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Portable Gas Chromatograph Calibration with Gases of Varying Viscosities
Abstract
The quantitative analysis of gas mixtures with gas chromatography is based on the calibration with certified standards and the determination of a response relationship for each species by regression analysis. The conventional assumption of a constant amount-of-substance injected onto the column, the sample size, for all standards analyzed represents one of the largest sources of uncertainties in this analysis technique. For systems using time-based micro injectors, the sample size injected onto the column is determined by the opening time of a pneumatically actuated micro valve and therefore depends upon the gas velocity developed in the injector's microchannel. For this reason, the sample size is not necessarily constant for a given opening time, but strongly correlates - according to fluid mechanic theory - with the gas mixture's dynamic viscosity. Neglecting these sample size variations leads to errors in the analysis up to 30%, depending on the diversity of the standards' viscosities, especially in processes with strongly changing hydrogen contents. A mathematical correction exploiting the inverse proportionality of the sample size and the sample's dynamic viscosity normalizes the injection amounts and minimizes the influence of the sample variations on the calibration regression. The data analysis based on the corrected normalized calibration is no longer viscosity dependent and can be applied to gas mixtures of any composition.
An important field of research is the miniaturization of analytical systems for laboratory applications and on-field analysis. In particular, gas chromatography (GC) has benefited from the recent advances in enabling technologies like photolithography, micromachining, hot embossing, and 3D-printing to improve sampling and sample preparation, microcolumn technologies, and detection. In this article, the developments and applications reported since 2015 were reviewed and summarized. Important applications using benchtop instruments, portable GCs, and micro-GCs (µGCs) were showcased to illustrate the current challenges associated with each miniaturized interfaces and systems. For instance, portable instruments need to be energy-efficient and ideally depend on renewable sources for carrier gas generation. Lastly, multidimensional separations were addressed using miniaturized systems to effectively improve the peak capacity of portable systems.
This paper reports on a gas sample injection system for gas chromatography, with the specificity of being potentially able to sustain the harsh environment of natural gas. This device integrates six low-leak microvalves made from polyetheretherketone (PEEK) membranes on silicon substrates. We report on the design, operation principle, fabrication, and intrinsic characteristics of the microinjector as well as its measured behavior in a compact gas chromatograph. The overall size of the microinjector——is among the smallest reported in the literature, and it permits the injection of a 250-nL sample volume. Characterization revealed a transition time of less than 35 ms between the sampling mode and the injection mode of the microinjector, with an expected potential of high precision on injection sample volume, owing to the very low leakage levels. Continuous operation and reproducible measurements are demonstrated when using the microinjector in a miniaturized chromatographic system. Comparison with commercially available injectors revealed similar quality of gas sample injection.\hfill [2011-0005]
In the paper, a portable gas chromatograph with silicon/glass integrated micromachined flushed injector and doubled thermal conductivity detectors (TCD) has been presented. The injector has two symmetrical dosing loops, each of 7μl volume and one fast injection microvalve, pneumatically operated. All parts of the injector are integrated on 35×35×2.5mm3 glass/silicon/glass die. Teflon coated thin polyimide foil is applied as the moving actuator of the microvalve of the injector. Three types of TCDs have been tested. In the first type of TCD, four thin-film Pt heaters/thermoresistors made on thin silicon nitride CVD deposited membrane suspended over deeply etched in a silicon substrate two channels, are applied. In the next two TCDs, miniature spirals made of thin Pt solid wire are assembled inside glass/silicon chip. Two configurations of spirals position have been tested: in-line serial and in-line parallel. Computer simulations, supported by experiments have indicated better parameters of TCD with parallel configuration of spirals. The TCDs with Pt spirals are resistant against carrier gas decays. In the portable gas chromatograph two capillary columns are used: active (molecular sieve) and passive. Columns and co-working TCD are thermally stabilized in an oven equipped with a thick-film heater made on polyimide foil. The instrument is equipped with built-in microcomputer with two 16-bit microprocessors and is operated (controlled) by an external portable PC. All important parameters of analysis (possible single or continuous mode of work) are adjusted electronically, as well as data acquisition and preparing of the complete analysis effect in the form of chromatograms and tables indicating qualitative and quantitative composition of gas mixture. The instrument is able to work in on-line continuous measurements, tests made for artificial atmosphere of deep mining industry have shown successful work of the instrument.
A miniature gas analysis system has been built based on the principles of gas chromatography (GC). The major components are fabricated in silicon using photolithography and chemical etching techniques, which allows size reductions of nearly three orders of magnitude compared to conventional laboratory instruments. The chromatography system consists of a sample injection valve and a 1.5-m-long separating capillary column, which are fabricated on a substrate silicon wafer. The output thermal conductivity detector is separately batch fabricated and integrably mounted on the substrate wafer. The theory of gas chromatography has been used to optimize the performance of the sensor so that separations of gaseous hydrocarbon mixtures are performed in less than 10 s. The system is expected to find application in the areas of portable ambient air quality monitors, implanted biological experiments, and planetary probes.
