A Numerical Technique for Total Site Sensitivity Analysis

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Applied Thermal Engineering 40 (2012) 397e408

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Applied Thermal Engineering

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A numerical technique for Total Site sensitivity analysis

Peng Yen Liewa, Sharifah Rafidah Wan Alwi a,*, Petar Sabev Varbanov b, Zainuddin Abdul Manan a,Ji�rí Jaromír Kleme�s b

a Process Systems Engineering Centre (PROSPECT), Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, MalaysiabCentre for Process Integration and Intensification e CPI2, Research Institute of Chemical and Process Engineering, Faculty of Information Technology, University of Pannonia,Egyetem u. 10, H-8200 Veszprém, Hungary

a r t i c l e i n f o

Article history:Received 19 October 2011Accepted 11 February 2012Available online 22 February 2012

Keywords:Total site problem table algorithm (TS-PTA)Total SiteHeat cascadeNumerical approachSite minimum utility targetsProcess integration

* Corresponding author. Tel.: þ60 07 5535533; faxE-mail address: [email protected] (S.R. Wan

1359-4311/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.applthermaleng.2012.02.026

a b s t r a c t

Total Site Heat Integration (TSHI) is a methodology for the integration of heat recovery among multipleprocesses and/or plants interconnected by common utilities on a site. Until now, it has not been used toanalyze a site’s overall sensitivity to plant maintenance shutdown and production changes. This featureis vital for allowing engineers to assess the sensitivity of a whole site with respect to operational changes,to determine the optimum utility generation system size, to assess the need for backup piping, toestimate the amount of external utilities that must be bought and stored, and to assess the impact ofsensitivity changes on a cogeneration system. This study presents four new contributions: (1) Total SiteSensitivity Table (TSST), a tool for exploring the effects of plant shutdown or production changes on heatdistribution and utility generation systems over a Total Site; (2) a new numerical tool for TSHI, the TotalSite Problem Table Algorithm (TS-PTA), which extends the well-established Problem Table Algorithm(PTA) to Total Site analysis; (3) a simple newmethod for calculating multiple utility levels in both the PTAand TS-PTA; and (4) the Total Site Utility Distribution (TSUD) table, which can be used to design a TotalSite utility distribution network. These key contributions are clearly highlighted via the application of thenumerical technique to two Case studies.

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1. Introduction

Pinch Analysis is an established technology for reducing energyconsumption that has been widely applied in various industries formore than 30 years. Dhole and Linnhoff [1], Raissi [2] and Kleme�set al. [3] extended traditional heat integration, which focuses ondirect heat transfer among process streams at a single site, to heatintegration for multiple sites. This is known as Total Site HeatIntegration (TSHI), sometimes called “site-wide integration”. Directheat transfer is not always suitable for inter-process heat recoverydue to the required high degree of operational flexibility and thelong-distance piping needed, which makes it very costly [4]. TSHIusing indirect heat transfer utilising existing utility systems istypically more cost effective because the existing plant pipingsystem can be used. TSHI heat integration is linked by a commoncentral or sectional utility system.

Dhole and Linnhoff [1] have introduced Site Sink and SourceProfiles (SSSP), a graphical tool that can be used to evaluate fuelconsumption, cogeneration, emissions and cooling needs foran integrated site. A simple exergy model was proposed for

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cogeneration capacity estimation based on SSSP, and themodel wasfurther extended by Raissi [2] and Kleme�s et al. [3]. Based on SSSP,Kleme�s et al. [3] developed the Total Site Profile (TSP) and the SiteUtility Grand Composite Curve, which can be used to evaluate TotalSite potential heat recovery. Subsequently, Maréchal and Kalit-ventzeff [5] introduced a mathematical programming tool forminimising Total Site energy costs. Their work also included anintegration of combined heat and power production using a steamnetwork. Matzuda et al. [6] have successfully studied the heatrecovery potential for a large steel plant using TSP analysis.

An advanced approach to these concepts, known as top-levelanalysis, is one that allows for “scoping”, i.e., selecting siteprocesses to target for heat integration improvements [7]. Theutility system is first optimised for the current steam and powerdemands. This is followed by an assessment of the potential benefitof reducing steam demands at various levels by successively opti-mising the system in steps of steam demand reduction. This resultsin a set of curves for steam marginal prices for the system underconsideration.

Perry et al. [8] extended the Total Site concept to a broaderspectrum of processes in addition to the industrial process.A potential for the integration of renewable energy sources wasintroduced to reduce the carbon footprint of a Locally Integrated

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P.Y. Liew et al. / Applied Thermal Engineering 40 (2012) 397e408398

Energy Sector (LIES). In a LIES, heat sources and sinks can be derivedfromsmall-scale industrial plants, largebuilding complexes (such ashotels and hospitals), offices and residential areas.

One of the major challenges in implementing the Total Siteconcept involving renewable energy is the variation of energysupply and demand with time and location. Therefore, Varbanovand Kleme�s [9] suggested performing Total Site targeting in a set oftime slices to maximise heat recovery within each time slice. Var-banov and Kleme�s [10] then further extended the concept to heatstorage, heat waste minimisation and carbon footprint reduction. ATotal Site heat cascade is also introduced in this work to illustratethese concepts.

