Isothermal reactions

Semibatch reactors are often used to mn highly exothermic reactions isothermally, to run gas-liquid(-solid) processes isobarically, and to prevent dangerous accumulation of some reactants in the reaction mixture. Contrary to batch of)eration, temperature and pressure in semibatch reactors can be varied independently. The liquid reaction mixture can be considered as ideally mixed, while it is assumed that the introduced gas flows up like a piston (certainly this is not entirely true). Kinetic modelling of semibatch experiments is as difficult as that of batch, non-isotherma experiments. 

Consecutive reactions, isothermal reactor cmi < cw2, otai = asi = 0. The course of reaction is shown in Fig. 5.4-71. Regardless the mode of operation, the final product after infinite time is always the undesired product S. Maximum yields of the desired product exist for non-complete conversion. A batch reactor or a plug-flow reactor performs better than a CSTR Ysbr.wux = 0.63, Ycstriiuix = 0.445 for kt/ki = 4). If continuous operation and intense mixing are needed (e.g. because a large inteifacial surface area or a high rate of heat transfer are required) a cascade of CSTRs is recommended. 

Since enthalpy changes are path-independent quantities, one is at liberty to choose a convenient path for making the calculation. If we carry out the reaction isothermally at the inlet temperature and then heat the products at constant composition to the effluent temperature, we find that... 

The reaction isotherm of classical thermodynamics applied to the formation of the transition state relates K to AG0, the standard Gibbs energy of formation of the activated... 

The equilibrium position for the monomer-polymer equilibrium in Eq. 3-174 will be dependent on the temperature with increased temperature, resulting in a shift to the left, since the forward reaction is exothermic. The reaction isotherm... 

Here is the chemical potential of i in its standard state and a, is its molal activity. This equation can be recast in the form known as the van t Hoff reaction isotherm ... 

Consider a tubular reactor where a chemical reaction changes the concentration of the fluid as it moves down the tube. Assuming first-order chemical reaction, isothermal reactor, and constant density, the modeling equation is... 

Curing reactions Vulcanisation reactions Isothermal ageing Product stability Thermal degradation Identification of processing aids Plasticisers Mould lubricants Blowing agents Antioxidants Flame retardants Safety concerns... 

The Gibbs free energy of a dissolved species varies with the activity, a, and in the case of a gas, with the partial pressure, p. Consider the equation for the reaction isotherm (16) for the general chemical reaction Equation (9) ... 

Analysis of polymerization-depolymerization equilibria makes use of the reaction isotherm... 

The energy balance for a CSTR can be derived from Equation (9.2.7) by again carrying out the reaction isothermally at the inlet temperature and then evaluating sensible heat effects at reactor outlet conditions, that is. 

Return to isothermal reactions —Suppose first the temperature kept constant or the reaction isothermal suppose that T, m, V refer to the instant t, and 7, m, v to the temperature f, later than t and very close to t we shall have... 

We will first Cany out the reaction isothermal at 175°C up to the time the cooling was turned offit 45 min. Combining and canceling yields... 

So far only the case of an isothermal reaction without volume change has been considered. A volume change during reaction (isothermal case) has no effect on if if only Knudsen diffusion is operative but has a small effect if bulk diffusion is... 

For reactions in solution, the reaction isotherm may take the form... 

The reaction isotherm is of particular value in the study of chemical processes, but it has also been used in connection with physical changes, to which it is equally applicable. The general criterion of a decrease of free energy for a process to be spontaneous is applicable, of course, to all processes, physical or chemical. 

At the present moment, the chief interest is the evaluation of the standard free energy AF of a process. It will be seen in Chapter XIX that this can be calculated from AF and the activities of the substances present in the cell, by utilizing a form of the reaction isotherm [equation (33.5)]. It can be stated, however, that if the substances involved in the cell reaction are... 

According to the reaction isotherm derived in 33a, the free energy change of the process is represented by... 

---