Stig Stenström

Evaluation of Equations Approximating Thermodynamic and Transport Properties of Water, Steam and Air for Use in CAD of Drying Processes

Over 70 equations approximating thermodynamic and transport properties of water and steam at pressure 1 bar and saturated pressure as well as air at pressure 1 bar were tested against reference data. Their accuracy and relative computing time were reported.

Extraction of cadmium from phosphoric acid solutions with amines. Part III. A thermodynamic extraction model

A thermodynamic model for cadmium extraction from phosphoric-hydrochloric acid mixtures has been developed. The model enables calculations of cadmium extraction as well as coextraction of phosphoric and hydrochloric acids to be made. In the aqueous phase Bromleys model for the activity coefficients is adopted. In the organic phase simple expressions for the non-ideal behaviour of phosphoric acid are used. The model is capable of calculating cadmium extraction from 0.02 M up to 6.7 M H3PO4 and chloride concentrations between 0.005 and 0.10 M HCl. For high amine concentrations the prediction for cadmium extraction is too high, indicating that high concentrations of phosphoric acid in the organic phase affect the activity coefficient for the extracted cadmium complex significantly. The thermodynamic constant for the extraction equilibrium has been determined as log K11 = 10.23.

Extraction of cadmium from phosphoric acid solutions with amines Part I. Extractant selection, stripping, scrubbing and effects of other components

Abstract Cadmium can be effectively removed from wet-process phosphoric acid (WPA) by solvent extraction using long-chain amines, preferably tertiary amines, as extractants. Stripping can be performed either with water or with acids such as phosphoric or sulphuric acid depending on the final conversion of the cadmium waste. Scrubbing could be necessary in some cases and can be performed with alkali. A factorial design experiment was performed to determine the effects of four different components (H2SO4, HCl, HF, H2SiF6) in WPA on cadmium extraction with Alamine 336. The largest influence on cadmium extraction is seen for the hydrochloric acid, drastically increasing the distribution coefficient. Variations of the concentrations of H3PO4, HCl, HF, Fe and Al were also studied. The extraction of cadmium was seen to be strongly dependent on phosphoric acid concentration. A simple extraction model is proposed indicating that CdCl42− ions are extracted.

THE DIFFUSION OF WATER VAPOUR THROUGH PULP AND PAPER

ABSTRACT The diffusion of water vapour through pulp and paper was investigated experimentally in a diffusion cell. The diffusion cell consisted of two chambers differing in relative humidity. Any given sample was placed as a banier between the two chambers. Two pulp qualities and eleven paper grades were included in the investigation. The diffusivities measured ranged from 2.1.10-8m2/s to 5.4.10-6m2/. Effective vapour diffusivity was found to correlate with the density of the sheet. For five of the paper or pulp grades measurements were performed a t different temperatures and relative humidities. In the range of relative humidities investigated, effective vapour diffusivity was found to be independent of relative humidity.

A study of the permeability of pulp and paper

A sheet of paper is modelled as a two-dimensional network of cellulose fibres. The fibres are assumed to be either cylindrical or band-shaped. Both well-ordered fibrous structures and fibrous structures in which a random arrangement of fibres is assumed are studied. The equations for creeping flow through such structures are solved, and the calculated permeabilities are compared with measured values. Flow through such paper structures as pulp sheets and handsheets of unbeaten sulphate pulp is found to be adequately described by a structural model that assumes cellulose fibres to be band-shaped when a fibre aspect ratio of 3.5 (well-ordered structure) or 5 (random fibre distribution) is used. These values of the fibre aspect ratio compare favourably with the values used when gas diffusion through the same sheets is modelled. For newsprint sheets the measured permeability is found to be lower than that predicted by the models when physically realistic values for the aspect ratio are taken. It is also found that for all the pulp and paper grades investigated (a total of 19) the measured permeabilities and effective diffusivities correlate with each other.

