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Scale-up of polymerization processes
Abstract
Scale-up and/or scale-down procedures are crucial during the development of chemical processes. These procedures should be integrated to form a multidisciplinary approach encompassing several branches of science and engineering. The challenge is to find a balance between chemistry, design, environment, hygiene and safety (EHS) compliance and economic factors. The iterative process of scale-up procedures also requires concept discrimination at early stages of process development. Scale-up studies should predict the reactor behavior and the quality and properties of a product from laboratory and pilot plant to full scale; however, not every pilot scale presents a valid industrial scaled-down size. A rational mathematical model describing the process reasonably well for large scale is important for designing industrial processing units, but this is only feasible if a phenomenological understanding is achieved, and if the necessary constants and parameters are estimated accurately. An accomplished process development procedure must lead to products presenting the same properties at both small and large scale.
... The mixing process has a great influence on the quality of products in the chemical industry [1,2]. For example, processes like polymerization, crystallization, and precipitation are very sensitive to the mixing process [3,4]. Micromixer is a kind of continuous flow chemical synthesis device with a feature dimension from micron to millimeter that may have a considerable potential to improve the mixing process of the chemical reaction [5]. ...
... Consequently, it has been proposed that the chemical kinetics is characterised through the undertaking of experiments at smaller scales, whilst transport phenomena are investigated at scales that are more closely aligned to those of commercial production (e.g. Mayer, 2002). However, this strategy of separation is not appropriate in some cases due to the difficulty in breaking the couplings between the chemical reaction, the mixing, and the transport of mass and energy (e.g. ...
Mathematical modelling is important for process development, but often requires a large amount of experimental effort to generate the necessary data. To reduce the effort, it is important to recognize that model parameters associated with transport phenomena may be less important than those of chemical kinetics in the early stage of development. This is because the characteristics of transport phenomena can change significantly between different process scales. Thus, the experimental effort in the early development stage can be reduced by applying optimal experimental design techniques which focus specifically on the precision of the subset of parameters pertining to the chemical kinetics. This idea, not reported previously in the literature, is tested through a simulated case study based on a toluene nitration process model. It is shown that subset parameter oriented designs outperform their full set counterpart in terms of achieving better precision of the chemical kinetic parameters with the same number of experiments, or requiring fewer experiments to achieve the same level of precision.
A continuous pilot-scale tubular reactor is designed for the polymerization of acrylic acid in aqueous solution. The reactor is equipped with CSE-X static mixers from Fluitec which strongly increase heat and mass transfer despite laminar flow. A mathematical model of the reactor is proposed assuming that it is a perfect plug-flow reactor and using kinetic and rheological equations previously established using a lab-scale rheo-Raman device. Calculated values of temperature and polymer concentration profile along the reactor as well as outlet average molar masses and viscosity are in good agreement with experimental results. Provided that the outlet viscosity of reaction medium does not exceed 0.5 Pa s, no reactor fouling occurred. On the contrary, fouling is detected after 6 to 8 h running with higher viscosities. This work is the first demonstration of applicability of CSE-X static mixer elements in a continuous polymerization process producing high molar mass water-soluble polymer. In addition, successful scale-up of polymerization reactor is reported up to pilot-scale as well as batch to continuous transposition, only on the basis of lab-scale rheo-Raman data. The interest of rheo-Raman device for the acquisition of primary data is established.
A continuous pilot-scale tubular reactor is designed for the polymerization of acrylic acid in aqueous solution. The reactor is equipped with CSE-X static mixers from Fluitec which strongly increase heat and mass transfer despite laminar flow. A mathematical model of the reactor is proposed assuming that it is a perfect plug-flow reactor and using kinetic and rheological equations previously established using a lab-scale rheo-Raman device. Calculated values of temperature and polymer concentration profile along the reactor as well as outlet average molar masses and viscosity are in good agreement with experimental results. Provided that the outlet viscosity of reaction medium does not exceed 0.5 Pa s, no reactor fouling occurred. On the contrary, fouling is detected after 6 to 8 h running with higher viscosities. This work is the first demonstration of applicability of CSE-X static mixer elements in a continuous polymerization process producing high molar mass water-soluble polymer. In addition, successful scale-up of polymerization reactor is reported up to pilot-scale as well as batch to continuous transposition, only on the basis of lab-scale rheo-Raman data. The interest of rheo-Raman device for the acquisition of primary data is established.
