Volume 6  Number 6                            Dennis R. Dinger                                1 April 2008

Updates

"... for Ceramists" Series Books

          Requests for Multiple Copies

I have had several recent inquiries about the purchase of multiple copies of these books.  Here are my two suggestions:  

          (1)  If you purchase downloadable versions, purchase the required number of copies (please be honest about the number) from the Books and Downloads page of this website.  Then download a single copy and distribute it (or print it and distribute it) to the people for whom you purchased the copies. ... or ... 

          (2) Purchase the required number of paperback copies from the Books and Downloads page of this websiteand distribute them to your people.  My books are priced $19.95, $24.95, and $29.95 with this in mind.  You won't find many other good ceramics books in this price range.  Most others start at $80 to $100 each and prices rise from there.  For example, our PPC book (when it was available) was $195 per copy.  (I had no input when that price was set.  During one phone conversation, after they made sure I was sitting down, they simply told me the price.)          

          Spanish Language Books

For those of you who speak Spanish as your primary language, a downloadable PDF version of Rheology for Ceramists in Spanish is currently in progress.  Reología para Ceramistas is currently being edited to be made available as soon as possible.  Best estimate at this time is that it will be available sometime in 2008 because the editing process is proceeding slowly.  The PDF file will be set up so it can be printed on your printer if you prefer a hard copy.  Depending on the reception this version receives, I will then consider translating the Particle Calculations book as well.  I will also then consider translating it into Portuguese.  Any thoughts, comments, and/or suggestions will be appreciated.

          English Language Books

The paperback version of Characterization Techniques for Ceramists is available on the Books and Downloads page at the web site!    Retail price is $29.95 plus shipping and handling. The book has 256 pages and it covers 34 different characterization techniques that are commonly used by ceramists.  Purchase a copy NOW!

The book sets on the web site have also been revised to include this new book.  A 3-book set of paperbacks, including one each of Particle Calculations for Ceramists, Rheology for Ceramists, and Characterization Techniques for Ceramists, is now available for $64.85 plus shipping and handling.  This is a $10 saving off the total retail price of the 3 paperback books.  A 3-book set of downloads is also available for $52.85.  This, too, represents a $10 saving off the total retail price of the 3 downloadable books.  

The E-Book version of Characterization Techniques for Ceramists is available for downloading at the Books and Downloads page of the website for $24.95.  The download is a 2.889 Mb self-extracting Zip® file for the Windows® environment which unzips to the 2.998 Mb book in PDF file format.  Those of you who order the downloadable book will want to know that the PDF book is formatted to print on 5.5" X 8.5" paper (i.e., 8.5" X 11" sheets cut in half.)

The other two books, Rheology for Ceramists and Particle Calculations for Ceramists, continue to be available for purchase as downloadable E-books and as paperback books at the Books and Downloads page of the web site.

The E-zine

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Two Distinguishing Features of Suspensions:  Settling and Abrasion

Introduction

There is a single, major distinguishing characteristic of suspensions that seems to be frequently ignored.  By their very nature, and by definition, suspensions contain particles which can (1) settle and (2) abrade anything into which they come into contact.  In this article, we will consider differences between suspensions and high viscosity Newtonian suspensions.

It Is Easy to Assume All Fluids Are Newtonian Fluids (But That Would Be Incorrect)

     Introductory Courses Cover Simple, Newtonian Fluids

Students who study fluid dynamics almost always begin with the study of Newtonian fluids:  most specifically, water.  Introductory courses in fluid mechanics cover water, its properties, and its performance, in great detail.  Then, moving on to more complicated issues, courses then cover other fluids and viscosities.  It is possible (and even probable) that many engineers graduate with BS degrees without ever having studied the differences between high viscosity simple fluids (e.g., molasses) and high viscosity suspensions (e.g., clay casting slips.)  The author has seen piping systems that were designed for high viscosity simple fluids that simply did not work properly with suspensions.  The designs would have been great for pumping molasses or other high viscosity simple fluids, but they were terrible designs for suspensions.

     The Easiest Equations to Apply Are for Simple, Newtonian Fluids

Many of the equations in textbooks are meant for simple fluids -- regardless of viscosity.  The fundamental assumption is that the fluids must be simple fluids.  When this is so, the equations are relatively simple, straightforward, and easy to use.  Many equations that apply to suspensions are much more complicated and much more difficult for practicing engineers to apply and/or understand.  

     Textbooks And Articles for non-Newtonian Fluids Are Complicated And Difficult to Understand

On his book shelf, the author has several advanced books on suspension systems that contains predictive equations which are very difficult to understand, apply, and interpret.  Some scientists understand presentations and predictive equations littered abundantly with differential equations, but to the average practicing engineer -- they make little sense.  The author himself has trouble understanding some of these, and he has done a lot over the years with non-Newtonian suspensions.

