A thermal‐flywheel approach to distributed temperature control in microchannel reactors

Microchannel reactors are a promising route for monetizing distributed natural gas resources. However, intensification and miniaturization represent a significant challenge for reactor control. Focusing on autothermal methane‐steam reforming reactors, a novel microchannel reactor temperature control strategy based on confining a layer of phase‐change material (PCM) between the reactor plates is introduced. Melting‐solidification cycles, which occur with latent heat exchange at constant temperature, allow the PCM layer to act as an energy storage buffer—a “thermal flywheel”—constituting a distributed controller that mitigates temperature excursions caused by fluctuations in feedstock quality. A novel stochastic optimization algorithm for selecting the PCM layer thickness (i.e., distributed controller “tuning”) is introduced. Furthermore, a hierarchical control structure, whereby the PCM layer is complemented by a supervisory controller that addresses persistent disturbances, is proposed. The proposed concepts are illustrated in a comprehensive case study using a detailed two‐dimensional reactor model. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2051–2061, 2013     

Flexible on‐line Modeling and Control of pH in Waste Neutralization Reactors

The control of pH in waste neutralization processes presents a challenging highly nonlinear and time‐varying problem in which the reactor also suffers from inaccessible state information. The ability to characterize the changing dynamics of such reactors is essential to the success of advanced control schemes for these applications. In this work, flexible on‐line modeling of a pH reactor simulating nonstationary behavior was studied. This entailed a comparison of the most popular connectionist learning algorithm, the “Widrow‐Hoff delta rule”, with a classical tool in adaptive identification and control, recursive least squares (RLS). The modeling was pursued within the framework of neural networks using the ADALINE neural network. Further, two heuristically defined first‐principles‐based transforms were investigated for providing “general globally linearizing” information to the ADALINE. The comparisons of the learning algorithms for different neural network information vectors has led to a critical understanding of the flexibility of each algorithm for on‐line learning of the diverse process gain characteristics encountered in pH reactors.     

Free radical polymerization engineering—II: Modeling of homogeneous polymerization of styrene in a batch reactor,influence of initiator

A “tendency” model of free radical polymerization is used to re-investigate batch polymerization of styrene in solution in cyclohexane with slow and fast decomposing initiators. It is impossible to account for experimental data when a conventional mechanism involving only transfer to the monomer is assumed. In order to sucessfully represent mononer consumption and average molecular weights by number and by weight as a function of time for both initiators, two additional processes are required, i.e. transfer to and induced decomposition of the initiator (if transfer sites are available). The coresponding set of rate constants was determined. When a fast initiator is used, this polymerization is sensitive to mixing effects and could be utilized as a test reaction in polymer reactors. The elucidation of the role of the initiator is a good example of the usefulness of the “tendency” model for studying complex polymerization mechanisms.     

Microcatalytic reactors: Kinetics and mechanisms with isotopic tracers

Pulsed microcatalytic reactors have found extensive application in the petroleum and chemical industries where rapid catalyst screening and evaluation are demanded. Automated, continuous-operation test units allow the accumulation of considerable amounts of data in a minimum of time. The technique is also useful in research where the small pulse size enables one to study “initial” interactions between surface and reactants. In this way, information about many kinetic parameters, such as intrinsic reaction rates, orders, poisoning effects, and catalyst deactivation, can be obtained. Both stable and radioactive isotopic tracers may be used economically in microcatalytic reactors to provide significant mechanistic information which cannot be obtained conveniently by other methods. For example, one can explore the chemical nature and number of active sites, as well as the fate of individual atoms and molecules as they interact with the catalyst. Although the technique may be applied to both “simple” and complex catalytic reaction systems, the discussion will be limited to a review of data obtained from (1) “unimolecular” reactions such as cyclopropane and butene isomerization and cumene dealkylation over the mixed oxides silica-alumina, silica-magnesia, and zeolites and (2) “bimolecular” reactions such as ethylene hydrogenation over alumina. Some of the limitations of microcatalytic reactors will also be given.     

