Showing papers in "Aiche Journal in 1996"

Theoretical model for drop and bubble breakup in turbulent dispersions

Abstract: A theoretical model for the prediction of drop and bubble (fluid-particle) breakup rates in turbulent dispersions was developed. The model is based on the theories of isotropic turbulence and probability and contains no unknown or adjustable parameters. Unlike previous work, this model predicts the breakage rate for original particles of a given size at a given combination of the daughter particle sizes and thus does not need a predefined daughter particle size distribution. The daughter particle size distribution is a result and can be calculated directly from the model. Predicted breakage fractions using the model for the air–water system in a high-intensity pipeline flow agree very well with the available 1991 experimental results of Hesketh et al. Comparisons of the developed model for specific particle breakage rate with earlier models show it to give breakage-rate values bracketed by other models. The spread in predictions is high, and improved experimental studies are recommended for verification.

Identification of faulty sensors using principal component analysis

Abstract: Even though there has been a recent interest in the use of principal component analysis (PCA) for sensor fault detection and identification, few identification schemes for faulty sensors have considered the possibility of an abnormal operating condition of the plant. This article presents the use of PCA for sensor fault identification via reconstruction. The principal component model captures measurement correlations and reconstructs each variable by using iterative substitution and optimization. The transient behavior of a number of sensor faults in various types of residuals is analyzed. A sensor validity index (SVI) is proposed to determine the status of each sensor. On-line implementation of the SVI is examined for different types of sensor faults. The way the index is filtered represents an important tuning parameter for sensor fault identification. An example using boiler process data demonstrates attractive features of the SVI.

Effect of droplet size on the rheology of emulsions

Abstract: The effect of droplet size on the rheological behavior of water-in-oil and oil-in-water emulsions was investigated using a controlled-stress rheometer. Results indicate that the droplet size has a dramatic influence on emulsion rheology. Fine emulsions (water-in-oil or oil-in-water) have much higher viscosities and storage moduli than the corresponding coarse emulsions. The shear-thinning effect is much stronger in the case of fine emulsions. When coarse droplets are replaced by fine droplets (keeping total volume fraction of the dispersed phase constant), the resulting emulsion exhibits a minimum in rheological properties (viscosity, storage and loss moduli, time constant) at a certain proportion of fine droplets. However, the minimum in viscosity occurs only at low shear stresses. At high stresses, the viscosity of the mixed emulsion increases as the proportion of fine droplets increases. The study of the aging effect on the rheological behavior shows that water-in-oil emulsions age much more rapidly than the oil-in-water emulsions.

A moving horizon-based approach for least-squares estimation

Abstract: A general formulation of the moving horizon estimator is presented. An algorithm with a fixed-size estimation window and constraints on states, disturbances, and measurement noise is developed, and a probabilistic interpretation is given. The moving horizon formulation requires only one more tuning parameter (horizon size) than many well-known approximate nonlinear filters such as extended Kalman filter (EFK), iterated EKF, Gaussian second-order filter, and statistically linearized filter. The choice of horizon size allows the user to achieve a compromise between the better performance of the batch least-squares solution and the reduced computational requirements of the approximate nonlinear filters. Specific issues relevant to linear and nonlinear systems are discussed with comparisons made to the Kalman filter, EKF, and other recursive and optimization-based estimation schemes.

Estimation of gas solubilities in salt solutions at temperatures from 273 K to 363 K

Abstract: The objective of this article is to extend the model of Schumpe (1993) to a wider temperature range. To this end, solubility data reported in the literature for temperatures from 273.15 to 363.15 K were analyzed to obtain a set of 892 Sechenov constants for different gas/salt/temperature combinations. Gas solubility was expressed in the form of the Bunsen coefficient (volume of gas, reduced to 273.15 K and 101.3 kPa, absorbed per unit volume of solvent at 101.3 kPa partial pressure). Only the relative solubility is considered and any out of various proportional measures (Schumpe et al., 1982) could be used.

