Liquid Transport Mechanism During Liquid-containing Fluidization

Liquid-containing fluidization technology has been widely used in industrial process.Such processes require that liquid forms uniform distribution among particles,so as to achieve the goal of stable operation of equipment and improvement of product quality.However,the liquid always enters the bed from the discrete position.The local high liquid content promotes the formation of the particle agglomerates.In the fluidization process,the agglomerates are broken and the liquid moves to the dry area.Therefore,the liquid can reach uniform distribution after the transport process.Therefore,it is of great significance to study the mechanism of liquid transport in a liquid-containing fluidized bed.In this paper,through the experimental method,the bed expansion experiment of wet particles is carried out,and the liquid transport pattern in the bed is identified,which paves the way for the study of liquid transport.On this basis,the study of liquid transport process in wet particle fluidized bed is carried out.The main result is as follow:(1)Based on the study of the bed expansion phenomenon in the fluidization process of liquidcontaining Geldart group B particles,a method for judging the liquid transport pattern in the bed by the average expansion ratio is proposed.It is pointed out that the extreme point of the average expansion ratiothe ratio of liquid to solid curve is the critical ratio of liquid to solid for distinguishing free liquid from trapped liquid.The effects of particle diameter,liquid viscosity and liquid surface tension on the critical ratio of liquid to solid are studied,It is found that the critical ratio of liquid to solid increases with the particle diameter and decreases with the surface tension.Viscosity has no obvious effect on the critical ratio of liquid to solid.On the basis of the above research results,the regime map of liquid transport pattern is drawn by combining Bond number and ratio of liquid to solid,and the correlation between the critical ratio of liquid to solid and Bond number is fitted.Thus,the method and guidance are provided for judging the liquid transport pattern in the liquid-containing fluidized bed.(2)In order to explain the rationality of the method of judging the liquid transport pattern by bed expansion process,and to further study the internal reason of the bed expansion process,the aeration mechanism of the bed under liquid bridge force is explored combined with the bed expansion phenomenon in free liquid regime,and the effects of particle diameter,liquid viscosity and liquid surface tension on the bed expansion ratio are explored;The expansion ratio increases with the increase of surface tension;The viscosity has no obvious effect on the expansion ratio.The mathematical model of bed expansion process is established.The external stress of gas acting on bed and the tensile strength produced by liquid bridge force are compared to calculate the theoretical value of complete fluidization velocity.The results are compared with the experimental results and they are in good agreement,thus the internal mechanism of enhancing the aeration capacity of emulsion phase in liuqid-containing fluidized bed is the balance of liquid bridge force and excess drag force,in addition,it shows that the liquid in the free liquid regime forms a uniform distribution between particles,thus theoretically explaining the rationality of the judgment method of liquid transport pattern.On this basis,the complete fluidization velocity of the bed in the free liquid regime is predicted,which increases rapidly with the ratio of liquid to solid and then remains stable.Through sensitivity analysis,it is found that the surface tension has the most obvious effect,followed by the superficial air velocity and particle diameter,while the ratio of liquid to solid has the least effect.(3)Based on the correlation between critical ratio of liquid to solid and Bond number,the free liquid regime and trapped liquid regime of the system of calcium chloride solution and glass bead are determined,and the experimental study of liquid transport process is carried out.In the trapped liquid regime,the spatially and temporally evolution process of free liquid content is reconstructed by conductance method.At the same time,combined with pressure fluctuation signal analysis and digital image processing method,the macro fluidization characteristics and particle circulation characteristics in the bed are obtained,and then the internal physical mechanism of liquid transport behavior in the bed is revealed.The results show that the conversion of trapped liquid to free liquid is the main mechanism of liquid transport in the bed,which is further confirmed by the comparison of attractors.The transport process of free liquid is from top to bottom,and from the center to both sides.The bubble diameter curve shows that the bubbles in the middle and upper part of the bed have stronger effect,and it is easier to break the agglomerates and release free liquid,so that the content of free liquid in the middle and upper part of the bed is higher.On this basis,the effects of superficial air velocity,particle diameter,liquid viscosity and liquid surface tension on liquid transport process are studied;It is easy to form stable agglomerates by reducing the particle diameter,in addition,at the same fluidization number,the decrease of bubble stress results in the decrease of free liquid content;Increasing the viscosity of liquid enhances the stability of agglomerates and reduces the uniformity of liquid distribution;Reducing the surface tension of the liquid reduces the strength of the particle agglomerates,thus promoting the release of free liquid.