On novel processes for removing sulphur from refinery streams

Abstract

This review discusses some of the processes that have been, or are being, developed as an alternative/addition to present-day hydrodesulphurization processes that are based on (selective) heterogeneous catalysis. The technologies applied in these alternatives range from reactive adsorption, oxidative routes (especially for diesel) and other chemical conversion methods, to “simple” physical separation methods (non-destructive adsorption, extraction, etc.). It is concluded that it appears that for the time being, as long as sulphur levels of 10 ppmw or slightly below are aimed at, the classical hydrotreating options and their off shoots still hold the field of transportation-fuel desulphurization, although a few of the possible alternatives do have achieved commercial status in the gasoline area. If, on the other hand, S in product levels should be as low as <1 ppmw, then polishing processes will come into their own.

Introduction

Recent developments in environmental legislation are inexorably moving us to a world of zero-sulphur, i.e., in this respect spectroscopically pure, transportation fuels. For the time being, “zero-sulphur” means <10 ppmw S, but it is not excluded that the upper limit will in time come down even further. It is also to be noted that 10 ppmw is not de rigueur everywhere yet, but this is certainly only a question of time.

In the wake of this legislation, especially in the case of gasoline desulphurization, quite a number of novel process options have been created, or revived, both catalytic and non-catalytic ones, some of which have indeed been successfully commercialized. The subject has been ably reviewed by Babich and Moulijn [1], Song and Ma [2], [3] and – for gasoline only – Brunet et al. [4]. Since after the first flurry of novelty creation things have quieted down to a certain extent, we will not go over the field in extenso again, but try and limit ourselves to a brief overview of ongoing developments.

Even a cursory glance at the list of ideas that have been canvassed to address the zero-sulphur issues will show that most of them have not (yet?) been implemented, i.e., not progressed beyond the pilot-plant stage, if indeed they did reach that stage. This is of course to be expected in this Darwinian world. Indeed, for a given design to be commercialized, it needs to meet quite a stringent set of criteria, some of the more important of which include:

• Capital cost: e.g., the less unit operations (less pieces of equipment) the better, no expensive materials; preferably a single product stream for diesel.

• Operation cost: e.g., minimize the consumption of hydrogen or expensive chemicals, minimize the generation of waste streams (esp. dilute ones).

• Product volume should be high, e.g., >99%.

• Process cycle life should be reasonably long, e.g., four plus years in general, and five plus years for FCC gasoline post treatment units in order to match the FCC cycle length.

• Technical complexity should be avoided as much as possible: e.g., no difficult to operate units (minimizing down time), no difficult catalyst(s) (tolerant of upsets).

• Feed flexibility.

• Overall value to refiner.

To be a possible winner, a particular process design needs to look good on all counts, and this one can easily imagine not being a trivial matter.

The next two sections will summarize the developments in the areas of gasoline, and diesel desulphurization, respectively, where the emphasis will be on the non-catalytic routes, after which some concluding remarks will be made.