Energy Fuels 2010, 24, 4668–4672 Published on Web 02/16/2010
: DOI:10.1021/ef901230r
New Process for the Production of Glycerol tert-Butyl Ethers† Martino Di Serio,* Luca Casale, Riccardo Tesser, and Elio Santacesaria Department of Chemistry, University of Naples Federico II, via Cintia 80126 Naples, Italy Received October 27, 2009. Revised Manuscript Received February 2, 2010
The increase in biodiesel production has enhanced the availability of its co-product, glycerol. Many researchers are therefore devoted to individuate new uses for glycerol. In this paper, the evaluation of the use of a heterogeneous acid catalyst (amberlyst 15) in the production of glycerol tert-butyl ethers (GTBEs) as a diesel additive is performed. The best experimental synthesis conditions are reported together with the proposal of a new process essentially based on the extraction of GTBE with biodiesel.
1. Introduction The increasing availability of glycerol, as a consequence of the increase in biodiesel production (of which glycerol is a byproduct) is rapidly saturating the market, and consequently, great interest is now addressed to the development of new process routes for alternative uses of glycerol. Among the various possibilities, our attention has been focused on glycerol etherification with isobutene to produce glycerol tert-butyl ethers (GTBEs), which can be used as diesel and biodiesel additives1 or as an octane booster for gasoline.2 The reaction is performed at 60-100 °C with isobutene excess (from 2:1 to 4:1 isobutene/glycerol molar ratio) under moderate pressure of 15-20 bar to have isobutene in the liquid phase. This is a complex multiphase system; isobutene is insoluble in the glycerol phase, but a unique phase could be formed during the reaction because isobutene is highly soluble in di- and tri-ethers of glycerol, which are products of the reaction. The kinetics of the reactions was studied in the presence of several acid catalysts.2-4 The best results among heterogeneous catalysts were achieved with commercial strong acid ion-exchange resin amberlyst 15, while the best results among homogeneous catalysts were achieved with p-toluenesulfonic acid.5 The main problem in the production of GTBE is that the product is a mixture of different molecules (see reactions 1-3).
The more desirable molecules are di-ethers. As a matter of fact, mono-ethers are less soluble in fuels, while for tri-ethers, there is the highest content of isobutene, which is not a biocomponent.1 Moreover, the cost of isobutene is higher than that of used glycerol. So, di-ethers are the preferred products also from economic reasons. It is opportune to point out that also secondary reactions are present, which reduce the yield in ethers with the formation of C8 and C12 hydrocarbons (isobutene oligomers; see reactions 4 and 5).
† This paper has been designated for the Bioenergy and Green Engineering special section. *To whom correspondence should be addressed. E-mail: diserio@ unina.it. (1) Jaecker-Voirol, A.; Durand, I.; Hillion, G.; Delfort, B.; Montagne, X. Glycerin for new biodiesel formulation. Oil Gas Sci. Technol. 2008, 63, 395–404. (2) Behr, A.; Obendorf, L. Development of a process for the acidcatalyzed etherification of glycerine and isobutene forming glycerine tertiary butyl ethers. Eng. Life Sci. 2003, 2, 185–189. (3) Klep acov a, K.; Mravec, D.; Bajus, M. tert-Butylation of glycerol catalysed by ion-exchange resins. Appl. Catal., A 2005, 294, 141–147. (4) Karinen, R. S.; Krause, A. O. I. New biocomponents from glycerol. Appl. Catal., A 2006, 306, 128–133. (5) Klep acov a, K.; Mravec, D.; Kaszonyi, A.; Bajus, M. Etherification of glycerol and ethylene glycol by isobutylene. Appl. Catal., A 2007, 328, 1–13.
r 2010 American Chemical Society
Therefore, in GTBE production, the main goals are to optimize both the synthesis and purification procedures. In the technical literature, two processes have been proposed up to day: the ARCO process6 and the Bhern and (6) Gupta, V. P. Glycerine ditertiary butyl ether preparation. U.S. Patent 5,476,971, 1995.
