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| 2. |
- Gourdon, Mathias, 1980, et al.
(författare)
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Scale formation and growth when evaporating black liquor with high carbonate to sulphate ratio
- 2008
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Ingår i: Nordic Pulp and Paper Research Journal. - 2000-0669 .- 0283-2631. ; 23:2, s. 231-239
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Tidskriftsartikel (refereegranskat)abstract
- Scaling in black liquor evaporators has for longbeen recognised as a problem. Most problems are connected tothe precipitation of sodium salts. In the black liquor evaporation,as the black liquor gets concentrated, sodium carbonateand sodium sulphate will saturate and precipitate from the solution.When precipitation occurs the salts may crystallise on theheat transfer surface or adhere to the surface as well as they cancrystallise in the bulk. If crystals remain on the heat transfersurface an insulating layer of scales can quickly build up causingthe heat flux to drop and eventually forcing the evaporatorto be shut down for cleaning. The research in this paper focused primarily on scaling forblack liquors with high ratio of carbonate to sulphate, a conditionshown to be important in industrial black liquor evaporators(Frederick et al. 2004). The scaling behaviour was investigatedexperimentally in a falling film evaporator. The aim wasto find favourable operating conditions decreasing or eliminatingscaling. The examined parameters were different heat flux,circulation flow rate, internal residence time and black liquorfeed concentration. For the investigated operating conditions themain parameters affecting scaling were the circulation flow rateand heat flux whereas the scaling rate was seen independent ofthe internal residence time.
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| 3. |
- Perdana, Indra, 1973, et al.
(författare)
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Effect of external mass transport on permeation in a Wicke-Kallenbach cell
- 2009
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Ingår i: Chemical Engineering Research and Design. - : Elsevier BV. - 0263-8762 .- 1744-3563. ; 87:10, s. 1438-1447
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Tidskriftsartikel (refereegranskat)abstract
- Wicke-Kallenbach (WK) cells are often used for studying separation and component transport in porous material. In order to obtain important transport parameters, mathematical models are frequently developed but they generally consider the fluid phase in the WK compartments to be free of concentration gradients and thus external mass transport resistance is neglected. The present work is a detailed study of how the external transport influences permeation through porous material in a WK cell. External mass transfer effects were demonstrated experimentally and they were further studied from three-dimensional (3-D) model simulations. Film theory was applied to develop an appropriate Sherwood number correlation for the external mass transport coefficient. It was shown that the flow in the compartment and geometrical factors of the WK cell had significant influences on the permeation. It was found that introducing the proposed external mass transport correlation into a simplified one-dimensional (1-D) model was adequate to reproduce results from both the experimental data and the 3-D model.
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| 4. |
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| 5. |
- Johansson, Miriam, 1978, et al.
(författare)
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Falling film hydrodynamics of black liquor under evaporative conditions
- 2009
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 2009:52, s. 2769-2778
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Tidskriftsartikel (refereegranskat)abstract
- Bubbles were observed in a thin, evaporating, falling film of black liquor (a fluid mixture generated during the pulp production) on the exterior wall of a research evaporator. Because the presence of bubbles could not be explained by nucleate boiling, a combination of turbulent vapour entrainment and effects due to surface-active compounds – surfactants – is proposed. Black liquor contains numerous surfactants, which are likely to enhance bubble formation and stabilization in the fluid and on the film interface. One observed important effect of bubble formation was fluid loss due to bubble-bursting aerosolization (sputtering). Also, bubbles and bubble processes probably alter the film velocity-profile and heat transfer resistance, thereby affecting heat transfer across the film and hence evaporator efficiency.
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| 6. |
- Johansson, Miriam, 1978
(författare)
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Heat Transfer and Hydrodynamics in Falling Film Evaporation of Black Liquor
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The evaporation plant is one of the major steam consumers in kraft pulp mills, which are the dominant type of chemical pulp mill in Sweden. Previous studies have shown potential to decrease the live steam demand for the pulp process, e.g. by increasing the number of effects in the evaporation plant, or by heat integration of the evaporation plant with other parts of the process. This study is intended to provide a scientific basis for cost estimations for the design changes needed to accomplish the increased energy efficiency. An important part of the study was measurement of black liquor evaporation behaviour under controlled conditions in a one-effect research evaporation plant built in cooperation with Metso Power AB. In the validation experiments with water as the evaporated fluid, the heat transfer coefficients obtained were mostly within ±10% of those calculated with the correlation for heat transfer in falling film evaporation from Schnabel and Schlünder (1980).From the experiments presented in this study, it is seen that the Nu number, for a specific Re number in the turbulent flow regime at each Pr number level, ceases to increase with increased Re number. Taking this observation into account, a new heat transfer correlation was developed, based on experimental data from the research evaporator. It was found that adding a dry solid content dependence to the new correlation improved the prediction for black liquor. Two comprehensive turbulence models for falling film hydrodynamics were studied, which provided increased qualitative understanding of the velocity, temperature and heat transfer profile in the evaporating black liquor falling film. The heat transfer coefficients calculated with the turbulence models were in agreement with the experimental heat transfer data for water. However, they did not agree with the experimental heat transfer coefficients for black liquor. Several possible reasons for the deviation were discussed. From studying the wave behaviour it was found that there were large local fluctuations in the film thickness. For the water falling film, existing correlations (Brötz’s and Brauer’s) were mostly within the 95% confidence interval of the measured average film thickness. The black liquor film tends to be thicker than the film thickness calculated with the correlations, especially for higher Re numbers. Bubbles were observed in the evaporating falling film of black liquor. A combination of turbulent wave-breaking and effects due to surface-active compounds – surfactants – was found to be the most likely explanation for the bubble creation.
