Comparison between liquefied natural gas，pipe natural gas and substitute natural gas

采用LCA方法对煤制天然气方案及其替代方案（俄罗斯进口管道天然气以及澳大利亚进口液化天然气）进行了评价,揭示了煤制天然气全生命周期各环节的环境效应。3种方案中,煤制天然气的CO₂等环境排放最高。煤制天然气对原材料价格的承受能力低下,随着褐煤价格的上涨,煤制天然气项目的经济性将受到较大的挑战。     

Horizontal gas and gas/solid jet penetration in a gas–solid fluidized bed

In this paper, the real time, dynamic phenomena of the three-dimensional horizontal gas and gas/solid mixture jetting in a 0.3 m (12 in) bubbling gas–solid fluidized bed are reported. The instantaneous properties of the shape of the jets and volumetric solids holdup are qualified and quantified using the three-dimensional electrical capacitance volume tomography (ECVT) recently developed in the authors’ group. It is found that the horizontal gas jet is almost symmetric along the horizontal axis during its penetration. As the jet width expands, the total volume of the gas jet increases. A mechanistic model is also developed to account for the experimental results obtained in this study. Comparison of jet penetration length and width between the model prediction and ECVT experiment shows that both the maximum penetration length and the maximum width of the horizontal gas jet increase with the superficial gas velocity. When the horizontal gas jet coalesces with a bubble rising from the bottom distributor, it loses its symmetric shape and can easily penetrate into the bed. For the horizontal gas/solid mixture jet penetration in the bed, the tail of the jet at the nozzle shrinks and the jet loses its jet shape immediately when the jet reaches its maximum penetration length, which are different from the characteristics exhibited by the gas jet. The solids holdup in the core region of the gas/solid mixture jet is higher than that in the gas jet. The penetration length of the horizontal gas/solid mixture jet is also larger than that of the gas jet.     

Performance of solid–gas reaction heat transformer system with gas valve control

Single-stage solid–gas reaction heat transformer system with the reactive salts of CaCl₂ and MnCl₂ was investigated. The system performances with gas valve control (closed protocol) were measured and compared with those without gas valve control (open protocol). The reasons of these differences were discussed. It was concluded that specific heating power (SHP), coefficient of performance (COP) and exergic COP (COP_ex) of the experimental set-up were improved with gas valve control. From the theoretical analysis, it was concluded that the improvement of system performances was mainly due to the difference of gas pressure in system operation, while not the multi-step reactions between CaCl₂ and NH₃. Further improvements of the performances of experimental set-up were also proposed. It was concluded that conducting heat recovery process would increase system COP and COP_ex, and converting to novel two-stage system with reactive salts of CaCl₂, MnCl₂ and FeCl₂ would increase temperature lift ΔT.     

Measuring gas hold-up in gas–liquid/gas–solid–liquid stirred tanks with an electrical resistance tomography linear probe

An electrical resistance tomography (ERT) linear probe was used to measure gas hold-up in a two-phase (gas–liquid) and three phase (gas–solid–liquid) stirred-tank system equipped with a Rushton turbine. The ERT linear probe was chosen rather than the more commonly used ring cage geometry to achieve higher resolution in the axial direction as well as its potential for use on manufacturing plant. Gas-phase distribution was measured as a function of flow regime by varying both impeller speed and gas flow rate. Global and local gas hold-up values were calculated using ERT data by applying Maxwell's equation for conduction through heterogeneous media. The results were compared with correlations, hard-field tomography data, and computational fluid dynamic simulations available in the literature, showing good agreement. This study thus demonstrates the capability of ERT using a linear probe to offer, besides qualitative tomographic images, reliable quantitative data regarding phase distribution in gas–liquid systems.     

Comparative study of gas–oil and gas–water two-phase flow in a vertical pipe

A wire-mesh sensor has been employed to study air/water and air/silicone oil two-phase flow in a vertical pipe of 67 mm diameter and 6 m length. The sensor was operated with a conductivity-measuring electronics for air/water flow and a permittivity-measuring one for air/silicone oil flow. The experimental setup enabled a direct comparison of both two-phase flow types for the given pipe geometry and volumetric flow rates of the flow constituents. The data have been interrogated at a number of levels. The time series of cross-sectionally averaged void fraction was used to determine characteristics in amplitude and frequency space. In a more three-dimensional examination, radial gas volume fraction profiles and bubble size distributions were processed from the wire-mesh sensor data and compared for both flow types. Information from time series and bubble size distribution data was used to identify flow patterns for each of the flow rates studied.     

