Isosteric Heat Of Sorption Research Articles

Experimental Determination and Modeling of Sorption Isotherms of Tropical Fruits: Banana, Mango, and Pineapple

Sorption isotherms of banana, mango, and pineapple have been experimentally found at 40, 50, and 60°C by use of the salt method for a range of water activities from 0.056 to 0.85. The sorption capacity of these fruits increases with the temperature for a given water activity. The experimental curves have been simulated by the GAB and BET models. The BET model ensures a better representation of the experimental results for water activity lower than 0.35 with maximum deviation of 1.0, 1.5, and 2.0 kgw · kgdm−1 for respectively banana, mango and pineapple. On the contrary, the GAB model enables the representation of the whole desorption isotherms and the estimation of the water content corresponding to monolayer saturation and of the isosteric heat of sorption. It also takes into account the temperature effect. The isosteric sorption heat of the three studied products are deduced from experimental results and empirical correlations are proposed leading to a satisfactorily representation.

Enthalpy-entropy compensation based on isotherms of mango

Moisture equilibrium data of mango pulp were determined using the static gravimetric method. Adsorption and desorption isotherms were obtained in the range of 30-70 ºC, to water activities (a w) from 0.02 to 0.97. The application of the GAB model to the experimental results, using direct nonlinear regression analysis, provided agreement between experimental and calculated values. The net isosteric heat of sorption was estimated from equilibrium sorption data, using the Clausius-Clapeyron equation. Isosteric heats of sorption were found to increase with increasing temperature and could be well adjusted by an exponential relationship. The enthalpy-entropy compensation theory was applied to sorption isotherms and plots of deltaH versus deltaS provided the isokinetic temperatures, indicating an enthalpy controlled sorption process.

The fitting of various models to water sorption isotherms of tea stored in a chamber under controlled temperature and humidity

The moisture sorption characteristics of tea stored in a chamber regulated by an atomizing humidification system were investigated at 25, 35 and 45 °C for water activity ranging from 0.1 to 0.9. The sorption isotherms of tea were typical type II sigmoidal curves according to BET classification. In both adsorption and desorption, an increase in temperature resulted in lower equilibrium moisture contents at corresponding values of water activity. The sorption isotherms exhibited hysteresis over the whole water activity range. GAB, BET, Henderson, Iglesias and Chirife, Oswin, Peleg, Smith and Caurie models were applied for analysing the experimental data. Nonlinear regression analysis was used for the determination of the parameters in the equations. Estimated parameters and fitting ability for sorption models were evaluated. The Peleg model was found to be the most suitable for describing the relationship between equilibrium moisture content and water activity for the whole range of temperatures and relative humidities studied. The surface area of monolayer was calculated. The BET equation was solved for the monolayer moisture content and the corresponding a w values at which monolayer forms were presented. Sorption isotherm data were used to determine the thermodynamic functions such as isosteric heat of sorption, sorption entropy, spreading pressure, net integral enthalpy and entropy. The Clausius–Clapeyron equation was used to evaluate the isosteric heats of sorption. The isosteric heats of sorption and sorption entropy decreased with increasing moisture content. The heat of desorption was little higher than that of adsorption at low moisture content. The enthalpy–entropy compensation theory could be successfully applied to water sorption by tea. This theory showed that the moisture sorption of tea was governed by enthalpy-controlled mechanisms. The spreading pressure increased with increase in water activity and decreased with increasing temperature. The net integral enthalpy decreased with moisture content while the net integral entropy increased.

Obtaining the isosteric sorption heat directly by sorption isotherm equations

The latent heat of vaporization of water in foodstuffs is an important factor for the design of drying equipments. The thermal properties can be calculated by equations that are derived directly from isotherm equations by the Clausius–Clapeyron thermodynamic theory. The sorption isosteric heat equation can be expressed as a function of temperature and moisture content. The sorption isosteric heat computed from the water activity ( A w) sorption isosteric model developed in this study showed good agreements with the values calculated by the Clausius–Clapeyron equation or from calorimetric method. As the A w model had the better fitting performance, the sorption isosteric heat values calculated from the water activity sorption isosteric model were more close to the sorption isosteric heat values obtained from other methods.

