A model of surface renewal with application to fluid bed coating of particles

A model of surface renewal is developed based upon the concept that praticles can move randomly between inactive and active domains within a contacting device. This type of model has application to particulate systems where reactions are occurring on the particle surface. Application is made of these concepts to fluid bed coating of particles. Fluid bed coating of particles is important as a process to enable time release capsules where time release behaviour is associated with the coating weight distribution.     

Mixing of particles in rotary drums: A comparison of discrete element simulations with experimental results and penetration models for thermal processes

The transverse mixing of free flowing particles in horizontal rotating drums without inlets has been simulated by means of the Discrete Element Method (DEM) in two dimensions. In the simulations the drum diameter has been varied from 0.2 to 0.57 m, and the rotational frequency of the drum from 9.1 to 19.1 rpm, for drum loadings of 20% or 30%, and average particle diameters of 2.5 and 3.4 mm. The choice of operating parameters allows for comparison with experimental data from literature. Though simple models for inter-particle interactions have been implemented, the overall agreement is good. The results are presented and discussed in terms of mixing times and mixing numbers that means numbers of revolutions necessary for uniform mixing of the solids. In this way, comparison with penetration models, as typically applied to modelling of thermal processes, is possible. The limitations of such continuum models are pointed out, along with the potential of DEM to replace them, in the long term.     

Inter-tablet coating variability: Residence times in a horizontal pan coater

This paper investigates inter-tablet coating variability, specifically, tablet residence times within the spray zone. Discrete element method computer simulations, experiments, and analytical investigations are performed to measure the residence time per pass, the circulation time, and appearance frequency of spherical shaped tablets for a range of pan speeds and tablet loads. In addition, the fractional residence time, defined as the ratio of time spent by a tablet in the spray zone to the total coating time, is measured. The average fractional residence time (averaged over all the tablets in the bed) is found to be equal to the ratio of the time-averaged number of tablets exposed to the spray to the total number of tablets in the pan, a result that is consistent with analyses. The average fractional residence time is observed to be independent of pan speed and total coating time. Furthermore, the fractional residence time is shown to be related to the residence time per pass and circulation time per pass. Appearance frequency is defined as the number of appearances a tablet makes in the spray zone per pan rotation. Simulations and analyses show that appearance frequency decreases with increasing pan speed. Circulation time per pass for a tablet is defined as the average time between successive appearances in the spray zone and residence time per pass is defined as the average time spent in the spray zone per pass. These various measures are all related, but from the standpoint of developing an analytical model for coating variability, fractional residence time is a more useful and intuitive parameter as it determines the fraction of total run time that a tablet spends in the spray. This paper concentrates on determining average fractional residence times and residence time per pass, while the inter-tablet variability is more closely related to the standard deviation of the fractional residence time.     

Experimental study on fluidization of fine powders in rotating drums with various wall friction and baffled rotating drums

Fine powders were found to be fluidized in a rotating drum by internal cycling gas by the drum rotation. It is essentially a fluidized bed without requiring any external fluidizing gas. Such a rotating drum can be regarded as a new gasless fluidized bed for fine powders in contrast to a traditional fluidized bed, possibly leading to a considerable amount of energy savings. In addition, the fluidization quality of fine powders was found to be further improved with the assistance of drum rotation because of the shearing movement among particles that eliminates channeling and cracks and possibly also breaks agglomerates. Five regimes were identified in the rotating drum including slipping, avalanching-sliding, aerated, fluidization and re-compacted regimes. It was also found that drum wall friction plays an important role to fluidize fine powders because the friction carries particles to the freeboard, leading to gas cycling that fluidizes the powders. As well, three types of specially designed baffles were utilized to promote powder fluidization in rotating drums. These baffles effectively bring an early onset of all the regimes in rotating drums by reducing powder-wall slipping, carrying particles and bringing additional gas to the powders.     

