Adsorption dryers satisfy tough demands
August 10th 2009

Frequently industrial applications expect mobility and flexibility from compressed air processing. One solution is the installation of complete compressed air stations in transportable sea containers.These make special demands on technology – in particular as far as compressed air drying is concerned, as Beko explains

Karl Uhl in Bochum has provided a German energy supplier with two interconnected sea containers that accommodate three oil-injected, air-cooled screw compressors with an operating energy input of 22kW each. Downstream, there is a 3000L compressed-air tank as a buffer tank, and three high-capacity adsorption dryers with the corresponding compressed-air filters, condensate drains and a dew point control. All installed resources are connected via signaling lines with an equipment cabinet that is also mounted in the container. Apart from the supply of electric power, the unit is an independently functioning platform for the generation of high-grade compressed air.

The mobile compressed-air station is used several hundred kilometres from the energy supplier’s ‘home’ control centre in the pump station of a natural gas pipeline between the gas field and the refinery. Special importance is attached to compressed-air drying as nothing must corrode, freeze up or fail. Uhl opted for cold-regenerated adsorption dryers. Adsorption drying is based upon a physical process, where the air humidity, through adhesive forces, is absorbed to a desiccant (e.g. alumina or silica gel) in the dryer container.

When the adhesive forces of the desiccant are neutralised by hydration the desiccant must be regenerated, meaning that the moisture needs to be removed. To allow this, continuously operating adsorption dryers have two separate pressure vessels: One of the containers actively dries compressed air; the second has the time to regenerate the previously moistened desiccant. Treatment of the desiccant takes place without the additional application of heat. Instead, part of the dried volume flow is diverted.

This partial flow, also referred to as purge air, expands to a pressure slightly above atmospheric pressure and is extremely dry. It is led through the compressed-air container to be regenerated. It absorbs the moisture accumulated in the desiccant and emerges into the environment via an outlet valve.

Advantages claimed include simple set-up, applicability even at high ambient temperatures, fast drying and regeneration times and regeneration without extra energy. However, a disadvantage of some adsorption dryers is their system-inherent compressed air pressure drop, resulting among other things from the divergence of purge air, and the function of the pre and after-filters.

This deficiency needs to be compensated for by increased compressor performance, involving a higher energy input. Uhl avoids this problem using dryer technology from Beko Technologies. The company chose three Drypoint AC adsorption dryers which limit pressure drop (including pre-and after-filter) to just 0.25 bar on average. The cost of the dryers will be paid back after three years on average via energy savings alone. Beko uses, for example, inclined-seat valves with a large orifice cross section to minimise the flow resistance.

Direct mounting of the valves at the pressure vessels, as well as rectilinear piping without redundant changes in direction, have the same effect. Flow-optimised Clearpoint compressed-air filters including a Bekomat 20 FM condensate drain with filter management are used as standard.

The large desiccant bed contributes to the low pressure drop. This results in additional capacity to maintain full dryer performance, for example at a sudden high volume flow or a low operating pressure. An adjustable regeneration-air nozzle allows changeovers to other operating conditions and provides additional reliability regarding the dosage of the correct and most profitable amount of regeneration air. The intelligent control with an accurately defined program run, maintained even after an interruption or a power outage, is capable of distinguishing between a planned interruption and a breakdown of power supply. The Drypoint AC prevents undesired loading of the dryer during a power interruption and ensures full operational capability; the compressed-air system remains dry. It is also suitable for discontinuous operation.

For the application at Uhl, the stable pressure dew point of -40°C of the Drypoint AC dryers is important. It ensures trouble-free operation even during the winter and protects the pneumatics of the gas pumping station in the outer area against freezing.

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