Intermittent drying of initially saturated porous materials

The aim of this paper is to examine the advantages of convective non-stationary (intermittent) drying and the possibility of its application to materials susceptible to crack formation (wood and ceramics). The notion “non-stationary” means here drying with periodically changeable parameters (air temperature and humidity). The drying time, energy consumption, and quality of dried samples are examined at different schedules of intermittent drying. The acoustic emission (AE) method is applied to monitor on line the material behavior and to detect the commencement of material cracking, and thus to find the moment at which the changes in drying conditions should be initiated. The mathematical model is developed to describe and imitate the intermittent drying kinetics determined experimentally. This model is used next for numerical calculation of the net energy used for drying, and to compare it with the measured total electric energy consumption. The tests were carried out on cylindrically shaped samples of wood and kaolin-clay. The benefits of non-stationary drying with respect to that performed in constant conditions were assessed. Such a formulated subject is a novelty in drying area.     

Influence of convective intermittent drying schemes on drying induced stress–strain of a 3D clay object

Drying process of industrial green in-process products especially those susceptible to cracking, need great care, and optimally arrangement of parameters of convective drying. Intermittent drying is a new technique in drying area and is a promising solution for product quality enhancement. The intermittent drying with variable air temperature and the intermittent drying with variable air humidity are the most used techniques. The current study is devoted to 3D modeling and simulation of intermittent drying with variations of both air humidity and temperature and it is then compared with each of the cases of the intermittent drying with variable air temperature and the intermittent drying with variable air humidity. It was observed that the best dried product quality was obtained in intermittent drying with periodic changes of air temperature. Vapor condensation in the intermittent drying with variable air humidity is an undesirable phenomenon that significantly reduces the effectiveness of this process.     

OPTIMIZATION OF OPERATING CONDITIONS IN TUNNEL DRYING OF FOOD

ABSTRACT

Food drying process in tunnel dryer was modeled from Keey's drying model and experimental drying curve, and optimized in operating conditions consisting of inlet air temperature, air recycle ratio and air flow rate. Radish was chosen as a typical food material to be dried, because it has a typical drying characteristics of food and quality indexes of ascorbic acid destruction and browning in the drying. Stricter quality retention constraint required higher energy consumption in minimizing the objective function of energy consumption under constraints of dried food quality. Optimization results of cocurrent and counter current tunnel drying showed higher inlet air temperature, lower recycle ratio and higher air flow rate with shorter total drying time. Compared with cocurrent operation counter current drying used lower air temperature, lower recycle ratio and lower air flow rate, and appeared to be more efficient in energy usage. Most of consumed energy was analyzed to be used for air heating and then escape from the dryer in form of exhaust air.     

Study of Intermittent Low-Pressure Superheated Steam and Vacuum Drying of a Heat-Sensitive Material

Low-pressure superheated steam drying (LPSSD) has recently been applied to drying of various heat-sensitive foods and bioproducts with success. Several studies have shown that the quality of LPSSD-dried products is superior to that obtained using conventional hot air or vacuum drying. However, drying time and energy consumption for LPSSD is generally greater than that for vacuum drying. Therefore, it is necessary to examine different methodologies to improve the energy efficiency of LPSSD. An intermittent drying scheme is one possible method to reduce the energy consumption of the process while maintaining the desired product quality. In this study, the effect of intermittent supply of energy (through an electric heater and steam injection to the dryer) and vacuum (through the use of a vacuum pump) at various intermittency values or on:off periods (10:5, 10:10 and 10:20 min in the case of intermittent supply of energy and 5:0, 5:5, and 5:10 min in the case of intermittent supply of vacuum) at the on-period setting temperatures of 70, 80, and 90°C on the drying kinetics and heat transfer behavior of the drying samples (banana chips) was studied. The effects of these intermittent drying schemes and conditions on the quality parameters of dried banana chips; i.e., color, shrinkage, texture, and ascorbic acid retention, were also studied. Finally, the energy consumption values for intermittent LPSSD and vacuum drying were monitored through the effective (or net) drying time at various intermittent drying conditions and compared with those using continuous LPSSD and vacuum drying.     

