Rising concentration of CO2 in the atmosphere has become a significant concern, paving the way for worldwide research on mitigation techniques like carbon capture and storage (CCS) and carbon capture and utilization (CCU). Zero Emission Fuel (ZEF), an aspiring startup based in the Netherlands, aims to develop a micro plant that produces methanol from just solar energy and air. Their process involves capturing CO2 and H2O directly from the air, splitting H2O to obtain H2 and producing methanol by reacting CO2 with H2. The focus of this research is on the absorption process of ZEF's direct air capture unit. Instead of capturing CO2 and H2O through a widely used batch direct air capture process, ZEF chooses to side with a novel continuous absorption and desorption process involving a flow of bulk polyamines without any conventional support structures. Polyethyleneimine (PEI-MW-600) and Tetraethylenepentamine (TEPA) are used as absorbents to achieve a target of capturing 825 grams of CO2 in 8 hours every day from a single direct air capture unit. Preliminary investigations show that the viscosity of PEI-600 is significantly higher than that of TEPA. An increase in CO2 concentration increases the viscosity of both polyamines significantly. In comparison, increasing H2O concentration leads to a maximum viscosity point in both polyamines and subsequent decrease in viscosity with a further increase in H2O concentration. Although, compared to CO2, H2O has a smaller effect on viscosity. Moreover, premixed samples of PEI-600 and TEPA with H2O show an increase in the CO2 absorption rate when pure CO2 is bubbled through the samples. In order to study and characterize the novel absorption process, an experimental setup facilitating a flow of absorbent is developed, and experiments are conducted following an experimental approach to calculate the mass transfer rates and average concentrations of CO2 and H2O. Fourier-transform infrared spectroscopy is used to estimate these concentrations. Experiments are performed on different initial concentrations of PEI and TEPA. TEPA is found to have a better absorption performance with an average CO2 absorption rate that is two times higher than PEI-600. Contrary to preliminary investigations done by bubbling CO2 into polyamines, absorbents with premixed water flowing down a channel were found to have lower CO2 absorption rates than pure polyamines. A multiple regression viscosity model, heat model, and a model of absorption column is developed using the data collected to estimate average viscosity, the heat of absorption of water, and characteristics of the absorption process. Diffusion of CO2 through the absorbent layers is found to be the primary limiting factor, while mass flow rates of absorbent and air are also found to influence the absorption process. Finally, a design of the absorption column is made to meet ZEF's requirement taking into account various performance characteristics studied during the thesis.

The dwindling of natural resources, mainly fossil fuels, and the alarming increase in the global average temperatures, are the biggest concerns of the engineering and scientific community of 21st century. Studies have found that known reserves of oil and natural gas will last another 50 years. Coal will last another 110 years, which points towards dire consequences. The experience will not be that of a car slowly running out of gas, but rather like driving off a cliff, since we innately depend on these resources. The realization of these concerns has lead to a rise in the investments all over the globe for Sustainability. The right questions are now being asked. How can we fulfill our demands without compromising the demands of our future generations? One of the keen areas of interest is the conversion of power into fuel. Synthesis of hydrocarbons from sequestrated carbon can produce carbon-neutral fuels that can replace some of the conventional fossil fuels. Methanol has been actively wiping the floor with the research community due to its simplicity of production and less toxic nature. While there are quite a lot of ways of producing methanol, very few of them are carbon neutral. Zero Emission Fuels B.V. has embarked on a venture to produce methanol by a totally carbon neutral process. They begin by capturing carbon dioxide and water from air and splitting the water into hydrogen and oxygen using solar power. Carbon dioxide and hydrogen are then combined in a reactor to form