Isosteric Heat Of Desorption Research Articles

Drying Kinetics, Desorption Isotherms and Apparent Diffusivity of Sage Leaves

The use of fresh herb is limited in the food and pharmaceutical industry thats why the dry form of the herb is the one commonly used. To save the quality of medicinal and aromatic plants it is very important to provide optimum drying and storage conditions. The aim of this study is to determine and model the drying kinetics of sage leaves. Initially the desorption isotherms are determined for different temperatures (30, 45 and 60°C). The drying experiments were carried out in a convection oven at the same temperature range. For the desorption isotherms and the drying kinetics various models reported in the literature were used and from the statistical view, the Yanniotis and Blahovec and the Fick models fit well the results of desorption isotherms and the oven drying, respectively. The net isosteric heat of desorption of the sage leaves ranged from 6.86 to 63.45 kJ/mol. The total time of oven drying reduced substantially with an increase of the drying temperature. Effective moisture diffusivity values ranged from 1.1x10-12 to 3.7x10-12 m2/s and significantly affected by temperature. An Arrhenius relation with an activation energy value of 66.87 kJ/mol expressed effect of temperature on the diffusivity. Keywords: Desorption isotherms, Drying kinetics, Modelling, Effective diffusivity, Sage leaves

Desorption isotherms and isosteric heat of anaerobic fermentation residues

Abstract This paper presents the equilibrium desorption isotherms and the isosteric heat of sorption of a mixture containing mechanically dewatered fermentation residue (obtained from a blend of chicken, swine and cattle manure) used in biogas plants and corn spoiled silage in a ratio of 2:1. The moisture desorption isotherms of the fermentation residue were determined at 32 °C, 40 °C and 80 °C and in the relative humidity range of 0.057 / 1 using static gravimetric method. Mathematical equations were used to analyze the desorption data of Modified Henderson, Modified Halsey, Modified Oswin, Modified Chung–Pfost and Modified GAB models. The constants of the model equations were calculated by non-linear regression analysis. The Modified Henderson model fitted to the desorption isotherm data well. Using the proposed function, the final moisture content of the material can be determined as long as it can be dried in infinite time with the drying gas in the given conditions. The isosteric heat of desorption was calculated by using the Modified Henderson model in the studied temperature range based on the Clausius–Clapeyron equation. The isosteric heat varied between 46 kJ·mol−1 and 67 kJ·mol−1 at moisture levels 1.91

Sorption isotherms and isosteric heats of sorption of mint variety (Mentha viridis) leaves and stems: Experimental and mathematical investigations

Knowledge of sorption isotherms of agricultural products is necessary to control rehydration/dehydration and storage processes. The aim of this work is to determine moisture adsorption and desorption isotherms of Moroccan mint leaves and stems (Mentha viridis). The sorption isotherms of the plants was determined within the range of 0.0549–0.964 relative air humidity at three different temperatures (40, 50 and 60 ∘C) using saturated salt solutions method. Guggenheim, Anderson and Boer (GAB) model was used to describe the experimental data. The isosteric heats of desorption and adsorption were determined. Then, the isosteric heat of desorption was compared to the isosteric heat of adsorption and both decreased continuously with increasing the equilibrium moisture content. Finally, the comparison between the sorption net isosteric of mint leaves and stems was studied.

Hygroscopicity of baru (Dipteryx alata Vogel) fruit

ABSTRACT With the knowledge on the hygroscopic equilibrium of the baru (Dipteryx alata Vogel) fruit, the product can be adequately handled to maintain the moisture content at the levels recommended for safe storage. Thus, this paper aimed to determine the water desorption isotherms of baru fruits at temperatures of 20, 25, 30 and 35 °C, and water activity between 0.14 and 0.80, and obtain the values of isosteric heat of desorption as a function of the equilibrium moisture content of the product. The equilibrium moisture content was obtained using the static-gravimetric method. Modified Halsey was the best model recommended to represent the hygroscopicity of baru fruits. The recommended moisture contents for safe storage of baru fruits are not more than 19.9, 19.3, 18.6 and 18.0 (%, d.b.) for the respective temperatures of 20, 25, 30 and 35 °C. The integral isosteric heat of desorption increases as the water content decreases, leading to an increment in the energy required to remove water from the product.

