International Journal of Pure and Applied Sciences and Technology

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1 Int. J. Pure Appl. Sci. Technol., 12(2) (212), pp International Journal of Pure and Applied Sciences and Technology ISSN Available online at Research Paper Using Simulation Modelling to Improve the Performance of Chemical Engineering Processes: Part 2 Retrofitting Heat Exchanger Network Kenneth Kekpugile Dagde 1,* and Beabu Kiasira Piagbo 2 1 Department of Chemical/Petrochemical Engineering, Rivers State University of Science and Technology, Port Harcourt, PMB 58, Port Harcourt, Rivers State, Nigeria 2 School of Science and Engineering, Teesside University Middlesbrough, TS1, 3BA, United Kingdom * Corresponding author, (Kenneth_dagde@yahoo.com) (Received: ; Accepted: ) Abstract: Business concerns continue to devise means for minimal energy utilization due to rising energy cost, environment concerns, and the need to maximise profit. For systems designed during the era of cheap energy, retrofitting becomes a viable option. This research applies heat integration techniques to monitored industrial plant data of a functional refinery to retrofit the preheat section of the Crude Distillation Unit (CDU), using Aspen Energy Analyser. The retrofit operations includes modifying utility heat exchangers; resequencing heat exchangers; repiping heat exchangers; addition of new heat exchangers; addition of new area; and eliminating cross pinch heat exchangers. The retrofit design eliminating cross exchangers showed about 14% reduction in total cost, with about 55.37% and 91.22% reduction in the cooling cost and cooling duty respectively. There were also about 18.97% reduction in the number of shells and 47.1% reduction in the number of exchanger units translating into about 16.57% and 2.74% reduction in operating cost and capital cost respectively, thus highlighting the benefits of retrofitting to a poorly designed network. Keywords: Retrofitting, Heat Exchanger Networks, Cross pinch heat exchangers.

2 Int. J. Pure Appl. Sci. Technol., 12(2) (212), Introduction Globalisation and its attendant consequences have led to increase competition among business concerns. Thus to remain viable businesses have to produce goods and services at affordable prices to consumers within minimal production cost. Refinery and petrochemical operations are both capital and energy intensive operations. One important way of saving production cost in the oil and gas industry is through process integration. Process integration ensures continuous improvement of the energy consumption and operation of the plant. It is widely applied to design and synthesis of heat exchanger networks For the grassroots design case with the advanced process integration knowledge available today, cost effective and energy efficient heat exchanger network can be easily designed. Regrettably, it is not that easy with the retrofit design case, where the topology of the existing network, the design optimisation objective function, and other factors imposes serious constraints on the possible retrofit design [1, 2]. A review of the literature shows that the heat exchanger retrofitting problem could be solved using either of or a combination of the following techniques: Pinch analysis technique [2, 3, 4, 5, 6]; Mathematical Programming Technique [7, 8, 9, 1, 11]; Combination of Pinch analysis & Mathematical programming technique (1, 12, ]; Simulated Annealing and Genetic Algorithm technique [13, 14, 15, 16, 17, 18, 19]; and Path analysis technique [2]. The reader may consult the cited authors for a full discussion of these techniques. Also, for a comprehensive review on the research in process integration and heat exchanger network synthesis, the reader may see the following review papers [21, 22, 23, 24, 25, and 26]. Following the analysis reported in part 1[27], retrofitting of the Heat Exchanger Network was recommended due to inefficient design evidenced by gross pinch rule violation. This paper explores the energy and cost saving opportunities available to the refinery by developing retrofit designs with the aid of Aspen Energy Analyser software. 2. Materials and Method The research reported in this paper uses monitored industrial plant data from the Crude Distillation Unit (CDU) of a functional refinery in Port Harcourt, Nigeria. The process map of the process, data collection and extraction, and analysis of the heat exchanger network is reported in the part 1 of the paper [27]. The automatic heat exchanger retrofit function of Aspen Energy Analyser was used for the retrofitting. To ensure that the software performs the retrofit efficiently, the scope of the problem was reduced by minimising stream segmentation, and reducing the number of heat exchangers in the network. This simplifies the network and increases the efficiency of the model. The simplified network design for retrofit purpose is shown in Figure 1. The objective of a retrofit design is to modify existing HENs to enable it meet up with process operating condition and provide a cost effective network design. This is achieved subject to any design and operating constraints

