Air-suspension coating in the food industry: Part II — micro-level process approach

A review of air-suspension particle coating concluded that, in order to speed product and process development, a phenomenological approach is necessary to develop generic guidelines for the selection of coating materials and process variables. This paper identifies 10 fundamental phenomena (micro-level processes) that occur during an air-suspension particle coating process: particle motion, atomisation, droplet-particle collision, droplet impact and adherence, droplet impact and spreading, infiltration, drying, film formation, layering and inter-particle agglomeration. Their relevance to the coating objectives is discussed and from these four are identified as key micro-level processes: drying, droplet impact and spreading, and stickiness which encompasses the two key micro-level processes of droplet impact and adherence and inter-particle agglomeration. It is believed that significant advances in particle coating research can be made through examination of these key micro-level processes.     

Granulation of Fe-Al-Ce nano-adsorbent for fluoride removal from drinking water by spray coating on sand in a fluidized bed

A technology for the granulation of Fe-Al-Ce nano-adsorbent (Fe-Al-Ce) in a fluidized bed was developed. The coating reagent, a mixture of Fe-Al-Ce and a polymer latex, was sprayed onto sand in a fluidized bed. The granule morphology, coating layer thickness, granule stability in water and adsorption capacity for fluoride was investigated by analyzing samples for different coating time. The coating amount was from 3% to 36%. With increasing coating amount, granule stability decreased and adsorption capacity increased. FTIR analysis showed that the latex can react with active hydroxyl on the Fe-Al-Ce adsorbent, which led to a decrease of the adsorption capacity. Coated granules with a coating amount of 27.5% had a fluoride adsorption capacity of 2.22 mg/g (coated granules) at pH 7 and initial fluoride concentration of 0.001 M. A column test showed that 300 bed volumes can be treated with the effluent under 1.0 mg/L at an initial fluoride concentration of 5.5 mg/L, space velocity of 5 h^− 1 and pH of 5.8. The coating granulation of the Fe-Al-Ce adsorbent can produce granules that can be used in a packed bed for the removal of fluoride from drinking water.     

Kinetics of fluidised bed melt granulation I: The effect of process variables

This paper is the first of a series to study the influence of operating conditions on the kinetics of fluidised bed melt granulation. First, we identify the rate processes responsible for the net growth in granule size in a top-sprayed fluidised bed granulator and propose a sequence of events based on these rate processes. The overall kinetics during the process is identified to be a combination of particle aggregation, binder solidification and granule breakage. By conducting experiments in a small-scale modified commercial fluidised bed granulator, the influence of various operating conditions (binder spray rate, bed temperature, atomising pressure, fluidising air velocity) on the granule growth behaviour was examined. The results indicate the granule growth rate to be directly dependent on the relative amount of binder sprayed into the bed, which essentially determines the speed of the aggregation process. The overall granule growth rate is observed to increase relatively with increased bed temperature for a more viscous PEG4000, while a maximum growth is seen for a lower viscosity PEG1500. A larger droplet size was also seen to have increased the overall growth rate, even though a smaller droplet seems to be able to induce a faster initial growth. The results also reveal the increase in fluidising air velocity to reduce the overall granule growth rate. The final granule size distribution was also observed to become narrower with increased bed temperature and fluidising air velocity. These observations are effectively explained using the proposed sequence of rate events.     

Continuous granulation in the pharmaceutical industry

Traditionally the manufacturing of pharmaceutical dosage forms has been a batch-wise process and continuous processes have limited applications in a pharmaceutical manufacturing plant. However, several factors (reduction of cost, improved process efficiency, optimal use of equipment, flexibility in production capacity) are stimulating the pharmaceutical industry to investigate the opportunities offered by continuous processes. This paper discusses some of the techniques which could be implemented in a continuous granulation process of pharmaceuticals.     

