bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface interactions in a shear field
Authors: Dietsche Laura J. , Denn Morton M. , Bell Alexis T.
Published: June 2004

The dynamics of chain exchange between flowing bulk melt and the channel wall were studied for oligomeric polyolefins using attenuated total reflectance Fourier transform infrared spectroscopy (ATR/FTIR). The dynamics are dominated by a surprisingly slow first‐order process which depends on flow rate and materials of construction of the channel wall. The picture which seems to emerge is an entropically‐driven flow‐rate‐ (or stress‐) dependent adsorption equilibrium which retards the exchange between the surface and bulk.
Simulation of hydrosol deposition in granular media
Authors: Choo Chang‐Upp , Tien Chi
Published: June 2004

An algorithm is developed for simulating particle deposition from liquid suspensions flowing through granular media, specifically the prediction of the extent of deposition and the change of media permeability on a local basis. The formulation is based on the premise that granular media may be considered as an assembly of collectors represented by a dual‐configuration—either spherical or capillaric — to be applied for deposition and permeability reduction considerations. Comparisons with experiments indicate that the simulation algorithm can indeed be used as a predictive tool for estimating the transport and distribution of particulate matter in granular media.
Cake formation in 2‐D cross‐flow filtration
Authors: Lu Wel‐Ming , Hwang Kuo‐Jen
Published: June 2004

The mechanism of cake formation is studied in 2–D cross‐flow filtrations. By analyzing the forces exerted on a depositing particle, the value of the critical friction angle, βc, which leads the particle to be stuck on the contact position, is determined. Not only the probability of particle deposition but also the size distribution of deposited particles can be calculated from the theoretical value of βc. Numerical programs are constructed to simulate the packing structure and size distribution of particles in a formed cake and the path of cake compression. The local cake properties in the cake, such as local porosity and local specific filtration resistance, are also estimated by applying the design program and the data of filtration volume vs. time. The predicted values of average cake porosity and average specific filtration resistance agree very well with the experimental data.
Thermodynamic modeling of concentrated aqueous electrolyte and nonelectrolyte solutions
Authors: Kolder Alexander , De Pablo Juan J.
Published: June 2004

A new approach is presented for calculation of activity coefficients in aqueous electrolyte solutions. This approach requires only standard thermodynamic properties for the pure components and for the solutes at infinite dilution. No adjustable mixture parameters are necessary. This new procedure has been applied to calculate: (1) the solubility of salts and the activity of water in binary aqueous solutions; (2) the temperature dependence of the solubility of salts; (3) the azeotropic composition of a system containing a volatile electrolyte, (H2HNO3); (4) the solubility of carbon dioxide in water at elevated pressures and at different temperatures.
Modeling of heat generation in ammonia‐treated solid rocket propellant
Authors: Raun Richard L. , Isom K. Bruce
Published: June 2004

One approach to demilitarize solid rocket propellants is treatment with ammonia. Ammonia extracts the oxidizers ammonium perchlorate and HMX, yielding a solid residue that is more suitable for incineration and less sensitive to impact and other modes of accidental initiation. Ammonia treatment of nitroglycerin‐containing propellants is complicated by an exothermic reaction between ammonia an nitroglycerin. If not removed, the heat generated by this reaction can cause propellant ignition. To help design safe treatment processes, a model for the ammonia‐propellant reaction was developed, which integrates transient energy and species conservation equations to simulate ammonia diffusion, heat generation, and heat flow in a propellant and in the solid residue resulting from ammonia treatment. It was calibrated using residue thickness and thermocouple data for one propellant. The calibrated model was used to predict conditions leading to ignition of thin propellant strips. The results agree well with experimental observations.
Potentialities and limitations of mixing simulations
Authors: Soulvaiotis A. , Jana S. C. , Ottino J. M.
Published: June 2004

As numerical simulations in mixing become pervasive, an analysis of errors becomes crucial. Purposely discretized examples with exact analytical solutions provide a reference point from which to judge the soundness of numerical solutions. Three types of errors are identified and examined: discretization, time integration, and round‐off, with emphasis on the first two. Theoretical derivations and numerical examples for 2‐D, steady (regular) and time‐periodic (chaotic) flows indicate that errors, in general, behave as material lines. In regular flows, their magnitude increases, on the average, with at most t2, while in chaotic flows it increases exponentially. Errors tend to align with the direction of the streamlines in regular flows and with manifolds in chaotic flows. As a result, even though exact and calculated trajectories diverge exponentially fast in chaotic flows, overall mixing patterns are reproduced, at least qualitatively, even when the velocity field is calculated using the main features of a line as it is deformed by the flow, although the error in its length may be more than 100%. It is concluded that accurate quantitative information, such as the location of periodic points or the length of a deformed line, can be obtained from numerical simulations. However, robust application of standard numerical analysis tools, such as mesh refinement, is necessary, which, in turn, can lead to nearly prohibitive computational costs.
Steady‐state drop‐size distributions in high holup fraction dispersion systems
Authors: Chatzi E. G. , Kiparssides C.
Published: June 2004

