Study Of Falling Film Evaporation Based On Multi-stage Flash Model

Falling film flow is widely used in various process industries due to its specialadvantages, such as small flow rate, small temperature difference and high heat andmass transfer coefficients. Multicomponent film flowing along a heated wall wouldpartially evaporate; thereby the liquid mixture could achieve separation to some extentafter falling through a certain length. The purpose of our research is to explore themass transfer process of falling film evaporation. Through theoretical analysis andexperiments, it is proved to be reasonable in explaining the mass transfer process offalling film evaporation by multi-stage flash model. Furthermore, the prospects of themodel are introduced.In this paper, a new multi-stage flash model is proposed to analyze thevapor-liquid mass transfer during falling film evaporation process. Through thesimulation of Aspen Plus, we found that the irreversibility of separation process wasreduced because of the uniform distribution of driving forces. Under the same heatingpower, mole fraction and molar flow rate of light component of top distillate increasewith the number of flash columns, which means multi-stage flash separation has ahigher thermodynamic efficiency.On the basis of multi-stage flash model, we assume that falling film evaporationis constituted of many connected flash units. Heating on the right side of the fallingfilm plate is equal to input energy to each flash unit. By contrasting molar flow rate oflight component of top distillate under uniform heating condition and concentratedheating condition, a better separation performance was obtained through uniformheating. It proves that a higher thermodynamic efficiency could be obtained in fallingfilm evaporation through equipartition of energy. According to the multi-stage flashmodel, uniform heating means increasing flash units and reducing the irreversibilityof the separation process, and consequently has a higher thermodynamic efficiency. Inaddition, a falling film heat integrated distillation tower is proposed by combiningfalling film evaporation and internal heat integrated distillation. In the tower, theliquid phase evaporation and the vapor condensation on either side of falling filmplate achieve energy coupling. In order to investigate the effects of different film flow pattern on falling filmevaporation, different separation performance of different falling film plates werecompared in experiments. The plate with fins obtained higher mole fraction and molarflow rate of light component of top distillate. The experimental results indicate thatmass transfer during falling film evaporation could be enhanced through optimizationof film flow pattern. At last, some measures to enhance falling film evaporation are asfollows: intensify the renewal of free surface of the liquid film, reduce the bias flowand shrinkage of liquid film, strengthen heat transfer through the wall of falling filmplate and increase in residence time of the liquid film on the wall.