Steam-heated stirred tank

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). 

Fig. 4.3-12 Temperature profile in a steam heated stirred tank at semi logarithmic illustration...

We have a steam-heated stirred tank in which an endothermic slurry reaction is being carried out. Develop an analytical solution to the temperature response of the tank when there is a step change in the steam temperature. 

For the steam-heated stirred-tank system modeled by Eqs. 2-51 and 2-52, assume that the steam temperature is constant. 

Determine Np and Npc for the steam-heated, stirred-tank system model in Eqs. 2-50 through 2-52 in Chapter 2. Assume that only steam pressure Pj can be manipulated. 

A common process task involves heating a slurry by pumping it through a well-stirred tank. It is useful to know the temperature profile of the slurry in the agitated vessel. This information can be used to optimize the heat transfer process by performing simple sensitivity studies with the formulas presented below. Defining the inlet temperature of the slurry as T, and the temperature of the outer surface of the steam coil as U then by a macroscopic mass and energy balance for the system, a simplified calculation method is developed. 

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. 

Liquid is fed continuously to a stirred tank, which is heated by internal steam coils (Fig. 1.21). The tank operates at constant volume conditions. The system is therefore modelled by means of a dynamic heat balance equation, combined with an expression for the rate of heat transfer from the coils to the tank liquid. 

Figure 1.21. Continuous stirred tank heated by internal steam coil.

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... 

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. 

How we model the stirred tank heater is subject to the actual situation. At a slightly more realistic level, we may assume that heat is provided by condensing steam and that the coil metal is at the same temperature as the condensing steam. The heat balance and the Laplace transform of the tank remains identical to Chapter 2 ... 

The use of an unnecessarily high-temperature hot utility or heating medium should be avoided. This may have been a major factor that led to the runaway reaction at Seveso in Italy in 1976, which released toxic material over a wide area. The reactor was liquid-phase and operated in a stirred tank, Figure 27.1. It was left containing an uncompleted batch at around 160°C, well below the temperature at which a runaway reaction could start. The temperature required for a runaway reaction was around 230°C5. The reaction was normally carried out under vacuum at about 160°C in a reactor heated by steam at about 300°C. The temperature of the liquid could not rise above its boiling point of 160°C at the operating pressure. 

The rates of heat transfer between the fermentation broth and the heat-transfer fluid (such as steam or cooling water flowing through the external jacket or the coil) can be estimated from the data provided in Chapter 5. For example, the film coefficient of heat transfer to or from the broth contained in a jacketed or coiled stirred-tank fermentor can be estimated using Equation 5.13. In the case of non-Newtonian liquids, the apparent viscosity, as defined by Equation 2.6, should be used. 

Sulfonation of p-nitrotoluene (PNT) is performed in a cascade of Continuous Stirred Tank Reactors (CSTR). The process is started by placing a quantity of converted mass in the first stage of the cascade, a 400-liter reactor, and heating to 85 °C with jacket steam (150°C). PNT melt and Oleum are then dosed in simultaneously (exothermal reaction). When 110°C is reached, cooling is switched on automatically. On the day of the accident, a rapid increase in pressure took place at 102 °C. The lid of the reactor burst open and the reaction mass, which was decomposing, flowed out like lava, causing considerable damage. 

Starch is dispersed in the paper mill in large stainless steel tanks by injection of steam or by heat transfer from a steam-heated jacket. The tanks are stirred and equipped with baffles to prevent formation of a single vortex at the agitator shaft. A minimum heating time of 20 minutes at 95°C is normally required. Steam injection dilutes the starch paste by condensate, which must be considered for concentration control. Pastes that are prone to retrogradation are held at a temperature above 91°C or quickly cooled to 66°C to prevent amylose formation. Attention to storage temperature and water balance is an essential requirement for the effective use of starch in a paper mill. 

The RCH/RP unit (Figure 15) [116] is essentially a continuously stirred tank reactor, followed by a phase separator and a strip column. The reactor (1), which contains the aqueous catalyst, is fed with propene and syngas. The crude aldehyde product passes into the decanter (2), where it is degassed and separated into the aqueous catalyst solution and the organic aldehyde phase. The catalyst solution moves to heat exchanger (3) and produces process steam. Water lost in the aqueous phase can be replaced at (3) after which it returns to reactor (1). 

The above endothermic reaction takes place in two continuous stirred tank reactors (CSTRs) in series, and the heat required by the reaction is supplied by steam (see... 

An endothermic continuous stirred tank reactor (CSTR) is shown schematically in Figure 15.2. The symbols used in the control schematics are listed in Table 15.1 The controlled variable is the temperature of the product leaving the reactor, and the manipulated variable is the flow rate of steam to the heat exchanger, which adds heat to the recycle line. The final control element is the control valve and associated equipment on the steam hue. The sensor is a temperature sensor/trans-mitter that measures the temperature of the product stream leaving the reactor. The controller compares the measured value of the product temperature with its desired temperature (setpoint) and makes changes to the control valve on the steam to the heat exchanger. The process is the... 

A simplified scheme of the RCH/RP unit is presented in Figure 2 [1, 10, 11], The reactor (1) is essentially a continuous stirred tank reactor equipped with a gas inlet, a stirrer, a heat exchanger and a catalyst recycle line. Catalyst and reactants are introduced at the bottom of the reactor. Vent gas is taken from the head of the reactor and from the phase separator. Control of the liquid volume inside the reactor is simple the liquid mixture composed of catalyst solution and aldehydes leaves via an overflow and is transferred to a phase separator (2), where it is partially degassed. The separation of the aqueous catalyst solution (density of the catalyst solution 1100 g/L) and the aldehydes occurs rapidly and completely, favored by the difference in densities (density of aldehyde layer 600 g/L due to dissolved gases). The catalyst solution passes a heat exchanger and produces process steam that is consumed in downstream operations. Some water is extracted from the catalyst solution by its physical solubility in the aldehydes (about 1.3% w/w) which may be replaced before the catalyst solution re-enters the reactor. 

The sulfur dioxide-rich citrate solution in the bottom of the absorber is fed by level control through a steam-heated exchanger to a three-stage continuous stirred tank reactor system countercurrent to a flow of hydrogen sulfide gas. For this installation the gas source is a tank of liquid hydrogen sulfide. 

Two streams 1 and 2 are being mixed in a well-stirred tank, producing a product stream 3 (Figure 4.8). Each of the two feed streams is composed of two components, A and B, with molar concentrations cA, cb, and ca2, cb2, respectively. Also let F, and F2 be the volumetric flow rates of the two streams (ft3/min, m3/min) and T i and T2 their corresponding temperatures. Finally, let cA, c b3, F3, and T3 be the concentrations, flow rate, and temperature of the product stream. A coil is also immersed in the liquid of the tank and it is used to supply heat to the system with steam, or remove heat with cooling water. 

Consider again the stirred tank heater, but now under feedback control (Figure 5.5). Control loop 1 maintains the liquid level at a desired value by measuring the level of the liquid and adjusting the value of the effluent flow rate. Therefore, control loop 1 introduces a relationship between F and h. Similarly, control loop 2 maintains the temperature of the liquid at the desired value by manipulating the flow of steam and thus the flow of heat Q. Consequently, control loop 2 introduces a relationship... 

II.4 Do the same work as in Problem II.3 for the stirred tank heaters system shown in Figure PII.4. For tank 1, the steam is injected directly in the liquid water. Water vapor is produced in the second tank. A i and A 2 are the cross-sectional areas of the two tanks. Assume that the effluent flow rates are proportional to the liquid static pressure that causes their flow. A, is the heat transfer area for the steam coil. 

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