usually released into the atmosphere from a safe location.
A slipstream from the bioreactor is directed to the sulfur recovery section, first to a gravity settler. The concentrated sulfur slurry from the bottom of the sulfur settler is pumped to a dewatering unit, in which water is removed to achieve about 70 wt% solids (95 - 98 wt% sulfur). The sulfur dewatering unit is typically either a decanter centrifuge or a filter press. Figure 2 shows a picture of a typical sulfur cake. The filtrate is collected and bled from the system for conductivity control, SO42- removal, or recycled back into the process.
Figure 2. Typical sulfur cake.
Another distinctive trait of the process is the fact that the produced sulfur, also known as biosulfur, has superior properties. Biosulfur is hydrophilic and, hence, does not show any of the hydrophobic behaviours that are typical for chemically produced sulfur, typically formed in Claus and/or redox processes. This has a positive effect on the process operation, as the solution behaves like a stable suspension and exhibits no clogging or other undesired occurrences.Biosulfur can be used as the basis for a range of new agricultural products designed to act as liquid fertilisers and liquid fungicides. The very small particle size adds to the appeal since it guarantees an even distribution over crops, as well as easy absorption by plants and soils. If so desired by the end user, biosulfur can also be melted to obtain conventional 'Claus specification' sulfur.
Read part two of this article here.
Written by Gijs van Heeringen, Bob van de Genderand Jan Klok, Paqell BV, the Netherlands. This is an abridged article taken from the April 2016 issue of Hydrocarbon Engineering. Subscribers can view the issue in full by logging in.
References
- VAN HEERINGEN, G., and VAN DIJK, J. H., Shell-Paques Desulfurization Process Matured into Breakthrough Technology, Touch Briefings, 2008.
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/01042016/nature-at-its-finest-part-one-2915/
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