Oxidative desulfurization represents an alternative route to diesel desulfurization that can complement HDS chemistry. An ODS unit—which operates under milder conditions (lower temperature and pressure) and requires no hydrogen—can be integrated with a conventional HDS unit to improve the overall economics.

ODS can easily convert difficult-to-desulfurize components under low temperature and pressure conditions to form sulfur-containing sulfones. The sulfones are highly polar compounds and are easily separated from the diesel product by either extraction or adsorption.

The main research trend in ODS has focused on the development of oxidation catalysts. Thus, comparatively less attention has been paid to the separation of sulfone from diesel. In this study, we focused on not only sulfone adsorption using adsorbents to produce ULSD but also the regeneration of the adsorbent to organize a continuous sulfone separation process. We also suggested a new process scheme for sulfone separation with energy efficiency by the addition of another adsorption step, in which the adsorbent was prepared by CO₂ activation.

—Lim et al.

The new KIER process consists of:

1. the separation of sulfone in diesel (S concentration > 150 ppm) by adsorption on silica and the subsequent regeneration of silica using a polar solvent;

2. the separation of sulfone in methanol by adsorption on activated carbon and the subsequent regeneration of activated carbon using a non-polar solvent; and

3. the distillation for the recovery of a non-polar solvent.

Initially, the team had looked at a process comprising an adsorption step with silica for the separation of sulfones from diesel and subsequent distillation for the recovery of the regeneration solvent, which removed adsorbed sulfones in silica and resultant separation of sulfones. They found this to be a simple way to organize a continuous sulfone separation process for the production of ULSD.

Methanol and acetone were found to be good solvents for the regeneration of the sulfone-saturated silica adsorbent. In the distillation step, however, they found that the separation of sulfones from methanol or acetone consumes a lot of energy, because the vaporization heats of methanol and acetone are 2−3 times higher than those of hydrocarbons.

Hydrocarbons with low vaporization heat, such as n-butane or n-pentane, seemed to be suitable solvents, but those did not regenerate the sulfone-saturated silica adsorbent effectively.

To combine the advantages of a polar solvent (silica regeneration) and that of a hydrocarbon (low heat of vaporization), the team modified the initial, simpler process scheme with the addition of another adsorption step between the adsorption and distillation steps.

In the second adsorption step, the activated carbon adsorbent prepared by CO₂ activation of a carbon molecular sieve (CMS-4K) selectively adsorbed sulfones in the polar solvent (methanol) and easily released sulfones by hydrocarbon (n-butane) washing. The silica adsorbent was successfully regenerated by methanol (polar solvent) and sulfones were separated with low energy consumption because n-butane, with low vaporization heat, was the solvent in the distillation step.

They found that the two-step adsorption process was able to substantially reduce the energy consumption during the distillation because the heat of vaporization (320 kJ/kg) of n-butane is much lower than that (1104 kJ/kg) of methanol. The energy consumption in the distillation step to produce 1 kg of ULSD for the initial scheme was more than twice as high (1413 kJ/kg ULSD vs 640 kJ/kg) as that for the two-step adsorption process.

Resources

• Sam Mok Lim, Jong-Nam Kim, Jihye Park, Sang Sup Han, Jong-Ho Park, Tae Sung Jung, Hyung Chul Yoon, Sung Hyun Kim, and Chang Hyun Ko (2012) Energy-Efficient Sulfone Separation Process for the Production of Ultra-low Sulfur Diesel by Two-Step Adsorption. Energy & Fuels doi: 10.1021/ef201964v

• Ron Gatan, Paul Barger, Visnja Gembicki, Agostino Cavanna and Daniele Molinari (2004) Oxidative Desulfurization: A New Technology For ULSD. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 49 (2), 577