Statistical process monitoring and disturbance diagnosis in multivariable continuous processes

Abstract: Detecting out-of-control status and diagnosing disturbances leading to the abnormal process operation early are crucial in minimizing product quality variations. Multivariate statistical techniques are used to develop detection methodology for abnormal process behavior and diagnosis of disturbances causing poor process performance. Principal components and discriminant analysis are applied to quantitatively describe and interpret step, ramp and random-variation disturbances. All disturbances require high-dimensional models for accurate description and cannot be discriminated by biplots. Diagnosis of simultaneous multiple faults is addressed by building quantitative measures of overlap between models of single faults and their combinations. These measures are used to identify the existence of secondary disturbances and distinguish their components. The methodology is illustrated by monitoring the Tennessee Eastman plant simulation benchmark problem subjected to different disturbances. Most of the disturbances can be diagnosed correctly, the success rate being higher for step and ramp disturbances than random-variation disturbances.

Coagulation and fragmentation: Universal steady-state particle-size distribution

Abstract: A population balance model presented describes simultaneous coagulation and fragmentation during shear-induced flocculation. Given sufficient time, a floc-size distribution reaches steady state that reflects the balance between coagulation and fragmentation. The model agrees with experimental data for the evolution of the average floc size. Higher shear shifts the steady-state size distribution to smaller sizes. When the steady-state size distributions obtained at various shear rates are scaled with the average floc size, however, they collapse onto a single line. This indicates that the steady-state floc-size distribution is self-preserving with respect to fluid shear. This distribution is universal for the employed coagulation and fragmentation rates provided that less than 5% (by number) of the particles remain unflocculated. This result is supported with experimental data on shear-induced flocculation of polystyrene particles, although a detailed quantitative comparison is limited by the irregular structure of the flocs.

Journal review. Azeotropic distillation

Abstract: Recent and ongoing research in the distillation of nonideal mixtures is reviewed focusing on advances in the methodologies for the synthesis, design, analysis and control of separation sequences involving homogeneous and heterogeneous azeotropic towers. Maps of residue curves and distillation lines are examined, as well as geometric methods for the synthesis and design of separation sequences, trends in the steady-state and dynamic analysis of homogeneous and heterogeneous towers, the nonlinear behavior of these towers, and strategies for their control. Emphasis is placed on the methods of computing all of the azeotropes associated with a multicomponent mixture, on the features that distinguish azeotropic distillations from their zeotropic counterparts, on the potential for steady-state multiplicity, and on the existence of maximum and minimum reflux bounds. Important considerations in the selection of entrainers are examined. For the synthesis of separation trains, when determining the feasible product compositions, the graphical methods are clarified, especially the conditions under which distillation boundaries can be crossed and bounding strategies under finite reflux. The application of geometric theory to locate the fixed points, at minimum reflux, is reviewed in connection with homotopy-continuation algorithms for this purpose. The use of homotopy-continuation algorithms, especially for the steady-state simulation of heterogeneous azeotropic distillations, is justified. Methods for phase stability analysis are reviewed in connection with the location of real bifurcation points at phase transitions, an important feature of algorithms for the dynamic simulation of heterogeneous azeotropic distillations.

Optimal design of dynamic systems under uncertainty

Abstract: Fundamental developments of a unified process design framework for obtaining integrated process and control systems design, which are economically optimal and can cope with parametric uncertainty and process disturbances, are described. Based on a dynamic mathematical model describing the process, including path constraints, interior and end-point constraints, a model that describes uncertain parameters and time-varying disturbances (for example, a probability distributions or lower/upper bounds), and a set of process design and control alternatives (together with a set of control objectives and types of controllers), the problem is posed as a mixed-integer stochastic optimal control formulation. An iterative decomposition algorithm proposed alternates between the solution of a multiperiod “design” subproblem, determining the process structure and design together with a suitable control structure (and its design characteristics) to satisfy a set of “critical” parameters/periods (for uncertainty disturbance) over time, and a time-varying feasibility analysis step, which identifies a new set of critical parameters for fixed design and control. Two examples are detailed, a mixing-tank problem to show the analytical steps of the procedure, and a ternary distillation design problem (featuring a rigorous tray-by-tray distillation model) to demonstrate the potential of the novel approach to reach solutions with significant cost savings over sequential techniques.

