Mathematical modeling and performance analysis of thin layer drying of bitter gourd in sensible storage based indirect solar dryer

An indirect forced convection solar dryer integrated with porous sensible heat storage medium was developed. The effect of porous thermal storage and mass flow rate of air on the performance of the system for drying bitter gourd was studied. The experimental setup consists of a blower, solar flat plate collector with corrugated absorber plate (2 m²) and a drying chamber. The thermal storage medium (pebble) is placed below the corrugated absorber plate, in the air passage as a porous medium. The experiments have been carried out with various mass flow rates of air and different drying models have been used for explaining the drying behaviour of sliced bitter gourd. The result shows that, (i) the initial moisture content 92% (w.b) of bitter gourd was reduced to 9% (w.b) in 7 h in the proposed drying system, while it was 10 h for open sun drying, (ii) the maximum specific moisture extraction rate was observed as 0.215 kg/kWh at the mass flow rate of 0.0636 kg/s and the corresponding specific energy consumption was 4.44 kWh/kg, (iii) the collector and drying efficiency of the system were 22% and 19% respectively. The two term model and Midilli–Kucuk model are most suitable for indirect solar dryer and open sun drying in terms of statistical parameter respectively. The drying inside sensible heat storage based indirect dryer was more consistent and produced better quality product as compared to open sun drying.Industrial relevance

• •Every year post harvest losses are increasing rapidly due to the lack of storage facilities.
• •Solar dryers are the most suitable technology that can be easily availed at low cost and in small scale and it can be used as an income generation option for farmers and women in rural areas.
• •The role of solar dryers in food processing industries is significant especially in developing countries in the following areas
  • oFor preserving fruits and vegetables
  • oDairy industries
  • oAgricultural crop drying
  • oTimber drying
  • oIndustrial waste drying
• •The utilization of thermal storage medium in the solar dryers is being focused much for the reasons of extending the availability of the system for operation and to achieve better quality of the products.

     

CHARACTERISTIC DRYING CURVE and MATHEMATICAL MODELING of THIN-LAYER SOLAR DRYING of PRICKLY PEAR CLADODE (OPUNTIA FICUS INDICA)

Thin‐layer solar drying experiments were conducted for the prickly pear cladode grown in Marrakech, Morocco. the experimental drying curves obtained show only a falling rate period. the results verified, with good reproducibility, that the drying air temperature is the main factor in controlling the drying rate. the expression of the drying rate equation was determined empirically from the characteristic drying curve. Eight different drying models were compared according to their correlation coefficient (r²) to estimate solar drying curves. the Page model could satisfactorily describe the solar drying curves of cladode with an r² of 0.9995. the coefficient of this model could be explained by the effect of drying air temperature with an r² of 1.0000.     

Fuzzy and Conventional Modeling of Open Sun Drying Kinetics for Roasted Green Wheat

Roasted green wheat is a high-value product made by roasting green (under-mature) wheat. Due to its high initial moisture content, drying is needed to avoid fast deterioration. Open sun drying of this product is by far the most common practice due to its simplicity, effectiveness, and low cost. The assumption of open sun drying adequacy to prevent deterioration was tested in this study. The drying data were fitted into eight common thin-layer drying models. Goodness of fit for each model was evaluated using coefficient of determination (R²) and root mean square error. The two-term exponential model was found to best describe open sun drying of roasted green wheat with R², and root mean square error values of 0.988 and 0.038, respectively. A fuzzy model of open sun drying for roasted green wheat was also developed and compared with conventional modeling. The results showed a much better performance of fuzzy model compared to conventional models with a much lower value of root mean square error (1.2?×?10^?6). The effective diffusivity was also evaluated for roasted green wheat kernels and found to be 1.7?×?10^?11 m²/s, which was in agreement with published data. The results showed that open sun drying for 5.5 h was effective and adequate to reduce moisture content to a safe level and to prevent deterioration of this product. These findings will provide valuable information for the design of a commercial roasted green wheat solar drying system.     

