Purge Operation Research Articles

Design Change and Operational Consideration of the HVAC System during Nuclear Power Plant Decommissioning

The heating, ventilation, and air conditioning (HVAC) system plays a crucial role in ensuring the safety of workers and preventing the release of gaseous radioactive materials into the environment during the decommissioning of a nuclear power plant (NPP). To establish an HVAC operation strategy, decommissioning phases were divided into four stages, and the HVAC systems were reclassified. In addition, assumptions have been made regarding design modifications and maintenance for the reactor containment building (RCB) HVAC, fuel handling building (FHB) HVAC, and main control room (MCR) HVAC. Based on these, for RCB HVAC operation, natural ventilation and RCB purge operation during the transition period are proposed. In the decommissioning stage, recirculation operation, entire ventilation operation consisting of continuous operation and purge operation, and finally partial ventilation operation to purify local space were proposed. Moreover, during the transition period, the FHB HVAC was proposed to operate as normal NPP, and the MCR HVAC was suggested to operate with safety-related equipment removed.

Analysis and control strategy design for PEMFC purging process

In this study, a lumped-parameter model for shutdown purging has been developed using MATLAB/Simulink®. Data gleaned from actual operational scenarios on a fuel cell experimental platform serve as the basis for parameter identification within the model. The study encompasses a comparative analysis of purging processes under various operational parameters, including stack temperatures, purging currents, and purging flow rates. Moreover, an innovative purging strategy is proposed, employing an open-loop control mechanism to modulate the purging flow rate. The analysis reveals that an elevated stack temperature and a reduced purging current markedly enhance purging efficiency and diminish the water content at equilibrium. A negative correlation is observed between purging time and purging flow rate, with energy consumption demonstrating a U-shaped pattern across different flow rates. According to the proposed purging strategy, the optimal purging flow rate is ascertained through a Multi-dimensional Array Programming (MAP) lookup. This lookup is contingent upon stack temperature, purging current, and initial membrane water content. Validation confirms that this strategy not only truncates the purging duration but also curtails the system energy expenditure. These analytical insights and the newly-formulated purging strategy hold both theoretical and pragmatic implications for optimizing stack shutdown purging operations and advancing future purging methodologies.

Construction of Nitrogen Content Observer for Fuel Cell Hydrogen Circuit Based on Anode Recirculation Mode

The anode recirculation mode is increasingly being adopted in today’s fuel cell systems. The recycling of hydrogen gas can effectively improve fuel utilization and the wider economy. However, using the purge strategy for the recirculation exhaust has a significant impact on the operational performance and economic efficiency of fuel cell systems.Experiments have shown that, when the purge interval increases from 6 s to 10 s, the recirculation pump power increases by about 20%, the nitrogen content in the exhaust gas increases, and the stack voltage shows a 10 V attenuation. The accumulation of nitrogen permeation in the anode circuit leads to the degradation of the fuel cell performance. Therefore, it is necessary to discharge the accumulated nitrogen through the purge valve in a timely manner. However, opening the exhaust valve with excessively high frequency can result in the unreacted hydrogen being discharged, which reduces the economic efficiency of the fuel cell. This paper is based on the principle of mass conservation and models each subsystem of the anode circuit in the recirculation pump mode of the fuel cell separately, including the proportional valve model, the hydrogen consumption model of the fuel cell, the nitrogen permeation model of the fuel cell, the neural network model of the circulating pump, and the purge valve model. These submodels are integrated to construct a nitrogen content observer for the hydrogen circuit, which can estimate the nitrogen content. The accuracy of the model is validated through experimental data. The estimation error is less than 5.5%. The nitrogen content in the anode circuit can be effectively estimated, providing a model reference for purge operations and improving hydrogen utilization.

