Environmental Engineering Reference
In-Depth Information
The improved high performance of HDM, HDM/HDS, and HDN catalysts has also been
marketed by the above-mentioned catalyst companies for residual oil hydroprocessing. The
HDM catalysts are designed to maximize metals (V and Ni) removal from the residual oil
feed. They have large pore volume with balanced amounts of wide pores and mesopores to
enhance the diffusion of the metals-containing large reactant molecules into the active surface
within the catalyst pores and to allow for even more deposition of the removed metals within
the pores. New generation HDM catalysts have a high capacity for storage of the metals
removed from the feed, while retaining high activity and stability for metals removal. They are
used in the front-end reactors, and in effect, they protect the valuable HDS and HDN catalysts
that follow in the second, third and, possibly, fourth reactors, from deactivation by metals
contamination. HDM/HDS catalysts that are used in the middle reactors (second and, possibly,
third) are designed with two functions. First, they remove some of the remaining metals, not
picked up by the front-end demetallization catalysts, and secondly, they have significant
activity for HDS. A third type of catalyst with very high surface area and a sharp narrow pore
distribution is usually placed in the last reactor. This catalyst is known as the tail-end catalyst.
It possesses the highest HYD activity. The major responsibilities of this catalyst are HDN,
hydroconversion and HYD in addition to HDS. New generation tail-end catalysts have higher
stability which is essential for the increased length of cycle at severe operations. By using a
combination of these improved catalysts in multiple reactor residue hydrotreating units, the
on-stream efficiency of the catalyst system has been increased considerably.
In addition to the development of highly active and more stable new generation hydrotreating
catalysts, improvements in the feed distribution in reactors by using better trays, better
catalysts loading and process revamps and optimization have been made in recent years to
improve hydrotreating reactor performance. These improvements have increased run-lengths
significantly and, thereby, reduced spent catalyst waste generation.
3.4 Hydroprocessing Reactors and Processes
The detailed reviews of the commercial and emerging processes used for hydroprocessing of
petroleum feeds were published elsewhere [11,120,121] . Simplified schematics of the
conventional and advanced refineries shown in Figs. 2.4 and 2.10 , respectively indicated the
presence of several catalytic units operating in a hydroprocessing mode on the site of the
petroleum refinery. It is evident that during the transition from the conventional refinery to
advanced refinery, the number of catalytic reactors has been further increased. Properties of
the feeds and those of the anticipated products after hydroprocessing of the former determine
the selection of catalysts and the extent of the process modifications. Thus, for heavier feeds,
revamping or modifications of reactors may not be sufficient, therefore, several reactors
operating in series may be needed to achieve a desirable conversion and quality of anticipated
products.
 
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