Moisture content distribution in semibatch drying processes,part II. Falling particle drying rate

This article describes the evolution of the moisture content distribution in a population of wet particles during a semibatch drying process. The population balance model that is defined for this process includes falling particle drying rate, that is, the drying rate of a particle depends on its moisture content. This model is solved analytically using the “method of moments” and “method of characteristics.” Simulation experiments give realistic and meaningful results. Drying experiments still have to be performed to test the validity of the assumptions made in this model. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Differences between constant and intermittent drying in surf clam: Dynamics of water mobility and distribution study

Low-field nuclear magnetic resonance (LF-NMR) has been increasingly popular as analytical tools for evaluating the dynamics of water mobility and distribution. In this study, dynamics of moisture mobility and constitution of surf clam during constant drying process and intermittent drying process were evaluated by LF-NMR, while the differences of physical and chemical indexes were measured. Intermittent drying improved the product quality of clam, such as moisture content, shear force, color indices, sugar content, peroxide value, thiobarbituric acid value, and the bulk water ratio, which were closely related with moisture distribution and microstructure. The moisture constitution of constant drying process and intermittent drying process were distinctly different. Tempering process reduced drying time and resulted in lower moisture content in dried surf clam. In the meanwhile, the boundary between A₂₁ and A₂₂ was acquired by LF-NMR, revealed that bound water and immobilized water transformed from each other. During tempering process, the myofibril stretched out, verifying that moisture approached a relatively homogeneous. In addition, R² value reached 0.9897 and 0.9926 for calibration and validation, respectively, displaying good linear correlations between the T₂₁ parameters and moisture content. This study interpreted the dynamics of water mobility and distribution on the proton level to explain the reason that tempering processes to improve physicochemical indexes of surf clam.     

Drying characteristics and water dynamics during microwave hot-air flow rolling drying of Pleurotus eryngii

Abstract

The aim of this study was to improve the drying uniformity and quality of Pleurotus eryngii by using the combination of microwave drying and hot-air flow rolling drying. The moisture content, drying uniformity, and water migration of P. eryngii during microwave hot-air flow rolling drying (MARD) were analyzed in detail. The temperature distribution images were obtained via infrared thermal imaging techniques and the relationship between relaxation time and signal amplitude were obtained via low-field nuclear magnetic resonance analysis and imaging (LF-NMR/MRI). The curves of signal amplitude with moisture content changes were fitted by a linear model with good linearity correlation. It was found that the hot-air and rolling bed could improve the drying uniformity of microwave drying. And the free water was found to transform into immobilized water and bound water during the drying process. Only a small amount of water was left in the dried P. eryngii in the final stage. The results could provide supportive information for improving the uniformity and quality of the drying processes of the edible fungi.     

Obtaining model parameters of drying kinetics for highly shrinkable materials without knowing the surface area a priori

Shrinkage parameters of highly shrinkable materials such as length, diameter and surface area during drying are difficult to quantify in situ. However, these are significant components of an accurate model. In this study, an attempt to isolate the surface area effect is reported in order to fetch the REA model (reaction engineering approach) parameters without knowing it a priori. Carrot cube and cabbage leaf were selected as experimental material and dried with hot air under a range of conditions. Shrinkages was calculated using an optical method which is used to qualitatively compare with that “calculated” using the current approach. By matching the experimental temperature and moisture content profiles against time after obtaining REA parameters for both samples without knowing the surface area, the surface areas can be “calculated” numerically. Surface area was found to be affected by sample temperature as well as the moisture content. Drying simulations can be well carried out when correlating the surface area against sample moisture content X and temperature T, and it provides the best accuracy in predicting data on T and X vs. time. In addition, carrot cube can shrink ideally while cabbage leaf cannot. The overall relative errors of predicted moisture content and temperature were less than 1%.     

