氧化铝生产蒸发工序的分析

Exergy analysis of evaporation process in an alumina refinery

为降低氧化铝生产蒸发工序的能耗,根据工业铝酸钠溶液的密度、比热容、各组分的活度因子和标准化学等性质,推导出工业铝酸钠溶液的计算式;对四效蒸发器-三级闪蒸器系统进行分析,计算蒸发系统及其各单元的效率和损系数。研究结果表明:蒸发系统的效率为13%～19%;三级闪蒸器的效率较高,均超过了90%;四效蒸发器的效率较低,几乎都低于80%,其中第4效蒸发器的效率最低,为9%～12%;冷凝水和乏汽形式的外部损失和蒸发器内传热过程引起的内部损失是蒸发系统的2类主要损失,其损系数分别为0.273～0.301和0.291～0.329;虽然预热器的混合损系数仅为0.016～0.030,但其用能过程不合理,因此,建议加强冷凝水和乏汽的余热回收利用,优化蒸发系统的传热温差分布和操作参数,改进预热器的使用方式。

To reduce the energy consumption of evaporation process in the alumina refinery,the formulas for calculating the exergy of industrial sodium aluminate solution were derived in terms of the density,specific heat capacity,activity coefficients and standard chemical exergies of its species.Then exergy analysis was applied to the four-effect evaporators and three-stage flashes evaporation system to calculate and analyze the exergy efficiencies and exergy loss coefficients in the evaporation system and its individual units.The results show that the exergy efficiency of the evaporation system is 13%-19%;the three-stage flashes have higher exergy efficiencies in excess of 90% while the four-effect evaporators have poor efficiencies,which are almost lower than 80%,especially the exergy efficiency of the 4th effect evaporator is only 9%-12%,approximately.The external exergy losses in the form of condensed water and exhausted vapor and the internal exergy losses due to heat transfer in evaporations are the two main types of exergy losses in the evaporation system,whose exergy coefficients are 0.273-0.301 and 0.291-0.329,respectively;though the coefficient due to combined process in preheaters is only 0.016-0.030,which is unreasonable in view of energy conversion.Thus it＇s proposed that waste heat recovery and utilization of condensed water and exhausted vapor should be enhanced;the distribution of temperature differences in the evaporation system and the operating parameters should be optimized and the method for using preheaters should be improved.