I would like to understand Distillation Column Top Pressure and Pressure in Reflux drum for total condenser. (Debutanizer Column)
I have designed many distillation columns in my life, so I know how it's done.
Every distillation column has a pressure controller (PC) that maintains the column pressure at its set point. A debutanizer has the PC usually installed on the reflux drum.
In case of total condensation the PC controls the cooling medium used in the condenser, or a hot vapour bypass around the condenser.
The designer of the debutanizer has determined that setpoint, following a few steps:
(1) Take a look at the design feedstock composition.
(2) The distillate of a debutanizer will consist of the C4 and lighter components of that feedstock.
(3) Take a look at the available utilities to be used as a cooling medium in the condenser: depending on the rest of the plant these can be ambient air, cooling water, chilled water, refrigerant, ...........
(4) We will assume here that the designer selects cooling water, available at a temperature of say 30 oC.
(5) Assuming the cooling water warms up from 30 to 40 oC in the condenser, the condenser outlet temperature can be 40 - 45 oC, depending on the exact composition (especially the amount of C3 in it) of the distillate. Let's assume 45 oC.
(6) Now the designer does a Bubble Point Pressure calculation of the distillate composition of step 2. That gives the required pressure in the reflux drum, and therefore the setpoint of the PC.
(7) The debutanizer column top pressure will be say 0.3 bar higher than the reflux drum pressure due to frictional pressure loss over condenser and associated piping.
(8) The debutanizer column top temperature can then be found by a Dew Point Temperature calculation of the distillate composition (from step 2), as in case of total condensation (which you mentioned) the distillate product, the reflux and the column vapour overhead all have the same composition.
(9) The bottoms product of the debutanizer will consist of the C5 and heavier components in the design feedstock.
(10) The debutanizer bottoms temperature, which is the reboiler outlet temperature, is then determined by a Bubble Point Temperature calculation of the bottoms product from step 9 at a pressure that is say 0.3 bar (to account for pressure drop over the trays in the column) higher than the column top pressure.
(11) Now the designer takes a look at the available utilities for use as a heating medium in the reboiler: steam, hot oil, a pump-around from one of the other distillation columns in the plant, ...............
(12) Obviously the selected heating medium has to be able to provide a temperature high enough to achieve the required reboiler outlet temperature determined in step 10. If none of the available utilities meets that requirement, a fired heater would be required as a reboiler.
(13) Usually nobody likes to have a fired reboiler, so in that case the process to determine the column pressure has to be reversed by starting with selecting a lower bottoms pressure that results in a lower bottoms temperature that can be achieved by the preferred heating medium.
(14) In that case also the reflux drum pressure will have to be lowered, resulting in a lower condenser outlet temperature, and then the selection of the cooling medium in the condenser has to be redone.
(15) Once all pressures have been set by the above steps, the designer simulates the column using a process simulator like Aspen Plus or Hysys or PRO/II or ..........
You could use freeware like DWsim.
Note that all the above is the procedure for total condensation of the distillate. In case of partial condensation (mixed distillate), or in case of a vapour distillate the procedure is somewhat different.
Answer to Q1, Q2, Q3 & Q4: the reflux drum and column top pressures are simply as I described above in step 7, no need to worry about equilibria or condensation or whatever inside the column.
Edited by PingPong, 29 June 2025 - 12:26 PM.
Forums
Blogs
Articles
File Library
FAQ
Search
FB
3
