C.J. Wormald

Excess enthalpies of gaseous mixtures of n-alkanes

Flow calorimetric measurements of the excess enthalpy HE of 13 binary mixtures of gaseous n-alkanes from ethane to n-octane are reported. The measurements lie in the temperature range 304.5 to 413.3 K and are at a pressure of 101.325 kPa. These measurements, together with the HEs for 7 methane + n-alkane mixtures reported previously, are used to obtain values of the cross term isothermal Joule-Thomson coefficient φ12, the cross term second virial coefficient B12, and the geometric-mean-rule parameter ξ for each mixture. The values of ξ obtained from the HEs are in close agreement with those obtained from critical temperatures of the mixtures. The B12s obtained from the HEs agree well with those obtained from compression measurements. Comparison with six combining rules is made. It is found that the rule ξ = 2(I1I2)12(I1 + I2)−1(V11cV22c)12(V12c)−1 fits all the measurements within experimental error.

A precision differential flow calorimeter the excess enthalpy of benzene + cyclohexane between 280.15 K and 393.15 K

Abstract A precision differential flow calorimeter for the measurement of enthalpies of mixing of liquids has been constructed. Measurements of the excess enthalpy for benzene + cyclohexane have been made at temperatures between 280.15 K and 393.15 K, and especially careful measurements have been made at 298.15 K. The standard deviation of the 298.15 K measurements is 0.21 J mol −1 , and agreement with the results of the best isothermal dilution calorimeters is excellent.

A differential-flow mixing calorimeter. The excess enthalpy of methane + benzene, methane + cyclohexane, and benzene + cyclohexane

A differential-flow mixing calorimeter for gases at low densities is described. It has been used to make measurements of the excess enthalpy of methane + benzene, methane + cyclohexane, and benzene + cyclohexane vapours at atmospheric pressure and at temperatures between 363.15 and 413.15 K. The excess enthalpies of the methane mixtures are accurate to ± 1.5 per cent. The excess enthalpies of benzene + cyclohexane are all less than 0.5 J mol−1, and are accurate to ± 0.2 J mol−1. Using the corresponding-states correlation of McGlashan and Potter, which fits the thermodynamic properties of both methane and benzene vapour, agreement between calculated and measured excess enthalpies for methane + benzene is within experimental error. As the above correlation fits the thermodynamic properties of cyclohexane only moderately well, the calculated excess enthalpies for methane + cyclohexane and benzene + cyclohexane are not in such close agreement with experiment.

The structure of liquid methanol by H/D substitution technique of neutron diffraction

Neutron diffraction (ND) measurements on liquid methanol (CD3OD,CD3O(H/D),CD3OH) under ambient conditions have been performed to obtain the total (intra-+intermolecular), Gdist(r) and intermolecular, Ginter(r) radial distribution functions (rdfs) for the three samples. The extent to which intermolecular structure is affected by using two different intramolecular models is discussed. The H/D substitution on hydroxyl–hydrogen (Ho) has been used to extract the partial distribution functions, GXHodist/inter(r) (X=C, O, and H—a methyl hydrogen) and GXXdist/inter(r) from the difference techniques of ND at both the distinct and intermolecular levels. The O–Ho bond length, which has been the subject of controversy in the past, is found purely from the partial distribution function, GXHodist(r) to be 0.98±0.01 A. The C–H distance obtained from the GXXdist(r) partial is 1.08±0.01 A. These distances determined by fitting an intramolecular model to the total distinct structure functions are 0.961±0.001 A and 1.096±0.001 A, respectively. The GXXinter(r) function, dominated by contributions from the methyl groups, apart from showing broad oscillations extending up to ∼14 A is featureless, mainly because of cancellation effects from six contributing pairs. The Ho⋯Ho partial pair distribution function (pdf), gHoHo(r), also determined from the second-order difference, shows that only one other Ho atom can be found within a mean Ho⋯Ho separation of 2.36 A. The average position of the O⋯Ho hydrogen bond determined purely from experimental GXHointer(r) partial distribution function, at 1.75±0.03 A is found to lie in the range (1.75–1.95 A) of values reported from computer simulation results.Neutron diffraction (ND) measurements on liquid methanol (CD3OD,CD3O(H/D),CD3OH) under ambient conditions have been performed to obtain the total (intra-+intermolecular), Gdist(r) and intermolecular, Ginter(r) radial distribution functions (rdfs) for the three samples. The extent to which intermolecular structure is affected by using two different intramolecular models is discussed. The H/D substitution on hydroxyl–hydrogen (Ho) has been used to extract the partial distribution functions, GXHodist/inter(r) (X=C, O, and H—a methyl hydrogen) and GXXdist/inter(r) from the difference techniques of ND at both the distinct and intermolecular levels. The O–Ho bond length, which has been the subject of controversy in the past, is found purely from the partial distribution function, GXHodist(r) to be 0.98±0.01 A. The C–H distance obtained from the GXXdist(r) partial is 1.08±0.01 A. These distances determined by fitting an intramolecular model to the total distinct structure functions are 0.961±0.001 A and 1.096±0....

