Adsorbers remove more oil from compressed air

Activated carbon adsorbers promise filter lifetimes up to ten times longer together with much better process reliability and reduced oil residues compared with conventional activated carbon filters.

Constant dripping hollows out a stone, and this is a saying that certainly applies to the process reliability and the profit calculations of compressed air systems, particularly where aerosols are involved, ie fine oil droplets and oil vapour in the compressed air.

A new generation of activated carbon adsorbers now promises filter lifetimes up to ten times longer together with much better process reliability and reduced oil residues compared with conventional activated carbon filters.

Removing solid contaminations, such as dust, soot, rust or unburned hydrocarbons, from compressed air can often be quite difficult.

The challenge is even greater when it comes to the effective reduction of the residual oil content, which ranges on average from 3 to 10mg/m3 and may, in extreme cases, be as much as 300mg/m3.

The energy carrier "compressed air" is basically produced by the compression of atmospheric ambient air containing all the usual pollutants.

Various types of compressors, eg piston compressors, screw compressors, rotary compressors or turbo compressors, perform this work.

During the compression of the ambient air, which is sucked into the compressor, it is unavoidable that the pollutants will also become concentrated - a compression to 8bar, for example, will lead to a ninefold concentration of the given pollution level.

And that's not all: the compressors themselves equally contribute to the contamination of the compressed air, especially through the input of oil.

The total result is a "cocktail" that is hard to assess and has far-reaching consequences, not least because of the likely interactions between the pollutants in the compressed air.

Dust particles combine with water or oil to form larger particles, oil and water can bond and turn into a stable emulsion, and sulphur dioxide sucked in from combustion residues mixes with condensate and becomes sulphurous acid.

The plant operator is then presented with the bill: problems at the pneumatic points of use and damage to the compressed air system itself.

Many sectors of industry, particularly pharmaceutical works, food processing plants or paint-spraying facilities, depend on the supply of oil-free compressed air of a high quality.

Here, the main focus is on the aerosols and oil vapours released in the compressor.

This residual oil can, for example, affect the function of sensitive tool parts at the points of use, wash out the basic lubrication on components, or even contaminate the products at the end of the process chain.

Inside the tools and machines, the oil residues in the compressed air will expand and escape into the environment - with various negative effects.

They may, for example, settle on surfaces and create a troublesome oil film that prevents the proper adhesion of paints or glues.

Furthermore, it is not rare for oil vapours to contain nitrosamines that are harmful to health.

And, at least, it is disagreeable that oil in inhaled air is noticeable as from a threshold value of 0.3mg/m3.

Under ISO8573-1, the residual oil content is classified according to quality, with maximum values ranging from 0.01 to 5mg/m3.

For many critical applications, coalescence filters are employed in the belief that this will meet the requirements of class 1 (maximum total oil content 0.01mg/m3).

However, coalescence filters are only able to remove oil aerosols, besides solid particles, whereas the ISO standard refers to the total oil content, which comprises not only oil aerosols but also oil vapours.

Where the compressed air temperature lies above 2C, the oil vapour content will clearly be higher than the oil aerosol content.

Seen comprehensively, class 1 quality can only be achieved by coupling coalescence filters and activated carbon filters or adsorbers.

For further information see VDMA standard sheet 15390 "Compressed air quality - list of recommended purity classes according to ISO 8573-1", status March 2004.

How does the oil get into the compressed air in the first place?.

There are basically three major sources: the ambient air that is sucked into the compressor, the compressor's cooling and lubrication system, and the oil-lubricated plant components.

If the compressor site has not been carefully chosen, this will, for a start, have a negative impact on the quality of the air entering the compressor.

In certain environments in particular - such as machine rooms, motor vehicle halls and large industrial estates - the air contains a high level of hydrocarbons.

These substances then become more "tightly packed", ie concentrated, in parallel to the air compression process.

The fact that the quality of the compressed air initially corresponds to the quality of the ambient air is of special relevance for the operation of oil-free compressors.

With this compressor type it is very important to ensure that the air in the compressor station does not contain any oil.

However, this will not be possible if oil vapours can escape during gearbox venting - a problem for which there is often no satisfactory solution.

These oil-free compressors are at present still relatively rare in German industrial facilities, where around 90% of all compressors are oil injection cooled.

And it is because of oil injection cooling that by far the largest amount of residual oil gets into the compressed air in the form of droplets and vapours.

