Shaun Reid, operations manager at TUNRA Bulk Solids, and Mitch Boots, PhD Student at the University of Newcastle, share details on their ongoing research into bulk bag performance. This project has been commissioned by the International Fine Particle Research Institute.
Flexible Intermediate Bulk Containers (FIBCs), commonly known as ‘bulk bags,’ perform a critical role in the production and supply of various materials, including food-grade products, volatile chemicals, pharmaceutical powders, and agrochemicals.
Despite their widespread application, issues often arise during the discharge of FIBCs, including arching, segregation, and erratic flow. When relied upon for delivery of fine and/or cohesive products, the performance of these systems is seen to be sensitive to storage and transport conditions, with FIBC conditioners (flow aids) commonly relied upon to promote material flow.
TUNRA Bulk Solids is collaborating with the International Fine Particle Research Institute (IFPRI) and the University of Newcastle to research the mechanics at play during the transport and discharge of FIBCs, so that their performance may then be improved. This project stemmed from IFPRI members’ interest in addressing common discharge and handling issues across a range of industries internationally. The collaboration aims to develop new methods to predict FIBC discharge characteristics, in consideration of the bulk material flow properties, the mechanical properties of the bag and the conditions it experiences during service.
FBICs
FIBCs are industrial containers typically made from woven polypropylene (PP) fabric, used for transporting a wide range of materials, from landscaping supplies to hazardous chemicals. Constructed according to international standards such as ISO 21898 or AS 3668, FIBCs must meet minimum load criteria.
However, these standards do not include specific guidelines on stitching, elasticity, stiffness, and fabric thickness, all of which influence the bag’s rigidity and performance. FIBCs vary in design, featuring different inlets, outlets, dimensions, and mounting loops.
The design of dischargers for FIBCs is wide ranging, often employing flow aids and conditioners to improve the discharge of cohesive granular materials.
The type rating of an FIBC bulk bag pertains to its antistatic properties and does not directly indicate the flexibility, size, or load-bearing capabilities of the bag. The classifications are as follows:
Type A: No static protection.
Type B: Surface breakdown voltage below 6 kV.
Type C: Electrically conductive or capable of being grounded.
Type D: Antistatic properties.
Recent studies have shown that FIBCs behave differently from conventional bins. Specifically, as the wall flexibility increases, vertical stresses rise while lateral stresses decrease. The dynamic nature of FIBC motion during discharge also affects performance, along with the stress history in transportation, lifting, and moving.
TUNRA’s approach
The team’s current approach involves application of experimental, theoretical, and numerical methods to understand and predict the discharge behaviour from FIBCs.
Drawing on over 50 years of expertise in bulk solids handling from TUNRA Bulk Solids, the team aims to apply recent advancements in bulk solid mechanics – such as hoop stress theory and numerical modelling tools – to develop flow models for discharge from containers with flexible walls. This approach will be supported by experimental testing to measure and analyse discharge patterns from FIBCs, focusing on specific bulk material types that will also undergo characterisation of their flow properties.
The team is in the process of commissioning a phase one experimental rig, to provide insights into flow regimes in a two-dimensional context, initially focusing on flow regimes and later measuring the consolidation stresses within the bulk material. The apparatus will serve to validate and refine simulation results and inform the development of a full-scale rig planned for commissioning in 2025.
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In parallel to the experimental work, Discrete Element Method (DEM) approaches are being developed to simulate the performance of varying bag/material combinations. DEM offers greater flexibility than continuum–based analyses by examining microscale interactions between particles and boundaries to predict flow through a system, though require care in their calibration and application.
Simulations have been conducted using ANSYS Rocky DEM, in which the bulk bag has been represented by an assembly of interconnected ‘flexible’ particles.
Flexible particles are composite structures made by connecting simple rigid elements with flexible joints. The prescribed joint mechanics define the reaction of the flexible particle assembly, in this case the reaction and deformation of the bulk bag during discharge.
Initial DEM observations reveal that the bag’s rigidity significantly impacts the discharge of the material, showing a substantial variation in discharged mass when the container’s stiffness is modified. As the container’s rigidity decreases, its ability to hold its shape is diminished. Initially, the bulk material largely maintains the container’s geometry during discharge.
However, as the bulk solid discharges, the container begins to rapidly lose the structural integrity provided by the bulk solid, leading to a notable transition in behaviour.
Summary
By further developing the approach and early observations presented within this article, the research team aims to explore and address the challenges associated with the discharge of FIBCs.
The insights gained from this study are intended to inform design and discharge strategies to reduce reliance on conditioners and enhance the efficiency and reliability of FIBCs across a range of applications, with the characteristics of the bulk material as a key focus.