First-order reactions examples

Mole Balances on the (1) Bubble, (2) Cloud, and (3) Emulsion Solution to the Balance Equations for a First Order Reaction Example CD12-3 Catalytic Oxidation of Ammonia Limiting Situations... 

Example 11.1 Diffusion with First-Order Reaction Example 11.2 Diffusion of Two Species Example 11.3 Diffusion through a Film with e = 1.0... 

Even complex chemical reaction mechanisms can be separated into several definite elementary reactions, i. e. the direct electronic interaction process between molecules and/or atoms when colliding. To understand the total process B-fot example the oxidation of sulfur dioxide to sulfate - it is often adequate to model and budget calculations in the climate system to describe the overall reaction, sometimes called the gross reaction, independent of whether the process A Bis going via a reaction chain A C D E. .. Z B. The complexity of mechanisms (and thereby the rate law) is significantly increased when parallel reactions occur A X beside A- C,E- X beside E F. Many air chemical processes are complex. If only one reactant (sometime called an educt) is involved in the reaction, we call it a unimolecular reaction, that is the reaction rate is proportional to the concentration of only one substance (first-order reaction). Examples are all radioactive decays, rare thermal decays (almost autocatalytic) such as PAN decomposition and all photolysis reactions, which are very important in air. The most frequent are... 

An example of a two-stage hydrolysis is that of the sequence shown in Eq. IV-69. The Idnetics, illustrated in Fig. IV-29, is approximately that of successive first-order reactions but complicated by the fact that the intermediate II is ionic [301]... 

Fixed-time integral methods are advantageous for systems in which the signal is a linear function of concentration. In this case it is not necessary to determine the concentration of the analyte or product at times ti or f2, because the relevant concentration terms can be replaced by the appropriate signal. For example, when a pseudo-first-order reaction is followed spectrophotometrically, when Beer s law... 

In a curve-fitting method the concentration of a reactant or product is monitored continuously as a function of time, and a regression analysis is used to fit an appropriate differential or integral rate equation to the data. Eor example, the initial concentration of analyte for a pseudo-first-order reaction, in which the concentration of a product is followed as a function of time, can be determined by fitting a rearranged form of equation 13.12... 

First-Order Reactions The simplest case is a first-order reaction in which the rate depends on the concentration of only one species. The best example of a first-order reaction is an irreversible thermal decomposition, which we can represent as... 

Reaction A5.5 is not the only possible form of a first-order reaction. Eor example, the reaction... 

For combustion of simple hydrocarbons, the oxidation reactions appear to foUow classical first-order reaction kinetics sufficiently closely that practical designs can be estabUshed by appHcation of the empirical theory (8). For example, the general reaction for a hydrocarbon ... 

Sets of first-order rate equations are solvable by Laplace transform (Rodiguin and Rodiguina, Consecutive Chemical Reactions, Van Nostrand, 1964). The methods of linear algebra are applied to large sets of coupled first-order reactions by Wei and Prater Adv. Catal., 1.3, 203 [1962]). Reactions of petroleum fractions are examples of this type. 

Example 10 Reactor Size with Recycle For first-order reaction with Cq/C<) = 10 and i = 5, C ICo = 2.5. The relative reactor sizes with recycle and without are... 

A parameter such as a rate constant is usually obtained as a consequence of various arithmetic manipulations, and in order to estimate the uncertainly (error) in the parameter we must know how this error is related to the uncertainties in the quantities that contribute to the parameter. For example, Eq. (2-33) for a pseudo-first-order reaction defines k, which can be determined by a semilogarithmic plot according to Eq. (2-6). By a method to be described later in this section the uncertainty in itobs (expressed as its variance associated with cb. Thus, we need to know how the errors in fcobs and cb are propagated into the rate constant k. 

Thus, if Ca and Cb can both be measured as functions of time, a plot of v/ca vs. Cb allows the rate constants to be estimated. (If it is known that B is also consumed in the first-order reaction, mass balance allows cb to be easily expressed in terms of Ca-) The rate v(Ca) is the tangent to the curve Ca = f(t) at concentration Ca-This can be determined graphically, analytically, or with computer processing of the concentration-time data. Mata-Perez and Perez-Benito show an example of this treatment for parallel uncatalyzed and autocatalyzed reactions. 

