Motionless Mixers in Bulk Solids Treatments – a Review†

Motionless Mixers in Bulk Solids Treatments – A Review†

J. Gyenis University of Kaposvar Research Institute of Chemical and Process Engineering*

Abstract

In this paper the general features, behavior and application possibilities of motionless mixers in mixing and treatments of bulk solids are overviewed, summarizing the results published during the last three decades. Working principles, mechanisms, performance, modeling and applications of these devices are described. Related topics, such as use in particle coating, pneumatic conveying, contacting, flow improvement, bulk volume reduction, and dust separation are also summarized.

recombining of different parts of materials are com- 1. Introduction mon mechanisms in this respect, both in fluids and Motionless or static mixers are widely known and bulk solids. applied in process technologies for the mixing of liq- Motionless mixers eliminate the need for mechani- uids, especially for highly viscous materials, and to cal stirrers and therefore have a number of benefits: contact different phases to enhance heat and mass No direct motive power, driving motor and electrical transfer. Producing dispersions in two- and multi- connections are necessary. The flow of materials phase systems such as emulsions, suspensions, (even particulate flow) through them may be induced foams, etc., is also the common aim of using motion- either by gravity, pressure difference or by utilizing less mixers. Far less information is known on their the existing potential or kinetic energy. The space behavior, capabilities and applications in bulk solids requirement is small, allowing a compact design of treatments. Although investigations started more equipment in bulk solids treatments. Installation is than thirty years ago in this field, research and devel- easy and quick, e.g. by simple replacement of a sec- opment is still under way even now. Recent studies tion of tube or by fixing inserts into a tube or vessel. and applications gave evidence on the beneficial fea- Set-up and operating costs are much lower than those tures of such devices in powder technology for mix- of mechanical mixers, while maintenance is practi- ing and other treatments of bulk solids. cally superfluous. Motionless mixers are available in Motionless or static mixers are flow-modifying a number of different types, shapes and geometries, inserts, built into a tube, duct or vessel. These tools made from a great variety of materials. The mixer can do not move themselves, but using the pressure dif- therefore easily be matched to process requirements ference or the kinetic and potential energy of the and to the features of the processed materials. Phys- treated materials, create predetermined flow patterns ical properties, e.g. flow behavior, particle size, and/or random movements, causing velocity differ- mechanical strength, abrasive effects, safe prescrip- ences and thus relative displacements of various parts tions. e.g. for food and pharmaceutical industries, can of the moving material. In this way, motionless mixers be taken into account by the proper design of mixers. can considerably improve the process to be carried Applications in powder technology are equally feasi- out. In fluids, motionless mixers work efficiently ble in gravity and pneumatic conveying tubes, in both in turbulent and laminar regions. Splitting, shift- chutes, hoppers and silos, or even in rotating, vi- ing, shearing, rotating, accelerating, decelerating and brated or shaken containers. The greatest advantages of motionless mixers in bulk solids treatment are: high performance, continu- * Veszprem, Egyetem u. 2, Hungary ous operation, energy and manpower savings, mini- † Accepted: June 28, 2002 mum space requirement, low maintenance costs,

KONA No.20 (2002) 9 trouble-free operation, easy measurement and con- requirement and a narrow residence time distribu- trol, and improvement of product quality. tion. In multiphase flows no deposits and blockages There are a number of different types of motionless occur, due to the high turbulence caused by the sharp mixers available from a number of manufacturers, edges and crossings. But, a drawback of this mixer most of them are applicable for bulk solids, too. The also comes from this, because sharp edges and the most widely known types are, e.g. Sulzer SMX and sudden changes in flow directions increases pressure SMF mixers, Ross ISG and LPD mixers, Komax drop. In bulk solids flow, troubles can arise, especially mixer, Kenics and FixMix mixers, etc. Fig. 1 shows for cohesive materials, for larger particles or broad some examples of these devices. size distributions. The Sulzer SMX mixer (Fig. 1a) is a typical form Fig. 1b shows the Ross LPD mixer which consists of lamellar mixer, composed of narrow strips or lamel- of a series of slanted semi-elliptical plates positioned las placed side by side within a tube section. These in a discriminatory manner in a tubular housing. strips decline from axial direction alternately by posi- When the material flows through this mixer, the input tive and negative angles, crossing the planes of each stream is split and diverted repeatedly in different other, and thus constituting a 3-D series of X forms. directions along the cross-section of the tube, until During flow, the material is split into several streams a homogeneous mixture is achieved. This type of or layers corresponding to the number of strips, motionless mixer is generally used for the turbulent shifted to opposite directions relative to each other. flow of low-viscosity liquids to enhance macro- and Flow cross-sections contract along its up-flow sides micro-mixing and/or to improve the heat transfer and expand at the down-flow sides. Thus, the material coefficient in heat exchangers. It is also feasible for is forced laterally from the contracting to the neigh- particulate flows. But, since the flow at a given tube boring expanding channels. One SMX element, i.e. section is divided into two streams only, shear and one series of crossing strips, mixes principally in two material exchange takes place in one plane only dimensions along the plane of the X forms. Therefore, between the two half-tube cross-sections, thus the the next series of X forms is aligned at 90° to ensure mixing effect along a given length is weaker than in three-dimensional mixing. Sulzer SMX is thus charac- SMX mixers, especially for viscous materials or bulk terized by excellent cross-sectional (transversal) mix- solids. Naturally, the pressure drop is also less. For ing and a high dispersing effect with a small space bulk solids, the maximal throughput in a Ross LPD

a

( f )

b

b

(a)(b)(c)(d)(e)(g)

Fig. 1 Examples for motionless mixers also applicable for bulk solids (a) lamellar (Sulzer SMX) mixer, (b) Ross LPD mixer, (c) Komax mixer, (d) Kenics static mixer, (e) FixMix mixer, ( f ) a - taper and (g) b - skewness of a FixMix element