Bandyopadhyay et al. [11] proposed a modification of the SiteGrand Composite Curve (SGCC) that incorporates assisted heattransfer. This type of heat transfer takes into account the non-monotonic parts or pockets of the process GCC, which were notconsidered by Dhole and Linnhoff [1], although this may not bepractical for many integrated sites. The results of their study showthat the modified SGCC tends to increase heat recovery potential,particularly those within each process, a feature which is notconsidered in the TSP. However, the economy of this design has tobe explored, as an increased integration using nonstandard steammains can be costly.

Kapil et al. [12] proposed the recovery and upgrading of low-grade heat from processes. The work has proposed a new meth-odology for estimating the cogeneration potential for a site utilitysystem via bottom-up and top-down procedures. Ghannadzadehet al. [13] presented Iterative Bottom-to-TopModel (IBTM) as a newshaftwork targeting model to estimate the cogeneration potentialfor site utility systems prior to the detailed design.

Fodor et al. [14] further developed a TSHI targeting method toallow for a variation of the minimum temperature difference(DTmin) among Total Site processes. Previous works by Dhole andLinnhoff [1] and Kleme�s et al. [3] assumed a uniform DTmin ona Total Site. Fodor et al. [14] and Varbanov et al. [15] proposed theuse of a utility and process-specific DTmin between utility andprocess streams, which is more realistic in practical applications.

The Total Site methodology and the concepts developed byDhole and Linnhoff [1] and used in recent studies are based ona graphical method, with the typical advantages and disadvantagesof such approaches. Numerical methodologies that provide similarbenefits such as the Problem Table Algorithm (PTA) for heat pinchand the Water Cascade Analysis (WCA) for water pinch are there-fore desirable.

The PTA is a numerical tool for intra-process heat integrationproposed by Linnhoff and Flower [16]. This tool is the equivalent tothe use of Composite Curves (CCs) and Grand Composite Curve(GCCs) in the graphical method and supports a more precisegraphical construction by providing exact values for the crucialpoints. The algorithmwas extended to multiple utility targeting byCosta and Queiroz [17]. The PTA was also recently extended to theUnified Targeting Algorithm (UTA) by Shenoy [18]. The UTA isa powerful tool for obtaining the maximum resource recovery forProcess Integration problems including heat and mass exchange,water, hydrogen, carbon emissions and material reuse networks.However, the method proposed by Costa and Queiroz [17] involvesrather complex calculations, whereas the UTA cannot be used forTSHI problems. To make the PTA a more powerful tool, simplermethod for multiple utility targeting would be beneficial. Addi-tionally, the PTA can also be extended to TSHI.

In the current study, a new numerical tool for targeting TSHI isproposed, known as the Total Site Problem Table Algorithm(TS-PTA). This numerical tool is an alternative to the graphical TSHIapproach and is suitable for both the uniform and non-uniformDTmin methods proposed by Dhole and Linnhoff [1], Kleme�s et al.

[3], Fodor et al. [14] and Varbanov et al. [15]. Although graphicalapproaches are advantageous in terms of providing valuable visualinsights, they are difficult to construct, especially for large prob-lems, and may yield some inaccuracies inherent in the graphicalnature of the method. The Problem Table Algorithm (PTA), which isa numerical tool introduced by Linnhoff and Flower [16] as analternative to the Composite Curves, has been among the preferredanalytical tools used to compensate for the limitations of thegraphical approaches. In this work, the PTA method is extended toinclude TSHI analysis.

The previous works cited have generally not deeply studied theflexibility of integrated plants. A numerical tool therefore offersa good opportunity to evaluate the sensitivity of each plant in TSintegration. The Total Site Sensitivity Table (TSST) can be used asa tool to explore site-wide sensitivities to various operationalchanges and variations. A typical case is when one site processmustbe closed down for regular maintenance or due to an accident.Using the TSST, the effect of a plant shutdown can be assessed, andsuitable measures can be taken during the design and operationalstages to ensure other site utility supplies are not disrupted.

2. Methodology

A summary of the procedure involving the four methodologiesis described in the following.

2.1. Tool 1: Total Site Problem Table Algorithm (MU-PTA)

The initial steps follow the same procedure as the PTA forindividual process. First, the shifted temperatures for the processstreams in each individual process are calculated as described inSmith [19], Kemp [20] and Kleme�s et al. [21]. The PTA is constructedas described by Linnhoff and Flower [16] and Smith [19]. Themultiple utility cascade procedure for each individual plant is asfollows:

a. Above the pinch region:i. Subtract half of the minimum temperature difference

within each process, DTmin,pp/2, from the shifted tempera-ture to return it to a normal temperature, and then add theminimum temperature difference between the utility andprocess stream (DTmin,up) [14,15].

ii. Cascade the heat available in each temperature intervalfrom the highest temperature to the pinch temperature.When a negative value results, an external heat enthalpyrepresenting the utility is added immediately to thetemperature interval during cascading.

iii. The amount of each utility type required can be determinedby summing the external heat enthalpies from below eachutility temperature to the next utility temperature.

b. Below the pinch region:i. DTmin,pp/2 is added, and DTmin,up is subtracted, to the shif-

ted temperatures [14,15].ii. The heat available in each temperature interval is cascaded

from the lowest temperature to the pinch temperature, andthe external cooling utility required is immediately addedto the temperature interval when there is positive value inthe cascade.

iii. The amount of each utility type to be generated is obtainedby adding the external cooling utility above each utilitytemperature but below the next-highest temperatureutility.

