Properties of real fluids in critical region: third virial coefficient

In present work, a perturbed virial expansion truncated at second order (third virial coefficient) is applied to calculate fluid critical points. We have used square-well (SW) potential model with constant width of the attractive well to obtain critical properties of several real fluids. Then, we have considered the width of attractive well as a function of temperature and studied its effect on critical properties of real fluids. Finally, we have studied the effect of different compressibility factors on critical properties of real fluids. Results reveal that critical temperatures and pressures calculated by using variable (temperature-dependent) width of the attractive well are in better agreement with experimental data. Selection of suitable compressibility factor has an important influence on our results. In general, we have theoretically improved critical points of some real fluids by using SW model with variable width of the attractive well and suitable compressibility factor. (author)