Abstract
This entry presents an overview of the methods for drying agricultural and forestry products. The need for the drying of agricultural and forestry products is presented, and the principles of drying operations are described. It is shown that a diversity of drying systems are currently used. The performance of these dryers varies significantly. Significant progress has been made in the research and development of drying technology to improve product quality, reduce cost, and improve environmental performance.
INTRODUCTION
Drying, the removal of water from products, is a necessary operation in many industries for the purpose of preserving product quality or adding value to the products. This entry focuses on the drying of agricultural and forestry products; in particular, grain drying and (solid) wood drying, which form the bulk of drying operations in these two industries.
THE NEED FOR DRYING OF AGRICULTURAL AND FORESTRY PRODUCTS
The main purpose of agricultural drying is often to reduce field loss and weather damage, and to maintain product quality during subsequent storage and delivery. This is in contrast to the drying of wood, whose main purpose is to have all the shrinkage and distortion take place before the wood is put into use. The reduction of insect and fungal attack is another reason for the need for wood drying.
In addition to the above difference, agricultural drying is also typically a seasonal activity, while wood drying is normally a year-long operation. This difference can have a significant impact on a number of aspects of drying operation and dryer design.
Overall, drying may be regarded as a risk management tool for the agricultural industry, and a value-adding tool for the forestry industry.
METHOD OF THERMAL DRYING
In most cases, drying involves the application of thermal energy. This is achieved by heating up the product and forcing hot air through it, therefore vaporizing and removing the moisture inside the product. In addition to the promotion of heat and mass transfer, the circulation of air also helps to carry the heat to and the moisture away from the product.
Significant energy is required in the drying process for several reasons:
• Raising the temperatures of air, the product, and water
• Vaporizing the water
• Compensating heat loss through radiation, convection, and operational losses (e.g., leaks)
• Compensating heat loss through the venting of heated humid air
Thermal drying, which involves water phase change, is a very energy intensive activity. For example, evaporating one cup (250 mL) of water would require approximately the same amount of energy as it would to heat a big pan of soup (3 L) from 25 to 70°C. Thus, the efficiency of energy use in drying processes is significant in the context of energy, economic, and environmental policy goals.
Because artificial drying normally offers the advantages of better control over product quality and higher productivity, this method has been widely used in the agricultural and forestry industries. Most artificial dryers also use the direct heat and vent method to drive the drying process.
HOW DRYING TAKE PLACE
During drying, evaporation may take place in two stages.[1'2] At first, there may be sufficient moisture within the product to replenish the moisture lost at the surface, until the critical point is reached and a dried surface forms. Evaporation is then principally dependent upon the rate of internal moisture diffusion. This is called the falling rate period or second period of drying, and is often a diffusion process. Compared with the first period of constant drying at the rate of liquid water evaporation, diffusion is typically a slow process, and is mainly controlled by internal moisture transport of the product. Diffusion processes may be considerably accelerated with increased temperatures. External mass transfer plays a relatively small role at this stage.
Fig. 1 Typical components and arrangement of a batch wood drying kiln.
Corresponding to the above process, initially, as the product surface is dried, it is restrained by the wet core so that it is subjected to a tensile stress. Later, as the core dries, it is in turn restrained by the drier surface, so that the stress profile inverts. At the end of drying, the product surface may be left with a residual compressive stress, whereas the core is subjected to a tensile stress. This is called case-hardening.[3] A drying schedule will therefore need to ensure that the stresses developed during any period of the drying process not exceed the strength of the material, so that stress damage of the product does not take place. At the end of drying, stress relief for the residual stresses may also be carried out.[4'5] This is particularly important when a product needs further processing or when a high-temperature fast drying schedule is employed. Cooling of agricultural products also minimizes the water condensation on the product surface.
TYPES OF DRYERS
A dryer generally consists of a chamber, a heating and air circulation system, and a control system.
Dryers may be classified in many ways, such as by modes of operation (e.g., batch or continuous dyers), by fuel sources, by drying temperature ranges, or by dryer throughputs. Further classifications are also possible, including heat transfer methods (e.g., direct or indirect heat transfer) and relative directions of the flows of product and air (e.g., cross-flow, counter-flow, and concurrent-flow).