Thermodynamics of Wax Precipitation in Petroleum Mixtures

Abstract: A thermodynamic framework is developed for calculating wax precipitation in petroleum mixtures over a wide temperature range. The framework uses the experimentally supported assumption that precipitated wax consists of several solid phases; each solid phase is described as a pure component or pseudocomponent that does not mix with other solid phases. Liquid-phase properties are obtained from an equation of state. Calculated wax-precipitation data are in excellent agreement with experimental results for binary and multicomponent hydrocarbon mixtures, including petroleum.

Asphalt flocculation and deposition: I. The onset of precipitation

Abstract: Formation of asphalt aggregates and their deposition on the pore surfaces of a porous medium, which alter the structure of the medium and its effective properties, is a critical problem to catalytic and oil recovery and refinery processes. Extensive new experimental data for the amount of precipitated asphalt formed with crude oil and various solvents are presented. Results indicate that, contrary to the previous assumptions, asphalt formation is at best partially reversible. A thermodynamic model based on the Flory–Huggins theory of polymer solutions is used, together with the Soave equation of state, to predict the data. Critical evaluation of the model shows that its predictions do not agree well with our data. As an alternative, we propose a new model that employs a scaling equation, somewhat similar to those encountered in aggregation and gelation phenomena. The scaling function takes on a very simple form, and its predictions are in very good agreement with the data. It also predicts that the onset of precipitation may obey a simple universal equation.

Effects of mixing on the composition and morphology of tissue‐engineered cartilage

Abstract: Cartilage constructs were grown using isolated chondrocytes and biodegradable polymer scaffolds made of fibrous polyglycolic acid in the form of 1-cm-dia × 5-mm-thick discs. The scaffolds were seeded in a mixed cell suspension and cultured for up to 8 weeks under static or mixed tissue culture conditions in petri dishes and spinner flasks. Turbulent mixing significantly improved the biochemical compositions and altered morphologies of the cartilage constructs, which were the thickest ones cultured to date in vitro. Constructs from mixed cultures were more regular in shape and contained up to 70% more cells, 60% more sulfated glycosaminoglycan, and 125% more total collagen when compared to constructs from static cultures. Mixing also induced the formation of an outer capsule with multiple layers of elongated cells and collagen fibrils around the inner tissue phase, while statically grown constructs consisted of round cells embedded in cartilaginous matrix. Mixing during cell seeding and tissue culture is thus an important parameter for the cultivation of tissue-engineered cartilage in a range of sizes, shapes and compositions for a variety of clinical applications (e.g., fibrous cartilage for reconstructive surgery or articular cartilage for joint resurfacing).

Preferentially oriented submicron silicalite membranes

Abstract: An oriented submicron silicalite membrane has been prepared by growing a layer of oriented silicalite crystals on a composite precursor nanocrystalline silicalite/alumina film using controlled secondary growth. The orientation of the crystals at the surface was such that both straight and sinusoidal channel networks run nearly parallel to the membrane surface. The membrane exhibits ideal selectivities for H{sub 2} over N{sub 2} as high as 60 at 150 C and O{sub 2} over N{sub 2} as high as 3.6 at 185 C, H{sub 2}, N{sub 2} and O{sub 2} permeances are 2.13, 0.05 and 0.17 cm{sup 3}(STP)/(cm{sup 2}{center_dot}min{center_dot}atm) at 185 C, respectively, and the corresponding apparent activation energies are 11, 26 and 32 kJ/mol. The permeation characteristics are attributed to the preferred orientation of the molecular sieving layer.

Comparison of isotherm models for hydrocarbon adsorption on activated carbon

Abstract: A number of equilibrium isotherm models can be utilized for correlating experimental equilibrium results. Seven different isotherm models are studied using equilibrium data of methane, ethane and propane in activated carbon. Besides comparing the goodness of data fit, the limiting behaviors as well as the pressure and temperature derivatives of the equilibrium isotherm models are also investigated. Performance of multicomponent extensions of these isotherm models and their combinations with the ideal adsorbed solution theory are also compared with experimental data. This systematic evaluation of the more important equilibrium isotherm models provides the general basis for making a preliminary selection of an effective model for a given application. Although an accurate and thermodynamically consistent model is desirable, these requirements may often be compromised for computational simplicity in dynamic process modeling studies.

