Simulated Natural Gas Mixtures Research Articles

Portable Mixed Potential Sensors for Natural Gas Emissions Monitoring

The US Environmental Protection Agency has estimated that emissions from natural gas and petroleum systems account for 32% of methane emissions in the US.[1] Natural gas (NG) is transported along hundreds of thousands of miles of pipeline in the US, and any plan to perform continuous monitoring of NG systems for leak detection will require the development of low-cost, field-deployable systems. Mixed potential systems based on ceramic electrolytes and metal or metal-oxide electrodes are expected to be capable of resolving CH4, C2H6 and other sub-components of natural gas mixtures and interferent mixes at the resolution required for buried pipeline monitoring. We have developed additively manufactured mixed potential electrochemical devices which are paired with artificial neural networks to perform mixture identification and discrimination.[2] Deployment of these devices outside of the laboratory setting requires the integration of the sensors into a portable sensor package (Figure 1(a)). We have produced a small form-factor housing for the four-electrode mixed potential device with an integrated heater. This is coupled with an Internet-of-Things data acquisition and networked transmission system to enable recording of the sensor response and transmission to the cloud for processing. The sensor system has demonstrated the capability to resolve simulated natural gas mixtures at the 40 ppm CH4 level (Figure 1(b)). Initial results of testing the sensing system outside the laboratory will also be presented. This work is supported by US Department of Energy, Office of Fossil Energy and Carbon Management award DE-FE0031864.

Classification and Regression of Binary Hydrocarbon Mixtures using Single Metal Oxide Semiconductor Sensor With Application to Natural Gas Detection

To detect natural gas pipeline leaks, ethane in the natural gas can be used to discriminate it from background methane emissions. Our approach classifies samples of methane and simulated natural gas based on time-series data collected from a low-cost microfluidic detector. This detector houses a single metal oxide semiconducting (MOS) gas sensor in a coated microchannel, which provides selectivity based on diffusion times of different gases. A data generation apparatus was designed and built, allowing us to create customized gas mixtures and collect data from our sensing apparatus in an automated fashion. We present a comparison of machine learning models and data representation methods, and demonstrate the feasibility of both discriminating methane from simulated natural gas mixtures, and also obtaining accurate concentration estimates in random binary mixtures of methane and ethane. We achieve a 98.75% classification rate between methane, ethane, and binary mixtures, and a 12.0% mean relative error in regression estimates for arbitrary mixtures. In addition, we discriminate samples of pure methane from simulated natural gas mixtures containing 1% and 3% ethane with 86.7% and 93.3% accuracy, respectively, and regress the concentrations of both the methane and ethane components with a maximum of a 19.3% mean relative error.

CeO2-Decorated ?-MnO2 Nanotubes: A Highly Efficient and Regenerable Sorbent for Elemental Mercury Removal from Natural Gas.

CeO2 nanoparticle-decorated ?-MnO2 nanotubes (NTs) were prepared and tested for elemental mercury (Hg0) vapor removal in simulated natural gas mixtures at ambient conditions. The composition which had the largest surface area and a relative Ce/Mn atomic weight ratio of around 35% exhibited a maximum Hg0 uptake capacity exceeding 20 mg?g?1 (2 wt %), as determined from measurements of mercury breakthrough which corresponded to 99.5% Hg0 removal efficiency over 96 h of exposure. This represents a significant improvement in the activity of pure metal oxides. Most importantly, the composite nanosorbent was repeatedly regenerated at 350 ?C and retained the 0.5% Hg0 breakthrough threshold. It was projected to be able to sustain 20 regeneration cycles, with the presence of acid gases, CO2, and H2S, not affecting its performance. This result is particularly important, considering that pure CeO2 manifests rather poor activity for Hg0 removal at ambient conditions, and hence, a synergistic effect in the composite nanomaterial was observed. This possibly results from the addition of facile oxygen vacancy formation at ?-MnO2 NTs and the increased amount of surface-adsorbed oxygen species.

Regenerable α-MnO2 nanotubes for elemental mercury removal from natural gas

The elemental mercury (Hg0) vapor removal ability of α-MnO2 nanotubes (NTs), nanorods (NRs) and nanowires (NWs) was evaluated in simulated natural gas mixtures. It was shown that NTs exhibited superior efficiency (~100%) at the temperature range of interest (25–100 °C). The Hg0 breakthrough (corresponding to 99.5% Hg0 removal efficiency) at ambient conditions and Hg0in = 870 μg∙m−3, occurred after 48 h, reflecting a Hg0 uptake of >10 mg∙g−1 (1 wt%). Most importantly, the developed nanosorbent was repeatedly, sufficiently regenerated at the relatively low temperature of 250 °C in a series of seven successive sorption-desorption tests, maintaining its capacity to remove all incoming Hg0in in each cycle. Interestingly, kinetic studies showed that at least 85% of total adhered Hg0 could be recovered within the first hour of a 3-h regeneration process. Moreover, the presence of CO2 and H2S did not affect its activity. The thermogravimetric analysis indicated significantly higher dehydration for NTs due to the loss of bound water inside its tunnels, likely indicating the presence of abundant surface adsorbed oxygen species which are believed to facilitate Hg0 adsorption. This was also confirmed by X-ray spectroscopy, possibly explaining the enhanced activity of NTs.

