Series Of Continuous Stirred Tank Reactors Research Articles

A Numerical Investigation Concerning the Effect of Step-Feeding on Performance of Constructed Wetlands Operating under Mediterranean Conditions

The effects of wastewater step-feeding (SF) on the performance of horizontal subsurface flow constructed wetlands (HSF CWs) are numerically investigated. The purpose is to check if this alternative feeding technique increases the ability of HSF CWs to remove pollutants. Two methodologies are used: Initially, the tanks-in-series (TIS) methodology, based on the finite volume method (FVM), is analyzed using the volumetric degradation coefficient λ. In this case, the operation of a CW is similar to a series of continuous stirred-tank reactors (CSTRs) operating under steady conditions. Then, the step-feeding (SF) procedure is presented, in which the CW is operated like a plug flow reactor (PFR). For the numerical investigation, the available experimental data for five existing HSF CWs are used. The results show that SF does not improve the performance of HSF CWs in removing biochemical oxygen demand (BOD) operating under Mediterranean conditions. When the HSF CWs operate without the SF procedure, the performance is between 55 and 81% for the TIS method and 60 and 89% for the PFR method, while the ability of the CW tank to remove the BOD decreases and varies from 48 to 79% (TIS) and from 54 to 86% (PFR), respectively.

Using Particle Residence Time Distributions as an Experimental Approach for Evaluating the Performance of Different Designs for a Pilot-Scale Spray Dryer

The performances of four different designs for a pilot-scale spray dryer have been evaluated and compared based on experimentally measured particle residence time distributions (RTD), recovery rates and physical properties of spray-dried fresh skim milk. The RTDs have been measured using a dye pulse injection method, and the measurements have been fitted to models using continuous stirred-tank reactors in series (CSTR-TIS) for quantitative performance evaluation and comparison. Conical drying chambers and a box connection design have been used in the latest dryer design to reduce the amount of wall deposition and provide a smoother gas flow pattern. The particle-to-gas mean residence time ratio for the latest design is significantly closer to unity (1.6 s/s to 1.0 s/s) compared with earlier designs (2.6 s/s to 1.5 s/s). The latest design has a wider spread of RTD (n = 5–8) compared with earlier designs (n = 13–18), which may be linked to the recirculation zone in the box connection. Although the latest design has a wider spread of RTD, the conical design has shown promising results compared with a cylindrical drying chamber in terms of overall wall deposition behaviours.

Modeling and Experimental Validation of Continuous Biocatalytic Oxidation in Two Continuous Stirred Tank Reactors in Series

The rate of oxygen transfer from the gas phase to the liquid phase is a critical process parameter for biocatalytic oxidations due to the poor water solubility of molecular oxygen and the low oxygen affinity of many of the relevant enzymes, such as oxidases. In gas–liquid systems, mechanical mixing can be used to increase the interfacial area available for mass transfer and thereby increase the volumetric mass transfer coefficient (kLa). As such, the operation of these reactions in a continuous stirred tank reactor (CSTR) may allow for better performance in a readily scalable way. Even so, achieving a high substrate conversion in a single reactor would require operation at high pressure, to improve the solubility of oxygen, as well as a high enzyme concentration. An alternative and more cost-effective means of improving the substrate conversion might be to operate a series of multiple CSTRs. As such, the oxidation of glucose to gluconic acid by glucose oxidase, coupled with catalase, was modeled in a series of identical well-mixed reactors. It was found that achieving full conversion would require an impractical number of reactors at atmospheric pressure. However, the overall conversion of the reaction could be doubled by simply using two CSTRs in series. Subsequently, experiments were carried out to validate this, and the results showed that the overall conversion was in fact tripled. This likely resulted from a higher kLa in the second reactor, which was potentially caused by the change in the media composition from the first reactor.

