Net Isosteric Heat Research Articles

Analysis of adsorption–desorption moisture isotherms of rosemary leaves

The moisture sorption isotherms data of rosemary leaves were investigated via the gravimetric static method at three different temperatures (30, 40 and 50°C). The sorption isotherms obtained were of sigmoid shape and of type II of Brunauer classification. From various mathematical models used for our experimental data, The GAB model best described the experimental data.The sorption data were analyzed for determination of monolayer moisture content (3.4–9.7%), and properties of sorbed water. Analysis of sorption isotherm by the model of the Yanniotis and Blahovec shows that the contribution of surface adsorption in relation to solution water at each water activity was higher in desorption than in adsorption The net isosteric heat of desorption and adsorption of the rosemary leaves ranged respectively from 1.77 to 47.37kJ/mol and from 0.85 to 26.86kJ/mol. The enthalpy–entropy compensation suggested that the sorption mechanism involved was enthalpy driven. The net integral enthalpy of adsorption and desorption decreased with increasing equilibrium moisture content.The integral entropy increased with moisture content, but was of negative value.

Experimental study of water desorption isotherms and thin-layer convective drying kinetics of bay laurel leaves

The aim of this work is to determine the desorption isotherms and the drying kinetics of bay laurel leaves (Laurus Nobilis L.). The desorption isotherms were performed at three temperature levels: 50, 60 and 70 °C and at water activity ranging from 0.057 to 0.88 using the statistic gravimetric method. Five sorption models were used to fit desorption experimental isotherm data. It was found that Kuhn model offers the best fitting of experimental moisture isotherms in the mentioned investigated ranges of temperature and water activity. The Net isosteric heat of water desorption was evaluated using The Clausius–Clapeyron equation and was then best correlated to equilibrium moisture content by the empirical Tsami’s equation. Thin layer convective drying curves of bay laurel leaves were obtained for temperatures of 45, 50, 60 and 70 °C, relative humidity of 5, 15, 30 and 45 % and air velocities of 1, 1.5 and 2 m/s. A non linear regression procedure of Levenberg–Marquardt was used to fit drying curves with five semi empirical mathematical models available in the literature, The R2 and χ2 were used to evaluate the goodness of fit of models to data. Based on the experimental drying curves the drying characteristic curve (DCC) has been established and fitted with a third degree polynomial function. It was found that the Midilli Kucuk model was the best semi-empirical model describing thin layer drying kinetics of bay laurel leaves. The bay laurel leaves effective moisture diffusivity and activation energy were also identified.

Moisture Adsorption Characteristics of Sandesh Powder

Sorption studies were carried out for sandesh powder at 10, 20 and 40 o C to determine its moisture adsorption characteristics. The adsorption curves obtained were observed to follow a sigmoid shape and corresponds to type II, with a definite inversion due to the temperature effect above 0.79 water activity. The GAB, BET and Caurie isotherm models were attempted to describe the experimental data and were found to give the best fit between experimental and predicted values as indicated by R 2 values. Monolayer moisture content was calculated using the BET and GAB model. Caurie model was used to compute the properties of sorbed water viz., monolayer water, number of adsorbed monolayer, density, and surface area of adsorption in sandesh powder. The net isosteric heat of sorption of water in sandesh powder estimated using Clausius-Clapeyron equation at different moisture contents indicated its strong dependence on moisture content.

Adsorption Studies of Radish Leaf Powder

Radish leaves ( Raphanus sativus ) powder fractions was subjected to moisture adsorption isotherms at different isothermal temperature conditions from 15-45°C with an equal interval of 10°C. The sorption data obtained in gravimetric static method under 0.11–0.90 water activity conditions were subjected for sorption isotherms and found to be typical sigmoid trend. Experimental data were assessed for the applicability in the prediction through sorption models fitting and found that Polynomial and GAB equations performed well over all fitted models in describing equilibrium moisture content – equilibrium relative humidity (EMC–ERH) relationships for shelf stable dehydrated radish leaf powder, over the entire range of temperatures condition under study. The net isosteric heat of sorption, differential entropy and free energy were determined at different temperatures and their dependence was seen with respect to equilibrium moisture content.

