Sugar-lignin bio-refineries using renewable lignocellulosic carbon as an input material could be used in the future to produce a variety of value added products including fuels and specialty chemicals. The bio-refinery aims to replace a proportion of goods currently produced using fossil fuels. Lignocellulosic material has a significant sugar potential in the form of cellulose and hemicellulose and this can be accessed using enzymatic hydrolysis. The lignocellulosic feedstock used in this research was paper pulp derived from municipal solid waste (MSW) and the aim of the work was to maximise the efficiency of producing a concentrated sugar solution from the cellulose (or glucan) component of MSW using commercial enzyme preparations. Analysis of the pulp by acid hydrolysis showed a ratio of 56: 12: 27: 5 of Glucan: Hemicellulosic sugar chains other than glucan: Lignin & pseudo lignin: Ash on total solids (TS). The hydrolysis behaviour of this pulp was similar to that of other lignocellulosic substrates even though the matrix of this material is perhaps more complex. Glucan conversion could be increased by 6% if the pulp was extracted with acetone to remove solvent soluble compounds. Using the additive PEG 6000 increased conversion by 15 % over 48 hours, and allowed a 40 % reduction in the enzyme requirement. PEG also increased the centrifugal dewaterability of the substrate by up to 13%. These results were obtained in single stage batch experiments. It was found, however, that both the glucose concentration in solution and the overall glucan conversion in the substrate could be improved by using a two-stage hydrolysis strategy. Using 50 mg enzyme g-1 pulp at high total solids content >18.5% TS singlestage enzyme hydrolysis gave a maximum glucan conversion of 68%. It was found that two-stage hydrolysis could give higher conversion if sugar inhibition was removed by an intermediate fermentation step between hydrolysis stages. This, however, was not as effective as direct removal of the sugar products, including xylose, by washing of the residual pulp at pH 5. This improved the water availability and allowed reactivation of the pulp-bound enzymes. Inhibition of enzyme activity could further be alleviated by replenishment of β-glucosidase which was shown to be removed during the wash step. The two-stage hydrolysis process developed could give an overall glucan conversion of 88%, with an average glucose concentration of 7.5 wt% in 4 days after combining the hydrolysates of the first and second stage of hydrolysis. The residual washwater from the two-stage hydrolysis with intermediate wash step process contained a dilute amount of sugar. It was found that this washwater could be used as dilution water for a new batch of hydrolysis without any detriment to conversion efficiency. Thus, to further the work above a washwater recycle strategy was applied to the two-stage hydrolysis process. Washwater at various pHs and with or without the addition of PEG 6000 was used as dilution water for a subsequent round of hydrolysis, where up to 6 rounds of 48-hour hydrolysis were completed to reach a steady stage configuration. In these strategies the enzyme dose was reduced to 30 mg C-Tec3 g-1 pulp. Use of a pH 5 or pH 9 wash resulted in an increase in conversion of up to 5% in the first-stage hydrolysis rounds, indicating that enzyme carryover was occurring. The sugar augmentation and enzyme carryover consistently resulted in glucose yields above 7.0 wt% in the first stage hydrolysate when using this lower enzyme dose. The best result achieved in this strategy was obtained when using 0.25 wt% PEG 6000 in the reaction medium and washwater. By reducing the amount of liquid in the second-stage of hydrolysis, it was found that an overall average glucan conversion of 81% could be achieved over the two hydrolysis stages with an average glucose concentration of over 8 wt% in a 4 or 5 day reaction period. This result is significant, as it meets the downstream processing requirements for bioethanol, a major bio-refinery product, and does this with a low enzyme loading. Furthermore, the waste discharge is minimised due to the high glucan conversion.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:655353 |
| Date | January 2014 |
| Creators | Puri, Dhivya Jyoti |
| Contributors | Banks, Charles |
| Publisher | University of Southampton |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://eprints.soton.ac.uk/376672/ |
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