ELECTRICAL-RESISTANCE ELEMENT ARRAYS OPERATING AT HIGH VOLTAGES FOR HEATING AND DRYING IN INDUSTRIAL APPLICATIONS

Abstract

Drying using ambient subzero temperatures is of potential interest for thermosensitive products. Existing theoretical drying models have been used to predict the response of the system to different aeration systems. The model is based on enthalpy balances and includes water freezing and deposition of water on the surface of the commodity. It uses thermophysical properties of the commodity (i.e., maize in this study) and ambient weather data collected from northeastern China. Water within the grain is modelled as bound, free or frozen. The physical state of water under subzero temperatures has been investigated using a differential scanning calorimeter and nuclear magnetic resonance spectrometry. It has been established that the quantity of bound water was around 17%. Thermophysical properties characterizing the drying behavior of maize kernels cv. Huangmo 417, the most common variety grown in northeastern China, were determined under a wide range of moisture contents and drying temperatures. Those were: particle and bulk density, porosity, thermal conductivity, specific heat, thin layer drying, and sorption isotherms. It could be established that the thermal conductivity and specific heat were strongly dependent on temperature and relative humidity and that the sorption isotherms followed the 5-term Guggenheim-Anderson-de-Boer model. The industrial-scale in-store drying experiments in northeastern China have demonstrated the feasibility of in-store drying under subzero conditions. Advantages in terms of reduced susceptibility of maize to mould formation have been established, resulting in improved quality and financial returns to the processor.     

Studies of Physical State of Water in Maize from Northeast China (cv. Huangmo 417) During Drying at Subzero Temperatures

In northeastern provinces of China, maize is one of the main sources of income in agriculture. After harvest, the grain is kept under subzero conditions for 3-4 months. Studies have been undertaken to assess the feasibility of drying at subzero temperatures during storage. Using a combination of NMR and MRI enabled a better understanding of physical states of water in the cv. Huangmo 417 during drying at subzero temperatures. The studies showed two water components with different relaxation times (long and short T₁) that were found when temperature was lowered from +25 to -30°C. T₁ was a minimum when the temperature was reduced to -20°C. T₂ was found to be much shorter than T₁. This was an indication that water mobility was decreasing at the same time as the temperature. The results from MRI revealed the location of frozen water in different areas of the maize kernel during subzero drying. This approach to characterizing the states of water in maize under subzero conditions will assist in devising drying strategies for northeastern China.     

Evaluation of the Sorption Equilibrium and Effect of Drying Temperature on the Antioxidant Capacity of the Jaboticaba (Myrciaria cauliflora)

In recent years, interest toward berries has increased (e.g., Myrciaria cauliflora or jaboticaba) because of their high phenolic content (phenolic acids, flavonoids, and anthocyanins) that has been associated with positive effects on consumer health and which play an important role in the antioxidant properties of food. This study analyzed the sorption isotherms, thermodynamic properties of sorption (isosteric heat and Gibbs free energy), and the evolution of the antioxidant capacity during the drying process. The effects of drying temperatures of 40°, 50°C, and 60°C on the antioxidant capacity and thermodynamic properties of sorption were evaluated. The gravimetric static method for sorption isotherm determination over a range of relative humidity levels from 0.10 to 0.90 was used. The sorption isotherms exhibited a Type II behavior, typical for many foods. The Guggenheim, Anderson, and Boer (GAB); Oswin; Peleg; and Lewicki models were used to fit the experimental data, and it was determined that the GAB and Peleg models were most appropriate for describing the sorption curves. The isosteric heat and Gibbs free energy were obtained from the experimental sorption equilibrium. The isosteric heat of adsorption decreased when the moisture content increased, while the Gibbs free energy increased. In addition, the phenolic content and antioxidant capacity increased while drying at 50°C and 60°C, whereas these factors decreased at 40°C. Our results provide the food industry with information concerning the best drying conditions to preserve antioxidant properties.     

Moisture sorption characteristics of waste activated sludge

Nowadays the treatment of activated sludge often includes a thermal drying step. Data such as water vapour sorption isotherms and heat of sorption are required for good design and control of the drying processes. An equilibrium method was used to measure these data on domestic activated sludge, between 39 °C and 80 °C. The experimental investigation indicated that the nature of the sludge and its evolution (bulking sludge, anaerobic storage) do not affect the sorption isotherms. Experimental data were fitted to different well‐known models; the Oswin relationship gave the better fit. Based on sorption data the heat of sorption as a function of moisture content was determined. © 2001 Society of Chemical Industry     

