Development of high performance falling film plastic tower for liquid desiccant application

Abstract

Simultaneous heat and mass transfer equipment (absorber/regenerator) is the heart of the liquid desiccant system. Apart from the driving potential difference, the system's overall process efficiency is steered by how liquid desiccant and air interact. The interaction pattern depends on the type of chemical tower used. Packed bed, spray and falling film towers are three conventional designs used for the above purpose. Falling film towers are popular for simultaneous heat and mass transfer applications due to modest air pressure drop and energy intensive liquid distribution. Air-liquid desiccant contacting surface in the case of metal suffer from corrosion problem, thus requiring frequent replacement, whereas non-metallic surfaces such as plastics have poor wetting characteristics. Two significant limitations of falling film towers for liquid desiccant application are the contraction of falling liquid film on the plastic surface (poor wetting) and uneven distribution of liquid across solid surfaces (maldistribution). In the preliminary work, wetting characteristics of Plain Al and Plain PP surfaces have been experimentally investigated to find the suitable alternative to a corrosion-prone metallic surface to be used as a gas liquid contacting surface in falling film towers. Further, two wetting characteristic improvement methods: surfactant addition and surface modification technique, have been used to improve the PP surface's wetting characteristic. Importantly, Modified PP plate C provides significant wetting factor improvement in the low flow rate range and more effectively than the surfactant addition method. For the maldistribution study, four types of liquid distributors: plain tube, spray nozzle, perforated plate, and branch tube distributor, have been tested. From the results, the branch tube distributor, fabricated based on equal area form inlet to multipoint outlet, stood better than other tested distributors. We have proposed a new distributor design (patent application filed), a gravity-flow distributor, for the vertical multi-plate falling film tower for further development. It is recommended that a gravity-flow distributor is suitable for multi-plate falling film towers. The actual performance augmentation by incorporation of modified PP plates is checked in a liquid desiccant system. It is found that the Modified PP plate C's regeneration rate is around 54.6% superior to the Plain PP plate for the tested conditions. The Modified PP plate C is optimized for water and in quest of finding the optimal solution for liquid desiccant (LiCl) as a working fluid. A new Modified PP plate E is fabricated based on thermophysical properties (capillary length) of LiCl at absorber operating conditions. It is found that under studied conditions, the dehumidification effectiveness of the best Modified PP plate E is almost 1.83 times superior to Plain PP plate. Further new correlations for the estimation of air humidity effectiveness of the falling film regenerator and absorber have been developed using wetting factor information. The developed correlations are suitable for accurate prediction of the metallic and non-metallic surface performance for both adiabatic and non-adiabatic conditions. The humidity effectiveness correlation for a dehumidifier is further generalized to predict the data of the liquid desiccant solutions other than LiCl.