Subject:- Applied Chemistry 2.

Topic:- Fuel

Difficulty:- High

Q3.3 Define cracking & discuss fixed bed and movable bed cracking[Difficulty level-high] Fluidized bed Catalytic cracking is process in which heavy oil is heated in presence of a catalyst. Generally used catalysts are crystalline substances e.g. bauxite, zeolite, crystalline aluminosilicate. And bentonite etc.The temperature is adjusted apt as where heavy oil gets vaporized. During the process heavy oil gets cracked and form lower hydrocarbon one saturated and one unsaturated and one unsaturated. C16H34 → C8H18 + C8H16 Hexadecane octane octene Fixed bed cracking In this method , vapours of the heavy oil are heated in the presence of catalyst due to which better yield of petrol is obtained. In this method heavy oil is vaporized by heating in an electrical heater. Then the vapours are passed over a series of trays containing catalyst.Generally catalysts used are bauxite, zeolite, crystalline alumina silicate. And bentonite etc The reaction chamber is maintained at T = 425 – 5400 C P = 1.5 kg / cm2 The cracked gases are taken out from the top of the reaction chamber and allowed to pass into fractionating tower, where gasoline fraction is collected. The octane value of Gasoline is about 80-85. During the cracking free carbon is also formed which deposits on catalyst then flow of vapours of heavy oil is passed over the second set of reaction chamber and the catalyst in earlier chamber is regenerated by burning the carbon deposits with the help of air and reused. Moving bed bed or fluidised bed catalytic cracking In fluidized bed catalytic cracking a fine powder of catalyst is circulated through the cracker along with the vapours of heavy oil. The catalyst accelerates and directs the cracking efficiently to form gasoline and other lower hydrocarbon C18H38 → C10H22 + C8H16 Octadecane decane octene

C16H34 → C6H14 + C10H20 Hexadecane hexane decane

C16H34 → C8H18 + C8H16 Hexadecane octane octane In modern refineries fludised bed catalytic cracking is used , The major part of process involves crackers and regenerators, which are kept side by side. In this method heavy oil and catalysts is heated in the still to convert heavy oil into vapours. These vapours along with hot catalyst are brought to the cracker . The cracker is maintained at, T= 550 – 5700 C P = 1 kg / cm2
In the cracker the vapours of heavy oil and hot catalyst come in intimate contact with each other and the breaking of high molecular weight hydrocarbons into lower hydrocarbons takes place. The low boiling hydrocarbon moved upto top of the cracker which are passed through cyclone separator while the catalyst remains in the cracker. These cracked gases are further passed through the fractionating tower to have three major fractions. a. Gasoline b. Middle oil c. Heavy oil The gasoline is further cooled and purified to remove impurities of sulphur ,unsaturated hydrocarbons an colouring matters, if present. The catalyst performs two functions a. To get better quality of gasoline during cracking process b. To carry heat during process Regeneration of exhausted catalyst Catalyst gets deactivated due to deposition of free carbon on catalyst. The deactivated catalyst is taken from the bottom of the cracker and brought into regenerator, where it is heated to about 5000 C in presence of hot air to burn carbon dioxide which is taken out from the top of the regenerator. The regenerated catalyst in hot condition is taken down to the vapours of heavy oil and recirculated in the cracker