A gas pressure gradient-dependent subgrid drift velocity model for drag prediction in fluidized gas–particle flows

Due to the linear correlation between the subgrid drift velocity and the filtered drag force, modeling the drift velocity would be an alternative way to obtain the filtered drag force for coarse-grid simulations. This work aims to improve the predictability of models for the drift velocity using a new effective marker, the filtered gas pressure gradient, which is identified by momentum balance analysis. New models are constructed based on conditional averaging of the results obtained from fine-grid two-fluid model simulations of three-dimensional unbounded fluidized systems. A priori assessment is presented with the comparison between the proposed models and the best available Smagorinsky-type model with dynamic adjustment technique proposed in the literature. Results show that the proposed models give satisfactory performance. More important, the proposed models are demonstrated to have a better adaptability for cases under various physical conditions than the Smagorinsky-type model.     

Local bubble size distribution,gas–liquid interfacial areas and gas holdups in an up-flow ejector

Particle image velocimetry (PIV) was used to measure local bubble size distributions (BSD), gas–liquid interfacial areas and gas holdups in an up-flow ejector, based on the water–air system with different liquid and gas flow rates under the presence/absence of the swirl body. The results show that the bubble flow patterns are different whether to add the swirl body into the nozzle, especially at low gas flow rate because the bubbles formed “bubble chain” in the ejector with swirl. The mean bubble sizes D₃₂ of the two are both related to the pressure drop between import and export, gas ratio and liquid flow. The interfacial area and D₃₂ are both mainly dependent on the local gas holdups. The mean bubble sizes in the absence of swirl body are smaller than that in the presence of swirl under different operating conditions. The gas holdups and interfacial area are larger with swirl than those without swirl. With the increase of the gas fraction, the differences of D₃₂, a_t and e_G become smaller.     

Total and gas convective heat transfer from a vertical tube to a mixed particle gas—solid fluidized bed

Experimental data were obtained for the average gas convective and total heat transfer coefficients for a vertical tube immersed in an air-fluidized bed of narrowly as well as widely distributed particle size mixtures. The gas convective heat transfer coefficient was determined by measuring the rate of mass loss from a vertical naphthalene tube 0.0262 m in diameter and 0.1012 m in length and using a heat and mass transfer analogy. These data were obtained at a bed temperature of about 330 K and superficial velocity of 0.1 to 1.1 m/s. The total heat transfer coefficients were measured under identical conditions using an electrically heated vertical tube. The total heat transfer coefficient decreased with an increase in particle diameter from 0.237 to 1.35 mm. The addition of fines was found to increase the total heat transfer coefficient. The gas convective heat transfer coefficient increased with increase in particle size and fluidizing velocity. The dependence of the gas convective heat transfer coefficient on gas velocity was more pronounced for large particles. The addition of fines resulted in decrease in gas convective coefficient. The relative contribution of the gas convective component of heat transfer coefficient was found to increase with increase in particle diameter. Its dependency on fluidizing velocity was found to be more complex. The experimental data were compared with the existing heat transfer models and correlations.     

Micromixing in a gas–liquid vortex reactor

The gas–liquid vortex reactor (GLVR) has substantial process intensification potential for multiphase processes. Essential in this respect is the micromixing efficiency, which is of great importance in fast reaction systems such as crystallization, polymerization, and synthesis of nanomaterials. By creating a vortex flow and taking advantage of the centrifugal force field, the liquid micromixing process can be intensified in the GLVR. Results show that introducing a liquid into a gas-only vortex unit results in suppression of primary and secondary gas flow. The Villermaux–Dushman protocol is applied to study the effects of the gas flow rate, liquid flow rate, and liquid viscosity based on a segregation index. Based on the incorporation model and reaction kinetics, the micromixing time of the GLVR is determined to be in the range of 10⁻⁴ ~ 10⁻³ s, which is comparable to the highly efficient rotating packed bed and substantially better than a static mixer.     

