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.     

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 DYNAMIC MODELING WITH A MECHANISTIC MODEL

ABSTRACT

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.     

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.     

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.     

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.     

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.     

Influence of Dryer Type on Surface Characteristics of Milk Powders

Reconstituted milk powders (skim milk, whole milk, and milk protein concentrate powders) were spray dried using research-scale (laboratory- and pilot-scale) spray dryers to investigate the influence of type on the dryer on the surface composition (protein, fat, lactose) and morphological characteristics (size and surface structure) of the powder particles. Milk powders produced by these research-scale dryers were compared to commercially produced and freeze-dried powders. The powders produced by the laboratory- and pilot-scale dryers were significantly different from the commercially dried powders in both surface composition and morphology. The milk powders produced by laboratory- and pilot-scale dryers provided reproducible results with similar surface morphologies between dryer types, despite varying surface compositions. The surface composition of the freeze-dried powder was also significantly different from than that of the spray-dried powders. The freeze-dried skim milk powder (SMP) was similar in surface protein to its bulk protein composition, indicating relative homogeneity of particle composition of the freeze-dried powder.     

Modeling and simulation of drying characteristics on flexible filamentous particles in rotary dryers

Experiments were conducted to demonstrate the effects of the drum wall temperature on the heat and mass transfer in rotary dryers. The drying characteristics of flexible filamentous particles in rotary dryers were further explored. In addition, the inlet and outlet temperatures and moisture contents of granular particles were measured. As a result, the good agreement between the simulations and experiments verified the rationale and feasibility of the numerical method. Therefore, the approach was adopted to evaluate the temperature and moisture content of wet granular particles in a rotary dryer in different conditions, for instance, drum wall temperature and rotational speed. The results revealed that the higher drum wall temperature led to hotter particles with lower outlet moisture content. Conversely, the higher rotational speed resulted in cooler particles with higher outlet moisture content due to the decrease of residence time in the rotary dryer.     

Spray Drying of Mannitol as a Drug Carrier—The Impact of Process Parameters on Product Properties

Powders intended for the use in dry powder inhalers have to fulfill specific product properties, which must be closely controlled in order to ensure reproducible and efficient dosing. Spray drying is an ideal technique for the preparation of such powders for several reasons. The aim of this work was to investigate the influence of spray-drying process parameters on relevant product properties, namely, surface topography, size, breaking strength, and polymorphism of mannitol carrier particles intended for the use in dry powder inhalers. In order to address this question, a full-factorial design with four factors at two levels was used. The four factors were feed concentration (10 and 20% [w/w]), gas heater temperature (170 and 190°C), feed rate (10 and 20 L/h), and atomizer rotation speed (6,300 and 8,100 rpm). The liquid spray was carefully analyzed to better understand the dependence of the particle size of the final product on the former droplet size. High gas heater temperatures and low feed rates, corresponding to high outlet temperatures of the dryer (96–98°C), led to smoother particles with surfaces consisting of smaller crystals compared to those achieved at low outlet temperatures (74–75°C), due to lower gas heater temperatures and higher feed rates. A high solution concentration of the feed also resulted in the formation of comparably rougher surfaces than a low feed concentration. Spray-dried particles showed a volume-weighted mean particle size of 71.4–90.0 µm and narrow particle size distributions. The mean particle size was influenced by the atomizer rotation speed and feed concentration. Higher rotation speeds and lower feed concentrations resulted in smaller particles. Breaking strength of the dried particles was significantly influenced by gas heater temperature and feed rate. High gas heater temperatures increased the breaking strength, whereas high feed rates decreased it. No influence of the process parameters on the polymorphism was observed. All products were crystalline, consisting of at least 96.9% of mannitol crystal modification I.     

Characterization of a Spray Drying System for Soymilk

ABSTRACT

Soymilk slurries were spray dried at various combinations of inlet air temperature (1255. 265, and 275°C), feed rate (0.38 and 0.57 Lpm), and atomizer speed (119,300 and 26,800 rpm). Monitored dryer parameters included dry and wet bulb temperaturea of ambient and exhaust air, drying chamber's inlet and outlet air temperatures, product and feed temperatures, velocity pressure at exhaust pipe, atomizer speed, and feed rate. Weather data was included in the analysis.

Collected data was used to estimate the state of the outlet air using psychrometrics and assuming adiabatic saturation. Equations to predict outlet and product temperatures were developed. Average thermal and eveporative efficiencies were 65.9 and 78.4%. respectively.     

