CUBING AND PELLETING ANIMAL FEED

Transcription

1 CUBING AND PELLETING ANIMAL FEED The majority of feed milling engineers can give information about obtaining the maximum output at minimum cost from their respective plants and their literature merits study.some of the factors which affect cube/pellet quality and output are: Moisture content of raw materials This can vary after receipt according to the relative humidity at the time of use. The lower the moisture content, the greater the quantity of steam required to produce satisfactory product and vice versa. Generally, the natural moisture content of the feed to the conditioner should be between 11 and 12%; this may necessitate the addition of water. This can be done in limited quantities (1 to 1.5%) in the mixer but by far the safest way is to add water to one or more of the ingredients at the earliest possible stage, e.g. cereal before grinding, wheatfeed, etc. If molasses is being added, then this ingredient has an appreciable effect on moisture addition and this must be taken into account. The moisture content of molasses can be as high as 24%, so if 10% is being added to the meal prior to the press, then the overall moisture content of the meal is Increased by 2.4%. Source of Supply Fairly large variations in analysis, colour, texture and density occur for a given ingredient whether received from one source or several sources. This is bound to affect the quality and level of output from the press if adjustments are not made. Wherever possible, more than one delivery of the same ingredient should be blended to the mixer to average out the differences. Formulation The ratio and quantity of protein, oil and fibre plays a large part in the ease or difficulty of producing good quality cubes or pellets. Bulk density is a major factor affecting press output. For example, a 100 HP pellet mill on a feed weighing 272kg/m 3 (l7lbs/ft 3 ) would produce from 3 to 4 tonnes per hour, while the same press on a feed weighing 640kg/m 3 (40lbs/ft 3 ) with fat added would give 9 to 10 tonnes per hour. For the most part, ingredients which are high in natural protein have a high density and good production rates can be expected. Ingredients with a low protein content usually are of low density and the press has more work to do to increase their bulk density by compression, hence one can expect lower outputs. The exception is that when processing some high protein concentrates a reduced output of 30 to 50% can be expected. The natural oil in an ingredient plays a significant role in the pelleting process. With pressure and heat, the oil comes to the surface and helps to lubricate the material through the die. If fat is added prior to the press to a formula with a high oil content then soft cubes or pellets can result. To overcome this problem, the use of a fat coater should be considered - the fat then being added to the finished pellet or cube. Fibre is a natural binder but it is difficult to compress and force through the holes of the die. Usually a high fibre feed produces a tough pellet but results in a low production rate. Die Life Die life is affected by abrasive action. Dehydrated alfalfa from sandy soils may contain a proportion of sand which is extremely harmful, whereas that from loamy soils may contain little or no sand. The ingredient itself may be abrasive, e.g. ground corn cobs. Some added minerals may also be abrasive. 1

2 If the material Is not going through the die as fast as it should be, vertical lines running lengthwise on the walls of the hole of the die, known as fluting or scoring, occur. Chemicals affect die life: free fatty acids are released from some ingredients when they are subjected to high steam temperatures. The worst offenders are gluten feed, rice bran, oatmeal, fish meal and some added animal fats. These acids pit the wall of the die. When the press is shut down after using such materials, even for a short while it is important to hand feed oily bran (use a mineral not a vegetable oil) through the die to flush the holes. Stainless steel dies do largely overcome the problem. With the high cost of dies and the importance of obtaining the maximum level of output in the shortest possible time, perhaps more attention should be paid to the above factors when formulating least cost grists with the aid of a computer. When a press is operating for lengthy periods on a particular product, consideration should be given to installing an automatic pellet controller. By monitoring and controlling the feeder speed, mill load and steam addition one should achieve maximum output with a good quality product. Table 8 gives the average protein, oil and fibre content of the main ingredients in common use in the U.K - variations can be expected, however, according to the source of supply. A guide is given on their pelletability and the degree of abrasiveness which obviously affects die life. Where known, the ability to absorb molasses is stated - good = over 20%, medium = 10 to 20%, poor = under 10%. Finally, the minimum and maximum percentage inclusions which one would expect to use in any one feed are given. These figures are given to assist in plant design. While it is essential that the end product gives the maximum conversion rate at the least cost, often a compromise has to be reached and it is hoped that Table 8 will prove useful to this end. Table 8 Protein, oil and fibre content of animal feed ingredients Ingredient Protein N x 6.25 Average Oil Fibre Pelletability Abrasive degree 2 Ability to absorb molasses * Inclusion % % % % Min Max Alflafa meal (dehydrated) Low High Good 1 50 Bagasse Low High 1 20 Barley meal Med Med Med Beet pulp Low Med 2 20 Blood meal Low Med 1 10 Bonemeal Low High 1 15 Bran Med Med Med 1 70 Brewers grains Low Med Poor 1 30 Buttermilk Low High 1 20 Citrus pulp Low Med 5 15 Coconut meal exp Med High Good 1 20 Coffee meal Med Med 1 6 Copra cake meal Med Med 1 20 %