Sorption isotherms, GAB parameters and isosteric heat of sorption

AbstractThe diffusion–sorption drying model has been developed as a physics‐based way to model the decreasing drying rate at low moisture contents. This new model is founded on the existence of different classes of water: free and bound water. The transition between these classes and the corresponding thermodynamics form distinct components of the drying model. This paper shows that the characteristics of the different classes of water and of the transition between them can be deduced from the GAB sorption isotherm. The parameters in the GAB sorption isotherm support the theory of localised sorption, establishing the existence of different classes of water. Moreover, the sorption mechanism retrieved from the GAB parameters is in accordance with the sorption mechanism, which is obtained from the moisture dependence of the net isosteric heat of sorption. This holds for experimental sorption data of corn and starch as well as for literature data on five vegetables and four fortified cassava products. An extremum in the net isosteric heat of sorption coincides with the transition between bound and free water, and the partition moisture content corresponds with the monolayer value derived from the GAB equation. This confirms that the GAB monolayer value can be chosen as model boundary between bound and free water. Moreover, it reveals that this method can be developed into a technique to estimate the bound water content in foods. Copyright © 2005 Society of Chemical Industry

Moisture Sorption Isotherms for Crushed Chillies

The moisture isotherms of crushed chillies stored in a chamber the relative humidity of which is controlled by an atomising humidifier were determined at 25, 35 and 45 °C and within the range of 0·1–0·8 water activity. The sorption isotherms of crushed chillies exhibited type III behaviour according to BET classification. Sorption capacity decreased with increasing temperature. The sorption isotherms of crushed chillies exhibited hysteresis. The moisture sorption data were used to fit Guggenheim–Anderson–de Boer, Brunauer–Emmett–Teller, Henderson, Oswin, Peleg, Crapiste and Rotstein, Caurie models, and constants of equations tested were determined by non-linear regression analysis. Among the various models tested to interpret sorption isotherms, one was selected which presented best statistical indicators. The Peleg and the Crapiste–Rotstein models correctly fit the whole set of data, for water activity ranging from 0·1 to 0·8, but the Crapiste and Rotstein model was preferred since the values of its parameter were more realistic. The surface area of the monolayer was calculated. The water activities corresponding to the monolayer values were determined. The thermodynamic functions such as isosteric heat of sorption, sorption entropy, spreading pressure, net integral enthalpy and entropy were determined by means of sorption isotherm data. The isosteric heats of desorption, calculated from the slope of plot of logarithm of water activity versus reciprocal of temperature, were higher than the isosteric heats of adsorption.

Sorption isotherms of Chenopodium ambrosioides leaves at three temperatures

Moisture equilibrium data of Ch. ambrosioides leaves by desorption and adsorption were determined at 30, 40, and 50 °C by the gravimetric static method. The equilibrium was achieved after 12 days for desorption and 12 days for adsorption at water activities ranging from 5% to 90%. The hysteresis effect was not distinctly observed in the range of temperatures tested. Six mathematical models were used to fit the experimental data. The modified Halsey model was the best model describing the equilibrium moisture data for desorption. The GAB and modified Halsey models were the most suitable models for estimating adsorption isotherms. The net isosteric heats of sorption of water were calculated by applying the Clausius–Clapeyron equation to the sorption isotherms at different temperatures.

Sorption isotherms of turnip top leaves and stems in the temperature range from 298 to 328 K

Data for desorption and adsorption isotherms at 298, 313 and 328 K of both fresh and vacuum dehydrated turnip tops were experimentally obtained and compared. Analysis of the isotherms for leaves and stems showed no significant differences. Sorption isotherms at 298 K can be classified as Type II and at higher temperatures as Type III. The experimental data were fitted by the Peleg model with satisfactory results. The desorption isotherms of leaves clearly showed the influence of temperature, decreasing the moisture content at a fixed water activity value with higher temperatures. With the other isotherms, the effect of the temperature was less important. The presence of hysteresis cycles was detected at 298.1 K, but at the higher temperatures the hysteresis cycles were not practically significant (for both stems and leaves). The isosteric heat of sorption was evaluated, being higher for desorption processes and at lower moisture contents and temperatures.

Effect of temperature on moisture desorption isotherms of kheer

Desorption isotherms of kheer, a rice based partially heat concentrated and sweetened Indian milk dessert, were obtained in the temperature range of 10–40°C. The desorption curves exhibited sigmoid shape corresponding to type II, typical of many foods. There was generally a negative temperature effect on EMC at low aw, but curves at 25°C and 40°C showed inversion above water activity of 0.60 implying a higher equilibrium moisture content at higher temperature. Of the five sorption models tested, the GAB model gave the best fit at all the three temperatures. Besides monolayer moisture, properties of sorbed water viz. number of adsorbed monolayers, bound or nonfreezable water, density of sorbed water and surface area of sorption were also obtained. Isosteric heat of sorption obtained by applying Clausius–Clapeyron equation decreased exponentially with the increasing moisture content.

