Equilibrium Moisture Content 0 INTRODUCTION/PURPOSE 1 SCOPE 2 FIELD OF APPLICATION 3 DEFINITIONS 4 THEORY 5 IMPORTANCE OF EQUILIBRIUM MOISTURE CONTENT 6 MEASUREMENT OF EQUILIBRIUM MOISTURE CONTENT 7 BIBLIOGRAPHY FIGURES 1 DYNAMIC METHOD : EQUILIBRIUM MOISTURE CONTENT RIG 2 DESORPTION ISOTHERMS

1. GBH Enterprises, Ltd.
Process Engineering Guide:
GBHE-PEG-DRY-002
Equilibrium Moisture Content
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2. Process Engineering Guide:
Equilibrium Moisture Content
CONTENTS
SECTION
0
INTRODUCTION/PURPOSE
2
1
SCOPE
2
2
FIELD OF APPLICATION
2
3
DEFINITIONS
2
4
THEORY
3
5
IMPORTANCE OF EQUILIBRIUM MOISTURE CONTENT
3
6
MEASUREMENT OF EQUILIBRIUM MOISTURE CONTENT 3
7
BIBLIOGRAPHY
4
FIGURES
1 DYNAMIC METHOD : EQUILIBRIUM MOISTURE
CONTENT RIG
4
2 DESORPTION ISOTHERMS
5
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3. 0
INTRODUCTION/PURPOSE
The drying of solids is a complex process. The drying rate depends on the nature
of the material in question, the particular mechanism (or mechanisms) involved in
the moisture transfer process and the external conditions to which the solids are
exposed. Knowledge of the drying process is a prerequisite to dryer design and
cannot be calculated with any degree of confidence using existing theoretical
models. In practice the drying rate should be measured under conditions
approximating as closely as possible to those envisaged for the full scale
dryer. The parameters which need characterizing are:
(a)
Drying kinetics;
(b)
Equilibrium Moisture Content;
(c)
Physical, thermal and chemical properties of both the feed and the dry
product.
1
SCOPE
This Guide describes the significance of the Equilibrium Moisture Content in
understanding the drying of solids.
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
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4. Equilibrium
Moisture
Content
The Equilibrium Moisture Content (Xe) is the
moisture content of a product when at equilibrium
with the partial water pressure of the surroundings.
Free Moisture
Content
The Free Moisture Content is the moisture in
excess of the Equilibrium Moisture content.
With the exception of terms used as proper nouns or titles, those terms with initial
capital letters which appear in this document and are not defined above are
defined in the Glossary of Engineering Terms.
4
THEORY
The moisture in a material to be dried can be present in two forms, bound and
unbound. The nature of the solids and their accompanying moisture determines
the drying characteristics. The bound moisture in a solid exerts an equilibrium
vapor pressure lower than that of pure water at the same temperature. Water
retained in small capillaries in the solid, adsorbed at solid surfaces, as solutions
in cell or fibre walls, or chemically combined with the solids (water of
crystallization) falls in the category of bound moisture. The unbound moisture in a
hygroscopic material is that moisture in excess of bound moisture. All water in a
non-hygroscopic material is unbound water which exerts an equilibrium vapor
pressure equal to that of pure water at the same temperature. The Free Moisture
Content consists of unbound and some bound moisture. Only Free Moisture
Content can be evaporated.
A useful measure of bound moisture is the Gibbs free energy change ΔG (J/mol)
which occurs when a mole of water is adsorbed or desorbed from the solid. The
Gibbs free energy change is defined as:
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5. Some typical values for ΔG are:
5
IMPORTANCE OF EQUILIBRIUM MOISTURE CONTENT
The Equilibrium Moisture Content of a solid is an important consideration in the
understanding of drying processes because:
(a)
When drying a solid there is a limit to the extent of water removal.
(b)
If the gas contacting the solids has a humidity greater than zero there will
be a certain solids moisture content below which no further drying will take
place, this is the Equilibrium Moisture Content.
(c)
The moisture content will never fall below the Equilibrium Moisture
Content however long the drying time.
In addition, the material's characteristics with respect to the nature of the residual
moisture will provide a better understanding of final product quality.
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6. 6
MEASUREMENT OF EQUILIBRIUM MOISTURE CONTENT
There are two basic experimental techniques for determining Equilibrium
Moisture Content:
(a)
Constant Volume (or static) Method
The constant volume or static method involves placing a sample of solid in
a constant volume system containing gas at fixed conditions. The normal
technique for maintaining the gas humidity constant is to place a saturated
salt solution in the system. The main disadvantage of this method is that it
is time consuming and at high humidifies can take several weeks to reach
equilibrium.
(b)
Dynamic Method
The other technique involves a dynamic method where gas at a fixed
temperature, humidity and flow is drawn over the solids until equilibrium is
attained. This technique overcomes many of the disadvantages of the
static method. The main features of the apparatus are shown
schematically in Figure 1. Such a rig is situated at the SPS in the UK and
is available for in-house experimentation (cost details can be obtained
upon enquiry). Measured Equilibrium Moisture Content values are
normally plotted in the form of isotherms against relative humidity.
Depending on whether the humidity is increasing (adsorption) or
decreasing (desorption) slightly different isotherms are found. Figure 2
shows desorption isotherms for a range of different materials and
indicates the nature of the bound moisture and the difficulty of the drying
task. The effect of temperature on sorption isotherms is not large and for
design purposes can be ignored for variations up to 10°C from the
experimental temperature.
7
BIBLIOGRAPHY
[1]
Spray Drying Handbook 4th edition p76, K Masters, George Godwin
[2]
SPS Industrial Drying Technology - Short Course Data
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7. FIGURE 1
DYNAMIC METHOD: EQUILIBRIUM MOISTURE CONTENT RIG
FIGURE 2
DESORPTION ISOTHERMS
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Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
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8. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com

9. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com