Bubble columns for condensation at high concentrations of noncondensable gas: Heat‐transfer model and experiments

This paper deals with the condensation of water vapour possessing a content of noncondensable gas in vertical tubes. The condensation of pure steam on a vertical surface is introduced by the Nusselt condensation model. However, the condensation of water vapour in a mixture with non-condensable gas differs from pure vapour condensation and is a much more complex process. The differences for the condensation of water vapour in a mixture containing a high concentration were theoretically analysed and evaluated. In order to investigate these effects, an experimental stand was built. Experiments were carried out in regards to the case of pure steam condensation and the condensation of water vapour with a non-condensable gas mixture to evaluate the influence of the variable non-condensable gas content during the process. A non-condensable gas in a mixture with steam decreases the intensity of the condensation and the condensation heat transfer coefficient. A gradual reduction of the volume and partial pressure of steam in the mixture causes a decrease in the condensation temperature of steam, and the temperature difference between steam and cooling water. The increasing non-condensable gas concentration restrains the transportation of steam towards the tube wall and this has a significant effect on the decrease in the condensation rate.

This article deals with the possibility of separating water vapour from flue gases after oxyfuel combustion using condensation processes. Those processes can generally be described as condensation of water vapour in the presence of non-condensable gases. Hence, the effect of noncondensable gas (NCG) on the condensation process has been theoretically and experimentally analysed in this study. The theoretical model was developed on the basis of the heat and mass transfer analogy with respect to the effect of the NCG, the flow mode of the condensate film, the shear stress of the flowing mixture, subcooling and superheating. Subsequently, an experimental analysis was carried out on a 1.5m long vertical pipe with an inner diameter of 23.7mm. The mixture of vapour and air flowed inside the inner tube with an air mass fraction ranging from 23% to 62%. The overall heat transfer coefficients (HTC) from the theoretical model and experimental measurement are significantly lower than the HTC obtained according to the Nusselt theory for the condensation of pure water vapour. The overall HTC decreases along the tube length as the gas concentration increases, which corresponds to a decrease in the local condensation rate. The highest values of the HTC are observed in the condenser inlet, although a strong decrease in HTC is also observed here. Meanwhile, there is a possibility for an HTC enhancement through turbulence increase of the condensing mixture in the condenser outlet. Results also showed that the heat resistance of the mixture is several times higher than the heat resistance of the condensate film. The developed theoretical model based on heat and mass transfer analogy is in good agreement with experimental results with the standard deviation within +25% and −5%. The model is more accurate for lower NCG concentrations.

The problem of laminar film condensation of a vapor from vapor-gas mixture in laminar flow in a vertical parallel plate channel is formulated theoretically. The flowing gas-vapor mixture contains a noncondensable gas in high concentration. An example of this case is the flow of humid air, in which air is present in high concentration. Vapor condenses at the dew point temperature corresponding to mass fraction of vapor in the gas-vapor mixture and the total pressure. The rate of condensation is controlled by the diffusion of the vapor through the noncondensable gas film. Thus the problem of convective condensation is treated as a combined problem of heat and mass transfer. The problem is governed by the mass, momentum and energy balance equations for the vapor-gas mixture flowing in a channel, and the diffusion equation for the vapor species. The flow of the falling film of condensate is governed by the momentum and energy balance equations for the condensate film. The boundary conditions for the gas phase and the condensate film are considered. The temperature at the gas-to-liquid interface is estimated by making use of the equations of heat and mass balance at the interface. The local condensation Nusselt number, condensation Reynolds number, and temperature at the gas-to-liquid interface are estimated from the numerical results for different values of the system parameters at the channel inlet, such as relative humidity, temperature of vapor-gas mixture, gas phase Reynolds number, and total pressure. The condensation heat transfer coefficients computed from the present theory are compared with the experimental data available in literature, and the agreement is found to be good. The present work is an extension of the earlier work, in which the problem of in-duct condensation of humid air in turbulent flow was solved theoretically. Humid air is considered as the gas-vapor mixture, since various physical and thermal properties have to be specified during the analysis.

