Design and analysis of the commercialized drier processing using a combined unsymmetrical double-feed microwave and vacuum system (tea leaves) (Kusturee Jeni, Mudtapha Yapa, Phadungsak Rattanadecho).pdf

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Contents lists available at ScienceDirect

Chemical Engineering and Processing:

Process Intensiﬁcation

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

Design and analysis of the commercialized drier processing using a combined

unsymmetrical double-feed microwave and vacuum system

(case study: tea leaves)

Kusturee Jeni, Mudtapha Yapa, Phadungsak Rattanadecho ∗

Research Center of Microwave Utilization in Engineering (R.C.M.E.), Department of Mechanical Engineering, Faculty of Engineering, Thammasat University (Rangsit Campus),

Pathumthani 12120, Thailand

article info

abstract

Article history:

Received 5 October 2009

Received in revised form 4 March 2010

Accepted 5 March 2010

Available online xxx

Combined microwave (MW) and vacuum drying of biomaterials has a promising potential for high-

quality dehydrated products. A better knowledge of the drying kinetics of biomaterial products could

improve the design and operation of efﬁcient dehydration systems. The experiments were carried out

on commercialized biomaterials drier using a combined unsymmetrical double-feed microwave and

vacuumsystem. Three kilograms of tea leaveswere appliedwith themicrowave power of 800 (single-feed

magnetron) and 1600W (unsymmetrical double-feed magnetrons) operating at 2450MHz frequency.

Rotation rates of the rotary drum were held constant at 10 rpm. Vacuum pressure was controlled at the

constant pressure of 385 Torr and 535 Torr, respectively. In this study, the system can be operated either

in continuous or pulse mode in each experiments. Experiments show that in the case of high power level

and continuous operating mode causes greater damage to the structure of tea leaves sample. Microwave

drying with pulse operating mode at 385 Torr ensured the shortest drying time and the best overall

quality of dried tea leaves, and thus was chosen as the most appropriate technique for tea leaves drying.

Keywords:

Microwave vacuum drying

Tea leaves

Heat and mass transfer

Quality

1. Introduction

acquires a wide range of application on the pharmaceutical and

food industries. Microwave–vacuum drying has been investigated

as a potential method for obtaining high-quality dried foodstuffs,

including fruits, vegetables and grains [9–16] . Drouzas et al. [17]

applied the vacuum–microwave technique to investigate the pro-

cess of model fruit gel drying. They studied drying kinetics under

different levels of pressure andmicrowave power. Sunjka et al. [18]

dried cranberries using vacuum–microwave and microwave–hot

air drying techniques, and demonstrated better quality of the prod-

uct obtained with vacuum–microwave drying.

An excellent review of the drying techniques in dielectric mate-

rials using microwave energy has been presented by Schiffmann

[19] , Metaxas andMeridith [20] andDatta andAnantheswaran [21] .

Although combinedMW–vacuumdrying has found some appli-

cation in the dehydration of fruit juices, more research and

development is needed before the process is used in large commer-

cial scale. In particular, the effect of vacuum and MWpower on the

drying kinetics should be known quantitatively, so that the drying

system can be optimized from the cost and quality standpoints.

The main objective of this research was to examine the feasibil-

ity of using MW–vacuum drier to dry biomaterials, i.e., tea leaves

and experimentally explore drying characteristics of tea leaves in

different drying conditions, including microwave radiation time,

microwave power level, vacuum level and typical microwave feed-

ing process. At the same time, the research results would be

Nowadays, the most important thing in industries, except for

producing the high quality products to the markets, is to increase

productivity and to reduce production cost. In general, several pro-

ductionprocesses of agricultural and industrial products are related

to drying either by a natural method or using energy from other

sources resulting in a low production rate or a high cost products.

Microwave drying is one of the most interesting methods in term

of mechanisms and economics for heating and drying in various

kinds of product [1–8] .

