Home » Energy-Efficient Drying

Energy-Efficient Drying

August 1, 2010

Reduced-oxygen drying uses dry superheated steam in place of hot air as the heated drying medium.

Figure 1. A typical R-O2 drying schematic.

Conventionally designed dryers-even technologically advanced models-are constrained by the hygrometric rules of humidity-controlled drying. Even when fitted with effective air circulation and precision control systems, conventional heat- and humidity-controlled drying has reached a point where only marginal improvements can be made.

Several alternative technologies, including microwave and vacuum drying, have been available for many years with varying degrees of success. Long-term research and development programs have continued to try to improve these high-tech and high-capital-cost technologies into large-scale, cost-effective industrial processes.

The objective of these technologies is to achieve much shorter drying cycles while reducing operational energy costs. Although limited success has been achieved both in the laboratory and the field, disadvantages remain, including high capital equipment costs, high energy consumption, process control challenges, and health and safety issues.

Reduced-Oxygen Drying

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 superheated 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 100øC (212øF). Think of a domestic kettle: When water in a kettle is boiling, it emits steam from the spout. The steam only becomes visible when it condenses on contact with the surrounding atmosphere as it exits the spout. At the immediate tip of the exit spout, a small space where the steam is invisible can be witnessed, because at that location the temperature is more than 100øC (212øF). This is dry superheated steam; Table 1 illustrates why it is a superior drying medium to air.

Because the specific heat capacity of steam is more than twice that of air, it can transfer more than twice the amount of heat for the same mass flow. Therefore, with the same temperature differential between the moist product and the drying medium, the fan power required to achieve a given heat transfer is more than halved.

The viscosity of steam is approximately half that of air at the same temperature, which enhances its ability to impinge on or percolate through a moist product (thus increasing its drying effect). In a superheated steam atmosphere, then, the product temperature quickly attains the steam's 100øC (212øF) saturation temperature, eliminating the moisture's surface tension and halving its viscosity. This absence of surface tension assists the surface moisture to evaporate quickly while the lower viscosity enables the internal moisture to migrate faster to the product's surface.

The absence of air within a drying process prevents oxidation of sensitive products while avoiding the contamination by combustion residues that occurs during directly heated conventional drying and humidity-controlled drying. The low levels of air/oxygen during the drying process also prevent the risk of material combustion of potentially flammable products (woods, plastics, paper, etc.) while they are dried at elevated temperatures.

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.

As the recirculation mix of steam/air and the product get hotter, 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 dimensions 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 120øC (248øF), the dryer ambient or off-product temperature will be around 100øC (212øF). The saturation temperature (or dew point) will then be at around 83øC (181øF), as will the surface 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 surfaces of 100øC are reached, the remaining air is displaced by the same process. The moisture evaporation rates increase, and the interior of the still-moist material is quickly heated to 100øC. As drying continues to its completion, the surplus of the remaining moisture retreats toward the center of the product and the temperature of the dry sections rises to above 100øC.

Figure 3 illustrates how the continual recirculation of the air/steam mixture causes the air-to-steam proportion in a dryer to rapidly decrease. During six air changes, the steam content progresses from 50, 75, 87.5, 93.75, and 96.87 to 98.44%.

Figure 2. Decreasing proportion of air and increasing proportion of steam in saturated air, at atmospheric pressure and temperature between 0-100°C (32-212°F).

R-O2 Drying Cycle

Unlike conventional drying cycles, which can consist of a series of ramps and dwells for both temperature and relative humidity control, the R-O2 drying cycle consists of only two phases: warm-up (between ambient and 100øC/212øF), and drying (above 100øC/212øF). All biomass, organic and inorganic products have different drying characteristics and an ideal or optimum safe temperature at which they can be safely dried. This applies whether the process is conventional or R-O2 technology. However, the R-O2 drying system requires that drying must be carried out at a temperature above 100øC. The R-O2 drying system is also different in that only the temperature profiles are controlled (not the relative humidity).

The R-O2 dryer operates under full recirculation conditions during the entire drying cycle and normally uses an indirect-fired form of heating, which prevents proportional combustion air volumes (required by direct-fired burners) from entering the drying process. During the warm-up phase, varying amounts of the product's initially contained moisture evaporates and effectively raises the dryer's humidity level. This process suppresses rapid moisture evaporation from the product, but it allows the product to be quickly heated to 100øC without rapid product shrinkage and subsequent distortion. Saturation of the air and consequent product condensation damage is avoided by raising the temperature of the air at a faster rate than its dew point temperature.

Figure 3. The continual recirculation of the air/steam mixture causes the air-to-steam proportion in a dryer to rapidly decrease.

At the start of the drying cycle, at a relatively low air dew point temperature, the percentage of steam contained in the air is relatively low. However, at around 60-70øC (140-160øF), the proportion of steam rapidly increases. As the temperature reaches around 90øC (194øF), almost the entire recirculating atmosphere is steam. This almost-saturated atmosphere prevents further evaporation from the surface of the product, which then allows the internal core of the product to heat up at the same rate as that of its surface. This reduces stresses in the product and allows the entire product to be rapidly but safely heated up to 100øC and above.

The peak evaporative drying phase begins when the saturation temperature reaches 100øC, although some drying does begin when it is 83øC (181øF); at this point, the dryer is half full of steam. Additional heating raises the temperature of the product and evaporates the remaining water, while the steam generated from the moist product during the drying phase continues to be vented from the dryer during the drying process. As the higher drying temperatures lower the viscosity of the water, the water flows to the surface more freely and quickly through the product's pore structure, thereby increasing the drying rate.

Heating continues until the product is dry. At this stage, the process is either stopped and the product removed from the dryer, or ambient air is reintroduced into the dryer to cool both product and structures. Cooling must occur at rates such that the product is not subjected to thermal shock and to allow, where applicable, safe access into the dryer for unloading of the product.

