Model-based scale-up of a continuously operated consolidated bioprocess based on a microbial consortium for the production of ethanol

This work demonstrated that consolidated bioprocessing is a promising concept for conversion of lignocellulose to ethanol at industrial scale. CBP offers great cost saving potential, is feasible to be operated continuously and may be scaled up due to extensive knowledge of the process from a chemical engineering point of view.Cost savings of up to 27.5% of the total costs compared to conventional bioethanol production from lignocellulose, as stated by a techno-economic assessment, make continuous CBP the strongest lever to reduce processing costs of lignocellulosic ethanol. A cost sensitivity analysis identified scale and yield as the main cost-pushers from a process point of view, whereas the price level of the plant location has the highest impact on the investment conditions.To prove the feasibility of continuous CBP, and therefore the mentioned cost savings, experiments with a maximum titer of 3.258±0.007 g/L, a productivity of 0.025 g/(Lh) and constant enzyme production over 750h were conducted. Furthermore, the continuous experiments showed that experiments with identical volumetric oxygen transfer rate k_L a, but different oxygen fluxes per membrane area, showed titer differences of ca. 80% (1.83 g/L vs 3.26 g/L) in favor of setups with the large membrane surface. A rigorous process model was developed for continuously operated CBP. 9 species were considered (oxygen, glucose, total concentration Trichoderma reesei, secondary mycelia concentration of T. reesei, enzymes, cellulose, cellobiose, yeast density and ethanol). 8 of these 9 species needed to be modelled spatially resolved to account properly for mass transfer limitations. The fungal biofilm thickness d_f was found to be a critical parameter with an optimum for every membrane configuration. Smaller d_f reduced the fungal biofilm volume and thus, the enzyme production and larger d_f increased the diffusion path length causing shortage in nutrient supply as well as lower enzyme concentrations in the bulk. The enzyme synthesis rate of the secondary mycelia was fitted by reducing it by ca. 40% (0.67 FPU/(mLd) vs 1.152 (FPU/(mL*d) compared to batch models.With the process model as centerpiece, a scale-up framework consisting of the model and a set of non-dimensional parameters was developed to scale CBP from 2.7L laboratory scale to 130L pilot scale. The majority of the non-dimensional parameters could be kept constant during scale-up as requested by the similarity paradigm. Necessary changes were simulated with the model. Again, the fungal biofilm thickness d_f proved to be a relevant parameter for scale-up considerations. In general, the relative loss of biofilm volume is compensated by longer residence times since the residence time has by far the least impact on the process economics.Finally, popular rate-controlled separation techniques were investigated regarding their potential with CBP, since they offer the potential to reduce yeast inhibition, to avoid separation limitations by the azeotrope and they are not bound to the vapor-liquid-equilibrium of highly diluted ethanol-water mixtures. However, most of the mechanisms fail to handle the solids of the fermentation broth. Pervaporation, being the most promising concept for in-situ product removal, would be a cost-saving alternative to distillation for batch operation, but is limited by an unfavorable vapor-liquid equilibrium due to low bulk concentrations during continuous operation.

Optimization of mixed conducting perovskites for the application in dense ceramic oxygen separation membranes, used in partial oxidation of natural gas to synthesis gas

Defne Bayraktar, Stefan Diethelm, Jan Van Herle

The aim in this project was the optimization of LaFeO3-based perovskite materials for oxygen separation membranes (OSM). The work was carried out by the Laboratory for High Performance Ceramics, Empa in collaboration with the Laboratory for Industrial Energy Systems, LENI, EPFL and the Ceramics Laboratory, LC, EPFL. The ceramic material of interest is a mixed conductor for oxygen ions and electrons at high temperature (800°-900°C) and therefore can separate oxygen from nitrogen with 100% selectivity when exposed to an oxygen partial pressure gradient (e.g. air vs. natural gas). In this capacity, such materials are attractive for the application of partial oxidation (POX) of natural gas (NG) to synthesis gas in an important and growing business. The crucial point in this developing technology is the chemical and mechanical stability of the membrane under the operating conditions (high temperature, PO2 gradient). Because of thermal and chemical expansion, large stress can be induced in the material, which can lead to fracturing. Additional destabilizing processes such as kinetic demixing (elemental segregation under influence of the oxygen gradient) or slow phase transformation can lead to decreased performance with time. The overall goal in this Ph.D. thesis was to address these stability problems from a combined experimental and theoretical point of view in order to develop a more detailed understanding and strategy for improving the materials properties. Experimentally, the ceramic membrane materials were synthesized with different B-site substitutions with the aim of decreasing chemical expansion. The focus lied on LaFeO3 materials substituted with Sr on A-site. Detailed characterizations of the most promising compositions were done including structural, transport, and mechanical properties.

