module b- ph control

8/4/2019 Module B- pH Control

1/17

PID pH Control Laboratory Manual

Page 1 Chemical Engineering Department

ChE 381-Process Lab ProjectspH Control:On/Off pH Control & Closed Loop PID pH Control Loop Tuning

Laboratory Manual

Dr Parviz Safaeefar

Department of Chemical Engineering

Curtin University of Technology


2/17



On/Off pH Control & Closed Loop PID pH Control Loop Tuning

Objective

1.

To demonstrate the characteristics of On/Off control in pH control.2. To demonstrate the PID loop tuning procedure on a pH control.

Background

PID Control

In operation, the proportional controller calculates the amount of error between themeasurement and set point, amplifies it, and positions the final control element toreduce the error. Integral action avoids the offset created in proportional onlycontrol by bringing the output back to the set point. Proportional-plus-integral (PI)control is generally used on processes where no amount of offset can be tolerated.However, some processes are so difficult to control or so critical to maintain at set point,that the use of all three modes will be helpful in achieving desired control. Derivative controlresponds to the rate of range of error. Hence, combined PID control responds to allaspects of process errordirection, magnitude, duration, and rate of change. The output ofa PID controller is a linear combination of P, I, and D modes of control.Processes that benefit most from PID control have rapid and large disturbances in whichderivative action can respond to the rapidity of the changes, and integral action canrespond to the duration of them.

Figure 1: System response to a process upset with different modes of analog

Control


3/17



GENERAL OPERATING PROCEDURES

General Start-up Procedures

1. Make sure that all valves are initially closed.

2. Prepare 20 liters of 0.02M Acid (HCL) and 20 liters of 0.02M Base (NaOH) with

deionised water in Acid Tank TN1 and Base Tank TN2, respectively (Pleaserefer to Section 5.3 to 5.4 for the preparation of acid and base solution).

3. Turn on control panel power supply.

4. Remove protective cover for sensor AE1.Note: To do this, the sensor has to be taken out from its PVC holder by loosen

the jam screw. Put back into its original position after it is done.

5. Fill the Buffer Tank TN3 with deionised water (Please refer to Section 5.2 onthe preparation of deionised water).

6. Open valves HV1 and HV4. Then fill up TN4 with deionised water by switchingon Buffer Pump (P3).

Note: Top up deionised water in TN3 as necessary to avoid it from empty.7. Stops pump P3 when water starts to overflow into TN5.

8. Switch on mixer M1 and adjust the speed to 30%.

9. Check that the stroke length of both Acid and Base pump is adjusted to 100%.Then, check that the frequency of the acid pump is set to 40%.

10. For Yokogawa controller and recorder operations please refer to Appendix A.Note: Yokogawa recorder chart trend speed has been preset at 600mm/hr.

Preparation of Deionised Water

1.

Check the deionizer battery level and its water quality to make sure that thedeionizer is still in good condition. Replace the resin if necessary.

2. Initially, make sure that the valves (HV1-HV4) are closed.

3. Supply the laboratory tab water to the deionizer by opening valve HV1.

4. Open valve V4 to get deionised water.

Preparation of Acid in Tank TN1

1. Fill up tank TN1 with 20 liters of deionised water.

2. To prepare a 0.02M acid solution, measure 33.5 ml of hydrochloric acid (36%

HCl) into TN1 and mix it well with the deionised water.

3. To top up TN1 with 0.02M acid solution, mix 17 ml of hydrochloric acid (36%HCl ) with 10 liters of deionised water.

Note: If hydrochloric acid with different concentration is used, recalculate theamount required.


4/17



Preparation of Base in Tank TN2

1. Fill up tank TN2 with 20 liters of deionised water.

2. To prepare a 0.02M base solution, weigh 16 grams of sodium hydroxide(NaOH) and dissolve into the deionised water in TN2.

3. To top up TN2 with 0.02M base solution, dissolve 8 grams of NaOH into the

each 10 liters of deionised water.

Discharging the Waste from Sump Tanks

1. Check the local authorities for the waste discharge standard.

2. Before discharging the waste, open valve HV2 and HV3. Then, switch onpump (P4) to reduce the conductivity to an allowable level.