The scale-up/-down of polymn. reactors has to deal with large viscosity changes during the process, addressing mass- and heat-transfer issues. A practical example on scale-up of styrene and Me methacrylate free radical bulk and soln. polymn. is presented. The main crit. parameters are mixing at mol. level (micromixing) and heat removal capacity. The operating parameters being kept const. are: reaction conditions (temp., pressure, chem.) and thus the reaction time. A pilot plant issued from scale-down of possible industrial sizes was developed to represent, at best, larger scales. The main parameter being scaled-up is the heat removal capacity, which has to be maintained const. among the different sizes. New concepts are adapted to dissipate the mixing energy where it is the most suitable, and the final step is the scale-up/-down strategy. Another issue addressed in this contribution is the need for in-line analytics that could operate at different plant scales and thus give important information for process control. Scale-up/-down strategy must include the whole process, not only the reaction stage but also what happens before, after, and simultaneously, i.e., upstream, downstream, and peripheral operations. Finally the measurable success of a scale-up/-down anal. could only be proved at industrial scale, where a good agreement between pilot- and larger scale should be obsd. The concepts in terms of transfer phenomena, analytics, sepn., product properties, feasibility, and economics should also be included in this anal.
For the development of eco-efficient processes needed for the manufacturing of pharmaceutical drug substances, Chemical Development in former Sandoz Pharma (the described procedures were developed and established in Sandoz Pharma, and they will have to be harmonized with
the procedures of former Ciba and will continue as redefined Novartis Pharma practice) has chosen a stepwise approach, whereby the solving of safety and ecological issues is synchronized with the total development process, which includes clinical, pharmacological, and toxicological
studies over many years. At the start, when toxicological studies are the prerequisites for any clinical program, and when only the research synthesis is available for scaleup, the emphasis is on speed to deliver the required amount of drug substance for such time-critical activities. Consequently,
only the 'unacceptables' (conditions or reagents which pose threats to the environment (safety, ecology, industrial hygiene)) are to be eliminated at this stage. Further down in development time scale, other improvements of the processes (short cuts, alternative synthesis routes, more cost-effective
reaction conditions and reagents), including the elimination of 'criticals' (conditions and reagents which are tolerable in smaller amounts but should be eliminated before the final manufacturing process is to be established), are to be undertaken. Before any process step is carried out in
the pilot plant, a tailor-made development risk analysis has to be performed where all aspects of safety and ecological considerations are analyzed and resolved. A Safety & Ecology Newsletter, a collection of such 'unacceptables' and 'criticals', as well as proven solutions/alternatives
for them is periodically published by the Process R&D Group of Chemical Development and is circulated to all chemists of the Novartis Group worldwide. This has helped to raise the awareness of such potential problems and their avoidance already at the research level.
Inline conversion measurements in chemical reactions are in general and especially in polymerization reactions necessary for process monitoring and control. For polymerization monitoring, the sound propagation (UPV) can be related to the texture of the bulk with a high accuracy. As
the monomer is transformed to polymer, both the density and adiabatic compressibility change, making the UPV closely related to the monomer conversion. This technique is based on the increase of UPV with rising elasticity of the medium due to the polymerization. The application of the UPV
technique to follow the composition evolution of the solution during the polymerization reaction in a recycle tubular reactor is presented. The sensor can be directly inserted in the tubular reactor and does not require a sampling circuit. This makes its use particularly simple and suitable
for laboratory and industrial purposes.
Scaleup's one of the most important and least talked about aspects of mixing, The literature's shrouded in mystery and laden with rules, This article cuts through to what's vital.
Recent progress in polymerization easily allows the control of precise structure of a base polymer and accelerates lower-cost production using larger-scale-plants. In addition to this, today's more important issue in our industry is the development of the key-technology corresponding to a sustainable society. Under these circumstances, the polymer industry in Japan has been focussing on down-stream products from upper-stream using advanced processing technology. To meet today's trends, the polymer industry should establish more sophisticated technology in both higher functionality and hybridization to survive in the 21st century. In particular, various hybridized polymerprocessing systems such as the combination of reactive extrusion and injection molding should be applied to create cost-effectiveness and less environmental burdens as well as higher performance.
