Rotary drum efficiency

Drying and cooling the products of ammonium phosphate production are conventionally achieved in a rotary drum, and a means must be provided to remove the dust particles from the air streams to be exhausted to the atmosphere. At the Minnesota plant, a high-efficiency dry cyclone recovery system followed by a wet scrubber was designed. In this way, material recovered from the dry collector (and recycled to the process) pays for the dry system and minimizes the load and disposal problem in the wet scrubber, because it eliminates the need for a system to recover the wet waste material that is discharged to the gypsum disposal pond for settling. 

Vertical mixers have become more commonly used due to low space requirements and high efficiency of operations. The vertical mixer shown in Fig. 36.42 utilizes high speed agitation to intimately mix and uniformly blend liquids and dry raw materials prior to discharge. The contact time is reduced to less than five seconds. The rotary drum units generally contain baffles that roll or lift the product... 

The system operates as a rotary drum screen with continuous washing to prevent clogging of the meshes (Fig. 3.48). Such systems are part of a technological line as a substitute for sedimentation in the case of waste-water purification they can replace flotation equipment. The efficiency of separation carried out in this way is about 60%. 

Corona-active electrode rotary drum handles wide range of particle diameters from 75-1000 pm high capacity < 0.75 kg/s m of drum width high efficiency 95% separates good from poor conductors. 0-40 kV DC, 0.5-1 mA/electrode insensitive to humidity and temperature often can recycle the middlings with recycling 10-30% OK. 

Application of the models to washing on fiiU-scale eqiiipment raises the question of washing efficiency. One approach involves the use of a strai tforward efficiency fector which is related principally to the type of filter to be used and to the appUcation, as mentioned above. Purchas and Wakeman [1986] noted the relationship between the washing efficiency and the type of system found by Choudhury and Dahlstrom for rotary drum vacuum filters and proposed a correction of the diversion parameter rather than the use of an efficiency ctor. This correction may be expressed as ... 

The efficiency of a rotary drum filter is expressed by the product of the average filtration rate and the ratio of filtrate volume to weight of filter aid spent. Efficiency reaches a maximum at different rates of knife motion and varies according to the properties of the specific filter aid. An increase in the thickness of a cake layer cut results in a ratio of filtrate volume to weight of filter aid spent. One should account for the volume weight of different filter aids since this parameter varies considerably even among different grades of the same material. 

Continuous rotary disk filter. This filter consists of concentric vertical disks mounted on a horizontal rotating shaft. The filter operates on the same principle as the vacuum rotary drum filter. Each disk is hollow and covered with a filter cloth and is partly submerged in the slurry. The cake is washed, dried, and scraped off when the disk is in the upper half of its rotation. Washing is less efficient than with a rotating drum type. 

Roller discharge (Figure 3.19) is limited to cakes of an adhesive nature, which will transfer from the main filter cloth to a separate roller. The cake is then released from the roller by a scraper blade, which can be in close contact with the roller without causing cloth wear problems. In effect this is a form of scraper discharge, but eliminating any contact between the scraper blade and the cloth. The proper discharge of cake from a rotary drum vacuum filter is of paramount importance if this type of filter is to continue working efficiently. 

Similar to the trough dryer is the so-called rotary dryer specified in Fig. 33. This may be used for either liquids or solids, and the capacity and efficiency are good. Both the outside jacket and the interior drum are heated by either steam, hot water or oil, and the heating surface is kept clean by the scrapers attached to the interior revolving drum. From 1.3 to 1.5 lb. of steam are required to evaporate 1 lb. of water. 

Conduction or indirect dryers are more appropriate for thin products or for very wet solids. Heat for evaporation is supplied through heated surfaces (stationary or moving) placed within the dryer to support, convey, or confine the solids. The evaporated moisture is carried away by vacuum operation or by a stream of gas that is mainly a carrier of moisture. Vacuum operation is recommended for heat-sensitive solids. Because the enthalpy lost with the drying air in convective dryers is large, their thermal efficiency tends to be low. For conduction dryers the thermal efficiency is higher. Paddle dryers for drying of pastes, rotary dryers with internal steam tubes, and drum dryers for drying thin slurries are examples of indirect dryers. 

The major limitation of rotary dryers is the relatively low volumetric load ratio of the drum, which typically ranges from 3% to 16%. Because sound energy is absorbed in the bulk of particulates, it is difficult to secure the efficient use of high-intensity sound energy in large rotary dryers. Much better energy utilization is in the case of spray dryers since sonic irradiation intensifies heat and mass transfer but also enhances Uquid atomization (see Figure 13.9). 

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