Study On Computational Mass Transfer In Chemical Engineering

A brief review of Computational Fluid Dynamic (CFD) and Computational Heat transfer (CHT) is firstly presented, including their development, the closure of transport equations and their applications in chemical engineering. An introduction of the computational Mass transfer (CMT) is subsequently given, in which the closing model for the differential mass transfer, i.e. the equations of concentration variance (c~2|-) and concentration variance dissipation ε_c, are precisely derived and simplified aswell as the model constants are ascertained.Based on the analysis of the momentum source terms and the research results on round jet, a distributing source term is proposed, involving the effect of momentum transfer and the sieve holes arrangement on the distillation tray. To testify its validity, the distributing source term and the uniform source term are both used in the momentum equation and the CMT model is applied to a 1.2-m-dia. sieve tray. The computed results are compared with the experimental data taken from the literatures. The comparison reveals that the results of applying distributing source term are in reasonable agreement with the experimental data, and gives predictions near the center line of the tray and the wall better than that by using uniform sources term. In addition, the velocity fields on the trays with different sieve arrangement are simulated. The results reveal again the advantage of using the distributing source term.The proposed CMT model is applied to two distillation columns, whose diameters are 1.2m and 0.429m respectively. The computed results of the 1.2-m-dia. tray, including the outlet concentration, the Murphree tray efficiency, the overall column efficiency and the turbulent mass transfer diffusivity under different operating conditions, are compared with the experimental data taken from the literatures. The computed results are reasonably agree with the exerimental. The predicted bottom concentrations and overall tray efficiencies under different F-factors of the 0.429-m-dia. column are confirmed reasonably with the experimental data in the literatures. The validity of the proposed model is demonstrated by the successful simulation of different scale distillation columns. In addition, the concentration fields of different trays with different sieve, hole arrangement, inlet and outlet weirs are simulated using the proposed model, in order to examine the effect of tray structures on the concentration fields. Finally, the effect of tray structure on the concentration field can be predicted by the proposed CMT model so as to avoid the use of experimental investigation.The concentration profiles of dissolved Oxygen were experimentally measured when the water was saturated with pure Oxygen flowed through a sieve tray. The measured results showed that the concentration eddy existed in the sector where the circulating flow was formed. With the proposed model, the distribution of concentration was simulated and compared with the experimental data. Reasonable agreement was found between them. By using Laser Doppler Anemometry (LDA), the local liquid velocity profiles of the single-phase flow and two-phase crossing flow were measured on a sieve tray simulator. The effect of gas on liquid under crossing flow was studied. The experimental study in this work is helpful for the verification of the proposed CMT model.