Simultaneous Production of tert-Amyl Ethyl Ether and tert-Amyl Alcohol

Department of Chemical Engineering, Middle East Technical University, Ankara 06531, Turkey tert-Amyl ethyl ether (TAEE) and tert-amyl alcohol (TAA), w...
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Ind. Eng. Chem. Res. 2005, 44, 5227-5232

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Simultaneous Production of tert-Amyl Ethyl Ether and tert-Amyl Alcohol from Isoamylene-Ethanol-Water Mixtures in a Batch-Reactive Distillation Column Dilek Varisli† and Timur Dogu* Department of Chemical Engineering, Middle East Technical University, Ankara 06531, Turkey

tert-Amyl ethyl ether (TAEE) and tert-amyl alcohol (TAA), which are the attractive alternatives to methyl tert-butyl ether as octane-enhancing gasoline-blending components, are produced by the simultaneous etherification and hydration of 2-methyl-2-butene (2M2B) in a batch-reactive distillation column. It was shown that, by changing the reboiler temperature in the range of 90-124 °C, significant increases of the overall fractional conversion of 2M2B, reaching values of 0.99, were achieved in this system. In the presence of water, the formation of TAA was also found to take place by the reactions of 2M2B and TAEE with water. Higher selectivities were observed for TAA than for TAEE, in the presence of water. This is due to the higher adsorption equilibrium constant of water than of ethanol on an Amberlyst-15 resin catalyst. A significant increase in the fractional conversion of 2M2B to TAEE was observed in the absence of water. Introduction Fuel oxygenates are known to improve the burning efficiency of gasoline and reduce carbon monoxide emissions. Because of their lower volatility and lower atmospheric reactivity as compared to hydrocarbon constituents of the refinery gasoline, they also help to reduce the formation of atmospheric ozone resulting from gasoline emissions. A significant number of kinetic studies are available in the literature1 for the production of methyl tert-butyl ether (MTBE), which is the most common oxygenate used in gasoline blending to improve the octane number. Water pollution problems created by the extensive use of MTBE diverted the attention of researchers and fuel producers to higher alcohols and also to ethanol-based tertiary ethers as alternative gasoline-blending oxygenates. Significant research was focused on the production of alternative oxygenates, such as tert-amyl ethyl ether (TAEE) and tert-amyl methyl ether (TAME), as octane-enhancing gasolineblending components. tert-Amyl alcohol (TAA) may also be considered as an attractive alternative to MTBE. These oxygenates may be produced by the reaction of C5 reactive olefins (isoamylenes, IAs), which are already present in light gasoline,2 with ethanol, methanol, or water, over acidic resin catalysts. In recent years, a significant number of kinetic studies related to the synthesis of higher ethers,3-12 like TAEE and TAME, were published in the literature. In the work of Linnekoski et al.,13 hydration and etherification kinetics of 2-methyl-2-butene (2M2B) to produce TAEE and TAA were investigated in stirred-tank and batch reactors. In recent years, reactive distillation (RD) processes attracted significant attention for the production of ethers such as MTBE and ethyl tert-butyl ether (ETBE) to enhance the conversion in such equilibrium-limited reactions. The most important advantage of the use of RD for equilibrium-controlled reactions is the elimina* To whom correspondence should be addressed. Tel.: 90312-2102631. Fax: 90-312-2101264. E-mail: [email protected]. † E-mail: [email protected].

