CHAPTER No . 4

AVIATION GASOLINE BASE STOCK FROM CONVERSION PROCESS:

Many conversion process have been develop for the production of high antiknock aviation gasoline. The greater percentage of the regular and premium grades of this type of fuel consists of conversion products.

The following general classes of conversion processes have been developed.

cracking
polymerization
reforming

CRACKING:

An average crude oils on distillation yields 20 – 25 % gasoline, 30 – 45 % intermediate oils (kerosene, naphtha etc) and 25% - 50% residual fuel oils. Increasing demand for high quality gasoline exceed the availability of gasoline by distillation alone. Hence, cracking of heavier fuel oils is done to produce mainly gasoline.

TYPES OF CRACKING:

There are two types of cracking thermal and catalytic cracking.

Thermal Cracking:

In this process, the fuel which may be gas oil fuel oil (heavy or light) atmospheric or vacuum residue is heated upto 450 – 750 ^oC at pressure ranging from 1 – 70 atm to produce gas gasoline etc. Octane number of gasoline obtained may be up to 75. The yield and quality of the gasoline obtained will depened upon the type of feed temperature and pressure.

Catalyhe Cracking:

In this process gasoline produced having in general a higher octane value and lower sulphur content than those non catalytic or straight thermal cracking processes depending upon the operating conditions the product may contain a much lower percentage of olefins. A typical yield pattern in cracking is given be low.

Feed residue of vacuum distillation of crude yields gasoline = 55% , Diesel = 20% , Fuel oil = 15%, Coke = 10% , Octane number of gasoline = 92.

(b) Polymerization:

The tem polymerization may be define as combination of smaller hydrocarbon molecules to form largers molecules. This method utilizes the waste gases from cracking processes to form hydrocarbons boiling in the gasoline range. This increase gasoline yield and also quality since the polymer gasoline has a relatively higher octane number.

Only unsaturated compounds will combine under polymerization conditions and the product is generally olefin in nature. Polymerization is carried out under elevated temperatures and pressures. High temperatures in general tend to crack rather than polymerize low temperature and higher pressure favour polymerization. Catalysts are used under modified temperature and pressure conditions to increase the yield.

(c) Reforming:

Reforming means arrangement of molecules with out much affecting the average molecular weight of feed which is generally naphtha of gasoline boiling range. Reforming is carried out to produce high quality gasoline by heating, with or with out catalyst, the naphtha.

Reforming can be thermal or catalytic as in the case of cracking. Catalyst a part from accelerating the process also enhance the yield and quality of gasoline. The gasoline produced by reforming is called reform gasoline or reformat. The reform gasoline has a wider boiling range than the feed and its volatility is increase.

Octane number increases in the order paraffin ® olefin ® naphthane ® isoparaffin ® aeromatic.

The main reaction in the reforming process is dehydrogenation of naphthene to produce aeromatics. Other reactions occurring during reforming are dehydrocylization of paraffins isomerization, hydrocracking, dehydroisomerization, dehydrogenation etc.

1) Dehydrogenation of naphthene to aromatic.

2. Dehydroisomerization:

In a typical reaction methyl eyclopentane isomerises to cyclohexane which dehydrogenates to yield benzene.

3. Dehydrocyclisation of Paraffins to Aromatic.

4. Hydrocracking of higher paraffins to lower paraffin’s.

Decane n – pentane Iso – pentane

Lower boiling point paraffins have higher octane number.

5. Isomerisation of paraffins:

Here , n – paraffins are converted into iso paraffin which have higher octane number.

though olefins have higher octane number than corresponding n-parffins still this reaction is undesirable b/c lower olefins polymerize to produce gummy materials which is un wanted.