Sulfuric Acid (H2SO4) in 9.2

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Introduction

Sulfuric acid (H2SO4) is a strong acid that is widely used in the hydrometallurgical industry. It has a strong affinity for water and is soluble at all concentrations.

Concentrated sulfuric acid, consists of 98.3% H2SO4(l) and 1.7% water. Any solution containing a lower concentration of acid, will contain the aqueous form - H2SO4(aq).

When water and sulfuric acid mix, or when streams with different concentrations of H2SO4 mix, there is an exothermic reaction and the resulting stream(s) is hotter. This is known as the Energy, or Heat, of Dilution.

SysCAD 9.2 has been designed to automatically handle the complexities of sulfuric acid dilution.

Adding or Removing Sulfuric Acid

SysCAD 9.2 will automatically add both H2SO4(l) and H2SO4(aq) to all SysCAD species databases.

  • If these species are not required in the project, then you simply do not add them to the configuration file.
  • If you wish to Add sulfuric acid to a project, you need to include both species in the configuration file. SysCAD will expect both H2SO4(l) and H2SO4(aq) in the configuration file. Therefore, if you select one of these species to add to the configuration file, then SysCAD will automatically add the other species.
  • If you wish to Remove sulfuric acid from the project, you may simply select either H2SO4(l) or H2SO4(aq) from the display list and click on the Remove button. SysCAD will automatically remove both the liquid and aqueous species from the display list.

Please see Adding or Removing a Species for further information.

Editing the Properties of Sulfuric Acid

SysCAD will automatically use the default values that are stored in SysCAD for sulfuric acid. The user can see this data if they edit the species database. See Editing SysCAD Database.

If the user has other data that they wish to use in the project, then they can type the required data into the fields in the species database, as described below.

Note: All fields that have default data are indicated by a button on the right hand side with a 'D' - as shown below:

Image:Specie Database 3.gif

When you first view the data for either H2SO4(l) or H2SO4(aq), the data fields that contain default data will be greyed out. The data in these fields is visible, but you may not be able to view the entire data set.

To edit the default data, or to override it with your own data:

  • Click on the 'D' button on the left hand side of the required data field
  • The field will change to a white background
  • View or change the default data or type in your own data
  • When changes are complete, click on the 'Update' button. To ignore any changes at this point, click on the 'Undo' button.
  • If you wish to return to the default data at any time you can simply click on the 'D' button and SysCAD will insert the default data into that field.

Default Data for Sulfuric Acid

SysCAD 9.2 has default data and functions to describe the physical properties of both liquid and aqueous sulfuric acid. The data has been obtained from a number of sources, and the functions were developed from reliable data. All of the data and functions are accessible in the species database.

The following sections describe the data that is provided for these two species:

Liquid Sulfuric Acid - H2SO4(l)

Liquid sulfuric acid only exists in solution at concentrations above 98.3% acid. Solutions containing lower concentrations of acid are considered to consist of the aqueous form of sulfuric acid.

Available Data:

  1. Density,
  2. Heat of Formation at 25°C (H25),
  3. Heat Capacity, Cp, and
  4. Acid Dissociation constants (Ka).

Aqueous Sulfuric Acid - H2SO4(aq)

Aqueous sulfuric acid exists in water at all concentrations below 98.3% acid. Solutions containing higher concentrations of acid are considered to consist of the liquid form of sulfuric acid. However, if you wish to adjust this value for the change from liquid to aqueous acid, you may simply change the value in the Phase Change field.

Available Data:

  1. Density correction function - Solution Density as a function of acid mass fraction in aqueous solution,
  2. Heat of Dilution - Heat of Formation at 25°C as a function of percentage acid mass in aqueous solution,
  3. Heat Capacity, Cp,
  4. Phase change between the liquid and aqueous form at a specific acid mass fraction in aqueous solution, and
  5. Acid Dissociation constants (Ka).

Please also see Species Table for further information on this data.

Sulfuric Acid in SysCAD Projects

If you have included sulfuric acid in the project configuration (cfg file), then SysCAD will use the data in the species database to calculate all of the normal properties of streams that contain sulfuric acid. In addition to this, SysCAD will do the following specific calculations:

Heat of Dilution

If the species database contains Heat of Dilution data (see Heat of Dilution in the Species Table) SysCAD will use this to calculate the temperature change when streams with different acid concentrations mix.

