Regulation drives innovation....Part 1 fuel filtration

I want to be very controversial today and suggest that, in spite of feelings to the contrary by those who believe that the free market should dominate the the technology trends in filtration, the function of government regulation is as important to driving innovation as any amount of customer needs. 

The reason for this statement is that in the last few years of working on filter media technology, areas where regulatory control has been strongest has driven the market to change and innovate. Where there has not been the drivers to make changes, the need for lower cost and commoditisation of products has led to a stagnation in technology. 

The two examples that I want to use are fuel media technology and HVAC. In this blog I will focus solely on fuel filtration. 

Emissions regulation driving the fuel filter market
The need for emissions control into the atmosphere is a major public health hazard globally. Couple this with the rapid growth in car and truck ownership in the Developing World we are facing a major challenge. 

Car and truck emissions of concern are:

• soot particles from diesel trucks and cars. We discussed the filtration of soot in an earlier blog. In this case the challenge is to avoid the emission of soot in the first place. These form potentially carcinogenic particles known as PM2.5's and contribute significantly to smog in the atmosphere.   
• NOx. The formation of nitrogen oxides is a by-product of high temperature combustion. Reducing the fuel consumption is a key route to reducing the levels of emissions. 

Both of these factors have led to increasing regulatory control globally. The leadership is driven by the Europeans through a series of emissions limits over the years known as Euro I-VI. These couple the particulate and NOx emissions and set stringent demand on new vehicles on the roads. 

Outside of emissions controls, many countries e.g. UK and Germany have used tax as a powerful lever to ensure that the cost of fuel remains high (In Germany the tax is €0,83/litre) and to drive manufacturers to be more fuel efficient and reduce the consumption of fuel. 

There are many technology requirements needed to achieve these targets including exhaust catalyst systems and with increased control on the dosing of fuel to the injection systems. The latter has driven a need for cleaner fuel in both petrol and diesel systems. 

In particular diesel powered vehicles (due to the lower fuel consumption but higher particulate emissions) has seen both a greater growth in usage and also a greater focus in terms of technology and innovation. 

Diesel fuel has a range of challenges to meet the requirements of the regulations due to its inherent dirtiness. The key issues are: 

• water content: this varies regionally over the globe with the highest levels in China, India and Brazil. The diesel filter has to shed the water to ensure that the droplets don't corrode the the injectors and inhibit clean ignition.
• waxes and broad chemical composition. Diesel, unlike petrol has a wider range of chemical composition with much higher molecular weight organic components. These are often waxy, particularly at lower temperatures leading to blockages of fuel filters requiring the fuel to be pre-heated to around 70C. However the sources of material for diesel also alter. Diesel is primarily a hydrocarbon from oil but, increasingly, bio-sourced components are being used for a portion of the diesel fuel e.g. rape seed oil or soya oil. The behaviour of these materials is much more aggressive to filters than traditional mineral oil based diesel.   

• inherent particulate concentration in the fuel. As with water  variability n the quality standards to which diesel is manufactured leads to different levels of free particulates as can be seen from the global map from Bosch. Higher levels of particulates demand longer lifetime elements in terms of dust holding capacity. 

Increased performance specifications for diesel fuel filtration

These increasing demands of performance have seen the market move significantly over the last 10 years. The demands for increased diesel fuel filtration performance has seen a move from media with 80-90% 4 micron efficiency to ISO 19438 to 99.5% 4 micron efficiency at the same level currently. This was achieved initially through the development of more efficient cellulose media, operating at the limits of a standard paper machine (1-2 cfm permeability). IN the US addition of glass increased this efficiency further. However the risks of glass fibres being washed from the filter into the fuel line, damaging the engine was not accepted in Europe. Increasing the efficiency decreases the lifetime of the element as the initial pressure drop increases. Thus composites have become required (see my previous blog on gradient density). Initially these were single layer cellulose meltblown composites with PBT (for chemical and heat resistance) but as the targets for efficiency have been driven higher by Bosch and others, the limits of cellulose have slowly been surpassed and multi layer all synthetic composites will be the requirement for the future or cellulose based composites where the cellulose acts solely as a pleatable backer to enable the material to be processable on standard pleating lines.    

The increasing need for water separation has led to the development of complex dual stage elements. Water is always present in all fuel. The water droplets in all fuel are created by the actions of pumps and are stabilised by the presence of surface active ingredients in the fuel such as lubricity additives and anti oxidants. With increasing use of bio diesel, the long chain fatty acids of natural oils also behave as surface active components, reducing the surface tension and both increasing the stability and decreasing the particle size of the droplets. The result is a need for a coalescing, water shedding pre-filter with an excellent water separation to ISO 16332. In many modern fuel filter assemblies the result is a two stage water and particle separation for diesel. 

