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Material Flow Characteristics
Hopper is a device used for storage of raw materials, intermediates products in the form of particulates in process plant. There are two types of flow characteristics of a storage vessel which are; mass flow and core/funnel flow. Mass flow means that all the materials inside the bin or silo is in motion whenever any of it is drawn from the outlet, whereas core or funnel flow is the flow that occurs when only a portion of the material flows when any material is withdrawn. Usually, there are rat-hole formations in core flow.
Mass flow has the best characteristics for material storage in vessel compared to core flow. The motion of material in mass flow is uniform and it can reached steady state condition easily. Mass flow has many advantages over core flow (Rhodes, 2008). Table 1.1 shows the principle characteristics of mass flow and core flow. Table 1.1: Principle Characteristics of Mass Flow and Core Flow (Perry’s Chemical Engineering Handbook) Mass flow
All solids moving solid
Funnel/Core flow
Moving solid
Stationary solid
Figure 1.1: Mass flow hopper (Courtesy of Chemical Engineering, Perry’s Chemical Engineering Handbook )
Figure 1.2 Core flow hopper (Courtesy of Chemical Engineering Perry’s Chemical Engineering Handbook)
Particles segregate, but remit on discharge
Particles segregate and remain segregated
Powders de-aerate and do not flood when the system discharges
First portion in, is the last one out
Flow is uniform
Flow is erratic
Density of flow is constant, independent of Density can vary height of bin/silo Stresses are generally low throughout the Product tends to bridge or arch, and then to mass of solids, low compacting of the powder rat-hole when discharging Level indicators work reliably
Level indicators must be placed in critical positions so they will work properly
Product does not remain in dead zones, where degradation can occur
Product can remain in dead zones until complete clean-out of the system
Bin can be designed to yield non-segregating storage, or to function as a blender
Bins perform satisfactorily with free-flowing, large-particles solid
No stagnant regions
Rhodes (2008) has been cited Jenike (1946) had come out with the rational design for hopper. The design criteria allow an engineering analysis of material storage. Jenike’s design can be used to determine whether the vessel will function with mass or funnel flow. Other than that, the outlet dimensions of the hopper to allow t he product to flow through also can be determined by Jenike’s design. The design is mainly focused on the conical hoppers and the slope angle required to ensure mass flow to occur is depending on the powder-powder friction and powder-wall friction.
The friction coefficient between powders and wall of the hopper is called kinematic friction (Φ). Jenike’s teaching on the flow through hopper is that materials that can be compacted (as opposed to being free-flowing) will be compacted because of storage-vessel shape and the packing characteristics of the product. When this happens, the material forms an arch that is capable of withstanding considerable stress. Since the arch transfers the load to the hopper walls and in doing so applies so much pressure to them, the kinematic coefficient (Φ) becomes great. The net result is that the “dome” or “bridge” that forms prevents any flow from the vessel. Force must then be applied to the arch so that it will collapse and flow will begin, even if erratically (Raymus, 1999).
In the early years, the research on hopper discharge is mainly focused on the gravity discharge from flat-bottomed hopper (Wanjie et al, 2009). The discharge of powder particles from hopper is mainly depend on the hopper shape, whether it is conical, cylindrical, or etc. The particle properties also lead to the discharging of the powder, such as particle size distribution, and the particle surface moisture flowability. As been cited by Wanjie et al, 2009; for the fine particle with diameter less than 500 micron, the discharge rate is decreases with the decrease of the particle diameter, while the coarse particle is just contrary (Fowler and Glastonbury, 1959). The relative motion of the particle and the air caused the characteristics of pressure gradient in the hopper and the finer particles have much greater pressure gradient (Nedderman et al, 1983). Changes on the pressure in the hopper will alter the discharging rates of the particles.
The flow of the powder through hopper is mainly focused in the field of pharmaceuticals and food processing.
Reference Grantges J. Raymus, “Section 21: Handling of Bulk Solids and Packaging of Solids and Liquids”, Perry’s Chemical Engineer Handbook , Seventh Edition, 1999 (page 21-1)
Martin Rhodes, “Chapter 10: Storage and Flow of Powders-Hopper Design ” Introduction to Particle Technology, Second Edition, 2008 (page 265)
R.M.Nadderman, U. Tuzun, R.B Thorpe, “The Effect of Intersitial Air Pressure Gradients on the Discharge from Bins”, Powder Technology, 35 (1983), 69-81.
R.T. Fowler, J.R. Glastonbury, “The Flow of Granular Solids through Orifices”, Chemical Engineering Science, 10 (1959), 150-156
Wanjie Huang, Xing Gong et al, “Discharge Characteristics of Cohesive Fine Coal from Aerated Hopper”, Powder Technology, 194 (2009) 126-131.
