Product Selectivity Control

During the long history of FT synthesis, various approaches have been developed to control product selectivity. Parameters like temperature, pressure, H. CO ratio, conversion, space velocity and alkalization of iron catalysts influence chain length, chain branching, (he ole fin/paraffin ratio and the alcohol selectivity as well as carbon deposition and methane selectivity. The effect of the different parameters can be obtained from Table IV 9]. As a typical example for the 

Sekclivitv control in Fischer Tropsch synthesis by reaction parameters and catalyst 

Tempetaiute Pressure Hj. CO ratio Conversion Space velocity Alkalis, of iron catalyst 

Increase with incteastng parameter Dccicase with increasing parameter Complex relation  

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Atobe, M., Sasahira, M. and Nonaka, T., 2000, Ultrasonic effects on Electro-organic Processes part 42. Product selectivity control in reductive homo and cross couplng of acrylonitrile . Paper presented on the 197" meeting of the Electrochemical Society May 2000, Toronto Camab Abstract no. 108. 

After a short review of the historical developments, the basic features of the Fischer-Tropsch reaction will be dealt with and special attention will be given to the possibilities of product selectivity control. Finally, the various mechanistic proposals known so far will be discussed in respect to recent studies of catalyst... 

Amemiya, F., Matsumoto, H., Fuse, K. et al. (2011) Product selectivity control induced by using hquid-liquid parallel laminar flow in a microreactor. Organic and Biomolecular Chemistry, 9, 4256-4265. 

Atobe M, Sasahita M, Nonaka T (2000) Ultrason Sonochem, Ultrasonic Effects on Electroorganic Processes. Part 17. Product Selectivity Control in Cathodic Reduction of Acrylonitrile 7 103-107... 

There are two general temperature poHcies increasing the temperature over time to compensate for loss of catalyst activity, or operating at the maximum allowable temperature. These temperature approaches tend to maximize destmction, yet may also lead to loss of product selectivity. Selectivity typically decreases with increasing temperature faster deactivation and increased costs for reactor materials, fabrication, and temperature controls. 

Advantages to Membrane Separation This subsertion covers the commercially important membrane applications. AU except electrodialysis are pressure driven. All except pervaporation involve no phase change. All tend to be inherently low-energy consumers in the-oiy if not in practice. They operate by a different mechanism than do other separation methods, so they have a unique profile of strengths and weaknesses. In some cases they provide unusual sharpness of separation, but in most cases they perform a separation at lower cost, provide more valuable products, and do so with fewer undesirable side effects than older separations methods. The membrane interposes a new phase between feed and product. It controls the transfer of mass between feed and product. It is a kinetic, not an equihbrium process. In a separation, a membrane will be selective because it passes some components much more rapidly than others. Many membranes are veiy selective. Membrane separations are often simpler than the alternatives. 

Catalyst cooler(s). Installing a catalyst cooler(s) is a way to control and vary regenerator heat removal and thus allow processing of a poor quality feedstock to achieve increased product selectivity. 

There are several available terminal oxidants for the transition metal-catalyzed epoxidation of olefins (Table 6.1). Typical oxidants compatible with most metal-based epoxidation systems are various alkyl hydroperoxides, hypochlorite, or iodo-sylbenzene. A problem associated with these oxidants is their low active oxygen content (Table 6.1), while there are further drawbacks with these oxidants from the point of view of the nature of the waste produced. Thus, from an environmental and economical perspective, molecular oxygen should be the preferred oxidant, because of its high active oxygen content and since no waste (or only water) is formed as a byproduct. One of the major limitations of the use of molecular oxygen as terminal oxidant for the formation of epoxides, however, is the poor product selectivity obtained in these processes [6]. Aerobic oxidations are often difficult to control and can sometimes result in combustion or in substrate overoxidation. In... 

Adequate PC and its associated instrumentation are essential for product quality control. The goal in some cases is precise adherence to a single control point. In other cases, maintaining the temperature within a comparatively small range is all that is necessary. For effortless controller tuning and the lowest initial cost, the processor should select the simplest controller (of temperature, time, pressure, melt-flow, rate, etc.) that will produce the desired results. 

