Catalytic cracking waste

The Phenox process (254) removes phenol (qv) from the efduent from catalytic cracking in the petroleum industry. Extraction of phenols from ammoniacal coke-oven Hquor may show a small profit. Acetic acid can be recovered by extraction from dilute waste streams (255). Oils are recovered by extraction from oily wastewater from petroleum and petrochemical operations. Solvent extraction is employed commercially for the removal of valuable... 

Reduced Emissions and Waste Minimization. Reducing harmful emissions and minimizing wastes within a process by inclusion of additional reaction and separation steps and catalyst modification may be substantially better than end-of-pipe cleanup or even simply improving maintenance, housekeeping, and process control practices. SO2 and NO reduction to their elemental products in fluid catalytic cracking units exemplifies the use of such a strategy (11). 

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. 

Carbon Monoxide Boilers Carbon monoxide boilers are used to recover waste heat generated from oil refining fluid catalytic cracking (FCC) processes. The FCC process produces copious volumes of by-product gas containing 5 to 8% carbon monoxide (CO), which has a heat content of about 150 Btu/lb. A 10,000 barrel (bbl) per day FCC unit produces 60,000 to 150,000 lb/hr of CO. 

Industrial Engineering Chemistry Research 36, No.ll, Nov.1997, p.4523-9 TRANSFORMATION OF SEVERAL PLASTIC WASTES INTO FUELS BY CATALYTIC CRACKING... 

The thermal and catalytic cracking of PP, PS, and SBR waste, dissolved in light cycle oil, was studied in a riser simulator. 19 refs. 

It has been shown, however, that such catalysts may contain protons, either by design or because of the difficulty in removing all traces of moisture, and these protons have been shown to be superacidic with Hammett acidities up to —18. These protons will also play some role in the catalytic activity of these ionic liquids in practical situations. Ionic liquids in which superacidic protons have deliberately been generated by addition of small amounts of water, HCl or H2SO4 have been used to catalytically crack polyethene under relatively mild conditions. The main products are mixed C3-C5 alkanes, which would be a useful feedstock from waste polyethene recycling. In contrast to other cracking procedures no aromatics or alkenes are produced, although small amounts of polycyclic compounds are obtained. 

Catalytic processes (finid catalytic cracking, catalytic hydrocracking, hydro-treating, isomerization, ether manufacture) also create some residuals in the form of spent catalysts and catalyst fines or particulates. The latter are sometimes separated from exiting gases by electrostatic precipitators or filters. These are collected and disposed of in landfills or may be recovered by off-site facilities. The potential for waste generation and hence leakage of emissions is discussed below for individual processes. 

From the chemical manufacturing industry, catalytic cracking and catalytic hydrogenation, gas absorption or scrubbing processes in which desired or waste products are removed from a waste stream, the nitration of benzene and toluene where the reactants have limited mutual solubility, and carbonylation processes using carbon monoxide. 

High-temperatnre pyrolysis and cracking of waste thermoplastic polymers, such as polyethylene, polypropylene and polystyrene is an environmentally acceptable method of recycling. These type of processes embrace both thermal pyrolysis and cracking, catalytic cracking and hydrocracking in the presence of hydrogen. Mainly polyethylene, polypropylene and polystyrene are used as the feedstock for pyrolysis since they have no heteroatom content and the liquid products are theoretically free of sulfur. 

The book also explores the application of various acidic catalysts, such as silica-alumina, zeolites (HY, HZSM-5, mordenite) or alkaline compounds such as zinc oxide. However, the main problem with catalytic cracking is that in the course of the cracking process all catalysts deactivate very quickly. Expensive zeolite catalysts increase the cost of waste plastics cracking process to the point where it becomes economically unacceptable since the catalyst becomes contained in coke residue and therefore cannot be recovered and regenerated. 

K. Gobin, D. Koumantaropoulos, and G. Manos, One stage catalytic cracking of plastic waste on zeolite-based catalysts. Stud. Surf. Sci Catal, 135, 4989-4994 (2001). 

Additionally, some particular catalytic cracking processes for recovering specific raw chemicals from plastic wastes can be found in the literature. This is the case of the process... 

An alternative process based on two sequential catalytic cracking stages aimed at obtaining gasoline and diesel from waste plastics or heavy oil/waste plastics mixtures is shown in Figure 3.16 [99]. The catalyst employed in the first step is made up of powder alumina, waterglass and HZSM-5 zeolite and is mixed up directly with the waste plastics in a screw reactor preferably at 600-700°C. The second catalytic step consists in a fixed... 

Figure 3.16 Scheme of a process for the direct catalytic cracking of plastic wastes in two steps [99]... 

Another approach for overcoming the problems posed by conventional cracking catalysts has been disclosed recently by Reverse et al. [101]. In this case, direct cracking is performed by using as catalyst a molten bed of pure metal or a metal mixture (mainly lead, zinc, tin) at a temperature of 460-550°C wherein the waste polymer is loaded inside the reactor at a certain depth. The authors point out that the products are indeed a result of the combination of both thermal and catalytic cracking. The catalyst composition may also include some acidic component such as metal silicates, metal carbonates and their mixtures. The process can be applied to pure and mixed polymers (PE, PET, PP, PVC), as well as to the plastic fraction of municipal solid wastes. 

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