Kinetics of fluidized bed melt granulation—II: Modelling the net rate of growth

Previous work [Tan, H.S., Goldschmidt, M.J.V., Boerefijn, R., Hounslow, M.J., Salman, A.D., Kuipers, J.A.M. (2004a). Building population balance for fluidized bed granulation: lessons from kinetic theory of granular flow. Powder Technology, 142, 103-109] shows that we can derive an aggregation kernel (equi-partition of kinetic energy (EKE)) on the basis of the kinetic theory of granular flow and use it effectively to describe the net granule growth in fluidized bed melt granulation (FBMG). In this paper, we incorporate the EKE kernel into a population balance model to extract the effective aggregation rate constant that accounts for the net granule growth for the series of FBMG experiments shown in Tan, et al. [(2004b). Kinetics of fluidized bed melt granulation I: effect of process variables, Chemical Engineering Science, to be submitted]. These extracted rate constants are subsequently expressed as a function of different operating condition. The results consistently show that the aggregation rate constant increases in direct proportion with binder spray rate, from where we conclude that the rate of granule formation is directly dependent on the amount of binder available for aggregation per unit time. The aggregation rate constant was also observed to increase with higher bed temperature when a higher viscosity binder was used, but showed a maximum value for a less viscous binder as a function of temperature. The aggregation rate was also seen to be faster when granulating using a larger droplet size and at a lower fluidizing air velocity. The observations in the rate constant plot can be effectively explained by the physical parameters in the EKE model and the sequence of rate events proposed in Tan, et al. [(2004b). Kinetics of fluidized bed melt granulation I: effect of process variables, Chemical Engineering Science, to be submitted].     

Selection of operational parameters for the production of instant soy protein isolate by pulsed fluid bed agglomeration

The objective of this study was to select the optimal operational conditions for the production of instant soy protein isolate (SPI) by pulsed fluid bed agglomeration. The spray-dried SPI was characterized as being a cohesive powder, presenting cracks and channeling formation during its fluidization (Geldart type A). The process was carried out in a pulsed fluid bed, and aqueous maltodextrin solution was used as liquid binder. Air pulsation, at a frequency of 600 rpm, was used to fluidize the cohesive SPI particles and to allow agglomeration to occur. Seventeen tests were performed according to a central composite design. Independent variables were (i) feed flow rate (0.5-3.5 g/min), (ii) atomizing air pressure (0.5-1.5 bar) and (iii) binder concentration (10-50%). Mean particle diameter, process yield and product moisture were analyzed as responses. Surface response analysis led to the selection of optimal operational parameters, following which larger granules with low moisture content and high process yield were produced. Product transformations were also evaluated by the analysis of size distribution, flowability, cohesiveness and wettability. When compared to raw material, agglomerated particles were more porous and had a more irregular shape, presenting a wetting time decrease, free-flow improvement and cohesiveness reduction.     

Liquid spray vs. gaseous precursor injection — Its influence on the performance of particle coating by CVD in the fluidized bed

Aluminum coatings were created onto glass beads by chemical vapor deposition in a fluidized bed reactor at different temperatures. Two different routes were examined. First, tri-isobutyl-aluminum (TIBA) vapor was enriched in nitrogen and thermally decomposed inside the fluidized bed to deposit elemental aluminum. On the other hand, liquid injection of TIBA via a two-fluid nozzle directly into the fluidized bed was tested. To ensure homogeneous coating on the bed material, the fluidizing conditions necessary to avoid agglomeration were investigated for a broad range of temperatures. Also, different glass types and pretreatments of the substrate surface were investigated to elucidate the influence of the surface chemistry on the growth and morphology of the layers deposited.     

Particle population modeling in fluidized bed-spray granulation—analysis of the steady state and unsteady behavior

Owing to its intensive mass and heat transfer ratios and its coupling of the process stages of drying, shaping and homogenization as well as classification, continuous fluidized bed-spray granulation drying has gained acceptance as a thermal treatment process for granular solids. In this study, a balance of the particle populations is completed for a continuous fluidized bed-spray granulation with external classification. Thus, it ought to be possible to describe the particle size distributions changing over time in the fluidized bed and in the product flow [Powder Technol. 82 (1995) 37; H. Uhlemann, L. Mörl, Wirbelschicht-Sprühgranulation, Springer Verlag, 2000, ISBN 3-540-66985-X.].     

