Recycle Flow Research Articles

Model-based control structure design of a full-scale WWTP under the retrofitting process.

The anoxic-oxic (A/O) municipal wastewater treatment plant (WWTP) of Manresa (Catalonia, Spain) was studied for a possible conversion to an anaerobic/anoxic/oxic (A2/O) configuration to promote enhanced biological phosphorus removal. The control structure had to be redesigned to satisfy the new necessity to control phosphorus concentration, besides ammonium and nitrate concentrations (main pollutant concentrations). Thereby, decentralized control structures with proportional-integral-derivative (PID) controllers and centralized control structures with model-predictive controllers (MPC) were designed and tested. All the designed control structures had their performance systematically tested regarding effluent quality and operating costs. The centralized control structure, A2/O-3-MPC, achieved the lowest operating costs with the best effluent quality using the A2/O plant configuration for the Manresa WWTP. The controlled variables used in this control structure were ammonium in the effluent, nitrate at the end of the anoxic zone and phosphate at the end of the anaerobic zone, while the manipulated variables were the internal and external recycle flow rates and the dissolved oxygen setpoint in the aerobic reactors.

Concentration of apricot juice using complex membrane technology

In this study, pressed apricot (Prunus armeniaca L.) juice was concentrated using complex membrane technology with different module combinations: UF-RO-OD, UF-RO-MD, UF-NF-OD and UF-NF-MD. In case of the best combination a cross-flow polyethylene ultrafiltration membrane (UF) was applied for clarification, after which preconcentration was done using reverse osmosis (RO) with a polyamide membrane, and the final concentration was completed by osmotic distillation (OD) using a polypropylene module. The UF-RO-OD procedure resulted in a final concentrate with a 65-70 °Brix dry solid content and an excellent quality juice with high polyphenol content and high antioxidant capacity.Nanofiltration (NF) and membrane distillation (MD) were not proper economic solutions.The influence of certain operation parameters was examined experimentally. Temperatures of UF and RO were: 25, 30, and 35 °C, and of OD 25 °C. Recycle flow rates were: UF: 1, 1.5, and 2 m3 h−1; RO: 200, 400, and 600 l h−1; OD: 20, 30 and 40 l h−1. The flow rates in the module were expressed by the Reynolds number, as well. Based on preliminary experiments, the transmembrane pressures of UF and RO filtration were 4 bar and 50 bar, respectively. Each experimental run was performed three times. The following optimal operation parameters provided the lowest total cost: UF: 35 °C, 2 m3 h−1, 4 bar; RO: 35 °C, 600 l h−1, 50 bar; OD: 20, 30 and 40 l h−1; temperature 25 °C.In addition, experiments were performed for apricot juice concentration by evaporation, which technique is widely applied in the industry using vacuum and low temperature.For description the UF filtration, a dynamic model and regression by SPSS 14.0 statistics software were applied.

Numerical study of a hybrid multi-stage vacuum membrane distillation and pressure-retarded osmosis system

In this study, we introduce a hybrid system that integrates a multi-stage vacuum membrane distillation (MVMD) with pressure-retarded osmosis (PRO). The MVMD system employs a recycling flow scheme (MVDM-R) for the continuous production of both distillate water and highly concentrated brine. The concentrated brine that is produced from the MVMD-R system is then used as a draw solution for power generation in the PRO system. We theoretically assessed the distillate and power production of the MVMD-R-PRO system with respect to inlet feed flow rate and recycling flow ratio in the MVMD-R system. When the inlet feed flow rate is constant, the production of distilled water increases slightly, with a decrease in the recycling flow. The maximum possible brine concentration from the MVMD-R system is 1.9M NaCl at an inlet feed flow rate of 3kg/min and a 90% recycling flow. A maximum power density of 9.7W/m2 is achieved when river water is used as a feed solution in the PRO system at feed and draw solution flow rates of 0.5kg/min and a constant hydraulic pressure difference.

