Direct Heat Transfer

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. 

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... 

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. 

Contactive (Direct) Heat Transfer Contactive heat-transfer equipment is so constructed that the particulate burden in solid phase is directly exposed to and permeated by the heating or cooling medium (Sec. 20). The carrier may either heat or cool the solids. A large amount of the industrial heat processing of sohds is effected by this mechanism. Physically, these can be classified into packed beds and various degrees of agitated beds from dilute to dense fluidized beds. 

Equipment commonly employed for the diying of sohds is described both in this subsection in Sec. 12, where indirect heat transfer devices are discussed, and in Sec. 17 where fluidized beds are covered. Diyer control is discussed in Sec. 8. Excluding fluid beds this subsection contains mainly descriptions of direct-heat-transfer equipment. It also includes some indirect units e.g., vacuum diyers, furnaces, steam-tube diyers, and rotaiy calciners. 

The One-Dimensional Pseudo Homogeneous Model of Fixed Bed Reactors. The design of tubular fixed bed catalytic reactors has generally been based on a one-dimensional model that assumes that species concentrations and fluid temperature vary only in the axial direction. Heat transfer between the reacting fluid and the reactor walls is considered by presuming that all of the resistance is contained within a very thin boundary layer next to the wall and by using a heat transfer coefficient based on the temperature difference between the fluid and the wall. Per unit area of the tube... 

Thus gas detonations initiate condensed explosives (if at all) by some direct heat transfer process. In oxy-acetylene detonations, the equilibrium temperature (CJ temp) is quite high 4500°K... 

The entropy generation for the direct heat-transfer process is ... 

ACETYU3VE lVU aJFACTURE FROM HYDROCARBONS. THERMAL PROCESSES WITH DIRECT HEAT TRANSFER... 

Thus the same result is found for adiabatic processes as for direct heat transfer AStotai is always positive, approaclring zero as a limit when tire process becomes reversible. This same conclnsion can be denronstratedfor airy process wlratever, leading to the general equation ... 

Considerthe direct heat transfer fro 111 a heat reservoir at Ti to another heat reservoir at temperature 72> where Ti > 72 > T,. It is not obvious why the lost work of this process should depend on 7, the temperature of the surroundings, because the surroundings are not involved ui the actual heat-transfer process. Through appropriate use of the Camot-engine formula, show for the transfer of an amount of heat equal to Q that... 

In both cases indirect heat transfer in involved. Direct heat transfer is discussed in Section 7.7. This is the case if the liquid is cooled by the addition of ice cubes or is heated by injection of steam. 

Related topics include direct heat transfer (Section 16.11.3.8), reactors (Section 16.11.6.27), mixing (Section 16.11.7.1), dryers (Section 16.11.5.5), and size enlargement (Section 16.11.9.5). 

Computer modeling of convection has had mixed success. Many convection problems, particularly those involving laminar flow, can readily be solved by special computer programs. However, in situations where turbulence and complex geometries are involved, computer analysis and modeling are still under development. Mass transfer analogies can play a key role in the study of convective heat transfer processes. Two mass transfer systems, the sublimation technique and the electrochemical technique, are of particular interest because of their convenience and advantages relative to direct heat transfer measurements. 

A comprehensive review of the naphthalene sublimation technique is given in Ref. 126. The naphthalene sublimation technique, commonly employed to measure convective transport phenomena, has several advantages over direct heat transfer measurement techniques. Tliese advantages are more detailed mass transfer distribution over the test piece (typically thousands of data measured points), avoidance of heat conduction and radiation loss, and better control on boundary conditions. 

As an alternative to direct heat transfer measurements it is possible to use changes in pressure drop brought about by the presence of the deposit. The pressure drop is increased for a given flow rate by virtue of the reduced flow area in the fouled condition and the rough character of the deposit. The shape of the curve relating pressure drop with time will in general, follow an asymptotic shape so that the time to reach the asymptotic fouling resistance may be determined. The method is often combined with the direct measurement of thickness of the deposit layer. 

---