Low-quality Natural Gas Research Articles

Carbon dioxide capture in gallate-based metal-organic frameworks

Carbon capture and sequestration are vital industrial processes that not only mitigate the greenhouse effect but also promote upgrading of low-quality natural gas. Porous adsorbent-based adsorption separation process appeals to be promising, while it still remains a great challenge to develop adsorbents with high adsorption uptake and selectivity of CO2 as well as high working capacity in practical applications. In this work, a group of gallate-based metal-organic frameworks with aperture size of 3.52 to 3.65 Å were reported for molecular sieving of CO2 from N2 and CH4. Remarkably, Mg-gallate exhibited a record-high uptake of CO2 (5.05 mmol g−1) among all reported molecular sieving adsorbents combined with ultrahigh uptake ratio of CO2/CH4 (29.7) and CO2/N2 (84.2) at 1 bar and 298 K. Multiple O-H…C and O…H-O interactions afford dense packing of CO2 molecules in the framework structure with a density up to 1.06 g cm−3 on Mg-gallate, verified by density function theory calculations. Experimental breakthrough experiments confirmed the superb separation performance of flue gas and natural gas over a wide range of temperature. Robust structure and cycling stability along with facile scale-up synthesis method further offer great potential of M−gallate in industrial applications.

Nitrogen rejection from low quality natural gas by pressure swing adsorption experiments and simulation using dynamic adsorption isotherms

In order to remove N2 from low quality natural gas, a mathematical model has been established by Aspen adsorption, using the CH4-selective sorbent silicalite-1 pellets. The dynamic adsorption isotherm was first simulated by breakthrough simulation of a CH4/N2 mixture at different adsorption pressures and feed flow rates based on breakthrough experiments. The resulting simulated CH4 dynamic adsorption amounts were very close to the experimental data at three different adsorption pressures (100, 200, and 300 kPa). Moreover, a single-bed, three-step pressure swing adsorption (PSA) experiment was performed, and the results were in good agreement with the simulated data, further corroborating the accuracy of the gas dynamic adsorption isotherm obtained by the simulation method. Finally, based on the simulated dynamic adsorption isotherm of CH4 and N2, a four-bed, eight-step PSA process has been designed, which enriched 75% (vol) CH4 and 80% (vol) CH4 to 95% (vol) and 99% (vol), and provided 99% (vol) recovery.

A robust two–dimensional layered metal–organic framework for efficient separation of methane from nitrogen

Selective capture and separation of methane (CH4) from nitrogen (N2) is a feasible approach to mitigate the effects of global warming and to improve the heating value of low-quality natural gas. However, the efficient separation of CH4/N2 is a challenge issue since their very close kinetic diameters and thermodynamic properties. In this work, a new type of two-dimensional (2D) layered metal–organic framework (MOF) with permanent rhomboid pore channels, denoted as Ni(4-DPDS)2CrO4, (4-DPDS = 4,4′-dipyridyldisulfide) was synthesized for the first time and used for CH4/N2 separation. This novel 2D MOF not only performs high stability as outstanding as or even better than previously reported 3D MOFs but also exhibits a relatively high CH4 adsorption capacity of 0.95 mmol/g. Ni(4-DPDS)2CrO4 has a high affinity towards CH4 with the highest reported Qst value of 28.4 kJ/mol as well as a high CH4/N2 selectivity of 7.3, which is comparable to the state-of-the-art MOF materials reported so far. DFT calculations revealed the energy favorable binding sites for methane molecules are located in the middle of the cavity decorated with CrO42- anion. Such an angular inorganic anion provides polar sites and makes guest–host interactions in close proximity, affording tight binding affinity. Breakthrough experiments and regenerability tests suggest it is a promising material in natural gas purification.

Microporous Carbon Adsorbents Prepared by Activating Reagent-Free Pyrolysis for Upgrading Low-Quality Natural Gas

Separation of methane and nitrogen is an essential matter of concern in upgrading low-quality natural gases such as biogas, coalbed methane, shale gas, and so forth. However, similar weak polaritie...

