Heat-transfer coefficients in agitated vessels

Heating or cooling of process fluids in a batch-operated vessel is common in the chemical process industries. The process is unsteady state in nature because the heat flow and/or the temperature vary with time at a fixed point. The time required for the heat transfer can be modified, by increasing the agitation of the batch fluid, the rate of circulation of the heat transfer medium in a jacket and/or coil, or the heat transfer area. Bondy and Lippa [45] and Dream [46] have compiled a collection of correlations of heat transfer coefficients in agitated vessels. Batch processes are sometimes disadvantageous because ... 

EXAMPLE 4.1 i-1. Heat-Transfer Coefficient in Agitated Vessel with Jacket... 

T. Chilton, T. Drew, R. Jebens, Heat transfer coefficients in agitated vessels, Ind. Eng. Chem. 36 (1944) 510. 

It is important to calculate U accurately to determine the required heat transfer area for a reactor. Typical expressions to calculate overall heat transfer coefficients for agitated vessels are presented in [174,180] and generally in standard chemical engineering texts and reference books. 

Based on initial heat flow calorimetry studies, a process development engineer must choose the appropriate reactor vessels for pilot plant studies. A pilot plant typically has vessels that range from 80 to 5000 L, some constructed of alloy and others that are glass lined. In addition some vessels may have half-pipe coils for heat transfer, while others have jackets with agitation nozzles. A process drawing for a typical glass-lined vessel is shown in Figure 4. In Sections 3.1.4.1 and 3.1.4.2 we review fundamental heat transfer relationships in order to predict overall heat transfer coefficients. In Section 3.1.4.3 we review experimental techniques to estimate heat transfer coefficients in process vessels. 

Heat transfer can occur in either batch or continuous configurations. Both types of processes require fluid motion to obtain an effective heat transfer to the bulk of the fluid. In batch processing using jacketed vessels, helical coils, or coils in a baffle configuration, for example, sufficient agitation is required for heat transfer through the medium while continuous systems rely on flow rate to achieve effective heat transfer to satisfy process requirements. Effective heat transfer in batch operations for structured liquid detergents may require scrapers or anchor-type impellers to increase heat transfer coefficients in jacketed vessels. 

Empirical dimensionless group correlations have been used in the scale-up process. In particular, the correlation for the inside film heat transfer coefficient for agitated, jacketed vessels has been employed for the scale-up to a larger vessel. Reaction calorimeters are often used to give some indication of heat transfer coefficients compared to water in the same unit. Correlation for plant heat transfer is of the general form... 

HEAT-TRANSFER COEFFICIENTS. In an agitated vessel, as shown in Chap. 9, the dimensionless group D np/fi is a Reynolds number useful in correlating data on power consumption. This same group has been found to be satisfactory as a correlating variable for heat transfer to jackets or coils in an agitated tank. The following equations are typical of those that have been offered for this purpose. 

Heat transfer coefficient to fluids in a vessel using mechanical agitated coils or jacket... 

Heat transfer in agitated vessels with internal coils containing the heat transfer fluid (process on outside of coil) is expressed by the outside coefficient on coils ... 

Toluene is continuously nitrated to niononitrotoluene in a cast-iron vessel of 1 m diameter lined with a propeller agitator of 0.3 m diameter driven at 2 Hz. The temperature is maintained at 310 K by circulating cooling water at 0.5 kg/s through a stainless steel coil of 25 mm outside diameter and 22 mm inside diameter wound in the form of a helix of 0.81 in diameter. The conditions are such that the reacting material may be considered to have the same physical properties as 75% sulphuric acid. If the mean water tcmpcralure is 290 K, what is the overall heat transfer coefficient ... 

When the reactor is scaled up to 60 cm radius, however, the operating point is between the two curves. This means that the reaction can be safely run at 50°C in a well-agitated process vessel of 60 cm radius with the heat transfer coefficient as stated above becauseerating point is below the Semenov curve. In case the agitation is lost, however, the Frank-Kamenetskii curve becomes the better predictor of runaway temperatures, and because the operating point is above this curve, the estimate is that the reaction will run away. The calculation of the Frank-Kamentskii method is available in ASTME-1231 [166]. 

Agitation of the reaction mass may also be critical in such a situation without agitation, cooling being provided by natural convection only, leads to a considerable reduction of the heat transfer coefficients (see Section 9.3.5). Generally, by natural convection, the heat transfer coefficient is reduced to 10% of its value with stirring [14], Nevertheless, this is only valid when natural convection is established, that is, for smaller vessels and contents with moderate viscosity (see Section... 

Determine the heat transfer coefficient from a coil immersed in an agitated vessel with a diameter of 10 ft (3.048 m). The agitator is a paddle measuring 3.5 ft (1.01 m) in diameter and revolving at 200 rev/min. The fluid properties are ... 

Table B.7 Overall heat-transfer coefficients for immersed coils in agitated vessels. ...

Gas flow has little effect on heat transfer in a mechanically agitated vessel containing power-law fluid. While for turbine stirrers the heat-transfer coefficient for a power-law fluid can be obtained from Eq. (7.7), a more generalized form Nu = a[Re /(m)]2/3 Pr1/3 should be preferred. Here the expression given by Metzner and Otto (1957) for Re /(m) should be used and the viscosity in Prandtl number must be the constant viscosity value at high shear rates. 

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