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?

 

[horacio Torres]

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/

 

[142846]

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.

 

[FMJalink]

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.

 

[142846]

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.

 

[FMJalink]

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.

 

[LittleInch]

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.



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

 

[142846]

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.)

 

[EdStainless]

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

 

[LittleInch]

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.

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

 

[FMJalink]

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.