Unstable Reactor Research Articles

Multifractal detrended fluctuation analysis of boiling water reactors

The aim of this work is to determine a relationship between multifractal parameters and Boiling Water Reactor (BWR) stability; therefore, a Multifractal Detrended Fluctuation Analysis (MF-DFA) of the data series obtained from the Average Power Range Monitor (APRM) and the Local Power Range Monitors (LPRM) during stable operation and during an instability event of a BWR-5, which concludes with the SCRAM, was performed. First, a much larger number of data than recommended was considered to calculate the multifractality for the instability event (4,200 data points) and for stable operation (5,700 data points) and determine if there was any difference among these analyses. Subsequently, the data were divided into periods of 50 s (500 data points) to detect the moment when the reactor begins to operate in an unstable manner, here it was not necessary to have enough information to calculate the multifractal parameters because it was only of interest to detect any trend or difference between the multifractal parameters that indicated instability in the operation. The results show that the data from the stable operation present a behavior with higher multifractality compared to the data from the unstable operation. In fact, the results indicate that the data from the unstable operation of the reactor tend to monofractality. This can be seen more clearly through the width of the multifractal spectrum, it is larger for the data corresponding to the stable operation. The proposed methodology was successfully applied to a challenging case study (Forsmark stability benchmark) as it is difficult to detect unstable reactor operation.

The performance and microbial community of anaerobic membrane bioreactor for high calcium papermaking wastewater treatment

High calcium papermaking wastewater challenges anaerobic biological technology applications, leading to sludge calcification and unstable reactor operation. This study employed an anaerobic membrane bioreactor (AnMBR) to treat high-calcium papermaking wastewater (1000 mg/L) and achieved an average chemical oxygen demand removal efficiency of 90.1%. No significant accumulation of volatile fatty acids was observed during the entire experimental period. The methane production capacity decreased with the increasing inlet calcium ion concentration, and the maximum methane production rate in Stage 4 was 115.4 mL·(gVSS·d)−1. The maximum transmembrane pressure reached was 4.33 kPa, and the sludge was primarily present in the form of flocs due to the hydraulic shear effects. Anaerolineaceae, Bacteroidetes_vadinHA17, Methanosaetaceae and Methanobacteriaceae were identified as the dominant bacterial species in Stage 4, correlating with the robust anaerobic digestion performance. In conclusion, the AnMBR exhibited a reliable and effective treatment method for high calcium papermaking wastewater.

Technical note: stable and unstable reactors

It is well known that a reactor is stable if the core reactivity decreases with the core power. This is the case for many types of reactors, including the PWR. However, this was not the case for the RBMK (Reaktor Bolshoy Moshchnosti Kanalniy) which could be unstable at low power. What does it mean precisely? By using a 2 × 2 system of non-linear ordinary differential equations we show that naturally (i.e. without using the control rods), with the same reactivity injection, if the initial power is lowered, then the final power may be higher, which is a rather unusual behaviour.

Effects of submicron magnetite particles on granulation of flocculent sludge and process stability in upflow anaerobic sludge blanket reactor

Promoting direct interspecies electron transfer (DIET) with conductive additives is considered a promising approach to enhance methanogenesis. This study investigated the effects of adding submicron magnetite particles on sludge granulation and methanogenic performance in upflow anaerobic sludge blanket reactors inoculated with flocculent sludge. The reactor supplemented with magnetite was more stable and resilient than the no-magnetite control, with higher degree of granulation (up to 26.6-fold) and biomass retention. Magnetite addition to unstable reactors improved the methane yield in both reactors (1.2–1.3-fold). Electroactive Deltaproteobacteria bacteria, including Geobacter and Syntrophobacter, were enriched in the presence of magnetite. Methanogenic functional genes involved in DIET-based syntrophy were more abundant under magnetite-supplemented conditions. However, the improvement of methanogenic performance and granulation was limited, and inducing the self-embedment of magnetite into mature sludge granules rather than granulating flocculent sludge with magnetite appears to be a better strategy for engineering DIET in anaerobic granular sludge systems.

