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Optimization of the ETBE (ethyl tert-butyl ether)

production process

Eliana Weber de Menezes, Renato Catalua

Department of Physical Chemistry, Institute of Chemistry, Federal University of Rio Grande do Sul, Avenida Bento Gonalves, 9500,

CEP-91501-970 Porto Alegre, RS, Brazil

A R T I C L E I N F O A B S T R A C T

Article history:

Received 14 August 2007

Received in revised form 14 May 2008

Accepted 14 May 2008

The synthesis of ETBE (ethyltert-butyl ether) from the reaction of ethanol with isobutene is

an exothermic reaction of equilibrium. To increase the conversion of isobutene requires

operating the reaction system at low temperatures and with excess ethanol in order to

displace theequilibriumtowards theproducts. ETBE andethanol form an azeotropicmixture

which hinders therecycling of nonreacted ethanol in theprocess. Thepurpose of this work is

to optimize the synthesis of ETBE eliminating the introduction of water into the system to

break the ETBE/Ethanol azeotrope. The production process model proposed here eliminates

the recycling of ethanoland suggests the use of the azeotropic mixture (ETBE/Ethanol) in the

formulation of gasolines. The direct use of the azeotrope in the formulation of automotive

gasolines reduces the implementation and production costs of ETBE.

2008 Elsevier B.V. All rights reserved.

Keywords:

ETBE

Azeotropic mixture (ETBE/EtOH)

Gasoline

1. Introduction

Oxygenated compounds are known to be important as

components in the formulation of automotive gasolines,

not only as enhancers of gasoline octane ratings[1,2]but also

as reducers of carbon monoxide (CO) and unburned hydro-

carbons (HC), minimizing the emission of volatile organic

compounds[36]. The introduction of a minimal percentage

of oxygenated compounds in the formulation of gasolines

has been required by law in most countries which have areas

of low air quality.

Alcohols and ethers are the oxygenated compounds most

commonly used as additives in automotive gasolines, since theypossess the desired characteristics of octane ratings and CO

emission reductions [7]. Some countries prefer ethers rather than

alcohols dueto their mixing characteristics, such as low volatility

and compatibility with the hydrocarbons of gasoline [8,9].

Alcohols are substantially more polar than the ethers and

hydrocarbons of gasoline, and may cause phase separation in

the presence of a small amount of water in the gasoline storage

and distribution system[10,11].

Tertiary ethers offer advantages over ethanol due to their low

Reid vapor pressure (RVP), low latent heat of vaporization, and

low solubility in water[7,12]. The most commonly used of these

ethers are MTBE and ETBE. It is worth pointing out that ETBE

is considered semi-renewable, since the raw material for its

production ethanol is derived from biomass[7].

ETBE is produced by reacting a C4 stream containing

isobutene with ethanol over an ion-exchange resin catalyst. On

an industrial scale, the conventional process of ETBE synthesis

consists basically of the following stages: pretreatment of the C4hydrocarbon feed flow, reaction, purification, and recovery of

nonreacted products[13,14]. Nowadays, to minimize implemen-

tation and operating costs, reactive distillation (also calledcatalytic distillation) is proposed as an alternative route for

ETBE synthesis, offering high conversion and low implementa-

tion/operating costs in comparison with conventional synthesis

[1517]. The reactive distillation process combines the reaction

and purification stages in a single unit of the process[18].

In the ETBE production process, nonreacted ethanol forms an

azeotropic mixture with ETBE, which cannot be separated by

distillation. The process of ETBE purification occurs through the

F U E L P R O C E S S I N G T E C H N O L O G Y 8 9 ( 2 0 0 8 ) 1 1 4 8 1 1 5 2

Corresponding author.Tel.: +55 51 3308 6306; fax: +55 51 3316 7304.E-mail address:rcv@ufrsgs.br(R. Catalua).

0378-3820/$ see front matter 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.fuproc.2008.05.006

a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

w w w . e l s e v i e r . c o m / l o c a t e / f u p r o c

mailto:rcv@ufrsgs.brhttp://dx.doi.org/10.1016/j.fuproc.2008.05.006http://dx.doi.org/10.1016/j.fuproc.2008.05.006mailto:rcv@ufrsgs.br

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introduction of water into the system and involves the separa-

tion of the ETBE, the C4 hydrocarbon mixture, ethanol and water.

