The utilization of supercritical fluids (SCF) in the Fischer-Tropsch Synthesis (FTS) further complicates the hydrocarbon products identification and analysis process due to the dilution of hydrocarbon peaks by the predominant solvent peak. Therefore, in this project, a custom-made Gas Chromatography (GC) analysis system was designed and implemented to identify and quantify SCF-FTS products. The FTS products were identified using two different methods. The first was through retention time matching by injecting standard solutions, and the second was through the use of the GC/MS system. The quantification of CO and CH 4 was achieved by using external standards, where the CO conversion was calculated by relating the peak area of CO to the peak area of an internal standard (argon) while the CH 4 selectivity was calculated by relating the peak area of CH 4 to that of CO. After setting and calibrating the GC system, two reaction conditions (gas phase: 240°C, 20 bar syngas with 2:1 H 2:CO molar feed ratio and for the supercritical fluids FTS (SCF-FTS): 240°C, 65 bar with 20 bar syngas partial pressure and 2:1 H 2:CO molar feed ratio) were used to compare the different FTS reaction media. The comparison between the gas phase FTS and the SCF-FTS showed the following: carbon monoxide conversion was improved by 14% in the SCF-FTS, while the hydrocarbon product profile SCF-FTS showed 78% reduction in light hydrocarbons (C 1 - C 4) products, 35% increase in middle distillates (C 11 - C 22) products compared to gas phase FTS. These improvements have resulted in higher chain growth probability for the SCF-FTS ( α = 0.85) compared to the gas phase FTS ( α = 0.76). These results are generally in agreement with previously reported enhancement in the SCF-FTS[1].
Escalating concerns about the unstable oil prices and oil supply insecurity coupled with stricter environmental regulations have catalyzed the interest in the production of synthetic fuels from alternative resources such as natural gas, coal and biomass via the X-to-Liquid (XTL) process or other process. The Fischer-Tropsch synthesis (FTS) is considered as the heart of the XTL process [
A forethought towards the desired product selectivity is an extremely important criterion in the design of FTS processes from an economic feasibility point of view. Fixed bed and slurry bubble reactors are the two main reactor types used in industrial Low-Temperature-Fischer-Tropsch (LTFT) [
The analysis and identification of FTS products is quite a cumbersome process since it begins with simple molecules (H2 and CO) and ends up with hundreds of molecules that can be normal paraffins, isomers, olefins and oxygenates. The introduction of supercritical solvents to the FTS feed stream further complicates the FTS product analysis process due to the dilution of products with the solvent. This dilution results in a large solvent peak that may absorb nearby product peaks and simultaneously result in relatively small hydrocarbons peaks. The groundbreaking study that investigated the application of SCF in the FTS was carried out by Yokota and Fujimoto in 1989 [
The FTS reported in this work was carried out under both conventional gas phase and non-conventional supercritical and near supercritical phase utilizing the newly commissioned bench-scale novel high-pressure fixed bed reactor unit. The reactions were performed over a cobalt catalyst supported on alumina (15 wt% Co/Al2O3) that was prepared using the impregnation technique, and then calcined and reduced under hydrogen flow at elevated temperatures. For the gas phase FTS, the reaction was carried out at 240˚C, 20 bar syngas with 2:1 H2:CO molar feed ratio. As for the SCF-FTS, the reaction was carried out at 240˚C, 65 bar with 20 bar syngas partial pressure and 2:1 H2:CO molar feed ratio. These conditions were selected to provide the basis for comparing the two reaction media. Each experimental set was allowed to reach steady state conditions in terms of activity (CO conversion) and selectivity (hydrocarbon product distribution), as measured by the on-line GC setup. After reaching steady state, liquid samples were collected for off-line analysis.
The cobalt catalyst supported on alumina (15 wt% Co/Al2O3) used for the FTS reactions was prepared using the impregnation technique. Firstly, the alumina support (surface area 255 m2/g, median pore 70 micron and 5000 Å, total pore volume 1.14 cm3/g, packing density 0.395 kg/L) was crushed, sieved (75 - 150) μm, washed with deionized water and then calcined under airflow at 700˚C. Secondly, an aqueous 2 molar cobalt solution was prepared by dissolving cobalt nitrate precursor (Co(NO3)2∙6H2O) in deionized water while stirring at room temperature. Then, the cobalt solution was gradually added to the treated support under constant stirring to avoid lumps formation. Afterwards, the formed catalyst paste was dried, crushed and sieved (150 - 250) µm. Thirdly, the obtained catalyst powder was calcined in a rotary oven under airflow and temperature ramp up to 350˚C. Then, the calcined catalyst was diluted with quartz sand by vigorously mixing the catalyst and quartz to assure homogenous catalyst distribution. Finally, different catalyst loadings were added into the reactor tube on top of a layer of glass wool and quartz.