Thermally rearranged polypyrrolone membranes for high-pressure natural gas separation applications

The gas permeability performance of thermally rearranged polypyrrolone (PPL-450) membranes have been experimentally evaluated under similar industrial conditions of gas mixtures, pressures, and stage-cuts. The membranes were synthesized from 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 3.3′-diaminobenzidine (DAB). The thermal treatment of the PPL-450 was accomplished under vacuum at 450 °C. Several gas mixtures (binary, ternary and quaternary) were utilized to study the performance of the PPL-450 membranes for CO2 removal and helium recovery from natural gas streams. The quaternary feed gas-mixture was composed of 59% CH4, 30% N2, 10% CO2 and 1% C2H6, which represent a low quality natural gas. Three pressures (400, 600, and 800 psig) and five stage-cuts (1.2, 2.8, 4.2, 6.4, and 9.8%) were considered for the study. It was observed that the permeability values of all gases were decreased for mixed-gas experiments when compared to that of the single-gas permeability. On the other hand, CO2/CH4 permeability ratio was increased from 29, for pure gas, to 45 for quaternary gas-mixture. Likewise, the He/CH4 permeability ratio was increased from 33, for pure gas, to 46 for ternary gas-mixture. Furthermore, at 800 psig and 9.8% stage-cut, PPL-450 was able to remove 80% of CO2 from the quaternary gas-mixture. In another study of a very dilute helium-concentration (0.14%) more than 80% helium was recovered at 800 psig and 5% stage-cut. Moreover, in all gas mixtures containing CO2, the PPL-450 membrane demonstrated an outstanding resistivity to CO2-induced plasticization at high pressures.

Removal of sulfur from sulfur-bearing natural gas to produce clean jet fuel

ABSTRACTDespite the low percentage of trace level sulfur in natural gas (NG), its use as a jet fuel for reduction from the ultra level is very important. Clean and efficient NG as a fuel has major advantages and importance. If the sulfur content of natural gas will decrease in how much it is substantially upgraded. The main problems and difficulties encountered method of removing minor amounts of sulfur remaining in gases after conversion of the major portion of the sulfur content of the gas to elemental sulfur by the Claus or Stretford methods, or any method. Low-quality NG contains hydrogen sulfide, carbon dioxide, and nitrogen, but is not suitable for treatment using current conventional gas treating methods due to economic and environmental challenges. To remove sulfur from natural gas as the most common on the Claus process is applied. However, the Claus process itself is not sufficient for removal of sulfur in the ultra level. The tail gas from the Claus reaction is then passed through a catalytic hydrogenation reactor together with a supply of hydrogen to reduce the sulfur and sulfur compounds to hydrogen sulfide. As a results, hydrogen sulfide is completely removed from the NG streams by washing with alkanolamine solutions.

A relaxation times coupling method to construct acoustic relaxation calibration for two-frequency measuring gas compositions

In our previous work (Hu et al., 2014), a method has been proposed to detect gas compositions by locating the acoustic spectral peaks, which can be detected only by two-frequency acoustic measurements in practice. However, as a ‘Detection Calibration’, the effective relaxation area (ERA) constructed by existing theoretical model cannot match the two-frequency measurements when there are more than one strong relaxational components in gas mixtures. This paper proposes a method to construct the ERA by coupling the decoupled single relaxation times together to a whole relaxation time. For gas mixtures with only one single relaxation process, the predicted ERA results match with the experimental data better than those predicted by the existing model. Moreover, for gas mixtures in which more than one relaxation process are significant, the ERA results predicted by the proposed method also match with the detection results of two-frequency measurements better than the existing model. This relaxation time coupling based ERA constructing method is validated by the application in low-quality natural gas detection.

