Vacuum Distillation Unit (VDU) – To further fractionate the topped crude/residue from the atmospheric distillation unit, higher temperatures are warranted which might produce cracking of crude/coking at the atmospheric distillation fractionation pressure. Hence, reduced pressure for more fractionation is required to prevent thermal cracking. This new fractionation step occurs in a vacuum distillation column. The vacuum column is a little different than atmospheric distillation column where larger diameter column is used to maintain comparable vapor velocities at the reduced pressures.
Vacuum Distillation Unit (VDU) | Energy Efficiency Guidelines in Refinery
The internals of the vacuum tower is different from the atmospheric tower where packing is used instead of trays. Typical vacuum tower may produce gas oils, lubricating-oil base stocks and heavy residue. Heavy residuals are the feed to the propane de-asphalting units. A second stage tower may be operating at even lower vacuum to fractionate the surplus residuum from the first vacuum tower, which is not used for de-asphalting.
In summary, vacuum distillation unit is the unit in oil refining facility that process the low value atmospheric distillation residue to produce more valuable products. These valuable products depend on the target of the unit either to mainly produce fuel or mainly produce lube oils. In either cases, such valuable products are light gases, light and heavy vacuum gas oil used as fuel and/or feedstock to other downstream units and lube oils.
Vacuum residues are either used in a gasification process to produce synthesis gas and for power plants fuel or used in viscosity breaking units (Vis-breaker) to upgrade it to more valuable products.
A typical configuration of the vacuum distillation unit (VDU) is shown in graph below.
It essentially consists of vacuum tower, furnace and preheat train.
Energy requirements for typical vacuum distillation unit include fuel for the furnace, steam, power and cooling water. The amount of energy consumption, via energy balance
calculation, in vacuum unit used to produce fuels is different than the one required
producing lube oils. The higher energy consumption for lube oils production-based
vacuum unit relative to fuels vacuum unit is attributed to the increased amount of steam
required in the side-stream stripping process and ejector steam. This extra amount of steam is necessary to make the stringent product specifications for the lube side-streams.
It is always advised to find the minimum amount of steam requirements/steam target via
simulation that matches exact desired product specification.
Again for the furnace, flue gas temperature of 300ºF or higher is recommended according to the type of fuel burned and hot streams going to air coolers and water coolers shall be fully utilized as much as possible before losing their heat content to air and water.
For efficient vacuum distillation unit, feed conditions shall be higher than 250ºF, the final/approach temperature of the bottoms product can be assumed 400ºF to account for the high viscosity of this stream and it is a major area of energy use in the system.
The energy balance for the vacuum distillation unit includes the feed and product streams, preheat furnace, pumparounds, steam for bottoms stripping and jet ejectors. The lube oil vacuum unit includes steam for the side streams besides previously mentioned items. Intra-process integration between hot streams to be cooled and cold streams to be heated, using pinch techniques, can be used to find minimum energy consumption or theoretically possible minimum at different approach temperatures among hot and cold streams.
Another benchmark/target can be also found via inter-process integration of vacuum distillation and atmospheric distillation units, for instance.
More involved inter-process integration is also possible to stretch the limit in energy consumption saving via the inter-process integration of more than two units in oil refinery.
The 10 major guidelines/compliance points:
– Heat integration shall be applied with global ΔT_min less than or equal to 30ºF and the right use of utility level.
– Process-Utilities system shall exhibit best possible synergy.
– Efficiency of rotating equipment shall be more than 80-85 %.
– Efficiency of fired heaters shall be more than 90-92%.
– Distillation columns shall be integrated with the rest of the process or among themselves and columns with long difference in its temperature profile shall be adapting inter-coolers or inter-heaters design features.
– Inlet feed to compressors shall be as cold as possible and to turbines as hot as possible.
– Turbines are used whenever possible and let down valves and let down drums shall be minimized.
– The process products’ temperatures shall not be higher than the feed temperatures, and process temperatures to air coolers shall be lower than 200ºF.
– “Source” Processes shall be integrated with adjacent ones and/or produce heating and/or cooling utilities.
– Heat transfer equipment shall always exhibit high U, as high as possible.