     Many, Therefore, Make the Simplifying Assumption -- All Fluids Are Newtonian

For the above mentioned reasons, it is much easier to assume that all fluids (including suspensions) behave as simple Newtonian fluids.  This is the wrong approach because about the only things suspensions have in common with simple fluids is that they flow and they have viscosities -- and even then, their behaviors are very different.  High viscosity simple fluids are generally Newtonian.  Most suspensions are non-Newtonian.  This is a huge difference that must be taken into account!

The Newtonian Assumption May Be Easiest to Utilize -- But It Is WRONG!!

When one is dealing with a suspension, one MUST not only assume non-Newtonian behavior, but one MUST utilize equations and designs meant for suspension systems.

     A Bad Example -- A Suspension System Designed for a High Viscosity Simple Fluid

One suspension piping system pumped a suspension from a tank on one side of a laboratory up to the ceiling, across the ceiling along a long straight run of pipe, and down to a processing tank on the other side of the lab.  The piping system had a single leg across the room with no return lines to the feed tank.  This was a GREAT design for a high viscosity, simple, Newtonian fluid!  It used a relatively large diameter stainless steel pipe to minimize fluid flow velocities (and pressure drops).  The pump ran intermittently when flow was needed -- so the fluid started and stopped 24/7 as it sent fluid to the process across the room.  It was a GREAT design!  But it didn't work!  Why?  The fluid was a suspension -- not a simple fluid.  

For a shear-thinning suspension, the large diameter pipe minimized flow velocities and therefore took least advantage of the benefits of shear-thinning rheological behavior (lower viscosity for higher shear rates.)  Since the large diameter pipe minimized flow velocities, it maximized the probability that settling would occur (and it did!)  The intermittent use of the pump also provided long time intervals with no flow at all, which further encouraged settling.   

This was a simple, elegant design -- which didn't work because the fluid was a high solids suspension, not a high viscosity simple fluid.

     A Good Example -- The Arizona Coal Slurry Pipeline  

In this case, the pipeline was designed for a suspension.  Not only did they go to a lot of effort to maintain sufficient flow velocities to keep turbulence in the pipeline which was utilized to keep all particles suspended, but they designed it with several holding ponds along the line so when the it had to be shut down for maintenance purposes, they could empty the line of suspension and fill it with water.  Then, to restart the line, the water was pumped out of the pipeline as the suspension refilled it and continued its flow to the destination.

The designers went to a lot of trouble to implement special procedures during operation to handle the unstable suspension.  It was unstable because it contained a relatively coarse particle size distribution and when the turbulence and flow rate were insufficient, particles settled (thereby blocking the line.)  Had this been a piping system for a simple Newtonian fluid, none of these procedures would need to have been implimented.  For a simple fluid, to shut down the line, turn off the pumps.  To restart, turn ON the pumps again.  Slurries don't work like that, however, and they had designed the system properly -- for a coal/water slurry.

The Two Biggest Problems That Occur in Suspensions But Not in Simple Fluids:  Settling and Abrasion

Simple fluids do not have particles that can settle, so the settling issue is non-existent in simple fluids.  Simple fluids do not have particles that can abrade, so the abrasion problem is also non-existent in simple fluids.  Yes -- cavitation in simple fluid systems can abrade/erode system components, but cavitation occurs during severe conditions -- high velocity flow, high speed pumps and propellors, etc.  Even cavitation problems pale in comparison to suspension abrasion problems.

     Settling

Settling occurs in suspensions when flow velocities are low, or during down times when no flow occurs.  The issue here is that suspensions contain particles and particles settle.  Simple fluids do not contain particles, so particle settling cannot occur.  All non-colloidal particles (sizes greater than about 1 micrometer are defined as non-colloidal) are affected by gravity.  They will settle.  The larger the particle, the faster it will settle.  Colloidal particles generally do not settle because they are small enough to be kicked around by the Brownian motion of the fluid molecules.

Whenever suspensions are flowing, settling can be minimized.  The faster the flow rate, the more turbulence will occur, and the less settling will occur.  But once a particle has successfully settled, it can be difficult (or even impossible) for it to be re-entrained into the flow stream again.  This depends on the nature of the chemical additives in the suspension.  This is another distinguishing feature between suspensions and simple fluids.  Suspended particles require chemical additives to control their surface charges to minimize/maximize their attraction/repulsion and their propensities to settle.  Simple fluids have no such requirements.  

One can easily tell whether a piping system has been designed for a suspension or for a simple fluid.  Simple fluid systems are generally single legs from the pump to the process that can be run continuously or intermittently as required.  Suspension systems are generally flow loops from pump to process (and return) which are designed to be run continuously to maintain all particles in suspension -- even when the process is not operating.  In simple fluids systems, larger diameter pipes are beneficial because they reduce flow velocities and corresponding pressure drops.  In suspension systems, smaller diameter pipes take advantage of shear-thinning rheologies.  Higher flow velocities can reduce flow viscosities under shear -- which is beneficial to the overall functioning of the system.  