The present work reports the MOS (metal/semiconductor/oxide) device application as a gas sensor to H2 and NH3 detection. For this propose we used the C x V curve behavior to study the sensor response by using the change of flat band voltage or capacitance near to flat band state. The sensor showed a nonlinear response as a function of H2 concentration with high sensibility at low gas concentration and low sensibility at high gas concentration. The chemical image response of sensor was obtained by scanning pulse light of small beam on the gate region area. The chemical images pointed out that the sensor response strongly depends on the carrier gas used in the experimental assay.The chemical images patter corresponding to H2 and NH3 didn’t showed significant differences however theirs intensities were clearly different. These results were discussed in the present work.

The effects of internal circulation velocity and the presence of noncondensable gas on vapor removal rate by condensation from a rising large vapor-gas bubble produced in a hypothetical core disruptive accident are investigated by solving the resulting transient heat and mass transfer problem of turbulent flow. Sample calculations are performed for the condensation of UO/sub 2/ and sodium vapors containing noncondensable fission gases. The time-averaged condensation heat transfer coefficients are presented for the condensation of UO/sub 2/ and sodium vapors for different internal circulation velocities and the concentration of noncondensable gas.

Introduction These days, the use of various industrial and personal equipment using hazardous gases is on the rising such as H2 for fuel cells. To utilize these gases, the low concentration sensor(~ppm) for leakage detection as well as the high concentration sensor (~%) for monitoring hazardous gasses in the industrial sites are essential. Semiconductor type and electrochemical type sensors are commonly used for gas detection but they are not suitable for high concentration because they are easily saturated in high concentration. In contrast, thermal conductivity type sensors are used as high concentration sensors because they measure resistance change of the heat loss of heated wire(heater) to a gas environment without sensor signal saturation in high concentration. However conventional thermal conductivity type sensors require large power (hundreds of milliwatts to watts) and relatively large size. In the previous study, a microscale bridge-type heater-based gas sensor was developed using MEMS technology for low power consumption and small size [1, 2]. This sensor used the 3ω-method to measure accurately heat loss to the gas with a high signal to noise ratio.Here, we developed a 3ω-method based high concentration gas sensor using a suspended nano-sized wire heater to minimize the required power and size. The suspended nano-sized wire heater consists of a suspended nanowire backbone, an eave structure and a thin gold heater on the suspended nanowire. The suspended nanowire backbone structure was fabricated by pyrolyzing suspended photoresist wire, and a thin gold layer as a heater material, which ensures high sensitivity due to its high-temperature coefficient of resistance, was selectively coated on the suspended carbon nanowire by virtue of the eave structure. The 3ω-method based gas sensor exhibited high sensitivity and wide linear range with ultra-small power consumption because of its suspended architecture, small size, high aspect ratio and high surface to volume ratio. In addition, all the processes of the 3D nanostructures were carried out at a wafer-level enabling cost-effective manufacturing owing to novel eave structures and carbon-MEMS processes (consisting of photolithography and pyrolysis) [3]. Method The suspended nanowire heaters were fabricated by a three-step process. First, eave structures for the selective metal coating on a suspended carbon nanowire was fabricated by oxide etching and isotropic silicon etching. Then, suspended microscale suspended polymer wires were patterned by two successive photolithography processes and the micro polymer wire was converted into a carbon nanowire by a dramatic volume reduction in pyrolysis. The pyrolysis temperature was set to 700℃ for low carbon electrical conductivity so that the electrical charge flows only through the gold. The last step was the deposition of gold as a heater line. A 50-nm-thick gold was deposited using evaporation. Owing to the eave structure and anisotropic evaporation, the gold layer is solely connected through the suspended wire as shown in Figure a. Results and Conclusions The gold-coated suspended carbon nanowire and the eave structure were well-defined as shown in Figure b. The gold-coated nanowire exhibited a very stable 3ω voltage output signal compared to bare carbon nanowire with high conductivity (Figure c). The thermal penetration depth reduces as the input frequency increases, the 3ω voltage decreases with increasing input frequency (Figure d). The 3ω voltage linearly changed with gas concentration depending on the relative thermal conductivity of target gas in comparison to that of N2 (Figure e). Therefore, selective gas detection is feasible. Figure f showed the response and recovery time of the sensor when it is exposed to 100% Ar and 5% H2. The sensor measures a gas concentration based on the thermal equilibrium between heater structure and gas environment only. Thus, very fast response and recovery time within 3s can be achieved even at high gas concentrations. In addition, the power consumption was only 0.107 mW due to suspended nanowire-type heater configuration.