Moisture sorption isotherms and thermodynamic properties of Oak wood (Quercus robur and Quercus canariensis): optimization of the processing parameters

The aim of this paper was to determine the moisture desorption isotherms and essentials thermodynamic properties of two Oak wood varieties. Desorption isotherms were measured using a static gravimetric method at 50, 60, 70 and 80 °C within the range of 5–90 % relative humidity. The equilibrium moisture content decreased with increasing temperature and decreased with decreasing relative humidity at a constant temperature. The ‘Thermodynamic’ sorption equation was found to be the best for describing the experimental moisture sorption isotherms of woods within the range of temperature and water activity investigated. The Fiber saturation point, deduced from the ‘Thermodynamic’ model parameters, depends on the temperature and varying from 22.6 to 54.4 (% kg water/kg dry matter). Isosteric heat of desorption and differential entropy were calculated by applying Clausius–Clapeyron equation to the desorption data fitted by the ‘Thermodynamic’ model. The isosteric heat of desorption and the differential entropy decreased with increasing moisture content according to an exponential law equation and varying from 2.03 to 31.14 kJ/mol and from 73.98 to 4.34 J/(mol K), respectively. The linear relationship between differential enthalpy and entropy satisfied the enthalpy–entropy compensation theory. The sign of Gibbs free energy was found to be positive (+283 J/mol) and (+97 J/mol) for Quercus robur and Quercus canariensis, respectively. The isokinetic temperature was found to be greater than the harmonic temperature. Based on the enthalpy–entropy compensation theory, it could be concluded that the moisture desorption isotherm of Oak wood is a non-spontaneous and enthalpy-controlled process.

Moisture Sorption Isotherms And Thermodynamic Properties Of Urtica Dioica Leaves

Urtica dioica is a Moroccon endemic plant of used for its virtues in traditional medicine. Thus, it is necessary to study the effect of preservation processes on the storage conditions of the plant. The static gravimetric method was used to determine sorption isotherms of Urtica dioica leaves at three temperatures (40, 50 and 60 °C) and in the range of water activity ( w a ) ranging from 0.0572 to 0.898. Six mathematical models were used to fit the experimental data. The Enderby and Peleg models were found to be the most suitable for describing the sorption curves. The optimal water activity for conservation of Urtica dioica leaves was determined. Isosteric heats of desorption and adsorption were calculated by applying the Clausius- Clapeyron equation to the sorption isotherms at different temperatures; it decreased with increasing moisture content. A linear relation exists between the enthalpy and entropy of the sorption reaction.

Higroscopicidade das sementes de sorgo-sacarino

RESUMO Objetivou-se, neste trabalho, determinar as isotermas de dessorção das sementes de sorgo-sacarino para diversas condições de temperatura e atividades de água e obter o calor isostérico de dessorção. As sementes foram submetidas à secagem em estufa com ventilação de ar forçada a 40 °C até atingir os teores de água de aproximadamente 24; 19; 15; 12 e 8 (% b.s.). As isotermas de dessorção das sementes de sorgo-sacarino foram obtidas pelo método estático indireto, sendo a atividade de água (aw) determinada por meio do equipamento Hygropalm Model Aw 1. Para o controle da temperatura, utilizou-se de B.O.D., regulada a 10; 20; 30 e 40 °C. Aos dados experimentais, foram ajustados modelos matemáticos frequentemente utilizados para a representação da higroscopicidade de produtos vegetais. Verificou-se que o modelo de Chung-pfost é o que melhor representa a higroscopicidade das sementes de sorgo-sacarino. O calor isostérico integral de dessorção aumenta com a redução do teor de água, sendo que os valores, para as sementes de sorgo-sacarino na faixa de teor de água de 7,9 a 28,0 (% b.s.), variam de 3.136,54 a 2.493,33 kJ.kg-1.

Desorption isotherms and isosteric heat of 'cajuzinho-do-cerrado' achenes

ABSTRACT The objective of this study was to determine the desorption isotherms of 'cajuzinho-do-cerrado' achenes (Anacardium humile St. Hil.) in various conditions of temperature and water activity, as well as to select the one that best represents the phenomenon and to determine the isosteric heat of desorption. The fruits were collected at the Emas National Park, in the municipality of Mineiros-GO, Brazil, pulped and then subjected to drying in silica gel at temperature of 25 ± 2 °C until the moisture contents of 17.6, 13.6, 11.1, 8.7 and 5.3 (d.b.%). After drying, the desorption isotherms were determined by the indirect static method. The water activity (Aw) was determined at different temperatures, and the achenes were placed in a B.O.D. chamber, regulated at 10, 20, 30 and 40 °C. Data of hygroscopic equilibrium moisture content were fitted to different mathematical models through non-linear regression analysis, using the Gauss-Newton method. The Copace model was the one that best represented the hygroscopicity of 'cajuzinho-do-cerrado' achenes, while the integral isosteric heat of desorption of 'cajuzinho-do-cerrado' achenes for the moisture content range of 4.51 to 13.40 (% d.b.) varied from 2,734.82 to 2,548.49 kJ kg-1.

