Process Integration for Efficient Use of Energyon for Efficient Use of Energy

Smith: Chemical Process Design and Integration (Chapters 16,18) Kemp: Pinch Analysis and Process Integration (Chapter 9)

Process Integration for Efficient Use of Energy

Cheng-Liang Chen

PSE

LABORATORY

Department of Chemical Engineering National TAIWAN University

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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 ➢ Concluding Remarks

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Heat Exchanger Network Design: The Pinch Design Method

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Design of Individual Processes for Maximum Energy Recovery Divide the process at the pinch

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Design of Individual Processes for Maximum Energy Recovery The feasibility of heat transfer

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Design of Individual Processes for Maximum Energy Recovery Cross-pinch heat transfer: Actual = Target + XP

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Design of Individual Processes for Maximum Energy Recovery Cold utility above the pinch

Hot utility below the pinch

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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 ➢ Do not recover process heat across

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Typical Grid Diagram

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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 =











—



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Where Is The Pinch ?

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Pinch Is Easily Shown

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Separate Above/Below-Pinch Regions

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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

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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 S L C

= = = =

# # # #

of of of of

matches or units (lines in graph theory) streams including utilities (points in a graph) independent loops components

➢ A single component and loop-free:

NUNITS = S − 1

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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 (NUNITS = (5 − 1) + (4 − 1) = 7)

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The Pinch Design Method

Stream 1. 2. 3. 4.

Reactor Reactor Reactor Reactor

1 1 2 2

feed prod feed prod

Type

Supply Temp. TS (oC)

Target Temp. TT (oC)

∆H (M W )

Heat Capacity Rate mC ˙ p(M W/oC)

Cold Hot Cold Hot

20 250 140 200

180 40 230 80

+32.0 −31.5 +27.0 −30.0

0.20 0.15 0.30 0.25

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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 curves:

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The Pinch Design Method Start at the Pinch (∆Tmin exists between all hot/cold streams, the most constrained region)

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The Pinch Design Method Divide at the pinch

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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 MW Tc = 140 + 0.23 MW/ o C ∆Tmin

= > = =

155oC Th − Tc 162 − 155 7oC

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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 MW Tc = 140 + 0.33 MW/ o C ∆Tmin

= < = =

150oC Th − Tc 162 − 150 12oC

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The Pinch Design Method CP Inequality for Individual Matches Below Pinch: if CPH = =

130oC Th − Tc 130 − 125 5oC

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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 MW Th = 150 − .253 MW/ o C ∆Tmin

= < = =

138oC Th − Tc 138 − 125 13oC

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The Pinch Design Method CP Inequalities: Summary for temperature differences to increase moving away from the pinch Above Pinch: CPH ≤ CPC

Below Pinch: CPH ≥ CPC

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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

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The Pinch Design Method The ”Tick-Off” Heuristic (above pinch) Now we have identified feasible matches ⇒ How big should we make them ?

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The Pinch Design Method The ”Tick-Off” Heuristic (above pinch) Maximize loads to ”tick off” streams ⇒ to keep capital costs down

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The Pinch Design Method The ”Tick-Off” Heuristic (above pinch) Then fill in the rest

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The Pinch Design Method The ”Tick-Off” Heuristic (above pinch) Then fill in the rest

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The Pinch Design Method The ”Tick-Off” Heuristic (below pinch) Maximize loads to ”tick off” streams ⇒ to keep capital costs down

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The Pinch Design Method The ”Tick-Off” Heuristic (below pinch) Maximize loads to ”tick off” streams ⇒ to keep capital costs down

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The Pinch Design Method The ”Tick-Off” Heuristic (below pinch) Then fill in the rest

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The Pinch Design Method The ”Tick-Off” Heuristic (below pinch) Then fill in the rest

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The Pinch Design Method The ”Tick-Off” Heuristic (below pinch) Note: one match violates CP rules But, it is away from the pinch and therefore feasible

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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

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The Pinch Design Method The Completed Design

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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

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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 Above Pinch: SH ≤ SC

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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 Below Pinch: SH ≥ SC

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Stream Splitting: CP Inequality

Above Pinch: CPH ≤ CPC Hot stream with larger CP values ⇒ Split into smaller parallel hot streams (opt flow rates ?)

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Stream Splitting: CP Inequality

Below Pinch: CPH ≥ CPC Cold stream with larger CP values ⇒ Split into smaller parallel cold streams (opt flow rates ?)

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Stream-Splitting Algorithms Above the Pinch

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Stream-Splitting Algorithms Below the Pinch

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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 ☞ Do not recover process heat across

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Thank You for Your Attention Questions Are Welcome