In the agriculture and forestry sector, the most common type of dryer is still the fixed-bed batch (bin, shed, or compartment) dryer (Fig. 1). In these dryers, the product remains stationary while the drying environments are successively varied. This drying mode is particularly suitable for small to medium operators, with the advantage of low capital cost requirement. However, this method is also generally of lower capacity and more labor intensive. In comparison, a continuous dryer would typically use higher temperatures, have much larger capacity, and be more suitable for large operations. This method, however, requires large capital outlay. It is also sometimes more difficult to achieve accurate product specification with continuous driers, because of the potential impact of process air leakage and ambient conditions.
In the past decade, there has been an increasing interest in the use of various drying facilities, particularly low-cost, low-temperature dryers, as more and more farmers have begun to appreciate the importance of drying in the total harvesting system. The rapid growth of the forestry plantation industry also promotes the widespread installation of various timber-drying kilns.
Other specialist drying methods are also available. These include fluidized-bed drying for moist particulate products (such as grains, peas, and sliced vegetables) and drying by the application of energy from microwave or dielectric sources. However, many of these methods may only be cost effective for particular high-value products or for obtaining specific attributes for specific products. Freeze drying is reported to be able to achieve smaller product shrinkage, longer product storage life, and better retention of biological activity, so it is popular with the food industry.
DRYING SCHEDULES
A drying schedule may be described as a series of temperature, humidity, and air velocity settings used to dry the product to a specific moisture content, and to produce consistent, defect-free dry products in as short a time as possible with the least amount of energy use. Many agricultural and forestry products are required to be dried to a final moisture content of around 10%—12%.
There are two ways to define moisture content.[6] In general, the moisture content of forestry products is often expressed in dry basis (MCdb), which is the fraction of the mass of water in comparison with the mass of the oven-dry product. By contrast, the moisture content of agricultural products is normally expressed in wet basis (MCwb), which is the amount of water in the product divided by the total product weight. These two definitions of MCdb and MCwb can be converted to each other by the relationship
During drying, agricultural and forestry products usually undergo considerable changes, including shrinkage, cracking (both externally and internally), nutrient loss, and color changes. By imposing harsh drying conditions, a high-temperature regime may bring in the benefit of shortened drying schedules. However, such a process may also increase the risk of quality degradation, so a right balance needs to be achieved between these two competing factors.
The required drying time varies greatly for different agricultural and forestry products, ranging from a few hours to several days or even several weeks, depending on the product characteristics and the specified quality grades. Many agricultural products also have to be dried within a certain time frame to avoid significant quality deterioration.1-7-1
At present, most commercial dryers operate at comparatively moderate drying schedules to avoid the risk of quality loss. Low-value and more permeable materials may be dried more rapidly. In general, it may be categorically found that a high-temperature, high-humidity schedule may be suitable for fruit products, while a low-temperature, low-humidity schedule would be good for high-value seed products. In the middle, grain and timber are reasonably robust and may be suitable for a high-temperature, low-humidity schedule.
Currently, simple staged temperature controls are preferred for drying agricultural and forestry products, particularly for grain drying. To minimize degrades, some additional pre- or post-treatments such as initial air drying and post-drying cooling and conditioning may also be employed. Additional quality control may be attained by presorting the material prior to kiln drying so that the properties of batches are relatively homogenous. Grain inverter or airflow reversal may also be adopted.
Current drying schedules have been largely derived from trial-and-error experiments over a number of years. However, this method could be lengthy and expensive. Recently, a number of theoretical models have been developed to simulate and optimize the drying process and to reduce the number of laboratory and field experiments.1-8’9-1
Typical drying schedules for several agricultural and forestry products are as follows:
Permeable softwoods (such as radiate pine) for structural uses:
• Dry straight through from green to an average moisture content of 4% at a dry-bulb temperature of 120°C, a wet-bulb temperature of 70°C, and an air velocity of 5 m/s.
• Cool outside under cover for 90 min.
• Steam for 2 h.
• Cool with weight on and de-fillet within 24 h of steaming.
Permeable softwoods (such as radiate pine) for furniture uses (appearance grades):
• Dry at a dry-bulb temperature of 90°C and a wet-bulb temperature of 60°C, with a total duration of 2-3 days. Final steaming is also required, in order to remove the residual stress generated during the drying process. This is necessary, as the dried timber will be further reprocessed during furniture making.
