Neutralisation modeling

I was trying to find a dynamic model to simulate the behaviour of a simple neutralisation reactor in the chemical industry, where an alkaline input flow is neutralised with acid in a continuously stirred tank reactor. I was unable to find a complete and simple model in the web, thus I propose two different but equal solutions to this problem. It will be seen that the modeling part is quite quickly done, but it is quite difficult to propose a stable control loop for this highly non-linear system. Enjoy the read!