Observations on the nature of bulk solids

Phil Black - PII Editor 13/11/2020

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Bulk solids come in all sizes and forms, so it is not surprising that their nature covers a vast range of conditions. Unlike liquids and gasses, it is not practical to construct a data base of conditions affecting their flow because a great many factors of the materials themselves and of ambient, operational and effects of time can interact to affect how a loose bulk material may behave.

Bulk density and stress history being major factors in influencing the flow potential. Fortunately, tools have been developed to measure specific handling characteristics and these can be related to the design of storage, handling and process plant. Wall friction is of ubiquitous application, its value invaluable for design and giving an indication of the optimum material of construction that could be adopted.

Some materials, such as biomass, remain very challenging and phenomenon such as segregation and ‘flushing’ can be awkward to control. Many industries have, by hard won experience, established designs that work and manufacturers standardised storage and conveying equipment suitable for ‘easy to handle’ products.

In addition to applying the technology to more difficult duties, specialised companies have developed techniques and best practice, with attention to the fine detail that bulk solids respect, to apply to applications that demand more individual attention.

A key feature in all bulk handling operations is to secure reliable flow at a required rate and condition. Basic hopper design procedure is well established. Less well understood are the interfacing of feeders with hoppers and the use of hopper inserts to improve flow performance.

These are inter-related items difficult to research, especially hopper insets, with the notable exception of ‘bullet’ and ‘Cone-in-Cone’ type because of the many variables involved and possible effect on structural integrity.

Loads acting on feeders is also a sensitive issue, as starting loads may be many times that of sustained running. Fitting large drive units would incur high capital and uneconomical running costs on large installation so steps are often taken to mitigate starting loads by operating procedure or design provision.

Hopper inserts are rarely considered other than as emergency retrofits, as it is counter-intuitive to think an obstacle in the flow path would improve flow. The fact is that an insert creates a different flow regime system that has to be considered separately, not only for its effect on flow, but what loads it imparts to the structure.

Apart from changing the flow pattern they are useful on reducing or redressing segregation and for reducing the loads on a feeder so should be considered as part of the basic hopper design by an experienced manufacturer as they can perform many functions. (1).

Another design area that tends to be overlooked, where inserts may have a role, is the

Importance of securing a uniform drawdown of the hopper contents to rectifying segregation and reducing a ‘flushing’ hazard. Screw feeders are often used to discharge hoppers and a common technique is to employ an increasing pitch to generate a progressive extraction along the hopper outlet slot.

However, this change alone leads to a highly preferential flow pattern as the first flight exposed to the hopper content will extract its full transfer capacity whereas subsequent flights can only extract the differential increase.

Further, a pitch increase reduces the transfer efficiency yet has to serve a longer section of the slot, so the rate of extraction progressively decreases per unit length. The result is a highly preferential flow path for the hopper contents that can to draw fresh, dilate material from the hopper surface and risk ‘flushing or selectively extract segregated product from the hopper contents. (2).

By contrast, uniform extraction maximises residence time for settlement and remixes the body of the contents on discharge. A registered design by Ajax addresses the ‘initial draw’ differential and special design techniques are adopted to compensate for differential extraction by screws.

The company also offers a unique method of extracting progressively from a slot outlet with a reversing screw feeder. This design has a large headroom saving on the traditional method of fitting a bifurcated valve and twin chutes whilst also enabling either or both receiving points to be served at the same time. (3).

Refs.

‘Using inserts to address solids flow problems’. Bates L. et al. Chem. Eng. www.che.com July 2010
‘The importance of uniform drawdown in by feeders’. Ajax publication.
‘A technique to improve the performance of a reversing screw feeder’. Bates. L. ICBMH. Newcastle. Aus. Jan.2013.