Issues regarding the definition of known conditions in ASPEN HYSYS
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In ASPEN HYSYS, given the composition of the known materials and any two of temperature, pressure, and gas phase fraction, HYSYS will perform a flash calculation on the stream. Assuming there are 5 components, A, B, C, D, and E, in the logistics system, three phases can be formed. With the phase fraction given by f = C – P + 2 = 5 – 3 + 2 = 4, the degree of freedom of this system is 4. If the pressure and temperature are known, and given that the fugacities of each component are equal in different phases due to phase equilibrium, it is possible to establish relationships regarding the composition of each component across different phases. Thus, for 5 components, if the composition of any 2 components in any phase is known, the composition of the remaining 3 components in those phases can also be determined. Therefore, in multi-component, multiphase systems, if the pressure and temperature are known, the fugacity of each component in any phase can be determined using the equation of state, which enables the calculation of the system’s flash point. But I’ve never understood why, given the known vapor fraction as well as the known pressure or temperature, it’s possible to calculate the system’s flash point. If the volume is known and either the temperature or pressure is known, a flash calculation can be performed. But the gas phase fraction is different from volume; I’ve never been able to understand this. I hope someone experienced can give me some guidance.
I don’t seem to understand what you’re saying. It was mentioned earlier that if the composition of any two components in any phase is known, then the composition of the remaining three components in each phase can also be determined. The last sentence states that if the volume is known, and either the temperature or pressure is known, a flash calculation can be performed
What you mentioned, determining the vapor fraction, can actually be understood in this way: once the vapor fraction is determined, the flow rates of the vapor and liquid phases are also determined, as are the inlet composition and flow rates. This ensures mass balance is satisfied; not only phase balance is achieved, but there is also an additional mass balance equation, which naturally reduces the number of degrees of freedom. And just by specifying temperature and pressure, it is still necessary to solve the mass balance equation