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Pneumatic conveying of sugar

Which pneumatic conveying systems to use for transporting sugar ?

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Section summary
1. Introduction : sugar in industry ?
2. Pneumatic conveying of sugar
3. Lean / Dilute phase conveying of sugar
4. 4. Dense phase conveying of sugar

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1. Introduction : sugar in industry ?

Sugar is ubiquitous in food industry. It is one of the most used ingredient in all sorts of products, like pastries, yogurt, drinks... Even if sugar has been on the spotlight recently as some pointed out its effect on health, it is still produced in between 170 and 190 millions tons every year. It means that companies involved in the field of bulk handling processing for food industry will likely have to handle sugar at one point or another. Those industries thus need to transport large quantities of sugar in their plants, and one of the most efficient way to do so it to use pneumatic conveying. To help in designing installations or troubleshooting them, this page is proposing an overview of the pneumatic conveying of sugar.

2. Pneumatic conveying systems for sugar

Pneumatic conveying present many advantages that make it a good choice for transporting powders such as sugar : it can reach high capacity, can convey over relatively long distance and with a pipe layout quite flexible, 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.

Sugar can be suited for both types of pneumatic conveying, the decision on the technology being taken depending on the process requirements for an application, and on the actual nature of the sugar to be processed as it can be of different particle sizes.

3. Lean / Dilute phase

When can it be used for sugar 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 (5-10 max). The product is transported in the form of a "cloud" in the conveying pipes. The transport can be done by pressure or by vacuum (shorter distance).

This technology is very often used for sugar, particularly for factories receiving sugar as an ingredient for their products in bags or Big bags. 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 sugar 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 sugar conveying system in dilute phase

Sugar is also very often transported in trucks, which must then be discharged to a silo. The conveying is then done thanks to a compressor on the truck and with the truck driver manipulating the valves to control the flow of sugar. As the truck operator will try to reduce at maximum the duration of discharge of the truck, the conveying system operates at relatively high solids / air ratio which makes the conveying being in between dilute and dense phase (see below). The conveying line must be well sized (sufficient diameter) to make sure the conveying will not block and the filter of the silo must also be designed with sufficient area in order to accept the large quantities of air from the compressor, especially at the end of the conveying when the pressurized truck tank suddenly decompresses in the line.

When conveying sugar, 2 key parameters should be controlled : the humidity of the air conveying sugar and the temperature of the air.

Humidity : A high humidity in the air may indeed transfer to the sugar which can lead to the caking of sugar in silo, thus, when the air is taken from an uncontrolled environment, such as outside, a dehumidifier of the conveying air should be installed.

Temperature : The temperature can be a concern for pressure conveying as, if the pressure drop is high in the line, the temperature at the outlet of the compressor will start getting high. Temperatures higher than 65-70c can create a softening of the sugar, which can lead to caking. It can even be a safety hazard if the temperature is so high that the sugar starts to burn. For this reason, vacuum conveying has many advantages when applicable (the air temperature is just the ambient air temperature when it enters the conveying pipe), and when pressure conveying must be applied, operators should at least monitor the temperature of the air and the pressure drop so that interlocks can be triggered if it gets too high, or, if the system is designed to work at relatively high pressure, an aftercooler should be installed. When using an aftercooler, it is highly recommended to dehumidify the air, as the cooler can create condensation and a mist that would be blown to the sugar.

4. Dense phase

When can it be used for sugar conveying ?

In some applications, it is required not to break the sugar granules, for example when the sugar is used as ornament on a pastry. For this purpose, it is possible to convey sugar in dense phase. Dense phase allows indeed to convey at much lower velocities than dilute phase. As the attrition of particles increases to the power ~2.5 of the conveying velocity, the breakage of the powder is much reduced.

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.

5. Storage of sugar in silos

The storage of sugar in silos must be particularly well studied. Indeed, sugar can create different problems, especially dust explosion which causes a significant safety hazard, and caking in the silo making the flow of the material difficult.

5.1 Sugar dust explosion

Particularly when hoppers and silos are located at the end of a pneumatic conveying line, dust explosion can happen as a dust cloud of sugar is often present during and after the conveying. One must also be careful to the risk of cone discharge explosion in large silos.

For this reason, as for every process handling bulk materials, a dust explosion risk analysis must be done on the pneumatic conveying system and the receiving silo. It is probable that protections, such as explosion panels, and some design adaptation (such as not exceeding a certain diameter for the silo to avoid the risk of cone discharge explosion) will be required.

5.2 Sugar caking in silo

Another risk to consider, this time for quality and process performance reasons, is the caking of sugar in the silo. Indeed, if sugar is submitted to a humid air, liquid bridges can form in between the crystals of sugar, leading to lumping. This phenomena can be avoided by conditioning the air in the silo at a relative humidity around 45-50% RH, and by isolating the silo to avoid strong variations of temperature (going below the dew point of the air in the silo can lead to a condensation, the water will then slightly dissolve sugar, and when the temperature increases again water will evaporate leaving a strong solid bridge in between the sugar crystals).