On the Physical Properties of Distillers Dried Grains with Solubles (DDGS)

Transcription

1 Iowa State University From the SelectedWorks of Kurt A. Rosentrater July, 2008 On the Physical Properties of Distillers Dried Grains with Solubles (DDGS) Klein Ileleji, Purdue University Kurt A. Rosentrater, United States Department of Agriculture Available at:

2 An ASABE Meeting Presentation Paper Number: On the physical properties of distillers dried grains with solubles (DDGS) Klein Ileleji, Ph.D., Assistant Professor and Extension Engineer Department of Agricultural and Biological Engineering Purdue University, 225 South University Street, West Lafayette, IN 47907, Kurt A. Rosentrater, Ph.D., Agricultural and Bioprocess Engineer USDA-ARS, North Central Agricultural Research Laboratory 2923 Medary Avenue, Brookings, SD, 57006, Written for presentation at the 2008 ASABE Annual International Meeting Sponsored by ASABE Rhode Island Convention Center Providence, Rhode Island June 29 July 2, 2008 Abstract. Distillers dried grains with solubles (DDGS) is a complex heterogeneous granular solid that exhibit a wide range of physical and chemical properties. This fact is one of the major reasons that livestock nutritionists often find it difficult to include it in their feed rations. The rapid growth of the corn ethanol industry, the primary generator of DDGS, has resulted in a huge influx of this product into the marketplace. Unfortunately, the lack of standard methods for product quality determination and reporting is becoming a bottleneck to expanding the market for this product. Challenges in the logistics of moving DDGS through the market chain, primarily via rail, is also a cause of concern. This paper discusses past and the current state of research on the physical properties of DDGS and their determination. In particular is examines the causes of variability in particle density (PD), bulk density, particle morphology (particle size and shape) and particle size distribution (PSD). It also highlights the importance of developing standards for DDGS physical characteristics, measurement methods, and reporting that will be useful for the design of handling and storage systems, as well as DDGS product trade. Keywords. DDGS, corn ethanol, physical properties, granular bulk, standard methods. 1

3 Introduction Distillers dried grains with solubles (DDGS) is a co-product of corn processing to ethanol using the dry-grind process. The number of corn ethanol plants, and their processing capacities, has been markedly increasing in recent years (Figure 1). As of April 2, 2008, 147 manufacturing plants in the U.S. produced approximately 32.2 billion L/yr (8.5 billion gal/yr). At least 55 additional plants are currently under construction, with 6 more undergoing renovation or expansion; these will contribute an additional 19.3 billion L/yr (5.1 billion gal/yr) (RFA, 2006). As the ethanol industry has grown, so too have the quantities of coproducts that have been generated (Figure 1) DDGS Ethanol DDGS (million metric tons) Ethanol (million gallons) Year 0 Figure 1. The ethanol industry has grown substantially over the years, as evidenced by both the fuel ethanol produced each year as well as DDGS generation. Most of the increase in ethanol production during the past decade is attributed to growth in the dry-grind process (Rausch and Belyea, 2006). An in-depth review of technologies of production, characteristics, market issues and strategies for modifying and improving processing methods of corn co-products was discussed by Rausch and Belyea (2006). While their article was a good overview of various issues of corn co-products from the various processes, it did not address why DDGS from dry-grind process was variable. Rosentrater (2006) reported some physical properties of DDGS which included moisture content (db), water activity, bulk density, angle of repose, thermal conductivity, thermal diffusivity, thermal resistivity, and color L, a and b values. This data showed that all physical properties exhibited statistically significant differences between the six dry-grind processing plants sampled (n = 72 per measured variable) from ethanol processing facilities in eastern South Dakota. While correlations were made between the variables measured, the overall goal of the study was to provide baseline data for typical DDGS that are produced in eastern South Dakota. Thus, no attempt was made to explain why the physical properties of DDGS from the plants sampled were significantly different. In 2

