To my knowledge you can make the recycle as large as you wish. It is a waste of energy though. You could look for a centrifugal compressor manufacturer with smaller size centrifugal compressors in his delivery program or switch to a screw type compressor for which the mentioned maximum flow rate is a more normal case.

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To my knowledge you can make the recycle as large as you wish.

Are you saying at steady-state condition we can make recycling flow rate larger than input flow rate? do you have any practical experience?

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It is a waste of energy though. You could look for a centrifugal compressor manufacturer with smaller size centrifugal compressors in his delivery program or switch to a screw type compressor for which the mentioned maximum flow rate is a more normal case.

I agree that recycling could cause waste of energy, but question is not about waste of energy or selection of the best type of compressor. Question is about the feasibility of the process mentioned in my first post.

The recycle flow has to brought back to the inlet pressure you mentioned and has to be cooled sufficiently to prevent heating up. The compressor itself does not care whether you added the recycle flow to the stream from the reservoir. It will be just the suction flow for the compressor. The recycle system will be somewhat larger then with a 20 000 kg/h system would be expected.

FMJalink thanks a lot for your explanation, but I couldn't understand how to make 80,000 kg/h in recycle line while we have only 20,000 kg/s supply reservoir and don't added any source of gas from any where to recycle line. I think the largest recycle line can make only 20,000 kg/h flow rate because supply reservoir can produce maximum 20,000 kg/h.
Therefor it is impossible supply more than 40,000 kg/h at inlet of compressor. Am I right?

regarding item #2, why is the vendor recommending a centrifugal compressor sized for 100,000 kg/h?

I did not look into the requirements for start-up and shutdown. Basically the recycle is separate from through flow, after the system is filled with gas. You can suee that with a black box approach moving the system boundary from inside the recycle to outside the recycle.

The problem lays, as you probably already knew, in the start-up. There the compressor wants the recycle flow, which still has to be compressed. I do not know whether is possible with your system, but it should work when the compressor is put on full recirculation first with an open connection to the suction reservoir and closed discharge. Then you create first the required recycle. With shutdown it will be the reverse. When this is direct on line application (constant speed) a suction buffer vessel might be required.
Good luck

Just do not understand why there is a recommendation like your number 2. You need to challenge this and understand why.

However no you are not correct.

In full recycle the compressor will flow around the system whatever it wants. In your case it's 100,000 kg/hr. Then what ever extra gas you insert into that closed loop will come out the other end.

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regarding item #2, why is the vendor recommending a centrifugal compressor sized for 100,000 kg/h?

You could consider at the first this compressor has been sized based on item #2 (100,000 kg/h), but after the years and years available gas decreases to maximum 20,000 kg/h). I want to know is it possible use this compressor at this condition (20,000 kg/h.)
In the other hand at zero recycle, with flow rate of 100,000 kg/h at reservoir outlet, compressor can be run/operate based on vendor advise.
The question is when the flow rate of reservoir decreasing to 20,000 kg/h, is it possible operate this compressor with 80,000 kg/h recycle? 80,000 kg/h comes from vendor advise to providing minimum flow rate of 100,000 kg/h at the inlet of compressor. Because 20,000 kg/h comes from reservoir and 80,000 kg/h shall be added to 20,000 kg/h to have totally 100,000 kg/h.

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but it should work when the compressor is put on full recirculation first with an open connection to the suction reservoir and closed discharge. Then you create first the required recycle. With shutdown it will be the reverse.

with closed discharge of compressor system what is the maximum recycle flow rate that could be happened when the suction buffer vessel flow rate is maximum 20,000 kg/h? Can we achieve 80,000 kg/h at recycle?

142846,

You don't seem to have read the posts properly.

"I want to know is it possible use this compressor at this condition (20,000 kg/h.)" - Do you mean with only 20,000 kg/hr going through it?
Don't know - please post the compressor curve, but it seems unlikely.

"The question is when the flow rate of reservoir decreasing to 20,000 kg/h, is it possible operate this compressor with 80,000 kg/h recycle? "
Yes it is possible, though it is very wasteful of energy as you're compressing and doing work on 100,000 kg/hr, but then recycling 80% of that back to the compressor inlet. Your discharge heat exchanger will be working hard, but it will work as a cost of increasing OPEX per kg of fuel gas.

