#### Overview

In this tutorial we examine the use of Unit Designers to achieve particular results. Unit Designers act as constraints to the mass and energy balance and ensures that particular results are obtained.

The tutorial focuses on the leaching of copper sulphide by oxygen. From practice we know that the exit concentration of acid must be 10 g/L. In addition, not all of the copper dissolves so that the exit concentration of copper in the solid phase is 10%.

In this tutorial we will use Unit Designers to constrain the solution of the mass balance so that these two conditions are met.

You will go through the following 12 steps in this tutorial:

#### Step 1: Draw the flowsheet

Draw the flowsheet using a tank so that it looks like that shown in Figure 1.

Figure 1. Flowsheet for a leaching reactor.

Label the streams and the stream flags:

• FEED            – 101
• STEAM         – 104
• SPENT          – 102
• OXYGEN       – 103
• VENT           – 105
• PRODUCT    – 106

#### Step 2: Add components to the mass balance

Add the following components to the balance:

 Cu2S (s) chalcocite H2O (l) water H2O (g) water vapour H2SO4 (aq) sulphuric acid CuSO4 (aq) copper sulphate N2 (g) nitrogen O2 (g) oxygen SiO2 (s) silica

#### Step 3: Setup stream compositions

Set the component flow rates, temperatures and pressures for each of the input streams so that they are the same as those represented in the table below from the Selected Stream tab in “Stream Manager” (not from the All Streams tab!).

 FEED SPENT OXYGEN STEAM Cu2S (s) 0.100 - - - H2O (l) 0.200 0.37736 - - H2O (g) - - - 0.2412 H2SO4 (aq) - 0.1132 - - CuSO4 (aq) - 0.1132 - - N2 (g) - - 0.0012 - O2 (g) - - 0.0688 - SiO2 (s) 0.100 - - - Temp C 75.0 70.0 25.0 *191.8 Pressure kPaG 0 0 0 1200.0

*Determined using “Set Steam Conditions” as shown in previously.

#### Step 4: Setup reactions

Set-up the following reactions:

Cu2S (s) + 2.5 O2 (g) + H2SO4 (aq) -> 2 CuSO4 (aq) + H2O (l)

ext = 0.99875

H2O (g) -> H2O (l)

ext = 1.0

#### Step 5: Choose energy balance properties

Choose to calculate the energy balance as a non-isothermal balance.

This is shown in Figure 2.

Figure 2. Choosing the energy balance properties.

#### Step 6: Set the evaporation rate

We do not want evaporation from the Tank at this point, although we may want to add it later.

In the "Calculation” section of the properties of the Tank, set the evaporation rate to 0.0001 t/hr.

This is shown in Figure 3.

Figure 3. Setting the evaporation rate to 0.0001 t/hr.

#### Step 7: Calculate

Run the project, as shown in Figure 4.

Figure 4. The Process Toolbar, with the cursor hovering over the Run button.

#### Step 8: Examine results

The flows and compositions of the product stream are shown in Figure 5 below.

Figure 5. Results for the PRODUCT stream.

The temperature has risen to 140°C and most of the copper has been removed.

The acid concentration has dropped from 29 g/L to 11 g/L and the copper concentration has risen to 27 g/L.

The gas concentrations in the VENT stream are shown in Figure 6.

Figure 6. Results for the VENT stream.

The oxygen concentration has decreased from 98% to 92%.

There is a small amount of water vapour in the gas stream, in line with the specification that the evaporation is 0.0001 t/hr.

#### Step 9: Adding a Unit Designer to constrain the acid concentration

Suppose that the design or the calculation requires that the concentration of acid is 10 g/L.

In this case we wish to adjust the flow rate of SPENT so that the acid concentration is 10 g/L.

This is achieved by using a Unit Designer, which adjusts the flow of the material in the SPENT stream to meet this criterion.

Note that in order to adjust flows the stream that is being adjusted must be a feed stream.

The calculation engine does not calculate backwards through units, so this strategy makes sense only if the stream is a feed to the entire flowsheet.

Add a Unit Designer and connect it to the measured and manipulated streams.

Activate the DataAndDesignPoints for the streams that you are going to link the Flowsheet Designer to if they are not already activated.

This can be done as follows: select the stream that you want to connect the designer to and in the Properties Viewer, press the ellipsis button (...) in the DataAndDesignPoints property shown in Figure 7.

This will result in a blue diamond and an orange circle appearing on the stream.

Figure 7. Activating the Data and Design Points on a stream.

Add the Unit Designer by dragging it from the Process Library and onto the page.

Connect the measurement point and design point to the Unit Designer using the DesignConnectorTool.