Assessing the effect of green tuff as a novel natural inorganic carrier on methane-producing activity of an anaerobic sludge microbiome

The upflow anaerobic sludge blanket (UASB) process is an anaerobic biological system that is widely used for the treatment of a variety of wastewater owing to its benefits such as low energy requirements and energy recovery as methane gas; however, unstable reactor performance is often observed. To solve this problem, several inert inorganic carriers and polymer materials have been studied for stabilization of the sludge microbial community and promotion of methane production. In this study, we demonstrate the effects of green tuff (GT) as a novel natural inorganic carrier on methane-producing activity (MPA) in combination with syntroph/methanogen-utilizable substrates and on the microbial community assembly of methanogenic batch cultures fed with granular sludge collected from UASB reactors treating milk- (MMW) or soy sauce- (SSW) manufacturing wastewater. The MPAs of MMW cultures with GT were 4.56- (propionate), 2.01- (n-butyrate), 1.37- (n-valerate), and 1.22- (benzoate) fold higher than those without GT. Likewise, SSW cultures with GT showed 1.34- (acetate) and 1.42- (propionate) fold higher MPA than those without GT. These results indicate that GT has potential to enhance MPAs during methanogenic degradation of these substrates. According to 16S rRNA transcripts-based community profiling, populations of syntrophic bacteria (Syntrophobacter and Smithella) and hydrogenotrophic methanogens (Methanobacterium and Methanolinea) in the cultures with GT were higher than in those without GT. Therefore, GT can be a potential solution for the activation of syntrophic partnerships and improvement/stabilization of reactor performance.

Rapid start-up strategy of 1 kWe diesel reformer by solid oxide fuel cell integration

A rapid start-up strategy of a diesel reformer for on-board fuel cell applications was developed by fuel cell integration. With the integration with metal-supported solid oxide fuel cell which has high thermal shock resistance, a simpler and faster start-up protocol of the diesel reformer was obtained compared to that of the independent reformer setup without considering fuel cell integration. A reformer without fuel cell integration showed unstable reactor temperatures during the start-up process, which affects the reforming catalyst durability. By utilizing waste heat from the fuel cell stack, steam required at the diesel autothermal reforming could be stably provided during the start-up process. The developed diesel reformer was thermally sustainable after the initial heat-up process. As a result, the overall start-up time of the reformer after the diesel supply was reduced to 9 min from the diesel supply compared to 22 min without fuel cell integration.

Stabilization of an unstable tubular reactor by nonlinear passive output feedback control

The multiple–input multiple–output (MIMO) output feedback (OF) control problem of an exothermic multi-jacket tubular open-loop unstable reactor is addressed. Over its axial length, the reactor has several equally sized cooling jackets. The controller must adjust the jacket temperatures on the basis of per jacket temperature measurements so that the closed-loop system is robustly stable. The problem is solved within a constructive framework, by combining notions and tools from chemical reactor engineering and partial differential equations (PDEs) control systems theory. The result is a MIMO nonlinear OF dynamic control design with (i) a decentralized MIMO passive state feedback (SF) controller implemented with a pointwise observer (PWO), (ii) closed-loop stability conditions in terms of sensor set and control gains, and (iii) efficient late lumping-based on-line implementation. The design is put in perspective with industrial PI and inventory control, and applied to a representative example through numerical simulation with favorable comparison against adaptive controllers.

Importance of heat-transfer area in the controllability of openloop unstable reactors

Openloop unstable reactors present probably the most interesting and challenging control problem in chemical engineering. They can occur when the reactions are exothermic and irreversible and when reactant per-pass conversion is not high so that there is “fuel” available in the reactor to cause a temperature runaway. An openloop unstable process cannot be operated without a feedback controller on automatic. In a typical CSTR process the reactor temperature is controlled by manipulating the flowrate of the cooling medium. Closedloop stability may be achieved if the reactor is designed with sufficient heat-transfer area. The temperature control loop exhibits “conditional stability” since the controller gain must lie inside some range of values whose span depends on the heat-transfer area selected in the design. The purpose of this paper is to demonstrate the critical design trade-off between heat-transfer area and dynamic controllability in openloop unstable CSTRs.