Excess enthalpies for (water + nitrogen)(g) up to 698.2 K and 12.6 MPa

Abstract A new flow mixing calorimeter has been used to make measurements of the excess enthalpy of (water + nitrogen)(g) at temperatures up to 698.2 K and pressures up to 12.6 MPa. Tests on the calorimeter, and consistency tests on the measurements, suggest that the results are free from significant errors. The measurements are fitted to an equation from which excess volumes and compression factors for the mixture are calculated.

The excess enthalpy of (water + nitrogen) vapour and (water + n-heptane) vapour

Abstract The excess enthalpy of (water + nitrogen) vapour and (water + n -heptane) vapour has been measured using a flow-mixing calorimeter. The measurements are at a pressure of 101.325 kPa and extend over the range 373.15 to 423.15 K. The accuracy of the measurements is ± 2 per cent. Cross-term second virial coefficients B 12 for (water + nitrogen) derived from the excess enthalpies are in good agreement with B 12 s obtained from measurements of the solubility of water in nitrogen.

The excess enthalpies of hydrogen + methane, hydrogen + nitrogen, methane + nitrogen, methane + argon, and nitrogen + argon at 298 and 201 K at pressures up to 10.2 MPa

Abstract A flow-calorimetric apparatus for the direct determination of the excess enthalpy of binary gaseous mixtures has been constructed. The apparatus will operate at pressures of up to 15 MPa and at temperatures from ambient down to the boiling temperature of liquid nitrogen. The calorimeter has a rapid response so that the amount of gas used for each run is small. Measurements on H2 + CH4, H2 + N2, CH4 + Ar, and N2 + Ar at 298 K and 201 K at pressures up to 10.2 MPa are reported. The accuracy of the measurements is ± 2 per cent. Comparison is made with other work, with the virial equation of state, and with the fluid theories of Scott.

Excess molar enthalpies and excess molar volumes of {xCO2+(1-x)C2H6} up to 308.4 K and 11.0 MPa

Abstract Measurements of the excess molar enthalpy H m E and excess molar volume V m E for { x CO 2 + (1 − x )C 2 H 6 } are described. The measurements cover the range 248.1 to 308.4 K at pressures from 4.0 to 11.0 MPa. The uncertainty of the H m E s is ±2 per cent, and of the V m E s is ±4 per cent. The Patel-Teja equation of state reproduces all the main features of H m E ( x ) and V m E ( x ), and fits the measurements better than expected. Agreement with results from other laboratories is within the combined experimental uncertainties.

Excess enthalpies of mixtures of methane + each of the n-alkanes from ethane to n-octane

Abstract Flow-calorimetric measurements of theexcess enthalpy HE of mixtures of methane + each of the n-alkanes from ethane to n-octane at a pressure of 101.325 kPa and over the temperature range 241.1 to 418.3 K are reported. The overall accuracy of the measurements is approximately ± 2 per cent. Comparison is made with HE calculated using a reduced equation of state which fits the isothermal Joule-Thomson coefficients of the n-alkanes together with the geometric mean and Hudson-McCoubrey combining rules. The results lie midway between the values calculated from the two rules.

A new enthalpy-increment calorimeter enthalpy increments for n-hexane

Abstract A water-cooled heat-exchange calorimeter for the measurement of enthalpy increments of fluids up to 700 K and 15 MPa has been constructed. Novel features of the design ensure that heat leaks are small, well controlled, and easily measured. The results of 90 test runs on steam gave enthalpy increments in agreement with steam tables to within 0.5 per cent. The calorimeter has been used to measure enthalpy increments of n -hexane up to 573.2 K and 12.67 MPa, and the results are compared with the Starling modification of the BWR equation of state. Overall agreement of 115 measurements with the BWRS equation is 0.97 per cent. In the critical region BWRS values differ from experiment by up to 4 per cent.