Flow Rate Disturbances Research Articles

Saddle-focus bifurcation and chaotic behavior of a continuous stirred tank reactor using PI control

The saddle-focus bifurcation and chaotic behavior of a continuous stirred tank reactor (CSTR) with two PI controllers (PI1 and PI2) to produce propylene glycol is analyzed in this paper. First, a simplified reactor model is used to show that disturbances in the inlet flow rate and variation of the parameters in the PI2 controller may give rise to a homoclinic orbit. Then, a complete reactor model is considered, for which the analysis of the saddle-focus bifurcation around the homoclinic orbit demonstrates the presence of chaotic dynamics. The chaotic behavior is corroborated from the sensitive dependence, Lyapunov exponents, power spectral density, Poincaré sections and bifurcation diagrams. Once the chaotic dynamics has been reached, it is possible to modify the strange attractor by changing the proportional PI2 constant, which in turn allows improving the reactor performance in chaotic regime. The results of the analytical calculations are verified throughout full numerical simulations.

Design and Control of Distillation System for Methylal/Methanol Separation. Part 1: Extractive Distillation Using DMF as an Entrainer

A method for methylal/methanol separation using extractive distillation with N,N-dimethylformamide as an entrainer is presented. Rigorous steady state and dynamic simulations for this process are implemented on commercial simulators (Aspen Plus and Aspen Dynamics). On the basis of global economic optimization, a design with optimized operation conditions for this process is developed. For dynamic simulations, feed flow rate and feed composition disturbances are used to evaluate the dynamic performance of several control structures. The dynamic simulation results reveal that the control structure with fixed reflux ratio can handle all disturbances well, except one kind of composition disturbance. An improved control structure with R/F (the reflux flow rate/the feed flow rate) ratio scheme is used to maintain the two products' purity requirements. Dynamic simulation results reveal that this control structure can handle feed flow rate and composition disturbances quite effective.

Remixing Control for Divided-Wall Columns

The divided-wall column is an energy-saving promising technology for the separation of ternary mixtures, but control is difficult because of its complexity structure and interactions of control loops. Although a few control methods have been reported, the general control method for all ternary mixtures is still pending because of the difference of relative volatilities. In this work, we explore a new control structure by control the remixing of the intermediate at the top trays in the prefractionator section for the separation of benzene, toluene, and o-xylene. It is found that avoiding the remixing of toluene helps the divided-wall column work very close to the optimal conditions. The remixing of toluene is studied at steady state first. Then, a remixing control structure with one remixing control loop and three composition control loops is presented. Minimized energy consumption is achieved by avoiding the remixing of toluene at the top of the prefractionator. Disturbances in feed flow rate and feed com...

The Impact on Drug Mass Flow Rate of Interrupting and Resuming Carrier Fluid Flow

Stopping and resuming carrier fluid flow can lead to potentially dangerous transient disturbances in drug mass flow rate. We compared the impact of 2 infusion sets, one with very low dead-space volume and the other with greater dead-space volume, on the amount of drug delivered during stop-and-go carrier fluid flows. Two infusion sets, both with antireflux, connected to an angiocatheter and with dead-space volumes of 6.185 mL and 0.071 mL, respectively, were assessed. Two protocols were studied: carrier fluid flow of 90 mL/h associated with noradrenaline infused at 7 mL/h and carrier fluid flow of 350 mL/h with a noradrenaline infusion flow of 65 mL/h. During both protocols, the carrier fluid was stopped and resumed at the same rate 30 minutes later. Effluent noradrenaline concentration was measured using UV spectrophotometry. Flow change efficiency was calculated from the ratio of the area under the experimental mass flow rate curve to the area under the theoretical instantaneous mass flow rate curve. For both flow rate conditions, flow change efficiency was significantly different for the 2 infusion sets during the 10-minute period after stopping carrier fluid flow and the 10-minute period after restarting it. The major phenomena were sudden decreases in drug delivery after stopping carrier flow and sudden, temporary increases when it was resumed. The very low dead-space volume infusion set resulted in significant reduction in changes in drug delivery compared with the standard set, even at high flow rates. The use of a very low dead-space volume set attenuates disturbances in drug delivery caused by interrupting and resuming carrier fluid flow.

