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Digital Re-print - July | August 2010 Factors affecting pelleting and energy consumption

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The prosperity of industrial feed compounding is based on the refining effect of mixed and

processed raw materials, especial-ly since the introduction of the pelleting process in the 1920s.

Compound feed complies with different quality attributes like hygienic and nutritional physiological aspects, technical characteristics, and environmental compatibility. Despite rais-ing additional costs for equipment as well as for electrical and thermal energy consumption, currently up to 70 percent of the compound feed produced in Europe and in the USA, are pelleted.

The requirements with respect to pellet quality, especially durability, differ according to the animals to be fed, to the kind of live-stock breeding and in some cases to the demand of the farmers too.

Pellets should be hard and resistant to all demands placed on them from the various stages in the production process to the feeding trough, be as dust-free as possible and be of good appearance in colour, texture and surface without fractured areas which may cause abrasion.

Advantages of pelletingPelleting compound feed offers sev-

eral advantages compared to just ground, mixed mealy feedstuff.

The hectolitre weight (tap density) of the bulk material is raised to some extent, so lower transportation and storage volume is required.

Generally, pellets are free-flowing; arching in bins and silos is reduced.

Emptying and dosing is quite easy with no segregation arising within the product.

Contamination risk from bins and hoppers or even conveying equipment is of minor inter-est because of the very small amount of resi-dues that arise when transporting stable pellets.

Heat treatment during the pelleting process

reduces microorganisms, the overall hygiene state is improved and storage periods may be extended - up to storage conditions, decay is deferred.

Benefits are not only given with respect to transport and handling but also to livestock management. Feed composition is guaranteed even in small units, all components of the mix-ture are incorporated homogeneously in the pellet, ensuring an even supply of ingredients and nutrients, selective feeding is impossible.

Due to market conditions, the composition or components may be changed without the risk of the feed being refused by the animals. Palatability is also improved. The integrated thermal process modifies starch and creates a certain taste and smell which, by the way, may be covered by special additives in order make the feedstuff distinctive.

Digestibility is increased, thus the nutrient utilisation is enhanced and there is less feed wastage.

The efficiency of the pelleting process is measured by a large production throughput at low power requirement and an output of pellets with low abrasion index, fulfilling high quality aspects and resulting in almost low specific energy consumption (kWh/t).

These are conflicting objectives of high demands and it is difficult to meet the require-

ments at same time, especially regarding the fact that almost 80 percent of the total costs are related to raw material input and a mere of 20 percent is on hand with respect to technical exercise of influence.

The production of firm and stable pellets requires high pressure implying high energy consumption and/or reduced throughput. Fewer throughputs stand for extended reten-tion time under pressure and thus produce better compaction.

The specific energy demand rises anyway. But of course, pelleting is not just a matter

of pressure and energy input. The pelleting process can be divided in

three main procedures:- conditioning the pre-processed com-

plete feed meal (complete except for sensitive micro components to

be added on top or end of line)- compacting and consolidat-ing the mash into pellets- cooling the moist and hot pellets to a storable and conveyable product

Besides technological items material properties are of important impact. Next to density, the particle size distribution is of special interest because it affects the pelleting process significantly: from both theoretical and practical experience it is well known that fine structured material almost allows trouble-free production of firm pellets, but causes high energy consumption for milling.

Coarsely grounding feed, at the recent request of nutritionists, tends to produce less stable pellets, because bigger particles pre-determine breaking points within the pellets, creating more cross sectional areas within the pellet and thus more abrasion.

Depending on material’s properties (hard-ness, brittleness or even flexibility) those par-ticles are crushed between rollers and die consuming more energy and causing probably more wear.

Factors affecting pelleting and energy consumption

by Rainer Löwe, Research Institute of Feed Technology of IFF, Braunschweig-Thune, Germany

Figure 1 illustrates a typical but simplified flow diagram of a feed production plant

Grain&feed millinG technoloGy14 | July - august 2010

FeatureEnergy consumption

The original (primary) particle size is changed.

This additional operating expense has to be set in relation to less energy input during milling. Commonly, the breakdown of energy consumption of compound feed production is 60 percent for pelleting and about 16 percent for milling (see Figure 3).

