Computer modeling of the kinetics of CO2 absorption in rebreather scrubber canisters

Despite the ubiquitous use of carbon dioxide (CO₂) scrubbers in life-support equipment, the kinetics of absorption in scrubbers is poorly understood. Furthermore, there are no dependable sensors to indicate when scrubber life has been expended in underwater breathing apparatus. It was thus our aim to model scrubber kinetics, and from that modeling determine whether thermal sensors could reliably indicate remaining scrubber life. To that end, we have exploited a stochastic method for the discrete simulation of CO₂ absorption reactions in scrubber canisters found in closed-circuit underwater breathing apparatus (rebreathers). The model, applicable to both sodalime and lithium hydroxide absorbents, allows us to test the effects of temperature, flow rates, CO₂ production, canister geometry, and granule size on absorption kinetics. The model exhibits some similarities to the finite-element method: the simulated bed contains a minimum of 200,000 volume elements or cells, within which are found the cell temperature and amount of CO₂ stored for each increment of time. However, unlike the usual application of partial differential equations in finite modeling, mass and heat transfer are determined stochastically. Although constrained by the overall physics, chemical absorption within each cell, with its resulting thermogenesis, and CO₂ flow path among cells are probabilistic. The result is a simulation rich in CO₂ absorption and thermal fluctuations. Diffusion of CO₂ within absorbent granules is impeded by diffusion resistance generated by the accumulation of reaction products within the granules. Thermal conductivity of the absorbent granules, granule size and volume, convective flow rate, and heat transfer between air and granules all influence the reaction kinetics. Gas flow can be directed evenly across the canister or toward a point source. We currently use the model to predict and explain the time- and space-varying thermal profiles occurring as CO₂ reaction fronts move through various canister shapes. We are exploring how knowledge of transport coefficients computed from fluctuation analysis would improve estimates of canister survival time. We also use the simulation to visualize the patterns of CO₂ absorption within a scrubber canister. Since the absorption process is inefficient, the model may ultimately aid optimum use of carbon dioxide absorbents and canister geometry