Crude Distillation Petroleum Refining Engineering 2012-2013
Crude Oil Receiving Crude oil is received and stored in a floating roof tank Roof is made floating over the oil to avoid loss of hydrocarbon vapors and low boiling factions The roof also floats up and down to compensate for the breathing of operation during pumping in and out of the tank Crude oil received from tank cars, pipelines or tanker ships may contain water This water must settle in the receiving tank before oil accounting is started
Floating Roof Tank
Crude Distillation The first process in the refinery Topping unit or atmospheric distillation unit Receives the highest flow rate Usually runs at 60% of the design feed rate Seasonal temperature changes should be accounted for Capacity range: 10,000 BPSD to 400,000 BPSD (1,400 56,000 tpd)
Distillation Products CDU produces products that have to be processed in downstream processes Typical distillation prodcuts Gases Light straight run naphtha (light gasoline) Heavy naphtha (heavy gasoline or military jet fuel) Kerosene (light distillate or jet fuel) Light gas oil (LGO or diesel) Heavy gas oil (HGO Atmospheric residue or topped crude
Distillation Process Crude products heat exchange Crude heating Distillation columns, 30 to 50 trays Steam function Stripping light hydrocarbons Lowering the partial pressure of hydrocarbon vapors Tower energy balance Condenser and reflux Pumparounds Cold reflux Heat removal Reduces vapor flow rate
Operation of CDU Degree of fractionation is determined by the gap or overlap between two adjacent side streams Ideally there should be no overlap between products Gap or overlap
Cut Points Cut points are controlled by the overhead vapor temperature and flow rate of products fro column or side strippers Amount of naphtha is determined by the dew point of naphtha at its partial pressure Changing the draw rate of product affects the cut point of the heavier product below it Lowering kerosene draw flow rate will lower its end point (lighter) and will affect the initial cut point of LGO Side stream rate affects the temperature at the withdrawal tray, lowers the internal reflux, affect the degree of fractionation
Pressure and Temperature Pressure is controlled by the back pressure of the reflux drum 0.2 0.34 barg (3-5 psig) Flash zone pressure is usualy 0.34 0.54 bar higher than the top tray Overhead temperature must be 14 17 o C higher than the dew point of water at columns overhead pressure
Crude Oil Desalting Some brine is associated with oil in form of fine water droplets emulsified in crude oil Salt content cab be as high as 2000 PTB and should be lowered to 2 5 PTB Salt can cause deposition in tubes in furnaces and HEX Corrosion problems Catalyst poisoning
Types of Salts in Crude Oil Free water and emulsified water Salts are mostly dissolved in fine water droplets Salts are mostly magnesium, calcium and sodium chlorides
Desalting Process Water-oil emulsion has to be broken Water washing Heating Coalescence Droplets size 1 10 micron Achieved by electrostatic field Settling
Electrostatic Desalter
Desalter Operation Temperature: increasing temperature lowers density and viscosity. settling rate is increased with temperature Washing water ratio Water level: raising water level reduces settling time for water droplets. If too high it can reach the electrode Washing water injection point: water is injected at the mixing valve. Sometimes to avoid salt deposition injection is done after the crude pump
Desalter Operation Demuslifier injection rate: added after the crude pump or before mixing valve at 3 10 ppm of the crude Type of washing water: soft water to prevent scaling. Slitely acidic (ph 6). Free from hydrogen sulfide and ammonia. Distillation overhead or process water can be used Pressure drop in mixing valve: compromise between better mixing and emulsion stabilization
Vacuum Distillation To extract more products from atmospheric reside Produces Light vacuum gas oil (LVGO) Medium vacuum gas oil (MVGO) Heavy vacuum gas oil (HVGO) Vacuum residue