Scaleup of the reaction step, which represents the heart of the flowsheet for a latest process-industries product, is discussed. A wide range of practical reaction-scaleup guidelines that previously have been diffused throughout the engineering literature is assembled. Intelligent scaleup entails finding and collating all the information and data required for the design, construction, and startup of the latest full-size plant. The success of any commercial operation depends on understanding the effect of scaleup on the process.
A discussion on the design and scaleup of hydrogenation reactors, where a slurry of catalyst in liquid that contains the substrate is exposed to hydrogen gas, covers the different design concerns, including rates of mass transfer and kinetics; chemical kinetics; overall rate of hydrogenation; practical aspects of design, which include catalyst selection, solvent use, and effect of pressure and temperature; batch and semi-batch units; continuous units; semi-continuous units; temperature control; batch units and runaway reactions; and procedure for scaleup of slurry reactors.
Evaporative cooling is an attractive mechanism for heat removal in polymerization reactors where fast, exothermic reactions and high viscosities complicate other means of heat removal. In this work we consider the effect that the mechanism has on the dynamics of a well mixed tank reactor for solution free-radical polymerization of styrene. Continuation and stability analysis reveals the existence of isola structures and oscillatory behavior at typical industrial conditions as well as strong coupling of the degree of evaporative cooling with important parameters like residence time, feed initiator concentration, jacket temperature, and reactor size. Further analysis is performed to try to understand the impact that this behavior has on practical aspects such as reactor design, scale-up, and operation.
Polymerization reactors usually exhibit complex nonlinear dynamic behavior because of the complexity of the physicochemical interactions and the kinetics of the polymerization reactions. In these reactors many important variables, often related to end-use polymer properties, cannot be measured on-line or can only be measured at very low sampling frequencies. Furthermore, end-use polymer properties are usually related to the molecular weight and composition distributions in the polymerization reactor. Finally, the typical industrial polymer reactor is used to manufacture a variety of grades of the same basic product necessitating frequent startups, on-line transitions, and shutdowns.
The purpose of this article is to present a case study of integrating statistical process control (SPC) and engineering process control (EPC) in an industrial polymerization process. We develop and compare the effectiveness of several regulation strategies to reduce polymer viscosity deviations from target. Controllers are derived using the constrained minimum variance criterion. Robustness properties and stability conditions of the controllers are discussed. The performance and the adequacy of the regulation schemes in a simulation of realistic assignable causes affecting the process are studied. In this context the benefits of implementing an integrated EPC/SPC system are compared with those of using an EPC system alone.
Application of the triple-probe method for volume-resolved monitoring of electron density and electron temperature during industrial-scale polymerization plasma process was tested. As a model process, simultaneous deposition of barrier film on the inner wall of six bottles using an asymmetrical r.f. discharge was investigated. The reproducibility of the process was better than 94%. The plasma density differed between particular bottles up to a factor of 2, and this difference increased with pressure. The barrier properties of the film increased with the product of the ion current and square root of the electron temperature
Nylon-6 polymerization in a VK tube (vertical column) reactor is largely not understood because of the complex internal structure of the reactor and the unavailability of essential industrial data on polymerization. In the present study, the hydrolytic polymerization of nylon-6 has been simulated in an industrial VK tube reactor for concentrations of various species. The model, which assumed a flat velocity profile in the radial direction, predicted the industrial results with high accuracy. The internal design of the reactor appears to be responsible for getting close to plug-flow conditions inside the VK tube. An empirical relation based on temperature and pressure was used to determine the water profile along the vertical axis of the reactor. Because the pressure increased along the axis of the reactor and the temperature first increased and then decreased, the model predicted a lowest water content (LWC) point at near the highest temperature point. The concentration of water at the LWC point was found to be critical in determining the properties of the end product. The model suggested three main zones in a VK tube reactor: a very small top turbulent zone, where most of the water was lost, followed by a large middle zone, which was a vaporizing plug-flow tubular zone, and a bottom zone, which was a nonvaporizing plug-flow tubular zone. The vaporizing zone ended and the nonvaporizing zone began where the LWC point was achieved. The small turbulent zone at the top did not appear to affect the end polymer properties of polymerization. This model was found to be useful in carrying out process optimization and suggesting improvements in VK tube designs for higher productivity.
