ETBE Simulations

3.4.1 ETBE Column Simulation Basis

The MTBE laboratory column that was used to validate the reactive distillation model was also used as the basis for the first simulations of an ETBE reactive distillation column. Smith's original column was assumed to contain ten ideal stages: a total condenser, two rectifying stages, one reactive stage, five stripping stages and a partial reboiler. Three reactive stages were specified for the ETBE column so that a higher loading of catalyst was possible to accommodate the lower reaction rate of ETBE compared with MTBE. The feed was specified to consist of a mixture of C4 hydrocarbons (40% isobutene, 60% 1-butene) and ethanol (at a stoichiometric excess of 5.0%) which had been pre-reacted to 80% isobutene conversion. This mixture was introduced to stage 6 of the column (immediately below the reactive stage). The feed rate and reboiler duty were based on a pilot scale column with an internal diameter of 154 mm. The three available degrees of freedom were satisfied with the following specifications:

• the column pressure was fixed at 950 kPag;

• the reboiler duty was optimised to maximise the isobutene conversion.

These specifications depart from those used for MTBE in the original catalytic distillation patent application (Smith, 1980). A higher pressure was specified for ETBE to increase the reaction zone temperature and, thereby, improve the reaction rate and reduce the catalyst requirement. A lower reflux ratio was specified to reduce the energy requirements. The original product specification (although not explicitly stated) was clearly unsuitable for ETBE synthesis due to differences in the phase behaviour which affect the product compositions. The complete simulation input is shown in Table 3.4.

Table 3.4 - ETBE Reactive Disti liai ion Column Simulation Input

Feed Specification

Column Specification

Temperature

30°C

Rectification Stages

2 (inc.condenser)

Rate

0.76 L/min

Reaction Stages

3

Composition (mole basis)

29.1% ETBE, 9.1% ethanol, 7.3% isobutene, 54.5% n-butenes

Stripping Stages

5 (inc.reboiler)

Total Stages

10

Overhead Pressure

950 kPa

Condenser

total

Composition (approx. weight basis)

43.3% ETBE, 6.1% ethanol, 6.0% isobutene, 44.6% n-butenes

Reflux Ratio

5.0

Reboiler

partial

Reboiler Duty

8.26 kW

Overall Excess Ethanol

5.0 mol%

3.4.2 ETBE Column Simulation Results

Simulations of this ETBE column were completed using both Pro/II and SpeedUp. The key results are presented in Table 3.5. The estimates of isobutene conversion and ETBE purity differ by less than 1% between the two simulators and the key indicator of bottoms temperature is only 1°C divergent. Figures 3.3 and 3.4 show the column temperature profile and composition profile as predicted by each simulator. Both sets of profiles match well with the only significant discrepancies occurring between estimates of ethanol concentration around the middle of the stripping section and the temperature profile near the top of the column.

The variation in predicted ethanol concentration in the stripping section can be attributed to the difference in vapour phase representation. Although the vapour phase non-ideality is generally low, the fugacity coefficients depart from unity when the ethanol concentration is greater than 10 mol% and can, consequently, produce a significant variation in the K factor (ratio of vapour to liquid concentrations) which affect the composition profiles. The differences in temperature profiles were deemed to be the result of the slightly different vapour pressure correlations that were used: Pro/II used SimSci database constants while published Antoine coefficients were used in the SpeedUp model. The lower isobutene conversion predicted by the SpeedUp model was attributed mostly to the kinetic model of the reaction, which produces a slightly lower isobutene conversion than the equilibrium model which was used in the Pro/II simulations. The overall agreement between the two simulators was excellent and is considered to be an indication of model validity.

Tabic 3.5 - ETBE Reactive Distillation Column Simulation Results

Property

Pro/II

SpeedUp

Condenser Temperature (°C)

74

79

Reaction Zone Temperatures (°C)

75-81

80-84

Reboiler Temperature (°C)

159

160

Isobutene Conversion (mol%)

98.3

97.4

Bottoms Composition (wt%)

96.1% ETBE,

96.5% ETBE,

2.1% ethanol,

2.8% ethanol,

1.7% butenes,

0.7% butenes,

0.06% DIB

0.04% DIB

Distillate Composition (wt%)

97.9% n-butenes,

97.6% n-butenes,

0.9% isobutene,

1.7% isobutene,

1.2% ethanol

0.7% ethanol

Condenser Duty (kW)

6.86

6.73

Reboiler Duty (kW)

8.26

8.33

Reaction Rates (mol/min)

0.16 (stage 3),

0.17 (stage 3),

0.19 (stage 4),

0.18 (stage 4),

0.15 (stage 5)

0.12 (stage 5)

Fractionation Column Reboiler
Figure 3.3 - Temperature Profile in the ETBE Column
Fractionation Column Reboiler
Figure 3.4 - Composition Profiles in the ETBE Column

Was this article helpful?

0 0

Post a comment