Showing papers in "Aiche Journal in 2003"

Advances in Biomaterials, Drug Delivery, and Bionanotechnology

Abstract: Biomaterials are widely used in numerous medical applications. Chemical engineering has played a central role in this research and development. Polymers as biomaterials, materials and approaches used in drug and protein delivery systems, materials used as scaffolds in tissue engineering, and nanotechnology and microfabrication techniques applied to biomaterials are reviewed.

Aggregation structure and thermal conductivity of nanofluids

Abstract: Nanofluids are obtained by suspending metallic nanoparticles in conventional base liquids. Such a new class of heat-transfer fluid is superior to the base liquids in energy-transport performance, which depends on the distribution, volume fraction and thermal properties of the suspended nanoparticles. The theory of Brownian motion and the diffusion-limited aggregation model are applied to simulate random motion and the aggregation process of the nanoparticles. A theoretical model is developed to predict the thermal conductivity of nanofluids. Comparison between the theoretical and experimental results shows the validity and accuracy of the theoretical model.

Disturbance models for offset‐free model‐predictive control

Abstract: Model predictive control algorithms achieve offset-free control objectives by adding integrating disturbances to the process model. The purpose of these additional disturbances is to lump the plant-model mismatch and/or unmodeled disturbances. Its effectiveness has been proven for particular square cases only. For systems with a number of measured variables (p) greater than the number of manipulated variables (m), it is clear that any controller can track without offset at most m controlled variables. One may think that m integrating disturbances are sufficient to guarantee offset-free control in the m controlled variables. We show this idea is incorrect and present general conditions that allow zero steady-state offset. In particular, a number of integrating disturbances equal to the number of measured variables are shown to be sufficient to guarantee zero offset in the controlled variables. These results apply to square and nonsquare, open-loop stable, integrating and unstable systems.

Chemical processing and micromixing in confined impinging jets

Abstract: Rapid processes such as certain organic reactions or precipitations at high supersaturation require the rapid mixing provided by jet mixers. Micromixing in a confined impinging jets (CIJ) mixer was characterized employing the Damkohler number to correlate processing. A scaling theory for the characteristic micromixing time, proportional to momentum diffusion starting at the Kolmogorov microscale, is shown as sufficient to express the micromixing performance of the CIJ mixer. A recently characterized second-order competitive reaction set is used as a “chemical ruler” to assign an absolute value to the mixing time in the CIJ mixer. A wide range of characteristic time (320 to 5 ms) is evaluated with hydrochloric acid competing for sodium hydroxide neutralization or 2,2-dimethoxypropane acid catalyzed hydrolysis. This reaction set was sensitive enough to detect the onset of a turbulent-like flow at a Re of 90 and was able to demonstrate a decrease in undesired products up to the highest Re tested, 3,800 or a jet velocity of 19 m/s. It represents a significant advancement to the reaction sets and techniques used for previous mixing studies, which are reviewed. Experiments verify the characteristic mixing time in a CIJ mixer scales as the inverse of the jet velocity to the three halves power, and the “mesomixing volume” (the volume over which the majority of flow energy was dissipated) is best approximated as proportional to the internozzle separation cubed. For each of the different jet diameters, chamber diameters and outlet configurations tested, the selectivity of the reaction scaled linearly with the Damkohler number, as determined from the known reaction kinetics and the calculated Kolmogorov diffusion time. The first full characterization is provided of micromixing in impinging jets that allows the prediction of mixing performance, reaction selectivity, and scale-up criteria.

Quadrature method of moments for population‐balance equations

Abstract: Although use of computational fluid dynamics (CFD) for simulating precipitation (and particulate systems in general) is becoming a standard approach, a number of issues still need to be addressed. One major problem is the computational expense of coupling a standard discretized population balance (DPB) with a CFD code, as this approach requires the solution of an intractably large number of transport equations. In this work the quadrature method of moments (QMOM) is tested for size-dependent growth and aggregation. The QMOM is validated by comparison with both Monte Carlo simulations and analytical solutions using several functional forms for the aggregation kernel. Moreover, model predictions are compared with a DPB to compare accuracy, computational time, and the number of scalars involved. Analysis of the relative performance of various methods for treating aggregation provides readers with useful information about the range of application and possible limitations.

