Heat and Mass Transfer During Fry-Drying of Sewage Sludge

Deep-frying, which consists of immersing a wet material in a large volume of hot oil, presents a process easily adaptable to dry rather than cook materials. A suitable material for drying is sewage sludge, which may be dried using recycled cooking oils (RCO) as frying oil. One advantage is that this prepares both materials for convenient disposal by incineration.

This study examines fry-drying of municipal sewage sludge using recycled cooking oil. The transport processes occurring during fry-drying were monitored through sample weight, temperature, and image analysis. Due to the thicker and wetter samples than the common fried foods, high residual moisture is observed in the sludge when the boiling front has reached the geometric center of the sample, suggesting that the operation is heat transfer controlled only during the first half of the process followed by the addition of other mechanisms that allow complete drying of the sample. A series of mechanisms comprising four stages (i.e., initial heating accompanied by a surface boiling onset, film vapor regime, transitional nucleate boiling, and bound water removal) is proposed. In order to study the effect of the operating conditions on the fry-drying kinetics, different oil temperatures (from 120 to 180°C), diameter (D = 15 to 25 mm), and initial moisture content of the sample (4.8 and 5.6 kg water·kg^?1 total dry solids) were investigated.     

Dimensionless Formulation of Convective Heat Transfer in Fry‐Drying of Sewage Sludge

Fry‐drying is an alternative for heat and mass transfer intensification. The process reuses waste oil as a heating medium for drying by contact with the wet sludge. At the end of the process, a stable derived fuel is obtained, a granular solid composed of the dried indigenous sewage solid and the impregnated oil. The fry‐dried sludge is storable and transportable without any pathogen elements. Knowledge about heat and mass transfer rates during the frying process is essential in order to assess the quality of the final product such as calorific value, oil uptake, porosity changes, etc. The heat transfer properties including transfer by free convection between the solid and the frying oil are fundamental for the process design and manufacturing of the fry‐dried product. The convective heat coefficient by temperature measurement and overall energy balance calculation is determined. The heat flux is calculated from the fry‐drying kinetics including moisture loss and oil intake kinetics. Various hydrodynamic regimes for convective heat transfer during the frying process are discussed (non‐boiling, boiling, and low‐boiling regime). A dimensionless formulation for estimating the convective transfer is proposed.     

Heat and Mass Transfer in Fry Drying of Wood

This article describes the coupled heat and mass (water, oil) transport phenomena in parallelepiped samples of beech (Fagus sylvatica) fried in peanut oil between 120 and 180°C. The aim was to evaluate the suitability of simultaneous fry drying and oil impregnation as an alternative wood treatment process. Water loss and oil impregnation were continuously assessed during the process. Temperature and pressure were measured at the center of the sample. The water in the peripheral layers of the wood vaporizes at atmospheric pressure. The water at the center of the wood vaporizes at overpressures of the order of 2.8 × 10⁵ Pa. High fluxes of water were recorded of about 0.006 kg/(m²s). The impregnated oil can amount to 20% of the mass of the removed water.     

Heat and Mass Transfer in Fry Drying of Wood

This article describes the coupled heat and mass (water, oil) transport phenomena in parallelepiped samples of beech (Fagus sylvatica) fried in peanut oil between 120 and 180°C. The aim was to evaluate the suitability of simultaneous fry drying and oil impregnation as an alternative wood treatment process. Water loss and oil impregnation were continuously assessed during the process. Temperature and pressure were measured at the center of the sample. The water in the peripheral layers of the wood vaporizes at atmospheric pressure. The water at the center of the wood vaporizes at overpressures of the order of 2.8 × 10⁵ Pa. High fluxes of water were recorded of about 0.006 kg/(m²s). The impregnated oil can amount to 20% of the mass of the removed water.     