Residence time dependence of molecular weight distributions in continuous polymerizations

Residence time distribution (RTD) is a spectral property of contiuous chemical reactors. Batch reactors may be viewed as having “monodisperse” residence time distributions. This article discusses molecular weight distributions (MWDs) of polymeic materials formed in continuous and in semicontinuos process and how they are affected by reaction time distributions. All synthetic high polymers, even those Prepared in batch reaction, possess a MWD which may sometimes, for a given monomer, be altered chemically by a proper choice of catalyst and diluent. An interesting concept suggested by the present work is the prospect of “tailoring” the MWD for a given monomer-catalyst-diluent system physically by selecting appropriate reactor conditions. Hence, althought this work involves analysis the results may provide a guide to synthesis.     

Improved operating scenarios for the production of acrylonitrile‐butadiene emulsions

This article focuses on important aspects related to the production of acrylonitrile‐butadiene rubber (nitrile rubber or NBR) emulsions using a train of eight continuously stirred tank reactors. The first aspect we discuss concerns miscellaneous operating procedures, targeted toward achieving desired properties of the NBR emulsion and at the same time reducing the amount of off‐spec material during the transient period. In one of the operating procedures discussed, the first reactor is started full of “batch recipe,” whereas the remaining reactors in the train are half full with all reaction ingredients. In another case, we discuss an alternative design scenario, where the first reactor in the train is of a much smaller volume compared with the following reactors. The benefits of these two operating (production) scenarios are illustrated by comparison of different polymer and latex properties obtained with the commonly used procedures. Second, we discuss a novel criterion that can quantitatively distinguish between Cases I and II kinetics emulsion polymerization behavior with respect to continuous reactor stability (oscillatory behavior). Finally, additional feed policies are addressed, such as splitting the more reactive monomer and/or other reaction ingredients among the reactors of the train for better control of certain desired properties. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Ozone treatment process for the removal of pharmaceuticals and personal care products in wastewater

This study investigated the degradability of pharmaceuticals and personal care products (PPCPs) by ozonation for the treatment of secondary effluent of a municipal wastewater treatment plant. A set of experiments were conducted in a laboratory using a pilot-scale process consisting of three flow-through reactors in series, by varying the ozone dose (1–9 mg L^?1), the hydraulic retention time (5–15 min), and the concentration of ozone injected into the reactors (14–42 mg L^?1). Thirty-seven PPCPs were detected in the secondary effluent, which belongs to the use categories of antibiotics, analgesics, antiarrhythmic agents, anticonvulsants, vasodilators, lipid modifying agents, anti-itch drugs, anti-psychotic drugs, insect repellents, bronchodilators, diuretics, peptic ulcer drugs, NMDA receptor antagonists, antifungal drugs, antimicrobial drugs, and antineoplastic agents. These PPCPs were broadly classified into five groups ranging from “sensitive” to ozone (O₃) or “unstable” in the ozonation process, to the group of “insensitive” to O₃ or “very stable” in the ozonation process. These groups are based on the PPCP concentrations after the ozone treatment and their limit of detection (LOD). Furthermore, this study examined comparatively the effects of the ozone dose, the retention (reaction) time, and the concentration of O₃ supplied to the reactors on the degradability of the PPCPs.     

Development of a dynamic circulation-interaction model for mechanically agitated,liquid-liquid reactors

A mathematical model based on the circulation-interaction concept was developed for the dynamic behaviour of continuous, mechanically-agitated, liquid-liquid reactors. The model is described by a set of ordinary, non-linear differential equations and a set of algebraic equations. These equations provide the relationships between the state variables of the system and the inputs to the system. The model permits the study of the dynamic response of the state variables of a liquid-liquid reactor to disturbances and manipulation of the operating conditions. The model can assist in understanding the dynamics of such a system and should be useful for reactor design and design of control systems. Two numerical examples with reasonable and realistic parameters and data values were studied. “Hot” spots can develop during steady-state operations and sensitive and dangerous cases can develop during the dynamic response.     

