Superheated steam-drying

Currently, fewer than 10 major dryer manufacturers around the world offer SSD technology on a commercial scale. Often, it is necessary to custom-design a superheated steam dryer for a new application, which may involve a new dryer type or an application to a new product. Mujumdar (1990) has presented an extensive review of principles, practice, industrial applications, potential new applications, market penetration potential, and R D needs for 

SSD technologies. Special consideration is given to potential applications for use of electricity and reuse of the exhaust steam from an SSD. Kumar and Mujumdar (1990) have provided an extensive bibliography and discussed the basic principles and applications of SSD. 

One of the obvious advantages of SSD is that the dryer exhaust is also steam, albeit at lower specific enthalpy. In air drying, the latent heat in the exhausted stream is generally difficult and expensive to recover. Indeed, at current world prices of energy sources, it is often more expensive to recover energy in the exhaust stream than to waste it in the stack gases for most direct dryers with low- to medium-temperature exhausts. 

No oxidative or combustion reactions are possible in SSDs. This means no fire or explosion hazards and often also a better quality product. 

For products containing toxic or expensive organic liquids that must be recovered, steam drying avoids the danger of fire or explosions while allowing condensation of the off-streams in relatively smaller condensers. 

---

Figure 16.34. Inversion temperature with superheated steam drying 54)...

Postfrying treatments, such as hot air (Nonaka et ah, 1977) and superheated steam drying (Kochhar, 1999) have been shown to be effective in... 

Pimpaporn, R, Devahastin, S., Chiewchan, N. (2007). Effects of combined pretreatments on drying kineties and quality of potato chips undergoing low-pressure superheated steam drying. J. Food Eng., 81, 318-329. 

Fyhr C., "Superheated Steam Drying of Wood Chips in Pneumatic Conveying Dryers , PhD Thesis, Department of Chemical Engineering Design, Chalmers University of Technology, Gdteborg, 1996. 

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. 

In general, superheated steam drying should be considered if one or more of the following conditions apply ... 

Such dryers are superior only for continuous operation because of the inherent problems associated with start-up and shut-down when water condensation on the product as well as presence of noncondensables (air) cause problems. See Mujumdar (1995a) for detailed information on superheated steam drying. 

The drying times may range from days to months. Wood, for example, is dried in hot air kilns from weeks to months, depending on the size of the pieces to be dried and the type of wood species. Superheated steam drying under vacuum conditions enhances drying rates as well as product quality. Only batch dryers are suited for these long drying time requirements. 

Mujumdar, A.S. 1995a. Superheated steam drying. Handbook of industrial drying. 2nd ed. Ed. A.S. Mujumdar, 1071-1086. New York Marcel Dekker. 

Superheated steam drying-finishing of the predried muscle bundles makes use of the potential drying properties of steam at temperatures above 100°C as explained by Chang et al. (1991). However, the relative humidity at the steam temperature range is rather higher than that of the ambient air heated to the same temperature. Thus, superheated steam... 

Mujumdar, A.S., Superheated steam drying, in Handbook of Industrial Drying, 2nd ed., Mujumdar, A.S. Ed., Marcel Dekker, New York, 1995, chap. 35. 

Pronyk, C., Cenkowski, S., and Muir, W.E., Drying foodstuffs with superheated steam. Drying TechnoL, 22(5), 889-916,2004. 

Although superheated steam drying was proposed one century ago, the potential of superheated steam as a drying medium was not exploited industrially for half a century. Some advantages of steam drying are summarized by Mujumdar [64] as follows ... 

More recently, Kast [18] has presented modified correction factors for high mass fluxes in laminar and turbulent flows. Although more complex in form, their effect on the calculated drying rates for superheated steam drying of paper was found by Loo and Mujumdar [19] to be negligible. The simpler forms given by Keey are therefore recommended for practical calculations. 

Extensive results have been reported in the literature on drying of wood as well. Again, all quality indicators are positive. Several vendors worldwide have already commercialized the vacuum-superheated steam-drying systems. It is noteworthy that besides enhancement of the product quality, the drying times are reduced two- to five-fold resulting in significant reduction in cost of inventory in the dryer. 

Table 20.3 summarizes the key factors contributing to the feasibility of superheated steam drying. Both product- and process-related factors must be considered. Laboratory testing is required before proceeding with any decision to consider superheated steam drying. 

David, M., 1987, Exploratory study of effect of superheated steam drying on properties of paper, ME thesis, McGill University, Montreal, Quebec, Canada, 1988. 

Kiiskinen, H.T. andEdelmann, K.E., 2002, Superheated steam drying of paper web. Developments in Chemical Engineering and Mineral Processing, 10(34) 349-365. 

Superheated steam drying technologies have emerged as the drying technologies of the future in view of the numerous advantages (and challenges) they offer. This area is important... 

Hosseinalipour, S.M. and Mujumdar, A.S. 1996. A model for superheated steam drying of particles in an impinging stream dryer. In Mathematical Modeling and Numerical Techniques in Drying Technology (I.W. Turner and A.S. Mujumdar, Eds.), Marcel Dekker, New York. 

A third intensification is usually needed to overcome this paradoxical situation. It is to replace Fick-type vapor diffusion process by a total pressure gradient (TPG) (Darcy-type process), which can be obtained through microwave MW heating, superheated steam drying (SHSD) or (MFD). 

Superheated steam drying has been credited with the following advantages ... 

---