Influence of Processing Conditions on the Physicochemical Properties and Shelf-Life of Spray-Dried Palm Sugar (Arenga pinnata) Powder

The influence of spray-drying conditions on some physicochemical properties of palm-sugar powder ( Arenga pinnata ) was studied. Twenty tests were carried out according to a central composite design. Independent variables were: inlet temperature (150–190°C), feed flow rate (9–21 g/min), and maltodextrin concentration (14–25%). Process yield, hygroscopicity, and outlet temperature were analyzed as responses. Results revealed that increasing inlet temperature resulted in higher process yield and outlet temperature and a lower hygroscopicity. Similarly, higher inlet temperature led to lower moisture content, dissolution rate, and total phenolic content of the powder. Conversely, the feed flow rate negatively influenced process yield and hygroscopicity, and positively influenced moisture content. Moreover, maltodextrin exhibited negative influence on process yield and hygroscopicity, respectively. Moreover, storage (30°C, six months) led to noticeable losses in flowability and wettability. Powder morphology was also influenced by the inlet temperature. Lower inlet temperature resulted in particles with shrivelled surfaces while higher temperature produced a greater number of larger particles with smooth surfaces.     

Influence of the Entrance Configuration on the Performance of a Non-Mechanical Solid Feeding Device for a Pneumatic Dryer

The solids feeder is an important component of a dryer, since it is responsible for introducing the moist material at controlled, specified rates. The purpose of this paper is to investigate the effects of solids feeding configuration on the fluid dynamic behavior of a 53-mm-diameter vertical pneumatic conveyor with a loop of 180°, aiming at further applications in drying granular materials. A non-mechanical solid feeding system constituted by a hopper connected to an inclined pipe was applied to feed type D particles in the conveying line. This simple feeding apparatus was modified through the insertion of different flow restriction devices at the air inlet, namely a reduction nozzle and a Venturi device. This was aimed at studying how the solids flow rates and the fluid dynamics of the whole conveying line are affected by the entrance configuration and inlet devices. The use of inlet devices combined with the non-mechanical inclined valve affected significantly the performance of the valve when operating with type D particles in a pneumatic conveying line. When using inlet devices, an increase in the conveyed solid flow rates at a given air velocity was observed. The reduction nozzle yielded a range of solids loading ratios similar to that of the inclined valve with no inlet device, and introduced some pressure instabilities at the entrance region. The Venturi device allowed operation at a wider range of solids loading ratios and no pressure instability was detected in the conveying line. For the conditions investigated, neither the gas velocity nor the loading ratio affected the extent of entrance length. The inlet devices may be successfully applied to modify and improve the performance of the inclined valve as a solids feeder in pneumatic dryers.     

Effects of skim milk concentrate dry matter and spray drying air temperature on formation of capsules with varying particle size and the survival microbial cultures in a microcapsule matrix

The impact of total solid (TS) content in combination with the feed rate and air inlet temperature on the survival of Lactobacillus paracasei ssp. paracasei F19 after spray drying in a skim milk matrix has been investigated and correlated with the capsule size. Depending on the experimental conditions, the survival rates ranged from 64 to 0.2%. The higher the air inlet temperature, the lower was the survival rate and an inversely correlation between the TS content and particle size has been determined. These results clearly indicate that process stress analyses and product-related characteristics must not be regarded separately.     

Spray Drying and Agglomeration of Instant Bayberry Powder

Optimum technology of spray-dried bayberry powder was studied using D-optimal experimental design. The operating conditions were varied within the following ranges: inlet air temperature 140–160°C, outlet air temperature 65–85°C, maltodextrin DE values 12 and 19, and feed concentrations of 7–17°B. The spray-dried bayberry powder was analyzed for moisture content and color. Moisture content of spray-dried powder was determined mainly by the inlet and outlet air temperatures, DE value, and the feed concentration. The inlet and outlet temperature had important effects on powder color. Finally, instant bayberry powder for beverages was produced by agglomeration of the spray-dried product.     

Characterization of a Spray Drying System for Soymilk

Soymilk slurries were spray dried at various combinations of inlet air temperature (1255. 265, and 275°C), feed rate (0.38 and 0.57 Lpm), and atomizer speed (119,300 and 26,800 rpm). Monitored dryer parameters included dry and wet bulb temperaturea of ambient and exhaust air, drying chamber's inlet and outlet air temperatures, product and feed temperatures, velocity pressure at exhaust pipe, atomizer speed, and feed rate. Weather data was included in the analysis.

