plant-wide control for through- put maximization

Talk at RIL Jamnagar, 17th Dec 2007

Plant-wide Control for Through-

put Maximization

Dr. Nitin Kaistha

Department of Chemical Engineering

Indian Institute of Technology Kanpur


Outline of presentation

• IntroductionIntroductionIntroductionIntroduction

• Case Study 1: Cumene ProcessCase Study 1: Cumene ProcessCase Study 1: Cumene ProcessCase Study 1: Cumene Process

• Case Study 2: HDA processCase Study 2: HDA processCase Study 2: HDA processCase Study 2: HDA process

• ConclusionsConclusionsConclusionsConclusions


Introduction

• Research in plant wide control gained momentum in recent years

– Availability of computational facilities

– Commercial plant-wide process simulators, eg. Aspen Plus, HYSYS

• Much of research on plantwide control is focused on synthesizing the effective decentralized regulatory Control Structure

CHEMICAL PLANTCHEMICAL PLANTCHEMICAL PLANTCHEMICAL PLANT

Raw materialsRaw materialsRaw materialsRaw materials ProductsProductsProductsProductsReactors

Columns

Separators

Heaters

Coolers

Furnaces

Pumps

Compressors

• Little work on plant-wide control for maximizing plant profit


Optimal Plant-wide Operation

• Optimization of key set-points for maximum plant profit

– Model Predictive Control

– Real Time Optimization

• Computationally intensive techniques requiring

high fidelity plant-wide process models

FOCUS OF THIS WORK

• Simple approach for improved process profitability


Process Constraints Limit Production

Column Flooding Maximum available flow rates (mass, energy)

Maximizing the production rate requires controlling Maximizing the production rate requires controlling Maximizing the production rate requires controlling Maximizing the production rate requires controlling the plant such that it operates close to the bottleneck constraithe plant such that it operates close to the bottleneck constraithe plant such that it operates close to the bottleneck constraithe plant such that it operates close to the bottleneck constraintntntnt

Maximum Maximum Maximum Maximum Plant ProfitabilityPlant ProfitabilityPlant ProfitabilityPlant Profitability

Maximum Maximum Maximum Maximum

Production Rate Production Rate Production Rate Production Rate

CONTINUOUS CHEMICAL PROCESSESCONTINUOUS CHEMICAL PROCESSESCONTINUOUS CHEMICAL PROCESSESCONTINUOUS CHEMICAL PROCESSES


Case Study-1: Design and Plant-wide Control of a Cumene Process

Students : Sadanand SinghShivangi Lal


PROCESS DETAILS

• Gas phase reaction

• Cooled packed bed reactor

• Three column separation section

Salient Process Features

• Benzene recycle with 2:1 excess benzene to reactor

• Cumene product specifications: • Purity 99.9 mol %• Production rate: 12.3 tonnes/hr

Primary reaction:C3H6 + C6H6 � C9H12 r1 = k1cpcc mole/g cat sec

propylene benzene cumene k1= 3.5 X 104 exp (-24.9/RT)

Secondary reaction:C3H6 + C9H12 � C12H18 r1 = k1cpcc mole/g cat sec

propylene cumene p-diisopropyl benzene k1= 2.9 X 106 exp (-35.1/RT)