methanol, making a truly carbon-neutral process. They aim to make fuel consumption, a completely circular process while keeping it economic. An experimental characterization of ZEF’s methanol reactor under varying pressures and H2/CO2compositions was done. The new reactor design focuses on heating the feed gases before catalyst bed, more heat integration, reliable sensor data, and the ability to mix gases in desired compositions. Methanol yield, quality, energy efficiency, reactor inlet and outlet temperatures, power requirements were experimentally determined in a transient analysis. It was seen that the reactor produces 4.84 mmol/gcat /hr of methanol at 50 Bar and reactor wall temperature of 250◦C with H2 : CO2 = 3:1 mol% feed gas. The catalyst exhibited partial deactivation during night after shutdown. Contrary to expectation, the reactor produced more methanol at lower pressure. The production was reported at 5.96 mmol/gcat /hr at 35 Bar. Although, only 4.36 mmol/gcat /hr at 25 Bar. Moreover, the pressure reduction caused the natural circulation to slow down, and increase the inlet temperature from 208◦C at 50 Bar to 234◦C at 35 Bar, which is the reason for the observed increase in yield. It can be concluded that the reactor yield under these conditions is limited by kinetics and not thermodynamics. The reduced mass flow rate allowed the reactor to consume less power and show higher energy efficiency. It increased from 34.7% at 50 Bar to 43.2% at 35 Bar. The rate of pressure decline in the reactor was correlated with the methanol yield using gas law by accounting for non-ideality with the compressibility factor. The predicted yield was seen to be in good agreement with experimentally determined. The methanol yield of the reactor with variable feed gas composition was obtained for 50 Bar and 250◦C reactor wall temperature and seen to decrease on either side of the ideal composition. It was concluded that the yield becomes both stoichiometrically and kinetically limited. It is recommended that similar experiments be done at increasing temperatures and equilibrium yield be determined experimentally. Additionally, the reactor needs to be operated with varying compositions and higher reactor wall temperatures at constant pressure, to obtain the operating line for maximum production of reactor.

CO2 concentration in the atmosphere is increasing leading to global warming and thus, climate change. Moreover, current renewable technologies such as wind or solar energy only cover the electricity market. A big part of the global energy market is still based on fossil fuels and difficult to electrify. Hence, there is an urgent need to produce renewable liquid hydrocarbons to replace fossil fuels. Zero Emission Fuels (ZEF) is a start-up that aims to develop a small-scale chemical plant to convert carbon dioxide (CO2) and water (H2O) from the air into methanol (MeOH) using photovoltaic energy. In this work, the absorption of CO2 using bulk polyamines was characterized experimentally, as well as through numerical modelling. In addition to this, guidelines for the design of the absorber, part of the Direct Air Capture (DAC) unit of ZEF’s micro-plant, are given. The focus of the study is placed on the kinetics and loading of CO2. To be able to model the absorption process an experimental approach has been developed regarding the uptake of CO2 and H2O. Firstly, pure H2O absorption experiments were performed in a climate chamber in order to obtain the H2O loadings using polyamines for different H2O content in the air. Secondly, air capture experiments were performed at lab conditions in order to obtain both H2O and CO2 loadings for different process conditions. Thirdly, constants relevant for the modelling part (reaction rate constant and Henry constant) were obtained experimentally. An absorption model, developed in MATLAB, was used to estimate the diffusion coefficient of both H2O and CO2 using the experimental H2O and CO2 loadings. Two main conclusions are obtained from the absorption model of CO2: the absorption of CO2 on polyamines is diffusion-limited and the diffusion coefficient of CO2 is two orders of magnitude lower than the diffusion coefficient of H2O. Based on the fact that the CO2 absorption process is diffusion-limited the mixing patterns should be introduced in the design of the continuous absorber. However, due to the fact that viscosity could influence the flowing behaviour of polyamines, more research is needed.

