Distillation column, real plates

Unfortunately, the fractionating column you usually get is not a bubble-plate type. You have an open tube that you fill with column packing (see Class 3 Fractional Distillation ) and noplates. The distillations up this type of column are not discreet, and the question of where one plate begins and another ends is meaningless. Yet, if you use this type of column, you do get a better separation than if you used no column at all. It s as if you had a column with some bubble-plates. And if your distilling column separates a mixture as well as a bubble-plate column with two real plates, you must have a column with two theoretical plates. 

So far, discussing distillation trajectories and their bundles, we proceeded from the fact, that separation stages are equilibrium ( theoretical plates). In real separation process at plates of distillation columns equilibrium is not achieved and the degree of nonequilibrium is different for different components. That leads to decrease of difference between compositions at neighboring plates and to change of curvature of distillation trajectories (Castillo Towler, 1998), but does not influence the location of stationary points of distillation trajectory bundles because in the vicinity of stationary points equilibrium and nonequilibrium trajectories behave equally. Therefore, implemented above analysis of the structure and of evolution of section trajectory bundles is also valid for nonequilibrium trajectory bundles. 

Data for packed columns cannot be compared with those of real plate columns except in absolute units of milliliters per theoretical plate (see Table 1). Dynamic hold-up increases with increasing distillation rate up to the flood point, and varies from one kind of fractionator to another. 

DISTRIBUTED AND UNDISTRIBUTED COMPONENTS. A distributed component is found in both the distillate and bottoms products, whereas an undistributed component is found in only one product. The light key and heavy key are always distributed, as are any components having volatilities between those two keys. Components more volatile than the light key are almost completely recovered in the distillate, and those less volatile than the heavy key are found almost completely in the bottoms. Whether such components are called distributed or undistributed depends on the interpretation of the definition. For a real column with a finite number of plates, all components are theoretically present in both products, though perhaps some are at concentrations below the detectable limit. If the mole fraction of a heavy nonkey component in the distillate is 10 or less, the component may be considered undistributed from a practical standpoint. However, in order to start a plate-by-plate calculation to get the number of plates for the column, this small but finite value needs to be estimated. 

This chapter is the central one of the book all previous chapters being introductory ones to it, and all posterior chapters arising from this one. Distillation process in inhnite column at finite refiux is the most similar to the real process in finite columns. The difference in results of finite and infinite column distillation can be made as small as one wants by increasing the number of plates. Therefore, the main practical questions of distillation unit creation are those of separation flowsheet synthesis and of optimal design parameters determination (i.e., the questions of conceptual design) that can be solved only on the basis of theory of distillation in infinite columns at finite reflux. 

---