ISSN 1000 7924 The Journal of the Association of Professional Engineers of Trinidad and Tobago Vol.40, No.2, October/November 2011, pp.43-48
Use of Pinch Analysis in Sizing and Integrating a Heat Exchanger into
an Existing Exchanger Network at a Gas Processing Plant
Kamel SinghaΨ and Raymond CrosbiebThe University of Trinidad and Tobago, Point Lisas Campus, Esperanza Road, Brechin Castle, Trinidad and Tobago, West Indies
aE-mail: [email protected] bE-mail: [email protected]
Ψ
Corresponding Author
(Received 29 April 2011; Revised 25 October 2011; Accepted 01 November 2011)
Abstract: This paper presents a case study of a gas processing plant which has been experiencing a problem of water carry-over in the sub-cooled inlet natural gas to its downstream process in Trinidad and Tobago. To solve the problem, a hot oil utility was proposed to exchange heat with the inlet natural gas and keep the water in its vapor state. The heat exchange was integrated into an existing heat exchanger network (HEN), and the integration was analysed using pinch analysis. The method allowed trade-offs in energy amongst the hot oil utility stream and four cold process streams of which the inlet natural gas stream was one. An Aspen Hysis simulation for the HEN using live plant data was exported to Aspen Energy Analyser in which the pinch analysis was performed. The size of hot utility, pinch temperature, and minimum temperature difference (∆Tmin)for
the HEN were, 347.78 kW, 215.85°C and 15.5°C respectively. A shell and tube heat exchanger having an overall heat transfer coefficient (U) of 71.3 kJ/h-m2-°C, area (A) of 60.3 m2, logarithmic mean temperature difference (LMTD) of 201°C and heat
duty of 239.9 kW was proposed for the heat exchange between the hot utility and the inlet gas. Hand calculations were performed to verify simulation results. A net capital outlay of USD0.3 million was expected to cover the plant retrofit. The savings was expected to be some USD1.74 million over a ten-year period.
Keywords: Heat exchanger network, sub-cooled, pinch analysis, Aspen Hysis, Aspen Energy Analyser, simulation 1. Introduction
A natural gas processing plant experienced a problem of water carry-over in the sub-cooled inlet natural gas to its downstream processes; free water deposited in the molecular sieves at the front end of the plant as a liquid hydrocarbon/water mixture reduced the lifespan of the sieves from four to two years. The cost of a molecular sieve change out was USD 1.2 million with an estimated production loss of USD 0.6 million. To solve the problem, a hot oil utility, was proposed to exchange heat with the inlet natural gas and keep the water in its vapor state. Pinch point analysis was used to find the optimum location for the proposed heat exchange (E-5705) within the existing heat exchanger network (HEN) served by a hot oil utility (heater H-5501). An Aspen Hysis process simulation of the hot oil heater, the HEN and new heat exchanger (E-5705), along with Aspen Energy Analyser software, was used to locate the pinch point and develop an engineering specification and rough cost estimate for E-5705.
Pinch analysis is a methodology for minimising energy consumption of chemical processes by calculating thermodynamically feasible energy targets (minimum energy consumption) and achieving them by optimising the heat recovery systems, energy supply methods, and
process operating conditions. This technology is useful when integrating heat exchanger networks in chemical plants as it reduces capital costs and decreases specific energy demands. The challenge for this work was to integrate the proposed heat exchange with the existing HEN and provide a thermodynamically acceptable design for the E-5705 heat exchanger.
The paper was divided into six sections. Section 1 is concerned with computer simulation in Aspen Hysis for the hot utility, the existing HEN, and the proposed feed gas heater E-5705. Determine the size of hot utility for the HEN including the heat duty for E-5705 (Integrate E-5705 as part of the existing HEN served by the hot oil utility heater H-5501). Section 2 is an explanation of pinch analysis. Section 3 describes the application of pinch analysis to determine the location of the minimum temperature difference in the HEN and size of hot utility required. Section 4 presents the heat exchanger calculations using results from Aspen Hysis and Aspen Energy Analyser software and compare to key hand calculations. The last two Sections 5 and 6 provide a discussion of results and a technical specification for the E-5705 feed gas heater, along with a rough cost estimate for E-5705.
