Airless drying appears to be a technique for energy conservation in drying as it enables the energy lost to the atmosphere with the water vapor in the exhaust gases in conventional air dryers to be recycled. Because ∼85% of the thermal energy in the water vapor produced by drying is the latent energy, only the remaining 15% can be potentially recovered, as the dew point at which the latent energy could be recovered from the vapor–air mixture at the typical dryer exhaust is∼40°C and therefore not industrially useful. Hence, replacing hot air in convective drying by water vapor as it is in airless drying would offer substantial energy savings, typically from 40 to 90%, depending on process conditions (Anonymous, 1994; Stubbing, 1994a–1994c, 1999). An example of an energy audit for a conventional and an airless dryer is given in Table 8.1 (Heat-Win Ltd, Bitterley, U.K.).
The concept of airless drying (Stubbing, 1987) relies on evaporating the moisture from the wet product to form an atmosphere of almost pure steam. Since steam is slightly superheated, this leads to energy savings by avoiding the requirement to heat large quantities of air. In addition, the water evaporated in the dryer can be used as a source of heat for other processes. This concept is similar to that of superheated steam (SHS) dry-ing (see Chapter 7), however, at a lower degree of superheatdry-ing, and when steam is generated by evaporation of water from the wet material.
In batch mode of operation, airless drying has three distinct stages.
These are described in the following:
1. Warm-up phase. During this stage, the drying chamber initially contains wet solids and air at ambient temperature and at atmo-spheric pressure. At the start of the process, the recirculation fan is turned on and the air is circulated through the drying chamber and the heat exchanger as it is in conventional convective drying.
No air is allowed to enter the dryer, but a fraction of air equal to its expansion volume is vented to the atmosphere. As evaporation progresses, a fraction of humid air is vented to the atmosphere before the remainder passes again through the heat exchanger and the recirculation fan. Now the drying medium is a mixture of air and water vapor. As the sequence of venting, heating, and dry-ing continues, the composition of the drydry-ing medium becomes
CRC_73877_Ch008.indd 123
CRC_73877_Ch008.indd 123 12/29/2008 11:28:10 AM12/29/2008 11:28:10 AM
richer in water vapor and poorer in air until finally it consists of
© 2009 by Taylor & Francis Group, LLC
124 Advanced Drying Technologies
almost pure water vapor. Because of heating in an internal heater, this water vapor is at a temperature of ∼120 to 150°C on entering the drying chamber and at ∼100°C at exit. Its pressure is slightly above atmospheric pressure. Therefore, the vapor at any point of the dryer is superheated (although the degree of superheating drops along the dryer) and thus is capable of transferring heat to the moisture in the solid and carrying away the vapor without any water condensing out.
2. Airless drying phase. This phase begins as soon as the last portion of air has been vented from the system and the drying medium consists essentially of superheated water vapor. The superheated vapor continues to circulate around the system while a continu-ous stream of excess vapor is vented so as to maintain the required pressure in the system. The vented steam has a pressure of 1 atm and is only slightly above saturation. Therefore, it can be used as a source of heat in a number of ways. It can be condensed by mix-ing with cold water to produce a supply of hot water at slightly
<100°C, or it can be mechanically compressed to increase its tem-perature before being used as a source of heat for the dryer itself.
TABLE 8.1
Energy Audit for Conventional and Airless Drying of Ceramic Materials in a Tray Dryer
kJ/kg %
Evaporation energy (from assumed 20°C ambient temperature)
2594 75.32
Energy for dry material heating
(50% moisture content; from assumed 20 to 60°C;
at 1.25 kJ/kg K)
50 1.45
Structural energy losses (from well-insulated dryer) 100 2.90 20 to 80°C exhaust temperature)
700 20.33
Total 3444 100
Energy use with an airless dryer
Evaporation energy (from assumed 20°C ambient temperature)
2594 74a
Energy for dry material heating (50% moisture content; from assumed 20 to 100°C; at 1.25 kJ/kg K)
100 4a
Structural energy losses (from well-insulated dryer) 150 4a 20 to 80°C exhaust temperature)
Nil Nil
Total 2844 83a
−2170 −63a
Equals 674 20a
a Percent of the conventional dryer’s thermal energy.
Source: Courtesy of Heat-Win Ltd., Bitterley, U.K.
CRC_73877_Ch008.indd 124
CRC_73877_Ch008.indd 124 12/29/2008 11:28:11 AM12/29/2008 11:28:11 AM
Energy use with an efficient conventional dryer
Air heating losses (to heat air through-flow from
Air heating losses (to heat air through-flow from
© 2009 by Taylor & Francis Group, LLC
Drying chamber
Compressor
Condensate
Burner Vent Flue Fan gases Indirect heater
Heat exchanger
FIGURE 8.1
Airless batch dryer with vapor recompression.
TABLE 8.2
Comparison of Energy Consumption
Type of Dryer kJ/kg
An airless dryer producing hot process water
626 An airless dryer with vented steam
compression to recycle energy
400 An airless double-dryer reusing energy
from its airless section in its air section
1900 3500 Source: Courtesy of Heat-Win Ltd., Bitterley, U.K.
3. Cool-down phase. When all of the liquid in the wet solid has been evaporated, a cool-down phase is initiated in which air is readmit-ted to the drying chamber to drive out the superheareadmit-ted vapor.
