Dryhouse Technologies DDGS Production

Dryhouse technologies and DDGS production 303

Chapter 21

Dryhouse technologies and DDGS production

D.A. MONCEAUX1 AND D. KUEHNER2

1 AdvanceBio, LLC, 5405 Dupont Circle, Milford OH 45150 USA ([email protected])2 Barr-Rosin, Maidenhead, Berkshire, UK ([email protected])

INTRODUCTION

The cereal grains used for starch-based ethanol production such as corn, wheat, barley, rye and grain sorghum (milo) typically contain from 50 to 75% w/w starch (dry solids) (Table 1). Between 25 and 50% w/w of the grain remain after fuel ethanol has been produced. The ‘whole stillage’, containing both dissolved and insoluble nonfermentables and nondistillable microbial by-products, is rich in nutrients, fibre, protein, lipids and yeast and has traditionally been incorporated into animal feed rations.

Table 1. Composition of cereal grains used in alcohol production.

Content (weight, % of dry matter) Component Corn Wheat Barley Rye Sorghum

Starch 65–72 67–70 52–64 55–65 72–75Protein 9–12 12–14 10–11 10–15 11–12Fat 4.5 3.0 2.5–3 2–3 3.6Ash 1.0 2.0 2.3 2.0 1.7

The dryhouse is where these valuable animal feed components are recovered from the ethanol production process. The sole purpose of dryhouse operations is to remove water from whole stillage in an efficient and mechanically reliable manner. This process yields a number of possible concentrated products – distillers’ wet grains (only some water is

removed by centrifugation), distillers’ dried grains (DDG), distillers’ dried grains with solubles (DDGS), thin stillage (backset) and/or syrup (concentrated thin stillage, which may be sold separately). Excepting wet grains and thin stillage that must be used near the site of production, these products can be cost-effectively stored and shipped to distant animal feed ration markets.

DRY-GRIND ETHANOL PROCESS OVERVIEW

Historically, ethanol production from starch-bearing cereal grains used one of two general categories of process technologies: wet milling or dry grind. Recently, dry-grain fractionation processes, similar to technologies that were in practise before the advent of corn wet milling, have been re-introduced to the dry-grind fuel ethanol industry. Dry-grain fractionation processes enable the ethanol producer to extract additional value from the non-ethanol coproduct by separating it into a broader spectrum of products for both conventional and developing markets.

Currently, the majority of fuel ethanol production facilities not using wet milling employ conventional dry-grind technologies, with variations of the following major unit operations:

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

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304 D.A. Monceaux and D. Kuehner

• Milling• Mashing• Cooking• Liquefaction• Saccharification• Fermentation

• Distillation• Alcoholdehydration

• Centrifugation*• Evaporation*• Drying**Dryhouseunitoperations

These eleven unit operations are typically grouped intothetwogeneralprocessareas,asshowninFigure1. The ‘front-end’ processes convert the starch-rich cereal grain into ethanol as efficiently as possible. The ethanol-rich beer that is produced in the front-end operations is recovered in the ‘back-end’ distillation process and dehydrated to produce fuel ethanol. The residual suspension, called whole stillage, rich in nonfermentable dissolved and suspended solids, is processed in back-end dryhouse operations, usually producing distillers’ dried grains with solubles (DDGS) and thin stillage/backset.

Ethanol process unit operations are often managed as separate front-end and back-end processes, a practise derived from the industry’s early association with the distilled spirits industry. Numerous process and energy-integration strategies continue to blur the distinction, as technology changes drive greater process efficiencies and capital cost reductions.

DRYHOUSE PROCESSES – OVERVIEW

Two basic processes (solids separation and dehydration) and three unit operations (centrifugation, evaporation and drying) make up the dryhouse in a modern, large-scale dry-grind fuel ethanol plant. Other unit operations have been used on a limited basis. The objective is to concentrate the solids fraction of whole stillage using a combination of mechanical separation and thermal processes. A wide variety of technologies are available.Those selected must deliver mechanical reliability and thermal efficiency at a moderate capital investment. The resultant product is 90% dry matter DDGS, made by maintaining properties of the original, approximately 15% dry matter, whole-stillage feedstock. Table 2 provides information on DDGS composition. In this discussion, DDGS is assumed to be the final product made in the dry mill.

