Training Paper

BÜCHI Labortechnik AG Training Papers Spray Drying 1 Training Papers Spray Drying Contents 1 What is spray drying 1. Introduction 2 Spray drying principle 2.1 Dispersion of the feed solution in small droplets 2.2 Mixing of spray and drying medium with heat and mass transfer 2.3 Open-cycle and closed-cycle system 2.4 Drying of spray 2.4.1 Separation of product and air 3 General applications 4 Mini Spray Dryer 4.1 Spray Drying with the BÜCHI Mini Spray Dryer B-290 4.2 Design of the Instrument 4.2.1 Diagram of the dry air flow 4.2.2 Diagram of the product flow and spray nozzle 4.3 Instrument settings 4.3.1 Interaction of the individual parameters 4.3.2 Inlet temperature / outlet temperature 4.3.3 Aspirator 4.3.4 Pump performance 4.3.5 Spray flow and concentration of solution 5 Applications 5.1 Spray drying 5.2 Micronization or structural change 5.3 Micro encapsulation 5.4 Englobing 5.5 Overview of selected applications Copyright© BÜCHI Labortechnik AG, 1997 - 2002 English, Version B (19 pages) Order Code Spray drying 97758 BÜCHI Labortechnik AG Training Papers Spray Drying 2 What is spray drying 1. Introduction 2. Spray drying principle Example: 100 ml of a solution are sprayed, resulting in approx. 8 x 108 = 800,000,000 drops (25 microns) representing approx. 12 m2 of sur- face area. This clearly demonstrates that the solvent (mainly water) is vaporized extremely quickly. Nozzle and product Spray drying is a very widely applied, technical method used to dry aqueous or organic solutions, emulsions etc., in industrial chemis- try and food industry. Dry milk powder, detergents and dyes are just a few spray dried products currently available. Spray drying can be used to preserve food or simply as a quick drying method. It also provides the advantage of weight and volume reduction. It is the transformation of feed from a fluid state into a dried particulate form by spraying the feed into a hot drying medium. Intensive re- search and development during the last two decades has resulted in spray drying becoming a highly competitive means of drying a wide variety of products. The range of product applications contin- ues to expand, so that today spray drying has connections with many things we use daily. Spray drying involves evaporation of moisture from an atomised feed by mixing the spray and the drying medium. The drying me- dium is typically air. The drying proceeds until the desired moisture content is reached in the sprayed particles and the product is then separated from the air. The mixture being sprayed can be a sol- vent, emulsion, suspension or dispersion. The dispersion can be achieved with a pressure nozzle, a two fluid nozzle, a rotary disk atomiser or an ultrasonic nozzle. So different kinds of energy can be used to disperse the liquid body into fine particles. The selection upon the atomiser type depends upon the nature and amount of feed and the desired characteristics of the dried product. The higher the energy for the dispersion, the smaller are the generated droplets. 1 The complete process of spray drying basically consists of a se- quence of four processes: 2.1 Dispersion of the feed solution in small drop- lets BÜCHI Labortechnik AG Training Papers Spray Drying 3 2.2 Mixing of spray and drying medium (air) with heat and mass transfer The manner in which spray contacts the drying air is an important factor in spray dryer design, as this has great bearing on dried prod- uct properties by influencing droplet behaviour during drying. This mixing is an important aspect and defines the method of spray drying: The material is sprayed in the opposite direction of the flow of hot air. The hot air flows upwards and the product falls through increas- ingly hot air into the collection tray. The residual moisture is elimi- nated, and the product becomes very hot. This method is suitable only for thermally stabile products. The material is sprayed in the same direction as the flow of hot air through the apparatus. The droplets come into contact with the hot drying air when they are the most moist. The product is treated with care due to the sudden vaporization. The advantages of both spraying methods are combined. The prod- uct is