10 KONA No.20 (2002) mixer is higher than in SMX, and the risk of plugging surfaces of the helices, the pressure drop along a for cohesive powders or large particle sizes is consid- Kenics mixer is very low, while it provides continuous erably reduced. Another difference is that, due to the and complete mixing and eliminates radial gradients non-uniform axial velocity profile, the longitudinal in temperature, velocity and composition. For bulk mixing effect of the LPD mixer may be higher than solids, because of the non-uniform axial velocity pro- that of the SMX mixer. file, a certain degree of longitudinal mixing also takes The Komax mixer (see Fig. 1c) consists of flat place. Due to the smooth surfaces and relatively wide plates arranged essentially in axial direction in a tube, flow channels, the risk of plugging or blockage is but at both ends of these plates they are hatched, very limited. rounded and bent in opposite directions. The neigh- The FixMix motionless mixer shown in Fig. 1e is boring mixer plates are arranged at 90° in radial very similar to the Kenics static mixer, with the essen- direction, touching each other with the tips of the tial difference that the individual elements are slanted bent flaps. This mixer is also called a “Triple-Action relative to the tube axis and are tapered along their Mixer”, because it provides (i) two-by-two division, length. It results in several benefits: the slightly (ii) cross-current mixing and (iii) back mixing of increasing gap between the mixer element and the counter-rotating vortices. Each mixing element set in tube wall eliminates the corners or contact points combination sweeps approximately two-thirds of the between them. Therefore, there are no dead zones, circumference of the pipe and directs the flow to the and deposition or blockage cannot occur. On the opposite side, providing very strong wall-to-wall radial other hand, the cross-section of the flow channels on transfer. Between the sets of generally four mixer ele- the two sides of a mixer element changes continu- ments, inter-set cavities provide space for intensive ously along its length: the cross-sectional area on one contacting of the sub-streams of material by strong side expands while on the other side it contracts. Due momentum reversal and flow impingement. For mul- to the tangential flow at the wall and the pressure dif- tiphase flow, this mixer is resistant to fouling or clog- ference between the two sides, an intensive cross- ging, because the flips of the mixer elements are flow takes place between the neighboring flow smoothly contoured with a large radius. Intersections channels. These features provide improved mixing between the element ends with the wall are all efficiency, lower pressure drop with suitable cross- oblique angles, eliminating corners that can trap solid sectional turbulence, and more uniform radial and or fibrous materials and promote material accumula- tangential velocity fields. The higher velocity and the tion. Momentum reversal and flow impingement pro- turbulence close to the tube wall results in higher vide a self-cleaning environment. heat transfer coefficients and a cleaner surface. In The Kenics static mixer shown in Fig. 1d pos- addition to this self-cleaning effect, the lack of cor- sesses almost all the advantages of the Komax mixer. ners makes the cleaning easier for difficult materials. It consists of a long cylindrical pipe containing a num- This mixer provides higher mixing efficiency per unit ber of helical elements twisted by 180° alternately in mixer length and reduces the risk of blockage in bulk left-hand and right-hand directions, perpendicular to solids treatment. flow direction. The adjacent elements are set by 90° During the past three decades, quite a number of in radial direction, therefore the outlet edge of a given papers were published on the application of motion- element and the inlet edge of the next one are perpen- less mixers in this latter field, and it is worth survey- dicular to each other. The smooth helical surface ing these results to initialize further studies and directs the flow of material towards the pipe wall and practical applications. back to the center, due to secondary vortices induced by the spiral-form twist of the flow channels. Addi- 2. Motionless Mixers in Bulk Solids Mixing tional velocity reversal and flow division results from shearing of the material along the tube cross-section The mixing of bulk solids is an important operation between the adjacent elements. The systematic divi- in many industries, such as in the chemical and phar- sion of streams and their recombination in another maceutical industries, mineral processing, food in- way enhance the mixing effect proportionally to 2n, dustry and for treatments of agricultural materials. where n is the number of the applied mixer elements. Uniform composition in these processes is of primary For fluids or in multiphase flows, a relatively narrow importance, as well as to eliminate segregation. Be- residence time is ensured, in addition to excellent sides mechanical mixers and silo blenders, motion- radial mixing. Due to the smooth and mildly bending less mixers represent a viable solution for this task,

KONA No.20 (2002) 11 especially if continuous operation is possible or nec- There is a general belief that motionless mixers per- essary. The application to homogenize solids was form mainly radial (transversal) mixing, and that axial already envisaged as early as in 1969 by Pattison [1]. (longitudinal) mixing is negligible. But, according to A crucial condition of such an application is the our experience, also seen from the results of other trouble-free flow of the bulk solids to be treated. workers, this is not entirely valid even for viscous liq- Therefore, motionless mixers can only be used for uids or plastic materials, due to non-uniform axial free-flowing particles, but cohesiveness to some ex- velocity profiles. Depending on their shape, arrange- tent is tolerable even in gravity flow of such materials. ment and operational conditions, and in addition to a In addition to the devices conventionally known as high radial dispersion [2], motionless mixers may motionless mixers, any other tools inserted into a cause effective axial mixing [3, 4], too. This latter fea- tube or vessel or the intentional changes of tube or ture is of crucial importance in equalizing concen- chute cross-sections can modify the flow pattern of tration variations in time and space in continuous solids. Therefore, they may also be considered as particle flows caused either by segregation or by non- some kind of motionless mixer. Various inserts in uniform feeding. silos or hoppers, or cascade chute assemblies causing the multiple division and recombination or modifica- 2.2 Mixing Kinetics and Performance tion of bulk solids flows can also be ranked here. The performance of a bulk solids mixer is charac- terized by its throughput, i.e. the treatable mass per 2.1 Mixing Mechanisms of Motionless Mixers unit volume and time, and by the degree of homo- Several workers investigated the mixing mecha- geneity achieved. These features are in close relation nisms of particulate solids in motionless mixers act- with the mixing kinetics, i.e. with the rate of homo- ing in transversal [2] and longitudinal directions [3]. geneity improvement, characterizing how fast the Observing the interactions between the particles and concentration uniformity is getting better as a func- mixer elements, four kinds of mixing actions were tion of time or mixer length. The mixing kinetics distinguished [3, 4]. Namely: (a) multiple division and determines the number of motionless mixers neces- recombination of the particle flow, (b) interaction of sary to achieve a given degree of homogeneity or its particles with other particles, with the surface of equilibrium. The latter may be the result of two com- mixer elements and the tube wall, (c) changes in flow peting processes: mixing and segregation. In other direction, and (d) differences in velocity profile. words: whilst the mixing mechanism tells us in what Three main mechanisms, namely diffusive, convec- manner, the mixing kinetics characterizes by which tive and shear mixing were attributed to these rate and how far the process is going on. actions. Shear: certain particle regions are sheared From a practical point of view, the performance of a during particle flow, creating velocity differences mixer is the most important feature, if we disregard between the adjacent layers. Convection: multiple divi- the actual mechanism of mixing. However, the mech- sion and recombination of the particle flow, as it is anism gives an explanation of the mixing kinetics split into sub-streams and diverted to different ducts experienced, thus also serving as a starting point for or directions, and then unified with other sub- further improvements. Therefore, almost all studies streams. Dispersive or diffusive-type mixing: stochastic carried out in this field aimed at determining the rate movements with interchanges of different particles. A of process [5-14], and only a few works dealt with the great deal of experience accumulated in the last mechanism. The kinetics of the mixing and segrega- decades indicates that these mixing mechanisms act tion processes competing with each other can be more or less together in any type of motionless mixer, characterized by the equation [14]: and therefore they are hardly separable from each dM other. K (1M)K Φ (1) dt m s The majority of experimental studies carried out until now used helical mixer elements such as Kenics where M denotes the actual degree of mixedness, Km or FixMix-type devices. In a few works, other types of and Ks are the kinetic constants of mixing and segre- motionless mixers such as Sulzer (Koch) mixers and gation, respectively, while Φ is the so-called segrega- mixer grids composed of helices, rods or lamellas, tion potential [14]. Although there are a number of were investigated. various definitions for the degree of mixedness [15], For certain types of mixers, the mixing mecha- the most simple and well applicable one was defined nisms influence the characteristic direction of mixing. by Rose [16] as

