Method for heavy oil displacement during the first shutdown of atmospheric and vacuum distillation units
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Dear experts: The atmospheric and vacuum distillation unit is started for the first time to supply materials for subsequent units such as coking units. When it comes to shutting down the unit, it is required that light oil be used to replace heavy oil without performing any purging process. How should this replacement be carried out? Please give me some advice! Thank you! Plan (replacement with light oil): First, shut down the vacuum distillation system, break the vacuum in the vacuum distillation tower, and reduce the temperature at the outlet of the vacuum furnace to 360 degrees, so as to ensure the appropriate steam supply temperature at the bottom of the atmospheric distillation tower. The atmospheric pressure system operates normally, with the non-condensable gases from the initial and atmospheric distillates being sent to the atmospheric furnace for combustion. By using normal first-line and second-line diesel, as well as normal third-grade oil (process adjustments are made to ensure that the freezing point of the blended diesel is around 5 degrees), the atmospheric pressure diesel is first removed from the plant’s pipeline; thereafter, the first reduced-pressure line is used to remove the wax oils from the second-, third-, and fourth-reduced-pressure lines, through neutralization and backflow into the vacuum distillation tower. At the same time, the crude oil headspace from the 2 desalination tanks is transported for processing via the backwash water line. The brine is sent to the secondary line after leaving the desalination tank. After sampling and analysis, if the freezing point of the diesel used for displacement in the wax oil pipeline meets the requirements, the displacement with light oil can be stopped. The reduced three-loop circuit can replace the conventional four-loop oil. Start shutting down the atmospheric pressure system; as the atmospheric pressure furnace is gradually cooled, replace the secondary circulation line. When the furnace outlet temperature drops to around 150, the diesel used in the initial operation process is introduced from the start-up line to replace the crude oil in the main circulation system, and this diesel is then sent to the tank area. Once completed, contact the crude oil tank farm and use startup diesel to push the crude oil into the pipeline leading to the crude oil tank farm.
This post was last edited by tanronghuiws on 2010-6-17 at 19:30. I have experienced this approach on many occasions; the key step is to fill the pressure-reduction side line system with diesel to prevent condensation； Operation steps: (1) Lower the temperature of the atmospheric pressure furnace, and reduce the dry points of lines Chang San and Chang Si ； Pressing light diesel into the vacuum system can also displace the side-stream heavy wax oil. (2) After the unit has been cooled down and the oil removed through circulation, diesel is supplied to the vacuum distillation tower using the diesel feed line (start-up diesel line). The pumps on the vacuum side lines are started sequentially (including the intermediate reflux pump, which operates from the top of the tower to its bottom). The condition of the oil is checked at the control valve for the discharge of wax oil from the vacuum side lines as well as at the intermediate reflux control valve (usually experience is sufficient; an experienced operator can estimate the dry point of the diesel by examining its color. If necessary, the laboratory can be contacted to determine the boiling range of the oil) ； Confirm that the wax oil systems are all filled with diesel. (3) Use diesel to flush and displace the large circulation system. Note: (1) Do not push the oil from the electrodesalination unit out; firstly, it takes a long time to shut down the system and remove the oil, and secondly, it is inconvenient to restart operations. Oil is usually removed by sending it back to the crude oil pipeline. If the diameter of this pipeline is DN500 and its length is 2000 meters, that amounts to 400 cubic meters of crude oil. During restart operations, the crude oil used is mostly recycled heavy oil and diesel from the plant, and there are no light fractions available, making it difficult to establish a proper mid-stage reflux. (2) If the pressure reduction system is reluctant to be filled with oil, steam purging can also be used, but be careful as there may be dead zones. (3) The downtime for this method is estimated to be 8 hours, while the startup time is estimated to be 16 hours.
Last winter, the atmospheric and vacuum distillation unit here was shut down without draining oil or performing purging. During this shutdown period, the method of pressure shifting was used: one side line was first stopped, allowing the light oil from that side line to flow into the other side line. The freezing point of the oil in the side lines was found to be within acceptable limits, and no freezing incidents occurred during the more than 40 days of shutdown in winter. During parking, be careful to reduce the pressure and maintain a slight overpressure; nitrogen can be used for this purpose. When performing steam purging on certain equipment such as heat exchangers, be careful to prevent the pipelines from expanding.
Reply to 2# tanronghuiws: Thank you for your answer! There’s one thing I’m not clear about. I’d like to ask: you mentioned to be careful not to let the oil in the electrodesalination unit escape – so what should be done with the crude oil inside the electrode tank?
The crude oil in the electrodialysis tank remains inside it; during circulation, the inlet valve of the electrodialysis system is closed while the outlet valve remains open, allowing the electrodialysis process to proceed via a bypass route.
Reply to 5# tanronghuiws: Can it be ensured that the crude oil in that electrodesalination tank will not solidify?
The normal freezing point of crude oil is between -20 and +20. It is advisable to check the evaluation of the crude oil to determine its freezing point. If the shutdown is short-term (within 4 days), there is no need to worry about the crude oil freezing. However, if a shutdown is required over the winter, this approach must be considered carefully; it is absolutely not feasible for refineries in the north, unless they are located in the south. If the freezing point is indeed high, the crude oil mixed with diesel can be used to displace the crude oil in the electrodialysis tank during the large-scale circulation process; however, this method makes it difficult to control the temperature at the top of the vacuum distillation tower during startup, and diesel leakage can easily occur if the water seal tank at the tower top is not properly controlled.
Thank you for your answer. We are from Beihai Refinery in Guangxi, South China, and we plan to start operations in September; the temperature there in September is around 30 degrees. We intend to process imported mixed crude oil, with an API value of 29.9. The freezing point isn’t yet known, but it’s likely that no displacement will be necessary. If you have any further questions, please let me know! Thank you again
Is there anything else anyone would like to add? Thank you all for your suggestions!
How should one proceed when restarting operation after interrupting the diesel replacement under reduced pressure, given that the diesel in that interval vaporizes completely to the top of the tower as a result of vacuum extraction under reduced pressure?
If electric desalination of crude oil doesn’t result in oil removal after a long time, there is an informal method that allows operation without shutting down the power supply.