Characterization of the Residence Time Distribution in Spray Dryers

In this work it is presented a study on the residence time distribution (RTD) of particles in a co-current pilot-plant spray dryer operated with a rotary atomization system. A nuclear technique is applied to investigate the RTD responses of spray dryers. The methodology is based on the injection of a radioisotope tracer in the feed stream followed by the monitoring of its concentration at the outlet stream. The experiments were performed during the drying of aqueous suspensions of gadolinium oxide. The RTD responses obtained experimentally presented good reproducibility, indicating that the technique applied is well suited to investigating fluid-dynamics of spray dryers. In addition to the experimental investigation, a mathematical model was used to describe the RTD experimental curves.     

SPOUTED AND SPOUT-FLUIDIZED BEDS FOR GRAM DRYING

The objectives of this work are to analyze the drying performance of conical-cylindrical spouted bed (CSB) dryers for three different grains (rice, corn and wheat), and to compare the drying efficiency of CSB dryers with that of spout-fluid bed (SFB) dryers. A PC-program was developed for: (I) -optimization of the CSB dryer dimensions; (2) -simulation of drying grains in the optimized CSB dryer (including start-up period); and, (3) -analysis of the drying performance in a similar SFB dryer.

The liquid diffusion model is used to describe the falling rate drying period. Semi-empirical correlations available in the literature as well as information obtained in the authors' laboratory for spouted and spout-fluidized beds of grains are used to describe the aerodynamic parameters.

The results are presented in terms of the size of the dryer, energy consumption, air handling requirement, drying characteristics etc for different drying conditions. The drying effeciency in a CSB is compared with that in a similar SFB for different grain feed rates and drying temperatures.     

Mathematical Modeling of Rotary Drying of Woody Biomass

Woody biomass in the form of forestry industry residues has been recognized as a promising resource for renewable energy and liquid fuels. Drying of the woody biomass is one of the key operations in development of the energy conversion technologies. Rotary drying is an effective method due to the enhanced contact between the solids and the drying medium (hot air). In this work, a mathematical model was developed to simulate the drying of the woody biomass as chips in a rotary dryer, based on energy and mass balance and transfer, experimental drying kinetics of the wood chips, and using literature correlations for the residence time. A new correlation between the theoretical maximum drying rate and the actual constant drying rate for the wood chips was obtained from the drying experiments, which was incorporated in the drying model. The model was applied both for cocurrent and countercurrent rotary dryers, and the simulation results are consistent with the observed trend. However, the accuracy of the model needs to be further investigated through experimental validation of the residence time correlation.     

SPOUTED AND SPOUT-FLUIDIZED BEDS FOR GRAM DRYING

The objectives of this work are to analyze the drying performance of conical-cylindrical spouted bed (CSB) dryers for three different grains (rice, corn and wheat), and to compare the drying efficiency of CSB dryers with that of spout-fluid bed (SFB) dryers. A PC-program was developed for: (I) -optimization of the CSB dryer dimensions; (2) -simulation of drying grains in the optimized CSB dryer (including start-up period); and, (3) -analysis of the drying performance in a similar SFB dryer.

The liquid diffusion model is used to describe the falling rate drying period. Semi-empirical correlations available in the literature as well as information obtained in the authors' laboratory for spouted and spout-fluidized beds of grains are used to describe the aerodynamic parameters.

The results are presented in terms of the size of the dryer, energy consumption, air handling requirement, drying characteristics etc for different drying conditions. The drying effeciency in a CSB is compared with that in a similar SFB for different grain feed rates and drying temperatures.     

Wheat drying in well-stirred-batch and continuous-moving-bed dryers

Earlier studies established the physical chemistry and diffusion mathematics of wheat drying. This knowledge was applied in a mathematical model of continuous drying in a well-stirred bed, and a good correlation of experimental results on spouted-bed drying was obtained. Now a model has been formulated for both well-stirred batch and continuous moving-bed dryers of the commercial types. The set of mass and energy balances and rate equations has been solved numerically by digital computation. Heating and cooling cycles are both treated. Good agreement is found with the experimental data of Stansfield and Cook on a dryer simulating the performance of commercial moving-bed dryers.     