Fe0 inhibits bio-foam generating in anaerobic digestion reactor under conditions of organic shock loading and re-startup

Bio-foaming during the anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) is a major cause of unstable reactor operations, especially under high organic loading rates (OLRs). Here, we used two 40-liter anaerobic reactors to deal with the actual organic fraction of municipal solid waste at OLRs of 5 and 7.5 kgVS/m3/d and tried to inhibit bio-foam generation by dosing with Fe0. One of reactors had 400 g of Fe0 added before operation and additional 200 g of Fe0 was added when the operating conditions changed; the other one was the control reactor with no Fe0 dosing. The results show that bio-foam was generated in the control reactor at an organic shock loading of 10 kgVS/m3/d and re-startup at 5 kgVS/m3/d, while the reactor containing Fe0 operated normally. Due to the surfactant attributes of volatile fatty acids (VFAs), the probability of bio-foam generation was positively correlated with the effluent’s degree of surface activity (SA) induced by VFAs, which is a parameter representing the foaming potential. Because VFA biodegradation was enhanced in the reactor with added Fe0, at the position of Fe0 accumulation, the SA was decreased by 49.7–59.2%. Furthermore, the effect of Fe0 dosing on inhibiting bio-foam was evaluated by comparison with traditional commercial anti-foam agents. The results suggest possible reasons for bio-foam generation in OFMSW AD and suggest the reasonable usage of Fe0 to inhibit bio-foam formation.

Treatment of cheese whey by a cross-flow anaerobic membrane bioreactor: Biological and filtration performance

Whey, produced in large quantities during cheese production, is a rapidly fermentable high strength wastewater characterized by a high biodegradability and low alkalinity. In this study, a lab-scale cross-flow anaerobic membrane bioreactor was used to address the commonly experienced difficulties such as unstable reactor performance and unexpected biomass losses when treating whey wastewater with conventional anaerobic reactors. The anaerobic membrane bioreactor provided a stable treatment performance, i.e. more than 90% chemical oxygen demand removal, and moderate membrane fluxes between 8 and 11 L m−2 h−1 could be obtained, applying a low cross-flow velocity of about 0.5 m s−1. Short term critical flux tests revealed that higher fluxes up to 36 L m−2 h−1 are possible at elevated cross-flow velocities and/or reduced mixed liquor suspended solids concentrations. Sludge filterability indicated by capillary suction time and specific resistance to filtration deteriorated throughout the study. Chemical cleaning efficiency gradually decreased, indicating irreversible membrane fouling during long term operation.

Direct treatment of high-strength soft drink wastewater using a down-flow hanging sponge reactor: performance and microbial community dynamics.

A stand-alone down-flow hanging sponge (DHS) system with a two-stage configuration was operated for 700days to treat synthetic soft drink wastewater at 3000mg/L chemical oxygen demand (COD). Throughout the operation, >90% COD and total organic carbon (TOC) removal efficiency was obtained by the first stage, and a final effluent of COD <60mg/L (TOC <20mg/L) was consistently maintained with the second stage. Lower organic removal efficiency was observed to closely correlate with lower pH, higher volatile fatty acid (VFA) concentration, and higher suspended solid (SS) in the effluent. Occasionally, biomass sloughing was observed as a cause to unstable reactor performance in the first stage. The microbial community of the retained biomass on the sponges differed significantly based on spatial locations of sponges, sampling time points, and loading shocks. In general, Proteobacteria were found to be more abundant in the reactor at an organic removal efficiency >80% than that at <50%. Specifically, operational taxonomic units closely related to Tolumonas auensis and Rivicola pingtungensis were identified as important populations that were responsible for degrading the major substrate in the soft drink wastewater toward to the end of the reactor operation. In addition, high abundance of Bacteroidetes in the reactor was speculated to be responsible for the VFA accumulation in the effluent. This study demonstrated that stand-alone DHS reactor could be used in treating high-strength wastewater efficiently.

