高校化学工程学报

载气蒸发是利用在垂直加热管的底部引入一种惰气作为载气,使液体的蒸发在气液两相界面上进行的过程。为减少热敏性物料蒸发过程中物料的停留时间而开发的升膜单程直通式载气蒸发器,要达到足够的浓缩效果,必须在低液速条件下进行,因而对低液速载气蒸发传热和操作特性的研究成为一个急需解决的问题。今在减压条件下,于φ27mm3mm,加热长度为1350mm的垂直不锈钢管中进行了低液速载气蒸发传热和操作特性的研究,实验中获知在一定的操作温度下,对于入口表观液速ul=0.032⁰.321m穝-1时,当采用较低的表观载气速时,蒸发传热膜系数随表观载气速的增加而增加,而且较对流沸腾有明显提高,加热壁面温度降低;但当表观载气速过大时,传热膜系数反而随着表观载气速的增加而减小。同时研究中对低液速载气蒸发的操作机理进行了进一步的分析。

Gas Carrying Evaporation is a process in which an inert gas is introduced at the bottom of the vertical heating tubes and serves as carrying gas, and the vaporization of the liquid occurs on the gas-liquid interface. In order to shorten the residence time of thermally sensitive solutions in evaporation, a single pass gas carrying evaporator was developed. The evaporator needs to be operated at lower liquid velocity for the sake of obtaining product concentration needed. Therefore, it is important to investigate heat transfer and operating characteristics of Gas Carrying Evaporation at low liquid velocity. The heat transfer and operating characteristics of Gas Carrying Evaporation was studied with air-water system in a vertical stainless steel tube with f 27mm3mm in diameter and 1500mm in heating length under vacuum condition. It is found that, at the fixed operating temperature and the superficial liquid velocity of the entrance (ul=0.032⁰.321m穝-1), when superficial carrying gas velocity is small, the heat transfer coefficient of Gas Carrying Evaporation increases with increasing of superficial carrying gas velocity. Comparing with convection boiling, the heating wall temperature of Gas Carrying Evaporation is lowered and the heat transfer coefficient is notably raised. However, when superficial carrying gas velocity is too large, the heat transfer coefficient reduces with increasing of superficial carrying gas velocity. Moreover, the operating mechanism of Gas Carrying Evaporation is analyzed.