The plate where feed is administered is called feed plate. In feed plate, the liquid rate or vapor rate or both may change depending upon thermal condition of feed.
Cold feed: the entire feed stream adds to the liquid flowing down the column. In addition, some vapor condenses to heat the feed to boiling point. This makes the liquid flow even greater in stripping section and decrease flow of vapor in rectifying section. V < V̅ and L̅ > L +F.
Bubble point: no condensation is required to heat the feed. Therefore, entire feed becomes part of liquid stream (L̅). V = V̅ and L̅ =F + L.
Partly vapor: liquid portion of feed becomes part of L̅ and vapor portion becomes part of L̅ and vapor portion becomes part of V. Here, L̅ = L + Lf and V = V̅ + Vf 
Saturated vapor: feed at dew point, entire feed becomes part of V. Therefore, L = L̅ and V = F + V̅.
Superheated vapor: at solely vapor state of feed, part of liquid from rectifying column is vaporized to cool the feed to state of saturated vapor. V > V̅ +F and L̅ < L
Then vapor in rectifying section consists of- Vapor from stripping section
- Feed
- Extra moles of liquid vaporized in cooling the feed
Liquid flow to stripping section is less than that in rectifying section by amount of additional vapor formed.
All five types of feed can be characterized by the use of single factor denoted by ‘q’. it is defined as moles of liquid flow in stripping section that result from the introduction of each mole of feed. Then ‘q’ has following numerical limits for various conditions.
- Cold feed, q > 1
- Feed at bubble point (saturated liquid) q = 1
- Feed partly vapor 0 < q < 1
- Feed at dew point (saturated vapor), q = 0
- Feed at superheated vapor q < 0
About Author
Name : Pratiksha Shrestha
pratiksha.shrestha2001@gmail.com
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.