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At least 73 records · Page 4Linked to original sources
PubMed2006

Optimal design and operation of a certain class of asynchronous simulated moving bed processes.

A compact representation of the cyclic operation of simulated moving-bed chromatography is established from the governing equations for the analogous single-column model that reproduces the cyclic steady-state (CSS) behavior of the multi-column process. A broad class of physically realizable asynchronous processes is then derived by dropping the integrality condition on the number of columns per zone, which now represents the average over a cycle. The steady periodic solution of the multi-column unit is computed by solving the analogous single-column model using a full-discretization method. The nonlinear algebraic system resulting from the simultaneous discretization of both spatial and temporal coordinates is solved using the gPROMS software. This solution strategy leads to shorter computational times than those previously reported in the literature. Process optimization is handled using single objective functions, to avoid competing effects, which are explicitly constrained by product quality and maximum allowable internal flow rates. The process is optimized for maximum feed throughput, with a possible upper bound on eluent consumption or flow rate, or minimum eluent consumption for a given feed flow rate. The nonlinear programming problem is solved by an external solver while still carrying out the CSS calculations in gPROMS. The feasibility of the approach is demonstrated on the chromatographic separation of an enantiomeric mixture with nonlinear competitive isotherm. Emphasis is given to the benefits that can be gained by upgrading an existing system to asynchronous operation. It is shown that eluent consumption for optimized asynchronous configurations in the higher feed-throughput region can be significantly reduced by modulation of eluent flow rate and selective product withdrawal.

Algorithms↗
PubMed2000

Quasi-planar nucleus structure in apoferritin crystallization.

First-order phase transitions of matter, such as condensation and crystallization, proceed through the formation and subsequent growth of 'critical nuclei' of the new phase. The thermodynamics and kinetics of the formation of these critical nuclei depend on their structure, which is often assumed to be a compact, three-dimensional arrangement of the constituent molecules or atoms. Recent molecular dynamics simulations have predicted compact nucleus structures for matter made up of building blocks with a spherical interaction field, whereas strongly anisotropic, dipolar molecules may form nuclei consisting of single chains of molecules. Here we show, using direct atomic force microscopy observations, that the near-critical-size clusters formed during the crystallization of apoferritin, a quasi-spherical protein, and which are representative of the critical nucleus of this system, consist of planar arrays of one or two monomolecular layers that contain 5-10 rods of up to 7 molecules each. We find that these clusters contain between 20 and 50 molecules each, and that the arrangement of the constituent molecules is identical to that found in apoferritin crystals. We anticipate that similarly unexpected critical nucleus structures may be quite common, particularly with anisotropic molecules, suggesting that advanced nucleation theories should treat the critical nucleus structure as a variable.

Apoferritins↗
PubMed2004

Using a deformable discrete-element technique to model the compaction behaviour of mixed ductile and brittle particulate systems.

This paper illustrates the application of a combined discrete- and finite-element simulation to the compaction of assemblies comprising both ductile and brittle particles. Through case studies, the results demonstrate the importance of using a fine mesh on the particle boundary, the effect of fragmentation and its impact on the form of the compression curve, and the effect of inclusion of ductile particles at ca. 25% by volume suppressing brittle failure mechanisms. Although, the calculations can be extended to three dimensions, the computational cost is a current limitation on such calculations. The novelty of this approach is in its ability to predict material yield surfaces for the compaction of a mixture of particles. The initial results are optimistic, but there is a need for model improvement, principally through the ability to capture the random packing of irregular particles since this will eliminate a key problem in defining an initial density for the simulation. The main advantage of this technology is in its ability to minimize the need for expensive triaxial testing of samples to develop the yield-surface history.

Chemistry, Pharmaceutical↗
PubMed1994

Cooperative structural transitions induced by non-homogeneous intramolecular interactions in compact globular proteins.

