Special Report: Innovative H2S methane reforming
Sawsan Ali from the American University of Sharjah looks at potentially ground-breaking territory in the treatment of hydrogen sulphide
In the future, producing hydrogen from hydrogen sulphide (H2S) in sulphur recovery units using an innovative H2S methane reforming process, in a similar fashion to the well-known water-gas shift reaction between water and methane, could be possible. Preliminary research has been based on a sour, highly rich H2S stream from an actual amine regeneration unit in a sweetening plant.
While in conventional sulphur recovery units, valuable hydrogen is wasted, methane reforming will allow the recovery of sulphur products and hydrogen simultaneously. This method will in the future be subject to intensive R&D but it has the potential to become a revolutionary idea of simultaneously recovering sulphur from H2S while generating hydrogen as a fuel.
In essence, the proposed design involves cryogenic separation of carbon dioxide and water from the hydrogen sulphide feed, implementation of a membrane reactor and product separation. A membrane reactor assisted in decreasing the required feed temperature for full conversion in the highly endothermic reforming reaction. It was found that carbon dioxide could be separated cryogenically, but further research must be done on new kinds of amine gas absorption units which can achieve a high degree of selectivity of H2S over carbon dioxide (CO2).
Also, the membrane reactor turned out to be quite effective in maximising the conversion, which warrants further examination. The high energy requirement of the procedure also makes the exploration of renewable energy sources worthwhile - such as the use of solar reactors for example.
Overall, the methane reforming process could possibly and ultimately replace the presently crucial and traditional Claus process units which have always been used as the conventional sulphur recovery method. Indeed, this also justifies the need for further exploration especially regarding reaction kinetics and their implementation in a membrane reactor. Additionally, an economic model needs to be generated to evaluate the overall economic feasibility so that the proposed schemes can be further optimised.
The production of hydrogen from hydrogen sulphide in sulphur recovery units, as a potential replacement to the Claus process, was investigated from various different perspectives. The process was analysed regarding the separation of H2S and CO2 from the sour gas leaving the amine unit, the optimum flash gas to hydrogen sulfide feed ratio, the reactor design and operating conditions and, lastly, the separation of sulphur products.
It was ultimately possible to attain a gaseous hydrogen production rate of 28,800 tonnes per year at a purity of 99.7% with full methane conversion. This production rate is quite high. Also, the separation of CO2 from the feed was accomplished almost completely, yielding a 99% purity for the H2S.
Apart from this, as the research focused in more closely, membrane reactor modelling was performed which decreased the required reactor temperature and simultaneously increased the hydrogen yield, when compared to a packed bed reactor. Lastly, energy integration and exergy analysis were conducted.
But, in the investigation methodology, the primary challenge was of course the separation of CO2 from H2S since the handled gas feed was equimolar in both. The separation scheme decided upon was cryogenic distillation. But there were several separation schemes explored throughout the design stage, starting from amine sweetening units through to using special solvents.
Moreover, one of the major obstacles encountered was the large flow rate handled by the entire units. In fact, the 3,018 Kmol per hour sour gas feed is generally considered to be quite high. Nevertheless, it was ultimately seen as crucial to proceed with the large feed levels, since the same feed is usually being directed to the Claus process - which is the method this proposed design project intends to replace.
This large flow rate also affected the number of reactors used, as two membrane reactors, in parallel, was seen as the best design option since membrane reactor modules are restricted by specific sizes, and catalyst weight, which would not necessarily be able handle such large flow rates.
When in fact large flow rates lead to greater equipment sizes, then this affects the capital, utility and operating costs required for operating the whole plant. It is suggested the possible implementation of multiple train configuration to cope with turndown conditions should be studied, as well as for capital expenditure (CAPEX) and operating expenditure implications.
Furthermore, as mentioned, the entire process of hydrogen sulphide-methane reforming is highly energy intensive. Looking to future delivery of this procedure, the employment and utilisation of solar reactors would contribute to developing a more sustainable energy supply.
One of the main accomplishments in this design is also the potential implementation of an innovative type of reactor - the plasma reactor.
Additionally, these plants handle toxic substances like hydrogen sulphide which are sour gases. H2S in particular imposes a dangerous risk of severe corrosion. Therefore, proper material selection must be carried out as part of the detailed design stage considering the requirements for any chemical treatment. Furthermore, the implemented membrane reactors require high thermal and mechanical stability. If a ceramic construction material was considered then this too would contribute to CAPEX costs.
Overall, this trial and study threw up many interesting findings and aspects for further attention and investigation. In general, the whole design project explored innovative schemes for separation and reaction.
Certainly, this research idea cannot be dismissed as it still might be feasible if the market for hydrogen improves, or if environmental consciousness becomes even more important. Dr Naif Darwish, Sawsan Ali, Zeina Jendi , Iman Ustadi , Sana Jaber, Thurayya Ghalayini and Seba Zein Al Abdeen of the American University of Sharjah all contributed to this research.