Product Yield and Selectivity

Distributors in industrial units typically have large numbers of injection points of quite diverse design characteristics, some of which are depicted in Eigure 16 for fluidized-bed appHcations. Flow variations through these parallel paths can lead to poor flow distributions within a reactor, thus reducing product yields and selectivity. In some circumstances, undesirable side products can foul portions of the distributor and further upset flow patterns. Where this is important, or where the possibiHties and consequences are insufficiently understood and independent means caimot be employed to assure adequate distribution, the pilot plant must be sized to accommodate such a distributor. Spacing should be comparable to those distributors that are anticipated to be... 

In recent years alkylations have been accompHshed with acidic zeoHte catalysts, most nobably ZSM-5. A ZSM-5 ethylbenzene process was commercialized joiatiy by Mobil Co. and Badger America ia 1976 (24). The vapor-phase reaction occurs at temperatures above 370°C over a fixed bed of catalyst at 1.4—2.8 MPa (200—400 psi) with high ethylene space velocities. A typical molar ethylene to benzene ratio is about 1—1.2. The conversion to ethylbenzene is quantitative. The principal advantages of zeoHte-based routes are easy recovery of products, elimination of corrosive or environmentally unacceptable by-products, high product yields and selectivities, and high process heat recovery (25,26). 

In the modern unit design, the main vessel elevations and catalyst transfer lines are typically set to achieve optimum pressure differentials because the process favors high regenerator pressure, to enhance power recovery from the flue gas and coke-burning kinetics, and low reactor pressure to enhance product yields and selectivities. 

For a profitable electrochemical process some general factors for success might be Hsted as high product yield and selectivity current efficiency >50%, electrolysis energy <8 kWh/kg product electrode, and membrane ia divided cells, lifetime >1000 hours simple recycle of electrolyte having >10% concentration of product simple isolation of end product and the product should be a key material and/or the company should be comfortable with the electroorganic method. 

Improvement of product yield and selectivity in microreactors by combining fluid segments of different concentrations and sizes... 

The limitation on this simple dioxan synthesis is that unbranched olefins are unreactive. Nevertheless, the method offers advantages over the alternative acid-catalyzed (Prins) reaction in regard to both product yield and selectivity. 

Panov, A.G., Larsen, R.G., Totah, N.I., Larsen, S.C. and Grassian, V.H. (2000). Photooxidation of toluene and p-xylene in cation-exchanged zeolites X, Y, ZSM-5, and beta the role of zeolite physicochemical properties in product yield and selectivity. J. Phys. Chem. B 104, 5706-5714... 

Notwithstanding the intellectual challenges posed by the subject, the main impetus behind the development of computational models for turbulent reacting flows has been the increasing awareness of the impact of such flows on the environment. For example, incomplete combustion of hydrocarbons in internal combustion engines is a major source of air pollution. Likewise, in the chemical process and pharmaceutical industries, inadequate control of product yields and selectivities can produce a host of undesirable byproducts. Even if such byproducts could all be successfully separated out and treated so that they are not released into the environment, the economic cost of doing so is often prohibitive. Hence, there is an ever-increasing incentive to improve industrial processes and devices in order for them to remain competitive in the marketplace. 

In hindsight, the primary factor in determining which approach is most applicable to a particular reacting flow is the characteristic time scales of the chemical reactions relative to the turbulence time scales. In the early applications of the CRE approach, the chemical time scales were larger than the turbulence time scales. In this case, one can safely ignore the details of the flow. Likewise, in early applications of the FM approach to combustion, all chemical time scales were assumed to be much smaller than the turbulence time scales. In this case, the details of the chemical kinetics are of no importance, and one is free to concentrate on how the heat released by the reactions interacts with the turbulent flow. More recently, the shortcomings of each of these approaches have become apparent when applied to systems wherein some of the chemical time scales overlap with the turbulence time scales. In this case, an accurate description of both the turbulent flow and the chemistry is required to predict product yields and selectivities accurately. 

Mechanisms of Product Yield and Selectivity Control with Octane Catalysts... 

SCHEME 44. Products [yields and (selectivities)] of the uncatalyzed epoxidation in trifluoroethanol... 

Table I shows the effects of Mel/DME and CO/DME ratios in the feed gas on product yields. With increasing Mel/DME ratio both methyl acetate yield and selectivity increased. The yield of methyl acetate increased with an increase in the CO/DME ratio whereas its selectivity decreased. In the case of methanol carbonylation on Ni/A.C. catalyst, the product yield and selectivity were strongly affected by CO/MeOH ratio but not by Mel/MeOH ratio (14-16). The promoting effect of methyl iodide on the methanol carbonylation reached a maximum at a very low partial pressure, that is 0.1 atm or lower. However, both CO/DME and Mel/DME ratios were important for regulating the product yield and selectivity of the dimethyl ether carbonylation. This suggests that the two steps, namely, the dissociative adsorption of methyl iodide on nickel (Equation 4) and the insertion of CO (Equation 5) are slow in the case of dimethyl ether reaction.

