Reactor/heat exchanger systems

Equation (5.11) represents a straight line in the diagram of fractional temperature rise versus reactor feed temperature. We show three such lines in Fig. 5.21. All lines intersect the temperature rise curve at least once (at a low temperature not shown in Fig. 5.21). It therefore appears that the reactor FEHE can have one, two, or three steady-state solutions for this particular set of reaction kinetics. Furthermore, the intermediate steady state, in the case of three solutions, is open-loop unstable due to the slope condition discussed in Chap. 4. This was verified by Douglas et al. (1962) in a control study of a reactor heat exchange system. 

Fig. 6.13 Example of a heavy-pilot microstructured heat exchanger integrated 5 kW i reactor-heat exchanger system for selective oxidation to achieve gas purification of H2-rich reformer gas for fuel cells. (Source IMM.)...

Some Reactor/Heat-Exchanger Systems 6.2.1 Autothermal Reactors... 

Figure II.5.e-2 Possible steady-slate operating points in reactor-heat exchanger system, for two inlet temperatures 7 j. 

Karakaya M, Avci AK. Simulation of on-board fuel conversion in catalytic microchannel reactor-heat exchanger systems. Topics in Catalysis 2009 52 2112-2116. 

Conventional industrial processes operate continuously and at steady state. Steady-state operation, however, is sometimes less economical, particularly in cases where considerable energy effects are encountered. For example, classical sulfur dioxide oxidation processes involve a large reactor-heat exchanger system to force conversion of the reactant to an acceptable level. By non-steady-state operation, the reactor volume can be considerably diminished. Unconventional operation modes are much more sensitive than conventional steady-state modeling. Very advanced dynamic modeling concepts are thus needed. 

Glycerol was to be ethoxylated at 115-125°C in a circulating reaction system with separate reactor, heat exchanger and catalyst units. The valve at the base of the reactor was still closed, but an inoperative flow indicator failed to indicate absence of circulation and a total of 3 tonnes of the oxide, plus glycerol, was charged to the reactor. Upon subsequent opening of the valve, the reaction mixture passed through the heater, now at 200° C, and a runaway reaction developed, the reactor burst and an explosion followed. 

The choice of the heat exchange system also depends on the overall purpose of the reactor. 

In this approach accident cases and design recommendations can be analysed level by level. In the database the knowledge of known processes is divided into categories of process, subprocess, system, subsystem, equipment and detail (Fig. 6). Process is an independent processing unit (e.g. hydrogenation unit). Subprocess is an independent part of a process such as reactor or separation section. System is an independent part of a subprocess such as a distillation column with its all auxiliary systems. Subsystem is a functional part of a system such as a reactor heat recovery system or a column overhead system including their control systems. Equipment is an unit operation or an unit process such as a heat exchanger, a reactor or a distillation column. Detail is an item in a pipe or a piece of equipment (e.g. a tray in a column, a control valve in a pipe). 

Pfiefer et al. are developing a methanol fuel processor system using steam reforming for a 200 Wg fuel cell based power supply. The researchers are currently working on the methanol reformer reactors, heat exchangers, combustors, and preferential oxidation reactors (Figure 23) for the system. The reactor bodies are either stainless steel or copper. 

We follow a three-step procedure First, we must find how equilibrium composition, rate of reaction, and product distribution are affected by changes in operating temperatures and pressures. This will allow us to determine the optimum temperature progression, and it is this that we strive to approximate with a real design. Second, chemical reactions are usually accompanied by heat effects, and we must know how these will change the temperature of the reacting mixture. With this information we are able to propose a number of favorable reactor and heat exchange systems—those which closely approach the optimum. Finally, economic considerations will select one of these favorable systems as the best. 

Hence only about 2% of the reaction energy can be removed by the heat exchange system. Thus, the reactor behaves quasi adiabatic. In other words, the reaction is so fast that the heat exchange system is unable to remove any significant heat. 

The heat exchange area (A) may vary with time due to the volume increase by the feed. This variation is determined by the geometry of the reactor, especially by its height covered by the heat exchange system (jacket, internal coils, or welded half-coils). In case there is a significant change in the physical chemical properties of the reaction mixture, the overall heat exchange coefficient (U) will also be a function of time. 

A reliable control of the reaction course can be obtained by isothermal operation. Nevertheless, to maintain a constant reaction medium temperature, the heat exchange system must be able to remove even the maximum heat release rate of the reaction. Strictly isothermal behavior is difficult to achieve due to the thermal inertia of the reactor. However, in actual practice, the reaction temperature (Tr) can be controlled within 2°C, by using a cascade temperature controller (see Section 9.2.3). Isothermal conditions may also be achieved by using reflux cooling (see Section 9.2.3.3), provided the boiling point of the reaction mass does not change with composition. 

The condition for the practical implementation of such a feed control is the availability of a computer controlled feed system and of an on-line measurement of the accumulation. The later condition can be achieved either by an on-line measurement of the reactant concentration, using analytical methods or indirectly, by using a heat balance of the reactor. The amount of reactant fed to the reactor corresponds to a certain energy of reaction and can be compared to the heat removed from the reaction mass by the heat exchange system. For such a measurement, the required data are the mass flow rate of the cooling medium, its inlet temperature, and its outlet temperature. The feed profile can also be simplified into three constant feed rates, which approximate the ideal profile. This kind of semi-batch process shortens the time-cycle of the process and maintains safe conditions during the whole process time. This procedure was shown to work with different reaction schemes [16, 19, 20], as long as the fed compound B does not enter parallel reactions. 

These equations calculate the temperature and conversion profiles in a polytropic tubular reactor. The term (a) represents the heat generation rate by the reaction and the term (b) the heat removal rate by the heat exchange system. This equation is similar to Equation 5.2, obtained for the batch reactor. Moreover, since the... 

In a reactor working under normal operating conditions, meaning the heat exchange system is working as designed, the mechanism of heat transfer is forced... 

The first term depends entirely on the physical properties of the reactor contents and degree of agitation. It represents resistance to heat transfer of the internal film and of eventual deposits at the wall, which may determine the overall heat transfer [3], Therefore, the reactor should be regularly cleaned with a high pressure cleaner. Both last terms depend on the reactor itself and on the heat exchange system, that is, reactor wall, fouling in the jacket, and external liquid film. They are often grouped under one term the equipment heat transfer coefficient (cp) [4, 5],... 

In case of total failure of the system, the heat exchange system will become inactive and the vacuum pump also stops the temperature of reactor contents will equilibrate with the reactor itself, thus the temperature would reach a level somewhat above 120 °C. The TMRad is longer than 24 hours. 

An endothermic reaction A — R is performed in three-stage, continuous flow stirred tank reactors (CFSTRs). An overall conversion of 95% of A is required, and the desired production rate is 0.95 x 10 3 kmol/sec of R. All three reactors, which must be of equal volume, are operated at 50°C. The reaction is first order, and the value of the rate constant at 50°C is 4 x 10-3 sec-1. The concentration of A in the feed is 1 kmol/m3 and the feed is available at 75°C. The contents of all three reactors are heated by steam condensing at 100°C inside the coils. The overall heat transfer coefficient for the heat-exchange system is 1,500 J/m2 sec °C, and the heat of reaction is +1.5 x 108 J/kmol of A reacted. 

Consider a batch reactor as shown in Figure 6-4 with a heat exchange system. There is no flow into or out of the reactor. It can be assumed that the total mass of the mixture, m, is constant and allowing for a change in volume, VR, from Equation 6-12 gives (no flow implies that... 

Figure 7.14 Energy flows of different magnitudes in a reactor-external-heat-exchanger system.

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... 

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