Showing papers in "Aiche Journal in 1989"

Methane steam reforming, methanation and water‐gas shift: I. Intrinsic kinetics

Abstract: Intrinsic rate equations were derived for the steam reforming of methane, accompanied by water-gas shift on a Ni/MgAl2O4 catalyst. A large number of detailed reaction mechanisms were considered. Thermodynamic analysis helped in reducing the number of possible mechanisms. Twenty one sets of three rate equations were retained and subjected to model discrimination and parameter estimation. The parameter estimates in the best model are statistically significant and thermodynamically consistent.

Synthesis of mass exchange networks

Abstract: Mass-exchange units are ubiquitously used in industrial facilities. A mass exchanger is any direct-contact mass-transfer unit that employs a mass-separating agent (MSA) or a lean phase (e.g., a solvent, an adsorbent, a stripping agent, an ion exchange resin) to selectively remove certain components (e.g., pollutants, products, byproducts) from a rich phase (e.g., a waste, a product). Examples of mass-exchange operations include absorption, adsorption, stripping, solvent extraction, leaching, and ion exchange. Appendix 2, Modeling of Mass-Exchange Units for Environmental Applications, provides an overview of the principles of modeling, designing, and operating individual mass-exchange units.

Gas-particle flow in a vertical pipe with particle-particle interactions

Abstract: A theory is presented for the fully-developed flow of gas and particles in a vertical pipe. The relation between gas pressure gradient and the flow rates of the two phases is predicted, over the whole range of cocurrent and countercurrent flows, together with velocity profiles for both phases and the radial concentration profile for the particles. The gas and the particles interact through a drag force depending on their relative velocity, and there are mutual interactions between pairs of particles through inelastic collisions. This model is shown to account for marked segregation of gas and particles in the radial direction, and the predicted relation between the pressure gradient and the flow rates of the two phases is surprisingly complex.

Methane steam reforming: II. Diffusional limitations and reactor simulation

Abstract: Given the intrinsic kinetics, the tortuosity factor of a Ni/MgAl2O4 catalyst was determined under reaction conditions by minimizing the sum of squares of residuals of the experimental and the simulated conversions. The parallel cross-linked pore model with uncorrelated pore size distribution and orientation was used in the calculation of the effective diffusivities. A modified collocation method was used to obtain the partial pressure profiles of the reacting components in the catalyst pellet. The simulation of the experimental reactor during the optimization of the tortuosity factor also yielded the effectiveness factors of the reactions. The results of the simulation of an industrial steam reformer are also discussed.

A neural network methodology for process fault diagnosis

Abstract: The ability of knowledge-based expert systems to facilitate the automation of difficult problems in process engineering that require symbolic reasoning and an efficient manipulation of diverse knowledge has generated considerable interest recently. Rapid deployment of these systems, however, has been difficult because of the tedious nature of knowledge acquisition, the inability of the system to learn or dynamically improve its performance, and the unpredictability of the system outside its domain of expertise. This paper proposes a neural-network-based methodology for providing a potential solution to the preceding problems in the area of process fault diagnosis. The potential of this approach is demonstrated with the aid of an oil refinery case study of the fluidized catalytic cracking process. The neural-network-based system successfully diagnoses the faults it is trained upon. It is able to generalize its knowledge to successfully diagnose novel fault combinations it is not explicitly trained upon. Furthermore, the network can also handle incomplete and uncertain data. In addition, this approach is compared with the knowledge-based approach.

Product yields and kinetics from the vapor phase cracking of wood pyrolysis tars

Abstract: The homogeneous vapor phase cracking of newly formed wood pyrolysis tar was studied at low molar concentrations as a function of temperature (773 - 1.073 K), at residence times of 0.9 - 2.2 s. Tar conversions ranged from about 5 to 88%. The tars were generated by low heating rate (0.2 K/s) pyrolysis of --2 cm deep beds of sweet gum hardwood, and then rapidly conveyed to an adjacent reactor for controlled thermal treatment. Quantitative yields and kinetics were obtained for tar cracking and resulting product formation. The major tar conversion product was carbon monoxide, which accounted for over two-thirds of the tar lost at high severities. Corresponding ethylene and methane yields were each about 10% of the converted tar. Coke formation was negligible and weight-average tar molecular weight declined with increasing tar conversion.