Monitoring independent components for fault detection

Abstract: A chemical process has a large number of measured variables, but it is usually driven by fewer essential variables, which may or may not be measured. Extracting such essential variables and monitoring them will improve the process-monitoring performance. Independent component analysis (ICA) is an emerging technique for finding several independent variables as linear combinations of measured variables. In this work, a new statistical process control method based on ICA is proposed. For investigating the feasibility of its method, its fault-detection performance is evaluated and compared with that of the conventional multivariate statistical process control (cMSPC) method using principal-component analysis by applying those methods to monitoring problems of a simple four-variable system and a continuous-stirred-tank-reactor process. The simulated results show the superiority of ICA-based SPC over cMSPC.

Modeling of CO2 capture by aqueous monoethanolamine

Abstract: The process for CO2 removal from flue gases was modeled with RateFrac. It consists of an absorber, a stripper, and a cross heat exchanger. The solvent used in the model contains about 30 wt % monoethanolamine (MEA) in water. MEA reacts with CO2 in the packed absorber. The finite reaction rate requires a kinetic characterization. The RateFrac absorber model was integrated with a FORTRAN user kinetic subroutine to make the model consistent with the interface pseudo-first-order model and with a regressed Electrolyte-NRTL equilibrium model. It was adjusted with laboratory wetted wall column data and field data from a commercial plant. Sensitivity analyses were performed on process variables to find operating conditions at low steam requirement. Many variables strongly affect the process performance, but an overall optimization shows that there are no economical ways to reduce the steam requirements by more than 10%. The reboiler duty can be reduced from that of a base case representing current industrial operating conditions, by 5% if acids are added to the solvent, by 10% if the absorber height is increased by 20%, and by 4% if the absorber is intercooled with a duty of one-third of the reboiler duty. The power plant lost work is affected by varying stripper pressure, but not significantly, so any convenient pressure can be chosen to operate the stripper.

Voidage variation in packed beds at small column to particle diameter ratio

Abstract: Randomly packed beds of equal spheres exhibit damped oscillatory voidage variation in the near wall region. Models reported in literature were found to be lacking in their ability to relate this variation to average bed porosity. The changing mean of the voidage oscillations and deviations from sinusoidal behavior were not described well either. Experimental data and a new radial bed porosity model are presented. Improved radial bed voidage and average bed voidage predictions were demonstrated. It has further been shown that multiple stable packing configurations exist within the same packing mode, which complicates modeling at a small column to particle diameter ratio.

Robust nonlinear model predictive control of batch processes

Abstract: NMPC explicitly addresses constraints and nonlinearities during the feedback control of batch processes. This NMPC algorithm also explicitly takes parameter uncertainty into account in the state estimation and state feedback controller designs. An extended Kalman filter estimates the process noise covariance matrix from the parameter uncertainty description and employs a sequential integration and correction strategy to reduce biases in the state estimates due to parameter uncertainty. The shrinking horizon NMPC algorithm minimizes a weighted sum of the nominal performance objective, an estimate of the variance of the performance objective, and an integral of the deviation of the control trajectory from the nominal optimal control trajectory. The robust performance is quantified by estimates of the distribution of the performance index along the batch run obtained by a series expansion about the control trajectory. The control and analysis approaches are applied to a simulated batch crystallization process with a realistic uncertainty description. The proposed robust NMPC algorithm improves the robust performance by a factor of six compared to open loop optimal control, and a factor of two compared to nominal NMPC. Monte Carlo simulations support the results obtained by the distributional robustness analysis technique.

