Starch and glucose production in large scale


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Starch and glucose production in large scale

  2. 2. 2 BONAFIDE CERTIFICATE Certified that this project report “STARCH MANUFACTURING PROCESS” is the bonafide work of “ALIF HOSSAIN” from KIIT University who carried out the project work under my supervision. SIGNATURE SIGNATURE HEAD OF THE DEPARTMENT SUPERVISOR
  3. 3. 3 Acknowledgment. “Gratitude is the hardest of emotion to express and often does not find adequate ways to convey the entire one feels” Summer training is the one of the important part of B.Tech/M.Tech (Dual degree) course, which helped me to learn a lot of experiences which will be beneficial in my succeeding career. For this ineffable sense of gratitude I take this opportunity to express, my deep sense of indebtedness to, Mr. Parimal Jena, Manager, Sukhjit Starch Industries, Malda who has provided me an opportunity to learn the industrial culture during my B.Tech/M.Tech (Dual degree) course. At the same time I want to thank all the other employee members. I am also very much thankful to Mr. Kapil Sharma, Supervisor, Starch Unit, Sukhjit Starch Industries, Malda for his interest, constructive criticism, persistent encouragement and untiring guidance throughout the development of the project. It has been my great privilege to work under his inspiring guidance.
  4. 4. 4 ABSTRACT Starch is basically a polymer of glucose sugar units. The long chain starch molecule itself is very versatile and is used as a thickening agent in foods, as a raw material for ethanol and plastics production, and as a coating agent in the paper and textile industries. The properties of starch depend on the extent and nature of the bonds in the molecule and can be changed by various preparation methods that involve acids, bases, sodium hypochlorite, and enzymes. These methods yield products such as modified starch, unmodified starch, dextrins, cyclodextrins, and starch derivatives. These products are generally dried to a powder as a final product. The starch molecule is also used as the raw material for making all kinds of sugar syrups. For these products, the starch slurry is combined with acid and enzymes called amylases that break the starch molecules into dextrose (glucose) and other sugars. The kind of enzyme, reaction time, and process conditions are varied depending upon the product requirements for different kinds of sugars. The cleaned corn kernels are “steeped” at 50 °C (120 °F) in a mild (0.1 to 0.2 %) sulfurous acid solution to loosen the hull, soften the gluten, and dissolve some constituents of the kernel. The steeping phase lasts between 24 and 48 hours at 4 pH and causes the kernels to swell to double their normal size. Sulfur dioxide gas is added to the water to break chemical bonds in the gluten and prevent the growth of bacteria in the steep tanks. Water used in steeping is later concentrated and used as animal feed. After steeping, the corn is coarsely ground to free the germ. The lighter corn germ is separated from the corn slurry by cyclone separation or by gravity in a settling tank. The germ, which contains 85 % of the oil, is further washed over screens to remove any starch that may be adhering to it. Mechanical pressure and solvent extraction are used to remove the corn oil. This oil is then clarified and filtered before delivery as a final product. The corn/water slurry (minus the germ) is then finely ground to release the starch and gluten from the kernel. This mixture flows over screens that capture the fibrous kernel but allow the starch and gluten to pass through. The fiber is collected, rinsed to reclaim starch, and then used for animal feed. The gluten is then removed by centrifuge, and the remaining starch is washed several times until reaching 99.5 % purity. At this point, the process will change depending on the desired final products.
  5. 5. 5 TABLE OF CONTENT 1.Inroduction 1.1. History………………………………………………………. 7 1.2. Botany………………………………………….…………… 8 1.3. Spectrum of Maize Variety…………………………………. 9 1.4. Composition………………………………………………… 9 1.5. Starch…………………………………………….………… 10 1.6. The Corn Refining Process………………………………... 10 2.The Layout………………………………………………………... 12 3.Production process for Starch…………..………………………... 13 3.1. Corn Starch Production………………………………....…. 13 3.1.1. Production of Starch Slurry……………………...…... 13 3.1.2. Production of Starch and Dextrin…………............... 14 3.1.3. Production of Glucose and Dextrose………..…...…. 16 4.Over View of The Process………………………………………… 17 4.1. Over View………………..…………………………………. 17 4.2. Manufacture of Corn Flakes…….…………………....….. 22 5.Diagrammatic Representation of the Process………..…………... 24 6.Critical factors in setting up a Maize Processing Plant………...... 25 6.1. Raw Material………..…………………………………...… 25 6.2. Land………………………….…………………………….. 25 6.3. Water………………………………….……………………. 25 6.4. Power………………………………………….…………… 26 6.5. Steam………………………………………………….…… 26 6.6. Technology……………….………………………………... 26 7.Step by Step Description of the process…….…………………… 27 7.1. Cleaning Steeping…………………………….…………… 28 7.1.2. Steeping Evaporation………………………………….... 29
  6. 6. 6 7.1.3. SO2 ……………………………………………………….30 7.2. Grinding 30 7.3. Fine Grinding and Screening………………………………… 32 7.4. Fibre Washing Section……………………………………….. 33 7.5. Primary Separation…………………………………………… 34 7.6. Thickening……………………………………………………. 35 7.7. Gluten Refining……………………………………………….. 35 7.8. Starch Rotary Vacuum Filter………………………………… 39 7.9. Drying Effluent Treatment Plant…………………………….. 40 7.10. Description Detail…………………………………………….. 41 8. Further Efficiency Implementation……………………………… 43 8.1. Process Related Energy Efficiency Measures………………... 45 8.2. Energy Management………………………………………….. 46 8.2.1. Drying Technology…………………………………… 45 8.2.2. Controls on Heaters between steps………………….. ..46 8.2.3. Reusing waste heat…………………………………….46 8.2.4. Replace dryers with more efficient dryers…………..... 48 8.2.5. Challenging customer demand……………………..… 49 8.3. Membranes……………………………………………….…. 49 8.4. Future Technologies………………………………….….….. 51 8.4.1. Steeping………………………………………………..52 8.4.2. Using enzymes during steeping to reduce steep time.....53 8.5. Intermittent Milling and Dynamic Steeping (IMDS)……..... 52 8.6. Alkali Steeping……………………………………………….53 8.7. Conclusion……………………………………………........... 54 9. References……………………………………………………….. 55
  7. 7. 7 INTRODUCTION HISTORY Maize is one of the oldest cereal varieties. It originates from a region in present-day Mexico. The original maize varieties which only grew in regions of tropical and subtropical climate evolved over the years by selective crossing and finally produced higher yields and obtained the ability to grow under moderate climatic conditions. Starting in America, maize soon spread all over the world. Especially in the 20th century yield and pest resistance of the so-called hybrid-maize varieties was further raised by newly developed breeding methods and improved pest control. Concerning the utilization of maize two different possibilities are distinguished: silo maize and grain maize. For silo maize production the whole plant is harvested when still not fully ripe, chopped and stored in silos. Within these silos, which are widely hermetic, lactic acid fermentation occurs. Chaff material is then converted into durable silage which is primarily used for animal feeding in winter. For grain maize only grains of fully ripe maize-plants are used. Due to storability reasons the grains have to be dried immediately after harvesting. Besides their utilization as animal feed the maize kernels are also important as raw material for ethanol- and starch production. Due to continuously increasing demands in Austria the share of grain maize used for starch production advanced from 3 % to more than 15 % during the last couple of years. Because of the increased need for grain maize, special breeding programmes focused on maize-varieties suitable for the starch industry. These maize-varieties offer improved attributes such as increased starch content, easy going processability and special qualities of the starch itself. Furthermore special maize-varieties were grown for specific technical applications of starch. Among these are waxy maize and high-amylose maize.