Top-spray fluid bed coating: Scale-up in terms of relative droplet size and drying force

Top-spray fluid bed coating scale-up experiments have been performed in three scales in order to test the validity of two parameters as possible scaling parameters: The drying force and the relative droplet size. The aim was to be able to reproduce the degree of agglomeration as well as the mechanical properties of the coated granules across scale. Two types of placebo enzyme granule cores were tested being non-porous glass ballotini cores (180–350 μm) and low porosity sodium sulphate cores (180–350 μm). Both types of core materials were coated with aqueous solutions of Na₂SO₄ using Dextrin as binder. Coating experiments were repeated for various drying force and relative droplet size values in three top-spray fluid bed scales being a small-scale (Type: GEA Aeromatic-Fielder Strea-1), medium-scale (Type: Niro MP-1) and large-scale (Type: GEA MP-2/3). The tendency of agglomeration was assessed in terms of particle size fractions larger than 425 μm determined by sieve analysis. Results indicated that the particle size distribution may be reproduced across scale with statistical valid precision by keeping the drying force and the relative droplet size constant across scale. It is also shown that none of the two parameters alone may be used for successful scaling. Morphology and microscope studies indicated that the coating layer is homogenous and has similar structures across scale only when both the drying force and the relative droplet size were fixed. Impact and attrition tests indicated that it is possible to produce granules with similar attrition and impact strength across scale and that the two types of mechanical properties are inversely related.     

Inhibition and promotion: The effect of earth alkali metals and operating temperature on particle agglomeration/defluidization during incineration in fluidized bed

Some studies have demonstrated that earth alkali metals promote agglomeration; however, others have shown that they inhibit the generation of agglomeration. The earth alkali metals (Mg and Ca) may cause both inhibition and promotion of agglomeration/defluidization. Therefore, this study focuses on the effect of Mg, Ca and the operating temperature on the agglomeration/defluidization of sodium during incineration in a fluidized bed reactor. The results indicate that the added Mg and Ca inhibit agglomeration and increase the defluidization time. At low Na/Mg and Na/Ca mole ratios, Mg and Ca inhibit defluidization significantly. However, the inhibition reduces as the concentration of Na increases. When the mole ratio (Na/Mg and Na/Ca) exceeds two, the inhibition of Mg and Ca is not apparent. Under these conditions, the operation temperature is the main factor with regard to agglomeration/defluidization. When Mg and Ca are added to reduce the agglomeration/defluidization, both the concentration of Na and operating temperature must be considered.     

Investigation of the particle flowpattern and segregation in tapered fluidized bed granulators

The particle flowpattern and granule segregation in tapered fluidized beds have been studied using two techniques. The first technique is to fluidize beds of varying total mass and granule fractions, then defluidize them suddenly to “freeze” the composition, section the bed in layers, and determine the composition of each layer by sieving. The second technique is to track a radioactive particle mimicking a granule as it moves in the bed. The results show that the segregation behaviour of granules is complex, their behaviour changing from flotsam at low granule concentrations to slightly jetsam at higher concentrations. The flow in the tapered bed is very different from what is expected based on relations derived for cylindrical beds. In the tapered bed a central region of high bubble activity and upward flow was a dominant feature. This “gulf streaming” became more pronounced as the total bed mass, and therefore the bed height, was increased, resulting in a bed turn-over time almost independent of the total bed mass. Quantitative data are given for upward and downward particle velocities and flows, bed turnover times, and axial granule concentration profiles.     

Triboelectrification of particulate flows on surfaces: Part I — Experiments

Triboelectrification is a ubiquitous feature of dry particulate processes. It generates hazards in many industrial systems, but is exploited in several other important applications, including electrophotographic toner charging and triboelectric separation. Unfortunately, the charging of continuous particulate flows during interaction with solid surfaces is poorly understood, and design of devices to maximise or minimize triboelectrification is usually qualitative or based on trial and error. This paper presents the results of an experimental case study of the relationship between the accumulated charge and the mechanical attributes of a particulate flow, including contact time, velocity, and the mode of contact. Silica particles of ∼ 1 mm diameter and a stainless steel slide tribocharger were used for this case study.     

