Selecting optimal mixer options in feed milling
by Detlef Bunzel, Evonik, Germany
Dosing and mixing are two of the major procedures in feed milling. Getting them right can have a significant impact on feed quality and the cost effectiveness of the process.
The design of this process depends on the number and properties of ingredients, dose rates and required output.Feed production ingredients are usually mixed in a batch, due to the amount of ingredients and frequent changes in feed formulation. Even though the number of ingredients may vary, different groups can be classified:
- Main ingredients (soy, corn and wheat etc., typically >5% of the formulation)
- Minerals and major additives (limestone, salt, phosphorus etc., 1-5% of the formulation)
- Micro ingredients (amino acids, vitamins, <1% of the formulation)
- Medication (<< 0.1%)
Most of these ingredients (typically > 95%) are added as dry bulk. Dry bulk ingredients are weighed on scales per batch and the accuracy of scales depends on their weighing ranges. Therefore, individual scales with corresponding and appropriate weighing ranges are needed to weigh main, minor and micro ingredients (Regulation (EC) No 183/2005 of the European Parliament, Annex II).
Dry bulk ingredients are loaded into the mixer, after dosing and weighing, at the beginning of the mixing cycle.
Certain ingredients are added as liquid (oil, fat, molasses, water, acids and other additives, usually < five percent. These are dosed via flow meters or on scales and sprayed onto the dry mash during the mixing cycle. In order to ensure continuous operation of the pellet mills, the capacity of the batch mixing line will be defined based on the design capacity of the pelletising line.
Mixing line capacity
Production capacity of the feed mill is defined based on continuous processes in tonnes per hour. For the mixing line equipment, this must be transferred into batches per hour as mixing is a batch process.Each given output can be met by varying batch size and batch frequency. Batch size is defined in weight units as bulk batches are assembled on scales.
Nevertheless, equipment for transport, storage, dosing and mixing of these bulk batches is primarily defined by batch volume:
- Dosing equipment must be designed to dose the volume of bulk ingredients in the assigned dosing time
- Scale hoppers must be designed to accommodate batch volume according to their weighing capacities
- Hoppers before and after the mixer and the mixer itself must be designed to accommodate the total batch volume
- Mixers must be designed to allow for an appropriate filling rate at full batch volume in order to get optimal mix homogeneity within the assigned mixing time
- Conveying equipment after the mixer must be designed to convey the total batch volume within the cycle time.
While mixer sizes in feed mills range from less than one tonne (2,000 litres) to 10 tonnes (20,000 litres), mixing time may vary from around 60 seconds to four-ro-five minutes (or even longer) depending on mixer design and on quality expectations.
A paddle mixer may achieve a comparable level of homogeneity in less than 60 seconds, while it takes more than 200 seconds to achieve a good Coefficient of Variation in a double ribbon mixer.
Considering time requirements for mixer filling and discharging, total cycle times of two-to-six minutes may result. Consequently, approximately 10-30 batches may be produced per hour.
Time sequence of batch process
The time sequence of the batch mixing process can roughly be structured in four blocks:
- Dosing time
- Mixer fill and discharge times
- Mixing time
- Discharge time of bin after mixer
A close look at the time sequence improves the understanding of the batch process:
- Dosing time is shorter than mixing time;so, the subsequentbatch will be ready to be filled intothe mixer as soon as the mixer isdischarged
- The liquid addition system is designedto allow for sufficient mixing of drycomponents before spraying of liquidsbegins and provides for a final mixingtime after spraying stops
- The conveying line after the mixer isdesigned to discharge the batchfrom the surge hopper after themixer within the mixing time of thefollowing batch
Process parametersand batch size
Changing the batch size affects thesize of the equipment involved:
- Mixer sizeincreases proportionallywith the batch size, just as the outputper hour will increase, assumingconstant number of batches per hour
- Dosing timeincreases proportionallywith batch size and dosing equipmentremaining unchanged
- If mixingtime is shortened in orderto increase output (rather thanincreasing batch size) this will resultin shortened dosing time. To dosethe same amounts in a shorter time,dosing equipment must be upsized,and dosing accuracy will decreaserelative to batch size. Doubling thediameter of a screw feeder wouldincrease output and dosing errorapproximately by the factor four
- Weighing accuracy is related directlyto batch size. E.g., for a scale with3,000 digits resolution and with aweighing range of three tonnes, thesmallest read out will be 1kg. Witha weighing range of six tonnes theaccording read out will be 2kg
- Mixing time is much more influencedby mixer design (besides productrelated parameters) than by batchsize. With the same basic mixerdesign, shortening mixing time mayhave a negative effect on mixhomogeneity
- Spraying time– just as dosing time –increases proportionally with batchsize with the same spraying equipment.Liquid addition has its restrictions,as liquids can only be sprayed ontothe surface of the mash inside themixer, while batch size increases withmixer volume.
- Withbatch size spraying time will increaserelative to mixing time with negativeeffects on mix quality. Such effects arestronger with a modern mixer designand shorter mixing time.
- Fill and dischargetime is relativelyshort and isn't greatly affected bybatch cycle time. Generally, with anincreasing number of batches perhour, time losses by changing batcheswill increase in relation to totalproduction time.