Continuous xylanase production with Aspergillus nidulans under pyridoxine limitation using a trickle bed reactor

A trickle bed reactor (TBR) with recycle was designed and tested using Aspergillus nidulans with a pyridoxine marker and over-expressing/secreting recombinant client xylanase B (XynB). The pyridoxine marker prevented the fungus from synthesizing its own pyridoxine and fungus was unable to grow when no pyridoxine was present in the medium; however, enzyme production was unaffected. Uncontrolled mycelia growth that led to clogging of the TBR was observed when fungus without a pyridoxine marker was used for XynB production. Using the fungus with pyridoxine marker, the TBR was operated continuously for 18days and achieved a XynB output of 41U/ml with an influent and effluent flow rate of 0.5ml/min and a recycle flow rate of 56ml/min. Production yields in the TBR were 1.4 times greater than a static tray culture and between 1.1 and 67 times greater than yields for SSF enzyme production stated in the literature.

On the flow direction effect in sequential modular simulations: A case study on fluidized bed biomass gasifiers

Feed location is a key parameter in the performance of hydrogen production multi-phase reactors. While the current sequential-based models successfully simulate multi-phase fluidized reactors (FR) with cocurrent feeding, no general countercurrent-feed sequential model is available to predict the behaviour of complex top-feeding systems. In this work, the modular process simulation approach is expanded to simulate the FB-based hydrogen production with opposite feed directions. The model consists of novel logically-ordered plug flow reactors (PFRs) and continuous stirred-tank reactors (CSTRs), accounting for the feed location using a recycle flow, termed as solid circulation factor (XC,i). As a case study, a fluidized bed gasifier for green fuel production is simulated, taking the hydrodynamic and reaction kinetic sub-models into consideration. The dynamic two-phase (DTP) model describes the physical phenomena in the hydrodynamic sub-model, and a comprehensive set of reaction kinetic models derived from literature, including tar conversion, provides the reaction sub-model for a biomass gasifier. By integrating these sub-models, the fluidized bed reactor is divided into two main regions, namely dense bed and freeboard. The dense bed is segmented into several sections, each of which treating the gas flow as a plug flow through the bubbles and as a completely-mixed flow in the emulsion phase. The freeboard section is well simulated employing a plug flow reactor. Satisfactory agreement between the model prediction and the experimental data is achieved when the countercurrent flow and full kinetic model is used, while cocurrent flow and/or equilibrium simulation bring about poor predictions. This work paves the way toward designing a comprehensive fluidized bed sequential module to be integrated with the industrial process simulators.

Design and Control of Di n-Pentyl Ether Process

Recycle of un-converted reactants is a common practice in industrial chemical processes. However, the material recycle induces a non-linear behaviour of the plant which often manifests as high sensitivity of the recycle flow rates with respect to disturbances such as changes in raw material quality, production rate and uncertain design parameters. This non-linear behaviour of the system is often the source of control difficulties. Thus the importance of appropriate control structure in reactor-separation-recycle is evident. The case study of di n-pentyl ether production illustrates two control strategies that can be applied to processes involving one reactant and one recycle. The strategy based on self-regulating reactant inventory uses the plant-inlet flow rate as the dominant variables which significantly affects the production rate. The strategy based on inventory feedback control uses the reactor inlet flow rate, the reactor holdup or the reaction temperature / pressure as throughput manipulator. For the di n-pentyl ether process, both strategies are applicable, as demonstrated by rigorous dynamic simulation.

An SO2 tolerant process for CO2 capture

The presence of SO2 in flue gas has a negative impact on typical CO2 capture processes utilising aqueous amines. For countries like Australia, that do not have flue gas desulphurisation, retrofitting such equipment to existing coal fired power stations is of the order of 100s of millions of dollars. In this work a new process configuration is described in which SO2 is absorbed into a fraction of the absorbent used for CO2 capture. SO2 absorption is carried out in the bottom of the absorber column into a bleed stream from the bulk solvent, and a recycle flow allows the absorbent to be near saturation in SO2. This high concentration allows a number of low cost options for sulphur removal from the absorbent such as chemical treatment and precipitation, with predicted capital and operating costs in the range of 100s of thousands for a full scale plant. The principles underlying the concept, specifically kinetic selectivity of SO2 absorption over CO2 and reactivity with amines, have been studied in the laboratory. Also a rate based model of an absorber column has been used to model the behaviour of SO2 from the application of the process concept to CSIRO's CO2 capture pilot plant at Loy Yang power station. The laboratory and modelling results support both its feasibility and utility.