Process Chemistry


Benzene

Propylene

Preheater

Furnace

Reactor Cooler

Recycle benzene

Fuel Gas Recovery Column

Fuel Gas

RecycleColumn

di-isopropyl benzene

ProductColumn

Cumene

Process Design


Benzene

Propylene

Preheater

Furnace

F2

F1

T1T2

Recycle benzene

T3

Reactor Cooler

HS

…

P1L1

T4

P2

T5

L3

FC

L2


Fuel Gas

RecycleColumn

P3

L4

T6

L5

RC Recycle benzene


ProductColumn

Cumene

L7

T7

P4

L6

RC

Recycle benzene

Benzene

Propylene

Preheater

Furnace

F2

F1

T1T2


ProductColumn

Cumene

L7

T7

P4

L6

RC

T3

Reactor Cooler

HS

…

P1L1

T4

P2

T5

L3

FC

L2


Fuel Gas

RecycleColumn

P3

L4

T6

L5

RC

Plant-wide Control Structure


F1

F2

T2

Fresh Benzene

Propylene

Preheater

T1

Furnace

L1

P1

Reactor

HS

…

T3

Recycle Benzene

Reactor Heat Management Schemes

CS1: Coolant duty controlled scheme

Production Rate Handles :• Propylene feed rate

SET

CS2: Furnace duty controlled scheme

Production Rate Handles:• Propylene feed rate

•Coolant duty

SET

CS3: Propylene feed controlled scheme

Production Rate Handles :• Coolant duty

SET


Comparison of Different Control

Schemes

Response for change in throughResponse for change in throughResponse for change in throughResponse for change in through----put by put by put by put by ±±±±10%10%10%10%



Schemes

Response for change in C3 feed purity by Response for change in C3 feed purity by Response for change in C3 feed purity by Response for change in C3 feed purity by ±±±±5%5%5%5%



Schemes

1.8410.5580.395Through-put by -10%

1.8360.9560.424Through-put by +10%

0.2700.1140.065C3 mol fraction by +5%

0.3170.1160.068C3 mol fraction by -5%

CS3CS2CS1Disturbance


Reactor Temperature Profile

Reactor temperature profile at final steady state using CS1, CS2Reactor temperature profile at final steady state using CS1, CS2Reactor temperature profile at final steady state using CS1, CS2Reactor temperature profile at final steady state using CS1, CS2 and CS3 for +10% throughand CS3 for +10% throughand CS3 for +10% throughand CS3 for +10% through----put put put put

changechangechangechange


Plant-Wide Response for CS1

Response for change in throughResponse for change in throughResponse for change in throughResponse for change in through----put by put by put by put by ±±±±10%10%10%10%


Through-put Maximization

BOTTLENECKBOTTLENECKBOTTLENECKBOTTLENECKMaximum Reactor Maximum Reactor Maximum Reactor Maximum Reactor Heat RemovalHeat RemovalHeat RemovalHeat Removal

CS1CS1CS1CS1

Reactor heat duty used for Reactor heat duty used for Reactor heat duty used for Reactor heat duty used for

controlling hotcontrolling hotcontrolling hotcontrolling hot----spotspotspotspot

Need sufficient trim away from Need sufficient trim away from Need sufficient trim away from Need sufficient trim away from

maximum duty for disturbance maximum duty for disturbance maximum duty for disturbance maximum duty for disturbance

rejectionrejectionrejectionrejection

THROUGHTHROUGHTHROUGHTHROUGH----PUT LOSSPUT LOSSPUT LOSSPUT LOSS

CS3CS3CS3CS3

Reactor heat duty NOT Reactor heat duty NOT Reactor heat duty NOT Reactor heat duty NOT

used for controlused for controlused for controlused for control

Can be fixed at maximumCan be fixed at maximumCan be fixed at maximumCan be fixed at maximum

Process operation AT Process operation AT Process operation AT Process operation AT

bottleneck constraintbottleneck constraintbottleneck constraintbottleneck constraint

MAXIMUM THROUGHPUTMAXIMUM THROUGHPUTMAXIMUM THROUGHPUTMAXIMUM THROUGHPUT


CS1CS1CS1CS1

CS3CS3CS3CS3

337

338

339

340

341

342

343

344

0 1 2 3 4 5 6

Time (hours)

Co

ola

nt

T (

C)

Trim for CS1Trim for CS1Trim for CS1Trim for CS1

Tc, minTc, minTc, minTc, min


SUMMARY

• CS1 gives tightest hot-spot temperature control

• CS2 susceptible to large temperature profile deviations

• CS3 provides acceptable hot-spot temperature

control

• 6% increased throughput in CS3

KEY ROLE OF CONTROL STRUCTURE ON MAX THRUPUTKEY ROLE OF CONTROL STRUCTURE ON MAX THRUPUTKEY ROLE OF CONTROL STRUCTURE ON MAX THRUPUTKEY ROLE OF CONTROL STRUCTURE ON MAX THRUPUT


