Hydrodesulfurization process variables

The hydrodesulfurization process variables (Chapter 5) usually require some modification to accommodate the various feedstocks that are submitted for this particular aspect of refinery processing. The main point of the text is to outline the hydrodesulfurization process with particular reference to the heavier oils and residua. However, some reference to the lighter feedstocks is warranted. This will serve as a base point to indicate the necessary requirements for heavy oil and residuum hydrodesulfurization (Table 6-6). 

It should be noted, however, that this reaction sequence may be different from what may actually be occurring in the reactor. The reactions proceed at different rates depending on the process variables. Hydrodesulfurization of complex sulfur compounds such as dibenzothiophene also occurs under these conditions. The desulfurized product may crack to give two benzene molecules ... 

In summary, protection of the catalyst and the reactor is essential if shutdowns are to be minimized, and this is particularly the case when the heavier feedstocks are used in the hydrodesulfurization process. Some measure of protection from the asphaltic constituents can be achieved by removal of these materials by means of a deasphalting step (Chapter 7). The overall relative merits of any particular total processing scheme should be assessed in terms of feedstock properties, product yields, process variables, and last but not least, process economics. 

The efficiency of the hydrodesulfurization process is measured by the degree of sulfur removal or, in other words, by the yields of sulfur-free products. However, there are several process variables (Table 5-8) that need special attention as any one of these variables can have a marked influence on the course and efficiency of the hydrodesulfurization process. 

The objective of the present work is to evaluate the effect of a wide range of process or reaction variables— reaction temperature, hydrogen partial pressure, catalyst loading, and reaction time—on hydrodesulfurization and hydrogenation of filtered liquid product (coal-derived liquid) obtained from the coal dissolution stage in the presence of a commercial presulfided Co-Mo-Al catalyst. The selectivity for desulfurization over hydrogenation (Se) is used to rate the effectiveness of the above mentioned process variables. Se is defined as the fraction of sulfur removal per unit (g) of hydrogen consumed, that is,... 

All commercially produced gasoline blends which are intended for vehicle use contain organo-sulfur compounds in concentrations ranging from several parts per million (ppm) to 1000 ppm [1]. These compounds are present in the crude oil feedstocks [2] and can be partially removed by a hydrodesulfurization process in refining [3,4]. Removal of nearly all of the sulfur from gasoline is not always economical due to a number of factors including the variability in feedstock properties and the cost of deep hydrodesulfurization (HDS)... 

Allied with design and preparation, catalyst testing is the exploratory screening of candidate catalysts. This phase does not yield either kinetics or process variables but merely ranks performance. Bench reactors used should be as simple and rapid as possible, for many samples are usually tested. For ease in operation and interpretation, model compound reactions are helpful. Thus, for example, cumene dealkylation is a model for catalytic cracking, and thiophene hydrogenolysts for hydrodesulfurization. Care must be taken to ensure that the model system does indeed parallel process performance. 

Different catalysts were used when the Claus process was reintroduced in refineries in 1940-1950. Bauxite, for example, which was already available in refineries to hydrodesulfurize straight-mn naphthas, is a variable mixture of gibbsite and boehmite with iron and silica impurities. When calcined to activate the alumina, it is converted to a catalyst with about 1-12% ferric oxide supported on y-alumina. Bauxite catalysts were successfully used in the Claus process, giving a sulfur conversion greater than 90%. ... 

---