gas solid mixing

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Process Engineering Guide: GBHE-PEG-MIX-708

Gas Solid Mixing Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Gas Solid Mixing CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 4 GAS-SOLID FLUIDIZED BED 3 5 MIXING IN FLUIDIZED BEDS 3 5.1 Group A Powders 3 5.2 Group B Powders 4 5.3 Group C Powders 4 5.4 Group D Powders 4 6 MECHANISMS OF MIXING AND SEGREGATION 4 6.1 Particle Segregation 5 6.2 Rate of Mixing 7 6.3 Solids Circulation 7



7 GRID DESIGN 7 7.1 Choice of Configuration 7 8 PLENUM CHAMBER DESIGN 7 9 SPOUTED BED 7 10 NOMENCLATURE 9 11 BIBLIOGRAPHY 9 FIGURES 1 POWDER CLASSIFICATION DIAGRAM FOR

FLUIDIZATION BY AIR 4 2 DIAGRAMMATIC REPRESENTATION OF MIXING BY A

SINGLE RISING BUBBLE IN A BED OF SMALL PARTICLES 5

3 SEGREGATION PATTERNS WITH 'PRACTICAL'

MATERIALS 6 4 SPOUTED BED – DIAGRAMMATIC 7 DOCUMENTS REFERRED TO IN THIS ENGINEERING GUIDE 10



0 INTRODUCTION/PURPOSE This Guide is one in a series of Mixing Guides produced for GBH Enterprises. 1 SCOPE This Guide provides general awareness of gas-solid mixing in fluidized and spouted beds. It does not deal with fluidized bed reactors or driers. 2 FIELD OF APPLICATION This Guide applies to Process Engineers in GBH Enterprises worldwide. 3 DEFINITIONS No specific definitions apply to this Guide. With the exception of terms used as proper nouns or titles, those terms with initial capital letters which appear in this document and are not defined above are defined in the Glossary of Engineering Terms. 4 GAS-SOLID FLUIDIZED BED The basic gas-solid fluidized bed has many applications in Process Engineering. However their design is mainly empirical, and a few specialized companies offer their designs based on their practical experience. Most of the academic work has been done with sand and air, and the industrial published work is related to specific use like coal combustion, fluidized bed driers and catalytic crackers. 5 MIXING IN FLUIDISED BEDS The degree of mixing or segregation achieved in a fluidized bed depends on the solids' properties i.e. particle size, density and shape.



A classification chart which divides the solids in four groups, A, B, C, and D was proposed by D Geldart 1973 [Ref. 2] (see Figure 1) and is the accepted method of describing powder properties in the fluidization field. However, when the properties of a powder fall near a boundary it will always be difficult to define its behavior. 5.1 Group A Powders

Group A powders are easy to fluidize and gross circulation of powder occurs, which produces a rapid mixing, with considerable gas recirculation.

5.2 Group B Powders

Group B powders are easy to fluidize but there is little gas recirculation. 5.3 Group C Powders

Group C powders are cohesive and 'normal' fluidization is extremely difficult, and particle mixing is very limited.

5.4 Group D Powders

Group D powders are difficult to mix and they are more suitable for processing in a spouted bed.

The A/C boundary has received much attention, because of the dramatic change in mixing behavior with small changes in particle size or gas density.



FIGURE 1 POWDER CLASSIFICATION DIAGRAM FOR FLUIDISATION BY AIR (Ambient Conditions)

6 MECHANISMS OF MIXING AND SEGREGATION Mixing is caused solely by the bubbles rising through the solids (P N Rowe 1965 [Ref. 3], A W Nienow 1978 [Ref. 4] and R R Cranfield, 1978 [Ref. 5]). Each bubble gathers a wake of material from near the bottom of the bed and carries it to the surface. Each bubble also draws up a spout of material below itself (see Figure 2).



FIGURE 2 DIAGRAMMATIC REPRESENTATION OF MIXING BY A SINGLE RISING BUBBLE IN A BED OF SMALL PARTICLES

No mixing occurs in the absence of bubbles, even if the bed is fluidized, and if by maldistribution of the gas a region of the bed is not bubbling, the particles there will be almost stagnant. Three separate zones are normally present in a fluidized bed. The distributor zone, with solid particles mixing with the incoming gas, normally in the turbulent regime. The freely bubbling zone with convection currents of solids. The zone adjacent to the wall is a layer of slowly descending particles which do not mix with the bubbling zone, and go down to just above the distributor zone. 6.1 Particle Segregation When particles of different sizes or densities are present, then segregation as well as mixing will occur, and the final equilibrium distribution of particles in each zone will be a function of their sizes, densities, and the relative proportions of each one. Rowe P N et al, 1972 [Ref. 6] showed that depending on the gas velocity it is possible to either mix or segregate two kinds of particles of different densities and even if sometimes complete mixing cannot be achieved, it is possible to get a middle region of uniform composition with segregated zones of the heavier one at the bottom and the lighter one at the top (see Figure 3).



FIGURE 3 SEGREGATION PATTERNS WITH 'PRACTICAL' MATERIALS



6.2 Rate of Mixing Except for beds of very fine particles (< 60 µm) all excess gas above minimum fluidization flow passes through the bed in the form of bubbles, that is:

Each bubble carries a wake of particles equal to about one third of its own volume [Ref. 5]. If εB is the fraction of bed filled with bubbles then the average velocity at which particles move downwards will be given approximately by:

6.3 Solids Circulation The actual flow patterns depend on the number of bubbles, the position of the jets on the distributor and the presence of gas maldistribution which will have a big influence in the solids movement, because of the stagnant zones which form where there are no gas bubbles. The walls always have a boundary gas layer which holds particles, hence the movement will always be downward in the walls and in between the position of the orifices and upward with the bubbles. Depending on the distance between the orifices in the distributor and also the gas velocity, stagnant zones can be present and a great number of alternative designs are available depending on the particular application.



7 GRID DESIGN 7.1 Choice of Configuration The method of Wen et al, 1986 [Ref. 7] is recommended. The grid in a fluidized bed reactor is intended to distribute the fluidizing medium uniformly over the bed cross-section. In practice, this has taken a variety of forms. Whatever the physical form all are fundamentally classifiable in terms of the direction of fluid entry, either upwards, laterally, or downwards. The choice is dependent on prevailing process conditions, mechanical feasibility and cost. 8 PLENUM CHAMBER DESIGN The design of the plenum chamber needs to be 'Compatible Compatible' with the grid to ensure uniform gas distribution. 9 SPOUTED BED In the case of large particle sizes which are difficult to fluidize (see 5.4 - Group D), all the gas is fed in a single orifice in the centre of the bed and a single circulation loop is stabilized, and the particles spend most of the time in the annular region slowly moving downward. This arrangement is suitable for slow gas-solid reactions or drying where a long residence time of solids in the (almost) stagnant gas is acceptable or required (see Figure 4). The Harwell report: SPS RR 26, shows that single size solids are well mixed. In the case of two sizes it was found that each component was well mixed. However both components did not have the same residence time.



FIGURE 4 SPOUTED BED – DIAGRAMMATIC