Figure 8.1 presents a schematic diagram of an airless batch dryer with vapor recompression. By compressing the vented steam and then condens-ing it in an internal heater of the airless dryer, most of the latent heat can be repeatedly recycled within the drying process. Since energy savings remain at ∼80% (see Table 8.1), the energy cost savings are lower because of the electrical energy needed for driving the compressor. Alternatively, the vented steam can be either condensed and used as hot process water or reused as a heating medium in another convective dryer. The net exter-nal thermal energy requirements to remove 1 kg of water in three possible hot air dryer are compared in Table 8.2.
CRC_73877_Ch008.indd 125
CRC_73877_Ch008.indd 125 12/29/2008 11:28:11 AM12/29/2008 11:28:11 AM
configurations of an airless dryer (see Stubbing, 1994b) and a conventional
A typical efficient conventional hot air dryer
© 2009 by Taylor & Francis Group, LLC
126 Advanced Drying Technologies
The main problem to be solved when evolving from the batch process to the continuous one is conveying the material through the dryer within the superheated atmosphere without allowing ambient air to enter the drying chamber. Since mechanical seals around the conveyor would be imprac-tical, a nonmechanical sealing method was developed (Stubbing, 1998).
With reference to Figure 8.2, this method utilizes the horizontal thermal tain level across the ducts that extend from the drying chamber and form the entry and exit ports for the conveyor with the material to be dried.
According to Stubbing (1994b), the substantial (55%) difference in density between air and steam at 100°C combined with the temperature difference itself give rise to the stability of the layer. As proven in pilot tests, conveyed
Once a part of the wet material is inside the drying chamber, air heat-ing, circulation, and progressive replacement with water vapor take place as in a batchwise operation until the process temperature reaches 100°C.
product inlet and outlet ducts. From that point on, the wet material can be conveyed through the drying chamber while evaporated water is continu-ously heated and recirculated at atmospheric pressure. To avoid possible oxidation of the product by the air contained in the drying chamber dur-the target temperature is attained.
FIGURE 8.2
Continuous airless dryer.
Drying chamber
Vent Burner
Fan Flue gases
Indirect heater Hopper
(preheater)
Conveyor
CRC_73877_Ch008.indd 126
CRC_73877_Ch008.indd 126 12/29/2008 11:28:12 AM12/29/2008 11:28:12 AM
and density differential stratification layers. This layer is located at a
cer-materials can move up and down through the stratification layer with no was detected through the dryer’s condenser (Anonymous, 1994).
apparent disturbance—this was confirmed by the fact that no vented air
At this stage, the stratification layer is formed at a certain level across the
ing the warm-up period, water may first be sprayed in the chamber until
Stratification layer
© 2009 by Taylor & Francis Group, LLC
Besides the typical advantages of drying in a steam/SHS atmosphere ing rates due to preferential properties of steam against air, such as viscos-to be energy savings and thus lower processing costs and reduced CO2 emissions. For example, the energy savings in high- humidity paper dry-ing with exhaust heat recovery in comparison with conventional paper drying are claimed to be up to 7.75 TOE (tons of oil equivalent) per 100 t of paper produced. This approximates to 10 t of fossil fuel savings and reduction of CO2 emissions by up to 30 t/100 t of dry paper produced.
The cost of thermal and compression energy inputs has been estimated at
£456 for airless dryers as compared to £1094 for conventional paper dryers (Stubbing, 1990). See the References section for more information about energy consumptions and cost estimates.
References
Anonymous. 1994. Paper sludge—from a disposal problem into a valuable prod-uct. DTI’s Environmental Management Options Scheme. Project Information Brochure No 3.
2,209,383.
Stubbing, T. J. 1990. Airless drying process saves energy and reduces emissions.
Paper Technol., 31(June): 36–39.
Stubbing, T. J. 1993. Airless drying: Its invention, method and application. Trans.
IChemE. Part A., 71(A5): 488–495.
Stubbing, T. J. 1994. Airless drying—a case study from the ceramic industry.
DEMOS Communicate. Heat-Win Ltd, Bitterley, U.K.
Stubbing, T. J. 1994a. Sludge Disposal: Moist Incineration or Dry Combustion—a Preliminary Comparison. Heat-Win Ltd., Bitterley, U.K., P. 8.
Stubbing, T. J. 1994b. Airless drying. Drying 94. Proc. 9th Int. Drying Symposium (IDS’94). Gold Coast, Australia, pp. 559–566.
Stubbing, T. J. 1994c. Airless drying for reducing the cost of drying materials, reducing the cost of sludge processing and reducing the cost of drying paper.
The DEMOS Progress Seminar on Paper Pulp Sludge from a Disposal Problem into a Valuable Product, Leicester, February 8.
Stubbing, T. J. 1998. Method and apparatus for continuous drying in superheated steam. US Patent 7,711,086.
Stubbing, T. J. 1999. Airless drying—development since IDS’94. Drying Technol., 17(7&8): 1639–1651.
CRC_73877_Ch008.indd 127
CRC_73877_Ch008.indd 127 12/29/2008 11:28:12 AM12/29/2008 11:28:12 AM
such as no oxidation, reduced fire and explosion hazard, and higher dry-ity and specific heat capacdry-ity, the main advantage of airless drying appears
Stubbing, T. J. 1987. Method and apparatus for energy efficient drying. U.K. Patent
© 2009 by Taylor & Francis Group, LLC
129