Epstein N., Grace J.R. Spouted and Spout-Fluid Beds: Fundamentals and Applications

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Drying of solutions, slurries, and pastes 217

the liquid bridge.

The viscous force (dynamic) is caused by the resistance offered to

liquid ﬂow in the radial direction of a liquid bridge, and, unlike the static force, always

opposes relative motion.

The magnitude of the interaction forces owing to the presence of a liquid depends

on the liquid saturation level, given by the ratio of liquid and particle volumes. The

interaction force is a function of a number of variables, such as the amount of liquid,

the surface tension, contact angle between liquid and particle, distance between the

particles, and particle geometry.

39,40

If a small amount of liquid is added to a system,

enough only to form a thin ﬁlm coating the particles, cohesiveness increases because the

liquid smoothes the particle surface roughness by ﬁlling irregularities and decreasing

the speciﬁc surface area. The liquid will also reduce the separation distances among

particles, allowing electrostatic forces to become more signiﬁcant. When the amount of

liquid is sufﬁcient to ﬁll all the interparticle voids, the system is in the capillary state,

in which particles are bonded by the capillary suction forces acting at the air–liquid

interfaces on the particle surfaces. When the particles are completely immersed in the

liquid, the system is in an overcapillary state, and the adhesion is caused by surface

tension on the liquid convex surfaces.

Solid bridges are stronger than liquid ones and may be generated by sintering, chemical

reaction, particle fusion, recrystallization, and evaporation.

39–41

Only those formed by

evaporation occur in the drying of pastes on inert particles.

Passos and Mujumdar

identiﬁed the types of interaction forces developed in wet

spouted and ﬂuidized beds by adding metered amounts of glycerol. They pointed out that

these forces can affect gas and solid ﬂows, leading to uncontrollable particle agglom-

eration and poor gas–solid contact. They also showed that spout formation and bed

collapse in wet beds differ for spherical and nonspherical particles. They proposed an

empirical model for predicting P

and U

in wet beds of nonspherical particles and

the minimum conditions for t he internal spouting regime in wet two-dimensional beds of

spherical particles. The models yielded good predictions of U

and P

in beds with

glycerol, but observed that the predicted trends contradict some literature data obtained

with conventional pastes.

To predict the minimum spouting velocity in wet beds, Spitzner Neto et al.

developed

a model based on the analysis of interaction forces in the annulus. They applied the model

to the drying of animal blood and homogenized egg paste. Their model assumes that the

particles in the annulus form a huge particle-agglomerate that offers the major resistance

to ﬂuid dynamic circulation. It also considers that the air velocity at minimum spouting

is the minimum value required to break particle bonds. At this condition, the force of the

airﬂow on the particle-agglomerate would represent the maximum resistance offered by

the agglomerate. This force has been estimated from the interparticle forces and liquid

saturation i n the bed. Another simplifying assumption is that the drag force required for

spouting the wet particles is given by the drag force in the dry bed plus the cohesive

forces acting on the wet particles. The pressure drop over the particle-agglomerate has

been estimated from the Ergun

equation, and only liquid bridges were considered. The

theory of Schubert et al.

was employed to estimate the magnitude of the liquid bridge

interaction forces.

218 Jos

e Teixeira Freire, Maria do Carmo Ferreira, and F

abio Bentes Freire

This modeling led to an equation for estimating U

in wet beds for a given saturation

rate, given the liquid saturation and U

ms0

(minimum spouting velocity for the dry bed).

in the ﬁnal equation is also a function of a number of other variables, such as

the bed height, bed surface diameter, air inlet diameter, resistive tension between two

particles, and total number of particles. The authors compared the model predictions

with experimental values obtained from drying egg paste at 60

◦

C and reported poor

agreement, with estimates always higher than experimental values. This poor perfor-

mance was attributed to overestimation of the magnitude of interaction forces by the

model, consistent with the conclusion that particle cohesion is associated mainly with

interaction forces among particles coated by a low-moisture powder, rather than by

liquid.

The discrepancies in the trends predicted by the model of Passos and Mujumdar

and

those veriﬁed experimentally in beds operating with real pastes indicate that glycerol

should not be used as a standard paste to simulate a wet bed. A real paste shows a very

different wetting pattern and suffers signiﬁcant changes in rheological properties during

the process. It is added into the bed either as a liquid or as a high-moisture-content slurry,

but its compositon changes dramatically during drying, with the liquid or paste turning

into a low-moisture-content powder. These changes are not accounted for in the model.