Reduction Kinetics of Ilmenite Ore for Pressurized Chemical Looping Combustion of Simulated Natural Gas

Reduction kinetics of ilmenite ore as an oxygen carrier for the chemical looping combustion of a simulated natural gas mixture under elevated pressure was studied using a pressurized thermogravimetric analyzer (PTGA). The fuel gas is a mixture of hydrocarbon, carbon dioxide, and nitrogen to simulate an actual combustion environment. The oxidation phase of the experiments was carried out in air. Effects of temperature (1023–1223 K), total pressure (0.6–1.6 MPa), fuel partial pressure (0.126–0.34 MPa), and CO2 partial pressure (pFuel/pCO2 = 0.5–1) were studied. The results showed that the presence of small amounts of ethane and propane clearly led to a higher ilmenite reactivity at temperatures below 1123 K, but this effect became less significant as temperature increased and completely disappeared above 1123 K. The results also showed that increasing CO2 partial pressure had little effect on ilmenite conversion rate, though it did have some slightly negative influence on ilmenite oxygen carrying capacity t...

Isoreticular rare earth fcu-MOFs for the selective removal of H2S from CO2 containing gases

In this work, we present the implementation of reticular chemistry and the molecular building block approach to unveil the appropriateness of Rare Earth (RE) based Metal-Organic Frameworks (MOFs) with fcu topology for H2S removal applications. Markedly, RE-fcu-MOFs, having different pore aperture sizes in the range of 4.7–6.0Å and different functionalities, showed excellent properties for the removal of H2S from CO2 and CH4 containing gases such as natural gas, biogas and landfill gas. A series of cyclic mixed gas breakthrough experiments were carried out on three isoreticular fcu-MOFs, containing linkers of different lengths (between 8.4 and 5Å), by using simulated natural gas mixture containing CO2/H2S/CH4 (5%/5%/90%) under different adsorption and regeneration conditions. The fcu-MOF platform has good H2S removal capacity with a high H2S/CO2 selectivity, outperforming benchmark materials like activated carbon and Zeolites in many aspects. The comparison of H2S removal performance with the related structures of the RE-fcu-MOFs provides insightful information to shed light on the relationship between the structural features of the MOF and its associated H2S separation properties. The excellent H2S/CO2 and H2S/CH4 selectivity of these materials offer great prospective for the production of pure H2S, with acceptable levels of CO2for Claus process to produce elemental sulfur.

Reverse selective glassy polymers for C3+ hydrocarbon recovery from natural gas

The separation properties of two novel triethoxy bearing polynorbornenes (PNBs)—both a ROMP and addition type—are compared to unmodified polydimethylsiloxane (PDMS) using multicomponent hydrocarbon mixtures up to 800psi. Partial pressures of C2H6, C3H8, C4H10 and CO2 as high as 120, 80, 32 and 80, respectively, were used in order to expose the membranes to simulated natural gas mixtures. All three materials displayed strong increasing permeability coefficients with pressure, which is indicative of swelling and plasticization. Despite this behavior, the PNBs showed stable selectivity for C3H8/CH4 and C4H10/CH4 up to 500psi. ROMP PNB's selectivity for both aforementioned gas pairs was effectively constant at 3 and 4 for C3H8/CH4 and C4H10/CH4, respectively, over the entire feed pressure range (200–800psi). Selectivity in Addition PNB fell by ~27% between 500–800psi. In the 5-component mixture at 800psi, Addition PNB displayed C3H8/CH4 and C4H10/CH4 selectivity of 4 and 6, respectively, which was the highest out of the three materials. PDMS selectivity declined significantly with feed pressure. The largest reduction was observed in the 5-component compared to the 3-component feed, which is consistent with penetrant-induced plasticization. These results demonstrate the strong dependence of selectivity on feed pressure for PDMS. Selectivity in the PNBs, despite apparent plasticization, showed much lower dependence on feed pressure, suggesting plasticization may not be as detrimental to these reverse selectivity glassy polymers compared to traditional diffusion selective glasses.

Capillary Force-Aided Gas Hydrate Growth in Shut-In Conditions

This paper presents a significant finding where a capillary force is observed for a gas hydrate growth under a 24-hours shut-in condition (static fluid) using a newly designed apparatus comprising six units of identical rocking cells within isochoric conditions. The gas hydrate growth (static condition) is compared to a gas hydrate formation under a rocking condition. Crystal growth with a simulated natural gas mixture at various time intervals is noted. The visual observations of nucleation and growth at a wetted surface are described. The obtained visual results suggest that capillary forces provide continuous mass-transfer of water towards gas hydrate growth, even within shut-in conditions.