Refrigeration cycles in low-temperature distillation processes for the purification of natural gas

The increasing energy demand has made low-quality natural gas reserves worthy of consideration for exploitation. As a consequence, industries have developed new process solutions in order to exploit these gas reservoirs in a profitable way. Most of these solutions are natural gas purification processes by distillation at low-temperature, involving or not solid CO2 formation. Due to the low-temperatures reached in this type of processes, the choice of the appropriate refrigeration cycle becomes of paramount importance for limiting their energy consumptions and, thus, their operating costs. The aim of this work is to compare the performances of different types of refrigeration cycles using the coefficient of performance (COP) as discriminating factor. Several compounds (such as nitrogen, light hydrocarbons and ethylene) and their mixtures have been considered as working fluids and both non-cascade and cascade systems have been taken into account. Simulations by means of Aspen Hysys® V7.3 have led to conclude that the propane-ethylene cascade refrigeration cycle allows to attain the best performances.

Synthesis and characterization of zeolitic imidazolate framework ZIF-7 for CO2 and CH4 separation

There are surging demands to separate carbon dioxide from methane in upgrading low-quality natural gas including biogas with an energy efficient process. In this work, adsorption equilibria and kinetics of CO2 and CH4 on a zeolitic imidazolate framework ZIF-7 were determined at various temperatures and gas pressures up to 1bar, to evaluate the feasibility of removing CO2 from CH4 in a vacuum swing adsorption process using ZIF-7 as adsorbent. The as-synthesized ZIF-7 sample was characterized with scanning electron microscopy for crystal structure, powder X-ray diffraction for phase structure, thermal gravimetric analysis for thermal stability, and carbon dioxide adsorption at 0°C for pore textural properties. To study the gate-opening of ZIF-7, CO2 adsorption was measured at five different temperatures (273, 298, 323, 348 and 373K) and pressures up to 10bar. A thermodynamic model was employed to estimate the free energy of phase transition between narrow pore and large pore, and a modified dual-site Sips equation could fit the S shape CO2 isotherms well. Adsorption equilibrium selectivity (α) and adsorbent selection parameter for pressure swing adsorption processes (S) were calculated from the adsorption equilibrium data. The relatively high values of adsorption selectivities suggest that ZIF-7 is a promising adsorbent candidate for CO2/CH4 separation in a pressure swing adsorption process.

Thermodynamic Simulations of Hydrate-Based Removal of Carbon Dioxide and Hydrogen Sulfide from Low-Quality Natural Gas

This paper aims at presenting a computational scheme to thermodynamically simulate a continuous multistage operation for separating, by forming clathrate hydrates, carbon dioxide (CO2) and hydrogen sulfide (H2S) from a low-quality natural gas and at showing the stage-to-stage changes in the gas-phase composition, the crystallographic structure and composition of the formed hydrate, and the gas/aqueous-liquid/hydrate equilibrium temperature (the higher temperature limit for hydrate formation). The paper first describes the fundamental concept and algorithm of the computational scheme and then applies the scheme to the processing of a specific natural gas modeled as a CH4 + C2H6 + C3H8 + N2 + H2S + CO2 mixture. It is demonstrated that the optimum number of stages should be determined by finding a compromise between the improving removal of CO2 and H2S and increasing losses of combustible substances, particularly C2H6 and C3H8, from the residual gas with an increasing number of stages.

Quantitative Acoustic Relaxational Spectroscopy for real-time monitoring of natural gas: A perspective on its potential

Although there are several methods for high sensitivity gas composition detection, typically off-line and laboratory-based, approaches for real-time, moderate-sensitivity detection are few. An approach based on Quantitative Acoustic Relaxation Spectroscopy (QARS) is proposed here. The methodology is based on the frequency-dependent acoustic propagation in a gas mixture, which depends on the concentration of contaminant gases in a known base gas. After describing the molecular acoustics model and QARS algorithm, we provide examples related to the identification of “mine gas” contamination in air, a mining safety application, and detecting impurities in low quality natural gas, an important issue in the natural gas processing industry. In both of these cases where moderate-sensitivity, continuous gas composition sensing would be useful, the QARS approach shows significant promise, at least theoretically, for realistic contaminant levels. Of course, further work is needed to refine several aspects of the molecular relaxation model that is necessary for the approach to be successful, not to mention challenges related to the development of a sensor based on the technique. Nevertheless, QARS holds potential to be a simple, robust, and inexpensive approach for moderate-sensitivity, continuous, real-time gas composition sensing.

