New Engineered Particles from Spray Dryers: Research Needs in Spray Drying

This article reviews developments in the simulations of spray dryer behavior, including the challenges in modeling the complex flow patterns inside the equipment, which are often highly transient and three-dimensional in nature. There appears to be considerable scope for using CFD simulations for investigating methods to reduce the rates of wall deposition and of thermal degradation for particles by modifying the air flow patterns in the chamber through small changes in the air inlet geometry. Challenges include building particle drying kinetics and reaction processes, as well as agglomeration behavior, into these simulations. The numerical simulations should be valuable supplements to pilot-scale testing, enabling more extensive and accurate optimization to be carried out than hitherto possible. New understanding of reaction processes and materials science, in combination with recent knowledge of the application of CFD to these problems, may enable new engineered powder products to be developed from the one-step spray-drying process.     

Stickiness, Functionality, and Microstructure of Food Powders

Three major aspects of food powder are described and discussed. Stickiness is one issue that can cause production and product handling difficulties. Stickiness has been interpreted in a number of ways and thus measured differently. Functionality of the primary (single) particles or agglomerated powders is of practical interest to the consumers. The desired quality will have to be matched or exceeded by the powder manufacturers. Finally, microstructure provides a key linkage between the production and the functionality. Its formation has impacts on both the stickiness and functionality.     

An Experimental Investigation of Lime Juice Drying in a Pilot Plant Spray Dryer

One of the major problems in spray drying of fruit juices such as lime juice is stickiness and thermoplasticity of their compositions. Lime juice consists of invert sugars and citric acid, which have low glass transition temperatures. Due to this characteristic, the particles stick on the dryer wall upon their collision with it. As a result, drying of these materials is very difficult. In order to solve this problem, various percent of silicon dioxide and maltodextrin (DE5), based on total soluble solid content of lime juice, have been used to reach a suitable drying condition. A cool chamber wall spray dryer was used in this investigation in order to decrease the probability of particles stickiness on the wall. Our investigation revealed that an addition of 10% silicon dioxide and 20% maltodextrin (DE5) to lime juice is the optimum amount for a complete and successful drying of lime juice. Sampling of particles from different longitudinal distances in the dryer tower is carried out to find the particle moisture contents as they fly downward in the dryer. The results show a very fast decrease in entrance moisture contents. Based on our experimental data, the variation of moisture contents are presented as a function of radial distance from central line and longitudinal distance from the entrance region.     

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

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.     

Attention web designers: You have 50 milliseconds to make a good first impression!

Three studies were conducted to ascertain how quickly people form an opinion about web page visual appeal. In the first study, participants twice rated the visual appeal of web homepages presented for 500 ms each. The second study replicated the first, but participants also rated each web page on seven specific design dimensions. Visual appeal was found to be closely related to most of these. Study 3 again replicated the 500 ms condition as well as adding a 50 ms condition using the same stimuli to determine whether the first impression may be interpreted as a 'mere exposure effect' (Zajonc 1980). Throughout, visual appeal ratings were highly correlated from one phase to the next as were the correlations between the 50 ms and 500 ms conditions. Thus, visual appeal can be assessed within 50 ms, suggesting that web designers have about 50 ms to make a good first impression.     

Adhesion and Cohesion Characteristics of Sewage Sludge During Drying

The stickiness of sludge during the drying process is an effect of its adhesion and cohesion. Furthermore, the adhesion and cohesion characteristics of sludge are comprehensive results of sludge components and their correlation with moisture. This work presents a simple method to test the adhesion and cohesion characteristics of sludge during the drying process based on the Jenike shear test. The results show that the most significant sludge adhesion and cohesion emerges at a moisture content of approximately 45 to 70% and 30 to 60%, respectively, and at a heating temperature of 120°C. An increase of the heating temperature to 200°C only brings a small movement to the sticky range location. The possible reasons for sludge adhesion and cohesion are discussed based on the results of sludge component analysis. The contributions of mineral materials with tiny particle sizes and metal salts to the adhesion and cohesion are discussed in this study because the contribution of organic matter has been widely demonstrated. The adhesion and cohesion nature of sludge exhibits a large effect on its drying characteristics. A remarkable fluctuation of both the drying rate and stirring power, located between the first and second falling rate period, is observed when the adhesion force of sludge reaches the maximum. When the sludge cohesion force decreases to approximately 3800 N/m², sludge bulks are broken with the stirring of paddles.     

