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

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Granulation and particle coating 227

Figure 13.4. Experimental setup for spouted bed coating or granulation.

This measure has been used by several researchers.

16,34–38

Several authors have veriﬁed

that particles grow linearly with time. A growth constant is then represented by

K =

− m

. (13.2)

Empirical correlations for η and K, obtained by data ﬁtting or from statistical analysis

of factorial experimental design, are available in the literature for spouted bed coating of

several products. For example, Kucharski and Kmiec

analyzed the coating of tablets

with sucrose solution and obtained:

= 0.3253Re

1.089





−0.0646



air



0.0393

(

1 − C

)

1.0074

−5.53

, (13.3)

where η

is the efﬁciency of inertial ﬁxation of the atomized droplets, given by

= St k

/(St k +0.25)

1.012

. (13.4)

Here the Stokes number is deﬁned as

St k =

H=0.002m

36μd

. (13.5)

The Sauter mean diameter of the atomized droplets, d

, can be estimated from the

correlation of Nukiyama and Tanasawa,

whereas gas velocity at the central (spout)

zone, u

H=0.002m

, is estimated from the equation proposed by Kmiec,

or it can be simply

taken as the oriﬁce velocity.

228 Sandra Cristina dos Santos Rocha and Osvaldir Pereira Taranto

In an investigation of coating alumina with aqueous solutions of sucrose and talc,

Oliveira et al.

developed correlations for the growth coefﬁcient and efﬁciency as a

function of operating conditions:

K = 0.143



air



0.440



air



0.225

− 0.00817, (13.6)

η = 6.05



air



− 1.944



air



+ 174.2



air



− 17980



air



− 4.204.

(13.7)

Other authors have also found that the atomization pressure and air temperature

inﬂuence process efﬁciency and particle growth kinetics. One correlation for process

efﬁciency, working with independent encoded variables X

to X

(each within the −1

to +1 range), corresponding to spouting gas mass ﬂowrate, spouting gas temperature,

atomization pressure, and coating suspension mass ﬂowrate, respectively, is

η = 67.80 − 6.15X

+ 2.22X

+ 2.72X

− 2.84X

+ 5.83X

−2.56X

+ 4.57X

+ 2.12X

− 3.29X

. (13.8)

This equation is based on a 2

factorial experimental design with four central points.

Statistical analysis was applied to quantify and correlate the effects of operating indepen-

dent variables on the process efﬁciency of urea coating with aqueous polymer suspension.

The ranges of the independent variables included when developing this correlation were

1.33 × 10

−2

≤ W

air

≤ 1.66 × 10

−2

kg/s; 50 ≤ T

air

≤ 70

◦

C; 1.38 × 10

≤ P

≤ 2.07 ×

kPa; and 1.077 × 10

−2

≤ W

≤ 1.425 × 10

−2

kg/s.

Empirical analyses of spouted bed coating operations for different products and a

large range of experimental conditions are available in the literature. Some studies have

also evaluated product quality and have provided results concerning such properties as

uniformity, particle protection, and controlled release. Table 13.1 summarizes the main

publications from which data are available.

High efﬁciencies are reported in all papers cited in Table 13.1, sometimes surpass-

ing 95 percent, showing the viability of spouted beds for coating of particles. Some

authors

16,47,49–52

have also conﬁrmed the ability of this technology to provide a product

of high quality.

Spouted bed coating of spherical submillimeter nuclear fuel kernels of uranium oxide

or uranium carbide with pyrolytic carbon (pyC) and/or pyrolytic silicon carbide (SiC),

to contain ﬁssion products generated in a high-temperature gas-cooled nuclear reactor,

has been studied intensively.

53,75,58

The pyrocarbon can be generated

by thermal

decomposition of methane

or of other hydrocarbons,

whereas the SiC is produced

by pyrolysis of methylsilane, various methylchlorosilanes, or mixtures of a hydrocarbon

with either SiH

(silane) or SiCl

. The coating process has typically been carried out in a

conical-cylindrical spouted bed furnace of 55 to 130 mm ID at temperatures of 1450

◦

for porous pyC coatings to 2150

◦

C for high-density isotropic pyC, and kernel charges

of around 1 kg f or each coating.

An earlier bistructural isotropic (BISO) fuel consisted

Granulation and particle coating 229

Table 13.1. Empirical analysis of granulation/coating in spouted beds.

