The objective for this tutorial is to explore recycle streams. Often there is production and consumption of a reagent, such as sulphuric acid. If you are not familiar with the circuit that you are modelling then you can get into a situation where a component is completely depleted or where it builds up uncontrollably. Of course these problems are also found on the plant. Thus, you cannot operate this software or a plant without understanding the effects of recycles.
In this tutorial we will examine how to converge recycles in Cycad Process.
The process that we are going to model in this tutorial is the following: Copper is leached from a sulphide ore to produce copper in solution. The solution is filtered and then the copper is removed from solution by electrowinning. The concentration of the copper and acid in solution after leaching must be in the correct range so that copper can be directly recovered by electrowinning.
You will go through the following steps in this tutorial:
- Draw the flowsheet
- Add the components
- Add the electrowinning reaction
- Select parameters for the filter press
- Examine the results
- Add a solution bleed
- Add process water
- Add a unit designer to constrain the acid concentrate
- Examine the results
As before, the first step is to draw the flowsheet. The easiest place to start is to use the flowsheet from Tutorial 6.
Add two new Process Units to the diagram: a filter press and an electrowinning cell house. Connect these units as shown in Figure 1. Notice that at this stage we have not yet recycled the material from the electrowinning cell house back to the mixing tank TK-010.
You can change the direction of the filter and the electrowinning cell by choosing Format>Flip>Flip Horizontally from the main menu.
Add a spillage stream to the mixing tank labelled TK-010. This will be used to provide process water if needed.
Figure 1. Adding the two additional Process Units to the results of Tutorial 6
Add the following components the your balance
- Cu2S (s)
- H2SO4 (aq)
- CuS (s)
- SiO2 (s)
- H2O (l)
- CuSO4 (aq)
- H2SO4 (l)
- H2O (g)
- N2 (g)
- O2 (g)
- Cu (s)
Step up the components for the Feed stream and spent return stream as shown in Figure 2
Figure 2. Setup of the input streams
Add the reaction shown in Figure 3 to the electrowinning Process Unit.
Figure 3. The copper electrowinning reaction.
The equation shown in Figure 3 is:
CuSO4 (aq) + H2O (l) -> Cu (s) + H2SO4 (aq) + 0.5 O2 (g)
ext = 0.45
Select the filter press by clicking the left mouse button when the cursor is hovering above the filter.
The properties of the filter are displayed in the Properties Viewer.
Navigate to the calculation properties and change the "Cake Moisture" property to 30%.
Select the following process units (mixing tank (TK-020), autoclave (AU-010) and electrowinning cell (EW-010)). Open the evaporation rate tab, in the Properties window. Then set the OffGasSaturated property to “Yes” for each unit.
As before, change the maximum number of iterations and tolerance for the calculation.
The Calculations Options window is shown in Figure 4.
Change the maximum number of iterations to 100 and the tolerance to 1e-9.
Figure 4: Change the calculation options.
Press the Run Cycad Project button on the calculate process toolbar shown in Figure 5.
Figure 5. The Process Toolbar, with the cursor hovering over the Run Cycad Project button.
Since the spent electrolyte and spent return stream have not been connected yet, this gives us the opportunity to check for the build-up of acid in our system.
Let us compare the acid flowrate in the spent return stream (119*) against the acid flowrate produced in the spent electrolyte stream (117).
Figure 6. Comparing the acid coming into the flowsheet in stream 119 to the acid leaving the flowsheet in stream 117
It is clear from Figure 6 that there is a net production of acid [H2SO4 (aq)] in our process. This means that we need to bleed acid from the circuit if we want to connect up the spent recycle.
We will add a bleed in the next step. Before proceeding to this next step, please ensure that your copper cathode production rate is 0.915 t/h.
*This stream number is 102 if working from the previous worked example.
Add a bleed to the circuit as shown in Figure 7. Use the Stream Split unit from the Process library to split out a bleed from the electrowinning discharge.
Note: If streams cross and you would like to indicate that they cross rather than mix, then select the stream that is horizontal at the crossing, and bring it to the front of the diagram drawing order.
This is done as follows: Choose Format>Order>Bring to Front from the main menu.
Figure 7. Add a recycle bleed to the process.
Select the Stream Split unit and then enter 0.7 as the Split Fraction for this unit.
Then Run the split on its own by selecting the Stream Split and then pressing on the "Run Selected Unit" button on the Process Toolbar. This will run only the selected unit.
Check that only 30% of stream 117 mass flow rate is bled from the circuit.
If it is the other way around, change the Split Fraction to a value of 0.3 instead.
Note: The “Spent” stream has now replaced by the recycle, which is representative of the operational process.
In practice, the spillage stream contains mostly wash water. In order to represent this in the model, select the spillage stream then click on stream manager to change the water [H2O (l)] flowrate of the spillage stream to 15 t/hr.
Add a unit designer, as shown in Figure 8, to control the fresh acid addition (stream 103) in order to maintain the concentration of acid entering the autoclave (stream 106) to 27 g/L. (Follow the steps in Tutorial 4 to do this).
Figure 8. Add a Unit Designer to constrain the acid concentration entering the autoclave to 27 g/L.
Press the Run Cycad Project button on the calculate process toolbar, shown in Figure 9.
Figure 9. The Process Toolbar, with the cursor hovering over the Run Cycad Project button.
Analyze the Results !!!!!!
The overall results can be examined by pressing the Production tab above the Page Viewer. The results that you should get are shown in Figures 10.
Figure 10. The production and reagent report summary