Laminar mixing in different interdigital micromixers: I. Experimental characterization

Abstract: Interdigital-type glass micromixers with alternating feed channels to periodically create liquid multilamellae, were fabricated for basic investigations of hydrodynamics for liquid mixing of two streams. Three interdigital designs developed, rectangular, triangular, and slit-type, differed in their flow-through mixing chamber. These designs are based on simple polygon geometries and their combinations. The flow patterns of an aqueous solution dyed with blue and uncolored water were investigated for different interdigital mixers. Since mixing in a nonfocused device, such as the rectangular mixer, was not completed, focusing techniques were applied. Geometric focusing was used to reduce lamellae width and to speed up mixing. In the special version of the triangular mixer, SuperFocus mixer, liquid mixing times are reduced to about 10 ms, as determined by iron-rhodanide reaction imaging. In the latter case, the lamellae are compressed by a factor of 40, from a width of 160 to 4 μm. For both the triangular and slit-type micromixer, flow patterns differed from regular multilamination ones. At high flow rates, lamellae tilted. When the flow cross-sectional area was expanded, jet and associated eddies formed further improved mixing. Mixing investigations also helped develop imaging techniques. The rhodanide imaging complemented water blue contrasting.

The quest for sustainability: Challenges for process systems engineering

Abstract: La new strategic significance to the term “sustainability.” No longer confined to the economic realm, sustainability now embraces a broad spectrum of company characteristics related to social and environmental responsibility. This shift in thinking (dare we say enlightenment?) is due to growing recognition among business executives that profitability alone is inadequate as a measure of success, and that many of the nonfinancial concerns associated with sustainability are fundamental drivers of long-term shareholder value. Conversely, failure to recognize these strategic issues threatens the very survival of a business enterprise. For example, mounting concerns over the ability to curb industrial emissions of CO2 and other global warming gases are only the tip of this proverbial iceberg. The original concept of “sustainable development” (SD), defined over 15 years ago by a U.N. commission, suggested pursuing development in a way that respects both human needs and global ecosystems, assuring quality of life for future generations (WCED, 1987). It was clear even then that current trends in population growth and economic development were not sustainable. Without dramatic changes in the patterns of human activity, there will be severe challenges to the continued growth of global industries. Examples of these challenges include (World Bank, 2001): • Adverse environmental impacts such as climate change; degradation of air, water, and land; depletion of natural resources, including fresh water and minerals; loss of agricultural land due to deforestation, soil erosion and urbanization; and threatened ecosystems. • Adverse socio-economic impacts such as widespread poverty, lack of potable water, proliferation of infectious diseases, social disintegration resulting from displacement of traditional lifestyles, growing income gaps, and lack of primary education. Rather than ignoring these ominous signals, a number of visionary business leaders have risen to the challenge and developed a new model of industrial progress that marries economic growth with social and environmental responsibility (Holliday et al, 2002). Many corporations are beginning to partner with governments and nongovernmental organizations to seek sustainable solutions that preserve their freedom to operate. Pragmatically speaking, such voluntary initiatives are certainly preferable to resistance or indifference, which would invite an increasingly onerous regulatory regime that limits industrial growth through economic or technological constraints. Responding to the challenges of sustainability requires insight into the characteristics of a sustainable system, and a fundamental rethinking of how all industrial products and processes are designed, built, operated, and evaluated. Qualitative definitions of sustainability such as that given by the U.N. are not particularly helpful for engineering decision-making. The following definition is more useful: A sustainable product or process is one that constrains resource consumption and waste generation to an acceptable level, makes a positive contribution to the satisfaction of human needs, and provides enduring economic value to the business enterprise. The determination of an “acceptable level” represents a technical challenge, but it is common to assert that resource utilization should not deplete existing capital, that is, resources should not be used at a rate faster than the rate of replenishment, and that waste generation should not exceed the carrying capacity of the surrounding ecosystem (Robèrt, 1997). Since sustainability is a property of the entire system, and not of an individual subsystem, incorporating sustainability into engineering requires the boundaries of “the process” to be greatly expanded—beyond the plant and even beyond the corporation. As shown in Figure 1, the analysis boundaries might extend to the economy and the ecosystem. Moreover, the scope of analysis needs to be expanded beyond cost and performance issues to include environmental integrity and socio-economic well being. Life cycle assessment (LCA), now an ISO-standardized methodology, is an important example of the effort to expand the traditional process boundary. LCA considers both the upstream and downstream processes associated with a given product in terms of energy use, material use, waste generation, and business value creation (Consoli et al., 1993). The life cycle stages, shown in Figure 1, may include resource extraction, procurement, transportation, manufacturing, product use, service, and end-of-life disposition or recovery. The feedback loop indicates the importance of recycling, reuse, and reverse logistics. Careful consideration of life cycle implications can sometimes yield surprising results. For example, efforts to develop “green” plastics, such as polylactides, seem appealing because the feedstocks are renewable and the plastics are The Quest for Sustainability: Challenges for Process Systems Engineering