4668
pubs.acs.org/EF
Energy Fuels 2010, 24, 4668–4672
: DOI:10.1021/ef901230r
Di Serio et al.
2
Obendorf process. p-Toluenesulfonic acid was used as the catalyst in both processes. In the ARCO process, a partial conversion of glycerol is achieved to realize phase separation after the reaction. The lower phase (containing unreacted glycerol, catalyst, and a high concentration of mono-ethers) is recycled in the reaction. Isobutene is then removed from the upper phase by a flash unit, and the residual liquid phase containing ethers is washed with water to remove mono-ethers and traces of glycerol and residual catalyst. Thus, 0.8 kg of mono-ether per 1 kg of “higher ethers” is lost in 1.8 kg of wastewater contaminated with organics.2,6 In the Bhern and Obendorf process,2 a high glycerol conversion is achieved. Isobutene is fed directly to the reactor, while the glycerol is fed to an extractor (five theoretical stages), extracting catalysts and mono-ether from the reaction products. This phase is then recycled to the reactor, while the raffinate is sent to a flash unit to recover the unreacted isobutene, which is also recycled to the reactor. The liquid product of the flash unit is sent to a vacuum rectification column (40 theoretical stages, 1.5 reflux ratio, 0.005 bar pressure overhead) to obtain a GTBE mixture with a high concentration of di- and tri-ethers. Vacuum rectification is necessary to have a low temperature and low residence time to prevent the cleavage of ethers because of the presence of traces of catalyst. The bottom product in the rectification column (mainly mono-ethers and catalyst traces) is recycled to the reactor. Bhern and Obendorf used p-toluensulfonic acid as the catalyst, because they claimed the absence of isobutene oligomers with that homogeneous catalyst.2 On the contrary, oligomers were present in the final product when sulfonic resins were used. However, this choice, as we have seen, has serious consequences on the process design. As a matter of fact, a vacuum rectification column, operating is severe conditions, is necessary to remove the catalyst traces from the final product. In this paper, we report our results on the evaluation of the use of a heterogeneous acid catalyst (amberlyst 15) in the production of GTBE as a diesel additive. We report the best experimental synthesis conditions found and two new process schemes, which are simpler and more economical than those described above. In both of the schemes, GTBEs are extracted using biodiesel as the extracting agent. In the first very simple scheme, we obtained a final product with a glycerol content greater that that required by the EN 14214:2003 specification, while in the second scheme, an additional washing step with water lowered this value to the desired level.
Figure 1. Composition of the final mixture after isobutene stripping at 1 bar and 25 °C. Reaction conditions: temperature, 92 °C; pressure, 15 bar; catalyst, amberlyst 15; reaction time, 480 min; g of catalyst/g of glycerol, 0.011; mol of isobutene/mol of glycerol, 2. Table 1. Calculated Equilibrium and Kinetic Constants of Reactions 1-5 equilibrium and kinetic constants 3.29 0.81 0.09 0.002 0.01
Ke1 Ke2 Ke3 k4 (L mol-1 min-1) k5 (L mol-1 min-1)
The best performances in terms of conversion and selectivity were obtained under the conditions described in the caption of Figure 1 (isobutene/glycerol ratio = 2, and T = 90 °C), where the composition of the product mixture at the end of the reaction after isobutene stripping is shown. Higher isobutene/ glycerol ratios and/or temperatures favor the formation of oligomers, in agreement with data reported by Karinen and Krause.4 2.2. Kinetic Analysis. Considering all second-order reactions, data of Table 1 were used to estimate both the equilibrium constants for the reactions 1-3 and the kinetic constants for the reactions 4 and 5. A more accurate kinetic analysis would require many more experiments to obtain the confidence intervals of the parameters. However, the obtained values can be used in the process design to calculate the composition of the mixture coming from the reactor, with this being the main scope of the paper to demonstrate the possibility of using biodiesel as the extracting agent.