Pilot scale continuous reactor for water treatment by electrochemical advanced oxidation processes: Development of a new hydrodynamic/reactive combined model

The development of continuous flow electrochemical reactors is required to overcome the limitations of conventional batch reactors for treatment of large flows of effluents. Therefore, the objective of this study was to develop and characterize a new pilot-scale reactor using BDD anode and carbon felt cathode operating in continuous mode. First, a “Design of Experiment” analysis was performed in order to identify the most critical operating parameters for the percentage of mineralization of 29.8 mg L−1 hydrochlorothiazide (HCT) solution. The liquid flow rate has been identified as the most critical parameter together with the configuration of the reactor (number of electrodes, distance between electrodes). Moreover the designed reactor was able to reach very high percentage of mineralization (97%) for a mean residence time of 83 min. To better understand the important role of the flow rate and the configuration, a hydrodynamic study was then performed. “Residence Time Distribution” curves were obtained and fitted well with the continuous-stirred tank reactor in series with dead zones (CSTR-DZ) model. The 28-electrodes configuration had a lower dead volume fraction whatever the liquid flow rate applied. By increasing the liquid flow rate the hydrodynamic behavior tends more to a plug flow reactor. Finally, a new mathematical model for the mineralization of HCT solution was proposed by combining mineralization kinetic with hydrodynamic CSTR-DZ model. This model was then compared to experimental data and the model was able to capture experimental trends. This approach opens up interesting perspectives for a successful scale-up for continuous electrochemical reactors.

Detailed modeling and advanced control for chemical disinfection of secondary effluent wastewater by peracetic acid

Advanced control of chemical disinfection processes is becoming increasingly important in view of balancing under-treatment (low pathogen inactivation) and over-treatment (excessive consumption of disinfectant and disinfection byproducts formation) thereby providing considerable environmental and economic benefits. Conventional control strategies such as flow pacing or residual trim ignore chemical demand/decay, inactivation kinetics, and other factors governing disinfection performance in continuous-flow reactors such as reactor hydraulics and process variability. This study presents the development, verification, and pilot-scale validation of a novel CT-based real-time disinfection control strategy, derived from first principles, and applied to peracetic acid disinfection of municipal secondary effluent wastewater. Validation experiments were carried out using a 3-m3 pilot contact basin of which the hydraulic performance was first characterized by means of tracer tests and then mathematically modeled using the well-established theoretical framework of continuous stirred-tank reactors in series. The analytical model describing hydraulic performance was subsequently extended to take into account disinfectant demand/decay and microbial inactivation kinetics. The integrated model was successfully used to predict, and control, residual peracetic acid as well as microbial concentration in the pilot effluent. Validation studies conclusively supported that the novel CT-based control strategy was superior in maintaining constant disinfection performance, desired microbial counts, and low residual disinfectant under variable flow and wastewater quality. When compared with flow pacing, the CT-based control required two times less the amount of chemical for the same treatment objective (<100 cfu/100 mL). Remarkably, the CT-based control strategy could be extended to other chemical disinfection processes such as chlorination and ozonation, alone or in combination with physical treatment technologies such as membranes and ultraviolet irradiation.

A Dynamic Simulator for Slurry‐Phase Catalytic Olefin Copolymerization in a Series of CSTRs: Prediction of Distributed Molecular and Rheological Properties

AbstractA comprehensive mathematical model is developed to simulate the dynamic Ziegler–Natta ethylene‐α‐olefins copolymerization in a series of slurry‐phase continuous stirred tank reactors. A generalized multisite kinetic mechanism is considered to describe the molecular and compositional developments (joint molecular weight‐copolymer composition distribution) in a series of continuous stirred tank reactors (CSTRs). Dynamic macroscopic mole balances are derived to calculate the dynamic evolution of all species concentrations in three phases (i.e., gas, liquid, and polymer) of the multiphase system. The polymer molecular properties (i.e., molecular weight distribution, short chain branching distribution, etc.) are determined by two different approaches, namely, the method of double moments and a Monte Carlo algorithm. Detailed thermodynamic calculations are carried out to calculate the solubility of all species in the various phases. Particular emphasis is given to the calculation of mass transfer rates from the gas to the continuous liquid phase and from the liquid to the polymer swollen phase. Comparison of model predictions with experimental measurements has shown that the present plant simulator is capable of simulating the dynamic operation of ethylene with α‐olefins copolymerization over a broad range of operating conditions and, thus, can be used in future process and product optimization and control of lab‐, pilot‐, and industrial‐scale plants.

Characterization of a continuous agitated cell reactor for oxygen dependent biocatalysis.

