Sunday 5th Jul, 2020

Not everyone wants to have their cake and eat it

Grant Wellwood, General Manager of specialist bulk materials engineering firm Jenike & Johansen, explains how to stop material from caking after it leaves the factory.

Grant Wellwood, General Manager of specialist bulk materials engineering firm Jenike & Johansen, explains how to stop material from caking after it leaves the factory.

Q. Our customers currently pay a premium for our high quality, free-flowing powder product, however, we have started getting reports of caking. When our powders are packaged into bags at our plant, they are definitely lump free, yet when customers de-bag the contents at their facilities, they see “an excessive amount of caked material”. 

This situation creates problems and rework for the customer, putting our longstanding and lucrative supply contract at risk. We suspect moisture is the cause, but we recently upgraded our process to fill directly from the upstream dryer to eliminate the holding step and moisture pick-up before packaging. Our quality control records show we have actually reduced the as-packed moisture content of our powder and our bags have vapour barriers, so these incidents of caking are perplexing.

We make a high-quality product that exceeds all its specifications when it leaves our facility, yet the customer is not happy. Although this problem occurs beyond our effective sphere of influence, we are held accountable for quality until the material is debagged and the customer is understandably not happy. Is there anything bulk solids science can do to help alleviate this problem even though the powder has left our facility (in perfect condition) and we can exert no influence?

Thanks in advance: “On-a-Diet” (no cake for me thanks)

A. Dear On-a-Diet (OaD),

Unwanted particle agglomeration (caking) is a major issue for many industries because of its negative impact on so many aspects of a manufacturing process including:

• Increased risk profile by resorting to non-standard procedures. These ad-hoc interventions often involve improvised tools which often have low integrity and put operators in the line of fire with respect to moving parts and/or collapsing cake structures.

• Reduced finished product quality from either the inability to properly disperse/spread or from good-intentioned interventions to break up caked particles that violate critical control point protocols and introduce impurities.

• Reduced productivity due to interruptions to flow in the value-chain arising from blockages, manual interventions, ingredient starvation or increased time for dispersion.

• Inhibited process capability arising from reduced “live” holding capacities in the flowsheet, for example.

• Increased operating cost because of rework or outright rejection of feed material batches.

Perversely, eliminating your post dryer cooling step in order to minimise opportunities for moisture pick-up is probably the root cause of your caking problem, OaD. Let me explain.

Even if your granular or powder product seems dry and free flowing going into its vapour barrier packaging, caking can still occur over time due to something called moisture migration.

Moisture migration occurs whenever there is a difference in a system’s water activity – the equilibrium relative humidity of the air in the void spaces between the powder particles – a phenomenon driven by temperature gradients.

Transient temperature gradients are created when a powder is packaged at elevated temperatures and then stored or transported under ambient conditions. If the bags are stored outdoors or in a warehouse that has poor temperature control, temperature cycling may occur, which sets up temperature gradients within the bulk material. This is why the packaging of many free-flowing food ingredients like salt, sugar, or flour sport the “store in a cool place” recommendation.

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Based on my understanding of your current situation, the powder is being packaged at a temperature higher than ambient. If so, as that pack cools, powder closest to the fabric (polymer, paper, cloth) will be colder than material at the centre of the bag. As a result, the relative humidity of the interstitial air will be highest nearest the fabric exposed to ambient temperatures. The moisture in that air will adsorb onto the surface of the particles in an effort to establish equilibrium. This lowers the absolute humidity of the air closest to the fabric, thereby providing a concentration driving force for further transfer of moisture from the centre of the bag towards the fabric.

In chemical engineering terms, each sealed vapour-barrier packet is a closed system, as energy but not mass can transfer across its boundary. Even in a closed system comprising visually dry and free-flowing powder, there can often be enough water trapped within to cause caking if it all migrates to one region of the packet.

In addition to the moisture content of the particles that make up the powder (as measured by your quality control tests), the air between them, which is typically 40 per cent of the bulk volume, will also contribute to the water content within the closed system.

Air at 25°C can carry 0.02 kilograms of water per kilogram of dry air and the carrying capacity goes up sharply with temperature. At 40°C air can carry 0.05 kilograms of water per kilogram of dry air. As the temperature of the air you are now packing is elevated, this could be a major contributor, especially if the drier unit is directly gas fired (products of combustion) or counter current.

Once moisture migration takes place within
the closed system, there are three mechanisms
that can cause the particles in a powder to bond, resulting in caking:

1. With many powders water can act as a plasticiser. This will soften the material, leading to an increase in inter-particle contact area, a greater number of contacts between neighbouring particles, and a decrease in distance between adjacent particles. The resulting increase in cohesive strength can be dramatic if the bulk material is one where plasticisation causes its glass transition temperature to fall below ambient, leading to the phenomenon known as sintering.

2. When some powders are exposed to high relative humidity air, the water adsorbed by the powder collects between powder particles due to capillary condensation, forming liquid bridges. If the entire void is filled with liquid, a negative capillary pressure will exist, resulting in additional attractive forces between particles. In general, the opportunity for incidental capillary formation between particles increases as particles size reduces, hence attrition and segregation can be important considerations.

3. Solid bridges between particles can form when soluble matter in liquid bridges precipitate during cooling or drying. In general, capillary condensation must first occur in order for solid bridging to take place.

While there are a range of options available, identifying the root cause involved is always the pathway to the best long-term fix. Shear cell tests that measure the powder’s gain in cohesive strength when stored at rest can be conducted under controlled conditions in a laboratory. Caking phenomena is controlled by microproperties of the bulk, so to fully inform the science and get the best solution, a suite of characterisation tests (below) are recommended to explore the materials.

