Non-condensable Gas Research Articles

A universal correlation development for film-wise condensation in the presence of non-condensable gases on vertical walls under turbulent free convection

Accurate prediction of steam condensation in the presence of non-condensable gases is essential for accident analysis in nuclear reactor containments. This study uses the steam–air mixture to illustrate the effect of the mixture's compressibility in analyzing steam condensation. Then, a consolidated database of steam condensation on vertical walls was established, and seven typical empirical and semi-empirical correlations were evaluated. Based on heat and mass transfer analogy and curve fitting, a new universal correlation for predicting steam condensation on vertical walls in the presence of non-condensable gases, including binary and ternary mixtures, under turbulent free convection is established, and this correlation can well handle the transition from binary to ternary mixtures using the same formula. More than 90% of predicted results compared with 1027 data points are within the ± 25% error band, and the mean absolute error is 10.6%. The similarity of helium and hydrogen in analyzing steam condensation is discussed.

Numerical simulation of condensation from isobutane vapour – air mixture

Hydrocarbon condensation has been a topic of interest in recent years due to its applications in refrigeration, chemical processing and air conditioning. The condensation of friendly fluids reduces the Global Warming Potential and Ozone Depletion Potential, which can be used as an alternative to many hydrofluorocarbon (HFC) refrigerants. The main objective of this study is to investigate the combined heat and mass transfer of laminar film condensation from isobutane vapour in the presence of a small percentage of non-condensable gas (air) inside a vertical tube. The external tube wall is subjected to a constant temperature. To achieve our goal, a mathematical model based on mass conservation, momentum, and energy in both gas and liquid phases is established. The discretization of equations in each region with the boundary conditions is realised using an implicit finite difference scheme. The validity of the numerical model developed is verified in comparison with the results of the literature. The obtained results show that the condensation process is enhanced by increasing the inlet velocity and the inlet-to-wall temperature difference. Moreover, the results indicate that the presence of non-condensable gas reduces the efficiency of heat and mass exchanges.

Thermal and hydrodynamic analysis of a self-purging hot reservoir variable conductance heat pipe

A new design of a hot reservoir variable conductance heat pipe (VCHP) is proposed and investigated in this study. While a hot reservoir VCHP can provide much tighter thermal control performance in comparison to a traditional cold reservoir VCHP, it was seen that the hot reservoir design is prone to more common operating failures due to the accumulation of the working fluid inside the reservoir. Once accumulated, this excessive working fluid in the reservoir can only be removed by diffusion which takes a long time. In this study, we propose and investigate a new design for the hot reservoir VCHP that would perform an additional function of working fluid vapor purging in addition to operating as a normal heat pipe. In the proposed design, the non-condensable gas (NCG) section of the heat pipe is connected to the reservoir by an external tube, forming a NCG flow-inducing loop configuration, hence the name self-purging hot reservoir VCHP (SPHR-VCHP). With proper placement of the NCG pipe in the VCHP a pressure-induced flow is generated through the loop from the NCG section towards the reservoir. It is shown that this loop flow can remove the excess working fluid vapor in the reservoir significantly faster than by diffusion. A Fortran code to simulate the two-phase flow in the heat pipe section combined with an ANSYS Fluent CFD model of the external loop is employed to predict the fluid flow, heat transfer, and mass transfer behaviors inside the SPHR-VCHP. An experiment is set up for the SPHR-VCHP that is made of a 291 mm long titanium tube with an inner diameter of 14.7 mm. The working fluid and the NCG are acetone and helium, respectively. The CFD predictions of wall temperature are validated by the experimental data with mean absolute error equal to 2.68%. A thorough CFD study is then performed to optimize the loop design in order to enhance purging rates by changing the size of the NCG tube to operate under microgravity conditions for space applications. Based on this study an optimum NCG pipe diameter equal to 40% of the inner diameter and pipe length equal to the length of the evaporator section is suggested. The wall temperature results show that the SPHR-VCHP design does not impact the thermal performance of the original hot reservoir heat pipe, while also having a self-purging capability that makes the design safe from working fluid accumulations issues.

