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.     

Microwave-Convective and Microwave-Vacuum Drying of Cranberries: A Comparative Study

Abstract

Two drying methods of cranberries (microwave-vacuum and microwave-convective) are reviewed, and their advantages and disadvantages regarding the quality of dried product and the process performance are presented. Mechanically and osmotically pretreated berries were subjected to drying and quality evaluation. Quality parameters are color (in Hunter L*a*b* coordinates), textural characteristics (toughness and Young's modulus), and organoleptic properties (color, texture, taste, and overall appearance). Special emphasis was given to the energy performance of the process, monitoring of the real-time temperature profile, and the total microwave power-on time. Two microwave power densities are assessed, as well as different microwave power-on/power-off cycling periods. In almost all observed parameters, microwave-vacuum drying exhibited enhanced characteristics when compared to microwave-convective drying. Drying performance results (defined as mass of evaporated water per unit of supplied energy) showed that microwave-vacuum drying is more energy-efficient than microwave-convective. Tasting panel results exhibited slight preference in all parameters for microwave-convective dried samples.     

Quality Evaluation of Microwave-Dried Packham's Triumph Pear

Abstract

The objectives of this work were to carry out the microwave and microwave-convective drying of pear, in order to preserve the quality by reducing the drying time of the fruit; to compare the kinetic with convective drying mode data and to fit the comparative data to models from literature. The effect of osmotic dehydration and the variation in the microwave power were analyzed at constant values of air temperature and velocity. The experimental data fit well the proposed kinetic model. The processed pear was evaluated with respect to water content, quantity of calcium, phosphorus and potassium, water activity and apparent density. The best results were found for the samples osmotically dehydrated and dried in the microwave-convective dryer. The osmotic dehydration and the short drying times were important to maintain sample characteristics.     

THE EFFECT OF PROCESS CONDITIONS ON THE DRYING KINETICS AND REHYDRATION CHARACTERISTICS OF SOME MW-VACUUM DEHYDRATED FRUITS

ABSTRACT

The effect of drying conditions, namely, microwave power, vacuum pressure, as well as sample geometry, on the drying kinetics and rehydration characteristics of five microwave vacuum dehydrated fruits (apple, avocado, mushroom, pear and strawberry) was studied. The investigation involved a wide range of microwave power, vacuum pressure and sample size levels. The drying rate and the rehydration ratio, a measure of rehydration characteristics, were found to depend on the drying conditions. An empirical mass transfer model, involving a characteristic parameter for each process (drying and rehydration( as a function of process variables, namely, microwave power, vacuum pressure and sample size, was also tested with the data obtained in a microwave oven equipped with vacuum apparatus. Furthermore, a comparison of rehydration properties of microwave vacuum and conventionally dehydrated products, was conducted.     

Primary energy demand and energy costs of fixed-bed drying using the example of chamomile flowers

Abstract

Energy use efficiency in the drying of medicinal and aromatic plants is largely determined by weather conditions and process parameters. While the former are beyond control, the latter can be optimized. In order to achieve such optimization, different energy supply variants, based on typical operating conditions of batch-type grate drying in Thuringia, Germany, were analyzed. It was found that partial air recirculation and integration of heat pumps allow substantial savings in primary energy. However, under the constraint of German energy prices, significant savings in energy costs can only be achieved if combined heat and power generation systems are applied at the same time.     

Energetic Analysis of Drying Biological Materials with High Moisture Content by Using Microwave Energy

One of the most important parameters in the drying process is the efficiency of energy transfer. Although it is well known that the absorbed component of the referred energy in the dehydration process under microwave conditions is theoretically lower—especially during the falling rate period of drying—by an order of magnitude than that of convective drying, the real efficiency of energy transfer in the microwave field depends on numerous—partly unknown—factors. In this study, the most significant relations between the energy transfer efficiency and the drying parameters were investigated. During the experiments, two power levels were used to study the effects of domain variables in apple, potato, and onion dehydration processes. The results showed that variations in microwave power level played an important role in overall drying kinetics.

