Article pubs.acs.org/EF
Inline Analysis of the Dry Reforming Process through Fourier Transform Infrared Spectroscopy and Use of Nitrogen as an Internal Standard for Online Gas Chromatography Analysis Roland Lee, Raynald Labrecque, and Jean-Michel Lavoie* Department of Chemical and Biotechnological Engineering, Université de Sherbrooke, Sherbrooke, Quebec J1K 2R1, Canada S Supporting Information *
ABSTRACT: In this work, Fourier transform infrared spectroscopy (FTIR) was used as a process analytical tool (PAT), allowing for quantification of the output gas from a dry reforming of methane (DRM) reaction. This PAT was correlated with gas chromatography (GC) analysis, where nitrogen was used as an internal standard. Because nitrogen does not react under dry reforming (DR) conditions (1 atm and 700−950 °C), it was found to be reliable and accurate, thus allowing for an easy mass balance of the reaction. N2 as an internal standard allowed for a more streamlined determination of the effect of the temperature and flow rate on conversion of CO2 and CH4. The DRM reaction is an example of thermal processes where FTIR could be used as an inline process analytical tool, allowing for fast carbon balance determination (quantification of CO2, CH4, and CO). Such a PAT could downstream be adapted to process as gasification, pyrolysis, and most of the methane-reforming processes.
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INTRODUCTION Gas chromatography (GC), is often the go-to technique for quantification of the output gas and adjustment of thermal processes during operations. Although very precise, this technique relies on lengthy and singular runs, making realtime monitoring of the output gas rather difficult. During process operations, results can be delayed by long periods, depending upon the types of compounds that are being assessed. Also, GC systems can end up as being very expensive if many systems are operated in stream and in series to cope for the run time.1,2 Furthermore, even if GC allows for quantification of a wide array of components in the gas mixture, a number of variables need to be known, among which a very important one is the total volume of gas analyzed.3 Internal standards are an easy method to avoid difficulties in this regard; however, they should be selected carefully, so that they do not interfere with the reaction.3 Although far less common in industry, Fourier transform infrared spectroscopy (FTIR) could be used for semicontinuous quantification of carbon-based species as methane, carbon monoxide, and carbon dioxide. Spectroscopy as compared to chromatography greatly improves response time because it only depends upon a single scan time (around 5 s).4,5 Although such technology could hardly be used for quantification of different chains of alkane, FTIR could easily be used as a first response at the output of reforming reactions [autothermal reforming (ATR), steam methane reforming (SMR), or dry reforming (DR)] or even a thermochemical biomass conversion process as gasification or pyrolysis.6,7 In this work, the possibility of using FTIR as a fast “real-time” measurement tool or process analytical tool (PAT) in the determination of the syngas of the DR with direct correlation to GC quantification was investigated. For the latter, the use of nitrogen as an internal standard for GC was studied to simplify calculations, leading to mass balance of gas components produced from a DR reaction using steel wool as a catalyst. © 2014 American Chemical Society
MATERIALS AND METHODS
Analysis. The gas mixture studied in this work was sampled directly at the output of a custom-designed DR reactor (experimental details provided below). Samples were analyzed online for GC and inline for FTIR. The latter was a Varian 640-IR equipped with a PIKE Technologies heated flow cell, allowing for quantification of CO2, CH4, and CO. The system was linked to the reactor using 1/4 in. piping, and the system was located 2 m from the DR reactor in line with the system. The inline configuration for FTIR allows for the total flow of the DR reactor to pass through the 100 mL heated gas cell (PIKE Technologies heated flow cell) at 120 °C to reduce water condensation. Calibration of the FTIR system was