The introduction of water into thepurification process augments

the costs of implementation and production of ether. For this

reason, some technologies use pervaporative separation of the

ethanol from the ETBE/alcohol mixture through special mem-

branes[1923].

It has been demonstrated that the azeotropic mixture (ETBE/

ethanol) is less volatile than ethanol and that its octane rating is

higher and its production cost lower than ETBE, thus presentingpromising potential for application in gasoline formulations[8].

The synthesis model proposed here eliminates the recycling

of ethanol and suggests the use of the azeotropic mixture (ETBE/

ethanol) as a direct additive in the formulation of automotive

gasolines.

2. Experimental

2.1. Reaction system and purification

2.1.1. Reaction

The ETBE production process was carried out in a flow, using

as reagents a mixture of C4 hydrocarbons with 36 mol% ofisobutene (i-C4) and 99.5 mol% of anhydrous ethyl alcohol.

Table 1 presents the mean molar composition of the

industrial load of C4 hydrocarbons. Amberlyst 15 resin

was used as catalyzer. The schematic diagram in Fig. 1

depicts the production process.

The reactionsystem consists of an adiabatic fixed bed reactor

fed by two cylinders, one containing the reagent ethanol (EtOH)

and the other the C4hydrocarbon mixture under a pressure of

20 bar. The composition of the reagent mixture and the reaction

system are controlled by two electronic liquid flow gauges, one

for ethanol, with a capacity of 405 mL/h, and the other for the C4hydrocarbons mixture, with a capacity of 1380 mL/h. These

gauges allow the EtOH/i-C4ratio and space velocity to be set asdesired. The reagent mixture is heated and fed into the reactor's

lower portion.The temperatureof thecatalytic streambedandat

the exit is monitored with thermocouples inside and outside the

reactortoensurethe reactionis in thesteady state condition.The

reactor's effluent is flashed into a distillation column under

Table 1Mean molar composition of the industrialhydrocarbons load of the C4cut

Compounds Concentration (molar%)

Isobutane 1.7

n-butane 7.6

2-transbutene 16.9

1-butene 33.2

Isobutene 36.0

2-cisbutene 4.6

Fig. 1 Flowchart of the ETBE synthesis. (1) Nitrogen; (2) and (3) Reagents; (4) Adiabatic fixed bed reactor; (5) Distillation column.

PI: Pressure Indicator; TI: Temperature Indicator; TR: Temperature Recorder; TIC: Temperature Indicator Controller; FIC: Flow

Indicator Controller.

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Fig. 2 Isobutene conversions as a function of the temperature at the exit from the reactor, considering the distinct EtOH/i-C4molar ratios (MR) and a space velocity of 0.52 h1.

atmospheric pressure, separating the C4 hydrocarbons into

vapor phase and the ethanol, ETBE and byproducts into liquid

phase.The isobuteneconversion was evaluatedas a function of

the composition of the C4 hydrocarbons in the vapor phase.

The concentration of liquid C4 at the bottom of the column is

negligible.

The conversion of isobutene was determined by gas chroma-

tography from the molar balance in the reactor. The calculation

methodology considered normalization of the isobutene in

relation to the saturated hydrocarbons (isobutane and n-butane),

which are considered inert and do not participate in the reaction.

The conversion of isobutene was calculated according to Eq. (1)):

The composition of the C4 hydrocarbon (reagent) load and the

C4in the vapor phase (reaction products) was determined by gas

chromatography using a thermal conductivity detector (GC-TCD,

Shimatzu 17A), a plottype fused silica capillary column with a

stationary phase of Al2O3/Na2SO4(50 m0.53 mm) and Helium

(5.0) as carrier gas. The analytical conditions were: isotherm at

40C for 20min, a heating ramp-up of20 C/minup to190C, and

holding at this temperature for 10 min. The injector and detector

temperatures were 180 C and 220 C.The splitratio was 1:20 and

the volume of injected sample was 20 L.

