Process Description
The VDU (Vacuum Distillation Unit) separates residue from the ADU (Atmospheric Distillation Unit) into light vacuum gas oil (LVGO), heavy vacuum gas oil (HVGO), slop wax, and vacuum residue using distillation under vacuum conditions. The hot feed from the ADU is heated even higher in the VDU Furnace to vaporize most of the heavier components. The vaporized components in the feed are condensed in the VDU by three circulating refluxes and removed as products. Sour gas, light slop oil and sour water are produced in the overhead vacuum system.
VDU Feed & Heating
Atmospheric residue feed from the bottom of the ADU Tower, T-100, is pumped to the VDU by ADU Bottoms Pumps, P-114/P-114B, which are electric motor-driven centrifugal pumps. The residue is sent to the Vacuum Furnace, F-200.
Feed from P-114/P-114B is partially vaporized in the Vacuum Furnace, F-200 using fuel gas. Normally, the feed is heated to 750 DEG F. The heated crude is sent to the bottom section of the Vacuum Distillation Tower, T-200, where it is separated into slop oil, light vacuum gas oil (LVGO), heavy vacuum gas oil (HVGO), slop wax, and bottoms vacuum residue.
Combustion air is provided to F-200 by Forced Draft Fan, K-202. F-200 is outfitted with an ignition system to perform an initial light off of the fuel gas during startup. The furnace draft and oxygen are assumed to be ideally regulated in F-200. An interlock system stops fuel gas in the event of low crude flow rate, loss of K-202 or loss of flame detection in F-200.
Vacuum Distillation Tower
The Vacuum Distillation Tower consists of 4 packed sections above the feed nozzle and a trayed stripping section below the feed nozzle. It is equipped with three side draws and pumparound sections for LVGO, HVGO, and slop wax products. The tower consists of variable cross-section areas to produce optimal gas velocities for good liquid-vapor contacting with a minimal pressure drop. Vacuum towers are typically designed this way because vacuum conditions result in low gas densities which, in turn, would result in high velocities in a conventional distillation tower.
Stripping Section
The liquid from the Vacuum Furnace, F-200, and excess wash oil from the flash zone above the feed nozzle into are collected and routed to the stripping section of Vacuum Distillation Tower, T-200. This combined liquid (vacuum residue) flows into the stripper section before being collected in the tower bottom, where it is collected and sent for further processing, typically in a Delayed Coker Unit. Steam is injected below bottom tray of the stripping section to reduce the hydrocarbon partial pressure by stripping some light boiling components from the bottoms liquid. The steam also serves to cool the residue to minimize cracking and coking of the vacuum residue. The vapors from the Vacuum Furnace are routed to the flash zone of the tower along with vapors produced by the stripping section.
Vacuum residue from the stripping section is collected in the bottom of T-200 and pumped to battery limits by VDU Bottoms Pumps, P-214/P-214B, which are electric motor-driven centrifugal pumps.
Flash Zone
The flash zone serves partially quench the hot vapor from the Vacuum Furnace, F-200, and to wash down heavy residue droplets that are entrained with the vapor. A portion of the circulating slop wax is used as the wash oil. Without the flash zone, these entrained droplets would pollute the circulating slop wax distillate and degrade its quality. The flow rate of wash oil needs to be sufficiently high at all times to ensure the packing of the flash zone is completely wetted. At low wash oil rates most of the wash oil will be vaporized by the hot vapor from the Vacuum Furnace leading to insufficient distribution of wash oil on the packing.
Slop Wax Section
The slop wax pan is located at the base of the slop wax packed section. The pan collects liquid from the packing of the slop wax section. Vapors from the flash zone flow through risers in the pan and up into the packing of the slop wax section. The vapor risers are capped to prevent slop wax liquid from falling through risers. Product, pumparound slop wax and wash oil for the flash zone are pumped from the slop wax pan by Slop Wax Pumps, P-213/P-213B, which are electric motor-driven centrifugal pumps.
The slop wax product and pumparound from P-213/P-213B are cooled in Slop Wax P/A Exchanger, E-213. The cooling stream of E-213 is assumed to be a cool thermal oil stream used for heating service outside the VDU (Vacuum Distillation Unit). Some of the cooled slop wax product is sent to storage while the pumparound stream is returned to the tower at the top of the slop wax section packing. Uncooled slop wax for wash oil is taken off upstream of E-213.