In the effort against climate change the company ZEF has as its target to make methanol (CH3OH) out of atmospheric CO2; a process which can be seen as power-to-fuel and/or power-to-x. In light of the developmental cycle within ZEF a new reactor with an increased size and enhanced mass manufacturability is required. The reactor contains three main components: reactor bed, condenser and heat exchanger. The heat exchanger design has been shown to be highly efficient as well as the underlying design philosophy [1]. However, an exact in depth evaluation of the condenser performance has not been performed yet. Furthermore, in light of reactor optimisation, an investigation on the reactor bed design for the use case of ZEF is seen as necessary. The condenser plays a dual role in the reactor: it acts as the separation mechanism of products from the recycle stream and as vital factor in generating mass flow. Due to the current modelling architecture/philosophy of ZEF computationally simple descriptions of condensation phenomena are desired. A method based on linear relations for the latent heat release is developed. In combination with a PT-flash plug-in for MatLab this allows for modelling of heat effects and liquid-gas separation due to condensation. According to literature local heat transfer of condensation is significantly hampered com- pared to ideal Nusselt Film Condensation theory. The reason for this degradation is the presence of Non-Condensable Gasses which will limit the heat flux of the condensing species to the condenser surface. A sub model is used to evaluate whether this effect is significant. A combined model and experimental approach is used for evaluation of these models. The reactor bed is the generation site of the methanol and in previous work it has been found there might be limiting effects in this bed [1]. Evaluation of literature on the causes indicates that mass transfer and temperature limitations are likely the cause. Furthermore literature suggests that the reactor bed might be able to attain a higher Space Time Yield. A set of models are made to describe the mass and heat transport of the reactor bed. These are based on 1-D heat transfer correlations, a linearized Thiele modulus, and the Bussche & Froment kinetic model. Furthermore, the reactor bed and condensation models are integrated into an existing overall model. This enables the simulation of synergy between these processes. A new reactor bed design is made based on these models which should increase the Space Time Yield, and is subject to experimental validation. For the experimental validation a new reactor was designed and build based upon the new models developed. Characterisation experiments indicate satisfactory qualitative behaviour of the condensation modelling. Quantitatively deviations are observed which are expected to be due the over prediction of methanol formation by the reactor bed models. The reactor bed model deviations are mainly attributed to a lower than predicted mass flow rate, and adverse flow fields in the reactor bed. A new reactor bed design is proposed which should significantly reduce the adverse flow field effects while increasing thermal performance. The reactor bed design used allows for a decrease in catalyst size without causing a significant decrease in mass flow rate. An increase of a factor 1.25 for the Space Time Yield compared to the previous design has been observed during experiments. Insulation performance is satisfactory with the insulation performing within 20 W as modelled. Thermal efficiency has decreased by a factor 1.8 and is attributed to the under performance of the reactor bed. Furthermore, the control of the reactor has been evaluated in terms of mass flow rate measurements, the prevention of the stalling of flow, and control. A new mass flow rate measurements device based on differential pressure, a new feed injection design, and further development of control have been experimentally validated. Designs for each of these subjects have been found to be satisfactory. Furthermore, it was found that reactor bed geometry also has an effect on the control of the reactor.   

Zero Emission Fuels B.V (ZEF) is currently in the process of designing and prototyping a sustainable methanol micro-plant, which captures water vapor and carbon dioxide from the atmosphere and uses those components to eventually produce methanol. In order to produce methanol, hydrogen is required which is produced from the captured water by electrolysis in a zero gap design, high pressure alkaline water electrolyser. The pressure in the electrolysis setup is regulated at a desired operating pressure of 50 bar by a pressure control system, designed by ZEF. Insight is required in how the flow of electrolyte and bubbles behaves in the electrolysis setup and the electrolyser cells, in order to implement design or operation condition changes to allow for an efficient operation in the eventual micro-plant. This study focuses on the experimental characterization of the flow and bubble phenomena in the ZEF electrolyser design, during operation at a desired pressure of 50 bar, with the use of visualization by video footage inside the electrolyser cells. Camera equipment used for the visualization consists of a microscope camera and a GoPro camera. An ultrasound Doppler velocimetry device (UDV) was used to measure the electrolyte flow velocity and direction in different parts of the electrolysis setup, in order to substantiate findings made by the camera equipment. Experiments were conducted with an electrolysis setup based on the current electrolyser design made by ZEF, with the addition of a transparent add-on to allow for visualization of the flow and bubbles inside an electrolyser cell on the oxygen side. First the operation of the electrolyser was characterized by means of IV curves and temperatures, followed by visualization experiments for varying operating pressures and current densities at different parts of the electrolyser cell. One of the main observations made during the visualization experiments is the periodic occurrence of stagnation of electrolyte and gas flow out of the electrolyser. Stagnation of the flow occurs inside the electrolyser cells on the oxygen side, at the top channels leading the flow out of the electrolyser cells, to eventually a flash tank. Different multiphase flow regimes were observed during active flow through the top channels, which vary based on the operating current density, pressure and temperature. Bubble behavior was observed at different positions on the electrode in the electrolyser cell and possible enhancement of gas crossover was detected in the electrolysis setup and investigated. The effect of the pressure control system on the operation of the electrolyser was also investigated and showed to affect the electrolyte flow direction in the electrolysis system. Models were made to validate and explain the findings from the visualization video footage and characterization of the electrolyser. A previous made model that characterizes the flow in the electrolysis system is used for comparison. Due to the lack of a temperature regulation system, a finned tube heat exchanger was modelled to be implemented in the experimental setup as for the eventual electrolysis setup. Based on the findings from the experiments, recommendations are proposed regarding the experimental and eventual design, and for further development of the high pressure alkaline water electrolyser and the electrolysis system.

