First-principles-based identification of the significance of heat and mass transfer in chemisorption systems

Adsorption systems have the potential to reduce overload on the electricity grid, provide uses for waste heat from industrial processes, and reduce gaps in the cold chain, helping limit food spoilage. However, translating models to real-world systems has faced extensive challenges. Adsorbent beds suffer from heat and mass transfer limitations that complicate the modeling and design. This results in bed designs requiring extensive experimentation or highly complex models to produce preliminary system designs. Thus, this work conducts a first order analysis to aid in system design and minimize the upfront computational and experimental efforts. An order-of-magnitude analysis for generic chemisorption systems is conducted, followed by a specific case study of a spiral finned pipe with a calcium chloride and ammonia working pair to exhibit the benefits of the scaling analysis in bed design. The study shows that for general systems, the permeability and adsorbent conductivity are the most likely limiting factors in performance. Furthermore, the analysis shows that adsorption systems will often be heat transfer limited unless conduction scales of <1 mm or a significant improvement in adsorbent thermal conductivity is achieved. For the specific spiral finned pipe system, a conduction scale of 0.2 mm is required to limit the heat transfer resistance of the system with a 10 °C driving temperature difference for conduction. The current fin radius and spacing should be reduced to improve the overall performance; however, application specific information must also be considered to ensure increases in thermal mass do not reduce system performance.