A Mettler RC-1 reaction calorimeter was utilized to investigate the thermochemistry of two aromatic nitrations prior to their scaleup. The calorimetry provided data that enabled the evaluation of alternative modes of operation and ultimately to intrinsically safer processes than were originally proposed. The results from this study and how they were used for pilot plant scale-up are discussed.
The two-phase model developed for the UNIPOL polyethylene process is improved by introducing polymer diffusion resistance, this means modelling of polyethylene fluidized bed reactors has been examined on two levels, at small scale of individual polymer particle, and macroscale of the whole reactor. The model utilizes the multigrain model that accounts for the reaction rate at catalyst surface to explore the static and dynamic bifurcation behavior of the fluidized bed catalytic reactor. Detailed bifurcation diagrams are developed and analyzed for the effect of polymer growth factor and Thiele modulus (the significance of the porous medium transport resistance is characterized by Thiele modulus) on reactor dense phase monomer concentration and reactor temperature as well as polyethylene production rate and reactor single pass conversion for the safe temperature region. The observations reveal that significant diffusion resistance to monomer transport exists, and this can mask the intrinsic rate constants of the catalyst. The investigation of polymer growth factor indicates that, the nascent stage of polymerization is highly gas phase diffusion influenced. Intraparticle temperature gradients would appear to be negligible under most normal operating conditions.
Recent theories of bifurcation and chaos are used to analyze the dynamic behavior of the UNIPOL process for the production of polyethylene in the gas phase using the Ziegler-Natta catalyst. Dynamic behavior covers wide regions of the design and operating parameters domain of this industrially important unit. A conventional proportional-integral (PI) controller was implemented to stabilize the desired operating point on the unstable steady-state branch. The presence of the PI controller stabilized the desired unstable steady-state regions to a certain range of catalyst injection rate, by contrast, it is found out that the controlled process can go through a period doubling sequence leading to chaotic strange attractors. The practical implications of this analysis can be very serious, since chaos is shown to exist right near the desired operating point where high polyethylene production rates can be achieved
The correlation analysis (CRA) theory is an important tool in the system identification field. Among its utilities, the correlation analysis enables the mathematical modeling of a process, through the estimate of its impulse response. This model can be used directly, as in some control algorithms, or, at least, can allow the attainment of important preliminary dynamic information that is valuable in constructing a parametric model of the system. This paper presents the mathematical development of the multivariable correlation analysis and its application to simulated and experimental data. Results show that the present method is effective in identifying an industrial polymerization reactor and overperforms the one commonly used, that suggests the use of the monovariable correlation analysis even for multivariable systems. A numeric analysis of the problem is also done and some guidelines are proposed to overcome possible problems.
A mathematical model was developed to simulate the industrial process of nylon-6,6 polymerization. The process is carried out mainly in a tubular reactor under two-phase flow conditions (“flasher tube”). The model included equations for the single phase flow and the two-phase flow. The beginning of the two-phase region is determined as the point where the vapor pressure equals the total pressure. Pressure drop calculations account for frictional, gravitational, and accelerational terms, as well as for the head losses in fittings and tube expansions. Available corrections for helical coiled tubes and non-Newtonian fluids are taken into account. After parameter adjustment, the model was validated by comparison with industrial plant data and then successfully applied to debottlenecking studies in the industrial plant.
A methodology, presented by Jaeckle and MacGregor [C.M. Jaeckle and J.F. MacGregor, Product Design Through Multivariate Statistical Analysis of Process Data, AIChE J. 44 (5) (1998) 1105–1118.], for finding a window of process operating conditions within which a product with specified quality characteristics can be produced is applied to two industrial polymerization processes. The approach uses historical data available on the process operating conditions and on the corresponding product quality for a range of existing product grades. Latent variable models — built using the existing data — are inverted to obtain a window of operating conditions, which are not only capable of yielding the desired product but are also consistent with past operating procedures and constraints. A semi-batch emulsion polymerization process for manufacturing various grades of a copolymer latex, and a batch solution polymerization process for manufacturing a range of polymer resins are considered.
Phase inversion during compounding of low- viscosity ratio polystyrene/polyethylene blends was studied in two different batch mixers. Using a constant maximum-shear-rate as the scale-up criterion, longer mixing times were required in the large mixer due to its lower specific area. A new triangular element blade design was used to ob tain different batch sizes in the same mixer. On scale-up with these blades, a constant specific area was maintained and equal mixing times to phase inversion were observed.
