[机械模具数控自动化专业毕业设计外文文献及翻译]【期刊】聚合反应器的测量和控制-外文文献

Computers and Chemical Engineering 30 (2006) 14471463Measurement and control of polymerization reactorsJohn R. Richards, JohnE. I. du Pont de Nemours and Company, Experimentalform25Abstractkinetics. to end-usemeasuredsequencereactors composition,hierarchical resemibatch, focusingpolymerization polymerandK Emulsion1.Consistent polymer properties are of paramount importanceto end-user manufacturers who must produce the polymer initspropertiescompositioninform,ofmereasuchthemusttionmainearlyThesurementscientists designing and operating polymer reactors and associ-ated equipment. We have attempted to summarize and update the0098-1354/$doi:final form and shape for the intended application. Theseare the result of complex polymer architecture andformed in reaction and perhaps further influencedisolation and extrusion processes. Producing consistent, uni-and in-specification polymer for the end-user are the tasksthe polymer process measurement and control systems. Poly-processes, whether batch or continuous, rarely run underxactly specified conditions; disturbances move the processway from desired trajectories. However, in order to operateprocesses safely and in order to set the characteristics ofproducts optimally, a set of process manipulated variablesbe kept constant or systematically modified over the dura-of the reaction or in the course of the various reaction steps.Ray, Soares, and Hutchinson (2004) recently reviewed thedevelopments of polymer reaction engineering from thedays of polymer science to the current challenges of today.Corresponding author. Tel.: +1 302 695 4059; fax: +1 302 695 8805.E-mail address: (J.R. Richards).information already provided in our previous work (Congalidis& Richards, 1998; Richards & Congalidis, 2005; Richards &Schnelle, 1988).The framework for our discussion is the hierarchical approachsummarized in Fig. 1, which has proved very useful in the suc-cessful application of process control in a complex industrialenvironment as shown in an earlier review by Richards andSchnelle (1988). It has been our own experience and learningthat the same hierarchical approach is particularly important inthe control of polymer reactors.Process knowledge, which is usually captured in an exper-imentally validated mathematical model, is the cornerstone ofa successful control strategy. This is particularly true for poly-merization reactors, where the in-depth knowledge of processoperation in terms of the effect of operating variables on poly-mer properties can be used to great advantage in the design of thecontrol system and can result in a much more straightforward(and therefore easy to maintain) strategy than would have beenpossible otherwise. This point will be illustrated in Section 3 andparticularly by using the examples referred to in Figs. 2 and 3. see front matter 2006 Elsevier Ltd. All rights reserved.10.1016/pchemeng.2006.05.021Received 9 February 2006; received in revisedAvailable onlineThe measurement and control of polymerization reactors is very challengingIn these reactors many important variables, which are relatedat low sampling frequencies. Furthermore, end-use polymer propertieslength, and branching distributions. This paper surveys the instrumentationwith emphasis on, for example, measurement of viscosity,approach to the control system design and reviews traditionaland continuous reactors. These approaches are illustrated byreactor. Finally, the paper captures some of the trends in thereactor control.2006 Elsevier Ltd. All rights reserved.eywords: Process control; Polymerization reactors; Solution copolymerization;IntroductionP. CongalidisStation, Wilmington, Delaware 19880, USA2 May 2006; accepted 16 May 2006July 2006due to the complexity of the physical mechanisms and polymerizationpolymer properties, cannot be measured on-line or can only beare related to the entire molecular weight, copolymer composition,technologies, which are of particular interest in polymerizationmolecular weight, and particle size. This paper presents agulatory techniques as well as advanced control strategies for batch,on the control of a commercial multiproduct continuous emulsionindustry, which may impact future development in measurementcopolymerization; Mathematical modeling; Reactor controlpurpose of this contribution is to discuss the various mea-and control techniques of importance to engineers and1448 J.R. Richards, J.P. Congalidis / Computers and Chemical