A reaction engineering approach to the problem of concrete carbonation

Abstract: Steel bars in reinforced concrete are protected from corrosion by the high pH environment of the surrounding concrete This alkaline environment is destroyed by the reaction of atmospheric CO2 with the Ca(OH)2 of the concrete mass When this process, called carbonation of concrete, reaches the reinforcing bars, corrosion of the latter may commence In this paper, the physiochemical processes in this phenomenon are presented and modeled mathematically The mathematical model is fairly complex, but certain simplifying assumptions are possible, which lead to the formation of a “carbonation front” and to a simple analytical expression for the evolution in time of this front, in terms of the composition parameters of cement and concrete and of the environmental conditions This simple expression is in very good agreement with experimental results obtained in this and in previous studies The effect of some parameters on the carbonation front propagation is also discussed

Phase equilibria for supercritical fluid process design

Abstract: Supercritical fluids (SCF's) are powerful solvents with many unique properties. They have great potential for many extraction processes, but reliable and versatile mathematical models of the phase equilibrium thermodynamics are needed for their use in process design and economic feasibility studies. This paper reviews existing experimental data and analytical methods and attempts to delineate their strengths and limitations. Also discussed are new data needs and possible new directions for a better fundamental understanding of the molecular processes in SCF solutions.

Incipient fault diagnosis of chemical processes via artificial neural networks

Abstract: Artificial neural networks have capacity to learn and store information about process faults via associative memory, and thus have an associative diagnostic ability with respect to faults that occur in a process. Knowledge of the faults to be learned by the network evolves from sets of data, namely values of steady-state process variables collected under normal operating condition and those collected under faulty conditions, together with information about the degree of the faults and their causes. Here, we describe how to apply artificial neural networks to fault diagnosis. A suitable two-stage multilayer neural network is proposed as the network to be used for diagnosis. The first stage of the network discriminates between the causes of faults when fed the noisy process measurements. Once the fault is identified, the second stage of the network estimates the degree of the fault. Thus, the diagnosis of incipient faults becomes possible.

Porosity distributions in a fluidized bed with an immersed obstacle

Abstract: Erosion in bubbling fluidized-bed combustors is a serious issue that may affect their reliability and economics. Available evidence suggests that the key to understanding this erosion is detailed knowledge of the coupled and complex phenomena of solids circulation and bubble motion. A thin transparent “two-dimensional” rectangular fluidized bed with an obstacle served as a rough model for a fluidized-bed combustor. This model was studied experimentally and computationally using two hydrodynamic equation sets. The computed hydrodynamic results agree reasonably well with experimental data. Bubble frequencies and sizes compare well with those obtained from analyzing a high-speed motion picture frame-by-frame. Time-averaged porosities computed from both models agree with time-averaged porosity distributions measured with a gamma-ray densitometer. The principal diferences between the data and the computations in this paper are due to asymmetries present in the experiment and to the simplified solids rheology used in the hydrodynamic models.

Heat and mass transfer during the dissociation of hydrates in porous media

Abstract: Presentation d'un modele physique decrivant la dissociation d'hydrates dans un milieu poreux lors d'une stimulation thermique

The rheology of suspensions containing polymerically stabilized particles

Abstract: The non-Newtonian viscosities of polymerically stabilized colloidal suspensions are usually predicted and correlated on the basis of data and scaling principles for Brownian hard spheres. Here, the specific effect of the stabilizer layer is investigated using suspensions of monodisperse PMMA particles with a chemically attached stabilizer layer. The ratio between particle radius and stabilizer layer thickness is changed between 5 and 61. At high values of this ratio the data show Brownian hard sphere behavior. At lower values deviations appear. As a first approximation, the “softness” of the particles can be characterized through the concentration at maximum packing. A more detailed comparison with hard sphere data provides a measure for softness that changes with concentration and shear rate. A theoretical estimate of the concentration effect is in line with the experiments. The critical shear stress (or Peclet number) is not a constant but goes through a maximum when the concentration is increased.

