Monte Carlo modeling of fluidized bed coating and layering processes

A stochastic modeling approach based on a Monte Carlo method for fluidized bed layering and coating is presented. In this method, the process is described by droplet deposition on the particle surface, droplet drying and the formation of a solid layer due to drying. The model is able to provide information about the coating coverage (fraction of the particle surface covered with coating), the particle‐size distribution, and the layer thickness distribution of single particles. Analytical solutions for simplified test cases are used to validate the model theoretically. The simulation results are compared with experimental data on particle‐size distributions and layer thickness distributions of single particles coated in a lab‐scale fluidized bed. Good agreement between the simulation results and the measured data is observed. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2670–2680, 2016     

Evaluation of the morphological and release characteristics of coated fertilizer granules produced in a Wurster fluidized bed

In the present study, coated fertilizer granules were produced in a Wurster chamber of a fluidized-bed equipment for controlled-release agrochemical applications. The coating material was a commercially available latex of poly(vinylidene-chloride). The influence of the process parameters (e.g., spray rate of the polymer latex, fluidization air velocity, distance of the partition from the air inlet and perforation percentage of the plate at the air inlet) on the morphology and quality of the coating was thoroughly investigated. The surface characteristics of the coated fertilizer granules were examined by scanning electron microscopy (SEM). The release rate of the fertilizer's nutrients from the coated granules was determined by kinetic-release experiments carried out in distilled water. Depending on the selected process conditions, the coating thickness could vary from 25 to 65 μm, while the surface of the coated granules changed from completely smooth to rough and porous. It was shown that by suitable selection of the coating conditions in the Wurster process, controlled-release fertilizers exhibiting prolonged release profiles could be produced.     

Measurement of particle concentration in a Wurster fluidized bed by electrical capacitance tomography sensors

It is essential to measure and monitor the particle flow characteristics in a Wurster fluidized bed to understand and optimize the coating processes. In this article, two electrical capacitance tomography (ECT) sensors are used to measure the particle concentration in different regions in a Wurster fluidized bed for the “cold” particle flows. One ECT sensor has a 12‐4 internal‐external electrodes and another has eight electrodes. The 12‐4‐electrode ECT sensor is used to measure the particle concentration in the annular fluidization region (outside of the Wurster tube) and the eight‐electrode ECT sensor is used to measure the particle flow in the central region (inside the Wurster tube). The effect of particle type, particle moisture, fluidization velocity, and geometrical parameters on the Wurster fluidization process is studied based on the two ECT measurements. The radial particle concentration profiles in the annular fluidization and central flow regions with different operation parameters are given. Fast Fourier Transform analysis of the particle concentration in the Wurster tube is performed with different superficial air velocities. The optimum operating ranges of the Wurster fluidization process for different particles are given. In the end of the article, computational fluids dynamics simulation results are given and used to compare with the measurement results by ECT for a typical Wurster fluidized bed. © 2014 American Institute of Chemical Engineers AIChE J 60: 4051–4064, 2014     

Helping to choose operating parameters for a coating fluid bed process

The feasibility of solid particles coating in a fluid bed with a Wurster tube is studied for several types of particles and aqueous coating solutions. The model products are wheat semolina, beads of glass, alumina, resin polystyrene, plastic PMMA, with a size range between 125 and 1250 μm and densities between 500 and 2500 kg m⁻³. The chosen coatings are representative of those used for the food products, such as maltodextrin, acacia gum, and sodium chloride in aqueous solution.

The air flow rate suitable for a regular circulation of particles in the reactor is determined for each particle type. For each coating solution, the flow rate leading to agglomeration is considered as the maximal limit flow rate to use for coating. Then comparative coating experiments were realized.

For a similar initial load of particles, the same mass of coating was atomized (13.5 g min⁻¹) at 50 °C. The mass of coating deposit on particle surface is increased linearly during an atomization sequence lasting 33 min. For example, for every 100 g of alumina particles, the rates are 0.48, 0.51, and 0.53 g min⁻¹ for sodium chloride, maltodextrin, and acacia gum, respectively. We then obtain a coating efficiency between 87% and 98%.

