Residuals, hydrodesulfurization

Koyama, H. Nagai, E., and Torii, H., Commercial Experience in Vacuum Residue Hydrodesulfurization, in NPRA Anual Meeting, 1995., March 19-21. AM-95-43. 

Shah and Paraskos47 applied their analysis to evaluate the importance of axial dispersion on pilot scale (a) residue hydrodesulfurization, (b) gas-oil hydrocracking, and (c) shale-oil denitrogenation reactor performances. The calculations indicated that the axial dispersion effect is less important in case (c) than in cases (a) and (b). Under certain pilot-scale operations, axial dispersion effects could be significant in cases (a) and (b). 

The Mizushima Oil Refinery of Japan Energy Corporation first implemented a high conversion operation of vacuum residue, versus a constant desulfurization operation, in the commercial residue hydrodesulfurization unit equipped with fixed-bed reactors, to produce more middle distillates as well as fuel oil with lower viscosity. The catalysts will be replaced when the sulfur content in the product oil reaches the allowable limit. Since we have believed that an increase in the residue conversion decreases the catalyst activity by coke deposition, we have been interested in controlling the coke deactivation to maximize the residue conversion during a scheduled operating period. 

Those deactivation models accounting for both coke and metal sulfides are rather simple. Coke and metals foul residue hydrodesulfurization catalysts simultaneously via different processes, and decrease both intrinsic reaction rate and effective diffusivity. They never uniformly distribute in the commercial reactors. We have examined the activity and diffusivity of the aged and regenerated catalysts which were used at the different conditions as well as during the different periods. This paper describes the effects of vacuum residue conversion, reactor position, and time on-stream on the catalyst deactivation. Two mechanisms of the catalyst deactivation, depending on residue conversion level and reactor position, are also proposed. 

A study on the residue hydrodesulfurization catalysts used in the commercial reactors has suggested that there exists two deactivation mechanism such as metal-controlled deactivation and coke-controlled deactivation, depending on a residue conversion level. In the second and third bed, the deactivation is controlled by metal deposition. However, in the fourth bed, a coke-controlled deactivation appears at a high residue conversion. We also have proposed that there exist two stages in the metal-controlled deactivation. During the first stage, metal sulfides partially poison the active sites and... 

COMMERCIAL EXPERIENCE IN VACUUM RESIDUE HYDRODESULFURIZATION Hiroki Koyama, Eiichi Nagai, Hidenobu Torii, Hideaki Kumagai... 

The Mizushima Oil Refinery of Japan Energy Corporation first implemented an operation of vacuum residue hydrodesulfiirization in the conventional fixed bed reactor system in 1980. We have also conducted a high conversion operation to produce more middle distillates as well as lower the viscosity of the product fuel oil to save valuable gas oil which is used to adjust the viscosity. Vacuum residue hydrodesulfurization in fixed bed reactors mvolves the characteristic problems such as hot spot occurrence and pressure-drop build-up. There has been very little literature available discussing these problems based on commercial results. JafiFe analyzed hot spot phenomena in a gas phase fixed bed reactor mathematically, assuming an existence of the local flow disturbance region [1]. However, no cause of flow disturbance was discussed. To seek for appropriate solutions, we postulated causes ofhot spot occurrence and pressure-drop build-up by conducting process data analysis, chemical analysis of the used catalysts, and cold flow model tests. This paper describes our solutions to these problems, which have been demonstrated in the commercial operations. 

Feed properties and operation conditions determine catalyst life in the residue hydrodesulfurization. In a high conversion operation of vacuum residue, catalyst deactivation due to coke is as important as the one due to metals. Though many researchers have worked on understanding and modelling deactivation of residue hydrodesulfurization catalysts, there has still been a controversy in a coke deactivation mechanism [2, 3]. Very few publications are available discussing an effect of a bed temperature profile on catalyst deactivation in large scale adiabatic commercial reactors. Most of the studies on deactivation of residue hydrodesulfiirization catalysts have been done with small-scale isothermal reactors [2,3,4,5]. The activity tests of the used catalysts were conducted to study the catalyst deactivation in the commercial reactors. This paper also describes an effect of a bed temperature profile on coke deactivation, which was tested in the commercial reactors. 

Effective solutions to the problems of the vacuum residue hydrodesulfurization unit equipped with the fixed bed reactors, such as a hot spot, pressure-drop buildup, and catalyst deactivation by coke fouling, were discussed. Improving liquid distribution can prevent hot spot occurrence. Dispersing inorganic solids throughout the reactors can control a pressure-drop increase in the first bed. For a high conversion operation, controlling the conversion in each bed can minimize the coke deactivation in the fourth bed. 

Since the late sixties, residue hydrodesulfurization plants have been constructed for the production of low sulfur fuel oil, specially in Japan. After the oil crisis, their role has changed significantly. Nowadays they are also used as ... 

The catalyst which has a larger pore diameter tends to show a lower deactivation rate, as well as lower HDS activity. Figure 1 shows one example of the results of residual hydrodesulfurization experiments testing three kinds of catalysts which have different pore diameters. The micro-reactors were operated under the same conditions, as shown in Figure 1. Catalyst A, Catalyst B and Catalyst C were the test catalysts which have the same properties with different pore diameters ( C > B > A). The activity and deactivation rate of each catalyst were shown to depend strongly on pore diameter, as shown in Figure 1. 

Beuther H, Schmid BK. Reaction mechanisms and rates in residue hydrodesulfurization. Sixth World Petroleum Congress June 1963 Frankfurt. Section 3 297-310. 

Montagna, A., Y. T. Shah. The Role of Liquid Holdup, Effective Catalyst Wetting, and Backmixing on the Performance of a Trickle-Bed Reactor for Residue Hydrodesulfurization. 

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