The conversions obtained in the reaction system were

evaluated as a function of the EtOH/i-C4molar ratio (MR) in the

load and the temperature at the reactor's exit. The molar ratios

evaluated were 1.0, 1.1, 1.2, 1.3, 1.4 and 1.5. The temperature

interval of the reaction was 48 C to 88 C, using a single space

velocity of 0.52 h1, which was chosen on the basis of previous

experiments, in order to ensure sufficient residence time of the

reactants in the catalytic stream bed to enable the products

leaving thereactor to meetthe equilibriumcondition. Thisspace

velocity corresponds to the minimum limit of operation of the

flow control of the reactants using a 340 cm3 reactor.

2.1.2. Purification of the reactor's effluent

The effluent from the reaction system was fractionated in a

distillation column to remove the light compounds (C4 excess

hydrocarbons of the reaction). In this first column that

receives the effluent from the reactor, the bottom flow

consists of a mixture (ETBE/EtOH) together with secondary

products of the reaction (tert-butyl alcohol and C8 hydro-

carbons). The product of this bottom flow column is directed

to a second distillation column (under identical conditions as

those of the first). The bottom flow consists of ETBE with a

high degree of purity, together with byproducts of the

reaction, while the top flow consists of the azeotropic ETBE/

EtOH mixture.

The composition of the bottom flow was analyzed by gas

chromatography with flame ionization detector (CG-FID, Varian

39XL), using a fused silica capillary column (CP sil PONA CB) with

a 100% dimethylpolysiloxane active phase (100 m 0.25 mm) and

Helium (5.0) as a carrier gas. The analytical conditions were

isotherm at40 C for 20 min,a heating ramp-up of 5 C/min up to

190 C, and holding at this temperature for 10 min. The injector

and detector temperatures were 250 C and 300 C, respectively.

The initialsplit ratio was of 1:300, passing on to 1:20 after 2 min of

analysis. The volume of injected sample was 20 L.

3. Results and discussion

3.1. Evaluation of the parameters of the reactional system

Fig. 2presents the isobutene conversion profiles adjusted as a

function of the temperature at the exit from the reactor and the

EtOH/i-C4molar ratios of the feed. The conversions shown here

represent the results of three consecutive assays for each

reaction condition evaluated.

iC4 conversion Normalization of the iC4 load Normalization of the iC4 reactors exit

Normalization of the iC4 load 100 1

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As indicated inFig. 2, at a space velocity of 0.52 h1, the

reaction attains the maximum conversion in the temperature

interval of 61 to 67 C. Because it is a reversible and exothermic

reaction, the increase in temperature exerts a negative effect

on the displacement of the chemical equilibrium; hence, the

higher the temperature the lower the conversion of isobutene

in equilibrium. At temperatures of 50 to 61 C,the conversion is

directly proportional to the increase in temperature due to the

faster reaction. At temperatures below 61 C, the conversion is

kinetically controlled while at higher temperatures, the con-

version is controlled by thermodynamic equilibrium.

The increase in ethanol concentration with the increase in

the EtOH/i-C4molar ratio in the system's feed directly reduces

the velocity of the reaction (according to the Eley-Riedel kinetic

mechanism), but increases isobutene conversion. These results

are compatible with the values reported by Franoisse & Thyrion

[24]. AsFig. 2indicates, for molar ratios (MR) of 1.0 to 1.2, the

maximum conversions vary from 88 to 90%, whileat molar ratios

of 1.3 to 1.5 the conversions vary from 91 to 92%. At a tem-

perature of 65 C, the molar ratios above 1.2 present practically

the same isobutene conversions.

For MR=1.0, the best operational temperature for maximum

conversion is 59 to 63 C. As the MR increases, so does the

temperature of maximum conversion. This behavior is causedby

the reaction mechanism. When the ethanol concentration

increases, the reaction rate decreases due to the adsorption of

ethanol in the active sites of the catalyst, making diffusion of the

isobutene inside the particle catalyst difficult, and thus present-

ing a negative reaction order for the ethanol concentration.

