Pc Module2

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ChE 4L02

Process Control Laboratory

pH SYSTEMS

FALL 2002

Office hours Telephone Email

Instructor: Y. Samyudia JHE 345A9:00-4:00, M-F

Ext. 23273 samyudi

Teaching Assistant: R. Baker JHE 370B Ext. 27429 bakerr

CONTENTS

I. Laboratory Relevance and GoalsII. Preparation Prior to the First LaboratoryIII. Safety and EquipmentIV. Start-up and Shut-down ProceduresV. Preliminary Tasks

VI. Process Modeling and ControlA. Empirical Model Identification and Pressure ControlB. Empirical Model Identification and Flow ControlC. pH ControlD. Self-Directed LearningE. Summary Report

VII. Major Laboratory Tasks and ScheduleResourcesReferences


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Laboratory Relevance and Goals

The pH of liquids affects important behavior, such as reaction rates, vapor-liquidequilibrium, and corrosivity. In addition, the instrumentation used in the laboratorydemonstrates a range of typical control equipment. Therefore, you will gain useful

experience in process control by working with this equipment. In addition, therequired tasks for this laboratory enable you to apply principles and practice thatyou learned in your process control course to a real (not simulated) physical process.

This laboratory has the following goals, which are in addition to the general goalspresented in the course outline.

Understand instrumentation in typical control loops Perform empirical modeling and controller tuning for a complex cascade and

multi-loop system.

Gain experience with digital equipment for implementing the PID feedbackalgorithm

Define and perform a Self-Directed Learning investigation

1. Preparation Prior to the First Laboratory Session

To make the best use of your time in the laboratory, you will need to prepare beforethe first session. This laboratory involves the following technologies andequipment.

Principles of pH Empirical modeling of process dynamics Feedback control using a proportional-integral-derivative controller in single-

loop and cascade control Measuring pressure, flow and pH Manipulating the opening of a control valve

You should (1) read this document thoroughly and (2) review material on the topicshighlighted above. You should review/learn the process reaction curve method anda statistical-based parameter estimation method, since you will use both in this

laboratory. Your group can share the responsibility for learning these topics andteaching all other members of the group.

Before the first laboratory session, you will need to answer a short quiz to testyour preparation. The quiz will be posted in the web and constitute 5% of thelaboratory grade.


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2. Safety and Equipment

Before you begin performing experiments, you need to understand the equipmentand how to operate it safely. To assist you, a schematic is given in Figure 1; pleasenote that this schematic is far too simple to serve as a piping and instrumentation

drawing. You should begin your first session by performing the following tasks.

A. Safety Learn the safety procedures from the TA, ask questions, and complete thesafety form when you are convinced that you can operate the equipmentsafely and know how to respond to likely emergencies. Note that eyeprotection is required in the laboratory.

B. Process equipment The TA will explain the basic equipment involved in the laboratory. It is your

responsibility to understand all equipment, i.e., be able to explain the purposeand physical principles for every element in the equipment. During the firstsession, you must collect information and prepare a sketch of the equipment.

C. Control Station A LabVIEW-based computer control has been developed for this experiment.

You just turn on the computer and double click the pH labViews icon so thatthe following diagram will appear in your computers monitor. The TA willexplain the main function of this station.

You are required to provide a piping and instrumentation (P&I) diagram of the

laboratory equipment in your report. Each group needs to prepare only one

drawing; a copy of this drawing must appear in the reports from each group

members.


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The drawing should include the process from the feed tanks to the effluentdrain. See Austin (1979), Weaver (1986) and Woods (1995) for examples ofthese drawings.

D. Control equipment Sensors, displays, valves, and control calculation

equipment are essential for automatic process control. You shouldthoroughly understand this equipment. As background, you can refer to thediscussion of the feedback control loop elements given in Chapter 7 of Marlin(2000) and much more detail provided in Andrew and Williams (1979-1982).

The digital controller is based on the following continuous PID formulation.

Idt

tdCVTdtEtE

TtEKKtMV

t

fd

I

csense +

+=

0

)(')()'(

1)()(

withE(t) = SP(t) CVf(t)CVf(t) = The filtered value of the measured variable. The filter time

constants are quite fast, so that the effect is about 1-2 seconds offiltering.

Ksense = determines the sign of the feedback; this is determined by thedirect (reverse) acting switch. (See Marlin (2000), Chapter 12)

KC = the controller gain. This must be positive. The units arecontroller output/CV in engineering units.

