DISSERTATIONS AND THESES IN DRYING WRITTEN IN CZECHOSLOVAKIA IN THE PERIOD 1975 - 84

Research and development of drying processes and equipmect has a long tradition in Czechoslovakia (1). There are at least 10 top reoearch centers where drying of varicus materials has been studied. Remarkable attention to drying theory and application is also paid at Czechoslovak universities which yields numerous theses dealing with various aspects of the drying process. The total of 52 M.Sc. theses oriented to drying and/or dryers had been published in the last decade. Moreover, 22 Ph.D. theses were carried out in the same period. This represents an important source of theoretical and experimental results. However all theses had been mitten in the Czech or Slovak language, and therefore their wide international application is rather limited. They are summari-zed in the following list.     

APPLICATION OF THE DISCRETE ELEMENT MODELLING IN AIR DRYING OF PARTICULATE SOLIDS

The Discrete Element Method (DEM) has been widely used as a mathematical tool for the study of flow characteristics involving particulate solids. One distinct advantage of this fast developing technique is the ability to compute trajectories of discrete particles. This provides the opportunity to evaluate the interactions between particle, fluid and boundary at the microscopic level using local gas parameters and properties, which is difficult to achieve using a continuum model. To date, most of these applications focus on the flow behaviour. This paper provides an overview of the application of DEM in gas-solids flow systems and discusses further development of this technique in the application of drying particulate solids. A number of sub-models, including momentum, energy and mass transfer, have been evaluated to describe the various transport phenomena. A numerical model has been developed to calculate the heat transfer in a gas-solids pneumatic transport line. This implementation has shown advantages of this method over conventional continuum approaches. Future application of this technique in drying technology is possible but experimental validation is crucial.     

STRATEGIES FOR DRYING TOMATOES IN A TUNNEL DEHYDRATOR

A tunnel dehydrator can be operated using batch or semi-batch modes and while a batch mode is the simplest, it has the inherent characteristic of producing non-uniform moisture content. Although semi-batch mode does not have this problem, because all product experiences the same drying conditions, it is not used when drying tomatoes in Australia. The operation of most dehydrators is based on operator experience and rules of thumb, methods that are not likely to give optimum performance. A model that is suitable for simulating the operation of a tunnel dehydrator with any fruit has been developed. This model has been applied to the development of strategies for drying tomatoes with the objective of achieving optimum throughput, colour and drying efficiency. The results of these simulations are presented in this paper.     

DRYING PAPER IN SUPERHEATED STEAM

The industrial potential for drying paper in superheated steam has been investigated in a series of studies designed to treat paper properties - drying process relations as well as process engineering aspects. Properties measurements show that for paper made from mechanical pulps, drying in superheated steam produces a better bonded sheet which is thereby stronger and has a lower scattering coefficient. Surface properties of such steam dried paper are improved, including reduced tinting. Desorption equilibrium shows that completely dry paper can be obtained at very low superheats. Impingement drying rates can be about twice as high for drying in superheated steam as in air. A hybrid cycle was developed in which drying is partially by superheated steam impingement drying and partially by conventional cylinder drying. Preliminary design and techno-economic evaluation indicates that three factors, paper properties, dryer size and energy efficiency, in various combinations specific to each case, may provide industrial potential for this new technology for drying paper.     

DRYING PAPER IN SUPERHEATED STEAM

ABSTRACT

The industrial potential for drying paper in superheated steam has been investigated in a series of studies designed to treat paper properties - drying process relations as well as process engineering aspects. Properties measurements show that for paper made from mechanical pulps, drying in superheated steam produces a better bonded sheet which is thereby stronger and has a lower scattering coefficient. Surface properties of such steam dried paper are improved, including reduced tinting. Desorption equilibrium shows that completely dry paper can be obtained at very low superheats. Impingement drying rates can be about twice as high for drying in superheated steam as in air. A hybrid cycle was developed in which drying is partially by superheated steam impingement drying and partially by conventional cylinder drying. Preliminary design and techno-economic evaluation indicates that three factors, paper properties, dryer size and energy efficiency, in various combinations specific to each case, may provide industrial potential for this new technology for drying paper.     

