Thodoris D. Karapantsios

Kinetic analysis for the removal of a reactive dye from aqueous solution onto hydrotalcite by adsorption

The removal of a reactive color, Cibacron Yellow LS-R, from aqueous solutions by adsorption onto hydrotalcite particles is investigated using batch rate experiments. Measurements are performed at various initial color concentrations, solid loads, pH values and ionic backgrounds (dissolved NaCl). The speed of agitation and the temperature inside the batch adsorber are also varied within a practical range of values. It is shown that the sorption capacity is relatively high for most experimental conditions so hydrotalcite may be considered as a suitable sorbent for this application. The probable mechanism of the process is investigated by a number of homogeneous and heterogeneous reaction kinetic models as well as diffusion kinetic models. It is found that no single kinetic model can fully describe the sorption process at all times. At least three independent rate-controlling mechanisms appear to compete each other and dominate the different stages of sorption.

A conductance probe for measuring liquid fraction in pipes and packed beds

Al~tract--The performance of ring electrodes measuring the conductance of gas--liquid mixtures in pipes and packed beds is studied experimentally and theoretically. With relatively closely spaced pairs of rings, one can detect liquid segregation, i.e. stratified and annular distribution, as well as a uniform liquid distribution if the mean liquid fraction is available. Conversely, reliable cross-sectionally-averaged holdup data can be obtained if the liquid distribution pattern is known. Measurements in packed beds and pipes are qualitatively very similar. Existing theoretical expressions are employed to interpret the conductance measurements. Moreover, a new analytical solution for an annular liquid distribution is presented, which is particularly helpful in assessing the spatial probe sensitivity in the axial direction. The latter is found to be quite satisfactory. In general, agreement between the data and theoretical predictions is fair to excellent, providing the necessary confidence for practical applications. Two successful applications of the technique to packed beds are reported.

STATISTICAL CHARACTERISTICS OF FREE FALLING FILMS AT HIGH REYNOLDS NUMBERS

Abstract The characteristics of films flowing inside a vertical pipe are studied experimentally. Using an accurate wire conductance technique the film thickness is measured over the Re range 509–13,090. The mean film thickness data are in good overall agreement with established relations. Extensive statistical analysis shows that film thickness fluctuations have a stochastic character. The description given by Dukler and co-workers, of a relatively well-defined substrate in between the large waves, is confirmed up to Re ≈ 7000. At higher Re the substrate tends to lose its identity, due to amplification of the small waves on its surface. Moreover, data on standard deviation, h max and h min indicate that at Re > 5000 the amplification of large waves essentially stops, and that the substrate thickness tends to increase. The calculated probability density functions of film thickness, the spectral density functions, the skewness and kurtosis provide new reliable information for the statistical description of film thickness fluctuations. Finally, by identifying the wave peaks it is found that the total number of waves is nearly constant, in the range of Re studied, and that the distribution of wave peaks becomes bimodal at Re ≳ 4000.

Rheological and physical characterization of pregelatinized maize starches

Abstract This work studies maize starches modified by a small-scale industrial double drum dryer. The impact of the processing thermal treatment is examined with respect to the morphology, texture and color of the dry product sheets. Moisture absorption/desoprtion tests are employed to evaluate the moisture exchange capability of the product sheets and also infer information about its bulk porosity. The variation of the intrinsic and apparent viscosity of the pregelatinized starches reconstituted with water to pastes is examined when the steam pressure, drum speed and level of the gelatinization pool between the drums are varied. Evidence is provided that the dispersed swollen starch granules control the rheological behavior of the starch pastes.

Longitudinal characteristics of wavy falling films

New data are presented on film thickness characteristics, over the Reynolds number range 370–11,020, and relatively long distances from liquid entry; i.e. 1.7–2.46 m. The mean film thickness δmean does not show a definite trend remaining roughly constant, while δmin has a tendency to slightly increase. Other quantities such as the standard deviation s, δmax and dominant wave velocity Vwave display a weak tendency to increase with longitudinal distance. This small variation of the aforementioned quantities suggests that the flow may not be fully developed, over the distances examined.