On the measurement of local gas hold-up and interfacial area in gas–liquid contactors via light sheet and image analysis

Particle image velocimetry techniques coupled with advanced image processing tools are receiving an increasing interest for measuring flow quantities and local bubble-size distributions in gas–liquid contactors.Aim of this work is that of providing a numerical procedure able to reconstruct local gas hold-up and specific interfacial area from images obtained by laser sheet illumination. A correction for measured quantities accounting for laser sheet thickness is proposed and tested by means of Monte Carlo simulations. The algorithms proposed are robust and independent of any measured parameters of the actual bubble size distribution.     

Local solid mixing in gas–solid fluidized beds

Diffusivity of the solid particles in a 152-mm ID gas–solid fluidized bed was determined at different regimes of fluidization. The gas was air at room temperature and atmospheric pressure and the solids were 385 μm sand or 70 μm FCC particles. The experiments were done at superficial gas velocities from 0.5 to 2.8 m/s for sand and 0.44 to 0.9 m/s for FCC (in both bubbling and turbulent regimes). Movement of a tracer was monitored by radioactive particle tracking (RPT) technique. Once the time-position data became available, local axial and radial diffusivity of solids were calculated from these data. Calculated diffusivities are in the range of 3.3×10⁻³ to 5.6×10⁻² m²/s for axial and 2.6×10⁻⁴ to 1.5×10⁻³ m²/s for radial direction. The results show that the diffusivities, both axial and radial, increase with superficial gas velocity and are linearly correlated to the axial solid velocity gradient. Solid diffusivity in a bed of FCC was found to be higher than that of a bed of sand at the same excess superficial gas velocity.     

Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system

This paper reports the performance of an yttria-stabilized zirconia fuel cell (YSZ) using five kinds of gas systems. The final target of this research is to establish the combined fuel cell systems which can produce a H₂ fuel and circulate CO₂ gas in the production process of electric power. A large electric power was measured in the H₂–O₂ gas system and the CO–O₂ gas system at 1073 K. The formation process of O²⁻ ions in the endothermic cathodic reaction (1/2O₂+2e⁻→O²⁻) controlled the cell performance in both the gas systems. The electric power of the H₂–CO₂ gas system, which allowed to change CO₂ gas into a CO fuel (H₂+CO₂→H₂O+CO) in the cathode, was 1/31–1/11 of the maximum electric power for the H₂–O₂ gas system. This result is related to the larger endothermic energy for the formation of O²⁻ ions from CO₂ molecules at the cathode (CO₂+2e⁻→CO+O²⁻) than from O₂ molecules. The CO–H₂O gas system and the H₂–H₂O gas system was expected to produce a H₂ fuel in the cathode (CO+H₂O→H₂+CO₂, H₂+H₂O→H₂+H₂O). Although relatively high OCV values (open circuit voltage) were measured in these gas systems, no electric power was measured. At this moment, it was difficult to apply H₂O vapor as an oxidant to the cathodic reaction in a YSZ fuel cell.     

Effect of the injection-nozzle geometry on the interaction between a gas–liquid jet and a gas–solid fluidized bed

In industrial fluid cokers, bitumen is first mixed with steam in a premixer, and then fed to the atomization nozzle. The objective of this work was to evaluate the impact of both the premixer and the nozzle geometrical configuration on the quality of the liquid–solid contact resulting from injections of liquid into a gas–solid fluidized bed. To assess the quality of the liquid–solid contact a method based on electric conductance measurements of the bed material previously developed by the authors [9] was used. Liquid atomization efficiency in open air, spray geometry, and spray stability were also characterized to evaluate their effects on the nozzle spraying performance within the fluidized bed. This study indicated that spray stability is highly beneficial to the liquid–solid contact efficiency. In particular, fluid constrictions such as the series of converging and diverging sections within the nozzle have a stabilizing effect on the spray. Future optimization of the existing liquid-injection systems should consider alternative gas–liquid premixers and nozzle geometries to enhance the jet stability.     

