ETBE Production in a Conventional MTBE Unit

The four main process steps in the conventional MTBE process are: (a) pretreatment to remove potential catalyst poisons from the feed streams; (b) reaction (usually two stages); (c) purification of the ether product; and (d) recovery of the unreacted alcohol for recycling. This configuration is shown in Figure 6.1 and is directly applicable to ETBE production.

6.1.2.1 Pretreatment

The pretreatment stage is common to all ether synthesis processes where a catalyst is used. The design of the pretreatment equipment and its operation is dependent on the concentration of potential poisons that are expected to be present, and also the catalyst cycle life required. Typical installations include water washing, selective dehydrogenation and/or dual ion exchange resin switch beds that are cycled (one on, one off) due to the fast deactivation of the resin.

Production Etbe

Figure 6.1 - Conventional Ether Synthesis Route

isothermal

adiabatic

reactor

reactor

Figure 6.1 - Conventional Ether Synthesis Route

6.1.2.2 Reaction

The ETBE reaction is kinetically controlled at low temperatures and thermodynamically controlled at higher temperatures. Operating with only a single reactor would require a very large reactor volume to attain a satisfactory conversion (the reaction rate is always less than 0.2 mol/hr/g catalyst at reaction temperatures below 60°C), or would be restricted to relatively low isobutene conversion (the maximum stoichiometric conversion at 80°C is only 80%). Therefore, there is a considerable incentive to install two reactors in series. The First reactor should operate in the thermodynamically controlled region at high temperature in order to perform the majority of the reaction with a relatively small catalyst volume. The second reactor should operate in the kinetically controlled region to increase the overall conversion.

In an adiabatic reactor, the temperature increases sharply when reaction rates are high due to the exothermic nature of the ETBE reaction. This creates two problems: firstly, the reaction can shift from the kinetically controlled region to the thermodynamically controlled region and reduce the overall, final conversion; and secondly, localised reactor hot spots can develop which deactivate and thermally degrade the catalyst (ion exchange resin is generally unstable at temperatures above 120°C). The preferred solution to these problems is to operate the first reactor isothermally by circulating water between the reactor tubes (Miracca et al., 1994). The second reactor can still be operated adiabatically (thereby reducing the capital cost of the reactor) as much less heat is released and the subsequent temperature rise across the reactor is small (normally 10°C or less).

The design and operation of a dual reactor system should be optimised to minimise the installed reactor volume and, therefore, to maximise the catalyst cycle life. The reaction rate at any point is dependent on the temperature and composition at that point. The concentration of all components must be known, not just those taking part in the reaction. The following equations apply to a two reactor system where the first reactor is operated isothermally at T, and the second reactor is operated adiabatically with an inlet temperature of T, and an outlet temperature of T2:

Specific Weight of Catalyst Required in a Dual Reactor System

0 0

Post a comment