Desired products series reactions

Intraparticle mass transport resistance can lead to disguises in selectivity. If a series reaction A — B — C takes place in a porous catalyst particle with a small effectiveness factor, the observed conversion to the intermediate B is less than what would be observed in the absence of a significant mass transport influence. This happens because as the resistance to transport of B in the pores increases, B is more likely to be converted to C rather than to be transported from the catalyst interior to the external surface. This result has important consequences in processes such as selective oxidations, in which the desired product is an intermediate and not the total oxidation product CO2. 

Preparation of 1 -(/3-D-arabinofuranosyl)-2-thiocytosine A solution of 2.0 g of 1 -(2, 3, 5 -0-triacetyl-/3-D-arabinofuranosyl)-2,4-dithiouracil in 100 ml of methanol is saturated with anhydrous ammonia at 0°C. The mixture, in a glass liner, is heated in a pressure bomb at 100°C for three hours. The reaction mixture is concentrated to a gum in vacuo, and most of the byproduct acetamide is removed by sublimation at 60°C/0.1 mm. The residue is chromatographed on 100 g of silica gel. Elution of the column with methylene chloride-methanol mixtures with methanol concentrationsof 2-25% gives fractions containing acetamide and a series of brown gums. The desired product is eluted with 30% methanol-methylene chloride to give a total yield of 0.386 g (30%), MP 175°-180°C (dec.). Recrystallization from methanol-iso-propanol furnishes an analytical sample, MP 180°-182°C (dec.). 

A series of new palladium complexes 26 bearing a phosphine-functionalised NHC ligand has been shown to be more efficient than typical catalytic systems (i.e. Pd(OAc), PPhj) for the reaction depicted in Scheme 7.10 [55], In fact, these conditions afforded the 9-benzylidene-97f-fluorene in higher yields when iodobenzene was used, whereas the reaction of the less reactive phenyl bromide with diphenylacetylene was achieved for the first time affording the desired product in moderate to good yields. 

The selectivity in a system of parallel reactions does not depend much on the catalyst size if effective diffusivities of reactants, intermediates, and products are similar. The same applies to consecutive reactions with the product desired being the final product in the series. In contrast with this, for consecutive reactions in which the intermediate is the desired product, the selectivity much depends on the catalyst size. This was proven by Edvinsson and Cybulski (1994, 1995) for. selective hydrogenations and also by Colen et al. (1988) for the hydrogenation of unsaturated fats. Diffusion limitations can also affect catalyst deactivation. Poisoning by deposition of impurities in the feed is usually slower for larger particles. However, if carbonaceous depositions are formed on the catalyst internal surface, ageing might not depend very much on the catalyst size. 

Suggest a short series of reactions that would be expected to transform the material on the right into the desired product shown on the left. 

It is proposed to react 1 t-h-1 of a pure liquid A to a desired product B. Byproducts C and D are formed through series and parallel reactions ... 

A V - W where V is the desired product. These liquid phase reactions are to be carried out in a cascade of two equal volume CSTR s in series. If the reactors are to be sized so as to maximize the concentration of species V in the effluent from the second reactor, determine the reactor volumes necessary to process 500 gal/hr of feed containing 6 moles/gal of species A. No V or W is present in the feed. What fraction of the A ends up as V The rate constants kx and k2 are both equal to 0.5 hr - L... 

Consecutive Reactions Where an Inter-mediate Is the Desired Product. Consecutive reactions in which an intermediate species (V) is the desired product are often represented as a series of pseudo first-order reactions... 

Extension of the Kunii-Levenspiel bubbling-bed model for first-order reactions to complex systems is of practical significance, since most of the processes conducted in fluidized-bed reactors involve such systems. Thus, the yield or selectivity to a desired product is a primary design issue which should be considered. As described in Chapter 5, reactions may occur in series or parallel, or a combination of both. Specific examples include the production of acrylonitrile from propylene, in which other nitriles may be formed, oxidation of butadiene and butene to produce maleic anhydride and other oxidation products, and the production of phthalic anhydride from naphthalene, in which phthalic anhydride may undergo further oxidation. 

These networks can be analyzed by breaking them down into their simple series and simple parallel components. For example, for the following elementary reactions, where R is the desired product, the breakdown is as follows ... 

When selectivity and yield of a given product need to be maximized, the design issues become more complicated. While rninimum T is frequently desired, it is usually more important to obtain maximum selectivity to a desired product and niinimiim selectivity to undesired products. For simple series and parallel reaction systems, we can fairly easily summarize the choices. 

For series reactions with an intermediate desired, there is always an optimum T for maximum yield, and the PFTR gives a higher maximum yield if both reactions have positive order, while the CSTR gives a higher maximum yield if the reactions are negative order (a rather rare occurrence). For series reactions with the final product desired, the PFTR requires the shorter time and gives less intermediate for positive-order kinetics. 

In all cases studied, the membrane reactor offered a lower yield of formaldehyde than a plug flow reactor if all species were constrained to Knudsen diffusivities. Thus the conclusion reached by Agarwalla and Lund for a series reaction network appears to be true for series-parallel networks, too. That is, the membrane reactor will outperform a plug flow reactor only when the membrane offers enhanced permeability of the desired intermediate product. Therefore, the relative permeability of HCHO was varied to determine how much enhancement of permeability is needed. From Figure 2 it is evident that a large permselectivity is not needed, usually on the order of two to four times as permeable as the methane. An asymptotically approached upper limit of... 

General conclusions In series reactions, as the concentration of the desired intermediate P builds up, so the rate of degradation to the second product Q increases. The best course would be to remove P continuously as soon as it was formed by distillation, extraction or a similar operation. If continuous removal is not feasible, the conversion attained in the reactor should be low if a high relative yield is required. As the results for the continuous stirred-tank reactor show, backmixing of a partially reacted mixture with fresh reactants should be avoided. 

---