Introduction adiabatic process

Here the subscripts i and / refer to the initial and final states of the system and the work w is defined as the work performed on the system (the opposite sign convention—with w as work done by the system on the surroundings—is also in common use). Note that a cyclic process (one in which the system is returned to its initial state) is not introduced as will be seen later, a cyclic adiabatic process is possible only if every step is reversible. Equation (A2.1.9), i.e. the introduction of f/as a state function, is an expression of the law of conservation of energy. 

Reactor heat carrier. As pointed out in Chapter 7, if adiabatic operation is not possible and it is not possible to control temperature by indirect heat transfer, then an inert material can be introduced to the reactor to increase its heat capacity flowrate (i.e. product of mass flowrate and specific heat capacity). This will reduce temperature rise for exothermic reactions or reduce temperature decrease for endothermic reactions. The introduction of an extraneous component as a heat carrier effects the recycle structure of the flowsheet. Figure 13.6a shows an example of the recycle structure for just such a process. 

The Marcus classical free energy of activation is AG , the adiabatic preexponential factor A may be taken from Eyring s Transition State Theory as (kg T /h), and Kel is a dimensionless transmission coefficient (0 < k l < 1) which includes the entire efiFect of electronic interactions between the donor and acceptor, and which becomes crucial at long range. With Kel set to unity the rate expression has only nuclear factors and in particular the inner sphere and outer sphere reorganization energies mentioned in the introduction are dominant parameters controlling AG and hence the rate. It is assumed here that the rate constant may be taken as a unimolecular rate constant, and if needed the associated bimolecular rate constant may be constructed by incorporation of diffusional processes as ... 

A new small-scale test has been developed which needs only a few g of thermally unstable material, which may be contained in an open cell version of the apparatus if aerobic processes are involved, otherwise in a closed cell apparatus, both with full temperature control and monitoring systems. Such materials may be divided into 2 types, depending on the behaviour of a sample after introduction into adiabatic storage at elevated temperature. The first type, which after attaining the adiabatic temperature shows a steady further increase in temperature in line with Frank-Kamenetski s thermal explosion theory, exhibit thermal combustion (TC) behaviour. In the second type, after adiabatic temperature has been reached, the sample shows a sudden rapid rate of rise, exhibiting autocatalytic (AC) behaviour as chain branching... 

Chapter 4 An adiabatic self-heating process recorder 4.1 Introduction... 

FIG. 4-2 Adiabatic expansion process in a turbine or expander. [Smith, Van Ness, and Abbott, Introduction to Chemical Engineering Thermodynamics, 7th ed.,p. 269, McGraw-Hill, New York (2005). ]... 

As already mentioned above, the derivation of the Butler-Volmer equation, especially the introduction of the transfer factor a, is mostly based on an empirical approach. On the other hand, the model of a transition state (Figs. 7.1 and 7.2) looks similar to the free energy profile derived for adiabatic reactions, i.e. for processes where a strong interaction between electrode and redox species exists (compare with Section 6.3.3). However, it should also be possible to apply the basic Marcus theory (Section 6.1) or the quantum mechanical theory for weak interactions (see Section 6.3.2) to the derivation of a current-potential. According to these models the activation energy is given by (see Eq. 6.10)... 

In the great majority of atom-transfer reactions between species in their ground electronic states, even if several non-degenerate states do correlate with the separated reactants, their existence is of limited importance. Chemical reaction usually proceeds via the potential hypersurface that is at all points lowest in energy. The process is then said to be electronically adiabatic. The only effect of there being more than one overall electronic state is the introduction of a statistical factor, which will be temperature dependent if the spin-orbit terms are split by to allow for the fact that not all collisions will occur on the lowest potential. In assessing the results of dynamical calculations, one should determine whether any allowance has been made for this effect as no standard practice has been established. 

As has already been mentioned in the Introduction, it is a common observation that there exists an energetical threshold above which an efficient non-radiative deactivation process takes place in ESIPT systems. The process can be safely identified as internal conversion to the ground state. The extremely high efficiency of the process knr — at the threshold indicates the presence of strong non-adiabatic... 

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