Sergio A. Giner

Wheat drying kinetics. Diffusivities for sphere and ellipsoid by finite elements

Abstract A two-dimensional finite element model was used to analyze isothermal thin layer drying of wheat representing the grains as axisymmetric ellipsoids. The effective diffusion coefficient was estimated by minimizing the sum of squares of the residuals between numerically predicted and experimental moistures. Estimations done for drying air temperatures of 35°C, 50°C, 60°C and 70°C and initial grain moistures of 0.2694, 0.2396, 0.2133 and 0.1891 d.b., were correlated by means of Arrhenius-type functions. These correlations were compared to those previously obtained with the same data by assuming grains as spheres. The results, all in the order of 10 −11 m 2 / s , indicated that the diffusion coefficients found for ellipsoidal geometry were some 9–13% lower than the values fitted for spheres. The ratio of diffusivities for ellipsoids to those for spheres was in the order of the wheat sphericity. However, further theoretical and experimental studies are required to verify this observation.

Cross-flow drying of wheat. A simulation program with a diffusion-based deep-bed model and a kinetic equation for viability loss estimations

ABSTRACT A mathematical model of heat and mass transfer for fixed beds was developed according to the modern theory of process simulation and standard laws of thermodynamics and transport phenomena. The mass transfer grain-air was predicted with simplified diffusional expressions together with an equation for the static equilibrium moisture content. Four differential equations were obtained for a grain layer and they were integrated along the bed depth and time with second and a fourth-order methods, respectively. The model was validated by comparing drying time predictions with experimental values, being the average error of 6%. The model was extended into a program for continuous cross-flow drying-cooling

Evaluation of the thermal efficiency of wheat drying in fluidized beds: Influence of air temperature and heat recovery

Abstract A study is reported of means of improving the thermal efficiency of wheat drying in fluidized beds. One means consists of the use of high air temperatures in the first zone of the drier. This resulted in only a small efficiency increase due to the restrictions imposed by the thermal sensitivity of the grain. However, it was found that the high air velocities used for fluidization lead to high-temperature low-humidity of the exhaust air, thus making it suitable for recirculation. To quantify this effect, an algorithm was developed which allows calculation of the recirculated fraction of the outlet air and the temperature of this fraction. The efficiency calculated for the new conditions showed a marked increase, reaching a value of about 53% (in drying with no air recirculation, it was 28%). An extension of the algorithm also made possible the calculation of the heat supplied by the air used to cool the grain, when it is added to the stream of recirculated air from the previous drying stage. Under such circumstances the efficiency was about 62%, which is comparable to that exhibited by cross-flow driers and some types of mixed-flow driers.

Water uptake by dehydrated soy protein isolates: Comparison of equilibrium vapour sorption and water imbibing methods

Abstract Two forms of the hygroscopic behaviour of soybean protein isolates (SPI): liquid water imbibing capacity (WIC) and equilibrium vapour adsorption were tested in non-dialysed native (N) and denatured (D) samples, and in corresponding dialysed isolates (ND and DD). Water activity ( a w ) was measured by the optical condensation dew point method. The GAB model adequately predicted the SPI experimental sorption isotherms. The greater number of polar groups exposed during denaturation along with the salt-induced protein aggregation cause the stronger adsorption on D isolate compared with N. No differences between the isotherms up to very high a w were observed between ND and DD isolates. WIC of isolate D was greater than in N and this correlated with higher moisture adsorbed in equilibrium. Denatured non-dialysed isolates may be stored safely at higher moisture contents, and constitute a suitable ingredient for processing nutritionally-enriched formulated foods requiring high water retention.

WATER - CORN EQUILIBRIUM: TEMPERATURE DEPENDENCY OF THE G.A.B. MODEL PARAMETERS AND CALCULATION OF THE HEAT OF SORPTION

ABSTRACT By using sorption data for com in the range of 4.4-60°C, the temperature (T) variations of the GAB. model parameters (Mm, CB and k) were studied. As only the monolayer moisture (Mm) varied (decreased) with temperature, a generalized GAB equation of four total parameters was obtained. This, together with the Clapeyron equation allowed a method to calculate the heat of sorption (Lb) as a function of T and moisture content (M). For each temperature, Lb presented a maxinuin some 30% above the heat of vaporization of water (Lf), which occurred around the Mm, in congruence with the sorption theory. A similar behavior of Lb was found using the Othmer method. In contrast, the Gallaher equation predicted an ever increasing Lb up to 3 Lf at very low moistures. The GAB-Clapeyron method is then proposed to calculate Lb as a function of M and T.