Reaction variables affecting product properties

Attempts to scale up the NS-300 membrane preparation from the laboratory scale to continuous machine production led to a high degree of variability in membrane properties ( ). The difference was attributed in part to the variability of machine-made polysulfone support film. Properties of the machine-made polysulfone supports differed from the laboratory cast support films. One of the major factors affecting this difference was that the machine-made support film was cast on a nonwoven polyester backing material which can vary in properties. Thus, the machine support film, which was quite adequate for NS-lOO type membranes using polymeric amine reactants, still remained a limitation for the monomeric amine reaction of NS-300. 

A contaminant can be defined as any material on the surface that counterflows with the film formation process, affects the film properties in an undesirable way, or influences the film stability in an undesirable way. In most cases, the concern is with both the type and amount of the contaminant. Contaminants can cover the whole surface, for example oxide reaction layers or adsorbed hydrocarbon layers, or they can be limited to restricted areas, for example particles or fingerprints. A major concern in processing is the variability of the contamination in such a manner as to affect product reproducibility. 

It is also common to measure by voltammetry the thermodynamic properties of purely chemical reactions that are in some way coupled to the electron transfer step. Examples include the determination of solubility products, acid dissociation constants, and metal-ligand complex formation constants for cases in which precipitation, proton transfer, and complexation reactions affect the measured formal potential. Also in these instances, studies at variable temperature will afford the thermodynamic parameters of these coupled chemical reactions. 

Such a description oversimplifies the overall process because it docs not correctly represent the molecular formulas of the intermediates and end products, nor does it depict the simultaneous occurrence of the two reactions. However, this oversimplification captures the key phenomenological idea that a three-dimensional gel network comes from the condensation of partially hydrolyzed species. Any parameters that affect either or both of these reactions are thus likely to impact on the properties of the product. In fact, Livage et al. [4] pointed out that the important variables arc the relative rates of hydrolysis and condensation. 

We consider finally a few obvious variables which contribute to the overall complexity of our system. (1) Phase hydrogenations may be carried out with the unsaturated molecule either as a vapor or as a liquid, with or without the presence of a solvent whose physico-chemical properties affect the rate and mechanism of reactions in a manner not yet understood. (2) Pressure where the organic reactant is in the liquid phase, the hydrogen pressure can be varied from subatmospheric to 600 atm, and although the principal effect is on the rate, the product distribution is sometimes changed. In gas-phase reactions, alteration of the ratio of the partial pressures of the reactants sometimes has a... 

Several factors affect enzymatic reactions in SCFs. The solubility of the substrates and the activity and stability of the enzyme are the most common parameters that are usually considered. Pressure and temperature can always be used to change the reaction mixture density and other transport properties, which will significantly affect the solubility of the substrates and products in the flnid (Randolph et al., 1991). On the other hand, it may be difficult to understand the absolute effect of process variables and their ability to enhance enzyme stability and activity. 

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