Superheated Steam Drying |
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Superheated Steam Drying is an environmental and energy saving drying technologySuperheated steam drying processClosed loop systemThe Superheated dryer is a closed loop pneumatic conveying type. The wet solids are fed into the flow of pressurised superheated transport steam by means of a pressure tight rotary valve, plug screw or similar. Indirect heatingThe transport steam is superheated indirectly via a tubular heat exchanger, by a heating media such as medium pressure steam, flue gases or thermal oil. Also, electrical heating can be applied. Fast dryingIn the subsequent drying ducts, moisture is vapourized from the product, forming excess transport steam and lowering its degree of superheat. Normally the residence time in the system is 5-60 seconds only. For some materials a second superheater is necessary to achieve the required dryness. Cyclone separationTransport steam and the dry material are separated in a high efficiency cyclone and the material is discharged from the dryer by means of another pressure tight rotary valve.
Circulation by a fanFrom the cyclone, the transport steam is recycled by a centrifugal fan to the inlet of the first heat exchanger. The excess steam generated is continuously bled off. Steam generationThe generated excess steam is normally at a pressure of 1-5 bar. It can be reused either directly or after a re-boiler generating clean steam. Steam recompressionIf there is no external use of steam from the dryer, it can be recompressed into 10-20 bar by using a screw or turbo steam compressor and used as the heating media in the superheater. This type of energy recovery is called Mechanical Vapour Recompression, MVR. Power consumption is normally 150-200 kWh/ton evaporated water. Products dried in Superheated Dryers:
Advantages of the Superheated Steam Dryer (SSD™)Energy recoveryPrimary consumption is about 750 kWh/ton evaporated water without any heat recovery. 70-90% of the energy is recoverable by re-using generated steam in another process, for heating purposes, or by using Mechanical Vapour Recompression. With a condensing turbine the generated steam can produce about 200 kWh electricity per ton evaporated water.
Safety and EnvironmentSteam atmosphere eliminates the risk of fires or explosions, since no oxygen is present. Using a closed pressurizied steam system there are no particles or volatile compounds vented to the atmosphere, nor any visible steam plume. The possible volatiles from the product can easily be handled or treated in the condensate, where they are collected by condensation of the generated steam.
Ease of operationRapid start up, shutdown and response are some benefits of using indirectly heated steam system. Also the control of a low and uniform moisture content is easily achieved.
Excellent product qualityThe unique drying conditions in the Superheated Steam Dryer (SSD™) are favourable for the drying of many products where quality is important. There are no air or flue gases that can oxidize or contaminate the product. Controlled temperatures and short residence times (5-60 seconds) make the technology suitable for drying of sensitive products. Due to the drying principle, a very uniform dryness of the dried product is achieved. The conditions in the dryer make it possible to achieve heat treatment and/or effective sterilisation of products.
Examples of added value with the Superheated Steam Dryer (SSD™)Increased protein valueIncreased by-pass protein value in cattle feed - more milk and meat per kg of feed.
Increased oil yieldIncreased yield and quality of vegetable oil from oil seeds.
Chemical recoveryStripping of chemicals from solids, for example acetylated fibers. Chemicals are recovered as condensate.
Less taste changeDrying and expansion of tobacco without change of taste and quality.
SterilisationComplete killing of all bacterias in animal feed, fertiliser and sludge.
Energy recoveryIntergration in CHP plant with Superheated Steam Dryer (SSD™) for biomass pellet production, 90-95% of the energy recoverable as electricity and district heating.
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