Preparation process and performance characterization of hollow fiber ceramic membranes
1. Preparation Process and Performance Characterization of Hollow Fiber Ceramic Membranes
Hollow fiber ceramic membranes not only possess the advantages of ceramic materials such as
high thermal and chemical stability and long-term durability, allowing them applicable in harsh
environment with strong acid, strong alkali and high temperature, the high specific area and
bulking density of hollow fibers reduce the space requirement, which is significant to practical
application due to the expensive commercial place. In this paper, usingα-Al2O3, SiO2, TiO2 and
kaolin as main body, a series of hollow fiber ceramic membranes were prepared via phase
inversion method combined with high-temperature sintering process. Thenα-Al2O3 hollow fiber
composite membranes were prepared by coating boehmite sol, silica sol, titanium sol,
a-Al2O3-H2O dispersion, Al powder-H2O dispersion, PA solution and its mixture with the
inorganic particles through different technologies and processes. These membranes were
characterized by means of dope solution viscosity, SEM, XRD, FTIR, TGA, mechanical properties,
water contact angle, porosity, density, pore size and pore size distribution, pure water flux and gas
separation performance. The main results are listed as follows:Firstly, the system withα-Al2O3
particles dispersed in poly(ether sulfone) (PES)/ 1-methyl-2-pyrrolidone (NMP) solution was used
as dope solution, Al2O3 hollow fiber membranes were prepared with the solution through
wet-spinning technology and sintering process. Solid and polymer content directly affected the
dope solution viscosity. Higher solid and polymer content brought higher viscosity and more
obvious characteristics of the pseudoplastic fluid. The existence of Al2O3 particles prevented the
formation of common finger-like pore structure, replaced by a large quantity of irregular
macroporous structure. The polymer layer on membrane surface was densified with PES content
increasing. Polymer chains completed thermal decomposition during sintering process.α-Al2O3
presented no crystal change before and after sintering process with different aimed sintering
temperatures. With Al2O3 content of 55 wt.%, the membrane mean pore size was about 1μm and
the bending strength was 49.5 MPa. There was an optimal value of PES content, viz.8～9 wt.%,
that the microstructure, flexural strength, average pore size and distribution of various parameters
were more prominent. Through the experiment process, applicably lower PES content and higher
solid content would help improve membrane performance.Secondly, the previous preferred Al2O3
hollow fiber membrane (with PES content of 8 wt.%) was used as substrate, coated by boehmite
sol and titanium sol. After sintering process, separation layers were obtained with macro-pore
modification function. The effect of coating times, coating technology and the amount of solvent
on membrane formation process were examined, and the results indicated that particle-type Al2O3
sol possessed better properties, and with more coating times, maximum pore size and mean pore
size decreased and rejection increased gradually. After coated by 4 times, separation layer of about
10μm was formed on the substrate surface. Aluminium was much easier to load on the substrate
after sintering because of its lower melting point. The main difference between leaching cake by
filtration and the adsorption layer of coating solution was in the thickness and surface binding
interaction, which was more obvious for Al2O3 particle dispersion. Membrane formation property
of polymer-TiO2 sol was largely influenced by the amount of solvent addition if DegOH was used
as solvent. When nDegOH:nTBT was at 8.0 or so, the membrane maximum and mean pore size
reached a minimum value after 4 times coating viz.1.1μm and 0.8μm respectively. Higher
sintering temperature helped grain production, but was not conducive to membrane layer
formation. When sol-gel method was used to prepare composite membrane, the sol property,
2. coating methods, drying and sintering environment parameters, heating rate and air atmosphere
during the sintering process were all required to be controlled.Then using polyamic acid (PA),
Al2O3 particles, SiO2 particles, aluminum and silica sol as raw materials, DMAc as the solvent,
five different coating solution were prepared and coated on the substrate surface. After thermal
imidization at 300℃vacuum calcination and then carbonized at 800℃vacuum, polyimide
(PI)-derived carbon membranes were obtained. After diluted by DMAc, the viscosity of PA
solution decreased and surface tension tended to stabilize. Natural drying in air was adopted, with
more easier operation and lower cost than water bath and heating process. After adding inorganic
material, coating solution viscosity reduced to 100 cP or less, which was good for coating layer
absorption. Inorganic components were conducive to improve membrane surface pore structure
and hydrophilicity, restraining gas permeation which still remained at a high flux level (0～600
GPU). The overall selectivity of O2/N2 significantly improved. The surface pore structure formed
by sol-gel method was more uniform than that of direct adding particles.Meanwhile, the reaction
in Al2O3-SiO2 and Al2O3-kaolin system at a high temperature was introduced to the phase
inversion method and the in situ reaction sintering process to prepare SiO2/Kaolin-Al2O3 hollow
fiber membranes containing mullite. By adding different sized particles into the dispersion system,
particle interaction could be controlled, with less aggregation and flocculation and more system
stability and availability. During the sintering process, some amount ofα-Al2O3 was consumed in
the reaction with SiO2/kaolin, and the residual still remained asα-Al2O3. Tridymite type of SiO2
first transformed into cristobalite and then reacted with α -Al2O3 to produce mullite
(3Al2O3·2SiO2). The reaction was in solid-state at 1450℃, and majorly in liquid-solid reaction at
1600℃. The presence of liquid phase greatly enhanced the reaction. Since the effective
component of reaction was limited in kaolin, Al2O3-kaolin system did not change much with the
sintering temperature, consistent with Al2O3 hollow fiber membrane preparation process, but
could also improve membrane morphology and structure. Properties of membranes prepared by
SiO2-Al2O3 system at a lower sintering temperature (1450℃) even could compete with Al2O3
membrane with higher temperature (1600℃). When the mass ratio of Al2O3 and kaolin was 1:1,
the mean pore size of obtained hollow fiber membrane sintered at 1600℃could be reduced to
about 0.5μm. To achieve required membrane performance, the preparation of SiO2/kaolin-Al2O3
hollow fiber membrane containing mullite could not only reduce the manufacturing cost of
membrane materials, but also lead to more savings in energy costs during the sintering process.
But how to maintain the sintering temperature and the reaction ratio of the materials is the
key.Finally, using the same process with the previous work, four different TiO2 hollow fiber gas
permeable membranes was obtained after sintered at 1000℃,1200℃,1450℃and 1600℃.
Different with Al2O3 hollow fiber membranes, as sintering temperature increased, membrane
properties were first improved then deteriorated, caused by lower melting point of TiO2 (1858℃)
than that of Al2O3 (2050℃). Based on the N2 flux curves under different pressures, the gas
permeation mechanism of TiO2 hollow fiber membrane was considered to be Knudsen diffusion.
Rutile phase of TiO2 remained stable before and after sintering at different temperatures and the
phenomenon of thermal expansion of particles was very significant. The addition of aluminum
powder in the membrane preparation process played a significant role in the pore structure
formation, resulted in the increment of membrane porosity and N2 flux. When the sintering
temperature was 1200℃, TiO2 hollow fiber membrane with solid content of 60 wt.% presented
the optimum performance.