Soft-sensors for prediction of impact energy in horizontal rotating drums

Knowledge of internal variables in horizontal rotating drums is useful for the optimisation and control of industrial processes including milling, mixing, coating and agglomeration. The energy of particle-particle (p-p) impacts, which is the focus of this paper, is a variable of particular interest because of its relevance to the efficiency of these processes. For example the comminution rate in a grinding mill or the structural and chemical transformations when alloying in mills are both determined by the impact energy of particles. There are two main obstructions that prevent the measurement of internal variables in horizontal rotating drums: the harsh internal environment, which makes internal installation of sensors for online measurement impractical, and the lack of sensors that can directly measure the internal variable of interest. The above difficulties warrant the implementation and development of soft-sensors to predict the particle-particle impact energy from variables that can be physically measured outside the drums. Based on a discrete element method (DEM) model, such a soft-sensor model is developed for horizontal rotating drums that can quantitatively predict p-p impact energy within 5% of its actual value.     

转台式三鼓成型机鼓轴换位重合度探讨

从成型机转台的结构和运动过程出发,提出了转台式三鼓成型机鼓轴换位重合度的精度概念,并详细讨论了重合度的研究对象以及设计了重合度的测量方法,文末讨论了重合度精度在实际过程中受到的影响。本文只研究重合度的对象和测量方法,不研究重合度的测量计量、测量精度设计、测量误差等内容。     

A study on the transition between neighbouring drum segregated bands and its application to functionally graded material production

The composition transitions between two neighbouring segregated bands in a rotating drum were studied using binary glass beads of 6 different size ratios. The concentration gradient of the coarser particles in the transition zone increased dramatically when the particle size ratio increased from 1.68 to 2.01 and then gradually decreased as the particle size ratio increased from 2.01 to 3.37. The concentration gradient profile could be explained by the differences of the dynamic angles of repose between the two components of the binary mixture. A novel, convenient and particulate-level composition controlled FGM processing method is proposed based on the transition zone structure between two neighbouring segregated bands. A Cu/Al₂O₃/Cu functionally graded material was successfully produced based on the proposed method using binary mixture of Cu and Al₂O₃ powders.     

A study of the hold-up in rotary drums with discharge end constrictions

This paper describes the results of a detailed investigation of operating conditions on the hold-up of material in a rotating drum. It was found that the hold-up increases linearly with increasing feed rate, decreases linearly with increasing slope of the drum, and behaves in a complex manner with increasing drum speed. The combined effect of some of these operating variables on the hold-up was also studied and interpreted in terms of the type of flow of material in the drum. A semi-empirical model was adopted to predict the hold-up of material in a horizontal rotating cylinder with an end constriction.     

Application of incomplete similarity theory for estimating maximum shear layer thickness of granular flows in rotating drums

Granular flow in a rotating drum provides a convenient system for investigating mixing, segregation and general properties of granular materials. This study is concerned with the maximum thickness of the flowing surface layer observed at the rolling regime. A scaling relation is first derived with the consideration of incomplete similarity associated with the drum-particle size ratio and the Froude number. Calibration is then carried out with published laboratory data, which were collected for the case of rotating drums half-filled with glass beads. The scaling relation is also compared with other kinds of datasets, showing good agreement and possibilities of the proposed approach to be further extended to more complex cases.     

Numerical Spray Model of the Fluidized Bed Coating Process

In this study, a new model for the batch top-spray fluidized bed coating process is presented. The model is based on the one-dimensional (axial) discretization of the bed volume into different control volumes, in which the dynamic heat and mass balances for air, water vapor, droplets, core particles, and coating material were established. The coupling of the droplet phase's mass and heat transfer terms with the gas and solid phases was established by means of a droplet submodel in which droplet trajectories were individually simulated.

The model calculation method combines a Monte Carlo technique for the simulation of the particle exchange with the first-order Euler's method for solving the heat and mass balances, enabling the prediction of both the dynamic coating mass distribution and the one-dimensional (axial) thermodynamic behavior of the fluidized bed during batch operation. The simulation results were validated using experimental two-dimensional spatial air temperature and air humidity distributions, which were measured in a fluidized bed pilot reactor using a scanning probe.