Although these figures are not statistically affirmed and based on feed producer survey in Germany, frequently asked coarser mixtures may cause a different ranking.

Factors affecting pellet stabilityFeed ingredients influence pellet stability in

different ways. While – depending on moisture content

and temperature – starch and protein may cause more stable pellets, fat as a lubricant reduces friction in the die holes and leads to weaker pellets.

Increasing the fat contents leads to partial coating of the feed particles, which prevents the penetration of the steam and thus the develop-ment of binding agents – moisture bridges as well as the gelatinisation of starch [Reference 1].

While formula-tion and preparation are beyond the press operator’s control, he is left with a number of other parameters to exercise influence on the pellet mill operation and hence on the product quality.

It is evident from the large number of influ-encing factors that there can be no standard for the plant setting.

The pelleting system has to be adapted to actual requirements by means of adjustment and it has to cover a very wide spectrum.

Manipulation is possible through die and roller selection, rotary speed, gap width and consequently pressure conditions, but also by throughput and steam regulation, always tak-ing due account of the motor load or current intake.

Longer die channels increase the prob-ability that binding will take place between the feed particles so that – supported by enhanced frictional forces – the pellet durabil-ity is adequately improved. The same effect

is reached when using small bore-hole diam-eters.

Normally, the energy consumption will rise linearly, while the pellet stability tends towards a final value the longer the die holes are.

A considerable influence on pellet stability takes the press throughput.

For fixed die dimensions and a given number of die holes, the load of each hole increases with higher throughput: the power intake rises while the pellet-stability decreases. This can be explained by the minor compac-tion of the thicker layer pressed into the holes at a time [Reference 2].

Double-stage pelletingAnother important factor is the precom-

paction of the feedstuff before entering the die

Table 1: Effects due to different feed processing measures

EffectShort-term

conditioning and pelleting

Long-term conditioning Expanding Extruding

Structure changes low no high very high

Hygienisation low very high very high very high

Matrix changes low low high very high

Grain&feed millinG technoloGy July - august 2010 | 15

Feature Energy consumption

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steam. Adding as much steam as possible dur-ing pre-conditioning is thus desirable.

Depending on absorbability and other fluids within the mixture, and taking into account that 0.6 percent of saturated steam will increase process temperature by approximately ΔT = 10 K (Figure 4), minimum content should be approximately three percent (pelleting) to five percent (expanding).

However, steam input is limited due to pressureless operation of the pre-conditioner by the maximum possible feed temperature and moisture with respect to pelleting (lubricat-ing effect) and storage stability (moulding).

To ensure thermal energy is used efficiently, loss minimisation and the maximum levels of steam that can be added need to be consid-ered. Measures for saving thermal energy are, for example, sufficient boiler pressure to allow a drying phase by pressure reduction directly in front of the conditioner, accompanied by an adequate number of steam traps, insulation of all steam pipes, conditioners etc and of course stringent avoidance of leakages.

The grinding processWith respect to grinding processes, the

specific energy requirement is determined by material properties, the required particle size reduction, the comminution equipment and the process and plant layout.

Generally, particles should be only as fine as

necessary and as coarse as possible, since the energy requirement rises exponentially with increasing fineness [Reference 5].

Measures to discharge the mill-motor load, such as the preliminary separating of fines, closed-circuit processes or multi-level size reduction, can achieve energy savings of between five and 25 percent.

Frequency controlled drives allow selec-tive circumferential speed to realise roughly aspired structures and particle size distributions.

establishing bonds between feed parti-cles improving pellet stability. However increasing the amount of steam is limited because of crossing admissible moisture levels reducing the friction in front of and within the die channels.

Factors affecting energy consumption

Considering potential energy savings, specific conditions of industrial compound feed produc-tion need to be taken into account. Increasing customer focused production of small batches as well as intensified use of hydrothermal-mechanical treatment (in consequence of highly demanded hygiene safety) ran up costs for energy consumption (Reference 4).

To assess the energy efficiency of a proc-ess, its specific electrical energy requirement (kWh/t) should be related to effect-specific energy consumption, for example with respect to the refinement process, refinement targets and capacity utilisation.

This would allow general conclusions regarding specific processability.

