Mass flow from bins

Number of particles Ny, total number N , number per unit mass Peclet number for axial mixing, UL/E, dimensionless Number of increments or screens number of particles in spot sample also, exponent in equation for flow from bins... 

Two other causes of flow problems that can occur due to problems at a hopper outlet are improper feeder design and upward gas pressure gradients. If a feeder is not designed to fully activate the outlet area, this will prevent mass flow from developing in the vessel, thereby potentially creating significant flow problems. Gas pressure gradients within a bin... 

Because mass flow bins have stable flow patterns that mimic the shape of the bin, permeabihty values can be used to calculate critical, steady-state discharge rates from mass flow hoppers. Permeabihty values can also be used to calculate the time required for fine powders to settle in bins and silos. In general, permeabihty is affected by particle size and shape, ie, permeabihty decreases as particle size decreases and the better the fit between individual particles, the lower the permeabihty moisture content, ie, as moisture content increases, many materials tend to agglomerate which increases permeabihty and temperature, ie, because the permeabihty factor, K, is inversely proportional to the viscosity of the air or gas in the void spaces, heating causes the gas to become more viscous, making the sohd less permeable. 

Usually the rate, Q, is far in excess of the required rate, especially if the bulk material consists primarily of coarse particles. Slowing down the discharge rate requires a feeder. Fine powders, on the other hand, have considerably lower maximum discharge rates when exiting from a mass flow bin, because of the interaction between air (or gas) and solid particles as reflected in the permeabiUty of the material. 

Most flow problems can be overcome by using a mass flow design if the mass flow pattern developed by the bin is not disturbed. Thus a properly designed feeder or discharger must be employed. A feeder is used whenever there is a requirement to transfer soflds at a controlled rate from the bin to a process or a tmck. A discharger is used when there is a need to discharge soflds, not control the rate of discharge. 

A potential problem for rotary valve usage is that they tend to pull material preferentially from the upside of the valve, which can affect the mass flow pattern. Another problem is that once soHd drops from the vane, the air or gas that replaces it is often pumped back up into the bin. In addition, air can leak around the valve rotor. Such air flows can decrease the soflds flow rates and/or cause flooding problems. A vertical section shown in Figure 13 can alleviate the preferential flow problem because the flow channel expands in this area, usually opening up to the full outlet. To rectify the countercurrent air flow problem, a vent line helps to take the air away to a dust collector or at least back into the top of the bin. 

Until Jenike developed the rationale for storage-vessel design, a common criterion was to measure the angle of repose, use this value as the hopper angle, and then fit the bin to whatever space was available. Too often, bins were designed from an architectural or structural-engineering viewpoint rather than from the role they were to play in a process. Economy of space is certainly one vahd criterion in bin design, but others must be considered equally as well. Table 21-14 compares the principal characteristics of mass-flow and funnel-flow bins. 

Testers are available to measure the permeability and compressibility of powders and other bulk solids (6). From such tests critical, steady-state flow rates through various outlet sizes in mass flow bins can be calculated. With this information, an engineer can determine the need for changing the outlet size and/or installing an air permeation system to increase the flow rate. Furthermore, the optimum number and location of air permeation levels can be determined, along with an estimate of air flow requirements. 

A non-uniform feed density is not a primary concern. Since tablet presses operate as volumetric feeders, variation of the feed density into the press feed frame can result in tablet weight variation. A funnel flow bin will typically have a more non-uniform feed density than a mass flow bin, since the blend in the funnel flow bin will be subjected to different consolidation pressures depending upon where in the bin it is discharged from. For instance, the blend located at the bottom of the bin at the hopper walls, which is outside the flow channel, may be more consolidated and have a higher density than the blend within the flow channel. 

Mass flow discharge from a bin occurs when the following two design criteria are meet ... 

Use a different interior surface finish with better wall friction properties (lower friction). Conduct wall friction tests on alternative wall surfaces to assess if changing the surface finish while still using the existing bin geometry (e.g., electro-polishing an exiting 2B finish) will convert the bin from funnel flow to mass flow. This is often one of the most cost-effective modifications to obtain mass flow. 

Raw materials and recycle (undersized, somewhat predensified material) are made available in day bins. It is essential that at all times enough material is available (controlled by level indicators) for continuous discharge from the silos. Activators and/or mass flow bin designs may be necessary to ensure constant flow. In most cases, the individual components of a mixture will be metered for correct analysis. It is important to take into consideration the recirculating material as it may influence the final analysis and, on the other hand, may be necessary to ensure acceptable densification. 

It is important to ensure that the maximum feed rate from the storage bin is always greater than the maximum expected operating rate of the feeder. Otherwise, the feeder will become starved and flow-rate control will be lost. This problem is particularly pronounced when handling fine powders because their maximum flow rate through an opening is significantly less than that of coarser-particle bulk solids whenever a mass-flow pattern is used [1]. 

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