A Study of the Limitations to Spray Dryer Outlet Performance

The concept of the product moisture locus was tested in this work using a pilot-scale modified Niro spray dryer (diameter 0.8 m, height 2 m), where the residence time of the particles inside this spray dryer is lower compared with larger industrial spray dryers. The moisture contents of skim milk powder produced from spray drying skim milk (solids content 8.8% w/v) at different operating conditions, namely different swirl vane angles (0°, 25°, 30°), inlet air temperatures (170°C, 200°C, 230°C) and process fluid flowrates (1.4 kg h^-1, 1.6 kg h^-1, 1.8 kg h^-1), were compared with the predicted equilibrium moisture contents. In addition, the residence time of the particles was also increased in the spray dryer by decreasing the inlet air mass flowrate from 0.016 to 0.013 kg s^-1. The outlet moisture contents of the skim milk powder for all the 23 runs carried out in this work were within 0.4% of the equilibrium values. Thus, the skim milk powder particles were in close equilibrium with the gas inside the drying chamber. These equilibrium limitations are confirmed by other literature data (Boonyai, P. Comparative Evaluation of Soymilk Drying in a Spray Dryer and Spouted Bed of Inert Particles. M.Sc. Thesis. Asian Institute of Technology: Bangkok, Thailand, 2000; 90 pp; Harvie, D.J.E.; Langrish, T.A.G.; Fletcher, D.F. A computational fluid dynamics study of a tall-form spray dryer. Trans IChemE 2002, in press). The use of this finding to predict spray dryer performance is demonstrated by mass and energy balance calculations.     

Fluxes and Patterns of Wall Deposits for Skim Milk in a Pilot-Scale Spray Dryer

The pattern of wall deposits in a pilot-scale spray dryer has been studied, using skim milk, by changing the flow rate to the nozzle and measuring the resultant deposition fluxes at different positions inside the dryer. The solids concentration was maintained at 30%. The deposition was measured at three locations of the conical section and in the cylindrical section of the spray dryer. Particle deposition can be either due to the inertia of the particles or turbulent diffusion, and it is not immediately obvious which of these mechanisms is dominant. Inertial deposition appeared to be present mainly at the bottom location of the conical section and was the largest amount in quantitative terms, being at least an order of magnitude larger compared with diffusion deposition, which seemed to be dominant on the side (cylindrical) walls of the spray dryer. In addition to the above observations, the deposition patterns in the conical section have been quantified. The relative deposition flux, in m^-2, which is the ratio of the deposition flux, in g m^-2 h^-1, to the solids flow rate into the dryer, in g h^-1, was between 0.04 and 0.09 m^-2 at a solids concentration of 8.8% and between 0.15 and 0.4 m^-2 at a solids concentration of 30%. The fused appearance of the microstructure in the wall deposits of skim milk powder, as seen in the Micro-CT study, suggests that re-entrainment of the wall deposits is unlikely.     

COMPARISON OF GLASS TRANSITION TEMPERATURE AND STICKY POINT TEMPERATURE FOR SKIM MILK POWDER

Wall deposition of particles in spray dryers is a key processing problem, and information about the glass transition temperature of the amorphous material that arises from spray drying can be used to guide the selection of operating conditions that may minimise wall deposition. The glass transition temperatures for skim milk powder with various moisture contents were determined using Differential Scanning Calorimetry (DSC), and a repeatable glass transition temperature diagram was established from these results. The glass transition temperature decreased as the moisture content increased, as expected (low moisture content 1.65 g/100 g of dry powder, glass transition temperature 87.7°C; high moisture content 4.52 g/100 g of dry powder, glass transition temperature 46.7°C). The glass transition temperature was found to be virtually the same as the sticky-point temperature measured using a thermo-mechanical test. The difference is essentially due to the difference between doing a mechanical test for viscosity (sticky-point) and a phase transition measurement (DSC).     