The bifurcation behavior of tubular reactors

Methods for studying the bifurcation behavior of tubular reactors have been developed. This involves the application of static and Hopf bifurcation theory for PDE's and the very precise determination of steady state profiles. Practical computational methods for carrying out this analysis are discussed in some detail. For the special case of a first order, irreversible reaction in a tubular reactor with axial dispersion, the bifurcation behavior is classified and summarized in parameter space plots. In particular the influence of the Lewis and Peclet numbers is investigated. It is shown that oscillations due to interaction of dispersion and reaction effects should not exist in fixed bed reactors and moreover, should only occur in very short “empty” tubular reactors. The parameter study not only brings together previously published examples of multiple and periodic solutions but also reveals a hitherto undiscovered wealth of bifurcation structures. Sixteen of these structures, which come about by combinations of as many as four bifurcations to multiple steady states and four bifurcations to periodic solutions, are illustrated with numerical examples. Although the analysis is based on the pseudohomogeneous axial dispersion model, it can readily be applied to other reaction diffusion equations such as the general two phase models for fixed bed reactors.     

Structure of operating domains of loop reactors

A loop reactor (LR), composed as an N-unit loop with step-wise shifted inlet and outlet ports, is one of suggested technological solutions for low-concentration volatile organic compounds (VOC) combustion. Such a scheme ensures a sufficiently high temperature with autothermal behavior and nearly uniform catalytic utilization. The main drawback of the LR is a very narrow window of switching velocities that sustain a stable “frozen” solution that exists if the switching and the pulse velocity are synchronized. In the present work we show the existence of many “finger”-like domains of complex frequency-locked solutions that allow to significantly extend the operation domain, rendering the LR scheme more attractive for practical implementation. A brief comparison with the other heat recuperation technologies (reverse flow and circular loop reactors) is presented. © 2008 American Institute of Chemical Engineers AIChE J, 2008     

Estimation and control in polymerization reactors. A review

The control of polymerization reactors is particularly arduous for several reasons; (a) difficulty in specification of the control objectives; (b) high exothermicities and viscosities involved: (c) complicated process dynamics: and (d) problems with measurement of the polymer structure. Computer control has now simplified many tasks such as sequential operations of batch reactors; but no less important, it has made feasible the implementation of elaborate state estimation and control algorithms. The tendency in the polymerization industry and in current research centers is centered on producing ‘tailor-made’ polymers from the view of its microscopic and/or macroscopic structures. To this end, several solutions have been proposed. For example, in the ‘chemical approach’, open loop methods have been developed based on the detailed dynamic models of the processes. Any control technique requires a knowledge of the process to be controlled through mathematical models. These models may be developed either from detailed mass and energy balances, or through empirical input-output or “black-box” techniques. The most promising control techniques are probably optimal control, adaptive control, and their combinations. Optimal control is powerful because it utilizes directly the detailed non-linear plant model. Adaptive control developed for linear plants has already proven satisfactory, in spite of the highly nonlinear nature of the polymerizations. Undoubtedly, improved solutions will be obtained when this technique is extended to non-linear systems.     

Viscoelastic study of ionomers

Stress-relaxation curves were obtained for ionomers containing different cations, per cents of ionization, and thermal treatments. Differences in the rheological behavior were found to depend more on the ionization level than on the ion. A recently proposed model for ionomers is discussed and found to be consistent with these results. In terms of this model the degree of ionization in the polymer acts as a regulator for the growth of small oriented lamellar (crystalline) regions. In the most general terms, the mechanical behavior and strength of ionomers appears dominated by the existence of “hard” regions interspersed among “soft” regions. In the polymers studied here there was some slight crystallinity; however, similar effects and explanations are probably suitable for amorphous “ionomers.” Toughness was also found in some completely amorphous carboxylcontaining copolymers without added ionic salts. The same explanation of “hard” regions interspersed among soft regions is also valid here. The “blocky” nature of the copolymerization may play a role in setting up this type of structure.     