4 another study by Shurson (2005) conducted at the University of Minnesota, the particle size distribution and chemical composition of DDGS for sixteen "new generation plants" varied among plants, and there were noticeable differences between "new" and "old" generation plants, especially with respect to their physical (color and particle size) and chemical (amino acid profiles) properties. In a study on particle segregation in corn DDGS induced by three handling scenarios, particle size distribution (PSD) and morphological characteristics of DDGS from an old generation ethanol plant was quite large and variable (Ileleji et al. 2007). They showed that the large PSD and morphological variability would induce segregation during handling and cause bulk property differences. Further studies by this group confirmed particle segregation in DDGS and its potential effect on chemical variability within the bulk (Clementson et al. 2008). The higher the PSD of DDGS, the higher the particle segregation during handling. In that same study (Clementson et al. 2008), it was also shown that the PSD from an old generation ethanol plant was also different and larger than the PSD from a new generation fuel ethanol plant. They recommended the need to develop standard sampling protocols, because error in sampling would cause errors in the measured properties. All these studies highlighted differences in physical properties of DDGS produced by various ethanol plants in the industry, but no detailed exposition for the causes of this variability has been presented in the literature. In a study by Ileleji et al. (2008), particle and bulk characteristics of DDGS were shown to be affected by process variables, specifically the ratios of distillers wet grains (DWG) and condensed corn distillers solubles (CCDS). Increasing the CDS increased the geometric mean particle size, PSD and produced a darker DDGS given the same drying temperature condition. While dry-grind ethanol plants for the most part have the same process technology producing the same product (ethanol, DDGS and CO 2 ), one would expect their products to be similar. This is only the case for ethanol which has to meet standard fuel specifications, while the other two DDGS and CO 2 are co-products sold as by-product streams of the production process and not necessarily produced to meet a standard specification. So variability would be expected a priori, but what are the causes and how would you control DDGS product quality to minimize product variability? Variability of DDGS in the industry has been one of the biggest issues preventing its acceptance as a livestock feed in rations of livestock. This is the important question that we attempted to investigate in this paper with respect to physical property of DDGS. More importantly, as we move toward the development of standard measurement methods for DDGS in ASABE, it would be important to understand the fundamentals of DDGS production so as to develop appropriate methods for measuring data. Of practical importance is the elucidation of issues during handling that can be attributed to physical characteristics of DDGS. Another major concern with DDGS is the handling/logistics of getting the product from production centers in the Midwest to utilization centers in the Southwest and Southeast where most of the cattle feedlots are located. While the major issue that has been pointed our in the literature has been the in ability of product to flow freely out of railcars, of similar importance has been optimizing product shipment by getting as much DDGS in a railcar. The bulk density, mass per volume give an indication of the quantities of material that can be loaded into a given volume of railcar. Obviously, the larger the bulk density the better for cost effective shipment of any product. Because the bulk density of DDGS can vary from to kg/m 3, the shipping cost for the product will be variable as well, and even can dictate from whom to buy a product. For example, a standard railcar with about m 3 (5700 ft 3 ) volume (Ginder, 2007) will hold 62.8 metric tons of kg/m 3 DDGS 3

5 compared to 80.9 metric tons of DDGS if the bulk density was up to kg/m 3. Corn with a bulk density of 720 kg/m3 (ASAE Standards, 2005) will hold metric tons of grain in the standard railcar; about 53.4 and 35.3 metric tons more than the low and high bulk density values reported by Rosentrater (2006). High bulk densities of up to kg/m 3 have been reported (Ileleji et al. 2007). Therefore, producing a higher bulk density DDGS product, while meeting nutritional and handling requirements would have a huge impact on the cost of shipping this product which is already higher than shipping commodity grain. Understanding bulk density variability and developing standard practices for the industry is important to the industry. To effectively utilize ethanol co-products in the marketplace, the physical and nutritional properties of these residual streams must be known, and customers demand that they are consistent over time. These data are essential for livestock diet formulation, design of equipment and processing facilities, optimization of unit operations and logistics and handling of product during shipment. While much chemical and nutritional data has been determined as a result of livestock feeding trials, little physical property information has been gathered. In particular, the in-depth investigation of the physical properties of DDGS and factors that affect their variability has not been adequately reported in the literature. The objectives of this paper are thus to discuss 1) important physical properties of DDGS; 2) variability in these physical properties; 3) gaps in the knowledge base; 4) lack of standards; and 5) the importance of establishing standards methods for the determination and reporting of physical properties. The physical properties of DDGS from corn that was investigated were particle density (PD), bulk density, particle morphology (particle size and shape) and particle size distribution (PSD). Distillers grains production processes and products Two main techniques are used to produce ethanol: wet milling and dry grind processing. Both processes will be discussed in this paper, both our focus will be on the physical properties of DDGS produced by the dry-grind process. The corn wet milling process has been thoroughly reviewed by May (1987) and more recently by Rausch and Belyea (2006). Briefly, the process (Figure 2) consists of steeping the raw corn to moisten and soften the kernels, milling, and then separation of the kernels components through various processes including washing, screening, filtering, and centrifuging. The primary end product obtained from corn wet milling is industrial corn starch, which is utilized for sweetener and ethanol production. Corn wet milling has drastically increased in scale since the early 1980s, due primarily to developments in ethanol and high fructose corn syrup markets, which are products based on corn starch (Wright, 1987). Byproducts from wet milling include corn oil and a host of feed byproducts, including corn gluten feed (CGF), corn gluten meal (CGM), corn germ meal (CGM), and condensed fermented corn extractives (CFCE). The byproduct stream from corn wet milling is significant, and in fact, accounts for approximately 35% of the raw corn input, while only about 66% of the corn kernel is actually converted into starch. These byproducts, however, are distinct entities compared to distillers grains, which are coproducts from dry grind processing. Because of its lower investment and operational requirements, and advances in fermentation technology, dry grind processing has become the primary method for 4