Calculation Steps for the Design of Different Components of Heat Pump Dryers Under Constant Drying Rate Condition

Heat pump–assisted dryers are an alternative method for drying heat-sensitive food products at low temperature and less relative humidity with lower energy consumption. The mathematical models of a heat pump dryer consist of three submodels; namely, drying models, heat pump models, and performance models. Heat and mass balance of both refrigerant and air circuits in all components of the system are used for development of mathematical models. The models are used for design of different components of heat pump dryers operating under constant drying rate condition. A simple stepwise design procedure for batch-type, closed-loop heat pump dryer is also presented.     

Review of: “Impregnation,Drying and Adhesive Coating Equipment”by Konovalov 7.1. and Koval A.M. Khimia,Moscow, 1989, 222 p. (in Russian)

Zeolites have potential to increase efficiency of medium-temperature drying in the food industry. This work concerns the comparison between conventional dryers and dryers using air dehumidified by zeolite. Steady-state mass and energy balances have been used and the work concerns drying temperatures ranging from 52 to 70°C. Process integration based on pinch analysis has been applied and nine different heat exchanger networks for energy recovery are compared. Results indicated that dryers using air dehumidifier by zeolites are 10–18% more efficient than conventional dryers.     

Experiments on Hot-Air Drying of Wheat in a Semi-Technical Mixed-Flow Dryer

Although mixed-flow grain dryers are widely used, there is still a need to optimize the process control as well as the dryer apparatus. Fluctuations of the grain moisture content at the dryer entrance are still a major problem resulting in quality and economic losses due to under- or overdrying. Therefore, a mathematical model for heat and mass transfer in a mixed-flow dryer has been developed. Practical drying experiments were carried out at a semi-technical dryer test station that was operated quasi-continuous. The measurements reveal the complexity of the mixed-flow drying process. First predicted results are in satisfactory agreement with data.     

SOLAR DRYING OF GRAPES

Mass losses and low quality are the most serious disadvantages of traditional grape drying methods. For the production of high quality raisins an increase in the drying rate is required and the grapes should be protected from rain, dust and insects during drying.

Under the terms of a joint German-Greek research program low-cost solar grape dryers were developed in the Institute of Agricultural Engineering of Hohenheim University and were tested in Greece in cooperation with the Crete Agricultural Research Center.

The required data basis for optimizing solar grape dryers was obtained by additional laboratory tests measuring the influence of various drying parameters on drying rate and quality.

Tests with the solar dryers have shown that it is possible to reduce the drying time and improve the quality significantly compared to the traditional drying methods. Also mass losses due to rain can be prevented.     

The Effect of Different Plasticizers on Lactose Crystallization During Spray Drying

Lactose crystallization may occur during spray drying, depending on the operating conditions for drying, and this work reviews previous and new evidence for the effects of different additives or plasticizers on retarding or accelerating the rate of crystallization. The effects of different operating conditions during spray drying are also considered in the experimental work reviewed and performed here, which was mainly carried out with Buchi B-290 laboratory-scale spray dryers (Buchi, Flawil, Switzerland), although some work on pilot-scale equipment is also discussed. The additives used and reviewed in this article include milk proteins, such as casein and whey protein isolate, polyethylene glycol, and ascorbic acid. The key physical properties, such as glass transition temperature and drying characteristics of the material, are discussed, allowing degrees of amorphicity in spray-dried lactose to be controlled over a wide range, from close to 0% to nearly 100%.     