Figure 1. Dry-grind block flow diagram.

Front-endprocessing

Back-endprocessing


Table 2. A typical analysis of corn DDGS.

Component Concentration

Dry matter 89.18%Crude protein 30.03%Fat 10.86%Acid hydrolysed fat 11.06%Fibre 7.22%Ash 5.97%Nitrogen-free extract 44.73%Carbohydrates 51.96%Acid detergent fibre 13.72%Total digestible nutrients 86.49%Calcium 0.07%Phosphorus 0.74%Potassium 1.01%Magnesium 0.3%Sulphur 0.67%Sodium 0.18%Chloride 0.19%Zinc 61.63 ppmManganese 18.40 ppmCopper 6.29 ppmIron 126.00 ppmArginine 1.32%Histidine 0.82%Isoleucine 1.17%Leucine 3.55%Lysine 0.91%Methionine 0.64%Cystine 0.66%Phenylalanine 1.51%Theonine 1.11%Tryptophan 0.24%Valine 1.57%

Anonymous (2008).Nutritional values expressed on 100% dry matter basis.

Dryhouse technologies have changed little over nearly half a century because

• thefuelethanolindustryhasexperiencedonlytwoperiods of significant growth between protracted periods of limited new plant construction (this has restricted technology development),

• alternativelarge-scaledryhouseprocesseshavenotbeen demonstrated on a commercial scale and

• project finance strategies have suppressed theimplementation of new, innovative technologies.

Most improvements in dryhouse operations and energy efficiency have been the result of incorporating interprocess unit operations, energy-integration strategies and technology breakthroughs into front-end processes.Foremosthasbeenthedramaticreductioninstillage volumes that has resulted from increasing beer ethanol content. Ongoing improvements in fermentation technology and yeast metabolism have brought about dramatic increases in per cent v/v ethanol content in fermentors. In the early 1980s, corn dry-grind fuel ethanol plants considered 8% v/v ethanol beer acceptable. Today, a number of plants claim consistent operation at ethanolconcentrationsabove20%v/v(Figure2).Theimpact of this ‘very high gravity fermentation’ is a significant reduction in the related decanter centrifuge and evaporator duty and in plant investment.

Dryhouse mass balance

The increased demand for renewable fuel has led to a corresponding increase in dry-grind fuel ethanol plant production capacity. While new plant annual production capacities averaged less than 30 million denatured gallons ten years ago, today most new dry-grind plant

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Volumetric beer ethanol content (%)

Figure 2. Relationship between beer ethanol content and stillage flow rate.


capacities range from 60 to 100 million US gallons per year, and above.

To successfully manage dryhouse operations, it is important to know the ‘mass balance’ (or ‘material balance’)–thequantitiesofmaterialsthatneedtobeprocessed. Process simulation modelling of a corn dry-grind, motor-fuel-grade ethanol plant that produces a nominal 100 million US gallons per year, generates the dryhouse material balance illustrated in Table 3. The stillage volumes and solids concentrations shown are representative of a plant producing a nominal 14% v/v ethanol beer and recycling approximately 30% of the thin stillage as dilution water (backset) in grain mashing. The evaporator condensate and dryer vent flow rates indicate the energy demand of the corresponding unit operations. Solids concentrations in the evaporator condensate and dryer vent are due to the presence of volatile organic compounds in the stillage.

SOLIDS SEPARATION

Dryhouse operations begin with partitioning the whole stillage suspension into fibre- and suspended-solids-rich wet cake and dissolved-solids-rich, thin-stillage fractions. A number of suspended-solids separation technologies have been used with varying degrees of success. These technologies include filtration systems that use inclined wedge wire or vibrating screens, or various types of belt or leaf filter presses. With whole-stillage substrates, filtration produces a low-solids cake as well as a filtrate with high suspended solids. Postfiltration presses improve the concentration of cake solids, but high sheer increases the concentration of suspended solids in the pressate. Applying organic polymers improves solids separation, but this process is generally not cost effective.

Intheseparationofliquidsolids,thecentrifugalforcegenerated by the rotating assembly replaces the weaker force of gravity. High gravitational forces generated at very high rotational speeds efficiently separate suspended solids across a range of particle sizes in addition to dewatering the wet cake. The optimum technology is one that is capable of processing high hydraulicflowrates,requiresmoderatecapitalcosts,

operates with low maintenance and is robust while delivering high suspended-solids capture rates in addition to a high total-solids wet cake.

