Thermophysical Properties of Natural Gas-I |ChemFam #27|

Greetings to everyone! In my previous post we have discussed about the sources and a general process overview of natural gas. Today, we shall be discussing about properties of natural gas. To be more specific, we shall be discussing the thermophysical properties. As we know that the properties plays a very important role in any system because they determine the way a system would behave and with the help of these properties we can analyze a system as well as design any kind of process.

alt

What shall we learn?

We shall be discussing about the various types of thermophysical properties that includes gas specific gravity, pseudocritical properties. We shall also be looking into the estimation of these two types of properties.

Properties of Natural Gas

The properties of natural gas can be broadly categorised into 4 categories. These are thermophysical, thermodynamic, chemical and combustion properties. We shall limit our discussion to thermophysical properties only in this particular post. In the thermophysical properties of natural gas, we shall be discussing about gas specific gravity, compressibility factor, boiling point, freezing point, density and viscosity etc.

It has been found that the natural gas is colourless, odourless non toxic and lighter than air. Since natural gas contains several components, therefore its properties vary with composition of natural gas. The properties are estimated by using some appropriate mixing rules.

Usefulness of Some of The Natural Gas Properties

Let us see some significance of some properties -

Properties	Used to determine
Compressibility factor	Density, Viscosity, quantity and flow in wells
Density and viscosity	Flow in wells and pipelines, compressor power
Heat capacity and thermal conductivity	Temperature in pipelines, insulation thickness
Calorific value	Wobble index

Gas Specific Gravity

We already know by specific gravity that it represents a kind of a ratio between the density of a substance to the density of a standard substance. Generally we use water as standard, but for gases we use air as standard. So, the gas specific gravity in case of natural gas is defined as the ratio of gas density to the density if air at the same pressure and temperature.

Mathematically,

γ_g = ρ_gas/ρ_air = (M/Z)_gas / (M/Z)_air

Whenever we say that the ratio of the density of gas to density of air, we have to understand that these referred to some specific temperature and pressure. Here, we find that we are representing the gas specific gravity in terms of the compressibility factor. So, here the M represents molecular weight and Z represents the compressibility factor.

At standard conditions (at STP), we know the compressibility factor of gas and air are almost the same.

Mathematically,

Z_gas = Z_air. Thus,

γ_g = M_gas / M_air

Generally, we take the molecular weight of air (M_air) to be around 29 (28.96 to be more specific). Therefore, we can find the gas specific gravity.

The gas specific gravity helps us in identifying or knowing how the gas would disperse in case of leakage which has a direct bearing on the safety measures. Except methane all other hydrocarbons are denser than air. It is because the natural gas is primarily methane and that is why generally natural gas is lighter than air. On the contrary, looking at the LPG which is basically iso-butane and some amount of propane which are heavier than air. So, if there is any kind of gas leakage from the gas cylinder at our home then it will settle down rather than moving up.

In case of natural gas leakage, methane will get dispersed in the atmosphere while others will settle on the ground. So, it will move away from the point of leakage.

Reduced Properties

Next we come to another very important set of properties before we proceed to find out the other properties including reduced properties.

Now the question is why we need reduced properties?

Whenever we are talking of any kind of density or other properties, there are so different components and we have different combinations of temperature, pressure according to which we shall be having different property values. It becomes a mind boggling affair to archive or to put all these to develop any kind of database. One way to ease out the representation of the properties is through some reduced properties.

These reduced properties represent state variables of a fluid tat are normalised using their values at the critical points of the fluid. Now if you don’t know what state variable is! Let me refresh it for you. State variables are those variables which represent the state of a system such as pressure, temperature, volume, entropy etc. Therefore, we shall be having reduced temperature, reduced pressure, reduced volume and so on as reduced properties.

Now, let us come to the general definition of reduced pressure and reduced temperature.

Mathematically these two are expressed as,

P_r = P/P_c, T_r = T/T_c

Where P_r and T_r are the reduced pressure and reduced temperature respectively and P_c and T_c are the critical pressure and critical temperature respectively.

Pseudo-reduced Properties

Now, when we talk of a mixture of gases like natural gas, we do not have a single critical value of any of the properties as it has several gases in it. In that case, we have to figure out that how to represent the reduced values to have the overall critical values.

Pseudo means that it is not actual. These pseudo-reduced properties are defined in similar manner as reduced properties for the pure components. These represent state variables of a fluid mixture that are normalised using their values at the critical points of the fluid mixture. Similar to the reduced properties, we have pseudo-reduced temperature, pseudo-reduced pressure, pseudo-reduced volume and so on.

Mathematically,

P_pr = p/p_c and T_pr = T/ T_pc

Where, p_c and T_pc are the pseudocritical pressure and pseudocritical temperature. P_pr And T_pr are the pseudoreduced pressure and pseudoreduced temperature.

Now, there are various types of mixing rule and very common mixing rule is given by ‘Kay’ and the pseudocritical temperature and pressure are given by Kay’s mixing rule as,

T_pc = ∑ y_iT_ci and P_pc = ∑ y_iP_ci

Kay gave a mixing rule as the pseudocritical properties as some kind of weighted average with respect to the mole fraction. So, this pseudocritical temperature is equal to the summation of the product of the mole fraction and the critical temperature of each of the components. Similarly, the pseudocritical pressure is equal to the summation of the product of the mole fraction and the critical pressure of each of the components.

Principle of Corresponding States

There is a very important principle which says that at the same reduced temperature and reduced pressure, all properties become the same irrespective of composition. This way it makes our life easier to maintain the database.

Pseudocritical Properties from Specific Gravity

If the gas composition is not given but gas specific gravity (gamma;_g) is known, then th pseudocritical properties may be estimated from semi-empirical correlations of Rojey and Jaffret.

P_pc[MPa] = 4.892 - 0.405γ_g,

T_pc[K] = 94.72 + 170.75γ_g

Sutton gave correlations which may be used for 0.57<γ_g<1.68

P_pc[psia] = 756.8 - 131γ_g - 3.6γ_g²

T_pc[R] = 169.2 + 349.5γ_g- 74.0γ_g²

B I B L I O G R A P H Y

Advanced Natural Gas Engineering

Handbook of Natural Gas Transmission and Processing

Fundamentals of Natural Gas Processing

Read my previous Blogs:

Sources and Process Overview of Natural Gas |ChemFam #26|

Recovery, Upgradation and Purification of Helium in Natural Gas |ChemFam #25|