12 replies to this topic

[chemks2012]

Dear all

We have an existing packed column with diameter of about 970mm and two packed beds each 6m high. The packing used is 2” metal pall rings. It is debottlenecking project i.e. client is operating at about 16.5T/hr feed rate i.e. 15T/hr waste water containing ammonia + 10% caustic solution. The feed flowrate of 16.5T/hr contains 15% by wt ammonia. Caustic is added to maintain the pH of 11.5. Ammonia is removed by steam supplied at 1.4barg pressure at 3200kg/hr giving ammonia top composition as 90% by wt to maintain the bottom concentration as 350ppm [by wt]. Client has tried increasing the feed rate from 15 T/hr to 18T/hr but column behaves badly, ammonia concentration in the bottoms increases, column temp increases etc.

I have looked at the original design datasheet [prepared back in 1993] of this column and it seems it was designed for feed rate of 18T/hr, steam supply at 1.4barg & flowrate of 4500kg/hr etc.

I have performed mass balance and now I know the top and bottom mass flowrates for water and NH3.

My queries are:
1) Before, I do anything else, I want to check whether the column height is sufficient for the feed rate of 15T/hr and 18T/hr. The original datasheet [prepared for the feed rate of 18T/hr] has 9 NTU and HTU as 1.26mm [I think this unit should be in meter i.e. 1.2m?] and as mentioned above 2off beds, each 6m high i.e. total height of column is 12m. I understand that HETP = NOG X HOG or HETP = NTU X HTU and therefore, HETP = 9 X 1.26 = 11.34m ~ 12m. Please let me know if my understanding is correct. If I want to check if this height is correct, how do I check it i.e. how do I calculate NTU and HTU?
2) If the column is designed for higher flowrate why it is not possible to operate it at higher feed rate?
3) I understand that the caustic is added to increase the ammonia solubility in the infeed waste water and thus to give uniform ammonia concentration in the feed etc but let’s say if I increase pH from 11.5 to 12.5, how will it make difference in terms of mass balance apart from increased caustic flowrate? Is it not more of chemistry side?

Any help would be much appreciated.
Thanks
KS

[Art Montemayor]

KS:

My comments are as follows:

• Do you have the stripper calculations on hand? If so, what is the overhead vapor flow rate?
• Please refer to a recent, similar application discussed in our Forum: http://www.cheresour...ate/#entry62592
• The other stripper has more bed height, about 1 meter more per bed. However, this may be due to the higher specs on the bottoms condensate product. Note the details given for the two beds and internals.
• The other stripper shows a superficial velocity of about 0.75 ft/sec. I consider the superficial velocity through the stripper as very important. Although we don’t know your superficial velocity, it appears to be higher.
• The height of the theoretical transfer unit should be approximately the same for both applications – within reason. Since you are debating a capacity change, it is the diameter (the superficial velocity) that is the critical parameter in my opinion – not the height of the transfer unit.

Can you please furnish your calculations for the heat and mass balance?

To address your queries:

• You state you want to check whether the column height is sufficient for the feed rate of 15 T/hr and 18 T/hr. I believe this is wrong logic and procedure. The stripper capacity is set by the diameter – not by the height. That is why I look at the superficial velocity through the column.
• We don’t know if the column was, indeed, correctly designed for the higher feed rate. That is why I request the calculations.
• I do not know the basic reason for adding caustic soda to the feed.

Attached Files

•  Ammonia Stripping Column.xlsx   37.61KB   344 downloads

[breizh]

http://nepis.epa.gov...ekPage=x

Consider the link above to support your query .
Caustic soda is added for the dissociation of the species .
Hope this helps
Breizh

Edited by breizh, 16 August 2012 - 06:50 PM.

[chemks2012]

Thanks Art,

Please find the attached spread sheet wherein I have performed Mass & Heat Balance. If you see the heat balance, there is some imbalance and it seems I have missed something important there. Not sure what is that I am missing here!!
Sorry, what do you mean by other stripper? The one discussed in the other thread? Any why superficial velocity is important and not the column height?

Thanks Breizh,
Yes, I came across this article before and is interesting. If I compare the steam to feed ratio suggested in the article it is 4lb/gal , ours is almost half i.e. 1.6lb/gal. Not sure if more steam could help or may be a feed preheater. At present client is cooling the bottom stream before discharging it effluent and we are thinking client should install a feed preheater and use the bottom stream for heating.

