In a chemical pilot plant or plant one cannot choose a reactor as easily as one chooses a 250, 500,1000 or 5000 mL flask. One has to work with what is available and that may be constrained not just by the equipment within those four walls by what other processes are planned to be run at the same time as yours.
A plant reactor can be characterized by two crucially important volumes: the Minimum Stirrable Volume and the Maximum Stirrable Volume. The former will be discussed here and the latter in another blog article.
The minimum stirrable volume is exactly what the name teaches. It is the minimum volume of liquid that needs to be in the reactor so that the turning stirrer paddles effectively stir the reactor contents. To some extent, this minimum stirrable volume depends upon what is being stirred and what will happen during the initial stage of the reaction of interest. For example, a homogeneous solution in which an exothermic refluxing occurs at the beginning of the reaction may only need the moving blades to touch the liquid surface because gas evolution and thermal convection are going to move the homogeneous liquid phase around. At the other extreme, if zinc powder, tin granules, or magnesium turning need to be swept up off the bottom of the reactor, the immersion of the stirrer blades probably needs to be complete and the stirring rapid.
The reason chemists, who are more accustomed to working in the laboratory, tend to forget the minimum stirring volume constraint is that two of the most common laboratory stirrer types are the magnetic stirrer and the crescent bladed overhead mechanical type. The magnetic stir bar sits on the bottom of the flask so that the minimum stirrable volume is very small while the crescent bladed stirrer is very often set up with its curved edge nestled up against the bottom of the round-bottomed flask in which it is installed so that again the minimum stirrable volume is close to zero.
In order to make use of this parameter, the chemist must examine the procedure which he intends to use and decide at what points in the process stirring will be required. The chemist estimates what the reactor volume will be throughout the process. The process cannot be run with less material than will allow stirring at the point of lowest volume where stirring is essential. This is particularly important in the very earliest pilot plant run when one does not want to risk more material than is required to see whether the scale-up proceeds without problems.
An example may clarify this. Suppose you want to run a process step for the first time in a 2000-liter reactor that is planned for production. The starting materials are expensive and you do not want to risk any more materials in this first run than are necessary. The minimum stirrable volume in this 2000L reactor is 200L. Your prototype process will produce 45 gm of the desired intermediate from 40 g of starting material in the laboratory. Twenty grams of insoluble anhydrous potassium carbonate must be stirred in the solvent during the reaction. The point in the process where stirring is essential and the total volume is lowest is right at the beginning when you start adding a reagent drop-by-drop. At this point, you have 40 gm of starting material, 20 gm of potassium carbonate, and 300 ml of solvent. Very roughly the total volume at this point of minimum volume is 360 ml.
The minimum amount of starting material you can use without any change in your lab procedure is 200/360 X40 =22.22kg. If you try using less after loading the reactor with the starting material, the potassium carbonate and the organic solvent the slurry will not touch the stirrer blades.
Suppose management is not happy with this. They don’t want to risk more than 10 kg of starting material in the first run. The best answer is to perform the first run in the smallest reactor that can be found that has the same configuration as the reactor planned for the full production. A smaller reactor will typically have a smaller minimum stirrable volume. This is one reason that a process is often scaled up in steps.
If you must use the 2000 liter reactor you will need to perform your reaction with a larger proportion of solvent in that first run. Run your reaction at this higher dilution at laboratory scale to be sure that the change makes essentially no difference. Then you are ready for that first plant run.
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