This tool also could be used for single process Heat Integrationwhich has different temperature shifting at the beginning.

Table 2Stream data for Plant B of Case study 1 with DTmin ¼ 10 �C; modified example fromKemp [20].

Stream Ts (�C) Tt (�C) DH(MW) mCp (kW/�C) Ts’ (�C) Tt’ (�C)

B1 Hot 200 50 0.450 3.0 195 45B2 Hot 240 100 0.210 1.5 235 95B3 Hot 200 119 1.863 23.0 195 114B4 Cold 30 200 0.680 4.0 35 205B5 Cold 50 250 0.400 2.0 55 255

P.Y. Liew et al. / Applied Thermal Engineering 40 (2012) 397e408 399

Specifically, at above pinch, DTmin,pp/2 is added to the shiftedtemperature in step a(i). At below pinch point, DTmin,pp/2 is sub-tracted in step b(i).

2.2. Tool 2: Total Site Problem Table Algorithm (TS-PTA)

The TS-PTA is a continuation of the PTA table with an extensionof four columns. This table represents an algebraic version of theSite Composite Curve (SCC) in a graphical TSHI analysis. Theprocedure is described as follows:

a. The heat sinks above the pinch region in each process areadded as ‘net heat sinks’ according to utility type. Similarly, theheat sources below the pinch region are added to become ‘netheat sources’ according to utility level.

b. The net heating requirement at each utility level is formulatedby deducting the net heat source from the net heat sink.

c. The net heating requirements are then cascaded from the topto the bottom.

d. Analogous to the PTA, the most negative value of the previouscascade is then used to initiate a new cascade, after firstchanging it to a positive value.

e. Similar steps to construct a PTA involving multiple utilities areperformed as follows:i. Above the Total Site Pinch, the net heat requirement is

cascaded from the top to the bottom. An external heatingutility is added into the system when there is a negativevalue to balance the heat deficit at different utility levels.

ii. Below the Total Site Pinch, the neat heat requirement iscascaded from the lowest temperature cooling utility to thepinch. A negative external cooling utility is added whena positive value occurs in the cascade.

2.3. Tool 3: Total Site Utility Distribution (TSUD) table

To visualise the site distribution network, a TSUD table can beconstructed as follows:

a. The table lists the heat sources and sinks of each site accordingto utility. The external heat requirement calculated in the TS-PTA is also recorded.

b. Arrows are used to indicate possible utility exchanges from onesite to another or from a utility plant to a site.

2.4. Tool 4: Total Site Sensitivity Table (TSST)

The Total Site Sensitivity Table (TSST) is a practical tool foranalysing the effects of variations in Total Site operating conditionson heat distribution and utility generation. The TSST is constructedas below:

a. The TS-PTA is used to determine the utilities necessary fordifferent operating conditions, e.g., when one of the plants is

Table 1Stream data for Plant A of Case study 1 with DTmin ¼ 20 �C; modified example fromCanmet ENERGY [22].

Stream Ts (�C) Tt (�C) DH(MW) mCp (kW/�C) Ts’ (�C) Tt’ (�C)

A1 Hot 200 100 2.00 20 190 90A2 Hot 150 60 3.60 40 140 50A3 Cold 50 120 4.90 70 60 130A4 Cold 50 220 2.55 15 60 230

shutdown. The findings are recorded in the table based on thedifferent types of utilities.

b. Variations of normal operation with various operating condi-tions are calculated by subtracting the utility requirements innormal operations from the utility requirements underdifferent operating conditions according to utility type.

A more detailed explanation of all the tools using different stepsis described below.

3. Demonstration Case study

The four tools are used to demonstrate their application to TotalSite sensitivity analysis.

3.1. Step 1: construct the Problem Table Algorithm (PTA) todetermine QHmin, QCmin and the pinch temperature for eachindividual plant

The temperature of cold streams (Tc) and the temperature of hotstreams (Th) in an individual plant are converted to shifted coldstream temperatures (Tc’) and shifted hot stream temperatures(Th’). Tc is shifted by adding half of the minimum temperaturedifference between processes, DTmin,pp, whereas Th is shifted bysubtracting half of DTmin,pp. Assuming a DTmin,pp of 20 �C for plant Aand a DTmin,pp of 10 �C for Plant B, Tables 1 and 2 show the shiftedtemperatures of all streams in Plants A and B of Case Study1. Table 3 shows the utility temperature levels available at theplants, which are used in the next step. The minimum utility/process temperature difference, DTmin,up, is 10 �C.

PTA are performed for both Plant A and Plant B. The completedPTAs for Plants A and B are shown in Tables 4a and 4b, respectively.As shown in Table 4, plant A requires 2250 kW of hot utility and400 kW of cold utility with a shifted pinch temperature of 60 �C.Plant B requires 100 kW of hot utility and 1543 kW of cold utilitywith a shifted pinch temperature of 195 �C. Figs. 1 and 2 are theGCCs for plants A and B. The results from these GCCs are similar tothe results obtained from the PTA given in Tables 4a and 4b.