3 Int. J. Pure Appl. Sci. Technol., 12(2) (212), Figure 1: Retrofit Design Grid Diagram The retrofit operation include modifying utility heat exchangers; resequencing heat exchangers; repiping heat exchangers; addition of new heat exchangers; and addition of new area to the heat exchanger network. It also included a manual retrofit design eliminating all cross pinch exchangers in the original design. Details on how the retrofit operations were performed are document in [28]. 3. Results and Discussion 3.1 Modifying Utility Exchangers The first automated retrofit operation performed was modifying the utility heat exchangers. The network design generated by modifying the utility heat exchangers is shown in Figure 2. This was accomplished as shown in Figure 2 by moving the utility end of the cooler on the kerosene pump around stream from the air utility stream to the cooling water stream. Table 6 summarise the effect of this modification in comparison with the original case study design. Improvements are seen in the reduction of the cooling cost index, the number of shells, and area. The modification of area will cost $296, while there is an operational saving cost of 4.129*1-2.

4 Int. J. Pure Appl. Sci. Technol., 12(2) (212), Figure 2 Retrofit Designs 1 Grid Diagram Modifying Utility Heat Exchangers Table 1 Comparison of Retrofit Designs 1 with Base Case Modifying Utility Heat Exchangers Parameter Retrofit Design 1 Base Case Deviation % Deviation Heating Cost Index Heating Value (KJ/h) Cooling Cost Index Cooling Value (KJ/h) Number of Shells Total Area (m 2 ) * * * * * * * * New Area Cost Index: $296 Operations Saving: 4.129*1-2 cost/sec 3.2 Re-Sequencing Heat Exchangers Aspen Energy Analyser automatically re-sequences the heat exchangers by moving one end of the heat exchangers. The automatic network design generated by re-sequencing the heat exchangers is shown in Figure 3.

5 Int. J. Pure Appl. Sci. Technol., 12(2) (212), Figure 3 Retrofit Design 2 Grid Diagram Re-sequencing Heat Exchangers Table 2 Comparison of Retrofit Designs 2 with Base Case Resequencing Heat Exchangers Parameter Retrofit Design 2 Base Case Deviation % Deviation Heating Cost Index Heating Value (KJ/h) Cooling Cost Index Cooling Value (KJ/h) Number of Shells Total Area (m 2 ) * * * *1 4 New Area Cost Index: 3.763*1 4 cost * * * * Operations Saving:. cost/sec The resequencing shifts the hot end of the HVGO/crude exchanger from the Heavy Gas Oil (HVGO) stream to the atmospheric residue stream. Thus the HVGO stream is eliminated from crude preheating. Table 2 summarises the effect of this retrofit design. From the Table, improvement is only seen in the reduction in area and number of shells. There is no saving in operating cost. Considering the $3.763*1 4 cost to effect the modification in area, this design is not economically viable. 3.3 Re-Piping Heat Exchangers The next retrofit operation involves re-piping the existing heat exchanger network. This is done by moving both ends of the heat exchangers. The retrofit design generated by re-piping the heat exchanger network is shown in Figure 4. The design moves one of the heat exchangers on the crude from storage stream to the pre-flashed crude stream. The moved heat exchanger utilises the additional heat still available in the atmospheric residue stream to further heat the crude before it enters the crude charged heater. The moved heat exchanger was originally heated by the stripped LDO stream. This modification increases the cooling cost index as more cost is required to cool the stripped LDO stream

6 Int. J. Pure Appl. Sci. Technol., 12(2) (212), before it is stored. Table 3 summarise the effect o this retrofit design. There is little modification in the number of shells and the area of the heat exchanger network. Considering the minimal savings in operating cost and the cost of effecting the modification in area, this too does not make much economic sense. Figure 4 Retrofit Designs 3 Grid Diagram Repiping Heat Exchanger Table 3 Comparison of Retrofit Designs 3 with Base Case Repiping Heat Exchangers Parameter Retrofit Design 3 Base Case Deviation % Deviation Heating Cost Index Heating Value (KJ/h) Cooling Cost Index Cooling Value (KJ/h) Number of Shells Total Area (m 2 ) * * * *1 4 New Area Cost Index: 1.135*1 5 cost * * * * Operations Saving: -1.27*1-2 cost/sec

7 Int. J. Pure Appl. Sci. Technol., 12(2) (212), Adding Heat Exchangers Adding a new heat exchanger into the existing network is another automated retrofit operation performed by the software. Figure 5 shows the generated retrofit design. A new heat exchanger is added as shown. Figure 5 Retrofit Designs 4 Grid Diagram - Adding a New Heat Exchanger This utilises the available heat of the atmospheric stream to further heat the crude before the crude is heated in the furnace. Table 4 summarises the effect of this design on the original design. There is virtually no effect except a marginal operating cost saving. Considering the cost of the additional heat exchanger and the small operating cost savings, this design too is not economically viable. Adding New Heat Exchanger Table 4 Comparison of Retrofit Designs 4 with Base Case Parameter Retrofit Design 4 Base Case Deviation % Deviation Heating Cost Index Heating Value (KJ/h) Cooling Cost Index Cooling Value (KJ/h) Number of Shells Total Area (m 2 ) * * * *1 4 New Area Cost Index: 4713 cost * * * *1 4 Operations Saving: 3.563*1-8 cost/sec

