4 replies to this topic

[ors1237]

Hello,

 

I have conducted a binary distillation of DCM and TCE using a distillation training rig, and have been asked to explain some relationships within my obtained data. I used a manometer to measure the pressure difference between the top and bottom of the column, and as the reboiler heating power, therefore boil up rate, increased, so did the pressure drop. I have a fairly vague understanding of why this is, due to increased vapour flow rate, however if anyone could provide a more detailed explanation for this i would be very grateful.

 

Thanks in advance,

 

ors1237

[Art Montemayor]

 ors1237:

 

It’s all very simple:

 

You have a fixed, distillation column that has fixed internals (trays, downcomers, weirs, etc..  all these internals represent a frictional or resistance force that acts against the liquid and vapor flows taking place inside your column.  The liquid flows are normally very small in comparison to the vapor flows and thus the resistance confronted by the vapor (measured in pressure drop) is very significant when the vapor flow rate is increased beyond its design value.  For example, when you have a constant liquid feed to the column and increase the heat input to the reboiler, this increases the vapor flow up the column against the same liquid flow that the column was designed for.  Therefore, the vapor flow generates more pressure drop due to more resistance resulting from excess vapor trying to ascend through a fixed path.  In other words, you are trying to “cram” more vapor through a resistance that was designed for less vapor flow.

 

[ors1237]

Thank you very much

[ors1237]

Further to my previous question, why does the number of theoretical plates, therefore the efficiency of the column, decrease as the heating power is increased? 

 

Thanks,

 

Ors1237

[katmar]

Referring to your first question - just to illustrate Art's answer I have attached a typical operating diagram (this one is from a packed column) which clearly shows Art's point that as the liquid flow changes there is a relatively small change in pressure drop, but large changes in pressure drop as the vapor flow changes.  The dot in the middle of the diagram is the design point.  The operating diagram for a trayed column will be similar.

 

Your second question is much more involved.  The answer depends on where you are in the operating range of the column. At low flow rates you will actually find the opposite effect, i.e. that as the flow increases the efficiency of the column improves.  This is because when the flows are low there is insufficient energy to ensure good mixing of the (separate) fluids, and the boundary layers will be relatively thick.  Increased heat input, and therefore increased flow and mixing improves the situation.

 

However, when you are above the normal operating point you are approaching flooding.  In this situation the rising gas physically carries (entrains) some of the liquid upwards, thus working against the expected separation of the components i.e. the liquid which is low in the volatile component moves up the column instead of downwards.

 

The usual behavior is to find the efficiency increasing with gas flow rate until a fairly broad plateau is reached, and then a rapid fall off in efficiency at very high rates as the column starts to flood.

 

You should read up on this sort of thing in a good book like Kister, Treybal, Perry or Coulson and Richardson.

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