|Click to view schematic display A||Click to view schematic display B||Click to view schematic display C||Click to view schematic display D|
The VDU (Vacuum Distillation Unit) takes the residuum from the ADU (Atmospheric Distillation Unit) and separates the heavier end products such as vacuum gas oil, vacuum distillate, slop wax, and residue.
Heavy crude oil is preheated by the bottoms feed exchanger, further preheated and partially vaporized in the feed furnace, and passed into the vacuum tower where it is separated into slop oil, vacuum gas oil, vacuum distillate, slop wax, and bottoms residue.
This tower contains a combination of 14 fractionation trays and beds. It is equipped with three side draws and pump around sections for vacuum gas oil, vacuum distillate, and slop wax products.
The liquid from the feed furnace enters the tower bottoms, where it is collected and sent for further processing. Steam is injected into the base of the tower to reduce the hydrocarbon partial pressure by stripping some light boiling components from the bottoms liquid. The vapors from the feed heater enter the tower below tray 14.
At tray 14, a draw pan is located from which slop wax product is drawn. The slop wax product and pump around are cooled, with the slop wax product going to storage, while the pump around is returned to the tower at tray 11.
The next product draw is located at tray 8, where the draw for vacuum distillate product is located. The vacuum distillate draw tray is a total draw tray, where the reflux from the tray is pumped under flow control to the tray below. The product and pump around are cooled, with the vacuum distillate product going to storage, while the pump around is returned to the tower at tray 7.
The last product draw is located at tray 4, where the draw for vacuum gas oil product is located. The vacuum gas oil draw tray is also a total draw tray, where the reflux from the tray is pumped under flow control to the tray below. The product and pump around are cooled with the vacuum gas oil product going to storage, while the pump around is returned to the tower at tray 1.
The overhead from the VDU is condensed and combined with the vacuum steam. The slop oil and water are separated by gravity in the vacuum drum. The water is drained to disposal, while the slop oil is accumulated and occasionally drained to slop collection.
The VDU feed is pumped by P-114 (HS-114) controlled by FIC-124. It is preheated by the bottoms feed exchanger E-200 before entering the Feed Furnace (F-200). TIC-200 controls the temperature of the feed entering the VDU (T-200) by adjusting fuel gas flow to the furnace.
Bottoms liquid is collected and sent to storage through pump P-214 (HS-214), controlled by LIC-214, and indicated by FI-224. This residue's 95% point is monitored by AI-224. Stripping steam is injected into the VDU bottoms by FIC-234.
Hot slop wax is pumped from the tower by pump P-213 (HS-213). The slop wax product flow to storage (FI-223) is controlled by LIC-213, and it's 95% point is monitored by AI-123. Cooled pump around is controlled by FIC-213 and returned to the tower above the slop wax draw tray.
Hot vacuum distillate is pumped from the tower by pump P-212 (HS-212). The vacuum distillate product flow to storage (FI-222) is controlled by LIC-212, and it's 95% point is monitored by AI-122. Cooled pump around is controlled by FIC-212 and returned to the tower above the vacuum distillate draw tray. Vacuum distillate reflux is controlled by FIC-232 and returned to the tower below the vacuum distillate draw tray.
Hot vacuum gas oil is pumped from the tower by pump P-211 (HS-211). The vacuum gas oil product flow to storage (FI-221) is controlled by LIC-211, and it's 95% point is monitored by AI-121. Cooled pump around is controlled by FIC-211 and returned to the tower above the vacuum gas oil draw tray. Vacuum gas oil reflux is controlled by FIC-231 and returned to the tower below the vacuum gas oil draw tray.
The VDU overhead vapor flows through the overhead condenser E-210 (HV-212) into the Overhead Vacuum Drum D-211. The hydrocarbons are fully condensed and mixed with the vacuum condensate flow from E-211.
The water separates from the hydrocarbon liquid by gravity, where the drum's water level is maintained by LIC-210 which sends the water to treatment via P-210 (HS-210). The hydrocarbon phase overflows from the water phase, and slowly accumulates (LI-209). When a sufficient level of slop oil has accumulated, the level can be drained via P-209 (HS-209).
The VDU vacuum pressure is maintained by the steam to the vacuum ejector (HV-211), the cooling water (HV-212) to the steam condenser E-211, and the hydrocarbon condenser E-210. The pressure is regulated by PIC-210, which reduces the vacuum by circulating water to the vacuum ejector. The VDU bottoms pressure is indicated by PI-214.
The VDU tower temperature profile is indicated by TI-210 (overhead), TI-211 (gas oil), TI-212 (vacuum distillate), TI-213 (slop wax), and TI-214 (VDU residuum).