Hydrogenation or Hydrocracking

Hydrogenation technologies are not that widely used in waste management. It is usually a two-stage process where plastic waste is first subjected to mild cracking by low-temperature pyrolysis and the condensable product is reacted with hydrogen 

Both pyrolysis and hydrocracking are particularly desirable waste management approaches from a sustainability standpoint, for several reasons. 

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Bitumen is a hydrogen-deficient oil that is upgraded by carbon removal (coking) or hydrogen addition (hydrocracking) (2,4). There are two methods by which bitumen conversion can be achieved by direct heating of mined tar sand and by thermal decomposition of separated bitumen. The latter is the method used commercially, but the former has potential for commercialization (see Fuels, synthetic). 

A combination of triflic acid with iodine was shown to be effective to liquefy three types of coal in toluene or tetralin under hydrogen pressure.141 The major role of acid was found to enhance coal depolymerization to asphaltenes, whereas the main function of iodine was to hydrogenate and hydrocrack asphaltenes to oils. The combined catalytic system removed 50% of the nitrogen and 90% of the sulfur of the coals (Illinois No. 6 and Pittsburg seam samples). 

In the platforming process (Fig. 1), the feedstock is usually a straight-run, thermally cracked, catalytically cracked, or hydrocracked C6 to 200°C naphtha. The feed is first hydrotreated to remove sulfur, nitrogen, or oxygen compounds that would foul the catalyst, and also to remove olefins present in cracked naphthas. The hydrotreated feed is then mixed with recycled hydrogen and preheated to 495 to 525°C at pressures of 116 to 725 psi (0.8 to 5 MPa). Typical hydrogen charge ratios of 4000 to 8000 standard cubic feet per barrel (scf/bbl) of feed are necessary. 

For example, Weisz et ah (8, 9,58) studied acid- and metal-catalyzed reactions of n-paraffins in the presence of branched paraffins and aromatics. They report that, by a proper balance of both shape-selective acid and metal functions, the n-paraffins can be hydrocracked from the mixture without simultaneously hydrocracking other paraffin isomers or by incurring the loss of aromatics through hydrogenation or hydro-genolysis. 

On the whole, the reactions involved are highly exothermic (AH = —190 to —230 kJ/ mol for hydrocracking), except for the production of heavy aromatics, which is slightly endothermic, and the decomposition of methane to carbon and hydrogen. They take place at elevated temperature, about 650°C, and, depending on each case, are favored (hydrogenation) or not (decomposition, coke formation) by an increase in pressure, which is optimally about 5 to 6.10s Pa absolute. 

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