Blends with lobed fibres like P84? constitute a mechanically less sensitive alternative to membrane filter media, writes Dr. Georg Rathwallner.
Fibre blends in dry filter applications are, when properly made, a way to increase filtration efficiency and process stability and reduce the risk of off times. These blends can achieve advantages concerning chemical stability and may offer an economically favorable solution superior to felts made of one blending partner alone. Felt manufacturers provide a wide range of different blended felts and it is not always easy to figure out for what purpose single components are added. Subsequently mentioned examples show some simple but proven ways to utilise fibre blends to gain some understanding of why special blending partners are used. Blending has a rather negative image as often cheaper materials have been used for blending just to offer felts for a lower price than competition. The list of examples is long, e.g., glass blended into polyphenylene sulphide (PPS) or m-aramide, yellow polyester or kermel (polyamidemide) into P84? (polyimide) felts. This kind of blending usually results in reduced felt quality concerning chemical stability and/or filtration efficiency. Premature failures as a result of fibre degradation or blinding and dust penetration are experienced.

This article is not dealing with possibilities to cheat customers by blending but with clearly declared felt compositions that improve quality and ways to find solutions that are economically and technically favourable by blending different fibre materials. After a short theoretical introduction samples for successful applications of fibre blends in different de-dusting applications of the cement industry will be given. When chosen properly, the blend can combine superior filtration behaviour and chemical resistance, and be better than felts made entirely of one sort of fibre. With fibre blends, it has to be noted that chemical and thermal stability are mainly determined by the major blending partner. Addition of smaller amounts of fibres with higher stability does not significantly improve the stability of the blend even an improvement of the filtration behaviour may be achieved. Scrims out of higher resistant material may prolong the bag life by 10-30 per cent and provide mechanical stability in case of beginning deterioration of the batt material. However, the fibre batt still has to provide its structural stability and the entanglement on the scrim.

Theoretical aspects of filtration efficiency for fibre blends
Different fibre cross sections (round, bean or bone-shaped, trilobal and multilobal (Fig.1) and the fibre titres have influence on the pore size and pore size distribution mainly. Furthermore, adhesion between fibres and dust particles play a major role in dust capture. The mechanical adhesion forces between fibres and dust remain mainly unchanged by blending different fibre materials, but another component – the distribution of static charges on the fibre material – varies between different fibre materials and can be influenced by blending of different materials. However, there is no general rule on how to blend fibre materials in order to optimise the distribution of charges on the fibres, and therefore the adhesion of dust due to these forces is available. Ways to increase the filtration efficiency are:

– Implementation of fine fibres:
which increase the fibre surface and decrease pore size. Blends of different fibre titres on the one hand make it possible to use finer fibres than processible on their own and on the other hand, result in derived, narrow pore size distribution.

– Structured fibre cross section: Trilobal and multilobal fibres increase the fibre surface (when the felt weight is kept constant). In addition low velocity areas exist between the lobes of the fibres where dust can be accumulated without an increase of the differential pressure drop. P84 is known for its membrane like filtration efficiency in combination with mechanical robustness. Pure 100 per cent P84 is an alternative to woven glass with PTFE membrane for kiln dedusting. In applications where chemically and thermally less stable fibres are suitable from a chemical point of view, the filtration efficiency of P84 can be utilised by using it as top cap on the dust side or blended into the surface of other base materials.

– Higher felt weight and denser needling: can improve the pressure drop along the bag life due to decreased penetration of particulate into the felt. By a weight increase, additional stability can be obtained for chemical or mechanical challenging applications.

An alternative approach is to increase the filtration efficiency, using a PTFE membrane. On the contrary to needle felts, the membrane is a thin and mechanically susceptible layer. If handling and/or operating conditions do not respect the limits of a membrane, it will be damaged leading to increased emissions and pressure drop, and reduced life.

Kiln/mill filter
In some kiln/mil dedusting systems, PPS is suitable from the chemical and thermal stability point of view (peaks below 200?C and continuous operation at 150?C or lower).

From the filtration efficiency point of view, it is not sufficient for typical design conditions premature failures due to blinding and mechanical damage are often observed far before the expected end of the service if just the chemical stability is considered. Therefore, blends with P84 fine fibres are becoming more common instead of 100 per cent PPS felts and result in a much better performance on a long term.