  8. 8. 8 BOTANY Like all cereals, maize (Zea mays L.) belongs to the family of Gramineae. Maize appears in many shapes and varieties which can be distinguished by plant height, growth time, grain shape and climatic needs. The climatic needs of each maize- variety are empirically determined and specified as a number (in German the so-called “Reifezahl” =RZ). This number serves as a guiding figure concerning the climatic requirements of the respective maize variety. Due to the fact that RZ is determined solely for the climatic region of Austria, it is only valid here. The higher the number, the more days with a minimum of a certain temperature are necessary for the maturation of the plants. Concerning grain shape one has to distinguish between flint maize and dent maize. As depicted in the figures typical dent maize has a rather longish grain shape with an indentation on its top side which looks just like a tooth. A flint maize grain on the other hand is smaller and has a rounder form. Most of the varieties which are commercially available are hybrids and show either more similarity to flint maize or to dent maize. The maize kernels grow on the maize plant in cobs. Every cob contains about 400-600 maize kernels. Determination of dry matter is an adequate method for the purchaser to estimate the value of corn. The higher the dry matters the better the starch yield and the better the storage stability. An even faster method for the purchaser is to measure weight density. Starch yield and other wet-milling properties are not appreciably affected unless corn test weight drops below about 61.7 kg/hl. dent maize flint maize
  9. 9. 9 Only dent corn is useful for the starch process - flint corn f.e. is extremely difficult to steep. Corn Composition (15% Moisture Basis) Number of Samples Protein (%) Oil (%) Starch (%) 151 Average Range Average Range Average Range 7.7 5.7-9.7 3.3 2.6-4.9 61.7 59.9-64.8 SPECTRUM OF MAIZE VARIETIES The number of maize varieties grows steadily. Besides agricultural utilizations they are bred to meet new applications. For instance, there are special crossbreeds for the food sector like sweet corn and popcorn. For industrial starch production varieties with high starch content were bred. COMPOSITION A mature maize kernel consists of the pericarp, the germ and the so-called endosperm in which the starch is stored. Maize is composed of approx. 70 % starch, 8 % protein and 4 % fat. The rest is composed of water, fibres, sugar and various mineral nutrients.
  10. 10. 10 STARCH Starch or amylum is a carbohydrate consisting of a large number of glucose units joined by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store. It is the most common carbohydrate in the human diet and is contained in large amounts in such staple foods as potatoes, wheat, maize (corn), rice, and cassava. Pure starch is a white, tasteless and odorless powder that is insoluble in cold water or alcohol. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin by weight. Glycogen, the glucose store of animals, is a more branched version of amylopectin. Starch is processed to produce many of the sugars in processed foods. Dissolving starch in warm water gives wheatpaste, which can be used as a thickening, stiffening or gluing agent. The biggest industrial non-food use of starch is as adhesive in the papermaking process. Starch can be applied to parts of some garments before ironing, to stiffen them; this is less usual now than in the past. The enzymes that break down or hydrolyze starch into the constituent sugars are known as amylases. Beta-amylase cuts starch into maltose units. If starch is subjected to dry heat, it breaks down to form dextrins, also called "pyrodextrins" in this context. This break down process is known as dextrinization. The Corn Refining Process For more than 150 years, corn refiners have been perfecting the process of separating corn into its component parts to create a myriad of value added products. The corn wet milling process separates corn into its four basic components: starch, germ, fiber, and protein.
  11. 11. 11 There are five basic steps to accomplish this process. First the incoming corn is inspected and cleaned. Then it is steeped for 30 to 40 hours to begin breaking the starch and protein bonds. The next step in the process involves a coarse grind to separate the germ from the rest of the kernel. The remaining slurry consisting of fiber, starch, and protein is finely ground and screened to separate the fiber from the starch and protein. The starch is separated from the remaining slurry in hydrocyclones. The starch then can be converted to syrup or it can be made into several other products through a fermentation process.
  12. 12. 12 THE LAYOUT
  13. 13. 13 PRODUCTION PROCESS FOR STARCH Although starch has many varieties, the three varieties that are primarily produced in India is wheat, potatoes and corn starch. Corn Starch Production The corn wet milling process begins with the production of starch slurry. This slurry can be further processed to produce starch, dextrin and glucose. Production of Starch Slurry: Figure 2-1 illustrates the production of starch slurry. First, shelled and cleaned kernels are placed in steep tanks and soaked in water containing small quantities of sulfur dioxide (SO2) for 24 to 48 hours at a temperature of approximately 51- 52o C. This process allows for the extraction of soluble materials from the kernel. The SO2 prevents fermentation and helps to separate the starch and protein. After steeping is completed, steepwater is drained from the kernels and concentrated. This concentrated steepwater is primarily used in producing animal feed products. Next, the kernels are ground in attrition mills to loosen the hull. Water is added to the mills, creating a mixture of macerated slurry and whole germ. This slurry is placed in hydrocyclone separators, which remove the lighter germ. The germ is then dried and either sold as is or further processed into corn oil and germ meal. The corn oil can be either refined to make a salad oil or cooking oil or a raw material input to margarine. The germ meal is used in the production of animal feed. The remainder of the kernel, including the hull, gluten, and starch components, is sent through an additional series of grinding mill, the hull particles are caught on screens, while the gluten and starch particles pass through. The hulls are later used to make animal feed or refined corn fiber. The remaining slurry of gluten and starch, or mill starch, is then separated by centrifugation. The gluten is dried and either sold as corn gluten meal (60 percent protein) or used in producing corn gluten feed (21 percent protein). The starch slurry is then washed and dewatered using filters or centrifuges. At this stage, the starch slurry can go through a number of processes that can yield starch and dextrins or glucose and liquid starch.
  14. 14. 14 Production of Starch and Dextrins: Figure 2-2 displays the process used to convert the starch slurry into starch and dextrins. Most of the slurry is passed through a starch dryer to produce unmodified corn starch. Alternatively, it can undergo treatment with chemicals or enzymes and then pass through a starch dryer to create a wide variety of modified starches.
  15. 15. 15 The industry produces many types of modified starches, including acid thinned (Mixed with water), oxidized, cationic, hydroxyethyl, acetate, succinate, and phosphate starches. Acid thinned starches are thinned by treatment with dilute mineral acid, resulting in pastes with decreasing viscosity. Oxidized starches have reduced viscosity due to oxidation, primarily with sodium hypochlorite. Cationic and hydroxyethyl starches are stabilized against felling by reacting with monofunctional reagents, giving the starch more strength. Hydroxyethyl starch is produced by adjusting the pH of the starch and adding a salt, increasing its stability and resulting in a clear paste. Starch acetates are produced by acetylating the slurry with acetic anhydride or vinyl acetate, reducing the tendency of the starch to congeal. Starch succinates are made by using succinic anhydride instead of acetic anhydride, thereby improving the thickening quality of the starch. Starch phosphates are produced by esterifying starch with orthophosphate or sodium tripolyphosphate to increase the stability of the starch. In addition, the starch slurry can be passed through a starch dryer and then be dry-heated or roasted, with or without an acid or alkaline catalyst, to produce dextrins. This process gives the dextrins a low viscosity, more cold water solubility, less tendency to gel, and more reducing power than common starch, leading to the use of dextrins as adhesives.