The role played by non-homogeneous interactions in stabilizing cooperative structural changes in proteins was investigated by exhaustive simulations of all compact conformations compatible with several well-defined globule-like shapes in three dimensions. Conformational free energies corresponding to the association of residues i and j were computed both for the unperturbed system, all subject to identical intramolecular interactions, and for the perturbed system in which a single pair of residues is probed by changing its interactions with an attractive or repulsive interaction. The high packing density leads to strong coupling between residues so that specific interactions between a given pair of residues are accompanied by considerable enthalpy changes. Relatively weak, about 1-2 kcal/mol, attractive interactions can exert a dramatic effect on the free energy distribution. Usually, central positions in the sequence most affect the conformational characteristics. Some of these interaction pairs appear to be capable of effecting major conformation transitions because of the high level of cooperativity in the dense state. Effects of repulsive interactions, however, do not depend so strongly on residue pair and cause more localized structural changes. This approach can suggest more, or less, sensitive loci for amino acid substitution.

Biophysical Phenomena↗
PubMed1998

A comparative evaluation of the DINAMAP 8100 and DINAMAP Compact TS using a non-invasive blood pressure simulator.

BACKGROUND: The Critikon DINAMAP 8100, although widely used in clinical practice, has been reported to measure the blood pressure with unacceptable systematic and non-systematic errors. The 8100 device has recently been succeeded by the Compact TS. OBJECTIVE: To evaluate two DINAMAP devices. METHODS: A non-invasive blood pressure simulator was used to evaluate the Compact TS in comparison with the 8100 over a range of pressures (60/30 to 200/150 mmHg), pulse rates (40-200 beats/min) and pulse strengths (down to 10% of the nominal pulse strength), with the addition of a simulated movement artefact. The ability of each monitor to measure the pressure consistently when presented with 30 repeated simulated waveforms at 120/80 mmHg was assessed. RESULTS: The Compact TS recorded more consistently than did the 8100 device, with an average SD over the pressure range of 1.45 mmHg compared with 2.22 mmHg. The average pressures recorded by the two monitors at the simulated 120/80 mmHg pressure were not statistically different. Although the two monitors recorded statistically significantly different pressures at some of the simulated pressures the differences were within 10 mmHg and hence not clinically significant. Both monitors underestimated the systolic blood pressure when they were presented with very weak oscillometric waveforms. The results suggest that the Compact TS may have lower non-systematic errors in clinical practice than does the 8100 device, but that systematic errors for the two monitors may be similar.

Journal Article↗
PubMed2003

Dynamics of polymer chain collapse into compact states.

Molecular dynamics simulation methods are used to study the folding of polymer chains into packed cubic states. The polymer model, based on a chain of linked sites moving in the continuum, includes both excluded volume and torsional interactions. Different native-state packing arrangements and chain lengths are explored; the organization of the native state is found to affect both the ability of the chain to fold successfully and the nature of the folding pathway as the system is gradually cooled. An order parameter based on contact counts is used to provide information about the folding process, with contacts additionally classified according to criteria such as core and surface sites or local and distant site pairs. Fully detailed contact maps and their evolution are also examined.

Journal Article↗
PubMed2006

Currently available simulators: ex vivo models.

The introduction of the ex vivo tissue endoscopy simulators represents a major advance in endoscopic training, particularly in therapeutic endoscopy. The simulators have been popular teaching tools from the start, and the data supporting their benefit are accruing, especially in hemostasis training. Simulators like the compact EASIE may prove most beneficial as training tools for interventional skills that require repetitive practice and a larger volume of procedures than may occur naturally during the course of standard endoscopy practice. More data are needed to confirm that hands-on simulator training improves outcomes in clinical endoscopic performance and to characterize better the influence of such simulator work on subsequent endoscopic practice. As ex vivo simulators become more available, it is likely that these models will enhance initial training and also allow practicing gastroenterologists to acquire new techniques, maintain their skills, and demonstrate proficiency for credentialing purposes.

Animals↗
PubMedDate not supplied

[Study of the compact denatured state of a protein by molecular dynamics simulation].

The native state can be considered as a unique conformation of the protein molecule with the lowest free energy of residue contacts. In this case, all other conformations correspond to the denatured state. The degree of their compactness varies significantly. Under folding conditions, the compact denatured state rather than the random coil is in equilibrium with native protein. The balance between the main forces of protein folding, the solvophobic interactions and conformational entropy, suggests that some properties of the compact denatured state are close to those of native protein, whereas other properties are close to those of the random coil. To investigate the molecular structure of the compact denatured state, the method of molecular dynamics simulation seems to be very useful.

