Humidity Effects in Solids Drying Processes

Abstract

Thermal drying from a wet solid to a dry solid is an essential intermediate step in many solids processing plants. Humidity has a major effect on dryer performance and ability to meet quality specifications, particularly for convective dryers. Humidity calculations using a psychrometric chart give an excellent short-cut method for dryer sizing. Ambient and inlet humidity affects both drying kinetics and equilibrium moisture content, especially at lower drying temperatures, and is also important in storage. Exhaust humidity measurement is also useful for tracking and controlling the progress of drying, especially as direct moisture content measurement is often difficult. However, accurate and reliable measurements are difficult due to the hot, damp and dusty environment. The recent revision of British Standard BS 1339 has helped to clarify definitions and improve calculation methods.

Full text

HUMIDITY EFFECTS IN SOLIDS DRYING PROCESSES
Ian C Kemp
Senior Technical Manager, GMS, Glaxo SmithKline plc,
Priory Street, Ware SG12 0DJ, United Kingdom
Abstract: Thermal drying from a wet solid to a dry solid is an essential intermediate step in
many solids processing plants. Humidity has a major effect on dryer performance and ability to
meet quality specifications, particularly for convective dryers. Humidity calculations using a
psychrometric chart give an excellent short-cut method for dryer sizing. Ambient and inlet
humidity affects both drying kinetics and equilibrium moisture content, especially at lower
drying temperatures, and is also important in storage. Exhaust humidity measurement is also
useful for tracking and controlling the progress of drying, especially as direct moisture content
measurement is often difficult. However, accurate and reliable measurements are difficult due
to the hot, damp and dusty environment. The recent revision of British Standard BS 1339 has
helped to clarify definitions and improve calculation methods.
INTRODUCTION
Drying of solids
Drying is an essential intermediate step in many manufacturing processes, e.g. for chemicals,
foods, paper, textiles, consumer products and miscellaneous particles and powders. It
accounts for 10-20% of the total energy consumption of many countries, because of the heat
which must be supplied to overcome the latent heat of evaporation. The vapour produced
emerges as a high humidity exhaust stream. Usually the liquid being removed is water, but it is
also common to be evaporating an organic solvent.
Most drying processes are either convective (with heat supplied by hot air) or conductive (with
heat transmitted from hot walls, pipes or surfaces). Radiation (e.g. infra-red), dielectric heating
(radiofrequency and microwaves) or combinations may also be used.
Humidity and its definition
Humidity is a key factor in drying processes. It expresses the amount of vapour in the carrier
gas stream (usually air). However, there is often confusion between relative and absolute
humidity. Absolute humidity is a measure of the mass of moisture in the air (g/kg, kg/kg or
g/m3); relative humidity (RH) is a percentage of the saturation humidity at the given
temperature. Either may be important in different situations. For example, absolute humidity
dictates the heat and mass balance, while relative humidity influences the equilibrium moisture
content. However, a change in temperature has a major effect on RH, whereas absolute
humidity is unchanged if it is quoted in mass units (e.g. g/kg). Ideally, both instrument displays
and plant control rooms should show both relative and absolute humidity.
Even for absolute humidity, there is confusion over terminology. Often, it is defined as a
volumetric concentration in g/m3 or kg/m3. This is acceptable in fields such as meteorology
and air-conditioning which deal with air-water systems close to atmospheric pressure and
ambient temperature. However, it is very inconvenient for dryer calculations because the
volume varies too much over the wide range of temperatures encountered. Hence, in drying
and other chemical engineering processes, “absolute humidity” is usually defined on a mass