Effect of wall subcooling on condensation in a steam-air mixture on a vertical tube

In this study, the effects of wall subcooling on the condensation heat transfer in steam-air mixture on a vertical tube was investigated. The effect of wall subcooling on condensation heat transfer was analyzed experimentally and using heat and mass transfer model. The experimental analysis indicated that wall subcooling effect varied depending on the degree of subcooling, i.e., the decrement of condensation heat transfer became less in low subcooling. In addition, the noncondensable gas fraction affected the wall subcooling effect. Such phenomenon was also found in the analysis using heat and mass transfer model. Through analysis using heat and mass transfer model, it was found that the accumulation of noncondensable gas and steam diffusion have competing effects. The former deteriorates and the latter enhances the condensation heat transfer with increasing wall subcooling. In low subcooling condition, both effects have higher rate of change, resulting in the different subcooling effect with that of high subcooling condition. We then developed an empirical correlation of condensation heat transfer over a wide range of wall subcooling; this predicted the experimental results reasonably well, particularly at low subcooling.

The influence of a noncondensable gas on the collapse of a bubble in a stratified steam explosion at NPP

In the present work, the effect of a noncondensable gas on the collapse of a hot bubble in cold water was studied. It is assumed that the bubble consists of a gas mixture of steam, which can condense on cold water, and a noncondensable gas. The dynamics of the bubble was modeled by the integral equations of mass and energy supplemented by the steam diffusion equation. The steam condensation rate was determined by heat fluxes on the interfacial surface from the steam and water sides. It was used a one-dimensional spherically symmetric approach. The water surrounding the bubble was considered as an incompressible fluid, the flow of which was described by the Rayleigh-Plesset. Heat transfer in water is described by a one-dimensional heat equation, which takes into account conductive and convective mechanisms of heat transfer. Calculations performed using this model showed that the presence of a noncondensable gas in a bubble does not significantly affect its collapse in cold water. This means that the noncondensable gas cannot significantly weaken the impact of water on the melt, which occurs when a bubble collapses near the interface, which leads to a stratified steam explosion.

Production and characterization of plastic oil from mixed plastic waste through the pyrolysis process

With increasing population, modernization, and industrialization, plastic usage is increasing daily, resulting in massive plastic waste. The current research investigated the generation of plastic oil from mixed plastic waste. The pyrolysis process is a thermochemical reaction between 340 °C and 440 °C. The process residence time was one hour per batch. Thermal breakdown of plastic waste at optimal temperature produces volatile matter, which passes through a series of condensation processes in a water-circulating heat exchanger to make plastic oil. The maximum yield of plastic oil was obtained at 425 °C. The heating value of plastic oil measured was around 42.5 MJ kg−1, which signifies the utilization of plastic oil in various process industries. Non-condensable gases were collected and analyzed. Pyro-char is a solid byproduct of the plastic pyrolysis process that is mostly employed in road laying.

Pyrolysis in the Chemical Industry and Its Major Industrial Applications

Pyrolysis is an irreversible thermochemical treatment process of materials at elevated temperatures in an inert atmosphere. Pyrolysis is used heavily in the chemical industry to produce many forms of carbon and other chemicals from petroleum, coal, wood, oil shale, biomass or organic waste materials, and it is the basis of several methods for producing fuel from biomass. The end products of pyrolysis include solid residual coproducts and ash, noncondensable gases and condensable liquids. These products can be controlled by optimizing pyrolysis parameters such as temperature and residence time.

Modeling of two-phase flow of high temperature geothermal production wells in the Yangbajing geothermal field, Tibet

Two-phase flow (flow of water in both liquid and gas phase, containing a non-condensable gas such as CO2) in the wellbore is one of the most important processes in the production performance and wellbore scaling evaluations of high temperature geothermal wells. This paper first describes the discharge tests in the Yangbajing geothermal field, Tibet. Next, a simple model for governing the two-phase flow in the presence of CO2 in the wellbore is constructed and a robust calculation method is presented. The model is applied to three production wells in the Yangbajing geothermal field. The results show that the velocity difference between the gas and liquid phase should be included in the model. Ignoring this difference (i.e., homogeneous model) would result in a significant deviation between measurements and calculations. A drift flux model (DFM) describes the velocity difference, where the specifics of the particular model can have significant effects on the results for the pressure and temperature profiles in the wellbore. Three commonly used DFMs are compared to estimate their performance. The calculated wellhead pressure and temperature are in the range of 2,3 bar and 125°C–135°C for all three wells at a production rate of about 20 kg/s. The estimated wellhead gas mass fraction is between 3% and 8%. Considering CO2 content, three different scenarios were evaluated, although the effect on the pressure and temperature profiles were limited. However, CO2 content has a much more significant effect on the flash depth, which is an important parameter for the estimation of calcite scaling.

