As of today, reducing the anthropogenic release of greenhouse gases might be considered as humankind’s most pressing challenge. Moving away from fossil fuels to renewable energy technologies is an essential step, but will not suffice. The chemical industry is responsible for a significant 6.3% of all CO2 emissions, thus fossil-free chemicals production is also required. To that end, carbon capture and utilization (CCU) offers a way to reduce emissions in industrial processes, while converting captured CO2 into commodity and specialty chemicals. Under the umbrella of CCU technologies, microbial electrosynthesis (MES) offers a sustainable solution with minimal water usage and the capacity to increase the value of electrical energy produced from renewable sources.

The ability of microorganisms to accept and utilize electrons from an electrode to catalyze the reduction of CO2 is the basis of MES processes. Whilst significant progress on understanding its fundamentals has been achieved, performance improvements have been modest. To reach industrial viability, a major breakthrough is needed. Unraveling multi-scale interactions between microbial, electrochemical and engineering parameters within MES systems will allow for the rational design of scalable bioreactors. Still, it remains widely unknown what is limiting current setups. The work presented in this thesis aims to identify, understand, and tackle major key process parameters, allowing for a step-by-step design approach to develop scalable MES bioreactors.

How microorganisms adapt to changing operational parameters and different reactor environments was investigated in Chapter 2. A general framework for modeling microbial kinetics within MES reactors was developed, and results showed that CO2 availability may be a limiting factor in existing systems. An insufficient mass transfer capability led to partially limited biomass growth under reported operational conditions, either because of a low gas partial pressure or an inefficient gas delivery strategy. The dynamic reactor-scale model also revealed that in biofilm-driven reactors, a continuous operational mode markedly improved microbial growth and potentially led to denser biofilms and higher current densities. Simulations indicated distinct correlations between operational process conditions and critical performance indicators (e.g., productivity), underscoring existing process limitations and paving the way for future system optimization.

A major knowledge gap in MES is that biomass-specific rates such as microbial growth rates had not been experimentally elucidated and were thus unknown to date. In Chapter 3, a method using nitrogen balances and optical density to determine the amount of microorganisms in biofilm and in suspension at any given time was developed. This was necessary to allow further complex computational attempts, since biomass concentration was one of the major unmeasured variables within biofilm-based MES processes. Measured growth rates during the colonization stage ranged from 0.12 to 0.16 days-1, values in accordance with the ones obtained in previous mathematical simulations. Interestingly, results showed that biomass-specific production rates were relatively low (0.37 molC molX-1 day-1) when compared to syngas and chain elongation studies (up to 10 molC molX-1 day-1). Thus, this comparative analysis highlighted that there is room to significantly improve metabolic rates in MES.

After gaining insight on what major factors limit MES performance, a novel directedflow- through bioelectrochemical reactor (DFBR) with a serpentine flow-pattern entirely filled with a 3D carbon-based electrode was developed in Chapter 4. The elimination of free-flowing liquid in the cathode chamber allowed the DFBR design to substantially increase mass transfer as well as carbon and hydrogen utilization efficiencies. Results demonstrated a 3-fold higher volumetric current density (-28 ± 7 mA cm-3cathode) and productivity (43 ± 24 kgC m-3cathode day-1) than previously reported in biofilm-based MES studies. Most notably, volumetric productivities obtained were now comparable to lab-scale syngas fermentation, a technology that has been successfully scaled up to an industrial level. These findings serve as a milestone in developing MES and emphasize key design parameters for efficient bioelectrochemical CO2 reduction. Furthermore, results obtained with the novel DFBR design proved that a knowledge-driven step-by step approach allows for successful MES reactor development.

Collectively, this dissertation shows that it is possible to unravel the main limitations in currently used MES reactors. The subsequent utilization of such knowledge to design scalable reactors able to achieve industrially relevant performance is also demonstrated. Nonetheless, new challenges are sure to arise while further developing MES as a technology. Extensive research, accounting for a multiscale and multidisciplinary approach is therefore a must in order to bring MES to industrial production.
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In 2021, the EU set a goal for climate neutrality by 2050, followed by a 2024 recommendation for a 90% reduction in net greenhouse gas (GHG) emissions by 2040 compared to 1990 levels. According to the 2022 IPCC Sixth Assessment Report, achieving the 1.5°C goal is still possible but requires immediate and significant changes across all sectors.

The chemical industry, responsible for 10% of industrial CO2 emissions in 2022, must accelerate its shift away from fossil fuels. This involves replacing fossil-based fuels and feedstocks with alternatives that have lower environmental impacts, such as using CO2 captured from the air or industrial emissions and renewable energy sources like solar, wind, and geothermal. This shift has led to the development of carbon capture and utilization powered by intermittent renewable electricity (IRE).