Summary

The increasing complexity of fuel filters over the last decade has not been driven solely by process economic but by regulation. The fact that the latest standards in fuel filtration have been set in Europe are related to the increasingly tight regulations in Europe. The fact that China is still only now implementing Euro IV standards of emissions cleanliness whilst the EU is at Euro VI show that the regulatory environment is driving the technological environment. If we had left this solely to the market, would Europe have developed the most advanced fuel filtration systems in the world? Probably not- this market, like others in filtration would have ended up with low cost static performance, not the vibrancy of technological innovation. 

The high levels of environmental cleanliness are the least that our children should expect from us. For once the regulators are not wrong and are setting the standards that drive innovation. 

Soot Filtration: How can you test for it?

In a recent blog on aerosols and dust I mentioned briefly soot testing. In this blog, I want to go into more detail about soot testing and some of the challenges and possible solutions that you can find out there. 

What is soot?
Soot is the result of the incomplete combustion of organic materials such as coal, wood, or liquid hydrocarbons. Soot particulates are associated with much of the urban pollution in both the developing and developed world and are often a major contributory factor to pollutants known commonly as "PM 2.5". As we will see, this is not technically correct for soot but the implications of these particulates on health cannot be underestimated, contributing significantly to premature deaths in extreme cases. 

The soot particle is not like a typical dust where the material is a defined material with a defined particle size range. Typically starting at about 50nm the hot soot particle from incomplete combustion will grow through particle coalescence over a short interval of time to around 100-120nm cooling from a hot, sticky particle to a cool, resinous material which is much harder. 

The result is that the behaviour of soot when interacting with filter materials can and does alter as a function of time so, a smaller hotter particle will stick to the surface of a filter whilst a cooler, harder particle will behave in a much more traditional way. 

The other issue to be aware of is that the chemical composition, it's softening point and particle size also depends upon the chemical nature of the combusting material as well as the level of oxygen at the point of combustion. This means that use of poorly defined materials such as kerosene can lead to significant differences in the nature of the soot. 

How does soot impact my filter? 
Soot in the air has a significant impact on the lifetime of filters, this is a well known and accepted fact. It leads to shorter operating lives of all air filters. Soot build up is also seen in lube/oil filtration where it enters into the oil and, again, can impact the lifetime of the element. 

Beyond this hot soot also passes through air filters and builds up on the airflow and other sensors in the air management system of a car or truck.   

How can I measure soot filtration? 
Simulating soot is one of the hardest challenges in filtration testing today. The inherent instability of soot particles makes standardising the testing protocols very challenging and this is, as of 2014, still in development. There are a number of different solutions in the market, the simplest being a small kerosene burner (Often known in the US as "Tiki Torches") feeding soot into an airflow. A typical example from Ahlstrom is shown below. 

Soot test stand set up (from Ahlstrom Particles Q1 2012 p3)

Other companies also have such a test set up including the test institute IBR. These systems can generate a very consistent particle size distribution between 90 and 150nm (if measured).

However the only problems that these set ups have is the probable inconsistency of the soot generated due to the reasons stated above due to the fuel, face velocity factors meaning that particle growth time and particle temperature at the surface mean that the nature of the particles can and will be different based on test scenario. Inconsistency and non traceability is a key issue that has to be addressed.

One final challenge with soot is the inherently long run times (30 minutes testing led to little or no pressure drop increase at 15cm/s on a typical flatsheet air filter sample). 

The issue of slow loading is a fundamental one in soot testing. If the concentration of soot is too high, it is not easy to measure the particle size because the number of particles per volume of air becomes too high and the actual particle size will be larger as the close proximity of the soot allows for more agglomeration of the particles. This is seen in all soot testing set ups. 

Exhausts
The obvious source of soot is the source that generates it. Some set ups use diesel engines as a source of soot (one ex colleague of mine once tried to use this as a justification for the company to buy a Bugatti but that didn't really work), funnelling the soot through to the test stand from there. Another, less subtle option used by a filter manufacturer with labs in central Stuttgart was to apparently syphon the air from the polluted street outside the building. However in both of these cases reproducibility elsewhere is the major challenge.    

Gas soot generators- Chilling off
In trying to standardise soot testing the ISO standards committee have tried to remove the inherent variabilities of such approaches and standardise the protocol. The current proposals are to generate a soot standard to ISO DIN 12103 which would be an annex to the current automotive air test standard (ISO 5011). The fuel here is ethylene or propylene burned in a controlled oxygen environment using nitrogen to control the degree of combustion. The soot is then quenched to fix the particle size and stop further coalescence and agglomeration of the particles. 

