Download - Heat Part3 Retrofit

•  Optimal Retrofit ≠ Optimal Grassroot §  Optimal Reuse of installed Heat Exchangers §  Requires accurate Modeling (“Rating”) §  Shorter Paybacks (especially Energy Projects)

•  Phases are the same, but Content is different §  Data Extraction has been discussed already §  Targeting with focus on Optimal Value for ΔTmin §  Process Modifications more difficult than in Grassroot §  Network Design is focused on reduced Heat Transfer

across the Pinch point (Process and Utility Pinches) §  Optimization is used to maximize the Utilization of

Existing Heat Exchangers through Loops and Paths

What about Retrofit Design of Heat Exchanger Networks ?

T. Gundersen Retro 1

Process, Energy and System

Heat Integration − Retrofit Design

Penalty Heat Flow Diagram

Pinch

T Hot Streams

Cold Streams

ST

Hot Streams

Cold Streams CW

QP = QPP + QPH + QPC Q: What Pinch ? Which ΔTmin ?




QPC

QPPQPH

Energy Target Plot

HRAT

QH,min

QH,exist

QH,new

HRATnew HRATexist

ΔE a

b

c

HRAT = Heat Recovery Approach Temperature




Savings vs. Investments

Investment (US$)

Savings (US$/yr)

Invmax

a

b d c

PB=1 PB=2

PB=3

min HRAT subject to Inv ≤ Invmax and PB ≤ PBmax




Examples of Cross-Pinch Heat Transfer




H

C

H

mCpH

mCpC

TH,in

TC,out TC,in

TH,out

TP,H

TP,C

, , , ,XP H H in P H C C out P CQ mCp T T mCp T T⎡ ⎤ ⎡ ⎤= ⋅ − − ⋅ −⎣ ⎦ ⎣ ⎦> = 0 <

”Shifting” in Retrofit Design




LP

C

HP QXP

QC

QH

LP

C

HP QXP

QC − QXP

QH − QXP

H

C

H

C

QH

QC

Example of an Existing Network Pinch180°

C2210° 160°

C1210° 50°

H2220° 60°

H1270° 160°

160°

Ca

2

2

H

1

1

1000 kW

2500 kW

Cb

980 kW

1320 kW

2200 kW

160°

214.4°

120°

mCp(kW/°C)

18.0

22.0

20.0

50.0

QPP = 22 • (220 - 180) = 880 kW QPC = 18 • (214.4 - 180) = 620 kW QP = 1500 kW = 2500 - 1000 QPH = 0 kW




Changing Operating Conditions (“shifting”)

Pinch 180°

C2 210° 160°

C1 210° 50°

H2

220° 60°

H1

270° 160°

160°

Ca

2

2

H

1

1

1000 kW

2500 kW

Cb

360 kW

440 kW

2200 kW

160°

214.4°

80°

mCp (kW/°C)

18.0

22.0

20.0

50.0

180°

620 kW

880 kW

180°

“Releases” Heat above the Pinch by changing Operating Conditions (Temperatures) for Exchanger 2 and Cooler Ca




Network After Modifications (Retrofit)

Pinch 180°

C2 210° 160°

C1 210° 50°

H2

220° 60°

H1 270° 160°

160°

Ca

2

2

H

1

1

1000 kW

1000 kW

Cb

360 kW

440 kW

2200 kW

160°

214.4°

80°

mCp (kW/°C)

18.0

22.0

20.0

50.0

180°

620 kW

880 kW

180° 4

3

4

3

190°

The Project requires Purchase of 2 new Units and additional Area (new shell ?) to Unit 2 (smaller ΔT )




A simpler Retrofit Solution

The Project requires Purchase of only 1 new Unit, while the Energy Savings is 620 kW (versus 1500)

T. Gundersen

Pinch 180°

C2 210° 160°

C1 210° 50°

H2 220° 60°

H1 270° 160°

160°

Ca

2

2

H

1

1

1000 kW

1880 kW

Cb

360 kW

1320 kW

2200 kW

160°

214.4°

120°

mCp (kW/°C)

18.0

22.0

20.0

50.0 3

3

620 kW

180°

172.4°

Retro 10



WS-7: A simple Retrofit Problem




H1

C1 220°C 70°C

50°C 250°C

120°C

4000 kW

H

C I

I

8000 kW

12000 kW

170°C

mCp (kW/ºC)

100

80

Given: ΔTmin = 5ºC U = 1.0 kW/(m2K) CST = 0.1 NOK/kWh CCW = 0 NOK/kWh

Further: Steam available at 250ºC, Cooling Water at 20ºC (constant) 8000 Operating Hours per Year Cost of new Exchanger: Chex = 0.5 + 0.01·A (m2 and MNOK) Cost of moving/repiping existing Exchanger: Chex = 0.5 MNOK Maximum Payback: PBmax = 3 years

Targeting by using Pro_Pi Software




Result: For ΔTmin ≤ 30ºC: QH,min = 0 kW, QC,min = 8000 kW

Demand Curves

0

2000

4000

6000

8000

10000

12000

0 10 20 30 40 50 60 Global temperature difference (K)

Q (k

W)

WS-7 (cont.): Alternative Retrofit Projects

T. Gundersen



Retro 13

H1

C1220°C 70°C

50°C250°C

120°C

4000 kW

H

CI

I

8000 kW

12000 kW

170°C

mCp(kW/ºC)

100

80

Existing Design: PB = n.a.