Macroscopic phenomena in suspension polymerization reactors are extremely complex, and breakage and coalescene of polymerizing monomer droplets are not well understood, especially for high dispersed‐phase volume fractions. Depending on the agitation, concentration and type of surface‐active agent, the droplet size can exhibit a U experiementally and theoretically as the balance between breakage and coalescence rates of monomer drops. Both processes are related to the drop surface energy, which is proportional to the interfacial tension and its variation with time. In this study, the most comprehensive models describing breakage and coalescene processes in a dispersion system were incorporated into a generalized numerical algorithm to predict the steady‐state drop‐size distributinos in a high holdup (50%) liquid‐liquid distributions in high holdup dispersion systems, experiments were carried out with a model system of 50% n‐butyl chloride in water in the presence of a surface‐active agent, poly(vinyl alcohol), at different concentrations and agitation rates. The theoretical model can predict reasonably well the drop‐size distribution for all experimental investigation elucidates the relationships between the changing structure of PVA molecules at the monomer/water interface and their effects on breakage and coalescence frequencies at different agitation times and rates.
New concept of gas purification by electron attachment
Authors: Tamon Hajime , Mizota Hirotoshi , Sano Noriaki , Schulze Stefan , Okazaki Morio
Published: June 2004

A new concept of gas purification by electron attachment is proposed. Low‐energy electrons generated in a corona‐discharge reactor are captured by electronegative impurities, producing negative ions. The ions drift to the anode in the electric field and are removed at the anode of the reactor. Two types of reactors were used to remove the negative ions: a deposition‐type reactor, which deposits negative ions at the anode surface; a sweep‐out‐type reactor, which sweeps out enriched electronegative impurities through the porous anode. Removals of dilute sulfur compounds, oxygen and iodine from nitrogen were conducted to verify the concept of gas purification. Simulation models were used to estimate removal efficiencies of these compounds, by taking into account electron attachment, and experiemental constants of the models were determined. The removal efficiency correlated by the models agreed well with the experiemental one.
Transient discharge of solid particles from upper outlet of vertical bubble columns
Authors: Hidaka Nobuyuki , Kakoi Kazuhiro , Matsumoto Toshitatsu , Morooka Shigeharu
Published: June 2004

Sieved glass beads of 230, 280 and 470 μm dia. were steadily suspended by upward concurrent gas and liquid flows in vertical bubble columns of 7 cm dia. and 4.85 m height or 15 cm dia. and 2.7 m height. Then the liquid velocity was suddenly increased to a higher value, which caused an unsteady‐state discharge of solid particles from the top of the column. The transient discharge rate was determined by collecting solid particles in the effluent, and transient axial distributions of gas and solid holdups along the column length were measured by sectioning the column with horizontal shutter plates just after stopping gas and liquid flows. A 1‐D sedimentation‐dispersion model was applied to describe the unsteady‐state discharge rate of solid particles as well as the change in gas and solid holdups in the columns. The model developed successfully expressed the transient behavior of solid particles over entire columns consisting of dense and dilute regions.
A theory for fast‐igniting catalytic converters
Authors: Leighton David T. , Chang Hsueh‐Chia
Published: June 2004

Using asymptotic expansion and numerical analysis, we demonstrate how the step‐response ignition time of an automobile catalytic converter depends on the ratio of the reaction rate to the interphase heat‐transfer rate, as measured by a key Damköhler parameter χ and the degree of monolith subcooling ŋ.In the region of low reactionrate at smallχ, the normalized ignition time tig scaled bythe homogeneous ignition time from the inlet gas temperature is (tigt) = 1 + 2 χ1/2∣ln( χ1/2/2 η)∣1/2, and the ignition takes place at a thermal front deep in themonolith. At large χ when the reaction rate is high, ignition occursat the leading edge of the monolith with (tig/t) = 2.50 + χ(ln η − 0.34).The delay in ignition time with increasing χ is due to a Taylor‐Aris dispersion mechanism induced by interphase heat transfer. Although the small‐ χ ignition mechanism is faster, its downstream ignition location leads to a very slow upstream propagation of the thermal front that follows ignition. An optimal converter system that ignites in 13 s, 25% of the current value in a standard step‐response test, is then designed by placing a small igniter, which ignites by the small‐ χ mechanism, upstream to preheat the current converter which then ignites by the large‐ χ mechanism. The length of the igniter is kept small by bypassing 2/3 of the exhaust since, from our theory, t is independent of the gas velocity.
Limiting cases and approximate solutions for fixed‐bed reactors with periodic flow reversal
Authors: Nieken Ulrich , Kolios Grigorios , Elgenberger Gerhart
Published: June 2004