HVGO Section
The HVGO (heavy vacuum gas oil) pan is located at the base of the HVGO packed section. The pan collects liquid from the packing of the HVGO section. Vapors from the slop wax section flow through risers in the pan and up into the packing of the slop wax section. The vapor risers are capped to prevent slop wax liquid from falling through risers. Product, pumparound HVGO and reflux for the slop wax section are pumped from the HVGO pan by HVGO Pumps, P-212/P-212B, which are electric motor-driven centrifugal pumps.
The HVGO product and pumparound from P-212/P-212B are cooled in HVGO P/A Exchanger, E-212. The cooling stream of E-212 is assumed to be a cool thermal oil stream used for heating service outside the VDU. Some of the cooled HVGO product is sent to storage while the pumparound stream is returned to the tower at the top of the HVGO section packing. Uncooled HVGO for pumpdown to the slop wax section is taken off upstream of E-212. There is normally no HVGO reflux sent to the slop wax section, but this line can be used to adjust product properties of the slop wax product if needed.
LVGO Section
The LVGO (light vacuum gas oil) pan is located at the base of the LVGO packed section. The pan collects liquid from the packing of the LVGO section. Vapors from the HVGO section flow through risers in the pan and up into the packing of the slop wax section. The vapor risers are capped to prevent slop wax liquid from falling through risers. Product, pumparound LVGO and reflux for the HVGO section are pumped from the LVGO pan by LVGO Pumps, P-211/P-211B, which are electric motor-driven centrifugal pumps.
The LVGO product and reflux for the HVGO section are taken directly from P-212/P-212B. The pumparound stream from P-212/P-212B is cooled in LVGO Air Cooler, E-211, and is returned to the tower at the top of the LVGO section packing. There is normally no LVGO reflux sent to the HVGO section, but this line can be used to adjust product properties of the HVGO product if needed.
Vacuum System
The overhead vapor from the Vacuum Distillation Tower is first cooled in Vacuum Pre-condenser, E-210. Condensed steam and hydrocarbons collected in the shell side of E-210 are routed by gravity to the Vacuum Drum, D-211. The Pre-Condenser reduces the vapor load on the Ejector, EJ-211.
Cooled vapor from E-210 is routed to the Ejector, EJ-211, which uses steam as a motive fluid to pull vapor from E-210 into the ejector. The effluent from EJ-211 is cooled and condensed in Vacuum Condenser, E-209, using cooling water. Cooling water from E-209 is used to cool E-210.
Non-condensable gas, water and slop oil from E-209 and E-210 are separated by gravity in Vacuum Drum, D-211, which is a three-phase separator. The sour water is pumped to disposal facilities by Sour Water Pumps, P-210/210B, which are electric motor-driven centrifugal pumps, while the slop oil is accumulated and occasionally manually pumped to slop collection facilities by Slop Oil Pumps, P-209/209B, which are electric motor-driven centrifugal pumps. Vapor from D-211 is recirculated to EJ-211 for capacity control. The net vapor from D-211 is routed to gas treating facilities for ultimate use as fuel gas. Some of the vapor from D-211 is recirculated to EJ-211 for pressure control. D-211 operates slightly above atmospheric pressure.
Product Specifications
The VDU (Vacuum Distillation Unit) fractionates 13.67 MBPD of Atmospheric Residuum to produce:
- 0.90 MBPD of LVGO (light vacuum gas oil)
- 5.69 MBPD of HVGO (heavy vacuum gas oil)
- 3.46 MBPD of Slop Wax
- 3.48 MBPD of Vacuum Residue
The VDU feed is heated to 750 DEG F before entering the Vacuum Distillation Tower which is maintained at 2.00 inHg. The top temperature is maintained at 213 DEG F which minimizes loss of hydrocarbons to the vacuum system. The vacuum residue normally leaves the bottom of Vacuum Distillation Tower, T-200, at 658 DEG F.
The following are the design draw temperatures from T-200:
- LVGO – 279 DEG F
- HVGO – 483 DEG F
- Slop Wax – 623 DEG F
Instrumentation
Basic Controls: VDU Feed
The VDU (vacuum distillation unit) feed is pumped by P-114/P-114B whose motors are controlled by switches, HS-114/HS-114B, respectively. The feed flow rate is controlled by FIC-124 which adjusts the position of FV-124. FAL-124 is an indicating switch of the low alarm status of FIC-124 and is a trip input to the VDU Furnace Firing Interlock, I-200. The feed temperature is indicated on TI-114 and the API density if the feed is indicated on AI-200.