2. Understanding Pinch Analysis
Pinch analysis was originally developed in the 1970s at the ETH Zurich and Leeds University by Professor Bodo Linnhoff (Linnhoff and Turner, 1981). The analysis was regarded as a powerful way to optimise thermal systems, the designer can track the energy flows more clearly and modify the process to reduce energy consumption. Pinch also enabled the design of an optimum interface between the process and the utility systems. At its onset, ICI Plc UK took note of this promising technique and realised energy savings of 30% on processes already considered to be optimised (Linnhoff and Turner, 1981). The close co-operation between researchers and industry led to rapid developments; new research findings were quickly tried out, while challenges encountered on real plants required novel analysis methods. Within a few years, further papers describing many of the key techniques were published (Linnhoff and Hindmarsh 1983) and the techniques were disseminated through a user guide (Linnhoff et al., 1982), three ESDU Data Items (1987-1990) and training courses. Applications in industry also forged ahead; Union Carbide, USA, reported even better results than ICI, mainly due to progress in the understanding of how to effect process change (Linnhoff and Vredevld, 1984) and BASF, Germany, reported completing over 150 projects and achieving site-wide energy saving of over 25% in retrofits in their main factory in Ludwigshafen (Kornor, 1988).
Pinch analysis was somewhat controversial in its early years. Its use of simple concepts rather than complex mathematical methods, and the energy savings and design improvements reported from early studies, caused some disbelief. Divided opinions resulted; Morgan (1992) reported that pinch analysis significantly improved both the process design and the design process, whereas Steinmeyer (1992) was concerned that pinch analysis might miss out on major opportunities for improvement. Nevertheless the techniques have been generally accepted, with extensive academic research and practical application in industry.
Pinch analysis gives composite curves for systems, one for all hot streams (releasing heat), hot composite curve and one for all cold streams (requiring heat), cold composite curve. The point of closest approach between the hot and cold composite curves is called the pinch temperature (pinch point or just pinch), and is where design is most constrained. Hence, by finding this point and starting design here, the energy targets can be achieved using heat exchangers to recover heat between hot and cold streams. In practice, during the pinch analysis, often cross-pinch exchanges of heat are found between a stream with its temperature above the pinch and one below the pinch. Removal of those exchanges by alternative matching makes the process reach its energy target (see Figure 1).
The design of any heat transfer equipment must
always adhere to the Second Law of Thermodynamics (SLT) that prohibits any temperature crossover between the hot and the cold stream. A minimum heat transfer driving force must always be allowed for a feasible heat transfer design. Thus the temperature of the hot and cold streams at any point in the exchanger must always have a minimum temperature difference (∆Tmin). This ∆Tmin
value represents the bottleneck in the heat recovery. Values for ∆Tmin were determined by the overall heat
transfer coefficients (U) and the geometry of the heat exchanger.
Figure 1. Pinch Analysis
Source: http://www.cheresources.com/pinchtech4.shtml For a given value of heat transfer load (Q), for smaller values of ∆Tmin, area requirements rise. For higher
values of ∆Tmin the heat recovery in the exchanger
decreases and demand for external utilities increases. Thus, the selection of ∆Tmin has implications for both
capital and energy costs.
Just as for a single heat exchanger, the choice of
∆Tmin (or approach temperature) is vital in the design of a
heat exchanger network. To begin the process an initial
∆Tmin value is chosen and pinch analysis is carried out.
Typical ∆Tmin values based on experience are available in
literature, ∆Tmin for chemical plants were in the range 10 -
20°C.
3. Sample Calculations for Pinch Analysis
The first step in performing a pinch analysis was to develop a problem data sheet, the thermal optimisation problem comprised five streams, the hot stream 1, referred to as the hot oil utility which gives up its heat to the four cold streams 2-5 (HEN), stream 5, is the sub cooled NG at entry that needed to be heated above its saturation temperature corresponding to its pressure to keep entrained water in vapor state, refer to Table 1. The end point temperatures for each stream were taken from the process flow diagram (pfd) and the enthalpy change calculated from flow rates specified on the pfd and the
Differential heat flow dq, when added to a process stream, will increase its enthalpy (dh) by cp x dt and such
physical properties of the material streams. Calculations were done in accordance with methodologies outlined by Kemp (2007) and Bejan et al (1996). The additional major
steps were as follows: Q = m x cp x T …..Eq.1
1) Thermal data extraction for process and utility
streams (see Table 1) 3.1 Temperature Interval Adjustment (see Table 2) Hot Stream Tint = T act - ∆T min /2
2) Temperature interval adjustments for hot and cold streams (∆Tmin of 15.5°C) (see Table 2 and Figure
2) Cold Stream T∆T int = Tact +∆T min /2
min = 15.5°C
3) Construct composite curves and grand composite
curve. (for industrial chemical processes ~ 12-20°C)
Stream (1) Hot Stream 215.85 = 223.6- 15.5/2 = 215.85°C The technique was dependent upon the representation
of the process streams on a temperature-enthalpy graph. This is possible once the supply and target temperatures, flows rates and physical properties for the process streams are known.
