Microwave Drying of Wood Strands

Characteristics of microwave drying of wood strands with different initial moisture contents and geometries were investigated using a commercial small microwave oven under different power inputs. Temperature and moisture changes along with the drying efficiency were examined at different drying scenarios. Extractives were analyzed using gas chromatography/mass spectrometry (GC/MS). The results showed that the microwave drying process consisted of three distinct periods (warm-up period, evaporation period, and heating-up period) during which the temperature, moisture change, and drying efficiency could vary. Most of the extractives were remnant during microwave drying. It was observed that with proper selections of power input, weight of drying material, and drying time, microwave drying could increase the drying rate, save up to 50% of energy consumption, and decrease volatile organic compound (VOC) emissions when compared with the conventional drying method.     

TEMPERATURE AND MOISTURE CHANGES DURING MICROWAVE DRYING OF SLICED FOOD

ABSTRACT

Microwave drying characteristics of sliced foods were investigated using potatoes (Solarium tuberosum) as a test model. Sliced samples were dried to 7-10% moisture content at microwave power levels between 2.2 W/g and 3.6 W/g (raw material). Moisture and temperature changes during drying were monitored. Semi-empirical models were developed that followed temperature and moisture changes during microwave drying. Sliced potatoes experienced three distinct periods: a warming-up period with little removal of moisture; a constant temperature period in which most of the drying took place; and a heating up period in which the drying rate decreased and sample temperature increased rapidly, often causing partial charring. Product temperature during the second period of microwave drying increased with sample thickness and microwave power. Drying rates were not affected by slice thickness, but increased with the microwave power/mass ratio. Product charring towards the end of drying may be avoided by reducing microwave power and increasing ambient air velocity.     

TEMPERATURE AND MOISTURE CHANGES DURING MICROWAVE DRYING OF SLICED FOOD

Microwave drying characteristics of sliced foods were investigated using potatoes (Solarium tuberosum) as a test model. Sliced samples were dried to 7-10% moisture content at microwave power levels between 2.2 W/g and 3.6 W/g (raw material). Moisture and temperature changes during drying were monitored. Semi-empirical models were developed that followed temperature and moisture changes during microwave drying. Sliced potatoes experienced three distinct periods: a warming-up period with little removal of moisture; a constant temperature period in which most of the drying took place; and a heating up period in which the drying rate decreased and sample temperature increased rapidly, often causing partial charring. Product temperature during the second period of microwave drying increased with sample thickness and microwave power. Drying rates were not affected by slice thickness, but increased with the microwave power/mass ratio. Product charring towards the end of drying may be avoided by reducing microwave power and increasing ambient air velocity.     

Drying characteristics and water dynamics during microwave hot-air flow rolling drying of Pleurotus eryngii

Abstract

The aim of this study was to improve the drying uniformity and quality of Pleurotus eryngii by using the combination of microwave drying and hot-air flow rolling drying. The moisture content, drying uniformity, and water migration of P. eryngii during microwave hot-air flow rolling drying (MARD) were analyzed in detail. The temperature distribution images were obtained via infrared thermal imaging techniques and the relationship between relaxation time and signal amplitude were obtained via low-field nuclear magnetic resonance analysis and imaging (LF-NMR/MRI). The curves of signal amplitude with moisture content changes were fitted by a linear model with good linearity correlation. It was found that the hot-air and rolling bed could improve the drying uniformity of microwave drying. And the free water was found to transform into immobilized water and bound water during the drying process. Only a small amount of water was left in the dried P. eryngii in the final stage. The results could provide supportive information for improving the uniformity and quality of the drying processes of the edible fungi.     

Modeling of Moisture Diffusion in Microwave Drying of Hardwood

A one-dimensional mathematical model was developed to predict temperature and moisture content profiles in red maple (Acer rubrum L.) and white oak (Quercus alba) during microwave drying. The model was solved using the finite element analysis with MATLAB software. The predictions for temperature and moisture content agreed favorably well with the experimental data. The diffusion coefficients of the red maple and the white oak in microwave drying conditions were calculated and analyzed. Equations of the diffusion coefficient in longitudinal and transverse directions based on input microwave power level are presented in this article. In microwave drying of hardwood, the red maple was heated more efficiently than the white oak because of higher absorbing efficiency of the microwave power.     

