Indirect Dryer Research Articles

A Review on Sludge Drying after High Pressure Filter Pressing

Sludge drying is a key step in municipal sludge treatment. After mechanical dewatering, the water content of sludge is usually between 50% and 70%, which needs further drying. The main drying technologies include thermal drying, solar drying, ultrasonic drying and microwave drying, among which thermal drying technology is the most mature. Research shows that low-temperature drying technology has obvious advantages in reducing the release of harmful substances, energy consumption and cost. In terms of technology application, domestic sludge drying process is mainly divided into direct drying and indirect drying. Direct drying directly evaporates water through high-temperature gas, while indirect drying makes use of heat conduction effect to contact with high-temperature media. At present, indirect drying technology is mainly used in China, such as paddle type, disc type and thin layer drying. In particular, the low-temperature heat pump drying technology excels in terms of safety, footprint and energy consumption, with its ability to operate at lower temperatures, reduce the release of hazardous gases, and achieve environmental and economic benefits in a closed-cycle mode. This technology is suitable for small and medium-sized wastewater treatment plants and shows clear advantages in cases of budgetary constraints.

A novel approach for the active‐mode indirect drying of food grains with high temperature thermal energy storage system

AbstractThis article introduces a novel approach for the active‐mode indirect drying of food grains using an independent high‐temperature thermal energy storage (TES) system. It uses commercial software to conduct numerical analysis of a solar drying unit fused with indirect TES. Employing commercial grade phase change material X‐180, solar drying unit incorporates scheffler dishes, receiver, TES, thermic fluid pump, air blower, and dryer fitted with a radiator. Heat transfer fluid Thermenol‐66 has been used to facilitate the heat transfer from receiver to TES and dryer. Two distinct TES designs, one without fins (case A) and the other with fins (case B), have been used to encapsulate phase change material and facilitate the flow of heat transfer fluid. The dynamic performance of independent solar drying unit components was examined at various mass flow rates during charging and discharging process. The results demonstrate that adding 42 fins, i.e. (case B), reduces the charging and discharging time to 22% and 24%, respectively. Due to high charging and discharging power, the rate of temperature change in case B is 19% higher compared to case A. To maintain drying quality, the dryer should have a continual flow of hot air at a constant temperature. Therefore, utilization of the TES, case A, provides a significantly higher uniformity index and a more extended drying period during the discharge process.

A hybrid solar/biomass active indirect kiln dryer for timber in the Democratic Republic of Congo

This paper presents a small-scale pilot project in the Democratic Republic of Congo of a hybrid kiln dryer for sawn timber powered by solar and biomass energy. The chamber’s thermal insulation comprised layers of vetiver grass, wood and clay, 100% biodegradable. Sun collectors and a supplementary furnace-burning biomass from the nearby sawmill provided the heating energy, while photovoltaic panels supplied electric energy for ventilation and electric devices. The climate was controlled through actuators operated by an electronic system for industrial kiln dryers. Despite relatively high production costs, primarily due to the photovoltaic system, the kiln allowed for good use of local labour and materials. Preliminary tests indicated an improved drying potential of the kiln compared external conditions with a consequent lower drying time and final MC. Data emphasised the influence of thermal energy in the drying rate: the extra power given by the furnace increased the drying rate from 0.4 to 1.4% day−1. The cost of energy ($/kWh) was calculated as 0.009 for the furnace, 0.05 for the solar collector and 0.14 for the photovoltaic. The conversion efficiency of solar collector resulted to be 48% against 14% of photovoltaic.

