化学反应工程英文课件Chapter 13.ppt

化 学 反 应 工 程 Figure 13.8 Tracer response curves for closed vessels and large deviation from plug flow. 化 学 反 应 工 程 Open Vessel. This represents a convenient and commonly used experimental device, a section of long pipe (see Fig. 13.9). It also happens to be the only physical situation (besides small D/uL) where the analytical expression for the E curve is not too complex. The results are given by the response curves shown in Fig. 13.10, and by the following equations, first derived by Levenspiel and Smith (1957). 化 学 反 应 工 程 Figure 13.9 The open－open vessel boundary condition. 化 学 反 应 工 程 (14) (15) 化 学 反 应 工 程 Figure 13.10 Tracer response curves for “open” vessels having large deviations from plug flow. 化 学 反 应 工 程 Comment For small D/uL the curve for the different boundary conditions all approach the “small deviation” curve of Eq. 8. At large D/uL the curves differ more and more from each other. (b) To evaluate D/uL either match the measured tracer curve or the measured to theory. Match is simplest, through not necessarily best; however, it is often used. But be sure to use the right boundary conditions. 化 学 反 应 工 程 (c) If the flow deviates greatly from plug (D/uL large) chances are that the real vessel doesn’t meet the assumption of the model (a lot of independent random fluctuations). Here it becomes questionable whether the model should even be used. I hesitate when D/uL ＞1。 (d) You must always ask whether the model should be used. You can always match values, but if the shape looks wrong, as shown in the accompanying sketches, don’t use this model, use some other model. 化 学 反 应 工 程 (e) For large D/uL the literature is profuse and conflicting, primarily because of the unstated and unclear assumptions about what is happening at the vessel boundaries. The treatment of end additivity of variances is questionable. Because of all this we should be very careful in using the dispersion model where backmixing is large, particularly if th