Passive Venting Research Articles

Testing and comparison of a thermodynamic vent system operating in different modes in a liquid nitrogen tank

• A multifunctional system was established for cryogen storage technology evaluation. • Four different pressure control modes were compared on the same testing system. • A new parameter was proposed for evaluating contribution factors to pressurization. • Ullage heating is more effective to self-pressurization than liquid evaporation. • Active TVS vents more fluid for depressurization than passive TVS does. Thermodynamic vent system (TVS) has been considered as a promising method for pressure control in cryogenic propellant tanks in microgravity environment. TVS could be operated in different modes, namely, the mixing-venting (MV) mode, the mixing-only (MO) mode, the mixing-only followed by mixing-venting (MOMV) mode, and the passive venting (PV) mode. An experimental investigation on the effect of operation mode on the TVS performance was conducted by using liquid nitrogen as the liquid oxygen simulant on a testing facility named Multi-functional Cryogenic Test System. Results of those four operating modes tests were compared regarding the ullage cooling effect, liquid temperature, pressure control capability, and mass loss. It was found that with the help of spraying, the MV mode and the MO mode cooled the ullage more effectively than the PV mode did. However, the PV mode is more effective in suppressing the increase of the liquid temperature. A non-dimensional parameter η has been proposed for comparison between the ullage heating and the liquid evaporation contribution to the pressurization in cryogenic fluid storage tanks. For the mixing-venting mode, the ullage heating is the main force that pressurizes the tank when the liquid is subcooled, while when the liquid is saturated, the evaporation takes over the role. Moreover, the MOMV mode performs the best in reducing mass loss within 0–1500 min. The results will be useful to make a control strategy for the operation of pressure control in on-orbit cryogenic tanks through thermodynamic venting method.

Life-Cycle Assessment of a Regulatory Compliant U.S. Municipal Solid Waste Landfill.

Landfills receive over half of all U.S. municipal solid waste (MSW) and are the third largest source of anthropogenic methane emissions. Life-cycle assessment (LCA) of landfills is complicated by the long duration of waste disposal, gas generation and control, and the time over which the engineered infrastructure must perform. The objective of this study is to develop an LCA model for a representative U.S. MSW landfill that is responsive to landfill size, regulatory thresholds for landfill gas (LFG) collection and control, practices for LFG management (i.e., passive venting, flare, combustion for energy recovery), and four alternative schedules for LFG collection well installation. Material production required for construction and operation contributes 68-75% to toxicity impacts, while LFG emissions contribute 50-99% to global warming, ozone depletion, and smog impacts. The current non-methane organic compound regulatory threshold (34 Mg yr-1) reduces methane emissions by <7% relative to the former threshold (50 Mg yr-1). Requiring landfills to continue collecting LFG until the flow rate is <10 m3 min-1 reduces emissions by 20-52%, depending on the waste decay rate. In general, for landfills already required to collect gas, collecting gas longer is more important than collecting gas earlier to reduce methane emissions.

Investigation of the Effectiveness of a Passive Device for Soiling Mitigation for Vehicle Side-Cameras

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;The development of modern autonomous automotive technology depends heavily on the reliable performance of external sensors that are vulnerable to soiling. Existing active cleaning devices, such as washers and wipers, are relatively complex and expensive. Furthermore, little research has been done on alternative soiling mitigation strategies and devices for sensors. With the emerging trend of replacing side-mirrors with camera monitor systems, it is important for such systems to stay clean in adverse weather in order to provide critical navigation information. To meet this need, a passive aerodynamics-based cleaning device was investigated. A converging vent device was integrated into the side-camera housing and the subsequent degree of soiling was estimated at a wind speed of 20 m/s (72 km/h), representing urban and suburban driving speeds. The vent outlet height and outlet jet angle of the vent device were varied and the variants were compared to the non-vented reference model. The degree of soiling was evaluated experimentally and computationally. The variants were 3D-printed and sprayed with water in wind tunnel tests, where the degree of soiling was quantified by water droplet counts. Numerical simulation was also performed using ANSYS® Fluent to visualize the airflow patterns. The results from wind tunnel tests and CFD suggest that the vent device is effective in reducing soiling for up to 94%. This investigation demonstrated that the outlet geometry of a passive vent device significantly affects the deposition aerodynamics and type of soiling on a sensor and provides insight for further design and development of such devices.&lt;/div&gt;&lt;/div&gt;

An Assessment of the Dynamic Global Warming Impact Associated with Long-Term Emissions from Landfills.

