Roland Wimmerstedt

Membrane distillation - A theoretical study of evaporation through microporous membranes

Abstract Membrane distillation is a process in which, for example, water in a heated salt solution, evaporates through a porous membrane. The vapour condenses on a coolant surface on the other side of the membrane. The two liquid streams, the salt solution and the condensate, are separated by a highly porous hydrophobic membrane. Surface tension forces withhold liquids from the pores and prevent contact between the two liquids. The temperature difference, causing a corresponding vapour pressure difference across the membrane, provides the driving force of the membrane distillation process. Evaporation will occur at the solution surface if the vapour pressure on the solution side is greater than the vapour pressure at the condensate surface. Vapours then diffuse through the pores to the cooler surface, where they condense. The dependence of mass and heat transport upon different process and membrane parameters involved in membrane distillation has been investigated theoretically.

Fractionation and concentration of kraft black liquor lignin with ultrafiltration

Kraft black liquor lignin is a biofuel that is separated from the cellulose during kraft pulping. Improved technology and energy integration in paper mills have led to an energy surplus at many mills. It is therefore of great interest to extract the lignin from the pulp mill and sell it as fuel to replace fossil fuel in other furnaces. The fractionation of kraft black liquor with a total dry matter content of about 15 wt% has been studied using ultrafiltration. The flux for three polymeric membranes with different cut-offs was investigated as well as their retention of lignin and other process specific substances. The retention of lignin for the three membranes with cut-offs of 4,8 and 20 kDa was 80%, 67% and 45%, respectively. The retention of sulphur and sodium was zero for all three membranes. The purity of the final lignin fuel is of importances as the ash content preferably should be as low as possible. The flux and retention during concentration and diafiltration of the black liquor were therefore studied. The dialfiltration operation was conducted in batch and semicontinuous mode. The lignin purity was 36% in the original kraft black liquor and 78% after semi-continuous dialfiltration.

Treatment of three types of landfill leachate with reverse osmosis

Treatment of landfill leachate has gained increasing attention during recent years, and several commercial RO leachate treatment plants have been installed in, e.g., Germany, the Netherlands and Switzerland. in this investigation the influence on membrane performance when treating new types of landfill leachate was studied. Three different types of landfill leachate were included in this study: leachate from a conventional landfill and two leachates from a new type of landfill. At the new landfill the waste is divided into different categories and deposed in waste cells with separate leachate collecting systems. Leachate from a cell with biodegradable waste and from a cell with special waste, containing mainly ashes, were included in this study. A linear correlation between flux and conductivity was found for leachate both from the conventional landfill and from the biodegradable waste cell. The flux varied, depending on the conductivity of the leachate, between 48 and 3 l/m(2)/h. The reduction of pollutants was high. The reduction of the chemical oxygen demand and NH4-N was more than 98% for leachate from both the conventional landfill and the biodegradable waste, for example. The salt concentration, and thus the osmotic pressure, was very high in the leachate from the cell containing special waste. The flux was therefore too low for RO to be a suitable treatment process for this leachate. (Less)

Ultrafiltration of kraft black liquor with a ceramic membrane

Emissions from the combustion of fossil fuels contribute to the greenhouse effect around the globe. It is therefore of great interest to reduce the use of fossil fuels and revert to some form of biofuel. Lignin, extracted from kraft black liquor, has the potential to become a significant contributor to the biofuel market. A modern pulp mill has an energy surplus that can amount to 7 GJ per tonne of pulp. One way to reduce this surplus is to extract lignin, which is the main source of energy in a kraft pulp mill, and convert it to a usable biofuel, which can be sold on the market. If membrane technology is used to extract lignin from kraft black liquor additional benefits in the overall operation of the mill can be achieved, such as, an improved pulp quality. In this investigation a tubular ceramic membrane manufactured by Orelis, France, with a cut-off of 15,000 Da was used to fractionate kraft black liquor. The experiments were performed at 60, 75 and 90°C at various pressures. The flux and retention during concentration to a volume reduction of 0.9 were studied. The fluxes were 90, 110 and 130 l/m2h at 60, 75 and 90°C and TMP 100 kPa. The lignin retention was about 30%. The retention of monovalent ions was virtually zero, whereas multivalent ions were retained to a high degree, probably due to their association with the organic material. During concentration to a volume reduction of 0.9 at 100 kPa and 90°C, the dry substance increased from 16 wt% to 22 wt% and the lignin concentration from 55 g/1 to 158 g/I. The average flux during the concentration was 90 1/m2h. The flux was 20 l/m2h at the end of the concentration.

The application of membrane technology in the pulp and paper industry

Abstract A state of the art review on industrial applications of membrane technology in the pulp and paper industry is presented. Applications have been arranged under three headlines, effluent treatment, concentration and fractionation. Among new and promising applications are treatment of waste paper white water, effluent after deresination and concentration of spent liquor from chemi-mechanical pulping.