Fixed‐bed reactors with periodic flow reversal are used to carry out reactions with low exothermicity in an autothermal way. Contrary to conventional autothermal reactors, the preheating of the feed gas is accomplished by regenerative heat exchange between the gas and fixed bed. The feedback of heat and direct coupling of heat exchange and chemical reaction give rise to an unusual reactor behavior. This behavior can be understood through consideration of the limiting cases of very long and very short cycle periods. Both cases can be calculated easily with the methods presented. Especially for the case of rapid flow reversal, simple equations can be derived that allow for a good physical understanding of the process and for a preliminary reactor design. In this case, it is also possible to compute the unstable solution, which is a lower bound of the temperature profile necessary for reactor startup.
Mass transfer with chemical reaction in thin‐layer electrochemical reactors
Authors: Devaux Régine , Bergel Alain , Comtat Maurice
Published: June 2004

Thin‐layer electrochemical reactors are analytical reactors where mass transfers are generally assumed not to affect the overall reaction rate. The validity of this hypothesis was tested for systems involving a second‐order or an enzymatic homogeneous reaction that consumed the product generated by the electrochemical heterogeneous reaction. The experiments were performed with the oxidation of hexacyanoferrate II into hexacyanoferrate III, coupled with the consumption of hexacyanoferrate III by the reduced form of nicotinamide adenine dinucleotide, catalyzed or not by a diaphorase. The tools commonly used for gas ‐ liquid interfaces were adapted to the description of mass transfer in a closed thin film. The theoretical model agreed well with the experimental data and served as the basis for testing the thin‐layer hypothesis. The validity of the hypothesis was plotted as a function of the Hatta and Damköhler numbers. The influence of concentration and diffusion parameters are discussed with focus on the validity criterion.
Binary mixture of monostearin‐distearin monolayers at the air–water interface
Authors: De la Fuente Feria Julia , Patino Juan M. Rodríguez
Published: June 2004

The study of mixed films has become of considerable interest both from theoretical and practical points of view. The π‐A isotherms of monostearin‐distearin (about 50% 1,3‐ and 50% 1,2‐isomers) mixed films spread on water as a function of emulsifier ratio and temperature were studied in an automated Langmuir‐type film balance. When molecules of both emulsifiers are spread on the air–water interface they are packed more closely together than when either emulsifier is present alone, and they enter into some form of association. This association can be demonstrated by surface pressure‐area isotherms and by thermodynamic studies. The characteristics of the mixed films, their elasticity and miscibility, which are of interest in emulsion or foam stability are discussed. A molecular model established can explain the interactions between film and subphase molecules and their relationships with variables of the process (pressure and temperature). This model fits the calculated thermodynamic parameters.
Cross‐flow ultrafiltration of micellar surfactant solutions
Authors: Markels John H. , Lynn Scott , Radke Clayton J.
Published: June 2004

A Steady‐state fouling‐resistance and osmotic‐pressure model is used to predict flux in the laminar, Cross‐flow ultrafiltration of micellar cetyl ( =hexadecyl)pyridinium chloride (CPC) solutions at 0.01‐M NaCl background electrolyte. The model assumes a constant overall hydraulic resistance including the effect of surfactant fouling and native membrane resistance. Measurements of osmotic pressures of CPC solutions at 0.01‐M NaCl as a function of surfactant concentration describe the effect of concentration polarization on permeate flux. Two types of asymmetric polythersulfone membranes are used : 5,000 molecular weight cutoff (MWCO) membranes that allow partial monomer permeation, but quantitatively reject all micelles; 50,000 MWCO membranes that allow some micelle permeation. For the former, the intrinsic refection coefifient for monomer, measured separately, is sufficient to describe surfactant rejection, without adjustable parameters. Predictions of the volumetric flux of the permeate, including the value of the limiting flux, agree well with the experimental results over the entire range of pressure drop, axial velocity, and bulk surfactant concentration. For the 50,000 MWCO membrances, the data are described using a best‐fit value of the overall surfactant rejection coefficient. For the first time, unusual behavior is observed experimentally in which the flux levels off with increasing pressure drop. Both effects are in accord with the proposed model. No gel layer need be postrulated to explain the flux behavior of either membrane type.
Multiple roll systems: Residence times and dynamic response
Authors: Benjamin D. F. , Anderson T. J. , Scriven L. E.
Published: June 2004

In roll coating as in other coating processes the coating liquid often suffers changes in properties on the time of the coating flow, that is, from fractions of a second upward depending on the amount of recirculation and recycling. The agents of change may be chemical reaction, colloidal aggregation, or evaporation. Hence the mean residence time and the residence time distribution of the liquid are important to designers and operators of coating processes. Here, building on the examination of roll‐coating systems by Benjamin et al. (1995), the residence times of liquid coated by representative arrays of multiple rolls in the “forward roll” mode and relatively starved feed condition (neglecting the possibly significant effects of “rolling banks” and other internal recirculations when they are present) are analyzed. The dynamic response of these transfer coaters to step changes in the feed gap and to periodic gap changes, as from roll and bearing run‐out, are also analyzed. No reports of operating or laboratory experiments are available for comparison. Nevertheless the results make plain how these quality‐limiting features may depend don the number of rolls used; their sizes, speed, and arrangement, and the properties of the coating liquid.
Analysis of multidimensional models of monolith catalysts for hybrid combustors
Authors: Groppi G. , Belloli A. , Tronconi E. , Forzatti P.
Published: June 2004