6 ethanol production. The dry grind process (Figure 3) includes several key steps, including grinding, cooking, liquefying, saccharifying, fermenting, and distilling. Generally entire corn kernels are ground into either a coarse meal or a flour. Then the corn starch is converted into dextrose and simple sugars by enzymes; these simple sugars are converted into ethanol using yeast during fermentation. In-depth information on this process can be found in Tibelius (1996), Weigel et al. (1997), Dien et al. (2003), Jaques et al. (2003), Bothast and Schlicher (2005) and Rausch and Belyea (2006). RAW CORN SCREEN EVAPORATORS STEEP WATER STEEP DEGERMINATOR GERM SEPARATORS WASHER CONCENTRATES (CFCE) GLUTEN FIBER GRINDING WASHING CENTRIFUGAL SEPARATORS DRYER CRUDE OIL, GERM MEAL DRYERS STARCH WASH SEPARATORS GLUTEN MEAL GLUTEN FEED CORN STARCH Figure 2. Flow chart for typical corn wet milling processing. Ethanol production from corn grain by the dry-grind process results in three main product streams: 1) fuel ethanol, the primary end product; 2) nonfermentable corn kernel constituents, which are primarily protein, fiber, oil, and ash (and are often sold as distillers grains); and 3) carbon dioxide. Anecdotally, the rule of thumb commonly used in industry states that for each 1 kg of corn processed, approximately 1/3 kg of each of these resultant streams will be produced. This process will yield approximately 10.2 L of ethanol, 8.2 kg of distillers grains, and 8.2 kg of carbon dioxide from 25.4 kg (1 bushel) of corn. The nonfermentable residues are removed from the process stream from the distillation column as whole stillage, and are processed into a range of distillers coproduct feed materials (Figure 4). They are centrifuged to remove water; this dewatered product is known as Distillers Wet Grains (DWG) and normally would be at about 70 to 75% moisture (wet basis, w.b.). The separated thin stillage with about 5% solids content is then evaporated of water to produce condensed corn distillers solubles (CCDS), often referred to as syrup. The solids content of CCDS is increased to about 35 to 40% during the evaporation process producing a thickened brown syrupy liquid. The CCDS are recombined with the DWG, dried to ensure a substantial shelf life, and then sold as distillers dried grains with solubles (DDGS). 5

7 Corn GRAIN STORAGE Grinding GRINDER MIXER Enzymes Slurry Tank COOKING TUBE VACUUM FLASH Enzymes Liquefaction Tank Saccharification & Propogation Enzymes MASH Enzymes COOLER Saccharification Tank Yeast Starter Tank Fermentation CO 2 FERMENTATION TANK Jet Cooker Cooking BACKSET CDS ETHANOL DENATURANT THIN STILLAGE EVAPORATOR SYRUP Liquefaction MOLECULAR SIEVE Centrate Centrifuge RECTIFIER STRIPPING COLUMN Distillation Centrifugation WHOLE STILLAGE BEER WELL DDGS DRYER DWG DDG DRYER Wet Cake Coproducts Processing Figure 3. Flow chart for typical corn dry-grind fuel ethanol processing. Stripper Column Backset (10 50% of total thin stillage) Whole Stillage 5 15% solids Evaporator Centrifuge Wet Cake 35 50% solids Thin Stillage (centrate) 5 10% solids Syrup (CDS) 25 55% solids Mixer Dryer Dryer Dryer DWG DDG DWGS DDGS CDS Dried Solubles Figure 4. Flow chart for typical dry-grind coproducts processing. 6

