Temperature hydroprocessing units

Liquid products contain sulfur and nitrogen and must be hydroprocessed to improve quality. Separate hydroprocessing units for upgrading the naphtha, kerosene, and gas oil fractions can be used to optimize the overall process. Refined gas oil or diesel fuel is aromatic in character and contains more cycloparaffins than conventional crude oil. The resulting fuel is low in cetane number, high in density, and typically has very good low-temperature handling properties. 

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. 

Figure 19-39 shows examples of gas-liquid-solid fluidized-bed reactors. Figure 19-39a illustrates a conventional gas-liquid-solid fluidized bed reactor. Figure 19-39h shows an ebuUating bed reactor for the hydroprocessing of heavy crude oil. A stable fluidized bed is maintained by recirculation of the mixed fluid through the bed and a draft tube. Reactor temperatures may range from 350 to 600°C (662 to 1112°F) and 200 atm (2940 psi). An external pump sometimes is used instead of the built-in impeller shown. Such units were developed for the liquefaction of coal. 

In addition to scale-up difficulties, there are a number of problems related to the stable operation of a bubble column associated with hydrodynamics. For example, consider the important commercial application of bubble columns in hydroprocessing of petroleum resids, heavy oils and synthetic crudes. Hydrodynamic cold flow and hot flow studies on the Exxon Donor Solvent coal liquefaction process (Tarmy et al., 1984) showed that much of the literature correlations for the hydrodynamic parameters (holdup, interfacial area and dispersion coefficients) obtained with cold flow units, at ambient conditions, are not applicable for commercial units operating at relatively higher pressures. In addition, the flow pattern in commercial units was considerably different. In the hydroprocessing of petroleum residues by the H-Oil and LC-Fining processes, refinery operations have experienced problems with nonuniform distribution of gas and liquid reactants across the distributor, maintaining stable fluidization and preventing temperature excursions (Beaton et al., 1986, Fan, 1989 and Embaby, 1990). Catalyst addition, withdrawal and elutriation have also been identified as problems in these hydrotreaters. 

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