Selectivity maximization

In summary one can view the ethylene epoxidation system as one where selectivity maximization requires the coexistence of the following two adsorption reactant states ... 

C. Karavasilis, S. Bebelis, and C.G. Vayenas, Selectivity Maximization of Ethylene Epoxidation via NEMCA with Zirconia and (3"-Al203 Solid Electrolytes, Ionics 1, 85-91 (1995). 

Vayenas, C. G. and S. Pavlou. 1987b. Optimal catalyst distribution for selectivity maximization in pellets parallel and consecutive reactions. Chem. Eng. Sci. 42(7) 1655-1666. 

For the various reaction mechanisms used in determining both instantaneous and the overall selectivities, selectivity depends on the energy of activation obtained from the Arrhenius equation [(k = k0exp(-E/RT)], the temperature, initial concentration, and the time of reaction. From the Arrhenius equation, the specific reaction rate k is an integral part of the selectivity expressions. Furthermore, analyzing selectivity expressions may indicate an enhanced effect of the temperature on selectivity. Maximizing the expressions for both instantaneous and overall selectivities may depend on the following ... 

Vayenas, C. G., and Pavlou, S., Optimal Catalyst Distribution for Selectivity Maximization in Pellets. Paper No. 72d, AIChE Annual Meeting, Washington, D.C., Nov. 27-Dec. 2, 1988. 

In the preceding section, the choice of reactor type was made on the basis of which gave the most appropriate concentration profile as the reaction progressed in order to minimize volume for single reactions or maximize selectivity for multiple reactions for a given conversion. However, after making the decision to choose one type of reactor or another, there are still important concentration effects to be considered. 

Multiple reactions in parallel producing byproducts. Once the reactor type is chosen to maximize selectivity, we are in a position to alter selectivity further in parallel reaction systems. Consider the parallel reaction system from Eq. (2.20). To maximize selectivity for this system, we minimize the ratio given by Eq. (2.21) ... 

Figure 2.10 Choosing the reactor to maximize selectivity for multiple reactions producing byproducts.

It should be emphasized that these recommendations for the initial settings of the reactor conversion will almost certainly change at a later stage, since reactor conversion is an extremely important optimization variable. When dealing with multiple reactions, selectivity is maximized for the chosen conversion. Thus a reactor type, temperature, pressure, and catalyst are chosen to this end. Figure 2.10 summarizes the basic decisions which must be made to maximize selectivity. ... 

In Chap. 2 the objective set was to maximize selectivity for a given conversion. This also will minimize waste generation in reactors for a given conversion. 

The thermal profile through the reactor will in most circumstances be carefully optimized to maximize selectivity, extend catalyst life, etc. Because of this, direct heat integration with other process streams is almost never carried out. The heat transfer to or from the reactor is instead usually carried out by a heat transfer intermediate. For example, in exothermic reactions, cooling might occur by boiling water to raise steam, which, in turn, can be used to heat cold streams elsewhere in the process. 

In two stages with recycle to the second stage, the conversion per pass is approximately 50 wt. % and the selectivity to middle distillates is maximal 75 to 80 wt. %. However, the investment is clearly higher and is justified only when feedstocks are difficult to convert and that their content in nitrogen is high. Figure 10.11 represents two variants of the hydrocracking process. 

In the particular framework for lubricating oil bases, the operation takes place batchwise, generally using distillates selected according to the desired base, so as to minimize by-products and to maximize lubricating oils and their qualities. 

The next step, therefore, is to address the question how is it possible to take advantage of the many additional available parameters pulse shaping, multiple pulse sequences, etc—m general an E(t) with arbitrary complexity—to maximize and perhaps obtain perfect selectivity Posing the problem mathematically, one seeks to maximize... 

High Peroxide Process. An alternative to maximizing selectivity to KA in the cyclohexane oxidation step is a process which seeks to maximize cyclohexyUiydroperoxide, also called P or CHHP. This peroxide is one of the first intermediates produced in the oxidation of cyclohexane. It is produced when a cyclohexyl radical reacts with an oxygen molecule (78) to form the cyclohexyUiydroperoxy radical. This radical can extract a hydrogen atom from a cyclohexane molecule, to produce CHHP and another cyclohexyl radical, which extends the free-radical reaction chain. 

Because RPSA is appHed to gain maximum product rate from minimum adsorbent, single beds are the norm. In such cycles where the steps take only a few seconds, flows to and from the bed are discontinuous. Therefore, surge vessels are usuaHy used on feed and product streams to provide unintermpted flow. Some RPSA cycles incorporate delay steps unique to these processes. During these steps, the adsorbent bed is completely isolated and any pressure gradient is aHowed to dissipate (68). The UOP Polybed PSA system uses five to ten beds to maximize the recovery of the less selectively adsorbed component and to extend the process to larger capacities (69). 

Relative potency alone does not determine dmg selection because maximal effectiveness is similar for all agents. A single daily dose of any sulfonylurea, except tolbutamide, is sometimes adequate to control blood glucose in NIDDM patients. 

The goal of the MRI scientist is to maximize the contrast-to-noise ratio between tissues. Examination of equation 13 reveals that by varyiag TR and TE, the clinician has a tremendous amount of flexibiHty to select the desired contrast between two tissues. 

Several variations of the above process are practiced. In the Sumitomo-Nippon Shokubai process, the effluent from the first-stage reactor containing methacrolein and methacrylic acid is fed directiy to the second-stage oxidation without isolation or purification (125,126). In this process, overall yields are maximized by optimizing selectivity to methacrolein plus methacrylic acid in the first stage. Conversion of isobutjiene or tert-huty alcohol must be high because no recycling of material is possible. In another variation, Asahi Chemical has reported the oxidative esterification of methacrolein directiy to MMA in 80% yield without isolation of the intermediate MAA (127,128). 

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