Conversion in a reversible reaction

With an irreversible reaction, virtually complete conversion can be achieved in principle, although a very long time may be required if the reaction is slow. With a reversible reaction, it is never possible to exceed the conversion corresponding to thermodynamic equilibrium under the prevailing conditions. Equilibrium calculations have been reviewed briefly in Chap. 1 and it will be recalled that, with an exothermic reversible reaction, the conversion falls as the temperature is raised. The reaction rate increases with temperature for any fixed value of VjF and there is therefore an optimum temperature for isothermal operation of the reactor. At this temperature, the rate of reaction is great enough for the equilibrium state to be approached reasonably closely and the conversion achieved in the reactor is greater than at any other temperature. 

The design equation (63) applies and the expression for the rate of reaction, r, is 

Substitution of this expression in the design equation (63), followed by integration gives 

This equation relates x, the fractional conversion achieved, in a reactor of volume V when the volumetric flow rate at inlet is and the temperature is such that the rate coefficient of the forward reaction is kf and the equilibrium conversion is x. The temperature dependence of kf is given by an Arrhenius equation, while that of Xp can be calculated by thermodynamics. 

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Allow vaporization of one of the components in a reversible reaction in order that removal increases maximum conversion. 

Diene 265, substituted by a bulky silyl ether to prevent cycloaddition before the metathesis process, produced in the presence of catalyst C the undesired furanophane 266 with a (Z) double bond as the sole reaction product in high yield. The same compound was obtained with Schrock s molybdenum catalyst B, while first-generation catalyst A led even under very high dilution only to an isomeric mixture of dimerized products. The (Z)-configured furanophane 266 after desilylation did not, in accordance with earlier observations, produce any TADA product. On the other hand, dienone 267 furnished the desired macrocycle (E)-268, though as minor component in a 2 1 isomeric mixture with (Z)-268. Alcohol 269 derived from E-268 then underwent the projected TADA reaction selectively to produce cycloadduct 270 (70% conversion) in a reversible process after 3 days. The final Lewis acid-mediated conversion to 272 however did not occur, delivering anhydrochatancin 271 instead. 

This type of periodic operation allows for conversion improvement in reversible exothermic reactions [9], A cycle average inlet temperature for the conditions of continuous temperature oscillation can be substantially lower than the inlet temperature under steady-state conditions. This leads to a lower outlet temperature and higher equilibrium conversion for a reversible reaction. Better performance is achieved if temperature oscillations attenuate sufficiently during the passage through the catalyst bed [9]. 

Co-current versus counter-current flows. It is noted that in the operation of a separation system, a counter-current flow has always given a larger average concentration gradient than a co-current flow. Thus, it is expected that counter-current flow configuration is preferred between the two in a membrane unit. In a membrane reactor, however, an additional factor needs to be considered. To obtain a high conversion of a reversible reaction, it is necessary to maintain a high forward reaction rate. 

Figure 11.24. Conversion of a reversible reaction A B in a perfect-mixing membrane reactor for an endothermic reaction [Mohan and Govind, 1988b]...

Figure 1.2 Exploitable features of membrane reactors, (a) Enhancing the conversion of a reversible reaction in a packed-bed inert membrane reactor, (b) Enhancing the conversion of a reversible reaction in a catalytic membrane reactor, (c) Preventing slip in a reaction requiring stoichiometric feeds, (d) Enhancing the rate of a multiphase reaction, (e) Energetic, thermodynamic, or kinetic coupling of two reactions run on opposite sides of a membrane, (f) Hybrid of fixed-bed reactor (PER) and selective inert membrane reactor (IMR-P) in series. 79...

Substitution Reactions on Side Chains. Because the benzyl carbon is the most reactive site on the propanoid side chain, many substitution reactions occur at this position. Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide stmeture. In a reversal of the ether cleavage reactions described, benzyl alcohols and ethers may be transformed to alkyl or aryl ethers by acid-catalyzed etherifications or transetherifications with alcohol or phenol. The conversion of a benzyl alcohol or ether to a sulfonic acid group is among the most important side chain modification reactions because it is essential to the solubilization of lignin in the sulfite pulping process (17). 

An example of a reversible reaction in the liquid phase is afforded by the esterification reaction between ethanol and acetic (ethanoic) acid forming ethyl acetate and water. Since, however, ethyl acetate undergoes conversion to acetic acid and ethanol when heated with water, the esterification reaction never proceeds to completion. 

Thus, in a reversible process that is both isothermal and isobaric, dG equals the work other than pressure-volume work that occurs in the process." Equation (3.96) is important in chemistry, since chemical processes such as chemical reactions or phase changes, occur at constant temperature and constant pressure. Equation (3.96) enables one to calculate work, other than pressure-volume work, for these processes. Conversely, it provides a method for incorporating the variables used to calculate these forms of work into the thermodynamic equations. 

For a redox reaction in an electrochemical cell the decrease in free enthalpy (- AG) is in accordance with the energy delivered by the transfer of electrons through an external circuit if this takes place in a reversible way, i.e., at a rate slow enough to allow complete attainment of equilibrium, the conversion of 1 gram mole will deliver an electrical energy of - AG = z FE. In total cell reaction mred, + n ox2 m ox, + nred2, where m81 = nS2 electrons are transfered (<5, and S2 represent the respective valence differences of the two redox systems), we have... 

One of the classic examples of a reversible reaction that is first-order in both directions is the conversion of y-hydroxybutyric acid into its lactone in aqueous solution. 

For the oxidation of SC>2(A), reaction (A) in Section 21.1, a reversible reaction, on a single plot of equilibrium conversion fA versus T (assume A to be limiting reactant), sketch each of the following three cases to show relative positions of the equihbrium curves ... 

A reversible reaction, A B, is conducted in a two stage CSTR with cooling between stages to 25 C which is also the fresh feed temperature. Inlet concentration Aq = 10, Other data follow. Compare residence times and other operating conditions for 80% conversion with and without intercooling. 

Thus, the role of zinc in the dehydrogenation reaction is to promote deprotonation of the alcohol, thereby enhancing hydride transfer from the zinc alkoxide intermediate. Conversely, in the reverse hydrogenation reaction, its role is to enhance the electrophilicity of the carbonyl carbon atom. Alcohol dehydrogenases are exquisitely stereo specific and by binding their substrate via a three-point attachment site (Figure 12.7), they can distinguish between the two-methylene protons of the prochiral ethanol molecule. 

The most important reaction of benzenesulphohydroxamic acid is its decomposition by alkalis. This decomposition does not consist in a reversal of the process of formation (i.e. conversion into benzenesul-phonic acid and hydroxylamine). An exchange of the state of oxidation takes place benzenesulphinic acid and nitroxyl are produced ... 

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