Design Of Micro-Nano Bubble Cavitation Reactor And Treatment Of Shale Gas Fracturing Flowback Fluid

Shale gas is an important unconventional natural gas resource.In the mining process of shale gas,the use of hydraulic fracturing technology produced a large number of shale gas fracturing flowback fluid with high organic matter,suspended matter and salt contents,which is difficult to deal with and seriously polluted.In this work,micro-nano bubbles cavitation effect is used for the primary oxidation of organic pollutants with easy degradation and moderate degradation performance in fracturing flowback fluid,providing a technical scheme for the oxidative pretreatment of shale gas fracturing flowback fluid.In this work,taking the shale gas fracturing flowback fluid as the research object,a new type of micro-nano bubbles cavitation reactor was designed by combining the ejector with the variable-pitch spiral cavitation.Firstly,numerical simulation and structural optimization of the key component,i.e.variable pitch spiral cavitation,were carried out,and the performance of the micro-nano bubbles cavitation reactor was evaluated,then the cavitation oxidation and co-processing experiments with Fenton were carried out.Moreover,the degradation law of shale gas fracturing flowback fluid and the degradation path of typical pollutants were speculated.(1)Structural design and optimization of micro-nano bubbles cavitation reactor.Based on variable pitch spiral blades,a variable pitch spiral cavitator was designed and a model of micro-nano bubbles cavitation reactor was established.Subsequently,the numerical simulation for the internal flow field of the variable pitch screw cavitation device was conducted using Fluent software.Through analyzing the internal variable pitch spiral cavitation velocity distribution,turbulence dissipation rate distribution and the axial static pressure distribution law,the optimized structure of variable pitch screw cavitation was constructed,i.e.the center cone angle α=1.0°,leaf number N=4,variable pitch spiral coefficient m=0.5.(2)Performance evaluation of micro-nano bubbles cavitation reactor.With bubble size,dissolved oxygen content and fluorescence intensity as performance parameters,the bubble size,number and morphology of micro-nano bubbles cavitation reactor were analyzed by automatic grain-shape analyzer,nano particle size and Zeta potential analyzer and ultra-depth of field 3D microscopy system.The results show that the bubble size mainly distributes in 339～509 nm and 1937～2562 nm,and the average size of micro-nano bubble is 976 nm.The dissolved oxygen content in water reached 9.5mg/L.In addition,the fluorescence intensity was used to reflect the free radical production indirectly,and the highest fluorescence intensity is 31725 a.u..(3)Synergistic experiment of cavitation oxidation and Fenton oxidation of shale gas fracturing flowback fluid.The results of water quality analysis show that the organic components in shale gas fracturing flowback fluid include polyacrylamide,long-chain alkane,benzene series and esters,etc.In the cavitation oxidation test with micro-nano bubbles reactor,the COD removal rate is 29.87%under the conditions of 80 mL/min inlet gas,0.2 MPa inlet pressure,35 ℃ system temperature and 30 min running time.Whereas,the COD removal rate reaches 71%in the collaborative Fenton oxidation test.The degradation effect is good for the esters and benzene series,but it is poor for the long-chain alkane in shale gas fracturing flowback fluid.The paper includes 60 pictures,34 tables and 121 references.