performed with a mixture of gases bought from Praxair (CO2, CH4, and CO). The chromatographic data were obtained using a Bruker 456 GC [thermal conductivity detector (TCD)−TCD−flame ionization detector (FID)]. GC ran on three channels; the first channel was calibrated for the analysis of H2, O2, N2, CH4, and CO using a TCD for detection. Separation was achieved using a molsieve 13 × 80/100 mesh, 1.5 m × 1/8 in. IS (BRP81071) column and a Hayesep N 80/ 100 mesh, 0.5 m × 1/8 in. IS (BRP1480) column. The second channel was calibrated for the analysis of CO2, ethylene, and acetylene, with separation achieved with dual Hayesep N 80/100 mesh, 0.5 m × 1/8 in. IS (BRP1480) columns. The actual setup did not allow for separation of ethane and ethylene, and thus, the response of the TCD for ethylene quantification also includes ethane. The last channel of GC was set up for the determination of the longer chain hydrocarbons (C3−C12) using a FID and a BR −1, 10 m × 0.15 mm, 2 μm (BR99150) column. GC was calibrated using pure mixtures of gases (CO2, CH4, CO, N2, H2, and propane) and also allowed determination of other trace organics. The method conditions are given in Table 1. Experimental Setup (DR). Figure 1 shows the schematic for the reactor used for DR (see eq 1). The reaction was used as a validation tool for both the GC and FTIR analytical tools. CH4 + CO2 → 2CO + 2H 2
(+ 247 kJ/mol)
(1)
Received: August 21, 2014 Revised: November 5, 2014 Published: November 6, 2014 7398
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Table 1. Method Parameters for Total Gas Analysis with Bruker 456 GC (TCD−TCD−FID)a channel injection volume (μL) carrier gas reference gas injectors split temperature initial pressure (psi) final pressure ramp (°C/min)
channel
TCD 1
TCD 2
FID
250 Ar Ar
250 H2 H2
250 H2 NA
NA NA 35 35 NA
NA NA 4 4 NA
150 200 6 16 10
flow (mL/min) make up gas flow (mL/min) reference flow (mL/min) flow H2 (mL/min) air (mL/min) column oven temperature (°C) detector temperature filament temperature polarity
TCD 1
TCD 2
FID
1 NA 20 (Ar) NA NA 70 150 200 positive
1 NA 15 (H2) NA NA 70 150 200 negative
1 30 (N2) NA 30 300 35 250 NA NA
a TCD 1 detects products separated with a molsieve 13 × 80/100 mesh, 1.5 m × 1/8 in. IS (BRP81071) column and a Hayesep N 80/100 mesh, 0.5 m × 1/8 in. IS (BRP1480) column. TCD 2 detects separation achieved with dual Hayesep N 80/100 mesh, 0.5 m × 1/8 in. IS (BRP1480) columns. FID detects with a BR −1, 10 m × 0.15 mm, 2 μm (BR99150) column.
gases used as a reactant were purchased from Praxair, including CO2 (98% purity), CH4 (98% purity), and N2 (99% purity).
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RESULTS AND DISCUSSION The increasing interest toward natural and shale gas has drawn significant attention toward DR as a greener approach for the conversion of methane to syngas and potentially fuels through downstream synthesis. With regard the process analysis, two major issues arise in this regard: (1) analysis accuracy and (2) analysis time for product verification and valuation. In both cases, high accuracy is required. In emerging technologies as DR, analytical data are also linked to the process because they allow for its optimization. In this regard, GC analysis is vitally important and allows for determination of the full mass balance on the system. However, given the types and volumes of products formed, it is essential to obtain fast and easy trend data to quickly alter process variables, thus reaching optimal results. FTIR is a versatile and easy-to-use analytical tool, and although it does not have the highest precision or the ability to detect diatomic compounds (as H2), it gives reliable and fast results that can be used to determine trends and find changes quickly during process operation. Figure 2 shows the calibration of GC for CO2, CH4, H2, N2, propane, and CO. From Figure 2, it can be seen that, for all gases, there is a wide linear range, going from 0 to 100% of the evaluated gas. The calibration curve (TCD response to
Figure 1. Process flow diagram for a continuous packed-bed thermal DR reactor used in this experiment, with GC used as an online analytical tool (5 min sampling rate) and FTIR used inline with the process (10 s sampling rate).
