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On-Line Gas Chromatographic Analysis of CO2 Hydrogenation Products
Studies in SurfaceScienceand Catalysis 153 S.-E. Park, J.-S. Changand K.-W.Lee (Editors) 9 2004ElsevierB.V. All rightsreserved.
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ON-LINE GAS CHROMATOGRAPHIC ANALYSIS OF CO2 H Y D R O G E N A T I O N P R O D U C T S M. Reinikainen* and M. Niemel& VTT Processes, P.O. Box 1602, FIN-02044 VTT, Finland *Correspondingauthor, e-mail: [email protected] I. B A C K G R O U N D
Greenhouse gas mitigation strategies emphasize improving efficiencies of energy conversion and utilization processes, but also introduce new means for capture, storage and fixation of the CO2 produced. Also chemical utilization of carbon dioxide has gained attention and among conversion processes, those for licluid automotive fuels are generally much larger in scale than processes producing chemicals.' Accordingly, liquid fuels production could act as a substantial CO2 sink provided hydrogen used in the conversion of CO2 is produced via noCO2-release basis and effective catalysts are available. Thus, the anticipated processes require catalytic materials, which activate CO2 at low or moderate temperatures and convert it to useful products such as olefins, alcohols or hydrocarbons. This work emphasizes in RWGS + FT synthesis reaction wherein a mixture of CO2 and hydrogen is converted in a tubular fixed bed reactor on an iron containing catalyst to a highly complex mixture of unsaturated and saturated hydrocarbons (C~-C20+), CI-C8§ oxygenates as well as water and carbon monoxide. The proportional share of various types of products and the overall conversion may substantially vary according to the catalyst used thus setting a great challenge for the on-line analysis system. On-line GC-systems have been used also previously for the analysis of the CO2 hydrogenation reaction product. For example, the analysis setup has consisted of a GC equipped with a TCD for the analysis of CO2, CO, Ar (internal standard) and CH4 using a carbosphere column, and a FID for the analysis of light (CI-C8) hydrocarbons using a GS-Q capillary column 2'3. This analysis has also been used in combination with an off-line analysis. Namely, the liquid hydrocarbons have been collected in a product trap (2~ and the oil phase has been analyzed off-line with a GC equipped with a SPB-1 capillary column and a FID. 3 In another experimental setup, the analysis of the gases H2, CO2, CO, CH4 and the internal standard Ar has been carried out on-line and the organic products (C1-C20) have been determined off-line using an ampoule sampling technique and an adapted GC equipped with a FID. 4 In this specific system developed by Schulz small glass ampoule samples (ampoule filling time < 0.1 s) are taken from the gaseous product stream providing the possibility to monitor the time resolution of its composition. 5 Yet another on-line setup for all products has comprised GCs equipped with Porapak Q for CO2, MS-13X for methane and CO, a PLOT column for hydrocarbons and a PEG 6000 (15%)+TCEP (8%) supported on Chromosorb WAW (60/80 mesh) for alcohols. 6
566 In our case the aim was to develop an economic analysis system based on one GC and yet capable of determining on-line all the essential CO2 hydrogenation products including the permanent gases, light hydrocarbons, higher hydrocarbons and oxygenates and thus to avoid the use of an additional off-line analysis which is both labor and time consuming. A fast and reliable on-line product analysis of this complex reaction product mixturein a singlerun is essential for efficient catalyst development. 2. RESULTS AND DISCUSSION 2.1. The analysis system The tailored analytical system for CO2 hydrogenation product consists of an Agilent-6890 gas chromatograph equipped with one FID, one TCD, three pneumatic valves and four different columns. Packed Porapak-Q (1 m precolumn) and Carboxen-1000 -columns were used to analyze the inorganic gases and CI-C3 hydrocarbons with the TCD. The other hydrocarbons (C3-C16) and oxygenates were separated with a combination of a polar Innowax (0.32 mm, 60 m) and a non-polar DB-1 -capillary column (0.32 mm, 30 m) in series. The analysis system was tuned using a true product stream: a continuous flow tubular reactor was run at 250300~ and 10 bar using a Fe-catalyst. The product thus formed was carried to on-line sampling valves via a heated (175~ line. The identification of the main products was based on the retention times obtained by injecting model compounds. Other components of the product stream were identified by collecting samples of the light gases and liquid products, which were analysed by a JEOL SX-102 mass spectrometer equipped with a similar combination of columns as used in the on-line GC. The MS-GC analysis of the light gases and the hydrocarbon phase of the liquid product allowed to identify typical Fischer-Tropsch -reaction products, consisting mainly of a homologous series of 1-olefins and n-alkanes with a high share of olefins as is typical for the promoted iron catalysts. ~ In addition a large number of other reaction products such as other olefins, branched, cyclic and aromatic products could be identified by MS. In a typical GCanalysis over 90 w-% of all the hydrocarbon products could be detected and identified. According to MS-GC, the water soluble phase of the liquid product consisted mainly of acetaldehyde, n-alcohols, 2-ketones and carboxylic acids with ethanol being the main product, see also Table 1. In addition to these products, other compounds such as heavier alcohols, ketones and acids as well as various isomers and esters were present in the product and their share could be estimated to be less than 2 w-%.