Biocatalytic oxidation reactions employing molecular oxygen as the electron acceptor are difficult to conduct in a continuous flow reactor because of the requirement for high oxygen transfer rates. In this paper, the oxidation of glucose to glucono-1,5-lactone by glucose oxidase was used as a model reaction to study a novel continuous agitated cell reactor (ACR). The ACR consists of ten cells interconnected by small channels. An agitator is placed in each cell, which mixes the content of the cell when the reactor body is shaken by lateral movement. Based on tracer experiments, a hydrodynamic model for the ACR was developed. The model consisted of ten tanks-in-series with back-mixing occurring within and between each cell. The back-mixing was a necessary addition to the model in order to explain the observed phenomenon that the ACR behaved as two continuous stirred tank reactors (CSTRs) at low flow rates, while it at high flow rates behaved as the expected ten CSTRs in series. The performance of the ACR was evaluated by comparing the steady state conversion at varying residence times with the conversion observed in a stirred batch reactor of comparable size. It was found that the ACR could more than double the overall reaction rate, which was solely due to an increased oxygen transfer rate in the ACR caused by the intense mixing as a result of the spring agitators. The volumetric oxygen transfer coefficient, kL a, was estimated to be 344 h-1 in the 100 mL ACR, opposed to only 104 h-1 in a batch reactor of comparable working volume. Interestingly, the large deviation from plug flow behavior seen in the tracer experiments was found to have little influence on the conversion in the ACR, since both a plug flow reactor (PFR) model and the backflow cell model described the data sufficiently well. Biotechnol. Bioeng. 2017;114: 1222-1230. © 2017 Wiley Periodicals, Inc.

The Spatial-Temporal Characteristic of Ideal Reactors and Their Inherent Relations

On the basis of spatial-temporal characteristic of ideal reactors, the inherent relations among the batch reactor, the plug flow reactor (PFR), and the continuous stirred tank reactor (CSTR) were discussed. Both the F and E functions of the recycle PFR reactor were derived. The role of CSTR in series or the recycle PFR as a bridge or a link among the ideal reactors was addressed. Based on the discussion, the relations among the ideal reactors were briefly summarized.

Performance of continuous stirred-tank reactors connected in series as a photocatalytic reactor system

Film-diffusion resistance significantly reduces the decomposition of organic compounds over an immobilized photocatalyst in the vicinity. To overcome this problem, the performance of a photocatalytic system consisting of five continuous stirred-tank reactors (CSTRs) in series was investigated. Each reactor was a square-shaped container with a glass plate immobilized with a film of TiO2 photocatalyst at the bottom. Two 4-W blacklight blue-fluorescent lamps fixed above the container provided UV light. A 10-ppm aqueous solution of 2,4-dinitrophenol (DNP) supplied continuously to the reactor was stirred by an impellor fixed to a shaft with a wind propeller at its upper end. The change in the DNP concentration in each reactor with increasing flow rate agreed with the result calculated by a mathematical model, constructed for the photocatalytic system, which included a mass-transfer coefficient expressed in terms of rotational speed. Hence, this model was used to simulate the performance of the reactor system. At sufficiently high rotational speeds, the conversion of DNP at the exit of the fifth CSTR was substantially higher than that at the exit of a single CSTR with the same liquid volume, and was close to the calculated value for a plug-flow reactor without film-diffusion resistance. At low rotational speeds, however, the DNP concentration remained remarkably high because of large film-diffusion resistance, demonstrating that the photocatalytic reaction requires sufficient liquid mixing to enhance the reaction rate. Thus, the system of five CSTRs in series is effective as a photocatalytic reactor system.

Model-Based Reactor Design in Free-Radical Polymerization with Simultaneous Long-Chain Branching and Scission

Polymers are the products of processes and their microstructure can be changed significantly by the reactor systems employed, especially for nonlinear polymers. The Monte Carlo simulation technique, based on the random sampling technique, is used to explore the effect of reactor types on the branched polymer structure, formed through free-radical polymerization with simultaneous long-chain branching and scission, as in the case of low-density polyethylene synthesis. As a simplified model for a tower-type multi-zone reactor, a series of continuous stirred-tank reactors, consisting of one big tank and the same N-1 small tanks is considered theoretically. By simply changing the tank arrangement, various types of branched polymers, from star-like globular structure to a more randomly branched structure, can be obtained, while keeping the following properties of the final products, the monomer conversion to polymer, the average branching and scission densities, and the relationship between the mean-square radius of gyration and molecular weight.