Experimental set up is reported elsewhere. Dry reforming was studied using a 230 cc internal volume reactor containing steel wool heated and activated with an electrical current flow reported elsewhere.8−10 The reactor was heated using a tubular furnace with a constant flow of nitrogen up to the operating temperature. Once the operating temperature was achieved, the reactive mixture (CO2/CH4/N2) was injected in the reactor at a steady flow (using a Brooke model SLA 5850S; 1 SLPM mass flow controllers) of 1:1:0.1 L/min. As a result, the molar composition for the respective gases at the inlet was of 47.6:47.6:4.8%. The working temperature was measured at the core of the reactor as well as at the gas input and output. Prior to the GC injections, the reactions were maintained at steady state (for a given flow rate and temperature) for 20 min, after which four GC samples were typically run before the flow rate and/or temperature were changed. At post-operation, the oven was turned off and the reactor was allowed to cool under N2. In this work, the DR reactions used for the analytical system validation were performed at total flow rates of 262.5, 525, 1050.0, and 2100.0 mL/min using the same CO2/CH4/N2 ratio as previously mentioned at 25 °C and 1 atm with 52.75 g of catalyst providing a gas hourly space velocity (GHSV) of 298.58, 597.16, 1194.31, and 2388.63 mL h−1 g−1, respectively, with a flow that was estimated to be laminar (Re < 1000) at any location within the reactor and for all operating conditions used (reactant gases are wellmixed prior to entering the reactor through the use of the static mixer). Porosity of the steel wool was evaluated to be 95%. The catalyst used for the experiments was a commercially available fine filament BullDog steel wool purchased from Canadian Tire hardware store in Sherbrooke, Quebec, Canada. Elemental analysis of the steel wool indicated that it was mostly composed of iron (98.5%), with 0.24% carbon content, with the rest being minor impurities. The
Figure 2. Calibration curves for online Bruker 456 GC (TCD−TCD− FID) of DR reactants (CO2 and CH4), products (CO, H2, and propane), and internal standard (N2) under ambient conditions from pure gases at a total flow of 1 L/min. 7399
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concentration) for CO2 was used for both ethylene and acetylene quantification, as reported by Jones et al.11 As for oxygen quantification, precise quantification was not possible. A qualitative estimation of the oxygen content could be achieved through the TCD response for hydrogen in the system. However, given the nature of the DR reaction (eq 1), it is rather unlikely for significant oxygen concentrations to occur in the mixture; thus, this information was used more as a marker for reactor-related issues and leaks. Longer chain hydrocarbons can be quantified according to the propane calibration curve given that the response factor for the FID is known to be linear with changes in the molecular mass of the compounds based on the fact that they are all aliphatic.11 Because both nitrogen and hydrogen are symmetrical, they are impossible to identify and quantify using FTIR. As mentioned above, the reactor output was linked to FTIR using a heated gas flow cell, which allowed for highperformance FTIR sampling of the flowing gas samples. Another positive of using FTIR is that it is a non-destructive technique; therefore, it can easily be paired downstream with another PAT if needed. Results show that the sampling rate can be varied down to 1 scan/10 s with no discernible change in the calibration curves, allowing for fast and accurate concentration determination of the gases, as reported in Figure 3.