2. Experimental Section 2.1. Chemicals, Apparatus, and Procedure. The commercial strong acid ion-exchange resin amberlyst 15 from Rohm and Haas (France) was used as the etherification catalyst. All used chemicals [isobutene (Air Liquid) and glycerol (Aldrich)] were of analytical-grade purity. Several synthesis tests were performed, varying operating conditions (isobutene/glycerol ratio and temperature). The tests were conducted in an autoclave (1 L) with 500 rpm stirring rates. The samples of the reaction products were analyzed on a capillary column EC WAX (30 m 0.32 mm 0.5 μm). Analyses were carried out with a temperature program from 50 to 220 °C (with a rate of 10 °C min-1) and at 220 °C for 10 min isothermally.
3. Process Design The process was designed and simulated with CHEMCAD 6. Owing to the presence of polar compounds in the process, the nonrandom two liquid (NRTL) thermodynamic/activity model was selected for use as the property package for the simulation 2 3 N N P P τji Gji xj τ G x ij ij i 7 N X Gji xj 6 j 6 7 i þ ln γi ¼ N 6τij - N 7 ð6Þ N 4 5 P P P j Gij xi Gij xi Gij xi i
4669
i
i
Energy Fuels 2010, 24, 4668–4672
: DOI:10.1021/ef901230r
Di Serio et al.
where γi is the activity coefficient of component i, N is the number of components, xi is the molar fraction of component i, τji = Aji/T, Gji = exp(-Rjiτji), and Rij = Rji. The binary interaction parameters determined by Bhern and Obendorf2 were used for the isobutene-glycerol-glycerol ether systems. Because some NRTL binary interaction parameters were not available in the Bhern and Obendorf2 data or CHEMCAD 6 library, they were estimated using the universal functional activity coefficient (UNIFAC) liquid-liquid equilibrium. The used NRTL parameters are reported in Table 2. The simplified flow sheet of the process is reported in Figure 2, while the operative conditions and the characteristics of the
unit operations are reported in Table 3. The composition of the streams of the flow sheet of Figure 2 are reported in Table 4. The new process for the production of diesel additives is based on the presence of an extraction column (E1; six theoretical stages) using biodiesel [mixture of fatty acid methyl esters (FAMEs)] as the extraction solvent (in the simulation, the oleic methyl ester is used as a representative compound of the FAME mixture). This approach solves the problem of the low solubility of mono-ethers. As a matter of fact, only the soluble amount of ethers is extracted by biodiesel, while the excess remains in the raffinate together with the nonreacted glycerol. The raffinate is recycled to the reactor (R1). It is necessary to remove the hydrocarbon components from the reaction products before extraction with biodiesel. This operation is performed using a flash unit (F1) operating at 73 °C and 0.1 bar. The gaseous current is fed to another separation unit (F3) operating at 1 bar and 25 °C to recover C8-C12 hydrocarbons in the liquid phase. This product can be
Table 2. NRTL Parametersa i
j
Aij
Aji
Rij
i
j
Aij
Aji
Rij
1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 3 3 3
2 3 4 5 6 7 8 9 3 4 5 6 7 8 9 4 5 6
937.02 1229.9 -465.14 -465.14 -88.42 -111.39 1401.79 -19.65 207.34 1573.3 1573.3 1215.89 2170.78 -274.35 2499.96 -630.83 -630.83 260.27
721.75 -310.24 -742.47 -742.47 56.87 24.91 1419.19 -522.16 79.22 528.53 528.53 2011.19 1826.74 258.11 1260.56 680.4 680.4 1248.24
0.2 0.2 0.2 0.2 0.329 0.376 0.255 0.249 0.2 0.2 0.2 0.2 0.2 1.011 0.2 0.2 0.2 0.275
3 3 3 4 4 4 4 4 5 5 5 5 6 6 6 7 7 8
7 8 9 5 6 7 8 9 6 7 8 9 7 8 9 8 9 9
751.6 -415.84 1527.38 170.22 -224.58 198.32 -20.32 673.67 -377.8 -241.76 -46.44 955.45 -12.43 1200.53 159.26 1047.02 30.12 2499.99
1000.98 1613.06 191.25 2.92 1000.56 692.87 2499.9 17.33 746.9 666.52 2499.99 -579.06 -3.23 2499.99 -435.29 2499.93 -65.5 273.01
0.228 0.392 0.201 0.33 0.286 0.296 0.345 0.317 0.398 0.297 0.202 0.209 0.298 0.2 0.382 0.2 0.311 0.2
Table 3. Operative Conditions and Characteristics of the Unit Operations of Flow Sheets Reported in Figures 2 and 3a
a 1, isobutene; 2, glycerol; 3, mono-ether; 4, di-ether; 5, tri-ether; 6, di-isobutene; 7, tri-isobutene; 8, water; 9, methyl oleate.
