Dew P Flash using PR EOS

Dew P flash calculation determine dew point pressure (P) and liquid mol fraction (Xi) for a mixture at given temperature (T) and vapor mol fraction (Yi). These calculations can be performed in excel spreadsheet using Peng Robinson Equation of State (PR EOS).

Estimate pressure P and liquid mol fraction (Xi). P can be estimated as following –

P = 1/ Σ Yi/ Pisat
Pisat = exp[ ln(Pc) + ln(10)(7/3)(1 + ω )(1-T/Tc)]

where Pc, Tc and ω are critical constants for a component i. Liquid mol fraction is estimated as following

Xi = Yi/Ki
Ki = exp[ ln(Pc/P) + ln(10)(7/3)(1 + ω )(1-T/Tc)]

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First iteration starts with estimated P and Xi. Parameters for Peng Robinson EOS are calculated for each component i.

κi = 0.37464 + 1.54226ω - 0.26992ω²
αi = [ 1 + κi (1 - (T/Tc)0.5)]²
ai = 0.45724 (RTc)²α / Pc
bi = 0.07780 RTc / Pc

Mixture parameters are calculated next

aij = [(ai.aj)0.5(1 - kij)] = aji
a = ΣiΣj aij.Yi.Yj
b = Σi bi.Yi
A = aP/(RT)²
B = bP/RT

where, kij’s are Binary Interaction Parameter available from literature.
Following cubic equation is solved to get ZV.

Z³ + (B-1)Z² + (A-3B² -2B)Z + (B³+B²-AB) = 0

Above equation can be written as

Z³ + C2.Z² + C1.Z + C0 = 0

Solving Cubic Equation

Cubic equation is solved using following procedure. Calculate Q1, P1 & D.

Q1 = C2.C1/6 - C0/2 - C2³/27
P1 = C2²/9 - C1/3
D = Q1² - P1³

If D >= 0, then equation has only one real root provided by

Z1 = (Q1 + D0.5)1/3 + (Q1 - D0.5)1/3 - C2/3

If D < 0, then equation has 3 real roots, following parameters are calculated

t1 = Q1² / P1³
t2 = (1 - t1)0.5 / t10.5. Q1/abs(Q1)
θ = atan(t2)

Roots are calculated as following –

Z0 = 2.P10.5.cos(θ/3) - C2/3
Z1 = 2.P10.5.cos((θ + 2*Π)/3) - C2/3
Z2 = 2.P10.5.cos((θ + 4*Π)/3) - C2/3

Roots thus calculated are arranged in descending order, highest root gives ZV and lowest root gives ZL.

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Fugacity

Based on ZV, vapor fugacity φiV is calculated for each component.

As a next step, Liquid phase fugacity is calculated. Mixture properties are estimated as following –

a = ΣiΣj aij.Xi.Xj
b = Σi bi.Xi
A = aP/(RT)²
B = bP/RT

Cubic equation is solved as per method shown above to get ZL.

Z³ + (B-1)Z² + (A-3B² -2B)Z + (B³+B²-AB) = 0

Based on ZL, liquid fugacity φiL is calculated for each component.

Liquid phase mol fraction is calculated as

Xi = Yi.φiViL

New values of Xi thus calculated are again used to estimate φiL and thereafter Xi. This iteration is repeated till there is no further change in Xi values. Typically, in 25 iterations change in Xi values become negligible.

At the end of iteration ΣXi is calculated, if it is close to 1, results are obtained. If not, new value of P is estimated such that ΣXi is close to 1. In excel it can be achieved by using GOAL SEEK function, in which P value is changed to make summation equal to 1.

Note

For some initial values of Pressure, Xi become equal to Yi and summation ΣXi becomes 1, it happens when initial guess for P falls in critical region. For such cases use lower value of pressure such that summation is not equal to 1 and then use Excel GOAL SEEK function to estimate Dew Point Pressure and liquid mol fractions Xi.