Microwave Negative Pressure Drying Characteristics Of Tobacco Stalk Particles

Tobacco stalk is a special by-product of commercial crop.Since tobacco stalk contains nicotine or carries tobacco germ,it can neither be suitable for being used as the fodder nor be directly discarded to the field.Tobacco stalk resource recycling can reduce environmental pollution,enhance utilization value of tobacco stalk,increase income for tobacco farmers and achieve sustainable development of tobacco.Due to semi-lignification characteristic of tobacco stalk,fresh tobacco stalk can be made into biomass fuel through smashing,drying and squeezing processes.In recent years,production of biomass fuel with tobacco stalk has become the research hotspot of tobacco stalk resource recycling.The moisture content of fresh tobacco stalk is about 75%.The research shows that,when the moisture content of raw materials is 12%~15%,the effect of biomass compression molding is optimal.So,drying treatment of tobacco stalk particles is one of important processes to prepare biomass fuel.Microwave drying has the advantages of rapidity,uniformity and high efficiency.Negative pressure can reduce boiling point of water evaporation and thus accelerate drying speed.Therefore,the combination of microwave and negative pressure has been applied to dry many materials.Based on the advantages of microwave and negative pressure,microwave and negative pressure were applied to dry tobacco stalk particles in this paper,and the drying characteristics of tobacco stalk particles were studied.The following work has been conducted:(1)The preliminary test was conducted for microwave drying of tobacco stalk particles.Loading capacity and microwave power were used as the experimental factors to study their influence on drying time,drying rate and unit energy consumption.The results showed that,the drying process includes three stages: rising speed,constant speed and declining speed.And the duration of constant speed is long.The five drying mathematical models were applied to carry out nonlinear regression fitting of the experimental data and solve.The comparison of determination coefficient and residual square root indicates that Logarithmic model is the optimal model to express microwave drying of tobacco stalk particles.And the drying coefficient k decreases with the increase of loading capacity.(2)Technological parameters of microwave drying were optimized.Microwave power,material thickness and pressure in the drying room were chosen as the experimental factors to study the influence of each factor on drying characteristics of tobacco stalk particles.Three-factor and three-level quadratic regression orthogonal experiment was adopted to analyze the influence of three factors on moisture content and drying uniformity of tobacco stalk particles.In addition,regression mathematical model between various indexes and experimental factors was established,and multi-objective nonlinear optimization method was utilized to confirm the optimal technological parameters to dry tobacco stalk particles with microwave and negative pressure.(3)Microwave drying equipment of tobacco stalk particles was designed.Crawler belt transport mode,size of drying box,the number of microwave generators and arrangement way of microwave generators were confirmed.Finite element simulation software HFSS was applied to simulate different arrangement of microwave units in the drying box and to optimize the arrangement mode of microwave generators in the drying box.Flow field simulation was conducted for the equipment under the conditions of no load and load.Internal temperature,air speed and pressure of drying equipment were analyzed.Rationality of drying equipment was also verified.At the same time,the control system of the equipment was developed.PLC was set up to control hardware platform.Automatic control strategy and program were designed.(4)The drying effect and energy consumption of the equipment were studied.The comparative experiment was done for the developed equipment with hot air drying and microwave drying equipment.The micro-structure of tobacco stalk particles,temperature uniformity score,appearance quality and equipment energy consumption were investigated.