Electrolytes

1. Basic understanding of process engineering
2. Aspen Plus Software installed on your computer
3. Have a Computer
4. Have a Desire

In this part the trainer describes a comprehensive and practical problem which is water desouring. He explains how such process could be modeled in Aspen Plus in a stepwise manner. At first how defining properties for electrolytes is different from that of normal processes; then he starts explaining the Electrolyte Wizard and how it could be used. He also elucidates different approaches-true component and apparent component approach- taken by Aspen Plus when it comes to simulation of electrolytes. Finally, he switches to simulation environment to simulate the process and uses RadFrac column to serve the purpose.

Overview

Electrolyte systems involve ionic components and reactions that you must define to complete the components specification and reaction chemistry. Specifying the appropriate reactions and species generally requires reasonable knowledge of solution chemistry. You can use the Electrolyte Wizard to simplify this task.

On this first screen, if you are using the enterprise database, select the reaction database to use.

The behavior of the electrolyte wizard depends on the selection for the activity coefficient basis for ionic components on the lower part of the initial screen of the wizard. This setting is copied into the generated Chemistry.

When this basis is Unsymmetric, activity coefficients of ions are based on infinite dilution in pure water. You must have already defined water as a component to use this wizard before you can use this wizard in this case.

When this basis is Symmetric, activity coefficients of ions are based on pure fused salts. Water is not necessary (though it may be included as a solvent).

For any of these options, you should have already defined molecular components that define the electrolyte system of interest.

Select the base components from which the ionic and salt species and all pertinent reactions will be generated.

You can represent the hydrogen ion as either:

• Hydrogen ion H+
• Hydronium ion H3O+

If you select H+ using the hydrogen ion type group box, the generated reactions will be in terms of H+.

You can also exclude salts formation from the species/reaction generation. If you exclude salts, the electrolyte problem is much simpler to solve, but the Aspen Physical Property System will not detect any salt precipitation that may occur in your flowsheet.

You can also include or exclude the water dissociation reaction from the list of generated reactions. By default, the water dissociation reaction is excluded from the list. However, if the included generated reactions involve the H3O+ (or H+) and the OH- ions, the water dissociation reaction will be included automatically, even with the option turned off. In other words, if there is a possibility of water being generated (from combining H3O+ with OH-), the water dissociation reaction is required and will be generated.

You must include the water dissociation reaction if you want to report any properties that require the knowledge of the H3O+ (or H+) or OH- ions, such as pH or pOH.

You can include ice formation reaction. The Aspen Physical Property System handles formation of ice as a salt precipitation reaction with appropriate K-Salt.

Remove any unwanted aqueous species, salts, or reactions by selecting them and clicking Remove.

By default, the Electrolyte Wizard will set up the global physical property specifications by:

Placing the generated species on the Components | Specifications | Selection sheet.

Placing the generated reactions on the Chemistry form with a Chemistry ID of GLOBAL.

Retrieving all available equilibrium constants for the generated reactions and placing them on the Chemistry form.

Placing any supercritical components (noncondensable components) on the Components | Henry-Comps form with a Henry-Component ID of GLOBAL.

Selecting the property method used as the global physical property method to be used for the calculation, based on your selection in this step. (The methods available are limited to those compatible with the selected activity coefficient basis for ions.) A Chemistry ID of GLOBAL and a Henry-component ID of GLOBAL are also specified on the Methods | Specifications | Global sheet.

Retrieving all available binary and pair parameters for the electrolyte NRTL model and placing them on the Methods | Parameters forms.

Select the calculation approach you want for your electrolyte calculations.

There are two ways to represent the compositions of electrolyte systems in the calculation:

• Apparent component
• True component

If you choose apparent component, the system compositions is represented in terms of molecular components prior to solving the solution chemistry. The feed streams can be given only in terms of apparent components. All calculated compositions will be given in terms of apparent components (solvents, molecular solutes, electrolyte; no ions or salts).

If you choose true component, the system compositions are represented in terms of all species that exist at chemical equilibrium. The feed streams can be given in terms of either true species or apparent components. All calculated liquid phase compositions will be reported in terms of true species (solvents, molecular solutes, electrolyte, ions, and salts).

The Aspen Physical Property System performs rigorous electrolyte equilibrium and property calculations in both approaches. Both approaches should give the same results. The default true component approach is generally preferred for calculation efficiency.

In the apparent component approach, the solution chemistry and the true species are transparent to the process flowsheet and the units operation computation algorithm, such as the flash and distillation algorithms. Use this approach when:

• The apparent components can be easily identified
• The compositions of the apparent components are of main interest in the calculation.
• This approach is generally preferred for weak electrolyte systems such as sour water stripping.

In the true component approach, the chemical equilibrium equations that describe the solution chemistry are solved simultaneously with the material balance, energy balance and phase equilibrium equations that describe the units operation model. Use this approach when:

• The apparent components cannot be easily identified.
• The compositions of ionic species are of main interest in the calculation.
• This approach is generally more convenient for complex electrolyte system such as waste water treatment.

In second part he follows the same procedure for new process but this time tries to investigate new aspects of electrolytes simulation in Aspen Plus. In this part he is more concerned with simulation in which there are both electrolyte and non-electrolyte processes. To tackle problems associated with such modeling, he proposes two solutions, one of which works and one of which does not work for this example.