7/30/2019 ChemEngEdu Common Plumbing and Control Errors in Plantwide Flowsheets W Luyben

1/6

__ curriculum )

COMMON PLUMBINGANO CONTROl ERRORSIN PLANTWIOE FLOWSHEETSWILLIAM L. LUYBENLehigh University Bethlehem, PA 19015A lmost all senior design courses discuss only thesteady-state economic aspects ofprocess design andexcIude any consideration of dynamic behavior. Veryfew design textbooks even mention dynamics and control.t'

7/30/2019 ChemEngEdu Common Plumbing and Control Errors in Plantwide Flowsheets W Luyben

2/6

Perhaps the most serious plumbing error, and one that isalarrningly common in student flowsheets, is to not have anyvalve in a line connecting process units that are operating atdifferent pressures. This is illustrated in Figure 2 where aprocess stream flows from a vessel operating at apressure of10 bar into a vessel operating at 2 bar. There must be a valvein this line to take the pressure drop and regulate the flow.The valve can be set by an ,..----------------------------------:L::CJ='--,upstream controller (e.g., LC2' __-{X .--- -- - ,i - - V i - 2 - - - - - - - - - - - - _ _ - - -- - - - -- Q o E : . - _ . . . - - - - - - - - . - . . .. _ .. ..levei or pressure control- .~- - - - - - ~( ~~- - - - - - - - - - - -lers), or it can be set by a I , " , o : '- -- -- - - - - - -- - . . -- - - -- - -- - - - - - - 'downstream controller. But & '- , r@-: :--:;;;-~""--,; :a valve is required. : ~E(}-I---.....~"_----_,Students often state that Fel:

the pressure can be reduced f - - - - ~ ! - - - :by just cooling the stream.They confuse a "closed"system having a fixedamount of material with the"open" flow system encoun-tered in a continuous-pro-cess flowsheet.

In my experience about 50% of the problems in designingand operating a real chemical plant involve hydraulics. Stu-dents need to have a solid understanding of practical fluidmechanics. Pressure-driven dynamic simulations provide auseful platform for developing this vital plumbing know-how.The following is a brief compilation of some of the most

common plumbing errors that students make in developingflowsheets. It might be useful to also state that 1 have seenmany of these same errors made by presumably experiencedengineers on real plants. So perhaps they are not quite asobvious as one might think.No Valve Instal led

location into a unit at higher pressure. Students often forgetto put in the necessary pumps or compressors.Two Valves in L iquid-Fi /l ed L ineThis is probably the most frequently made error. Since a

liquid is essentially incompressible, its flowrate is the sameat any point in a liquid-fil led line. Therefore the flowrate canbe manipulated at only one location.This means there should be only one valve in the line thatis regulating the flowrate of liquido It is physically possibleto install two valves in series in a line, but these two valvescannot function independently.Figure 3 shows several examples of this type of "forbid-

den" plumbing arrangement. When a stream is split into twostreams at a tee in the line, the flow through each branch canbe independently set by two valves. The same is true whentwo streams are combined.Note that we are talking about liquid-filled lines. For gas

systems, valves can be used in a line at severallocations.

PC3

Stream Flowing"Uphi/l"Equally distressing is to

see a flowsheet in which aprocess stream is shown asflowing from a low-pressure

~ .. __ ... '1 3 1

03

. lC21, o , ~-- , __

[:@-J :L l2 ,U ,

. I ~ i~ - - - - - - - - - - - - : : : : : : :: ; - - - j1 dead2 .v "\... . ; P21'-~-:--"JTC2LCJ:[---_'_ '_' -.0; 1 dead3

Butyl acetateproduct

Figure 1. Example of plantwide contral structure.

f------I 2bar

CoolingWater10bar

Figure 2. Missing valve. Figure 3. Forbidden plumbing: two valves in liquid-filled line.

