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In the early days of oilseed production, functions were often far removed, and actions taken by one operation were done for optimization of its own performance with little consideration on impacts made on subsequent processes. For example, the elevator dryer operator, in order to get the maximum grain throughput during busy harvest periods, might dry grain at an excessively hot temperature without considering the impact on the oil quality. The degumming operator may set the centrifuge to take advantage of trading rule limits without regard of the downstream impact on the refining operation.
Within the last few years, the emphasis has changed from stand-alone operations toward the integrated manufacturing facility, producing a more complete range of value-added products from the raw seed to the dinner table. During this transition, operations have become more dependent on each other, as the individual functions involved must now consider the impact of their actions on the total process. At the same time, the scope of knowledge each operation must have of other functions has expanded, and it is important that at least a basic understanding of the "big picture" be available to thedecision maker. The purpose of this Chapter is to provide an overview of the typical processes and interrelations associated with a total integrated facility. It is also hoped this basic overview will prove to be beneficial to those new in the industry. It must be stressed that this Chapter attempts to touch in a limited number of pages subjects that are the basis for volumes of books and lifetimes of knowledge. It must also be emphasized that this Chapter does not expand the many viable options of each type of process, nor does it intend to provide details for proper operation of each of the unit operations. For a more detailed discussion, the reader is encouraged to refer to the many fine chapters within this series and other publications dealing in much greater detail with each of the individual functions of the integrated facility.
It is useful to consider the modern manufacturing operation as a set of unit operations and develop a block diagram representing the facility. Figure 1 illustrates the processes involved with the subsequent sections of this Chapter reviewing these unit operations.
The typical operations associated with storage include receiving, sampling, drying, storage, and cleaning prior to processing. Figure 2 illustrates the common functions of this operation. There are many variants; for example, some processors may clean the grain both before and after the drying operations. In any case, the basic operations are designed to accomplish the same task, which is to provide a safe haven for the grain and deliver it at the proper time and condition to the processing facility.
As the product is received fresh from the farmer's field, the grain will contain foreign material, consisting of naturally occurring sticks and pods, metal and rock accumulated during handling, and contamination from weed seeds and other grains. The elevator manager will sample the grain and make adjustments in the price paid based on the moisture, splits, heat damage, and other factors. Typically grain receipts are segregated based on these quality factors, with the moisture content being one of the prime factors for separation. For proper storage and subsequent processing, the contaminants must be removed and the grain must be dried prior to storage. As grain freshly harvested may have a moisture content of up to 20% (although many farm operations are equipped with dryers), the grain must generally be dried to around 13% moisture for safe extended storage. High moisture damage typically results in reduced oil content, decreased protein, and increased color and refining loss of the extracted oil. A precursor to this damage is often indicated by a rising grain temperature, and many storage facilities are equipped with a series of temperature cables embedded in the grain with indicating and recording equipment located in the manager's office. If left unchecked, the grain will spontaneously heat and become damaged and under extreme conditions, a serious fire may develop. It is routine practice to monitor the temperature of the grain daily and, if heating is occurring, immediately process the grain. If this is not possible, or the degree of heating is not severe, the elevator manager may simply "rotate" the grain by moving it from one location to another.
Drying is usually accomplished in a vertical column, direct fired unit, although steam, and even solar energy have been used as a heat source for certain installations. Naturally, the hotter the drying temperature, the faster the drying operation will be. However, drying temperatures must be closely monitored as excessive temperatures will damage the seed. It has been found that temperatures in excess of 63°C will significantly increase the color of both the meal and the oil, denature the protein, increase the nonhydratable phosphatide levels in the crude oil, and result in greater potential for grain dryer fires.