tion of conversion limitations by the continuous removal of products from the reaction zone. Apart from the increased conversion, other advantages of RD can be summarized as the improved product selectivities and reduced capital investment. In works of De Garmo et al.,14 Jacobs and Krishna,15 Nijhuis et al.,16 Isla and Irazoqui,17 Hauan et al.,18 Higler et al.,19 and Quitain et al.,20 experimental and modeling studies were reported for the production of MTBE and ETBE in a RD column. Higler et al.19 proposed a nonequilibrium RD model for MTBE synthesis, which was considered as an advancement over the pioneering equilibrium model of Jacobs and Krishna.15 Sneesby et al.21 studied RD for the production of ETBE from isobutylene and ethanol over an acidic catalyst, namely, Amberlyst-15. In the work of Thiel et al.,22 a batch-RD column was used in the production of MTBE and TAME. Multiple chemical reactions, which were always present in the industrial production of MTBE and TAME, were theoretically analyzed and experimentally examined by Sundmacher et al.23 The hydration of IAs to produce TAA was studied in a RD column by Gonza´lez et al.24 This reaction, like etherification of isoolefins, is moderately exothermic and is limited by chemical equilibrium. Using RD, high yields of TAA were reported under conditions approaching total reflux. In the kinetic study of Aiouache and Goto,25 etherification of TAA with ethanol, dehydration of TAA to IA, and etherification of the produced IA to TAEE were investigated in a batch reactor. More recently, a novel RD-pervaporation process was proposed by the same authors32 for the etherification of TAA with ethanol to produce TAEE. In our recent study, a batch reflux-recycle reactor was proposed to achieve high product yields and selectivities in the production of tertiary ethers.26 The possible use of bioethanol (containing some water) in the production of TAEE may result in significant economic advantages. In such a process, simultaneous production of TAEE and TAA was expected. Kinetic studies for the production of TAEE and TAA are

10.1021/ie049241w CCC: $30.25 © 2005 American Chemical Society Published on Web 01/20/2005

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Figure 1. Schematic diagram of the experimental setup.

limited, and RD applications for the simultaneous production of TAEE and TAA are missing in the literature. The major objective of the present study was to investigate the simultaneous production of TAA and TAEE in a RD column from ethanol-water-IA mixtures and to investigate the effects of water on the total conversion and TAEE selectivity in TAEE synthesis. Experimental Work Experiments were carried out in a batch-RD column shown in Figure 1, under total reflux conditions. The column, having an inner diameter of 5 cm and a height of 40 cm, had eight perforated plates, and the space between successive plates was 5 cm. Two of these plates (the fourth and fifth plates from the bottom) were selected as the reaction zone. Amberlyst-15 catalyst packages (10 g/plate) were placed over these two plates. Each package contained 1 g of catalyst.27,28 The catalyst used in this study (Amberlyst-15) is a strongly acidic macroreticular ion-exchange resin. The physical properties of Amberlyst-15 were reported in our earlier publications.5,29 It has a macroporosity of 0.32, and its apparent density is close to unity. The surface area determined from mercury-intrusion porosimetry is 59.2 m2/g. These resin catalyst particles are composed of gel-like micrograins and macropores. The radii of

catalyst particles and gel-like micrograins were reported as 3.7 × 10-4 and 4.1 × 10-8 m, respectively. Its pore size distribution is rather narrow, but in a polar medium, it is reported to swell appreciably. About 50% volume swelling of the micrograins was reported in the presence of water.29 The active sites on the catalyst are sulfonic groups (-SO3H) attached to the polystyrene chain. The average diameter of the macropores was reported as 2.28 × 10-8 m, which was 4 orders of magnitude smaller than the diameter of the catalyst particles. In the experiments, 2M2B (Merck) and two different grades of ethanol [99.8 vol % from Sigma Aldrich and 96 vol % ethanol (the amount remaining is water) from Birpa] were used. IA used in our work contained 95% 2M2B [with the amount remaining being majorly 2-methyl-1-butene (2M1B)]. 2M2B is the major isomer of IAs, and the isomerization reaction between 2M2B and 2M1B was usually assumed to be in equilibrium over acidic resin catalysts. Although IA could be represented by the mixture of 2M2B and 2M1B, they could be lumped together as IA with the properties of the major isomer.25 In the chromatographic analysis, TAME (>97%) from Aldrich and TAA from Merck were used for calibration. In most of the experiments, the waterethanol mole ratio in the liquid feed was 0.134, while the initial mole fraction of 2M2B was kept at 0.05.

Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005 5229 Table 1. Reboiler and Reaction Zone Temperatures and System Pressures experimental system pressure (P, bar) Tboiler,avg (°C)

Trxn (°C)

initial

final (t g 300 min)

90 ( 2 101 ( 2 111 ( 2 119 ( 1 124 ( 4

57 ( 3 64 ( 4 75 ( 2 84 ( 2 94 ( 5

2.1 3.3 3.8 4.9 5. 6

1.9 2.2 3.3 4.3 4.6

However, in some experiments, the water-ethanol mole ratio was adjusted to 0.0065. Experiments were repeated in a reboiler temperature range between 90 and 124 °C. Temperatures were measured by seven thermocouples located at different positions within the apparatus. Two of these thermocouples were at the top and at the bottom of the column. The temperatures of the reflux, the condensate, and the reboiler were also measured by thermocouples. The samples taken from the reboiler were analyzed using a gas chromatograph (Varian Aeograph GC). The GC had a flame ionization detector. A column packed with 15% FFAP on Chromosorb AW was used during GC analysis. During the experiments, the reboiler temperatures were kept constant at the desired values. The fluctuation of the reboiler temperature in a single set of experiments was only about (2 °C. The reboiler temperatures in the range of 90-124 °C corresponded to initial system pressures of 2.1-5.6 bar, respectively. Because of changes in the composition of the mixture in the RD column, a decrease of the system pressure was observed as a function of time. In each experiment conducted at a fixed reboiler temperature, the values of the reboiler temperature, reaction zone temperature (as estimated from the total reflux assumption), and system pressure are reported in Table 1. It took about 20 min to increase the temperature of the mixture in the reboiler to the desired value and to start reflux to the column from the condenser. After the desired temperature was reached in the reboiler, it was kept constant by continuous regulation of the heat input to the reboiler and also by adjustment of the cooling water flow rate in the condenser. The time at which the reflux was started was noted as the start of the experiment in presenting the conversion and selectivity values reported in this work. Results and Discussion Chemical analysis of the contents of the reboiler indicated the formation of a significant amount of TAA in addition to the formation of TAEE.27 Instantaneous fractional conversion of 2M2B to the products was evaluated from the reboiler compositions using the following definitions:

XTAEE ) xTAEE/(xTAEE + xTAA + xIA) XTAA ) xTAA/(xTAEE + xTAA + xIA) Here, xTAEE, xTAA, and xIA correspond to the instantaneous mole fractions of TAEE, TAA, and IA (2M2B + 2M1B) in the reboiler, respectively. Fractional conversion of 2M2B to TAA and TAEE and the total conversion values obtained at a reboiler

Figure 2. Fractional conversion of 2M2B to TAEE and TAA at a reboiler temperature of 111 °C (feed compositions of 5 mol % 2M2B, 83.8% ethanol, and 11.2% H2O; water-ethanol, 0.134): [, XTAEE; 9, XTAA; 2, Xtotal.

Figure 3. Variation of the total fractional conversion of 2M2B at different reboiler temperatures (feed compositions of 5 mol % 2M2B, 83.8% ethanol, and 11.2% H2O; water-ethanol, 0.134). Table 2. Experimental (for Long Times, t g 350 min) and Equilibrium Conversion Values of IA Evaluated at Different Reboiler Temperatures Treboiler (°C) 90 101 111 119 124

experimental fractional conversions XTAEE XTAA Xtotal 0.235 0.236 0.121 0.208 0.084

0.533 0.720 0.868 0.776 0.912

0.768 0.956 0.989 0.984 0.996

equilibrium conversions XTAEE,eqb XTAA,eqb Xtotal,eqb 0.502 0.441 0.390 0.342 0.318