An Advanced Centrifugal Technique to Characterize the Sticking Properties of Tomato Pulp during Drying

An advanced centrifugal technique was developed to characterize the stickiness of tomato pulp at various moisture contents and temperatures. To provide tomato pulp samples with different moisture contents, tomato powder produced by a laboratory spray dryer was wetted to six different moisture levels. By noting the rotational speed for a tomato pulp droplet detachment and weighing the mass of the droplet remaining on the surface it is possible to gauge the approximate attachment forces holding the droplet to the substrate. The effects of droplet mass and centrifugation time after reaching the desired rotor speed on the detachment rate were also studied. The detachment force was found to increase at the beginning and then started decreasing as the moisture content decreased presenting a maximum at moisture content of about 34%. In addition, the higher the droplet temperature, the higher the attachment force.     

NEW MEASUREMENTS OF THE STICKY BEHAVIOR OF SKIM MILK POWDER

The drying of sugar-rich foodstuffs is often complicated by depositions on the walls of spray dryers, due to the stickiness of the products. This material property has been found to depend on both product temperature and moisture content and undergoes a rapid change from non-sticky behaviour to sticky behaviour with only a small change in these parameters. In this investigation, this so-called sticky point is measured for skim milk powder by measuring the cohesive force between stirred particles in a heated flask. The line separating the sticky and the non-sticky regions is given as a function of bulk temperature and moisture content. For the temperature range from 25 to 95°C investigated here, the line shows good agreement with the predicted glass transition temperature for lactose, shifted up by 23.3 K. This information can then be used in CFD simulations carried out to model the build-up of wall depositions inside spray drying chambers. As a first order approximation to estimate the behaviour of a particle impacting on a wall, sticky particles can be assumed to adhere to the wall, whereas non-sticky particles can be considered to bounce off it.     

Agglomeration in Spray Drying Installations (The EDECAD Project): Stickiness Measurements and Simulation Results

Spray drying is used for the manufacture of many consumer and industrial products such as instant dairy and food products, laundry detergents, pharmaceuticals, ceramics, and agrochemicals. During spray drying, agglomerates of powder particles are formed that determine the instant properties of the powder. Agglomeration during spray drying is considered to be a difficult process to control. The main cause of this is the complex interaction of the process variables: the atomization process, the mixing of spray and hot air, the drying of suspension droplets, and the collision of particles, which might lead to coalescence or agglomeration. As a consequence, agglomeration during spray drying is operated by trial and error. In an EC-sponsored project, named the EDECAD project and coordinated by NIZO food research, an industrially validated computer model, using CFD technology, to predict agglomeration processes in spray drying machines is developed. A Euler-Lagrange approach with appropriate elementary models for drying, collision, coalescence, and agglomeration of the dispersed phase is used. The main result of the EDECAD project is a so-called design tool, which establishes relations between the configuration of the drying installation (geometry, nozzle selection), process conditions, product composition, and final powder properties. The design tool has been validated on pilot plant scale and industrial scale. This article presents the setup and results of dynamic stickiness tests and some CFD simulation and validation results.     

Spray Drying of Raisin Juice Concentrate

Raisin juice concentrate is a natural sweetener in syrup or paste form and it is produced from second-grade dry raisins by leaching them with water. Dried raisin juice, although is easier to handle and has more potential applications than the syrup, is not available in the market. In the present study, raisin juice powder was produced with a lab-scale spray dryer. The problem of stickiness in the drying chamber was overcome through the use of 21 DE, 12 DE, and 6 DE maltodextrins as drying aid agents. For each type of maltodextrin, the dryer operating conditions and the minimum concentration of maltodextrin in the feed, necessary for successful powder production, were determined. The maximum ratio of (raisin juice solids)/(maltodextrin solids) achieved was 67/33 and was made possible with the use of 6 DE maltodextrin. The inlet and outlet drying air temperatures were 110 and 77°C respectively, while the feed contained 40% w/w total solids. The physical and sensory properties of all powders produced were determined and found to be satisfactory, the only exception being their high hygroscopicity.