sprayed upwards and only remains in the hot zone for a short time to eliminate the residual moisture. Gravity then pulls the prod- uct into the cooler zone. Due to the fact that the product is only in the hot zone for a short time, the product is treated with care. Combined Co-Current flow Counter-Current flow BÜCHI Labortechnik AG Training Papers Spray Drying 4 The material to be sprayed flows onto a rapidly rotating atomizing disk and is converted to a fine mist. The drying air flows in the same direction. The product is treated with care, just as in the co-current flow method. As soon as droplets of the spray come into contact with the drying air, evaporation takes place from the saturated vapour film which is quickly established at the droplet surface. Due to the high specific surface area and the existing temperature and moisture gradients, an intense heat and mass transfer results in an efficient drying. The evaporation leads to a cooling of the droplet and thus to a small thermal load. Drying chamber design and air flow rate provide a droplet residence time in the chamber, so that the desired droplet moisture removal is completed and product removed from the dryer before product temperatures can rise to the outlet drying air tem- perature. Hence, there is little likelihood of heat damage to the prod- uct. 2.4 Drying of spray (removal of moisture) Disk atomizer (rotary wheel) Air is mostly used as drying medium. The air stream is heated elec- trically or in a burner and after the process exhausted to atmos- phere. This is a open-cycle system. If the heating medium is recy- cled and reused, typically an inert gas such as nitrogen, this is a closed-cycle system. These layout is typically chosen, when flam- mable solvents, toxic products or oxygen sensitive products are processed. The most common type of spray dryer is the open-cycle, co-cur- rent spray dryer. In such a design, the atomised feed and the drying air is simultaneously injected into a spray drying chamber from the same direction. 2.3 Open-cycle and closed cycle system BÜCHI Labortechnik AG Training Papers Spray Drying 5 In principal, two system are used to separate the product from the drying medium: 1 Primary separation of the drying product takes place at the base of the drying chamber 2 Total recovery of the dried product in the separation 2.4.1 Separation of product and air 3. General applications equipment Most common separation equipment is the cyclone. Based on inertial forces, the particles are separated to the cyclone wall as a down-going strain and removed. Other systems are electrostatic precipitators , textile (bag) filters or wet collectors like scrubbers. Possible applications The list of materials which are successfully spray dried is enormous, so only general principles should be listed hereby: Application Goal / use Practicalapplication Spray drying Drying of inorganic and organic products corn starch pigments dried milk Micronization Reduction of a product’s par- ticle size salt dyes Micro encapsulation A liquid product is embed- ded in a solid matrix perfumes strawberry aroma peach oil Englobing A solid product is embed- ded in another solid or a mixture of solids carotenoids in gelatins BÜCHI Labortechnik AG Training Papers Spray Drying 6 1 Air intake 2 Heater 3 Flow stabilizer intake into the drying chamber 4 Cyclone, the product is separated from the air flow here 5 Aspirator 6 Temperature sensor, air inlet 7 Temperature sensor, air outlet 8 Container for collecting finished product A Solution, emulsion or dispersion of the product B Peristaltic feed pump C Two fluid nozzle (spray mist, spray cone) D Compressed air or inert gas supply connection E Cooling water connection F Nozzle cleaning device, consisting of needle pneumatically pushed trough nozzle 4.2.2 Diagram of the product flow and spray nozzle 4 Mini Spray Dryer 4.2.1 Diagram of the dry air flow 4.2 Design of the Instrument The Mini Spray Dryer B-290 is a laboratory scale system to perform spray drying processes down to 50 ml batch volume and up to 1 litre solution per hour. Due to the glassware, the complete drying process from the two- fluid nozzle down to the collection vessel is visible. Even a lot of fundamen- tal investigations of the spray drying process has been undertaken, it still remains a step with some uncertainties and difficulties to model. One reason is the big influence of material properties and drying behaviour of the product and another is the complex fluid dynamics in a spray dryer. 