Drying Kinetics of Tomato by Using Electric Resistance and Heat Pump Dryers

Abstract

Drying kinetics of tomato was studied by using heat pump dryer (HPD) and electric resistance dryers with parallel and crossed airflow. The performance of both systems was evaluated and compared and the influence of temperature, air velocity, and tomato type on the drying kinetics was analyzed. The use of HPD showed to be adequate in the drying process of tomatoes, mainly in relation to the conversion rate of electric energy into thermal energy. The heat pump effective coefficient of performance (COP_HT,EF) was between 2.56 and 2.68, with an energy economy of about 40% when compared to the drying system with electric resistance. The Page model could be used to predict drying time of tomato and statistical analysis showed that the model parameters were mainly affected by drying temperature.     

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％。     

Drying Kinetics and Quality of Shrimp Undergoing Different Two-Stage Drying Processes

An innovative two-stage drying concept is presented in this article. The work considered drying of shrimp using a superheated steam dryer followed by a heat pump (SSD/HPD) or a hot air dryer (SSD/AD) both from drying kinetics and dried product quality points of view. The experiments were performed using the first-stage superheated steam drying temperature of 140°C while the second-stage heat pump drying (or hot air drying) was performed at 50°C. The moisture content of shrimp at the end of the superheated steam drying stage was varied between 30 and 40% (w.b.). The effect of tempering between SSD/HPD was also investigated. Shrinkage, color, rehydration behavior, texture (toughness and hardness), and microstructure of dried shrimp were measured. The results showed that SSD/HPD dried shrimp had much lower degree of shrinkage, higher degree of rehydration, better color, less tough and softer, and more porous than single-stage SSD dried shrimp. It was also found that SSD/AD gave redder shrimp compared to shrimp dried in a single-stage superheated steam dryer. No improvement in terms of shrinkage and rehydration behavior was observed, however.     

Drying Kinetics and Quality of Shrimp Undergoing Different Two-Stage Drying Processes

Abstract

An innovative two-stage drying concept is presented in this article. The work considered drying of shrimp using a superheated steam dryer followed by a heat pump (SSD/HPD) or a hot air dryer (SSD/AD) both from drying kinetics and dried product quality points of view. The experiments were performed using the first-stage superheated steam drying temperature of 140°C while the second-stage heat pump drying (or hot air drying) was performed at 50°C. The moisture content of shrimp at the end of the superheated steam drying stage was varied between 30 and 40% (w.b.). The effect of tempering between SSD/HPD was also investigated. Shrinkage, color, rehydration behavior, texture (toughness and hardness), and microstructure of dried shrimp were measured. The results showed that SSD/HPD dried shrimp had much lower degree of shrinkage, higher degree of rehydration, better color, less tough and softer, and more porous than single-stage SSD dried shrimp. It was also found that SSD/AD gave redder shrimp compared to shrimp dried in a single-stage superheated steam dryer. No improvement in terms of shrinkage and rehydration behavior was observed, however.     

HEAT PUMP DRYING: RECENT DEVELOPMENTS AND FUTURE TRENDS

This review paper focuses on the recent progress made in heat pump drying (HPD). An introduction is given on the operating and working principle of the heat pump drying system. Advantages and limitations of HPD are outlined. Two industrial applications of HPD, food and timber, are discussed. New developments in HPD are presented. A section discussing the potential of incorporating advanced heat pump cycles for drying application is also presented. Some possible new hybrid HPD technologies, e.g., radio frequency and infrared assisted HPD are discussed in terms of their industrial potential. Opportunities for further R&D to achieve better product quality and energy efficiency are identified. A new heat pump dryer design is also described to demonstrate the new trend in versatile heat pump design.     

HEAT PUMP DRYERS: RESEARCH AND DEVELOPMENT NEEDS AND OPPORTUNITIES

As some previous research works on heat pump dryers (HPD) gave contradictory results, there is a need to review and identify R&D needs and opportunities in HPD. It was found that mathematical modeling cannot ignore the interdependence of the heat pump working fluid and the process air of the dryer. The performances of various HPD configurations with respect to all operating varibles need further investigation. A well defined mathematical model of combined dryer-drying material characteristic is required for the system modeling. The relative specific drying cost and relative useful energy were introduced as the dryer selection criteria. The role of the heat pump in the HPD system, CFC alternatives and non-conventional heat pump cycles using air or steam as working fluids deserve further investigation.     