IntroductionThese days, the use of various industrial and personal equipment using hazardous gases is on the rising such as H2 for fuel cells. To utilize these gases, the low concentration sensor(~ppm) for leakage detection as well as the high concentration sensor (~%) for monitoring hazardous gasses in the industrial sites are essential. Semiconductor type and electrochemical type sensors are commonly used for gas detection but they are not suitable for high concentration because they are easily saturated in high concentration. In contrast, thermal conductivity type sensors are used as high concentration sensors because they measure resistance change of the heat loss of heated wire(heater) to a gas environment without sensor signal saturation in high concentration. However conventional thermal conductivity type sensors require large power (hundreds of milliwatts to watts) and relatively large size. In the previous study, a microscale bridge-type heater-based gas sensor was developed using MEMS technology for low power consumption and small size [1, 2]. This sensor used the 3ω-method to measure accurately heat loss to the gas with a high signal to noise ratio.Here, we developed a 3ω-method based high concentration gas sensor using a suspended nano-sized wire heater to minimize the required power and size. The suspended nano-sized wire heater consists of a suspended nanowire backbone, an eave structure and a thin gold heater on the suspended nanowire. The suspended nanowire backbone structure was fabricated by pyrolyzing suspended photoresist wire, and a thin gold layer as a heater material, which ensures high sensitivity due to its high-temperature coefficient of resistance, was selectively coated on the suspended carbon nanowire by virtue of the eave structure. The 3ω-method based gas sensor exhibited high sensitivity and wide linear range with ultra-small power consumption because of its suspended architecture, small size, high aspect ratio and high surface to volume ratio. In addition, all the processes of the 3D nanostructures were carried out at a wafer-level enabling cost-effective manufacturing owing to novel eave structures and carbon-MEMS processes (consisting of photolithography and pyrolysis) [3]. Method The suspended nanowire heaters were fabricated by a three-step process. First, eave structures for the selective metal coating on a suspended carbon nanowire was fabricated by oxide etching and isotropic silicon etching. Then, suspended microscale suspended polymer wires were patterned by two successive photolithography processes and the micro polymer wire was converted into a carbon nanowire by a dramatic volume reduction in pyrolysis. The pyrolysis temperature was set to 700℃ for low carbon electrical conductivity so that the electrical charge flows only through the gold. The last step was the deposition of gold as a heater line. A 50-nm-thick gold was deposited using evaporation. Owing to the eave structure and anisotropic evaporation, the gold layer is solely connected through the suspended wire as shown in Figure a. Results and Conclusions The gold-coated suspended carbon nanowire and the eave structure were well-defined as shown in Figure b. The gold-coated nanowire exhibited a very stable 3ω voltage output signal compared to bare carbon nanowire with high conductivity (Figure c). The thermal penetration depth reduces as the input frequency increases, the 3ω voltage decreases with increasing input frequency (Figure d). The 3ω voltage linearly changed with gas concentration depending on the relative thermal conductivity of target gas in comparison to that of N2 (Figure e). Therefore, selective gas detection is feasible. Figure f showed the response and recovery time of the sensor when it is exposed to 100% Ar and 5% H2. The sensor measures a gas concentration based on the thermal equilibrium between heater structure and gas environment only. Thus, very fast response and recovery time within 3s can be achieved even at high gas concentrations. In addition, the power consumption was only 0.107 mW due to suspended nanowire-type heater configuration.