Centrifugation

Continuous decanter or solid-bowl centrifuges are generally used for whole-stillage solids separation, clarifying the centrate as well as dewatering the fibrous wet cake.

The eight major components of the decanter centrifuge areshowninFigure3:

Figure 3.Decantercentrifugecomponents

1. Solid bowl, the primary rotating assembly that contains the fluid and applies gravitational force for separating the suspended solids.

2. Main drive, which provides the rotational energy to the solid bowl, producing gravitational force.

3. Scroll conveyor, the internal rotating assembly that conveys the suspended-solids-rich cake from the rotating solid bowl.

4. Differential back drive, which provides breaking energy that reduces the speed of the scroll conveyor, producing a conveying effect and scraping the suspended solids along the internal wall, up the beach and out of the solids discharge port of the solid bowl.

5. Feed zone, which introduces the feed into the rotating solid bowl with minimal turbulence.

6. Beach, the inclined section of the solid bowl that extendsabovethepoolofliquidandprovidesanarea where gravitational force compresses and partially dewaters the suspended-solids-rich cake.

7. Solids discharge ports, the discharge points for suspended-solids-rich cake near the top of the beach.

Table 3. Dryhouse mass balance (for a dry-grind facility producing 100 million US gallons per year).

Whole Thin Wet Backset Evaporator Evaporator Evaporator DDGS Dryer stillage stillage cake feed condensate syrup vent

Solids, tph1 41.12 23.64 17.48 6.50 17.14 0.07 17.07 34.52 0.02Water, tph1 279.88 246.30 33.59 67.73 178.56 145.76 32.80 4.25 62.141 Tons per hour.


8. Filtrate ports with adjustable weirs, which are openings in the end of the solid bowl opposite the beach that provide control of fluid pool depth, inventory, residence time and a discharge point for clarifiedliquid.

The design of the decanter centrifuge, in conjunction with centrifuge settings, determines the settling velocity of particles as well as the particle critical diameter, which together affect the capture efficiency ofthesuspendedsolids.Forwhole-stillagesolutionsthat contain suspended solids with a wide range of particle sizes, the settling velocity is defined by Stokes’slaw(Figure4),where

ut =

settling velocity

ω2 = rate of rotation (radians per second)r = radial distanceρ

p = solid density

ρ = liquiddensityD

p = particle diameter

µ = fluid viscosity

= 18µ

t u ω2r(ρp–ρ)Dp

2

Figure 4.Stokes’slawequationfordeterminingsettlingvelocity.

With a given decanter centrifuge and whole-stillage flow rate, particle critical diameter, or ‘cut point’, is definedbyFigure5,where

Dp c

= particle critical diameter, or the smallest particle diameter capable of being separated,

Q = flow rateµ = liquidviscositys = liquiddepthρ

p = solid density

ρ = liquiddensityV = liquidvolumeω = rate of rotation (radians per second) r = radial distance

= (ρp–ρ)Vω2r pc D

9Qµs

Figure 5.‘Cutpoint’equationfordeterminingparticlecriticaldiameter.

Decanter centrifuges provide three basic controls to the plant operator:

• Feedrate,whichadjuststheeffectiveresidencetime

• Weir height, which controls the liquid volumeand pool depth

• Back-drivetorque,whichadjuststhedifferentialspeed between the solid bowl and the scroll conveyor

The effects of back-drive torque adjustments oncake solids and centrate clarity are illustrated in the following figures. In Figure 6, increasing torqueby reducing the solid-bowl and scroll-conveyor differential speed results in an increased suspended-solids residence time in the dewatering beach as well asinhigherwet-caketotalsolids.Figure7illustratesthecapture efficiency of suspended solids as a function of wet-caketotalsolids.Together,Figures6and7show

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cak

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Figure 6.Effectsofback-drivetorqueadjustmentofadecanter centrifuge on suspended solids.


that centrifugation is a compromise between cake solids and centrate clarity.