Development of infrared-assisted hybrid solar dryer for drying pineapple slices: Investigation of drying characteristics,mass transfer parameters,and quality attributes

The research aims to the development and performance evaluation of an infrared-assisted hybrid solar dryer (IR-HSD) for drying pineapple slices. The IR-HSD contains an evacuated tube solar collector, blower assembly, drying chamber, infrared heater, phase change material (PCM) chamber, etc. The study investigates and compares the thermal profiling, drying kinetics, mass transfer parameters and quality attributes of pineapple slices dried using different methods, such as direct drying, phase change material (PCM)-assisted drying, PCM + IR-assisted drying, and sun drying. The average temperatures achieved inside the drying chamber for direct drying and PCM-assisted drying were 60.16 °C and 57.29 °C, respectively, while the temperature reached 60 °C for PCM + IR assisted drying. The average drying rate in PCM + IR drying was higher than in direct and PCM-assisted drying methods. The best-fitting models were the Modified Page model for direct and PCM + IR-assisted drying and the Midilli-Kucuk model for PCM-assisted and sun drying. The higher effective moisture diffusivity for PCM + IR-assisted drying was 2.59 × 10⁻⁹ m²/s, followed by direct and PCM-assisted drying. Similarly, the convective mass transfer coefficient obtained was 1.085 × 10⁻⁷ m/s, 8.321 × 10⁻⁸ m/s, and 1.381 × 10⁻⁷ m/s for direct, PCM-assisted, and PCM + IR-assisted drying, respectively. The superior quality attributes such as total flavonoid content (TFC), total phenolic content (TPC), and antioxidant activity were observed in all drying approaches with retention of colour except open sun drying, which decreased significantly. FTIR spectroscopy analysis confirmed the presence of different bioactive, including aromatic compounds, phenols and hydrocarbon functional groups. The infrared-assisted hybrid solar drying of pineapple slices showed a better drying process and quality attributes.     

Thin-layer drying characteristics of kurut, a Turkish dried dairy by-product

Kurut, which is made in villages of Eastern part of Turkey, is a sun‐dried dairy product. Thin‐layer drying behaviour of kurut at a temperature range of 35–60 °C, with 5 °C increments, and constant thickness of 0.5 cm and drying air velocity of 1.5 m s^?1 was determined in a convective type dryer. The data of sample weight, dry and wet‐bulb temperatures were recorded continuously during each experiment and drying curves obtained. The drying process took place in the falling rate period. Drying curves were then fitted to eleven mathematical models available in the literature to estimate a suitable model for drying of kurut. Two‐term model gave better predictions than other models and satisfactorily described the thin‐layer characteristics of kurut. The effective diffusivity varied from 2.444 × 10^?9 to 3.597 × 10^?9 m² s^?1 over the temperature range. The temperature dependence of diffusivity coefficient was described by the Arrhenius‐type relationship. The activation energy for moisture diffusivity was found to be 19.88 kJ mol^?1.     

Analysis of giant pumpkin (Cucurbita maxima) drying kinetics in various technologies of convective drying

The paper analyses the results of the research on drying a variety of giant pumpkin (Justynka – 957). The research involved drying kinetics of pumpkin as well as changes in the volume of 10 mm pumpkin cubes, dried at the temperature of 80 °C, using free (natural) convection and in the forced convection in the tunnel dryer, with the drying factor speed of 1.2 m s⁻¹ and in the fluid bed dryer. Drying kinetics of pumpkin pieces dried in the tunnel dryer, fluid bed dryer and during two-stage initial drying in the tunnel, and in the fluid bed dryer were examined. Measurements results were used to verify the theoretical models of drying kinetics of the first period taking volume shrinkage into consideration, and the second period of the examined processes.     