Pyrolysis of low-value waste Trapa natans peels: An exploration of thermal decomposition characteristics, kinetic behaviour, and pyrolytic liquid product

The current work seeks to maximise the bio-oil yield by optimizing the experimental conditions and process parameters for the pyrolysis of water chestnut peels (WCP). The pyrolysis process was carried out in a stainless steel reactor to produce the pyrolysis product. The kinetic analysis was carried out using the model-free techniques such as Ozawa-Flynn-Wall (OFW), Vyazovkin (VZK), and Kissinger-Akahira-Sunose (KAS), while the thermal degradation profile was measured using a thermogravimetric analyzer (TGA) at dynamic heating rates (10, 15, and 20 °C min−1). Additionally, Gas Chromatography (GC) was used to study the non-condensable gases, whereas biochar was characterized according to its physicochemical properties. The results of the kinetic analysis exhibited that utilising the KAS, OFW, and VZK, the average apparent activation energies of WCP were obtained to be 344.37, 337.10, and 325.10 kJ mol−1, respectively. The thermal pyrolysis results revealed that the highest bio-oil yield (42.20 wt%) was achieved at 600 °C temperature, 25 °C min−1 heating rate, and 6 cm bed height. According to GC results, the hydrocarbon levels increased as the temperature increased. Biochar's suitability for usage in various industrial applications was validated by the findings of its characterization.

Influence of operating conditions on in-tube vapour condensation heat transfer characteristics in the presence of noncondensable gases at high pressure

The in-tube vapour condensation heat characteristics in the presence of noncondensable gas at high pressures are experimentally validated with an error of 20%. Numerical simulations are then conducted to explore heat transfer characteristics at different operating pressures, wall surface temperatures and mass flow rates. The flow and heat transfer characteristics are thoroughly discussed and the buoyancy effects induce a secondary circulation altering flow patterns, changing the water film and heat transfer coefficient distribution. In addition, increasing operating pressures is found to reduce the wall heat transfer rate. Even though the flow pattern is significantly different caused by the increased gas mixture density, the total condensate flow rate is approximately the same at different operating pressures due to the same inlet conditions and temperature difference. The difference in the wall heat transfer rate is attributed to the change of latent heat of vapourisation and falling film at different pressures and is evident at high pressures. At the high temperature difference, a nonuniform film height towards the bottom is emerged, sweeping the condensate upward. Similarly, increasing the mass flow rate can also increase the total wall heat transfer rate and total condensate flow rate.

A new cavitation model for simulating steady and unsteady cavitating flows

Cavitation is a widespread hydrodynamic phenomenon. Accurate prediction of cavitating flow is beneficial to the design and maintenance of the hydrodynamic machinery. A new cavitation model reducing the number of parameters needed to obtain satisfactory results is presented and validated. Based on the multiphase flow equation, the mass transfer due to cavitation is expressed as a source term in the gas-phase continuity equation. The mass transfer rate is derived from the assumption that the number of cavitation nuclei per unit volume in the mixture varies linearly with the vapor volume fraction during the evaporation stage. The mass transfer rate also incorporates the simplified Rayleigh–Plesset equation and the critical radius of cavitation nuclei. The new cavitation model considers the influence of non-condensable gas and the molar density of non-condensable gas is the only physical parameter that must be specified. The cavitating flow over a hydrofoil, cavitating flow in a sharp-edged orifice, and the unsteady cavitating flow in a venturi tube were predicted to verify the validity of the new cavitation model. The simulation results of the new model are in good agreement with the experimental results. The new cavitation model predicts cavitation regions and periodic unsteady cavitating flows.