A Study of the Limitations to Spray Dryer Outlet Performance

Abstract

The concept of the product moisture locus was tested in this work using a pilot-scale modified Niro spray dryer (diameter 0.8 m, height 2 m), where the residence time of the particles inside this spray dryer is lower compared with larger industrial spray dryers. The moisture contents of skim milk powder produced from spray drying skim milk (solids content 8.8% w/v) at different operating conditions, namely different swirl vane angles (0°, 25°, 30°), inlet air temperatures (170°C, 200°C, 230°C) and process fluid flowrates (1.4 kg h^?1, 1.6 kg h^?1, 1.8 kg h^?1), were compared with the predicted equilibrium moisture contents. In addition, the residence time of the particles was also increased in the spray dryer by decreasing the inlet air mass flowrate from 0.016 to 0.013 kg s^?1. The outlet moisture contents of the skim milk powder for all the 23 runs carried out in this work were within 0.4% of the equilibrium values. Thus, the skim milk powder particles were in close equilibrium with the gas inside the drying chamber. These equilibrium limitations are confirmed by other literature data (Boonyai, P. Comparative Evaluation of Soymilk Drying in a Spray Dryer and Spouted Bed of Inert Particles. M.Sc. Thesis. Asian Institute of Technology: Bangkok, Thailand, 2000; 90 pp; Harvie, D.J.E.; Langrish, T.A.G.; Fletcher, D.F. A computational fluid dynamics study of a tall-form spray dryer. Trans IChemE 2002, in press). The use of this finding to predict spray dryer performance is demonstrated by mass and energy balance calculations.     

Spray Drying of Skim Milk Mixed with Milk Permeate: Effect on Drying Behavior, Physicochemical Properties, and Storage Stability of Powder

The possibility of using milk permeate (MP) to lower the protein level of skim milk powder (SMP) in producing powders of 34% and lower protein is explored. Skim milk suspensions with various levels of MP were prepared by mixing SMP and MP powder (MPP) at the ratios of 1:0, 7:3, 3:7, and 0:1: from 34 to 5.3% protein. The suspensions were dried in a spray dryer with inlet and outlet temperatures of 180 and 80°C, respectively. Increasing permeate concentration in the mixture showed a greater tendency to stickiness manifested by lowered the cyclone recovery of the powder as more powder stuck on the wall of the dryer. Increasing permeate concentration in the resultant powder did not significantly affect the bulk density but led to a reduction in the particle size and also made the powder slight green and yellowish in color. It also found to lower the glass transition temperature (T_g) of the skim milk powder (SMP) and induce crystallization of lactose at lower water activity (a_w ≥ 0.328 for SMP:MPP of 3:7 and 0:1 compared to a_w ≥ 0.0.432 for SMP:MPP of 1:0 and 3:7). Addition of MP in SMP lowered the T_g values of the resulting powders. The permeate fraction in spray-dried SMP/MPP mixtures found to lower the critical a_w and moisture content, suggesting the SMP mixed with MPP is more likely to become sticky than SMP alone (at 34% protein) when stored at a similar water activity and moisture content.     

An Investigation and Quantitative Assessment of Particle Shape in Milk Powders from a Laboratory-Scale Spray Dryer

Seven samples of spray-dried milk were prepared using a miniature-scale Buchi Mini Spray Dryer B-290 (diameter 0.5 m, height 1.1 m). For each run, all inlet conditions were held constant except for feed type, inlet solids concentration, and inlet temperature. Skim milk at a solids concentration of 8.8% and whole milk at a solids concentration of 11% were dried at two inlet temperatures, 120 and 200°C. Lactose-free skim milk (8.8% solids concentration) and skim milk at a solids concentration of 41.2% were also dried at an inlet temperature of 200°C to assess the effects of milk feed type and inlet concentration, respectively. Equilibrium between the outlet product moisture content and the outlet gas conditions in the miniature spray dryer was not reached, unlike previous results for pilot-scale and larger dryers, so it appears that, in small (miniature-scale) dryers, the outlet moisture content is limited by kinetics and not by equilibrium. Calculated yields ranged from 10.4 to 82.7%, with whole milk giving significantly lower yields than skim milk, due to the sticky nature of fat found in whole milk. Lactose-free skim milk produced lower yields than skim milk dried at the same conditions, indicating that the lower glass-transition temperatures of the converted lactose sugars make these powders stickier. This sugar effect was not as large as that of the fat content. Comparisons between the SEM images, fractal dimensions, particle size distributions, and bulk densities showed that lower inlet temperatures produced particles of a more collapsed nature with a higher bulk density and lower fractal dimension than milk particles dried at higher temperatures. Also, feed solids concentration heavily influenced the shape of the particles, with high concentrations producing more spherical, less broken or shriveled particles with higher fractal dimensions, as the shell walls are thicker under these conditions. The fractal dimensions appeared to give generally consistent results for quantifying the average particle shapes.     

