The influence of intra-droplet heat and mass transfer limitations in evaporation of binary hydrocarbon mixtures

The influence of intra-droplet heat and mass transfer limitations during the evaporation of binary hydrocarbon mixtures is investigated. Liquid transport processes are described with a 1D formulation (diffusion limit model - DLM) taking into account temporal and spatial gradients of the substance properties. The vapor liquid equilibrium is described with a gamma-phi model and the activity coefficients are calculated using the NRTL approach. A suitably normalized segregation factor for each component based on the time-integrated evaporation rate is introduced and used for the analysis of n-heptane/n-decane and n-heptane/iso-octane droplets varying the ambient conditions and the initial liquid compositions. Simulations with the DLM approach are compared to results obtained with a OD model (rapid mixing model - RMM), which assumes homogeneous droplets in the infinite-diffusivity limit. Simulation results are also compared to experimental data where available. Biot numbers for heat and mass transfer as well as Lewis numbers for the liquid phase are evaluated and their differences are analyzed. A constant Lewis number based approach to calculate the liquid diffusion coefficients is investigated in detail as an alternative to the computationally expensive calculation of the individual diffusion coefficients at each inner droplet location. A suitable strategy to precompute the Lewis number as a function of temperature and composition is presented and the results are compared to the detailed calculations. (C) 2013 Elsevier Ltd. All rights reserved.