12 KONA No.20 (2002) 2.3 Modeling and Simulations s M1 (2) To describe a process or equipment theoretically, or s 0 to predict the characteristic features and results of where s and s0 are the estimated standard deviations their applications under different conditions, math- of the sample concentrations taken from the actual ematical modeling and simulation proved to be widely mixture and in a totally segregated state, respectively. applied and useful tools. To investigate motionless Among the earliest works carried out in this field, mixers in solids mixing, various modeling principles the mixing performance of the Kenics [2-4, 8] and and simulation methods were used till now. Chen et Sulzer (Koch) motionless mixers [6, 7] was reported al. [4] adapted the concepts of an axially dispersed by the team of L. T. Fan. In some studies, the mixing plug-flow model, commonly used at that time to of wheat, sorghum grains and flour was determined describe the mixing of fluids in various unit opera- both in axial and radial directions. Among different tions. In their work, a quasi-continuous deterministic experimental methods, the radioactive tracer tech- model was applied to describe the axial dispersion of nique was used to observe the rate of the process. particles in a gravity mixer tube containing motion- The practical purpose of these investigations was to less mixer elements. Residence time distributions upgrade low-quality products by adding high-quality were measured by the stimulus-response technique, ones during continuous blending. Motionless mixers and apparent Peclet numbers and axial dispersion were proposed, e.g. to feed mills with a controlled coefficients were determined for three different kinds quality of raw material. The kinetics of simultaneous of solid particles, after different number of passes mixing and segregation processes was investigated through the mixer tube. experimentally with free-flowing particles which dif- Apparent Peclet numbers (Pe′) and axial dispersion fered in particle size or in particle density, or both. coefficients (K′ax) have similar definitions here to Under given conditions, comparison showed that those used in fluids to characterize the intensity of differences both in particle size and density gave longitudinal mixing. Their values can be determined substantially faster mixing and segregation rates from the residence time distribution of tracer parti- compared to systems where differences took place in cles in the mixer tube determined by discrete sam- either particle size or particle density alone [8]. pling at the outlet at different times after they are The mixing of materials of different particle sizes, introduced at the inlet in the form of a plug [17]. The shapes and densities is generally accompanied by second central moment m2 of the residence time dis- segregation. After quite a long mixing time, these tribution is as follows: concurrent processes result in a balanced degree of — 2 mixedness which is lower than it would be in a totally m2∑(tit) · xi (3) random distribution of the component particles. i

Depending on the initial positions of the components, where ti is the residence time of tracer particles in equilibrium is approached gradually from below, or the mixer tube taken off by the i-th sample, and xi is very often, after going through a maximum. In this the mass fraction of tracer particles in the i-th sample. latter case, the uniformity of a mixture decreases in The mean residence time of all tracer particles along the final period of the process, in spite of continued the mixer tube corresponds to the first moment of the mixing action. Boss and his co-workers [10, 11] inves- residence time distribution as: tigated the balanced degree of mixedness in systems — of different particle sizes. Two types of motionless t∑ti · xi (4) mixers were tested: (a) lamellar mixers composed of i slanted metal strips crossing each other similarly to The apparent Peclet number is determined from Sulzer SMX mixers, and (b) roof mixers consisting of the mean residence time t— and the second central grids of angle profiles arranged in horizontal rows at moment m2 of the residence time distribution, cor- different cross-sections. Similarly to the works of Fan rected by m2,0, which is the second central moment and his co-workers, these experiments showed signif- obtained in a plain tube, i.e. without motionless mixer icant segregation after certain passes through the elements: motionless mixer. (In these experiments, increasing 2t— 2 mixer lengths were simulated by repeated flows Pe′ (5) m m through a given length of mixer tube.) 2 2,0 and the axial dispersion coefficient:

KONA No.20 (2002) 13 tively revolving tumbler mixer, with and without L 2 K′ mix (6) motionless mixer grids. Some years later, this model ax t— · Pe′ was simplified for simulation purposes [21], and was where Lmix is the length of the studied mixer section. then made more exact by Mihalyko and his co-worker The intensity of mixing, i.e. the achievable mixing [22], introducing the concept of a so-called double effect of motionless mixer elements per unit length is stochastic model. the higher the lower Pe′ is or the higher K′ax is. DEM simulations based on a Lagrangian approach Chen et al. [4] pointed out that smaller particles also helped to understand the features and working had a higher apparent axial dispersion coefficient mechanisms of motionless mixers in particulate sys- than the bigger ones. By increasing the linear velocity tems [23]. of solids through the motionless mixers, an almost exponential improvement was obtained in the axial 2.4 Application Studies for Bulk Solids Mixing dispersion coefficient. A similar technique and model The mixing of free-flowing particle systems in grav- was used by Gyenis et al. [17] to investigate the influ- ity flow through motionless mixers was investigated ence of different flow regimes, particle properties by several workers [2-12]. Most of these studies and length-to-diameter ratios of helical motionless resulted in suitable homogeneity after a few passes, mixers on the resultant axial dispersion coefficient. but in some cases, depending on the physical proper- In mixing processes, especially in particulate sys- ties of the components, considerable segregation also tems, stochastic behavior is a common feature which emerged. Herbig and Gottschalk [24] applied hori- causes random variations both in flow characteristics zontal bars serving as motionless mixers built into a and in the local concentrations of components. Such gravity mixer. It was found that assuring the smallest behavior in gravity mixer tubes containing motionless possible shear action is essential to suppress segrega- mixers was already recognized by the team of L. T. tion. Fan [3, 7]. To interpret the experimental findings, a A special, alternatively revolving bulk solids mixer discrete stochastic model was applied [3, 7, 9, 18]. For equipped with motionless mixer elements shown in this, prior knowledge of the one-step transition proba- Fig. 2 was studied by Gyenis and Arva [13, 14, 25, bilities of the particles was necessary, which were 26] and provides practically segregation-free mixing. determined experimentally. This model seemed to be This device consists of two cylindrical containers (1) suitable to predict concentration distributions after and a mixer section between them containing hori- different passes through the mixers. However, on zontal mixer grids (2) composed of a number of examining the results of experiments and those obtained by the stochastic model, two kinds of dis- crepancies can be recognized. One is that, in spite of 1 identical particle properties, segregation and a spa- tially non-uniform mixing rate could be recognized 4 2 [3]. Gyenis and Blickle proposed a possible theoreti- cal explanation of this behavior [19]. It was assumed that spatially non-uniform transitions of particles between the adjacent layers were caused by the changing conditions during the non-steady-state flow. A correlation was proposed between the mixing rate and the energy dissipation caused by the interactions 3 of the particles and the mixer elements. The other discrepancy came from the fact that ear- 1 lier stochastic models used only expected values to (a) (b) characterize the one-step transition probabilities, to- tally ignoring their possible random variation. This can be quite high if large-scale flow instabilities occur, Fig. 2 Schematic diagram of the alternating revolving bulk solids especially in larger mixer volumes. Gyenis and Katai mixer proposed [20] a new type of stochastic model to (a) outline of the mixer, 1 - containers, 2 - grids composed of motionless mixer elements, 3 - components to be mixed, explain and describe the high random variations 4 - mount with rotating shaft, (b) helical motionless mixers experienced in repeated experiments in an alterna- arranged in grids

