M.J. Urbicain

OPTIMUM OPERATING CONDITIONS IN DRYING FOODSTUFFS WITH SUPERHEATED STEAM

ABSTRACT It is inferred from experimental data that in drying foodstuffs with superheated steam, the initial drying rate has a direct effect on the rate at which the overall drying takes place. That is, the faster the initial drying rate, the shorter the overall drying time. This criterion is very convenient because at the beginning, water moistens the sample external surface so evaporation does not depend on internal sample characteristics, but only on external convective heat and mass transfer rates. Mass and energy balance equations are solved and the result converted into a general initial drying rate equation, in which all dryer characteristics are grouped into one dimensionless parameter. The initial drying rate equation is mathematically maximized and the optimum working conditions determined. The result shows that initial drying rate always increase with increases of either the superheated steam temperature or velocity, but once these two variables are fixed, there exists at least one “optimum” pressure at which the initial drying rate is a maximum. Finally, the initial drying rate and optimum condition equations are applied to three model dryers, a dryer for a flat sheet, a fixed bed dryer and a rotary dryer. In each case, numeric values are computed and plotted as drying rate versus pressure curves, in which the optimum drying rate is also included. Also presented is a chart to compare the optimum pressures as functions of temperature and steam velocity for the three dryers.

New thermal conductivity probe design based on the analysis of error sources

The thermal conductivity determination by means of the thermal probe is a classical non-stationary method, suitable for foodstuffs because of the relatively short period of time and the small temperature rise required during experimental measurements. Even though its simplicity, this method has theoretical and practical implicit errors, their main sources being evaluated in this paper. The theoretical analysis shows that experimental errors can be kept within acceptable values, if the testing time period is within certain limits, out of which the errors grow quite rapidly. The error arising from the calculation of the thermal conductivity by means of an approximate equation is studied and a new way to express it is presented. Also, testing times were calculated for different probe dimensions. Finally, on the basis of two former probe which exhibits some advantages upon each other, a new probe design is proposed which recoveries those features.

A model for the dehydration rate of onions

Abstract A simple model for dehydration of onion pieces is presented to take into account the change of interfacial surface area undergone by the samples as drying proceeds, as well as the different water removal rates through surfaces of different characteristics. It is assumed that the drying rate is linearly proportional to the instantaneous water content whatever the surface involved; either the natural boundaries of the leaves (cataphila) or those produced by cutting them normally along the main dimension. Numerical integration of the instantaneous rates allows the calculation of water removed during a given time and, conversely, the drying time required to reduce the water content to a given extent as a function of the sample size. This information is relevant in order to design the cutting technique which would produce particles of adequate size for practical drying.

Design of a semi-continuous rotary drier for vegetables

In this work the experimental performance of a drying module is presented as the first stage in the design of a modular rotary semi-batch drier for vegetables using hot air as a drying agent. The equipment will be made of a series of such modules, each one working as a batch stage where the material is dried up to a certain extent before passing to the next one, where the operation is repeated. Hence, the whole set-up operates in a semi-continuous mode, such that the first module receives the raw material load at regular time intervals, while the last one delivers the dry product. The operation has been modelled in order to calculate the instantaneous value of the drying rate as a function of the air conditions and the material water content. Integration of this equation with suitable heat and material balances provides a complete description of the air and solid properties at any time, which in turn allows the simulation and optimization of the complete set of modules. Experiments with onions, cut in small cubic pieces, have been carried out for constant values of other operating conditions, namely air velocity, particle size, speed of rotation and onion load, to determine the model parameters.

Drying with superheated steam: maximum drying rate as a linear function of pressure

Drying with superheated steam (SS) as heating agent is controlled through three operative variables, the steam temperature, the recycle velocity, and the system pressure. Once the other conditions are fixed, there is an optimum pressure at which drying rate reaches its maximum value. The exact optimum conditions can be found through a differential mass and energy balance over the heat exchange area, resulting in two equations. The first one to calculate the drying rate as function of the operating conditions and the second one to find the values of those conditions which make the operation optimum. Then, using these equations, this paper shows that the relationship between maximum drying rate and pressure can be rewritten in a unique linear equation. Experiments were performed in a model system to obtain a family of drying rates versus pressure curves, and in accordance with the predicted behaviour, it was observed that all maximum drying rates lay on a straight line.

Rotary semi-continuous drier for vegetables: effect of air recycling

Abstract In this work, the model of the rate of water removal in a stage of a modular vegetables drier, proposed by the authors in a former work ( Pelegrina, A., Elustondo, M. P., & Urbicain, M. J., 1998. Design of a semi-continuous rotary drier for vegetables. Journal of Food Engineering, 37, 293–304 ) is applied to simulate the effect of the air recirculation rate on the unit performance, in particular the time taken and the heat requirements to attain a given final solid water content. Experimental data was obtained previously in the equipment described the cited work ( Pelegrina et al., 1998 ) consisting of a rotary drying device made of two concentric wire mesh cylinders, located in a closed box where hot air is blown. The drier cage has specially designed vanes and baffles to induce the solid circulation. In this work the air heat and mass balances equations, together with the drying velocity equation are solved so that the complete operation can be simulated for any working time, if the initial conditions are known. The simulation assumes that part of the exhaust air is made to recycle and mix with the fresh air supply in controlled proportions, such that the conditions of the gas mixture blown to the drier can be set and the influence of the amount of air recycled on the drier performance is calculated. It is shown that there is, for given working conditions, an optimal mixing proportion which makes the energy delivered a minimum. It is also shown the influence of the recycle on the final water contents if the drying time is the independent variable.

Setting the operating conditions of a vegetables rotary drier by the response surface method

Abstract The temperature and relative humidity of the air are the operating conditions that determine the drier performance, since the time involved and the final water content depend directly on them, and in turn determine the operation costs and the product quality. In this work, the relevant variables governing the drying operation are presented as surfaces, such that once some variables are fixed, the remaining are univocally set. In our case we have fixed final water content and drying time, usually the variables of commercial importance, and the air conditions result from the calculation. We have worked with Valenciana-type onions in a rotary drier and the result is a working zone, in terms of humidity and temperature, that yields a final water content of 5% of the original one, for drying times not larger than 8 h.

Critical hydration in a series of presses for apple juice manufacture

Abstract The usual practice of wetting the exhausted pomace of a press in a fruit juice production plant, and repressing it in a second unit, can be repeated several times in a series arrangement of presses, such that the juice in one press is used to wet the pomace entering the preceding one. If presses have different efficiencies, the operation feasibility depends on the hydration regime, which, in accordance with the relative location of the units, has a minimum or critical value which guarantees that all the presses actually produce juice. Otherwise, one or more could be receiving liquid just to wet the pomace leaving the unit. In this work a criterion is presented for calculation of the value which must be surpassed to ensure that all the presses are active. The final hydration regime adopted for a given press arrangement will be dictated by a simulation model in which this algorithm should be incorporated.