At higher temperatures, a material with more resistance against oxidation, e.g., P84 which can cover higher peak temperatures, is the economically favourable solution. A P84 felt will pay back the higher price due to the higher bag life in comparison to PPS, especially at continuous temperatures of 130-160?C or if peaks above 200?C (maximum 260?C) have to be considered. Operation at 220-240?C is acceptable when the raw mill is shut down as long as the weighted average of the temperature remains within certain limits. If the average temperature exceeds approximately 180?C, PTFE/P84 blends have an advantage over 100 per cent P84 because oxidation would limit the life of 100 per cent P84.

Finish mills
The typical material for dedusting of finish mills is polyester, besides polyacrylic. The latter has advantages at elevated water contents. The humidity can reach 20 per cent in case the clinker is not stored before grinding. To keep the temperature in the mill in a typical range of 80-90?C, direct cooling with water is used and polyester suffers from hydrolysis under such conditions. Polyacrylic and polyacrylic/polyester composites are suitable at higher humidity.

With the high dust content up to a range of a1 kg/Nm3, the bags often have to be changed after less than one year. Even polyester is not affected chemically at temperatures around 100?C in the air-like environment, bag failures as a result of increased mechanical damage due to blinding can be observed. Due to the accumulation of dust inside the felt, an increasing pressure drop and increased dust emissions through the felt make a change of the bags necessary. The implementation of polyester or acrylic fine fibres does not always bring the desired results.

In a finish mill with low humidity, P84-polyester blends, which consist of a blend of polyester and P84 (fine fibres of both materials are used) in the filtration sided fibre batt, have achieved bag lives of up to five years due to a surface-oriented dust cake build-up (Table 1 and Fig 2). As a result, a low and stable pressure drop was achieved. Therefore, besides the longer bag life, which over-compensates the higher price, fan energy could be saved. Fairly low susceptibility against grinding aids, which considerably effect the filtration behaviour of the cement dust, or peaks in dust content and a/c ratio could be observed. Even more challenging conditions lead to an increased pressure drop but regeneration of the felt after a normalisation of the filtration conditions could be observed.

Clinker cooler
Clinker cooler is not a major challenge chemically, but for most fibre materials concern peak temperatures and abrasive character of the dust. P84 is advantageous due to its temperature and peak temperature stability. m-aramide and polyamide-imide are suitable from the chemical point of view when continuous and peak temperature can be kept on a sufficiently low level. With the latter two materials, the high dust content and peak a/c ratio often lead to dust penetration into the felt and, as a result of increased mechanical burden, leads to bag failures. Blends with P84 can increase the filtration efficiency and reduce the sensibility against upset conditions (in particular, peaks of gas flow and dust load). A similar kind of blends with P84 can be utilised in case cooling is made down to temperatures where polyester and polyacrylic become suitable.

Alkali bypass filter
PTFE is often used besides fibre glass with PTFE membrane. P84/PTFE blends can ensure a lower pressure drop than 100 per cent PTFE felts, even if 100 per cent PTFE felts usually operate to the satisfaction of the end-user with the relatively sticky and easy-to-filter dust.

The 100 per cent P84 is limited to an average operating temperature of approximately 180?C (depending on the life expectation). Blends from PTFE and P84 are suitable to improve filtration efficiency without a restriction of peak temperatures in comparison to PTFE (Fig 3 and 4 and Table 2). Such a blend is suitable for high average temperatures above 180-190?C.

Due to the higher filtration efficiency, mechanical burden is reduced (less differential pressure, longer cleaning cycle times and less abrasion by incorporated dust) and often a higher bag life than with 100 per cent PTFE can be achieved even a chemical less resistant material is used for blending.

Summary
Fibre blends in dry filter applications are, when properly made, a way to increase filtration efficiency and process stability and reduce the risk of off times. Blends with lobed fibres like P84 constitute a mechanically less sensitive alternative to membrane filter media.

Felt manufacturers provide a wide range of different blended felts and it is not always easy to figure out for what purpose single components are added. The above-mentioned examples show some simple but proven ways to utilise fibre blends to get the understanding of why special blending partners are used.

Higher filtration efficiency is required not only to reduce emissions, but also to enable more stable filtration (less penetration of dust into the felt) which can extend the bag life significantly and reduce the energy consumption because of a comparably low and stable pressure drop throughout the life. In many cases, the total costs can be reduced with higher quality filter media.

Even reasonable improvements can be achieved by optimization of the felt material concerning chemical stability and filtration efficiency, conscientiously no miracles are possible. General misfits of filter layout cannot be compensated by the fabric only. To achieve a good performance, a proper cleaning system, air flow and adequate a/c ratios have to be provided.