Sometimes you need to mix two fluids to obtain a required component ratio in the outlet stream. For feed streams with a fixed composition, this can be a one time manual exercise to calculate the required flow ratio and then you use a SET operation to maintain your flow ratio. However, if at least one of your streams is in the middle of your flowsheet, you need to recalculate your ratio all the time. Here is the list of solutions in order of increasing efficiency:

- Use an Adjust to change the flow rate of Flow-2 until you get the desired component ratio

- Use a SET operation and Adjust the flow ratio until you get the desired component ratio

- Use a spreadsheet to calculate the required flow ratio

- Use a balance block to obtain the required flow ratio

The last option might be an unfamiliar one.

You would probably put the balance block in parallel with a normal mixer as shown below:

- Use an Adjust to change the flow rate of Flow-2 until you get the desired component ratio

- Use a SET operation and Adjust the flow ratio until you get the desired component ratio

- Use a spreadsheet to calculate the required flow ratio

- Use a balance block to obtain the required flow ratio

The last option might be an unfamiliar one.

You would probably put the balance block in parallel with a normal mixer as shown below:

Figure 1: Balance block and mixer in parallel

The goal is to make the normal mixer do what you'd normally expect from it and let the balance block do the component ratio calculation. In the above case, the composition of the two feed streams is known and the flow rate of one of them is also known. The requirement you have could be to have a ratio of 1 / 4 between methane and n-Butane. On the parameters page of the Balance block you can select "General" as the type. Now you can add a ratio between two or more components.

The Ratio definition screen looks like the below picture:

You can choose if you want a mass or mole or volume basis.

The key advantage is that you do not have to work out how to impose this ratio and there are no iterative calculations (outside the balance block anyways).