Transport between phases

Improvement of rates is mainly the result of biocatalyst engineering, while improvement of yields result from the biocatalyst selectivity and from mass transport between phases. This last phenomenon is also a key feature for environmental aspects. Hence, most of the impacts of a biological process deal with carbon release in the environment. This release takes place in the form of VOCs, including CO2. If it is difficult to avoid CO2 production when microorganisms are involved (it is still the same with enzymes because they were preliminary produced by cell cultivation), care can be taken for other organic compounds. 

The technique has drawbacks associated with the establishment of equilibrium between the gas and the other phase, typically a liquid. If the gas is a reactant, its rate of dissolving in the liquid phase should be faster than the chemical rate-determining step. If it is a product, its release should be sufficiently fast that supersaturation in the liquid phase does not become a problem. (The relationships between rates of transport between phases and chemical processes are explored in Chapter 5.)... 

Diffusive transport between phases can be described mathematically as the product of the departure from equilibrium and a kinetic term ... 

The usefulness of the ratio of the concentration of a solute between water and octanol as a model for its transport between phases in a physical or biological system has long been recognized (Leo 1971, 1981). It is expressed as Poct = C0/Cw= Kow This ratio is essentially... 

Equilibrium partitioning and mass transfer relationships that control the fate of HOPs in CRM and in different phases in the environment were presented in this chapter. Partitioning relationships were derived from thermodynamic principles for air, liquid, and solid phases, and they were used to determine the driving force for mass transfer. Diffusion coefficients were examined and those in water were much greater than those in air. Mass transfer relationships were developed for both transport within phases, and transport between phases. Several analytical solutions for mass transfer were examined and applied to relevant problems using calculated diffusion coefficients or mass transfer rate constants obtained from the literature. The equations and approaches used in this chapter can be used to evaluate partitioning and transport of HOP in CRM and the environment. 

If matter is transported between phases A and B, assuming the system is isolated from the rest of the universe (a closed system), the free energy of the system (G) is given by... 

According to Wikipedia [1], a membrane is a thin, typically planar structure or material that separates two enviromnents or phases and has a finite volume. It can be referred to as an interphase rather than an interface. Membranes selectively control mass transport between phases or environments. Again, according to Wikipedia, membranes can be divided into three groups (1) biological membranes, (2) artificial membranes, and (3) theoretical membranes. 

The basic principles of mass transfer are discussed in detail in [1.95-1.97]. Thermal separation processes are actually mass transfer processes matter is transported between phases and across phase interfaces. Mass transfer is caused by differences in concentration within a phase and by disturbances of the phase equilibrium. The time taken to return to the phase equilibrium depends mainly on mass transfer, but also on heat transfer (heat is transported not only by convection and radiation at higher temperature, but also by mass). For the design of thermal separation processes, along with a knowledge of phase equilibria, it is also important to have a detailed understanding of how equilibrium is reached and the time required, taking into account restrictions in the mass transfer rate. 

The next step is to identify the source(s) of a chemical and the nature of the release(s) of that chemical, including the amount and to what media. The story continues with a description of how the chemical moves through the environment, considering advection and transport between phases, the reactions it undergoes, and the potential to bioaccumulate. Finally, the exposure scenario describes who or what is exposed (i.e., the receptor, ecosystem, or habitat). 

Part II Building on Fundamentals is devoted to skill building, particularly in the area of catalysis and catalytic reactions. It covers chemical thermodynamics, emphasizing the thermodynamics of adsorption and complex reactions the fundamentals of chemical kinetics, with special emphasis on microkinetic analysis and heat and mass transfer effects in catalysis, including transport between phases, transfer across interfaces, and effects of external heat and mass transfer. It also contains a chapter that provides readers with tooisfor making accurate kinetic measurements and analyzing the data obtained. 

In the energy or thermodynamic domain, energy is to be transferred from somce to recipient in the required form and amount, at the required moment and position. Application of first and second principles in this domain is seen in microwave-based reactions, photochemical reactions, etc., where the selectivity as well as imiformity are enhanced. Alternate forms of energies such as electric, magnetic and acoustic fields illustrate the applications of third principle by enhancing mass and heat transport between phases. 

Recall that the mass balance equations of Eqs. (1.1a) and (1.1b) incorporate not only terms for internal chemical reactions but also terms for physical mass transport across the boundaries of the control volume. Often, useful control volume boundaries coincide with boundaries between phases, such as between air and water or between water and solid bottom sediment, as discussed for the lake control volume in Section 1.3.1. Note, however, that the terms "environmental media" and "phases" are not interchangeable. For example, chemicals in the gas phase can refer to chemicals present in gaseous form in the atmosphere or in air bubbles in surface waters or in air-filled spaces in the subsurface environment. Chemicals in the aqueous phase are chemicals dissolved in water. Chemicals in the solid phase include chemicals sorbed to solid particles suspended in air or water, chemicals sorbed to soil grains, and solid chemicals themselves. In addition, an immiscible liquid (i.e., a liquid such as oil or gasoline that does not mix freely with water) can occur as its own nonaqueous phase liquid (NAPL, pronounced "napple"). Some examples of mass transport between phases are the dissolution of oxygen from the air into a river (gas phase to aqueous phase), evaporation of solvent from an open can of paint (nonaqueous liquid phase to gas phase), and the release of gases from new synthetic carpet (solid phase to gas phase). Mass transport between phases is affected both by physics and by the properties of the chemical involved. Thus, it is important to imderstand both the types of chemical reactions that are common in the environment, and the relative affinities that various chemicals have for gas, liquid, and solid phases. 

The fust pre-treatment procedure for non-aqueous formulations is represented by the homogenisation of the sample that is obtained by stirring, ultrasonication or centrifugation and filtration. Sometimes, heating or cooling processes are employed, to favour substance transport between phases or to induce precipitation. 

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