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

8/12/2019 Process Integration for Efficient Use of Energyon for Efficient Use of Energy

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

Process Integrationfor Efficient Use of Energy

Cheng-Liang ChenP SELABORATORY

Department of Chemical EngineeringNational TAIWAN University


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Chen CL 1

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 Processesfor Maximum Energy Recovery

Divide the process at the pinch


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Cross-pinch heat transfer: Actual = Target + XP


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Cold utility above the pinch Hot utility below the pinch

h


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Design RuleDo 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 DiagramRules 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 eachother

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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 forma third loop by deleting the common line (BGCEB + CGDHC BGDHCEB)

The number of independent loops for a graph:

N UNITS = S + L C

N UNITS = # of matches or units (lines in graph theory)S = # of streams including utilities (points in a graph)L = # of independent loopsC = # of components

A single component and loop-free: N UNITS = S 1

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Number of Heat Exchanger Units

If the problem has a pinch:

N UNITS = ( S above pinch 1) (S below pinch 1)

To target the number of units for pinched problems, the streams

above and below the pinch must be counted separately(N UNITS = (5 1) + (4 1) = 7 )

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The Pinch Design Method

Stream Type Supply Temp. Target Temp. H Heat Capacity RateT S (oC ) T T (oC ) (MW ) mC p(MW/ o C )

1. Reactor 1 feed Cold 20 180 +32 .0 0.202. Reactor 1 prod Hot 250 40 31.5 0.153. Reactor 2 feed Cold 140 230 +27 .0 0.304. Reactor 2 prod Hot 200 80 30.0 0.25

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The Pinch Design MethodKnown

No exchanger should have a temp diff. smaller than T min No heat transfer across the pinch by

process-to-process heat transfer inappropriate use of utilities

Compositecurves:

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The Pinch Design MethodStart at the Pinch

( T min exists between all hot/cold streams, the most constrained region )

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The Pinch Design MethodDivide at the pinch

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The Pinch Design MethodCP Inequality for Individual Matches

Above Pinch: if CPH > CPC infeasible!

T h = 162 o (suppose) H h = 0 .25(162 150)

= 3 MWT c = 140 +

3 MW0 .2

MW/o

C= 155 oC T min > T h T c

= 162 155= 7 o C

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Above Pinch: if CPH CPC feasible

T h = 162 o (suppose) H h = 0 .25(162 150)

= 3 MWT c = 140 +

3 MW0 .3

MW/o

C= 150 oC T min < T h T c

= 162 150= 12 oC

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Below Pinch: if CPH < CPC infeasible!

T c = 125 o (suppose) H c = 0 .2(140 120)

= 3 MWT h = 150

3 MW. 15

MW/o

C= 130 oC T min > T h T c

= 130 125= 5 o C

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Below Pinch: if CPH CPC feasible

T c = 125 o (suppose) H c = 0 .2(140 120)

= 3 MWT h = 150

3 MW. 25

MW/o

C= 138 oC T min < T h T c

= 138 125= 13 oC

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The Pinch Design MethodCP 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 MethodThe 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 MethodThe Tick-Off Heuristic (above pinch)

Now we have identied feasible matches How big should we make them ?

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Maximize loads to tick off streams to keep capital costs down

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Then ll in the rest

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Then ll in the rest

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The Pinch Design MethodThe Tick-Off Heuristic (below pinch)


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Then ll in the rest

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Then ll in the rest

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Note: one match violates CP rulesBut, it is away from the pinch and therefore feasible

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The Pinch Design MethodThe 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 MethodThe 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 besatised for matches between streams at the pinch

Loads on individual units are determined using the kick-off heuristicto minimize # of units

Away from the pinch: more freedom, use judgment and processknowledge

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Stream Splitting: # of Streams

Cold utility must not be usedabove the pinch All hot streams must be cooled to

pinch temperature by heat recovery

Splitting cold streamsAbove Pinch: S H S C

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Stream Splitting: # of Streams

Hot utility must not be usedbelow the pinch All cold streams must be heated to

pinch temperature by heat recovery

Splitting hot streamsBelow Pinch: S H S C

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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 ow 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 ow rates ?)

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l l h


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Stream-Splitting AlgorithmsAbove the Pinch

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S S li i l i h


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Stream-Splitting AlgorithmsBelow the Pinch

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D i f I di id l P


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Pinch Design RuleDo 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

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

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