Thermal processes

High-flow aqueous process solutions are foundthroughout the dry-grind fuel ethanol plant. Extensive movement of energy from streams needing to be heated or cooled in the various unit operations is needed – occurring by one of the following mechanisms:

• Conduction, which is heat transfer by means of molecular agitation within a material without any motion of the material as a whole

• Convection, which is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it

• Radiation, which is heat transfer by electromagnetic radiation

Dryhouse thermal processes remove water from various stillage streams, increasing whole-stillage solids from approximately 15% w/v total solids and producing a 90% total solids DDGS coproduct. The dryhouse of the nominal 100-million-US gallon-per-year facility representedinTable3requirestheremovalof218tonsof water per hour.

Figure8presentstheenthalpy(thequantityofheatcontained in one kilogram of water at the selected temperature), or energy available in water as a saturated liquidandvapour,andtheassociatedtemperatureatvarious pressures. The ability to use water as an energy-transport mechanism is critical to efficient dryhouse operations.

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85 86 87 88 89 90 91 92 93 94Suspended solids capture efficiency (%)

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cak

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(% w

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Figure 7. Cake dryness versus the capture efficiency (recovery) of suspended solids.

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Figure 8. Water enthalpy chart.


Evaporation

Evaporation technology uses convective heat transfer to concentrate nonvolatile substances in solution or suspension, producing higher-solids-concentration products. The energy-driving evaporation process uses steam or other process streams. In evaporation, energy is applied to a liquid at constant pressure,raising the temperature to saturation – the point where it holds as much energy as possible without boiling. As additional energy is applied, the vapour pressureoftheliquidreachesthevapourpressureofthesurroundingenvironment,andtheliquidbeginsto vaporise. The heat of vaporisation is the amount ofenergyrequiredfortheliquidtochangestatetoavapour without a change in temperature. The resulting vapourseparatesfromtheresidualliquid,increasingthe concentration of the nonvolatile fraction.

Theheat-transferprocessisdefinedbyFourier’slaw(Figure9),where

Q = the rate of heat to be transferred

UO = the overall coefficient of heat transfer

AO = the area of heat-transfer surface

∆Tlm

= the logarithmic mean temperature difference

Q = UO

AO∆T

lm

Figure 9.Fourier’slawequationdefiningthe heat-transfer process.

Thin stillage is predominantly an aqueoussuspension of soluble and insoluble grain solids

and nondistillable fermentation end products. During stillage evaporation, thin stillage, which contains from 5 to 10% total solids, is concentrated to produce anominal30to50%totalsolids.Figure10illustratesthe temperature, enthalpy and state relationship of water,showingthequantityofenergyrequiredandavailableaswaterchangesfromaliquidtoavapourduring the evaporation process.

Evaporator systems

Asimpleindustrialevaporatorsystem(Figure11)will contain the following:

• Calandria, or heat exchanger, which transfers energy from the source stream to the solids-containing fluid, raising the fluid’s temperature to the boiling point

• Circulation,orfeed,pump, which supplies feed to the evaporator’s heat exchanger

• Distributor, which distributes feed or circulating fluid evenly across the faces of the tube sheets of the tubular evaporator calandrias, ensuring that the surfaces of the gravity-fed tubes are thoroughly wetted

• Transfer pump, which moves enriched-solids-containing fluid from the evaporator calandria

• Vapour separator, which separates the water vapour from the enriched-solids-containing fluid

• Condenser, which removes energy from the evaporator via heat transfer with another fluid

• Vacuumsource, which removes noncondensable components in the vapour

212°

32°

Heatingsystem

Heating ice

144 BTU/lbto melt ice

at 32°Fheat of fusion

180 BTU/lb to heat water from 32°F to 212°F

972 BTU/lbto vaporise water

at 212°Fheat of vaporisation

Energy addition

Tem

pera

ture

(F)

Figure 10. The relationship of water temperature, enthalpy and the state of water.


Figure 11. Components of a simple evaporator.

The design of a simple evaporator is shown in Figure11,whereapproximatelyoneunitofsteamiscondensed on the shell of the calandria, transferring the heat of condensation to feed located in the tubes, evaporating one unit of water. When the feed temperature is below saturation, additional energy willberequiredtoraisetheliquidtotheboilingpointat the system pressure. The vapour produced in the evaporator flows through the separator, removing entrainedliquidbeforecondensingandtransferringthe energy to cooling water, with the concentrated product pumped to storage. The uncontaminated steam condensate is returned to the boiler for reuse. Together, the components are referred to as an evaporator ‘effect’.

