Concentration reduced, consecutive reaction

The conditions are substantially more favorable for the microporous catalytic membrane reactor concept. In this case the membrane wall consists of catalyti-cally active, microporous material. If a simple reaction A -> B takes place and no permeate is withdrawn, the concentration profiles are identical to those in a catalyst slab (Fig. 29a). By purging the permeate side with an inert gas or by applying a small total pressure difference, a permeate with a composition similar to that in the center of the catalyst pellet can be obtained (Fig. 29b). In this case almost 100% conversion over a reaction length of only a few millimeters is possible. The advantages are even more pronounced, if a selectivity-limited reaction is considered. This is shown with the simple consecutive reaction A- B- C where B is the desired product. Pore diffusion reduces the yield of B since in a catalyst slab B has to diffuse backwards from the place where it was formed, thereby being partly converted to C (Fig. 29c). This is the reason why in practice rapid consecutive reactions like partial oxidations are often run in pellets composed of a thin shell of active catalyst on an inert support [30],... 

A schematic of a CD process for cumene production is shown in Fig. 3. The CDTECH process uses a zeolite catalyst in one of its patented CD structures and the product yield exceeds 99.5% purity with 99.9% selectivity to cumene. The high selectivity to cumene is achieved by controlling a low propylene concentration in the reaction section using a combination of process parameters such as system pressure, location of catalyst zone, and feedpoint. A low propylene concentration will result in a low propylene oligomerization rate and hence will reduce the amount of diisopropylbenzene and triisopropylbenzene produced by the consecutive reactions. One interesting aspect of this process is to recycle the diisopropylbenzene and triisopropyl benzene where transalkylation with benzene produces more cumene. 

For a consecutive reaction that is pseudo-first order in R, the effect of crystallization of R is to reduce the rate of reaction to the undesired over-reaction product S by reducing the concentration of R in the reaction mixture as the reaction proceeds. The topic of increasing reaction selectivity in the presence of, or by the creation of, a second phase, e.g., by crystallization, has been discussed by Sharma (1988) and Paul et al. (2003, chapter 13). 

Thus, Sp is equal to the ratio of the intrinsic rate constants just the same as though diffusion was not involved. For consecutive reactions, such as A B C, diffusion has an adverse effect on the selectivity of B with respect to C. This is because diffusion resistance reduces the surface concentration of A (from which B is produced) and increases the surface concentration of B (from which C is produced), with respect to bulk concentrations. 

This greatly simplifies analysis of the system since the two rates differ only in the fundamental rate constant and the concentration of the reactants. Computer simulation of the consecutive reaction mechanism allows one to determine the 2/ 1 needed to achieve the desired yield of C2H4. Figure 6 shows how the selectivity to B varies as a function of conversion of A for various 2/ 1 ratios. (Since oxidative activation of ethane to give an ethyl radical results in an ethylene product, it only serves to consume oxidant and can be ignored. If direct ethane conversion to CO2 occurs this will reduce C2 yield.) The results in Figure 4 are fit well by a 2/ 1 about 6. 

In general, the solubility of a compound containing a basic anion (that is, the anion of a weak acid) increases as the solution becomes more acidic. As we have seen, the solubility of Mg(OH)2 greatly increases as the acidity of the solution increases. The solubility of Pbp2 increases as the solution becomes more acidic, too, because F is a weak base (it is the conjugate base of the weak acid HF). As a result, the solubility equilibrium of Pbp2 is shifted to the right as the concentration of F is reduced by protonation to form HF. Thus, the solution process can be understood in terms of two consecutive reactions ... 

