Solubility solution process

Although the data for the silver halides suggest that silver(I) fluoride is likely to be more soluble than the other silver halides (which is in fact the case), the hydration enthalpies for the sodium halides almost exactly balance the lattice energies. What then is the driving force which makes these salts soluble, and which indeed must be responsible for the solution process where this is endothermic We have seen on p. 66 the relationship AG = — TAS and... 

Solution Polymerization. Two solution polymerization technologies ate practiced. Processes of the first type utilize heavy solvents those of the second use molten PE as the polymerization medium (57). Polyethylene becomes soluble ia saturated C —hydrocarbons above 120—130°C. Because the viscosity of HDPE solutions rapidly iacrease with molecular weight, solution polymerization is employed primarily for the production of low mol wt resias. Solution process plants were first constmcted for the low pressure manufacture of PE resias ia the late 1950s they were later exteasively modified to make their operatioa economically competitive. 

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

In the early 1990s, solution processes acquired new importance because of their shorter residence times and abiUty to accommodate metallocene catalysts. Many heterogeneous multicenter Ziegler catalysts produce superior LLDPE resins with a better branching uniformity if the catalyst residence time in a reactor is short. Solution processes usually operate at residence times of around 5—10 min or less and are ideal for this catalyst behavior. Solution processes, both in heavy solvents and in the polymer melt, are inherently suitable to accommodate soluble metallocene catalysts (52). For this reason, these processes were the first to employ metallocene catalysts for LLDPE and VLDPE manufacture. 

Vitreous sihca does not react significantly with water under ambient conditions. The solution process involves the formation of monosilicic acid, Si(OH)4. Solubihty is fairly constant at low pH but increases rapidly when the pH exceeds 9 (84—86). Above a pH of 10.7 sihca dissolves mainly as soluble sihcates. Solubihty also increases with higher temperatures and pressures. At 200—400°C and 1—30 MPa (<10 300 atm), for example, the solubihty, S, of Si02 in g/kg H2O can be expressed as foUows, where d ls the density of the vapor phase and T is the absolute temperature in Kelvin. 

The effect of a temperature change on solubility equilibria such as these can be predicted by applying a simple principle. An increase in temperature always shifts the position of an equilibrium to favor an endothermic process. This means that if the solution process absorbs heat (AHsoin. > 0), an increase in temperature increases the solubility. Conversely, if the solution process is exothermic (AH < 0), an increase in temperature decreases the solubility. 

These processes differ from other solution processes in that the solute undergoes a chemical transformation. The aqueous medium dissolves the metal by a chemical reaction that converts the insoluble metal into soluble cations. 

Solutizer process. In this process, caustic alkali of 25% strength containing a small amount of solutizer agent such as potassium isobutyrate is used. This makes the mercaptans more soluble in caustic solution and easily removable. Solutizer solution is boiled for regeneration. 

Metallocenes are homogeneous catalysts that are often soluble in organic solvents. Therefore, polymerization can occur via a solution process with a non-polar diluent dissolving the propylene gas, the catalyst, and the co-catalyst system. They can also be adsorbed onto an inert substrate which acts as part of the fluidized bed for gas phase polymerization processes. 

In this book, we have examined the chemistry behind the possible new approaches to solving the separation problem and have discussed process designs that could be used to implement the Chemistry. In this chapter, we have addressed what we believe to be the biggest problems remaining to be solved for the different approaches described in the rest of the book and have attempted to provide pointers towards possible solutions. These should provide impetus for further research in this important area aimed at improving the chemistry, phase behaviour, differential solubility and process design and eventually lead to the commercialisation of many more homogeneous catalytic processes so as to make chemicals production much more environmentally acceptable. 

Particularly desirable among film deposition processes are solution-based techniques, because of the relative simplicity and potential economy of these approaches. However, the covalent character of the metal chalcogenides, which provides the benefit of the desired electronic properties (e.g., high electrical mobility), represents an important barrier for solution processing. Several methods have been developed to overcome the solubility problem, including spray deposition, bath-based techniques, and electrochemical routes, each of which will be discussed in later chapters. In this chapter, a very simple dimensional reduction approach will be considered as a means of achieving a convenient solution-based route to film deposition. 

Figure 3.2. Film formation using a dimensional reduction approach involves three steps 1) breaking up the insoluble extended inorganic framework (a) into more soluble-isolated anionic species, which are separated by some small and volatile cationic species (b). 2) Solution-processing thin films of the precursor (b). 3) Heating the precursor films such that the cationic species and corresponding chalcogen anions are dissociated, leaving behind the targeted inorganic semiconductor (c).

Increasing the size of PAHs makes their deposition on surfaces difficult because they can neither be sublimed nor made sufficiently soluble for solution processing. A precursor route has thus been designed according to which molecules are deposited on a surface and transformed into the final disc-type adsorbate structures in a thermal solid-state reaction with the substrate surface acting as a template.1261 An exciting example is the hexaether 41 (scheme 11) which is sublimed onto a Cu-(1U) sur-... 

Dow Chemicals group and coworkers [276,350] synthesized similar triarylamine-fluorene copolymers 251 and 252, possessing carboxylic acid substituents, via hydrolysis of the corresponding ethyl ester polymers, prepared by Suzuki polymerization. Due to the very polar substituents, the copolymers 251 and 252 are only soluble in polar solvents such as DMF but not in aromatic hydrocarbons as toluene or xylene, which allowed simple fabrication of multilayer PLEDs by solution processes (Chart 2.65). 

Table III gives values of the changes in Gibbs energy, enthalpy, entropy, and heat capacity of the solution process as calculated from the equations of Table I. Figure 1 shows the recommended noble gas mole fraction solubilities at unit gas partial pressure (atm) as a function of temperature. The temperature of minimum solubility is marked.

Commerical polymerizations of ethylene, propene, and other a-olefins are carried out as slurry (suspension) and gas-phase processes [Beach and Kissin, 1986 Diedrich, 1975 Lieberman and Barbe, 1988 Magovern, 1979 Vandenberg and Repka, 1977 Weissermel et al., 1975]. Solution polymerization has been used in the past for ethylene polymerization at 140-150°C, pressures of up to 8 MPa (1 MPa = 145 psi = 9.869 atm), using a solvent such as cyclohexane. The solution process with its higher temperatures was employed for polymerization with the relatively low efficiency early Phillips initiators. (Polyethylene, but not the initiator, is soluble in the reaction medium under the process conditions.) The development of a variety of high-efficiency initiators has allowed their use in lower-temperature suspension and gas-phase processes, which are more advantageous from many... 

The oxidation and hydrolysis of Fe leads to Fe " oxides either directly or via soluble green rust complexes, solid green rusts or Fe(OH)2. The latter convert to the oxides either by a solid state reaction or a via solution (reconstructive) transformation. Generally, where there is a difference between the structure of the precursor and that of the final oxide, a via solution process seems more likely, but internal rearrangement during topochemical oxidation to the new phase, may also take place. 

The details of enzymatic magnetite formation in bacteria, especially the valence and chemical form in which the Fe enters the cell, are still not fully understood. At low oxygen concentrations in the bacterial habitats dissolved Fe may exist in bivalent form, but Fe added as a soluble Fe " complex, such as Fe " citrate (Schuler Bauerlein, 1996) can also function as an Fe source. Within the cell, part of the Fe will then form a highly reactive Fe "oxide, probably ferrihydrite, which in turn, reacts with the dissolved Fe to form magnetite (Mann et al. 1989) by a via-solution process (Fig. 17.6) ... 

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