Gas-Fractionation Plant

an installation used for the separation of mixtures of light hydrocarbons into individual, or industrially pure, substances.

Gas-fractionation plants are part of natural gasoline plants, gas refineries, and chemical and petrochemical processing plants. The capacity of gas-fractionation plants may be as high as 750,000 tons of raw material per year, including natural gasolines (which are produced from natural and refinery gases), petroleum stabilization products, and pyrolysis and cracking gases. The raw materials are composed mainly of hydrocarbons containing one to eight carbon atoms per molecule. The separation of the hydrocarbon mixtures is performed by fractional distillation in column distillers.

The method of separating natural gasoline in a gas fractionation plant includes preheating of the natural gasoline in a heat exchanger and feeding it to a propane column (see Figure 1). The propane vapors, which are condensed in a condenser-cooler and fed to a reflux vessel, are drawn off from the top part of the column. Some of the propane is returned to the top of the column as reflux; the excess is drawn off in the form of a finished product. After preheating, the liquid from the bottom of the column is fed for further separation by the same method into the next column, where a mixture of butanes is separated from the liquid in the form of overflow, and gasoline is drawn off from the lower part. The separation of butanes into isobutane and normal butane, and gasoline into isopentane, normal pentane, and hexanes, and so on, is performed in a similar manner. The approximate pure-substance content after processing of natural gasoline is

Figure 1. Diagram of a gas-fractionation installation: (1) propane column, (2) stabilization column, (3) isobutane column, (4) condenser-coolers, (5) preheaters for bottom of column, (6) heat exchangers, (7) coolers

as follows: propane 96 percent, isobutane 95 percent, normal butane 96 percent, isopentane 95 percent, and stable gasoline 74 percent (in commercial products with these names).

Improvements in the technological system of gasfractionation plants are designed to reduce capital expenditures and power costs and to automate monitoring and control systems by means of electronic computers and by the installation of chromatographic product quality analyzers on flow lines.