Recycle Flow Research Articles

Model predictive control of continuous drum granulation

This paper details a methodology for the design of a model predictive controller for a continuous granulation plant. The work is based on a non-linear one-dimensional population balance model (1D-PBM), which was parameterized using experimental step test data generated at a continuous granulation pilot plant installed at the University of Queensland, Australia. The main objective was to operate the granulator under optimal conditions while off-specification material was fed back into the granulator to increase the economy of the process. The final algorithm design combines elements of model predictive control (MPC) with gain scheduling to cancel non-linearities in the recycle flow. A model directly identified from the step test data was the basis for testing a model predictive controller. Simulations show that the efficiency and robustness of this granulation process can be improved by applying the proposed control strategy. Ongoing work focuses on the implementation of the proposed control strategy on a full scale industrial plant.

Effect of Entrainer Loss on Plant-Wide Design and Control of an Isopropanol Dehydration Process

Azeotropic distillation is an important technology for separating azeotropes or minimizing the energy expenditure of separating close boiling mixtures. In heterogeneous azeotropic distillation, an entrainer is usually added, which is recovered and reused in the subsequent stripping process. Depending on the design, there may be various degrees of entrainer loss. In this work, the plant-wide design and control of an isopropanol dehydration system using cyclohexane as an entrainer via heterogeneous azeotropic distillation is conducted. When there is negligible entrainer loss, a special case of input multiplicity is found, in which a set of design specifications can be achieved by an infinite but bounded set of operating conditions. The optimal condition with the minimum reboiler duty in this bounded set is achieved when the organic recycle flow is the smallest. Due to the existence of these multiple steady states, the entrainer inventory control is self-regulatory over long time operations if a PI control is used for the level of the organic phase in the decanter. If large offsets are allowed in the entrainer inventory control, the system may exhibit a long period of pseudosteady state followed by catastrophic failure. Alternatively, one can allow a small amount of entrainer loss to the bottom of the heterogeneous azeotropic distillation column, and provide a small amount of entrainer makeup. In this case, a control loop is added to guard against process drift due to loss of entrainer. However, because continuous maintenance of entrainer inventory is difficult, an on-off control scheme of entrainer makeup feed is used.

Fluidized bed reactor for the biological treatment of ion-exchange brine containing perchlorate and nitrate

The removal of perchlorate and nitrate from contaminated drinking water using regenerable ion-exchange processes produces a high salt brine (3–10% NaCl) laden with high concentrations of perchlorate and nitrate. This bench-scale research describes the operation of acetate-fed granular activated carbon (GAC) based fluidized bed reactors (FBR) for perchlorate-only, and combined nitrate and perchlorate removal from synthetic brine (6% NaCl). The GAC was inoculated with a salt-tolerant culture developed by the authors and used previously in batch systems. An FBR was an effective design for perchlorate reduction and exhibited first-order degradation kinetics with respect to perchlorate concentrations. Nitrate was also removed by the organisms in the column and had no negative effects on the removal of perchlorate using the FBR design. However, at higher concentrations of nitrate the FBR was more difficult to operate due to loss of carbon and biomass from the formation of nitrogen bubbles and the high recycle flow rates needed.

Predicted Distributed State Effects on Enhanced Biological Phosphorus Removal in a 5‐Stage Bardenpho Wastewater Treatment Configuration

Conventional wastewater treatment simulation programs use a "lumped" approach, where process rates are calculated using bulk concentrations of biomass and microbial storage products. A recently developed distributed, or agent-based, approach, where individual bacteria are modeled to account for their potentially variable hydraulic experiences, was applied to the 5-stage Bardenpho process, a type of enhanced biological phosphorus removal (EBPR) that includes internal recycle flows, which were hypothesized to affect distributed state development. Consistent with previous results, the EBPR predicted performance was worse according to the distributed approach than the lumped approach. In addition, increasing the internal recycle rate increased the anoxic reactor nitrate concentrations, tending to decrease EBPR performance. However, in the distributed approach, differences in the state distributions in internal recycle-linked reactors decreased with increasing recycle flow, tending to improve EBPR. These phenomena tend to have simultaneous and opposite effects on EBPR. The net effect will depend largely on the specific systems and the nitrate concentration in anoxic reactors.

