Vapour-liquid equilibria in the ternary mixture N2-CH4-CO2 and the quaternary mixture N2-CH4-C2H6-C3H8

Vapour-liquid equilibria in the ternary mixture Nz-CH4-CO2 and the quaternary mixture N,.-CH4-Cz He-C3 H8 G. Trappehl and H. Knapp Institute of Thermodynamics and Plant Design, Technical University of Berlin-West, Strasse des 17 Juni 135, D-1000 Berlin 12, FRG

Received 28 December 1987 A systematic study was performed with mixtures consisting of N2, CHa, C2Hs, C3Hs and

CO2 to investigate experimentally phase equilibria and caloric properties and to test the accuracy of thermodynamic correlations. This Paper (the third in a series) reports new results of T-p-x-y measurements on ternary and quaternary systems in the range 20 < p < 120 bar and 200 < T < 220 K. The results are compared with data calculated by generalized equations of state.

Keywords" phase equilibria; hydrocarbons; low temperature studies

In production plants of the chemical, oil or gas industry often multicomponent mixtures have to be separated into pure components or fractions of specified composition. Design ofseparation processes requires information about the distribution factors, Kl, for the components in coexisting phases. For high pressure liquid-vapour phase equilibria equations of state can be used to calculate fugacity coefficients and K values K i = Y~ = ~__~

Acceptable accuracy can be achieved with binary parameters, ko that were previouslyfitted to experimental binary vapour-liquid equilibria (VLE) data. In binary mixtures 1I, 22 and 12, interaction must be considered; the mixing effect (I1 + 2 2 - 2 x 12) can be determined by experiment and be indicated by excess functions g E = h E - T s E or by binary interaction parameters, ko, in mixing and combination rules. In ternary mixtures in addition to the binary interactions 11, 22, 33, 12, 13 and 23, ternary interactions 111, 222, 333, 112, 113, 223, 122, 133, 233 and 123 should be considered. The more components contained in a mixture, the more possible combinations exist. It is also interesting to count the number of combinations of r components that are possible if n different components are available (see Table 1). It is obvious from the table that the number of possible polynary combinations that could be assembled with a higher number of substances increases astronomically. We can investigate the conditions in a ternary mixture of specific interest. However, we can not study systematically all possible polynary mixtures of a pool that provides, for example, only 50 different substances not even if we limit our interest to ternary mixtures. -

0011-2275/89/010042-09 $03.00 © 1989 Butterworth & Co (Publishers) Ltd

42

Cryogenics 1989 Vol 29 January

"" Especially if we work with generalized process calculation methods we have to restrict the list ofempirical interaction parameters to binary combinations. It might be a justification or consolation if we learn from experts of molecular and statistical thermodynamics that the effect of a third neighbour on the interaction energy bctw~n two close neighbours is less than 15 % of the undisturbed binary interaction energy. After having admitted our practical - but also our theoretical - limitation, we can at least compare experimental data of polynary mixtures with calculations that are based on binary parameters. Several ternary and quaternary mixtures were investigated in a phase equilibrium cell and in a calorimeter. The equilibrium apparatus has been explained in the first part of this paper (see Reference 1). The results of this work are presented in the Appendix.

Experimental

programme

The ternary system N2--CH4--CO2 and the quaternary system N2-CH4-C:Hr--C3H 8 were investigated in high 1 Combinations possible with components

Table

various numbers of

Number of components, n

Binary Ternary Quaternary

3

10

50

100

6 10 -

55 220 715

1275 22100 292 825

5050 171 700 4 421 275

Vapour-liquid equilibria: G. Trappehl and H. Knapp Experimental VLE data for systems containing N 2, CH 4, C 2He, C3 H8 or CO 2 - review of published data and range of this work's data

Table 2

Train- Tmax

Pmin-Pmax

System

(K)

(bar)

Experimental points

Source

N2-CH4-CO 2

270.00 233.15-273.15 220.00-233.1 5 220.00

45.596-111.458 60.800-101.330 60.800-121.590 20.0-120.0

48 53 30 56

Somait et aL 2 Sarashina et aL 3 AI-Sahhaf et al 4 This work a

35

This work b

no data could be found in literature

N2-CH4-C2H6-C3H8 200.00

20.0-120.0

aSee Tables A I - A 4 (in the Appendix) and Figures 1 and 2 bSee Tables A 5 - A 8 (in the Appendix) and Figures 3 and 4

pressure, low temperature phase equilibrium apparatus. A review of data published by other workers and of this work's operating range is given in Table 2.

process calculations. The procedures used for the calculation of the deviation in Dp/p, Dy, DK/K and D f / f are explained in Reference 1.

Comparison: data

Summary

experimental

and c o r r e l a t e d

As mentioned above there is merit in presenting the deviation between vapour pressures, K values, vapour concentrations determined in the experiment and those calculated with popular generalized equations of state. The engineer responsible for process design can then judge the accuracy that can be expected in multicomponent 0.0

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This paper concludes a series of measurements of T, p, x and y data in two-, three- and four-component mixtures at low temperatures and high pressures. To test the accuracy of recommended correlations, ternary and quaternary VLE were calculated with generalized equations of state that refer to binary parameters fitted previously to binary VLE data. The inaccuracies do not

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Cryogenics 1989 Vol 29 January

43

Vapour-liquid equilibria: G. Trappehland H. Knapp C3H8

CH4

7 C2H6

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Figure 3 N2-CH4-C2Hs-C3H8systemat 200 K and 20 bar NZ

Figure 4

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Nz-CH4-C2Hs-C3H s system at 200 K and 80 bar

increase drastically, only near critical conditions where generalized equations of state also often fail in binary mixtures.

References Acknowledgements The authors are grateful to the Deutsche Forschungsgemeinschaft for financial support. The experimental work was carried out with the assistance of J. yon Dabrowski (mechanic) and C. Eichenauer (technician).

44

Cryogenics 1989 Vol 29 January

1 Trappehl,G. and Knapp, H. Cryogenics (1987) 27 696-716 2 Sonutit,F.A. and Kidnay,AJ. J Chem Eng Data (1978)23 301-305 3 $arashina, IF..,Arai, Y. and Saito, S. J. Chem Eng Japan (1974) 7 421--425 4 Al-Sahhaf,T.A.,Kidnay,AJ. and $1mln, E.D. Ind Eng Chem Fandam (1983) 22 372-380

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