3 replies to this topic

[JAVABOY]

Hi friends;
Basically i am electrical engineer. But from last three months i work on to develop the Gas properties simple calculator. I am happy i solved many things after reading from the net or help by some friends.
But now i am stucked and want a help on the following subject.
I have following data
Mole %age of each gas in our natural gas fuel. LIke Methene 87.1446, I butane=0.0712 and other gases.
I calculate sucessfully LHV,HHV, Specifiq gravity and density as well by using combustion constants chart.
But when i visit a person he advised me it is little bit wrong because u not correct the values by using Z.
He display one paper in which i saw that if LHV is 820.47 BTU/SCf after correction it becomes 818.2 BTU/SCF
So he said me use pressure coreection factor and Z for each gas. I think i used the combustion constant values which are at 14.73 PSIA. So i sum it and multiply by the delta which is= 14.65/14.7
so i correct LHV at that value but still i have error becuse of correction of Z. Then he told me if you do not know about how u calculate then use 1.0019 its almost same for natural gas.He also not knowing how to calculate and correct the final HHV, LHV and SG values.
Now i want that how i calculate the 1/Z of each gas i only knowing the mol% of each gas which contains in our natural gas?
I study some material and do not know how i apply z=Vm P/RT
How i calculate Vm for mathene,Ibutane,Hexane,P-Butane,I-pentane and so on.
Thanks advance for help me

Regards

[kkala]

The queries arise because you have to calculate gas heating value per unit volume. Following may be useful.
1. Ideal gas law states that pV=nRT (see http://en.wikipedia....i/Ideal_gas_law), where n=number of kgmoles (kg of gas/ MW). For real gases the compressibility factor z is inserted, so pV=nzRT. For n=1 the equation becomes pVm=zRT, where Vm=volume of 1 kgmole (also called molar volume) of the gas. Knowing z and conditions, you can calculate Vm=zRT/p, also gas density=MW/Vm (MW=molecular weight of the gas). Use wikipedia to find universal gas constant R, if not known, pay special attention to units.
2. Pressure correction looks unclear to me. Reference gas volumes usually are Nm3, or std m3, based on atmospheric pressure = 14.6959 psia ~ 14.7 psia (14.73 psia??). A pressure of 14.65 psia (??) is also mentioned. You are interested in finding precise values, I see. Density calculated as above could cover pressure correction.
Probably a way out could be as below.
(α). Find heating values of each gas based on 1 kgmol or unit mass from Tables. For instant, look at Perry's "Chemical Engineers' handbook", 7th edition (McGraw-Hill, 1997), Chapter 2, Table 2-221, "Heats of Combustion"; or at http://en.wikipedia....t_of_combustion (limited data); or at http://www.engineeri...lues-d_420.html.
(β) Convert above heating values per unit mass of gas, if needed. E.g. CH4 has a LHV of 802600 kJ/kgmol and MW=16.043, so LHV=802600/16.043 kJ/kg = 50028 kJ/kg.
(γ) Find gas density at desired conditions as above (e.g. in kg/m3).
(δ) Heating value of gas per unit volume will be (β)*(γ).
There may be some difficulty in unit conversions, but you have the feeling of magnitude of calculated values. Also look at http://www.cheresour...7710#entry47710, indicating attention to latent heats and some more data.
In a gas mixture (like Natural gas) you can consider each gas at total gas pressure and add their volumes to find total gas volume (Amagat's low).
3. Although you can find the compressibility factor z of a gas (at the conditions you want) in Perry and other sources (knowing its critical temperature & pressure), better to go to http://www.cheresour...h__1#entry49288 and use the spreadsheet proposed by breizh (z factor & specific gravity real gas.xls) in the post dated 27 Jun 2011. This spreadsheet is very convenient; and I think more precise than a diagram. If a gas is missing (see sheet named "data", 1st in the spreadsheet), you can add it, along with its necessary data (critical conditions and accentric factor).For accentric factor look at http://en.wikipedia....Acentric_factor. It may be difficult to find it for a specific substance; you can first look into http://www.iupac.org...f/6108x1413.pdf, then use goggling for a specific substance.
4. Hope above is helpful. As a general law says, a little more precision requires a lot of extra effort.

Edited by kkala, 17 July 2011 - 04:08 AM.

[katmar]

You have expressed your heat values in the units BTU/SCF. An SCF (standard cubic foot) is actually NOT a unit of volume. It is a unit of mass or matter because it is corrected to a known pressure and temperature standard. If you know the pressure, temperature and molecular weight then you know how much mass (or matter) you have - so you are working on a mass basis and not a volumetric basis.

Your heat value should not change for changes in pressure if you express it in these terms. If you want to express the heat value on an ACF (Actual Cubic foot) basis then the density and compressibility might be relevant. You have not stated your actual pressure and temperature so it is not possible to say whether Z will influence things. Certainly if you are trying to correct between P = 14.65 psi and P = 14.73 psi there will be no difference in Z. If you use breizh's spreadsheet as recommended by kkala you will be able to prove this for yourself.

[S.AHMAD]

Dear JAVABOY

What your friend said about using z is correct. I most simulation software e.g. HYSIS, Equation of state (EOS) is being used to compute z. However, the method is tedious and requires iteration. Z is a function of pressure, temperature and composition. For example you can use Peng-Robinson EOS. You can refer to most Thermodynamic textbook for the method of computing z value for a mixture. However, for most engineering calculation, we do not bother to make the correction. If you are using it for custody transfer, then better to have that correction. The method I normally used is by using published data in molar basis (BTU/lb-mole of MJ/k-mole) for each component and sum the product of LHV or HHV with the mole fraction. Convert to SCF or NM3 basis by dividing with 379.48 scf/lb-mole or 22.414 Nm3/k-mol to arrive at the unit of BTU/SCF or MJ/Nm3. The SCF unit is refer to 60F and 1 atm. The Nm3 is refer to 0C and 1 atm. If you want to make the correction, then divide the figure with z.If the composition of gas is fairly consistent, I suggest you calculate the z once and use it again and again. The error will be very small. For SG,no correction required, but if you are calculating density then you need to incorporate z in the calculation. I hope this helps.