8 If the CCDS is not added before the DWG is dried, then distillers dried grains (DDG) is produced. Most plants are set up to produce the dried coproducts, which are used for livestock feed, either locally or shipped via truck or rail for use by distant customers. Lately there has been growing interest in local use of DWG. The sale of all types of distillers products as livestock feed contributes to the economic viability of ethanol manufacturing, and are thus vital components to each plant's operations (Rosentrater, 2006). AAFCO (2006) has established official definitions (Table 1) for these co-product streams in order to facilitate the sale of these co-products in the marketplace. Note that DDGS from new fractionated technologies would not be compatible to the definitions listed in Table 1, and inclusions will need to be made as newer process technologies are developed that would modify DDGS from a typical dry-grind process. Table 1. Official coproduct names and definitions, as delineated by AAFCO (2006). Common Acronym DDGS DDG DWG CCDS CDDS Official Name Corn Distillers Dried Grains with Solubles Corn Distillers Dried Grains Corn Distillers Wet Grains Corn Condensed Distillers Solubles Corn Distillers Dried Solubles Official Definition for Trade Is the product obtained after the removal of ethyl alcohol by distillation from the yeast fermentation of a grain or a grain mixture by condensing and drying at least ¾ of the solids of the resultant whole stillage by methods employed in the grain distilling industry. The predominating grain shall be declared as the first word in the name. Is obtained after the removal of ethyl alcohol by distillation from the yeast fermentation of a grain or a grain mixture by separating the resulting coarse grain fraction of the whole stillage and drying it by methods employed in the grain distilling industry. The predominating grain shall be declared as the first word in the name. Is the product obtained after the removal of ethyl alcohol by distillation from the yeast fermentation of a grain mixture. The guaranteed analysis shall include the maximum moisture. Is obtained after the removal of ethyl alcohol by distillation from the yeast fermentation of a grain or a grain mixture by condensing the thin stillage fraction to a semi-solid. The predominating grain must be declared as the first word in the name. Is obtained after the removal of ethyl alcohol by distillation from the yeast fermentation of a grain mixture by condensing the thin stillage fraction and drying it by methods employed in the grain distilling industry. The predominating grain must be declared as the first word in the name. 7

9 Variability of DDGS in the industry It has been noted that DDGS variations and inconsistencies have been problematic for the livestock industry (Applegate and Adeola, 2006; Corzo, 2007). Rosentrater (2006) examined physical properties from several commercial fuel ethanol plants, and many exhibited substantial differences between processing plants. Samples had moisture contents ranging from to 21.16%; water activity values between and 0.634, thermal conductivity values between 0.06 and 0.08 W/m C; thermal diffusivity between 0.13 and 0.15 mm 2 /s; bulk density between and kg/m 3 ; angle of repose between and o. Color also varied between plants: Hunter L ranged between and 49.82; Hunter a ranged from 8.00 and 9.81; and Hunter b was between and Bhadra et al. (2007) also examined various physical properties from several commercial fuel ethanol plants, and once again, differences between processing plants were considerable, as were differences for each plant over time. Samples had moisture contents ranging from 3.54 to 8.21%; water activity values between 0.42 and 0.53, thermal conductivity values between 0.05 and 0.07 W/m C; thermal diffusivity between 0.10 and 0.17 mm 2 /s; bulk density between and kg/m 3 ; angle of repose between 25.7 and o. Color also varied between plants: Hunter L ranged between and 50.17; Hunter a ranged from 5.20 and 10.79; and Hunter b was between and Spiehs et al. (2002) investigated nutrient compositions of DDGS from 10 commercial ethanol plants, and found substantial variation among plants. For example, crude protein varied between 28.1 and 31.6 % (db); crude fat varied between 8.2 and 11.7% (db); crude fiber varied between 7.1 and 9.7 % (db); and ash content varied between 5.2 and 6.7% (db). Other nutrients, such as amino acids and minerals, similarly exhibited substantial variation. There may be a number of factors that lead to variations in DDGS, not only from batch to batch, but also between plants. These include differences in the raw corn itself; size reduction (i.e., grinding) prior to cooking; cooking, conversion, and fermentation procedures, conditions, and additives; completeness of fermentation when the slurries are discharged from fermentation tanks (which could lead to higher levels of residual sugars or starch); the addition level of CDS before/during the process of drying DWG into DDGS; and the drying process itself (i.e., temperatures, times, ambient temperature and humidity) (US Grains Council, 2007). Influence of process variables of DDGS physical properties DDGS is a granular bulk consisting of the unfermentable components of the grain (corn) from which it originated. In comparison to corn where all the components of the grain, starch, protein, fiber and oil are locked in the various structures (endosperm, pericarp, germ and tip cap) of a single corn kernel, DDGS particles consists of the broken up structures residual endosperm, pericarp, germ and tip cap that hold the residual starch, protein, fiber and oil (Fig. 5). In a study on the morphological characteristics of DDGS, Ileleji et al (2007) found that the variation in the composition of the DDGS particles in three sub-groups based on particle size distribution was due to the variation in the proportion of the different kernel components (pericarp, germ and endosperm) which have different chemical compositions. For example, while the pericarp consisting of primarily fiber would be glued as part of a single corn kernel, in DDGS, the pericarp 8

