The effect of turbulence on the rate of evaporation of LNG on water

The present study provides new measurements of the rate of evaporation of cryogenic liquids, liquefied natural gas (LNG) and liquid nitrogen (LN₂), floating on a water surface with different levels of turbulence intensity. The turbulent water surface is generated with an upward-pointing submerged jet with controlled jet velocity, an approach which has often been used in studies of free-surface turbulence. Direct measurements of the rate of evaporation were carried out for different pool thicknesses and turbulence intensities of the water surface. These tests reveal a strong dependence of the evaporation rate on the turbulence intensity, as well as a dependence on the thickness of the cryogenic liquid layer above the water surface. Models of LNG spills on water currently use a single rate of evaporation; these findings show that this approach is inadequate. Future models should incorporate the water turbulence intensity, and possibly the LNG spill thickness for improved accuracy.     

A new categorization of release models using a semi-analytic solution

The spread of cryogenic liquid due to a limited period of release is investigated for the first time to clarify the unclear conventional concept regarding two release types: continuous and instantaneous release. The physical phenomenon is described by equations involving the volume, radius and height of the liquid pool, and there are three governing parameters: the evaporation rate per unit area, a release time, and a spill volume. As a result of the perturbation solutions, the combined model, which consists of the continuous model and the subsequent instantaneous model, is necessary for a large spill source rate, whereas the continuous model is only required for a small spill source rate. This combined release model is more realistic than the instantaneous release model, and it is shown that the combined model and the continuous model are clearly distinguished in the coordinate system of the release time and the spill volume using the analytical feature of the perturbation solution.     

Development of heat transfer and evaporation model of LNG covered by Hi-Ex foam

Natural gas is a kind of clean, efficient green energy source, which is used widely. Liquefied natural gas (LNG) is produced by cooling natural gas to −161 °C, at which it becomes the liquid. Once LNG was released, fire or explosion would happen when ignition source existed nearby. The high expansion foam (Hi-Ex foam) is believed to quickly blanket on the top of LNG spillage pool and warm the LNG vapor to lower the vapor cloud density at the ground level and raising vapor buoyancy. To identify the physical structure after it contacted with LN₂ and to develop heat transfer model, the small-scale field test with liquid nitrogen (LN₂) was designed. In experiment, three layers including frozen ice layer, frozen Hi-Ex layer and soft layer of Hi-Ex foam were observed at the steady state. By characterizing physical structure of the foam, formulas for calculating the surface of single foam bubble and counting foam film thickness were deduced. The micro heat transfer and evaporation model between cryogenic liquid and Hi-Ex foam was established. Indicating the physical structure of the frozen ice layer, there were a certain number of icicles below it. The heat transfer and evaporation mathematical model between the frozen ice layer and LNG was derived. Combining models above with the heat transfer between LNG, ground and cofferdam, the heat transfer and evaporation mathematical model of LNG covered by Hi-Ex foam was developed eventually. Finally, LN₂ evaporation rate calculated by this model was compared with the measured evaporation rate. The calculated results are 1.2–2.1 times of experimental results, which were acceptable in engineering and proved the model was reliable.     

Configuration predictions of large liquefied petroleum gas (LPG) pool fires using CFD method

Liquefied petroleum gas (LPG) has potential pool fire risks due to its flammability. The configuration of pool fires plays a significant role when applying the solid flame model or point source model to assess the risks from heat radiation. However, no existing correlations can precisely predict the configuration of large LPG (100% propane) pool fires. To enhance the fundamental understanding on how pool diameter and wind velocity can influence the configuration of large LPG pool fires, an experimentally validated Computational Fluid Dynamics (CFD) model is employed to simulate fires using different burning rate models. Fire temperature profiles, flame heights, and flame tilts predicted by the CFD model were compared with empirical models and experimental data. Accordingly, new correlations for flame height and flame tilt as functions of pool diameter D and wind velocity u_w have been developed. The comparisons demonstrate that the new correlations have the best overall accuracy in the prediction of flame height and tilt for large LPG pool fires under different conditions (10 m ≤ D ≤ 20 m, 0 ≤ u_w ≤ 3 m·s⁻¹).     