Impact of tank geometry on the maximum turbulence energy dissipation rate for impellers

Abstract: The maximum turbulence energy dissipation rate per unit mass, emax, is an important variable in dispersion systems, particularly for drop breakup and coalescence, and for gas dispersion. The effect of tank geometry (number of baffles, impeller diameter, and off-bottom clearance) on emax for four impellers (the Rushton turbine, RT; the pitched blade turbine, PBT; the fluidfoil turbine, A310; and the high-efficiency turbine, HE3) is examined. Mean and fluctuating velocity profiles close to the impellers were measured in a cylindrical baffled tank using laser doppler velocimetry. Local and maximum turbulence energy dissipation rates in the impeller region were estimated using e = Av3/L with A = 1 and L = D/10 for all four impellers. Factorial designs were used to test for the effects of single geometric variables under widely varying conditions and interactions between variables. Several factorial designs were used to ensure that real effects were separated from effects that appeared as an artifact of the experimental design. Results show that the tank geometry has a significant effect on emax, primarily with respect to variations in impeller diameter and interactions between the off-bottom clearance and impeller diameter. For the same power input and tank geometry, the RT consistently produces the largest emax and/or emax scaled with N3D2.

Data reconciliation and gross‐error detection for dynamic systems

Abstract: Gross-error detection plays a vital role in parameter estimation and data reconciliation for dynamic and steady-state systems. Data errors due to miscalibrated or faulty sensors or just random events nonrepresentative of the underlying statistical distribution can induce heavy biases in parameter estimates and reconciled data. Robust estimators and exploratory statistical methods for the detection of gross errors as the data reconciliation is performed are discussed. These methods have the property insensitive to departures from ideal statistical distributions and to the presence of outliers. Once the regression is done, the outliers can be detected readily by using exploratory statistical techniques. Optimization algorithm and reconciled data offer the ability to classify variables according to their observability and redundancy properties. In this article, an observable variable is an unmeasured quantity that can be estimated from the measured variables through the physical model, while a nonredundant variable is a measured variable that cannot be estimated other than through its measurement. Variable classification can be used to help design instrumentation schemes. An efficient method for this classification of dynamic systems is developed. Variable classification and gross-error detection have important connections, and gross-error detection on nonredundant variables has to be performed with caution.

Sorption-enhanced reaction process

Abstract: A process for carrying out simultaneous reaction and separation of desired products in a single unit operation is described. It uses a fixed packed column of an admixture of a catalyst and a sorbent that selectively removes a reaction by-product from the reaction zone. The sorbent is periodically regenerated by using the principles of pressure-swing adsorption. The process steps allow direct production of the desired product at high purity and at the reaction pressure. High conversion of the reactants to products in an endothermic, equilibrium-controlled reaction can be achieved while operating the reaction at a substantially lower temperature than would be necessary by a plug-flow reactor packed with the catalyst alone. The equilibrium-controlled reverse water-gas shift reaction for the production of carbon monoxide is experimentally evaluated as a proof of the concept.

Three‐phase reactor model for hydrotreating in pilot trickle‐bed reactors

Abstract: A three-phase reactor model for describing the hydrotreating reactions in a trickle-bed reactor was developed. It includes correlations for determining mass-transfer coefficients, solubility data, and properties of the compounds under process conditions. The model, based on the two-film theory, was tested with regard to the hydrodesulfurization of vacuum gas oil in a new high-pressure pilot plant operated under isothermal conditions. The sulfur content of the product oil was found to depend strongly on the gas/oil flow ratio within the reactor. This is due to the inhibiting effect of hydrogen sulfide on the chemical reaction rates described by Langmuir-Hinshelwood kinetics. The poor conversion which, in contrast to industrial plants, is often observed in pilot plant reactors can be explained by incomplete catalyst wetting produced by low liquid velocities. The simulation shows a good agreement with the experiments carried out in a wide range of temperature, pressure, space velocity and gas/oil ratio.

Feedback Control of Hyperbolic PDE Systems

Abstract: Transport-reaction processes in which the diffusive and dispersive phenomena are negligible compared to the convective phenomena can be adequately described by systems of first-order hyperbolic PDEs. Representative chemical processes modeled by such systems include heat exchangers [115], plug-flow reactors [115], fixed-bed reactors [130], pressure swing adsorption processes [119], and so forth.

