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16/05/2011
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Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Desalters are the Key equipment in CDUs and does the functions of Desalting as well as Dehdyration. There are a few parameters that indicate the efficienty of Desalters, viz., Desalting efficiency, Dehydration efficiency, Brine quality, desalting water consumption, chemical consumption. While desalting helps minimising corrosion, dehydration helps in preventing fouling of down stream preheat exchangers (improve preheat recovery and hence minimise the Fuel consumption) and Brine quality helps in ensuring effluent quality in Waste water treatment facility at a low expense. Desalting is said to be efficient when 90% of Desalting is done in each stage (target of <1.0 PTB salt and <500 PPM of Oil in Brine). NO water carry over into Desalted Crude is the other parameter that judges the desalter performance. All the above need to be achieved with lowest Desalting water and Chemical consumption. Remember water is not cheap. High oil carry over with Desalter Brine increases slops production and hence increase recycling costs.
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12/07/2010
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Jayaraj Jayam, Chennai Petroleum Corporation Limited, njayaraj@gmail.com
The foremost objective of the desalter is to remove inorganic salt such as chlorides of sodium and magnesium from the crude oil. Also it acts as a surge drum to give sufficient residence time to help settling of suspended particles. It should remove all water including micro size particles of water using the high electric field.
Efficiency is calculated as follows: (Quantity of salt removed/Total salt in crude before desalter)*100.
The expected removal percentage is 95 to 98%. Removal of water can be identified by closely observing the variation in sour water make from atmospheric column overhead product drum or reflux drum.
Efficiency of desalter can be improved by adding correct quantity of water (3 to 5% of feed), maintaining differential pressure across the mix valve (approx 1.0kg/cm2), desalter temperature to improve solubility and desalter operating pressure to give proper residence time for settling of water. Desalter temperature also plays a role for settling of water by reducing the viscosity of crude oil. As the settling velocity is inversely proportional to viscosity, it helps to settle water.
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08/07/2010
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Berthold Otzisk, Kurita Europe GmbH, Berthold.Otzisk@kurita-water.com
Very good indicators for the desalting performance are the desalting efficiency (%) and dehydration efficiency (%). A single stage desalter system should give a desalting efficiency >90%. With a double stage desalter system a desalting efficiency >98% is expected. The inorganic salts of NaCl, MgCl2 and CaCl2 are typically expressed as ppm NaCl.
The desalting efficiency is calculated with this formula: Desalting efficiency = ((ppm NaCl in crude oil before desalter – ppm NaCl in crude oil after desalter) / ppm NaCl NaCl in crude oil before desalter) * 100
To calculate the dehydration efficiency the amount of wash water must be included: Dehydration efficiency =((%water in crude oil before desalter + %wash water to desalter - %water in crude oil after desalter) / (%water in crude oil before desalter + %wash water to desalter)) * 100
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07/07/2010
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keith bowers, B and B Consulting, kebowers47@gmail.com
In order to best judge the 'efficiency' of a desalting unit (whatever efficiency means) one compares the quality of the product with the feed, and estimates that improvement compared to the maximum theoretically possible. In the practical world, one measures the un-removed contaminants in the de-salter product stream and the oil lost in the de-salter water product. Salt content in pounds (kilograms) per thousand barrels is a commonly used metric. Also important is the BS&W to determine how much dispersed water is not being removed. Then a Karl-Fischer water analysis (ppm) to determine the dissolved water remaining in the processed crude is helpful.
The process objective of 'de-salting' is to:
a) remove water soluble chlorides and sulfates and any other 'salts' so they do not cause corrosion as they decompose and form mineral acids in the distillation equipment
b) remove any particulates such as sand, iron oxides, clays, and any other 'sediment' that would accumulate on the distillation trays or packing and cause both plugging and corrosion under the deposits.
The goal is a perfectly 'clean' feed to the distillation equipment. 'Efficiency' can be considered as a calculation of just how far short of perfection the unit and operations achieved.
One might also be concerned about the quantity of chemicals used, electricity consumed, pumping horsepower, water use versus theoretical minimum for the measured performance, hydrocarbon loss in the water effluent, total treating cost per barrel, etc. The BIG MONEY savings are in reducing residual salts and sediments as much as reasonably possible to reduce corrosion and fouling of the equipment.
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07/07/2010
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
KW per Barrel
% salt removal per stage
oil in brine
Required transformer size varies with crude source, so care is required in comparing with other desalters.
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07/07/2010
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Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
The simplest way is to measure the residual salt, i.e. 'Salt = ~ 1 pound per thousand barrels is good target for single stage desalting of conventional crude oils, while 'Salt' = ~0.5 ptb is good target for recommended two stage desalter system.
Other aspects to consider are:
(a) Residual water in crude oil [should be down in the few tenth weight percent via BS&W testing]
(b) Residual dirt and rust and any carbonaceous materials in crude oil [Most of these should have collected in the bottom of the desalter vessels and been blown down to suitable external decanting and transfer receiver via well engineered 'mud drain system'
(c) whether emulsions have been mostly 'broken' such that there is only a small 'rag' layer inside the desalter vessels [with reasonable expenditures on demulsification chemicals]
(d) Ensuring that desalting is at optimal crude oil outlet temperature year around [regardless of whether crude oil in tankage is cold and encon exchanger network is clean or fouled].
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