Selectivity in Exothermic Reactions

The Hammond Postulate implies that the transition stah of a fast exothermic reaction resembles the reactants (se( reaction energy diagram at left). This means that it wil be hard to predict the selectivity of competing exothermi( reactions both barriers may be small and similar even i one reaction is more exothermic than the other. 

Consider abstraction of a hydrogen atom from propan( by fluorine atom. This can generate either of two propy radicals, depending on which hydrogen is attacked. 

Add the energies of propane and fluorine atom (at left (the reactants), and then the energies of 1-propyl radica (or 2-propyl radical) and hydrogen fluoride (th( products). Are these reactions exothermic or endothermic If the former, then calculate the relative concentrations 0 1-propyl radical and 2-propyl radical that would exist ii an equilibrium mixture at 298 K. Use equation (1). 

Obtain the partial CH and HF bond distances in eacl transition state, and compare them to the CH and HF bon( distances in propane and hydrogen fluoride, respectively Does the Hammond Postulate correctly predict whicl bond distances will be most similar Explain. 

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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. 

Thus our first conclusion if the harmful effects of thermal shock, or sintering of the catalyst surface, or drop in selectivity, do not occur with hot particles, then we would encourage nonisothermal behavior in exothermic reactions. On the other hand, we would like to depress such behavior for endothermic reactions. 

In view of the extremely exothermic nature of the reaction of elemental fluorine with hydrocarbons, inorganic hydrides, and so on, it might seem obvious that there should be little or no selectivity in the reactions of molecular or atomic fluorine. 

Fixed-bed catalytic reactors and reactive distillation columns are widely used in many industrial processes. Recently, structured packing (e.g., monoliths, katapak, mella-pak etc.) has been suggested for various chemical processes [1-4,14].One of the major challenges in the design and operation of reactors with structured packing is the prevention of liquid flow maldistribution, which could cause portions of the bed to be incompletely wetted. Such maldistribution, when it occurs, causes severe under-performance of reactors or catalytic distillation columns. It also can lead to hot spot formation, reactor runaway in exothermic reactions, decreased selectivity to desired products, in addition to the general underutilization of the catalyst bed. 

In the foregoing illustration the temperature rise at the catalyst surface had a beneficial effect on selectivity. This is because the activation energy for the desired reaction was greater than that for the reaction producing by-product C. If were less than E2, external heat-transfer resistance would have reduced the selectivity for exothermic reactions. 

Besides selectivity, energy requirements will grow more essential in the future. One advantage of homogeneous catalysis lies in its mild reaction temperatures needed for reactions to occur. Also working in solution makes heat recovery especially in exothermic reactions — often much easier. For instance, reflux of a solvent can be used to control the temperature in a reaction vessel. 

Distributor The controlled addition of reactant(s) limits side reactions to increase selectivity by optimizing the reactant concentration profile to mitigate the temperature rise in exothermic reactions Partial oxidation oxidative dehydrogenation of hydrocarbons oxidative coupling of methane... 

Radicals also exhibit high selectivity in addition reactions. For example, the per-oxyl radical of oxidizing styrene adds to the double bond of styrene with the rate constant k= 68 l/(mol s), and the oxygen molecule adds with k= 5.610 l/(mol s) (298 K). As in the case of abstraction reactions, the distinction is resulted by the feet that the first reaction is exothermic (A// = -100 kJ/mol), and the second reaction is endothermic (A// =125 kJ/mol). In this case, the differences are due to the fact that the chemical energy is stored in the free radical. To illustrate this, below we present the A// values for RH molecules and radicals R- formed ftem them. It is seen that this difference ranges from 180 to 280 kJ/mol, that is, very significant... 

In the petrochemical industry close to 80% of reactions are oxidations and hydrogenations, and consequently very exothermic. In addition, profitability requires fast and selective reactions. Fortunately these can be studied nowadays in gradientless reactors. The slightly exothermic reactions and many endothermic processes of the petroleum industry still can use various tubular reactors, as will be shown later. 

