8 replies to this topic

[ncarrascob]

Dear all,
I had used Aspen HYSYS to estimate the injection rate of ethylene glycol required to prevent hydrate formation in a natural gas stream. Getting 1163 lb / hr of 80 wt% EG for a decrease in the hydrate formation temperature from 42.7 º F to -20 ºF for a current 11.6 MMscfd of natural gas saturated with water at 109.6 ° F and 272.3 psia.

However, in my eagerness to check how accurate is the estimate of Aspen HYSYS, I made calculations according to Equations 20.6 and 20.8 of the GSPA manual giving me with surprise that this value is almost 200% less than that estimated by the software.

In Aspen Hysys simulation I used the Adjust tool to estimate the EG rate that make the hydrate formation utility calculates a hydrate formation temperature equal to the temperature of the mixture gas + condensate + EG aqueous solution leaving the propane chiller. So, as a result, the adjust tool gives a value of 1163lb/hr of 80 wt% EG. However, GSPA correlations give 400 lb/hr o that inhibitor.


The gas composition is as follows:

COMPONENT mole fraction.

c1 0.92976044
c2 3.00 E-02
c3 1.20 E-02
i-c4 4.26 E-03
n-c4 5.59 E-03
n-c5- 7.85 E-04
I-C5 3.27 E-03
N-C5 1.82 E-03
n-C6 3.35 E-03
CO2 2.39 E-03
O2 8.15 E-05
N2 1.77 E-03
H2O 4.91 E-03

Please, anyone of yours help me understand what I'm doing wrong.

Edited by Neizer, 16 February 2011 - 05:11 PM.

[ankur2061]

Joaco,

I have made a very comprehensive spreadsheet for hydrate inhibitor injection calculations using the Hammerschmidt equation for both methanol and mono ethylene glycol injection and following the GPSA method.

I can check your calculations and inform you the result if you provide me the following information:

1. Molecular weight of the gas (in the gas composition provided by you O2 is repeated twice)

2. Upstream (higher) Pressure and temperature in the system / pipeline

3. Downstream (lower) Pressure and temperature of the system / pipeline

Additionally have a look at the following two links for posts on the forum which have discussed hydrate formation and hydrate inhibition calculations:

http://www.cheresour...h__1#entry41204

http://www.cheresour...h__1#entry34068

Regards,
Ankur

Edited by ankur2061, 30 January 2011 - 01:12 AM.

[sheiko]

Dear Ankur,

Could this spreadsheet be available to ChE Plus subscribers?

Thanks.

[ankur2061]

Sheiko,

I will have a word with Chris Haselgo regarding this matter.

Regards,
Ankur.

[ncarrascob]

Dear ankur,

Thank you very much for the really good links related to natural gas hydrate formation.

I really want to apologize for having absented myself these days

Hopping that is not too late, i'm providing required information:

-natural gas specific gravity= 0,84 @ 109,6 ºF and 272,3 psia.

-The last value is the water mole fraction.

-UPSTREAM CONDITIONS ARE: 120ºF and 50psig (low pressure separator).
-DOWNSTREAM CONDITIONS ARE: -25ºF and 250 psia (straight refrigeration system)

[Propacket]

Find attached a DOS based program for Hydrates prediction. It was provided by someone in this forum.

Attached Files

•  CSMHyd.zip   186.25KB   122 downloads

[ncarrascob]

Dear P. Engr,

Thank you very much for attach the program

[ankur2061]

DOWNSTREAM CONDITIONS ARE: -25ºF and 250 psia (straight refrigeration system)

The downstream data does not seem to be right. How is the downstream pressure higher than the upstream pressure? The drop in temperature also from 120 deg F to -25 deg F just by cooling due to pressure drop seems unlikely.

If you are cooling gas through a turbo-expander or a propane chilling unit than the water condensed should be removed at that stage only.

Please check your input data once again.

Regards,
Ankur.

[ncarrascob]

Dear Ankur,

Maybe I didn't give you a clear explanation of the simulated proccess, so hopping could have your help I'll describe the straight refrigerated process that i have simulated using aspen hysys.

In the field separation of associated natural gas (ng), we have 12MMscfd of a ng stream leaving the low pressure separator (which work at 50psi and 120 ºF), this stream satured whit water is compressed until 278 psia, after that is cooled until 110ºF and the free water is decanted in a adiabatic flash tank. The gas stream leaving this tank is further cooled in two heat exchanger (the first: Gas-gas exchanger and the sencond: Propane chiller stage), in the first heat exchanger the inlet gas stream is cooled aproximately until 12ºF and in the second heat exchanger the gas is chilled until -25ºF. Before entering to the cooling stages etylene glycol is injected to pevent hydrate formation. For better understanding please refer to the attached picture.

Thank you very much.

Attached Files

•  EG-injection hysys simulation.bmp   2.93MB   76 downloads

Edited by Neizer, 18 February 2011 - 03:26 PM.