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LANDSBERG - Milk powder with his own and other fats is produced in dairy plants and is used to make a variety of food and feed.
Picture: solids Disc valve in Hygienic Design
Tasks for the storage and pneumatic conveying arise both at the production in milk handling operation between production and shipping, as well as at the converter between delivery and processing.
Here it is about milk powder with high fat content. The investigation by Hausner (1) showed a very high degree of compaction from what results that the product has to be categorized in the deposited state as "non-flowing “ and that the assessment of storage capacity in silos requires further investigations.
Regarding the pneumatic conveying an assessment acc. to Geldart (2) and a classification in Group C was carried out, meaning cohesive, bad to not fluidised.
Hygiene requirements and operating conditions:
Primarily, there was to meet compliance with the Machinery Directive 2006/42/EC Annex 1 Clause 2.1 including food processing equipment as well as DIN EN1672 / 2 hygiene requirements.
Furthermore had to be followed the EHEDG guidelines as well as the GMP- and FDA recommendations. The entire system had to be free of dead space, completely emptied and easily cleanable. The cleaning was as far as possible, to strive as a dry cleaning by air flushing. Wet cleaning should have been possible because of the unavoidable greasy film. During storage, particularly during the pneumatic conveying, was to maintain the primary grain size, bulk density and appearance of the product and to avoid a certainly loss of quality.
To ensure the flowability and thus the storage capacity further investigations were carried out according to "Jenike" (3). Because of the influence of the fat content the investigations had to be extended to various storage temperatures and storage periods.
Higher fat contents lead to significant deterioration of the flowability. Decreasing temperatures in the product during storage result in an increase of the bulk resistance and thus lead to the deterioration of the flowability and storability.
At very high levels of fat occur in storage times of a few hours already bulk material strengths in the vicinity of the consolidation stress and thus flow without discharge aids is no longer possible. Higher temperatures worsen the wall friction values and this leads to distinctive adhesion affinity.
To make the process storage and pneumatic conveying despite the variety of dependencies controllable, a distinction was made in terms of fat content:
- Milk powder with a fat content of <40% and about 25° C, we classify as storable, respecting the required outlet diameter "no recirculation" to about 24 hours
- Milk powder with a fat content > 40%, <60%, we classify under otherwise identical conditions in "storable only with recirculation."
The higher fat content leads to a high time solidification. After just a few hours storage time a minimum outlet diameter that is larger than the silo diameter would be necessary.
In the recirculation (reconveying) of the bulk material is removed at regular intervals in the mass flow a little bulk and then refilled into the silo. The movement of the bulk material prevents excessive time solidification and allows a feasible dimensioning in compliance with the mass flow criteria.
To contain the influence of temperature the outside silos were constructed as double wall construction with insulation. For the interpretation of mass flow regulation, the required wall inclination of the conical part and its smallest still admissible outlet diameters were determined for the prevention of bridging.
All connections for feeding and outgoing pipes and fittings were carried out dead space free. For the support and assurance of the mass flow at a reasonable outlet diameter were provided vibration bin dischargers.
Storage in mass flow silo:
Our mass flow in hygienic design is characterized by:
Horizontal drop of the product level.
The product in the entire silo cross section is in motion during the discharge.
There are no dead zones, and thus no product residue.
The total mass has a uniform residence time and thus a uniform aging.
The premise of "first-in first-out" is upheld.
During the discharge over the entire cross-section occurs back mixing of the segregation that has probably happened during the filling process.
All connections for feeding and outgoing pipes and fittings are made free of dead space and centered.
Any kind of connection nozzle and inside corners and angles have been avoided.
For the support and assurance of the mass flow at a reasonable outlet diameter vibration bin dischargers were provided.
All surfaces which are in contact with the product have an average surface roughness of Ra <0.8m.
The connection of the vibration bin discharger to the vibrating mass flow takes place without a gap with a collar made of FDA compliant elastomer.
To meet the quality criteria in the pneumatic conveying, further experiments and practical tests have been carried out. It was confirmed that the product is not fluidised, has no air retention and, because of its cohesiveness, even a short accumulation of material (plugs) can cause in the conveying pipeline a complete line closure.
The conveying in dense phase with a velocity of < 6m/s, which is required to meet the quality criteria, is merely possible with a pressurized accompanying line system and compressed air feeds into the delivery lines at appropriate intervals.
The connection of the pneumatic conveyor occurs with centered connecting components with gap-free sleeves. The material inlet valve is a disc valve in hygienic design, easily disassembled and cleaned by a divided housing and gap-free connection by centering flanges and FDA-conform seal.
The pneumatic conveyor in Hygienic Design is absolutely free of dead zones, has gap-free centered mounting connections with FDA-conform gaskets.
Mass flow during emptying with vibration support and air flood cleaning minimize the need for cleaning.
For wet cleaning, the pneumatic conveyor with the entire conveying line system is CIP-able and piggable.
The conveying line in hygienic design with pressurized accompanying line system is made of stainless steel pipe in the hygiene class H3 with an average surface roughness Ra <0.8 m.
Centered clamp connections, shock-free, without gaps, pig-and CIP-able are minimizing the need for cleaning.
The slow conveying in plug flow, which is required to meet the quality criteria is realized with the help of the accompanying line system with ventilation facilities (so-called relay stations) in the conveying line.
The relay stations are easily to dismantle by centered clamp connections and include a filter unit made of micropore -stainless steel.
The cleaning of the filter unit is carried out automatically with compressed air from the pressurized accompanying line system.
The entire system is also CIP-able.
The flowability of the product is already influenced significantly by moderate fat content. For security reasons, unforeseen longer periods of storage or temperature variations, even when fat content is < 40% the reconveying of partial lots is recommended at regular intervals.
The high cohesiveness of the product requires a high surface quality of all materials which are in contact with the product, offset-free centered connections, no dead spaces and rigorous mass flow.
Complex investigations of materials for storage and transportability, partially under simulation of the operating conditions, led to processing and apparatus technical convincing solutions.
Hausner-Zahl nach Grandison, A.S. & M.J. Lewis, Ed. (1996) Separation in the Food and Biotechnology Industries, Cambridge: Woodhead Publishing Ltd.
Geldart, D., Types of gas fluidization. Powder Technol. 7 (1973), S. 285/292
Jenike, A. W., Storage and Flow of Solids. Bull. Nr. 123, Utah Engng, Exp. Station, Univ. Of Utah, Salt Lake City (1964)
Schwedes + Schulze, Ermittlung von Fließeigenschaften von Walzen-Sahnepulver im Auftrag der Firma system-technik GmbH Landsberg, 2010
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