Drying Characteristics of Barley Grain Dried in a Spouted-Bed and Combined IR-Convection Dryers

A study was performed to determine the drying characteristics and quality of barley grain dried in a laboratory scale spouted-bed dryer at 30, 35, 40, and 45°C and an inlet air velocity of 23 m/s^-1, and in an IR-convection dryer under an infrared radiation intensity of 0.048, 0.061, 0.073, and 0.107 W cm^-2 at an air velocity of 0.5 m/s^-1. The results show that the first, relatively short, phase of a sharp decrease in the drying rate was followed by the phase of a slow decrease. The time of barley drying depended on temperature of inlet air in a spouted-bed dryer and on radiation intensities in an IR-convection dryer. Barley drying at 45°C in a spouted-bed dryer was accompanied by the lowest total energy consumption. The average specific energy consumption was lower and the average efficiency of drying was higher for drying in a spouted-bed dryer. The effective diffusivities were in the range 2.20-4.52 × 10^-11 m² s^-1 and 3.04-4.79 × 10^-11 m²/s^-1 for barley dried in a spouted-bed and in an IR-convection dryer, respectively. There were no significant differences in kernel germination energy and capacity between the two drying methods tested.     

SPRAY DRYING DYNAMIC MODELING WITH A MECHANISTIC MODEL

In this work we suggest the dynamic modeling of a spray dryer considered as a series of well-stirred dryers. That is, a series of dryers in which the output variables are equal to the state variables. The state equations were obtained from the heat and water mass balances in product and air. Additionally, heat and water mass balances in interface jointly with water equilibrium relation between product and air were considered. A pilot spray dryer was modeled assuming one, two, five and 20 well stirred steps. Low-fat milk with 10-20% of solids was dried at different inlet air temperatures (120-160°C), air flow rate of 0.19 kg dry air s^-1 and different feed rates (1.4 - 4.2 × 10^-4 kg dry solids s^-1). Stationary result showed that the model predicts the experimental air outlet temperature, at different inlet conditions with a maximum deviation of 6°C. The dynamic simulation reproduce the experimental one with moderate accuracy. Experimental dynamic showed that the pilot plant spray dryer has a well-stirred process behavior. The model represents a method for estimate outlet product moisture as function of the outlet air temperature. This has application for automatic control because there is not an easy way to measure on-line measure the outlet product moisture content.     

Fluxes and Patterns of Wall Deposits for Skim Milk in a Pilot-Scale Spray Dryer

The pattern of wall deposits in a pilot-scale spray dryer has been studied, using skim milk, by changing the flow rate to the nozzle and measuring the resultant deposition fluxes at different positions inside the dryer. The solids concentration was maintained at 30%. The deposition was measured at three locations of the conical section and in the cylindrical section of the spray dryer. Particle deposition can be either due to the inertia of the particles or turbulent diffusion, and it is not immediately obvious which of these mechanisms is dominant. Inertial deposition appeared to be present mainly at the bottom location of the conical section and was the largest amount in quantitative terms, being at least an order of magnitude larger compared with diffusion deposition, which seemed to be dominant on the side (cylindrical) walls of the spray dryer. In addition to the above observations, the deposition patterns in the conical section have been quantified. The relative deposition flux, in m^?2, which is the ratio of the deposition flux, in g m^?2 h^?1, to the solids flow rate into the dryer, in g h^?1, was between 0.04 and 0.09 m^?2 at a solids concentration of 8.8% and between 0.15 and 0.4 m^?2 at a solids concentration of 30%. The fused appearance of the microstructure in the wall deposits of skim milk powder, as seen in the Micro-CT study, suggests that re-entrainment of the wall deposits is unlikely.     

