Reactors for conversion or formation of inorganic solids

Low-temperature solid/liquid processes 11.2.1. Chemical dissolution of solid reactants 

When the starting material is porous, the course of the chemical dissolution process may be different. When the rate of the process is completely determined by 

When the solid particles do fall apart after a certain reaction time, the total dissolution time will be reduced. If diis time can be measured and if it turns out to be reproducible, one may expect it to be the same on the larger scale. If one wants to be on the safe side, it is better to extrapolate the conversion as a function of time, measured before the particles fall apart, and extrapolate this to jc 1. This value of t can be used widi confidence on the larger scde. 

In a continuous reactor with pure plug flow, the residence time x should he  

However, it is very difficult to suspend solid particles effectively and still maintain plug flow. An agitated or pulsed column reactor may be applicable (see section 43,1,5), but then the effects of the residence time distribution have to be taken into account. For large scale operations a series of CSTR s is often more practical, even if the residence time distribution is greater, since it offers better possibilities for effective suspension of coarse particles, and also for heat transfer to the vessel wall. However, one should be aware of the fact that in a continuous reactor with backmixing the conversion of solid particles can never be complete. Because of the residence time distribution, a fraction of the solid particles, those with an individual residence time shorter than the dissolution time, will leave the reactor. For obtaining a complete conversion of the solid, a tubular reactor, that guarantees a certain minimum residence time, will have to be installed after the last CSTR. An alternative is to separate the unconverted solid and return it to the first reactor. 

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Residence time for supercritical water oxidation systems may be as short as several minutes at temperatures of 600 to 650°C. More than 99.9 percent conversion of EPA priority pollutants such as chlorinated solvents has been achieved in a pilot-scale plant with retention time less than 5 minutes. The system is limited to treatment of liquid wastes or solids less than 200 microns in diameter. Char formation during reaction may impact the oxidation time of the organics, while separation of inorganic salts during the process may be a problem. Typical materials for the reactor are Hastelloy C-276 and Iconel 625 (high nickel alloys), which can withstand high temperatures and pressmes and the corrosive conditions. 

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