Karin Östergren

One-dimensional syneresis of rennet-induced gels.

The kinetics of one-dimensional syneresis of rennet-induced, skim milk gels of different thicknesses (3, 6.5, 10.5, 20 and 35 mm) at pH 6.40 and 6.00 were studied. Using a laser displacement transducer, it was possible to follow the initial stages of syneresis within 10 s of surface wetting. The syneresis rate was about 50% faster at pH 6.00 than at pH 6.40 and the initial syneresis rate increased linearly with gel thickness, an effect which was not predicted by theories based on elastic deformation (syneresis pressure) of the network during syneresis.

Syneresis of submerged single curd grains and curd rheology

Syneresis of a single curd grain submerged in ultrafiltrated milk permeate was determined over time by progressive dilution of an added tracer, blue dextran. The influence of pH, rennet concentration and the size of the curd grain was investigated. Dynamic rheological measurements were performed with renneted milk under the same conditions. Analysis of variance showed that grain size was the most important factor influencing shrinkage of the grain in the initial stage of syneresis. Smaller curd grain size resulted in more intense syneresis. The influence of pH and rennet concentration on shrinkage were of lesser importance. At a later stage of syneresis, pH was shown to be the dominating factor with the shrinkage being more pronounced at a lower pH. Inclusion of the storage modulus of curd in the syneresis model did not improve the prediction.

Modelling and analysis of axial flow through and compression of a non-rigid chromatographic bed

Abstract Compression and axial flow through a chromatographic column packed with a non-rigid gel have been characterized experimentally and theoretically. A two-dimensional model based on the assumption of elastic deformation of the solid phase and steady-state Darcy flow was modified to include the mechanical forces exerted by the end pieces of the column. Flow–pressure curves were successfully predicted by the model. Pressure drop and radial distribution of the linear velocity at different flow rates, wall friction coefficients and column radii were investigated by computer simulations. It was shown that significant radial variations in the linear velocity may arise near both the outlet and the inlet of the column.

NUMERICAL STUDY OF TWO-DIMENSIONAL COMPACTION, FLOW, AND DISPERSION IN A CHROMATOGRAPHIC COLUMN

Mathematical and numerical modeling of the complex interplay between compaction, flow, and hydrodynamic dispersion in a chromatographic column under steady state conditions has been examined. The mathematical model was based on pure elastic analysis and Darcy flow. A two-dimensional control volume approach was applied to solve the coupled nonlinear equations describing the flow and compaction. The two-dimensional equation describing the hydrodynamic dispersion was solved using an explicit control volume algorithm. Numerical diffusion was eliminated by using a window of moving points following the concentration front.

An improved numerical method of calculating the striation thinning in static mixers

An improved method of calculating the rate of striation thinning for laminar flow in a static mixer has been developed. The shapes of the objects studied are reconstructed using elliptic Fourier descriptors, allowing discrimination between true changes in shape and noise due to discretization errors. The method also allows one to choose the level of complexity in the objects to be reconstructed which faciliates optimization of the computations without loss of information. The evaluation of the rate of striation thinning is based on local velocities giving a result closely related to the volumetric flow in different regions of the static mixer.

A study of permeability and hydrodynamic dispersion under conditions of chromatographic flow

The influence of particle size and particle size distribution on permeability and hydrodynamic dispersion was studied as the first stage in the development of a numerical model of chromatographic processes. Permeability and hydrodynamic dispersion were measured and models were tested against the data obtained. The influence of the chromatographic column wall was minimized by using a wide column (d = 0.113 m). Nonporous glass beads of different sizes (d p(mean) = 125–250 µm) and size distributions (SD/d p(mean) = 0.06–0.21) were used as packing material. The longitudinal dispersion was measured between Pep numbers 3 and 430 by recording the response of a sodium chloride step input at four radial positions at each of three levels of electrodes inside the column. Permeability was measured at the same time; the Rep numbers lay between 0.004 and 0.6.

Use of a simultaneous PLIF-PLIF technique to overcome optical problems in concentration measurements in two-phase bubbly systems

An experimental technique based on double planar laser-induced fluorescence (PLIF/PLIF) using two tracers for the measurement of the local concentration of liquid phase in a two-phase system with large bubbles is presented. Fluorescence emission intensity calibration was performed to validate the use of the laser and tracer combinations at each point in the images on two CCD cameras, separately. By normalizing the fluorescence emission intensity of the first tracer with a known concentration, with the fluorescence emission of the second tracer, the optical problems related to the bubbly two-phase flow were minimized. Concentrations measured in a bubbly system are shown to be in good agreement with the true concentration values

Characterization of hydrodynamic dispersion in a chromatographic column under compression

The efficiency of a compressible, packed chromatographic bed has been characterized experimentally and by computer simulations. The experimental measurements were performed in situ by monitoring the propagation of a tracer using three sets of concentrically located electrodes. The results obtained were compared with results obtained for the same column packed with rigid, non-porous glass beads. Both columns featured a core, characterized by a flat velocity profile and a nearly constant hydrodynamic dispersion. The velocity variations, as well as hydrodynamic dispersion anomalies, in the wall region were found to be smaller in the compressed bed than in the column packed with rigid glass beads. The hydrodynamic dispersion in the compressed column was found to be approximately twice that expected for a column packed with rigid particles under the same conditions. The computed velocity variations in the compressible bed were found to agree qualitatively with experimental data.