Water desorption isotherms and net isosteric heat of desorption on lignite

Water vapor desorption isotherm studies were performed on lignite at the temperature 30, 40 and 50°C to investigate the behavior of water on coal. For a description of desorption process, a Guggenheim–Anderson–de-Boer (GAB) model provided a good fit. At the monolayer region, the overlapping isotherms at different temperatures could not provide valid data to calculate the isosteric heat by applying the Clausius–Clapeyron equation, and hence the isosteric heat was only determined at the moisture content from 0.125 to 0.280mg/mg dry coal. Then, the Brunner–Emmet–Teller (BET) theory was applied to calculate the isosteric heat at the moisture content below 0.125mg/mg dry coal. Net isosteric heat of desorption determined from the isotherms increases from 1.88kJ/mol in the capillary water to 8.17kJ/mol at the monolayer region. This increase is most likely that the forces from coal surface act on all types of water in raw lignite, and as the moisture content decreases, the forces are becoming stronger. Besides, with the effect of the interaction among adsorbed water molecules and the bonding force between water molecules and active sites, at the monolayer region, a maximum energy peak appears on the curve of isosteric heat.

Water Sorption Isotherm of Pea Starch Edible Films and Prediction Models.

The moisture sorption isotherm of pea starch films prepared with various glycerol contents as plasticizer was investigated at different storage relative humidities (11%–96% RH) and at 5 ± 1, 15 ± 1, 25 ± 1 and 40 ± 1 °C by using gravimetric method. The results showed that the equilibrium moisture content of all films increased substantially above aw = 0.6. Films plasticized with glycerol, under all temperatures and RH conditions (11%–96%), adsorbed more moisture resulting in higher equilibrium moisture contents. Reduction of the temperature enhanced the equilibrium moisture content and monolayer water of the films. The obtained experimental data were fitted to different models including two-parameter equations (Oswin, Henderson, Brunauer–Emmitt–Teller (BET), Flory–Huggins, and Iglesias–Chirife), three-parameter equations Guggenhiem–Anderson–deBoer (GAB), Ferro–Fontan, and Lewicki) and a four-parameter equation (Peleg). The three-parameter Lewicki model was found to be the best-fitted model for representing the experimental data within the studied temperatures and whole range of relative humidities (11%–98%). Addition of glycerol increased the net isosteric heat of moisture sorption of pea starch film. The results provide important information with estimating of stability and functional characteristics of the films in various environments.

Effect of Temperature on Sorption Isotherms and Thermodynamics of Intermediate Moisture Category Indian Milk Product Cham-cham

Cham-cham, a traditional Indian milk product prepared by heat and acid coagulation of milk, has a shelf life of a few days. Its storage stability could be studied by monitoring the changes in water activity at different temperatures and humidity. Hence, moisture sorption characteristics of cham-cham were determined at three temperatures i.e., 5, 25 and 40C to obtain sorption isotherms. The data obtained were fitted to several models in which BET (Brunauer–Emmett–Teller), Modified Mizrahi and Henderson model gave best fit. From the BET equation, monolayer moisture content calculated at 5, 25 and 40C were 24.27, 15.60 and 18.25%, respectively. Density of sorbed water, number of adsorbed monolayers, percent bound water and surface area of adsorbent were calculated. The porosity of cham-cham increased with increase in temperature. The net isosteric heat of sorption decrease exponentially with increasing moisture content and enthalpy–entropy compensation theory confirmed the process to be spontaneous and enthalpy driven. Practical Applications The equilibrium moisture content, monolayer moisture content, net isosteric heat of sorption and related properties calculated can be used to represent the state of water present in cham-cham at different temperatures for physicochemical and microbiological changes. These results could successfully be utilized in predicting water activity values of cham-cham at different temperature and moisture content. The predicted water activity values may prove successful in studying and enhancing the shelf life of cham-cham by designing the production process and packaging material for cham-cham making and packaging.

Moisture sorption isotherms and heat of sorption of Algerian bay leaves (Laurus nobilis)

The moisture sorption isotherms of Algerian bay leaves (Laurus nobilis) were determined experimentally in this work. The equilibrium moisture contents of the leaves were measured at 40, 50, and 60 °C using static gravimetric method. Six mathematical models were tested to fit the experimental data of sorption isotherms and predict the hygroscopic behavior during storage or drying. Peleg model was found to be the best fitting model for describing the sorption curves. The net isosteric heat of sorption was computed from the equilibrium data at different temperatures by applying the integrated form of the Clausius-Clapeyron equation. The net isosteric heat of sorption is inversely proportional to the equilibrium moisture content and is found to be an exponential function of moisture content.

Moisture Sorption and Thermodynamic Properties of Vacuum-Dried Capsosiphon fulvescens Powder.