unit operation
temperature (°C)
pressure (bar)
number of stages or volume (m3)
R1 E1 E2 E3 F1 F2 F3 F4 F5 F6
90.0 25.0 25.0 25.0 76.7 25.0 110.0 130.0 25.0 25.0
20.0 1.0 1.0 1.0 0.1 1.0 0.02 0.02 1.0 1.0
16 6 2 2 1 1 1 1 1 1
a
R, reactor; E, extraction column; F, flash unit.
Figure 2. Simplified flow sheet of the GTBE process giving a product FAME þ GTBE, donating about 0.09% of glycerol, which is a value greater than the EN 14214:2003 specification.
4670
Energy Fuels 2010, 24, 4668–4672
: DOI:10.1021/ef901230r
Di Serio et al.
Table 4. Composition of the Streams of the Flow Sheet of Figure 2 stream number
1
2
3
stream name
isobutene glycerol FAME
temperature (°C) pressure (bar) vapor mass fraction total (kg/h)
90 20 0 550
isobutene 100 glycerol mono-ether glycerol di-ether glycerol tri-ether glycerol 1-di-isobutylene tri-isobutylene methyl oleate
90 20 0 500
4
5
6
7
8
9
10
12
13
gasoline
25 1 0 12400
90 20 0 1386
73 0.1 0 1225
73 0.1 1 161
25 1 0 38
100
9.48 8.59 31.72 40.68 5.22 1.94 2.36 0.02
0.59 9.71 35.89 46.02 5.91 0.29 1.58 0.02
77.11 0.03 0.02 0.05 0.01 14.52 8.27 0.00
Component Mass % 18.35 0.59 0.59 0.00 0.12 9.71 9.71 50.17 0.07 35.89 35.89 49.13 0.21 46.02 46.02 0.60 0.05 5.91 5.91 0.00 45.90 0.29 0.29 0.00 35.31 1.58 1.58 0.00 0.00 0.02 0.02 0.10
100
11
73 1 0 1225
25 1 0 1225
25 1 0 212
14
15
16
17
25 1 0 9073
25 1 1 123
103 20 0 673
FAME þ GTBE 15 25 25 25 1 1 1 1 0 0 0 0 1385 4340 8060 13413
0.08 0.14 3.70 6.20 0.80 0.04 0.21 88.84
94.97 0.00 0.00 0.00 0.00 4.98 0.05 0.00
99.08 0.00 0.00 0.00 0.00 0.91 0.01 0.00
48.16 43.77 7.52 0.09 0.00 0.44 0.00 0.02 100
100
0.05 0.09 2.50 4.19 0.54 0.03 0.14 92.45
Figure 3. Simplified flow sheet of the GTBE process with final water washing to respect the limit imposed by the EN 14214:2003 specification.