TI = the integral time (minutes)Td = the derivative time (minutes)

MV(t) = the controller output in units of (1) 0-100% if sent to a valve or(2) the secondary CV range if sent to a secondary in cascade.

The integral mode differs from the standard algorithm by the term

(E), which is unique to the LabView software. The term is defined in thefollowing equation.

+

=

range2

2

SP)t(E

1

1)E( with SPrange = Spmax SPmin

This term has the effect of reducing the integral mode when the error is large.

The controller is executed at a constant period of 1 second. Since this is veryfast, the controller functions essentially as a continuous controller.


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Also, the data is stored at a period of 1 second. You must remember to begindata storage before an experiment. Should you decide that the data isworthwhile, you can store in on the disk. Later, you will be able to import thedata into Excel for plotting and calculations.

You will operate the equipment using the displays provided by the LabViewsoftware. You have an overview display that enables you to observe all keyprocess variables and make most required changes to process operation. Inaddition, you have a separate display for every controller; you can changecontroller parameters, such as tuning constants, using these screens.

3. Start-up and Shut-down Procedures

The TA will instruct you on how to startup and shutdown the equipment.The procedures are summarized in Tables 1 and 2. Please be sure that youunderstand these instructions.


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Table 1. Instructions for Starting up the pH Experiment.

1. Verify that both feed tanks contain sufficient liquid for the duration of the experiment, whichis 50L of acid and 100L of base.

2. Verify that the water inlet valves and the drain outlet valves to/from the feed tanks areclosed.

3. Open the valve on the instrument airline at the back of the equipment.

4. Open the water valve to dilute the effluent as it enters the drain.5. Switch on the power to the experimental equipment; the orange LED will light up.6. Open the pH Control program by double clicking the pH Control.exe icon on the desktop.

The program will load and begin to run. There is no reason to stop the program at anytime during the experiment.

7. Perform a 3-point calibration on the pH sensor using the buffer solutions provided.8. Set both Bronco II DC drives, which control the pump speeds, on manual and stop

modes. Set the dials to about 50%. These are located at the top of the equipment panel.9. Make sure all controllers are in the manual mode (OFF).10. Make changes to the valve positions to ensure they are working properly by changing the

controller outputs (both FCs and both VCs) in manual and observing the stempositions; then, set them all to 50%.

11. Flip the switch on one of the Bronco II drives from stop to start (the switch must be heldup until the pump starts spinning). When the pump is running, flip the manual switchto automatic, at which point the pump will stop. Repeat for the other pump.

12. After determining the controller parameter, switch the pressure controllers to automatic oron, and enter the desired set point values for each (see Table 3). Allow them toachieve steady state.

13. Adjust the acid and base large control valves (VCs outputs) to achieve nearly the desired

flow rates.14. Close the bottom outlet valves on both tanks, and close the valve on the second

neutralization tank about halfway, so the liquid level is high enough for the probe to beinserted. Turn on the mixers once the liquid level is at least 1 inch.

15. Clean off the pH probe with distilledwater and insert it into the secondtank.

16. Manually adjust both large valves (VC outputs) until you are close to the desired flows.Then, switch both flow controllers into automatic.

Table 2. Instructions for Shutting Down the pH Experiment.

1. Place all controllers in manual or off

2. Set the pressure controller outputs (motor speeds) to zero percent.3. After the pressures have decreased, close the acid and base large and small control

valves.

4. Switch the pumps off5. Turn off the tank mixers.

6. Open the bottom outlet valves from both neutralization tanks.7. Close the instrument air valve.8. Close the water dilution valve to drain.

9. Remove the pH probe and place it in its storage medium.10. Exit the LabView program using the x in the top right corner of the window, and turn

off the display.11. Turn off the power to the experimental equipment; the orange LED with go off.12. Clean the areas around the computer and experimental equipment.


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4. Preliminary Tasks

One of your important learning goals is to understand all elements in a control loop.Control equipment is not perfect, so that you should understand its actual behaviorand how the behavior influences the dynamic performance of control systems. In

this section, you will investigate the behavior of individual elements before youconsider the integrated control loop. To answer these questions, you will have toreview the equipment carefully, locate information in references, and performsimple empirical tests.