ethylene and acetylene have the same form of C−H stretching, occurring in the range of 3000 cm−1. This peak overlaps as well with the peak for methane, making them impossible to quantify at high concentrations. Although ethylene and acetylene do not occur significantly during the DR reaction, they could affect the methane quantification. Carbon monoxide and carbon dioxide have similar IR absorbance, which results as well in peak overlap. It is therefore important to select carefully the quantification wavelengths for each component that is least affected. While being a constraining factor, it would only represent a slight error if FTIR was used as an analytical tool for processes as steam reforming, gasification (to a certain extent), or methane reforming. Because the DR system investigated in this work did not lead to conversion of nitrogen, it is possible to assume that the volume of nitrogen should be constant in and out of the reactor. On the basis of this assumption, the percentage of N2 from GC could be used directly to determine the volume of gas exiting the reactor via eq 2 below
VT =
VN2 % N2
(2)
where VT is the total volume in the stream, VN2 is the volume of N2 injected into the reactor, and % N2 is the percentage of N2 determined by GC. From this, the determination of the volume of the various gases following the reaction is determined from their volume percentage, after which they are correlated with the GC values. Table 2 shows mass balance for the dry reforming of methane (DRM) reaction operated at 720 °C with total flow rates varying from 262.5 to 2100 mL/min (volumetric flows at 20 °C, 1 atm). Results shows that the use of nitrogen as an internal standard (eq 2) permits calculation of the mass balance for carbon, oxygen, and hydrogen. The loss of all three elements increases with an increasing flow through the reactor. The number of moles of each is directly proportional to the gas fed to the reactor. The loss of carbon can be associated with the formation of char at the outlet of the reactor as the gases cools, while the loss of hydrogen can be related to the reverse water− gas shift and oxygen to water condensation at the output of the reactor. This hypothesis was confirmed by Banville et al.,8 who showed, with similar catalyst and conditions, that a steel wool catalyst could react with CO2, forming various forms of iron oxide at the surface. It can also be seen that, as a result of the decreased conversion of methane at higher flow rates, produced H2 decreases, while, on the other hand, given the comparative
Figure 3. Calibration curves for inline Varian 640-IR equipped with a PIKE Technologies heated flow cell at 120 °C of DR reactants (CO2 and CH4) and product (CO) under ambient conditions from pure gases at a total flow of 1 L/min.
In combination with the inability for FTIR to identify or quantify nitrogen and hydrogen, overlapping also occurs for other organic compounds. As an example, the peaks for
Table 2. GC Quantification of the Gas Mixture Produced from Methane DR at a Temperature of 720 °C (Measurement Taken on Gases Exiting the Catalyst) with GHSVs between 298.58 and 2388.63 mL h−1 g−1 (with an Input Ratio of 1:1:0.1 CO2/CH4/ N2) over 52.75 g of Coarse BullDog Steel Wool reactants (mL/min)
percent conversion (%)
products (GC) (mL/min)
selectivity
loss (mmol/min)
temperature (°C)
CO2
CH4
CH4
CO2
H2
CO
CH4
CO2
CO/H2
carbon
hydrogen
oxygen
720 720 720 720 720 720 720 720
125 125 250 250 500 500 1000 1000
125 125 250 250 500 500 1000 1000
89 90 204 205 436 437 920 908
77 77 170 172 379 380 824 814
54 54 51 51 46 46 40 40
87 86 116 117 145 143 171 171
29 28 18 18 13 13 8 9
39 38 32 31 24 24 18 19
2 2 2 2 3 3 4 4
−0.1 −0.1 0.4 0.3 1.8 1.8 3.8 4.7
1.5 1.5 3.7 3.4 7.4 7.1 10.7 12.7
0.4 0.4 1.9 1.8 4.4 4.3 8.0 9.0
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which it was shown to increase slightly. The variation could also result from the FTIR-heated flow cell size because, at low flow rates, the equilibration time for this cell diameter could be too lengthy with regard to the experimental period. Therefore, optimization of the FTIR parameters including but not limited to a variation in the scan time could lead to a reduction of this error. The variations shown in Figure 4 and Table 3 could also be a result of primarily peak overlap because, at high conversion rates, the carbon monoxide FTIR response overlaps with the carbon dioxide peak, resulting in an apparent increase in the concentration of CO2. Conversely, at very low conversions, the overlap between the carbon dioxide and carbon monoxide peaks results in an apparent increased concentration of carbon monoxide in the product gases (see Table 3).
