Hydrodesulfurization Process and Reaction Engineering Aspects

Hydrodesulfurization is technically performed either in the gas phase or in a trickle bed reactor, that is, in a three-phase system with gaseous H2, liquid oil, and a solid catalyst. Trickle bed reactors are more complicated with respect to hydrodynamics and partial wetting of the catalyst that is unwanted but has to be considered, for example, for reactor modeling. Based on the above given reaction conditions, we can easily determine which feedstock has to be desulfurized in a trickle bed reactor and which feedstock completely forms a gas phase (vapor). The molar ratio of hydrogen (fresh and recycled) to oil (for a mean carbon number of about 20) is about two. Thus the partial pressure that the oil has to build up to be completely vaporized is about 3 of the total pressure. 

Based on the normal boiling point T oi, (at 1 bar) of a hydrocarbon, we can estimate the vapor pressure at the temperature of hydrodesulfurization (Thds) by the vapor pressure equation, Eq. (3.1.44), and Trouton s rule [Eq. (3.1.46)]  

Trickle-bed reactors have the following problems (i) In general they are complicated with respect to the mass transfer (gas-liquid and liquid-solid). In addition, proper hydrodynamic conditions (wetting of catalyst, distribution of gas and liquid phase) are hard to realize. Scale-up is therefore difficult and pilot plants are often still needed, (ii) The amount of H2 fed to the reactor - above all in the case of a second stage deep desulfurization of an already hydrotreated fuel - is much higher than the amount chemically needed. Therefore, a costly recycle compressor for the (unconsumed) hydrogen has to be installed. 

There are several options to improve (deep) desulfurization of fuels (i) better catalysts for deep HDS based on noble metals are on the market and still the focus of research (e.g., Fujikawa et al., 2001 Navarro et al., 1996 Reinhoudt et al, 1999 Robinson et al, 1999) (ii) alternative processes have been developed like the adsorption process SZorb (Sughrue, 2003) and (iii) extraction of organic S-compounds by novel extraction media like ionic liquids is now also considered (Boesmann et al, 2001 Esser et al, 2004). 

For deep desulfurization, trickle-bed reactor technology may also be improved by using a two-phase reactor. Thereby, the oil is externally pre-saturated with H2 and only the liquid phase is passed over the catalytic fixed bed. To discuss the 

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