Unified separation science.docVIP

均是精品，欢迎下载学习！！！ 欢迎下载百度文库资源 资料均是本人搜集 均是精品，欢迎下载学习！！！ Unified Separation Science - J Calvin Giddings Chapter 2 Equilibrium the driving force for separative displacement All isolated systems move, rapidly or slowly, by one path or another, towards equilibrium. In fact essentially all motion stems from the universal drift of eventual equilibrium. Therefore, if we wish to obtain a certain displacement of a component through some medium, we must generally establish equilibrium conditions that favor the desired displacement. Clearly, knowledge of that equilibrium state is indispensable to the study of the displacements leading to separation. In many separation processes (chromatography, countercurrent distribution, field-flow fractionation, extraction, etc.), the transport of components, in one dimension at least, occurs almost to the point of reaching equilibrium. The equilibrium concentrations often constitute a good approximation to the actual distribution of components bound within such systems. Equilibrium concepts are especially crucial in these cases in predicting separation behavior and efficacy. 2.1 MECHANICAL VERSUS MOLECUALR EQUILIBRIUM We can identify two important classes of equilibria: Mechanical – defines the resting place of macroscopic bodies. Molecular – defines the spatial distribution of molecules and colloids at equilibrium. Of the two, (a) is more simple. With macroscopic bodies, it is unnecessary to worry about thermal (Brownian) motion, which greatly complicates equilibrium in molecular systems. This is equivalent to stating that entropy is unimportant. This is not to say that entropy terms are diminished for large bodies, but only that energy changes for displacements in macroscopic systems are enormous compared to those for molecules, and the swollen energy terms completely dominate the small entropy terms, which do not inherently depend on particle size. Without entropy consideration, equilibrium along any given coordinate x is found very simp