14 KONA No.20 (2002) ordered FixMix elements. The material to be mixed 1,0 (3) is filled into the lower container, then the mixer is rotated intermittently. In other words, the mixer body 0,8 is turned through 180° in one direction, then stopped 0,6 wheat flour - polypropylene granules to allow the bulk solids that are meanwhile at the top quartz sand - polypropylene granules enough time to flow down through the mixer ele- 0,4 ments. After this, the mixer is turned again through quartz sand - sodium chloride 0,2

180° but in the opposite direction. The whole proce- Degree of mixedness, M dure is then repeated till the required homogeneity is 0,0 obtained. Because of the periodically reversing direc- 0 2 4 6 8 10 12 14 16 18 20 22 24 tions of particle flow and the changing direction of Number of mixer turns acceleration and deceleration of the particles, segre- gation can be almost totally avoided. Fig. 3 Kinetic curves obtained by the mixer shown in Figure 2 By this method, a considerable improvement of the with extreme particle systems (particle sizes, densities, mixing rate was achieved, resulting in a short mixing and their ratios are given in Tables 1 and 2) time and a high homogeneity at a reasonably low power consumption [27]. This was evidenced by examining the mixing kinetics of particle systems pling method was given by Gyenis and Arva [13]. The composed of materials with extremely different physi- mass of each sample was 20 g, and the compositions cal properties, listed in Table 1. The density ratios of of samples were determined from 32.5 g material the component particles were varied from 1.2 : 1 to taken off from each sample, by dissolving the sodium 2.9 : 1, while the particle size ratios were changed chloride tracer particles and measuring the conduc- from 1 : 2.7 to 1 : 110 as seen from Table 2. Kinetic tivity of the solution, or in other cases, by weighing curves, i.e. the variation of the degree of mixedness the polypropylene tracer particles after mechanical as a function of mixer turns are plotted in Fig. 3. It separation of the samples. shows that this mixer provides rapid and segregation- Theoretical considerations gave an explanation of free mixing. The degrees of mixedness were deter- this segregation-free operation and of the high achiev- mined from the concentration variations of 65 spot able equilibrium homogeneity [14]. samples taken after different numbers of mixer turns Bauman [29] studied three different types of mo- or mixing times by metal templates according to a tionless mixers (Kenics, Komacs and Sulzer SMX) to regular pattern. A detailed description of this sam- mix particulate materials differing in mean particle size and size distribution, in bulk density and in mass ratio. It was concluded that the mass ratio of the com- ponents, as well as the shape and number of motion- Table 1 Particle sizes and densities of the constituents in mixes shown in Fig. 3. less mixers played a significant role in the achievable homogeneity. Kenics-type mixers proved to be the Mean particle size, Density, Component mm kg/m3 best when mass ratios were equal. But, when these differed significantly, Komacs and Sulzer SMX mixers Quartz sand 0.17 2650 gave much better results. Sodium chloride 0.46 2160 Motionless mixers are now increasingly applied Wheat flour 0.05 1510 in various process technologies to mix particulate Polypropylene granules 5.500910 solids. An interesting example for this is, e.g. the indus- trial standard of Tennessee [30], which mandates the use of Komax, Ross, Koch (Sulzer) or similar com-

Table 2 Size and density ratios of the constituent particles in monly accepted motionless mixers to manufacture mixes shown in Fig. 3. hydraulic cement.

Size ratio, Density ratio, Component 3. Bulk Solids Flow through Motionless Mixers Quartz sand - Sodium chloride 1 : 2.7.. 1.2 : 1 3.1 Gravity Flow Studies Quartz sand - Polypropylene 1 : 32.4 2.9 : 1 The continuous flow of bulk solids in vertical tubes Wheat flour - Polypropylene 1 : 110. 1.7 : 1 through Kenics-type motionless mixers was studied