Table 1. Oxygen-containing products in a typical GC analysis. Type of product Alcohols Ketones Carboxylic acids Others
ComLpound methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol meth acetone, 2-butanone, 2-pentanone, 2-hexanone acetic acid, propionic acid, butanoic acid, pentanoic acid acetaldehyde, water .......
567 The obtained GC detector responses were corrected using response factors published by Dietz 7. When data for a certain compound (such as acetaldehyde) was not available in the literature, it was either estimated or measured by injecting standard samples. Methane, which could be detected by both detectors, was used for the calibration of the sensitivities of the column/detector responses. In all, the total analysis time for one sample could be reduced to 30 min at its minimum and the tuned analysis system allowed to detect more than one hundred compounds out of the complex product mixture of CO2 hydrogenation reaction. The analysis data from the report files of an Agilent ChemStation (one for each detector) were collected by a command macro on the final Excel-template to calculate CO2 conversion, product selectivities and other characteristic features as well as to generate figures for catalyst evaluation and development. 2.2. Experimental data for Fe-Cu-A1-K The developed analysis system has successfully been used to investigate the reactivity of different catalysts in CO2 hydrogenation. The comparative experiments have been carried out using 1g of an in situ reduced catalyst in a fixed bed tubular reactor, initially pressurized by hydrogen to 10 bar at 300~ and then switched to a gas flow of 2 1/h of CO2:H2:Ar in ratio 3:6:1. Since it is well known that FT synthesis requires long reaction periods to attain steady state performance, the experimental runs were carried out for six days. The reaction parameters have been continuously monitored and samples have been withdrawn for analysis at desired times manually or automatically, see the results in Figure 1.
Figure 1. The performance of the Fe-Cu-A1-K2 catalyst (1 g) in CO2 hydrogenation at 10 bar and 300~ gas flow 2 1/h of CO2:H2:Ar (30:60:10). The products (CO + HC + OXY) have been normalized to 100%.
568 Apparently, the mass balance increased with time on stream when the reaction product displaced the hydrogen originally present in the pressurized reactor pipelines, and the liquid products gradually diffused from the fixed bed and were carried to analysis. In about 24 hours the catalyst reached a fairly constant level of conversion of approx. 23-26 %; a result in agreement with earlier findings 4. The proportion of different types of products changed in the course of a run, see Figure 1. In general the product was highly olefinic as observed also before 4, and the product distributions obeyed the Schulz-Flory polymerization law as shown illustratively in Figure 2. Consequently also significant amounts of heavy, high boiling hydrocarbons were formed and gradually also condensed in the lines and valves causing baseline shift and inflated heavy hydrocarbon peaks. This is indicated also by a change in the slope of the Schulz-Flory plot at higher hydrocarbon numbers in samples after several hours on stream, see Figure 2. The system could be, however, easily reconditioned by rinsing the lines with a solvent and baking the columns aider each run. The product stream also contains a significant amount of carboxylic acids and therefore the reaction product is highly corrosive especially at elevated temperatures. Accordingly, frequent inspection and maintenance of the reactor and analysis system components is necessary.
