Selective hydrocracking

Hydrodewaxing Selective hydrocracking of paraffins from oil fractions or products to prevent wax precipitation (negative influence on cold flow properties) ... 

Dewaxing is an important specific hydrocracking process used to improve diesel and heating oils by pour-point reduction.88 This is achieved by shape selectivity of certain zeolites allowing selective hydrocracking of long-chain paraffinic waxes to C3—C5 alkanes in the presence of other paraffins.89 Platinum H-mordenite is used in an industrial process.90... 

For materials selection, hydrocrackers are treated the same as hydrodesulfurizers, particularly in the first stage. From a materials standpoint, the demarcation between low-pressure units (hydrodesulfurizers) and high-pressure units (usually hydrocrackers) is 650 psia (4,480 kPa). 

N. Y. Chen (Mobil Research Development Corp., Princeton, N. J. 08540) It might be of interest to the audience, particularly to those who are not familiar with the application of zeolites in industrial catalytic processes, to mention that since the discovery of catalysis over shape-selective zeolite first published by Weisz and Frilette in I960, a commercial process based on selective hydrocracking reactions similar to that reported in this paper has been in operation on a large scale in more than four of our refineries since 1967. A technical paper describing this process, known as the Selectoforming process, was published in 1968. 

Ni-loaded H-erionite catalystshave been used commercially for the selective hydrocracking of n-paraffins in naphthas ( Selectoforming ). Chen and Garwoodhave extended its use to the selective removal of n-paraffins from jet fuels. The conversion of n-paraffins was inhibited by sulphur and/or the absence of high hydrogen partial pressure, as had also been found with lighter paraffins. 

In catalytic de-waxing, special shape-selective catalysts are used to selectively hydrocrack only the straight-chain alkanes to low boiling point by-products. Since it is the linear alkanes that comprise the bulk of the waxy components in the lube boiling range, the oil is effectively de-waxed. There are differences in the composition and properties of base oil de-waxed by catalytic solvent processes because of different selectivities. 

Zeolitic catalysts are commonly used for such purposes however, the modification of the acidity of the perovskite by addition of a second acidic oxide such as silica or alumina yields materials with balanced hy-drogenating-cracking functions that may be effective for selective hydrocracking. Barium zirconates were found to be materials particularly suited for use in catalytic hydrocracking of residua, especially for their ability for removal of carbonaceous deposits from the coked catalysts... 

Source C. J. Egan, G. E. Langlois, and R. J. Watts, Selective Hydrocracking of C9- to C12-Alkylcyclohexanes on Acidic Catalysts. Evidence for the Paring Reaction, Journal of the American Chemical Society 84 1204-1212 (1962). With permission. 

The Mobil process for catalytic dewaxing by selective hydrocracking of wax molecules arose from that company s development work on zeolites and the discovery of the remarkable selectivity exhibited by these catalysts some 20 years prior to first commercialization. In 1960 Weise and Frilette, of the then Socony Mobil Research and Development Laboratories,1 reported that n-decane cracked readily to lighter paraffins over the sodium form of a zeolite known as 13X, whereas the bulkier molecules, a-pinene and isopropylbenzene, underwent no reaction (Figure 10.1). 

Source N. Y. Chen and W. E. Garwood, Selective Hydrocracking of n-Paraffins in Jet Fuels, Industrial and Engineering Chemistry Process Design and Development 19 315-318 (1978). With... 

The catalysts used in studies were prepared on the basis of ZSM-5 zeolite and industrial catalyst of selective hydrocracking (SHC-1) prepared on the basis of pentasyl type zeolite... 

Some gas oils and lube oil base stocks, particularly those produced by hydrocracking, have cold flow properties that do not comply with specifications. The molecules responsible for these problems are mainly the long straight-chain alkyls, including those linked to a naphthene or aromatic ring (Table I). There are two ways to remove these chains—by selective hydrocracking or by selective hydroisomerization. 

Two distinct kinds of Catalytic Dewaxing are in use today. One involves the boiling point conversion of the paraffin components of waxy feeds by selective hydrocracking and the other by selective hydroisomerization. 

Catalyst selectivity is a measure of the rate of formation of a desired product relative to the rate of conversion of the feed (or formation of other products). Hydrocracking selectivity is expressed as the yield of desired product at a specific conversion. Yield is determined by the rate of formation of the desired product relative to the feed rate. At 100% conversion, catalyst yield equals catalyst selectivity. Hydrocracking selectivity is affected by operating conditions. In general, more severe operating conditions cause higher selectivity for secondary products. 

Engineering aspects of the SMDS process are reviewed here. They include the manufacture of synthesis gas, the production of paraffinic Fischer-Tropsch waxes and the control of the chain length distribution by a selective hydrocracking step. The close interaction between the properties of the individual catalyst particles, the choice of the reactor technology and the overall process performance is discussed in detail. 

It appears, therefore, that fluid-bed technology is not suitable for the SMDS process, which is based on the concept of synthesizing a very heavy product in combination with selective hydrocracking. Figure 6 shows reactor systems which are suitable, in principle. 

Figure 11. Carbon-distribution of a Fischer-Tropsch product before and after selective hydrocracking.

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