  16. 16. 16 Production of Glucose Syrup and Dextrose: Figure 2-3 highlights the production process for glucose and liquid starch. To produce glucose, the starch slurry is first treated with acid or enzymes and heated in a conversion process to break down the starch molecule, yielding degrees, ultimately resulting in producing a wide variety of glucose. Next, the corn syrup is refined using carbon to remove residual color, odor, taste, or flavor bodies. At this point, some of the corn syrup has the water removed from it to produce some types of glucose syrup (regular corn syrup). The remainder of the corn syrup goes through a process called ion exchange to remove additional flavor and color bodies that were missed during the previous stages of production. In this process, the syrup passes through anion resin and cation resin vessels. In the case of fructose syrups, additional ion exchange steps may be necessary to remove certain additional substances. Finally, the water from this corn syrup, is evaporated to yield some additional types of glucose syrup, dextrose, and high fructose corn syrup (HFCS). To produce liquid glucose, the original starch slurry is simply fermented and distilled.
  17. 17. 17 OVERVIEW OF THE PROCESS This section describes the main production processes and the products derived from corn wet milling. The flowchart depicted in Figure 3 provides a brief overview of the main processes. In the discussion below, more attention is given to the drying and dewatering steps because these are the main energy consumers in the milling process. Figure 3: Overview of the processes and products of corn wet milling The goal of corn wet milling is to separate corn into its four main components S starch, germ, fiber and protein (gluten) S and then convert the components into useful products. The two main products, ethanol and sweeteners, are made by further processing starch. Corn starch is another main product, along with corn oil, made from the germ component. The fiber contains proteins and it, along with other byproducts, is generally used in animal feed. The corn wet
  18. 18. 18 milling process begins with corn that has been removed from the cobs and cleared of all debris and foreign materials. The corn enters the steeping stage, where the kernels are soaked in large stainless steel tanks in mildly acidic water at about 12°F (50°C) normally for 20-36 hours. There is a tradeoff between operating and capital costs in increasing steep times. Longer steep times require more capital and energy but less sulfur dioxide (SO2) and later cleanup (scrubbing) of the SO2. In energy terms, small amounts of energy are lost due to thermal radiation and increased circulation pump power, because of increased steep times. However, these are traded for decreased power at the grind mills. During steeping, the kernels absorb water and more than double in size. The gluten bonds in the corn loosen and starch is released. The corn is then coarsely ground to release the germ from the kernel. The water that the corn soaked in, referred to as steepwater, contains much of the soluble material from the corn, including a significant percentage of proteins and sugars. In addition to most of the soluble matter from the corn, water drawn off from the steeping stage contains the products of lactic fermentation and bacterial cells. The solids content is in the range of 5-10% and must be evaporated to 45-50% solids (called heavy steepwater or corn steep liquor) for commercial sale or for blending with corn fiber to produce corn gluten feed. To maximize efficiency, steepwater evaporation has usually been done in multiple effect evaporators (3-5 effects). Corn steep liquor is used for commercial purposes as an ingredient in animal feed or as a nutrient source in industrial fermentation. The slurry generated by the coarse grinding that follows steeping subsequently undergoes degermination (separation) to separate the germ from the other components. The corn germ contains most of the oil present in the corn kernel and is separated to recover the oil on- or offsite. Since this fraction contains most of the corn’s oil, it is less dense than water and can be efficiently separated using hydrocyclone separators. The hydrocyclones spin out the low- density germ from the remainder of the slurry. The germ is pumped into a series of screens to remove the loose gluten and starch and then washed repeatedly to recover and return all starch to the main process stream. The germ is dewatered using a screw press, which has a conical
  19. 19. 19 screw revolving slowly within a cone of perforated mesh. It is squeezed and passes to the narrow end where it is discharged, while water passes out through the mesh. This results in germ with a water content of 50-60%. To achieve a moisture content of 2-4%, the germ is then dried, typically using a rotary steam tube dryer. This dryer consists of a large rotating cylinder that has numerous tubes running inside it. These tubes are heated internally by steam. As the cylinder rotates, the moist germ is tumbled around and falls over the heated tubes. The internal walls of the rotating cylinder are designed to scoop the material and move it along from one end to the other. A countercurrent flow of air carries away the moisture. The steam consumption is about 120% of the weight of water evaporated. Because of its regular shape and close particle size distribution, corn germ can also be dried using a fluidized bed dryer. It is only economic to process the corn germ on a large scale, as there are a small number of centralized plants that purchase dried germ from many corn wet milling sites. Hence, smaller corn wet milling plants ship germ out to be processed offsite. After the germ is dried, corn oil is extracted through a combination of chemical and mechanical processes. The extracted oil then undergoes a series of steps to remove all impurities and to prepare the product for market and consumption. The underflow from the hydrocyclones is the corn-water slurry that remains after recovering the oil in degermination. This slurry undergoes fine grinding and screening to liberate all starch and gluten from the fiber. A thorough grinding breaks up all the components in the kernel. A series of screens allows the gluten and starch to pass but retain the fiber. The fiber wash water is then used to recover as much starch and gluten to the main stream as possible. Fiber is then dewatered in two steps. First, a screen centrifuge, which has a conical basket fitted with a perforated plate screen, is used to reduce the moisture to 65-75%. Next, a screw press similar to that used for germ dewatering reduces moisture content by a further 10%. Corn steep liquor is added to the moist fiber and the mixture is dried to yield corn gluten feed. This drying is done using various dryer designs. Commonly used is a rotary dryer consisting of a rotary drum through which the feed passes and is dried by a co-current hot air stream, usually generated by
  20. 20. 20 direct firing. In many areas it is now more economical to use steam tube dryers, also used for drying germ, although the economics strongly depend on the availability of fuels and the cost of steam. The fiber becomes the major component of animal feed. It is combined with the corn steep liquor to make corn gluten feed for cattle and other animals. In most cases, the cattle feed is dried and exported. If local farms are present, this feed could be sold wet as a feed product to those farmers, thus avoiding long-distance shipping costs, drying costs and the energy of drying. The solution remaining after the fiber is separated and dewatered contains mostly starch and gluten. This starch-gluten mixture undergoes starch gluten separation (centrifugation). Since the gluten has a lower density than the starch, centrifuges can be used to spin out the gluten from the solution. After removing the gluten slurry, the gluten is dewatered using filter or centrifuge technologies. The first option is a filter. The standard technique for dewatering gluten is the belt vacuum filter, which requires energy to maintain a vacuum using a pump. Another filtering option is the rotary drum filter, in which a rotating drum with a filtering surface and an internal vacuum passes through a stream of thickened gluten. A gluten cake forms on the drum and can be collected; it has a solids content of 40-43%. The second option is a decanter centrifuge, which results in a gluten mud of 30-40% solids content. This works well when the material being dried has not been treated with SO2. While the decanter centrifuge is a simpler installation for the plant, the filter system produces a dried intermediate product that reduces the cost of final drying. After dewatering, the material is still too wet to be handled well by a dryer, so some previously dried material is blended in to bring the moisture content down to around 30%. Then a number of dryer options exist. Rotary dryers are cheap and simple to operate, but can scorch the product. The typical dryer choice is a flash dryer. In this system, the moist material is dispersed in a stream of hot air, generally heated by direct firing of gas. The hot air stream conveys the material along a drying column with a residence time of a few seconds and delivers the product to cyclones to collect it. The flash dryer has a low thermal efficiency because 20-30% of the