Summer 2005 203

7/30/2019 ChemEngEdu Common Plumbing and Control Errors in Plantwide Flowsheets W Luyben

3/6

Figure 4 illustrates this situation. The pressure in the first vessel is regu-lated by valve V 1. The pressure in the second vessel is regulated by valveV2. This is workable because gas is compressible, so the instantaneousflowrates through the two valves do not have to be equal as is the casewith liquids. The gas pressure in the process units can vary between thetwo valves.Valve ln Sucton of PumpPumps are used to raise the pressure of a liquid stream. Compressors are

used for the same purpose in gas systems. In this section we are consideringliquid flows using centrifugal pumps.Although students have leamed about net positive suction head (NPSH)

requirements for pumps, they frequently forget about this concept and installa control valve in the suction of a pump. Figure 5 illustrates this forbiddenplumbing. Suppose the liquid is coming from the base of a distillation col-umn. This liquid is at its bubblepoint under the conditions in the column. Thebase of the column must be located at an elevation high enough to provideadequate pressure at the pump suction to prevent the formation of vapor inthe pump. This is the NPSH requirement.If a control valve is installed between the column and the pump suction,the pressure drop over the valve will create a pump suction pressure that

violates the NPSH requirements. So controI valves in Iiquid systems shouldbe located downstream of centrifugal pumps. The exact opposite is true forgas systems with compressors, as discussed in the next section.It should also be remembered that no valves should be used for positive

displacement pumps. The flowrate of the liquid can only be regulated bychanging the stroke or speed of the pump or by bypassing liquid from thepump discharge back to some upstream location. The lower part of Figure 5illustrates this forbidden plumbing with a positive displacement pump. Throt-tling a valve in the pump discharge will not change the flowrate of liquidthrough the pump. It will just increase the pump discharge pressure and raisethe power requirement of the motor driving the pump.Valve Downstream of Centrfugal CompressorCentrifugal rotary compressors are positive displacement devices. At a

fixed speed they compress a fixed volume of gas per time (ft3/minute).The mass flowrate of gas depends on the density of the gas at the com-

pressor suction, so changing the suction pressure will change the massflowrate.

V1,

@ --- - - - - - - - - - - - - - - - - - - - J

V2

Figure 4. Two valves in gas-filled line.204

Throttling a valve in the compressor suctionchanges the compressor suction pressure, so it canbe used to control the gas flowrate. But throttlinga valve in the compressor discharge, as shown inFigure 6, does not change the gas flowrate. Itjustincreases the compressor discharge pressure andpower requirements.There are three viable ways to regulate the flow-

rate of gas in a compression system:1. Suction throttling2. Bypass or spill-back from discharge to

suction3. Change compressor speedThe last option is the most energy efficient but

requires a variable-speed drive, which is typicallya steam turbine if high-pressure steam is avail-able in the plant. Variable-speed electric motorsare also available. In compressor simulations thisvariable-speed option can be easily simulated bymanipulating compressor work.In the discussion above we have considered cen-

trifugal compressors. Regulation offlow througha reciprocating compressor can be adjusted bythrottling a valve in the suction, by changing

CentrifugalPump

PositiveDisplacementPump

Figure 5. Forbidden pump plumbing.

n .9 Gas StreamV------[X---CentrifugalCompressor

Figure 6. Forbidden compressor plumbing.Chemical Engineering Education

7/30/2019 ChemEngEdu Common Plumbing and Control Errors in Plantwide Flowsheets W Luyben

4/6

peed, or by changing the length of the stroke=-but not by throt-ling a valve in the discharge.Reciprocating gas compressors usually have clearance pocketsat change the flowrate slightly, and therefore only provide minorjustments in flow.OMMON CONTROl STRUCTURE ERRORSMost students in a senior design course have had a course inntrol fundamentals. They have been exposed to the mathematicsnd to the tuning of single-input, single-output feedback controlops with specified variables to be controlled and manipulated.To develop a control scheme for a typical process, however,any controlloops are required. Decisions must be made abouthat to control and what to manipulate. Students have had littlexposure to this more complex and more realistic situation.The most practical way to learn how to develop a plantwide con-ol system is to examine several realistic examples and step throughlogical plantwide design procedure. [5J At Lehigh, severallecturesre given early in the second semester discussing reactor control,stillation control, and plantwide controI. Then the design groups

Reactor

Vaporizer

Coo l i n gWater

Figure 7. Flows fixed in and OUt.A. Recycle = 10 kg-mol/h

1000 kg-mollh 01 A-----~!:-.---,-~~~~~:~~1000 kg-mollh 01 C

1000 kg-mol/h 01 B

: . _ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - -_ !