The high-pressure bench-scale reactor unit used in this project consists primarily of five sections (
At the feed delivery section syngas (33.4% CO and 66.6 H2), carbon monoxide (99.97% CO), hydrogen (99.999% H2) and argon (99.999% Ar) were introduced into the system at controlled flow rates (0 - 500 nmL/min) using four mass flow controllers (MFCs). For the supercritical phase testing, liquid solvent, n-hexane (98.5% n-hexane) was introduced at controlled volumetric flow rates using a High Pressure Liquid Chromatography (HPLC) pump. A feed purification system composed of a cylindrical vessel filled with three layers of adsorbers/catalysts (BASF Selexsorb COS, BASF PuriStar R3-15 and BASF E-315) and an inline gas filter were employed at the solvent stream and at each gas stream for the removal of COS, H2S, CS2, CO2, acetylene, O2, arsine, sulfur compounds, moisture and particulates. The liquid solvent is then passed through a vaporizer before it is mixed with the feed gases in the static mixer that ensure proper mixing of the feed gases and the SCF.
The reaction mixture then enters the custom made tubular fixed bed reactor (Autoclave Engineers, 16" overall length, 12" heated length, 2/3" internal diameter, 1.0" outer diameter, 73 cm3 net volume in heated zone). The reactor tube was vertically embedded in a hollow ceramic insulated electric furnace with three heating zones that allow reactor temperature control within ±2.0˚C. The pressure control valve located between the reactor and the
hot trap controlled the reactor pressure.
Two flash separation columns were placed downstream of the high-pressure reactor. The first one is the hot trap surrounded by a heating tape and insulated by glass wool and aluminum foil. The hot trap is operated at 150˚C and 0.9 barg to facilitate the condensation of heavy hydrocarbons (wax) prior to the on-line GC system. The formed wax is collected in a special pressure vessel for future analysis. All the lines connecting the reactor with the hot trap, the wax collection vessel and the on-line GC were heated to 150˚C to prevent any wax condensation.
The remaining hot trap gas stream goes towards the cold trap used to separate liquid hydrocarbons and permanent gases. The cold trap is internally cooled using cooling coils with chilled water flow at 4.0˚C. Hydrocarbons and water condensed by the cold trap were collected in a tank and liquid samples were periodically collected for off-line GC analysis.
In a typical experimental run, the system was first flushed with n-hexane and then purged with nitrogen. Secondly, the catalyst was activated in-situ under hydrogen flow (100 - 350 nmL/min) at a temperature range between 180˚C to 350˚C. Thirdly, the system was purged and flushed again. Fourthly, the solvent was introduced (2.43 - 2.74 nmL/min) and the reactor pressure (20 - 80 bar) and temperature (230˚C - 240˚C) were slowly ramped to their desired set points. After the temperature and pressure had been stabilized syngas was allowed (50 - 150 nmL/min) at the desired flow rate. Each experimental set was allowed to reach steady state conditions in terms of conversion and product distribution, as indicated by the on-line GC. After reaching steady state, wax and liquid samples were collected for further off-line analysis.
A fraction of the gases leaving from the top of the hot trap is directed towards the on-line GC system through an air actuated 8 way selecting valve that is used to inject samples at specified intervals. Operation of the sampling valves was controlled by a custom-made Shimadzu GC Postrun software that acquires samples at preset times and record TCD and FID data. This system has been designed to ensure that the same sample to be injected simultaneously to two combined GCs with three detectors. This unique on-line analytical system consists of dual gas chromatographs with three detectors, TCD-1, FID and TCD-2 (
gas at 5.0 mL/min and 4.0 bar. The FID detector analyzes (C3 - C13) hydrocarbons by utilizing a single fused silica capillary column to separate the hydrocarbons. The capillary column also uses helium as a carrier gas at 5.0 mL/min and 4.0 bar, while air (350 mL/min, 4.0 bar) and H2 (35 mL/min, 4.0 bar) were used to light the detector flame and N2 as makeup gas at 25 mL/min and 4.0 bar. Hydrocarbon products heavier than C15 were not detected by the on-line FID. One more detector TCD-2 is used with a single packed column to detect H2, where in this column N2 was used as a carrier gas at 25 mL/min and 4.0 bar.