Engineering 30 (2006) 14471463Fig. 1. The process control hierarchy (Richards & Congalidis, 2005).The use of polymerization reactor modeling in conjunctionwith control design was discussed by the authors in an ear-lier publication (Congalidis, Richards, & Ray, 1989). Processknowledge together with the appropriate sensors, transmitters,and analyzers are the prerequisites for the design of the basiccontrol system to regulate pressure, temperature, level, and flow(PTLF). Only when the elements of the regulatory control sys-tem are in place and are properly designed and maintained canthe control engineer attempt, in increasing order of complex-ity, the implementation of more advanced regulatory controlstrategies, multivariable model based control algorithms, andon-line scheduling and optimization strategies to compute setpoints for the regulatory controls. In many instances advancedcontrol applications have failed in an industrial environmentnot because the algorithms were necessarily faulty but becausethe basic regulatory control system performed poorly, eitherbecause of inadequate design (leading to operation in an open-loop mode) or because one of the critical measurements (i.e. aprocess analyzer) was poorly maintained. In other instances thebasic regulatory control may have been in place but some of theelements of advanced regulatory control (for example cascadecontrol and ratio control) were not being implemented result-ing in degradation of reactor performance in terms of consistentpolymer properties.In this contribution we have attempted to cover control topicsthat we believe should be of interest to a wide spectrum of engi-neers and scientists in the polymer industry. We have thereforeelected to discuss all elements of the process control hierarchyas they apply to polymer reactor control fully realizing that thelower levels of control would be obvious to the academic com-munity or to experienced industrial practitioners.2. Measurement techniquesThe measurement technique to be chosen is principally deter-mined by the measured quantity and by the accuracy by whichthe variable must be measured. The measuring instrument pro-duces a signal, which must be transformed in such a way thatit can be registered by an indicator or recorder and further pro-cessed. This requirement is fulfilled directly by some measuringmethods; however, in most cases a measurement transmitteris operated between the sensor and the measurement device.Electrical signals are much more commonly used today thanpneumatic signals. We will concentrate our discussions on theFig. 2. Solution copolymerization with recycle loop (Congalidis et al., 1989).J.R. Richards, J.P. Congalidis / Computers and Chemical Engineering 30 (2006) 14471463 1449with recmeasurementcesses.duced.minimizedgalidis,mentorewtors2.1.tanttheadvCongalidis2.2.float-typeometers.radiation,instrumentstocourseforbasedmonomers.changesFig. 3. Emulsion terpolymerization processtechniques that are more specific to polymer pro-During a measurement stochastic errors can be intro-The effects of process and measurement noise can beby signal conditioning or filtering (Richards & Con-2005; Seborg, Edgar, & Mellichamp, 2004).All of the techniques to follow represent polymer measure-state-of-the-art and are listed here due to either their noveltytheir frequent utilization, with multiple techniques presentven in a single installation. A summary table of on-line hard-are sensor classification techniques for polymerization reac-can be found in Kammona, Chatzi, and Kiparissides (1999).PTLF measurementsPressure, temperature, level, flow and weight are very impor-basic measurements for polymer processes. They formcornerstone for all control strategies both regulatory andanced. These measurements are reviewed in Richards and(2005) and Liptak (2003).Densitometry, dilatometry and gravimetryThe density of liquids is monitored by displacement anddensitometers, hydrometers, and hydrostatic densit-More advanced instruments are oscillating Coriolis,vibration, and ultrasonic densitometers. Many of thesecan be connected photometrically or mechanicallyproduce a usable electrical signal (Liptak, 2003).themonitoredthe(Rodriguez,determinedloss.Thenremodetermined.2.3.follocontinuouslystant(pseudoplastic)ortheal.,cometersycle loop (Congalidis & Richards, 1998).Dilatometers measure the volume shrinkage during theof liquid polymerization reactions and are mainly usedlaboratory measurement of monomer conversion. They