B. Recycle = 10,000 kg-mol/h1000 kg-mollh 01 A-------:-r.....--.--- i ~ : ~ , ~ ~ - ~ - ~ ; / ~ -

1000 kgmollh 01 C1000 kg-mollh 01 B

: _ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - !Figure 8. Recycle independent of fresh feed.

Sur nr ner 2005

attempt to develop a control structure for their individualflowsheets. Despite these lectures and reading assign-ments in the textbook, the students' first efforts at de-veloping a plantwide control system often contain manycontrol-structure errors. Some of the more common arelisted below.Fixing Flows 80th In and OutFigure 7 shows a process in which two liquid streams,

containing reactants A and B, are fed into a vaporizer.Each stream is flow controlled..The liquid feeds are vaporized and preheated before

entering an adiabatic tubular reactor. Reactor effluent iscooled and fed into a downstream distillation column.The flowrate to the distillation column is flow controlled.It is obvious that this structure is unworkable. But con-

trol schemes like this are proposed year after year byseveral groups of very capable students. They getwrapped up in the individual unit operations and neglectto look at the big picture.Similar conceptual issues often occur in specifying

recycle streams. Students often have trouble realizingthat the flowrate of a recycle stream is completely inde-pendent of the flowrate of a fresh-feed stream. Fresh-feed flowrates are set by the production requirements.To produce 1000 kg-mol/h of a product C in a processwith the reaction A +B -7 C, the fresh feed of each ofthe reactants must be 1000 kg-mol/h. Of course, if anyreactants are lost as impurities in the streams leavingthe unit, the fresh feeds must be appropriately larger.But inside the process we could have a recycle streamof reactant A, for example. As illustrated in Figure 8,the flowrate of this recycle can be anything we want itto be: 10 kg-mol/h or 100,000 kg-mol/h,Recycle flowrate is completely independent of fresh-

feed flowrate.Liquid Levels and Gas Pressures NotControlledStudents frequently submit flowsheets in which there

is no control of liquid levels in vessels or no control ofpressure in gas-filled systems. Allliquid levels must becontrolled in some way. They can be controlled by ma-nipulating a downstream valve orby manipulating an up-stream valve. Of course, the level control schemes for theindividual units must be consistent with the plantwide in-ventory control scheme that connects all the units.There are very few exceptions to this requirement for

controlling all levels. The most common exception iswhen a solvent is circulating around inside a pro~essand there are no losses of this .solvent, In this case threwill be a liquid level somewhere in the process that"floats" up and down as the solvent circulation-ratechanges. This level is not controlled.

20 5

7/30/2019 ChemEngEdu Common Plumbing and Control Errors in Plantwide Flowsheets W Luyben

5/6

The pressure in a gas-filled system must also be con-trolled. Gas pressure ean be eontrolled by regulatingthe flow of gas into or out of the system. It ean also beeontrolled by regulating the rate of generation of gas(e.g., in a vaporizer, in a distillation eolumn reboiler,or in a boiling exothermie reaetor). Pressure ean alsobe eontrolled by regulating the rate of eondensation ofgas (e.g., in the eondenser of a distillation eolumn).The system ean eonsist of several gas-filled ves-

sels with vapor flowing in series through the vessels.Figure 9 illustrates some of these ideas. In this flow-sheet the pressure in the gas loop is eontrolled by therate of addition of a gas fresh-feed stream. The pres-sures in all of the vessels float up and down together,but differ slightly due to pressure drops (whieh aretypieally kept quite small to reduce compressioneosts). The flowrate of the gas recycle stream is flowcontrolIed, using a cascade system: Flow controlIeroutput adjusts the setpoint of the turbine speed eontroller, whose out-put manipulates high-pressure steam to the turbine.There are rare occasions when pressure is allowed to float. Theseoccur when it is desirable to keep pressure as low as possible for someprocess optirnization reason (e.g., in some distillation eolumns whererel ative volatilities inerease with decreasing pressure). In these sys-tems heat removal is maximized to keep pressure as low as possible.