The same shortcomings apply to the model of Sptizner Neto et al.,

which considered

neither the paste composition changes nor the existence of dynamic forces linking the

particles.

Employing interparticle forces to model the drying of pastes in beds of inert particles

is appealing, but considerable further investigation is required to achieve consistent

modeling and reasonable results. Microscopic measurements of interaction forces, such

as those reported by Pagliai et al.

for wet ﬂuidized beds, are still lacking for wet spouted

beds. The evidence obtained so far is all based on macroscopic measurements of bed

dynamic parameters, such as P

and U

. The evidence that in steady-state operation

the particles are coated by a low-moisture powder, rather than by a liquid, must also be

incorporated into the modeling.

12.5 Final remarks

In this chapter, a review of the published developments in paste dr ying on inert particles

since 1974 has been presented. An extensive list of materials that have been dried

successfully has been found, which is encouraging, but it is obvious that considerable

further work is required to investigate paste drying and design of efﬁcient dryers. An

analysis on drying of sludge from metals ﬁnishing wastewater treatment plants showed

that a spouted bed is cost-competitive with conventional technology.

The main focus

of recent research is to study how paste composition affects drying behavior, and a few

contributions on this topic have already been published.

15,23

Another aspect demanding

attention is scaleup: most studies have been conducted in small dryers, with diameters

from 0.15 to 0.5 m, and very few investigators have operated dryers of diameter greater

than 0.5 m. Scaleup, however, is challenging.

Drying of solutions, slurries, and pastes 219

As far as transport parameters are concerned, it is clear that the paste affects not

only the dynamic behavior of the bed, but also heat and mass transfer phenomena.

Most experimental investigations, however, still focus on ﬂuid dynamic parameters

(P

, U

, particle circulation, etc.). Thermal and mass transfer parameters need further

investigation.

The modeling of drying on inert particles has not yet been successful in faithfully

describing the process. Simulations are far from providing good predictions of process

variables, and no model may be safely recommended. No model has succeeded in

incorporating the effects of the presence of a paste on the bed dynamics or in describing

the complex changes observed as the liquid becomes a powder during the drying. Models

based on interparticle forces are promising, but require more realistic assumptions.

Finally, there is some evidence that the particles used as a support for drying do not really

constitute an inert material. This is another aspect that demands further investigation.

Chapter-speciﬁc nomenclature

Volumetric airﬂow rate, m

Volumetric paste or solution feed rate, m

s Saturation degree, deﬁned as (1 – ε)V

/(εV

)

V Bed volume, m

Solution or paste volume, m

Volume of particles in the bed, m

Volume of the static bed of particles, m

ϕ Particle sphericity

Subscripts

0 Property of dry bed

i Inlet conditions

N Nozzle

o Outlet conditions

References

1. C. Strumillo, A. Markowiski, and W. Kaminski. Modern developments in drying of pastelike

materials. In Advances in Drying, vol. 2, ed. A. S. Mujumdar (Washington, DC: McGraw-Hill,

1983), pp. 193–231.

2. K. B. Mathur and N. Epstein. Spouted Beds (New York: Academic Press, 1974).

3. C. R. F. Souza and W. P. Oliveira. Spouted bed drying of Bauhinia forticata link extract: the

effects of feed atomizer position and operating conditions on equipment performance and

product properties. Braz. J. Chem. Eng., 22 (2005), 239–247.

4. T. Schneider and J. Bridgwater. The stability of wet spouted beds. Drying Technol., 11 (1993),

277–301.

220 Jos

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abio Bentes Freire

5. T. Szentmarjay and E. Pallai. Drying of suspensions in a modiﬁed spouted bed dryer with an

inert packing. Drying Technol., 7 (1989), 523–536.

6. A. Reyes, G. D

ıaz, and R. Blasco. Slurry drying in gas-particle contactors: ﬂuid-dynamics

and capacity analysis. Drying Technol., 16 (1998), 217–233.

7. A. S. Markowski. Quality interaction in a jet spouted bed dryer for bio-products. Drying

Technol., 11 (1993), 369–387.

8. Q. T. Pham. Behavior of a conical spouted-bed dryer for animal blood. Can. J. Chem. Eng.,

61 (1983), 426–434.

9. M. I. R

e and J. T. Freire. Drying of pastelike materials in spouted beds. In Drying ’89,

ed. A. S. Mujumdar and M. A. Roques (New York: Hemisphere, 1989), pp. 426–432.

