Report the process on the project
Report of current process on the project
PH value is a very common chemical on-line measurement in process industry. A good pH control performance is the final target. However, there are many different elements that all contribute to pH control problem, including the basic understanding of what pH value actually is, pH measurement, pH modeling and simulation and finally the actual pH control. All these elements are tried to look into, at least superficially.
In order to understand better the principles of modeling and controlling the pH value, and achieve the actual pH value, these steps are proposed to follow which are shown as below:
The first step is to be familiar with the basic properties of liquids, ionic reactions and what is their relation to pH value. The study includes the properties of liquids, chemical reactions, acid and bases neutralization and the definition of pH.
Second step is pH measuring, simple specification based on the titration collect data from real site. In many cases the quality of the pH measurement is the reason for poor pH control performance. Different pH measuring principles solutions and weaknesses are learnt.
Third step is pH modelling: comperation of static and dynamic model, the traditional pH dynamic models: wiener models, reaction invariants and physico-chemical models that separate the dynamics and highly nonlinear static ph mapping are learnt and analysis. Define the variable data in the pH neutralization process; build the mechanistic model for the real pH neutralization tank.
Forth step is pH control diagram design: The basic control paradigms, including feedback and feed forward structures, applied to pH process are compared and leant. Linearised the feedback control by using an inverse of the titration curve.
Model based PI controller design.
Closed-loop control system simulations.
Analysis and conclusion.
pH models are crudely divided into static and dynamic models
Static models are valid when the system has reached the equilibrium. The acid-base unit reactions can be considered instantaneous and therefore static modeling is a very natural approach. Static models include titration curves and distribution diagrams.
Titration curves are traditional classical pH-models. They describe the pH-value as a function of acid-base content difference. The shape of the titration curve is determined by the participating chemical components.
Titration curve shows the dependency between pH (function of oxonium ion activity) and acid/base concentration. Strong acids and bases dissociate completely and their ion concentrations are total concentrations as well. Weak components only dissociate partially and their ion concentrations have to be calculated from equilibrium constants.
Dynamic pH process modeling
Dynamic modeling is fundamentally more difficult than static modeling. Static modeling concentrates only on the...