In the specific case of sodium chloride on glass beads, the deposit of crystallized salt was linear during 10 min then stopped. Addition of acacia gum (50%) to the NaCl coating solution leads again to a linear deposit over 65 min.     

Fine particle coating by a novel rotating fluidized bed coater

Fine particle coating has been conducted by using a novel rotating fluidized bed coater. The coater consists of a plenum chamber and a horizontal porous cylindrical air distributor, which rotates around its axis of symmetry inside the plenum chamber. Cohesive fine cornstarch (mass median diameter of 15 μm), a Geldart Group C powder, was used as core particle and an aqueous solution of hydroxypropylcellulose (HPC-L) was sprayed onto the cornstarch to generate a film coating. Fine particle coating was conducted under various coating levels (wt.% HPC-L) and the particle size distribution of the coated particles, release rate of an aqueous pigment (food blue No. 1), which had been pre-coated onto the initial cornstarch, and the degree of agglomeration were investigated. The relationship between the coating level and the physical properties of the coated particles was analyzed. The results indicated that coating of cohesive fine cornstarch with HPC-L could be achieved, producing a favorable prolonged release property with almost maintaining the individual single particle.     

Application of nano-particle coatings to carrier particles using an integrated fluidized bed supercritical fluid precipitation process

Production of active ingredients such as pharmaceuticals in nano-particulate form is highly desirable but the resulting product is difficult to handle and to use in applications. A novel process is described for coating nanoparticles onto excipient particles of c. 300 μm by rapid expansion of a supercritical solution (RESS) into two types of modified proprietary equipment: a Wurster coater and a fluidized bed. This novel approach has been demonstrated through the successful deposition of six mimics for active ingredients (benzoic acid, adamantane, ferrocene, phenanthrene, stearic acid and vitamin K3) on carrier excipient particles of microcrystalline cellulose (MCC). Evidence from SEM, EDX and Confocal Raman microscopy suggests that the coating particles are below 30 nm in size. Unlike most conventional coating processes, this approach avoids the use of liquids and high temperatures. As a wide range of actives and excipients can potentially be employed, the approach is applicable across the process and product industries, in particular pharmaceutical, household goods, personal care and catalyst industries.     

多层筛板流化床料层高度均匀性研究

通过测定流化床料层压差,研究了多层流化床顶层进料均匀性、进气及排气方式对多层流化床流化料层高度均匀性的影响。结果表明,布料越均匀,上中二料层高度越接近,且在低气速下易于形成较良好的流化层,但随着气速增大,其影响逐渐减弱;在低气速下进气方式对中下二料层均匀性有较大影响,但随气速增大影响也减弱,底部进气方式更易达到较好的料层均匀性;流化床的排气方式对流化料层基本无影响。因此均匀的布料和较均匀的进气预分布有助于均化各料层高度,并拓宽多层流化床的操作弹性。     

Monte Carlo simulation of particle breakage process during grinding

The Monte Carlo method is quite useful in the modeling of particulate systems. It is used here to simulate the particle brekage process during grinding that can be represented by a population balance equation. The simulation technique is free from discretization of time or size. The results of simulation under restricted conditions of grinding compare very well with the available analytical solution of the population balance equation. The procedure is extended to simulate the grinding process in its entirety. This method provides an alternative to the modeling of the grinding process where the governing population balance equation cannot be readily solved.     

Effect of solid circulation rate on coating efficiency and agglomeration in circulating fluidized bed type coater

Circulating fluidized bed was proposed to be used as a coater, and coating experiments of glass beads with silica powder were performed in a circulating fluidized bed. Glass beads and silica powder were chosen as model particles, because their shape was almost spherical. The respective effects of gas flow rates supplied from a distributor and from an air nozzle for solid circulation, feed rate of powder suspension and particle content in the bed on coating efficiency and agglomeration are mainly discussed. Coating efficiency in circulating fluidized bed coater was correlated well with solid circulation time rather than with gas flow rates or solid circulation rate, while the agglomeration among core particles was mainly governed by solid circulation rate.     