Adjustment of mathematical models and quality of soybean grains in the drying with high temperatures

ABSTRACT The aim of this study was to evaluate the influence of the initial moisture content of soybeans and the drying air temperatures on drying kinetics and grain quality, and find the best mathematical model that fit the experimental data of drying, effective diffusivity and isosteric heat of desorption. The experimental design was completely randomized (CRD), with a factorial scheme (4 x 2), four drying temperatures (75, 90, 105 and 120 ºC) and two initial moisture contents (25 and 19% d.b.), with three replicates. The initial moisture content of the product interferes with the drying time. The model of Wang and Singh proved to be more suitable to describe the drying of soybeans to temperature ranges of the drying air of 75, 90, 105 and 120 °C and initial moisture contents of 19 and 25% (d.b.). The effective diffusivity obtained from the drying of soybeans was higher (2.5 x 10-11 m2 s-1) for a temperature of 120 °C and water content of 25% (d.b.). Drying of soybeans at higher temperatures (above 105 °C) and higher initial water content (25% d.b.) also increases the amount of energy (3894.57 kJ kg-1), i.e., the isosteric heat of desorption necessary to perform the process. Drying air temperature and different initial moisture contents affected the quality of soybean along the drying time (electrical conductivity of 540.35 µS cm-1g-1); however, not affect the final yield of the oil extracted from soybean grains (15.69%).

Determinação do equilíbrio higroscópico e do calor isostérico do bagaço de cana-de-açúcar

Com o intuito de analisar a dinâmica de sorção e a energia necessária para a vaporização da água na desidratação do bagaço da cana-de-açúcar, o presente trabalho teve como objetivo a determinação de seu equilíbrio higroscópico e calor isostérico, a partir de diferentes condições controladas de temperatura (20; 40; 55 e 70 ºC) e umidade relativa do ar (10; 12; 17; 18; 30; 40; 45; 50; 60 e 85%), assim como avaliar, entre diversos modelos descritos na literatura, o que melhor represente o comportamento de sorção. Como resultado, foi verificado que o modelo que melhor se ajustou aos dados empíricos do processo de sorção do bagaço de cana-de-açúcar foi o Modelo de Henderson Modificado, com coeficiente de determinação de 0,972, enquanto o calor isostérico integral de dessorção variou de 2524,09 a 2951,95 kJ kg-1, para os teores de água de equilíbrio de 0,014 a 0,170 b.s., respectivamente.

Desorption Isotherms and Isosteric Heats of Fermented Cocoa Beans (Theobroma cocoa)

<p>Water desorption isotherms of fermented cocoa beans from Ivory Coast were determined using the gravimetric static method of saturated salts solutions at 30 °C, 40 °C and 60 °C, and isosteric heats of desorption were calculated from Clausius-Clapeyron equation. The experimental data were fitted to several isotherm equations. The best fittings were obtained for the BET equation for aw<0.50 (with an average mean relative deviation (MRD) value of 1.56%) and for the Harkins-Jura equation for aw?0.50 (with an average value MRD equation of 4.17%). The isotherms obtained at 30 °C and 40 °C are practically coincident and overlapped for aw below 0.40. Fermented cocoa beans presented a monolayer moisture content of 0.083 dry basis (d.b.) at 30 °C and this value decreases with increasing temperature. The net isostheric heats of desorption for fermented cocoa beans for the temperature range studied (30-60 °C) were estimated as a function of moisture content. The maximum net isosteric heat of desorption for fermented cocoa beans was estimated at around 13.51 kJ/mol corresponding to a moisture content value of 2.85%. The energy requirement for maintaining the moisture content low of 8.7% (d.b.) or 8% wet basis (w.b.) for safe storage of this product or for reducing the moisture content during drying was estimated at around 9.58 kJ/mol.</p>

