Study And Application Of Phase Equilibrium For The Hydrocarbon/Water/Methanol System

In recent years, with improvement of the living standards of the people, not only energy needs is increasing, but also much attention has been devoted to the problem of environmental pollution that energy development and utilization caused. However, it is universally acknowledged that natural gas is the cleanest energy. So in many years to come, developing natural gas in a big way is important to social effects and environmental effects. For the development, storage and application of natural gases, the better purifying technique is very important. Natural gases purification includes removing water, getting rid of H₂S and CO₂ generally. Removing water is the key of the technique. For developing accurate and efficient removing water technique, the precondition is the exact vapor-liquid-liquid phase equilibrium calculation of hydrocarbon/water/methanol's multicomponent system. In this paper, a new method of phase equilibrium calculations and a modified EOS are adopted to study thermodynamic properties for the hydrocarbon/water/methanol system and the removing water technique of natural gas using methanol.Subroutines of three classical cubic EOS and a modified EOS were established, and those can calculate hydrocarbon/water/methanol system. Vapor-liquid-liquid phase equilibrium calculations often involve sequential procedures for the phase identification and the phase equilibrium calculation. A new method for vapor-liquid-liquid phase equilibrium calculations, based on establishing FORTRAN subroutines for simulation software PRO/II, was adopted for calculation of multiphase equilibria using EOS. The method is simple, reliable.When the subroutine was called, it requires only adding subroutines of thermodynamic EOS to PRO/II. The subroutines can provide phase number present at phase equilibrium, their quantities and compositions simultaneously. In addition, it requires thermodynamic EOS that can calculate accurately for hydrocarbon/water/methanol system. Based on classical cubic EOS, a modified Peng-Robinson EOS was built up. It laid the groundwork for phase equilibrium calculations for hydrocarbon/water/methanol system and the removing water technique of natural gas with methanol.Phase equilibrium calculations were verified with several typical systems, including hydrocarbon, hydrocarbon/water and hydrocarbon/water/methanol, using different classical cubic EOS and a modified EOS. The fugacity coefficients for the former two systems were calculated using the Soave-Redlich-Kwong, Peng-Robinson and Peng-Robinson-Stryjek-VeraEOS and those for the latter one were calculated using the modified Peng-Robinson, Soave-Redlich-Kwong, Peng-Robinson and Peng-Robinson-Stryjek-Vera EOS. The curves diagrams were drawn and the results show that a modified Peng-Robinson is accurate and reliable for hydrocarbon/water/methanol system. After analyzing the removing water technique for natural gas using methanol by modified Peng-Robinson, to test if each variable is changed, the technique could remove the water in natural gas to the permission and under the limit of environmental protection.