Impact of decreasing overhead pressure & temperture at Vacuum Column 1

[gustoha]

Hi, everyone,

I'm asking about vacuum column, as study about energy saving in column.

Our vacuum column is operated at 75mmHgA and temperature is 80degC. We CAN decrease the top temperature by increasing cooler duty. So I think it's possible to decrease reboiler duty by decrease overhead pressure caused by decreasing overhead temperature.

Is there anything needed to be considered? I think capacity factor (Cf) is not changed seriously because both temperature and pressure is decreased.

I'd like to hear your guy's opinion.
Thanks.

 

[alisaffari]

hi, gustoha
At first, i apologize for my English language.
I think, you shall consider amount of reboiler duty decrease against cooler duty increase for energy saving comparison.
if you decrease duty of reboiler, vapor production in tower reboiler will be decrease and reflux rate will be decrease relatively. therefore, according to distillate and bottom product specification (composition and flowrate) you shall take in account reflux ratio.
in addition to reboiler and cooler duty you shall consider other parameter for product specification.

 

[RayA1004]

Hello Gustoha,
A few things come to mind here...
Assuming your overhead composition does not change, the only way I can see a lower top temperature is by lowering the head pressure - you'll have to be sure your vacuum system is capable of achieving this.
Your overall reboiler duty will be set (primarily) by the latent heat of your total boil-up; the sensible heat savings and lower heat losses from equipment are typically minimal compared to this (in my experience).
If you lower the system pressure, the vapor volume will increase (maybe dramatically as you are already at 75 mm Hg abs). You should check how close you are to flooding in the column.

good luck
Ray

 

[Montemayor]

My star goes to RayA1004 and my hat is off to him as well. He has hit the main point right on the head.

Chemical engineers are supposed to know that the pressure of a distillation column (always measured at the top head) is set by the VAPOR PRESSURE of the liquid in the top tray. In other words, as RayA1004 has correctly pointed out, the subject vacuum tower will not achieve the desired lower temperature if that temperature does not correspond to an achievable vacuum - which, in practical terms means that the capacity of the vacuum jet or pump being used has to achieve the vapor pressure (partial vacuum) related to the desired temperature. Otherwise, it ain't gonna work.

'Way to go Ray. You get an "A" in Unit Operations.

 

[Millerd1]

Your overhead pressure will likely be determined by the nearest bottleneck in your overhead vacuum system, or some combination of any side cooling you have. Typically the bottleneck is some combination of the cooling media temperature in the summer time, and the approach temperature of the lowest pressure condenser, but it could be the vacuum system pumping capacity itself. However, in order to get there your top most side cooler and probably any lower side coolers must increase their heat removal. Otherwise if the side cooling is not increased as your column pressure reduces heavier and heavier molecules will vaporize and boil off over head, adding their mass flow to the vacuum device and adding more condensing duty from the current overhead cooling needs. It is like either a condenser or vacuum device is already rate limiting your column performance, so you need to find a way to reduce that incoming vapor load so the pressure can go down further

If you increase the side cooling, any side draw products you are currently producing will become lighter in average composition. Especially the front end of their boiling curve.

Get a precision distillation of the feed stock to this column paying special attention as your labs is capable of doing to define the front end boiling point, the compounds in that initial boiling mass. Get a component definition of the first 15% if you can. Collect flow an composition data on the vacuum system exhaust gaseous and liquids leaving the condensers and compare that to what your lab results are.

Then simulate the full distillation curve to your corrent conditions, adjusting the condenser duty, and pressure to replicate what you see happening today. Note the vacuum device mass flow momentum and the condenser duites. Then figure out where the cooling has to be increased while those bottlenecks are held constant or reduced. Look for ways to increase side stream heat removal, this accomplishes two things it pulls light ends out before they tax the overhead vacuum system, reduces the overhead flow and condensing duty as well.

A word of warning, it is possible to get low enough in pressure that the system could start vaporizing compounds that must then condense and some will even solidify in the condensers. Paraffins are a particular problem for vacuum condensers. Once paraffin or high viscosity compounds coat the condenser cooling surface your cooling capacity is greatly reduced and the vacuum system could start surging due to the hotter, then colder cycles as the paraffin is melted off, then replaced anew.

 

[Millerd1]

One final point, if your compounds can tolerate a trace presence of water, of a light hydrocarbon stripping agent you can get a much improved bottoms product and upper streams yield by placing a very small amount of either compound in your feed or any bottoms stripper assembly your column might have.

This stripping effect in a vacuum column can be very dramatic, so use a very small amount at first. Remembering at all times whatever you put in extra, has to be leave the column ovehead via a limited cpacity overhead vacuum system. That extra duty and mass you add probably has to be taken out by side cooler(s).

 

[ajs1972]

With respect to your proposal (energy savings), I don´t think this change can saves energy in reboiler´s steam.

 

[25362]

There are cases in which the effect on a pre-condenser may be just the opposite.
If you are dealing with a multicomponent hydrocarbon system, a decrease in tower top temperature may actually increase your top pressure.
This can happen when the reduced amount of heavier liquid components in the overhead condensed stream would, in fact, lessen their "absorbing" capability increasing the load of lights going to the vacuum-generating equipment.