The principle of using distillation method to determine the water content of crude oil
Under the condensation of circulating water, a certain amount of oil sample is mixed with an insoluble solvent and slowly heated. The distillate is controlled to drip into the receiver at a flow rate of 2-5 drops/s. The oil sample, solvent, and water are distilled together. Due to the relative density of water being higher than the solvent, they settle to the lower part of the receiver marking tube. The solvent and oil enter the flask again to continue distillation and separation until the water content in the lower part of the receiver marking tube no longer increases. When the upper part of the receiver clarifies, heating stops. Calculate the moisture content based on the amount of sample used and the volume of distilled water.
2. Factors affecting the determination of moisture content
2.1 Sampling and Processing of Samples
The crude oil emulsion belongs to an unstable system. As the storage time increases, oil and water will continuously separate, and the water content of the upper oil sample will decrease while the water content of the lower oil sample will increase. Therefore, the water content of the crude oil in any part of the oil sample cannot represent the water content of the entire tank. This requires strict three-level sampling to ensure the representativeness of the oil sample. The absence of free water at the bottom of the sample indicates that the crude oil emulsion belongs to a stable system of crude oil. After shaking, it has a certain degree of stability and will not separate quickly in a short period of time. Therefore, the sample can be weighed after vigorous shaking to reduce errors; The lower part of the oil sample contains free water, and after vigorous shaking, the crude oil emulsion still belongs to an unstable system. Therefore, the upper emulsified crude oil and lower free water are calculated separately, and the emulsified crude oil with free water removed is vigorously shaken and distilled to determine the water content of the crude oil.
2.2 Preparation work for measurement
The measuring instrument should be clean and dry to prevent water droplets from hanging on the wall. Heat the sample to the lowest temperature with sufficient fluidity, and shake vigorously before weighing to mix oil and water evenly. Plug the upper part of the condenser with cotton or desiccant to prevent external water from entering the condenser. The instrument has good airtightness, preventing the loss of moisture in the system.
2.3 Selection of solvents
In the application of GB/T8929 and GB/T260 standards for measuring the water content of crude oil, an insoluble solvent needs to be added. The solvent plays a role in dissolving, diluting, dispersing, washing, and carrying the crude oil throughout the entire process. The poor fluidity of crude oil results in water in oil and water in oil phenomena. The high water content and impurities leading to wall sticking and water hanging determine higher requirements for solvent solubility.
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