used as a component of the gasoline pool. The gaseous phase exiting from the unit (3) contains mainly isobutene, which is recycled to the reactor R1. As can be seen (stream 17 in Table 4), this new process design allowed us to obtain the desired biodiesel-GTBE mixture (about 7.5% of GTBE1), but this cannot be directly used as a diesel additive. As a matter of fact, the content of glycerol is higher that the specification (0.020%) required by the EN 14214:2003 specification. This problem can be solved by washing the obtained product with water. Obviously, this solution requires a complication of the previously reported process, and the best configuration obtained by us is reported in Figure 3, while the best operative conditions and the characteristics of the unit operations are summarized in Table 3. The composition of the streams of the flow sheet of Figure 3 are reported in parts a and b of Table 5. As can be seen, the first part of the plant remains the same as that reported in Figure 2. In the new configuration, two new
extraction columns (two theoretical stages) and four flash separation units have been introduced. In the column E2, the mixture of biodiesel and GTBE is washed with water, obtaining as output a stream (37) with lower glycerol content. In the column E3, the raffinate (38) exiting from column E2 is treated with another portion of fresh biodiesel (15). This operation is necessary to recover the di-ethers contained in this stream. The biodiesel þ GTBE current (35) exiting from the E3 column is mixed with the same current type (37) exiting from the E2 column. The obtained stream (25) is then fed to the flash unit F3 (P = 0.02 bar, and T = 110 °C) to remove water. The purified stream (21) is then mixed with fresh biodiesel (20) to obtain a final product (33) with the required GTBE concentration. The flash unit F4 (P = 0.02, and T = 130 °C) has the scope to remove water from the raffinate current (36) exiting from the E3 extraction column, and the obtained liquid phase is then recycled to the R1 reactor. The flash units F5 (P = 1 bar, and 4671
Energy Fuels 2010, 24, 4668–4672
: DOI:10.1021/ef901230r
Di Serio et al.
Table 5. Composition of Some Streams of the Flow Sheet of Figure 3 a stream number
1
2
3
stream name temperature (°C) pressure (bar) vapor mass fraction total (kg/h)
isobutene 90 20 0 550
glycerol 90 20 0 500
FAME 25 1 0 12400
90 20 0 1706
76.7 0.1 0 1549
76.7 0.1 1 156
25 1 0 28.5
100
7.78 10.86 36.29 38.39 4.00 1.39 1.19 0.04 0.07
0.42 11.95 39.96 42.26 4.40 0.19 0.72 0.02 0.08
80.58 0.04 0.02 0.06 0.01 13.25 5.84 0.20 0.00
Component Mass % 18.38 0 0.06 94.44 0.21 42.44 0.14 0.00 0.13 56.06 3.60 0.00 0.36 1.31 5.93 0.00 0.06 0.00 0.62 0.00 48.95 0.00 0.03 5.29 31.87 0.00 0.10 0.04 0.03 0.05 0.00 0.23 0.00 0.14 89.51 0.00
isobutene 100 glycerol mono-ether glycerol di-ether glycerorol tri-ether glycerol 1-di-isobutylene tri-isobutylene water methyl oleate
100
4
5
6
7
10 25. 1 0 400
11
12
25 1 0 10949
25 1 1 128
13
14
15
16
18
19
103. 20 0 682
water 85 25 25 25 20 1 1 1 0 0 0 0 1705 1500 9800 40
17
50.8 20 0 524
108 20 0 132
98.93 0.00 0.00 0.00 0.00 1.02 0.01 0.05 0.00
39.52 39.97 18.44 1.54 0.00 0.41 0.00 0.04 0.07 100
