•The equilibrium data in THI solution based formation water is first investigated.•The 0.55 mass fraction concentration of EG 0.55 mass fraction fills the vacancy of this area.•The testing pressure range from 4.22 MPa to 34.72 MPa was rare in published data.In this paper, the three-phase coexistence points are generated for multicomponent gas hydrate in methanol (MeOH) solution for (0.05, 0.10, 0.15, and 0.35) mass fraction and ethylene glycol (EG) solution for (0.05, 0.10, 0.15, 0.35, 0.40 and 0.55) mass fraction. The phase equilibrium curves of different system were obtained by an isochoric pressure-search method on high pressure apparatus. The phase equilibrium regions of multicomponent gas hydrate were measured using the same composition of natural gas distributed in the South China Sea. And the different concentration solutions were prepared based formation water. The experimental data were measured in a wide range temperature from 267.74 to 298.53 K and a wide range pressure from 4.22 MPa to 34.72 MPa. The results showed that the hydrate phase equilibrium curves shifted to the inhibition region in accordance with the increased inhibitor concentration. In addition, the equilibrium temperature would decrease about 2.7 K when the concentration of MeOH increased 0.05 mass fraction. Besides, the suppression temperature was 1.25 K with the 0.05 mass fraction increase of EG concentration in the range of 0.05 mass fraction to 0.15 mass fraction. While in high EG concentration region, the suppression temperature was 3.3 K with the same increase of EG concentration (0.05 mass fraction).

•Glycine had weak inhibitory effect while cloud enhance the performance of PVCap.•The induction time of PVCap increased 16-fold with the help of glycine.•The rapid growth region of PVCap was avoided in the presence of glycine.•Glycine significantly decrease the cost and improve biodegradability of PVCap.The inhibitory performance of glycine and its synergistic potentiality for poly N-vinylcaprolactam (PVCap) was studied by evaluating subcooling temperature, induction time and crystal growth inhibition respectively. Glycine could not inhibit CH4 hydrate formation alone but it could enhance the inhibitory performance of PVCap. The subcooling temperature of PVCap increased by 4.1 °C and the induction time also increased by 16-fold after blending the glycine with PVCap. Simultaneously, the performance of PVCap inhibiting hydrate crystal growth became more powerful in the presence of glycine. The rapid growth region of PVCap was totally avoided even at 13.5 °C subcooling with the help of glycine, leading crystal growth rate decreasing by 80%. The biggest difference between glycine and common synergists was that 1.0% mass fraction glycine could equivalently replace PVCap in the same amount, leading 40.8% lower cost and 23.4% higher biodegradability. Furthermore, the relationship between outstanding synergistic effect of glycine and its hydrophilic structure was studied.Download high-res image (62KB)Download full-size image

•The recovery rate of MEG was affected by the temperature rather than HI-121.•HI-121 could be recovered along with MEG under proper recovery conditions.•HI-121 with various polymer chain lengths reduced the TIE of the recovered MEG.•The solution recovered under milder conditions retained good inhibitory performance.•Adding 5 wt% MEG restored the recovered solution performance to original levels.Kinetic hydrate inhibitors (KHIs) combined with thermodynamic inhibitors (THIs) such as monoethylene glycol (MEG) have been good additives for the prevention of hydrate blockages in oil and gas industry operations. The regeneration and recycling of MEG are conventional process steps used to reduce costs. However, the recovery of THIs in the presence of KHIs or the recovery of the KHIs alone has rarely been investigated. In this paper, a series of experiments was designed to study the recovery of both a KHI based poly (N-vinylcaprolactam) and MEG. The results showed that the MEG recovery rate was closely related to the recovery temperature, but was not influenced by the KHI. The MEG recovery rate from solutions consisting of MEG and the KHI was as high as 94.52%, and the KHI was recovered along with the MEG. The polymer structure of the KHI was rarely changed when the recovery temperature was close to its polymerization temperature. The presence of the KHI had a negative impact on the thermodynamic inhibition efficiency of the MEG. The KHI performance of the recovered solution obtained at the KHI polymerization temperature could reach the level of the fresh combination inhibitor, but the recovered solutions obtained at temperatures far above the KHI polymerization temperature demonstrated worse inhibitory performance. The kinetic performance could be restored by adding 5.0 wt% fresh MEG. MEG enabled a subcooling temperature decrease into the range in which KHI which could play its role effectively, leading to the improved kinetic performance of the recovered solution.Download high-res image (175KB)Download full-size image