Kinetics of the reaction of CO2 with aqueous potassium salt of taurine and glycine

Abstract: The kinetics of the reaction between CO2 and aqueous potassium salts of taurine and glycine was measured at 295 K in a stirred-cell reactor with a flat gas-liquid interface. For aqueous potassium taurate solutions, the temperature effect on the reaction kinetics was measured at 285 and 305 K. Unlike aqueous primary alkanolamines, the partial reaction order in amino acid salt changes from one at low salt concentration to approximately 1.5 at salt concentrations as high as 3,000 mol·m-3. At low salt concentrations, the measured apparent rate constant (kapp) for potassium glycinate is comparable to the values in literature. In the absence of reliable information in the literature on the kinetics and mechanism of the reaction, the applicability of the zwitterion and termolecular mechanism (proposed originally for alkanolamines) was explored. For the zwitterion mechanism, the forward second-order reaction rate constant (k2) of the CO2 reaction with amino acid salt seems to be much higher than for alkanolamines of similar basicity, indicating that the Bronsted plot for amino acid salts might differ from that of alkanolamines. The contribution of water to the deprotonation of zwitterion seems to be more significant than reported values for aqueous secondary alkanolamines.

Nucleation regime map for liquid bound granules

Abstract: Nucleation is the first step in granulation where the powder and liquid first contact. Two types of nucleation in wet granulation processes are proposed. Drop controlled nucleation, where one drop forms one nucleus, occurs when drops hitting the powder surface do not overlap (low spray flux Psi(a)) and the drop must wet quickly into the bed (short drop penetration time t(p)). If either criterion is not met, powder mixing characteristics will dominate (mechanical dispersion regime). Granulation experiments were performed with lactose powder, water, PEG200, and 7% HPC solution in a 6 L and a 25 L mixer granulator. Size distributions were measured as the drop penetration time and spray flux were varied. At short penetration times, decreasing Psi(a) caused the nuclei distribution to become narrower. When drop penetration time was high, the nuclei size distribution was broad independent of changes in dimensionless spray flux. Nucleation regime maps were plotted for each set of experiments in each mixer as a function of the dimensionless distribution width delta. The nucleation regime map demonstrates the interaction between drop penetration time and spray flux in nucleation. The narrowest distribution consistently occurred at low spray flux and low penetration time, proving the existence of the drop controlled regime. The nucleation regime map provides a rational basis for design and scale-up of nucleation and wetting in wet granulation.

