Residuum hydrodesulfurization

In Section III, commercial residuum hydroprocessing technology is discussed to establish the role and requirements of hydroprocessing in the overall refinery residuum conversion scheme. Commercial residuum hydroprocessing catalysts and residuum hydrodesulfurization (RDS)-hydrodemetallation (HDM) technology are reviewed briefly. 

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). 

Hydrodemetallization and subsequent asphaltene cracking with moderate hydrodesulfurization take place simultaneously in the reactor under conditions similar to residuum hydrodesulfurization. The reactor effluent gas is cooled, cleaned up and recycled to the reactor section, while the separated liquid is distilled into distillate fractions and vacuum residue which is further separated into deasphalted oil and asphalt using butane or pentane. 

Data obtained using this equation showed that a change in hydrogen sulfide concentration from 1% to 12% (by volume) could reduce by 50% the rate constants for the easy-to-desulfurize and the difficult-to-desulfurize reactions. On the basis of the data available from kinetic investigations, the kinetics of residuum hydrodesulfurization may be represented by the following general equation ... 

RDSA RDS. RDS is used for attnospheric residuum hydrotreating, and VRDS for vacuum residuum hydrodesulfurization. The feedstock is put in contact with catalyst and hydrogen at moderate temperatures and pressures, consuming about 700-1300 SCF H2/bbl of feed (Lars et al., 1984 Otterstedt et al., 1986). 

RDS Isomax [Residuum desulphurization] A hydrodesulfurization process for removing sulfur compounds from petroleum residues, while converting the residues to fuel oil. Developed by Chevron Research Company in the early 1970s. Ten units were operating in 1988. See also VGO Isomax, VRDS Isomax. 

RESID-fining [Residuum refining] A hydrodesulfurization process adapted for petroleum residues. Developed by Esso Research Engineering Company and licensed by them... 

Figure 5. Hydrodesulfurization kinetics for an Arabian light atmospheric residuum...

There is a variety of sulfur-containing molecules in a residuum or heavy crude oil that produce different products as a result of hydrodesulfurization reaction. Although the deficiencies of current analytical techniques dictate that the actual mechanism of desulfurization remain largely speculative, some attempt... 

Removal of the metal contaminants is not usually economical, or efficient, during rapid regeneration. In fact, the deposited metals are believed to form sulfates during removal of carbon and sulfur compounds by combustion that produce a permanent poisoning effect. Thus, if fixed-bed reactors are to be used for residuum or heavy oil hydrodesulfurization (in place of the more usual distillate hydro-desulfurization) it may be necessary to first process the heavier feedstocks to remove the metals (especially vanadium and nickel) and so decrease the extent of catalyst bed plugging. Precautions should also be taken to ensure that plugging of the bed does not lead to the formation of channels within the catalyst bed which will also reduce the efficiency of the process and may even lead to pressure variances within the reactor because of the distorted flow patterns with eventual damage. 

Figure 6-10 Carbon deposit on the catalyst as a function of time in the hydrodesulfurization of a residuum. 

Thus, any processing sequence devised to hydrodesulfurize a heavy oil or residuum must be capable of accommodating the constituents which adversely affect the ability of the catalyst to function in the most efficient manner possible. 

Table 6-13 Hydrodesulfurization Rates for a Vacuum Residuum and its Deasphalted Products... 

High amounts of asphaltenes and resins require high hydrogen partial pressures and may actually limit the maximum level of hydrodesulfurization, or final traces of sulfur in the residuum may only be eliminated under extremely severe reaction conditions where hydrocracking is the predominant reaction in the process. High asphaltene and resin contents are also responsible for high viscosity (Figure 6-7) which may increase the resistance to mass transfer of the reactants... 

Residua and heavy oils contain impurities other than sulfur, nitrogen, and oxygen, and the most troublesome of these impurities are the organometallic compounds of nickel and vanadium. The metal content of a residuum or heavy oil can vary from several parts per million (ppm) to more than 1000 parts per million (Table 6-15), and there does seem to be more than a chance relationship between the metals content of a feedstock and its physical properties (Reynolds, 1997 Speight, 1999). In the hydrodesulfurization of the heavier feedstocks the metals (nickel plus vanadium) are an important factor since large amounts (over 150 ppm) will cause rapid deterioration of the catalyst. The free metals, or the sulfides, deposit on the surface of the catalyst and within the pores of the catalyst, thereby... 

In the process, a residuum is desulfurized and the nonvolatile fraction from the hydrodesulfurizer is charged to the residuum fluid catalytic cracking unit. The reaction system is an external vertical riser terminating in a closed cyclone system. Dispersion steam in amounts higher than that used for gas oils is used to assist in the vaporization of any volatile constituents of heavy feedstocks. 

The Demex process is a solvent extraction demetallizing process that separates high metal vacuum residuum into demetallized oil of relatively low metal content and asphaltene of high metal content (Table 8-5) (Houde, 1997). The asphaltene and condensed aromatic contents of the demetallized oil are very low. The demetallized oil is a desirable feedstock for fixed-bed hydrodesulfurization and, in cases where the metals and carbon residues are sufficiently low, is a desirable feedstock for fluid catalytic cracking and hydrocracking units. 

This source of hydrogen is being effectively utilized with the aid of the cryogenic hydrogen upgrader to recover and purify hydrogen for return to such refinery applications as residuum hydrocracking and hydrodesulfurization. 

RESID-fining [RESIDuum refining] A hydrodesulfurization process adapted for petroleum residues. Developed by Esso Research Engineering Company and licensed by them and Union Oil Company of California. A proprietary catalyst is used in a fixed bed. As of 1988, eight plants had been designed. 

Crude oil contains a number of elements other than carbon and hydrogen, which produce severe environmental impacts and must be removed. [1, 2] The principal method of their removal is catalytic hydrotreating. Many of the important crude oils have sulfur contents 1-6% by weight and nitrogen up to one percent. Ttic most important metals are vanadium and nickel, which may be as high as 1000 parts per million. These elements tend to be much more concentrated in the heavier portions in the crude oil, particularly in the residuum after distillation atmospheric resid boils above 650° F and vacuum resid boils above 1020° F. The process of hydrodesulfurization (HDS) is often carried out in parallel or in series with the process of hydrodemetallation (HDM). 

Application of this model to a residuum desulfurization gave a linear relationship. However, it is difficult to accept the desulfurization reaction as a reaction that requires the interaction of two sulfur-containing molecules (as dictated by the second-order kinetics). To accommodate this anomaly, it has been suggested that, as there are many different types of sulfur compounds in residua and each may react at a different rate, the differences in reaction rates offered a reasonable explanation for the apparent second-order behavior. For example, an investigation of the hydrodesulfurization of an Arabian light-atmospheric residuum showed that the overall reaction could not be adequately represented by a first-order relationship. However, the reaction could be represented as the sum of two competing first-order reactions and the rates of desulfurization of the two fractions (the oil fraction and the asphaltene fraction) could be well represented as an overall second-order reaction. 

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