Dynamic Measurement of Carbon Dioxide Volumetric Mass Transfer Coefficient in a Well-Mixed Reactor Using a pH Probe: Analysis of the Salt and Supersaturation Effects

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• 作者：M. Kordasrc="http ; //pubs.acs.org/images/entities/ccaron.gif" border="0"> ; V. Linek
• 刊名：Industrial & Engineering Chemistry Research
• 出版年：2008
• 出版时间：February 20, 2008
• 年：2008
• 卷：47
• 期：4
• 页码：1310 - 1317
• 全文大小：616K
• 年卷期：v.47,no.4(February 20, 2008)
• ISSN：1520-5045

文摘

This paper presents a new method for the evaluation of volumetric mass transfer coefficients (k_La) using apH probe response recorded during the absorption of carbon dioxide in a well-mixed reactor. For this methodof evaluation, it is not necessary to know the reaction equilibrium constant, the experiment start time, or theinitial and final steady-state pH probe readings. The experimental procedure is based on Hill's recent publication(Ind. Eng. Chem. Res. 2006, 45, 5796). Hill reported a decrease in k_La following the addition of salt duringcarbon dioxide absorption in a well-mixed reactor and explained it as the result of ionic charge effects reducingthe ability of carbon dioxide molecules to diffuse away from the surface. Generally, oxygen absorption fromair is preferred because the solubility of oxygen is low and, thus, so will be the gas-phase depletion. In suchexperiments, the mass transfer coefficients are less affected by gas-phase residence time distribution and bybubble-size distribution. The solubility of carbon dioxide is 26 times higher than that of oxygen, which canlead to significant gas concentration changes. Thus, the k_La values, measured using absorption of dilutedcarbon dioxide, are more likely to be distorted by driving force errors caused by the use of an inappropriategas-mixing model. Using a stirred cell, the interfacial area of which is known, the mass transfer coefficientsof oxygen and carbon dioxide in water and in a salt solution were compared. The mass transfer coefficientsobtained for oxygen agreed with, or were slightly superior to, the coefficients obtained for carbon dioxide,which corresponds with the lower molecular diffusivity of CO₂. The mass transfer coefficients of carbondioxide obtained in salt solution were not significantly lower than those obtained in pure water, which is instrong agreement with the literature but contradicts the results reported by Hill. It is shown that Hill's findingsmay be the result of his use of both an inaccurate gas-mixing model (no depletion of gas) and an imprecisereaction term in his equations.