HEAT EXCHANGER NETWORK SYNTHESIS

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A technique to do process integration in industry in order to minimize energy consumption in the process.

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HEAT EXCHANGER NETWORK SYNTHESIS:

HEAT EXCHANGER NETWORK SYNTHESIS Shakti Bhardwaj

What is pinch technology?:

What is pinch technology? Pinch Technology, a technique for designing a process to minimise energy consumption and maximise heat recovery. The technique calculates thermodynamically attainable energy targets for a given process and identifies how to achieve them. A key insight is the P inch T emprature , which is the most constrained point in the process.

Advantages of performing HENS:

Advantages of performing HENS Financial savings by better process heat integration. Optimize the heat transfer equipment during the design of the equipment. Analyzing heat flows Establishing targets for energy use Identifying inefficiencies in processes Defining process improvements Debottlenecking Optimizing utility consumption

How to perform HENS?:

How to perform HENS? A Simple Two-Stream Heat Recovery Problem STREAM TYPE SUPPLY TEMP. (°C) TARGET TEMP. (°C) H (MW) Heat Capacity Rate (MW/°C) 1 COLD 40 110 +14 0.2 2 HOT 160 40 -12 0.1

A Simple Two-Stream Heat Recovery Problem Direct Heating and Cooling:

A Simple Two-Stream Heat Recovery Problem Direct Heating and Cooling Required Hot Utility: QH = 14 MW, Cold Utility: QC = 12 MW

Using Heat Exchanger to Recover Heat:

Using Heat Exchanger to Recover Heat Tmin = 10°C : Q REC = 11 MW Q Hmin = 3 MW Qcmin = 1 MW

A Simple Two-Stream Heat Recovery Problem Using Heat Exchanger to Recover Heat:

A Simple Two-Stream Heat Recovery Problem Using Heat Exchanger to Recover Heat Tmin = 20°C : Q REC = 10 MW Q Hmin = 4 MW Q Cmin = 2 MW

A Simple Two-Stream Heat Recovery Problem Two Basic Facts :

A Simple Two-Stream Heat Recovery Problem Two Basic Facts 1. Correlation between Tmin , QHmin , QCmin 2. More In, More Out!! QHmin + X = QCmin + X D T min 10 20 Qexch 11 10 QHmin 3 4 QCmin 1 2

A Flowsheet with Two-Hot-Two-Cold Streams:

A Flowsheet with Two-Hot-Two-Cold Streams Stream Type Supply Temp. Target Temp. H Heat Capacity Rate T S (C) T T (C) (MW) m˙ Cp(MW/C) 1. Cold 20 180 +32.0 0.20 2. Hot 250 40 −31.5 0.15 3. Cold 140 230 +27.0 0.30 4. Hot 200 80 −30.0 0.25

Composite Hot Stream:

Composite Hot Stream Add the heat available in each temperature interval to form a composite curve

Composite Cold Stream:

Composite Cold Stream Add the heat available in each temperature interval to form a composite curve

Obtain Hot/Cold Utilities from Composite Curves:

Obtain Hot/Cold Utilities from Composite Curves Plotting the hot and cold composite curves together gives the targets for hot and cold utilities. Tmin = 10°C : QREC = 51.5, QCmin = 10, QHmin = 7.5; Tmin = 20 C : QREC = 47.5, QCmin = 14 , QHmin = 11.5

Correct Setting for Tmin Is Fixed by Economic Trade-Offs:

Correct Setting for Tmin Is Fixed by Economic Trade-Offs

The Pinch:

The “ pinch” separates the HEN problem into two parts: Heat sink - above the pinch, where at least Q Hmin utility must be used Heat source - below the pinch, where at least Q Cmi n utility must be used. The Pinch

The Heat Recovery Pinch :

The Heat Recovery Pinch Above the pinch, process is in heat balance with the min hot utility QHmin Below the pinch, process is in heat balance with the min cold utility Qcmin

Three Forms of Cross-Pinch Heat Transfer:

Three Forms of Cross-Pinch Heat Transfer Pinch gives three rules that must be obeyed in order to achieve the minimum energy targets for a process : • Heat must not be transferred across the pinch • There must be no external cooling above the pinch • There must be no external heating below the pinch Violating any of these rules will lead to cross-pinch heat transfer resulting in an increase in the energy requirement beyond the target.

HEN Representation with the Pinch :

HEN Representation with the Pinch The pinch divides the HEN into two parts:  the left hand side (above the pinch)  the right hand side (below the pinch) At the pinch, ALL hot streams are hotter than ALL cold streams by  T min .

The Heat Recovery Pinch: Summary :

The Heat Recovery Pinch: Summary Temperature-enthalpy diagrams can be used to determine heat recovery potential Composite curves can be used to target for many hot/cold streams Energy targets set from material/energy balances and Tmin If maximum heat recovery is to be achieved, then no heat transfer across the pinch --- process-to-process --- inappropriate use of utilities

Energy Targeting for Individual Process :

Energy Targeting for Individual Process Process stream data can be extracted to produce composite curves and the problem table algorithm to target for process heat recovery Composite curves allow energy targets to be set PTA gives:- 1: Adjust for Tmin 2: Set up temperature intervals 3: Calculate interval heat balances 4: Cascade for positive heat flows QHmin , QCmin and pinch location without drawing graphs

Typical Grid Diagram:

Typical Grid Diagram

Pinch Is Easily Shown:

Pinch Is Easily Shown

Number of Heat Exchanger Units:

Number of Heat Exchanger Units The number of independent loops for a graph: N UNITS = S + L − C N UNITS = No. of matches or units S = No. of streams including utilities L = No. of independent loops C = No. of components For a pinch problem: N UNITS = (S above pinch − 1) − (S below pinch − 1)

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