Numerical study of the deep removal of R134a from non-condensable gas mixture by cryogenic condensation and de-sublimation

Nowadays, the limits on greenhouse gas emissions are becoming increasingly stringent. In present research, a two-dimensional numerical model was established to simulate the deep removal of 1,1,1,2-tetrafluoroethane (R134a) from the non-condensable gas (NCG) mixture by cryogenic condensation and de-sublimation. The wall condensation method was compiled into the Fluent software to calculate the condensation of R134a from the gas mixture. Besides, the saturated thermodynamic properties of R134a under its triple point were extrapolated by the equation of state. The simulation of the steam condensation with NCG was conducted to verify the validity of the model, the results matched well with the experimental data. Subsequently, the condensation characteristics of R134a with NCG and the thermodynamic parameters affecting condensation were studied. The results show that the section with relatively higher removal efficiency is usually near the inlet. The cold wall temperature has a great influence on the R134a removal performance, e.g., a 15 K reduction of the wall temperature brings a reduction in the outlet R134a molar fraction by 85.43%. The effect of changing mass flow rate on R134a removal is mainly reflected at the outlet, where an increase in mass flow rate of 12.6% can aggravate the outlet molar fraction to 210.3% of the original. The research can provide a valuable reference for the simulation of the deep removal of various low-concentration gas using condensation and de-sublimation methods.

A numerical study on film condensation of steam with non-condensable gas on a vertical plate

The film condensation of steam is very common in several industrial areas, such as condensers in power plants, seawater desalination, and air-conditioning systems. In some studies, the non-condensable gas and liquid film are overlooked for the sake of simplicity. To provide an integral computational scheme, in the present study, the film condensation of steam in the presence of non-condensable gas on a vertical plate has been simulated using a two-dimensional CFD model combining a wall condensation model and volume of fluid (VOF) model. After verification, the proposed computational scheme is used to simulate the steam condensation process, with the mass fractions of non-condensable gas varying from 5% to 45%. The results indicate that the concentration of non-condensable gas in the boundary layer decreases gradually with the condensation process, resulting in a decline in the synergy between temperature and velocity field. It can also be found that the fluctuation of the liquid film can influence the concentration distribution of the non-condensable gas layer. For cases with high concentrations of steam, the thermal resistance of liquid film can reach more than 20% of the total thermal resistance, which should not be ignored.

A transient geothermal wellbore simulator

A new transient geothermal wellbore simulator is presented. It is based on a mathematical model consisting of three conservation equations (mass, momentum and energy) and a slip model describing how the liquid and vapour phases move differently. The three conservation equations are discretised using the finite volume method with thermodynamic variables such as pressure and temperature defined at cell centres and flow variables such as volume flux of liquid and vapour velocity defined at cell boundaries. Implicit differencing in time is used and upstream weighting in space is essential.The three discrete conservation equations are solved together with the slip equation, giving a system of 4N nonlinear algebraic equations to be solved for 4N unknowns for a numerical model containing N blocks. This system of equations is solved using the Newton-Raphson method.For solving difficult transient problems e.g., starting and stopping production in a high temperature well, some subtle modifications of the standard upwinding approach were introduced. The simulator was then able to solve all the test cases we tried, including completion tests, opening and closing of wells (with some high enthalpy cases) and well stimulation by air-lifting and air-compression. The well stimulation simulations used a version of the simulator modified to handle a mixture of water and a noncondensible gas (NCG) by including two mass conservation equations, one for water and one for the NCG.Our simulator appears to be very robust and can solve a wide range of interesting transient flows in geothermal wells.

