CHAPTER 3
HEAT INTEGRATION AND PINCH ANALYSIS
3.1
INTRODUCTION
Clearly, it is possible to heat the cold stream using steam and cool the hot stream using cooling water. However, this would incur excessive energy cost. It is also
3-2
1.
It is mainly a graphical method which allows engineer to keep physical approach of involved phenomenon while other optimization techniques are purely numerical.
2.
It takes into account the whole process or the whole plant, providing a systematic approach instead of focusing on a specific unit or equipment.
3.
It is demonstrated that its use can reduce both capital and operating costs. Emissions are consequently consequently also minimized. minimized.
4.
The energy minimization is performed without any knowledge of the heat exchanger network which is design afterwards.
5.
A very deep knowledge of the analysed process is not required to
3-3
3.3
IDENTIFICATION IDENTIFICATION OF THE HOT, COLD AND AND UTILITY STREAMS STREAMS IN THE
PROCESS
Hot streams are those that must be cooled or are available to be cooled. Cold streams in the other hands are those that must be heated or need to be heated to a certain desire temperature. Hot streams and cold streams can exchange heat between one another.
Utility streams are used to heat or cool process streams, when heat exchange between process streams is not practical or economic. A number of different hot utilities such as steam, hot water and flue gas while a number of different cold utilities such as cooling water, air and refrigerant are used in industry worldwide.
3.4
THERMAL DATA EXTRACTION EXT RACTION FOR PROCESS AND UTILITY STREAMS
3-4
Table 3.1: Summary of hot streams Equipment
Type
Cooler DC1
Hot Hot
373.65 302.12
298.15 301.69
0.01 5.82
∆H (MW) 0.61 2.50
DC2 DC3
Hot Hot
370.49 336.46
364.69 334.90
0.66 2.06
3.84 3.21
Total
10.17
Tsupply(K) Ttarget(K)
FCp(MW/K)
Table 3.2: Summary of cold streams
3.5
Equipment
Type
Heater
Cold
Tsupply(K) Ttarget(K) FCp(MW/K) 302.60
SELECTION OF INITIAL TMIN VALUE
348.15
0.01 Total
∆H
(MW) 0.54 0.54
3-5
When carrying out problem using Problem Table Algorithm (PTA), the temperatures were shifted according to
Tmin/2
being added to the cold streams and
subtracted from the hot streams. This value of contribution to the overall
Tmin between
Tmin /2
can be considered to be a
the hot and the cold streams. Rather than
making the Tmin contribution equal for all streams, it could be made stream-specific (Robin Smith (2005), Chemical Process Design and Integration):
T*H,i = TH,i- Tmin, cont, i T*C,j = TC,j+ Tmin, cont, j
Where T*H,i, TH,I are the shifted and actual temperatures for Hot Stream i , T*C,i, TC,jare shifted and actual temperatures for Cold Stream j , and cont, j
are the contributions contributions to
above example if
Tmin for
Tmin contribution
Tmin, cont, i
and
Tmin,
Hot Streams i and and Cold Streams j Streams j . Thus, for the
for liquid streams is taken to be 5ºC and for gas
streams 10ºC, then a liquid-liquid match would lead to
T
= 10ºC , a gas-gas
3-6
For heat exchange to occur from the hot stream to the cold stream, the hot stream cooling curve must lie above the cold stream-heating curve.Because of the „kinked‟ nature of the composite curves, they approach each other most closely at one point defined as the minimum approach temperature which is
Tmin. Tmin can
be measured directly from the T-H profiles as being the minimum vertical difference between the hot and cold curves. At a particular
Tmin value,
the overlap shows the
maximum possible possible scope for heat recovery recovery within the process. process. The hot end and and cold end overshoots indicate minimum hot utility requirement (Q H min) and minimum cold utility requirement (Q C min), of the process for the chosen
Tmin.
From composite curves, the energy requirement for a process is supplied via process to process heat exchange and/or exchange with several utility levels such as steam levels, refrigeration refrigeration levels, hot oil circuit, and furnace flue gas.
