Continuous-flow drier

Continuous-flow drier. The principle is that hot air removes the excess moisture and ambient air and then cools the grain to 10-15 °C. However, in very warm weather this may not be possible and night air rrtay have to be used to cool the heap after drying. These driers are rated as x t/horrr taking out 5% moismre in wheat. 

Grain can be stored in bulk for up to one month at 16-17% me, but for longer term storage it should be dried down to 14% me. Only fully ripe grain in a dry period is likely to be harvested in the UK at 14% moisture. In a wet season, the moisture content may be over 20% and the grain may have to be dried in two or three stages if a continuous-flow drier is used. 

Thorpe, G.R., The thermodynamic performance of a continuous-flow fluidized bed grain disinfestor and drier, /. Agric. Eng. Res., 37 (1987) 27-41. 

After interesterification has proceeded to the desired point, the catalyst is destroyed by adding water and carbon dioxide. Carbon dioxide buffers the caustic to a lower pH and minimizes saponification of the lard. The neutralized lard is heated to melt the trisaturated glyceride crystals. Soaps are removed by conventional water washing and centrifuging, and the lard is dried in a continuous-vacuum drier. A basic flow chart for the process is shown in Fig. 12-8. 

Mobile grain driers. These have become popular in recent years. Most are of the recirculating batch type, but portable versions of well-established static batch or continuous flow machines are also in use. 

Hanni et al. (1976) described a continuous fluidized bed drier using the centrifugal principle. It consisted of a perforated horizontal cylinder rotating inside a plenum with hot air blown across the outside of the cylinder and perpendicular to the axis of rotation (Figure 1.20). Vanes placed in the air inlet allowed the incident angle of the air flow to be varied from 0° to 45° to the perpendicular. Particles were fluidized inside the Teflon-coated stainless steel cylinder which could be tilted by up to 6° in order to control particle residence time. The cylinder, of... 

A continuous process for polymerization of nylon 6,6 in which a fluidized bed solid state polymerization reactor is used as the high polymerizer is represented schematically in Figure 3 (26). In this process the low molecular weight polymer is produced in a filled pipe reactor located just upstream of the spray drier. The liquid product of this step is then sprayed into a hot inert gas atmosphere where the water is flashed off and a fine powder is produced. This powder is fed into an opposed-flow, fluidized bed reactor at 200 °C where the high molecular weight polymer powder is generated at temperatures well below the 255 °C melting point of nylon 6,6. The powder is then melted in the extruder and converted into fiber or chip. 

In the continuous process, a 10k oxidized Courtelle tow was pulled at 7.2 mh under tension, first through a furnace at 200° C used as a drier, flushed with a counter-current of N2 and then, through a second furnace with temperature varying in the range 250-1000°C, with a fixed concurrent N2 flow. When steady conditions had been reached in the second furnace, a gas sample was taken and results reported as volumes of each gas removed in 10 ml (NTP) of N2 purge gas. 

In plug-flow fluidized bed dryers the bed usually has a length-to-width ratio in the range 5 1 to 30 1 the solids flow continuously as a plug through the channel from the inlet to the exit. This ensures approximately equal residence time for all particles, regardless of their size. For nearly monodisperse particles this ensures uniformity of product moisture content. The main operational problems occur at the feed end where wet feedstock must be fluidized directly rather than mixed with drier material as in a well-mixed unit. To handle this problem, several alternative strategies may be employed, e.g.. 

It is characteristic of unsteady-state operation that concentrations at any point in the apparatus change with time. This may result from changes in concentrations of feed materials, flow rates, or conditions of temperature or pressure. In any case, batch operations are always of the unsteady-state type. In purely batch operations, all the phases are stationary from a point of view outside the apparatus, i.e., no flow in or out, even though there may be relative motion within. The familiar laboratory extraction procedure of shaking a solution with an immiscible solvent is an example. In semibatch operations, one phase is stationary while the other flows continuously in and out of the apparatus. As an example, we may cite the case of a drier where a quantity of wet solid is contacted continuously with fresh air, which carries away the vaporized moisture until the solid is dry. 

A new process involves a product wet with methanol, and it is planned to dry the product by a continuous evaporation of the methanol into a stream of hot carbon dioxide (CO2 rather than air in order to avoid an explosive mixture). One of the schemes being considered is that of Fig. 7.16. The tower will be packed with 25-mm ceramic Raschig rinp. The C02-methanol mixture from the drier, at the rate 0.70 kg/(m tower cross section) s (516 Ib/ft - h) total flow, 1 std atm, 80 C, will contain 10% methanol vapor by volume. The refrigerated liquid methanol wiU enter the tower at — 15 C at a rate of 4.75 kg/m s (3503 Ib/ft h). The methanol of the effluent gas is to be reduced to 2.0% by volume. 

A general flow diagram for a continuous drier, arranged for countercurrent flow, is shown in Fig. 12.27. Solid enters at the rate mass dry solid/(area)(time),f is... 

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