Landfills are a major contributor of anthropogenic CH4 emissions. Since the greenhouse gas (GHG) emissions associated with landfilling waste can occur over decades to centuries, the standard static approach to estimating global warming impacts may not accurately represent the global warming impacts of landfills. The objective of this study is to assess the implications of using 100 yr and 20 yr static and dynamic global warming potential (GWP) approaches to estimate the global warming impacts from municipal solid waste landfills. A life-cycle model was developed to estimate GHG emissions for three gas treatment cases (passive venting, flare, CH4 conversion to electricity) and four decay rates. For the 100 yr GWP, other model uncertainties (e.g., static GWP values, decay rate, moisture content, or gas collection efficiency) generally had a larger effect on the estimated global warming impact than the choice of static versus dynamic GWP methods. This shows that when comparing single-point GWP values, the choice of static versus dynamic is relatively unimportant for most landfills. While dynamic GWPs consider temporal variance and provide useful estimates for the warming over a set time horizon, for most comparative analyses, static values provide reasonable bounds for the actual 100 yr warming impact.

The effectiveness of passive gas ventilation on methane emission reduction in a semi-aerobic test cell operated in the tropics

Two landfill test cells, with and without gas vents, were used to investigate the effectiveness of passive aeration, through basal leachate pipes, in mitigating methane emissions from municipal solid waste disposal in the tropical climate of Thailand. Surface methane emission rate, as well as methane content in the landfill gas, were determined for a period of three years. The results indicate that the average methane emission rate from the test cell with passive gas vents (42.13 g/t dry wt./d) was about half of that from the test cell without gas vents (90.33 g/t dry wt./d). Methane emission rates from both test cells fluctuated and were influenced by precipitation. The emission rate during the wet period in the test cell with gas vents (61.67 g/t dry wt./d) was 3 times as much as that observed during the dry period (20.95 g/t dry wt./d). The emission rate during the wet period in the test cell without gas vents (120.33 g/t dry wt./d), was twice the value of that observed during the dry period (60.32 g/t dry wt./d). The measurements also revealed the formation of methane hotspots in the test cell with passive vents after rainfall events, leading to higher localized surface emissions. Introduction of gas vents helped reduce methane emissions from solid waste landfills in a tropical region. However, rainfall should be limited to avoid turning semi-aerobic conditions into anaerobic conditions.

N2 gas egress from patients’ airways during LN2 spray cryotherapy

Spray cryotherapy using liquid nitrogen (LN2) is a general surgical tool used to ablate benign or malignant lesions. Adequate egress of the gaseous nitrogen (N2) generated during this process must be provided for safe use when LN2 is used within the body rather than topically. When delivered to either the gastrointestinal tract (requiring active venting via a suction tube) or body cavities open to room barometric pressure (such as lung airways) allowing for passive venting, the N2 gas generated from the boiling process must be evacuated. This work will examine the egress of N2 during procedures requiring passive venting from human airways undergoing liquid nitrogen spray cryotherapy. Venting characteristics for safe N2 egress will be presented and discussed based on analytical modeling using fluid mechanics simulations and experimental studies of N2 venting with laboratory and porcine models.

Spray cryotherapy (SCT): institutional evolution of techniques and clinical practice from early experience in the treatment of malignant airway disease.

