Research On Water Content Measurement Of The Oil Sludge Produced During Storage And Transportation With A Capacitance Sensor

With the rapid development of China’s economy, the amount of oil exploitation, processing, and storage continued to increase; there are a large number of oil tanks in the oilfield, oil transportation ports and oil refineries. After long-term storage, large amounts of oil sludge generate at the bottom of the tanks, causing a waste of resource and a serious threat to the environment. The oil sludge produced during storage and transportation has high recycling value because of the high oil content and the low sand content. The water content and the oil content need to be measured to estimate the recycling value of the oil sludge. On the other hand, the oil sludge recycling companies need to determine the recycling process and adjust the amount of processing reagents according to the composition of the oil sludge. Therefore, the water content measurement of the oil sludge has significant application value. So far, there is no specific standard for the water content measurement of the oil sludge produced during storage and transportation. There is also no instrument available in the market. The methods now used in laboratories have lots of disadvantages, such as complex operations, long time costing, large sampling errors and so on. Those methods are not entirely applicable to the oil sludge produced during storage and transportation. So a method which is fast and suitable for field applications for measuring the water content of the oil sludge is needed.In this paper, the forming process of the oil sludge in the storage tanks was studied and the physical&chemical properties of the oil sludge were analyzed. The advantages and disadvantages of several common methods for the water content measurement were summarized, and then the method of measuring the water content with a capacitance sensor was proposed. The theoretical model of the dielectric constant for mixture was studied, and then was corrected according to experimental data to establish a more precise and practical model for the measurement. The measurement system was designed based on a coaxial cylindrical capacitance sensor. The entire system was based on an ultra-low-power MSP430microcontroller. The capacitance detection circuit based on an AD7746capacitance-to-digital converter was presented. The main research work and innovations are as follows:(1) For the oil sludge produced during storage and transportation, which consists of three phases:oil, water and sand, a dielectric constant model is established. Oil and sand are mixed first in theory according to the two-phase model, and then they are mixed with water to establish a three-phase model. By theoretical calculations, the feasibility of measuring water content with the dielectric constant method in the case of unknowing the sand content was analyzed.(2) The effect of the temperature change on this method was analyzed; with considerations of great viscosity and poor mobility of the oil sludge, the heating-stir pretreatment was proposed to make the samples easier to measure.(3) According to related literatures, a coaxial cylindrical capacitance sensor was designed in consideration of edge effects, sensitivity and the physical&chemical characteristics of the oil sludge.(4) By the analysis of the experimental data of the prototype system, the most suitable theoretical model for the oil sludge produced during storage and transportation was selected—the logarithmic model. In order to establish a more precise and practical model for measurement, the theory model was corrected with the fitting method according to experimental data.