We have used a simplified model of emulsion polymerisation, along with an adaptive calorimetric approach and a set of non-linear state estimators to monitor the individual monomer conversions in a pilot scale polymerisation reactor for the production of methyl methacrylate/butyl acrylate/carboxylic acid polymers. The usefulness of reaction calorimetry for the monitoring of such systems is demonstrated. It is also shown that non-linear, high gain estimators can be used to monitor such systems even if they are based on simplified models that ignore polymerisation of the two principal monomers in the aqueous phase, and of the carboxylic acid. The advantages and disadvantages of several different types of on-line sensors for use in industrial situations are also reviewed.
The end point, rates of reaction, and heat of reaction have been determined for the reaction of 1,10-dibromodecane and 1,6-hexamethylenediamine, which forms a moderately cross-linked polymer useful as a bile acid sequestrant in the treatment of elevated cholesterol. The parameters have been used to predict scale-up of the polymerization from the laboratory to pilot plant and full-scale production. A novel application of a bromide-specific electrode analytical method was developed to determine the extent of polymerization.
To perform on-line monitoring of the absolute weight-averaged mass, Mw, of the polymers produced in a polymerization reaction, refractometer (RI), ultraviolet absorbance (UV), and time-dependent static light scattering (TDSLS) detectors were placed in series, and a diluted stream of reactant solution was made to flow through them. The technique allows rapid determination of time-dependent reaction “signatures” and end-product masses. Hence the effects of changing reaction conditions such as reactant concentrations, temperature, and initiators can be quickly assessed. Such a technique is expected to be of wide utility in characterizing polymerization reactions, both on the laboratory scale, where new polymers are synthesized and conditions optimized, as well as on the industrial scale, where on-line quality control can be performed. For stepwise reactions, the RI and TDSLS detectors are sufficient for determination of Mw, whereas for free radical reactions, the polymer concentration must be measured in order to obtain the traditionally defined Mw (i.e. without monomer taken into consideration). The latter was achieved for poly(vinyl pyrrolidinone) polymerization by measuring the monomer concentration with the UV detector. As a further measure of characterization, a single capillary viscometer was also placed in series with the other instruments. This allowed the reduced viscosity to be monitored simultaneously.
We present a theory of non-steady state free radical polymerization kinetics at high conversions where entanglements are present. Our immediate aim is to explain apparently infinite experimental living chain lifetimes at conversions that are high, but very far from the onset of glassy behavior. In these “posteffect” studies, the time dependence of the total number of living chains is measured, after the steady state is interrupted by switching off primary radical production at t = 0. We find that infinite lifetimes are inevitable in posteffect when entanglements are present. Our starting point is a previous theoretical study of steady state entangled polymerizations according to which the principle termination mechanism for the majority long entangled chains is provided by the small population of short mobile unentangled chains. In posteffect, we find that the entire short living chain population disappears after a time scale τshort ≈ z/vp, where z is the conversion-dependent threshold for entanglement-dominated reaction kinetics and vp is the rate at which monomers add to a living chain. For t < τshort the situation is essentially unchanged from steady state and the terminated fraction Rt grows linearly in time t. But by τshort all short chains have either grown to become long or have terminated through interpolymeric radical−radical reactions. Consequently, the net termination rate is drastically suppressed for t > τshort, decaying as 1/t1/2. Correspondingly, R(t) increases as t1/2. At the longest times, t > τrad, where τrad is the mean steady state living chain lifetime, termination saturates: in the presence of entanglements, the living population is infinitely long-lived and the final terminated fraction is of order (z/N̄)1/2 1, where N̄ is the steady state living chain length. Our intermediate time prediction, R(t) t1/2, is consistent with experiment.
The power of multivariate statistical methodologies, namely, MPCA (multiway principal component analysis) and MPLS (multiway projection to latent structures or multiway partial least squares), for batch process analysis, monitoring, fault diagnosis, product quality prediction, and improved process insight is illustrated. These techniques were successfully applied to an industrial emulsion polymerization batch process. One key feature of this work is that reaction extent was used as the common reference scale to align batches with varying time durations. MPCA/MPLS technology (1) detected potential process abnormalities, (2) determined the time an abnormal event occurred, and (3) indicated the likely variable or variables which caused the abnormality. The results also indicated that variations in an ingredient trajectory and heat removal variables were primarily associated with viscosity variability. The resultant PLS model predicted the product viscosities within measurement error, thereby improving our workflow process. Process knowledge played a key role in variable selection and interpretation of the results.
