Reactor temperature heat carrier

Different technical solutions are used in the temperature control of industrial reactors. The heat carriers mentioned in Section 9.2.f may be used by different technical means the direct way whereby the heat carrier is directly mixed with the reaction mass, internal or external coils, jacket, simple circuits, and indirect systems with a double circulating system. These techniques with their advantages and drawbacks, in terms or process safety, are reviewed in the following sections. 

Reactor heat carrier. Also as pointed out in Sec. 2.6, if adiabatic operation is not possible and it is not possible to control temperature by direct heat transfer, then an inert material can be introduced to the reactor to increase its heat capacity flow rate (i.e., product of mass flow rate and specific heat capacity) and to reduce... 

Heat carriers. If adiabatic operation produces an unacceptable rise or fall in temperature, then the option discussed in Chap. 2 is to introduce a heat carrier. The operation is still adiabatic, but an inert material is introduced with the reactor feed as a heat carrier. The heat integration characteristics are as before. The reactor feed is a cold stream and the reactor efiluent a hot stream. The heat carrier serves to increase the heat capacity fiow rate of both streams. 

Advanced Cracking Reactor. The selectivity to olefins is increased by reducing the residence time. This requires high temperature or reduction of the hydrocarbon partial pressure. An advanced cracking reactor (ACR) was developed jointly by Union Carbide with Kureha Chemical Industry and Chiyoda Chemical Constmction Co. (72). A schematic of this reactor is shown in Figure 6. The key to this process is high temperature, short residence time, and low hydrocarbon partial pressure. Superheated steam is used as the heat carrier to provide the heat of reaction. The burning of fuel... 

Before injecting the pulses into the reactor, the reactor is supplied with the carrier gas nitrogen and allowed to reach steady state. For the high-temperature experiments, the reactor was heated for at least one hour and the steady state observed with a temperature sensor on the reaction plate. The reactor temperature was mostly 450 °C, at close to atmospheric pressure. 

The influence of the reactor temperature on the conversion of methane was examined during pulsed operation. The heat performance of the foil heater was slowly increased while the reactor was continuously supplied with gas pulses consisting of pure oxygen at 129.5 ml min-1 together with a flow of methane (0.5 ml min-1). The volume flow of the carrier gas nitrogen was adjusted to 130 ml min-1 at atmospheric pressure, delivering a residence time in the coated spiral of 0.4 s. The cat-... 

There are several possible mechanisms for the heat exchange between a reacting medium and a heat carrier radiation, conduction and forced or natural convection. Here we shall consider convection only. Other mechanisms are considered in the chapter on heat accumulation. The heat exchanged with a heat carrier (q ) across the reactor wall by forced convection is proportional to the heat exchange area (A) and to the driving force, that is, the temperature difference between the reaction medium and the heat carrier. The proportionality coefficient is the overall heat transfer coefficient (U) ... 

Initial temperature (T0) Before starting the feed, the reactor has to be heated up to its initial temperature. If only the heat carrier s temperature can be actively... 

The most common heat carrier for heating industrial reactors is steam, providing an efficient and simple means. The efficiency of steam is due to its high latent heat of condensation (AHv = 2260 kj kg1 at 100 °C). For saturated steam, the temperature can be controlled by its pressure. Some values are presented in Table 9.1. The pressure and latent heat of evaporation corresponding to a given temperature may easily be estimated using Regnault law ... 

This is the simplest system for temperature control of a reactor only the jacket temperature is controlled and maintained constant, leaving the reaction medium following its temperature course as a result of the heat balance between the heat flow across the wall and the heat release rate due to the reaction (Figure 9.9). This simplicity has a price in terms of reaction control, as analysed in Sections 6.7 and 7.6. Isoperibolic temperature control can be achieved with a single heat carrier circuit, as well as with the more sophisticated secondary circulation loop. 

Performing a reaction under isothermal conditions is somewhat more complex. It requires two temperature probes, one for the measurement of the reaction mass temperature and a second for the jacket temperature. Depending on the internal reactor temperature, the jacket temperature is adjustable. The simplest method is to use a single heat carrier circuit to act either on the flow rate of cooling water or on the steam valve. With a secondary heat carrier circulation loop, the temperature controller acts directly on the heating and cooling valves by using a conventional... 

If we also consider that the reactor is heated to a constant heat carrier temperature (Tc), the heat balance in Equation 9.2 becomes... 

The thermal time constant is only one aspect of reactor dynamics. In practice, the heat carrier temperature cannot be adjusted instantaneously at industrial scale, as it has its own dynamics, depending on the equipment and the temperature control algorithm. These aspects of the dynamics of the heat exchange and temperature control systems are considered in the next sections. 

Figure 9.13 Heating curve of a reactor heated with a constant heat carrier temperature of 100°C.

This expresses that the reactor contents temperature approaches the heat carrier temperature asymptotically, following an exponential law (Figure 9.13). 

Calculation of the reactor contents temperature (T) as a function of time, when heated with a heat carrier at a constant temperature (Tc) ... 

In order to achieve an accurate control of the internal reactor temperature, a cascade controller can be used. In this type of controller, temperature control is managed by two controllers arranged in cascade, that is, in two nested loops (Figure 9.14). The external loop, called the master, controls the temperature of the reaction mixture by delivering a set value to the slave, the inner loop, which controls the temperature of the heat carrier (Tc). 

The set point of the heat carrier temperature is calculated proportionally to the deviation of the reactor temperature from its set point ... 

A more accurate method determines the thermal time constant from a plot of the natural logarithm of temperature difference between the reactor contents (Tr) and the heat carrier (Tc) as a function of time. This is an application of Equation 9.11 ... 

The reactor is draft-loaded with corresponding amounts of butylbromide, tin and butyl alcohol. Then the apparatus is closed with a lid fashioned with an agitator, moved into the operation chamber and connected to all piping. After that the jacket of the reactor is filled with a heat carrier with a temperature of 90-95 °C, the contents of the apparatus are heated at agitation to 85-90 °C, the operation chamber is closed with a protective plug and sources of y-radiation are introduced into the chamber. After certain periods of time the mixture in the reactor is sampled with the help of a special sampler. 8-10 hours after the radiation has started, the reaction is completed and the sources of radiation are taken out of the chamber. The reactive mixture is loaded off and the apparatus is prepared for the next operation. 

For a highly exothermic reaction the optimization of the temperature profile is the key factor in maximizing the reactor productivity. For endothermic reactions maximizing the reaction temperature and employing a heat carrier is often the best solution. Table 2.11 summarizes the guidelines. 

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. 

In the case of a solid heat carrier, the preheated feedstock is placed in contact with a refractory mass raised to a hi temperature. Cracking lowers the temperature and generates coke deposits that must be removed. The state of the solid and the inifiai operating condidons are restored by combustion. These operations can take place m the same reactor c> clically on a hxed r ractory (Wulff process) or in distinct units in which the solid exists in the form of moving or fluidized particle beds. In this case the hydrocarbon feedstock is injected in the combustion gases. 

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