L KA aEt0H

1323.1 T

ETBE

krate=L209xl0"exp[

The dimensation of isobutene to form di-isobutene (DIB) is an unavoidable side reaction in both the ETBE and MTBE reaction systems:

This reaction is also equilibrium limited and an expression for the equilibrium constant of the dimensation has been estimated from the free energies of formation (Columbo et al., 1983):

In K - 95.2633 + 5819.8644/T - 17.2 In T - 0.0356 T (3.10)

If any water is present in the reaction environment, one further, undesirable reaction (the hydration of isobutene to form of isobutanol) is also possible:

The ETBE rate equations clearly show that ethanol has a retarding effect on the reaction. However, in practice, some ethanol excess is required to limit the side-reactions involving isobutene. The LHHW reaction model predicts a large adsorption equilibrium constant for ethanol that implies that, at ethanol excesses of 4 mol% and above, the catalyst surface is largely covered with ethanol. Under these conditions, the dimensation and oligomensation of isobutene are essentially eliminated (Kitchaiya and Datta, 1995).

The reaction rate also depends on the activity of the catalyst that is susceptible to both deactivation (slow ageing) and poisoning (fast ageing). Poisoning is potentially a senous problem as water and, especially salts, neutralise active catalyst sites. Deactivation occurs over a much longer period and is accelerated by thermal degradation caused by hot spots due to inadequate mixing. In situ regeneration has generally been unsuccessful and a regular catalyst changeover is requiied for most reactors (Flato and Hoffman, 1992).

3.1.2 Physical Properties

The reaction components have distinctly different physical properties and form a very non-ideal mixture, The most relevant properties are summarised in Table 3.1 (SimSci, 1994; Furzer, 1994; Brockwell et al., 1991; Lide, 1995). In an industrial context, isobutene is only likely to be available as a component within a mixture of other hydrocarbons (mostly C4s) but for most practical purposes, the physical properties of the mixture can be lumped and approximated with the properties of isobutene. While both ethanol and ETBE can be stored at atmospheric pressure, neither isobutene nor mixtures of the three components can unless the percentage of ETBE in the mixture is substantial.

Table 3.1 - Key Physical Properties of the Reaction Components

Property

Ethanol

Isobutene

ETBE

Molecular Weight

46

56

102

Specific Gravity

0.795

0.600

0 746

Normal Boiling Point (°C)

78

-7

73

Boiling Point @ 1000 kPa (°C)

155

75

174

Blending RVP (kPa)

122

440

27

Specific Heat (kJ/kg)

2.46

1.27

2.10

Octane Rating ((R™+MOf%)

115

n/a

111

Energy Content (MJ/kg)

26.7

44.6

36.3

The values of octane are approximate (literature reports vary but ETBE is generally claimed to have an average octane rating one number higher than MTBE: Furzer, 1994; Brockwell et al. 1991; Piel and Thomas, 1990; Unzelman, 1989) and vary with the composition of the mixture. Knock engine testing is required to confirm the octane properties of a mixture. An octane rating is not generally cited for isobutene due to its high volatility but a high blending value (approximately 100) is anticipated based on iso-olefin trends.

3.1.3 Phase Behaviour

The combination of an alcohol, an olefin and an ether forms a highly non-ideal liquid phase and azeotropes have been detected experimentally in the ETBE reaction system and other similar mixtures (Gmehling, 1994). The UNIFAC model predicts the presence of these azeotropes and was found to be accurate in estimating their compositions. This is useful to expand on the limited experimental data and to determine the effect of pressure on the azeotropic compositions. Table 3.2 describes the phase behaviour of three binary pairs in the ETBE system. Importantly, the azeotropes between ethanol and butenes appear to only exist at high pressure. All of the azeotropes are predicted to be homogenous.

Table 3.2 - Compositions of Binary Azeotropes in (he ETBE Reaction System

Binary Pair ethanol-isobutene

Ethanol Cone, at 0 kPag (mol%)

Ethanol Cone, at 950 kPa (mol%)

Etbanol Cone, at 1400 kPa (mol%)

Exp't n/a

UNIFAC

Exp't n/a

UNIFAC

Exp't 0.94%

UNIFAC 1.25%

ethanol-1 -butene

n/a

-

n/a

-

n/a

1.45%

ethanol-ETBE

37%

38%

n/a

59%

n/a

66%

The phase behaviour of the MTBE system departs from the above in several important aspects: the azeotropes between methanol and the various butenes exist at nearly all pressures; these azeotropes contain a much larger percentage of methanol (7-15 mol%); and UNIFAC predicts phase splits between methanol and the various butenes at atmospheric pressure. Such a phase split has not been reported but direct experimentation is made difficult by the low temperatures required. However, this uncertainty is relatively unimportant as the model is still consistent with industrial experience of homogeneity at pressure and the system can be easily constrained to a single liquid phase by specifying vapour-liquid-equilibrium (VLE) rather than vapour-liquid-liquid-equilibrium (VLLE).

The MTBE system has been modelled with other thermodynamic packages (e.g. the Wilson equation, Bravo et al., 1993; and the UNIQUAC method, Abufares and Douglas, 1995) but the UNIFAC method is recommended, regardless of the accuracy of the predictions of any liquid phase splits, for its superior accuracy in predicting azeotropic compositions. Although the choice of VLE or VLLE makes little difference for the ETBE system, a VLE model is recommended for MTBE as the azeotropic compositions are predicted much more precisely than with a VLLE model.

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