Separation space heat transfer

The thorough mixing of the solid leads to effective gas-solid heat exchange with an excellent heat-transfer characteristic and hence a uniform temperature distribution in the reaction space. Heat-transfer coefficients are typically 100-400 kJm h K and for small particles can be as high as 800 kJm h K. For fine particles and at high reaction rates, circulating fluidized-bed reactors with separation and recycling of the soUd are particularly suitable. 

Solution Polymerization. In this process an inert solvent is added to the reaction mass. The solvent adds its heat capacity and reduces the viscosity, faciUtating convective heat transfer. The solvent can also be refluxed to remove heat. On the other hand, the solvent wastes reactor space and reduces both rate and molecular weight as compared to bulk polymerisation. Additional technology is needed to separate the polymer product and to recover and store the solvent. Both batch and continuous processes are used. 

A fluidized-bed reactor consists of three main sections (Figure 23.1) (1) the fluidizing gas entry or distributor section at the bottom, essentially a perforated metal plate that allows entry of the gas through a number of holes (2) the fluidized-bed itself, which, unless the operation is adiabatic, includes heat transfer surface to control T (3) the freeboard section above the bed, essentially empty space to allow disengagement of entrained solid particles from the rising exit gas stream this section may be provided internally (at the top) or externally with cyclones to aid in the gas-solid separation. A reactor model, as discussed here, is concerned primarily with the bed itself, in order to determine, for example, the required holdup of solid particles for a specified rate of production. The solid may be a catalyst or a reactant, but we assume the former for the purpose of the development. 

The free-convection flow phenomena inside an enclosed space are interesting examples of very complex fluid systems that may yield to analytical, empirical, and numerical solutions. Consider the system shown in Fig. 7-10, where a fluid is contained between two vertical plates separated by the distance 5. As a temperature difference AT,. = T - T> is impressed on the fluid, a heat transfer will be experienced with the approximate flow regions shown in Fig. 7-11, according to MacGregor and Emery [18]. In this figure, the Grashof number is calculated as... 

Heat transfer in horizontal enclosed spaces involves two distinct situations. If the upper plate is maintained at a higher temperature than the lower plate, the lower-density fluid is above the higher-density fluid and no convection currents will be experienced. In this case the heat transfer across the space will be by conduction alone and Nus = 1.0, where 8 is still the separation distance between the plates. The second, and more interesting, case is experienced when the lower plate has a higher temperature than the upper plate. For values of Grs below about 1700, pure conduction is still observed and Nu = 1.0. As convection begins, a pattern of hexagonal cells is formed as shown in Fig. 7-12. These patterns are called Benard cells [33]. Turbulence begins at about Gr6 = 50,000 and destroys the cellular pattern. 

Air at atmospheric pressure is contained between two 0.5-m-square vertical plates separated by a distance of 15 mm. The temperatures of the plates are 100 and 40°C, respectively. Calculate the free-convection heat transfer across the air space. 

Two 30-cm-square vertical plates are separated by a distance of 1.25 cm, and the space between them is filled with water. A constant-heat-flux condition is imposed on the plates such that the average temperature is 38°C for one and 60 for the other. Calculate the heat-transfer rate under these conditions. Evaluate properties at the mean temperature. 

A horizontal air space is separated by a distance of 1.6 mm. Estimate the heat-transfer rate per unit area for a temperature difference of I65°C, with one plate temperature at 90°C. 

Two vertical plates 50 by 50 cm are separated by a space of 4 cm which is filled with water. The plate temperatures are 50 and 20°C. Calculate the heat transfer across the space. 

A special section of insulating glass is constructed of two glass plates 30 cm square separated by an air space of 1 cm. Calculate the percent reduction in heat transfer of this arrangement compared to free convection from a vertical plate with a temperature difference of 30°C. 

A superinsulating material is to be constructed of polished aluminum sheets separated by a distance of 0.8 mm. The space between the sheets is sealed and evacuated to a pressure of 10"5 atm. Four sheets are used. The two outer sheets are maintained at 35 and 90°C and have a thickness of 0.75 mm, whereas the inner sheets have a thickness of 0.18 mm. Calculate the conduction and radiation transfer across the layered section per unit area. For this calculation, allow the inner sheets to float in the determination of the radiation heat transfer. Evaluate properties at 65°C. 

Inadequate heat transfer can be caused not only by an air space, but to an even greater extent by salt crust formation on the inside of the vessel. Hence, mixtures from which salts separate must always be stirred, and the stirrer should come as near to the walls as possible in order to keep them free from incrustation. When large amounts of salt separate, even the best stirrers are inadequate. One case is known where a salt crust only 4 cm. thick resulted in overheating an autoclave to the point where it was red hot. At an internal temperature of 240°C. and pressure of 48 atmospheres, the autoclave was blown out like a balloon and the bottom split. The escaping gas cooled the steel enough so that no further danger was involved. It is almost certain, however, that cast iron would have exploded. (Fusions such as those in the preparation of -naphthol, if done without a metal bath, would certainly ruin any pressure vessel.)... 

Some of these zones have been divided into isothermal regions. This is shown in Figure 2 which shows that the preheater consists simply of the above three zones whereas the reactor distributor and separation space have been represented by three and five regions respectively. The reactor was cooled by forced air from a fan controlled in an on-off manner. Heat transfer to the cooling air was modelled as either forced or natural convection depending on whether the fan was on or off. 

The relationship of Pohlhausen was used for the heat transfer in the separation space (22) ... 

C The upper and lower comparlinenis of a well-insulated container are separated by two parallel sheets of glass with an air space between them. One of the compartments is to be iilled with a hot fluid and the other with a cold fluid. If it is desired that heat transfer between the two compartments be minimal, would you recommend puUing the hot fluid into the upper or the lower compartinenl of the container Wiry ... 

Consider a 1.2-m-high and 2-m-wide double-pane window consisting of two 3-mm thick layers of glass (i = 0.78 V/ra °C) separated by a 3-cm-wide air space. Determine the steady rate nf heat transfer through this window and the temperature of its inner surface for a day during which the room is maintained at 20 C while the tempera-lure of the outdoors is 0 C. Thke the heat transfer coefficients on the inner and outer sur ces of the window to be h, = 10 W/m °C and Aj = 25 W/in °C and disregard any heat transfer by radiation. 

---