Study On Heat Transfer Enhancement And Fouling Prevention & Removing For A Three-Phase Circulating Fluidized Bed Wheat Straw Black Liquor Evaporator

Pulp and paper industry is one of the major industries of national economy. A comparatively large number of Chinese pulping and paper-making mills are straw-based, as forest resources are relatively limited whereas non-wood fiber plants are abundant. Wheat straw black liquor is blamed for contributing serious pollution to the environment. It is treated by chemical recovery to reclaim alkali and energy. But the problem lies in the wheat straw black liquor evaporation, especially in evaporation, due to its inferior physical properties, which means that evaporation is a "bottle-neck" problem in chemical recovery for wheat straw black liquor. The three-phase circulating fluidized bed evaporator has been applied for the first time in the evaporation process of wheat straw black liquor in this paper. Thorough studies have been carried out for the mechanisms of preventing and removing fouling and enhancing heat transfer in the novel evaporator from both theories and experiments. The main contents of the paper are as follows:First, comprehensive summary and analysis have been carried out, including the situations of wheat straw black liquor's evaporation, the forming process and mechanisms of fouling in heat exchangers, as well as the novel technique of three-phase circulating fluidized bed evaporator.Second, the chemical composition and fouling mechanisms of foulants from alkaline wheat straw black liquor have been analyzed. Fractal theory has been used to study the structure of the foulants. Third, the mechanisms of fouling prevention and remove in three-phase circulating fluidized bed have been theoretically studied. The three-phase shearing stress plays a primary role on extending induction period of fouling. It can delay adhesion time before fouling and increase denudation rate. A collision model between particles and wall has been established, the crack formed by the collision stress may expand by thermal stress which leads to peeling of foulant . Forth, the mechanisms of heat transfer enhancement of solid particles have been systematically studied. The mechanisms of solid particle strengthening single phase convective heat transfer and nucleate boiling heat transfer have been illustrated on theories. Fifth, a heat transfer model has been established based on the artificial neural network by the Matlab neural network toolbox. Rapid learning speed, few training steps and high fidelity are advantages of the model. The maximal deviation is no more than 5%. Sixth, the effects of fouling prevention & remove and heat transfer enhancement have been experimentally studied.