Bromine (128 g., 0.80 mole) is added dropwise to the well-stirred mixture over a period of 40 minutes (Note 4). After all the bromine has been added, the molten mixture is stirred at 80-85° on a steam bath for 1 hour, or until it solidifies if that happens first (Note 5). The complex is added in portions to a well-stirred mixture of 1.3 1. of cracked ice and 100 ml. of concentrated hydrochloric acid in a 2-1. beaker (Note 6). Part of the cold aqueous layer is added to the reaction flask to decompose whatever part of the reaction mixture remains there, and the resulting mixture is added to the beaker. The dark oil that settles out is extracted from the mixture with four 150-ml. portions of ether. The extracts are combined, washed consecutively with 100 ml. of water and 100 ml. of 5% aqueous sodium bicarbonate solution, dried with anhydrous sodium sulfate, and transferred to a short-necked distillation flask. The ether is removed by distillation at atmospheric pressure, and crude 3-bromo-acetophenone is stripped from a few grams of heavy dark residue by distillation at reduced pressure. The colorless distillate is carefully fractionated in a column 20 cm. long and 1.5 cm. in diameter that is filled with Carborundum or Heli-Pak filling. 4 hc combined middle fractions of constant refractive index are taken as 3-l)romoaccto])lu iu)nc weight, 94 -100 g. (70-75%) l).p. 75 76°/0.5 mm. tif 1.57,38 1.5742 m.]). 7 8° (Notes 7 and 8). 

The higher activity of the catalyst [(mall)Ni(dppmo)][SbFg] in [BMIM][PFg] (TOF = 25,425 h ) relative to the reaction under identical conditions in CFF2C12 (TOF = 7591 h ) can be explained by the fast extraction of products and side products out of the catalyst layer and into the organic phase. A high concentration of internal olefins (from oligomerization and consecutive isomerization) at the catalyst is known to reduce catalytic activity, due to the formation of fairly stable Ni-olefin complexes. 

The experimental data is concentration-time data, and integrated rate expressions give the observed rate constants. The technique can be extended to multi-step equilibria and to equilibrium reactions with consecutive steps. For multi-step equilibria, analysis results in a sum of exponentials, which may or may not reduce to first order. 

Figure 4.6 presents kinetic curves of cyclohexadiene accumulation at an optimal temperature of 580 °C. It is shown that in the initial period cyclohexadiene accumulation as an intermediate product is intensified until its consumption and accumulation rates are equalized and cyclohexadiene concentration reaches the maximum. Further increase of conditional contact time reduces cyclohexadiene concentration. For example, at 580 °C and r = 0.7 h the cyclohexadiene yield is 17.5%, decreasing to 9.5% with r increased to 2.5 h. Under optimal conditions (T = 580 °C, cyclohexene volume rate 1.4ml/(mlh), cyclohexene 20% aqueous H202 = 1 3), yields were 17.1% for cyclohexadiene and 5.8% for benzene. The reaction selectivity approached 100%. The entire process was of a consecutive autocatalytic type. 

A 5 L, 3-neck round bottom flask equipped with a mechanical stirrer was charged with the N-acetyl-3-(4-thiazolyl)-DL-alanine ethyl ester (210.0 g, 0.87 mol), distilled water (1.6 L), and 1 M aqueous KCI (0.8 L). The homogeneous solution was adjusted to pH 7.0 with 0.1 M NaOH and then was treated with Subtilisin Carlsberg (1.8 g) dissolved in 0.1 M aqueous KCI (25 ml). The reaction mixture was stirred at room temperature with 1.0 M NaOH added as required to maintain the pH at 6.25-7.25. After 4 h, 430 ml of base had been consumed and the reaction was judged to be complete. The reaction mixture was then extracted with chloroform (4x1.5 L), the aqueous phase was carefully acidified to pH 4 with 2 M HCI and then was concentrated under reduced pressure. Residual water was removed by consecutive evaporation... 

Care must be taken in order to ensure that the effective rates of the consecutive radical reactions are higher than the rate of hydrogen abstraction from (TMS)3SiH. Apart from the standard synthetic planning based on known rate constants, this is usually effected either by controlling the concentration of the reducing agent in the reaction medium... 

Preservatives are important causes of allergic contact dermatitis in cosmetics. In a 10-year analysis in 16 centers in 11 countries, 73 818 consecutive patients were patch-tested for the preservatives listed above. There were several cases of contact allergy to formaldehyde and MCI/MI. These preservatives are currently avoided in cosmetics. However, the frequency of positive reactions to MDBGN has risen, from 0.7% in 1991 to 3.5% in 2000. The authors suggested that the concentration of this preservative should be reduced in leave-on cosmetic products (1). 

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