Publicação
Integration of cellulases recycling on 2nd generation bioethanol production from recycled paper sludge
| Resumo: | Finding low-cost cellulosic materials that can provide appropriate amounts of sugars is one of the present challenges for cellulosic ethanol production. The valorization of residues from different sources represent in this context an attractive option. One of these residues is recycled paper sludge (RPS), which is generated in high amounts on the paper recycling process, being usually disposed at landfills. Another critical point concerning the economy of 2G bioethanol is the cost of enzymes. Despite the important cost-reduction achieved lately, enzymes are still very expensive. The recovery and reutilization of enzymes is one of the most promising strategies for a reduction on enzyme cost. The general aim of this thesis is thus to provide relevant insights on the feasibility to integrate enzyme recycling in the process of bioethanol production from RPS. Despite several studies on enzyme binding to cellulosic materials, no such study exists for RPS. Thus, the first aim of this thesis was to evaluate the hydrolytic performance of cellulases and their adsorption on RPS, since this is very important concerning the definition of a strategy for enzyme recycling. Cellulases efficiently convert RPS, no visible toxic effects being detected. The hydrolysate was also easily fermented by yeast cells, no additional nutrients supplement being required. At the end of the process, a large fraction of Cel7A activity was found soluble on the liquid, the solid-bound fraction being efficiently recovered through alkaline elution. Four rounds of hydrolysis and fermentation were successively conducted, both fractions of the enzymes being recovered after each round, using only 30 % of the original enzyme load used in the first stage. This strategy enabled steady levels of enzyme activity while also allowing important levels of solid conversion. Targeting the economy of the process, high solid loadings are required for higher ethanol titers to be achieved. Additionally, different enzymes can present distinct performance and binding affinities towards RPS. On a second part of this thesis we aimed to investigate the performance of different enzyme cocktails and process conditions and their impact on the feasibility of enzyme recycling under intensified conditions. Distinct cocktails were assessed for thermostability, hydrolysis performance and activity partition between phases of the solid-liquid system. Celluclast showed an inferior thermostability, nevertheless, its performance at moderate temperatures was slightly superior to other cocktails (ACCELLERASE®1500 and Cellic®CTec2). Also the enzyme distribution in the solid-liquid medium was more favorable in the case of Celluclast, enabling the recovery of 88 % of the final activity. Using Celluclast, a Central Composite Design was designed to study the influence of solids and enzyme dosage on RPS conversion. Solids loading showed a significant effect on glucose production, no major limitations being found for a concentration under 22 % of solids. Furthermore, an increase on enzyme loading from 20 to 30 FPU/gcellulose showed no significant additional effect on sugars production, thus 22 % solids and 20 FPU/gcellulose were identified as the best operational conditions towards an intensified process. Applying these conditions, a system of multiple rounds of hydrolysis with enzyme recycling was analyzed. Steady levels of activity from one round to another were obtained with only 50 % of fresh enzyme being added at each cycle, enabling interesting levels of solid conversion (70-81 %) in the subsequent rounds. Finally, an economic study was conducted to analyze the viability of RPS conversion into ethanol, under the intensified conditions and enzyme recycling. Overall, this process was found to be economically viable even though the moderate levels of final ethanol critically affected production costs. On a scenario of enzyme recycling, despite the increase on production costs due to the recycling operations (0.15 Million US$/year), a reduced enzyme consumption and a superior ethanol production enabled a better economic output. The exclusive recycling of the liquid fraction allowed lower production costs; however, total ethanol production decreased leading to an inferior economic output. A sensitivity analysis has further suggested that enzyme cost may represent a critical factor on the economic viability of enzyme recycling, with reductions on its cost above a level of 33 % resulting on a scenario where is economically unattractive. Summarizing, this work elucidates the important role of the enzyme cocktail and its interaction with the cellulosic material on enzymes recyclability, thus highlighting the high specificity of the presented results. Overall, the technical and economic feasibility of enzyme recycling in the process of bioethanol production from RPS was demonstrated. |
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| Autores principais: | Gomes, Daniel Gonçalves |
| Assunto: | Engenharia e Tecnologia::Biotecnologia Industrial |