3.2. Step 2: construct a Multiple Utility Problem Table Algorithm(MU-PTA) for each individual plant to obtain targets for multipleutility levels as heat sources and sinks for TSHI

MU-PTA are constructed to target the amounts of various utilitylevels selected as potential sinks and sources for use in Total Site

Table 3Site utility data for Case study 1.

Utility Temperature (�C)

High-pressure steam (HPS) 270Medium pressure steam (MPS) 179.93Low-pressure steam (LPS) 133.59Cooling water (CW) 15e20

Table 4aSingle utility cascade table for Plant A of Case study 1.

1 2 3 4 5 6 7

T’ ( C)( C)

mCp (kW/ C)

(kW/ C)(kW)

Initial Heat

Cascade

Single Utility Heat

Cascade20 40 70 15

230 0 2250

40 -15 -600

190 -600 1650

50 5 250

140 -350 1900

10 45 450

130 100 2350

40 -25 -1000

90 -900 1350

30 -45 -1350

60 -2250 0

10 40 400

50 -1850 400


integration. The multiple utility cascade methodology is an exten-sion of the PTA with an additional 4 columns. The multiple utilitycascade calculations are similar to GCCs and can be used identifypockets and target the exact amounts of utilities needed withina given utility temperature interval. Note that multiple utilitycascades must be performed based on the pinch regions for eachplant that were determined in Step 1.

3.2.1. Multiple utility cascades in the region above the pinch of eachindividual plant

All shifted temperatures (T0) in the region above the pinch(column 1, Table 5) from Table 4 PTA are reduced by DTmin,pp/2 toreturn them to normal temperatures and then the minimumtemperature difference between the utility and the process(DTmin,up) is added, as shown in column 2, Table 5; the resultingtemperature is labeled T00. The utility temperatures listed in Table 3were also added into Table 5 to make it easier to determine theutility distribution at a later stage.

Heat is again cascaded starting from the highest temperaturesegment to the pinch temperature, as shown in column 7, Tables 5aand 5b. Note that there are no changes in the calculations of ‘summCp’ and ‘sum DH’ for each temperature level. This cascade isknown as a ‘multiple utility heat cascade’; it differs from theprevious heat cascade in the PTA (column 6, Tables 4a and 4b)because it is performed interval-by-interval. If a negative value isencountered while cascading one of the temperatures, externalutilities are immediately added at that point (the amount ofexternal utility added is listed in column 8) equal to the negativevalue. The cascade then becomes zero at that temperature, e.g., at

a shifted temperature of 190 �C, the cascade initially gives a valueof �600 kW at column 7. Therefore, 600 kW of external utility isadded at this interval, as listed at column 8. The cascade nowbecomes zero here, as shown in column 7, Table 5. The cascade isthen continued, and the procedure is repeated.

Once the multiple utility heat cascades are completed, theamounts of each type of utility consumed in the process are ob-tained by adding the utility consumed below the utility tempera-ture (from column 8, Table 5) to before the next utility temperature.For example, Table 5a shows that 600 kW of high-pressure steam(HPS) at a temperature of 270 �C is consumed in plant A between270 �C and 179.93 �C. Thus, 1650 kWof low-pressure steam (LPS) isused between 133.59 and 60 �C for plant A. The same procedure isrepeated for plant B to yield a requirement of 100 kW of high-pressure steam.

3.2.2. Multiple utility cascades for the region below the pinch ofeach individual plant

A similar methodology is used for multiple utility cascadingbelow the pinch temperature. All temperatures available below thepinch are shifted by adding DTmin,pp/2 and then subtracting theminimum temperature difference between the utility and process,DTmin,up (see the region below the pinch in column 2, Table 5) toobtain the temperatures in the utility temperature scale. Utilitytemperatures are then added to the temperature list, as in column2, Table 5.

However, multiple utilities are instead cascaded starting fromthe bottom temperature to the pinch temperature, and any positiveheat value encountered while cascading must be zeroed out by

Fig. 2. Grand Composite Curve for Plant B of Case study 1 [15].

Table 4bSingle utility cascade table for Plant B of Case study 1.

1 2 3 4 5 6 7

T' ( C)( C)

mCp (kW/ C)

(kW/ C) (kW)Initial

Cascade

Single Utility

Cascade3 1.5 23 4 2

255 0 100

20 -2 -40

235 -40 60

30 -0.5 -15

205 -55 45

10 -4.5 -45

195 -100 0

81 21.5 1741.5

114 1641.5 1741.5

19 -1.5 -28.5

95 1613 1713

40 -3 -120

55 1493 1593

10 -1 -10

45 1483 1583

10 -4 -40

35 1443 1543

Fig. 1. Grand Composite Curve for Plant A of Case study 1 [15].


Table 5aPTA with multiple utility heat cascades for Plant A of Case study 1.