Drying characteristics and kinetics of bottle gourd using stand-alone indirect solar dryer

Drying food is an effective method to preserve fruits and vegetables, increasing their shelf life and preventing germination. Bottle gourds, known for their high nutritional value, can rot quickly due to their high moisture content, causing biochemical and microbial changes. To improve shelf life, moisture can be removed to prevent microbial activity. Different drying methods are used to preserve gourd family vegetables for long periods and off-season. Indirect solar drying shields foods from UV rays, dust, insects, and other contamination by avoiding direct exposure. This study aimed to utilize an indirect solar dryer to research the potential for drying bottle gourds and conduct physicochemical analyses for the dried products to compare the drying techniques. In this experiment, different samples of bottle gourd slices were dried using an indirect solar dryer and compared with sun and shade drying. Under physicochemical properties, moisture content (MC), retention of color, thickness, and shrinkage of dried samples were analyzed and compared. The result showed that the time taken to reach 10% moisture was 13 h for samples dried with sun drying. While shade and indirect solar drying took 17 h. The Midilli-Kucuk model outperforms other approaches for drying bottle gourd, with the greatest R2 values (0.9967, 0.9950, 0.9936) and the lowest RMSE values (0.0182, 0.0220, 0.0277) for direct sun, shade, and indirect solar drying, respectively. The indirect sun dryer showed good color retention compared to the fresh bottle gourd samples with L* value of 44.08, b* value of 15.40 and a* value of 7.01. The volumetric shrinkage was 97.64% in the direct sun drying method after 15 h, which is higher compared to sun and shade dryers due to the effect of time and temperature. In addition, there was a significant difference (p-value <0.05) between the thickness of the three drying methods. The use of indirect solar drying methods can be highly recommended to get high-quality dried food with zero energy cost. For future studies it is recommended to enhance the function and design of the indirect solar dryer to speed up drying times and improve the quality of dried samples.

An experimental evaluation of drying banana slices using a novel indirect solar dryer under variable conditions

Novel thermal characteristics of drying banana slices in an indirect dryer are presented for four different experimental drying conditions in the forced convection mode. The novel characteristics include measuring the airflow velocity in the drying chamber, measuring the thermal profiles in different trays comprehensively and measuring the relative humidity under different conditions. Two tests are carried for 16 h in two consecutive days (8 h per day for each test). The first test is on acloudy day followed by a sunny day, while the second test is carried out on two consecutive sunny days. Tests 3 and 4 are 24 h tests with high (0.23 m/s) and low (0.11 m/s) average drying chamber airflow velocities under good solar radiation conditions. The maximum temperatures obtained in the collector and the drying chamber are around 80 and 48 °C, respectively, for the 16 h tests. Significantly lower collector and drying chamber temperatures are obtained due to cloudy conditions. Maximum collector temperatures are around 84 and 95 °C for the high and low average airflow chamber velocities for the 24 h tests. The corresponding maximum temperatures in the drying chamber are around 50 °C for the 24 tests. The final moisture ratios are 0.26 (cloudy and sunny days) and 0.20 (two sunny days), respectively, for the 16 h tests. These final moisture ratios are lower than those obtained for the 24 h tests which are 0.32 and 0.28, respectively. Increasing the drying chamber airflow velocity results in faster moisture removal during sunshine hours for the 24 h tests. For tests 1, 2, 3 and 4, the maximum average collector efficiencies during the sunshine period are around 60, 80, 40 and 10 %, respectively. The average drying efficiencies for the total solar drying period for tests 1,2,3 and 4 on day 1 are 6.9, 6.9, 5.5 and 5.7 % respectively. These values are comparable, suggesting that the average collector powers, airflow velocities and efficiencies have a very small effect on the average solar drying efficiency for the whole drying period. The quality of the bananas slices mainly in terms of the colour and shape is also compared with previous studies and commercially available products. A reasonably acceptable quality product is obtained.

