Jacketed vessels heat transfer coefficient

This chapter reviews the various types of impellers, die flow patterns generated by diese agitators, correlation of die dimensionless parameters (i.e., Reynolds number, Froude number, and Power number), scale-up of mixers, heat transfer coefficients of jacketed agitated vessels, and die time required for heating or cooling diese vessels. 

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

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 ... 

Approximate Overall Heat Transfer Coefficients for Jacketed Mixing Vessels... 

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... 

The heat transfer coefficient to the vessel wall can be estimated using the correlations for forced convection in conduits, such as equation 12.11. The fluid velocity and the path length can be calculated from the geometry of the jacket arrangement. The hydraulic mean diameter (equivalent diameter, de) of the channel or half-pipe should be used as the characteristic dimension in the Reynolds and Nusselt numbers see Section 12.8.1. 

The temperature of the reactor could theoretically be controlled by changing the flow rate or the temperature of the water in the jacket. It will now be shown that the former is impractical. The over-all heat transfer coefficient is given in the major equipment section as around 50 BTU/hr ft2°F or greater. This means that the major resistance to heat transfer is the film on the inside of the reaction vessel. 

U = over-all heat transfer coefficient for the jacketed vessel... 

Three different principles govern the design of bench-scale calorimetric units heat flow, heat balance, and power consumption. The RC1 [184], for example, is based on the heat-flow principle, by measuring the temperature difference between the reaction mixture and the heat transfer fluid in the reactor jacket. In order to determine the heat release rate, the heat transfer coefficient and area must be known. The Contalab [185], as originally marketed by Contraves, is based on the heat balance principle, by measuring the difference between the temperature of the heat transfer fluid at the jacket inlet and the outlet. Knowledge of the characteristics of the heat transfer fluid, such as mass flow rates and the specific heat, is required. ThermoMetric instruments, such as the CPA [188], are designed on the power compensation principle (i.e., the supply or removal of heat to or from the reactor vessel to maintain reactor contents at a prescribed temperature is measured). 

This value of the overall heat transfer coefficient h is rather greater than that normally found in jacketed vessels [7] and some additional cooling, perhaps by means of coils, would probably be required. In any event, a much more careful analysis of the heat transfer situation is required, but this lies outside the scope of the present chapter. 

TABLE 17.11. Jacketed Vessels Overall Heat Transfer Coefficients... 

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. 

Before we leave this example, let us take a look at the issue of heat transfer. In setting up the simulation, we have specified the reactor temperature (430 K) and volume (100 m3) but have said nothing about how the heat of reaction is removed. The simulation calculates a heat removal rate of 12.46 x 106 W. If the aspect ratio of the vessel is 2, a 100-m3 vessel is 4 m in diameter and 8 m in length, giving a jacket heat transfer area of 100.5 m2. If we select a reasonable 30 K differential temperature between the reactor and the coolant in the jacket, the jacket temperature would be 400 K. Selecting a typical overall heat transfer coefficient of 851 W K-1 m-2 gives a required heat transfer area of 488 m2, which is almost 5 times the available jacket area. Aspen Plus does not consider the issue of area. It simply calculates the required heat transfer rate. 

The ethylbenzene CSTR considered in Chapter 2 (Section 2.8) is used in this section as an example to illustrate how dynamic controllability can be studied using Aspen Dynamics. In the numerical example the 100-m3 reactor operates at 430 K with two feedstreams 0.2 kmol/s of ethylene and 0.4 kmol/s of benzene. The vessel is jacket-cooled with a jacket heat transfer area of 100.5 m2 and a heat transfer rate of 13.46 x 106 W. As we will see in the discussion below, the steady-state simulator Aspen Plus does not consider heat transfer area or heat transfer coefficients, but simply calculates a required UA given the type of heat removal specified. 

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... 

Table B.6 Overall heat-transfer coefficients for jacketed vessels. ...

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. 

Eq. (2) can also be used to calculate the heat-up time for nonisothennal heating (such as hot-water jacketing), provided that the difference between the outlet and inlet jacket temperatures is not greater than 10% of the difference between the batch and average jacket water temperature [21 The heat transfer area, reaction mass and heat capacity of the vessel contents are generally known. The overall heat transfer coefficient, however, is a function of five resistances and can be difficult to estimate. 

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