10 would be broken individual particles, that have the potential to be entrained in air and segregate out during handling. This possibility holds true for other individual particles consisting of various compositions and having different particle and bulk densities. The particle densities of the three sub-groups of DDGS bulk in Ileleji et al. (2007) were 940.7, and kg/m 3 for Groups I, II and III, respectively. The categorization based on particle size is shown in Fig. 6 from Ileleji et al. (2007). The larger particles in Group I (retained on U.S. sieve 4 12) were the least dense of the three and the smallest particles Group III (retained on U.S. sieve ) was the densest of the three. The reasons given for there density differences were was related to the structural composition and more exposition on this will be mentioned later. Components locked up in whole kernel -Starch Components broken into individual particles during ethanol processing Starch Oil Sugar Protein Fiber Figure 5. A comparison of corn and DDGS with respect to composition and structure. 100 % Cumulative Weight Group I Group II Group III, ~ 68% Group II, ~ 24% Group III Group I, ~ 8% US Sieve Opening (d, microns) Figure 6. Particle size distribution of the DDGS bulk sample used for morphological analysis grouped into I, II and III (after Ileleji et al., 2007). 9

11 Another factor to consider in examining DDGS is the process conditions in which it was formed. In DDGS production, as previously mentioned two product streams, DWG and CCDS are blended in a screw conveyor or paddle mixer before conveying to rotary drum dryers to reduce the mixture moisture from 70 75% (w.b.) to about 10 to 12% (w.b.). It is in the rotary drum dryer that DDGS particles are formed and its characteristics will depend on the ratio of DWG to CCDS, the process configuration for blending and dying to DDGS (i.e. where in the process and how are the products blended), drying characteristics (drying temperatures, air flow and drum rotary speed). Two drying process configurations, Figures 7 and 8, that have been observed in ethanol plants in the Midwest installed by two different technology providers will be discussed with respect to the particle characteristics of DDGS produced from them. DDGS 10% MC & 300 o F + wet cake (DWG) at 65-75% MC + Syrup (CCDS) with 35 40% solids Recycled DDGS stream routed to blend with WDG & Syrup 35%MC 350 o F product temp % MC & 300 o F About 50-70% of DDGS from dryer is routed to storage pad for curing before transport Figure 7. Drying process for DDGS using one high-capacity rotary drum dryer (configuration common in old generation corn ethanol plant). Figure 7 is DDGS process that uses a high-capacity rotary drum steam tube dryer to dry blended DWG and CCDS into DDGS as shown in the schematic diagram. This configuration is common in old generation corn ethanol plants built in the early 1980s and usually more than one high-capacity dryer is available to be used parallel for drying DDGS. The process starts by blending DWG at 65-75% MC with CCDS of 35 40% solids content in addition to freshly dried DDGS at 10% from the rotary drum dryer. This ratio of DWG, CCDS and freshly dried DDGS is such that the moisture content of the blend should be about 35% on entering the rotary drum dryer. The product temperature in the rotary drum steam tube dryer can be up to 149 o C (300 o F) with very high inlet temperatures of up to 538 o C (1000 o F). After the product is dried to10-12% moisture (w.b.) about 50 to 70% of the dried batch is routed to the storage pad where it sits in a pile to cool before loading onto railcars or trucks for shipment. The other half is routed back to the front end of the drier where is blending with more DWG and CCDS and the process continued again. The recycling of dried product to reduce energy requirements for drying, as well as speed up drying also causes product deterioration due to 10

