Usually, in concentration polarization models, the mass transfer coefficient is an unknown parameter. Also, its variation with changing experimental circumstances is in question. In the literature, many relationships can be found to describe the mass transfer coefficient under various conditions, as well as various corrections for deviating behaviour during ultrafiltration. To obtain reliable mass transfer coefficient relations directly from experimental data, two methods were tested: a method using the osmotic pressure difference during an ultrafiltration experiment, and a method based on the variation in observed retention when cross-flow velocities are changed. The osmotic pressure method appeared to be too insensitive for changing experimental circumstances (according to theoretical considerations). The velocity variation method appeared to be much more useful, although the error in the mass transfer coefficients obtained can be rather large owing to experimental and fitting uncertainties. Therefore the traditional mass transfer relations used in ultrafiltration may be as reliable as (and much more easy to use then) the velocity variation method. The velocity variation method can probably still be used in practice, however, when one or more of the parameters needed in the conventional mass transfer coefficient relations is unknown.

Six new ecdysteroids have been isolated from SERRATULA TINCTORIA; these are: the 2,22- and 3,22-diacetates of 20-hydroxyecdysone, 5beta-hydroxyrubrosterone, 3-epi-poststerone, 3-epi-rubrosterone, and 22-oxo-20-hydroxyecdysone. These minor compounds were found together with the known ecdysteroids, 20-hydroxyecdysone, its 2-, 3-, and 22-monoacetates, rubrosterone, poststerone, polypodine B (5beta,20-dihydroxyecdysone), pterosterone (25-deoxy-20,24-dihydroxyecdysone), and makisterone C (24-ethyl-20-hydroxyecdysone). All these ecdysteroids were isolated by a combination of several chromatographic techniques (liquid chromatography on alumina, DCCC, and HPLC), then identified using standard mass spectrometric and 2D (1)H-NMR techniques.

SummaryThe application of corrugations as turbulence promoters in membrane filtration was studied. This study showed that it is possible to deform an originally flat membrane to a corrugated shape without damaging it. In hyperfiltration experiments using corrugated cellulose acetate membranes it was found that the corrugations improve mass transfer, provided they are not too close together. At a given value of the mass-transfer coefficient, the presence of corrugations can lead to lower energy consumption in hyperfiltration than possible with flat membranes.

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SUMMARYConcentration polarisation, decreasing the efficiency in membrane separation processes, can be reduced by increasing mass transfer between membrane surface and bulk of the feed stream. Analogous to techniques used in plate heat exchangers efforts have been made to enhance mass transfer in a plate hyperfiltration module by using a corrugated membrane in stead of a flat one. The corrugations are pressed into an originally flat membrane. These corrugations do not only have an influence on the mass transfer, but also on such membrane properties as salt and water permeability. Corrugations enhance mass transfer in a more effective way than increase of flow rate does.The effect of the corrugations on membrane properties shows a large spread. For corrugated membranes prepared by our group, flux increases of 100% at almost the same or even slightly higher retentions have been obtained.