The moisture sorption isotherms of vacuum-dried edible green alga (Capsosiphon fulvescens) powders were determined at 25, 35, and 45°C and water activity (a w) in the range of 0.11~0.94. An inversion effect of temperature was found at high water activity (>0.75). Various mathematical models were fitted to the experimental data, and Brunauer, Emmett, and Teller model was found to be the most suitable model describing the relationship between equilibrium moisture content and water activity (<0.45). Henderson model could also provide excellent agreement between the experimental and predicted values despite of the intersection point. Net isosteric heat of adsorption decreased from 15.77 to 9.08 kJ/mol with an increase in equilibrium moisture content from 0.055 to 0.090 kg H2O/kg solids. The isokinetic temperature (T β) was 434.79 K, at which all the adsorption reactions took place at the same rate. The enthalpy-entropy compensation suggested that the mechanism of the adsorption process was shown to be enthalpy-driven.

Moisture desorption isotherms of cucumber seeds: modeling and thermodynamic properties

Hygroscopic equilibrium curves or sorption isotherms are important in order to define dehydration limits of the product, estimate moisture content alterations under environment conditions and to acquire moisture content values for safe storage. This work aimed to determine desorption isotherms of cucumber seeds stored at different temperature (10, 20, 30, 40 and 50 ºC) and relative humidity (0.11 to 0.96) and the thermodynamic properties of this process. Mathematical models were fitted to experimental data in order to represent the agricultural products hygroscopicity. Net isosteric heat of desorption was performed based on the Clausius-Clapeyron thermodynamic relation and Gibbs free energy through the Van't Hoff equation. Based on statistical parameters, Modified Henderson model was the one that best represented hygroscopicity of cucumber seeds. Equilibrium moisture content of cucumber seeds decreased with temperature increment at a constant value of relative humidity. Differential enthalpy values, representing the energy required to remove moisture from the cucumber seeds, ranged from 2923 to 2469 kJ. kg-1. Values of differential entropy and Gibbs free energy decreased with moisture content increase. Enthalpy-entropy compensation theory is valid for the sorption of cucumber seeds, in which the water sorption mechanism in cucumber seeds can be considered to be enthalpy controlled.

Assessment of drying temperature of sugarcane bagasse on sorption of reactive blue 5G dye

We studied the effect of drying temperature of sugarcane bagasse (SB) on sorption of reactive blue 5G (RB5G) dye to test whether SB is a suitable biosorbent for industrial dye removal. First we assessed a set of SB characteristics that could influence the sorption capacity of dried biomass. Kinetic drying of the SB biomass was measured at different drying temperatures, ranging from 30 °C to 80 °C. We used a static gravimetric method to perform equilibrium moisture sorption experiments at drying temperatures of 35–65 °C. In addition, we measured activation energy and net isosteric heat of sorption. SB biomass was characterized as a heterogeneous low-porous and hygroscopic material, with a series of functional groups on its surface. A Lewis thin-layer model best represented the kinetic drying data. In addition, moisture diffusion into the SB biomass was characterized by a rate-controlling step in the drying process. Equilibrium moisture sorption data showed the expected effects of temperature controlling the sorption capacity and a multilayer sorption process. A sigmoidal sorption isotherm data was best represented by a BET model. A high energy demand is required for moisture removal from biomass as suggested by the measured isosteric heat of moisture sorption. Drying temperature did not influence the RB5G adsorption capacity of bagasse, therefore any drying temperature in the range of 30 °C to 80 °C could be used to make this material feasible as an industrial adsorbent.

Structural and thermodynamic characterization of modified cellulose fiber-based materials and related interactions with water vapor

In this study, the surface characteristics, water vapor interactions, and state of water adsorbed on unmodified and coated paper samples were investigated in an attempt to obtain a better understanding of the fundamental principles related to thermodynamics of this process, as well as to provide essential insight that could be used for further improvement of the papers’ barrier properties. Based on the BET measurement, the coated paper samples showed higher specific surface areas than unmodified paper; however, their mean pore diameters are smaller. The BJH method was used for pore size distribution analysis. Hydrophobic properties of the paper samples were determined from experimental isotherms, e.g., monolayer moisture content, and these results have been related to the water vapor transfer rates (WVTRs) showing a complex nature of these relations. The highest peak corresponding to the modified samples with smaller pore sizes was found to be in the range of 1–30nm, while it was in the 30–100nm pore size range for unmodified paper. The net isosteric heats of sorption for different unmodified and modified paper samples were determined from water vapor adsorption isotherms measured at 15, 25, and 35°C. The net isosteric heats of sorption decreased with an increase of moisture content after reaching the maximum values at 12.53, 15.25, 14.71, 23.2, and 22.77kJ/mol for unmodified, zein grafted, calendered coated, PLA, and PHBV coated papers, respectively. The state of adsorbed water and water-vapor interaction on paper surface were also studied by TGA–DSC and FT-IR spectroscopic techniques. The FT-IR results revealed the formation of water vapor clusters due to poly-layer formation at the higher relative humidity. This has a profound effect on the equilibrium properties, e.g., the intensity of interactions between water molecules and the paper surface decreases as the relative humidity (moisture content) increases. Percentage of bound and unbound water formation and also the dehydration energy at various relative humidities were determined for the paper samples using the TGA–DSC technique.