Basic Controls: VDU Furnace
TIC-200 controls the temperature of feed leaving the radiant coils of VDU Furnace, F-200, by adjusting the position of fuel gas control valve, TV-200. The flow of fuel gas to F-200 is indicated on FI-201. TIC-200 is locked in manual mode with an output of 0% when F-200 Interlock, I-200, is in the TRIP state as indicated on XA-200.
Switch, HS-202, controls the motor of Forced Draft Fan, K-202. The motor status is a trip input to I-200. BS-201 detects a flame/flameout condition of the fuel gas burners of F-200 and is also a trip input to I-200. Refer to VDU Interlocks below for the operation of interlock, I-200.
Basic Controls: VDU Bottom
Bottoms liquid (vacuum residue) is collected and sent to storage by VDU Bottoms Pumps, P-214/P-214B, whose motors are controlled by switches, HS-214/HS-214B, respectively. LIC-214 controls the bottom level by adjusting the position of LV-214. The bottoms flow to storage is indicated by FI-224 and its temperature is indicated by TI-214. The vacuum residue’s 95% point is indicated by AI-224.
Stripping steam is injected into the bottom of T-200 by FIC-234 which adjusts the position of FV-234. The pressure drop across the trays of the stripping section is indicated on PDI-215. The VDU (vacuum distillation unit) bottom pressure is indicated by PI-214.
The flow of wash oil (slop wax) to the flash zone of T-200 is controlled by FIC-233 which adjusts the position of FV-233. The pressure drop across the flash zone packing is indicated on PDI-214. A low differential pressure suggests insufficient wash oil flow.
Basic Controls: VDU Slop Wax System
Hot slop wax is pumped from T-200 by Slop Wax Pumps, P-213/P-213B, whose motors are operated by HS-213/HS-213B, respectively. The slop wax product flow to storage is indicated on FI-223. The flow is controlled by LIC-213 which controls the slop wax pan level by adjusting the position of LV-213. The 95% volume boiling point of the slop wax product is indicated by AI-223. The temperature of cooled slop wax from E-213 is indicated by TI-223. The slop wax pumparound flow is controlled by FIC-213 which adjusts the position of FV-213. FV-213 will fully open on a loss of instrument air. Wash Oil to the flash zone is controlled by FIC-233 by adjusting the position of FV-233.
The temperature of slop wax on the slop wax pan is indicated by TI-213. The pressure drop across the packing of the slop wax section is indicated on PDI-213.
Basic Controls: VDU HVGO System
Hot HVGO (heavy vacuum gas oil) is pumped from T-200 by HVGO Pumps, P-212/P-212B, whose motors are operated by HS-212/HS-212B, respectively. The HVGO product flow to storage is indicated on FI-222. The flow is controlled by LIC-212 which controls the HVGO pan level by adjusting the position of LV-212. The 95% volume boiling point of the HVGO product is indicated by AI-222. The temperature of cooled HVGO from E-212 is indicated by TI-222. The HVGO pumparound flow is controlled by FIC-212 which adjusts the position of FV-212. FV-212 will fully open on a loss of instrument air. HVGO reflux to the slop wax system is controlled by FIC-232 by adjusting the position of FV-232.
The temperature of HVGO on the HVGO pan is indicated by TI-212. The pressure drop across the packing of the HVGO section is indicated on PDI-212.
Basic Controls: VDU LVGO System
Hot LVGO (light vacuum gas oil) is pumped from T-200 by LVGO Pumps, P-211/P-211B, whose motors are operated by HS-211/HS-211B, respectively. The LVGO product flow to storage is indicated on FI-221. The flow is controlled by LIC-211 which controls the LVGO pan level by adjusting the position of LV-211. The 95% volume boiling point of the LVGO product is indicated by AI-221. The LVGO pumparound flow is controlled by FIC-211 which adjusts the position of FV-211. FV-211 will fully open on a loss of instrument air. LVGO reflux to the HVGO system is controlled by FIC-231 by adjusting the position of FV-231.
The temperature of cooled LVGO from E-211 is indicated by TI-221. TIC-231 controls the LVGO pumparound return temperature by adjusting the position of three-way valve, TV-231. TV-231 will open fully toward E-211 on a loss of instrument air.
