Title: A1256655490GzYRT
1CHE 448 Chemical Engineering Design
Spring 2006
2Example of Heat Exchanger Network
Stream T source F T target F M Cp Btu/h F Q MBtu/h
C1 120 235 20,000 2.3
C2 180 240 40,000 2.4
H1 260 160 30,000 3.0
H2 250 130 15,000 1.8
No network 4 units, 4.8 MBtu/h of cold and 4.7
MBtu/h of hot utilities.
3Temperature interval methodLinnhoff Flower
(1978)
Temperature intervals are determined by source
and target temperatures of streams.
4How do we choose the temperature Intervals?
First, we must define Dtmin 10 F
Temperature Hot side Cold side
260/250 Source H1 None
250/240 Source H2 Target C2
245/235 None Target C1
190/180 None Source C2
160/150 Target H1 None
130/120 Target H2 Source C1
5Temperature interval Example
6Minimum utilities in reversible network DT0
7The pinch depends on the value of DTmin
8Eliminate pinch by adding hot utilities.
9Minimum energy requirements
- Minimum from energy balance 0.10 Mbtu/h
- Minimum for reversible network DT0, 0.15
Mbtu/h - Minimum for small approach temp DT 10 F -gt
0.6 Mbtu/hr - Minimum for large approach temp DT 20 F -gt
1.125 Mbtu/h
10Conclusions from TI analysis
- The Choice of Dtmin results from an economic
between capital costs of heat exchangers and cost
of utilities. - Any amount of heat introduced above the minimum
balance will have to be removed by a cold
utility. - A pinch indicates the presence of two subsystems,
one that lacks energy (above) and one that has
too much energy (below). - Any energy crossing the pinch, will have to be
removed by a cold utility.
11Finding the Heat Exchange NetworkApproach
temperature 10 F
Stream T source F T target F M Cp Btu/h F Q MBtu/h
C1 180 235 20,000 1.1
C2 180 240 40,000 2.4
H1 260 190 30,000 2.1
H2 250 190 15,000 0.9
Stream T source F T target F M Cp Btu/h F Q MBtu/h
C1 120 180 20,000 1.2
H1 190 160 30,000 0.9
H2 190 130 15,000 0.9
12Subsystems on sides of pinch
13Finding the Heat Exchange NetworkApproach
temperature 10 F
Stream T source F T target F M Cp Btu/h F Q MBtu/h
C1 180 235 20,000 1.1
C2 180 240 40,000 2.4
H1 260 190 30,000 2.1
H2 250 190 15,000 0.9
Stream T source F T target F M Cp Btu/h F Q MBtu/h
C1 120 180 20,000 1.2
H1 190 160 30,000 0.9
H2 190 130 15,000 0.9
14Hueristics for HEN design
- Exchange the hottest hot stream with the cold
stream that has the largest target temperature.
Exchange the coolest cold stream with the hot
stream that has the lowest target temperature.
15Heat Load Feasibility
- If a stream or service is matched only once, its
partner must have an equal or larger heat load.
Thus, the stream with the largest heat load must
have at least two matches. - The stream with the second largest heat load must
have at least two matches unless it is matched
against the largest stream.
16Target Temperature
- Each match bringing a stream to its target
temperature must be a stream or service whose
supply temperature is compatible with that target
temperature. - Each stream or service must be used in at least
one match.
17Use heuristics to find stream matches
Alternative I
18Alternative I 7 units
19Alternative I can we reduce the number of units?
20Alternative II Smaller number of units
21Comparison of alternatives with smaller number of
units
Number of units 6
Hot Util 1.08 MBtu/h
Cold Util 1.18 MBtu/h
Number of units 6
Hot Util 0.575 MBtu/h
Cold Util 0.675 MBtu/h
22Minimum number of unitsBipartite graph
23Minimum number of units for pinch subsystems
24Use of bipartite graph to reduce number of units
25Smaller number of units with heat transfer across
the pinch
Second Law constraints prevents from having the
minimum number of units
26Other issues
Target temperatures of cold stream1 and hot
stream 1 cannot be controlled in a simple way.
How do we provide energy for start ups?
27Summary of pinch method for designing HENs
- List all streams including utilities and define
the value of Dtmin - Use the temperature interval method to uncover
the presence of a pinch. - If there is a pinch, separate two sub-networks
and balance them. - Determine minimum number of units above and below
the pinch. - Determine feasible network using heuristics.
- Reduce number of units by small evolutionary
changes to original netwok.
28Networks with phase changes
29Stream splitting
- Stream splitting must be used when
- The number of hot streams on the cold side of the
pinch is smaller than the number of cold streams. - The number of cold streams on the hot side of the
pinch is smaller than the number of hot streams - Stream splitting helps to reduce the number of
exchangers without increasing use of utilities.
30Example stream splitting
Stream Ts C Tt C mCp kW/C Q kW
H1 200 100 5 500
H2 150 100 4 200
C1 90 190 10 1000
31Heat Integration of a Reactor System
Stream Ts F Tt F mCp Btu/s F Duty Btu/s Cost/Btu
C1 100 580 1 -480 0
C2 100 580 2 -960 0
H1 600 200 3 1200 0
Steam 650 650 2/106
Hot Wat 250 gt 130 1.5/106
Cooling W 80 lt 125 1/106
32Temperature IntervalReversible (Ideal) System
33Temperature interval I
34Temperature Interval II
35Cost of heat exchanger network system
36Heat integrated system I
37Heat Integrated System II
38Cost of system without heat integration
Unit Str 1 Str 2 Duty Btu/s A(ft2) Cost utility Cost unit
HU-1 1 Steam 480 825 28300 62000
HU-2 2 Steam 960 1650 56700 82000
CU-1 3 CW 1200 4650 35500 145000
39Cost of Integrated System
Unit Stream 1 Stream 2 Duty Btu/s Area ft2 Cost utility Chex
HU-1 C1 Steam 80 138 4700 45000
HU-2 C2 Steam 160 276 9400 48000
PU-1 C1 H1/3 400 1550 0 80000
PU-2 C2 2H1/3 800 3100 0 114000