Fixed-Bed Hydroprocessing

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). 

Residue Hydrocracking. The catalysts used for HCR should have dual functionality, cracking and hydrogenation (HYD) functions. The process scheme of a typical HCR fixed-bed system contains two reactors  

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Alvarez A, Ancheyta J, Munoz JAD. Comparison of quench systems in commercial fixed-bed hydroprocessing reactors. Energy Fuels 2007 1 1133-1144. 

Alvarez A, Ancheyta J, Centeno G, Marroqufn G. A modeling study on the effect of reactor configuration on the cycle length of heavy oil fixed-bed hydroprocessing. Fuel 2011 90 3551-3560. 

For fixed-bed hydroprocessing units, the process conditions - pressure, temperature, space velocity, and catalyst - are determined by feed quality and process objectives. Table 12 shows typical process conditions for the hydrotreating of different feeds in fixed-bed hydrotreating units. The values shown are approximate. 

Fig. 14. Schematic flow diagram showing multiple fixed-bed reactor residuum hydroprocessing coupled with distillation equipment for high-severity RDS (Paraskos et al., 1974).

Reactor design is a key element in each process listed in Table XX. The method of feed introduction, the arrangement of the catalyst bed, and the mode of operation have an impact on the ability to process residua. For this reason, classification by reactor type provides a convenient and appropriate distinction for discussing hydroprocessing technology. The most common reactor designs include fixed beds, ebullated or expanded beds and slurry beds, and moving-bed reactors. These classifications are discussed in more detail next. 

Fic. 18. Schematic flow diagram of residuum hydroprocessing unit utilizing Shell bunker reactor upstream of fixed-bed reactor (Speight, 1981). 

Hydroprocessing studies were carried out in continuous fixed-bed tubular reactors containing 50-200 cm3 of catalyst. All reactors were operated in downflow mode. To facilitate mass balance data acquisition,... 

Generally, an amount of coke on the catalyst increases from the entrance to the exit of the fixed bed reactors in residue hydroprocessing (1, 6, 7). Tamm et al. showed the highest remained catalyst activity at the outlet of the bench-scale fixed-bed reactor after a constant desulfurization operation (1), while Myers et al. found the highest catalyst deactivation rate in the last stage of three-stage pilot-scale expanded-bed reactors after a 60 - 70% vacuum residue conversion operation (7). These results from two typical reactor operations support that the catalyst deactivation in a lower... 

Industrially, hydrodesulfurization of oil fractions, like aU hydroprocessing, is carried out catalytically in a fixed bed trickle flow unit. The catalyst is stacked in a packed bed and gas (hydrogen) and liquid (oil) are fed downstream concurrently. The reactor operates in the trickle-flow regime, in which the catalyst pellets are fully wetted with the liquid and both gas and liquid flow along the external surface. 

Alvarez A, Ancheyta J. Modeling residue hydroprocessing in a multi-fixed-bed reactor system. Appl. Catal. A Gen. 2008 351 148-158. 

Most hydrotreaters and hydrocrackers are trickle-bed units. A classic article by Satterfield " describes the fundamental behaviour of such units, in which mixtures of liquid and gaseous reactants pass down over fixed beds of catalyst. In hydroprocessing units, the liquid reactants are petroleum fractions, and the gaseous reactant is hydrogen. 

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