"E"
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Figure 2. Schulz-Flory plot of hydrocarbons and oxygenates (data point Figure 1, time 49 h). REFERENCES 1 T. Riedel, M. Clayes, H. Schulz, G. Schaub, S-S. Nam, K-W. Jun, M-J. Choi, G. Kishan and K-W. Lee, Appl. Catal. A:General, 186 (1999) 201-213 2 J-S. Hong, J.S. Hwang, K-W. Jun, J.C. Sur and K-W. Lee, Appl. Catal. A:General 218 (2001) 53-59 3 S-R- Yan, K-W. Jun, J-S. Hong, M-J. Choi and K-W. Lee, Appl. Catal. A:General 194-195 (2000) 63-70 4 T. Riedel, G. Schaub, K-W. Jun and K.W. Lee, Ind. Eng. Chem. Res. 40 (2001) 1355-1363 5H. Schulz, S. Nehren, Erd61Kohle Erdgas Petrochem 39 (1986) 93-94 6 H. Ando, Y. Matsumura and Y. Souma, J. Mol. Catal A: Chemical 154 (2000) 23-29 7j. Dietz, Gas Chromatography,February, 1967, 68-70
565
ON-LINE GAS CHROMATOGRAPHIC ANALYSIS OF CO2 H Y D R O G E N A T I O N P R O D U C T S M. Reinikainen* and M. Niemel& VTT Processes, P.O. Box 1602, FIN-02044 VTT, Finland *Correspondingauthor, e-mail: [email protected] I. B A C K G R O U N D
Greenhouse gas mitigation strategies emphasize improving efficiencies of energy conversion and utilization processes, but also introduce new means for capture, storage and fixation of the CO2 produced. Also chemical utilization of carbon dioxide has gained attention and among conversion processes, those for licluid automotive fuels are generally much larger in scale than processes producing chemicals.' Accordingly, liquid fuels production could act as a substantial CO2 sink provided hydrogen used in the conversion of CO2 is produced via noCO2-release basis and effective catalysts are available. Thus, the anticipated processes require catalytic materials, which activate CO2 at low or moderate temperatures and convert it to useful products such as olefins, alcohols or hydrocarbons. This work emphasizes in RWGS + FT synthesis reaction wherein a mixture of CO2 and hydrogen is converted in a tubular fixed bed reactor on an iron containing catalyst to a highly complex mixture of unsaturated and saturated hydrocarbons (C~-C20+), CI-C8§ oxygenates as well as water and carbon monoxide. The proportional share of various types of products and the overall conversion may substantially vary according to the catalyst used thus setting a great challenge for the on-line analysis system. On-line GC-systems have been used also previously for the analysis of the CO2 hydrogenation reaction product. For example, the analysis setup has consisted of a GC equipped with a TCD for the analysis of CO2, CO, Ar (internal standard) and CH4 using a carbosphere column, and a FID for the analysis of light (CI-C8) hydrocarbons using a GS-Q capillary column 2'3. This analysis has also been used in combination with an off-line analysis. Namely, the liquid hydrocarbons have been collected in a product trap (2~ and the oil phase has been analyzed off-line with a GC equipped with a SPB-1 capillary column and a FID. 3 In another experimental setup, the analysis of the gases H2, CO2, CO, CH4 and the internal standard Ar has been carried out on-line and the organic products (C1-C20) have been determined off-line using an ampoule sampling technique and an adapted GC equipped with a FID. 4 In this specific system developed by Schulz small glass ampoule samples (ampoule filling time < 0.1 s) are taken from the gaseous product stream providing the possibility to monitor the time resolution of its composition. 5 Yet another on-line setup for all products has comprised GCs equipped with Porapak Q for CO2, MS-13X for methane and CO, a PLOT column for hydrocarbons and a PEG 6000 (15%)+TCEP (8%) supported on Chromosorb WAW (60/80 mesh) for alcohols. 6
566 In our case the aim was to develop an economic analysis system based on one GC and yet capable of determining on-line all the essential CO2 hydrogenation products including the permanent gases, light hydrocarbons, higher hydrocarbons and oxygenates and thus to avoid the use of an additional off-line analysis which is both labor and time consuming. A fast and reliable on-line product analysis of this complex reaction product mixturein a singlerun is essential for efficient catalyst development. 2. RESULTS AND DISCUSSION 2.1. The analysis system The tailored analytical system for CO2 hydrogenation product consists of an Agilent-6890 gas chromatograph equipped with one FID, one TCD, three pneumatic valves and four different columns. Packed Porapak-Q (1 m precolumn) and Carboxen-1000 -columns were used to analyze the inorganic gases and CI-C3 hydrocarbons with the TCD. The other hydrocarbons (C3-C16) and oxygenates were separated with a combination of a polar Innowax (0.32 mm, 60 m) and a non-polar DB-1 -capillary column (0.32 mm, 30 m) in series. The analysis system was tuned using a true product stream: a continuous flow tubular reactor was run at 250300~ and 10 bar using a Fe-catalyst. The product thus formed was carried to on-line sampling valves via a heated (175~ line. The identification of the main products was based on the retention times obtained by injecting model compounds. Other components of the product stream were identified by collecting samples of the light gases and liquid products, which were analysed by a JEOL SX-102 mass spectrometer equipped with a similar combination of columns as used in the on-line GC. The MS-GC analysis of the light gases and the hydrocarbon phase of the liquid product allowed to identify typical Fischer-Tropsch -reaction products, consisting mainly of a homologous series of 1-olefins and n-alkanes with a high share of olefins as is typical for the promoted iron catalysts. ~ In addition a large number of other reaction products such as other olefins, branched, cyclic and aromatic products could be identified by MS. In a typical GCanalysis over 90 w-% of all the hydrocarbon products could be detected and identified. According to MS-GC, the water soluble phase of the liquid product consisted mainly of acetaldehyde, n-alcohols, 2-ketones and carboxylic acids with ethanol being the main product, see also Table 1. In addition to these products, other compounds such as heavier alcohols, ketones and acids as well as various isomers and esters were present in the product and their share could be estimated to be less than 2 w-%.