12 denaturing of protein and other components which are repeatedly exposed to high temperatures. The drying process of particulate solids like DDGS in rotary drum dryers is also a granulation process where wet solids are broken up and pulverized to a granulation flowable product. Having a liquid, CCDS in the blend during drying induces particle agglomeration to form spherical ball-shaped particles sometimes termed as syrup balls (Ileleji et al., 2007). The growth and size of agglomerated particles of DDGS is the subject of a current investigation by one of the authors. The quantity of CCDS might be one of the major factors influencing DDGS particle agglomerates size and the particle distribution of the bulk (Ileleji et al., 2008). Their preliminary findings indicate that particle size of agglomerates depends on the level of CCDS, the process of introduction and blending of CCDS with DWG and DDGS, drum rotation speed and the total number of revolutions a batch blended feedstock experiences in the drum dryer (Probst and Ileleji, 2008). DDGS from old generation plants with this process typically are darker in color, have a larger geometric mean particle size, d gw (3 mm or more) and a larger spread of particle size distribution. The larger sized agglomerates is because all the CCDS is introduced into the blend at once, thus producing a blend with a higher propensity for agglomeration. The other dryer configuration is currently what is installed in newer dry-grind ethanol plants. It consists of a pair of rotary drum dryers with two making a pair connected in series as shown in the schematic of Fig. 8. A blend of DWG, CCD and freshly recycled DDGS is blended in a screw conveyor trough just before it enters Dryer I which dries the product to about 35% moisture or less. The dried DDGS from Dryer two is split into two streams. One stream is recycled back to blend with DWG and CCDS in Dryer I and the other stream is blended with more syrup and DDGS as 10% moisture to be dried down to 10% moisture in a second Dryer II. About four times more CCDS is usually added in Dryer II than in Dryer I. This newer configuration produces DDGS that is lighter (golden brown) in color, have a smaller particle size (d gw is about 1 mm or less), and a narrower particle size distribution. The reason for the decrease in particle size, which is correlated to a decrease in particle agglomeration during drying is due to the addition of the syrup which acts an agglomeration agent in two steps of a two stage (Dryer I and II) drying process. This process reduces agglomeration formation and enhances bulk pulverization. DWG CCDS CCDS DRYER I DRYER II DDGS Recycle 25-35%MC DDGS Recycle 10-12%MC 10% & o F Figure 8. Drying process for DDGS using two rotary drum dryers connected in series (configuration common in new generation corn ethanol plant). 11

