3 replies to this topic

[nSam]

I have a series of questions regarding the air separation unit:

 

1. Why is air compressed to a pressure of 5-8 bars?

2. What is the purpose of the reboiler/condenser found in the low-pressure distillation column?

3. What stage of the unit is the impure gaseous nitrogen released?

4. Does the unit itself produce hazardous waste?

5. Is water used in the cooling tower (direct contact cooler)? If yes, is the water just waste water? If no, does it have to undergo waste water treatment?

6. In case of leakages in the pipes, how are the operators able to detect these (and thus, preventing asphyxiation in the proess)?

[Art Montemayor]

nSam:

 

You fail to identify what specific design or type of air separation column you are writing about.  For the sake of stopping a lengthy question and answer series with you, I am going to assume that you refer to a Linde double column.

1. Air fed into an air separation plant can be at various pressures – all dependent on what is the design of the process for that unit.  The air pressure employed is determined by the degree of refrigeration required by the process selected.
2. The purpose of the reboiler/condenser found in the low-pressure distillation column is segregate the low pressure column from the high pressure column, to condense the nitrogen vapor produced in the high pressure column and serve as the reboiler for the low pressure column.  This is shown in the sketch I have made in the attached Excel workbook.
3. The relatively impure gaseous nitrogen is produced as the overheads of the low pressure column.
4. All that goes into a conventional air separation column is atmospheric air.  Lubricating oil is used for the compressors and other machinery; solid adsorbent is used in the air dryers.  Cooling water is used for the air coolers.  All that is produced is the components of the incoming air: oxygen, nitrogen and maybe argon.  Do you call this hazardous waste material?  I am also attaching a paper on environmental impact factors involving air separation units.
5. Of course water is used in the cooling water tower.  That is why it is called “cooling water tower”.  The water is recirculated and the water that is evaporated into the atmosphere is made up by city water supply or other sources.
6. Most, if not all, air separation units are housed in ultra-ventilated buildings where there are conventional and convenient convection air currents.  This type of design gives good ventilation in case of nitrogen leaks.  However, seldom if ever are there any leaks in an air separation unit.  The fluids are totally non-corrosive to the materials of construction (usually copper and stainless steel).  Some leaks do occur with time due to stress inside the cold box – and these are usually in the welded joints.  However, these are easily spotted because of the extreme cold spots that appear outside the cold box.

 Air Separation ProcessRev1.xls   959.5KB   247 downloads

 Air Separation Units Environmental Impacts.pdf   197.01KB   245 downloads

[Bodhisatya]

The high pressure gradient created by the Compressed air is the Driving force of the entire Air separation unit.The Compressor drives the air into the downstream equipment such as Wash Towers ,Dryers and Finally Cold Box.

 

Secondly during PreBid stage of any Air Separation Unit ,there is a clear mandate that at Battery Limit Client needs products at specified Pressure say 7 kg/cm2(g).This Pressure can be met at Battery Limit only when the Process Air fed to the cold box is at a pressure higher than 7 kg/cm2(g).

 

And most importantly depending upon the cycle chosen  the Pressure is chosen so.Greater the pressure difference across the Cold Expander..more efficient refrigeration will be achieved.

 

The Reboiler Condenser is perhaps the heart of an Air Separation Unit,as pointed out by Art Sir, the upcoming vapor (enriched Nitrogen) condenses against the boiling Oxygen.A part of this condensed N2 serves as a reflux for the HP Column and another part for LP Column .So it can clearly be fathomed that if Condenser doesn't perform we can't obtain product with specified purity.Also the pressure of HP Column tend to rise.In sort Process severely suffers.

 

Generally the Waste N2 is obtained from the top of LP Column.If Pure Nitrogen is obtained from the 1st stage of the Column,waste N2 might be collected from the 25th stage.

 

As the Unit handles 99.5 % (by Volume) of O2 ,in the reboiler -Condenser ,any sort of hydrocarbon accumulation may lead to Fire ignition inside the Cold Box.U should refer to EIGA  aided handbook for better understanding.Generally ASUs doesn't produce any Hazardous waste.

 

If U are referring to Direct Contact Coolers as the Unit where hot compressed Air is cooled down to a lower temperature by counter-current Water then U need either service water or Raw water.It's better to examine the Cooling water fed to Wash Towers once in a while ,if excessive Foaming ingredient is there in water ,it might led to Carryover of water along with Air ,hence damaging Downstream. 

 

Regards

Bodhisatya

[AZIZ_MN]

nSam,

Assume linde double column

ans.1- most of utility supplied to process plant like utility air, instrument air, nitrogen pressure range between 7-8 barg and to operate coldbox turbine certain delta pressure is required

 

ans.2-reboiler or condenser in LP column to cool down gas nitrogen to liquid nitrogen(liquid nitrogen feed as reflux to LP column and part of the stream goes to storage tank) and boil out liquid oxygen to gaseous oxygen and supply to users through compressor

 

ans.3- impure nitrogen released from top tray of LP column (tray No i forgot) and waste gas from top of the column

 

ans.4-waste gas which  coming out from top of LP colum normally used as regenration gas for mole sieve dryer

and outlet of regenration vents to atmosphere(out side of ASU plant closed building) so surrounding of 2-3 meter area will have low oxygen level

 

anw.5- yes, any waste water from process plant must under go through treatment

 

ans.6-Air sep.unit inside close building, hood exhaust fans provides contineous ventilation of building will not allow to acumalate rich oxygen gas or nitrogen inside the building  for more safety online oxygen analyser installed inside building will give alarm incase of high or low oxygen level

 

*inside Air sep.Unit building  noise level is very high so additinally red blinking light also provided to alert the operators visiting the plant 

*cold box is contineously purge with little amount of nitrogen to avoid leaked gas accumalation inside the cold box

Best regards

azzu