NOx conversion properties of a novel material: Iron nanoparticles stabilized in carbon

Nitrogen oxides (NOx) belong to the most common pollutants from combustion processes and are a major threat to human health. Carbon-based catalysts exhibit strong advantages for NOx removal like low-toxic application and easy handling. However, gasification of the carbon matrix at elevated temperatures is still one of the greatest concerns. Hence, we have directed our focus on especially low temperature NOx-removal using a novel material, iron nanoparticles stabilized in a carbon matrix (nano-Fe/C). The investigations included NO2 uptake properties and catalytic conversion of NO2 in recycle flow at 425K and 328K, scanning transmission electron microscopy and 77K-N2-adsorption.Nano-Fe/C exhibits superior NOx-removal properties compared with untreated or iron-infiltrated activated carbon or magnetite reference catalysts. No severe catalyst deactivation or catalyst aging at 425K is observed. Even at 328K nano-Fe/C still exhibits NO2-conversion, although without converting the product NO. NO2 adsorption at 297K is suggested to occur in three stages with different kinetics: (1) NO2 adsorption and reduction to NO, (2) physisorption on the oxidized catalyst surface and (3) saturation of the catalyst and diffusion into the substrate matrix. At 425K, NO2 is quickly reduced to NO and the resulting NO is further converted to N2O. After complete consumption of NO, the residual NO2 is also converted to N2O. A possible reaction mechanism is suggested based on the conversion kinetics.

Through Control of Wetting and Drying Conditions of Monolithic Supports Toward a Uniform Catalyst Distribution

This article presents an optimized wetting and drying procedure for the preparation of monolithic catalysts. The catalyst precursor consumption was minimized by applying a tailored incipient wetness impregnation technique. The homogeneous distribution of the active phase within the monolithic catalysts was achieved by drying of the impregnated monoliths under a forced flow in a specially designed dryer. The dryer operated at ambient conditions with high recycle flow of drying gas. The direction of recycle flow was periodically alternated by a pneumatically driven valve assembly. The effect of the proposed impregnation method is demonstrated with an analysis of the Ni distribution in the impregnated Al2O3 monolith supports.

Transparency in the Extractive Industries: Time to Ask for More

The quest for transparency spans countries, policymakers, NGOs, and industries.Transparency can be defined as disclosing to the public, in a timely and reliable manner, information that governments and/or corporations previously con-sidered confidential. Recent examples include the Carbon Disclosure Project,the Aarhus convention on access to environmental information, the Cartagena Protocol on Biosafety and its provisions on global genetically modified organism flows, and a wide array of financial information (e.g., in the G8 declaration of Lough Erne in 2013). Stemming from the “right to know,”advocates from NGOs and development organizations view transparency as a cure for corruption and a benefit for democratic accountability; transparency should lead to stakeholder empowerment and improve legitimacy, learning, investment certainty, and better governance. In this article I look at the efforts to establish financial transparency as a norm for the extractive sector. This sector is important because its activities are accompanied by a high level of corruption, especially in resource-rich developing countries. I show that those efforts are not enough and there is good evidence to demand more. I argue that such transparency norms should be extended to environmental pressures in order to facilitate progress on the circular economy and resource efficiency. My conclusions point at synergies between knowledge generation across financial and environmental information.

Development and performance of bench-scale reactor for the photocatalytic generation of hydrogen

In this study, a new novel bench-scale (5 L) tubular photocatalytic reactor was developed and its feasibility studies were conducted for optimizing the operating variables, namely concentration of sulfide ion, concentration of sulfite ion, pH, catalyst concentration, lamp power, volume of wastewater and recycle flow rates at batch recycle mode for the generation of hydrogen from aqueous sodium sulfide using CdS–ZnS/TiO2 core–shell NPs (nanoparticles). The maximum H2 generation was found at 0.05 M concentration of sulfide ion, 0.2 M concentration of sulfite ion, pH 11.3, 0.5 g/L catalyst concentration and recycle flow rate of 18 L/h. Reusability studies were conducted for analyzing stability of photocatalyst. The results showed that the generation of hydrogen depends on light intensity, photoreactor used, nature of photocatalysts and the operating conditions.