basis, and expressed in units of kg (moisture) per kg (dry gas). In other disciplines this is often
called the “mixing ratio”. Users should always look at the stated units when interpreting
humidity data!
Likewise, confusion often arises because volumetric flowrates differ substantially depending on
whether they are stated at operating temperature (maybe 100-2000C, or even higher) or
ambient temperature. Even for so called “standard” flows (e.g. SCFM, standard cubic feet per
minute) several different values are used for the “standard” temperature (ranging from 00C to
250C), and calculation errors can result. Hence, in drying calculations it is generally best to use
mass flowrate in kg/s (or kg/h) and mass velocity in kg/m2s (mass flux) in preference to
volumetric flowrate (m3/s) and velocity (m/s) respectively. Air flowmeters should preferably
display both volumetric and mass flowrates
Dewpoint is directly related to the absolute humidity; for example, an air-water mixture with
7.5 g water vapour per kg of air has a dewpoint of approximately 100C (at typical atmospheric
pressures).
An important step forward in reducing confusion and standardising best practice is the recent
updating of British Standard BS 1339 on Humidity and Dewpoint. Part 1 (2002) contains
rigorous definitions and formulae, both for the air-water system and other solvent-gas systems,
including interconversions between all the parameters given above. Part 2 (2007) covers
calculation methods, based on a spreadsheet with inbuilt functions for all the key humidity
transformations. This allows users to generate calculations or tables for any desired set of
conditions, and replaces several reams of lookup tables. Finally, Part 3 (2004) is a practical
descriptive guide to the measurement of humidity and dewpoint.
Humidity effects in dryers
Humidity affects the dryness of the solids it is in contact with. A dynamic equilibrium is set up
between the moisture in the solids and the vapour in the air. Solids left in contact with moist
air for long periods will reach their equilibrium moisture content, XE, which increases as
relative humidity increases or temperature falls. As a result, solids in a cool damp atmosphere
will pick up more moisture than those stored in warm dry conditions. XE is often plotted as an
adsorption or desorption isotherm.
Equilibrium relative humidity (ERH) uses the principle in reverse. For a given solids moisture
content, the relative humidity of the air in contact with the damp solid at equilibrium will be
constant. This provides a useful indirect method of measuring solids moisture content.
Temperature or humidity cycling often affects solids in storage. For example, the air in an
uncontrolled storage area will be at a relatively high temperature and absolute humidity during
the day. At night the temperature falls and the relative humidity increases until the dewpoint is
reached and air condenses on the walls and the solid surface (“silo rain”). This surface
moisture then cakes the solid together into large lumps so that it will not flow freely on
discharge. A similar problem can arise if solids are discharged from the dryer without cooling,
and stored or bagged when hot. Further moisture is evaporated, giving high local humidity;
when the air cools down to ambient, condensation again occurs. Humidity should always be
carefully considered whenever there are caking and lump formation problems in storage and
transport. High relative humidity may also encourage product spoilage, e.g. by promoting
biological reactions and mould growth.

APPLYING HUMIDITY IN CALCULATIONS
The needs of the practitioner involved in industrial drying are completely different to those of
the standards expert. He is not generally interested in predicting or measuring values of
humidity and temperature accurate to several decimal places; in the challenging environment of
a dryer exhaust he is pleased to have any instrument which keeps working and gives a reading
accurate to the nearest degree or a few % RH. Humidity helps with shortcut calculations for
dryer design and performance and in identifying trends as operating conditions vary.
Psychrometric charts and dryer sizing
The familiar Grosvenor temperature-humidity psychrometric chart is not the only one used in
drying. Enthalpy-humidity psychrometric charts are also extremely useful. The enthalpy
plotted is the total of the sensible heat and latent heat of the gas-vapour mixture. There are
two types; Mollier (non-orthogonal, used in UK and Europe) and Bowen (orthogonal, used in
USA). The constant-enthalpy lines on the Mollier chart slope downwards with a gradient
proportional to the latent heat. Though less intuitive initially, it is clearer to read in practice as
the information tends to be cramped together on the Bowen chart. The charts provide a useful
visual representation of gas-phase conditions during drying processes.
Psychrometric charts are very useful for rapid sizing of convective (air heated) dryers, by
finding what airflow will be needed. The sensible heat lost from the gas as it is cooled
becomes latent heat of vapour which returns to the mixture, and the total gas-vapour mixture
enthalpy remains roughly constant. Hence, the air conditions during drying roughly follow a
constant-enthalpy line. This provides a simple visual way of determining dryer outlet
conditions and performing a short-cut design calculation. Allowing a suitable safety margin
above the dewpoint, say 200C, the exhaust temperature and the corresponding humidity can be
read off from the chart. A simple mass balance on the moisture in the gas gives the required
airflow.
For example, suppose we want to dry 1 tonne/hr of wet solid from 15% moisture to 3%. The
required evaporation rate is 12% of 1 te/h, or 120 kg/h. If we use air at 1500C with an inlet
humidity of 10 g/kg, the chart (Figure 1) shows the wet bulb temperature is about 420C.
Assuming an outlet temperature of 600C, we find that the dewpoint is 400C and exhaust
humidity 46 g/kg. The humidity change over the dryer is 36 g/kg (0.036 kg/kg) so to
evaporate 120 kg/h we will need (120/0.036) or 3330 kg/h of air. A safety margin of about
10% can be added to allow for heat losses.