Drying performance and economic analysis of novel hybrid passive-mode and active-mode solar dryers for drying fruits in East Africa

In East Africa, post-harvest loss of fresh perishable fruits is enormous, causing recurrent seasonal deficits. In response, a modified passive-mode hybrid solar dryer hereafter called; ‘an Improved Solar Dryer (ISD)’, was developed as a potential low-cost technology alternative for drying fruits in Uganda. An improved PV-assisted hot air dryer using active-mode hybrid solar dryer hereafter called; ‘Solar Photovoltaic and Electric (SPE)’ was also fabricated. The ISD dryer utilises freely-available solar energy to dry the produce. In contrast, the SPE dryer uses both solar photovoltaic and electricity sources. In this study, the drying performance of the ISD and SPE dryers were performed and compared to that of the traditional open sun drying (OSD) method. Relative to the OSD method, the economic performance of the ISD and SPE dryers was analysed. The drying performance results show that the mean drying air temperatures achieved by the ISD and SPE dryers were 31.9 and 41.1 °C respectively; relative to the 27.6 °C for the OSD method. On average, the thermal energy attained by the ISD and SPE dryers were 3551 and 5757 Watts (W) respectively, as compared to 2952 (W) obtained for the OSD method. It took the ISD and SPE dryers 10 and 18 h to effectively dry the fruit (pineapple) products, respectively as opposed to the 30 h taken by the OSD method. The results confirm the superior performance of the ISD and SPE dryers than the OSD method. Results for the economic performance analysis show more reduced costs of drying the products using the ISD than the SPE dryer. Similarly, the pay-back period for the ISD was 2.4 times shorter than that of the SPE dryer. Joint results for economic analysis and drying performance confirm the ISD as a better drying technology than the SPE dryer in the context of the resource-constrained farmers/agro-processors without electricity in Uganda and East Africa.     

Thin layer drying model for simulating the drying of Tilapia fish (Oreochromis niloticus) in a solar tunnel dryer

Mathematical models for predicting the plenum chamber temperatures developed by a solar tunnel dryer and the drying of Tilapia fish (Oreochromis niloticus) in the solar tunnel dryer was developed, and simulated in Visual Basic 6 (Microsoft Visual Basic 6.0™). Based on Student’s t-test, the simulated and actual data for both plenum chamber temperature and moisture ratio did not differ significant at 5% level of significance. In addition, the simulated and actual moisture ratios showed similar trends, and reduced exponentially with drying time. Further, the performances of models at 10% residual error interval were 83% and 81% for plenum chamber temperature and moisture ratio, respectively. Finally, strong linear correlations existed between simulated and actual data for plenum chamber temperature (R² = 0.961), and for moisture ratio (R² = 0.995). Therefore, the model can be used to predict the drying of Tilapia fish in a solar tunnel dryer.     

SELECTION OF CHEMICAL AND THERMAL PRETREATMENT COMBINATION TO REDUCE THE DEHYDRATION TIME OF SOUR CHERRY (Prunus cerasus L.)

Sour cherry (Prunus cerasus L.) is a good source of anthocyanins showing antioxidant and anti‐inflammatory activities. Contemporary drying processes of fruits and vegetables should be fast and at relatively low drying temperatures to minimize the energy consumption and the thermal degradation of their nutritional composition. Therefore, sour cherries placed into sample bags were dipped in one of eight different pretreatment combinations consisting of chemical solutions (2% ethyl oleate, 2% K₂CO₃, 1% citric acid or water) and dipping temperature (23 or 60C) for 1 min to accelerate the skin moisture diffusivity by breaking down the waxy cuticular surface of sour cherry. Pretreated sour cherry samples were dehydrated by three different methods (artificial drying, solar drying and open sun drying). Sour cherries treated with 2% ethyl oleate solution at room temperature (23C) were the fastest to reach the final weight loss percentage (75%). They dried in 24 h in the laboratory tray dryer providing 45.2C drying air. The untreated samples reached the same weight loss percentage of 75% at a drying time of 92 h. A high dipping temperature (60C) eliminated the positive effect of ethyl oleate. Solar drying and open sun drying of sour cherries treated with 2% ethyl oleate at room temperature finished approximately in 149 h and 372 h, respectively. The color of dried sour cherries was used as a quality indicator. The increase of dehydration time decreased redness values as seen in solar drying and open sun drying. The recommended pretreatment and drying processes for sour cherries are dipping them into 2% ethyl oleate solution at room temperature for 1 min and then drying them artificially at 45.2C air temperature to effectively retain their original color.     

