Physical separation of crude oil was not sufficient to meet the demand for motor fuels that had increased significantly with the increasing number of automobiles. Thermal cracking was therefore introduced as the first conversion process to produce more distillate fuels from petroleum refining. Thermal cracking, starting typically with breaking the C-C bonds in alkanes, proceeds with free radical chain reactions. One of the common outcomes of thermal cracking is to make light or medium distillates from the heavier fractions of the crude oil. Visbreaking, a mild (low-severity) thermal cracking process, reduces the viscosity of the VDR by rejecting a small quantity of coke as deposits on reactor surfaces. The principal product of visbreaking from VDR is a heavy fuel oil. On the other end of the thermal conversion spectrum, coking, the most severe thermal cracking process, converts VDR to light distillates and gaseous products by rejecting carbon in large quantities in the form of petroleum coke. If the sulfur and metal contents of VDR are sufficiently low, the petroleum coke can be a valuable by-product that is used for producing anode coke for electrolysis of alumina to produce metallic aluminum.
You should now be able to:
- summarize the chemistry of thermal cracking and free radical chain reactions;
- apply thermal cracking of gas oil to produce lighter distillates and examine how thermal reactivity affects process configuration;
- appraise thermal cracking for upgrading of residual fractions (visbreaking and coking) and interpret thermal severity to compare visbreaking processes;
- analyze and compare different coking processes: Delayed Coking, Fluid Coking and Flexicoking.
Reminder - Complete all of the Lesson 6 tasks!
You have reached the end of Lesson 6! Double-check the to-do list on the Lesson 6 Overview page to make sure you have completed all of the activities listed there before you begin Lesson 7.
|Readings||J. H. Gary, G. E. Handwerk, Mark J. Kaiser, Chapter 5|
|Assignments||Exercise 5: A coil visbreaker operates at 500°C for 1 min. How long will it take to achieve the same thermal severity at 450°C in a soaker visbreaking process? An apparent Arrhenius activation energy for thermal cracking is given as 50 kcal/mol.|
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