HEAT PUMP DRYING: RECENT DEVELOPMENTS AND FUTURE TRENDS

ABSTRACT

This review paper focuses on the recent progress made in heat pump drying (HPD). An introduction is given on the operating and working principle of the heat pump drying system. Advantages and limitations of HPD are outlined. Two industrial applications of HPD, food and timber, are discussed. New developments in HPD are presented. A section discussing the potential of incorporating advanced heat pump cycles for drying application is also presented. Some possible new hybrid HPD technologies, e.g., radio frequency and infrared assisted HPD are discussed in terms of their industrial potential. Opportunities for further R&D to achieve better product quality and energy efficiency are identified. A new heat pump dryer design is also described to demonstrate the new trend in versatile heat pump design.     

Condenser Coil Optimization and Component Matching of Heat Pump Dryer

The heat pump dryer is an energy-efficient piece of drying equipment, but due to its complicated system of the two interactive working fluids (refrigerant and drying air), the optimum design remains the question. A heat exchanger is the major component influencing the heat pump dryer (HPD) performance. This article reports a study to optimize the condenser coil of the HPD and the component matching in order to obtain optimum performance. The study was carried out by a mathematical model for system simulation and followed by experimental verification. Five HPD configurations were studied, including different designs in air and refrigerant flow paths. It was found that the closed-loop HPD with air bypassing over the evaporator is the most appropriate configuration. The proper coil design is 2-row, 2-circuit configuration with optimum refrigerant mass flow rate of 16-20 g/s/circuit. The optimum air flow rate was found to be in the range of 0.6-0.8 kg/s. The corresponding number of coil modules is 8.     

Condenser Coil Optimization and Component Matching of Heat Pump Dryer

The heat pump dryer is an energy-efficient piece of drying equipment, but due to its complicated system of the two interactive working fluids (refrigerant and drying air), the optimum design remains the question. A heat exchanger is the major component influencing the heat pump dryer (HPD) performance. This article reports a study to optimize the condenser coil of the HPD and the component matching in order to obtain optimum performance. The study was carried out by a mathematical model for system simulation and followed by experimental verification. Five HPD configurations were studied, including different designs in air and refrigerant flow paths. It was found that the closed-loop HPD with air bypassing over the evaporator is the most appropriate configuration. The proper coil design is 2-row, 2-circuit configuration with optimum refrigerant mass flow rate of 16–20 g/s/circuit. The optimum air flow rate was found to be in the range of 0.6–0.8 kg/s. The corresponding number of coil modules is 8.     

HEAT PUMP DRYERS: RESEARCH AND DEVELOPMENT NEEDS AND OPPORTUNITIES

ABSTRACT

As some previous research works on heat pump dryers (HPD) gave contradictory results, there is a need to review and identify R&D needs and opportunities in HPD. It was found that mathematical modeling cannot ignore the interdependence of the heat pump working fluid and the process air of the dryer. The performances of various HPD configurations with respect to all operating varibles need further investigation. A well defined mathematical model of combined dryer-drying material characteristic is required for the system modeling. The relative specific drying cost and relative useful energy were introduced as the dryer selection criteria. The role of the heat pump in the HPD system, CFC alternatives and non-conventional heat pump cycles using air or steam as working fluids deserve further investigation.     

Drying of Guava and Papaya: Impact of Different Drying Methods

Heat pump dryers (HPD) are known as high-energy-efficiency devices with low economic cost. As it is usually a closed system, the drying media can be substituted by inert gases. In this study, the effect of nitrogen and carbon dioxide on guava and papaya were investigated. Both drying kinetics and quality of these dried fruits resulting from the two methods were compared with normal air HPD, vacuum dryer, and freeze dryer. When using CO₂, the effective diffusivity during the drying process was 44% higher in guava and 16.34% higher in papaya. There was less browning, faster rehydration, and more vitamin C retention in the final products. All these reveal the great potential of modified atmosphere heat pump dryer in the food drying industry.     

