PTQ Q3 2023 Issue

Revamp

Existing

4”

Coil steam

3,200 lb/hr

Vacuum column

12”

8” 8”/10“/12”

8” 10”

5” 5” 5” 6” 6”

5” 5” 5” 6”

Coil steam

29,000 lb/hr

4” 4” 4” 4” 4” 4” 4” 4”

Fuel gas

Figure 7 Vacuum heater modification

a single system so that the heater outlet pressure can be calculated based on the transfer line geometry. The cal- culation is done backwards from column flash zone. The calculation must account for critical velocity. Most, if not all, transfer lines will approach critical velocity in one or two locations. The pressure is limited by critical (sonic) veloc- ity. It is essential that pressure drop calculations properly account for critical velocity so that an accurate vapourisa- tion profile can be generated. Oil film temperature depends on bulk oil temperature, oil mass velocity, and localised heat flux. The oil film layer is where coke forms. The film temperature can be more than 100°F higher than the bulk oil in the larger outlet tubes. This is because the mass velocity in the outlet tubes is much lower compared to the inlet radiant tubes, which are typically smaller diameter. The last few tubes of a vacuum heater increase in size as oil begins to vaporise. This is nec - essary to manage pressure drop. A vacuum heater ideally has an inlet tube mass velocity of 450 lb/sec-ft 2 . Outlet tubes can have mass velocities as low as 100 lb/sec-ft 2 and lower in some cases. The inlet radiant section tubes were 4 in, with sizes increasing to 5 in, 6 in, 8 in, 10 in, with the last outlet tube 12 in. A rigorous tube-by-tube analysis of oil film temper - atures and residence time⁷ concluded that the outlet tube sizes were not ideal. There were too many large tubes. The larger-than-necessary tube sizes decreased oil mass velocity, which in turn increased oil film temperature and oil residence time. The low outlet tube mass velocity was compensated for using a large amount of coil steam in the original design. Because the steam rate was so large, it was injected near the outlet tubes. Modifying the last few outlet tubes in each pass presented an opportunity to reduce coil steam injection rate and location. The last three tubes of each pass were 8 in, 10 in, and 12 in diameter. The revamp replaced these tubes with 6 in,

8 in, and a combination of 8 in/10 in/12 in for the last tube (see Figure 7 ). This change better managed oil mass flux, oil film temperature, and residence time. Because the mass velocity was now higher in the outlet tubes, the coil steam could be reduced from 29,000 lb/hr to 3,200 lb/hr. The large reduction in steam rate allowed the injection location to be moved to the inlet of the radiant section while staying within overall heater pressure drop constraints. The overall residence time of oil in the heater was reduced since all the tubes would now have steam flow. Because of heater optimisation, lower ejector loads enabled the first-stage condensers to be replaced within the footprint of the exist - ing condensers. Increasing crude and vacuum column distillate yields Prior to the revamp, the crude and vacuum heaters oper- ated at maximum firing. Expanding heater firing was not an option. Optimisation of crude preheat increased inlet temperatures and permitted the outlet temperatures to be increased without increasing the firing rate. Additional optimisation was needed to maximise distillate recover - ies. Stripping section efficiency in both columns was poor, hence they represented a very low capital opportunity to increase flash zone vaporisation. High-efficiency plug-flow stripping trays⁸ were installed in the atmospheric column, and stripping steam flow rate increased from 8,600 lb/h to 30,000 lb/h. Additional diesel recovery was realised by increasing reflux ratios in both columns. Pumparound heat balances were adjusted to shift heat up the column. Prior to the revamp, all the RGO was processed in the vacuum column. RGO processing was increased by processing 15kBPD in the atmospheric column. Atmospheric column heat balance The CV1 atmospheric tower fractionates crude into over- head naphtha, kerosene, LGO, HGO, and atmospheric

PTQ Q3 2023