Improved frequency response model identification method for processes with initial cyclic‐steady‐state

A new nonparametric process identification method is proposed to obtain the frequency response model from given process input and output data. The proposed algorithm can estimate exact models for all desired frequencies. It is applicable to various process conditions (initial/final steady‐state, initial steady‐state/final cyclic‐steady‐state, and initial/final cyclic‐steady‐state) and requires a smaller amount of memory than previous methods. Also, it provides the exact models even in the presence of a static disturbance and shows an acceptable robustness to measurement noises. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Improved fourier transform to estimate frequency responses

For the automatic tuning of PID controllers, a new identification method is proposed to estimate the frequency responses of the process from the activated process input and output. It can extract many more frequency responses as well as guarantee better accuracy compared with the previous describing function analysis algorithm. Also, the proposed method can be applied to the case that the initial part of the activated process input and output is periodic (cyclic-steady-state), whereas the previous method using the modified Fourier transform cannot incorporate the case.     

Relay feedback identification for processes under drift and noisy environments

Relay feedback identification methods are widely used to find the process ultimate information and tune proportional‐integral‐derivative controllers. The conventional relay feedback method has several disadvantages, which include poor estimates of the process ultimate information for low‐order processes, chattering of relay for noisy environments, and asymmetric relay responses for constant biases or slow drifts in the process outputs. Methods to mitigate each of the above disadvantages are available. However, a systematic method to treat all of them has not been studied yet. Here, simple relay feedback methods that resolve these problems by introducing band‐pass filters in the feedback loop are proposed. The high‐pass filter part in band‐pass filter removes a constant bias or low frequency drift, and the low‐pass filter part removes high frequency noise and high‐order harmonic terms in the relay feedback oscillation, resulting better estimates of the process ultimate information. Because filters used for the proposed methods are able to reject constant biases, the process steady state gains can be estimated without disturbing the relay feedback oscillations and first order plus time delay (FOPTD) models can be obtained by combining the process steady state gains with the relay oscillation information. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Constrained receding‐horizon experiment design and parameter estimation in the presence of poor initial conditions

An optimal experiment design assumes the existence of an initial or nominal process model. The efficiency of this procedure depends on how the initial model is chosen. This creates a practical dilemma as estimating the model is precisely what the experiment tries to achieve. A novel approach to experiment design for identification of nonlinear systems is developed, with the purpose of reducing the influence of poor initial values. The experiment design and the parameter estimation are conducted iteratively under a receding‐horizon framework. By taking steady‐state prior knowledge into account, constraints on the parameters can be derived. Such constraints help reduce influence of poor initial models. The proposed algorithm is illustrated through examples to demonstrate its efficiency. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Two‐step principal component analysis for dynamic processes monitoring

In this study, a two‐step principal component analysis (TS‐PCA) is proposed to handle the dynamic characteristics of chemical industrial processes in both steady state and unsteady state. Differently from the traditional dynamic PCA (DPCA) dealing with the static cross‐correlation structure and dynamic auto‐correlation structure in process data simultaneously, TS‐PCA handles them in two steps: it first identifies the dynamic structure by using the least squares algorithm, and then monitors the innovation component by using PCA. The innovation component is time uncorrelated and independent of the initial state of the process. As a result, TS‐PCA can monitor the process in both steady state and unsteady state, whereas all other reported dynamic approaches are limited to only processes in steady state. Even tested in steady state, TS‐PCA still can achieve better performance than the existing dynamic approaches.     

Error‐triggered on‐line model identification for model‐based feedback control

In industry, it may be difficult in many applications to obtain a first‐principles model of the process, in which case a linear empirical model constructed using process data may be used in the design of a feedback controller. However, linear empirical models may not capture the nonlinear dynamics over a wide region of state‐space and may also perform poorly when significant plant variations and disturbances occur. In the present work, an error‐triggered on‐line model identification approach is introduced for closed‐loop systems under model‐based feedback control strategies. The linear models are re‐identified on‐line when significant prediction errors occur. A moving horizon error detector is used to quantify the model accuracy and to trigger the model re‐identification on‐line when necessary. The proposed approach is demonstrated through two chemical process examples using a model‐based feedback control strategy termed Lyapunov‐based economic model predictive control (LEMPC). The chemical process examples illustrate that the proposed error‐triggered on‐line model identification strategy can be used to obtain more accurate state predictions to improve process economics while maintaining closed‐loop stability of the process under LEMPC. © 2016 American Institute of Chemical Engineers AIChE J, 63: 949–966, 2017     

