Soutenance mémoire
Powders exist in a wide variety of industries as chemicals, pharmaceutics, cosmetics, agriculture and food, like plastics beads, ceramic materials, detergents, fertilizers or medecines. In food industry powders represent stable dried products, ingredients, with a reduced weight and volume for transport, able to be designed for easy dosage and dissolution, while retaining nutritional and functional properties. Food powders may be added directly to a dish in a small quantity (salt, pepper, spices, sugar, aromas), or may be consumed or processed with other constituents in a solvent (water, milk, oil). Examples of powders include milk and derivatives, flour, cocoa, sugar, coffee, soup, vegetables, meat, fish, sauce mix, vending machine powders and ingredients (colourings, enzymes, yeasts). Depending on the desired end-use, specific properties of individual particles are required with or without interaction with other particles (e.g. for powders mixing), or with solvent (e.g. instant dissolution); and the whole powder should behave as a “fluid” for easy transport and dosage. In some cases specific shape and surface of particles could be requested for improving the aspect (coating) and the attractiveness of a commercial product. Several processes are used for production of powders like crystallisation, precipitation, freeze drying, extrusion, milling, roller or spray drying. They differ mainly by the material to transform, by the operating conditions and the energy to use (heating, cooling, mechanical forces), and by the characteristics of the final product (size, shape, crystallized or amorphous structure, solubility, stability…). A final drying step is often required to control the required powder properties. For powder utilisation, one important step will be (very often) to be disintegrated again in a solvent either in a preliminary mixture or directly as a food substance. Whatever the user, an industrial or a consumer, one of the main properties will be the ability of powder to be dosed and to dissolve. Besides adapted composition, that means a good flowability without interaction between particles (no sticking, smooth surface) and a structure favourable to the penetration of solvent. Individual particles aggregated in solid agglomerates have shown such properties and the spray drying process coupled with fluidised bed was adapted to produce such agglomeration. Spray drying is a continuous process which transforms a concentrated liquid in a powder, limiting the possible modifications of composition during the process. The industrial equipments are usually high towers more or less sophisticated, with high powder flow rates and consumption of air and energy. Some are dedicated, others are flexible and used for different products. The principle is to atomize the liquid feed (solution, suspension or emulsion) in small drops (10-20 µm) to increase the surface of exchange with the drying agent, usually (constituted by) hot air. Each drop is quickly dried (by air), until obtaining a solid dry particle, with size close to the initial liquid drop size.
Bases of spray drying
The spray drying process transforms a pumpable liquid into a powder, i.e. individual dry solid particles. The liquid feed is made of a solvent, usually water, and constituents which are soluble (solution) and/or insoluble (emulsion, suspension). Soluble components correspond to short or long chain polymers (polysaccharides, proteins,…) and to small molecules as salts. The dry particles will be made of these dry constituents with still some traces of water (or solvent). So, to get a powder state it is necessary to remove, to extract, to evaporate water which is more or less strongly linked, adsorbed onto constituents, in pores, which means using special drying conditions (Bimbenet and Dumoulin, 1999).
In spray drying, a gas, usually hot air, is used to bring to the liquid the energy for evaporation of water and to transport the water vapour. The liquid flow (thickness) is first reduced by forming a thin film that is then broken in small drops to increase the exchange surface and to improve the heat and mass transfers with hot air. The transfer coefficients are also enhanced by creating a turbulent air flow around the liquid drops.
These conditions lead to some important parameters to take into consideration:
• The time necessary to effectively realize the drying till obtaining a dry particle must be minimized. That means an optimized air/drop contact and a limited (reduced) quantity of water to extract: the liquid must be previously concentrated.
• The formation of a film of liquid then its breakage in drops (atomization) with a regular size and shape to control the drying process must be facilitated: choice of the atomization device, physical properties of the liquid feed (viscosity, surface tension). Anyway, the size of drops must be small for easy drying but not too small to obtain powder easy to handle and to use afterwards.