increase in conversion of CO2 over CH4, the amount of CO exiting the reactor increases. Figure 4 shows the correlation for the mole percentage of CO, H2, CH4, and CO2 for GC and FTIR analyses at the
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CONCLUSION FTIR has been found to be a fast and accurate method for inline quantification of products generated during DR of methane. Correlation to GC results indicates that low flow rates induce an error of typically 1% for CH4 compared to GC quantification, while higher variations were observed for both CO2 and CO. Calibrations for both GC and FTIR have been shown over a wide mole percentage range. As a first response PAT, FTIR is considered a fast accurate technique with great potential in industrial gasification or synthesis gas production. It is noteworthy that, for “real” gasification samples, the existence of aromatics and polyaromatics may result in difficulties associated with windows and mirror systems and should be taken into account. The use of nitrogen as an internal standard for GC quantification has been shown to be an effective and fast method to provide an accurate mass balance of the reactant and products. Use of nitrogen could additionally be applied in numerous other systems and will work well under the provision that the reactants in the reactor are in gaseous form and are injected in known ratios/volumes.
Figure 4. Comparison of the molar ratio of gases produced from methane DR (CO, CH4, and CO2) as detected by FTIR versus GC at 720 °C with GHSVs between 298.58 and 2388.63 mL h−1 g−1 (with an input ratio of 1:1:0.1 CO2/CH4/N2) over 52.75 g of coarse BullDog steel wool.
output of the DR reactor (720 °C) at a GHSV varying from 298.58 to 2388.63 mL h−1 g−1 (with an input ratio of 1:1:0.1 CO2/CH4/N2) over 52.75 g of coarse BullDog steel wool. The apparent decrease in conversion with regard to the increasing flow rate at the input of the reactor is most likely a result of the decreasing residency time within the reactor. Results from FTIR seem to correlate to a certain extent with the results obtained from GC. However, Figure 4 shows difference in quantification when using FTIR compared to GC under the same conditions. The variation increases at high and low concentrations of CO2 and CO, in the gas as depicted in Figure 4 and Table 3. Table 3 shows the different results obtained for chromatography compared to spectroscopy. Among the compounds quantified, CH4 shows only a slight variation in conversion as determined by either GC or FTIR, with variation ranging from 0.2 to 2% (FTIR/GC). However, both CO2 and CO showed an apparently large variation in the results, as determined by FTIR compared to GC, with a minimum variation of 1.6%, increasing to 14% at low flows. The variation for both CO2 and CO decreases with an increasing flow rate up to 1050 mL/min (total flow rate), after
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ASSOCIATED CONTENT
S Supporting Information *
FTIR spectra for reaction media during the DR process (CH4, CO2, and CO) (Figure S1) and GC chromatograms for the three channels for the DR reaction during reaction (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.
Table 3. Comparison of the Gas Molar Ratio Produced from Methane DR (CO, CH4, and CO2) as Detected by FTIR versus GC, with Percent Error in Detection between the Detection Methods [at 720 °C with GHSVs between 298.58 and 2388.63 mL h−1 g−1 (with an Input Ratio of 1:1:0.1 CO2/CH4/N2)] reactants (mL/min)
products (GC) (mL/min)
products (FTIR) (mL/min)
percent error (%) (FTIR to GC)
temperature (°C)
CO2
CH4
CH4
CO2
H2
CO
CH4
CO2
CO
CH4
CO2
CO
720 720 720 720 720 720 720 720
125 125 250 250 500 500 1000 1000
125 125 250 250 500 500 1000 1000
89 90 204 205 436 437 920 908
77 77 170 172 379 380 824 814
54 54 51 51 46 46 40 40
87 86 116 117 145 143 171 171
91 91 204 205 439 439 922 922
87 87 177 176 370 370 768 768
75 75 108 108 141 141 181 182
2.0 1.0 0.2 0.3 0.8 0.4 0.3 1.5
14.0 13.0 3.6 2.7 2.4 2.7 6.8 5.7
13.8 13.1 6.8 8.0 2.6 1.6 5.7 6.3
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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