Decolorization of synthetic Methyl Orange wastewater by electrocoagulation with periodic reversal of electrodes and optimization by RSM

Treatment of Methyl Orange (MO), an azo dye, synthetic wastewater by electrocoagulation with periodic reversal of the electrodes (PREC) was examined. Response Surface Methodology (RSM) was used to optimize the influence of experimental conditions for color removal (CR), energy consumption (ENC), electrode consumption (ELC) and sludge production (SP) per kg MO removed (kg(MO_r)) with optimal conditions being found to be pH 7.4, solution conductivity (к) 9.4 mS cm⁻¹, cell voltage (U) 4.4 V, current density (j) 185 mA cm⁻², electrocoagulation time (T) 14 min, cycle of periodic reversal of electrodes (t) 15 s, inter-electrode distance (d) 3.5 cm and initial MO concentration of 125 mg L⁻¹. Under these conditions, 97 ± 2% color was removed and ENC, ELC and SP were 44 ± 3 kWh kg(MO_r)⁻¹, 4.1 ± 0.2 kg(Al) kg(MO_r)⁻¹ and 17.2 ± 0.9 kg(sludge) kg(MO_r)⁻¹, respectively. With the enhanced electrochemical efficiency resulting from the periodic electrode reversal, the coefficients of increased resistance and decreased current density between the two electrodes in the PREC setup were 2.48 × 10⁻⁴ Ω cm⁻² min⁻¹ and 0.29 mA cm⁻² min⁻¹, respectively, as compared to 7.72 × 10⁻⁴ Ω cm⁻² min⁻¹ and 0.79 mA cm⁻² min⁻¹ as measured for the traditional electrocoagulation process. The rate constant of decolorization was also enhanced by 20.4% from 0.152 min⁻¹ in the traditional electrocoagulation process to 0.183 min⁻¹ in the PREC process. These performance characteristics indicate that the PREC approach may be more promising in terms of practical application, as a cost-effective treatment, than conventional electrocoagulation for textile dye removals.     

Improved research-scale foam generator design and performance characterization

The release of a cryogenic, flammable liquid, such as LNG, poses a threat to individuals in the area of the release as well as responders who attempt to limit the damage of the release. The most common mitigation technique is high-expansion foam which can be used to blanket the liquid, reducing the accumulation of flammable vapor above the pool through a number of different mechanisms. Despite the effectiveness of high-expansion foam blanketing, there are many aspects of the interaction between foam and LNG that are unknown. A lab-scale high-expansion foam generator has been developed to allow the study of those interactions. Additionally, the novel foam generator design addresses many of the drawbacks of industrial-scale foam generators and allows researchers better control of the foam, while producing foam at rates that are conducive to lab applications. Foam was produced using the generator and expansion ratio and foam stability were measured to determine the quality. The generator was able to produce foam with expansion ratio between 298 and 892 that collapsed at an average rate of 0.4 cm per minute. This quality of the foam is comparable to industrial-scale foam generators and the foam production rate is between 1.2 and 2.2 m³/min, which fits lab-scale needs. The foam generator can also be used with other types of non-firefighting foam, such as decontamination foam for chemical, biological, or nuclear decontamination.     

Dynamic safety analysis of process systems with an application to a cryogenic ammonia storage tank

A method for calculating the dynamic reliability of safety systems and its application to a refrigerated liquid cryogenic ammonia storage tank is presented. The method is based on the theory of Markov chains and can model dynamic phenomena of the process and its safety systems. It offers the capability of modelling realistically the competing process of repairing failed safety systems and the exceeding of safe limits by some critical physical parameters of the process. The results of the Markovian analysis are compared to those of the classical Fault Tree/Event Tree methods and it is shown that the proposed method offers a substantial improvement over the classical approach. The probability of failure from overpressure of a cryogenic ammonia storage tank depends in general on the level of the ammonia in the tank at the time of accident initiation. Assuming a uniform distribution for the ammonia level in the tank, the average upper and lower limits for the failure probability over a year provided by the FT/ET methods span three orders of magnitude [1.4×10⁻¹–1.0×10⁻⁴] depending on whether repair is considered or not. The proposed approach realistically determines this failure probability at 3.3×10⁻³. Additional results from specific levels of ammonia are also provided.     

