How to accurately measure water in crude oil and why is it so important
Nov 17 2020
The accurate analysis of a crude oil sample to determine the water content is important in the refining, purchase, sale and transfer of crude oils as contracts are based on net dry oil, in other words, no one wants to be paying crude oil prices for barrels of water.
How do we measure the water content?
Historically different methods have been employed to determine water in crude oil. In the distillation method, commonly known as the Dean & Stark technique, a solvent having similar distillation characteristics as water is added to the sample. This is then heated under reflux conditions so that the water and solvent co-distil. They are then condensed and the condensate is collected in a graduated trap where the water droplets, being heavier than the solvent, sink to the bottom of the trap and are measured against the graduations.
The centrifuge method for water in crude oil, known as BS & W (Basic Sediment & Water), involves spinning a specified amount of sample for a specified time in a heated centrifuge until all the water and sediment settle in the bottom of the centrifuge tube.
By far the most widely accepted method for the determination of low concentrations of water in crude oil is the Karl Fischer titration. The Karl Fischer method offers increased accuracy and much faster analysis over these other techniques. Aquamax KF Coulometric Karl Fischer instruments can also be used in the field as well as in the laboratory.
Who needs to do this?
Water content determination by Karl Fischer titration is no longer restricted to being used by a chemist in a laboratory – now the technique is used by engineers, plant operators, tanker drivers, distribution/maintenance engineers and other non- laboratory personnel.
As well as in the laboratory Karl Fischer titrations can now be performed in many different locations such as the tailgate of trucks, mobile laboratories, offshore installations, tankers, engineering workshops and dockside cabins.
“ACE” Control System (Automatically Compensated Errors)
The standard test methods, ASTM D4928, MPMS Chapter 10.9, IP 386, etc, for water content determination of crude oil samples by coulometric Karl Fischer titration, stipulate that the Karl Fischer anode reagent be mixed 60:40 with xylene to improve sample miscibility and reduce waxy deposits and asphaltenes from dropping out and thereby causing problems with the generator electrode. This modification to the reagent, and subsequent addition of the oil samples being analysed, affects the resistance / capacitance of the titration vessel.
When this resistance in the electrolysis cell is increased beyond certain limits the coulometer is unable to operate efficiently and provides an incorrect result, usually falsely high.
For this reason, although in principle standardisation of a coulometer is not necessary since the water ‘titrated’ is a direct function of the coulombs of electricity consumed, the above mentioned methods suggest that the performance of the coulometer be regularly monitored by injecting 10 μl of pure water. The suggested interval is after every 10 determinations and the result obtained should be 10,000 ± 200μg. The reagents should be changed if the result is outside these limits.
The “ACE” control system guarantees that the electrolysis current produced by the coulometer and the count rate displayed are always correctly synchronised regardless of changes to the electrolysis cell resistance. Aquamax KF Reagent A is a coulometric KF anolyte for use with generator electrodes which incorporate a frit (diaphragm) to separate the anode and cathode chambers. Used in conjunction with Reagent C this formulation is supplied in a pack of 8 x 100ml bottles, 8 x 5ml cathode vials, all conveniently located in a single carton.
Aquamax KF Reagent has been specially formulated with an optimum blend of solubilizers suitable for oil samples, to eliminate side reactions, so that the operator does not have to store or mix chemicals.
Aquamax KF have been the most widely used water determination tool for water in crude at the well head, on shore, off shore, in labs and at the sample point on the back of a truck.
A recent BBC documentary shows you how important and how easy Aquamax KF are to use when needing to accurately test for water in crude oil.
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