ethane compression from 21 to 120 bar 5

[mamed]

Is it possible to compress ethane from 21 bar to 120 bar in one compression stage??

More overall, in your thoughts it is better to transport ethane in gas phase or dense phase? which one is more practical and common?
What are the operating pressure and temperature??

Thank you for your time

 

[BNiessing]

Too much ratio for one stage in a recip. You'll run into discharge temperature issues and/or discharge volumetric efficiency problems. Needs two stages.

 

[rotw]

Your mass flow should be sufficient to be handled by a centrifugal compressor first. Then consider that very low to low mass flow with centrifugal will lead to poor efficiency and affect the discharge temperature.

Your suction temperature should stay below a certain limit so that you don't get a too high discharge.
At the same time you should have also sufficient margin to dew point before entering the compressor. It depends from the gas composition: is it a pure Ethane, does it contain some heavy hydrocarbons etc. ?

Assuming pure Ethane, dew point temperature at 20 bar suction pressure should be around -10 deg C or so. With an inlet gas temperature to compressor of 10-15 deg C, it maybe doable with one compression (thermodynamic) stage depending on poly-tropic efficiency of the compressor.

 

[rotw]

Hope some specialist of pumps will provide feedback and share their experience on ethane transportation in Liquid phase so we have an idea as well of the typical operating conditions (pump inlet temperature and pressure for example).

 

[apetri]

1) pure ethane has a critical pressure of about 48.7 Bar.a , at 120 Bar.a you are well above critical point

2) supposing you have to design a pipeline at about ambient temperature (to avoid expensive insulation) and assuming fluid temperature of 15 C (about 288 K) the density of pure ethane at 120 Bar.a and 288 K would be about 422 Kg/M3 (values calculated with PRODE PROPERTIES, extended Peng Robinson model),
that value (422) looks not too different from the density of a liquid phase

3) by solving a single polytropic stage for pure ethane, polytropic efficiency 0.75 and pin 20 Bar.a Tin 270 K, Pout 120 Bar.a the calculated temperature (again with PRODE PROPERTIES and Peng Robinson extended) is about 400 K which makes the single stage design difficult, however there are other equipments as for example screws which perhaps could do the work but you should consult an expert in this field to see if it's a practicable solutiion.

 

[rotw]

Quoted
3) by solving a single polytropic stage for pure ethane, polytropic efficiency 0.75 and pin 20 Bar.a Tin 270 K, Pout 120 Bar.a the calculated temperature (again with PRODE PROPERTIES and Peng Robinson extended) is about 400 K which makes the single stage design difficult, however there are other equipments as for example screws which perhaps could do the work but you should consult an expert in this field to see if it's a practicable solutiion.
Unquoted

Assuming 75 % poly efficiency as you did and an inlet temperature of 15 deg C, a discharge temperature of 165-170 deg C is still descent for a centrifugal and therefore a single stage will possible without too much issues.

We need to know if the gas composition contains pure ethane and what is the flow involved.

 

[mamed]

Dear Apetri
Thank you for your time.

Dear rotaryworld
The gas contains about 99% methane and the flow rate is about 400000 tons per year.

 

[rotw]

''99% methane''
Is it Methane or Ethane ??

 

[BigInch]

Ethene, Ethane, Ethylene???

Independent events are seldomly independent.

 

[mamed]

Ethane, I'm sorry.

 

[rotw]

ok Ethane.
The problem is not your 99% but the remaining 1%.
Heavy hydrocarbon may shift the dew point line.

Assuming (lot of assumptions here) we stay with the pure ethane dew point curve, it should be feasible with a small centrifugal.

 

[mamed]

The remaining 1 perc is .95 Methane and .05 Propane. The assumptions are right!

 

[gaswell]

you may contact Dresser

 

[dcasto]

At 15 C the discharge temp will be 203C at 75% polytropic eff. To hot for most compressors, the cost of the special compressor would not justify going to a two stage unit. As for the 1% other, its a nonevent, that 1% won't do anything, even if it were C8.

 

[OldDoc]

dcasto,
out of curiosity, how have you calculated those values ?
for pure ethane,
single polytropic stage with polytropic eff. 75%
tin 15 C pin 20 Bar.a
pout 120 bar.a
I get a tout of about 149 C
(about 50 C less)

 

[apetri]

Prode (with Peng Robinson) calculates 149 C as tout
(Pin 20 Bar.a tin 15 C , Pout 120 Bar.a)

 

[rotw]

quoted
As for the 1% other, its a nonevent, that 1% won't do anything, even if it were C8.
Unquoted

not agree..

running 1% with and without heavy hydrocarbon here

http://www2.questconsult.com/cgi-bin/jrm_dewbub

6 = Isopentane, C5H12 0.3%
7 = n-Pentane, C5H12 0.3%
8 = n-Hexane, C6H14 0.3%

Calculating dew point temperature at 2000.0 kPa absolute.
The dew point temperature is 284.3 degrees Kelvin.

6 = Isopentane, C5H12 0.0%
7 = n-Pentane, C5H12 0.0%
8 = n-Hexane, C6H14 0.0%


Calculating dew point temperature at 2000.0 kPa absolute.
The dew point temperature is 265.9 degrees Kelvin.


enough delta to compromise single stage option
I don't want to try in the commercial software, I know it be the same outcome

 

[apetri]

the mixture given by mamed

C2 0.99
C1 0.0095
C3 0.0005

shows little difference at 20 Bar.a from bubble (-8.895) to dew (-7.408 C) lines
in this case the impact of light components (C1, C3) is limited.

Of course if you add small fractions of heavy components as C7 and above thinks would be different.

However in some cases it could be acceptable to have a very limited amount of liquid
entering the compressor providing it doesn't generate mechanical problems
(one should ask the manufacturer for a specific simulation),

we have had a compressor working with very small liquid fractions without problems.

from process simulation point of view,
you can solve polytropic stages with any amount of liquid
(polytropic stage with phase equilibria),
I have used this option (available in PRODE) to model screw compressors,
you can even solve a pump (liquid in, liquid out) as polytropic stage.

 

[rotw]

Yes the composition is in our hand and the impact is limited. Clear.
But something posted above seems to mean that 1% will not matter "Anyhow". We know now that this is not true.

 

[dcasto]

I used peng Robinson [tt][/tt]