Such systems are already in commercial production by companies such as Matter  (known as REXS) and are used for both filtration testing and for testing of other systems (such as DPF catalysts by Bosch for instance).  This system can generate between 50mg/hour at 80nm and 3g/hour at the larger 200nm by increasing the concentration of the particles. 

The advantages of such a system is clear. The fuel is well known and chemically controllable, using a controlled quenching enables the particle size to be fixed easily. The biggest disadvantage however is the cost of the generator itself. 

Graphite erosion
One proposal from Palas is for a spark erosion of graphite. Such a system would use a high voltage system to erode a graphite rod in an inert atmosphere to generate particles of average particle size 98.9nm. The only disadvantage of this system is the relatively low rate of soot generation (0.06mg-7mg/hour) but this can be rectified by use of a small sample size. A concept developed by me shows the layout of such a system. 

concept soot test stand using spark erosion (Tony Lawson)

No soot at all? 
A more traditional approach to evaluating soot is to find an alternative aerosol with a similar particle size such as KCl. This has been used successfully to replicate soot. The biggest disadvantage is that it isn't soot. 

Soot Testing to EN779
A variant of the ISO 12103 proposed protocol was demonstrated to me recently by Unifil and really impressed me. Using propane to generate a quenched sample of average particle size 95nm which is able to test flat sheet samples very effectively to 450Pa. A typical sample on a F8 rated media showed the impact of soot with a DHC of around 2gsm to soot compared with a typical DHC of 237gsm using SAE ISO fine test dust on the same material using a flatsheet test stand. This is a significant demonstration of how  rapidly soot cant lead to a loss in performance. 

So in summary, soot is a nasty material that we have to remove from air. However it is a difficult material to simulate in a lab compared to real life and whilst innovative solutions exist to test soot these are yet to be formalised into documented, approved ISO standards. 

Thanks again for following my blog...until next time

Tony

Aerosols and Dust.....

In my blog on filtration testing, I alluded to the use of challenge dusts and aerosols for testing. In this blog I'd like to address this issue a little further. 

All filtration testing requires a predefined standardised material of known particle size to effectively determine the efficiency of the filter whether by arrestance (gravimetric efficiency) or by fractional efficiency using optical particle counters or Scanning Mobility ParticleSizer.

The selection of test aerosol or dust is determined by the ISO standard selected but also depends upon the nature of the filter being tested. Finally the selection of the aerosol has to align with the environment that the final filter will operate in. For instance, having a good filter for sand and dust will not give a good prediction of lifetime in a city where soot in the atmosphere will shorten the lifetime of the filter. 

The testing protocols used widely in the filtration world generally, not perfectly, reflect this. ISO committees all over the world spend many days drinking coffee and debating the detail of how filters should be tested. The resultant standards reflect the real life status and the standards continue to alter over time. 


Aerosols

….in automotive air

For lower efficiency media such as automotive air and heavy duty air, a dedicated, controlled particle size aerosol would not have the level of efficiency to make the measurement of fractional efficiency effective.  Consequently it is much more effective to measure the fractional efficiency of the challenge test dust.

...in lube, fuel and hydraulic applications

The major limitation in liquid filtration testing currently is the resolution of the OPC (Optical particle counter) technology. In air these go down to nanometer scale but in liquid filtration the technology only has a resolution to the micron scale. The current limit specified by all the ISO standards for liquid multipass filtration is 4 microns. More recent technology from Pamas in Germany has taken the limit down to 1.5 microns. This relatively poor resolution favours a relatively coarse test dust and the ISO standards all state the use of SAE ISO Medium test dust both for loading and as a measure of fractional efficiency. 

…in HVAC and industrial applications

For higher efficiency filters such as HVAC air cleaners, the selection of an aerosol is essential to measure the filtration effectiveness as measuring the fractional efficiency of the contamination dust would be ineffective. Consequently dedicated aerosols based on DEHS for EN779 and KCl are used to measure the fractional efficiency whilst a separate contaminant dust is used for loading. The particle size range is dictated by the test standard used and typically range from 0.3 microns to 10 microns.

The aerosols are statically discharged to remove the effect of static build on the aerosol particles up that can artificially raise the efficiency of the media.

…in HEPA and ULPA

For the highest level of filter efficiency the choice of aerosol is driven by the need to determine not simply the efficiency but the penetration of the particles. In a filter with efficiencies >99.95%, this means that <5 particles out of 10,000 are penetrating the media. Consequently the tests are carried out at a particle size known as MPPS (Most Penetrating Particle Size). The definition of this comes from the fundamental theories of filtration.