  • By default, SysCAD will contain this data in the form of Spline data with Enthalpy of Formation at 25°C as a function of percentage acid mass in solution.
  • If you do NOT want to include this functionality (enthalpy of mixing) in a project, then you must edit the species database and carry out the following steps:
  1. Click on the green default button on the right hand side of Ht of Dilution - the field changes from grey to white.
  2. Click on the '?' button and select None() from the drop down list.
  3. Click on 'Update'.
  4. When you reload your project, the enthalpy of mixing will no longer occur.
  • If you wish to re-enable the heat of mixing, then you must edit the species database and carry out the following steps:
  1. Click on the green default button on the right hand side of Ht of Dilution
  2. The default data will be inserted into this field.
  3. Click on 'Update'.
  4. When you reload your project, the enthalpy of mixing will be enabled.

Note: In an open project in SysCAD to see the Enthalpy of Formation at various mass fractions of H2SO4, go to the Species Data page and adjust the Mass Fraction. See Species Data for more information.

Automatic Phase Change between Liquid and Aqueous Forms

SysCAD will use the Phase Change data to automatically change H2SO4 from the liquid form to the aqueous form (and visa versa, although this seldom occurs in a hydrometallurgical facility).

  • When you enter H2SO4 (aqueous or liquid) mass fraction data into SysCAD, for example in a Feeder, SysCAD will compare the mass fraction to the value in the species database. If the mass fraction is equal to or above the mass fraction of phase change, then SysCAD will use the Liquid form. If the mass fraction is less than the mass fraction of phase change, then SysCAD will use the Aqueous form.
  • The default SysCAD data for sulfuric acid specifies that the phase change occurs at a mass percentage in water of 98.3% H2SO4. Above this mass percentage, H2SO4 occurs in the liquid form, and below this it will occur as the aqueous form.

Example

User Input
SysCAD Result
Description
Specie
Mass Percentage
Specie
Mass Percentage
H2SO4(aq) 98.5% H2SO4(l) 98.5% Automatic change from Aqueous to Liquid form, 98.5% > 98.3%
H2SO4(l) 98% H2SO4(aq) 98% Automatic change from Liquid to Aqueous form, 98% < 98.3%
H2SO4(aq) 55% H2SO4(aq) 55% No change
H2SO4(l) 98.3% H2SO4(l) 98.3% No change


You project must meet 2 conditions for automatic phase change between H2SO4(l) and H2SO4(aq) to occur:

  1. The species database must contain Phase Change data - see Phase Change in the Species Table. (By default, SysCAD will contain the phase change data in the H2SO4(aq) species.); and
  2. The field PhaseChange@MF on the Plant Model access window is set to ON.

If the first condition is met, i.e. Phase Change data is present in the species database, but the PhaseChange@MF field is set to OFF, SysCAD will generate a single message every time that the project is solved - PhaseChange@MF switched off. This is to alert the user to the fact that while data does exist for the automatic phase change to occur, it has been disabled in the project.


Enabling H2SO4 Phase Change in a Project

  1. In a NEW SysCAD 9.2 project the user does not have to do anything to obtain this functionality for sulfuric acid. SysCAD will automatically detect if Phase Change data exists in the species database and will set the field PhaseChange@MF on the Plant Model access window to ON.
  2. For a project that has been upgraded from an earlier version of SysCAD, the field PhaseChange@MF on the Plant Model access window will by default be set to OFF (You will receive a message as described above every time you solve the project).
    To enable the Phase Change functionality, go to View - PlantModel and change the PhaseChange@MF field to ON.
  3. If the default Phase Change data for H2SO4 was removed from the species database by a previous user, then it may be re-instated by editing H2SO4(aq) in the specie database and clicking on the green default buttons in the Phase Change fields. (see Editing the Properties of Sulfuric Acid).


Disabling H2SO4 Phase Change in a Project

There are 2 methods for disabling this functionality in a project. The first method is simpler and does not involve closing the project or removing any data:

  1. Go to View - PlantModel and change the PhaseChange@MF field to OFF.
  2. Edit the species database as follows:
    • Go to H2SO4(aq),
    • Click on the green default buttons in the three Phase Change fields (see Editing the Properties of Sulfuric Acid),
    • Select the blank option, ' ', from the drop down list for all three fields,
    • Click on the Update Button,
    • Click on the OK button and reload the project.


Changing H2SO4 Phase Change Data

The default data for H2SO4 will change sulfuric acid from the liquid to the aqueous form at 98.3% H2SO4. However, some users may wish to use a different value for the phase change, such as 98%. To do this edit the species database as follows:

  1. Go to H2SO4(aq),
  2. Click on the green default button next to the 'Function' field in the Phase Change area (see Editing the Properties of Sulfuric Acid),
  3. Change the value from the Default of 0.983 to the required fraction, i.e 0.98,
  4. Click on the Update Button,
  5. Click on the OK button and reload the project.
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