化学反应工程英文课件Chapter 11

化学反应工程

Chapter 11 Basics of Non-Ideal FlowSo far we have treated two flow patterns, plug flow and mixed flow. These can give very different behavior (size of reactor, distribution of products). We like these flow patterns and in most cases we try to design equipment to approach one or the other because one or the other often is optimum no matter what we are designing for. these two patterns are simple to treat.

化学反应工程But real equipment always deviates from these ideals. How to account for this? That is what this and following chapters are about. Overall three somewhat interrelated factors make up the contacting or flow pattern: 1. the RTD or residence time distribution of material which is flowing through the vessel; 2. the state of aggregation of the flowing material, its tendency to clump and for a group of molecules to move about together; 3. the earliness and lateness of mixing of material in the vessel.

化学反应工程Let us discuss these three factors in a qualitative way at first. Then, this and the next few chapters treat these factors and show how they affect reactor behavior. The Residence Time Distribution, RTD Deviation from the two ideal flow patterns can be caused by channeling of fluid, by recycling of fluid, or by creation of stagnant regions in the vessel. Figure 11.1 shows this behavior. In all types of process equipment, such as heat exchangers, packed columns, and reactors, this type of flow should be avoided since it always lowers the performance of the unit.

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化学反应工程Setting aside this goal of complete knowledge about the flow, let us be less ambitious and see what it is that we actually need to know. In many cases we really do not need to know very much, simply how long the individual molecules stay in the vessel, or more precisely, the distribution of residence times of the flowing fluid. This information can be determined easily and directly by a widely used method of inquiry, the stimulus-response experiment. This chapter deals in large part with the residence time distribution (or RTD) approach to nonideal flow. We show when it may legitimately(合理地) be used, how to use it, and when it is not applicable what alternatives to turn to.

化学反应工程In developing the “language” for this treatment of nonideal flow (see Danckwerts, 1953), we will only consider the steady-state flow, without reaction and without density change, of a single fluid through a vessel.State of Aggregation of the Flowing Stream Flowing materials is in some particular state of aggregation, depending on its nature. In the extremes these states can be called microfluids and macrofluids, as sketched in Figure. 11.2.

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Microfluid

Macroflui d

Figure 11.2 Two extremes of aggregation of fluid

化学反应工程Single-Phase Systems. These lie somewhere between the extremes of macro- and microfluids. Two-Phase Systems. A stream of solids alway

s behaves as a macrofluid, but for gas reacting with liquid, either phase can be a macro-or microfluid depending on the contacting scheme being used. The sketches of Figure. 11.3 show completely opposite behavior.

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Figure 11.3

Examples of macro- and microfluid behavior.

化学反应工程 Earliness of MixingThe fluid elements of a single flowing stream can mix with each other either early or late in their flow through the vessel. For example, see Fig. 11.4. Usually this factor has little effect on overall behavior for a single flowing fluid. However, for a system with two entering reactant streams it can be very important. For example, see Fig. 11.5.

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Figure 11.4 Examples of early and of late mixing of fluid.

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Figure 11.5 Early or late mixing affects reactor behavior.

化学反应工程Role of RTD, State of Aggregation, and Earliness of Mixing in Determining Reactor Behavior In some situations one of these factors can be ignored; in others it can become crucial. Often, much depends on the time for reaction, t rec the time for mixing t mix , and the time for stay in the vessel t stay . In many cases t stay has a meaning somewhat like t mix but somewhat broader(更广的).

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11.1 E, THE AGE DISTRIBUTION OF FLUID, THE RTDIt is evident that elements of fluid taking different routes through the reactor may take different lengths of time to pass through the vessel. The distribution of these times for the stream of fluid leaving the vessel is called the exit age distribution E, or the residence time distribution RTD of fluid. E has the units of time-1.

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停留时间分布密度函数 — E(t) 函数对于同时进入反应器入口的 N 个流体粒子，若在出口处 进行检测，则其中停留时间介于 t ~ t + dt 之间的流体粒 子个数 dN 所占的分率为 E(t) dt = dN / N —— 我们定义 E(t) 为停留时间分布密度函数。 如：在某时刻进入反应器入口的 100 个流体粒子，到达出 口时停留时间为 5 ~ 6 min 的粒子有 8 个，若取 t = 5 min, dt = t = 1 min, 则此时 E(t) dt = dN / N = N / N = 8 /100 = 0.08; E(t) = dN / (dt*N) = N / ( t *N) = 0.08 min-1 当 dt 0,则 E(t) dt 是一个瞬时( 如t = 5 min 时 )的分率

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停留时间分布函数 — F(t) 函数对于同时进入反应器入口的 N 个流体粒子，若在出口 处进行检测，则其中停留时间介于 0 ~ t 之间的流体粒 子所占的分率为 F(t) —— 我们定义 F(t) 为停留时间分 布函数。 如：在某时刻进入反应器入口的 100 个流体粒子，到 达出口时停留时间为 0 ~ 5 min 的粒子有 20 个，若取 t = 5 min,则此时 F(t) = 20 /100 = 0.2。 F(t) 是一个累积(如 t = 0~5 min )的分率。