J. A. Schouten

Stability of doubly occupied N2 clathrate hydrates investigated by molecular dynamics simulations

Classical molecular dynamics calculations were performed for a structure II clathrate hydrate with N2 guest molecules in order to investigate the possibility of double occupancy, i.e., two N2 molecules inside one large cage. For all of the pressures, temperatures, and compositions at which the simulations have been performed, the doubly occupied clathrate remained stable. The structure of the host lattice is indistinguishable from that of a singly occupied clathrate hydrate. The volumes and energies are linearly dependent on the filling fraction. The range of values are the same for both the singly as well as doubly occupied clathrates. In the doubly occupied cages, the O–N2 radial distribution function, and therefore the structure in the vicinity of the N2 molecule, is similar to that of the mixed fluid. An extensive investigation of the distances in the cages shows a large similarity between singly and doubly occupied clathrates. All these results indicate that, upon filling the large cages with pairs o...

The two-component lattice-gas model

Abstract In this article a description is presented of the two-component lattice-gas model. This model combines the essential features of the lattice theory of Guggenheim for a two-component system and the lattice-gas model for a one-component system. Computer calculations of the critical lines and the spinodal curves have been performed for various combinations of the energy parameters. The coordinates of the double plait points have been determined analytically.

Optimum performance characteristics of an irreversible absorption refrigeration system

On the basis of the endoreversible model of absorption refrigeration cycles, a new cycle model, which includes the heat leak from the heat sink to the cooled space and irreversibilities due to the internal dissipation of the working fluid besides the finite-rate heat transfer between the working fluid and the external heat reservoirs, is established and used to calculate the maximum coefficient of performance and cooling rate of the absorption refrigeration system for a given total heat-transfer area of the heat exchangers. The temperatures of the working fluid and the distribution of the heat-transfer areas of the heat exchangers are optimized for the two states of maximum coefficient of performance and maximum cooling rate. The behavior of the dimensionless specific cooling rate as a function of the coefficient of performance is presented, which may be used to describe the general performance characteristics of an irreversible absorption refrigerator. The practical operating regions of the cycle system are determined and the new bounds of the primary performance parameters are given.

On the temperature correction to the ruby pressure scale

Since there is much confusion about the temperature correction to the ruby high‐pressure scale, we have measured the temperature dependence of the ruby R1 and R2 lines for several ruby samples in the range 150–400 K. The results show a variation of 0.3 A, corresponding to a pressure of 1 kbar. The results do not depend on chromium concentration. We find that the error in pressure can be reduced to about 0.4 kbar by measuring the temperature dependence of both the R1 and R2 lines of the ruby sample that are used in the high‐pressure experiment.

Vapour-liquid and gas-gas equilibria in simple systems

The vapour-liquid equilibrium of the system argon-krypton has been investigated. The co-existing liquid and gas phases have been determined as a function of pressure and temperature. The experimental critical line is compared with theoretical results obtained from perturbation theory and the Van der Waals equation of state.

Measurements of the densities of liquid benzene, cyclohexane, methanol, and ethanol as functions of temperature at 0.1 MPa

Abstract The densities of liquid benzene and cyclohexane from 283 to 333 K and of liquid methanol and ethanol from 203 to 333 K and 193 to 333 K, respectively, have been measured at 0.1 MPa with an accuracy better than 0.02 per cent using a calibrated glass pyknometer. The experimental densities for each of the liquids can be fitted to a polynomial function of temperature within the accuracy of the measurements. In the region of overlap, the present results are in good agreement with most of the previous measurements.

Improved phase diagram of nitrogen up to 85 kbar

A quasi‐isochoric scanning method has been used to study the phase diagram of nitrogen from 150 to 550 K and up to 85 kbar in a diamond anvil cell in order to make a comparison with previous measurements of the binary phase diagram He–N2. It has been confirmed that there is only one solid‐solid‐fluid triple point in N2 up to 85 kbar. However, both the δ–β transition line and the melting line have shifted appreciably towards lower pressures. The present experiment shows that, as a result of this, the triple point is located at 555±5 K and 80±2 kbar, which is 20% lower in pressure than previous data.

Molecular dynamics simulation study of the properties of doubly occupied N2 clathrate hydrates

We have performed molecular dynamics calculations to obtain a number of properties of a doubly occupied structure II N2 clathrate hydrate, in particular to study its behavior under higher pressures. The calculated neutron diffraction pattern is in agreement with the experimental result. The effect of the presence of the filling of the small cages and of the large cages (in either single or double occupancy) on the calculated pattern is demonstrated and discussed. The calculated Raman spectra show that the average vibrational frequency of the N2 molecules decreases in going from the singly occupied small cages to the doubly occupied large cages and then to the singly occupied large cages, respectively. The frequency distributions are explained in terms of radial distribution functions. When applying large pressures at low temperatures, a clathrate-amorphous transition occurs for a partially doubly occupied clathrate. The transition occurs at about the same pressure as for single occupations, but the densif...

Gas—gas equilibrium and the two-component lattice-gas model

Abstract A two-component lattice-gas model is developed to explain the phase equilibria in binary systems.

Clathrate hydrates in the system H2O–Ar at pressures and temperatures up to 30 kbar and 140 °C

Studies of the binary mixture H2O–Ar—by means of a quasi-isochoric scanning method—have revealed the range of stability of clathrate hydrates in the high-pressure and high-temperature region. The results obtained show an extension of the decomposition curve above the melting curve of pure argon and up to the melting curve of pure ice VII. At low pressures the argon decomposition temperature first increases and then decreases with pressure, showing a local maximum and minimum temperature. At higher pressures the slope of the decomposition curve remains positive but undergoes three more breaks. Two new quadruple points have been determined; at 105 °C, 20 kbar (Q4) and at 136 °C, 29 kbar (Q5). In the lower region (below 10 kbar) the decomposition curve shows two breaks; at 31.5 °C, 6.2 kbar (Q2) and at 37.5 °C, 9.6 kbar (Q3), indicating that three different clathrate structures are formed in this region. The positions of the three-phase lines separating the two clathrate structures at lower pressure could be...