Aspen Plus was developed by AspenTech and today is one of the most used steady-state process modeling and simulation software in process engineering, especially it is used in the study of chemical processes such as refinery ones. It is used in both academia and industry.
Aspen Plus offers the possibility to model chemical processes, from classic distillation columns to different types of reactors with the possibility of adding other typical equipment of the industry such as valves, pumps, compressors, mixers and pipelines. Furthermore, the software offers the possibility to access a database with the physical properties of the chemical components that will be studied. In Aspen Plus it is not necessary to enter the mathematical equations of the model to save time, obviously the correct model must be chosen to describe the process. Each block contains the mass and energy balances that describe the operation of a unit. Within Aspen Plus, two environments can be distinguished:
• Properties environment: the components of the system are defined as the thermodynamic properties are chosen.
• Simulation environment: flows and equipment are drawn in a worksheet, then the operating parameters are defined.
3.2.1
Properties environment
In the case under consideration, the interacting chemical components are added. There are two phases in the CO2-MEA-H2O system: liquid and gaseous. Aspen Plus offers the possibility of recognizing ions through one of the following two approaches:
• Apparent components: in this case the components in the liquid phase are considered undissociated.
• True components: the dissociation of components is considered. This approach is the most faithful to reality.
Eleven components are then automatically generated through Elec Wizard. Figure 3.6 shows them.
Chapter 3
Once the components have been defined, it is important to choose the model for the calculation of the thermodynamic properties. The model must consider the strong non-ideality of the liquid as there are ions. From the literature, the best models are Electrolyte Non-Random Two Liquid (ELECNRTL) and ENRTL-RK. These models are coupled with the Redlich-Kwong equation of state for the computation of the non-idealities of the vapor/gas phase. Once these models are selected, Aspen will automatically calculate the parameters through its database.
3.2.2
Simulation environment
Once you have entered the properties, you can move on to the simulation environment.
In this environment it is possible to place the equipment and the streams of the process. In this work I will build the CO2 capture process through the following equipment:
• Absorber;
• Stripper;
• Pumps;
• Heat Exchangers;
• Splitters;
• Mixers;
• Flash.
Figure 3.6: Chemical components participating in the CO2 capture plant.
All these equipments are connected by the different streams. To simulate the absorption and stripping processes, the RadFrac model present in Aspen Plus is used because the processes involve multistage gas-liquid distillation operation. The process scheme used in both models is shown in the Figure 3.7 and is the classic CO2 capture system that will be described in the Chapter 4.
In the carbon capture process, there are equilibrium and kinetic reactions in the absorption process. In Aspen there are two methods to define the equilibrium constants:
• Standard Gibbs free-energy change: the equilibrium constant has the following expression, and ∆𝐺0 is calculated from Aspen Properties database:
𝐾𝑒𝑞 = 𝑒𝑥𝑝 (∆𝐺0 𝑅𝑇𝐿)
• Parameter-based correlation: the equilibrium constant has the following form where A, B, C and D comes from different sources:
𝑙𝑛(𝐾𝑒𝑞) = 𝐴 + 𝐵
𝑇𝐿 + 𝐶𝑙𝑛(𝑇𝐿) + 𝐷𝑇𝐿
As for the kinetic equations, the kinetic constant is calculated by Arrhenius' law:
𝑘 = 𝑘0(− 𝐸𝑎 𝑅𝑇𝐿)
Let’s see what happen in the columns. The absorption and regeneration processes are very complex from a mathematical point of view as it is necessary to describe at the same time the following processes: thermodynamics is not ideal, the reactions that occur, the interphase transfer, the transport of the components in two phases and the fluid dynamics of the system
Figure 3.7: CO2 capture process modeled in Aspen Plus.
Chapter 3
must be described. Using the RadFrac model you can have two different approaches: the approach with equilibrium stages and the rate-based approach. In both models the height of the column is discretized in a certain number of parts which are called stages. Therefore, I will not simulate a column with plates but rather a packed column; the plates are therefore not the real plates of a plate column but are the steps along the spatial dimension in which Aspen Plus divides the calculation.
In the case in which absorber is chosen to work at equilibrium, it is assumed that the liquid phase and the gas phase are in contact for the time necessary to reach thermodynamic equilibrium at each stage. In the case study, however, it is not possible to use this approach as there is at the same time transfer of matter and chemical reactions occur. This can be circumvented by introducing the Murphree efficiency (around 0.3) which considers the deviation from the equilibrium phase.
Instead of using a low Murphree efficiency value, it is more convenient to choose the rate-based model. Using this approach, it is possible to consider the mass transfer that occurs during chemical reactions. The rate-based stage is divided into two phases divided by an interface. The first area is the bulk while the second is the film. Figure 3.8 shows a graphical representation of what is happening.
Fluid dynamics also plays a very important role in the modeling of the process. Usually reactive absorpion-stripping columns are modeled as ideal plug-flows in which there is no axial dispersion. In the case of Aspen Plus, the RadFrac model approximates the plug-flow as a series of n-CSTRs because each block contains algrebral equations. It is therefore
Figure 3.8: Rate-based stage representation for the absorption process. [9]
necessary to verify that this column behaves as an ideal plug-flow. There are two phenomena that make fluid dynamics move away from the desired one and they are: axial diffusion and backmixing due to the countercurrent.
To verify the axial diffusion, it is sufficient to calculate the Peclet number which is defined as the ratio between the rate transferred by convection inside a fluid and that transferred by diffusion. The Peclet number is calculated by Equation 3.1:
𝑃𝑒 = 𝐹𝐿𝐶
For high values of Pe, the column behaves like an ideal plug-flow; for low values instead, the diffusion is felt and cannot be neglected. The Peclet number can be defined for the gas phase and for the liquid phase and also for two characteristic lengths which can be the height of the column (it provides information at the global level of the column) and the packing equivalent diameter (it provides information on diffusion at local level).
As for the backmixing due to the countercurrent, this phenomenon cannot be evaluated by the Peclet number. However, backmixing is implicitly included in the matter and energy balance and therefore to understand if it affects the process, it is first necessary to numerically solve the system of equations.