| Ano: | 2018 |
| País: | Portugal |
| Tipo de documento: | tese de doutoramento |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade do Minho |
| Idioma: | inglês |
| Origem: | RepositóriUM - Universidade do Minho |
| Resumo: | Finding low-cost cellulosic materials that can provide appropriate amounts of sugars is one of the present challenges for cellulosic ethanol production. The valorization of residues from different sources represent in this context an attractive option. One of these residues is recycled paper sludge (RPS), which is generated in high amounts on the paper recycling process, being usually disposed at landfills. Another critical point concerning the economy of 2G bioethanol is the cost of enzymes. Despite the important cost-reduction achieved lately, enzymes are still very expensive. The recovery and reutilization of enzymes is one of the most promising strategies for a reduction on enzyme cost. The general aim of this thesis is thus to provide relevant insights on the feasibility to integrate enzyme recycling in the process of bioethanol production from RPS. Despite several studies on enzyme binding to cellulosic materials, no such study exists for RPS. Thus, the first aim of this thesis was to evaluate the hydrolytic performance of cellulases and their adsorption on RPS, since this is very important concerning the definition of a strategy for enzyme recycling. Cellulases efficiently convert RPS, no visible toxic effects being detected. The hydrolysate was also easily fermented by yeast cells, no additional nutrients supplement being required. At the end of the process, a large fraction of Cel7A activity was found soluble on the liquid, the solid-bound fraction being efficiently recovered through alkaline elution. Four rounds of hydrolysis and fermentation were successively conducted, both fractions of the enzymes being recovered after each round, using only 30 % of the original enzyme load used in the first stage. This strategy enabled steady levels of enzyme activity while also allowing important levels of solid conversion. Targeting the economy of the process, high solid loadings are required for higher ethanol titers to be achieved. Additionally, different enzymes can present distinct performance and binding affinities towards RPS. On a second part of this thesis we aimed to investigate the performance of different enzyme cocktails and process conditions and their impact on the feasibility of enzyme recycling under intensified conditions. Distinct cocktails were assessed for thermostability, hydrolysis performance and activity partition between phases of the solid-liquid system. Celluclast showed an inferior thermostability, nevertheless, its performance at moderate temperatures was slightly superior to other cocktails (ACCELLERASE®1500 and Cellic®CTec2). Also the enzyme distribution in the solid-liquid medium was more favorable in the case of Celluclast, enabling the recovery of 88 % of the final activity. Using Celluclast, a Central Composite Design was designed to study the influence of solids and enzyme dosage on RPS conversion. Solids loading showed a significant effect on glucose production, no major limitations being found for a concentration under 22 % of solids. Furthermore, an increase on enzyme loading from 20 to 30 FPU/gcellulose showed no significant additional effect on sugars production, thus 22 % solids and 20 FPU/gcellulose were identified as the best operational conditions towards an intensified process. Applying these conditions, a system of multiple rounds of hydrolysis with enzyme recycling was analyzed. Steady levels of activity from one round to another were obtained with only 50 % of fresh enzyme being added at each cycle, enabling interesting levels of solid conversion (70-81 %) in the subsequent rounds. Finally, an economic study was conducted to analyze the viability of RPS conversion into ethanol, under the intensified conditions and enzyme recycling. Overall, this process was found to be economically viable even though the moderate levels of final ethanol critically affected production costs. On a scenario of enzyme recycling, despite the increase on production costs due to the recycling operations (0.15 Million US$/year), a reduced enzyme consumption and a superior ethanol production enabled a better economic output. The exclusive recycling of the liquid fraction allowed lower production costs; however, total ethanol production decreased leading to an inferior economic output. A sensitivity analysis has further suggested that enzyme cost may represent a critical factor on the economic viability of enzyme recycling, with reductions on its cost above a level of 33 % resulting on a scenario where is economically unattractive. Summarizing, this work elucidates the important role of the enzyme cocktail and its interaction with the cellulosic material on enzymes recyclability, thus highlighting the high specificity of the presented results. Overall, the technical and economic feasibility of enzyme recycling in the process of bioethanol production from RPS was demonstrated. |
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