  21. 21. 21 input heat is discharged with the spent air. Another option is the use of steam-tube dryers, as is used for germ drying. The tube dryers are more efficient than the flash dryers that use large amounts of air, but since they use steam rather than direct firing, inefficiencies are introduced. The byproduct made from the gluten, corn gluten meal, is also used as animal feed, commonly for poultry. The remaining starch solution is then diluted and spun many times to yield highly pure starch, typically more than 99.5% pure. For starch that will be sold directly (instead of being converted into syrups and/or ethanol), the product (pearl starch) must be completely dried to a powder. The set of hydrocyclones or the filtering system is the final step used to separate the starch from the gluten. This results in raw starch slurry with a solids content of about 33 to 40% starch. (Chemical modification at this stage can lower concentrations due to the addition of chemicals and centrifuges can be used to raise those concentrations back to 33 to 37% solids.) Any higher concentration cannot be pumped, as the slurry becomes dilatant. Starch slurries cannot be held for too long because microorganisms begin to develop, affecting the color, odor and physical properties of the starch. The remaining starch slurry present after the final washing stage is then dewatered to 33-42% moisture content. The most commonly used dryer for this material is the flash dryer described above for gluten drying. Starch slurry can also be dried in a spray dryer or a film dryer. For small batch starches, other drying systems may be more appropriate, such as conveyer belt dryers, fluid bed dryers or drum dryers. Starch can also be physically modified to produce a broad range of products with varying functionality. It can be modified in several ways to produce products for different applications. Processes used to physically modify starch include conveyor belt drying, drum drying, flash drying, thermal processing, compact granulation or jet cooking. Dried starch can also be chemically modified to produce starch with functional substituent groups. The starch that is not dried and prepared for sale as various starch products goes on to the saccharification or starch conversion stage to convert the solution to sugar syrups. In starch conversion, the starch solution is treated with acid and/or enzymes to convert the starch into various sugars such as dextrose and maltose. These syrups are then refined or further processed to make a variety of
  22. 22. 22 final products, including high fructose corn syrup (HFCS). Evaporation is a critical part of the syrup refining step. Much of the energy consumed in a syrup-producing corn wet mill is done in the syrup evaporation area. Some of the largest opportunities for energy saving appear in the syrup refinery through process integration S i.e. recovery of waste heat from jet conversion to be used in syrup evaporation, etc. Manufacture of Corn Flakes The corn flakes are one of the important value added products manufactured out of yellow and white maize. It is generally eaten as a breakfast cereal but the demand for this product is limited to hotels and big cities. It is a product of dry milling, which is manufactured by flaking of the major grain after extraction of germ. The flaked grain is either roasted for manufacture of corn flakes, breakfast cereal or fried to manufacture corn flakes served as snack foods. The raw flakes are also used for manufacture of beer. A brief process flow of corn flakes is given as under: Receiving => Cleaning & Polishing => Milling => Bran => Cooking under pressure => flavoring => Agitation (lump breaking) => Drying => Sweating => Flaking => Roasting/ Frying => Grading => Packing The corn grains after cleaning and polishing are milled to remove the germ and bran. Germ is utilised for extraction of corn oil. The bran is cooked under pressure in rotary steam cooker and mixed with flavoring material. The cooking is completed when the material turns out to a uniform translucent colour. The cooked material is carried to an agitator or lump breaker and finally dried in drier to moisture level of about 15 per cent to 20 per cent. The dried material is kept in tempering tanks for few hours to permit the residual moisture to become equally distributed, which is known as sweating. This is very essential to have uniform pressing for the flakes. The tempered material is next passed though a heavy duty flaking machine. The flakes are then immediately transferred to gas fired rotary ovens for roasting. While rotating, the flakes are continuously carried forward until they are dropped into conveyer. The roasted flakes are subjected to inspection, preferably on conveyers or tables. Then properly roasted flakes are graded and transferred to the packing bins immediately because flakes are hygroscopic. They
  23. 23. 23 are then packed in water resistant polythene packages or food grade waxed paper packages. The plant and machinery is available in India. As the demand for this product in the state is limited, the units may have to market their products mainly outside the state.
  25. 25. 25 CRITICAL FACTORS IN SETTING UP A MAIZE PROCESSING PLANT RAW MATERIALS The viability of a maize processing plant depends upon the availability and uninterrupted supply of raw material to the unit. On an average, a unit with a crushing capacity of 100 MT/ day will require about 30000 MT of maize per year (assuming 300 days of operation of the plant). Hence, the availability of raw material is one of the important considerations in deciding the location of maize processing unit. The plant will be able to procure major portion of its raw material requirement with in the radius of 200 km. The state produces mainly yellow dent corn which is most suitable for wet milling for manufacture of starch and other byproducts . LAND Land requirement of starch manufacturing unit is very high, as it requires large area to set up plant and machinery and effluent treatment plant. There should be enough land for disposal of treated waste water. A unit with crushing capacity of 100MT/day should have at least 10 acres of land. However, if available at reasonable price, the unit may acquire upto 15 acre of land to meet future expansion requirements. WATER The water requirement for the wet milling industry is relatively large with an average use of 4 cum per MT of crushing per day. For a wet milling unit of 100 MT capacity, therefore about 4 lakh litre of water/ day is required. The site where wet milling units are set up should have a good source of water, preferably a perennial river. As the unit also generate high amount of sewage water, which require to be disposed off properly. In case the water is to be sourced from ground, the water table should be high and the areas should fall in white category of unrestricted use.
  26. 26. 26 POWER The average power requirement is about 170-250 units per day per MT of maize crushing. The milling unit requires uninterrupted power supply and hence a DG set is required as standby arrangement. STEAM The steam requirement is 1 ton / MT of maize crushing. The units manufacturing starch by wet milling in states like Maharashtra and Gujarat, use coal for production of steam. TECHNOLOGY The technology is indigenous except for starch-gluten separation and starch washing unit which is imported through mostly companies based in China. Starch is usually manufactured from maize by a process known as wet milling. The wet milling process is a complex process, which involves a series of operations, by which the corn is separated into three parts, the outer hull or bran, the germ (the source of most of the corn oil) and the endosperm (the source of gluten and starch). The critical operations which have a direct bearing on the quality of the final product are - Good quality yellow dent corn without various impurities will increase the quality of the final product. - Germination of maize and the microbial growth are controlled by steeping. - The simultaneous washing and concentration of starch to the desired moisture and solid level increases the quality and marketability of the finished product. The different steps involved in the wet milling are presented below.
  27. 27. 27 STEP BY STEP DESCRIPTION OF THE PROCESSES CLEANING The corn is received in the facility and held in storage silos prior to cleaning. Small and large foreign matter in the corn is then removed to prevent clogging the screens, increasing viscosity during the process and affecting the quality of the finished products. This is represented in our model as a waste of 2.4% debris of total capacity. The silo in our model is sized to hold enough corn for 3 days of operation. Included in this area are also weighing and handling equipment. The corn is transported to the unit in trucks in gunny bags and offloaded in receiving area or in silos. The receiving area is designed in such a manner that there is enough space for smooth movement of expected number of vehicles. The grain is fed to the belt conveyor which takes the maize grains to cleaning section. The raw material for wet milling is shelled dent corn delivered in bulk. The cleaning is normally done twice before wet processing. First, supplied maize has to pass the incoming inspection. If it meets with the specifications of the respective starch factory, it is coarsely sieved to separate contaminations, e.g. stones, cobs, dust particles, foreign grain material, and fine material. The grain is passed over perforated metals sheets, air blowers, electromagnets to remove the impurities. After cleaning, the maize kernels are stored and then conveyed into steeping tanks.