Heat pump drying of grape pomace: Performance and product quality analysis

Abstract

Grape is one of the most popular fruits and various types of grape have been cultivated by more than 100 countries around the World. The wine and juice industry produces large quantities of by-product, called grape pomace (GP) as an industrial waste and it consists of skins, seeds, and stems. Various processes such as separation, pressing, drying, and milling are applied to benefit from its health effects. In this study, the seeded black GP Kalecik karas? (Vitis vinifera) was dried in an assisted closed cycle heat pump dryer (HPD) designed for high-moisture products to investigate the drying behaviors of GP. The effects of drying air temperature on bioactive properties and the drying characteristics of GP, and performance of system have discussed. Experiments were carried out at two different temperatures (45 and 50°C) and air velocity of 1.0 m/s. It was seen that increasing temperature decreased the drying time, coefficient of performance of whole system (COP_ws), and specific energy consumption (SEC). The average values of COP_ws for temperatures 45°C and 50°C were calculated as 3.28 and 3.10, respectively. The drying efficiencies (DE) at drying air temperature of 45°C and 50°C ranged from 2 to 12% and from 2 to 15%, respectively. Additionally, result of analysis has indicated that using a HPD at lower temperatures increases performance of system despite of higher energy input. Bioactive properties of dried samples at drying air temperature of 45°C are better than 50°C. The results show that drying the GP at low temperature is more suitable for product quality. For this reason, heat pump may be preferred. It shows that this drying system with higher capacities in the future can be recommended as an alternative technique in terms of energy usage, drying time, and performance of system.     

The Effects of Predrying Treatments and Different Drying Methods on Phytochemical Compound Retention and Drying Characteristics of Moringa Leaves (Moringa oleifera Lam.)

The most appropriate maturity stage of Moringa oleifera leaves was selected for drying based on phytochemical content, including quercetin and kaempferol. Desorption isotherms were developed and were best fit by the modified Henderson model. Prior to drying, samples were left untreated, blanched in boiling water, and blanched in NaHCO₃/MgO. The leaves were dried by hot air tray drying (TD) and heat pump–dehumidified drying air (HPD) at air temperatures of 40, 50, and 60°C. Alternatively, leaves were subject to microwave drying (MWD) at 150, 450, and 900 W and to freeze drying (FD). The moisture versus time data were fitted to five drying models. In general, a three-parameter model gave the best fit. The drying constant was related to the drying temperature or microwave power using an Arrhenius model. Effective moisture diffusivity (D _eff) increased with higher drying temperature, higher microwave power, or blanching treatments. Structural changes in the leaves after drying and upon rehydration were observed by scanning electron microscopy (SEM). Leaves blanched and dried using HPD at 50°C and fresh and dried using FD showed a partial breakdown of the tissue structure upon rehydration. HPD and blanching reduced the drying time by 8.3% and increased quercetin and kaempferol levels by 42.1 and 51.4%, respectively, compared to TD at 50°C. MWD provided the quickest drying followed by HPD and TD, respectively. HPD drying of M. oleifera after blanching resulted in relatively greater quality compared to TD and MWD.     

Low-Temperature Drying Using a Versatile Heat Pump Dehumidifier

A drying system incorporating a commercial 2.3 kW heat pump dehumidifier was designed and constructed. The HPD was equipped with an external water-cooled condenser that rejected excess heat out of the system. The design of the system allowed for conducting drying with recirculation of air as well as use of electrical heaters. In an open mode, the drying could be carried out simultaneously with room dehumidification and water heating in the secondary condenser. Drying experiments were conducted with apple and comparisons were made between HPD assisted drying (partial and complete) and hot air drying (at 45 and 65°C). The HPD dried fruit exhibited better rehydration properties than the hot air dried samples. Water activity of the HPD dried samples was noticeably lower than that of the hot air dried samples at the same water content, indicating that the residual moisture was probably held under higher tension in the former. In terms of energy consumption, the process of HPD assisted drying is more expensive as much of the energy input is rejected at the secondary condenser as excess heat.     

