Reactor Feed-Effluent Exchange Systems

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. 

While introducing a heat carrier solves the reactor problem, it unfortunately creates some other concerns. First, we increase the size of the separation section since we have to separate the products from a large amount of heat carrier. Second, we make the plant thermally inefficient by significantly increasing the plant s energy load due to repeated heating and cooling of the heat carrier. To solve the efficiency problem we 

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. 

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Listing C.l. Symbolic derivation of reduced-order model of the slow dynamics, and of the input-output linearizing temperature controller for the reactor-feed effluent heat exchanger system in Section 6.6... 

Douglas, J. M.. Orcutt, J. C., and Berthiaume. P. W. Design and Control of Feed-Effluent, Exchanger-Reactor Systems. Ind. Eng. Chem. Fundam., 1, 253-257 (1962). 

Description The para-depleted liquid Cg aromatics raffinate stream from the PX separation unit, along with hydrogen-rich recycle gas are pumped through feed/effluent exchangers and the charge heater (1) and into the reactor (2). Vapor then flows down through the fixed, dualbed catalyst system. 

Consider the simplified flow diagram of a reactor system shown in Fig. 20. In this example, we have included the possibility of several input streams, a product recycle loop, and a feed-effluent heat exchanger so as to again represent a very general reactor system. The possible control variables or disturbances to the process are the flow rates of the input gases, the recycle... 

Fig. 20. Packed bed reactor system including feed-effluent heat exchanger.

If the system consists of a series of adiabatic reactors, there are two basic configurations. The first has heat exchangers or furnaces between each of the reactors to cool or heat the reactor effluent before it enters the next reactor. The second configuration uses cold shot cooling. Some of the cold reactor feed is bypassed around the upstream reac-tor(s) and mixed with the hot effluent from the reactor to lower the inlet temperature to the downstream catalyst bed. 

In Chapters 5 and 6, high-temperature exothermic tubular reactor systems were considered. All of these systems used feed-effluent heat exchangers (FEHE) to preheat the feed to the desired reactor inlet temperature by recovering heat from the hot reactor exit stream. Some of the systems also used a trim furnace to add additional heat if needed. 

We considered two scenarios that are typical for the operation of reactors with feed-effluent heat exchange. The first set of simuiations traced the response of the ciosed-ioop system to a 10% increase in the production rate, imposed at t = 1 h by increasing the feed flow rate. Subsequently, we analyzed the response of the same situation, but with the added complexity of an unmeasured 10 K increase in the feed temperature occurring at t = 1 h. In both cases, the setpoint of the reactor temperature controller Tjqset was increased by 2 K at t = 1 h in order to maintain reactor conversion at the higher production rate. 

Figure 4.11 present the complete flowsheet together with the control structure. The reaction takes place in an adiabatic tubular reactor. To avoid fouling, the temperature of the reactor-outlet stream is reduced by quenching. A feed-effluent heat exchanger (FEHE) recovers part of the reaction heat. For control purposes, a furnace is included in the loop. The heat-integrated reaction system is stabilized... 

Step 3. The exothermic heat of reaction must be removed, and the reactor feed must be heated to a high enough temperature to initiate the reaction. Since the heat of reaction is not large and complete one-pass conversion is not achieved, the reactor exit temperature is only 32°F higher than the reactor inlet temperature. Since heat transfer coefficients in gas-to-gas systems are typically quite low, this small temperature differential would require a very large heat exchanger if only the reactor effluent is used to heat the reactor feed and no furnace... 

Fig. 10 shows a typical SO2 multistage reactor system manufactured by Zieren-Chemiebau. This particular design has feed-effluent heat exchangers and cold-shot interstage cooling. 

Figure 25.3 Alternative control configurations needed for a feed-effluent heat exchanger system around a reactor.

As we have seen in Example 1.2, operation at Pi is inherently unstable and we need to design a stabilizing controller, which should be very robust to maintain stable operation. The design of such controller may not always be possible because either the reaction is extremely sensitive or its mechanism and parameters are poorly known. In such a case we need to redesign the reactor system and make it inherently stable (Figure 25.2c) or eliminate the feed-effluent heat exchange. 

Now, some difficulties may appear around the reactor. The inspection of the heat recovery system reveals a rather complicated structure. To simplify this first analysis, the whole heat recovery system is lumped in a single unit, called FEHE, which is the abbreviation for Feed-Effluent-Heat-Exchanger. However, coding the reactor remains difficult. Selecting a Plug Flow Reactor (PFR) model, close to physical reality, requires kinetics. Again, we can simplify the analysis by assuming the main reactions close to equilibrium in a first unit R1 (REQUIL) followed by a second unit R2 (RSTOIC) that... 

The steady state of the complete system—reactor and heat exchanger—has to satisfy both Eqs. 11.5.e-5 and 11.5.e-7. But it is easily seen that, depending on the location of 7t and on the slope, the straight line can have up to three intersections with (i.e., three steady-state operating conditions are possible for the system reactor -f heat exchanger, whereas the reactor on itself has a unique steady state). The multiplicity of steady states in the complete system is a consequence of the thermal feedback realized in the heat exchanger between the feed and the effluent. 

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