A. Control valve Answer the following question about the control valve thatcan be manipulated by the feedback controller.5. What type of valve body (globe, ball, diaphragm, etc.) is used for the large

acid? Is this a good choice?6. How much stiction and hysteresis exists in the large acid valve

performance?7. Describe the dynamic response of the large acid valve.8. The acid flow is influenced by two valves, large and small. What is the

advantage of this approach? Why is it especially advantageous for pHcontrol?

B. The pH is measured.1. What is the physical principle of the sensor? What reproducibility and

accuracy should we expect?2. Where is the sensor located? Discuss the advantages and disadvantages

for this location.C. Flow Measurement

Acid and base flows are measured for use in flow controllers.1. The acid and base flows are measured by sensors using different

principles. Describe the principles for each.2. Which sensor do we expect to provide better accuracy? What is the

expected accuracy of each? What is the cost for each?

You are required to provide written answers to these questions after the firstlab session. These answers will count for 5% of your laboratory grade.


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5. Process Modeling and Control

The goal of this laboratory is to develop a control system to maintain the effluent pHnear its set point. The typical operating conditions in the process are given in the

following. The flow of the base represents the product stream that must beneutralized; you are not allowed to adjust this stream to achieve pH control. Theacid flow is the utility stream that can be adjusted to achieve pH control.

When you have completed the startup procedure, the process will be in operationnear the desired conditions in Table 3. All automatic control will be off or inmanual, and the data storage will be off.

You will perform typical tasks to

i. Determine the dynamic behavior of the system and tune the pressurecontroller,

ii. Determine the dynamic behavior of the system and tune the flow controlleriii. Implement pH control, andiv. Perform a self-directed study proposed by your group and accepted by the

instructor.

However, chemical engineers should always apply their process knowledge wheninvolved in process operations and control. Therefore, you should review theprinciples of the pH process before beginning this experiment.

A. Empirical Model Identification and Pressure Control

Engineers use models, either qualitative or quantitative, for nearly every task inplant design and operation. Since fundamental models are time consuming todevelop and require data that might not be available, we often build models based

Table 3. Typical operating conditions for the pH Experiment

Measurement Controller outputAcid Pressure, P1 6 psig Acid pump 50%

Base Pressure, P2 3 psig Base Pump 80%

Acid flow, F1 250 mL/min Acid vales large 20%small 50%

Base flow, F2 500 mL/min Base valves large 55%Small 50%


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on experimental data; these are termed empirical models. You should review/learnthe process reaction curve, since you will use it in this laboratory. When performingthese experiments, manipulate the valve using the PC-based LabView system;observe the results on the screen graphs, and save worthwhile data for later use.

1. For each flow system, determine the models between the pump speed andpressure using the process reaction curve method.a. Design the experiment and justify the designb. Perform the design and evaluate the data (review 3P03 for data

evaluation)c. Estimate the model parameters and discuss the accuracy of your results

2. For each flow system, tune the feedback controller.a. Calculate the tuning constantsb. Implement, test, and fine tune

Important Data storage

You can observe the dynamic responses on the real-time screen plots provided by the LabViewprogram. However, you cannot plot these directly or manipulate the data for calculations.

Therefore, we have provided the ability to save data to a file. Please remember to follow thefollowing instructions.

1. The data storage is initially off and will not store data until you take action2. You can begin to store data by changing the save data switch to ON. (You should

not leave this on at all times, because you will store a very large file and you will notbe able to easily find the relevant data.)

3. After you have completed an experiment, you can stop saving data. The program willprompt you to store the data. If you believe that the data will be useful, save the data.We recommend that you use the .xls extension, so that the data in saved as an Excel

spreadsheet.4. Be sure to give the file a meaningful name so that you distinguish the data from

different experiments.5. Do not run any other programs at the same time that you are running LabView. You

can copy to a floppy if you want to use the file in Excel during the lab period.

6. You can open the Excel file and plot and perform calculations. When you save the fileafter modification, BE SURE to save as an Excel spreadsheet this will require you to

change the default Save as type!7. You should not include the data listings in your lab report. However, you must retain

the data in case the instructor has questions.


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B. Empirical Model Identification and Flow Control

For each flow system, determine the models between the small valve and the flow,and tune the flow controller using the same method as in (1) and (2) above.

C. pH Control

After parts A and B, we have stabilized the flows, but we have no automatic controlof the effluent pH. In this laboratory, you will implement a feedback controller tocontrol pH bymanipulating the acid flow rate. This is a cascade design! (Its time to reviewcascade control if its a little fuzzy after the summer.)

