Improvement of porous polyvinylidene fluoride-co-hexafluropropylene hollow fiber membranes for sweeping gas membrane distillation of ethylene glycol solution

Porous polyvinylidene fluoride-co-hexafluropropylene(PVDF-HFP)hollow fiber membranes were fabricated through a wet spinning process.In order to improve the membrane structure,composition of the polymer solution was adjusted by studying ternary phase diagrams of polymer/solvent/non-solvent.The prepared membranes were used for sweeping gas membrane distillation(SGMD)of 20 wt% ethylene glycol(EG)aqueous solution.The membranes were characterized by different tests such as N2 permeation,overall porosity,critical water entry pressure(CEPw),water contact angle and collapsing pressure.From FESEM examination,addition of 3 wt% glycerol in the PVDF-HFP solution,produced membranes with smaller finger-likes cavities,higher surface porosity and smaller pore sizes.Increasing the polymer concentration up to 21 wt% resulted in a dense spongy structure which could significantly reduce the N2 permeance.The membrane prepared by 3 wt% glycerol and 17 wt% polymer demonstrated an improved structure with mean pore size of 18 nm and a high surface porosity of 872 m^−1.CEPw of 350 kPa and overall porosity of 84% were also obtained for the improved membrane.Collapsing pressure of the membranes relatively improved by increasing the polymer concentration.From the SGMDtest,the developed membrane represented a maximumpermeate flux of 28 kg·m^−2·h^−1 which is almost 19% higher than the flux of plain membrane.During 120 h of a long-termSGMD operation,a gradual flux reduction of 30% was noticed.In addition,EG rejection reduced from 100% to around 99.5% during 120 h of the operation.

CO_2-triggered gelation for mobility control and channeling blocking during CO_2 flooding processes

CO2 flooding is regarded as an important method for enhanced oil recovery （EOR） and greenhouse gas control. However, the heterogeneity prevalently dis- tributed in reservoirs inhibits the performance of this technology. The sweep efficiency can be significantly reduced especially in the presence of ＂thief zones＂. Hence, gas channeling blocking and mobility control are important technical issues for the success of CO2 injection. Normally, crosslinked gels have the potential to block gas channels, but the gelation time control poses challenges to this method. In this study, a new method for selectively blocking CO2 channeling is proposed, which is based on a type of CO2-sensitive gel system （modified polyacry- lamide-methenamine-resorcinol gel system） to form gel in situ. A CO2-sensitive gel system is when gelation or solidification will be triggered by CO2 in the reservoir to block gas channels. The CO2-sensitivity of the gel system was demonstrated in parallel bottle tests of gel in N2 and CO2 atmospheres. Sand pack flow experiments were con- ducted to investigate the shutoff capacity of the gel system under different conditions. The injectivity of the gel system was studied via viscosity measurements. The results indi- cate that this gel system was sensitive to CO2 and had good performance of channeling blocking in porous media. Advantageous viscosity-temperature characteristics were achieved in this work. The effectiveness for EOR in heterogeneous formations based on this gel system was demonstrated using displacement tests conducted in double sand packs. The experimental results can provide guideli- nes for the deployment of theCO2-sensitive gel system for field applications.