Process intensification of VAc–VeoVa10 latex production

Process intensification of vinyl acetate–VeoVa10 latexes production is investigated by synthesizing commercial-like 57wt% solids waterborne dispersions stabilized with poly(vinyl alcohol) in 2 CSTRs in series. The effect of several process variables (temperature, initiator concentration and emulsifier distribution along continuous stirred tank reactors (CSTRs)) on monomer conversion and polymer microstructure was analyzed. A mathematical model that considers the segregation of the polymer among particles and the effect of the residence time distribution was developed and a good agreement between the experimental data and model predictions was achieved.

Optimal design of multi-stage bioreactors performing wastewater treatment using the MATLAB optimisation function (fmincon)

Optimum design of continuous stirred tank reactors (CSTRs) in series to biodegrade organics in wastewater was carried out. It was assumed that cell growth kinetics follows Contois model with endogenous decay. The optimal design was based on the minimum overall reactors volume required for a certain degree of substrate removal. The effect of operating parameters such as substrate inlet feed concentration to the first reactor, substrate removal efficiency, and number of CSTRs in series on the optimum design were investigated. The non-linear constrained optimisation problem was solved using the MATLAB function fmincon . For equal CSTRs, the system of non-linear equations was solved by ‘fsolve’ MATLAB function. Results indicate that the optimum multi-stage bioreactors are beneficial only at high substrate conversion. The substrate concentration in the feed to the first reactor has little effect on the total reactors volume. Recycle of biomass to the first reactor reduces the required total reactors volume. In general, at high percentage of substrate removal the optimum reactors design requires smaller volume compared to equal-size reactors.

Process modeling and thermodynamic analysis of Lurgi fixed-bed coal gasifier in an SNG plant

This paper presents a comprehensive steady state kinetic model of a commercial-scale pressurized Lurgi fixed-bed dry bottom coal gasifier. The model is developed using the simulator Aspen Plus. Five sequential modules: drying zone, pyrolysis zone, gasification zone, combustion zone and overall heat recovery unit, are considered in the main process model. A non-linear programming (NLP) model is employed to estimate the pyrolysis products, which include char, coal gas and high-weight hydrocarbons/distillable liquids (tar, phenol, naphtha and oil). To accelerate solution convergence, an external FORTRAN subroutine is used to simulate the kinetics of the combustion and gasification processes which are formulated in terms of a series of continuous stirred-tank reactors. The model is validated with industrial data. The effects of two key operating parameters, namely oxygen/coal mass ratio and steam/coal mass ratio, on the thermodynamic efficiencies of the Lurgi gasifier and the gasification system as a whole are investigated via extensive simulation studies.

Tracer experiment and RTD analysis of DAF separator with bar-type baffles

This paper describes the development of a new dissolved air flotation (DAF) separator with a flow streamlining baffle to improve solid separation efficiency. The analysis of the RTD (residence time distribution) curves indicated that the parameter θ(10) (dimensionless time at which 10% of tracer has discharged) increased from 0.38 for control reactor to 0.54 for the test reactor, suggesting significant reduction in short circuit flow. The RTD curves were also used to develop a compartment model for white water (rich in micro-bubbles and water flow is turbulent) and clear water (little or no air content and water flow is quiescent) zones in the reactor using a series of CSTR (continuous stirred tank reactors) and plug flow regime respectively. The proportion of the volume occupied by the white water zone was different in control and test configurations. In the test reactor, the fraction of the clear water zone was found to increase from 6 to 37%, resulting in improvement of the suspended solid (SS) removal efficiency from 97 to 99%.

The influence of the degree of back-mixing on hydrogen production in an anaerobic fixed-bed reactor

This paper reports on results obtained from experiments carried out in an acidogenic anaerobic reactor aiming at the optimization of hydrogen production by altering the degree of back-mixing. It was hypothesized that there is an optimum operating point that maximizes the hydrogen yield. Experiments were performed in a packed-bed bioreactor by covering a broad range of recycle ratios (R) and the optimum point was obtained for an R value of 0.6. In this operating condition the reactor behaved as 8 continuous stirred-tank reactors in series and the maximum yield was 4.22 mol H2 mol sucrose−1. Such optimum point was estimated by deriving a polynomial function fitted to experimental data and it was obtained as the conjugation of three factors related to the various degrees of back-mixing applied to the reactor: mass transfer from the bulk liquid to the biocatalyst, liquid-to-gas mass transfer and the kinetic behavior of irreversible reactions in series.

