To determine the suitable operating conditions for the hydrate-based CO 2 separation process from a fuel gas mixture, the hydrate nucleation and growth kinetics of the simulated fuel gas (39.2 mol % CO 2 /H 2 gas mixture) in the presence of tetra-n-butyl ammonium bromide (TBAB) are investigated. The experiments were conducted at the TBAB concentration range of 0.14-1.00 mol %, the temperature range of 275.15-282.45 K, the driving force range of 1.00-4.50 MPa, the gas/liquid phase ratio range of 0.86-6.47, and the hydrate growth time of 15-120 min. It is found that the addition of TBAB not only shortens the induction time and accelerates the hydrate growth rate, but also enhances CO 2 encaged into the hydrate. However, the number of total moles of gas consumed and the number of moles of CO 2 transferred into the hydrate slurry phase decrease with the increase of the TBAB concentration when the TBAB concentration is above 0.29 mol %. The induction time reduces, and the number of moles of gas consumed, the hydrate formation rate, and the number of moles of CO 2 encaged into hydrate phase increase with the increase of the driving force. However, when the driving force is more than 2.5 MPa, H 2 prefers to go into the hydrate phase with the increase of the driving force, as compared to CO 2. In addition, the temperature has little effect on the hydrate formation process.

The equilibrium hydrate formation conditions for the gas mixture of CO 2 and H 2 with tetrabutyl ammonium bromide (TBAB) are measured. The data show that TBAB can reduce the gas hydrate formation pressure as an additive with the mole fraction of (0.14, 0.21, 0.29, 0.50, 1.00, and 2.67) %. The experiments were carried out in the temperature range of (274.05 to 288.55) K and the pressure range of (0.25 to 7.26) MPa. The equilibrium hydrate formation pressure of the CO 2 + H 2 + TBAB mixture is remarkably lower than that of the CO 2 + H 2 mixture at the same temperature and decreases with the increase in the concentration of TBAB. In addition, to avoid the formation of pure TBAB hydrate, in which there are no CO 2 or H 2 , for the above gas mixture, the formation conditions of the pure TBAB hydrate with TBAB mole fraction from (0.14 to 2.67) % were also measured.

The behavior of hydrate formation in porous sediment has been widely studied because of its importance in the investigation of reservoirs and in the drilling of natural gas hydrate. However, it is difficult to understand the hydrate nucleation and growth mechanism on the surface and in the nanopores of porous media by experimental and numerical simulation methods. In this work, molecular dynamics simulations of the nucleation and growth of CH4 hydrate in the presence of the surface and nanopores of clay are carried out. The molecular configurations and microstructure properties are analyzed for systems containing one H2O hydrate layer (System A), three H2O hydrate layers (System B), and six H2O hydrate layers (System C) in both clay and the bulk solution. It is found that hydrate formation is more complex in porous media than in the pure bulk solution and that there is cooperativity between hydrate growth and molecular diffusion in clay nanopores. The hydroxylated edge sites of the clay surface could serve as a source of CH4 molecules to facilitate hydrate nucleation. The diffusion velocity of molecules is influenced by the growth of the hydrate that forms a block in the throats of the clay nanopore. Comparing hydrate growth in different clay pore sizes reveals that the pore size plays an important role in hydrate growth and molecular diffusion in clay. This simulation study provides the microscopic mechanism of hydrate nucleation and growth in porous media, which can be favorable for the investigation of the formation of natural gas hydrate in sediments.

Trapping CO2 in hydrates is a promising approach to reduce the greenhouse gas emissions. This work presents the efficient separation process of CO2 from the simulated fuel gas (39.2 mol % CO2/H2 binary mixture) based on the hydrate crystallization in the presence of tetra-n-butylammonium bromide (TBAB). The experiments were carried out in the TBAB concentration range of 0.14−1.00 mol %, the temperature range of 275.15−285.15 K, the driving force range of 1.00−4.50 MPa, the gas/liquid phase ratio range of 0.86−6.47, and the hydrate growth time from 15 to 120 min. The results indicate that the increase of the TBAB concentration or the driving force can enhance the separation efficiency, except when the TBAB concentration is above 0.29 mol % or the driving force is above 2.50 MPa. The lower gas/liquid phase volume ratio and the hydrate growth time can also promote gas consumption. However, H2 more competitively encages into the hydrate phase with time. In addition, the temperature change has little effect on the CO2 separation efficiency with the fixed driving force. It is worth noting that the one-stage hydrate formation/decomposition process for the fuel gas in the presence of 0.29 mol % TBAB at 278.15 K and 2.50 MPa driving force could obtain a 96.85 CO2-rich gas and a 81.57 mol % H2-rich gas. The split fraction (SFr) and separation factor (SF) of CO2 are 67.16% and 136.08, respectively. On the basis of the data of the separation efficiency, a hybrid conceptual process for precombustion capture based on only one hydrate formation/decomposition stage coupled with membrane separation is presented.

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Nonparametric regression analysis when the regression function is discontinuous has broad applications. Existing methods for estimating a discontinuous regression curve usually assume that the number of jumps in the regression curve is known beforehand, which is unrealistic in certain cases. Although there has been research on estimation of a discontinuous regression curve when the number of jumps is unknown, this problem is still mostly open because such research often requires assumptions on other related quantities such as a known minimum jump size. In this paper, we propose a jump information criterion, which consists of a term measuring the fidelity of the estimated regression curve to the observed data and a penalty related to the number of jumps and jump sizes. Then, the number of jumps can be determined by minimizing our criterion. Theoretical and numerical work shows that our method works well in practice.