In this study, CO2 capture from (67.00 mol %) CH4/CO2 mixed gas with tetra-n-butylammonium bromide (TBAB) solution at 281.3 K through a pressure recovery of hydrate separation method were studied. During the experiment, pressure was recovered by TBAB solution injected into the cell to improve the separation efficiencies of CO2, and the effects of the concentration of TBAB solution and the operating conditions was investigated. The results showed that the CH4 concentration in the gas phase could achieve 93.52 mol % with a pressure recovery method at 1.14 MPa and 0.293 mol % TBAB. Under the pressure of 1.14 MPa and 0.1 mol % TBAB, the maximum CO2 separation factor was 52.87 and the CH4 separation and recovery factor was 2.050. The results demonstrated that the pressure recovery method can significant enhance the separation efficiency of hydrate separation. It is an effective method to cut down energy consumption of hydrate-based gas separation.

Molecular dynamics simulation was used to examine the growth of methane hydrate in the presence of natural product pectin at different concentrations, including the mass fractions 2.46% and 3.62%. Snapshots of the system configurations with time, radial distribution functions of the carbon atoms, and the total energy of the system were employed to characterize the effect of pectin on methane hydrate growth. Results indicated that pectin is a good inhibitor of methane hydrate. The higher the concentration of pectin is, the better the effect of inhibition is. The double-bonded oxygen atoms of pectin combine with hydrogen atoms of water, and the hydrogen atoms of hydroxyl in pectin combine with oxygen atoms of water through hydrogen bonds, which disturbed the further growth of the methane hydrate. The role of the pectin’s active groups in hydrogen bonds with water both as proton donor and as electron acceptor makes pectin have a better inhibitory effect on the growth of methane hydrate.

We first measured the hydrate equilibrium of two real gases from the South China Sea in the pressure range of 1.85 MPa to 9.32 MPa. Then, three kinetic hydrate inhibitors (poly(N-vinyl) pyrrolidone, Inhibex501, and synthesized KHI-HY4) were tested from 275.15 K to 283.15 K and the pressure range from 2 MPa to10 MPa in the case of two real gases under 11.6 K to 14.0 K subcooling. The results show that the maximum subcooling temperature of three hydrate inhibitors increased with the increase of the pressure for the real gases, and hydrate formation induction time increased more than 1.5 times with the inhibitor concentration decreasing for HY4 or Inhibex501 for the real gases at a certain subcooling range. It was believed that each inhibitor has its optimal subcooling for usage in real gas. When the subcooling reached the optimal value, remarkable inhibitory performance could be displayed.

In this paper, hydrate phase equilibrium data for the CO2/CH4 + water system, CO2/CH4 + tetrabutylammonium bromide (TBAB) + water system, CO2/CH4 + tetrabutylammonium chloride (TBAC) + water system, and CO2/CH4 + tetrabutylammonium fluoride (TBAF) + water system were measured at temperatures from 280.2 K to 291.3 K and pressures from 0.61 MPa to 9.45 MPa with the 2.93·10–3 mole fraction of tetrabutylammonium halide. The equilibrium hydrate formation conditions were measured by an isochoric pressure-search method. The mole fractions of the mixture gas used in this work were 0.33 CO2 and 0.67 CH4. The experimental data for the CH4 + water system were contrasted with the published equilibrium data in the literature. Both have a good consistency, which demonstrates that the experimental method and the apparatus used in this paper are feasible and reliable. The experiment results show that the hydrate stable region was enlarged by adding TBAB, TBAC, or TBAF. Among the three additives, TBAF is the best and the enlarged extent order of three additives is TBAF > TBAC > TBAB. The three-phase equilibrium pressure of the CO2/CH4 + TBAF + water system is 0.61 MPa at 284.2 K.