Improving mixing in microbioreactors

This paper describes a possible active mixing method for a 30 μ l microbioreactor that was designed, simulated and tested. Pressure based recycle flow was investigated in a cylindrical microreactor for mixing efficiency. Based on the computational fluid dynamics (CFD) simulation results and the requirements of the application, the recycle flow mixing method proved to be suitable as a method to induce sufficient mixing in the microbioreactor. This was verified experimentally using image analysis of dye distribution behavior.

Process alternatives for methyl acetate conversion using reactive distillation. 1. Hydrolysis

In a polyvinyl alcohol (PVA) plant, reaction stoichiometry indicates that equal molar of methyl acetate is generated for every mole of PVA produced. This work explores an alternative to convert methyl acetate back to acetic acid (raw materials of PVA plant), methyl acetate (MeAc) hydrolysis. The design and control of methyl acetate hydrolysis using reactive distillation is studied. Because of the small chemical equilibrium constant ( ∼ 0.013 ) and unfavorable boiling point ranking (MeAc being the lightest boiler), the reactive distillation exhibits the following characteristics: (1) total reflux operation and (2) excess reactant (water) design. The proposed flowsheet consists of one reactive distillation column with a reactive reflux drum, two separation columns, and one water-rich recycle stream. A systematic design procedure is used to generate the flowsheet based on the total annual cost (TAC). Two dominate design variables are: recycle flow rate (for the degree of excess in water) and the overhead impurity level of acetic acid in the product column (to avoid tangent pinch). Finally, the operability of the hydrolysis plant is evaluated. A plantwide control structure is developed followed by process identification and controller tuning. The results show that reasonable control performance can be achieved using simple temperature control for feed flow and feed composition disturbances.

Effect of the Internal Recycles on the Phosphorus Removal Efficiency of a WWTP

The aim of this work was to study the effects of the internal recycle flow rates on a University of Cape Town (UCT) process. The study was carried out by continuously feeding urban sewage to a UCT pilot-scale plant and by using batch tests. In the pilot-scale experiments, it was observed that the maximum nitrogen removal was obtained when both internal recycle flow rates ranged from 200 to 250% and that the maximum phosphorus removal was obtained when the aerobic-to-anoxic flow rate ranged from 100 to 150% and the anoxic-to-anaerobic recycle flow rate ranged from 75 to 100%. In order to obtain a deeper insight on the anaerobic and anoxic processes, batch tests were carried out and the results were modeled. From the results obtained in the batch test, it was concluded that the nitrate−nitrogen concentration in the anaerobic compartment should be always lower than 1 mg L-1. Taking into account these observations and the opposite effects of the internal flow rates on the nitrogen and phosphorus removals, equilibrium internal recycle flow rates were selected in the pilot-scale plant in order to obtain the best effluent characteristics. These values ranged from 100 to 150% for the aerobic-to-anoxic compartment recycle flow rate and from 75 to 100% for the anoxic-to-anaerobic compartment recycle flow rate.

Photocatalytic oxidation of organic pollutants on titania–clay composites

TiO 2/Ca-montmorillonite composites were prepared by wet grinding in an agate mill. Positively charged TiO 2 nanoparticles are bound to the surface of the negatively charged montmorillonite layers via heterocoagulation; the clay mineral is used as adsorbent and support for the photooxidation process. Aquatic solution of 0.5 mM phenol was degraded by irradiation with UV–VIS light ( λ = 250–440 and 540–590 nm) in suspensions of TiO 2–clay composites and significant photodegradation was observed at 40–60% TiO 2/Ca-montmorillonite compositions. Synergistic effect was detected at solid/liquid interface for degradation of phenol and at solid/gas interface in the recycling flow reactors for photooxidation of ethanol and toluene vapors.

On the modeling and solution algorithm for the reverse logistics recycling flow equilibrium problem

This paper presents a study that characterizes, formulates, and solves the reverse logistic recycling flow equilibrium (RLRFE) problem. The RLRFE problem is concerned with the recycling channel in which recyclable collectors, processors, landfills, and demand markets form a multi-tiered network to process the recycled material flows from sources destined either for landfills or demand markets. Motivated by a government policy making or enterprise conglomerate recycling system design and operation needs, the RLRFE problem is elaborated from a system-optimal perspective using the variational inequality (VI) approach. For each origin–destination (OD) pair, the corresponding equilibrium conditions are established as a variation of the Wardrop second principle. In light of demand and cost function interactions, a nested diagonalization solution (ND) algorithm is proposed that gradually transforms the RLRFE problem into a traffic assignment model. To address multiple landfills in the recycling network and to understand how a variable-demand problem can be analyzed as a fixed-demand problem, we propose a supernetwork representation of the RLRFE problem. A numerical analysis on a test case illustrates the model formulation and the proposed algorithm.

