Cracking is the process of dissociating or separating higher molecular hydrocarbon to lower hydrocarbon at elevated temperature. This is known as thermal conversion process. Cracking can be catalytic or non catalytic process. The opposite of cracking is reforming. This has become reality in petroleum refineries in order to bridge the gap of some petroleum products or fractions production. Cracking was discovered by Jesse Dubbs in 1913.
Thermal cracking reaction
At temperature above 400˚c, long chain hydrocarbon (paraffin) decomposes. Alkane compounds easily split into fragments as the temperature increases. The degree of splitting depends on the size of the compound. Also, the process adopted for dissociation employed.
In thermal cracking, radicals are formed. Theses radicals combined with other hydrocarbons to form olefins. The general formula for cracking is given as
The cracking reaction follows the reactivity order of
n-paraffin> Iso-paraffin> cycloparaffin> aromatics> naphthenes> polynuclear aromatics. The procedure used for saturated aromatics and unsaturated aromatics differs. Example includes:
Thermal cracking was the common method used to obtain distillates from crude oil. Temperature and residence time are the major system parameter considered for greater yield. Introduction of thermal catalytic cracking enhanced the process. Thermal cracking facilitated the production of petroleum product (Naphtha and heavier distillate). The gaseous product of cracking process is propylene and butylenes which are used to produce polymer gasoline of high octane number. The thermal cracking can be summarized as below:
|Temperature (˚c)||Nature of operation||product|
|560-520||Thermal cracking||Gas, gasoline etc|
|520-600||Low temperature coking||Gasoline soft etc|
|600-800||Gas production||Gas & unsaturated compounds|
|800-1000||High temperature coking||Heavy aromatics|
|Above 1000||Decomposition||H2 gas, carbon black|
This is the process of converting heavy gas oil into light and middle distillate. These heavy gas oils are recovered from vacuum distillation unit. One of the aims of visbreaking is to reduce the residue viscosity (fuel oil). The product of visbreaking can be used as fuel for boilers and heaters.
In visbreaking operation, the temperature of the heat exchanger is above 485˚c with sufficient residence time. The pressure is between 10-15Kg/cm2 which aid in keeping the coke formation reaction dormant. at lower pressure, the yield becomes higher of gas oil and naphtha. The flow diagram is shown below
Properties of cracked products
Cracked products should possess the following:
- Octane number increased
- Oxidation stability increased
- Characterization factor decreased
- Unsaturation and aromatization increased
- Increase in sulphur content
- Boiling, pour point and viscosity decreased.
Dr.Ram Prasad. Petroleum refining technology, first edition by KHANNA publishers 2012. P.192-198
Bhaskara Rao.B.K. modern petroleum refining process, fifth edition.vijay primlani publishing company LTD.