Atmospheric-Pressure Superheated Steam Drying

The basic fiindamentals of both conventional atmospheric-pressure Superheated steam drying and more novel low-pressure Superheated steam drying are presented in this chapter. The application of these processes to the drying of a wide variety of foods and biomaterials is illustrated and discussed, with emphasis placed on the use of LPSSD to preserve or even improve the properties of the drying materials. 

As noted earlier, a majority of dryers are of the direct (or convective) type. In other words, hot air is used both to supply the heat for evaporation and to carry away the evaporated moisture from the product. Notable exceptions are freeze and vacuum dryers, which are used almost exclusively for drying heat-sensitive products, tend to be significantly more expensive than dryers operated near atmospheric pressure. Another exception is the emerging technology of superheated steam drying (Mujumdar 1995a). In certain cases, such as the drum drying of pasty foods, some or all of the heat is supplied indirectly by conduction. 

The superheated steam at atmospheric pressure carries the dried fuel together with the hot bed material to a cyclone where the solids are separated... 

Superheated steam drying at atmospheric (or near atmospheric) pressure is the process that has been most extensively studied compared to SSD at other pressure ranges - that is, at a pressure higher or lower than the atmospheric pressure. In this section, some examples are given to explain the consequences of SSD, from both the drying (heat/mass transfer) and product quality changes points of view. This overview will be limited to foods and biomaterials, and hence will exclude products such as wood, paper, ceramics, sludge, coal, and peat that can also be dried in a superheated steam environment. 

Eor printing on polyester, the fixation conditions are more rigorous than on other disperse dyeable fibers, owing to the slower diffusion of disperse dyes in polyester. Eor continuous fixation the prints are exposed at atmospheric pressure to superheated steam of 170—180°C for 6—8 min. A carrier may be added to the print paste for accelerated and fliU fixation. Dry-heat fixation conditions of 170—215°C for 1—8 min are less popular for printed fabrics, but are sometimes employed because of lack of other equipment. 

Phenylacetaldehyde (120 g, 1 mol) and 0.5 mol of another aldehyde component were dissolved in 350 mL saturated solution of ammonia in absolute ethanol. The solution was sealed in a 1-L stainless-steel bomb and heated at 225-230°C for 6 h with constant rocking. The pressure was 1000 psi. The bomb was then cooled and opened the contents was transferred to a distilling flask. As much ethanol as possible was distilled on the steam bath at atmospheric pressure. The residue was subjected to a preliminary distillation in which everything boiling up to 280°C (ImmHg) was collected except in the case of the acetaldehyde condensation, in which a preliminary distillation with superheated steam at 200°C was resorted to (omission of this preliminary treatment led to difficulties in the subsequent extraction step). The distillate was taken up in ether and extracted 15 times with 2 N HCl. The residual ether layer was discarded, and the aqueous layer plus a third layer—which invariably formed—were made basic with an excess of concentrated ammonium hydroxide. The bases liberated were extracted with ether and dried over potassium carbonate the solution was then concentrated. 

At normal atmospheric pressure, the boiling point of water equals 100°C. Consequently, in order to obtain an internal overpressure, the temperature of the porous medium must be above that level during at least one part of the process. This is the aim of convective drying at high temperature (moist air or superheated steam) and a possible aim of contact drying or drying with an electromagnetic field (microwave or radio frequency). 

Since atmospheric-pressure (or near atmospheric-pressure) SSD involves the use of a high-temperature drying medium (i.e., superheated steam at a temperature higher than 100 °C), this type of process may not be appropriate for heat-sensitive materials such as fruits, vegetables, herbs and spices, and many other biomaterials. For this reason, an alternative drying technique has been developed to combine the advantages of SSD with an ability to conduct drying at a lower temperature. The proposed alternative is based on the fact that water vaporizes at a lower temperature if the pressure of the system is lower superheated steam can then be produced at a lower temperature under such a lower pressure. 

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