3. Open valve HV9 to drain the waste.

Experiments

Experiment 1: On/Off pH Control

Objective: To demonstrate the Characteristics of On/Off Control in pH

Control

1. Switch the control selector to PID control.

2. Set the controller into Manual mode and adjust set point (SV) to pH 7.0.

3. Switch on the Acid Metering Pump P1 follows by the Base Metering Pump P2.

4. Adjust the output (MV) gradually so that the pH measurement (PV) in the tankTN4 matches the set point (SV) of pH 7.0.

5. Enter the following values in pH controller and put the controller into AutoMode and start recording the PV and MV values.

PB 2%

I 1s

D 0s

6. Simulate a load change by increasing the Acid Metering Pump P1 stroking rateto 100% for 30 seconds and restore P1 to its initial rate (40%).

7. Stop the experiment once the measurement exhibits a uniform pattern.

Note:For Yokogawa controller and recorder operations please refer to Appendix A.Yokogawa recorder chart speed has been preset at 600mm/hr.


5/17



Experiment 2: Closed Loop PID pH Control Loop Tuning (Ziegler and NicholsMethod)

Objective: To demonstrate the PID loop tuning procedure on a pH control.

1. Switch the control selector to PID control.

2. Set the controller into Manual mode and adjust set point (SV) to pH 7.0.

3. Switch on the Acid Metering Pump P1 follows by the Base Metering Pump P2.

4. Adjust the output (MV) gradually so that the pH measurement (PV) of the tankTN4 matches the set point (SV) of pH 7.0.

5. Enter the following values in pH controller and put the controller back intoAuto Mode and start recording.

PB 100%

I 9999sD 0s

6. Simulate a set point by changing SV value to pH 7.5. Wait until themeasurement exhibits a uniform pattern and observe if any equal amplitudeoscillation occurs.

7. Maintain the value of I and D, and repeat step 2 to step 4 with reduced PBvalue until the measurement oscillates at a constant amplitude about the setpoint. Suggested values are as follows: -

PB*

50

30

20

10

8. Determine the natural period using the following method:

Natural Period, s3600TrendSpeed

DT =

Where D = Distance in mm between successive crests or valleys

9. Set new PB to 1.66PB*, new I to 0.5T and new D to 0.125T. Test the

new PID values with load change and observe the response.


6/17



General Shut Down Procedures

1. Switch off all pumps.

2. Switch off mixers M1.

3. Turn off control panel power supply.

4. Switch off the main power. Then, put back caps for both of the pens in thechart recorder.Note: Always put back the caps to the pen after end of experiments, as the

ink will dry rapidly with the tip exposed. Make sure to use pen caps ofthe same ink color. If a pen cap of a different ink color is used on thepen, the remaining ink in the cap may be absorbed through the pen tipand the ink may change its color.

5. Drain all the tanks completely by opening drain valves (HV5-HV9).

6. Fill the protective cover for pH sensor with distilled water and replace it if thesensor is not in use.

EQUIPMENT MAINTENANCE

1. ALWAYS put back the caps on the respective pen after the end of experiments, as theink will dry rapidly with the tip exposed.

2. Drain all the tanks completely when the unit is not in use.

3. The pH sensor AE1 should always be capped with its cover filled with distilled waterwhenever the mixing tank TN4 is empty.

4. The pH sensor electrodes are susceptible to coating by many substances and theresponse time can deteriorate dramatically. If they are mechanically intact they canoften be restored to normal performance by one of the following procedures:

Salt deposits: HCl 0.1M (5min) then NaOH 0.1M (5 min)

Oil/grease films: detergent and water

Clogged reference junction: KCl 0.1M (60-800C during 10 min)

5. Wipe off any spillage on the unit immediately.

6. Always check that all indicators are functioning.

7. Always check that all the pumps are working properly.

8. Clean the equipment with damp cloth every month.

9. Replace the chart paper and the felt pens of the chart recorder whenever necessary.

Note: Please refer to Appendix B on how to replace them.