Microwave–vacuum (MV) drying is a novel alternative method

of drying, allowing to obtain products of acceptable quality. It per-

mits a shorter drying time and a substantial improvement in the

quality of dried materials, in relation to those dried with hot air

and microwave drying methods. Furthermore, other advantages

including environmental friendliness at low temperature, which

not only over-comes the limitation of low thermal conductiv-

ity of the material under vacuum due to the absence of drying

medium, but also avoids the defect of internal crack and interior

burning caused by excessive heating in microwave drying, and

∗ Corresponding author. Tel.: +66 2564 3001 9; fax: +66 2564 3010.

E-mail address: ratphadu@engr.tu.ac.th (P. Rattanadecho).

doi: 10.1016/j.cep.2010.03.003

Please cite this article in press as: K. Jeni, et al., Design and analysis of the commercialized drier processing using a combined unsymmetrical

double-feed microwave and vacuum system (case study: tea leaves), Chem. Eng. Process. (2010), doi: 10.1016/j.cep.2010.03.003

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beneﬁcial to present a theory basis for further study and indus-

trial application of a combined microwave and vacuum technology

in biomaterials in the future.

comes to improving the electric ﬁeld as well as heating uniformity

and low cross-coupling using multiple feeds, the obvious question

to ask is where and how to position the feeds. The disadvantage

with multiple feeds is the mutual coupling that exists if they are

improperly fed. The feed positioning requires an understanding

and knowledge of the electric and magnetic ﬁeld directions. As

described above, the simulation is needed to determine the posi-

tioning of feeds in cavity [24] .

The principle behind the design of feed multimode cavities for

improved heating and low cross-coupling can be demonstrated

here in various cases. With the details of calculations omitted, the

simulation results in various caseswill be summarized in Fig. 1 with

single-feed cavity and double-feed cavity having different orienta-

tion, i.e. symmetrical double-feed and unsymmetrical double-feed

magnetrons.

Fig. 1 a shows the simulation of electric ﬁeld arrowplot of single-

feed wall on which the TE10 waveguide is to be connected and

microwave power of 800W is applied. It is observed that a single

source and feed will certainly create patchy regions of ﬁeld max-

ima and minima. Fig. 1 b and c shows the simulation of electric

ﬁeld arrow plot in case of symmetrical double-feeds magnetrons

and unsymmetrical double-feeds magnetrons on simultaneously,

respectively, assuming that the double feeds provide double the

amount of power.

The simulation results show that it is the excitation of similar

modes in symmetrical double-feeds magnetrons that increases the

cross-coupling which leads to uneven ﬁeld distribution. The reason

for high coupling between feeds that because they are symmetri-

cally positioned on the cavity wall and therefore excite the same

mode. The reduction of this coupling is done by using the unsym-

metrical feeds placement where each port excites a different set of

modes. The conclusion drawn from these results is that the electric

ﬁeld uniformity is better with double-feeds sources as compared

to one-feed source. Furthermore, a better energy spread through-

out the entire cavity is obtained with the unsymmetrical placed

sources with each one exciting discrete modes. Therefore, the

2. Design of unsymmetrical double-feed magnetrons in

multi-mode cavity

It iswell known that the unevenﬁelddistribution creates the hot

and cold spots. Hot spot could contribute to the phenomena of run-

away. For food products, cold spots are unwelcome as they allow

bacteria to thrive if the temperature is not sufﬁcient high enough

to kill them, which could cause food poisoning [22] . This reason

explains why a more uniform heating is generally desirable. In the

past, many researchers have devisedways of improving the electric

ﬁeld as well as heating distributionwith varying the degrees of suc-

cess by changing either the source, the microwave feeding system,

shape of cavity or the environment surrounding the load. Some

base their ideas on empty cavities, which in a practical situation

are meaningless [23,24] . In analysis, electromagnetic waves in the

cavity were simulated to design the microwave–vacuum system.

