15.1 Introduction
In industrial processes, hydrocarbons are contacted with H 2 O, depending upon the desired effect. When hydrocarbon vapors at very high pressures are contacted with water, water which has a very high latent heat of vaporization quenches the hydrocarbon vapors and transforms into steam. In such an operation, chemical transformations would not be dominant and energy lost from the hydrocarbons would be gained by water to generate steam. The quenching process refers to direct contact heat transfer operations and therefore has maximum energy transfer effeiciency. This is due to the fact that no heat transfer medium is used that would accompany heat losses. The steam cracking of hydrocarbons is an anti-quenching operation, and will involve the participation of water molecule in reactions in addition to teh cracking of the bnydriocarbond on their own. Since steam and the hydrocarbons react in the vapour phase the reaction products can be formed very fast. Therefore cracking of the hydrocarbons on their own as well as by steam in principle is very effective.
When steam cracking is carried out, in addition to the energy supplied by the direct contact of steam with the hydrocarbons, steam also takes part in the reaction to produce wider choices of hydrocarbon distribution along with the generation of H2 and CO.
- • Hydrocarbons such as Naphtha and LPG have lighter compounds.
• When they are subjected to steam pyrolysis, then good number of petrochemicals can be produced.
• These include primarily ethyelene and acetylene along with other compounds such as propylene, butadiene, aromatics (benzene, toluene and xylene) and heavy oil residues.
• The reaction is of paramount importance to India as India petrochemical market is dominated by this single process.
15.2 Reaction
CxHy + H2O + O2 → C2H4 + C2H6 + C2H2 + H2 + CO + CO2 + CH4 + C3H6 + C3H8 + C4H10 + C4H8 + C6H6 + C + Heavy oils
- • The reaction is pretty complex as we produce about 10 to 12 compounds in one go.
• The flowsheet will be reaction-separation-recycle system only in its topology. But the separation system will be pretty complex.
• Almost all basic principles of separation appears to be accommodated from a preliminary look.
• Important separation tasks: Elimination of CO and CO2, Purification of all products such as ethylene, acetylene etc.
• The process can be easily understood if we follow the basic fundamental principles of process technology.
• Typical feed stocks are Naphtha & LPG.
• Reaction temperature is about 700 – 800°C (Vapor phase reaction).