(142b) Oxidative Desulphurisation Using Vortex Based Hydrodynamic Cavitation with Water Dispersed in Organic Phase

Abatement of sulphur from diesel is a growing concern, displayed by implementation of legislation in many countries around the globe, the EU, China and USA have all enforced a sulphur concentration limit of 10 – 15 ppm for diesel ^[1]. To refine diesel to these standards, hydrodesulphurisation (HDS) the industry standard, is being operated at extreme conditions, (100 – 200 atm, 320 – 400 ^oC) for several cycles, significantly driving up cost as consumption of H₂ and MoS₂ catalyst is augmented ^[2]. Even with these conditions HDS struggles to reduce the sulphur concentration under 30 ppm, considering this coupled with the cost of an additional HDS unit, (approx. $50 – 80 million) an alternative process is widely sought after ^[3].

Thus, various other desulphurisation technologies are currently being researched and developed: Adsorptive Desulphurisation (ADS), Bio-Desulphurisation (BDS), Extractive Desulphurisation (EDS) and Oxidative Desulphurisation (ODS) are the four of the main processes under discussion and test. Of these, ODS has been identified as the more effective process from a performance and economical viewpoint, due to mild operating conditions; along with usage of common refinery equipment and complimentary chemistry to HDS ^[3].

ODS traditionally uses an oxidant/catalyst often coupled with an extractant to aid in separation of Sulphur containing compounds from fuels. Ultrasound Assisted Oxidative Desulphurisation (UAOD) has received attention from several research groups, consisting of exposing the reaction mixture to an ultrasonic wave which enhances the formation of OH· due to inception of cavities via ‘Acoustic Cavitation’ (AC). AC is known to intensify a process by inducing micro-bubbles or cavities which develop extreme localised conditions (> 10,000K & 1000 atm) following bubble growth and collapse, forming the aforementioned radicals which dissociate water molecules existing in the liquid medium ^[4].

AC has successfully enhanced ODS on a laboratory scale, however, there are serious issues with scale-up and associated cost limitations ^[5]. Another form of cavitation, ‘Hydrodynamic Cavitation’ (HC), demonstrates high potential to induce cavitation in a more energy efficient manner. HC exhibits extreme potential due to its effectiveness without the use of external oxidant or catalyst.

Research carried out by Suryawanshi et al. ^[6] have shown that HC can effectively remove ≥ 99% of refractory sulphur compounds from both model oil and diesel when low organic volume fractions (2.5%) are instigated. Their work also investigated important parameters such as identification of cavitation inception point, influence of pressure drop, initial sulphur concentration, volume fraction of organic phase and effect of model-oil species/sulphur species amongst others. The concept exhibits high potential since it does not use any external oxidant or catalyst. This work utilises only a small volume fraction of organic phase suspended in aqueous phase. Considering the large volumes of transportation fuels, this approach may demand excessively large reactors. In this research, we have developed the concept of using hydrodynamic cavitation for oxidative desulphurisation further by operating it as aqueous phase dispersed in organic phase. This approach has a potential to significantly improve the overall productivity.

The process of cavitation in a two-phase system with aqueous phase suspended in an organic phase will be investigated systematically. Model oil composed of a heavy hydrocarbon, such as dodecane, will make-up the organic phase. Model sulphur compounds thiophene, dibenzothiophene (DBT) and 4,6 – dimethyldibenzothiophene (4,6 – DMDBT) are considered as the sulphur compounds in this work as they have been identified as key S containing compounds in transportation fuels.

Experiments will be carried out for investigating:

• The influence of pressure drop, volume fraction of aqueous phase and initial sulphur concentration.
• Effect of suspended aqueous phase on cavitation inception, yield and collapse intensity.
• Overall desulphurisation efficiency for the above parameters.

Benchmarking of results will be carried out and compared to a batch system treated by AC, utilising an acoustic horn. The AC process will be carried out utilising a fluid volume of 200ml, a frequency set to 20kHZ, amplitude of 80% and reaction time of 1 hour, whilst maintaining a temperature beneath 50^oC. All results will be analysed by an Agilent 7890A GC with FPD and HP – 1 (30m x 0.32mm x 0.3mm) column.

Cavity dynamics models will be adapted for the liquid-liquid dispersed systems. The cavity dynamics models will be used to interpret the experimental results. This will greatly facilitate further development and optimisation of oxidative desulphurisation using hydrodynamic cavitation.

A main feature of the research is utilisation of a 3D-printed vortex-based cavitation device. This design shields the walls from collapsing cavities and offers significantly better performance than conventional hydrodynamic cavitation devices (Surywanshi et al. ^[7]). This design incorporated into this research aims to continue development of a highly effective oxidative desulphurisation process by bridging the gaps in fundamental understanding of cavitation in multiphase flow and its influence on ODS. The results presented here will provide a sound basis for further work on development of optimal and industrially sustainable process oxidative desulphurisation process.

References

1. https://www.dieselforum.org. (2016). Diesel Sulfur Limits Worldwide and the Need for ULSD. [online] Available at: https://www.dieselforum.org/policyinsider/diesel-sulfur-limits-worldwide....

2. Babich, I. (2003). Science and technology of novel processes for deep desulfurization of oil refinery streams: a review⋆. Fuel, 82(6), pp.607-631.

3. Ibrahim, M., Hayyan, M., Hashim, M. and Hayyan, A. (2017). The role of ionic liquids in desulfurization of fuels: A review. Renewable and Sustainable Energy Reviews, 76, pp.1534-1549.

4. Ja'fari, M., Ebrahimi, S. and Khosravi-Nikou, M. (2018). Ultrasound-assisted oxidative desulfurization and denitrogenation of liquid hydrocarbon fuels: A critical review. Ultrasonics Sonochemistry, 40, pp.955-968.

5. Anderson, K.; Atkins, M.P.; Borges, P.; Chan, Z.P; Rafeen, M.S.; Sebran, N.H; van der Pool, E.; Vleeming, J.H. Economic analysis of ultrasound-assisted oxidative desulfurization, Energy Source, Part B: Economics, Planning and Policy, 2017, 12 (4), 305–311.

6. Suryawanshi, N., Bhandari, V., Sorokhaibam, L. and Ranade, V. (2016). A Non-catalytic Deep Desulphurization Process using Hydrodynamic Cavitation. Scientific Reports, 6(1).

7. Developing techno-economically sustainable methodologies for deep desulfurization using hydrodynamic cavitation./ Suryawanshi, Nalinee B.; Bhandari, Vinay M.; Sorokhaibam, Laxmi Gayatri; Ranade, Vivek V. In: Fuel, Vol. 210, 15.12.2017, p. 482-490.