1 2 3 4 5 6 7 8 9

T’( C)

T’’( C) ( C)

mCp (kW/ C)

(kW/ C) (kW)

Multiple Utility Heat

Cascade

UtilityConsumed/ Generated

(kW)

Heat Sink/ Source20 40 70 15

270 0 HPS 600

40 0 0

230 230 0

40 -15 -600 600

190 190 0

10.07 5 50.35

179.93 50.35 MPS 0

39.93 5 199.65

140 140 250

6.41 45 288.45

133.59 538.45 LPS 1650

3.59 45 161.55

130 130 700

40 -25 -1000 300

90 90 0

30 -45 -1350 1350

6060 0

Pinch60 0

10 40 400-400

50 50 0

35 0 0

15 0 CW 400


generating utilities (see the lower part of column 7 and 8 in Table 5).For the region below the pinch, the negative values encounteredduring multiple utility cascading represent pockets in the GCC.

The amount of utility that can be generated can be determinedby adding the amounts of excess heat from above the utilitytemperature to the next utility temperature level. For example,plant A can generate 400 kW of CW using process heat between 50and 10 �C. For plant B, 216.50 kW of medium pressure steam (MPS)at 190 to 179.93 �C and 996.31 kW of LPS between 179.93 and133.59 �C can be generated, whereas 330.19 kWof CW is consumed.

The proposed method differs from the one developed by Costaand Queiroz [17]. The method in this study was developed througha detailed observation of multiple utility targeting in the GCC. Inaddition, the method proposed herein is a direct continuation ofthe PTA, in which the multiple utility cascade actually uses most ofthe information from the PTA. The method proposed by Costa andQueiroz [17] includes an interpolation step for finding the upperand lower temperature boundaries after utility targeting. However,

the proposed methodology targets utilities according to tempera-ture intervals, with the utility temperatures becoming temperatureboundaries, to distinguish the amounts of each utility type. Thecalculations involved in this proposedmethod are also simpler thanthose of the previously proposed method.

3.3. Step 3: construct the Total Site Problem Table Algorithm(TS-PTA) to determine the amounts of utilities that can beexchanged among processes

This part is an extension of the PTA to represent the Site CC inTSHI. The utilities available from each plant are arranged fromhighest to lowest temperature. The utilities generated below thepinch temperature for all sites, as determined in Step 3, are addedtogether to represent the net heat source (see column 3, Table 6).The utilities consumed above the pinch temperature for all sites, asdetermined in Step 2, are added together to represent the net heatsink (see column 4, Table 6). Fig. 3 shows the TSP and the Site

Table 6Total site Problem Table algorithm (TS-PTA) for Case study 1.

1 2 3 4 5 6 7 8 9

Utility Utility Temp. (�C) Net heatsource (kW)

Net heatsink (kW)

Net heatrequirement (kW)

Initial heatcascade

Final single heatcascade

Multiple utilityheat cascade

External utilityrequirement (kW)

0 1137.19 0HPS 270 0 700 �700 700

�700 437.19 0MPS 179.93 216.50 0 216.50 0

�483.50 653.69 216.50LPS 133.59 996.31 1650 �653.69 437.19

�1137.19 0 (Pinch) 0CW 15e20 730.19 0 730.19 L730.19

�407 730.19 0

Table 5bPTA with multiple utility heat cascade for Plant B of Case study 1.

T’( C)

T’’( C) ( C)

mCp (kW/ C)

(kW/ C) (kW)

Multiple Utility Heat

Cascade

Utility Consumed/ Generated

(kW)

Heat Sink/ Source

3 1.5 23 4 2

270 0 HPS 100

10 0 0 0

255 260 0

20 -2 -40 40

235 240 0

30 -0.5 -15 15

205 210 0

10 -4.5 -45 45

195

200 0Pinch

190 0

10.07 21.5 216.51 -216.50

179.93 0 MPS 216.50

46.34 21.5 996.31 -996.31

133.59 0 LPS 996.31

24.59 21.5 528.69 -330.19

114 109 -198.5

19 -1.5 -28.5 0

95 90 -170

40 -3 -120 0

55 50 -50

10 -1 -10 0

45 40 -40

10 -4 -40 0

35 30 0

15 0 0 0

15 0 CW 330.19


Fig. 3. TSP and SCC for Case study 1 [15].


Composite Curve for Case study 1, as proposed by Varbanov et al.[15]. The net heat sink and the net heat source from Fig. 3 are thesame as in the TS-PTA (columns 3 and 4, Table 6). The net heat sinkis subtracted from the net heat source to obtain the net heatrequirement (column 5, Table 6). The locations with negativeamounts of net heat indicate heat deficits, whereas the locationswith positive values indicate heat surpluses. The Second Law ofThermodynamics specifies that heat can only be transferred froma higher temperature to a lower temperature. Therefore, the heatsurplus at higher temperature utilities can be transferred to utilitieswith lower temperatures that have heat deficits. For example, the217 kWof MPS in Case study 1 can be transferred to LPS, which hasheat deficit of 654 kW, instead of disposing of this excess heat withan external cooling utility. As a result, the net heat requirement iscascaded from top to bottom, starting with an initial value of zero.The most negative value in the initial heat cascade (column 6,Table 6) is then used to determine the amount of external heating

Table 7Total site Utility Distribution (TSUD) table for Case study 1

utility needed for the system by making it positive and cascadingcolumn 5 again (see column 7, Table 6). This gives a value of1137.19 kW of external heating needed. The value at the bottom ofthe cascade represents the total cooling utility needed by thesystem, which is 730.19 kW. The location at which the valuebecomes zero is the Total Site Pinch Point, which is between the LPSand CW temperatures.