Mollier Chart for Air/Water at 101.325 kPa
0
20
40
60
80
100
120
140
160
Enthalpy (kJ/kg)
0 20 40 60 80 100
Gas humidity (g/kg)
0
20
40
60
80
100
120
140
Gas Temperature (C)
180 200 220 240 260 280 300 320 340 360 380 400 420
Boiling Pt
Triple Pt
Sat. Line
Rel Humid
Adiabat Sat
Spot Point
Figure 1 Mollier psychrometric chart with typical operating line for adiabatic dryer
(Courtesy Aspen Technology Inc.)
Temperature driving force and drying rate
If surface moisture is present, the outer surface of a solid will be at the wet bulb temperature
Twb as long as any surface moisture is present. Twb is affected by humidity as well as
temperature, and the driving force for drying is the difference between dry bulb and wet bulb
temperatures, (Tg-Twb).
Obtaining wet bulb temperature from dry bulb temperature and humidity previously required a
tedious iterative calculation. The availability of functions such as those in BS 1339 Part 2 is a
major help, and it is also convenient to have direct approximate relationships for wet bulb
temperature based on inlet temperature and humidity. The best fit depends on the temperature
and humidity range selected. For example, for pharmaceutical drying, the range of interest is
typically 40-800C inlet temperature and 0.005-0.015 g/kg absolute humidity (Y). The
following relationship gives Twb to within 10C over this range and, equally important, gives the
driving force (Tg-Twb) to within 1%:
Twb = 0.24Tg +500Y + 8
However, at the higher inlet temperature ranges typically found for dryers in the chemical
industry, the following fit applies for 80<Tg <150 and Y<0.02 kg/kg:
Twb = 3.18 √(Tg +2400Yi )

How much difference does humidity make to wet bulb temperature and driving force? The
answer is that it depends on temperature; it is a small effect for a dry bulb temperature of
1500C, but is significant at 800C and substantial at 400C.
Table 1 has been generated using the humidity spreadsheet functions in British Standard BS
1339 Part 2, and shows that if the dewpoint changes from 100C to 200C at an inlet temperature
of 1500C it makes only a 2% difference in driving force (and hence drying time). However, at
800C the difference is 7% and at 400C, no less than 26%. Moreover, if one is drying close to
equilibrium moisture content and the product ERH is significant – say 50% - an increase in
inlet humidity will increase exhaust humidity and RH substantially, and make drying even
slower.
Table 1 Wet bulb temperature and driving force for varying humidity and temperature
Absolute humidity g/kg 2 7.5 15 20
Absolute humidity
kg/k
g 0.002 0.0075 0.015 0.02
Dewpoint Tdew 0C -7 10 20 25
Dry bulb temperature: 150 0C
Wet bulb temperature 41.1 42.8 45.1 46.4
Driving force (Tg-Twb) 108.9 107.2 104.9 103.6
Difference from Tdew=10 1.7% 0.0% -2.1% -3.3%
Dry bulb temperature: 80 0C
Wet bulb temperature 28.3 31.1 34.6 36.6
Driving force (Tg-Twb) 51.7 48.9 45.4 43.4
Difference from Tdew=10 5.9% 0.0% -7.0% -11.1%
Dry bulb temperature: 40 0C
Wet bulb temperature 16.6 21.0 25.9 28.6
Driving force (Tg-Twb) 23.4 19.0 14.1 11.4
Difference from Tdew=10
23.0
% 0.0% -25.7% -40.2%
INLET AND OUTLET HUMIDITY IN DRYERS
In dryers, inlet humidity is a controlled parameter which affects the process, whereas outlet
humidity is a monitoring parameter which is affected by the process.
Inlet humidity
Variations in inlet humidity will naturally occur as the ambient conditions fluctuate, with
day/night and winter/summer variations. For high drying temperatures (e.g. 1500C or above)
the effect on drying is generally small. However, at the lower temperatures (less than 1000C)
used for temperature-sensitive materials, such as foods and pharmaceuticals, changes in inlet
humidity significantly affect the wet bulb temperature (and hence driving forces and drying