I = 0 MNOK, ΔE = 0 MNOK/yr

H1

C1220°C 70°C

50°C250°C

922.9 kW

H

CI

I

11077.1 kW

8922.9 kW

139.23°C

mCp(kW/ºC)

100

80208.46°C

Project # 1: PB = 0.20 yr = 2.4 months

I = 0.5 MNOK, ΔE = 2.46 MNOK/yr



H1

C1220°C 70°C

50°C250°C

120°C

4000 kW

CI

I

8000 kW

8000 kW

170°C

mCp(kW/ºC)

100

80

II

II

130°C



H1

C1220°C 70°C

50°C250°C

120°C

4000 kW

CI

I

8000 kW

8000 kW

170°C

mCp(kW/ºC)

100

80

II

II

130°CH

H

WS-7 (cont.): Alternative Retrofit Projects

T. Gundersen



Savings MNOK/yr

Investment MNOK 0

1.0

2.0

3.0

0.5 1.0 1.5 0

PB = 2.4 months

PB = 4.6 months

ΔPB = 11.8 months

Retro 14

The Optimum

WS-10: Retrofit

Optimization with Loops and Paths




Stream Ts Tt mCp ΔH °C °C kW/°C kW

H1 250 120 40 5200 H2 200 180 80 1600 C1 130 290 50 8000 C2 140 240 20 2000

Steam 320°C (condensing) Cooling Water 20°C à 30°C

ΔTmin = 10ºC QH,min = 4000 kW QC,min = 800 kW

Grand Composite Curve

100

150

200

250

300

350

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Q (kW)

T (°C) Grand Composite Curve

100

150

200

250

300

350

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Q (kW)

T (°C)

TPinch = 200ºC/190ºC and 140ºC/130ºC

WS-10: Existing Network




mCp (kW/ºC)

[40]

[80]

[50]

[20]

H1

H2

C2

C1 Ha

C

200º

190º 130º

140º

130º

120º 165º 200º

200º 180º

158º 190º 290º

250º

240º 140º Q=5000

Q=2000

Q=1600 Q=1400

Q=1800

I

III

II

Targeting for ΔTmin = 10ºC: QH,min = 4000 kW , QC,min = 800 kW

Cross Pinch Heat Transfer: QXP,I = 1000 kW , QXP,C = 1000 kW

H1 C

WS-10: Retrofit Network




mCp (kW/ºC)

[40]

[80]

[50]

[20]

H2

C2

C1 Ha

130º

120º 140º 175º

200º 180º

158º 190º 290º

250º

240º 140º Q=1600 Q=1400 [1400]

UA=106.1 [36.5]

Q=800 [1800]

I

III

II 200º

Q=1000 [2000] UA=50.1 [71.7]

Q=3000 [5000]

Q=2000 UA=138.6

Hb

Q=1000 UA=9.7

230º

190º

IV

Investments: New Exchangers IV and Hb and Additional Area for Existing Exch. II

Savings: 1000 kW Reduction in Steam and Cooling Water

WS-10: Summary




a) Unchanged Energy Consumption Loop A: Ha (+x) → IV (-x) → I (+x) → Hb (-x) b) Better Use of Existing Ha and I

c) Reduces the Area for new Hb and IV

a) Unchanged Energy Consumption Loop B: IV (+y) → II (-y) b) Better Use of Existing I

c) Area increase in IV > Area saved in II

a) Increased Energy Consumption Path C: Ha (+z) → IV (-z) → C (+z) b) Reduces Area for Exchangers IV & II

c) Less (!!) Use of Existing I

a) Increased Energy Consumption Path D: Ha (+w) → II (-w) → C (+w) b) Reduces Area for Exchangers II & IV (This path is dependent – Combine B and C) c) Less (!!) Use of Existing III

a) Reduced (!!) Energy Consumption Path E: Hb (-v) → I (+v) → C (-v) b) Better Use of Existing I

c) Increased Additional Area for II

Loop A most promising, possibly combined with Path C if Existing Exchanger I becomes limiting

Optimization with 4 DOFs

T. Gundersen

Heat Recovery and Iterative Design

R S H U

R = Reactor System S = Separation System H = Heat Integration U = Utility System

Decomposition

R

S

H

U

Interactions

Proc. Mods. 1


Process Modifications


The “ Plus / Minus “ - Principle

T

Q QC,min

QH,min

T. Gundersen Proc. Mods. 2



Download - Heat Part3 Retrofit

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