Storage facilities come in many sizes and varieties, and storage methods are often limited to the imagination of and the assumption of risk by the elevator manager. It is noteworthy that the design is always concerned with protection against the elements, and, despite the design, the condition of the material put into storage always determines the success of the storage program. While on-farm storage is generally limited to aerated steel or stave silo tanks, storage at the local elevator and the processing plant may take several forms. Complexes are situated to serve rail, road, ship, or barge traffic as the locality requires, and the functionality is determined by the success of movement of products into, out of, and within the complex. A popular storage method is traditional concrete or steel storage tanks, with typical sizes of 100,000-750,000 bushels per tank. Structures of this type are normally very visible in many small towns throughout North America. Muskogee houses, which are large warehouse bulk storage facilities, have long been popular for cottonseed and other seeds that are difficult to handle. Undercover "tents" and inflatable warehouses with capacities in excess of 2 million bushels are often used, and even caves with huge capacities are sometimes employed. In small towns, it is common during harvest to see parking lots filled with grain, with the elevator's manager and stockholders praying for the rain to hold off until sufficient storage becomes available in more permanent facilities.
In some crushing operations producing a high-protein meal, it is common to dry the grain from its 13% moisture storage conditions to 10.5% processing conditions. This is necessary to shrink the meat away from the hull and to remove excess moisture that would end up in the extracted meal. After process drying, it is desirable to temper the grain for an additional 4-10 days prior to processing to allow moisture to migrate evenly throughout the grain. Even when the storage conditions have been at low moisture, it remains common practice to pass the grain through the dryer to help shrink the hull from the meat, allowing the subsequent dehulling step to be performed more effectively. This additional preprocessing step does increase operating costs, not only because of the energy spent to heat the grain, but also because this represents one additional unit operating with associated depreciation, operating, and handling losses. There are new technologies available for dehulling integrated in the preparation process that largely eliminate the need for a process dryer.
Cleaning methods vary greatly depending on the seed received, but typically consist of a magnet designed to remove tramp metal, a scalper designed to remove large and heavy materials, and a sizing screener designed to remove fine and oversized materials. Aspiration may also be employed to assist in removal of light foreign material. While contaminants removed by the magnet and scalper are normally discharged as waste, contaminants separated in the cleaner may be ground and reintroduced into the meal stream, or may be used as fillers in feed rations. In addition to cleaning, cottonseed is often delinted prior to preparation, with the lint fibers removed by a series of saw cutters prior to processing.
The function of the preparation process is to properly prepare the seeds for extraction of the oil, either by solvent or mechanical methods and, if applicable, remove the hulls and other materials from the seed kernel or meat. While a particular seed may contain from 20 to 50% oil, the oil is tightly bound within the cell and mechanical action must be taken to either forcefully remove the oil or to make the oil more accessible to subsequent solvent extraction. The unit operations typically involved are illustrated in Figure 3, and usually involve scaling, cleaning, cracking, conditioning (or cooking), and flaking. Depending on the process and the oilseed in question, process drying, and dehulling (or decorticating) may be employed, as may be expanders and collet dryer/coolers. After the preparation process, the prepared flakes or collets are delivered to the extraction operation.
Once arriving in the preparation facility, the seed is usually scaled through a weighing device or other control means. The scale is often used to check the physical inventory of seed against the production accumulated, with the difference reconciled as shrinkage. After weighing the seed, it is then delivered to the cleaning process, which generally follows the same path as that described in the receiving operation. After cleaning, cottonseed and sunflower seeds may be dehulled by impacting the seed, breaking and separating the hulls. The hulls may be used as a solid fuel source for boiler operations or for animal feed supplement. The traditional process continues with the cracking rolls, which are a set of two- or three-high corrugated rolls turning at relatively high speeds that break the grain into several pieces. For the soybean processor producing high-protein meal, the cracking breaks the bond between the meats and the hulls, and in the traditional process the hulls are then removed by aspiration. After dehulling, the meats are delivered to the conditioner (or cooker) where heat is gently applied to make the cracks soft and pliable for the subsequent flaking operation described later in this section.