Quantification of source-level turbulence during LNG spills onto a water pond

The use of computational fluid dynamics (CFD) models to simulate LNG vapor dispersion scenarios has been growing steadily over the last few years, with applications to LNG spills on land as well as on water. Before a CFD model may be used to predict the vapor dispersion hazard distances for a hypothetical LNG spill scenario, it is necessary for the model to be validated with respect to relevant experimental data. As part of a joint-industry project aimed at validating the CFD methodology, the LNG vapor source term, including the turbulence level associated with the evaporation process vapors was quantified for one of the Falcon tests.This paper presents the method that was used to quantify the turbulent intensity of evaporating LNG, by analyzing the video images of one of the Falcon tests, which involved LNG spills onto a water pond. The measured rate of LNG pool growth and spreading and the quantified turbulence intensity that were obtained from the image analysis were used as the LNG vapor source term in the CFD model to simulate the Falcon-1 LNG spill test. Several CFD simulations were performed, using a vaporization flux of 0.127 kg/m² s, radial and outward spreading velocities of 1.53 and 0.55 m/s respectively, and a range of turbulence kinetic energy values between 2.9 and 28.8 m²/s². The resulting growth and spread of the vapor cloud within the impounded area and outside of it were found to match the observed behavior and the experimental measured data.The results of the analysis presented in this paper demonstrate that a detailed and accurate definition of the LNG vapor source term is critical in order for any vapor cloud dispersion simulation to provide useful and reliable results.     

过程液氯贮槽装置封闭厂房及事故氯吸收塔联用安全效用分析

分析、设立液氯贮槽液相泄漏的几种典型事故情景,并利用液相泄漏、液池蒸发、重气扩散和人员中毒死亡概率等模型对比研究封闭厂房及事故氯吸收塔等安全措施对液氯贮槽液相泄漏扩散中毒后果的影响,给出不同事故情景下液氯泄漏速率、液池半径、液池蒸发速率、室外氯气中毒死亡概率等事故后果特征值。对封闭厂房及事故氯吸收塔安全效用进行定量分析和比较研究。结果表明,液氯贮槽的封闭厂房对抑制液氯泄漏扩散中毒事故后果效用明显;事故氯吸收塔能消除液氯贮槽微小孔泄漏所对应的小事故情景,还能对封闭厂房最严重泄漏事故后果起到初期削峰作用。显然,封闭厂房及事故氯吸收塔联用可以降低液氯贮槽事故影响后果,具有良好安全效用。     

Numerical simulation of evaporation of volatile liquids

The paper presents the results of the validation of the developed pool evaporation model using literature and our own experimental data. The proposed model was used to examine the effect of wind velocity and pool sizes on the evaporation rate of volatile liquid (hexane). Contrary to the semi-empirical evaporation model widely used in hazard assessment, stronger dependence of evaporation rate on pool size at low wind speeds is obtained.     

Experimental analysis of the evaporation process for gasoline

This paper presents the findings from a study on the evaporation process of 93 RON (research octane number) unleaded gasoline. The parameters measured in the experiment included the weight, the RVP (Reid vapor pressure) and the viscosity of gasoline, the concentration of NMHC (non-methane total hydrocarbon) in the oil vapor and the concentration of the main vapor constituent. Results showed that the parameters changed significantly as evaporation processed. The weight loss reached 86.36% after 300 days and presented a logarithmic curve with time. The RVP decreased from 38 kPa to 9.6 kPa. The viscosity of gasoline increased from 8.6 × 10^?4 Pa s to 1.51 × 10^?3 Pa s. All the concentrations of NMHC and the main constituent of vapor decreased in varying amounts. Most of the changes might be attributed to the evaporation of volatile hydrocarbons.     