Substantial oxygen enrichment is observed in the natural air hydrates formed in Arctic and Antarctic ice sheets. Inspired by this phenomenon, a novel air separation method, utilizing hydrate crystallization, is proposed in our work. The three-phase equilibrium pressure for the air + water system is greater than 15 MPa as the temperature is upon ice point, which makes the air separation impractical by hydrate formation under mild temperature conditions. So, some additives were selected to reduce the phase equilibrium pressure of air hydrates at a fixed temperature. In this work, hydrate dissociation conditions for the air + tetrahydrofuran (THF) + water system (x = 0.0300 and 0.0500), the air + cyclopentane (CP) + water system (x = 0.0300 and 0.0556), and the air + tetra-n-butyl ammonium bromide (TBAB) + water system (w = 0.20 and 0.30) were measured in the temperature ranges of (279.7 to 290.5) K, (284.0 to 296.2) K, and (283.4 to 288.0) K, respectively. The comparison result shows that, in the pressure range of (0 to 1.5) MPa, the promotion effect of these three additives on the air + water system follows TBAB > CP > THF.

The effects of aluminum foam (AF, average pore size of 1000 μm) on formation and growth kinetic behaviors of methane hydrate with 0.03 wt % sodium dodecyl sulfate (SDS) were investigated in a 300 cm3 stainless steel vessel without stirring under 4.2, 6.0, and 8.3 MPa and 273.15 K. AF is a porous metal medium possessing large rough surface and excellent thermal conductivity. The experimental results demonstrated that porous AF played an acceleration role in the initial formation and further growth of methane hydrate by promoting hydrate nucleation and facilitating the removal of hydration heat. When AF was used, not only was the induction time reduced but the formation and growth were also sped up significantly, compared to conditions without it. In addition, under the above three pressures, the maximum formation rates (Rf,max) were increased by enormous times, 52% and 23%, with the help of AF, respectively. The relatively low increment of Rf,max under high pressure most likely was caused by AF’s own limitations (pore size). AF with smaller pore size can be selected for further study.

In this work, hydrate phase equilibrium data for oxygen + tetrahydrofuran (THF) + water, nitrogen + THF + water, and air + THF + water systems were measured in the temperature range of (281.84 to 303.63) K and the pressure range of (0.981 to 29.527) MPa at (5.13 or 5) mol % of THF. All hydrate phase equilibrium data were measured using an isochoric method. The hydrate dissociation conditions for methane + sodium dodecyl sulfate (SDS) + water system and oxygen + SDS +water system were measured and compared with the data reported in the literature, and the good agreement between them demonstrates the reliability of method and the experimental apparatus used in this work. The results show that THF can enlarge the hydrate stability zone compared with the corresponding single gas hydrates system. In the air + water system, the hydrate equilibrium pressure can be reduced to 0.981 MPa at the temperature of 281.84 K by addition of 5 mol % of THF, which provides valuable information for research on the air separation by hydrate crystallization.

Three gas separation technologies, chemical absorption, membrane separation and pressure swing adsorption, are usually applied for CO2 capture from flue gas in coal-fired power plants. In this work, the costs of the three technologies are analyzed and compared. The cost for chemical absorption is mainly from $30 to $60 per ton (based on CO2 avoided), while the minimum value is $10 per ton (based on CO2 avoided). As for membrane separation and pressure swing adsorption, the costs are $50 to $78 and $40 to $63 per ton (based on CO2 avoided), respectively. Measures are proposed to reduce the cost of the three technologies. For CO2 capture and storage process, the CO2 recovery and purity should be greater than 90%. Based on the cost, recovery, and purity, it seems that chemical absorption is currently the most cost-effective technology for CO2 capture from flue gas from power plants. However, membrane gas separation is the most promising alternative approach in the future, provided that membrane performance is further improved.