SAFETY PRECAUTIONS

1. The unit must be operated under the supervision of trained personnel.

2. Students should be briefed on the safety precautions in handling any hazardouschemicals. Should there be any skin contact the affected parts should be washedquickly with a lot of water without any delay.


7/17



3. Proper attire such as laboratory coat, fully covered shoes (no sandals), gloves and eyegoggles or safety spectacles should be worn when handling concentrated acids oralkali.

4. Avoid metal contact with chemicals to avoid any corrosion. If there is any spillage ofchemicals on the floor or platform, use absorbent material to absorb and wipe it out,then use water hose to flush the spillage area.

5. Before draining tanks by using drain valves mounted at the bottom of each raised tankfrom the platform, always make sure drainage system is properly connected into thedesignated discharge area and always rinse the inside tank with water. Make sure allanalyzer electrodes are always be kept wet and immersed in water or solution.

6. Never use aqueous ammonia because it may attack the Viton O-rings used in somepumps.

7. All operating manuals supplied with the unit must be completely read and understandbefore conducting any experiment on the unit.

References

1. Perrys Chemical Engineers Handbook, 7th Edition, (Ed: R.H. Perry, Green, andJ.O. Maloney), McGraw-Hill, New York, 1997

2. Principles and Practice of Automatic Process Control, 2nd Edition, Carlos A. Smithand Armando B. Corripio, Wiley, 1997

3. CE Refresher: Process Automation, McGraw0Hill, Inc. New York, 1985


8/17




9/17



Figure 1: Unit Construction for pH Control

5

4

32

67

8

9

1

6


10/17



Table 1: Instruments Description for pH Control

No. Instrument Tag. No. Description Range

1 PID Controller AIC-416Microprocessor based PIDcontroller, controlling solution pH

-

2 Recorder AR1 Continuous 2pen recorder forpH recording.

-

3 Agitator M1Electrical stirrer to ensure goodmixing in mixing tank (TN4)

0 500 RPM

4 PH Sensor + Transmitter AE1 pH sensor for Mixing tank 0 14

5 Process Tanks

TN1TN2TN3TN4TN5

Acid TankBase TankBuffer TankMixing TankSump Tank

30 L30 L30 L50 L50 L

6 Hand Valves HV1-HV9Input/Output Isolation valves to

direct the process flow-

7 Process Pumps

P1P2P3P4

Acid dosing pumpBase dosing pumpBuffer solution dosing pumpCirculation pump

0 19 LPH0 19 LPH0 20 LPM0 20 LPM

8 Deionizer E1Packed column filled with cationand anion exchange resin forwater demineralisation purpose

-

9 Control Panel -

Monitoring/installation ofcontroller, chart recorder, stirrerspeed controller, power supplyand pump switches and change

over switch for PID and Non-Linear control

-

Set Point Change from pH 7.0 to pH 9.5

Set Point Change from pH 9.5 to pH 7.0Set Point Change from pH 7.0 to pH 4.0Set Point Change from pH 4.0 to pH 7.0


11/17



Appendix AInstrument Operation Guide


12/17



Appendix A.1: YOKOGAWA Loop Controller Operation

i. Selection of Operation Panel Group1. When power is turned on, initialization of the controller is carried out. Then, the controller

shows the operation panel group.

2. Press the key while holding down the key, the LCD display panel group isselected one after another in the following sequence:- Operation Panel Group- Tuning Panel Group- Engineering Panel Group

The controller shows the operation panel group again by pressing the key while

holding down the key.

ii. Selection Operation of the Operation Panel1. Press the key while holding down the key to select the Operation Panel

Group, and an operation panel will be displayed.

2. The operation panel changes from current to next one every time you press thebutton as shown below:

Loop 1 & Loop 2 Trend 1 & Trend 2 Alarm


13/17



iii. Auto/Manual Mode Selection1. Select the Loop 1 panel (as described in Section ii).

2. Press / mode key to select the desired control operation mode. The LEDindicator in the key which corresponds to the selected operation mode key is lit.