The distributions of electric ﬁeld strength and mode generation

were investigated in the simulation. COMSOL software was used

for constructing domain meshes while Finite Element Method was

used to solve the problems. Generated resonant modes inside the

multimode cavity, where the reﬂections from the walls of cavity

constructively reinforce each other to produce a standing wave,

were calculated by determining the number of half-wavelengths

in each of the principal directions [21] . The quality factor (Q-factor)

and the maximum electric ﬁeld strength (E max ) were calculated by

using the equations found in [20] . The time-average complex power

ﬂow through a deﬁned closed surface is calculated from Poynting’s

theorem [4] when a microwave source is connected to the cavity.

In this study, the idea of using the multiple sources is presented.

This idea is providing certain advantages such as lower inventory

of spare parts and less maintenance downtime, the method has

the added beneﬁt of creating a good uniformity. However, when it

Fig. 1. The simulation of electric ﬁled distribution (V/m) in multimode cavity (Slice Plot Type and Arrow Plot Type). (a) Simulated electric ﬁeld distribution and arrow

plot in multimode cavity with one-feed single magnetron. (b) Simulated electric ﬁeld distribution and arrow plot in multimode cavity with symmetrical double-feed two

magnetrons. (c) Simulated electric ﬁeld distribution and arrow plot in multimode cavity with unsymmetrical double-feed two magnetrons.

Please cite this article in press as: K. Jeni, et al., Design and analysis of the commercialized drier processing using a combined unsymmetrical

double-feed microwave and vacuum system (case study: tea leaves), Chem. Eng. Process. (2010), doi: 10.1016/j.cep.2010.03.003

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concept designed by using unsymmetrical double-feed sourceswas

performed in this study.

at ambient air temperature. The initial moisture content of material

was 172% (dry basis). All tea leaves used for drying were from the

same batch.

3. Materials and methods

3.2. Experimental program

3.1. Materials

The experimental materials comprised tea leaves from a farm

located North Part of Thailand, stored at 10 ◦ C until sample prepa-

ration. Three hours before drying the bulk of tea leaves were placed

An experimental stand for the commercialized biomaterials

drier using a combined unsymmetrical double-feeds microwave

and vacuum system was shown in Fig. 2 a. The microwave power

was generated by means of unsymmetrical double-feeds mag-

Fig. 2. The commercialized biomaterials drier. (a) Detail of the system. (b) MW drying experiments were carried out at the Research Center of Microwave Utilization in

Engineering (R.C.M.E.) Department of Mechanical Engineering, Faculty of Engineering, Thammasat University.

Please cite this article in press as: K. Jeni, et al., Design and analysis of the commercialized drier processing using a combined unsymmetrical

double-feed microwave and vacuum system (case study: tea leaves), Chem. Eng. Process. (2010), doi: 10.1016/j.cep.2010.03.003

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netrons according to design concept as shown in Section 2 (2

compressed air-cooled magnetrons of 800W each for a maximum

of 1.6 kW) operating at a frequency of 2450MHz. The power setting

could be adjusted individually in 800W steps. The microwave was

conveyed through a series of rectangular (11.0 cm

5.5 cm) wave

0.72m) in

which thematerials to be dried can be rotated by rotary drum in the

cavity. The rotary drum was made of polypropylene with dimen-

sions approximately of 30 cm radius and 50 cm length and the

rotation speed of the rotary drum was controlled about 10 rpm in

order to enhanced the interaction between microwave and dielec-

tric load. The maximum vacuum degree was about 50 Torr. The

Microwave–vacuum drying experiments were carried out for two

levels of microwave power (800W-one magnetron turned on and

1600W-two magnetrons turned on) and two levels of vacuum

pressure (535 Torr and 385 Torr). In this study, the system can be

operated either in continuous or pulse mode in each experiment.

In the intermittent mode or pulsedmicrowave operatingmode, the

magnetron was alternately turned on and off for pre-determined

set times. The pulsed microwave operating mode of 60 s on/60 s off

was performed in each experiment.