In larger particle sizes the key drivers of filtration mechanisms are driven by the fluid flow this leads to a linear particle flow into the filter where particle capture through inertia and interception dominate. As the particle size decreases, the probability of these events happening decreases.

At a point of a particle size of about 0.13-0.18 microns, the particles are so light and small that the macro effects of the fluid flow are overtaken by molecular buffeting and the particle starts to be moved sideways as opposed to forwards, this creates diffusion filtration mechanisms and below these particle sizes the efficiency effectively starts to rise (see figure below).

MPPS Efficiency Curve

The inflexion point is known as the MPPS, most penetrating particle size. Typically this lies between 0.13 and 0.18 microns and at this point the maximum aerosol penetration occurs.

Measurement of efficiency at this point focuses on a single particle size leading to a different type of particle counter which is range specific and typically a SMPS type design. The aerosol for HEPA and ULPA testing are typically oil based DOP (Dioctyl phthalate) or DEHS (Di ethyl hexyl sebacate).

Contaminant Dusts

The measurement of a filter lifetime is determined by the time to load up the filter with a contaminant to a known pressure drop. In order to achieve this, the filter has to be challenged with a realistic test that represents the environment in which the filter will operate. The will depend upon a whole host of environmental issues related to the location. Some examples are:

•         Deserts and arid areas: Dust and sand
•         Major conurbations, China and India: Hydrocarbons, soot and dust
•         Sea: salt spray
•         Mines: silicate and coal dust
•         Building Interiors: Household dust

Each environment has a major challenge associated with it. The nature of the contaminants, particle size and the chemical properties associated with it. Each has an ability to significantly shorten the lifetime of the filter element and impact the overall performance through pore blocking and rapid pressure rise.

Each of these topics is a major subject in itself but in essence the ISO tests, to a degree try to reflect the environmental challenges in which the final filter will operate.

SAE ISO Test Dusts

The most common standard contaminants used for loading studies are based on the SAE ISO test dusts. There are 4 common dusts from Arizona in the US specified to ISO 12103-1. Details of the compositions of these dusts by volume (not weight) are shown in the Table below.

SAE ISO Test dust compositions by volume

Of these, the most commonly used test dusts are:

• A2 ISO fine test dust is widely used for automotive air filtration with some limited use of A1 ultrafine test dust for higher efficiency filter media.
• A3 ISO medium test dust is specified for liquid filtration where the particle counters are unable to operate effectively below 4 microns.

ASHRAE Test Dust

For higher efficiency HVAC applications, the specially formulated ASHRAE 52.1 test dust is specified. This is a blend of:

• 72% SAE ISO A2 Test Dust
• 23% Carbon black powder
• 5% Milled cotton linters

The cotton linters can limit the ability of standard dust feeders to cope with the long fibres and the standard feeder for ASHRAE 52.2 and EN779 is in fact a conveyor belt with a vacuum slot to suck the dust in pre-cut segments into the airflow. For large elements this is suitable however for smaller flatsheet stands this is inadequate as it is too crude for the much lower airflow. Consequently the flatsheet testing in H&V has to be undertaken with SAE ISO A2 medium test dust which makes a suitable alternative.

KCl or NaCl loading

For most HEPA and ULPA applications testing, the loading capacity is not a measured outcome of filtration testing. However for some applications such as face mask, or higher efficiency air applications, NaCl or KCl loading can be seen as a viable test of element lifetime. However the testing time is extremely long and therefore the test requirements look at loading to a limited pressure drop increase or to a limited mass (Facemasks)

Soot

The growth in pollution in developing countries and conurbations in Western cities has led to a significant growth in trying to understand the impact of soot. Unlike a standard dry, hard particle such as Arizona test dust, soot is a complex organic material that doesn’t have a specific, controllable particle size.  Typically starting at about 50nm the hot soot particle from incomplete combustion will grow over an interval of time to around 100-120nm cooling from a hot, sticky particle to a cool, resinous material which is much harder. This inherent instability makes standardising the testing protocols very challenging and this is, as of 2014, still in development. Use of KCl as a viable alternative in terms of particle size distribution has been validated but not fully accepted in the market. There are commercially available soot generators from Matter Aerosol and some excellent new ideas from Palas but many test systems use home made soot generators using kerosene as a basis. 
As with KCl/NaCl loading, the real challenge is to be able to effectively load with soot in a realistic test timeframe. 

As usual, if you have any comments about this or any other blogs, feel free to comment and contact me.