  28. 28. 28 STEEPING The purpose of steeping is to soften and condition the corn kernel for subsequent milling and to prevent germination and fermentation. A proper steeping is essential for high yields and high starch quality. The steeping is carried out in a continuous counter-current process. At first the purified maize kernels are transferred into a tank containing steep water. The clean corn is soaked in a dilute SO2 solution (steep acid), under controlled conditions of time and temperature. Steeping is the chemical processing step where the protein matrix is broken down to release the starch granules so they can be separated during subsequently milling. The objective of steeping is to facilitate the separation process by softening the kernel, increasing the moisture content of the grains and removing soluble matter. The overall efficiency of the wet milling process is dependent on the proper steeping of the corn. In practice, the steeping is done in a semi-continuous counter current system. During the steeping process the corn entering steeping is in contact with the most diluted, oldest SO2 solution (light steepwater) and the oldest steeped corn is soaking in the most concentrated,
  29. 29. 29 freshest steepwater (steep acid). The corn does not move but the steepwater is transferred through the different tanks to go from the oldest steeped corn to the freshest. The water used for steeping is not fresh water but comes from downstream in the process; the SO2 concentration is adjusted prior to steeping. The sulfur dioxide in the industry is produced from burning elemental sulfur. During the steeping process, most of the soluble solids (about 69%) are removed and carried in the steepwater. This light steepwater (also called steep liquor) is concentrated to 50% solids, mixed with the corn fiber and dried to produce an animal feed as corn gluten feed. The SO2 concentration is 2000ppm for the steep acid and 600ppm for the light steepwater. The moisture content in the corn increases from 15 to 45% during steeping. The grain is fed into large steep tanks with hot water at 52 0 C and steeped for 70 hr. Generally, RCC steep tanks are used by the existing units in India. However, steep tanks can also be fabricated by stainless steel but it increase the capital cost. The RCC tanks should be designed in such a manner that it withstands the gravitational force, as well as the weight of the material. Steeping mixture containing sulfur dioxide (SO2) in hot water is added in the steeping tanks to prevent germination and bacteria. The steeping conditions the grain for later steps by softening of the maize kernels and loosens the bonds between germ, husk and endosperm. During the soaking process, nutrients are absorbed into the water and this water is later evaporated to concentrate the nutrients to get corn steep liquor or condensed corn fermented extractives. STEEPWATER EVAPORATION The steepwater containing approximately 10% dry substance is drained from the kernels and condensed on a multi-stage evaporator. Most organic acids formed during the fermentation are volatile and evaporate with the water. The condensate from the first evaporator stage will therefore be discharged after the heat is recovered by preheating the entering steepwater. The steepwater is condensed to an auto-sterile product - a valuable nutrient in the fermentation industry - or concentrated to approximate 48% dry matter and mixed and dried with the fibre fraction.
  30. 30. 30 SO2 The sulphur dioxide may be prepared by burning sulphur and absorbing the gas in water. Because modern processes call for stricter and narrow dosage, a supply of sulphur dioxide gas under pressure is preferred or SO2 is replaced by sodium hydrogen bisulphite where no local gas supply is available. GRINDING The grinding process is completed in 2 stages. The grinders are made of stainless steel with adjustable RPM with or without pneumatic settings. There are a number of manufacturers of grinding machines in India. In first stage, the steeped maize grains are ground coarsely to loosen the husk and germ. The second stage grinding, known as fine grinding, help in detaching the germ from the grain. The Grind Mill, which consists of studded drums rotating in opposite directions, is designed to crack the corn kernel separating the starch without damaging the corn germ. Kernels not fully opened are reground in a second mill as required. GERM SEPARATION (DEGERMINATION) The pasty mix obtained after fine grinding is pumped to water filled settling troughs, known as germ separators or degerminators. It is a 3 stage process where the slurry containing soluble
  31. 31. 31 husk, gluten and starch are separated from germ. The lighter density rubbery germ float on the top and is skimmed off. The oil-bearing germ is lighter than other particles and is segregated in a series of cyclone separators. A combination of mechanical and solvent processes extracts oil from the germ. The oil is then refined and filtered into finished corn oil. The remaining mixture of corn, starch and husks is filtered, to remove husks, and processed into cattle feed. The germ is passed to germ drier which is finally sent to oil extraction unit. The germ contains 45 per cent oil and the rest is crude fibre and moisture. The starch manufacturers generally prefer to sell germ rather than own oil extraction unit. Oil constitutes half the weight of the germ at this stage, and the germ is easy to separate by centrifugal force. The lightweight germs are separated from the ground slurry by hydrocyclones in a two step separation with regrinding in between. The germs are washed repeatedly counter-currently on a three-stage screen to remove starch. Process wash water is added at the last stage. The germ is separated from the rest of the kernel after a coarse grinding. The swollen kernel is ground (first degermination) and the oil-rich corn germ is separated from the starchy slurry using four sets of hydrocyclones. The separation is
  32. 32. 32 based on the lower density of the germ, due to its high oil content, compared to the density of the slurry. The germ is retrieved from the overflow of the first set of hydrocyclones while the underflow continues the separation in the second set. After the first and second separations, the remaining slurry is ground again (second degermination) and any remaining germ is recovered by the last two sets of hydrocyclones. The overflows of all hydrocyclones, with the exception of the first set, are recycled to grind tanks to optimize the purity of the germ recovered. In order to achieve the desired separation, adjustments to the underflow-to-feed ratio (U/F) for the hydrocyclones are set. In our model, the U/F ratio has been set as 80, 70, 80 and 60% for the first, second, third and fourth hydrocyclones separations, respectively. The control of the specific gravity during germ separation is critical to proper recovery. Although it is possible to get a very clean separation of the germ from other components, the specific gravity is typically adjusted to allow some pericarp fiber (coarse fiber) to be co-recovered with the germ. This is done to aid in the oil extraction process because clean germ “slips” during expelling and decreases the oil extraction efficiency. The pure germ (overflow, separation 1) is washed in a series of screens using process water, dewatered to about 50% solids in a screw press and dried to a final moisture content of 3% in a fluidized bed dryer. In the model, the screens are represented by two-way component splitters and the screw press by a plate and frame filter. The underflow of the last set of hydrocyclone continues the co-product separation process. FINE GRINDING AND SCREENING After germ separation the mill flow is finely ground in impact or attrition mills to release starch and gluten from the endosperm cell walls (fibres). The degerminated mill starch leaving the fine mill is pumped to the first stage of a fibre washing system, where starch and gluten is screened off. The overs, hull and larger fibres, are washed free from adhering starch and gluten (insoluble protein) on screens in counter-current with process wash water added at the last stage. The last fibre washing stage has a slightly courser screen for pre-dewatering the fibre prior to a tapered screw press.