4.1 Spray Drying with the BÜCHI Mini Spray Dryer B-290 Thus, small scale feasibility studies and trials are an often used approach to win some experience with a certain product to spray dry. Even the direct scale-up from a lab-bench unit to a big system cannot be easily made, it helps to understand and quantify the drying behaviour. For small batch sizes e.g. in pharmaceutical applications, a small spray dryer is particularly interesting to win small product volumes within a short time. Thermolabile components such as enzymes or antibiotics remain fully active. The Mini Spray Dryer B-290 functions according to the same principle as the co-current flow atomizer, i.e. the sprayed product and drying air flow are in the same direction. BÜCHI Labortechnik AG Training Papers Spray Drying 7 Spray Drying is a method where the result strongly depends upon the material properties. Thus, the instrument settings, namely inlet tempera- ture, feed rate, spray air flow and aspirator flow are in a combined system influencing the product parameters: - Temperature load - Final humidity - Particle size - Yield • Larger temperature differences between the inlet and outlet temperatures result in a larger amount of residual moisture. • A high aspirator speed means a shorter residence time in the device and results in a larger amount of residual moisture. • A high aspirator speed results in a higher degree of separa- tion in the cyclone. • Higher spray flow rates tend to result in smaller particles. • Higher spray concentrations result in larger particles. • Higher pump speed, result in a lower outlet temperature. 4.3.1 Interaction of the individual parameters The optimisation of these parameters are usually made in a ”Trial & Error” process. Some initial conditions can be found in the application database for equal or similar products. 4.3 Instrument settings aspirator rate ? air humidity ? inlet tempe- rature ? spray air flow ? solvent ins- tead of wa- ter concen- tration ? feed rate ? outlet tempera- ture ?? less heat losses based on total inlet of energy ? more energy stored in humidity ??? direct proportion ? more cool air to be heated up ?? more solvent to be evapo- rated ??? less heat of en- ergy of sol- vent ?? less water to be evaporated particle size - ??? more energy for fluid disper- sion (?) more fluid to dis- perse (?) less surface tension ??? more remaining product final humidity of product ??lower par- tial pressure of evapo- rated water ?? higher partial pres- sure of drying air ?? lower relative humidity in air - ?? more wa- ter leads to higher particel pressure ??? no wa- ter in feed leads to very dry product ? less water evaporated, lower partial pressure yield ?? better separation rate in cy- clone (?) more humidity can lead to sticking pro- duct (?) eventually dryer pro- duct prevent sticking (??) de- pends on application ?? no hygroscopic behaviour leads to easier dying ? bigger particles lead to higher separation - - - parameter dependence BÜCHI Labortechnik AG Training Papers Spray Drying 8 The outlet temperature is the result of the combination of the fol- lowing parameters: • Inlet temperature • Aspirator flow rate (quantity of air) • Peristaltic pump setting • Concentration of the material being sprayed The optimal choice for the temperature difference between the inlet and the outlet temperature is one of the most important points to consider when spray drying. Of course, other product specific fac- tors, such as the melting point or decay temperature, must be taken into consideration. In spite of this, there is still some room for ad- justment. The throughput of the device as well as the residual mois- ture content can be influenced within this temperature difference range. The following table shows the interaction between the inlet and outlet temperatures, depending on the pump throughput. The fol- lowing guidelines can be derived