How to add natural gas or X gas to another one and have properties?

[Saver2008]

Hi everybody!!

I have a certain acid gas and I know its composition and I want to add it another gas (natural gas) and also I know its composition. The reason that I want to add this natural gas is for giving the acid gas more heat liberation in order it can be burned. I have heard that with a software like HYSIS is possible to do that but I do not have this software. Does it exist a manual method that I can use with EXCEL in order to solve that?

Thanks!!!
R

 

[TD2K]

Take both streams and their compositions in either a mass or mol basis. Using the heating value of the components, calculate the individual components contribution. Heating values are available from GPSA data book and many other sources.

Let's say you are blending equal volumes of methane and nitrogen. The LHV of methane is about 911 BTU/scf. The LHV of nitrogen is 0.

So, 0.5 scf * 911 BTU/scf + 0.5 scf * 0 BTU/scf = 455.5 BTU/scf. Just extend the concept to your number of components.

You can use HHV or LHV depending what you need. The same approach can be used on a mass basis but using BTU/lb for each component.

 

[apetri]

The procedure is very simple, just sum up the heating values of all components multiplied by relative mass fraction of the component in mixture.
This is very easy in Excel once you have collected the properties (heating values) for the different components in the mixture, GPSA is a good source, you may consider also engineeringtoolbox.com and other sources.
Finally if you prefer a ready to use solution there are Excel add-ons (see for example Prode Properties) which can do the work for you, with these tools you simply write a macro in Excel's cell and the add-on calculates the required values, the advantage being the possibility to calculate a large number of properties (transport properties, phase equilibria etc.) which may be useful in your work.

 

[Saver2008]

Dear TD2K and apetri.
Thank you so much. But theres something I do not get it. Taking your example, suppose that I have 0.5 scf of a gas that has 0.6 % of CH4 and another gas that has 0.6 scf of it and 0.2% de CH4.

0.5 scf x 0.6 x 911 btu/scf + 0.6 scf x 0.2 x 911 btu/scf = 382.62 btu/scf btu/scf?

That`s correct? and also taking that example, suppose we have this composition: 0.6 CH4 + 0.2 CH4 = 0.8 CH4...is that correct?

Thank you very much!!!!

 

[TD2K]

If you have 0.5 scf of a gas containing 0.6% (we'll assume vol%) of methane, then you have 0.5 * 0.006 = 0.003 scf of methane. You would do a similar calculation for the second gas.

Your example is correct if you change the 0.6% to 60%.

Remember that 1 scf is a certain number of moles and from the ideal gas law, it's the same number of moles regardless what type of gas it is.

 

[Saver2008]

@TD2K
Thank you. I have done it and then I sum all the components and gives me a datum very high.....I`m goint to write a little of this:

Acid Gas = 333,342.77 lb/hr
CH4 - 0.0028 HV= 21433.00 btu/lb

Natural Gas = 129,634.88 lb/hr
CH4 - 0.617 HV = 21433.00 btu/lb

For CH4 --> (333342.77 x 0.0028 x 21433) + (129634.88 x 0.617 x 21433.00) = 1,734,317,224 btu/hr-mol and when I sum all componentes gives me 4581411242 btu/hr-mol....it is too big but I do not know. did I make something wrong?

Thanks again!!!

 

[TD2K]

Assuming the 0.0028 and 0.617 are weight fractions, your first calculation is correct except for the units, it should be BTU/hr, there's no 'mol' in it.

 

[Saver2008]

@TD2K,

Thankyou, just one question. As acid gas has .0028 of CH4 and natural gas 0.617 of CH4, the mixed gas how much CH4 will have...0.6198? or for this case what shoul we do?

Thanks again.!!!

 

[TD2K]

You're making this a lot harder than it has to be. Think about what you are doing rather than just plugging numbers into a formula or I guarantee you that you'll use the wrong numbers at some point.

So, in this case, you have 333,342.77 lb/hr of acid gas with a methane weight fraction of 0.0028 or 933 lb/hr of methane. You also have 129,634.88 lb/hr of natural gas with a methane weight fraction of 0.617 or 79985 lb/hr of methane. Grand total methane 80918 lb/hr. Total acid gas and natural gas is 462978 lb/hr.

Methane weight fraction in the combined stream is therefore 0.1748.

 

[Saver2008]

@TD2K
Thank you for your support and for your comments. You are right in all ways. First I have to understand what I´m doing. But I jus have one question about your last post: How did you get the "0.1748" datum for Methane in the combined stream?

Thanks!!!
RS

 

[hacksaw]

the usual method is to use separate burners for acid gas and your supplemental fuel, but that is just the start

then you can adjust your supplemental fuel quickly to keep up with fluctuations in acid gas compositon

the same btu calcs apply