|M.Sc Student||Tankus Karen Adie|
|Subject||Oil-Water Separation using Electrochemmically Modified|
Carbon Nanotube (CNT) Membranes
|Department||Department of Chemical Engineering||Supervisor||Professor Viatcheslav Freger|
|Full Thesis text|
In many industries, mainly the oil and gas extraction industry, oil-water separation is a major economic concern. Popular methods in the industry include skimmers and other coagulation methods, which fail to completely remove oily species in droplet form, creating large amounts of wastewater. Current wastewater treatments are either expensive or not sufficient to meet environmental regulations. One way to increase efficiency and reduce expenses is to develop membrane technology. This technology is very effective, attractive in its simplicity and is very economical.
To allow efficient oil removal, membranes should have an optimal pore size, and be sufficiently hydrophilic and oleophobic. Existing membranes are usually based on polymers, which are inherently hydrophobic, even when containing hydrophilic groups. Novel non-woven carbon nanotube (CNT) sheets have many advantages, such as exceptional mechanical, thermal and chemical durability, and optimal pore size for oil removal. However, native CNTs are hydrophobic, hence not suitable for oil-water separation. This study focuses on developing methods to render CNT sheets hydrophilic in order to convert them into efficient membranes for oily wastewater treatment.
This study takes advantage of the CNT's high conductivity, to convert surface characteristics form hydrophobic to hydrophilic. Three different approaches were explored: (1) generating hydrophilic groups on the surface using Electro-Oxidation (EO), (2) changing the membrane's wetting properties using electrowetting (EW), and (3) coating the membrane with hydrophilic polymers using electro-polymerization (EP).
EO modification was done by exposing membranes in situ to various DC voltages. Surface characterization showed an increase in the degree of oxidation upon increasing applied voltage and/or oxidation time. Filtration experiments conducted in a dead-end cell using oil-water emulsions showed that, even when adding a surfactant, only dissolved oil permeated through the membrane, while oil droplets were completely rejected. It was verified that the separation mechanism was rejection, rather than oil adsorption on the CNTs.
The effect of EW was tested by conducting filtration experiments and simultaneously applying AC voltage between the membrane and solution. No increase water filtration rate or oil rejection was found. Apparently, the relatively low voltage used was unable to produce a significant effect. Applying higher voltages was not feasible, due to feed limitations caused by high levels of salinity that are commonly found in produced water.
To examine EP feasibility, polymer coating was produced using electrochemically initiated grafting of PEG-methacrylate. This drastically increased water permeability, however, it was found that polymer leaching prevented proper membrane performance evaluation.
In summary, EO showed the most promising results of all methods, while EW was limited by solution conductivity and did not improve water permeability or oil rejection and EP indeed increased water permeability, but leaching-out of the coated polymer was a problem, which did not allow properly measuring oil rejection. Overall, the used methods show great potential in modifying CNT sheets to efficient filters for oil removal.