Effectiveness of hybrid drying (S.J. Kowalski, K. Rajewska).pdf

Chemical Engineering and Processing 48 (2009) 1302–1309

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Chemical Engineering and Processing:

Process Intensiﬁcation

journal homepage: www.elsevier.com/locate/cep

Effectiveness of hybrid drying

S.J. Kowalski, K. Rajewska ∗

Department of Process Engineering, Institute of Technology and Chemical Engineering, Poznan University of Technology, pl. Marii Skłodowskiej Curie 2, 60-965 Poznan, Poland

article info

abstract

Article history:

Received 16 February 2009

Received in revised form 22 May 2009

Accepted 27 May 2009

Available online 6 June 2009

The aim of this paper is to examine the effectiveness of convective drying enhanced with microwave

and infrared radiation. Four drying methods: convective, convective-microwave, convective-infrared and

convective-microwave-infrared, were analyzed with respect to the length of the drying time by securing

a good quality of dried products. To this aim the drying curves, the temperature of drying bodies, and the

drying rates were determined experimentally and the appraisal of the dried product quality was made

optically. The experiments were carried out on the kaolin samples in the form of cylinders. It was stated

that a proper combination of these three drying methods may result in a very high drying rate and at the

same time in a very good quality of the dried product.

Keywords:

Combined convective-microwave-infrared

drying

Drying time

Quality of dried products

1. Introduction

Combined drying methods like convective-microwave [16–22] ,

microwave-vacuum [23,24] and other methods [25–28] are pro-

posed by many authors to rise the effectiveness of drying, i.e. to

minimize the drying time and the consumption of energy as well

as to improve the quality of dried products.

The presented here studies dealt with an intensiﬁcation of con-

vective drying by application of microwave and infrared radiation.

An attempt was made to ﬁnd out a hybrid drying method ensuring

minimal drying time at simultaneously preserving good quality of

dried products. It was concluded that a combination of these three

methods gives very positive results, however, a suitable application

of the individual drying methods and a reasonable portioning of the

energy sources is necessary. Otherwise the fracture of the material

may occur.

The drying kinetics, i.e. the drying curves, the drying rates,

and the dried body temperature evolutions, as well as the qual-

ity of the dried products were analyzed for the four different

schedules of drying, namely, convective, convective enhanced

with microwaves, convective enhanced with infrared radiation,

and convective enhanced with microwave and infrared radiation.

The tests were carried out on the cylindrical sample made of

kaolin.

The main aim of this paper is to present the experimental stud-

ies showing that combined drying processes consisted of different

drying methods may beneﬁt by very positive results as it concerns

both shortening of the drying time and a good quality of the dried

product.

The idea of combined drying is examined earlier by some

authors [1,2] . Garcia and Bueno [3] considered the efﬁciency of

combined convective-microwave drying of agar gel and Gelidium

seaweeds under different operating conditions. Glouannec et al.

[4] and Salagnac at al. [5] presented the kinetics of drying allow-

ing a combination of convection as well as infrared and microwave

radiation. Zhang et al. [6] presented a comprehensive review of

recent progress in microwave-related combined drying research

and recommendation for future research to bridge the gap between

laboratory research and industrial application. Itaya et al. [7] ana-

lyzed the behavior of drying-induced stresses in a ceramic molded

slab by convective drying enhanced with microwave heating.

There are also a number of papers concerning both modeling and

experimental methodology of combined convective-microwave or

radiant-convective drying, as e.g. [8–15] . Among these there are

papers, which take into account mechanical effects like deforma-

tions and drying-induced stresses, as e.g. [8–10,14,15] , and those

which do not consider these effects, as e.g. [11–13] .

2. Experimental methods and materials

∗ Corresponding author. Tel.: +48 61 665 3690; fax: +48 61 665 3649.

E-mail addresses: stefan.j.kowalski@put.poznan.pl (S.J. Kowalski),

kinga.rajewska@put.poznan.pl (K. Rajewska).

Convective, microwave, and infrared drying differ from each

other in the way of energy supply. In the convective drying the

energy is supplied through the body surface from the ambient hot

air. In microwave drying heat is supplied volumetrically due to high

frequency polarization of the dipole molecules, mainly of water

molecules. In infrared drying the energy is supplied to the body due

doi: 10.1016/j.cep.2009.05.009

S.J. Kowalski, K. Rajewska / Chemical Engineering and Processing 48 (2009) 1302–1309

1303

Table 1

Drying parameters for the individual programs.

0.5 [%]

m db

0.01 [g]

FDP

5 [min]

Max. drying rate

0.1 [g/min]

0.01 [cm 2 ]

224.21

140

0.4

18.5

227.73

1.1

141.37

230.21

0.8

227.91

2.2

to surface radiations and can be absorbed, reﬂected or permeated

through the body.

