Chemical equilibrium with applications

CEA Chemical Equilibrium with Applications. Computer Program for Calculation of Complex Chemical Equilibrium Compositions and Analysis, Gordon, S. and McBride, B. J. NASA Lewis Research Center, NASA Report... 

Denbigh, K. G. (1966). The Principles of Chemical Equilibrium with Applications in Chemistry and Chemical Engineering. Cambridge Univ. Press, London and New York. 

G2. K. G. Denbigh, The principles of chemical equilibrium with applications in chemistry and chemical engineering, Cambridge University Press, 4th ed., 1981, ISBN 0521236827. 

Chemical Equilibrium with Applications (CEA) is an expert system developed by NASA for determining compositions in chemical equilibria for deriving thermodynamic properties of a chemical system in propulsion jet engines. 

NASA Chemical Equilibrium with Applications (CEA) downloadable software, www.grc.nasa.gov/WWW /CEAWeb (accessed 15 May 2014). 

NASA/Glenn s CEA2 (chemical equilibrium with applications) data base (Ref. 1)... 

In this mode of separation, active compounds that form ion pairs, metal complexes, inclusion complexes, or affinity complexes are added to the mobile phase to induce enantioselectivity to an achiral column. The addition of an active compound into the mobile phase contributes to a specific secondary chemical equilibrium with the target analyte. This affects the overall distribution of the analyte between the stationary and the mobile phases, affecting its retention and separation at the same time. The chiral mobile phase approach utilizes achiral stationary phases for the separation. Table 1 lists several common chiral additives and applications. 

At first sight, this may seem like a very simple question with a simple answer, yes. The problem is, of course, that many organic and inorganic ions are in chemical equilibrium with neutral species. Depending on the position of this equilibrium -and the latter is a function of temperature and pressure - even a pure ionic liquid may contain significant amounts of neutral molecules. Of course, this will greatly infiuence all properties of the substance. Volatility, viscosity, chemical reactivity etc. will greatly differ from the hypothetical mixture of the individual ions if free molecular species such as amines, phosphines, Bronsted-acids or acid esters form as neutral molecules in an equilibrium reaction under the conditions of the ionic liquid application. 

Many reactions of industrial importance are limited by chemical equilibrium, with partial conversion of the limiting reactant and, with the rate of the reverse reaction equal to the rate of the forward reaction. For a specified feed composition and final temperature and pressure, the product composition at chemical equilibrium can be computed by either of two methods (1) chemical equilibrium constants (K-values) computed from the Gibbs energy of reaction combined with material balance equations for a set of independent reactions, or (2) the minimization of the Gibbs energy of the reacting system. The first method is applicable when the stoichiometry can be specified for all reactions being considered. The second method requires only a list of the possible products. 

As mentioned in Section 7.1, Langmuir s monolayers are not in chemical equilibrium with the solution, and, as a consequence, the Gibbs adsorption equation is not applicable to such monolayers. However, relations between n, T, and T are completely determined by the number of molecules, and the interactions between them, in the monolayer, irrespective of the way the monolayer has been formed. Equations of state are therefore identical for Gibbs and Langmuir s monolayers. 

The most frequent application of phase-equilibrium calculations in chemical process design and analysis is probably in treatment of equilibrium separations. In these operations, often called flash processes, a feed stream (or several feed streams) enters a separation stage where it is split into two streams of different composition that are in equilibrium with each other. 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

A catalyst cannot change the ultimate equilibrium point set by thermodynamics, but it can affect the rate at which this point is approached. However, it can facilitate approach to equilibrium with respect to a desired reaction while not influencing the rates of other less desirable reactions. In optimizing yields of desired products, chemical engineers are very concerned with the selectivity or specificity of a catalyst. For commercial applications, selectivity is often more important than activity per se. 

Fowle and Fein (1999) measured the sorption of Cd, Cu, and Pb by B. subtilis and B. licheniformis using the batch technique with single or mixed metals and one or both bacterial species. The sorption parameters estimated from the model were in excellent agreement with those measured experimentally, indicating that chemical equilibrium modeling of aqueous metal sorption by bacterial surfaces could accurately predict the distribution of metals in complex multicomponent systems. Fein and Delea (1999) also tested the applicability of a chemical equilibrium approach to describing aqueous and surface complexation reactions in a Cd-EDTA-Z . subtilis system. The experimental values were consistent with those derived from chemical modeling. 

In many reacting flows, the reactants are introduced into the reactor with an integral scale L that is significantly different from the turbulence integral scale Lu. For example, in a CSTR, Lu is determined primarily by the actions of the impeller. However, is fixed by the feed tube diameter and feed flow rate. Thus, near the feed point the scalar energy spectrum will not be in equilibrium with the velocity spectrum. A relaxation period of duration on the order of xu is required before equilibrium is attained. In a reacting flow, because the relaxation period is relatively long, most of the fast chemical reactions can occur before the equilibrium model, (4.93), is applicable. 

At a given temperature, a reaction will reach equilibrium with the production of a certain amount of product. If the equilibrium constant is small, that means that not much product will be formed. But is there anything that can be done to produce more Yes, there is— through the application of Le Chatelier s principle. Le Chatelier, a French scientist, discovered that if a chemical system at equilibrium is stressed (disturbed) it will reestablish equilibrium by shifting the reactions involved. This means that the amounts of the reactants and products will change, but the final ratio will remain the same. The equilibrium may be stressed in a number of ways changes in concentration, pressure, and temperature. Many times the use of a catalyst is mentioned. However, a catalyst will have no effect on the equilibrium amounts, because it affects both the forward and reverse reactions equally. It will, however, cause the reaction to reach equilibrium faster. 

The chapter starts with a brief review of thermodynamic principles as they apply to the concept of the chemical equilibrium. That section is followed by a short review of the use of statistical thermodynamics for the numerical calculation of thermodynamic equilibrium constants in terms of the chemical potential (often designated as (i). Lastly, this statistical mechanical development is applied to the calculation of isotope effects on equilibrium constants, and then extended to treat kinetic isotope effects using the transition state model. These applications will concentrate on equilibrium constants in the ideal gas phase with the molecules considered in the rigid rotor, harmonic oscillator approximation. 

Since natural waters are generally in a dynamic rather than an equilibrium condition, even the concept of a single oxidation-reduction potential characteristic of the aqueous system cannot be maintained. At best, measurement can reveal an Eh value applicable to a particular system or systems in partial chemical equilibrium and then only if the systems are electrochemically reversible at the electrode surface at a rate that is rapid compared with the electron drain or supply by way of the measuring electrode. Electrochemical reversibility can be characterized... 

---