Solids drying

Reprinted from The Alcohol Textbook 5th Edition 2009 with permission from Lallemand Ethanol Technology and Nottingham University Press

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Like evaporation, drying is a mass-transfer process resulting in the removal of water or moisture from a process stream. While evaporation increases the concentration of nonvolatile components in solution, in drying processes the final product is a solid. Drying processes reduce the solute or moisture level to

• improve the storage and handling characteristics of the product,
• maintain product quality during storage and transportation and
• reduce freight cost (less water to ship).

Industrial drying applications use conductive and/or convective heat-transfer processes to reduce the concentration of residual volatile components in process streams that are rich in nonvolatile compounds. The principles of solids drying are similar to those of other thermal processes such as evaporation. Consequently, industrial evaporators and drying systems have many functional similarities, including

• an energy source,
• mechanisms for introducing feed into the drying system,
• a conditioning system to ensure that feed and product flow freely in the dryer,
• heat-transfer mechanisms and
• vapour-product separation equipment.

Figure 20. Encapsulated, or bound, moisture

In addition to the thermodynamic principles of Fourier's law such as heat duty, heat-transfer rate and temperature differentials, dryer design and operations must also consider three interrelated factors that impact dryer selection and operations: particle residence time, temperature sensitivity of the product and bound moisture. The presence of bound, or encapsulated, moisture (Figure 20) - the water that is chemically bound to cellulose, hemicellulose, lignin or similar compounds and is difficult to remove - increases the residence time in the dryer. In many cases temperature must also be increased, adversely affecting the quality of temperature-sensitive products.

Dryer categories and selection

Table 4 categorises continuous industrial dryer technologies by their methods of heat transfer and product conveyance. Several of these technologies have been used in the dry-grind ethanol industry, with varying degrees of success.

Table 4. Continuous dryer technologies.
		Heat-transfer mechanism		Vaporised liquid conveyance		Material conveyance		Dryer types
Direct dryers		Direct contact between wet solids and hot gases		By the drying media		Mechanical and/or pneumatic		Tray, sheet, rotary*, tunnel, through circulation, SSD** rotary
						Pneumatic by drying media		Pneumatic conveyor*, ring*, fluid-bed, spray, SSD** ring
Indirect dryers		To wet solids through retaining wall		Independently from heating media		Mechanical		Cylinder, drum, steam-retaining wall belt, screw conveyor, steam tube*
* Application in dry-grind ethanol production. ** SSD: Super heat steam dryer.

Dryer technology selection criteria includes a combination of external factors and drying systems issues (Table 5), several of which are common to both categories.

Table 5. Comparison of dryer issues.
External factors		Drying system issues
Plant location		Reliability and operability
Local rules and regulations		Energy efficiency
Energy source		Energy source
Emission requirements		Emission requirements
DDGS market		Product quality
Upstream processes		Capital investment
Energy recovery		Energy recovery

Dryer exhaust

Increasing fuel prices have led the ethanol industry to pursue strategies that reduce the net energy cost of producing DDGS. This has resulted in improvements in dryer efficiency as well as in combustion systems capable of using lower-priced fuels such as coal, biomass and forest products. In addition, process technology providers continue to pursue opportunities to utilise the dryer exhaust as a source of energy in other ethanol plant operations, reducing overall plant energy consumption. The properties of the dryer exhaust are a function of the dryer technology used and affect the ability to recover the energy value of the dryer. The ability to effectively recover energy contained in dryer exhaust is primarily a function of the dew point of the vapour. Dew point is defined as the temperature at which water will begin to condense from the exhaust under constant pressure. In dryer systems, the dew point of the exhaust stream is influenced by its composition. Aside from water vapour, the dryer exhaust is composed of air, products of fuel combustion, particulate matter and volatile compounds present in or resulting from the thermal decomposition of the dryer feed. The concentrations of the various nonwater components are a function of dryer design, maintenance and operations. Air is generally introduced into dryers during the fuel combustion process, as a sweep gas to assist in conveying moisture and solids or as a result of leakage. Dryer design selection is impacted by air moisture concentration and associated dew point, as illustrated in Figure 21.

