Influence of Polymeric Suspension Characteristics on the Particle Coating in a Spouted Bed

Abstract

The influence of the coating suspensions and particle properties on the coating process in a conventional spouted bed is presented. Glass beads were coated at fixed operating conditions with different formulations of aqueous polymeric coating suspensions in a spouted bed of laboratory scale. The wettability of the solids by the liquid was quantified by the contact angle and surface tension of the coating suspensions. The coating efficiency and particle growth were correlated with the adhesion of the coating suspension to the solid particle, which is a function of the solids and liquid characteristics. The physical properties of the coated particles—particle mean diameter, sphericity, bulk, absolute and apparent densities, porosity and flow velocity—were determined and compared to the properties of uncoated particles.     

Effect of cohesive forces on fluidized and spouted beds of wet particles

Fluid beds are now being used for processing pasty materials including production of fine powders through drying suspensions in beds of inert particles; coating of tablets or pellets; granulation, etc. In such processes, the fluid bed operation becomes more complex due to the development of cohesive forces resulting from liquid bridges between particles. Such forces can affect gas and solids flow leading to uncontrollable particle agglomeration and to poor gas–solid contact. This work is aimed at analyzing and quantifying the differences of flow behavior in fluidized and spouted beds of wet and dry particles. Experimentally, surface stickiness is induced by application of metered amounts of glycerol. Based on pressure drop vs. fluid flow rate curves, solids circulation rates and bed porosity variations, two types of particle–particle interaction forces are identified and their effect on air–solid flow is quantified as a function of glycerol concentration. Implications of these results in coating, granulation and drying of suspensions in these beds are also discussed.     

Developments in spouted bed technology

The spouted bed, though originally developed for drying of coarse granular solids, has, over the last decade, proved to be of interest for carrying out a remarkably wide variety of operations. These include not only other diffusional and/or thermal operations such as heating or cooling of solids, drying of solutions and suspensions onto inert bed particles, particle coating, and granulation, but also certain mechanical functions like solids blending, comminution and aerosol collection from a gas stream. The use of spouted bed reactors for several chemical processes — coal carbonization, shale pyrolysis, ore roasting, cement clinker production, and thermal cracking of petroleum — has also received attention. This paper presents a review of the above developments, supplemented in a few cases by recent results of research being carried out at the University of British Columbia. The particular features of a spouted bed which play the key role in each type of application are identified, some new applications are suggested and consideration given to the future industrial potential of individual processes.     

THE USE OF A SPOUTED BED IN THE FERTILIZER COATING OF SOYBEAN SEEDS

ABSTRACT

Soybean seeds were coated with fertilizer by the atomization of a fertilizer suspension using a spouted bed. The efficiency of coating adhesion was measured as the coating mass adhering to the seeds and the sprayed solids mass. The change in moisture mass between coated and uncoated seeds was considered in the evaluation of yield. A study of binder concentration and process time was conducted in order to discover conditions under which germination of coated seeds was feasible. Final experiments were done in order to quantify the influences of spouted bed air temperature and fertilizer suspension flowrate on the process yields and the germination capacity of coated seeds. Two empirical equations resulted from the use of Myers' surface technique (1976). These equations predict yield and germination for the range of variables studied.     

THE USE OF A SPOUTED BED IN THE FERTILIZER COATING OF SOYBEAN SEEDS

Soybean seeds were coated with fertilizer by the atomization of a fertilizer suspension using a spouted bed. The efficiency of coating adhesion was measured as the coating mass adhering to the seeds and the sprayed solids mass. The change in moisture mass between coated and uncoated seeds was considered in the evaluation of yield. A study of binder concentration and process time was conducted in order to discover conditions under which germination of coated seeds was feasible. Final experiments were done in order to quantify the influences of spouted bed air temperature and fertilizer suspension flowrate on the process yields and the germination capacity of coated seeds. Two empirical equations resulted from the use of Myers' surface technique (1976). These equations predict yield and germination for the range of variables studied.     

Statistical modelling of the spouted bed coating process using positron emission particle tracking (PEPT) data

Coating of particles larger than about 1 mm can be achieved in a spouted bed, a particle mobilization device in which a strong particle circulation occurs, rapidly upwards in a lean central “spout” region and downwards in a slowly moving annular settled bed. In a spouted bed coater, a spray nozzle is placed at the base of the spout, spraying upwards into a distinct coating zone. The coating formation in a spouted bed is inter alia a function of (i) the particle motion, that is, how often and where particles enter and traverse the coating zone and (ii) the extent of droplet collection by individual particles passing through the coating zone. The coating model proposed here is based on the statistical history of individual particles, whose projected area governs the collection of spray droplets in the coating zone. Positron emission particle tracking (PEPT) has been used to determine the particle trajectories, the distribution of cycle times and the size and voidage of the spout. Whilst the model is not capable of delivering absolute values of coating mass a priori, it can predict deviations from a mean, which can itself be determined from an overall mass balance. To validate the model, a spouted bed coating process was studied in which coarse PVC spheres were coated with the hot‐melt coating material polyethylene glycol (PEG) 1500. Coating mass distributions, derived from the weight data of individual particles before and after manual coating removal, compared (for the studied conditions) very well with the predictions of the model.     

