Reaction stages

This reaction is an undesirable side reaction in the manufacture of hydrogen but utilised as a means of removing traces of carbon monoxide left at the end of the second stage reaction. The gases are passed over a nickel catalyst at 450 K when traces of carbon monoxide form methane. (Methane does not poison the catalyst in the Haber process -carbon monoxide Joes.)... 

The two stage reaction sequence is called ozonolysis and is represented by the gen eral equation... 

Staged reactions, where only part of the initial reactants are added, either to consecutive reactors or with a time lag to the same reactor, maybe used to reduce dipentaerythritol content. This technique increases the effective formaldehyde-to-acetaldehyde mole ratio, maintaining the original stoichiometric one. It also permits easier thermal control of the reaction (66,67). Both batch and continuous reaction systems are used. The former have greater flexibiHty whereas the product of the latter has improved consistency (55,68). 

Polyestetification involving insoluble reactants such as isophthaUc acid is normally carried out in two-stage reactions, in which isophthaUc acid reacts first with the glycol to form a cleat melt. The balance of the reactants, including maleic anhydride, is then added to complete the polyester polymer, thus avoiding longer cycle times and some discoloration. 

In these processes, the starch slurry is prepared in the same manner as in the low temperature process. In a dual-enzyme/dual-heating process, the steps ate the same as the low temperature process until the completion of the second-stage reaction. Then, a 2—5-min heat treatment foUowed by a second enzyme addition and another reaction step is employed. In a dual-enzyme/single-heating process, the starch slurry is immediately heated to 145—150°C for one minute or less. Although the enzyme is rapidly inactivated, sufficient hydrolysis takes place to provide a partially thinned hydrolyzate that can be pumped to a second stage where additional enzyme is added and the reaction continued at 95—100°C for 20—30 minutes. The temperature is then lowered for the remainder of the reaction. 

Reactions tend to deposit carbon on the catalyst, which ultimately leads to loss of activity. At this stage reactions are stopped, the catalyst purged and regenerated by a controlled oxygenating vapor. AH the vapors from the regeneration process are also passed to the incinerator. A catalyst can undergo many regenerations before replacement becomes necessary. 

FIG. 29-29 Velocity and pressure distribution in a three-stage reaction turbine. 

Aldiough expander designers are capable of controlling die stage reaction, diey must also pay attention to constraints dial are imposed by gas condidons. 

Because of the insolubility of cellulose it is not possible to carry out uniform esterification with the lower organic acids (acetic acid, propionic acid etc.) and in those cases where incompletely substituted derivatives are required a two-stage reaction is employed. This involves total esterification in a medium in... 

Urea-formaldehyde resins are usually prepared by a two-stage reaction. The first stage involves the reaction of urea and formaldehyde under neutral or mildly alkaline conditions, leading to the production of mono and dimethylol ureas (Figure 24.1). The ratio of mono to dimethylol compounds will depend on the urea-to-formaldehyde ratio and it is important that there should be enough formaldehyde to allow some dimethylol urea formation. 

Refers to the elapsed time after an adhesive is applied until pressure effects curing. Intermediate-stage reaction step for various thermosetting resins. During this stage the material swells when in contact with certain liquids and... 

Sulfanuric halides contain the characteristic group -N=S(0)X- (X = Cl, F). Unlike the isoelectronic cyclophosphazenes (NPClajx (x = 3-17), " only six-membered rings have been well characterized. The sulfanuric halides are colourless solids (X = Cl) or liquids (X = F), which are stable in dry air. Sulfanuric chloride [NS(0)C1]3 is best prepared by treatment of SOCI2 with sodium azide in acetonitrile at -35°C (Eq. 8.16). It may also be obtained as a mixture of a- and yS-isomers in a two-stage reaction from H2NSO3H and PCls. The fluoride [NS(0)F]3 is formed as a mixture of isomers by the fluorination of [NS(0)C1]3 with Sbp3. ... 

Available information on the mechanism of cyclocondensation is rather contradictory. According to one hypothesis, both the condensation of aryl halides with copper acetylides and the cyclization occur in the same copper complex (63JOC2163 63JOC3313). An alternative two-stage reaction route has also been considered condensation followed by cyclization (66JOC4071 69JA6464). However, there is no clear evidence for this assumption in the literature and information on the reaction of acetylenyl-substituted acids in conditions of acetylide synthesis is absent. 

Relaxation kinetics in single-stage reactions. Derive expressions for the reciprocal relaxation times of the following single-stage reactions ... 

The sulfonylation mechanism involves a two-stage reaction, as shown in Scheme 6.4. The sulfonylium cation ArS02+ attacks the carbon on the aromatic nucleus to generate an intermediate complex, which subsequently decomposes to afford the final product by eliminating a proton. It is postulated that the effective sulfonylating agent is the sulfonylium salt generated by action of the Lewis-acid catalyst on the sulfonyl halide.18... 

Similarly, the acylation mechanism also involves a two-stage reaction.23 Poly-phosphoric acid, A1C13, HF/BF3, and CF3S03H. are typical catalysts. 

Fig. 10. Concentration curves fitted by exponential regression for the reaction of l,2,4,5r tetrabromobenzene (A) with surface bound chloride, a consecutive four-stage reaction with three intermediates (B,C,D) and the product E.

This would ignore the conditions of the two reactions, and in particular the special circumstances under which the endothemtic photosynthesis reaction takes place. This also ignores how multi-stage reactions are being summarised in a single equation. It is not unusual to see energy included as a term, especially in the respiration equation ... 

The P-alanyl dipeptides carnosine and anserine (A -methylcarnosine) (Figure 31-2) activate myosin ATPase, chelate copper, and enhance copper uptake. P-Alanyl-imidazole buffers the pH of anaerobically contracting skeletal muscle. Biosynthesis of carnosine is catalyzed by carnosine synthetase in a two-stage reaction that involves initial formation of an enzyme-bound acyl-adenylate of P-alanine and subsequent transfer of the P-alanyl moiety to L-histidine. 

Attempts have been made to apply the structure-activity concept (Hansch and Leo 1995) to environmental problems, and this has been successfully applied to the rates of hydrolysis of carbamate pesticides (Wolfe et al. 1978), and of esters of chlorinated carboxylic acids (Paris et al. 1984). This has been extended to correlating rates of biotransformation with the structure of the substrates and has been illustrated with a number of single-stage reactions. Clearly, this approach can be refined with the increased understanding of the structure and function of the relevant degradative enzymes. Some examples illustrate the application of this procedure ... 

In another concrete example of use, the Ugi reaction served as a prominent example to test the performance of one micro mixer and to test specifically the effect of micro mixing on such a multi-stage reaction [25]. 

The reaction time for complete cyanide oxidation is rapid in a reactor system with 10-30 min retention times being typical. The second-stage reaction is much slower than the first-stage reaction. The reaction is typically carried out in the pH range of 10-12 where the reaction rate is relatively constant. Temperature does not influence the reaction rate significantly. 

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