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Ind. Eng. Chem. Res. 1996, 35, 982-984
Reactive Distillation for Synthesizing Ethyl tert-Butyl Ether from Low-Grade Alcohol Catalyzed by Potassium Hydrogen Sulfate Mohammed Matouq, Amando T. Quitain, Katsuroku Takahashi, and Shigeo Goto* Department of Chemical Engineering, Nagoya University, Nagoya 464-01, Japan
Synthesis of ethyl tert-butyl ether (ETBE) from the reaction between ethanol (EtOH) and tertbutyl alcohol (TBA) in the presence of different acid catalysts (KHSO4, NaHSO4, H2SO4, and Amberlyst 15) was investigated at low alcohol grade (mixture of 80 mol % water). Potassium hydrogen sulfate (KHSO4) showed the highest selectivity among the tested catalysts. Other catalysts caused the dehydration of TBA into water (H2O) and isobutene (IB). In the top of the reactive distillation column with total reflux, the condensate was split into two layers. The upper layer contained ETBE with a more than 60 mole fraction. Introduction
Table 1. Ion-Exchange Capacity
Ethers such as methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE) have been recently recognized as high-octane boosters with low-bending Reid vapor pressures (Rvp) and no pollutant effect (Rvp values for MTBE and ETBE are 8 and 4 psi, respectively). Among these ethers, MTBE has dominated the market. MTBE is generally produced by the reaction between methanol (MeOH) and isobutene (IB). ETBE may become a more attractive option since ethanol (EtOH) is produced from renewable resources such as biomass. New trends to produce ETBE from bioethanol is now under consideration. Here one of the chief objects of this research is to synthesize ETBE from tert-butyl alcohol (TBA) and EtOH given by eq 1.
TBA + EtOH w ETBE + H2O
(1)
The grade of alcohol was chosen to be as low as that for EtOH produced from biomass. The ion-exchange resin in the H+ form has generally been used for the production of MTBE and ETBE. Unfortunately, there are two main drawbacks to employing this catalyst. First, the dehydration of TBA into IB (eq 2) has been reported to be significant (Matouq and Goto, 1993).
TBA w IB + H2O
(2)
Second, the presence of water has a strong inhibition effect on the catalytic activity (Cunill et al., 1993). Therefore, the ion-exchange resin might not be a suitable catalyst for synthesis of ETBE from low alcohol concentration in an aqueous solution. Sodium hydrogen sulfate (NaHSO4) and sulfuric acid (H2SO4) were active to produce mixed ethers from two alcohols in a 15% aqueous solution (Norris and Rigby, 1932). Thus, these catalysts and potassium hydrogen sulfate (KHSO4) will be examined for the reaction of TBA and EtOH in this study. Reactive distillation for the production of MTBE from TBA and MeOH was effective (Matouq et al., 1994). The high initial water content in the feed mixture increases the attractiveness of reactive distillation in this study. Since synthesis of ETBE is a reversible reaction, the existence of water will shift the reaction toward the * To whom all correspondence should be addressed. Telephone and FAX: +8152-789-3261. e-mail:
[email protected]. nagoya-u.ac.jp.
0888-5885/96/2635-0982$12.00/0
catalyst
mol H+/kg
KHSO4 NaHSO4 H2SO4 Amberlyst 15 (pellet)
7.34 8.33 20.41 3.98
dissociation of ETBE. Therefore, if ETBE can be separated from an aqueous solution as a condensate, the reactive distillation will be promising. Experimental Section Catalyst. Potassium hydrogen sulfate (KHSO4), sodium hydrogen sulfate (NaHSO4), and sulfuric acid (H2SO4) of reagent grade were adopted without any further purification. The ion-exchange resin, Amberlyst 15, in the pellet form was used to compare with other catalysts. The pellet was prepared by mixing the ion-exchange particles with 15 wt % polyethylene powder, placed in a mold of 8 mm, and heated for 1 h at 413 K to bind the particles (Matouq et al., 1994). Table 1 presents the ion-exchange capacity for these catalysts. Apparatus. A schematic diagram of a reactive distillation column is shown in Figure 1. The glass column with 3.5 cm i.d. and 70 cm height equipped with six sieve plates was used to perform the experiment. A Teflon plate was perforated with holes of 3 mm diameter. Every plate was equipped with a U-shaped glass tube as the downcomer. Thermocouples were connected to the column to record the temperature profile inside the column. A three-neck, round-bottomed flask (reboiler) was connected to the column through its central opening. The other two openings were connected to the thermocouple and the line for recycling a portion of the solution from the bottom to the top part of the reaction zone in the column to maintain a specific dissolved amount of liquid catalyst on the plate. The heat for the reboiler was supplied from an electric heater. To prevent heat loss, the column was insulated with a fiber glass. A gas meter was connected through the condenser to measure the gas flowrate. Procedure. A measured amount of catalyst was added to the mixture of EtOH, TBA, and H2O (1, 1, and 8 mol), and the mixture was placed in the boiler. The mixture was heated up to its boiling point (354 K). At intervals of 1 h, samples were taken from the condensate and analyzed. In the case of Amberlyst 15, © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 983 Table 2. Experimental Results in the Condensate after 6 h with Total Refluxa upper layer (mol %)
lower layer (mol %)
run no.
