Stirred tank heat flow

Regenass, W. (1997) The development od Stirred-Tank Heat Flow Calorimetry as a Tool for Process Optimization and Proces safety. Chimia, 51,189-200. 

Chemical reactors, particularly for continuous processes, are often custom designed to involve multiple phases (e.g., vapor, liquid, reacting solid, and solid catalyst), different geometries (e.g., stirred tanks, tubular flows, converging and diverging nozzles, spiral flows, and membrane transport), and various regimes of momentum, heat, and mass transfer (e.g., viscous flow, turbulent flow, conduction, radiation, di sion, and dispersion). There... 

Consider the stirred-tank heating system shown in Fig. 2.3. The liquid inlet stream consists of a single component with a mass flow rate Wf and an inlet temperature Ti, The tank contents are agitated and heated using an electrical heater that provides a heating rate, Q, A dynamic model will be developed based on the following assumptions ... 

A completely enclosed stirred-tank heating process is used to heat an incoming stream whose flow rate varies. 

Functions that exhibit time delay play an important role in process modeling and control. Time delays commonly occur as a result of the transport time required for a fluid to flow through piping. Consider the stirred-tank heating system example presented in Chapter 2. Suppose one thermocouple is located at the outflow point of the stirred tank, and a second thermocouple is immersed in the fluid a short distance (L= 10 m) downstream. The heating system is off initially, and at time zero it is turned on. If there is no fluid mixing in the pipe (the fluid is in plug flow) and if no heat losses occur from the pipe, the shapes of the two temperature responses should be identical. However, the second sensor response will be translated in time that is, it will exhibit a time delay. If the fluid velocity is 1 m/s, the time delay ( o = L/v) is 10 s. If we denote /( ) as the transient temperature response at the first sensor and fd(t) as the temperature response at the second sensor. Fig. 3.3 shows how they are related. The function/ = 0 for t < to. Therefore,/ and/are related by... 

A stirred-tank heating system described by Eq. 4-37 is used to preheat a reactant containing a suspended solid catalyst at a constant flow rate of 1000 kg/h. The volume in the tank is 2 m, and the density and specific heat of the suspended mixture are, respectively, 900 kg/m and 1 cal/g °C. The process initially is operating with inlet and outlet temperatures of 100 and 130 °C. The following questions concerning process operations are posed ... 

A feedforward-only control system is to be designed for the stirred-tank heating system shown in Fig. E15.il. Exit temperature T will be controlled by adjusting coolant flow rate, qc. The chief disturbance variable is the inlet temperature Ti which can be measured on-line. Design a feedforward-only control system based on a dynamic model of this process and the following assumptions ... 

A stirred-tank heat exchanger with a bypass stream is shown in Fig. E18.17 with the available control valves. The possible manipulated variables are mass flow rate W2, valve stem positions Xc and and /, the fraction of mass flow rate wi that bypasses the tank before being added to the exit stream. Using the information given here, do the following ... 

Fig. 5. Hoechst/Rhc ne-Poulenc oxo flow scheme A, stirred tank reactor B, separator C, phase separator D, stripping column E, heat exchanger and F,...

A useful classification of lands of reaclors is in terms of their concentration distributions. The concentration profiles of certain limiting cases are illustrated in Fig. 7-3 namely, of batch reactors, continuously stirred tanks, and tubular flow reactors. Basic types of flow reactors are illustrated in Fig. 7-4. Many others, employing granular catalysts and for multiphase reactions, are illustratea throughout Sec. 23. The present material deals with the sizes, performances and heat effects of these ideal types. They afford standards of comparison. 

A continuous flow stirred tank reactor (CFSTR) differs from the batch reactor in that the feed mixture continuously enters and the outlet mixture is continuously withdrawn. There is intense mixing in the reactor to destroy any concentration and temperature differences. Heat transfer must be extremely efficient to keep the temperature of the reaction mixture equal to the temperature of the heat transfer medium. The CFSTR can either be used alone or as part of a series of battery CFSTRs as shown in Figure 4-5. If several vessels are used in series, the net effect is partial backmixing. 

In many eases, the heat flow (Q) to the reaetor is given in terms of the overall heat transfer eoeffieient U, the heat exehange area A, and the differenee between the ambient temperature, T, and the reaetion temperature, T. For a eontinuous flow stirred tank reaetor (CFSTR) in whieh both fluid temperatures (i.e., inside and outside the exehanger) are eonstant (e.g., eondensing steam), Q is expressed as... 