Performance analysis of indirect solar dryer with natural heat energy retention substances for drying red chilli

The world is grappling with substantial food waste after cultivation, mostly due to increased moisture contents. Therefore, the objective of this study concentrates on developing an innovative indirect methodsolar dryer designed to eliminate moisture content by incorporating glass pieces and paraffin wax as Natural Heat Energy Retention Substances (NHERS) in the solar collector. Three distinct dryer setupshave been designed to assess the effectiveness of the suggested innovation namely a collector without NHERS, a collector with glass pieces as NHERS and a collector with combined glass pieces and paraffin wax as NHERS for drying red chilli. Along with these three different dryer setups, open solar drying was done. From the experimental observations, the drying kinetics, energy, exergy and colour analyses were conducted. Red chilli's wetnesscontent dropped from 83.8% (wet basis) in the beginning to 7.3% at the end in 28, 44, 54 and 72 h under the dryer with combined glass pieces and paraffin wax as NHERS, dryer with glass pieces as NHERS, dryer without NHERS and traditional sun drying respectively. The maximum thermal efficiency of the solar collector is determined to be 57.9% and the highest dryer efficiency is 67.2% in the same setup. The maximum exergy efficiency in the dryer with combined glass pieces and paraffin wax as NHERS is 53.7%. Moreover, ten drying models were employed to forecast the MR and the optimal model to describe red chillies drying properties wasthe logarithmic model with the lowest χ2 (0.0008843) and maximum R2 (0.9682) value. The colour of dried red chilli was best in the dryer with combined glass pieces and paraffin wax as NHERS. From these investigations, the developed indirect solar dryer with combined glass pieces and paraffin wax NHERS indicates sustainable drying time and improved product quality.

A Study for Estimating the Overall Heat Transfer Coefficient in a Pilot-Scale Indirect Rotary Dryer

An experimental study and dimensional analysis of the effective heat transfer coefficient in a continuous-indirect rotatube dryer using forest biomass as the granular material isare developed in the present work. The study employed a factorial design 33 to investigate the effects of feed flow frequency (20–35–50 (Hz)), drum rotational velocity (6–8–10 (rpm)), and saturated vapor pressure (4–5–6 (bar)) on the heat transfer coefficient. During steady state conditions, the moisture content profiles and inlet and outlet temperatures were measured within the experimental region, and parameters, such as the effective heat transfer coefficient, solid retention change, and moisture content ratio were studied. The results showed that heat transfer was optimized with high solid feeding rates, low pressure, and low rotation, with solid feeding being the predominant factor. The moisture content profiles revealed a change in the hydrodynamic behavior, with the center point of the experimental region being the least optimal. The dimensional analysis yielded a Nu number as a function of Pe, Fr, and the feeding dimensionless number. A new dimensionless energy efficiency number improved the coefficient correlation from 85.88 (%) to 94.46 (%), indicating the developed model potential to predict dimensionless variables and scale continuous-indirect rotatube dryers.

A Comparative Management Analysis on the Performance of Different Solar Drying Methods for Drying Vegetables and Fruits

Drying is the process of moisture removal which is applied to many foodstuffs including fruits and vegetables for preservation and storage purposes. Since solar energy is one type of environmentally friendly renewable energy, open-type solar dryers, natural convective type solar dryers and greenhouse type solar dryers were designed and fabricated in this study for the preservation of fruits and vegetables. A comparative study among various solar drying methods was performed to study the drying performance by maintaining the quality and texture of the dried foodstuffs. Factors such as solar radiation, ambient temperature, moisture in the air, materials used for glazing, inclination, etc., were considered during the fabrication of the solar dryer so that a better estimate of the performance of the solar dryer could be obtained. The lowest drying rate was observed when convective drying was used as an indirect mode of heating. The maximum drying rate observed in open drying was 0.088 kg/kgh, whereas in convective drying under the same conditions, it was 0.03 kg/kgh, which was almost 65% less. This in turn also resulted in the dried samples displaying a better texture and better color. The shrinkage effect on the samples was less pronounced for those samples in the convective dryer than it was for those in the open and greenhouse dryers, as the method uses indirect drying. Comparing convective and greenhouse drying, more shrinkage and a greater browning effect were observed for the open drying method. Out of three types of solar dryers, the greenhouse dryer was selected to study thermal performance because of its better drying rate. DHT11 sensors controlled through Arduino programming were employed in this study to record the temperature and moisture at various locations in the greenhouse dryer setup. The range of energy efficiency of the greenhouse solar dryer was estimated to be from around 15% to 25% on average. This might be due to a greater extent of energy losses. No significant difference was observed in the energy efficiency with respect to the samples used for drying.