  33. 33. 33 FIBRE WASHING SCETION The slurry of husk, starch and gluten is ground for better recovery of starch. The fibre washing is a 6 stage process which is carried out by DSM box. The husk is separated from the soluble starch and gluten slurry by a counter current flow system. The husk is sent to either drying section or used as animal feed in wet form. The husk is mainly carbohydrate which also contains 8 per cent protein. The germ removal step is followed by fine grinding in an impact mill to completely disrupt the cells of the endosperm and release the starch granules. The resulting suspension is led over bend green cascades for separation from fibre and other maize components. The starch milk, which contains the protein fraction, the so-called gluten, passes through. The bend screen cascades are connected in series. For complete washing out of the starch and separation of the fibres they are operated by counter flow principle. Additionally, washing water is added to the last process stage. The separated residues are dehydrated and dried for use as an animal feed component referred to as maize feed. The dewatered fibres from the dewatering press
  34. 34. 34 may be mixed with concentrated steepwater and cakes from the oil press and dried to approximate 12% moisture. The dried fibre are pelletized to reduce bulkiness and pneumatically transported to a silo ready for shipping. The fibre fraction is a valuable constituent of animal feeds. The degermed corn slurry from the germ separation is passed over the grit screen to separate water and loose starch and gluten (mill starch) from the fiber and bound starch and gluten. The mill starch is sent further in the process, for the separation of gluten and starch. The remaining solids (fiber and bound starch and gluten) are finely ground (third grind mill) to complete the dispersion of the starch; this milling is intended to free the starch with minimum fiber breakup. The ground slurry is then washed and separated in a series of tanks and fiber wash screens (six in our model), in a countercurrent fashion. The wash water (process water from the gluten thickener) is introduced in the last stage and it flows in a countercurrent fashion, finally coming out in the first screen with the free starch and gluten. The clean fiber is recovered in the last stage, dewatered first by a screen to a moisture content of 75% and then by a screw press to a final moisture of 60%. This fiber is usually combined with the concentrated steep liquor, dried in a rotary drum drier to 10% moisture and sold as corn gluten feed. The final corn gluten feed has a protein content of approximately 20% on a dry weight basis. PRIMARY SEPARATION The thickened slurry is passed through a high speed centrifuge to separate the heavier starch from the lighter protein (Gluten).
  35. 35. 35 THICKENING The slurry of starch and protein is passed through a centrifugal concentrator to get the concentrated slurry. This machine is also called as milk stream thickener. GLUTEN REFINING The protein slurry is passed through a centrifuge to get concentrated slurry of gluten. The gluten contains 65 per cent protein and is a good source of protein for the animals and is used in animal feed preparation. The gluten slurry is dewatered on a vacuum belt filter or decanter. The decanter removes more water, but requires strict pH-adjustment to the iso-electrical point of the gluten. Dewatering splits the gluten stream in: • Process water • Gluten (moist)
  36. 36. 36 The dewatered gluten is dried in a rotary steam tube bundle dryer to approximately 10% moisture and disintegrated in a hammer mill. Drying is facilitated by powder recycling. The dried gluten is sold as corn gluten meal with 60% protein. It is a valuable source of methionine. The high xanthophyll content - typically 500 ppm - makes it an efficient pigmenting ingredient in poultry feeds. The crude starch milk still contains all the dissolved proteins. This fraction is called gluten, and
  37. 37. 37 most of it is separated off by means of two successive nozzle type continuous centrifugal separators. The process utilises density differences between starch and protein. The protein fraction is dehydrated by means of a rotary drum filter, then dried and used as a high protein feed additive. It is mostly given to chicken, since its high xantophyll shares positively affect egg yolk pigmentation. The gluten is separated from the starch by density differences in a disk stack centrifuge. Prior to the separation, the mill starch is degritted to remove any foreign particles such as sand, rust or pipe scale that might interfere with the centrifuges later in the process. The mill starch is then concentrated, to facilitate the separation, in a centrifuge called the mill-starch thickener. The thickened mill starch stream is passed to the primary centrifuge where the less-dense gluten is separated from the starch. The purpose of the primary centrifuge is to obtain high-quality gluten in the overflow. The underflow, which contains the starch, some gluten and other impurities, is sent to the starch washing process. The gluten is then dewatered in three succeeding steps using a centrifuge (gluten thickener), a rotary vacuum belt filter and a ring dryer to a final moisture concentration of 10%. The gluten is sold, usually for animal feed, as corn gluten meal. The final corn gluten meal has a protein content of approximately 60% on a dry weight basis and contains xanthophylls that give it a yellow color. STARCH Washing with fresh clean water refines the crude starch milk. With hydrocyclones it is feasible to reduce fibre and solubles including soluble protein to low levels with a minimum of fresh water. To save water the wash is done counter currently, i.e. the incoming fresh water is used on the very last step and the overflow is reused for dilution on the previous step, and so on. By using multi stage hydrocyclones all soluble materials and fine cell residues are removed in a water saving process. The refined starch milk contains an almost 100% pure starch slurred in pure water. With a middling separator the overflow from the starch refining hydrocyclones may be refined into: • Process water Overflow • Starch Underflow
  38. 38. 38 In the strong gravitational fields of a hydrocyclones and a centrifuge, the starch settles quickly. Refining is based on the differences in weight density between water, fibres and starch: Although some impurities go with the starch in the underflow, there is - by means of a sieve - a last chance to remove the larger particles. Impurities not removed this way are not removable by any known technique. The starch milk, which still contains approximately 2 % of protein and fibres after separation, is then refined in a multi-step cyclone plant. The last stage of the multi-step cyclone plant is the one and only step of the wet milling process where fresh water is added. By optimal construction and adjustment of the plant it is possible to reduce the protein content in the starch below 0.3 % on dry matter. Hydro-cyclone plants have become accepted for starch refining for their high performance, their low water consumption, and their low maintenance efforts. ROTARY VACUUM FILTER The thickened gluten slurry is further concentrated to get gluten cake with 40 per cent solids through a rotary vacuum filter. The cake is further dried by hot air and / or sun to bring down the final moisture content to 12 per cent.
  39. 39. 39 DRYING The concentrated starch slurry is then dried by hot air application (175 0 C) to 11-12 per cent moisture content level. The main product of wet-milling of maize is starch. Besides, it produces four major co-products for the feed industry namely the steep water, husk (hulls or bran), germ and gluten. These co-products represent about 25-30 % of the processed maize. The starch is raw material for various ancillary industries like dextrose monohydrate, dextrins, saccharin etc. For manufacture of further derivatives of starch, ancillary units need to be attached with starch manufacturing units. The wet milling has developed into an industry that seeks optimum use and maximum value from each constituent of the maize kernel. In addition to starch and
  40. 40. 40 the various other products, and edible corn oil, the industry has become an important source of well-defined specialised ingredients used in feed formulation industry. The gluten slurry is dewatered on a vacuum belt filter or decanter. The decanter removes more water, but requires strict pH-adjustment to the iso-electrical point of the gluten. Dewatering splits the gluten stream in: • Process water • Gluten (moist) EFFLUENT TREATMENT PLANT Effluent treatment plant is an essential component of a starch industry. It should be set up as per the norms of State Pollution Control Board. It has been made mandatory to set up a ETP in all starch manufacturing units.