Regular Solution Model for Asphaltene Precipitation from Bitumens and Solvents

Abstract: A regular solution theory liquid-liquid equilibrium model was developed to predict asphaltene precipitation from Western Canadian bitumens. The input parameters for the model are the mole fraction, molar volume, and solubility parameters for each component. Bitumens were divided into four main pseudo-components corresponding to SARA fractions: saturates, aromatics, resins, and asphaltenes. Asphaltenes were divided into fractions of different associated molar mass based on a Schultz-Zimm molar mass distribution. Asphaltene self-association was accounted for through the average molar mass of the distribution. The molar volumes and solubility parameters of the pseudo-components were calculated using solubility, density, and molar mass measurements. The model successfully predicted the effect of solvent type and associated molar mass on asphaltene precipitation for model oil and n-alkane systems. The model also predicted the onset and amount of asphaltene precipitation from bitumens.

Numerical Simulation of Solids Suspension in a Stirred Tank

Abstract: Large-eddy simulations of the turbulent flow dri®en by a Rushton turbine ha®e been coupled to a Lagrangian description of spherical, solid particles immersed in the flow. The working fluid was water, whereas the solid particles had the properties of glass ( y 23 ) beads. Simulations were restricted to a lab-scale tank ®olume 10 m , and relati®ely () low solids ®olume fractions up to 3.6% . Two sets of particles were considered with particle dia. of 0.30 mm and 0.47 mm, respecti®ely. It has been in®estigated to what le®el of detail the particle motion needs to be modeled in order to meet Zwietering’s just suspended criterion. It appeared to be essential to take particle-particle collisions into account, mainly because of their exclusion effect that pre®ents unrealistic buildup of particle concentrations closely abo®e the bottom. The simulations gi®e detailed insight in the beha®ior of the particles, and in the way that the liquid flow is altered by the presence of the particles. The frequency and intensity of particle-particle collisions, and particle-impeller collisions, ha®e been in®estigated. Furthermore, it will be demonstrated () that the rotational Reynolds numbers of the big 0.47 mm particles were of the same order of magnitude as their translational counterparts.

Laminar mixing in different interdigital micromixers: II. Numerical simulations

Abstract: Flow patterns and mixing properties of micromixing devices described in Part I are investigated by computational fluid dynamics (CFD) and semianalytical methods. The CFD results provide qualitative information on mixing, but, due to numerical artifacts, no quantitative information can be derived in most cases. The geometrical arrangement of liquid lamellae predicted by the CFD simulations explain transmitted-light experiments reported in Part I. In contrast, the semianalytical method provides a simple model for the mixing devices presented and allows to assess their mixing properties. For two of the four micromixers, mixing times were in the range of milliseconds, thus lending themselves to fast, mass-transfer-limited reactions. Experimental data obtained for a specific micromixer support the semianalytical model. Both the model and experiments suggest that geometric focusing of a large number of liquid streams is a powerful micromixing principle.

Diagnostic Tool to Detect Electrode Flooding in Proton-Exchange-Membrane Fuel Cells

Abstract: An electrode flooding monitoring device designed for PEM fuel cells with interdigitated flow distributors was proposed and tested. The pressure drop between the inlet and outlet channels can be used as a diagnostic signal to monitor the liquid water content in the porous electrodes, because of the strong dependence of the gas permeability of the porous electrodes on liquid water content. It can be used with no modification to existing fuel-cell assembly to give real-time flooding information in the electrode backing layers during operation. The device has been employed to investigate the correlation between the fuel-cell performance and the liquid water saturation level in the backing layers, the effects of various operating parameters, and the dynamics and hysteresis behavior of liquid water in the backing layers. The results provide, for the first time, direct evidence to show that inadequate water removal causes liquid water build up in the cathode, which in excessive amounts can severely reduce the performance of a PEM fuel cell. It was observed that more than 30 min. were needed for a PEM fuel cell to reach a new steady state when subjected to current-density changes, and this was attributed to the slow liquid water transport process. The results confirmed that increasing air flow rate or cell temperature increased the liquid water removal rate from the backing layers. The hysteresis behavior of fuel-cell performance was related to the water imbibition and drainage cycles in the electrode backing layers and was attributed to the difference in the water removal rate by capillary force and the difference in membrane conductivity.

