Upload
phamliem
View
221
Download
0
Embed Size (px)
Citation preview
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%