Similarly to Step 2, the utilities in Table 6 can be separated intotwo parts, i.e., the regions above and below the Total Site Pinchregion. Multiple utility cascades above the Total Site Pinch point usethe same method as in Step 2(a) (see column 8 and 9, Table 6). Thenet heat requirement (column 5, Table 6) is cascaded (column 8 and9) from the top to the pinch point by assuming that there is no heatsupplied at a temperature above the HPS. The same amount ofexternal heating utility is added when there is a negative value inthe cascade, e.g., 700 kWof HPS and 437.19 kWof LPS are needed inCase study 1 as heating utilities. Step 2(b) is similar for the regionbelow the Total Site Pinch, as shown below the pinch in columns 8and 9 of Table 6. Multiple utilities are cascaded (columns 8 and 9)from the bottom to the pinch point, and cooling utility is addedwhen there is a positive value in the cascade until it reaches zero.Note that cooling utilities below the Total Site Pinch are repre-sented by negative numbers. For Case study 1, 730.19 kW ofexternal cooling water (CW) is required to dispose of the excessheat.

The effect of multiple utility cascading above the Total Site Pinchin Table 6 is clearly evident in Fig. 3. The heat sources at MP and LPtemperatures are provided to the heat sink at LP. A heatingrequirement is necessary for LP instead of MP, which is lesseconomical. Fig. 3 clearly shows that the heat requirement of437.19 kW also can be fulfilled by using Hot Water (HW) at a rangebetween 50 and 60 �C.

3.4. Step 4: construct a Total Site Utility Distribution (TSUD) tableto visualise the utility flow in the sites

The SCC does not adequately display the utility distributionwhen there are several processes involved on the integrated site.The amounts of utility distribution for each site from on-site utilitysystems can be visualised using the TSUD table (Table 7). All the

.

Table 8Stream data for Plant A [8] with DTmin,pp ¼ 20 �C.

Stream Ts (�C) Tt (�C) DH (kW) mCp (kW/�C) Ts’ (�C) Tt’ (�C)

A1 Hot 170 80 5000 55.5556 160 70A2 Hot 150 55 6477 68.1818 140 45A3 Cold 25 100 1500 20.0000 35 110A4 Cold 70 100 1050 35.0000 80 110A5 Cold 30 65 5250 150.0000 40 75

Table 9Stream data for Plant B [8] with DTmin,pp ¼ 10 �C.


B1 Hot 200 80 10,000 83.3333 195 75B2 Cold 20 100 4000 50.0000 25 105B3 Cold 100 120 10,000 500.0000 105 125B4 Hot 150 40 8443 76.7575 145 35B5 Cold 60 110 1000 20.0000 65 115B6 Cold 75 150 7000 93.3333 80 155

Table 10Stream data for Plant C [8] with DTmin,pp ¼ 20 �C.


C1 Hot 85 40 23.85 0.5300 75 30C2 Hot 80 40 96.40 2.4100 70 30C3 Cold 25 55 17.70 0.5900 35 65C4 Cold 55 85 77.40 2.5800 65 95C5 Cold 33 60 6.48 0.2400 43 70C6 Cold 25 60 77.00 2.2000 35 70C7 Cold 30 121 12.74 0.1400 40 131C8 Cold 25 28 151.68 50.5600 35 38C9 Cold 30 100 59.50 0.8500 40 110C10 Cold 18 25 100.80 14.4000 28 35C11 Cold 21 121 5.00 0.0500 31 131

Table 11Stream data for Plant D [8] with DTmin,pp ¼ 10 �C.


D1 Cold 15 60 6000 133.3333 20 65D2 Cold 15 80 5000 76.9232 20 85

Table 12Site utility temperatures.

Utility Temperature (�C)

High-pressure steam (HPS) 170Steam (ST) 125Hot water (HW) 50e60Cooling water (CW) 20

Table 13Total Site Problem Table algorithm (TS-PTA) during normal operation.

Utility Utility Temp.(�C) Net heatsource (kW)

Net heatsink (kW)

Net heatrequirement (

HP 170 3.04 �3.04

ST 125 2967.17 7769.31 �4802.14

HW 50e60 1495.71 7606.92 �6111.22

CW 10 35.15 35.15


heat sources and heat sinks in the various plants are listed sepa-rately according to utility type, as shown in columns 3 and 4. Theexternal utilities calculated from Step 4 are also listed in Table 7.Arrows within the table show that heat sources can be transferredto heat sinks for the same type of utility. If there are extra heatsources, heat can be transferred to the lower utility levels.