Process optimization via simulated annealing: Application to network design

Abstract: Simulated annealing is a multivariable optimization technique based on the Monte Carlo method used in statistical mechanical studies of condensed systems and follows by drawing an analogy between energy minimization in physical systems and costs minimization in design applications. In this paper, simulated annealing is introduced and reviewed. The utility of the method for optimization of chemical processes is illustrated by applying it to the design of pressure relief header networks and heat exchanger networks.

Diffusion limitations in fischer‐tropsch catalysts

Abstract: The extent of diffusion limitations in the catalytic conversion of synthesis gas to hydrocarbons by the Fischer-Tropsch reaction has been established for a number of iron- and cobalt-based catalysts. The studies were performed in a fixed-bed microreactor system at temperatures in the range 473-523 {Kappa}. Variation of catalyst particle size in the range 0.2.-2.6 mm shows that the conversion of synthesis gas decreases considerably when the average particle size is increased. The effects of variation of particle size and pore diameter have been quantified with the Thiele model for diffusion limitations. Evidence has accumulated that the limited mobility of reactant molecules in the liquid-filled pores of Fischer-Tropsch catalysts is the main cause of retardation of the reaction rates. The experimentally determined reaction rates with various catalysts operated under different conditions show an excellent fit with the theoretical model.

Deadtime compensation for nonlinear processes

Abstract: Many industrially important processes feature both nonlinear system dynamics and a process deadtime. Powerful deadtime compensation methods, such as the Smith predictor, are available for linear systems represented by transfer functions. A Smith predictor structure in state space for linear systems is presented first and then directly extended to nonlinear systems. When combined with input /output linearizing state feedback, this Smith-like predictor makes a nonlinear system with deadtime behave like a linear system with deadtime. The control structure is completed by adding an external linear controller, which provides integral action and compensates for the deadtime in the input/output linear system, and an open-loop state observer. Conditions for robust stability with respect to errors in the deadtime and more general linear unstructured multiplicative uncertainties are given. Computer simulations for an example system demonstrate the high controller performance that can be obtained using the proposed method.

Flow in packed tubes with a small tube to particle diameter ratio

Abstract: La simulation porte sur un ecoulement monophasique dans un petit tube garni de particules spheriques de taille quasi uniforme. On distingue dans ce milieu poreux simule des unites structurales pentaedriques pres des parois et tetraedriques au centre du tube

Precipitation of microsize organic particles from supercritical fluids

Abstract: The precipitation of organic particles from supercritical fluids (SF) by expansion (SFX) has become an interesting alternative to milling without thermal decomposition. The authors report how through the rapid expansion process, a dramatic change of the solute supersaturation ratio was created causing subsequent precipitation with a narrow particle-size distribution. It was found that {beta}-carotene precipitates from SF ethylene and ethane have the feed material crystallinity. However, SF CO{sub 2} reacted with {beta}-carotene and did not give characteristic {beta}-carotene X-ray spectra. The mean particle sizes of these precipitates were in the submicron range (ca 0.3 {mu}m). Increased solubility was obtained by addition of toluene as cosolvent in SF ethylene. It was found that the mean size of {beta}-carotene particles, generally remained unchanged if the toluene concentration was less than 1 1/2 mol %. The SFX process appears to be in a single fluid phase when up to 1 1/2 mol % toluene cosolvent is used.

Hindered transport of spherical macromolecules in fibrous membranes and gels

Abstract: Methods are described for calculating the effects of hydrodynamic interactions on the hindered transport of solid spherical macromolecules in ordered or disordered fibrous media. These methods are applied to a medium made up of a square lattice of straight, bead-and-string-type fibers. Hydraulic permeabilities and coefficients governing hindered diffusion and convection are obtained from a detailed hydrodynamic model, and the hindered transport coefficients are shown to be in very good agreement with an effective medium approach based on Brinkman's equation. The use of Brinkman's equation for the calculation of hindered transport rates in fibrous membranes and gels is validated further by comparing with experimental data from the literature.

