416 Rules of Thumb for Chemical Engineers

Table 1Gibbs Free Energy of Formation of Gas, DGf = A + BT + CT2

Properties 417

418 Rules of Thumb for Chemical Engineers

Properties 419

420 Rules of Thumb for Chemical Engineers

Properties 421

422 Rules of Thumb for Chemical Engineers

Properties 423

(table continued)

424 Rules of Thumb for Chemical Engineers

DGf,CH2O = -115.972 + 1.663*10-2(600)+1.138*10-5(6002) = -101.9

DGf,CH2O = -241.74 + 4.174*10-2(600)+7.428*10-6(6002) = -214.02

DGf,CH4O = -201.860 + 1.254*10-1(600)+2.035*10-5(6002) = -119.28

DGf,O2 = 0

The change in Gibbs free energy for the reaction isdetermined from Equation 2 and the Gibbs free energy offormation for the products and reactants:

DGr = DGf,H2O + DGf,H2O - DGf,CH4O - 0.5*DGf,O2

= -101.9 + (-214.02) - (-119.28) - 0= -196.64kjoule/g-mol

Repeating the calculations at the other temperaturesgives these results:

T, °K 600 700 800 900 1,000Gr, kJ/g-mol -197 -204 -210 -217 -224

Since the change in Gibbs free energy for the reactionis highly negative, the thermodynamics for the reactionat these temperatures are favorable (reaction promising).

Data Source. The correlation constants were determinedfrom a least-squares fit of data from the literature.2–13 Inmost cases, average deviations between calculated andreported data were less than 0.6kjoules/g-mol.

A copy (51/4 inch floppy disk) of a menu-driven com-puter program to calculate Gibbs free energy of forma-tion and change in Gibbs free energy for reactions(including random access data file of compound coeffi-cients) is available for a nominal fee. For details, contact:C. L. Yaws, Dept. of Chem. Eng. Lamar University, P.O.Box 10053, Beaumont, Texas 77710, USA.

Literature Cited

1. Yaws, C. L. and Chiang, P.-Y., Chemical Engineer-ing, Vol. 95, No. 13, Sept. 26, 1988, pp. 81–88.

2. Gallant, R. W., Physical Properties of Hydrocarbons,Vols. 1 and 2, Gulf Publishing Co., Houston, 1968and 1970.

3. Gallant, R. W., and Railey, J. M., Physical Propertiesof Hydrocarbons, Vol. 2, 2nd ed., Gulf PublishingCo., Houston, 1984.

NOTE: 1. Temperature range is 298–1,000K except for S, Br, and Icompounds.

2. Temperature range for S, Br, and I compounds is 298–717.2K, 332.6–1,000K, and 458.4–1,000K.

3. Additional compounds are: H2O(g) (A = -2.4174E+02, B =4.1740E-02, C = 7.4281E-06) And HCL(g) (A = -9.2209E+01, B = -1.1226E-02, C = 2.6966E-06).

(text continued from page 415)

The equation for any of 700 major organic compounds isgiven as temperature coefficients. Then the reaction canbe tested at various temperature levels beyond the stan-dard 298°K conditions imposed by many other data tab-ulations. Data for water and hydrogen chloride are alsoincluded.

Gibbs free energy of formation of ideal gas (DGf,kjoule/g-mol) is calculated from the tabulated coefficients(A, B, C) and the temperature (T, °K) using the follow-ing equation:

DGf = A + BT + CT2 (1)

Chemical equilibrium for a reaction is associated withthe change in Gibbs free energy (DGr) calculated asfollows:

DGr = DGf,products - DGf,reactants (2)

If the change in Gibbs free energy is negative, the ther-modynamics for the reaction are favorable. On the otherhand, if the change in Gibbs free energy is highly positive, the thermodynamics for the reaction are notfavorable and may be feasible only under special circumstances. Rough criteria for screening chemicalreactions are as follows:

DGr < 0kjoules/g-mol favorable0 < dGr < 50kjoules/g-mol possibly favorableDGr > 50kjoules/g-mol not favorable

Example. Calculate the change in Gibbs free energy for the reaction of methanol and oxygen to produceformaldehyde and water at reaction temperatures of 600,700, 800, 900, and 1,000°K:

Using correlation constants from Table 1 and Equation1 at temperature of 600°K, we obtain:

CH O g O g CH O g H O g4 2 2 20 5( ) + ( ) Æ ( ) + ( ).

Properties 425

10. Stull, D. R. and Prophet, H., Project Directors,JANAF Thermochemical Tables, 2nd ed., NSRDS-NBS 37, U.S. Govt. Printing Office, Washington,D.C.. 1971.

11. Weast, R. C., ed., CRC Handbook of Chemistry andPhysics, CRC Press, Boca Raton, Fla., 1985.

12. Yaws, C. L., Physical Properties, McGraw-Hill BookCo., New York, 1977.

13. Reid, R. C., et al., The Properties of Gases and Liquids,4th ed., McGraw-Hill Book Co., New York, 1987.

Source

Yaws, C. L. and Chiang, P., “Find Favorable ReactionsFaster,” Hydrocarbon Processing, November 1988, p.81.

4. Braker, W., and Mossman, A. L., Matheson Gas DataBook, 6th ed., Matheson, Lyndhurst, N.J., 1980.

5. Dean, John A., Handbook of Organic Chemistry,McGraw-Hill Book Co., New York, 1987.

6. Perry, R. H., ed., Perry’s Chemical Engineers’ Hand-book, 6th ed., McGraw-Hill Book Co., 1984.

7. Selected Values of Properties of Hydrocarbons andRelated Compounds, Vol. 7, TRC Tables (u–y), Ther-modynamic Research Center, Texas A&M Univ.,College Station, Texas, 1977 and 1987.

8. Selected Values of Properties of Chemical Com-pounds, Vol. 4, TRC Tables (u–y), Thermodynamic:Research Center, Texas A&M Univ., College Station,Texas, 1977 and 1987.

9. Stull, D. R., et al., The Chemical Thermodynamics ofOrganic Compounds, Wiley, New York, 1969.

New Refrigerants

New hydrofluorocarbons (HFCs) are replacing thechlorofluorocarbons (CFCs) and hydrochlorofluorocar-bons (HCFCs) phased out to lessen damage to the ozonelayer. The DuPont website lists physical properties for anumber of their refrigerants.Go to:

www.dupont.com/suva/412.html

These physical property tables give three vapor pressurepoints that can be connected on your favorite Cox typevapor pressure chart for a quick approximation of thewhole range. These three points are:

normal boiling pointcritical propertiesvapor pressures at 25°C

If more accurate vapor pressure data across the range areneeded, DuPont can supply them. I received full-rangevapor pressure data for many products on request fromtheir very helpful SUVA“ Refrigerants Division.

Source

E. I. DuPont deNemours and Company website, “Physi-cal properties and ASHRAE standard 34 classificationof SUVA“ refrigerants.”