Continuous controlled radical polymerization of methyl acrylate with copper wire in a CSTR

The controlled radical polymerization of methyl acrylate in DMSO mediated by copper wire in a continuous stirred tank reactor (CSTR) was successfully demonstrated. Copper wire proved to be an easy to handle and reusable catalyst source, mediating chain growth through the SET-LRP mechanism. Polymerizations were conducted at 30 °C for three different residence times at varying copper surface area and ligand concentration. Doubling the available copper surface area led to an approximately 30% increase in polymerization rate at the same residence time, in agreement with published literature. Experiments conducted at low ligand concentration showed only a slight drop in polymerization rate with no adverse effect on molecular weight control. Chain extensions were conducted using methyl acrylate to simulate a second polymer block; no significant dead chain fraction was observed, indicating that the polymer produced retained living characteristics. The life time of copper wire as a catalyst source was estimated from experimental data on copper consumption rates. Deviation from expected polydispersities and the effects of utilizing two CSTRs in series on the molecular weight distribution are discussed.

Globally optimal reactor network synthesis via the combination of linear programming and stochastic optimization approach

Reactor network synthesis based on superstructure optimization is often a complex non-linear programming problem (NLP), which is very difficult to solve by means of the traditional optimization approaches. To solve this problem, a double-level new optimization method which combines linear programming and stochastic optimization approach is proposed in this paper. In addition, a superstructure network that includes continuous stirred-tank reactor (CSTR) and plug flow reactor (PFR) which is approximated as a series of CSTRs is constructed. By the proposed method and the superstructure network, the NLP is divided into a linear programming in flow rate and reactor volume space, and a stochastic optimization problem in concentration space. We solve two cases to illustrate the feasibility of the proposed method. The results show that this new optimization method can reduce the scales and difficulties of the problem and give more suitable structure of the reactor network, as well as better reactor size than those reported in the literature.

Transesterification of vegetable oils: Simulating the replacement of batch reactors with continuous reactors

A kinetic model was employed to represent biodiesel production via transesterification of vegetable oils. Reaction rate constants found in the open literature were used in order to compare the behavior of batch and continuous processes. A single continuous stirred tank reactor (CSTR) under the usual operation conditions was not capable of achieving the same productivity as a batch process. However, when reactors in series were used, the continuous process presented a behavior similar to batch processes. As a result, it was evidenced that a series of CSTRs can be an industrially feasible choice for replacing batch transesterification reactors in large scale biodiesel plants. Further, it was shown that the loss in productivity caused by changing from batch to continuous process can be compensated by means of using higher catalyst concentrations.

Analysis of conversion and operation strategies for enzymatic hydrolysis of lignocellulosic biomass in a series of CSTRs with distributed feeding

Design considerations for enzymatic hydrolysis of lignocellulosic biomass in two and three continuous stirred tank reactors (CSTRs) in series with distributed feeding of substrate and enzyme, followed by a series of CSTRs, are discussed. A previously developed, fitted, and validated kinetic model is extended to accommodate distributed feeding and used along with the micromixing limiting situations of macrofluid and microfluid to describe the reaction system. The capabilities of the reaction system proposed are explored for a range of cumulative substrate concentration from 5 to 20% w/w (dry basis). Continuous distributed feeding does not show advantages in terms of cellulose conversion when compared with the operation where an equivalent mass of substrate is added at the first reactor of the series, but the potential to increase substrate concentration beyond the concentrations that can be handled in conventional CSTRs, and therefore, the volumetric productivity of reactors, is evident.

MWD design in a series of CSTRs with living polymerisation reactions

In previous papers of this work methodologies have been established i) to predict the molecular weight distribution (MWD) of living polymerisation processes in various reactor configurations, ii) and - in a reversed calculation process - to design reactor parameters and feed profiles for a single CSTR or a tubular reactor to meet a target MWD. In this paper a model of a series of continuous stirred tank reactors (CSTRs) with various initiator and monomer feed strategies have been used to establish i) the possible MWD shapes with constant feed, ii) the MWD with the lowest possible polydispersity index, and iii) a methodology to design multimodal MWDs with a continuous steady state process. Both the MWD prediction and the design methodologies use a simplified, very fast, direct algorithm, well suited for control purposes.