Based on the recorded data, we determined the typical energy efficiency factors for microwave drying of apple, potato, and onion. The specific energy consumption was calculated to compare the energy efficiency of a single measurement series. It was evident that by applying higher power levels, the energy utilization was more efficient. Furthermore, we theoretically proved and experimentally showed the nonthermal effects of the microwave energy.     

Microwave Drying of Wood Strands

Abstract

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.     

THE INFLUENCE OF PRESSURE AND TEMPERATURE ON THE KINETICS OF VACUUM DRYING OF KETOPROFEN

ABSTRACT

This paper explores the influence of temperature and pressure on drying kinetics of 2-(3-benzoylphenil propionic acid) ketoprofen, in a vacuum dryer on laboratory scale, Experimentally determined relations between moisture content and drying rate vs time, were approximated with an exponential model. Model parameters were correlated with drying conditions (temperature, pressure) and defined by functions of their potentions.

From an energy balance of the process, a mathematical model for simulating dependence of sample temperature vs drying time, and moisture content of material, has been developed.

Simulation of the drying kinetics and sample temperature, by use of those functional dependencies shows good agreement with experimental results.     

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.     

COMBINED CONVECTIVE AND INFRARED DRYING OF A CAPILLARP POROUS BODY

Abstract

This work investigates the effect of spray drying conditions on some properties of tomato powder prepared by spray drying of tomato pulp. A pilot scale spray dryer (Buchi, B-191) with cocurrent regime and a two-fluid nozzle atomizer was employed. Sixty-four different experiments were conducted keeping constant the feed rate, the feed temperature, and the atomizer pressure, and varying the compressed air flow rate, the flow rate of drying air, and the air inlet temperature. Tomato powders were analyzed for moisture, solubility, density (bulk and packed), and hygroscopicity. Analysis of experimental data yielded correlations between powder properties and the above-mentioned variable operating conditions. Regression analysis was used to fit a full second order polynomial, reduced second order polynomials and linear models to the data of each of the properties evaluated. F values for all reduced and linear models with an R ² ≥ 0.70 were calculated to determine if the models could be used in place of full second order polynomials.     

Thin layer drying kinetics,characteristics and modeling of plaster of paris

Drying behavior of thin slabs of plaster of paris was investigated in microwave, convective and microwave assisted convective drier. The process parameters involved in the study include the microwave output power, air temperature used for drying, the initial moisture content of the sample, thickness and geometry of the sample. A typical drying profile, with no constant rate period followed by a falling rate period was obtained for all the three methodologies used. The mechanism of moisture movement within the sample during the falling period, as expressed by effective moisture diffusivity, was between 1.166 × 10⁻⁸ and 2.6927 × 10⁻⁸ (m² s⁻¹) for the least microwave output power of 180 W. The specific energy consumption and the drying efficiency involved in the microwave process, estimated from the energy supplied to the drier and the total time was found to be 0.4557 MJ kg⁻¹ and 82.56% for a sample thickness of 0.013 m. The microstructure of the fractured surfaces and qualitative phase analysis of microwave activated plaster of paris were extensively investigated by Scanning Electron Microscopy, Fourier Transform Infra-Red spectroscopy and X-Ray Diffraction techniques. The experimental moisture loss data with time was fitted to nine semiempirical thin layer equations. The suitability of the models was validated by comparing it with the predicted moisture ratio for any given set of operating conditions. The statistical analyses of the available literature correlations, for their applicability to the data of the present study showed greater deviation attributing their restricted range of applicability of individual correlations. As a result, a third order polynomial equation was developed from this study.     

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.     

