Day22 - CE 561 Lecture Notes Fall 2009 Day 22: Batch and...

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CE 561 Lecture Notes Fall 2009 p. 1 of 22 Day 22: Batch and Semi-batch reactors Batch reactors are, in many ways, the simplest reactors. We dump reactants in (instantaneously) stir them enough to keep them well mixed, and take them out when they are done. For large volume chemical production, continuous processing is almost always economically preferable to batch processing. However, there are industries in which batch processing is widely used, and there are good reasons for using it. One case where it is advantageous is when small quantities of different materials or grades of material are to be made using the same equipment. It may be better to use large-scale equipment for a small fraction of the time (for each product) than to use smaller scale equipment continuously. Another is when the product is highly regulated, as in the food and pharmaceutical industries. If something is made in a batch reactor, it can be tested and certified batch by batch, and from a regulatory point of view, it is easier to deal with. These are also high value, low volume industries where the cost savings attainable by switching to continuous processing would be minor compared to overall production costs. Finally, batch reactors are sometimes used because they are relatively simple to scale up. Anything that works in a batch reactor in the lab will also work in a bigger batch reactor, if both are sufficiently well mixed and if the same temperatures can be achieved in both (though of course this is not always possible). This simple scale-up is beneficial when time-to-market is important (as in the pharmaceutical industry), or when the process of interest is a small part of a larger system, so that operability problems of this small process could have adverse effects on a much larger scale (a high-risk, low benefit situation). We have already learned quite a bit about batch reactors during the kinetics portion of the course, but we will briefly consider a few additional topics here. For an isothermal, constant volume batch reactor, the equations governing the species concentrations are the rate equations that we have been working with all semester. These are 1 M k ik i i dC r dt α = = We spent substantial time working with these equations and learning methods for solving them. The non-constant-volume batch reactor is treated in Froment and Bischoff, but we won’t bother with it here. If the reactants are liquids, then it is unlikely that reaction will cause significant volume change. If the reactants are gases, we will probably contain them in a constant volume reactor, so the total concentration will change as the total number of moles changes, but the reactor volume will not. If we really wanted to, we could have some sort of piston reactor that has variable volume and constant pressure, but this is unlikely to be the case.
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Day22 - CE 561 Lecture Notes Fall 2009 Day 22: Batch and...

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