STRATEGIES FOR DRYING TOMATOES IN A TUNNEL DEHYDRATOR

ABSTRACT

A tunnel dehydrator can be operated using batch or semi-batch modes and while a batch mode is the simplest, it has the inherent characteristic of producing non-uniform moisture content. Although semi-batch mode does not have this problem, because all product experiences the same drying conditions, it is not used when drying tomatoes in Australia. The operation of most dehydrators is based on operator experience and rules of thumb, methods that are not likely to give optimum performance. A model that is suitable for simulating the operation of a tunnel dehydrator with any fruit has been developed. This model has been applied to the development of strategies for drying tomatoes with the objective of achieving optimum throughput, colour and drying efficiency. The results of these simulations are presented in this paper.     

IMPROVING ENERGY EFFICIENCY IN COMBINED MICROWAVE-CONVECITVE DRYING

Convective-microwave drying has been recognized as a convenient option for drying of valuable materials. However, the advantages of the method must be carefully evaluated in order to establish the limits for full scale operating conditions because, as demonstrated in this work, the reduction in the absolute drying time as a function of the applied microwave power is devalued by an increase in the unabsorbed microwave energy, decreasing the overall energy efficiency of the drying process.

Experimental study of the energy efficiency of combined microwave-convective drying of agar gel and Gelidium seaweeds was carried out in a laboratory scale microwave-convective dryer by continuously following the absorption of microwave energy by the sample as well as the change of the sample mass during the drying process, under different operating conditions. Several drying strategies based on the on-off application of the microwave power were proposed and evaluated from the point of view of both drying kinetics and energy efficiency in order to program and rationalize the use of microwave energy in the combined microwave-convective drying process.     

MOISTURE MOVEMENT IN SOFTWOOD TIMBER AT ELEVATED TEMPERATURES

The anatomy of a common plantation-grown softwood in New Zealand, Pinna radiata, is reviewed and the differences between the drying behaviour of softwoods and hardwoods are discussed. These differences have significant effects on the paths for moisture movement, and this review of the timber physiology has been used as the basis for a model of the moisture movement at elevated temperatures which distinguishes between the drying behaviour of sapwood and heartwood. It is postulated for the drying of heartwood that liquid flow does not occur within a board due to the aspirated state of the pits, so an evaporative plane at the boiling point of water sweeps through the timber until the centre of the board is reached. In the case of the much wetter sapwood where the pits are not initially aspirated, we assume that liquid flow initially maintains an evaporative plane just below the surface until the moisture is no longer funicular. Thereafter, drying proceeds in a similar way to heartwood. Independent experimental measurements of the temperature profiles at the centre and surface of heartwood and sapwood bkards (100 × 50 mm) have been compared with model predictions, and the close agreement gives some confidence in the model. The model predicts that the drying process for this timber can be divided into three periods, marked by transitions in the gradients of the drying rates when plotted against the average moisture contents. This behaviour has been observed experimentally for Pinus radiata, giving us further confidence that our drying model includes the important physical processes for moisture movement in this timber.     

聚合物涂膜干燥研究进展

评述了人们对于聚合物涂膜形成过程及干燥机理的认识 ,并以涂膜干燥过程的模拟、涂膜缺陷的形成、扩散系数的测定及估算等方面的研究进展为依据 ,讨论了聚合物涂膜干燥过程中需要进一步研究和开发的领域     

OPTIMUM OPERATING CONDITIONS IN DRYING FOODSTUFFS WITH SUPERHEATED STEAM

It is inferred from experimental data that in drying foodstuffs with superheated steam, the initial drying rate has a direct effect on the rate at which the overall drying takes place. That is, the faster the initial drying rate, the shorter the overall drying time. This criterion is very convenient because at the beginning, water moistens the sample external surface so evaporation does not depend on internal sample characteristics, but only on external convective heat and mass transfer rates. Mass and energy balance equations are solved and the result converted into a general initial drying rate equation, in which all dryer characteristics are grouped into one dimensionless parameter. The initial drying rate equation is mathematically maximized and the optimum working conditions determined. The result shows that initial drying rate always increase with increases of either the superheated steam temperature or velocity, but once these two variables are fixed, there exists at least one “optimum” pressure at which the initial drying rate is a maximum. Finally, the initial drying rate and optimum condition equations are applied to three model dryers, a dryer for a flat sheet, a fixed bed dryer and a rotary dryer. In each case, numeric values are computed and plotted as drying rate versus pressure curves, in which the optimum drying rate is also included. Also presented is a chart to compare the optimum pressures as functions of temperature and steam velocity for the three dryers.     

