10 replies to this topic

[Light]

Hello to everyone out here.

I want to know the significance of using a double column rectifier for air fractionation. Please explain why can't the separation be achieved in a single column. I have been searching for an answer since a long time but in vain. Please help.

 

[Art Montemayor]

Samkha:

 

You have to be more specific in your questions when you deal with engineers.  For example, you ask a general question without identifying what – exactly – it is that you want to produce from air fractionation: liquid oxygen, liquid nitrogen, liquid argon, gaseous oxygen, gaseous nitrogen, or gaseous argon.  It makes a big difference as to whether you can apply a single or double coulumn.

 

Please specify what it is that you want to produce with an air separation unit (ASU).

[Light]

Art Sir

 

My apologies for not being specific. I am refering to the process producing pure nitrogen and pure oxygen as well as consisting of a crude argon column and pure argon column. Some of the oxygen is withdrawn as liquid while the rest of it along with all of nitrogen product are vaporized to cool countercurrently the incoming air. In the process, the incoming feed air after being cooled to its liquefaction temperature is fed to the high presure column (lower column) and separated into almost pure nitrogen at the top and oxygen rich liquid at the bottom. All of the bottom product and some of the top product (after providing necessary reflux for the High Pressure Column) are fed to the low pressure column (upper column) at a suitable point. The top product is pure nitrogen and bottom product is almost pure oxygen (from the low pressure column). The feed for the crude argon column is withdrawn from a 'suitable' tray of the low pressure column. Also a waste gas stream is withdrawn from near the top of the low pressure column. 

Here are my questions:

1. Why is the separation carried out in a double column (i.e. low and high pressure column)? Why can't it be carried out in a single column, i.e. low pressure column?

2.How is the 'suitable' tray for crude argon feed decided?

3. Why is it necessary to withdraw a waste gas stream?

[Art Montemayor]

 

Samkha:

 

You are still not being specific.  You did not address my specific question(s).  I specifically cited liquid and gaseous products.  You respond with: “pure nitrogen and pure oxygen as well as consisting of a crude argon column and pure argon column”.  So you fail to respond in an accurate, specific manner.  This thread is going to suffer from excessive writing, back-and-forth, and this a waste of good time.

 

All I can respond with is a general response to a general specification.

 

Pure oxygen (99.5+% vol.) can be produced in both a single and double column.  However, the single column acts as a pure stripping column and lacks a rectification section.  Therefore, the single column can only strip out the oxygen as a gaseous product – with no liquid production of anything else.  Pay careful attention to the fact that a single column cannot produce a reflux stream of pure liquid nitrogen.  Without this reflux stream, you cannot produce liquid nitrogen or gaseous nitrogen at 99.5+% vol. purity.

 

The “suitable” tray for a crude argon draw off is decided by the trays’ efficiency and the location of the appropriate tray containing the optimum composition.

 

I can’t comment on the necessity to withdraw a waste gas stream if you don’t identify the waste stream you refer to.

 

I really wish you would have replied with what I requested.  That way I could have furnished a specific and probably more valuable response.  The quality of the response can only be as good as the quality of the query.

 

[DB Shah]

Attaching a simplified ppt, It is a break through technology by Carl Von Linde. Kindly note that the bottom column pressure is such that the bottom column top temperature is higher than top column bottom temperature. Thus the bottom column serves as a heat source (reboiler) for top column and top column bottom serves as a heat sink (condenser) for bottom column.

Attached Files

•  ASU-00.pptx   653.19KB   177 downloads

[Art Montemayor]

^{Thank you very much, DB Shah.  This is a Linde Presentation that I have been missing.}

 

^{Although very simplified, it makes its point.  Viewers should be alerted to the fact that the incoming air into the single column has to be liquid phase, and the only way to achieve that is to use the liquid oxygen bottom product as the coolant in a counter-current heat exchanger with the incoming gaseous ambient air feed to the cold box.}

[Light]

Thank you very much Art Sir and DB Shah sir for your help.