Micromixing effects in a two-impinging-jets precipitator

Abstract: To carry out rapid precipitation on a practical scale, the use of a two-impinging-jets (TIJ) mixer was explored in terms of its ability to deliver rapid micromixing. The two-step Bourne reaction scheme between 1-naphthol and diazosulfanilic acid was used to characterize micromixing in the TIJ mixer as a function of hydrodynamics and mixer geometry. The characteristic time for micromixing in this device was at least as small as 65 ms. The larger the jet diameter, the higher the jet Reynolds number required to achieve the same micromixing quality. The micromixing time in the TIJ mixer correlated well as a function of a Damkohler number that led to a scale-up criterion giving an inverse relationship between the micromixing time constant and the 1.5 power of the jet velocity. Finally, the precipitation of Lovastatin was carried out in the TIJ mixer under well and poorly micromixed conditions. The level of micromixing in the precipitator affected the crystal size distribution of the precipitated product only when the time constant for micromixing was comparable to or larger than the induction time for nucleation of the solute.

Thermodynamic micellizatin model of asphaltene precipitation from petroleum fluids

Abstract: A thermodynamic micellization model is proposed for the description of asphaltene precipitation from petroleum fluids. It describes the solubilization of asphaltene polar species by resin bipolar molecules in the micelles. A simple form of the standard Gibbs free energy of micellization is used. The petroleum fluid is assumed to be a dilute solution with respect to the monomeric asphaltenes, resins, and micelles. The Peng-Robinson equation of state (PR-EOS) is applied to describe the fugacity of monomeric asphaltene in the bulk of the petroleum fluid. Intermicellar interactions as well as osmotic pressure effects are neglected. The proposed model shows promising results to describe asphaltene deposition from crude mixtures. It predicts the change in precipitation power of different alkane precipitants and the effect of pressure on asphaltene precipitation. The amount and the onset of predicted asphaltene precipitation are sensitive to the amount of resins in the crude. All these results are in line with laboratory observations and oil-field data.

Molar mass distribution and solubility modeling of asphaltenes

Abstract: Attempts to model asphaltene solubility with Scatchard-Hildebrand theory were hampered by uncertainty in molar volume and solubility parameter distribution within the asphaltenes. By considering asphaltenes as a series of polyaromatic hydrocarbons with randomly distributed associated functional groups, molar volume and solubility parameter distributions are calculated from experimental measurements of molar mass and density. The molar mass distribution of Athabasca asphaltenes is determined from interfacial tension and vapor pressure osmometry measurements together with plasma desorption mass spectrometry determinations from the literature. Asphaltene densities are calculated indirectly from mixtures of known concentration of asphaltene in toluene. Asphaltene density, molar volume, and solubility parameter are correlated with molar mass. Solid-liquid equilibrium calculations based on solubility theory and the asphaltene property correlations successfully predict experimental data for both the precipitation point and the amount of precipitated asphaltenes in toluene-hexane solvent mixtures.

Generalized modular representation framework for process synthesis

Abstract: A generalized modeling framework for process synthesis alternatives is proposed, based on fundamental mass/heat-transfer principles. A multipurpose mass/heat-transfer module is introduced as the building block of the framework, whereas basic block-superstructure rules (such as splitting, mixing, and bypassing) are used to develop a systematic representation of process units and process structures, conventional or not. Process synthesis procedures are explored within this modeling framework, where synthesis alternatives are not postulated as process unit networks, but explored simultaneously without predefining synthesis schemes, as combinations of mass/heat-exchange stream matches. The representation potential of this framework is illustrated with examples from unit operations, whereas synthesis example problems are presented to show the broad range of process alternatives that can be modeled, identified, and optimized.