Moisture sorption isotherms and thermodynamic properties of whole black peppercorns (Piper nigrum L.)

Adsorption and desorption isotherms for whole black peppercorns (Piper nigrum L.) were developed at 22, 30 and 37 °C using standard static gravimetric method. The sorption isotherms exhibited a type III behaviour according to Brunauer-Emmett-Teller (BET) classifications. The equilibrium moisture content (EMC) decreased with increasing temperature at a particular water activity (aw). Hysteresis existed over the entire aw range at 30 and 37 °C. At 22 °C, an intersection of the curves was found at aw closer to 0.75. The goodness of fit of the models was evaluated using several statistical indices and residual plots. The Guggenheim-Anderson-de Boer (GAB) and Peleg models were determined to best describe the experimental sorption data. The monolayer moisture content (M0) were 4.78, 3.56 and 3.49 g/100 g at 22, 30 and 37 °C for adsorption. The maximum isosteric heat of sorption was 28.1 and 73.3 kJ/mol for adsorption and desorption, respectively. The net isosteric heat of sorption and sorption entropy increased with increase in EMC; both curves dropped off exponentially after M0. The Gibbs free energy change for adsorption ranged from 100.6 to 9370.6 J/mol. The black peppercorns can be stored at 22, 30 and 37 °C, by reducing their MC to 10, 8 and 7 g/100 g, respectively, with aw below 0.60.

Modeling of Water Sorption Isotherms Characteristics of Spray-Dried Cherimoya (Annona cherimola) Purée

Spray drying is a solution to preserve food products from spoiling and moisture adsorption isotherms of dry products can be used to predict their shelf life during storage and distribution. Sorption isotherms of spray-dried cherimoya purée were determined using a gravimetric method between 10°C and 50°C and water activities from 0.05 to 0.96. The data obtained were fitted to Guggenheim, Anderson, and De Boer (GAB), GAB modified, Henderson modified, Oswin modified, Chung–Pfost modified, and Hasley isotherms and the Henderson model best fitted experimental data. The net isosteric heat of sorption values determined using Clausius–Clapeyron ranged from 3.04 and 0.92 kJ/mol between 5% and 30% moisture content in dry basis, respectively. Differential entropy also decreased with moisture content. The compensation theory was applied to sorption isotherms and higher value of isokinetic temperature than harmonic mean temperature denotes that adsorption process was enthalpy controlled and a negative value of Gibbs free energy reflected a spontaneous process.

Thermodynamic Properties of Water Desorption of Papaya

Thermodynamic properties were obtained from desorption isotherm data for papaya at 40, 50, 60 and 70C. The specific objectives were to model desorption isotherms by GAB and BET models and to apply the enthalpy-entropy compensation theory to desorption isotherm. The GAB model fitted well the data with monolayer moisture content varying from 0.086 to 0.103 g water/g solids. Equilibrium moisture content decreased with increasing temperature. The net isosteric heat of desorption was determined using Clausius–Clapeyron equation and decreased with increasing moisture content varying from 2.52 to 14.26 kJ/mol. The differential entropy values decreased with moisture content from 0.037 to 0.007 J/mol K. The enthalpy-entropy theory proved to be valid since the relationship between enthalpy and entropy was linear and the harmonic temperature (327.6 K) differed from the sample isokinetic temperature (383.4 K). Through the entropy-enthalpy theory, it could be concluded that the water desorption isotherm of papaya is a nonspontaneous process and enthalpy controlled. Practical Applications This work presents information about the moisture sorption properties of papaya fruit. Moisture sorption isotherms are important to estimate drying time, predict ingredients behavior upon mixing, select packaging, model moisture changes that occur during storage and estimate shelf life stability. In addition, moisture isotherm supplies thermodynamic properties that permit estimation of energy requirement of drying process. Because papaya is extremely perishable, dehydration could be a necessary step in the processing. Therefore, this work intends to contribute in providing useful information for designing drying process of papaya.