GasA

L iq uid B

Gas Recyele

---~

Reactor

Vapo r i za r

LC - -- - -- -- ":- - - @CoolingWater

Dist il lat ion Columns with a Fixed Product FlowrateThe first law of distillation control says that you cannot fix the dis-

tillate-to-feed ratio in a distillation column and also control any com-position (or temperature) in the column. This law is a result of the verystrong impact of the overall component balance on compositions andthe relatively smaller effect of fractionation (reflux ratio, steam-to-feed ratio, etc.) on compositions.Figure 10 illustrates the effect of fixing the distillate and bottomsflowrates when changes in feed composition occur. Initially the feedcontains 50 mol/h of A and 50 mol/h of B. The distillate contains 49mol/h of A and 1 mol/h of B, and the bottoms contains 1 mol/h of Aand 49 mol/h of B. So product purities are 98 mol%. Then the feedcomposition is changed so there are 55 mol/h of A and 45 mol/h of B.The distillate and bottoms flowrates are kept eonstant at 50 mol/hr.Now the distillate will be essentially 50 mol/h of A, and the bottomswill be 5 mol/h of A and 45 mol/h of B. Thus the bottoms purity willdrop from 98 mol% Bto 90 mol% B. No matter what reflux ratio orreboiler heat input is used, this purity cannot be changed. Controllinga composition or a temperature in the column is not possible.There are columns in which a produet stream is fixed. These are

called "purge columns" because the purpose is to remove a smallamount of some component in the feed. In these eolumns, temperatureor composition is not controlled. The flowrate of the purge stream issimply ratioed to the feed flowrate.A somewhat more eomplex situation occurs when the purging is done

in a sidestream column that has three product streams. Consider thesidestream columns shown in Figure 11. The feed stream is a temary206

Figure 9. Pressure in gas loop.49A1B 50AOB

55A45B50ASOB

1A49B

5A45B

Figure 10. Fixing product streamin distillation column.

mixture. Two cases are shown. In the eolumn on theleft the feed contains a small amount of the lightesteomponent, and it is purged in the distillate stream.The intermediate component is removed in the liq-uid sidestream.The distillate is flow eontrolled, and reflux-drum

leveI is eontrolled by manipulating reflux flowrate. Theissue here is how to manipulate the sidestream flow-rate. It eannot be fixed but must change in response tofeed eomposition and flowrate disturbanees. Theseheme shown in the left of Figure 11achieves this byratioing the sidestream flowrate to the reflux flowrate.Temperature or composition can be controlled in thiscolumn because the separation between the interme-diate and heavy components can be adjusted.In the column on the right in Figure 11, the feed

contains a small amount of the heaviest component,and it is purged in the bottoms stream.The intermediate component is removed in the va-

por sidestream. The bottoms stream is flow con-trolled, and base leveI is controlled by manipulating

Chemical Engineering Education

7/30/2019 ChemEngEdu Common Plumbing and Control Errors in Plantwide Flowsheets W Luyben

