Liquid Fuels: Petroleum

II. LIQUID FUELS

PETROLEUM

Petroleum or crude oil is naturally occuring liquid fuel. It is a dark brown or black coloured viscous oil found deep in earth's crust. The oil is usually floating over a brine solution and above the oil, natural gas is present. Crude oil is a mixture of paraffinic, olefinic and aromatic hydrocarbons with small amounts of organic compounds like N, O and S.

The average composition of crude oil is as follows

1. Classification of Petroleum

Petroleum is classified into three types.

1. Paraffinic-Base type crude oil

It contains saturated hydrocarbons from CH₄ to C₃₅ H₇₂ with a smaller amount of naphthenes and aromatics.

2. Naphthenic (or) Asphaltic Base type crude oil

It contains cycloparaffins or naphthenes with a smaller amount of paraffins and aromatics.

3. Mixed Base type crude oil

It contains both paraffinic and asphaltic hydrocarbons.

2. Refining of Petroleum or Crude Oil

The crude oil obtained from the earth is a mixture of oil, water and unwanted impurities. After the removal of water and other impurities, the crude oil is subjected to fractional distillation. During fractional distillation, the crude oil is separated into various fractions.

Thus, the process of removing impurities and separating the crude oil into various fractions having different boiling points is called Refining of Petroleum. The process of refining involves the following steps.

Step 1:  Separation of water (Cottrell's process)

The crude oil from oil well is an extremely stable emulsion of oil and salt water. The crude oil is allowed to flow between two highly charged electrodes, where colloidal water droplets combine to form large drops, which is then separated out from the oil.

Step 2 : Removal of harmful sulphur compounds

Sulphur compounds are removed by treating the crude oil with copper oxide. The copper sulphide formed is separated out by filtration.

Step 3 : Fractional distillation

The purified crude oil is then heated to about 400°C in an iron retort, where the oil gets vapourised. The hot vapours are then passed into the bottom of a “fractionating column (Fig 5.2). The fractionating column is a tall cylindrical tower containing a number of horizontal stainless steel trays at short distances. Each tray is provided with small chimney covered with a loose cap.

Fig 5.2 Fractional distillation of crude petroleum

When the vapours of the oil go up in the fractionating column, they become cooler and get condensed at different trays. The fractions having higher boiling points condense at lower trays whereas the fractions having lower boiling points condense at higher trays. The gasoline obtained by this fractional distillation is called straight-run gasoline. Various fractions obtained at different trays are given in table 5.3.

Table 5.3 Various fractions, compositions and their uses