13 Particle characteristics of DDGS and their effect on the bulk physical and flow properties and handling Some investigations have examined the role of moisture content and CDS levels on the resulting frictional and flowability properties of DDGS (Ganesan et al., 2007, 2008a, 2008b), and have found that as moisture content and CDS addition increases, flowability of DDGS can decrease. But, the physical properties of DDGS from corn: particle density (PD), bulk density, particle morphology (particle size and shape) and particle size distribution (PSD) have not been adequately researched in the literature. From the work reported by Rosentrater (2006) and Ileleji et al., (2007), there exists a large variation in physical properties from ethanol plants across the U.S. even variability could be seen from batches within the plant. So what causes the variability in physical properties and how can it be controlled? How should data on product be reported and what data should be included to give a better understanding of the origin/process of production from which the DDGS product in question came? These are two important questions worth investigating that have important practical marketing implications to the industry. The particle density measures the true density of powders (Zhou and Ileleji, 2005) and measures only the density of the particle matter excluding all air pores. Normally, the particle density of particulates (powders) increase as particle size is reduced. This is because the smaller the particle size, the lesser the air pores in the particle. This phenomena has been confirmed for DDGS particles (Ileleji et al., 2007), but more work needs to be conducted by linking particle density with structural components of fiber, germ, spent endosperm, etc. Larger particle agglomerates would have more air spaces, but their particle densities would vary depending on the amount of syrup blended in to form DDGS. A higher syrup level would promote more agglomeration and syrup would tend to fill air pores as agglomerates grow during the balling processes (also known as pelletization or granulation processes) of nucleation, coalescence and layering. A complete description of these processes can be found in Sastry and Fuerstenau (1973). Finer particles of DDGS with a higher particle density will have a higher bulk density than coarser agglomerated particles. However, this is not a generalized condition, because higher CCDS levels could also create denser particles that would yield a higher bulk density. The bulk density variability in the industry, especially with respect to product shipment is an issue of interest to ethanol plants and DDGS marketers. Producing DDGS with higher bulk density would save cost in shipping a lower density product. The relationship of process parameters to particle density and bulk density is still subject of investigation. Varying process parameters to achieve uniform sized spherical pellets has been demonstrated by Probst and Ileleji (2008) and the process patented. Current studies by them on the mechanisms and kinetics of particle agglomeration during drying of DDGS in a rotary drum would provide more light on the primary factors that determine particle and bulk density, and ways to increase bulk density of DDGS. Particle morphology (particle size and shape) of DDGS was recently study by Ileleji et al. (2008). In their study, only a sample of DDGS from an old generation ethanol plant was used. Particle size distribution of the DDGS sample (Fig. 6) was grouped into three groups, Group I, II and III categorized according to U.S. sieves 4 12, 16 30, and , respectively. Images of individual DDGS particles from these three groups were captured with a light microscope equipped with a digital camera and the acquired particle geometry was used to determine the aspect ratio (AR), elongation (E), 12

14 equivalent circle diameter (ECD), roundness (R) and form factor (FF) (Ileleji et al., 2007) according to specifications in ASTM F , Standard Practice for Characterization of Particles (ASTM Standard 2003). Their results showed that DDGS particles shape and sizes were related to particle structural components (fiber, germ and or agglomerates). The were the ones to first suggest that spherical particles of DDGS are the agglomeration of individual heterogeneous particles of corn kernel components (germ, fiber, tip cap and other solids) that are shaped into spherical balls and cemented firmly together with the aid of the condensed solubles and drying (Ileleji et al., 2007). The term of syrup balls as used by other authors (Shurson, 2005) and in the industry appear to indicate these particles are only made up of syrup (CCDS) which is not the case. Also, particle size can be correlated to bulk density and moisture content determination. A larger particle size with a low particle density will results in overall low bulk density. Studies by Ileleji et al. (2008) also showed that particle size affects the moisture holding capacity of a particle as well as its moisture content determination. Particle agglomerates will normally have moisture trapped in them from the syrup acting as a binder to glue particles together. The trapped moisture is not released as fast as an individual particle going through the rotary drum dryer, hence particle agglomerates have a higher moisture than more pulverized individual particles (Ileleji et al., 2008). The implication is that a bulk having a large number of agglomerated particles with trapped moisture if not steeped well to allow cooling would cake and bridge more readily during transport. Moisture content determination can be also affected with large particle sizes of agglomerates, especially for standard methods that require moisture determination of unground samples. Moisture contents determined with unground and ground samples from the same bulk can be vary by as much as 2 percent points (Ileleji et al, 2008c). Therefore, it is advisable to perform moisture determination and even moisture sorption studies with ground samples of DDGS to take out the effect of particle size. Finally, particle size distribution (PSD) is very important with respect to bulk handling particulate bulk. It was shown by Ileleji et al. (2007) and Clementson et al. (2008) that particles from a DDGS with a large PSD would segregate during handling. Because individual particles carry specific chemical components, particle segregation could also lead to segregation of nutrients in the bulk. Findings from a recent study on DDGS particle segregation by Clementson et al. (2008) advocate the development of a standard sampling protocol for obtaining representative samples during production and in shipment. When particles segregate during hopper filling, the lighter particles tend to congregate at the center of the filling spout and compact over time. When this phenomenon occur, the hopper becomes very difficult to discharge even if the product was free flowing during loading. Furthermore, because particle size affects particle and bulk densities, PSD also affect both variables. Quantifying the relationship between particle size and particle and bulk density would be a beneficial tool for bulk characterization of the physical properties of DDGS. This is especially important where there is so much variability in DDGS industry wide. On standard methods for DDGS To some extent, the lack of standard methodologies for product quality determination and reporting has become a bottleneck to expanding the market for DDGS. Towards that end, the Renewable Fuels Association, National Corn Growers Association, and American Feed Industry Association s Analytical Methods Sub Working Group recently 13

