Continuous stirred tank reactor equilibrium reactions

CONTINUOUS STIRRED TANK REACTOR EQUILIBRIUM REACTION AND JACKET COOLING A < = > B ... 

Example 4.8 Chemical reactions and reacting flows The extension of the theory of linear nonequilibrium thermodynamics to nonlinear systems can describe systems far from equilibrium, such as open chemical reactions. Some chemical reactions may include multiple stationary states, periodic and nonperiodic oscillations, chemical waves, and spatial patterns. The determination of entropy of stationary states in a continuously stirred tank reactor may provide insight into the thermodynamics of open nonlinear systems and the optimum operating conditions of multiphase combustion. These conditions may be achieved by minimizing entropy production and the lost available work, which may lead to the maximum net energy output per unit mass of the flow at the reactor exit. 

Flush The flush reaction path model is analogous to the perfectly mixed-flow reactor or the continuously stirred tank reactor in chemical engineering (Figure 2.5). Conceptually, the model tracks the chemical evolution of a solid mass through which fresh, unreacted fluid passes through incrementally. In a flush model, the initial conditions include a set of minerals and a fluid that is at equilibrium with the minerals. At each step of reaction progress, an increment of unreacted fluid is added into the system. An equal amount of water mass and the solutes it contains is displaced out of the system. Environmental applications of the flush model can be found in simulations of sequential batch tests. In the experiments, a volume of rock reacts each time with a packet of fresh, unreacted fluids. Additionally, this type of model can also be used to simulate mineral carbonation experiments. 

The rates of product formation in parallel first-order steps are proportional to the fractional distance from equilibrium, which is the same for all participants. In reactions with parallel steps of different reaction orders, the selectivity to the product formed by the parallel step of higher order is higher in batch or plug-flow than in continuous stirred-tank reactors, and decreases with progressing conversion in any type of reactor. 

These are systems that exchange both energy and matter with the environment through their boundaries. The simplest chemical reaction engineering example is the continuous stirred tank reactor. These systems do not tend toward their thermodynamic equilibrium, but rather towards a state called stationary non-equilibrium state and is characterized by minimum entropy production. Open systems near equilibrium have unique stationary non-equilibrium state, regardless of the initial conditions. However far from equilibrium these systems may exhibit multiplicity of stationary states and may also exhibit periodic states. 

For a closed chemical system with a mass action rate law satisfying detailed balance these kinetic equations have a unique stable (thermodynamic) equilibrium, lim c( )=Cgq. In general, however, we shall be concerned with chemical reactions that are maintained far from chemical equilibrium by flows of reagents intoand out of a continuously stirred tank reactor (CSTR). In this case the chemical kinetic equation (C3.6.1) must be supplemented with flow terms... 

Analogously to batch distillation and the RCM, the simplest means of reactive distillation occurs in a still where reaction and phase separation simultaneously take place in the same unit. Additionally, we can choose to add a mixing stream to this still, and the overall process thus consists of three different phenomena chemical reaction, vapor liquid equilibrium, and mixing. Such a system is referred to as a simple reactive distillation setup. This setup is shown in Figure 8.1 where a stream of flowrate F and composition Xp enters a continuously stirred tank reactor (CSTR) in which one or more chemical reaction(s) take place in the liquid phase with a certain reaction rate r =f(kf, x, v) where v represents the stoichiometric coefficients of the reaction. Reactants generally have negative stoichiometric coefficients, while products have positive coefficients. For example, the reaction 2A + B 3C can... 

The reaction-diffusion dynamics of the acid autocatalytic Chlorite-Tetra-thionate (CT) reaction was thoroughly investigated (2). Like other autocatalytic reactions, the CT reaction exhibits a more or less long induction period followed by a rapid switch to thermodynamic equilibrium. In a continuous stirred tank reactor (CSTR), this reaction can exhibit bistability. One state is obtained at high flow rates or at highly alkaline feed flows, when the induction time of the reaction is much longer than the residence time of the reactor. The reaction mixture then remains at a very low extent of reaction and this state is often named the Flow (F) or the Unreacted state. In our experimental conditions, the F state is akaline (pH 10). The other state is obtained for low flow rates or for weakly alkaline feed flows, when the induction time of the chemical mixture is shorter than the residence time of the reactor. It is often called a Thermodynamic (T) or Reacted state because the reaction is almost completed in the CSTR. In our experimental conditions, the T state is acidic (pH 2). The domains of stability of these two states overlap over a finite range of parameter. 

Attractive in its simplicity, yet complex in its behavior, the Continuous Stirred Tank Reactor has, for the better part of a century, presented the research community with a rich paradigm for nonlinear dynamics and complexity. The root of complex behavior in this system stems from the combination of its open system feature of maintaining a state far from equilibrium and the nonlinear non-monotonic feedback of various variables on the rate of reaction. Its behavior has been studied under various designs, chemistries and configurations and has exhibited almost every known nonlinear dynamics phenomenon. The polymerization chemistry has especially proven fruitful as concerns complex dynamics in a CSTR, as attested to by the numerous studies reviewed in this chapter. All indications are that this simple paradigm will continue to surprise us with many more complex discoveries to come. 

