Dynamic analysis and characterization of a desorption column for a continuous air capture process

Abstract

The world is currently in the middle of an energy crisis, with a growing demand for energy playing catch up with an increasing population. The problem stems from the fact that we rely heavily on fossil fuels to meet our energy needs, and the combustion of these fuels are the primary source of greenhouse gas emissions. An accelerated rate of emissions of greenhouse gases has led and continues to lead to an increase in the average temperature of the planet, stated: Global Warming.
Shifting the balance in our favour requires arresting and lowering our emissions. Direct air capture of Carbon dioxide is one solution that has garnered massive traction in the global scientific community. Zero Emission Fuels is a visionary start-up operating out of Delft that aims to build a micro-plant capable of producing methanol using energy derived from the sun and raw material (CO2and H2O)harnessed from the atmosphere. The heart of their concept is liquid amine-based direct air capture. ZEF has pioneered the continuous process that involves the simultaneous absorption and desorption of CO2and water as the amine circulates from the absorber to the desorption column. The raw material and the energy to drive the process is harnessed from the environment. Thus these inputs remain outside the control of the ZEF system and are treated as external disturbances. This research aims to analyse the impact of the varying environmental conditions, i.e. the ambient temperature, absolute humidity and incident solar radiation, on the performance of the desorption column. Following which a control scheme is developed to ensure the system meets the requirements of ZEF, i.e. production of CO2and water in a 3:1 molar ratio and an energy consumption limit of 450 kJ/mole of CO2desorbed.Firstly, a set of experiments with a trayed stripping column were performed to understand the start-up and shut-down behaviour of the column. Based on the observations, a simplistic model of the reboiler was developed to predict the transient behaviour of the column during start-up. A sensitivity analysis was carried out to gain insights into the parameters influencing start-up time and energy demand. Furthermore, different scenarios to start up a column were identified and based on the results, the batch mode is adopted as the efficient way to start up a column. The model predicts that start-up and shut-down account for less than 10% of the total operating time available. Moreover, start-up accounts for a maximum of 6% of the total available energy for production.
Secondly, a set of single-stage kinetic experiments were performed at different temperatures to understand the limitations of the desorption process inside the column. A vapour-liquid equilibrium based stage-by-stage model of a desorption column integrated with a varying space-time yield based absorber model. Design parameters of the integrated DAC model were tweaked, and a base case was developed, to understand the impact of a varying sorbent composition and PV panel output on the performance of the DAC subsystem. It was clear the open model was not capable of meeting the 3:1top ratio specifications of ZEF. Which prompted the implementation of control structures. Single loop mass-flow control, pressure control and cascade mass flow-temperature control schemes were individually tested with the aid of the integrated DAC model. Finally, based on the performance of the individual control schemes, a final parallel control scheme was developed. Wherein the temperature and the pressure of the system adjusts according to the varying power input and the absolute humidity conditions that impact the top ratio of products. The parallel control scheme was found to be adequate in maintaining the top ratio at desired levels.