Performance analysis of two industrial dryers (cross flow and rotary) for ligno-cellulosic biomass desiccation

Artificial drying, using industrial devices (dryers), helps to reduce the residual humidity content in biomass in a relatively short time. Convection is one of the most common mode of drying (referred to as direct drying). Heat is supplied by hot air/gas flowing over the surface of the solid. The heat for evaporation is supplied by convection to the exposed surface of the material; the evaporated humidity is carried away by the drying fluid. Indirect dryers (working by conduction) are more appropriate for particulate and granular materials or for very wet solids; while radiative dryers use various sources of electromagnetic radiation with wavelengths ranging from the infrared to microwaves. In this work, two mathematical models of cross flow and rotary dryers (both convective dryers) have been proposed. Both dryers treat wood chips. The two models allow to calculate the thermal efficiency and residence time of wet solid wood chips, as a function of the residual moisture content, as well as the analysis of the behaviour of the outlet wet solid and drying gas, in consideration of the dryer length and of the feeding material conditions in the dryer. The models have been developed in the Mathcad software environment.

Drying characteristic, sustainability, and 4E (energy, exergy, and enviro-economic) analysis of dried date fruits using indirect solar-electric dryer: An experimental investigation

Drying characteristic, sustainability, and 4E (energy, exergy, and enviro-economic) analysis of dried date fruits using indirect solar-electric dryer: An experimental investigation

Drying of Sultana seedless (Vitis vinifera L.) grape variety in indirect drying chamber using solar air collector with conic dimpled absorber: The case of end-season drying

The main functions of mechanical drying are to extend the drying season, prevent reduction in dry product mass due to immature harvest, and obtain high-quality products without exposure to rainy weather conditions. A SAC with a conic dimpled absorber-assisted indirect solar dryer was designed, manufactured, and tested for the Sultana Seedless grapes under the end-season conditions. Also, the effect of medicinal chamomile oils on the color quality of raisins was investigated. While the first three days of the drying experiment were reminiscent of summer, the remaining days were characteristic of autumn. The thermal efficiency of the collector at a flow rate of 0.05 kg/s was realized as 66.33 % and 48.27 % on average during the days (between hours of 10:00–15:00), depending on the psychrometric conditions. The moisture content of the grapes (22.5 brix) decreased from 3.37 to 0.22 kgw/kgsm and lasted for 272 h/12 days. It has been revealed that the collector efficiency and drying time were significantly affected by the psychrometric conditions, and the use of medicinal chamomile oil in dipping pretreatment increased the color quality of raisins. Effective use of the SAC in the end-season drying process should be increased by techniques such as serial connection and heat storage.

Modeling of the process of solar drying of grapes in indirect type installations with natural air convection

The paper models the process of natural convection of an indirect type dryer mathematically for the accumulation and absorption of heat which uses water. The data from the experiment conducted by the authors of the work is used as the initial data. The Reynolds equations and the temperature distribution equations are used for the mathematical model considering the Boussinesq hypothesis. The SIMPLE control volume method is used in the paper for the difference approximation of the initial equations. The temperature and velocity fields in the drying chamber are determined. It is revealed that the maximum speed in this mode is reached in the upper part of the dryer and it will be equal to 0.02-0.03 m/s.

