Micro fluidised bed technology for the screening of carbon capture adsorbents(Chemical Engineering)

Abstract

To accomplish global Net Zero emissions goals, a diverse portfolio of technologies, regulations, frameworks, and changes in behaviour will be required. Within this landscape, carbon capture is still relevant in reducing or entirely offsetting greenhouse gas emissions, particularly emissions that cannot be avoided in the immediate or long-term. Adsorption-based processes using solid sorbents are appealing due to their high flexibility, non-volatility and low energy regeneration burden. Ultimately, the success of adsorption-based processes depends on the development of the materials (rapid kinetics, high capacities, high stabilities, high selectivities, etc.) and gas-solid contacting technology (good mixing, low-pressure drop, etc.). This thesis proposes that 3D-printed Micro Fluidised Bed Reactor (MFBR) technology can meet these requirements due to cost-effectiveness, high-throughput capability, minimal energy requirement, efficient heat/mass transfer characteristics, and enhanced safety measures. In the early phases of material development, only small volumes of samples are typically synthesised; it would be inefficient to mass-produce potentially poor-performing materials before their complete characterisation. Thus, the MFBR's primary function is to facilitate data collection of new materials under relevant operating conditions, thereby enabling informed decision-making. Accordingly, this thesis aims to develop the MFBR as a platform for low-cost and rapid screening of novel CO2 adsorbents. To demonstrate this approach, 3D-printed micro fluidised beds are used to screen the performance of a commercially available hydrotalcite product (PURAL MG70, Sasol). In its raw as-supplied state, this powdered hydrotalcite has significant cohesive characteristics that prevent fluidisation. Accordingly, detailed hydrodynamic experiments were first performed in order to find feasible MFBR designs and operating conditions for these Geldart C powders by studying the pressure drop characteristics. The hydrodynamic studies demonstrate that the fluidisation quality was significantly enhanced by employing a straightforward removal of fines through pre-sieving, specifically retaining particles larger than 53 μm (density of 2 g/cm3 ), followed by pre-fluidisation. This improved quality included removing a significant hysteresis between increasing and decreasing the gas velocity, minimising the amplitude of the pressure drop overshoot prior to fluidisation, and ensuring that the whole bed was fluidised without gas bypassing (slugging). Furthermore, addition of a secondary inert Geldart A type particle (silica, with a mean particle size of 93 ± 10 μm and density of 2.65 g/cm3 ) to the hydrotalcite powder ii resulted in similar improved fluidisation quality. These treatments were valid in three different 3D-printed MFBRs (bed diameters of 𝐷௧ = 10–15 mm) at all bed heights tested (𝐻௦/𝐷௧ = 1–3). Following this, the adsorption process was studied using CO2 breakthrough experiments, validated against independent TGA measurements. These breakthrough tests were conducted for a 10 mm bed diameter at various bed heights (𝐻௦/𝐷௧ = 2–3), CO2 concentration (8–16 vol%), superficial gas velocities (1.5–6 𝑈௠௙) and operating temperatures (25–60 °C). The results indicate that the measured CO2 adsorption capacity increases as the gas velocity increases in the bubbling regime before decreasing again in the slugging regime. A maximum capacity of 0.76 mmol/g was measured when operating at 4𝑈௠௙, 16 vol%, and 40 °C. The capacity declined at lower velocities (<3 𝑈௠௙) because of inadequate gas-solid mixing and declined at higher velocities (>4 𝑈௠௙) because of gas bypassing due to slugging. The maximum capacities observed agreed with independent TGA measurements at all conditions. This agreement defined the operating window for studying the adsorption kinetics (corresponding to operating under kinetically limited conditions). At 16 vol% CO2 concentration, the desirable kinetically limited velocity range was 3 – 4 𝑈௠௙. At a lower CO2 concentration of 8 vol%, the process was mainly diffusion-limited, which reduced the width of the operating window; only 4𝑈௠௙ achieved the kinetically limited state. This highlights the importance of including hydrodynamic screening in the workflow of materials development using the MFBR platform. Finally, the desorption kinetics were studied through the implementation of temperature swing adsorption, where adsorption was performed at 40 °C (which gave the highest capacities) and desorption was performed between 40 °C and 90 °C. At desorption temperatures of 40 °C, expectedly, a low 7% CO2 recovery was observed. At 90 °C, the CO2 recovery increased to 33% of the adsorbed CO2. Raising the desorption temperature changes the thermodynamic equilibrium, destabilising the affinity of the CO2 to the hydrotalcite by overcoming the activation energy of weak Van der Waals forces or chemical interactions with the surface. The maximum ‘desorbed capacity’ was measured to be 0.24 mmol/g at 90 °C. Based on the results obtained in this thesis, it can be concluded that 3D-printed micro fluidised beds can be used for the development of carbon capture sorbents, offering insights for decision-making and design.