Efficient drying and oxygen-containing functional groups characteristics of lignite during microwave irradiation process

To remove the high moisture of ZhaoTong lignite, the efficient drying characteristics and oxygen-containing functional groups changes in lignite during microwave irradiation process were highlighted in this study. As the microwave absorbers, lignite char and NaNO₃ were added to microwave drying of ZhaoTong lignite. The minimum chemical oxygen demand of waste water generated from microwave drying process of lignite was 99.89?mg?O₂?L^?1. The effects of microwave power, lignite mass, the weight ratio of lignite char to lignite and NaNO₃ content on the drying rate, and moisture diffusion coefficient of lignite were investigated during lignite microwave irradiation process. It was found that the drying rate and moisture diffusion coefficient of lignite increased with increasing microwave power, the weight ratio of lignite char to lignite and NaNO₃ content, but decreased with increasing lignite mass. Lignite char and NaNO₃ were mixed with lignite that can enhance the instantaneous surface temperature of lignite sample under microwave irradiation. Compared with addition of lignite char to lignite, the addition of NaNO₃ to lignite can decrease the unit electric power consumption of moisture evaporating. And the minimum unit electric power consumption of moisture evaporating was 9.44?Wh?g^?1. The FTIR technology was used to investigate the oxygen-containing functional groups changes in lignite during microwave drying process. The oxygen-containing functional groups of lignite were effectively removed with increasing microwave power.     

Microwave Finish Drying of Osmotically Dehydrated Cranberries

Abstract

Combined microwave and hot-air drying characteristics was studied for the drying of cranberries that had been previously partially dehydrated by osmosis in a high fructose corn syrup (76°Brix). A 750 W 2450 MHz microwave oven was used to dry cranberry samples from 57% to 15% moisture content using three different power densities (0.75, 1.0, 1.25 W/g of initial cranberries) and two different power cycles (30 s On/30 s Off and 30 s On/60 s Off). All combinations of these variables were tested in triplicate. Quality of the cranberries was measured using a universal testing machine, chromameter, and with the use of a taste test panel. Drying times ranged from 2.2 to 5.0 h. Power times and power cycles affected the drying time and the quality of the dried cranberries. Lower power densities resulted in cranberries with higher quality. High power densities (125 W/g) resulted in the burning of some cranberries.     

Shrinkage Characteristics of Potatos Dehydrated Under Combined Microwave and Convective Air Conditions

ABSTRACT

A study of the pysical structure and quality potatoes during microwave and air drying war undertaken. The effect of various experimental variables on volumetric shrinkage including microwove power levels, air temperature and velocity as well as sample geometry was investigated. In both microwave and air drying, the shrinkage of potatoes showed a linear behaviour in relation to different moisture content levels. Experimental data showedthat air velocity and microwave power had a major effect on the degree of shrinkage, also the shrinkage behaviour was independent of sample geometry and air temperature. The use of microwave produced smaller changer in volume (less shrinkage) and henee a better quality finished product.     

LF-NMR online detection of water dynamics in apple cubes during microwave vacuum drying

ABSTRACT

In this study, changes in various states of water in apple cubes were assessed using low-field nuclear magnetic resonance (LF-NMR) during microwave vacuum drying. Apple cubes were dried at different microwave power levels (100, 150, and 200?W). Indicators including moisture content and water activity were measured. The Carr–Purcell–Meiboom–Gill (CPMG) sequence was used to measure transverse relaxation times (T₂). The results showed that three water fractions with different T₂ relaxation times (around 11, 126, and 1335 ms) were detected in fresh apples, which corresponded to different cell compartments. The transverse relaxation time of bulk water (T₂₃) and the signal per mass of the bulk water (A₂₃/g) decreased significantly with increasing drying time at different microwave power levels. The signal per mass of the total water (A_Total/g) had significant correlation with the total moisture content (R²?=?0.9919). Furthermore, good correlation (R²?=?0.9799) between water activity and NMR parameters based on partial least square regression model were observed. This research revealed that LF-NMR spectroscopy, a nondestructive technique, can detect the changes of water in different populations in the matrix.     

Moisture flow characteristics and variation patterns of dielectric constants in bitter gourd slices during microwave hot-airflow vibrating drying

Abstract

The uniformity of microwave-dried bitter gourd slices was poor, and a microwave hot-airflow vibrating drying (MHAVD) apparatus was used to carry out an in-depth elucidation of moisture flow characteristics underlying the variation in dielectric characteristics. Low-field nuclear magnetic resonance and imaging (LF-NMR/MRI) was used to measure the moisture state and distribution during different drying stages, and the relationship model between moisture semaphore and moisture ratio were analyzed. A vector network analyzer was used to measure the patterns in the variation of dielectric characteristics in bitter gourd slices at different drying stages. As a result, the temperature and moisture distribution of bitter gourd slices during MHAVD was uniform caused by hot air and vibration, and the water semaphore in NMR gradually decreased, but the bound water semaphore increased during certain drying stages. There is a significant linear relationship between the total semaphore A₂ and the moisture ratio M_R in bitter gourd slices per unit dry basis mass in the NMR spectrum (R² = 0.99). As moisture decreases, the ability of the material to store microwaves and convert electromagnetic energy to heat energy decreases, the ability of the material to absorb microwaves decreases, and the microwave penetration significantly increases. The moisture flow characteristics in bitter gourd slices is related to the variation of dielectric characteristics during MHAVD, and drying uniformity was improved.     

