Particle Formation
The process of disintegration of liquid/solid suspension jets and sheets by atomization is analysed in a fundamental manner and visualized by suitable measurement methods, which allow qualitative and quantitative evaluation of the process. Supporting numerical analysis and theoretical derivations will contribute to basic understanding and control of the suspension atomization process. Model suspensions will be atomized by means of conventional and specifically designed atomizers.
The fifth year activities that are reported here include:
- Further Experimental investigations of suspension atomization in twin-fluid atomizer
- Experimental investigations of atomization suspension sheet in flat pressure nozzle
Model suspensions based on water and glycerol/water-mixtures with different kind of suspended particles (Kaolin, Polymer, Glass) have been atomized by means of twin-fluid atomizer. Model suspensions based on water and Glycerol/water-mixture with suspended particles (glass and Kaolin) have been atomized by means of a flat pressure nozzle.
The main aim of the project carried out in the years-1997-2003 was to deliver complex and profound, experimental and theoretical, description of co- and counter-current spray drying process including determination of drying and degradation kinetics, determination of final product properties and elaboration of our own CFD code for reliable scaling up of spray drying process. One of the most significant outcomes of the project was designing and building a 9 m long, 0.5 m in diameter spray drying tower. The tower capacity enabled identification of the effect of various drying process parameters, like the effect of feed properties, feed rate and feed temperature, drying agent temperature and air flow rate on drying and degradation kinetics, particle residence time, particle morphology, etc. The column was equipped with a 72 kW heating system, waste air cooling system, dust collecting system and optical glass windows to perform measurements using laser techniques (LDA, PDA). Feed was delivered from a stainless steel tank (equipped with a cooling/heating steam/water jacket, 150 liter volume) to the nozzle by a progressive-cavity pump. Two steam generators were used to deliver heating agent to the jacket to maintain the required temperature of the drying material. Each pipe and the nozzle were equipped with a water heated jacket. The construction of the tower enabled taking samples at subsequent time intervals and making laboratory analysis of moisture content, size distribution, etc., as well as the quality index specific for a given product at a different distance from the atomizer. To determine the flowfield in the spray tower and structure of the spray in the cross-sectional area and along the length of the tower, the laser technique was employed. The tower walls contain numerous portholes to allow laser measurements at different vertical locations. A transport system of laser device comprising hoists and pulleys enables the laser unit to move easily between levels and carry out measurements at an arbitrary height of the column and in selected points in a given cross section. First three years of the project were devoted to development and improvement of experimental equipment to perform in situ measurements of drying process parameters using Particle Dynamic Analysis (PDA) to determine the structure of spray and microseparator to find drying agent and product temperatures and humidities. This is definitely, the most sophisticated spray drying system ever developed for research of this process. Microseparator technique, significantly modified during the project realization can be easily used by industry people to find accurate temperature of a drying agent undisturbed by the presence of product particles moving in the air. Cocurrent spray drying operation mode was subjected to exhaustive experimental analysis in first three years of project realization. All relationships between initial drying process parameters and behaviors of continuous and discrete phase were determined. The results obtained throw a new light on the mechanism of the process which takes place inside the spray tower during drying of various materials. Most of the results were presented for the first time in literature. One of the most important findings of this work, in our opinion, was lack of aerodynamic segregation of particles in the drying chamber. The analysis of the results shows that in each point along the spray axis there is practically an identical particle size distribution. Spray is mixed very well from the very beginning of the process. This finding is of great importance for understanding the phenomenon of simultaneous momentum, heat and mass transfer during spray drying. One of the challenges of the project was to find drying and degradation kinetics in spray drying process. Substantial degradation of the products was found for most of the trials. Experiments showed a rapid decrease of baker’s yeast activity in the vicinity of the atomizer, the finer atomization the highest degradation of the product. Spray drying of heat sensitive products requires careful selection and control of process parameters.