Research on Nonlinear Dynamic Characteristics of Fluid-Conveying Pipes System

Lateral vibration equations of fluid-conveying pipes system are high order partial differential equations, and the analytic solution is difficult to obtain, so in this paper the numerical solution is obtained by the finite element method. Firstly, the finite element equations of lateral vibration of fluid-conveying pipes were set up, and four kinds of boundary constraints were proposed. The modal analysis of vibration system was carried out by using mode decomposition method, and the system responses were solved by using Newmark method. The impact of pipe span, flow rate, fluid pressure, flow rate disturbance and fluid pressure disturbance on the modes and responses of vibration system were studied. The research results provided theoretical support for the vibration reduction research of fluid-conveying pipes.

A Computational Study of the Flow Through a Vitreous Cutter

Vitrectomy is an ophthalmic microsurgical procedure that removes part or all of the vitreous humor from the eye. The procedure uses a vitreous cutter consisting of a narrow shaft with a small orifice at the end through which the humor is aspirated by an applied suction. An internal guillotine oscillates back and forth across the orifice to alter the local shear response of the humor. In this work, a computational study of the flow in a vitreous cutter is conducted in order to gain better understanding of the vitreous behavior and provide guidelines for a new vitreous cutter design. The flow of a Newtonian surrogate of vitreous in a two-dimensional analog geometry is investigated using a finite difference-based immersed boundary method with an algebraically formulated fractional-step method. A series of numerical experiments is performed to evaluate the impact of cutting rate, aspiration pressure, and opening/closing transition on the vitreous cutter flow rate and transorifice pressure variation during vitrectomy. The mean flow rate is observed to increase approximately linearly with aspiration pressure and also increase nearly linearly with duty cycle. A study of time-varying flow rate, velocity field, and vorticity illuminates the flow behavior during each phase of the cutting cycle and shows that the opening/closing transition plays a key role in improving the vitreous cutter's efficacy and minimizing the potential damage to surrounding tissue. The numerical results show similar trend in flow rate as previous in vitro experiments using water and balanced saline solution and also demonstrate that high duty cycle and slow opening/closing phases lead to high flow rate and minor disturbance to the eye during vitrectomy, which are the design requirements of an ideal vitreous cutter.

A Controllable Membrane-Type Humidifier for Fuel Cell Applications—Part II: Controller Design, Analysis and Implementation

A membrane-based gas humidification apparatus was employed to actively manage the amount of water vapor entrained in the reactant gas supplied to a fuel cell stack. The humidification system utilizes a gas bypass and a series of heaters to achieve accurate and fast humidity and temperature control. A change in fuel cell load induces a reactant mass flow rate disturbance to this humidification system. If not well regulated, this disturbance creates undesirable condensation and evaporation dynamics, both to the humidification system and the fuel cell stack. Therefore, controllers were devised, tuned, and employed for temperature reference tracking and disturbance rejection. Two heater controller types were explored: on-off (thermostatic) and variable (proportional integral), to examine the ability of the feedback system to achieve the control objectives with minimal hardware and software complexities. The coordination of the heaters and the bypass valve is challenging during fast transients due to the different time scales, the actuator constraints, and the sensor responsiveness.

P controller with partial feed forward compensation and decoupling control for the steam generator water level

In this paper, a P controller with partial feed forward compensation and decoupling control for the steam generator water level is presented. While taking the steam flowrate as a disturbance to water level, the controller design can be completed in three stages. (1) Main circuit controller is designed without regard to disturbance. Since the transfer function of the steam generator model contains integrate element and differential element, the proportional (P) controller can selected as main circuit controller instead of PID controller for steam generator water level. (2) Partial feed forward compensation is introduced to remove the disturbance from the steam flowrate. If disregarding the differential element, the partial feed forward compensation's designing turns to be very simple. Partial feed forward compensation coefficient is set as reciprocal of P controller gain. (3) The coupling effects between the water level regulating and steam flowrate disturbance can be decreased by model reference decoupling control. The proposed methodology shows satisfactory transient responses, disturbance rejection and robustness.