In a recent review, Tao etal. [34] describe the partial fluorination and the perfluorination of organics with particular emphasis on medically important compounds and pharmaceuticals. The selective electrofluorination (SEF) of olefins and active methylene groups is reviewed by Noel et al. [35] In the case of heterocycles, nuclear fluorination is known to be the predominant process. However, in aromatic compounds, nuclear substitution as well as addition proceeds simultaneously, leading to the formation of a mixture of products. The influence of solvents, supporting electrolytes, and adsorption on product yield and selectivity is summarized and evaluated. Dimethoxyethane is found to be a superior solvent for SEF processes. Redox mediators have been employed to minimize anode passivation and to achieve better current efficiencies. 

Free radical brominations can be conducted effectively in SC-CO2 as solvent. The high product yields and selectivities usually found for brominations in conventional solvents are not compromised by the use of this nontoxic, less environmentally threatening medium. The results demonstrate that supercritical fluid solvents retain virtually all of the chemical advantages associated with conventional organic solvents (Tanko et al., 1994). 

As an example, let us consider the stoichiometric oxidation of diphenylmethanol to benzophenone, one of the most commonly used photosensitizers in photochemistry (Figure 1.3). We will evaluate this reaction using the measures of product yield, product selectivity, E-factor, and atom economy. In this reaction, three equivalents of diphenylmethanol react with two equivalents of chromium trioxide and three equivalents of sulfuric acid, giving three equivalents of benzophenone. First, let us see how the reaction measures with respect to product yield and selectivity. Assume that this is an ideal chemical reaction which goes to completion, so one obtains 100% yield of the product, benzophenone. If no other (organic) by-product is obtained, the product selectivity is also 100%. This is all well and good, and indeed for many years this has been the way that chemical processes were evaluated, both in academia and in the (fine-) chemical industry. 

By focussing on the above points, cat cracking experiments can be performed in laboratory equipment with residue-containing feedstocks. With proper choice of operating conditions, the resulting product yields and selectivity simulate those in commercial units quite well. 

Figure 4.10 Temperature dependencies of reaction product yields and selectivity at methane oxidation molar ratio CH4 25% H202 = 1 1, t= 1.2s (1 methanol 2 CO + C02 3 formaldehyde 4 selectivity by formaldehyde and 5 total methane conversion).

Figure 4.13 Temperature dependence of reaction product yields and selectivity at methanol oxidation molar ratio CH3OH 25% aqueous H20, vCH3oH = 1.44ml/h, vH2q2 = 2.32ml/h (1 CO 2 C02 3 formaldehyde 4 selectivity and 5 total methanol conversion).

Of special attention is one more reagent containing Fe2+, H202 (or 02), ascorbic and ethyl diamino tetraacetic acids (the Udenfried system) [29], In this system, Fe2+ andFe3+ display equal catalytic activity. The authors of the review [34] conclude that purely hydrox-ylating catalytic systems containing Fe2+ possess a limited activity and, consequently, reaction product yields and selectivity are low. 

The results of the demonstration plant are compared with those of the bench unit in Table I. The two units were operated at identical conditions except for gas velocity. The product yields and selectivities over the first cycle were remarkably similar. The adiabatic temperature rise and the band aging behavior were also the same. However, the demonstration unit performed considerably better with respect to cycle length. Typical cycles were about 50% longer. 

Table 3 Products, yield, and selectivity for alkyne/alkene insertion into EBTHI zirconaaziridines...

Figure 11.17 C2 product yield and selectivity of oxidative coupling of methane of a 0.5-m long plug-flow membrane reactor as a function of (ratio of oxygen permeation rate to methane flowrate at the reactor entrance) [Wang and Lin, 1995]...

The test run used commercially-produced zeolite catalyst, and the unit was a modified, commercial wax hydrofinisher. Charge stock was an LPG mixture of propane/propene/butanes/butenes (62% olefins) from an FCC unit. The test run lasted 70 days and product yields and selectivities were the same as in our smaller pilot plants. 

For fast chemical reactions, the characteristic times of the chemical reaction and the physical processes are of the same order of magnitude and mass and heat transfer phenomena affect reactor performance including product yield and selectivity. For very fast reactions, the transformation rates become completely limited by transport phenomena. As a result, the reactor performance is diminished relative to the maximal performance attainable in the kinetic regime, and product yield and selectivity may be considerably reduced. 

---