  41. 41. 41 Description Detail: Belt conveyer 55.556 kg/s m loading rate/belt width Cleaning 2.4% removed as debris Steeping tanks 15 tanks 60 hr residence time 90% volume 51o C Sulfur burner 2000ppm of SO2 in steeping tanks Evaporator Mechanical vapor recompression 50% solids steep liquor Dewatering screen DSM screen 50% overflow moisture content Bauer mill— 1st grind 0.0087 kJ/s/(kg/h) specific power Hydrocyclones 1/2 Primary germ separation Bauer mill— 2nd grind 0.0016 kJ/s/(kg/h) specific power Hydrocyclones 3/4 Secondary germ separation Screw press Germ dewatering 50% final moisture content Fluidized bed dryer Germ dryer 0.07 kg natural gas/kg evaporated, 3% moisture content Disc mill— 3rd grind 0.0116 kJ/s/(kg/h) specific power Dewatering screen Fiber wash DSM screens 76% overflow moisture content
  42. 42. 43 FURTHER EFFICIENCY MEASURES IMPLEMENTATION: PROCESS-RELATED ENERGY EFFICIENCY MEASURES: Feed Streams Corn hybrids Steeping Intermittent milling and dynamic steeping Use of gaseous SO2 Alkali steeping Use enzymes during steeping to reduce time Fermentation and Ethanol Production Improving yeast fermentation Bacterial fermentation Molecular sieves Enzymes used for hydrolysis Mechanical dewatering Germ dewatering Starch dewatering filters Fiber dewatering Drying Technology Process integration/pinch technology Controls on heaters between steps Multiple effect evaporators Thermal and mechanical vapor General Equipment/Utility Approaches Energy management systems and programs Combined heat and power Alternative fuels Anaerobic wastewater treatment HVAC Electronic controls Non-production hours setback temperatures Ventilation and cooling design improvements Heat and steam distribution—Distribution Improve insulation Maintain insulation Recover flash steam Maintain steam traps Monitor steam traps automatically Repair leaks Improve steam traps
  43. 43. 44 Reusing waste heat Replacing dryer with more efficient one Regenerative thermal oxidizers Direct use of gas turbine offgases for drying recompression Operations and maintenance Improve process control Reduction in air supply rate Challenging customer requirements Membranes Reverse osmosis (RO) for steepwater concentration Microfiltration (MF) for steepwater concentration RO or other membrane to concentrate syrups Membranes to purify syrups Membrane filtration or RO to recycle starch wash water or reduce water in steeping
  44. 44. 45 Although technological changes in equipment can help to reduce energy use, changes in staff behavior and attitude may have a greater impact. Staff should be trained in both skills and the company‘s general approach to energy efficiency in their day-to-day practices. Personnel at all levels should be aware of energy use and objectives for energy efficiency improvement. Often this information is acquired by lower level managers but not passed to upper management or down to staff. Programs with regular feedback on staff behavior, such as reward systems, have had the best results. Though changes in staff behavior, such as switching off lights or closing windows and doors, often save only small amounts of energy at one time, taken continuously over longer periods they may have a much greater effect than more costly technological improvements. Most importantly, THE INDUSTRY need to institute strong energy management programs that oversee energy efficiency improvement across the corporation. A bi-monthly energy management program should be organized to cater all employees actively contribute to energy efficiency improvements. Participation in voluntary programs or implementing an environmental management system such as ISO 14001 can help companies track energy and implement energy efficiency measures. ENERGY MANAGEMENT: Drying technology: Because of the temperatures used in steeping and the extensive drying and evaporation required in corn wet milling, drying technology, as well as heat and steam distribution systems are an important part of the plant. For evaporators, the major sources of heat loss are excessive venting, radiation or
  45. 45. 46 convection losses, poor vacuum system performance, air leakage, water leakage, fouling and poor separator efficiency. Controls on heaters between steps: When properly controlled, most of the time these heaters should be unnecessary and turned off. It is important, therefore, that good controls are used in conjunction with the heaters. Reusing waste heat: Waste heat from the starch dryer can be reused. There are two ways of recovering this heat: recuperation via heat exchangers or direct recycle back into the dryer. When using heat exchangers, a number of factors should be considered in addition to savings from energy efficiency: extra fan power may be required, dust may damage the heat exchanger surfaces and energy integration may increase the complexity of start-up and shutdown operations and limit flexibility. In exhaust recycling, the recycle gas must be completely free of dust if there is a risk of it being ignited in the heat source. The changed humidity-temperature profiles should be checked to ensure they do not adversely affect the quality of the product. Simple gas burners may have problems igniting or with stability in direct systems if high levels of recycling cause high humidity and low oxygen levels. Equilibrium levels of impurities may build up, and if sulfur is carried over from earlier steeping, dew point corrosion may be a risk. However, odors are concentrated in direct or indirect recycling and are easier to remove. If the problems can be overcome, recycling may be a cost-effective way to save energy that would otherwise be wasted. GEA Barr-Rosin (Barr-Rosin), a process engineering company has developed a number of dryers and dryer systems that recycle waste heat. One such system integrates the gluten dryers with a waste heat evaporator (WHE) and a regenerative
  46. 46. 47 thermal oxidizer (RTO). Generally, this system reuses the energy put into some of the steepwater evaporators and the heat input to the RTO to dry the light steepwater and gluten. The WHE uses the latent heat given up by the vapor in the steepwater to evaporate light steepwater in earlier stages or in the RTO. By combining the WHE with the RTO, the exhaust contains less water vapor and the size of the RTO is reduced. The hot RTO exhaust provides 50% of the heat for the gluten dryer. In addition to saving energy, Barr-Rosin claims the system achieves great flexibility of operation, eliminates odor and reduces exhaust volume. In the Barr-Rosin system, convention dryers were chosen because they produced the best product quality by minimizing heat damage. However, other types can be used and Barr-Rosin provides detail for each dryer type available for reusing waste heat, including direct fired feed and corn gluten meal dryers, rotary feed dryer and fluidized bed dryer (for germ drying), as well as WHE and RTO. Another system developed by Barr-Rosin is a closed exhaust gas recycle on the direct gas fired rotary dryer treating the dewatered corn fiber mixed with steep liquor ("heavy steepwater"). In this system, the dryer exhaust is recycled back to the air heater as dilution air; the only fresh air used is that for combustion. According to Barr-Rosin, in addition to saving energy by recovering the heat in the recycled exhaust gas, other benefits are accrued. Total mass of emissions and final exhaust are less because the exhaust gas is recycled, emission control equipment is less costly and the operation is safer because the oxygen content of the drying medium is low. Barr-Rosin has also created a similar recycle system for germ drying. They use a direct gas fired fluidized bed dryer. The dryer recycles most of the exhaust gases as
  47. 47. 48 "dilution air" to supplement drying medium; the only fresh air inlets to the system are burner combustion air, cooling air and possible leaks. Replace dryer with more efficient dryer: Some dryers are more efficient than others, particularly when they can recapture otherwise lost waste heat. Direct dryers are typically more efficient than indirect dryers. However, water formed during combustion in direct heating may decrease the efficiency of drying. Fluidized bed dryers are mainly suitable for drying granular solids and operated best on solids with a narrow particle size distribution. For this reason, they are particularly suitable for drying germ. In CPC (the Netherlands) replaced a conventional steam-heated rotating pipe bundle dryer (indirect heated rotary dryer) with a fluidized bed dryer for germ drying. The dryer in this project used flue gases from a combined heat and power unit already installed. The hot exhaust gases from the CHP-unit are blown through a bed of the corn germ. A perforated grid supports the germ and ensures an even distribution of the gases. The dried product is continuously removed and the suspended residue is extracted using cyclonic separators. The CHP unit provides gases at 266 O F (130 O C) to the fluidized bed dryer, which are extracted by fans using 260kW of power. No additional heat is required. The energy saved due to the difference in energy saved and additional energy required by the fans is 63%, over 2.5 GJ/tonne (LHV) of germ produced, equivalent to 2.4 MBtu/ton germ (HHV). Improvements could also be made in the process to better utilize the heat by optimizing the relationship between bed surface area, bed height, gas flow and the average residence time of the germ in the bed. Heat from the dryer exhaust at 190°F (88°C) could also be recovered again for use elsewhere in the milling process.