Use of the isolation or pseudo-order technique. This approach is discussed in Chapter 2, where it was shown how a second-order reaction could be converted to a pseudo-first-order reaction by maintaining one of the reactant concentrations at an essentially eonstant level. The same method may be usefully applied to eomplex reactions. In this way, for example. Scheme XI can be studied under conditions such that it functions as Scheme IX. A corollary that must be kept in mind is that a reaction system that is observed to behave in accordance with (as an example) Scheme IX may actually be more complex than it appears to be, if an unsuspected reactant is present under pseudo-order conditions. 

Concentration-time curves. Much of Sections 3.1 and 3.2 was devoted to mathematical techniques for describing or simulating concentration as a function of time. Experimental concentration-time curves for reactants, intermediates, and products can be compared with computed curves for reasonable kinetic schemes. Absolute concentrations are most useful, but even instrument responses (such as absorbances) are very helpful. One hopes to identify characteristic features such as the formation and decay of intermediates, approach to an equilibrium state, induction periods, an autocatalytic growth phase, or simple kinetic behavior of certain phases of the reaction. Recall, for example, that for a series first-order reaction scheme, the loss of the initial reactant is simple first-order. Approximations to simple behavior may suggest justifiable mathematical assumptions that can simplify the quantitative description. 

The analysis of Example 11.3c reveals an important feature of a first-order reaction The time required for one half of a reactant to decompose via a first-order reaction has a fixed value, independent of concentration. This quantity, called the half-life, is given by the expression... 

Notice that for a first-order reaction the rate constant has the units of reciprocal time, for example, min-1. This suggests a simple physical interpretation of k (at least where k is small) it is the fraction of reactant decomposing in unit time. For a first-order reaction in which... 

One may be able to design experiments with concentration choices that reduce the system to the case of opposing first-order reactions. For Eq. (3-19), for example, this will be so if [B]o [A](j. A plot of In ([A]r - [A]f) versus time will be linear, with... 

The concentration of the reactant does not appear in Eq. 7 for a first-order reaction, the half-life is independent of the initial concentration of the reactant. That is, it is constant regardless of the initial concentration of reactant, half the reactant will have been consumed in the time given by Eq. 7. It follows that we can take the initial concentration of A to be its concentration at any stage of the reaction if at some stage the concentration of A happens to be A], then after a further time tv2, the concentration of A will have fallen to 2[AJ, after a further tU2 it will have fallen to [A], and so on (Fig. 13.13). In general, the concentration remaining after n half-lives is equal to (t)" A 0. For example, in Example 13.6, because 30 days corresponds to 5 half-lives, after that interval [A ( = (j)5 A]0, or [A]0/32, which evaluates to 3%, the same as the result obtained in the example. 

Since S/t has units of moles per volume per time and a has units of moles per volume, the rate constant for a first-order reaction has units of reciprocal time e.g., s. The best example of a truly first-order reaction is radioactive decay for example,... 

Example 1.3 Find the outlet concentration of component A from a piston flow reactor assuming that A is consumed by a first-order reaction. 

Example 1.6 Apply Equation (1.54) to calculate the mean residence time needed to achieve 90% conversion in a CSTR for (a) a first-order reaction, (b) a second-order reaction of the type A - - B — Products. The rate constant... 

Within the strictly chemical realm, sequences of pseudo-first-order reactions are quite common. The usually cited examples are hydrations carried out in water and slow oxidations carried out in air, where one of the reactants... 

Equations (2.22) and (2.23) become indeterminate if ks = k. Special forms are needed for the analytical solution of a set of consecutive, first-order reactions whenever a rate constant is repeated. The derivation of the solution can be repeated for the special case or L Hospital s rule can be applied to the general solution. As a practical matter, identical rate constants are rare, except for multifunctional molecules where reactions at physically different but chemically similar sites can have the same rate constant. Polymerizations are an important example. Numerical solutions to the governing set of simultaneous ODEs have no difficulty with repeated rate constants, but such solutions can become computationally challenging when the rate constants differ greatly in magnitude. Table 2.1 provides a dramatic example of reactions that lead to stiff equations. A method for finding analytical approximations to stiff equations is described in the next section. 

