Ind. Eng. Chem. Res. 2004, 43, 6349-6354
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Etherification of C6 Fischer-Tropsch Material for Linear r-Olefin Recovery Arno de Klerk† Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and Development, P.O. Box 1, Sasolburg 1947, South Africa
Purification of 1-pentene and 1-hexene by distillation from Fischer-Tropsch products is hampered by the close-boiling compounds 2-methyl-1-butene, 2-methyl-1-pentene, and 2-ethyl1-butene. Etherification and double-bond isomerization of these compounds in the presence of the linear R-olefin, with little loss of the R-olefin, were demonstrated. The focus of this study is on the hexene isomers. The effects of the feed composition, methanol to tertiary olefin ratio, space velocity, temperature, and different sulfonic acid resin catalysts (Amberlyst 15 and 35) were investigated. Blending octane numbers for the ethers in standard petrol and FischerTropsch petrol are given, showing 2-methyl-2-methoxypentane to be a poor fuel additive. Experimental equilibrium data for the reaction network to produce 2-methyl-2-methoxypentane are also presented. Introduction In a Fischer-Tropsch refining environment, etherification can be found in two roles. The role that is familiar to most is the production of octane boosters for automotive fuel, like methyl tert-butyl ether, ethyl tertbutyl ether, and tert-amyl methyl ether (TAME). The role that is less well-known is that of enabling technology for the purification of linear R-olefins. This is interestingly enough its traditional role. Etherification was initially developed as a necessary step in the purification of 1-butene for use as comonomer in the manufacturing of plastic. Because Fischer-Tropsch products are rich in linear R-olefins, like 1-pentene and 1-hexene, etherification is of importance if these chemicals are to be purified for comonomer use. An etherification-based process for the purification of 1-pentene and 1-hexene from high-temperature Fischer-Tropsch products was commercialized in 1994 at the Sasol refineries in Secunda, South Africa. Presently the installed capacity is 220 000 tons/year, and it is used mostly for 1-hexene production.1 There are three troublesome olefins preventing the purification of 1-pentene and 1-hexene by distillation only (Table 1).2 There is only a 1.2 °C boiling point difference between 2-methyl-1-butene (2M1B) and 1-pentene. Likewise, 2-methyl-1-pentene (2M1P) and 2-ethyl1-butene (2E1B) straddles 1-hexene with boiling point differences of 2.6 and 1.4 °C, respectively. The two easiest ways of getting rid of these molecules are by double-bond isomerization and etherification. Both transformations take place over an acidic catalyst, and both will be considered in this paper. There are also paraffins in the same boiling range that cannot be removed. These molecules are not reactive and are allowed as contaminants in the final comonomer grade product. However, the FischerTropsch product contains mostly linear hydrocarbons and has a low paraffin-to-olefin ratio.3 The isoparaffin content is consequently low. † Tel.: +27 16 960-2549. Fax: +27 16 960-2826. E-mail:
[email protected].
Table 1. Boiling Point of Hydrocarbons Close Boiling to 1-Pentene and 1-Hexene compound
normal boiling point (°C)
3-methyl-1-butene 2-methylbutane 1-pentene 2M1B n-pentane cis-2-pentene trans-4-methyl-2-pentene 2M1P 2-methylpentane 3-methylpentane 1-hexene 2E1B cis-3-hexene
20 27.8 30 31.2 36.1 36.9 58.5 60.7 60.3 63.3 63.3 64.7 66.4
Literature on the etherification of 2M1B is plentiful.4 Data on the etherification of C5 Fischer-Tropsch material would therefore arguably not add much new knowledge to the field, despite the present study having a different focus. Experimental work on C5 FischerTropsch material has indeed shown etherification and isomerization of 2M1B in the presence of 1-pentene to be quite facile. Even at more than 95% conversion of 2M1B, etherification selectivity in the order of 75-80% could be achieved, with little loss of 1-pentene due to double-bond isomerization. Both products from the linear R-olefin purification process, TAME and 2-methyl-2-butene, are high octane fuel components. This paper will focus on the etherification and isomerization of 2M1P and 2E1B in C6 Fischer-Tropsch material. Although the aim was to study these reactions for linear R-olefin purification, there is little in the published literature5-15 available on the etherification of C6 olefins in general. Some discussion on fuel properties is included. From the discussion, it will be clear that the reaction of methanol with 2M1P and 2E1B over an acidic catalyst is an efficient way of removing these species from the C6 matrix, but that the etherification products have limited value as fuel additives for improving octane.
