August 2010 | Volume 160 | Issue Number 8
Special Section | Brick & Clay Record
➤ Green Building
➤ Glass Life Cycle Assessment
➤ Energy-Efficient Drying
➤ Reduced-oxygen drying uses dry superheated
steam in place of hot air as the heated drying medium.
by Andrew J. Hall, President, CDS Inc., Boonton, N.J.
onventionally designed dry- the spout. The steam only becomes visible increasing its drying effect). In a super-
ers—even technologically when it condenses on contact with the sur- heated steam atmosphere, then, the prod-
advanced models—are con- rounding atmosphere as it exits the spout. At uct temperature quickly attains the steam’s
strained by the hygrometric the immediate tip of the exit spout, a small 100°C (212°F) saturation temperature,
rules of humidity-controlled drying. Even space where the steam is invisible can be wit- eliminating the moisture’s surface tension
when fitted with effective air circulation nessed, because at that location the tempera- and halving its viscosity. This absence of
and precision control systems, conven- ture is more than 100°C (212°F). This is dry surface tension assists the surface moisture
tional heat- and humidity-controlled dry- superheated steam; Table 1 (p. 22) illustrates to evaporate quickly while the lower viscos-
ing has reached a point where only mar- why it is a superior drying medium to air. ity enables the internal moisture to migrate
ginal improvements can be made. Because the specific heat capacity of faster to the product’s surface.
Several alternative technologies, includ- steam is more than twice that of air, it can The absence of air within a drying pro-
ing microwave and vacuum drying, have transfer more than twice the amount of cess prevents oxidation of sensitive prod-
been available for many years with vary- heat for the same mass flow. Therefore, ucts while avoiding the contamination by
ing degrees of success. Long-term research with the same temperature differential combustion residues that occurs during
and development programs have contin- between the moist product and the drying directly heated conventional drying and
ued to try to improve these high-tech and medium, the fan power required to achieve humidity-controlled drying. The low lev-
high-capital-cost technologies into large- a given heat transfer is more than halved. els of air/oxygen during the drying process
scale, cost-effective industrial processes. The viscosity of steam is approximately also prevent the risk of material combus-
The objective of these technologies is to half that of air at the same temperature, tion of potentially flammable products
achieve much shorter drying cycles while which enhances its ability to impinge on (woods, plastics, paper, etc.) while they are
reducing operational energy costs. Although or percolate through a moist product (thus dried at elevated temperatures.
limited success has been achieved both in
the laboratory and the field, disadvantages
remain, including high capital equipment
costs, high energy consumption, process con-
trol challenges, and health and safety issues.
A new method, termed reduced-oxygen
(R-O2) drying, uses dry superheated steam
in place of hot air as the heated drying
medium (see Figure 1). This dry super-
heated steam is created solely from moisture
contained within the materials to be dried.
Superheated steam at atmospheric
pressure is basically an invisible dry
gas with a temperature of above 100°C
(212°F). Think of a domestic kettle: When
water in a kettle is boiling, it emits steam from Figure 1. A typical R-O2 drying schematic.
CERAMIC INDUSTRY ➤ August 2010 21
Table 1. Characteristics of steam vs. air as a drying medium.
Density Specific Heat Capacity Viscosity
Kg/m3 J/Kg/K Ns/m2
Air 1.29 993 18.30
Steam 0.80 2020 8.70
When water is boiled and becomes steam at atmospheric
pressure, its volume is increased by 1670 times. For example, 1 l
(1 Kg) of water becomes 1.67 m3 of superheated gas by volume.