The Application of Heat Pump in Drying of Biomaterials

ABSTRACT

The UN estimates that 6.4 billion people will be living on Earth by the year 2000. As a consequence, dried biomaterials demand will rise and equipments will have to be built or redesigned to accomadate changes. The design and innovations on industrial dryers require fundamental knowledge conceived in laboratory units. The new heat pump dryer operates at a wide relative humidity and the air temperature is adjustable from –20 to 100'C. Above and belw freezing point drying modes are done in the same plant which allows the control of biomaterial properties. The technique consists of two stages, unbound weakly held water removal by freeze drying. and tightly bound moisture desorption by medium temperature drying. The fluidized and shelf heat pump dryers have the required features to dry heat sensitive biomaterials. The dryers were built and extensive research on biomaterials has been conducted at The Norwegian Institute of Technology. Many emeriments were done on effect of heat pump drying conditions on–enrime and biological active bacteria solutions, fruits, cod fish, fish feed and shrimps. At optimum heat drying conditions, the bacteria Rhodococcus reached 100% viability and dried biomolecules attained full biolosical activity. The heat pump dryer allowed adjustment of the bio–mterial properties and quality according to its phase, shape and size. Dried samples showed improvements on survival rate. rehydration, colour, hardness and mono–multimolecular sorption isotherms over poorly dried products. Beneficial results were achieved for nearly all tests. This indicates that the heat pump dryer application has high potential in processing heat sensitive biomaterials.     

The Application of Heat Pump in Drying of Biomaterials

The UN estimates that 6.4 billion people will be living on Earth by the year 2000. As a consequence, dried biomaterials demand will rise and equipments will have to be built or redesigned to accomadate changes. The design and innovations on industrial dryers require fundamental knowledge conceived in laboratory units. The new heat pump dryer operates at a wide relative humidity and the air temperature is adjustable from -20 to 100'C. Above and belw freezing point drying modes are done in the same plant which allows the control of biomaterial properties. The technique consists of two stages, unbound weakly held water removal by freeze drying. and tightly bound moisture desorption by medium temperature drying. The fluidized and shelf heat pump dryers have the required features to dry heat sensitive biomaterials. The dryers were built and extensive research on biomaterials has been conducted at The Norwegian Institute of Technology. Many emeriments were done on effect of heat pump drying conditions on-enrime and biological active bacteria solutions, fruits, cod fish, fish feed and shrimps. At optimum heat drying conditions, the bacteria Rhodococcus reached 100% viability and dried biomolecules attained full biolosical activity. The heat pump dryer allowed adjustment of the bio-mterial properties and quality according to its phase, shape and size. Dried samples showed improvements on survival rate. rehydration, colour, hardness and mono-multimolecular sorption isotherms over poorly dried products. Beneficial results were achieved for nearly all tests. This indicates that the heat pump dryer application has high potential in processing heat sensitive biomaterials.     

Temperature dependence of the sorption isotherms of cement-based materials: Heat of sorption and Clausius–Clapeyron formula

In the field of radioactive waste management the concrete structures are expected to undergo significant heating due to the waste thermal power and significant drying (in the French design, the temperature is not expected to exceed 80 °C). The durability assessment of such structures thus requires the knowledge of the evolution of the water vapour sorption properties versus temperature. The latter can be easily estimated using the Clausius–Clapeyron (CC) equation: the approach presented hereafter requires the knowledge of the isosteric heat of adsorption and one desorption isotherm (at ambient or any other temperature). Moreover, it is shown that using the CC equation and the desorption isotherms at two different temperatures, it is possible to estimate accurately the desorption isotherm at any other temperature.     

Heats and Entropies of Sorption of Cereal Grains : A Comparison Between Integral and Differential Quantities

Several thermodynamic properties for maize, rough rice and wheat has been calculated using desorption isotherms available in the literature. It was obtained an analytical expression to predict the differential heat (isosteric) as function of moisture content using a three parameters equilibrium model, based on enthalpy-entropy compensation effect, which takes into account the effet of temperature. Two integral heats of sorption were calculated, the first from the slopes of the iso-spreading pressure lines and the second from integration of the differential heat. This last one was used to estimate the energy requirement to remove water from initial moisture content to different final moisture levels, which is useful in drying area. Finally it was calculated the differential and integral entropies of sorption as function of moisture content.     

Kaffir Lime Leaf (Citrus hystric DC.) Drying Using Tray and Heat Pump Dehumidified Drying

Desorption isotherms of mature kaffir lime leaves are determined and a nonlinear regression program was applied to the experimental data to fit with any of the four moisture sorption isotherm models. It was found that the modified Halsey model could fit the best. Tray and heat pump–dehumidified drying of kaffir lime leaves were conducted and it was found that the modified Page model was the most effective one. The drying constant was fitted to drying air temperature using the Arrhenius model. Effective moisture diffusivities were determined using the drying data. Heat pump–dehumidified drying reduced drying time and provided dried kaffir lime leaves with higher amount of citronellal than tray drying.     

