Purchas D. Handbook of Filter Media

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Handbook of Filter Media

by Derek B. Purchas, K. Sutherland (Editor)

• ISBN: 1856173755

• Publisher: Elsevier Science & Technology Books

• Pub. Date: November 2002

Preface

The appearance of a second edition of a technical handbook is a sign of the

publisher's confidence in its value to its readers. This confidence is bred from the

success of the first edition, but it must also involve the changes and additions to

the second edition that make it an advancement upon the first. A purpose of this

preface is to highlight some of those changes.

The first edition was written entirely by Derek Purchas, the doyen of

consultants in the technology of filtration, certainly in the UK, and probably in

the world. In its preparation Derek had the assistance of inputs from media

manufacturers of all kinds, sizes and geographical bases. That assistance is no

less a part of the second edition, and another purpose of this preface is to

acknowledge that assistance.

The Handbook remains very much as Derek first wrote it, in terms of style,

structure and content. Partly this is because the technology of filter media is very

well established, so that a great deal of the material of the first edition embodied

timeless features of the technology. The basic designs of filter have been in

existence for over a century, and the six or so years since the first edition have not

seen any major changes in filtration equipment.

However, there are significant changes in this second edition, of both structure

and content, and, not surprisingly with a different main author, of style. The least

of these is that the chance has been taken to make some textual corrections. Some

additional emphasis is placed on air and gas filtration throughout the Handbook,

especially in the more utility uses for such techniques.

The arrangement of the Handbook is similar to the first edition, although there

are now two extra chapters, one formed by splitting one of the earlier chapters

('Textiles') into two, and the other being new ('Standards').

In terms of filter media technology, this second edition notes the following

trends:

9 the increased requirements for hot gas cleaning, involving advanced

polymeric, metallic and ceramic media;

9 the continuing development of membrane media, with an ever wider

range of polymer compositions available;

9 the rapid growth of cross-flow filtration as a process option;

9 the increased use of composite media;

xviii Handbook of Filter Media

the growth in availability of the various non-woven media made by

spinning molten polymers; and

the expansion of those media once thought of as restricted to reverse

osmosis and ultrafiltration, i.e. those used for molecular separations, into

the microfiltration range (and the parallel appearance of nanofiltration as

a separate technology).

A technological change of a very different kind has been the appearance of the

World Wide Web on the Internet, with many suppliers taking advantage of its

existence to design and publish Web pages of their own. These sites vary

enormously in quality, from little more than a company's name and address, to a

complete inclusion of a product catalogue running to hundreds of pages and

being a joy to read.

Although not a technological change, mention must also be made of the far-

reaching changes in the corporate structure of the filter media marketplace.

Many of the companies named in the first edition no longer exist as separate

entities, if they exist at all. Of the 7 3 companies that advertised in the first edition

over one-quarter no longer exist under those names (or have undergone major

changes in their interest in filtration media).

Many of the specified media in the first edition are now available from a

different supplier (or differently named at least). To some extent, this process is

limiting the range of media available to the end user or filter OEM, although small

companies are also appearing to make up the numbers.

A quick skim through the Handbook will show that it includes a considerable

amount of proprietary information. This has been made available by the various

manufacturers, in the interests of value to the reader, and the authors and publisher

are exceedingly grateful to the manufacturers concerned. Acknowledgement is

duly made in the text of the source of such material. Acknowledgement is also made

to those members of staff of filter media manufacturers who have given their time

in the identification of important new ideas in materials and media.

Some suppliers ignored requests to help, and some even expressly declined to

contribute. To those companies, it is hoped that this edition will provoke regret

that they did not take part in the exercise. Perhaps the appearance of a second

edition may spur other suppliers to take an interest in the future.

There is no wish by authors or publisher to make this Handbook into a buyers'

guide of filter media. Accordingly, no attempt is made to mention or even list all

the suppliers of any particular type of medium. Those suppliers and their media

referred to are deemed sufficiently representative of the type under discussion -

but they are also among the limited range of suppliers who responded favourably

to a request for help for this edition.

