Delayed coker production

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). 

Vapors from the top of the drum are directed to the fractionator where they are separated into gases, naphtha, kerosine, and gas oil. Table 3-3 shows products from a delayed coker using different feeds. ... 

Feeds and products from a delayed coker unit (using different feeds) ... 

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. 

During the past 25 years United States petroleum coke production has increased from less than 1,000,-000 tons per year to 3,400,000. Most of the 1,000,-000 tons were produced in externally fired shell stills and manually removed from the stills. Almost all of the 3,400,000 tons were produced in large "delayed coker" type coke drums and removed mechanically. As was true 25 years ago, the major use for petroleum coke is as a fuel, although the proportion so used is declining. Its use in aluminum production is growing rapidly and currently takes about 18% of the production. 

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. 

Three operating control variables in a delayed coker dictate the product quality and yields for a given feedstock. These variables are the heater outlet temperature, coke drum pressure and the ratio of recycle to fresh feed. 

A decrease in pressure has the effect of vaporizing more heavy hydrocarbons. As the production of desirable liquid hydrocarbons is increased at low pressures, the coke yield is correspondingly decreased. Thus, most modern delayed cokers have been designed to operate at a low coke drum pressure. 

Illustrated in Table I are the estimated yields and product qualities of several representative delayed coker feedstocks. The yields were established from generalized correlations using typical operating conditions for the operation noted. 

One of the largest uses of petroleum coke is for anodes employed in the production of aluminum. This usage demands a somewhat premium feedstock to produce sponge coke that is low in metal and sulfur content in order to meet product quality specifications. After production in a delayed coker, anode quality coke must be calcined to remove VCM and moisture. 

A specialized application of petroleum coke is the production of electrodes for the steel industry. For this application, it is necessary to use needle coke because its low coefficient of thermal expansion and low resistivity. The needle coke must have low sulfur and low metals content. After production in a delayed coker, needle coke is crushed and calcined in preparation for electrode production. 

The product, called fluid coke (FC), is produced by coking of liquid feedstock coating tiny coke seeds as they are agitated on a fluid bed, where the temperature is 100-150°C above that for delayed cokers. Relatively little fluid coke is used in the aluminum industry because it is often high in impurities (sulfur and metals), and available only as submillimeter particles which are harder to grind and bind into the anode. 

In present-day refineries, the fluid catalytic cracking (FCC) unit has become the major gasoline-producing unit. The FCC s major purpose is to upgrade heavy fractions, that is, gas oil from the atmospheric and vacuum distillation columns and delayed coker, into light products. Atmospheric gas oil has a boiling range of between 650-725°F.9... 

The catalytic cracking unit is often referred to as the gasoline workhorse of a refining unit. As shown in Fig. 18.9, feeds to the catalytic cracking unit are gas oils from the atmospheric and vacuum distillation columns and delayed coker. These heavier fractions also carry metals such as nickel, vanadium, and iron. More important, sulfur compounds concentrate in the heavier product fractions. Table 18.8 lists a typical mass balance for sulfur.25 FCC blend-stocks comprise 36 percent of the volume of the gasoline pool. However, this stream also contributes 98 percent of the sulfur concentration to blended procucts.25 As specifications on sulfur concentrations in diesel and gasoline tighten, more efforts are focused on how feeds and product streams from the FCC are pre- and posttreated for sulfur concentrations. 

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. 

Fluid coking and fluid catalytic cracking (FCC) are mechanically similar The products of fluid coking and delayed coking are the same (i.e., coke and distillate products), but the equipment is physically different. Alkylation of the three- and four-carbon molecule products from these units is commonly performed to convert them to branched chain gasoline, which increases the octane rating. As can be seen from Figure 1.1 in Chapter One, the feed to fluid catalytic crackers is a gas-oil distillate. For delayed cokers and fluid cokers, the feed is residium. 

After the reactions have occurred in the catalytic crackers, cokers, and alkylation plants, the effluents are separated by fractionation to obtain the various products desired. The materials for the fractionation equipment are selected using basically the same criteria as those used for crude units, as discussed in Chapter One [i.e., carbon steel below 550 to 600°F (288 to 315°C) and 5Cr-V6Mo piping and 12Cr clad vessels above]. However, there are some slight differences from crude units. An example is furnace tubes in delayed cokers, which are usually 9Cr-t Mo (because of the high-fluid temperatures) rather than 5Cr-V Mo. 

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. 

In refinery applications such as the delayed coker, linear poly-(dimethylsiloxane)s of intermediate molecular weight are used. These products are normally described in terms of their viscosity in centistokes. Thus a 60,000- or 100,000-centistoke silicone fluid would be the typical recommendation for use in a delayed coker. More viscous and consequently higher molecular weight silicones have been used as described in U.S. Patent 3 700 587 (11). 

Figure 5 is a diagram of a typical delayed coker, including the distillation tower and light ends section. The feedstock is preheated using some of the heavy and light gas-oil product streams. It then enters the lower portion of the fractionation tower, above the point where the product vapors from the coke drums enter the tower. This introduction of the... 

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. 

Figure 13.9 shows the four delayed coker units operated by Conoco, Immingham/England, which have a production capacity for 300,000 tpa of premium coke. 

A delayed coker combination tower is similar in design and function to an ordinary crude distillation tower heat is removed and products are fractionated. Referring to Figure 3-1, four products are made wet gas, unstabilized naphtha or wild gasoline, furnace oil, and heavy gas oil. 

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