Figure 21. Impact of exhaust dew point and moisture content in relation to dryer design

As the fuel ethanol industry has grown, it has attracted the attention of environmental regulatory agencies. During the 1990s it was determined that DDGS dryer exhaust was a major source of priority pollutants. This included volatile organic compounds such as acetic acid, ethanol and furfural, as well as particulate matter and products of the drying process. In addition, as the ethanol production capacity of plants increased, products of combustion such as nitrogen oxides, sulphur oxides and carbon monoxide became a factor in permitting new plants. The industry and technology suppliers responded with the addition of emissions control devices like thermal oxidisers to conventional dryer technologies as well as the development of new dryer technologies.

Recovering dryer energy in other processes results in the condensation of substantial amounts of water from the exhaust stream. This has the effect of reducing the organic load to and thermal duty on the thermal oxidisers at the expense of generating a high biochemical oxygen demand (BOD) in the wastewater stream.

DDGS drying systems

Various dryer technologies have been used during the course of the development of the dry-grind ethanol industry. Early in the industry's history, most dryhouses were more or less based upon beverage distillery industry standards, with DDGS drying operations that predominantly used steam-tube dryers. As the industry matured, average plant capacity increased and DDGS transformed from a by-product to a value-added source for animal nutrition. Rotary and ring-dryer technologies began to displace steam-driven systems. Of the numerous options that have been implemented during the past three decades, three basic configurations dominate the industry today - steam-tube dryers, rotary dryers and ring dryers. The fundamental design for a DDGS dryer incorporates the following basic process steps:

• Furnaces combust fuels, generating hot gases that are used directly or indirectly (SSD) as a source of heat for drying. In the case of steam-tube dryers, the energy source lies in the boiler, generally independent of the dryer.
• Solids handling system and pumps continuously feed, convey and discharge wet cake, solubles and DDGS.
• Feed conditioning and mixers blend a portion of the dry DDGS product and 'wet' incoming feed, changing the physical properties and handling characteristics of the feed streams and reducing the agglomeration of solids and plugging of dryer internals.
• Dryer body moves solids in rotating flighted drums or vertical ducts to contact the solids and hot gas streams or surfaces.
• Product recovery separators and cyclones remove DDGS solids and fine particulate matter from the gas/vapour streams.
• Product coolers reduce the temperature of the dry DDGS product to near-ambient temperature, improving product handling and reducing the opportunity of spontaneous combustion during storage and transit.
• Emissions-control cyclones, scrubbers and thermal oxidisers reduce the emission of particulate matter, carbon monoxide and volatile organic compounds (VOCs).
• Air handling system moves vapour and hot gases during the drying process.

Various approaches that use these systems are compared in Table 6 and illustrated in the figures that follow.

Table 6. Characteristics of DDGS dryer technologies Please click here to enlarge this table

Rotary direct-fired PGR dryers

To date, rotary direct-fired PGR dryers (RDFDs), shown in Figure 22, have the greatest market penetration in North America. This is primarily due to a significant base that was installed before increased attention from regulatory agencies and the mandated application of thermal oxidation systems. Of the four dryer technologies illustrated, a lower construction cost has kept the RDFD in high demand. The lower cost is a result of the dryer's ability to operate at higher inlet gas temperatures. Since higher inlet gas temperatures reduce the time that solids must reside in the dryer body, the size of the dryer's rotating drum can be reduced, with a comparable reduction in equipment cost. However, higher temperatures result in reduced product quality, increased VOC emissions and higher equipment maintenance. Modern RDFDs are equipped with partial recycle of the exhaust (partial gas recycle), resulting in reduced fire risk (lower oxygen content) as well as improved energy efficiency and reduced dryer emissions. End-of-pipe thermal oxidation systems are generally used for emissions control. Alternatively, some RDFD installations have integrated the dryer exhaust thermal oxidisation process with waste-heat steam generation to improve upon overall plant energy balance. Unfortunately, this has linked dryer emissions treatment with steam generation for the ethanol plant, but these two processes do not always operate synchronously.

