Feasibility analysis of using a bubble column evaporator as a solvent swap device

Abstract

A feasibility analysis was performed on the use of a bubble column evaporator (BCE) as a solvent swap device. Batch BCE simulations successfully proved that the BCE was capable of performing a solvent swap for both the dcm - methanol and the water - methanol systems with the aid of computational fluid dynamics (CFD). This was achieved by saturating the inlet gas with the new solvent which condensed inside the column. The old solvent then evaporated into the gas phase which exited via the column outlet. Progression of the solvent swap therefore caused the liquid mass fraction of the species to vary. The effect of bubble size, gas flowrate, gas inlet temperature and liquid mass fraction on the solvent swap efficiency was also investigated. The liquid mass fraction was found to have the greatest influence on the solvent swap efficiency. The evaporation rate decreased proportionately with the mass fraction of the evaporating species in the liquid phase which was a result of Raoult’s law. The thermodynamic model was used to validate the CFD solution and estimate the time required for a batch solvent swap. A smaller bubble size was found to enhance mass transfer in the solvent swap by increasing the evaporation and condensation thermodynamic efficiencies. The higher gas flowrate also improved the mass transfer in the system and reduced the time required for a solvent swap as there was a greater throughput of gas, however it resulted in a decreased evaporation efficiency due to a lower gas residence time in the column. The higher gas inlet temperature increased the rate of condensation and the time required for the a solvent swap. The increased condensation of the new solvent had a negative effect on the evaporation of the old solvent as a result of raoults law which resulted in a longer solvent swap operation. The increased gas inlet temperature also caused the evaporation rate to decrease because of the relationship between the density of an ideal gas and temperature. An increased inlet gas temperature resulted in a lower inlet gas density. There was therefore a reduced mass throughput of gas into the column and consequentially the rate of evaporation decreased. The liquid mass fraction was found to have the greatest influence on the solvent swap efficiency. The evaporation rate decreased proportionately with the mass fraction of the evaporating species in the liquid phase which was a result of Raoult’s law.