Stream (1) Hot Stream 157.35 = 165.1-15.5/2 = 157.3°C Stream (2) Cold Stream 165 = 165 + 15.5/2 = 172.75°C Stream (2) Cold Stream 219.8 = 219.8 +15.5/2 = 227.55°C Typical for Streams 3, 4, and 5 (not shown)
Table 1. Plant Data
Stream # Inlet Temp (C) Outlet Temp (C) m(f) (kg/hr) cp (kJ/kg C) Heat Capacity Rate (m(f)*Cp) (kW/C)
1(hot) 223.6 165.1 65000 2.762 49.869
2(co) 165 219.8 42890 2.495 29.725
3(co) 152 162.8 13720 3.198 12.188
4(co) 120.9 136.6 19590 3.26 17.740
5(co) 17 25 43250 2.447 29.398
Table 2. Adjusted Temperatures
Stream # Inlet Temp (C) Outlet Temp (C) Heat Capacity Rate (m(f)*Cp) (KW/C)
1(hot) 215.85 157.35 49.869
2(co) 172.75 227.55 29.725
3(co) 159.75 170.55 12.188
4(co) 128.65 144.35 17.740
5(co) 24.75 32.75 29.398
Figure 2. Temperature Intervals Profile (from Table 2)
24.75 32.75 128.6 144.35 157.35 159.75 170.5 172.75 215.85 227.5 <Str1 Str2> Str3> Str4> Str5> Note: Str1: 157.35-215.85°C; Str2: 172.75-227.5°C; Str3: 159.75-170.5°C; Str4: 128.6-144.35°C; Str5: 24.75-32.75°C
3.2 Calculation for net heat transfer (∆Q (i)) (kW) for each temperature interval
For TIN 2 = (29.725 -49.869) x (215.85- 172.75) = -20.144 x 43.1
= -868.2 kW (hot and cold stream crossing) Temperature Interval Number (TIN)
For TIN 3 = (49.869) x (172.75 - 170.55) = (Heat Capacity Rate of Cold Stream – Heat Capacity
Rate of Hot Stream x (Temperature Difference for Interval).
= -49.86 x 2.2 = -109.7 kW
For TIN 4 = (12.188 – 49.869) x (170.5 – 159.75) = -405.1 kW
Sample calculations done for TIN 1, 2, 3 and 4 starting
with the highest. Calculation for (See Table 3 for results). ∆Q(i) (KW) – cumulative net heat transfer For TIN 1 = (29.725) x (227.55- 215.85) i
= 29.725 x 11.7 ∆Q(i) = ∑ Qj
For TIN 1 = (0 + 347.7) = 347.7 kW For TIN 2 = (347.7 + -868.2) = -520.43 kW For TIN 3 = (-520.43 + -109.713) = - 630.14 kW For TIN 4 = -630.14 + -406.96 = -1037.10 kW Calculation for ∆Q*(i) (KW) = ∆Q(i) – Q hu, min. (see Tables 4 and 5 and Figure 3 for results)
For TIN 1 = (0 - 347.7) = -347.7 kW For TIN 2 = (347.7 - 347.7) = 0 kW
For TIN 3 = (-520.435 - 347.784) = - 868.21 kW For TIN 4 = -630.147 – 347.7 = -977.932 kW
Table 3. Pinch Point
Q(hu min) 347.784
T(pinch) 215.85
T(hc pinch) 223.6
T(cc pinch) 208.1
Table 4. Calculation of Utility Loads
TIN
(i) Temp (C) Adjust Q(i) (KW) ∆Q(i) (KW) ∆(KW) Q*(i)
227.55 0 -347.784 1 347.784 215.85 347.784 0.000 2 -868.219 172.75 -520.435 -868.219 3 -109.713 170.55 -630.147 -977.932 4 -406.960 159.75 -1037.108 -1384.892 5 -122.180 157.3 -1159.288 -1507.072 6 0.000 144.35 -1159.288 -1507.072 7 278.515 128.65 -880.773 -1228.557 8 0.00 32.75 -880.773 -1228.557 9 235.184 24.75 -645.589 -993.373
Table 5. Data Points for Plotting Grand Composite Curve
-∆Q(i) *(KW) 347.7 0.000 868.21 977.93 1384.89 T(C) Shift 227.5 215 172.57 170.55 159.75 -∆Q(i) *(KW) 1507.07 1507.07 1228.55 1228.5 993.3 T(C) Shift 157.3 144.3 128.6 32.75 24.75
Figure 3. Grand Composite Curve
4. E5705 Heat Exchanger Design
A shell and tube heat exchanger was chosen to provide heat integration and exchange between the hot oil utility and the sub cooled inlet natural gas (Stream 5). The prime objective in the design of the heat exchanger was to determine the surface area required for the specified duty (rate of heat transfer) using the temperature difference of 15.5°C from the pinch. The heat exchanger calculations were performed using an Excel template (see Table 6). The calculations were made in accordance with the flowchart from Figure 4.
