Weight hydrocracking

For more aromatics yield, the end point of the feed may be raised to include higher molecular weight hydrocarbons in favor of hydrocracking and dehydrocyclization. However, excessive hydrocracking is not desirable because it lowers liquid yields. 

Pore size optimization is one area where developmental efforts have been focused. Unimodal pore (NiMo) catalysts were found highly active for asphaltene conversion from resids but a large formation of coke-like sediments. Meanwhile, a macroporous catalyst showed lower activity but almost no sediments. The decrease of pore size increases the molecular weight of the asphaltenes in the hydrocracked product. An effective catalyst for VR is that for which average pores size and pore size distribution, and active phase distribution have been optimized. Therefore, the pore size distribution must be wide and contain predominantly meso-pores, but along with some micro- and macro-pores. However, the asphaltene conversion phase has to be localized in the larger pores to avoid sediment formation [134],... 

HC Unibon [Hydrocracking] A version of the hydrocracking process for simultaneously hydrogenating and cracking various liquid petroleum fractions to form branched-chain hydrocarbon mixtures of lower molecular weight. The catalyst is dual-functional, typically silica and alumina with a base metal, in a fixed bed. Developed by UOP. By 1988,46 licenses had been granted. Currently offered under the name Unicracking. 

MHC Unibon [Mild hydrocracking] A mild hydrocracking process for desulfurizing gas oil and converting it to lower molecular weight hydrocarbons, suitable for further processing by catalytic cracking. Developed by UOP. 

In the hydrocracking process, this phenomenon is exploited to shift catalyst selectivity from the naphtha to the distillate products. Here the wide separation of sites is exploited to minimize the potential for secondary cracking in initial products and intermediates. This, along with the introduction of escape routes for the primary product tends to preserve the higher molecular weight hydrocarbons, thereby producing more dishllates [49, 61, 62]. 

Catalytic Dewaxing Also called CDW. A hydrocracking process for removing waxes (linear aliphatic hydrocarbons) from petroleum streams by converting them to lower molecular weight hydrocarbons. The catalyst is a synthetic mordenite. Developed by BP two units were operating in 1988. 

An alternative to adjusting process conditions is to have a secondary solvent precipitation stage. Toluene was found to be the best solvent tried and, again, low levels of trace elements were obtained. Using this method, however, there is a loss of product, but this is mainly high molecular weight species, which are difficult to hydrocrack and are probably responsible for deposition of carbonaceous material on the catalyst. 

An ft catalyst was prepared containing 3.1 wt % Pd, and used to hydrocrack a refractory gas oil (25.0° API gravity molar weight 216 53 vol % FIA aromatics) that had been hydrotreated to 4.2 ppm N, and fortified with dimethyl disulfide to a total sulfur content of 1130 ppm. 

During the last decade, there has been a definite trend towards heavier hydrocracker feedstocks (2). Recognition of the interactions between feed molecular weight and catalyst chemical and physical properties has been necessary to support this trend. 

Figure 7 shows the effect of feed molecular weight on the reaction rates observed with strongly acidic hydrocracking catalysts. These data were obtained with an early version of an amorphous catalyst. They may, however, be used to illustrate general trends involving feed character and molecular weight. 

Besides influencing over-all reaction rates, pore diffusion can cause changes in selectivity. An extreme example of this was observed (26) when a high molecular weight California solvent-deasphalted oil was hydrocracked over a small pore size palladium zeolite catalyst at high temperatures. The feedstock gravity was 16.4° API, and 70% boiled above 966°F. The resulting product distribution is compared with that... 

Figure 10. Interaction between catalyst pore size and feed molecular weight in hydrocracking a California solvent deasphalted oil...

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