20 replies to this topic

[krishanan]

Dear All,

 

Greetings,

 

Before putting my queries i just want to explain the background for the same.

 

I have a natural gas coming from source at 40 kg/cm2(g) & 10 Deg.C. Methane is in the natural gas is 93%.

 

I have a pressure reducing station where i am reducing natural gas pressure from 40 kg/cm2(g) to 9.5 kg/cm2(g).

 

Also note that i am using this natural gas as a fuel for water tube boiler through already provided KOD

 

My queries are

 

1. what is the temperature of natural gas at 9.5 kg/cm2(g) i.e after pressure reduction?

 

2. Are there in spread sheets for calculating the above temperature (I know its a Isenthalpic process) or equation to be used to calculate the same?

 

3. If i want to maintain the same 10 Deg.c after pressure reduction may be i need to provide the heater (I am prefer to have only steam heater). In that case please help me out how to design the steam heater, it's very much appreciable if i have spreadsheet for the same?

 

I am desperately looking for your help in this regard.

 

Thanks in advance.

Krishna Ananthaneni

[Art Montemayor]

I have to assume you have no simulation program on which you can rely to obtain the answer for the resulting temperature after isenthalpic expansion.  I also assume that your calculation doesn't have to be accurate to the exact resulting temperature and that you need a practical approximation on which to size your steam heater.  (By the way, where are you going to obtain steam to heat the initial flow of natural gas fuel?)

 

Based on the above I also assume the fuel gas is essentially 100% methane for calculation purposes.  That being the case, refer to my attached workbook to see how I would estimate the resulting fuel gas temperature entering a steam heater.  For design purposes, I would use -10 oC as the fuel gas inlet temperature to be conservative.  Any excess heat input will be favorably recovered in part in the resulting combustion.

 

Refer to Kern's "Process Heat Transfer" for the method on how to design the gas heater.

 

 Free Expansion of Natural Gas.xlsx   29.95KB   8178 downloads

[PingPong]

As Art indicated, pure methane temperature would drop from 10 ^oC (50 ^oF) to -5 ^oC (23 ^oF) due to the indicated pressure reduction, and if one intends to install a heater downstream the pressure reduction, then one best sizes that heater for an inlet temperature of -10 ^oC, just to be sure.

 

I have the impression however that Krishna intends to install a heater upstream the pressure reduction, to assure a temperature of 10 ^oC downstream the pressure reduction. In that case the heater needs to warm the natural gas from 10 to about 30 ^oC. It does not require steam to do that, any sufficiently warm stream, even cooling water return, could do the trick. Note however that one needs to protect the heating system for possible tube leakage, or even tube burst, especially if the heater is installed upstream the pressure reduction at 40 kg/cm²(g).

 

It depends on the actual gas composition what the exact temperature drop will be.

 

Note that the temperature inside the pressure reduction valve will drop much more than 15 - 20 degrees, so if the natural gas would contain too much water vapor there could be a risk of hydrate formation.

[JAVilleg]

I have done this on ProII a 100% methane flow at 40 kg/cm2(g) & 10° C expands (on an expander) to an outet pressure of 9.5 kg/cm2(g), the result temperature is -74.806 °C which is way too low.

 

Now, using a valve, and setting the outlet pressure of 9.5 kg/cm2(g), the result temperature is -6.705 °C which is very similar to what Mr. Art Montemayor predicted.

Edited by JAVilleg, 28 July 2014 - 12:48 PM.

[Art Montemayor]

Jose:

 

The flow across a pressure control valve is very close to a free, irreversible adiabatic expansion - which makes it a an isenthalpic process.  This is why I resort to the thermo data at constant enthalpy.

 

However, when you are applying an expansion engine (a turbo expander, in your case) you are going through a reversible process (which is isentropic and much more efficient than an isenthalpic process).   The reversible isentropic process, although much more efficient,  is not related to the topic in this thread.  The isenthalpic process of pressure reduction is used in industry because it is more cost efficient and better controlled.  I doubt if you could justify a turbo expander on this application.

[Bobby Strain]

krishna,

       There are some decent process simulators available for free. You should get one or two. I don't use them, but I'm certain if you ask, you will get some good recommendations and advice.

 

Bobby

[krishanan]

Dear Art,

 

Thank you very much for your response.

 

Regarding to your assumptions

 

1. Yes, i don't have simulation program to work out the outlet temperature of gas.

 

2. I am looking for the accurate solution since i am executing in one of our project

 

3. Actual composition if natural gas is as enclosed here

 

 Natural gas analysis.xlsx   14.04KB   445 downloads

 

4. I have fuel oil & other fuel gases with that i will start the boiler & generate steam for heating the natural gas

 

Also can you please provide the scheme which shows auxiliaries/Accessories, Instruments to be provide for gas heater system for safety & reliable operation

 

 

I have one more clarification upon your excel sheet as follows

 

Based on your calculations at pressure 135 Psia i could see the phase of natural gas is still vapour. Now my query is

 

1. Do i need to provide heater to heat the gas

 

2. Any possibility of hydrate formation though H20 % is zero as per analysis

 

3. KOD at downstream of pressure control valve will not serve the purpose(Which was already there in scheme)

 

 

Rgds

krishna

Edited by krishanan, 29 July 2014 - 12:03 AM.

[gegio1960]

krishanan,

1) Both Art and Pingpong (and Bobby, too) have already given to you good hints.

2) You should apply your engineering knowledge to the given advices to obtain your appropriate solution.

3) No one will do your work for you but several experts in this forum can comment/improve your ideas.

4) Engineers always use approximations and margins... otherwise they aren't engineers.