Distributed parameter models of the single channel of a monolith combustor have been derived from progressively simplified assumptions. Simulation results are compared to assess the importance of the different physicochemical phenomena occurring in the combustor and to identify the simplest adequate model. For typical operating conditions of the hybrid combustor (gas and wall temperature not exceeding 1,073 and 1,273 K, respectively, high flow rate and pressure, natural gas as fuel), the results show that variations of gas properties have to be considered while homogeneous combustion can be neglected. Assumption of the approximate radial profile of axial velocity with invariant parabolic shape, rather than the rigorous solution of momentum balance and the continuity equation, provides accurate results. Moreover, for simulation of ceramic monoliths, backward heat transmission by wall conduction can be neglected with a substantial saving of computational labor.
Adaptive multirate state and parameter estimation strategies with application to a bioreactor
Authors: Gudi Ravindra D. , Shah Sirish L. , Gray Murray R.
Published: June 2004

The design and development of a mu8ltirate software sensor for use in the chemical process industry are presented. The measurements of process outputs that arrive at different sampling rates are formally accommodated into the estimation strategy by using rhwe multirate formulation of the iterated extended kalman filter. Measurement delays associated with some of the process outputs are included in the system description by addition of delayed states. Observability issues associated with state and parameter estimation in a multirate framework are discussed and modified measurement equations are proposed for systems with delayed measurements to ensure relatively “strong” system observability. The evaluation of the proposed multirate state and parameter estimator through simulations and an experimental application on afed batch fermentation system gave satisfactory performance.
Mechanistic description and experimental studies of electrochromatography of proteins
Authors: Basak Subir K. , Ladisch Michael R.
Published: June 2004

Electrochromatography is a form of gradient liquid chromatography in which an axial electric potential is applied to columns packed with gel‐filtration media. Experimental methodology and a mechanistic model are further developed for a system that minimizes Joule heating at electric field strengths of 100 V/cm by dissipating heat through a cooling jacket and use of a cooled, low ionic strength eluting buffer. Focusing of proteins can be achieved in a 15‐mm‐dia. Column by the interplay of eluent velocity, electrophoretic migration rate, and electrically induced concentration polarization when the stationary phase is more conductive than the mobile phase. Voltage gradients of up to 125 V/cm for eluent velocities at 18–25 cm/h separate binary protein mixtures of Bhb‐α‐lactalbumin, BSA‐myoglobin, and α‐lactalbumin‐myoglobin over Sephadex G‐100 and G‐50. Retention times are consistent with values obtained from a mechanistic nonlinear model.
Robust output regulation for nonlinear chemical processes with unmeasurable disturbances
Authors: Wu Wei , Chou Yi‐Shyong
Published: June 2004

It becomes a problem when the input‐output feedback linearization of nonlinear processes presents unmeasurable disturbances that appear linear in the model and are not in the span of control input space. The prcsented hybrid nonlinear controller, which consists of a nonlinear static state fredback with a series of a adjustable parameters and an internal model, can achieve disturbance attenuation and offset‐free performance. The major advantage of the proposed control system design is to reject disturbance without producing a vigorous control action. A simple estimation method of computing a single parameter that replaces a series of adjustable parameters of the controller is developed. The proposed estimation strategy of computing a single parameter is based on the Lyapunov stability theory. These control schemes are illustrated by an example of a chemical reactor to obtain satisfactory control.
Separation devices for gas mixing
Authors: Agrawal Rakesh , Xu Jianguo
Published: June 2004

When two fluid streams with different compositions mix, a significant amount of exergy is lost Utilizing the mixing exergy of gaseous streams of different compositions can reduce equipment cost and energy consumption as well as generate electric power. Suitable modification of known separation processes such as distillation, membranes, adsorption, and absorption to convert them into efficient mixing processes is discussed in detail Mixing may also be combined with a separation process such that the same device is used for both purposes which results in the use of the mixing exergy. This combined separation/mixing process is a single and relatively simple device with efficiencies not typically attainable in the generally used two‐step method of separation followed by mixing. For such applications, some is∼ teresting results are presented for the processes using membrane, adsorption and absoqrion devices.
Bubble dissolution viscous liquids in simple shear flow
Authors: Favelukis Moshe , Tadmor Zehev , Talmon Yeshayahu
Published: June 2004