Artificial Neural Network Modeling of the Deposition Rate of Lactose Powder in Spray Dryers

A spray dryer is the ideal equipment for the production of food powders because it can easily impart well-defined end product characteristics such as moisture content, particle size, porosity, and bulk density. Wall deposition of particles in spray dryers is a key processing problem and an understanding of wall deposition can guide the selection of operating conditions to minimize this problem. The stickiness of powders causes the deposition of particles on the wall. Operating parameters such as inlet air temperature and feed flow rate affect the air temperature and humidity inside the dryer, which together with the addition of drying aids can affect the stickiness and moisture content of the product and hence its deposition on the wall. In this article, an artificial neural network (ANN) method was used to model the effects of inlet air temperature, feed flow rate, and maltodextrin ratio on wall deposition flux and moisture content of lactose-rich products. An ANN trained by back-propagation algorithms was developed to predict two performance indices based on the three input variables. The results showed good agreement between predicted results using the ANN and the measured data taken under the same conditions. The optimum condition found by the ANN for minimum moisture content and minimum wall deposition rate for lactose-rich feed was inlet air temperature of 140°C, feed rate of 23 mL/min, and maltodextrin ratio of 45%. The ANN technology has been shown to be an excellent investigative and predictive tool for spray drying of lactose-rich products.     

Activity-loss characteristics of spores of Bacillus thuringiensis during spray drying

Survival of spores of Bacillus thuringiensis was determined under various processing conditions for spray drying. The results indicated that the viable spores number of B. thuringiensis decreased with increased inlet air temperature, outlet air temperature and atomising air pressure. When the inlet air temperature ranged from 170 °C to 250 °C, the outlet air temperature (75 °C) and atomising air pressure (0.15 MPa) were fixed, the pseudo-z value (one logarithmic cycle reduction) was 238.1 °C; when the outlet air temperature ranged from 65 °C to 95 °C, the inlet air temperature (200 °C) and atomising air pressure (0.15 MPa) were fixed, the pseudo-z value was 85.5 °C, the activation energy calculated according to the outlet air temperature was 59.96 kJ mol⁻¹; when the atomising air pressure ranged from 0.10 MPa to 0.25 MPa, the inlet air temperature (200 °C) and outlet air temperature (75 °C) were fixed, the pseudo-z value was 3.49 MPa, the variance analysis showed that the atomising air pressure has no significant influence to the spores. The diluting solutions between Tween-80 solution and phosphate buffer have significant influence on the plate count of spores.

The B. thuringiensis powder prepared by spray drying with inlet air temperature 250 °C, outlet air temperature 97 °C, the spores count of powder decreased obviously. But when the inlet air temperature of 155–165 °C, outlet air temperature of 66–70 °C were employed, the spores count of powder approaches to that of freeze drying powder. The spores count of oven drying powder was lower than that of the freeze drying powder, but close to the spray drying powder which inlet air temperature was 200 °C, outlet air temperature was 75 °C.     

Dairy Milk Particles Made with a Mono-Disperse Droplet Spray Dryer (MDDSD) Investigated for the Effect of Fat

Mono-disperse droplet generation and subsequent drying in a spray-drying chamber, i.e., mono-disperse droplet spray dryer (MDDSD), provides a better-defined “flight experience” for liquid droplets. The related particle formation can be investigated more easily than that in the usual poly-disperse droplet spray dryer (PDDSD). Previously, skim milk, which is of high protein and high lactose content and is one of the two main dairy fluids that are processed into powder form for consumer markets, was subjected to this kind of investigation in Australia. Here, whole milk, which is the other main dairy fluid, has been spray-dried in a MDDSD set-up at Xiamen University (China). Because the initial droplet size is uniform, measurable, and the particle morphology after drying is consistent, it was possible to investigate the initial solids content effect upon shrinkage and inlet air temperature effect upon shrinkage. In contrast to what had been found for skim milk particles, the formation of the fat-containing (whole milk) particles does not follow the perfect shrinkage model as the skim milk does. This work has improved our quantitative understanding of the whole milk drying process. A fundamental analysis invoked with a modified one-dimensional modeling of spray drying has been given that has shown some further insight about the process.     