Protein Conformation↗
PubMed2006

How does canal taper affect root stresses?

AIM: To examine the effect of specific tapers on root stresses and thus vertical root fracture. METHODOLOGY: The effect of taper on root stresses was calculated during simulated warm vertical compaction of gutta-percha in a straight rooted premolar for three tapers (0.04, 0.06 and 0.12 mm mm(-1)) using finite element analysis. Stresses in the dentine were observed whilst the root was filled with three subsequent gutta-percha increments. Each increment was compacted at 10 or 15 N and the gutta-percha cooled down to 37 degrees C. After filling, composite was polymerized in the access space. A functional occlusal load of 50 N was then applied on the buccal cusp incline. The stress distribution in the root during the occlusal loading was compared with the stresses during filling. RESULTS: During filling, the highest stresses were found: (a) at the canal surface; (b) using the smallest taper; (c) in the apical third; and (d) during the first gutta-percha increment. The root stress distribution changed when the functional post-filling load was applied. It generated the highest stresses at the external root surface, with a tensile stress concentration at the lingual surface of the cervical third. Since the stresses during simulated masticatory loading concentrated on the external surface, an increased taper size caused only slightly higher root stress levels. CONCLUSIONS: With increasing taper, root stresses decreased during root filling but tended to increase for masticatory loading. Root fracture originating at the apical third is likely initiated during filling, whilst fracture originating in the cervical portion is likely caused by occlusal loads.

Computer Simulation↗
PubMed2005

Introducing variable cell shape methods in field theory simulations of polymers.

We propose a new method for carrying out field-theoretic simulations of polymer systems under conditions of prescribed external stress, allowing for shape changes in the simulation box. A compact expression for the deviatoric stress tensor is derived in terms of the chain propagator, and it is used to monitor changes in the box shape according to a simple relaxation scheme. The method allows fully relaxed, stress free configurations to be obtained even in nontrivial morphologies, and it enables the study of morphology transitions induced by external stresses.

Journal Article↗
PubMed2006

Emergence of artificial photons in an optical lattice.

We establish the theoretical feasibility of direct analog simulation of the compact U(1) lattice gauge theories in optical lattices with dipolar bosons. We discuss the realizability of the topological Coulomb phase in extended Bose-Hubbard models in several optical lattice geometries. We predict the testable signatures of this emergent phase in noise correlation measurements, thus suggesting the possible emergence of artificial light in optical lattices.

Journal Article↗
PubMed2005

Granular dynamics in compaction and stress relaxation.

Elastic and dissipative properties of granular assemblies under uniaxial compression are studied both experimentally and by numerical simulations. Following a novel compaction procedure at varying oscillatory pressures, the stress response to a step strain reveals an exponential relaxation followed by a slow logarithmic decay. Simulations indicate that the latter arises from the coupling between damping and collective grain motion predominantly through sliding. We characterize an analogous "glass transition" for packed grains, below which the system shows aging in time-dependent sliding correlation functions.

Journal Article↗
PubMed2004

Porous Ti-6Al-4V alloy fabricated by spark plasma sintering for biomimetic surface modification.

Porous compacts with both biological and biomechanical compatibilities and high strength were developed. Spherical powders of Ti-6Al-4V alloy, which were either as received or surface modified with the use of calcium ions by hydrothermal treatment (HTT), were fabricated by a spark plasma sintering process. The porous compacts of pure Ti were used as reference materials. Porosity was approximately 30%, and compressive strengths were 113 and 125 MPa for the as-received Ti alloy powders and those modified by the HTT process, respectively. The bending strength and elastic modulus of as-received Ti alloy powders were 128-178 MPa and 16-18 GPa, respectively. Each of the compacts was immersed in simulated body fluid (SBF). The amount of adsorption/precipitation of calcium phosphate through the compacts was measured by weight change and was observed by SEM. The compacts were covered with calcium phosphate after 2 weeks of immersion in SBF. The compacts of Ti alloy had plenty of precipitated apatite crystals, and modification by HTT accumulated more precipitation. Because calcium phosphate is a mineral component of bone, apatite, which is precipitated on the surface of the compacts, could adsorb proteins and/or drugs such as antibiotics. It is expected that a large amount of proteins and/or drugs could be impregnated when the porous compacts developed are used.

