Delayed coker process

Thermal Cracking. Heavy petroleum fractions such as resid are thermally cracked in delayed cokers or flexicokers (44,56,57). The main products from the process are petroleum coke and off-gas which contain light olefins and butylenes. This stream also contains a considerable amount of butane. Process conditions for the flexicoker are more severe than for the delayed coker, about 550°C versus 450°C. Both are operated at low pressures, around 300—600 kPa (43—87 psi). Flexicokers produce much more linear butenes, particularly 2-butene, than delayed cokers and about half the amount of isobutylene (Table 7). This is attributed to high severity of operation for the flexicoker (43). 

The gas oil feed for the conventional cat cracker comes primarily from the atmospheric column, the vacuum tower, and the delayed coker. In addition, a number of refiners blend some atmospheric or vacuum resid into the feedstocks to be processed in the FCC unit. 

A test run is conducted to evaluate the performance of a 50,000 bpd (331 m /hr) FCC unit. The feed to the unit is gas oil from the vacuum unit. No recycle stream is processed however, the off-gas from the delayed coker is sent to the gas recovery section. Products from the unit are fuel gas, LPG, gasoline, LCO, and decanted oil (DO). Tables 5-2 and 5-3 contain stream flow rates, operating data, and laboratory analyses. The meter factors have been adjusted for actual operating conditions. 

The process was first demonstrated at Cities Service s Lake Charles refinery with the construction of a 2500 BPD unit. The unit was later expanded to 6000 BPD with no increase in the size of the catalytic reactors. Maximum sulfur removal from delayed coker feedstock was achieved by this expansion. 

A typical delayed coker unit consists of coking, fractionation and blowdown sections, along with coke handling facilities. Coker gas is either processed in a dedicated vapor recovery unit or may be sent for processing, together with other gases, to a centralized vapor recovery unit. 

Because of the increased sulfur and impurity levels in crudes currently being processed, refiners in recent years have been considering residue desulfurization units upstream of the delayed coker. In addition to the reduction in sulfur content, residue desulfurization units also lower the metals and carbon residue contents. Due to the reduction in the carbon residue, the liquid product yield is increased and the coke yield reduced. In addition, the coke produced from a desulfurized residue may be suitable for use as anode grade coke. Table I shows the yields and product properties after coking Medium Arabian vacuum residue, with and without upstream residue desulfurization. 

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. 

For convenience, the discussion of materials for these various processes is divided into five chapters. Crude units and utilities are discussed in this chapter. FCCs, fluid cokers, delayed cokers, sour water strippers, and sulfur plants are covered in Chapter Two. Desulfurizers, reformers, hydrocrackers, and flue gas are discussed in Chapter Three. Hydrogen plants, methanol plants, ammonia plants, and gas treating are discussed in Chapter Four. Underground piping, pipelines, production equipment, and tankage associated with the refinery industry are covered in Chapter Five. Discussed throughout these chapters are many common environments and equipment (e.g., sour or foul water, distillation, etc.) that appear in the various types of refinery process plants. 

Visbreaking is a relatively mild thermal (noncatalytic) cracking process that is used to reduce the viscosity of residua. A visbreaker reactor may be similar to a delayed coker with a furnace tube followed by a soaker drum. However, the drum is much smaller in volume to limit the residence time with the entire liquid product flowing overhead. Alternatively, the entire visbreaker may be a long tube coiled within a furnace. Coke formation can occur and the coke accumulates on visbreaker walls periodic decoking (cleaning) is necessary. 

The process of coke formation in a delayed coker has been studied. Rates of reaction and selectivities have been determined from which an overall sequence of coke development is suggested. 

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. 

Figure 5. Schematic of a delayed coker showing the process flow.

Zachary, Bryan 2005. Applying SIS Standards to Coker Processes, 10 Annual Universal Delayed Coking Seminar, Long Beach, CA, August 1 3, 2005. 

The cracker tar which is generated by this process is suitable as feedstock for production of premium coke in the delayed coker the gasoline quality is somewhat better than that of coker naphtha. 

A delayed coker consists of a furnace to heat the feedstock to around 500 °C, two pressure vessels (coke drums) with a diameter of 4 to 7 m and 20 to 30 m high, together with a distillation system to separate the volatile components. Figure 13.8 shows the flow diagram for the delayed coking process. 

FIGURE 2-1 A simplified process flow diagram of a delayed coker... 

I was called to Axxoco Oil Co. lo help increase the capacity of their delayed coker. The refinery was bottlenecked by resid processing capacity so that any incremental increase in coking capacity would permit the plant to increase crude runs substantially. 

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