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Plant and Equipment Wellness, Part 1: Observing Variability

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ChE Plus Newsletter Volume 3, Issue 1

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MS Excel Spreadsheet Contest, $100 Cash Prize!

In this issue:

New Article Available

vol3iss1_refinery.gif (9033 bytes)Petroleum Refinery Planning and Optimization Using Linear Programming
The petroleum refining industry has effectively embraced the software solutions to optimize the business supply chain to maximize the profit margins and create order in the chaos of numerous opportunities and challenges.  Find out how they're doing it, read the full article.

The Leaf Filter Screen Cake Trap

The problem of an improperly constructed filter cake caused my company to lose $250,000 profit over a two month period. It is a factor of leaf filtration poorly understood by plant operators and maintainers.

Successful leaf filtration requires a properly formed and structured filter cake. It is the cake that does the filtering. The screen simply supports the cake. The cake is intentionally built-up on the screen to have certain properties. If those properties are not present, you will not get a good flow, or a clean product, from the filter.

The filter cake must trap the material to be removed, yet still pass the fluid being filtered. Hence the passage ways through the filter cake must be large enough to allow a reasonable flow rate, yet be narrow and twisted to stop foreign material getting through.

vol3iss1_leaffilter.gif (82680 bytes)

Over a period time the removed material build's-up and grows thick on the screen. Once the passage ways through the filter cake get blocked the flow rate drops and the pressure difference across the cake rises. At that point the flow rate is too slow the cake must be discharged and a new filter cake applied on the clean screen.

In creating the cake it may be necessary to first apply a precoat on the screen. Especially when removing very fine particles. The precoat is usually a medium of fine fibres that layer in criss-cross fashion over the screen. Diatomaceous Earth (skeleton of diatoms - tiny planktonic organisms) is a common precoat. The precoat bridges over the screen openings and stops the screened material, forcing it to build-up and thicken on the precoat.

"The sand particle size range had altered! The passages that used to naturally occur when the sand cake developed had disappeared! The answer to the problem was to find out why the particle range had altered."

When using precoat you do not want to blind the screens by filling up the holes. Select a screen hole size that is suitable to the particle size range to be removed, while still allowing any precoat to sit over the holes without blocking them. It may be necessary to conduct some trials and to get advice from both the filter manufacturer and the precoat material supplier when sizing the screen and precoat medium.

There are also situations when a body feed is continuously added into the steam to be filtered. The intention being to continuously create fine passage ways through the growing filter cake. By making the cake porous, it is possible to get longer filter runs before the flow drops too low, and the cake has to be removed.

The problem that occurred at my company was a classic misunderstanding of how to form good filter cakes. We filter sand out of a viscous liquid. The sand is used to make the filter cake. No precoat is required. If we take what we now know about creating a filter cake, it is clear that there must be sufficient numbers of small passages through the cake to pass the liquid.

Typically the sand cake was 5 mm thick at the start of a filter run and grew to 15 mm thick before cleaning the screens. The sand needed to pack together, yet leave minuscule passages around the sand particles. To do this it was necessary to have sizes and shapes of sand that left gaps in-between when they pack together.

The symptom we suffered was very low flow rates no matter what pump pressure was applied. We just could not get liquid through those screens!

From what you now know about making good filter cakes, can you guess what had happened?

The sand particle size range had altered! The passages that used to naturally occur when the sand cake developed had disappeared! The answer to the problem was to find out why the particle range had altered.

We found the problem to be a tank agitator that had worn down its blades from stirring action against the sand. With the blades reduced in diameter there was not enough agitation to suspend the bigger, heavier grains of sand. They remained at the bottom of the mixing tank and only the smaller, lighter grains were transferred into the process.

When the smaller sand particles were used to make the filter cake they packed together closer than ever before and produced very small passages between the grains. So small in size that little liquid could pass between them.

Be careful what happens when your filter cake is constructed. Because the cavities in the cake must be sufficient in quantity and size to get the flow through the cake, yet still filter-out your contaminant.


FREE ASTM Viscosity Calculator

By utlizing the ASTM D341 equation, this calculator will extrapolate a visocity curve based on the recommended method.  The user enters two viscosity points at two known temperatures and a curve is generated.   Grab this free spreadsheet here.

MS Excel Spreadsheet Calculation Contest

vol3iss1_spreadsheet.gif (5149 bytes)We're looking for the best process engineering calcuation spreadsheets out there.  Enter our contest and a chance to win a $100 prize!   The spreadsheets will be judged on the following criteria:

  • Usefulness of the calculation(s) performed

  • User-friendly interface

  • Accuracy of the calculation

Email us your spreadsheet today.  The winner will receive a $100 cash prize!

Who are the Best and Brightest Suppliers?

As a process engineer, you've most likely utilized a supplier as a technical resource to help you solve problems in your plant.   A supplier with the technical expertise that you need can be an extremely valuable resource and often the advice is free! 

But, have you had a supplier go above and beyond your expectations?  Maybe a supplier spent days at your plant site to help you solve a nagging problem.  Or, perhaps a supplier recognized a process change or modification that saved your company thousands of dollars. 

Tell us about your experiences and we'll share them in our upcoming newsletters.

Tell Us About Your Favorite Supplier:
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Supplier's Company Name:
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What did they supply?
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New Software Titles Available

We've released a brand new software titles in our online store.

Ezze Sim

EZZE SIM is an Excel “Monte Carlo Simulator” Add-in which contains two working Dryer (fluid bed & spray) simulations. The simulations can be used for your drying application or as a template to develop your own process simulation using the step by step instructions contained in the online users guide. If your process can be described mathematically (mass - energy balances and process correlations) you can simulate the process!

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