As no “new” air is allowed to enter the R-O2 dryer, each Kg of
water evaporated from the product (in steam form) occupies a
volume of 1.67m3. The air initially contained in the dryer, and
then the air/steam mix, is continually recirculated through the
dryer’s heat source and the materials being dried. Figure 2. Decreasing proportion of air and increasing proportion of steam
in saturated air, at atmospheric pressure and temperature between
As the recirculation mix of steam/air and the product get hot- 0-100°C (32-212°F).
ter, 1.67 m3 of increasingly humid air is vented as each Kg of water
is evaporated. This continual reduction of air progresses until the
dryer becomes virtually free of air/oxygen and contains only the
dry superheated steam. Thus, in a typical batch dryer with dimen-
sions of 4.0 x 3.0 x 2.5 m (30 m3), the atmosphere will contain
as little as 2% of air by volume after 108 Kg of water has been
removed from the products (see Figure 2).
When the exhaust temperature of the air from the heater is
120°C (248°F), the dryer ambient or off-product temperature
will be around 100°C (212°F). The saturation temperature (or
dew point) will then be at around 83°C (181°F), as will the sur-
face temperature of the moist product.
At this stage, half the original air will have been vented to
atmosphere, and the dryer will contain 50% air and 50% steam.
If additional heating continues and saturation and product sur- Figure 3. The continual recirculation of the air/steam mixture causes the
faces of 100°C are reached, the remaining air is displaced by the air-to-steam proportion in a dryer to rapidly decrease.
same process. The moisture evaporation rates increase, and the
interior of the still-moist material is quickly heated to 100°C. As The R-O2 dryer operates under full recirculation conditions dur-
drying continues to its completion, the surplus of the remaining ing the entire drying cycle and normally uses an indirect-fired form of
moisture retreats toward the center of the product and the tem- heating, which prevents proportional combustion air volumes (required
perature of the dry sections rises to above 100°C. by direct-fired burners) from entering the drying process. During the
Figure 3 illustrates how the continual recirculation of the air/ warm-up phase, varying amounts of the product’s initially contained
steam mixture causes the air-to-steam proportion in a dryer to moisture evaporates and effectively raises the dryer’s humidity level. This
rapidly decrease. During six air changes, the steam content pro- process suppresses rapid moisture evaporation from the product, but it
gresses from 50, 75, 87.5, 93.75, and 96.87 to 98.44%. allows the product to be quickly heated to 100°C without rapid product
shrinkage and subsequent distortion. Saturation of the air and conse-
R-O2 Drying Cycle quent product condensation damage is avoided by raising the tempera-
Unlike conventional drying cycles, which can consist of a series ture of the air at a faster rate than its dew point temperature.
of ramps and dwells for both temperature and relative humid- At the start of the drying cycle, at a relatively low air dew
ity control, the R-O2 drying cycle consists of only two phases: point temperature, the percentage of steam contained in the air is
warm-up (between ambient and 100°C/212°F), and drying (above relatively low. However, at around 60-70°C (140-160°F), the pro-
100°C/212°F). All biomass, organic and inorganic products have portion of steam rapidly increases. As the temperature reaches
different drying characteristics and an ideal or optimum safe tem- around 90°C (194°F), almost the entire recirculating atmosphere
perature at which they can be safely dried. This applies whether the is steam. This almost-saturated atmosphere prevents further
process is conventional or R-O2 technology. However, the R-O2 dry- evaporation from the surface of the product, which then allows the
ing system requires that drying must be carried out at a temperature internal core of the product to heat up at the same rate as that of its
above 100°C. The R-O2 drying system is also different in that only surface. This reduces stresses in the product and allows the entire
the temperature profiles are controlled (not the relative humidity). product to be rapidly but safely heated up to 100°C and above.
