CRUDE OIL DISTILLATION
Refining of crude oils or petroleum essentially consists of primary separation
processes and secondary conversion processes. The petroleum refining process is the
separation of the different hydrocarbons present in the crude oil into useful fractions and the
conversion of some of the hydrocarbons into products having higher quality performance.
Atmospheric and vacuum distillation of crude oils is the main primary separation processes
producing various straight run products, e.g., gasoline to lube oils/vacuum gas oils (VGO).
These products, particularly the light and middle distillates, i.e., gasoline, kerosene and diesel
are more in demand than their direct availability from crude oils, all over the world.
PRETREATMENT OF CRUDE OILS
Crude oil comes from the ground, which contains variety of substances like gases,
water, dirt (minerals) etc. Pretreatment of the crude oil is important if the crude oil is to be
transported effectively and to be processed without causing fouling and corrosion in the
subsequent operation starting from distillation, catalytic reforming and secondary conversion
processes.
PRETREATMENT TAKES PLACE IN TWO WAYS:
Field separation
Crude desalting
Field separation is the first step to remove the gases, water and dirt that accompany
crude oil coming from the ground and is located in the field near the site of the oil wells. The
field separator is often no more than a large vessel, which gives a quieting zone to permit
gravity separation of three phases: gases, crude oil and water (with entrained dirt).
CRUDE DESALTING
It is a water washing operation performed at the refinery site to get additional crude
oil clean up. Crude Oil Desalting consists of:
Purifying process
Remove salts, inorganic particles and residual water from crude oil
Reduces corrosion and fouling
Desalting process is used for removal of the salts, like chlorides of calcium, magnesium and
sodium and other impurities as these are corrosive in nature. The crude oil coming from field
separator will continue to have some water/brine and dirt entrained with it. Water washing
removes much of the water-soluble minerals and entrained solids (impurities). There are two
types of desalting: single & multistage desalting. Commercial crudes, salt contents 10-200
ppb, earlier 10-20 ppb were considered satisfactorily low. However, many refiners now aim at
5 ppb or less (1-2 ppb) which is not possible through single stage desalting, hence two stage
desalting is required. Desalting process consists of three main stage: heating, mixing and
settling. Crude oil is heated upto 135-141o C in the train of heat exchanger operating in two
parallel section. The temperature in desalting is maintained by operating bypass valve of heat
exchanger. Single stage desalting with water recycle is usually justified if salt content in
crude is less than 40 ppb. Two stage desalting involves dehydration followed by desalting.
Double stage desalting is better for residuum hydrotreating. Fuel oil quality is better.
Desalting process is two stage process: forming emulsion of crude and water and
demulsification in which emulsion is broken by means of electric field and demulsifying
chemicals.Desalting is 203 carried out by emulsifying the crude oil and then separating the
salt dissolved in water. Two phases water/oil is separated either by using chemicals to break
down the emulsion or by passing high potential electric current. By injecting water the salts
dissolved in the water and solution are separated from the crude by means of electrostatic
separating in a large vessel.