Dielectric Properties and Optimization of Parameters for Microwave Drying of Petroleum Coke Using Response Surface Methodology

The dielectric properties of petroleum coke at five temperatures between 20 to 100 ^° C, covering different moisture content levels at 2.45 GHz, were measured using an open-ended coaxial probe dielectric measurement system. The effects of drying temperature, duration of drying, and sample mass on the moisture content and dehydration rate of petroleum coke was assessed utilizing the response surface methodology. The dielectric constant, loss factor, and loss tangent were all found to increase nearly linearly with increase in moisture content. Three predictive empirical models were developed to relate the dielectric constant, loss factor, and loss tangent of petroleum coke as a linear function of moisture content from 3–10%. An increase in temperature between 20 to 100 ^° C was found to increase the dielectric properties. The penetration depth was observed to increase linearly with decrease in moisture content in the range of 3 to 10%. A predictive empirical model was developed to calculate penetration depth for petroleum coke. Two mathematical models were established and analyzed using RSM to describe the relationship between the microwave drying conditions and the responses, moisture content, and dehydration rate. Statistical analysis with response surface regression showed that microwave drying temperature, duration of drying, and sample mass were significantly related to moisture content and dehydration rate. Based on the RSM analysis, the optimum process conditions were estimated to be a microwave drying temperature of 75 ^° C, drying duration of 10 sec, and sample mass of 60 g, with the resultant moisture content being 0.34 at a dehydration rate of 2.94 g/min.     

An Empirical Model for the Estimation of Moisture Ratio During Microwave Drying of Plaster of Paris

The drying characteristics of plaster of Paris (POP) under microwave irradiation were studied for different shapes of materials through various drying parameters like microwave power, initial moisture content, and drying time. An empirical model for the estimation of moisture ratio was developed using the drying kinetic data of POP. Further, the experimental data on moisture ratio of POP for different operating conditions were fitted with the nine basic drying model equations. Based on the observations, the constants and coefficients of the literature models were rewritten in the form of Arrhenius and logarithmic expressions considering microwave power as input variable. Fifty-eight new model expressions were derived by changing the constants and coefficients and tested using the present experimental data. From the analysis of RMSE, χ², and EF parameters for the derived models, a suitable empirical model (Model No. 55, RMSE = 0.0874; χ² = 0.0020; EF = 0.9999) was established to represent the present experimental data on microwave drying of POP.     

THE INFLUENCE OF POLARIZATION ON THE MICROWAVE DRYING PROCESS OF SHELLED CORN

ABSTRACT

In this paper the effect of electromagnetic polarization of the microwave field on drying of shelled corn was analyzed. The drying process was investigated in a microwave field of 2.45 GHz. A number of corn kernels with the same orientation were stocked on plastic plates and these plates with corn kernels on their surfaces were fixed to each other, creating the corn matrixes. Plastic plates and glue used to the experiments were inert, related to the applied microwave power. By rotating test materials the influence of different polarization angles of electromagnetic field could be registered. Furthermore, setting test material in different positions the effect of distance between the antenna and the corn matrix were observed

The experiments carried out showed that in moist crops placed in microwave field ion polarization is dominant and the drying rate depends on relationship between the electric field strength vector and the overall moisture gradient in the kernel.     

COMBINED MICROWAVE AND SPOUTED BED DRYING OF DICED APPLES: EFFECT OF DRYING CONDITIONS ON DRYING KINETICS AND PRODUCT TEMPERATURE

ABSTRACT

The influence of microwave power (0 to 8.0 W/g, dry basis) and hot air temperature (25°C to 95 °C) on drying rate and product temperature of diced apples (from 31 % to 5% moisture content, dry basis) in a laboratory microwave and spouted-bed combined dryer was investigated. Product temperature initially increased sharply to a plateau about 12 to 15°C above the spouted bed air temperature at a microwave input power 6.4 W/g. This temperature remained almost constant thereafter. Uniform microwave heating was achieved as evidenced by uniform product color and product temperature. Drying rates increased with increasing spouted-bed air temperature or microwave power level, But higher microwave power caused more darkening of the product. Drying of the diced apples in the microwave and spouted bed drying system exhibited two falling rates periods. The influence of air temperature on effective moisture diffusivity followed an Arrhenius type equation. The activation energies were 23.7 kJ/mol and 26.7 kJ/mol for the first and second falling rate periods, respectively.     