Dynamics and stability of ethylene polymerization in multizone circulating reactors

Multizone circulating bed reactors (MZCR) have the exclusive characteristics of producing polymers of different molecular weights in a single particle. Traditional fluidized bed reactors, on the other hand, can produce only one kind of molecular weight with relatively narrow distribution. A dynamic model for the MZCR is used to illustrate the basic dynamic behavior of the new reactor design used for polyethylene production. The model is used to study the copolymerization of ethylene with butene. Several parameter sensitivity analyses are performed to show the computer-simulated time responses for reactor temperature, number-average molecular weight, weight-average molecular weight, catalyst feed rate and the monomer/comonomer concentration along the reactor length. At certain operating conditions dynamic instability is observed and the results for the effect of cooling water temperature, catalyst feed rate, monomer and comonomer initial feed concentration on the reactor temperature and polymer molecular weight reveal that the system is very sensitive to disturbances in the heat exchanger coolant temperature. Also, at some operating conditions, the reactor temperature oscillates above the polymer melting temperature. Temperature runaway above polymer softening point is a serious problem which may cause polymer melting and hence reactor shutdown. The oscillatory behavior of the reactor temperature necessitates a suitable temperature control scheme to be installed.     

SOME ASPECTS OF DESIGN OF COMMERCIAL REACTORS FOR DIRECT COAL LIQUEFACTION

In recent years, a number of direct coal liquefaction processes have been developed. All processes use a slurry type reactor. Although for lab-scale reactors of large length-to-diameter ratio the use of highly sophisticated slurry reactor model may be justified, simple considerations can meaningfully elucidate the behavior of industrial reactors. A simple analysis shows that the coal liquefaction is controlled by intrinsic kinetics. Both gas and slurry phases can be assumed to be completely backmixed in large diameter reactors. A simple analysis of the thermal behavior revealed multiplicity for a fairly wide range of operating conditions. In most cases, the intermediate unstable steady state is close to the temperature observed in adiabatic coal liquefaction reactors (with and without quench). Due to the unstable character of the operation, point pathological phenomena like runaway may be possible and a close feedback control of the commercial reactor may be required.     

Intensified energetically enhanced steam methane reforming through the use of membrane reactors

This work focuses on the implementation of membrane reactors (MRs) in the production of hydrogen through steam–methane reforming (SMR). A novel equilibrium MR model featuring Gibbs Free Energy Minimization is introduced and applied to the SMR-MR process. In addition, the concept of “energetically enhanced steam methane reforming (EER),” which allows for the use of a hybrid (methane combustion/renewable energy) energy supply in the production of hydrogen, is intensified. The UNISIM software (Honeywell™) is used to create a range of intensified flowsheets depicting the proposed IEER-MR process as well as two baseline flowsheets depicting “a standard SMR-MR process” and “a fully exothermic EER process.” Heat integration studies are carried out on the developed flowsheets, and the baseline designs are compared to the IEER-MR designs to identify energetic intensification.     

Cocurrent downflow circulating fluidized bed (downer) reactors — A state of the art review

In the last decade, cocurrent downflow circulating fluidized bed reactors, called “downer” reactors, have been proposed as an alternative to upflow circulating fluidized bed, or “riser”, reactors. In this paper, published results on downer studies are summarized and future directions of research are recommended. Downer reactors are shown to have several distinct advantages over upflow circulating fluidized bed reactors and can potentially be used in many industrial processes, mainly due to a more uniform gas and solids flow structure compared with risers.     

