Crude Oil Desalting and Residual Water Issues

Residual water, which is dispersed in crude oil as very small droplets, typically contains substantial amounts of dissolved salts as chlorides. The salts are picked up in the reservoir or during transportation. A typical oil reservoir contains gas, oil and water with the water often being salty.  The salt in crude oil can lead to higher corrosion rates, catalyst poisoning in down stream units and fouling of heat exchangers. The crude can also contain suspended solids. The crude oil is typically desalted before being sent to the distillation train. Intertek PARC has a two-stage electrical (AC) desalter which can be operated either in the co-current or counter-current mode. For more information on desalting, visit:  http://www.intertek.com/testing/pilot-plant/desalting-crude-oil/.

The salt content of the desalted oil is usually reduced to a level of 1-2 lbs/1000 bbls. It can be determined using electrometric methods ASTM D 3230 and IP 265 by measuring the conductivity of the crude oil in an alcohol solvent mixture. These two tests measure the conductivity of the oil due to the presence of inorganic chlorides, such as sodium, calcium and magnesium. A calibration curve is generated relating current vs. chloride concentration. A 10: 20: 70 CaCl2: MgCl2: NaCl mixture, which presents a number of common crudes, is used in the calibration. Species other than chlorides must not contribute significantly to conductivity for accurate results to be obtained. ASTM D 3230 and IP 265 testing is provided by Intertek (http://www.intertek.com/petroleum/test-directory/s/).

Crude conductivity can be significantly impacted by blending of two or more crudes to meet total acid number (TAN) specifications, typically <0.5. Blending two low conductivity crudes can lead to a blend with conductivity higher than those of the individual crudes [1]. When desalting high conductivity crude either due to higher temperature operation or to the presence of other conducting species, more power is consumed [2]. ASTM D 3230 and IP 265 are not reliable with high conductivity crudes due to interference as pointed out in the method D 3230.  Potentiometric method ASTM D 6470 or back titration method IP 77 can be used to measure chloride concentrations.  ASTM D 6470 and IP 77 testing is provided by Intertek (http://www.intertek.com/petroleum/test-directory/s/).

Regards;

Rob Absil, Intertek PARC Pilot Plant Technology Center

Topics of interest include:

•    Crude oil blending effects on crude oil conductivity.
•    Problems experienced with crude salt content measurements.

References:

1.    Potter, A.C., Crude Oil Conductivity Presentation, February 2007, retrieved from Internet.
2.    Pruneda, E. F. et al., J. Mex. Chem. Soc.,2005, 49 (1), 14-19, retrieved from the Internet.