Simulation analysis of gas–solid flow characteristics and water evaporation in flue gas semi-dry desulphurization process based on CPFD method

The gas–solids flow in an industrial-scale semi-dry method desulphurization tower is simulated by the computational particle fluid dynamics (CPFD) approach. Compared with previous studies on desulphurization towers, this study focuses on analyzing particle distribution characteristics such as particle volume fraction, temperature distribution, and residence time. The simulation fully considered the particle–fluid, particle–particle, and particle–wall interactions in the desulphurization tower. Based on these considerations, the effects of flue gas inlet velocity and temperature on the gas–solid distribution characteristics of the desulphurization tower are simulated. An optimization scheme for adjusting the gas–solid flow in the desulphurization tower is proposed. The research results show that the error between the CPFD simulation data and experimental data is small and the changing trend is consistent. The particles in the bed of the desulphurization tower show a typical core–annulus flow. The distribution of gas and particles in the bed has a serious deviation with the increase of the flue gas inlet velocity and temperature. As the axial height of the desulphurization tower increases, the flue gas velocity, temperature, particle concentration, and water vapour distribution in the bed become more uniform. The relatively stable operating conditions for the gas–solid flow in the desulphurization tower is that the flue gas inlet velocity and temperature are 15 m/s and 393 K, respectively. Under these operating conditions, the pressure loss caused by the venturi accounted for 73.6% of the total pressure loss of the desulphurization tower. When the particle radius is between 0–150 μm, the particle size and the flue gas inlet velocity have the greatest influence on the particle residence time. Finally, the distribution of gas and particles before and after the adjustment of the desulphurization tower is compared, which showed that adjusting the bottom structure of the desulphurization tower could optimize the gas–solid flow.     

Supersonic attrition nozzles in gas–solid fluidized beds

Gas–solid fluidized beds are used in both catalytic and non-catalytic processes, and some of the industrial applications are fluid catalytic cracking, polyethylene production, drying and classification, coating, and granulation. In some applications, the size distribution of the bed particles must be controlled in order to maintain good fluidization, and attrition nozzles can be used for this purpose. Supersonic attrition nozzles are more efficient than subsonic nozzles, and, in this study, different geometries of the Laval nozzle, a convergent–divergent (C–D) nozzle, have been investigated. The geometry of this type of nozzles gives supersonic velocities under the right operating conditions.     

Stability and growth of gas hydrates below the ice–hydrate–gas equilibrium line on the P–T phase diagram

Using a previously developed experimental technique, the behavior of small methane and propane hydrate samples formed from water droplets between 0.25 and 2.5 mm in size has been studied in the pressure–temperature area between the ice–hydrate–gas equilibrium line and the supercooled water–hydrate–gas metastable equilibrium line, where ice is a stable phase. The unusual persistence of the hydrates within the area bounded by these lines and the isotherms at T=253 K for methane hydrate or at T=263 K for propane hydrates was observed. This behavior has not previously been reported. For example, in the experiment carried out at 1.9 MPa and 268 K, the methane hydrates existed in a metastable state (the equilibrium pressure at 268 K is 2.17 MPa) for 2 weeks, then immediately dissociated into liquid supercooled water and gas after the pressure was isothermally decreased slightly below the supercooled water–hydrate–gas metastable equilibrium pressure. It was found that dissociation of metastable hydrate into supercooled water and gas was reversible. The lateral hydrate film growth rates of metastable methane and propane hydrates on the surface of supercooled water at a pressure below the ice–hydrate–gas equilibrium pressure were measured. The temperature range within which supercooled water formed during hydrate dissociation can exist and a role of supercooled water in hydrate self-preservation is discussed.     

Environmental high resolution electron microscopy of gas‐catalyst reactions

Environmental electron microscopy has become an important scientific method for fundamental studies of dynamic chemical reaction processes in heterogeneous catalysis and of catalytic growth of carbon nanotubes. Outstanding contributions are resulting from the ability to observe gas‐catalyst surface reactions in situ, on the atomic scale. A great deal of structural and chemical information including lattice modification of working catalysts is possible. This is key to understanding novel reaction processes, including release mechanism of structural oxygen in oxide catalysts in selective oxidation of hydrocarbons and to designing improved catalysts. This brief survey of the recent spectacular developments in environmental high resolution electron microscopy shows that new opportunities are being opened up in catalysis. This revised version was published online in August 2006 with corrections to the Cover Date.