1. PINCH ANALYSIS
1.1. Introduction
Pinch Analysis is a systematic methodology to identify and overcome the performance limiting constraint (or “pinch”) in any process system. The prime objective of pinch analysis is to achieve financial savings by better process heat integration (maximizing process-to-process heat recovery and reducing the external utility loads).
Pinch technology presents a simple methodology for systematically analysing chemical processes and the surrounding utility systems with the help of the First and Second Laws of Thermodynamics. The First Law of Thermodynamics provides the energy equation for calculating the enthalpy changes (dH) in the streams passing through a heat exchanger. The Second Law determines the direction of heat flow. 1.2. Data Requirement for Pinch Analysis
1. Identification of the Hot, Cold and Utility Streams in the Process
• ‘Hot Streams’ are those that must be cooled or are available to be cooled. e.g. product cooling before storage
• ‘Cold Streams’ are those that must be heated e.g. feed preheat before a reactor.
• ‘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 (steam, hot water, flue gas, etc.) and cold utilities (cooling water, air, refrigerant, etc.) are used in industry.
2. Thermal Data Extraction for Process & Utility Streams
For each hot, cold and utility stream identified, the following thermal data is extracted from the process material and heat balance flow sheet:
• Supply temperature (TS, o
C) : the temperature at which the stream is available. • Target temperature (TT,
o
C) : the temperature to which the stream must be taken to.
• Heat capacity flow rate (CP, kW/ oC) : the product of flow rate (m) in kg/sec and specific heat (Cp, kJ/kg 0C). CP = m x Cp
• Enthalpy Change (dH) associated with a stream passing through the exchanger is given by the First Law of Thermodynamics:
Enthalpy Change, dH = CP x (TS - TT)
Consider the example of a process with four streams, two cold feeds (to be heated) and two hot product (to be cooled), as illustrated in figure-1. Feed to reactors is heated before inlet to the reactor and the product stream is to be cooled. The heating and cooling are done by use of steam and cooling water respectively. The Temperature (T) vs. Enthalpy flow (H) plot for the feed and product streams depicts the hot (Steam) and cold (CW) utility loads.
Cold stream-1 Hot stream-1
Cold stream-2 Hot stream-2
Figure 1: Hot & Cold streams
Table 1: Summary of temperature & heat flow
Stream Inlet temp Outlet temp Sp. Heat Mass
flow Heat duty CP
C C kJ/kg-C kg/h kJ/h kW kW/C
Cold Stream-1 50 220 3.6 15000 9180000 2550 15.00
Cold Stream-2 80 120 3.6 80000 11520000 3200 80.00
Hot Stream-3 200 100 3.6 20000 7200000 2000 20.00
Hot Stream-4 150 60 3.6 40000 12960000 3600 40.00
The problem can be represented on a Temperature-Enthalpy (T-H) diagram given in figure-2. Enthalpy is a measure of thermal energy. The temperature axis shows the available driving forces for heat transfer, while the enthalpy axis shows the demand for and availability of heat. In this case, both hot and cold duties are supplied by utilities (e.g., steam and cooling water).
From Table-1,
Total hot utility required = 2000 + 3600 = 5600 kW Total cold utility required = 2550 + 3200 = 5750 kW
This would have been the total utility load of the systems discussed above without considering any heat recovery. Now we analyze the system by pinch technology to understand how much of hot and cold utility would be required to carry out the same process.
In the following figure, the green line indicate cold curve of each of the two cold streams. That is, the product cooled from 200 C to 100 C in reactor-1 and 150 C to 60C in reactor-2.
H
Reactor2
C
80 ºC
120 ºC
150 ºC60 ºC
H
Reactor1
C
50 ºC
220 ºC
200 ºC100 ºC
Figure 2: Cold stream curves
Enthalpy flow at starting point is estimated by multiplying CP with initial temperature . i.e. 15 x 50 = 750 kW. Enthalpy at final point is 15 x final temperature = 15 x 220 = 3300 kW. Similarly, the enthalpy at Cold stream-2 inlet is 80 x 80 = 6400 kW and at final point is 80 x 120 = 9600 kW.
Similarly, hot stream values are also plotted as shown below in figure-3.