areon the principle that polymers are denser than theirAs monomer is converted to polymer, volumeare monitored by following the change in height ofsolution inside a graduated capillary tube. Conversion iswith a computer-linked photodetector that tracksmeniscus in the capillary and records the height changesCohen, Ober, & Archer, 2003).The percentage of total solids in a polymer sample can beby the gravimetric method through moisture weightThe sample is loaded onto a pan and the weight determined.it is put into an oven at high temperature for a time tove all volatiles. It is then reweighed and the percent solidsViscosity measurementViscosities are of interest in polymer technology in order tow the course of a polymerization reaction or to monitorthe quality of a product. Viscosity may be con-(Newtonian), shear thickening (dilatant), or shear thinningwith shear rate. For polymer systems, solutionmelt, the viscosity can be related to the molecular weight ofpolymer (Kammona et al., 1999; Liptak, 2003; Rodriguez et2003). In most cases viscosity is measured by capillary vis-or rotating viscometers. The capillary viscometer may1450 J.R. Richards, J.P. Congalidis / Computers and Chemical Engineering 30 (2006) 14471463also be employed in-line for monitoring of molecular weight inpolymerizations as described in Vega, Lima, and Pinto (2001).An indirect method to obtain a measure of molecular size thatis quick and inexpensive is the Melt Indexer (Rodriguez et al.,2003). The Melt Index is defined as the number of grams of poly-mer extruded in 10 min through a capillary 2.1 mm in diameterandD1238).portionallyrotatingcommonlyvitcoaxialthisofthefromviscometerofa2.4.samplesatanalyticalconsumingofordericallyobtain2003)casesorquencbondstheMorehastionalpolymerparticularlyNearreactorularOpticalinspectrathesecomposition(Kammonausuallyters (Liptak, 2003). A differential refractometer is commonlyused as a concentration detector in the effluent of a gel per-meation chromatography (GPC) column for molecular weightdetermination. Raman spectroscopy is dependent on the colli-sion of incident light quanta with the molecule, inducing themolecule to undergo a change (Rodriguez et al., 2003).Itisnowbeingvibrationalbymolecular(Elizalde,etcanlatterofizationofhydrocarbonspHthatneticabsorbsThiscopolymeralsoasandnototherAmong(Kammonawereate(helium)plefromtionbeingof(Liptaboutionsratio.stepfraction2.5.esttensionpresent,critical8 mm in length at a certain temperature and pressure (ASTMIt is evident that the Melt Index varies inversely pro-to the polymer molecular weight.Among the different possibilities to measure viscosities inviscometers, the coaxial cylinder apparatus is the mostused in practice. The measurement of the angularelocity of the cup and the angular deflection of the bob makespossible to determine the viscosity (Liptak, 2003). Beside thedevice the cone-and-plate viscometer is also used. Indevice, an inverted cone faces a solid plate and the apexthe cone just touches the plate. The measured liquid is infree gap. The viscosity of the measured fluid is computedthe torque on the driving shaft (Liptak, 2003). The Mooney, particularly used in the rubber industry, is a variantthe cone-and-plate viscometer, which restricts the sample todisc-shaped cavity (ASTM D1646) (Liptak, 2003).Measurement of compositionThe composition of raw materials, finished products, andof the various steps of a reaction is normally measuredthe laboratory using the appropriate physical and chemicalmethods. However, sampling and analysis are timeand, in many cases, the result of the analysis is onlycurrent interest and too late for control decisions to be made. Into monitor compositions continuously, one needs automat-functioning analytical instruments that can continuouslythe composition of a mixture.Optical methods are common (Kammona et al., 1999; Liptak,as infrared spectrographic analysis (IR) permits in manyto follow the appearance or the disappearance of onemore characteristic absorption frequency bands. These fre-y bands correspond to frequencies of vibrations of thein the molecules. One must first analyze the spectrum ofIR radiation and then measure the corresponding frequencies.recently the Fourier transform infrared technique (FTIR)been used for faster data acquisition and handling than tradi-IR spectrographic analysis. IR and FTIR can be applied tosolutions or solid films for composition analysis and areuseful for copolymer composition determination.IR