Spray cryotherapy (SCT) was initially developed for gastroenterology (GI) endoscopic use in the esophagus. In some institutions where a device has been utilized by GI, transition to use in the airways by pulmonologists and thoracic surgeons occurred. Significant differences exist, however, in the techniques for safely using SCT in the airways. We describe the early experience at Walter Reed National Military Medical Center from 2011 to 2013 using SCT in patients with malignant airway disease and the evolution of our current techniques and clinical practice patterns for SCT use in patients. In November 2013 enrollment began in a multi-institutional prospective SCT registry in which we are still enrolling and will be reported on separately. Twenty-seven patients that underwent 80 procedures (2.96 procedures/patient). The average age was 63 years with a range of 20 to 87 years old. The average Eastern Cooperative Oncology Group (ECOG) status was 1.26. All malignancies were advanced stage disease. All procedures were performed in the central airways. Other modalities were used in combination with SCT in 31 (39%) of procedures. Additionally 45 of the 80 (56%) procedures were performed in proximity to a silicone, hybrid, or metal stent. Three complications occurred out of the 80 procedures. All three were transient hypoxia that limited continued SCT treatments. These patients were all discharged from the bronchoscopy recovery room to their pre-surgical state. SCT can be safely used for treatment of malignant airway tumor (MAT) in the airways. Understanding passive venting of the nitrogen gas produced as the liquid nitrogen changes to gas is important for safe use of the device. Complications can be minimized by adopting strict protocols to maximize passive venting and to allow for adequate oxygenation in between sprays.

Onsite survey on the mechanism of passive aeration and air flow path in a semi-aerobic landfill

The semi-aerobic landfill is a widely accepted landfill concept in Japan because it promotes stabilization of leachates and waste via passive aeration without using any type of mechanical equipment. Ambient air is thought to be supplied to the landfill through a perforated pipe network made of leachate collection pipe laid along the bottom and a vertically erected gas vent. However, its underlying air flow path and driving forces are unclear because empirical data from real-world landfills is inadequate. The objective of this study is to establish scientific evidence about the aeration mechanisms and air flow path by an on-site survey of a full-scale, semi-aerobic landfill.First, all passive vents located in the landfill were monitored with respect to temperature level and gas velocity in different seasons. We found a linear correlation between the outflow rate and gas temperature, suggesting that air flow is driven by a buoyancy force caused by the temperature difference between waste in the landfill and the ambient temperature. Some vents located near the landfill bottom acted as air inflow vents. Second, we conducted a tracer test to determine the air flow path between two vents, by injecting tracer gas from an air sucking vent. The resulting slowly increasing gas concentration at the neighboring vent suggested that fresh air flow passes through the waste layer toward the gas vents from leachate collection pipes, as well as directly flowing through the pipe network. Third, we monitored the temperature of gas flowing out of a vent at night. Since the temperature drop of the gas was much smaller than that of the environment, the air collected at the gas vents was estimated to flow mostly through the waste layer, i.e., the semi-aerobic landfill has considerable aeration ability under the appropriate conditions.

ESTIMATION OF LANDFILL GAS PRODUCTION

Landfills accepting biodegradable organic waste produce significant amounts of methane richlandfill gas. Landfill gas, or LFG, constitutes approximately equal quantities of methane andcarbon dioxide. The global emissions of methane from landfills are estimated to be about 10%oftotal anthropogenic emissions. Methane, a greenhouse gas, has a global warming potential(GWP) 21 times that of carbon dioxide over a laO-year time horizon. Therefore, control ofmethane from anthropogenic sources could substantially mitigate global warming. Consideringthese factors, there is renewed interest in controlling methane emissions from landfills.Methane emissions from landfills can be controlled by extracting LFG for energy recovery,passive venting and flaring, or by modifying the landfill cover to enhance passive oxidation ofmethane to carbon dioxide. To design any of the LFG control techniques prior knowledge of theamount of gas available is necessary.LFG production is site specific and depends on factors such as climatic conditions, wastecharacteristics, spatial heterogeneity, age of landfill, geometry of the landfill etc. The currenttechniques used to estimate LFG production are: theoretical calculations, generation models,experimental studies, LFG pumping tests and direct flux measurement. This paper discusses theadvantages and disadvantages in using these methods to estimate LFG production. The paperconcludes with a newly developed method at the University of Calgary, Canada to estimate LFGproduction. The method incorporates a geostatistical technique and a l-D numerical modelwithin a Geographic Information System (GIS).

