Title: CHE 448 Chemical Engineering Design
1CHE 448 Chemical Engineering Design
Spring 2006
2Question?
- Would you like to postpone the 2nd midterm to
April 6? - If so, April 4 you turn in second report and have
a class by Mr. Garnett on Human Error
3Example of Heat Exchanger Network
No network 4 units, 4.8 MBtu/h of cold and 4.7
MBtu/h of hot utilities.
4Temperature interval methodLinnhoff Flower
(1978)
Temperature intervals are determined by source
and target temperatures of streams.
n
5How do we choose the temperature Intervals?
First, we must define Dtmin 10 F
6Temperature interval Example
7Minimum utilities in reversible network DT0
8The pinch depends on the value of DTmin
9Eliminate pinch by adding hot utilities.
10Minimum 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
11Conclusions from TI analysis
- The Choice of Dtmin results from an economic
trade-off 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.
12Minimum use of utilities estimate QF(Dmin).
13Finding the Heat Exchange NetworkApproach
temperature 10 F
14Subsystems on sides of pinch
15Hueristics 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.
16Heat 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.
17Target 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.
18Finding the Heat Exchange NetworkApproach
temperature 10 F
19Use heuristics to find stream matches
Alternative I
20Alternative I 7 units
21Alternative I can we reduce the number of units?
22Alternative II Smaller number of units
23Comparison of alternatives with smaller number of
units
Number of units 6
Hot Util 0.98 MBtu/h
Cold Util 1.08 MBtu/h
Number of units 6
Hot Util 0.575 MBtu/h
Cold Util 0.675 MBtu/h
24Minimum number of unitsBipartite graph
25Minimum number of units for pinch subsystems
26Use of bipartite graph to reduce number of units
27Smaller number of units with heat transfer across
the pinch
Second Law constraints prevents from having the
minimum number of units
28Other 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?
29Summary 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.