Dryers materials

FIG. 12-73 Link-Belt Roto-Louvre dryer. (Material Handling Systems Division, FMC Corp.)... 

Tunnel Dryers. Material to be dried is placed on trucks or carts which are moved through a tunnel in which there is a How of hot gases. 

Through-circulation Dryers. Material is supported on a conveying screen that moves continuously. Hoi gases from below or above conveyor pass through the material and pick up moisture. 

Rotary Dryers. Material (liquid) is pumped to and showered within n rotating cylinder through which hot gases flow. 

Tray Dryers. Material is placed on vibrating trays over or under which hot gases flow. 

Sheering Dryers. Material in sheet form passes continuously through a hot chamber. Depending upon product, sheet may be taut (as pinned to a frame), or it may pass through dryer in festoon manner. 

Pneumatic Conveyor Dryers. Material is moved in a stream of gas ul high-velocity and high temperature and finally collected by a cyclone separator... 

Steam-tube Rotary Dryers. Material is passed through a long rotating cylinder A shell around the cylinder enmams steam, hot water, or other heating medium. 

Vacuum Rotary Dryers. Material is subjected to agitation within a stationary, horizontal shell tinder vacuum. The agitation may be heated to increase drying effectiveness. 

Agitated-pan Dryers. Material is placed in covered shallow1 puns, Pans arc jacketed for heating. An agitator stirs the material constantly. This design may be operated at atmospheric pressure or under vacuum. 

For the drying of materials in sheets or masses, dryers capable of supporting the material in the desired atmosphere for an adequate period of time are necessary. In batch dryers, material is held in place and subjected to the desired conditions or cycle of conditions. In continuous dryers, the material is conveyed through the dryer, generally through a number of zones where different conditions are maintained. 

FIGURE 6.1 Schematic diagram of a typical indirect dryer material to be dried is separated from heat source by a conducting waU. 

A tee is required to split the product material into two streams. One stream goes through a recycle and mixes with the fresh material in a mixer and then is introduced into the dryer material inlet. The established flowsheet is displayed in Figure 60.10. The simulated result is shown in Figure 50.11. 

Liquid material of 4000 kg/h flow rate is initially at a solid mass content of 13% and a temperature of 3°C. It needs to be concentrated to a solid content (mass concentration) of about 57% before it is sent to a spray dryer. Material density is 720 kg/m at initial temperature. Concentration process needs to be performed at around atmospheric pressure. Specific heat of the material without moisture... 

Although drum dryers are easy to operate and have high energy efficiency, they have relatively low throughput compared to spray dryers. Materials may also be unevenly dried and become scorched, resulting in the generation of unacceptable colors or flavors, or the loss of functionality. Thus, drum dryers may be better suited for the drying of lower cost protein extracts (e.g., food and feed applications) than for pharmaceutical or nutraceutical products. 

Tunnel dryers are shown in Fig. 3.15a. Wet material on trays or a conveyor belt is passed through a tunnel, and drying takes place by hot air. The airflow can be countercurrent, cocurrent, or a mixture of both. This method is usually used when the product is not free flowing. 

Another important class of dryer is the fluidized-bed dryers. Some designs combine spray and fluidized-bed dryers. Choice between dryers is usually based on practicalities such as the materials handling characteristics, product decomposition, product physical form (e.g., if a porous granular material is required), etc. Also, dryer efficiency can be used to compare the performance of different dryer designs. This is usually defined as follows -. 

From the dryer, the strands are screened to remove fines and small particles, which would detract from board quaHty and economy. These fines are burned for fuel or possibly sold to a nearby particleboard mill as raw material. The larger strands are used as surface material and the smaller strands are core material. 

Fig. 1. Fine chemicals plant design showing successive additions of processing equipment, where A represents the reaction vessel with agitator B, centrifuge C, dryer D, crystaUi2ation vessel E, raw material feed tanks F, centrifuge which may have an automatic discharge G, mother Hquor tank H,...

Reaction times can be as short as 10 minutes in a continuous flow reactor (1). In a typical batch cycle, the slurry is heated to the reaction temperature and held for up to 24 hours, although hold times can be less than an hour for many processes. After reaction is complete, the material is cooled, either by batch cooling or by pumping the product slurry through a double-pipe heat exchanger. Once the temperature is reduced below approximately 100°C, the slurry can be released through a pressure letdown system to ambient pressure. The product is then recovered by filtration (qv). A series of wash steps may be required to remove any salts that are formed as by-products. The clean filter cake is then dried in a tray or tunnel dryer or reslurried with water and spray dried. 

FIOR Process. In the FIOR process, shown in Figure 5, sized iron ore fines (0.04—12 mm) are dried in a gas-fired rotary dryer. A skip hoist dehvers the dry fines to lock hoppers for pressurizing. The fines pass through four fluidized-bed reactors in series. Reactor 1 preheats the ore to 760°C in a nonreducing atmosphere. Reactors 2, 3, and 4 reduce the ore at 690—780°C. At higher (ca 810°C) temperatures there is a tendency for the beds to defluidize as a result of sticking or hogging of the reduced material. 

When the dryer is seen as a heat exchanger, the obvious perspective is to cut down on the enthalpy of the air purged with the evaporated water. Minimum enthalpy is achieved by using the minimum amount of air and cooling as low as possible. A simple heat balance shows that for a given heat input, minimum air means a high inlet temperature. However, this often presents problems with heat-sensitive material and sometimes with materials of constmction, heat source, or other process needs. AH can be countered somewhat by exhaust-air recirculation. 

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