Hydrodynamic Modeling of Gas/Particle Flows in Riser Reactors

Abstract: Complex hydrodynamic behavior of circulating fluidized beds makes their scale-up very complicated. In particular, large-scale lateral solids segregation causes a complex two-phase flow pattern which influences significantly their performance. Lateral solids segregation has been attributed to direct collisional interactions between particles as well as to interaction between gas-phase eddies and dispersed particles. However, these phenomena have not been investigated thoroughly. This article discusses an advanced 2-D hydrodynamic model developed for circulating fluidized beds based on the two-fluid concept. Because theory to model the interaction between gas-phase eddies and dispersed particles is not available, turbulence was modeled on a macroscopic scale using a modified Prandtl mixing length model. To model the influence of direct particle-particle collisions the kinetic theory for granular flow was applied based on the Chapman-Enskog theory of dense gases. For model validation purposes, a cold flow circulating fluidized bed was employed in which sand was transported with air as fluidizing agent. The column is equipped with pressure transducers to measure the axial pressure profile and with a reflective optical fiber probe to measure the local solids concentration and axial solids velocity. Theoretically calculated solids concentration and axial solids velocity agree satisfactorily with experiment, especially when one realizes that the model contains no adjustable parameters. In general, however, the model slightly underpredicted the experimentally observed lateral solids segregation and yielded a more peaked velocity profile compared to its experimental counterpart

Gas holdup in bubble column reactors operating in the churn-turbulent flow regime

Abstract: A comprehensive experimental study of gas holdup in bubble columns of varying diameters, fitted with different distributor types, using several liquids is presented. Air was used as the gas phase. Experiments to test the influence of gas density were also carried out with He, Ar, and SF6. A generalization of the two-phase model for gas–solid fluidized beds was used to interpret the experimental data where the “dilute” phase is identified with the “large” bubble population and the “dense” phase with the liquid phase where the “small” bubble population is entrained. Gas holdups in dilute and dense phases were determined from dynamic gas disengagement experiments. In the churn-turbulent regime of operation, voidage of the gas in the dense phase was independent of the superficial gas velocity. Reilly et al.'s correlations for the gas holdup and superficial gas velocity at the regime transition point estimate the gas voidage of the dense phase and the superficial gas velocity well through this phase. Corresponding correlations of Wilkinson et al. significantly underpredict dense-phase parameters. The experiment showed that the dilute phase or large bubble holdup in bubble columns, operating at superficial gas velocities > 0.1 m/s, is independent of liquid properties, how the gas is distributed and the density of the gas phase. But it is affected significantly by the column diameter. Relying on hydrodynamic analogies with a gas–solid–fluid bed, a simple correlation was developed that is considerably more accurate than the Wilkinson correlation that significantly overpredicts large bubble holdup.

Use of biased-relay feedback for system identification

Abstract: The purpose of this work is to devise a relay feedback experiment that can identify two points on the Nyquist curve from a single test at the frequencies ω=0 and ω=ω u . An identification procedure is devised accordingly for this parameter identification. The theory of equivalent gain is described, the biased relay feedback system is analyzed, and the potential problems are explored. Parametric system identification is discussed followed by the conclusion

Analysis and design of metabolic reaction networks via mixed‐integer linear optimization

Abstract: Improvements in bioprocess performance can be achieved by genetic modifications of metabolic control structures. A novel optimization problem helps quantitative understanding and rational metabolic engineering of metabolic reaction pathways. Maximizing the performance of a metabolic reaction pathway is treated as a mixed-integer linear programming formulation to identify changes in regulatory structure and strength and in cellular content of pertinent enzymes which should be implemented to optimize a particular metabolic process. A regulatory superstructure proposed contains all alternative regulatory structures that can be considered for a given pathway. This approach is followed to find the optimal regulatory structure for maximization of phenylalanine selectivity in the microbial aromatic amino acid synthesis pathway. The solution suggests that from the eight feedback inhibitory loops in the original regulatory structure of this pathway, inactivation of at least three loops and overexpression of three enzymes will increase phenylalanine selectivity by 42%. Moreover, novel regulatory structures with only two loops, none of which exists in the original pathway, could result in a selectivity up to 95%.

Decomposition techniques for the solution of large-scale scheduling problems

Abstract: With increased product specialization within the chemical-processing industries, the ability to obtain production schedules for complex facilities is at a premium. This article discusses ways of quickly obtaining solutions for industrially relevant, large-scale scheduling problems. A number of time-based decomposition approaches are presented along with their associated strengths and weaknesses. It is shown that the most promising of the approaches utilizes a reverse rolling window in conjunction with a disaggregation heuristic. In this method, only a small subsection of the horizon is dealt with at a time, thus reducing the combinatorial complexity of the problem. Resource- and task-unit-based decompositions are also discussed as possible approaches to reduce the problem to manageable proportions. A number of examples are presented throughout to clarify the discussion.