However, electrochemical processes struggle with fluctuating electricity supplies. Variability in electricity can lower production rates or damage electrolysers. Managing these fluctuations to maintain steady production is difficult with control systems alone, emphasizing the need for flexible operation in electrochemical plants.

Feasibility studies are essential for understanding the market competitiveness of new technologies, identifying potential technical, investment, and environmental challenges.

Electrochemical processes using renewable electricity to produce chemicals are gaining traction. While industrial-scale water electrolysis has been widely studied, less research has focused on electrosynthesis under intermittent electricity supply. This dissertation explores process designs and conditions for scaling up a novel CO2 electrochemical plant using intermittent renewable electricity. The research addresses three sub-questions:

What does flexibility mean in designing novel chemical processes?
How does intermittency affect the techno-economic and environmental performance of a novel CO2 electrochemical plant?
What factors influence the competitiveness of novel CO2 electrochemical technology?

For the first sub-question, a systematic literature review on flexibility was conducted, resulting in a conceptual framework for defining, designing, and evaluating flexibility in novel chemical processes.

For the second sub-question, a microbial electrosynthesis (MES) plant producing hexanoic acid from CO2 was designed and modeled in Aspen Plus. The plant's volume flexibility was explored by varying throughput rates. The plant was coupled with IRE profiles and buffering units using Python scripts, and optimization was performed to enhance economic potential. The study assessed how intermittency impacted the plant’s performance and carbon footprint.

For the third sub-question, the techno-economic and environmental performances of hexanoic acid from MES were compared to its competitors. Two future value chains were considered: one producing hexanoic acid and another upgrading it to n-alkanes for sustainable aviation fuel (SAF). Competitors included plant-based and fermentative hexanoic acid, and certified SAFs.

Technical findings indicate that improving electrolysers’ productivity and product concentration is crucial. Inflexibility in downstream processing (DSP) technologies can affect production and economic outcomes. Designing flexible DSP technologies and sizing equipment to handle fluctuations is essential. Buffering units can mitigate intermittency impacts, and smaller parallel units can maintain production during low electricity supply.

Economic findings suggest that novel electrochemical plants driven by constant grid electricity might struggle in today’s market due to high CAPEX and electricity costs. Intermittency worsens this issue. Enhancing technology, securing cheaper electricity, and optimizing buffering units can improve economic outcomes. Future market conditions and demand shifts could also impact competitiveness.

Environmental findings show that combining electrolysis with renewable electricity has lower impacts than using grid electricity but might not be cleaner than competitors. DSP-related utilities and chemicals can be environmental bottlenecks, linked to selected DSP technologies and CO2 feedstock's carbon intensity.

This dissertation provides an ex-ante assessment of novel technology, acknowledging limitations such as not considering ramping rates. Future research should explore more flexible DSP technologies, modular units, and interactions between different flexibility types. Upgrading hexanoic acid to n-alkanes is currently unattractive environmentally; alternative products like adipic acid could be considered.

Joint efforts between the industrial sector and power suppliers are necessary for significant defossilisation through electricity-based chemical processes.@en

Presented works describe a novel approach to assess mixing in a stirred vessel using light-based tomography. The study is driven by key research questions: What are the obstacles in three-dimensional dynamic tracer distribution reconstruction? How should the experimental equipment be constructed to obtain the best possible data from three cameras recording the back-lit stirred tank? How can raw images be processed to isolate the ray-dye interaction? And, how can relevant mixing information be obtained from projections and reconstructed volumetric data? The research begins with a short introduction of traditional mixing measurement techniques, establishing the context and relevance of the work. The theoretical background of the study is then presented, including the principles of tomographic reconstruction and the main algorithms used in the process. The methodology involves the use of synthetic data obtained from LES for the framework and baseline creation followed by the acquisition of experimental data. A significant part of the methodology is dedicated to image pre-processing, which incorporates as main steps the inverted grayscale transformation, brightness normalization, background removal using image similarity metrics and object removal with the use of a neural network. The use of the simplistic forward model based on the Lambert-Beer law is described, followed by the implementation of the projection matrix-free Simultaneous Algebraic Reconstruction Technique. The outcomes of both synthetic and experimental data reconstruction are presented and despite the shortcomings of the used experimental setup the 2D and 3D mixing maps were created, supported by the local Coefficient of Variance calculation to gain further insight into the process. The conclusions highlight the potential of light-based tomography for evaluating mixing while acknowledging the need for significant refinement and validation of the methodology. Recommendations include the improvement of imaging, equipment modifications, and reconstruction implementation. ii

