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Melt-blown cartridge technology
HNOLOGY a OFMELT-BLOWN CARTRIDGES
Melt-blown cartridge technology Historically, thermally bonded or meltblown depth cartridges were defined as nominal filters, and the term 'absolute filter' was reserved for surface cartridge types. With the arrival of improved manufacturing techniques, however, absolute ratings can now be achieved with depth filters. In this article, Chris FauIder, European product manager for Parker Hannifin's Fulflo Division, explains the changes in definition, and outlines the benefits of both types of filter for the chemical process industry. h e range of fluids used in chemical
processing covers a v a s t s p e c t r u m -T from p a i n t t o food, t h r o u g h to fine chemicals. C o n t a m i n a n t s w i t h i n t h e s e fluids fall into t h r e e categories: [ ] Solids such as sand grit and pipe scale. [ ] Gelatinous particles formed in the mixing process. [] Fibres from hairs and packing materials.
These c o n t a m i n a n t s all b e h a v e differently, a n d need to be filtered by different m e c h a n i s m s . There are two filter types for c a p t u r i n g t h e s e particles. The surface filter t r a p s c o n t a m i n a n t s on t h e outside of t h e filter medium; to increase t h e performance, t h e m e d i u m is pleated to increase t h e surface area. The second type is t h e d e p t h filter, which t r a p s contamin a n t s within t h e m e d i u m (Figure 1). The surface filter is particularly good a t removing solids. However, gelatinous a n d fibrous particles can -- given certain conditions, such as high p r e s s u r e differentials -- be forced t h r o u g h t h e pores, e i t h e r by changing direction in t h e case of fibres, or c h a n g i n g s h a p e in t h e case of a gel. The d e p t h filter c a n remove all t h r e e types effectively, b u t until t h e arrival of new technology, it could not a p p r o a c h t h e efficiency of t h e surface type. There is a t r a d i t i o n a l a r g u m e n t in t h e filter world between t h e t e r m s 'absolute' a n d 2aominal'. Historically, d e p t h filters were were t h o u g h t of as having n o m i n a l
efficiency, while surface filters h a d absolute efficiency. This h a s changed, with t h e i n t r o d u c t i o n by Parker Hannifin of t h e MBC (melt-blown cartridge) absoluter a t e d d e p t h filter medium. The absolute rating, as generally recognised by filter manufacturers, can m e a n a n y t h i n g between 98% ( B e t a ratio 50) a n d 99.98% efficiency ( B e t a ratio 5000). Nominal filtration is typically described in percentage t e r m s as between 80% a n d 90% efficiency. A good w o u n d cartridge ( t h e t r a d i t i o n a l d e p t h filter) can remove 90% of t h e particles of a specified size a p p r o a c h i n g t h e matrix. The hydraulics filter i n d u s t r y only expresses filtration removal characteristics in t e r m s of t h e Beta ratio. On t h e o t h e r h a n d , t h e processing i n d u s t r i e s use p e r c e n t a g e r e m o v a l efficiency ratings, which are j u s t a different description of the s a m e thing. The Beta ratio is t h e u p s t r e a m particle c o u n t divided by t h e d o w n s t r e a m particle c o u n t a t a specified particle size a n d larger. Thus % Removal efficiency = Beta ratio - 1 x 10O Beta ratio With t h e i n t r o d u c t i o n of w h a t is believed to be a u n i q u e process for producing very small-diameter, c o n t i n u o u s l y p r o d u c e d fibres, P a r k e r c a n n o w c o n t r o l t h e capability of d e p t h cartridges to remove particles absolutely, i.e. to t h e 99.98% ( B e t a 5000) rating. This is claimed to b r e a k t h e m o u l d for d e p t h filtration, as it is n o w possible to remove t h e t h r e e types of c o n t a m i n a n t s with d e p t h filtration, a n d does n o t necessarily require t h e higher-cost surface filtration solution. However, surface filters still have t h e i r place. Depending on t h e particle size a n d c o n t a m i n a n t , a d e p t h filter may n o t filter t h r o u g h o u t t h e complete matrix, a n d m a y only t r a p particles on t h e surface. In t h i s case, a surface filter c a n give u p to six times the surface area, giving significantly longer life, dirt-holding capacity a n d flow. A f u r t h e r benefit, w h e n t h e surface filter is designed a n d built correctly, is t h a t it c a n be baekwashed i n s/tu; Parker's PXD filter is designed to do this. A surface filter can also achieve even finer ratings. The Fulfio Advantage can go to 99.99% absolute rating at 0.2 /~m. The difference between 99.98% a n d 99.99% m a y look very small, b u t 99.99% is a Beta 10,000 r a t i n g c o m p a r e d to a B e t a 5000 rating. This higher rating is necessary w h e r e h i g h e r quality is essential, such as in reduction of microorganisms, or in fine chemical p r o d u c t i o n for t h e electronics industry.