3 Cottonseed meal exp Med Med 1 10 Cottonseed meal extr Med Med 1 10 Cottonseed meal undec Low Med Poor 1 10 Distillers grains Low Med 1 20 Distillers solubles Med Med Feather meal Med Med 1 30 Fish meal Med Low Poor 1 15 Grass meal Low High Good 2 30 Groundnut exp High Med 1 15 Groundnut extr High Med 1 15 Herring meal Med Med 1 15 Hominy High Low Good 1 30 Kaffir corn (Dari) Med Med Med Limestone Low High Linseed meal exp High Low Poor 2 15 Linseed meal extr Low Med Poor 2 15 Locust bean meal Low Med 1 10 Lucerne meal Low High 1 50 Maize meal High Med Med Maize gluten meal Med Med Poor 2 30 Meat meal Med Med 1 30 Meat and bone meal Med Med Poor 1 40 Milk powder Low High 1 <5/td> Millet Med Med 1 60 Milo Med Med Med Oatfeed meal Low High 1 40 Oats ground Med Med Good Olive pulp meal Med High 2 15 Palm kernel meal exp Med Med 5 20 Rape seed meal extr Med Med 1 15 Rice bran exp Med Med 2 20 Rice bran extr Med Med 2 20 Screenings grain Med High Good Sorghum meal Med Med Med Soya bean meal exp High Low Med 1 50 Soya bean meal extr High Low 1 33 Sunflower seed meal extr Med Med 1 20 Wheatfeed High Med Good Wheat ground Med Med Med Whey powder Low High 1 15 White fish meal Med Low Poor 1 15 Yeast Med Med 1 10 * Ability to absorb molasses, Good - over 20%, Medium %, Poor - under 10% Texture The type of grind, coarse, medium or fine, plays an important part in press output and quality of product. Very coarse grinds have the added disadvantage of providing breaking points in the cubes or pellets, thus producing more fines to and after the cooler. Medium and fine grinds generally will result in higher pelleting capacity than a coarse grind as the former grinds provide a greater surface area for 3

4 absorption of moisture from the steam and thus better lubrication through the die. The finer the grind the higher the density thus giving the press less work to do. It has been found in practice that better pelleting capacity and quality can be achieved by using a mixture of grinds of differing fineness. If a number of ingredients are mixed together and then ground to a uniform size, the capacity and quality will be lowered. Other factors Apart from the points already mentioned, there are many other factors which affect output and quality - e.g. the temperature of the meal ex the conditioner to the press (this should be taken as near to the point of entry to die as possible), the moisture content of the meal feeding the conditioner die thickness, die speed, roll setting and type of roll face. Processing Different compound foods require varying treatment but in an endeavour to simplify matters, they can be broken down into five main categories, as shown in Table 9 Suggested conditions for each category are intended to act as a guide only - optimum setting must be left to the operator. These conditions may vary according to the type of press used and the manufacturer. Table 9 Category Moisture range in feed to die % Compound food categories Temperature to die Steam Pressure 1. Heat sensitive feeds containing sugar, milk powder or whey High cereal content feeds 50-80% grain, protein under 25% High natural protein feeds and concentrates. Natural protein 25-45% Poultry and dairy feeds. Natural protein 12-16%, low grain but high infillers o C o F High urea 6% and/or high molasses 10-20% ambient bar Steam supply Steam plays a very important role in the cubing/pelleting process. It largely controls the moisture content of the feed to the die, the added water providing a lubricant to the die thus reducing friction through the die resulting in less wear. It is the sole means of controlling the temperature of the meal to the die. The temperature of the meal to the die should equate, as nearly as possible, to the temperature of the cube/pellet ex the die - the greater the difference, the greater the friction through the die. For proper utilisation of steam, a uniform flow of feed is essential. In providing a steam supply, there are three major considerations:- 1. Sufficient volume 2. Constant pressure 3. Provision of dry saturated steam carrying no water. 4