Operatable and Efficient Mitigation Strategies for H2S Poisoning in Proton Exchange Membrane Fuel Cells: Releasing Pt Reactive Sites for Hydrogen Oxidation

The anode feed for hydrogen fuel cell electric vehicles (FCEVs) currently still relies on the reformed fuel that inevitably contains contaminants (e.g., CO, H2S, and NH3). As one of the most sensitive impurities, trace H2S in hydrogen significantly deteriorates the durability of proton exchange membrane fuel cells (PEMFCs), while there is still a lack of effective strategies to mitigate H2S poisoning in PEMFCs. Herein, we present two kinds of facile, operatable, and efficient strategies to mitigate a H2S-poisoned anode, with the aim of improving the durability of PEMFCs by releasing more Pt reactive sites for hydrogen oxidation reaction (HOR) catalysis. The first mitigation strategy is conducted by increasing the cell temperature temporarily when purging with pure hydrogen after H2S poisoning by means of accelerating H2S desorption on Pt sites under high temperatures and thus releasing more available Pt reactive sites for HOR catalysis. A higher temporary temperature leads to a larger recovery percentage of performance, outperforming the effect of the generally adopted pure hydrogen purging operation. Another mitigation strategy is called “internal oxygen permeation”, based on the oxidation of sulfur-adsorbed anode by compelling air diffusion through thin PEM via constructing a pressure differential between the cathode and the anode. The cell was found to be recovered more at a high pressure differential due to the fact that more sulfur species are oxidized by a larger content of permeated oxygen, showing ∼88.7% performance recovery for 0.40 bar pressure differential. The proposed strategies with simplicity, operability, and resilience show promising potential in the application of long-term PEMFCs, which is critical for the durability improvement of FCEVs and cost reduction of hydrogen purification.

Scheduling of time-constrained single-arm cluster tools with purge operations in wafer fabrications

Purge operations are widely used to remove residual chemicals and impurities in chambers of wafer fabrication systems to ensure wafer quality. However, such operations lead to more start-up transient processes and wafer delays. In addition, wafer residency time constraints must be met to ensure the wafer quality. Thus, the system scheduling problem with such considerations will become more complicated. This work aims to optimize the start-up transient process of a single-arm cluster tool with purge operations and wafer residency time constraints by using resource-oriented Petri net (ROPN) models. Based on the models, we dynamically depict the behavior of the steady state and start-up transient processes of a single-arm cluster tool with purge operations. Then, a scheduling method that dynamically adjusts the robot waiting time is proposed to find an optimal schedule for such systems with purge operations and wafer residency time constraints. Finally, some examples are tested to illustrate the effectiveness of our method.

Nonlinear model predictive control of PEMFC anode hydrogen circulation system based on dynamic coupling analysis

Anode hydrogen circulation system with purge valve has become one of the most effective ways for proton exchange membrane fuel cell (PEMFC) to alleviate performance degradation caused by nitrogen permeation and improve the hydrogen utilization rate. However, nonlinearity and multivariable coupling of the system make it difficult for PEMFC to achieve pressure and hydrogen supply synchronous tracking under complex operating conditions. To solve the problem, a novel nonlinear model predictive control scheme based on coupling analysis is proposed in this paper. Firstly, coupling characteristics of the hydrogen circulation system and the control pairing of the multivariable system are analyzed based on the relative gain array method. Then a novel control scheme based on an adaptive model predictive controller and a nonlinear model predictive controller is designed to keep the anode pressure stable and sufficient hydrogen supply, which use nonlinear observers to estimate the internal states online. Finally, the proposed controllers are implemented in control experiments of a 50 kW PEMFC anode hydrogen circulation system. The results demonstrate that the proposed control approach has great dynamic performance, anti-disturbance ability and can maintain a high hydrogen utilization rate under purge operation, current disturbance and uncertain intake pressure.

Model-Based Analysis of a Highly Efficient CO2 Separation Process Using Flexible Metal–Organic Frameworks with Isotherm Hysteresis

To realize large-scale CO2 separation to prevent global warming, energy and space saving separation technologies are required. Flexible metal–organic frameworks (flexible MOFs) with gate-opening properties have been found to be promising adsorbents, but the unconventional sigmoidal shapes and hysteresis of the isotherms have hindered quantitative evaluation of their applications in separation processes. This paper is the first to evaluate ELM-11, a flexible MOF that exhibits sigmoidal isotherms with hysteresis, in a process model and quantify its advantages over a conventional adsorbent. Based on the experimental uptake data, isotherm models for adsorption and desorption were developed, and their parameters were estimated. The resulting isotherm models are incorporated into a rigorous dynamic model of partial differential algebraic equations (PDAEs) to simulate a vacuum pressure swing adsorption (VPSA) process to evaluate the process performance. Numerical challenges to solve the PDAEs, including sigmoidal and hysteresis isotherm models, were resolved with the proposed numerical approaches. Sensitivity analysis for feed pressure and temperature was performed to identify the optimal operating strategy. A comparison with a conventional adsorbent, zeolite 13X, showed that high selectivity and sigmoidal isotherms of ELM-11 give higher productivity and CO2 product purity exceeding 99% without rinse and purge operations as well as lower power consumption for the compressor and vacuum pump.

