Motionless Mixers Research Articles

Enhancement of heat transfer and thermal homogenity with motionless mixers

AbstractMotionless mixers are well suited to augment heat transfer and to provide a uniform thermal environment in laminar flow reactors. Heat transfer performance can be characterized with the Nusselt number, while the degree of thermal homogeneity can be characterized using the thermal time distribution which is the nonisothermal analogue of the residence time distribution. Theoretical results, calculations, and design strategies are presented for a variety of mixer types including flow inverters as well as the more common flow dividers. Two‐channel, partial flow inversion is an excellent, near optimal, strategy for cooling or other situations such as polymerizations which give elongated velocity profiles. For flattened velocity profiles, perfect mixing can sometimes give better results. In practice, however, partial inverters can be expected to outperform those motionless mixers which are primarily flow dividers.

Oil drop size and gas hold‐up in an oil–water airlift system with motionless mixers

AbstractThe degree of emulsification, measured as surface area of oil generated, was studied. The effect of interfacial tension, volume fraction of oil, and power per unit volume on the Sauter mean diameter of the oil drops was determined in an airlift system with motionless mixers. A mathematical expression to predict the Sauter mean diameter was developed using regression techniques. From this equation another equation, which will predict the surface area of oil in terms of the same variables, was derived. The effects of water air surface tension and power per unit volume on the gas hold‐up were obtained using similar techniques. The results show that the interfacial tension and the surface tension are important variables when hydrocarbon fermentations are carried out in airlift systems.

Mass transfer in bubble columns packed with motionless mixers

The cocurrent upward mode was employed to absorb pure oxygen into water in bubble columns packed with Koch (Sulzer) motionless mixers. The liquid-side volumetric mass transfer coefficient, K La , in the packed bubble column was found to be always larger than that in the unpacked bubble column. In the range of liquid velocities from 6.7 cm/sec to 39.9 cm/sec, the value of K La in the packed bubble column increased with the increasing liquid velocity while that in the unpacked bubble column was almost independent of the liquid velocity. The equation of the form K La = mν l βϵ was successfully adopted to correlate the K La data.

Oxygen transfer and axial dispersion in an aeration tower containing static mixers.

Oxygen transfer from gas to liquid under steady-state cocurrent flow conditions was modeled using the dispersion model, and the oxygen transfer coefficients were estimated from available data for a column with Koch motionless mixers. The dispersion in the column was estimated for several different gas and liquid flow rates using steady-state tracer experiments. The estimated oxygen transfer coefficients were compared with those estimated using complete mixing and plug flow models. The results indicate that the dispersion model is the most appropriate model for estimating the mass transfer coefficient from the available data.

A modified coalescence—dispersion model for the axial mixing of segregating particles in a motionless mixer

A modified coalescence—dispersion model has been developed for the axial mixing of segregating particle systems in a motionless mixer (Kenics mixer). This model is capable of generating the concentration distribution as a function of time and its applicability is not constrained by the initial concentration distribution in the mixture. Two important parameters in this model are the coalescence rate and the distribution ratio. It has been found that the former can be correlated as a linear function of the number of helices in the mixer, and the latter depends heavily upon the physical properties of the individual particles. The validity of the present model has been tested against the available experimental data [16].

Pressure drop, gas hold‐up, and oxygen transfer in tower systems

AbstractPressure drop, gas hold‐up, and oxygen transfer were investigated in a sieve tray column, a column with Koch motionless mixers, and a bubble column. The oxygen transfer experiments were conducted using cocurrent flow of gas and liquid under steady‐state conditions with oxygen transfer from the gas to the liquid phase. The oxygen transfer rates and efficiencies of the sieve tray column and the column with Koch mixers were found to be superior to those of the conventional bubble column. Gas hold‐up was also greater when sieve trays or Koch mixers were inserted in the tower. The pressure drop was found to be primarily due to the liquid head in all three columns.

Stochastic modeling of segregation in a motionless mixer

The mixing of particles with different characteristics is generally accompanied by segregation which prevents the particles from attaining the randomly mixed state. A discrete steady-state Markov chain model was employed to model the axial segregation of solid particles in a motionless mixer. Three systems containing particles of different sizes and/or densities were studied. One step transition probabilities of the model were experimentally determined. The model can predict the concentration profiles, the degrees of mixedness, and the equilibrium states of these particle systems blended by passing the particles through a motionless mixer. Experimental results are in good agreement with those predicted from the model.