• The temperature of drying air must be high compared to the liquid one to accelerate the initial transfer of water from drop to air; but maintaining the product at a reasonable temperature during a short time to avoid deterioration: the spray drying process must be a fast drying process.
Therefore, the spray drying process is a continuous process with three main operations:
• The atomization of a well formulated liquid feed, to produce a continuous spray of drops (some microns), with a great surface of exchange with drying .
• The drying of liquid drops due to an efficient contact between drops and moving hot air. The liquid solvent (water) is evaporated from the drop surface till obtaining a dry particle. The circulation of air and the transport of particles need a space and distance/time provided by the geometry of the drying chamber.
• The separation of the final dry powder from cooled and humidified air, and its recovery with possible further processing to modify powder properties.
Which characteristics for powders?
The main characteristics of final spray dried powders are related to end-use properties: water content and water activity for stability, size and size distribution for powder mixing and handling, bulk density and flowability for transportation, and wettability and dispersibility for instant properties (Huntington, 2004; Melcion et al., 2003; Aguilera, 2008). The initial composition of the liquid feed has an influence on both drying and powder rehydration, and also on the final repartition of components into particles or on the surface (i.e. fat for flowability). Therefore the initial liquid composition is closely in relation with the powder properties (Shrestha et al., 2008; Adhikari et al., 2004; Goula et al., 2008).
Powder water content and water activity
Water content X is generally expressed on dry basis (kg water.kg-1 total solids), total solids (dry matter) being constant during drying process (if no losses). But, it may also be expressed on wet basis Xwet as a function of total weight (evolving during drying). Water content of a commercial product is often the most important specification for industrial powders. It is often fixed by the maximal value allowed by the law (e.g. 3% or inferior to 3%), to avoid reduction of shelf life due to biochemical reactions and development of bacterial activity. From an economic point of view, it is convenient to operate close to the allowed limit, because each percent of humidity to remove could represent an important energy cost on year basis. Spray dried powders water content is affected by several factors including properties of liquid feed (concentration, temperature, viscosity, surface tension), type of atomizer (nozzle or rotary), type of spray dryer and drying air characteristics (flow rate, inlet and exit temperature, relative humidity) .
Density
Powder density is generally described by the bulk density ρbulk (kg.m-3), defined as the ratio between the mass of many particles of the material and the total (bulk) volume they occupy. The total volume includes individual particles volume, inter-particle voids volume and internal pores volume. It represents an important property for powder handling, for marketing and economics, even if the different needs can be in contrast between them. As an example, a high bulk density is required for reducing transportation costs that depend on total volume. But a low bulk density may correspond to a more attractive product with improved instant properties (Pisecky, 1997). Bulk density is not an intrinsic property of a material; it can change depending on how the material is handled. For this reason, the bulk density of powders is usually reported both as « freely settled » and « tapped » density. The tapped density refers to the bulk density of the powder after a specified compaction process, usually involving controlled vibration of the container (in relation with transportation, shocks of packages). Bulk density depends on the density ρs of the solid composing the powder, on air volume inside the particles (occluded and in open pores) and on the shape of the particles that influences the amount of interstitial air between the particles. A regular spherical particle shape minimizes the amount of interstitial air. Controlling the amount of occluded air could lead to a higher or lower bulk density. For example, stirring of the liquid feed solution may result in the creation of air bubbles inside the liquid, then in the drops and in final powder particles. A lower bulk density is usually obtained with particles agglomeration. By assembling several small particles with solid bridges, a porous structure is formed (agglomerate) with voids between particles, increasing the mean size, decreasing the fines proportion, and leading to specific instant properties.
Flowability and instant properties
Flowability is defined as the ability of a powder to exhibit a free-flow behaviour. Good flowability of final powders is required for easy transportation in pipes (i.e. to fill packages) and for various uses like in vending machines. Several factors affect powder flowability, like size, shape and composition of the surface of the particles (Teunou et al., 1999; Fitzpatrick et al., 2004). Large mean particle size, narrow particle size distribution, spherical shape and smooth surfaces with no sticky or fat components contribute to a better flowability.
