Presentation

This session is the second part of an introductory course on heat integration or pinch method.

It presents in details a practical example of this method, with the heat exchanger network determination.

If you are not yet familiar with the basic concepts of heat integration (pinch, composites, algorithm for minimizing the pinch), we suggest you should first study session IT1En.

Gourlia case study

• Build the composite curves by hand
• Build the heat exchanger network by hand
• Solve the problem with Thermoptim

Problem data

• Heat availability

Stream 1	220 – 40 °C	1,800 kW
Stream 2	320 – 200 °C	2,400 kW
Stream 3	140 – 40 °C	800 kW
Stream 4	100 °C	1,200 kW
Boiler	600 °C	1,900 kW

Problem data

• Needs

Stream 5	40 – 320 °C	6,000 kW
Stream 6	220 °C	600 kW

Problem Table Algorithm PTA

• 1) shift minimal and maximal temperatures by ΔTmin/2

  • decrease for hot streams
  • increase for cold streams
• 2) determine the temperature intervals of the problem by sorting the bounds of the upper and lower temperatures of each stream and build the enthalpy balances of each temperature interval which determines the hot and cold composites
• 3) sum up the enthalpy deficits
• 4) bring as hot utility the equivalent value of the maximum enthalpy deficit

PTA: 1) temperature shift

• before

  stream	m Cp	Ts	Tt	H
  	(kW/K)	(°C)	(°C)	(kW)
  1	10	40	220	1,800
  2	20	200	320	2,400
  3	8	40	140	800
  4	1200	100	101	1,200
  5	21	40	320	6,000
  6	600	220	221	600

• after

  stream	m Cp	type	Tinf	Tsup	Δh
  	(kW/K)		(°C)	(°C)	(kW)
  1	10	chaud	35	215	1,800
  2	20	chaud	195	315	2,400
  3	8	chaud	35	135	800
  4	1200	chaud	95	96	1,200
  5	21	froid	45	325	6,000
  6	600	froid	225	226	600

PTA: 2) temperature intervals

• 1) shift minimal and maximal temperatures by ΔTmin/2

  • decrease for hot streams
  • increase for cold streams
• 2) determine the temperature intervals of the problem by sorting the bounds of the upper and lower temperatures of each stream and build the enthalpy balances of each temperature interval which determines the hot and cold composites
• 3) sum up the enthalpy deficits
• 4) bring as hot utility the equivalent value of the maximum enthalpy deficit

PTA: 2) temperature intervals

• Δh_bd = (T_i - T_i+1) (ΣmCp_b - ΣmCp_d) = Δh_b - Δh_d

PTA: 3) cumulative net enthalpy balances

• 1) shift minimal and maximal temperatures by ΔTmin/2

  • decrease for hot streams
  • increase for cold streams
• 2) determine the temperature intervals of the problem by sorting the bounds of the upper and lower temperatures of each stream and build the enthalpy balances of each temperature interval which determines the hot and cold composites
• 3) sum up the enthalpy deficits
• 4) bring as hot utility the equivalent value of the maximum enthalpy deficit

PTA: 3) cumulative net enthalpy balances

PTA: 4) enthalpy balances with input

• 1) shift minimal and maximal temperatures by ΔTmin/2

  • decrease for hot streams
  • increase for cold streams
• 2) determine the temperature intervals of the problem by sorting the bounds of the upper and lower temperatures of each stream and build the enthalpy balances of each temperature interval which determines the hot and cold composites
• 3) sum up the enthalpy deficits
• 4) bring as hot utility the equivalent value of the maximum enthalpy deficit

PTA: 4) enthalpy balances with input

Stream pairing

• Notations

Composite curves

• The pinch separates the problem in two parts
• Each zone is thermally independant
• Heat exchanger networks can be built separately

Conditions for a pinch to appear

• Pinch:
• either appearance of a new cold stream (need)
• or disappearance of a hot stream (availability)
• or simultaneity of both events: onset of a cold stream and disappearance of a hot stream

Rules

• separately endo- and exothermic zones study
• begin the design at the pinch and depart gradually from it, to deal first with the most constrained problem
• maximize the load of heat exchangers (to minimize their number)
• above the pinchmCph ≤ mCpc
• below the pinch mCph ≥ mCpc

Stream pairing

• Reminder of data

Above the pinch

• the first interval of the endothermic zone: hot stream 4 disappearance
• 1 cold stream (5), and 2 hot streams (1 and 3)
• mCp constraints are met
• we must then split (5) in two

Above the pinch

• interval above:

  number	mCp	ΔH
  	(kW/K)	(kW)
  1-10	600
  5	21.4	1,285.7

• disappearance of stream 3
• the corresponding load (320 kW) must be supplied to stream 5: mCp₅₂ = mCp₃ = 8

Above the pinch

• we deduce mCp₅₁ = 13.4
• to exhaust stream 1 (1,200 kW), the exit temperature of heat exchanger 1 is equal to 179°C

Above the pinch

• streams 1 and 3 being exhausted, only 2 still contains availability (2,400 kW), which must be shared between the divided stream 5 and stream 6
• we can transfer the entire load in 6 (600 kW), and part of the residual load on the first branch of 5 (1,800 kW of the 1,888 required). However, this scenario leads to a failure in terms of T
• better to remix both stream 5 branches at the outlet of heat exchangers 1 and 2, and to have a hot utility (1,608 kW)

Heat exchanger network

• Hot end

Below the pinch

• only one cold stream: 5
• constraint mCp_h ≥ mCp_c met if and only if paired to 4
• by maximizing the load of this exchanger (5), and knowing that the inlet T of 5 is 40 °C:
• Δh₅ = mCp₅ (90 - 40) = 1,071 kW

Below the pinch

• 5 totally heated
• cannot cool 1, 3, and4
• to be done by cold utilities (1,209 kW)

Final heat exchanger network

Modeling with Thermoptim

• Assumptions
• fluids : perfect air and steam

• estimated flow rates :

  

  	m	ΔH
  	(kg/s)	(kW)
  stream 1	9.946	-1,800
  stream 2	19.892	-2,400
  stream 3	7.9568	-800
  stream 4	0.5317	-1,200
  stream 5	21.312	6,000
  stream 6	0.3232	600

Modeling tips

• enter data in both the inlet and outlet tabs
• pinch method fluid
• ΔT_pinc = 10 K

Optimization screen

Composites

Heat exchanger network

• 1) divide all streams that cross the pinch, in order to separate both sub-networks

Heat exchanger network

• 2) analyze the heat capacity rates mCp of the streams

Heat exchanger network

• 3) build exchange blocks to test possible matches:
• parallelization of 5 above the pinch

Heat exchanger network

• 4) build the exchangers :
• beware of streams duplicated
• exchangers are not automatically built in the diagram editor

Small practical difficulties

• 1) model modification:
• division of streams across the pinch
• exchangers not built in the diagram editor
• 2) default parallelizations may not be satisfactory
• 3) in exchangers, the quality of recalculated outlet points should not be between 0 and 1 (evaporation and condensation)
• Therefore: operate step by step creating backup project and diagram files

Full heat exchanger network

Practical use of the pinch method

In this session, we have illustrated the heat integration method by treating an example proposed in 1989 by Gourlia in the Revue Générale de Thermique.

Thermoptim files of the example can be downloaded from the link below.

We now suggest that you refer to the specialized temperature, and in particular to publications of Prof. B. Linnhoff.

We recommend that you read the following synthesis:

Introduction to Pinch Technology, Copyright 1998 Linnhoff March.

• Thermoptim files