Reference Column geometry Particle/process Results

Weiss and

Meisen

Cone-cylindrical Urea/coating with hot-melt sulfur Process efﬁciency and

product quality

Kucharski and

Kmiec

Cone-cylindrical Tablets/coating with sucrose aqueous

solution

Process efﬁciency and

mass distribution on

particles

Kucharski and

Kmiec

Cone-cylindrical Tablets/coating with sucrose aqueous

solution

Particle growth kinetics

Oliveira et al.

Cone-cylindrical Alumina/coating with sucrose and talc

aqueous solution

Process efﬁciency and

particle growth

kinetics

Taranto et al.

Cone-cylindrical Tablets/ﬁlm coating with aqueous

polymeric suspension

Fluid dynamics and heat

transfer

Barrozo et al.

Cone-cylindrical Soybean seed/coating with fertilizers Particle growth

Oliveira et al.

Cone-cylindrical Tablets/ﬁlm coating with aqueous

polymeric suspension

Fluid dynamics and

particle growth

kinetics

Ayub et al.

Two-dimensional Urea/coating with hot-melt sulfur Growth kinetics and

process efﬁciency

Donida and

Rocha

Two-dimensional Urea/ﬁlm coating with aqueous

polymeric suspension

Growth kinetics and

process efﬁciency

Silva et al.

Cone-cylindrical Tablets/ﬁlm coating with aqueous

polymeric suspension

Process efﬁciency

Donida and

Rocha

Two-dimensional Urea/ﬁlm coating with aqueous

polymeric suspension

Process efﬁciency and

coating quality

Almeida

Cone-cylindrical

spouted and

ﬂuidized beds

Broccoli seeds/ﬁlm coating with

aqueous polymeric suspension

Process efﬁciency and

coating quality

Pissinati and

Oliveira

Cone-cylindrical Gelatinous capsules/ﬁlm coating with

aqueous polymeric suspension

Process efﬁciency and

coating quality

Publio and

Oliveira

Cone-cylindrical Tablets/ﬁlm coating with aqueous

polymeric suspension

Process efﬁciency and

coating quality

Kfuri and Freitas

Spout-ﬂuid bed Tablets/ﬁlm coating with aqueous

polymeric suspension

Process efﬁciency

Oliveira et al.

Cone-cylindrical Gelatinous capsules/ﬁlm coating with

aqueous polymeric suspension

Process efﬁciency

Martins et al.

Cone-cylindrical Gelatinous capsules/ﬁlm coating with

aqueous polymeric suspension

Process efﬁciency and

coating quality

Borini et al.

Cone-cylindrical Excipient and dosage with

acetaminophen/hot melt granulation

with polyethylene glycol

(MW 4000)

Particle growth and

product quality

of ∼0.8 mm kernels, each coated with an inner buffer layer of porous pyC under an

outer layer of high-density pyC, to a ﬁnal diameter of ∼1 mm. Later, operation at higher

temperature resulted in a tristructural isotropic (TRISO) fuel consisting of 0.5-mm or

smaller kernels, each uniformly coated ﬁrst by a thin layer of gas, then a porous buffer

layer of low-density pyC, followed by a sealing layer of graphite powder treated with

230 Sandra Cristina dos Santos Rocha and Osvaldir Pereira Taranto

thermosetting resins, an inner isotropic SiC interlayer, and ﬁnally an outer envelope

of high-density isotropic pyC, with the overall diameter approaching 1 mm.

53,58

recent efforts

are exploring fuel designs that incorporate zirconium carbide, either as

a supplementary layer or to replace the SiC.

The choice of spouting over ﬂuidization for this coating process is dictated partly by

its more systematic particle cycling, but mainly by the absence of a distributor plate,

on which deposition of pyrolysis products could cause blockage.

Essentially the same

technique has been used in pyrolytic coating of prosthetic devices, such as aortic heart

valves and hip joints, with a biocompatible silicon–pyrocarbon alloy.

These devices are

much larger and more complex in shape than nuclear fuel particles. They are distributed

in a spouted bed of smaller inert particles, and their coatings are much thicker than for

nuclear fuel particles. This bioengineering process, even more than nuclear fuel coating,

thus gives rise to considerable particle segr egation (Chapter 8), and has generated study

of segregation,

as well as an account of the differences between thermochemical

coating in the nuclear and bioengineering areas.

Simulation and modeling have also been applied to spouted bed coating and gran-

ulation. Models have been based both on basic conservation equations

56–59

and on

population balances.