As far as the enhancement of convective drying with microwave

energy is concerned, we have tested the following schedules:

Apart from combining of the convective drying (hot air drying)

with the microwave drying the authors used additionally the third

method, namely, the infrared radiation applied in the ﬁrst period of

drying. In this period the surface is covered with the thin ﬁlm of liq-

uid since the moisture transport from the material interior towards

the surface is sufﬁcient enough to create such a ﬁlm. The evapora-

tion of this liquid is similar to that from an open liquid surface. The

temperature of the material becomes constant during this period,

and the rate of drying depends mainly on the air physical parame-

ters. Application of the infrared radiation in our tests was beneﬁted

by shortening of the drying time without unfavorable inﬂuence on

the material quality.

The authors tested a great number of various combinations of

the three mentioned methods of drying. In this paper, however,

there are presented only those test, which yielded the best results.

We have to mention that a thoughtless enhancement of the con-

vective drying with the microwaves may sometimes bring more

harm than advantage. For example, application of the microwave

enhancement by drying of brittle materials during the second

period of drying, i.e. when the surface is dry and the core wet,

causes rapid increase of the temperature and the pressure inside,

which burst the material. Therefore, in the four drying schedules

presented in this paper the convective drying is enhancement with

the microwaves and the infrared radiation only in the heating and

in the constant drying rate periods.

1. Application of microwave energy at the beginning of drying. In

such a case the interior of the material was heated intensively

which caused expulsion of moisture towards the surface, where

it evaporated and was evacuated outside the dryer by the stream

of air. The drying rate increased radically and the drying time

reduced signiﬁcantly.

2. Microwave energy was applied at the moment when the rate of

convective drying started to decrease. The surface of the mate-

rial at this moment was nearly dry and the rest of moisture was

concentrated inside the body. Additional electromagnetic energy

generated heat inside the body and the produced vapor pushed

out the moisture towards the surface where it was removed out-

side the dryer. The drying rate increased rapidly at the moment

of application of microwaves. Supplying of microwave energy at

this stage of drying seems to be the most effective in the case of

thick and hardly heating bodies.

3. The smallest efﬁciency of convective drying take place close the

end of drying. Therefore, application of microwave enhancement

at this stage of drying can be justiﬁable for some materials.

Fig. 1. Scheme of laboratory hybrid drier.

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S.J. Kowalski, K. Rajewska / Chemical Engineering and Processing 48 (2009) 1302–1309

Fig. 2. Convective drying: (a) drying curve and the body temperature; (b) relative rate of convective drying; (c) photo of the sample after drying.

In order to show how the different drying methods inﬂuence

the drying time and the quality of the dried products, the following

schedules of drying are presented:

The cylindrical kaolin samples were seated in the drier cham-

ber on a special ceramic thimble with mandrel embedded on the

balance located beyond the chamber. In this way a continuous mea-

surement of the sample weight was possible during all methods of

drying, also during the microwave drying.

The hybrid drier enabled combined drying consisted of the three

mentioned methods. The instrumentation enable programming

and control of the velocity and the temperature of the air supplied

to the drier chamber, control of the microwave power, two-step

control of the infrared heater, and the measurement of the sample

surface temperature with the help of the optical pyrometer.

In all the above deﬁned processes, the temperature of the air

supplied to the drier was 80 ◦ C, the velocity of the air inﬂux

1. 0

1. Convective drying.

2. Convective drying enhanced with microwaves at the beginning

up to 30 min.

3. Convective drying enhanced with infrared radiation in the con-

stant drying rate period.

4. Convective drying enhanced with microwaves and infrared radi-

ation through the ﬁrst 70 min of drying.

The material used for studies was the granulated kaolin, from

which a greasy paste of moisture content 0.4 kg/kg db (dry basis)

was prepared. From this greasy paste the cylindrical samples of

diameter 30 mm and height 60 mm were molded. The air relative

humidity H , the dry mass of the samples m db , the evaluated contact

surface area with the air A , the maximum of drying rate, and the

duration of the falling drying rate period FDP are given in Table 1 .

The drying tests were realized in the laboratory hybrid drier,

whose scheme is presented in Fig. 1 .

0.1 m/s, and the microwave power was 100 W. The exposition

of the microwave and infrared energy was chosen in such a way

to not damage the dried sample. The kinetics of drying in all these

processes was analyzed, that is, drying curves, the drying rate, and

the temperature of dried material T s . The duration of each process

was set up to the moment when the ﬁnal moisture content of the

sample c.a. 0.06 kg/kg db was reached.

