(98a) Modelling of Novel Experimental Apparatus or Direct Air Capture (DAC) Machinery with Emphasis on Sorbent Performance.

The direct capture of carbon dioxide from ambient air, known as direct air capture (DAC), may prove to be an important tool in fighting climate change. DAC currently is too expensive. It requires innovative process design and better sorbent materials to become economically viable. A major challenge is an energy-efficient implementation of the regeneration step post-capture, which can involve a combination of heating, water application, and evacuation. How best to combine these Thermal-Swing, Moisture Swing, and Pressure Swing mechanisms critically depends on the choice of the sorbent. Unfortunately, experimental tools to test the interaction of water and CO₂ sorption and their thermal consequences are rather limited. Furthermore, practical implementations of any regeneration systems will likely operate under near-vacuum conditions where the gas phase over the sorbent is completely dominated by CO₂ and water vapor.

We, therefore, have designed an experimental apparatus to test the behaviour of sorbents when exposed to low-pressure water vapor. The adsorption of water vapor delivers heat in excess of the heat of condensation, resulting in an autothermal heating of the sorbent, which in response to this temperature rise starts to release the CO₂. A vacuum pump removes the gas and thereby lets the process run to completion. The experimental apparatus, which was presented at the AIChE 2024 annual meeting in the poster session, is designed to quantify the temperature, flow rate, and steam entering the experimental chamber and to measure the rate of outflow and the composition of the exit gas stream. The experiment can also characterize the heat release during water adsorption. In addition, it is possible to add other components to the input gas streams as e.g. CO₂ and O₂, as long as the total pressure in the sample chamber does not exceed a few kPa.

This new instrument, of which several iterations have been built, is extremely well suited to characterize sorbents that bind both CO₂ and H₂O, where the two sorbates strongly interact when bound to the sorbent. However, the design is sufficiently complex to require careful modelling of the process to take advantage of the wealth of data it generates. For example, in interpreting the temperature changes in terms of heats of sorption and reaction kinetics, it is important to model heat transfer in the system. The scope of the model is such that it can easily be extended to numerical simulations of production systems that incorporate these types of sorbents and these types of sorbent regeneration.

The modelling code is object-oriented and written in Python 3. The objects map into the components of the experimental apparatus or, in the future, into the units of operation of the DAC device. For example, one object represents a water tank maintained at a certain temperature and with a headspace filled exclusively with water vapor. In a second object representing the sorbent chamber, this water vapor is pulled onto the sorbent material and thereby releases heat, which in turn kicks off the CO₂. Heat losses from the sorbent chamber are considered in the model. Other modules include a low-pressure nitrogen swept chamber with a calibrated N₂ flow that dilutes the water vapor arriving from the sorbent chamber. After this module, there is a vacuum compressor module that raises the gas mixture pressure to 1 atm without water condensation. By measuring the H₂O and CO₂ concentrations in this recompressed N₂ stream, one obtains an accurate measure of the gas flow exiting the sorbent chamber.

The models are augmented by various sorbent models that can be fit to the experimental data. Once a sorbent has been characterized, the model can be reformulated to capture the features of a potential DAC device and, therefore, characterize its overall performance. Here, we show model results in comparison to the bench scale experiments. We will emphasize the power of the experimental apparatus to differentiate between different sorbent characteristics, e.g., in terms of isotherms and reaction kinetics.