2007

Process modeling and integration of fuel ethanol production from lignocellulosic biomass based on double acid hydrolysis

Daniel Favrat, Martin Gassner, François Maréchal, Zoé Périn-Levasseur, Suping Zhang

Lignocellulosic biomass is considered as one of the most promising feedstocks for producing fuel ethanol because of its global availability and environmental benefits of its use. In this paper, the process of lignocellulosic ethanol production was investigated at its present state of development. The experimental data from the East China University of Science and Technology were used to develop a process model and evaluate the performance of the whole process design. For the process simulation, all relevant information about the process streams, physical properties, and mass and energy balances were also considered. Energy integration is investigated to identify the best ways to supply heat to the process, realizing also combined heat and power production from wastewater and residue treatment. The sensitivity on ethanol yield and the overall system performance are also investigated.

2009

Bioethanol production from oil palm empty fruit bunches

Simone Chiesa

Face to the increasing demand for energy, the declining availability of fossil fuels and the periodical tensions in the geopolitical context of oil-producing countries have raised serious concerns in the governments worldwide as well as in the public opinion. Similarly, awareness has been increasing of the environmental harms caused by burning fossil fuels and of the associated adverse effects on human health. A number of different potential sources of renewable energy have been investigated over the last decades, including wind power, solar energy and biomass, among others. In particular, concerning the use of biomass, the production of ethanol as a transportation fuel has been considered a particularly attractive and feasible option. Production of ethanol from feedstocks containing easily fermentable sugars such as corn or sugar cane has been actually implemented, especially in the US and in Brazil, but it has been harshly debated especially on the ground of the induced competition between energy and food uses. Hence, a so-called second generation production has been envisaged, which makes use of lignocellulosic materials, namely biomass residues from different agricultural and industrial processes, which appear particularly interesting due to their abundance and the lack of possible use for human consumption. Unfortunately, the production of ethanol from cellulosic feedstocks still presents technical challenges to be overcome before production at the industrial scale could become profitable. In a number of tropical countries in Africa, Asia and South America, important amounts of agricultural residues are generated during the process of oil extraction from the fruits of the palm tree Elaeis guineensis. Among these residues, the empty bunches that carry the fruitlets are at present severely underused, despite their high content of cellulose (of roughly 30%) and the presence of other saccharides. In the scientific literature, relatively limited attention has been given to this feedstock, when compared to other residues such as, for instance, corn stover. Moreover, different studies can be difficult to compare because of the many dissimilar factors involved, as the diverse geographical origin of the residues, and different techniques and analytical protocols used. This thesis deals with the comparison of possible alternatives for processing empty fruit bunches to produce ethanol. The attention has been focused on the pretreatment step, the first operation performed on the feedstock, which is crucial because of its cost and impact on downstream processing (enzymatic saccharification and fermentation). Thus, three pretreatment techniques generally accepted as among the most promising have been chosen: dilute acid, dilute alkali and liquid hot water treatment. They have been performed on the same feedstock under controlled laboratory conditions. For each pretreatment, experimental parameters (such as temperature, time, pH, etc.) have been varied according to a statistical experiment design. Pretreated samples have been enzymatically hydrolyzed to assess the degree of conversion of cellulose into fermentable glucose. The influence of the conditions employed during pretreatment have been evaluated using surface response analysis, which also enabled to define optimal conditions with respect to the total amount of glucose liberated upon enzymatic saccharification. Rigorous compositional analyses using wet chemistry protocols have been carried out on the samples before and after pretreatment, allowing their complete characterization. The fate of each type of compound of the biomass could thus be followed during the processing including pretreatment and enzymatic saccharification. Additionally, IR spectroscopy has been used to retrieve precious information about the nature of the solid substrate. The liquids produced during pretreatment have been similarly analyzed to detect the presence of solubilized sugars as well as that of a number of degradation products which could be inhibitors for microorganisms used in a successive fermentation stage. Taken together, all these elements allowed to investigate the mode of action of the pretreatment techniques employed and establishing a meaningful comparison among the options when empty fruit bunches are used as a feedstock. An in-depth discussion is presented concerning the effectiveness of each treatment with respect to the disruption of the recalcitrant lignocellulosic structure, the improvements in glucose conversions which are attainable upon pretreatment, and other significant criteria such as the production of inhibitors and the possible use of the liquid streams. All these factors, in fact, may have a strong impact on a possible extension from the laboratory to the pilot-plant or industrial scale. Treatments using either a dilute sulfuric acid solution or only pure water have proved overall more efficient than the dilute alkali treatment in attacking the lignocellulosic matrix and relevant amounts of sugars could be liberated under optimized conditions. Oppositely, the alkali treatment under the conditions explored shows limitations probably attributable to the feedstock considered, especially given its relevant lignin content. Each pretreatment, however, has characteristic strengths and weaknesses that must be considered carefully before adopting a choice, especially in relation with the possible products of interest: an approach including the production of different chemicals along with ethanol could probably be a realistic option for possible future scale-up. One of the major contributions of this thesis is hence to provide a comparative study of different techniques performed under standardized conditions, and to investigate a feedstock of interest for many countries for which however information about its response to pretreatment techniques is still fragmented.