For energy transfer to occur, a temperaturedifferential must exist across the heat-transfer surface area. Because steam and stillage are both predominately water, a temperature differential must be accompanied by a corresponding pressure differential,asillustratedinFigure8.

The efficiency of the simple evaporator system in Figure 12 results in about one unit of steamremoving one unit of water with a near-equalquantityofenergytransferredtothecoolingwater.Figures13through15illustrateoptionsforfurther

improvement in system efficiency. In these designs, the vapour directed to the condenser (shown in the simpleevaporatorillustratedinFigure11)isroutedto succeeding evaporation stages, or effects. So, the first evaporator effect condenses the incoming steam,producinganear-equalamountofvapourthatcondenses in the second effect.

Figure 12. Single-effect evaporator.

As seen in these figures, additional evaporator effects remove more water per unit of steam supplied, and system efficiency improves. From this, it appearsthat evaporation systems could be infinitely efficient through the addition of effects, but design and operating parameters dictate otherwise. Critical issues include• theminimalpracticalcondensingtemperatureof

the final vapour, which is a function of the cooling water temperature,

• themaximumpracticalproductsidetemperatureinthe first effect, which is a function of the thermal stability and fouling potential of the feed,

• the practical differential temperature acrossthe individual effects, considering operating parameters such as fouling of heat-transfer surface area and boiling-point elevation of the product and

• cleaningfrequency.


Figure 13. Two-effect evaporator.

Figure 14. Three-effect evaporator.


A multi-effect stillage evaporation system, with a first-effectproductsidetemperatureof210°F and a final condensatetemperatureof130°F,thatoperateswitha15°Fdifferentialtemperatureacrosseacheffectcouldbe designed with approximately five effects.

Thermocompression evaporators

Alternatives exist to improve evaporator efficiency without the continued, capital-intensive addition of effects. Such evaporator systems have been used in dryhouse designs for over 25 years. These systems increase efficiency by recycling vapour from later effects to preceding effects in the evaporator. To accomplish this, the pressure of the vapour must be increasedbythermocompression(Figure16)tooffsetthe design-basis pressure drop of the system.

Evaporator vapour is generally boosted by taking a portion of the vapour from one of the effects and directingittoasteamejector(Figure17).Thisdeviceproduces a Venturi effect, where ejector fluid under high pressure is converted into a high-velocity jet at the throat of the nozzle, which creates a low pressure at that point. The low pressure draws the suction fluid

into the nozzle, where it mixes with the motive fluid, resulting in an intermediate pressure–vapour mixture. Thequantityofvapourrecycledisafunctionofthedesign of the ejector, the motive steam pressure and the pressure of the evaporator vapour. A disadvantage of steam ejector systems is that the motive steam is often contaminated with impurities present in the evaporator vapour. In stillage evaporators, the condensate contains measurable concentrations of ethanol and organic acids and cannot be reused as boiler feed makeup water.

Mechanical compression evaporators

Another variant of vapour-compression technology uses electrical or steam-turbine-driven devices such as fans, blowers or compressors to boost the pressure and recycle the evaporator vapour. In mechanical vapourrecompression(MVR)evaporation(Figure18), the vapour from the separator, free of entrained liquid, is compressed, elevating the condensingtemperature. The vapour directed to the shell of the evaporator body condenses, transferring energy back to the circulating fluid. A small amount of

Figure 15.Four-effectevaporator.


additionalenergyisrequiredtobalancethesystem’senthalpy,‘replacing’energythatisrequiredtoraisethe temperature of the incoming feed to the operating conditions of the evaporator.

Figure 17. Steam ejector (Croll Reynolds Company, Inc.).

Design-basis considerations for MVR systems are a compromise between

• compressor power (increasing the pressuredifferential increases power demand, resulting in lower system reliability) and

• exchanger surfacearea (higherboostpressuresincrease vapour temperatures, providing greater differential temperatures across evaporator bodies, reducing heat-transfer surface areas).

In stillage evaporation, compressing large vapour flows translates to high power demand. MVR systems are best suited for applications where

high-pressure steam is available for an exhaust, •

an extractive turbine-drive is applied and•

low-cost power allows for an electrical drive.•

Fluid properties and evaporator system design

Fluid physical properties must be taken intoconsideration during the design of evaporator systems. Stillage is a complex mixture of inorganic salts, organic acids, soluble and insoluble proteins, peptides and amino acids, carbohydrates, sugar alcohols such as glycerol, lipids and fibre fines.