OPTIMIZATION OF OPERATING CONDITIONS IN TUNNEL DRYING OF FOOD

Food drying process in tunnel dryer was modeled from Keey's drying model and experimental drying curve, and optimized in operating conditions consisting of inlet air temperature, air recycle ratio and air flow rate. Radish was chosen as a typical food material to be dried, because it has a typical drying characteristics of food and quality indexes of ascorbic acid destruction and browning in the drying. Stricter quality retention constraint required higher energy consumption in minimizing the objective function of energy consumption under constraints of dried food quality. Optimization results of cocurrent and counter current tunnel drying showed higher inlet air temperature, lower recycle ratio and higher air flow rate with shorter total drying time. Compared with cocurrent operation counter current drying used lower air temperature, lower recycle ratio and lower air flow rate, and appeared to be more efficient in energy usage. Most of consumed energy was analyzed to be used for air heating and then escape from the dryer in form of exhaust air.     

Strain–Stress Formation During Stationary and Intermittent Drying of Deformable Media

The goal of this work is to study the effect of different drying conditions on the induced stresses within deformable media, the drying kinetics, and the energy consumption. A comparison between stationary and intermittent drying with periodically changing air temperature was performed. A theoretical formulation of the coupled heat, mass, and momentum transfers in saturated porous media was established. The model is based on the averaging theory. The thermo-hydro-mechanical coupling was closed using the effective stress theory of Terzaghi. In this approach, the viscoelastic behavior of the medium was considered. A bi-dimensional-shaped bentonite sample was used for numerical tests. The evolution of drying kinetics and stresses within the material during drying at constant and intermittent conditions was presented. It was observed that a non-stationary drying with smaller period applied at the end of the constant drying rate phase has the best effects on the product quality and energy gain without considerably extending the drying time.     

Application of the reaction engineering approach (REA) for modeling intermittent drying under time-varying humidity and temperature

A ‘good’ drying model is important for the design of dryer, evaluation of dryer performance and prediction of product quality. Among the available models, the reaction engineering approach (REA) is a lumped model, proven to be simple, robust and accurate to model drying of several materials. In this paper, the REA is implemented to model intermittent drying, which is usually practiced for saving energy consumption and maintaining product quality during drying, under time-varying drying air temperature and humidity, which is a challenging drying case to model. For this purpose, the equilibrium activation energy (ΔE_v,b) is defined according to the drying settings in each time period and combined with the relative activation energy (ΔE_v/ΔE_v,b) generated from the convective drying experimental data obtained under constant drying conditions. The mass and heat balances also implement the corresponding drying settings in each time period during the intermittent drying. The results indicate that the REA can describe both the moisture content and temperature profiles of the intermittent drying under time-varying drying air temperature and humidity well. The accuracy, simplicity and robustness of the REA for the intermittent drying under time-varying drying air temperature and humidity are proven here. This has provided a major and significant extension of the REA on modeling challenging drying cases.     

INTERMITTENT THIN LAYER CORN DRYING

ABSTRACT

In this study a block oriented modelling technique was introduced in order to simulate the intermittent drying technology process. First, the relevant blocks of the parameters concerning to drying air and the properties of material to be dried were developed. These blocks were then used to set up the full modell of a fix bed dryer including the temperature and moisture distribution of drying air and The material along the depth of the dryer. The simulation results were compared to Bakker-Arkema model with a good agreement. Finally, several measurements were performed for the case of thin layer of com to prove the advantage of the intermittent drying technology. It has been founded that in both energy saving and quality of dried product the intermittent technology has a significant contribution.     

Optimization study of soybean intermittent drying in fixed-bed drying technology

Intermittent drying of materials is an alternative operation that aims at reducing energy consumption, improve the preservation of dried products or decrease effective drying time. Intermittent drying supplies the system with time-varying input air properties that are opposite to traditional operations, where air properties are constant at the dryer inlet. The major objective of this study is to establish the most satisfactory patterns of air temperature and velocity modulation at the dryer entrance to reduce energy consumption. This optimization study was based on a heterogeneous model for the drying of grains in fixed bed validated with experimental data. Intermittent and conventional operation experiments were conducted using equal energy consumption, and the influence of air temperature and velocity modulation on the drying rates related to the percentage of evaporated water were assessed. Results indicated that higher drying rates can be achieved under intermittent operation, and the validated model based on these results could reasonably predict temperature and moisture content profiles. Simulations pointed out that the best modulation patterns of air properties is a function of a variety of system conditions such as initial temperature and moisture content of both soybean and drying air. However, a tendency to reduce energy consumption was observed when the system operation is initially at high temperature and constantly at low velocity.     