Art Sir, I was referring to the process in which gaseous Nitrogen and liquid oxygen are produced with a purity of 99.5+ %. 

The waste gas which I am referring to is all nitrogen with a small quantity of oxygen (less than 1%).

Again my apologies, I would be more specific in the future.

[AZIZ_MN]

Art Sir

 

3. Why is it necessary to withdraw a waste gas stream?

Answer for your third question probably is,waste gas withdrawal is to maintain low pressure column pressure 1.2 to 1.3 mbar and same waste gas stream further used as regeneration gas for moleseive dryer,

regards

azzu

[DB Shah]

Dear Art,

The ppt was made for my fresh engineer as well as one Instrument engineer friend, hence very simple. Kindly feel free to add something to the ppt. This is just Rev 00, and we enggs are found of adding revisions  

 

 

Dear Samkha,

As you see there are two columns each each giving one pure and one impure stream-

 

Bottom column

Top product -Pure N2

Bottom product - impure liq N2+O2

 

Top Column

Bottom product - Pure O2

Top product -Impure N2

 

As you can see in the 9th slide of the ppt, bottom column bottom impure liq N2/O2 is used as reflux of top column.

This leaves you with three outlet streams-

Pure O2 from top column bottom

Pure N2 from bottom column top

Impure N2 from top column top.

[Bodhisatya]

. 

Here are my questions:

1. Why is the separation carried out in a double column (i.e. low and high pressure column)? Why can't it be carried out in a single column, i.e. low pressure column?

2.How is the 'suitable' tray for crude argon feed decided?

3. Why is it necessary to withdraw a waste gas stream?

 

Below are my answers to your 3 questions:

 

1. The HP column is just a stripping Column, where Oxygen will be stripped and can be collected at the bottom with an average Composition of 30 %, which normally is abbreviated as RL (Rich Liquid ), provided that you have a Liquid nitrogen reflux, which happens in a Simple Nitrogen generator, where a Column is aided with a total condenser.  But when you aspire for Oxygen of purity of 99 % and above, you have to opt for Two columns.  PL (Poor Liquid) and Rich Liquid (RL) serve as reflux and feed to LP Column in addition to LP Air ex an Expansion turbine (Only for the Lachmann Air cycle, not for the Claude cycle).  Here, the incoming RL is vaporized against the condensing  GAN (Gaseous Nitrogen).  As the vapor starts moving up the column, PL as well as RL start stripping the Oxygen in the upcoming vapor, and as a result the vapor becomes rich in N2 and the sump Liquid rich in O2.  There is an energy integration process working where the re-boiler and condenser load are balancing each other and perhaps this explains the requirement of Double Column, primarily to produce Liquid N2 reflux to be used in both columns, while in HP column it will strip O2 from incoming air, in LP column it provides the necessary rectification for upcoming vapor.  The reason for operating the Column at different pressure is N2 at atmospheric pressure is colder than O2, so at elevated pressure it becomes warm enough to release enough heat for the RL to boil.

 

2. Argon take off is generally governed through studying the Column profile.  The point of take is generally 10 % argon content.  As higher you go, the Argon percent might increase, but at the cost of N2 % increase, which might cause N2 binding in Crude and Pure Argon Column, so the desirable take off point is 10 %.

 

3. As per product spec, LP Column delivers GAN, GOX and LOX, which are much higher than the generated RL vapor, so if you don't withdraw that extra amount, the LP Column pressure will increase as a result of the saturation temperature of Pure GAN and LOX, which will result in imbalance.  So it must be withdrawn.  It does serve another purpose for the regeneration of Pre Purification Units. 

 

Bodhisatya.

[Light]

Thanks to all who replied.

Thank you Bodhisatya for your detailed explanation. All my questions are answered satisfactorily and the distillation process is now clear to me.