KONA No.20 (2002) 15 by Gyenis and his co-workers [12, 31, 32] under dif- Another flow regime evolves when the solids flow ferent conditions. These mixer tubes consisted of rate is controlled at the inlet of the tube, with free out- three sections: (1) a plain feeding tube at the top, (2) flow at the bottom. In this case, the flow rate of bulk motionless mixers in the middle, and (3) another solids fed into the tube must be lower than the maxi- plain tube below the motionless mixers. The mass mal attainable throughput of the motionless mixers. flow rate of solids was controlled by two means: (a) This 3rd flow regime, shown in Fig. 4c, is character- by adjusting the feeding rate at the inlet, or (b) by ized by almost free-falling particles in the upper and controlling the particle discharge at the bottom. Local bottom plain tube sections with very low and progres- particle velocities were measured by fiber-optical sively decreasing solids concentration. In the mixer probes that captured light reflection signals. Average section, however, a rapid sliding particle flow takes and local solids hold-ups were determined by weigh- place along the surface of the mixer elements with a ing the material remaining in the tube after closing much higher solids hold-up compared to the upper both ends, and by gamma-ray absorption during oper- and lower tube sections. When the solids flow rate in ation at different cross-sections, respectively. The this flow regime is increased from zero to the maxi- axial mixing intensity was characterized by apparent mal attainable capacity, the average solids volume Peclet numbers and dispersion coefficients calculated fraction in the tube also increases from zero to a maxi- from residence time distributions of tracer particles. mal value. Reaching this latter stage, the upper tube The details of experiments and the applied experi- mental devices were described in several papers of Gyenis and his co-workers [12, 31, 32]. unlimited inflow unlimited inflow limited inflow Experiments [12, 31, 32] and DEM simulations [23] equally revealed that, depending on the feeding and discharging conditions, three characteristic flow regimes could be distinguished in such gravity mixer tubes, shown schematically by visualized simulation results in Fig. 4. A first type of flow regime takes place, shown in Fig. 4a, if a solids flow withdrawn from the bottom, e.g. by a belt or screw conveyor, is less than the maxi- mal possible throughput of the gravity mixer tube, determined by its diameter and the configuration of (a) (b) (c) motionless mixers. Naturally, the inflow of particles at the tube inlet must be unlimited in this case, e.g. directly from a hopper. This 1st flow regime is charac- terized by dense flow, i.e. high solids hold-up in all the three tube sections. st By increasing the discharged flow rate in this 1 limited discharge free outflow free outflow flow regime, the solids hold-up in the mixer tube decreases slightly, mainly due to the growing “gas bubbles” just below the lower surface of the motion- less mixer elements. After reaching the maximal throughput, which is achieved by unlimited outflow, the solids hold-up suddenly drops to a well-deter- (d) mined value, resulting in a second flow regime shown in Fig. 4b. It is characterized by a dense sliding parti- cle bed in the upper tube, accelerating particle flow along the mixer elements and almost free-falling parti- 0.0s 0.4s 0.8s 1.2s 1.4s 1.8s cles in the lower plain tube section below the mixer section. The transition between the 1st and 2nd flow regimes can be well recognized from the snap-shots Fig. 4 Characteristic flow regimes in gravity mixer tubes with motionless mixers in the middle tube section in Fig. 4d, obtained by DEM simulation, showing the (a) 1st flow regime, (b) 2nd flow regime, (c) 3rd flow regime, actual situations at successive moments in time. (d) transition between the 1st and 2nd flow regimes

16 KONA No.20 (2002) section above the motionless mixers suddenly these curves steeper. becomes choked with this sliding particle bed. This Achieving the maximum throughput of the given causes a rapid transformation from the 3rd to the 2nd mixer tube, the average solids volume fraction sud- flow regime. denly drops to a distinct point shown in Fig. 5, which Visual observations and experimental data revealed characterizes the 2nd flow regime in the given system. that in these flow regimes, a different and well- It should be emphasized in this respect that local defined correlation exists between the solids hold-up solids volume fractions in this flow regime changes in the mixer tube and the mass flow rate of particles from place to place, generally decreasing in down- taken off at the bottom or fed into the tube. wards direction from the inlet of the mixer section, As seen from Figs. 4a-d, the particle concentration and especially in the plain tube below the mixer ele- and thus solids volume fraction changes from place to ments. However, the average solids volume fraction is place along the length of the tube. However, since the a well-determined distinct value, belonging to the usual mixer tubes are composed of several sections maximal possible mass flow rate of particles under (e.g. feeding, mixing and post-mixing sections) and, this condition. This mass flow rate and average solids very frequently, free tube sections (inter-element dis- volume fraction depend on the diameter of tube, on tances) can be present between the individual mixer the physical properties of material, e.g. particle size, elements, it is reasonable to characterize operation density, shape, surface properties, particle-particle conditions by the mean solids volume fraction aver- and particle-wall frictions, as well as on the geometri- aged within the whole tube. cal and other properties of the motionless mixer ele- Plotting this mean solids volume fraction vs. the ments, on the length ratios of feeding, mixing and mass flow rate, important characteristics of the post-mixing sections, on the distance between the above-described flow regimes can be recognized, also mixer elements, etc. From Fig. 5, it is clearly seen indicating the conditions of transitions between them, that by decreasing the l/d ratio of the helical mixer as is shown in a general status diagram in Fig. 5. elements, the maximal throughput of the tube belong- The upper, slightly slanted curves of this diagram ing to the 2nd flow regime also decreases. The gap correspond to the changes in the 1st flow regime. between the solids volume fraction at the end point of Namely: by increasing the discharged mass flow rate the 1st flow regime curves and at the 2nd flow regime at the tube bottom, the solids volume fraction aver- mainly depends on the length of the plain tube below aged within the whole mixer tube also decreases the mixing section and on the distances between the slightly, mainly due to the increasing void fraction mixer elements: the gap diminishes if these plain tube within the section containing motionless mixers. sections are shorter. Decreasing the l/d ratio of the mixer elements also In the 3rd flow regime, shown by the lower curves decreases the mean solids volume fraction, making in Fig. 5, the mean solids volume fraction increases as the mass flow rate of solids fed into the tube inlet is increased. The higher the retaining effect of the mixer elements against the particulate flow, i.e. the 0,7 lower their l/d ratio, the higher the mean solids vol- 1st flow regime )

ε 0,6 ume fraction in the tube will be: i.e. the slope of the corresponding curves becomes steeper. After achiev- l/d2 0,5 l/d 1 l/d 1.5 ing the maximum throughput of the mixer tube from

0,4 this side, the feeding section becomes choked and the mean solids volume fraction in the tube suddenly 2nd flow regime rd 0,3 l/d1.5 increases: i.e. the 3 flow regime transforms to the l/d2 2nd flow regime. This transformation is reversible, but 0,2 l/d1 some hysteresis loop was experienced here. The gaps 0,1 between the ends of the 3rd flow regime curves and Mean solids volume fraction, (1- 3rd flow regime the data point of the 2nd flow regime mainly depend 0,0 on the length of the feeding section. 0 500 1000 1500 2000 2500 It should be noted that Fig. 5 is a general diagram Mass flow rate, kg/h only and therefore does not give precise quantitative

Fig. 5 General status diagram of flow regimes in gravity mixer data on given particle systems or mixer tube configu- tubes containing motionless mixers rations. It is a summary of our experiences and mea-

KONA No.20 (2002) 17 surements carried out with different particle systems (a) ) and mixer tubes under different conditions [12, 31, ε 0,7 1st flow regime 32]. However, this is a quite realistic diagram regard- 0,6 ing the scales of its axes and tendencies of its curves, 0,5 since they are close to the results obtained for the gravity flow of quartz sand of 0.17 mm mean particle 0,4 0,3 size in a gravity mixer tube of 1.6 m length and 0.05 m feeding post mixing inner diameter, with about 0.6 m total length of helical 0,2 section motionless mixers section mixer elements. Very similar curves were obtained by 0,1 recent DEM simulations for polymer particles with 3

