13 replies to this topic

[samrat]

Forum Members,

The following is the natural gas composition on DRY BASIS.
1. Nitrogen: 0.1378 Mol. Percent
2. CO2: 3.4757 ---DO----
3. C1: 52.2984
4. C2: 14.5107
5. C3: 15.2966
6. iC4: 3.8737
7. nC4: 5.2511
8. iC5: 1.8311
9. nC5: 1.9799
10. C6: 1.345
Suction Temperature of Gas: 45DegC
Suction Pressure of Gas: 0.085Kg/CM2g
The Gas is SATURATED AT 45DEG C WITH WATER
The Gas is compressed to 7Kg/CM2g in a wet screw(Oil Flooded with CPI Synthetic Oil with 100CST Viscocity)
The Oil injection is controlled in flooded screw to get the average oil temperature of 60DEG C.
The adiabatic efficiency of compressor can be taken as 75 to 77 Percent

[samrat]

Forum Members,

The following is the natural gas composition on DRY BASIS.
1. Nitrogen: 0.1378 Mol. Percent
2. CO2: 3.4757 ---DO----
3. C1: 52.2984
4. C2: 14.5107
5. C3: 15.2966
6. iC4: 3.8737
7. nC4: 5.2511
8. iC5: 1.8311
9. nC5: 1.9799
10. C6: 1.345
Suction Temperature of Gas: 45DegC
Suction Pressure of Gas: 0.085Kg/CM2g
The Gas is SATURATED AT 45DEG C WITH WATER
The Gas is compressed to 7Kg/CM2g in a wet screw(Oil Flooded with CPI Synthetic Oil with 100CST Viscocity)
The Oil injection is controlled in flooded screw to get the average oil temperature of 60DEG C.
The adiabatic efficiency of compressor can be taken as 75 to 77 Percent

Due to some manual mistake I couldn't complete my question. Now I am continuing.
The flow of Gas 70,000SM3/Hr(Standard Meter Cube Per Hour)
The questions:
1. Conversion from Dry base to Wet base
2. The dew point of the gas at suction and Discharge
3. If the Gas is hot by-passed to reach the suction temperature to 65DEG C, the effect of this on discharge if the discharge pressure is still maintained at 7KG/CM2g.
How do we take care the above feed and the compression in either Hysys or any other Simulation software?
4. Will there by a condensation of heavy hydrocarbons and water vapor in case the gas is flashed in the suction and cooled to 55Deg C in discharge and flashed in discharge?This will be at normal operating suction temperature of 45Deg C without the hot by-pass.Request quantum of each constituent of the gas if dumped in KODs?
Appreciate your response
samrat

[ankur2061]

Samrat,

Some tips on the engineering design of the compressor system you have described:

A. Even though your compressor is an oil injected wet compressor you require a proper knockout drum at the compressor inlet to remove as far as possible heavier hydrocarbon liquids and water vapor. The reason is that the mixture of liquid water, liquid hydrocarbons and lubricating oil can cause serious fouling and / or corrosion problems in your compressor which may lead to serious performance deterioration of the compressor.

B. Obviously since this is a wet, oil injected screw compressor you require a high efficeincy filter/coalescer at the discharge of the compressor for >99.9 oil /hydrocarbon mixture removal efficiency for downstream applications. This filter/coalescer should be installed downstream of a gas cooler on the discharge side.

C. HYSYS or any similar process simulation software will definitely model the entire compressor system including a suction KO drum and a discharge side cooler (generally Air cooled HEs are employed nowadays). HYSYS provides two options: centrifugal and reciprocating for modeling a compressor. There is no 3rd option for a compressor type in HYSYS. HYSYS will also predict the dew point of the individual gas stream using the utilities tool in HYSYS. The oil injection and it's quantity in the discharge side is something which you will have to provide as inputs. Also, you will not be able to model any filtration / oil removal system in HYSYS.

Hope this helps.

Regards,
Ankur.

[samrat]

Ankur Srivastavaji,
Thanks for your quick guidance. I am further elaborating based on your answers.
1. There is a suction knock out. As a matter of fact there is also a pre-cooler in the suction.
The flare gas enters at 60Deg C and gets cooled to 45Deg C. The pre-cooler is a shell
tube design.
2. After the OTS there is also a Coalser filer
3. The gas is cooled at discharge with Air X-changer
4. There is also KOD at discharge after the Air-X-changer.
5. Believe the Wet Screw characteristics in a simulation is the same as Positive
Displacement Recip.
6. The Lubrizol Oil Grade is CP-4601-100 and the rate of injection is 169LPM. The Oil
feed is thermostatically controlled at 60Deg C.
7. At the drain of Coalser OR Secondary Oil Separator there is "RO", size: 0.125", which
takes oil back to the suction of the compressor.
8. At the suction after the KOD, there is also a suction filter to remove any impurities. The
Pressure drop is insignificant.
My question still are:
1. How do I configure the Dry Gas to Wet Gas composition?
2. For the given case with all descriptions, Suction KOD liquid drop out composition?
3. The discharge Gas stream compostion?
4. The discharge KOD liquid gas stream composition?
5. The dew points of the constituents at Suction KOD and Discharge KOD?
6. Would you please elaborate about Tools--->Utilities?
Thank you very much for your active participation.
Sincerely,
samrat

[ankur2061]

Samrat,

Answers provided as per the numbered questions

1. Refer the link provided for the wet to dry basis and vice versa converter:

http://www.epa.gov/a...le2/dry/dry.htm

2. Liquid dropout from suction KOD will depend on the design of the KOD including internal such as mist eliminator which prevent entrainment of the liquid in the outlet gas. KOD sizing calculations can be done based on the Souders-Brown equation. Refer the links below:
http://en.citizendiu...-Brown_equation

http://chemsof.com/s...l_Separator.xls

3. The discharge stream composition will be the same as the stream composition of the gas leaving the suction KOD plus the injected oil in the screw compressor. .

