| Polypropylene is the second most commonly used synthetic resin in the world.In recent years,with the continuous release of new polypropylene production capacity in China,the polypropylene industry is facing the situation of supply and demand imbalance,the competition between middle and low-end products is fierce,and the high-end products such as high impact copolymer polypropylene,high melt strength polypropylene and metallocene polypropylene are still heavily dependent on imports,so they are the key development direction of high performance polypropylene products.Among them,anti-impact copolymer polypropylene is a polymer alloy with propylene homopolymer as the matrix and dispersed phase such as ethylene-propylene random copolymer and ethylene-propylene multi-block copolymer embedded in its particle pores.Due to the excellent toughness and impact resistance,impact copolymer polypropylene is widely used in automobile,household appliances,pipe and other fields.How to further improve the content of ethylene-propylene random copolymer in impact copolymer polypropylene and obtain the uniformly distributed rubber phase is one of the current research hotspots,which has important scientific significance and practical application value.Among the polypropylene production processes,the Innovene process and Horizon process both adopt two unique horizontal stirred bed reactors to generate a flow state close to plug flow,allowing different polypropylene particles to have similar residence time,and thereby to improving the uniformity of rubber phase content in the product.However,there are still some common problems in these processes.When two horizontal stirred bed reactors are operated in series to produce impact polypropylene,there are some problems in the first reactor producing homopolymer,such as large amount of backmixing and long residence time of some particles.In the second reactor producting impact copolymer,the agglomeration of polypropylene particles with high rubber phase content or their adhesion to the agitator will affect the flow of materials,which directly affects the long-term operation stability and product quality of the device,and also limit the further improvement of rubber phase content in impact copolymer polypropylene.In view of the above problems,this paper takes polypropylene horizontal stirred bed reactor as the research object.According to the different requirements of the first and second reactor for the mixing ability of agitator,the blade structures of industrial T-type agitator and door-type agitator are improved.Through the systematic experiment of single-group agitator,double-group agitator and multiple-group agitator,the effect of mixing agitator structure and its combination on the mixing characteristics and residence time distribution of polypropylene particles is investigated to obtain the optimal mixing impeller structure and develop a new type of horizontal agitator to improve its mixing performance.At the same time,by measuring the startup torque and stable torque of different agitators,combine with the force analysis of the interaction between polypropylene particles and agitator,the prediction model of horizontal agitator torque is established,which is expected to provide a basis for the scale up design of horizontal agitator reactor and motor selection of the horizontal stirred bed reactor.The main research contents and conclusions of this paper are as follows:1.The optimization idea of blade structure is proposed of reducing the blade width of T-type agitator to weaken axial diffusion,setting inclined blade in door-type agitator to strengthen radial mixing,and keeping the blade top structure unchanged to maintain wall scraping ability.Four T-type agitators with different blade widths and four doortype agitators with different structures were designed respectively.Then,the effects of single-group agitator with different structures on the radial and axial mixing of polypropylene particles were investigated by using magnetic particle tracer-loadingsampling analysis in a cold model experimental device of a horizontal stirred bed.The results show that the radial and axial mixing ability of T-type agitator is better than that of door-type agitator.Under the same conditions,the radial mixing time of T-type agitator is about 60 s,while that of door-type agitator is about 120 s.With the decrease of blade width,the radial mixing ability and axial mixing ability of T-type agitator decrease,and the comprehensive performance is optimal when the blade width is reduced to 40 mm.Compared with other door-type agitator,the door-type agitator with two 135° oblique blades have stronger axial mixing ability,and the door-type agitator with two 45°oblique blades have the best radial mixing ability and the best comprehensive performance.2.The radial mixing and axial diffusion abilities of double-group agitator and single-group agitator are compared.The results show that,compare with single-group T-type agitator,double-group door-type agitator enhances the radial mixing of materials on the surface of the bed,has little effect on the radial mixing degree of materials inside the bed,and did not change the axial diffusion amount on the section.Compare with single-group door-type agitator,double-group door-type agitator can enhance the radial mixing of materials inside the bed,but do not change the radial mixing effect of materials on the surface of the bed.Furthermore,six kind of horizontal agitators including multiple-group agitator in three categories,including full T-type agitator combination,full door-type agitator combination,and T-type agitator+door-type agitator combination,are constructed to investigate their effects on the residence time distribution of polypropylene particles.The number of full mixing kettle N is calculated by the series model of multistage full mixing kettle to represent the degree of material flow towards flat pushing flow.It is found that the order of N values of the full mixing kettle is:T-type agitator+door-type agitator combination<T-type agitator combination<door-type agitator combination.The whole door-type agitator can make the material flow state more inclined to the horizontal push flow,but the agitator composed of Ttype agitator and door-type agitator makes the material flow state more close to the full mixing flow,which is not suitable for the horizontal agitator bed reactor.The optimal combination paddle agitators were determined as follows:full T-type agitators composed of 24 groups of T-type agitators of 40 mm width,and full T-type agitators composed of 12 groups of T-type agitators with two 45° oblique blades.3.By measuring and analyzing the change of torque signal in the stirring process,the influence law of operating conditions and agitator structure on startup torque and stable torque is revealed,and the prediction model of horizontal agitator torque is established based on the force analysis of agitator paddle.The results show that the startup torque and stable torque of T-type agitator decrease with the decrease of blade width,and blade width does not affect the ratio of startup torque and stable torque under high feeding coefficient.With the increase of the number of inclined blades,the startup torque of the door-type agitator decreases obviously,and the stable torque increases with the increase of the projected area of the blade in the direction of motion.For different types of agitators,the stable torques of T-type agitator and door-type agitator are close to each other under high feeding coefficient,but the startup torques and stable torques of T-type agitator are higher than those of door-type agitator due to the larger blade loading area.The ratio of startup torque to stable torque for T-type agitator is between 1.6 and 1.8,while that for door-type agitator is between 1.4 and 1.5.When increasing the number of agitators,the startup torque of double-group agitator is lower than that of single-group agitator with the same structure.Compared with double-group door-type agitator,the interaction of double-group T-type agitator in stable state has more obvious effect on the reduction of stable torque.The torque prediction model of horizontal agitator is established,and the torque variation during the stirring process is calculated by analyzing the force sources of blades.It is found that the main source of torque is shear resistance of wedge material on blade surface and inside the bed as well as gravity of the bed above the blade.The model is verified by the experimental results under the same conditions.The relative deviation is ±20%.The model can calculate the stable torque of horizontal agitator under different operating conditions,which provides a basis for motor selection. |
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