According to our chromatographic analysis, the reaction

products of ethanol with isobutene are ETBE, C4 hydrocarbons

(nonreacted), ethanol (nonreacted), TBA (tert-butyl alcohol), SBA

(sec-butyl alcohol), C8 hydrocarbons and, in lesser proportion, C12hydrocarbons. Higher temperatures favor the formation of

reaction byproducts, leading to the increased production of

compounds with higher molar masses, such as isobutene dimers

(C8) and isobutene trimers (C12). The increase in ethanol

concentration in the load requires a higher temperature to

activate the reaction. This fact, allied with the presence of water

in the ethanol, favors the formation of TBA and, at a lower

concentration, SBA, due to the reaction of the water with the C4olefins. Based on our experimental results, we found that the

highest formation of secondary products was obtained with a

molar ratio of 1.5 and at a reaction temperature of 87 C.

3.2. Optimization of the production process

Based on the experimental results summarized in Fig. 2, the

highest production of ETBE (or the greatest conversion of i-C4)

was found to occur with MR 1.5. However, this led to a higher

production of the azeotropic ETBE/EtOH mixture. Table 2

presents the mass balance as a function of the molar ratios of

1.0 and 1.5 in the feed and a temperature of 62 C (correspondingto the maximum conversion temperature for MR =1.0), consider-

ing as base load 100 kg of C4hydrocarbons (0.66 mol of i-C4).

Accordingto the results presented in Table 2,asthemolar

ratio of EtOH/i-C4increases, so too does the conversion and

the production of the ETBE/EtOH azeotropic mixture. At a

molar ratio equal to or higher than 1.4, the concentration of

ethanol in the reactor's effluent is higher than in the

composition of the azeotropic mixture. Thus, all the ETBE

produce in the reaction system is concentrated in the top

flow of thefractionation columnin theformof azeotropeand

the bottom flow is composed of ethanol plus the secondary

products of the reaction.

As the data inTable 2 indicate, the stoichiometric molar

ratio allows for the highest ETBE production of high grade

purity, minimizing the production of theazeotropic mixture. To

increase the production of ETBE with a high degree of purity,

minimizing or preventing the formation of the azeotropic

mixture, it is necessary to use water in the system. However,

this increases the installation cost of the production plant.

Moreover, the introduction of water leads to the formation of

the azeotropic EtOH/H2O mixture, which makes it difficult to

recycle the ethanol. Some technologies use pervaporative

separation of the ethanol in the azeotropic mixture (ETBE/

EtOH) by means of special membranes. The use of ETBE in

azeotropic form would eliminate the costs related to the

purification stage of the ETBE production process.

In high purity ETBE productionunits whichuse water tobreak

the ETBE/EtOH azeotrope, the recycled ethanol contains water in

its composition, increasing the formation of TBA and SBA

alcohols and reducing the activity of the catalyst.

4. Conclusions

In the synthesis of ETBE using an adiabatic reactor and a space

velocity of 0.52 h1, the highest isobutene conversion is obtained

at reaction temperatures ranging from 61 to 67 C. When the

concentration of EtOH in the load increases, the conversion of i-

C4in the equilibrium also increases, but the reaction rate toward

ETBE formation decreases.

The azeotropic mixture possesses a potential for application

in gasoline formulations, offering advantages over the use of

ethanol (such as lowervolatilityand lower solubility in water)and

ETBE (higher octane rating and lower production costs). The

production system without ethanol recycling, considering the

ETBE/EtOH azeotropic mixture as an end product of the system,

minimizes production costs since it does not require the ethanol

purification unit.

The maximum ETBE production with a high degree of

purity and minimal production of the ETBE/EtOH azeotropic

mixture is attained using a stoichiometric molar ratio of

EtOH/i-C4.

Table 2Mass balance of ETBE production with a 100 kg ofC4hydrocarbons load for the molar ratios (MR) of 1.0 and1.5 at a temperature of 62 C

MR i-C4conversion, (%) Load (kg) Products (kg)

mEtOH mAzeotrope mETBE

1.0 88 30 20 43

1.1 89 34 36 301.2 90 36 50 20

1.3 91 40 66 8

1.4 91.5 42 80

1.5 92 46 97

Results extracted fromFig. 2.

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Acknowledgements

The authors acknowledge to the Petrochemical Company of the

Rio Grande do Sul (COPESUL), Brazil, for supplying the raw

material (C4cut) for the production of the ETBE and thanks the

financial support of the CNPq.

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