As to the authors: Derek Purchas, now effectively retired, was introduced

earlier as the author of the first edition of this Handbook. He also wrote a classic

textbook on solid/liquid separation processes, and co-edited a textbook on

equipment scale-up. Ken Sutherland is a consultant, mainly in industrial

marketing, specializing in the filtration equipment and materials sector. He is a

co-author of Elsevier's Decanter Centrifuge Handbook (and of a textbook on

Preface

xix

industrial market research). So that the reader will know where to place the

blame for any errors in this edition, it is only fair that this Preface should be

signed by

Ken Sutherland

1 May 2002

Foreword to the First Edition

'I worked hard at altering the arrangement of the first three chapters of the third

volume. What labour it is to make a tolerable book, and how little readers know

how much the ordering of the parts has cost the writer!'

These heart-felt words were written in his diary on 6 February 1854 by the

eminent historian Lord Macaulay, at a time when he was struggling to make

further progress with his monumental (and highly readable) multi-volume book

The History of England.

Almost a century and a half later, that entry in Macaulay's diary struck a

sympathetic chord as I sought relaxation at the end of another gruelling day's

wrestling with the dry and unromantic desiderata of filter media. So apposite did

it at once appear that I noted it for inclusion, in due course, in this brief foreword

which was then no more than a one-word title, to be expanded when the pain

and agony of the serious writing had been completed. In quoting it now I am

emboldened to couple it with a few lines from An Essay on Criticism by that

earlier bitingly witty author Alexander Pope:

The perfect Judge will read each word of Wit

With the same spirit that its author writ:

Survey the Whole, nor seek slight faults to find

Where nature moves, and rapture warms the mind.

In reality, a technical book is inevitably and very properly likely to be

sub)ected to critical assessment and comment at all levels, including both its

breadth of concept and its minutiae - the more so where the book is offered as the

first attempt to provide a comprehensive overview of the entire spectrum of

industrial filter media. A corollary of this touch of realism is recognition that the

resultant book must regrettably contain a variety of errors, oversights or

deficiencies; notification of any of these would be welcomed, so that corrective

action can be taken in future editions.

As will be evident from the very many acknowledgements throughout the

book, compiling it has been made possible only by the generous cooperation of

many companies and of the even more numerous individuals, some of whom

have patiently responded to an almost endless flow of faxes, letters and

telephone calls. In expressing my thanks to all of these, I would like to add an

apology for any errors which, due to faults of misinterpretation, omission or

commission on my part, may have crept into the printed text derived from the

information they supplied.

xxii Handbook of Filter Media

Amidst the legion of those who have contributed to my endeavours since the

first words were entered into my word computer on 8 February 1995, there is

one person to whom special acknowledgement must be made. This is my wife

Margaret, whose patient support has once again enabled us happily to survive

the trials and tribulations which are the inescapable birth pangs associated with

producing a book.

Derek B Purchas

5 September 1996

Table of Contents

Preface

Foreword to the First Edition

Ch. 1 An Introduction to Filter Media 1

Ch. 2 Woven Fabric Media 35

Ch. 3 Non-woven Fabric Media 81

Ch. 4 Wet-laid Fibrous Media 117

Ch. 5 Air and Gas Filter Media 153

Ch. 6 Screens and Meshes 201

Ch. 7 Coarse Porous Sheets and Tubes 261

Ch. 8 Membranes 307

Ch. 9 Replaceable Filter Elements 365

Ch. 10 Packed Beds 411

Ch. 11 Testing Filter Media 453

Ch. 12 Filter Media Standards 497

Glossary 505

Index of Advertisers 527

Index

CHAPTER

An Introduction to Filter Media

The process of filtration is widely used throughout industry, commerce and

domestic life. It covers the production of potable water by reverse osmosis,

through the protection of delicate components from the impact of large solid

particles, and the clarification of beer, to the separation of waste sewage sludges.

Filtration involves the physical separation of one or more components from a

suspension or solution in a fluid, by passage of that fluid through or across a

barrier that is permeable only to some of these components.

The key element in this description is the barrier, permeable only to part of the

feed suspension or solution. This barrier is the filter medium, and a filter is then

any mechanical structure that holds the medium in the most efficient way.

1.1 Definition of Filter Medium

It has rightly been said that the heart of any filter is the filter medium. Unless it is

fitted with an adequate medium, even the most ingenious filter is useless. So

what exactly is a filter medium?

The, now ageing,

Filtration Dictionary and Glossary t 1)

defined a filter medium as

'any permeable material used in filtration and upon which, or within which, the

solids are deposited'. This definition was not broad enough for the first edition of

this Handbook, because it surprisingly assumed that only solid particles are

relevant, and therefore excluded the many instances where particles comprise

droplets of liquid. The following improved definition was then suggested as

sufficiently all embracing: 'A filter medium is any permeable material upon

which, or within which, particles are deposited by the process of filtration'.