The dissolution of a gas bubble in a viscous Newtonian liquid in simple shear and creeping flow was studied experimentally A simple theoretical model for bubble growth or dissolution at high Peclet numbers is also presented. The main result of the model shows that for high capillary numbers (slender bubbles) the dissolution or growth rate is proportional to the shear rate to the 3/8 power The experimental work was carried out in a Couette apparatus. Air and carbon dioxide bubbles were injected into polymer liquids (low‐molecular‐weight polyisobutylenes) and photographed under shear. No “tip‐streaming” was observed, and experimental results compared favorably with predictions of the simple model, based on a mechanism that combines diffusion and convection.
Predicting drying in coatings that react and gel: Drying regime maps
Authors: Cairncross Richard A. , Francis Lorraine F. , Scriven L. E.
Published: June 2004

The competition between drying and reactions in a liquid coating containing precursors to a random network polymer can give rise to a variety of drying phenomena. Solidification, or gelation, of the polymer may occur before, after, or during the removal of solvents from the coating. Rates of drying and reaction are probed by solving the equations of mass transfer by diffusion along with chemical reaction in one dimension. Solutions to this system of equations are obtained by Galerkin's method with finite‐element basis functions and entail large‐scale computation. Skinning, or solidification at the surface of the coating while the bulk is still liquid, occurs in thick coatings when the diffusional resistance to drying is significant, that is, at high mass‐transfer coefficients. Homogeneous solidification occurs in thin coatings at low mass‐transfer coefficients. Drying regime maps represent these solidification phenomena as regions in parameter space.
Fluidization characteristics of biobone particles used for biocatalysts
Authors: Ellis N. , Margaritis A. , Briens C. L. , Bergougnou M. A.
Published: June 2004

Liquid–solid fluidization characteristics of irregularly shaped Biobone particles were studied in a fluidized bioreactor column 2.35 m high and 0.1 m dia. The wet biobone particles had a density of 1,890 kg/m3. Two size ranges were studied: 1.70–2.36 mm and 1.25–2.36 mm. The Biobone is a natural cheap material composed of collagen, which is a strong protein structure, embedded with microcrystals of hydroxyapatite and calcium phosphate, and it is an excellent matrix of commercial importance used for the immobilization of enzymes, whole cells and other biocatalysts. Fluidization characteristics of Biobone particles, which include measurements of pressure drops, holdups, minimum fluidization velocities, particle entrainment, and residence time distributions at different water superficial velocities, are reported. Because of the highly irregular shape of Biobone particles, new fluidization behavior was observed, which was much different from the fluidization behavior of spherical particles reported in the literature. A new channeling index, Ic, is proposed to quantify the channeling characteristics of fluidized Biobone particles, and a new parameter, α, was developed to distinguish the transition between fixed and fluidized states of the Biobone particles.
Dean vortices with wall flux in a curved channel membrane system: 2. The velocity field
Authors: Chung Kun Yong , Brewster Mary E. , Belfort Georges
Published: June 2004

The velocity and pressure fields and the effect of wall flux on these fields in a spiral channel are presented. As fluid flows inward through a spiral channel with constant gap and permeable walls, the streamwise flux decreases while the curvature increases. Thus, by balancing the stabilizing effect of wall suction with the destabilizing effect of increasing curvature, established vortices can be maintained along the spiral channel. This approach is used to prescribe spiral geometries with different wall fluxes. Using a weakly nonlinear stability analysis, the influence of wall flux on the characteristics of Dean vortices is obtained. The critical Dean number is reduced when suction is through the inner wall only, is slightly reduced when suction is equal through both walls, and is increased when suction is through the outer wall only. The magnitude of change is proportional to a ratio of small numbers that measures the importance of the effect of curvature. In membrane filtration applications the wall flux is typically 2 to 5 orders of magnitude less than the streamwise flow. If the radius of curvature of the channel is of the order of 100 times the channel gap, the effect on the critical Dean number is within 2% of the no‐wall flux case. If the radius of curvature is sufficiently large, however, it is possible to observe effects on the critical Dean number that approach O(1) in magnitude for certain parameter ranges.
Solving linear diffusion‐reaction networks in porous catalyst particles using BEM
Authors: Karur Sriganesh R. , Ramachandran P. A.
Published: June 2004

Multiple diffusion reactions are frequently encountered in the modeling of heterogeneous catalytic reactors. Obtaining an accurate estimate of the yield and selectivity in such reactions is crucial for an optimal design of reactors. Due to the inadequacy of analytical techniques in handling nonuniform catalyst shapes and mixed boundary conditions, numerical techniques are often employed to compute these design parameters. Among other numerical techniques, the boundary element method (BEM) is a superior method to solve linear diffusion reaction problems. The integral nature of the BEM formulation allows for boundary‐only discretization of the particle, thus reducing the computer execution time and the data preparation effort. A boundary element algorithm is developed to solve a network of linear diffusion reactions in porous catalyst particles in two dimensions. For this purpose, a matrix of fundamental solutions is defined and derived. The developed algorithm is applied to complex reaction networks to obtain the yield of intermediates for nonregular catalyst shapes and nonuniform boundary conditions. The method can be used as a design tool to study particle scale modeling in detail and can be incorporated into an overall reactor model.
Prediction of electrodeposition rates from an impinging jet
Authors: Li Quan , Walker J. D. A.
Published: June 2004

