Characterization and Design of a Stripper for a Continuous Direct Air Capture System

Abstract

Capturing CO2 directly from the air has gained wide attention as it is one of the possible solutions to mitigate the risks of climate change. Zero Emission Fuels (ZEF) is a start-up in Delft that develops a small-scale plant to produce methanol from sunlight and air only. CO2 and H2O are captured from the air by a direct air capture unit that operates continuously by means of an absorption and stripping column. Liquid amines are investigated as chemical sorbent. In this work, the stripping column is characterized and optimized for the liquid amine tetra-ethylenepentamine (TEPA). A vapor-liquid equilibrium based stage-by-stage stripper model was established using mass and energy balances for each stage individually. A number of input parameters was specified to understand the effect of those parameters on the performance of the stripping column. The parameters include; composition, temperature and mass flow rate of the feed, number of stages, H2O reflux ratio, temperature of the reboiler and absolute pressure of the column. The mass balance was solved according to the Rachford-Rice equation while using bisection as numerical root finder. To validate the stripper model, experiments were performed for varying configurations regarding the input parameters mentioned above. For this, a trayed stripping column with bubble caps, was build and adjusted according to the experimental plan. Furthermore, single stage kinetic experiments were performed at 115 °C and 950 mbar to find the limitations of the desorption process inside the column. Subsequently, the Damköhler number was estimated to understand the effects of reaction rate and diffusion during the process. Sensitivity analyses were performed to find the effect of input parameters on the performance of the stripping column. The effect was measured in terms of; CO2 concentration in the outlet stream, cyclic capacity of the liquid solvent, CO2 and H2O vapor ratio in the top stage of the column and energy demand per desorbed mol of CO2. Based on the results, a tool to predict the performance of the stripper in an elementary way was produced. Based on the kinetic experiments, it was found that a typical hold up time that is required for the system to reach equilibrium was measured at 600 s. This number was used to estimate the liquid hold up volume per stage in the final stripper design. The experimental and model results were combined in a new stripper design considering the operating conditions stated by ZEF. The 5 stage column operates at 1000 mbar and a reboiler temperature of 120 °C. The feed was preheated up to 105 °C and the mass flow rate was determined at 0.31 g/s resulting in a hold up volume of 187 ml per stage. The cyclic capacity of the system equals 3.3 mol CO2 per kg TEPA and the energy demand was found to be 279 kJ per mol CO2. In addition, a new design for ZEF's direct air capture system was presented where the absorption column is modelled as a black box. An heat exchanger was implemented to minimize the energy demand of the system, which resulted in a lowest energy demand of 2319 kWh per ton of CO2. This is slightly higher than the DAC energy demand of companies like Climeworks, Carbon Engineering and Global Thermostat, but could potentially decrease when optimizing the system.