Several patents exist on carrying out exothermic reactions for manufacture of reactive intermediates where high selectivity is essential. Even this author has a patent to make ethylene oxide in a transport line reactor (Berty 1959). Yet no fluidized bed technology is in use today. Mostly fixed bed, cooled tubular reactors are used for that purpose. 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

Chemical reactions are sometimes conducted in a dilute solution to moderate reaction rates, to provide a heat sink for an exothermic reaction, or to limit maximum reaction temperature by tempering the reaction. In this example there are conflicting inherent safety goals—the solvent moderates the chemical reaction, but the dilute system will be significantly larger for a given production volume. Careful evaluation of all of the process risks is required to select the best overall system. 

So far, consideration has been limited to chemistry physical constraints such as heat transfer may also dictate the way in which reactions are performed. Oxidation reactions are highly exothermic and effectively there are only two types of reactor in which selective oxidation can be achieved on a practical scale multitubular fixed bed reactors with fused salt cooling on the outside of the tubes and fluid bed reactors. Each has its own characteristics and constraints. Multitubular reactors have an effective upper size limit and if a plant is required which is too large to allow the use of a single reactor, two reactors must be used in parallel. 

A. Available Design Methods for Vent Sizing. Several methods are available to size the vent with a wide range of sophistication. The FIA chart, Fig. 1 prepared by the Factory Insurance Association in the mid 1960 s is a simple chart summarizing a wealth of experience. Reactions are classed by the degree of exothermic reaction. With vessel size and a judgment of reaction type a vent size range can be selected. 

In the feed pretreatment section oil and water are removed from the recovered or converted CCI2F2. The reactor type will be a multi-tubular fixed bed reactor because of the exothermic reaction (standard heat of reaction -150 kJ/mol). After the reactor the acids are selectively removed and collected as products of the reaction. In the light removal section the CFCs are condensed and the excess hydrogen is separated and recycled. The product CH2F2 is separated from the waste such as other CFCs produced and unconverted CCI2F2. The waste will be catalytically converted or incinerated. A preliminary process design has shown that such a CFC-destruction process would be both technically and economically feasible. 

Worz et al. stress a gain in reaction selectivity as one main chemical benefits of micro-reactor operation [110] (see also [5]). They define criteria that allow one to select particularly suitable reactions for this - fast, exothermic (endothermic), complex and especially multi-phase. They even state that by reaching regimes so far not accessible, maximum selectivity can be obtained [110], Although not explicitly said, maximum refers to the intrinsic possibilities provided by the elemental reactions of a process under conditions defined as ideal this means exhibiting isothermicity and high mass transport. 

The parameter p (= 7(5 ) in gas-liquid sy.stems plays the same role as V/Aex in catalytic reactions. This parameter amounts to 10-40 for a gas and liquid in film contact, and increases to lO -lO" for gas bubbles dispersed in a liquid. If the Hatta number (see section 5.4.3) is low (below I) this indicates a slow reaction, and high values of p (e.g. bubble columns) should be chosen. For instantaneous reactions Ha > 100, enhancement factor E = 10-50) a low p should be selected with a high degree of gas-phase turbulence. The sulphonation of aromatics with gaseous SO3 is an instantaneous reaction and is controlled by gas-phase mass transfer. In commercial thin-film sulphonators, the liquid reactant flows down as a thin film (low p) in contact with a highly turbulent gas stream (high ka). A thin-film reactor was chosen instead of a liquid droplet system due to the desire to remove heat generated in the liquid phase as a result of the exothermic reaction. Similar considerations are valid for liquid-liquid systems. Sometimes, practical considerations prevail over the decisions dictated from a transport-reaction analysis. Corrosive liquids should always be in the dispersed phase to reduce contact with the reactor walls. Hazardous liquids are usually dispensed to reduce their hold-up, i.e. their inventory inside the reactor. 

The side-chain substitution of toluene, p-chlorotoluene, etc. is industrially practised. This reaction is carried out in a photochemical reactor. It is an exothermic reaction in which HCl is produced. The reaction is consecutive, and hence CL first reacts with toluene reacts to form the desired benzyl chloride, which is then converted to benzal chloride, and finally benzotrichloride. We may, however, well be interested in the selectivity to benzyl chloride. An additional complication arises due to nuclear chlorination, which is most undesirable. A distillation-column reactor can offer advantages (Xu and Dudukovic, 1999). 

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