Pressurized superheated steam dryer

High-pressure superheated steam dryers need to be pressure vessels and hence are capital intensive. They are recommended only for special applications. For example, a pressurized, fluid-... 

Drying of pulps, bagasse, sludges, hog fuel, spent grain from breweries, etc., in a pressurized superheated steam dryer (3-5 bars) manufactured by Niro A/S, Denmark (Jensen, 1992)... 

FIGURE 23.31 Classification of superheated steam dryers based on their operating pressures. 

Figure 7.4 Pressurized superheated steam multicompartment fluidized bed dryer (Courtesy of Niro A/S, Denmark).

Figure 7.7 Simplified process flow diagram of superheated steam dryer configuration to recover energy from dryer exhaust a) excess steam with recuperation by high-pressure steam b) excess steam recompressed to superheat c) excess steam with recuperating by electricity.

FIGURE 8.23 Pressurized superheated steam fluidized bed dryer. 

Figure 19.1 classifies superheated steam dryers based on their operating pressure. The product temperature necessarily exceeds the saturation temperature of steam at the corresponding operating pressure. So, for products that may undergo undesirable physical transformations (e.g., melting) or chemical transformations (e.g., hydrolysis) at elevated temperatures, a low-pressure operation is desirable. 

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. 

Products that may melt, undergo glass transitions, or be otherwise damaged at the saturation temperature of steam at the dryer operating pressure cannot clearly be dried in superheated steam even if they contain only surface moisture. Operation at reduced pressure, however, is a feasible option that may also enhance the drying rate. 

Recently, Niro A/S of Denmark has successfully commercialized a pressurized-steam-fluid bed dryer for particulate and sludgelike or pulpy materials [9]. The fluid bed has a special cellular design and operates at 3 bar. The fluid bed is driven by superheated steam flow that is recycled through a heat exchanger by a fan and blown up through the fluid bed. Capacities of 2-40 ton/h water evaporation are available. Initiated at pilot scale in 1982, the full-industrial-scale dryer (6-m diameter) for drying beet pulp came on stream in Denmark in 1985. 

The pressurized-steam flash dryer originally developed at the Chalmers University of Technology, Gothenburg, Sweden, in the early 1970s is ideal for drying peat as well as pulp, bark, and so on. This dryer is a closed, pressurized system in which the peat is exposed to indirectly heated superheated steam. The dryer consists of transport ducts, heat exchangers, a cyclone, and flans. The superheated steam recirculates at a pressure of 2-6 bar. The primary heating steam is condensed (usually 8-15 bar) on the shell side. 

Dryers suitable for drying of the biomass include batch through-circulation dryer (perforated floor bin dryer), PMB or continuous through-circulation dryer, direct rotary dryer (rotary cascade dryer), indirect rotary dryer (steam-tube rotary dryer), fluidized bed dryer (pressurized steam fluid bed dryer), and pneumatic conveying dryer (flash dryer) (Amos 1998 Brammer and Bridgwater, 1999 Wimmerstedt, 2006 Roos, 2008). Drying media can be flue gas, hot air, or superheated steam. 

Peat is usually dried at power plants and briquette factories with flue gases (300-600°C) from the boiler. More recently, steam flash dryers have been commissioned in Sweden using back-pressure steam. The first industrial pulp dryer utilizing superheated steam as the carrier for conveying the pulp to be dried was... 

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