0.00 34.69 60.00 5.02 0.01 0.00 0.00 0.06 0.22
94.44 0.00 0.00 0.00 0.00 5.28 0.04 0.25 0.00
37
38
100 100
b stream number
20
21
23
24
25
26
27
28
29
31
25 1 1 3.6
waste water 25 1 0 45
stream name temperature (°C) pressure (bar) vapor mass fraction total (kg/h) isobutene glycerol mono-ether glyce di-ether glycero tri-ether glycer 1-di-isobutylene tri-isobutylene water methyl oleate
32
25 1 0 1100
110 0.02 0 12306
25 1 0 9.8
130 0.02 1 7.4
25 1 0 12357
110 0.02 1 51
130 0.02 0 124
25 1 0 42
100
0.00 0.02 2.43 5.08 0.55 0.01 0.08 0.01 91.81
24.79 0.00 0.69 22.54 1.20 17.20 15.52 0.42 17.64
0.00 8.08 30.94 8.1 0.02 0.00 0.00 50.93 1.93
0.05 0.02 2.44 5.08 0.55 0.02 0.09 0.29 91.44
11.68 0.55 4.83 4.32 0.23 3.48 2.97 68.57 3.37
0.00 9.78 72.63 16.96 0.04 0.00 0.00 0.1 0.49
Component Mass % 26.58 96.2 0.21 20.47 0.15 0.00 2.05 0.03 0.25 0.00 10.38 0.25 5.5 0.00 1.35 6.01 0.32 0.00 0.00 0.35 37.57 2.74 0.01 40.64 25.31 0.04 0.01 27.67 0.21 1.02 85.66 0.08 4.12 0.00 0.32 4.5
T = 25 °C) and F6 (P = 1 bar, and T = 25 °C) allow for a better recovery of isobutene, which is recycled to the R1 reactor. The new process simulation gives rise to a final biodieselGTBE product, with a concentration of GTBE equal to about 7.5% by weight as in the first configuration, but mainly gives a final concentration of glycerol (0.02%) in agreement with the EN 14214:2003 specification. Therefore, this product can be used as the biodiesel additive, in agreement with the results of Jaecker-Voirol et al.1 Moreover, the obtained gasoline (stream 32) is of better quality than the one obtained in the previous plant configuration. As a matter of fact, now, the glycerol concentration is much lower (0.03 instead 0.12%). The use of water as the extracting agent has as a consequence the presence of a wastewater stream (31). However, the amount of this stream is very low (45 kg/h) and is much more convenient that the ones used in the Arco process. As a matter
33
gasoline FAME þ GTBE 25 103 1 1 0 0 38 13406 0 0.02 2.23 4.67 0.51 0.01 0.07 0.01 92.48
34
35
36
25 1 0 38
25 1 0 1633
25 1 0 132
25 1 0 10724
25 1 0 265
0.25 0.75 6.35 0.04 0.00 0.01 0.01 92.59 0.01
0.00 0.06 3.59 4.11 0.14 0.00 0.00 0.11 92.00
0.00 9.68 70.29 16.46 0.04 0.00 0.00 2.95 0.57
0.06 0.02 2.27 5.23 0.61 0.03 0.10 0.32 91.36
0.01 5.16 57.15 33.53 0.86 0.00 0.00 2.13 1.15
of fact, in our case, 0.006 kg of ether per 1 kg of GTBE is lost in 0.045 kg of wastewater contaminated with organics. 4. Conclusions The possibility to use an acid sulfonic resin as catalysts for the production of GTBE has been demonstrated. A new process using biodiesel as the extracting agent for GTBE has been proposed. The performances of this process have been evaluated on the basis of a preliminary kinetic analysis, showing that the new process is simpler than the previous ones proposed in the literature and gives mixtures of biodiesel and GTBE that can be used directly as diesel additives. Acknowledgment. The authors are grateful to the Italian Ministry of Foreign Affairs (MAE, Code 269/P/0127728), Direzione Generale per la Promozione e la Cooperazione Culturale, CN/ ASIA PRO ECO/11 (109087), and Merloni Progetti SpA for the financial support in developing the present work.
4672