The estimation of initial points and final points of enthalpy flow is done as explained below. Hot stream-1: Initial temperature = 200 C
CP = 20 kW/C
Enthalpy flow = 200 x 20 = 4000 kW Final temperature = 100 C
CP = 20 kW/C
Enthalpy flow = 100 x 20 = 2000 kW Hot stream-2: Initial temperature = 150 C
CP = 40 kW/C Enthalpy flow = 150 x 40 = 6000 kW Final temperature = 60 C CP = 40 kW/C Enthalpy flow = 60 x 40 = 2400 kW 0 50 100 150 200 250 300 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Enthapy flow, kW T e m p e ra tu re , C Cold stream-1 Cold stream-2 CP=15 CP=80
Figure 3: Hot Stream Curves
1.3. Construction of composite curves
Any stream with a constant heat capacity (CP) value is represented on a T - H diagram by a straight line running from stream supply temperature to stream target temperature. When there are a number of hot and cold streams, the construction of hot and cold composite curves simply involves the addition of the enthalpy changes of the streams in the respective temperature intervals. An example of cold composite curve construction is shown in Figure 4. A complete hot or cold composite curve consists of a series of connected straight lines, each change in slope represents a change in overall hot stream heat capacity flow rate (CP).
The figure -4 shown below indicates construction of cold composite curve.
From temperature 50 to 80 C only Cold stream-1 exist. Hence during this temperature interval, the cold composite curve overlaps the Cold stream-1 curve. The initial point of enthalpy flow is 50 x 15 = 750 kW and the final point is 80 x 15 = 1200 kW.
From temperature 80 to 120 C, both cold stream-1 & 2 are in existence. Hence the CP values of both stream are to be added. i.e. CP in the zone is 15 + 80 = 95 kW/C. The change in enthalpy during 80 to 120 C is thus 95 x ( 120-80) = 3800 kW. The composite curve in this region hence begins at final point of previous zone, i.e. 1200 kW and extends to 1200 + 3800 = 5000 kW.
From 120 C to 220 C, only cold stream-1 exists. Hence CP= 15 and final point would be 5000 + 15 x (220 – 120) = 6500 kW. 0 50 100 150 200 250 300 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Enthapy flow, kW T e m p e ra tu re , C Hot stream-1 Hot stream-2 CP=20 CP=40
Figure 4: Construction of Cold Composite Curve
Similarly, a hot composite curve is also constructed as shown in figure-5.
Figure 5: Construction of hot composite curve Cold curve-construction procedure
0 50 100 150 200 250 300 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Enthapy flow, kW Temperature, C
Cold composite curve Cold stream-1 Cold stream-2 CP=15 CP=80 CP= (15 + 80) = 95 CP=15 0 50 100 150 200 250 300 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Enthapy flow, kW Temperature, C CP=40 CP=20 CP=40 CP=40+20=60 CP=20 Hot composite curve
Figure-6 shows combined hot and cold composite curves in one schematic.
Figure 6: Hot and Cold composite curves
For identifying heat recovery potential, from hot to cold stream, we now shift the hot stream to begin at x-axis corresponding to a temperature of 60 C. Note that the shift of curve along a horizontal path does not affect the energy calculations; we are only interested in difference of enthalpy flows and not the initial or final points. This is given in figure -7
Figure 7: Shifted composite curves 0 50 100 150 200 250 300 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Enthapy flow, kW T e m p e ra tu re , C
Cold composite curve
0 50 100 150 200 250 300 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Enthapy flow, kW T e m p e ra tu re , C
Maximum Heat Recovery
QCmin QHmin
The hot end and cold end overshoots indicate minimum hot utility requirement (QHmin) and minimum
cold utility requirement (QCmin), of the process for the chosen ∆Tmin.
From figure-7, we are now able to understand the heat recovery potential, minimum cold utility required (cooling water) and minimum hot utility required ( steam).
Heat recovery potential = 5600 – 750 = 4850 kW Minimum hot utility = 6500 – 5600 = 900 kW Minimum cold utility = 1200 – 0 = 1200 kW.