ANALYSIS AND OPTIMIZATION OF THE CONDITIONING PHASE IN TIMBER DRYING

The conditioning phase of the conventional timber drying process has been investigated. Due to wood shrinkage, tensile stress will develop in the surface layer of the board during drying. This stress combined with the change in MC causes mechano-sorptive creep. If the process is abruptly terminated when the target MC is reached, then there will be a considerable internal MC gradient and the creep deformation will cause distortions in subsequent machining. A conditioning phase is often introduced at the end, in order to reduce these problems. The conditioning phase has been analyzed with a wood drying simulation model. It is shown that traditional conditioning cannot completely reverse the deformations, even at very long conditioning times. An optimizing procedure has thus been used to create a new conditioning “schedule” which gives the best possible result within a given time. Features of such optimized schedules are presented. As the surface layer is elongated during drying, there is a risk that considerable stress develops as the moisture profile levels out. In some cases internal checking will be the result. The optimization procedure is thus modified to avoid such situations and the impact on the conditioning schedule is discussed.     

DRYING TECHNOLOGY AND EQUIPMENT IN INDIA

India produces over 200 million tonnes of food grains annually. There is need of postproduction management system to match with the increasing production rate. Drying is the primary need of crop's postproduction system. Sun drying is a popular mode of crops drying in India. The use of mechanical dryers are almost nil at arm level. With ever-growing population, the food grain production will increase to meet the demand of people. However, the agriculture is taking the shape of industry. Therefore, quality maintenance of the food grain is the primary concern. Thus, the crop drying cannot be dependent on sun drying. A good amount of research and development have been carried out on drying technology during the last two decades. So far very little attention have been given on the development of high capacity dryers, which are presently needed. High capacity dryers are also needed in food processing, chemicals and other industries, which are at growing stage in India. Hence, in nut and shell, there is a big demand for high capacity mechanical dryers and advanced drying technology. This paper presents the status of drying equipment and technology in India and its perspectives in future. To overcome the problems of drying and dryer development, the measures taken by the Government have been also mentioned. Potential of drying equipment business in international and domestic market is also highlighted in this paper.     

IMPROVING ENERGY EFFICIENCY IN COMBINED MICROWAVE-CONVECITVE DRYING

ABSTRACT

Convective-microwave drying has been recognized as a convenient option for drying of valuable materials. However, the advantages of the method must be carefully evaluated in order to establish the limits for full scale operating conditions because, as demonstrated in this work, the reduction in the absolute drying time as a function of the applied microwave power is devalued by an increase in the unabsorbed microwave energy, decreasing the overall energy efficiency of the drying process.

Experimental study of the energy efficiency of combined microwave-convective drying of agar gel and Gelidium seaweeds was carried out in a laboratory scale microwave-convective dryer by continuously following the absorption of microwave energy by the sample as well as the change of the sample mass during the drying process, under different operating conditions. Several drying strategies based on the on-off application of the microwave power were proposed and evaluated from the point of view of both drying kinetics and energy efficiency in order to program and rationalize the use of microwave energy in the combined microwave-convective drying process.     

SUGGESTED NEW METHODS IN THE PREPARATION OF DUST-PRESSED BODIES1

Milling. —A continuous pebble mill of the self-feeding type is recommended. Segregation due to difference in density is not to be feared. Drying. —Instead of drying completely, wetting down, tempering, aging and grinding the author recommends drying the press cake only down to a water content of 16%, followed immediately by disintegrating and pressing. The pressed ware produced by this shortened procedure is very satisfactory. A Combined Disintegrator and Dryer. —The author suggests the application of a hot blast to the clay as it leaves the mill for the disintegrator thus drying and disintegrating it in one operation. This suggestion has not, however, been tried out.     