Example 3.1 Find the fraction unreacted for a first-order reaction in a variable density, variable-cross-section PER. 

Example 3.9 Solve Equation (3.46) for the case of a first-order reaction where p, q and otrans are constant. Then take limits as 0 and see what happens. Also take the limit as 0. 

Example 4.8 Find the yield for a first-order reaction in a composite reactor that consists of a CSTR followed by a piston flow reactor. Assume that the mean residence time is ij in the CSTR and ti in the piston flow reactor. 

Example 5.5 Ingredients are quickly charged to a jacketed batch reactor at an initial temperature of 25°C. The jacket temperature is 80°C. A pseudo-first-order reaction occurs. Determine the reaction temperature and the fraction unreacted as a function of time. The following data are available ... 

Example 5.7 A CSTR is commonly used for the bulk pol5anerization of styrene. Assume a mean residence time of 2 h, cold monomer feed (300 K), adiabatic operation UAgxt = ), and a pseudo-first-order reaction with rate constant... 

The Merrill and Hamrin criterion was derived for a first-order reaction. It should apply reasonably well to other simple reactions, but reactions exist that are quite sensitive to diffusion. Examples include the decomposition of free-radical initiators where a few initial events can cause a large number of propagation reactions, and coupling or cross-linking reactions where a few events can have a large effect on product properties. 

Example 8.1 Find the mixing-cup average outlet concentration for an isothermal, first-order reaction with rate constant k that is occurring in a laminar flow reactor with a parabolic velocity profile as given by Equation (8.1). 

Example 8.6 Generalize Example 8.5 to determine the fraction unreacted for a first-order reaction in a laminar flow reactor as a function of the dimensionless groups and kt. Treat the case of a parabolic velocity profile. 

Repeat Example 8.1 and obtain an analytical solution for the case of first-order reaction and pressure-driven flow between flat plates. Feel free to use software for the S5anbolic manipulations, but do substantiate your results. 

Example 9.4 Use forward shooting to solve Equation (9.15) for a first-order reaction with Pe = 16 and kt = 2. Compare the result with the analytical solution, Equation (9.20). 

Example 9.6 Compare the nonisothermal axial dispersion model with piston flow for a first-order reaction in turbulent pipeline flow with Re= 10,000. Pick the reaction parameters so that the reactor is at or near a region of thermal runaway. 

Example 11.10 Determine phase concentrations for a liquid-liquid reaction in a packed-bed reactor. The reactive component is dilute in both phases. It enters the reactor in one phase but undergoes a pseudo-first-order reaction in the other phase. All parameters are constant. 

FIGURE 11.7 A pseudo-first-order reaction in one phase with reactant supplied from the other phase. See Example 11.10. 

Example 11.12 Solve Equations (11.31) and (11.32) for the simple case of constant parameters and a pseudo-first-order reaction occurring in the liquid phase of a component supplied from the gas phase. The gas-phase film resistance is negligible. The inlet concentration of the reactive component is... 

Example 14.1 Consider a first-order reaction occurring in a CSTR where the inlet concentration of reactant has been held constant at uq for f < 0. At time f = 0, the inlet concentration is changed to Up Find the outlet response for t > 0 assuming isothermal, constant-volume, constant-density operation. 

Example 14.1 shows how an isothermal CSTR with first-order reaction responds to an abrupt change in inlet concentration. The outlet concentration moves from an initial steady state to a final steady state in a gradual fashion. If the inlet concentration is returned to its original value, the outlet concentration returns to its original value. If the time period for an input disturbance is small, the outlet response is small. The magnitude of the outlet disturbance will never be larger than the magnitude of the inlet disturbance. The system is stable. Indeed, it is open-loop stable, which means that steady-state operation can be achieved without resort to a feedback control system. This is the usual but not inevitable case for isothermal reactors. 

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