Chemical reactors reaction stoichiometry

Fermentation systems obey the same fundamental mass and energy balance relationships as do chemical reaction systems, but special difficulties arise in biological reactor modelling, owing to uncertainties in the kinetic rate expression and the reaction stoichiometry. In what follows, material balance equations are derived for the total mass, the mass of substrate and the cell mass for the case of the stirred tank bioreactor system (Dunn et ah, 2003). 

The design of chemical reactors encompasses at least three fields of chemical engineering thermodynamics, kinetics, and heat transfer. For example, if a reaction is run in a typical batch reactor, a simple mixing vessel, what is the maximum conversion expected This is a thermodynamic question answered with knowledge of chemical equilibrium. Also, we might like to know how long the reaction should proceed to achieve a desired conversion. This is a kinetic question. We must know not only the stoichiometry of the reaction but also the rates of the forward and the reverse reactions. We might also wish to know how much heat must be transferred to or from the reactor to maintain isothermal conditions. This is a heat transfer problem in combination with a thermodynamic problem. We must know whether the reaction is endothermic or exothermic. 

For the designer, understanding the mass balance of the plant is a key requirement that can be fulfilled only when the reactor/separation/recycle structure is analyzed. The main idea is that all chemical species that are introduced in the process (reactants, impurities) or are formed in the reactions (products and byproducts) must find a way to exit the plant or to be transformed into other species [4]. Usually, the separation units take care that the products are removed from the process. This is also valid for byproducts and impurities, although is some cases inclusion of an additional chemical conversion step is necessary [5, 6]. The mass balance of the reactants is more difficult to maintain, because the reactants are not allowed to leave the plant but are recycled to the reaction section. If a certain amount of reactant is fed to the plant but the reactor does not have the capacity of transforming it into products, reactant accumulation occurs and no steady state can be reached. The reaction stoichiometry sets an additional constraint on the mass balance. For example, a reaction of the type A + B —> products requires that the reactants A and B are fed in exactly one-to-one ratio. Any imbalance will result in the accumulation of the reactant in excess, while the other reactant will be depleted. In practice, feeding the reactants in the correct stoichiometric ratio is not trivial, because there are always measurement and control implementation errors. 

Sulphur is detrimental to the sjmthesis and trace amounts of sulphur are removed using zine oxide prior to synthesis. After the production of synthesis gas, the methanol sjmthesis requires compression to about lOObar. The methanol synthesis loop con rises a reactor, a separator and recompression of the reeyele gas. A purge gas can be used to produce power supplemented by steam raised in the methanol reactor and the coal gasifier. The crude methanol produced can be upgraded to chemical grade product by distillation. The intermediate methanol is passed into storage. The reaction stoichiometry is ... 

Stage 3. Reaction The reaction stage is the heart of a chemical manufacturing process. In the reactor the raw materials are brought together under conditions that promote the production of the desired product almost invariably, some byproducts will also be formed, either through the reaction stoichiometry, by side reactions, or from reactions of impurities present in the feed. 

The concept of independent reactions, or, more accurately, independent stoichiometric relations, is an important concept in stoichiometry and reactor analysis. The number of independent reactions indicates the smallest number of stoichiometric relations needed to describe the chemical transformations that take place and to determine all the state quantities of a chemical reactor (species composition, temperature, enthalpy, etc.). As will be seen later, the number of independent reactions also indicates the smallest number of design equations needed to describe the reactor operation. Since state quantities are independent of the path, we can select different sets of independent reactions to determine the change from one state to another. Below, we discuss the roles of independent and dependent reactions in describing reactor operations. We also describe a procedure to determine the number of independent reactions and how to identify a set of independent reactions. 

This novel reactor type has specific advantages for chemical processes requiring strict adherence of the feed rates to the reaction stoichiometry. The reaction plane within the catalyst membrane would shift in such a... 

The chemical reactor has a determinant role on both the material balance and the structure of the whole flowsheet. It is important to stress that the downstream levels in the Hierarchical Approach, as the separation system and heat integration, depend entirely on the composition of the reactor exit stream. However, a comprehensive kinetic model of the reaction network is hardly available at an early conceptual stage. To overcome this shortcoming, in a first attempt we may neglect the interaction between the reactor and the rest of the process, and use an analysis based on stoichiometry. A reliable quantitative relationship between the input and the output molar flow rates of components would be sufficient. This information is usually available from laboratory studies on chemistry. Kinetics requires much more effort, which may be justified only after proving that the process is feasible. Note that the detailed description of stoichiometry, taking into account the formation of sub-products and impurities is not a trivial task. The effort is necessary, because otherwise the separation system will be largely underestimated. 