Alloys↗
PubMed1995

Deformation of the Stokes B2 rotary tablet press: quantitation and influence on tablet compaction.

Deformations that affect the vertical punch displacement of a Stokes B2 rotary tablet press were characterized with a cathetometer. The press deformation was found to be elastic for both the upper and lower compression roller assemblies. However, the upper and lower compression roller assemblies have different Hookian spring constants: 8.58 x 10(4) and 5.18 x 10(4) kN/m for the upper and lower assemblies, respectively. Using two-way analysis of variance, the Hookian spring constants were shown to be independent of compaction phases and lower punch penetration setting. To study the influence of press deformation on tablet compaction, the Hookian spring constants were factored into the caculation of the incremental work of compaction for dibasic calcium phosphate dihydrate and microcrystalline cellulose. As the peak compression pressure increases, the force-displacement work done on the tablet during the loading phase decreases relative to calculations that neglect press deformation. This decrease in force-displacement work was attributed to elastic press deformation, which absorbs energy during the loading phase and then releases this energy later in the compaction cycle, altering the punch-displacement profile. The rate at which the press stores and releases elastic energy depends in part upon the viscoelastic properties of the tablet. Based upon these results, the coupling between press elasticity and a tablet's viscoelastic properties should be accounted for when analyzing tablet compaction or trying to simulate the punch-displacement profile of a tablet press that deforms during compaction.

Chemistry, Pharmaceutical↗
PubMed2005

Simulation of radionuclides 99Tc and 243Am migration in compacted bentonite.

The distribution coefficients of radionuclides derived from batch experiments are used to evaluate the migration behavior of radionuclides (99)Tc and (243)Am in compacted bentonite after different times. The effects of times, dispersion coefficient, distribution coefficient and pore water velocity on the concentration distribution of (99)Tc and (243)Am in the environment are also calculated from the theoretical migration equation. (99)Tc is poorly sorbed to bentonite and moves quickly into the environment. Migration of radionuclide increases with increasing dispersion coefficient and decreasing distribution coefficient. The effects of water velocity on the migration of Tc and Am are obvious. Most (243)Am is retained in the first several meters, and 20 m of the backfill-compacted bentonites are sufficient to prevent the migration of radionuclide Am from the repository to the environment.

Journal Article↗
PubMed2005

Effect of the variation in the ambient moisture on the compaction behavior of powder undergoing roller-compaction and on the characteristics of tablets produced from the post-milled granules.

Effect of variation in the ambient moisture levels on the compaction behavior of a 10% acetaminophen (APAP) powder blend in microcrystalline cellulose (MCC) powder was studied by comparing the physical and mechanical properties of ribbons prepared by roller compaction with those of simulated ribbons, i.e., tablets prepared under uni-axial compression. Relative density, moisture content, tensile strength, and Young's modulus were used as key compact properties for comparison. Moisture was found to facilitate the particle rearrangement of both, the APAP and the MCC particles, as well as the deformation of the MCC particles. The tensile strength of the simulated ribbons also showed an increase with increasing moisture content. An interesting observation was that the tensile strength of the roller compacted samples first increased and then decreased with increasing moisture content. Variation in the ambient moisture during roller compaction was also found to influence the characteristics of tablets produced from the granules obtained post-milling the ribbons. A method to study this influence is also reported.

Acetaminophen↗
PubMed2003

Flexible endoscopy simulators.

Training in flexible endoscopy is becoming increasingly complex. In an effort to improve the efficiency of endoscopic education, physicians are turning to simulation technology to provide a platform for training away from the endoscopy suite. The concept of medical simulation is not new, but the recent addition of powerful computer-generated virtual reality simulation has revolutionized the field. These compact computers are now able to generate a simulated environment that not only mimics the movement of the endoscope, but also recreates the sounds of the endoscopy suite, the feel of the movement of the scope, the reaction of intestinal tissue, and the response of a patient experiencing discomfort. Within this life-like simulated environment, a wide variety of diagnostic and therapeutic endoscopic procedures can be performed. This article reviews the history of flexible endoscopy simulators and details the most advanced models currently available. The literature supporting the use of these simulators is also presented, and issues involving the incorporation of simulation technology into endoscopic education and credentialing are discussed.

Animals↗