Table 6. Hand Calculations and Software Values for U, Q and ∆Tmin Over all U (kJ/h-m2-C) Duty (Q) kW LMTD Pinch Point Aspen Hysis Calculation 71.3 239.9 200.7 Hand Calculation 70.2 235.1 200.8 15.50 Aspen Energy Analyser Calculation 15.50
Figure 4. Design Procedure for Shell-and-Tube Heat Exchangers
Source: Abstracted from Sinnott (2005, p.845)
Key parameters for specifying the exchanger are given in Table 7. Several assumptions were made for the heat exchanger calculations below:
1) Negligible heat losses to the surroundings;
2) Negligible transformation of kinetic and potential energy to thermal energy;
3) Constant properties;
4) In this instance, negligible tube wall thermal resistances and fouling factors; and
5) The overall heat transfer rate is constant along the length of the heat exchanger (fully developed flow).
4.1. Heat Exchanger Cost Estimates
Equipment costs were calculated based on the 2010 Chemical Engineering Plant Cost Index (CI) (Gitman, 2005). Table 8 provides the plant cost data. A list of the molecular sieve replacement costs is given below:
1) Material cost for molecular sieves: $ 900,000.00 2) Labour: $300,000.00
3) Production loss / day: $ 150,000.00 4.2. Net Present Value Analysis
Presumed that the number of years under consideration is 10 year, an economic analysis of the Pinch design method
was attempted. The workings of net present value (NPV) calculations are given in Table 10.
Using NPV = C* ( 1+i)-n NPV Cost Option A: = -105000*(1+.1)-2 -105000*(1+.1)-4--105000*(1+.1)-6 -105000*(1+.1)-8 -105000*(1+.1)-10 = 867.77-717.416- 592.698 -489.833 – 0. =-3,072,285 USD NPV Cost Option B: = -300000-900000*(1+.1)-4-900000*(1+.1)-8 = -300000 -641.712 - 419.867 = -1,334,579 USD
Resultant Cost Saving: = -3,072,285 - (-1,334,579) =-$ 1,737,706 USD
From the NPV calculations, Option B is more feasible as it is the smaller cost value.