Performance of a solar-heated,forced-air,fully automated lumber dryer

A 3.5 m³, external collector type kiln has been designed and constructed to dry green lumber to M.C. low enough for use in furniture. The kiln uses a hot air transfer-circulation system and its ratio of collector area to lumber capacity is 2.9 m²/m³. The kiln is equipped with a fully automated supervisor system. The energy efficiency and performance of the solar kiln have been evaluated through a series of operational tests. The solar collectors operate at 71 to 75% efficiency, while the drying chamber at an efficiency of 46%. The kiln has been found able to use 33% of the available solar energy for lumber drying, Control system has been proved able to run the process to the drying gradient required by any given species of wood. The performance of the dryer, evaluated by direct comparison with air drying, is satisfactory in respect of the final moisture content and drying time. A final M.C. of 10% was attainable at all times of the year.     

THIN-LAYER DRYING KINETICS OF SESAME HULLS UNDER FORCED CONVECTION AND OPEN SUN DRYING

Sesame hulls are a useful by‐product of the sesame processing industry. The sesame hulls are produced at a high moisture content (68% wet basis) and need further drying to prevent deterioration. In this study, both open sun drying (OSD) and forced convection drying (FCD) at 42, 55, and 76C and 1.2 m/s air velocity were investigated. Six common thin‐layer drying models were fitted to the experimental data. Several statistical parameters were used to evaluate the performance of thin‐layer drying models, including r², x², root mean square error (RMSE) and residuals. Sesame hull drying was found to take place completely in the falling rate region. The modified Page model was found to describe OSD data well, while the Wang and Singh model was the best model for describing FCD. Effective diffusivity was found to be 1.89 × 10^‐8 m²/s and 7.36 × 10^‐10 to 1.20 × 10^‐9 m²/s for OSD and FCD, respectively. Activation energy was also found to be 12.95 kJ/mol for FCD.     

EFFECT OF SOLAR DRYING ON THE COMPOSITION OF ESSENTIAL OIL OF SACHA CULANTRO (ERYNGIUM FOETIDUM L.) GROWN IN THE PERUVIAN AMAZON

Two solar drying methods (direct cabinet solar dryer and indirect cabinet solar dryer) were tested under tropical conditions for drying aerial parts of sacha culantro ( Eryngium foetidum L.) in Pucallpa City (Peruvian Amazon). The drying behavior was monitored during all experimental runs. Dried samples and fresh leaves were hydrodistilled and isolated oils were analyzed using gas chromatography with flame ionization detector and gas chromatography-mass spectrometry. ( E )-2-dodecenal was determined as the main constituent of the sacha culantro essential oil, averaging 61.8–62.2%, followed by n -dodecanal (10.9–15.5%), ( E )-2-tetradecenal (6.7–7.6%) and 1-tetradecene (3.6–5.7%). When comparing both solar drying methods, the indirect method was found as more suitable for drying E. foetidum since the dried product resembled the fresh herb more closely in its chemicalcomposition and had better appearance. However, a better drying efficiency of 10.3% was achieved when drying in the direct solar dryer compared with 5.8% for the indirect solar dryer.

PRACTICAL APPLICATIONS

Harvested leaves of sacha culantro are widely used as a condiment in a range of Peruvian, Latin American and Caribbean foods, including vegetable and meat dishes, canned food and sauces. There is lack of any postharvest processing of this herb although it may extend possibilities for use of the herb and facilitate the product export from local production areas and might be a good economic source for poor local farmers. This study shows solar drying as a reasonable preservation technique of sacha culantro leaves which may have a practical application in case of postharvest processing of the herb in the target region of Pucallpa City in Peruvian Amazon.     

Sun drying of figs: an experimental study

In this work, the sun drying behaviour of figs was investigated. Drying experiments were conducted for figs (Ficus carica) grown in Iskenderun-Hatay, Turkey. The constant rate period is absent from the drying curve. The drying process took place in the falling rate period. The drying data were fitted to the different mathematical models such as Lewis, Henderson and Pabis, Page, Logarithmic, Two-term, Two-term exponential, Verma et al. and Wang and Singh models. The performance of these models was investigated by comparing the determination of coefficient (R²), reduced chi-square (χ²) and root mean square error (RMSE) between the observed and predicted moisture ratios. The results showed that the Verma et al. model was found to satisfactorily describe the sun drying curve of figs with a R² of 0.9944, χ² of 0.000483 and RMSE of 0.062857.     