Characteristics of hydrocarbon uptake in cultures with two liquid phases

In hydrocarbon fermentation, the efficiency of hydrocarbon uptake by cells is one of the keys to the economical production of single-cell protein. This work is concerned with characterization of cultures with two liquid phases for understanding the hydrocarbon uptake process by cells. Batch cultivation of Candida lipolytica was carried out in shaking flasks and in a tower fermentor with motionless mixers. Microscopic observation and cell and hydrocarbon concentration distribution in batch cultivation showed that some cells are attached to the large oil drops and others are free from them. Interfacila tension between oil and water and Sauter mean drop size decreased as cultivation proceeded. On the basis of the experimental results, the process of hydrocarbon uptake by cells is discussed.

A mechanistic kinetic model of the rate of mixing segregating solid particles

Axial mixing and demixing of three diffrent free-flowing segregating particle systems of Kenics motionless mixers were investigated. The first particle systems contained particles differing in size only, the second in density only, and the third in both size and density. Experiments were conducted in an effort to visualize the kinetics of the mixing—demixing process and to examine the influence of size and/or density differences on the rate of the process. Two previous models proposed by Rose and by Weydanz were tested with the experimental data. Neither model was satisfactory. A new model was proposed in this work and was found to give a good description of the mixing—demixing process in the motionless mixer. Experimental results of the three different cases were compared with the case of identical size and density. The comparison showed that the differences in both particle density and size gave rise to substantially faster mixing and demixing rates than the difference in either particle size or particle density alone.

Oxygen transfer to mixed cultures in tower systems

AbstractA modified dynamic method is introduced to determine the oxygen transfer coefficient, KL a, in aerobic fermentation systems which are not mechanically agitated. The dissolved oxygen concentration is measured continuously following a step down or a step up in aeration rate. The response curve is analyzed to obtain the value of KLaExperiments were carried out at several different air flow rates using mixed culture in concurrent tower fermentors with motionless mixers. The effect of sieve trays and Koch motionless mixers on oxygen transfer was investigated using a 3 in. diameter column. The values of KL aobtained at the bottom of each column were found to be higher than those obtained at the top. Comparison of the results showed that the values ofKL a were higher when the Koch mixers were used than when the sieve trays were employed.The oxygen uptake rate by the organisms rX, is also calculated by using the KL a values obtained. They compare favorably withthe experimentally measured values.

The mixing of solid particles in a motionless mixer—a stochastic approach

AbstractA Markov chain model was used to model the axial mixing of solid particles in a motionless mixer having no moving parts. One step transition probabilities were determined experimentally for the model. Based on these transition probabilities, the model was able to predict spatial distribution of tracer particles up to seven steps of the Markov chain, which was equivalent to seven consecutive passes of the mixture through the mixer. Experimental results were in good agreement with those predicted from the Markov chain model.

Evaluation of a motionless mixer using a radioactive tracer technique

The efficiency of motionless mixers for mixing free flowing solid particles has been studied by using a radioactive tracer technique. The tracer is produced by irradiating the particles (grains of wheat) with neutrons. Consequently, the surface and physical properties of the tracer particles are identical with those of the other particles. For the operating conditions studied in this work it appears that for a given number of passes there is a number of helices beyond which improved dispersion does not occur. Sampling and analysis of samples was achieved by gamma-ray spectroscopy without any disturbance to the final mixture.

EFFECT OF HANDLING METHODS ON BULK VOLUME AND HOMOGENEITY OF SOLID MATERIALS

SUMMARY Bulk volume expands or contracts when solid particles of fairly uniform size flow out of a container by gravity and are loaded into another container. Expansion or contraction depends on the initial void fraction of the bulk volume. In this study, only expansion of the bulk volume was studied. The expansion was measured after wheat and spherical Lucite particles flowed separately through a plain tube and through a motionless mixer. It was found that expansion of the bulk volume was independent both of the number of helices in the mixer and the length of the plain tube. Passing wheat particles through the mixer expanded their bulk 3–4% less than did passing them through a plain tube of the same distance. Using 3/16‐in. particles, the mixer gave 3% less bulk volume expansion. For a given weight range of particles (using the expanded volume in the plain tube as a basis) the mixer reduced the expanded volume of wheat 20% and reduced more than 40% of the expanded volume of 3/16‐in. Lucite particles. The degree of mixedness, a measure of radial mixing, increased exponentially as the number of helices in a motionless mixer increased. Use of four helices for mixing 3/16‐in. Lucite particles, six helices for mixing 1/8‐in. particles and six helices for mixing red and white wheat appears to be optimum for the operating conditions in this study.