Introduction:  Enceladus’s erupting plume likely originates from a subsurface ocean, and thus represents an avenue for revealing the ocean composition. The plume composition was measured by two mass spectrometers on Cassini during flythroughs of the plume. These measurements currently provide our best means of estimating Enceladus’s ocean chemistry and the moon’s potential to host life. The Cosmic Dust Analyzer (CDA; Srama et al. 2004) measured the composition of ice grains in the plume and Saturn’s E ring, finding a variety salts (including biologically useful phosphate) and organic molecules that could be hydrothermal, primordial, or possibly biological in origin (e.g., Postberg et al. 2018, 2023; Khawaja et al. 2019). The Ion and Neutral Mass Spectrometer (INMS; Waite et al. 2006) analyzed the gases in the plume and detected CO2, NH3, H2, CH4 and HCN, which suggests conditions in the ocean are likely favorable for chemotrophy (e.g., methanogenesis) and possibly prebiotic chemistry (e.g., Waite et al. 2017, Peter et al. 2023).Motivation: However, the relative abundances of key molecules in the plume may be quite different in the ocean due to fractionating processes during eruption (Fifer et al. 2022). The effects of fractionation are important when considering how the plume gas represents (or misrepresents) the abundances of gases in the ocean. For instance, condensation of water vapor onto the icy walls of the tiger stripe fissures can cause gases like CO2 to have much higher abundances (relative to water) in the plume than in the ocean (Glein et al. 2015; Glein & Waite 2020; Fifer et al. 2022). In a competing fractionation, the differential exsolution of gases from Enceladus’s ocean will tend to enrich water vapor in the plume relative to other gases (Fifer et al. 2022). While CDA in situ measurements suggest that the ocean’s pH is ~8.5 – 10.5 (e.g., Postberg et al. 2009, Postberg et al. 2023), studies to account for fractionation and estimate ocean gas content and pH from the plume measurements have produced a wide range of possible ocean compositions, with pH ~6–13 (e.g., Marion et al. 2012; Glein et al. 2015). Thus, quantifying fractionation in the plume gas during eruption can better determine the ocean composition.Here, we used laboratory experiments to constrain a key fractionation process: the exsolution of gases at the liquid-gas interface.Methods:  In a stainless steel vessel at 0°C, we added pure water or saline solutions and degassed them under vacuum. We then introduced a single gas (e.g., CO2) and allowed it to dissolve to equilibrium. We monitored the headspace pressure and partially evacuated the headspace gas, driving gas exsolution in an analogous process to how the plumes may form from a water-filled fissure on Enceladus. We can calculate a mass transfer coefficient associated with exsolution by monitoring the increasing headspace pressure during exsolution and deriving the concentration remaining in solution. We also used a stir bar to investigate the effects of stirring or mixing on exsolution.Results:  We find a positive linear correlation between stir rate and mass transfer coefficient for CO2 (Figure 1) consistent with previous experiments investigating gas transfer in water (Nishimura et al. 1991). Notably, our mass transfer coefficients are comparable to those derived for ocean-atmosphere exchange on Earth (Broecker & Peng 1982). In trials using a 0.2 NaCl solution, we found a reduction in the mass transfer coefficient of CO2 by ~25% compared to pure water, which is larger than in previous studies (~10%) for CO2 diffusion in NaCl solutions (Zhang et al. 2015).Figure 1: Mass transfer coefficient for CO2 exsolution from pure water as a function of stir rate in solution. Conclusions: We find that the mass transfer coefficient of CO2 strongly depends on the rate of stirring. For Enceladus’s plume formation, this means that an observed flux of erupting gas could originate from either a well-mixed ocean with low gas concentrations or a poorly-mixed ocean with higher gas concentrations. Therefore, it is important to quantify the degree of mixing in the surface ocean where the plume gas is likely sourced. 