Effects of Drying Methods on Functionality of a Native Potato Protein Concentrate

Pulp and dilute fruit water (PFW) are by-products from the potato starch processing industry. Potato protein obtained from an expanded bed adsorption (EBA) process is a value-added protein concentrate that can offer special technical properties in food systems. The influence of drying techniques on the physiochemical, quality, and functional properties (color, water content, bulk density, rehydration properties, sorption isotherms, specific enzyme activity, solubility, protein denaturation) of dehydrated potato water effluent was investigated. The results indicate that atmospheric freeze drying (AFD) is a more gentle drying process than spray drying and vacuum freeze drying. Both enthalpy measurements and sorption isotherms indicate reduced protein denaturation of AFD samples, while specific enzyme activity is at the same level for all dried samples.     

Effects of Drying Methods on Functionality of a Native Potato Protein Concentrate

Pulp and dilute fruit water (PFW) are by-products from the potato starch processing industry. Potato protein obtained from an expanded bed adsorption (EBA) process is a value-added protein concentrate that can offer special technical properties in food systems. The influence of drying techniques on the physiochemical, quality, and functional properties (color, water content, bulk density, rehydration properties, sorption isotherms, specific enzyme activity, solubility, protein denaturation) of dehydrated potato water effluent was investigated. The results indicate that atmospheric freeze drying (AFD) is a more gentle drying process than spray drying and vacuum freeze drying. Both enthalpy measurements and sorption isotherms indicate reduced protein denaturation of AFD samples, while specific enzyme activity is at the same level for all dried samples.     

Drying Kinetics and Sorption Isotherms of the Nejayote

The nixtamalization is an alkaline cooking process in corn tortillas industry that uses large quantities of water (5:1 corn weight) and producing a byproduct called nejayote. In this work, drying kinetic of insoluble fraction from nejayote was obtained using a hot air cabinet dryer at 60, 75, and 90°C. The air rate and water content were 1.5 m/s and 0.028 kg of water/kg of dry air, respectively. Sorption isotherms were measured by the COST method with sulfuric acid solutions at 30, 45 and 60°C. Results showed a difference in drying kinetics/drying time between drying temperatures of 60 and 75°C, and practically no difference between 75 and 90°C. This was probably due to the development of a superficial crust on the product. The shape of sorption curves was as expected; no temperature effect was identified but it is a common behavior for products with high sugar content. Finally the sorption isotherm was predicted based on primary chemical compounds (carbohydrates, proteins, crude fiber, and calcium hydroxide).     

稻谷就仓低温通风干燥系统生产性实验

中国是稻谷的生产和消费大国，总产量占世界总产的33％左右，居世界第一位。传统的稻谷高温烘干方法介质温度高、干燥生产能耗大、费用高，烘干后稻谷惊纹率高，整精米率降低，严重影响稻谷加工和食用品质。在探索稻谷低温通风干燥特性、综合分析国内外稻谷低温通风干燥特性试验研究成果的基础上，研制了稻谷深层通风干燥系统，确定了立管式多环径向通风干燥工艺路线，通过稻谷就仓通风干燥系统的生产性实验，对通风干燥仓结构性能、稻谷通风前后质量变化规律等进行了测试，验证了稻谷低温通风干燥的科学性与可行性。     

Drying Kinetics and Sorption Isotherms of the Nejayote

Abstract

The nixtamalization is an alkaline cooking process in corn tortillas industry that uses large quantities of water (5:1 corn weight) and producing a byproduct called nejayote. In this work, drying kinetic of insoluble fraction from nejayote was obtained using a hot air cabinet dryer at 60, 75, and 90°C. The air rate and water content were 1.5 m/s and 0.028 kg of water/kg of dry air, respectively. Sorption isotherms were measured by the COST method with sulfuric acid solutions at 30, 45 and 60°C. Results showed a difference in drying kinetics/drying time between drying temperatures of 60 and 75°C, and practically no difference between 75 and 90°C. This was probably due to the development of a superficial crust on the product. The shape of sorption curves was as expected; no temperature effect was identified but it is a common behavior for products with high sugar content. Finally the sorption isotherm was predicted based on primary chemical compounds (carbohydrates, proteins, crude fiber, and calcium hydroxide).     