Cross-sectional solids hold-up, (1- 0,0 3 mm diameter and 1190 kg/m density [23], with the 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 difference that the corresponding mass flow rates Length from the inlet, l with similar mean solids fractions are somewhat lower compared to those of quartz sand. (b)

Measurements [12] and DEM simulations [23] ) ε 0,7 revealed a periodic variation of the local solids volume 0,6 2nd flow regime fractions along the motionless mixers in the middle 0,5 tube section. As typical examples, Figs. 6a,b,c show the results obtained by the DEM simulation of partic- 0,4 ulate flow in a mixer tube of 0.05 m ID and 0.80 m 0,3 feeding motionless post mixing length, composed of a feeding, a mixing and a post- 0,2 section mixers section mixing section of 0.20, 0.30 and 0.30 m length, respec- 0,1 tively. The number of spherical model particles in the 0,0 Cross-sectional solids hold-up, (1- tube used for the DEM simulation was changed from 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 10,000 to 65,000, with 3.0 mm diameter and 1190 Length from the inlet, l kg/m3 density. Other parameters such as stiffness, restitution coefficients, particle-particle and particle- (c) ) wall frictions, and inlet velocity were described in ε 0,7 3rd flow regime detail by Szepvolgyi [23]. The mass flow rate con- 0,6 trolled at the inlet or at the outlet of the tube to gener- feeding post mixing 0,5 section motionless mixers section ate different flow regimes was varied between 300 0,4 and 1500 kg/h. From this diagram, it is seen that peri- odicity of local solids volume fractions took place in 0,3 all the three flow regimes, due to the multiple interac- 0,2 tion with the motionless mixers, which caused peri- 0,1 odic deceleration and acceleration of the flow along 0,0 Cross-sectional solids hold-up, (1- the mixer lengths. These diagrams also show signifi- 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 cant differences between the three flow regimes Length from the inlet, l regarding the change of local solids volume fraction along the various sections of the mixer tube. Fig. 6 Variation of the local cross-sectional solids volume frac- Experimental studies revealed that these flow tions along the tube length in the three characteristic flow regimes regimes had a great influence on the mixing perfor- (a) 1st flow regime, (b) 2nd flow regime, (c) 3rd flow regime mance, too. Fig. 7 shows some examples for resi- dence time distributions of tracer particles measured during gravity flows through Kenics-type motionless especially for smaller length-to-diameter (l/d) ratios. mixers [12]. The model material for these experi- However, due to the higher throughput and lower ments was quartz sand, the same as used for the mean residence time, the apparent Peclet numbers solids volume fraction measurements, and a short and axial dispersion coefficients were more beneficial plug of sodium chloride or polypropylene granules in the 2nd and 3rd flow regimes, mainly for higher l/d was used as the tracer. It was found that residence ratios. The best homogeneity values were obtained in time distributions were somewhat broader in the 1st the 2nd flow regime and, at higher mass flow rates in flow regime relative to the 2nd or 3rd flow regimes, the 3rd flow regime, close to its transition point [12].

18 KONA No.20 (2002) (a) of in-line mixers and other operations by effective gas- 3,0 solids contacting. no mixer, G830 kg/h 2,5 Motionless mixers can also be applied in counter- l/d2, G830 kg/h

) current gas-solids two-phase flow for more effective τ l/d1, G710 kg/h 2,0 phase contacting. It is known that fluidized bed 1,5 processes often use various types of inserts in the

1,0 particle bed to improve the fluidization behavior [33]. Frequency, f( Frequency, 1st flow regime Motionless mixers can replace the usual inserts, 0,5 favorably influencing the minimum fluidization gas 0,0 velocity, solids hold-up, and heat and mass transfer 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0 between the phases by enhancing the relative veloci- Residence time, τ ties and avoiding fluidization abnormalities.

(b) 3,0 4. Other Applications in Bulk Solids Handling

2,5 without motionless mixer, G2000 kg/h 4.1 Improvement of Bulk Solids Flow and

) l/d 2, G 2300 kg/h τ 2,0 l/d1.5, G1500 kg/h Reduction of Bulk Volumes l/d1, G950 kg/h In handling bulk solids, motionless mixers are well 1,5 suited to eliminate problems in the bulk solids flow, 1,0 and to decrease the bulk density in storage and trans- Frequency, f( Frequency, 2nd flow regime 0,5 port. In silos, inserts are frequently used to enhance flow 0,0 0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 uniformity in space and/or time [34], avoiding pulsa- Residence time, τ tion and bridging, which may totally stop the flow. Concentric, inverted or double cones, slanted plates (c) or rods are frequently used for this purpose. In this 3,0 way, funnel flow can be transformed to mass flow, 2,5 without motionless mixer, G 2000 kg/h avoiding segregation during discharge. For this pur- l/d2, G1760 kg/h ) pose, various forms of motionless mixers can also be τ 2,0 l/d1.5, G470 kg/h used, but before their application, careful design work l/d1.5, G1700 kg/h 1,5 l/d1, G700 kg/h is necessary with preliminary investigation of mater- 1,0 ial properties and its flow behavior through the Frequency, f( Frequency, 3rd flow regime motionless mixers to avoid potential troubles. 0,5 According to our experiences, motionless mixers in 0,0 gravity tubes make the flow of fine powders more sta- 0,0 1,0 2,0 3,0 4,0 5,0 6,0 ble, even if they are cohesive to some extent, such as Residence time, τ flour or ground coffee. It should be noticed, however, that this is true only for continuous or non-inter- Fig. 7 Residence time distribution curves obtained with Kenics- type motionless mixers in different flow regimes rupted flows. If continuous discharge is stopped, trou- (a) 1st flow regime, (b) 2nd flow regime, (c) 3rd flow regime bles can arise in re-starting the flow. This difficulty can be avoided by applying quasi-motionless mixers, connected to each other elastically, joining them by springs, thus making the individual mixer elements mobile to a certain extent [35]. Stresses inside the 3.2 Gas-solids two-phase flows bulk solids column cause some passive movements of Experiments were carried out by Gyenis et al. [12] the mixer elements, which is enough to start or to sta- in a concurrent downward gas-solids two-phase flow bilize the flow, therefore avoiding choking. in a test device where the flow rates of both phases In loading a container, silo, truck or railroad boxcar could be controlled more or less independently. By from a spout or hopper, the bulk volume of solids may increasing the gas flow rate, the mass flow rate and expand. Therefore, during storage or transportation, thus the solids hold-up could be enhanced consider- a considerable part of the available space is occupied ably, which is generally beneficial to the performance by air. However, if particulate materials are passed

