Viscosity and density of isobutane measured in wide ranges of temperature and pressure including the near‐critical region

Accurate ηρpT data for isobutane were measured for nine isotherms between 298.15 and 498.15 K using simultaneously a vibrating‐wire viscometer and a single‐sinker densimeter. The maximum pressure was 93% of the saturated for subcritical isotherms and 30 MPa for supercritical isotherms. The density measurements are generally characterized by an uncertainty of ≤0.1%. Allocation errors for temperature and pressure influence significantly their uncertainty in the near‐critical region. A comparison with the equation of state by Bücker and Wagner shows agreement normally within ±0.1%. The near‐critical isotherm 410.15 K reveals differences to −3.7% exceeding the uncertainty of 1.9%. The uncertainty in viscosity is ≤0.3%. The comparison with the correlation of Vogel et al. yields deviations exceeding the uncertainty of the correlation (3%). The critical enhancement becomes evident for the near‐critical isotherm amounting to 1.4%. The new data will improve the viscosity surface correlation. © 2015 American Institute of Chemical Engineers AIChE J , 61: 3116–3137, 2015