Operating Variables in Desalter
Some of the variables in the desalter operation are crude charge rate, temperature,
pressure, mixing valve pressure drop and wash water rate, temperature, and quality, desalting
voltage. Crude oil temperature charged to the desalter is very important for the efficient
operation of desalter. Lower temperature reduce desalting efficiency because of increased
viscosity of oil while higher temperature reduce desalting efficiency due to greater electrical
conductivity of the crude. Pressure in the vessel must be maintained at a high value to avoid
vaporization of crude oil pressure which result in hazardous ondition, erratic operation and a
loss of desalting efficiency. Flow diagram for crude oil desalting is given Figutre MIV
CRUDE OIL DISTILLATION process
Desalted crude flows to atmospheric and vacuum distillation through crude pre
flashing section. Atmospheric distillation column (ADU) and Vacuum distillation column
(VDU) are the main primary separation processes producing various straight run products,
e.g., gasoline to lube oils/vacuum gas oils (VGO). These products, particularly the light and
middle distillates, i.e., gasoline, kerosene and diesel are more in demand than their direct
availability from crude oils, all over the world. Two stage Desalter Desalted Crude Oil
Effluent Water Process Water Unrefined Crude Oil Demulsifier Preheat heat exchanger
Mixing Unit 204 Crude oil distillation consists of atmospheric and vacuum distillation. The
heavier fraction of crude oil obtained from atmospheric column requires high temperature. In
order to avoid cracking at higher temperature the heavier fraction are fractionated under
vacuum. Typical flow diagram of crude oil distillation is given in Figure M-VI 2.2. Various
Streams From Atmospheric And Vacuum Distillation Column is given in Table M-VI 2.2
Table M-VI 2.2: Various Streams From Atmospheric And Vacuum Distillation Column
Column Fraction Temperature Carbon range Uses
Atmospheric Fuel Gases >40 C1-C2 Fuel
Column LPG C3-C4 Domestic fuel
Straight run 20-90 C6-C10 Gasoline pool
gasoline
Naphtha 130-180 C6-C10 Catalytic reforming
(Medium and and aromatic plant feed
heavy) stock Steam cracker,
synthesis gas
manufacture
Kerosene 150-270 C11-12 Aviation turbine fuel,
Domestic fuel, LAB
feed stock (paraffin
source)
Light gas oil 230-320 C13-C17 High speed diesel
component
Heavy gas oil 320-380 C18-C25 High speed diesel
component
ATMOSPHERIC COLUMN
Various steps in atmospheric crude oil distillation are
Preheating of Desalted crude
Preflash
Distillation
Stabilization of Naphtha
The desalted crude oil from the second stage desalting process is heated in two parallel heat
exchanger. The preheated crude having temperature of about 180o C is goes to pre flash drum
where about 3-4percent of light ends are removed. The preheated crude from the preheater
section is further 206 heated and partially vaporized in the furnace containing tubular heater.
The furnace has two zones: radiant section and convection section. The radiant zone forms
the combustion zone and contains the burners. In convection zone the crude is further heated
(inside the tube) by the hot flue gases from the radiant section. Heated and partially vaporized
crude from the fired heaters enters the flash zone of the column and fractionated in the
atmospheric column. The distillation section consist of overhead section, heavy naphtha
section, kerosene section, light gas oil section, heavy gas oil section and reduced crude
section each section contains circulating reflux system. Naphtha stabilizer, caustic wash and
naphtha splitting section: The unstablished naphtha from the atmospheric distillation column
is pumped to the naphtha stabilizer section for separation of stabilized overhead vapours
which is condensed to recover LPG which is treated in caustic and amine treating unit. The
stabilized naphtha is further separated into light, medium and heavy naphtha.
PRODUCTS of Atmospheric Distillation Unit
Unstabilized Naphtha consists of LPG, naphtha and light gases (C-5 115 oC)
Intermediate Naphtha (Bombay High) (135oC) Solvent Naphtha
Heavy Naphtha (130-150 oC) routed to diesel or naphtha.
Kero/ATF (140-270/250oC)
Light Gas Oil (250/270-320oC)
Heavy Gas Oil (320-380oC)
Reduced Crude Oil
Operating Variables in ADU unit are
Furnace coil outlet temperature
Crude distillation Column top pressure and top temperature
Stripping Steam flow
Product withdrawal Temperature
IMPURITIES
OLEOPHOBIC IMPURITIES
Oleophobic impurities include salt, mainly chloride & impurities of Na, K, Ca& Mg,
sediments such as salt, sand, mud, iron oxide, iron sulphide etc. and water present as soluble
emulsified and /or finely dispersed water.
OLEOPHILIC IMPURITIES
Oleophilic impurities are soluble and are sulphur compounds, organometallic compounds, Ni,
V, Fe and As etc, naphthenic acids and nitrogen compounds.
Reflux (distillation)
Reflux is a distillation technique involving the condensation of the distilled product
vapours and the return of a portion of the condensate to the distillation system from which it
originated. Reflux is widely used in large-scale, industrial continuous distillation columns as
well as in laboratory distillation apparatus setups. It is also used in laboratories to supply heat
to reactions over a long period of time.
Reflux in industrial distillation
