5 replies to this topic

[jdch07]

hello every one.

I m not getting understood the role of booster air compressor in the air separation unite when you want oxygen at high pressure.
Is it requirement of Main heat exchanger for certain high pressure air to boil up high pressure liquied oxygen? if yes then why do so?

Can any body please answer.

I want to design a model air separation plant with high pressure gasious oxygen say more than 60 bar at the rate of 3000 ton per day with cheapest energy per unite. nitrogen required is approx. 20 bar and same rate

Also please suggest the article or ebook for designing of unite operations of air separation plant and whole air separation plant.

[jdch07]

Mr. Padmakar can you pls help me to get the rid of the question above?

[Art Montemayor]

JDCH07:

You say you want to design a "model" air separation unit, but do not understand the process of how the air separation unit works.

I don't know what you mean by "model", but you can't do one without dominating the other. In other words, if you don't know how an air separation plant works, you haven't any business trying to design one.

What you propose to do is very simple - when viewed and done by someone who has operated, modified, installed, and maintained air separation units producing 99.99% gaseous oxygen at 200 bar pressure. I know because I have done that.

What specifically are you trying to do, for what reason, and what is your background in this engineering discipline?

[jdch07]

sir, I m chemical engineer with 8 yrs of experience for operating of effluent treatment plant/desalination plant/nitrogen generation plant with cryogenic distillation/and air compressors.

Presenlty i m reviewing the desing supplied by ASU vendors for new facilities coming to us.
Till date what we operated is only nitrogen generation at 8 kg/cm2 (5 ppm O2 purity).
Now new facility consisting of various products like 76 kg/cm2 of Gasious Oxygen 99 % mole , 64 kg/cm2 Gasious N2 (99% mole) and we are going for internal compression cycle( taking liquid from column and pumping it upto desired product pressure passing it through HE ).

My question is why it calls for booster air compressor. After so many reading and searching i am able to find out only that : If you want to have product at high pressure you must pass air - higher than your product pressure, in HE to evaporate it fully(due to thermodynamic reason).

For getting the air higher pressure only we require Booster air compressor?
Why we need air higher pressure than product pressure in HE to get desired gasious high pressure product. what quantity of air we need to pass it HE if we want to have 1000 TPD Gasious oxygen at 76 kg/cm2 ? and what pressure.

Please explain me the fundamental for booster air compressor in Air separation plant.

[Art Montemayor]

JDCH07:

Thank you for responding with positive information. Now I have a clearer understanding of what you are trying to understand. I think I can help.

I believe you have been presently producing high purity nitrogen (99.9% vol.) and are now starting to purchase additional equipment that will enable you to produce both oxygen and nitrogen simultaneously from the same Air Separation Unit (ASU). You don’t tell us if you are planning on modifying the existing unit or if you are installing a completely new unit that will replace the existing one. Either way, 1,000 tonnes of oxygen per day is a huge amount of production.

Your basic query centers on the need for a “booster compressor”. I have used booster compressors on air separation columns – but only to feed more air into the unit for additional production. I believe what you call a “booster” is a compressor that raises the initial compressed air feed to a higher pressure level. Correct me if I am wrong.

I don’t know what ASU firm you are dealing with and what their separation cycle looks like, but the basic design for air separation has not changed since the original concept was put forth by Linde, well over a hundred years ago. The very basic equipment employed to separate air into its 3 major components is the Linde Double Column – so-called because it consists of two distillation columns, one installed on top of another. Today, because of improvements and large sizes, the two columns may tend to be separated, with a nitrogen condenser/reboiler in between them. But the original Linde design still holds.

The “bottom” column is called the high pressure column and usually operates at approximately 60 -75 psig (4.2 – 5.27 kg/cm2g). the “top” column is called the low pressure column and usually operates at approximately 5-7 psig (0.35-0.5 kg/cm2g). The HP column produces crude Oxygen feed (45% vol) and pure nitrogen reflux for the LP column. It can also serve as the source for Argon production using a side column. You basically are only interested in oxygen and nitrogen, so we’ll stick to that.