Three‐dimensional wave dynamics on a falling film and associated mass transfer

Abstract: The evolution of solitary waves into three-dimensional (3-D) waves was experimentally observed on a vertically falling water film at mainly Re = 10–100. At Re greater than 40, 2-D solitary waves are very unstable in the face of spanwise perturbations of approximately 2-cm wavelengths and disintegrate into isolated horseshoe-shaped solitary waves and clusters of dimples between the horseshoes, whereas wavefront modulations are limited to low levels at Re below 40. Horseshoes of larger velocities have larger curvature heads, and extend longer oblique legs upward. Curving capillary ripples preceding each horseshoe widen their wavelengths with an increase in the wavefront inclination, showing that the ripples possess the characteristics of the shallow-water capillary waves. The horseshoes may hold vortices inside, and they have similarities in shape and size to hairpin vortices observed in laminar–turbulent transition regions of boundary layers on walls. The disintegration of waves into dimples is caused by a capillary instability similar to the one for breakup of a cylindrical soap film. This transition of wave dynamics at Re≈40 is associated with a drastic change in the mass transfer from the surface into the film.

Fluid dynamic simulation of O3 decomposition in a bubbling fluidized bed

Abstract: Recent advances in dense, multiphase, computational fluid dynamics (CFD) have allowed accurate simulation of the gas and particle motion in bubbling and circulating fluidized beds. Since fluidized-bed reactors are used for many chemical processes, a simulation must also be able to accurately couple chemical reactions to bed hydrodynamics. The catalytic decomposition of ozone (O 3 ) often has been used to study experimentally the contacting behavior of catalytic reactors. Simulations of laboratory-scale experiments of premixed O 3 decomposition in a bubbling fluidized bed using the multiphase CFD code MFIX were conducted. The grid-independent results are in very good agreement with reported experimental data on total conversion over a range of fluidization velocities and initial bed heights. This confirms the ability of multiphase hydrodynamic models to capture quantitatively the effect of hydrodynamics on chemical reactions in a bubbling fluidized bed.

Morphology of methane and carbon dioxide hydrates formed from water droplets

Abstract: Methane and carbon dioxide hydrate crystals were formed on nearly spherical water droplets at 274.6 K and 2,150 kPa or 1,000 kPa above the corresponding three-phase hydrate equilibrium pressure. Each experiment was performed with two droplets 5 mm and 2.5 mm in diameter or three droplets with a diameter of 2.5 mm. At the higher pressure the water droplets quickly became jagged and exhibited many needlelike or hairlike crystals extruding from the droplet, whereas at the lower pressure the surface was smooth. In almost all experiments, a depression or collapse of the hydrate layer was observed to occur. This collapse was interpreted as evidence of a continuing hydrate formation after the droplet surface was covered by the hydrate layer. The type of hydrate-forming gas and the size of the droplet was observed not to influence the macroscopic hydrate crystal morphology. The decomposition of the methane and carbon dioxide hydrate layers was also observed. Reformation was also experimented, and the effect of memory on the morphology of hydrate crystal growth was determined.

Separation performance predictions of a Stairmand high‐efficiency cyclone

Abstract: () Large-eddy simulations LES were performed on the gas flow in a Stairmand highefficiency cyclone at Res 280,000. The LES realistically represent the 3-D, time-dependent flow, including phenomena such as ®ortex core precession, and ®ortex breakdown. Quantitati®ely good agreement with measured ®elocity profiles is obser®ed, for both the time-a®eraged ®elocities and the ®elocity fluctuation le®els. The single-phase LES formed the starting point for modeling solid particle motion in the cyclone based on one-way coupling between the gas flow and the particles. It is shown that some details of the flow in the relati®ely small region in the ®icinity of the inlet ha®e strong influence on the separation process. Due to the long residence times of particles inside the cyclone, the concurrent simulation of gas flow and particle motion is a lengthy computational process. Therefore, three alternati®e ways of modeling particle motion were explored: a frozen-field approach, an eddy-lifetime model, and a periodic-flow approach. The results of these approaches are compared to the results of the concurrent simulation.