Different from the softwoods, hardwoods are generally less permeable and more difficult to dry, so they are usually kiln dried by moisture content schedules.[10] This means that the dry- and wet-bulb temperatures are changed when the timber (lumber) reaches certain moisture contents. Hardwood is also often air dried first, before the kiln drying. Depending on the species and thickness of the lumber, the drying times may vary from one to a few weeks, with the final temperature being gradually raised from the ambient temperature to between 45 and 65°C. The air velocity is typically maintained at 1-1.5 m/s.
For grain drying (milling grade), it is generally recommended that the maximum drying temperature be limited to 70°C to minimize the heat damage, particularly at initial period of high moisture content. Feedstock grades can be dried at much higher temperatures. Seed drying is usually limited to 30°C-40°C. The common airflow rate for grain dryers is in the range of 200-1000 L/s/t of grain.
DRYER PERFORMANCE AND ENERGY EFFICIENCY
A dryer may be regarded as an energy system. Various energy sources may be used in the drying process, including electricity and various primary fuels such as coal, diesel, and gas. These energy sources all have different heating values, costs, and environmental impacts. Although electricity is a convenient and “clean” energy source, it is a high-grade energy because the typical efficiency of thermal generation of electricity is only 35%-50%. Overall, electricity is generally more expensive, particularly after taking account of the associated supply and transmission charges.
Typically, the energy cost for a small or medium drying operation may range from a few thousand dollars to over twenty thousand dollars; depending on and significantly influenced by the quantity and initial moisture content of the product, and operation practice such as drying schedules and controls. Assuming a 5% moisture removal, the total amount of water being removed from one ton of product is about 50 kg. Currently, in a commercial dryer,the energy required to evaporate 1 kg of moisture from a product ranges from 3.5 to 7.0 MJ.[11,12]
In many cases, it has been found that there is little correlation between the dryer energy performance and product process requirement. Lower process requirements do not necessarily lead to higher energy efficiency. This indicates that there is a significant potential to improve the dryer energy performance.
Although the technology is currently available, it is noted that there are significant barriers for the uptake of energy-efficient technology in the drying industry. This is because present production methods have historically been based on considerations of process throughput, reliability, and capital cost. Energy costs, although comprising a significant part of total operating costs in the drying of agricultural and forestry products, typically represent only 2%-5% of product value, and are therefore often of low priority. This is further reinforced by the factor that drying may be a secondary activity for many farmers and operators. This is particularly the case for agricultural dryers, as agricultural drying is typically a highly seasonal activity. Most agricultural dryers are only utilized for one to three months.
DRYER DESIGN AND SELECTION
Established procedures are now available for the design of agricultural and forestry product dryers. In spite of this, the actual performance of different industries and different dryers still varies considerably. Poor drying systems can lead to significant penalties in terms of lost production and lost income, including increased energy cost and degraded or non-uniform product. In comparison with the “seasonal” agricultural drying industry, the timber industry is typically a year-long operation, and hence more expensive technology and personnel training may be justified. For example, in the timber drying industry, automatic kiln monitoring, management, and control systems have now been routinely implemented to improve the dryer performance. This is relatively rare for the drying of agricultural products. Because of the short period used, most agricultural crop dryers are also not insulated.
To obtain the maximum performance and the desired product quality, it is important that a suitable drying system be selected, with correct system sizing, matching of subsystems, and operating procedures. These factors are often interlinked, so an integrated and holistic approach is required. For example, when employing a high-temperature regime, more water vapor is produced, and higher airflow rates will be required to carry away this vapor. When contemplating increasing the dryer capacity or adopting a new dryer, it is also important to consider the impact on all the other components of crop harvesting and storage systems. A decision should not be made on the basis of the effects of that particular facility alone.
To save energy, some large sites may have several dryers so that a cascade arrangement for exhaust energy recovery is justified. For a large production, it may be possible to carry out electricity tendering or form an electricity user “club” to reduce the electricity tariff. Optimization of fan sizing and operation is also important, as fan laws stipulate that a 50% fan speed reduction can result in a reduction of fan power to only one-eighth of the original power requirement. Less fan power may be needed during the later stages of the diffusion drying process.