The following calculation, taking a typical compressor station as an example, may help to visualise the extent of contamination.

A compressor with oil injection cooling produces approximately 900m3/h of compressed air.

With compressed air temperatures between 25 and 35C, one can assume an average residual oil content of 10mg/m3: 0.01g/m3 x 900m3/h x 24h x 30d = 6.48kg.

This means even with optimum maintenance, an oil-lubricated compressor will export 6.48kg or approx.

7.2 litre of oil into the compressed air every single month.

In this context, it should be realised that the residual oil content stated by compressor manufacturers is normally related to a temperature of 20C.

However, the actual temperature of the compressed air is often significantly higher, resulting in much greater oil vapour content per cubic metre.

Here, synthetic oils have an advantage over mineral oils because the content of volatile oil vapours emitted is lower than for mineral oils of the same viscosity class.

Moreover, synthetic oils support the cold-start performance of the compressor, while oil vapour formation at high temperatures is reduced.

Even the finest coalescence filter is not able to adsorb oil vapour.

Besides separating out solid particles, this type of filter is designed to remove oil aerosols as listed by the manufacturer, for example, for concentrations of 0.01mg/m3 (at 20C and 1bar(a)).

With a compressed air temperature of 35C, the total oil content downstream of the same super fine filter will then amount to 0.1mg/m3 (0.01mg/m3 aerosol plus 0.09mg/m3 oil vapour).

For a residual oil content that satisfies at least quality class 1, let alone for obtaining absolutely oil free compressed air, it will therefore be necessary to incorporate a more effective system: an activated carbon stage.

The type of filter usually employed for this purpose contains cloth elements embedded with activated carbon particles.

The ratio of carrier cloth to activated carbon is approximately 2:1.

In the filter housing, the incoming air is channelled from the inside through the cloth that is wound around a carrier structure.

During this process, the entrained oil is adsorbed on the activated carbon within the cloth.

Although these filter elements have certain advantages - favourable purchase price and low-pressure differential - there are also various drawbacks.

For example, high maintenance expenditure due to short filter lifetimes, fairly low process reliability due to difficult assessment of the actual filter efficiency, relatively short contact time of compressed air and adsorbent during the passage of air through the filter, and filter overrun during a pressureless startup of the compressor.

But there is also good news in this connection: a new generation of activated carbon adsorbers, developed by compressed air specialist Beko Technologies in the German city of Neuss, has now been launched to solve all the above problems.

The innovative "Clearpoint V" cartridge system replaces conventional filter elements wherever long filter lifetimes, minimal residual oil content and high process reliability are absolutely essential.

In contrast to filter elements containing cloth ingrained with activated carbon particles, the Clearpoint cartridge is filled completely with activated carbon.

The amount of activated carbon in the Beko cartridge is at least twenty times as much as in the normal type of filter element.

This results in a number of significant benefits: as the compressed air flows through the cartridge, it continually moves across activated carbon.

The contact time of air and activated carbon is considerably longer than in a filter.

If a super fine filter with an outlet aerosol concentration of 0.01mg/m3 is installed upstream of the Clearpoint V, the total residual oil content (vapour and aerosol) sinks down to 0.001mg/m3 - three times less as with activated carbon filters.

As a result of the generous quantity of activated carbon in the cartridge, the lifetime of the filter is prolonged to an impressive extent.

Whereas the usual filter elements "break through" - ie show a marked deterioration in filter performance - after approximately 250 operating hours, this new filter cartridge can have a service life of around 2500 hours, according to the calculations of the Beko engineers.

A tenfold increase of the usual lifetime.

An oil check indicator, available as an option, allows precise monitoring of the degree of saturation of the cartridge.

The indicator comprises a glass tube mounted on the filter head via an adapter plate.

A chemical inside the glass tube will progressively change colour in relation to the actual oil content in the filtered compressed air.

Treatment components are normally very efficient when they are operated within certain flow velocities.

However, in the case of abrupt air withdrawal or during startup of the compressor with a pressureless network, flow velocities may be so high that the contact time for oil vapour adsorption will be too short, thus allowing oil to break through the filter barrier.

This "oil breakthrough" represents a serious risk to process reliability.

Here too, Clearpoint V offers a valuable advantage: Its specific design considerably extends the contact time inside the cartridge, so that filter overrun due to short peaks in the flow velocity is safely excluded, irrespective of the cause.

