Temperature cooled tubular reactors

The need to keep a concave temperature profile for a tubular reactor can be derived from the former multi-stage adiabatic reactor example. For this, the total catalyst volume is divided into more and more stages, keeping the flow cross-section and mass flow rate unchanged. It is not too difficult to realize that at multiple small stages and with similar small intercoolers this should become something like a cooled tubular reactor. Mathematically the requirement for a multi-stage reactor can be manipulated to a different form ... 

In designing a wall-cooled tubular reactor, we want to operate such that the trajectory stays near the maximum rate for all temperatures. Thus for an exothermic reversible reaction the temperature should increase initially while the conversion is low and decrease as the conversion increases to stay away from the equilibrium constraint. One can easily program a computer to compute conversion and T versus t to attain a desired conversion for rninimum T in a PFTR. These curves are shown in Figure 5-17 for the three situations. 

Figure 5-23 Plots of reactant and temperature profiles versus axial position z and radial position JC in a wall-cooled tubular reactor. The reactor can exhibit a hot spot near the center where the rate is high and cooling is least.

The notable feature of the wall-cooled tubular reactor is that there can exist a hot spot near the center of the reactor and near the entrance. We saw this for the lumped model, which allowed only for variations in the direction, but when radial variations are allowed, the effect can become even more severe as both temperature and concentration vary radially. 

We used the wall temperature in the boundary condition, and this may be different from the coolant temperature T. There may be temperature variations across the wall as well as through the coolant. These are described through the overall heat transfer coefficient U, but in practice all these effects must be considered for a detailed description of the wall-cooled tubular reactor. 

There are five fundamental differences between CSTRs and tubular reactors. The first is the variation in properties with axial position down the length of the reactor. For example, in an adiabatic reactor with an exothermic irreversible reaction, the maximum temperature occurs at the exit of the reactor under steady-state conditions. However, in a cooled tubular reactor, the peak temperature usually occurs at an intermediate axial position in the reactor. To control this peak temperature, we must be able to measure a number of temperatures along the reactor length. 

Figure 5.20 shows the temperature profiles in the cooled tubular reactor for the optimum designs with the two catalysts. The optimum recycle flowrate is larger with the expensive catalyst, as expected, which yields an optimum inlet temperature that is higher. 

Cooled Tubular Reactor with Constant-Temperature Coolant... 

Figure 5.28 shows the flowsheet and the Specifications page tab for a cooled tubular reactor with co-current flow of coolant. The flowrate and temperature of the coolant... 

Cooled Tubular Reactor with Coolant Temperature Manipulated... 

One of the three options considered in Chapter 5 was a cooled tubular reactor with a coolant temperature that is the same down the length of the bed. With this type of system, the temperature of the coolant can be used as the manipulated variable to control some variable in the reactor. We will illustrate the control of the peak temperature by using several temperature measurements at different locations and selecting the highest to feed to a temperature controller as the process variable PV signal. The output signal OP of this controller will be the coolant temperature. 

These results show that the peak temperature controller works well for a variety of disturbances in this cooled tubular reactor system. 

Bilous and Amundson [1] were the first to describe the phenomenon of parametric sensitivity in cooled tubular reactors. This parametric sensitivity was used by Barkelew [2] to develop design criteria for cooled tubular reactors in which first order, second order and product- inhibited reactions take place. He presented diagrams from which for a certain tube diameter dt the required combination of CAO and Tc can be derived to avoid runaway or vice versa. Later van Welsenaere and Froment [3] did the same for first order reactions, but they also used the inflexion points in the reactor temperature T versus relative conversion XA trajectories, which describe the course of the reaction in the tubular reactor. With these trajectories they derived a less conservative criterion. Morbidelli and Varma [4] recently devised a method for single order reactions based on the isoclines in a temperature conversion plot as proposed by Oroskar and Stern [5]. 

The second issue for cooled tubular reactors is how to introduce the coolant. One option is to provide a large flowrate of nearly constant temperature, as in a recirculation loop for a jacketed CSTR. Another option is to use a moderate coolant flowrate in countercurrent operation as in a regular heat exchanger. A third choice is to introduce the coolant cocurrently with the reacting fluids (Borio et al., 1989). This option has some definite benefits for control as shown by Bucala et al. (1992). One of the reasons cocurrent flow is advantageous is that it does not introduce thermal feedback through the coolant. It is always good to avoid positive feedback since it creates nonmonotonic exit temperature responses and the possibility for open-loop unstable steady states. 