Temperature Control of the BTX Divided-Wall Column

The control of a divided-wall column is more difficult than the control of a conventional two-column separation sequence for the separation of ternary mixtures because there is more interaction among control loops. In a previous paper, a control structure using four composition loops was shown to provide effective control of the purities of the three product streams and also achieve minimum energy consumption for both feed flow rate and feed composition disturbances. The numerical example studied the separation of benzene, toluene, and o-xylene. The four manipulated variables were reflux flow rate (R), side-stream flow rate (S), reboiler heat input (QR), and liquid split (βL) at the top of the wall. In this paper we explore the use of temperatures to avoid expensive and high-maintenance composition analyzers. Two types of temperature control structures are studied. In the first, three temperatures located in the main column and one temperature on the prefractionator side of the wall are used to adjust the...

Comparison of Control Strategies for Dividing-Wall Columns

Conventional ternary separations progressed via thermally coupled columns such as Petlyuk configuration to a novel design that integrates two distillation columns into one shell—a setup known today as dividing-wall column (DWC). The DWC concept is a major breakthrough in distillation technology, as it brings significant reduction in the capital invested as well as savings in the operating costs. However, the integration of two columns into one shell leads also to changes in the operating mode and ultimately in the controllability of the system. Although much of the literature focuses on the control of binary distillation columns, there are just a few studies on the controllability of DWC. In this work we explore the DWC control issues and make a comparison of various control strategies based on PID loops, within a multiloop framework (DB/LSV, DV/LSB, LB/DSV, LV/DSB) versus more advanced controllers such as LQG/LQR, GMC, and high order controllers obtained by H∞ controller synthesis and μ-synthesis. The controllers are applied to a dividing-wall column used in an industrial case study—the ternary separation of benzene−toluene−xylene. The performances of these control strategies and the dynamic response of the DWC is investigated in terms of product composition and flow rates, for various persistent disturbances in the feed flow rate and composition. Significantly shorter settling times can be achieved using the advanced controllers based on LQG/LQR, H∞ controller synthesis, and μ-synthesis.

Control of an oil-heated, fractal-like branching microchannel desorber

A PID controller was designed to improve the performance of an oil-heated, fractal-like branching microchannel desorber for use in an ammonia-absorption refrigeration system. System identification techniques were employed to develop single-input, single-output reduced order models relating the oil mass flow rate and the rectified circulation ratio ( f * ) for four different operating conditions. PID controllers were designed for each of these reduced order system models and the closed-loop response of the desorber was simulated for various operating conditions and controller combinations. The preliminary controller designs were refined by examining controlled desorber performance in an experimental flow loop. The resulting PID controller yielded a closed-loop response with a faster rise time and lower overshoot than the simulated controllers. The tuned controller regulated the desired rectified circulation ratio under the operating conditions studied, controlling both the amount and concentration of the ammonia refrigerant generated. Prescribed variations in f * were tracked by the controlled system with a settling time of approximately 10 min. Controlled performance was robust to external disturbances in the strong solution flow rate and the oil temperature, regulating f * about the desired value with a settling time similar to that observed during the tracking tests.

Blood flow rate measurements with indicator techniques revisited

In view of the emerging role, disturbances in regional blood flow rate seem to play in the pathogenesis of the metabolic syndrome; we review the concepts of the classical indicator dilution and washout techniques used for determinations of regional blood flow rate. Prerequisites, assumptions, necessary precautions for the application of these experimental techniques are emphasized. Special attention has been carried out to elucidate the consequence of a choice of indicators having a large distribution volume in the tissues.

