Heat liquid-vessel wall

Although they are termed homogeneous, most industrial gas-phase reactions take place in contact with solids, either the vessel wall or particles as heat carriers or catalysts. With catalysts, mass diffusional resistances are present with inert solids, the only complication is with heat transfer. A few of the reactions in Table 23-1 are gas-phase type, mostly catalytic. Usually a system of industrial interest is liquefiea to take advantage of the higher rates of liquid reactions, or to utihze liquid homogeneous cat ysts, or simply to keep equipment size down. In this section, some important noncatalytic gas reactions are described. 

Just inside the shell of the tube bundle is a cylindrical baffle F that extends nearly to the top of the heating element. The steam rises between this baffle and the wall of the healing element and then flows downward around the tubes. This displaces non-condensed gases to the bottom, where they are removed at G. Condensate is removed from the bottom of the heating element at H. This evaporator is especially suited for foamy liquids, for viscous liquids, and for those liquids which tend to deposit scale or crystals on the heating surfaces. Vessel J is a salt separator. 

Consider a vessel containing an agitated liquid. Heat transfer occurs mainly through forced convection in the liquid, conduction through the vessel wall, and forced convection in the jacket media. The heat flow may be based on the basic film theory equation and can be expressed by... 

In this section, the phenomenon of BLEVE is discussed according to theories proposed by Reid (1976), Board (1975), and Venart (1990). Reid (1979, 1980) based a theory about the BLEVE mechanism on the phenomenon of superheated liquids. When heat is transferred to a liquid, the temperature of the liquid rises. When the boiling point is reached, the liquid starts to form vapor bubbles at active sites. These active sites occur at interfaces with solids, including vessel walls. 

The most damaging BLEVE occurs when a vessel contains a flammable liquid stored at a temperature above its normal boiling point. The vessel walls below the liquid level are maintained at a low temperature due to the rapid heat transfer to the liquid. However, the vessel walls exposed to the fire above the liquid level will heat rapidly due to the much lower heat transfer to the vapor. The vessel wall temperature... 

States or Australia. In some cases, pot stills, arranged in cascade, are still used. The more sophisticated plants employ one or more carbon steel or cast-iron vessels heated electrically and equipped with temperature controls for both the bulk liquid and the vessel walls. Contact time is usually 6—10 h. However, modem pitches are vacuum-distilled, producing no secondary quinoline insolubles, to improve the rheological properties. 

Case 1 Data on heat transfer between liquid and the vessel wall and between liquid and the surface of helical coil in the vessels stirred with flat-blade paddle stirrers were correlated as [4] ... 

Several wick structures are in common use. First is a fine-pore (0.14—0.25 mm (100-60 mesh) wire spacing) woven screen which is rolled into an annular structure consisting of one or more wraps inserted into the heat pipe bore. The mesh wick is a satisfactory compromise, in many cases, between cost and performance. Where high heat transfer in a given diameter is of paramount importance, a fine-pore screen is placed over longitudinal slots in the vessel wall. Such a composite structure provides low viscous drag for liquid flow in the channels and a small pore size in the screen for maximum pumping pressure. 

Spiral baffles, which are sometimes installed for liquid services to improve heat transfer and prevent channeling, can Be designed to serve as reinforcements. A spiral-wound channel welded to the vessel wall is an alternative to the spiral baffle which is more predictable in performance, since cross-baffle leakage is eliminated, and is reportedly lower in cost [Feichtinger, Chem. Eng., 67, 197 (Sept. 5, I960)]. 

Microwaves heat the liquid phase, whereas vapors do not absorb microwave energy. The temperature of the vapor phase is therefore lower than the temperature of the liquid phase and vapor condenses on cool vessel walls. As a result, the actual vapor pressure is lower than the predicted vapor pressure. This sort of sustained dynamic, thermal non-equilibrium is a key advantage of microwave technology, because very high temperatures (and, in turn, short digestion times) can be reached at relatively low pressures. 

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