fuel gas centrifugal compressor system 1

[142846]

I'm postulating a scenario, perhaps this is a thermodynamic question! Any way I do need your help.

A fuel gas centrifugal compressor system consist as follow:
1. A gas supply reservoir with constant pressure of 10 barg that we can get maximum. 20,000 kg/h from it.
2. Centrifugal compressor that based on vendor recommendation we should supply minimum 100,000 kg/h in compressor inlet. For 100,000 kg/h, gas pressure increases from 10 barg to 25 barg.
3. Air cooler for cooling the outlet gas from compressor. We have not any limitation to select the air cooler with respect of sizing and heat duty.
4. recycle line with a control valve. the recycle line is branched from outlet of air cooler and ended to inlet scrubber.
5. finally 20,000 kg/h gas goes the outlet scrubber at 25 barg.

Because of having only 20,000 kg/h at supply reservoir we have to have 80,000 kg/h on the recycle line to meet the minimum flow rate of compressor. (100,000 kg/h)

Is it possible to have this system? If the answer is yes, we always could supply the minimum compressor inlet gas with recycling system!?

In the other hand is it possible having recycling mass flow greater than the supply mass flow?

 

[FMJalink]

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.

 

[142846]

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?

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.

 

[FMJalink]

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.

 

[142846]

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?

 

[pmover]

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

 

[FMJalink]

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

 

[LittleInch]

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.

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

 

[142846]

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.

 

[142846]

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?

 

[LittleInch]

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.

 

[142846]

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?

 

[142846]

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:

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

 

[142846]

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.

 

[LittleInch]

"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.

 

[pmover]

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.

 

[LittleInch]

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%.

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

 

[pmover]

LI, Thanks for the clarification.

 

[142846]

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?

 

[LittleInch]

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.