Food powders are often reconstituted as liquid solutions (emulsions, suspensions) by adding water (or a liquid, e.g. milk). If the result is an homogeneous liquid obtained rapidly without unsolved lumps, the powder has a good instant behaviour. The particles size, porosity, composition and the presence of some components (hydrophobe) on the surface will play an important role in the behaviour of the powder during reconstitution.
The instant properties are described as sinkability, wettability, dispersibility and solubility (Schubert, 1993; Fang et al., 2008). The sinkability expresses how particles penetrate the liquid surface. The wettability determines the time necessary for liquid penetration in the porous structure of powder, thanks to capillarity; it is often the rate determining step. Wettability is measured as the wetting time of a fixed amount of powder in contact with a water surface, till the last particles of powder penetrate the water surface. A quick wetting is enhanced by a good porosity of powder. Particle size is another parameter: for example, for milk powders particles diameter should be between 200 and 300 µm, with fines (d < 100 µm) fraction lower than 20%. Too small particles are difficult to wet due to high surface tension of liquid compared to particles weight (Pisecky, 1997). Dispersibility is the aptitude for the powder to be re distributed as single particles in the reconstituting liquid. Solubility refers to the rate and extent to which the components of the powder particles dissolve in the liquid. Dispersibility and solubility determine if the powder is completely dissolved or not, by evaluation of powder residuals observed in the liquid solution after filtration. The percent of dissolved powder compared to the total solids concentration give a dispersibility index (Pisecky, 1997). To improve instant properties of spray dried powders, an agglomeration step is often performed (Buffo et al., 2002).
