On Multi-layer Adsorption Theory and Chromatographic Model for Gas-solid system

• Main Authors: Cheng-Hsing Wang, 王振興
• Other Authors: Bing Joe Hwang
• Format: Others
• Language: zh-TW
• Published: 2000
• Online Access: http://ndltd.ncl.edu.tw/handle/71436530545816805392

博士 === 國立臺灣科技大學 === 化學工程系 === 88 === Abstract The traditional adsorption models can only provide some of the properties of the adsorption systems, such as the specific surface area and porosity of adsorbents, adsorption energy etc. The developed multi-layer adsorption theory can provide a deeper insight into the adsorption behavior and more characteristics of adsorption systems. This is essential for the improvement of absorbent''''s performance. The multi-layer adsorption theory is applied to the development of a complete gas-solid chromatographic model. The optimal operation conditions for the chromatographic system can be obtained from the simulation of this model. Furthermore, a novel method based on the proposed chromatographic model is suggested to obtain the characteristics of an adsorption system. A novel adsorption isotherm was derived from the combination of the multi-layer adsorption reactions and the heterogeneity of adsorbents in this study. Furthermore, a competitive adsorption model based on the criteria of the multi-layer adsorption was developed for the multi-component adsorption systems of alkanes. The unloading fraction, effective coverage, layer thickness, equilibrium constants and the adsorption characteristics were obtained from the fittings of the experimental data with the proposed model. Meanwhile, several kinds of adsorption systems involving the adsorbents, activated carbons and zeolites, as well as the adsorbates, (e.g., alkanes, alkene, hydrogen sulfide, carbon dioxide and carbon monoxide) were discussed. From the results, the adsorption or the desorption behaviors depend on both the properties of adsorbents and adsorbates. The traditional monolayer models are not adequate to describe their adsorption behavior, especially at higher pressures. The proposed multi-layer model is suitable not only for a multi-layer adsorption but also for a monolayer adsorption. The prediction results were well coincided with the experimental data over a wide range of temperatures for the binary alkane adsorptions. For alkanes, the primary competitive adsorption occurs in the first adsorption layer. The difference between the isosteric heats would influence the selectivity in the alkane competitive adsorption system. In addition, the behaviors of the active site replacement were also explored. The cross- adsorption behaviors occur in the competitive adsorption systems, which consist of polar adsorbates. The cross-adsorption behaviors for H2S were changeable due to steric hindrance in the competitive adsorption systems. A complete model involving fluid mechanics, mass transfer, and adsorption reactions was proposed for an isothermal gas chromatographic system. Introducing the proposed multi-layer adsorption model, a non-isothermal chromatographic model was developed. The chromatography behaviors consisting of three kinds of adsorption isotherms (the linear, Langmuir and multi-layer adsorption models) are compared. Based on the proposed model, the effects of equilibrium constants, packing porosity and flow rate on the chromatography behaviors were analyzed. An optimal chromatographic strategy was suggested for the temperature programming. From the simulation results, the shape of chromatographic peak for the multi-layer adsorption becomes more sharp but tailing than that for the other two isotherms. It showed the multi-layer adsorption is close to the real situation in a gas-solid chromatographic system. For isothermal chromatography, the simulation results showed that the peak high and the column efficiency increase with the packing porosity. For most of gas chromatography systems (Kp1 < 50), the column efficiency is consistent with van Deemter''''s law. However, the column efficiency decreases with flow rate at low flow rates if strong adsorption occurs (e.g. Kp1 > 300 and Kp2 = 1). This is opposite to van Deemter'''' law but consistent with Gaspar''''s theorem. The column efficiency for the isothermal chromatography cannot be improved by elevated temperature. Using the method of temperature programming, the reduction of retention time and peak width improve the column efficiency. A novel method, which is easy and time saving, was successfully developed for characterizing a multi-layer adsorption isotherm. Based on the established optimal strategies, the chromatographic behavior was fitted and the characteristic parameters of adsorption system were obtained for the system of cyclohexane-helium through ChromSorb 102. The good prediction results were used to identify the general chromatographic model. Since the column temperature is not the same as oven temperature and the temperature distribution in column is non-uniform, the real peak shape is border than the prediction one and the errors for the rear-half peak are larger than those for the front-half peak. The prediction error for peak shape decreases with temperature. However, the predicted retention time is consistent with experimental data. The adsorption equilibrium isotherm can be obtained by the proposed strategy. In addition, the simulation results show that the equilibrium constant ratio (Kp2/Kp1) is about 1 % for the system of cyclohexane adsorbed on ChromSorb 102.