Garlic dehydration inside heat exchanger-evacuated tube assisted drying system: Thermal performance,drying kinetic and color index

An experimental investigation was done on an advanced evacuated tube-assisted solar drying system without and with load conditions at various water flow rates (10 L/h, 20 L/h, and 30 L/h) to evaluate its performance analysis. 79.56 °C maximum greenhouse air temperature was recorded without load at 30 L/h water flow rate with an average solar intensity of 850 W/m². Highest value of drying rate (DR) is 1.48 kgH₂O/kg dry solid/h and the maximum efficiency of solar collector (SC) and solar dryer (SD) is 43.62% and 55.28%, respectively, at 30 L/h water flow rate. Garlic was dehydrating from 70% to 8% (wb) moisture content (MC). The maximum exergy efficiency (EE) and minimum exergy loss were 57.64% at 30 L/h water flow rate and 4.58 W at 10 L/h water flow rate. Quality assessment is also carried out for dried garlic samples in the heat exchanger –evacuated tube assisted drying system (HE-ETADS). Color conservation (indices) of dehydrated garlic sample is best in HE-ETADS (L_o = 60.42, a_o = −0.92, and b_o = 11.54) in comparison to old-style (traditional) drying process (L_o = 58.89, a_o = - 0.67, and b_o = 5.99). Therefore, the developed drying system represented not only good financial returns but also better product quality. The present system provides interesting options for the entry of this type of collectors in medium-scale applications in the agricultural and industrial sectors.     

不同干燥方法对干湿梅理化性质的影响研究

采用强制对流干燥、自然对流干燥、烘箱干燥、温室干燥、自然日晒干燥对干湿梅进行干燥,研究干燥过程及产品的理化性质。结果表明:强制对流、自然对流、与烘箱干燥较温室与自然日晒干燥耗时缩短70%以上;烘箱干燥缺少太阳能干燥物料的特殊香气;温室与自然日晒干燥由于夜间缓苏导致组织结构疏松,嚼感降低,硬度值变化减缓,感官评价较差;强制对流干燥色泽变化适度,感官性能最佳。     

Mathematical Modelling of Thin Layer Dried Cashew Kernels

In this paper mathematical models describing thin layer drying of cashew kernels in a batch dryer were presented. The range of drying air temperature was 70 – 110°C. The initial moisture content of the cashew kernels was 9.29% (d.b.) and the final moisture content was in the range of 3.5 to 4.6% dry-basis. Seven different thin layer mathematical drying models were compared according to their coefficients of determination (R²) mean square error (MSE) and mean relative deviation modulus (P) to estimate drying curves. The effects of the drying air temperature and time on the drying model constants and coefficients were predicted by multiple regression analysis using linear and non-linear type models. The results have shown that among the models, the Page model was found to be the best for describing the drying behaviour of cashew kernels with R², MSE and P values of 0.9830, 0.00311 and 5.046 respectively.     

Microwave assisted fluidized bed drying of nutmeg mace for essential oil enriched extracts: An assessment of drying kinetics,process optimization and quality

The drying mechanism of the myristicin enriched nutmeg mace has been optimized in a microwave assisted fluidization bed dryer (MWFBD) through multiple linear regression (MLR) and artificial neural network (ANN) modeling. The developed drying technique overcomes the non-uniformity heating problems in microwave heating and prolonged drying in fluidized bed drying. During the novel method of drying selected input variables were drying air temperature (DT) (40–50 °C) and microwave power (MP) (480–800 W) and output variables involving colour, oil yield, and myristicin quantity have been investigated by a continuous air velocity of 5.1 m/s. Six mathematical models about one to four conditions have fitted with an experimental design. Suitable enforcement of such models was evaluated through statistical measures. The coefficient of determination (R²) of MLR varied from 0.89 to 0.98, and the sum of squared error (SSE) varied within 6 × 10⁻³ to 158.18, while R² of ANN fluctuated from 0.82 to 0.95, and the mean squared error (MSE) varied between 0.006 and 0.1450, which shows MLR design performance superior than ANN design. The processing conditions of 48.24 °C DT and 637.431 W MP with a drying time of 1.3 h were identified as optimum conditions with a desirability value of 0.98 to obtain maximum oil yield (13.38%) and good colour (L* (20.83), a* (17.34), b* (8.62)) of nutmug mace. Moreover, no myristicin (5.92%) degradation was observed compared with the sun and convective drying. Among the tested models, page and logarithmic models gave a better prediction of moisture ratio.     