2,32,60,61

For example, Pi

na et al.

proposed a model to describe

a silicon chemical vapor deposition (CVD) spouted-bed reactor. A phenomenological

model including hydrodynamics, interphase mass exchange between the different regions

of the bed, and kinetic rates for heterogeneous and homogeneous reactions was imple-

mented, together with a population balance accounting for growth and agglomeration

of silicon particles. Such models contribute to understanding the complex simultaneous

phenomena involved in particle granulation/coating in a spouted bed. They may also

assist in achieving reliable scaleup from pilot to industrial units.

13.4 Surface properties inﬂuencing granulation/coating processes

Recent studies on granulation in ﬂuidized beds and particle coating in spouted beds have

evaluated the inﬂuence of particle and suspension properties on adhesion and, therefore,

on the characteristics of the ﬁnal product and process efﬁciency. In addition to liquid

viscosity, density, and surface tension, which directly inﬂuence the coating by affecting

atomization, the suspension-particle wettability is an important factor in determining

process performance.

20,63–68

For liquids, surface tension plays a crucial role with respect to adhesion. If the liquid

wets the surface, it spreads on the solid surface. An adhesive of low energy levels or

low surface tension is readily adsorbed by high-energy-level s olids; the contact angle

decreases and the wetting is effective. If the solid surface has a lower energy, the contact

angle is high and wetting is poor.

Wettability, the ability of a drop to wet and spread

on the surface of a particle, governs the adhesion and growth rate of particles in coating

processes.

66,70–72

It can be quantiﬁed by the contact angle formed by the three phases:

solid, liquid, and gas. The contact angle is a property of the solid–liquid–gas system,

Granulation and particle coating 231

Table 13.2. Suspension compositions (% by mass) in coating study of Rocha et al.

Suspension

composition

(mass %) HEC

Polyethylene

glycol (PEG)

Magnesium

stearate

Titanium

dioxide

Tween



Colorant Talc Water

1 5.5 0.75 1.0 1.25 – 1.0 3.5 87.0

2 3.5 0.75 – 1.25 1.0 1.0 3.5 89.0

3 3.5 0.75 1.0 1.25 – 1.0 3.5 89.0

HEC = Hydroxyethylcellulose

Table 13.3. Contact angles of particle-suspensions in coating study of Rocha et al.

Contact angle (degrees)

Material Glass ABS



∗

Suspension 1 37.8 ± 1.4 70.4 ± 1.4 76.7 ± 2.4 79.0 ± 2.3

Suspension 2 34.9 ± 2.4 54.3 ± 3.5 57.5 ± 3.3 70.9 ± 2.4

Suspension 3 37.9 ± 2.3 70.0 ± 1.5 79.5 ± 1.6 80.9 ± 1.2

∗

PS = polystyrene; PP = polypropylene

depending on characteristics of both the liquid (e.g., surface tension) and the solid (e.g.,

hygroscopicity, roughness, and surface tension).

In the study of Rocha and Donida,

glass beads, ABS



, polypropylene (PP), and

polystyrene (PS) of different sizes, shapes, densities, and surface tensions, as well as

three suspensions of different formulations, were used to analyze the inﬂuence of

particle-suspension wettability when atomizing suspensions over beds of spouted par-

ticles. The suspension compositions are shown in Table 13.2, and the system contact

angles are presented in Table 13.3.

For each type of particle, suspensions 1 and 3 supplied similar contact angles, owing

to their similar surface tensions. On the other hand, for suspension 2, the contact angles

were smaller because of its lower surface tension. The variability in glass wettability (or

contact angles) for the three suspensions was small. The effect of the high glass surface

energy (85 × 10

−3

N/m) is predominant when seeking good wettability (low contact

angles), which was found to be nearly independent of the liquid surface tension in the

range analyzed (from 46.3 to 67.5 × 10

−3

N/m).

For ABS and PS, which have intermediate surface energies (34 × 10

−3

and 32 ×

−3

N/m, respectively), the variability of contact angles was great, showing the strong

inﬂuence of the liquid surface tension in this range of solid surface energies. PP has the

lowest surface energy (30 × 10

−3

N/m) and, as in the case of the glass, the wettability

variation was small for the three suspensions. In this case of low solid surface energy,

contrary to the glass results, wettability was poor for all three cases with high contact

angles for the three suspensions.

232 Sandra Cristina dos Santos Rocha and Osvaldir Pereira Taranto

Tests were carried out in a typical spouted bed coating system shown in Figure 13.4,

including an acrylic cone-cylindrical column.