Case Study-2: PLANT-WIDE CONTROL OF

HDA PROCESS FOR IMPROVED PROFITABILITY

Student : Sanjay Kr. Jha


Valve Positioning Control Approach

• Adjust through-put manipulator till bottleneck

constraint is approached closely

ThroughputThroughputThroughputThroughputManipulatorManipulatorManipulatorManipulator

BottleneckBottleneckBottleneckBottleneckConstraintConstraintConstraintConstraintMeasurementMeasurementMeasurementMeasurement

PROCESSPROCESSPROCESSPROCESS

SP 95%SP 95%SP 95%SP 95%

VPCVPCVPCVPC

SPSPSPSP

FC


PCPCPCPC

SP = 95%SP = 95%SP = 95%SP = 95%

SPSPSPSP

ShinskeyShinskeyShinskeyShinskey’’’’ssss valve positioning control for distillation columnsvalve positioning control for distillation columnsvalve positioning control for distillation columnsvalve positioning control for distillation columns

VPCVPCVPCVPC

FLOATING PRESSURE CONTROLFLOATING PRESSURE CONTROLFLOATING PRESSURE CONTROLFLOATING PRESSURE CONTROL


CondensateCondensateCondensateCondensate

FeedFeedFeedFeed

SteamSteamSteamSteam

SP = 95%SP = 95%SP = 95%SP = 95%

TCTCTCTC

VPCVPCVPCVPC

SPSPSPSP

FCFCFCFC

FeedFeedFeedFeed

SteamSteamSteamSteam95%95%95%95%

TC

CondensateCondensateCondensateCondensate

Equivalence of VPC scheme to a conventional control structureEquivalence of VPC scheme to a conventional control structureEquivalence of VPC scheme to a conventional control structureEquivalence of VPC scheme to a conventional control structure


HDA ProcessHDA ProcessHDA ProcessHDA Process

Salient process featuresSalient process featuresSalient process featuresSalient process features

� Exothermic reaction in adiabatic plug flow reactorExothermic reaction in adiabatic plug flow reactorExothermic reaction in adiabatic plug flow reactorExothermic reaction in adiabatic plug flow reactor

� 5:1 H5:1 H5:1 H5:1 H2222 to Toluene fed to reactor (suppresses side to Toluene fed to reactor (suppresses side to Toluene fed to reactor (suppresses side to Toluene fed to reactor (suppresses side rxnrxnrxnrxn))))

� Unreacted HUnreacted HUnreacted HUnreacted H2222 and toluene recycled backand toluene recycled backand toluene recycled backand toluene recycled back

� CHCHCHCH4444 formed in reaction is purgedformed in reaction is purgedformed in reaction is purgedformed in reaction is purged

� Hot reactor effluent used to preheat cold feed to reactorHot reactor effluent used to preheat cold feed to reactorHot reactor effluent used to preheat cold feed to reactorHot reactor effluent used to preheat cold feed to reactor

� Three column separation trainThree column separation trainThree column separation trainThree column separation train

Chemistry (Chemistry (Chemistry (Chemistry (DealkylationDealkylationDealkylationDealkylation of toluene): of toluene): of toluene): of toluene):

6 5 3 2 6 6 4C H CH (Toluene)+H C H +CH→

6 6 6 5 6 5 22C H C H -C H + H↔


HDA Process Flow Sheet and PlantHDA Process Flow Sheet and PlantHDA Process Flow Sheet and PlantHDA Process Flow Sheet and Plant----wide Control Structurewide Control Structurewide Control Structurewide Control Structure

J. M. Douglas, Conceptual Design of Chemical Processes, McGraw Hill, New York (1988).


• A rigorous Dynamic Simulator is developed in HYSYS 3.2, a commercial plant-wide control software package

• This flow sheet is used to investigate the enhancement in the plant through-put using VPC based approach

BottleBottleBottleBottle----necks considered for HDA Processnecks considered for HDA Processnecks considered for HDA Processnecks considered for HDA Process

