Performance Degradation Of PVDF Membrane And Improvement Of Membrane Preparation During CO₂ Absorption Process

Greenhouse effect is one of the most important environmental problems in the world,and there is no doubt that the continuous emission of greenhouse gases is the major reason for global warming.The reduction of carbon dioxide emission is an important objective at the present stage since CO₂ is the main greenhouse gases.Conventional decarbonization methods all have their shortcomings.At the same time,membrane absorption technology is a novel gas separating technology that combines membrane separation technology with traditional gas absorption technology.In the membrane absorption process,the membrane material plays an important role so that the membrane degradation problem has become the main problem of membrane absorption applications.In this thesis,the performance changes of polyvinylidene fluoride?PVDF?membrane during the process of membrane absorption of CO₂ have been investigated,and the PVDF membrane modified improves the service life of the membrane.In order to study the degradation of commercial PVDF membrane during membrane absorption,the membrane has been immersed in NaOH,ammonia and MDEA for 300 hours.The results show that after immersed in the alkali solution,the surfacehydrophobic performance of PVDF membrane declines,permeability decrease,the membrane pores are wetted,and the mass transfer resistance increases.NaOH has the most obvious damage on membrane,followed by ammonium hydroxide and MDEA.With the increasing of the alkali solution concentration,the change in the performance of PVDF membrane is more obvious.In order to modify the PVDF membrane,the same experiment was carried out on the homemade PVDF membrane.The results show that after immersed in the alkali solution,the surface hydrophobic performance of PVDF membrane declines,permeability decrease,and the mass transfer resistance increases.NaOH has the most obvious damage on membrane,followed by ammonium hydroxide.With the increasing of the alkali solution concentration,the change in the performance of PVDF membrane is more obvious.Aiming at the degradation of PVDF membrane under alkali solution,the PVDF membrane was modified.?1-butyl-3-methylimidazolium hexafluorophosphate?was added to the conventional casting solution,and membrane modified by the ionic liquid was prepared using immersion precipitation.It was found that the addition of ionic liquid improves thesurface hydrophobicity and structural properties of PVDF membrane.The surface properties of PVDF membranes are the best when the concentration of ionic liquid is 3%.The least loss of ionic liquid in modified membrane in the least in water at 4?.The surface membrane was prepared by immersion precipitation method and solvent evaporation method.A layer of nano-CaCO₃ coating solution was coated on the base membrane,and membrane was etched with hydrochloric acid solution after dried.PVDF membrane modified by nano-CaCO₃ surface etching was obtained,which was analyzed by infrared spectroscopy.The characteristic absorption peak of nano-CaCO₃ was found.Thermal stability experiments show that the maximum temperature PVDF membrane modified can withstand is 440 ?.The membrane has the best surface properties,structural properties and alkali resistance when the immersion time is 15 h.The performance of nano-CaCO₃ surface-modified membrane is the worst when the immersion time is 5h.The membrane has the best surface properties,structural properties and alkali resistance when the etching time is 90 min.The membrane performance is the worst when the etching time is 30 min.Compared with the membrane prepared by solvent evaporation method,the membrane prepared by immersion precipitation method is more suitable for the membrane absorption process.