A review on environmental effect of various designs and types of drying methods

AbstractFruits and vegetables are nutritious and good in minerals, vitamins, and fib. Most fresh fruits and vegetables contain a minimum of 70% of moisture content which causes the formation of bacteria, fungi, and viruses to affect and cause the items to rot. But dried fruits contain only less than 20% of moisture content. So drying the fruits and vegetables helps to preserve the items for longer use without causing them to rot. Dried fruits have a great source of antioxidants and help in health benefits like improving blood flow, and reducing the effects of many diseases. Compared to fresh fruits, dried fruits are good in nutrition and have less weight for packaging and transportation usage. For drying materials, various types of solar dryers are used. Natural or open sun drying methods comparatively consume more than 12 h to dry material, whereas, direct and indirect drying methods would enhance the efficiency and dry the products quicker than open drying. For the consideration of quality drying and to increase efficiency, various modifications of solar dryers are designed and fabricated. In this review, we discussed the most appropriate developments in the drying of food items, required phase change materials (PCMs), performance, and environmental analysis of SD, and also reported the various designs of the solar drying techniques.

Experimental forced convection greenhouse and indirect cabinet drying of date fruits: a comparative study

Experimental forced convection greenhouse and indirect cabinet drying of date fruits: a comparative study

Environmental sustainability and exergy return on investment of selected solar dryer designs based on standard and extended exergy approaches

ABSTRACT Different solar dryer designs are available in the literature. However, it is essential to understand how the performance of the dryer designs contributes to environmental sustainability by reducing waste in the exergy-loop and mitigate CO2 emission into the atmosphere. The exergy-based sustainability indices include waste exergy ratio, lack of productivity (LOP), sustainability index, environmental destruction coefficient and exergy recovery ratio. The paper also introduces exergy return on investment (ExRoI) by applying an extended exergy accounting approach with the inclusion of exergy influx of material resources, capital and labor in the exegetic computations of the sustainability index. Four solar dryers consisting of an indirect solar-cabinet dryer, mix-mode solar dryer, hybrid solar dryer and a double-partitioned single-pass-collector solar dryer with a wind generator were used as a case study. Comparatively, the ExRoI is lower than the standard exergy-efficiency for the four dryers by 6.87 to 91.61%. In contrast, the standard exergy-based sustainability index was higher by 5.4 to 61.2%. Total invested exergy ranged from 0.04TJ to 0.081TJ. The values of waste exergy ratio, LOP, ERR, EDC, and carbon credit ranged from 0.47 to 0.79, 0.566 to 16.45, 5.44 × 10−4 to 1.37 × 10−1, 1.57 to 17.45 and 231.45 to $2908, respectively.

Pre-Drying of Chlorine-Organic-Contaminated Soil in a Rotary Dryer for Energy Efficient Thermal Remediation.

In response to the current problem of the high energy consumption of direct thermal desorption systems when treating soils with a high moisture content, we propose using the waste heat of the system to pre-dry soil to reduce its moisture. Taking chlorine-organic-contaminated soil as an object, an experimental study on the drying and pollutant desorption characteristics of soil in an indirect rotary dryer was carried out. The results show that the non-isothermal drying process was divided into warm-up and falling rate periods, and no constant period was observed. The higher the rotation speed, the lower the soil outlet temperature and the higher the drying tail gas temperature. Soil outlet and dry tail gas temperatures were lower for soils with a higher moisture content. Benzene and cis-1,2-dichloroethylene are easily desorbed. Therefore, the disposal of dry tail gas should be determined according to the type and concentration of soil pollutants present. The volumetric heat transfer coefficient was found to be 85-100 W m-3 °C-1, which provides a key parameter for the size design of a rotary dryer.