- Another quality related aspectis the 'product carry-over'. Depending on equipment design and maintenance, whenever changingbatches, product from the one being discharged will remainand becarried over into the following batch.
While keeping output per hourconstant, there is a tendency for increasedcarry-over when batch size decreasesand the number of batches per hourincreases.A further consideration is that, with an increasingnumber of batches per hour, wear andtear will increase on parts thatare used when changing batches. These include slide gates, pneumatic pistons,and drives and electric drives.
Batch size and cycle time
The most critical consequence of increasingthe number of batches per hour is theshortened mixing time relative to batchcycle time.If the fill and dischargetime of the mixer is 30 seconds in total, then increasing the number of batch cycleswould have a negative impact onnet-mixing time.
With 10 batches per hour, 300 secondsor five minutes per hour would be needed tofill and discharge the mixer. 55 minuteswould be net mixing time to produce goodmixhomogeneity.With 30 batches per hour, 15 minuteseach hour would be needed to change batchesmeaning only 45 minutes would remain toproduce a good quality mixture.
Increasing batch frequency above15 batches per hour causes excessivedowntime for changing batches at theexpense of productive dosing, sprayingand mixing time.
Segmentation ofmixing time
In order to get good mix homogeneitywith a balanced batch cycle segmentation, it is important to considerthe order and timingin which ingredients are added. Mixing time starts after all dry ingredientshave been added.
When filling the mixer, macro ingredients should be added first.This ensures a good distribution in the mash. In somemixers, there are dead zones that don't mixclose to the bottom. These wouldfill up with micros if those were addedfirst. Losses of micros perbatch due to mixer gates not being 100 percent tight when closed will also be reducedifthey are added on top ofmacro ingredients.For micro ingredients, like amino acidsand vitamins, a good distribution as wellas exact dosing and avoidance of carry-overiscritical for high feed quality, to ensuregood growth performance and the healthstatus of animals.
Adding liquids too early within themixing cycle will affect the homogeneityof all theingredients. Once liquidsare sprayed into the mixer particle size increases, due to adhesionbetween liquid droplets and dry particles.
When adding fluids, water-soluble liquids should be sprayed before fat-soluble liquids. The fat-soluble liquids will coat thesurface of dry mash granules and thusprevent the absorption of furtherliquids. This means liquids will stayon the surface and form lumps and cakingon the mixer surface and mixing tools(paddles and ribbons).
Mixer filling rate
Mixer manufacturers generallyrecommend a filling rate of 70-to-85 pe cent.Specifically, with double ribbon mixers,it is important to note that the innerribbon must always be covered with mash. If the filling rate fallsbelow this level, the mixing dynamics ofthe inner ribbon is negatively affected,and product will accumulate to oneside of the mixer by the outer ribbon.
If mixers are overfilled, thedistance from the spraying nozzles to themash is diminished and the liquids will cover a smaller surface. Lumping will occur because the volume of liquidswill exceed the specific absorptioncapacity of the mash.With a low filling rate,liquids sprayed onto mixer tools andside walls will cause caking that willeventually be carried over into followingbatches.
Therefore, itis recommended a minimumfilling rate of over 60 percent is used, even thoughmanufacturers may claim that suchmixers perform well below 50 percent. The mash will only absorbthe liquids and keep mixer tools clean,if mixer tools are well covered beneaththe spraying array.
Working precision test ofthe feed production
Compound feed ensures animals aresupplied with adequate levels of energyand nutrients. In modern animalproduction, compound feed is also usedto apply prophylactic medical treatmentsto maintain animal health.
Consumers give food safetya lot of attention and, as part of the food chain, feed production hasto live up to certain standards and comply withbasic rules and best practices.Many countries have issued specificregulations with additional guidelinesfor the feed industry to safeguard thiscompliance. Traceability, hygiene andworking precision are the key wordsaddressed by these guidelines.
Different concepts have evolved tomeet customers' needs and to cope withregulatory requirements to not onlyproduce an adequate quality but alsoverify it on a regular basis. The focus of such concepts is to evaluatemix homogeneity of the batch mixer bymeasuring the 'coefficient of variation' ofa tracer added into the mash. For example, ourEvonik's AMINOBatch®Working Precision Test uses supplemental amino acids astracers to evaluate the coefficient of variationin the batch mixing process.
Dosing and mixing are core processes inthe feed mill with high impact on outputand feed quality. Conflicting aspects ofequipment and process design must bereconciled to find an acceptable balancebetween cost effectiveness and goodquality production.
With batch cycle times below fourminutes, the ratio between time neededfor changing batches and net mixing timegets out of balance at the expense ofdosing accuracy, mix homogeneity andprocess stability, causing higher risks forfeed quality.
The industry trend of speeding up thebatching process requires special attentionregarding the segmentationof the dosing and mixing cycle. Namelyliquid addition systems may become abottleneck: shorter batch time results inshorter mixing and spraying time. Thelimiting factor for liquid spraying is thecapability of the mash to absorb liquidsadded at high dose rates.
With liquid addition, it is very importantto check and maintain a good filling rateof the mixer. Otherwise, lumping andcaking on mixer walls and tools will result.
In feed production it is important tocheck working precision of the dosingand mixing process to ensure quality production.If well executed, such tests giveopportunities to identify optimizationpotential in the process and equipmentoperation as well as in maintenance.