times), and the exhaust humidity (influencing product moisture content). Therefore, inlet
humidity control must be considered, with the following options:
- No control. If the effect on the drying process is unimportant.
- Capping. A chilled water coil is used to ensure the inlet dewpoint is no higher than a certain
value, e.g. 100C (7-8 g/kg).
- Dehumidification. If very dry air is needed for moisture-sensitive materials, a chemical
absorption wheel can be used to give humidities of 2 g/kg or less.
- Rehumidification. If the product could dry out too fast or too far, steam or water injection
can be used to increase humidity.
High humidities can also be achieved by recycling exhaust air. For example, when drying
leather in tunnel dryers, air recycle is used to maintain relative humidity in the circuit at 40-
50% at temperatures of 40-80oC. Leather cracks and denatures if dried too far, and these
conditions keep the equilibrium moisture content at about 15% and prevent further drying.
To control humidity within a tight range (e.g. 5 to 8 g/kg), a combination of capping and
rehumidification can be used.
Exhaust humidity
Outlet humidity is a good indicator of the progress of drying. For continuous drying, exhaust
humidity monitoring shows whether drying is consistent, and can give early warning of
fluctuations in solids inlet moisture content or mass flowrate (which can be difficult to measure
accurately on-line). For batch drying, exhaust humidity can give an indirect measure of how
close the solids are to their desired final moisture content (endpoint).
Outlet moisture content is a key part of the dryer product specification, and ideally should be
measured on-line. For continuous dryers, this can be done using near infra-red on the product
stream, but batch dryers present a greater problem. In-vessel measurements suffer from
fouling and non-uniform solids mixing. Hence product moisture is normally deduced from
indirect measurements. Product or exhaust gas temperature are normally used as indicators of
the endpoint, but exhaust humidity provides a useful alternative or additional information.
Figure 2 shows results from a drying kinetics rig at SPS, Harwell, where the outlet humidity
was monitored using a fast-response spectroscopic infra-red gas analyser. A mass balance
over the dryer module is used to convert humidity to evaporation rate from the solids, giving
Figure 3. Then, integration over time gives the drying curve (moisture content variation with
time), Figure 4. Note that the integration smoothes out the scatter on the humidity data.
However, the result is sensitive to errors in flowrate and humidity measurement, so it is
essential to cross-check the drying curve with the initial and final solids moisture contents
obtained from an accurate laboratory method, usually an oven test. The mass balance should
agree to within 5%.

Figure 2 Experimental humidity-time plot
Figure 3 Derived plot of drying rate against time
Figure 4 Drying curve (moisture content v. time)
MEASUREMENT AND CONTROL
Dryer control