22 August 2010 ➤ WWW.CERAMICINDUSTRY.COM
Figure 4. A typical R-O2 technology drying cycle illustrates the relation- Figure 5. The relationship between the dryer’s internal temperature and
ship between the dryer’s internal temperatures and the actual product relative humidity for the same typical drying schedule.
process equipment, such as a primary fan, heater and ductwork sys-
The peak evaporative drying phase begins when the satura- tem. However, significant differences exist in the engineering build,
tion temperature reaches 100°C, although some drying does equipment selection, and process operation and control.
begin when it is 83°C (181°F); at this point, the dryer is half full The system operates on 100% full recirculation principles
of steam. Additional heating raises the temperature of the prod- rather than a combination of recirculation air mixed with the
uct and evaporates the remaining water, while the steam gener- introduction of volumes of ambient fresh air. In addition, indi-
ated from the moist product during the drying phase continues rect-fired heat exchangers are always used to prevent the ingress
to be vented from the dryer during the drying process. As the of combustion air. The high-quality engineering build provides
higher drying temperatures lower the viscosity of the water, the a level of air tightness that prevents any steam leakage or air
water flows to the surface more freely and quickly through the ingress. Special attention is paid to thermal efficiencies and the
product’s pore structure, thereby increasing the drying rate. airtight sealing of the dryer structures and ductwork.
Heating continues until the product is dry. At this stage, the Because of the generally higher operating temperatures used
process is either stopped and the product removed from the dryer, with R-O2 drying processes, the thickness, density and type of
or ambient air is reintroduced into the dryer to cool both product structural insulation is also greater than normally used in con-
and structures. Cooling must occur at rates such that the product ventional hot air dryers. Finally, as the R-O2 dryer is controlled
is not subjected to thermal shock and to allow, where applicable, by temperature profiles only, there is no need for costly humid-
safe access into the dryer for unloading of the product. ity-control equipment.
The maximum steam temperature determines the evaporation Before drying, ceramic articles contain around 15-20% of water
rate and provides a simple method of controlling the drying pro- by weight and (because the specific gravity of clay is approximately
cess. Every product has its maximum safe warm-up rate and peak 3.5) around 40-50% water by volume. During the first phase of con-
temperature tolerance, which determine the drying cycle. In order ventional drying, the ceramic shapes cease to be “plastic” and shrink
to achieve a successful R-O2 technology drying cycle for a particular by up to 10% dimensionally and 27% by volume as the moisture
product/material, both the maximum safe warm-up rate and peak separating the clay particles is removed.
temperature must first be determined. This valuation enables the During the second phase of drying, known as the falling rate
fastest drying cycle to be attained without damage to the product. period, drying is completed without further shrinkage by remov-
It is a combination of the rapid heating that is possible dur- ing the remaining moisture contained in the interstices between
ing the warm-up phase and the rapid moisture removal during the clay particles. While in the shrinkage phase of conventional
the peak drying phase that allows the R-O2 drying process to humidity-controlled drying, the product cannot be heated above
achieve substantial reductions in drying times and higher energy the saturation temperature (typically around 60°C/140°F). At this
efficiencies compared to conventional drying techniques. Fig- temperature, both the moisture surface tension and its viscos-
ure 4 illustrates a typical R-O2 drying cycle and the relationship ity are relatively high. In consequence, distortion or cracking may
between the dryer’s internal temperatures and the actual product result unless the moisture in the outer layers of the product is
temperatures, while Figure 5 shows the relationship between the slowly removed, giving time for the moisture in the core to migrate
dryer’s internal temperature and relative humidity. toward the product surface and allow uniform shrinkage to occur.
By comparison, during the shrinkage phase of R-O2 drying,
Process Differences the entire article is rapidly and safely heated to 100°C by recir-
At first glance, R-O2 dryers do not appear to be significantly different culation of the air volume that is initially contained in the dryer
than conventional hot air dryers. They incorporate a similar array of at start-up. This occurs without significant evaporation from the
CERAMIC INDUSTRY ➤ August 2010 23
Table 2. Comparison of R-O2 drying times vs. conventional drying Potential Applications
(in hours, unless otherwise specified). Any product or material that can safely tolerate a temperature of
Conventional R-O2 Percent Reduction above 100°C (212°F) can be dried in the R-O2 dryer, including:
• Refractory products • Plaster molds for casting
Economy-style 8-10 4-6 50%
• Ceramic insulating brick • Tableware items (all types
Luxury-style 24 10 58%
• Insulation fibers and of ceramic bodies)
One-piece WCs 96 12 90%
materials • Clay pipe, roof tile and brick
Fireclay sinks 96 16 83%
• Solvent-based binder ceramics • Pottery castware figurines
Plaster molds 72 16 77%
• Minerals and ornamentals
• Slurries, colors and glazes • HT electrical porcelain
• Sanitaryware/bathroom insulators
Leatherhard state 30 min. 3-4 min. 87%
products • Specialty ceramics
Whitehard state 30 min. 5-6 min. 80%
Plaster molds 24 6 75%
substantially greater weight of humid air than of remaining mois-
Insulators ture must first penetrate the porous, dry outer layer of clay in order
Large 60-90 20-30 66% to evaporate the core moisture and transport it away.