When comparing different sites within one company, the balance sheets should be checked as to whether initial conditions are in fact comparable. An assessment always needs to consider not only energy consumption, but also investments and amortisation, mainte-nance, wear and tear and the cost of spare parts [Reference 4].

Influences on energy consumption are manifold: plant diagram, processing capacity and its utilisation are essential to realise aspired processing aims.

Handling hundreds of formulations and producing batch sizes between 1.5 tonne and 50 tonne is economically challenging. Besides structure of components and mixture formulations technical condition of machinery and equipment as well as maintenance affect energy consumption.

Last but not least technological disci-pline and education of the staff take effect (Reference 4).

Besides material, machine and process parameters, the processing aims determine the energy input: hygienisation efforts, reduction of antinutritive substances, decomposition of certain ingredients, pellet stability.

It is essential to understand that the higher the targeted processing level, the higher the energy input. Qualitative effects of refinement processes are shown in Table 1.

In refinement processes, part of the electri-cal energy inputs can be provided by cost-effi-cient thermal energy in the form of saturated

hole – caused by double-stage pelleting, the gap width between roller and die (automatic gap control) or the roller speed. The higher the speed (or the number of rollers), the smaller the material plug being pushed through the bore, the better the compaction inside the plug.

The addition of pelleting aids can bind, for example fatty contents, by increasing the surface area; this may reduce pellet abrasion. But the effect of auxiliary agents depends to a high extend on feed mixture recipes. Friction reducing tensides or surface active agents may improve pelleting behaviour, too.

By facilitating the easier transfer of heat (vapour) through the meal, which is con-ditioned more intensively, surfactants may reduce pellet press energy consumption.

Another effect is the possible reduction of undesired moisture loss (shrink) which under certain conditions may give support to raise the production rate [Reference 3].

Finer grinding of the compound-feed com-ponents leads to larger specific surfaces and thus to more favourable conditions for the absorption of liquids and the effectiveness of the saturated steam and results in more firm pellets.

Application of the saturated steam creates more favourable conditions for

Figure 2: compiles compacting affecting factors

Figure 3: Breakdown of energy consumption

Figure 4: Increase of temperature when conditioning compound feed by means of

saturated steam

Grain&feed millinG technoloGy16 | July - august 2010

FeatureEnergy consumption

References[1] Heidenreich, E. - Löwe, R. -

Pelleting technology: looking to the future - International Milling Directory 2000, pp. 55-60

[2] Löwe, R. - Judging pellet stability as part of pellet quality - Feed Tech 9 (2005), pp. 15-19

[3] Birchmore, J. - Surfactant’s benefits to feed manufacturers - Feed Magazine/Kraftfutter 3-4 (2010),pp. 17-21

[4] Feil, A - Is it possible to cut energy costs for the production process? - IFF colloquium Braunschweig May 2005 International Research Institute of Feed Technology

[5] Löwe, R. - Vermahlungstechniken für Futtermittel - Tierernährung für Tierärzte, TiHo Hannover April 2007, Proceedings pp. 33-34; Editor J. Kamphues and P. Wolf

Heat recoveryAdditionally possibilities for heat recovery

from production processes for heating circuit water for social rooms or offices should be reviewed.

Because of low enthalpy and possibly hygienic contamination direct use of thermal energy from cooling or drying air for raw or feeder water is not recommended. A benefit from this energy recovery would rather be a reduction of odour emissions. Checking the possibilities of combined heat and power cycle may be useful.

The total energy requirement of a feedmill is decisively determined by plant layout, for-mulation texture, type and technical state of the equipment, plant capacity, load factor and technological discipline among the staff.

In addition to the measures mentioned, the prevention of energy losses (next to steam accommodation pressure-air supply) and the use of low-cost electricity rates basically re-present further optimisation potentials; in view of energy costs developments, these should certainly be taken into account.

Development trends include larger designs for hammer mills, and the increased use of hammer mills with vertical rotors and roller mills with two or more assembly levels (roller pairs with different gaps), either instead or combined.

Both machines are preferable in terms of energy use, but with roller mills material-specific processing restrictions need to be taken into account.

More information:Website: www.iff-braunschweig.de

Figure 5: Influence of saturated steam addition on energy requirement, pellet

durability and process temperatures

Grain&feed millinG technoloGy July - august 2010 | 17

Feature Energy consumption

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