Storage and flow of bulk solids
By Greg Mehos, Ph.D., P.E.
About this book
When I became an adjunct professor at the University of Rhode Island, I realized that most books on the subject of bulk solids testing and hopper design were either very terse or rather intense for readers who did not have a mechanical engineering background. I began preparing my course notes when I worked at Cabot Corporation and improved them when I joined Jenike & Johanson, Inc. While at both Cabot and J&J, I wrote a number of articles for Chemical Engineering, Chemical Engineering Progress, and other rags. They really came in handy when I was asked to write the subsection on powder flow and hopper design of the ninth edition of Perry’s Chemical Engineers’ Handbook. Jenike & Johanson encouraged me to write, provided that I only disclosed design methodsthat were published in the open literature. I carefully adhered to those guidelines when writing this book. Yes, some of the text was taken from my prior publications, but I figure that it isn’t plagiarism when you Ctrl+C and Ctrl+V your own material. A few of the formulas I derived on my own, but they were all based on fundamental engineering principles. Consequently, some of the analyses that I present may be slightly different from what you might find published elsewhere, so use them with caution.
I always advise my students to keep this book in the bathroom. That way they can read about hoppers when they are sitting on the hopper, and if they run out of toilet paper, there isn’t a crisis.
I have found that when teaching, it is best to start with the fundamentals and then use them to derive the equations that can be used to predict bulk solids flow behavior and design systems for reliable flow. When you read this work, I encourage you to understand the first fundamental equation, know how to apply the final one, and then appreciate that someone who was much smarter than us was able to come up with all the equations in between.
I’ve always said that handling powders is a lot like electricity – sometimes a little knowledge is more dangerous than none at all. In the real world, there is almost always more than one answer to a problem. For challenging problems, I encourage you to contact me, Jenike & Johanson, Solids Handling Technologies, or other engineering firms that specialize in the storage and handling of bulk solids. Andrew Jenike developed his test and design methods in the 1960’s; yet his principles have withstood the test of time and are still used today. When analyses are based on fundamentals rather than empiricism, an engineer can have great confidence in his or her designs, and formulators can be confident that their powders can be handled reliably in their available equipment.
Engineers are adept at solving equations, and as a consultant, I rely on the following formula:
Happiness Equals Reality Minus Expectations
Note that there are three terms. If the last term is larger than the middle one, the first one is negative. My goal as a consultant is to understand the reality of bulk solids handling. That way I can exceed my clients’ expectations, they’ll be happy and eager to pay me, and I can eke out a miserable existence. Greg Mehos, Ph.D., P.E
More of Greg Mehos: https://mehos.net/downloads
(click and scroll down the page)
Table of Content
1. INTRODUCTION
Designing systems for reliable handling of bulk solids or determining if a vessel is appropriate for a bulk solid that is to be handled can pose challenges that do not typically arise when tackling assignments that involve the transport of fluids. More often than not, the information needed for predicting fluid behavior is readily accessible. A fluid’s viscosity and density can usually be found in a reference book or a website; otherwise, correlations, estimation methods, or equations of state detailed in textbooks can be used to calculate the necessary physical properties. Given the diameter of a transfer line, the fluid’s flow behavior, e.g., laminar or turbulent flow, can then be confidently predicted. From the length and layout of the line and knowing the roughness of the pipe, information that is also readily found in print, the pump required to transfer the fluid at the desired rate can be specified. If cavitation is a concern, the pump’s net positive suction head requirements can be readily determined as the fluid’s vapor pressure is likely available from data or correlations. You know the drill. Gather the physical properties, specify a velocity, assume a pipe diameter, and then calculate a Reynolds number. Then calculate a ΔP, which will allow you to calculate an hp and size your pump. Easy as π!
Designing a system for handling solids, however, may be more trying as the fundamental properties required to predict flow behavior may not be immediately obvious and any necessary data may not be readily available. In fact, a property as simple as a material’s bulk density is highly dependent on the particles’ shape, size, and porosity, and therefore any published data providing the bulk density of a powder may not necessarily be representative of the material that will be handled. In addition, because bulk solids are compressible, the bulk density of a material inside a hopper, bin, or silo will vary due to consolidation stresses. Without proper training, one may be resigned to select a conical hopper that has an aesthetically pleasing slope or 2 recommend a pyramidal vessel that is inexpensive to fabricate, size a feeder that conforms to the supplier’s data sheet, and propose the installation of vibrating equipment to promote flow. Perhaps that is why identifying equipment and lines that handle bulk solids is often easy – they are the ones with the hammer marks (see Figure 1.1).
Many geometries are used in the design of hoppers, bins, and silos, including conical, pyramidal, wedge, chisel, and transition (round to rectangular). Common designs are shown in Figure 1.2.
Frequently, the size and geometry of a hopper or bin are based on ease of fabrication rather than with consideration of the solids’ flow behavior. Sometimes, bulk solids are stored in flat-bottomed vessels, some equipped with agitators. These vessels are appropriate for storing liquids, but bulk solids behave differently.
(click and scroll down the page)