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Pneumatic conveying of milk powder

Which pneumatic conveying systems to use for transporting milk powder ?

Milk powder pneumatic conveying : dilute phase and dense phase systems

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Section summary
1. Introduction : milk powder in industry ?
2. Pneumatic conveying systems for milk powder
3. Lean / Dilute phase pneumatic conveying of milk powder
4. Dense phase pneumatic conveying of milk powder

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1. Introduction : milk powder in industry ?

Milk powder is a widespread food commodity but also a raw material used by industrials, representing a market of more than 30 billion USD worldwide. It is produced through a process that combines evaporation (concentration) and spray drying by processing raw milk. Milk powder is then sold as such (it can however be mixed with some minerals and vitamins prior to packaging, for example for producing infant formulas) or can be used by industrials to manufacture other products, like cheese, cereals mixes, chocolate...etc... For industrial producing or using milk powder, it is thus key to be able to convey large quantities of milk powder reliably, the only technology that allows to do that is pneumatic conveying.

This page is reviewing how milk powder can be conveyed pneumatically.

2. Pneumatic conveying systems for milk powder

Pneumatic conveying presents many advantages that make it a good choice for transporting milk powder : it can reach high capacity, can convey over relatively long distances and with a flexible pipe layout, allows to contain well the dust as well as dust explosion risks.

In a pneumatic conveying system, air is blown on one side of the process and the mixture gas + powder flows in conveying pipes until the its final destination. There are however different ways to blow the air and introduce the powder in the conveying pipe. As a consequence different transport technologies have been developed over time : typically dilute phase and dense phase conveying.

Milk powder relatively fine (< 200-300 microns) and can be suited for both types of pneumatic conveying, the decision on the technology being taken depending on the process requirements for an application.

3. Lean / Dilute phase pneumatic conveying of milk powder

When can it be used for milk powder conveying ?

A very common way to transport powder is to transport it in a dilute phase. The powder is diluted by the air which means that the ratio (kg product/kg air) is relatively low. The product is transported in the form of a suspension in air in the conveying pipes. The transport can be done by pressure or by vacuum (shorter distance).

This technology is very often used for milk powder, particularly for factories receiving milk powder as an ingredient for their products (for example when mixing with cereals, or to produce ready to drink beverages) in bags or Big bags (Super Sacks). The factory operators must then install the following equipment :

• Bag tipping station, or Big Bag tipping station
• Sieve and magnet to remove foreign bodies
• Product infeed to the pneumatic conveying line, for example with an airlock rotary valve for pressure conveying (the airlock rotary valve may be blow-through if the milk powder is not flowing well)
• The conveying system
• A receiver (a hopper / silo) equipped with a filter and explosion protection devices if the need is pointed out by a dust hazard analysis (DHA)

Figure 1 : typical milk powder conveying system

4. Dense phase pneumatic conveying of milk powder

When can it be used for milk powder conveying ?

Milk powder can be conveyed in dense phase which means that the transport is done at a much higher concentration of the product.

In pressure dense phase, air must be compressed at higher pressure than in lean phase. It can be done by installing a specific compressor, or simply by using air from the compressed air network of the factory. Pressures reached will range from around 1 bar g to several bag g. To introduce the powder in the transport pipe, some tanks, designed to withstand pressure, are often used. They are called pressure tank or pressure sender hoppers.

 Figure 2 : Simplified flowsheet of a dense phase pneumatic conveying system for milk powder after spray dryer and fluid bed

Dense phase conveying is often used to convey materials sensitive to breakage or sensitive to demixing. This technology is thus very often used by producers of milk powder that will package and sell directly the milk powder after production. The milk powder, after spray drying and usually after having gone through a fluid bed dryer (to dry further and cool the powder) is conveyed in dense phase to filling machines located nearby. The granulometry of milk powder is indeed very important to ensure a good reconstitution of the product when mixed with water. A too high breakage of the material in dilute phase conveying can lead to the production of fines that will make the dissolution of the powder difficult, implementing a dense phase pneumatic conveying line which is conveying the powder at reduced velocity is then preferred since it will limit the particles degradation and keep the original PSD as much as possible.

Another advantage of dense phase pneumatic conveying systems of milk powder is to convey high quantities of product with small volumes of air thus small energy requirement. This technology is thus interesting to consider for very high capacity lines (>10-15 t/h), which is often the case with spray dryers producing milk powder.