Single-phase reactions

Single-phase reactions within the ground water and... 

Silicate glasses, physical properties, 150-54 Single-phase reactions, ground-water 335-44 Site, ground-water leaching,... 

This chapter is restricted to homogeneous, single-phase reactions, but the restriction can sometimes be relaxed. The formation of a second phase as a consequence of an irreversible reaction will not affect the kinetics, except for a possible density change. If the second phase is solid or liquid, the density change will be moderate. If the new phase is a gas, its formation can have a major effect. Specialized models are needed. Two-phase ffows of air-water and steam-water have been extensively studied, but few data are available for chemically reactive systems. 

The difference between complete segregation and maximum mixedness is largest when the reactor is a stirred tank and is zero when the reactor is a PFR. Even for the stirred tank case, it has been difficult to find experimental evidence of segregation for single-phase reactions. Real CSTRs approximate perfect mixing when observed on the time and distance scales appropriate to industrial reactions, provided that the feed is premixed. Even with unmixed... 

Allyl alcohol isomerization is typically conducted as a single-phase reaction, needing efforts for separation of the catalyst [110, 113], One driver was to exploit a catalyzed liquid/liquid route with aqueous (catalytic) and organic phases as commonly employed in the chemical industry. 

Despite affecting conversion, mass transfer is known to impact enantio- and regioselectivity for many reactions [63]. For this reason, conventional micro-titration apparatus, typically employed in combinatorial chemistry of single-phase reactions, also often suffer from insufficient mixing when dealing with multi-phases [63, 66]. 

Higher selectivity, easier processing, use of inexpensive solvents, use of cheaper chemicals, and ease of heat removal have been realized through phase-transfer catalysis (PTC). It appears that no catalytic method has made such an impact as PTC on the manufacture of fine chemicals (Sharma, 1996). Many times we benefit by deliberately converting a single-phase reaction to a two-phase reaction. Consider catalysis by. sodium methoxide in a dry organic. solvent. This can invariably be made cheaper and safer by using a two-pha.se. system with a PT catalyst. 

The resulting expressions for cA/cAo for several values of n are given in the second column in Table 4.1. Results are given for n = 0 and n = 3, although single-phase reactions of the type (A) are not known for these orders. 

This chapter provides an introduction to several types of homogeneous (single-phase) reaction mechanisms and the rate laws which result from them. The concept of a reaction mechanism as a sequence of elementary processes involving both analytically detectable species (normal reactants and products) and transient reactive intermediates is introduced in Section 6.1.2. In constructing the rate laws, we use the fact that the elementary steps which make up the mechanism have individual rate laws predicted by the simple theories discussed in Chapter 6. The resulting rate law for an overall reaction often differs significantly from the type discussed in Chapters 3 and 4. 

Equations similar to 12.3-10 to -15 may be written in terms of internal energy, U, with Cv, the heat capacity at constant volume, replacing CP. For liquid-phase reactions, the difference between the two treatments is small. Since most single-phase reactions carried out in a BR involve liquids, we continue to write the energy balance in terms of H, but, if required, it can be written in terms of U. In the latter case, it is usually necessary to calculate AU from AH and Cv from CP, since AH and CP are the quantities listed in a database. Furthermore, regardless of which treatment is used, it may be necessary to take into account the dependence of AH (or AU) and CP (or C,) on T ... 

For a first-order, single-phase reaction (A - products, (—rA) = a a) taking place in the tanks, calculate the fiactional conversion of A(/a) leaving the second tank based on the SFM, if q = 0.5 m3 min-1 (constant-density flow), V = 10 m3, and jfcA = 0.1 min-1. 

For maximum efficiency, methylation should be a single-phase reaction and so those methods employing aqueous alkali are most suitable for water-soluble compounds and the Purdie method for non-polar compounds. A carbohydrate which is initially soluble in water e.g., starch, dextran) may be less soluble when partially methylated and for this reason an inert solvent e.g., acetone, dioxane, carbon tetrachloride) is often introduced during the later stages of a methylation with dimethyl sulfate. It is interesting to note that methyl a-D-glucopyranoside forms a trithallium derivative which is insoluble in water and consequently the introduction of more than three methoxyl groups by the Menzies method... 

The results of this analysis of the product and catalyst distribution show that only a limited range of systems may be apphcable for the telomeriza-tion of butadiene and carbon dioxide. The reaction was performed in the biphasic systems EC/2-octanol, EC/cyclohexane and EC/p-xylene. The yield of 5-lactone reached only 3% after a reaction time of 4 hours at 80 °C. hi the solvent system EC/2-octanol triphenylphosphine was used as the hgand. With the ligand bisadamantyl-n-butyl-phosphine even lower yields were achieved in a single-phase reaction in EC or in the biphasic system EC/cyclohexane. The use of tricyclohexylphosphine led to a similar result, only 1% of the desired product was obtained in the solvent system EC/p-xylene, which forms one homogeneous phase at the reaction temperature of 80 °C. Even at a higher temperature of 100 °C and a longer reaction time of 20 hours no improvement could be observed. Therefore, we turned our interest to another telomerization-type process. 

Upon variation of the stirring velocity between 500 and 1500 rpm the conversion of the olefin remained at the same high level and the selectivity to the linear aldehyde also remained constant. Obviously there is no mass transfer limitation in this two-phase reaction system. In comparison to the single-phase reaction in propylene carbonate as the only solvent [23], the selectivity decreases from 95% to 70%, which can be explained by the high concentration of the non-electron-donating solvent dodecane in the propylene carbonate phase. The presence of the dodecane leads to a decrease of the isomerization velocity, which results in a lower hnearity of the formed aldehydes. 

Suppose a single-phase reaction aA + bB of reaction rate for reactant A is then... 

One of the main advantages of the use of dense gases as a solvent for enzyme-catalysed reactions is the simple downstream processing. The physico-chemical properties of dense gases are determined by their pressure and temperature, and are especially sensitive near the critical point. Usually the single-phase reaction mixture is contacted with enzyme preparation, and later changes in P and/or T cause the less soluble substance to precipitate. 

The concentration profiles for the supercritical single-phase reaction are similar to those in gas-phase reactions ( cf., Fig. 9.3-3 and Fig. 9.3-1). The difference is that, with the solvent, these conditions can be achieved for much larger molecules. These supercritical single-phase conditions can be achieved for suitable solvents and at certain conditions. The catalyst used determines the temperature-range for the reaction, as previously discussed. 

Fig. 3. Fluorous-phase reactions single-phase reaction versus reaction with phase changes.

Significantly, these reactions were not homogeneous single-phase reaction systems as neither reactant was soluble in the aqueous alkaline reaction medium. The workers postulated that selective absorption of microwaves by polar molecules and intermediates in a multi-phase system could substitute as a phase transfer catalyst without using any phase transfer reagent, thereby providing the observed acceleration similar to ultrasound irradiation [92],... 

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