Experimental and theoretical investigation of anaerobic fluidized bed biofilm reactors

This work presents an experimental and theoretical investigation of anaerobic fluidized bed reactors (AFBRs). The bioreactors are modeled as dynamic three-phase systems. Biochemical transformations are assumed to occur only in the fluidized bed zone. The biofilm process model is coupled to the system hydrodynamic model through the biofilm detachment rate; which is assumed to be a first-order function of the energy dissipation parameter and a second order function of biofilm thickness. Non-active biomass is considered to be particulate material subject to hydrolysis. The model includes the anaerobic conversion for complex substrate degradation and kinetic parameters selected from the literature. The experimental set-up consisted of two mesophilic (36±1ºC) lab-scale AFBRs (R1 and R2) loaded with sand as inert support for biofilm development. The reactor start-up policy was based on gradual increments in the organic loading rate (OLR), over a four month period. Step-type disturbances were applied on the inlet (glucose and acetic acid) substrate concentration (chemical oxygen demand (COD) from 0.85 to 2.66 g L-1) and on the feed flow rate (from 3.2 up to 6.0 L d-1) considering the maximum efficiency as the reactor loading rate switching. The predicted and measured responses of the total and soluble COD, volatile fatty acid (VFA) concentrations, biogas production rate and pH were investigated. Regarding hydrodynamic and fluidization aspects, variations of the bed expansion due to disturbances in the inlet flow rate and the biofilm growth were measured. As rate coefficients for the biofilm detachment model, empirical values of 3.73⋅10(4) and 0.75⋅10(4) s² kg-1 m-1 for R1 and R2, respectively, were estimated.

Effect of wall slip on the shear flow of a polymer in a channel with a wavy bottom

The effect of stick and wall slip boundary conditions on the specific features of the shear flow of viscous polymers in a confined two-dimensional channel with a wavy bottom is studied. The distribution of flow-rate disturbances across the transverse cross section of the channel is calculated by the numerical simulation of the Navier-Stokes equation for an incompressible fluid at arbitrary amplitudes and an arbitrary wave number of the wall. The wall slip is modeled by the introduction of a thin layer of a low-viscosity fluid at the bottom face. Slippage leads to a marked enhancement of flow rate disturbances including inertial advection. The results agree with the known analytical solutions for the low-amplitude wall wave.

Development of a Model Predictive Controller for Tankless Water Heaters

This paper describes the development of a novel approach to temperature control of tankless water heaters (TWHs). Conventional methods of controlling TWHs typically allow a high degree of error in the outlet water temperature when subjected to disturbances in flow rate or inlet water temperature. Poor control affects comfort, safety, and energy consumption associated with TWHs. The novel control approach presented here uses model predictive control (MPC) to minimize the outlet temperature error. A dynamic heat transfer model of an electric TWH is developed and validated and used within a MPC-based controller. The controller is interfaced to a physical prototype TWH and tested using an experimental test bed developed for this project. An integral performance criterion is used to compare performance and to tune the MPC in a standardized series of tests, also developed for this project. MPC is shown to provide excellent control of the prototype TWH.

Overall control strategy of a coupled reactor/columns process for the production of ethyl acrylate

Ethyl acrylate (EA) is widely used in industry as a precursor for varnishes, adhesive, and finishes of papers and textiles. This important ester can be produced directly from ethanol (EtOH) and acrylic acid (AA) via esterification reaction with the presence of sulphuric acid as homogeneous catalyst. The proposed design flowsheet of this process includes a CSTR reactor coupled with a rectifier and an overhead decanter. In order to further purify the final EA product, another stripper is needed with its top vapor recycled back to decanter. The simplest and industrial easily applicable overall control strategy will be investigated with only one tray temperature control loop in each of the two columns. The final proposed overall control strategy of this process is found to be different than another similar coupled reactor/columns process published earlier [I-L. Chien, Y.P. Teng, H.P. Huang, Y.T. Tang, Design and control of an ethyl acetate process: coupled reactor/column configuration, J. Proc. Cont. 15 (2005) 435–449]. Both EtOH and AA feed flow rates are used as manipulated variables in the overall control strategy with CSTR heat duty left as throughput manipulator for the overall process. The final EA product with stringent specifications of 0.1 wt% EtOH and 0.005 wt% AA impurities can be achieved with this proposed overall control strategy despite feed flow rate and feed composition disturbances.