Figure 16. Thermocompression evaporator.


Figure 18. Mechanical vapour recompression (MVR) evaporator.

A key property is fluid boiling-point elevation. When a solute is added to a solvent, the vapour pressure of the solvent (above the resulting solution) is less than the vapour pressure above the pure solvent. The resulting boiling point of the solution will be greater than the boiling point of the pure solvent. This is because the solution (which has a lower vapour pressure) must be heated to a higher temperature in order for the vapour pressure tobecomeequal to the externalpressure.During evaporation, as the solids concentration of the stillage increases, boiling-point elevation reduces the

effective differential temperature and increases the requiredheat-transfersurfacearea.

Another physical property having a significant impact on evaporator design and performance is fluid viscosity. As dilute feed streams concentrate, fluid viscosities increase, exhibiting both Newtonian and non-Newtonian properties:

• ANewtonianfluidisafluidwhoseviscositydoesnot change with the rate of flow or shear stress.

• Anon-Newtonianfluidisafluidwhoseviscositychanges with the rate of flow or shear stress.

Figure 19 illustrates the shear–temperaturerelationship in thick-stillage streams that contain high concentrations of suspended solids.

Elevated fluid viscosities interfere with film formation in falling-film evaporation systems, resulting in uneven wetting of heat-transfer surfaces. In high-velocity forced circulation evaporation systems such as finishing evaporators, increasing viscosities generate laminar flow, reduced Reynolds numbers and corresponding lower heat-transfer coefficients. Increasing fluid velocity in an effort to improve heat transfer has the adverse effect of increasing system pressure drop and power demand. In all instances, increased fluid viscosities accelerate fouling of heat-transfer surfaces, with fouling being most severe in high solids effects.

Evaporator clean-in-place (CIP) systems

The design-basis criteria of a stillage evaporation system must consider fouling rate and the associated cleaning process. Most evaporation systems are equipped with CIP systems that chemically

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Figure 19. Concentrated stillage viscosity profile over a range of temperature and revolutions per minute during testing.


clean heat exchangers, separators and associated product-sidepipingwithoutopeningtheequipment.Heat-transfer-surface fouling rate and the associated CIPfrequencyandCIPcycletimedetractfromtheonstream time of the evaporator system. In heavily fouled stillage evaporators, one finds complex, mixed organic and inorganic deposits on the heat-transfer surface.Cleaningstillageevaporators requires thefollowing steps, which can require up to twentyhours to complete:

• Initial rinse. Hot water flushes stillage from the system and rinses away easily removable deposits, reducing chemical consumption during subsequentCIPsteps.

• Caustic wash. Dilute caustic solution attacks and partially solubilises the deposit’s organic matrix.

• Intermediate rinse. Hot water rinses dilute caustic from the system in advance of the ensuing acid wash.

• Acid wash. A dilution acid solution, such as sulfamic acid, attacks and partially solubilises the calcium oxalate and calcium-sulphate-rich inorganic matrix of deposits.

• Final rinse. Hot water flushes residual acid and dislodged deposits from the system.

Solids drying

Likeevaporation,dryingisamass-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

• improvethestorageandhandlingcharacteristicsof the product,

• maintain product quality during storage andtransportation and

• reducefreightcost(lesswatertoship).

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

• anenergysource,• mechanismsforintroducingfeedintothedrying

system,• a conditioning system to ensure that feed and

product flow freely in the dryer,• heat-transfermechanismsand• vapour–productseparationequipment.

In addition to the thermodynamic principles of Fourier’slawsuchasheatduty,heat-transferrateandtemperature 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(Figure20)–thewaterthatischemicallybound to cellulose, hemicellulose, lignin or similar compounds and is difficult to remove – increases the residence time in the dryer. In many cases temperature mustalsobeincreased,adverselyaffectingthequalityof temperature-sensitive products.

Figure 20. Encapsulated, or bound, moisture.

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.

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


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

Table 4. Continuous dryer technologies.