Zero Emission Fuels B.V. (ZEF) is developing an integrated micro-plant to produce methanol from captured CO2 and green Hydrogen produced via alkaline water electrolysis. The electrolysis unit represents one of the largest financial and energetic costs in the system and is required to operate at 166 mA/cm2 under 2 V. In order to decrease the costs, ZEF is looking to synthesize highly efficient nickel-based electrodes to avoid using expensive noble-metal ones. The objective of this work was the synthesis of efficient and stable nickel-based electrodes. Two electrodes were synthesized through electrodeposition (Raney nickel and NiFe) and one was synthesized through hydrothermal treatment (NiFe-LDH) and were compared to a RuO2-containing (Permascand) electrode and to a smooth nickel electrode. On/off cycles for 2.5 hours were carried out to measure the stability of the electrodes. Additionally, performance tests in a pressure range of 1-5 bar were carried out to measure the effect of increasing pressure. The experiments were carried out in an in-house designed and built zero-gap alkaline electrolysis cell. Raney nickel, with a measured roughness factor of 150 was the best performing of the synthesized electrodes. After the stability tests, it was able to produce 166 mA/cm2 at 1.9 V (1.73 and 2.12 V for Permascand and smooth nickel, respectively). Raney nickel as anode material presented significant degradation. Raney nickel is the most promising material for HER, none of the synthesized materials presented significant stability for OER. From the nickel-iron electrodes, NiFe with a roughness factor of 19.5 presented the best performance of the synthesized materials with 2.04 V for the mentioned current density. NiFe presented significant degradation, especially as anode material. NiFe presented a relative high performance considering its low electrochemical active surface area attributed to the presence of highly efficient active sites. NiFe-LDH was quickly degraded during the tests as cathode and anode material. The pressure tests showed an inverse relationship between voltage and pressure. This hints that a decrease in bubble size with pressure is the cause behind the decrease in voltage. A simple model, based on experimental data and thermodynamic considerations, estimated that operating the cell at 50 bar reduces the voltage by 0.25 V compared to operation at 1 bar. The estimated reduction in voltage at a pressure of 50 bar would allow to operate the electrolysis cell under 2 V even with smooth nickel mesh as electrode.

With the rising renewable energy demand, sustainable energy storage systems become more important. Zero Emission Fuels (ZEF) is developing a solar powered methanol micro-plant that produces methanol, using water and carbon dioxide, out of the air. ZEF’s focus this far was on developing the subsystems for the micro-plant. This report is about the integration design of the different subsystems. The main integration challenge was to design a functional micro-plant concept that produces methanol at a low cost. The state-of-the-art subsystem designs were researched, and a base-case cost estimation was conducted. Next, in a scaling research it appeared optimal to implement one micro-plant per three solar panels, instead of one plant per panel. A functional configuration architecture was designed and conceptual mass producible subsystem designs were developed. These designs were used for building a scale 1:1 integration prototype and for conducting a cost analysis of the concept. The microplant concept would produce methanol at a significantly lower cost compared to the base-case. Consequently, the micro-plant reached the target cost. Design guidelines and recommendations are given for further optimising the micro-plant's integration design and costs.