Effect of wall growth on the kinetic modeling of nitrite oxidation in a CSTR

A simple kinetic model was developed for describing nitrite oxidation by autotrophic aerobic nitrifiers in a continuous stirred tank reactor (CSTR), in which mixed (suspended and attached) growth conditions prevail. The CSTR system was operated under conditions of constant nitrite feed concentration and varying volumetric flow rates. Experimental data from steady-state conditions in the CSTR system and from batch experiments were used for the determination of the model's kinetic parameters. Model predictions were verified against experimental data obtained under transient operating conditions, when volumetric flow rate and nitrite feed concentration disturbances were imposed on the CSTR. The presented kinetic modeling procedure is quite simple and general and therefore can also be applied to other mixed growth biological systems.

Design of an Auto-Tuning PID Controller by a Generalized Predictive Control Method

In this work, an auto-tuning PID controller based on the generalized predictive control (GPC) method is proposed for on-line tuning of the controller parameters due to the changes in operating conditions. The proposed scheme is applied to the control of a distillation column, with identification of the parameters of an ARMAX model determined from the operating data of an industrial column. An evaluation of the maximum likelihood (ML) and Kalman filter (KF) methods of identification showed that at a 10-second sample interval, reliable model parameters could be predicted using either the ML or KF method. However, at a 60-second sample interval simulation results showed that only the KF method generated reliable parameters. Column control was studied for an objective of maintaining constant liquid temperature on an intermediate tray, in the presence of a disturbance in the feed flow rate, with the top product flow rate being a manipulated variable. The simulation results for the auto-tuning PID control scheme using the GPC formulation are compared to the control performance which was obtained using a tuned conventional PID controller and also GPC, at a 60-second sample interval.

Design and control of an ethyl acetate process: coupled reactor/column configuration

In this paper, design and control of a realistic coupled reactor/column process to produce ethyl acetate is studied. The process design is more complicated because the ethyl acetate product is neither the lightest nor the heaviest component in the system. A search procedure is proposed to obtain the optimum process design and operating condition of this process. The optimum process design is the one that minimize the Total Annual Cost (TAC) of this process while satisfying the stringent product impurity specifications. The optimum overall process design includes a continuous-stirred tank reactor (CSTR) coupled with a rectifier, a decanter, another stripper, and a recycle stream. After the process design is established, the next step is to use dynamic simulation to test the appropriate control strategy for this process. Sensitivity analysis is performed to obtain the suitable temperature control points for the columns. The proposed control strategy is very simple containing only one temperature control loop in each column. This recommended simpler control strategy uses the ratio of acetic acid feed rate to ethanol feed rate to control the 5th stage temperature of the rectifier and uses the stripper reboiler duty to control the 5th stage temperature of the stripper. The proposed control strategy does not need any on-line composition measurements and can properly hold product purity in spite of feed flow rate and feed composition disturbances. For small deviations of the product impurity compositions during disturbances, a slow cascade outer composition loop structure can be implemented using off-line composition measurements from the quality lab.

Avaliação de controles PID adaptativos para um sistema de aquecimento resistivo de água

The aim of this work is to implement a conventional PID/SISO feedback control to obtain a fine adjustment of the water inlet temperature at a pasteurization process. For that, a resistance of 2500 Watts and a Pt100 to measure the temperature were installed in the water inlet line of the pasteurizer heating section. As the water temperature behavior according to the same step change on the potency of the resistance depends on the working flow rate, a single controller was designed to keep this temperature at its desirable set-point, for the water flow rate, within the range of 300 to 700L/h. Three different tunings for the PID were tested: the first consisted on the implementation of a function for the calculation of the PID parameters fitted to individual values obtained from each flow rate, according to process reaction curve methodology; the second consisted on using the PID parameters calculated as the average of these individual values; at the third tuning, an adaptive function fitted with the individual parameters obtained with Astrom & Hagglund methodology was used. The performance evaluation of the configured PID controllers was carried out by comparing the error index values, obtained after disturbances in the water flow rate in the closed loop system. The error indexes calculated after step changes in the water flow rate were used to evaluate the tunings. The results have shown that the third tuning, called Bang presented minor oscillations and smaller error indexes compared to the other two methods.