ISOTERMAS E CALOR ISOSTÉRICO DAS SEMENTES DE PINHÃO-MANSO - DOI: 10.13083/1414-3984/reveng.v23n1p9-18

O presente trabalho foi conduzido no Laboratório de Pós-Colheita de Produtos Vegetais do Instituto Federal Goiano, Câmpus Rio Verde, com os objetivos de obter as isotermas de dessorção e os valores de calor isostérico integral de dessorção das sementes de pinhão-manso. Os frutos foram colhidos e, posteriormente, procedeu-se ao beneficiamento das sementes, obtendo-se teor de água inicial de 20,9 % (b.s.). Em seguida, foram submetidas à secagem em estufa com ventilação de ar forçada a 40 °C até atingir os teores de água de 13,6; 11,1; 8,7; 6,4 % (b.s.). As isotermas de dessorção das sementes de pinhão-manso foram obtidas pelo método estático indireto, sendo a atividade de água (aw) determinada por meio do equipamento Hygropalm Model Aw 1. Para o controle da temperatura, utilizou-se uma B.O.D., regulada a 10, 20, 30 e 40 °C. Aos dados experimentais, foram ajustados os modelos matemáticos frequentemente utilizados para representação da higroscopicidade de produtos agrícolas. O modelo de Sigma Copace é o que melhor representa a higroscopicidade das sementes de pinhão-manso. O calor isostérico integral de dessorção aumenta com a redução do teor de água, sendo que os valores, para as sementes de pinhão-manso, na faixa de teor de água de 5,6 a 13,4 % (b.s.), variam de 3035,61 a 2631,89 kJ kg-1.

<b>Equilibrium isotherms and isosteric heat of pepper variety bico (<i>Capsicum chinense</i> Jacq.)

During processing, the storage requires attention, since the moisture content is a major factor that influences the quality of raw materials and manufactured products. Equilibrium isotherm represents the relationship between the material moisture content and water activity for a given temperature, providing important information for use in industrial operations such as drying and storage. This study aimed to determine the equilibrium isotherms of peppers variety bico (Capsicum chinense Jacq.) in whole and paste forms, calculate the isosteric heat of desorption and determine its behavior. To this end, we used six mathematical models to fit experimental data of desorption isotherms. The GAB model fitted well the experimental desorption data of pepper variety bico, in paste and whole forms. The isosteric heat obtained for the whole form was higher than for the paste form.

<b>Thermodynamic properties of water desorption of forage turnip seeds

The purpose of this study was to determine the thermodynamic properties of the process of water sorption in forage turnip seeds. The equilibrium moisture content of forage turnip seeds was determined by the gravimetric-dynamic method for different values of temperature and water activity. According to the results, increasing the moisture content increases the energy required for the evaporation of water in forage turnip seeds, and the values of integral isosteric heat of desorption, within the moisture content range of 3.33 to 11.30 (% d.b.), varies from 4,222.70 to 2,870.34 kJ kg-1. With the elevation in the equilibrium moisture content, there is an increase in differential entropy and Gibbs free energy, which has positive values, demonstrating non-spontaneity in the process of desorption in the seeds. The theory of enthalpy-entropy compensation can be satisfactorily applied to the sorption phenomenon, and the process of water desorption of forage turnip seeds is controlled by enthalpy.

Effect of convective-, vacuum- and freeze drying on sorption behaviour and bioactive compounds of lemon balm (Melissa officinalis L.)

The influence of drying method on the moisture sorption isotherms of lemon balm leaves (Melissa officinalis L.) was investigated at 25 and 50°C using the hygrometric method. Equilibrium relative humidity (aw) in the head space of the samples was measured at several moisture contents (0.04≤Xe≤0.45kg/kg dry basis) developed by convective-, vacuum- and freeze drying. The essential oil constituents were determined by GC/FID, while rosmarinic acid was analysed by HPLC. The experimental sorption data was modelled by the modified Halsey equation. The isosteric heat of desorption (Qst) was calculated as a function of moisture content and temperature directly by the model in which the Clausius–Clapeyron equation has been incorporated. The different sorption characteristics were ascribed to modifications in the matrix structure caused by the mode of drying. Sublimation of water vapour during freeze drying resulted in a higher sorption capacity for 0.1≤aw<0.8. Qst increased as moisture content and temperature decreased, increasing sharply for Xe below 0.2kg/kg. Qst of freeze dried leaves at Xe of 0.10kg/kg was significantly higher than that of vacuum- and air dried material. Geranial content was higher after freeze- or convective drying, while vacuum drying led to lower contents of neral and geranial. Freeze- and vacuum dried samples exhibited greater rosmarinic acid content than the hot-air dried leaves of M. officinalis.