10. H. Barrett and A. Fane. Drying of liquid materials in a spouted bed. In Drying ’89,ed.A.S.

Mujumdar and M. A. Roques (New York: Hemisphere, 1990), pp. 415–420.

11. C. Brereton and C. J. Lim. Spouted bed drying of sludge from metals processing industry

wastewater treatment plants. Drying Technol., 11 (1993), 389–399.

12. Q. Guo, S. Hikida, Y. Takahashi, N. Nakagawa, and K. Kato. Drying of microparticle slurry

and salt-water solution by a powder-par ticle spouted bed. J. Chem. Eng. of Japan, 29 (1996),

152–158.

13. J. Xu, S. Osada, and K. Kato. Limiting efﬁciency for continuous drying of micro-particle

slurries in a powder-particle spouted bed. J. Chem. Eng. of Japan, 31 (1998), 35–40.

14. R. L. Cunha, K. G. Maialle, and F. C. Menegalli. Evaluation of drying process in spouted

bed and spout ﬂuidized bed of xanthan gum: focus on product quality. Powder Technol., 107

(2000), 234–242.

15. M. F. D. Medeiros, S. C. S. Rocha, O. L. S. Alsina, C. E. M. Jer

onimo, U. K. L. Medeiros, and

A. L. M. L. da Mata. Drying of pulps of tropical fruits in spouted bed: effect of composition

on drying performance. Drying Technol., 20 (2002), 855–881.

16. P. I. Spitzner Neto, F. O. Cunha, and J. T. Freire. Effect of the presence of paste in a conical

spouted bed dryer with continuous feeding. Drying Technol., 20 (2002), 789–811.

17. M. L. Passos, A. L. G. Trindade, J. V. H. d’Angelo, and M. Cardoso. Drying of black liquor

in spouted bed of inert particles. Drying Technol., 22 (2004), 1041–1067.

18. D. S. Cordeiro and W. P. Oliveira. Technical aspects of the production of dried extract of

Maytenus ilicifolia leaves by jet spouted bed drying. Int. J. Pharmac., 299 (2005), 115–126.

19. M. S. Bacelos, P. I. Spitzner Neto, A. M. Silveira, and J. T. Freire. Analysis of ﬂuid dynamics

behavior of conical spouted beds. Drying Technol., 23 (2005), 427–453.

20. M. Benali and M. Amazou. Drying of vegetable starch solutions on inert particles: quality

and energy aspects. J. Food Eng., 74 (2006), 484–489.

21. E. Pallai-Vars

anyi, J. T

oth, and J. Gyenis. Dying of s uspensions and solutions on inert particle

surface in mechanically spouted bed dryer. China Particuology, 5 (2007), 337–344.

22. S. Wachiraphansakul and S. Devahastin. Drying kinetics and quality of okara dried in a jet

spouted bed. LW, 40 (2007), 207–219.

23. A. R. F. Almeida. Analysis of paste drying in a spouted bed. PhD thesis, Federal Univ. of S

Carlos, Brazil (in Por tuguese) (2009).

24. K. W. Allen. “At forty comes understanding” – a review of some basics of adhesion over the

past four decades. Int. J. Adhesion & Adhesive, 23 (2003), 87–93.

25. C. Gay. Stickiness – some fundamentals of adhesion. Integr. Comp. Biol., 42 (2002), 1123–

1126.

26. F. P. Collares, J. R. D. Finzer, and T. G. Kieckbush. Glass transition control of the detachment

of food pastes dried over glass plates. J. Food Eng., 61 (2004), 261–267.

Drying of solutions, slurries, and pastes 221

27. C. C. Rodrigues. Analysis of the drying of suspensions in spouted bed with inert particles.

M.Sc. dissertation, Federal Univ. of S

ao Carlos, Brazil (in Portuguese) (1993).

28. K. Patel, J. Bridgwater, C. G. J. Baker, and T. Schneider. Spouting behavior of wet solids.

In Drying ’86, ed. A. S. Mujumdar and M. A. Roques (New York: Hemisphere, 1986),

pp. 183–189.

29. M. L. Passos and A. S. Mujumdar. Effect of cohesive forces on ﬂuidized and spouted beds

on wet particles. Powder Technol., 110 (2000), 222–238.

30. M. L. Passos, A. S. Mujumdar, and G. S. V. Raghavan. Spouting and spout-ﬂuidization of

dry-wet particles in a two-dimensional bed. In Drying of Solids, ed. A. S. Mujumdar ( New

Delhi: Sarita, Prakaschan, 1990), pp. 211–220.