l~gure 1. Depth medium (top) and surface medium (bottom).
326
0015-1882/94/US$7.00 © 1994 Elsevier Science Ltd
June 1994
Filb'ation & Separation
@OLOGYOFMELT-BLOWCNARTRIDGES Ttble 1. ~
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.
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Typical filtration requirement Absolute rated depth medium, MBC
Absolute medium
X
X X X X X X
jeJ~
Total solution D e p t h a n d surface filtration can complem e n t each o t h e r in t h e same system, where t h e r e is a large s p r e a d of particle sizes. In this case it is ideal to r e c o m m e n d d e p t h filters as prefilters, a n d surface filters as a final polishing medium, or as a prefilter to critical applications where a m e m b r a n e filter m a y be required.
Comparison of the benefits of depth and surface filters For example, Parker's MBC d e p t h filter -now challenging t h e definition of absolute ratings previously reserved for surface filters -- achieves its efficiency t h r o u g h a graded pore construction. Larger particles are t r a p p e d in t h e o u t e r layers, a n d smaller ones on t h e i n n e r layers (see t h e SEM images in Figure 2). Fulflo technology uses a one-piece g r a d e d p o r e s t r u c t u r e of t h e r m a l l y bonded, c o n t i n u o u s microfine polypropylene fibres. The pore size v a r i a t i o n is
a c h i e v e d by u s i n g fibres of differing diameters, and maintaining uniform density t h r o u g h o u t t h e cartridge. Pore sizes do not change as t h e differential pressure increases d u r i n g service. Thus, once a particle is stopped, it stays there. In t h e past, t h e d e p t h filter would be followed by a surface or pleated cartridge. Now, in m a n y cases, this can b e achieved in one step with t h e MBC technology. The graded construction p e r m i t s t h e removal of a wide range of particle sizes which provides a longer life. The efficiency ratings range from 1 to 70 pm. The filters have a c a r t r i d g e - c o m p a t i b l e polyolefin g a s k e t h e a t - s e a l e d to b o t h ends, or alternatively a double O-ring e n d c a p seal to fit i n d u s t r y - s t a n d a r d housings. The surface filter is b e t t e r for higherviscosity fluids, i.e. u p to 3000 centistokes. Parker's Polymate, for example, gives high surface area, high flow a n d long life where t h e c o n t a m i n a n t s are in a n a r r o w particle size range. Table 1 shows t h e best application for each type of filter.
Conclusion Parker's d e p t h a n d surface filters b o t h use melt-blown thermally b o n d e d polypropylene fibres to give a secure, non-fibrereleasing matrix, using FDA-approved materials. A Polymate surface filter h a s a t h i n n e r medium, pleated to give high surface area, while t h e MBC h a s a m u c h greater depth, a n d is graded to remove different particle sizes t h r o u g h t h e depth, with a new, unique m a n u f a c t u r i n g process to produce c o n t i n u o u s finely controlled d i a m e t e r fibres which now achieve absolute r a t e d efficiency at low cost. [ ]
~ g u r e 2. Scanning electron micrographs of the outer (left), middle (centre) a n d i n n e r (right) layers of the graded pore size construction of Parker's MBC depth filter cartridge ( x 500 magnification). With constant fibre packing density throughout the cartridge, the larger fibres near the outside diameter of the cartridge create larger pores, which trap the biggest particles while passing the smaller ones. As the fibre diameter decreases moving towards the inside diameter, the pores become smaller, thus removing smaller and smaller particles. The innermost layer is the 'final filter', a n d defines the ultimate particle removal rating of the filter.