Crack formation in the drainage layer caused by excessive flow velocity is yet another risk associated with filter elements, but eliminated with the Clearpoint V cartridge design.

Finally, but not least important, the activated carbon fill of the cartridge is protected by an elastic material provided both at the air inlet and outlet.

On the one hand, this prevents abrasion due to mechanical friction inside the activated carbon granulate, and, on the other, the material acts as a flow distributor on the inlet side to achieve uniform loading of the activated carbon.

In order to obtain a neutral assessment in accordance with the ISO standard of these new activated carbon adsorbers, Beko asked the Institute of Energy and Environmental Technology (Institut fur Energie und Umwelt) For an independent validation of the filter lifetime and efficiency.

All the values have been confirmed.

A fair share of the success is also due to the very high quality of the activated carbon chosen for the Clearpoint V cartridges.

It is worthwhile to have a closer look at this very interesting filtration material.

One could almost suspect that activated carbon possesses certain intelligence.

Small molecules, such as oil vapour, are adsorbed in micropores, whereas larger particles, such as oil droplets, are "directed" to larger pores.

Therefore, the ratio of small to large pores decides which molecules are preferably retained by the activated carbon employed.

The given size distribution of the pores depends on the type of carbon processing.

Activated carbon is produced by chemical or steam activation of carbonaceous materials, such as peat, wood, bituminous coal, petroleum coke or lignite, but it can also be derived from coconut shells, fruit stones or sugar.

During chemical activation, the raw material is dried and mixed with certain substances, for example, phosphoric acid, potassium hydroxide, sodium carbonate, and zinc chloride or sodium sulphate.

The mass is heated, in the absence of air, to 400 to 600C, and during this process the chemicals act on the basic material and remove its hydrogen and oxygen atoms.

The resulting product - after washing and drying - consists of almost pure carbon with a more or less large pore volume depending on the actual reaction conditions.

During steam activation, the carbon of the raw material is partly gasified in a nitrogen atmosphere with temperatures between 800 and 1000C.

The quality of the final product is determined by the reaction temperature and time as well as by the concentration of O2, CO2, H2O, CO and H2.

With both methods, the finished activated carbon is characterised by a huge surface area.

It ranges from approximately 400 to 2000m2/g, ie the same mass can have an adsorption capacity that is five times greater.

Just to visualise the size, 1g of good quality activated carbon has about the same surface area as three tennis courts.

Incidentally, the fill of a large Clearpoint V cartridge - consisting of coal-based, high quality activated carbon with a grain size of 1mm - has a surface area equal to 4130 tennis courts or 171 football fields.

The cartridges are available in three sizes for filtering compressed air at 50, 100 or 200m3/h.

The adsorption behaviour of the activated carbon is not only determined by its quality - it is also important to ensure suitable working conditions.

The temperature is a major factor here.

This is because the adsorption capacity, contrary to the adsorption velocity, decreases as the temperature goes up.

In fact, activated carbon that has been used for relatively cool compressed air and is then exposed to higher temperatures even has a tendency to release substances previously adsorbed, ie there is a risk of desorption.

In addition, the adsorption behaviour of the activated carbon is influenced by the aerosol and oil vapour concentration in the compressed air.

With a low concentration, the activated carbon takes up a relatively small amount of contaminants.

With a greater concentration, it becomes more "retentive" - ie the adsorption capacity increases in line with rising adsorptive concentration.

But this is only true up to a certain point when the effect may be dangerously reversed: If the adsorption capacity of the activated carbon is fully exploited with high residual oil concentrations in the compressed air, and if the same activated carbon is then subjected to continued use with lower concentrations, desorption is very likely to occur.

The substances previously adsorbed will be released again and will thus recontaminate the compressed air.

For this reason, it is extremely important in practice that the adsorption capacity is never exploited to the limit.

Clearpoint V cartridges are protected against this risk by the optional oil check indicator described above.

Better monitoring, greater process reliability, longer lifetimes, optimised filter efficiency, less maintenance: the new oil vapour adsorption technique really seems to be quite a quality leap in the field of compressed air treatment.

It is also very pleasing that this progress does not come at an excessive price.

The extra costs are maximal 33% higher than for conventional filter elements and - calculated in relation to the benefits - this means that Clearpoint V cartridges are also a very interesting alternative in economic terms.

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