A novel process integration, optimization and design approach for large-scale implementation of oxy-fired coal power plants with CO 2 capture

The widespread use of fossil fuels within the current energy infrastructure is considered as the largest source of anthropogenic emissions of carbon dioxide, which is largely blamed for global warming and climate change. At the current state of development, the risks and costs of non-fossil energy alternatives, such as nuclear, biomass, solar, and wind energy, are so high that they cannot replace the entire share of fossil fuels in the near future timeframe. Additionally, any rapid change towards non-fossil energy sources, even if possible, would result in large disruptions to the existing energy supply infrastructure. As an alternative, the existing and new fossil fuel-based plants can be modified or designed to be either “capture” or “capture-ready” plants in order to reduce their emission intensity through the capture and permanent storage of carbon dioxide in geological formations. This would give the coal-fired power generation units the option to sustain their operations for longer time, while meeting the stringent environmental regulations on air pollutants and carbon emissions in years to come. Currently, there are three main approaches to capturing CO 2 from the combustion of fossil fuels, namely, pre-combustion capture, post-combustion capture, and oxy-fuel combustion. Among these technology options, oxy-fuel combustion provides an elegant approach to CO 2 capture. In this approach, by replacing air with oxygen in the combustion process, a CO 2-rich flue gas stream is produced that can be readily compressed for pipeline transport and storage. In this paper, we propose a new approach that allows air to be partially used in the oxy-fired coal power plants. In this novel approach, the air can be used to carry the coal from the mills to the boiler (similar to the conventional air-fired coal power plants), while O 2 is added to the secondary recycle flow as well as directly to the combustion zone (if needed). From a practical point of view, this approach eliminates problems with the primary recycle and also lessens concerns about the air leakage into the system. At the same time, it allows the boiler and its back-end piping to operate under slight suction; this avoids the potential danger to the plant operators and equipment due to possible exposure to hot combustion gases, CO 2 and particulates. As well, by integrating oxy-fuel system components and optimizing the overall process over a wide range of operating conditions, an optimum or near-optimum design can be achieved that is both cost-effective and practical for large-scale implementation of oxy-fired coal power plants.

Effects of Operational Variables on Nitrogen Removal Performances and Its Control in a Pre‐Denitrification Plant

AbstractThe aim of this work is to study the effects of six operational variables, i.e., dissolved oxygen (DO), nitrate recirculation flow, sludge recycle flow, sludge wastage flow, external carbon dosage, and anoxic volume fraction, on the performance of nitrogen removal and its control in a pre‐denitrification plant. The results obtained show that the six operational variables have a significant influence on nitrogen removal in such a system, while the utilization of the control strategies can improve the situation to a significant extent. The control of DO concentration should be correlated with the influent ammonia load, the effluent requirement and nitrification type. The anoxic effluent nitrate concentration should be controlled at ca. 2 mg/L or the ORP value at the end of the anoxic zone should be controlled at ca. –90 mV. The control of the sludge recycling flow by online monitoring of the sludge blanket height (SBH), is an alternative to the conventional control of the constant sludge recycle flow. It may be possible to achieve the automatic control of sludge wastage flow by online measuring of the ammonia concentration and the nitrification capacity of the sludge. The recirculation of nitrate and external carbon dosage should be simultaneously controlled to optimize nitrogen removal. The anoxic volume fraction should also be optimized, to ensure a good balance between nitrification and denitrification.

CONTROL STRUCTURE DESIGN FOR A REACTOR/SEPARATOR PROCESS WITH TWO RECYCLES

A hierarchical controller is designed for a reactor/separator system which consists of a CSTR and two distillation columns with two material recycles. In designing the lower level regulatory control layer, the overall system is divided into subunits, for which regulatory control for inventories and product compositions is designed relatively independently. The higher level coordination controller is designed so that the steady state performance is near-optimal. The coordination controller manipulates some of the interconnecting flows between the subunits, namely the two recycle flows, looking at the separator loads. Control performance is demonstrated through simulations.