Zero Emission Fuels (ZEF B.V.) is a start-up working to build a sustainable methanol micro-plant. Carbon dioxide and water are obtained from atmosphere. Hydrogen is obtained from captured water through alkaline electrolysis and the hydrogen is used for methanol production. The alkaline electrolyser should run at 50 bar and 90°C with 30% KOH as the electrolyte. It is a multi-cell design and the electricity supplied to the electrolyser leaks within the system through the electrolyte. This is the energy supplied to the system not being used for splitting water into hydrogen and oxygen. Leaking current and flow scale with the length and the radius of any channel in the system and contradict each other in terms of requirements. This study focuses on characterising the electrolyser to validate a modified version of the existing Matlab model at a channel in the system for two different dimensions of the channel. The Matlab model predicts the flow and the leaking currents in the system. Estimating the flow in the system is essential to validate the model and using the model, the leaking currents are estimated. Experiments were performed on a single cell version of the electrolyser at atmospheric pressure to obtain the flow and thermal characteristics. Comsol simulations were ran on the single cell system to support the Matlab results. The heating curve of the system was obtained at four different points in the system to check for the necessity of insulation. The system went up to 56°C at the hottest point in the system after 2 hours. This confirmed for the necessity of insulation. The diameter of the bubbles at the electrode were measured and compared to the estimated diameter in Matlab. They were 34% off. The flow part of the experiments were done at a lower current density to simulate 50 bar flow at 1 bar. The flow rate of the electrolyte was estimated using high speed cameras and tracking fluorescent particles at one of the channels in the system. The values of the flow rate from Comsol, Matlab and Comsol were 25% away from each other whereas Matlab and experiments were 65% away from each other. The mass flow rate predicted by Comsol was in between Matlab and experiments. The absence of a temperature network to estimate temperature at every part of the system, the approximate geometry and ignoring smaller resistances to flow in the Matlab model are responsible for the offset. Leaking currents were estimated in the single and multi-cell system using Matlab. The power lost due to leaking currents were higher in the multi-cell system as compared to the single cell system. The reduction in leaking currents by using gas slugs was estimated and the current design achieves the electrical efficiency of 99%. Recommendations were to make the channel diameter larger and remove the part of the channel that is in the cell. Further modifications to the Matlab and Comsol model were suggested to improve the prediction. Experiments related to multi-cell setup have to be done to estimate the actual electrical efficiency of the system and validate the Matlab model.

With the influx of the industrial revolution of the past centuries, the global energy demand has grown proportional to the global economical growth. Consequently, an enormous rise in greenhouse gas emissions has been observed, where the major contribution to global warming comes from the rising concentration of CO2. Therefore, the concept of capturing CO2 directly from the air (DAC) has gained world wide attention as it can reduce the carbon footprint of men kind. Zero Emission Fuels (ZEF), an inspiring start-up based in Delft, aims to develop a micro plant that utilizes the DAC concept to produce methanol as a fuel, using only the sun as its energy source. CO2 and H2O is captured directly from the atmosphere by the DAC unit which operates continuously with the help of an amine sorbent that flows through an absorption and stripping column. Previous research shows pure tetraethylenepentamine (TEPA) having great potential as a CO2 capturing sorbent. However, it has some limitations regarding slow absorption and desorption kinetics due to a high sorbent viscosity, that prevent ZEF from reaching their goal of capturing 825 grams of CO2 per DAC unit in eight hours of operation. The focus of this research is on the sorbent selection process of ZEF’s DAC system. Research has shown that adding a diluent to TEPA could potentially improve the performance of the ZEF DAC unit. Therefore, a total of nine diluents, DEG, PEG-200, PEG-400, PEG-600, selexol-250, selexol-500, glycerol, 1,4-butanediol, sulfolane, mixed in different ratio’s with TEPA, have been put to the test through the developed framework of sorbent selection. In the framework of sorbent selection all the sorbents are tested