Multicomponent distillation introduction

The next task is to set up the equations that model a complete distillation column. As noted in the introduction to this chapter, simulation of multicomponent distillation operations usually is carried out using the equilibrium stage model introduced below. 

Understand the importance of the introduction of computers as tools for rigorous analysis and design of multicomponent distillation equipment. 

The previous chapters served as an introduction to multicomponent distillation. Matrix methods are efficient, but they still require a fair amount of time even on a fast conputer. In addition, they are simulation methods and require a known number of stages and a specified feed plate location. Fairly rapid approximate methods are required for preliminary economic estimates, for recycle calculations where the distillation is only a small portion of the entire system, for calculations for control systems, and as a first estimate for more detailed simulation calculations. 

To perform multicomponent distillation calculations requires the introduction of the concept of key components. These are the components between which the split between components is required and must be present in appreciable amounts in the feed, and effectively all of one will appear in the distillate (light key) and all of the other in the bottoms (heavy key). More correctiy, the light key is the lightest component in the bottoms whose composition is to be specified. Some simplified calculations assume that all LLK (lighter than light key) components are absent from the bottoms and all HHK (heavier than heavy key) from the distillate. The key components effectively imlockthe problem. 

Shortly after the introduction of the bismuth molybdate catalysts, SOHIO developed and commercialized an even more selective catalyst, the uranium antimonate system (4). At about the same time, Distillers Company, Ltd. developed an oxidation catalyst which was a combination of tin and antimony oxides (5). These earlier catalyst systems have essentially been replaced on a commercial scale by multicomponent catalysts which were introduced in 1970 by SOHIO. As their name implies, these catalysts contain a number of elements, the most commonly reported being nickel, cobalt, iron, bismuth, molybdenum, potassium, manganese, and silica (6-8). 

The Ki, values for each species i and the enthalpies used in the energy balance equations for any stage ra are obtained from conventional approaches used in multistage distillation analysis. However, the species flux is expressed in terms of the sum of a convective component and a diffusive component. The diffusive component is modeled using the Maxwell-Stefan approach (Section 3.1.5.1) for this complex multicomponent system in a matrix framework. For an illustrative introduction, see Sender and Henley (1998). 

A major feature of the three techniques of continuous chromatography in the crossflow configuration is that the feed stream is introduced only over a small section of the flow cross section of the carrier fluid. Had the feed stream been introduced throughout the flow cross section in the carrier fluid flow direction, multicomponent separation would not have heen possible feed introduction mode is important. We see in Section 8.3.2 that introduction of the feed fluid across the whole flow cross section for this fluid in a crossflow system is a common feature of separation in a plate in a distillation column which can produce only a hinary separation. 

The basic assumption of the Fenske-Underwood relation is that the ratio of the equilibrium constants or the relative volatility, as defined by Eq. (6.19), in a binary mixture or the two key components present in a multicomponent mixture remain constant over the temperatures encountered in the distillation column. If this can be assumed without the introduction of excessive error, the minimum number of plates at total reflux can be determined from... 

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