While the ultimate purpose of operating a refinery is to produce gasoline, diesel or asphalt, key objectives include improving return on investment (ROI), net profitability and cash flow. (1)
Great strides have been made in improving plant efficiency and productivity by implementing online, interactive computer controls. Key segments of refineries have been upgraded, such as vacuum tower steam ejectors and vacuum condensers. Tower resid stripping trays have also been replaced with new types of structured packing.
But has this improved profitability or cash flow? How can the first-line management level know that its cash flow or ROI has been improved?
Setting goals. Process engineers try to keep products on spec and minimize utility consumption and pump outages. What other goals can be set, and what other guidelines can be employed? At the first-line management level, cash flow, net profitability or ROI cannot be easily measured, but they can be determined if things are really improving.
To gauge directionally if cash flow is improving, the following variables, which are listed in order of economic importance, should be monitored:
1. Minimum of 1,050[degrees]F (566[degrees]C) minus gasoil in vacuum resid, based on ASTM D-1160 test
2. Minimum of 650[degrees]F (343[degrees]C) minus diesel in fluid catalytic cracking unit (FCCU) feed, based on ASTM D-86 test
3. Minimum of 340[degrees]F (171[degrees]C) jet fuel (sim distillation) in naphtha
4. Minimum of [C.sub.3]-[C.sub.4] liquefied petroleum gas (LPG) in fuel gas
5. Minimum [C.sub.7] toluene precursors in isomerization unit feed
6. Minimum [C.sub.6] (i.e., benzene precursors) in reformer feed
7. Minimum of amylenes ([C.sub.5]) in alkylation feed
8. Minimum butylenes (alky feed) in fluid coking unit (FCU) naphtha
9. Minimize Ni + Va in gasoil to FCU feed hydrotreater
10. Minimize slop production.
Vacuum resid gasoil quality. Considering the diesel diluent needed to add to vacuum resid to meet Goal #6 oil viscosity specs, each barrel of gasoil left in vacuum tower bottoms is a $20 loss. This is particularly true with the new, 5,000-ppm-sulfur spec on bunker fuel oil for ocean-going ships.
However, many plants do not sample the vacuum tower bottoms stream for fear of a fire at the sample point. The auto ignition temperature of the vacuum resid is 320[degrees]F (160[degrees]C). The sample point is on the run-down stream at 450[degrees]F (232[degrees]C). The plant safety department may prohibit the use of such a sample station.
However, a sample can be obtained without any hazard, even though the resid solidifies at temperatures below the 320[degrees]F (160[degrees]C) auto-ignition temperature. The sample can be obtained at the bottom's pump discharge temperature of 680[degrees]F (360[degrees]C) using a steel bottle, as shown in FIG. 1. The bottle should be allowed to fill with resid, blocked in and cooled, and then unscrewed and sent to the lab.
The seemingly simplest way to reduce gasoil lost to vacuum resid would be to cut back on the wash oil flowrate (FIG. 2) above the grid from approximately 5,000 bpd to 3,000 bpd. However, this will not yield 2,000 bpd of incremental heavy gasoil product. About 80% of the wash oil evaporates in the...