Liquid‐particle surface properties on spouted bed coating and drying performance

Polymeric coating has been applied to particles for several reasons, which goes from enhancing product esthetics to control the release of chemicals. Spouted beds are among the equipments widely used to coat particles. Its choice is justified by the cyclic movement of the particles in stable spouting resulting in product homogeneity and good solid–fluid contact with high heat and mass transfer rates. Also, the spouted bed with inert particles is an alternative to drying pastes due to its low cost and applicability to low scale production. This work presents results of coating and drying efficiencies for different systems that combine 04 different solids and 03 formulations of suspensions. Materials having different surface tension were chosen and suspensions with varying surface tensions were formulated, resulting in different wettabilities (quantified by contact angles measurements). The results show the direct influence of these properties on the coating and drying performances.     

Coating and Drying in Spouted Bed: Influence of the Liquid-Particle Work of Adhesion

The aim of this work is to analyze the resulting process: coating or drying, and the respective fluid dynamic behavior, by bottom-spraying polymeric suspensions on a spouted bed of inert particles. Glass beads, ABS®, polypropylene (PP), and polystyrene (PS) were chosen as inert particles and were analyzed and characterized by their physicochemical properties. The polymeric suspensions were characterized by density, surface tension, rheology, and wettability. The fluid dynamic behavior of the bed was correlated with the particles and suspensions characteristics for each process: coating and drying. Each process performance was correlated with the polymeric suspension-particle work of adhesion.     

Coating and Drying in Spouted Bed: Influence of the Liquid-Particle Work of Adhesion

The aim of this work is to analyze the resulting process: coating or drying, and the respective fluid dynamic behavior, by bottom-spraying polymeric suspensions on a spouted bed of inert particles. Glass beads, ABS®, polypropylene (PP), and polystyrene (PS) were chosen as inert particles and were analyzed and characterized by their physicochemical properties. The polymeric suspensions were characterized by density, surface tension, rheology, and wettability. The fluid dynamic behavior of the bed was correlated with the particles and suspensions characteristics for each process: coating and drying. Each process performance was correlated with the polymeric suspension-particle work of adhesion.     

EFFECT OF PARTIAL PROCESSES OF DRYING ON INERT PARTICLES ON PRODUCT QUALITY

Particulate materials of high moisture content (suspensions, pulps) can be advantageously dried in the mechanically spouted bed (MSB)-dryer with inert packing. Due to the flow characteristics the partial processes of the inert bed drying (coating, drying, wearing of the dried coat) can be well controlled. The particle size of the product, which is one of the most important quality requirement is controlled e.g. by the wearing time in the inner conveyor screw, that is by its rotation speed and conveying length. However, this latter can be increased only simultaneously with the spouted bed height together with pressure drop increase. To avoid this drawback a modified MSB-dryer was developed. By this way the particle-size can be controlled in vider range, independently from the bed height. The results were demonstrated by drying tests carried out with microwave pretreated potato pulp.     

Particle velocity and solids circulation rate in a jet-spouted bed

Radial distribution of particle velocities, solids circulation rate and radial variation in bed voidage in a jet-spouted bed were investigated in a 0.2 m diameter conical-cylindrical column. The upward particle velocity is strongly dependent on gas velocity and its radial profile is the Gaussian. The solids circulation rates are of the same order of magnitude as those in a conventional spouted bed and are proportional to the gas velocity. Estimation of the variation in bed voidage in the radial direction revealed that the annulus voidage of about 0.90 was greater than the spout voidage of around 0.70. This tendency is contrary to that in a conventional spouted bed.     

COATING OF UREA WITH AN AQUEOUS POLYMERIC SUSPENSION IN A TWO-DIMENSIONAL SPOUTED BED

In this work, a process of coating of urea with polymeric film was developed and analyzed experimentally in a two-dimensional spouted bed. An experimental design was utilized to investigate the effects of operational variables, e.g., air flow rate, air temperature, atomizing air pressure and coating suspension flow rate, on the efficiency of the process, particles growth, particles growth kinetics and physical properties. The results showed that the efficiency of the process and particle growth characteristics were strongly influenced by the operational variables. Coating of the polymeric suspension of Eudragit® improved up to four times the moisture holding capacity of coated urea in relation to uncoated urea. The bulk density, grain size and sphericity were found to be independent of operational conditions.     

