Computer simulations of vapor–liquid phase equilibria of n-alkanes

Abstract

For petrochemical applications knowledge of the critical properties of the n-alkanes is of interest even at temperatures where these molecules are thermally unstable. Computer simulations can determine the vapor-liquid coexistence curve of a large number of n-alkanes ranging from pentane ͑C 5 ͒ through octatetracontane ͑C 48 ͒. We have compared the predicted phase diagrams of various models with experimental data. Models which give nearly identical properties of liquid alkanes at standard conditions may have critical temperatures that differ by more than 100 K. A new n-alkane model has been developed by us that gives a good description of the phase behavior over a large temperature range. For modeling vapor-liquid coexistence a relatively simple united atom model was sufficient to obtain a very good agreement with experimental data; thus it appears not necessary to take the hydrogen atoms explicitly into account. The model developed in this work has been used to determine the critical properties of the long-chain alkanes for which experiments turned out to be difficult and contradictory. We found that for the long-chain alkanes ͑C 8 -C 48 ͒ the critical density decreases as a function of the carbon number. These simulations were made possible by the use of a recently developed simulation technique, which is a combination of the Gibbs-ensemble technique and the configurational-bias Monte Carlo method. Compared with the conventional Gibbs-ensemble technique, this method is several orders of magnitude more efficient for pentane and up to a hundred orders of magnitude for octatetracontane. This recent development makes it possible to perform routinely phase equilibrium calculations of complex molecules.

Applications of Molecular Simulation in Oil and Gas Production and Processing

“…located on the main atom of the group, the model is said "United Atoms" [14][15][16]. Alternatively, the force center may be located at an intermediate position between the atoms belonging to the group (Fig.…”

“…It must be stressed that alternate expressions can be used instead of Equations (9) and (10), which are just empirical correlations. For instance, the bending energy is often written u bend = 1/2 k bend (θ -θ o ) 2 [11,[14][15][16] and the main reason for preferring Equation (9) is that its evaluation from atomic coordinates takes less computer time [17,19].…”

“…to generate more frequently favorable configurations that have an increased probability to be accepted. These statistical bias methods are now standard in Monte Carlo simulation to handle either systems with preferred orientation [21], long flexible chains [14,22] or branched flexible molecules [23]. Indeed, it is a common problem when complex molecules or dense phases are considered that the acceptance probability of some Monte Carlo moves is very low.…”

Non-Equilibrium Molecular Dynamics Used to Obtain Soret Coefficients of Binary Hydrocarbon Mixtures

“…Those differences and deviations compared to the experimental data are possibly due to the different NEMD methodology employed and to the force field used to model the n-alkanes. While used the SKS force field (Smit et al, 1995) with the HeX (BD-NEMD) and the S-NEMD algorithms, we tested the R-NEMD algorithm (also a BD-NEMD algorithm) of Müller-Plathe (1997) with the TraPPE-UA force field (Martin and Siepmann, 1998). Better agreement with experimental data was found for the system with N exch equal to 300 fs and a time step of 1 fs, showing deviations of 17.5% from experimental data, which can be related to a smaller energy drift.…”

Liquid-crystal phase equilibria of Lennard-Jones chains

“…Simulations based on coarse-grained models can access longer time and length scales than their atomistic counterparts, allowing a bulk description of fluids. Coarse-grained models are commonly used to represent a simplified picture of large molecules, such as biomolecules [12][13][14][15][16], polymers [17][18][19][20][21][22], or liquid crystals [23][24][25][26][27][28]. Moreover, simulation results obtained from coarse-grained models can be directly compared with theoretical predictions that are based on a well-defined Hamiltonian, such as the family of perturbation theories developed from the statistical association fluid theory (SAFT) [29][30][31][32][33][34][35][36].…”