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

&lt;p&gt;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&amp;lt;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.&lt;/p&gt;

Water sorption isotherms and air drying kinetics modelling of the brown seaweed Bifurcaria bifurcata

Air drying kinetics at 35, 50, 60 and 75 °C and water adsorption and desorption isotherms at 5, 25, 40 and 55 °C of the brown seaweed Bifurcaria bifurcata were obtained. Water sorption isotherms were modelled by means of the two-parameter Halsey model. A model to estimate equilibrium moisture content of adsorption and desorption processes as function of water activity and temperature simultaneously was obtained by means of linear correlations of Halsey model parameters with temperature. Water sorption isotherms were analysed by a thermodynamic approach to obtain properties such as net isosteric heat of sorption, net equilibrium heat, differential and integral entropy. Monolayer moisture content evaluated by BET model corresponded to the moisture content at which local minimum integral entropy value was observed for sorption processes. Air drying kinetics at different temperatures were modelled using the Page model. The two parameters of the Page model were linearly correlated with temperature. The goodness of the fits was evaluated by evaluation of the coefficients of determination and root mean square errors.

Thermodynamic study of water sorption and water barrier properties of nanocomposite films based on brea gum

Water transport in edible films based on hydrophilic polymers is a complex phenomenon due to the strong interactions of sorbed water molecules with the polymeric structure. The effect of montmorillonite (Mt) nanoparticle incorporation into brea gum (BG) based films was studied through thermodynamic and phenomenological analyses. Moisture adsorption isotherms at three different temperatures of BG based films and BG/Mt nanocomposite films were obtained. Thermodynamic parameters showed an exothermic process that results less favourable when Mt is incorporated into the BG matrix, reducing the water uptake. Entropy change and net isosteric heat of adsorption showed a peak at monolayer water content, which is greater in composite films, indicating a more stable and ordered structure when Mt was added. Gibb's energy was indicative of process spontaneity and of the lower affinity to water composite films. Water vapour permeability depends on the tortuosity of the pathway formed by Mt nanoparticles and solubility of water molecules into the BG film matrix. Mt incorporation reduces the hydrophilic character of BG based films and thus their water vapour permeability.

Desorption isotherms and thermodynamic properties of prickly pear seeds

The objective of this work was to determine the desorption isotherms and thermodynamic properties of prickly pear seeds. These properties are indispensable, particularly in regard to seed drying and storage in order to guarantee its quality prior to its industrial use as an important source of essential oil and meal for medicinal and cosmetic purposes. Desorption isotherms were determined using a static gravimetric method at three temperatures: 40°C, 60°C and 70°C over a relative humidity range of 5–90%. All the curves exhibited type III behavior, according to Brunauer’s classification. Equilibrium moisture contents decreased as the temperature increased. The GAB model fitted well the desorption isotherm data for prickly pear seeds with the monolayer moisture content depending on the temperature and varying between 0.433 and 0.032kg water/kg dry matter. The Clausius–Clapeyron equation was used to evaluate the net isosteric heat of water desorption by using the GAB theoretical model in the studied temperature range. The net isosteric heat and the differential entropy decreased strongly as the moisture content increased, varying from 0.585 to 20.88kJ/mol and from 0.391 to 56.70J/mol/K, respectively, and could be well adjusted by an empirical exponential relationship. The enthalpy–entropy theory proved to be valid since the relationship between enthalpy and entropy was linear. The value of Gibbs free energy was found to be positive and the harmonic temperature (329.18K) differed from the isokinetic temperature (361.32K). Through the entropy–enthalpy theory, it could be concluded that the water desorption isotherm of prickly pear seeds is a non-spontaneous and enthalpy-controlled process.

Moisture Sorption Isotherm of Preconditioned Pressure Parboiled Brown Rice

The pressure parboiling of paddy was carried out at 294.204 kPa for 7 min and preconditioning of brown rice was carried out in fluidized bed dryer at 60-80°C. The moisture sorption isotherms of pressure parboiled preconditioned brown rice at different salt concentrations (0, 2, 3, 3.5 and 4%) were obtained at 20 ± 1°C, 25 ± 1°C, and 30 ± 1°C. The experimental data of sorption isotherm were fitted with some of sorption models (GAB, MGAB, MCPE, MOSE, MHEE, and MHAE models). According to the statistical results, the MGAB model gave the best fit to the experimental sorption data and MHAE model was the least adequate. Sorption isotherm data were used to determine the some thermodynamic functions. The net isosteric heat of sorption was determined from the best fitting equation using the Claussius-Calpeyron equation. The net isosteric heat of sorption decreased with increasing moisture content and increased with increasing salt concentration same trend was observed in entropy of sorption. The spreading pressure increased with increasing water activity and salt concentration and decreased with increasing temperature. The net integral enthalpy decreased with increasing moisture content and increased with increasing salt concentration and reverse trend was observed in integral entropy.