At present, a number of computer models have been developed and used in the dryer design. The main advantage of this method is to achieve a precision sizing of the equipment and to produce a predictable design to reduce the customer’s business risk. For agricultural drying, climate-based models[13] have also been developed to ensure optimal design and integration between various agricultural machinery, crop performance, and perceived weather risk. Together with local historical weather data and future climate forecasts, these models have been used to assist in the decisions of both long-term investment in drying facilities and short-term tactical operation decisions (e.g., by adjusting the crop planting and harvesting schedule, by crop choice and crop diversification, or by early negotiation with harvest and drying contractors).
RECENT RESEARCH AND DEVELOPMENT
Significant research has been carried out in the area of agricultural and forestry product drying. This has included research on dryer design, the impact of drying on product quality, improvements of drying energy efficiency, new methods of drying, and applications of new technologies.
Dryer Design and Operation
Because most of the current agricultural dryers are of small to medium throughputs and are operated by rural family businesses, the main constraints for agricultural drying are often the capital expenditure and the technical competence and skills of farmers and local dryer manufacturers. It has been found that the common reasons for poor dryer performance are poor dryer design, inappropriate equipment selection and installation, and bad control and operating practice. Practical information on the best operating practice also tends to be fragmented and not readily available in a useable form.
One of the current research priorities is therefore to demonstrate and establish appropriate technologies to overcome the above barriers and to improve the integration of heat and mass transfer processes and the matching of subsystems.[14]
For commercial operators, uneven drying is also a significant problem. This may be difficult to eliminate, because drying environment is inherently dynamic and varies with locations inside the dryer. Furthermore, dryers may also be required to handle variable resources, including feed materials of different species, non-uniform initial moisture contents, and different sizes. Computational fluid dynamics (CFD) has now been widely used to improve the dryer design and to minimize air recirculation loss. The latest research is also focusing on the development of new sensor techniques and enhanced machine vision tools for collecting quality control information and developing expert systems for rapid problem diagnosis.
Considerable effort is also being made to investigate the effect of drying conditions on the shrinkage, stress development, and quality of products.
New Methods of Drying and New Technologies
A number of innovative methods and drying technologies are being developed. Among them, drying with a modified atmosphere has shown significant promise. Drying and storing fruit and foods in a controlled atmosphere (CA) can lead to improved product quality, because displacing oxygen with other gases such as N2 and CO2 retards the oxidation process. Considerable commercial success has been achieved in the area of CA fruit storage and transport. Similar opportunities have also been identified in the area of fruit drying, particularly in terms of eliminating the use of chemical preservatives or other additives. Recent experiments have shown that the use of innovative CA, oxygen-free drying for apples can significantly improve the product attributes, in particular reducing brown staining and avoiding the requirement of using sulfur chemical pretreatment.[15] This can lead to more healthy products, with the additional benefits of improved taste (no acid) and better texture.
Since drying is an energy-intensive operation, much attention is also given to the development of an energy-efficient drying process. Heat-pump drying is one such technology, because the heat normally vented to the atmosphere is recovered. A heat-pump dryer (HPD) is essentially an industrial adaptation of a normal air conditioning system. Energy (electricity) inputs to the dryer include those to the compressor and the fans. For each unit of electrical energy used by the heat pump, generally three to four units of energy are available for drying the product. In a heat-pump dehumidifier, most of the moisture is also removed from the kiln as liquid rather than moist warm air.
Due to the limits of currently applied working fluids (refrigerants), the HPD normally has to operate at low to medium temperatures, so the drying rates are also slower. This is suitable for a number of heat-sensitive products, but also makes it difficult to compete with alternative mainstream technologies, where the emphasis is often put on the fast drying rate and quick return of plant capital costs.[16] Solar-assisted HPDs are also being developed[17] to accelerate the drying process.
CONCLUSION
Drying is a significant operation in the agricultural and forestry industries. Considerable progress has been made in the research and development of drying technology to improve product quality, reduce cost, and improve environmental performance. It has been shown that a diversity of drying systems are used. The efficiency and performance of different driers vary significantly. Energy consumption is strongly influenced by the dryer design, the particular operation practice, and the individual skills of the operator.
A number of innovative methods and drying technologies are being developed. Among them, drying with a modified atmosphere and improvements in sensor and control technology have shown significant promise.