• Feed gas compressorFeed gas compressorFeed gas compressorFeed gas compressor

• FurnaceFurnaceFurnaceFurnace

Compressors and Furnaces are expensiveCompressors and Furnaces are expensiveCompressors and Furnaces are expensiveCompressors and Furnaces are expensive&&&&

Marginally overMarginally overMarginally overMarginally over----designeddesigneddesigneddesigned


VPC for compressor bottleneckVPC for compressor bottleneckVPC for compressor bottleneckVPC for compressor bottleneck

1. Toluene fresh feed

2. Hydrogen feed

7. Gas recycle8. Toluene recycle

VPCVPCVPCVPC

SPSPSPSP

PC1

HHHH2222 FeedFeedFeedFeed

7

1

2

FCFCFCFC

Toluene FeedToluene FeedToluene FeedToluene Feed

8

CompressorCompressorCompressorCompressor

Flash

Drum

Pressure


Variation in benzene production and total toluene rate with feedVariation in benzene production and total toluene rate with feedVariation in benzene production and total toluene rate with feedVariation in benzene production and total toluene rate with feed gas composition gas composition gas composition gas composition

for VPC based and defor VPC based and defor VPC based and defor VPC based and de----rated operation with feed compressor bottleneck . rated operation with feed compressor bottleneck . rated operation with feed compressor bottleneck . rated operation with feed compressor bottleneck . (a) Total (a) Total (a) Total (a) Total

benzene production rate, (b) Total toluene production rate. benzene production rate, (b) Total toluene production rate. benzene production rate, (b) Total toluene production rate. benzene production rate, (b) Total toluene production rate.

145

150

155

160

165

170

175

180

185

190

90 92 94 96 98 100

H2 mole fraction of fresh gas feed (%)

Ben

zene

pro

duct

ion

rat

e (k

mo

l/hr)

WITH VPC

NO VPC

(a)(a)(a)(a)

235

240

245

250

255

260

265

270

275

280

90 92 94 96 98 100

H2 mole fraction of fresh gas feed (%)

To

tal

tolu

ene

flo

w r

ate

(km

ol/

hr)

With VPCNO VPC

(b)(b)(b)(b)


Variation in plant operating profit with fresh hydrogen Variation in plant operating profit with fresh hydrogen Variation in plant operating profit with fresh hydrogen Variation in plant operating profit with fresh hydrogen feed composition feed composition feed composition feed composition

for feed gas compressor bottleneckfor feed gas compressor bottleneckfor feed gas compressor bottleneckfor feed gas compressor bottleneck

200

220

240

260

280

300

320

340

360

380

400

90 92 94 96 98 100

H2 mole fraction of fresh gas feed ( % )

Pla

nt

op

erat

ing

pro

fit

($/h

r)

WITH VPC NO VPC


Dynamic response to input disturbance profile in the fresh hydroDynamic response to input disturbance profile in the fresh hydroDynamic response to input disturbance profile in the fresh hydroDynamic response to input disturbance profile in the fresh hydrogen feed composition gen feed composition gen feed composition gen feed composition

(a)(a)(a)(a) Benzene production rate Benzene production rate Benzene production rate Benzene production rate (b) Gas recycle pressure control loop (b) Gas recycle pressure control loop (b) Gas recycle pressure control loop (b) Gas recycle pressure control loop

(c) Compressor duty(c) Compressor duty(c) Compressor duty(c) Compressor duty (d) Total toluene feed rate (d) Total toluene feed rate (d) Total toluene feed rate (d) Total toluene feed rate


110.98255.48556.67812.15366.46583.25949.711005

81.27254.4557.12811.52335.67576.35912.0297.54

55.74249.88558.58808.46305.62570.67876.29953

26.94248.12557.53805.74275.06564.99840.0592.52

0245.07558803.07245.07558.44803.22901

Incremental

Profit*

($/hr)

Operating

Profit

($/hr)

Utility Cost

($/hr)

Product Price –

Raw

Material

Cost ($/hr)

Operating

Profit

($/hr)

Utility Cost

($/hr)

Product Price –

Raw

Material

Cost

($/hr)

Fresh H2 m.f.

