Ruggiero, Giacomo
(2021)
Optimized Biofuels production by syngas fermentation.
[Tesi di dottorato]
| Item Type: |
Tesi di dottorato
|
| Resource language: |
English |
| Title: |
Optimized Biofuels production by syngas fermentation |
| Creators: |
| Creators | Email |
|---|
| Ruggiero, Giacomo | giacomo.ruggiero@unina.it |
|
| Date: |
2021 |
| Institution: |
Università degli Studi di Napoli Federico II |
| Department: |
Biologia |
| Dottorato: |
Biotecnologie |
| Ciclo di dottorato: |
33 |
| Coordinatore del Corso di dottorato: |
| nome | email |
|---|
| Moracci, Marco | marco.moracci@unina.it |
|
| Tutor: |
| nome | email |
|---|
| Marzocchella, Antonio | UNSPECIFIED | | Salatino, Piero | UNSPECIFIED |
|
| Date: |
2021 |
| Keywords: |
Syngas, biofuels, clostridium carboxidivorans, syngas fermentation, modelling |
| Settori scientifico-disciplinari del MIUR: |
Area 03 - Scienze chimiche > CHIM/11 - Chimica e biotecnologia delle fermentazioni |
[error in script]
[error in script]
| Date Deposited: |
22 Jul 2021 16:13 |
| Last Modified: |
07 Jun 2023 11:16 |
| URI: |
http://www.fedoa.unina.it/id/eprint/14070 |
Collection description
The study carried out during the present Ph.D. program aimed at investigating the fermentation of syngas by Clostridium carboxidivorans to produce biofuels, with special attention on butanol. C. carboxidivorans is an acetogenic bacteria able to fix C-1 gases into biomass and to produce extracellular metabolites of interest. The work was carried out at the "Dipartimento di Ingegneria Chimica, dei Materiale e della Produzione Industriale" of the Università degli Studi di Napoli 'Federico II'. The activities were focused on the study of the kinetic characterization of the selected strain, using as a carbon source a gaseous stream of sole CO, the investigation of the experimental conditions that promote the highest production of cells and metabolites, and the strategies to develop an effective reactor configuration, that allows to optimize the gas exploiting and increase the concentration of butanol and ethanol in the culture medium. The experimental work was coupled with the development of a mathematical model that describes faithfully the behavior of C. carboxidivorans in a fermentation process. The model soundness was also assessed. The proposed model was used to simulate a particular reactor system, with biofilm growth on non-porous carrier particles, and to investigate the performance of the bioreactor.
The experimental activity required an accurate design and set-up of the laboratory to operate with syngas. Indeed, syngas is a toxic and inflammable gas. The safe operation for operators and the environment requires to adopt controlled feeding system, sensors to detect any leakage, devices to shut-down the feeding and fermenter system.
The fermentation of C. carboxidivorans was carried out in serum bottles of 280 mL volume. The tests were performed under "batch" conditions, without any supplement during the experiments. The bottles, after the medium load, were sealed with rubber stops and aluminium crimps and boiled to guarantee sterile conditions. The gas was loaded by mean of a needle and a 2-way valve, up to the desired pressure. Tests were carried out at different initial pressure. As the pressure increased the substrate amount increased and the mass transfer rate increased as a consequence of the large driving force. Indeed, the CO concentration in the liquid phase under equilibrium conditions increases with the CO partial pressure, according to Henry's law. It was assessed, by absorption tests, that the time required to reach equilibrium gas/liquid conditions was much lower than the characteristic fermentation time.
The partial pressure of CO, PCO, ranged between 0.5 and 2.5 bar, to assess its effect on the growth. The process was characterized in terms of CO uptake, estimated by the pressure drop and the gas composition, biomass growth and metabolites production. The decrease of pH from the initial value (5.75) was observed and it was typically associated with acid production.
The test campaign pointed out that the cell growth was substrate inhibited. The best fermentation performance - in terms of solvent production - was measured at initial PCO = 1.7 bar. Under these operating conditions, cells reached a concentration of 0.68 g/L, while 400 mg/L of ethanol and 130 of butanol were produced. Batch fermentation didn't show any reassimilation of acids, even at low pH (around 4.5). As this reaction it is not energetically favourable, the depletion of substrate prohibited this mechanism.
Fermentation tests were carried out in a mini-bioreactor, 250 mL volume, equipped with device to feed CO continuously, to monitor the pH in the time and the redox potential. The reactor was kept at constant temperature (35 °C) and under constant agitation by rotating impellers (250 rpm), to favour the mass transfer between gas and liquid phase. A first test campaign was carried out at pH set to 5.75. A second campaign was carried out to observe the natural acidification of the medium, and try to push the system to solventogenesis by setting the pH to 5, turning on the control when the medium naturally reached that value.
The continuous system was characterized by fermentation performance - in terms of solvent production - better than those assessed for batch tests. Indeed, the final concentration of ethanol and butanol were double than that measured during the batch tests, 800 mg/L of ethanol and 327 mg/L of butanol. Moreover, during the experiment at variable pH, 784 mg/L of hexanol were detected, after 140 hours of fermentation. The maximum cell concentration didn't vary significantly for bottle and gas-fed fermentation. The pH decrease seemed to affect acid production, allowing the cells to reach a higher concentration. However, a higher number of experiments is required to give more consistency to the results of this section.
The complex metabolism and the growth of C. carboxidivorans was described with three lumped reactions. The main reaction is the cell growth characterized by total inhibition (substrate and products). The second reaction is the conversion of acids in alcohols. The third reaction is the direct production of solvents, not associated to cell growth. A mathematical model was based on mass balances referred to species involved in the fermentation, coupled with the kinetics and mass transfer rate between the gas and liquid phases. Model parameters were assessed by regression of data available in the literature - Fernández-Naveira et al. (2016a) and present investigation included) according to the proposed model. A vector of 31 kinetic/yield parameters to describe the fermentation system were assessed.
The model was to catch dynamics phenomena quite well (overall R2 = 0.88). The soundness of the model was assessed by measuring the effect of a perturbation on the parameters, in terms of final concentration of cells and metabolites. Finally, the effect of some operating conditions (gas/liquid ratio, gas flow rate and agitation speed) of industrial interest was addressed. It was pointed out that a high transport rate for gas to liquid – expressed in terms of kLa - has a negative effect on the production. It was speculated that this is due to substrate inhibition on cell growth acts.
The proposed mathematical model implemented was extended to assess the potential of a biofilm bioreactor. The biofilm can be described as a layer of aggregated cells adhered on the surface of a carrier. Biofilm thickness may change as a consequence of cell growth, attachment of free-cell from the liquid phase, detachment and mechanical stress.
Promoting the formation of biofilm is one of the most effective strategies for process intensification. The study was carried out by developing a model in MATLAB®
7
environment. Simulation were carried out to assess the bioreactor performance under steady state and transient conditions. A comparison between the system with and without biofilm was carried out to assess the contribute of the biofilm. The presence of biofilm yielded an enhancement of butanol concentration of 85%, large CO consumption with an increase of about 20 % of the conversion of the CO stream fed to the reactor. The effect of the liquid flow rate and the detachment rate were also investigated.
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