Structure optimization of diamond personal dosimeters based on Monte Carlo simulations

We have studied the usefulness of diamond personal dosimeters for X-rays and gamma-rays by using a Monte Carlo simulation method. On one hand, the energy responses (ERs) calculated for both diamond and silicon with various thicknesses as a function of incident photon energy indicate that diamond yields much more suitable ERs for personal dosimeters in the energy range from 30 keV to 1.25 MeV than silicon when used in the current measuring mode. This is because the atomic number of diamond is close to the effective atomic number of the human soft tissue. On the other hand, we have found that diamond dosimeters with allowable energy-independent responses in the pulse counting mode can be fabricated for the photon energies ranging from 60 keV to 1.5 MeV by optimizing the layered structures of various materials such as epoxy resin, polyimide, Al and Cu located on the diamond layer sensing the incident photons. The advantage of diamond in terms of dosimeter performance is discussed.     

流化床甲烷化反应动力学模拟

利用AspenPlus结合Fortran编写的动力学子程序对流化床甲烷化反应进行建模,结果显示该模型与实验值吻合较好。基于该模型考察了不同工艺条件对cH4收率影响,研究表明：随着反应温度升高,CH4收率逐渐降低,压力越小,cH4收率下降越明显;原料气流量对甲烷化反应影响较小,增加流量,CH4收率呈现降低趋势;当氢碳比在2．7—3．0范围内,CH4收率增幅较大,氢碳比大于3．0时,CH4收率增加缓慢。     

Full‐physics simulations of spray‐particle interaction in a bubbling fluidized bed

Numerical simulations of a gas‐particle‐droplet system were performed using an Euler‐Lagrange approach. Models accounting for (1) the interaction between droplets and particles, (2) evaporation from the droplet spray, as well as (3) evaporation of liquid from the surface of non‐porous particles were considered. The implemented models were verified for a packed bed, as well as other standard flow configurations. The developed models were then applied for the simulation of flow, as well as heat and mass transfer in a fluidized bed with droplet injection. The relative importance of droplet evaporation vs. evaporation from the particle surface was quantified. It was proved that spray evaporation competes with droplet deposition and evaporation from the particle surface. Moreover, we show that adopting a suitable surface coverage model is vital when attempting to make accurate predictions of the particle's liquid content. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2569–2587, 2017     

A multistep method for steady-state Monte Carlo simulations of polymerization processes

Identifying the microscopic information of polymers is of great significance for polymerization processes. Monte Carlo (MC) simulation is a powerful tool to predict the microscopic structure of polymers. Currently, most MC methods are designed for dynamic polymerization processes based on time evolution. The study on MC simulation for steady-state processes is scarce and current approaches face challenges in addressing complex mechanisms. In this work, a multistep method is proposed for the steady-state MC simulation. By introducing the “buffer pool” concept, the proposed method is computationally efficient and flexible to derive accurate predictions for processes with various polymerization mechanisms. Three applications with increasing complexity in the kinetic mechanisms, including both linear and branching polymerizations, are presented to demonstrate the applicability of the proposed method.     

Study of tablet-coating parameters for a pan coater through video imaging and Monte Carlo simulation

The movement of tablets in a pan coater and the exposure of different surfaces of tablets for deposition of coatings by spray-coating have been studied by video imaging and Monte Carlo simulation techniques. A representative variety of tablets of different shapes and sizes were used at different pan loads and at various pan speeds. A single “tracer” tablet was used to track the motion of tablets and coating variables such as circulation time, surface time, projected surface area and surface velocity of a tablet were determined from the video imaging experiments. The coating uniformity is described in terms of the coating variation from tablet to tablet CV(tt) and a new parameter CV(st) the coating variation on a single tablet. The effect of shape of tablets on coating uniformity was analyzed by introducing a “sphericity” of tablet (φ_s) into the CV models. The methodology, new models and the analysis developed here incorporating the additional parameters will help users to optimize the coating process in pan-coating operations.     

