PART 8

RETROFITRETROFIT

RETROFIT

RETROFIT GOAL: Reduce Energy Consumption

RETROFIT MECHANISMS:• Addition of one or more new heat exchangersAddition of one or more new heat exchangers

(in series or parallel) • Relocation of existing exchangers • Area addition to existing heat exchangers• Area addition to existing heat exchangers

–Adding a shell–Exchanging the bank of tubes by one more efficient (Brown Fintube, Houston, TX) , , )

• Area reduction to existing heat exchangers

PDM TARGETING

XSmaller ΔTmin

Existing network

AX

Optimumgrassrootdesign

CB

Smaller Area

CInfeasible

Energy requirementgy q

LIKELY PATH

Existing network

Likely pathof retrofit

AX

Existing network

Area

Retrofit shouldnot discard existing area(questionable!!)

BOptimum grassroot design

alreadyinvested

(q )

Infeasible

Energy requirementEnergy requirement

TARGETINGCONSTANT AREA EFFICIENCYCONSTANT AREA EFFICIENCY

Are

a

Target Constant α

Ay

, ,t Y t X

Y X

A AA A

α= =Y

y

Existing design

At,Y

Ax

AAt,x

Ey Ex

TARGETINGCONSTANT AREA EFFICIENCYCONSTANT AREA EFFICIENCY

PROCEDURESt t ith i ti t Obt i it id l ( A

rea

, ,t Y t XA Aα= =• Start with existing system. Obtain its ideal area (use

vertical heat transfer) (At,X)• Compute • Pick a value of EY<EX

Obt i it id l ( ti l h t t f ) (A )

A

Existing

Ay

At,Y

Y XA AY

, /t X XA Aα =

• Obtain its ideal area (use vertical heat transfer) (At,Y)• Obtain “real” area estimate • Obtain the number of exchangers (use minimum

number of exchangers) • Comp te the capital cost Cost of additional area

Existing design

AxAt,x

Ey Ex

, /Y t YA A α=

• Compute the capital cost= Cost of additional area (AY - AX) and fixed cost new exchangers (NY – NX)

• Obtain Payout (ROI and NPV could also be used)(Payout= Capital investment/ savings over a year)

TARGETING Determinee retrofit area based onDeterminee retrofit area based on

Payout

HEN Retrofit

NEXT STEP:

• Obtain a new grassroots design and maximize the use of existing exchangers

• Correct Cross-Pinch Exchangers• Correct Cross-Pinch Exchangers• Use Network Loops and Paths to minimize area reduction

HEN Retrofit Identify Cross Pinch ExchangersIdentify Cross-Pinch Exchangers

This pinch is the one corresponding to AY

FCp=229

FCp=20 4FCp=20.4

FCp=53.5

FCp=93

FCp=200

HEN Retrofit Remove Cross Pinch ExchangersRemove Cross-Pinch Exchangers

HEN Retrofit Add exchangers to meet energy targetsAdd exchangers to meet energy targets

1) New exchanger: Fixes cross i h f h 1 d 4pinch of exchanger 1 and 4

Coolers have smaller duty

Heater has ll d t

2) Split is needed after cross pinchfor exchangers 1 and 4 is removed.

smaller duty

We note that areas needed in 1 and 4 may be much larger than existing areas

3) Cross pinch exchanger 2 is fixed when area was added . Load of exchanger 1 increased. However, a split is now needed.

HEN Retrofit Loop and Path AnalysisLoop and Path Analysis

• Choose a value for EMAT (Exchanger Minimum Approach Temperature).

• Increase the duty of the process-process exchangers along a loop until one or more of the exchangers becomes a bottleneck

• If the full savings potential has not been achieved try to remove the bottleneck by• If the full savings potential has not been achieved, try to remove the bottleneck by(1) modifying branch flow fractions in split regions.(2) Re-sequence exchangers if one has excess driving force and another is

the bottleneck or insert a split so that the driving forces are more even.

• Increase the duty of exchangers on the loop again until the full savings potential has been achieved.

• Use heat load loops to re-distribute the driving forces in the network, thereby reducing overall area requirement.

• Explore the use of paths to perform energy relaxations to also adjust area.

HEN Retrofit Loops and PathsLoops and Paths

-X

+X

The loop is used to shift loads so that areas in exchangers 1 and 4 can be utilized without

X

+X

addition. We reduce the load of exchanger 1 by X and adjust all others accordingly. Optimum X is obtained when area is enough. 4 and 3 will change its duty accordingly

-X

HEN Retrofit Loop Analysis ResultLoop Analysis Result

N dNeed area Addition

HEN Retrofit CONCLUSIONS for PDM MethodCONCLUSIONS for PDM Method

• A Targeting procedure is based on uncertain i l di ieconomical predictions

• Changes are made by inspection• Loops and Path anal sis req ire to b ild• Loops and Path analysis require to build expertise• Not completely systematic and not automatic• Not completely systematic and not automatic• May work well for small problems• Combinatorial choices in large problems mayCombinatorial choices in large problems may increase substantially.