Thermal curing of glass‐epoxy prepregs by luminescence spectroscopy

In this study, we investigated the application of the luminescence spectroscopy technique in steady‐state conditions to study glass fiber‐epoxy F161 prepregs. We conducted this study by comparing the results obtained from the intrinsic fluorescence with Fourier transform near infrared spectroscopy. The extrinsic fluorescence of 9‐anthroic acid (9‐AA) was also used. Fourier transform infrared spectroscopy was also used to characterize the epoxide resin. The prepregs containing 9‐AA and those that did not were heat‐treated at 177°C (F161) for 1100 min at a 2°C/min heating rate. The results obtained by both methods indicated that the crosslinking reaction could be monitored by analysis of the spectral changes of the emission bands of the prepreg and 9‐AA. The intrinsic emission at 320 nm was attributed to the fluorophore group containing the epoxy ring and was used to calculate the conversion degree. The photophysical behavior of the 9‐AA probe indicated a reduction of free volume of the polymeric matrix with curing process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

A globally convergent method for finding all steady‐state solutions of distillation columns

A globally convergent method is proposed that either returns all solutions to steady‐state models of distillation columns or proves their infeasibility. Initial estimates are not required. The method requires a specific but fairly general block‐sparsity pattern; in return, the computational efforts grow linearly with the number of stages in the column. The well‐known stage‐by‐stage (and the sequential modular) approach also reduces the task of solving high‐dimensional steady‐state models to that of solving a sequence of low‐dimensional ones. Unfortunately, these low‐dimensional systems are extremely sensitive to the initial estimates, so that solving them can be notoriously difficult or even impossible. The proposed algorithm overcomes these numerical difficulties by a new reparameterization technique. The successful solution of a numerically challenging reactive distillation column with seven steady‐states shows the robustness of the method. No published software known to the authors could compute all solutions to this difficult model without expert tuning. © 2013 American Institute of Chemical Engineers AIChE J 60: 410–414, 2014     

Process identification method using relay feedback and backward integrals

We propose a new process identification method that combines the two methods of the relay feedback to activate the process and the backward integrals to estimate the model parameters. Novel deviation variables are introduced to incorporate the case that the initial part of the process is unsteady-state without sacrificing the dynamic information included in the initial part, while the previous approaches assign zero-weighting to the initial parts, resulting in loss of the dynamic information included in the initial part. The final cyclic-steady-state part of the process input and output data is chosen as the reference of the deviation variables. The proposed method can estimate the model parameters analytically by using the backward integrals and the least squares method.     

Gas‐solid flow distribution through identical vertical passages: Modeling and stability analysis

As previous evidence shows, the distribution of gas‐solid flow traveling through identical parallel paths can be significantly nonuniform, often with harmful operating consequences in practice. A fundamental analytical and numerical study is performed on the distribution of gas‐solid pneumatic flow passing through a “Y branch”. While many steady‐state gas‐solid distribution solutions, including a uniform distribution, satisfy the governing equations, linear stability analysis indicates that the uniform distribution is stable the most likely solution of the system. Both 2‐D (two‐dimensional) and 3‐D multiphase computational fluid dynamic simulations and stability analyses confirm the analytical conclusions. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Free‐radical solution polymerization of styrene in a tubular reactor—effects of recycling

The styrene free‐radical solution polymerization reaction in a tubular loop reactor is studied here both experimentally and through simulation. An attempt is made to compare the performances of tubular loop reactors when the recycle ratio is varied, based on steady‐state and dynamic responses and on the quality of the polymer produced at different conditions. It is shown here that steady‐state responses of loop reactors and traditional tubular reactors are very similar as far as the quality of the polymer obtained is concerned. Therefore, the recycle ratio cannot be used as a fundamental operation parameter for grade transitions at plant site. However, it is also shown that the recycling of polymer material is very important to accelerate the attainment of the final steady‐state in tubular reactor configurations, because recirculation of material homogenizes the distorted radial profiles of the axial flow velocities.     