0.076 0.076 0.076 0.075 0.074

0.578 0.517 0.466 0.417 0.392

temperature of 111 °C and with an initial reboiler composition of 5% (mole) 2M2B, 11.2% H2O, and 83.8% ethanol (with the H2O-ethanol mole ratio being 0.134) are shown in Figure 2. As is shown in this figure, very high total conversion values reaching to almost complete conversion of 2M2B are achieved. As expected, the total conversion of 2M2B and the fractional conversion to TAA increased with time. However, the fractional conversion to TAEE passed through a maximum. This observation indicated the presence of a consecutive reaction between TAEE and water. This is the reverse of the reaction investigated by Aiouache and Goto.32 Total conversion values obtained at different temperatures are illustrated in Figure 3. An increase in the temperature resulted in an increase in the total conversion.27 At a reboiler temperature of 124 °C, which corresponded to a reaction zone temperature of 94 °C, a total fractional conversion value of 0.996 was achieved in the reboiler for reaction times greater than 250 min. However, at a reboiler temperature of 90 °C, the total fractional conversion value reached at 400 min was only 0.768 (Table 2). Fractional conversion values of 2M2B to TAA and TAEE, obtained at different temperatures, are shown in Figures 4 and 5, respectively. As was shown in these figures and also in Figure 2, fractional conversion of

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Figure 4. Variation of the fractional conversion of 2M2B to TAA at different reboiler temperatures (feed compositions of 5 mol % 2M2B, 83.8% ethanol, and 11.2% H2O; water-ethanol, 0.134).

Figure 5. Variation of the fractional conversion of 2M2B to TAEE at different reboiler temperatures (feed compositions of 5 mol % 2M2B, 83.8% ethanol, and 11.2% H2O; water-ethanol, 0.134).

experimental reboiler temperatures. 2M2B to TAA was much higher than fractional conversion to TAEE at all temperatures. In our earlier works,5,6 it was shown from the DRIFTS (diffuse reflectance infrared Fourier transform spectra) analysis of the surface species involved in such etherification reactions that alcohols and isoolefins were both adsorbed on the -SO3H sites of the catalyst during etherification reactions. Adsorbed alcohol molecules are bridged by hydrogen bonds to each other and also to the -SO3H sites of the catalyst. Polar molecules, like alcohols, adsorb on the catalyst and also cause swelling. Even stronger adsorption of water molecules than alcohols is expected. In fact, the adsorption equilibrium constants of water and ethanol evaluated from the relationships proposed by Aiouache and Goto25 also showed that the adsorption equilibrium constant of water (KH2O) was more than 3 times bigger than the adsorption equilibrium constant of ethanol (KEtOH). For instance, at a reaction zone temperature of 94 °C, the ratio of the adsorption equilibrium constants of water and ethanol (on Amberlyst15) was estimated to be about 3.5. In the presence of water, a major fraction of the surface of the catalyst was expected to be covered by water. Water acts as a poison in the etherification reaction between ethanol and IAs. However, a significant amount of TAA was formed by the reaction of adsorbed water with 2M2B. On the basis of experimental results and sample analysis, the following reactions were assumed to take place in the RD column.

The experimental values of fractional conversions of 2M2B to TAA and TAEE and also the total conversion values evaluated at longer times (t g 350 min) in the reboiler are reported in Table 2. In the same table, fractional equilibrium conversion of 2M2B charged to the reboiler is also given. In the evaluation of equilibrium conversions, the equation derived by Kitchaiya and Datta30 was used to estimate the equilibrium constant of the TAEE formation reaction (reaction I), at the

2078.6 - 6.5925 ln T + 0.0231T T 1.126 × 10-5T 2 - 1.414 × 10-8T 3 (1)

ln K1 ) 26.779 +

The experimental equilibrium constant expression derived by Aiouache and Goto25 was used to estimate the equilibrium constant of the TAA synthesis reaction (reaction II).

ln K2 ) -5.75 + 1376/T

(2)

