Carbon nanotube/polysulfone nano composite membranes for the filtration of crude oil/water mixture
Date
2015
Authors
Owusu-Ansah, Kwame
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Abstract
Membrane technology is widely used for water treatment purposes in a wide array of industries. This
research focused on the use of this technology in the oil and gas industry. This is primarily because of
its low cost and low energy requirements, its environmental friendliness and its ability to treat large
quantities of waste water. The ability to scale membranes makes them very attractive for use on
offshore platforms where space is limited. This research focused on the use of polymeric membranes
in the treatment of produced water primarily from enhanced oil recovery (EOR) processes. This is
when the energy of the oil or gas reservoir is boosted using chemicals, heat, biological or technical
techniques. Water is a highly used resource in this process and this contributes to the large
quantities of produced water waste from this industry.
Carbon Nanotubes (CNTs) were first produced and characterized. CNTs are widely studied
nano-materials due to their good qualities and vast applications. Numerous techniques have been
developed for the synthesis of this material with chemical vapor deposition method (CVD) being
most common. This involves the growth of CNTs, usually in the presence of a catalyst at a high
temperature. The effect of the catalyst support (zeolite and CaCO3) on the synthesis of CNTs
was first studied. It was observed that CNTs that were grown from zeolite supported catalysts
(CNT-z) had smaller outer diameters ranging from 10nm to 50nm, while CNTs grown from CaCO3
supported catalysts (CNT-c) had larger outer diameter of 20nm to 80nm. This disparity in the
outer wall diameter was still evident after functionalization with HNO3. Another difference between
the two supports was the CNT purity achieved after synthesis. Functionalized CNT-z (fCNT-z )
had more catalytic impurities than functionalized CNT-c (fCNT-z), which was almost clean of any
residual catalyst. The impurities in the fCNT-zs were found to be SiO2 . This was removed by
HF treatment to give very pristine CNT-zs. The CNTs were characterized with Raman, Fourier
transform infrared radiation (FTIR), thermogravimetric/derivate weight loss analysis (TGA/DTG
), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The fCNT-cs
were used in the membrane preparation because of their larger size, which meant better interaction
with the membrane structure.
Two techniques were used in the production of the membranes: the solution casting method
(SC method), which produced non-porous, dense and isotropic membranes, and the phase inversion
method (PI method ), which produced micro-porous anisotropic membranes. The membranes were
modified using fCNTs and a polyester non-woven fabric (NWF). The membranes were characterized
using the SEM, atomic force microscopy (AFM), contact angles, FTIR, TGA/DTG and tensile strength analysis. The surface morphologies clearly revealed the surface pores while the cross
sections showed voids. The pore sizes ranged from 1.598μm for the membranes with the NWF,
to 0.191mm for the CNT imbedded membranes without the NWF. The surface pores were not as
evident in the solution cast membranes. The dense nature of these membranes meant they had
significantly higher tensile strengths and Young’s moduli. Rougher membranes were produced with
the PI method and this confirmed the presence of pores with lower contact angles. TGA/DTG
showed that SC membranes were more thermally stable at higher temperatures than the membranes
produced via PI.
Functionalized CNTs interacted with the hydrophobic membranes to enhance its physical, chemical
and mechanical properties. CNTs reduced the contact angles, increased the surface roughness,
pore and void sizes and the tensile strengths for both kinds of membranes. It however reduced the
thermal stability and rigidity of the membranes. The addition of the NWF support also decreased
the pore sizes of the membranes by PI and introduced voids into the membranes by SC.
The PI membranes were tested for performance using a synthetically made produced water. It
was shown that increasing pressure increased permeate flux. Functionalized CNTs also increased
permeate flux while controlling fouling via pore blockage. The addition of NWF also resulted in
flux decline by providing additional resistance to the flow of permeate through the membrane.
Water and oil permeates were then collected and tested for oil concentrations. The results showed
oil rejections ranging between 78% and 90% with the mixed matrix PI membranes supported on a
polyester NWF having the highest oil rejections.