Abstract: In this work, carbon molecular sieve (CMS) membranes were prepared based on a polymer of intrinsic microporosity, named PIM-6FDA-OH. The goal of this work was to examine the effect of the fabrication parameters of the CMS membranes on the gas separation performance of the final CMS membranes produced. Furthermore, the performance changes are reported for membranes physically aged over 7, 30, 60, and 90 days. The membranes prepared consisted of thin-film (about 3 !m thick) CMS selective layers supported by a stainless-steel tube. The experiments were split into four projects.
The first project aimed to determine the effect the layer thickness had on the final performance of the produced CMS membranes. Five pairs of membranes were prepared using different coating solution concentrations, and different number of layers. The concentrations used were 5 (1 layer), 7.5 (1 layer), and 9 wt% (1, 2, and 3 layers) polymer in THF. The membranes had the same soak time of 15 minutes and pyrolysis temperature of 650 °C. The results showed that the increase in number of layers did not provide any benefits and was unnecessary. Moreover, the decrease in concentration produced membranes with higher permeances but with a greater loss in selectivity. Therefore, the 9 wt% concentration solution with one layer was chosen for the remaining experiments. The second project examined the effect of the pyrolysis temperature on the performance of the final membranes produced. All membranes were made with the 9 wt% solution and the soak time was held constant at 15 minutes. The soak temperatures tested were: 700, 750, 850, and 950. °C. The membranes pyrolyzed at temperatures above 650 °C were severely defective. This suggests that either the precursor polymer could not form defect-free thin membranes using high soak temperatures, or another potential reason is related to interfacial defect formation between the CMS layer and the porous stainless-steel support. Further experiments are required to fully understand the soak temperature effect on the formation of thin CMS films on porous supports. The third project examined the effect of the soak time (i.e. time the membranes are held isothermally at the pyrolysis temperature) on the final performance of the membranes. The same 9 wt% solution was used, and the pyrolysis temperature was 650 °C. The pyrolysis soak times were 15 minutes, 1 hour, 3 hours, and 10 hours, respectively. The results showed that as the soak time increased the membranes became denser and provided higher selectivities and lower permeances. Furthermore, the membranes with longer soak times became more size-sieving earlier during physical aging than the membranes made with shorter soak times. Physical aging was accelerated with an increase in soak time, i.e., membranes made by soaking over 10 hours reached stable permeance over time starting at day 7. The fourth project aimed to investigate the preparation process, as well as to test the performance of the membranes under different environments. Two types of polyimide precursor membranes were made, one set with the pristine polyimide and the other one with a PDMS top coating. The results showed that the membranes with PDMS had similar selectivities but far slower permeances than the CMS membranes, the membranes made without PDMS coating had much lower selectivities and permeances. CMS membranes soaked for 15 minutes and 3 hours, respectively, were tested to check the permeances of all the five gases (H2, O2, N2, CH4, and CO2) under pressure cycles from 2 to 8 bar. The membranes passed the tests and their permeances were not affected by exposing them to high pressures and back, except for the membranes soaked for 3 hours when tested with CO2.
|Date of Award||Jun 2021|
|Original language||English (US)|
|Supervisor||Ingo Pinnau (Supervisor)|
- Carbon Molecular Sieves
- Gas separation
- Polymers of Instrinsic Microporosity