A robustly stabilizing model predictive control strategy of stable and unstable processes

This paper deals with the development of a robust model predictive control strategy with guarantee of stability, applicable to the stable and unstable processes. The model uncertainty is assumed to be described by a discrete set of linear models (multi-plant uncertainty), and the robustness is achieved by assembling cost-contracting constraints for all the possible models in the uncertainty domain. On the basis of a suitable state-space model description, an offset free control law is obtained by means of a one-step optimization formulation. The usefulness of the method proposed here is illustrated with control simulations of an unstable reactor system taken from the literature.

Design a Lyapunov control under input constraint for stabilisation of a CSTR

In this paper, a new nonlinear controller synthesis approach is presented. This approach has been developed for determination of a Lyapunov function which is computed by solving Zubov's partial differential equation of the studied controlled system. A nonlinear control law with adjustable parameters of attraction regions is derived. A Lyapunov-based control synthesis is implemented to construct an input control that guarantees stability and high precision along a steady-state. The characterisation of parameters of attraction regions can cause serious practical problems as saturation behaviour. The performance of the developed technique has been examined for temperature control of an unstable reactor and its effectiveness has been demonstrated through numerical simulation. A genetic algorithm optimisation based on linear matrix inequality are implemented to solve such problem of the proposed approach. The performance of the developed technique has been examined for temperature control of an unstable reactor and its effectiveness has been demonstrated through numerical simulation.

High temperature biogas reactors to treat stillage from an industrial bioethanol process: Metabolic and microbial characterization

Stillage derived from bioethanol production process is a side stream conventionally used as feed additive after a cost‐intensive dehydration step. From economical and ecological points of view, it also represents an appealing substrate for biogas production. In this work, we examined the biomethanization of thin stillage in a stirred bioreactor under thermophilic conditions (55°C). Different organic loading rates and hydraulic residence times (HRTs) were tested over a long period of operation. Using thin stillage as a mono‐substrate, the maximum loading rate reached was 2.1 goTS/L/day (oTS, organic total solid). However, with the addition of a commercially available iron hydroxide additive, a maximum organic loading rate of 5.9 goTS/L/day was achieved. GC‐MS and denaturing gradient gel electrophoresis were used to study the metabolites and the microbial population dynamics within the biogas reactor under different process conditions. For all organic loading rates studied, volatile fatty acids were shown to give a clear indication of reactor instability. Products of aromatic amino acid degradation, especially phenyl acetic acid (PAA), were detected earlier in reactors even at very low organic loading rates. PAA concentration above 0.25 g/L indicated an unstable reactor performance and values above 0.5 g/L were found to be inhibitory to the biogas production in batch cultures.

Organizational Strategy in a Gentrifying Neighborhood

This article explores how human service providers in a gentrifying neighborhood have responded to change. Using Miles and Snow’s (1978/2003) typology for organizational strategy, it analyzes how these providers have responded to the external environment and have planned strategically for their futures while serving the low-income residents and homeless in the neighborhood. Analysis shows that the providers’ strategic responses closely align with the framework and expected behaviors. However, notable differences were found between organizations in Heartside that are stable (Defender, Prospector, Analyzer) and those that are unstable (Reactor) with regard to impact of neighborhood changes and organizational strategic responses.

New forms of Riccati equations and the further results of the optimal control for linear discrete-time systems

For linear discrete-time systems, the traditional finite horizon optimal controller is proved to render the closed-loop systems asymptotically stable under some assumptions in literature. In this paper, a new form of finite horizon discrete-time Riccati equation is proposed. It is proved that the new form of finite horizon discrete-time Riccati equation is equivalent to the other three old ones. Based on this new form of finite horizon discrete-time Riccati equation, the finite horizon optimal controller of linear discrete time systems is proved to render the closed-loop system exponentially stable without any assumptions. At the same time, a new form of infinite horizon discrete-time Riccati equation is proposed when the discrete system is controllable or stabilizable. It is proved that the new form of infinite horizon discrete-time Riccati equation is equivalent to the other three old ones too. Based on this new form of infinite horizon discrete-time Riccati equation, the infinite horizon optimal controller of linear discrete-time systems is proved to render the closed-loop system exponentially stable when the open-loop system is either controllable or stabilizable. Finally an unstable batch reactor and an unstable inverted pendulum are used to verify the theory results of this paper.

