Plug flow reactor adiabatic

Adiabatic plug flow reactors operate under the condition that there is no heat input to the reactor (i.e., Q = 0). The heat released in the reaction is retained in the reaction mixture so that the temperature rise along the reactor parallels the extent of the conversion. Adiabatic operation is important in heterogeneous tubular reactors. 

Find the size of adiabatic plug flow reactor to react the feed of Example 9.5 (F q = 1000 mol/min and AO 4 mol/liter) to 80% conversion. 

EXAMPLE 9J ADIABATIC PLUG FLOW REACTOR WITH RECYCLE... 

Industrial reactors generally operate adiabatically. Cholette and Blanchet [8] compared adiabatic plug flow reactor to the CSTRmm. For exothermic reactions, they inferred that the performance of a CSTRmm is better than that of a plug flow reactor at low values of conversion, and vice-versa at high values of conversion. They further showed that the design considerations for endothermic reactions are similar to those for isothermal reactions. 

Inverse response creates control difficulties. Assume, for example, that we wish to control the exit temperature of an adiabatic plug-flow reactor by manipulating the inlet temperature as shown in Fig. 4.13. From a steady-state viewpoint this is a perfectly reasonable thing to consider, since there are no issues of output multiplicity or open-loop instability, assuming the fluid is in perfect plug flow and there is no... 

Figure 4.13 Proposed temperature control of adiabatic plug-flow reactor.

In Chap. 4 we mentioned that the simplest reactor type from a control viewpoint is the adiabatic plug-flow reactor. It does not suffer from output multiplicity, open-loop instability, or hot-spot sensitivity. Furthermore, it is dominated by the inlet temperature that is easy to control for an isolated unit. The only major issue with this reactor type is the risk of achieving high exit temperatures due to a large adiabatic temperature rise. As we recall from Chap. 4, the adiabatic temperature rise is proportional to the inlet concentration of the reactants and inversely proportional to the heat capacity of the feed stream. WTe can therefore limit the temperature rise by diluting the reactants with a heat carrier. 

It now looks as if we have achieved the best of all worlds a thermally efficient process with an easy-to-control reactor Can this be true Not quite. What we forget are the undesirable effects on the reactor that thermal feedback introduces. In Chap. 4 we explained in detail how7 process feedback is responsible for the same issues we tried to avoid in the first place by selecting an adiabatic plug-flow reactor. It is necessary that we take a close look at the steady-state and dynamic characteristics of FEHE systems. 

Just as we approached reactor control in Chap. 4, we will start by exploring the open-loop effects of thermal feedback. Consider Fig. 5.19, which shows an adiabatic plug-flow reactor with an FEHE system. We have also included two manipulated variables that wall later turn out to be useful to control the reactor. One of these manipulated variables is the heat load to the furnace and the other is the bypass around the preheater. It is clear that the reactor feed temperature is affected by the bypass valve position and the furnace heat load but also by the reactor exit temperature through the heat exchanger. This creates the possibility for multiple steady states. We can visualize the different... 

Figure 5.19 Adiabatic plug-flow reactor with feed-effluent heat exchanger and trim heater.

We start by plotting the temperature rise in the reactor. This is done by integrating the steady-state differential equations that describe the composition and heat effects as functions of the axial position in the reactor. The adiabatic plug-flow reactor gives a unique exit temperature for a given feed temperature. This also means that we get a unique difference between the exit and feed temperatures. The temperature difference has to be less than or equal to the adiabatic temperature rise at a given, constant feed composition. Figure 5.20 show s the fractional temperature rise as a function of the reactor feed temperature for a typical system. 

Figure 5.20 Normalized temperature rise in adiabatic plug-flow reactor as function...

Figure 5.2S Control of packed adiabatic plug-flow reactor with FEHE and bypass control.

The advantages of the ring-shaped particles are also found for other type of reactions. To demonstrate this, consider an adiabatic plug flow reactor assuming that the external mass and heat transfer limitations are negligible. Equations for fluid-phase concentration and temperature (which are equal to the concentration and temperature at the surface of the pellet) are... 

For adiabatic plug-flow reactor, we solve (r), (s), and (u) simultaneously, sub-jeet to the initial conditions that Zi(0) = Z2(0) = 0, 0(0) = 1. Figure E9.3.3 shows a comparison of the V production between the adiabatic distributed-feed reactor and the adiabatic plug flow reactor with = 300°C. Figure E9.3.4 compares the production of V. 

For comparison, for adiabatic plug-flow reactor (R = 0) of the same volume, the outlet extent is Zout — 0.742, and 9out =1.135. The production rate of product B is 1,131 mol/min. For adiabatic CSTR R = oo) of the same volume, the outlet extent is Zout = 0.841, and 9out= 1.132. The production rate of product B is 1260 mol/min. Note that for both isothermal and adiabatic operation, a recycle reactor provides a higher production rate of product B than a corresponding plug-flow reactor and a CSTR. 

The reaction. is carried out in two long, adiabatic, plug-flow reactors with an interstage cooler between them, as shown. in Figure 633. (Refer to the discussion in Section 1.4,2 for more on interstage cooling.) The feed consists of component A diluted in an inert N2 stream, j =... 

The reaction, globally exothermic, takes place in an adiabatic Plug Flow Reactor at pressures of 25 to 35 bars and temperatures between 620 and 720 °C. Large excess of hydrogen, typically 5 1 molar ratio, prevents the formation of coke. The reaction conversion is typically 60-80%, because at higher value the selectivity drops rapidly. 

Figure 17.1 presents a simplified flowsheet, as developed in Chapter 7. Fresh raw materials mixed with recycled toluene and hydrogen is heated up to the reaction temperature. The reaction takes place in an adiabatic plug flow reactor. The reactor outlet is quenched with recycled hydrocarbon to prevent coke formation. Finally, the reaction mixture is cooled at 33 °C, and separated in a flash vessel in gas and liquid. 

Table 4.4 Example Analytical Solutions for Adiabatic Plug-Flow Reactors at Constant Pressure ...

Figure 3-1 Reactor volume vs. nonequilibrium conversion of CO in a singie-stage adiabatic plug-flow reactor that produces methanol from CO and H2. The steep increase in reactor volume near 9% CO conversion is a consequence of near-equiUbrium conditions when the feed enters at 340 K.

Once again, it is outside the scope of this paper to enter in this fascinating world, fix>m both chemical and numerical points of view. We will focus on a simple ideal case, the adiabatic plug flow reactor to put in tiie light the most prominent features of such models. 

The utility of the reactor point elSectiveness can be best illustrated through an example. Consider an adiabatic plug-flow reactor in which a catalytic reaction takes place. Suppose that 4>a is greater than 3 and that the intrinsic rate expression is ... 

Figure 10.11 Calculation procedures for adiabatic, plug-flow reactors using the equations in Table 10.6.

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