Apr-2002

Rigorous hydrotreater simulation

The authors describe an integrated approach to dealing with the complexities of producing ultra low sulphur diesel, involving analytical support and process research as well as computer simulation

Michael C Hu, KBC Advanced Technologies
Zbigniew Ring, Jenny Briker and Mure Te, National Centre for Upgrading Technology

Article Summary

The need to be able to assess the process alternatives for future production of low and ultra-low sulphur diesel (ULSD) has resulted in the development of a rigorous hydrotreater simulator, capable of predicting feedstock and process condition effects on sulphur removal below 10ppmw. How low the sulphur level will eventually go throughout the world is still a subject of debate, but the trend points to a near zero sulphur requirement.

Although the technologies required to produce low and ultra-low sulphur gasoline and diesel are somewhat different, a significant increase in overall refinery hydrotreating capacity – or, more specifically, the degree of hydrodesulphurisation – is inevitable. The additional sulphur reduction will have to be achieved either by feed hydrotreatment for catalytic cracking conversion units or hydrofinishing of final fuel products.  This article is concerned with  the subject of ULSD.

There is no common strategy for refiners to plan their future capital investment. They must select the most economical process option and plan their investment strategy based on available technical know-how, refinery complexity and local market economics. Usually, refineries will work closely with process licensors and catalyst vendors to accomplish this task.  Nevertheless, it is extremely useful to have a process simulator capable of predicting hydrotreater performance under ultra-low sulphur mode of operation for various feedstocks and process conditions.

The development of this simulator requires a good understanding of diesel feed and product characterisation, comprehensive and realistic process data, and rigorous kinetics modelling.  Recently, Canada’s National Centre for Upgrading Technology (NCUT) and KBC Advanced Technologies, based in California, have been cooperating in analytical research, pilot plant testing, and process modelling to provide  an integrated tool for assessing alternative process options for future low and ULSD production. 

After a brief literature review, the analytical and hydroprocessing pilot plant facility at NCUT is presented here, along with selected results of pilot plant testing in terms of sulphur type analysis.  The technical basis of KBC Profimatics’ hydrotreater model, HTR-SIM is described in detail.

Technical issues
To meet the current 500ppmw diesel sulphur specification, refineries have successfully increased their hydro- desulphurisation (HDS) capacity by either installing more active HDS catalysts, raising reactor temperature (at a cost of shorter catalyst cycle life), lowering reactor space velocity (eg, dense bed loading, lower throughput, or adding a new reactor), or improving reactor internals for good catalyst utilisation. 

Adjustments in other process variables, such as increased hydrogen partial pressure and reduced hydrogen sulphide concentration, in the treat gas recycle loop have also become more important.  To reduce diesel sulphur content from 500ppmw to 15ppmw or less, substantial further increases in operating severity and reactor volume will be unavoidable. For example, Lamourelle and co-workers [Lamourelle, McKnight and Nelson; Clean fuels: route to low-sulphur low aromatic diesel; NPRA annual meeting, 2001] reported a required 200 per cent increase in catalyst volume to switch from 500ppmw to 10ppmw S. 

Selection of the right type of HDS catalyst, cobalt/molybdenum (Co/Mo) versus nickel/molybdenum (Ni/Mo), and the catalyst from within these two groups that is most suitable for the feedstock at hand, also become much more important for ultra-low sulphur production. Some authors have reported the so-called sulphur “wall effect” [Mayo et al; Elegant solutions for ultra-low sulphur diesel; NPRA annual meeting, 2001. Nash; Meeting the challenge of low aromatics diesel; NPRA annual meeting, 1989].

This effect limits the lowest sulphur level in the diesel stream that is achievable by raising hydrotreater reactor temperature. The lowest achievable sulphur level depends on the catalyst type and hydrogen partial pressure. Under realistic hydrotreater conditions, it could be as high as 20ppmw. The wall effect can be explained by the hydrodesulphurisation mechanism.

It is well known that there are two distinct routes for sulphur removal by hydrotreating (Figure 1). The first is direct hydrogenolysis. Almost all the sulphur present in the forms of mercaptans, sulphides, disulphides and thiophenes, as well as a majority of benzothiophenes and unsubstituted dibenzothiophenes, is removed by this route.

The relative reactivity of the various sulphur compounds differs significantly and within each compound group. It has also been reported that the reactivity of sulphur compounds also decreases monotonically with true boiling point [Sau, Narasimhan and Verma; Studies in surface science and catalysis, 1997].

The Co/Mo HDS catalysts are most effective in removing sulphur via this route, even under modest pressures. There is a small, positive effect for higher hydrogen partial pressure and a negative effect of hydrogen sulphide in the recycle gas.

The second route, particularly effective with Ni/Mo catalysts, requires partial hydrogenation of aromatic rings in the dibenzothiophene structure prior to the removal of sulphur by hydrogenolysis.  This route is much slower than the direct hydrogenolysis route.  It is strongly influenced by hydrogen partial pressure and subjected to thermodynamic equilibrium limitation. 

The sulphur “wall effect” is believed to be the result of the equilibrium limitation at high reactor temperature. Due to steric hindrance, some of the substituted dibenzothiophenes, particularly those with alkyl-substituents at the 4 and 6 positions, must be desulphurised through the hydrogenation route.  One of the most unreactive sulphur compounds in the diesel range is the 4,6-dimethyl-dibenzothiophene.  When the diesel sulphur level is lowered to below 100ppmw, almost all of the remaining sulphur belongs to the dibenzothiophene class.