Color Change Kinetics of Okra Undergoing Microwave Drying

In this present study, the effect of microwave output power and sample amount on color change kinetics of Turkey okra (Hibiscus esculenta L.) were investigated by using microwave drying technique. The color parameters for the color change of the materials were quantified by Hunter L (whiteness/darkness), a (redness/greenness), and b (yellowness/blueness) values. These values were also used for calculation of the total color change (ΔE), chroma, hue angle and browning index. The microwave drying process changed color parameters of L, a, and b, causing a color shift toward the darker region. The values of L and b decreased, whereas values of a and total color change (ΔE) increased during microwave drying. The mathematical modeling study of color change kinetics showed that L and b fit a first-order kinetic model, whereas a and total color change (ΔE) followed a zero-order kinetic model. However, chroma and browning index (BI) followed a first-order kinetic model, whereas hue angle followed a zero-order kinetic model. On the other hand, the data of the total color change (ΔE), chroma, hue angle, and browning index depending on the ratio of the microwave output power to sample amount were adequately fitted to a quadratic model. For calculation of the activation energy for color change kinetics parameters, the exponential expression based on Arrhenius equation was used.     

Microwave Drymg of Grapes in a Single Mode Cavity at 2450 Mhz - i: Drying Kinetics

ABSTRACT

Grapes were dried into raisins using combined microwave and convective drying in a single mode cavity specially designed for the studies. Effect of processing parameters, viz.microwave power density (P) air temperature (T) and velocity (u) on drying kinetics was studied in an effort to optimize the operating conditions. The individual and interactive effects of these variables were srudied using three levels of P, four levels of T and two levels of u. Higher drying rates observed with the higher P and T were atuibuted to the increased product temperature and higher partial vapour pressure difference between the surface of the fruit and the air stream. Increase of air velocity increased the drying times. Low drying rate at higher air velocity was attributed to decreased fruit temperature due to rapid convective heat mnsfer at the surface.     

Analysis of Energy Consumption in Drying Process of Non-Hygroscopic Porous Packed Bed Using a Combined Multi-Feed Microwave-Convective Air and Continuous Belt System (CMCB)

In this study, the analysis of energy consumption during the drying of non-hygroscopic porous packed bed by combined multi-feed microwave-convective air and continuous belt system (CMCB) was investigated experimentally. By using a combined multi-feed microwave-convective air and continuous belt system drier, the microwave power was generated by means of 12 compressed air-cooled magnetrons of 800 W each that give a maximum of 9.6 kW. The power setting could be adjusted individually in 800 W steps. Hot air with the maximum working temperature of 240°C was generated using 24 units of electric heater where the total power capacity is 10.8 kW. Most importantly, this work focused on the investigation of drying phenomena under industrialized microwave processing. In this analysis, the effects of the drying time, hot air temperature, porous structure (F-Bed and C-Bed), and location of magnetrons on overall drying kinetics and energy consumption were evaluated in detail. The results showed that the overall drying and energy consumption depend upon the porous structure, hot air temperature, and location of magnetrons. Furthermore, using the continuous microwave application technique had several advantages over the conventional method, such as shorter processing times, volumetric dissipation of energy throughout a product, and less energy consumption. The results presented here provided fundamental understanding for the drying process using a combined multi-feed microwave-convective air and continuous belt system in industrial size.     

MICROWAVE VACUUM DRYING KINETICS OF SOME FRUITS

ABSTRACT

Microwave vacuum drying kinetics of three fruits (namely, apple, kiwi and pear) were studied by introducing an one-parameter empirical mass transfer model, involving a characteristic parameter (drying constant), as a function of process variables. The model was tested with data produced in a microwave oven equipped with vacuum apparatus, using non-linear regression analysis. The investigation involved a wide range of microwave power and vacuum pressure levels. The parameters of the model considered were found to be greatly affected by the microwave power level while vacuum pressure affected slightly the process.     

