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how to set up pH control with Ca(OH)2 instead of Cl-


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In a case where lime, Ca(OH)2, is used to control pH, do I still have to use Cl- in initial system constraints in X1t? In this scenario, a fixed amount of lime is added to the medium initially and more is added if pH falls lower than a target value. For example, initial lime amount = 0.5 g/kg, in a volume of 40 liters of the medium (column) and the target pH = 10.5. The target pH is not always strictly met.

Can I directly model the use of lime as pH control in this case? If not, then how would I determine the equivalent Cl- to use in the model?


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You do not have to include Cl- in your basis input if it is not in your system. X1t (also React, Phase2, and X2t) by default looks for Cl- as the balancing component unless otherwise specified. This is typical as chloride is an abundant species where most commercial labs would report its concentration based on a charge balance calculation. You can change to another abundant species for charge balancing in your run. To do so, click on the unit of the component that you want to balance species on and select "Balance species".

I am not quite sure what you mean by using the equivalent Cl- to control pH. It sounds like you are adjusting your fluid's pH by adding an initial dose of lime and then progressively more with to meet the target pH. You can add lime as a Reactant to see how the pH changes as it is titrated in your system. 

Another method to control pH with lime is to swap it in for lime in the Basis. This would tell the program to calculate the pH of the system in equilibrium with the lime. From your brief description, this doesn't sound like it describes the scenario. For more information on swapping in the basis, please see section 2.2 Setting and constraining the basis in the GWB Essentials User Guide. 

It might be helpful if you set up a React simulation to make sure that pH is behaving as intended before adding transport in X1t. 

Hope this helps,
Jia Wang
Aqueous Solutions LLC

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