Palm oil mill effluent (POME) is a high organic pollution produced during the palm oil mill process, with a brownish color and stingy odor at high temperatures. Given the popularity in palm oil output over the years, the massive amount of POME causes growing concern. The enforcement of wastewater discharge standards and laws, as well as energy recycling of sustainability goals have facilitated the development of POME treatment processes. Several lab-scale studies have looked into the treatment of industrial wastewater using anaerobic membrane bioreactor (AnMBR) which has received considerable research interest due to its demonstrated potential for POME treatment. In this study, the synthetic POME was treated by a lab-scale crossflow anaerobic membrane bioreactor system. This study tested the feasibility of thermophilic PVDF-AnMBR systems for synthetic POME treatment, and meanwhile evaluated the biological and filtration performance of AnMBR treating lipid-rich wastewater at different sludge retention times (SRTs = 60 days, 90 days, and 140 days). AnMBR showed an adequate biological performance during the stabilizing state. The synthetic POME could be treated with over 98% of COD removal efficiency in all operational conditions. Plus, better digestion efficiency could be achieved at higher SRT (140 days). However, this study stresses that even though the membrane ensures biomass retention, the AnMBR process is still dodged by long-chain fatty acid (LCFA) accumulation and inhibition problems, especially at short SRT (60 days). The continuous reduction of biomass concentration during the stabilizing process of SRT at 60 days eventually resulted in the decreased methane production and system instability. Under all operational conditions, sufficient filtration performance and net permeate fluxes between 8 and 11 LMH were achieved. The trans-membrane pressure (TMP) was under 200 mbar throughout operating process. No membrane cleaning was needed. The results showed that better sludge filterability could be achieved at SRT of 90 days. The sludge filterability was compared as per the standard methods, including specific resistance to filtration and capillary suction time, which did not show a linear relationship with SRTs. Meanwhile, the physical-chemical characteristics of the sludge during the operational phases, including TSS concentrations and SMP, have a close correlation with sludge filterability parameters, such as capillary suction time and supernatant filterability.

Chiral amines are valuable compounds for their use as building blocks for pharmaceutical and fine chemical industries. Previously, chiral amines were made with metals as catalysts, but these are unsustainable and difficult to remove from the product. Organocatalysis is a sustainable alternative, but requires chiral ligands, which are costly. Over the last decade, biocatalytic methods have been developed as well. Our goal was to produce chiral amines with our designed bi-enzymatic cascades, containing an ene reductase and amine dehydrogenase. We produced and purified ene reductases and assayed the asymmetric reduction of our proposed substrate scope, consisting of unsaturated ketones and aldehydes. We also investigated the impact of multiple reaction conditions on these performances that are optimal for amine dehydrogenases. Starting with 10 mM of substrate, we obtained concentrations up to 9.7 mM amine. Also, we obtained 3-methylcyclohexylamine with an enantiomeric and diastereomeric excess up to 99%. Therefore, we conclude that this bi-enzymatic cascade is capable of producing chiral amines with both high enantiomeric and diastereomeric excess. Further research to perform cascades on a larger scale is recommended.

5-hydroxymethylfurfural (HMF) is a versatile renewable base chemical, which can be produced by the acid-catalyzed dehydration of fructose. At this time HMF cannot yet compete with oil-based analogues, such as p-xylene, due to the high costs of production. Future incentives to encourage the transition to a bio-based economy may increase the feasibility of investing in a HMF production process, but only if an efficient and cost-effective production method is designed. Previous research showed promising results using a reactive extraction unit. However, the selection of the organic solvent has a large influence on both the overall yield, as well as on the energy requirements in the final recovery of the HMF in a subsequent separation step. In this work, an intrinsically better, but hard to separate solvent, tri-butyl phosphate (TBP), was compared to methyl isobutyl ketone (MIBK), a solvent which is easy to separate from the HMF by distillation. The chosen method of separation for the TBP-HMF reaction mixture is back-extraction, using an anti-solvent to transfer the HMF back to an aqueous phase. By using backextraction, it was possible to lower the energy (steam) requirements of a TBP-based process under the benchmark calculated for a MIBK-based process by 18%. A drawback of the TBP-based process is its increased complexity, leading to higher capital costs. A model was developed to assess the balance between reduced steam demands and increased capital and operational costs. It was found that even if an optimistic scenario for the effectiveness of the anti-solvent was used, the MIBK-based process was superior across nearly all key performance indicators. Most indicative of the relative feasibility is the minimum selling price of HMF (at a production of 20 kton/year), which is €1.85/kg or €1.64/kg, for a TBP- and MIBK-based process respectively. The only area where the TBP-based process performs slightly better is sustainability, with calculated emissions of 1.90 kgCO2eq/kg HMF compared to 1.97 kgCO2eq/kg HMF for a MIBK-based process.