Multiscale resolution of fluidized‐bed pressure fluctuations

Abstract: Pressure fluctuation signals measured from four different axial locations in a bubbling bed 0.3 m in diameter and 3 m in height were analyzed using multiple approaches, including wavelet transform, Hurst analysis, multiscale resolution, and time-delay embedding. After examining decomposition residuals using different compact support Daubechies wavelets, the Daubechies second-order wavelet was chosen as an optimal wavelet for decomposing pressure signals. Hurst analysis of the decomposed signals shows that the measured pressure fluctuations can be resolved to three characteristic scales: bifractal mesoscale signals with two distinct Hurst exponents; monofractal micro- and macroscale signals with only one characteristic Hurst exponent. Energy profiles of the three scale components confirm that the measured pressure signals mainly reflect the mesoscale component. Time-delay embedding analysis of three scale signals demonstrates that the microscale dynamics is more complex than the mesoscale dynamics, and the mesoscale dynamics is more complex than the macroscale dynamics. That this result cannot be found solely from Hurst analysis shows the importance of integrating multiple approaches for characterizing the complexity of fluidized systems.

Thermoresponsive transport through porous membranes with grafted PNIPAM gates

Abstract: Both thermoresponsive flat membranes and core-shell microcapsule membranes, with a porous membrane substrate and grafted poly(N-isopropylacrylamide) (PNIPAM) gates, were successfully prepared using a plasma-graft pore-filling polymerization method. PNIPAM was proven to be grafted homogeneously onto the porous membrane substrates, in the direction of both the membrane thickness and surface. Regardless of the solute molecular size, temperature had an opposite effect on diffusion coefficients of the solute across the PNIPAM-grafted membranes with low graft yields as opposed to those with high graft yields. The PE-g-PNIPAM membranes change from positive thermo-response to negative thermoresponse types with increasing pore-filling ratios at around 30%. Phenomenological models were developed for predicting the diffusion coefficient of the solute across PNIPAM-grafted membranes at temperatures, both above and below the lower critical solution temperature (LCST). Predicted diffusional coefficients of solutes across both the PNIPAM-grafted flat and PNIPAM-grafted microcapsule membranes fit the experimental values. To obtain an ideal result for the diffusional thermoresponsive controlled release through PNIPAM-grafted membranes, the substrates strong enough to prevent any conformation changes are more suitable for preparing thermoresponsive membranes than weak ones.

Simulation and optimization of pressure-swing adsorption systems for air separation

Abstract: Over the past 20 years, PSA processes have gained increasing commercial acceptance as an energy-efficient separation technique. After a startup time, the system reaches cyclic steady state (CSS), at which the conditions in each bed at the start and end of each cycle are identical. Contrary to the traditional successive substitution method, we apply a direct determination approach using a Newton-based method with accurate sensitivities to achieve fast and robust convergence to CSS. Trust region methods and scaling are used to handle ill-conditioning and model nonlinearities. When design specifications are imposed, this approach is easily extended to include design constraints. This eliminates trial-and-error experiments and determines all the operating parameters simultaneously. In addition, we modify a standard flux limiter in order to deal with nondifferentiable terms and ensure computational accuracy and efficiency. Optimal PSA processes are designed by means of state-of-the-art rSQP-based optimization algorithms. The simultaneous tailored approach incorporates detailed adsorption models and specialized solution methods. Here, CSS convergence is achieved only at the optimal solution and the time-consuming CSS convergence loop is eliminated. Applications of several nonisothermal VSA O2 bulk gas separation processes are presented to illustrate all of these approaches.