Sensitivity analysis was carried out to study the effect of controllable process variables, such as fluidization air and atomization air properties, as well as the properties of the spraying liquid upon the simulated dynamic temperature and humidity distributions. Also, the effects of relevant process variables on growth rate uniformity and process yield were studied. Based on these sensitivity studies it was concluded that nozzle parameters, such as air pressure and positioning with respect to the bed, are as important as the fluidization air properties (humidity, temperature, and flow rate) for the coating growth rate uniformity and process yield.     

Numerical Spray Model of the Fluidized Bed Coating Process

In this study, a new model for the batch top-spray fluidized bed coating process is presented. The model is based on the one-dimensional (axial) discretization of the bed volume into different control volumes, in which the dynamic heat and mass balances for air, water vapor, droplets, core particles, and coating material were established. The coupling of the droplet phase's mass and heat transfer terms with the gas and solid phases was established by means of a droplet submodel in which droplet trajectories were individually simulated.

The model calculation method combines a Monte Carlo technique for the simulation of the particle exchange with the first-order Euler's method for solving the heat and mass balances, enabling the prediction of both the dynamic coating mass distribution and the one-dimensional (axial) thermodynamic behavior of the fluidized bed during batch operation. The simulation results were validated using experimental two-dimensional spatial air temperature and air humidity distributions, which were measured in a fluidized bed pilot reactor using a scanning probe.

Sensitivity analysis was carried out to study the effect of controllable process variables, such as fluidization air and atomization air properties, as well as the properties of the spraying liquid upon the simulated dynamic temperature and humidity distributions. Also, the effects of relevant process variables on growth rate uniformity and process yield were studied. Based on these sensitivity studies it was concluded that nozzle parameters, such as air pressure and positioning with respect to the bed, are as important as the fluidization air properties (humidity, temperature, and flow rate) for the coating growth rate uniformity and process yield.     

A flowsheet model for the development of a continuous process for pharmaceutical tablets: An industrial perspective

A dynamic model of a continuous direct compression process for pharmaceutical tablets is presented. The objective is to assess the impact of the variability from the feeder system on the concentration of drug in the powder in the feed frame of a tablet press. The model is based on principles of dispersed flow from the reaction engineering field. An estimability analysis was performed to understand the impact of the available measurements on the estimated parameters and suggest better ways to approach the parametrization. Predictions are successfully contrasted with experimental data. The model is used to produce residence time distributions at different process conditions and a graphical representation of the allowable range of disturbances in the feeders that can be mitigated by the process. The model was used in support of the method development for an online near infrared sensor. © 2017 American Institute of Chemical Engineers AIChE J, 64: 511–525, 2018     

A mathematical model developed to predict the chemistry and corrosion rate in a crevice of variable gap

A model is developed to predict the chemistry, corrosion potential and rate of pipeline steels in a crevice formed when a coating disbonds from a pipe surface. The gap of the coating disbonded region is assumed to vary with distance from the mouth. The effect of this gap variation on the chemistry and corrosion rate in the coating disbonded region is investigated in this study through modeling. The preliminary model results suggest that overall, the variation of the disbondment gap with distance has an insignificant effect on the pH, corrosion potential and rate in the coating disbonded region. Unlike some conventional crevice corrosion often associated with a large cathode-to-anode area ratio, the area ratio here is rather relatively small and the pH commonly falls in the neutral or alkaline range. Within this pH range, even if the pH varies within a few units across the crevice length, the variation of the crevice corrosion rate is not significant. This paper reports on and discusses the fundamental principles used for the model, some key model results, and the practical implications of the results.     