Observation of trapped gas during electrical resistance heating of trichloroethylene under passive venting conditions

A two-dimensional experiment employing a heterogeneous sand pack incorporating two pools of trichloroethylene (TCE) was performed to assess the efficacy of electrical resistance heating (ERH) under passive venting conditions. Temperature monitoring displayed the existence of a TCE-water co-boiling plateau at 73.4 °C, followed by continued heating to 100 °C. A 5 cm thick gas accumulation formed beneath a fine-grained capillary barrier during and after co-boiling. The capillary barrier did not desaturate during the course of the experiment; the only pathway for gas escape being through perforated wells traversing the barrier. The thickness of the accumulation was dictated by the entry pressure of the perforated well. The theoretical maximum TCE soil concentration within the region of gas accumulation, following gas collapse, was estimated to be 888 mg/kg. Post-heating soil sampling revealed TCE concentrations in this region ranging from 27 mg/kg to 96.7 mg/kg, indicating removal of aqueous and gas phase TCE following co-boiling as a result of subsequent boiling of water. The equilibrium concentrations of TCE in water corresponding to the range of post-treatment concentrations in soil (6.11 mg/kg to 136 mg/kg) are calculated to range from 19.8 mg/l to 440 mg/l. The results of this experiment illustrate the importance of providing gas phase venting during the application of ERH in heterogeneous porous media.

Mitigating methane emissions from passive landfill vents: a viable option for older closed landfills

This study investigated the use of biofilters to reduce methane emissions from landfill passive gas vents. Two biofilter designs were evaluated. The two filter designs achieved similar percent oxidation averages. The radial biofilter design, however, obtained a much higher methane oxidation rate. The higher surface area of flow in the radial biofilters decreased the methane influx leading to greater oxygen penetration into the biofilters. An average percent oxidation of 20% and higher were obtained when air temperature was 20-36°C, indicating the optimal soil temperature for methanotrophs to oxidise methane.

Carbon – Making the right choice for waste management in developing countries

Due to initiatives such as the clean development mechanism (CDM), reducing greenhouse gas emissions for a developing country can offer an important route to attracting investment in a variety of qualifying project areas, including waste management. To date CDM projects have been largely confined to schemes that control emission from landfill, but projects that avoid landfilling are beginning to be submitted. In considering the waste options which might be suitable for developing countries certain ones, such as energy from waste, have been discounted for a range of reasons related primarily to the lack of technical and other support services required for these more sophisticated process trains. The paper focuses on six options: the base case of open dumping; three options for landfill (passive venting, gas capture with flaring, and gas capture with energy production), composting and anaerobic digestion with electricity production and composting of the digestate. A range of assumptions were necessary for making the comparisons based on the effective carbon emissions, and these assumptions will change from project to project. The highest impact in terms of carbon emissions was from using a sanitary landfill without either gas flaring or electricity production; this was worse than the baseline case using open dumpsites. Landfills with either flaring or energy production from the collected gas both produced similar positive carbon emissions, but these were substantially lower than both open dumping and sanitary landfill without flaring or energy production. Composting or anaerobic digestion with energy production and composting of the digestate were the two best options with composting being neutral in terms of carbon emissions and anaerobic digestion being carbon negative. These generic conclusions were tested for sensitivity by modifying the input waste composition and were found to be robust, suggesting that subject to local study to confirm assumptions made, the opportunity for developing CDM projects to attract investment to improved waste management infrastructure is significant. Kyoto credits in excess of 1 tCO2e/t of waste could be realised.

The more closed the bypass system the better: a pilot study on the effects of reduction of cardiotomy suction and passive venting on hemostatic activation during on-pump coronary artery bypass grafting

Cardiac surgery with cardiopulmonary bypass (CPB) leads to a powerful activation of the hemostatic system. We assessed to what extent this activation can be attenuated by comparing three different perfusion regimens for on-pump coronary artery bypass grafting (CABG): 1) use of a closed CPB system with aspiration of blood from the operation field via the cardiotomy suction line and active venting of the heart via a roller pump; 2) use of a closed CPB system avoiding aspiration of blood from the operation field via the cardiotomy suction line, but with active venting of the heart; and 3) use of a closed system, avoidance of aspiration of blood from the operation field via the cardiotomy suction line and with passive venting of the heart into the collapsible venous reservoir. Our data show that avoidance of aspiration of blood via the cardiotomy suction line significantly reduces hemostatic activation during on-pump CABG. However, further attenuation of hemostatic activation can be achieved by further closing the system and minimizing the blood/air interface by passive venting of the heart.