from the data: 4.3.2 Inlet temperature / outlet temperature Inlet temperature is understood as being the temperature of the heated drying air. The drying air is sucked or blowed in over a heater by the aspirator. The heated air temperature is measured prior to flowing into the drying chamber. When spray drying a solution, emulsion or dispersion the solvent is removed by vaporization. The temperature of the air flow does not have to be higher than the boiling point of water to evaporate the individual drops during the short residence time. The gradient between wet surface and not saturated gas leads to an evaporation at low temperatures. The final product is separated and has no further thermal load. The temperature of the air with the solid particles before entering the cyclone is designated as the outlet temperature. This tempera- ture is the resultig temperature of the heat and mass balance in the drying cylinder and thus cannot be regulated. Due to the intese heat and mass transfer and the loss of humidity, the particles can be regarded to have the same temperature as the gas. Thus, as a rule of thumb is: outlet temperature = max. product temperature. BÜCHI Labortechnik AG Training Papers Spray Drying 9 Outlet temperature de- pending on the pump per- formance for various inlet temperatures. For a final product with a very small amount of residual moisture, the inlet temperature must be as high as possible and the tem- perature difference must be as small as possible. Increasing the temperature difference while holding the inlet tem- perature constant increases the residual moisture content in the final product as well as the spray flow rate of the device. The drying air is sucked or blown through the device by the aspira- tor motor creating under pressure conditions. By regulating the aspirator speed, the amount of heated drying air can be increased or decreased. If the system is running in the sucking mode, a slight underpressure will take effect in the spray dryer. Because the amount of energy available for vaporization changes when the amount of drying air is increased or decreased, the aspirator speed setting has a significant effect on the drying performance of the device. The optimum setting must be determined experimentally using the following guidelines: High aspirator speed ? higher degree of separa- tion in the cyclone Lower aspirator speed ? lower residual moisture content Spray medium: distilled water 4.3.3 Aspirator BÜCHI Labortechnik AG Training Papers Spray Drying 10 Tube used: Silicon tube, inner diameter 2.0 mm 4.3.4 Pump performance The peristaltic pump feeds the spray solution to the nozzle. The pump’s speed affects the temperature difference between the inlet temperature and the outlet temperature. The pump rate directly corresponds to the inlet mass. The higher the throughput of solu- tion, the more energy is needed to evaporate the droplet to par- ticles. Thus, the outlet temperature decreases. The limitation of the pump is when the particules are not dry enough resulting in sticky prod- uct or wet walls in the cylinder. The pump throughput is also de- pendent upon various factors such as the viscosity of the spray solution and tubing diameter. The following guidelines can be derived from the facts described above as they relate to the pump rate: Increasing the pump rate lowers the outlet temperature and thus increases the temperature difference between the inlet tempera- ture and the outlet temperature. Reducing the pump rate while holding the inlet temperature and aspirator flow rate constant increases the dry content of the final product. B-290 Spray flow perfor- mance Spray medium: distilled water BÜCHI Labortechnik AG Training Papers Spray Drying 11 4.3.5 Spray flow and concentration of solution The spray flow rate is the amount of compressed air needed to disperse the solution, emulsion or suspension. A gas other than compressed air can be used. The spray flow rate can be set to between 300 and 800 l/h on the device. A rotameter indicates the spray flow throughput. The table below gives a correlation of the flow meter and the gas throughput. The particle size of the final product can be influenced by the spray flow