Joback method

The Joback method works quite well in methane substitutes for boiling points, except for the —H, the —F and —NO2 molecules see figure 5.14. However, the melting points are much less satisfactory, as they depend not only on the properties of the groups added, but also on the architecture and the shapes of the molecules. 

The compilations of CRC (1-2), Daubert and Danner (3), Dechema (15), TRC (13-14), Vargaftik (18), and Yaws (19-36) were used extensively for critical properties. Estimates of critical temperature, pressure, and volume were primarily based on the Joback method (10-12) and proprietary techniques of the author. Critical density was determined from dividing molecular weight by critical volume. Critical compressibility factor was ascertained from application of the gas law at the critical point. Estimates for acentric factor were primarily made by using the Antoine equation for vapor pressure (11-12). 

The compilations of CRC (1-2), Daubed and Danner (3), TRC (11-12), and Yaws (15-33) were used extensively. Estimates for enthalpy of vaporization at the boiling point were primarily based on the Kistiakowsky rule (9) and Riedel method (9). The Joback method (9) was used for estimating critical temperature. Estimates of liquid density were primarily based on Schroeder correlation (9, group contribution), Le Bas method (9, group contribution), and Tyn-Calus equation (9, critical volume). Liquid volume was ascedained from molecular weight and liquid density (molecular weight/density). 

Organic molecules The Ambrose method is recommended for all three critical properties of hydrocarbons and n-alcohols. The Joback method is recommended for r and of all other organic molecules. Fedors method is recommended for of these compounds, but the Joback method can also be used. 

Example Estimate the critical constants of o-xylene by using the Joback method. 

Input data Molecular structure and one measured vapor pressure value P)),eas, (often at a low pressure). The method requires estimation of and using a group contribution method. The original Pailhes method used the Lytrerson method [Lydersen, A. L., AlChE 21 (1975) 510], but the Joback method is illustrated here for consistency with the previous examples on critical constant estimation. 

The Joback method was modified in the MPBPVP program to include the same 85 groups used by the Stein and Brown [117] method for BP calculation. The MPBPVP program has a calculated MAE = 44 K for a set of 2045 compounds. 

Norma/Bo//wg Point—Free ng Point. One method s deviation for boiling and freezing point data is reported to be less than 2 K for 80% of a 600 compound set and less than 5 K for an additional 18% (201). However, this method is limited to organic compounds having only one functional group and a short base group such as methji, vinji, and phenji. In contrast, the Joback method (188) has much wider apphcabflity and a reported average error of 12.9 K. 

Considering the fact that the Rihani-Doraiswamy and Joback methods are applicable to a wide range of organic compounds and provide Cp as a continuous function of T (a very useful feature in reactor design), they are clearly the recommended methods. 

Although it is very simple, it can be regarded as the basis of other methods like the Joback method, where the ratio between the normal boiling point and critical point in the vicinity of 2/3 can be found as well. 

The Joback method needs only the structural formula as input information except for the estimation of the critical temperature, where additionally the normal boiling point is required. In case it is unknown, it can be estimated with the Joback method as well. The Joback relationships are... 

The average errors for the Joback method reported by Nannoolal [8] are slightly larger 1,41% for the critical temperature, 7.11% for the critical pressure, and 3.73% for the critical volume. A detailed review can be found in [7, 8]. 

In all cases, the statements for the average errors refer to molecules with more than three carbon atoms, as for simpler substances experimental data are usually available and estimations do not make sense. As well, the Joback method gives unreasonable results for large molecules with more than 20 C-atoms (7, 8],... 

Estimate the critical data of 7n-xylene using the Joback method (Figure 3.1). The normal boiling point is given to be Ty = 412.25 K. 

The normal boiling point is an easily accessible physical property and has been measured for a large number of substances. In case that it is not available, the normal boiling point can be estimated with group contribution methods, for example, Joback and Reid [4] and Constantinou and Gani [5], analogously to the estimation of the critical point. The estimation formula for the Joback method is... 

The experimental value for pentylcyclohexane is Tb = 476.75 K. The Joback method provides an excellent result in this case. The use of a reference substance is not a guarantee to obtain a more accurate result but helps to avoid large errors. 

Vi is the number of occurrences of group i in the molecule. The values for the particular group contributions are listed on www.ddbst.com. The results for the representation of the data sets are reported to be superior to those obtained with other methods [12]. An evaluation with more than 2000 substances yielded average deviations of 16.5 K for the Joback method and only 1.5 K for the Rarey/Nannoolal method, which demonstrates the superiority of the latter one. 

Estimate the ideal gas specific isobaric heat capacity of ethyl acetate (CH3-COO-CH2-CH3) at T = 298.15 K using the joback method. The molar mass of ethyl acetate is M = 88.11 g/mol. 

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