catalyst
wt (g)
XIB (%)
ETBE
H2O
TBA
EtOH
1 2 3 4 5 6
KHSO4 KHSO4 NaHSO4 H2SO4 A15 A15
5.0 10.0 8.8 1.8 9.2 9.2
0 0 14.5 16.3 26.7 95.3
10 62 trace 70 65 71
16 13 26 10 11 9
40 10 23 7 9 4
34 15 35 13 15 16
a
ETBE
H2O
TBA
EtOH
1.4
84
4.6
10
5 0.7 trace
77 88 94
7 3.3 0.6
11 9 5.4
A15, Amberlyst 15. Molar ratio TBA:EtOH:H2O ) 1:1:8 (run nos. 1-5) ) 5:5:0 (run no. 6).
a
b
Figure 1. Reactive distillation column setup.
since it was difficult to hold the pellet on the sieve plate, the pellets were placed in a closed pocket of cotton cloth. The analysis was conducted in a thermal conductivity gas chromatograph with a 2.5 m column packed with Gaskuropack 54 at 423 K. Ethers produced by intramolecular dehydration of alcohols (di-tert-butyl ether and diethyl ether) were not detected. Results and Discussion Table 2 summarizes the experimental results obtained in the condensates after 6 h of using the reactive distillation column with total reflux. The conversion of TBA to IB (eq 2), XIB, could be determined from the gas flowrate of IB through the condenser. Run no. 1 presents standard conditions with 5 g of KHSO4. Figure 2a shows concentration profiles of the condensates with time for run no. 1. ETBE and EtOH increase with time while H2O is decreasing. All values become almost constant after 5 h. Figure 2b shows the temperature profile inside the column for run no. 1. The temperature decreases from the bottom (354 K) to the top (351 K) monotonously. When the weight of KHSO4 increases from 5 to 10 g (run no. 2 in Table 2), concentration profiles of the condensates with time are dramatically changed as shown in Figure 3. The behaviors before 4 h (thick solid lines in Figure 3) are almost the same as those of Figure 2a, except for EtOH which has a peak. The production of ETBE
Figure 2. (a) Concentration profiles of condensates for run no. 1. (b) Temperature profile for run no. 1. Table 3. Binary Azeotropic Mixture ETBE-H2O ETBE-EtOH EtOH-H2O TBA-H2O
composition (mol %)
temperature (K)
73.4-26.6 62.9-37.1 78.7-21.3 64.5-35.5
338 340 352 353
increases suddenly around 3 h, and then the condensates are split into two layers after 4 h. ETBE is rich in the upper layer (thin solid lines in Figure 3), and H2O is rich in the lower layer (thin broken lines). Other components are less than 20 mol %. The binary mixtures have azeotropic compositions as indicated in Table 3. Then, pure ETBE cannot be separated in the distillation. However, rich ETBE in the nearly azeotropic mixture can be obtained in the upper layer of condensate for the reactive distillation. The effect of alkali ion has been examined by using NaHSO4 in run no. 3 of Table 2. This catalyst failed to synthesize ETBE because the dehydration of TBA to IB (eq 2) was dominant. When H2SO4 (run no. 4) and Amberlyst 15 (run nos. 5 and 6) were used, two layers appeared in the condensate. For these cases, IB gas has been produced
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Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996
TBA was dominant. The acid strength for these catalysts may be too strong to produce ETBE, while KHSO4 has a moderate acid strength. Nomenclature XIB ) conversion of ethanol (TBA) to isobutene (IB) Abbreviations ETBE ) ethyl tert-butyl ether EtOH ) ethanol IB ) isobutene TBA ) tert-butyl alcohol
Literature Cited
Figure 3. Concentration profiles of condensates for run no. 2.
significantly. Especially, in the case without initial water (run no. 6), most of TBA would be dehydrated into IB. The desired product, ETBE, cannot be produced.
Cunill, F.; Vila, M.; Izquierdo, J. F.; Iborra, M.; Tejero, J. Effect of Water Presence on Methyl tert-Butyl Ether and Ethyl tertButyl Ether Liquid-Phase Synthesis. Ind. Eng. Chem. Res. 1993, 32, 564-569. Matouq, M.; Goto, S. Kinetics of Liquid Phase Synthesis of Methyl tert-Butyl Ether from tert-Butyl Alcohol and Methanol Catalyzed by Ion Exchange Resin. Int. J. Chem. Kinet. 1993, 25, 825-831. Matouq, M.; Tagawa, T.; Goto, S. Combined Process for Production of Methyl tert-Butyl Ether from tert-Butyl Alcohol and Methanol. J. Chem. Eng. Jpn. 1994, 27, 302-306. Norris, J. F.; Rigby, G. W. The Reactivity of Atoms and Groups in Organic Compounds. J. Am. Chem. Soc. 1932, 54, 2088-2100.
Conclusion ETBE could be produced from the reaction between TBA and EtOH at low grade in the presence of KHSO4 as a catalyst. A reactive distillation column was a good choice to separate ETBE from an aqueous solution and a homogeneous catalyst. Other catalysts (NaHSO4, H2SO4, and Amberlyst 15) failed to synthesize ETBE because the dehydration of
Received for review July 25, 1995 Revised manuscript received December 14, 1995 Accepted December 27, 1995X IE9504664
Abstract published in Advance ACS Abstracts, February 1, 1996. X