Modelling of water sorption isotherms of macaroni stored in a chamber under controlled humidity and thermodynamic approach

The moisture sorption isotherm data of macaroni stored in a chamber the relative humidity of which is regulated by an atomizing humidifier were determined at 25, 35 and 45 °C and relative humidity ranging from 10% to 90%. The sorption isotherms were sigmoidal in shape (Type II). At a given water activity, moisture content decreased with increasing temperature. Hysteresis between adsorption and desorption isotherms was shown over almost the range of 0.1–0.9 water activity. The experimental sorption data were applied to various isotherm equations (GAB, BET, Henderson, Iglesias and Chirife, Oswin, Peleg, Smith and Caurie). Several statistical tests such as r 2, P%, average residual and RMSE were adopted to evaluate the fitting performance of the models. The Peleg equation was best for characterizing the sorption behaviour of macaroni for the whole range of temperatures and relative humidities studied. The surface area of monolayer and the water activities corresponding to the monolayer values were determined. The thermodynamic functions determined in this study included isosteric heat of sorption, sorption entropy, spreading pressure, net integral enthalpy and entropy. The Clausius–Clapeyron equation facilitated satisfactory determination of the isosteric heats of sorption, which were found to increase with decreasing moisture content. The spreading pressures increased with increasing water activity. The net integral enthalpy decreased with moisture content. Net integral entropy was negative in value and increased with increase in moisture content.

Moisture sorption characteristics of chickpea flour

Moisture equilibrium data (adsorption and desorption) of chickpea flour were determined using the static gravimetric method of saturated salt solutions at four storage temperatures: 10, 20, 30 and 40 °C. The range of water activities for each temperature was between 0.11 and 0.85. Equilibrium moisture content decreased with the increase in storage temperature at any given water activity. The experimental data were fitted to four mathematical models (modified Chung–Pfost, modified Oswin, modified Halsey and modified Henderson). The monolayer moisture content was estimated from sorption data using the Brunauer–Emmett–Teller (BET) equation. The isosteric heats of sorption were evaluated using Clausius–Clapeyron equation.

Sorption of Benzene in CoAlPO4-5 Molecular Sieves

The sorption isotherms of benzene in CoAlPO4-5 at temperatures of 323–399 K were measured by the gravimetric technique. The sorption capacity and isosteric heat were estimated and interpreted on the basis of the pore topologies of the molecular sieves. The isotherms followed the typical Type-I shape, and could be well fitted with Langmuir and Dubinin-Radushkevich equations. The initial heats of sorption at zero coverage could be well explained by the surface curvature model. The isosteric heats of sorption varied with the coverage, which may suggest a shift of molecular packing manner in the sorbed phase.

Moisture sorption isotherms of black gram nuggets ( bori) at varied temperatures

Adsorption–desorption behaviours of bori nuggets prepared from black gram pulse were studied at different relative humidity (ERH) ranging from 0% and 85% and temperatures at 25, 35, 45 and 55 °C following the static equilibriation technique using the saturated solutions of various salts. These data were analysed and fitted Guggenheim–Anderson–de Boer (GAB) sorption model. The temperature dependence of the respective coefficients revealed that the GAB model is acceptable ( r 2⩾0.99; M RES ⩽ 0.481) in describing the EMC–ERH relationships of the nuggets. The M 0, C and K values were in the range of 8.35–8.24, 82.28–41.04 and 0.74–0.72 for adsorption. The corresponding values for desorption are 9.52–8.77, 107.83–39.68 and 0.74–0.73. The net isosteric heats of sorption estimated from the Clausius–Clapeyron equation, decreased exponentially from 5.94 to 2.52 kJ mol −1 with the increase in moisture content of the sample 5–24% (db) for adsorption. The corresponding values were 8.42 to 3.42 kJ mol −1 for desorption.

Moisture sorption isotherms and heat of sorption of bitter orange leaves ( Citrus aurantium)

Moisture adsorption–desorption isotherms (EMC/e.r.h.) of Citrus aurantium leaves were determined at 30, 40, and 50 °C using the standard gravimetric static method over a range of relative humidity from 0.05 to 0.9. The experimental sorption curves were fitted by six equations: modified Chung–Pfost, modified Halsey, modified Oswin, modified Henderson, modified BET, and GAB. The modified Halsey and the modified BET models were found to satisfactorily describe the desorption isotherms of Citrus aurantium leaves. For adsorption, GAB and modified BET models were the best fit. The net isosteric heats of sorption of water were determined from the equilibrium data at different temperatures.

Moisture sorption characteristics of rice flour.