Polymer-zeolite composite membranes for direct methanol fuel cells

Abstract: Direct methanol fuel cells require membranes with the dual properties of high proton conductivity and low methanol crossover. Such membranes have a high selectivity, that is a high ratio of proton conductivity to methanol permeability. This research reports such a membrane made of polyvinylalcohol and loaded with mordenite, a proton conducting, methanol impermeable zeolite. Protons travel directly through both the polymer and zeolite phases, but methanol has a more tortuous path around the zeolite particles. The composite membranes show up to twenty times higher selectivity than Nafion, the current benchmark. The improved behavior, a result of the proper tailoring of the polymer and dispersion phase, is predicted using Maxwell's theory for diffusion in composite media.

Novel multifunctional optical‐fiber probe: I. Development and validation

Abstract: By combining aspects of optical fiber probes used to determine local particle concentrations and local particle velocities and by rapid signal processing, it is shown that a single probe can determine local instantaneous particle fluxes. Providing a transparent cover to prevent a “blind zone” is shown to be critical in improving the probe performance and linearity. A simple mechanistic model is successful in predicting the performance of the optical system. The results of the probe are validated using particles glued onto both a rotating disk and a flat vibrating surface.

Microstreaming Effects on Particle Concentration in an Ultrasonic Standing Wave

Abstract: It is shown that the magnitude of Rayleigh microstreaming convective drag on microparticles in water in a 3.2-MHz ultrasonic standing wave can be comparable to the lateral direct radiation force in the nodal plane (DRF 1 ) and can significantly influence the microparticle aggregation. The transducer of a single half-wavelength chamber was excited to give a single particle aggregate. The estimated sound pressure amplitude was 0.5 MPa. Particle image velocimetry (PIV) measurements gave the average microstreaming velocity in the nodal plane as 450 μm.s -1 , which is comparable to the 340- μm.s -1 value calculated from Rayleigh's theory. Movement of 25-μm latex particles was primarily determined by DRF 1 , while that of smaller 1.0 μm, particles was determined by Rayleigh microstreaming. A 15-μm latex particle velocity map, simulated from microstreaming data, the measured velocity map of 25-μm particles, and the cube-dependent relationship between DRF 1 's on particles of different sizes, was in reasonable agreement with a measured velocity map. Further evidence for the importance of microstreaming came from the result that velocities for 1- and 25-μm particles were of similar magnitude, but were opposite in direction.

Inertial flow structures in a simple‐packed bed of spheres

Abstract: The detailed characteristics of the interstitial velocity distributions in pores are critical to understanding heat and mass transfer in a packed bed of granular material. The velocity distributions in a pore that models a simple-packed bed were measured directly by a magnetic resonance imaging technique. With an increase in the Reynolds number from 12.17 to 59.78-204.74, the increase and decrease in main flow velocity did not correspond to the local pore geometry, as is the case with a creeping flow. This indicated that inertial forces dominate over viscous forces. For example, at the Reynolds number of 204.74, the fluid penetrated through the center of the pores like a jet with negligible change of velocity. Circulation in the surrounding stagnant spaces generated eight symmetrical eddies in the plane perpendicular to the main flow direction.

Finite‐element scheme for solution of the dynamic population balance equation

Abstract: A first-order finite-element scheme is proposed for solving the 1-D dynamic population balance equation. The model includes the kinetic processes of nucleation, growth, aggregation, and breakage in a spatially uniform system with or without flow. The numerical scheme features improved stability by taking into account the convective nature of the growth term, and enhanced conservation of moments at nonuniform grids by identifying and mitigating the sources of conservation error in the evaluation of the aggregation source term. Exhaustive benchmarking is carried out against analytical solutions for size-dependent growth and nucleation, size-dependent aggregation with a variety of kernels, binary and multiple breakage with uniform and parabolic daughter particle distributions, and combinations of these, respectively. In all cases tested the method proves capable of producing accurate results at a reasonable CPU time, while being relatively simple and easy to implement.