污泥干燥焚烧一体化中热量计算的探讨

介绍了几种常用的污泥干燥焚烧一体化技术及其设备,分析了污泥在转筒干燥-循环流化床焚烧一体化设备中的热量输入、输出过程,对污泥干燥以及焚烧设备的设计有一定的参考价值。     

Simultaneous Heat and Mass Transfer in Shrinkable Apple Slab During Drying

The goal of the study was to determine the influence of drying shrinkage on the kinetics of convection apple slab drying. The arbitrary Lagrange-Eulerian (ALE) method was used to enter a problem with moving boundaries. It was found that drying shrinkage had a major influence on the both simulated temperature and water content in the material. The lower the moisture content in particles during drying, the more pronounced the effect of shrinkage on simulation of heat and mass transfer. It was found that application of the arbitrary Lagrange-Eulerian method for shrinkage modeling leads to a relatively simple mathematical model of the drying kinetics of shrinkable materials.     

Simultaneous Heat and Mass Transfer in Shrinkable Apple Slab During Drying

The goal of the study was to determine the influence of drying shrinkage on the kinetics of convection apple slab drying. The arbitrary Lagrange-Eulerian (ALE) method was used to enter a problem with moving boundaries. It was found that drying shrinkage had a major influence on the both simulated temperature and water content in the material. The lower the moisture content in particles during drying, the more pronounced the effect of shrinkage on simulation of heat and mass transfer. It was found that application of the arbitrary Lagrange-Eulerian method for shrinkage modeling leads to a relatively simple mathematical model of the drying kinetics of shrinkable materials.     

A Generalized Correlation for Heat and Mass Transfer in Freezing, Drying, Frying, and Freeze Drying

Experiments on air freezing, deep frying, freeze drying, and air drying on similar samples of 12 mm slice of potato were carried out. The temperature change with time was measured at three locations across the thickness of the potato. All the measurements done on frying, freeze-drying, and air-drying showed the existence of a moving interface that recedes towards the center of the sample as time progresses. A one-dimensional model, based on quasi-steady state analysis was developed to predict drying/frying processes. The analytical model was found to predict well the temperature distribution and the interface movement in the potato slices in all the three applications, and expected to predict many other applications yet to be tested. Three dimensionless groups were used to characterize the model.     

Analysis of the Shrinkage Effect on Mass Transfer During Convective Drying of Sawdust/Sludge Mixtures

Convective drying of wastewater sludges and sawdust/sludge mixtures was studied. The first part of this work was an experimental study performed in a cross-flow convective dryer using 500 g of wet material extruded through a disk with circular dies of 12 mm. The results showed that the sawdust addition has a positive impact on the drying process from a mass ratio of 2/8, on a dry basis, with observed drying rates higher than the original sludge. The second part of this work consisted of developing a drying model in order to identify the internal diffusion coefficient and convective mass transfer coefficient from the experimental data. A comparison was made between fitted drying curves, well represented by the Newton's model, and the analytical solutions of the diffusion equation applied to a finite cylinder. Variations of dimensional characteristics, such as the volume and exchange surface of the sample bed, were obtained by X-ray tomography. This technique allowed us to confirm that shrinkage, which is an important phenomenon occurring during sludge and sawdust/sludge mixture drying, must be taken into account. The results showed that both the internal diffusion coefficient and convective mass transfer coefficient were affected by mixing and sawdust addition. The internal diffusion coefficient changed from 7.77 × 10^?9 m²/s for the original sludge to 7.01 × 10^?9 m²/s for the mixed sludge and then increased to 8.35 × 10^?9 m²/s for the mixture of a mass ratio of 4/6. The convective mass transfer coefficient changed from 9.70 × 10^?8 m/s for the original sludge to 8.67 × 10^?8 m/s for the mixed sludge and then increased to 12.09 × 10^?8 m/s for the mixture of a mass ratio of 4/6. These results confirmed that sawdust addition was beneficial to the sludge drying process as the mass transfer efficiency between the air and material increased. Reinforcing the texture of sludge by adding sawdust can increase the drying rate and decrease the drying time, and then the heat energy supply will be reduced significantly. The study also showed that neglecting shrinkage phenomenon resulted in an overestimation for the internal diffusion coefficient for the convective drying of sludges and sawdust/sludge mixtures.     