Liquid products from the cold trap are periodically collected for off-line analysis using GC/MS system equipped with FID and Mass Spectroscopy (MS) that uses a capillary column to separate (C5 - C30) hydrocarbons. The column uses helium as carrier gas at 5.0 nmL/min and 4.0 bar, air at 350 nmL/min and 4.0 bar, and H2 at 35 nmL/min and 4.0 bar to light the FID flame and N2 as makeup gas at 25 nmL/min and 4.0 bar.
The CO inlet flow rate to the FTS reactor was measured and controlled by the MFC; however, CO participates in the FTS reaction, and thus its outlet flow rate cannot be known directly. In order to be able to calculate the outlet CO flow rate, it was necessary to use an inert (argon) as an internal standard. The method adopted herein is similar to that previously used by Nijs and Jacobs [
where QCO,out is the outlet flow rate of CO, “a” is the slope and “b” is the intercept. For example, syngas with H2:CO of 2:1 (CO is 1/3 of syngas flow) was allowed into the system at different flow rates (10 - 200) nmL/min. Argon was allowed at constant flow rate (10 or 15) nmL/min. The resulting gas mixture was then directed toward the on-line GC/TCD, where CO and Ar peak areas were determined. CO and Ar peak ratio (PACO/PAAr) was then calculated and plotted against CO flow rate.
The material balance calculation was based on carbon, where the amount of carbon (as CO) entering the reactor is equal to the amount of carbon (as unreacted CO and produced hydrocarbons) leaving it. Therefore, the CO consumption rate and conversion throughout the experimental run were calculated based on the difference between moles of CO entering the reactor and moles of unreacted CO leaving the reactor as shown in Equation (3)
and Equation (4). Note that syngas H2:CO ratio of 2:1 was normally used unless CO, H2 were introduced separately.
where xCO is the consumption rate of CO and XCO is the conversion percent of CO.
In order to calculate the CH4 outlet flow rate, a calibration gas with 10.09 CO mol∙% and 4.00 CH4 mol∙% (
CH4 selectivity was then calculated as follows:
where
The hydrocarbon product distribution was determined utilizing the on-line and off-line FID GC setups. The hydrocarbon formation rate (g/gcat∙h) was calculated as the sum of formation rate of all species (isomers, normal paraffins, olefins, alcohols) with “n” carbon number. Then, the normalized weight percentages (Wn) of each carbon number was calculated from the ratio of the hydrocarbon formation rate of carbon number “n” to the total weight of hydrocarbons formed as detected by the FID.
It was difficult to determine the exact amount of C6 hydrocarbons produced when running the FTS reaction under near and supercritical conditions since large quantities of n-hexane were present in the feed. This resulted in a very large n-hexane peak absorbing nearby peaks within the C6 range. Thus, calculating the formation rate of C6 hydrocarbons was impractical. Hence, the hydrocarbons within the C6 product range were eliminated from the product distribution and were not included in the Anderson-Schulz-Flory (ASF) calculations. Additionally, for consistency purposes, C6 hydrocarbons were also eliminated in the gas phase FTS when comparing the performance under the two reaction mediums.
To start the hydrocarbon product distribution calculations, a reference or starting point was needed. For the on-line GC, this was the CH4 formation rate (g/gcat∙h), previously determined using the GC TCD. The TCD CH4 formation rate with the CH4 peak area given by the FID was used as a reference ratio and held constant for each sample. The ratio was then used to calculate the formation rate of hydrocarbons with different carbon numbers. This method is similar to that previously used by Snavely and Subramaniam [
As discussed below in section 3.1, each hydrocarbon present in the FID spectrum was identified and assigned with its distinct carbon number and type (isomer, normal, olefin, alcohol). After that, the peak area for each carbon number with the same type was summed, and then the total peak area for each carbon number was summed. The weighted molecular weight for each carbon number (n) was then estimated as follows:
The total hydrocarbon formation rate for each carbon number (n), was then calculated as follows:
where FHC is the hydrocarbon formation rate.
The weight fraction of hydrocarbons containing “n” carbon atoms (Wn) was then calculated as follows:
where FHC,n is the hydrocarbon formation rate for “n” carbon number.
The natural logarithm of the weight fraction of carbon number “n” over the carbon number [ln(Wn/n)] was calculated in order to plot the ASF distribution, where [ln(Wn/n)] was plotted against “n”. The slope of the trend line can then be used to find the chain growth probability as follows:
where ln(α) is the slope of the trend line and the chain growth probability is equal to the exponential of the slope (α = eslope).
