Can Aspen solve problems in the non-equilibrium state?
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Today I came across a very simple example in a thermodynamics book: 100 L of nitrogen is compressed from state T1, P1 to 10 L at temperature T2; the task is to find P2, with no phase change occurring. This is a problem that can be solved manually, but I wanted to use Aspen to handle it; however, I couldn’t find a suitable method for this purpose. There are two reasons for this: 1) In Aspen Plus, the modules I’ve come across so far all involve flow, meaning there is a flow rate involved, whereas this problem involves a closed system, so no appropriate module exists. 2. The PVT relationships for Aspen seem to be based on equilibrium states; for example, in the case of component analysis of pure components, the points on the PT diagram represent their saturated vapor pressures, which corresponds to a thermodynamically stable state. And it is clear that for this problem, no such corresponding relationship exists between P and T; that is, the point corresponding to the P-T relationship is not on the saturated vapor pressure curve. Otherwise, P2 could be directly determined by looking up the value at T2 in the chart ； This system has 2 degrees of freedom, with P and T being able to vary independently. How should such problems be solved using Aspen? It’s a minor issue, but since it involves fundamental data, its implications are quite broad. By solving this problem, it becomes possible to determine the pVT relationships and thermodynamic properties of non-equilibrium systems in closed systems using Aspen.
It seems this can be calculated using Aspen; by establishing some design specifications, the result can be obtained
This post was last edited by superduckly on 2012-5-23 09:46. First of all, thank you for your answer. The design specifies that it can only be used in specified models; however, which model has no inlets or outlets and no equilibrium stages? Such a model is static, unlike fluid flow which is dynamic. Therefore, the volume unit should be m3, rather than m3/hr or similar – it is used solely to calculate changes in thermodynamic properties and PVT properties resulting from changes in the system’s state I still haven’t figured out user-defined models; for the rest, I haven’t found anything that allows this yet
You’re overcomplicating things. In this problem, only two states are of interest. A nitrogen stream with parameters T1 and P1, and a flow rate of 100 L/hr, can be defined. This stream then passes through a heat exchanger module, where its temperature is adjusted to T2; the pressure in the heat exchanger is also set. The goal is to compress this stream to a flow rate of 10 L/hr. By doing so, all the properties of interest can be determined
This post was last edited by ljjt on 2012-5-23 21:09. OP, I’d like to see your manual calculation process to confirm whether it was done using the principles of thermodynamics for irreversible processes (that is, non-equilibrium thermodynamics)
Well, I’ve done the calculations using the heat exchanger; it comes down to defining the flow rate. Defining it as 100 L/hr versus 100 L/min yields completely different results, because flow rate is related to time. Meanwhile, for 100 L of material moving from P1T1 to P2T2, its thermodynamic properties depend only on the initial and final states, not on the process itself.
The purpose of this question in the book is to enable students to master the use of numerical integration; otherwise, given T2V2 and T1P1V1, the solution can be obtained directly by using the appropriate equation of state. I have a certain degree of misunderstanding, but this misunderstanding has no substantial impact on this discussion, as the problem still exists. Well, let’s calculate the enthalpy change from T1P1 to T2P2. The manual calculation process involves going from the actual state of T1P1 to the ideal state of T1P1, then through a constant-temperature and constant-pressure process to the ideal state of T1P2; followed by a constant-pressure and constant-temperature process to the ideal state of T2P2, and finally to the actual state of T2P2. By calculating the enthalpy change for each step, the total enthalpy change is obtained. But this process does not involve a phase transition, so there is no phase equilibrium, right?
Just as in my reply to another responder, I have a slight misunderstanding regarding this question itself. If this problem is rephrased as finding the enthalpy change from T1P1 to T2P2, excluding the enthalpy of phase change, it becomes easier to understand why a heat exchanger is not suitable. In heat exchangers, the enthalpy change is related to the flow rate; however, in reality, there is no such concept as a flow rate for a material system. It is either 100 L/HR or 100 L/MIN, or it represents the enthalpy change resulting from temperature and pressure changes as 100 L of fluid passes through the exchanger.
I think what you have calculated are still the thermodynamic state functions of a thermodynamic equilibrium state.