15 concluded an investigation of several standard methods for the analysis of chemical composition in DDGS, in order to determine which are most appropriate for the industry to use. Their recommendations, which the industry is now implementing, are summarized in Table 2 below. Table 2. Industry-recommended methods for determination of DDGS composition (AFIA, 2007). Property Method Title Moisture NFTA Lab Dry Matter (105 ºC / 3 hr) Crude Fat AOAC Official Oil in Cereal Adjuncts (Petroleum Ether) Method Crude Protein AOAC Official Method Protein (Crude) in Animal Feed Combustion Crude Fiber AOAC Official Method Fiber (Crude) in Animal Feed and Pet Food Ash 1 AOAC Official Ash of Animal Feed (600 C for 2 hours) Method This standard method was not actually investigated by the coproducts working group, but it is the primary method used in the industry to determine DDGS ash content, which is why it is included here. In terms of physical properties, however, no work has yet been pursued toward the recommendation or development of standard methodologies. Nevertheless, ASABE already has a few published standards that could be applicable to the determination of physical properties of DDGS. These are summarized below in Table 3. Table 3. ASABE standards that may be appropriate for determination of DDGS physical properties. Property Method Title Particle Size Distribution S319.3 Method of Determining and Expressing Fineness of Feed Materials by Sieving Drying S448.1 Thin-Layer Drying of Agricultural Crops There is thus a dire need for additional information on the physical properties of DDGS. Other standards could be developed to fill this need. Additionally, research could be performed to establish property information, and relationships between these properties, for typical DDGS streams. This information must include information relevant to the design of storage structures and processing operations. Because process design and variables affect DDGS properties, the inclusion of some basic physical characteristics such as particle size and particle size distribution could shed some light on their origin. Standard immediately impacted by this data could include those listed in Table 4 below. Developing standards for the physical characteristics of DDGS, including specific measurement methods and reporting procedures, will be useful for the design of handling and storage systems, as well as for product trade. ASABE is well positioned to pursue standards development, and thus can play a strong role in the developing biofuels industry. 14

16 Table 4. ASABE standards immediately impacted by studies and data on DDGS physical properties. Method Title D241.4 Density, Specific Gravity, and Mass-Moisture Relationships of Grain for Storage D243.4 Thermal Properties of Grain and Grain Products D245.5 Moisture Relationships of Plant-based Agricultural Products D252.1 Tower Silos: Unit Weight of Silage and Silo Capacities D274.1 Flow of Grain and Seeds Through Orifices D293.2 Dielectric Properties of Grain and Seed EP433 Loads Exerted by Free-Flowing Grain on Bins EP545 Loads Exerted by Free-Flowing Grain on Shallow Storage Structures S413.1 Procedure for Establishing Volumetric Capacities of Cylindrical Grain Bins Conclusions DDGS is a complex heterogeneous bulk particulate quite different from the source feedstock it was produced from. Unlike corn that contain all the compositions of starch, fiver, protein, and fat all locked up in individual corn kernels, in DDGS, a composition can occur in an individual particle or a group of compositions in agglomerated particles. These facts make particles of DDGS prone to segregation during handling, especially when the particle size distribution of the bulk is large. Process parameters, in particular the ratio of DWG and CCDS in the blend and process design (how the ratio are blended and dried) affect the granulation of DDGS in the dryer and ultimately DDGS particle properties. While we have attempted to describe the distinct differences of DDGS produced by two process designs seen in the U.S. fuel ethanol industry (primarily in older plants, old generation and newer plants, new generation ), the lack of operational control to produce a DDGS product with desired attributes ultimately results in the large product variability reported in the industry. As the profitability margin for ethanol due to increased corn feedstock cost becomes tighter, plants would be forced to understand ad desire a more uniform and better DDGS product to be competitive. The particle density (PD), bulk density, particle morphology (particle size and shape) and particle size distribution (PSD) of DDGS were reviewed. What is clear is that more work needs to be conducted to elucidate the relationship between these variables. This need is also tied with the need to develop standard methods for physical properties determination and practices for their reporting. Fundamental studies on understanding physical properties of DDGS currently being conducted by the authors would inform the standards development work by ASABE. The efforts would provide the industry with more confidence about the production and utilization of DDGS. 15

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