| Petroleum sludge is a major waste generated during crude oil production, transportation, storage, and refining. Because of its high content (30wt%-80wt%) of petroleum hydrocarbons (PHCs), oil recovery is considered to be the most economical and environmental choice for handling this industrial waste. As a relatively clean and well commercialized technology, centrifugal separation is suitable for engineering applications of resource utilization of oily sludge. Petroleum sludge can generally be considered a complex emulsion of aqueous droplets dispersed in petroleum liquid that typically contains oil, water, and solids. However, the presence of emulsified water makes it difficult to achieve high dwatering efficiency by centrifugation.The primary aim of this study is to improve oil-water separation efficiency in petroleum sludge. A series of work are performed to investigate the distribution characteristics of emulsified droplets in oily sludge, the motion law of water droplets in the centrifugal field, and the high-efficient pre-treatment technology.First, physical and chemical properties of petroleum sludge were analyzed. The results showed that oily sludge contains a variety of heavy metals and polycyclic aromatic hydrocarbon (PAHs). The oil/water/solids content in oily sludge from different sources are quite different, and this is mainly related to sludge formation conditions. Relative content, morphology, existence forms and the particle size of each component can be determined from the microscopy observation. The calorific values of oily sludge are proportional to carbon content and the H/C is about 2. According to the results of chromatographic simulated distillation, oily sludge contains high content of heavy oil fractions.Differential scanning calorimetry (DSC) was successfully employed to characterize the emulsified water droplets and salt in petroleum sludge. Samples of three different types were evaluated before and after centrifugation. The results indicated that water content and eutectic of salt hydrates could be identified during DSC controlled heating from -60℃ to room temperature. Moreover, a new equation to retrieve water droplets size distribution according to the cooling phase of the DSC thermogram from 0℃ to -60℃ is proposed. This is the first time DSC was employed to characterize emulsified water and deduce water droplets size distribution in petroleum sludge. These results are essential and useful for optimization of petroleum sludge treatment.The parameters affecting centrifugal separation of emulsified water in petroleum sludge was analyzed. The key factors in improvement of oil-water separation efficiency were determined through the orthogonal experiments. For petroleum sludge with high viscosity and wide droplet size distribution, the key factors to improve dewatering rate are temperature and centrifugal speed. Heating can effectively reduce the viscosity of oily sludge, and lifting centrifugal speed can reduce the critical diameter of separated droplets. For petroleum sludge with tiny water droplets(<5μm), the key factor to improve dewatering rate was rotational speed of centrifuge. And in order to improve the performance of mechanical dewatering, demulsification pre-treatment prior to water/oil/solid phase separation is essential to obtain high-purity oil resources. Increase in centrifugal time can improve dehydration rate of oily sludge with large diameters(> 0μm).The Monte Carlo method was used to predict the settling behavior of different-sized water droplets, based on the Navier-Stokes equation. Numerical estimation and experimental analysis both show that the water removal rate increases with rising centrifugal speed. When the centrifugal speed increased from 2000 to 10,000 rpm, the water removal rate increased from 28% to 99%. Meanwhile, the critical separation size of emulsified water droplet decreased from over 14μm at 2000 rpm to around 3μm at 10,000 rpm. The Monte Carlo method was also successfully applied to predict the dehydration rate and the critical separation size of petroleum sludge with different viscosity under specific centrifugal condition. The simulations are in good agreement with experimental results. This calculation model can provide quantitative information and guidance for optimizing the centrifugation of petroleum sludge for oil recovery.Pre-treatment technologies, such as ultrasound, mixing light oil, and addition of microemulsion, were applied to optimize separation characteristics of oil-water in petroleum sludge. And the coupling of various pre-treatment technologies was proposed to reduce the viscosity of oily sludge and promote demulsification. The results showed that 25 kHz and 30 kHz ultrasound could directly remove water by 26.3 wt% and 28.1 wt% respectively, but resulted in decrease of droplet diameter in remaining water. Although there is little or no separation of water by high-frequency ultrasounds,35 kHz and 40 kHz strongly affected the emulsified water, and increased the mean droplet size. These differences could be primarily explained by acoustic resonance and cavitation effects. Mixing light oil can effectively reduce the viscosity of oily sludge and significantly improve the dehydration rate. Microemulsion has the ultra-low interfacial tension, and it can effectively break down the water-in-oil structure of petroleum emulsion.In order to promote engineering applications of resource utilization of oily sludge, improve the quality of recovered oil, and reduce secondary pollution, the integrated techniques of two-stage separation system of oily sludge was developed. A typical petroleum sludge sample was selected to carry out process optimization experiment in a laboratory scale. The results showed that with 20 wt% addition of kerosene in the High-Solid Pretreatment Unit, solid particles larger than 15.1μm in oily sludge can be removed. And by addition of 7 wt% microemulsion in Chemical-Conditioning second-order separation, it is effective to separate water and solids from oily sludge. And the recovered oil contains less than 3 wt% residual water and solids. This system will be applied in an oily sludge disposal demonstration project which has a treatment capacity of 100 t/d. |
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