Boiling point diagram for ideal plate is shown in the figure below. The concentration of fluid entering and leaving the nth plate is shown in in the figure. By definition of ideal plate, vapor and liquid leaving the plate n are in equilibrium. So Xn and Yn represents equilibrium concentration.
Since concentration of both phase increase with height of column, Xn-1 is greater than Xn and Y1 is greater than Yn+1. Although the streams leaving the plates are in equilibrium, those entering are not.
When the vapor from plate n+1 and liquid from plate n-1 are brought into intimate contact their concentration tends to move towards an equilibrium state as shown by arrows in above figure.
Here, liquid streams are at their dew point. Heat necessary to vaporize component A is sullied from heat released in condensation of component B. Thus each plate act as an interchange apparatus in which component A is transferred to vapor stream and component B to liquid stream.
Since concentration of A in both liquid and vapor increases with column height, temperature decreases. Temperature of plate n is greater than that of plate n-1 and less than that of plate n+1.
Equilibrium concentration doesn’t necessarily means that reactants and products are present in equal amount. It means that reaction has reached a point where concentration of reactant and product are unchanging with time because forward and backward reaction have same rate.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.