However, even that breadth of coverage is no longer enough, because a large

proportion of filtration operations now concern molecular and ionic species in

solution. Accordingly, a revised definition is suggested in this new edition of the

Handbook, to take note of this additional need:

A filter medium is any material that, under the operating conditions of the filter, is

permeable to one or more components of a mixture, solution or suspension, and is

impermeable to the remaining components.

2 Handbook of Filter Media

The retained components may be particles of solid, droplets of liquid, colloidal

material, or molecular or ionic species in solution, while the permeate (or filtrate)

will normally be the suspending fluid or solvent, possibly together with some of

the other components.

The purpose of this Handbook is to describe the materials used to make filter

media, to highlight their main characteristics, and to advise upon their selection

and use.

1.2 Filters and Their Media

In the definition of filter medium given above, the nature of the filter itself is not

defined. A filter is any device in which a separation is achieved among other

components of a suspension or solution, in a fluid - which may be a liquid or a

gas - where the separation is caused by mechanical means, without the

involvement of a change in phase (such as the melting of a solid, or the

evaporation of a liquid). Filtration is almost entirely a characteristic of the size of

the particle, droplet or molecule being separated, whereas relative particle

density is a more important feature in sedimentation.

As is often the case in attempting a perfect definition or classification, the

boundaries involved in defining filtration in this way are less than absolutely

precise. Thus, some membrane separations do involve a change in phase: some

filtration processes involve electrical as well as mechanical forces: and some

processes involve chemical forces as well as physical ones. Most significantly, the

process of dry screening (sieving or sifting) involves the passage of part of a

mixture of granular solids through a screen - the filter medium - with little or no

passage of a fluid at all, yet this is clearly a filtration process.

That said, the bulk of filtration processes involve the removal of particles,

droplets or molecules from a fluid, by means of a physical barrier, the filter

medium, through which they will not pass by virtue of their size. A particular

form of filter may be able to use a wide variety of filter media, to achieve the same,

or different, separations.

1.2.1 Purposes of filtration

There are two main purposes in filtration:

9 to remove impurities from a fluid, and

9 to recover valuable materials from suspension in a fluid (usually a liquid).

The first of these ('clarification') normally employs finely porous filter media,

and aims to remove as much of the impurity as possible, and preferably all of it.

The second ('harvesting') also aims for as complete a recovery as possible of the

wanted material, but uses coarser media, mainly because the cake of recovered

solid does most of the filtering, and also is less concerned with the clarity of the

filtrate.

An Introduction to Filter Media 3

Here again the division is not exhaustively precise: some harvesting processes

remove waste solids for subsequent treatment, while some clarification removes

only some of the suspended solids (ahead, for example, of a finer filtration

process). Filtration may also be used to classify a suspended solid into two

separate size fractions.

The choice of filtration equipment involves selection both of the right medium,

and of the best type of filter in which to mount it. The harvesting processes are

more limited in equipment choice, because of the need to remove accumulated

solids in relatively large quantity, often leading to the need for complex types of

filter. Clarification duties are much less concerned about solid removal, and so

can be satisfied by simpler (and, therefore, cheaper) filters, with correspondingly

simpler media formats.

1.2.2 Filtration mechanisms

In terms of the way in which a particle, say, is trapped by a filter medium, and so

removed from the fluid, four basic mechanisms can be distinguished. Their

distinctive characteristics are briefly outlined as follows.

(a) Surface straining.

Here the particle that is larger in size than the pores of the

medium deposits on the surface, and stays there until it is removed. Particles that

are smaller in size than the pores pass through the medium. As shown

schematically in Figure 1.1, this is the main operating mechanism for bar

screens, and plain woven monofilament mesh. It also plays a major role in

filtration with membranes.

(b) Depth straining.

For media that are relatively thick by comparison with

their pore diameters, particles will travel along the pore until they reach a point

where the pore narrows down to a size too small for the particle to go any further,

so that it becomes trapped. Felts and other non-woven fabrics utilize this

mechanism, which is illustrated in Figure 1.2.

Now a particle can also be trapped in the depth of the

medium, even though it is smaller in diameter than the pore at that point. Such

Figure 1.1. Filtration by surface straining ~ 2 ~