Bubble columns for condensation at high concentrations of noncondensable gas: Heat‐transfer model and experiments

Abstract

Carrier gas based thermodynamic cycles are common in water desalination applications.These cycles often require condensation of water vapor out of the carrier gas stream. Since the carrier gas is most likely a non-condensable gas present in very high concentrations (60-95%), a large additional resistance to heat transfer is present. We propose to reduce the aforementioned thermal resistance by condensing the vapor-gas mixture in a column of cold liquid rather than on a cold surface by using a bubble column heat exchanger. A theoretical predictive model for estimating the heat transfer rates and new experimental data to validate this model are described. The model is purely physics based without the need for any adjustable parameters, and it is shown to predict heat rates within 0% to -20% of the experimental values. The experiments demonstrate that heat transfer rates in the proposed device are up to an order magnitude higher than those achieved in existing state-of-the-art dehumidifiers.Keywords: condensation, bubble column, non-condensable gas, thermal resistance model, dehumidification, moist air, carrier gas * Corresponding author c p specific heat capacity at constant pressure (J/kg·K) c p,g average specific heat capacity at constant pressure of the vapor-air mixture (J/kg·K) T air local energy-averaged temperature of the air-vapor bubble (T coil local temperature of the coil surface (T column local energy-averaged temperature of the liquid in the column (T coolant local energy-averaged temperature of the coolant in the coil (

Experiments and modeling of bubble column dehumidifier performance

“…Dehumidification in bubble columns has previously been shown to reduce device volume and condenser area by an order of magnitude [5]. A schematic diagram of the bubble column dehumidifier used in this work is shown in However, the key advantage of the bubble column dehumidifier lies in moving the condensation process off an expensive solid surface and onto the surface of a swarm of bubbles.…”

“…Additionally, practical bubble column dehumidifiers for HDH desalination need to be very short to ensure a minimal gas-side pressure drop, and several researchers have noted that the gas and liquid phases behave differently near the gas inlet [10,11,12,13]. Most bubble column reactors are significantly taller than those used for dehumidification [5,10], so the entry region is often neglected in the reactor modeling and design literature.…”

“…A model by Narayan et al [5] proposes a thermal resistance network for the bubble column dehumidifier with transport mechanisms taken from the bubble column reactor literature. This model predicts the heat flux with moderate accuracy in simple configurations, but it calls for refinement.…”

A Numerical Solution Algorithm for a Heat and Mass Transfer Model of a Desalination System Based on Packed-Bed Humidification and Bubble Column dehumidification

“…The use of a short bubble column for dehumidification was first introduced by Narayan et al [6] and was found to be very promising since it reduces the negative effect of the noncondensable gases on condensation. In traditional dehumidification systems, water vapor has to diffuse through an air layer, which increases the resistance to the mass transfer.…”

“…In traditional dehumidification systems, water vapor has to diffuse through an air layer, which increases the resistance to the mass transfer. In a bubble column, the location of condensation is the transiently formed bubble-water interface which has a very large specific area, giving it superior efficiency compared to alternative dehumidification systems [6,7].…”