Portable instruments provide high-quality data and deserve more respect than they get
Valves are the most important components of lab-on-a-chip systems for the manipulation of small-volume samples. Various types of valves have been developed, but most of them require external actuators or complicated control lines (displacement chambers) to deform microchannels for valve functionalization. The development of a simple microfluidic valve has thus far been quite a technical challenge. Herein is presented the development of a simple, normally-closed passive valve on a microchip using a new patterning technique involving oxygen plasma with aluminum sacrificial lines, where surface activation is not induced. Only the valve sections are inactivated which produces a non-covalent, reversible seal between the polydimethylsiloxane (PDMS) lid and the glass substrate. When a fluid pressure in excess of the burst pressure is applied to this non-covalent seal area, the PDMS detaches slightly from the glass so that the fluid can pass through. Valve function was demonstrated by observation of the microchannels with a fluorescence microscope. A fluorescent dye was injected through the valve, and the burst pressure was measured to be 20 kPa. The opened valve was calculated to have a 2.1 μm gap (maximum) via the Hagen–Poiseuille law. Injection from multiple channels was also demonstrated, and the burst pressure did not change significantly over 6 injection cycles. This extremely simple structure can be utilized practically as an easy-to-implement and back-flow-free check valve, so long as the pressure in the outlet channel does not exceed the burst pressure threshold.
Dimensional analysis has been applied for the correlation of the thermal conductivity of a gas to its temperature, molecular weight, heat capacity, and critical constants. This approach indicates that the group k*λ/Cp should be a function of zc and TR, where λ = M1/2Tc1/6/Pc2/3. Experimental thermal conductivities of hydrocarbons at normal pressures (approximately 0.2 to 5 atm.) have been used to develop two relationships. The first is applicable to all types of hydrocarbons for 0.6 < TR < 3.0 with the exception of methane and the cyclic hydrocarbons below TR = 1.0, for which the other relationship is applicable. These two relationships have been used to calculate thermal conductivities for twenty-eight gaseous hydrocarbons for which experimental data are available. Calculated values for normal paraffins, isoparaffins, olefins, diolefins, acetylenes, naphthenes, and aromatics produce an average deviation of 2.4% from experimental values for 154 points considered.
Abstract The integration of solid oxide fuel cells (SOFCs) with gasification systems have theoretically been shown to have a great potential to provide highly efficient distributed generation energy systems that can be fuelled by biomass including municipal solid waste. The syngas produced from the gasification of carbonaceous material is rich in hydrogen, carbon monoxide and methane that can fuel SOFCs. However, other constituents such as tar can cause catalyst deactivation, and blockage of the diffusion pathways. This work examines the impact of increasing concentrations of toluene as a model tar in a typical syngas composition fed to a NiO-GDC/TZ3Y/8YSZ/LSM–LSM SOFC membrane electrode assembly operating at 850°C and atmospheric pressure. Results suggest that up to 20 g/Nm3 of toluene and a low fuel utilisation factor (c.a. 17%) does not negatively impact cell performance and rather acts to increase the available hydrogen by undergoing reformation. At these conditions carbon deposition does occur, detected through EDS analysis, but serves to decrease the ASR rather than degrade the cell. Alternatively, the cell operating with 32 g/Nm3 toluene and with a fuel utilisation of 66.7% is dramatically affected through increased ASR which is assumed to be caused by increased carbon deposition. In order to test for the presence of tar products at the anode exhaust samples have been captured using an absorbing filter with results from HS-GC/MS analysis showing the presence of toluene only.
The synthesis step in the production of synthetic natural gas from wood, i.e. the methanation, was investigated by systematic experiments with commercial nickel catalyst in a micro-fluidised bed reactor. Ethylene in the feed is always converted completely; dominantly serial reactions of ethylene to ethane and further to methane under isothermal fluidised bed methanation conditions could be shown. Lower temperatures favour the production of the intermediate ethane while high temperatures cause the formation of carbon depositions and carbon whiskers. Applying optimal operation conditions, the hydrogenation of the unsaturated olefin not only avoids the deposition of carbon or coke, but also leads to an increase of the higher heating value (HHV) of the produced (raw) SNG.
Thermal conductivities of the six sets of binary mixtures of n-pentane, diethyl ether, n-hexane and diethyl peroxide (5- and 6-membered chain species) with one another, and of their sixteen sets of binary mixtures with helium, neon or argon (monatomic species), or nitrogen (diatomic species), have been measured at 50 and 100°C. Marked similarities between all the polyatomic molecules (chain species) are shown by their individual thermal conductivities and by their behaviour in mixtures with monatomic and diatomic molecules.
The paper introduces the new ASME measurement uncertainty methodology which is the basis for two new ASME/ANSI standards and the ASME short course of the same name. Some background and history that led to the selection of this methodology are discussed as well as its application in current SAE, ISA, JANNAF, NRC, USAF, NATO, and ISO Standards documents and short courses.