5) Engineers should be as much practical as possible... if you have a lot of 0, you could/should avoid to list them.

6) If you want to have accurate results, you must use accurate tools (...and you have to pay for them).

[gegio1960]

after your edit ;-)

7) case 2 has an important amount of "heavy" components....

[krishanan]

Dear Mr. Gegio,

 

Thanks for your precious reply.

[PingPong]

Art's calculations were based on pure methane, as no composition was available at that time.

Obviously pure methane will not form liquid due to pressure reduction.

 

Natural gas 1 temperature will drop from 10 ^oC to - 8 ^oC after pressure reduction from 40 to 9.5 kg/cm²(g).

Natural gas 2 temperature will drop from 10 ^oC to - 11 ^oC after pressure reduction from 40 to 9.5 kg/cm²(g).

 

Natural gas 2 dewpoint will be - 30 ^oC at 9.5 kg/cm²(g).

 

In the throat (vena contacta) of the control valve the temperature will, in theory, be low enough to form cold liquid droplets, that revaporise further downstream when velocity decreases from sonic to normal and as a consequence temperature increases to -11 ^oC. However do not remove KOD.

 

When no water vapor is present there is no risk of hydrate formation.

 

What is the natural gas flowrate? Or boiler design duty?

Edited by PingPong, 29 July 2014 - 06:33 AM.

[krishanan]

Dear PingPong,

 

Thank you very much for your reply.

 

Natural gas we are sending to both our boilers(2 Nos) & client's HRSG GT. hence total natural gas requirement for boilers & clients HRSG is 46000 kg/hr. Particular to boiler(1 No.) natural gas flow rate required is 17000 kg/hr.

 

Based on your reply i understood that no need of providing gas heater.

 

I will also retain the KOD at downstream of pressure reducing station.

 

Rgds

krishna

[JAVilleg]

I have run once again a simulation on PRO/II, using now the composition provided (weight %), I chose the Peng Robinson EOS to this case, acording to the simulation, the results are as follows:

 

Case 1

Pressure drops from 40 kg/cm2 gauge to 9.5 kg/cm2 gauge

Temperature drops from 10 °C to -7.857 °C

No liquid reported

 

Case 2

Pressure drops from 40 kg/cm2 gauge to 9.5 kg/cm2 gauge

Temperature drops from 10 °C to -10.510 °C

No liquid reported

Edited by JAVilleg, 29 July 2014 - 01:07 PM.

[JAVilleg]

krishna,

       There are some decent process simulators available for free. You should get one or two. I don't use them, but I'm certain if you ask, you will get some good recommendations and advice.

 

Bobby

 

Could you please provide some links to download such free simulators? I am very interested on them, thank you.

[krishanan]

Dear J A Villeg,

 

Thank you very much for you reply.

[PingPong]

Based on your reply i understood that no need of providing gas heater.

Provided that the downstream piping and KOD (plus level instruments) can handle the low temperature. And provided that the C5+ fraction of the natural gas does not contain benzene, which would solidify.

 

Note also that if the same KOD is used for the fuel gas, that drum may contain water and/or benzene condensed from the fuel gas.

 

Moreover the pressure reduction control valve material shall be suitable for the much lower temperature inside due to sonic velocity (choked flow) which means that enthalpy drops to increase kinetic energy of the gas in the throat of the valve.

[PingPong]

Case 2

Pressure drops from 40 kg/cm2 gauge to 9.5 kg/cm2 gauge

Temperature drops from 10 °C to 4.071 °C

I also used PRO/II and Peng Robinson but find -11 ^oC for case 2.

Please verify your calculation.

 

Could you please provide some links to download such free simulators?

Just google DWSim

and ChemSep

[Bobby Strain]

And Prode, COCO, too.

 

Bobby

[JAVilleg]

 

Case 2

Pressure drops from 40 kg/cm2 gauge to 9.5 kg/cm2 gauge

Temperature drops from 10 °C to 4.071 °C

I also used PRO/II and Peng Robinson but find -11 ^oC for case 2.

Please verify your calculation.

 

 

Dear PingPong, you're right, I have verified the simulation, I made a mistake I activated the pressure drop button instead of the outlet pressure button, the result temperature por case 2 is -10.510 °C and not as implied before, I apologize for any inconvenience it may have caused, I shall correct my last post and be more careful next time.

 

 

Could you please provide some links to download such free simulators?

Just google DWSim

and ChemSep

 

Thank you so much , I will.

[krishanan]

Thank you Ping for your valuable precautions,

 

We are taking care of vessel(KOD & Flare KOD) & piping materials. We are using separate KOD system for Both Fuel gas & Natural gas, However we are combining both KOD outlets. Will this create any problems? Also note that both Natural gas & Fuel gas lines are steam traced right from battery limit. With this steam tracing do you think still problems occurs by combining both natural & Fuel gas?

Edited by krishanan, 29 July 2014 - 10:07 PM.

[PingPong]

Steam tracing will not warm up the natural gas noticably; it is only intended to prevent liquid condensation from the fuel gas due to heat loss to the ambient air.

 

Steam tracing the natural gas line may even result in freezing of the tracing tubing, especially at the pressure reduction valve which will be much colder than -11 ^oC (as I explained before).

 

Natural gas at -11 ^oC and 9.5 kg/cm²(g) will, in theory, form hydrates when water vapor (coming with the fuel gas) is added. It depends on the fuel gas temperature, composition and mix ratio of fuel gas and natural gas, what the mix temperature will be, and what the hydrate formation temperature will be.

Edited by PingPong, 30 July 2014 - 06:04 AM.