Reactor heat integration adiabatic operation

The heat integration characteristics of reactors depend both on the decisions made for the removal or addition of heat and the reactor mixing characteristics. In the first instance, adiabatic operation is considered, since this gives the simplest design. 

Adiabatic operation. If adiabatic operation leads to an acceptable temperature rise for exothermic reactors or an acceptable fall for endothermic reactors, then this is the option normally chosen. If this is the case, then the feed stream to the reactor requires heating and the efiluent stream requires cooling. The heat integration characteristics are thus a cold stream (the reactor feed) and a hot stream (the reactor efiluent). The heat of reaction appears as elevated temperature of the efiluent stream in the case of exothermic reaction or reduced temperature in the case of endothermic reaction. 

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. 

Cold shot. Injection of cold fresh feed for exothermic reactions or preheated feed for endothermic reactions to intermediate points in the reactor can be used to control the temperature in the reactor. Again, the heat integration characteristics are similar to adiabatic operation. The feed is a cold stream if it needs to be increased in temperature or vaporized and the product a hot stream if it needs to be decreased in temperature or condensed. If heat is provided to the cold shot or hot shot streams, these are additional cold streams. 

Case (2) CSTR T0 for Vmin with adiabatic operation and specified fA, FAo and q The result given by 18.4-4 requires only that the operating temperature within the CSTR be Topt, and implies nothing about the mode of operation to obtain this, that is, nothing about the feed temperature (TJ, or heat transfer either within the reactor or upstream of it. If the CSTR is operated adiabatically without internal heat transfer, T0 must be adjusted accordingly to a value obtained from the energy balance, which, in its simplest integrated form, is, from equation 14.3-10,... 

The influence of heat losses through the reactor wall have been studied [5,23]. Radial temperature gradients inside the monolith material can often be neglected, because the operation is usually adiabatic. This means that modeling of one single channel is adequate. Any nonuniform flow distribution may be incorporated into a reactor model by integration of the single channel performance over the whole cross section of the reactor. 

The second stage of development is typically aimed at mimicking commercial operations by employing recycle streams to achieve realistic simulations of the integrated process. Isothermal conditions are usually maintained in the reactor, but if heat release is a concern, such as residuum hydrotreating, then it is wise to run adiabatically so that the adiabatic reaction temperature can be established and also how much heat must be removed in the final commercial design. Defining catalyst deactivation, yield patterns, and how various feed types influence the process are typical aspects to explore. 

By using integrated heat exchangers, researchers at PNNL [27] were able to control the temperature of a microstmctured WGS reactor and impose a near-optimal temperature profile, rather than operating the reactor as two sequential adiabatic units (Figure 26.5). The first section of the reactor, operated adiabatically, consisted of increasing the reactor temperature, while the second section was cooled to follow as closely as possible the optimal temperature profile. This way of operating the reactor resulted in a reduction in the reactor size by a factor of 2. If the reaction rate is expressed in the form of a power law ... 

Giroux et al. performed a comparison of an adiabatic water-gas shift reactor, which had a temperature rise from 300 to 360 °C, isothermal operation at 350 °C achieved by integrated heat-exchange and a reactor with a declining temperature profile from 550 to 300 °C [57]. In each case the same degree of carbon monoxide conversion was achieved in the reactors, but the space velocity increased from 35 000 h for the adiabatic reactor to 50 000 h for the isothermal reactor and even up to 70 000 h for the reactor with the declining temperature profile, which means that the last reactor would require only half the size of the adiabatic counterpart. 

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