Heat transfer coefficient tanks

Horizontal-Tank Type This type (Fig. ll-56a) is used to transfer heat for melting or cooking diy powdered solids, rendering lard from meat-scrap solids, and drying divided solids. Heat-transfer coefficients are 17 to 85 W/(m °C) [3 to 15 Btu/(h fF °F)] for drying and 28 to 140 W/(m °C) [5 to 25 Btu/(h fF °F)] for vacuum and/or solvent recovery. 

These devices are replacing the older tank and spiral-conveyor devices. Better provisions for speed and ease of fill and discharge (without powered rotation) minimize downtime to make this batch-operated device attractive. Heat-transfer coefficients ranging from 28 to 200 W/(m °C) [5 to 35 Btu/(h fF °F)] are obtained. However, if caking on the heat-transfer walls is serious, then values may drop to 5.5 or 11 W/(m °C) [1 or 2 Btu/(h fH °F)], constituting a misapplication. The double cone is available in a fairly wide range of sizes and construction materials. The users are the fine-chemical, pharmaceutical, and biological-preparation industries. 

Polymerization processes are characterized by extremes. Industrial products are mixtures with molecular weights of lO" to 10. In a particular polymerization of styrene the viscosity increased by a fac tor of lO " as conversion went from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1,800 K (3,240 R). Heat transfer coefficients in stirred tanks with high viscosities can be as low as 25 W/(m °C) (16.2 Btu/[h fH °F]). Reaction times for butadiene-styrene rubbers are 8 to 12 h polyethylene molecules continue to grow lor 30 min whereas ethyl acrylate in 20% emulsion reacts in less than 1 min, so monomer must be added gradually to keep the temperature within hmits. Initiators of the chain reactions have concentration of 10" g mol/L so they are highly sensitive to poisons and impurities. 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

The following equation can be used to predict heat transfer coefficients from coils to tank walls in agitated tanks. 

Tank diameter, ft, or L (consistent units). Figure 5-34 = Residence or holding time, sec, or time of mixing = Overall heat transfer coefficient, bulk mixing liquid to transfer fluid on opposite side of heat transfer wall (coil, plate, jacket), Btti/hr/sq ft/°F = Velocity of mixed fluids through mixer, ft/sec = Volume, consistent units... 

A jacketed reaction vessel containing 0.25 nv1 of liquid of specific gravity 0.9 and specific heat 3.3 kJ/kg K is heated by means of steam fed to a jacket on the walls. The contents of the tank are agitated by a stirrer rotating at 3 Hz. The heat transfer area is 2.5 nr ami the steam temperature is 380 K. The outside film heat transfer coefficient is 1.7 kW/m2 K and the 10 mm thick wall of the tank has a thermal conductivity of 6.0 W/m K... 

An open cylindrical tank 500 mm diameter and I m deep is three quarters filled with a liquid ol density 980 kg/mJ and of specific heat capacity 3 kj/kg K. If the heat transfer coefficient from the cylindrical walls and the base of the tank is 10 W/m2 K and front the surface is 20 W/m3 K, what area of heating coil, fed with steam at 383 K. is required to heat the contents from 288 K to 368 K in a half hour The overall heat transfer coefficient for the coil may be taken as 100 W/m2 K, the surroundings we at 288 K and the heal capacity of the tank itself may be neglected. 

A reaction vessel is heated by steam at 393 K supplied to a coil immersed in the liquid in the tank. It takes. 1800 s to heat the contents from 293 K to 373 K when the outside temperature is 293 K. When the outside and initial temperatures are only 278 K, it takes 2700 s to heat the contents to 373 K. The area of the steam coil is 2.5 rn2 and of the external surface is 40 m2. If the overall heat transfer coefficient from the coil to the liquid in the vessel is 400 W/m2 K, show that the overall coefficient for transfer from the vessel to the surroundings is about 5 W/m2 K. 

Ii takes 1800 s to heat a tank of liquid from 293 to 333 K using steam supplied to an immersed coii when the steam temperature is 383 K. How long will it take when the steam temperature is raised to 393 K The overall heat transfer coefficient from the steam coil to the tank is 10 times the coefficient from the tank to surroundings at a temperature of 293 K and the area of the steam coil is equal to the outside area of the tank... 

