# What are D_b and D_a in the Generalized Fugacity Coefficients table?

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:

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?

• Have you checked the text in the book? – Chet Miller Oct 5 '19 at 15:47
• I wasn't sure what $D$ stands for. But, I assume $a$ and $b$ are van der Waal coefficients in real gas equation. – Mathew Mahindaratne Oct 5 '19 at 16:49
• See page 574 in your book. – Chet Miller Oct 6 '19 at 18:27

$$D_b$$ 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 $$z_c < 0.27$$.