GAS AND GASOLINE FRACTIONATING

◆ GAS AND GASOLINE FRACTIONATING ◆

1. PURPOSE AND PRINCIPLE OF THE INSTALLATION

The purpose of the gas and gasoline fractionating installation is to fractionate the gas + gasoline cut obtained at the top of the atmospheric distillation column. It generally consists of several conventional 2 products distillation columns and the scheme below shows an example of the separation obtained.

2. UNIT FLOW SCHEME - OPERATING CONDITIONS

A simplified flow scheme of a gas and gasoline fractionating unit it shown in Figure 6. Prior of this unit, a hydrogen catalytic desulfurizing installation converts the sulfur present in the atmospheric distillation gas + gasoline cut into volatile hydrogen sulfide (H2S) and it is then largely eliminated in the acid gas.

2.1 Gasoline Stabilization

The initial separation is carried out in a gasoline stabilization column or debutanizer which separates the gas + gasoline cut into gas(C4 and more volatile products) obtained at the top of the column, and into stabilized total gasoline which is removed from the bottom of the column. The operating pressure of this column is around 10 bars which results in a temperature of around 70 °C at the top and generally over 200 °C at the bottom.

2.2 Gas Fractionating

The gas is cut subsequently fractionated in the deethanizer and the depropanizer. The combustible gas cut or fuel gas (C1+C2) is separated in gaseous form in the upper part of the debutanizer. A relatively high operating pressure (20 to 30 bars) is required to condense the reflux. The propane-butane mixture obtained at the bottom of the column is separated in the depropanizer, giving a propane cut at the top and a butane cut at the bottom. The column operates at a pressure of approximately 17 bars. The gas fractionating flow scheme calls for two comments:    - the very volatile vapors at the top of the deethanizer are very difficult to condense and this leads to inevitable losses of propane       in the combustible gas network,    - the presence of H2S whose volatility is between that of ethane and propane involves an elimination treatment (amine washing) of the       gas cut obtained at the top of the debutanizer.

2.3 Gasoline Fractionating

The stabilized total gasoline obtained at the bottom of the debutanizer is circulated to the gasoline fractionating column (gasoline splitter) where light gasoline is obtained at the top and heavy gasoline at the bottom. The operating pressure of this column is slightly higher than atmospheric pressure (around 1.5 bars).

2.4 Quality of the Cuts Obtained

The quality of the products obtained is controlled by the following standard tests:    - vapor pressure and evaporation for LPG,    - ASTM distillation for gasoline Modern units are often equipped with continuous analysis devices (chromatography in the gaseous phase, for example) which replace the tests mentioned above.

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