Effects of Component Arrangement and Ambient and Drying Conditions on the Performance of Heat Pump Dryers

Performances of five heat pump dryer (HPD) configurations were studied by computer simulation. The component arrangement covers the fully open, the partially open, and the fully closed systems with external condenser or external cooler. The HPD performance was investigated for high and low drying rate and within the ambient temperature range of 20°C to 40°C. The best operating mode of the HPD depends on both drying rate and ambient condition. For the case of tropical climate and high drying rate, the partially open system is the optimum operating mode in general, except at some particular conditions where the fully open and fully closed systems are recommended. However, the operation for the low drying rate is more complicated, as the optimum configuration is sensitive to the change of ambient temperature.     

Effects of different drying methods on the physical characteristics and flavor of dried hawthorns (Crataegus spp.)

ABSTRACT

The objective of this study was to compare the physical characteristics and flavor of dried hawthorns obtained by freeze-drying (FD), microwave freeze-drying (MFD), atmospheric freeze-drying (AFD), and heat pump drying (HPD). The parameters including moisture content, product temperature, rehydration ratio, chroma, hue angle, yellowness index, total color difference, and energy consumption were investigated. The use of fuzzy reasoning for the sensory evaluation of hawthorn quality was also performed. As expected, AFD, FD, and MFD produced better accepted dried hawthorn products than did HPD. Nevertheless, FD consumed the highest energy and had a long drying time, but its product was the best; AFD had a similar energy cost as HPD, but its drying time was the longest; and MFD had a higher energy cost and longer drying time than HPD, but its product quality was similar to that of FD products. As a result, MFD and AFD had potential to replace FD to yield dried hawthorns with high product quality and relatively low cost.     

Drying of Orthosiphon aristatus leaves: Mathematical modeling,drying characteristics,and quality aspects

The stage of maturity for Orthosiphon aritatus (OA) leaves revealed that stage I (young leaves) provided the highest bioactive compounds. Vacuum blanching (VB) for 75?s gave the highest sinensetin (28.4% increment) and eupatorin (21.0% increment) compared with heated water blanching (HWB). The modified Henderson model was the most suitable desorption isotherm model for the OA leaves. The VB and unblanched OA leaves were dried by different drying methods, including convection tray drying (CTD, 40–60°C), heat pump dehumidify drying (HPD, 40–60°C), mixed mode solar drying (64.6°C), and freeze-drying. Three-parameter model (TP) was the best model to explain all drying curves. The drying constant, K in the TP and activation energy were fitted to the Arrhenius model. Effective moisture diffusivities were increased with the VB, drying temperatures, and HPD. The highest specific moisture extraction rate was obtained from the VB and dried in the HPD at 60°C. The quality aspects of sinensetin (10.2% retention), eupatorin (10.7% retention), total phenolics, and antioxidant activity revealed the best quality for the OA leaves pretreated by the VB and dried in the HPD at 60°C and could reduce drying time by 44.8% compared with the CTD. The VB of the OA leaves and dried using the HPD at 60°C were recommended.     

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.     

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.     

HEAT PUMP DRYING OF AGRICULTURAL MATERIALS

Heat pump-assisted drying is an energy-efficient process because the heat is recoverable. However, the economic feasibility of the heat pump dryer (HPD) is debatable because high grade energy (electricity) is used. In this study, sawn rubber wood and bananas were dried in an experimental HPD. The moisture extraction rates (MER) and the specific moisture extraction rates (SMER) of wood drying and banana drying decreased rapidly with the drying time but the compressor power was relatively constant. If the final moisture content of wood is less than 10%, the maximum average MER and SMER were 2.854 kg/h and 0.572 kg/kWh, respectively. Banana drying yielded the highest average MER of 2.710 kg/h when the drying load was highest. The corresponding SMER was 0.540 kg/kWh. Economic analyses among the HPD, an electrically-heated hot air dryer and a direct-fired dryer revealed that the HPD had the lowest operating cost.