Steam cracking coke yields

The reaction temperature in DCC is higher than that of conventional FCC but much lower than that of steam cracking. Propylene yields over 20 wt% are achievable with paraffinic feeds. Ethylene yield is much higher than the conventional FCC process. The DCC-mixed C s stream also contains increased amounts of butylenes and iso-C s as compared to an FCC. The high olefin yields are achieved by deeper cracking into the aliphatic components of the naphtha and ECO. The dry gas produced from the DCC process contains approximately 50% ethylene. The cracking reactions are endothermic, and compared to FCC, a higher coke make is required to satisfy the heat balance. Table 1 summarizes typical olefins yields for DCC with FCC. 

Butylene yield Gas oil Residue Delayed coking Elexicoking Steam cracking of naphtha... 

The catalytic behaviour of steam/acid leached Y zeolites appears to be similar to that of USY zeolites. Bremer et al. (64) have reported that at equal conversions, cracking of gas oil over a DAY zeolite results in lower coke yields as compared to those obtained over REY zeolites. 

Table 10.2 presents the total coke yields and the nonvaporized hydrocarbons produced over a spent catalyst obtained with different feedstocks. The catalyst used was deactivated for 20 hours, 30 ReDox cycles, and 50% steam. When the 100% vacuum gas oil (VGO) is replaced with a mixture of 5%w DMO-VGO and/or 30%w DMO-VGO an increase of 30% and 120% in the coke yields was observed. While the spent catalyst from VGO cracking does not have adsorbed hydrocarbons, the mixture with DM0 does, becoming almost 1% for the mixture with 30%w DM0. The SARA (saturates, aromatics, resins, and asphaltenes) analysis of these hydrocarbons showed a high concentration of asphaltenes. 

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 effect of the decoking operation on coke formation in subsequent cracking runs was also studied and the results shown pictorially in Figure 10. On an uncoated Incoloy 800 tube, the high coke yield remained approximately constant throughout four successive steam cracking/air decoking cycles. This tube was subsequently coated with silica to produce an immediate xlO... 

Figure 10. Coke yields at 850°C in successive steam cracking/air decoking cycles...

Improved catalyst/oil mixing achieved by good atomizing nozzles results in less thermal cracking and yields that are more selective to gasoline. The presence of the large quantities of diluent in the riser (the steam) reduces hydrocarbon partial pressure and hence reduces coke make (see Table 2). 

Fluid coke is produced in a fluidized bed reactor where the heavy oil feedstock is sprayed onto a bed of fluidized coke. The oil feedstock is cracked by steam introduced into the bottom of the fluidized bed reactor. Vapor product is drawn off the top of the reactor while the coke descends to the bottom of the reactor and is transported to a burner where a portion of the coke is burned to operate the process. Fluid coke reactors are operated at about 510 - 540°C (950 - 1000 F). Flexicoke is a variant cm fluid coke, where a gasifier is added to the process to increase coke yields. Fluid coker installations tend to have yields that are lower than delayed coker installations, while flexicoker installaticms have yields that can be significantly greater than delayed coker installations. Fluid cokers produce layered and non-layered cokes. Both delayed and fluid coke installations produce amorphous, incipient, and mesophase cokes with the amorphous cokes having higher volatility and mesophase cokes having the lowest volatility. 

In the second step, the dioxanes are vaporized, superheated, and then cracked on a solid catalyst (supported phosphoric acid) in the presence of steam. The endothermic reaction takes place a about 200 to 2S0°C and 0.1 to OJ. 10 Pa absolute. The heat required is supplied by the introduction of superheated steam, or by heating the support of the catalyst, which operates in a moving, fluidized or fixed bed, and, in this case, implies cyclic operation to remove the coke deposits formed. Isoprene selectivity is about SO to 90 mole per cent with once-through conversion of 50 to 60 per cent The 4-4 DMD produces the isoprene. The other dioxanes present are decomposed into isomers of isoprene (piperylene etc.), while the r-butyl alcohol, also present in small amounts, yields isobutene. A separation train, consisting of scrubbers, extractors and distillation columns, serves to recycle the unconverted DMD, isobutene and fonnol, and to produce isoprene to commercial specifications. 

Gasification Kinetics of Coke Deposited on Silica-Alumina. Within the temperature range 1400 to 1600°F and in the presence of excess steam, the gasification reaction of coke deposited on the silica-alumina cracking catalyst closely followed first-order kinetics with respect to unreacted carbon (Figure 1). First-order rate constants were calculated from the slopes of these plots (Table III), and yielded an activation energy of 55.5 Kcal/mole. 

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