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I've been referring the book: Chemical Process Principles: Part-II Thermodynamics. I found a table that gives me fugacity coefficients in a tabular form corresponding to various reduced pressures and reduced temperatures. This is it:

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I dont know what the $D_b$ and $D_a$ in the table. In the bottom of the table, I see:

$$\left(\frac{f}{p}\right)^{'} = \left(\frac{f}{p}\right)(10^{D({z_c}-0.27)})$$

Is the $D$ in this equation related to the $D_b$ and $D_a$ in the table?

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    $\begingroup$ Have you checked the text in the book? $\endgroup$ – Chet Miller Oct 5 '19 at 15:47
  • $\begingroup$ I wasn't sure what $D$ stands for. But, I assume $a$ and $b$ are van der Waal coefficients in real gas equation. $\endgroup$ – Mathew Mahindaratne Oct 5 '19 at 16:49
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    $\begingroup$ See page 574 in your book. $\endgroup$ – Chet Miller Oct 6 '19 at 18:27
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$D_b$ and $D_a$ are deviation terms which must be used in the place of $D$ to obtain the fugacity coefficients for fluids whose critical compressibility factor is not 0.27. Replace $D$ with $D_a$ if $z_c > 0.27$ and with $D_b$ if $z_c < 0.27$.

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