Polylactides (PLA), biodegradable aliphatic polyesters, produced from renewable resources might substitute petrochemically based polymers in a broad range of applications in the near future, if we manage to produce them at lower cost and higher efficiency than nowadays. Possible applications include food packaging for meat and soft drinks, films for agro-industry and non-wovens in hygienic products. The authors developed, based on a new catalytic system, a reactive extrusion polymerisation process, which can be used to produce PLA continuously in larger quantities and at lower costs than before. This extrusion polymerisation process has been developed and tested with laboratory scale machines and has to be transferred to industrial processing equipment. This paper aims to address the problems attached with this transfer and to discuss the chances to finally achieve low cost PLA at industrial scale.
The interest for the stereospecific polymerization of olefins discovered by prof. Natta in 1954 was so high that only a few years elapsed between the discovery and the commercialization of polypropylene. The polypropylene discovery generated a tremendous scientific and technological effort for the development of the catalyst, the process, the products. The fundamental achievements in the polypropylene technology had a significant impact over the polyolefin technology in general. Thanks to it, polyolefinic polymers, copolymers and rubbers, polyefinic based materials and alloys via heterogeneous Ziegler-Natta catalyst polymerization are today largerly the most important family of plastic materials.
The ability to vary melt flow and the broad properties range of these materials, the excellent thermal and physical-mechanical properties of isotactic polyolefins together with the favorable economics and their full and easy recyclability widen the concept of monomaterial application fuelling their dynamic expansion since the early sixties.
A systematic procedure is formulated for the development of liquid-phase agitated reactors. The procedure has three components: synthesis, simulation, and scaleup. Synthesis leads the user to the reactor geometry, agitator type and speed, number and location of feed addition ports, feed addition or residence time, and heat-transfer policy that would achieve the desired performance objectives. It is based on an analysis of the interplay of reaction and mixing at various length scales, and a knowledge base of the capacities of heat-transfer equipment. Simulation assesses the performance of the reactors thus synthesized, taking into account detailed flow properties obtained through computational fluid dynamic simulations, experimental data, or a combination of both. Scaleup provides fundamentally based scaleup rules for direct development of these reactors from laboratory scale to production scale. A comparison of these derived scaleup rules with published empirical rules reveals the underlying physics, applicability, and limitations of these empirical rules. Three examples are presented to illustrate the use of this procedure.
In recent years, industrial interest in condensation copolymers with controlled microstructures has been increasing as these systems add an additional dimension to the design and manipulation of product properties without requiring completely new routes for monomer or polymer synthesis. The techniques used to control the compositional microstructure in condensation systems differ greatly from those in vinyl polymerization, as condensation polymers are continuously broken apart and reformed during the course of the polymerization. Blocky copolymers may be produced in a melt blending process only by limiting the contact time at reaction temperatures because the ultimate result of the polymerization and interchange reactions is complete randomization of the copolymer with a structure similar to that obtained in vinyl polymerization with all reactivity ratios equal to one. The design of processes yielding the desired product microstructure therefore requires a quantitative understanding of the effect of each reaction on the copolymer composition. As typical copolymer recipes include multiple monomers with different functionalities, in this paper a general copolycondensation model is presented that can accommodate an arbitrary number of monomers of differing reactivities. In this paper, only monofunctional and bifunctional monomers are considered; the extension to the case of gelating systems is left for a future paper. The use of this framework and the validity of the approach is demonstrated for an example situation in which a polyarylate is melt blended with PBT to produce a copolymer whose average sequence length may be controlled by limiting the extent of reaction. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 246–265, 2001
This article describes the application of neural networks and hybrid models to the finishing stage of nylon-6,6 polycondensation in a twin-screw extruder reactor. A planned experiment in the industrial and in the pilot plant was employed to build the neural network and the hybrid model. The hybrid model combines information calculated from the phenomenological model with the neural network model. The comparison of experimental with calculated data shows good agreement. During two years, industrial data were collected. The comparisons of the models' prediction with these data were performed and reasonable results are achieved from the industrial point of view. These models help an increase of industrial production of about 20%. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 905–912, 1999
A recently introduced online monitoring technique allows monomer conversion f, weight average polymer mass M̄w, and reduced viscosity ηr to be continuously monitored without any chromatographic columns during polymerization reactions. This technique was adapted to a Homogeneous Continuous Stirred Tank Reactor (HCSTR) to verify the quantitative predictions concerning f,M̄w, and ηr, as a function of the flow and kinetic parameters, to determine the kinetic parameters themselves, to ascertain the ideality of mixing in the reactor, to assess the effects of feed and reactor fluctuations, and to approximate a fully continuous tube type reactor. The synthesis of polyacrylamide was chosen as a practical system for the investigation. The online method should be a useful experimental technique for basic kinetic studies, development of new materials, assessment of reactor performance, verification of model studies, and monitoring of industrial scale reactors.