: A Mode KeySelects the A (Automatic) control operation mode

: M Mode KeySelects the M (Manual) control operation mode

iv. Change of Output/Manipulated Value (MV)1. Select the (Manual) control mode (as described in Section iii), use the

and keys to operate the output / Manipulated Value (MV) manually.

2. An increase or decrease of the MV value can be accelerated by holding down thekey, then pressing the MV operation key.

: MV Operation Increasing KeyIncreases the MV value

: MV Operation Decreasing KeyDecreases the MV value

v. Change of Set Point/Set Value (SV)1. Use the / key to change the set value (SV) of the control loop. This function is

available in the A (Automatic) and M (Manual) control operation mode.

: SV Operation Increasing KeyIncreases the SV value

: SV Operation Decreasing Key

Decreases the SV value


14/17



vi. Change of PB, I and D Value1. Press the key while holding down the key to display the Tuning Panel

Group.

2. Press the push button to observe PID 1 pull-down menu. The PID menus providedetailed information regarding the PID function blocks.

3. The and keys (adjacent to the & keys) allow you to stepupward or downward through the menu.

4. The and keys permit changes to the selected line.

5. To change the PID value, first toggle the key until PB1 (Proportional Band Value) ishighlighted. PB1 is shown at the top of the display below the PID1 title. The PB1 value can

be changed by holding down the or keys until the desired value is set. Then

toggle the key to select TI1 and TD1 to change the value of the Integral Action andDerivative Action, respectively.

6. Pressing the button transfers the display to the Tuning Menu Group menu.

7. Finally, press the key while holding down the key again to get back to theOperation Panel.


15/17



vii.Auto Tuning/Self-Tuning Control (STC)1. First, set the control operation mode to (Manual Mode).

2. Press the key while holding down the key to display the Tuning PanelGroup.

3. Press the push button to observe STC 1 pull-down menu. The STC menus providedetailed information regarding the PID function blocks.

4. Toggle the key until MI 1 (Signal Defection applied to MV Value) is highlighted. MI1is shown at the top of the display below the STC 1 title. Change the MI 1 to 20 % by

holding down the or keys. The MI 1 value designated will shift the processvariable by about 20 % during the auto tuning process.

5. Toggle the key until the STC is highlighted. Set the STC mode to ATSTUP by

holding down the key for 3 seconds.

6. Pressing the button transfers the display to the Tuning Menu Group menu.

7. Then, press the key while holding down the key again to get back to the

Operation Panel. Set the control operation mode to the (Automatic Mode) and now,the automatic tuning begins.ATSTUP appears on the control status display position of the loop panel. With theautomatic tuning, PID control has not been started yet. After 30 seconds, the controllerapplies the step variation (MI %) automatically to the manipulated output.

8. When the process variable has been stabilized, the controller will automatically set themanipulated output back to the previous setting.

9. When all setting items are prepared, the mode is automatically set to STC=ON, and thePID control is started. At the same time, the control status display of the loop panel

changes to STC-ON.10.To turn off the STC mode, go to the STC1 pull down menu and set the STC mode to OFF.11.Another way to turn off the STC mode is to change the PB, TI and TD value to any desired

value.


16/17



Appendix A.2: YOKOGAWA Chart Recorder Operation

i. Changing the Chart Speed1. Hold down the MENU key for 3 seconds to enter the Setting Mode.

2. Press the key to show Chart and then press the key.

3. Press the key to select the desired speed. Then press to enter the setvalue.

4. When the Setting complete screen appears, the new chart speed is applied.5. Press the ESC key to change the chart speed again or hold down the MENU key for 3

seconds to return to Operation Mode.

ii. Starting/Stopping the Recorder1. Turn on the Power Switch of the recorder.2. Check that the recorder pens and chart paper have been installed properly.3. Press the RCD key to start recording. The status display shows the word RECORD.

4.

While the recording is in progress, press the RCD key to stop recording. The wordRECORD on the status display clears.


17/17



Appendix A.3: Temperature Controller Operation

iii. Changing the Set Point1. Press button to select the set point setting mode.

2. / button to set the desired temperature. The set point is indicated on the SV

display (yellow digit LED display)3. Press button again to register the setting value.

module b- ph control

Documents