8 Int. J. Pure Appl. Sci. Technol., 12(2) (212), Adding New Area The next automatic retrofit operation was adding new area to the existing heat exchanger network. The retrofitted design generated is shown in Figure 6. Table 5 summarises the effect of this design. The.6% reduction in area produces very small operational cost saving at a high modification cost. Adding New Area Table 5 Comparison of Retrofit Designs 5 with Base Case Parameter Retrofit Design 5 Base Case Deviation % Deviation Heating Cost Index Heating Value (KJ/h) Cooling Cost Index Cooling Value (KJ/h) Number of Shells Total Area (m 2 ) * * * *1 4 New Area Cost Index: 4859 cost * * * * Operations Saving: 2.776*1-17 cost/sec Figure 6 Retrofit Designs 5 Grid Diagram Adding New Area 3.6 Eliminating Cross Pinch Exchangers The retrofit design with no cross pinch violation is shown in Figure 7. The same number of heat exchangers is used to accomplish the heating of the crude but some of the process stream

9 Int. J. Pure Appl. Sci. Technol., 12(2) (212), temperatures are altered to avoid transferring heat across the pinch. By altering the temperature of the process streams, the cross pinches were eliminated. The modifications made to the process streams to achieve this are shown in [27]. Table 6 compares the network cost indices and the network performance of the retrofit design eliminating all cross pinch exchangers and the original case study design. From the table, the value of the retrofit design eliminating all cross pinch exchangers is clearly seen. Despite the 12.3% increase in heating cost and heating value respectively, the retrofit design operates at about 14% reduced total cost compared with the case study design. There is significant reduction in the cooling cost and cooling duty by 55.37% and 91.22% respectively. The 18.97% reduction in the number of shells and 47.1% reduction in the number of exchanger units translate into a 16.57% and 2.74% reduction in operating cost and capital cost respectively. The 4.4% increase in total area of the retrofit design over the original design is understandable because pinch principle violation and misapplication of the driving force principle leads to reduced area in the network design [1]. Figure 7 Retrofit Designs 6 Grid Diagram Eliminating Cross Pinch Exchangers Table 6 Comparison of Retrofit Designs 6 with Base Case Eliminating Cross Pinch Exchangers Network Cost Indexes Parameter Retrofit Design Heating (Cost/sec).1313 Cooling (Cost/sec) 3.887*1 2 Operating (Cost/sec).172 Capital (Cost) 4.554*1 6 Total Cost (Cost/sec).2166 Network Performance Parameter Heating Value (KJ/h) Base Case * * Deviation % Deviation Retrofit Design Base Case Deviation % Deviation 1.491* * *

10 Int. J. Pure Appl. Sci. Technol., 12(2) (212), Cooling Value (KJ/h) Number of Units Number of Shells Total Area (m 2 ) 2.413* * * * * Comparison of the Retrofit Designs Table 8 compares the retrofit designs with the original design in terms of percentage deviation Design Original Design Modifying Utility Exchangers Re-sequencing Heat Exchangers Re-piping Heat Exchangers Adding New Heat Exchanger Adding New Area Eliminating Cross Pinch Exchangers Area (m 2 ) 1.735* % -9.28% -.63% % -.6% 4.4% Heating Value (KJ/h) 1.328*1 8 % % % % % 12.37% Cooling Value (KJ/h) 2.748*1 8 % % % % % -91% Operating Cost (Cost/s) % -.5%.5% % % % No. of Shells 5-4% -8% -2% % % -6% As seen from the table, the design modifying utility heat exchanger reduces the operating cost by.21%, the number of shell by 4%, and the total heat exchanger area also reduces by 4.9%. The design which re-sequences the heat exchanger network results in an operational cost saving of.5%. The number of shells also reduces by 8%, while 9.28% of the area was made available for heat exchange. Repiping the heat exchangers network results in a.5% increase in the operating cost. There is also a 2% reduction in the number of shells used, and.63% of the area was made available for heat transfer. Adding new heat exchanger to the network did not produce any significant effect. This is understandable because too many heat exchangers are already used in the network based on the target report. Adding new area to the heat exchanger network results in a.6% reduction in the area, with no operational cost saving. The manually generated retrofit design eliminating cross pinch exchangers provides huge energy and cost savings as can be seen in the reduction in cooling value, operating cost, capital cost and total cost. There is a 16.57% reduction in the operating cost. The design is promising, however, the cost implication involved in the modification of process temperatures and areas of the heat exchanger network have to considered in implementing this design. In summary, modifying utility heat exchangers and re-sequencing heat exchangers was able to produce some cost savings in the existing network. Re-piping the heat exchangers, adding additional heat exchanger and new area to the network provided modifications that were not economically viable. Also these modifications did not eliminate the cross pinch violations present in the network. The retrofit design eliminating pinch violation was the most promising of the retrofit designs. Thus supporting the fact that for a HEN with gross cross pinch violation, eliminating the cross pinches provides a viable retrofit design.

