Effect of Solvent on Controllability

The solvent used in the discussions presented so far in this chapter was water. In this section we look at the possibility of using other solvents. Solvent selection is critical to the successful design and operation of extractive distillation systems. Different solvents affect the phase equilibrium in different ways. 

An informative paper by Kossack et al. discusses in detail the steady-state economic design issues and presents several examples that use a variety of solvents for the separation of acetone and methanol. Some solvents drive the acetone overhead in the extractive column, while others drive the methanol overhead in the extractive column. The authors discuss the effect of solvent selection on issues such as selectivity, capacity, and boiling point. In this section we extend this work to compare the dynamic performance of the acetone-methanol system with different solvents. 

The numerical cases are based on those presented by Kossack et al. Three solvents are explored water, DMSO, and chlorobenzene. The first and second solvents drive the acetone overhead in the extractive column. The last solvent drives the methanol overhead in the extractive column. 

Control structures are developed for each system. Dynamic simulation results show that all systems are controllable, but product quality variabUity is poorest when the methanol is 

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As discussed in section 2.4.4 the coordinating ability of a solvent will often affect the rate of nucleation and crystal growth differently between two polymorphs. This can be used as an effective means of process control and information on solvent effects can often be obtained from polymorph screening experiments. There are no theoretical methods available at the present time which accurately predict the effect of solvents on nucleation rates in the industrial environment. 

Solvents may have neurotoxic effects on memory, learning, and performance which can be permanent. These effects are less well understood. The schedule-controlled operant behavior test has typically been required as a second tier test... it is proposed as a first-tier test... because of EPA s desire to obtain data on the effects of solvents on learning, memory, and performance. 

There is still a considerable amount of work to be done before a full understanding of tlie effects of solvents on the various steps in radical polymerization can be achieved. We expect these effects to be of greater significance as a further degree of sophistication is introduced into the control of the various steps. 

Determining the effect of solvent on the rate or course of a reaction can often provide insight into the reaction mechanism. One solvent property that may be important in extremely fast reactions is viscosity. If a reaction is encounter-controlled (also termed diffusion-controlled), the rate constant for the reaction is limited by the ability of the reacting species to reach other. For example, in aqueous solution, the second-order rate constant for the encoxmter of two species is ca. 10 °Lmol In such cases, changing fromalower viscosity... 

A relation between A jjv and molecular weight controlling mechanisms was discovered and the effect of initiator system, solvent and temperature on a mv was explained. The present work has led to an understanding of the effect of counteranion on PIB molecular weight. These studies provide better insight into the detailed mechanism of isobutylene polymerization, in particular into the initiation and the molecular weight controlling events. 

Temperature variation may also be a relevant factor in flowrate stability. Since the viscosity of the solvent is temperature dependent, wide swings in the ambient temperature can directly affect pump performance. The direct effects of temperature on pump performance usually are far smaller, however, than the effects on retention and selectivity therefore, control of column temperature is generally sufficient to obtain high reproducibility. 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

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