A similar technique is applied to low-density polyethylene reactors. Some of these systems operate in cooled tubular reactors at a very high pressure. Since the reactor has a thick tube wall, the temperature response to changes in the coolant is slow. Instead, the reaction rate (and thereby temperature.) is controlled by injecting initiator at select places along the length of the reactor tube (see Fig. 4.28). 

A countercurrent cooled tubular reactor such as that in Fig. 9.15 is said to be operating autothemically when the heat of reaction is sufficient to raise the temperature of the incoming stream from 7 to Tq. If it is not autothermic it might be necessary to add heat at z = 0 by means of an electric heater and this in fact is often done during start-up. By solving the equations... 

A high conversion level of isobutene (99%) can be reached with a double-stage configuration where, in both stages, water-cooled tubular reactors (WCTR), (1,2), are used for the isobutene dimerization to maintain optimal temperature control inside the catalytic bed. 

Fig. 3.6. Temperature profiles in methanol reactors. Left) WatCT cooled tubular reactor right) quench reactor (saw tooth profile)...

If we use Eq. (4.10.71), we have to keep in mind that pronounced radial temperature gradients may be present in cooled tubular reactors, even if the gradient is small or confined to a small region near the wall. Thus, Eq. (4.10.71) is strictly speaking only valid for an ideal PER with a uniform radial temperature, but for the subsequent examination of the basic principles of the behavior of non-isothermal tubular reactors we neglect this aspect and use an overall heat transfer coefficient Uh. The more complicated radial heat transfer in the case of pronounced radial temperature gradients in tubular reactors such as packed bed reactors will be treated in Section 4.10.7.3. Subsequently, we inspect the adiabatic operation of a tubular reactor first. Thereafter, we take a closer look at a wall[Pg.329]

Figure 4.10.41 Evolution of temperature and conversion in a cooled tubular reactor for the optimum path to minimize the reactor volume for a reversible exothermic reaction (example taken from Levenspiel 1999).

An interesting example of a cooled tubular reactor is the falling film reactor (or wetted wall column), that was described in section 4,63.1. " en the film is sufficiently thin and the evolution of heat is not too excessive, a very uniform temperature can be obtained. 

Hydrochloric acid may conveniently be prepared by combustion of hydrogen with chlorine. In a typical process dry hydrogen chloride is passed into a vapour blender to be mixed with an equimolar proportion of dry acetylene. The presence of chlorine may cause an explosion and thus a device is used to detect any sudden rise in temperature. In such circumstances the hydrogen chloride is automatically diverted to the atmosphere. The mixture of gases is then led to a multi-tubular reactor, each tube of which is packed with a mercuric chloride catalyst on an activated carbon support. The reaction is initiated by heat but once it has started cooling has to be applied to control the highly exothermic reaction at about 90-100°C. In addition to the main reaction the side reactions shown in Figure 12.6 may occur. 

In a tubular reactor system, the temperature rises along the reactor length for an exothermic reaction unless effective cooling is maintained. For multiple steady states to appear, it is necessary that a... 

Specific surface areas of the catalysts used were determined by nitrogen adsorption (77.4 K) employing BET method via Sorptomatic 1900 (Carlo-Erba). X-ray difiraction (XRD) patterns of powdered catalysts were carried out on a Siemens D500 (0 / 20) dififactometer with Cu K monochromatic radiation. For the temperature-programmed desorption (TPD) experiments the catalyst (0.3 g) was pre-treated at diflferent temperatures (100-700 °C) under helium flow (5-20 Nml min ) in a micro-catalytic tubular reactor for 3 hours. The treated sample was exposed to methanol vapor (0.01-0.10 kPa) for 2 hours at 260 °C. The system was cooled at room temperature under helium for 30 minutes and then heated at the rate of 4 °C min . Effluents were continuously analyzed using a quadruple mass spectrometer (type QMG420, Balzers AG). 

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