31. J. D. A. Santana, A. L. T. Charbel, M. L. Passos, and G. Massarani. Effect of interparticle

forces on the spouted bed ﬂow behavior. In Proceedings of IADC ’97 (Itu, SP, Brazil: Univ.

of Campinas Press, 1997), vol. A, pp. 135–142.

32. P. I. Spitzner Neto. Study of paste drying and ﬂuid dynamics of spouted beds in the presence

of pastes and liquids. Ph.D. thesis, Federal Univ. of S

ao Carlos, Brazil (in Portuguese) (2001).

33. M. F. D. Medeiros. Inﬂuence of the material properties on the drying of pulp fruit in spouted

beds. Ph.D. thesis, Univ. of Campinas, Brazil (in Portuguese) (2001).

34. I. Taruna and V. K. Jindal. Drying of soy pulp (okara) in a bed of inert particles. Drying

Technol., 20 (2002), 1035–1051.

35. A. Kmiec. Simultaneous heat and mass transfer in spouted beds. Can. J. Chem. Eng., 53

(1975), 18–24.

36. I. M. Oliveira and M. L. Passos. Simulation of drying suspension in a conical spouted bed.

Drying Technol., 15 (1997), 593–604.

37. W. P. Oliveira. Study of paste drying in conical spouted bed. Ph.D. thesis, Federal Univ. of

ao Carlos, Brazil (in Portuguese) (1996).

38. J. P. K. Seville, C. D. Willet, and P. C. Knight. Interparticle forces in ﬂuidization: a review.

Powder Technol., 113 (2000), 261–268.

39. H. Schubert, W. Herrmann, and H. Rumpf. Deformation behaviour of agglomerates under

tensile stress. Powder Technol., 11 (1975), 121–131.

40. H. Schubert. Principles of agglomeration. Int. Chem. Engng., 21 (1981), 363–377.

41. P. J. Sherrington and R. Oliver. Granulation (London: Heyden and Sons, 1981).

42. S. Ergun. Fluid ﬂow through packed columns. Chem. Eng. Progr., 48:2 (1952), 89–94.

43. P. Pagliai, S. J. R. Simons, and D. Kudra. Towards a fundamental understanding of deﬂu-

idisation at high temperatures: a micro-mechanistic approach. Powder Technol., 148 (2004),

106–112.

13 Granulation and particle coating

Sandra Cristina dos Santos Rocha and Osvaldir Pereira Taranto

13.1 Introduction

Particle coating and granulation have attracted increasing attention in the past decade,

with the objective of modifying particle physical and physicochemical properties. The

increasing interest has appeared in several industrial sectors, such as the chemical, food,

pharmaceutical, iron ore, agricultural, and nuclear industries. Cosmetics, ﬂavorings,

essences, enzymes, proteins, vegetables, seeds, fertilizers, sweets and candies, drugs,

pigments, and nuclear fuel microspheres are examples of products that have been mod-

iﬁed by coating or g ranulation processes.

1–4

Until the 1950s, rotary panels or drums were the predominant types of equipment

for particle coating and granulation. Since then, new equipment and processes have

been implemented by the pharmaceutical industry, owing to the replacement of tablets

coated with sugar solutions by those coated with polymer ﬁlms. In this new equipment,

a suspension or solution is atomized on particles suspended by hot air. A thin ﬁlm is

deposited on the particle surfaces and dried by the hot air as the particles circulate

through the chamber. Among the equipment, spouted beds, including Wurster coaters

(Chapter 14) and other designs, are intended to improve the process performance and

ﬂuid dynamics – for example, by applying vibration and adding draft tubes.

5–12

The choice of the most adequate equipment depends on the physical properties of

the particles to be coated, as well as on the coating material. The process conditions

are also critical to obtain a good-quality coated product. Fluidized and spouted beds

1,13

have been extensively used recently, mainly for the ﬁlm coating of particles.

14–21

This

application of spouted beds is justiﬁed by its ﬂuid dynamic characteristics, such as good

particle circulation, guaranteeing product homogeneity and effective solid–ﬂuid contact,

as well as high rates of heat and mass transfer. Spouted beds also offer the possibility

of process automation and use of the same equipment for other applications such as

granulation, drying, and coating of other products.

In the coating and granulation processes, the proper ties of the ﬁnal product are deter-

mined by the equipment geometry, operating conditions, coating material formulation,

and characteristics of the material to be coated.

22–25

The gas distributor and the column

geometry determine the particle dynamics in the bed, thereby inﬂuencing the process

Spouted and Spout-Fluid Beds: Fundamentals and Applications, eds. Norman Epstein and John R. Grace.

