Atmospheric and vacuum distillation
Distillationof crude oil is the first process of petroleum refining and atmospheric and reducedpressure distillation of crude oil is to cut crude oil into petrol, kerosene,diesel, lubricating oil distillate, raw materials and residual oil forsecondary processing in atmospheric tower and vacuum tower respectively accordingto its boiling range.
II. Process introduction
Atmosphericand reduced pressure distillation are mainly composed of electric desalting,atmospheric pressure kiln, vacuum furnace and vacuum tower and a primary toweris usually established and this device is called 3-stage gasification anddistillation.
Desalting anddewatering shall be performed to crude oil in oilfield after oilfieldexploitation and shall be performed again before distillation to protectequipment, stabilize operation and guarantee product quality since they cannotmeet the distillation requirements. The desalted and dewatered crude oil (watercontent is 0.1-0.2% and salt content is 10mg/l) is heated to 230-240° and thenenter primary tower to produce light petrol or implement catalytic reforming ofraw oil at the top of primary power and part of them will return to the towertop for top reflux. Products are not produced at side stream of primary tower,but it can be drawn out to constitute the fraction similar to heavy petrolfraction; after heat exchange, part of them will be sent to backflow entrancein the middle of atmospheric column and the other part is sent to primary towerfor circulating reflux.
Bottom oil ofprimary tower, also called topped crude oil, is usually heated to 360-370 °through atmospheric pressure kiln before entering atmospheric tower; affectedby cold reflux at tower top and circulating reflux in the middle, the oil goesto tower top from evaporation section with temperature gradually decreased andcomponent being lighter and the tower top will steam out petrol; theatmospheric tower is generally provided with 3-4 slide streams to distil offkerosene, light diesel and heavy diesel in liquid phase. These slide streamswill be sent out the device after they are stripped through stripping tower,pumped out with pump, provided with heat exchange with crude oil, partiallyrecovered and cooled to certain temperature. The atmospheric heavy oil withoutvaporization at tower bottom will be used as feed materials of vacuum towerafter the light components are blown out through stream stripping.
The bottomoil will be pumped out by pump, heated to about 400° and sent to vacuum furnaceand then enter atmospheric cooler after the tower top has separatednon-condensable gas and steam. The is extracted by Level 3-4 evacuator aftercooling in atmospheric cooler and the residual voltage in the tower ismaintained at 0.027-0.1MPA, which is good for full distillation of distillateoil. Generally, the vacuum tower has 3-4 side streams to extract distillate oilwith different weight as raw materials for lubricating oil stock, catalyticcracking or hydrocracking. It will partially return to tower for circulatingreflux after steam stripping, heat exchange and cooling. The bottom residual oilcan be used for raw materials of propane deasphalting, oxidized asphalt,visbreaking and coking devices after steam stripping, pumping of pump and heatexchange.
III. Introduction of control plan
(I) Cascade control and single throw of primary ring
1. Outlettemperature control of atmosphere pressure kiln TIC1019 and controller PID TIC1028of auxiliary ring
Outlettemperature of atmosphere pressure kiln is generally provided with cascadecontrol adjustment with furnace box temperature TIC1028; the residual errorwill be eliminated when furnace box temperature fluctuates to keep stableoutlet temperature, but it’s difficult to control since there is large lag andthe integral time and derivative time are about 5min. The single throw of primaryring shall be considered when cascade is difficult to control. By using thiscontrol plan, the outlet temperature of boiler has a straight curve and thedeviation is less than 1°.
2. Outlettemperature control of vacuum furnace TIC1075
Outlettemperature of vacuum furnace is generally provided with cascade controladjustment with furnace box temperature TIC1084; the residual error will beeliminated when furnace box temperature fluctuates to keep stable outlettemperature, but it’s difficult to control since there is large lag and theintegral time and derivative time are about 5min. The single throw of primaryring shall be considered when cascade is difficult to control. By using thiscontrol plan, the outlet temperature of boiler has a straight curve and thedeviation is less than 1°.
All levels ofreflux and temperature are controlled through single loop PID.
(III) Start/stop control of electric desalting
Controlled byswitching value, the two electrical desalters have start and stop commands andcan detect execution conditions of command and give out alarms. The similarcontrol includes cascade control between bottom liquid level of primary towerLIC1006 and raw oil feed device FIC1065; temperature of atmospheric tower topTIC1009 and quantity of reflux of tower top FIC1009; temperature of vacuumtower top TIC1038 and quantity of reflux of tower top FIC1021;
(II) 44 conventional single loop controls
1. High gaspressure PIC1701
Conventionalsingle loop control is adopted at entrance of high pressure gas tank; the gatevalve will be opened in case of low pressure and pet valve will be closed incase of high pressure.