compressed biogas expansion temperature drop heating

[askerix]

I have been given the task of making an installation expanding biogas(97% CH4) from 200 down to 4 bar.

Is it sufficient to calculate the temperature drop with the standard isentropic equation V1/V2=(P1/P2)^k-1/k?

I´m in doubt because I once heard that CNG(ca 94%CH4) drops approximately 1 kelvin pr. 2 bar pressure drop, witch would give me 93 kelvin drop in temperature, but I have calculated 140K with the standard equation -this is way to much difference that I can let it pass.

I guess the regulator will raise the temperature because of friction, but how can I calculate this in my equation? -I haven´t selected regulator yet ..

And:
Obviously heating is needed because of the temperature drop. My idea is to use a heat exchanger using R-134 in countercurrent.
In winter time the surrounding temperature can be as low as -30 degrees celsius(243K), witch could represent a problem.
I need about 7kW to heat the gas. Anyone have a good/better solution for heating?

 

[zekeman]

Generally speaking you can't use adiabatic isentropic expansion, unless you know that reversible work is performed.

State how you expand the gas and whether the process is adiabatic or not.
If it is throttled then no work is done and it is not reversible.

If this is homework, we can not answer it.

 

[MrBTU]

We've sold many of these heaters to the gas industry, and we use the rule of thumb of 1 deg F per atmosphere drop. They use a glycol loop with a boiler, pump, and head tank at the letdown station.

 

[askerix]

The pressure regulator works like most pressure regulators, where the fluid is doing work on a spring.

Why do my general isentropic equations differ with 40K from "the rule of thumb"?
Do I have to use CFD-analysis of each regulator to get the exact temperature drop?


However, thank you for your answers. I have now heard from different sources that 1 deg F per atmosphere drop is "good enough."

Do you know a link to any manufacturers of these heat exchangers?

 

[zekeman]

quote"The pressure regulator works like most pressure regulators, where the fluid is doing work on a spring."

What work?

 

[zekeman]

quote"We've sold many of these heaters to the gas industry, and we use the rule of thumb of 1 deg F per atmosphere drop. They use a glycol loop with a boiler, pump, and head tank at the letdown station."

Do you really think someone should use this socalled "rule of thumb" to offer management a "solution". Where is the empirical data and for which gases,what pressures, etc,etc.

My rule of thumb is to be skeptical of any rule of thumb.

I have no personal experience in this but my sense is that there must be some rational basis for this remarkable result, starting with the energy equation for a flow process.

 

[dcasto]

Yes, the rule of thumb is correct to use as long as it's the correct one. The 1 degree per 15 psi is very conservative and since you cannot predict to the neasrest .0001 moles per hour let alone the nearest 10% of the flow, the resulting equipment design based on the rule of thumb works well.

I'll give this caveat, if its a BCFD of gas, better get out the process simulator to get that last BTU required.

before you continue on you'd better tell us the remaining componets, like CO2 and water content. You will form solids if you do not watch those carefully.

 

[askerix]

It varies a bit, but I guess it is appropriate to assume that there is 2% Co2 and 1%N2. No water, very small amounts of ammonia and CO.

the flow is quite small; about 105nm3/h. nonemetric it´s like 66 scfh ?

 

[dcasto]

well, NIST says there will be a 91 K drop in temperature and CO2 will not freeze out at that low of a concentration. If the CO2 goes above 10 of 15%, you could form solids.