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Table des matières
INTRODUCTION
PART I – BIBLIOGRAPHY
1. The spray drying process: from liquid to powder
1.1. Bases of spray drying
1.2. Which characteristics for powders ?
1.2.1. Powder water content and water activity
1.2.2. Density
1.2.3. Flowability and instant properties
2. Drying of liquid drops
2.1. Atomization of liquid feed in drops
2.1.1. Rotary atomizer
2.1.2. Pressure nozzle
2.1.3. Pneumatic nozzle
2.1.4. Sonic nozzle
2.2. Drying of drops in air
2.2.1. Evolution of air properties along drying (Mollier diagram)
2.2.2. Drying rate: heat and mass transfer equations
2.2.3. Evolution of drops properties along drying
2.2.4. Configuration for air/drops contact
3. Particle sticky behaviour along spray drying
3.1. The glass transition phenomenon in drying
3.2. Characterization of stickiness of powders
3.2.1. Methods using powder “in bulk”
3.2.2. Methods using drying drops or fluidized particles
4. Agglomeration and spray drying process
4.1. Agglomerates: structure and formation
4.2. Equipments for spray drying and agglomeration
4.2.1. Multistage spray drying
4.2.2. Fines return
5. Process control for spray drying
5.1. Energetic considerations in spray drying
5.2. Measurements on air and product
5.3. Control
6. Modeling of spray drying and agglomeration
6.1. Principle
6.2. CFD models of spray drying
6.2.1. Simulation of turbulence
6.2.2. Simulation of drop transport
6.2.3. Drying kinetics of drops
6.3. Simulation of agglomeration
6.3.1. Collisions between particles
6.3.2. Collision result
6.4. Conclusion on CFD simulation of spray dying and agglomeration
Conclusion
PART II – MATHERIALS AND METHODS
1. Products
1.1. Maltodextrin solutions
1.2. Protein hydrolysate aqueous solution
2. Equipments and instrumentation
2.1. Niro Minor spray dryer
2.2. Niro FSD 4.0 spray dryer
3. Measurements on liquid feed, air and products
3.1. Measurements on liquid feed solutions
3.2. Measurements on air
3.2.1. Air flow rate
3.2.2. Air temperature
3.2.3. Air relative humidity
3.3. Measurements on powders
3.3.1. Water content
3.3.2. Water activity
3.3.3. Powder size distribution
3.3.4. Sorption isotherms and glass transition
3.3.5. Wettability
3.3.6. Bulk and tapped density
3.3.7 SEM microscopy
3.3.8. Conductivity of solutions for DTS measurements
3.3.9. Colour
4. Conditions of spray drying trials
4.1. Trials in Niro Minor
4.2. Trials in Niro FSD 4.0.
4.3. Residence time distribution measurements in Niro Minor
PART III – Results and discussion
1. Definition of process operating conditions
1.1. Measurements on air without liquid atomization
1.1.1. Heating of the chamber
1.1.2. Determination of heat losses
1.2. Water spray drying
1.2.1. Time to reach steady state inside the chamber
1.2.2. Positions for air properties measurements
1.2.3. Choice of operating parameters
1.2.4. Mass and heat balances on drying air
1.2.5. Estimation of possible secondary ambient air flow rate
1.3. Operating conditions for maltodextrin solutions drying
1.3.1. Choice of maltodextrin solution flow rates
1.3.2. Residence time distribution inside Niro Minor
Conclusion
2. Drying behaviour and stickiness development for maltodextrin solutions
2.1. Drying of maltodextrin DE12 solutions and comparison with water
2.1.1. Considerations on drying behavior of water and maltodextrin solutions
2.1.2. Effect of process parameters on spray drying behavior of liquid solutions
2.2. Comparison between MD21, MD12 and water drying behavior
2.3. Stickiness of maltodextrin DE12 and DE21 particles along spray drying
Conclusion
3. Spray drying CFD simulation
3.1. The model
3.1.1. Equations for continuous phase: air
3.1.2. Equations for the discrete phase: drops/particles
3.1.3. Definition of the dryer geometry
3.1.4. Boundary conditions
3.1.5. Numerical solution
3.2. Determination of simulation parameters
3.2.1. Validation of the global heat transfer coefficient h and TIN, with water drying
3.2.2. Sensibility to inlet air turbulence intensity and grid size effect
3.2.3. Sensibility to initial drop diameter
3.2.4. Effect of drop vapour pressure Pv
3.3. Application of simulation
3.3.1. Model validation
3.3.2. Continuous phase at steady state: air
3.3.3. Discrete phase at steady state: drying particles
3.4. Determination of powder stickiness inside the chamber
3.4.1. Effect of liquid feed flow rate on particle stickiness for MD12,and MD21
3.4.2. Effect of inlet air temperature
Conclusion
4. Powder insertion inside Niro Minor to perform agglomeration
4.1. Theoretical considerations: factors affecting agglomeration inside spray dryer
4.1.1. Stickiness of colliding particles
4.1.2. Collision probability between drying particles and inserted particles
4.1.3. Force of the impact between particles
4.2. Powder insertion system
4.2.1. Design of powder insertion device
4.2.2. Choice of insertion positions and powder jet shape
4.2.3. Choice of operating conditions
4.3. Results of powder insertion trials
4.3.1. Discussion on obtained agglomeration
Conclusion
5. Industrial application: spray drying of protein hydrolysate
5.1. General conditions of trials
5.2. Water trials without fluid bed: drying behavior and measurements feasibility
5.3. Protein hydrolysate solution drying trials feasibility
5.3.1. Drying trials for TIN 180°C
5.3.2. Effect of inlet air temperature (160°C and 200°C)
5.4. Discussion about the drying behavior, the role of fines return and fluid bed during protein hydrolysate solution spray drying
5.4.1. Spray drying behavior: air properties evolution inside the chamber
5.4.2. Position of fines return (top and bottom)
5.4.2. Role of the fluid bed (with top fines return)
CONCLUSION
LIST OF SYMBOLS
REFERENCES
ANNEX I Modeling of residence time distribution function in Niro Minor
ANNEX II Properties of air and water for CFD simulation
ANNEX III Articles, congresses, industrial visits
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