Kinetic separation of carbon dioxide and methane on a copper metal–organic framework

Separation of carbon dioxide and methane is an important issue in upgrading low-quality natural gas. Adsorption equilibria and kinetics of CO2 and CH4 on a copper metal–organic framework (MOF), Cu(hfipbb)(H2hfipbb)0.5 [H2hfipbb=4,4′-(hexafluoroisopropylidene) bis(benzoic acid)], were investigated to evaluate the feasibility of removing CO2 from CH4 in a pressure swing adsorption process using this new MOF adsorbent. The heat of adsorption of CO2 on the Cu–MOF at zero-coverage (29.7kJ/mol) is much lower than those on a carbon molecular sieve and a zeolite 5A adsorbent; and the heat of adsorption of CH4 on the Cu–MOF (21.4kJ/mol) is similar to that on the zeolite 5A adsorbent and smaller than that on a carbon molecular sieve. The Cu–MOF being investigated has apertures of (∼3.5×3.5Å), which favors the kinetically controlled separation of CO2 and CH4. The kinetic selectivity is found to be 26 at 298K, and the overall selectivity (combining the equilibrium and kinetic effects) is about 25 for an adsorption separation process. These results suggest that the Cu–MOF adsorbent is an attractive alternative adsorbent for the CO2/CH4 separation.

Upgrading low-quality natural gas with H 2S- and CO 2-selective polymer membranes : Part II. Process design, economics, and sensitivity study of membrane stages with recycle streams

The cost of membrane separation processes for removing CO 2 and H 2S from low-quality natural gas can be reduced for some concentration ranges of CO 2 and H 2S by utilizing concurrently two different types of polymer membranes, one with a high CO 2/CH 4 selectivity and the other with a high H 2S/CH 4 selectivity. The polymers considered in this exploratory study were 6FDA-HAB polyimide for the removal of CO 2 and [poly(ether urethane urea)] (PEUU) for the removal of H 2S. It was required that the concentrations of CO 2 and H 2S in low-quality natural gas be reduced to US pipeline specifications (≤2 mol% CO 2 and ≤4 ppm H 2S). Low-quality natural gas was simulated in this study by CH 4/CO 2/H 2S mixtures containing up to 40 mol% CO 2 and 10 mol% H 2S. Twenty-seven membrane process configurations (PCs) were examined by computer simulations and optimized in order to determine the most economical configurations. Part I of this study considered only PCs without recycle streams [J. Hao, P.A. Rice, S.A. Stern, Upgrading low-quality natural gas with H 2S- and CO 2-selective polymer membranes. Part I. Process design and economics of membrane stages without recycle streams, J. Membr. Sci. 209 (2002) 177–206]. In Part II, reported below, the study was extended to two- and three-stage PCs with various recycle options. A sensitivity analysis was also made to determine the effects of variations in feed flow rate, feed pressure, membrane module cost, and wellhead price of natural gas on process economics. The economically optimal PCs were found to be either two membrane stages connected in series with or without recycle streams or single stages without recycle, depending on feed composition and selected operating conditions. The optimal two-stage PCs with recycle streams would utilize the H 2S/CH 4-selective membranes in the first stage and either the CO 2/CH 4 or the H 2S/CH 4-selective membranes, or both, in the second stage. Three-stage membrane PCs were not found to be economically competitive under the conditions assumed in this study.

