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.
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!
Start with a flow sheet without heat exchangers installed
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?).
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.
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.
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!
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!
Separate the network design task at the pinch and treat the two designs separately
Design task above the pinch.
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
Option #1: one heater for the ingredients (1) + two heat exchangers [product (2) – (1) and containers (4) – (2)] C 70 C
Option #2: one heat exchanger [ingredients (1) – products] + two heaters [ingredients (1) and containers (4)] C
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!
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.