Microwave and Radio Frequency Drying

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-DRY-008

Microwave and Radio Frequency
Drying
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Process Engineering Guide:

CONTENTS

Microwave and Radio
Frequency Drying
SECTION

0

INTRODUCTION/PURPOSE

2

1

SCOPE

2

2

FIELD OF APPLICATION

2

3

DEFINITIONS

2

4

MICROWAVES

2

4.1
4.2
4.3
4.4
4.5

Microwaves General
Microwaves Heating
Microwaves Generation
Microwaves Transmission
Microwaves Hazards

2
2
3
3
3

5

MICROWAVE HAZARDS

4

5.1
5.2

Microwaves Drying
Microwaves Drying Systems

4
4

6

CRITERIA FOR CONSIDERING MICROWAVE DRYING

5


0

INTRODUCTION / PURPOSE

Microwave drying utilizes the volumetric adsorption of microwave radiation
throughout the body of the wet material. The principle is that of the domestic
microwave cooker. Present technology favors small-scale applications.
Radio frequency drying uses the same principle, but operates at a different wave
length of radiation. Its application is rarer than that of microwave drying.

1

SCOPE

This Process Engineering Guide summarizes the pertinent features of microwave
dryers, their range of operations and use within industry. It covers the general
principles of microwave radiation and its application to drying. For more detailed
information it directs the reader to the relevant external literature and
summarizes its content.
Radio frequency drying is referred to in the literature only.

2

FIELD OF APPLICATION

This Guide applies to process engineers in GBH Enterprises worldwide.

3

DEFINITIONS

For the purposes of this Guide, the following definitions apply:
SPS

The Separation Processes Service (SPS) is a research and
consultancy organization, based in the UK. It is active
in the main operations related to separation, including
comprehensive coverage of drying.

Microwaves

These are electromagnetic waves with the electric and
magnetic fields at right angles to each other and to the
direction of propagation of the waves. They are a form of
radiant energy similar to light, infrared and radio waves.


4

MICROWAVES

4.1

Microwaves General

Microwaves have wavelengths shorter than radio waves and longer than infrared
waves. UHF and VHF are in the microwave region. The frequencies allocated for
industrial scientific and medical use are 896 MHz, 2450 MHz, 5800 MHz and
22125 MHz. The frequencies used for industrial heating are 896 MHz and 2450
MHz. These have wave lengths of 0.33 m and 0.122 m respectively.
Microwaves are not "ionizing radiation" as the wavelengths are greater than that
for visible light and the wave quantum energy is very small.

4.2

Microwave Heating

Most molecules, although electrically neutral, have an asymmetrical distribution
of electrons and may be electrically positive at one end and negative at the other.
For example, the water molecule exists as a dipole due to the strong connection
of electron pairs to the oxygen atom. In an electric field these polar molecules will
align themselves in a specific direction, and when the field is removed will return
to their original position. The electric field of microwave radiation changes
millions of times per second and the resulting molecular agitation produces heat.
Materials react differently depending on their molecular structure. Water and
most solvents are heated very rapidly by microwaves whereas PTFE,
polypropylene and glass are unaffected. The heating effect of microwaves on a
material can be characterized by measuring its dielectric loss factor. The heating
power(p) is related to the frequency (f) the electric field strength (e) and the
dielectric loss factor of the material (k), by the following relationship:

4.3

Microwave Generation

Microwaves are generated by vacuum oscillators, the most common being the
magnetron and the klystron. The former are most frequently used for industrial
heat generation while the latter are more likely to be used in communications
applications. Magnetrons are only approximately 70% efficient in conversion of
the electrical energy into microwaves.

Cooling is therefore needed to remove the heat generated by the remaining
electrical energy. At 896 MHz, magnetrons are available that can produce up to
60 kW of microwave energy while magnetrons at 2450 MHz are only capable of
producing up to 5 kW of microwave energy.