Mould pressure control in rotational moulding

The technology of the rotational moulding of plastics has improved dramatically in recent years due to the recognition that measuring the temperature inside the mould is fundamental to process control. The next major advance will be made when the benefits of controlling the pressure inside the mould are fully utilized. A drawback of the rotational moulding process has always been the surface pinholes and internal bubbles that occur in the moulded part. These occur because, as the powder particles melt and coalesce, they trap pockets of air and it takes a considerable time for these to disappear. It is known that factors such as the viscosity of the polymer melt, the shape of the powder particles, the particle size and size distribution, the mould release agent and the metal used for the mould all affect the pinholing problem. However, the major factor that influences pinhole removal is mould pressure. If pressure is applied at a strategic point in the heating cycle, it can force trapped air out of the melted plastic. Indeed the use of mould pressurization can offer additional benefits such as cycle time reduction and significant improvements in the mechanical properties of the moulded part. However, few moulders make any attempt to monitor or control the mould pressure, mainly because its effect is not understood. This paper explains why mould pressure control throughout the moulding cycle is crucial to the rotational moulding industry in terms of, firstly, achieving good part quality and, secondly, reducing cycle times. It is also suggested that a fundamental change from passive venting to active venting of the mould is an important development that must occur in this industry.

Numerical simulation of gas flow around a passive vent in a sanitary landfill

A numerical model, based on the Darcy law, was used to simulate the two-dimensional gas flow around a passive vent in a sanitary landfill. We follow Findikakis and Leckie [ASCE J. Environ. Eng. 105 (1979) 927] in modeling the biodegradation of the solid waste and assume the first-order biodegradation kinetics. The numerical results from the Fresh Kills landfill, New York, show that the well’s ability in extracting the landfill gas by the passive vent decays quickly with the increase of the radial distance from the well. The influence radius of the well is generally less than 20 m. The effects from the final soil thickness, well depth, and other parameters on the gas flow are also discussed.

Kinetics of microbial landfill methane oxidation in biofilters

A methane oxidizing biofilter system fitted to the passive venting system of a harbor sludge landfill in Germany was characterized with respect to the the methanotrophic population, methane oxidizing capacity, and reaction kinetics. Methanotrophic cell counts stabilized on a high level with 1.3×10 8 to 7.1×10 9 cells g dw −1 about one year after first biofilter operation, and a maximum of 1.2×10 11 cells g dw −1. Potential methane oxidizing activity varied between 5.3 and 10.7 μg h −1 g dw −1. Cell numbers correlated well with methane oxidation activities. Extrapolation of potential activities gave methane removal rates between 35 and 109 g CH 4 h −1 m −3, calculated for 30 °C. Optimum temperature was 38 °C for freshly sampled biofilter material and 22 °C for a methanotrophic enrichment culture grown at 10 °C incubation temperature. Substrate kinetics revealed the presence of a low-affinity methane oxidizing community with a high V max of 1.78 μmol CH 4 h −1 g ww −1 and a high K M of 15.1 μM. K MO2 for methane oxidation was 58 μM. No substantial methane oxidizing activity was detected below 1.7–2.6 vol.-% O 2 in the gaseous phase. Methane deprivation led to a decrease in methane oxidation activity within 5–9 weeks but could still be detected after 25 weeks of substrate deprivation and was fully restored within 3 weeks of continuous methane supply. Very high salt loads are leached from the novel biofilter material, expanded clay, yielding electric conductivity values of up to 15 mS cm −1 in the leachate. Values>6 mS cm −1 were shown to depress methane consumption. Water retention characteristics of the material proved to be favourable for methane oxidizing systems with a gas permeable volume of 78% of bulk volume at field capacity water content. Correspondingly, no influence of water content on methane oxidation activity could be detected at water contents between 2.5 and 20 vol.-%.