Thanks.
KS

Attached Files

•  Mass & Heat Balance.xls   363KB   219 downloads

[breizh]

Yes indeed the best way is to use the hot water to preheat the feed .
Breizh

[Art Montemayor]

KS:

I don’t have the time at this moment to check your calculations; however, I believe our members will normally be doing this. I question why you are obtaining the heat of vaporization for ammonia at 70 ^oC in one of your worksheets. Are you assuming that the ammonia in the stripper exists as liquefied ammonia instead of associated ammonia with water? I don’t think that is the case.

Are you stripping out the dissolved (associated) ammonia out of the feed stream at 1.4 barg inside the stripper?

Since this is a stripping operation (and I don’t think you have any reflux) the overhead vapor stream must be a mixture of water vapor and ammonia gas that is saturated at 1.4 barg. Is that correct? Could you furnish a detailed sketch of the stripper operation with the identification of the flow streams and their properties? That would be a lot simpler than having to logically check out your material balance calculations.

I totally agree with Breizh and am very surprised that you are not capitalizing on the bottoms heat content to preheat your feed stream. This could contribute large energy savings to the operation.

The other stripper is the one shown in the other thread. Note the outline drawing and the dimensions shown there.

It is the column diameter that controls the column capacity – not the column height. The height will change with different feed and product compositions – but the change will be small compared with the difference in the column diameter. It is the column diameter that determines the flooding, channeling, entrainment, residence time, and equilibrium at each stage. Since your concern is a capacity increase over what you have experienced in the past (I think that is what you state), then it is the column diameter that should be checked for capacity increase capability – not the column height. More feed stream input means more sparging steam input at the bottom and more vapor +liquid engagement within the column. Viewed from a common sense approach, that should mean you need more space within the column to handle the additional liquid and vapor inventory.

[chemks2012]

Hello Art,

Thanks for your reply.
1) Sorry, what is the difference between yliquefied and associated ammonia? And how will I know whether its liquefied or associated ammonia?
2) The stripper has only one pressure measuring point and that is at the stripper bottom shows the pressure in the range of 0.1 to 0.3barg.

However, 1.4barg is the steam supply pressure. Client has confirmed later today that steam supply pressure could be in the range of 1.4 to 2.4barg depending on steam demand by other plants
3) The column does not have reflux
4) The column top temperature is in the range of 60 to 70 deg C at unknown column top pressure as no pressure measuring point on the column top and column top composition is 90% NH3, 10% H2O.
5) Will furnish a sketch shortly.
6)Thanks for your explanation on importance of column dia over height but I am not sure how to calculate/check column diameter
7) Energy saving point of view, it will be one of our suggestions to reuse to column bottom for feed preheating. Thanks Art and Breizh

Thanks

[breizh]

A reason why the column is not performing well could be dirty packing ( DP measurement will help) . A couple of years ago ( 15) in the soda ash plant I was working , we were using distillation column to strip ammonia ( I remember the steam pressure was 0.3 b g and the bottom temp was 103 C) and we were using trays ( around 20) . Now regarding the diameter or the hydraulic of the column , consider Eckert/lobo correlation .
Hope this helps .

Breizh

[S.R.Shah]

KS:

Please also check for channeling in the packed columns.

ratio of Packing diameter and Tower Diameter seems OK.

If channeling is there;Please try to keep one good liquid re distributor between 2 columns of 6 meter.

This may enhance mass transfer and capacity.

I observed channeling in Packed column without liquid re-distributor

Please check at constant pressure of Steam as higher pressure shall increase mass flow of steam.

Though there is not direct reflux ;internal reflux due to self condensation may also there..

I hope these checks also ease your problem

SRShah

[chemks2012]

Thanks Breizh,

Yes, I did check Eckert Lobo correlation, very helpful.

Thanks SRShah,

Agree, the column is missing a key item, a liquid redistributor but how would I know if channeling is occurring?

[breizh]

A few more info (practices ) about packed column .

Breizh

[chemks2012]

Thanks Breizh,
It's quite useful.

[S.R.Shah]

Thanks Breizh,

Yes, I did check Eckert Lobo correlation, very helpful.

Thanks SRShah,

Agree, the column is missing a key item, a liquid redistributor but how would I know if channeling is occurring?

Minimum packed height is to be maintained to prevent channeling.As per attached file of Breizh ;it is to be maintained minimum 6.5 meter.

I have come across small diameter Packed column absorption and distillation up to 600DN. I kept 1.5 meter. in addition proper design of feed redisributor and redistributor Design plays part in channeling and performance of overall column.