Experimental Results Contributed to Early Detection of Smouldering Milk Powder as an Integrated Part of Maintaining Spray Drying Plant Safety

Smoldering milk powder lumps deposited on dryer walls or falling through the hot air could potentially start a fire or even an explosion in milk powder dryers. Explosive behavior is more frequently experienced in the modern industry, where high production throughputs are desired. It has been known that powder at such a state emits carbon monoxide (CO). The work reported in this article is the study of the characteristics of CO generation during the incomplete combustion of milk powders. The powders used in the study were whole milk and skim milk. Samples placed in a reactor were heated using air at 300°C. The exhaust air of the reactor was continuously measured for CO concentration using a highly sensitive CO analyzer. Temperatures were monitored, and the smoke produced during the combustion of whole milk powder was collected and analyzed using gas-chromatography mass-spectroscopy (GC-MS). Whole milk powder was found to produce significantly higher amounts of CO than skim milk powder. Hydrocarbons, alcohols, and carboxylic acids were tentatively identified during the GC-MS analyses. Results from the study are definitely useful in optimizing the use of an early fire detection system in milk powder plants.     

A Parametric Study of the Gas Flow Patterns and Drying Performance of Co-current Spray Dryer: Results of a Computational Fluid Dynamics Study

A computational fluid dynamic study was carried out to investigate airflow pattern, temperature, and humidity profile at different levels in the drying chamber. Good agreement was obtained with published experimental data. The effects of operating pressure, heat loss from the chamber wall and inlet air conditions on the gas flow pattern, droplet trajectories, and overall dryer performance also were investigated. Results are presented and discussed in terms of the gas velocity, temperature, and humidity profiles within the chambers. The volumetric evaporation values, heat transfer intensity, and thermal energy consumption per unit evaporation rate were computed and compared for drying of a 42.5% solids solution in a spray chamber 2.215 m in diameter with a cylindrical top section 2.005 m high and a bottom cone 1.725 m high. Wall regions subject to formation of undesirable deposits are also identified.     

Spray Drying of Chicken Meat Protein Hydrolysate: Influence of Process Conditions on Powder Property and Dryer Performance

The influence of operational conditions of a spray dryer on powder properties and equipment performance during spray drying of chicken meat hydrolysate was evaluated by a central composite rotatable design. The independent variables were inlet air temperature (120 to 200°C) and feed flow (0.1 to 0.38 kg/h). Spray dryer performance was assessed through estimation of product recovery, outlet air temperature, thermal efficiency, and energy on the dryer, obtained by mass and heat balance in the dryer system. Powder property was characterized in respect to antioxidant activity. The stable free radical diphenylpicrylhydrazyl (DPPH) was used to estimate the antioxidant activity of protein hydrolysate powder. This response varied from 38.7 to 59.4% and was only affected by inlet air temperature. Moreover, the results demonstrate a significant effect of the processing conditions on dryer performance. The increase of feed flow results in higher thermal efficiency and lower energy on dryer. Higher product recovery values were obtained at lower inlet air temperature and feed flow.     

STABILITY AND SENSORY QUALITY OF SPRAY DRIED AVOCADO PASTE

Avocado (Persea americand) paste was spray dried at inlet air temperature of 180°C, 80°C outlet air temperature, air velocity of 27 m/s and a feed flow rate of 0.642 l/min. Lipids in the paste were emulsified using 10 % Monoacylglyceride (MAG). Treatments were applied according to the following antioxidants mixtures: 1) BHA (butylated hydroxyanisole) + BHT (butylated hydroxytoluene) (0.05 % each); 2) TBHQ (Tertiary butylated hydroxyquinone) + Citric Acid (0.05 + 0.1 %); 3) BHA + BHT + Citric Acid (0.05 + 0.05 + 0.1 %); 4) BHA + Propyl gallate (0.05 + 0.05 %); or 5) BHA + Propyl gallate + Citric acid (0.05 + 0.05 + 0.1 %). Samples were stored at 6, 12, 25, 28 and 40 °C. Peroxide values were determined periodically. Development of rancidity was detected by sensory evaluation of the samples. For samples kept at 6 and 12 °C, an antioxidant mixture containing BHA and propyl gallate at 0.05% gave the least protection to the stored avocado powder. The mixture containing TBHQ and citric acid yielded the lowest rancidity development.     

