Drying at low pressure
Publication Date: 2016-Apr-07
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DRYING AT LOW PRESSURE
For decades GEA Process Engineering A/S has dried thermoplastic, heat sensitive, hygroscopic and many more products with or without solvents at low pressures in order to be able to dry liquid feeds, pastes and solids in a more gentle way than atmospheric drying.
Fast drying at low temperature requires very large product surface area and high heat transfer rates. Many different types of drying equipment may provide just that since the drying process itself is not dependent on the type of equipment used. To name some suitable drying equipment types: spray dryers, fluid beds, SWIRL FLUIDIZER™, flash dryers or FSD™ (Fluidized Spray Dryer) are good candidates, either on their own or in any number of combinations.
The drying time of droplets in a dryer can be calculated by computational fluid dynamics (CFD). CFD can also reveal parameters for designing drying system components and processes.
For a range of products the onset of the thermoplastic behavior may occur at temperatures that do not allow drying in a spray dryer. In such cases low pressure drying is an option. Operating a dryer at 0.1 - 0.9 bar absolute allows for significant reduction of the drying temperature. Also drying processes operating below 0.1 bar is well-known.
Usually the cost of such a drying system will be higher than a system running at atmospheric pressure, but the resulting powder product will often be of a much higher quality and yield.
Throughout this document the terms "gas" will refer to a gas (or mixture), such as e.g. nitrogen, carbon dioxide, argon or superheated steam, as well as air.
Hereafter, low pressure spray drying is described, but as already mentioned other types of equipment/processes could also be used.
Spray drying is the most widely used industrial process involving particle formation and drying. It is highly suited for the continuous production of dry solids in either powder, granulate or agglomerate form from liquid feed stocks as solutions, emulsions and pumpable suspensions. Therefore, spray drying is an ideal process where the end-product must comply with precise quality standards regarding particle size distribution, residual moisture content, bulk density, and/or particle shape.
Spray drying involves the atomization of a liquid feedstock into a spray of droplets and contacting the droplets with drying gas in a drying chamber, even at inlet temperatures as low as 5°C or even lower in special cases.
Every spray dryer consists of a feed pump, atomizer, gas supply, gas heater, gas disperser, drying chamber, and systems for exhaust gas cleaning are combined into a system that meets individual operational safety, environmental protection, heat recovery, and powder recovery. All systems can be provided with pre- and post-treatment equipment, for example fluid bed dryer/cooler, agglomerator, de-duster and conveyor. Also the systems are available in open cycle, closed cycle, semi-closed cycle and asept...