Hydrocracking Process


Hydrocracking is one of the most versatile of all petroleum refining processes. Any fraction from naphtha to nondistillables can be processed to produce almost any desired product with a molecular weight lower than that of the chargestock. At the same time that hydrocracking takes place, sulfur, nitrogen, and oxygen are almost completely removed, and olefins are saturated so that products are a mixture of essentially pure paraffins, naphthenes, and aromatics.

This flexibility gives the hydrocracking process a particularly important role as refineries attempt to meet the challenges of today’s economic climate. The combined influences of low-quality feed sources, capital spending limitations, hydrogen limitations, environmental regulatory pressures, and intense competition have created a complex optimization problem for refiners. The hydrocracking process is uniquely suited, with proper optimization, to assist in solving these problems.

The UOP Unicracking process is carried out at moderate temperatures and pressures over a fixed catalyst bed in which the fresh feed is cracked in a hydrogen atmosphere. Exact process conditions vary widely, depending on the feedstock properties and the products desired. However, pressures usually range between 35 and 219 kg/cm2 (500 and 3000 lb/in2 gage) and temperatures between 280 and 475°C (536 and 887°F).


Single-Stage. The single-stage flow scheme involves full conversion through recycling of unconverted product and is the most widely used because of its efficient design resulting in minimum cost for a full-conversion operation. This scheme can employ a combination of hydrotreating and cracking catalysts or simply amorphous cracking catalysts depending on the final product required.
Once-Through. Unlike the single-stage flow scheme, the once-through flow scheme is a partial conversion option that results in some yield of unconverted material. This material is highly saturated and free of feed contaminants but is similar in molecular weight to the feed. If a refinery has a use for this unconverted product, such as FCC feed or high-quality lube base oil, this flow scheme may be preferred.
Two-Stage. In the two-stage flow scheme, feedstock is treated and partially converted once-through across a first reactor section. Products from this section are then separated by fractionation. The bottoms from the fractionation step are sent to a second reactor stage for complete conversion. This flow scheme is most widely used for large units where the conversion in the once-through first stage allows high feed rates without parallel reactor trains and the added expense of duplicate equipment.
Separate-Hydrotreat. The separate-hydrotreat flow scheme is similar to single-stage, but is configured to send reactor effluent that has been stripped of hydrogen sulfide and ammonia to the cracking catalyst. This configuration allows the processing of feedstocks with very high contaminant levels or the use of contaminant-sensitive catalysts in the cracking reactor if dictated by product demands.

Figure above illustrates a typical single-stage flow scheme. Feedstock, recycle oil, and recycle gas are exchanged against reactor effluent to recover process heat and are then sent through a final charge heater and into the reactor section. The reactor section contains catalysts that allow maximum production of the desired product slate. In virtually all hydrocracking systems, the combined reactions are highly exothermic and require cold hydrogen quench injection into the reactors to control reactor temperatures. This injection is accomplished at quench injection points with sophisticated reactor internals that both mix reactants and quench and redistribute the mixture. Proper mixing and redistribution are critical to ensure good temperature control in the reactor and good catalyst utilization through acceptable vapor or liquid distribution.
In this typical configuration, reactor effluent is sent through exchange to a hot separator, where conversion products are flashed overhead and heavy unconverted products are taken as hot liquid bottoms. The use of a hot separator improves the energy efficiency of the process by allowing hot liquid to go to the fractionation train and prevents polynuclear aromatic (PNA) fouling of cold parts of the plant. The overhead from the hot separator goes to a cold separator, where recycle gas is separated from the product. The product is then sent to fractionation, and recycle gas is returned to the reactor via the recycle compressor.
The fractionation train typically starts with a stripper column to remove hydrogen sulfide, which is in solution with the products. The removal ensures a relatively clean product in the main fractionator column, thus reducing column costs and metallurgy requirements. The stripper is followed by a main fractionating column with appropriate stages and sidedraws to remove the desired products. The bottoms from this main column is recycled back to the reactor section for complete feed conversion.
To allow complete conversion without PNA fouling or excessive catalyst coking, UOP has developed several techniques to selectively remove PNAs from the recycle oil stream. Some PNA removal is critical for successful operation at complete conversion. In earlier designs, the unit was simply purged of PNAs by taking a bottoms drag stream. In newer units, PNAs may be selectively removed by either fractionation or adsorption. The result is an increased yield of valuable liquid product.

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