Mass and Heat Transfer in a Continuous Plate Dryer

Abstract

A mathematical model for the mass-heat transfer in a continuous plate dryer has been developed. Along with a new mass transfer model, the formulas for several important parameters, such as height, volume of each granular heap and retention time, are provided. According to the penetration model of particle heat transfer, the average drying rate ([mdot]) is predicted together with the mean bulk temperature (T _out) and moisture content (X _out) by a straightforward stepwise calculation procedure. The only empirical parameter N _mix can be predicted by the method, provided that experimental data with various initial moisture contents (X _in) are given. According to the model, the optimization of a plate dryer should aim at the maximizing of the effective covering ratio (μ) and the total area-averaged heat transfer coefficient (α). The model and equations were applied in an experimental plant. In the result, the theoretical predictions are shown to be in satisfactory agreement with experimental data.     

Fate and Effects of Fry-Drying Application on Municipal Dewatered Sludge

The present experimental investigation examined the applicability of fry-drying technology for municipal dewatered sludge by using waste engine oil for frying. The process was studied in a 100–180°C temperature range for 2 to 20 min frying times. The cakes were shaped in a diameter range of 1 to 3 cm with 3-cm-length prior to the frying. Experimental points were determined by the Box–Wilson experimental method. Statistical analysis was employed for determining the response function coefficients for variables. The response functions were as dry solids content (DS), volatile solids content (VS), and low heating value (LHV). Experimental and predicted results indicated good agreement with high correlation coefficients. The results showed that fry-drying is an alternative, fast process for sludge drying. In addition, it makes fry-dried sludge as a product with noteworthy LHV. This study provides further insight into the fry-drying of municipal dewatered sludges.     

Physical Model of Heat and Mass Transfer in a Spouted Bed Coffee Roaster

A key step in coffee processing is roasting, where green coffee beans are heated at high temperatures (over 190°C), initiating a series of complex chemical reactions. This process can be divided in two phases (drying < 160°C and roasting > 160°C). Therefore, the quality of coffee is highly correlated with bean temperature during roasting. The aim of this work is to measure and model coffee bean temperature and moisture content during the roasting process, in order to determine on-line quality of the product allowing the control of the process. A spouted bed roaster, using hot air flow as heating medium, was developed. The surface and center bean temperature, as well as input and output air temperatures, were measured on-line. At each minute interval of roasting, samples of coffee beans were taken to measure the moisture content.

A dynamical model, which takes in account heat and mass transfer at the surface and inside of the beans, is proposed. This model, where only the water diffusivity in the bean was adjusted, gave a good prediction of bean temperature (center and surface), average moisture content, and output air temperature for all the experiments. However, in the industry, only the output air temperature can be measured. Consequently, this model could assist in on-line determination of the bean temperature and hence constitutes the first stage in developing a smart sensor for on-line checking including quality control.     

Discrete and Continuous Modeling of Heat and Mass Transport in Drying of a Bed of Iron Ore Pellets

Drying of a porous bed of iron ore pellets is here considered by modeling a discrete two-dimensional system of round pellets. As a complement to the two-dimensional model, a continuous one-dimensional model enabling fast calculations is developed. Results from the discrete model show that the temperature front advances faster in areas with large distances between the pellets. In areas with low flow speed, the temperature of the pellets increases with a relatively slow rate. The water inside these pellets will therefore remain for a long time. The continuous model fits the discrete model very well for a regular distribution of equal-sized particles. A discrete model with irregular packing will, compared to the continuous model, show a larger variation in the distribution of temperature and moisture content in the final phase of drying.     