Medium 24 8-10 58% With R-O2 drying, the core moisture is already heated to
Small 10-12 4-6 60% 100°C, so no air is required to transport it out of the article once
it has been evaporated by transfer of thermal energy at above
Brick 100°C from the dryer’s recirculating superheated steam atmo-
Specials 60 24 60% sphere (typically at 130°C/266°F). As a consequence, the remain-
Perforated 48 20 50% ing core moisture evaporates and simply becomes steam—
expanding by a factor of 1670 (1.67 m3/Kg)—and emerges from
Roof tile 192 72 62% the article through its already dry and porous outer layer.
Care is still needed, however, to avoid heating too rapidly, which
Refractories can cause an internal pressure sufficiently high enough to cause blis-
Insulating brick 90 35 61% tering on the article’s smooth outer surface. This should be consid-
Perforated brick 120 42 64% ered when designing the appropriate drying curve for the product.
For products that have solvent binders, the binders do evaporate at
Colors/glazes 16 8 50% different temperatures than water. However, using this type of sys-
tem is extremely safe and the emissions are kept to the absolute min-
article and without further air being added, while heating the imum. The risk of fire or explosion is reduced with R-O2 technology.
article to 100°C results in the moisture surface tension becoming However, the system incorporates PrevEx sensing and RTO equip-
virtually nil and its viscosity being substantially reduced. ment safeguards as necessary.
At 100°C, the internal moisture can migrate to the surface The cost of heating air in conventional ovens is high, and the
more easily, and rapid evaporation and shrinkage can take place heating value of solvents in air is very high once the concentra-
without damaging the product. At the same time, the steam gen- tion is above roughly 10% LFL. Excessive solvent ventilation
erated by the moisture evaporation quickly displaces and replaces increases the amount of air that must be heated, wasting fuel.
the dryer’s originally contained air. Once the shrinkage phase is
complete, the surface of the moisture retreats inwardly, and the Energy-Efficient Alternative
outer layer of clay becomes dry and porous. Because the highly R-O2 drying is a safe and more energy-efficient drying method,
turbulent atmosphere within the dryer penetrates the porous and it offers reduced drying times compared to conventional
structure of the product, the temperature of the dry clay increases drying processes, as shown in Table 2. Though variations will be
toward that of the dryer. required depending on specific operations and body preparations,
Conversely, with humidity-controlled drying, because the these drying times can be regarded as typical. The main thrust of
temperature of the remaining moisture is around 60°C (140°F), this article is based on drying in a batch form, but readers should
it will not evaporate unless air that is above 60°C at a saturation also consider continuous drying, which offers even greater poten-
temperature at or below 60°C is present at its surface. The air must tial to improve most production processes.
therefore penetrate the already dry outer layer of clay in order to
evaporate the inner moisture and transport it to the surface. In For additional information regarding reduced-oxygen drying,
practice, because the air (at above 60°C/140°F and a high RH) can- contact the author at (973) 641-6857 or firstname.lastname@example.org, or visit
not absorb much additional moisture before becoming saturated, a www.cds-group.co.uk.
24 August 2010 ➤ WWW.CERAMICINDUSTRY.COM