To obtain a full picture of the mechanism of spray drying process, extensive experimental trials were extended to counter-current spray drying process. Opposite to the co-current spray drying process, due to complex hydrodynamics of continuous phase, an aerodynamic segregation of particles (more bigger particles close to the column wall) was found. Analysis of the results confirms literature suggestions about strong couplings of momentum transfer between continuous and discrete phases which cannot be neglected in modeling of counter-current spray drying process. An increase of mean particle size with the distance from the nozzle caused by agglomeration process in recirculation zones in the column was observed. The results proved high sensitivity of counter-current spray drying process to initial drying and atomization parameters and a position of the nozzle in the dryer. Generally, in relation to practical applications, we can conclude that a performance of the counter-current spray drying process is stable in a narrow range of process and atomization parameters which makes such a system difficult to control. Summarizing, we could conclude that complexity of spray drying process is an outcome of three factors: parameters of a drying agent (temperature, flow rate), atomization parameters and particle residence time in the column. Drying kinetics of spray drying process comes from difficult to predict influence of the above mentioned parameters. One of the leading ideas in the project extension was to develop a CFD model of spray drying process to point out why the existing codes, in a certain range of process parameters and given geometries of the spray drying chamber fail to predict the spray drying process. A spray drying column developed at Lodz Technical University, was employed to collect a database to verify a CFD model of spray drying process. A comparison of experimental and theoretical results of CFD modeling enabled us to formulate the conclusions which might be important for industry people to scale up or check the performance of spray drying process under different operating conditions.
Every CFD model of spray drying process which can be found in the literature or developed individually enables a relatively correct determination of the continuous phase parameters (e.g. distributions of drying air temperature and humidity) regardless a number of simplified assumption in initial and boundary conditions assuming that heat losses to environment and effect of atomizing air were taken into account and proper model of flow turbulence was selected. However, to obtain reliable results of CFD simulations concerning also the discrete phase parameters, it is necessary to introduce to the model real initial particle size distributions and mass flow rates of the disperse phase and real evaporation kinetics. This data are often difficult to obtain in industrial conditions. One of the main successes of the project was to construct and test a device for determination of disperse drying kinetics on a laboratory scale. Our main idea was to elaborate a system where we could reach evaporation rates (and drying times) similar to those obtained in a spray drying column. A laboratory drying tunnel equipped with effective heating system and stabile weight measuring system has been designed and built. The tests on drying of maltodextrin solutions proved repeatability and correctness of this method. In the developed unit, we finally gained evaporation rates and drying times similar to the conditions obtained in a drying column. An effect of the initial moisture content on the critical moisture content was observed which is related to a decrease of the equilibrium vapor pressure over the solution and a decrease of the driving force of evaporation and drying rate of the process. Results of the experiments proved that the generalized drying curves determined in the lab scale could be used in scaling up of spray drying process if the critical moisture content of the material is known. Outcome of this part of the work offers to practitioners a cheap, fast and precise technique to determine realistic spray drying kinetics from small scale experiments.
The last part of the project which delivered a complementary picture of the mechanism of the process consisted of complex experimental studies on the effect of drying and atomization parameters on the properties of selected materials during spray drying process. Materials from two groups were chosen for investigations, skin-forming (maltodextrin) and agglomerate-like (detergent and cacao), that are most often subjected to spray drying in industrial conditions. In the study, 352 experimental tests of spray drying of maltodextrin, detergent and cacao powder as a function of temperatures and air flow rate, atomization conditions as well as temperature and dry matter content in the feed were carried out (in the last two years). We found that for all powders the mean particle diameter was smaller than the mean droplet diameter in the spray which was caused by shrinkage of the particles during moisture evaporation. We also proved that, depending on drying and atomization conditions, each of the tested materials could be cohesive, slightly cohesive or loose. We determined quantitative relationships and explained effect of the initial distribution of particle diameters and their morphology on product bulk density. Determination of these conditions in the frame of this work has an important practical meaning. The work delivers a profound and complementary picture of the mechanism of the process which takes place inside the spray tower during drying of various materials. The behavior of dispersed phase, drying and degradation kinetics, quantitative relations between initial parameters of drying and atomization and physical properties of spray-dried products are presented for the first time in literature. Determination of the local particle size and velocity distribution, detection of the uniformity of spray structure in the dryer, elaboration of quantitative relations describing drying and degradation kinetics are among the most important outcomes of the project.
Executive Summary
This report summarises the work done over six years in IFPRI project 37 Quantitative Analysis of Powder-Binder Agglomeration. This is a large body of work undertaken by ten researchers and IFPRI funds were substantially leveraged with funds from other sources. The main focus of the report is the analysis, in turn, of each of the three classes of granulation rate processes that dictate the product granule attributes:
- Wetting, binder dispersion and nucleation
- Consolidation and growth
- Wet granule breakage
We now know the key formulation properties and process parameters that control the rate processes of (1) nucleation and wetting, and (2) consolidation and growth. For both these rate processes, regime maps have been developed and validated based on the controlling dimensionless groups: a and tp for wetting and nucleation, Stv, Stdef, and s for growth and consolidation. This quantitative understanding of granulation rate processes is now at the point where it can be directly used in scaling granulation processes (eg. keeping dimensionless spray flux constant to maintain nucleation conditions) and characterising formulations for their granulation behaviour (eg measuring the dynamic yield stress of a new formulation).