高倍泡沫加速泄漏LNG扩散的有效性模拟试验

泡沫灭火剂在扑灭液体火灾中起到重要作用,关于低温液体蒸气云扩散控制的研究也逐渐得到应用。通过小尺寸模拟试验验证高倍泡沫加速泄漏LNG扩散的有效性,设计并进行了低温液体自然蒸发和高倍泡沫覆盖低温液体两个对照试验,测量了竖直方向上10个高度处的温度及装置整体质量,从而获取了低温液体蒸气到达泡沫层顶端时温度及蒸发速率的变化情况。结果表明,与未添加泡沫的情况对比,高倍泡沫的覆盖使泄漏低温液体在1 800 s内的蒸发量减少了6.4%,如果时间更长则减少的比例更多,且蒸发出的低温液体穿过泡沫层后蒸气温度可达0℃左右,而未添加泡沫时同等高度处蒸气温度为-75℃左右。0℃时,LNG蒸气密度已明显小于空气密度,此温度下LNG蒸气会迅速向上扩散,而不至在地表积聚,由此证明高倍泡沫能够加速泄漏低温液体蒸气向上扩散,减小了低温液体蒸气在地面积聚并引发火灾爆炸事故的可能性,从而证实了高倍泡沫加速泄漏LNG扩散的有效性。     

Comparing tools of varying complexity for calculating the gas dispersion

When handling flammable and/or toxic liquids or gases, the gas dispersion following a release of substance is a scenario to be considered in the risk assessment to determine the lower flammability distance (LFD) and toxicity thresholds. In this work a comparison of different gas dispersion tools of varying complexity ranging from a simple Gaussian model over a boundary layer model (BLM) and a Lagrangian model to CFD (in this case ANSYS CFX v14) is presented. The BLM covers the special case of liquid releases with formation of a pool. It does not only solve the gas dispersion but also calculates the evaporating mass flow out of the pool. The simulation values are compared to each other and to experimental data resulting mainly from our own open air experiments covering the near field and carried out on the Test Site Technical Safety of BAM (BAM-TTS) for different release types (pool evaporation, gas release) and topologies. Other validation data were taken from literature and cover large scale experiments in the range of several 100 m.     

An Experimental Study on Pyrolysis of Biomass

In this study, pyrolysis of sugarcane bagasse was performed in fixed bed tubular reactor under the conditions of nitrogen atmosphere, by varying temperature and different particle sizes. The effect of final pyrolysis temperature from 400 to 500°C and the nitrogen flow rate from 50 to 200 cc min⁻¹ on the pyrolysis product yields from sugarcane bagasse have been investigated. The Maximum bio-oil yield obtained is 24.12 wt% at the final pyrolysis temperature of 450°C, N₂ flow rate of 50 cc min⁻¹ and particle size of mesh number −8 + 12. The yield of bio-oil decreases with increase in temperature from 450 to 550°C and N₂ flow rate from 50 to 200 cc min⁻¹. The various characteristics of pyrolysis oil obtained under these conditions were identified on the basis of standard test methods. The empirical formula of pyrolysis oil with a heating value of 37.01 MJ Kg⁻¹ was established as CH_1.434 O_0.555 N_0.004. The results from the pyrolysis show the potential of sugarcane bagasse as an important source of liquid hydrocarbon fuel.     

Quantification of turbulence in cryogenic liquid using high speed flow visualization

A high speed flow visualization experiment was conducted to characterize the boiling induced turbulence when a cryogenic liquid is released on water. The advective transport of turbulent structures traversing through the liquid was captured and reconstructed using image processing software to obtain information on velocity components. The numerical results obtained from image processing were used to determine turbulence parameters like turbulent intensity, turbulent kinetic energy and eddy dissipation rate. An interesting aspect of the study was the formation of wavy structures called ‘thermals’ which were characteristic of turbulent convection. The thermals were found to act as a catalyst in increasing heat transfer and turbulence between water and cryogenic pool. The turbulent intensity was influenced by the turbulent velocity and had direct effects on the vaporization flux. Among the turbulence parameters, increase in turbulent kinetic energy resulted in faster vaporization of cryogenic liquid through enhanced mixing, whereas variations in the eddy dissipation rate had weak dependence on vaporization. Additionally, the initial height of cryogenic liquid was also found to strongly affect the vaporization mass flux.     

