Residue demetallization

Residues containing high levels of heavy metals are not suitable for catalytic cracking units. These feedstocks may be subjected to a demetallization process to reduce their metal contents. For example, the metal content of vacuum residues could be substantially reduced by using a selective organic solvent such as pentane or hexane, which separates the residue into an oil (with a low metal and asphaltene content) and asphalt (with high metal content). Demetallized oils could be processed by direct hydrocatalysis. 

Solvent extraction may also be used to reduce asphaltenes and metals from heavy fractions and residues before using them in catalytic cracking. The organic solvent separates the resids into demetallized oil with lower metal and asphaltene content than the feed, and asphalt with high metal content. Figure 3-2 shows the IFP deasphalting process and Table 3-2 shows the analysis of feed before and after solvent treatment. Solvent extraction is used extensively in the petroleum refining industry. Each process uses its selective solvent, but, the basic principle is the same as above. 

Because of the ease with which product 4 isomerizes to the conjugated enone in the presence of base, it is imperative that the demetallation and subsequent purification steps be carried out in glassware that is free of basic residues. 

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. 

The Demex process selectively rejects asphaltenes, metals, and high molecular weight aromatics from vacuum residues. The resulting demetallized oil can then be combined with vacuum gas oil to give a greater availability of acceptable feed to subsequent conversion units. 

In the once-through studies reported in the literature, a downflow reactor scheme was used for catalytic hydrocracking (9) in contrast to an upflow reactor scheme used in this study. It has been reported in the literature that an upflow reactor scheme is superior to the usual trickle-bed operation for residual feedstocks (18,19). Desulfurization, denitrogena-tion, and demetallization conversions were better in an upflow reactor. 

Removal of Ni and V from residual oils is diffusionally limited [8], and therefore Ni and V are deposited in the catalyst pores in a characteristic deposition profile [7, 11, 131 which is either U- or M-shaped, i.e. the maximum metal deposition is either located at the catalyst pellet surface or inside the pellet. These phenomena have been investigated for both V and Ni using various V/Ni porphyrins as model compounds [1, 2, 3, 4, 5], It has been concluded that the removal of V and Ni porphyrins proceeds via a sequential reaction network, where the porphyrins are hydrogenated in the first step and deposited in a subsequent one. These studies have also indicated that the degree of complexity of reaction pattern depends on the types of porphyrins used. In these studies, no HjS was present In a recent study [6] of demetallization of Ni and V porphyrins, the reaction path of HDV and HDNi reactions has been investigated in the presence of H S. 

Figure 6. Comparison of catalyst deactivation in desulfurizing virgin and demetallized Venezuelan atmospheric residues...

Technical interest has concentrated on hydroprocessing heavy and residual oils and coal. The catalyst is required to be effective in hds and also in hydrocracking, hdn and hdo (Section 6), and demetallization. A particular problem in these applications is catalyst by carbon and metals deposition. 

Vanadium. A residual oil was desulphurized (673 K, 115 atm) with a non-stoicheiometric vanadium sulphide (S/V, 0.8-1.8) formed in situ from VS4 Vanadium sulphide catalysts have been prepared by in situ sulphiding of vanadium complexes, e.g., bis(acetylacetonato)oxovanadium(IV), dissolved in crude petroleum.Vanadium compounds occurring in heavy oils have been activated as desulphurization and demetallization catalysts by treatment with triethylaluminium. Catalysts consisting of vanadium promoted by nickel can be prepared in situ by deposition of the metals from heavy crude oils. Ni-V hds and hdm catalysts on silica or carbon have been claimed. 

The main objective of OCR process is demetallization of residues. To decrease reactor volume and to make an effective use of the catalyst Chevron developed a technology in which oil and catalyst are fed to the reactor in counter-current flow (figure 5). The beaded catalyst is added once or several times a week to the top of the reactor and moves in plug flow to the bottom of the reactor, where the spent catalyst is removed. Chevron carried out extensive experiments to develop a safe and reliable system to feed fresh and to withdraw spent catalyst. 

In mild hydrocracking of atmospheric residue for maximum middle distillates production, the metal tolerance of the demetallization catalyst is the most important factor determining the catalyst life. The deactivation by metals is the prevailing mechanism in this mode of operation. 

The catalyst used was Topsoe TK-751. It is a general purpose HDS catalyst for residual feedstocks. Their functions are desulphurization, demetallation and asphaltenes and Conradson carbon reduction. It is recommended for HDS of residua with moderate metals content and for 2 stage catalyst in composite fillings. It has good HDS activity, good HDM selectivity and capacity for metals uptake. It is Ni/Mo type. 

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