Moisture sorption isotherms and thermodynamic properties of Jack pine and palm wood: Comparative study

Currently, the impact of the controlled climate on moisture sorption isotherms of Jack pine and palm wood still remains unclear. The aim of this paper was to determine and compare the moisture sorption isotherms and thermodynamic properties of Jack pine and palm wood. The equilibrium moisture content of wood samples was measured within the range of 5 to 90% relative humidity at 50°C, 60°C, 70°C and 80°C. For both wood varieties, the equilibrium moisture content decreased with increasing temperature, however the effect of temperature is very remarkable for palm wood. The isosteric heat of desorption, the enthalpy and the entropy decreased with the increase of the moisture content. A good straight line correlated differential enthalpy and entropy, according to the enthalpy–entropy compensation theory. The spreading pressure decreased with increasing temperature at a given water activity, and increased with increasing water activity at a constant temperature. Predictive correlations of the two wood properties, based on experimental methods and physical laws, were established and compared.

Desorption isotherms for fresh beef: An experimental and modeling approach

Desorption isotherms for fresh beef were determined at 30, 40 and 50°C by the static gravimetric method. The resulting isotherms exhibited a type II sigmoid shape. The BET, GAB and Halsey models were used to fit these experimental data. The GAB model was most accurate for all temperatures and all levels of water activity, followed by the BET and Halsey models. The temperature dependence of GAB constants was estimated. The isosteric heat of desorption and its evolution in relation to moisture content were calculated using Clausius–Clapeyron equations. The monolayer moisture content was determined using the GAB model: it decreased as the temperature increased. The density of bound water, the number of adsorption sites, the sorption surface area and the percentage of bound water were calculated using the Caurie equation: all these quantities decreased as the temperature increased. The Kelvin and Halsey equations were used for calculation of pore size, which increases with an increase in moisture levels and sorption temperature.

Effect of sodium chloride, sodium nitrite and temperature on desorption isotherms of previously frozen beef

Moisture desorption isotherms of beef were determined in the relative humidity range of 23 to 90% at 5, 15 and 25 °C and at 2.5% NaCl and 2.5% NaCl + 150 ppm NaNO 2 content. Desorption isotherms were found to be typical type II sigmoid. The water content at equilibrium was higher in beef with NaCl and NaCl + NaNO 2 than control samples. Experimental data were fitted to various mathematical models and it was found that the Peleg model was best in describing the equilibrium moisture content relationship for beef samples over the entire range of temperatures. The net isosteric heat of sorption was estimated from equilibrium desorption data, using the Clausius–Clapeyron equation. Isosteric heats of desorption were found to increase with decreasing moisture content.

Moisture Isotherms and Heat of Desorption of Pistachio (Kaleghochi Var.)

Moisture desorption isotherms (EMC/ERH) of Iranian pistachio variety of Kaleghochi were determined at 45, 60, 75 and 90°C using the standard gravimetric static method over a range of relative humidity from 10% to 82%. The experimental desorption curves were fitted under the following different equations: modified Henderson, modified Chung-Pfost, modified Halsaey and modified Oswin. The modified Oswin model was found to be a suitable model for describing the desorption isotherms of pistachio. The net isosteric heat of desorption of pistachio was also determined from the equilibrium data at different temperatures. Furthermore, the net isosteric heat of desorption of pistachio varied between 13.2 – 2.2kJ/mol at moisture varying between 2.5 – 15% (d.b).

Isotermas de dessorção de filé de bonito (Sarda sarda) desidratado osmoticamente e defumado

O presente trabalho teve como objetivo a obtenção de isotermas de dessorção de filés de bonito (Sarda sarda), previamente salgados a vácuo e defumados com fumaça líquida. As isotermas foram obtidas a quatro temperaturas (5, 25, 40 e 60 ºC) em condições de dessorção, através do método gravimétrico estático, com soluções salinas saturadas. Os dados experimentais foram ajustados a quatro modelos da literatura (BET linearizado, GAB, Henderson e Oswin modificado). Os resultados mostraram que as isotermas tomaram forma sigmoidal de tipo II e que o modelo de Guggenheim-Anderson-deBoer (GAB) foi aceitável para modelar os dados experimentais. O calor isostérico de dessorção, um parâmetro necessário para simular e projetar adequadamente o secador, também foi calculado e pode ser representado através de um modelo matemático simples, em função da umidade de equilíbrio.