Extent of reaction specified Two-phase, chemical equilibrium Multiphase, chemical equilibrium Continuous-stirred tank reactor Plug-flow tubular reactor Pump or hydraulic turbine Compressor or turbine Pressure drop in a pipe Stream multiplier Stream duplicator... 

Introduction of membranes may, in some cases, lead to more flexibility in the design and study of chemical oscillators. The continuous-stirred tank reactor (CSTR) configuration, which is often used to study chemical oscillators because it maintains reaction and product concentrations away from equilibrium [1, 2], controls the transport of reactants, intermediates, and products by fluid flow, and does not discriminate among species. Membrane selectivity between chemical species can provide a basis for selection of dynamical behaviors that are unavailable with a CSTR. 

For the selective nitrogenation of benzene with ammonia, Hodlerich et al. investigated the catalytic effect of a series of Group Vlll metals (e.g., Ru, Rh, Pd, Pt, Cu, and Ni) as the catalyst supported on carriers such as alumina, silica and zeolite in a plug slow reactor or in a continuously stirred tank reactor. Oxygen or carbon monoxide was employed respectively to shift the reaction equilibrium towards aniline formation [69]. Durante et al. developed a process for catalytic oxidative amination of aromatic hydrocarbons in which aromatic feedstock contacts with oxygen under suitable reaction conditions in the presence of a catalyst... 

The various types of reactors employed in the processing of fluids in the chemical process industries (CPI) were reviewed in Chapter 4. Design equations were also derived (Chapters 5 and 6) for ideal reactors, namely the continuous flow stirred tank reactor (CFSTR), batch, and plug flow under isothermal and non-isothermal conditions, which established equilibrium conversions for reversible reactions and optimum temperature progressions of industrial reactions. 

The elegant work of Roux, Simoyi, Wolf, and Swinney established the reality of chemical chaos (Simoyi et al. 1982, Roux et al, 1983). They conducted an experiment on the BZ reaction in a continuous flow stirred tank reactor. In this standard set-up, fresh chemicals are pumped through the reactor at a constant rate to replenish the reactants and to keep the system far from equilibrium. The flow rate acts as a control parameter. The reaction is also stirred continuously to mix the chemicals. This enforces spatial homogeneity, thereby reducing the effective number of degrees of freedom. The behavior of the reaction is monitored by measuring S( ), the concentration of bromide ions. 

The mode of reactor operation can be classified as batchwise or continuous . Batch reactions are started by filling a reactor with the reaction mixture and stopped after reaching the desired conversion. A steady state is only reached at equilibrium conversion of the reaction. A typical batch reactor is represented by the stirred tank reactor. 

When this reaction is run in an open system—a so-called continuous-flow stirred tank reactor, or CSTR (fig. 4.4), with continuous influx of reactants and outflow products and unreacted reactants—then for certain influx conditions the system may be in one of two stationary states far from equilibrium one of high I2 concentration, made visibly blue by addition of some starch, and one of low I2 concentration, a colorless solution. Measurements of bistability and chemical hysteresis in this system are shown in fig. 4.5. Bistable reaction systems have some similarities with bistable electronic switches, as pointed out some years ago by Roessler (see cited references in [1-3]). With bistable electronic switches it is possible to build an electronic computer, and now... 

This section contains several models whose spatiotemporal behavior we analyze later. Nontrivial dynamical behavior requires nonequilibrium conditions. Such conditions can only be sustained in open systems. Experimental studies of nonequilibrium chemical reactions typically use so-called continuous-flow stirred tank reactors (CSTRs). As the name implies, a CSTR consists of a vessel into which fresh reactants are pumped at a constant rate and material is removed at the same rate to maintain a constant volume. The reactor is stirred to achieve a spatially homogeneous system. Most chemical models account for the flow in a simplified way, using the so-called pool chemical assumption. This idealization assumes that the concentrations of the reactants do not change. Strict time independence of the reactant concentrations cannot be achieved in practice, but the pool chemical assumption is a convenient modeling tool. It captures the essential fact that the system is open and maintained at a fixed distance from equilibrium. We will discuss one model that uses CSTR equations. All other models rely on the pool chemical assumption. We will denote pool chemicals using capital letters from the start of the alphabet. A, B, etc. Species whose concentration is allowed to vary are denoted by capital letters... 

In the slurry process, the hydrolysis is accomplished using two stirred tank reactors in series (225). Solutions of poly(vinyl acetate) and catalyst are continuously added to the first reactor, where 90% of the conversion occurs, and then transferred to the second reactor to reach full conversion. Alkyl acetate and alcohols are continuously distilled off in order to drive the equilibrium of the reaction. The resulting PVA particles tend to be very fine, resulting in a dusty product. The process has been modified to jdeld a less dusty product through process changes... 

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