Suggested approach for development and Implementation details
We shall undertake trials to establish optimum operating conditions in two steps. Step 1 shall be to gradually reduce the operating pressure in naptha drum # 42D004 in steps of 0.02 kg/cm2g to as low as possible by closing the spill back valve # PV-420906. This can be done by decreasing the pressure set point gradually in steps of 0.02 kg/cm2g. The HGO draw flowrate shall be fixed during the trials and overflash shall be allowed to increase. After pressure is stablilized, the bottom stripping steam flow shall be reduced in steps of 50 kg/hr andoriginal value of overflash flowrate (13m3/hr) shall be restored. We shall continue this exercise until the spill back valve # PV-420906 is completely closed. After that we shall operate the column with a dead band of +/- 0.1 kg/cm2g in set point of naphtha drum operating pressure. The first step would ascertain the minimum pressure at which ADU-2 crude distillation column can be operated. The operator shall also keep a watch on the differential pressure between column top and bottom trays to ensure that reduction in operating pressure does not cause any flooding in trays. The second step is to determine the "true" overflash required in the crude column. The stripping steam will be reduced in steps of 200 kg/hr while maintaining the HGO draw at pre-trial levels. The colour of HGO and its 95% and 100% ASTM D-2887 temperature will be closely monitored. The stripping steam rate shall be reduced until the HGO stream gets a coloured tinge indicating breakthrough of ATB through the wash bed. This would indicate the lower limit of wash oil flow and B57 we must operate above this limit. Stripping steam flow rate shall be fixed at 300-500 kg/hr that the lower limit determined by the trials. The wash rate keeps changing with crude quality and it is important to establish the optimun stripping steam flowrates for three variety of crudes - heaviest, medium and lightest so that operator can alter the set points accordingly. The pressure will be optimized within the dead band of the set point automatically. The HGO 95% and 100% ASTM D-86 distillation points will increase from present values but that should not cause any concern because the stream is a part of VGO feed to DCC. The optimum HGO 95% point will be determined by this exercise and in future an inferential control block may be built in the APC to control predicted ASTM D-86 95% point for HGO at optimum levels.