It would be interesting to know where 100,000 kg/hr sits on the compressor curve - mid point / duty point or close to the surge line?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

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In full recycle the compressor will flow around the system whatever it wants. In your case it's 100,000 kg/hr. Then what ever extra gas you insert into that closed loop will come out the other end.

Thanks. You hit the target exactly. But I think we can not achieve to 80,000 kg/h at recycle when we have only 20,000 kg/h at suction buffer/reservoir. I think at the best condition we have maximum 20,000 kg/h at recycle. (equal to buffer/reservoir flow rate). Am I right?

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It would be interesting to know where 100,000 kg/hr sits on the compressor curve - mid point / duty point or close to the surge line?

Answer is surge, because at the first post I had bolded the word of "minimum" and I said that:

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2. Centrifugal compressor that based on vendor recommendation we should supply minimum 100,000 kg/h in compressor inlet.

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Don't know - please post the compressor curve, but it seems unlikely.

Unfortunately I haven't compressor curve, but I know that the minimum amount of 100,000 kg/h shall be fed at the inlet of compressor.

"Am I right?"

No.

I don't know how to say this any more than I and everyone is telling you, but you keep coming back with the same question when someone gives you an answer you don't want to accept for some reason.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

142846,

simply stating, having a centrifugal compressor sized with a driver that expends >60% of its power to recycle gas is very poor engineering and/or operation.

either the respondents do not understand the full scope of the situation or otherwise . . .

Perhaps you can clarify.

if this is an existing installation, a simplified P&ID/PFD sketch would be helpful. additionally, a curve using existing operating data can be created. it won't be perfect, but you can use existing data to project other operating scenarios.

Pmover,

The gas field originally produced 100,000 kg/ hr so compressor was sized for that. Over time production has dropped but compressor still needs 100,000 kg/hr apparently to avoid surge.

So instead of replacing the compressor with one of a smaller size, they still want to use the big one with 80% recycle. The OP for some reason doesn't think this is possible when you could recycle 100%.

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LI, Thanks for the clarification.

I am sorry for boring question but I have not gotten even one reason.
My concern is not about capex, opex, good design, compressor selection, ... because free electrical power is available.
Please say just one reason that why we couldn't have 80,000 kg/h as recycle flow to achieve totally 100,000 kg/h fed to compressor inlet to prevent the surge?

There is no physical reason why you can't do that.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

It is a terrible waste of energy--- and MONEY

If a understand well, the NG production deceased to 20.000 kg/hr. You can still working recycling 80.000 kg/hr gas... But it is not efficient due the energy cost. Consider the cost for a small compressor compared with the energy waste in 8 years, for sure you will loss a plenty amount of money. Your manager will not be pleased for it.


Horacio

https://www.linkedin.com/company/lagotuy/

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It is a terrible waste of energy--- and MONEY

Yes, but at the above posts I clearly said that money is not my concern. my question is about possibility/not possibility of compressor operation with 80,000 kg/h recycling while there is only 20,000 kg/h available gas at buffer inlet vessel.

Dear 142846, most (or at least a lot them) industrial centrifugal compressors systems are equipped with a full 100% recycle system. That means that once filled with gas they can run without any flow additional to the recycle flow. Your 100 000 kg/h compressor can, if equipped with such recycle, recycle the 100 000 kg/h from the discharge back to the suction.

When you want 20 000 kg/h to be compressed from suction to discharge, you can reduce to recycle flow to e.g. 80 000 kg/h, what would sum up again to your minimal flow rate of 100 000 kg/h.

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Dear 142846, most (or at least a lot them) industrial centrifugal compressors systems are equipped with a full 100% recycle system.

Yes. I agree. But dear FMJalink, when we have only 20,000 kg/h at buffer (supply), how could achieve to for example 80,000 kg/h in recycling line?. In the other hand compressor system designed, manufactured and equipped with 100,000 kg/h, but in this operation we haven't sufficient flow rate to reach 80,000 kg/h in recycle.
for example you consider we have a storage tank with capacity of 100,000 m3 water, but we have only 20,000 m3 water. therefor it is not possible full filling this tank.
I want to say that if If we have 100,000 kg/h at buffer vessel (inlet scrubber) then we could achieve 100,000 kg/h in recycle. at this example because we have only 20,000 kg/h we could achieve max. 20,000 kg/h in recycle not 80,000 kg/h.