Extraction of cadmium from phosphoric acid solutions with amines Part II. Diluent selection, flow-sheet configuration, pilot plant operation

A large number of diluents were screened to determine the optimum combination between diluent and extractant. Combinations of a C8 tertiary amine and aromatic diluents or of a C12 tertiary amine and aliphatic diluents do not lead to third-phase formation. A flow-sheet for a solvent-extraction process for treatment of various qualities of wet-process phosphoric acid solutions is proposed. Based on a cadmium purification level of 1 mg/kg, extraction, stripping and scrubbing sections of the process would contain two, two and one stages, respectively. A solvent pre-treatment stage may be necessary in some cases. Co-extraction of acids such as H3PO4, H2SO4, HCI, HF and H2SiF6 can be decreased to very low values by manipulating process variables such as pre-equilibration, phase ratios, diluent and amine concentration. Co-extraction of cations other than cadmium was also investigated. Iron, aluminium and magnesium are not co-extracted while zinc and calcium are co-extracted to some extent. The proposed process can be used either for pollution abatement or in combination with other purification processes to recover some of the valuable cations such as uranium, or to produce more refined phosphoric acid streams. (Less)

Gas diffusion through sheets of fibrous porous media

Using a cell model to describe a fibrous porous medium theoretical expressions ale deduced relating effective gas diffusivity to the volume fraction of the fibres. Cylindrical as well as band-shaped fibres are studied. The new models, which are based on solutions to the diffusion equation, agree well with existing correlations. The models are compared with experimental data for water vapour diffusion through sheets of nonwoven fabric and of paper. Diffusion through a nonwoven material consisting of viscose fibres is predicted well by the present model for diffusion around cylindrical fibres. Diffusion through some paper structures such as pulp sheets and handsheets of unbeaten pulp is adequately described by the present model for band-shaped fibres. For newsprint sheets the measured rate of diffusion is much slower than that predicted by either of the models. One explanation for the disagreement between experiment and model for newsprint could be the presence of fine material and fillers in the pores. (Less)

Water transport during the drying of cardboard studied by NMR imaging and diffusion techniques

An understanding of the distribution of water in wood pulp during drying is of fundamental importance to the paper industry. In this study nuclear magnetic resonance (NMR) imaging is used to measure quantitative water profiles during the drying of liquid packaging board with initial water ratio (water: dry mass) of approximately 1.5. The drying was carried out in situ within the magnet, under a range of air temperatures (20-60 degreesC), flow rates (135-400 1 h(-1)) and humidity conditions. One dimensional (1D) slice selective profiles were acquired along the thickness direction of the cardboard with a spatial resolution of 15 mum pixel(-1) at regular time points during the drying process. To give further insight into the distribution of water within the cellulose fibre matrix pulsed gradient spin echo (PGSE) diffusion measurements were acquired on samples equilibrated to water ratios between 0.2 and 2.0. The water profiles show that the initial water concentration varies between the three layers in the composite cardboard. At faster drying conditions the initial water loss is predominantly from the two outer layers, while with slower drying conditions the water loss is more homogeneous throughout all layers. The diffusion coefficient of the fibre water was found to decrease significantly with water content and was slower across the cardboard thickness (thus across the fibres) than parallel to the cardboard surface

Modelling of an electric IR heater at transient and steady state conditions Part I: model and validation

A model for an electric infrared (IR) heater has been developed. The model includes non-grey radiative heat transfer between the different parts of the IR heater, as well as conduction in reflector material and convective cooling of surfaces. The geometry is simplified into one dimension. Using IR module voltage as the only input, the model predicts the temperature of heater components and cooling air, as well as the net radiation heat transfer to the surroundings at steady state and transient conditions. The model has been validated against both steady state and transient experimental results from a small electric IR heater. The model predictions are in good agreement with experimental data both regarding steady state results and the transient response over a wide range of voltages.

Solvent Extraction of Phosphoric Acid with Long Chain Tertiary Amines

Solvent extraction of phosphoric acid with Alamine 336 and tri-n-octylamine in toluene has been investigated. The activity of phosphoric acid is expressed as a polynomial of ionic strength and Bromleys model for electrolyte solutions is employed for calculation of ionic activity coefficients. Three species R3N (H3P04) n with n = 1, 2 and 3 in the organic phase are taken into account for explanation of the distribution equilibria. The extraction constants have been determined as log K11 = 2.05, log K12 = 3.61 and log K13 = 3.84 respectively. Aggregation and other non-ideal behaviour in the organic phase are described by letting the corresponding activity coefficient quotients be a function of the total concentration of acid in the organic phase.