Moist Convection Is Most Vigorous at Intermediate Atmospheric Humidity

In Earth’s current climate, moist convective updraft speeds increase with surface warming. This trend suggests that very vigorous convection might be the norm in extremely hot and humid atmospheres, such as those undergoing a runaway greenhouse transition. However, theoretical and numerical evidence suggests that convection is actually gentle in water-vapor-dominated atmospheres, implying that convective vigor may peak at some intermediate humidity level. Here, we perform small-domain convection-resolving simulations of an Earth-like atmosphere over a wide range of surface temperatures and confirm that there is indeed a peak in convective vigor, which we show occurs near T s ≃ 330 K. We show that a similar peak in convective vigor exists when the relative abundance of water vapor is changed by varying the amount of background (noncondensing) gas at fixed T s , which may have implications for Earth’s climate and atmospheric chemistry during the Hadean and Archean eons. We also show that Titan-like thermodynamics (i.e., a thick nitrogen atmosphere with condensing methane and low gravity) produce a peak in convective vigor at T s ≃ 95 K, which is curiously close to the current surface temperature of Titan. Plotted as functions of the saturation-specific humidity at cloud base, metrics of convective vigor from both Earth-like and Titan-like experiments all peak when cloud-base air contains roughly 10% of the condensible gas by mass. Our results point to a potentially common phenomenon in terrestrial atmospheres: that moist convection is most vigorous when the condensible component is between dilute and nondilute abundance.

Life Cycle Analysis and Operating Cost Assessment of a Carbon Negative Catalytic Pyrolysis Technique Using a Spent Aluminum Hydroxide Nanoparticle Adsorbent-Derived Catalyst: Insights into Coproduct Utilization and Sustainability

The current research is intended toward the life cycle analysis (LCA) and operating cost assessment of catalytic and noncatalytic pyrolysis. The study assessed the impact of waste material-based catalysts (spent aluminum hydroxide nanoparticle (AHNP) adsorbent-derived catalysts) on the process’ overall environmental load and operating cost. Incorporating the utilization of process coproducts through recycling both noncondensable gases (NCGs) and biochar in the pyrolysis process, as well as exporting biochar as a coal substitute, reduced the values of the environmental impact and operating cost. Results revealed that biochar market export as coal replacement (for earning coal credits) is comparatively more beneficial than its back-recycling in the pyrolysis process (for offsetting natural gas requirements) from the environmental perspective. However, the reverse is true from the economic viewpoint in terms of operating cost. Among different considered processes, nickel-doped AHNP (Ni/Al)-catalyzed pyrolysis with scenario 2 (wherein all the produced biochar was coal-credited and all the NCGs were back-recycled) portrayed the highest environmental benefits owing to the maximum negative emissions (∼0.031 kg CO2 equivalent GHG emission saving). Low standard deviations (<10%), as obtained through Monte Carlo uncertainty analysis, indicated toward the reliability and robustness of the obtained LCA impacts. The present study exemplified the renewability and environmental friendliness of the AHNP-catalyzed pyrolytic biofuel over conventional petroleum-derived diesel and gasoline fuels.

An improved cavitation model with thermodynamic effect and multiple cavitation regimes

Mechanical feed pumps in organic Rankine cycle (ORC) power plants can suffer from cavitation to lose their normal feeding performance or even damage. Cavitation models for organic fluids in ORC systems are lacking presently. Hence, a new cavitation model with thermodynamic effect was proposed. Surface tension-controlled, inertia-controlled, intermediate and heat transfer-controlled cavitation regimes, and two key elements: vapour bubble growth rate and vapour bubble number density are included in the model. A known air or non-condensable gas concentration in the liquid was employed to determine cavitation nuclei number density. The model was coded in ANSYS CFX as user defined model and validated with cavitating flows of organic fluid R114 in a venturi, liquid nitrogen and liquid hydrogen on a tapered hydrofoil and warm water around a hydrofoil NACA 0015 in cavitation tunnels based on visualised cavity length. Two model constants, temperature depression, and minimal cavitation number were correlated to bulk liquid temperature, Reynolds number, and Jakob number. The temperature and pressure profiles of liquid nitrogen and hydrogen on hydrofoil surface were examined against the experimental data. The model was applied to simulate unsteady cavitating flows of organic fluid R245fa in a diaphragm pump. It was shown that the temperature depression and minimal cavitation number cannot be correlated to bulk liquid temperature, Reynolds number and Jakob number. Two model constants can be correlated fairly to Reynolds number. The model underestimates the thermodynamic effect by 43% for R114, 18.6% for liquid nitrogen and 32.6% for liquid hydrogen based on temperature depression. The predicted temperature and pressure profiles on hydrofoil surface agree with the experimental data for liquid nitrogen. The model can produce an expected curve of mean pump flow rate against net positive suction head available.

