This document discusses process integration and heat exchanger network design. It introduces key concepts like identifying hot and cold streams, computing minimum heating and cooling needs, determining the theoretical minimum energy using composite curves, establishing a minimum approach temperature called the "pinch", and applying "Golden Rules" to design the network around the pinch. The goals are to establish the minimum energy required for a process and develop an optimal design with the lowest energy intensity and investment costs by making conscious design decisions.
1. Heat Exchanger Network Design
one aspect of process integration
J. M. Shaw
Instructor CHE 465
I would happily credit the authors who provided
the example but am unable to do so.
2. Introduction
• Process integration provides a discipline which
allows designers to establish
– The minimum energy to operate a process
– A process design with the lowest energy intensity
– An optimal investment strategy
• Design decisions are made consciously and
consistently. Global capital and operating cost
implications and trade-offs become visible.
• We will focus on the first bullet only and will
introduce the terminology of the discipline as we
go along!
3. Start with a flow sheet without heat
exchangers installed
4. Identify streams that require heating called
“cold streams” and streams that require cooling
“hot streams.”
Without heat recovery we require 750 units for heating cold streams
(steam?) and 660 units for cooling hot streams (cooling water?).
5. Next compute the minimum energy
required to operate the process
Compute summary heating
and cooling needs for the
process. Do this step by
step to avoid errors!
These are called
“composite curves”
for heating and
cooling requirements.
6. What is the theoretical minimum energy
required to operate the process as designed?
Think of your process as a
single giant counter current heat
exchanger with an infinite
surface area!
The minimum approach
temperature of the hot and cold
composite streams is 0 C! The
temperature at which this occurs
is called the “pinch” – 70 C in
this case.
Energy that cannot be supplied
by exchange must be supplied
by utilities: ~ 200 units for
heating and ~ 110 units for
cooling.
7. How closely can this minimum energy
requirement be approached in practice?
Our analogy with a heat exchanger still holds and we establish a minimum approach
temperature for the composite streams, in this case 20 C, that arises around the pinch.
300 units for
heating and
210 units for
cooling!
Pay twice for
inefficiencies!
Heating and
cooling
requirements
rise together!
8. How do we translate these concepts into
practical heat exchanger network designs?
“Golden Rules”
1. Avoid exchanging heat between streams where one is
above and one is below the pinch.
2. Avoid cooling streams above the pinch using utilities.
3. Avoid heating streams below the pinch using utilities.
Violating the golden rules may be convenient and smaller heat exchangers
will certainly result but because of the excessive entropy generated you
will pay twice for this violation for as long as the plant operates!
9. Separate the network design task at the pinch
and treat the two designs separately
11. Stream Data
Stream 1 is heated from 60 C to 120 C
Stream 2 is cooled from 100 C to 80 C
Stream 4 is heated from 60 C to 80 C
Heat Exchanger Network Design Data
12. Option #1: one heater for the ingredients (1) + two heat
exchangers [product (2) – (1) and containers (4) – (2)]
60
300
40
92 C
70 C
13. Option #2: one heat exchanger [ingredients (1) – products]
+ two heaters [ingredients (1) and containers (4)]
100
77 C
40
260
14. If several options are equivalent from an
energy perspective, then what?
• Capital cost
– One larger heater vs two smaller ones.
• Layout
– In large chemical plants, distances between streams may be a factor.
• Materials properties and physical states
– Condensing vapour – liquid and liquid-liquid heat exchange are both
easier and therefore cheaper than gas-gas heat exchange!
– Corrosion.
• Risk with respect to process or product safety
– Is exchanging heat between raw ingredients and finished products
wise?
• Consider start-up, shutdown, control and operability issues
– A heater for the container washer is a good idea!
Make decisions consciously and clearly!
15. Summary
Follow this up with
a reality check!
Iterate until an acceptable
design is obtained!
Install heat exchangers,
coolers, and heaters on
that none or few of the
constraints are violated.