4. Application of the TS-PTA to TS sensitivity with changesand variations

As mentioned previously, the TS-PTA can be beneficial for ana-lysing the sensitivity of the TSHI to plant shutdowns due to main-tenance or upsets and to design mitigation strategies. This isillustrated using Case study 2 from Perry et al. [8]. Here, there arefour sites considered in Locally Integrated Energy Sectors (LIES):two industrial process plants, a hospital complex and a combinedresidential and office complex. The stream data for the four plantsare listed in Table 8e11 . Plants A and C are assumed to have thesame DTmin,pp of 20 �C, whereas Plants B and D both have a DTmin,ppof 10C. Table 12 shows the types of utilities serving the area, witha DTmin,up of 10 �C.

Steps 1 to 4 were performed for the processes in Case study 2.The final TS-PTA values for the standard operation of the plantscomprising the TS are listed in Table 13. Due to its numerical nature,it is very convenient tomanipulate data in the TS-PTA to obtain newvalues for various cases. For example, to consider a plant shutdown,we omit the contributions from the shutdown plant from the heatsinks and sources in columns 3 and 4 of Table 13. The new externalutility requirements are then obtained. Table 14 summarises theexternal utility variations when one of the plants is shutdown. Werefer to Table 14 as the proposed Total Site Sensitivity Table (TSST),which can be used to gain many insights into utility system design.The variance in Table 14 is calculated by subtracting the amounts ofexternal utilities during plant shutdowns from the values neededduring normal operation. A positive variance above the Total SitePinch indicates that the central utility has a heat surplus that is notused in any sinks. The utility systems have the following options:

(i) Fewer utilities can be generated, if permitted by the turn downratio.

(ii) The heat surplus can be disposed of using an external coolingutility, which would incur a penalty cost.

(iii) The heat surplus can be sold to other plants.(iv) For HP or MP steam, if a plant has a combined heat and power

system (CHP) with a double-stage extraction turbine, the heatsurplus can be used to generate extra electricity for the plant.

(v) The heat surplus can be cascaded downwards to locationswithnegative variances provided they are still located in the sameTS-PTA pinch region.

A positive variance below the Total Site Pinch represents surpluscooling utility produced by the utility plant, and it can be cascaded

kW)Initial heatcascade

Final heatcascade

Multiple utilitycascade

Amount ofutility needed

0 11,937.90 03.04

�3.04 11,934.86 04802.14

�4805.18 6484.70 06111.22

�10,916.40 0 0L227.27

�10,881.25 35.15 0

Table 14Total Site Sensitivity Table (TSST).

Utility Total Site external utility requirement, kW

Normaloperation

Plant Ashutdown

Variance fromnormal operation

Plant Bshutdown


Plant Cshutdown


Plant Dshutdown


HP 3.04 3.04 0 3.04 0 0 3.04 3.04 0

ST 4802.14 7769.31 �2967.17 810.77 3991.37 4665.19 136.95 1161.15 3640.99Pinch

HW 6111.22 6821.32 �710.08 6896.82 �758.60 5863.17 248.06 1247.65 4863.57Pinch Pinch Pinch Pinch

CW 35.15 35.15 0 0 35.15 35.15 0 35.15 0

STEP 1: Perform Problem Table Algorithm (PTA) for all individual process

STEP 2: Construct multiple utility cascade for each individual process

Above pinch temp. (heat sink) Below pinch temp. (heat source)

Cascade the heat available from the highest temperature towards pinch temperature, external utility

added when there is negative value in the cascade

Shift all the temperatures by deduct ΔTmin,pp/2 and add with ΔTmin,up

Shift all the temperatures by add ΔTmin,pp/2 and deduct with ΔTmin,up

Sum the external heat enthalpy below the utility temperature until before the next utility temperature

Cascade the heat available from the lowest temperature towards pinch temperature, external utility

added when there is positive value in the cascade

Sum the external heat enthalpy above the utility temperature until before the next utility temperature

STEP 3: Perform Total Site Problem Table Algorithm (TS-PTA)

Formulate ‘Net heat sink’ and ‘Net heat source’ by adding heat sink from above pinch region at each processes and heat source from below pinch region according to utility level

Calculate ‘Net heat requirement’ by deducting net heat source with net heat sink

Cascade the net heat requirement from top to bottom by assuming no hot utility provided

Cascade the net heat requirement from top to bottom by taking the most negative value in the previous cascade as hot utility provided

Above Total Site Pinch Below Total Site Pinch

Cascade the heat available from the highest temperature towards pinch temperature, external utility

added when there is negative value in the cascade

Cascade the heat available from the lowest temperature towards pinch temperature, external utility

added when there is positive value in the cascade

STEP 4: Construct Total Site Utility Distribution

(TSUD) Table

Record all the heat sinks and sources of different processes according to types of utility. Record also site

utility requirement as calculated in STEP 4

Represent the heat flows from one process to another or from utility to a process

Application: Construct Total Site Sensitivity Table

(TSST)

Om

it the heat sink and source from

process shutdown.

Record all the utility requirement calculated in STEP 4

Calculate the variance of normal operation with situation when one of the plant shutdown

Fig. 4. Summary of the proposed methodology.



upwards to serve as an extra heat source at higher temperature. Anegative variance indicates the central utility has a heat deficit, andmore external utility must be generated. Based on this deficit, thedesigners can determine the maximum size of utility system thatmust be built.