Thermodynamics of aqueous systems containing hydrophilic polymers or gels

Abstract: A quasichemical partition function is applied to represent the thermodynamic properties of aqueous solutions of nonelectrolytes, including linear polymers and crosslinked polymers (gels). The partition function extends conventional lattice theory; to take into account strong specific interactions (hydrogen bonds) as encountered in aqueous solutions, each molecule (polymer segment) may possess three energetically different types of contact sites. We distinguish between sites that interact through dispersion forces and sites that can participate in a hydrogen bond; hydrogen-bonding sites are divided into electron-pair donating sites and electron-pair accepting sites. The Helmholtz energy of the mixture is obtained using an oriented quasichemical approximation. The final equation contains three independent adjustable binary parameters; these are the exchange energies for different types of contact paris. To represent quantitatively upper or lower critical solution phenomena, we include the semitheoretical fluctuation correction recently proposed by de Pablo. Comparison with experimental data indicates that the proposed molecular-thermodynamic model may be useful for representing phase equilibria for a variety of aqueous systems including swelling equilibria for hydrophilic gels.

Feedforward and feedback control of a solution copolymerization reactor

Abstract: A control system is designed for a copolymerization reactor using a combination of feedforward, ratio, and feedback control to regulate polymer production rate, copolymer composition, molecular weight, and reactor temperature. The solution copolymerization of methyl methacrylate and vinyl acetate in a continuous stirred tank reactor is used as an illustrative example with the kinetic parameters and reactor operating conditions obtained from the literature. The process includes equipment to recycle unreacted monomers and solvent. The recycle stream introduces disturbances to the reactor feed, which perturb the polymer properties. A feedforward control strategy is proposed to counter these disturbances, and its effectiveness is demonstrated using the model. The mathematical model is used to investigate input/output control pairings in order to identify the fundamental nature of the solution copolymerization control problem and to determine the best control system structure. The combined feedforward/feedback strategy is tested on a nonlinear model of the process for set point changes and compensation of unmeasured reactor disturbances. The performance of the control system design was quite good, and such designs have been found successful in plant operations.

A state space formulation for model predictive control

Abstract: Model predictive control (MPC) schemes such as MOCCA, DMC, MAC, MPHC, and IMC use discrete step (or impulse) response data rather than a parametric model. They predict the future output trajectory of the process {ŷ(k + i), i = 1, …, P}, then the controller calculates the required control action {Δu(k + i), i = 0, 1, …, M − 1} so that the difference between the predicted trajectory and user-specified (setpoint) trajectory is minimized. This paper shows how the step (impulse) response model can be put into state space form thus reducing computation time and permitting the use of state space theorems and techniques with any of the above-mentioned MPC schemes. A series of experimental runs on a simple pilot plant shows that a Kalman filter based on the proposed state space model gives better performance that direct use of the step response data for prediction.

An experimental and modeling study of softening coal pyrolysis

Abstract: A quantitative model of softening coal pryolysis is developed and tested against observed behavior. The model treats intraparticle transport of gases and metaplast throughout the softening stage of pyrolysis via growth of bubbles uniformly dispersed in the molten coal. The model quantitatively describes the transient softening behavior including the initiation, duration, and magnitude of plasticity and swelling of the particle. The model kinetic parameters are derived from experiments, in which a highly softening Pittsburgh No. 8 bituminous coal was pyrolyzed, and total weight loss, and yields and molecular weight distributions of tars and pyridine extractables of cooled chars were measured. Model predictions of effects of pressure, particle diameter and temperature on tar yields and weight losses, of effects of pressure on swelling ratio, and of effects of heating rate on plasticity are presented. The model provides good fits to the laboratory data and predicts trends in pyrolysis behavior in good qualitative accord with expectations based on published data.

Modeling phenol degradation in a fluidized-bed bioreactor

Abstract: A study was made of phenol degradation by bacteria immobilized onto particles of calcined diatomaceous earth in a draft-tube, three-phase fluidized-bed reactor. A mathematical model is used to describe simultaneous diffusion and reaction of oxygen and phenol in the reactor. Kinetic parameters for the growth of nonsupported cells were obtained in batch and chemostat experiments. Liquid-solid mass transfer coefficients were determined experimentally and showed good agreement with literature values for conventional three-phase fluidized beds. Experimental steady-state degradation data were used to calculate biofilm substrate diffusivities. These were found to decrease as the biofilm density increased. The transition from phenol to oxygen-limiting biofilm kinetics predicted by the model was shown to exist experimentally. A critical ratio of phenol/dissolved oxygen concentration was found at which this transition occurred. This provides a criterion for establishing whether increased aeration will increase the volumetric degradation rate.