By application of the kinetic theory, with several simplifying assumptions, the previous equation of Buddenberg and the author has been modified to give a general equation for viscosity as a function of molecular weights and viscosities of the pure components of the mixture. Agreement of the equation with experimental data is demonstrated for a number of highly irregular binary gas systems and mixtures of three to seven components.
In the present work, the influence of reaction and mass transfer on the fluidized-bed methanation have been investigated by both experiments and modeling. By applying spatially resolved gas concentration and temperature measurements in a bench-scale fluidized-bed reactor, it was shown that most of the reaction proceeds in the first 20 mm while the temperature increases by 74 K in the first 2 mm of the bed. A CO conversion of practically 100% is achieved. The experimental data indicate that the measured gas composition represents mainly the dense phase and that mass transfer between bubble and dense phase is the dominant effect in the upper part of the bed. A fluidized-bed model is proposed based on the two-phase model approach, hydrodynamic correlations from the literature and kinetic parameters previously determined. Although it was not possible to reproduce all measured phenomena within the methanation reactor, this first attempt to model the fluidized bed provides a better understanding of the behavior of the reactor and the reactions.
In the fuel cycle system of fusion reactors, analysis of hydrogen isotopes is very important from the view point of system control. The gas chromatograph (GC) with cryogenic separation column (cryogenic GC) is one of the most extensively used methods for the analysis of hydrogen isotopes. The micro GC with cryogenic column is expected to improve analysis time, that is a major disadvantage of conventional GC. The present authors have modified the micro GC to use its separation column at cryogenic temperature for H2, HD and D2 mixture analysis. Obtained retention time of H2, HD and D2 was about 85, 100 and 130 s, respectively. Peak resolution between H2 and HD, these are nearest each other, was about 1.0. These result suggests that the column developed in this work attained the practical level for the separation of hydrogen isotopes without tritium. Present detection limit of hydrogen isotopes was about 100–200 p.p.m., and it can be improved further by adjustment of separation column.
The importance of natural gas as an international trading commodity and the cost to consumers has made the accuracy of determinations for the components of natural gas very important. Pricing of natural gas is based on the heating value of the gas determined from either calorimetry measurements or calculations based on individual component concentrations determined by gas chromatography (GC). Due to the expense of accurate calibration standards, many analysts and laboratories will use a single calibration standard to perform natural gas determinations. Therefore, the purpose of this study was to determine whether an analyst could accurately measure the components of natural gas, in particular methane, using a single standard, or whether a suite of standards is necessary to calibrate the analytical instrument. A suite of eight gravimetric primary standards was prepared covering a concentration range for methane of 64-94 mol%, with uncertainties of +/-0.05% relative (95% confidence interval). These natural gas primary standards also contained nitrogen, carbon dioxide, ethane, propane, iso-butane, n-butane, iso-pentane, n-pentane, and n-hexane with varying concentrations from 0.02 to 14%. A single analytical method was used in which only the amount of sample injected onto the column was altered. The results show that when injecting a 0.5 ml sample volume a second-order regression through the standards is necessary for the determination of methane. The results for nitrogen, ethane and propane also show the same trend. Only those individual standards whose methane concentration is within 1% of the test mixture predicted a concentration within 0.05% of the regression value. Those individual primary standards whose methane concentration is different by more than +/-1% of the test mixture predicted values differing by +/-0.5 to +/-2.0% from the regression value. These differences lie well outside the predicted concentration uncertainty interval of +/-0.20%. A smaller sample volume, 0.1 ml, resulted in a set of data that could be fit using linear regression. Each of the eight primary standards individually predicted the methane in the test mixture to be within +/-0.11% of the predicted value from linear regression. The data confirm that it is imperative to fully characterize the analytical system before proceeding with an analysis.
The results of gas chromatographic analysis of natural gas mixtures reveal strong correlations (Pearson correlation coefficient of >0.96) between the uncertainty of each component and variations in the ambient pressure. Although correction for ambient pressure variations can reduce this variability, normalisation of the results of each analysis using the assumption that the sum of all component amount fractions is unity provides significantly greater reductions in the uncertainty of each measured component. We show that the uncertainty in normalised components can be estimated approximately using the correlation coefficient as a measure of the correlation present in the measurements, or exactly using a full calculation of the variance/covariance (V/C) structure of the data.
Quantitative analysis of natural gas depends on the calibration of a gas chromatograph with certified gas mixtures and the determination of a response relationship for each species by regression analysis. The uncertainty in this calibration is dominated by variations in the amount of the sample used for each analysis that are strongly correlated for all species measured in the same run. The "harmonisation" method described here minimises the influence of these correlations on the calculated calibration curves and leads to a reduction in the root-mean-square residual deviations from the fitted curve of a factor between 2 and 5. Consequently, it removes the requirement for each run in the calibration procedure to be carried out under the same external conditions, and opens the possibility that new data, measured under different environmental or instrumental conditions, can be appended to an existing calibration database.