2 replies to this topic

[dilova2011]

Hi all,

In my final project, specifically in the equipment design, I have a system with three distillation columns.  How I can justify and select the condenser ?  air cooler? water cooled condenser?

 

 

Considering that this is a PILOT PLANT, flows and energy requirements are not large.

 

 

Condensation data: (all columns condense organic vapor mixture and atmospheric pressures) 

 

FIRST COLUMN (height 3 m, diameter 0.25 m): 

Duty of condenser : 60 kW  Temperature: 88ºC

 

SECOND COLUMN: (height 3 m, diameter 0.25 m): 

Duty of condenser : 33 kW  Temperature: 80ºC

 

THIRD COLUMN: (height 2 m, diameter 0.15 m): 

Duty of condenser : 12 kW  Temperature: 58ºC

 

 

 I suppose that the first two would use an air cooler, (this saves water costs) and the third column with cold water.

 

But I do not know if specifically in DISTILLATION COLUMNS OF PILOT PLANTS using air cooler is limited (large area respect to the column) because I haven't seen any pilot distillation column with air cooler, most with cooled water.

 

 

I would appreciate a justified response to the selection of the equipment

 

 

Updated: Also I need to justify for my pilot plant, what is the height above ground of condensers and reflux drums.

 

 

On one hand, the cost of elevating these equipments is not excesed (low weight, a lot of support or structure available9

 

Furthermore, the cost of pumping flows from the ground to the head are not high because they are not very high towers,

 

So I do not know what the two options is more viable

Edited by dilova2011, 13 June 2013 - 01:20 PM.

[GimliXJ]

Air coolers are typically chosen for process temperatures significantly higher than ambient for two main reasons: 1) High temperature difference in a water cooler can cause vaporization in your condenser and possibly vapor lock your process.  2) The U of an air cooler is much less than that of a water cooler.  Therefore, for the same duty, the air cooler will be a larger area and possibly a significantly higher cost.  Check your sizing and pricing.  The air coolers may cost more such that the cooling water condensers are the favorable option.

 

The height of your reflux drum and condenser are going to determine your pump sizing.  The closer the reflux drum is to the ground the less NPSHa and therefore the pumps will cost more (this is also dependant on your pump flow rates).  If the condenser is at the same elevation as the reflux drum, you may sub-cool your reflux if the condensed liquid can come into contact with the exchanger tubes.  Most of the time, the condenser is at a higher elevation than the reflux drum so that the liquid can free flow into the drum without sub-cooling.

 

I hope this helps.  Good Luck.

[Art Montemayor]

 

You are considering some important points in trying to arrive at a valued decision, but you are confusing the issue by not sticking to the basics of any type of engineering design problem:  identify the scope of what you are asked to furnish and stick to it.  This is something I see in a lot of young graduate engineers I have had working under me.  For example:

• You know the unit is going to be used as a Pilot Plant.  That should alert you to the fact that consistent and accurate data is going to be required from these units  - and with a minimum of having to make process adjustments and variations.  That means you must supply a unit that can not only do this in the summer time, but also in the winter time.
• You also know that a varying of the reflux ratios is going to be a big factor as is the varying of the feed rates and capacities.  That is what Pilot Plants are used for – to find out process information at different levels of operation.  But the information must be credible and dependable.  In other words, the plant must operate at constant and steady conditions with a minimum of upsets or variances.  And you can’t be subject to atmospheric conditions affecting your distillation runs.  That, I believe, makes air cooling a questionable selection.
• You give us a duty and a temperature for each condenser.  This is bad reporting, bad communication, and a lack of specifics.  Engineers can work like this.  You must identify what your temperature is: the temperature of the overhead vapor?  The temperature of the reflux?  Or the temperature of the cooling medium?
• However, if your given temperatures are those of the saturated reflux, then you are going to have problems controlling them with ordinary water cooling.  Remember, you should not allow your exiting water temperature from a condenser or cooler go over 120 ^oF (49 ^oC).  Therefore, you are going to have to circulate a large flow of cooling water and instrument to control the reflux temperature and the condensation.  The condenser will have to be carefully designed to allow rapid condensation and liquid separation.  (you don't state if there will be total condensation or partial; I've assumed the reflux drum will handle saturated liquid in all cases)
• If you have structural steel available to support the watercooled condenser and reflux drum, than do that.  Normally, this is a very expensive option.  However, it has its advantages in ensuring a good, positive NPSH for the reflux pump that can be installed and operated at grade.  This makes for good access for the operators in varying reflux ratios.

 I hope these recommendations help out.