Delayed coking coke formation

The feedstock is sprayed onto a fluidized bed of hot coke particles which is agitated by the gaseous products rising through the bed in the reactor. The fluidized solids technique permits the use of higher temperatures (than delayed coking) but without the usual overabundance of coke formation because of the shorter contact times with the result that higher yields of liquid products are produced. 

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 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. 

Figure S shows the ratio r1 1 / t, for H ZSM-b crystals of different morphology as a function of the amount of n-hexane coke deposited t.33.> poly-crystal line spherical particles and polyhedral crystals. Once again, two stages of coke iormation can be distinguished. The beginning of the second stage is virtually the same for all polyhedral crystals however, a distinct delay of this onset is to be seen with the poiycrystailine grain. This, experimental finding can be explained by the existence of the secondary pore system represented by the free space between the crystallites. Thus, an additional amount oi coke may be deposited on "neutral" spots outside tne zeolite channel network, causing a delay oi the onset oi the second period oi coke formation.

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. 

The delayed coking unit is where heavy oil residues are converted into coke, which can then be used, say, as fuel for electricity generation. The heavy oil residues are passed through a furnace - a thermal conversion unit where the long chain molecules are cracked - and into large coke drums where the coke formation actually takes place. The term delayed is used to indicate the coke formation does not take place in the furnace (which would lead to a plant shutdown) but, instead, the coke crystallizes in the large coke drums after the furnace. 

On the basis of this discussion, the mechanisms of mesophase carbon fiber formation are closely related to those of needle coke, the principal differences being the extent to which the deformation and relaxation mechanisms are able to act. Because delayed coking involves relatively gentle but random deformation processes by bubble percolation and the long dwell times in the coke drum afford opportunity for extensive disclination annihilation and micro-structural relaxation, the structure of needle coke can be well defined by polarized-light microscopy (2,36). 

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. 

Formation of C8K from Calcined Petroleum Coke. Materials that are less crystalline than metaanthracite do form intercalation compounds. Calcined petroleum cokes are formed by thermolysis at approximately 1400 °C of green petroleum cokes, which in turn are made by thermolysis of aromatic petroleum streams in delayed cokers operating in the range of 400-500 °C (29-32). (Immature cokes of higher H/C ratio are termed green they are not green in color.) The crystallite size, measured both in the direction of aromatic stacking (c axis) and in the in-plane direction (a axis) is of the order 50-60 A, which is less than half the crystallite size of the metaanthracite. However, literature reports indicate that such materials do form intercalation compounds with potassium metal (33, 34). 

---