Production and characterisation of poly(vinyl chloride) (PVC) hollow fibre membranes for gas separation applications

  • Alexandra Costa

Student thesis: Doctoral Thesis

Abstract

The present work was focused in the development of polyvinylchloride (PVC) hollow fibre membranes suitable for gas separation applications using the dry/wet phase inversion technique.The first part of this work involved the preparation of quaternary polymer solutions containing polyvinylchloride (PVC), dimethylacetamide (DMAc), tetrahydrofuran(THF) and ethanol (EtOH) to study their phase inversion properties.Different solutions, with different compositions, were prepared and a visual evaluation was made in order to understand the properties of the ternary phase diagram for this system and the possible location of the binodal boundary that separates the one phase homogeneous region from the two phase region (nonhomogeneous).For all the solutions prepared, there was one composition that is suggested to be inside the non-homogeneous region, thus indication the potential position of the binodal curve in the region studied.During the spinning process, a polymer solution is subjected to shear stresses and deformation; therefore, the rheology of the quaternary PVC solutions was studied in detail. Oscillatory, creep and recovery and flow experiments were performed to fully characterize these solutions. Temperature and composition showed to have a big influence in the viscoelastic properties of the solutions investigated. The preparation of the solution used to spin the hollow fibre membranes followed a different procedure due to a larger quantity of solution needed and a different "ageing" time. Because of that, a rheological evaluation of this solution was also carried out and the effect of procedural differences on the viscoelastic properties of the spinning solution was analysed.After the rheological studies, different spinning conditions were established to better understand the impact of the solutions' rheology features on the final performance of the PVC hollow fibre membranes. Spinning dope temperature, dope extrusion rate and external bath temperature were the parameters studied. The design of the experiments was done using the Taguchi method and a set of nine experiments with different experimental conditions were performed. Gas permeation results obtained for the different membranes showed poor selectivities, even after the two coating cycles. The analysis of variance (ANOVA) showed that the external bath temperature was the parameter that most contributed to the variability of the results.Scanning electron microscopy surface images revealed defects in the membranes surface (tearing). However, the Knudsen selectivity obtained for the uncoated membranes and the significant decline in the gas permeation through the membrane after coating cycles suggests that the fractures are not the main reason for the poor selectivities obtained.Mass transfer and resistance models available in the literature were used to predict the active layers thickness and the surface porosity of the membranes. Mass transfer model predicts thick active layers if high temperatures are used and is in accordance with the values obtained by the resistance modelling that also predict the existence of thick active layers with high surface porosity. The high surface porosity obtained is suggested to be due to the incomplete coalescence of the polymer nodules during the formation of the active layer in the dry gap and ispointed out as the possible reason for the poor selectivities.
Date of Award1 Nov 2017
LanguageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorVitor Magueijo (Supervisor) & (Supervisor)

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