The synthesis problem of a chemical reactor network may be defined as follows. Given the reaction stoichiometry and kinetic expressions, initial feeds, reactor targets (productivity, selectivity, flexibility), technological constraints, the optimal reactor network structure, as well as sub-optimal alternatives. The following elements should be determined ... 

Reaction mechanisms consist of a set of so-called elementary reactions that describe the chemical steps that occur at the molecular level. Once a kinetic mechanism is postulated, all the relevant chemical species should be listed these will be those that participate as reactants and/or products in at least one of the elementary reactions of the mechanism. Note that intermediate species, not appearing as reactants entering the reactor or products leaving it in the global reaction stoichiometry, are often relevant chemical species that must be considered (such as growing radicals or ionic propagating species). 

This book assumes a basic understanding of chemical reaction stoichiometry and material balances, equivalent to that administered in an undergraduate course in chemical engineering or chemistry. Knowledge of introductory chemical reactor theory is also beneficial, although it is not a requirement. 

The main problem in applying stoichiometric considerations to bioprocessing (beyond quantification in non-open-reactor systems) arises from the complex metabolic reaction network. In simple reactions stoichiometry is trivial, and complex reactions can only be handled with the aid of a formal mathematical approach analogous to the approach for complex chemical reactions (Schubert and Hofmann, 1975). In such a situation, an elementary balance equation must be set up. Due to complexity, it is not surprising that the approach first used in the quantification of bioprocesses was much simpler— the concept of yield factors Y. This macroscopic parameter Y cannot be considered a biological constant. 

Choosing a reactor for a given reaction is based on several considerations and combines reaction analysis with reactor analysis. Thus, we consider in this chapter the following aspects of reactions and reactors, much of which should serve as an introduction to chemists and a refresher to chemical engineers reaction rates, stoichiometry, rate equations, and the basic reactor types. 

An important feature of the non-stoichiometric formulation is that no information about the reaction stoichiometry is required. However, the species that the mixture is composed of must be specified. Note also that this type of chemical reaction equilibrium calculations is sometimes referred to as a Gibbs reactor simulation. 

Th e next few chapters will illustrate how the behavior of chemical reactors can be predicted, and how the size of reactor required for a givrai job, can be determined. These calculations will make use of the principles of reaction stoichiometry and reaction kinetics that were developed in Chapters 1 and 2. 

Use of Real Chemistry Real chemistry is used in many of the examples and problems. Generally, there is a brief discussion of the practical significance of each reaction that is introduced. Thus, the book tries to teach a little industrial chemistry along with chemical kinetics and chemical reactor analysis. Unfortunately, it is difficult to find real-life examples to illustrate all of the important concepts. This is particularly true in a discussion of reactors in which only one reaction takes place. There are several important principles that must be illustrated in such a discussion, including how to handle reactions with different stoichiometries and how to handle changes in the mass density as the reaction takes place. It was not efficient to deal with all of these variations through real... 

Aspen Plus includes the world s largest database of pure component and phase-equilibrium data for conventional chemicals, electrolytes, solids, and polymers. In this example the mixer, conversion reactor, compressor, cooler, flash, shortcut column mixer, splitter, throttling valve, and heater are connected as shown in Figure 9.9. In the mixer no information is required. Double click on the reactor and specify the reaction stoichiometry the fractional conversion of ethylene is 0.9. The reactor product is compressed to 20 bar and then cooled to 20°C before flashed. 

In this introductory chapter, we first consider what chemical kinetics and chemical reaction engineering (CRE) are about, and how they are interrelated. We then introduce some important aspects of kinetics and CRE, including the involvement of chemical stoichiometry, thermodynamics and equilibrium, and various other rate processes. Since the rate of reaction is of primary importance, we must pay attention to how it is defined, measured, and represented, and to the parameters that affect it. We also introduce some of the main considerations in reactor design, and parameters affecting reactor performance. These considerations lead to a plan of treatment for the following chapters. 

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