4.4

Microwave Transmission

Microwaves are transmitted along wave guides designed to operate above a
certain cut off frequency with very low losses. Waveguides are simply rectangular
pipes usually of brass or aluminium (approximately 5 cm by 10 cm for 2450 MHz
and 12.5 cm by 25 cm for 896 MHz). Microwaves will radiate out from an open
waveguide in a dispersed beam like a light beam from a torch.

4.5

Microwave Hazards

The information below is given to provide general guidance. However, users of
this Guide should ascertain the current state of advice on prevention of exposure
to hazards, including any National, European and International Guides and
Standards.
There are two basic hazards of microwaves:
(a)

Tissue damage due to exposure to high levels of microwave energy:
Several criteria exist, e.g. 100 Wm-2 at 50 mm from the microwave
source, maximum current densities in the body of f/100 mA/m-2 (f in Hz),
specific absorption rates of 0.4 W/kg body weight. Users should ensure
they are using the latest recommended criteria. Note also that people
carrying metallic implants such as cardiac pacemakers may be at risk.

(b)

Electric field effects causing corona discharge and arcing:
Problems with corona discharge are not usually present on industrial
heating applications because the field strengths are much lower than
those needed to cause breakdown. The field strength for a particular
application should, however, be calculated to confirm this. Arcing can be
eliminated by good equipment design with effective earthing of all
components.


5

MICROWAVE DRYING

5.1

General

For a water wet system, the microwaves will selectively heat up the water and at
atmospheric pressure the product mass will heat up to 100°C. To prevent
overheating of the product, it is necessary to operate under vacuum and so
suppress the liquid boiling point. Once all the free moisture has been evaporated,
the bound water will be heated and even under vacuum the product temperature
will rise. Ultimately, the product will be overheated and even burnt unless
microwave heating is stopped.

5.2

Microwave Drying Systems

Microwave equipment can be divided into two broad types:
(a)

belt - where continuous operation is the norm;
and

(b)

oven - where batch processing is usual.

In a microwave oven, the microwaves will bounce about by reflection and set up
a field pattern which may or may not be uniform depending on the relationship
between the microwave frequency and the cavity dimensions and shape. The
more patterns of reflection which can be set up within the cavity, the more
uniform will be the microwave heating.

6

CRITERIA FOR CONSIDERING MICROWAVE DRYING

When microwave drying is being considered a number of factors should be taken
into account. These are:
(a)

Scale:
Microwave drying is not ideal for large tonnage applications purely on cost
grounds - both the cost of the installation and electrical running costs. For
this reason, the applications tend to be in the fine Chemical /
pharmaceutical industries where scale tends to be small and products
have a high added value.


(b)

Start moisture:
Microwaves are not economic for the drying of materials with a high
moisture content as conventional drying methods are more cost effective
for this duty.

(c)

Final moisture:
Microwaves are not effective in drying product down to less than 1.0% as
when the solvent has been removed the microwave energy will tend to be
absorbed by the product causing it to heat up.

(d)

Temperature sensitivity:
Microwaves will heat up the product to be dried to the boiling point of the
solvent being evaporated. If the product needs to be kept below that
temperature then operation under vacuum is necessary. This is the case
with most pharmaceutical applications. As an alternative to vacuum
operation, a high gas flow through the product will allow the heat
generated by the air to be transferred away.

(e)

Microwave absorbance:
Microwaves are only effective for drying if the solvent which needs to be
evaporated absorbs microwave energy. The microwave absorbance is
proportional to the "loss Factor". The ideal system for microwave drying
should have a product (solid) with a low loss factor and a solvent with a
high loss factor.
Products which have a high loss factor can be prone to heating by
microwave absorbance when the moisture content
is low, i.e. as the product becomes dry. Hence the comment on final
moisture in (c).

Therefore if the product is manufactured on a small scale, has a start moisture
less than 20%, a final moisture greater than 1%, has a low microwave
absorbance and is being dried from a solvent with high microwave absorbance it
will be suitable for microwave drying. If it is temperature sensitive, vacuum
operation will be needed. If the product does not meet all of the above criteria but
there is no other alternative, then microwave drying may still be a viable option.


Microwave and Radio Frequency Drying

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