Multi-component granulation in a fast fluidized bed

The granulation of multi-component particles was conducted in a fast fluidized bed with an atomizing binder solution. The effects of gas velocity and binder droplet diameter on granulation rate, granule size distribution and granule composition were studied. The granulation rate and granule yield were determined by the balance between the agglomeration rate of feed particles and the disintegration rate of granules because there was no secondary granulation. With the increase in gas velocity and the reduction in binder droplet size, the agglomeration rate of feed particles decreased but the disintegration rate of granules increased, resulting in a reduced granule yield. Despite the larger fraction of small particles in the granules, the homogenous granulation of multi-component particles was achieved.     

Mixing and combustion in a shallow coal-limestone fluidized bed combustor

A model based on the Monte Carlo approach was developed to simulate the mixing and combustion behavior of a shallow coal-limestone fluidized bed combustor. The model involved the coupling of two sub-models: a combustion sub-model based on the two-phase concept of fluidization and a mixing sub-model based on our previously developed dynamic mixing model. The combustion sub-model considered both the volatile and char combustion. It assumed that the combustor consisted of three distinct phases, i.e., jet, bubble and emulsion, with combustion occurring only in the emulsion phase. The mixing sub-model considered the upward or downward movement of a coal particle in the bed as being governed by certain probability laws; these laws were, in turn, affected by the bubbling hydrodynamics. In all, the combustor simulation model took into consideration the effects of coal feed rate, coal size distribution, limestone size, air flow rate and combustor temperature on the combustor behavior. The simulation results included the dynamic response of coal concentration profile, coal size distribution, coal particle elutriation rate as well as the mixing status between the coal and limestone particles.     

Using stereo XPTV to determine cylindrical particle distribution and velocity in a binary fluidized bed

Nonspherical particles are commonly found when processing biomass or municipal solid waste. In this study, cylindrical particles are used as generic nonspherical particles and are co-fluidized with small spherical particles. X-ray particle tracking velocimetry is used to track the three-dimensional particle position and velocity of a single tagged cylindrical particle over a long time period in the binary fluidized bed. The effects of superficial gas velocity (u _f), cylindrical particle mass fraction (α), particle sphericity (Φ), and bed material size on the cylindrical tracer particle location and velocity are investigated. Overall, the cylindrical particles are found in the near-wall region more often than in the bed center region. Increasing the superficial gas velocity u _f provide a slight improvement in the uniformity of the vertical and horizontal distributions. Increasing the cylindrical particle mass fraction α causes the bed mixing conditions to transition from complete mixing into partial mixing. © 2018 American Institute of Chemical Engineers AIChE J, 65: 520–535, 2019     

Modelling and simulation of a coal gasification process in pressurized fluidized bed

A coal gasification mathematical model that can predict temperature, converted fraction and particle size distribution for solids have been developed for a high pressure fluidized bed. For gases in both emulsion and bubble phase, it can predict temperature profiles, gas composition, velocities and other fluid-dynamic parameters. In the feed zone, it could be considered a Gaussian distribution or any other distribution for the solid particle size. Experimental data from literature have been used to validate the model. Finally, the model can be used to optimize the gasification process changing several parameters, such as excess of air, particle size distribution, coal type and reactor geometry.     

考虑颗粒旋转的流化床流动和气化反应模拟

摒弃传统颗粒动力学模型中颗粒绝对光滑的假设,以粗糙颗粒为研究对象,同时考虑颗粒碰撞过程中的对心和切向分力建立了粗糙颗粒动力学模型,采用近似求解给出了相关本构关系式。结合粉煤气化反应模型模拟研究了鼓泡流化床内粉煤颗粒的流动-反应过程,获得了床内粗糙颗粒时均速度和浓度的径向分布。与光滑颗粒的计算结果相比,粗糙颗粒的脉动能量增大,床内不均匀特性进一步增强。同时得到的各气体组分的浓度分布与他人的实验结果相吻合。