5 Steam at 5.17 bar (75 p.s.i.g.) has a temperature of 160 o C (320 o F) and at bar (15 p.s.i.g.) a temperature of 121 o C (250 o F). The steam is in contact with the meal in the conditioner for 7 to 8 seconds only and as it has to drop below 100 o C (212 o F) to revert to water, in the higher pressure ranges 100% condensation may not have taken place. Generally, the higher the pressure, the less the moisture addition. To ensure that the correct quantity of steam at the right pressure is delivered to the conditioner, careful planning of the pipe installation must take place. The main points to watch are:- 1. Properly sized pipes to give sufficient volume (see Steam-flow through pipes) 2. Provision of frequent traps to ensure dry saturated steam. 3. Steam take off to be from top of pipe or header. 4. Provision of a pressure regulator, readily accessible to the operator, to smooth out fluctuations of pressure and to permit changes from high to low pressure steam (low pressure under bar - 30 p.s.i.g.) 5. Care must be taken over the correct selection of pressure regulator. For example, a 32 mm (1¼ inch) regulator will carry more volume of steam than a 50 nm (2 inch) pipe, so a reducer must be fitted on the high pressure side just before entering the regulator. On the low pressure side of the regulator a 50 mm (2 inch) pipe would be insufficient and this must be increased to 64 mm (2½ inches) to match the capacity of the pipe carrying high pressure steam. It should also be noted that the expansion of steam (drop in pressure) does not occur in the regulator but in the 1.2 to 1.5 metres (4 to 5 feet) downstream from the regulator. The regulator should therefore be placed about 2 metres (6 feet) from the entry point to the conditioner. 6. Provision of two steam pressure gauges, either side of the pressure regulator. 7. Provision of a strainer. A typical arrangement for steam services to a pellet mill is shown in Fig 4. Fig. 4: Pellet mill steam services The following formula gives a rough guide to the quantity of steam required for cubing/pelleting: Temperature of meal ex Conditioner minus ambient meal temperature x tonnes per hour x 0.6 = kg steam per hour. Example 5

6 Ambient meal temperature Meal ex conditioner Throughput Steam consumption = 20 o C = 80 o C 10 tonnes per = hour (80-20) x 10 x = 0.6 = 360 kg per hour = 36 kg per tonne Some millers prefer to use a kettle for conditioning purposes or even a kettle and a conditioner. With the kettle, the meal takes five minutes or so to pass through giving considerably more time for the steam and molasses (if added at this point) to penetrate even the most fibrous and difficult materials. Live steam can be injected into the meal either by a mushroom at the base of the kettle or through steam arms in direct contact with the meal. A float switch maintains a full level of meal in the kettle. Power requirements vary from 7.5 kw (10 HP) to 11 kw (15 HP) according to kettle size. The kettle is claimed to be particularly advantageous for rations with a high grain content and for reducing bacteria such as salmonella, to a safe level. Die Specification The strength of a die depends on its thickness and the hole pattern. Die thickness can vary from 32 m (1¼ inches) to 125 mm (5 inches). The hole area rather than the hole size affects the capacity of the mill, the smaller the hole area, the lower the capacity. The land area is less on the inside of the die than on the outside and the ratio between the two increases with the die thickness. There are a number of alternatives to obtain firm pellets from materials which are difficult to compress and trials would have to be made to ascertain which of the methods listed below gave the optimum result. 1. The die can be drilled from the outside at a slightly larger diameter than the hole size required (for about a quarter of its length). This is known as counterboring. It is used when pelleting difficult material where to obtain die strength, the overall die thickness is greater than the effective pelleting length required. 2. The die can be countersunk from the inside. Most straight hole dies are countersunk from the inside to reduce the amount of land and give the meal a lead into the hole. Natural wear will gradually increase the countersunk area until eventually the holes are adjacent to one another and there is no land. 3. The die can be tapered from the inside. The thickness of the die should be varied according to the reamer angle and taper depth. Taper angles vary from 1:16 down to 1:4 and affect the hole count - there are far less holes if the taper is 1:.4 against 1:16. With extreme reamer angles, which are required for bulky materials, the die thickness should be increased so the materials being compressed are held in the die for a longer period of time. Alternatively, well drilling can be used. A straight step is drilled on the inside of the die, the diameter being 9.5 nm (3/8 inch) for a 8nm 6