Filtration & Separation
June 1994
327
Melt-blown cartridge technology Historically, thermally bonded or meltblown depth cartridges were defined as nominal filters, and the term 'absolute filter' was reserved for surface cartridge types. With the arrival of improved manufacturing techniques, however, absolute ratings can now be achieved with depth filters. In this article, Chris FauIder, European product manager for Parker Hannifin's Fulflo Division, explains the changes in definition, and outlines the benefits of both types of filter for the chemical process industry. h e range of fluids used in chemical
processing covers a v a s t s p e c t r u m -T from p a i n t t o food, t h r o u g h to fine chemicals. C o n t a m i n a n t s w i t h i n t h e s e fluids fall into t h r e e categories: [ ] Solids such as sand grit and pipe scale. [ ] Gelatinous particles formed in the mixing process. [] Fibres from hairs and packing materials.
These c o n t a m i n a n t s all b e h a v e differently, a n d need to be filtered by different m e c h a n i s m s . There are two filter types for c a p t u r i n g t h e s e particles. The surface filter t r a p s c o n t a m i n a n t s on t h e outside of t h e filter medium; to increase t h e performance, t h e m e d i u m is pleated to increase t h e surface area. The second type is t h e d e p t h filter, which t r a p s contamin a n t s within t h e m e d i u m (Figure 1). The surface filter is particularly good a t removing solids. However, gelatinous a n d fibrous particles can -- given certain conditions, such as high p r e s s u r e differentials -- be forced t h r o u g h t h e pores, e i t h e r by changing direction in t h e case of fibres, or c h a n g i n g s h a p e in t h e case of a gel. The d e p t h filter c a n remove all t h r e e types effectively, b u t until t h e arrival of new technology, it could not a p p r o a c h t h e efficiency of t h e surface type. There is a t r a d i t i o n a l a r g u m e n t in t h e filter world between t h e t e r m s 'absolute' a n d 2aominal'. Historically, d e p t h filters were were t h o u g h t of as having n o m i n a l
efficiency, while surface filters h a d absolute efficiency. This h a s changed, with t h e i n t r o d u c t i o n by Parker Hannifin of t h e MBC (melt-blown cartridge) absoluter a t e d d e p t h filter medium. The absolute rating, as generally recognised by filter manufacturers, can m e a n a n y t h i n g between 98% ( B e t a ratio 50) a n d 99.98% efficiency ( B e t a ratio 5000). Nominal filtration is typically described in percentage t e r m s as between 80% a n d 90% efficiency. A good w o u n d cartridge ( t h e t r a d i t i o n a l d e p t h filter) can remove 90% of t h e particles of a specified size a p p r o a c h i n g t h e matrix. The hydraulics filter i n d u s t r y only expresses filtration removal characteristics in t e r m s of t h e Beta ratio. On t h e o t h e r h a n d , t h e processing i n d u s t r i e s use p e r c e n t a g e r e m o v a l efficiency ratings, which are j u s t a different description of the s a m e thing. The Beta ratio is t h e u p s t r e a m particle c o u n t divided by t h e d o w n s t r e a m particle c o u n t a t a specified particle size a n d larger. Thus % Removal efficiency = Beta ratio - 1 x 10O Beta ratio With t h e i n t r o d u c t i o n of w h a t is believed to be a u n i q u e process for producing very small-diameter, c o n t i n u o u s l y p r o d u c e d fibres, P a r k e r c a n n o w c o n t r o l t h e capability of d e p t h cartridges to remove particles absolutely, i.e. to t h e 99.98% ( B e t a 5000) rating. This is claimed to b r e a k t h e m o u l d for d e p t h filtration, as it is n o w possible to remove t h e t h r e e types of c o n t a m i n a n t s with d e p t h filtration, a n d does n o t necessarily require t h e higher-cost surface filtration solution. However, surface filters still have t h e i r place. Depending on t h e particle size a n d c o n t a m i n a n t , a d e p t h filter may n o t filter t h r o u g h o u t t h e complete matrix, a n d m a y only t r a p particles on t h e surface. In t h i s case, a surface filter c a n give u p to six times the surface area, giving significantly longer life, dirt-holding capacity a n d flow. A f u r t h e r benefit, w h e n t h e surface filter is designed a n d built correctly, is t h a t it c a n be baekwashed i n s/tu; Parker's PXD filter is designed to do this. A surface filter can also achieve even finer ratings. The Fulfio Advantage can go to 99.99% absolute rating at 0.2 /~m. The difference between 99.98% a n d 99.99% m a y look very small, b u t 99.99% is a Beta 10,000 r a t i n g c o m p a r e d to a B e t a 5000 rating. This higher rating is necessary w h e r e h i g h e r quality is essential, such as in reduction of microorganisms, or in fine chemical p r o d u c t i o n for t h e electronics industry.
l~gure 1. Depth medium (top) and surface medium (bottom).