Recent advances in biofuel drying

Sweden must be regarded as one of the leading countries to utilize biofuels for heating purposes. A national survey of technique and economy for biofuel drying systems has therefore been deemed to be of wider interest. As well flue gas dryers as steam dryers are used but in different applications. Operating experiences from both types are reported. A thermodynamic analysis of various system solutions is also performed. Rotary flue gas dryers dominate the market in fuel factories, which are not colocated with other industrial activities. Steam dryers are best suited for integrated systems, where their capability for heat recovery can be exploited. A new approach is to recover electricity instead of heat. This is accomplished in a system integrated in a district heating plant with power cogeneration. The environmental issues are of ever increasing concern and the release of volatile organic compounds (VOC) during drying and influence of the fuel moisture content on emissions from the combustion are discussed.

Modelling steam drying of a single porous ceramic sphere: Experiments and simulations

In order to investigate and describe the drying kinetics occurring in a porous material during superheated steam drying, a model was developed based on the fundamental transport equations for mass and heat. The driving forces in the model are gradients of moisture, temperature and pressure. The transport coefficients employed were either measured experimentally or were derived theoretically from the pore size distribution of the material. The porous material was viewed as having cylindrical pores that wind according to a tortuosity factor, which was the only adjustable parameter used in the simulations. Local thermodynamic equilibrium was assumed to be present throughout the material. To describe this, an experimental sorption isobar was measured. Single ceramic spheres (10 mm diameter), which served as the porous model material, were dried in a thermobalance which enabled weight changes during drying to be determined very accurately. All the experiments were carried out under atmospheric conditions, but both the steam temperature and the steam mass flux were varied within a broad range. Internal temperatures in the sphere were also measured by use of thin thermocouples (0.5 mm), in order to determine the shape of the internal temperature rise and obtain an indication of the temperature gradient. Good agreement was obtained between results of the experiments and of the simulations, both for the drying rate and for internal temperature under varying external conditions, providing support for the model. The model was also used to study the magnitude and influence of the different transport mechanisms during steam drying

Influence of the concentration of a low-molecular organic solute on the flux reduction of a polyethersulphone ultrafiltration membrane

Drastic flux reductions are sometimes encountered during ultrafiltration of solutes much smaller than the membrane pores. This usually occurs during ultrafiltration of hydrophobic, low-molecular solutes, such as fatty acids, alcohols and alkanes. The influence of the concentration of a carboxylic acid, octanoic acid, on the flux of a polyethersulphone membrane was studied in this investigation. The concentration was found to have a marked influence on the flux. The flux reduction was moderate at low concentrations, but became severe above a certain, critical concentration. Two flux-reducing mechanisms were evaluated; reduction of the effective pore radius by adsorption of solute molecules on the pore walls, and blocking of pores by capillary condensation. The adsorption of octanoic acid on a hydrophobic solid surface was studied by null ellipsometry.

Steam drying: modelling and applications

ABSTRACT The concept of steam drying originates from the mid of the last century. However, a broad industrial acceptance of the technique has so far not taken place. The paper deals with modelling the steam drying process and applications of steam drying with in certain industrial sectors where the technique has been deemed to hove special opponunities. In the modelling scction the mass and heat transfer proceases are described along with equilibrium, capillarity and sorption phenomena occurring in porous materials during the steam drying process. In addition existing models in the literslure are presented. The applications discussed involve drying of fuels with high moisture contcna, cattle feed exemplified by sugar beet pulp. lumber. paper pulp. paper and sludges. Steam drying is compared to flue gas drying of biofuels prior to combustion in a boiler. With reference to a current insrallation in Sweden. the exergy losses. as manifested by loss of co-generation cupacity. are discussed. The energy saving p...

The influence of a low-molecular hydrophobic solute on the flux of poly-sulphone ultrafiltration membranes with different cut-off

Abstract In this study the influence of a low-molecular hydrophobic solute on the flux of a hydrophilic regenerated cellulose membrane and hydrophobic polysulphone membranes with different nominal molecular weight cut-off, NMWCO, was investigated. Octanoic acid was used as a model substance because it is considerably smaller than the NMWCO of all the membranes used in the investigation. Octanoic acid was found to decrease the flux of the polysulphone membranes considerably. The decrease was dependent on the NMWCO, although in all cases the NMWCO is many times larger than the size of the octanoic acid molecules. It is also shown that the flux of the hydrophilic membrane, with a NMWCO of the same size as one of the denser hydrophobic membranes, is unaffected by the octanoic acid. The cross-sections of both used and unused membranes were examined in a scanning electron microscope (SEM). No visual difference was observed between the used and unused membrane samples. Analysis of the membranes with Fourier transform infrared spectroscopy (FTIR) showed that octanoic acid remained in the membranes even after thorough cleaning.