An endothermie reaetion A —> R is performed in three-stage, eon-tinuous flow stirred tank reaetors (CFSTRs). An overall eonversion of 95% of A is required, and the desired produetion rate is 0.95 x 10 kmol/see of R. All three reaetors, whieh must be of equal volume, are operated at 50°C. The reaetion is first order, and the value of the rate eonstant at 50°C is 4 x 10 see The eoneentration of A in the feed is 1 kmol/m and the feed is available at 75°C. The eontents of all three reaetors are heated by steam eondensing at 100°C inside the eoils. The overall heat transfer eoeffieient for the heat-exehange system is 1,500 J/m see °C, and the heat of reaetion is -1-1.5 x 10 J/kmol of A reaeted. 

Recycling of partially reacted feed streams is usually carried out after the product is separated and recovered. Unreacted feedstock can be separated and recycled to (ultimate) extinction. Figure 4.2 shows a different situation. It is a loop reactor where some of the reaction mass is returned to the inlet without separation. Internal recycle exists in every stirred tank reactor. An external recycle loop as shown in Figure 4.2 is less common, but is used, particularly in large plants where a conventional stirred tank would have heat transfer limitations. The net throughput for the system is Q = but an amount q is recycled back to the reactor inlet so that the flow through the reactor is Qin + q- Performance of this loop reactor system depends on the recycle ratio qlQin and on the type of reactor that is in the loop. Fast external recycle has... 

There is one significant difference between batch and continuous-flow stirred tanks. The heat balance for a CSTR depends on the inlet temperature, and Tin can be adjusted to achieve a desired steady state. As discussed in Section 5.3.1, this can eliminate scaleup problems. 

Continuous-flow stirred tank reactors are widely used for free-radical polymerizations. They have two main advantages the solvent or monomer can be boiled to remove the heat of polymerization, and fairly narrow molecular weight and copolymer composition distributions can be achieved. Stirred tanks or... 

Fig. 5.4-23 shows a sketch drawing of a BSC (Brogli et al., 1981). The stirred-tank reactor made of glass (a metal version is also available) is surrounded by a jacket through which a heat-transfer fluid flows at a very high rate the jacket is not insulated. The temperature of the circulation loop is regulated by a cascaded controller so that the heat evolution in the reactor is equilibrated by heat transfer through the reactor wall. The temperature in the loop is adjusted by injection of thermostatted hot or cold fluid. 

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... 

The temperature of a continuous flow of material through a steam-heated stirred tank is controlled by regulating the flow of steam. The tank temperature is measured by a thermocouple set inside a thermowell, giving a delayed temperature measurement response. This example is based on that of Robinson (1975). 

Chapter 3 concerns the dynamic characteristics of stagewise types of equipment, based on the concept of the well-stirred tank. In this, the various types of stirred-tank chemical reactor operation are considered, together with allowance for heat effects, non-ideal flow, control and safety. Also included is the modelling of stagewise mass transfer applications, based on liquid-liquid extraction, gas absorption and distillation. 

When we developed the model for the stirred tank heater, we ignored the dynamics of the heating coil. Provide a slightly more realistic model which takes into consideration the flow rate of condensing steam. 

Heat can be added to or removed from stirred-tank reactors via external jackets (Figure 7.5a), internal coils (Figure 7.5b) or separate heat exchangers by means of a flow loop (Figure 7.5c). Figure 7.5d shows vaporization of the contents being condensed and refluxed to remove heat. A variation on Figure 7.5d would not reflux the evaporated... 

Continuous Flow Reactors—Stirred Tanks. The continuous flow stirred tank reactor is used extensively in chemical process industries. Both single tanks and batteries of tanks connected in series are used. In many respects the mechanical and heat transfer aspects of these reactors closely resemble the stirred tank batch reactors treated in the previous subsection. However, in the present case, one must also provide for continuous addition of reactants and continuous withdrawal of the product stream. 

In continuous reactor systems, all reactants are continuously fed to the reactor, and the products are continuously withdrawn. Typical continuous reactors are stirred tanks (either single or in cascades) and plug flow tubes. Continuous reactors are characterized by stationary conditions in that both heat generation and composition profiles remain constant during operation (provided that operating conditions remain unchanged ). 

In some cases, where the wall of the reactor has an appreciable thermal capacity, the dynamics of the wall can be of importance (Luyben, 1973). The simplest approach is to assume the whole wall material has a uniform temperature and therefore can be treated as a single lumped parameter system or, in effect, as a single well-stirred tank. The heat flow through the jacket wall is represented in... 

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