KONA No.20 (2002) 19 through motionless mixers, the expansion of bulk vol- cles. ume can be reduced by 20-40 percent compared to Motionless mixers may be useful tools in pneumatic loading via plain tube, as was proven by experiments conveying lines, e.g. in horizontal tube sections. DEM with wheat grains [36]. When bulk solids, which were simulation studies revealed that particles that tended expanded already, are passed through motionless to settle downwards could be re-dispersed into the mixers, absolute reduction can also be achieved. By gas stream again by suitably designed motionless this method, as much as 4-10 percent more bulk mixer elements [23, 40]. This may reduce the salta- solids can be stored or transported in a given volume tion velocity, thus diminishing the required gas flow of a container or ship. To this end, motionless mixer rate. By using motionless mixers, a given section of elements should be well designed to avoid attrition or pneumatic conveying system can also serve as an breakage of the grains or particles. effective gas-solids contactor, too, or to realize other types of solids treatments simultaneously with con- 4.2 Coating, Size Enlargement, Size Reduction veying. Caution and preliminary experiments men- Motionless mixers are applicable not only to blend tioned above are also recommended here. particulate solids, but also to contact different parti- cles effectively with each other. In a suitably designed 4.4 Heat Treatment of Particulate Solids gravity mixer tube supplied continuously with two Heat transfer processes in particulate solids can be particulate solids differing in size, a coating of the big- improved by motionless mixers built in a cooler or ger particles with the smaller ones can be realized if heater, due to the multiple transmissions of particles suitable binding material is also supplied. Gyenis et from the bulk material to the heating or cooling sur- al. [37, 38] reported on a coating process of jelly face and back. In some cases, heat-transmitting tubes bon-bons with crystalline sugar, applying motionless or lamellas themselves, arranged, e.g. in a hopper, mixers. Similar equipment was used for coating can modify the flow pattern of the solids, similarly to ammonium nitrate fertilizer granules with limestone motionless mixers. powder to avoid sticking during storage. Granulation Very often, heat treatment is carried out in rotary or controlled agglomeration of particulate solids is heater or kiln, as is frequently used in the cement also conceivable by this method, but a crucial point is industry or, in smaller dimensions, in food process- to ensure proper conditions to avoid sticking of solids ing. Motionless mixers fixed inside a rotating unit, or deposition of the binding material onto the surface shown in Fig. 8, enhance the heat transfer between of the mixers. Disintegration or size reduction, as well particles and a streaming gas, or between the par- as controlled attrition [39], can also take place during ticles and the heated surface, as was patented by interaction between the particles and motionless mix- Bucsky et al. [41]. It also ensures uniform tempera- ers, especially at higher velocities ensured by gas- ture distribution throughout the cross-section of the solids two-phase flows. particle bed, which is of crucial importance in the treatment of heat-sensitive materials. Such a device is 4.3 Applications in Pneumatic Conveying also applicable for drying particulate solids. As was mentioned above, concurrent gas-solids flow in vertical tubes containing motionless mixers increases the flow rate and solids hold-up, also enhancing the mixing process compared to simple gravity flows of particles [12]. Because of the retain- ing effect of motionless mixers, the velocity differ- ence between the phases and also the residence time motionless of the particles can be increased considerably, com- gas mixers out pared to a plain tube. Such conditions are favorable grains for heat and mass transfer processes or chemical in hot gas in roasted reactions in gas-solids contactors, thus decreasing the product necessary dimensions. This makes the realization of out various operations during pneumatic conveying [39] conceivable. But for this, carefully planned pilot-scale experiments and caution in design are needed to Fig. 8 Rotary heater with motionless mixers to roast agriculture avoid troubles, e.g. choking or damage of the parti- materials

20 KONA No.20 (2002) 4.5 Drying Concluding Remarks Godoi et al. [42] used a vertical tube equipped with helical motionless mixers with perforations on their As was seen from this review, motionless mixers surface for the continuous drying of agricultural are useful tools for process improvements: not only grains. The particulate materials to be dried were fed for fluid treatments, but also in bulk solids technolo- continuously at the top, and slid or rolled down a- gies. In this latter field, the most detailed knowledge long the surface of the mixer elements. Heated gas is available in bulk solids mixing, discussed in a great streamed up along the flow channel between the number of papers. Investigations started more than mixer elements and through their perforations. Due thirty years ago, elucidating the kinetics and mecha- to the interactions between the grains, the motionless nisms of this operation, mainly in gravity mixing mixers and the gas, an effective mass and heat trans- tubes, but also in a special alternatively rotating bulk fer was achieved. The rotary equipment in Fig. 8 also solids mixer. Modeling and simulations helped to can be used for such a purpose. understand experimental findings and to predict the behavior and results of such equipment. 4.6 Dust Separation Particulate flows in motionless mixer tubes show Motionless mixers are applicable for the separation exiting phenomena which greatly influence the of particles from gases. The dust or volatile solids processes taking place in these devices. Other appli- content of hot industrial gases often causes troubles cations such as to improve the bulk solids flow in in pipeline operation due to deposition onto the tube tubes, chutes and silos, or to reduce the bulk volume wall, especially at critical sections. Based on labora- expansion led to significant results. In gas-solids two- tory- and pilot-scale experiments, Ujhidy et al. [43] phase flows, namely in pneumatic conveying and described a new gas purification method and equip- simultaneous treatment of bulk solids, their use offers ment applying helical motionless mixers shown in new possibilities for realization and process improve- Fig. 9. Solid particles are captured by a liquid film ment. Coating, size enlargement, size reduction and trickling down along the surface of mixer elements, attrition, heat treatment, drying, wet dust removal totally avoiding plugging and thus extra maintenance and dry particle separation are also promising fields of the gas pipeline system. Applying proper condi- of applications. tions, i.e. optimal superficial gas velocity and suitable motionless mixer geometry, the dry separation of Acknowledgement solids is also feasible, especially above one hundred or several hundred microns particle size. This effect The author wishes to acknowledge the support of is due to the centrifuging and collision of particles the Hungarian Scientific Research Fund (OTKA with the motionless mixers. T29313).