A modern well-instrumented dryer should have sensors for air temperature, humidity and
flowrate, and solids temperature and (where possible) moisture content, as shown in Figure 5.
Also, for continuous dryers, solids flowrate should be monitored.
Figure 5 Instrumentation for typical dryer
Direct feedback loops are normally used to control inlet air temperature and, less commonly,
inlet air humidity. Exhaust air temperature and humidity, and product temperature and solids
moisture content, show how drying is progressing and give warning of any fluctuation. If they
vary from their setpoint, inlet temperature can be altered. Automated feedback is possible, but
as drying is a complex process and reliable modelling is difficult, manual operator input is more
common. For dryers with a long solids residence time, an excursion in outlet humidity and
temperature can provide early warning of problems and allow inlet conditions to be varied to
restore the solids to the desired product conditions before they emerge.
Humidity measurement
Inlet humidity is relatively easy to measure. The airflow is clean (often filtered) and at
relatively low humidity. A sensor placed after the heater may have to work at high
temperature and low relative humidity (1% or less), or one after a chiller (to maintain a given
dewpoint) may be at 100% saturation, but it is not difficult to procure reliable and inexpensive
instruments for these conditions. For example, capacitance RH sensors or chilled-mirror
dewpoint sensors can be used.
In contrast, measurement of outlet (exhaust) humidity has been a difficult challenge for many
years. The hot, wet, dusty environment has frequently led to instrument fouling and
breakdown. The ideal specification for an exhaust humidity sensor would be:
- Robust; able to deal with near-saturation conditions at moderate temperatures, or full
dryer inlet temperature
- Reliable; able to operate for several months with little or no maintenance
- Reproducible; giving consistent output without needing regular recalibration
- Resistant to or protected from dust fouling
- Rapid response, preferably in seconds rather than minutes, to allow effective
monitoring and endpoint detection in batch dryers.
The requirements may be conflicting, for example, a perforated dust shield around a sensor can
slow down response time.
Inlet
air
H
Dryer
Heater
Exhaust gas
Y
Y
TF
T
XT
C
Chiller
TCYC
F = air flow rate, T = temperature, X = moisture content, Y = humidity, TC/YC = controllers
Solids

Exhaust humidity sensors should preferably be mounted after air filters which collect the worst
of the dust, but some fine dust penetration is inevitable.
Effective use of exhaust humidity for control and monitoring has been hamstrung by a lack of
proven, robust, reliable sensors. Comparing this with the complexity of control software,
Stuart Gardiner (ex ICI) commented in the 1990’s; “The brain of the plant has outstripped its
eyes and ears”. Although modern RH capacitance and chilled mirror sensors are considerably
improved over previous versions, there is still a greater need for verification and testing in
practical situations, particularly the exhaust of real industrial dryers.
Output should be in the form of absolute humidity or dewpoint; therefore, for a sensor
measuring relative humidity, a co-located temperature measurement is also needed.
OTHER ASPECTS
Humidity does not simply apply to water vapour in air. Many drying processes involve the
removal of solvents, and the same considerations apply for drying kinetics and equilibria.
BS1339 Part 1 provided generalised equations for any solvent-gas system.
In some cases, trace quantities of a solvent are being removed as well as large quantities of
water, and even a small solvent concentration in the drying gas may prevent the reduction of
the solvent component to the very low levels frequently required in product specifications. In a
gas recycle system, even with a condenser on the loop, a trace organic component may build
up as its concentration is too low to be condensed out at the condenser temperature
appropriate to recovery of the main component (water). In one case, the purge rate had to be
increased from an original estimate of 5-10% to at least 20% to keep a solvent below a
concentration of 0.25% in the final product.
Humidity requirements can also affect dryer selection. The curves in Figure 2-4 were for a
crossflow dryer where the inlet gas contacting the solids is at the same temperature throughout
the dryer. However, in cocurrent dryers, the gas temperature falls steadily throughout the
dryer. This is good for heat-sensitive materials (dry solids are only exposed to cool gas and
are unlikely to overheat) but the cool wet exhaust gives a high equilibrium moisture content
and this may make low outlet moisture contents difficult to obtain. Conversely, if a very low
outlet moisture content is desired, a countercurrent dryer may be used; the exiting solids will
be in contact with the hot dry inlet gas, minimising product moisture and maximising local
driving forces for drying. The problem here can come at the inlet; the wet gas contacts the
cold incoming solids and can condense on the surface, which may give local stickiness
problems.
CONCLUSIONS
Humidity is a key concept in understanding and controlling industrial drying processes. Despite
recent advances in instrumentation, there is a continuing need for proven, robust and reliable
sensors, particularly for exhaust humidity measurement.