10.1021/ie049704y CCC: $27.50 © 2004 American Chemical Society Published on Web 09/01/2004
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Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004
Table 2. Catalyst Characterization Data property particle size (mm) hydrogen ion concentration (mol/kg of dry resin) surface area (m2/g) porosity (mL/g) average pore diameter (nm)
Amberlyst 15
Amberlyst 35
0.4-1.2 4.7
0.4-1.25 5.4
50 0.36 24
44 0.35 30
Table 3. Characterization of C6-Cut Fischer-Tropsch-Derived Feed Material (mass %) compound
feed 1
feed 2
feed 3
C5 and lighter material 4-methyl-1-pentene 3-methyl-1-pentene isohexanes 2,3-dimethylbutenes 4-methyl-cis-2-pentene 2M1P 1-hexene n-hexane 2E1B 2/3-hexenes methylcyclopentenes 2-methyl-2-pentene C7 and heavier material oxygenates and dienes (mostly ketones)
0.0 0.2 0.2 4.3 0.0 0.0 5.8 86.6 0.2 0.7 0.1 0.0 0.9 0.6 0.4
0.0 3.4 3.4 5.4 0.7 0.0 5.3 68.7 2.0 0.7 0.8 0.6 0.1 0.1 8.8
1.2 9.2 10.9 4.5 0.8 0.2 4.2 55.7 6.5 0.5 2.0 0.6 0.8 2.2 0.7
Experimental Section Apparatus and Operating Procedure. The experimental work was done in a metal fixed bed reactor with an internal diameter of 28 mm and an internal thermocouple sheath of 6 mm diameter. The catalyst was supported on a wire mesh and was loaded in its original physical form. Before the catalyst was loaded, it was washed at ambient conditions with 10 bed volumes of methanol to remove residual water. It was found that failure to do so resulted in a long period of stabilization, during which conversion was suppressed. This was not a novel observation, and the influence of water has been discussed in the literature.16 The liquid feed was premixed and introduced by means of a pump at the bottom of the reactor. The reactor was operated in upflow mode to ensure proper wetting of the catalyst. Three independently operated heaters along the length of the reactor were used to control the temperature and to ensure isothermal conditions in the catalyst bed. The reaction is very exothermic, and despite the low reactive olefin concentration in the feed, deviations of 2-3 °C from perfect isothermal behavior were noted. The bottom of the reactor, below the catalyst bed, was used as a preheating zone. All experiments were done in the liquid phase at a pressure of 400 kPa gauge. The atmospheric pressure was 85 ( 1 kPa. Catalysts and Chemicals. Two sulfonic acid resin catalysts produced by Rohm & Haas, namely, Amberlyst 15 and 35 (Table 2), were used during the test work. The methanol used was an analytical analysis grade supplied by AECI, Sandton, South Africa. The methanol was characterized and found to be 99.85% pure, with the main impurity being water (0.1%) and the remainder being some oxygenates (ethanol, acetone, etc.). The olefinic feed material has been prepared by distillation from commercial high-temperature Fischer-Tropsch product streams from the Sasol Synfuels refineries in Secunda, South Africa (Table 3). Feeds 1 and 3 were taken from the Fischer-Tropsch condensates, and feed 2 was taken from the Fischer-
Tropsch stabilized light oil (SLO). The condensates are obtained from stepwise condensation of the vapor remaining after the Fischer-Tropsch product has been cooled and the oil and aqueous products have been knocked out. SLO is obtained from the oil product after stabilization by removal of the dissolved volatile material. Product Characterization. All product analyses were done by gas chromatography (GC). For quantitative data, a flame ionization detector was used, and for characterization, a mass spectrometer with electron impact fragmentation was used. The