Heat Pump Drying of Green Sweet Pepper

Thin-layer drying experiments under controlled conditions were conducted for green sweet pepper in heat pump dryer at 30, 35, and 40°C and hot air dryer at 45°C with relative humidities ranging from 19 to 55%. The moisture content of sweet pepper slices reduced exponentially with drying time. As the temperature increased, the drying curve exhibited a steeper slope, thus exhibiting an increase in drying rate. Drying of green sweet pepper took place mainly under the falling-rate period. The Page equation was found to be better than the Lewis equation to describe the thin-layer drying of green sweet pepper with higher coefficient of determination and lower root mean square error. Drying in heat pump dryer at 40°C took less time with higher drying rate and specific moisture extraction rate as compared to hot air drying at 45°C due to lower relative humidity of the drying air in a heat pump dryer though the drying air temperature was less. The retention of total chlorophyll content and ascorbic acid content was observed to be more in heat pump–dried samples with higher rehydration ratios and sensory scores. The quality parameters showed a declining trend with increase in drying air temperature from 30 to 45°C. Keeping in view the energy consumption and quality attributes of dehydrated products, it is proposed to dry green sweet pepper at 35°C in heat pump dryer.     

Ultrasound-assisted convective drying of apples at different process conditions

ABSTRACT

The aim of this article was to study the influence of drying parameters like air temperature and velocity on the effectiveness of convective (CV) and ultrasound-assisted convective drying. The apple samples were used as the testing material by drying at air temperatures of 313 and 323?K and three different air velocities: 2, 3, and 4?m/s. The drying kinetics, the quality of dried products, and the energy consumption in each drying process were analyzed. The model of drying elaborated by authors was performed to estimate numerically the effect of ultrasound enhancement of convective drying. It was found that application of ultrasound significantly accelerates the CV drying, mainly due to “vibration effect,” and the values of drying parameters like air temperature and velocity affect the drying effectiveness in a different way.     

Modeling Intermittent Drying of Wood under Rapidly Varying Temperature and Humidity Conditions with the Lumped Reaction Engineering Approach (L-REA)

For improving product quality and minimizing energy consumption during drying, intermittent drying is often recommended. The mathematical models that are used to describe intermittent drying are usually transport phenomena based, complex models. In this study, the lumped reaction engineering approach (L-REA) is implemented to model wood drying under rapid periodically changed drying air temperature and humidity with high number of cycles of intermittency. The equilibrium activation energy (ΔE _v,b ), an important parameter for REA approach, is evaluated according to the corresponding drying air temperature and humidity in each drying section. The results of modeling suggest the L-REA works well with the experimental data. The simplicity of the L-REA is obvious and is hoped to be used in an industrial setting more readily. The L-REA can be used for sustainable processing in industries to assist in energy audit and management.     

On the Influence of Glass Transition on Shrinkage in Convective Drying of Fruits: A Case Study of Banana Drying

The ideal shrinkage model assumes that the extent of shrinkage is equal to the volume of liquid water removed from the dried medium. Generally if a material undergoes glass transition during the drying process, shrinkage will no longer be ideal. The aim of this study was to observe how the glass transition temperature influences the shrinkage kinetics. Cylindrical banana samples were dried. Shrinkage extent was significant for all drying conditions (temperature: 30–50°C, relative humidity: 0–80%). Deviation from linearity was found to be affected not only by drying air temperature but also by its relative humidity.     

On the Influence of Glass Transition on Shrinkage in Convective Drying of Fruits: A Case Study of Banana Drying

The ideal shrinkage model assumes that the extent of shrinkage is equal to the volume of liquid water removed from the dried medium. Generally if a material undergoes glass transition during the drying process, shrinkage will no longer be ideal. The aim of this study was to observe how the glass transition temperature influences the shrinkage kinetics. Cylindrical banana samples were dried. Shrinkage extent was significant for all drying conditions (temperature: 30-50°C, relative humidity: 0-80%). Deviation from linearity was found to be affected not only by drying air temperature but also by its relative humidity.     

Drying Kinetics and Antioxidant Phytochemicals Retention of Salak Fruit under Different Drying and Pretreatment Conditions

Kinetics of hot air drying and heat pump drying were studied by performing various drying trials on salak slices. Isothermal drying trials were conducted in hot air drying and heat pump drying at a temperature range of 40–90°C and 26–37°C, respectively. Intermittent drying trials were carried out in heat pump drying with two different modes: periodic heat air flow supply and step-up air temperature. It was observed that the effects of relative humidity and air velocity on drying rate were significant when moisture content in salak slices was high, whereas the effects of temperature prevailed when the moisture content was low. As such, it was proposed that drying conditions should be manipulated according to the moisture transport mechanisms at different stages of drying in order to optimize the intermittent drying and improve the product quality. Generally, loss of ascorbic acid during drying was attributed to thermal degradation and enzymatic oxidation, whereas the loss of phenolic compounds was mainly due to thermal degradation. Experimental results showed that heat pump drying with low-temperature dehumidified air not only enhanced the drying kinetics but produced a stable final product. Heat pump–dried samples retained a high concentration of ascorbic acid and total phenolic compounds when an appropriate drying mode was selected.     