326
0015-1882/94/US$7.00 © 1994 Elsevier Science Ltd
June 1994
Filb'ation & Separation
@OLOGYOFMELT-BLOWCNARTRIDGES Ttble 1. ~
~*
~
~ ,
Mlttor~ t o ~
~
medlo ~
hir ~
glwm ~
.
i
Typical filtration requirement Absolute rated depth medium, MBC
Absolute medium
X
X X X X X X
jeJ~
Total solution D e p t h a n d surface filtration can complem e n t each o t h e r in t h e same system, where t h e r e is a large s p r e a d of particle sizes. In this case it is ideal to r e c o m m e n d d e p t h filters as prefilters, a n d surface filters as a final polishing medium, or as a prefilter to critical applications where a m e m b r a n e filter m a y be required.
Comparison of the benefits of depth and surface filters For example, Parker's MBC d e p t h filter -now challenging t h e definition of absolute ratings previously reserved for surface filters -- achieves its efficiency t h r o u g h a graded pore construction. Larger particles are t r a p p e d in t h e o u t e r layers, a n d smaller ones on t h e i n n e r layers (see t h e SEM images in Figure 2). Fulflo technology uses a one-piece g r a d e d p o r e s t r u c t u r e of t h e r m a l l y bonded, c o n t i n u o u s microfine polypropylene fibres. The pore size v a r i a t i o n is
a c h i e v e d by u s i n g fibres of differing diameters, and maintaining uniform density t h r o u g h o u t t h e cartridge. Pore sizes do not change as t h e differential pressure increases d u r i n g service. Thus, once a particle is stopped, it stays there. In t h e past, t h e d e p t h filter would be followed by a surface or pleated cartridge. Now, in m a n y cases, this can b e achieved in one step with t h e MBC technology. The graded construction p e r m i t s t h e removal of a wide range of particle sizes which provides a longer life. The efficiency ratings range from 1 to 70 pm. The filters have a c a r t r i d g e - c o m p a t i b l e polyolefin g a s k e t h e a t - s e a l e d to b o t h ends, or alternatively a double O-ring e n d c a p seal to fit i n d u s t r y - s t a n d a r d housings. The surface filter is b e t t e r for higherviscosity fluids, i.e. u p to 3000 centistokes. Parker's Polymate, for example, gives high surface area, high flow a n d long life where t h e c o n t a m i n a n t s are in a n a r r o w particle size range. Table 1 shows t h e best application for each type of filter.
Conclusion Parker's d e p t h a n d surface filters b o t h use melt-blown thermally b o n d e d polypropylene fibres to give a secure, non-fibrereleasing matrix, using FDA-approved materials. A Polymate surface filter h a s a t h i n n e r medium, pleated to give high surface area, while t h e MBC h a s a m u c h greater depth, a n d is graded to remove different particle sizes t h r o u g h t h e depth, with a new, unique m a n u f a c t u r i n g process to produce c o n t i n u o u s finely controlled d i a m e t e r fibres which now achieve absolute r a t e d efficiency at low cost. [ ]
~ g u r e 2. Scanning electron micrographs of the outer (left), middle (centre) a n d i n n e r (right) layers of the graded pore size construction of Parker's MBC depth filter cartridge ( x 500 magnification). With constant fibre packing density throughout the cartridge, the larger fibres near the outside diameter of the cartridge create larger pores, which trap the biggest particles while passing the smaller ones. As the fibre diameter decreases moving towards the inside diameter, the pores become smaller, thus removing smaller and smaller particles. The innermost layer is the 'final filter', a n d defines the ultimate particle removal rating of the filter.
Filtration & Separation
June 1994
327