Dusty gas inlet Nomenclature Purified gas outlet d diameter or width of motionless mixer element m Washing liquid inlet 2 K′ax axial dispersion coefficient, Eqn.6 m /s

Km kinetic constant of mixing

Dust separation Ks kinetic constant of segregation tubes with wetted Droplet separation Lmix total length of the studied mixer section m motionless mixers unit with motionless mixers L length, measured from the inlet m l length of one motionless mixer element m l/d length-to-diameter ratio of a motionless mixer element Slurry collecting basin M degree of mixedness, defined by Eqn.2 [15, 16] Slurry outlet s estimated standard deviations of sample concentrations taken from a mixture

s0 estimated standard deviations of sample Fig. 9 Dust removal unit applying motionless mixers concentrations taken in a totally segregated

KONA No.20 (2002) 21 state 13) Gyenis J.; Arva J.: Improvement of Mixing Rate of xi mass fraction of the tracer particles within Solids by Motionless Mixer Grids in Alternately Re- the i-th sample taken at the outlet of the volving Mixers, Powder Handling and Processing, 1 (2), mixer tube 1989, 165-171. 14) Gyenis J.: Segregation-Free Particle Mixing, Handbook Pe′ apparent Peclet number, Eqn.5 of Powder Technology, Vol. 10 [Eds. A. Levy and H. m2 second central moment of the residence Kalman]. Elsevier, Amsterdam-Tokyo, 2001, 631-645. time distribution of the tracer particles, 15) Fan L, T.; Wang R, H.: On Mixing Indices, Powder Tech- Eqn.3 s2 nol., 11, 1975, 27-32. ti the residence time of tracer particles 16) Rose H, H.: A Suggested Equation Relating to the Mix- within the i-th sample s ing of Powders and its Application to the Study of Per- t— the mean residence time of all tracer formance Certain Types of Machines, Trans. Instn. particles in the mixer tube, Eqn.4 s Chem. Engrs., 37 (2), 1959, 47-56. 17) Gyenis J.; Nemeth J.; Arva J.: Gravity Mixing of Solid Φ segregation potential [14] Particle Systems in Steady State Static Mixer Tubes, Swiss Chem., 13 (5), 1991, 51-55. Bibliography 18) Wang R, H.; Fan L, T.: Stochastic Modeling of Segrega- tion in a Motionless Mixer, Chem. Eng. Sci., 32, 1977, 1) Pattison D, A.: Motionless Inline Mixers Stir Up Broad 695-701. Interest, Chem. Eng., May 19, 1969, 94-96. 19) Gyenis J.; Blickle T.: Simulation of Mixing During Non- 2) Wang R, H.; Fan L, T.: Contact Number as an Index of Steady State Particle Flow in Static Mixer Tubes, Acta Transverse Mixing in a Motionless Mixer, Particulate Chimica Hungarica Models in Chemistry, 129 (5), Science and Technology, 1, 1983, 269-280. 1992, 647-659. 3) Chen S, J.; Fan L, T.; Watson C, A.: The Mixing of Solid 20) Gyenis J.; Katai F.: Determination and Randomness in Particles in a Motionless Mixer A Stochastic Approach, Mixing of Particulate Solids, Chem. Eng. 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22 KONA No.20 (2002) tion for Road and Bridge Construction. State of Ten- ing Coating of Grains with Smaller Particles, Magyar nessee. Section 900. March 3, 1995. Sheet of 5 of 19. Kemikusok Lapja, 44 (1), 1989, 16-21. (in Hungarian) 31) Gyenis J.: Strömungseigenschaften von Schüttgütern 38) Gyenis J.; Arva J.; Nemeth J.: Coating of Grains with im statischen Mischrohr, Chem.-Ing.-Tech. 64 (3), 1992, Fine Particles, Preprints of 9th International Congress of 306-307. Chemical Engineering, CHISA’ 87, Prague, 30 August 32) Nemeth J.; Gyenis J.; Nemeth J.: Gravity and Gas-Solid 4 September, 1987, 1-10. Flow in Static Mixer Tubes, PARTEC 95, Preprints of 39) Kalman H.: Personal communications. 3rd European Symposium Storage and Flow of Particu- 40) Gyenis J.; Ulbert Zs.; Szepvolgyi J.; Tsuji Y.: Discrete late Solids (Janssen Centennial), 21-23 March 1995, Particle Simulation of Flow Regimes in Bulk Solids Mix- Nürnberg, 459-467. ing and Conveying, Powder Technol., 104, 1999, 248- 33) Hilal N.; Ghannam M, T.; Anabtawi M, Z.: Effect of Bed 257. Diameter, Distributor and Inserts on Minimum Fluidiza- 41) Bucsky Gy.; et al.: Method and Equipment for Continu- tion Velocity, Chem. Eng. Technol., 24 (2), 2001, 161- ous Gentle Heat Treatment of Heat Sensitive Particulate 165. Materials, Hung. Pat. No. 209 661 A, 1995, 1-7. (in Hun- 34) Yang S, C.; Hsiau S, S.: The Simulation and Experimen- garian) tal Study of Granular Materials Discharged from a Silo 42) Godoi L, F, G.; Park K, J.; Alonso L, F, T.; Corre˛a W, A, with the Placement of Inserts, Powder Technology, 120 Jr.: Particle Flow in an Annular Static Mixer Dryer, (3), 2001, 244-255. Proc. 5th World Congress of Chemical Engineers, Vol. 35) Bucsky Gy.; et al.: Method and Equipment for Mixing of VI, 14-18 July 1996, San Diego, CA, U.S.A., 356-361. Continuous Streams of Particulate Materials, Hung. Pat. 43) Ujhidy A.; Bucsky Gy.; Gyenis J.: Application of Motion- No. 22/90, 1990. (in Hungarian) less Mixers in Gas Purification A Case Study, Proc. 36) Chen S, J.; Fan L, T.; Chung D, S.; Watson C, A.: Effect Joint Symposium (Korea/Hungary) on Energy and of Handling Methods on Bulk Volume and Homogene- Environmental Technology for 21st Century, November ity of Solid Materials, J. Food Science, 36, 1971, 688-691. 16, 2001, Taejon, Korea, 3-15. 37) Gyenis J.; Farkas I.; Németh J.; Hollósi S.: Energy Sav-

Author’s short biography

Janos Gyenis Professor Janos Gyenis graduated in Chemical Engineering at the University of Veszprem, Hungary. He received Dr. Habil and Ph.D. degrees at the Technical Uni- versity of Budapest, and a D.Sc. title at the Hungarian Academy of Sciences. At present, he is a full professor at the University of Kaposvar, Department of Engi- neering, and director of the Research Institute of Chemical and Process Engineer- ing. Since 1971, he has been involved in a number of collaborative research works with leading laboratories and university departments in Europe, the United States and Asia. He is now working with the mixing and flow of particulate solids and the application of motionless mixers for various purposes. Up to now, he has published about 120 papers and he is co-inventor of 22 patents.

KONA No.20 (2002) 23