  48. 48. 49 Challenging customer demand: Feed products are usually dried to a certain specification before being shipped to customers. MEMBRANES: Removing water at various stages of the corn wet milling process is a major energy consumer in the corn wet milling industry. Membrane separation processes could potentially be a suitable energy-efficient alternative in various stages of the production process. Membranes are selective barriers that allow the passage of certain species in a fluid, based on the size of the molecules. In practice, the feed solution is pumped across a membrane at a given pressure. Some of the solution (only aqueous solutions are discussed) passes through the membrane and is called the filtrate or permeate; the remaining solution, which will have a higher concentration of the retained materials, is called the concentrate or retenate. In the most restrictive use of membrane separation, nearly all ions and solutes are retained and only water passes. This is called reverse osmosis. Membranes offer a method of removing water from solution that has important advantages over evaporation or distillation. Not only is there no heat required for the separation (except in pervaporation systems), but there is no change of phase, so latent heat requirements are avoided. Therefore, significant energy savings are observed; even up to 90% savings have been reported. However, in some cases, high pressures and recirculation rates require a significant amount of energy and may not result in net savings. Fortunately, membranes generally offer also other benefits in addition to energy savings. Since exposure to heat is reduced, nutritional quality and digestibility is conventionally thought to be improved. Membranes give
  49. 49. 50 plants greater control over the production of alcohol and separation of the various products. They often produce better yields and with fewer moving parts, are generally more reliable. Recovery and reuse of water generally reduces overall plant water needs. For example, a company Kollacks and Rekers (1988) reported on a reverse osmosis (RO) system used for steepwater concentration. This system saved as much as 175 liters per ton of corn (1.3 gallons per bushel) and 53 to 59% steam. The use of ultrafiltration in conjunction with enzymes for production of starch conversion products has been shown to be more efficient than enzymatic batch processes alone. Membranes may experience biofouling or fragility of the membrane surface, although they may prevent surface fouling of evaporators downstream. In addition, for facility retrofits, capital costs may be a deterrent. Other disadvantages include limits on operating pressures that prevent implementation in certain processes and potential delays in bringing membranes online dues to problems with documentation for food safety by regulators. In addition to the above-mentioned benefits, membranes offer other advantages to standard filtration methods like rotary vacuum precoat filters (RVPF) using diatomaceous earth. Membranes are less costly, do not have large disposal requirements and have lower labor and maintenance costs than these filtration methods. Membranes do not need to be replaced as often as the filters and the wear and tear created by abrasive materials like diatomaceous earth does not exist for membranes. Other benefits like increased product recovery and reduced carbon requirements downstream have also been discussed.
  50. 50. 51 There are numerous applications of membranes in the corn wet milling process, such as steepwater concentration (or supplemental pre-concentration) by reverse osmosis (RO), reducing chemical oxygen demand (COD) in evaporator overhead, solvent recovery and oil purification in the corn oil refining stage, production of protein concentrates and isolates, enzyme modification in membrane reactors, starch washing, concentrating starch and recovering fresh water by RO. FUTURE TECHNOLOGIES:. This section includes a description of some technologies that may hold promise for the future but are still currently in the research, development or demonstration phases. Steeping The main requirement for the steeping stage is to get the incoming corn up to steep temperature, which can be done by heating hydraulic transport water with low pressure waste steam. Energy saving measures in the steeping stage generally focus on reducing the time the corn spends in the steeping tanks. Corn is soaked in the steeping stage at high temperatures (120°F or 50°C) for up to 50 hours. Reducing the steep time reduces the energy required to heat and maintain the corn and steepwater at 120 0 F. Steeping requirements are a small portion of total primary energy us (3%), however changes to the first step of the process could have a greater effect downstream. Using enzymes during steeping to reduce steep time. The use of enzymes during steeping is a technology that is currently being developed. Recent research indicates a two-stage steeping process using proteases may reduce conventional steep time by 67 to 83%. It was found that the corn kernel requires a hydration step of two to four
  51. 51. 52 hours at 48°C to 52°C (118°C to 126°F) before grinding so that the germ is penetrable and pliable enough that it does not break when the corn is coarsely ground. Without the hydration step, steep times were not significantly reduced. In addition to shortening the steep time and decreasing energy, other benefits are likely for this technology. One important benefit is that SO2 is not required for this process, eliminating the associated environmental control problems. Shorter processing times will either increase plant capacity or decrease capital investment in steep tanks. Broken as well as unbroken grains can be processed by either soaking broken kernels for less time than the unbroken kernels, or by simply adding the broken kernels to the unbroken ones in the second stage of the steeping process. The reduced processing time may ultimately significantly increase the productivity of the input corn over conventional steeping for the same corn input Finally, soaking water from the modified process contains a relatively low level of dissolved solids compared to conventional steep water, up to 90% less, and can potentially be reused after filtration, eliminating the need for evaporators and reducing water usage. The enzymatic steeping process is currently being researched and tested in conjunction with corn wet milling plants and hence, no cost or energy savings data for actual implementation was found at this time. Enzymes are already extensively being used and perfected in wheat starch separation processes. Intermittent Milling and Dynamic Steeping (IMDS). In this process, the corn kernels are soaked for a short time, and then ground to reduce particle size and reduce diffusional limitations before continued soaking. The steeping time can be reduced significantly (to 3-7 hours total), and the overall starch yields observed can
  52. 52. 53 be higher if lactic acid and SO2 are used. Recovered starch may be 1% more and recovered protein 3-4% more than in the conventional wet milling process. There are some energy savings since the steep water is not maintained at approximately 50°C (120°F) for as long. Additional energy requirements for grinding are unknown. The main benefits, in addition to reduced energy, are the increased yield and the reduced capital requirements, since fewer tanks are needed for steeping. This capital cost is significant because these tanks are high quality tanks to avoid corrosion from the sulfur dioxide used in the process. One drawback to IMDS, however, is that the percentage of oil in the germ is reduced by 5%, and there is slight germ damage (5- 10%). Table 1 summarizes the data for IMDS. Table 1: Intermittent Milling and Dynamic Steeping (IMDS) compared to conventional steeping. IMDS compared to conventional steeping Starch yield 101% Protein yields 103-104% Germ oil yields 95% Operating costs ` 9600000 savings (approx.) Capital costs ` 4,27,00,00,000 savings (approx.) Alkali steeping. Using alkali (such as NaOH) instead of SO2 to steep corn can shorten steep time and reduce capital cost and energy. Alkali wet milling of corn involves the removal of the pericarp (the outer hard layer of the kernel), cracking the pericarp-free corn in a roller mill, and then steeping the corn in an alkali
  53. 53. 54 solution. Alkali solutions remove the pericarp by dissolving the connecting material between the pericarp and the inner layer. Without the pericarp, diffusion of water and chemicals is made possible, shortening steeping times. Cracking the pericarp- free corn in a roller mill further reduces kernel size and steeping time. Following steeping, the remainder of the alkali corn wet milling procedure is the same as the conventional corn wet milling process. CONCLUSION For process-specific measures, some new technologies reduce energy and improve product quality consistency or yield. Six of these measures are still being developed and are included in the future technology section. Implementation of most of the other measures will be part of strategic investments and innovation at the corn wet milling plants. Selected technologies will have large additional benefits including product quality improvement.
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