The industrial manufacturing of poly(vinyl chloride) (PVC) by suspension polymerization is carried out in batch reactors. The productivity increase of these processes is directly related to the reduction of the time required to complete each batch. The reactor cooling system is designed such that it is capable to compensate for the maximum rate of heat release by the exothermic polymerization, so that the reactor cooling capacity is underutilized. An attractive mode of operation is to run the industrial process isothermally and to use a mixture (“cocktail”) of different initiators, which is able to spread the polymerization rate over the batch time. In this work, the optimal formulation of an initiator mixture is studied as a dynamic optimization problem, which makes use of a representative mathematical model for the batch suspension vinyl chloride polymerization process. Results show that by choosing the optimal amounts of initiators in the cocktail, a significant reduction of the total processing time for a given polymer specification can be obtained, as compared to the case in which only one initiator is optimally chosen.
Continuous solution copolymerization is an important industrial process in the manufacture of commodity and engineering plastics. The addition of comonomers and solvent, and the rate of heat exchange must be simultaneously manipulated to maintain safety, operability, and the product quality adequately, yielding a process with nonlinear behavior, strong and asymmetric input–output multivariable coupling, and potential for open-loop instability and state multiplicity, as shown in earlier dynamics and control studies. Accordingly, the key control objectives of the copolymerization reactor are: the compensation of interaction, the preclusion of input multiplicity and the robustness (i.e., tolerance to modeling and tuning errors) of the controller. In principle, these control issues should be considered within a nonlinear setting. Otherwise, the reactor may have to be operated with a conversion that is conservatively below what can be handled by standard mixing and heat-exchange equipment. To assess the inherent control possibilities and limitations of a given copolymerization reactor, a methodology to address the control problem is proposed such that the nonlinearity, interaction, input multiplicity, and robustness issues are explicitly confronted. The result is a linear multivariable interaction compensator whose tuning can be done with notions and tools from conventional control. This method is tested with the copolymerization of vinyl acetate with methyl methacrylate, dissolved in ethyl acetate.
A one-dimensional nonequilibrium model for multicomponent condensation is used to simulate a vertical single-pass shell-and-tube heat exchanger in an industrial gas-phase polyethylene reactor system. Starting the calculation at the top of the exchanger, the model can predict temperatures at the bottom of the exchanger within an accuracy of ±5 K as compared to three sets of industrial data. Sensitivities of model predictions were analyzed, including uncertainties associated with physical and transport property estimates, step size, and convergence criterion. Model predictions are not particularly sensitive to the estimation errors of physical and transport properties if K values are calculated using an equation of state applicable to both liquid and vapor phases. Effects of operating conditions on heat removal from polyethylene reactors were investigated for an existing process. It was quantitatively demonstrated why and how severely noncondensable gases impede condensation heat transfer. The level of noncondensable gases and the cooling water temperature are the two most important factors influencing the heat-removal rate. Replacing a portion of noncondensable gas, such as N2, with a condensable fluid that is inert to polymerization reactions can substantially increase the heat-removal rate from the reactor, thereby allowing for an increase in polymer production rate.
