May 12, 2009

Viscosity of Pure Gases at High Pressure

In the previous posts, I discussed about methods called Chapman & Enskog and also Yoon & Thodos. Both were applicable for finding out viscosities of pure gases at low pressures up to 5 atm. What about the viscosity at high pressure because gas properties change significantly with pressure and behaviour becomes more & more real deviating from ideal gas condition.

So in this part I will discuss the methods which are apllicable for gases at high pressure.

In such cases, mostly correlations are linked with reduced density i.e. based on the standard definition of reduced property is density at given condition / density at critical point.

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Also, you must remember that mostly the properties at high pressure are reported in terms of residual properties which is either consider as the differential value of property at given pressure and atmospheric pressure (vis1 - vis0) or as their ratio i.e. vis1/vis0. So in both the cases you must know vis0 at atm pressure, which you can find out in my previous articles Viscosities of Pure Gases at low pressure - Part-I and, Viscosities of Pure Gases at low pressure - Part-II.

The method is called Jossi, Stiel & Thodos method.

So for Non Polar Gas the equation is -

[(Vis1 - vis0)*zeta +1]^0.25 = 1.0230 + 0.23364 Dr + 0.58533 Dr^2 - 0.40758 Dr^3 + 0.0933 Dr^4

Dr = Reduced density with its usual definition i.e Dr = D / Dc
vis0 = already given in previous post as viscosity at atm pressure.
zeta is given by the following equation.

zeta = [Tc ^ (1/6)]/ [ M ^ 0.5 * Pc ^ (2/3) ]

Its valid for 0.1 < Dr < 3.0

Now for Polar Gases there are three parts depending on Dr value. The equations from Stiel & Thodos are -

Dr <= 0.1

(Vis - vis0) x zeta = 1.656 Dr^ 1.111

0.1< Dr <= 0.9

(vis - vis0) x zeta = 0.0607 * (9.045 Dr + 0.63)^ 1.739

0.9< Dr <= 2.6

log [4 - log {mod (vis-vis0)*zeta}] = 0.6439 - 0.1005 Dr - Delta

Where Delta is as below

Delta = 0 if 0.9 < Dr < 2.2
Delta = 0.000475 (Dr^3 -10.65)^2 if 2.2 < Dr < 2.6

Yes again the unit of viscosity here is micropoise.

List of other property estimation methods on this Blog.

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