The future of feed: Extrusion technology (part 1)

by Emily Wilson
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The act of forcing something through an orifice is defined simply as "extrusion." The accompanying actions, which constitute a continuous process line, define extrusion processing.

For many animal and pet food applications, a result of that processing is the transformation of raw materials into cooked and formed products. The extent of the cooking reactions depends on a number of factors, especially the method in which each process in the line is operated. A cooked product can include benefits such as enhanced durability, stability in an aqueous environment and improved palatability, digestibility and animal performance.

Extrusion cooking technology refers to screw-type extruders. Inherent in the design of these extruders is a continuous flow of material from the feed section of the screw to the discharge end of the screw. Other continuous systems often are employed upstream in the process flow to supply the extruder screw with feedstock materials. Feeding or delivery systems and conditioners usually are used in the process flow and have become standard equipment for many extruder manufacturers.

Feedstock for an extrusion process line can include particulate solids, powdery materials, liquids and slurries. Use of quality raw materials, proper mixing and particle size reduction practices can be advantageous to achieve process and product uniformity.

Knowledge of the proximate composition of the raw materials often is necessary to meet nutritional composition requirements and to make process adjustments. Unknown differences or alterations in the functionality of raw materials prior to extrusion can result in undesirable processing behavior or finished product quality.

Screening or particle size reduction ensures a consistent distribution of particle sizes to the process. As with most processes, better control of the input materials allows for more successful control of the output.

Consistency in the finished product does not depend solely on the input streams but also on the rate at which those streams are metered into the process flow. Gross variation in an input stream can cause large and unacceptable changes in bulk density of the finished product. Moreover, accumulation of raw materials in any of the process flow operations can lead to blockage and downtime in the extrusion process line.

Typically, feedstock material contains most of the dry ingredients and possibly some minor liquid ingredients, which have been properly blended prior to placement in a feed hopper or bin. In a bin, a live or agitated bottom often is used to achieve a more uniform mass flow of material into the process flow. The live bottom prevents bridging above the bin's delivery screw or screws.

For a more consistent flow of material, a certain feedstock level or range may need to be maintained in the live-bottom bin. This is especially true for volumetric feeding systems and some gravimetric or loss-in-weight feeding systems.

Proper use, calibration and tuning of a loss-in-weight feeding system for a given formulation are needed for product consistency. Volumetric systems may have a narrower range of levels for operation and cannot offer direct mass flow rate information for process monitoring and control. Variations in the feedstock rate may be observed at the die as a pressure range, especially on a single-screw extruder operating with a starved or incompletely filled extruder screw.

The delivery screw drag conveys the feedstock to either the feed throat of the extruder barrel or an intermediate unit operation, often referred to as a preconditioner. In general, this type of conditioning is a continuous operation whereby the feedstock material can be mixed with other dry materials, liquids and condensable gases.

Continuous conditioning systems are available on the market as either atmospheric or pressurized systems. Pressurized conditioning systems are not as popular as atmospheric units partly because of their higher cost. Conditioner costs also can be higher for units with variable speed motors.

The particulate flow behavior within the conditioner can be modified by varying the shaft rpm. Depending on the conditioning system, a low rotational rate can resemble a stirred bed, whereas a higher rate behaves like a fluidized bed.

The average residence time distribution and the amount of heat and moisture incorporated into the feedstock are dependent on the feedstock and the process parameters of the preconditioner.

A majority of the processing options, such as adding water, steam or oil, add mass to the feedstock stream and alter its composition. Moreover, control of these set-points may be necessary to achieve product consistency.

Estimating changes to the conditioned mash with respect to the compositional effect on the thermal properties, such as the heat capacity, may be warranted. In addition to the composition of the feedstock, process conditions affect the extent to which reactions such as gelatinization or protein denaturation occur.

Similar reactions occur in an extruder barrel and can be calculated as part of an overall energy balance. The barrel contains the "heart" of an extruder or extrusion cooking operation — the pumping or transport action derived, in part, by the rotation of the screw or screws inside the barrel of the extruder.