COD AND VFA'S CONTROL IN A TWO-PHASE ANAEROBIC DIGESTION PROCESS

In this paper, a two-phase anaerobic digestion system is considered to control the outlet concentration of both the Volatile Fatty Acids (VFA's) and the organic matter (other than the VFA's) characterized by its Chemical Oxygen Demand. The two-phase system consists of two fixed bed reactors whose dynamics are described by Partial Differential Equations. The recycle flow rates in each reactor are used as the manipulated variables to reach the control objective. The control laws designed here are decoupled and they are tested by means of simulation runs obtaining satisfactory results in spite of system input disturbances and changes on the set points.

Cell Separator Operation within Temperature Ranges To Minimize Effects on Chinese Hamster Ovary Cell Perfusion Culture

A cell retention device that provides reliable high-separation efficiency with minimal negative effects on the cell culture is essential for robust perfusion culture processes. External separation devices generally expose cells to periodic variations in temperature, most commonly temperatures below 37 degrees C, while the cells are outside the bioreactor. To examine this phenomenon, aliquots of approximately 5% of a CHO cell culture were exposed to 60 s cyclic variations of temperature simulating an acoustic separator environment. It was found that, for average exposure temperatures between 31.5 and 38.5 degrees C, there were no significant impacts on the rates of growth, glucose consumption, or t-PA production, defining an acceptable range of operating temperatures. These results were subsequently confirmed in perfusion culture experiments for average exposure temperatures between 31.6 and 38.1 degrees C. A 2(5-1) central composite factorial design experiment was then performed to systematically evaluate the effects of different operating variables on the inlet and outlet temperatures of a 10L acoustic separator. The power input, ambient temperature, as well as the perfusion and recycle flow rates significantly influenced the temperature, while the cell concentration did not. An empirical model was developed that predicted the temperature changes between the inlet and the outlet of the acoustic separator within +/-0.5 degrees C. A series of perfusion experiments determined the ranges of the significant operational settings that maintained the acoustic separator inlet and outlet temperatures within the acceptable range. For example, these objectives were always met by using the manufacturer-recommended operational settings as long as the recirculation flow rate was maintained above 15 L day(-1) and the ambient temperature was near 22 degrees C.

Concentration of red wine by nanofiltration

Red wine contains many compounds which have a beneficial effect on human health. The aims of this study were to concentrate the valuable components of wine by nanofiltration, to describe the mathematical model of the permeate flux and to estimate the cost of the process. Laboratory-size equipment with a nanofiltration membrane was used for the concentration. The experiments were carried out at constant pressure, different temperatures and different recycle flow rates. The concentration of compounds was analyzed in the permeate and retentate including total acid, total extract, alcohol, sugar, free sulphurous acid, total sulphurous acid, sugarless extract content and volatile acid content. A mathematical modelling and cost analysis of the process were also performed.

Minimizing the entropy production in a chemical process for dehydrogenation of propane

We minimize the total entropy production of a process designed for dehydrogenation of propane . The process consists of 21 units, including a plug-flow reactor, a partial condenser, two tray distillation columns and a handful of heat exchangers and compressors. The units were modeled in a manner that made them relatively insensitive to changes in the molar flow rates , to make the optimization more flexible. The operating conditions, as well as to some degree the design of selected units, which minimized the total entropy production of the process, were found. The most important variables were the amount of recycled propane and propylene , conversion and selectivity in the reactor, as well as the number of tubes in the reactor. The optimal conversion, selectivity and recycle flows were results of a very clear trade-off among the entropy produced in the reactor, the partial condenser and the two distillation columns. Although several simplifying assumptions were made for computational reasons, this shows for the first time that it is also meaningful to use the entropy production as an objective function in chemical engineering process optimization studies.

