Hydrotreating waste

Fluid catalytic cracking units present formidable emission control problems. Contaminants are present in both reactor product gas and regenerator flue gas. The reactor product contains hydrogen sulfide, ammonia, and cyanides, plus combined sulfur and nitrogen in the liquid products. Hydrogen sulfide, ammonia and cyanides are handled as part of the overall refinery waste water cleanup. The combined sulfur and nitrogen may be removed by hydrotreating. 

Consider the oil-recycling plant shown in Fig. 3.16. In this plant, two types of waste oil are handled gas oil and lube oil. The two streams are first deashed and demetallized. Next, atmospheric distillation is used to obtain light gases, gas oil, and a heavy product. The heavy product is distilled under vacuum to yield lube oil. Both the gas oil and the lube oil should be further processed to attain desired properties. The gas oil is steam stripped to remove light and sulfur impurities, then hydrotreated. The lube oil is dewaxed/deasphalted using solvent extraction followed by steam stripping. 

Elliot, D.C., Baker, E.G., Hydrotreating biomass liquids to produce hydrocarbon fuels, In Klass, D.L. (ed.), 1987, Energy from Biomass and Waste, publ. IGT, Chicago, p. 765. 

Hydrotreating also produces some residuals in the form of spent catalyst fines, usually consisting of aluminum silicate and some metals (e.g., cobalt, molybdenum, nickel, tungsten). Spent hydrotreating catalyst is now listed as a hazardous waste (K171) (except for most support material). Hazardous constituents of this waste include benzene and arsenia (arsenic oxide, AS2O3). The support material for these catalysts is usually an inert ceramic (e.g., alumina, AI2O3). 

A number of refinery processes require the use of a fixed-bed catalyst These processes include catalytic reforming, hydrodesulfurization, hydrotreating, hydro-cracking, and others. These catalysts become inactive in six months to three years and are eventually replaced in the reactors with fresh catalyst during a unit shutdown. Many of these catalysts contain valuable metals which can be recovered economically. Some of these metals, such as platinum and palladium, represent the active catalytic component other metals such as nickel and vanadium are contaminants in the feed which are deposited on the catalyst during use. After valuable metals are recovered (a service usually performed by the outside companies), the residuals are expected to be disposed of as solid waste. 

Hydrous metal oxide powders, such as sodium titanate, NaT O, can be prepared by treating TYZOR TPT with sodium hydroxide in methanol solvent to form a soluble intermediate, which is hydrolyzed in acetone—water to form an ion-exchange material useful in treating radioactive waste (158). Exchange of the sodium ion with an active metal such as Ni, Pt, or Pd gives heterogeneous catalysts useful in olefin polymerization, coal liquification, and hydrotreating. 

The name trickle-bed reactor is usually applied in reference to a fixed bed in which a liquid phase and a gas phase flow concurrently throughout a bed of catalyst. By far the most important application, and hence much of the work, on these reactors has been in the hydrotreating of heavy feedstocks in the petroleum industry (hydrocracking, hydrodesulfurization, hydrodenitrogenation). However, this seems a very versatile processing method, and has not been exploited nearly to its potential in other areas such as waste water treatment—at least as the scientific literature would indicate. 

Elliott, D. C. and E. G. Baker. "Hydrotreating Biomass Liquids to Produce Hydrocarbon Fuels," in Energy from Biomass and Wastes X, IGT, Chicago, 1987 p 765-784. 

E.g. in the so-called "pseudo-equilibrium model, developed by Sylvester [53-56], the same design procedure is used as in a single phase catalytic gas phase reaction, where the mass transfer resistance is replaced by a suitable overall term. Bulk flow and dispersion of the liquid phase are neglected and the whole transport mechanisms are lumped into the equilibrium of the reactant concentrations between gas-, liquid- and particle phase. It is an application of the same principle used successfully in fluid/fluid reactions [57], But the necessary precondition is that the rate of reaction is slow compared to the transfer rate across the phase boundaries, so that equilibrium can really by assured. This might be justified in some of the hydrotreating processes, but certainly not in case of an aqueous liquid phase, existing in waste water treating. Earlier models used in petroleum industry have taken in-... 

To recover this oil means thinking big. It requires about 2 milhon tons of oil sand to produce one million barrels of synthetic crude oil, (equivalent to about 285,223 tons of coal) and engaging in this activity has already reshaped 420 km (152.5 mi ) of territory in the oil sand areas north of Fort McMurray, including the removal of more than 50 km (19.1 mi ) of boreal forest (Woynillowicz Severson-Baker, 2006) (Fig. 27.3). Another obvious impact comes from the accumulation of waste-products from hydrotreating, primarily waste sulfur stripped from the oil during processing. Presently, it is compressed into yellow blocks and stored on site as a massive sulfur mountain (Fig. 27.4). 

---