Question

In the continuously stirred tank, see Figure, an acid process flow is neutralized by adding a concentrated NaOH solution. The acid process flow has a tendency to vary its pH with time, which gives undesired variation of the pH in the outflow. In an effort to reduce these variations one has decided to use control.

a) Classify the different signals as input, output, and disturbance signal. b) Draw a block diagram of the system with a control strategy..

Acid process flow NaOH solution Flow with desired pH

Answer 1

a) In the simplest terms, the given diagram and description tells us that we wish to control the pH value of the effluent from the tank. Since it is continuously sutirred we can assume that the temperature and pH throughout the tank are uniform. Since we wish to control pH value this becomes the controlled variable (or process variable) this needs to be measured (if we are planning to use feedback control strategy).

Looking at the two inlets, we can see that the acid inflow is not controllable as it does not have a control valve. Whereas, the NaOH inlet has a control valve such that flow of base can be manipulated to regulate the pH to a desired value. Therefore acid inlfow is disturbance to the process as it will make the pH deviate from desired value and base inflow is the manipulated variable which we will use the bring back the deviated pH to our desired value.

b)We can use the following control strategies to control pH.

1. Feedback control— we measure pH using analytical transducer and give that information to the controller which will adjust the valve position to change to base inflow in order to compensate for disturbance (acid inflow).

2. Feedforward control—in this strategy, we don't measure pH, instead we measure acid inflow and give this information (assuming pH of this liquidtliquid isais constant)feedforward controller which will manipulate base inflow. This type of controller anticipates the effect of disturbance and makes amends before it has affected the control variable. Feedforward control is faster than feedback control but fails when unmeasured disturbance is present (like temperature change or pH change of acid influent).

3. Ratio control— this is also a type of feedforward control where we measure both inlfows and give this information to a ratio calculator with set point this ratio calculator will obtain the live ratio of acid and base inflow and compare it to a desired value and forward this information to a flow controller which will adjust the valve position to manipulate base inflow thereby maintaing acid/base ratio for pH control.

ED FAQ ® Ratio calculator FOJ FC Flow controller Acid base *FT for base flow outlet flow Ratio control Inference is placed after consideration of adequate pike straight runs necessary for accurate Measurements

Acid Base (fok Set point Flow ly I controllar PHT PH Transmilta outlet Feed-back confered strategy feed forward flow FC Flow Aad Transmitter Basc outlet feed Joward contred strategy