The moisture content (dry basis) and dry matter content were

measured according to the AOAC (1995) standards [25] , using a

laboratory scale system to an accuracy of 0.01 g. Optical ﬁber (LUX-

TRON Fluroptic Thermometer, model 790, accurate to

¶ ×

0.24 2

Fig. 3. Variations ofmoisture contentwithdrying time using continuousmicrowave

operating mode and pulsed microwave operating mode at 1600W magnetron

power.

0.5 ◦ C) was

employed for measuring the averaged temperature of bulk load in

cavity. Optical ﬁbers were used instead of conventional thermo-

couples because the latter absorb microwave energy and produce

erroneous temperature indications. An infrared camerawas used to

control the temperature the cavity. A Multimeter TM Series Digital

with PC interface was used to monitor the temperature inside the

cavity and to facilitate feedback control of process. An infrared cam-

era was used to measure the surface temperature of the samples

(accurate to

wall and therefore excite the same mode. The reduced this cou-

pling is done by using the unsymmetrical feeds placement where

each port excites a different set of modes. The conclusion drawn

from these results is that the electric ﬁeld uniformity is better

with double-feeds sources as compared to one-feed source. Fur-

thermore, a better energy spread throughout the entire cavity is

obtained with the unsymmetrical placed sources with each one

exciting discrete modes.

The experiments were carried out on commercialized bio-

materials drier using a combined unsymmetrical double-feed

microwave and vacuumsystem. Three kilograms of tea leaves were

applied with the microwave power of 800 (single-feed magnetron)

and 1600W(unsymmetrical double-feedmagnetrons) operating at

2450MHz frequency. Rotation rates of the rotary drum were held

constant at 10 rpm. Vacuum Pressure was controlled at the con-

stant pressure of 385 Torr and 535 Torr, respectively. In this study,

the system can be operated either in continuous or pulse mode in

each experiments.

Figs. 3 and 4 show the drying curves for tea leaves with contin-

uous and pulsed microwave operating modes. It is clearly evident

from these curves that the averagedmoisture content proﬁles with

respect to elapsed times were depended on microwave operating

modes and vacuum pressure levels. Furthermore, the total drying

times were reduced substantially with changing the microwave

0.5 ◦ C).

In MW–vacuum process, the leakage of microwaves was pre-

vented by the countermeasure in double with a combination of

mechanical blocking ﬁlter andmicrowave absorber zone ﬁlter to be

provided each at the both covers end. The microwave leakage was

controlled below the DHHS (US Department of Health and Human

Services) standard of 5 mW/cm 2 .

The dielectric properties for tea leaves samples were mea-

sured at 25 ◦ C using a portable dielectric measurement (Network

Analyzer) over a frequency band of 1.5GHz to 2.6GHz as shown

in Fig. 2 b. The portable dielectric measurement kit allows for

measurements of the complex permittivity over a wide range of

solid, semi-solid, granular and liquid materials. It performs all of

the necessary control functions, treatment of the microwave sig-

nals, calculation, data processing, and results representation. The

software controls the microwave reﬂectometer to measure the

complex reﬂection coefﬁcient of the material under test (MUT).

Then it detects the cavity resonant frequency and quality factor

and converts the information into the complex permittivity of the

MUT. Finally, the measurement results are displayed in a variety of

graphical formats, or saved to disk.

4. Results and discussion

According to the electric ﬁeld simulation of the drier cavity,

a single feed magnetron will certainly create patchy regions of

ﬁeld maxima and minima. The using of multiple sources is pre-

sented to create a good uniform electric ﬁeld. The simulated result

is that it is the excitation of similar modes in symmetrical double-

feeds magnetrons that increases the cross-coupling which leads to

uneven ﬁeld distribution. The reason for high coupling between

feeds is because they are symmetrically positioned on the cavity

Fig. 4. Variations ofmoisture contentwithdrying time using continuousmicrowave

operatingmode and pulsedmicrowave operatingmode at 800Wmagnetron power.