spectroscopy has been used to control a polymerizationto produce solution polymers with well-defined molec-weight (Othman, Fevotte, Peycelon, Egraz, & Suau, 2004).analytical devices are also built for measuring radiationthe ultraviolet (UV) and the visible spectral region, but theabsorption bands obtained here are usually so broad thatdevices are only of limited use.The refractive index (RI) of a mixture is a function of theof the mixture and their respective refractive indiceset al., 1999). Operational measuring instruments aredifferential refractometers or critical angle refractome-used to provide a means of studying pure rotational andtransitions in molecules. Raman scattering of lightmolecules may be used to provide chemical composition andstructure and is currently being applied to polymersLeiza, & Asua, 2004; Kammona et al., 1999; Leffewal., 2005; Reis, Araujo, Sayer, & Giudici, 2004).Apart from optical methods, magnetic and electrical methodsalso be used for composition measurement. Examples of theare conductivity measurements (of ionic liquids, e.g. purityboiler feeding water), ionization methods (e.g. the flame ion-detector in gas chromatographs or the photo-ionizationgases with UV light as tracking measuring instrument ofin air), electrochemical potential methods (e.g.measurements), and occasionally polarographic methods.Nuclear magnetic resonance (NMR) is based on the principlewhen a hydrogen containing compound is in a strong mag-field and exposed to radio frequency signals, the compoundenergy at discrete frequencies (Rodriguez et al., 2003).technique can be used to measure chain molecular structure,composition, and copolymer sequence lengths. It candeduce isotacticatactic ratios and other structure variationsshown for example in Wyzgoski, Rinaldi, McCord, Stewart,Marshall (2004). Mass spectrometry and NMR are currentlyin routine on-line process use but can be used to calibrateon-line methods.Many methods depend on the separation of a fluid mixture.these the process gas chromatography (GC) stands outet al., 1999). Suitable devices for online controldeveloped from laboratory gas chromatographs and oper-very reliably. The principle of the GC is that a carrier gasis passed over a tubular column of a fine solid. A sam-is injected into the carrier gas stream and the gas effluentthe column is run past a detector such as a flame ioniza-detector. Calibration is based on the fact that all conditionsequal, a given hydrocarbon will require the same lengthtime to pass through the column to the detector (elution time)ak, 2003).A mass spectrometer source produces ions and informationa sample may be obtained by analyzing the dispersion ofwhen they interact with the sample using the mass-to-chargeSometimes mass spectrometers are used after a separationsuch as gas chromatography or liquid chromatography foridentification.Surface tensionIn emulsion polymerizations, particularly it may be of inter-to measure the surface tension of the emulsion. The surfacecan give an indication of whether or not micelles arewhich is important in particle nucleation above themicelle concentration (CMC) (Schork, 1993; Schork,J.R. Richards, J.P. Congalidis / Computers and Chemical Engineering 30 (2006) 14471463 1451Deshpande, & Leffew, 1993). The online method used is usu-ally the bubble pressure method (Schork et al., 1993).2.6. Molecular weight distribution (MWD)It is widely recognized that a reliable method of monitoringmolecularaofation(SEC)2003;tionporoustionsize,accessandFmolecularibratemolecularofofatednumbermerciallyhaathatobtainedbratedtionedweightcializingwith2.7.onmermaysizehacontrolled.niquesdifsedimentation,(SEM)twablestaticsurementphoton correlation spectroscopy (PCS). Dynamic light scatter-ing provides a relatively fast and simple method for submicronparticle sizing (Kammona et al., 1999). Turbidimetry, which is ameasure of the attenuation of a beam of light passing through asuspended particle sample, has been used traditionally in indus-try to obtain a measure of average particle size and even theentireneedofitsizenamicthiscapillary15atedAstomancess3.3.1.operatingthemalarecificIncontrollers,placecontrolfromorple,in3.2.uffinalhacontrol:(1)(2)weight distribution, and the various molecular weightverages (Mn, Mw, and Mz) during the polymerization process isimportance to final polymer quality. Traditionally gel perme-chromatography (GPC) or size-exclusion chromatographyhave been used to determine MWD (Rodriguez et al.,Kammona et