The evaluation of competing chemistries and subsequent product selection may be difficult, and feed rates for the wide array of available phosphonates and novel homo-, co-, and terpolymers available vary considerably. Polymers do not control all types of contaminants at an equal performance level, and product selection depends on the type, level, and ratios of contaminants present. 

Easy availability of ultrafast high intensity lasers has fuelled the dream of their use as molecular scissors to cleave selected bonds (1-3). Theoretical approaches to laser assisted control of chemical reactions have kept pace and demonstrated remarkable success (4,5) with experimental results (6-9) buttressing the theoretical claims. The different tablished theoretical approaches to control have been reviewed recently (10). While the focus of these theoretical approaches has been on field design, the photodissociation yield has also been found to be extremely sensitive to the initial vibrational state from which photolysis is induced and results for (11), HI (12,13), HCl (14) and HOD (2,3,15,16) reveal a crucial role for the initial state of the system in product selectivity and enhancement. This critical dependence on initial vibrational state indicates that a suitably optimized linear superposition of the field free vibrational states may be another route to selective control of photodissociation. 

Figure 10.5 shows the basic concept of the particle-level MR that gives (i) selective addition of reactants to the reaction zone and (ii) selective removal of products from the reaction zone. In the first case, if the diffusivity of one reactant (A) is much higher than that of the other components (B), the reactant (A) selectively diffuses into a catalyst particle through a membrane. Undesired reactions or the adsorption of poisons on the catalysts can be prevented. In the second case, the reaction has a hmited yield or is selectivity controlled by thermodynamics. The selective removal of the desired product from the catalyst particle gives enhancement of selectivity when the diffusivity of one product (R) is much greater than that of the other products (S). 

On-line GC analysis (Shimadzu GC 14A) was used to measure product selectivity and methane conversion. Details on the analysis procedure used for batch and continuous-flow operation are given elsewhere [12]. The molecular sieve trap was found to trap practically all ethylene, COj and HjO produced a significant, and controllable via the adsorbent mass, percentage of ethane and practically no methane, oxygen or CO, for temperatures 50-70 C. The trap was heated to -300°C in order to release all trapped products into the recirculating gas phase (in the case of batch operation), or in a slow He stream (in the case of continuous flow operation). 

A. Selectivity Control for the Reaction Channel Affording C8-Cycio-Oligomer Products... 

Catalytic reactions, both homogeneous and heterogeneous, are usually characterized by a diversity of reaction paths and consequently of reaction products. One of the problems encountered in catalytic reactions is that of selectivity control, i.e., how to achieve a high selectivity with respect to a desired reaction product. 

In contrast to the examples of selectivity control discussed in the previous sections, the problem here is control of the regioselectivity of the individual reaction steps. This is evident from the Scheme 5. In the first reaction step the nickel-hydride species adds to propene forming a propyl- or isopropyl-nickel intermediate this step is reversible, and the ratio of the two species can be controlled both thermodynamically and kinetically. In the second step, a second molecule of propene reacts to give four alkylnickel intermediates from which, after j8-H elimination, 8 primary products are produced (Scheme 5). 2-Hexene and 4-methyl-2-pentene could be the products of either isomerization or the primary reaction. Isomerization leads to 3-hexene, 2-methyl-2-pentene (the common isomerization product of 2-methyl-1-pentene and 4-methyl-2-pen-tene), and 2.3-dimethyl-2-butene. It can be seen from the Scheme 5 that, if the isomerization to 2-methyl-2-pentene can be neglected, the distribution of the products enables an estimate to be made of the direction of... 

Industrial carbon anodes and artificial graphites are not a single material but are rather members of a broad family of essentially pure carbon. Fortunately, artificial graphites can be tailored to vary widely in their strength, density, conductivity, pore structure, and crystalline development. These attributes contribute to their widespread applicability. Specific characteristics are imparted to the fmished product by controlling the selection of precursor materials and the method of processing [19]... 

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