Delayed coking processes

Petroleum coke is the residue left by the destructive distillation of petroleum residua in processes such as the delayed coking process (Figure 11.4). That formed in catalytic cracking operations is usually nonrecoverable, as it is often employed as fuel for the process. 

When compared to the delayed coking process, higher yields of liquid products are typically produced by fluid coking. This continuous process utilizes a fluidized reaction zone of hot coke particles held in motion by steam. The coke particles are first heated in a burner to temperatures ranging from 1,100°F to 1,200°F (593.3°C to 648.9°C). The hot coke particles then are blown into the reactor by steam. The residual fuel is fed into the reactor and cracks on the hot surface of the fluidized coke particles. 

The ASCOT process is a residual oil upgrading process which integrates the delayed coking process and the deep solvent deasphalting process (low energy deasphalting, LED A) (Bonilla, 1985 Bonilla and Elliott, 1987 Hydrocarbon Processing, 1996). 

Properties of the petroleum cokes used in the present test are shown in Table III. The symbol MPC represents petroleum coke manufactured with the use of Minas heavy oil by the delayed coking process, and DPC and FPC are, respectively, petroleum cokes provided by a delayed coker and a fluid coker commercially available in Japan. 

J. Zhon and X. Zhang, Co-processing of waste polyethylene with vacuum residue in delayed coking process. Preprints, 43(1), 194-198 (1998). 

The delayed coking process is the oldest and the most popular choice for resid upgrading. It is a semicontin-uous process that can be applied to the conversion of most types of resids and residua. 

Small particles of coke made in the process circulate in a fluidized state between the vessels and are the heat transfer medium. Thus the process requires no high-temperature preheat furnace. Fluid coking is carried out at essentially atmospheric pressure and temperatures in excess of 485°C (900°F) with residence times on the order of 15-30 seconds. The longer residence time is in direct contrast to the delayed coking process, in which the eoking reactions are allowed to proceed to completion. This is evident... 

Delayed coke (Table 16.1) is produced during the delayed coking process—a batch process—from vacuum residua (Chapter 2) (Speight and Ozum, 2002). The carbonization (thermal decomposition) reactions involve dehydrogenation, rearrangement, and condensation. Two of the common feedstocks are vacuum residues and aromatic oils. 

A second set of comparisons refers to the H/C ratio of the coke obtained in a delayed coking process. This material remains inside the coking drums for about 24 h and its final H/C ratio becomes about 0.5 (de Freitas Sugaya, 1999). The model is able to predict this ratio both by maintaining about 700 K for 24 h and also by decreasing the temperature profile (from 715 K to 690 K) according to the temperature evolution of the coking drum. 

This chapter presents the industrial applications and validations of certain detailed models which refer to the kinetics analysed earlier. The steam cracking process will be analysed first followed by visbreaking and delayed coking processes. Last of all, the method will be applied to the thermal degradation of plastic waste. 

These reactions are very important, principally in the delayed coking process because of the high residence time in the drum of up to 24 h. There is, therefore, a clearer need for appropriate lumping in the case of delayed coking. 

The kinetic aspect common to all the topics discussed in this chapter is the pyrolysis reactions. The same kinetic approach and similar lumping techniques are conveniently applied moving from the simpler system of ethane dehydrogenation to produce ethylene, up to the coke formation in delayed coking processes or to soot formation in combustion environments. The principles of reliable kinetic models are then presented to simulate pyrolysis of hydrocarbon mixtures in gas and condensed phase. The thermal degradation of plastics is a further example of these kinetic schemes. Furthermore, mechanistic models are also available for the formation and progressive evolution of both carbon deposits in pyrolysis units and soot particles in diffusion flames. 

In a delayed coking process only the volatiles are removed while feed is continuously added to the carbonizing mass. Both the fast and slow reactions axe consecutive for the incremental feed and concurrent with the material already in the coker which is in the process of coking. Thus, when the coker is being fed the amount of volatile products measured is the sum of the products produced by the fast reaction and by the slow reaction. After the feed is stopped and the rate drops, another constant but slow rate of product formation is observed. From the calculated slow rate of product formation Rs, the proportion contributed by the slow reaction during conventional operation could be backed-out and the rate of the fast reaction, alone, calculated. 

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