Equation 2 was derived25 using the data obtained between 50 and 80 °C. However, some of our experimental temperature values were higher than 80 °C. Consequently, because of the extrapolation of temperatures outside the range given in the literature,25 there could be minor errors introduced when estimating the equilibrium conversion values reported in Table 2. In the evaluation of equilibrium conversions, necessary activity coefficients were estimated from the UNIFAC method.31 The experimental value of the total fractional conversion of 2M2B obtained at a reboiler temperature of 124 °C was 0.996. However, if we had used a batch reactor instead of a RD column, the total fractional conversion that would be reached at that temperature would be only 0.392 (Table 2). These results indicated the advantage of using a batch-RD column system as compared to a batch reactor. It was interesting to note that experimental conversion values to TAA were much higher than the corresponding equilibrium values evaluated at the reboiler temperature. However, experimental conversions to TAEE were less than the corresponding equilibrium values (Table 2). This is majorly due to the exhaustion of 2M2B by the reaction with H2O. IAs are much more volatile than ethanol, TAEE, and TAA. This causes a significant increase in the 2M2B concentration with a corresponding decrease in the ethanol concentration along the distillation column. Although the mole ratio of 2M2B to ethanol was about 0.06 in the initial feed mixture charged into the reboiler, stage-by-stage calculations showed that this ratio became over 3 in the reaction zone (fourth and fifth stages) in most of the experiments. TAEE and TAA formed within the reaction zone were expected to be condensed and returned back to the reboiler. No further reaction is expected within the reboiler. As a result of this process, experimental fractional conversion values evaluated from the reboiler compositions reached values over the corresponding equilibrium conversions evaluated at the reboiler temperatures.

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This increase in the total fractional conversion was accompanied with a significant increase in the fractional conversion to TAEE (Figure 7). These results clearly indicated a significant increase in the etherification rate of IA to TAEE in the absence of water. As was also discussed before, when water is present, the formation of TAA through parallel and consecutive reactions reduced the selectivity of TAEE. In this work, experiments were carried out in a batch-RD column. However, continuous operation with a feed point below the reaction zone may be recommended for large-scale applications. Figure 6. Total fractional conversion of 2M2B at different waterethanol ratios in the feed (Treboiler, 110 °C; feed composition of 5 mol % 2M2B in an ethanol-water mixture).

Figure 7. Fractional conversion of 2M2B to TAEE for different water-ethanol ratios in the feed (Treboiler, 110 °C; feed composition of 5 mol % 2M2B in an ethanol-water mixture).

The maximum observed in the fractional conversion of 2M2B to TAEE indicated the presence of a consecutive reaction between the product of reaction I, namely, TAEE and water.

Concluding Remarks This study showed that very high IA fractional conversions over 0.99 could be achieved in the reaction with ethanol-water mixtures, in a RD column. The overall conversion of 2M2B was significantly increased with an increase in the temperature. In the presence of water, a significant amount of TAA production was observed, which was accompanied with a decrease in the fractional conversion to TAEE. Water acted as a poison for the production of TAEE. Unlike TAA, the fractional conversion to TAEE gave a maximum, indicating the presence of a consecutive reaction between TAEE and water. Because both TAEE and TAA could be considered as possible alternative oxygenates for improving the octane number of gasoline, the reaction of IAs with ethanol-water mixtures in a RD column was shown to be an attractive process. Acknowledgment Turkish State Planning Organisation Research Grant BAP-03-04-DPT-2002 K120540-19 and METU Research Fund Grant AFP-2001-07-02-00-25 are gratefully acknowledged. Literature Cited

The vapor pressure of TAEE is higher than the vapor pressure of TAA at the reaction zone temperatures of this work (T < 95 °C). As a result, the mole fraction of TAEE was estimated to be higher than the mole fraction of TAA in the reaction zone. Thermodynamics of reaction III become favorable in the reaction zone of the column. As seen in Figure 5, the maximum observed in the conversion to TAEE shifted to earlier times as the reboiler temperature was increased. This indicated that the rate of reaction III significantly increased with an increase in the temperature. To see the effect of the water-ethanol mole ratio on the product distribution and also on the total conversion of 2M2B, experiments were carried out at two different water-ethanol ratios at a reboiler temperature of about 110 °C. In both of these experiments, the initial mole fraction of 2M2B in the reboiler was 5%. However, the ratio of water-ethanol was adjusted as 0.134 and 0.0065 in these experiments. The total fractional conversion values of 2M2B obtained in these experiments are reported in Figure 6. As is seen in this figure, when the water-ethanol ratio was decreased, a significant increase in the total fractional conversion was observed.

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Received for review August 19, 2004 Revised manuscript received October 21, 2004 Accepted October 25, 2004 IE049241W