The exact and detailed identification of FTS products can be hindered by the large number of compounds involved [
The FTS products under both conventional gas phase and supercritical phase were found to be qualitatively similar (but not quantitatively, as will be discussed later). The main difference between the GC product distributions under the two reaction mediums is the dilution of the FTS products in the SCF-FTS, as illustrated and compared in
For the identification of C1 - C4 hydrocarbons and permanent gases (CO, CO2, H2, Ar) a number of standard gas mixtures with known molar percentages were used. The standard gas mixtures included common FTS products within the (C1 - C4) range. These hydrocarbons include methane, ethylene, ethane, propylene, propane, 1-butene, n-butane, iso-butane, butylene, butane, as well as H2 and CO2. In addition, different solutions containing known concentrations of (C1 - C6) normal alcohols (methanol, ethanol, propanol, butanol, pentanol and hexanol) were injected into the on-line GC for identifying the alcohol peaks. After that, a calibration table was prepared by entering the retention time for each identified species into the GC data collection software. The identification of permanent gases is shown in
The identification of C5-C34 hydrocarbon compounds were carried out off-line by injecting liquid FTS product samples into the GC/MS (
For the product distribution and selectivity studies, FTS products were divided into five groups (alcohols, isomers, α-olefins, normal paraffins and alkenes). As previously mentioned, the FTS products followed a similar pattern for each hydrocarbon group (CX), where it begins with (CX) isomers (increasing in number as X increases). The peaks follow the aforementioned CX of α-olefins (from 1 - 3), the normal (CX) alkane (normally the peak with the highest area, which is common for cobalt-based catalyst). After the normal alkane peak comes, the cis and then the trans (CX) alkenes peaks, as demonstrated in
peak was located before the CX isomers, for (C8 - C13) alcohols, the alcohol peaks were located between CX isomers and for (C14 - C20) alcohols, the alcohol peaks were found after the isomers and before the α-olefins peaks. It is also noticeable that, α-olefins were not detectable by the Agilent GC/MS beyond (C17), alcohols beyond 1-eicosanol (C20 normal alcohol) were not detectable, while cis and trans alkenes were not detectable beyond (C22). All observations mentioned earlier agree well with the profile of FTS hydrocarbon product distribution obtained from cobalt-based catalysts tested under similar conditions [
During the off-line GC/MS FTS product identification minor aromatic, cyclic and acidic compounds were identified in the organic phase. This is expected since these compounds have higher affinity and would be more dissolved in the aqueous phase. Since these compounds were not prevalent in the FTS product and the error resulting by ignoring them would be minimal, they were not included in the FTS product distribution. For instance, the combined total weight percent of aromatics, acids and esters in the organic FTS phase was estimated by Hackett et al. [
In this section, a comparison study between the conventional gas phase FTS and the SCF-FTS was conducted to
FID Retention Time (min) | Peak Area (a.u.) | MS Retention Time (min) | Compound Name | Compound Group | Compound Branch |
---|---|---|---|---|---|
3.218 | 612,711 | 2.228 | 1-butanol | C4 | Alcohol |
3.258 | 2,065,458 | 2.252 | 2-methyl hexane | C7 | Isomer |
3.367 | 2,459,938 | 2.328 | 3-methyl hexane | C7 | Isomer |
3.572 | 10,665,767 | 2.471 | 1-heptene | C7 | α-Olefin |
3.708 | 154,553,134 | 2.559 | n-heptane | C7 | Normal |
3.766 | 855,344 | 2.605 | 3-methyl-2-hexane-Cis | C7 | Alkene |
3.812 | 7,000,586 | 2.638 | 2-heptene-Cis | C7 | Alkene |
3.957 | 4,094,594 | 2.739 | 2-heptene-Trans | C7 | Alkene |
validate the FTS product identification and analysis process. This was carried by investigating the enhancements in terms of activity, selectivity and chain growth probability (α-value). The reactor was operated at 20 bar and 240˚C for the gas phase FTS. On the other hand, for the SCF-FTS the supercritical solvent (n-hexane) was introduced at 240˚C and the total reactor pressure was maintained at 65 bar. In both cases, the syngas partial pressure was kept at 20 bar with syngas with H2:CO of 2:1 feed ratio.
The on-line and off-line GC FTS product analysis confirmed a yield of a complex mixture of C1 to C34 hydrocarbons and C1 to C17 oxygenates, in addition to H2O, CO2 and unreacted syngas. The predominant FTS products were linear alkanes (n-paraffins), while branched (isomers), oxygenates (alcohols), α-olefins, and alkenes (cis and trans) hydrocarbons were produced to a lesser extent, as previously shown in Figures 8-10; this product distribution is typical for cobalt-based catalysts.
