Cryogenic Heat-Exchanger Design for Freeze-out Removal of Carbon Dioxide from Landfill Gas

Abstract

A cryogenic heat exchanger to remove carbon dioxide from landfill gas (LFG) is proposed and designed for applications to LNG production in distributed-scale. Since the major components of LFG are methane and carbon dioxide, CO₂ removal is a significant pre-process in the liquefaction systems. A new and simple approach is proposed to directly remove carbon dioxide as frost on the surface wall along the cooling passage in a liquefying heat exchanger and to install two identical heat exchangers in parallel for alternative switching. As a first step of feasibility study, combined heat and mass transfer analysis is performed on the freeze-out process of CO₂ in a counterflow heat exchanger, where CH₄-CO₂ mixture is cooled below its frost temperature in thermal contact with cold refrigerant. Engineering correlations for the analogy of heat and mass transfer are incorporated into numerical heat exchanger analysis with detailed fluid properties. The developed analytical model is used to estimate the distribution of CO₂ accumulation and the required heat exchanger size with latent thermal load for the cryogenic CO₂ removal in various operating conditions.