Sorption of moisture by resorbable surgical sutures in a wide temperature range

Methods are proposed for calculating the equilibrium moisture content and finding the sorption isotherms of resorbable sutures made from polyglycolic acid and polydioxanone based on their chemical structure, which allow modeling and optimizing the conditions for drying them. The calculations were performed with a thermal sorption equation and the Van Krevelen method. The calculated sorption isotherms were confirmed experimentally. The constants of the thermal sorption equation obtained and the proposed method of estimating the thermal sorption coefficient allowed theoretically calculating the sorption values in a wide temperature range and the heat consumption for the drying process. __________ Translated from Khimicheskie Volokna, No. 5, pp. 44–45, September–October, 2007.     

Mathematical Simulation of Batch Convective Drying of Food with Sorption Isotherms Calculated by Ross Equation

Mathematical simulations of the batch convective drying of cassava and carrot were developed employing predicted sorption isotherm. Sorption isotherms of carrot and cassava were predicted by Ross equation in conjunction with Raoult's Law and Norrish equations. The predictions considered the average chemical composition of the main solutes and nonsolutes on dry basis through a FORTRAN computational program that calculated the water activity at different moisture contents. The predicted and experimental water activity values were fitted by multiple linear regression to obtain empirical parameters from the Henderson modified equation. No difference was found between drying curves of these foods developed by simulation with sorption isotherms obtained experimentally and predicted through chemical composition. Therefore, it is possible generalize the simulation of food drying without necessity to have experimental data of water activity with saving time of operation.     

Prediction of sorption equilibrium both in air and superheated steam drying of energetic variety of willow Salix viminalis in a wide temperature range

Sorption isotherms and isobars of energetic variety of willow Salix viminalis in a wide range of temperature and water activity were determined. In order to investigate sorption isobars a specially constructed set-up was built. The sorption isotherms were determined in the water bath and CIsorp water sorption analyser. The experimental data were compared with the literature data. It was found out that water activity of willow as a function of temperature and moisture content can be described by one generalized equation. The equation can be then used for the purpose of prediction of sorption equilibrium during wood drying process in a wide range of temperatures and moisture contents.     

Optimal Control of the Secondary Drying Stage of Freeze Drying of Solutions in Vials Using Variational Calculus

Abstract:

The problem of operating freeze drying of pharmaceutical products in vials loaded on trays of freeze dryer to obtain a desired final bound water content in minimum time is formulated as an optimal control problem. Two different types of freeze dryer designs were considered. In the type I freeze dryer design, upper and lower plate temperatures were controlled together, while in the type II freeze dryer design, upper and lower plate temperatures were controlled independently. The heat input to the material being dried and the drying chamber pressure were considered as control variables. Only the scorch temperature was considered as a constraint on the system state variables during the secondary drying stage, because all the free water content (frozen water) is removed from the solid matrix during the primary drying stage of freeze drying. Necessary conditions of optimality for the secondary drying stage of freeze drying process in vials were derived and presented by using rigorous multidimensional unsteady-state mathematical models. The theoretical approach presented in this work was applied in the freeze drying of skim milk. Significant reductions in drying times of the secondary drying stage of the freeze drying process in vials were observed and more uniform bound water and temperature distributions in the material being dried were obtained compared to the conventional operational policies.     

CONVECTIVE DRYING OF PAPER CALCULATED WITH A NEW MODEL OF THE PAPER STRUCTURE

Paper drying models are based on the assumption of a rigid solid matrix. Because of the interaction of water molecules and paper fibre the dimensions of the sheet are changing, especially the thickness. These changes influence the moisture transport within the material. A model of the paper structure has been developed taking into account different kinds of pores in the paper.

By the assistance of this model the dependence between water content and thickness or transport parameters as permeability and thermal conductivity can be predicted. The only informations which are needed are sorption isotherms, apparent paper density and permeability of the single phase flow. Further informations as the pore size distribution are useful but not necessary.

The transport parameters are generated in the suggested way for wood pulp. It is shown that in the case of convective drying, the agreement between experimental results and model simulations is good.     

CONVECTIVE DRYING OF PAPER CALCULATED WITH A NEW MODEL OF THE PAPER STRUCTURE

Paper drying models are based on the assumption of a rigid solid matrix. Because of the interaction of water molecules and paper fibre the dimensions of the sheet are changing, especially the thickness. These changes influence the moisture transport within the material. A model of the paper structure has been developed taking into account different kinds of pores in the paper.

By the assistance of this model the dependence between water content and thickness or transport parameters as permeability and thermal conductivity can be predicted. The only informations which are needed are sorption isotherms, apparent paper density and permeability of the single phase flow. Further informations as the pore size distribution are useful but not necessary.

The transport parameters are generated in the suggested way for wood pulp. It is shown that in the case of convective drying, the agreement between experimental results and model simulations is good.