Starting with
The overall hydrocarbon product distributions from C1 - C32 under the previously mentioned conditions were compared as shown in
Higher α-value indicates higher selectivity towards heavier hydrocarbon production and vice versa.
supercritical hexane resulted in a significant increase in chain growth probability from 0.76 to 0.85. Similar incensement were observed in previous literature [
The aforementioned enhancements in CO conversion, reduced selectivity towards light hydrocarbons and increased selectivity towards heavy hydrocarbons is explained in literature and is discussed briefly in this paper. The increased conversion is attributed to the increased reactants accessibility to the active sites after the in-situ wax extraction from the catalyst pores by the SCF [
During the off-line FTS product peak identification using the (GC/FID/MS) system, considerable amounts of several alcohols were identified in the organic FTS product samples. This is uncommon for cobalt-based low- temperature FTS; thus, further investigation was required.
A typical liquid sample of the gas phase FTS (240˚C, 20 bar, 65 nmL/min syngas flow with 2:1 H2:CO molar feed ratio) was collected from the bottom of the cold trap. Then the organic phase from that sample was injected into the GC/MS for analysis. The alcohols identified within the sample’s organic phase are given in
To further investigate these results, the aqueous phase from the same sample was injected into the GC/MS, where the resulting MS spectrum is shown in
As a double check and to confirm the MS results, a reference sample was prepared by mixing a small amount (~5 μL) of a mixture of C1 - C6 alcohols diluted with 1.5 mL of n-hexane (Sigma Aldrich, ≥95% purity, with the rest composed of small amounts of C6 iso and cyclic hydrocarbons). The reference sample was then injected into the GC/MS, and the obtained MS spectrum is shown in
The FTS produces a wide range of products including n-paraffins, olefins oxygenates and aromatics. Pei et al. [
Alcohol | Retention time (min) | Response (a.u.) |
---|---|---|
1-Propanol | 2.457 | 276,259 |
1-Butanol | 3.203 | 467,519 |
1-Hexanol | 9.666 | 4,132,448 |
1-Heptanol | 16.473 | 25,329,552 |
1-Octanol | 22.644 | 36,927,076 |
1-Nonanol | 28.049 | 41,910,385 |
1-Decanol | 32.923 | 33,100,899 |
1-Undecanol | 37.417 | 32,615,101 |
1-Dodecanol | 41.619 | 31,647,273 |
1-Tridecanol | 45.578 | 29,650,492 |
1-Tetradecanol | 49.331 | 27,495,369 |
1-Pentadecanol | 52.892 | 22,176,038 |
1-Hexadecanol | 56.279 | 13,716,154 |
molybdenum based catalyst is used under the operating conditions of 307˚C, 75 bar and syngas ratio H2/CO = 2.
From the above discussion, it can be seen that while iron based catalyst are known to be selective towards alcohol production, cobalt based catalyst can also produce alcohols if the catalyst and reactions conditions were adjusted. Nevertheless, this finding requires further detailed investigation, which is currently being performed by the research team.
The dilution of FTS product with the solvent complicated the identification and analysis of the reactor effluent since the solvent peak was extremely dominant over the product spectrum. Therefore, a sophisticated custom- made on-line and off-line GC setup was developed. The GC analysis system and method implemented successfully enabled the identification and characterization of FTS hydrocarbons within (C1 - C32) range as well as permanent gases (CO, H2, CO2, Ar). After setting the analysis system, a set of reaction conditions were applied to verify the functionality of the GC analysis setup. Through the utilization of the custom-made analysis system, reliable activity and product distribution were obtained, which enabled the formulation of conversion, product distribution and ASF curves under both gas phase and supercritical phase. These curves were able to compare the cobalt catalyst performance under the distinct reaction media. The experimental findings provided us with accurate measurements of the hydrocarbon product distribution both the conventional gas-phase and in the supercritical phase, which are in good agreement with the product profiles reported in literature. This unique analytical setup has as well provided us the opportunity to identify and confirm the presence of significant alcohol amounts in the product profile of a cobalt-based catalyst, which are not common products for this type of catalyst. This phenomenon is currently under thorough investigation in our lab to understand the mechanistic of reaction that could lead to such products.
The authors would like to thank the Qatar National Research Fund (QNRF) (a member of Qatar Foundation) for financial support of this project under the National Priorities Research Program (NPRP) (grant # 4-144-2-590). The statements made herein are solely the responsibility of the authors.
AmroKasht,RehanHussain,MinhajGhouri,JanBlank,Nimir O.Elbashir, (2015) Product Analysis of Supercritical Fischer-Tropsch Synthesis: Utilizing a Unique On-Line and Off-Line Gas Chromatographs Setup in a Bench-Scale Reactor Unit. American Journal of Analytical Chemistry,06,659-676. doi: 10.4236/ajac.2015.68064