Published by Cambridge University Press.



Cambridge University Press 2011.

Granulation and particle coating 223

performance. The type and position of the spray nozzle are also important, as they govern

droplet for mation and drying of agglomerates, consequently affecting process efﬁciency.

Operating variables such as particle loading and spouting airﬂow rate also have a

direct impact on the formation of agglomerates and on ﬁlm drying because of their

inﬂuence on bed ﬂuid ﬂow, particle dynamics, and regime stability. Another key process

parameter is the air temperature, which affects the drying, and hence agglomerate

formation. Atomization conditions, such as air and coating material, must be chosen to

avoid the formation of large droplets, which are difﬁcult to dry and favor the formation

of unwanted agglomerates. At the same time, it is important to avoid the formation

of very small droplets, which dry too quickly, even before reaching the particles. The

coating material ﬂowrate must also be balanced with the solvent evaporation capacity,

controlling formation of agglomerates and premature drying of suspended droplets.

The ﬁlm-forming material is usually a polymer for ﬁlm-type coating processes and

therefore a f actor limiting the process thermal conditions. The choice of polymer depends

on the speciﬁc coating objectives; its concentration affects the processing time and coat-

ing quality; its temperature inﬂuences the viscosity, and consequently droplet formation

and particle agglomeration; its surface tension and contact angle, responsible for adhe-

sion of droplets to the solid surface, directly affect the process performance. The s olvent

determines the thermal conditions and the requirement for safety.

Physical properties of the particulate material to be coated, such as shape, size, and

cohesiveness, directly affect the ﬂuid dynamics and, as a consequence, help to govern the

drying and coating distribution, thus inﬂuencing the process and the ﬁnal product quality.

It is also known that the success of the coating depends on the wettability of the solution,

suspension, or hot melt on the particle surface. The wettability (characterized by the

contact angle) depends on the solid surface characteristics and the liquid surface tension.

13.2 Granulating and coating mechanisms

Because the “industrial art” of coating and granulation originated in several relatively

independent industries, terminology tends to be confusing. The term coating is some-

times applied indiscriminately to all processes in which material is added to a solid

surface. For instance, extruded pellets, containing several particles, may be called coated

particles. The terms coating, granulation, balling, pelletization, and agglomeration are

sometimes used interchangeably, and the particles resulting from those processes may be

called granules, balls, pellets,oragglomerates.

However, the terms more commonly

used are agglomeration or granulation, coating (or ﬁlm coating), and hot melt coating,

depending on the deﬁnitions and mechanisms described in the following subsections.

22,26

13.2.1 Granulation or agglomeration

In granulation or agglomeration, granules are formed by atomization, followed by dry-

ing of a bonding liquid on their surface. This bonding solution or suspension (e.g., of

sugar or starch) spreads over the particles and builds liquid bridges between the original

224 Sandra Cristina dos Santos Rocha and Osvaldir Pereira Taranto

Figure 13.1. Formation of agglomerates.

particles, as shown in Figure 13.1. The liquid bridges are then dried by hot air, resulting

in agglomerates. When the liquid is atomized continuously, the particles continue to

agglomerate, causing growth of the granule. During the growth process, smaller par-

ticles may also be produced because of attrition among particles, or even among the

agglomerates themselves.

Granulation or agglomeration is carried out to achieve one or several of the following

objectives: improved ﬂowability, better handling and compression, desired particle size

distribution, preferred shape, modiﬁed particle density, improved appearance, better dis-

solution time, or simply elimination of ﬁne particles. The desirable characteristics of an

agglomerated product are dust-free granules of optimum size distribution depending on

the application, low rigidity, low apparent density, porous structure, good dispersibility,

and excellent solubility.

13.2.2 Film coating

Fluidized and spouted beds with top, bottom, or tangential atomization can be used to

perform ﬁlm coating on the surface of particles of a wide range of diameters. Objectives

are to improve the particle appearance; mask unpleasant taste and/or odor; improve ﬂow

properties; reduce the tendency to agglomerate; prevent formation of cakes or lumps;

improve product dispersion in solvents; avoid generation of dust or powder; reduce

losses and handling hazards; protect the product from atmospheric humidity, light, or

heat; isolate incompatible substances (by keeping one in the particle nucleus and the

other in the coating); control the release of active components; and diminish side effects,

especially for pharmaceutical agents.

