Crossflow plate in a distillation column

We have seen in Section 8.1.3 that multistage distillation is generally carried out in a vertical multiplate column in which each plate has crossflow. Vapor bubbles rise vertically through a liquid layer flowing horizontally in cross-flow over the plate (Section 8.1.3.5) the same two-phase flow scheme is employed in a plate for gas-liquid absorption stripping. We were introduced to the notion of stage efficiency, plate efficiency, tray efficiency, etc., in Section 8.I.3.4. We will introduce here the models used to 

An additional point is important vis-k-vis relating E v to the mass-transfer process between the vapor and the liquid phases. It is carried out via a point efficiency Eog at any location on the plate  

This quantity illustrates the local vapor-phase composition change achieved between the vapor coming in from the bottom plate (ra -i-l) and the vapor going out from plate n at that location, with respect to the change that would have been achieved if the vapor phase leaving plate n at that location were in equilibrium with the local liquid-phase composition x,-2 ioc (i-e. x i i ,. =/e,( anioc))- We will see later how Egc is related to the local mass-transfer process. 

We will now write down the local species i balance equations for phase 1 flowing up in the y-direction and for phase 2 flowing axially in the z-direction on plate n in the manner of equation (6.2.33) however, for the convective transport term, we will have to use the particular coordinate component of the V [ vk) Ca) term from equation (6.2.30) since the vk) values are changing in both the y- and z-directions due to mass transport. 

Here we have neglected any dispersion/diffusion in the X- and z-directions for phase 1 in upflow. 

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Figure 8.3.5. Crossflow plate in a distillation column (a) gross flow pattern in a cross-sectional view (b) gross flow pattern in a plan view.

On a large plate, there are two stagnant regions (shown in Figure 8.3.5(b)) where there is no liquid replenishment. Ideally therefore, the crossflow plate in a distillation column may be analyzed via the simultaneous solution of equations (8.3.18) and (8.3.20) coupled via appropriate boundary conditions (Porter et at, 1972). 

There are a number of phase equilibrium driven separation processes where the separation devices are such that crossflow of two bulk phases exists. Crossflow is utilized to enable continuous contacting between two immiscible phases, vapor and liquid, in an efficient fashion, as in a plate located in a distillation column. In chromatographic processes, crossflow of the solid adsorbent particles and the mobile fluid phase (liquid or gas) can lead to continuous separation of a multicomponent feed mixture introduced at one location of the mobile fluid (eluent) phase. We will illustrate first how crossflow of adsorbent particles or the adsorbent bed and the mobile fluid phase overcomes the batch nature of multicomponent separation in elution chromatography. Then we will focus on the cross-flow plate in a distillation column. 

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. 

Fig. 15. Flow pattern in a crossflow plate distillation column.

Perforated plates such as sieve trays used in absorption, distillation or extraction columns. The holes can be covered by caps or valves to avoid weeping in the range of low superficial gas or vapor velocities. The two phases are moving in a crossflow on a tray. 

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