7 (5/16 inch) pellet. The straight section after the well or taper must be at least twice the depth of the well or taper. The die capacity can be considerably reduced due to the reduced number of holes. A taper or well is normally used when pelleting low protein/high fibre/low bulk density feeds. 4. A thick parallel bore die. This die will give the maximum number of holes thus increasing output. It would be suitable for high oil content feeds. Variable relief The outer three rings of holes from both sides are counter-bored from the outside to a depth of 6 mm (¼ inch). This feature has been patented by California Pellet Mill. It is claimed that by reducing the effective pellet length on the extremities of the die, the force exerted by the rolls is more even over the internal die face, thus giving a consistent product length. Fig. 5: Die characteristics Die speed is also important. For pellets within the range 3 nm (1/8 inch) to 6 m (1/4 inch) a periphery speed on the die of mm/sec. (2000 feet/minute) is ideal in most instances. For cubes within the range 16mm to 19 mm (5/8 inch to 3/4 inch) a periphery speed of about 6.35 mm/sec. (1250 feet/minute) will produce the best quality. Another point to watch is the correct setting of the rolls, if they are in contact with the inside face of the die, then the holes are likely to burr over. A gap of about 20 thou. is required. Metal It is essential that an efficient magnet is positioned just prior to the press to protect the die. Molasses addition It is not easy to add molasses to a cold meal so the best place to add molasses is at the conditioner. High pressure steam is introduced into the molasses line raising its temperature to about 93 C (200 F), so that it caramelises as it hits the meal. The steam must atomise the molasses and get its temperature up quickly, therefore the length of the pipe carrying the molasses from the point where steam is injected to that point where the molasses is released into the conditioner, is critical. It should be within the range mm (18-24 inches). Fig. 6: Molasses injection with steam Cooling 7

8 To get the maximum benefit from the cooler, the product ex the press should be sieved to remove the fines before going to the cooler. If a choice has to be made whereby only one sieve can be used then this should be positioned after the cooler. Ideally, the cooler should be placed directly under the press with no intermediate conveying or elevating. As the cooler both dries and cools through surface air exchange, the size of the cube/pellet determines the length of time it should be exposed to the air stream in the cooler. The larger the diameter of the product, the longer it takes for the heat and moisture to move from the surface where it can be removed. An efficient cooler should be capable of bringing the product to within 6 C (10 F) of ambient temperature. Selection of vertical or horizontal coolers are often dictated by space considerations. Horizontal coolers are advantageous on high molasses, high fat and urea pellets as these tend to stick or cake in a vertical cooler. Table 10 gives details of cooler retention time and air requirements for varying pellet and cube diameters. Table 10 Die Diameter Metric Imperial Cooler retention times and air requirements Retention time in minutes Air requirements per tonne/hour M 3 /min ft 3 /min 3mm 1/8" / / / / / / / / / / / Sieving Sieving is necessary after the press and before going to the packer or to bins and usually prior to delivery in bulk. Reciprocating sieves are often used and to remove fines from pellets a 8 to 12 wire Heavy Gauge screen would be satisfactory and on cubes a slotted perforated metal screen 7 or 8 mm x 28 or 30 mm. Vibratory screens are also well suited to this work and can have the following advantages. 1. A high capacity for their size (range 10 to 140 tonnes/hour) 2. Suitable for up to five divisions. 3. Low HP and maintenance costs. 4. Dust proof and quiet. 5. Sieves can be changed within about 15 minutes. Crumblers 8

9 Crumbs are used primarily in the poultry industry. They are reduced from a 4.75 mm or 4 nm (3/16 inch or 5/32 inch) cold pellet into small pellet particles usually known as crumbs, chips or crumbles. The crumbler is a rollermill with either 152 mm (6 inch) or 228 nm (9 inch) diameter rolls and a differential of 2½ to 1. The top roll has five flutes to the Inch and the bottom roll seven flutes. The flutes must be kept sharp. Some millers prefer the top roll to be cut longitudinally and the bottom roll circumferentially. Ideally, the roll should be positioned directly under the cooler and it is essential that the feed is spread evenly over the whole width of the roll. The pellet to the roll must be tough and hard - a soft pellet will result in excess fines which have to be reprocessed. The product ex the roll feeds a double deck sieve, the top deck for scalping and the lower deck for the extraction of fines, the overtails of this screen being the finished crumbs. The overtails of the top deck can be returned to the roll rather than the press. The fines, both from the sieve and the cyclone, should be returned to the front of the pellet mill feeder. The mesh size must be related to the type of product required but as a guide, a 7 wire on the top sieve and an 18 wire on the bottom sieve will, in most instances, give a satisfactory product. Automatic pellet mill controller For long runs at high capacity on the same product, an automatic controller will achieve maximum production with minimum attention. The system monitors mill load and steam addition to control the feeder speed. By using two temperature probes, the difference in temperature caused by the addition of steam is measured and the controller automatically positions the modulating steam valve to maintain the proper steam addition. Mill load is indicated in percentage of pellet mill motor load. The mill load is controlled by increasing or decreasing the feed rate In conjunction with automatic steam control. Feeder speed In controlled by driving an electrical variable speed drive using a speed control servo. Mill overload and roll slip cause the controller to stop the feeder and conditioner and close the steam valve. At the end of the run, the controller decreases the feeder speed and closes the steam valve. 9