Oil-desalting units

Description

When extraction, crude oil is always accompanied by oil-field (stratal) water (from <1 up to 80-90% of total mass); dispersing in oil, stratal water emulsifies into ‘water-in-oil’ emulsion types (continuous phase- oil, internal phase- water). Their formation and stability are conditioned by the presence of natural emulsifying agents (pyrobitumen, naphthene, tars) as well as by dispersed mechanical impurities (clay, sand, limestone and metal particles). As a rule, stratal water is highly mineralized by sodium, magnesium and calcium chlorides (up to 2 500 mg/l of chlorides even at water content of only 1%); moreover, it is rich in sulfates and hydrocarbonates and mechanical impurities.

The presence of the above mentioned substances and mechanical impurities in oil has a negative impact on oil-refining equipment:

due to high water content pressure builds up in oil-refining units, resulting in output capacity loss and increase in power consumption;
due to salt deposition furnace and heat-exchanger pipes require frequent cleaning, calorific efficiency is decreased; additionally, salts are strong corrosion agents (Ca and Mg chlorides hydrolyze giving HCl). Besides salts and mechanical impurities build up in residual oil such as masut and tar, reducing their quality.

Oil dehydration is carried out by breaking down (layering) of water-oil emulsion using demulsifying agents- various surface-active substances. Adsorbing on the interphase boundary these substances stimulate breaking of droplets (globules) of water dispersed in oil. However even drastic oil dehydration resulting in max. 0,1-0,3% strata water content (which is quite a technological challenge already) does not grant optimal output parameters. Due to high mineralization of strata water the content of chlorides is still rather high: 100-300 mg/l (on NaCl basis), in the presence of crystallized salts the percentage is even higher. That is why dehydrating alone is not enough to prepare oil from most oil-fields for further refining. Salts and water remaining in oil are removed by ‘desalting’, an operation not much different form dehydrating. The operation consists in mixing oil with fresh water and emulsion breaking followed by separating oil from flush water now containing salts and mechanical impurities.

Max. salt content in oil at unit output: 5 mg/l