Reactors atmospheric pressure

This type of reactor has developed more recertcly. In Europe, the first examples of implementation were designed for tannery and kraft paper pulp water, i,e. less concentrated in S than spent caustic. Operation at atmospheric pressure and at the only temperature allowed by the exothermic nature of the reactions naturally defines longer reaction times. However, the oxidation tanks built for this type of process are less sensitive to the maintenance and corrosion problems which may affect reactors working at high temperature and pressure. 

Because of the low pressures (0.4 to 0.6 bar) and temperatures (20 to 50 C), oxidation kinetics are slow and limited by the oxygen transfer velocity. The orders of magnitude for operating the tanks, which are limited to a water depth of 5 m and to blowered air flows of 100 Std m h per square meter, are as follows  

As for pressurized reactors, no provision has to be made for adding a catalyst since the spent caustic almost always contains enough metallic oxides of iron, nickel or cobalt to achieve expected oxidation kinetics. 

Depending on local conditions, phenol removal can be carried out in two main ways  

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Some reactants in atmospheric-pressure reactors must be highly diluted with inert gases to prevent vapor-phase precipitation, while generally no dilution is necessary at low pressure. However, atmospheric pressure reactors are simpler and cheaper. They can operate faster, on a continuous basis and, with recent design improvements, the quality of the deposits has been upgraded considerably and satisfactory deposits of many materials, such as oxides, are obtained. 

In our design considerations we have extrapolated the global rate expression for CO oxidation outside the conditions for which it was derived, and this extrapolation leads to erronous results. Experimental results on oxidation of CO in a flow reactor at varying pressure are shown in Fig. 13.3. The results clearly show that in the medium temperature range around 1000 K, an increased pressure acts to lower, not increase, the rate of CO oxidation. To secure adequate oxidation of CO, we would probably need to increase the postflame residence time in a high-pressure reactor compared to an atmospheric pressure reactor. 

In the commercial atmospheric pressure reactors with three gauzes, the load is about 200 m3 NH3 (NTP)/hr m2. It follows from (398) that under these conditions the fraction of unreacted ammonia is practically nil. 

Dimensionless Quantities and Reactor Types. Transport phenomena in CVD reactors can be described in terms of two broad groups (1) hot-wall, low-pressure reactors and (2) cold-wall, reduced- and atmospheric-pressure reactors. 

Numerous modeling studies of CVD reactors have been made and are summarized in recent review papers (I, 212). Table 3 in reference 212 lists major examples of CVD models up to mid-1986. Therefore, rather than giving an exhaustive list of previous work, Table V presents a summary of the major modeling approaches and forms the basis for the ensuing discussion, which is most appropriately handled in terms of two groups (1) hot-wall LPCVD systems and (2) cold-wall, near-atmospheric-pressure reactors. In LPCVD reactors, diffusion and surface reaction effects dominate, whereas in cold-wall reactors operated at near-atmospheric pressures, fluid flow and gas-phase reactions are important in predicting performance, as discussed earlier in relation to transport phenomena. 

In the process (Fig. 1), anhydrous hydrogen fluoride and carbon tetrachloride (or chloroform) are bubbled through molten antimony pentachloride catalyst in a steam-jacketed atmospheric pressure reactor at 65 to 95°C. The gaseous mixture of fluorocarbon and unreacted chlorocarbon is distilled to separate and recycle the chlorocarbon to the reaction. Waste hydrogen chloride is recycled by use of water absorption and the last traces of hydrogen chloride and chlorine are removed in a caustic scrubbing tower. 

Due to the high deposition rates possible at atmospheric pressure, approximately 1000 A/mtn, wafer throughput can be as high as 200 to 400 per hour. Also, since this is an atmospheric pressure reactor, there is no expensive vacuum system, and the capital cost of the reactor system is modest. These two facts contribute to a low cost per wafer processed, and has allowed this system to remain in commercial use for over 13 years. 

To obtain a good mass balance, it is advisable to dissolve a small amount of an inert compound in the reaction mixture so as to use it as an internal standard during chromatographic analysis. Of course, the internal standard has to be inert under the operation conditions. Moreover it should not exert a solvation effect and should not compete with the active molecules for adsorption within the zeolite micropores and on the active sites. Using a good internal standard is generally the only way to evidence immediately the elimination of very volatile products from atmospheric pressure reactors or the formation of heavy products not detectable by chromatographic analysis. 

Most reactions in scale-up facilities are conducted at or slightly above atmospheric pressure. Reactors are generally fitted with rupture disks set to release at about 25 psi. This allows a reactor to be sealed and contain toxic or irritating components, thus protecting operators and maintaining suitable levels of volatile reagents... 

As discussed above, it can be more efficient to have a single computer interfaced with several of these systems. Figure 11 shows a picture of the screen of a computer interfaced with three of these automated atmospheric pressure reactor systems. Each data point represents the time of introduction of the pulse of reactant gas. For room temperature reactions the standard low-pressure reactors shown in Figure 6 can be used with these systems. When temperature control is needed, jacketed versions of these reactors are used with the temperature maintained by a constant temperature recirculating bath. 

Figure 11 Computer screen showing reaction progress for three atmospheric pressure reactors. 

Rheinpreussen at Moers operated 90 conventional Ruhrchemie atmospheric pressure reactors, of which 60 were normally in the first stage and 30 in the second stage. The operating temperature was in the range 195-... 

Figure 12.3a illustrates an atmospheric pressure fixed-bed reactor used in German FT plants. The catalyst is located between vertical cooling plates, interconnected by horizontal cooling-water pipes. The heat of the reaction is led away from the catalyst by boiling water inside the pipes. The large-scale plants in Germany, France, and Japan were usually operated with these atmospheric pressure reactors. 

Fig. 7. Chemical vapor deposition continuous, atmospheric pressure reactor process ...

The higher reaction temperature results in more CO and less CO2 and CHi, as compared to the Lurgi or Winker reactors. The K-T is an atmospheric pressure reactor, but Shell is working with Koppers... 

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