This paper presents the optimal control policy of an industrial low-density polyethylene (LDPE) plant. Based on a dynamic model of the whole plant, optimal feed profiles are determined to minimize the transient states generated during the switching between different steady states. The industrial process under study produces LDPE by high-pressure polymerization of ethylene in a tubular reactor using oxygen and organic peroxides as initiators. The plant produces polyethylene of different grades that require continuous changes from one steady state to another, in order to switch among the different final products. These changes generate disturbances that keep the product out of specifications during the transient states, with a consequent economic loss. The plant model consists of two parts; the first one corresponds to the tubular reactor. Here, the partial differential equations corresponding to the mass and energy dynamic balances are discretized along the distance coordinate by using finite differences. The resulting ordinary differential equations include the energy balance and individual mass balances for oxygen, peroxides, ethylene, butane, free radicals and polymer. Although, methane is also present in the plant, in the reactor model it is considered as a nonreacting impurity along with the other impurities coming from the rest of the process. The second part of the model corresponds to the rest of the plant. Here we considered four components: ethylene, butane, methane and impurities. An interesting aspect of this process is the presence of many time delays that are incorporated in the optimization model. The resulting differential algebraic equation (DAE) plant model includes over five hundred equations. The dynamic optimization problem is solved using a simultaneous nonlinear programming (NLP) approach. The continuous state and control variables are discretized, by applying orthogonal collocation on finite elements. The resulting NLP is solved with a reduced space Interior Point Algorithm, which is applied directly to the NLP. In addition, a new mesh refinement strategy is applied to this model to confirm that no further improvement can be found in the optimal control profiles. The paper studies two cases of switching among different polymer grades, determining the optimal profiles of butane fed to the plant, in order to minimize the time to reach the steady state operation corresponding to the desired new product quality. The results are also compared with simpler model where reactor was considered as a black-box with the conversion level taken as constant data for each polymer grade. As a result, the dynamic model we developed and the solution methodology used is a flexible and practical tool to help process engineers for taking decisions during the plant operation.
A nonlinear model predictive control has been developed and applied to an industrial polypropylene semi-batch reactor and a high density polyethylene continuous stirred tank reactor. The control system consists of two-tier algorithms: at the first tier, an LQI is formulated based on a successively linearized nonlinear first principles process model. At the second tier, actual control actions are determined in consideration of process constraints by solving a QP problem, which is formulated by linearizing the nonlinear model around the LQI trajectory. A simple state estimator, which is capable of providing offset-free estimates of the outputs at steady states, has been designed for each application. In the semi-batch reactor process, the controller was able to maximize the monomer feed while satisfying the heat removal constraint. In the high density polyethylene process, the performance of the grade transition control was greatly improved.
Polymers, with their large spatial extent and chemical variety, afford materials scientists the opportunity to be architects at the molecular level. Once the architecture is decided, however, the task of construction moves to the chemical engineer to build the material in a faithful, stable and efficient manner. This involves assembling not only the molecular structure but also the larger scale internal micro- and meso-structure of the material. New processes are continually becoming available to the chemical engineer to accomplish these ends. New catalysts, new macromolecular building blocks, reactive processing, self-assembly, manipulation of phase behavior, applications of strong orienting fields and genetic engineering of materials are among the processing of polymers is the ability to measure product characteristics. Scattering and imaging methods have been the most powerful and the fastest growing techniques giving insight into polymer structure. Chemical engineering is as central as it always has been to leadership in the development and processing of polymers and other soft materials.
For the polymer production industries, the competitive edge will come from the technology that excels in controlling the polymer properties in a consistent way over the entire plant and in maximizing the production performance while keeping safety regulations. Based on the experience in applying advanced process control and scheduling schemes to industrial polyolefin polymerization plants, the state of the art in quality control systems for providing the polymer production plant with an enlarged capacity for product discrimination and flexibility is reviewed. On-line soft-sensing and optimal grade changeover control problems are the main focus of this paper. A quality control system for polymer production plants, which integrates optimal control with on-line sensing and scheduling techniques, is discussed making reference to an application of a prototype system to an industrial plant.
Fast time-to-market is an important issue in the development of chemical processes for fine chemicals and drug production. However, this needs to be balanced against the equally important issue of process safety. Developing a safe process within a short time frame is a demanding challenge but advances in: (i) risk analysis methods; (ii) procurement and interpretation of safety and scale-up data; and (iii) process control have opened up new perspectives in this field. The thermal stability of chemicals during storage and transportation is another field of interest, and data obtained from research in this area should allow the simplification of certain tedious procedures.
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