3-7
2.
The duplicate temperature temperature is bracket. The interval temperature is ranked in order of magnitude, showing the duplicate temperatures only once in order.
3.
A heat balance is carried out for the streams falling within each temperature interval:
Hn = ( Cpc- CpH) ( Tn)
4.
The heat surplus from one interval to the next down the column of interval temperature is “cascade”. It is implies that the heat can be transferred between the hot and cold streams. This is possible because any excess heat available from the hot streams in an interval is hot enough to supply a deficit in the cold streams in the next interval down. This will gives some of the heat flows negative value, which is infeasible.
3-8
3.7.1
Data for PTA of 2-Ethylhexyl Acrylate Production
Table 3.4 shows the shifted temperature of the streams according to pinch technology. Table 3.4: Shifted temperature for hot and cold streams FCp
Stream
Type
Ts(K)
Tt(K)
Tss (K)
Tst (K)
∆T
∆H
H1
HOT
373.65
298.15
368.65
293.15
75.5
6.14E-01
8.13
H2
HOT
302.12
301.69
297.12
296.69
0.43
2.50E+00
5816.28
H3
HOT
370.49
364.69
365.49
359.69
5.8
3.84E+00
661.4
H4
HOT
336.46
334.9
331.46
329.9
1.56
3.21E+00
2060.4
C1
COLD
302.6
348.15
307.6
353.15
45.55
5.37E-01
11.8
T S = Saturated Temperature, T t t = Target Temperature, T SS SS = SaturatedTemperature, SaturatedTemperature, T St St = Shifted Target Temperature, ΔH = Enthalpy
(MW/K)
Shifted
3-9
3.7.2
Ranked order of interval interval temperature to create create heat balances balances
From the data of the Global Temperature Interval table, cascade heat is then calculated and shown in the following figure (Figure 3.2) to determine the pinch temperature, minimum heating and cooling requirements. The result obtained will then be used to design the heat exchanger network using the maximum energy recovery method.
3-10
From the heat cascade table,
i.
The pinch temperature, Tpinch
=
368.65 K,
Hot pinch temperature, temperature, Tpinch, hot
=
373.65 K
Cold pinch temperature, temperature, Tpinch, cold
=
363.65 K
ii.
The minimum heating requirement, requirement , Q H,min
=
0 MW
iii.
The minimum cooling requirement, Q C,min
=
9.6283 MW
3.8
GRAND COMPOSITE CURVE
where the
The new tool, the Grand Composite Curve (GCC), was introduced in 1982 by Itoh, Shiroko and Umeda. The GCC shows the variation of heat supply and demand within the process. Using GCC diagram, the designer can find which utilities are to
3-11
Table 3.6: Data for Grand Composite Curve T(K)
∆H interval
368.65
0.00
365.49
0.03
359.69
3.91
353.15
3.96
331.46
3.88
329.90
7.09
307.60
7.01
297.12
7.09
296.69
9.60
293.15
9.63
Grand Composite Curve 400
3-12
Each match match should should not violate the the rules of thermodynamics thermodynamics where the the achieved temperature profile of each streams have a minimum difference of Tmin.
The matching matching process process should should start at the pinch point since since the pinch is is the most constrained part of the network.
3.9.1
Heat Exchanger Network Diagram
3-13
Total Saving For Hot Stream, Q HOT = (0.54 - 0) / 0.54) 0.54) x 100% = 100 %
Total Saving For Cold Stream, Q COLD = (10.17 – (10.17 – 9.62) 9.62) / 10.17 x 100% = 5.37%
From the comparison above, it is clear that the design of heat exchanger network reduces the utility requirements for both the hot and cold streams. By matching the appropriate streams within the plant, a large amount of heat can be transferred and thus, reducing the usage of external utilities for heating and cooling purposes. purposes.
The process flow diagram after heat integration integration is shown next page. p age.
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