Flow rates : Known Obtain : heat capacities (Cp) heat of vaporization/condensation Estimate : vapor loads in the column (design) Obtain heat loads of all streams which change temperature/phase Q. Is it possible to combine/integrate/match them? Energy integration Heat Exchanger Network(HEN) Energy Balance Level 5

Synthesis of Heat Exchanger Networks Questions 1.Why is it important to make a particular study of HENs ? Considerable reductions can be made in the energy requirements of chemical process plants by the application of recently developed techniques ( often of the order of 20 ~ 302 ) The reductions in energy requirements can be achieved with reduced capital costs 2.Is it possible to predict the minimum utility requirements ( e.g. cooling water, steam, fuel, etc. ) in a HEN 3.Is it possible to predict the minimum number of HEN s ? ( Heat exchange matches required are a network if the maximum amount of energy is to be required ) 4.Given affirmative answers to questions 2 and 3, can optimal networks be designed ? 5.How can the design procedures be extended to incorporate other terms of equipment such as gas turbines, distillation columns : HRAT (Heat Recovery Approach Temperature 냉각장치 : = 1~ 2 ℃ 싼 utility 필요시 : = 30 ℃ Pinch - sequential MIP - concurrent

Minimum utility requirements Simple example – one hot and one cold stream FC p T supply T target Stream (kw/ ℃ ) ( ℃ ) ( ℃ ) (1) Hot (2) Cold F : mass flow rate of stream ( kg / s ) C p ; heat capacity of stream ( kJ / kg ℃ ) FC p [ kw / ℃ ] 300 ℃ QHQH steam 180 ℃ 100 ℃ 90 ℃ QcQc Cooling water

Temperature – Enthalpy Plot T℃T℃ 180 ℃ Hot stream 10 ℃ Cold stream 0 T℃T℃ QcQc Q EX PINCH QHQH “ PINCH “ THOT – TCOLD =

For Increasing cold stream curve moving to the right Q vs Plot Q H ( or Q C if Q C ＞ Q H ) Q C ( or Q H if Q C ＞ Q H ) Q(kw) ( ℃ )  Q H and Q C are dependent upon Q H increase Q C increase ( more in more out ) Q H + X Q C + X

Multiple stream HENs Construct composite curves for the combined hot And cold streams Example – two streams FC P T supply T target -ΔH Streams C( kw / ℃ ) ( ℃ ) ( ℃ ) (kw) (1) (2) ΔH(kw) Individual streams Composite stream (1) (2) C=1.5 (1)+(2) C=4.5 C=3 C=

General Procedure 1.Composite hot stream curve on T~ ΔH plot 2.Subtract ΔT min everywhere and draw T H – ΔT min curve 3.Move the composite cold stream curve to right until curve for T H – ΔT min and T C touch at “pinch “ T ΔH THTH T H – ΔT min Composite cold curve ( move to right ) T ΔH QCQC Q EX QHQH Fig 8.1-8

Situations in which the composite hot and cold curves are ΔT min apart at some point are referred to as “pinched” Note that both heating and cooling utilities are required On the “ hot side “ of the pinch only heating utilities are required ( the addition of a cooler simply means that additional heating is necessary ) On the “ cold side “ only cooling utilities are required If more energy is added above the pinch than is necessary, then more energy has to be removed below the pinch. ( effectively increases ΔT min ). This situation corresponds to transferring energy across the pinch

Note In some cases unless ΔT min is made unrealistically large only one utility is required ( heating or cooling ) For example T ΔH Q EX QHQH Heater only ΔT min QCQC T ΔH Q EX THTH Cooler only Q C ( or Q H ) Q H ( or Q C ) QHQH QCQC ΔT min : value of ΔT min at which second utility is required

Situations in which the composite hot and cold curves are always greater than ΔT min apart are referred to as “ unpinched “. Pinched Problem Do not use cooling utilities above the pinch Do not use heating utilities below the pinch Avoid transferring energy across the pinch Consider a simple design problem Stream T supply T target C(FC P ) ( ℃ ) ( ℃ ) (kw / ℃ ) (1) (2) (3) (4) ΔT min = 10 ℃

First find the minimum heating and cooling requirements and the pinch temperature. Rather than construct composite curve it is easier to construct a table of energy excesses in terms of cold stream temperature. Subtract ΔT min from hot stream temperature to give the following table. Stream T supply T target FC P (1)(cold) (2)(hot) (3)(cold) (4)(hot) Convenient representation : FCP

Construct table Stream temp energy surplus Interval or deficit 160 – (surplus) (surplus) 135 – (deficit) 80 – (surplus) 50 – Energy surpluses may be cascaded down to lover temperatures as may be shown diagrammatically as follows : – – Hot utility “ cascade diagram “ Deficit of 20 kw hence this is not a feasible heat exchange process ∴ add 20kw from hot stility Cold utilit

– – Hot utility Zero energy cascaded at cold stream temp. 80 ℃ (hot stream 90 ℃ ) Hence pinch is at cold stream temp. of 80 ℃ Cold utility 20 Pinch at 90 ℃ ( hot streams ) 80 ℃ ( cold streams ) QH ( total hot utilities ) : 20 kw QC ( total cold utilities ) : 60 kw Note If more than 20 kw is added through the hot utility then the additional energy is cascaded through the entire process to the cold utility

Problem representation ( Linnhoff ) C H Hot stream Cold stream Stream numbers load cooler load Heater Heat exchanger FC P Direction of increasing temp. Advantage : easy to change configuration of network without re-routing streams. For the problem under consideration start at pinch pinchQ H = 20 Q C = 60kw

No energy should be transferred across the pinch, The design problem is effectively decomposed into two separate problems : one above the pinch and one below the pinch. Consider a heat exchanger on stream 2 or the hot side of the pinch. Two possibilities FC P (1)(2) T ΔH T ΔT min FC Phot ＞ FC Pcold FC Phot ≤FC Pcold infeasible feasible

A heat exchanger may be placed above the pinch connecting streams 2 and 3 with a load of 240 kw. Similarly a heat exchanger may be placed connecting streams 4 and 1 with a load of 90 kw leaving 20 kw (QH) to be provided by a heater to stream 1 Cold End Design FC P ΔT min TT FC Phot ≥FC Pcold FC Phot ＜ FC Pcold infeasible feasible A heat exchanger with a load of 90 kw may be placed connecting streams 2 and 1

Complete Design H C 170˚ 90˚ 60˚ 150˚90˚ 70˚30˚ 130˚ 20˚ 120˚ 80˚ 35˚ 60˚ 20˚ 90˚ 140˚ 240˚ 80˚ FC P The above design achieves maximum possible energy recovery for ΔT min = 10 ℃ 1.Start at the pinch and make matches “ outwards “ 2.Immediately above pinch make matches that meet the requirement 3.Immediately below pinch make matches that meet the requirement FC Phot ≤FC Pcold FC Phot ≥FC Pcold

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