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 oC 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