Spray Drying of Skim Milk Mixed with Milk Permeate: Effect on Drying Behavior,Physicochemical Properties,and Storage Stability of Powder

The possibility of using milk permeate (MP) to lower the protein level of skim milk powder (SMP) in producing powders of 34% and lower protein is explored. Skim milk suspensions with various levels of MP were prepared by mixing SMP and MP powder (MPP) at the ratios of 1:0, 7:3, 3:7, and 0:1: from 34 to 5.3% protein. The suspensions were dried in a spray dryer with inlet and outlet temperatures of 180 and 80°C, respectively. Increasing permeate concentration in the mixture showed a greater tendency to stickiness manifested by lowered the cyclone recovery of the powder as more powder stuck on the wall of the dryer. Increasing permeate concentration in the resultant powder did not significantly affect the bulk density but led to a reduction in the particle size and also made the powder slight green and yellowish in color. It also found to lower the glass transition temperature (T_g ) of the skim milk powder (SMP) and induce crystallization of lactose at lower water activity (a_w  ≥ 0.328 for SMP:MPP of 3:7 and 0:1 compared to a_w  ≥ 0.0.432 for SMP:MPP of 1:0 and 3:7). Addition of MP in SMP lowered the T_g values of the resulting powders. The permeate fraction in spray-dried SMP/MPP mixtures found to lower the critical a_w and moisture content, suggesting the SMP mixed with MPP is more likely to become sticky than SMP alone (at 34% protein) when stored at a similar water activity and moisture content.     

Partial Crystallization Behavior during Spray Drying: Simulations and Experiments

Simulations of crystallization behavior in a spray dryer have been performed using modifications to a model developed by previous workers and applied to a Buchi-290 laboratory-scale dryer. The potential crystallization behavior has been modeled using Williams-Landel-Ferry kinetics. Explorations using the model have suggested that the air inlet temperature is an important variable affecting crystallization in the dryer. The explorations suggested drying conditions that permitted reasonable drying while controlling the degree of crystallization. These conditions were examined and tested experimentally, also showing that the apparent degree of crystallinity was affected by the inlet air temperature over the range of inlet temperatures from 134°C (∼55% crystalline) to 210°C (∼76% crystalline). The simulation also predicted the trends in the experimental results from previous workers where their experimental results are considered in terms of the measurement techniques used in each case, suggesting that the simulation is a reasonable tool for developing operating conditions for drying equipment to give low or high degrees of crystallinity in the products.     

Drying of Probiotics: Optimization of Formulation and Process to Enhance Storage Survival

This work describes how probiotic bacteria can be dried at low temperature in two steps, combining spray drying and vacuum drying, in order to enhance their survival during storage. A sufficient number of dried probiotics survived storage for more than 3 months at 30°C, if an appropriate combination of protein and carbohydrate was selected as carrier and storage conditions were maintained optimal. The use of soy protein and maltodextrin or skim milk and arabic gum resulted in the best survival rates of probiotics during storage. No evident difference was found between different spray dryer configurations, although a cocurrent flow was preferred.     

Drying Kinetics and Color Retention of Dehydrated Rosehips

Freshly harvested rosehips (Rosa canina L.) were dehydrated in a parallel flow type air dryer at six air temperatures (30, 40, 50, 60, and 70°C) at air velocities of 0.5, 1.0, and 1.5 m/s. Drying air temperature and velocity significantly influenced drying time and energy requirement. Minimum and maximum energy requirement for drying of rosehips were determined as 6.69 kWh/kg for 70°C at 0.5 m/s, and 42.46 kWh/kg for 50°C, 1.5 m/s. In order to reduce drying energy consumption, it is recommended that the drying air velocity must not be more than 0.5 m/s and drying air temperature should be 70°C. In addition, the influence of drying air temperature and air velocity on the color of dried rosehip has been studied. Hunter L, a, b values were used to evaluate changes in the total color difference (ΔE) on dried rosehips. 70°C drying air temperature and 1 m/s air velocity were found to yield better quality product.