The maximum steam temperature determines the evaporation rate and provides a simple method of controlling the drying process. Every product has its maximum safe warm-up rate and peak temperature tolerance, which determine the drying cycle. In order to achieve a successful R-O2 technology drying cycle for a particular product/material, both the maximum safe warm-up rate and peak temperature must first be determined. This valuation enables the fastest drying cycle to be attained without damage to the product.

It is a combination of the rapid heating that is possible during the warm-up phase and the rapid moisture removal during the peak drying phase that allows the R-O2 drying process to achieve substantial reductions in drying times and higher energy efficiencies compared to conventional drying techniques. Figure 4 illustrates a typical R-O2 drying cycle and the relationship between the dryer's internal temperatures and the actual product temperatures, while Figure 5 shows the relationship between the dryer's internal temperature and relative humidity.

Figure 4. A typical R-O2 technology drying cycle illustrates the relationship between the dryer's internal temperatures and the actual product temperatures.

Process Differences

At first glance, R-O2 dryers do not appear to be significantly different than conventional hot air dryers. They incorporate a similar array of process equipment, such as a primary fan, heater and ductwork system. However, significant differences exist in the engineering build, equipment selection, and process operation and control.

The system operates on 100% full recirculation principles rather than a combination of recirculation air mixed with the introduction of volumes of ambient fresh air. In addition, indirect-fired heat exchangers are always used to prevent the ingress of combustion air. The high-quality engineering build provides a level of air tightness that prevents any steam leakage or air ingress. Special attention is paid to thermal efficiencies and the airtight sealing of the dryer structures and ductwork.

Because of the generally higher operating temperatures used with R-O2 drying processes, the thickness, density and type of structural insulation is also greater than normally used in conventional hot air dryers. Finally, as the R-O2 dryer is controlled by temperature profiles only, there is no need for costly humidity-control equipment.

Before drying, ceramic articles contain around 15-20% of water by weight and (because the specific gravity of clay is approximately 3.5) around 40-50% water by volume. During the first phase of conventional drying, the ceramic shapes cease to be "plastic" and shrink by up to 10% dimensionally and 27% by volume as the moisture separating the clay particles is removed.

Figure 5. The relationship between the dryer's internal temperature and relative humidity for the same typical drying schedule.

During the second phase of drying, known as the falling rate period, drying is completed without further shrinkage by removing the remaining moisture contained in the interstices between the clay particles. While in the shrinkage phase of conventional humidity-controlled drying, the product cannot be heated above the saturation temperature (typically around 60øC/140øF). At this temperature, both the moisture surface tension and its viscosity are relatively high. In consequence, distortion or cracking may result unless the moisture in the outer layers of the product is slowly removed, giving time for the moisture in the core to migrate toward the product surface and allow uniform shrinkage to occur.

By comparison, during the shrinkage phase of R-O2 drying, the entire article is rapidly and safely heated to 100øC by recirculation of the air volume that is initially contained in the dryer at start-up. This occurs without significant evaporation from the article and without further air being added, while heating the article to 100øC results in the moisture surface tension becoming virtually nil and its viscosity being substantially reduced.

At 100øC, the internal moisture can migrate to the surface more easily, and rapid evaporation and shrinkage can take place without damaging the product. At the same time, the steam generated by the moisture evaporation quickly displaces and replaces the dryer's originally contained air. Once the shrinkage phase is complete, the surface of the moisture retreats inwardly, and the outer layer of clay becomes dry and porous. Because the highly turbulent atmosphere within the dryer penetrates the porous structure of the product, the temperature of the dry clay increases toward that of the dryer.

Conversely, with humidity-controlled drying, because the temperature of the remaining moisture is around 60øC (140øF), it will not evaporate unless air that is above 60øC at a saturation temperature at or below 60øC is present at its surface. The air must therefore penetrate the already dry outer layer of clay in order to evaporate the inner moisture and transport it to the surface. In practice, because the air (at above 60øC/140øF and a high RH) cannot absorb much additional moisture before becoming saturated, a substantially greater weight of humid air than of remaining moisture must first penetrate the porous, dry outer layer of clay in order to evaporate the core moisture and transport it away.

With R-O2 drying, the core moisture is already heated to 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 100°C from the dryer's recirculating superheated steam atmosphere (typically at 130°C/266°F). As a consequence, the remaining core moisture evaporates and simply becomes steam-expanding by a factor of 1670 (1.67 m3/Kg)-and emerges from the article through its already dry and porous outer layer.

Care is still needed, however, to avoid heating too rapidly, which can cause an internal pressure sufficiently high enough to cause blistering on the article's smooth outer surface. This should be considered when designing the appropriate drying curve for the product. For products that have solvent binders, the binders do evaporate at different temperatures than water. However, using this type of system is extremely safe and the emissions are kept to the absolute minimum. The risk of fire or explosion is reduced with R-O2 technology. However, the system incorporates PrevEx sensing and RTO equipment safeguards as necessary.

The cost of heating air in conventional ovens is high, and the heating value of solvents in air is very high once the concentration is above roughly 10% LFL. Excessive solvent ventilation increases the amount of air that must be heated, wasting fuel.

Energy-Efficient Alternative

R-O2 drying is a safe and more energy-efficient drying method, and it offers reduced drying times compared to conventional drying processes, as shown in Table 2. Though variations will be required depending on specific operations and body preparations, these drying times can be regarded as typical. The main thrust of this article is based on drying in a batch form, but readers should also consider continuous drying, which offers even greater potential to improve most production processes.

For additional information regarding reduced-oxygen drying, contact the author at (973) 641-6857 or andrew@cds-inc.us.com, or visit www.cds-group.co.uk.