A mathematical model and numerical algorithm are developed to predict electrodeposition rates from a 2‐D jet of electrolyte that impinges on a flat surface. The principal situations of interest are for applied voltages that produce current densities below the limiting current. The motion is assumed to be at high speed with the jet inducing a thin laminar boundary layer on the surface; a progressively thinner concentration layer and an electrochemical double layer near the surface are accounted for. Two cases, corresponding to a submerged and unsubmerged jet, are considered. A boundary integral method is used to compute the current density along the plate in a general iterative numerical procedure coupled to the solution of the hydrodynamic, concentration and electrochemical boundary layers. The results show that relatively high deposition rates occur near the point of impingement and that altering the jet angle relative to the surface influences local electrodeposition rates significantly.
Hollow‐fiber membrane‐based rapid pressure swing absorption
Authors: Bhaumik S. , Majumdar S. , Sirkar K. K.
Published: June 2004

A novel gas purification technique called rapid pressure swing absorption (RAPSAB) was developed by integrating the best features of membrane contacting, gas‐liquid absorption, and pressure swing adsorption (PSA). In this cyclic separation process, a well‐packed microporous hydrophobic hollow‐fiber module was used to achieve nondispersive gas absorption from a high‐pressure feed gas into a stationary absorbent liquid on the module shell side during a certain part of the cycle followed by desorption of absorbed gases from the liquid in the rest of the cycle. The total cycle time varies between 20 s and upwards. Separation of mixtures of N2 and CO2 (around 10%) where CO2 is the impurity to be removed was studied using absorbent liquids such as pure water and a 19.5% aqueous solution of diethanolamine (DEA). Three RAPSAB cycles studied differ in the absorption part. Virtually pure N2 streams were obtained with DEA as absorbent demonstrating the capability of bulk separation to very high levels of purification. Numerical models developed predict the extent of purification for pure water and the DEA solution for one of the simpler cycles. Model simulations describe the observed behavior well.
Crystallization of phenanthrene from toluene with carbon dioxide by the GAS process
Authors: Berends Edwin M. , Bruinsma Odolf S. L. , De Graauw Jan , van Rosmalen Gerda M.
Published: June 2004

The crystallization of phenanthrene from toluene with carbon dioxide as the antisolvent gas is described. In the GAS process, a pressurized gas is dissolved into a liquid solvent, where it causes a volumetric expansion and lowers the solubility of the solute. Theoretical models are presented for the liquid‐phase expansion and the solubility as a function of pressure and temperature. The Nývlt theory for batch crystallization is adapted to predict the pressure profile in the crystallizer needed to maintain a constant supersaturation and growth rate. Generation of seeds is accomplished via a pressure pulse at the saturation pressure. The average particle size of the phenanthrene could be varied from 160 to 540 μm. Creation of seeds doubles the particle size and reduces the coefficient of variation significantly. The residual amount of toluene in the crystals without treatment is approximately 70 ppm. The particles are agglomerates of phenanthrene crystals.
On‐line parameter estimation in a high‐pressure low‐density polyethylene tubular reactor
Authors: Kiparissides C. , Verros G. , Pertsinidis A. , Goossens I.
Published: June 2004

The high throughput and operating cost of the reactor provide the economic incentive in considering the on‐line optimization of the LDPE tubular reactor. For an on‐line optimization procedure it is crucial to develop a flexible and adaptive on‐line parameter estimator, which based on the appropriate measurements from the reactor will adjust certain key model parameters so that the model can capture the actual reactor operation and reliably be used for the real‐time optimization of the reactor performance. Various issues involved in developing the on‐line estimator for such applications as the mathematical modeling of the reactor, the selection of the adjustable model parameters and reactor measurements, the decomposition of the problem according to the characteristic time constants of various disturbances, and the efficient and robust solution of the mathematical problem are discussed. The tools developed were successfully used on an EXXON high‐pressure LDPE tubular reactor. Results from the numerical experiments that are compared with scaled data from a real reactor show that the estimated temperature profiles agree remarkably with those observed at various phases of the reactor operation.
One‐sided single‐pellet technique for adsorption and intraparticle diffusion
Authors: Dog̈u Timur , Yasyerli Nail , Dog̈u Gülsen , McCoy Ben J. , Smith J. M.
Published: June 2004

