How Material Traits Shape Food Processing
A storage vessel (silos, bins, and hoppers, collectively referred to as silos in this article) that stores and discharges material flawlessly with granulated sugar would likely struggle the moment it stores powdered sugar. Intuitively, this doesn’t make sense – chemically, the two materials are identical (C12H22O11). The reason why is not the silo itself, but rather the difference in flow properties between those materials inside it. Food ingredients range from free-flowing to highly cohesive, and these flow characteristics dictate how they behave during storage and discharge, positioning flow properties as the most crucial consideration to make for silo design.
Free-Flowing vs. Cohesive
Raw materials and ingredients display a wide range of flow behavior across food processing operations. Products such as whole grains, granulated sugar, and dry coffee beans are considered free-flowing because individual particles move readily under gravity with minimal resistance during typical operating conditions. Even so, such materials can experience particle segregation during filling and discharge, allowing coarse and fine particles to separate and affect product uniformity. Additionally, some materials (like granulated sugar) can go from free-flowing to completely caked based on temperature, humidity, and storage time at rest.
Conversely, powdered milk, cocoa powder, and flour blends behave very differently. Particle size, fat concentration, moisture concentration, and other factors can significantly impact the cohesiveness of the material. As cohesive strength increases, materials become progressively more resistant to discharge out of silos, producing challenges throughout storage and discharge.
Common Issues in Food Industry Silos and The Impact of Flow Pattern
There are two discharge patterns that can occur out of silos: funnel flow and mass flow. In funnel flow, material moves to the outlet through a funnel-shaped flow channel surrounded by stagnant material along the hopper walls. This allows material to discharge through a central channel, leaving stagnant regions against the hopper walls, resulting in a first-in, last-out (FILO) flow sequence. Mass flow keeps every particle moving whenever material exits the vessel, maintaining first-in, first-out (FIFO) inventory movement and eliminating stagnant zones.
Outside of flow pattern evaluation, other common failures include:
- Cohesive arching/bridging- cohesive powders form a bridge above the hopper outlet fully restricting flow. This can occur in funnel flow or mass flow silos.
- Ratholing- a stable, empty vertical channel forms within a cohesive powder in a funnel flow silo. By definition, ratholing cannot occur in mass flow silos.
- Flooding- a condition in which fine powders are not allowed to fully de aerate within a silo, and the powder behaves like a liquid flushing through the outlet or feeder. This problem is exacerbated in funnel flow silos due to smaller active flow channels.
- Caking- unwanted agglomeration of particles due to bonding forces induced by moisture, pressure, Van der Waal forces, or static electricity. Caking can be so severe that the hopper outlet is completely blocked due to the increased size of the agglomerated material.
- Segregation- the separation of particles by differences in particle size, shape or density. Often, segregation includes side-to-side variation within a silo, which significantly impacts product uniformity in discharge if operating in a funnel flow.
- Material degradation/quality- because funnel flow silos operate in a FILO flow sequence, food materials lose freshness, cake, spoil, and increase batch-to-batch cross-contamination in funnel flow silos.
Many facilities attempt to solve these issues with vibrators or air cannons. Although discharge aids have their place, they rarely resolve a silo geometry that is fundamentally incompatible with the stored material.
Material Characterization: Relating Material Properties to Silo Geometry
Successful silo design requires knowledge of how a material behaves under expected operating conditions. Every measured property contributes to decisions that determine long-term storage performance.
First, the interaction between the stored material and interior surface of the silo (wall friction) must be measured. Understanding this relationship is how a determination of flow pattern is made. Materials with high wall friction need steeper hopper angles to operate in mass flow. Highly frictional food powders may also demand specially engineered transition geometries that promote reliable mass flow throughout discharge. Friction between the material stored in the silo and the silo’s interior surface must be measured through wall friction shear testing.
Next, the outlet dimensions of the silo must account for the cohesive strength of the material stored. Cohesive strength shear testing must be performed to determine the outlet dimensions needed to prevent arching and ratholing.
Material testing should also include compressibility of the material (bulk density as a function of consolidation pressure) to establish silo storage capacity, discharge rates, and material induced loads in the silo/feeder.
Other tests that should be considered depending on the type of material stored include:
- Permeability tests (evaluation of two phase flow affects)
- Sifting/fluidization segregation tests
- Particle attrition tests
- Moisture adsorption/desorption tests (equilibrium moisture content as a function of relative humidity to evaluate caking tendencies).
Additionally, it is important to note that testing should be conducted at process temperature and humidity conditions to accurately represent the operating environment.
Finally, discharge equipment needs to complement a material’s behavior. To promote proper flow, a feeder should be used to meter material out of the silo above. Ultimately, a feeder must accomplish the following:
- Provide reliable and uninterrupted flow of the material from the silo above.
- Control the discharge rate from the silo above, achieving the required rate while preventing uncontrolled flooding of fine powders.
- Remove material from the entire cross section of the hopper outlet in order to avoid interfering with reliable mass flow discharge in the silo above.
Reliable Silo Solutions
Jenike & Johanson provides a complete range of bulk material handling services for the food industry, from laboratory testing and material characterization to functional silo design, feeder engineering, and process retrofits. Measured flow property data, supported by decades of practical engineering experience, enables our team to develop storage systems that improve discharge reliability, protect product quality, and support efficient production. Get in touch with Jenike & Johanson to develop an engineering-led design that can help solve your flow challenges.
