Experimental and numerical study of an air lock purging system

High concentrations of H2S in offshore wells represent a major concern for personnel safety: if a significant external H2S contamination occurs, depending on wells and process conditions, it may prove impossible an effective evacuation, and thus Temporary Refuge (TR) provisions must be set-up to provide prompt availability of safe and reliable protection to personnel. Air Locks (ALs) to enter the TR may be necessary to ensure isolation of the safe internal environment when entering into the TR. ALs modelling is essential to verify that sufficient time for entering the TR is available to all personnel in case of accident. Nevertheless, due to the extreme conditions (high toxicity, short characteristic times, high purging air velocity etc.), experimental modeling of the AL can prove difficult and very expensive. Given the importance of ALs efficiency in a real emergency situation, simulation of its performances in realistic condition and optimization of the design of air purges to ensure the required efficiency is a factor of extreme importance for the overall safety of the installation. In this work, the purging efficiency of a typical AL has been analyzed through a combined approach of experimental tests and CFD simulations, to prove the capability of CFD modeling to analyse real AL conditions. A scaled model have been realized and analyzed using CO2 as tracing gas to determine the concentration field; even if the realization constraints above mentioned do not allow for a full scaling of all the involved variables, fluid-dynamic conditions have been set to reproduce real AL purging capabilities as close as possible. Experimental results have been used for a fine tuning and validation of the CFD tool in an operative range close to a real configuration, through the comparison of the obtained flow and concentration fields with those predicted by the CFD simulations of the experimental set-up. Subsequently, the tuned CFD approach has been used to simulate a typical AL and to check its performances

Influence of Ground on Jet Fire Extension

A common accident in the industrial process industry is the puncturing of storage tanks or rupture of process pipelines containing gases. In these scenarios, the gas will escape the piece of equipment producing a single-phase gas jet. If the fluid is flammable, an ignition source is most probably encountered during the accidental scenario and a jet-fire can follow the leak. Free jets of hazardous gases and free jet-fires have been extensively analyzed in the past literature to assess their shape and extension for safety purposes. Similar analyses have been conducted to observe the effect on shape/extension of neutral jets if obstacles were present. Also, the effect of the ground proximity to the jet source has been studied. In general, the presence of obstacles and the proximity to the ground lead to enlarged hazardous areas, mainly because of the Coanda effect. In this work, flammable jets igniting and forming a jet-fire were considered. The effect of the ground proximity was analyzed, to observe the extension of the flame. Two opposed phenomena were supposed to act on the fire, differently from non-ignited jets: the Coanda effect having an attractive nature towards the ground and the buoyancy effect on the opposite direction. The relevant methane jet-fires case study was considered carrying out computational fluid dynamics (CFD) simulations using the Fire Dynamics Simulator software. The study considered both the jet source height from the ground and the gas relief flowrate effects. CFD results were summarized basing on simple dimensionless parameters to determine the eventual variation of jet-fire extension for preliminary safety analyses

THERMOCOUPLES POSITIONING FOR EARLY-WARNING DETECTION OF THERMAL RUNAWAY

Runaway reactions have always been a serious issue for the chemical industry. Failures that may lead to this type of accident are different: block of the impeller, loss of the reactor temperature control, error in the loading of reagents, just to name a few. The rapid detection of this phenomena is crucial. One of the most widely used preventive systems is the so-called early warning system, which allows to give an early warning at the beginning of the fugitive reaction. Due to non-homogeneity of the temperature inside the reactor, the positioning of the sensors is of crucial importance. In fact, an incorrect localization of the temperature probe could lead to a false alarm, which would undermine the early warning system. The objective of this work is the computational fluid dynamic (CFD) simulation of different failure scenarios, in order to determine the best location of the temperature sensors

Predictive Models for the Estimation of the Minimum Ignition Energy of Polydisperse Organic Dusts

The process industry is a sector characterized by the sale of 50 % of its products in the form of powder and in which 80 % of the goods generated are made through a production system that involves the use of a powder. This sector massively employs solid materials and, using operations such as material transport, crushing, screening, sanding, trimming, feeding tanks and bins, storage of granular materials and many other activities, is very often characterized by the collateral emission of dusts. A similar scenario makes the risk of a dust explosion one of the major concerns of the process industry. In this context, to ensure the safety of people and infrastructures, it is crucial to obtain the parameters that characterize the explosiveness of the dust. Actually, these parameters are all determined experimentally, involving large economic costs, technical difficulties, and long dead times. This work focused on the estimation of one of these parameters, the Minimum Ignition Energy (MIE), which is considered to be one of the most important to assess the probability of having a dust explosion. Therefore, starting from the experimental test within a 1.2 L Hartmann tube, two new versions of a mathematical model capable of predicting the MIE for an organic powder were proposed. The models characterize the powder analysed through its particle size distribution and a few chemical-physical characteristics obtained from literature. Six organic powders were selected to validate the model (aspirin, cork, corn starch, sugar d50=135 µm, sugar d50=34 µm and wheat flour), with the intention of comparing the theoretical data obtained with literature experimental ones

