刮膜式分子蒸馏器内传质的数值模拟

Numerical Simulation of Mass Transfer in a Wiped Film Molecular Distillator

在刮膜式分子蒸馏器中,研究液膜内传质传热特性对改进蒸馏器内流场结构和优化操作参数具有重要意义。今采用计算流体力学(Computational Fluid Dynamics,CFD)软件,建立刮膜式分子蒸馏器三维模型,研究了液膜表面温度、浓度、蒸发速率沿三维方向变化,并探究了进料量和加热温度对传质传热特性的影响。结果表明:头波前液膜表面温度随着远离头波沿周向和径向逐渐减小,沿轴向逐渐增加,头波处、壁面附近和液膜表面处温度梯度较大;壁面附近处易挥发组分浓度几乎不变,但沿三维均呈减小趋势;蒸发速率沿三维的变化和温度呈相同的趋势;进料量越小,加热温度越高,液膜表面温度、易挥发组分蒸发速率和液相总传质系数越大,易挥发组分浓度减少的也越快,有利于传质。但较高的加热温度会增加生产成本,且易使物料分解,故应选择合适的加热温度。

Mass and heat transfer on liquid films are important parameters for the modification and optimization of wiped film molecular distillators. In this paper, a 3-D CFD model of a wiped film molecular distillator was established, which was used to study variations of temperature, concentration, evaporation rate and liquid phase mass transfer rate in three-dimensional direction and explore effects of flow rate and heat evaporationon heat and mass transfer. The results indicate that surface temperature of head wave liquid films gradually reduces along the circumferential and radial direction, while it gradually increases along the axial direction. The temperature gradients are larger at positions including first wave head, wall surface and liquid film surface. The concentration of volatile components keeps unchanged near wall surface, but it decreases in three-dimensional direction. The evaporation rate shows a similar trend as that of temperature in three-dimensional direction. Smaller flow rate, higher heating temperature and higher surface temperature can result in higher evaporation rate and total liquid phase mass transfer coefficient, which is beneficial to mass transfer. However, higher heating temperature will increase production cost and material decomposition. Therefore, appropriate heating temperature should be used.