The one‐sided single‐pellet dynamic technique introduced in this article is shown to be a very effective and simple method for the analysis of adsorption and diffusion in porous solids. Moment expressions are presented for reversible and irreversible adsorption processes. Since the adsorption equilibrium constant is the only unknown parameter in the first moment expression, this parameter can be evaluated from a single set of pulse‐response experiments. This technique to evaluate the adsorption equilibrium constant is illustrated using ethylbromide as tracer. It is also shown that for a very strongly adsorbing tracer, such as 1,2‐dichloroethane in soil, modified moment expressions derived with a semiinfinite boundary condition are necessary unless a criterion developed in this work is satisfied.
Coke effect in mass transport and morphology of Pt‐Al2O3 and Ni‐Mo‐Al2O3 catalysts
Authors: García‐Ochoa F. , Santos A.
Published: June 2004

The effect of high levels of coke deposit on catalyst properties and mass transport through the pores space was studied using commercial catalysts, Pt‐Al2O3 and Ni‐Mo‐Al2O3. Changes in porosity, pore‐size distribution and internal surface area of several aged catalysts were compared to those of fresh catalyst. Effective diffusivity was also measured by pulse chromatography. For Pt‐Al2O3, which shows a high initial porosity (0.93) and a relative high mean pore radius (300 Å), catalyst properties remain unchanged while coke content increased. For the Ni‐Mo‐Al2O3 catalyst (with initial porosity of 0.53 and mean pore radius of 50 Å), however, with an increase of 15% in coke content, isolated void regions appear, and porosity, internal surface area and effective diffusivity decrease sharply. Pore and stochastic models were used to interpret the effective diffusivity experimentally measured. A structural parameter in each case, tortuosity factor in pore models, and coordination number in the percolation model were calculated. When coke produces significant changes in catalyst morphology, the tortuosity factor almost doubles its initial value for a coke content of 20%. On the contrary, the percolation model seems to predict changes accurately in porosity, mass trasport coefficient, and an isolated prosity growing from the coordination number matched for a fresh catalyst.
Solvation thermodynamics of gas solubility at sub‐ and near‐critical conditions
Authors: Chialvo Ariel A. , Kalyuzhnyi Yu V. , Cummings Peter T.
Published: June 2004

The recently developed formalism for describing the solute‐induced effect in dilute near‐critical mixtures is extended to fluids composed of anisotropic molecules through the statistical mechanical interpretation of the derivative (∂P/∂x2) whose critical value is the Krichevskii parameter.

Rigorous expressions for Henry's constant and the solute distribution factor along the orthobaric curve are derived in terms of the volumetric and entropic solute‐induced local effects, and the quasi‐linear behavior of their orthobaric density dependence away from the solvent's critical point is rationalized.

The formalism is illustrated with integral equation calculations of the orthobaric density dependence of several solvation thermodynamic quantities for an infinitely dilute volatile solute in near‐critical solutions of hard‐sphere Yukawa fluids.
Importance of macromixing in batch cooling crystallization
Authors: Bohlin Martin , Rasmuson Åke C.
Published: June 2004

The importance of spatial variations of the conditions in industrial‐scale, agitated, batch cooling crystallizers is investigated by computer simulations. A three‐compartment model is developed considering primary and magma density‐dependent secondary nucleation. An increasing crystallizer size is described by an increasing suspension turnover time. It is shown that accumulation of larger crystals in the bottom region, localized supersaturation generation and variations in the secondary nucleation rate due to varying local mixing intensity will, under normal conditions, exert a low and often negligible influence on the product‐size distribution of an industrial unit. In a batch process, the product‐size distribution is governed mainly by the conditions early in the process. During this period the supersaturation half‐life is much longer than the suspension turnover time, and the influence of local variations becomes weak.
Reverse micellar mass‐transfer processes: Spray column extraction of lysozyme
Authors: Lye Gary J. , Asenjo Juan A. , Pyle D. Leo
Published: June 2004

Protein partitioning kinetics was measured for the semibatch extraction of lysozyme in a laboratory‐scale, liquid–liquid spray column. The organic, isooctane phase contained reverse micelles formed from the anionic surfactant, sodium di‐2‐ethylhexyl sulfosuccinate. For the extraction of protein from aqueous to reverse micellar phases, experiments were performed over a range of dispersed‐phase flow rates for cases of the organic‐ or aqueous‐phase dispersion. The influence of aqueous‐phase pH and ionic strength, which influence electrostatic interactions between protein and reverse micelles, was also investigated. Results were interpreted in terms of a two‐film model of mass transfer. The nature of the dispersed pahse could significantly influence the partitioning kinetics, while study of the droplet hydrodynamics suggested that stagnant drops were formed regardless of which phase was dispersed. Literature correlations for describing the droplet‐formation process and droplet hydrodynamics predicted measured values satisfactorily. Attempts wer also made to predict overall mass‐transfer coefficients based on existing correlations describing mass transfer during droplet formation, free rise (or fall), and coalescene. Predicted values of KL were 2–10 times greater than measured values, probably because of large concentrations of surfactant used to formulate the reverse micelle phases. This approach did, however, provide detailed information on the quantity of protein transferred during the successive processes of droplet formation, free rise (or fall) and coalescence.
Wavy‐to‐slug flow transition in slightly inclined gas–liquid pipe flow
Authors: Grolman Eric , Commandeur Niels C. J. , de Baat Eduard C. , Fortuin Jan M. H.
Published: June 2004