Collected data was used to estimate the state of the outlet air using psychrometrics and assuming adiabatic saturation. Equations to predict outlet and product temperatures were developed. Average thermal and eveporative efficiencies were 65.9 and 78.4%. respectively.     

An Experimental Investigation of the Wall Deposition of Milk Powder in a Pilot-Scale Spray Dryer

A pilot-scale, co-current spray dryer has been used to investigate the effect of varying the swirl vane angle for the inlet air, inlet air temperature and liquid feed flowrate on the wall deposition flux of skim milk powder. The spray dryer was a cylinder-on-cone unit with a diameter of 0.8 m and a height of 2 m. It was fitted with adjustable swirl vanes surrounding a Delavan GA1 two-fluid atomizer. Swirl vane angles of 0, 25, and 30°, inlet air temperatures of 170, 200, and 230°C and feed flowrates of 1.4, 1.6 and 1.8 kg h^-1 were used. Inlet air swirl was found to significantly influence the wall deposition flux, with the highest swirl vane angle of 30° giving rise to the largest wall deposition flux. The difference between the particle and sticky-point temperatures of the skim milk powder was also found to be important in influencing the wall deposition flux. The wall deposition flux was the highest, at 16 g m^-2 h^-1, when the outlet particle temperature was furthest above the sticky-point temperature, and this occurred at the lowest value of the inlet air temperature (170°C) and highest product moisture content. No significant effect on the wall deposition flux was evident when using a nonstick food grade material (nylon), adhesive tape or stainless steel as the surface materials for the wall deposition tests. Therefore, it is likely that cohesion occurs at a similar rate to adhesion in the wall deposition of milk powder. Grounding the spray dryer also did not have a significant effect on the wall deposition flux. Relating the wall deposition flux to the sticky-point curve in this way suggests that the same trends (increased wall deposition fluxes above the sticky-point curve) may apply for other materials as well.     

热泵干燥装置的特性方程及其应用分析

以应用最广泛的封闭式热泵干燥装置为例,建立了其SMER计算方程。在此基础上,对SMER随干燥器进口空气温度、干燥器出口空气温度和蒸发器出口空气温度而变化的规律进行了计算和分析。结果表明,其他两个参数一定时,SMER随干燥器出口空气温度上升而降低,随蒸发器出口空气温度上升而增加,但当干燥器进口空气温度取适宜值时,SMER存在最大值。     

在配置喷嘴雾化器的低速喷雾塔中空气一液滴接触的强化

环形空气进口能够显著强化低速喷雾干燥器的干燥阶段。更精殉一点,加入环形喷射进口能够延长有效干燥时间．但不能提高干燥速度。计算流体动力学分析进一步表明这是因为同轴环形被加热射流内部的“弯曲”,导致在喷嘴雾化器下部区域产生明显的穿流和逆流。对大液滴干燥这种改善更加明显。以此构想为基础,在莫纳什大学开发了一种空气室原型,使得同轴环形射流用于低速喷雾塔中。此样机成功将酪蛋白、脱脂牛奶和乳糖溶液喷雾干燥成自由流动粉体。     

Spray Drying Performance of a Laboratory Spray Dryer for Tomato Powder Preparation

Abstract

This work investigates the performance of a spray dryer for tomato powder preparation by spray drying of tomato pulp. Samples of tomato pulp with a 14% constant total solids concentration were used, and a pilot scale spray dryer (Buchi, B-191) with cocurrent operation and a two-fluid nozzle atomizer was employed for the spray drying process. Twenty-four different experiments were conducted keeping constant the feed rate, the feed temperature, and the atomizer pressure, and varying the compressed air flow rate, the flow rate of drying rate, and the air inlet temperature. In each experiment the air outlet temperature was recorded. Data for the residue remaining in the chamber and cyclone walls was gathered and two types of efficiencies were calculated as an indication of the spray dryer performance. Analysis of experimental data yielded correlations between residue accumulation and the variable operating conditions. The same operating parameters had a great influence on the air outlet temperature whereas temperature deviations were observed comparing measured air outlet temperatures with corresponding outlet adiabatic saturation temperatures.