rate setting. The higher the concentration of the spray solution, the larger and more porous the dried particles. The higher the spray flow rate, the smaller the size of the par- ticles in the final product. The spray concentration influences the particle size. A guideline is: Height (mm) Normlitre/hour Pressure drop Volume flow (real) 5 84 10 138 15 192 20 246 0.15 282.9 25 301 0.18 355.18 30 357 0.23 439.11 35 414 0.3 538.2 40 473 0.41 666.93 45 536 0.55 830.8 50 601 0.75 1051.75 55 670 1.05 1373.5 60 742 1.35 1743.7 65 819 1.8 2293.2 BÜCHI Labortechnik AG Training Papers Spray Drying 12 Applications Examples Product Inlet °C Outlet °C Spray concentration % Foodstuffs Low-fat milk 174 102 50 Yeast 95 55 60 Aroma/Cosmetics Beer concentrate 150 110 30-40 Olive leaf extract 150 90 36 Medical/pharmaceutical Blood plasma 180 100 5 Peptides 110 70 2 Chemical products Dispersion dyes 150 95 20 Spray drying is suited for most real or colloidal solutions, for emul- sions and dispersions as long as the dried product behaves like a solid. Diagram of spray drying inorganic or organic products An aqueous solution of the product (A) is dispersed into fine drop- lets (B) using a two fluid nozzle. The solvent evaporates immedi- ately surrounding the product in a vapor cloud that protects the product from thermal load. As soon as the critical concentration is exceeded, nucleation starts forming a solid shell. After the solvent is dryed away from the surface, the interface moves into the core (second step of drying).The final product (C) is a fine, amorphous or crystallized material. Spraying a highly concentrated solution re- sults in a more porous final product. 5 5.1 Spray drying BÜCHI Labortechnik AG Training Papers Spray Drying 13 5.2 Micronization or structural change A C D E B The crystalline product (A) is dissolved in a solvent (B) and this solu- tion (C) is dispersed into small droplets (D). The result is a final prod- uct (E) just like the final product described in the section on spray drying. Examples Product Inlet °C Outlet °C Spray concentration % Foodstuffs Lactose 160 105 30 Corn starch 130 70 40 Aroma/Cosmetics Metalsoap 165 122 60 Detergent 200 110 40 Medical/pharmaceutical Mixed products of 180 80 37 fructose-amino acid compounds Chemical products Calcium carbonate 220 100 10 Sodium citrate 160 90 20 The micronization or structural change is the change of morphol- ogy, e.g. if a fine powder is needed. This has a positive effect on the solubility or measurability of the final product. The main advantage of micronization is that a very regular particle size is achieved. Diagram of the micronization or structural change process BÜCHI Labortechnik AG Training Papers Spray Drying 14 5.3 Micro encpsulation An emulsion (D) is created from the liquid product to be treated (A), a carrier substance (B) such as maltodextrin and a filmogen solution (C) such as gum arabic in water. This emulsion is then sprayed into small droplets (E). The solvent evaporates leading to a solid matrix around the dispersed second phase (F). The result is that the small droplets of the product (A) are stored in the carrier substance (B) and embedded in the filmogen (C). Diagram of the micro encapsulation process Examples Product Inlet °C Outlet °C Spray concentration % Foodstuffs Soyabean oil in maltodextrin/gelatins 150 90 30 Aroma/Cosmetics Aroma, strawberry in maltodextrin/gelatins arabic 1,5 : 1,5 : 3 150 90 35 Medical/pharmaceutical Guajazulene in maltodextrin/gum arabic 1 : 2 : 1 120 70 ca. 30 BÜCHI Labortechnik AG Training Papers Spray Drying 15 5.4 Englobing Examples Product Inlet °C Outlet °C Spray concentration % Foodstuffs Inverted sugar (date pulp) in 100 80 20 Lactose 1:1 Medical/pharmaceutical Streptococci in low-fat milk powder/ glucose/gelatin 1:1:1:3 90 70 40 The englobing process is analogous to the micro encapsulation process, whereby a solid material is used instead of a liquid product. A solution or dispersion (D) is created from the product to be treated (A), a matrix (B) and water eventually with additional filmogen (C). This solution is then sprayed into small droplets (E).The matrix and / or filmogen lead to an agglomeration or coating of the suspended