Experiments on Hot-Air Drying of Wheat in a Semi-Technical Mixed-Flow Dryer

Although mixed-flow grain dryers are widely used, there is still a need to optimize the process control as well as the dryer apparatus. Fluctuations of the grain moisture content at the dryer entrance are still a major problem resulting in quality and economic losses due to under- or overdrying. Therefore, a mathematical model for heat and mass transfer in a mixed-flow dryer has been developed. Practical drying experiments were carried out at a semi-technical dryer test station that was operated quasi-continuous. The measurements reveal the complexity of the mixed-flow drying process. First predicted results are in satisfactory agreement with data.     

MODELING AND SIMULATION OF CROSSFLOW MOVING BED GRAIN DRYERS

The drying of grain in dryers of a crossflow moving bed type was theoretically and experimentally studied. Two different dryer configurations were analyzed, a dryer with central air distribution and another with multiple air duels. Experimental information was obtained in pilot-size dryers. A mathematical model to simulate the process was developed. Hindered drying was accounted for by using the concept of relative drying rate. An adjustable factor, specific to the dryers, was used to account for the uncertainties of the contact area and the transfer coefficients encountered in the literature. Agreement between experimental results and simulations was fairly good. Simulations showed that distance between inlet air and outlet devices, air to solid flow ratio and dryer height to cross section ratio have great influence on the process. The mathematical model may be a useful tool for process exploration and optimization of this type of dryers.     

Inversion Temperature and Pinch Analysis,Ways to Thermally Optimize Drying Processes

This work studies the compatibility and suitability of a combined inversion temperature and pinch analysis with the process selection for air and superheated steam spray drying of milk solids. The inversion temperature is a good starting point for an energy analysis because it is a simplified rate-based approach to comparing the steam and air drying systems. pinch analysis enables process integration, at least on a heat recovery and heat exchanger network level.

The resulting inversion temperature for the studied system was estimated as 182°C for the dryer inlet temperature. However, mass and energy balances showed that a minimum inlet temperature for spray drying of 184°C was required for the superheated steam dryer in order to ensure that the outlet solids temperature above the dew point temperature.

The inversion temperature is still very relevant in the early stages of a design process because it allows a quick assessment of which drying medium should result in a smaller dryer. It was evident that the steam system is better from an energy perspective because of the recoverable latent heat of the water vapor carried out of the dryer with the recycled steam. The steam system has between 82 and 92% of thermal energy recovery potential as condensable steam, compared with 13–30% energy recovery of the air system. However, other important design and operational factors are not discussed here in detail.

Combining the inversion temperature and pinch analysis suggests that superheated steam drying both gives better energy recovery and is likely to give smaller dryers for all operational conditions.     

MODELING AND SIMULATION OF CROSSFLOW MOVING BED GRAIN DRYERS

ABSTRACT

The drying of grain in dryers of a crossflow moving bed type was theoretically and experimentally studied. Two different dryer configurations were analyzed, a dryer with central air distribution and another with multiple air duels. Experimental information was obtained in pilot-size dryers. A mathematical model to simulate the process was developed. Hindered drying was accounted for by using the concept of relative drying rate. An adjustable factor, specific to the dryers, was used to account for the uncertainties of the contact area and the transfer coefficients encountered in the literature. Agreement between experimental results and simulations was fairly good. Simulations showed that distance between inlet air and outlet devices, air to solid flow ratio and dryer height to cross section ratio have great influence on the process. The mathematical model may be a useful tool for process exploration and optimization of this type of dryers.     

