Temperature and Heat Transfer

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. 

This paper describes work on equipment and instrumentation aimed at a computer-assisted lab-scale resin prep, facility. The approach has been to focus on hardware modules which could be developed and used incrementally on route to system integration. Thus, a primary split of process parameters was made into heat transfer and temperature control, and mass transfer and agitation. In the first of these the paper reports work on a range of temperature measurement, indicators and control units. On the mass transfer side most attention has been on liquid delivery systems with a little work on stirrer drives. Following a general analysis of different pump types the paper describes a programmable micro-computer multi-pump unit and gives results of its use. 

Another industrially important reaction of propylene, related to the one above, is its partial oxidation in the presence of ammonia, resulting in acrylonitrile, H2C=CHCN. This ammoxidation reaction is also catalyzed by mixed metal oxide catalysts, such as bismuth-molybdate or iron antimonate, to which a large number of promoters is added (Fig. 9.19). Being strongly exothermic, ammoxidation is carried out in a fluidized-bed reactor to enable sufficient heat transfer and temperature control (400-500 °C). 

Rosenow WM (1956) Heat Transfer and Temperature Distribution in Laminar Film Condensation, Trans ASME, 78 1645. 

Supplementary fired HRSG. Supplementary (or auxiliary) bring raises the temperature by bring fuel to use a portion of the oxygen in the exhaust. Supplementary bring uses convective heat transfer, and temperatures are limited to a maximum of around 850°C by ducting materials. 

The heat pipe achieves its high performance through the process of vapor state heat transfer. A volatile liquid employed as the heat-transfer medium absorbs its latent heat of vaporization in the evaporator (input) area. The vapor thus formed moves to the heat output area, where condensation takes place. Energy is stored in the vapor at the input and released at the condenser. The liquid is selected to have a substantial vapor pressure, generally greater than 2.7 kPa (20 mm Hg), at the minimum desired operating temperature. The highest possible latent heat of vaporization is desirable to achieve maximum heat transfer and temperature uniformity with minimum vapor mass flow. 

Physical situations that involve radiation with conduction are fairly common indeed. Examples include heat transfer through superinsulation made up of separated layers of very reflective material, heat transfer and temperature distributions in satellite and spacecraft structures, and heat transfer through the walls of a vacuum flask. 

A frequent operation in the chemical field is the removal of heat from a material in a molten state to effect its conversion to the solid state. When the operation is carried on batchwise, it is termed casting, but when done continuously, it is termed flaking. Because of rapid heat transfer and temperature variations, jacketed types are limited to an initial melt temperature of 232°C (450°F). Higher temperatures [to 316°C (600°F)] require extreme care in jacket design and cooling-liquid flow pattern. Best performance and greatest capacity are... 

Rohsenow, W.M. Heat transfer and temperature distribution in laminar film condensation. Trans. Am. Soc. Mech. Eng., Ser. C. J. Heat Transfer 78 (1956) 1645-1648... 

After the dispersion leaving the reactor is separated in the settler (decanter), the liquid hydrocarbon product stream is partially flashed, forming a vapor phase, mainly isobutane. The remaining liquid is hence cooled, and it is used as the coolant in the tube bundle of the reactor. As heat is transferred there, more hydrocarbons vaporize, forming a liquid-gas mixture. Obviously, temperature gradients occur on both sides of the tube bundle as a function of reactor length. Heat transfer (and temperature control) is an important design consideration in contactors. 

A.V. ZHUKOV, YU.A. KUZINA, V. P. SMIRNOV, A. P. SOROKIN, Heat Transfer and Temperature Distribution in Rod Bundles for LCFR. Proc. Scientific Session MEPhI-2000, Moscow, Ed. MEPhI, v.8, pp. 108-110. 

How can we use the fact that any spontaneous process is irreversible to make predictions about the spontaneity of an unfamiliar process Understanding spontaneity requires us to examine the thermodynamic quantity called entropy, which was first mentioned in Section 13.1. In general, entropy is associated either with the extent of randomness in a system or with the extent to which energy is distributed among the various motions of the molecules of the system. In this section we consider how we can relate entropy changes to heat transfer and temperature. Our analysis will bring us to a profound statement about spontaneity that we call the second law of thermodynamics. 

If the production rate is limited by the heat capacity and velocity of the D2O and slurry in the pile, it will not be in a better position than the heavy water cooling. The reason for this is that the specific heat of the slurry is hardly greater than that of water and that the permissible temperatures will be similar. If the limiting factors are heat transfer and temperature in the U and if the heat capacities play no role at all, it may have as high a production per ton D2 0 as the homogeneous pile. Actually, the situation may be expected to be intermediate between the above extremes. 

Flow measurement using thermoelectric devices and sensors implies the use of heat transfer and temperature measurements in microchatmels to determine the near-wall velocity. With appropriate calibration procedures, the mean fluid flow velocity or mass flow rate can be determined by measurements of the local wall temperature. Thermoelectric temperatrue probes and sensors, also known as thermocouples, rely on the Seebeck effect, where a temperatrue difference between two different metal contacts induces a voltage drop which can be electrically mea-strred. An electrical resistance heater introduces a heat flux into the fluid flow. The temperature is measured directly either at the heater, in its vicinity, or at the wall downstream of the heater. Often, the upstream mean temperature of the fluid flow is also measured to provide a comparison. Thermoelectric flow rate measurement is a very common measuring technique and for laminar flow one of the most accurate, reliable, and cost effective. 

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