through four different experiments, keeping efficiency and costs in consideration. The Airfarm experiments are used to evaluate absorption capacity and viscosity of the sorbent at specific ambient conditions. The Vapor-Liquid-Equilibrium (VLE) experiments are utilized to evaluate the desorption characteristics of the sorbent from where the regeneration energy demand can be calculated with the help of a mathematical model. The re-pumping experiments are used to evaluate the absorption kinetics of the sorbent and lastly, the degradation experiments are used to evaluate the sorbent life time performance. After the experimental procedure the sorbents can be judged on the key performance indicators regarding the ZEF DAC unit operated in a specific climate. It was concluded through the literature research and proved by an extensive experimental research that selexol, glycerol and sulfolane did not improve the characteristics of the sorbent. However, adding DEG, PEG or 1,4-butanediol to TEPA did have a profound positive effect on the overall performance of the DAC unit. Since the diluents reduce the sorbent viscosity, and therefore, increase the absorption and desorption kinetics. Based on the experimental results and the design specifications set by ZEF, PEG-200 proved the most promising out of all diluents in the mixing ratio TEPA:PEG-200 2:5. This sorbent has been tested for two different climates; the dry Sahara climate and the more humid Mediterranean climate to see how the sorbent would have to be changed for different climates. It was concluded, for a more humid climate, it requires less diluent for the sorbent to stay within the 2 푃푎 ⋅ 푠 viscosity limit. The experimental results were included in a full DAC model to design the DAC unit utilizing the optimized sorbent for the two different climates considering the design specifications set by ZEF. Following a sensitivity analysis an optimized conceptual DAC design was obtained for both climates. Where it was concluded that the optimized sorbent resulted in reduction in energy requirement for regeneration of the sorbent, where 1554 푘푊ℎ/푡표푛 of CO2 is needed for the Sahara climate and 1636 푘푊ℎ/푡표푛 of CO2 for the Mediterranean climate. Finally, a cost analysis has been performed regarding the operational and capital costs of the newly designed conceptual DAC units for both climates.

Increasing renewable electricity production calls for innovative methods to store electricity as well as a desire for electrification of processes that currently rely on fossil fuels. Zero Emission Fuels is a company that is developing a system to convert carbon dioxide and water from the air into methanol, a liquid hydrocarbon fuel, using photovoltaic energy. The scale of the system is fit for a single solar PV panel. The desired methanol output is 25 grams per hour. In this work, a new design for the methanol synthesis reactor of the system is developed, built and experimentally characterized. Knowledge from work by Basarkar and Gutierrez on a previous prototype is used as the starting point. The focus of the new design is placed on the heat integration network, the natural circulation effects, and tilting of the reactor. The heat exchanger makes use of heat pipes to transfer heat. Natural circulation is increased by increasing the channel dimensions of the system. This led to an increase in mass flow rate of over 3000%, making the mass flow rate the limiting factor for reactor performance. The mass flow rate is roughly 800% higher than assumed in the design phase; the resulting energy flows are too high for the heat exchanger to work effectively. Nevertheless, the heat exchanger heating duty relative to total heating is 230% higher than Basarkar. Tilting the reactor 20 degrees reduced the mass flow rate by 46% to 0.41 g/s, improving almost all aspects of reactor performance. Productivity increased by 58% to 15.7 g/h; 182% higher than Basarkar, though the space time yield is lower (4.1 vs 6.8 mmol/gcat/h). The energy efficiency of the system is close to Basarkar at 36.5% (vs. 37.5%). It is clear from experimental correlations that reducing mass flow rate will increase productivity, energy efficiency and heat exchanger performance. By simulating the requirements for an autothermal reactor it is found that the catalyst bed dimensions should be increased in terms of diameter and length to increase to residence time in the catalyst bed. The results agree with the experimental conclusion that the mass flow rate should be reduced. Furthermore, it is recommended to heat the fluid by convection instead of conduction to ensure a more homogeneous temperature profile in the catalyst bed. Also, the heat exchanger should be expanded by adding more heat pipes and increasing the heat transfer surface area.

