Study On Residence Time Distribution In The Modified Scheibel Extraction Column

The residence time distribution (RTD) has a direct impact on extraction efficiency in liquid-liquid extraction equipments. The study on RTD not only helps obtain hydrodynamic information and non-ideal flow such as the short-circuit, channeling, recycling of fluid to find out the factor which causes backmixing and offer the evidence to modify the structure of extractor to reduce backmixing, but also lays a foundation for the research of mass transfer. So it is essential to study the RTD in the modified Scheibel extractor.The residence time distribution under different operation conditions was studied in three type of modified Scheibel extractors and the backmixing model was proposed to describe the hydrodynamic behavior in this paper. Furthermore, the extraction efficiency of the modified Scheibel extractor Ⅲ which had light backmixing was determinated.Results showed that the experimental RTD curves were better fitted by backmixing model than tanks-in-series model, and the two-parameter backmixing model which considered inter-stage backmixing (the axial backmixing coefficient a) and intra-stage non-ideal flow (the correction coefficient of concentration β) was consistent with the experimental value and could be used to simulate experimental RTD curves of the modified Scheibel extractors.The results of different extractors showed that the axial backmixing of continuous phase was serious, while the axial backmixing of dispersed phase was significantly smaller in the modified Scheibel extractor Ⅰ which had some defects in structure. The improvement of the modified Scheibel extractor Ⅱ, Ⅲ reduced backmixing of continuous phase to a large extent. The value of continuous phase’s axial backmixing coefficient a of heptane-water system in the modified Scheibel extractor Ⅱ、Ⅲ was obviously smaller than that in the modified Scheibel extractor Ⅰ, and the value of a in the modified Scheibel extractor Ⅲ was smaller than that in the modified Scheibel extractor Ⅱ under most conditions. While the value of axial backmixing coefficient a of octanol-water system in the modified Scheibel extractor Ⅲ was obviously smaller than that in the modified Scheibel extractor Ⅱ, and the value of a could reach a small value when the agitation speed was low.The results of different systems showed that the properties of systems had great impact on the axial backmixing of continuous phase and intra-stage non-ideal flow. The axial backmixing decreased with an increase in viscosity of continuous phase and interfacial tension. In the modified Scheibel extractor Ⅱ, the β value of the system with lower interfacial tension was lower than 1, so it was easy to develop recycling of fluid in the stage; the β value of higher interfacial tension system was close to 1, so it was close to perfect mixing in the stage; the β value of the system with higher viscosity of continuous phase was higher than 1 in most operating conditions, so it was easy to develop short-circuit and channeling of fluid in the stage. In the modified Scheibel extractor Ⅲ, the β value was lower than 1 under most operating conditions, so it was easy to develop recycling of fluid in the stage, but the deviation from perfect mixing of higher interfacial tension system was smaller than that of lower interfacial tension system.The extraction experiment result of propanol heptane solution in the modified Scheibel extractor Ⅲ which had light backmixing showed that the extraction yield was improved obviously. Therefore, the structure modification in Scheibel extractor reduced the adverse impact of backmixing on extraction and extraction efficiency was improved evidently.