Ni/ZnO Nano Sorbent for Reactive Adsorption Desulfurization of Refinery Oil Streams

Introduction

Desulfurization is one of the most important processes in the petroleum refining industry. The conventional measure is by hydrodesulfurization (HDS) where molybdenum or tungsten supported on an alumina carrier with addition of cobalt or nickel is used as catalyst (Babich and Moulijn 2003). For eliminating the detrimental impact of acid rain, many governments have made more stringent regulations controlling the sulfur content in fuels. For example, in the new Europe V standard, the sulfur content in gasoline and diesel is limited below 10 ppm. But the HDS process faces challenge to achieve this objective. Thus other desulfurization technologies are heavily investigated. Among them, reactive adsorption desulfurization (RADS) emerges as a perspective direction, which uses a sorbent to remove the sulfur impurity from oil stream and keeps the sulfur in the sorbent. The great challenge for RADS technology is to develop a sorbent with high desulfurization activity and high sulfur capacity. Nano Ni/ZnO sorbent is found an ideal candidate for this objective, where metallic Ni catalyzes the HDS of sulfur-containing compounds, meanwhile ZnO component as a sulfur acceptor regenerates Ni species from the sulfided to the metallic (Tawara et al. 2000, 2001a).

Ryzhikov et al. (2008) observed that NiO/ZnO can be reduced in situ and shows better RADS of thiophene than pre-reduced counterpart. It was suggested that the H₂ pretreatment results in the formation of Ni-Zn alloy and agglomeration of nano particles, leading the decrease of activity. However, the pre-reduction for NiO/ZnO-SiO₂-Al₂O₃ sorbent improves the desulfurization capability (Fan et al. 2010), which is attributed to the additives of alumina and silica stabilizing the particles (Wen et al. 2012, Meng et al. 2013). Decreasing the size of ZnO nano particles increases the efficient contact between Ni and ZnO particles and enhances the desulfurization ability and sulfur adsorption capacity (Zhang et al. 2012a). But ZnO particles are not stable under the calcination or reaction conditions, the sinter of particles lead a noticeable drop of activity (Bezverkhyy et al. 2008). Thus structure additives, such as diatomite, perlite, attapulgite, silicasol, pseudoboehmite or their mixtures are often added to reinforce the texture of nano sorbent (Shangguan et al. 2013, Zhou et al. 2013).

Babich and Moulijn (2003) proposed a RADS mechanism of Ni/ZnO sorbent. H₂S as a sulfur carrier transfers the sulfur from Ni surface to ZnO bulk. It has been accepted by a lot of authors, but the recent DFT calculation showed that the direct sulfur transfer from Ni site to Zn site has lower energy barrier. This suggests a new mechanism that sulfur may be transferred through the interface between Ni and ZnO phases (Zhang et al. 2012b). The sulfur accumulation in the sorbent leads a gradual deactivation. Regeneration at above 500 ^oC can partly recover the RADS activity. However, the formation of new species, such as Ni₂SO₄, ZnSO₄, NiAl or AlZn spinel, influences the performance of regenerated sorbent (Wen et al. 2012, Meng et al. 2012).

The RADS of Ni/ZnO nano sorbent has been successfully applied in the desulfurization of gasoline. The S-Zorb technology is the representative one. It was first developed by Conoco Philips Petroleum Co. and then was bought out by China Petroleum & Chemical Corp. (Sinopec). This RADS process was demonstrated to possess some valuable features like low hydrogen consumption and low loss in octane number (Khare 1999, Gyanesh 2001). The first commercial S-Zorb unit was run at Borger refinery in April of 2001. And over thirty units have been built and operated by the end of 2014 (Jia et al. 2014).