Space–time convergence analysis on BWR stability using TRACE/PARCS

Unstable behavior of Boiling Water Reactors (BWRs) is known to occur during operation at certain power and flow conditions. Even though BWR instability is not a severe safety concern, it could cause reactor scram and significantly decrease the economic performance of the plant. This paper aims to (a) quantify TRACE/PARCS space–time discretization error for simulation of BWR stability, (b) establish space (nodalization) and time discretization necessary for space–time converged model and (c) show that the space–time converged model gives more reliable results for both stable and unstable reactor. The space–time converged model is obtained when further refinement of numerical discretization parameters (nodalization and time step) has negligible effect on the solution. The study is significant because performing a space–time convergence analysis is a necessary step of qualification of the TRACE/PARCS model, and use of the space–time converged model increases confidence in the prediction of BWR stability.

Feedforward output-feedback control for continuous exothermic reactors with isotonic kinetics

The robust feedforward (FF) output-feedback (OF) control problem of (possibly open-loop unstable) continuous exothermic jacketed reactors with isotonic kinetics with respect to reactant concentration is considered. The volume, temperature, and concentration must be regulated by manipulating the feed, exit and coolant flowrates on the basis of volume and temperature measurements. The problem is addressed as an interlaced controller-observer design within a constructive control framework. The result is a quasi linear-decentralized (qLD) FF-OF dynamic controller which: (i) recovers (up to observer convergence) the behavior of a robust nonlinear FF-SF controller, (ii) has closed-loop stability conditions coupled with conventional-like tuning guidelines, and (iii) constitutes an add-on to the PI control scheme commonly employed in industrial reactors. The approach is tested with an open-loop unstable reactor example through simulations.

Application of an extended IHMPC to an unstable reactor system: Study of feasibility and performance

Abstract Almost all the theoretical aspects of model predictive control (MPC), such as stability, recursive feasibility and even the optimality are now well established for both, the nominal and the robust case. The stability and recursive feasibility are usually guaranteed by means of additional terminal constraints, while the optimality is achieved considering closed-loop predictions. However, these significant improvements are not always applicable to real processes. An interesting case is the control of open-loop unstable reactor systems. There, the traditional infinite horizon MPC (IHMPC), which constitutes the simplest strategy ensuring stability, needs to include an additional terminal constraint to cancel the unstable modes, producing in this way feasibility problems. The terminal constraint could be an equality or an inclusion constraint, depending on the local controller assumed for predictions. In both cases, however, a prohibitive length of the control horizon is necessary to produce a reasonable domain of attraction for real applications. In this work, we study the application of an IHMPC formulation that has maximal domain of attraction (i.e., the domain of attraction is determined by the system and the constraints, and not by the controller) to an unstable reactor system. It is shown that the method is suitable for real applications in the sense that it accounts for the case of output tracking and it is offset free if the output target is reachable, and minimizes the offset if some of the constraints become active at steady-state.

Application of IHMPC to an unstable reactor system: study of feasibility and performance

Almost all the theoretical aspects of Model Predictive Control (MPC), such as stability, recursive feasibility and even the optimality are now well established for both, the nominal and the robust case. The stability and recursive feasibility are usually guaranteed by means of additional terminal constraints, while the optimality is achieved considering closed-loop predictions. However, these significant improvements are not always applicable to real processes. An interesting case is the control of open-loop unstable reactor systems. There, the traditional infinite horizon MPC (IHMPC), which constitutes the simplest strategy ensuring stability, needs to include an additional terminal constraint to cancel the unstable modes, producing in this way feasibility problems. The terminal constraint could be an equality or an inclusion constraint, depending on the local controller assumed for predictions. In both cases, however, a prohibitive length of the control horizon is necessary to produce a reasonable domain of attraction for real applications. In this work, we propose an IHMPC formulation that has maximal domain of attraction (i.e. the domain of attraction is determined by the system and the constraints, and not by the controller) and is suitable for real applications in the sense that it accounts for the case of output tracking, it is offset free if the output target is reachable, and minimizes the offset if some of the constraints become active at steady-state.