(%)

De-rated Operation (No VPC)With VPC

S No

*: Incremental Profit = Operating Profit with VPC – Operating Profit for De-rated Operation

Steady state plant operating cost and profit data for VPC based and de-rated

operation as the mole fraction of H2 feed is varied with the feed compressor as

bottleneck


VPC for furnace bottleneckVPC for furnace bottleneckVPC for furnace bottleneckVPC for furnace bottleneck

1. Toluene fresh feed

2. Hydrogen feed

7. Gas recycle8. Toluene recycle

9. Furnace inlet

10.Reactor inlet

13. FEHE hot in

14. FEHE hot outFuelFuelFuelFuel

FurnaceFurnaceFurnaceFurnace

TCTCTCTC3333FCFCFCFC

FEHEFEHEFEHEFEHE

VPCVPCVPCVPC

SPSPSPSP

14

2M

13

10

9

7

1

8


160

162

164

166

168

170

172

174

0 5 10 15 20

Temperature increment in fresh feed temperature (oC)

Ben

zen

e pro

duct

ion

rat

e (k

mo

l/h

r)

WITH VPC

NO VPC

(a)(a)(a)(a)

255

256

257

258

259

260

261

262

263

264

265

0 5 10 15 20


To

tal

tolu

ene

flow

rat

e (k

mol/

hr)

WITH VPCNO VPC

(b)(b)(b)(b)

Variation in benzene production and total toluene rate with feedVariation in benzene production and total toluene rate with feedVariation in benzene production and total toluene rate with feedVariation in benzene production and total toluene rate with feed temperature temperature temperature temperature

for VPC based and defor VPC based and defor VPC based and defor VPC based and de----rated operation with furnace bottleneck rated operation with furnace bottleneck rated operation with furnace bottleneck rated operation with furnace bottleneck (a) Benzene (a) Benzene (a) Benzene (a) Benzene

production rateproduction rateproduction rateproduction rate

(b) Total toluene rate(b) Total toluene rate(b) Total toluene rate(b) Total toluene rate


272

277

282

287

292

297

302

307

312

317

0 5 10 15 20


Pla

nt

op

erat

ing

pro

fit

($/h

r)

WITH VPCNO VPC

Variation in plant operating profit with temperature Increment of fresh feeds

for furnace bottleneck


Dynamic response to input disturbance profile in the fresh feed temperature

(a) Furnace duty

(b) Reactor inlet temperature

(c) Total toluene flow rate

(d) Benzene production rate


34.58278565.5843.50312.58872.09884.671005

25.4277.80565.45843.25303.20568.85872.05154

16.03277.67565.5843.17293.70568.85862.55103

8.48277.45565.70843.15285.93567.31853.2452

0277.28565.73843.01277.28556.70842.9801

Incremental

Profit*

($/hr)

Operating

Profit

($/hr)

Utility Cost

($/hr)

Product Price –

Raw

Material

Cost ($/hr)

Operating

Profit

($/hr)

Utility Cost

($/hr)

Product Price –

Raw

Material

Cost

($/hr)

Temp.

Increment

(oC)

De-rated Operation (No VPC)With VPC

S No

Steady state plant operating cost and profit data for VPC based Steady state plant operating cost and profit data for VPC based Steady state plant operating cost and profit data for VPC based Steady state plant operating cost and profit data for VPC based

and deand deand deand de----rated operation as the fresh feed temperature rated operation as the fresh feed temperature rated operation as the fresh feed temperature rated operation as the fresh feed temperature

is varied with the furnace as bottleneckis varied with the furnace as bottleneckis varied with the furnace as bottleneckis varied with the furnace as bottleneck

*: Incremental Profit = Operating Profit with VPC – Operating Profit for De-rated Operation


SUMMARY

• Plant operating profit can be increased by 10% or

more using VPC

• VPC is a simple and effective way for improving plant

profitability by maximizing plant throughput


CONCLUSIONS

plant-wide control for through- put maximization

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