Process configurations for the production of the 2-methoxy-2,4,4-trimethylpentane—a novel gasoline oxygenate

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Abstract

A comparison of various process configurations was made for the production of 2-methoxy-2,4,4-trimethylpentane, which is a promising gasoline component owing low solubility to water and high octane value. The ether is synthesised by reacting methanol with a mixture of 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene, which are obtained in large scale from the dimerisation of isobutene. The reaction equilibria were determined experimentally at temperature range 323–383 K in liquid phase using a commercial cation exchange resin Amberlyst 35 as catalyst. A first order kinetic model was developed and utilised in the process simulations. The production of the ether was found to be inefficient in a once through tubular reactor, because the conversion is strongly limited by the reaction equilibrium. Considerably higher conversion and more economical process are obtained by using reactive distillation in the process schema.

Introduction

Tertiary ethers have had a major role in gasoline development during the last two decades. The ethers–2-ethoxy-2-methylpropane (ETBE), 2-methoxy-2-methylbutane (TAME) and especially 2-methoxy-2-methylpropane (MTBE)—were the most economical solutions to increase the octane level of gasoline, to promote cleaner burning of gasoline and thus decreasing harmful emissions from vehicles. Later, however, because of leakage of gasoline storage tanks, MTBE was detected in ground water. This led to the ban of MTBE from gasoline pool by the end of year 2002 in California [1].

Refineries in the USA and Canada are interested in new alternatives for the existing MTBE plants and isobutene (2-methylpropene) feed stocks. Some companies have announced new process configurations to produce high octane gasoline components from the isobutene [2], [3], [4]. In these processes isobutene is first dimerised to isooctenes (mainly 2,4,4-trimethyl-1-pentene (TMP-1) and 2,4,4-trimethyl-2-pentene (TMP-2)), which can thereafter be hydrogenated to isooctane (2,2,4-trimethylpentane), as shown in Fig. 1 as the upper reaction route. n-Butenes have also been studied as a potential feed for the dimerisation [5].

The isooctene process neglects the oxygenate raw material methanol that would be available in large scale particularly if MTBE production is reduced. The alkenes TMP-1 and TMP-2 have a double bond attached in a tertiary carbon and are, therefore, reactive in etherification [6]. The reaction schema is illustrated in Fig. 2. The resulting ether, 2-methoxy-2,4,4-trimethyl pentane (C8ME) has high octane ratings reported in literature: a motor octane number of 147 [7], the blending motor octane number 99 and the blending research octane number 110 [8]. So, the etherification of alkenes can increase the octane value, which would also improve the quality of the product [9].

Ethers having nine carbons have not been studied in detail so far, but more is known about the properties of tertiary ethers consisting of up to seven carbons [10]. Some trends can be found in the properties of these ethers as a function of molecular size: e.g. vapour pressure and water solubility decrease with the increasing molecular size. The water solubility of C8ME is 0.014 wt.%, which is significantly lower compared with that of MTBE (4.3 wt.%). Thus the solubility problems encountered with MTBE are not expected this ether [8]. Lower water solubility reduces the risk for ground water contamination. The low vapour pressure is advantageous for gasoline blending. The higher boiling point enables replacement of some higher boiling components like aromatics, the amount of which should be reduced according to the tightening regulations. Thus heavier ethers are potential fuel components. In addition, the well-known benefits of implementation of oxygen into gasoline pool—reduction of hydrocarbon and carbon monoxide emissions and increase of octane rating—can be obtained with all these ethers.

The rate of etherification reaction slows down and the reaction equilibrium decreases as the size of the alkenes increases [6], [11], [12], which makes the process design demanding. The traditional reactors are not efficient in the production and more innovative reactor configurations are required. In this paper we report the results of the simulations of various process configurations.

Section snippets

Equipment

The reaction equilibrium measurements of the etherification were carried out in a 80 cm3 stainless steel batch reactor equipped with a magnetic stirrer. The temperature (323–383 K) was controlled by immersing the reactor in a water or oil bath. The reaction pressure was maintained at 0.8 MPa with nitrogen to ensure that the reaction mixture remained in the liquid phase. The catalyst, 1–1.5 g, was placed in a metal gauze basket. The samples from the reaction mixture were taken manually via a

Reaction equilibrium

The reaction equilibrium constants KC8ME1 and KC8ME2 for the etherification of TMP-1 and TMP-2 were calculated in terms of activities, ai=xiγi, where the activity coefficients γi have been calculated by the UNIFAC method [13]:KC8MEi=γC8MEγMeOHγTMP&0x00AD;ixC8MExMeOHxTMP&0x00AD;i

The enthalpy change for the reaction, ΔHRi, was assumed to be independent of the temperature in the investigation range. Thus, the temperature dependence of the equilibrium constant was determined by the Eq. (2).lnKC8MEi=

Conclusions

The applicability of a few process configurations for the production of new promising gasoline oxygenate, 2-methoxy-2,4,4-trimethyl pentane were studied. The etherification process was simulated with a simple first-order kinetic model. The rate of etherification reaction of methanol with 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene was found to be significantly slower compared with the formation rates of lighter ethers, which are used in gasoline. The reaction was strongly

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