For sustainably produced fuels and chemicals to become viable resources they need to be cost comparable with crude oil based products. Microbial fermentation is a promising alternative route to a variety of sustainable fuels and chemicals; however, due to increased production costs, it is not currently as economical as crude oil extraction and refining. The recovery of the dilute fermentation products from the broth can be incredibly energy intensive due to distillation still being the main separation process utilised. For high boiling organics, such as 2,3 butanediol, purification from the fermentation broth can contribute to over half of the cost of production. Therefore, there is a clear need for lower energy technologies for the separation and purification of fermentation products. This thesis presents an investigation into the application of membrane processes for the lower energy recovery of a number of different fermentation products:
Nanofiltration (NF) and Reverse Osmosis (RO) membranes have been investigated for the purification and concentration of 2,3 butanediol and acetate within the broth of a gas fermentation process, developed and commercialised by LanzaTech, USA. A number of commercial NF and RO membranes were screened for their separation properties of 2,3 butanediol and acetate with BW30 identified as the highest performing membrane with a rejection of 2,3 butanediol and acetate of 96.1 % and 94.6 % respectively at pH 6.5 within the gas fermentation broth. A membrane series was then developed to purify the broth before concentration to limit the reduction in permeability due to fouling. The membrane series was studied at LanzaTech, USA and consisted of a microfiltration membrane for retention of cells, nanofiltration for the retention of salts and macromolecules and NF / RO for the concentration of 2,3 butanediol and acetate within the broth. 2,3 butanediol and acetate were successfully concentrated to > 5 times that of the fermentation broth.
The effect of 2,3 butanediol and acetate on the pervaporative recovery of ethanol from a gas fermentation broth using a MMM of ZIF-8 and PDMS has been investigated. ZIF 8 was shown to increase the uptake of 2,3 butanediol by the MMM. The as-synthesised membranes were also shown to be sensitive to the pH of an acetate solution. Increasing 2,3 butanediol concentration was shown to increase the total flux and decrease the separation factor and has been attributed to the increased uptake. When pervaporation of a fermentation broth was conducted, an increase in total flux with only a small decrease in separation factor was observed. This led to an increased average PSI of 1281 compared to the model solution of 805. This has been attributed to some beneficial effects of components of the fermentation broth.
An investigation into MMMs of two analogous MOFs composed of copper, glutarate and a bipyridine was undertaken. The two MOFs exhibit similar pore environments but different crystal morphologies depending on the solvent used in the synthesis and have been related to the overall performance of these novel MMMs. The Cu MOFs of larger plate-like crystals blocked permeation through the membrane. This has been shown to be due to the non-ideal orientation of the 1-dimensional pores of the Cu-MOF within these membranes. However, the smaller crystals exhibited better performance as inorganic filler and exhibited increased performance with a maximum at a 15 wt% loading for pervaporation of acetone from water. A total flux of 0.061 kg m-2 h 1 and separation factor of 10.3 were observed for a 5 wt% acetone solution at 30 ºC.
Finally a novel method for the determination of the molecular weight cut-off (MWCO) of organic solvent nanofiltration (OSN) membranes utilising polypropylene glycols has been developed. The method overcomes the limitations of previous methods that utilise polystyrene or polyethylene glycols. Advantages include low cost of polypropylene glycol, high resolution of oligomers (58 g mol 1) and being suitable for use within polar, non-polar and polar aprotic solvents.
Overall this thesis presents investigations into a number of different membrane separation processes relevant to the recovery of dilute organics from fermentation broths. Specifically:
1. NF and RO have been identified as suitable for the partial purification and concentration of 2,3 butanediol and acetate within a gas fermentation broth.
2. The effect of 2,3 butanediol and acetate on the pervaporation of ethanol using a ZIF-8-PDMS MMM has been investigated.
3. Two novel MMMs of PDMS and Cu-MOFs have been developed for the removal of acetone from an aqueous feed and the relationship between structure and performance studied.
4. Finally a novel method for characterising the MWCO of OSN membranes has also been developed utilising polypropylene glycols.
|Date of Award||3 Mar 2017|
|Supervisor||Darrell Patterson (Supervisor) & David Leak (Supervisor)|