ETBE Production

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Optimization of the ETBE (ethyl tert-butyl ether) Production Process Article in Fuel Processing Technology · November 2008 DOI: 10.1016/j.fuproc.2008.05.006

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F U E L PR O CE SS I N G TE CH N O LOG Y 89 ( 20 0 8 ) 1 1 48 –1 1 5 2

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

Optimization of the ETBE (ethyl tert-butyl ether) production process Eliana Weber de Menezes, Renato Cataluña⁎ Department of Physical Chemistry, Institute of Chemistry, Federal University of Rio Grande do Sul, Avenida Bento Gonçalves, 9500, CEP-91501-970 Porto Alegre, RS, Brazil

AR TIC LE I N FO

ABS TR ACT

Article history:

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

Received 14 August 2007

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

Received in revised form 14 May 2008

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

Accepted 14 May 2008

displace the equilibrium towards the products. ETBE and ethanol form an azeotropic mixture which hinders the recycling of nonreacted ethanol in the process. The purpose of this work is

Keywords:

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

ETBE

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

Azeotropic mixture (ETBE/EtOH)

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

Gasoline

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.

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 hydrocarbons (HC), minimizing the emission of volatile organic compounds [3–6]. 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 they possess the desired characteristics of octane ratings and CO emission reductions [7]. Some countries prefer ethers rather than alcohols due to 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 C4 hydrocarbon feed flow, reaction, purification, and recovery of nonreacted products [13,14]. Nowadays, to minimize implementation and operating costs, reactive distillation (also called catalytic distillation) is proposed as an alternative route for ETBE synthesis, offering high conversion and low implementation/operating costs in comparison with conventional synthesis [15–17]. 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

⁎ Corresponding author. Tel.: +55 51 3308 6306; fax: +55 51 3316 7304. E-mail address: [email protected] (R. Cataluña). 0378-3820/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2008.05.006

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Table 1 – Mean molar composition of the industrial hydrocarbons load of the C4 cut Compounds

Concentration (molar%)

Isobutane n-butane 2-transbutene 1-butene Isobutene 2-cisbutene

1.7 7.6 16.9 33.2 36.0 4.6

introduction of water into the system and involves the separation of the ETBE, the C4 hydrocarbon mixture, ethanol and water. The introduction of water into the purification 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 membranes [19–23]. 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 presenting promising 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

1149

The ETBE production process was carried out in a flow, using as reagents a mixture of C4 hydrocarbons with 36 mol% of isobutene (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 reaction system consists of an adiabatic fixed bed reactor fed by two cylinders, one containing the reagent ethanol (EtOH) and the other the C4 hydrocarbon 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 C4 hydrocarbons mixture, with a capacity of 1380 mL/h. These gauges allow the EtOH/i-C4 ratio and space velocity to be set as desired. The reagent mixture is heated and fed into the reactor's lower portion. The temperature of the catalytic stream bed and at the exit is monitored with thermocouples inside and outside the reactor to ensure the reaction is in the steady state condition. The reactor's effluent is flashed into a distillation column under

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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atmospheric pressure, separating the C4 hydrocarbons into vapor phase and the ethanol, ETBE and byproducts into liquid phase. The isobutene conversion was evaluated as 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.

i
C4 conversion ¼

Normalization of the i
C4 ðloadÞ
Normalization of the i
C4 ðreactors exitÞ  100 Normalization of the i
C4 ðloadÞ

The composition of the C4 hydrocarbon (reagent) load and the C4 in the vapor phase (reaction products) was determined by gas chromatography using a thermal conductivity detector (GC-TCD, Shimatzu 17A), a “plot” type fused silica capillary column with a stationary phase of Al2O3/Na2SO4 (50 m×0.53 mm) and Helium (5.0) as carrier gas. The analytical conditions were: isotherm at 40 °C for 20 min, a heating ramp-up of 20 °C/min up to 190 °C, and holding at this temperature for 10 min. The injector and detector temperatures were 180 °C and 220 °C. The split ratio was 1:20 and the volume of injected sample was 20 µL. The conversions obtained in the reaction system were evaluated as a function of the EtOH/i-C4 molar 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 h− 1, 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 the reactor to meet the equilibrium condition. This space velocity corresponds to the minimum limit of operation of the flow control of the reactants using a 340 cm3 reactor.

2.1.2.

The conversion of isobutene was determined by gas chromatography 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)):

ð1Þ

products of the reaction (tert-butyl alcohol and C8 hydrocarbons). 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 at 40 °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 initial split ratio was of 1:300, passing on to 1:20 after 2 min of analysis. The volume of injected sample was 20 µL.

3.

Results and discussion

3.1.

Evaluation of the parameters of the reactional system

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

Fig. 2 presents the isobutene conversion profiles adjusted as a function of the temperature at the exit from the reactor and the EtOH/i-C4 molar ratios of the feed. The conversions shown here represent the results of three consecutive assays for each reaction condition evaluated.

Fig. 2 – Isobutene conversions as a function of the temperature at the exit from the reactor, considering the distinct EtOH/i-C4 molar ratios (MR) and a space velocity of 0.52 h− 1.

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Table 2 – Mass balance of ETBE production with a 100 kg of C4 hydrocarbons load for the molar ratios (MR) of 1.0 and 1.5 at a temperature of 62 °C MR i-C4 conversion, (%)⁎ Load (kg)

1.0 1.1 1.2 1.3 1.4 1.5

88 89 90 91 91.5 92

Products (kg)

mEtOH

mAzeotrope

mETBE

30 34 36 40 42 46

20 36 50 66 80 97

43 30 20 8 – –

⁎ Results extracted from Fig. 2.

As indicated in Fig. 2, at a space velocity of 0.52 h− 1, 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 conversion is controlled by thermodynamic equilibrium. The increase in ethanol concentration with the increase in the EtOH/i-C4 molar 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 Françoisse & Thyrion [24]. As Fig. 2 indicates, for molar ratios (MR) of 1.0 to 1.2, the maximum conversions vary from 88 to 90%, while at molar ratios of 1.3 to 1.5 the conversions vary from 91 to 92%. At a temperature 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 caused by 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 presenting 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, C12 hydrocarbons. 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 C4 olefins. 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.

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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 (corresponding to the maximum conversion temperature for MR=1.0), considering as base load 100 kg of C4 hydrocarbons (0.66 mol of i-C4). According to the results presented in Table 2, as the molar ratio of EtOH/i-C4 increases, 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 the fractionation column in the form of azeotrope and the bottom flow is composed of ethanol plus the secondary products of the reaction. As the data in Table 2 indicate, the stoichiometric molar ratio allows for the highest ETBE production of high grade purity, minimizing the production of the azeotropic 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 production units which use water to break 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 h− 1, 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 iC4 in 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 lower volatility and 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.

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Acknowledgements [12]

The authors acknowledge to the Petrochemical Company of the Rio Grande do Sul (COPESUL), Brazil, for supplying the raw material (C4 cut) for the production of the ETBE and thanks the financial support of the CNPq.

[13]

[14]

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