STOICHIOMETRY AND CONVERSION VARIABLES

Two subjects of concern to the practicing engineer involved with chemical reactions and/or chemical teactors are stoichiometry and (a topic briefly inboduced last chapter) conversion variables. The former topic is primarily concamed with the balancing of the chemical reaction of concern. The latter topic provides information on the extent of the reaction, i.e., the degree to which the reactant has been converted in the product formed. Interestingly, the two topics are interrelated, and to a certain degree, complement each other. It is for this reason that both are discussed in this chapter. 

Intertwined with these two topics is equilibrium. If a chemical reaction is conducted in which reactants are converted to fMxxlucts, the products will be formed at a rate governed (in part) by the concentration of the reactants and conditions such as temperature and pressure. Eventually, as the reactants form products and the products react to form reactants, the net rate of reaction must equal zero. At this point, equilibrium will have been achieved, How does this relate to chemical reactor analysis The chemical industry is usually concerned with the attainment of a product or jwo-ducts. [It should also be noted that the environmental industry centers on the destruction (or removal) of a reactant, often referred to as a waste—see Chapter 16.J Chemical reaction equilibrium principles allow the engineer/scientist to determine the end-products of a chemical reaction for a given set of operation conditions and initial reac-tant(s) if the final state is at equilibrium. However, from the standpoint of obtaining sufficient product(s) of economic value, a final state of equilibrium is almost always undesirable. 

The upper-case letters once again represent chemical species and the lower-case letters represent stoichiometric coefficients, Taking species A as the basis of calculation, the reaction expression is divided through by the stoichiometric coefficient of species A in order to arrange the reaction expression in the following form  

Chemical Reactor Analysis and Applications for the Practicing Engineer. By Louis ITieodorc (0 2012 John Wiley Sons, Inc. Published 2012 by John Wiley Sons, Inc. 

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This chapter serves to introduce the general subject of stoichiometry and conversion variables, which in a very real sense, is an extension of the material discussed in the previous chapter. To simplify the presentation to follow, some of the textual matter and illustrative examples will focus on combustion reactions because of the author s experience in this field. Topics to be reviewed include ... 

CHAPTERS STOICHIOMETRY AND CONVERSION VARIABLES 7. Assuming an ideal gas,... 

CHAPTER 5 STOICHIOMETRY AND CONVERSION VARIABLES The rate expression is now... 

Stoichiometry Conversion Variables Volume Correction Factor Yield and Selectively... 

In the following, the mass balance for substrate S and hydrogen in the liquid phase are written, considering that assumptions 1 to 4 hold. For a more illustrative view, mass balance is proposed with the concentrations as variables. In general, if the reaction stoichiometry is known, then the conversion number is used as the unified single variable. 

The mass flow of the conversion gas, its molecular composition, temperature and stoichiometry, are a complex function of volume flux of primary air, primary air temperature, type of solid fuel, conversion concept, etc. Several workers have tried to mathematically model these relationships, which are commonly referred to as bed models [12,33,14,51,52]. It is an extremely difficult task to obtain a predictive bed model, which is discussed in the introduction of this ew. The review of the thermochemical conversion processes below will outline the complex relationships between these variables and their effect on the conversion gas in sections B 4.4-B 4.6. 

The most important design variables of the conversion system are the mass flow and the empirical stoichiometry of the conversion gas. The conversion gas is the primary product of the thermochemical conversion process in the conversion system. The... 

The composition of poplar wood was usedasamodel for the feedstock composition however, as used in this simulation, the poplar is modeled as consisting of only cellulose, xylan, and lignin, with compositions of 49.47, 27.26, and 23.27%, respectively. Laboratory results for carbonic acid pretreatment are relatively scarce, so for the purpose of this comparative study, stoichiometry of pretreatment reactions was assumed to be equal to those used in the comparison model (3) cellulose conversion to glucose 6.5% xylan conversion to xylose 75 and lignins solubilized 5%. Thus, economic comparisons made with this model assess different equipment and operating costs but not product yields. For the successful convergence of the carbonic acid model, the simulation required initial specification of several variables. These variables included initial estimates for stream variables and inputs for the unit operation blocks. 

The identity of the product may be confirmed by its infrared and n.m.r. spectra, since the stoichiometry of the dioxanate is variable. The infrared spectrum shows prominent bands at ca. 2440, 2400, 2370, 2320, 2130, 2080, 1140, and 1010 cm."1. The XH n.m.r. of the product in D20 gave the expected decet for the [B3H8] ion with Jb—h = 33 Hz. Conversion to the cesium salt gives a moderately stable material of constant composition suitable for analysis ... 

Molar flow rates are mostly useful because using moles instead of mass allows you to write material balances in terms of reaction conversion and stoichiometry. In other words, there are a lot less unknowns when you use a mole balance, since the stoichiometry allows you to consolidate all of the changes in the reactant and product concentrations in terms of one variable. This will be discussed more in a later chapter. 

Only the concentrations of reactants and products appear in the final rate equation. The concentrations of active centers were eliminated in Step 3. All of the concentrations in the final rate equation are related through stoichiometry. For a single reaction, each concentration in the rate equation can be expressed in terms of one stoichiometric variable, e.g., extent of reaction, fractional conversion of a reactant, or the concentration of a single species, e.g., the concentration of the limiting reactant. 

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