low 2E1B content in the feed and the fact that it has two double-bond isomers made it difficult to obtain accurate selectivity data for this species. The selectivity values reported are for 2M1P conversion only. An internal standard was not used, and results show a bias of up to 5% (relative percent deviation) due to an increase in the product mass. This is within the experimental uncertainty of the GC results and has not been adjusted. Results Initial tests in the absence of any alcohol showed that Amberlyst 15 was quite active for double-bond isomerization of 2M1P (85% conversion) and 2E1B (100% conversion) but that about 10% of 1-hexene was also converted. This line of experimentation was consequently dropped in favor of etherification, and all subsequent tests were done in the presence of an alcohol. The first test run (run A) with methanol was done to establish a sensible operating envelope for the etherification reaction. Feed 1, a clean condensate-derived feed, was reacted over Amberlyst 15. The next test run (run B) over Amberlyst 15 was done with feed 2, an oxygenate-rich SLO-derived feed (Table 4). Few side reactions took place in either run apart from doublebond isomerization. The last set of test runs was done with feed 3, which is very similar to the commercial Secunda hexene train 1 feed (Table 5). Only the first test (run C) used Amberlyst 15 as the catalyst, and subsequent tests (runs D and E) used Amberlyst 35 as the catalyst. Run D was started at a low temperature (55 °C) and showed suppressed catalyst activity during the whole run. This was not found during run E, which was started at a higher temperature (70 °C). Some oligomerization (gum formation) was suspected but not seen in the product. The reason for the difference between runs D and E was not resolved. During all runs, few side reactions were observed apart from double-bond isomerization. Discussion Reaction Equilibrium. Experimental equilibrium data for the etherification of tertiary hexene isomers are limited. Guin and co-workers11 presented data for the etherification of 2,3-dimethyl-1-butene (23DM1B) and 2,3-dimethyl-2-butene (23DM2B) with methanol, while Zhang and Datta8,10 presented equilibrium data for the etherification of 2M1P and 2M2P, 23DM1B and 23DB2B, as well as 2E1B, cis-3-methyl-2-pentene, and trans-3methyl-2-pentene with ethanol. Unfortunately, these studies were concerned with hexyl ethers different from those of the present study, and only the olefin equilibrium data were applicable. Because maximum removal
Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004 6351 Table 4. Results with Feed 1 (Run A) and Feed 2 (Run B) over Amberlyst 15 run
feed
temp (°C)
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 B1 B2 B3 B4 B5 B6 B7
1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2
69 67 67 66 67 80 88 59 65 65 69 70 69 70 80 75 70 65 80 70
conversion (%)
LHSV (h-1)
methanol:tert-olefin (mol/mol)
1-hexene
2M1P
2E1B
etherification selectivity (%)
1 1 1 1 1 2 2 2 2 2.8 2 2.8 3.5 0.5 1 1 1 1 1 1
0.3 0.7 1.0 2.2 5.7 1.8 1.8 2.2 2.2 2.2 2.2 2.2 2.2 2.5 2.5 2.5 2.5 2.5 5.5 5.5
8 2 3 2 3 3 3 2 2 2 2 2 2 4 99 >99 >99 >99 >99 88 93 96 97 98 98 97 94 96 94 90 75 96 94
33 57 61 76 86 66 60 81 78 78 76 76 76 45 42 45 44 41 58 56
Table 5. Results with Feed 3 over Amberlyst 15 (Run C) and Amberlyst 35 (Runs D and E) run
feed
temp (°C)
C1 D1 D2 D3 D4 E1 E2 E3
3 3 3 3 3 3 3 3
70 55 60 66 70 70 68 65
conversion (%)
LHSV (h-1)
methanol:tert-olefin (mol/mol)
1-hexene
2M1P
2E1B
etherification selectivity (%)
1 1 1 1 1 1 1 1
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
