Process Integration
for Efficient Use of Energy
Cheng-Liang Chen
P
S
E
LABORATORY
Department of Chemical Engineering National TAIWAN University
Outline
➢ Systematic Approach for Chemical Process Design How do we go about the design of a chemical process?
➢ What Is Process Integration?
Onion model for process integration
➢ Pinch Analysis: Targeting Heat Recovery in Processes
➢ The Pinch Design Method for Heat Recovery Systems
➢ A Pinch Study Performed on A Major Operating Plant
➢ Utility Selection for Individual Processes
➢ Putting It into Practice
Heat Exchanger Network Design:
Design of Individual Processes
for Maximum Energy Recovery
Design of Individual Processes
for Maximum Energy Recovery
Design of Individual Processes
for Maximum Energy Recovery
Design of Individual Processes
for Maximum Energy Recovery
Design of Individual Processes
for Maximum Energy Recovery
Design Rule
Do Not Transfer Heat Across the Pinch
➢ Do not use steam below
➢ Do not use cooling water above
Typical Grid Diagram
Rules for Construction
➢ Hot streams run left to right
➢ Cold streams run right to left
➢ Hot streams on top; Cold streams on bottom
➢ Hot utility = H ➢ Cold utility = C
➢ Heat exchanger between streams =
—
Number of Heat Exchanger Units
➢ Graph any collection of points in which some pairs of points are connected by lines
➢ Path a sequence of distinct lines which are connected to each other
Number of Heat Exchanger Units
➢ A graph forms a single component if any two points are joined by a path
➢ Loop a path which begins and ends at the same point (CGDHC)
➢ If two loops have a line in common, they can be linked to form a third loop by deleting the common line (BGCEB + CGDHC → BGDHCEB)
➢ The number of independent loops for a graph: NUNITS = S + L − C
NUNITS = # of matches or units (lines in graph theory)
S = # of streams including utilities (points in a graph) L = # of independent loops
C = # of components
Number of Heat Exchanger Units
➢ If the problem has a pinch:
NUNITS = (Sabove pinch − 1) − (Sbelow pinch − 1)
➢ To target the number of units for pinched problems, the streams above and below the pinch must be counted separately
The Pinch Design Method
Stream Type Supply Temp. Target Temp. ∆H Heat Capacity Rate
TS(oC) TT(oC) (M W ) mC˙ p(M W/oC)
1. Reactor 1 feed Cold 20 180 +32.0 0.20
2. Reactor 1 prod Hot 250 40 −31.5 0.15
3. Reactor 2 feed Cold 140 230 +27.0 0.30
The Pinch Design Method
Known
➢ No exchanger should have a temp diff. smaller than ∆Tmin
➢ No heat transfer across the pinch by ☞ process-to-process heat transfer
☞ inappropriate use of utilities ➢ Composite
The Pinch Design Method
Start at the Pinch
The Pinch Design Method
Divide at the pinch
The Pinch Design Method
CP Inequality for Individual Matches
Above Pinch: if CPH>CPC ⇒ infeasible!
Th = 162o (suppose) ∆Hh = 0.25(162−150) = 3 MW Tc = 140 + 0.23 MWMW/oC = 155oC ∆Tmin > Th − Tc = 162 − 155 = 7oC
The Pinch Design Method
CP Inequality for Individual Matches
Above Pinch: if CPH≤CPC ⇒ feasible
Th = 162o (suppose) ∆Hh = 0.25(162−150) = 3 MW Tc = 140 + 0.33 MWMW/oC = 150oC ∆Tmin < Th − Tc = 162 − 150 = 12oC
The Pinch Design Method
CP Inequality for Individual Matches
Below Pinch: if CPH<CPC ⇒ infeasible!
Tc = 125o (suppose) ∆Hc = 0.2(140−120) = 3 MW Th = 150 − .153 MWMW/oC = 130oC ∆Tmin > Th − Tc = 130 − 125 = 5oC
The Pinch Design Method
CP Inequality for Individual Matches
Below Pinch: if CPH≥CPC ⇒ feasible
Tc = 125o (suppose) ∆Hc = 0.2(140−120) = 3 MW Th = 150 − .253 MWMW/oC = 138oC ∆Tmin < Th − Tc = 138 − 125 = 13oC
The Pinch Design Method
CP Inequalities: Summary
for temperature differences to increase moving away from the pinch
The Pinch Design Method
The CP Table
Cold utility must not be used above the pinch
⇒ hot streams must be cooled to pinch temp. by recovery hot utility can be used on cold streams above the pinch
The Pinch Design Method
The ”Tick-Off” Heuristic (above pinch)
Now we have identified feasible matches ⇒ How big should we make them ?
The Pinch Design Method
The ”Tick-Off” Heuristic (above pinch)
Maximize loads to ”tick off” streams ⇒ to keep capital costs down
The Pinch Design Method
The ”Tick-Off” Heuristic (above pinch)
The Pinch Design Method
The ”Tick-Off” Heuristic (above pinch)
The Pinch Design Method
The ”Tick-Off” Heuristic (below pinch)
Maximize loads to ”tick off” streams ⇒ to keep capital costs down
The Pinch Design Method
The ”Tick-Off” Heuristic (below pinch)
Maximize loads to ”tick off” streams ⇒ to keep capital costs down
The Pinch Design Method
The ”Tick-Off” Heuristic (below pinch)
The Pinch Design Method
The ”Tick-Off” Heuristic (below pinch)
The Pinch Design Method
The ”Tick-Off” Heuristic (below pinch)
Note: one match violates CP rules
The Pinch Design Method
The ”Tick-Off” Heuristic: Summary
To tick off a stream, individual units are made as large as possible ⇒ the smaller of the two heat duties on the streams being matched
The Pinch Design Method
The Completed Design
The Pinch Design Method: Summary
➢ Divide the problem at the pinch into separate problems
➢ Design separate problems, started at the pinch, moving away
➢ Temperature feasibility requires constraints on CP values to be satisfied for matches between streams at the pinch
➢ Loads on individual units are determined using the kick-off heuristic to minimize # of units
➢ Away from the pinch: more freedom, use judgment and process knowledge
Stream Splitting: # of Streams
Cold utility must not be used above the pinch
⇒ All hot streams must be cooled to pinch temperature by heat recovery ⇒ Splitting cold streams
Stream Splitting: # of Streams
Hot utility must not be used below the pinch
⇒ All cold streams must be heated to pinch temperature by heat recovery ⇒ Splitting hot streams
Stream Splitting: CP Inequality
Above Pinch: CPH ≤ CPC
Hot stream with larger CP values ⇒ Split into smaller parallel
hot streams
Stream Splitting: CP Inequality
Below Pinch: CPH ≥ CPC
Cold stream with larger CP values ⇒ Split into smaller parallel
cold streams
Stream-Splitting Algorithms
Above the Pinch
Stream-Splitting Algorithms
Below the Pinch
Design of Individual Processes
for Maximum Energy Recovery
Pinch Design Rule
Do Not Transfer Heat Across the Pinch
➢ Divide at the PINCH
➢ Start at the PINCH and move away
➢ Observe the PINCH rules:
☞ Do not use steam below
☞ Do not use cooling water above
