Electrostatic charging is a widely known natural phenomenon that has been observed sinceancient times. This phenomenon is also reported in solids handling and processing industrieswith varying extent of its interference in established processes and operations. Particlecollisions lead to electrostatic charge generation through triboelectrification. Sustainedcharging leads to particle agglomeration and adhesion, or even explosions. Hence, the presenceof charged particles are seen as a hindrance and a risk in certain processes. The focus of thisthesis is directed at gas-solid fluidization and pneumatic conveying where this holds true.Polyethylene is commonly manufactured in catalytic gas-solid fluidized bed reactors. Theinsulative nature of the polymer particles, the catalyst particles and the surrounding gas set upa conducive environment for electrostatic charge generation. Charged particles adhere on tonearby surfaces forming fused masses of polymer sheets. Presence of sheets in the reactorhinders the reactor productivity, thus warranting reactor shutdown and maintenance. On theother hand, catalyst introduction into the polyethylene reactor is performed through pneumaticconveying systems. In general, solids pneumatic conveying is known to cause the largestdegree of triboelectrification among many gas-solid systems. Therefore, the charging ofcatalyst particles may also contribute to operational challenges faced by this industry.Numerous studies have attributed particle characteristics, system variables, and operatingparameters as probable sources contributing to electrostatic charging in both fluidizationprocess and pneumatic conveying systems. However, a comprehensive consensus explainingparticle charging in real-world scenarios and suitable methods to mitigate or prevent chargingstill require further investigation. Thus, desirable control of charging in affected industrialsectors is still not present.Beyond the scope of fluidization and pneumatic conveying, certain studies have investigatedthe influence of dielectric strength of gases on the charging behaviour of solids. The worksclaim that gases with low dielectric strength perform better in minimizing electrostatic chargeof solids in controlled environments due to gas discharge and subsequent charge dissipation.Thus, applicability of such gases in dynamic processes like fluidization and pneumaticconveying must be investigated in hopes of reproducing similar observations. The principalaim of this thesis was to uncover a functional method to limit charging and particle adhesionin fluidization and other solids handling systems. As a means of accomplishing this, theobjective of this thesis was directed to study the efficacy of argon, which has a low dielectricstrength, against nitrogen in reducing triboelectrification of polyethylene particles influidization and pneumatic conveying operations.A stainless-steel fluidization apparatus was used to study the charging behaviour of acommercially produced polyethylene resin at atmospheric pressure. Results were drawn forpure argon and compared against pure nitrogen. Aiming to minimize the quantity of argonwhile simultaneously retaining as much efficacy as possible, binary mixtures of nitrogen andargon were also tested along with successive fluidization trials. Pure argon resulted in 90%reduction in fouling compared to pure nitrogen. Even binary mixture of 10 vol % argon showeda reduction of 50% in fouling values. Successive fluidization resulted in fouling valuescomparable to pure argon trials. Multiple pulse pneumatic conveying was carried out in astainless-steel tube with dehydrated amorphous silica that is a commonly used catalyst base inpolyethylene process. The net specific charge of the particles and the fouling inside the tubewere smaller under argon in the first injections. Subsequent injections were not as significant.Results from the above operations were validated through bench-scale shake tests performedunder controlled gaseous environment. Single large polyethylene particle charging was firsttested in nitrogen and argon atmosphere followed by multiple smaller particles. Bench-scaleshake tests showed argon influenced the saturation charges, reducing it and reaching it earliercompared to nitrogen. However, the degree of charge and fouling reduction was not assignificant as observed in fluidization trials.The thesis concludes that argon is indeed influential in reducing particle charge and particleadhesion in applicable systems. Influence of argon was observed in all operations withfluidization exhibiting the greatest degree of reduction in charging and fouling values.Furthermore, even small quantities of argon can make a non-linear impact on said parameters.The results also suggest that the majority of gas discharge and subsequent charge dissipationoccurs in areas of considerable electric fields. These are observed to entertain large number ofparticles contact and separation, providing plenty of opportunities for gas molecules to ionize.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/45100 |
| Date | 27 June 2023 |
| Creators | Sridhar, Nikhil |
| Contributors | Mehrani, Poupak |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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