QUANTIFICATION OF THROUGH DRYING RATE DATA

ABSTRACT

For through drying there are three distinct drying rate periods, increasing rate, constant rate and falling rate. The increasing rate period is so important that nearly half of the drying is completed in this period only. A drying rate - moisture content relationship for this period was obtained based on theoretical analysis. It was verified with experimental data. A quantitative representation of the complete drying rate curve was established using this relationship and a modified power law equation for the falling rate period drying rate - moisture content relation. It needs five parameters to quantify the through drying from wet to dry: moisture content at the end of the increasing rate period; exponent for the drying rate - moisture relationship during the increasing rate period; constant drying rate; critical moisture content and the power-law exponent for the falling rate period.     

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.     

DRYTNG OF PEPPER SEEDS USING A COMBINED MICROWAVE/FLUIDIZED BED DRYER

Abstract

The drying rates curves in terms of moisture content versus elapsed drying time for white pepper seeds were obtained experimentally using a fluidized bed and a combined microwave/fluidized bed. The combined microwave/fluidized bed employs a microwave field to assist convective drying. For both procedures, the drying rates were found to be dependent on the inlet air temperature and velocity. Significantly improved drying rates were achieved utilized a combined microwave/fluidized bed drying compared with a conventional fluidized bed.     

Modeling of Moisture Diffusion in Microwave Drying of Hardwood

A one-dimensional mathematical model was developed to predict temperature and moisture content profiles in red maple (Acer rubrum L.) and white oak (Quercus alba) during microwave drying. The model was solved using the finite element analysis with MATLAB software. The predictions for temperature and moisture content agreed favorably well with the experimental data. The diffusion coefficients of the red maple and the white oak in microwave drying conditions were calculated and analyzed. Equations of the diffusion coefficient in longitudinal and transverse directions based on input microwave power level are presented in this article. In microwave drying of hardwood, the red maple was heated more efficiently than the white oak because of higher absorbing efficiency of the microwave power.     

Three-stage hybrid osmotic–intermittent microwave–convective drying of apple at low temperature and short time

ABSTRACT

In recent years, intermittent microwave coupled with hot air-drying has been used increasingly, thanks to considerable improvements observed in drying properties. The aim of this study was to investigate the effect of process of drying apple pretreated osmotically with sucrose solution at five concentrations of 0 (control), 10, 30, 50, and 70% (w/w), using intermittent microwave at four power levels of 0 (control), 360, 600, and 900?W, four pulse ratios of 1, 2, 3, and 4, and convective hot air (40°C) on drying kinetics, effective moisture diffusion coefficient, shrinkage, bulk density, rehydration ratio, and energy consumption. Results showed that the three-stage hybrid osmotic–intermittent microwave–convective drying of apple at low temperature yielded higher drying rates (with 41.5% decrease in drying time) and improved quality of final product. The effective moisture diffusion coefficient increased with an increase in power, pulse ratio, and the concentration of osmotic solution. Furthermore, shrinkage, bulk density, and energy consumption of the samples decreased with an increase in power, pulse ratio, and the concentration of osmotic solution. In summary, the use of intermittent microwave coupled with forced convection of hot air (at low temperature) in drying of apple pretreated by sucrose osmotic solution led to products with improved properties in terms of both quality and quantity.     

Comparison of four pretreatments on the drying behavior and quality of taro (Colocasia esculenta L. Schott) slices during intermittent microwave vacuum-assisted drying

ABSTRACT

Effect of different pretreatments on drying behavior and quality of taro slices during intermittent microwave vacuum-assisted drying were investigated. Taro slices were subjected to the following pretreatments: blanching (B), blanching and freezing (B?+?F), blanching and immersion in maltodextrin solution (B?+?M), and blanching and immersion in maltodextrin solution and freezing (B?+?M?+?F). The reduction in amylose content, the increase in amylose to amylopectin ratio, the change of mechanical properties, the increase in sample bulk density and reduction in sample porosity and structure collapse during pretreatment processes facilitated heat and moisture transfer and quality changes during drying of taro slices. During drying process, all the pretreatment reduced the drying time of samples. The drying rate included a warming-up period and a falling rate period. The values of effective moisture diffusivity increased gradually as the moisture content decreased. The B?+?M?+?F pretreatment had the shortest drying time, the highest drying rate and the biggest effective moisture diffusivity. Moreover, for B?+?M?+?F pretreatment, the parameters including hardness, crispness, color, and microstructure had remarkable changes as compared with the other pretreatments.