ANALYSIS AND OPTIMIZATION OF THE CONDITIONING PHASE IN TIMBER DRYING

The conditioning phase of the conventional timber drying process has been investigated. Due to wood shrinkage, tensile stress will develop in the surface layer of the board during drying. This stress combined with the change in MC causes mechano-sorptive creep. If the process is abruptly terminated when the target MC is reached, then there will be a considerable internal MC gradient and the creep deformation will cause distortions in subsequent machining. A conditioning phase is often introduced at the end, in order to reduce these problems. The conditioning phase has been analyzed with a wood drying simulation model. It is shown that traditional conditioning cannot completely reverse the deformations, even at very long conditioning times. An optimizing procedure has thus been used to create a new conditioning “schedule” which gives the best possible result within a given time. Features of such optimized schedules are presented. As the surface layer is elongated during drying, there is a risk that considerable stress develops as the moisture profile levels out. In some cases internal checking will be the result. The optimization procedure is thus modified to avoid such situations and the impact on the conditioning schedule is discussed.     

AN OUTLINE METHOD FOR TROUBLESHOOTING AND PROBLEM-SOLVING IN DRYERS

Dryer troubleshooting is of great practical importance but has been neglected in the academic literature. Problems are often case-specific, non-quantitative and difficult to categorise. This paper outlines a systematic method for problem-solving in dryers. Problems tend to fall into one of three main categories; underperformance, materials handling, and product quality (non-moisture). Other problems include mechanical breakdown and SHE (safety/health/environment). Further categorisation may be useful; for example, the causes of dryer underperformance can be subdivided into heat and mass balance problems, drying kinetics and drying equilibria. The possible causes of the problem can then be listed, and process and equipment data is collected to allow these to be confirmed or dismissed, and to identify the root cause. Possible solutions can be considered, casting the net as widely as possible; some apparently unlikely suggestions may turn out to be feasible, or lead to a novel and better idea. The overall algorithm follows a convergent-divergent pattern (brainstorming technique). Certain types of dryer are particularly susceptible to certain types of problem. A range of case studies illustrates the application of the techniques in many different circumstances.     

BIXIN POWDER PRODUCTION IN CONICAL SPOUTED BED UNITS

Recent work has demonstrated that the red carotenoid bixin is easily extracted from Bixa orellana seeds by particle attrition and impact rather than by solvent extraction. This technique may require a previous step for drying seeds at a safe temperature to preserve pigment quality. A spouted bed (SB) with a draft tube is the most appropriate equipment to process both seed drying and bixin extraction at low operational costs. Operational parameters of the SB unit are optimized as a function of the powder production rate. The maximum air inlet temperature for drying the seeds is also specified. A high performance liquid chromatography method has been developed for the semi-quantitative determination of bixin content in the powder produced. Results show that this SB unit can be more competitive than ball mill equipment.     

NEW APPLICATIONS OF HEAT PUMPS IN DRYING PROCESSES

Heat pumps in combination with fluidized bed driers have been investigated extensively at The Norwegian Institute of Technology. A laboratory plant was constructed and a series of experiments on different types of temperature sensitive products were executed. By using heat pump, good temperature and humidity regulation is achieved. Drying temperatures can be regulated from -20°C to 50°C. This gives the possibility of freeze-drying at atmospheric pressure, and drying at temperatures above 0°C in the same plant. By using a temperature program, including an initial period of freeze drying, it is possible to regulate the products' physical properties. Examples are: the sinking velocity of fish fry feed, rehydration ability, colour and taste of dried fish, meat and vegetable products. Further, biotechnological products like bacterial cells and biomolecules/enzymes were dried with improved biological activity of the products compared to traditional drying methods like vacuum freeze drying or spray drying.     

喷雾闪蒸干燥在重质微细碳酸钙中的应用

介绍了重质微细碳酸钙的用途及生产方法，阐述了新技术喷雾闪蒸干燥的干燥原理、特点、工艺流程、设备及重质微细碳酸钙的产品质量指标，指明喷雾闪蒸干燥传热传质效果好，干燥速度快，自动化程度高，产品质量优，发展前景广阔。