6/6

HaavyPurga

LlquldSldastraam Vapor Sidastraam should be obvious. Yet this type of error crops up on severalflowsheets every year.Sometimes students correctly insert a valve in a line to satisfy

plumbing requirements, but fail to connect it to a controller. Allvalves must be positioned by some controller.Ratio ing Reactant FeedsOne of the most important aspects of plantwide control is themanipulation of the fresh-feed streams. A common error is to sim-

ply ratio the flowrates of the reactants so as to satisfy the reactionstoichiometry. Although this will work in a simulation stdy, itwill not work in reality.Flowrates cannot be measured accurately enough to guarantee

an absolute matching of the number of molecules of the variousreactants. The separation section typically prevents the loss of any

of the reactants. Therefore simply ratioing reactants inevitably results in a gradualbuildup inside the process of the reactant that is in slight excessoSome indication of the inventory of the reactants inside the system must be found

so that the flowrates of the fresh-feed streams can be appropriately adjusted. Ulti-mately these flows must satisfy the reaction stoichiometry down to the last molecule.But this much accuracy is way beyond our ability to measure flowrates.The plantwide control structure in Figure 1 illustrates this principIe. The chemistry

in this exampIe is the reaction of methyl acetate and butanol to produce butyI acetateand methanol. The reaction occurs in a reactive distillation colurnn (C2). There aretwo recyeIe streams. The "LTREC"-the distillate D2 from the reactive colurnn-isan azeotropic mixture of methyl acetate and methanol. The "HVYREC" is the distil-late D3 from the third colurnn, which is mostly recyeIed butanol.The fresh butanol is added tothis recyeIe stream to control the reflux-drum leveI in

the third column (level controller LC32). This leveI gives an accurate measurementof the amount of butanol in the system. If more butanol is reacting than is being fed,this leveI will decrease. On the methyl acetate side, the leveI in the reflux drum of thefirst colurnn is controlled by manipulating the fresh-feed stream, which containsmethyl acetate and methanol (levei controller LCI2). This level provides a measure-ment of the methyl acetate in the system.Note that the production rate in this plant is set by the flow controller FCI, which

controls the feed flowrate DI to the second colurnn. If more production is desired,the operator increases the setpoint of this flow controller. The increase in DI alsoresults in an increase in the flowrate of the heavy recyeIe because of the ratio.

LlghtPurga

Figure 11. Purge column with sidestream.: c ! tJ f- - - -~ IL. +~--------~

CoolingWater

Figure 12. Herron Heresy.reboiler heat input. The vapor sidestreamflowrate, which cannot be fixed, is ma-nipulated to control a temperature in thecolumn. Note that when a small amountof light impurity is present in the temaryfeed, a liquid sidestream of the interme-diate component is used with its drawofflocation above the feed location. This con-figuration is used because the liquid at thesidestream tray has a lower concentrationof the lightest component than the vapor.When a small amount of heavy impurityis present in the temary feed, a vaporsidestream of the intermediate componentis used with its drawoff location belowthe feed location because the vapor atthe sidestream tray has a lower concen-tration of the heaviest component thanthe liquidoIncorrect Sensor Locat ion andValves Wi thou t Inpu t SignalsFigure 12 shows what we call at Lehighthe "Herron Heresy" (after a senior stu-

dent in the design course who made thesame mistake twice). The diagram showsthat the temperature upstream of thecooler is controlled by the flowrate ofcooling water to the heat exchanger.This, of course, is impossible andSummer 2005

CONCLUSIONCommon plumbing and control concept errors have been discussed and illus-

trated. It is hoped that this paper will help students and engineers avoid theseproblems in their design projects, and more importantly, in reallife. Most of theseerrors are obvious and can be avoided by using some common sense and notgetting alI wrapped up in the computer simuIation aspects of the probIem.REFERENCES1. Seider, W.O., 1.0. Seader, and O.R. Lewin, Product and Process Design Principies, Wiley (2004)2. Dimian, A.C., Integrated Design and Simulation ofChemical Processes, Elsivier (2003)3. Luyben, W.L., Chapter AI, "The need for simultaneous design education," in The Integration of

Process Design and Control, P. Seferlis and M.C. Georgiadis, editors, Wiley (2004)4. Luyben, W.L., Plantwide Dynamic Simulatorsfor Chemical Processing and Control, MareeI Dekker

(2002)5. Luyben, Wl..; B.D. Tyreus, and M.L. Luyben, Plantwide Process Control, MeGraw-Hill (1999) O

207