A nomogram method for estimating the energy produced by wind turbine generators

A simple nomogram is constructed to estimate the power generated by a wind turbine generator (WTG) operated at near maximum efficiency using optimum tip-speed ratio between cut-in and rated wind speed, and at constant power using optimum pitch control between rated and cut-out wind speed. The nomogram is based on information that is readily available for commercial WTGs as well as some simple statistical quantities for the wind at the site. When the wind speed is described by a Weibull distribution, the power of a WTG is estimated in terms of three generalized non-dimensional parameters. When a Rayleigh distribution is employed only two parameters are necessary. A second nomogram is also developed for those less common cases where a small correction of the results of the first nomogram is needed. A mathematical analysis is presented which allows for the construction of single chart nomograms without sacrificing the necessary accuracy. Two application examples demonstrate the degree of accuracy achieved by the nomograms and the advantages they offer for parametric analyses as regards convenience and labor.

Performance of a double drum dryer for producing pregelatinized maize starches

The response of an industrial scale double drum dryer to variation of steam pressure, drums rotation speed and level (height) of the gelatinization pool between the drums is presented. To our knowledge, this is the first time that the gelatinization pool level is treated as an input variable. The output variables are the products moisture content, mass flow rate and specific load (equivalent to the products film thickness). The effect of the drum surface temperature and width of the gap between the drums on the behavior of the output variables is examined. A theoretical analysis is presented for the qualitative assessment of the basic process variables that control the film thickness of the product. The role of the thermal inertia of the drum wall to the response of the dryer is discussed. Changes in the thermal efficiency of the dryer are inferred from overall heat transfer coefficients.

DESIGN AND TESTING OF A NEW SOLAR TRAY DRYER

ABSTRACT A novel low cost tray dryer equipped with a solar air collector, a heat storage cabinet and a solar chimney is designed and tested. The design is based on energy balances and on an hourly-averaged radiation data reduction procedure for tilted surfaces. Measurements of total solar radiation on an horizontal plane, ambient temperature and humidity, air speed, temperature and relative humidity inside the dryer as well as solids moisture loss-in-weight data are employed as a means to study the performance of the dryer. First, detailed diagnostic experiments are carried out with no drying material on the trays. Next, a number of experiments is conducted using a controlled reference material whose reproducible dehydration pattern allows comparisons among runs. Drying is also tested during night operation and under adverse weather conditions. For all the employed conditions, the material gets completely dehydrated at a satisfactory rate and with an encouraging systems efficiency.

Mass transfer limitations during starch gelatinization

Abstract A relatively large volume of maize starch suspension (2.5–20% w/w solids) is heated to above its gelatinization temperature by two means: a microwave oven meant to provide a uniform global thermalization of the sample, and a conventional local electrical heater which, depending on agitation, yields different heating patterns on the sample. The progress of gelatinization is registered on-line by an electrical conductance technique. Contrary to what is observed in the conventionally heated samples, gelatinization is not completed in the microwave irradiated samples although the temperatures reached are as high as with conventional heating. This is attributed to poor mass transfer of water molecules during microwave irradiation as a result of the short processing period and the absence of mixing of water with starch components. The latter is in line with observations regarding the role of agitation in gelatinization under conventional heating.

Water dispersion kinetics during starch gelatinization

The kinetics of water dispersion during the gelatinization of dilute suspensions of maize starch was studied by analyzing changes in electrical conductance data recorded continuously with time. Several analytical methods were compared for the preliminary study of the activation energy of gelatinization. The probable mechanism of the process was investigated by a number of homogeneous and heterogeneous reaction kinetic models. A modified composite methodology coupled with a reduced-plot method was employed to fit the kinetic data. Two simultaneous elementary reactions, expressed by an autocatalytic kinetic model and a 3D moving phase-boundary rate model, predicted the overall kinetic behavior with appreciable success.