Continuous belt reactors

Except for continuous weighing, control of the flow of solids is less precise than that of fluids. Several devices used for control of feed rates are shown schematically in Figure 3.7. They all employ variable speed drives and are individually calibrated to relate speed and flow rate. Ordinarily these devices are in effect manually set, but if the solid material is being fed to a reactor, some property of the mixture could be used for feed back control. The continuous belt weigher is capable ordinarily of 1% accuracy and even 0.1% when necessary. For processes such as neutralizations with lime, addition of the solid to process in slurry form is acceptable. The slurry is prepared as a batch of definite concentration and charged with a pump under flow control, often with a diaphragm pump whose stroke can be put under feedback control. For some applications it is adequate or necessary to feed weighed amounts of solids to a process on a timed basis. 

In this section we will review the various types of CVD reactors scientists and engineers have used for the development of thermal CVD processes. This will be distinct from the commercial reactors used for production which will be covered in a later chapter. A similar review of reactors for development of plasma-enhanced CVD processes will be made at the end of the next chapter. We will cover the so-called cold wall systems for either single or multiple wafers first, followed by a discussion of continuous belt systems. Finally, we will review the hot wall reactor approach. 

A continuous belt xanthator (CBX) [239] is used at some facilities. In this equipment, the white crumb is fed through a vacuum gate where air is removed and then onto a conveyor belt where it forms a bed that can be several feet thick. The belt moves slowly through a chamber as the white crumb is sprayed with CS2. As there is no stirring of the reaction mass, the bed is usually dropped onto a second belt traveling in the opposite direction in the chamber. In this way, the xanthation is made more uniform. The xanthated crumb is discharged from the reactor through a sump filled with alkali. 

Beside continuous horizontal kilns, numerous other methods for dry pyrolysis of urea have been described, eg, use of stirred batch or continuous reactors, ribbon mixers, ball mills, etc (109), heated metal surfaces such as moving belts, screws, rotating dmms, etc (110), molten tin or its alloys (111), dielectric heating (112), and fluidized beds (with performed urea cyanurate) (113). AH of these modifications yield impure CA. 

In addition to secondarv resistance control, other devices such as reactors and thyristors (solid-state controllable rectifiers) are used to control wound-rotor motors. Fixed secondary reactors combined with resistors can provide veiy constant accelerating torque with a minimum number of accelerating steps. The change in slip frequency with speed continually changes the effective reac tance and hence the value of resistance associated with the reactor. The secondaiy reactors, resistors, and contacts can be varied in design to provide the proper accelerating speed-torque curve for the protection of belt conveyors and similar loads. 

Another example of a cold-wall reactor is shown in Fig. 5.9. It uses a hot plate and a conveyor belt for continuous operation at atmospheric pressure. Preheating and cooling zones reduce the possibility of thermal shock. The system is used extensively for high-volume production of silicon-dioxide coatings for semiconductor passivation and interlayer dielectrics. 

Reachons such as these usuahy take place in continuous reactors such as chip (memory or potato) processing lines, retorts, kilns, and conveyer belts in which the sohds pass through an oven with reactive gases such as air or perhaps in a fluidized sohd bed reactor as in the shot from guns puffed rice and other grain cereals or in a slurry reactor such as a vegetable oil or fat fryer. Some typical sohds reactors are sketched in Figure 9-2. 

This model bears the imprint of its own limitations as a good model should. It is assumed that there is no movement, yet algae are growing in the upper layers of the water and presumably making the medium more dense. This suggest that sedimentation or convective roll-over should be considered. We shall not go into such generalizations here, but merely leave this example with the comment that the same equations turn up in the theory of a continuous reactor in which a belt of material is to be impregnated with a solution, whose solute is immobilized on sites in the belt. 

A specialized variation of a cold wall reactor is the continuous reactor shown schematically in Figure 10. In this system, the surface to be coated moves underneath a set of gas injectors and is heated from below. In some cases, the substrates (wafers) are placed on a belt moving... 

Figure 9.36 Methane chemical ionization spectra at 2.5Torr. (A) l,2-Distearoyl-3-hexadecylglycerol reactor 430°C. (B) 1-Hexadecylglycerol reactor 430°C. (C) Palmitoylstearate reactor 380°C. Samples were applied to the belt continuously (Privett and Erdahl, 1978).

In the cross-flow conveyor-type reactors (Figure 4.5), the solid particles are fed continuously from a hopper on to a conveyor belt and a gas is passed in the cross-flow direction. This type of reactor is used for the oxidation of metallic ores in metallurgical industries. 

Figure 6.23. A continuous series of batch reactors on a conveyor belt.

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