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Desalter Operation Optimization

By Adhi Budhiarto - Rabigh, KSA

Desalter is used to eliminate salts, water, sediment, and suspended solid. We need tohave desalter for some reasons, as follows :

1. To avoid corrosion (effected by salts) it is usually main taret of desalteroperation.

!. To avoid foulin and co"in (effected by salts, sediment, and suspended solid).

#. To avoid erosion inside pipin$e%uipment (effected by sediment).

&. To avoid hih enery consumption (effected by salts, water, sediment, and

suspended solid).

'. To avoid catalyst poisonin (effected by salts).

. To avoid hih cost of chemical used for handlin foulin and corrosion (effectedby salts, sediment, and suspended solid).

As my experience, there are some factors influencing emulsion stability of liquid insideof the desalter, as follows :

! "rude density ,#mulsion between crude and water can be bro$en by density by

following Stoke formulae : V = K*(rhow-rhoo)*G*d!"no%which are : & 'water droplet fall down (elocity ) K ' constant) rhow ' water density) rhoo ' oildensity) * ' gra(ity acceleration) d ' water droplet diameter) no ' oil bul$(iscosity+! #(enthough crude density gi(es big impact to the emulsion stability,but it is not a operating (ariable!

! ! "rude inlet temperature , "rude (iscosity dereases with increasing temperature,so that water droplet can be easier to be separated! igher desalter temperaturecan also increase water droplet combination efficiency by : a+ .ncreasing thedilution of natural emulsion stabili/er, such as naphthenic acid! b+ .ncreasingcrashing frequency between water droplets!c+ Accelerating the diffusion of

demulsifier into oil-water interface!igher desalter temperature can alsoinfluence crude density! "rude density decreases with increasing temperature, sothat it can increase efficiency %see Stro$e formulae+! 0et, there is a maximumlimitation for desalter temperature! 1oo high desalter temperature can e(aporatelight ends which can ma$e turbulence! 1his turbulence can increase can decreasedesalter efficiency!

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2! #lectrical grid of desalter %if the type of desalter is bielectric+ , electrical gridoperation is usually trouble-free %e(enthough there is a possibility of shortcircuit+! 3roblems can be anticipated by ha(ing (oltage and ampere indicators! .nnormal operation, these indicators should not be fluctuated!

4! 5ash water in6ection rate, 5ash water in6ection rate should be moderate, not toolow or too high! 1oo low wash water in6ection rate can decrease desalterefficiency, because water droplets in emulsion established by mixing (al(e can betoo far to combine each other in the electrical field! 0et, too high wash waterin6ection rate can ma$e emulsion to be too conducti(e which can increaseelectrical current and decrease (oltage, so that it can decrease dri(ing force forpolari/ing droplets, combining each other in electrical field, and decreasingdesalter efficiency! 5ash water in6ection rate (aries depending on crudeproperties, usually 4 to 7 8 (olume of crude %for crude ha(ing high specificgra(ity, wash water in6ection rate can be increased to 7 9 8 (olume of crude+!1he influence of water droplet population;density in the process of water dropletcombination in electrical field can be described by formulae as follows : # =K*$!*a%"d& %which are : < ' pulling force among droplets in electricalfield) K ' constant) # ' (oltage gradient among grids) a ' radius of nearest waterdroplets) d ' distance among droplets in electrical field+! .f wash water in6ectionrate is low, the water droplet population;density is also low! .f the distanceamong droplets in electrical field is wider, the dri(ing force for polari/ingdroplets in electrical field will decrease, so that desalter efficiency will alsodecrease!

=! p of 5ash water in6ection , >ptimum p of wash water in6ection is between =!=to ?!=! .f p is less than =!=, it will cause corrosion in desalter! .f p is o(er then?!=, it will stabili/e emulsion, much oil will brea$through as effluent water! .n

high p en(ironment, naphthenic acid contained in crude will be ioni/ed andma$e sodium soap or potassium naphthenate which is a powerful emulsifier! .fwater in emulsion contains calcium and magnesium carbonate or sulfate, highp of wash water can ma$e sludge in desalter and scale in effluent water pipingor downstream heat exchanger!

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desalter, water le(el should be maintained to be -4 under lower electricalgrid!

! Cesalter pressure, Cesalter operating pressure is usually between =@ to =@ psigdepending on discharge pressure of feed pump and desalter location in the

preheater system! Cesalter operating pressure should be enough to a(oid crudee(aporation within normal operation temperature! owe suggested to ha(edesalter operating pressure at least @ psi abo(e (apor pressure of oil-watercompound at desalter normal operating pressure! igh e(aporation can cause(apor space in the upper side of desalter which can automatically cut electriccurrent supply to electrical grid!

! Cemulsifier, 1he ob6ecti(e of introducing demulsifier is to decrease oil content ofdesalter effluent water and to decrease salts and solids! By ha(ing demulsifier,pressure drop of mixing (al(e can be set higher, so that salt remo(al efficiencycan be increased! 1he usual type of demulsifier is al$oxylated al$yl-phenol;formaldehyde resin! Cemulsifier can increase desalter performance insome ways as follows :

Surface acti(e components will replace emulsifying agent in oil-water interfacewhich results in the combination of water droplets!

5 etting agent ma$es the solid particle surface wet, so that the solid can beremo(ed from oil phase or from oil-water interface and it will disperse into waterphase! 1his will decrease solid contained in desalted crude and results in waterdroplet combination!