Feed hydroprocessing units

In residue hydroprocessing units, heat release is high, but some licensors avoid using intra-reactor quench because residue feeds often form lumps of coked-bonded catalyst in fixed-bed units. In reactors with complex internals, such lumps are very hard to remove during a catalyst change-out. Therefore, fixed-bed residue units often comprise three or more 1-bed reactors in series with quench in between. In many cases, the first reactor is guard bed filled with one or catalysts designed to remove metals. 

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. 

With hydroprocessing units, most refiners try to maximize feed rate while (a) meeting other process objectives and (b) maintaining a high on-stream factor. Some try to maximize conversion, while others just want to hit a key process target at minimum cost. 

Desulfurization of FCC feedstocks reduces the sulfur content of FCC products and SOX emissions. In the United States, road diesel sulfur can be 500 ppm (0.05 wt%). In some European countries, for example in Sweden, the sulfur of road diesel is 50 ppm or less. In California, the gasoline sulfur is required to be less than 40 ppm. The EPA s complex model uses sulfur as a controlling parameter to reduce toxic emissions. With hydroprocessed FCC feeds, about 5% of feed sulfur is in the FCC gasoline. For non-hydroprocessed feeds, the FCC gasoline sulfur is typically 10% of the feed sulfur. 

Marathon s strategy was to leverage future clean fuels projects to minimize capital investment and maximize emission reductions. Units with feed hydrotreating capacity typically used catalyst additives. Units with no hydroprocessing typically relied upon other technology options in conjunction with catalyst additives. Since each application was unique, implementation was required over a period of time. Figure 14.5 shows the timeline when significant events occur for the Marathon CD. 

As in the traditional acid extraction process, the feedstock is generally dewaxed solvent refined base stock, since levels of the aromatics, polynuclear aromatics, and nitrogen and sulfur compounds are already reduced relative to a straight-run gas oil. This facilitates hydroprocessing by lightening the load on the catalysts and extending their lives. Equally important is that this is an already dewaxed feed, so the white oil producer does not have to bear the capital costs of crude fractionation and dewax units. 

Reactors, Catalyst Beds and Quench Zones. As shown in Table 4, most hydroprocessing reactions are exothermic. The heat released in naphtha and kerosene hydrotreaters is relatively low, so units designed for these feeds may use just one reactor that contains a single catalyst bed. However, for heavier feeds and/or feeds that contain large amounts of sulfur, aromatics or... 

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