11 Int. J. Pure Appl. Sci. Technol., 12(2) (212), Conclusion The original case study design had serious pinch violations and transfer of energy above the minimum temperature approach. The automatic retrofit designs were not able to resolve the cross pinches and area underutilisation. The retrofit design eliminating all cross pinch exchangers provides a more economical viable retrofit design. This agrees with the fact that for HEN with gross cross pinch violation or misapplication of the driving force principle, providing a retrofit design that eliminates the cross pinch and proper application of the minimum driving force provides viable retrofitting option [1, 3, 4]. References [1] N.D.K. Asante and X.X. Zhu, An automated and interactive approach for heat exchanger network retrofit, Chemical Engineering Research and Design, 75(1997), [2] T.N. Tjoe and B. Linnhoff, Using pinch technology for process retrofit, Chemical Engineering, 93(1986), [3] T.N. Tjoe, Retrofit of heat exchanger networks, Ph.D. Thesis, UMIST England, (1986). [4] B. Linhoff and E. Hindmarch, The pinch design method for heat exchanger networks, Chemical Engineering Science, 38(5) (1983), [5] K.K. Trivedi, B.K.R. O'Neill and R.M. Roach, A new dual-temperature design method for the synthesis of heat exchanger networks, Computers & Chemical Engineering, 13(1989), [6] A. Carlsson, P.A. Franck and T. Berntsson, Design better heat exchanger network retrofits, Chemical Engineering Progress, March (1993), [7] A.R. Ciric and C.A. Floudas, A retrofit approach for heat exchanger networks, Computers & Chemical Engineering, 13(1989), [8] A.R. Ciric and C.A. Floudas, A comprehensive optimization model of the heat exchanger network retrofit problem, Heat Recovery Systems and CHPI, 1(199), [9] T.F. Yee and I.E. Grossmann, A screening and optimization approach for the retrofit of heatexchanger networks, Industrial & Engineering Chemistry Research, 3(1991), [1] X.X. Zhu, Automated design method for heat exchanger network using block decomposition and Heuristic rules, Computers and Chemical Engineering, 21(1) (1997), [11] K. Ma, C. Hui and T.F. Yee, Constant approach temperature model for HEN retrofit, Applied Thermal Engineering, 2(2), [12] V. Briones and A.A. Kokossis, New approach for the optimal retrofit of heat exchanger networks, Computers & Chemical Engineering, 2(1996), S43-S48. [13] G. Athier, P. Floquet, L. Pibouleau and S. Domenech, Synthesis of heat-exchanger network by simulated annealing and NLP procedures, AICHE Journal, 43(1997), [14] D. Bertsimas and J. Tsitsiklis, Simulated annealing, Statistical Science, 8(1993), [15] M. Ravagnani, A.P. Silva, P.A. Arroyo and A.A. Constantino, Heat exchanger network synthesis and optimisation using genetic algorithm, Applied Thermal Engineering, 25(25), [16] J. Jeżowski, R. Bochenek and G. Poplewski, On application of stochastic optimization techniques to designing heat exchanger- and water networks, Chemical Engineering and Processing: Process Intensification, 46(27), [17] B. Allen, M. Savard-Goguen and L. Gosselin, Optimizing heat exchanger networks with genetic algorithms for designing each heat exchanger including condensers, Applied Thermal Engineering, 29(29), [18] E. Rezaei and S. Shafiei, Heat exchanger networks retrofit by coupling genetic algorithm with NLP and ILP methods, Computers & Chemical Engineering, 33(29), [19] H. Soltani and S. Shafiei, Heat exchanger networks retrofit with considering pressure drop by coupling genetic algorithm with LP (linear programming) and ILP (integer linear programming) methods, Energy, 36(211),

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