A process‐engineering model is presented for the stratified‐wavy‐to‐intermittent (SW–I) flow‐pattern transition in slightly inclined gas–liquid pipe flow. The main parameter for predicting (in) stability of wavy flow in inclined pipes is the average liquid holdup, which was found to reach a maximum, critical value at flow‐pattern transition. Observed values of the critical liquid holdup vary between 0.07 and 0.42, depending on pipe diameter, angle of inclination and transport properties of the gas–liquid system. Measurements were performed in transparent glass pipes of 26‐ and 51‐mm dia., at ten angles of inclination (0.1° ≤ β ≤ 6.0°), using air/water and air/tetradecane (n‐C14H30) systems at atmospheric pressure. Flow‐pattern maps are presented for selected angles of inclination, showing excellent agreement between predicted and observed flow‐pattern boundaries.
Prediction of sedimentation and consolidation of fine tails
Authors: Eckert William F. , Masliyah Jacob H. , Gray Murray R. , Fedorak Phillip M.
Published: June 2004

Sedimentation and consolidation of suspensions of fine particles were analyzed by integrating experimental measurement of properties in a centrifuge with a comprehensive numerical model. The yield stress and settling velocity for tailings from tar sands extraction were determined experimentally as a function of the volume fraction of solids. The evaluated state functions were used to simulate batch settling and consolidation, and the results compare well with long‐term settling tube tests. This approach is very attractive where gravity sedimentation may take many years, and it allows prediction of the rate of clear water production, total time for sedimentation and consolidation, and the maximum concentration of solids.

Scaling of the sedimentation between centrifuge and field conditions is discussed. Conversion of permeability‐void ratio relationships from geotechnical experiments to state functions of hindered settling velocity is demonstrated, allowing the use of data derived from a variety of experimental techniques.
Model for secondary nucleation in a suspension crystallizer
Authors: Meadhra Ruairí Ó. , Kramer Herman J. M. , Van Rosmalen Gerda M.
Published: June 2004

A model that makes it possible to evaluate the secondary nucleation rate in a pilot‐scale industrial crystallizer is presented. It relates the secondary nucleation rate directly to the volume rate of attrition of large parent crystals. Two other terms are included in the equality, one that expresses the distribution formed by the attrition fragments, and the other that expresses a survival efficiency accounting for the percentage of attrition fragments that grow out after the attrition step. The model is further tested on a 970‐L draft‐tube‐baffled evaporative crystallizer with ammonium sulfate as the model material used for crystallization and having an on‐line crystal size‐distribution measuring device.
Generalized modular representation framework for process synthesis
Authors: Papalexandri Katerina P. , Pistikopoulos Efstratios N.
Published: June 2004

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.
Spatiotemporal patterns in catalytic reactors
Authors: Sheintuch Moshe , Shvartsman Stanislav
Published: June 2004

The study of spatial structures in heterogeneous reactors is a challenging academic topic, revealing patterns that differ from those known to exist in reaction‐diffusion systems exposed to uniform conditions, as well as a practical problem that should affect design and operation procedures of commercial reactors like the catalytic convertor. Experimental observations and mathematical models of spatiotemporal patterns in high‐pressure catalytic reactors are reviewed. Patterns in high‐pressure reactors, in which thermal effects provide the positive feedback, as well as the long‐range communication, usually emerge due to global interaction. Patterns are classified comprehensively by considering reactors of increasing degree of complexity: a wire or ribbon exposed to uniform conditions, a globally coupled catalyst in a mixed reactor or in a control loop, and a fixed bed in which interaction by convection occurs only in one direction. Catalytic wires are not expected to exhibit sustained patterns in the absence of global interaction. Global interactions by external control or gas‐phase coupling are shown experimentally and analytically to induce a rich plethora of patterns. Complex motions were simulated to occur due to the interaction of convection, conduction and reaction in a fixed‐bed; only a few of these patterns were experimentally observed. Directions for future research are suggested.
Reactor of vapor‐phase graft polymerization of reactive monomer onto porous hollow fiber
Authors: Uezu Kazuya , Saito Kyoichi , Sugo Takanobu , Aramaki Satoru
Published: June 2004

A novel reactor for the modification of a porous hollow‐fiber membrane up to 1 m in length is presented. The polyethylene hollow fiber irradiated with an electron beam was exposed to the vapor of an epoxy‐group‐containing vinyl monomer (glycidyl methacrylate) in a cylindrical reactor rotated at 1 rpm. Construction of the hollow fibers resulted in a diffusion‐controlled reaction system, which led to the nonuniformity of degree of grafting (dg), whereas the shuffling of the hollow fibers by rotating the cylindrical reactor provided a higher reactivity (dg = 110% for 100 min at 300 K) and uniformity (standard deviation = 4.5%) of dg.