A distributed activation energy model for cellulose pyrolysis in a fluidized bed reactor

Cellulose pyrolysis is used to give solid char, condensable vapors and non-condensable gases. This is a complex process and to model this using CFD simulations, Euler-Euler approach with multi-fluid is applied. The objective of the present study is to develop a CFD model to combine the reaction kinetics with the hydrodynamics to study the cellulose pyrolysis in the fluidized bed reactor. The distributed activation energy model to compare the product yield with and without distribution of activation energy has been incorporated. Simulations of cellulose pyrolysis in a fluidized bed reactor using two different kinetic schemes have been conducted. Model has been validated using global kinetic scheme and then optimized kinetic parameters are introduced with a distribution of activation energy. The predicted values of activation energies, frequency factor and standard deviation impact the overall pyrolysis product yield. The yield of tar and char increases, however the fraction of gases decreases significantly because of higher activation energy. Tar yield is 82.1 wt% in the presence of DAEM, while the gas yield is 8 wt% and char yield comes out to be 9.9 wt%.

Self-circulation of oily spent hydrodesulphurization (HDS) catalyst by catalytic pyrolysis for high quality oil recovery

In this study, roasted spent HDS ash (sHDSc-A) was used for the first time to catalytically pyrolyze oily spent HDS catalysts (sHDSc) to improve the yield and quality of pyrolysis oil. The results showed that sHDSc-A promoted the decomposition of coke in oily sHDSc, resulting in the recovery of more oil and gas. Meanwhile, sHDSc-A significantly improved the quality of the pyrolysis oil. They inhibited the aromatization of alkanes to increase the saturation of the pyrolysis oil from 59.39% to 74.25% and the H/C radio from 1.62 to 1.72; promoted the decomposition of long-chain alkanes to increase the content of C11–C22 from 41.97% to 61.99%; enhanced the conversion of carboxylic acids to ketones led to the reduction of heteroatomic compounds such as N (56.10%–45.39%), S (66.95%–56.59%), and O (45.26%–26.70%) in the pyrolysis oil. The promotion of sHDSc-A in the pyrolysis process is attributed to the catalytic effect of the metal oxides in sHDSc-A. Among them, Al2O3 and Fe2O3 can promote decarboxylation of carboxylic acids and reduce O mobility, while MoO3 and Fe2O3 play a significant role in reducing coke and increasing pyrolysis oil. NiO can also promote methane vapor reforming, and thus increase the production of H2 in non-condensable gas. This study achieves self-circulation of oily sHDSc with a “waste-treatment-waste” strategy that presents the advantage of value-added energy recovery and waste reuse.

Volatile-char interactions during biomass pyrolysis: Effect of biomass acid-washing pretreatment

The purpose of this study is to investigate the effect of acid-washing pretreatment on the poplar wood (PW) pyrolysis under different volatile-char interactions, the experiment was conducted in a self-designed fixed-bed pyrolysis reactor. It was found that acid-washing pretreatment could alter the surface morphology of PW, increase the cellulose crystallinity and reduce the inorganic ash components (mainly Alkali and Alkaline Earth Metal species, AAEMs). AAEMs acted as catalysts for sugar ring fragmentation and charring reactions during both primary pyrolysis of biomass and secondary volatile-char interactions. The removal of AAEMs would increase the bio-oil yield and decrease the biochar and non-condensable gas yields, same trend was also observed with the decreasing volatile-char interactions. The removal of AAEMs and weakened volatile-char interactions both resulted in an increase of C content and a decrease of O content of biochar with more pore structures generated, especially micropores, the content of anhydrosugars in bio-oil was also largely enhanced. In addition, the decrease of volatile-char interactions would significantly reduce the effect of AAEMs in biomass on the pyrolysis process. A highest bio-oil yield of 63.37 wt% with 34.38% of anhydrosugars could be obtained even for a packed fixed-bed reactor by intentionally reducing volatile-char interactions and removing AAEMs from biomass during the process.