Figure 22. Rotary direct-fired PGR dryer

Ring dryers

Ring dryers (RDs) follow rotary direct-fired dryers in market penetration (Figure 23). Compared to rotary direct-fired dryers, these systems show similar installed capital investment but improved primary energy efficiency and product quality. When compared to a rotary dryer, pneumatic transport of the product in the ring dryer body increases electrical energy consumption. The primary energy efficiency of an RD is due, to a large extent, to the high hot-gas-recycle rate and well-sealed design. These features result in low air entrainment, producing a high dew point exhaust gas and offering greater opportunity for waste-heat recovery applications. The design reduces the time that the DDGS solids are subjected to heat, improving product quality. This is especially important when processing high-protein feeds. The short residence time is possible because of the application of separation, classification and particle-size reduction technologies. Combined, these serve to control particle size, selectively removing dry product from the system while retaining heavier and larger particles. End-of-pipe thermal oxidation systems are generally used for emissions control. Due to low air infiltration, the size and operating cost of the TO is reduced.

Figure 23. Ring PGR dryer

Indirect-fired SSD dryers

Indirect-fired SSD dryers (Figure 24a) follow RDFDs and RDs in market share. The system employs full gas recycle, but instead of introducing hot combustion gases directly into exhaust recycle, energy is applied indirectly via a heat exchanger. The exchanger transfers heat from the furnace combustion gases to the recirculating exhaust, superheating the stream. As the superheated dryer exhaust is reintroduced into the dryer, the energy is transferred to the product, vaporising water without condensing. For this reason, the technology is often referred to as superheated steam drying, or SSD. One major feature of the indirect-fired SSD dryer is the integration of emissions control technology provided by operating the furnace under conditions suitable for thermal oxidation of organic compounds in the dryer purge. In addition, the design of the closed SSD loop increases the purge gas energy recovery potential, providing even greater opportunity for waste-heat recovery applications. Due to reduced air entrainment, the ring SSD (Figure 24b) produces the highest dew point, and, therefore, the highest energy recovery potential. Furthermore, ring SSD systems can be pressurised, which increases energy recovery potential. Because rotating drums are difficult to completely seal, air infiltration is increased, and the exhaust dew point is reduced. The installed cost of indirect-fired SSDs is comparable with that of similar-sized RDFDs or RDs complete with thermal oxidiser systems. Reduced operating cost when running the indirect-fired SSD with energy recovery often results in better economics.

Figure 24a. Indirect-fired SSD dryer - rotary

Figure 24b. Indirect-fired SSD dryer - ring

Rotary steam-tube dryers

Rotary steam-tube dryers (RSTD) continue to be used where applications warrant (Figure 25). These include circumstances where appropriately priced steam is available, where fuel selection does not allow for direct-fired applications or where the fuel is incompatible with the use of indirect-fired SSD dryer heat exchangers. Major impediments to greater market penetration include low energy efficiency, high capital cost and reduced product quality. In the case of RSTDs, energy consumption is a function of both dryer and boiler efficiency. Consequently, energy demand per unit of water evaporated in RSTDs is generally higher than that of other dryer technologies. Likewise, capital investment increases due to the considerable heat-transfer surface area in the dryer and the associated steam generation system.

Figure 25. Rotary steam-tube dryer

Extended contact of the product with the surface of the steam-containing tubes results in protein denaturation and reduced product quality. RSTDs can be designed with relatively low air infiltration rates, but because the rotating drum is not as well-sealed as a ring dryer's duct, air infiltration is increased and the exhaust dew point is reduced.

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