The quality of dried samples was assessed visually. Our aim was

to inspect whether there appear any damages on the sample in a

S.J. Kowalski, K. Rajewska / Chemical Engineering and Processing 48 (2009) 1302–1309

1305

Fig. 3. Convective drying enhanced with microwaves: (a) drying curve and the body temperature; (b) relative rate of convective drying enhanced with microwaves; (c) photo

of the sample after drying.

given drying strategy. When so, then in which place and what was

their character: whether the cracks were circumferential or radial,

in upper or in lower part of the sample, whether they were like

surface scratch or rather deep ﬂaws, ﬁssure or even burst, etc?

Analyzing the inﬂuence of the drying parameters on the qual-

ity of dried products the authors used in their earlier studies the

acoustic emission (AE) method [29,30] . In that method a special

piezoelectric detector fasten to the dried sample registered the

number of acoustic signals and the energy of acoustic waves gen-

erated by micro- or macro-cracks arising in the material. In this

way it was possible to monitor the development of crack forma-

tion in dried material, which ﬁnally lead to the material damage.

In our opinion the visual appraisal of the product quality is sufﬁ-

cient enough for the statement whether the given drying schedule

aimed at the shortest drying time still securing a good quality of

dried products.

3.1. Convective drying

Fig. 2 a presents the drying curve and the body temperature vs.

time for the pure convective drying: the temperature of drying air

was 80 ◦ C and the air velocity 1.0 m/s. Drying curve presents the

decrease of moisture content X [kg H 2 O/kg dry mass], deﬁned as

mass of water referred to the mass of dry sample. Fig. 2 b presents

the relative rate of convective drying as a function of moisture con-

tent X , i.e. the real drying rate referred to the rate of convective

drying in the constant drying rate period (CDRP).

The plots in Fig. 2 a expose the characteristic periods of convec-

tive drying, that is, the preheating, the constant (CDRP) and the

falling (FDRP) drying rate periods.

The temperature of the sample in the CDRP was about 40 ◦ C, and

in the FDRP was increased since the critical point and reached 65 ◦ C

after 5 h of drying. The ﬁnal moisture content in the sample attained

0.06 kg/kg db .

Although the curve of drying rate is very ragged (what fol-

lows from the automatic differentiation by computer of the drying

curve), it shows clearly how the rate changes in time. One can distin-

guish not only the heating period and the CDRP but also the splitting

of the FDRP in two parts: constant and non-constant falling drying

3. Results and discussion

The results of the individual drying tests described above are

discussed below.

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S.J. Kowalski, K. Rajewska / Chemical Engineering and Processing 48 (2009) 1302–1309

Fig. 4. Convective drying enhanced with infrared radiation in the CDRP: (a) drying curve and the body temperature; (b) relative rate of convective drying enhanced with

infrared radiation in the CDRP; (c) photo of the sample after drying.

rate periods, a characteristic feature for the clay-like materials (see

e.g. [31] ).

Fig. 2 c presents the photo of the cylindrical sample after convec-

tive drying. The quality of the sample seems quite good, however,

the process took much time (5 h).

ple surface became more wet than it was initially. It means that the

moisture was pushed out from the sample interior in the form of

liquid due to thermal expansion and thermodiffusion effect. This

ejected moisture should be quickly evaporated, for example, by an

application of the infrared radiation. This was not the case in this

drying schedule and therefore there was not much proﬁt in shorten-

ing of the drying time with respect to the pure convective drying.

Also the ﬁnal appearance of the sample ( Fig. 3 c) was very simi-

lar to that presented in Fig. 2 c. At the end of this process after 5 h

drying the ﬁnal moisture content was 0.06 kg/kg db and the sample

temperature 68 ◦ C.

3.2. Convective drying enhanced with microwaves

In this drying schedule the microwaves were used in the initial

stage of drying for 30 min. Fig. 3 a illustrates the drying curve and the

sample temperature change, and Fig. 3 b presents the relative rate

of drying for this drying schedule, that is, the drying rate referred

to the drying rate of convective drying in the CDRP.

The temperature of the sample at the end of the microwave

heating reached 55 ◦ C and the rate of drying gained maximal value

1.2 g/min. After switching off the microwave magnetron the sample

temperature decreased rapidly down to 40 ◦ C and the drying rate

down to 0.4 g/min. The drying parameters reached the same values

as those in the pure convective drying.

The only difference in this drying schedule with respect to the

pure convective drying was that during microwave heating the sam-

3.3. Convective drying enhanced with infrared radiation in the

CDRP

The kinetics of drying corresponding to the convective drying

enhanced with the infrared radiation in the CDRP is presented in

Fig. 4 a and b.

After switching on the infrared heater the sample temperature

increased up to 70 ◦ C and the rate of drying in the CDRP reached

0.8 g/min, that is, twice so much as in the pure convective drying.

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