Connect from the Measurement Point on the Process Stream to the Unit Designer and from the Unit Designer to the Design Point on the “spent” stream. This is shown in Figure 8.

Figure 8. Adding the unit designer.

#### Step 10: Setting the properties for the Unit Designer

Set the properties for the Unit Designer as shown in Figure 9.

Figure 9. Unit Designer properties.

Follow these steps in changing and checking the Designer properties:

Take care to attach the Unit Designer to the correct ProcessUnit. Setting the value of the ProcessUnitID, highlighted, in Figure 10, does this.

Look up the ProcessUnitId by selecting the ProcessUnit and then navigating to the ProcessUnitID property.

Check that the measurement connector is properly connected. Look to see if the MeasurePoint and MeasurementPoinID have values in their property boxes.

Check that the design connector is properly connected. Look to see if the DesignPoint and DesignPointID have values in their property boxes.

Set the Measurement Variable. To set the measurement properties, expand the MeasurementVariable option by clicking on the + sign in the left margin of the Properties Viewer. Under the "Variable" heading, select "Composition" from the drop down menu. Under the "Value" heading, type in the number 10.

In the "Component", click on the ellipsis button and from the Component Selection Form that is launched select the H2SO4 (aq) component.

Set the Design Variable. Expand the DesignVariable option by clicking on the + sign in the left margin of the Properties Viewer. Under Variable, select "Flow".

Reviewing the properties that have been selected and displayed in the Properties Viewer is important.

These properties are interpreted as follows: Adjust the flow in the stream with a ProcessStreamID of 2 (this is the spent stream) between values of 2 and 5 (t/hr) so that the concentration of the component H2SO4(aq) in the process stream with a ProcessStreamID of 5 (this is the product stream) has a value of 10 g/L.

This must be done in conjunction with the calculation of the Process Unit that has a ProcessUnitID of 1. (this is our reaction tank TK-010)

#### Step 11: Adding a Unit Designer to constrain the copper in the residue

It is also known that the copper in the solid residue is 10%. This constraint affects the amount of leaching or dissolution that occurs so a Unit Designer must be added to adjust the reaction conversion.

Add a Unit Designer that connects to the Tank itself.

This is shown in Figure 10.

Figure 10. Adding a Unit Designer that will adjust the reaction conversion.

Select the properties for the Unit Designer so that they are the same as those shown in Figure 11.

Figure 11. Unit Designer Properties for ensuring that the copper in the residue is 10%.

Follow these steps in changing and checking the Designer properties:

Take care to attach the Unit Designer to the correct ProcessUnit.

Setting the value of the ProcessUnitID, highlighted in Figure 12, does this.

Look up the ProcessUnitId by selecting the ProcessUnit and then navigating to the ProcessUnitID property.

Check that the measurement connector is properly connected. Look to see if the MeasurePoint and MeasurementPoinID have values in their property boxes.

Check that the design connector is properly connected. Look to see if the DesignPoint and DesignPoinID have values in their property boxes.

Set the Measurement Variable. To set the measurement properties, expand the MeasurementVariable option by clicking on the + sign in the left margin of the Properties Viewer.

Under the "Variable" heading, select "Element Composition" from the drop down menu. Under the "Value" heading, type in the number 10.

In the "Component", click on the ellipsis button, and, from the Element Selection Form that is launched, select the Cu(s) element.

Set the Design Variable. Expand the DesignVariable option by clicking on the + sign in the left margin of the Properties Viewer.

Under Variable, select "Reaction". In the Component box, click on the ellipsis which launches a Reaction Selection Form. From this form, select the copper leaching reaction.

Under DesignMax set the maximum value for the reaction extent to a value 1. Similarly, under the DesignMin set the minimum value for the reaction extent to a value of 0.75.

Please take care to correctly setup the unit designers, as incorrect inputs in their setup will lead to the wrong results

A summary of what each unit designer should be doing is presented in Table 1.

Table 1: Unit Designer’s measured and controlled variables

 Designer Measurement Value Control Variable Value 1 [H2SO4(aq)] 10 g/L Flow of spent 2-5 t/hr 2 [Cu(s)] 10% Cu reaction extent 0.75-1

Also, remember to set the “Convergence Method” to “Secant”.

#### Step 12: Calculate and examine the results

Run the flowsheet calculation as before.

The results for the PRODUCT stream are shown in Figure 12.

Figure 12. The results for the PRODUCT stream, indicating that both the acid concentration and copper residue constraint have been met.

What should be seen in Figure 12 is that both unit designers achieved their objective. The first unit designer changed the spent flowrate to achieve a 10 g/L free acid concentration in the product stream. The second unit designer changed the reaction extent to ensure the residue contained 10% copper. These value are indicated by the arrows in Figure 12.