The report gives details of the development of characterisation tools, models and regime maps used to quantify the rate processes and experimental verification of models and regime maps. The impact of both process parameters and formulation properties on the granulation rate processes is studied in detail.
We are applying the same approach to quantify wet granule breakage, although this work will not be complete till the end of 2004. We report an approach to characterize the mode of failure of the granule matrix (brittle crack propagation or plastic deformation) for different formulations using an Instron Dynamite testing rig and detail the design and methodology for studying wet granule breakage in a breakage only granulator (BOG).
The report also gives summaries of related research in two areas, not directly funded by IFPRI, but very relevant to our multiscale approach to granulation modeling:
- Using positron emission particle tracking (PEPT) to characterize powder flow fields in granulators and using this information in conjunction with rate process models to predict granulator performance;
- Design and modeling of regime separated granulators for significant improvement in the control of granule attributes. Finally, a discussion of research and development gaps and opportunities in the field of granulation is presented.
Background and Project Overview
Atomization is a chemical engineering unit operation, which disintegrates a continuous liquid into a dispersed system of drops within a spray. Common atomizers for disintegration of liquids are producing droplet spectra to be characterized by:
- droplet size distribution,
- droplet size/velocity correlation and
- local and overall concentration and mass flux distribution.
These major describing parameters of a droplet spray are mainly influenced by relevant parameters such as:
- the atomizer design and working principle,
- fluid material properties and
- mass flow rates.
Within an existing atomization process, manipulation of the spray properties and droplet spectra often only is possible by means of changing the energy input into the atomizer, which simultaneously alters several of the relevant spray parameters with a minimum degree of control. This research project aims to analyse the influence of solid particles on the droplet characteristics in the liquid atomization with suspended particles and to develop atomizer strategies and design technical equipment in order to produce controlled droplet characteristics within a wide range of applications. Various model suspensions based on water, water/glycerol and/or water/Non-Newtonian fluid mixtures with particles will be atomized by means of twin-fluid atomizer, rotary atomizer and pressure jet nozzle. The influence of particle characteristics and loading of particles on the liquid jet and liquid film disintegration are analysed.
Summary of 3rd year
In this part of the project work a new mixing/extrusion concept was further developed to study the microstructure changes of a model system, which undergoes a transition from a 3- phase wet powder to a 2-phase concentrated suspension system (3P2S Transition) (Arancio and Windhab, 2003).
The further developed model system used consists of:
- powder/binder: liquid fat melt/oil (e.g. cocoa butter)(i)
- powder/solid filler: hydrophobic silica particles (Sip.D17, Degussa, (D)) (ii)
The first advantage of using a fat melt, which is in the fully liquid state at moderately elevated temperatures (eg. 40°C for cocoa butter), was given by the ease of solidification close to room temperature. This made investigations of the wet powder/suspension microstructure possible without additional structural changes during sampling from the process and during sample preparation. The second advantage proved was the good homogeneous mixing ability of the hydrophobic silica powder particles within the fat binder, even at extremely low binder fraction due to the fact that the binder fat melt was cold sprayed, thus producing small solidified binder particles. These solidified binder particles were mixed with the filler particles under NIR-based homogeneity control. If this powder mixture was heated up to 40°C, a significantly better homogeneity of the mixture of powder and liquid binder was obtained compared to the conventional/alternative process of mixing by spraying the low fraction of liquid binder into the filler powder (formation of local lumps).
