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Pharmaceutical Manufacturing and Packing Sourcer

Pure and Simple

Compressed air is a safe and reliable power source that is widely used in a variety of industries. Sometimes known as the fourth utility, compressed air is used by approximately 90% of all manufacturing companies in some aspect of their operations. However, unlike other utilities, it is generated on-site, and its quality and cost are the responsibility of the user. This means businesses need to be aware of potential pitfalls that can affect the quality of air delivered – the major one being the presence of contaminants.

Contamination Sources

In applications such as pharmaceutical, medical, laboratory, dental, automotive, electronics, telecommunications and laser cutting, compressed air purity is critical and contaminants must be removed or reduced to acceptable levels. In the UK, this is governed by ISO 8573-1:2010, which specifies requirements for air dryness and purity. Contamination sources include:

Water Vapour
In a typical compressed air system, there are 10 major contaminants coming from four different sources: atmospheric air, the air compressor itself, the air receiver and the distribution pipework. Atmospheric air contains water vapour, and its ability to hold this is dependent on temperature and pressure.

Just 1m³ of atmospheric air at 100% relative humidity contains 24g (0.02L) of water vapour at 25°C. As pressure grows, a smaller amount of water vapour can be held by the air, while, as temperature rises, a larger amount can be held. During compression, the air temperature increases, allowing the air to easily retain the moisture; but, postcompression, air is typically cooled to a more usable temperature that reduces its ability to retain water vapour. This results in the condensation of vapour into liquid water.

Saturated air, water aerosols and liquid water within the compressed air system can cause corrosion to the storage and distribution system, as well as damage to valves, cylinders, tools and production equipment. This results in higher maintenance costs and reduced production efficiency. Water in the compressed air can also cause damage to any products and packaging in direct contact with it.

Dirt Particles
As well as water, atmospheric air contains particles – typically between 140 and 150 million dirt particles in each cubic metre. Some 80% of these are less than two microns in size, and so are too small to be captured by the compressor intake filter – which can trap, on average, particles of over 25 microns – meaning they will travel unrestricted into the compressed air stream.

Atmospheric air can also contain up to 100 million microorganisms – bacteria, viruses, fungi and spores – per cubic metre of air, which are drawn into the intake of the air compressor. The moist environment of the compressed air system, particularly in the air receiver, is ideal for the growth of such microorganisms.

A further source of contamination from atmospheric air is oil in the form of hydrocarbons. Typical oil vapour concentrations can vary between 0.05mg and 0.5mg per cubic metre of ambient air. Once in the compressed air distribution system, oil vapours can cool and condense to form liquid oil. Both liquid and aerosolised oil mix with water in the system to form a thick, acidic condensate, which causes damage not only in the compressed air system itself, but also to production equipment, products and packaging.

Added Problems

By the time the air is compressed, cooled and exits an oil-lubricated compressor, an additional four contaminants have been added. These are liquid oil and aerosols, oil vapour from compressor oil, condensed liquid water and water aerosols. Finally, both the air receiver and the distribution system can add a further two contaminants in the form of rust and pipescale caused by warm, moist air in the system.

Of all the contaminants present within a compressed air system, the common perception is that oil causes the most problems, principally because it can be seen emanating from open drain points and exhaust valves. In reality, however, most issues in a compressed air system can be directly attributed to water in one form or another. In fact, up to 99.9% of the total liquid contamination found in a compressed air system is water.

To put this in context, a 2.8m3/min (100 cubic foot per metre) compressor and refrigeration dryer combination, operating for 4,000 hours in typical UK climatic conditions, can produce approximately 10,000L of liquid condensate annually. In hotter climates, this volume will increase significantly.

Purification Technologies

The contaminants outlined above need to be reduced or removed for efficient operation of a compressed air system, and it takes a combination of different technologies to achieve this. Oil and particulates can be dealt with by filtration. Water vapour, however, will pass as easily as air itself through water separators and coalescing filters, so needs to be removed by the use of a dryer.

The water vapour removal capability of a dryer is expressed as a pressure dewpoint (PDP). Dewpoint refers to the temperature at which condensation will occur, while PDP refers to the dewpoint of air above atmospheric pressure. High-efficiency compressed air dryers are used in critical applications where humidity levels, or PDPs, are specified to ISO 8573.1:2010. This standard specifies the purity of classes of compressed air with respect to particles, water and oil, independent of the location in the compressed air system at which the air is specified or measured. It also identifies gaseous and microbiological contaminants.

In critical applications, refrigeration dryers are unsuitable since they cannot produce a dewpoint below freezing, having dewpoints of +3°C, +7°C or +10°C. Most of the time, a PDP of -40°C is recommended, as one lower than -26°C will not only stop corrosion, it will also inhibit the growth of microorganisms. Some applications, like electronics, require a dewpoint of -70°C. Refrigeration systems also typically use ozone-depleting chlorofluorocarbon gases, and can be expensive and bulky, as well as requiring specialist maintenance.

Meanwhile, membrane dryers are usually limited to low-capacity applications, and their purge air requirements are usually higher than desiccant dryers. Membrane life is limited, especially in stop-start applications, and a high level of inlet filtration is needed. They are also prone to chemical attack and can suffer catastrophic failure due to shock and vibration.