Numerical modeling assessment of mechanical effect in bovine leather drying process

Leather manufacturing involves a crucial energy-intensive drying stage in the finishing process to remove its residual moisture and generates important heat gradients. The numerical model presented in this study has been developed to describe the drying process of porous medium: bovine leather that undergoes deformation due to shrinkage. The mathematical formulation of fundamental heat, mass and momentum transfers’ phenomena during drying summarizes a two-dimensional model considering elastic behavior of bovine leather. The evolution of moisture content, temperature, and mechanical stresses during drying was discussed. The model was validated with experimental results. Numerical simulations show good agreement with experimental results. The study shows that the elastic model keeps the stress sign at the final stage of drying. The deformations induce tensional stresses near the surface equilibrated by compressive stresses within the product. They reached their maximum for normal stresses equal to 5.97 and 3.52?MPa at around 2145 and 868?s, respectively, for normal stresses along x and y directions and then decrease.     

Experimental investigation of the morphology of salt deposits from drying sessile droplets by white‐light interferometry

This study presents a new approach to investigate the drying behavior and the structure of deposit resulting from drying of solid containing micro droplets. It is shown that deposit structure (porosity and “footprint”) depends on drying conditions. This dependency may contribute to better understanding of particle‐forming processes, such as fluidized bed coating. In the framework of this study, sessile droplets containing sodium benzoate dissolved in water were dried on thin glass plates in a small drying chamber. The drying conditions (temperature, moisture content and flow rate of drying gas) and material parameters (solid content of solution) were systematically varied. The drying rate of droplets was determined from the moisture balance of the drying gas. The final three‐dimensional shape of dried sessile droplets was measured using white‐light interferometry and transformed into a two‐dimensional profile using a Monte Carlo method. Moreover the mean porosity of dried droplets was calculated. By comparison of structural information and process conditions it is shown that the drying process may have a large influence on deposit structure. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2002–2016, 2018     

Development of a porosity prediction model based on shrinkage velocity and glass transition temperature

Abstract

Pore formation and evolution is a common physical phenomenon observed in food materials during different dehydration processes. This change affects heat and mass transfer process and many quality attributes of dried product. Many mathematical models ranging from emperical to classical models proposed in the literature for predicting porosity during drying of food materials. Classical model is in its infancy as the required materials properties during drying are not avaiable for the material charecterisation. Empirical and semi-empirical models are reasonably well developed in establishing relationships between pore evolution and moisture content and determining experimental based coefficients. However, there are no simplistic models that considered process conditions and material properties together to predict the porosity. The purpose of this work is to develop a simplistic theoretical model for pore formation taking both process parameters and changing material properties during drying into consideration. A new “shrinkage velocity” approach has been introduced and the model has been developed based on this shrinkage velocity taking into account the main factors that influence the porosity including the glass transition temperature. Experimental results show good agreement with simulated results and thus validated the model. This study is expected to enhance the current understanding of pore formation of deformable materials during drying.     

Analysis of D2 law in case of Shengli lignite drying under inert and uninert environment

Shengli lignite coal, originated from inner Mongolia China, contains significantly high amount of moisture (more than 30%) which can cause spontaneous combustion or other application problems. Thus, it is of interest to understand the heat and mass transfer mechanism of the low-rank lignite drying under different drying environments such as N₂, CO₂, air, argon, and helium. In this study, fundamental drying experiments with different drying agents were performed on coal samples using thermogravimetric analysis (TGA) method. Lignites with size of 0.045–0.075?mm were heated up from ambient temperature to a target temperature of 175°C under different environments at heating rates of 5, 10, 20, 40, and 80 °C/min, respectively. It was found that thermal conductivity of drying media, heating rate, and initial moisture content are three most significant factors affecting lignite drying process. The highest moisture release rate and the lowest T_peak (when maximum moisture release rate occurred) were observed when drying with helium due to its highest evaporation constant (i.e., highest thermal conductivity). Moreover, higher heating rate and moisture content resulted in higher evaporation rate and T_peak. In the meantime, the classical D² law, which is used to simulate the liquid fuel droplet combustion, was further developed to describe the “group effect” of moisture evaporation process of solid fuel during drying. The D² law well explains the experiment results. Finally, the structures of the dried lignite samples under different drying mediums were investigated through scanning electron microscopy studies. It was found that lignite coals shrank and became more compact when dried out, especially with drying agent CO₂.     