Experimental and kinetic modeling study of the oxy-fuel oxidation of natural gas, CH4 and C2H6

The oxidation of simulated natural gas mixtures, methane and ethane under oxy-fuel conditions has been studied experimentally in a flow reactor, at atmospheric pressure and in the 773–1673K temperature range. The influence of the stoichiometry has been analyzed by varying the excess air coefficient (λ) from fuel-rich (λ=0.2) to fuel-lean (λ=2) conditions.The results are interpreted in terms of a chemical kinetic model for the oxidation of small hydrocarbons. The updated kinetic mechanism in relation to the CH4 and C2H6 subsets, together with the interactions between CO2 and C1–C2 hydrocarbon radicals, is able to reproduce satisfactorily the experimental trends. Concerning C1 radicals, only CH2 is able to consume significant quantities of CO2, whereas the reactions with C2 radicals are insignificant under the present conditions.The effect of CO2 as collision partner on important third-body reactions is evaluated and the major pathways of the oxidation of natural gas, methane and ethane are identified. The reaction paths for all fuels tested are found to be similar to those in air combustion, except for the interactions of CO2 with H and CH2 radicals and the lower importance of reactions between H radicals and hydrocarbons in oxy-fuel combustion.

Catastrophic Crystal Growth of Clathrate Hydrate with a Simulated Natural Gas System during a Pipeline Shut-In Condition

This paper presents a significant finding where capillary-aided catastrophic gas hydrate growth is observed under a shut-in condition (static fluid) for an uninhibited system. With the use of a newly designed apparatus, consisting of six units of identical rocking cell, nucleation and growth are observed. All experiments are conducted under isochoric conditions using 11.0 K subcooling (ΔTsub) as a driving force. A simulated natural gas mixture of 90.0 mol % methane + 10.0% mol % propane is used as a gas phase. The liquid phase used is 3% (Na+, Cl–) brine. The observations of the capillary-aided mass-transport mechanism for a catastrophic gas hydrate growth are presented in this work. Our observations show hydrate crystal formation at a brine wetted area close to the brine/gas interface initiates capillary-aided catastrophic hydrate growth. Moreover, no mass transfer restriction of brine toward catastrophic gas hydrate growth is observed. Furthermore, our results show the denser (less porous appearance) hydrate structures are continuously displaced from the hydrate–water interface (hydrate growth side). This displacement allowed new hydrate crystals to be formed uninterruptedly until the exhaustion of water in the cells. On the basis of macroscopic observations, the capillary-aided mass-transport mechanism for a catastrophic gas hydrate growth is presented.

Accurate density measurements for a 91% methane natural gas-like mixture

Accurate density data are necessary to validate equations of state (EOS) used in custody transfer of natural gas. The American Gas Association (AGA) AGA8-DC92 EOS, currently the industry standard, has been validated against a databank of gas mixtures composed mostly of binary data. Accurate density data for multicomponent mixtures are necessary to check the ability of AGA8-DC92 to cover the entire range of pressure, temperature, and compositions encountered in custody transfer. This paper uses a magnetic suspension densimeter to measure the densities of mixtures containing nine components with n-pentane being the heaviest. We have used a high pressure, high temperature, compact single-sinker magnetic suspension densimeter to measure densities of a simulated natural gas mixture having 91% methane composition after validating its operation by measuring densities of pure argon, nitrogen, and methane in the range (270 to 340) K and (3.447 to 34.474) MPa. The agreement with the reference EOS for the pure components is about 0.03%. The estimated 2 σ uncertainty of the densities after compensating for apparatus and fluid effects is 0.12% with 93% of that value coming from the composition uncertainty. The measured data indicate the advisability of performing an extensive study of the AGA8-DC92 EOS for natural gas mixtures.

Measurements of enthalpy and phase equilibrium for simulated natural gas mixtures and correlation of the results by a modified Starling equation

New direct measurements of isobaric enthalpy change are reported for a number of 6-component simulated natural gas mixtures containing from 70 to 97% CH 4, 4 to 28% N 2 and higher hydrocarbons to C-4. The pressures explored were in the range of 1.3–5.7 MPa and the temperatures embraced a range starting from about 370K down to a minimum exit temperature of 153K. When phase separation occurred the measured compositions of the co-existing phases are reported. A modified version of the generalised Starling equation was developed in which the parameter γ is made composition dependent since it exerts a major influence on liquid phase behaviour. The resulting interaction parameter was evaluated for 17 binary mixtures of hydrocarbons from methane to n-decane and was expressed as a function of reduced pressure and temperature. Using the appropriate combination rules to go from binary to multicomponent behaviour, this version of the Starling equation was tested against the new experimental data.

Measured enthalpy changes for a simulated natural gas mixture and their prediction using a modified BWR equation

AbstractA new method of incorporating binary interaction data into the BWR equation is proposed. It is developed to predict phase equilibria and enthalpies for multicomponent mixtures using interaction data for 18 binaries.A calorimeter is described, together with experiments made upon it, for a 6‐component simulated North Sea Gas mixture at three pressure levels from 18 to 36 bar, and over a temperature range from 373 to 188K. The experimental enthalpy results are claimed to have an accuracy of ± 0.9%. These results are compared with the predictions of the modified BWR equation. The average deviation is 1.1%, the values being negative at about 18 bar and generally positive for the higher pressures tested.