INSTRUCTIONS FOR PREPARATION OF MANUSCRIPTS FOR DIRECT REPRODUCTION (REVISED, 1992)

ABSTRACT

A software for simulating the drying operation of diluted suspensions in conical spouted beds of inert particles has been developed. This computer program combines the fluid flow and the solids circulation models with the drying kinetic equations. The simulated results for drying animal blood in a spouted bed of inert particles compare with data reported in the literature.     

Hydrodynamic behaviour of a draft tube gas-liquid-solid spouted bed

Experiments were conducted using various types of solid particles to investigate the hydrodynamic properties of a gas-liquid-solid spouted bed with a draft tube. The hydrodynamic properties under study include flow modes, pressure profile and pressure drop, bubble penetration depth, overall gas holdup, apparent liquid circulation rate and bubble size distribution. Three flow modes were classified: a packed bed mode, a fluidized bed mode and a circulated bed mode. It was found that the friction factor accounting for the friction loss in the bed varies linearly on a logarithmic scale with the Reynolds number defined based on the apparent liquid circulation rate. The bubble penetration depth in the annular region, overall gas holdup and apparent liquid circulation rate increase with an increase in gas or liquid velocity. At high gas flow conditions an optimal solids loading exists which yields a maximum apparent liquid circulation rate. A model was proposed to describe the liquid circulation behaviour in the draft tube three-phas spouted bed. The average bubble size in the draft tube region is higher than that in the annular region for both the dispersed bubble regime and the coalesced bubble regime in the draft tube region.     

超细粉在导向管喷动床中的固体循环速率

Ultra-fine powders are difficult to be fluidized due to the strong particle to particle cohesiveness.However, the authors‘ experiments showed that the ultra-fine powder CaCO3 could be stably fluidized in a spouted bed with a draft tube. The effects of geometric and operating parameters on solid circulation rate of ultra-fine powder CaCO3 were investigated in a 120 mm diameter transparent semicircular spouted bed with a draft tube. Three draft tubes with different sizes were used in this study. It was found that the solids circulation rate was mainly dependent on the drawing rate of the gas jet from the nozzle, then on the gas transport capacity in the draft tube. With increasing gas feed rate, distance between the nozzle and the draft tube inlet and draft tube diameter, the solids circulation rate could be increased. Based on the jet theory, a quantitative correlation was proposed for predicting the solid circulation rate of ultra-fine powders in a spouted bed with a draft tube by taking into account the gas transport capacity in the draft tube.     

喷动床内气固二相流体动力行为颗粒动理学

应用气固二相双流体模型数值模拟了喷动床内流体动力行为,模型中采用稠密固相动力-摩擦应力模型。模型中同时考虑了动理学理论和摩擦应力理论。应用贴体坐标系使得网格与喷动床的倒锥体边界符合良好。模拟得到的喷动床内颗粒速度和浓度实验数据吻合较好。     

Heat transfer in a novel type spouted bed

A new method for spouted bed drying of coarse and even fine particulate solids involves introducing the gas tangentially and maintaining the uniform particle recirculation by a conveyor screw. Some flow characteristics are compared with those of a conventional spouted bed. The particle-fluid heat transfer was investigated in the annulus and spout respectively.     

Pressure drop,bed expansion and liquid holdup in a three phase spouted bed contactor

Data on the pressure drop, bed expansion and liquid holdup in a three phase spouted bed contactor with an initial bed height of 243 mm. were obtained as a function of the gas and liquid flowrate. Polyethylene spheres 10 mm. in diameter with a density of 320 kg/m³ were spouted in a 194 mm. column using a 30 mm. nozzle. The spouted bed contactor with gas and liquid mass flow-rates of 2.18 and 1.88 kg/m² sec, respectively had similar pressure drop per unit area of particle surface, total liquid holdup per unit volume of operating bed, and “active” holdup, as a fluidized contactor.     

Simulation Of Drying Suspension In A Conical Spouted Bed

A software for simulating the drying operation of diluted suspensions in conical spouted beds of inert particles has been developed. This computer program combines the fluid flow and the solids circulation models with the drying kinetic equations. The simulated results for drying animal blood in a spouted bed of inert particles compare with data reported in the literature.     

Boehmite Coating as a Consolidation and Forming Aid in Aqueous Silicon Nitride Processing

Silicon nitride powders were coated by boehmite via a solgel process to improve the consolidation and forming of aqueous silicon nitride suspensions. The coated silicon nitride suspensions had a significantly higher solids loading than the pure silicon nitride in water. Viscosity measurements and centrifugation showed that the coating changed the long-range interaction between the silicon nitride particles, whereas the pressure filtration study indicated that the oxide coating modified the short-range interaction at particle contacts. Preliminary rheological studies indicated that at pH 3, the boehmite-coated silicon nitride suspensions can gel, as indicated by a wide linear viscoelastic region in dynamic rheological tests. The gelation of coated silicon nitride at pH 3 offers the potential for net-shape forming. Our study establishes a water-based green body consolidation and forming technique of silicon nitride without polymeric additives or organic solvents.