4. Again I am assuming that the air cooled HE is upstream of the discharge KOD. When the cooled gas from the ACHE enters the discharge KOD and expands there will be liquid dropout. You can model this as a separator vessel in HYSYS to give you the vapor and liquid composition from the vessel.

5. HYSYS has the following tabs:
Tools---->Utilities---->Envelope Utility---->Performance----->Dew Point

After selecting the stream in the "envelope utility" you can then view the dewpoint as a plot or in tabular form.

6. Refer 5 above.

Regards,
Ankur.

[samrat]

Dear Ankurji,
Thanks for your response. I am also commenting based on the above points.
1. I did not understand even after going to the particular site. for conversions from dry to
wet base. I would be obliged if you can give me the wet base gas analysis.
2. The KODs have been sized based on experiences gained so far and both the KODs
have De-mister pads at appropriate heights based on the inlet nozzle. However I thank
you for the sites you have furnished, which refines further.
3. The comments are the same as point no 2 above.
4. The discharge KOD is positioned after the A/C air exchanger
5. Thank you for the path given.
6. I am attaching a pdf file of P&ID for your kind reference.
Sincerely,
samrat

Attached Files

•  PFD.pdf   173.01KB   125 downloads

[samrat]

Dear Ankurji,
If I enter dry base composition in Hysys, but the gas is saturated at the suction temperature, then where in Hysys would I play around to get the gas out put as WET. Alternatively, where can I insert the Relative Humidity in Hysys Program??
samrat

[ankur2061]

samrat,

In HYSYS v7.3 (the one which I have) you can use the unit operation option in flowsheet to get the "Saturate with water" operation.

The path is as follows:

Flowsheet----->Add Operation----->Extensions (select radio button)------>"Saturate with water------>Add

Alternatively in the unit operations pallette that pops when you enter flowsheet environment, there is the symbol showing "Extensions" which when you click gives some HYSYS unit operations extensions including "Saturate with water".

Use the help file of HYSYS to understand how the operation "saturate with water" should be used.

Regards,
Ankur.

[samrat]

Dear Ankur,
I tried to check but unable to find "Saturate with water" in any tab. I am enclosing the relevant word documents for clarity.
samrat

Attached Files

•  AllUnitOperations.doc   108.5KB   49 downloads
•  Extentions.doc   104KB   49 downloads

[ankur2061]

Samrat,

Yes I can see that in your version the "Extensions" does not have "saturate with water". Ask your local AspenTECH support office (in India they are located in Pune) for providing these extensions. This may require paying addtional license fees. The "saturate with water" extension is very useful for upstream oil and gas where you are modeling a natural gas stream and do not know the exact composition of the gas in terms of it's water content.

Regards,
Ankur.

[samrat]

Thanks Ankur for your support. I will do the needful.
samrat

[S.AHMAD]

1. Use Dalton's Law and Roult's Law to get the mole fraction of water vapor in the gas stream. Then normalized the hydrocarbon composition to get the wet basis.

mole fraction of water vapor = vapor pressure/total pressure

From steam table at 45C, the vapor pressure of water is 0.095 bara. The suction pressure is 0.085 kg/cm2 or 0.083 barg or 1.096 bara. This gives water vapor mole fraction of 0.095/1.096 = 0.0867.

The dry gas mole fraction = 1 - 0.0867 = 0.9133. Normalized the wet gas composition.

2. There are wo dew points namely water vapor and hydrocarbon gas dew points. I believe you are interested on the water dew point.

The suction water vapor dew point is 45C since the gas is saturated with water vapor.

For the discharge, you can apply the same laws.

Partial pressure water = 0.0867 x (6.86 + 1.01325) = .0.68 bara = vapor pressurre of water

From steam table, determine the saturated temperature which is 89C.

3. If the suction pressure is maintained at 0.085 kg/cm2g and temperature is kept above 45C, no water condensation.

4. Comments is the same as 3 above. Maintain temperature above 45C. For Hydrocarbon dew point, use HYSIS or do a flash calculation. Hydrocarbon dew point is lower than water dew point. However, use partial pressure of hydrocarbon as the total pressure instead the system pressure of 0.085 kg/cm2.

Edited by S.AHMAD, 25 January 2012 - 03:25 AM.

[S.AHMAD]

Continuation from post#12.

For those who has HYSIS or similar software, you can determine the wet gas composition by mixing two streams of dry gas and water vapor. Do a flash calculation at the pressure and temperature. Ensure that the water vapor stream is enough such that water is drained from the flash drum. The composition of vapor from the flash operation is the wet gas composition.

Edited by S.AHMAD, 25 January 2012 - 03:49 AM.

[samrat]

Dear S.Ahmed,
Thanks for the explanation. For the above gas composition given as dry base, I had done similiar calculation and got water vapor in wet gas as 8.28%. I am attaching for your kind info. Thanks once again for the efforts in explaining.
samrat

Attached Files

•  GAS CALCUALTION Site..pdf   9.83KB   186 downloads