Heat transfer media Water, cooling

In suspension polymerization each droplet can be considered as a small batch reactor. The water phase serves as a suspension medium, and also as a heat transfer medium. A cooling jacket takes care of the heat removal from the reactor. The polymerization rate in the droplets is mostly determined by the kinetics of initiation, propagation and termination reactions. At a certain degree of conversion, the Trommsdorff or gel-effect may occur (see section 13.3.1). Towards the end of the reaction, also the propagation step may be hindered by diffusion... 

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. 

Traditionally, sodium dichromate dihydrate is mixed with 66° Bh (specific gravity = 1.84) sulfuric acid in a heavy-walled cast-iron or steel reactor. The mixture is heated externally, and the reactor is provided with a sweep agitator. Water is driven off and the hydrous bisulfate melts at about 160°C. As the temperature is slowly increased, the molten bisulfate provides an excellent heat-transfer medium for melting the chromic acid at 197°C without appreciable decomposition. As soon as the chromic acid melts, the agitator is stopped and the mixture separates into a heavy layer of molten chromic acid and a light layer of molten bisulfate. The chromic acid is tapped and flaked on water cooled roUs to produce the customary commercial form. The bisulfate contains dissolved CrO and soluble and insoluble chromic sulfates. Environmental considerations dictate purification and return of the bisulfate to the treating operation. 

The refrigeration system is required to cool the ice condenser and the heat exchanger for the heat transfer medium. The system consists of four separate two-stage compressors with water-cooled liquehers. 

Fig. 8. Longitudinal section of an experimental waste-heat greenhouse in which temperature control in all seasons is provided by evaporation and heat transfer as air passes through a fiber pad soaked with power station cooling water or by heat transfer as air passes through a finned-tube heat exchanger that carries cooling water. A false ceiling provides for recycle of air through the heat-transfer medium. Reproduced by permission (31).

With reaction temperatures above 300 °C intermediate cooling can still be performed directly with boiling water, whereas in a fixed bed a high-temperature heat-transfer medium must be used as coolant (see next section). 

Application To produce aqueous formaldehyde (AF) or urea formaldehyde precondensate (UFC) from methanol using Haldor Tbpspe A/S FK-Series iron/molybdenum-oxide catalysts. Description The process is carried out in a recirculation loop at low pressure (0 to 6 psig) (1 to 1.5 bar abs). Vaporized methanol is mixed with air and recycle gas that were boosted by the blower (1). The mixture may be preheated to about 480°F (250°C) in the optional heat exchanger (2) or it may be sent directly to the reactor (3). In the reactor, methanol and oxygen react in the catalyst-filled tubes to make formaldehyde. Reaction heat is removed by an oil heat transfer medium (HTM). The reacted gas exits the reactor at approximately 540°F (290°C) and is cooled in the LP steam boiler (4) to 260°F (130°C) before entering the absorber (5). In the absorber, the formaldehyde is absorbed in water or urea solution. Heat is removed by one or two cooling circuits (6, 7). From the lower circuit (6)... 

Gas cooled reactors use carbon dioxide under pressure as a recirculating heat transfer medium (coolant) between the hot nuclear reactor core and water in a secondary circuit in order to raise steam and electrical power in an otherwise conventional high pressure steam generator/turbine/condenser loop. The role played by ion exchange is denoted by systems A-D in Figure 8.22. 

Use Corrosion inhibitor in alcohol-base antifreezes and water-cooled reactors. Oxidizing agent for organic material, especially in the presence of light heat-transfer medium. 

Progress has also been made in part cooling. Portable and centralized chillers have been developed to cool the part in a minimum time. These closed loop systems use water or water-ethylene glycol combinations as the heat transfer medium. The application of heat transfer principles allows one to computer model the cooling process (35). For the new engineering resins such as polyamide-imide and polyphenylene sulfide, one must use mold heating. The use... 

If the heat-transfer medium is not at a constant temperature but is a liquid (such as cooling water) of specific heat Cp entering at temperature 7 and flowing at a constant rate m,., the corresponding equation for the liquid temperature is... 

USE Corrosion inhibitor for water-cooled atomic reactors. Soln as low temp heat transfer medium. 

The most common cooling medium in the undergraduate laboratory is an ice-water bath. Liquid water is a more efficient heat transfer medium than ice, because it covers the entire surface area of the portion of the vessel that is immersed in the bath. Consequently, when preparing this type of bath, do not use ice alone. 

For strong exothermic or endothermic reactions with an adiabatic temperature rise of several hundred degrees, the rack type reactor is not sufficient. Then, a multi-tubular reactor is used, where the catalyst is located in up to 30 000 individual tubes, the outside of which is exposed to the flow of a heat transfer medium. In many cases, cooling is provided by boiling water, and the cooling temperature can easily be controlled by the pressure. For elevated temperatures molten salts can be employed as cooling or heating medium. 

In many cases, the thermal resistance of the reactor wall (dwaii/7waii) as well as the external heat transfer to the cooling medium (l/aw,ex) are negligible (the latter, for example, if the wall is cooled externally by boiling water). We then have a... 

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