Heat-transfer Vaporised liquid Material Dryer types mechanism conveyance conveyance

Direct dryers Direct contact between By the drying media Mechanical and/or Tray, sheet, rotary*, wet solids and hot gases pneumatic tunnel, through circulation, SSD** rotary

Pneumatic by drying Pneumatic conveyor*, media ring*, fluid-bed, spray, SSD** ring

Indirect dryers To wet solids through a Independently from Mechanical Cylinder, drum, steam- retaining wall heating media belt, screw conveyor, steam tube*

* Application in dry-grind ethanol production. ** SSD: Super heat steam dryer.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 inFigure21.

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

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 qualityUpstream processes Capital investment Energy recovery Energy recovery


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.

Rotary direct-fired PGR dryers

To date, rotary direct-fired PGR dryers (RDFDs),shown in Figure 22, have the greatest marketpenetration 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 costhaskepttheRDFDinhighdemand.Thelowercost 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 bereduced,withacomparablereductioninequipmentcost. However, higher temperatures result in reduced productquality,increasedVOCemissionsandhigherequipmentmaintenance.ModernRDFDsareequipped

30405060708090

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0 2 4 6 8 10 12Water (lbs)/dry air (lbs)

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poi

nt (t

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ratu

re °

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Partial gasrecycle and

steam tube dryer

Superheated steamring dryerSuperheated steam

rotary drumdryer

Opencircuitdryer

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


Figure 22. Rotary direct-fired PGR dryer.

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,

Table 6. Characteristics of DDGS dryer technologies.

Rotary direct-fired Ring PGR* Rotary indirect-fired Rotary steam-tube PGR3 (RDFD) SSD (RIFD)/Ring SSD dryer (TSTD)

Figure 22 23 24 25

Type Direct-fired Direct-fired Indirect-fired Indirect

Heat source Combustion gas Combustion gas Combustion gas Steam

Heat exchange Direct hot gas – Direct hot gas – Direct superheated Hot tube wall – solids contact solids contact steam – solids contact solids contact

Solids transport Mechanical/pneumatic Pneumatic in high- Mechanical/pneumatic Mechanical in in rotating drum velocity gas recycle in rotating drum in rotating drum or pneumatic in high- velocity gas recycle

Emission control EOS TO*** EOS TO*** Internal – purge gas EOS TO*** directed to furnace

Miscellaneous Disintegrator required Heat exchanger between Heat-transfer surface to control particulate combustion gas and required in dryer and size of solids drying media boiler

* PGR: Partial gas recycle. ** SSD: Superheated steam dryer. *** EOS TO: End-of-stack thermal oxidiser.

someRDFDinstallationshave integrated thedryerexhaust 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


Figure 23. Ring PGR dryer.

ethanol plant, but these two processes do not always operate synchronously.

Ring dryers

Ring dryers (RDs) follow rotary direct-fired dryers in market penetration (Figure 23). Compared torotary direct-fired dryers, these systems show similar installed capital investment but improved primary energyefficiencyandproductquality.Whencomparedto 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, improvingproductquality.Thisisespeciallyimportantwhen 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.

Indirect-fired SSD dryers

Indirect-firedSSDdryers(Figure24a)followRDFDsand 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, vaporisingwaterwithoutcondensing.Forthisreason,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


Figure 24a. Indirect-fired SSD dryer – rotary.

Figure 24b. Indirect-fired SSD dryer – ring.


opportunity for waste-heat recovery applications. Due toreducedairentrainment,theringSSD(Figure24b)produces the highest dew point, and, therefore, the highestenergyrecoverypotential.Furthermore,ringSSD 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-sizedRDFDsorRDscompletewiththermaloxidiser systems. Reduced operating cost when running the indirect-fired SSD with energy recovery often results in better economics.

Rotary steam-tube dryers

Rotary steam-tube dryers (RSTD) continue to be usedwhereapplicationswarrant(Figure25).Theseinclude 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, highcapitalcostandreducedproductquality.Inthecase of RSTDs, energy consumption is a function of bothdryerandboilerefficiency.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 theconsiderable heat-transfer surface area in the dryer and the associated steam generation system.

Extended contact of the product with the surface of the steam-containing tubes results in protein denaturationandreducedproductquality.RSTDscanbe 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.

