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Numerical Evaluation of Heat and Mass Transfer in Falling Film Distillation of Air

Ekrani, Mahdi | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 57357 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Afshin, Hossein
  7. Abstract:
  8. Cryogenic distillation of air is the only method of large-scale production of high-purity oxygen and nitrogen. The falling film column, due to its lower pressure drop and higher heat transfer, is gaining attention. In this method, the heat and mass transfer take place simultaneously on a vertical wall. The present study aims at providing a numerical framework based on a finite volume approach to simulate the falling film distillation column, focusing on the high-pressure column of an air separation unit. Due to the heat integration, filmwise condensation is expected to happen on the surface of the column. An innovative sub-model is developed to evaluate condensation heat transfer coefficient. The validation of falling film flow in the wavy-laminar regime is done for the velocity profile, the equilibrium concentrations, the Nusselt number, and the Sherwood number. The effect of heat transfer coefficient, column length, and feed ratio on product purities and heat integration is investigated in a Taguchi analysis. Then, a comparative study is carried out between falling film and packed column for air separation. Finally, some case studies are provided to assess the enhancement role of working pressure adjustment, insertion of gas-phase turbulator, and using a wavy wall. The challenge of interface smearing and its effect on heat and mass transfer calculations is also explored. The results of this study show that column length and feed ratio are the most sensitive design variables. Heat integration at its best can boost nitrogen purity by 9 percent. With the aid of the two-film theory, the mass transfer resistance of the gas phase is compared with that of the liquid phase. To reduce the gas-phase resistance by 60% turbulators are added in the gas-phase, which results in a 10% improvement of the product's purity and a 46% reduction of HETP. Using wavy walls also augmented heat transfer and product purity by 49% and 10%, respectively. With this enhancement in heat and mass transfer, HETP reduces by 56%, which results in a shorter column. Investigation of the interface smearing problem shows that the results without interface capturing might have an error as high as 10-17% for heat and mass transfer estimation
  9. Keywords:
  10. Distillation ; Falling Film ; Multiphase Flow ; Numerical Simulation ; Cryogenic ; Film Condensation ; Heat Integration

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