A specific pilot-scale membrane hybrid treatment system for municipal wastewater treatment

A specifically designed pilot-scale hybrid wastewater treatment system integrating an innovative equalizing reactor (EQ), rotating hanging media bioreactor (RHMBR) and submerged flat sheet membrane bioreactor (SMBR) was evaluated for its effectiveness in practical, long-term, real-world applications. The pilot system was operated at a constant flux, but with different internal recycle flow rates (Q) over a long-term operating of 475days. At 4Q internal recycle flow rate, BOD5, CODCr, NH4+-N, T-N, T-P and TSS was highly removed with efficiencies up to 99.88±0.05%, 95.01±1.62%, 100%, 90.42±2.43%, 73.44±6.03%, and 99.93±0.28%, respectively. Furthermore, the effluent quality was also superior in terms of turbidity (<1NTU), color (<15TCU) and taste (inoffensive). The results indicated that with providing only chemically cleaned-in-place (CIP) during the entire period of operation, the membrane could continuously maintain a constant permeate flux of 22.77±2.19L/m2h. In addition, the power consumption was also found to be reasonably low (0.92–1.62kWh/m3).

Adaptive Fuzzy Inference Control of the Recycle Compression System

The industry is increasing technology setting to improve manufacturing quality products and provide greater security for people to best use materials, leaving tiring or boring tasks to the machines. Every centrifugal or axial compressor has a characteristic combination of maximum discharge pressure and minimum flow beyond which it will surge. Preventing this damaging phenomenon is one of the most important tasks of a compressor control system. The most common way to prevent surge is to recycle a portion of the flow to keep the compressor away from its surge limit. Unfortunately, such recycling extracts an economic penalty due to the cost of compressing this extra flow. So the control system must be able to determine accurately how close the compressor is to surging so that it can maintain an adequate but not excessive recycle flow rate. The integrated control and protection systems are thus extremely important to companies and industries using turbo-compressors. To insure the functioning of the compression system it is necessary to develop a theory of command and control based on physical laws. In this paper, we are going to explore the ANFIS identification method integrated to PID controller in the recycle compression system. The results of simulation show a good estimation and control of the recycle compression system.

Transesterification of Methyl Acetate with n-Propanol: Reaction Kinetics and Simulation in Reactive Distillation Process

The reaction kinetics of the transesterification of methyl acetate with n-propanol is described by pseudohomogeneous (PH) kinetic model, which can successfully correlate with the experimental data. The influence of temperature as well as catalyst loading and initial molar ratio of n-propanol to methyl acetate on this reaction is studied under the condition of eliminating both internal and external mass transfer resistances. On the basis of the PH kinetic model, the influences of design parameters are studied, such as recycle flow rate, the number of stages of the reactive distillation column (RD), and theoretical stages of the conventional distillation column. These parameters are further optimized to minimize total annual cost in the RD process.

Performance simulation of a multi-VMD desalination process including the recycle flow

In this paper, a performance evaluation of a multi-vacuum membrane distillation (VMD) module was conducted using a one-dimensional model. The mathematical model consisted of momentum, mass and energy balance equation using the water permeate flux model and heat flux model. The simulation results were in good agreement with the experimental results from previous literature. The validated VMD model was implemented into Aspen Plus. Then, a multi-VMD module with one-through flow was simulated. As a result, in the high velocity region, the hydraulic pressure was used as the constraint to determine the number of membrane modules. In the low velocity region, the number of membrane module was determined based on the feed temperature. To improve water recovery and thermal efficiency of the multi-VMD module, the recycle flow was considered and the waste heat included in the discharge brine was recovered. As a result, it was possible to achieve water recovery over 40%. In addition, as the recycle flow ratio increased, thermal efficiency also improved since heat duty and thermal consumption per unit water production decreased.

Simultaneous C and N removal from saline salmon effluents in filter reactors comprising anoxic-anaerobic-aerobic processes: Effect of recycle ratio

Salmon processing generates saline effluents with high protein load. To treat these effluents, three compact tubular filter reactors were installed and an integrated anoxic/anaerobic/aerobic process was developed with recycling flow from the reactor's exit to the inlet stream in order to save organic matter (OM) for denitrification. The reactors were aerated in the upper section with recycle ratios (RR) of 0, 2, and 10, respectively, at 30°C. A tubular reactor behave as a plug flow reactor when RR = 0, and as a mixed flow reactor when recycle increases, thus, different RR values were used to evaluate how it affects the product distribution and the global performance. Diluted salmon process effluent was prepared as substrate. Using loads of 1.0 kg COD m−3d−1 and 0.15 kg total Kjeldahl nitrogen (TKN) m−3d−1 at HRT of 2 d, 100% removal efficiencies for nitrite and nitrate were achieved in the anoxic-denitrifying section without effect of the dissolved oxygen in the recycled flow on denitrification. Removals >98% for total organic carbon (TOC) was achieved in the three reactors. The RR had no effect on the TOC removal; nevertheless a higher efficiency in total nitrogen removal in the reactor with the highest recycle ratio was observed: 94.3% for RR = 10 and 46.6% for RR = 2. Results showed that the proposed layout with an alternative distribution in a compact reactor can efficiently treat high organic carbon and nitrogen concentrations from a saline fish effluent with OM savings in denitrification.

