1. Overview
Pinch analysis (also called pinch technology or process integration) is a systematic methodology for minimizing energy consumption in process plants. Developed by Bodo Linnhoff in the late 1970s, it determines the thermodynamic minimum heating and cooling utility requirements for a given set of process streams, and guides the design of heat exchanger networks to approach these targets.
Gas Processing
Amine & Dehy Units
Heat integration between reboilers, condensers, and process streams in gas treating.
NGL Fractionation
Column Integration
Feed/product heat exchange and condenser/reboiler integration in NGL trains.
Crude Stabilization
Preheat Trains
Crude oil preheat using hot product streams before the fired heater.
Utilities
Steam & Cooling Water
Minimize fired heater duty and cooling water by maximizing heat recovery.
The pinch concept: At the pinch point, the available driving force for heat transfer is at its minimum (ΔT_min). The pinch divides the process into a heat sink (above the pinch, needs heating) and a heat source (below the pinch, needs cooling). Transferring heat across the pinch increases both heating and cooling requirements simultaneously.
2. Stream Data Extraction
The first step in pinch analysis is identifying all process streams that require heating or cooling. Each stream is characterized by its supply temperature, target temperature, and heat capacity flow rate (CP).
Stream Classification:
Hot streams: Streams that need to be COOLED
Supply temperature > Target temperature
Release heat to the system
Cold streams: Streams that need to be HEATED
Supply temperature < Target temperature
Absorb heat from the system
Heat Capacity Flow Rate:
CP = m_dot × C_p (kW/°C or BTU/hr/°F)
Where:
m_dot = Mass flow rate (lb/hr or kg/s)
C_p = Specific heat capacity
Stream Duty:
Q = CP × |T_supply - T_target|
Where Q = Total heat load of the stream (BTU/hr or kW)
Example Stream Data
| Stream | Type | T_supply (°F) | T_target (°F) | CP (BTU/hr/°F) | Duty (MMBTU/hr) |
|---|---|---|---|---|---|
| H1: Amine lean | Hot | 250 | 120 | 15,000 | 1.95 |
| H2: Product gas | Hot | 200 | 100 | 20,000 | 2.00 |
| C1: Amine rich | Cold | 110 | 230 | 12,000 | 1.44 |
| C2: Reboiler feed | Cold | 100 | 300 | 8,000 | 1.60 |
3. Composite Curves
Composite curves are the graphical representation of pinch analysis. The hot composite curve combines all hot streams on a temperature-enthalpy (T-H) diagram, and the cold composite curve combines all cold streams. The overlap between the curves represents the maximum heat recovery possible.
Constructing Composite Curves:
1. List all temperature intervals defined by stream
supply and target temperatures
2. For each interval, sum the CP values of all
streams present in that interval:
Hot composite: CP_hot = Σ CP of hot streams
Cold composite: CP_cold = Σ CP of cold streams
3. Calculate enthalpy change for each interval:
ΔH = CP_composite × ΔT
4. Plot cumulative enthalpy vs temperature
Hot composite: starts at highest temperature
Cold composite: starts at lowest temperature
5. Slide cold curve horizontally until minimum
vertical distance = ΔT_min
This horizontal overlap = Maximum heat recovery
Reading the Composite Curves
Information from Composite Curves:
Overlap region:
Q_recovery = Maximum process-to-process heat exchange
Hot end gap (right side):
Q_H_min = Minimum hot utility (heating) required
This is the thermodynamic minimum - cannot be reduced
Cold end gap (left side):
Q_C_min = Minimum cold utility (cooling) required
This is also a thermodynamic minimum
Pinch point:
Location where curves are closest (ΔT_min apart)
Hot pinch temperature = T_pinch + ΔT_min/2
Cold pinch temperature = T_pinch - ΔT_min/2
Energy balance:
Q_H_min + ΣQ_hot = Q_C_min + ΣQ_cold
4. Problem Table Algorithm
The Problem Table Algorithm (PTA) is the numerical method for calculating minimum utility targets and identifying the pinch point. It provides exact results without the need for graphical construction.
Problem Table Algorithm Steps:
Step 1: Shift temperatures
Hot streams: T_shifted = T_actual - ΔT_min/2
Cold streams: T_shifted = T_actual + ΔT_min/2
This ensures ΔT_min is maintained automatically
Step 2: Create temperature intervals
Sort all shifted temperatures in descending order
Each pair of adjacent temperatures defines an interval
Step 3: Calculate interval heat balance
ΔH_i = (ΣCP_hot - ΣCP_cold)_i × ΔT_i
Positive ΔH: Interval has net heat surplus
Negative ΔH: Interval has net heat deficit
Step 4: Cascade heat
Start with zero heat input at top
Cascade heat surplus/deficit downward
Find minimum (most negative) cascade value
Step 5: Add minimum hot utility
Q_H_min = |minimum cascade value|
Add this to top of cascade
All cascade values become non-negative
Zero cascade point = Pinch location
Bottom cascade value = Q_C_min
ΔTmin Selection
| Application | Typical ΔT_min | Rationale |
|---|---|---|
| Gas-gas exchange | 20-40°F | Low heat transfer coefficients |
| Gas-liquid exchange | 15-30°F | Moderate coefficients |
| Liquid-liquid exchange | 10-20°F | Good heat transfer coefficients |
| Boiling/condensing | 5-15°F | High coefficients, phase change |
| Cryogenic | 3-10°F | Expensive cold utility, worth the area |
Trade-off: Smaller ΔT_min reduces utility costs (more heat recovery) but increases heat exchanger area (capital cost). The optimal ΔT_min balances these competing costs. For most gas processing applications, ΔT_min = 15-25°F is a good starting point.