Rheological study on high‐density polyethylene/organoclay composites

Maleic anhydride‐grafted polyethylene (MAPE) is investigated as a compatibilizer of polyethylene/organoclay nanocomposite. With MAPE help, partial exfoliation of the organoclay occurs in the nanocomposites with the melt compounding method for organoclay loading up to 8.0 wt%. Investigation of the rheological behaviors shows that at high frequencies or shear rates, the viscosity is essentially unaffected by the presence of organoclay; however, at low frequencies or shear rates, viscoelastic behavior alters dramatically, and this is attributed to the presence of anisotropic stacks of randomly oriented organoclay sheets and the formation of network structures. The important observations are firstly the initial stress overshooting observed in steady shear. At low shear rates, stress is much greater at the initial stage than the stress at the steady state; however, it can be eliminated by preshear at low shear rates, which means that preshearing can effectively break down the network structures and align the organoclay. Second, the normalized stress at the overshoot point is a function of the critical strain unit. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Integrated simulations of structural performance,molding process,and warpage for gas‐assisted injection‐molded parts. III. Simulation of cyclic,transient variations in mold wall temperatures

Whether it is feasible to perform an integrated simulation for structural analysis, process simulation, as well as warpage calculation based on a unified CAE model for gas‐assisted injection molding (GAIM) is a great concern. In the present study, numerical algorithms based on the same CAE model used for process simulation regarding filling and packing stages were developed to simulate the cooling phase of GAIM considering the influence of the cooling system. The cycle‐averaged mold cavity surface temperature distribution within a steady cycle is first calculated based on a steady‐state approach to count for overall heat balance using three‐dimensional modified boundary element technique. The part temperature distribution and profiles, as well as the associated transient heat flux on plastic–mold interface, are then computed by a finite difference method in a decoupled manner. Finally, the difference between cycle‐averaged heat flux and transient heat flux is analyzed to obtain the cyclic, transient mold cavity surface temperatures. The analysis results for GAIM plates with semicircular gas channel design are illustrated and discussed. It was found that the difference in cycle‐averaged mold wall temperatures may be as high as 10°C and within a steady cycle, part temperatures may also vary ∼ 15°C. The conversion of gas channel into equivalent circular pipe and further simplified to two‐node elements using a line source approach not only affects the mold wall temperature calculation very slightly, but also reduces the computer time by 95%. This investigation indicates that it is feasible to achieve an integrated process simulation for GAIM under one CAE model, resulting in great computational efficiency for industrial application. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 339–351, 1999     

Purification of Wet‐Process Phosphoric Acid by Hydrogen Peroxide Oxidation,Activated Carbon Adsorption and Electrooxidation

In this work, three technologies are studied for the purification of phosphoric acid produced by the wet process: chemical oxidation with hydrogen peroxide, adsorption onto activated carbon, and electrochemical oxidation by boron‐doped diamond anodes. The treatment of wet‐process phosphoric acid by chemical oxidation with H₂O₂ as oxidizing agent can remove 75 % of the initial TOC as maximum, indicating that this wet‐process phosphoric acid contains an important amount of organics that cannot be oxidized by hydrogen peroxide under the operation conditions used. High temperatures and hydrogen peroxide/TOC ratios close to 150 g H₂O₂/g TOC allow obtaining the best chemical oxidation results. The adsorption onto activated carbon can remove between 40 and 60 % of the initial TOC as maximum. Adsorption times of 2 hours and activated carbon/WPA ratios close to 12 g AC/Kg WTP assure both steady state and maximum adsorption of organics. The electrochemical process is the only technique by which complete mineralization of WPA organics can be achieved. Operating at 60 mA cm^–2 and at room temperature, high current efficiencies are achieved which only seem to decrease by mass transport limitations.     

Effect of steady shear on the microstructural evolution of melt‐intercalated polymer/clay nanocomposites

A sheet sample composed of poly(butylene terephthalate) and clay prepared by solid‐state compression was melt‐annealed in a rheometer under steady shear flow to investigate the whole hybridization process. The results of the offline morphology and thermogravimetric analysis as well as Fourier transform infrared characterization show that shear flow can reduce the dynamic process of hybridization, facilitating the formation of an intercalated nanoscale structure. With an increase in the shear intensity, the detachment level of clay increases more remarkably than the swollen degree. However, an increase in the shear intensity does not induce an exfoliated structure but can decrease the average thickness of the clay tactoids, leading to a remarkable enhancement in the thermal stability due to the increase in the effective filling volume of the clay. Furthermore, those intercalated nanocomposites annealed at a high shear rate still present a distinct hierarchical structure, which suggests that steady shear is not as good as dynamic or complex shear for promoting hybridization effectively. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