Online measurement of moisture content,moisture distribution,and state of water in corn kernels during microwave vacuum drying using novel smart NMR/MRI detection system

Moisture content is unevenly distributed and hard to measure when agricultural products are dried using microwave drying. A low-field nuclear magnetic resonance/imaging (NMR/MRI) and microwave vacuum drying (MVD) combination equipment was developed. The residual moisture content, distribution, and state of water (free, immobilized, and bound) in fresh corn kernels during MVD were quickly measured in real time. NMR results indicated that the amplitude of free and immobilized water decreased very rapidly at the early stage of MVD, while the amplitude of bound water experienced a similar rapid decrease at the last stage. MRI results indicated that the moisture content was always distributed unevenly during MVD, especially at the early stage. The moisture distribution tended to become uniform when drying progressed and the bound water became dominant. The residual moisture content of corn kernels and integral (total) amplitude of NMR were found to fit well with a linear model (R²?>?0.991, P?相似文献   

Review of heat Conduction,Second Edition,S. Kakac and Y. Yener

In some species, such as subalpine fir (Abies lasiocarpa [Hook] Nutt), the water content of the confined zones in heartwood is as high as or greater than that of sapwood. Such wet zones of heartwood are referred to as “wetpocket” or “wetwood.” Wood products from subalpine fir forests are adversely affected by the wetwood-associated problems, particularly during the drying process. The objectives of the study were as follows: (1) to investigate feasibility of a high X-ray energy industrial computed tomography (ICT) scanner for imaging wetwood; and (2) to determine changes of the 2-D and 3-D moisture profiles (from core to shell) at different drying times.

Although medical CT scanning has been used for attaining signal intensity profiles of typical wood at different drying times, the technology has not, to date, been used for the study of wetwood phenomenon. This study presents, for the first time, results from the ICT imaging of the wetwood phenomenon. The results indicate that the ICT imaging system provides a powerful technique for imaging wetwood at different drying times. In addition, the results show that during the initial phase of drying, almost flat moisture profiles were observed in all wood types except for the wetwood, which showed a relatively higher moisture profile. A much slower (sluggish) drying development pattern at each increment from core to shell was found within the wetwood zone than normal wood regions along the width, thickness, and length of the board.     

Use of low-field-NMR and MRI to characterize water mobility and distribution in pacific oyster (Crassostrea gigas) during drying process

Low-field nuclear magnetic resonance (LF-NMR) and magnetic resonance imaging (MRI) were used to monitor the water mobility and distribution of Pacific oyster during drying process. The results show the mobilities of bulk, immobilized, and free water were reduced, and the immobilized water was removed dramatically. T₂-weighted images displayed the water decrease from the external surface to inner center of oyster during drying. In addition, excellent correlations between the total moisture content and T₂₂ and A₂₂ were observed with coefficients 0.9777 and 0.9832, respectively. Principal component analysis showed the drying degree of oyster could be monitored based on raw relaxation data. Thus, the result revealed that LF-NMR and MRI have great potential in assessing water mobility and distribution in oyster during drying process.     

Kiln drying behavior utilizing drying rate of lumber from six fast-growth plantation species in Costa Rica

The objective was to study the effect of the drying time and moisture content variations on the drying rate during kiln drying. The moisture content is a good predictor of the variation in drying rate; to determine the drying rate, the model DR =a * t + b (where DR is the drying rate, t is the drying time, and a and b are constants) was used. Cordia alliodora, Dipterix panamensis, Hieronyma alchorneoides, and Vochysia ferruginea show a unique tendency, whereas Enterololium cyclocarpum and Samanea saman showed two tendencies. Drying rate in relation to time showed that it is possible to define the various stages during drying and the inflexion points in time where a change in the drying rate occurs. Two types of tendencies are shown in the drying rate vs. time relation in C. alliodora, E. cyclocarpum, and S. saman, whereas three tendencies appear in D. panamensis, H. alchorneoides, V. ferruginea. C. alliodora, E. cyclocarpum, and S. saman presented only one inflexion in the drying rate, which can be maintained at an average of 24.5, 11.64, and 7.83%/day, respectively, for those species. On the other hand, D. panamensis, H. alchorneoides, and V. ferruginea showed two inflexions.     