Fitting semi‐empirical drying models using a tool based on wavelet neural networks: Modeling a maize drying process

Maize drying is an important process, especially for storage and conservation. For this study, the experimental stage was carried out using a forced convection dryer with air heated at different temperature conditions (306.05–441.85 K) and flow (0.13–0.256 m³/hr), totalizing 15 drying curves. Then the performances of the classic drying kinetics methodology and the approach proposed in this paper, in which the increase in moisture content of the product with time was represented combining exponential models and neural networks based on wavelets, were compared. Good performance was obtained in predictions using the proposed approach. One of the main differentials of the methodology adopted was the obtainment of a model that has a global predictive capacity, within the range of tested operating conditions, which can be used in predicting drying curves for different operating conditions.

Practical applications

The drying process is also one of the most widely used methods for preserving food, and has the advantage of reducing the costs of storage and transport because of the low volume and weight of the end product. During the last years, this topic has attracted a broad industrial interest, resulting in many research studies investigating the drying process. Usually, with regard to the classic approach for modeling of the drying process, the kinetics of drying curves obtained in different operating conditions is affected separately, that is, the parameters are estimated independently, resulting in different regression problems. With the classical approach, in general, it is not possible to obtain a comprehensive prediction model with regards to operating conditions. We have proposed an alternative modeling method. Aiming to obtain a modeling tool with an overall predictive ability, an approach for drying kinetics prediction that combines exponential models and neural networks was proposed. The proposed modeling method was able to predict drying curves for different operating conditions.     

Effect of sulphiting and drying methods on physico-chemical and sensorial quality of dried apricots during ambient storage

Fresh mature and ripe apricots after preparations were pre-treated with potassium meta-bisulphite (KMS) at concentrations of 2–8% for 30, 45 and 60 min followed by drying in solar tunnel dryer and in open air. After completion of drying, the samples were packed in polyethylene pouches and evaluated for various quality parameters at 0, 6 and 12 months of ambient storage. KMS pre-treatment at concentration of 6% for 60 min prior to drying helped significantly in improving and maintaining the quality of dried apricots for up to 12 months of ambient storage. Solar tunnel drying resulted in hygienic product and reduced the drying time to 3–4 days compared with 14–16 days in open air drying. Residual free sulphur dioxide of apricots pre-treated at KMS concentration of 6% for 60 min was within the prescribed limits of the Fruit Product Order (1955) specifications for dried products.     

Food dehydration by solar energy

Solar driers that are currently being investigated for drying of agricultural products can be divided into two major divisions, depending upon how they transfer the incident solar energy to the product to be dried. These two divisions are direct and indirect drying, with some work also being done on combination drying procedures. In direct solar driers, the product to be dried is usually either inside a tent, greenhouse, or a glass‐topped box, where the product to be dried is heated by the direct rays from the sun and the moist air is removed by ambient wind movement. These dryers do accelerate moisture loss rate and the product is usually safe from inclement weather. These dryers usually do not require fans for forced air circulation. With indirect drying, the opposite is true, where most require powered fans for forced air circulation. With this type of dryer, both flat‐plate and inflated tube solar heat absorbers are used, with each offering certain advantages. Also, combination dryers have been built that utilize both direct and indirect principles. Product evaluation of solar dried foods indicate that in most cases the physical properties, flavor, and vitamin A and C retention were as good as, or better than, conventional dried foods. The economics of the solar systems indicate that most drying procedures are economically feasible for use in small‐scale operations only, with the exception of grain drying.