Modeling of pool spreading of LNG on land

This paper presents a source term model for estimating the rate of spreading of LNG and other cryogenic mixtures on unconfined land. The model takes into account the composition changes of a boiling mixture, the varying thermodynamic properties due to preferential boiling within the mixture and the effect of the various boiling regimes on conductive heat transfer. A sensitivity analysis is conducted to determine the relative effect of each of these phenomena on pool spread. The model is applied to continuous and instantaneous spills. The model is compared to literature experimental data on cryogenic pool spreading.     

Small-scale experimental study of vaporization flux of liquid nitrogen released on ice

One of the LNG accident scenarios is the collision of an LNG carrier on an iceberg during marine transportation. A collision can result in damages to the vessel and lead to the leakage of the contents on ice or an ice-water mixture. When cryogenic liquid comes in contact with ice, it undergoes rapid vaporization due to the difference in temperature between the ice and cryogenic liquid. This process is different from the heat transfer between water and cryogenic liquid as ice is a solid and thus heat transfer to the pool occurs primarily through conduction. In this paper, the heat transfer phenomenon between ice and cryogenic liquid was studied through a small-scale experiment and the resulting vaporization mass fluxes were reported. The experiment involved six spills with varying amount of liquid nitrogen on different ice temperature to determine its effect on vaporization mass flux. The vaporization mass fluxes were determined by direct measurement of the mass loss during the experiment. The results indicated that the vaporization mass flux was a function of release rate and ice temperature. When the release rate and ice temperature was high, the vaporization mass flux follows a decreasing trend. With further reduction in release rate and ice temperature, the vaporization mass flux was found to be independent with time. The one dimensional conduction model was validated against experimental results. The predicted temperatures and heat flux were found to be in good agreement with the experimental data.     

On the effectiveness of mitigation strategies for cryogenic applications

The need for sustainable energy sources, as well as the current energetic crisis involving the majority of markets, has promoted the use of cryogenic liquefaction for the transportation and storage of natural gas (i.e., LNG). To guarantee the development of a robust and safe infrastructure, a complete understanding of the main phenomena occurring at low temperatures is paramount. In this sense, the largest grey areas are the characterization of the combustion at low-initial temperature and the interactions between water and cryogenic liquid. For these reasons, this work presents an experimental campaign on the possible mitigation strategies for the mitigation of consequences related to the accidental release of LNG. Particular emphasis was posed on the direct and indirect effects of water on cryogenic pool fire. The former resulted in a significant increase in the dimensions of fire (∼+50%) and burning rate (∼300%) with respect to the case with no direct contact between water and LNG, whereas the latter generated an abrupt decrease in the measured temperatures (<100 °C). The use of an emergency flare to empty an LNG tank was tested, as well. The spatial distribution of temperature was monitored along with the time to guarantee the safe operability of this equipment in the case of LNG combustion. The explanations for the observed phenomena and trends were provided, allowing for the development of safe procedures for the emergency response related to cryogenic fuels.     

Achieving stable partial nitritation using endpoint pH control in an SBR treating landfill leachate

The feasibility of using endpoint pH control to achieve stable partial nitritation (PN) in an SBR for landfill leachate treatment was investigated. By imposing a fixed-time anoxia followed by variable-time aeration in an SBR cycle, successful partial nitritation was maintained for 182 days at a nitrogen loading rate of 0.30–0.89 kg/m³/day. The effluent NO₂⁻-N/NH₄⁺-N ratio and the effluent NO₃⁻-N concentration were 1.30 ± 0.22 and 16 ± 9 mg/L, respectively. High free ammonia (FA) and low dissolved oxygen (DO) concentrations were inhibition factors of nitrate formation. The termination of aeration at a suitable endpoint pH was the key to achieve an effluent NO₂⁻-N/NH₄⁺-N ratio close to the stoichiometric value. This endpoint pH control strategy represents practical potentials in the engineered application of combined PN–ANAMMOX processes.