The temperature of LVGO on the LVGO pan is indicated by TI-211. The pressure drop across the packing of the LVGO section is indicated on PDI-211. The temperature of vapor leaving the top of T-200 is indicated on TI-210.
Basic Controls: VDU Overhead
The VDU (vacuum distillation unit) overhead vapor from T-200 flows through the Vacuum Pre-condenser, E-210, where a portion of the steam and any residual hydrocarbons are condensed before entering Ejector, EJ-210. These liquids are drained by gravity to the Vacuum Drum. A seal loop in the condensate piping prevents vapor blow-through to the Vacuum Drum. Cooling water flow through E-210 and Vacuum Condenser, E-209, is regulated by HV-212.
Steam to Ejector, EJ-210, is regulated by HV-211. The Ejector compresses the vapor from E-210 using steam as a motive fluid. The vapor and motive steam combine and are sent to Vacuum Condenser, E-209, to condense the steam and any residual hydrocarbons from E-210.
The liquids and vapor from E-209 and E-210 are separated in Vacuum Drum, D-211. Hydrocarbons are separated from the water by gravity and float on the water side and fall over the internal weir. The water interface level LIC-210 is normally controlled at 50% by pumping the water to treatment facilities with P-210/P-210B whose motors are controlled by HS-210/HS-210B, respectively. LIC-210 adjusts the position of LV-210. The top of the weir is reached at 80% level indication on LIC-210.
The hydrocarbon phase on the opposite side of the weir in D-211 slowly accumulates and its level is indicated on LI-209. When a sufficient level of slop oil has accumulated, the level can be drained to slop facilities via P-209/P-209B whose motors are operated by HS-209/HS-209B, respectively.
The VDU vacuum pressure is controlled by PIC-210 which recirculates cooled vapor from D-211 to the suction of Ejector, EJ-211, by adjusting the position of PV-210.
F-200 Firing Interlock
Protection Logic
I-200 protects the VDU Furnace, F-200, from potential damage by stopping fuel gas to the furnace if any of the following occurs:
- Low feed flow (FIC-124 will alarm on low flow, FAL-124 will move to the TRIP state)
- Loss of flame detection (BS-201 is in the OUT state, XA-201 will alarm)
- Forced Draft Fan, K-202’s motor is off (HS-202 is in the OFF state, XA-202 will alarm)
If any if these conditions occur, switch XI-200 will indicate the TRIP state and cause I-200 to activate (XA-200 will alarm and indicate the TRIP state). When I-200 activates it will lock F-200 outlet temperature controller, TIC-200, in manual mode with an output of 0% to close TV-200, thereby stopping fuel gas to F-200.
Starting Fuel Firing
To start fuel gas to F-200, all the trip inputs to I-200 as indicated on XI-200 must be cleared first. Because there is no fuel gas to F-200 and, therefore, no flame is possible, the flame detection switch BS-201 must be bypassed by placing bypass switch HS-201 in the BYPAS position. Additionally, feed flow must be above the low alarm setting of FIC-124 and the motor for K-202 must be running to start fuel gas.
In order to reset the interlock, F-200 must be purged with air for a preset time (normally 120 seconds). The purge timer will start counting down when K-202’s motor is running and XI-200 indicates no trip inputs are active. The countdown time is indicated on XI-202T. When this time becomes zero, XI-202 will indicate the purge of F-200 is complete and will reset I-200 (XA-200 will indicate OK) and controller TIC-200 will be unlocked for control by the operator.
Before starting fuel gas using TIC-200, the ignition system for F-200 must be turned on using switch HS-203 to light off added fuel gas. When HS-203 is turned to the ON state, it will generate an alarm as a reminder that the ignition system should be turned off after F-200 firing is established.
After turning the ignition system on, adjust the output of TIC-200 to 5% to establish a small flow of fuel gas as indicated on FI-201. Verify the burners light off by watching BS-201 change from the OUT state to the FLAME state.
Once a fuel gas flow is detected, I-200 will automatically change the flame detection bypass switch HS-201 from the BYPAS state to the NORM state after 10 seconds to re-enable protection of F-200 from a flameout. If the burners fail to stay ignited after 10 seconds, I-200 will activate and HS-201 will have to be set to the BYPAS position again in order to start the purge timer which will, in turn, reset I-200.
Make sure the burner ignition system switch HS-203 is in the ON position when fuel gas flow is established. If F-200 firing cannot be established after two attempts using the correct procedure, then it is likely the flame sensor needs to be checked.