The coating material can be a solution, suspension, emulsion, latex, or hot melt (as

described below), resulting in a ﬁlm. The key components of a ﬁlm-coating formulation

are a polymer, a plasticizer, pigments, and a solvent. The most common polymers

are cellulose and acrylic derivatives, usually dissolved in water or organic solvents.

Polymers that are insoluble in water, such as methylcellulose, can still be used as

aqueous dispersions.

The desired characteristics of ﬁlm coatings are small variation

of the product weight (usually from 2% to 8%), size, and shape; short processing times;

high process efﬁciency; ﬂexibility in coating formulation; and good resistance of the

ﬁlm to attrition conditions or exposure to light and humidity.

Film coating is a complex dynamic process that involves formation of coating material

droplets, contact and spreading of droplets on the particle surface (providing that the

Granulation and particle coating 225

Figure 13.2. Film coating mechanism. Adapted from Glatt.

Figure 13.3. Coating by layer formation.

solid surface is wetted by the liquid), and evaporation of the solvent (Figure 13.2). In

addition to interfacial properties, the spreading of the liquid on the solid surf ace is

facilitated by low liquid viscosity and by small droplets.

The ﬁnal product is obtained by deposition of several layers on each particle, as shown

schematically in Figure 13.3.

Coated materials are expected to exhibit good stability, good ﬂowability, smooth

surfaces, prescribed colors, and other speciﬁc properties, deﬁned for each case.

13.2.3 Hot melt coating

When coating with hot melt material, the coating agent is applied in its molten state,

and solidiﬁes on the surface of the particles. This type of process produces a desirable

coverage, eliminating the solvent. The process mechanism is similar to ﬁlm coating,

except that solvent evaporation is replaced by cooling solidiﬁcation of the coating

material on the particle surface.

13.3 Advances in spouted bed granulation/coating

Granulation/coating in spouted beds is regarded in the literature as a single process

consisting of injecting a liquid phase into a bed of particles or seeds to be granulated or

226 Sandra Cristina dos Santos Rocha and Osvaldir Pereira Taranto

coated, together with hot air to promote spouting. A thin layer of the liquid deposits on

the particle surfaces as they pass through the atomization zone; this layer is then dried

by hot air, while the particles travel up the central spout and then descend through the

annulus.

If liquid bridges form and then dry, leading to bonding of particles and growth by

agglomeration, the product will be either a granule or an agglomerate. By controlling the

process conditions, the process may result in layer-by-layer growth, leading to a coated

product. The ﬁrst patents covering this type of spouted bed application were obtained

by Berquin

and Nichols.

Spouted bed granulation/coating may be carried out in batches or continuously. Atom-

ization of the suspension or solution can be performed at the top, at the bottom, or tan-

gential to t he bed. One coating/granulation equipment manufacturer

lists the following

capacities for ﬁlm coating:

Batch systems:

From 2 g to 1,500 kg/batch (top spray coating)

From 50 g to 700 kg/batch (bottom spray coating)

From 500 g to 250 kg/batch (tangential coating)

Continuous spouted bed coating: bottom spray – from 2 g to 3 metric tonnes per hour.

Brazilian Petrol S.A. described the operation of the ﬁrst commercial urea granulation

plant in Brazil, based on a multistage spout-ﬂuid granulator.

This process is owned

by Toyo Engineering Corporation (TOYO). This spout-ﬂuid bed granulation process is

applied in 17 urea plants worldwide, varying from 50 to 3250 tonnes/day production

capacities.

The existence of a region of high porosity, high gas velocity, and high gas temperature

at the bottom of the spouted bed makes it ideal for spraying liquids in continuous coating

or granulation processes. The rapid evaporation in this zone causes the temperature of

the gas to drop before the gas contacts the particles, making it possible to operate

at high gas temperatures without thermal damage, but promoting high evaporation

rates. The most important advantage of spouted beds for this type of operation is the

cyclic and systematic particle movement, which results in a homogeneously coated

product.

Figure 13.4 shows a schematic of a spouted bed batch granulation or coating sys-

tem

with atomization at the bottom of the bed. Although this is the most popular

conﬁguration, top spray is also common, especially in laboratory-scale units.

32,17

Experimental studies on spouted bed coating processes often analyze the inﬂuence of

the operating conditions on the efﬁciency of ﬁlm adhesion to the particle surface, to ﬁnd

appropriate conditions for a ﬁnal product of good quality.

The efﬁciency of adhesion in a spouted bed coating process, deﬁned as the ratio of

the mass of solids adhering to the particles to the mass of solids supplied to the bed,

given by

η =

− M

. (13.1)