Delayed coking operating conditions

The yields and product distribution obtained in delayed coking are functions of the character of the feed stock and the operating conditions. Gas oil yields of 60 to 85 volume % with coke yields of 10 to 30 weight % of the feed are reasonably representative 5). 

There are basic differences in coke sizing and structure for delayed and fluid cokes because of differences in the two coking operations. Sometimes, fluidization occurs in delayed coke drums when the feed has a high asphaltene content and the recycle ratio is low. Such conditions tend to produce millimeter-size "shot" coke, which is very isotropic, and yields a calcined coke which is very hard with low porosity and high density. Such coke is hard to... 

Apart from these rules of thumb, various correlations have been reported in the literature to calculate the yields, API gravity, and snlfur content of the products of delayed coking. Most of them only use the CCR content of the feed, while only a few incorporate the effect of pressure and temperature. The correlations have been developed using a wide range of type of feeds, whose properties can be found in the respective literature. Due to the empirical nature of correlafions, applying them beyond the range of feed properties, operating conditions, and product yields from which they were derived is risky, and the calculated values should be taken only as approximate estimates. 

Old correlations were derived from commercial data obtained in delayed cokers operating at different conditions, particularly temperature and pressure, than those used in current coking units. That is the main reason why they fail when applied to other reaction conditions. 

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). 

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