Des Alter

Process Description:Crude distillation unit is often referred as an atmospheric distillation unit because it is operated at slightly above atmosphere pressure. Pressure is highest at bottom which gradually dec reases till the top part of the column Crude oil is pumped from storage tanks where it is freed from sediments and free water by gravity. It goes through a series of heat exchangers where it is heated with hot products coming out from the distillation column. he temperature of the crude feed can reach !"#$!%# #C. he crude oil contains salt in the form of dissolved salt in the tiny droplet of water which forms a water-in oil emulsion and suspended solids. his water can be separated by gravity Chemical& 'echanical and (lectrical approach. )enerally electrical method is preferred in refinery where water is separated through electrostatic water separation. his process is called desalting. he crude is further heated in product produc t heat exchangers. his preheating is done b y using the hot  products which are coming out from side stripper of distillation distillation column. *e *e need to cool down the products to the desired temperature for pumping to the storage tanks. his is essential for the economics of the unit in terms of energy conservation and utili+ation. Preheating of crude oil is not done enough till this stage. Crude has to be vapori+ed partially  because all the products have to be in vapor form& except for the bottom product ,C residue crude oil/ before enters the atmospheric distillation column. hus a furnace is re0uired to boost the temperature to between 11# and 12%oC depending on the crude composition. 3fter heated by furnace now the partially vapori+ed crude is transferred to the flash +one of the column which is located at a point lower down the column and above what is called the stripping section. he crude distillation tower is typically 4%-5# m high. Distillation column is e0uipped with around 1#$%# valve or bubble cap trays. trays. 3s crude is is entered to the column lighter lighter component start to vapori+e due to their higher volatility. volatility. he vapor goes up rises up to the top of the column at a high h igh flow rate. he lightest hydrocarbon goes to the condenser at the column overhead. his condensed part goes to overhead reflux drum. In the reflux drum& gases and li0uid naphtha/ are separated. 6eparated )ases C!-C4 gases/ goes to the flare header and li0uid naphtha divided into two streams. ne stream drawn as a product stream and another one is returned to the column as a reflux. his reflux li0uid flows down through the series of column trays& flowing counter current to the hot vapo rs coming from bottom and condensing some of of those vapors along the way.

he products are also drawn from different trays of the column. hese are called side draw-offs. hese side cut products are straight run naphtha & kerosene or 7et fuel& 8i0uid gas oil 8)/ and

9eavy gas oil 9)/ .he lightest side draw-off from the fractionating column is a fraction called kerosene& boiling in the range !%#-"2#oC& which flows into a smaller column called sidestripper. he purpose of the side stripper is to remove some light hydrocarbons by using steam in7ection or an external heater called reboiler.  reboiler. It essentially helps to meet the properties specified for kerosene& since in a multi-component distillation there is overlap of constituents of various cuts. 3fter leaving the tower& bottom product residue crude oil goes to the vacuum distillation unit for further processing. verhead and side cuts products streams go to holding& intermediate storage tanks or for further units of refinery depending on their composition and refinery re0uirements.

Process Flow Diagram Atmospheric Crude Distillation Distillatio n Unit

Desalter:'ost crude contain traces of salt which can decompose in the heater to from hydrochloric h ydrochloric acid and cause corrosion of the fractionators overhead e0uipment. In order to remove the salt& water is in7ected into the  partially preheated crude crude and the stream is thoroughly thoroughly mixed. he mixture of oil and water is separated in a desalter& which is a large vessel in which ma y be accelerated by the addition of chemicals or by electrical devices If the oil entering the desalter is not enough heated& it may be too viscous to permit proper mixing and complete separation of the water and the oil& and some of the water may be carried into the fractionators. 3pproaches removal of brine suspensions/ ; 'echanical ; chemical ; (lectrical 'echanical 9eating of crude oil to !"#-!4#oC under pressure to check vapour loss loss followed by setting in large vessels

C9('IC38 ;9eating of crude oil& mixing wash water < chemical demulsifier / and settling ;=o more used being less efficient and unable to give constant product 0uality

(lectrical Desalting his desalting approach is largely used at present refinery. P,I=CIP8(

>*hen two immiscible li0uids one electrically conductive and one non conductive& pass through a high intensity electric field& the conductive li0uid coalesces into large droplets? In crude oil- water emulsion system water is electrically conductive hence coalesces into large drops and falls to bottom being heavier than oil.

Deasalting : Problems&Causes and 6olutions: Problems 9igh salt content in desalted crude oil

Causes a/ @eed salt content high b/ *ash water in7ection low c/ Crude oil flow rate more than design value d/ Insufficient mixing of  crude oil and wash water 

6olutions a/ Increase wash water  rate b/ ,educe crude oil flow rate c/ Increase the mixing valve pressure drop

il in the desalter effluent water 

9igh water carry over in desalted crude oil

a/ Interface level too low b/ (xcessive crude oil wash water mixing c/ Poor wash water 0uality d/ Crude oil temp too low a/ *ash water flow rate too high b/ Interface level too high

Source : Handbook of Petroleum Processing, !!"

a/ Increase the interface level b/ ,educe the mixing valve pressure drop a/ ,educe the wash water flow rate or  increase chemical in7ection b/ ,educe the interface level