There have been several novel approaches applied to preparation in the past few years. One concept that has been widely accepted, especially for soybean processing, is hot dehulling. After traditional cleaning, the seed may be delivered directly to the crackers or may enter the hot dehulling operation. As mentioned before, use of this technology generally eliminates the process drying step traditionally identified with the storage function. The basic principle shared by the three commonly used hot dehulling systems is to dry the bean from storage moisture to process moisture, dehull the seed while still hot, and deliver the conditioned cracks to the flakers without allowing the seed to subsequently cool. This not only saves the energy of one heating step, as much of the air is recycled reducing the energy required for the integrated facility, but reduces the fines generated compared with the traditional system where the grain is cracked cold. One system that has gained wide acceptance is the Escher-Wyss system, which uses a fluid-bed dryer-heater to perform the drying process. After the dryer-heater the grain is discharged to specially designed high-speed cracking rolls, where the seed is cracked while still hot, and then delivered to special high-shear impactors to separate the meats and hulls. The product is then delivered to aspirators, where the hulls are removed, and then to the conditioner, which allows the meats to cool slightly and to temper prior to flaking. Another process that has gained acceptance is the Buhler hot dehulling system, which uses a conditioning column with steam-heated elements to slowly bring the beans to 65°C. The beans are then subjected to a short treatment in a fluid-bed popper where the hull-meat bond is broken. The beans are then broken in half by impact splitters, the hulls removed in an aspirator, and the splits further cracked and sent to the flaking rolls. The Crown hot dehulling system uses a similar conditioning column followed by a jet dryer to crisp the soybean hull and free it from the meat. A proprietary Hulloosenator then splits the bean and rolls the hulls free where they are aspirated from the meats. The splits are then cracked to the final size for flaking and sent to the Crown Cascade Conditioner for additional aspiration with temperature and moisture adjustment. In addition to the obvious energy savings, these types of systems are reported to reduce residual oil content, improve extractability, and reduce refining loss. In all cases, the comments on drying temperatures presented during discussion of storage drying are valid with hot dehulling systems.
While having received the greatest attention, cottonseed, sunflower, and soybeans are not the only oilseeds suitable for dehulling operations. There is research underway to produce a dehulled canola seed. Removing the hulls will increase the protein content and reduce the fiber content of the meal making the product more attractive for feed formulation. A variety of methods have been tested, with encouraging results, although no commercial system has yet been installed.
After conditioning, the meats are generally passed to the flaking rolls where the cell wall is distorted, making the oil more accessible. The rolls being relatively large (70 × 157 cm or larger) are held together with hydraulic or mechanical pressure, squeeze the meats into flakes of approximately 0.30 mm thickness. For grains with lower oil content such as soybeans, the flakes are typically delivered directly to the solvent extraction plant. For oilseeds with higher oil concentrations, such as sunflower or canola, or installations where solvent extraction is not employed, the flakes are typically sent to mechanical pressing equipment.
A number of processes have been applied to enhance oil extractability and to improve conditions for consistent physical refining. One of the greatest problems associated with physical refining of high-phosphorous oils (such as soybean or corn) is that nonhydratable phosphatides generally cannot be removed without extensive bleaching clays and acid treatments. Because of variances in the crop year, growing conditions, and seed varieties, consistency in the oil is a major factor affecting successful application of physical refining. It is postulated that the presence of an enzyme during conditions associated with certain storage conditions and the subsequent extraction process causes water-hydratable phosphatides to become nonhydratable. Activity of this enzyme is directly impacted by the seed and growing conditions. Lurgi's Alcon process is said to inactivate this enzyme immediately after the flaking step, and provide an oil consistently acceptable for physical refining. This process is said to also reduce hexane carryover, although the characteristics of the meal are somewhat different from that obtained from a conventional process. Lecithin produced from degumming the oil is also affected.
Excerpted from Bailey's Industrial Oil and Fat Products, Edible Oil and Fat Products Copyright © 2005 by John Wiley & Sons, Inc.. Excerpted by permission.
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