Zero Emission Fuels B.V. (ZEF) is a start-up company developing a fully autonomous methanol synthesis micro-plant that will be energy driven by solar panels. The process implements an alkaline water electrolyzer, which supplies the methanol synthesis reactor with hydrogen. The electrolysis system includes a small-scale stack of cells and is designed to operate at 90 oC and the high pressure of 50 bar, with a 30% potassium hydroxide electrolyte. The system is also self-pressurized through the continuous accumulation of the electrolysis gases in the closed flash separation vessels. To control the process, the company has designed a novel system that aims to maintain the gas pressure at 50 bar and the liquid electrolyte level inside the flash separators at a fixed point.  The present work has two main objectives. The first is to characterize the transient dynamic response of the company's current experimental electrolysis setup under the effect of the operating conditions and control. The second objective is to predict the level of gas crossover that is induced during the system's operation and evaluate the risk of explosive mixtures formation. Two different models were developed to fulfill the research targets. The first model is based on a 1-d transient hydraulic network analysis. Simulations were conducted using Simulink for a current density range of 500-5000 A/m2. The model predicts the electrolyte flow and pressure response in the various elements and locations of the network respectively, indicating also the oscillatory behavior induced by the operation of the valves. A key finding is the high differential pressure between the stack anodes-cathodes that is caused when the valves open, posing a danger for the integrity of the separators between the half-cells. The second model was developed in MATLAB and uses the predicted flow response of the first model to estimate the crossovers by solving numerically the unsteady 1-d advection-diffusion equation in the corresponding elements. The effect of the supersaturated electrode boundary layer was also considered as an enhancing parameter of the mass transport through the separators of the cells and a sensitivity analysis was conducted. The main finding is that this parameter has an important effect, especially on the hydrogen crossover inducing 1% higher hydrogen impurity when it is increased by a factor of 10. The crossovers of both hydrogen and oxygen are found to gradually increase and reach an almost stable value under the effect of the control system. A comparison of the model with preliminary experimental data indicates a supersaturation intensity between 5 and 15 times higher than the solubility of the components. In this range, the system will safely operate above 2000 A/m2 without the formation of explosive mixtures. 

The present-day industrialized nations reached high standards of living using cheap fossil fuel energy. The high CO2 emissions as a result of burning these fuels over the years have started outpacing the natural carbon cycle, resulting in climate changes around the globe. We have reached a point in our history where merely reducing our carbon emissions would not solve the problem, rather carbon has to be captured from the atmosphere and either stored or converted to fuels. Converting the captured CO2 into methanol has been gaining traction in recent years as it is not only an excellent fuel but also serves as the building block to manufacture other important chemicals like dimethyl ether (DME), paraffin, olefins, plastics and polymers. This thesis focusses on the complete experimental characterization of a small scale, energy efficient methanol synthesis reactor modelled on the concept developed by Wim Brilman of the University of Twente, with respect to feed flow rates, methanol production and overall efficiency. The problem of comparative energy inefficiency of the Brilman reactor was solved by carrying out the reactions in a novel, natural circulation loop (NCL) fixed packed bed reactor with internal heat recovery using Cu/ZnO/Al2O3 as the catalyst. A mixture of H2 and CO2 in the molar ratio of 3:1-the optimum ratio for methanol production was fed into the reactor. A sensitivity analysis was carried out with regards to the sampling time of the liquids at the outlet and the reaction temperature. Maximum methanol productivity of 6.8 mmole (millimoles) CH3OH/gcat/h was obtained at 228 °C reactor wall temperature and 62 °C condenser wall temperature using 5 mm diameter pellets compared to 4.3 mmole CH3OH/gcat/h obtained by Brilman at 210 °C reaction temperature and 85 °C condenser temperature. Also, a high carbon conversion of 99.2% and methanol selectivity of 99.0% was achieved. The energy demand (in MJ/kgCH3OH) was reduced from 75 in the Brilman reactor to 24 using the current design. From these results, it was established that methanol could be synthesized using a small, lab-scale reactor in an energy efficient manner. It was further observed that reduced size of the catalyst did not contribute much to methanol yield due to the high-pressure drop. Finally, a brief analysis of the heat losses in the system led to the conclusion that an additional 14.6 W of heat would have enabled autothermal operation.