A ONE STEP SYSTEMATIC & AUTOMATIC METHOD IS SHOWN NEXT

HEN Retrofit Additional equations to Stages ModelAdditional equations to Stages Model

Some exchangers exist (in red) So e e c a ge s e st ( ed)

HEN Retrofit Additional equations to Stages ModelAdditional equations to Stages Model

Some exchangers exist (in red). Order must be respected. g ( ) p

Adding Area, Shells and new units0

, , , , , , , ,A AN

i j k i j k i j k i j kA A A A≤ + +

, , , ,A A

i j k i j kA Z≤ Γ ⋅, , , ,j j

, , , ,AN AN

i j k i j kA Z≤ Γ⋅

Reducing Area

0, , , , , ,

Ri j k i j k i j kA A A≥ −

, , , ,3, , , , , , , , , , 0

2i j k i j k

i j k i j k i j k i j k i j k

Th TcQ A U Th Tc

⎡ ⎤Δ + Δ⎣ ⎦− Δ Δ ≤

HEN Retrofit MILP Transportation ModelMILP Transportation Model

THIS IS A LINEAR MODEL THAT CONSIDERS

• Exchanger addition• Exchanger addition• Area addition through new shells. • Area addition through new tube arrangements.Area addition through new tube arrangements.• Relocation and re-piping

HEN Retrofit MILP Transportation Model ExampleMILP Transportation Model -Example

Before

HEN Retrofit MILP Transportation Model ExampleMILP Transportation Model- Example

After J5 125˚C 124˚CJ5 125˚C 124˚C

J6 175˚C 174˚CJ6 175˚C 174˚C

I1319.4˚C 244.10˚C

I3347.3˚C 45˚C

I3347.3˚C 45˚C

I5I5

J425˚C 20˚C

J425˚C 20˚C

44

1159060.593

1159060.593

1936917.414

104045.644

104045.644

15

0.020.02 0

1155868.897

1155868.897

3373.44612

3373.44612

I4263.5˚C 180.2˚C

I4263.5˚C 180.2˚C

I5297.4˚C 110˚C

I5297.4˚C 110˚C

I6248˚C 50˚C

I6248˚C 50˚C

I8 231 8˚C 120˚CI8 231 8˚C 120˚C

14

36903.233

3

4

6560.99

4

6560.99

86042.968

86042.968

1010

69902.575

69902.575

2038981.575

2038981.575

55 1212

I273.24˚C 30˚C

I273.24˚C 30˚C

231.8 C 0 C231.8 C 0 C

I9167.1˚C 69.55˚C

I9167.1˚C 69.55˚C

I10146.7˚C 73.24˚CI10146.7˚C 73.24˚C

9628.069

7

1065 039

7

1065 039

1358042.284

1358042.28458042.284

16

67053.083

16

67053.083

I773.24˚C 40˚C

I773.24˚C 40˚C

J1232.2˚C 30˚C

J2 343.3˚C 232.2˚C

1065.0391065.039

26903.091

26903.091

I11250˚ 249˚I11250˚ 249˚

I12 1000˚C 500˚CI12 1000˚C 500˚C

J3231.8˚C 226.2˚C

J3231.8˚C 226.2˚C

18

95207.4919

63561.2489

63561.24817

7913.77117

7913.771

HEN Retrofit MILP Transportation Model ExampleMILP Transportation Model-Example

HE Original Load Retrofit Load Original Area Retrofit Area Area Addition Shell Addition CostM J/hr M J/hr m2 m2 m2 $

1 160,311.20 155,868.90 4,303.20 3,926.252 6,903.09 6,903.09 59.40 63.80 4.40 342.033 17,118.40 9,628.07 33.40 21.534 658.00 6,560.99 2.30 16.63 14.33 YES 6,406.845 2,554.70 0.02 26.30 28.93 2.63 204.586 2,410.70 9,902.58 24.60 398.53 373.93 YES 34,379.017 1,065.04 1,065.04 5.50 5.87 0.37 28.708 45,024.40 6,042.97 145.00 41.669 100,642.70 63,561.25 1,212.70 962.01

10 4 473 60 4 045 64 93 70 93 7010 4,473.60 4,045.64 93.70 93.7011 54,618.70 59,060.59 685.70 1,239.90 554.20 YES 48,402.0912 6,293.80 3,373.45 40.00 44.00 4.00 311.1513 58,044.30 58,042.28 183.30 182.3914 36 903 20 36 903 23 101 60 101 4714 36,903.20 36,903.23 101.60 101.4715 36,917.40 0 93.90 016 67,053.08 67,053.08 278.10 288.97 10.87 845.3217 7,913.77 7,913.77 53.50 52.2418 136,138.80 95,207.49 976.40 709.0019 36,917.41 727.96 61,918.5320 38,981.58 651.93 56,004.54

8,318.60 9,556.76 14.88% 3 208,842.80

HEN Retrofit MILP Transportation Model ExampleMILP Transportation Model-Example

Costs Existing Retrofitted

Total utility cost $ 6 865 616/yr $ 5 004 800/yrTotal utility cost $ 6,865,616/yr $ 5,004,800/yrTotal fixed and area cost - $ 208,842/yr Total cost $ 5,213,643/yr $ 6,865,616/yrSavings ~24%