Comparing these figures with the initial estimation of 5600 kW for hot utility and 5750 kW for cold utility, we can see that there is a marked reduction in utility requirements if we analyse the system carefully. Note that the cold utility curve can be shifted horizontally to achieve a minimum temperature difference between both curves, called ‘pinch’. Less the value of ‘pinch’, heat recovery potential is more, heat transfer area required and hence cost of equipment also will be more. In the above example, ‘pinch’ or ∆Tmin = 10 ºC. This can be selected to be higher or lower depending on the overall cost reduction ( first
cost + running cost) criteria. The following values of DT min (∆Tmin ) is generally followed by industry.
To summarize, the composite curves provide overall energy targets but do not clearly indicate how much energy must be supplied by different utility levels. The utility mix is determined by the Grand Composite Curve.
1.4. Grand Composite Curve (GCC):
The GCC (Figure 8) shows the variation of heat supply and demand within the process. Using this diagram, the designer can determine which utilities are to be used. The designer aims to maximize the use of the cheaper utility levels and minimize the use of the expensive utility levels. Low-pressure steam and cooling water are preferred instead of high-pressure steam and refrigeration, respectively. The method involves shifting (along the temperature [Y] axis) of the hot composite curve down by ½ ∆Tmin and that of cold composite curve up by ½ ∆Tmin The vertical axis on the shifted composite curves
The Grand Composite Curve is then constructed from the enthalpy (horizontal) differences between the shifted composite curves at different temperatures. On the GCC, the horizontal distance separating the curve from the vertical axis at the top of the temperature scale shows the overall hot utility consumption of the process.
Figure 8: Grand Composite Curve
Figure 8 shows that it is not necessary to supply the hot utility at the top temperature level. The GCC indicates that we can supply the hot utility over two temperature levels TH1 (HP steam) and TH2 (hot
water).
The total minimum hot utility requirement remains the same: QHmin = H1 (HP steam) + H2 (Hot water). Similarly, QCmin = Cooling water. The points TH2 where the H2 level touch the grand composite curve
are called the "Utility Pinch."
In summary, the grand composite curve is one of the most basic tools used in pinch analysis for the selection of the appropriate utility levels and for targeting of a given set of multiple utility levels. The targeting involves setting appropriate loads for the various utility levels by maximizing the least expensive utility loads and minimizing the loads on the most expensive utilities.
Plus/Minus Principle: The overall energy needs of a process can be further reduced by introducing
process changes (changes in the process heat and material balance). There are several parameters
that could be changed such as reactor conversions, distillation column operating pressures and reflux ratios, feed vaporization pressures, or pump-around flow rates. The number of possible process changes is nearly infinite. By applying the pinch rules as discussed above, it is possible to identify changes in the appropriate process parameter that will have a favorable impact on energy consumption. This is called the "Plus/Minus Principle."
0 50 100 150 200 250 300 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Enthapy flow, kW T e m p e ra tu re , C
Grand Composite Curve
QC min QH min HP Steam requirement Hot water requirement TH1 TH2 Tc1 Cooling water requirement
Applying the pinch rules to study of composite curves provide us the following guidelines: • Increase (+) in hot stream duty above the pinch.
• Decrease (-) in cold stream duty above the pinch. will result in a reduced hot utility target,
and any
• Decrease (-) in hot stream duty below the pinch. • Increase (+) in cold stream duty below the pinch will result in a reduced cold utility target.
To summarise, the understanding of the pinch gives three rules that must be obeyed in order to achieve the minimum energy targets for a process:
• Heat must not be transferred across the pinch • There must be no external cooling above the pinch • There must be no external heating below the pinch
Violating any of these rules will lead to cross-pinch heat transfer resulting in an increase the utility energy supply requirement.
1.5. Concluding remarks
Pinch Technology is a procedure which helps in quantifying the maximum potential for heat recovery by integration of process heat exchangers. It also indicates the minimum utility energy supply requirements. It is ideally applicable to new plants at the design stage. It can also be applied to working plants in selected process sections or to the entire plant during major revamps.
The maximum potential for heat recovery indicated by pinch analysis may not be achievable practically due to physical constraints and economics; hence the process designers may have to take a call on what percentage of the heat recovery potential they should factor into the design.
The methodology of Pinch Technology is now also used for water conservation, conservation of process inputs like hydrogen etc.