Table 7. E5705 Specifications
Tube Side Data Shell side Data
Tube Pressure Drop 43.47Kpa Shell Pressure Drop 8.30Kpa
Tube length 6.00m Shell Pass 1
Tube O.D 20.00mm Shell side 1
Tube Thickness 2.0000mm Shell Parallel 1
Tube Pitch 50.000mm Baffle Type Single
Orientation Horizontal Baffle Cut (% Area) 20.00
Passes Per Shell 2 Baffle Orientation Horizontal
Tube Per Shell 160 Spacing 800.0000mm
Layout Angle Triangle (30 degrees) Diameter 739.0488mm
TEAM Type A E L Area 60.3m2
Table 8. Plant Cost Data 2010
Component Plant Cost Index
(2002) Plant Cost Index (2010) Assumed Cost (2002) (USD) Estimated Cost (2010) (USD)
E5705–Heat Exchanger 252.7 571.9 $ 125,000.00 $ 282,894.70 Piping 135 331 $ 220.00 /m $ 539.40 Valves 130 221.1 $ 280.00 $ 476.20 Fittings 148.7 221.1 $ 550.00 $ 817.70 Purchased Cost -- -- -- $ 284,727.30 Installation Cost -- -- -- $ 15,272.70 Grand Total: $ 300,000.00
Table 10. The workings of NPV calculations
Years 0 1 2 3 4 5 6 7 8 9 10 A Down time = 0 - -900000 150000 1050000 -900000 150000 1050000 -900000 150000 105000 -900000 150000 105000 -900000 150000 105000 B Down time= 300000 0 900000 900000
5. Discussion
The HEN was effectively analysed to reach its energy targets using pinch. The method was used to allow trade-offs in energy amongst 5 streams. The Aspen Hysis simulation for the HEN used live plant data and was exported to Aspen Energy Analyser which extracted the hot and cold stream data and generated the composite curves, the shift composite curves, and the grand composite curve. The size of hot utility, pinch temperature, and minimum temperature difference (∆Tmin)
for the HEN were 347.78 kW, 215.85°C and 15.5°C, respectively (see Table 3).
The pinch represented the point around which opportunities were looked at for energy savings within the HEN and the best location for the proposed heat exchanger E-5705. A shell and tube heat exchanger having an overall heat transfer coefficient (U) of 71.3 kJ/h-m2-°C, area (A) of 60.3 m2, logarithmic mean
temperature difference (LMTD) of 201°C and heat duty of 239.9 kW was proposed for the heat exchange between the hot utility and the inlet gas. Hand calculations were performed to verify key simulation results. A net capital outlay of USD 0.3 million was expected to cover the plant retrofit. The savings was expected to be USD1.74 million over a ten-year period. The introduction of this heat exchanger is expected to 1) eliminate the water carrying over problem to the dehydrators and 2) extend the life of the molecular sieves from 2 to 4 years.
6. Conclusions
In conclusion, pinch analysis was shown to be an effective analytical tool for analysing and solving a heat integration problem at a local gas plant that could lead to extending the life of its molecular sieves from two to four years. Heat exchanger networks can be effectively designed to reach the energy targets obtained by the problem analysis using the Pinch design method. The method was successfully used to allow trade-offs in energy between a number of heat medium units with little net capital outlay. The existing problem with water carryover at the inlet of the plant could be eliminated through the use of Pinch analysis, demonstrating its versatility and usefulness in exploring process improvement opportunities for heat exchanger networks.
To solve the problem, a hot oil utility, was proposed to exchange heat with the inlet natural gas and keep the water in its vapor state. Pinch point analysis was used to find the optimum location for the proposed heat exchange (E-5705) within the existing heat exchanger network (HEN) served by a hot oil utility (heater H-5501). An Aspen Hysis process simulation of the hot oil heater, the HEN and new heat exchanger (E-5705), along with Aspen Energy Analyser software, was used to locate the pinch point and develop an engineering specification and rough cost estimate for E-5705.
Future research could be conducted in using pinch
analysis to perform energy management studies to achieve financial savings by better process heat integration (maximising process-to-process heat recovery and reducing the external utility loads) and analyse mass heat exchanger networks to gain further insight into its thermal performance and provide opportunities for improvement, this work would benefit the local thermo-dynamicists in mastering key aspects of design and optimisation of thermal systems.
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Authors’ Biographical Notes:
Kamel Singh is a Senior Instructor and Program Leader for the
Bachelor of Applied Technology and Bachelor of Engineering in Applied Process and Utilities Technology Programs at The University of Trinidad and Tobago. He has eighteen years of industrial experience in the design and construction of process plant, pressure vessel, storage tank, pipeline, offshore production facilities, welding engineering, quality control and maintenance management. He was the Fabrication Manager at Carillion Caribbean Limited (CCL) and worked on several construction projects in energy, marine and commercial building sectors. His research interests include - modern energy systems, engineering curricula development and machine design.
Raymond Crosbie completed his Bachelor of Engineering (B.Eng)
Degree in Applied Process and Utilities Technology (Mechanical Option) at The University of Trinidad and Tobago, 2011 and is employed at Phoenix Park Gas Processors Limited (PPGPL) as a Mechanical Maintenance Technician. His interest is in thermal design and optimisation of process plant using pinch analysis.