This section is concerned with the UA xtiT — Text) term in the energy balance for a stirred tank. The usual and simplest case is heat transfer from a jacket. Then A xt refers to the inside surface area of the tank that is jacketed on the outside and in contact with the fluid on the inside. The temperature difference, T - Text, is between the bulk fluid in the tank and the heat transfer medium in the jacket. The overall heat transfer coefficient includes the usual contributions from wall resistance and jacket-side coefficient, but the inside coefficient is normally limiting. A correlation applicable to turbine, paddle, and propeller agitators is... 

Fig. 3.2 shows the case of a jacketed, stirred-tank reactor, in which either heating by steam or cooling medium can be applied to the jacket. Here V is volume, Cp is specific heat capacity, p is density, Q is the rate of heat transfer, U is the overall heat transfer coefficient, A is the area for heat transfer, T is temperature, H is enthalpy of vapour, h is liquid enthalpy, F is volumetric flow... 

Fick s Law 62, 223, 226, 637 Filling and emptying tanks 512 Film heat transfer coefficient 140, 627 Filter bed 579... 

The burglar trips and breaks his flashlight and also a large bottle of methanol that spills into a holding tank of 1.8 m diameter. He then uses a cigarette lighter to see. Use property data from the tables in Chapter 6 the gas specific heat is constant at 1.2 J/g K, the density for the gas in the room can be considered constant at 1.18 kg/m3 and the heat transfer coefficient at the methanol surface is 15 W/m2 K. 

The overall heat transfer coefficient is normally made up of several terms arising from the various resistances to the flow of heat. In the simple example mentioned above, there will be terms for heat transfer through the liquid by conduction and convection, for conduction through the metal wall of the tank and through any layers of insulating material and for heat loss from the outer skin to the surrounding air. 

Therefore, even a moderate error in the mixer scale-up will have only a small effect on the agitator-side heat transfer coefficient. Other factors that include heat transfer area per unit volume are considerably more significant. For instance, in the jacketed tank, the heat transfer area per unit volume decreases upon scale-up. In order to assure the same proportionate heat removal or addition per unit batch size, additional heat transfer area (e.g., coils) may be required. Additionally, other variables such as temperature driving force may have to be adjusted to compensate for decreased heat... 

Heat transfer coefficients in stirred tank operations are discussed in Section 17.7. 

TABLE 17.10. Overall Heat Transfer Coefficients in Agitated Tanks IU Btu/(hr)(sqft)(°R]... 

A vertical cylindrical tank is filled with well water at 65° F. The tank is insulated at the top and bottom, but is exposed at its vertical sides to cold night air at 10°F. The tank s diameter is 2 ft and its height 3 ft. The overall heat transfer coefficient is 20 Btu/ h °F ft2). Neglect the metal wall of the tank and assume that the water in the tank is perfectly mixed. 

A reaction A——>P is to be performed in a PFR. The reaction follows first-order kinetics, and at 50 °C in the batch mode, the conversion reaches 99% in 60 seconds. Pure plug flow behavior is assumed. The flow velocity should be 1 m s"1 and the overall heat transfer coefficient 1000Wm 2 K"1. (Why is it higher than in stirred tank reactors ). The maximum temperature difference with the cooling system is 50 K. 

A 2.5 m3 stainless steel stirred tank reactor is to be used for a reaction with a batch volume of 2 m3 performed at 65 °C. The heat transfer coefficient of the reaction mass is determined in a reaction calorimeter by the Wilson plot as y = 1600Wnr2KA The reactor is equipped with an anchor stirrer operated at 45 rpm. Water, used as a coolant, enters the jacket at 13 °C. With a contents volume of 2 m3, the heat exchange area is 4.6 m2. The internal diameter of the reactor is 1.6 m. The stirrer diameter is 1.53 m. A cooling experiment was carried out in the temperature range around 70 °C, with the vessel containing 2000 kg water. The results are represented in Figure 9.16. 

An intermediate product with a melting point of 50 °C should be stored during two months in a cylindrical tank at 60 °C. The tank is a cylinder with vertical axis equipped with a jacket on the vertical wall allowing hot water circulation, but no agitator. The bottom and the lid are not heated. The volume is 4 m3, the height 1.8 m, and the diameter 1.2 m. The corresponding shape factor is 8( ri, = 2.37. The overall heat transfer coefficient of the jacket is 50Wrn 2 K 1. 

With a stirrer, the overall heat transfer coefficient could be increased to 200Wm 2K 1. Since the heat transfer area for the filled tank is 2.26 m2, the thermal time constant is... 

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