5. Pinch Design Rules
The Three Golden Rules of Pinch Analysis:
Rule 1: Do NOT transfer heat across the pinch
Heat transfer from above to below the pinch
increases BOTH Q_H and Q_C by the same amount.
Every unit of cross-pinch transfer wastes energy.
Rule 2: Do NOT use cold utility above the pinch
The region above the pinch is a net heat SINK.
Using cooling above the pinch creates additional
heating demand that must be met by hot utility.
Rule 3: Do NOT use hot utility below the pinch
The region below the pinch is a net heat SOURCE.
Using heating below the pinch creates additional
cooling demand that must be met by cold utility.
Following all three rules achieves the minimum
utility targets identified by the Problem Table.
Violations and Their Cost
| Violation | Effect on Q_H | Effect on Q_C | Common Example |
|---|---|---|---|
| Heat across pinch (+X) | +X increase | +X increase | Lean/rich amine exchanger spanning pinch |
| Cold utility above pinch (+Y) | +Y increase | No change | Air cooler on hot stream above pinch |
| Hot utility below pinch (+Z) | No change | +Z increase | Steam heater on cold stream below pinch |
6. Heat Exchanger Network Design
After establishing the minimum utility targets and pinch location, the heat exchanger network (HEN) is designed to achieve (or approach) these targets. The network is designed separately above and below the pinch, then connected.
Feasibility Criteria at the Pinch:
Above the pinch (at the pinch point):
Number of hot streams ≤ Number of cold streams
CP_hot ≤ CP_cold (for each match)
Below the pinch (at the pinch point):
Number of cold streams ≤ Number of hot streams
CP_cold ≤ CP_hot (for each match)
If these criteria are not met:
- Split a stream to create additional matches
- Stream splitting adds a degree of freedom
Minimum Number of Units:
N_min = N_streams + N_utilities - N_subsystems
For a connected network (1 subsystem):
N_min = N_hot + N_cold + N_utilities - 1
Fewer units = simpler network but potentially
larger exchangers. More units = more flexibility
but higher piping and installation cost.
Grand Composite Curve
Grand Composite Curve (GCC):
The GCC plots the net heat flow at each temperature
level (from the Problem Table cascade). It shows:
1. The pinch point (where GCC touches zero)
2. Heat pockets (opportunities for integration)
3. Utility placement levels (where to add Q_H, Q_C)
Utility Placement from GCC:
Hot utility: Place at highest practical temperature
Cold utility: Place at lowest practical temperature
Multiple utility levels: Read from GCC shape
The GCC allows selection of the most cost-effective
utility levels (e.g., HP steam vs LP steam vs furnace).
7. Practical Applications in Gas Processing
Amine Unit Heat Integration
Typical Amine Unit Heat Recovery:
Hot streams:
- Lean amine from reboiler (~250°F to ~120°F)
- Overhead condenser (~220°F to ~120°F)
Cold streams:
- Rich amine to regenerator (~110°F to ~230°F)
- Reboiler duty (~240°F to ~260°F)
The lean/rich amine exchanger is the primary heat
recovery element. Pinch analysis can identify
additional opportunities:
- Reflux heat recovery
- Flash gas heat recovery
- Integration with adjacent units
Common Energy Savings Opportunities
| Opportunity | Typical Savings | Implementation |
|---|---|---|
| Lean/rich amine cross-exchanger | 40-60% of reboiler duty | Standard practice, optimize area |
| Feed/effluent exchange | 20-40% of heater duty | Additional exchanger in preheat train |
| Column integration (reboiler/condenser) | 10-30% of column utility | Heat pump or pressure-shifted columns |
| Waste heat recovery | 5-15% of total plant energy | WHR from compressor exhaust, flare |
Limitations and Considerations
- Pinch analysis assumes constant CP (may need segmentation for large temperature ranges)
- Phase changes require special handling (latent heat as separate stream)
- Practical constraints (operability, control, fouling) may prevent achieving minimum targets
- Retrofit projects face layout and piping constraints not present in grassroots design
- Safety considerations may prevent certain stream pairings (e.g., hydrocarbon vs. hot utility)
- Multiple utilities at different temperature levels add complexity to the analysis
Typical results: In gas processing plants, pinch analysis typically identifies 20-40% energy savings compared to a non-integrated design. In practice, 60-80% of the theoretical savings can be captured with reasonable heat exchanger network complexity. The remaining savings may require uneconomic heat exchanger area or impractical stream pairings.
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