Electrolytic oxygen generation for subsurface delivery: effects of precipitation at the cathode and an assessment of side reactions

This research investigated the oxygen-generating characteristics and side reactions of an electrolytic cell assembly that could be used to remediate sites with contaminants that are amenable to aerobic biodegradation. The oxygen-generating capabilities of new electrolytic cells and cells with light and heavy calcium carbonate precipitates on the cathode were evaluated in the laboratory under current densities ranging from 0.5 to 5.0 mA/cm 2. Higher current densities resulted in higher mass transfer coefficients ( K L a) and greater saturation oxygen concentrations ( C sat). As the cathodic deposits increased, the K L a tended to decrease and the C sat tended to increase. The oxygen transfer efficiency (OTE) did not vary as a function of current density or cathode coating, while the average OTE for all the tests was 67%. Laboratory column tests showed that chlorine production increased with current density and depended on chloride levels in the water. Hydrogen peroxide was generated at low concentrations (<1 mg/L) and at higher levels when chloride was absent in the feed solution. Calcium removal from solution increased with current density and resulted in a decrease in solution pH. Tests at a field monitoring well showed average C sat levels of 16.9 mg/L after 14 days of operation, no chlorine production because of low chloride levels in the well, artificially elevated hydrogen peroxide levels because of background interferences, and a pH decrease of 2.4 units. With passive venting, the average hydrogen gas levels at the headspace of the well were less than 1%.

Fate and Transport of Fuel Components Below Slightly Leaking Underground Storage TanksTechnical Note

Fuel leaks from underground storage tanks (USTs) and piping have been a major source of groundwater contamination. In the U.S. and Europe, regulations requiring upgrading of USTs to meet specific standards have significantly reduced instances of fuel contamination. Leak detection is primarily dependent on physical measurement systems that are generally capable of detecting leak rates as small as 0.2 L/h. Fuel leaks that are smaller than this detection threshold may remain undetected for long periods of time, posing a risk of contamination to shallow groundwater resources in sensitive areas. This risk was evaluated by modeling fate and transport of fuel components from small UST leaks under a variety of subsurface conditions and assuming that secondary containment does not exist. It was found that small leaks do have the potential to impact shallow groundwater, particularly if subsurface conditions are not conducive to natural attenuation processes. This may explain situations where groundwater contamination has been found below service stations in virgin areas that have upgraded leak detection systems. Modeling indicates that passive venting of tank and piping backfill could virtually eliminate the volatile components of fuel resulting from small leaks. Monitoring the tank and piping backfill for persistent gasoline vapor under very low vapor extraction conditions may be the best way to detect small chronic fuel leaks. Routine monitoring of shallow groundwater should be a component of a leak detection program, particularly in high-risk areas.

Numerical simulation of gas emission in a sanitary landfill equipped with a passive venting system

The gas emission in a sanitary landfill equipped with a passive venting system was investigated numerically. The Darcy's law was employed to simulate the gas flow in the landfill. We used the first order biodegradation rate in modeling the waste biodegradability. The results show that more than 43.3% gases produced from the waste will emit from the landfill surface for R = 30 m, where R is the half of the well spacing. These indicate that the influence radii (between 30 to 50 m) were overestimated in the designing of the gas collection systems in the Sanjuku and the Taichung City sanitary landfills in Taiwan. This might pose a potential of air pollution and raise chances of fires under such designing.

Handling of Small Volume Droplets using Hydrophobic Microcapillary Vent

To realize integrated multi-step Micro Total Analysis Systems (μTAS), pneumatic manipulation of reagent droplets is a promising scheme. In this scheme, passive vent valves are required for positioning and mixing of the droplets. This paper presents a novel microstructure named Hydrophobic Microcapillary Vent (HMCV). An array of hydrophobic microcapillaries allows air passing through it, whereas it stops liquid by negative capillary action. Test structures have been fabricated by molding technique of silicone elastomer. Positioning and mixing functions of droplets with pL-nL volume are demonstrated.