Moisture equilibrium data (adsorption and desorption) of rice flour were determined using the static gravimetric method of saturated salt solutions at three storage temperatures: 10, 20 and 30 degrees C. The range of water activities for each temperature was from 0.11 to 0.85. Equilibrium moisture content decreased with increase in storage temperature at any given water activity. The experimental data were fitted to four mathematical models (modified Oswin, modified Halsey, modified Chung-Pfost and modified Henderson). The monolayer moisture content was estimated from sorption data using the Brunauer-Emmett-Teller (BET) equation. The isosteric heats of sorption were evaluated using Clausius-Clapeyron equation.

Moisture sorption isotherms and modeling for pre-cooked flours of pigeon pea ( Cajanus cajans L millsp) and lima bean ( Canavalia ensiformis)

The moisture sorption isotherms for pre-cooked flours of pigeon pea and lima bean were determined using the static method of saturated salt solution in a range of water activity from 0.20 to 0.97 at 18, 28, 38, and 48 °C and fitted by the Henderson, Chung and Pfost, and Ratti, Crapiste and Rotstein models. The moisture content of the material tested varied from 8% to 60% (dry basis). The agreement between experimental and predicted values is good for three models; nevertheless the Ratti, Crapiste and Rotstein (1989) model gave a better fit of the experimental data. The net isosteric heat of water sorption, estimated by applying Claussius-Clapeyron equation to sorption isotherms at mean temperatures of 23, 33 and 43 °C, was found to be different for pigeon pea flour and lima bean flour. The net isosteric heat of sorption for two flours increased with increasing moisture content and decreased with increasing temperature.

Sorption isotherms and isosteric heat of sorption for grapes, apricots, apples and potatoes

Moisture sorption isotherms of grapes, apricots, apples and potatoes were determined at 30°C, 45°C, and 60°C using the standard, static-gravimetric method. Six two-parameter and five three-parameter sorption models were tested to fit the experimental data. A nonlinear regression analysis method was used to evaluate the constants of the sorption equations. The Halsey equation gave the best fit to the experimental sorption data for all materials tested over the range of temperatures and water activities investigated. The GAB model gave also the closest fit to the sorption data for potatoes and grapes. The agreement between experimental and predicted values of these models was found to be satisfactory. The isosteric heat of desorption and adsorption of water determined from the equilibrium data using the Clausius–Clapeyron equation.

Moisture Sorption Isotherms of Walnut Flour at Several Temperatures

ABSTRACTMoisture equilibrium data (adsorption and desorption) of walnut flour were determined using the static gravimetric method of saturated salt solutions at three temperatures 10, 25, and 40 °C. The range of water activities for each temperature was between 0.11 to 0.85. Equilibrium moisture content decreased with the increase in storage temperature at any given water activity. The experimental data were fitted to four mathematical models (modified Oswin, modified Halsey, modified Chung-Pfost and modified Henderson). The monolayer moisture content was estimated from sorption data using the Brunauer-Emmett-Teller (BET) equation. The isosteric heats of sorption were evaluated using Clausius-Clapeyron equation.

Temperature effect on phosphate sorption by iron hydroxide

The ion exchange sorption of phosphate on Fe(OH)3 is studied as a function of temperature (25–55°C) and concentration (1.49–3.23 mmol1‐1). The mechanism of sorption is observed to be the exchange of OH anions from the adsorbent surface by those of H2FO4 ‐ and HPO4 2‐ from the aqueous solution. An equation developed in the Langmuir formalism is used to explain the ion exchange sorption of phosphate on Fe(OH)3 The isosteric heat of sorption illustrates that the uptake of phosphate on Fe(OH)3 takes place through an anion exchange process, involving both the phosphate anions H2PO4 ‐ and HPO4 2‐. Further, at higher temperature the solid prefers the HPO4 2‐ anions forming a binuclear bridging complex.

Gas sorption and transport in syndiotactic polystyrene with nanoporous crystalline phase

In this contribution we analyse sorption and transport of several gases in semicrystalline syndiotactic polystyrene with nanoporous crystalline δ form. Investigation was performed on amorphous samples and on samples characterized by different degrees of crystallinity. Sorption isotherms of carbon dioxide, nitrogen and oxygen in the crystalline phase have been determined starting from experimental results obtained for semicrystalline and amorphous samples. Corresponding isosteric heats of sorption were evaluated for the crystalline and amorphous phase. Permeation tests were also performed to gather information on mass transport properties of semicrystalline samples, evaluating average diffusivities of carbon dioxide and oxygen, in the limit of small concentrations as function of degree of crystallinity.