     Abrasion

As mentioned above, the abrasion phenomenon does not apply to simple fluids.  Generally speaking, there is nothing in a simple fluid that can abrade piping, pumps, or other process devices.  This is certainly not true in suspension systems.  We installed an in-line mixer and an in-line viscosity probe into a coal slurry pipe line.  Both devices had been proven to work well in simple fluid systems.  Both devices disappeared within the first hour of use in the suspension system.  They disappeared because the abrasion was so severe that the probe and mixing head were destroyed, they broke off, and both pieces traveled down the piping system until they got stuck in an elbow or at some other pipe fitting.  

Even when mixing clay water suspensions, abrasion is severe.  We have recommended using HID (high intensity dispersion) in all suspension systems for mixing purposes.  Jim Funk had designed mixing blades with carbide tips for use in HID mixers.  One company tried replacing the carbide tips with stainless steel blocks.  After a relatively short time of use, the stainless steel blocks were reshaped into smooth, teardrop shapes.  It appeared that one could almost touch the spinning impellor without damaging a finger because the blocks were all perfectly smooth -- and they no longer resembled their original, sharp-edged rectangular shapes.  

In pneumatic conveying systems, the dry powders will abrade the piping rather quickly as well.  Flow in chutes and hoppers will also abrade the walls.  All flow systems which contain powders will be subject to abrasion problems.  This MUST be taken into account.

Most Instruments and Gauges Are Not Designed for Suspension Systems

The in-line mixer and viscosity probe, mentioned above, were not strong stand up to suspensions.  Most other common instruments, probes, gauges, valves, pumps, etc., are not build for use in suspension systems.  For example, if one tries to measure suspension pressure with a normal pressure gauge, it will clog with particles and be ruined in relatively short order.  There are pipe devices designed to measure pressures in suspension lines, but they are more expensive.  To measure a pressure in a suspension line, sections of oversized pipe have been designed containing rubber sleeves that are in contact with the suspension on the inside of the sleeve, and in contact with oil as a backing on the outside of the sleeve.  A normal pressure gauge can then be used with the oil to measure pressures.  The suspension pressure is transmitted through the rubber sleeve to the oil and to the pressure gauge.  It is a simple design, but it is much more complicated and costly than using a pressure gauge directly.  

Valves, pump impellors, etc., are also not designed for suspension systems.  High speed pumps with high speed impellors are not suitable for suspension systems.  They WILL work, but not for long, and not with continuous performance levels.  Valves are subject to severe abrasion because they are subject to the high suspension flow rates.  Pipe fittings are also subject to abrasion.  To minimize abrasion, one can increase pipe sizes or use sweeping elbows, but that brings settling problems back into play -- so there is a trade-off.  Too large a pipe diameter minimizes abrasion and encourages settling.  Too small a pipe diameter maximizes abrasion and minimizes settling (IF the suspension is shear-thinning.)  If the suspension tends to be dilatant, abrasion is always severe.

Beware When Trying to Apply Finite Element Analysis Packages for Fluids to Suspension Systems

Most early versions of finite element packages were designed for simple fluid systems.  Some recent versions may be designed to work with suspension systems.  In the author's opinion, an appropriate system for suspensions must track individual particle paths to be useful.  Even in today's world, computers are generally not big enough to handle the large numbers of particles and the large numbers of variables that must be applied to such predictive systems.  (I am willing to stand corrected on this point, but my experience has shown that to be truly accurate, all particles in a relatively large volume must be individually tracked for a model to be truly useful.  This requires a huge computer and long computation times)

The author has seen FEA packages applied to ceramic extrusion systems.  The problem was that the FEA software was incapable of handling particles and therefor could not truly predict the behavior of non-Newtonian rheologies in high solids extrusion systems.  Ceramists were encouraged to use such packages, but they were not accurate because they could not predict dilatant conditions, for example.  If a software package today can truly predict suspension behavior, it will be able to model both shear-thinning and dilatant behaviors and take both chemistry and collisions into account.  

A Systems Approach Must Be Used

One final suggestion:  use a systems approach when dealing with suspensions.  Years ago, we approached a company with lots of experience pumping simple fluids.  We suggested they could begin to practice with and learn about suspensions to begin to build a data base for the pumping of non-Newtonian fluids.  We were told, however, that the two types of fluids (simple fluids and suspensions) were similar and since they already knew how to pump simple fluids, they would also be able to pump suspensions without problem.  Needless to say, when they actually tried to pump suspensions, they had innumerable problems.

Companies using suspensions need to take a systems approach to the process.  It isn't enough to know how the suspension behaves during forming and other major processing steps without knowing how it behaves during simple flow and storage operations.  For example for the simple process of storage, simple fluids do not settle in storage tanks while suspensions do.  For an engineer to assume that a suspension stores, pumps, and generally behaves in like manner to a simple fluid will get that engineer into all sorts of nasty situations and trouble.

Summary  

Suspensions are not simple fluids.  They do not behave as simple fluids and procedures that are useful for simple fluids are often not applicable to the suspensions (and vice versa.)  Suspensions settle, and suspended particles cause major abrasion problems.  These points need to be in the forefront of an engineer's mind when undertaking to design a suspension system.