Gerrald Bargeman

Can nanofiltration be fully predicted by a model

There is an increasing need for model-based tools to design membrane processes for new industrial applications or to optimise existing membrane installations. The advantage of such tools is that costs can be saved by reducing the number of expermiments. In this study, the requirements for a membrane filtration model, suitable for practical use, are summarised. It is investigated to what extent it is possible to set-up such a model with the current available literature and knowledge. A membrane filtration model has been set-up based on the Maxwell–Stefan transport equations. A Freundlich equation is used to describe the membrane charge by means of adsorption of ions. With the model the permeate flux and rejections of multi-component liquid feeds can be calculated as a function of membrane properties (mean pore size, porosity, thickness, surface charge characteristic) and feed pressure. With two NF-membranes (Desal 5DK and a prototype capillary type 2 membrane) rejection experiments have been carried out with glucose, single salt solutions (NaCl, CaCl2, Na2SO4) and ternary ion mixtures of these salts. With the model the experimental flux-rejection curves can be fitted reasonably well. However, each salt mixture needs its own set of fitted parameters for the membrane charge isotherms. Furthermore, the fitted membrane charges are in contradiction with values from the literature obtained by electrokinetic measurements. Obviously, the membrane charge parameters have lost their physical meaning and are used to compensate for physical phenomena not included in the model. Extending the model with an electrostatic free energy term will be a step forward in development. Further research is needed to fulfil all requirements for the wide scope of industrial applications.

The development of electro-membrane filtration for the isolation of bioactive peptides: the effect of membrane selection and operating parameters on the transport rate

The ability to produce functional food ingredients from natural sources becomes increasingly attractive to the food industry. Antimicrobial (bioactive) ingredients, like peptides and proteins, can be isolated from hydrolysates with membrane filtration and/or chromatography. Electro-membrane filtration (EMF) is an alternative for the isolation of these usually strongly charged components. It is believed to be more selective than membrane filtration and less costly than chromatography. The isolation of bioactive peptides from a hydrolysate of αs2-casein, a protein originating from milk, was studied as a model separation for the development of EMF. This separation can be used as an example application for the isolation of other charged components from complex feedstocks in sseveral industries. After 4 h EMF the product consisted for 100% of proven or anticipated charged bioactive components. Diffusion and convection were negligible in relation to electrophoretic transport, since only charged components were recovered in the permeate product. The most important peptide (26% on total protein, starting from 7.5% in the feed) was αs2-casein ƒ(183–207), a very potent peptide against Gram positive and Gram negative microorganisms. The transport rate of αs2-casein ƒ(183–207) was reduced strongly when a polysulphone membrane with a molecular weight cut-off below 20 kDa was used. The amount of αs2-casein ƒ(183–207) transported increased practically linearly with the concentration and the applied potential difference. The use of desalinated feeds to further increase the electrical field strength in the feed compartment resulted in higher transport rates, but this increase was lower than expected probably due to the lower electrophoretic mobility. An average transport rate of 2.5 and 4 g/m2.h at maximum was achieved during 4 h EMF using GR60PP (25 kDa) and GR41PP (100 kDa) membranes respectively.

Capillary hollow fiber nanofiltration membranes

Spiral-wound modules generally have high packing densities, low costs, but require extensive feedwater pretreatment and have a high fouling potential. Tubular membrane modules have beneficial fouling properties, can be backflushed, but have low packing densities and are expensive. Both module types exist for the nanofiltration process. However, the membrane module type combining the superior properties of both types, namely capillary hollow fiber membrane modules, have not been developed yet. This paper presents the flux and retention performance data of new developed nanofiltration capillary hollow fiber membrane modules. The new modules show retention performances comparable with those of best-performing spiral-wound modules. They can typically be applied for water softening and decoloring. Continuous experiments on surface water and nanofiltration whey permeate demonstrate that the fouling behavior of the new capillary modules is indeed better than the spiral-wound modules due to its well-defined feed channel geometry.

Enhanced Process for Methanol Production by CO2 Hydrogenation

In the quest for a green chemical industry, much effort is devoted to the development of technologies for methanol synthesis by hydrogenation of CO2 – available from many sources. Low-cost sources of H2 are less frequently found, but an additional source at industrial scale is the wet hydrogen by-product of chlorine production. This study presents an enhanced process for methanol synthesis by CO2 hydrogenation using wet hydrogen by-product from salt electrolysis. This process uses a stripping unit where the wet hydrogen flows counter-currently to the condensed methanol-water mixture from the flash separator after reactor. This operation removes CO2 from the methanol-water mixture thus allowing a complete recycle of CO2 while also removing the water from wet hydrogen thus avoiding its negative impact on chemical equilibrium conversion. Consumption figures are 550 kWh electricity and 0.48-1.16 ton steam per ton methanol.

Transport phenomena during nanofiltration of concentrated solutions

In most scientific studies on nanofiltration either the development of new membrane materials or the characterization of membranes is reported. In the latter case most studies use single solute salt or sugar solutions and/or investigate nanofiltration of solutions with mixtures of ions at low concentrations relative to solution concentrations often used in industrial applications. Furthermore, several of these studies have tried to predict retention performance of nanofiltration membranes for salt solutions containing two different salts, on the basis of these characterization experiments and derived model parameters, often with limited success. Only limited knowledge is available in open literature on the effect of salt ions in an aqueous feed solution on retention of neutral solutes such as glucose and vice versa. A better insight in these phenomena is needed, since several nanofiltration applications treat solutions containing a combination of salts and (neutral) components such as sugars, amino acids, peptides or proteins. In addition, there has been limited attention in open literature for nanofiltration membrane performance during treatment of more concentrated salt solutions, such as depleted brine in chlor/alkali production and saturated brines in the production of salt crystals, despite the fact that a substantial amount of (potential) nanofiltration applications deals with these types of solutions. A better understanding of the phenomena occurring during nanofiltration of these types of solutions is a pre-requisite for proper design of membrane units for these types of applications. These research questions form the basis for the work presented in this thesis.