Design Basis and Expected Operating Conditions

11.2.1 Design Basis

The pilot plant was originally designed for a feed that duplicates industrial conditions: a pre-reacted mixture of C4 hydrocarbon, ethanol and ETBE (see Chapter 6). Specifically, the source hydrcarbon was assumed to contain 40% isobutene (i.e. typical of a steam cracking unit: Parra et al., 1994), a stoichiometric excess of 5.0% ethanol was nominated and the upstream reactor was assumed to operate at an outlet temperature which provided 80% isobutene conversion. The design feed rate was selected based on flooding estimates for typical small-scale packings and a column internal diameter of 155 mm. Column operation was simulated and optimised on both Pro/II and SpeedUp in order to maximise isobutene conversion without compromising the ETBE product purity. Table 11.1 indicates the design basis, the predicted results and the expected operating conditions.

The design basis compares favourably with reactive distillation equipment described in the recent literature. Two columns are being actively used at present for pilot scale synthesis of ethers: (a) in Finland, Neste have a column at Abo Akademi, which they have used to produce TAME but at a conversion of only around 30% (Bravo et al., 1995); and, (b) in Germany, Technische Universität Clausthal have a bench-top column (1600 mm x 53 mm i.d.) without a rectification section, which they have used to produce MTBE (Flato and Hoffman, 1992) and demonstrate oscillatory behaviour (Sundmacher and Hoffman, 1995).

Table 11.1- Original Design Basis

Feed Conditions

Feed rate

760 ml/min

Feed composition (wt%)

6.1% ethanol, 6.0% isobutene, 43.3% ETBE,

44.6% n-butene

Product Specification

Bottoms product rate

380 ml/min (50% vol. yield)

Overhead product rate

370 ml/min (49% vol. yield)

Distillate composition (wt%)

98.2% n-butene, 1.0% ethanol, 0.8%

isobutene, 0.1% ETBE

Bottoms composition (wt%)

96.1% ETBE, 2.0% ethanol, 1.8% butenes,

0.1% DIB

Isobutene conversion

98.5 mol%

Operating Conditions (Simulation Data)

Overhead pressure

950 kPag

Condenser duty

6.7 kW

Reflux rate

1850 ml/min

Reflux ratio

5.0

Reboiler duty

9.1 kW (including heat losses of 1.0 kW)

Condenser outlet temperature (Tl)

40°C

Overhead temperature (T2)

74°C

Temperature above reaction section (T3)

75°C

Temperature above stripping section (T4)

90°C

Temperature mid-stripping section (T5)

127°C

Bottoms temperature (T6)

159°C

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