Three electrodes gas sensor based on ITO thin film

Condensation of a vapor in the presence of non-condensable gas occurs frequently in process industry. For example in compact condensers for heat recovery, in extraction of toxic components from exhaust gases, in cooling systems of nuclear power plants, seawater desalination systems, air conditioning and petrochemical industry. It is well known that even small concentrations of non-condensable gas have a detrimental effect on condensation heat transfer rates. The key difference between the condensation of pure vapor and vapor in the presence of non-condensable gas compounds is that mass diffusion in the gas phase instead of heat diffusion in the condensate layer dominates heat transfer in the heat exchanger in the latter case. In condensation heat transfer with non-condensable gases, vapor-sided heat and mass transfer are essential. If possible, the diffusion resistance in the gas-liquid boundary layer should be reduced in order to enhance heat and mass transfer. A way to augment heat transfer is by introducing dropwise condensation instead of filmwise condensation. Heat transfer by dropwise condensation is possibly a factor 8 to 10 higher than filmwise condensation [115]. The exact reasons for this difference are still not fully understood. Where filmwise condensation is characteristic of metal heat exchangers with clean uncontaminated surfaces, dropwise condensation is, for example, achieved by applying a fluoropolymer heat exchanging surface. This study aims to elucidate the importance of the initial phase of dropwise condensation after drainage on heat transfer, when diffusion is not yet limiting. The effects of growth, coalescence and drainage of droplets with surface refreshing on airsteam condensation heat transfer enhancement are quantified. For this reason, a new small scale condenser setup is designed and applied. To supply a gas flow at well defined conditions, existing infrastructure is combined with an acoustic relative humidity sensor tailored to the required flow conditions. Also other measures are taken to increase accuracy of the heat exchanger test rig. An apparatus with controlled removal of condensate droplets from the condenser plates is designed and applied. The dropwise condensation process is frequently interrupted upon which nucleation restarts upon each sweep. Condensate growth and surface temperatures are assessed by simultaneous video and infrared recordings. Software is developed to automatically extract the positions and radii of condensate droplets from images quickly and reliably. Cold wakes downstream of big drops on the condenser plate were observed. A single controlled droplet removal action enables a ’reset’ of the condenser surface. This allows measurement of droplet growth histories. It is found that droplet growth follows a power law with the exponent increasing with increasing inlet vapor mass fraction. Direct contact condensation on drops at condenser plate dominates drop growth. The main finding is that that the total heat transfer resistance decreases with increasing droplet removal frequency, while two measures for mass transfer simultaneously increase. Increasing diffusion limitation is one explanation for the observed decreasing mass transfer rate with time. After initial fast growth of drops, the slight increase in interfacial temperature observed offers another explanation. Furthermore, the effect of a structured heterogeneous plate surface on droplet drainage and heat transfer in dropwise condensation is investigated. A structured coating of the condenser plates is applied to create two coexisting dropwise condensation patterns. The structured coating constrains drainage and introduces directed surface energy gradients. The condenser with the structured coating is compared with two equally sized condensers: a non-coated pvdf and a fully coated pvdf condenser. It is found that drop drainage is promoted by oriented Ti-coated tracks to such a degree that the maximum obtainable heat transfer performance is practically reached. Design recommendations are given.

Oxidative pyrolysis of plywood waste: Effect of oxygen concentration and other parameters on product yield and composition

The article presents a numerical study of vapor condensation appearing in an ejector. The ejector is a part of a device developed by the authors and used to purify air emissions. The article considers condensation of pure water vapor and condensation of vapor in a mixture of vapor and carrier gas. The system of axisymmetric Navier-Stokes equations and the system of moment equations are considered as a mathematical model for describing two-dimensional non-stationary gas-dynamic processes and processes of phase transitions in the device. The general system of equations is numerically solved using the finite volume method.

We investigate the issue of applicability of the solid phase microextraction (SPME) in the analysis of volatile organic compounds (VOCs) destruction products in the gliding arc discharge. Our research is focused on the measurements with the simple one stage gliding arc reactor, applied voltage was varied in the range of 3.5-4 kV. As a carrier gas, the dry air and its mixtures with nitrogen and oxygen, enriched by toluene, with flow rate of 1000-3500 ml/min was used. Total decomposition of toluene of 97 % was achieved at the oxygen content in carrier gas of 60 %. For measurements with air as a carrier gas, the highest efficiency was 95 %. We also tested the SPME technique suitability for the quantitative analysis of exhausts gases and if this technique can be used efficiently in the field to extract byproducts. Carbowax/divinylbenzene and Carboxen/polydimethylsiloxane/divinylbenzene fibres were chosen for sampling. Tens of various high-molecular substances were observed, especially a large number of oxygenous compounds and further several nitrogenous and CxHy compounds. The concentrations of various generated compounds strongly depend on the oxygen content in gas mixture composition. The results showed that the fiber coated by Carbowax/divinylbenzene can extract more products independently on the used VOC compound. The Carboxen/polydimethylsiloxane/divinylbenzene fiber is useful for the analysis of oxygenous compounds and its use will be recommended especially when the destruction is done in the oxygen rich atmosphere. With the higher ratio of oxygen in the carrier gas a distinctive decline of CxHy compounds amount have been observed. We also tried to describe the significant production of some compounds like benzyl alcohol, benzeneacetaldehyde, even in oxygen content is proximate 0 %. Experimental data demonstrated that it is necessary to use several SPME fibres for full-scale high-molecular products analysis.

Influence of defects on rare-gas diffusion in solids

Convective condensation of vapor in the presence of a non-condensable gas of high concentration in laminar flow in a vertical pipe