Gas solubility and Henry's law near the solvent's critical point

Abstract: It has been experimentally observed, for water and nonaqueous solvents alike, that Henry's constant passes through a maximum and then declines as the temperature is raised from the triple point to the critical point. From classical and nonclassical models, we derive exact relations for the value of Henry's constant and its temperature dependence at the solvent's critical point, showing that the decline of this constant is a universal phenomenon. We demonstrate that the limiting temperature dependence of Henry's constant can be predicted from the thermodynamic properties of the pure solvent and the initial slope of the critical line. The validity of our prediction is tested by comparing it with experimental solubility data for several gases in high-temperature water and benzene. Our predictive model appears valid over a temperature range of at least 15% in temperature below the critical point of the solvent.

Modeling of desorption at supercritical conditions

Abstract: Etude de l'interet de calculer les energies d'activation de desorption a densite fixe plutot qu'a pression fixe. Presentation d'une relation entre la constante de vitesse de desorption et la densite

Effects of process conditions on crystals obtained from supercritical mixtures

Abstract: Etude de la dimension des grains obtenus en fonction des conditions de temperature et de pression de pre et post-expansion. On etudie la cristallisation d'une poudre de naphtalene

Periodic operation of a trickle‐bed reactor

Abstract: The influence of periodic water flow on SO2 oxidation in a trickle bed of activated carbon catalyst was investigated, whereby gaseous reactants were introduced into the trickle-bed reactor continuously, but water was turned on and off. Mean liquid superficial velocities of 0.86 and 1.65 mm/s were used. At the latter, an increase in the oxidation rate of about 30 to 45% was found within a range of cycle periods from 2 to 80 min. A temperature change of up to 7°C was observed in the bed during periodic operation. An explanation of the improved average oxidation rate under periodic operation is developed in terms of the steady-state rates with and without water flow for symmetrical and asymmetrical cycles.

High-temperature adsorption of alkali vapors on solid sorbents

Abstract: Vapors of alkali metal compounds can be removed from coal combustion and gasification flue gases using high-temperature aluminosilicate sorbents. The fundamentals of alkali adsorption on kaolinite, bauxite, and emathlite are compared and analyzed both experimentally and through theoretical modeling. The results show that the process is not a simple physical condensation, but a complex combination of diffusion and reaction. The kinetics of adsorption on these sorbents have similarities: the process is diffusion-influenced, the rate decreases with time, and there is a final saturation limit. There are, however, differences in reaction mechanisms leading to potentially different applications for each sorbent. In adsorbing alkali chloride vapors, kaolinite and emathlite release all the chlorine back to the gas phase while bauxite retains some of the chlorine. Moreover, the products of reaction with emathlite have a melting point significantly lower than those for kaolinite and bauxite. Therefore, emathlite is more suitable for lower-temperature sorption systems downstream of the combustors/gasifiers, while kaolinite and bauxite are suitable as in-situ additives.

The optimal design of dynamic experiments

Abstract: A robust and efficient algorithm is developed to calculate dynamic inputs for optimal experimental designs. Different objective functions are presented to allow designs for both model discrimination and the improvement of parameter precision. Time-varying inputs are calculated by reformulating the optimal design problem as an optimal control problem. This approach can provide large improvements in the ability to discriminate among a series of models, and then increase the accuracy of the resulting parameters.

Uptake of phenylalanine and tyrosine by a strong-acid cation exchanger

Abstract: The equilibrium and rate of uptake of the amino acids phenylalanine and tyrosine by Amberlite 252, a strongly acidic, cation-exchange resin, have been investigated. Uptake of the amino acids by the hydrogen form of the resin occurs primarily by the stoichiometric exchange of hydrogen ions and amino acid cations. The amount of amino acid taken up by the resin can be calculated as a function of solution pH and amino acid concentration from a model that takes into account both solution and ion-exchange equilibria. The rates of uptake of the two amino acids have been determined experimentally for a closed batch system. The results of experiments in which the resin particle size, the flow rate, and the concentration were varied show that intraparticle transport is dominated by the slow diffusion of amino acid cations through the macroreticular polymer structure of the resin, with some contribution from macropore transport of amino acid cations and zwitterions. An intraparticle diffusion model has been developed to describe these results and predict the performance of fixed-bed operations.