142846, appears you are confused between absolute volumes/masses and volume/mass flow rates. Maybe following example helps.

When you have 2 cups. One full with liquid and the other empty. Then you lift the full cup and poor the liquid in the empty cup. There after switch the cups, left to the right hand, and the right to the left hand, Then raise the full cup and poor it in the empty cup. Continue this for a long time.

This could be considered to be a liquid stream making a cycle, simular to the recycle of the compressor. The total mass involved is just 1 cup of liquid. You can increase the flow rate by doing the pouring and cup switching faster. No addition (suction flow) is required.

In reality there will be a lot of splashing in the example and it will not get near to mass f!ow rates mentioned with the compressor, but I hope it makes the situation clear.

142846,

Stop getting hung up on the numbers in flow. Your water example is not the same thing as you're comparing volume to flow

Think of it like this maybe.

In the compressor, the recycle system and cooler there are let's say 10 kg of gas. In full recycle the mass of gas going round in a circle is the same actual mass (10kg), but it's going round that loop 2.8 times per second. (10 x 2.8 x 3600 = 100,000 kg/hr). So the compressor thinks its shifting 10 kg 2.7 times per second. It doesn't care that all the gas is the same actual molecules as it saw a fraction of a second ago.

Then you add in 0.56kg/sec at the inlet and remove 0.56 kg/sec at the outlet. Your 10kg is still going around the loop 2.7 times per second remember.

So you don't actually ever need 100,000 kg./hr Yu only need 10 kg going round in a circle very fast for the compressor to think it is working at 100,000 kg/hr

Does that help?

For a water system this is more like you have PD pump with a supply line rated at 20m3/hr. The pump is rated at 100m3/hr. the outlet only uses 20 m3/hr.

The pump just pumps most of the water round in a circle.



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Also: If you get a response it's polite to respond to it.

dear FMJalink and LittleInch
I think the conclusion of your explanations is that for 20,000 kg/h at buffer scrubber and 80,000 kg/h in recycle line, this compressor could be operable.
If I accept it, then generate some another question for me. what is your idea for example for 10,000 kg/h and 90,000 kg/h recycle? what about the 5,000 and 95,000 kg/h recycle? what about for example 1,000 kg/h and 99,000 kg/h recycle and finally for example 1 kg/h and 99,999 kg/h recycle (almost closed cycle).
you are saying if we have a huge compressor (100,000 kg/h), with regulating of recycling flow, we could run it for any flowrate without any problem.(except waste of energy.)

This is a start up question. After all unless you have a buffer tank you cannot supply more than 20k, so it will take 5 hrs for the system to begin producing usable output.
Will the compressor run partially starved for 5 hours?
Who knows.
When I have seen high recerc (about 40% of total flow) they had a tank that filled at outlet pressure.
This tank was dumped into the inlet along with new flow at start up in order to get to full flow faster.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

Ed, I don't agree with you. You only need enough gas to fill up the compressor and the recycle line.

142846 - basically you've finally got it. I wouldn't go as low as 1kg/ hr, but 1000 would probably work. And v very inefficiently.

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It is not that bad with a start up with a suction vessel. I will give only approximations, as the exact solution is a lot more elaborate.

Presume the total of compressed gas at 26 bar abs in the compressor and recycle system is 30 kg. Maybe it is somewhat more, but the approach will stay the same.

Connected to the front is a suction buffer vessel already filled with 11 bar abs fuel gas. Let's allow the suction pressure to decrease from 11 bar abs to 10 bar abs during the startup with closed discharge.

The compressor needs only 30 kg from the suction buffer vessel. Assuming the fuel gas is propane with MW = 44 kg/kmol, than approximately 15.3 Nm3 is required.

This would lead to a suction vessel of 15.3 m3, which could be something like a 2 m diam x 5 m cyl length vessel.

The compressor at 100 000 kg/h would suck the 15.3 Nm3 from this buffer within approx 0.6 sec and the suction pressure will brought up to 11 bar abs, pending on the suction system resistance, after some seconds.

Advice is contact the original supplier, let them work it out and let them guarantee the working of the system.