Upgrading low-quality natural gas with H 2S- and CO 2-selective polymer membranes : Part I. Process design and economics of membrane stages without recycle streams

The upgrading of low-quality natural gas by membrane separation processes utilizing two different types of polymer membranes was investigated by process design, optimization, and economic assessment studies. The membranes considered for these processes exhibit high CO 2/CH 4 and H 2S/CH 4 selectivities, respectively. Low-quality natural gas was simulated by ternary CH 4/CO 2/H 2S mixtures containing up to 40 mol% CO 2 and 10 mol% H 2S. The membrane processes were designed to reduce the concentrations of CO 2 and H 2S in the natural gas to US pipeline specifications (≤2 mol% CO 2 and ≤4 ppm H 2S). This study has identified the membrane process configurations (single stage or two stages connected in series) and types of membranes (CO 2-selective, H 2S-selective, or both) that would yield the most economical processes for the upgrading of natural gas containing different ranges of CO 2 and H 2S concentrations. The upgrading costs of process configurations without recycle streams are dominated by the cost of CH 4 lost in the permeate stream. The effects of recycle streams, variations in feed flow rate and pressure, membrane module cost, and wellhead cost of natural gas on the upgrading costs will be reported elsewhere.

Poly(ether urethane) and poly(ether urethane urea) membranes with high H 2S/CH 4 selectivity

The objective of this study was to synthesize rubbery polymers with a high H 2S/CH 4 selectivity for possible use as membrane materials for the separation of H 2S from ‘low-quality’ natural gas. Two poly(ether urethanes), designated hereafter PU1 and PU3, and two poly(ether urethane ureas), designated PU2 and PU4, were synthesized and cast in the form of ‘dense’ (homogeneous) membranes. PU1 and PU2 contained poly(propylene oxide) whereas PU3 and PU4 contained poly(ethylene oxide) as the polyether component. The permeability of these membranes to two ternary mixtures of CH 4, CO 2, and H 2S was measured at 35°C, and for a PU4 membrane also at 20°C, in the pressure range from 4 to 13.6 atm (4.05–13.78×10 5 Pa). PU4 is a very promising membrane material for H 2S separation from mixtures with CH 4 and CO 2, having a H 2S/CH 4 selectivity greater than 100 at 20°C as well as a very high permeability to H 2S. Permeability measurements were also made with commercial PEBAX TM membranes for comparison. The possibility of upgrading low-quality natural gas to US pipeline specifications for H 2S and CO 2 by means of membrane processes utilizing both highly H 2S-selective and CO 2-selective polymer membranes is discussed.

Field tests of membrane modules for the separation of carbon dioxide from low-quality natural gas

Field tests of membrane modules for the separation of carbon dioxide from low-quality natural gas

Field tests of membrane modules for the separation of carbon dioxide from low-quality natural gas

A commercial-scale, single-stage, spiral-wound membrane system has been operated for approximately 20 months to upgrade low-quality natural gas from a well in East Texas. Throughout the test period the retentate product (“sales” gas) met pipeline specifications. Data were obtained on two membrane modules containing two types of asymmetric cellulose acetate membranes, one “standard” and the other one of higher density. A summary of the field test data shows the effects of the operating variables of pressure, feed flow rate, and CO2 concentration in the feed (from 3 to 25 mole percent). Concentrations greater than 6 mole percent were obtained by adding pure CO2 to the feed gas. In addition, computer models for the separation of gases under “perfect mixing” and cross-flow conditions were applied to the analysis of the field data. In general, the field test data were consistent with a flow regime which was intermediate between perfect mixing and cross-flow.

Subquality Natural Gas Reserves

Abstract There are significant quantities of low-quality or subquality natural gas in the United States that have not been entered into the resource data base. The fields encountered have been shut in or abandoned for lack of a market. Such gas fields notably contain nitrogen in higher concentrations and also may contain carbon dioxide and/or hydrogen sulfide. There are, in fact, high-nitrogen-content natural gas occurrences in southwest Arkansas, northern Texas, West Texas and the Panhandle region, eastern New Mexico and eastern Colorado, western Kansas and north central Oklahoma—not to mention central California and other locales. A detailing of this subquality reserve would be most useful, as would new and improved methods for gas separation, particularly of nitrogen.