In the design of a distillation column, it is necessary to determine the column diameter and the column height for economic considerations. When determining the column height, the required theoretical number of stages is generally determined from the results of vapor-liquid equilibrium calculations, but in order to determine the actual column height, it is necessary to determine the column efficiency and the stage efficiency. Efforts have been made to estimate this efficiency empirically or theoretically (semi- theoretically), but highly accurate correlations have not yet been established. Here, the main approaches in considering the efficiency of tray columns will be summarized and a brief comparison will be made with actually reported examples. (H. Taguchi)
When examining a distillation column using a general-purpose simulator, it is common practice to calculate the condenser and the reboiler as one with the distillation column. This is because the number of recycle loops is reduced, and improvement in convergence can be expected. There are roughly two types of classification for reboilers: the kettle type and the thermosiphon type. In the case of a thermosiphon type in which vapor and liquid return to the distillation column in a mixed phase state, the distillation separation effect is difficult to understand. In this study, we will create simple distillation column models to examine the distillation separation effect of the thermosiphon type and compare it with the kettle type. (H. Taguchi)
The required number of theoretical stages and the reflux ratio of a distillation column can be determined using the Fenske equation (Tips to Scale-up & Design #1009), the Underwood equation (Tips to Scale-up & Design #1011), the Gilliland correlation or the Brown-Martin correlation. In order to calculate the diameter of a continuous distillation column and to study energy savings, the vapor-liquid composition and temperature distributions in the column are required, but they cannot be obtained by calculation using the above equations. In this study, we will introduce a sequential step calculation method using the Thiele-Geddes method, which is one of the shortcut distillation calculation methods. (H. Taguchi)
In multicomponent distillation calculations, the approximate theoretical number of stages can be obtained when it is possible to find the minimum reflux ratio. Whereas for binary systems it can be easily obtained by a graphical method, elaborate calculations are required for multicomponent systems. Among calculation methods of the minimum reflux ratio worthy of mention are the Colburn method, the Gilliland method, the May method and the Scheibel method, but here it is the Underwood method which is the best known method that will be introduced. (H. Taguchi)
Distillation calculation is the unit operation that benefited the most of the development of computers. Until the 1970’s, sequential stage calculations by large computers (mainly engineering companies) and the McCabe-Thiele method (chemical companies) were mainstream, but now the use of PC based process simulators is widespread. Using a simulator makes it easy to calculate, but the conditions for starting case studies can be quite different depending on individual skills. Whereas experienced engineers may undertake efficient investigations close to the solution, it is also true that beginners do not know where to start from. In this sense, having guidelines for determining the number of stages is important for efficient examination. Here, we will introduce these guidelines for continuous distillation and batch distillation. (Y. Kumagae)
Previously we introduced how the state of a distillation tower can be considered from the thermodynamic minimum energy by using the column targeting method. This time, improvements to a distillation tower will be concretely considered by using the column targeting method. In addition, the optimal reflux ratio that minimizes the total of the equipment and operation costs of the distillation tower and peripheral equipment will be discussed. (H. Taguchi)
When performing a new distillation calculation, it is necessary to determine the theoretical number of stages (and/or the reflux ratio) after determining the operating pressure. A plotting method using the McCabe-Thiele method is also effective, but it is difficult to apply to multicomponent systems. On the other hand, there is a shortcut method that is often used as a guideline for multicomponent distillation calculation. In this shortcut method, the minimum number of theoretical stages and the minimum reflux ratio are obtained, and then the theoretical number of stages and the reflux ratio can be determined by using Gilliland’s correlation. For systems that are not strongly non-ideal, it is an effective method to estimate rough values. Here, we will introduce the Fenske equation for finding the minimum number of theoretical stages. (H. Taguchi)
Generally, it is known that, for distillation, separation tends to be enhanced by lower pressures. For two components, the ease of distillation separation can be judged from the value of the relative volatility (α). Here, we will take typical substances as examples and examine the effect of pressure changes on their relative volatilities. (H. Taguchi)