Please cite this article in press as: K. Jeni, et al., Design and analysis of the commercialized drier processing using a combined unsymmetrical

double-feed microwave and vacuum system (case study: tea leaves), Chem. Eng. Process. (2010), doi: 10.1016/j.cep.2010.03.003

guides to ametallic vacuumcavity of 0.13m 3 (

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Fig. 5. Variations of cavity temperature with drying time using continuous

microwave operating mode and pulsedmicrowave operating mode at 1600Wmag-

netron power.

Fig. 6. Variations of cavity temperature with drying time using continuous

microwave operating mode and pulsed microwave operating mode at 800Wmag-

netron power.

operating modes as compared to the variation of vacuum pres-

sure levels. Particularly, In case of continuousmicrowave operating

mode where the supplying microwave energy continuously gives

the more microwave energy absorbed rates which leads to higher

temperature and moisture transferred rates within the dried sam-

ples. In case of continuous microwave operating mode at 1600W

magnetron power, the drying time of tea leaves was 50min in rela-

tion to the vacuum pressure of 385 Torr, and the drying times was

60min in relation to the vacuum pressure of 535 Torr. Namely,

in case of the continuous microwave operating mode at 800W

magnetron power the drying time was 60min in relation to the

vacuum pressure of 385 Torr and the drying times was 70min for

the vacuum pressure of 535 Torr. In the same ﬁgures, in case of

pulsed microwave operating mode at 1600W magnetron power,

the drying time of tea leaves was 120min in relation to the vacuum

pressure of 385 Torr and the drying time was 140min for the vac-

uum pressure of 535 Torr. Namely, in case of the pulsed operating

mode at 800Wmagnetron power, the drying time was 120min in

relation to the vacuum pressure of 385 Torr and the drying time

was 140min for the vacuum pressure of 535 Torr.

In analysis the effect of and vacuum pressure levels, the total

drying times were reduced slightly with variation of vacuum

pressure levels as compared to the case of changing microwave

operating modes. The drying process at lower vacuum pressure

(stronger vacuum pressure) easily allows water to evaporate at a

lower temperature. The drying curves show that the drying time

at lower vacuum (385 Torr) is shorter than the drying time at

higher vacuum pressure (535 Torr) when microwave power is kept

the same. Strong vacuum pressure leads to induce the faster of

the drying rate. The continuous microwave operating mode with

vacuum pressure of 385 Torr has shorter drying time than continu-

ous microwave operating mode with vacuum pressure of 535 Torr

by 16.7% at 1600W magnetron power and 14.3% at 800W mag-

netron power, respectively. At lower vacuumpressure, drying time

is shorter due to the reduction of boiling temperature of the dried

samples which leads to enhance themoisture transport in the dried

samples. The latter arises from the fact that the microwave energy

being absorbed as well as the temperature in the dried samples was

decreased. It is due to the changing dielectric properties of the tea

leaves which are proportionally dependent on moisture content.

Next presentation refers to the discussion on averaged temper-

atures of bulk load (cavity temperature) with respect to elapsed

times with various testing conditions. In Figs. 5–8 , the averaged

temperature of bulk load (i.e., tea leaves) is inﬂuenced by applied

vacuumpressure andmicrowave power level andmicrowave oper-

Fig. 7. Variations of cavity temperature with drying time using continuous

microwave operating mode and pulsed microwave operating mode at 385 Torr

vacuum pressure.

ating modes. Magnetron power as well as microwave operating

mode are strongly effects on the internal heat generation and

drying rate of dried samples. Furthermore, the higher microwave

power level as well as continuous microwave operating mode can

increase the temperature and drying rate by providingmore energy

for vaporizing water thus accelerating moisture removal at greater

Fig. 8. Variations of cavity temperature with drying time using continuous

microwave operating mode and pulsed microwave operating mode at 535 Torr

vacuum pressure.

Please cite this article in press as: K. Jeni, et al., Design and analysis of the commercialized drier processing using a combined unsymmetrical

double-feed microwave and vacuum system (case study: tea leaves), Chem. Eng. Process. (2010), doi: 10.1016/j.cep.2010.03.003

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