The MTBE reactive distillation process was patented several decades ago, and the process was widely used in the petroleum industry. Many reactive columns were installed around the world to produce MTBE, which was blended into gasoline. This process was probably the largest application of reactive distillation in terms of the number of columns and total production capacity. Because MTBE presents groundwater contamination problems, it is gradually being phased out of use in gasoline.
The reactive distillation column is essentially a ternary system with inerts. The liquidphase reversible reaction is isobutene + MeOH , MTBE (9.1)
Reactive Distillation Design and Control. By William L. Luyben and Cheng-Ching Yu Copyright # 2008 John Wiley & Sons, Inc.
The heavy component is MTBE, which leaves the reactive distillation column in the bottoms. The isobutene feed is contained in a mixed C4 stream from an upstream refinery unit. This stream contains a number of other C4 hydrocarbons because of the difficulty of separating the various components with very similar relative volatilities. In the numerical example, we assume that n-butene is the chemically inert component. Most of this inert component leaves in the distillate stream.
After the isobutene is consumed by the reaction, the resulting mixture of n-butene, methanol, and MTBE has vapor-liquid equilibrium properties that are nonideal with two binary azeotropes. At a pressure of 11 bar, which is the operating pressure of the column, the pure component boiling points are 76.1, 140.6, and 152.8 °C, respectively.
The two binary azeotropes at 11 bar are homogeneous and minimum boiling. The first is 89.3 mol% n-butene and 10.7 mol% methanol at 73.2 °C. The second is 42.3 mol% MTBE and 57.7 mol% methanol at 132.4 °C. The UNIQUAC physical properties model is used in the numerical example presented in this chapter.
The reaction is reported to be equilibrium limited, not kinetically limited, and most authors assume chemical equilibrium on each reactive tray. For example, Huang and Wang1 use the following equation for the temperature dependence of the chemical equilibrium constant:
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