Chemical reactors fractional conversion

Yield-Based Reactor Fractional Conversion Reactor Combined Specification Model Well-Stirred Reactor Model Plug Flow Reactor Model Two Phase Chemical Equilibrium General Phase and Chemical Equilibrium... 

Chemical reactions do not take place instantaneously, and indeed often proceed rather slowly. In such cases, it is not practical to design the reactor for complete conversion of the limiting reactant instead, the reactor effluent emerges with some of the limiting reactant still present and is then usually subjected to a separation process to remove the unconverted reactant from the product. The separated reactant is then recycled to the reactor inlet. The fractional conversion of a reactant is the ratio... 

A little thought about equation (1-50) will convince one that X defined in this way has the same value for each species. Thus, given A, o and X, all values of A,-may be calculated directly. Remember that the molar extent of reaction X is not the same as conversion x, but it is also a number that is constrained to be between zero and one. The use of X is particularly convenient in dealing with the mathematics that describe large numbers of reactions occurring simultaneously (see R. Aris, Introduction to the Analysis of Chemical Reactors, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1965), and to problems in chemical equilibrium. Since the convention of fractional conversion is still most often encountered, however, we by and large use it in this book. 

When the feed rate is increased, first the feed fraction (reactant) will increase. This increased fraction leads to a higher rate of reaction, and consequently the production will increase as well as the heat production. Consequently, the temperature will rise, as a result of which the rate of reaction will increase, leading to a lower feed fraction. Eventually, a stationary point is found, in which the conversion has increased. As can be seen, there is an inverse response of the composition. First, it increases subsequently it decreases to a value lower than the initial value. This type of behavior is often found in chemical reactors. 

One of Cauldron Chemical Company s most profitable products is made via a homogeneous, irreversible, liquid-phase reaction A —> products using a tubular, isothermal reactor packed with noncatalytic, nonporous spheres of uniform diameter dp. The spheres are believed to promote radial mixing and heat transfer. The fractional conversion of A in the effluent frxjm the current reactor is 99%. 

Aspen is capable of modeling chemical reactions. It can handle single and multiple reactions. Material balance can be done in the stoichiometric reactor, Rsto/c from Reactors in the model library. Click on Material Streams, and connect the inlet and product streams. Click on Components and choose the components involved. Peng-Robinson EOS is selected as the thermodynamic fluid package. Doubleclick on the conversion reaction block. Click on the Specification tab enter pressure as 1 atm and temperature as 25°C. Then click on the Reactions tab, click on New and enter the components involved in the reaction, stoichiometric coefficient, and fractional conversion as shown in Figure 3.13. Close the stoichiometric windows and then double click on the inlet stream, specify temperature, pressure, flow rate, and composition. Click Run and then generate the stream table as shown in Figure 3.14. 

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. 

Given k fit) for nny reactor, you automatically have an expression for the fraction unreacted for a first-order reaction with rate constant k. Alternatively, given ttoutik), you also know the Laplace transform of the differential distribution of residence time (e.g., k[f(t)] = exp(—t/t) for a PER). This fact resolves what was long a mystery in chemical engineering science. What is f i) for an open system governed by the axial dispersion model Chapter 9 shows that the conversion in an open system is identical to that of a closed system. Thus, the residence time distributions must be the same. It cannot be directly measured in an open system because time spent outside the system boundaries does not count as residence but does affect the tracer measurements. 

Fig. 4.27 represents the velocity profiles v(r) and degrees of conversion P(r) at the exit of a reactor for different values of Da/Da and constant [A] = 0.7 mol%. At Da = 0.5 (a rather low degree of conversion at the axis of the reactor), a low-viscosity stream flows out (breaks through) into the central zone (Fig. 4.27 b, curves 1 and 2). This means that the end-product leaving the reactor is a mixture consisting of two species (fractions) with very different molecular weights, leading to the appearance of a pronounced bimodal MWD-H, which is not due to the chemical process but is a direct consequence of the hydrodynamic situation in the reactor. 

Another question is important for the safety assessment At which instant is the accumulation at maximum In semi-batch operations the degree of accumulation of reactants is determined by the reactant with the lowest concentration. For single irreversible second-order reactions, it is easy to determine directly the degree of accumulation by a simple material balance of the added reactant. For bimolecular elementary reactions, the maximum of accumulation is reached at the instant when the stoichiometric amount of the reactant has been added. The amount of reactant fed into the reactor (Xp) normalized to stoichiometry minus the converted fraction (A), obtained from the experimental conversion curve delivered by a reaction calorimeter (X = Xth) or by chemical analysis, gives the degree of accumulation as a function of time (Equation 7.18). Afterwards, it is easy to determine the maximum of accumulation XaCfmax and the MTSR can be obtained by Equation 7.21 calculated for the instant where the maximum accumulation occurs [7] ... 

Win(ts,t) = 1 - F , fraction of material entering at time t which will remain in the reactor for a duration greater than tg, and Wout (ts,t) = 1 - Fout, fraction of material leaving at time t which remained in the reactor for a duration greater than tg. From these functions, two RTD can be defined, namely E n = 9F n/3tg and out = 3Fout/ats which have all the classical properties of steady state RTD except that they vary with time. In particular, chemical conversion can be calculated in the two limits of mixing earliness (see next Section). For minimal mixedness ... 

The feedstock consists of a mixture of C8 aromatics typically derived from catalytically reformed naphtha, hydrotreated pyrolysis gasoline oran LPG aromatization unit. The feed may contain up to 40% ethylbenzene, which is converted either to xylenes or benzene by the Isomar reactor at a high-conversion rate per pass. Feedstocks may be pure solvent extracts or fractional heartcuts containing up to 25% nonaromatics. Hydrogen may be supplied from a catalytic reforming unit or any suitable source. Chemical hydrogen consumption is minimal. 

---