Specific energy input

The specific energy input is the most important characteristic value. It is characteristic to each product and applies irrespectively of the size of the extruder. Generally, only measurable value is the specific mechanical energy input Pslx,tif lnCci, via the screw shafts. This is calculated using the following equation  

Specific energy consumption may be expressed as the ratio of the required power input to the effective material throughput and is expressed in kWh.kg [41]. Since mixture uniformity is sensitive to specific energy input, measurement of this parameter can provide an indirect indication of overall product quality for a particular compounding process. 

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Figure 8.7 Mean particle size versus specific energy input for different feed point positions if.p.p.) (CaOx, Rushton turbine, 40 min feed time, total concentration 0.008 M.) After Zauner and Jones, 2000b)...

The conventional scale-up criteria scale-up with constant stirrer speed , scale-up with constant tip speed and scale-up with constant specific energy input are all based on the assumption that only one mixing process is limiting. If, for example, the specific energy input is kept constant with scale-up, the same micromixing behaviour could be expected on different scales. The mesomixing time, however, will change with scale-up as a result, the kinetic rates and particle properties will be different and scale-up will fail. 

Vitamins, microorganisms, and enzymes are susceptible to inactivation or destruction in an extruder. Removal of microorganisms and enzymes is desirable in most cases, but vitamin retention is important for nutritional considerations (Bjorck and Asp, 1983). Survival of vitamins increases if moisture is increased and if temperature, screw speed, and specific energy input decrease (Killeit, 1994). Vitamin loss may be compensated by adding more than the necessary amount of preextrusion or by applying a vitamin coating, filling, or spray postextrusion. 

E specific energy input from the screw to the resin in J/g ... 

Figures 4.20 to 4.22 show the relationship between screw speed, throughput, specific mechanical energy input, and melt temperature. It is apparent that melt temperature and specific mechanical input increase as screw speed increases and decrease as the throughput increases (see Fig. 4.20 and Fig. 4.21), while the temperature increase in relation to the screw speed is greater than the temperature decrease in relation to the throughput. Therefore, according to the above equation, the melt temperature ultimately rises as the specific energy input rises (see Fig. 4.22).

Using characteristic values is a widespread and helpful means. An example for material characteristics is the specific energy input based measured in kWh/kg (see Section 6.5.1), while the available torque/axis distance measured in Nm/m3 is a machine characteristic. Dimensionless parameters (with the unit 1) play an important role for scale down/scale up considerations. The Reynolds number is a known dimensionless parameter, determining whether flow is laminar or turbulent. However, it is of little importance for extruder scale downs/scale ups. 

Measuring the drive power enables us to identify the specific energy input e=drive power/ throughput, which is also known as the specific drive power. This is a characteristic value for many processes and is also used for comparisons [1]. 

An enthalpy diagram also shows that the temperature increase in the example shown is directly linked to the specific energy input. Figure 6.6 gives the specific enthalpies for different products and converts them from kj/kg (left) to kWh/kg (right). In this example, the drive power of the extruder is converted into the dissipative product heat increase . 

The equations given in Fig. 6.5a and/or 6.5b can also be applied to an extruder section. In general, however, we do not know the power input to the shafts for an extruder section or, therefore, the specific energy input for this section. The pumping efficiency can be of use here it is determined in advance for the screw geometry in question (measurements taken with a model fluid are adequate, see Section 6.7) or determined by 3-dimensional calculations (as shown in Fig. 6.7). 

The specific energy input can be cancelled out by the pumping efficiency in the equation for the temperature increase in the back-pressure zone ATS, Fig. 6.8 a. The overall temperature increase is therefore obtained together with the temperature increase at the nozzle ATd-... 

Even without software support, the process engineer can still obtain certain predictions by a precise analysis of the processes involved. In this case, process-specific diagrams are very helpful. These illustrate, for example, the specific energy input (Fig. 11.9) or other quality-related characteristics as a function of viscosity, throughput, speed, or discharge pressure. With the aid of enthalpy (Fig. 11.10) as a physical, process-independent value, initial forecasts can be obtained as to the energy that will be required to melt a resin and to extrude at a specified end temperature. 

Specific energy input depending on melt index... 

The discharge pressure to be generated by the screws at the screw tip has a substantial influence on the specific energy input and ultimately on the melt temperature and product quality. It is extremely important, therefore, that it remains constant. 

Figure 15.3 shows the influence of specific energy input SEI on the WSA for an extruder with melt pump and the new ZSK-NT system. It is clear that the new processing system has benefits in terms of quality and homogenization without requiring higher energy inputs. 

FIG. 21-72 Influence of stress intensity on the size of limestone for a specific energy input of 1000 kj/kg. [From A. Kwade et al, Powder Technol. 86 (1996).]... 

Twin-screw extruders (with the exception of most counterrotating profile extruders) are designed to be starve fed. Therefore, throughput is independent of screw speed. This permits the processor to control residence time, degree of fill, and specific energy input (kw/kg). 

Conical mixers are also known as Nauta mixers (Fig. 18-53). Material placed in the conical bin is lifted by the rotation of the helical screw, which in turn is rotated around the wall of the cone. The lifting actions of the screw combined with motion around the cone provide bulk mixing for flowable dry powders, paste materials, and even viscous fluids. The specific energy input is relatively small, and the large volume of the mixers can even provide storage capacity The mixers may have multiple screws, tapered screws, and high-speed dispersers for different applications. At constant speed, both the mixing time and power scale up with the square root of volume. Sizes from 0.1 to 20 m3 (3.3 to 700 ft3) are available. 

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