Two-stage drying

A vacuum source this can be a water pump (see p. 29) with an anti-suck-back trap producing a vacuum of 15-25 mm Hg, at best or a rotary vacuum pump (consult your instructor about its use since it is an expensive piece of equipment and easily contaminated or damaged), which will evacuate down to 0.1 mm Hg. Two-stage dry vacuum pumps produce a vacuum of 1-5 mm Hg, are resistant to organic and acid vapours and are easy to use. 

Note that tfs > tcR. Likewise, to convert to a two-stage drying process with constant-rate drying down to X,r and first-order falling-rate drying beyond, the equation is... 

The drying model for a single wet particle/slurry droplet is based upon a two-stage drying process [50,51]. In the first drying period, the gas phase resistance controls the evaporation rate. Similarly to heat transfer, this resistance is between the gas and the wet envelope of the particle. This may be expressed by... 

The two-fluid model has been used for modeling the flow of particulate materials through pneumatic dryer. The model was solved numerically for a one-dimensional steady-state condition and was applied to the drying process of wet sand in a pneumatic dryer. A two-stage drying process was implemented. The predictions of the numerical simulations were compared with the experimental results of Rocha [35] (presented by Silva and Correa [34]), which were obtained in a 4 m high pneumatic dryer with diameter of 5.25 cm. 

Drying of ABS has been commercially successful in a two-stage drying system with the combination of direct and indirect heat transfer. Since ABS reqnires both snrface and bound moisture removal, a two-stage drying systan is recommended. 

Five examples and two applications are selected here to demonstrate the applications of Simprosys. The first example is a two-stage drying flowsheet with the exhaust gas from the second dryer mixed with fresh air as the first dryer s inlet gas. The second one is a typical drying flowsheet with a recycled material stream. The third is a combined two-effect evaporation and two-stage drying flowsheet. The fourth is a simple combustion drying flowsheet. The fifth one is a closed-loop... 

Sometimes, a long residence time is needed to accomplish some physical or chemical reactions, which are much slower than the drying kinetics, e.g., crystallization of PET (polyethylene terephthalate resin) is accomplished at a tall tower while the initial drying of surface moisture is done in a small fluid bed dryer in a two-stage drying-crystallization process. 

Fig. 5-22. Two-stage drying unit, process scheme (a) and /i,A -diagram (b). BL Blower GH Gas heater DR Dryer...

Yabu, H., Sakai, C., Fujiwara, T., Nishio, N., Nakashimada, Y. (2011). Thermophilic two-stage dry anaerobic digestion of model garbage with ammonia stripping. Journal of Bioscience and Bioengineering, 111, 312—319. 

Thakur, A. K., Gupta, A. K., 2006. Two stage drying of high moisture paddy with intervening rest period. Energy Convers. Manag. 47(18-19) 3069-3083. 

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