鱿鱼热泵-热风联合干燥节能研究

以热泵干燥阶段的温度、分阶段干燥的水分转换点、后期热风干燥阶段的温度为影响因素,以单位能耗除湿值（SMER）和总挥发性盐基氮值（T—VBN）两者的综合指标Y为最终评价指标进行响应面优化分析,得出了联合干燥最佳工艺参数：热泵干燥温度（X1=54．3℃）、水分转换点（X2=32．3％）和热风干燥温度（X3=43．5℃）。在最优工艺条件下比较HPD、HPD＋AD、AD三种干燥方式下的样品品质和耗能,联合干燥得到的鱿鱼干品质高于热风干燥,而且降低了干燥能耗38．67％。     

SOLAR DRYING OF RAISING

The drying of Sultana seedless grapes was investigated under intermittent and continuous operating conditions in a laboratory solar installation involving a thermal storage bed and an auxiliary heater. The effect of pretreatment, and of the a i r velocity on the drying rate of the grapes at constant temperature was also studied in relation t o the quality of the dried product.

Solar drying of the grapes was accomplished in 30.5 to 60.5 h of intermittent operation, or 19 to 60 h of continuous drying, involving the thermal storage bed and the auxiliary heater. The shortest drying time (19 h ) and the highest quality dried product were obtained with grapes dipped in a hot (80 C ) solution of sodium hydroxide and ethyl oleate, which were dried continuously at 42°C and 2m/s, air temperature and velocity respectively.

The mean apparent diffusivity of moisture in raisins at 6o°C and air velocity 2m/s was estimated as 1.0.10-10 m2/s.     

SOLAR DRYING OF RAISING

The drying of Sultana seedless grapes was investigated under intermittent and continuous operating conditions in a laboratory solar installation involving a thermal storage bed and an auxiliary heater. The effect of pretreatment, and of the a i r velocity on the drying rate of the grapes at constant temperature was also studied in relation t o the quality of the dried product.

Solar drying of the grapes was accomplished in 30.5 to 60.5 h of intermittent operation, or 19 to 60 h of continuous drying, involving the thermal storage bed and the auxiliary heater. The shortest drying time (19 h ) and the highest quality dried product were obtained with grapes dipped in a hot (80 C ) solution of sodium hydroxide and ethyl oleate, which were dried continuously at 42°C and 2m/s, air temperature and velocity respectively.

The mean apparent diffusivity of moisture in raisins at 6o°C and air velocity 2m/s was estimated as 1.0.10-10 m2/s.     

Quality evaluation of pineapple fruit during drying process

Pineapple (Anana comosus) slices were dried by hot-air convective drying technique at fixed temperature (45, 60 and 75 °C) and constant air velocity of 1.5 m/s. The effect of drying conditions (drying time and air temperature) on the pineapple quality was evaluated. The quality of dehydrated pineapple was analyzed by color and texture changes, l-ascorbic acid loss and the ability of water uptake during rehydration procedure. Water uptake during rehydration was described by Page model. Statistical analysis of data revealed not significant difference (p > 0.05) among color and mechanical characteristics of pineapple samples dried at different drying temperatures to preset moisture content. Pineapple samples dried at 45 °C had better rehydration ability and more l-ascorbic acid retention than those obtained by air drying 75 °C. Hence, 45 °C drying temperature was best condition for pineapple quality preservation.     

Drying Kinetics and Quality of Soy Residue (Okara) Dried in a Jet Spouted-Bed Dryer

The objective of this study was to investigate experimentally the effects of various drying parameters, i.e., inlet air velocity, inlet air temperature, initial bed height and heating duration, on both the drying kinetics and various quality attributes of dried okara viz. percentage changes of the total protein content, color, urease index, as well as the specific energy consumption during drying in a jet spouted-bed dryer. It was observed that all drying conditions tested could reduce the amount of urease to an acceptable level and increasing the heating duration, air velocity, and hot air temperature led to a significantly higher rate of reduction of urease activity. The percentage change of the total protein content of okara undergoing different drying conditions was not significantly different, however. After drying, the redness of okara was the highest changing color index, but its absolute value was still much lower than those observed for the lightness and yellowness. Hence, dried okara appeared light-brown. The specific energy consumption of the process was found to be in the range of 3.69 to 5.89 MJ/kg evaporated water.