INTRODUCTION

Thermal cracking processes include visbreaking and coking, which breaks heavy oil molecules by exposing them to high temperatures. Market for HRFO (Heavy Residuals Fuel Oil) has been decreasing. HRFO have been burned to produce electric power and to supply the energy needs of heavy industry. Severe environmental restrictions have caused many of these users to switch to natural gas. The increasing of HR in the crude there is more difficulty in economically disposing of them. Nowadays demand for the heavier residual fuel oil has decreased due to environmental regulations. This requires refiners to decrease their residual fuel yield and produce more light products. This is achieved using a severe thermal cracking process known as coking.

COKING

Coking, as its name indicates, is the most extreme form of thermal cracking in which the ultimate of total extinction of fuel oil is practiced, the heavy residue feedstock being pyrolysed to produce lighter boiling liquids which can be processed further into motor gasoline blend components and middle distillate, with a remaining yield of by-product coke which can be disposed of as solid fuel. A yield case for a refinery with fluid coker could be 36% gasoline, 42% middle distillate, fuel oil nil and coke 8%- the ultimate in fuel oil reduction. In the two stage flexi-coking process, there is an added gasifier stage in which the coke product can be turned into a low BTU gas. The coking process, of which there are a number of forms, is also usable for such feedstock as bitumen, tar sands and heavy non-conventional crude that are non-fluid at ambient conditions (when the latter come into production).

Coking processes are relatively severe cracking operations designed to convert residual oil products, such as straight-run atmospheric and vacuum residue, tars and pitches, into more valuable lighter products, such as naphtha and diesel oil. In addition, gas and coke are produced. The yield of each product is dependent on the coking conditions and the quality of the feedstock fed to the process. The C5-180°C naphtha may be used, following hydro treatment, as a petrochemical feedstock, or after splitting into C5-80°C and 80°C-180°C fractions as a gasoline blending component and catalytic reformer feedstock, respectively. The 180°C-330°C gas oil fraction, also after hydrogenation, can be a suitable automotive diesel component if derived from a more paraffinic type feedstock, or alternatively can be used as heating oil. The heavy gas oil fraction (330°C upwards) may be used as a fuel oil blending component, fluid catalytic cracker feedstock or, after further processing, as a needle coke feedstock. The coke produced from suitable feedstock may be used for electrode manufacture and the production of chemicals, thus considerably increasing its value. Otherwise this material is used as a fuel. With the current diminishing demand for fuel oils, coking processes are being examined again too see if they will meet individual refiner's requirements. Several large cokers are currently being constructed, particularly in the United States. There are two main types of coking process available, namely, delayed and fluid coking. The latter process has further been developed into a process known as flexi-coking, whereby the fluid coke produced is gasified to produce low thermal value gas.