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Catalytic Reactor: Hydrogenation of Ethylene

Overview

Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA

The hydrogenation of ethylene (C₂H₄) to ethane (C₂H₆) has often been studied as a model reduction reaction in characterizing new metal catalysts.^1-2 While supported nickel is not the most active metal catalyst for this reaction, it is active enough that reaction can take place at < 200°C.

The reaction typically involves adsorbed, dissociated hydrogen (H₂) reacting with adsorbed ethylene. In other words, both hydrogen-atoms and ethylene molecules form bonds with a metal site (here denoted "S"). The strong bonding of ethylene with S weakens the double bond sufficiently to allow hydrogen atoms to add to ethylene, forming ethane, which is not adsorbed.

The purpose of this experiment is, first, to convert raw composition measurements to limiting reactant fractional conversions.³ These conversions can then be used in a plug-flow reactor (PFR) to fit the data to a standard power-law kinetics model by the "Integral Method".³ A comparison of the experimental orders of reaction for both ethylene and hydrogen with the theoretical orders reveals in this case that the reaction is kinetically controlled rather than mass-transfer controlled.

Procedure

The system is controlled through a commercial distributed control system; there is only one operator interface.

1. Reactor Startup

To start up the real-time process history view, navigate to Start > DeltaV > Operator > Process History View, and then open CATUnitOverview. Chart scales can be compressed or expanded by clicking those buttons on the menu bar. Procedures to download data from the control system to an Excel spreadsheet are available on the computer.

Results

Nonlinear regression to obtain best estimates (using Eqs. 8 - 9) of the reaction orders m and n, and the rate constant k, can be tedious. Such a solution algorithm requires one numerical integration per data point per iteration of m and n, leading to many thousands of numerical integrations. An alternative technique that is almost as good, but much less computationally expensive, is to formulate trial pairs of m, n based on the structure of Eq. 6. Any values within the range of the theore

Application and Summary

The method described here is called the "Integral Method" in most books on kinetics and reactor design.³ While it is mathematically far more difficult to apply than differential methods, it is also better adapted to analyze the kinds of data that are easy to obtain in most pilot-scale reactor systems, where the reactant and product partial pressures and fractional conversions can vary over wide ranges. Because we are not relying upon batch reactors or low reactant conversions, such "in

References

O. Beeck, Discuss. Faraday Soc.8, 118 (1950).
J.B. Butt, AIChE J22, 1 (1976).
H.S. Fogler, "Elements of Chemical Reaction Engineering," 4^th Ed., Prentice-Hall, Upper Saddle River, NJ, 2006, Ch. 2-4; O. Levenspiel, "Chemical Reaction Engineering," 3^rd Ed., John Wiley, New York, 1999, Ch. 4-6; C.G. Hill, Jr. and T.W. Root, "Introduction to Chemical Engineering Kinetics and Reactor Design," 2^nd Ed., John Wiley, New York, 2014, Ch. 8.
B. Peri, Discuss. Faraday Soc., 41, 121 (1966).
Basic chemical kinetics - Fogler, Ch. 3, Levenspiel, Ch. 2, Hill and Root, Ch. 3.
N. Bartknecht, "Explosions: Course, Prevention, Protection", Springer-Verlag, 1981.
G.F. Froment, K.B. Bischoff and J. De Wilde, "Chemical Reactor Analysis and Design," 3^rd Ed., John Wiley, Hoboken, Ch. 11.
J.R.H. Ross, "Heterogeneous Catalysis: Fundamentals and Applications," Elsevier, Amsterdam, 2012, Ch. 8.