Effect on high frequency resistance behavior of proton exchange membrane fuel cell during storage process

The high frequency resistance (HFR) is used to characterize the water content in the proton exchange membrane fuel cell (PEMFC), and the change of cell HFR during the storage process after gas purge with different storage and purge conditions is studied in this paper. The repeatability of the experiment is verified firstly. Then, the changes of cell HFR with different storage conditions include cell temperature, environment temperature, initial cell HFR are analyzed. Finally, the effect of purge conditions in the preparation stage on the cell HFR is studied by change the purge flow rate, purge gas type, purge methods. The results show that the cell HFR is affected by both the storage conditions and the purge operation conditions. The time for the PEMFC to reach the maximum HFR increases with the decrease of the environment temperature and the increase of the initial cell temperature. The final stable HFR value decreases with the increases of the environment temperature.

Scheduling of Time-Constrained Single-Arm Cluster Tools with Purge Operations in Wafer Fabrications

Scheduling of Time-Constrained Single-Arm Cluster Tools with Purge Operations in Wafer Fabrications

Numerical studies of CO separation and enrichment from blast furnace gas by using a CuCl/Y fixed bed

ABSTRACT Emission amount of blast furnace gas (BFG) is huge per year and it cannot be directly combusted due to its low heat value. Separating and Enriching CO from blast furnace gas provides an effective way for its utilization. This paper proposes an improved adsorption isotherm model of CO on CuCl/Y adsorbent. A CO/CO2/N2 multi-component adsorption equilibrium model and a set of mathematical models for CO separation and enrichment from BFG by using a fixed bed are established. Detailed characteristics of CO/CO2/N2 adsorption/desorption during Vacuum pressure swing adsorption (VPSA) process are revealed. Effects of adsorption/desorption pressure, cycle time, flow rate, purge operation, and thermal desorption temperature on CO separation and enrichment are further investigated. Optimized operation parameters are provided based on energy consumption analysis. For the purpose of purity >55% and lower energy consumption, conditions of <300 kPa adsorption pressure, 5–20 kPa desorption pressure, and 343–363 K desorption temperature are recommended.

Efficient scheduling approaches to time-constrained single-armed cluster tools with condition-based chamber cleaning operations

In semiconductor manufacturing, an extremely stringent quality control is enforced for wafer fabrication processes. Consequently, cleaning operations that clear residual chemical gases and heat in the chambers are frequently performed. As one kind of cleaning operations, a purge operation which executes a cleaning operation each time a wafer is removed from a chamber is widely adopted in leading semiconductor fabrication plants. Such an operation can definitely improve the quality of wafer. However, it results in lower productivity. To make a tradeoff between quality and productivity, a condition-based chamber cleaning operation that performs a cleaning operation with the consideration of the actual state of a chamber is introduced in practice. Aiming to address the scheduling problem of time-constrained single-armed cluster tools with condition-based chamber cleaning operations, efficient scheduling approaches are proposed in this work for the first time and, algorithms for searching for a feasible schedule are also derived. Two illustrative examples are given to show the power of the approach.