The performed process consisted of the following steps:
- spray cooling of the binder-fluid phase in order to obtain a solid fine binder powder
- mixing of this binder powder with the filler powder and mixing quality control by near infrared spectroscopy
- pre-compaction of the model system “powder/binder powder mixture (PBPM)” in a mechanical testing machine (Zwick)
- ram extrusion of the pre-compacted system
- micro-structural investigation of the extruded product
The powder/binder mixture in the solid powder state of the binder (A) was used to investigate the local stresses/stress distribution leading to deformation of the solid cocoa butter (binder) particles. This allows identifying the zones where the transformation from a three phase wet powder to a two phase concentrated suspension (3P2S) preferably happens. The solid liquid mixture above the melting temperature of the binder (cocoa butter) (B) reflects the powder binder system in the normally applied state, with air additionally entrapped. The acting pressure and shear stresses lead to a synergistic reduction/compression of the air included until the 3P2S transformation takes place. The impact of pressure, shear and pressure + shear on the resulting powder/binder microstructure were investigated in detail and 3P2S phase diagrams derived, which form a new basis for design calculations of extruders/extrusion processes and extrudate property optimization.
The aim of this project is to enhance our understanding of how solvents affect the polymorphic forms of compounds. Polymorphism is very important to the pharmaceutical industry as a compound’s polymorph can exhibit different physical properties, some more desirable than others. Being able to control which polymorphic form is nucleated is thus very useful.
To study the effects of solvent type on polymorphism, enantiotropic compounds will be used. These compounds have multiple polymorphs but the distinguishing feature about them is that there is a transition temperature where the solubilities of the polymorphs are equal. This transition temperature is constant regardless of the solvent type as at this temperature the free energies of the polymorphs are equal. This allows us to study the effects of solvent type by decoupling it from the effects of the supersaturation.
The work done in this first year is primarily aimed at identifying the compounds which are enantiotropic in nature, selecting a model compound, characterizing it and determining its exact transition temperature. Subsequently, we will perform experiments using different solvents at the transition temperature and measure both induction times and nucleation rate.
Project Overview
In the first year of this project, a multidimensional population balance model has been developed for a general granulation process. The model details the evolution of particles with respect to distributions in size, porosity, and binder (moisture) content the three critical properties that are related to processing as well as end product properties.
Model Utilization
The model has been utilized to determine the control-relevant properties of the granulation process. The influence of changes in the binder addition rate as well as the agitator speed were determined using sensitivity analysis of the population balance model.
Results and Insights
The results provide insight into the degree to which size, porosity, and moisture content can be adjusted in a feedback control model of operation. The present analysis is flexible enough to address both batch and continuous granulation.
Implications
The insights generated from this analysis point to more effective strategies for the improved operation of industrial granulators, notably with regard to reduced recycle streams (both ¯nes and oversized particles).
This project has an overall goal of designing model-based control algorithms using multi-scale descriptions of particle attributes in high-medium shear granulators. The first objective of the second year of this project was to design a reduced order model that is appropriate for real time applications. The second objective involved a feasibility study of applying a reduced order model in a Model Predictive Control (MPC) algorithm. The methods used to reach these objectives have prepared us for the next stage, the objective for the third year: application of MPC algorithms on a pilot plant granulator. During the scope of this project four different granulation models have been considered (in order of complexity):
- Transfer function model (Pottmann, et al., 2000; Gatzke and Doyle, 2001)
- One dimensional well mixed discretized Population Balance Model (PBM) (Sanders, et al., 2005)
- One dimensional three compartment discretized PBM (Wang, et al., 2006)
- Three dimensional PBM (Immanuel and Doyle, 2005)
The models were either used as a plant model to simulate a real plant or as the basis for a linear controller model. This report described the details of the research with the second model. The fourth model has been described in our first year report (IFPRI # ARR51-01). The third model has been developed by the group in Queensland and had been adjusted to model their pilot plant, which will be used in future studies in this project. The main differences between model 2 and model 4 are the recycle flow (+ screens and crusher) and the introduction of three separate regimes in the granulator. More details about the UQ model and pilot plant are described in section 6: Summary and Future Work.
Abstract
The first part of this report (Chapter 3 and 4) present the framework that was used to setup and test an MPC controller on a simulated plant model. The plant model is based on earlier work done at The University of Sheffield and Organon, The Netherlands (Sanders, et al., 2005). The granulation kinetics were measured in a 10 liter batch granulator with an experimental design that included four process variables. The aggregation rates were extracted with a Discretized Population Balance model. Knowledge of the kinetics was used to model a continuous (well mixed) granulator. The controller model is a linearized state space model, derived from the nonlinear DPB model. It has the four process variables from the experimental design and a feed ratio as input variables. Since the DPB model describes the whole size distribution (GSD), different sets of output variables were chosen and compared. Preliminary results show successful implementation of Model Predictive Control on a continuous granulator. Further research is needed to test the linear model on an actual pilot plant.