• Adsorption isotherm

• Segregation and attrition tendencies

• Uniaxial compressibility

• Particle size distribution

• Shape and form via scanning electron microscopy (particle shape [points, flat surfaces…], surface cracks and fissures, chemical impurities

  Thermal expansion behaviour-via dilatometry,

Such analysis will help reveal the characteristics and moisture contents that must be avoided to prevent caking and reveal the dominant bonding mechanism involved.

Once the root cause and associated moisture threshold for unique packaging circumstances are known, you can confidently move into solution mode.

• Go to your customers’ operations in search of links between bad packets (containing unacceptable levels of caked material) and your customers’ handling processes. In the warehouse, look for things like rough handling on receival (attrition, segregation and bag damage/puncturing traces of powder on the floor?), high packet stacking and extended storage in uncontrolled temperatures, and elevated temperatures in the roof-space.
In the operation itself, first take a look at the debagging station and observe the caking problems first-hand. Speak to front-line staff about the issue and lookout for mechanistic clues. For example, is the caking more prevalent within in packages at the bottom of a stack (they are the deformed ones) or perhaps the ones one top (influenced by roof temperature)? Is the caked material within a packet adjacent to a fabric surface or perhaps always on the bottom surface (suggesting segregation issues)? If so, what type? Is the caked powder moist? Is it friable and easy to disintegrate or has it set like rock? Can you see bridges between individual particles?
Most operators will appreciate your interest in their problem, while the line managers and engineering staff will savour the opportunity to share their pain while appreciating your determination to identify the root cause and solve the problem.

• Look at your powder’s production-to-customer transport journey and if possible, eliminate ambient temperature variations/cycling (don’t forget distribution centres), excessive compressive forces (bag stacking) and sharp movements likely to cause segregation.

• If you don’t already do so, start collecting and logging data at your packet filling point against batch numbers (moisture content of the powder, ambient temperature, relative humidity, time of day, fill temperature) in order to shine some light on the issue. Are the packets subject to compressive forces in your warehouse or distribution centre? If so, over what time? Ask your customer to keep a log of bad packet batch numbers, so you can map them to your fill data and look for discernible patterns.

Within your own sphere of influence as a powder producer, consider the following changes in order to reduce the risk of post-dispatch caking:

1. Cooling the powder to ambient temperatures before packaging to avoid setting up a temperature gradient at the packaging stage.

2. Conditioning the powder prior to packaging by injecting low-humidity air as the powder flows through a mass-flow silo to purge moisture. Passing air will also improve the rate and quality of the cooling.

3. Over-drying the powder to a sufficiently low level such that if moisture migration takes place, any local increase in powder moisture content will not cause agglomeration due to plasticisation or liquid or solid bridging. Dryer powders are generally harder and therefore less prone to sintering.

4. Adding a parting agent such as magnesium oxide or silicon dioxide to your powder to minimise inter-particle contact area.

5. Adding desiccant pouches to the closed system packets in order to preferentially take up migrating moisture.

6. Reducing the normal pressure on individual packets (imparted by stacking) before they leave your operations. Consider limiting stack heights and using load distributing sheets.

7. Handling induced attrition during filling or dispatch on your side which can lead to the creation of moisture trapping capillaries within the bulk (due to attrition and/or segregation) and an increase in the number of inter-particle contact points and surface area.

8. Consider the impact of shape and size distribution of the particles and if there is potential (use shear testing to determine), modifying your process to produce a “caking-resistant formulation” (sold at a premium over competitor’s product?).

9. Identify any impurities (for example salts) that may promote caking, and if possible, eliminate them.

In your case, it would appear that “warm” filling is the root cause, so you could cut to the chase and revert to the previous workflow of holding the dried material before packing it off. However, rather than simply recommissioning the previous holding hopper, you may wish to consider an engineered solution featuring a counter current flow of dry cooling air. This approach would not only eliminate the temperature gradient within your packets, but also reduce the required holding time and inventory to
do so. Actively drying the air used will minimise
the moisture carried into each packet with the interstitial air.

Some materials are more prone to caking than others, but instead of fighting thermodynamics, can you make caking actually work for you?

For example, sugar is prone to caking but this tendency has actually been turned into an advantage. Cubed sugar is nothing more than engineered caking, but the resulting product is marketed as having many benefits including:

• Convenience. They can be picked up by hand and hold their form until mixed into liquids where they dissolve quickly without dust, spilling and/or wet spoons going back into the bowl.

• Portion control. Each cube is approximately equal to one and a half teaspoons of sugar and has approximately 25 calories.

• Higher transport intensity. Cubed sugar in a square packet on a square pellet equals higher packing density equals a smaller transport impact footprint.

Jakub Krystof Rad invented the first sugar cubes in the 1840s at a sugar refining plant in Dacice, a small Moravian town in what is now the Czech Republic. A combination of sugar crystals and sugar syrup are mixed together and placed in moulds until dry. The dried cubes emerge from this engineered caking process as a 100 per cent sugar product hard enough to retain their shape yet dissolve instantly on contact with water.

So, understanding and applying the science in the context of some marketing expertise has created a premium product. How much of a premium? Cubed sugar sells at 10 times the price of its free-flowing precursor, a 1000 per cent mark-up, the value of science and innovation.

Do you have a bulk solids handling question? Jenike & Johanson has developed the science of bulk solids flow and specialises in applying it to solving the most challenging bulk solids handling problems. So why not put them to the test with your question? The harder, the better.