PERFORMANCE OF SPRAY DRYER WITH INTEGRATED FILTER AND FLUID BED

Spray dryers featuring a fluid bed integrated into the base of a spray drying chamber have proved one of the most significant developments introduced into industry during the last decade, producing dust-free particulates under low product temperature conditions. The latest design development involves all particulate collection and exhaust air cleaning within the drying chamber with the use of integrated metallic filter elements (with CIP capability). This eliminates handling of fines outside the drying chamber and simplifies the exhaust air system contributing to lower pressure drop losses and lower overall energy consumption. This paper describes the performance of a pilot plant sized spray dryer featuring a drying chamber with both integrated particulate filters and fluid bed. Various products were tested. The results showed that the placing of particulate filters inside the drying chamber does not adversely affect the agglomeration process and that the powder quality compared with that achieved in a standard Fluidized Spray Dryer can be reproduced in this new design concept, with every possibility for improved quality due to no powder handling outside the drying chamber. The work also showed that by containing the powder within the drying chamber, notable operational advantages are apparent and that scale-up of the design concept represents no apparent difficulties.     

热力干燥技术：新发展和未来研发潜力

热力脱水是一个能源高度密集型的过程，几乎应用于所有工业部门。在发达国家，热力脱水的能耗占到国家所有工业能耗的10％～20％。随着能源成本的逐步升高和对减轻由石化燃料燃烧排放产生的环境污染的需求，开发新型干燥技术变得愈加重要。此外，由于在传质传热过程中会产生物理和（或）化学变化，干燥还会影响到干燥产品的质量。成千上万的干燥产品、数百种不同类型的干燥器使得开发适用范围较广的干燥器的设计及放大相当困难。过去的30年来，对各种形式的湿固态物料、糊浆和液体的干燥，已经有人做了大量的关于传递现象和材料科学方面的基础和应用研究。将简述目前干燥技术发展水平及其理论认识，并提供一些正在开发的新技术实例，指出一些为提高干燥技术而挑战基础和模拟研究的机会。还将给出实例说明如何对喷雾、脉冲燃烧干燥装置进行数学建模，以此进行创新型概念的设计。     

Fortschritte in der Zerstäubungstrocknungs-Technologie

Progress in the technology of spray drying . Stringent quality demands on spray-dried goods have led to increasing use of nozzle spray dryers. In contrast to disk spray dryers, nozzle dryers yield microgranules and low-dust products directly. Although the process of nozzle spray drying is over 50 years old, the volume of available calculation material necessary for general application is sparse and limited mainly to special products or problems. This article presents new advances in the technology of nozzle spray dryers. An analytical formulation of the drying of a drop within the first drying phase decisive for granulation constitutes the basis of the general applicability of the fresh insights gained.     

Thin-layer drying of porous materials: Selection of the appropriate mathematical model and relationships between thin-layer models parameters

Experimental investigation of the drying kinetics of various types of materials was carried out in laboratory-scale dryers under different conditions of temperature, microwave heating power and pressure. Leather samples (mechanically and vacuum-dewatered bull napa and wet blue cutting), paperboards (grafopack, testliner), wood (alder, birch, willow) and two pharmaceutical powders (chlorpropamide and hydrochlorotiazide) were dried in a microwave dryer. Thin clay slabs, Al–Ni catalyst and chlorpropamide were dried in a convection dryer, while chlorpropamide and ketoprofen were dried in a vacuum dryer. In order to compare drying kinetics, experimentally obtained data, X = f(t), were correlated with the Lewis “thin-layer” equation, the modified Page equation and Fick's second law. The drying constant, effective diffusion coefficient, mass transfer coefficient and modified Page model parameters were estimated by fitting the selected mathematical models to experimental data. High levels of correlation between measured and calculated data were obtained for all materials and dryers using modified Page model. The application of the Lewis and Fick's equation is justified only for drying of clay, catalyst and leather. Mass transfer coefficient depends linearly on the drying constant. The relation between the modified Page model parameter and the drying constant can be represented by a unique power function.     

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.     

Air-Drying of Seeds: A Review

Beginning with harvesting, seeds are usually subjected to a series of processes which include drying for immediate or future use. Seed quality can be influenced by several factors during drying. This article presents a review of the seed air-drying process, including mathematical models based on differential equations derived from mass and energy balances for seeds and air in fixed and moving bed dryers. The article concludes with an overview of several potential drying technologies that can be applied to seeds.