Some new operational modes and parameters of stress relaxation for the viscoelastic characterization of solid polymers. II. The “vertical‐shift” mode and intrinsic “time–strain clock”

As in part I of this study, in the same manner in the present part II as well, by the same modus operandi way, an attempt was made to introduce, by means of a given operational mode, some further practical parameters for a “by‐eye” but well‐proven experimental viscoelastic characterization of a polymeric solid. Thus, through consideration of the peculiar vertical shift behavior of the apparent modulus (?) of isotactic polypropylene (iPP) and based on the KWW model, it is shown that, in an empirical and a formalistic sense, a relevant effective or equivalent (single) characteristic relaxation time can be introduced which can give some new interpretations for the linear and nonlinear viscoelastic behavior of a polymeric material, as that of a “time–strain clock,” which, as an intrinsic function, is responsable for a functional time–strain shift of the relaxation time and, at the same time, for a shift toward to a more linear or to a more nonlinear behavior. In the above context of attempts, another functional relationship was shown, the so‐called spectral shift function and its corresponding parameter of nonlinearity strength, through which some further interpretations and characterizations connected with the existence of the permanent internal stress could be made. Finally, the introduction of the so‐called spectral isostrains and their corresponding “spectral inversion point” complete the “set” of the operative parameters proposed for the above‐mentioned purpose. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 138–148, 2003     

高聚物扩散体系中的“超调”现象

利用在EIT理论基础上的Jou模型研究了高聚物扩散体系中的“超调”现象.经过对Jou模型的简化处理，得到了双曲型的扩散动力学方程，通过对模型中3个组合参数的仿真分析，研究了参数的变化可能导致的不同的扩散行为.结果表明，Jou模型成功描述了长时间吸附中发现的“超调”现象，从理论上说明了引起“超调”现象的原因：扩散的瞬态过程引起的滞后是导致“超调”扩散行为的必要条件，而在扩散滞后存在的情况下，松弛时间大于扩散时间是引起“超调”的充分条件.     

Product sampling during transient continuous countercurrent hydrolysis of canola oil and development of a kinetic model

A chemical kinetic model has been developed for the transient stage of the continuous countercurrent hydrolysis of triglycerides to free fatty acids and glycerol. Departure functions and group contribution methods were applied to determine the equilibrium constants of the four reversible reactions in the kinetic model. Continuous countercurrent hydrolysis of canola oil in subcritical water was conducted experimentally in a lab-scale reactor over a range of temperatures and the concentrations of all neutral components were quantified. Several of the rate constants in the model were obtained by modeling this experimental data, with the remaining determined from calculated equilibrium constants. Some reactions not included in the present, or previous, hydrolysis modeling efforts were identified from glycerolysis kinetic studies and may explain the slight discrepancy between model and experiment. The rate constants determined in this paper indicate that diglycerides in the feedstock accelerate the transition from “emulsive hydrolysis” to “rapid hydrolysis”.     

Parametric sensitivity in tubular polymerization reactors

A recent technique for studying the parametric sensitivity of chemical reactors is applied to tubular chain homopolymerization reactors. The sensitivities of the temperature maxima with respect to various parameters of the model are computed. Using conditions typically encountered for high-pressure polyethylene systems, it is found that the temperature sensitivities with respect to all the nine parameters have their maxima at approximately the same value of the feed initiator concentration, thus leading to a generalized sensitivity-based constraint for design. It is also found that, under usual conditions of operation, no significant design constraints on the feed temperature are indicated. Detailed sensitivity plots are presented, which could be used to obtain “safe” operating conditions. The effects of changing the most important parameters, the dimensionless heat of reaction and the dimensionless activation energy (ϵ), on the sensitivity envelope are also investigated. Our studies reveal that better estimates of ϵ than are available presently are required. Sensitivities of the number-average chain length maxima with respect to the same nine parameters are also computed. Under conditions where the steady-state hypothesis applies, estimates of these sensitivities can be obtained analytically. However, for the usual values of the parameters, and close to “sensitive” values of the feed initiator concentration, this hypothesis does not apply, and the chain length sensitivities need to be obtained numerically. In the absence of the gel effect, chain length sensitivities do not usually provide design constraints because of the very low monomer conversions encountered.