Table 1. Oxygen-containing products in a typical GC analysis. Type of product Alcohols Ketones Carboxylic acids Others
ComLpound methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol meth acetone, 2-butanone, 2-pentanone, 2-hexanone acetic acid, propionic acid, butanoic acid, pentanoic acid acetaldehyde, water .......
567 The obtained GC detector responses were corrected using response factors published by Dietz 7. When data for a certain compound (such as acetaldehyde) was not available in the literature, it was either estimated or measured by injecting standard samples. Methane, which could be detected by both detectors, was used for the calibration of the sensitivities of the column/detector responses. In all, the total analysis time for one sample could be reduced to 30 min at its minimum and the tuned analysis system allowed to detect more than one hundred compounds out of the complex product mixture of CO2 hydrogenation reaction. The analysis data from the report files of an Agilent ChemStation (one for each detector) were collected by a command macro on the final Excel-template to calculate CO2 conversion, product selectivities and other characteristic features as well as to generate figures for catalyst evaluation and development. 2.2. Experimental data for Fe-Cu-A1-K The developed analysis system has successfully been used to investigate the reactivity of different catalysts in CO2 hydrogenation. The comparative experiments have been carried out using 1g of an in situ reduced catalyst in a fixed bed tubular reactor, initially pressurized by hydrogen to 10 bar at 300~ and then switched to a gas flow of 2 1/h of CO2:H2:Ar in ratio 3:6:1. Since it is well known that FT synthesis requires long reaction periods to attain steady state performance, the experimental runs were carried out for six days. The reaction parameters have been continuously monitored and samples have been withdrawn for analysis at desired times manually or automatically, see the results in Figure 1.
Figure 1. The performance of the Fe-Cu-A1-K2 catalyst (1 g) in CO2 hydrogenation at 10 bar and 300~ gas flow 2 1/h of CO2:H2:Ar (30:60:10). The products (CO + HC + OXY) have been normalized to 100%.
568 Apparently, the mass balance increased with time on stream when the reaction product displaced the hydrogen originally present in the pressurized reactor pipelines, and the liquid products gradually diffused from the fixed bed and were carried to analysis. In about 24 hours the catalyst reached a fairly constant level of conversion of approx. 23-26 %; a result in agreement with earlier findings 4. The proportion of different types of products changed in the course of a run, see Figure 1. In general the product was highly olefinic as observed also before 4, and the product distributions obeyed the Schulz-Flory polymerization law as shown illustratively in Figure 2. Consequently also significant amounts of heavy, high boiling hydrocarbons were formed and gradually also condensed in the lines and valves causing baseline shift and inflated heavy hydrocarbon peaks. This is indicated also by a change in the slope of the Schulz-Flory plot at higher hydrocarbon numbers in samples after several hours on stream, see Figure 2. The system could be, however, easily reconditioned by rinsing the lines with a solvent and baking the columns aider each run. The product stream also contains a significant amount of carboxylic acids and therefore the reaction product is highly corrosive especially at elevated temperatures. Accordingly, frequent inspection and maintenance of the reactor and analysis system components is necessary.
"E"
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-I-- OXY
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Figure 2. Schulz-Flory plot of hydrocarbons and oxygenates (data point Figure 1, time 49 h). REFERENCES 1 T. Riedel, M. Clayes, H. Schulz, G. Schaub, S-S. Nam, K-W. Jun, M-J. Choi, G. Kishan and K-W. Lee, Appl. Catal. A:General, 186 (1999) 201-213 2 J-S. Hong, J.S. Hwang, K-W. Jun, J.C. Sur and K-W. Lee, Appl. Catal. A:General 218 (2001) 53-59 3 S-R- Yan, K-W. Jun, J-S. Hong, M-J. Choi and K-W. Lee, Appl. Catal. A:General 194-195 (2000) 63-70 4 T. Riedel, G. Schaub, K-W. Jun and K.W. Lee, Ind. Eng. Chem. Res. 40 (2001) 1355-1363 5H. Schulz, S. Nehren, Erd61Kohle Erdgas Petrochem 39 (1986) 93-94 6 H. Ando, Y. Matsumura and Y. Souma, J. Mol. Catal A: Chemical 154 (2000) 23-29 7j. Dietz, Gas Chromatography,February, 1967, 68-70