10 replies to this topic

[ayche]

Can someone explain the concept of NTU.

When the height of the packed tower after calculations comes out to be some value. Now the type of  packing is changed and its seen that NTU this time is lower (the height of the tower and rest every condition remains the same as for the first type of packing). What if the effect on the absorption rate?.

 

Ayche

[breizh]

http://www.separatio...n/GA_Chp04c.htm

 

hope this helps

Breizh

[ayche]

thanks a lot sir… i read the entire page.. got the concepts ..

so that means if NTU for new packing is less … and since NTU is inversely proportional to driving force …. it measn rate of absorption should increase..

am i correct sir?.

 

Ayche

[siretb]

NO, I do not agree. 

The NTU is defined as an integral of the inverse of a driving force, over the separation required.  This means that for a given separation, under given operating conditions, the NTU is defined. At this stage the packing data is not considered

 

For a newpacking, the efficiency, the quality, of the packing may change, and the HTU (heigth of a transfer unit) will be different, usually smaller, than for older packings. This means that a given length of packing will achieve more, and that, overall, a smaller bed is needed. 

 

Of course, you may also decide to change the operating conditions, i e increasing the L/V ratio, this canges the distance between the operating line(s) to the equilibrium line. And thereby changes the NTU. But this is different. The design path is the same.  You define the need, calculate the NTU, and then the packing heigth.

[ayche]

Sir

I agree with all your points,

but where I saw this question it said inlet conditions and the height of the packing remains same for the new condition.

so NTU decreases , height of the packing and inlet conditions are same

what should be the absorption rate now?
 

Ayche

[siretb]

^{I do not understand your last question. Please re state your problem and what you do not understand. Thank you.   Bernard}

[ayche]

i will post the question as it is.. it came in one of the exams held in my country

the question is:

 

The packing of an existing absorption tower is replaced with a new type of packing. The height of the packing and the inlet conditions are maintained the same as before. Tests reveal that the number of transfer units is lower than before. This indicates that the tower with the new packing when compared to that with the old packing will

 

( A ) have a higher rate of absorption of the solute from the gas stream

( B ) have a lower rate of absorption of the solute from the gas stream

( C ) have the same rate of absorption of the solute from the gas stream

( D ) have a lower height of transfer unit

Edited by ayche, 14 January 2014 - 06:14 AM.

[PingPong]

Less NTU's means less mass (and heat) transfer. That should be rather obvious.

[ayche]

sir..wat i have read till now is that NTU is inversely proportional to driving force..

so if NTU is less that means mass transfer should be more..!

can you help me here

 

 

ayche

[PingPong]

You have to distinguish between required NTU's and available NTU's.

 

For example: suppose that absorber originally was required to have 6 NTU's, and the originally selected packing had an HTU = 1.0 meter, so the column was sized to have 6 * 1.0 = 6 meters of packing.

Now the original packing is replaced by a new packing (with higher hydraulic capacity, but) with an HTU = 1.2 meter. So the column now has only 6 / 1.2 = 5 NTU's

Less available NTU's means less absorption performance. That is rather obvious.

Edited by PingPong, 14 January 2014 - 02:50 PM.

[ayche]

Got the point ..understood ..!

Thanks a lot sir

Plus one more thing which I was neglecting was that the inlet conditions remain the same here(as mentioned in the question), so I can't apply the driving force logic…as it is same in both the cases..for both the packing

Thank You 

 

Ayche