Pulsed vacuum drying (PVD) of wolfberry: Drying kinetics and quality attributes

The effects of drying temperature (50, 53, 56, 59, 62, and 65°C) and pulsed vacuum ratio defined as the vacuum pressure duration versus atmosphere pressure duration (3:3, 6:6, 9:2, 12:5, 15:1, 18:4?min/min) on pulsed vacuum drying (PVD) characteristics and quality attributes of wolfberry in terms of polysaccharide content, color parameters (L^*, a^*, b^*, ΔE, and C), rehydration ratio and microstructure were investigated. Results revealed that appropriate PVD can reduce drying time by 73.2% compared to hot air drying at the same drying temperature. The moisture effective diffusivity (D_eff) ranged from 5.23?×?10^?10 to 9.73?×?10^?10?m²/s, calculated using the Weibull distribution model. The polysaccharide content, L^* (lightness), a^* (redness/greenness) of the PVD products were higher than those of the hot air-dried samples at the same drying temperature. The total color difference (ΔE) and color intensity (C) of PVD samples were close to those of the fresh ones. The retention rate of total polysaccharide content of PVD samples was about 49–77%, which was significantly higher than 30% of the hot air-dried samples. The surface of PVD wolfberry was highly porous, which may enhance moisture transfer during drying as well as rehydration processes. The results of current work indicate that PVD is a promising technology for wolfberry process, for the reason that PVD can reduce drying time significantly as well as enhance the quality attributes in terms of the total polysaccharide content, color parameters and rehydration ratio.     

Walnut structure and its influence on the hydration and drying characteristics

Abstract

Fresh harvested walnuts are dehulled, washed, and then dried by hot air (HA) as a continuous process in the industry. The objective of the current work was to study the walnut structure and investigate its effect on the moisture transfer characteristics during the walnut soaking and drying processes. Moisture transport pathways into the walnuts were determined using fluorescence tracer approach, and the hydration kinetics of walnuts under different soaking temperatures (15, 25, and 35?°C) was studied using Peleg model. HA drying experiments in single layer in a self-designed automatic HA dryer at 43?°C and air velocity of 1.41?m/s. The influence of the stem pore (sealed and non-sealed) and the soaking process (0, 2- and 5-min soaking time) on the walnut drying characteristics were investigated systematically. The results indicated that both the presence of the stem pore and the soaking time had significant influence (p < 0.05) on the hydration and drying characteristics of walnuts. Moisture absorptions through the stem pore and the shell were equally important during the soaking process. Two to five minutes soaking process led to 2–4?h additional drying time. This study contributed valuable knowledge for the simulation and prediction of moisture transfer characteristics during the walnut soaking and drying processes. The findings from this study could potentially be applied to the walnut drying industry for more efficient processing     

Comparison of four pretreatments on the drying behavior and quality of taro (Colocasia esculenta L. Schott) slices during intermittent microwave vacuum-assisted drying

ABSTRACT

Effect of different pretreatments on drying behavior and quality of taro slices during intermittent microwave vacuum-assisted drying were investigated. Taro slices were subjected to the following pretreatments: blanching (B), blanching and freezing (B?+?F), blanching and immersion in maltodextrin solution (B?+?M), and blanching and immersion in maltodextrin solution and freezing (B?+?M?+?F). The reduction in amylose content, the increase in amylose to amylopectin ratio, the change of mechanical properties, the increase in sample bulk density and reduction in sample porosity and structure collapse during pretreatment processes facilitated heat and moisture transfer and quality changes during drying of taro slices. During drying process, all the pretreatment reduced the drying time of samples. The drying rate included a warming-up period and a falling rate period. The values of effective moisture diffusivity increased gradually as the moisture content decreased. The B?+?M?+?F pretreatment had the shortest drying time, the highest drying rate and the biggest effective moisture diffusivity. Moreover, for B?+?M?+?F pretreatment, the parameters including hardness, crispness, color, and microstructure had remarkable changes as compared with the other pretreatments.     

DRYING KINETICS AND PARTICLE RESIDENCE TIME IN SPRAY DRYING

ABSTRACT

A unique experimental equipment for extensive trials on the spray drying kinetics and particles residence time involving “in situ” analysis of the properties of continuous and dispersed phases was designed, built, and tested. Advanced experimental techniques (including laser techniques) to determine current parameters of spray drying process (temperature, humidity, moisture content) and current structure of spray (particle size distribution, particle velocities, etc.) were employed. Full scale spray drying tests of baker's yeast and maltodextrin enabled identification of the effect of process parameters on drying kinetics and spray residence time in the tower. Quantitative relationship describing spray drying kinetics as a function of atomization ratio and drying agent temperature were determined. The experimental results proved that spray residence time was controlled by atomization ratio and airflow rate. Drying kinetics in spray drying process is presented for the first time in the literature.     