Based on Case study 2, the following conclusions can be drawn:

(i) HP - If Plant C is shutdown, there will be excess HP. Because allthe above pinch variance for utilities below HP (ST and HW)are also positive, this heat cannot be cascaded downwards. TheHP must be diverted for electricity generation if a CHP systemis available, disposed of using cooling utilities or sold to otherplants. For other plant shutdowns, there is no effect on HPconsumption.

(ii) ST - For ST, more ST is needed if Plant A is shutdown, andexcess ST is generated if Plants B, C and D are shutdown. Theboiler generating ST should have a maximum design capacitythat can reach 7770 kW and the boiler could be turn down to810 kW because if Plant B is shutdown, part of the surplus STavailable can be cascaded downwards to satisfy the HWdemand, a negative variance).

(iii) HW- ForHW,moreHWis needed if Plant A andB are shutdownand excess HW is generated if Plant C and D are shutdown.Hence, the boiler/heater generating HW should haveamaximumdesign capacity that can reach 6897 kWand a turndown of notmore than 4863 kW. If the turn down ismore than4863 kW, extra cooling utilities will be needed or the extra HWcan be sold to other plants. The HWutility requirement if PlantB is shutdown can be obtained from the surplus ST available.

(iv) CW - If Plant B is shutdown, there will be 35 kW of extracooling water capacity available. This extra cooling water canbe used to remove the surplus heat from ST, or the coolingtower can be shutdown.

5. Methodology summary

Fig. 4 presents a summary of the overall procedure for the fouruseful tools proposed in this study: the Problem Table Algorithmwith multiple utility targets, the Total Site Problem Table Algorithm(TS-PTA), the Total Site Utility Distribution (TSUD) table, and theTotal Site Sensitivity Table (TSST).

6. Conclusions

In the following, we present a summary of the contributions ofthis work:

1) A new method was developed for calculating multiple utilitylevels in the PTA that is simpler than that presented by Costaand Queiroz [17]. This work introduced the use of multipleutility cascades to determine multiple utility levels for indi-vidual PTAs and TS-PTAs. This tool enables the multiple utilitytargeting for individual processes to be done effectively usingthe numerical approach which produces more accurate results.

2) The TS-PTA was introduced for TSHI. We further demonstratedthat the TS-PTA yields more accurate results for TSHI analysiswhen compared with a graphical approach, which is prone toinaccuracies. The tool saves time and effort in determiningamounts of heat interchange among plants compared withgraphically constructed CCs, GCCs, TSPs and SCCs. This toolcould be explored further for the variable supply and demandTotal Site problem as proposed by Varbanov and Kleme�s [9].Also, TS-PTA could be used for continuous and batch processesthat may not be conveniently solved using graphical tools.

3) The Total Site Utility Distribution (TSUD) table can be beneficialfor the design of a Total Site utility distribution network. Thistool can be used to visualise and design the heat transfernetwork in the system, between utility streams and processstreams.

4) The Total Site Sensitivity Table (TSST) is introduced to analyseTotal Site sensitivity. A typical example is, TSST can be use foranalysing the variation in a plant’s utility requirements whenone of the integrated site plants is shutdown for reasons suchas scheduled maintenance (e.g., for repairing faulty parts orclearing unwantedmaterial in the reactor), periodic shutdowns(e.g., summer district heating shutdowns in the northernhemisphere), operability problems or unpredicted accidents.TSST results can also be used for utility design and productionplanning.

The present research can be extended for the optimisation ofcogeneration potential. A prior study on assisted heat transfer [11]can also be integrated into the TS-PTA. These developments shouldbe especially useful in increasing the applicability of the TS-PTA.Heat storage in Total Site system also could be explored throughthe mathematical tool proposed.

Acknowledgements

The authors would like to thank the Universiti TeknologiMalaysia for providing financial support through the UTM Inter-national Education Experience Fund and the financial support fromthe Hungarian project TÁMOP-4.2.2/B-10/1-2010-0025 and to theUniversity of Pannonia in Hungary for supporting the collaboration.

Nomenclature

Ts Initial Supply Temperature (�C)Tt Final Target Temperature (�C)T0 Shifted Temperature (�C)T00 Double-shifted Temperature (�C)CC Composite CurveGCC Grand Composite CurveCW Cooling WaterHP High-Pressure SteamHW Hot WaterLIES Locally Integrated Energy SectorLPS Low-Pressure SteammCp Heat Capacity Flowrate (kW/�C)PTA Problem Table AlgorithmQcmin Minimum Cooling Requirement (kW)Qhmin Minimum Heating Requirement (kW)SCC Site Composite CurveSGCC Site Grand Composite CurveSSSP Site SinkeSource ProfileTS Total SiteTSP Total Site ProfileTSHI Total Site Heat IntegrationTSST Total Site Sensitivity TableTSUD Total Site Utility DistributionTS-PTA Total Site Problem Table AlgorithmUTA Unified Targeting AlgorithmDH Stream Heat Load (kW)DTmin,pp Minimum Temperature Difference Between Process

Stream (�C)DTmin,up Minimum Temperature Difference Between Utility And

Process Streams (�C)


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Documents

A Numerical Technique for Total Site Sensitivity Analysis