DRYHOUSE OPERATIONAL ISSUES

Major unit operations comprising the dryhouse incorporate numerous control points to enable plant personnel to optimise operations. Aside from the designated control mechanisms, numerous ‘external’ parameters can have a major impact on dryhouse operations. These parameters primarily relate to two general categories of process fluid physical properties: particle size distribution and fluid viscosity. These properties are, to a degree, controllable by plant design and/or operator decisions.

Figure 25. Rotary steam-tube dryer.


Solids distribution studies reveal that fine-grind milling results in high concentrations of fine particulate matter in process streams. This serves to reduce decanter centrifuge separation efficiency, increases non-Newtonian viscosity of thin stillage and stillage concentrates and adversely impacts DDGS dryer operations by disrupting wet cake, syrup and DDGS mixing. Dryhouse operations are further compromised by high fermentation and stillage acidity concentrations. The presence of elevated populations of acid-producing microbes or the use of excessive quantities of mineral acids increasesprocess-fluid acidity and degrades fibre structure, producing fibre fines that reduce the separation efficiency of decanter centrifuge solids. Because fibre destruction is a function of acid concentration, temperature and time, technology providers must be concerned with beer-still operation temperatures as well as with whole-stillage storage conditions.

In addition to non-Newtonian viscosity generated by the presence of suspended solids, numerous front-end processes can serve as sources of Newtonian viscosity. Sources of non-Newtonian viscosity include

• unconvertedandretrogradedstarchresultingfrominefficientcookingandliquefactionoperations,

• unfermenteddextrinandglucoseconcentrationsresulting from poor temperature control and microbial activity,

• elevatedlacticacidconcentrationsresultingfrommicrobial contaminant activity and

• elevatedglycerolconcentrationsresultingfromenvironmental stress on yeast.

As the concentration of undesirable solids in the stillage increases during dryhouse operations, so does fluid viscosity. These additional solids not only detract from ethanol yield but also increase dryhouse duty and associated energy consumption. Therefore, optimisation of dryhouse operations is greatly dependent on nondryhouse processes.

CONCLUSION

The dryhouses found in today’s dry-grind fuel ethanol facilities are not significantly different from those found in early whisky distilleries. Decanter centrifuges, thin-stillage evaporators and thermal dryers continue to be widely used to convert whole stillage to DDGS. Recent growth in grain-based fuel

ethanol production has focused more attention on the industry. Opportunities are being explored to improve production economics, focus research activities and develop traditionally undervalued stillage streams.

These efforts include the application of dry-grind corn fractionation technologies that recover higher value protein and lipid-rich concentrates in advance of the ethanol process. This reduces mass flow through the ethanol process as well as solids loading on dryhouse operations. In addition, groups continue to research stillage-refining technologies that promise to recover and concentrate higher-value constituents of whole stillage such as glycerol, organic acids, amino acids, peptides, proteins and lipids.

The application of these new technologies, individually or in various combinations, will change the technical definition of DDGS as well as the unit operations and processes used in their production. As the industry continues to grow, the dryhouses of the past will not be easy to find in the ethanol production facilities of the future.

REFERENCES

Anonymous(2008).CollegeofFood,Agriculturaland Natural Resource Sciences, University of Minnesota, St. Paul, MN.

BelyeaRL,RauschKDandTumblesonME(2004)Composition of corn and distillers dried grains with solubles from dry grind ethanol processing. Bioresource Technology 94 293-298.

Croll Reynolds (2008) Process Vacuum and Power Systems Thermocompressor Theory. http://www.croll.com/_website/pr/thermhome.asp (accessed April 2008).

Green DW and Perry RH (2007) Perry’s Engineering Handbook 8th ed. McGraw-Hill, New York, NY.

Minton PE (1986) Handbook of Evaporation Technology. Noyes Publications, USA.

RauschKDandBelyeaRL(2005)Coproductsfrombioprocessing of corn. ASAE Annual International Mtg.,Tampa,Fl.July17-20.PaperNo.057041.

Shurman J (2008) Distillers Grains By-productsin Livestock and Poultry Feeds. University ofMinnesota, Department of Animal Science. http://www.ddgs.umn.edu/profiles.htm (accessed 10 April 2008).

Woon-Feung Leung Wallace (1998) Industrial Centrifugation Technology 1st ed. McGraw-Hill, New York, NY.

Dryhouse Technologies DDGS Production

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