Experimental Study On Microbubble-Enhanced Microfine Particle Flocculation Mechanism And Treatment Of Shale Gas Fracturing Flowback Fluid

Shale gas fracturing flowback fluid(FFF)is characterized by complex composition,high volume,viscosity,COD content and suspended fine solids,which is harmful to the environment and needs to be discharged or reused after treatment.This thesis is based on the study of microfine particles in shale gas FFF.A multi-fluid gradient-enhanced flocculation reactor was used,and introduce microbubbles(MBs)to improve flocculation performance.From the perspective of bubble-particle properties and floc properties,the mechanism of the action of MBs with microparticles and fine flocs in the flocculation process is analyzed,which provides a theoretical support for the microfine particle flocculation technology.Firstly,the thesis investigated the effect of MBs sizes on flocculation effect.By examining the changes of mean particle size of flocs,fractal dimension and floc strength and turbidity removal rates,it was found that the best flocculation effect was achieved only when the microbubble size was similar to that of micro-particle.Further,the effects of particle surface hydrophobicity,roughness,surfactant(hexadecyl trimethyl ammonium bromide,CTAB)and interfacial bubbles on the adhesion performance of bubble-particle were studied.It was found that the more hydrophobic and the larger roughness,the easier it was to achieve stable adhesion.The appropriate dosage of CTAB and the presence of interfacial bubbles were beneficial to bubble adhesion.Secondly,the effect of microfine particle size on flocculation effect was investigated to reveal the critical scale of anisotropic flocculation for this flocculation reaction and analyzed the flocculation mechanism in each region of the multi-flow gradient flocculation reactor.The effect of process conditions on the flocculation effect was also systematically investigated.The experimental results showed that the flocculation performance showed significant differences at the particle size of 0.20 μm.Therefore,0.20 μm was determined as the critical size for anisotropic flocculation.Meanwhile,it was found that when the particle size was 0.20 μm,which matched with the micro-vortex size in the micro-vortex flocculation reaction zone of the reactor,so as to obtain the best flocculation effect.Finally,the mechanism of microbubble-enhanced microfine particle flocculation was analyzed.The optimized process conditions for the treatment of shale gas FFF in a multi-flow gradient flocculation reactor were obtained through systematic condition tests.By comparing the effect of flocculation on shale gas FFF samples with and without the participation of MBs,it was found that the mechanism of microbubbleenhanced microfine particle flocculation mainly includes:(1)In the presence of MBs,the number of colliding particles in the flocculation process increased,the collision probability increased,the reaction time is shortened and the amount of flocculant is reduced;(2)In the presence of MBs,the apparent particle size of microfine particles increased and the adhesion efficiency increased;(3)MBs could play the role of trapping particles,bridging between flocs and filling flocs,improving floc strength and compactness.The high turbulence intensity in the cyclonic flocculation reaction zone of the multi-flow gradient flocculation reactor enhanced the particle-bubble-flocculant diffusion collision.The small vortex flocculation reaction zone promoted the growth of floc particles into large flocs.The micro-vortex flocculation reaction zone broke the loose flocs and formed dense flocs after secondary flocculation,which improved the overall flocculation effect.There are 63 figures,7 tables and 123 references included in this thesis.