Enthalpy CSTR reactors

For a continuous-flow reactor, such as a CSTR, the energy balance is an enthalpy (H) balance, if we neglect any differences in kinetic and potential energy of the flowing stream, and any shaft work between inlet and outlet. However, in comparison with a BR, the balance must include the input and output of H by the flowing stream, in addition to any heat transfer to or from the control volume, and generation or loss of enthalpy by reaction within the control volume. Then the energy (enthalpy) equation in words is... 

The special process feature for case 3 is a relatively high reaction enthalpy in combination with a low maximum permissible temperature Texo- An alternative safety solution would be to control both these two parameters. For example by adding a pump to the reactor and with solvent makeup the process can be made continuous (CSTR). This allows the adoption of a higher maximum permissible temperature Texo, because of the short residence time and the dilution effect, and a reduction of the adiabatic temperature increase ATadiab because of the dilution effect. Such a (drastic) process and facility change will always require an iterative safety-technical reaction PHA furthermore additional may become necessary. 

Example 2.7. To show what form the energy equation takes for a two-phase system, consider the CSTR process shown in Fig. 2.6. Both a liquid product stream f and a vapor product stream F (volumetric flow) are withdrawn from the vessel. The pressure in the reactor is P. Vapor and liquid volumes are and V. The density and temperature of the vapor phase are and L. The mole fraction of A in the vapor is y. If the phases are in thermal equilibrium, the vapor and liquid temperatures are equal (T = T ). If the phases are in phase equilihrium, the liquid and vapor compositions are related by Raoult s law, a relative volatility relationship or some other vapor-liquid equilibrium relationship (see Sec. 2.2.6). The enthalpy of the vapor phase H (Btu/lb or cal/g) is a function of composition y, temperature T , and pressure P. Neglecting kinetic-energy and potential-energy terms and the work term,... 

The last term is not present in the mass balance (unless the reactor leaks), but heat can be carried in and out not only with flow but also by heat transfer through the walls. An enthalpy balance on the contents of this CSTR gives... 

Nonisothermal stirred tanks are governed by an enthalpy balance that contains the heat of reaction as a significant term. If the heat of reaction is unimportant so that a desired Tout can be imposed on the system regardless of the extent of reaction, then the reactor dynamics can be analyzed by the methods of the previous section. This section focuses on situations where Equation 14.3 must be considered as part of the design. Even for these situations, it is usually possible to control a steady-state CSTR at a desired temperature. If temperature control can be achieved rapidly, then isothermal design techniques again become applicable. Rapid means on a time scale that is fast compared to reaction times and composition changes. 

When water is supplied in excess, the reaction is second order with respect to the propylene oxide concentration and zero order with respect to the water concentration. Its rate constant exhibits an Arrhenius dependence on temperature, with Aq = 3.294 X 10 mV(kmol-hr) and E = 1.556 X 10 kJ/kmol. Furthermore, it is customary to dilute the PO feed with methanol (MeOH), while the H2SO4 catalyst enters the reactor with the feed. Operating conditions are sought for carrying out this liquid-phase reaction in a 47-ft CSTR, with the liquid holdup at 85% of its total volume (1.135 m ). The liquid feeds are fed at 23.9 C, with one consisting of 18.712 kmol/hr of PO and 32.73 fanol/hrof MeOH. The water feed rate is from 160 to 500 kmol/hr (2.84-8.88 m /hr), selected to moderate the reactor temperature. To reduce the risk of vaporization, the reactor is operated at a pressure of 3 bar. Under these conditions, the transients for the PO concentration, Cpo (kmol/m ), and temperature, T (°C), are determined by solving the following species and enthalpy balances ... 

Problem 8-8 (Level 1) The homogeneous reaction of A with B takes place in an organic solution. An ideal CSTR with a volume of 10,0001 is being used. The molar feed rate of A to the CSTR (Fao) is 20(X) mol/h and the mole fraction of A in the feed is 0.20. The temperature of the feed to the reactor is 50 °C. As a first approximation, you may assume that the molar heat capacity for all species is 60 J/mol-°C. The enthalpy change on reaction ( A77r) is approximately constant and is equal to -37 kJ/mol A. 

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