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- Volume 17, Issue 1, 2012
South African Journal of Chemical Engineering - Volume 17, Issue 1, 2012
Volume 17, Issue 1, 2012
Source: South African Journal of Chemical Engineering 17, pp 1 –12 (2012)More Less
Depending on the rate of coalescence above a gas distributor in gas fluidized beds, bubbles of diameter approaching the column diameter can be formed. This phenomenon often occurs in columns with a large bed height to bed diameter ratio. In this event, "slugging" is said to occur. The transition from a bubbling regime to a slugging regime is accompanied by a drop in bubble/slug rising velocity. Slug formation is in most cases an undesirable phenomenon: slugs lead to a drop in efficiency of mass transfer processes due to a poor gas-solid contact and in the case of laboratory and pilot plant operation may lead to serious scaling problems. Experiments were performed in a 150 mm diameter column, with three closely sized fractions of river sand particles (diameter of 0.6-0.8 mm, 0.8-1 mm, and 1-1.4 mm). All these experiments were initially conducted using air at ambient temperature and pressure. Air velocities were chosen such that a transition from a bubbling to slugging regime could take place. Different experiments were then performed using a mixture of sand particles (29% of a fraction 0.6-0.8 mm, 23% of a fraction 0.8-1.mm, and 48% of a fraction 1-1.4 mm) at higher fluidization velocities with fully developed slugging under ambient and increased temperatures of 25, 200, 300, and 400°C. The detection system consisted of two pressure-sensing probes spaced 5 cm vertically apart. The shape of the ∆P-time trace from such a pair of pressure probes not only gives a good indication of the presence of a bubble, but also can be used to determine its velocity and size. Initially, correlations taken from the literature for bubble and slug velocities were compared with the experimental data. In turn, the experimental data was then used to train a neural network. A feed-forward back-propagation neural network was employed to model the relations among particle size, temperature, air fluidization velocity, bubble/slug size and rising velocity. Results indicate the neural network model was capable of predicting the velocities of bubbles and/or slugs and identifying the transition from a bubbling fluidized bed to a slugging bed. An improvement was evident if compared with a previously used criterion for the onset of slugging.
Dual fluidized-bed steam gasification of solid feedstock : matching syngas requirements with fuel mixturesSource: South African Journal of Chemical Engineering 17, pp 13 –24 (2012)More Less
Gasification of solid fuels has attracted increasing interest within the power industry for synthesis processes for liquid and gaseous fuel production. In addition to a high purity of syngas, the tar content and minor impurities (NH3, H2S, HCl), the gas composition has a major influence on the performance of any downstream utilization. The dual fluidized bed gasification technology developed at the Vienna University of Technology has been investigated for biomass use on a pilot scale over the last 15 years. Moreover, this process has also been demonstrated on an industrial scale. Allothermal (indirect) gasification with a solid heat carrier to transport the desired heat for gasification to the gasifier and the utilization of steam as a gasification agent ensures a high quality syngas that is nearly free of nitrogen. Originally designed for wood chips, the system can also handle a large number of alternative fuels. The advantage of fuel flexibility has turned out to be a key issue for the commercial breakthrough of this technology. To point out the influence on the system performance of fuels that have a different origin, wood pellets, as the designated feedstock and hard coal as an example fossil fuel were fed into the DFB gasifier in different blends to generate operating figures for coal ratios from 0% to 100%.
Author T.M. KnowltonSource: South African Journal of Chemical Engineering 17, pp 25 –49 (2012)More Less
In most gas solids processes involving substantial solids circulation around one or two loops, the circulation rate around the loop is controlled by a valve - typically a slide valve. However, many new process such as chemical looping processes, operate at higher pressures and pressures than the previous processes, and solids flow control using a slide valve is either to expensive or not possible. This is because slide valve technology does not presently extend to the high temperatures and pressures used in many of the new processes.
Therefore, developers are turning to nonmechanical means to control the solids flow around the solids flow loops. This paper describes the principles involved in nonmechanical control of solids around a loop, and the advantages and disadvantages of the various types of nonmechanical solids flow control systems.
A novel method for NOx and Hg emission control in power plants using existing wet limestone scrubbersAuthor R. KrzyzynskaSource: South African Journal of Chemical Engineering 17, pp 50 –57 (2012)More Less
Research was performed in a simulated wet flue gas desulphurization scrubber to enhance NOx (NO and NO2) and Hg (Hg2+ and Hg0) removals. Research showed that is a possibility to achieve a multi-pollutant (SO2, NOx and Hg) removal capacity in a wet flue gas desulphurization scrubber fed by limestone and sodium chlorite solution. Sodium chlorite additive was effective in increasing of NOx and Hg capture. In this paper, investigation is focused mainly on NO2 capture in the scrubber, which is the most demanding pollutant in respect to control. It was showed, that changing conditions in the wet scrubber could change the process effectiveness. Research showed also how NOx (as NO2) absorption could be improved in a multi-pollutant scrubber without changing other pollutants (SO2, Hg) removals capacity.
Effect of fines content on bubble properties in a two-dimensional fluidized bed by digital image analysisSource: South African Journal of Chemical Engineering 17, pp 58 –66 (2012)More Less
The effect of fines content in fluid catalytic cracking (FCC) catalyst on bubble properties was investigated by considering two catalyst batches. One contained fewer fines and had a Sauter mean diameter of 83 Î¼m while the other had a higher fines content with a 74 µm mean diameter. The hydrodynamics were analyzed in a two-dimensional fluidized bed with superficial gas velocities in the bubbling regime. Photos of the bed were analysed by isolating bubble images, after which colour images were converted to binary images by using MATLAB® software. These binary images were used to determine properties such as equivalent bubble diameter, gas fraction, circularity and bubble hold-up in the bed as function of superficial gas velocity and fines content. The results showed that bubble diameter increases with gas velocity for both catalyst batches. A smaller bubble size was observed at all except the highest superficial velocities for the catalyst batch with the higher fines content. However, the high fines content catalyst had a higher gas fraction than the low fines content batch at all gas velocities. This average value was the same regardless of gas velocity, while an increase in superficial velocity resulted in an increase in gas fraction for the lower fines content catalyst. The bubble hold-up for the catalyst batch with the lower fines content was also higher than that of the high fines content catalyst. For both batches an increase in bubble hold-up with velocity was observed. Similar circularity values for both catalyst types were calculated at all superficial velocities.
Particle fluidization and reaction engineering activities at the Institute of Chemical Engineering, TU WienSource: South African Journal of Chemical Engineering 17, pp 67 –87 (2012)More Less
This paper gives an overview of the research activities and highlights on particle fluidization and reaction engineering at the Institute of Chemical Engineering at Vienna University of Technology. It is structured into steam gasification in dual fluidized bed, CO2 capture including oxy-fuel combustion, chemical looping combustion and reforming, synthetic biofuels including synthetic natural gas, Fischer-Tropsch synthesis, mixed alcohols and the production of hydrogen. The chapter is followed by the studies of agglomeration under fluidized bed combustion conditions and catalytic cracking of feeds from biological sources. The paper concludes with iron ore reduction kinetics.