COKING IN BENZENE VAPOUR

[aberta]

We ahve a By-Products (BPP) plant in our integerated steel mill. The Coke Oven gas is cleaned at BPP and then exported to the Hot Strip Mill Furnace. BPP operations are (for Coke Oven Gas cleaning, Primary Cooler, Exhauster, Electrostatic Tar Precipitator, Ammonia Absorber (used Phosphoric acid), Final Cooler (wash oil), Light Oil Scrubbers (2) and Booster for exdport of gas. The wash oil is used to absorb Light Oils, BTX. The is recirculated after removing Light Oils in our Light Oil plant (LOP). It is in this LOP that we have a problem with our bezene collection system. The LOP consists of Wash OIL Still, Rectifier. The light fraction (from the top) goes to the Light Oil Condenser and Primary Light Oil Separator, where water and oil are separated. The lower fraction goes to the Secondary Light Oil separator. We have a Benzene Emission control system (BEC) on the Primary Light Oil Separator, Secondary Light Oil Separator, Hot wash oil decanter, cold wash oil Decanter, Rercirculating (WO) tank, Five Storage Tanks, Light Oil Loading system. The BEC system collects fumes from all these tanks and is sucked by the negative pressure of the Collection Main which has coke oven gas under hegative pressure (-350mm WC). We have a hot nitrogen purge in the main headers of BEC system. All tanks and overflows are sealed upto plus and minus 100 mm WC. The Bec System is controlled to about -30mmWC. The system has operated well for about three years, but now it is plugged. We found large amounts of black deposites under the final vertical run and also in the long section of horizontal piping. The deposites are being analysed, but initial tests seem to indicate that most of them are insoluble in hot water and appear to be carbon or cike type material which cannot be dissolved.
My question is: can wash oil (or light oils) in the Wash Oil still or The rectifier be COKING. The still top temperature is 115 degress C (uses steam at 250 kPa, at about 200 C) and rectifier (top temperature 75 C).