Analysis of parameters important for indirect drying of biomass fuel

This paper focuses on biomass drying for the design and operation of an indirect dryer used in a biomass power plant. Indirect biomass drying is not as well described process as direct drying, especially when used for the preparation of biomass in energy processes, such as combustion or gasification. Therefore, it is necessary to choose a suitable model describing the drying process and evaluate its applicability for this purpose. The aim of this paper is to identify parameters that most significantly affect the indirect drying process of biomass for precise targeting of future experiments. For this purpose, the penetration model was chosen. The penetration model describes indirect drying through 21 parameters. To run a series of experiments focused on all parameters would be time consuming. Therefore, the easier way is to select the most important parameters through a sensitivity analysis, and then perform experiments focused only on the significant parameters The parameters evaluated as significant are the temperature of the heated wall, operating pressure in the drying chamber, surface coverage factor, emissivity of the heated wall, emissivity of the bed, diameter of the particle, and particle surface roughness. Due to the presumption of perfect mixing of the material being dried, stirrer speed is added into important parameters. Based on these findings, it will be possible to reduce the scope of experiments necessary to verify the applicability of the penetration model for the description of indirect biomass drying and the design of dryers for a practical use.

Drying Microalgae Using an Industrial Solar Dryer: A Biomass Quality Assessment.

Microalgae are considered a promising resource of proteins, lipids, carbohydrates, and other functional biomolecules for food and feed markets. Competitive drying solutions are required to meet future demands for high-quality algal biomass while ensuring proper preservation at reduced costs. Since often used drying methods, such as freeze or spray drying, are energy and time consuming, more sustainable processes remain to be developed. This study tested an indirect and hybrid solar dryer as an alternative to conventional freeze drying of industrially produced Tetraselmis chui and Nannochloropsis oceanica wet paste. The effects of the drying method on biomass quality parameters, including biochemical profiles, functional properties, and microbial safety, were assessed. No significant differences were found between the applied drying technologies for total proteins, carbohydrates, lipids, and fatty acid profiles. On the other hand, some pigments showed significant differences, displaying up to 44.5% higher contents in freeze-dried samples. Minor differences were also registered in the mineral profiles (<10%). Analyses of microbial safety and functional properties of the solar-dried biomass appear adequate for food and feed products. In conclusion, industrial solar drying is a sustainable technology with a high potential to preserve high-quality microalgal biomass for various markets at expected lower costs.

Cinética de secado de Cecina de Yecapixtla usando un secador solar indirecto de flujo forzado

This study presents the drying behavior of cecina from Yecapixtla Morelos through an indirect solar dryer to obtain the drying kinetics. The behavior of the drying kinetics was examined, observing significant effects related to the levels of pH, salinity, and density of the cecina. The effective diffusion coefficients Deff were 5.673x10-9, 2.644x10-9, 2.133x10-8 for three samples corresponding to different days of elaboration. Artisanal dry cecina samples are compared versus the dry cecina of this study. The results showed a higher concentration of salinity and a lower pH of the dry cecina of this study versus the dry artisanal cecina. Solar drying through the indirect dryer present in this work is a viable alternative to give added value to the production of cecina from Yecapixtla in Morelos, Mexico.

Thin Layer Modelling of Indirect and Mixed-Mode on-Farm Drying of Selected Vegetables

The thin layer drying characteristics of tomato and okra slices dried using mixed-mode on-farm solar dryer, indirect mode on-farm solar dryer and open sun drying. The tomato and okra slices dried faster when dried under the mixed-mode on-farm solar dryer. Drying time was reduced considerably using the on-farm solar dryers. The drying data were fitted into Lewis, Henderson and Pabis, and page equations. The Page model (R2=0.9365, 0.9623; X2= 0.0067, 0.0000579; RMSE= 0.0086, 0.0020 and MBE= -0.008, -0.002) gave the best prediction for the mixed-mode drying and indirect mode drying of tomato slices respectively. In the same vein Page model (R2=0.9202, 0.933o; X2= 0.00091, 0.000730; RMSE= 0.0265, 0.0244 and MBE= -0.0088, -0.0074) gave the best prediction for the mixed-mode drying and indirect mode drying of okra slices respectively. Effective moisture diffusivity of tomato slices varied between -7.4724 X 10-08 and -1.6439 X 10-07 while that of okra varied between -3.12 X 10-07 and -8.08 X 10-07. The indirect mode dryer gave the best quality of dried tomato and okra.