The following conditions were main-

tained constant in each run: ﬁxed bed height 155 mm; ratio of air velocity to minimum

spouting velocity 1.15; spouting air temperature 60

◦

C; suspension ﬂow rate 6.0 mL/min;

atomization pressure 239 kPa; processing time 30 min.

By analysis of the process behavior, it was possible to sort the particles into two

groups: the ﬁrst group included glass particles and ABS, which, because of their wetting

characteristics (contact angle ≤ 70

◦

), were coated by the suspensions; the second group

included PS and PP particles, which, also because of their superﬁcial characteristics

(contact angle > 76

◦

), promoted drying of the suspension, resulting in a powdery

product collected in the cyclone. High contact angles indicate poor wettability and weak

adhesion; the interparticle friction inside the bed tends to strip off any dry ﬁlm adhering

weakly t o the particle surface, resulting in suspension drying.

These results are in agreement with those of Vieira et al.,

who investigated the

adhesion of an aqueous polymeric suspension to several types of inert particles (glass,

ABS, placebo, PP, PS, and low-density polyethylene [LDPE]) in a spouted bed with top

atomization of the suspension.

The process performance was evaluated by particle relative growth, coating efﬁciency,

and solids loss from elutriation, which was found to be also related to the wetting and

adhesion characteristics:

The smallest growth was obtained with a suspension of glass and ABS particles,

associated with a glass-only suspension and for the suspension with the greatest

polymer (HEC) concentration.

The glass coating efﬁciencies for all three suspensions were high and very similar. In

these cases, the high glass surface energy led to strong adhesion of the ﬁlm formed

on the particle, regardless of the suspension surface tension, for the range analyzed.

For ABS particles, the efﬁciency was of the same order as for the glass-only suspension,

which presented better wettability and therefore greater adhesion. All efﬁciencies were

in the range considered satisfactory for pharmaceutical industry processes, η>65%.

As for the coating process of glass and ABS, aside from material adhering to the

solids surface and the powder collected by the cyclone, another part of the suspension

remained in the bed. The loss by elutriation was smaller, but the process efﬁciency

also diminished. For conditions that result in poor wettability and, consequently, weak

adhesion, the ﬁlm tended to detach from the particles and form small dry plates that

remained in the bed. This can cause instabilities in the bed ﬂuid dynamics, interfering

with the solids circulation and also with the pressure drop,

in the same way as

reported for drying of pastes in spouted beds

73,74

(see Chapter 12).

To summarize, adhesion characteristics (solid surface energy and liquid–solid wetta-

bility) signiﬁcantly inﬂuence particle coating in spouted beds. Particles of high surface

energy and suspensions giving low contact angles favor the process, resulting in high

particle growth, high efﬁciency, and steady spouting.

Granulation and particle coating 233

13.5 Concluding remarks

Particle granulation and coating in spouted beds have aroused industrial and research

interest since their development in the 1960s. A considerable number of studies have been

reported in the literature. The results of these studies indicate t hat it is possible to obtain

high process efﬁciencies by controlling the variables that inﬂuence spouting dynamics

and the granulation or coating mechanisms. Conditions that provide strong adhesion of

the liquid to the particle surface (low contact angle) and choosing adequate operating

conditions can lead to products of superior quality compared with competing equipment,

in some cases with lower process times. Modeling of particle coating and granulation

has contributed to understanding process mechanisms, as well as in predicting important

process and equipment parameters, assisting scaleup.

This evolution in the development of spouted bed particle granulation and coating

has resulted in medium- and large-scale units, mainly in the chemical and pharmaceu-

tical sectors. Equipment is available to produce up to 3 tonnes/h of coated particles

continuously. For granulation, TOYO’s spout-ﬂuid bed urea granulation technology has

been applied to plants in different countries with production capacities between 50 and

3250 tonnes/day. Spouted beds have been proven to provide reliable granulation and

coating processes, resulting in round uniform products with high drying and adhesion

efﬁciencies. This ﬁeld is open to investigation to enhance the process fundamentals and

to provide new products.

Chapter-speciﬁc nomenclature

C solids concentration, kg of solids/kg of suspension

K growth coefﬁcient, kg/s

W mass ﬂow rate, kg/s

Greek letters

η adhesion efﬁciency

efﬁciency of inertial ﬁxation of atomized droplets

φ sphericity

Subscripts

0 initial

air air

at atomization

sp suspension

t at time t

234 Sandra Cristina dos Santos Rocha and Osvaldir Pereira Taranto

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