Reactor heat transfer

Heat transfer. Reactor heat removal preheater, reboiler, and condenser heat transfer areas temperature levels of steam and cooling water... 

Fluid Dynamics Fluid Dynamics (Chemical Engineering) Fluid Inclusions Heat Transfer Reactors in Process Engineering Solvent Extraction... 

The AHTR has the potential to provide a highly robust safety case because of various inherent and passive safety characteristics. Inherent safety characteristics include a low-core-power density, high-heat-capacity core, and high-temperature-margin fuel. Other inherent safety characteristics of the AHTR include atmospheric pressure operation and efficient liquid-coolant heat transfer. Reactor physics for the AHTR are similar to other graphite based, coated-particle fuel systems (GT-MHR) where negative feedback comes from the high-temperature Doppler effect within the fuel. 

In Chapter 1 it was shown which aspects of a chemical process have to be studied when it is to be developed from a laboratory synthesis to an industrial process. These include reaction rates, selectivities, mixing and mass transfer, heat evolution and heat transfer, reactor safety, waste purification, etc,. To this list we have to add scaling up. The scale-up factor from a laboratory reactor to a plant reactor is usually on the order of 10 - 10, That means that all the aspects mentioned above have to be considered as a function of the scale of operation, and that extrapolations have to be made to predict the behaviour of a process on a larger scale. 

Even if the reactor temperature is controlled within acceptable limits, the reactor effluent may need to be cooled rapidly, or quenched, to stop the reaction quickly to prevent excessive byproduct formation. This quench can be accomplished by indirect heat transfer using conventional heat transfer equipment or by direct heat transfer by mixing with another fluid. A commonly encountered situation is... 

In fact, cooling of the reactor effluent by direct heat transfer can be used for a variety of reasons ... 

The liquid used for the direct heat transfer should be chosen such that it can be separated easily from the reactor product and so recycled with the minimum expense. Use of extraneous materials, i.e., materials that do not already exist in the process, should be avoided because it is often difficult to separate and recycle them with high efficiency. Extraneous material not recycled becomes an effluent problem. As we shall discuss later, the best way to deal with effluent problems is not to create them in the first place. 

Because the characteristic of tubular reactors approximates plug-flow, they are used if careful control of residence time is important, as in the case where there are multiple reactions in series. High surface area to volume ratios are possible, which is an advantage if high rates of heat transfer are required. It is sometimes possible to approach isothermal conditions or a predetermined temperature profile by careful design of the heat transfer arrangements. 

Fluidized-bed catalytic reactors. In fluidized-bed reactors, solid material in the form of fine particles is held in suspension by the upward flow of the reacting fluid. The effect of the rapid motion of the particles is good heat transfer and temperature uniformity. This prevents the formation of the hot spots that can occur with fixed-bed reactors. 

One disadvantage of fluidized heds is that attrition of the catalyst can cause the generation of catalyst flnes, which are then carried over from the hed and lost from the system. This carryover of catalyst flnes sometimes necessitates cooling the reactor effluent through direct-contact heat transfer hy mixing with a cold fluid, since the fines tend to foul conventional heat exchangers. 

Heat transfer. Once the basic reactor type and conditions have been chosen, heat transfer can be a major problem. Figure 2.11 summarizes the basic decisions which must be made regarding heat transfer. If the reactor product is to be cooled by direct contact with a cold fluid, then use of extraneous materials should be avoided. 

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... 

Indirect heat transfer with the reactor. Although indirect heat transfer with the reactor tends to bring about the most complex reactor design options, it is often preferable to the use of a heat carrier. A heat carrier creates complications elsewhere in the flowsheet. A number of options for indirect heat transfer were discussed earlier in Chap. 2. 

The first distinction to be drawn, as far as heat transfer is concerned, is between the plug-flow and continuous well-mixed reactor. In the plug-flow reactor shown in Fig. 13.1, the heat transfer can take place over a range of temperatures. The shape of the profile depends on... 

Figure 13.1 The heat transfer characteristics of plug-flow reactors.

The thermal profile through the reactor will in most circumstances be carefully optimized to maximize selectivity, extend catalyst life, etc. Because of this, direct heat integration with other process streams is almost never carried out. The heat transfer to or from the reactor is instead usually carried out by a heat transfer intermediate. For example, in exothermic reactions, cooling might occur by boiling water to raise steam, which, in turn, can be used to heat cold streams elsewhere in the process. 

For indirect heat transfer, the heat integration characteristics of the reactor can be broken down into three cases ... 

Fixing the rate of heat transfer in a batch reactor is often not the best way to control the reaction. The heating or cooling characteristics can be varied with time to suit the characteristics of the reaction. Because of the complexity of hatch operation and the fact that operation is usually small scale, it is rare for any attempt to be made... 

If indirect heat transfer is used with a large temperature difference to promote high rates of cooling, then the cooling fluid (e.g., boiling water) is fixed by process requirements. In this case, the heat of reaction is not available at the temperature of the reactor effluent. Rather, the heat of reaction becomes available at the temperature of the quench fluid. Thus the feed stream to the reactor is a cold stream, the quench fluid is a hot stream, and the reactor effluent after the quench is also a hot stream. 

The reactor effluent might require cooling by direct heat transfer because the reaction needs to be stopped quickly, or a conventional exchanger would foul, or the reactor products are too hot or corrosive to pass to a conventional heat exchanger. The reactor product is mixed with a liquid that can be recycled, cooled product, or an inert material such as water. The liquid vaporizes partially or totally and cools the reactor effluent. Here, the reactor Teed is a cold stream, and the vapor and any liquid from the quench are hot streams. 

The reaction is exothermic, and multitubular reactors are employed with indirect cooling of the reactor via a heat transfer medium. A number of heat transfer media have been proposed to carry out the reactor cooling, such as hot oil circuits, water, sulfur, mercury, etc. However, the favored heat transfer medium is usually a molten heat transfer salt which is a eutectic mixture of sodium-potassium nitrate-nitrite. 

The appropriate placement of reactors, as far as heat integration is concerned, is that exothermic reactors should be integrated above the pinch and endothermic reactors below the pinch. Care should be taken when reactor feeds are preheated by heat of reaction within the reactor for exothermic reactions. This can constitute cross-pinch heat transfer. The feeds should be preheated to pinch temperature by heat recovery before being fed to the reactor. 

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