Adsorption Dryers

With dewpoints of -20°C, -40°C or -70°C, adsorption dryers usually employ the heatless pressure swing adsorption (PSA) method, often referred to as heatless regeneration.

Adsorption is a process whereby specific molecules (the adsorbate) adhere to the surface of a highly porous solid (the adsorbent) by electrostatic and molecular forces. The adsorbent has a specific pore structure that will be a combination of larger (macropores), slightly smaller (mesopores), or very small pores (micropores). The adsorbent is normally made into granules or beads with a clay binder to form packed beds, through which the adsorbate is passed and the process of adsorption can take place.

To continuously supply dry air, an adsorption dryer must regenerate the desiccant material. The simplest way to achieve this is to pass clean, dry and expanded air, known as purge air, over the wet adsorbent material. To enable this process, PSAs are designed with two vessels, so that while one is on-line drying the compressed air, another is off-line undergoing regeneration. Although using the same principles, there are two types of PSA design – twin tower and modular – and the differences between them can substantially affect performance and cost-effectiveness.

Twin tower PSAs consist of a pair of pressure vessels, and installation may require extensive pipework and plant modification, as well as inlet and outlet filtration. They also demand a large volume of desiccant, which may result in high operational and maintenance costs. The fill method used on twin tower designs can lead to inconsistencies in drying, desiccant regeneration and dewpoint, while, over time, desiccant attrition reduces the absorption capacity of the dryer and leads to blocked outlet filters and frequent servicing. Being based on pressure vessels, they also fall under the requirements of the Pressure Equipment Directive for annual inspection and certification. While their design is simple, traditional desiccant dryers can also suffer from purge air losses of up to 25%.

Modular PSAs use a replaceable desiccant cartridge, rather than pressure vessels, in a heatless twin chamber configuration contained within a housing. Wet air from the compressor after-cooler enters the dryer and is directed into one chamber. Water and particulates are removed by the filtration stage and water is retained within the dryer until the column is regenerated, when it will be vented into the atmosphere as it is depressurised. Following the filtration stage, air passes through the desiccant bed where any remaining moisture is adsorbed. Finally, the dry air passes through a particle filter, which retains any remaining desiccant particles that may have been carried through the system. At the same time, a small amount of dry air is counter-flowed down through the second chamber and exhausted to atmosphere, removing moisture and regenerating the desiccant. Electronic controls periodically reverse the function of each column to ensure a continuous supply of dry air.

Desiccant Performance

There are many different types of desiccant on the market, and the wrong choice can influence dewpoint performance. Attrition – the loss of drying efficiency resulting from the breakdown of the optimised clay/crystal structure within the dryer – is another factor that can affect dryer performance.

Of key importance is the method used to fill the cartridge. ‘Tip and pour’ methods result in a loose fill and can hinder the efficient channelling of air through the desiccant, preventing some of the material from being used for drying, while also allowing the desiccant beads a higher degree of movement relative to each other.

One way to help counter these problems is known as snowstorm filling. This requires the use of a specialist filling device that is optimised for the diameter of the cartridge being filled and the diameter of the desiccant bead. The method maximises packing density and minimises fluidisation of the desiccant bed to allow 100% of the available desiccant to be used for drying. In restricting the relative movement between the desiccant beads, it will also lead to a reduced rate of attrition.

Another prime cause of attrition is uneven flow across the drying surfaces. Failure to ensure even flow will cause greater friction in some areas of the drying bed, accelerating its degeneration. Excessively high flow can also result in shock loading or overloading the drying bed. Not only can this have potentially disastrous physical effects on the dryer bed structure through the creation of voids, but the resulting loss of dewpoint will impact heavily on drying efficiency and, more importantly, on the application – possibly leading to faulty, damaged or spoilt product. A further issue is the build-up of dust from the drying bed as it starts to break down, which, again, impacts on efficiency, and can lead to premature blocking of filters and silencers.

The problem of excessive flow velocity can be addressed by repressurising the drying bed to negate the possibility of having a high pressure differential between the two ends of the drying bed on column change-over. Additionally, the introduction of a flow limiter or sonic nozzle will ensure the total flow of air through the dryer is not excessive – in doing so, ensuring that the outlet dewpoint is not reduced. Meanwhile, excess dust can be prevented by adding an extra air filtration stage after one column is regenerated and before regeneration of the other column commences. This avoids passing air that is heavy with dust into the second column. To minimise the impact of attrition, and so ensure a longer life and maintain dewpoint performance, all of these elements should be incorporated into dryer design.

Finding a Solution

Desiccant air dryers can provide a simple solution in applications where air quality is a critical factor. Designs vary significantly and it is important to ensure that the dryer is sized correctly, based on its outlet flow. Maintenance of silencers is critical to operation, since increasing back pressure as they block with desiccant dust will create a continually reducing volumetric flow of the purge air, leading to incomplete regeneration, sub-optimal performance and possible failure. Modern dryers using the replaceable desiccant cartridge system typically have additional safeguards built into them in the areas of flow management and filtration to prevent accidental misuse, and deliver consistent and reliable operation.

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Dave Whelan has nearly 30 years of experience in the pneumatics sector, equipping him with extensive product and industry knowledge in the industrial automation sector. One of the world leaders in fluid and motion control, Norgren was acquired by IMI in 1972 and has effectively operated as the group's fluid power division. In 2015, this division was rebranded IMI Precision Engineering.
Dave Whelan
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