Ultrasonic Alternative to Hydrodesulfurization
Oil refineries are facing increasingly sulfurous (sour) crude supplies and environmental regulatory pressure on sulfur content of gasoline. At the same time, the costs of conventional hydrodesulfurization (HDS) are rising because of the hydrogen needed. Ultrasonic cavitation treatment is an effective alternative method.
Fossil fuels contain sulfur compounds. These result from the degradation of biological matter containing sulfur during the natural formation of fossil fuels.
Vehicles, such as cars, aircrafts and marine vessels or power plants cause sulfur dioxide (SO2) emissions as a result of the petroleum fuel combustion. The same sulfur – even in very low concentrations – causes damages to noble metal catalysts in the downstream catalytic reforming in petroleum refineries. Latest environmental regulations require a very deep desulfurization to meet the ultra-low sulfur diesel (ULSD) specifications.
Hydrodesulfurization (HDS) is the standard catalytic process for the removal of sulfur from petroleum products. In this process, the sulfurous fractions of the crude oil are mixed with hydrogen and a catalyst to react to hydrogen sulfide. Typically, the catalyst consists of an alumina base impregnated with cobalt and molybdenum. As the oil supplies get more sour, higher pressures and alternative catalysts are required for the desulfurization. Recalcitrant aromatic sulfur compounds (e.g. 4,6-dimethyldibenzothiophene) cannot be removed using hydrodesulfurization, due to their low reactivity (see Deshpande 2004).
An alternative to hydrodesulfurization is the ultrasonically assisted desulfurization. The exposure of liquids to ultrasonic waves of high intensity causes acoustic cavitation. This is the formation and subsequent violent collapse of small vacuum (cavitation) bubbles. Locally, extreme conditions arise from the violent collapse of each bubble:
- Temperature: up to 5000 Kelvin
- Pressure: up to 2000 Atmospheres
- Liquid Jets: up to 1000km/hr.
Such conditions promote a better surface chemistry of catalysts by enhanced micro-mixing. In particular the high local temperatures change the chemical reaction kinetics of the desulfurization process. (see sonochemistry). This effect allows for alternative – less expensive – catalysts or alternative desulfurization chemistry to be used. Deshpande et al. (2004) investigates an oxidative system composed of sodium carbonate and hydrogen peroxide in a biphasic system of diesel and acetonitrile. Ultrasonication was applied to the biphasic system. The study achieved a reduction of the DMDBT content by more than 90% in the diesel samples.
Hielscher is the leading supplier of high capacity ultrasonic devices, worldwide. As Hielscher makes ultrasonic processors of up to 16kW power per single device, there is no limit in plant size or processing capacity. Clusters of several 16kW systems are being used the processing of larger volume flows. Industrial fuel processing does not need much ultrasonic energy. The actual energy requirement can be determined using a 1kW ultrasonic processor in bench-top scale. All results from such bench-top trials can be scaled up easily.
If required, FM and ATEX certified ultrasonic devices (e.g. UIP1000-Exd) are available for the sonication in hazardous environments.
Ultrasonication is an effective processing technology. Ultrasonic processing costs result mainly from the investment
for ultrasonic devices, utility costs and maintenance. The outstanding energy efficiency (see chart) of Hielscher ultrasonic devices helps to reduce the utility costs.
Deshpande, A., Bassi, A., Prakash, A. (2004): Ultrasound-Assisted, Base-Catalyzed Oxidation of 4,6-Dimethyldibenzothiophene in a Biphasic Diesel-Acetonitrile System; in: Energy Fuels, 19 (1), 28 -34, 2005.
Mei H., Mei B.W., Yen T.F. (2003): A new method for obtaining ultra-low sulfur diesel fuel via ultrasound assisted oxidative desulfurization; in: Fuel, Volume 82, Number 4, March 2003, pp. 405-414(10), 2003.