A pattern matching method for large‐scale multipurpose process scheduling

This article proposes a novel pattern matching method for the large‐scale multipurpose process scheduling with variable or constant processing times. For the commonly used mathematical programming models, large‐scale scheduling with long‐time horizons implies a large number of binary variables and time sequence constraints, which makes the models intractable. Hence, decomposition and cyclic scheduling are often applied to such scheduling. In this work, a long‐time horizon of scheduling is divided into two phases. Phase one is duplicated from a pattern schedule constructed according to the principle that crucial units work continuously, in parallel and/or with full load as possible, exclusive of time‐consuming optimization. Phase two involves a small‐size subproblem that can be optimized easily by a heuristic method. The computational effort of the proposed method does not increase with the problem size. The pattern schedule can be not only used for production/profit maximization but also for makespan estimation and minimization. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Dispersion and Phase Separation of Water‐Oil‐Amphiphile Systems in Stirred Tanks

Drop size distributions and phase separation behavior of water‐oil‐nonionic amphiphile systems are investigated using an in situ endoscope measurement technique and an external camera in stirred tanks in batch mode. The fitting procedure and the simulation results of a phase separation model are analyzed under the condition that either the swarm sedimentation speed or the mean drop size during sedimentation is known. The steady‐state drop size distributions are self‐similar over the whole range of process parameters, but not in the decaying turbulence field after agitation stop. The coalescence rate in the first seconds after agitation stop clearly affects the separation behavior, so that a prediction of the separation time based on the initial conditions in steady state is not trivial.     

The ‐relation and a simple method to predict how many data points are needed for relevant steady‐state detection

Steady‐state detection is of vital importance for experiments and simulations in chemical engineering, as well as also other fields of science, engineering, and finance—particularly when the full timescale of interest cannot be measured or simulated. We present a breakthrough for estimating the number of data points required before successful steady‐state detection is feasible. Using an initial window of data, the method enables predicting the prerequisites for steady state detection (ppSSD), given as a number of data points. The method is shown to be accurate for data with realistic distributions (uniform, normal, and sine‐wave), and data from actual kinetic Monte Carlo simulations. Users need only to use the algebraic equations derived and provided in this work to estimate the required number of data points for relevant steady‐state detection. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3354–3359, 2018     

Consolidation of net‐shape random fiber thermoplastic composite preforms

A novel thermoplastic composite preforming process has been developed. This thermoplastic programmable powdered preforming process (TP‐P4) uses commingled glass and polypropylene yarns, which are chopped to a desired length and deposited onto a vacuum screen. The as‐placed fibers are then heat‐set for improved handling, before potential preconsolidation, and final conversion with preheating and press forming. This work investigated the effect of using either a double belt lamination preconsolidation stage or using an improved heat‐setting stage. Polymer degradation was examined using gel permeation chromatography analysis, and the void content evolution tracked using image analysis techniques from the heat setting stage until the final part. It was shown that without preconsolidation, preforms prepared for a 2 mm thick final pressed part could be compression molded into a substantially void free, non degraded part. By using the lamination route, this limit could be increased to 4 mm, but without allowing local thickness changes in the preform. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Between‐phase calibration modeling and transition analysis for phase‐based quality interpretation and prediction

The quality‐concerned between‐phase transition analysis is performed and an improved calibration modeling strategy is designed for quality prediction and interpretation in multiphase batch processes. From the between‐phase viewpoint, the quality‐related phase behaviors are decomposed and two subspaces are separated. In common subspace, the underlying quality‐relevant variation stays invariable between the neighboring phases, showing the common contribution to quality. The other part changes with the alternation of phases and has the different influences on quality interpretation, termed specific subspace here. Based on subspace separation, between‐phase transition regions are distinguished from steady phases. Different models are developed in steady phases and transition regions respectively for online quality prediction. Offline quality analyses are also conducted in two subspaces to explore the time cumulative effects. The proposed method gives an interesting insight into the phase behaviors and between‐phase transitions for quality prediction. The feasibility and performance of the proposed method are illustrated with a typical multiphase batch process. © 2012 American Institute of Chemical Engineers AIChE J, 59: 108–119, 2013