Counterflow Columns

Heat and mass transfer in counterflow phases usually only takes place within the area of the trays. 

The theory of separation stages is also valid for packed columns. Since single stages cannot be distinguished, an evaluation number n, or its reciprocal value HETP or HETS is introduced 

Graphical methods for the determination of the required number of theoretical stages discussed so far, are easy to use and are of practical importance for first approximations for separation processes of binary mixtures, or for systems with inert carrier mixtures and a single component to be transferred. If several components are transferred between the phases in contact, the number of separation stages can only be calculated with the aid of a computer graphical methods can only be partially applied. 

Methods to calculate counterflow columns exactly are based on a column model. The column model gives a system of nonlinear equations, the task of each method is to solve this system. The equation system contains an overall mass balance and also a balance for each component, enthalpy balances, equilibrium relationships and the stoichiometric conditions for the sum of the concentrations for the theoretical stages of a counterflow column (see Fig. 1-60). 

L Flow rate of phase I (downflowing phase) SL Flow rate of side stream phase I 

---

Extraction involves the transfer of components between two liquid phases, much as absorption or stripping involves the transfer of components from liquid to vapor phase or vice versa. As in vapor-liquid multistage separation processes, the device employed to carry out liquid-liquid extraction is usually a counterflow column that performs the function of a number of equilibrium stages interconnected in counterflow configuration. In each stage, two inlet liquid streams mix, reach equilibrium, and separate into two outlet liquid streams. As in vapor-liquid columns, the lack of complete equilibrium in liquid-liquid extractors is accounted for by some form of tray efficiency. Liquid-liquid extraction may also be carried out in a cascade of mixing vessels connected in series in counterflow. 

For packed columns, which are classified as counterflow columns (most trays are crossflow devices), mass transfer is related to transfer tassa in connterflow vapor-liquid cou(acting Hie most-used criterion of efficiency is the height equivalent to a theoretical plate (HETP) thet was first introduced by Peters 1... 

If phase I is generated by phase transition or phase reversal from phase II in a heat exchanger on top of a counterflow column, a reflux principle is used instead of the simple flow-through principle (see Fig. 1-51). Phase I is no longer a fresh feed to the column but is the reflux generated by converting phase II into phase I this is the case in countercurrent distillation or rectification. 

A discussion of a method of determining the number of theoretical separation stages required to separate a mixture in a counterflow column follows. 

A graphical method to determine the theoretical stages of counterflow columns which is easy to use was introduced by McCabe and TIiiele [1.98]. 

Since 1960, several methods for the exact calculation of a counterflow column have... 

The following approximations are made for the calculation of a counterflow column ... 

In absorption and extraction processes, the assumption of phase equilibrium of the phases leaving the stage is less valid than in rectification processes. The exact calculation for a counterflow column using the theory of separation stages is generally carried out for rectification processes. The following discussion of a system of nonlinear equations, based on the column model, is particularly valid for rectification, but this may be applied to all other counterflow processes. Before the equation system can be formulated the variables used to describe the state of the counterflow system must be expressed. According to the column model in Fig. 1-60 they are... 

The solution of the equation system gives the profile of the molar fluxes, the concentration profile and the temperature profile of a counterflow column. Finally the dimensions of the counterflow column need to be determined and the column must be integrated into the separation plant. 

Carrier distillation using water vapor as the carrier is common in the oil and fat industry. In the oil and petrochemical industries, carrier distillation is used to strip loaded solvent from absorption units, using water vapor or an inert carrier in continuous counterflow columns. 

The method of Ponchon and Savarit [2.61] is a graphical determination of the number of theoretical separation stages in counterflow columns. It is particularly applicable to binary mixtures in rectification processes. Exact results are obtained when the enthalpy-concentration diagram of Pon-... 

The following discussion of the pressure drop in counterflow columns which is applied to rectification columns is also valid for absorption and desorption columns. 

An important process parameter is the pressure drop of a gas flowing through a counterflow column. Therefore, under steady-state conditions the pressure at the column head and the pressure at the column bottom P), are fixed... 

Internals for counterflow columns in distillation and absorption are currently being developed. The use of this type of column on an industrial scale is not yet known. 

---