A carbon activation model with application to longan seed char gasification

In this paper a new structural model is presented to describe the evolution of porosity of char during the gasification process. The model assumes the char structure to be composed of bundles of parallel graphite layers, and the reactivities of each layer with the gasification agent are assumed to be different to represent the different degree of heterogeneity of each layer (i.e. each layer will react with the gasification agent at a different rate). It is this difference in the reactivity that allows micropores to be created during the course of gasification. This simple structural model enables the evolution of pore volume, pore geometrical surface area and the pore size distribution to be described with respect to the extent of char burn-off. The model is tested against the experimental data of gasification of longan seed-derived char with carbon dioxide and it is found that the agreement between the model and the data is reasonably satisfactory, especially the evolution of surface area and pore volume with burn-off.     

In-situ generation and application of nanocomposites on steel surface for anti-corrosion coating

Thin organic coatings directly on steel sheets provide excellent barrier protection in saline environment and meet deformability demands, but fail in providing active corrosion protection. We have put an effort to solve this problem by formulating composite coatings using in-situ generation of metal oxide nanoparticles (NPs) in the polymer matrix. Here we present a new synthesis method of high performance polyetherimide composite with TiO₂, MgO, and Al₂O₃ nanoparticles and their application for anti-corrosion coatings in saline environment. We observed that in-situ synthesis of these metal oxide NPs in the polymer curing process leads to evenly distribution and uniform size of nanoparticles. Thermo-mechanical property was analyzed for these three kinds of free-standing composite film to assess elasto-plastic behaviour and compared to mother polymer film. Results indicated that thermal stability and elastic behaviour of composites film are not affected to the great extent by the presence of NPs. The potentiodynamic and the electrochemical impedance studies on these composite coated steel panels were carried out to identify active–passive behaviour. Results showed active corrosion protection from nanocomposite coating based on TiO₂ and barrier protection was noticed from nanocomposite coating based on MgO and Al₂O₃.     

星点设计-效应面优化法优化阿司匹林肠溶片的包衣工艺

目的：采用星点设计一效应面优化法优化传统包衣锅制备阿司匹林肠溶片包衣工艺,提高其包衣效率。方法：通过筛选雾化压力、蠕动泵转速及枪一床距离3个工艺参数确定各因素水平,安排实验,结果以包衣效率为指标考察。结果：优化后的包衣工艺可有效的进行包衣。结论：星点设计一效应面优化法可利用最少的实验次数得到优化后的包衣工艺。     

Analysis of granule breakage in a rotary mixing drum: Experimental study and distinct element analysis

Rotary drums are commonly used in particulate solid industries for mixing, coating and reactions. The process is often accompanied by undesirable breakage of granules. For this reason, a scaled-down version is sometimes used as an attrition testing device. In this work, the attrition of granules inside a rotary drum at 18, 35 and 52 rpm drum rotation speeds for 4000 cycles is studied. The granules used in this study have been produced by extrusion and spheronisation with a size range of 500 to 1000 μm. The rotary drum has an internal diameter of 0.39 m, axial length of 0.3 m and a single baffle. The extent of breakage is quantified by sieving out fine debris which is two sieve sizes smaller than the feed particles. To relate the extent of breakage in the drum to granule characteristics, single granule impact tests have been performed on one type of granule at several velocities. The effects of particle size and impact velocity are analysed and a power-law relationship is fitted between impact velocity and single granule breakage. This information is then used to simulate granule breakage in a rotary drum by Distinct Element Method (DEM). The drum is simulated for 5 rotations at the rotational speeds stated above and the breakage rate is extrapolated to 4000 cycles where it is compared to experimental results obtained. The trends for particle breakage in both experiments (determined by sieving) and extrapolated DEM simulations are in agreement however the orders of magnitudes are different. The comparison shows that the extent of breakage obtained from extrapolated simulations is overestimated at drum speed of 35 and 52 rpm and underestimated at 18 rpm. There is close agreement between experiments and extrapolated DEM simulations for particle breakage at 18 rpm only after 4000. Furthermore, the effect of air drag on the attrition of granules by impact at a drum rotation speed of 52 rpm is investigated, where it is found to significantly reduce the breakage results.