Influence of blanching treatment and drying methods on the drying characteristics and quality changes of dried sardine (Sardinella gibbosa) during storage

The aim of this study is to examine the drying characteristics of blanched and unblanched sardine during indoor and open sun drying processes. Changes in temperature and relative humidity of the air during drying were recorded. The color, peroxide value (PV), thiobarbituric acid reactive substances (TBARS), free fatty acid (FFA) content, fatty acid composition, and sensory attributes of dried samples were also evaluated once a month for 5 months of storage. High drying rates were obtained in all samples at the start of drying and then decreased with increasing drying time. The highest drying rate and effective water diffusivity (D_eff) were observed in blanched sardine during open sun drying. Blanching treatment slowed down the FFA progression during product storage but adversely affected the color, PV, and TBARS content as well as sensory properties. Although sardine dried for a longer time under indoor drying conditions, it attained a stable moisture ratio that was lower than in open sun-dried samples. Indoor drying produced a quality stable product with less lipid oxidation and the desired moisture content, higher polyunsaturated fatty acids and sensory properties. Blanching treatment negatively affected the fish quality and is therefore not recommended for commercial sardine drying.     

Moisture transfer and stress development during high-temperature drying of Chinese fir

Abstract

The knowledge of moisture content (MC) and drying stress are crucial parameters to control the drying process and maintain the quality of dried wood. Herein, we investigated the pattern of moisture transfer and stress development in Chinese fir during the high-temperature drying process. The MC in each layer of lumber was separated into bound water and free water via nuclear magnetic resonance (NMR), and the drying and residual stress were measured using prong test method. There was different MC in each layer along the thickness, resulting in an MC gradient that initially increased and then decreased, which is consistent with the trend of drying stress. The T₂₁ peak indicating bound water shifted to the left especially when MC was below the fiber saturation point, signifying that the discharge of moisture became difficult with prolonged drying time. The ratio of bound water to free water was different in each layer, indicating that the moisture transfer was different along the thickness. Furthermore, the residual stress was greater than the corresponding drying stress though the disparity reduced gradually, which suggests that the MC gradient was the largest affecting factor for drying stress at high MC stages but decreased to some extent as the drying process continued.     

Effects of gluten and moisture content on water mobility during the drying process for Chinese dried noodles

To investigate the water mobility during a drying process, noodles were prepared with different gluten contents (10.0%, 12.5%, 15.0%, 17.5%, 20.0%, 22.5%, or 25.0%) and moisture contents (30%, 32%, or 34%), and dried on a food moisture analysis technology platform. Three types of water were deduced from relaxation signals measured by low-field nuclear magnetic resonance (LF-NMR) spectroscopy. Peak time T₂₂ increased with gluten and moisture content but decreased with drying time from 4–6 to 1.0–1.7?ms after 1.5?h. Gluten content mainly affected the drying rate (DR) in the middle drying period, whereas initial moisture content had an influence in the middle drying period and final drying period.     

Spouted bed drying of agricultural grains

It is shown that the presence of a “slotted draft tube” results in reduced air requirements for spouting and improved drying performance. Experimental data are presented on batch as well as continuous spouted bed drying of wheat, paddy, maize and peas. The variables studied are feed moisture content (Q_o), inlet air temperature (T_o), bed mass hold-up (M_p), inlet superficial air velocity (u_o) and bed diameter (D_c) in batch drying, and the above variables and solids feed rate (F_s) in continuous drying. The data on average overall drying rate, ?_m, in kg moisture evaporated per unit time per kg bed solids, is found to be correlatable as ?_m, = k (50Q_o + 0.118T_o ? 12.5) 10^?5, and the single parameter k is presented for wheat, paddy, maize and peas for both batch and continuous modes of spouted bed drying. The correlation obtained should be useful in dryer design for the grains studied as well as for other similar materials.