Experimental investigation on novel desalination system via gas hydrate

Freshwater scarcity has been troubling the high-quality development of many countries and regions, and seawater desalination is a vital source of freshwater. Recently, gas hydrate based desalination (GHBD) technology attracts much attention because of bringing no extra pollution to water. We developed a novel multifunctional desalination apparatus via gas hydrate with various operation modes and separation methods. In this study, the desalination characters under different separation modes were conducted using simulated seawater, and the hydrate-purging method with a desalination efficiency of more than 80% was selected to desalt the natural seawater sample. In addition, the purging method can greatly improve the desalination effects of both squeezed hydrate column and loose hydrates. The experimental results indicate that the removal efficiencies of different ions in the seawater were similar and their difference was related to the strength of ionic hydration. And above all, we designed and conducted a continuous desalination process including multiple injection, separation, washing and purging operations. The ultimate desalination efficiency using hydrate-purging method was over 80%, and the freshwater recovery was above 30% with 200 mL initial seawater in this study. The experimental proposal and results of this study are of great significance to the development of GHBD technology.

Evaluation on 14C emission patterns of various PWR types in South Korea and effectiveness of inventory reduction driving (IRD)

In this research, we investigated to examine the emission characteristics of radioactive carbon emitted from three types of pressurized water reactors. And we examined the possibility of 14C inventory reduction by changing operating conditions of one of the nuclear power plants. Results showed that 14C inventory in the reactor coolant system (RCS) remained constant before the overhaul (O/H) period and more than 90% of the radioactive carbon was found to be organic form. At spent fuel pool, the 14C chemical form of the liquid phase during the O/H period was also dominated by the inorganic (14CO2) form as in the case of the RCS due to the acid oxidized state. We applied gas stripper (G/S) and VCT purge operation to perform the 14C inventory reduction operation for Combustion Engineering type. In the case of gas strippers, approximately 50% of 14C inventory was reduced in the first year and then about 97% in the second year. Unlike our expectations, in case of VCT purge operation, 14C reduction effect was not showed greatly.

Tearing and related field distortions in deep-depletion charge-coupled devices

Tearing patterns affecting flat field frames in CCDs are a visually striking obstacle to performing Pixel Response Non-Uniformity corrections. These patterns can be explained by lateral field distortions, caused by the non-uniform distribution of holes in the channel stops between sensor columns. Over the course of LSST camera development, a number of practical fixes have been suggested to get rid of tearing. But applying these fixes to our 16-channel Teledyne-e2v sensors leaves at best a distortion pattern at the vertical edges of every segment. Our working hypothesis is that the origin of the tearing is the parallel clocking itself, which moves the holes that are present in the channel stops regions. The efficiency of these transfers depends strongly on the details of the clocking operations, resulting in the observed variety of distortion patterns. Removal of most of the distortion patterns can therefore be achieved by executing a purge operation, which flattens back the hole distribution in the channel stops, immediately before acquiring a frame. A more effective solution is to switch all clocking operations to use a bipolar voltage set.

Evaluation of dry acid gas removal process on bench scale test facility coupled with syngas produced by O2/CO2-blown gasifier

Dry acid gas removal test facility that comprises of halide and sulfur control units was coupled with O2/CO2-blown gasifier to demonstrate performance and operability of the dry gas purification process for the oxy-fuel IGCC power generation with efficient carbon dioxide separation. The facility was operated for initial five desulfurization cycles of sequential operation of sulfidation, regeneration, and reduction. The duration of each step was fixed to three hours which includes the purge operation to change the gas condition from syngas to oxidative gas, and vice versa. The test facility has the sulfur removal unit equipped with three fixed-bed reactors. Honeycomb shaped desulfurization sorbent that was improved to prevent harmful carbon deposition was installed in those reactors. The cyclic operation results revealed that the unit could reasonably remove sulfur compounds in the sulfidation steps and released the absorbed sulfur during succeeding regeneration steps for the initial five cycles of operation. The spent sorbent did not suffered with carbon deposition and retained their performance on sulfur removal. The absorbed sulfur in the spent sorbent was adequately released in regeneration steps for further operation of desulfurization cycles. The performance and operability of the dry acid gas removal process was successfully demonstrated in the actual syngas operation test.