Superstructure of Alternative Configurations of the Multistage Flash Desalination Process

This paper addresses the optimal synthesis and design of multistage flash (MSF) evaporator systems. A detailed nonlinear programming (NLP) model based on a superstructure developed previously by Mussati et al. [Ind. Eng. Chem. Res. 2003, 42, 4828-4829] has been appropriately reformulated to include more alternative configurations for the process. The new superstructure includes the number of stages and the stream flow patterns (distillate, feed and discharge brine, extraction points, and recycle flow patterns). Therefore, the superstructure simultaneously embeds an enormous number of alternatives, including, of course, the three commonly operating modes for the evaporator: MSF-BR, MSF-M, and MSF-OT systems and their combinations, which, to date, have not been analyzed systematically. In addition, a rigorous mathematical model by stage and component, which also includes the geometric design of each stage (length, height, and width), number of tubes in the preheaters, fluid-dynamic equations for the streams among others, is applied. An attractive configuration for the MSF system resulted from the proposed superstructure. This structure differs from the conventional structure, because it considers distillate extractions, two recycle streams, and new allocations for the cooling seawater, blow-down brine, and recycle. The mathematical model and the solution procedure have been implemented in GAMS [Brooke et al., CAMS: A User's Guide; Scientific Press: 1992]. Two study cases are presented, to illustrate the model and solution procedure capabilities. A complete description of the novel configuration, detailed comparison between different sub-optimal structures, and a sensitivity analysis on the main process variables are summarized.

A Hierarchical Controller Design for a Reactor/Separator System with Recycle

A hierarchical control structure that comprises regulatory and coordination control layers is designed for a reactor/separator process with recycle. At the regulatory control layer, the process is divided into subunits on the basis of the controlled group unit approach. In defining inventory controls for the subunits, Luyben's rule (fixing one of the flows on the recycle path) is employed to reduce dynamic interaction between subunits. The higher level coordination control layer is designed on the basis of the time-scale separation principle, which slowly manipulates the recycle flow between the subunits. A control structure that results in a wider operation range and small dynamic interaction is proposed, and its control performance is compared with that of a conventional control structure through simulations.

Effect of the Minimum Flux Condition in the Settler on the Nonlinear Behavior of the Activated Sludge Process

In this work, we analyze the nonlinear behavior of an activated sludge process (ASP). In an ASP, the wastewater stream is fed to an aerator which sustains an aerobic biochemical reaction. The effluent from the aerator is taken to a downstream settler. The overflow from the settler is clear and is withdrawn from the system while the underflow which is rich in biomass is recycled. The aerator and the settler are coupled through this recycle stream. The accumulation of the biomass in the system is prevented by periodic sludge withdrawal from the system and control of the concentration of the suspension in the aerator. Earlier efforts have considered the settler to be perfect and have considered the recycle ratio (ratio of recycle flow to fresh feed flow) “R” as a parameter. Our approach focuses on capturing the essential physics of the interaction between the aerator and the settler and views R as a state variable. In this work, we propose a simple model of the separator incorporating a minimum flux or the limiting flux condition in the model of the settler. This uniquely determines the recycle rate in contrast to earlier studies where the recycle rate was fixed as a parameter. We compare two modes of sludge withdrawal (i) from the aerator and (ii) from the settler. The nonlinear behavior of this system is analyzed, and we discuss the new features which arise due to the incorporation of the limiting flux condition. The system of equations governing the system behavior is a differential algebraic system. For the scheme of purging from the aerator, we observed that the recycle ratio is very sensitive to the variation of the sludge withdrawal fraction. The system exhibits isolas, which develop unstable steady-state branches giving complex behavior. Here, for low values of the sludge withdrawal fraction, the system has two steady-state branches and there is a region of no steady states in between. For high values of the sludge withdrawal fraction, it is shown that the system can admit as many as six steady states.

Control of sludge height in a secondary settler using fuzzy algorithms

During the last years, the efficiency of pollution removal and sludge treatment processes are major concerns because of the more restrictive legislation related to nutrient discharge. A key point in the field of activated sludge wastewater treatment plants (WWTPs) is the secondary settler efficiency. In this paper, the COST 624 Simulation Benchmark (model and data) is used to develop a fuzzy controller of the sludge height in the secondary settler. The presented control strategy is based on simple on-line data (influent, removal and recycle flows) and daily analytical values of the sludge volume index (SVI) allowing the fuzzy algorithm to reduce sludge height variations and thus to increase the settling process efficiency. The developed controller has then been adapted and applied to the Cassà de la Selva activated sludge WWTP (Spain) model, with satisfying results.