Carbon Dioxide Absorption And Channelling In Closed Circuit Rebreather Scrubbers
Authors:   ,   Cork Institute of Technology, Cunningham Shona 
Published: September 2024

"Closed Circuit Rebreathers (CCRs) are a type of Self Contained Underwater Breathing Apparatus (SCUBA) used predominantly by the diving community. CCR technologies have become prevalent within the commercial, military and technical diving communities over recent years. CCRs work by recycling breathing gas, by-passing the gas through a carbon dioxide (CO2)chemical scrubber and topping the resultant scrubbed gas up with pure oxygen to maintain the oxygen content at a level which supports life. One of the main dangers currently faced by divers using this equipment is the risk of CO2 poisoning if the scrubber fails. This failure occurs due to CO2 breakthrough - where breakthrough is defined as the time until the canister effluent passes through the chemical absorbent unscrubbed. There are a variety of reasons for CO2 breakthrough, and not all of the reasons can be identified by the operator. A better understanding of CO2 absorption within CCRs would lead to a reduction in failures. There are a variety of potential causes of CO2 breakthrough, where breakthrough is defined as the time until the canister effluent passes through the chemical absorbent unscrubbed. Not all of the causes can be compensated for by the operator. A transient computational fluid dynamic (CFD) model investigating the kinetics of flow and CO2 absorption focusing on the chemical reaction between the absorbent soda lime and CO2 is presented in the thesis. It presents the first reported mixture model theory simulation of the CCR scrubber using Ansys CFX 13.0. An extensive review of the design of CCRs, their applications and current modelling techniques identified a need to enhance the simulation of CCRs using the mixture model theory to simulate the chemical reactions occurring within the scrubber system of the CCR. The application of CFD techniques on CCRs allows the in depth analysis of the reaction between soda lime and CO2 using the graphical display provided by the simulated model. The simulation studies undertaken present a transient model with the ability to investigate seven key parameters affecting scrubber performance; geometry, wall temperature, inlet velocity, pressure, CO2 absorption, granule size and material selection. Analyses of the results demonstrate the sensitivity of the parameters to their behaviour on CO2 breakthrough. Due to the requirement of strong coupling between the phases of CO2 and soda lime, the mixture model is the best suited model for the liquid-particle mixture. Analysis of mesh size, mesh type and inflation are made to independently characterise the accuracy of the presented model by means of convergence before further comparisons with experimental data. The importance of mesh refinement is demonstrated as well as the contribution of inflation and grid independence to the accuracy of the model. A versatile model simulating chemical reactions within a CCR canister for different geometrical scrubber designs has been shown to be capable of analysing the design parameters of interest. A CCR scrubber model will bring new learning into the kinetics of CO2 absorption. It may also further the technology of monitoring scrubber duration in real time for a user. Data collected from a user may be fed into the system to allow a visual analysis of the scrubber. The first section of this thesis describes the operational requirements of the CCR system and in order to validate the CFD model, an experimental program was designed where an experimental test rig was manufactured to undertake experiments to study the impact of CO2 breakthrough of various parameters. The system incorporated CO2, O2, humidity and temperature sensors where the rig tested the scrubber’s capacity to absorb CO2 which is pumped in a controlled environment to benchmark the initial designs against existing literature. A review of the state of the art identified five feasible parameters important to scrubber kinetics; (i) geometry, (ii) material selection, (iii) inlet gas velocity, (iv) granule size and (v) packaging of the granules. These results gave an indication to the scrubber’s capacity to absorb CO2. CO2 absorption can be further inferred by placing thermocouples at various places within the canister and using relationship between the reaction and temperature to give an indication of absorption within the scrubber. As the chemical reaction occurs, the temperature rises and water vapour is produced. Through measuring both temperature and humidity, it was found that temperature gave the most consistent indication of CO2> absorption rather than humidity as the higher the humidity levels, the more erratic the humidity results became. The result of the experimental testing is a body of data benchmarked off current literature for validation which is used to compare with a transient CFD model to further the knowledge of CO2 absorption. The development of a CFD model for the CCR scrubber allows the simulation to assist in future designs of CCR system. It allows the simulation of alternative scrubber geometries which may be subjected to external parameters to assess their performance. The advantage to implementing a design through CFD allows the user to simulate alternative CCR scrubber designs before incorporating a manufacturing cost to assess the performance of a given design. This reduces cost, time and can assist the decision of manufacturing a given CCR scrubber. The results demonstrate that the CFD simulation using the mixture model theory is sufficiently reliable to simulate the reaction between CO and soda lime in CCR scrubbers."