1. Discuss advantages for the cascade design.

2. Perform a process reaction curve experiments at several ranges of the effluent pH, e.g., 4,

5.5, 7, 8.5 and 10. Why would we want to investigate various operating conditions?Compare your results with the theoretical titration curve.

3. Calculate the tuning at three conditions and explain differences.4. Implement the pH controller and evaluate the dynamic performance for the following

changes.

5. A set point change of 0.50 pH units.

6. A change in the base flow rate of -75 liters per minute.

7. Discuss strengths and weaknesses of the control system that you have designed. Theseissues should provide some ideas for the SDL exercise.

D. Self-Directed Learning

In this section, you will define a knowledge-based learning goal that is related tothis laboratory. By knowledge-based, we mean a goal directed to improving yourunderstanding of process dynamics and control technology (not group skills,communication skills, etc.).

Your plan should include a program of investigation to achieve the goal. Theprogram can include experimental, theoretical, and literature tasks; it must includesome experimentation. Prepare a clear and concise explanation of your goal and theprogram for investigation.

In your final laboratory report, briefly explain why you selected these goals and howyou designed the experimental program, as well as specific conclusions based on theinvestigations.

You must submit your proposed SDL goal to the instructor or TA for approval before

you begin this part of the laboratory. The explanation must be presented during the mid-

laboratory review (2nd

session), if not discussed and approved earlier.


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E. Summary Report

3. Major Laboratory Tasks and Schedule

Guidance on the key tasks and when they should be completed is given in Table 4.Students can rearrange some of the tasks; however, the graded activities cannot berescheduled. Students are encouraged to discuss topics with the TA and instructorat any time. They can propose their SDL project before the second session, if theywant.

Table 4. Guidance for pH-system laboratoryTiming Ungraded activity Graded activity Tasks by completion

Before 1st session Read Laboratorydocumentation

Review relevanttechnology

5% Quiz onpreparation,which is postedin the web (*)

Prepare for 5%Quiz

Experimentalplant

1st session Safety training Equipment

orientation

5% Quiz as theanswers to thequestions in thismodule

Sketch ofequipment

Collect data for

dynamic modelsBetween 1st and 2nd Calculate modelparameters

Complete P&IDrawing

2nd session Mid-lab reviewmeeting

Define SDL task

5% Quiz (**) 5% Mid-Lab

Review

Feedback controlexperiments

3rd session 5% summaryreports

Complete SDLtask

One week after 3rdsession

Submit finalreport

* Cover understanding of this document, relevant theory, and initial experimental plan** Cover equipment principles, theory, and initial results from modeling experiments

At the end of each your Lab session, you must submit a preliminary reportthat summarizes your experiment design and collected data. The reports (3

preliminary reports for 3-lab sessions) are submitted to the TA and willcount for 5% of your laboratory grade.


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Resources

The basic references for this laboratory are given in the References section of thisdocument. As you encounter questions during the laboratory, you will need togather information from other published material in the library, on the WEB, and

from suppliers of industrial control equipment. Naturally, you are encouraged toask questions of the TA and instructor, but you will be expected to search and findinformation available in public references.

References

Andrew, W. and H. Williams, Applied Instrumentation in the Process Industries, GulfPublishing, Houston, Volume I, 1979; Volume II, 1980; Volume III, 1982.

Austin, D., Chemical Engineering Drawing Symbols, Halsted Press, London, 1979.Hough, M., E. Wood, and T. Marlin, Instrumentation Principles, available at

www.pc-education.mcmaster.ca (March 2001).Hoyle, D., Designing for pH Control, Chemical Engineering, p. 121, November 8, 1976.Marlin, T., Process Control (2nd Edition), McGraw-Hill, New York, 2000.

(or, Marlin, T., Process Control (1st Edition), McGraw-Hill, New York, 1995)McMillan, G., pH Control, A Magical Mystery Tour, InTech, 69-76, Sept. 1984.Vendors, Notebook on laboratory equipment available in laboratory room (Do not

remove!).Weaver, R., Process Piping Drafting (3rd Edition), Gulf Publishing, Houston, 1986.Woods, D., Process Design and Engineering Practice, Prentice Hall, Englewood Cliffs,

1995.
http://www.pc-education.mcmaster.ca/http://www.pc-education.mcmaster.ca/http://www.pc-education.mcmaster.ca/

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