Nonlinear dynamic mechanism modeling of a polymer electrolyte membrane fuel cell with dead-ended anode considering mass transport and actuator properties

A dead-ended anode (DEA) has advantages such as simple structure, high reliability, and low price, and is widely utilized in polymer electrolyte membrane fuel cell (PEMFC) systems. Empirical parameters are commonly adopted in control-oriented models for such systems, and detailed information about mass transport processes is usually not available. Such models are neither helpful for understanding the internal processes within fuel cells, nor for designing control algorithms to improve system performance. A control-oriented model considering the mass transport processes and actuator properties is still lacking. This paper proposes a nonlinear dynamic mechanism model for the DEA system that can describe the dynamic voltage drop during water flooding with a large current density. The properties of the major components are explained in details, and the procedure of how the purging valves affects the mass transport and cell voltage is revealed quantitatively. The relationship between the minimum cell voltage and purging operations is summarized. The results show that (1) the proposed model can capture the stable and dynamic properties of a fuel cell with a DEA, (2) the cell voltage loss during closing of the purging valve is mainly caused by a decrease in oxygen and hydrogen partial pressures on the catalyst layers and an increase in the liquid water saturation ratio in the gas diffusion layers (GDLs); (3) the most important internal states that affect the stack voltage during purging is the liquid water saturation ratio in the GDLs.

Performance degradation and recovery characteristics during gas purging in a proton exchange membrane fuel cell with a dead-ended anode

Proton exchange membrane fuel cells operated in the dead-ended anode mode can improve the utilization of hydrogen and simplify the fuel cell system. Gas purging was an effective way to remove the excess water to avoid water flooding and performance degradation in PEMFC. Effective back diffusion of water from the cathode to the anode was critical for performance recovery after gas purging. The purging operation was the process of water rebalancing in the electrode. The water back diffusion rate across the membrane was mainly related to temperature, pressure and current density. The performance degradation and recovery characteristics during the purging process in a proton exchange membrane fuel cell with a dead-ended anode were investigated under various operating conditions in this paper. Performance decreased rapidly during the duration of purging, after which it increased immediately at first and then gradually reached its original level a few seconds after gas purging. Nevertheless, performance would eventually stabilize before the next purging operation was begun.

Modeling of U-shaped Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ hollow-fiber membrane for oxygen permeation

Abstract A mathematic model is developed for the perovskite-type mixed ionic-electronic conducting (MIEC) membrane, which makes it possible to simulate the process of oxygen separation in the U-shaped Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 − δ hollow-fiber membrane. The model correlates the oxygen permeation flux to the measurable variables. The trends of calculated results for purge operation coincide well with the experimental data, therefore the model is considerable for flux prediction under vacuum operation. Higher oxygen separation efficiency can be achieved with vacuum operation than purge operation. Parameter study with vacuum operation reveals that oxygen permeation flux increases with higher vacuum levels, and vacuum pressure of around 1.013 × 10 3 Pa is the optimal. Also, vacuum operation on the lumen side is much more efficient to achieve higher oxygen permeation flux compared with compression mode on the shell side.

NaBH&lt;sub&gt;4&lt;/sub&gt;を水素源とする燃料電池システムにおけるパージによる圧力変動の抑制法

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Model-based fuel pressure regulation algorithm for a hydrogen-injected PEM fuel cell engine

In Polymer Electrolyte Membrane Fuel Cells (PEMFCs) with Dead-Ended Anode (DEA) configuration, precise control of hydrogen pressure in anode is beneficial for better performance and durability of fuel cell systems. In most studies, mechanical pressure regulator is implemented to modulate the hydrogen pressure. But when load changes or purge operation activates, hydrogen pressure downstream the regulator varies with hydrogen flow fluctuation. In this study, a hydrogen injection system is developed utilizing gaseous fuel injectors. Based on a fuel cell simulation test bench, the performance of the hydrogen injection system is evaluated. A physics-based nonlinear model of the simulation test bench is presented in this paper. Experimental results validate the model. The model is further linearized and integrated in a Robust Predictive Controller (RPC), a Model-Based Controller (MBC), and an H∞ controller, respectively, in order to compare the control effects on the hydrogen pressure, especially the mitigation of the pressure swing during the purge operation. A comparative study is carried out under different scenarios, i.e. target pressure step response, hydrogen consumption mass flow step response, and periodical pulse response during the purge operation. According to the results, MBC is chosen for injection control, since the system demonstrates preferable performance in comparison with other controllers. Finally, pressure fluctuation data of both simulation test bench with hydrogen injection and conventional mechanical pressure regulator is compared to elucidate the effectiveness.