Research On Mechanism And Technology Of The Oil Recovery From Oily Sludge Via CO₂+ Water Composite Extraction

Oily sludge is one of the main wastes generated in crude oil exploration,exploitation,refining,tank clearing,storage and transportation due to the accidents,dripping,natural sedimentation and other reasons.It contains not only waste water and solid particles with traces of heavy metals potentially harmful to the environment,but also considerable content of recyclable oil with high value.It is reported that about 1 million tons of oily sludge containing 150,000 to 500,000 tons of crude oil have not been treated in China.Therefore,developing effective technology for treating oily sludge and recovering oil is of practical significance.In this work,a new technology named CO₂+Water Composite Extraction(CWCE)is developed for efficient recovery of oil from oily sludge.With respect to this new technology,a series of experimental and theoretical studies were performed to optimize operation conditions and disclose the working mechanism of CWCE method.The main contents are summarized as follows:(1)The analysises on the characteristics of two oily sludge samples ZHY and SLWG respectively from the China National Offshore Oil Corporation and Shengli Wugang were performed.The results show that:(I)the oil sludge samples contain 10?56 wt.%of recyclable oil,and there are big differences in the contents of water,oil and solid for oily sludge samples from different sources;(II)the oil and solid particles are cemented with complex microstructures;(III)oily ludge sample ZHY is mainly composed of medium and light oil while SLWG is rich in heavy oil.The extraction efficiency for oil sludge by supercritical CO₂ and liquid CO₂ were low,and the highest oil recovery ratio was only6.32%.(2)With respect to the oil recovery from oily sludge sample ZHY by using CWCE,systematically experiments were performed to investigate the influences of operation conditions such as temperature,pressure,stirring time and mass ratio of oily sludge to water upon the oil recovery ratio.The most suitable operation temperature,pressure and stirring time were found to be 60?,3 MPa and 90 minutes respectively,under which the oil recovery ratio could reach 84 wt.%,it is much higher than those obtained by using traditional methods such as ultrasonic treatment,solvent extraction,high temperature supercritical extraction and low temperature supercritical extraction.(3)The thermodynamic and kinetic mechanisms for CWCE method enhancing the oil recovery from oily sludge were investigated.The solubility of CO₂ in the liquid phases of the slurry formed with oily sludge sample ZHY and water as well as the expansion factor describing the volume expansion behavior of oil caused by the dissolution of CO₂in it were measured systemtically and the corresponding calculations by using thermodynamic model were performed.These measurements and calculations show that the mechanism that CWCE enhances the oil recovery from oil sludge lies in two aspects.At first,the dissolution of CO₂ in oily sludge decreased the density and viscosity of oil phase and increased the oil-solid interfacial tension.These behaviors could enhance the separation of oil phase from solid phase and favor upwards floating of oil drops in water.Secondly,mass of gas bubbles were produced in the depressurization process and adhere to oil drops,that phenomenon further enhanced the separation and upwards floating of oil phase.(4)The charateristics of waste water and oil sludge residue generated in the oily sludge treatment process by using CWCE methods were analyzed and the further treatment methods for them were studied.The results show that,compared with using simple water extraction method,the quality of sewage produced in oily sludge treatment could be improved remarkably by using CWCE technology.Deep deoiling for oil sludge residue can be achieved by indroducing ultrasound and detergent to CWCE method.It has been proved that the residual oil content in the oily sludge residue for ZHY can be reduced to 1.76 wt.%by using CWCE aided by the ultrasond with a frequency of 40 k Hz and a power of 50 W and the detergent(a mixture of Na₂Si O₃·9H₂O and BR-F201 in a mass ratio of 2:1)added in a dosage of 5.0 g·L^-1.The residual oil content could be further reduced to 1.06 wt.%after the secondary ultrasonic treatment.(5)An elementary economic evaluation on CWCE plus ultrasonic cleaning technology was made and a process was designed for oily sludge treatment in a scale of500 kg/h.In conclusion,the novel CWCE technology can not only increase the recovery ratio of oil,but also improve the quality of sewage produced in oily sludge treatment compared with conventional water extraction approaches.It is in agreement with the idea of green utilization of oily sludge.