Hydrodesulfurization reactor design

Reactor designs for hydrodesulfurization of various feedstocks vary in the way in which the feedstock is introduced into the reactor and in the arrangement, as well as the physical nature, of the catalyst bed. The conditions under which the hydrodesulfurization process operates (i.e., high temperatures and high pressures)... 

The downflow fixed-bed reactor has been used widely for hydrodesulfurization processes and is so called because of the feedstock entry at the top of the reactor while the product stream is discharged from the base of the reactor (Figure 5-6). The catalyst is contained in the reactor as stationary beds with the feedstock and hydrogen passing through the bed in a downward direction. The exothermic nature of the reaction and the subsequent marked temperature rise from the inlet to the outlet of each catalyst bed require that the reaction mix be quenched by cold recycle gas at various points in the reactor. Hence the incorporation of separate catalyst beds as part of the reactor design. 

However, there are more chances of localized heating in the catalyst bed and (in addition to the more expensive reactor design per unit volume of catalyst bed) it may be more difficult to remove contaminants from the bed as part of the catalyst regeneration sequence. For this reason alone, it is preferable that this type of reactor is limited to hydrodesulfurization of low-boiling feedstocks such as naphtha and kerosene and application to the higher-boiling heavy oils and residua is usually not recommended. 

Your supervisor at Kleen Petrochemical wishes to use a hydrodesulfurization reaction to produce ethylbenzene from a process waste stream. You have been assigned the task of designing a reactor for the hydrodesulfurization reaction. Focus reactor design. 

Reactors are stationary vessels that are classified as batch, semi-batch, or continuous. Some reactors use mixers to blend the individual components. Reactor design depends on the type of service the reactor will be used in. Some of the reactor processes (among many others) include alkylation, catcracking, hydrodesulfurization, hydrocracking, fluid coking, reforming, polyethylene, and mixed-xylene. Figure 7-14 shows the standard symbols for reactors. 

With these criteria in mind, various reactors have been designed to satisfy the needs of the hydroprocesses, including hydrodesulfurization (McEvoy, 1996). Thus, reactors may vary from as little as 4 ft. in diameter to as much as 20 ft. in diameter and have a wall thickness anywhere from 4.5 to 10 in. or so. These vessels may weigh from 150 tons to as much as 1000 tons. Obviously, before selecting a suitable reactor, shipping and handling requirements (in addition to the more conventional process economics) must be given serious consideration. 

Trickle-bed reactors are widely used in hydrotreating processes, i.e., hydrodesulfurization of gasoline and diesel fuel, in petroleum refining, chemical, petrochemical, and biochemical processes. The knowledge of hydrodynamic parameters is vital in the design of a TBR because the conversion of reactants, reaction yield, and selectivity depend not only on reaction kinetics, operating pressure, and temperature, but also on the hydrodynamics of the reactor. Special care is also required to prevent flow maldistribution, which can cause incomplete catalyst wetting in some parts... 

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. 

There is a trend in recent environmental legislation to lower sulfur specifications in both gasoline and diesel fuels. You work for a refinery in the Delaware Valley that anticipates a new diesel specification requiring an order of magnitude lower sulfur than currently allowed. In fact, legislation is already in the works in Europe to lower the allowable sulfur to this new level by the year 2005. To achieve these low sulfur levels, you are to design a new catalytic hydrodesulfurization (HDS) system. This type of reactor has been in use in industry for a long time, but never for such severe service. 

Reactions carried out in three-phase fixed-bed reactors such as hydrogenation, oxidation, and hydrodesulfurization can be highly exothermic. Such situations require incorporation of an efficient heat removal system in order to avoid hot spots or catalyst deactivation as much as possible [13, 92]. A good knowledge of the packed-bed heat transfer parameters is necessary for the design of the reactor and heat removal system. 

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