Authors

Abstract

To investigate the concentration of molasses ethanol wastewater, a two-dimensional numerical model of falling-film evaporation over horizontal tubes was developed. The model used the Volume of Fluid (VOF) method to capture the liquid-gas interface and was coupled with the Lee phase-change model. The model was validated by comparing the predicted liquid-film thickness and average heat-transfer coefficient with experimental data reported in the literature. The validated model was then used to systematically investigate the effects of spray density (0.10 ~ 0.30 kg/(m·s)) and heat-transfer temperature difference (2 ~ 8 °C) on the falling-film flow and evaporation performance of molasses ethanol wastewater. The results showed that the liquid-film thickness initially decreased and subsequently increased along the circumferential direction, reaching a minimum within the angular range of 90° ~ 120°. Increasing the spray density produced a thicker liquid film; nevertheless, the average heat-transfer coefficient increased monotonically by 23.0% ~ 27.2%, whereas the mass-transfer rate decreased by 19.3% ~ 25.1%. Increasing the heat-transfer temperature difference increased both the average heat-transfer coefficient and the mass-transfer rate. When the temperature difference increased from 2 to 8 °C, the mass-transfer rate increased 2.84 ~ 3.07-fold, and its effect on evaporation performance was substantially greater than that of spray density. Therefore, where operating conditions permit, a lower spray density combined with a higher heat-transfer temperature difference is recommended for the falling-film evaporative concentration of molasses ethanol wastewater.

Abstract in Chinese

为了处理糖蜜酒精废液的浓缩问题，基于流体体积法（VOF）和Lee相变模型，针对水平管外的降膜蒸发过程建立了一个二维数值模型。首先，通过将模拟得到的液膜厚度和平均传热系数与文献实验数据对比，验证了模型的可靠性。随后，利用已验证的模型系统研究了喷淋密度（0.10 ~ 0.30 kg/(m·s)）和传热温差（2~8 ℃）对糖蜜酒精废液降膜流动与蒸发性能的影响。结果表明：液膜厚度沿周向先减小后增大，在90°~120°的圆周角范围内达到最小值。喷淋密度增大使液膜变厚，但平均传热系数单调上升（增幅为23.0% ~ 27.2%），而质量传递速率则下降了19.3% ~ 25.1%。提高传热温差同时增大了平均传热系数和质量传递速率。值得注意的是，当温差从2℃升至8 ℃时，质量传递速率增大了2.84 ~ 3.07倍，其对蒸发效率的影响远超喷雾密度。因此，建议在糖蜜酒厂废水的降膜蒸发浓缩过程中，在条件允许的情况下，可以采用低喷雾密度与高温差相结合的操作策略。