Grinding in an air classifier mill — Part I: Characterisation of the one-phase flow

In this and the related second paper [1], we present an in-depth study of the two-phase flow and the stressing conditions of particles in an air classifier hammer mill. This type of mill belongs to the mostly used mills at all. In order to develop a predictive grinding model not only the material's reaction to the applied stress but also the stressing conditions within the mill, e.g. impact velocity, incidence angle, number of stress events, have to be known. The latter are strongly affected by the interactions between the fluid and the solid phase within the mill. Systematic flow investigations in the vicinity of the impact elements and in the region of the internal classifier have been performed by Particle Image Velocimetry (PIV) and by numerical predictions of the fluid flow in the complete mill using a commercial CFD solver. Different pin geometries have been studied at various peripheral velocities of the grinding disk and the classifier. The classifier velocity does not influence the velocity profiles near the impact elements in the main flow direction and vice versa, the flow in the grinding zone has little influence on the classification. The velocity profile in front of the impact element, where the comminution process takes place, is constant with time and preserves a characteristic form independent of the operational conditions.     

Model for biomass char combustion in the riser of a dual fluidized bed gasification unit: Part II — Model validation and parameter variation

The two-phase combustion model for biomass char combustion in a riser of a dual fluidized bed gasification unit that has been presented in part I is validated using the data obtained from the 8 MWth dual fluidized bed reactor at Guessing/Austria. The model is capable of calculating the average temperatures in all zones, the gas phase composition, solid hold up, char feed rates and air ratio. The model predictions for the temperature profile along the riser and for the exiting gas composition are in good agreement with the measured values. The simulation results show that the residual char from the gasifier is only partly converted in the riser for char particles larger than 0.6 mm. Un-combusted char is circulated back into the gasification reactor. Parameter variations show that the exact location where additional liquid fuels are introduced in the middle zone of the riser does not affect the global behaviour of the combustion reactor. Based on the simulation results it is proposed that external supply of char (additional) may be a very effective method for reducing producer gas recycling to the riser, which is currently necessary to obtain the desired gasification temperatures.     

Prediction of the dispersion of particle clusters in the nano-scale—Part I: Steady shearing responses

A modeling approach to predict and enhance understanding of the dispersion phenomenon is presented. The discrete/distinct element method is adopted to study the behavior of single spherical agglomerates, immersed in a simple shear flow field, in response to shearing under steady or dynamic/oscillatory flow conditions. The effects of hydrodynamic forces, which result from both the straining and rotating components of the flow, and cohesive forces of interaction, comprised of short-range van der Waals attractive and Born repulsive forces, are considered. The results of simulated distortion and dispersion of nano-size silica agglomerates in response to steady shearing are used to demonstrate the functionality of the three-dimensional simulation. Simulated results are found to be in good agreement with reported experimental trends. The current model allows us to probe and predict the dispersion phenomenon as a function of processing conditions, agglomerate structure/morphology, and material properties and interaction forces.     

Cocurrent downflow circulating fluidized bed (downer) reactors — A state of the art review

In the last decade, cocurrent downflow circulating fluidized bed reactors, called “downer” reactors, have been proposed as an alternative to upflow circulating fluidized bed, or “riser”, reactors. In this paper, published results on downer studies are summarized and future directions of research are recommended. Downer reactors are shown to have several distinct advantages over upflow circulating fluidized bed reactors and can potentially be used in many industrial processes, mainly due to a more uniform gas and solids flow structure compared with risers.     

Experimental study and optimization of the agglomeration of acerola powder in a conical fluid bed

The aim of this work was to study the physical property modifications of acerola powder produced by a wet-agglomeration process in a fluid bed. A 2² full experimental design was used to evaluate the influence of the main operational conditions and to optimize the agglomeration process. The product transformations were determined by an analysis of the particle diameter, size distribution, morphology, moisture content and solubility. The agglomeration of small granules results in an increase in the instant properties, increasing particle solubility.     

Kraft black liquor recovery in a fluidized bed: Part I — A review

The Tomlinson recovery boiler has been the mainstay of the kraft pulping industry for over fifty years. It is clear, however, that the main drawbacks to this process are the large capital costs of new plant and the smelt-water explosion hazard. This paper examines some of the fundamental scientific information which supports the concept of fluid bed gasification as an alternative to the Tomlinson boiler. It is shown that a knowledge of thermodynamics is useful but insufficient to completely understand the behaviour of the inorganic sulfur species during pyrolysis and gasification of kraft black liquor. Recent key experimental investigations at McGill have demonstrated that solid state reduction of sodium sulfate to sodium sulfide is feasible.