Heat transfer from an optionally heated or cooled barrel section may make only small positive or negative contributions to the overall temperature. This is especially true for larger extruders with greater ratios of volume to heat transfer surface area. Heat transfer from or to the interior barrel surface may influence the properties of the material at or near the barrel wall.

Moving past the discharge end of the screw in the direction of the overall desired flow, the transformed mass of material enters the die assembly. The die assembly can be as simple as a single orifice that influences the final shape of the emerging extrudate.

For greater flexibility in process design and reduced cost, a die insert can be changed with another insert to create a new extrudate shape rather than replacing the entire die assembly. Multiple orifice dies can have a number of die inserts and often are used on larger extruders for greater mass throughput. The die offers the last resistance to flow in the process.

A common objective of many extrusion cooking operations is the creation of a pumpable mass just before the die. Raw materials enter into the extruder, are conveyed through the barrel and are transformed eventually into a melted mass somewhere near the die.

For some extrusion cooking and forming operations, a melted mass can recover from the constrained region of the die upon exit and enlarge, with no cellularity discernible to the naked eye. This phenomenon is referred to as die swell.

With the aid of a cutting device at or near the die face, distinct extrudate pieces can be sliced from a continuous strand of material streaming from the die. The mass throughput, velocity distribution, cutter speed, cuts per revolution or cycle and the longitudinal expansion of expanded products combine to define the length of the extrudate pieces.

For sticky products, cutting can be performed with an alternate cutting device moments later downstream after the product has cooled. The stickiness can be formulation dependent or a result of process shear and discharge temperature. For amylose stickiness, monoglycerides often are used for complexing with this starch fraction, reducing the undesirable tactile properties of the hot extrudate.

Success of extrusion processing depends largely on the configuration and rotation of the screw or screws. The mechanical energy for screw rotation originates with the extruder drive motor.

Under varying loads caused by the transport and transformation of the feedstock materials, the drive system attempts to maintain a constant screw rotational rate of the screw. During the transformation of feedstock material into a pumpable mass, material and flow properties develop and change at various, but specific, locations along the length of the rapidly rotating screw. These locations are determined usually by design and achieved by incorporating restrictive devices such as shear or kneading blocks in the screw configuration.

The shear at any one location is compounded along the screw and die configurations. The rate of shear imposed on the material also may be of interest. For example, with regards to screw speed, a 100-mm diameter screw rotating at 500 rpm would have a maximum tip velocity of 157,000 mm per minute, whereas a tip speed of 314,000 mm per minute could be attained at 1,000 rpm. Both of these screw speeds are in the realm of modern extruder capabilities.

High rates of shear would be associated with these processes. Assuming no slippage of the material at the barrel wall, the high rates of shear in the screw may not be desirable for all processes.

Excess fragmentation of large molecules such as starch and protein in the feedstock can result from the high shear forces, elevated temperatures and pressures involved in this type of extrusion. Resistance to flow often is associated with friction and an elevation in material temperature. Thus, viscous dissipation of the mechanical energy from the rotating screws accounts for some of the temperature rise in the material, reducing the viscous behavior or altering the rheological properties of the melt.

Each piece of equipment, along with the type of material processed and processing conditions, influence the quality of the finished product. Changing any one of these often can affect the characteristics that define the product. For instance, variations in feedstock rate can influence product bulk density.

In principle, enough changes can occur given enough time to establish a new steady operating condition, which would cause no distinguishable differences from some of the observed or desired product attributes. It will be interesting to see how our understanding of extrusion cooking evolves with the introduction of new sensing technologies, especially those that incorporate rheological characterization and analytical capabilities.

All extruders are not the same and should not be expected to operate the same way. Therefore, an understanding of the operation of the various extruder types, whether single-screw, self-wiping co-rotating twin-screw, closely inter-meshing co-rotating twin-screw or counter-rotating twin-screw, will facilitate selection of the more appropriate operation for the task at hand.

Next month: Single-screw versus twin-screw extruders.

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