Study of the Interaction Between a High-pressure Jet and Horizontal Tanks Using CFD

Accidental high-pressure flammable gas releases are among the most relevant hazards in the process safety, and consequences could be severe. In the recent decades, there have been numerous efforts to study high-pressure jets in open field (i.e., free jets). Easy-to-use mathematical models have been developed, to rapidly assess the main physical variables involved in safety evaluations. However, in a realistic scenario, the accidental leak may involve either the ground or a piece of equipment. As demonstrated by recent works, when a jet interacts with an obstacle, its behavior can significantly change. Therefore, the mathematical models extrapolated for the free jet scenario could be a source of incorrect predictions. Focusing on the scenario of an accidental high-pressure unignited flammable jet, this work shows how the presence of one or two obstacles, placed at a different distance from the source of the leak, can influence the lower flammability limit cloud extent of methane. Varying the height of the source term, the effect of the interaction among the jet, both the obstacles, and the ground was systematically studied through a Computational Fluid Dynamics analysis

From semi-batch to continuous tubular reactors: A kinetics-free approach

A methodology, which does not require any kinetic information, for the rigorous transformation of an isothermal, homogeneous semi-batch process into an equivalent continuous side-fed tubular reactor was developed. Once the semi-batch process parameters are known, the proposed methodology allows for easily defining all the process parameters of a side-fed tubular reactor that guarantees the same performances as the original semi-batch process, in terms of conversion and product characteristics. Two different case studies were selected to investigate the potential of the proposed approach: a copolymer synthesis and the production of a fine chemical, clearly showing the need of a rigorous transformation of the semi-batch process into the continuous one since productivity and product quality are strongly affected by the feeding policy

Statistical analysis of modelling approaches for CFD simulations of high-pressure natural gas releases

The use of computational fluid dynamics (CFD) in process safety to estimate the risk of a given incidental scenario has become ever more present in common industry practice. The simulation of high-pressure, compressible natural gas jets is often performed by modelling its source with a simpler notional diameter approach, such that the highly computationally expensive nearfield zone need not to be simulated; this is particularly determining when simulating a gas release in complex scenario like liquid natural gas (LNG) regasification plants. In this study, we analysed the structure of compressible and incompressible jets, using Birch 1984 (B84) and Birch 1987 (B87) models. In this work, a study on the positioning of the notional diameter with respect to the real orifice of the released gas is performed, along with a statistical analysis to assess the limits of the simpler model approaches.It was found that no spacing is needed between the virtual and real sources, as the potential core generated by the simpler model is as large as the fully simulated nearfield zone by the compressible model. Additionally, an end-of-transition zone position correlation is reported. The incompressible models can be used instead of the fully compressible model for a wide range of release conditions, with both models providing accurate predictions of axisymmetrical mole fraction, temperature, and velocity profiles between 2.5 and 130 bar of storage pressure at a 1-inch orifice diameter. However, as the diameter increases, B84 is not a viable model for a “full bore” (10-inch diameter size) release at 65 bar. While B84 is reliable, B87 is the superior model for its ability to account for the compressible effects of the expansion. Therefore, B87 should be used when simulating cases where temperature is of particular interest to the user

Influence of the shape of mitigation barriers on heavy gas dispersion

Regasification plants have become an emerging risk because their numbers are increasing and concern from the general population towards these systems has grown. Consequently, there is increased interest in investigating the effect of mitigation measures to limit the impact of large accidents on the population living close to the plant. Among the various possible mitigation measures, physical barriers present several advantages; however, it is known that the necessary barrier height can became impracticably large to be effective in mitigating the consequences of a large LNG release. Therefore, computational fluid dynamics models were used in this work to analyze the performance of mitigation barriers with different shapes to investigate the possibility of increasing mitigation barrier efficiency by simply changing the main geometrical characteristics of the barrier such as roughness, battlements, or even holes

Mixing Efficiency and Residence Time Distributions of a Side-Injection Tubular Reactor Equipped with Static Mixers

Plug flow behavior in tubular reactors is often highly desirable in industry, since it can ensure high productivity, good selectivity, and enhanced heat transfer. To achieve this, good radial mixing combined with poor axial mixing is required: these conditions are quite easy to obtain if the flow regime is turbulent, but they are much more challenging to achieve if the flow is laminar. In this work radial mixing and residence time distributions in a side-injected tubular reactor equipped with a series of Sulzer SMX static mixers were investigated using Computational Fluid Dynamics. It was found that even at low values of Reynolds number the reactor can efficiently satisfy the plug flow conditions, and operative diagrams were determined to foresee the reactor behavior