Gaseous Carbon Dioxide as the Heat and Mass Transfer Medium in Drying

Using available correlations for heat transfer, a comparative analysis of drying rates in CO₂ and in air was performed for several basic types of dryers. Higher heat transfer rates were found for dryers with active hydrodynamics, which translates into shorter drying time for materials dried in the first drying period. These results were validated by experiments on drying wheat kernels fluidized by air and by CO₂. Shorter drying times by about 20% were confirmed for CO₂, which offers energy savings of about 3% of the heat input to the dryer. Additional energy savings of 4% of the heat load can be expected for drying at temperatures below 100°C because of the lower wet-bulb temperature for CO₂ than that for air. The potential for CO₂ abatement was evaluated based on a case study for drying of distillers' spent grain.     

Gaseous Carbon Dioxide as the Heat and Mass Transfer Medium in Drying

Using available correlations for heat transfer, a comparative analysis of drying rates in CO₂ and in air was performed for several basic types of dryers. Higher heat transfer rates were found for dryers with active hydrodynamics, which translates into shorter drying time for materials dried in the first drying period. These results were validated by experiments on drying wheat kernels fluidized by air and by CO₂. Shorter drying times by about 20% were confirmed for CO₂, which offers energy savings of about 3% of the heat input to the dryer. Additional energy savings of 4% of the heat load can be expected for drying at temperatures below 100°C because of the lower wet-bulb temperature for CO₂ than that for air. The potential for CO₂ abatement was evaluated based on a case study for drying of distillers' spent grain.     

A Study of Coupled Heat and Mass Transfer in Composite Food Products during Convective Drying

A two-dimensional liquid diffusion model was developed to predict the variation of transient moisture content and temperature distribution within model composite food products. Rectangular-shaped potato and apple slices were used to form the model composite samples for experimental tests. To verify the predictions of the theoretical model, a series of experiments were conducted on the model composite samples using a heat pump-assisted convective dryer. Effects of using different adhesives to hold the two slices forming the composite products, flipping of the products at selected intervals of time and variation of individual slice thickness were investigated both experimentally and numerically. Predictions of the numerical model are in good agreement with the experimental results.     

Heat and Mass Transfer Mechanisms in Drying of a Suspension Droplet: A New Computational Model

A new computational single-droplet drying model is presented. The model considers heat and mass transfer simultaneously together with the receding evaporation front approach. A spherical droplet under constant drying conditions is considered. Computations are performed to predict the drying of colloidal silica-water suspension and skimmed milk. It is shown that the results agree well with those of experimental observations available in the literature.     

Heat and Mass Transfer Mechanisms in Drying of a Suspension Droplet: A New Computational Model

A new computational single-droplet drying model is presented. The model considers heat and mass transfer simultaneously together with the receding evaporation front approach. A spherical droplet under constant drying conditions is considered. Computations are performed to predict the drying of colloidal silica-water suspension and skimmed milk. It is shown that the results agree well with those of experimental observations available in the literature.     

Accumulation of 5-Hydroxymethylfurfural in Oil During Frying of Model Dough

5-Hydroxymethylfurfural (HMF), a thermal process contaminant, forms in food during frying as a result of the Maillard reaction and caramelization. Owing to its chemical properties, HMF formed in foods during frying partially transfers into frying oil. This study aimed to investigate the accumulation of HMF in oil during repetitive frying operations. A model dough composed of 25 % of glucose was fried at 160, 170, 180 °C for 50 frying cycles. Apart from total polar compounds (TPC), accumulation of HMF was determined in oils during repetitive frying operations. Increasing frying temperature also increased the amount of HMF formed in dough, and those transferred to oil. Prolonging frying cycles to 150, increasing amount of dough being fried to 100 g and frying time to 10 min caused the TPC content to reach 25 % at the 130th frying cycle at 180 °C. Under the same frying conditions, the concentration of HMF showed a rapid increase at the first 10th frying cycle. Its increase was at a slower rate until the 50th frying cycle reaching a plateau level exceeding 5.0 mg/L. The results revealed that HMF transferred and accumulated in the frying oil during repetitive frying. The HMF concentration exceeding 5.0 mg/L in oil may be considered as an indicator for heavily used frying oil. Correlation between TPC and HMF contents of frying oil showed no linear correlation.