A Novel Process Concept for the Capture of CO2 and SO2 Using a Single Solvent and Column

The presence of SO2 in flue gas has a negative impact on typical CO2 capture processes utilising aqueous amines. For countries like Australia, that do not have flue gas desulfurisation, retrofitting such equipment to existing coal fired power stations is of the order of 100's of millions of dollars. In this work a new process configuration is described in which SO2 is absorbed into a fraction of the absorbent used for CO2 capture. SO2 absorption is carried out in the bottom of the absorber column into a bleed stream from the bulk solvent, and a recycle flow allows the absorbent to be near saturation in SO2. This high concentration allows a number of low cost options for sulfur removal from the absorbent such as chemical treatment and precipitation. The principles underlying the concept, have been studied in the laboratory, and a rate based model of an absorber column has been used to model the SO2 removal column section. The laboratory and modelling results support both the feasibility and utility of the process concept.

Dynamic Modeling, Validation, and Time Scale Decomposition of an Advanced Post-combustion Amine Scrubbing Process

Post-combustion amine scrubbing is a highly integrated process that uses extensive material and energy recycling to reduce costs and increase efficiency. As a result, the process variables exhibit a fast time scale at the unit level associated with the large recycle flows and a slow time scale at the plant level associated with the small feed and product flows. A reduced order model was developed for the system, and the material balances were demonstrated to be in nonstandard singularly perturbed form. Only the stripper vapor mole fractions evolve exclusively on the slow time scale in this model form; all other model states have both slow and fast components. By applying a variable transformation, the model is arranged into standard singularly perturbed form. The slow states in this form are the overall process material hold-ups as well as the stripper vapor mole fractions. An effective control strategy for the process should control the CO2 removal rate and overall system inventory on the slow time scale using the small stripper overhead flowrate and a cascaded level setpoint controller, respectively. We show that attempting to control CO2 removal rate with the large solvent recycle rate will likely lead to an ill-conditioned controller.

Applying Milp/Heuristic Algorithms to Automated Job-Shop Scheduling Problems in Aircraft-Part Manufacturing

This work presents efficient algorithms based on Mixed-Integer Linear Programming (MILP) and heuristic strategies for complex job-shop scheduling problems raised in Automated Manufacturing Systems. The aim of this work is to find alternative a solution approach of production and transportation operations in a multi-product multi-stage production system that can be used to solve industrial-scale problems with a reasonable computational effort. The MILP model developed must take into account; heterogeneous recipes, single unit per stage, possible recycle flows, sequence-dependent free transferring times and load transfer movements in a single automated material-handling device. In addition, heuristic-based strategies are proposed to iteratively find and improve the solutions generated over time. These approaches were tested in different real-world problems arising in the surface-treatment process of metal components in the aircraft manufacturing industry.

Li-ion battery recycling and cobalt flow analysis in Japan

Batteries sometimes contain precious or toxic substances (e.g. nickel, cobalt, lead, mercury, cadmium). However, the collection and recycling rate of small batteries were low in Japan.We focus on cobalt in lithium ion (Li-ion) batteries and conduct chemical analysis, questioner survey and flow analysis in Japan.Results of chemical analysis showed that the concentration of cobalt in Li-ion batteries was around 20% regardless of the year manufactured or the manufacturer. As a result of the consumer questionnaire survey, it became clear that 70% or more of the small batteries are not being removed when small electronic products are finally disposed. The survey also revealed that recognition of the law and system for collection and recycling of small rechargeable batteries is approximately 30–40%. Substance flow analysis showed that both production and demand for Li-ion batteries (cobalt) have increased during 2002–2010. The collection rate for used Li-ion batteries was about 10% during this period; uncollected batteries were either stored or disposed through incineration and landfill as municipal solid waste.