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| 7. |
- Johansson, Miriam, 1978, et al.
(författare)
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Heat transfer in evaporating black liquor falling film
- 2009
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 2009:52, s. 2759-2768
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Tidskriftsartikel (refereegranskat)abstract
- Black liquor is a fluid with high viscosity, implying high Pr numbers. Most of the previously developed correlations for falling film evaporation heat transfer were developed for fluids with relatively low Pr numbers, and therefore are not valid for most black liquor evaporation conditions. Experimental heat transfer data from black liquor evaporation are presented here. They show that the Nu number, as expected, increases in the turbulent region. However, at a specific Re number for each Pr number level, the Nu number ceases to increase with increasing Re number. A new correlation, taking this observation into account, has been developed on the basis of experimental data in the region of 4.7
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| 8. |
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| 9. |
- Sjöblom, Jonas, 1968, et al.
(författare)
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Latent variable projections of sensitivity data for experimental screening and kinetic modeling
- 2008
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Ingår i: Computers and Chemical Engineering. - : Elsevier BV. - 0098-1354. ; 32:12, s. 3121-3129
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Tidskriftsartikel (refereegranskat)abstract
- In microkinetic modeling the number of kinetic parameters is large and the precision of the “known” parameters is often very low. The standard approach is then to fit only the most uncertain parameters while regarding the fixed parameter as “true”. This assumption will have consequences also on the fitted parameters since the correlation structure often is quite significant. In this study we have taken the approach to fit many parameters and then try to use more efficient experimental designs to break the correlation structure and thus obtain more precise parameter estimation despite the large number of fitted parameters. After performing sensitivity analysis of many candidate experiments, a latent variable model (PCA) is made from the resulting sensitivity matrix and the score matrix is used as a candidate set prior to experiment selection. Due to the correlation structure in the sensitivity matrix, the number of components from the PCA model is fewer than the number of parameters. The columns in the score matrix are furthermore orthogonal whereas the columns in the original sensitivity matrix are not. Different designs were generated using the original sensitivity matrix, the score matrix as well as using a space-filling design and performing a sequential approach. Both steady state and transient experiments were evaluated. These different designs were used to fit kinetic parameters to a simulated dataset made using published parameter values. The results show no significant difference when using the original sensitivity matrix or the score matrix. However, since the score matrix has fewer columns than the full sensitivity matrix, the use of designs based on the score matrix enables more efficient designs when few experiments are required. The number of components in the PCA model also gives the rank of the parameter space induced by the candidate experiments. This is useful information when fitting many parameters in a microkinetic model and provides an assessment of the value of every candidate experiment before it is even performed.
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| 10. |
- Ström, Henrik, 1981, et al.
(författare)
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Choice of urea-spray models in CFD simulations of urea-SCR systems
- 2009
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Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 150:1, s. 69-82
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Tidskriftsartikel (refereegranskat)abstract
- The sensitivity of modeling choices to obtained results for Eulerian–Lagrangian CFD simulations of urea-SCR systems has been investigated for a system consisting of an AdBlue-spray located at the exhaust pipe wall, directed into the exhaust gas flow. The decomposition of urea is modeled as being heat transfer limited and taking place at a constant temperature (425 K). It is shown that modeling choices may affect the predicted extent of wall hit, which types of droplets that are predicted to hit the wall, and also where they will do so.The influence of the different forces due to drag, buoyancy, lift effects, thermophoresis and history effects was investigated, proving that only the forces due to drag and buoyancy are necessary to correctly describe droplet motion within this system. It is necessary to use a droplet drag coefficient that takes the current level of droplet distortion into account.A stochastic particle tracking model will describe the effects of turbulent dispersion, but also make the simulation results sensitive to the quality of the turbulence model's prediction of the turbulent fluctuating velocities. Using such a model will also resolve some of the enhancement of heat and mass transfer caused by the continuous acceleration/deceleration of droplets by turbulent eddies.
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