Coke removal

Air emissions from coking operations include the process heater flue gas emissions, fugitive emissions, and emissions that may arise from the removal of the coke from the coke drum. The injected steam is condensed and the remaining vapors are typically flared. Wastewater is generated from the coke removal and cooling operations and from the steam injection. In addition, the removal of coke from the drum can release particulate emissions and any remaining hydrocarbons to the atmosphere. 

The problems in fixed bed cracking reactors are (1) heat must be supplied to heat the reactants to the desired temperature and overcome the endothermicities of the cracking reactions and (2) the reactor must be shut down periodically for coke removal. Both of these problems were overcome by the development in the 1940s and 1950s of a fluidized... 

The first reaction is the isomerization from a zero-octane molecule to an alkane with 100 octane the second is the dehydrocyclization of heptane to toluene with 120 octane, while the third is the rmdesired formation of coke. To reduce the rate of cracking and coke formation, the reactor is run with a high partial pressure of H2 that promotes the reverse reactions, especially the coke removal reaction. Modem catalytic reforming reactors operate at 500 to 550°C in typically a 20 1 mole excess of H2 at pressures of 20-50 atm. These reactions are fairly endothermic, and interstage heating between fixed-bed reactors or periodic withdrawal and heating of feed are used to maintain the desired temperatures as reaction proceeds. These reactors are sketched in Figure 2-16. 

Typically, steam cracking reactors operate Ifom 10 to 40 days between coke removal cycles. 

The feed stock, usually topped or reduced crude oil, is heated in pipe coils (Figure 1) from about 900° to 950° F. The oil is then fed to one of two or more vertical, insulated coke drums. The coke drums are connected by valves so that they can be switched onstream for filling, then switched off-stream for coke removal. The temperature in the drum will ordinarily be 775° to 850° F. and the pressure 4 to 60 pounds per square inch gage. Hot, coke-still vapors from the coke drum pass to a fractionator where gas and gasoline, intermediate gas oil, and heavy gas oil are separated. More or less of the heavy gas oil is recycled. The ratio of recycled heavy gas oil to fresh feed is usually less than 1 but may go up to about 1.6 (5,15,28, 40). 

Fig. 1.17. Schematic picture of front propagation during oxidative coke removal, (a) Oxygen concentration in the regeneration gas. e.g., air. (b) Coke loading of the solid phase.

Fig. 1. 18. Effective temperature rise during oxidative coke removal as a function of the oxygen mole fraction of the regeneration feed gas.

Fig. 1.19. Front propagation during oxidative coke removal with low inlet temperature.

Figure 6. Coking and Coke removal by Fb from 0,3wt%Pt-0.3%wtRe/AbO3.

Figure 7. Kinetic expression of coke removal by hydrogen, experimental data model —.

Coking is another matter. It is a severe form of thermal cracking in which coke formation is tolerated to attain additional lighter liquids from the heavier, dirtier fractions of crude oil. In this process the metals that would foul catalysts are laid down with the coke. The coke settles out in large coke drums that are removed from service frequently (about once a day) to have the coke removed by hydraulic methods. Several coke drums are used to make the process continuous thus, one drum is online while the other is being emptied and readied for the next cycle.12... 

The coke was removed predominantly from pores in the range of 4-12 nm, resulting in a bimodal pore size distribution and an increase in the pore volume and surface area. The amount of coke removed depended on the extraction temperature, pressure and duration. In the most severe extraction conditions, the silica foulant of the catalyst could also be removed as fine particles. Pyridine poisoned the catalyst during extraction, however its removal by acetone wash could restore the catalyst activity. 

In summary, extraction with carbon dioxide, pyridine and sulfur dioxide can remove the coke from catalyst. The amount of coke removed depends on the extraction temperature, pressure and duration. Consecutive extractions with two solvents appear to remove more coke than the individual solvents do. Adsorption of certain solvents on the catalyst during extraction can poison the catalyst. Therefore, if poisoning solvents are used for decoking, their remains must be removed from the extracted catalyst to restore the catalyst activity. 

Tetrahydrofuran is not a good solvent for coke removal, but pyridine, carbon dioxide, and sulfur dioxide, can remove significant amounts of coke from the catalysts. The extent of coke removal depends on duration and severity of extraction. 

In addition to coke removal, in two severely extracted cases, the silica foulant of the catalyst could also be removed from the catalyst. 

It seems that fluid-bed cracking reactor (thermal or catalytic) is the best solution for industrial scale. However, regeneration and circulation of so-called equilibrium cracking catalyst is possible for relatively pure feeds, for instance crude oil derived from vacuum gas oils. Municipal waste plastics contain different mineral impurities, trace of products and additives that can quickly deactivate the catalyst. In many cases regeneration of catalyst can be impossible. Therefore in waste plastics cracking cheap, disposable catalysts should be preferably applied. Expensive and sophisticated zeolite and other molecular sieves or noble-metal-based catalysts will find presumably limited application in this kind of process. The other solution is thermal process, with inert fluidization agent and a coke removal section or multi-tube reactor with internal mixers for smaller plants. 

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