particles (F). Diagram of the englobing process BÜCHI Labortechnik AG Training Papers Spray Drying 16 5.5 Overview of selected applications Product Inlet °C Outlet °C Spray concentration % Baby food 160 95 40 Beer 180 108 Casein 150 90 6 Yeast 95 55 60 Krill 180 80 10 Lactose 160 105 30 Low-fat milk 174 102 50 Corn starch 130 70 40 Milk 110 70 15 Whey 180 80 6 /45 Soyabean extract (suspension) 130 75 80 Tofu 110 60 17 Englobing/micro encapsulation The application depends on the kind of product used, such as viscosity, density, additives etc. Therefore, the given parameters can not previsely be overtaken Foodstuffs Spray drying Product Inlet °C Outlet °C Spray concentration % Fruit concentrate, raspberry, in maltodextrin,2:8 150 90 30 Fruit concentrate, orange, in maltodextrin,2:8 150 90 40 Inverted sugar (date pulp) in lactose 1:1 100 80 20 Black currant juice in maltodextrin 170 100 47 Soybean oil in maltodextrin/gelatin 150 90 30 Sugar/fat mixture in maltodextrin/ 160 90 22 gum arabic 25:15:50:10 BÜCHI Labortechnik AG Training Papers Spray Drying 17 Aromas, cosmetics, cleaners and detergents Spray drying Product Inlet °C Outlet °C Spray concentration % Valerian extract 150 100 25 Beer concentrate 150 110 30 - 40 Chicory extract 130 75 38 Pine bark extract 120 85 4 Chestnut extract 200 130 20 Metal soap 165 122 60 Microfoam beads 160 114 3 Sodium citrate 160 90 20 Sodium orthophosphate 180 110 40 Olive leaf extract 150 90 36 Liquorice extract 100 75 36 Detergent 200 110 40 Fabric softener 125 75 20 Xanthane mixture 130 70 - Zeolite 180 120 10 Englobing/micro encapsulation Product Inlet °C Outlet °C Spray concentration % Aroma, strawberry in maltodextrin/ 150 90 35 gum arabic 1,5 : 1,5 : 3 Aroma, orange in maltodextrin/ 150 90 17 gum arabic 1,5 : 1,5 : 3 Cardamom oil in maltodextrin/ 170 100 20 gum arabic 1 : 20 : 39 Date juice in maltodextrin/ 120 90 30 gum arabic 25 : 25 : 1 Caraway oil in maidex/ 140 100 50 gum arabic Perfume oil in maltodextrin/ 130 70 gum arabic 1 : 2 : 1 Peach oil in maltodextrin 150 100 20 Bubble bath in sodium chloride1 : 2 160 100 15 Cinnamon oil in maltodextrin/ 170 100 20 gum arabic 1 : 3 : 6 Lemon oil in maltodextrin/ 130 90 20 gum arabic BÜCHI Labortechnik AG Training Papers Spray Drying 18 Medical/Pharmaceutical Spray drying Product Inlet °C Outlet °C Spray concentration % Albumin 110 60 5 Lyophilized anti-progresterone serum 80 60 1 Blood plasma 180 100 5 Dextran 154 120 20 Enzymes / coenzymes 80 55 12 Fructose-amino acid compounds 180 80 37 Galactomannan 200 115 5 Gelatin capsule dispersions 105 80 20 Glucose / amino acid compounds 1:1 130 80 10 Mannitol with enzymes 100 55 15 Combination vaccines 190 140 - Organ extracts with tetra-Na-diphosphate 1 : 0,43 150 88 11 Peptides 110 70 2 Vitamin A + E / gelatin-emulsion 100 55 - Cell suspension (bacteria cultures) 90 60 ca. 50 Englobing/micro encapsulation Carotinoid in gelatin 40 : 60 170 100 25 Guajazulene in maltodextrin/ 120 70 ca. 30 gum arabic 1 : 2 : 1 Streptococci in low-fat milk powder / 90 70 40 glucose/gelatin 1 : 1 : 1 : 3 Product Inlet °C Outlet °C Spray concentration % BÜCHI Labortechnik AG Training Papers Spray Drying 19 Chemical products Spray drying Product Inlet °C Outlet °C Spray concentration % Acrylamide 125 69 50 Albigen 180 90 10 Ammonium chloride 180 75 20 Ammonium nitrate 180 100 20 Lead oxide 150 90 — Calciumhydrogen citrate 200 110 50 Calcium carbonate 220 100 10 Calcium phosphate 190 100 — Dicalcium phosphate 170 90 20 Disodium phospate 200 140 50 Dispersion dyes 150 95 20 Iron oxide 170 125 — Pigments 130 110 36 Glass powder 120 90 20 Latex rubber 120 70 20 Indigo-sodium sulfate compound 150 90 30 Potassium hydrogen citrate 200 110 50 China clay 180 130 33 Various ceramics 150 120 46 Synthetic glues 100 70 20 Latex 160 90 31 Lignin 130 55 7/4 Magnesium phosphate 120 90 15 Melamine resin 120 80 — Metal oxide 210 135 — Sodium citrate 160 90 20 Sodium orthophosphate 180 110 40 Sodium sulfite 180 90 20 Cermaic oxide 100 80 26 Phenolic resin 135 105 50 Polyacrylamide 204 111 3 PVC-latex 160 90 31 Clay suspension 200 100 1,2 Peat extract 120 80 1,5 Vinyl acetate polymer 90 50 25 Zeolite 180 120 10 Tin oxide 230 170 — Zirconium oxide 180 100 —