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processed. The composition of the material and the particle size distribution of the feed both have a strong influence. The film resistance: This relates to the External Resistance for Mass Transfer and/or Heat Transfer to the material being processed. The type and the uniformity of the mixing within the system have the greatest influence upon this characteristic. For most food/feed materials, the rate of heat transfer (measured

via the Thermal Diffusivity) is much faster (typically around 100 times faster) than the rate of mass transfer (measured via the Mass Diffusivity). As a result of this, the Preconditioning process is described as being a “mass diffusion-controlled process” – the particles heat relatively easily, so the operation limits tend to be determined by the need for hydration/wetting. So for a chosen formulation, we should maximize the mass transfer

(hydration) capability of the system. The following points should be considered: • To the extent possible, ensure particles are small and have a uniform particle size distribution (the mean particle diameter and the standard deviation should both be as small as possible) • Ensure an intimate contact between the steam and the particles (ensure a good mixing action) • Ensure adequate residence time in the preconditioner, with narrow residence time distribution (see following for discussion of these aspects). While heating may be considered “easier” to achieve, it is still,

of course, critical. To ensure heating is carried out effectively and efficiently:

• Ensure the quantity of steam is sufficient (but not excessive, which is inefficient and can cause other problems). For preconditioners operating at atmospheric pressure, target discharge temperature of 85 to 95 o

preconditioner, to ensure consistent high quality (dry) steam – variations in the quality of the steam will cause variations in temperature and/or moisture content. Figure 1 shows the results of measurements of degree of cook

for various residence times in a preconditioner (for the same discharge temperature and moisture content in each case). Note that initially, increasing the residence time has a major effect on cook, but then the curve flattens out. As a rule of thumb, a mean residence time of at least 60 seconds is recommended. Residence time of more than 90 seconds will deliver very little additional benefit.

Residence Time and Mixing in the Preconditioner We have previously illustrated the importance of sufficient Mean Residence Time. Mean residence time in a preconditioner can be determined quite simply:

Mean Res. Time = Volume of Product in Chamber/Volumetric Throughput Rate.

That is, the Mean Residence Time for a given preconditioner (and

configuration of that preconditioner) is inversely proportional to the throughput rate. Thus, with other things unchanged, the cook achieved in the preconditioner will reduce as the rate is increased – expect the product to change as production rate is increased. We will discuss later the type and configuration of the preconditioner and their impact on residence time. Until this point we have focused primarily upon the Mean

Residence Time. In reality, the system behaviour cannot be described by a single value, rather the Residence Time Distribution (or RTD) must be considered. A distribution of times stems from the non-uniform flow of material through the system. This type of behaviour is typically represented graphically (see Figure 2).

C is practical. If you try to

achieve higher temperature, it is likely that uncondensed steam is exiting through feed inlet, which is both wasteful and can condense there, causing blockage. (If a temperature greater than 97 o

C is being achieved, then it is likely that uncondensed

steam also exists in the product stream, and this two-phase flow is likely to cause problems in the extruder.) • Design the system with a condensate trap just prior to the

Figure 1: The relationship between the Degree of Cook and the Mean Residence Time

Figure 2: Residence Time Distribution Curve

The data is often also represented as an “F(t) Curve”, where the

“Fraction of material that has exited the system at time t” (F(t) is plotted vs. “t”. This is shown in Figure 3. Obviously, a preconditioner with a poor RTD will cause non-

uniformity in the output – material which exits the preconditioner too quickly will be poorly hydrated and undercooked. That is, the RTD should be as narrow, and the Mean Residence Time as long, as practicable.


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