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|1. Sugar flow
problems in silo
|2. Root causes of
sugar lumping ?
|3. How to solve
sugar flow problems in silos ?
|4. Dust explosion|
Crystal sugar is very often stored in large silos >30 m3 that are supplied by trucks or railcars. Many sugar lumping, caking, more generally flow problems out of the silo are however experiences by factories. This page focuses on the root causes of sugar flow blockages in silos of factory operators receiving sugar from a supplier, and how to solve and prevent them. This page is not dealing with conditioning silos used by sugar producing factories (refineries) although some of the concepts presented can also be applied to those particular silos.
The most widespread issue reported by factories around the world operating sugar silos is the caking (lumping) of the sugar stored which leads to flow problem, quality problems, reduction in storage capacity and factory output, and in some cases, safety problems.
Sugar, in certain conditions, has the ability to form lumps, which can harden and get bigger with time. In extreme cases, blocks of several dozens of cm can be found in silos, typically along the silos walls. This hard sugar cannot easily be taken out of the silo. The presence of these large lumps is reducing the useful volume of the silo, can lead to blockage if lumps are falling down and obstruct the outlet of the silo. In certain cases, if a very large block suddenly falls, it can even damage the structure of the silo when impacting the cone.
Figure 1 : example of the presence of sugar blocks in a silo
Understanding how sugar is lumping in a silo, and preventing it, is thus crucial for factory operators to ensure a safe and reliable production.
The main root cause of sugar caking in silos is humidity transfer within the bulk, happening especially near the walls of the silos, and is compounded by other aggravating factors such as an incorrect flow pattern in the silo.
Sugar loaded to silo has a certain humidity. The level of humidity is usually not a problem for the sugar itself, but once loaded in a silo, if the silos walls are cooled down enough for example at night during the cold season, temperature near the sugar silo walls can fall below the dew point [Linek] and condensation can happen in the area close to the wall. As sugar is soluble in water, tiny parts of it dissolve, when it dries again, for example when the sun lights and heat up the silo again, the water evaporates and the dissolved sugar cristallize creating solid bonds in between sugar particles, which leads to lumping, often adhering to the silo walls [Linek].
Repeating this cycle can lead to harder and bigger sugar lumps.
There are 2 types of flow in silos : mass flow, for which the whole mass of material in the silo is in movement when the silo discharge, and funnel flow, where only the center part of the silo is in movement, leaving large areas in the sides of the silos with basically no flow.
In case the sugar silo has a funnel flow, it means that the residence time of the sugar crystals which can be the most prone to caking, as explained in paragraph 2.1, will be very long, and they will endure consolidation loads (compression) for a long time, which can generate very large lumps, or rather blocks, of sugar.
Those blocks of sugar can weight several dozens or hundred kg, which make them a hazard : if they suddenly fall down while the silo is mostly empty, they can hit and damage the structure of the silo. If the block is not too hard, another phenomena can be experience when the block falls down : flooding. In this case the arrival at the outlet of the silo of a large amount of product cannot be controlled and lead to a high flowrate of fluidized material out of the silo.
Related the paragraph 2.2, if the silo cone has not been properly designed, issues such as bridging and ratholing, which can happen even in the absence of lumps, may be experienced.
The proposals presented below should be considered for all new silos aimed at storing sugar in large quantities, factory operators should discuss them with their suppliers in order to select the appropriate ones. However it can be possible to retrofit existing silos, in case the considered silo is not equipped with those features and is experiencing sugar lumping, caking and flow problems.
The conditioning of the air inside the silo can help in alleviating some lumping issues with sugar. Dry air blanketing will indeed avoid that humid air from the external environment is penetrating the silo and transfer water to the sugar that can lead to caking (see paragraph 2). It can also help in removing excess moisture in the sugar stored but, if the air conditionning is only introduced at the top of the silo, this will have a limited effect.
To air condition the silo in the range 20-25c and 50-60% RH, a standalone dehumidifying system can be used and connected via pipe to the top of the silo.
Temperature changes as the wall of the silos can cause condensation, formation of liquid bridges in between sugar cristals that dissolves a little bit of sugar, then evaporation of the liquid water when the temperature increases again leading to the formation of solid bridges in between sugar particles. This cycle repeats with the changes in temperature and thus reinforces the lumps formed, which can adhere to the wall and finally create huge hazardous sugar blocks in the silo. The moisture migration from the bulk of the sugar stored is also providing the water required for this phenomena to happen.
One way to reduce the extent of the sugar lumping at the walls of the silos is to insulate it. The temperature changes will be less and condensation will be eliminated or reduced. It is particularly important to insulate sugar silos in areas where the temperature gradient is important in between day and night.
Lumps are developping and hardening over time, all the more if the material is submitted for a long time to a strong consolidation load (it is especially in the case in silos at transition in between the cylindrical shell and the cone). If the residence time of sugar in the silo is long then lumps will be likely to reach big sizes and be hardened. On the other hand, if the sugar is kept in movement, most of the solid bridges in between particles can be broken, preventing the grow and hardening of the sugar blocks.
The way to ensure the whole mass of sugar is in movement is to design a mass flow silo. As all the sugar, including and especially at the wall, moves down, it is submitted to shear forces and mixing that allow to keep it flowable or at least to prevent uncontrolled growth of the lumps. Mass flow silos can be achieved by designing the silo with the method of Jenike or by retrofitting the silo. Key design parameters to ensure mass flow are :
If may not be possible to ensure mass flow, especially in existing silos which have been designed differently, in this case the following can be applied :
Implementing one of the solutions mentioned above can help, but may not be sufficient. Factory operators should opt for combining each of those solutions to make sure sugar is not lumping in the silo, this adds CAPEX costs, but the money invested allows a reliable production that pays back the investment.
Figure 2 : sugar silo design aspects to ensure good flow and operation
To conclude, a reminder that sugar silos can suffer from dust explosion. Factory operators MUST assess the dust explosion risks in their installation and especially their sugar silo and run the mandatory risk assessment (DHA, DSEAR, ATEX) in their country. Typically sugar silos should be protected against explosion with explosion panels and quick acting valves, the exact type of protection and their sizing will come from the risk assessment and further studies.
To reduce the risks of dust explosion, the relative humidity in
the silo should not be too low (the value is variable depending on
authors, in the area 40-50% [Linek] or 55-60% [Kochergin]), this
helps to reduce dustiness and static electricity while keeping a
good flow [Linek].
[Linek] The right approach how to design the conditioning system
of a sugar silo, Linek and Rosch, Sugar Industry, 2017
[Kochergin] Sugar Storage Silos : A Slow Conditioning Approach