Coal tar is a dark viscous material obtained as a by-product during the process of destructive distillation of coal (to produce coke oven gas and coke).
Coal when heated to 1050oC, coke oven gas is liberated; it contains valuable chemical compounds. Hot coke oven gas from the battery is cooled to separate coal tar along with by-products like phenol, light oil, wash-oil, anthracene, pitch and naphthalene. The tar and ammonical liquor condense from the gas stream. Purification of tar takes place in the coal tar treatment plant and it contains a coal tar distillation section to separate the other chemical components. Initially, tar with water content, not more than 4% is supplied by a pump to the decanter of the tar distillation section, then it is delivered by the plunger pump to the 1st stage of the pipe still furnace, where it is heated up to a temperature of 115oC-130oC.
The heated coal tar is delivered to the evaporator of the 1st stage and separated to liquid and vapor. The light oil contains traces of naphthalene. They are removed from the top of the 1st stage evaporator and sent to the condenser and cooler where they are cooled down and further flow to a separator.
Ammonia water discharged from the bottom of the separating tank to a collecting tank. The light oil from the upper part of the separator is delivered to a collecting tank then it is mixed with light oil obtained from the separator of the fractionating column, after which this oil is pumped up to the fractionating column for reflux with the help of a pump.
Ammonia water from collecting tank is delivered to the pump house of tar. Light oils are sent to a collecting tank and then it is further sent to the section of gas condensation pump houses.
Dewatered coal tar from the 1st stage evaporator flows down to dewatered tar collecting tank. Excessive quantity of the dewatered tar from collecting tank overflows through a pipeline to the decanter. It is pumped by the plunger pumps to the 2nd stage evaporator of the pipe still furnace. The tar is heated up to 380-400oC and sent to 2nd stage evaporator for separation of vapors and pitch.
The pitch from the bottom part of the 2nd stage evaporator is discharged to the head tank or to the reactors of pitch liquid and cooling area (PLCA).
Vapour containing the mixture of different tar fractions, removed from the upper part of the 2nd stage evaporator and enter the third plate of the fractionating column for separation of fractions. The fractionating column consists of 59 plates and has an inlet for superheated steam.
Chemical products separated by fractionating column in coal tar distillation section
Anthracene – 1:
Anthracene–1 fraction is obtained in a relatively pure liquid state at a temperature between 280oC–290oC. This temperature is maintained on trays 6 and 8. Therefore, Anthracene–1 fraction can be tapped either from tray no. 6 or from 8 depending on the purity of the final fraction.
There are 5 tapings provided for wash oil on tray no’s 20, 18, 16, 14 and 12. Normally wash oil can be tapped at a temperature of 230oC-240oC which is optimum and is maintained on tray no – 12.
Naphthalene fraction tapped from tray no’s 30, 28, 26 and 24. The optimum temperature for Naphthalene tapping is 190oC – 200oC.
Phenol fraction separated in a pure state at temperatures about 160oC-180oC.
The light oil obtained from the top of the distillation column. The top of the distillation column is maintained at a temperature between 110oC–120oC. This light oil consists of smaller amounts of undistilled phenol and water. It is then condensed and cooled in twin condenser coolers, which are shell and tube type heat exchangers where vapor is passed to the shell side and water on the tube side. The condensed light oil along with small quantities of phenol and water is fed to separator – 2.
Separators are used to separate light oils from other fractions ( especially NH3-Water, phenol and water) obtained from the tops of the 1st stage evaporator and distillation column are sent to separator-1 and separator-2 respectively. The working and construction of each separator are same. Light oil is obtained from the top of the two separators, flows to the light oil tank by gravity. At the bottom of separator-1, ammonia water will be settled. At the bottom of separator-2, phenol-water will settle and both are removed through dip tubes to prevent light oils mixing with them.
Reflux for distillation column:
Part of the light oil from the light oil tank is used as a reflux to the distillation column. A reflux pump is used for this purpose. In order to regulate the flow of oil to the distillation column, a control valve is installed. Based on the temperature of phenol on tray no 48, this control valve operates at the temperature of phenol (185oC), reflux is sent back to the distillation column and the control valve of the light oil tank will be closed.