Pinch Analysis: Heat Integration and Utility Optimization
Pinch Analysis Problem 1
Given the following stream and utility data, determine:
- (a) The pinch point and corresponding pinch temperatures.
- (b) The minimum hot and cold utility requirements.
- (c) The total heat transfer area needed for a MER network, assuming counter-current heat exchangers.
Take minimum temperature difference: ΔTmin = 20°C
Stream Data
| Stream | Supply Temp (°C) | Target Temp (°C) | mCp (kW/°C) |
|---|---|---|---|
| H1 | 120 | 60 | 8 |
| H2 | 160 | 40 | 10 |
| C1 | 10 | 100 | 2 |
| C2 | 80 | 115 | 60 |
Utility Data
| Utility | Tin (°C) | Tout (°C) | Cost |
|---|---|---|---|
| HU | 180 | 179 | 0.2 |
| CU | 10 | 20 | 0.2 |
Solution
Step 1: Apply Temperature Shift
ΔTmin/2 = 20/2 = 10°C
| Stream | Supply (°C) | Target (°C) | Shifted Supply (°C) | Shifted Target (°C) | mCp |
|---|---|---|---|---|---|
| H1 | 120 | 60 | 110 | 50 | 8 |
| H2 | 160 | 40 | 150 | 30 | 10 |
| C1 | 10 | 100 | 20 | 110 | 2 |
| C2 | 80 | 115 | 90 | 125 | 60 |
Step 2: Construct Temperature Intervals
Intervals: 150, 125, 110, 90, 50, 30, 20
[Draw Temperature Interval Diagram Here]
Step 3: Calculate Net Heat Capacity
| Interval | Net Cp | ΔT (°C) | ΔH (kW) |
|---|---|---|---|
| 150-125 | -10 | 25 | -250 |
| 125-110 | 50 | 15 | +750 |
| 110-90 | 44 | 20 | +880 |
| 90-50 | -16 | 40 | -640 |
| 50-30 | -8 | 20 | -160 |
| 30-20 | +2 | 10 | +20 |
Step 4: Heat Cascade Balance
Pinch Point: 90°C (Shifted)
Minimum Utility Requirements:
- QH,min = 1380 kW
- QC,min = 780 kW
Pinch Analysis Problem 2
For the following stream data determine:
- (i) Pinch point and pinch temperature.
- (ii) Minimum hot and cold utilities requirements.
- (iii) Heat transfer area required for a MER network.
- (iv) Minimum units required for MER network.
Given: ΔTmin = 20°C
Stream Data
| Stream | Ts (°C) | Tt (°C) | mCp (kW/°C) |
|---|---|---|---|
| H1 | 175 | 45 | 10 |
| H2 | 125 | 65 | 40 |
| C1 | 20 | 155 | 20 |
| C2 | 40 | 112 | 15 |
Solution
Step 1: Temperature Shift
Shifted Pinch Temperature = 122°C
Pinch Temperatures:
- Hot Pinch: 132°C
- Cold Pinch: 112°C
Step 2: Utility Requirements
- QH,min = 430 kW
- QC,min = 630 kW
Step 3: MER Network Metrics
- Heat Transfer Area (A) ≈ 41.8 m²
- Minimum Units (Nmin) = 5
Process Engineering FAQs
(i) Basic criteria of Pinch design?
The basic criteria are: No heat transfer across the pinch, no external heating below the pinch, and no external cooling above the pinch.
(ii) Water loss in cooling towers?
Losses include: Evaporation, drift, and blowdown (bleed-off).
(iii) Why avoid cold utility above the pinch?
It increases both hot utility requirements and operating costs, violating pinch design rules.
(iv) Cooling tower approach calculation?
Approach = Cold water outlet temperature − Wet bulb temperature = 30°C − 23°C = 7°C.
(v) Distillation key components?
Key components define the separation (Light/Heavy). Non-key components are outside this range. Intermediate keys have volatility between the light and heavy keys.