The reboiler for the LP column is actually the HP nitrogen condenser – which serves a dual purpose. The LP column produces high purity liquid oxygen in its bottom sump and gaseous nitrogen in its overheads. Both these product streams are at the nominal LP column pressures. If you require the high purity oxygen at higher pressures (in the liquid or gaseous state) you must furnish external energy in a pump or compressor. If you require pure nitrogen – either as a liquid or a gas – you usually take it from the HP column overheads condenser and also furnish external energy in a pump or compressor if you want it at higher pressures. Note that the two distillation columns are inter-connected and rely on one another. Therefore, if you require simultaneous production of both nitrogen and oxygen, you must compensate for the re-distribution of the heat and mass transfer taking place inside the column. And this is usually done with the addition of more required energy in order to produce both pure products at the same time.

The Linde cycle – like any other cryogenic cycle – requires energy, and if more efficient separation is required, additional energy must be added – in order to furnish the additional cryogenic liquefaction refrigeration taking place. How this is done depends on your specific plant and / or process. Most efficient Linde distillation cycles employ the Claude cycle to obtain efficient refrigeration of the incoming feed air using the basic principle of isentropic expansion of a portion of the incoming feed air. For the capacity you have cited, you most probably have to use expansion turbines (Claude’s cycle) to maximize the economic operation of the plant. In order to expand and liquefy the incoming feed air for refrigeration and distillation needs, you must have a pre-determined feed air pressure that allows you to operate the expansion – and obtain the required process refrigeration. This need sets the required feed air pressure that is entering the ASU and is used to obtain refrigeration, liquefaction, heat transfer, and mass transfer.

You state that you have found out that if you want to have product (gaseous nitrogen and oxygen) at high pressures, you must pass air - higher than your product pressure - through the cryogenic heat exchanger to evaporate the liquid product fully. I do not believe this is a true statement as written. The reason(s) for the higher feed air pressure is due to the reasons I have cited. Note that you can substitute the need for a higher air feed pressure with an external refrigeration process – one that pre-cools the incoming air. I have done this application in some ASU units in order to reduce the start-up time of the plant. This external refrigeration was only used during start-up. Once I established operating liquid levels in both HP and LP columns, I would turn the external pre-cooling unit off because although it is faster in obtaining the desired result, it is much less efficient that the Claude cycle.

Please tell us whose process you are evaluating and furnish at least a PFD of the proposed cycle and process that you are describing.

I hope this explanation helps you out.

[jdch07]

Art Montemayor

Thank you very much for explanation and certainly it helps me very much.

I want to tell that I am operating presently ASU with nitrogen production only.
Now new ASU we want to purchase is for entirely different plant. and it will be Completely new ASU, it doesnt have to deal any thing with existing ones.

We are reviewing various ASU suppliers basic documents.

Every where it is written that putting product gas compressor for getting product at desired high pressure is not advisable. instead we should put internal pump inside cold box pressureised liquied upto desired level of pressure then passed it through HE.

So basically we have been suggested by everybody, internal compression cycle.(instead of external compression).

But to have internal compression cycle you have to have booster air compressor.
Booster air compressor combined with liquid pump (LOX) will make u enable to have your product at desired pressure. also it will be energy saving option when you compare with external gas compressors for product.

I am unable to give PFD at this time but i can give you description.

1. Main air compressor outlet goes to DCAC(Direct contact after cooler)------->TSA(Temperature swing adsorption for removal of contaminants)----------->certain amount of TSA outlet is going directly to HE and then from HE to HP Column: whereas balance amount is going to suction of Booster air compressor then going to compander unit (compressor---->HE--------> expander) and then in HP Column.
2. From LP column liquid oxygen is taken and inside coldbox only it is pressurised by pump to 76 kg/cm2 and then passed through HE. same thing is with nitrogen.

Rest of the things are like typical Double column ASU.

Now I want to understand why we are raising pressure of air upto certain quantity by booster air compressor?
What quantity of air we need to boost and why? if we want to have 1000 tpd liquid Oxygen at 76 kg / cm2

My base querry is why we need booster air compressor when we go for internal compression liqid pump cycle.

Pls help me to getting it understood.
Regards
JDCH07