Agitated vessels dissolution time

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. 

Lime feeders have a transportation delay that is proportional to the length of the feeder divided by its speed. This transportation delay may be several minutes. The residence time of a vertical tank must be at least 20 times as large as the sum of this delay, lime dissolution time, and turnover time for the vessel to be classified as well-mixed. To eliminate the need to increase the vessel size, and correspondingly the agitation power, the lime rotary valve speed can be base loaded and the pH controller can manipulate the conveyor speed or the influent flow. If the pH controller manipulates the waste flow, the dissolution time associated with an increase in lime delivery rate is also eliminated. The level controller on the... 

Conditions of dissolution in aqueous acidic (H2SO4) solution was optimised by carrying it out in a specially fabricated rotating ampule holder submerged in an oil bath. It holds 16 ampules and keeps the content vertically agitated. Ampules were withdrawn from the rotating blocks, quenched and analysed for Fe Cr from time to time. For larger stock, dissolution was carried out in 8 lit. (in laboratory) and 100 litre (pilot plant) M.S. vessels. 

The next step in the scale-up procedure is to carry out, at three different scales, experiments at each step to determine the agitator speed required to create a constant process result for that step. For example, if it is a dissolution step, introducing solids to the top of the vessel, then the rate or time of dissolution can be chosen. In this way, the scale-up exponent can be established for each stage. Figure 14.A.3 illustrates the scale-up exponents found for the various process steps under consideration. 

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