Heat removal

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)  

In the case of significant change of the physical chemical properties of the reaction mixture, the overall heat exchange coefficient (U) will also be a function of time. The heat transfer properties are usually a function of temperature, where changes in the viscosity of the reaction mass play a dominant role. 

Scale Reactor volume m3 Heat exchange area m2 Specific area m 1 

the specific cooling capacity of reaction vessels varies by approximately two orders of magnitude, when scaling up from laboratory scale to production scale. This has a great practical importance, because if an exothermal effect is not detected at laboratory scale, this does not mean that the reaction is safe at a larger scale. At laboratory scale, the cooling capacity may be as high as 1000 W kg 1, whereas at plant scale it is only in the order of 20-50 W kg 1 (Table 2.5). This also means that the heat of reaction can be measured only in calorimetric devices and cannot be deduced from the measurement of a temperature difference between the reaction medium and the coolant. 

In Equation 2.18, the heat transfer coefficient U plays an important role. Therefore, some methods to estimate or even to measure this coefficient are presented 

For dry storage systems, heat removal from the fuel occurs by conduction, radiation, and natural or, in some cases, forced air convection. For these facilities operational controls should consist of verifying that there are no impairments to air flow. If heat removal requires forced convection, additional operational controls and maintenance will be required on air moving systems. 

Heat transfer considerations may increase in importance as fuel is moved from low density storage to high density storage. 

The operating organization shall consider categories of dropped loads such as casks or lids, fuel and fuel storage racks. 

Dropping fuel during transfer from the cask to the storage rack (or vice versa in the case of cask loading for dry storage) might result in  

Dropping a fuel storage rack or basket during transfer, in isolation or among other loaded racks, may result in  

In the case of certain modular facilities such as vaults, the faet that the heat produced from the decay of fuel fission products decreases with time can be taken into account. For example, in some facilities forced cooling is initially provided, after which natural cooling is adequate. 

Redundant and/or diverse heat removal systems might be appropriate, depending on the type of storage system used, the potential for fuel overheating over an extended time and the level of conservatism necessary to provide accident mitigation. 

The design of heat removal systems for spent fuel storage facilities should include any appropriate provisions to maintain fuel temperatures at acceptable limits during the transfer of fuel. 

The design of a facility for the storage of spent nuclear fuel should be based upon a specified operational lifetime. This design life should include provision for routine inspection, refurbishment and replacement of parts. 

Safety related components of a spent fuel storage facility shall be designed to preserve their function during the lifetime of the facility. Where this is not possible, the design shall allow for the safe replacement of such components. 

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Figure 13.1a shows two possible thermal profiles for exothermic plug-fiow reactors. If the rate of heat removal is low and/or the heat of reaction is high, then the temperature of the reacting stream will increase along the length of the reactor. If the rate of heat removal is high and/or the heat of reaction is low, then the temperature will fall. Under conditions between the two profiles shown in Fig. 13.1a, a maximum can occur in the temperature at an intermediate point between the reactor inlet and exit. 

Both the reboiling and condensing processes normally take place over a range of temperature. Practical considerations, however, usually dictate that the heat to the reboiler must be supplied at a temperature above the dew point of the vapor leaving the reboiler and that the heat removed in the condenser must be removed at a temperature lower than the bubble point of the liquid. Hence, in preliminary design at least, both reboiling and condensing can be assumed to take place at constant temperatures. ... 

This produces even greater increases in viscosity, with the attendant increase in the difficulty of heat removal and processing. 

Isothermal polymerizations are carried out in thin films so that heat removal is efficient. In a typical isothermal polymerization, aqueous acrylamide is sparged with nitrogen for 1 h at 25°C and EDTA (C2QH2 N20g) is then added to complex the copper inhibitor. Polymerization can then be initiated as above with the ammonium persulfate—sodium bisulfite redox couple. The batch temperature is allowed to rise slowly to 40°C and is then cooled to maintain the temperature at 40°C. The polymerization is complete after several hours, at which time additional sodium bisulfite is added to reduce residual acrylamide. 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

In some cases particles have been added to electrical systems to improve heat removal, for example with an SF -fluidized particulate bed to be used in transformers (47). This process appears feasible, using polytetrafluoroethylene (PTFE) particles of low dielectric constant. For a successful appHcation, practical problems such as fluidizing narrow gaps must be solved. 

Catalytic methanation processes include (/) fixed or fluidized catalyst-bed reactors where temperature rise is controlled by heat exchange or by direct cooling using product gas recycle (2) through wall-cooled reactor where temperature is controlled by heat removal through the walls of catalyst-filled tubes (J) tube-wall reactors where a nickel—aluminum alloy is flame-sprayed and treated to form a Raney-nickel catalyst bonded to the reactor tube heat-exchange surface and (4) slurry or Hquid-phase (oil) methanation. 

Traditionally, production of metallic glasses requites rapid heat removal from the material (Fig. 2) which normally involves a combination of a cooling process that has a high heat-transfer coefficient at the interface of the Hquid and quenching medium, and a thin cross section in at least one-dimension. Besides rapid cooling, a variety of techniques are available to produce metallic glasses. Processes not dependent on rapid solidification include plastic deformation (38), mechanical alloying (7,8), and diffusional transformations (10). 

Eig. 11. Liquid-cooled cold plates or heat sinks have been developed as thermal management solutions to cool components for Hquid-cooled computer systems and other electronic systems where heat removal becomes one of the important design criteria. 

The highly exothermic nature of the butane-to-maleic anhydride reaction and the principal by-product reactions require substantial heat removal from the reactor. Thus the reaction is carried out in what is effectively a large multitubular heat exchanger which circulates a mixture of 53% potassium nitrate [7757-79-1/, KNO 40% sodium nitrite [7632-00-0], NaN02 and 7% sodium nitrate [7631-99-4], NaNO. Reaction tube diameters are kept at a minimum 25—30 mm in outside diameter to faciUtate heat removal. Reactor tube lengths are between 3 and 6 meters. The exothermic heat of reaction is removed from the salt mixture by the production of steam in an external salt cooler. Reactor temperatures are in the range of 390 to 430°C. Despite the rapid circulation of salt on the shell side of the reactor, catalyst temperatures can be 40 to 60°C higher than the salt temperature. The butane to maleic anhydride reaction typically reaches its maximum efficiency (maximum yield) at about 85% butane conversion. Reported molar yields are typically 50 to 60%. 

More generally, the neutron number density and the reactor power distribution are both time- and space-dependent. Also, there is a complex relation between reactor power, heat removal, and reactivity. 

Storage and Handling. Plutonium can be stored safely in dry air. Because of self-heating, storage accompanied by heat removal is advisable. The metal can be machined in moisture-free air containing at least 70 vol % Ar or He. Casting and foundry operations that requite melting of the metal must be carried out in vacuum or inert atmospheres and special containers. 

Suspension Polymerization. In this process the organic reaction mass is dispersed in the form of droplets 0.01—1 mm in diameter in a continuous aqueous phase. Each droplet is a tiny bulk reactor. Heat is readily transferred from the droplets to the water, which has a large heat capacity and a low viscosity, faciUtating heat removal through a cooling jacket. 

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