Study on quality attributes and drying kinetics of instant parboiled rice fortified with turmeric using hot air and microwave-assisted hot air drying

Abstract

This study investigated the quality and drying kinetics of instant parboiled rice fortified with turmeric (IPRFT) by using hot air (HA) and microwave-assisted hot air (MWHA) drying. The cooked long grain parboiled rice (LGPR) fortified with turmeric was dried with HA at temperatures of 65, 80, 95, and 110?°C. The microwave power density of 0.588 Wg^?1 was incorporated for drying with MWHA. Drying was performed until the dried IPRFT reached 16% (d.b.) of moisture content. The quality of the dried IPRFT was evaluated in terms of color, total phenolics content (TPC), total antioxidant capacity (TAC), rehydration ratio, volume expansion ratio, texture and microstructure. The results showed that the incorporation of microwave power with HA drying helped to reduce the drying time by 50% compared to conventional HA drying. A prediction of the moisture ratio by using the Page model provided the best R² and RMSE in drying kinetics. The drying conditions had small effects on the color, TPC, TAC, and microstructure of the dried IPFRT. The rehydration ratio, volume expansion ratio and texture of the rehydrated IPFRT showed minimal variations from changes in the drying conditions. The TPC and TAC of the dried IPRFT clearly increased compared to the TPC and TAC of the initial LGPR.     

Temperature Changes during Microwave-Vacuum Drying of Sliced Carrots

Abstract

The temperature changes during microwave-vacuum drying of sliced carrots were investigated. Sliced samples were dried to 7–10% moisture content (wet basis) at a wide range of microwave power and vacuum pressure levels. The experiments showed that for sample thickness less than 8 mm, the core temperature of the sample was the same as its surface temperature, with uniform temperature distribution within the sample. However, for sample thickness more than 8 mm, temperature gradient developed along the thickness of the sample. The experiments also showed that, with the decrease of moisture content X _w (dry basis), for samples with thickness ≤ 8 mm, the drying process of sliced carrots experienced three distinct periods: a warming-up period (X _w  = 7.68) without removal of moisture when the product temperature increased linearly with drying time until it reached the corresponding saturation temperature of water in the food at the vacuum pressure; a constant temperature period (2 ≤ X _w  < 7.68) in which most of moisture evaporated and flowed out of the sample efficiently with little resistance; and a heating-up period (X _w  < 2) in which the drying rate decreased and sample temperature increased rapidly. The mathematical models for predicting sliced sample temperature were also developed based on the energy conservation and regression of the experimental date.     

Microwave-Assisted Atmospheric Freeze Drying of Green Peas: A Case Study

Atmospheric freeze drying (AFD) is based on the sublimation of ice due to a pressure gradient (convective drying), and is a dehydration process for temperature-sensitive products. Since the process is slow in general, microwave radiation (MW) was applied in order to increase the sublimation in fluid and fixed bed conditions at drying temperatures of ?6°C, ?3°C, and 0°C. The modified Weibull model was used to describe the drying behavior for all investigations. With 280 Watt power supplied to the magnetrons, it was possible to reduce drying time by approximately 50%. The drying efficiency was approximately 30%, while the SMER was increased by 0.1 to 0.3 kg_waterkWh^?1, which gives better energy efficiency for the microwave drying system used in this investigation. The product quality (color reduction and particle size/porosity) was well preserved in fixed bed drying at ?6°C and ?3°C, while the product quality was reduced significantly in microwave AFD experiments at 0°C and in a fluid bed. The drying rates of AFD in a fluid bed condition were not as high as those in a fixed bed. MW-AFD in a fixed bed condition at temperatures of ?6°C and ?3°C performed best regarding product quality, drying time, and process control.     

MICROWAVE DRYING OF MULTICOMPONENT SOLS

ABSTRACT

Extensive studies on microwave drying of multicomponent materials have been carried out in order to determine dielectric properties of metalsiloxane sols, define the healing and drying characteristics of metalsiloxane sols and gels, develop the mathematical model of thermal processing, and design the continuous microwave dryer that could be used in an industrial scale