Steam Power Plant Retrofit in an Oil Refinery Plant

Transcription

1 Proceedings of the 6th International Conference on Process Systems Engineering (PSE ASIA) June 2013, Kuala Lumpur. Steam Power Plant Retrofit in an Oil Refinery Plant C. L. Chen, C. Y. Lin, H. C. Chen Department of Chemical Engineering, National Taiwan University, Taipei Taiwan Abstract A systematic methodology is presented for the analysis and design of steam systems. The main objective is to find the economic way of reducing energy consumption in a refinery, where the existing steam power plant is redesigned and retrofit is taken into account to improve site performance. The work includes modification of units and installation of new equipment. Furthermore, to prompt energy integration in industrial parks, energy exchange (e.g. steam, electricity) among adjacent companies is considered. The problem is formulated as a mixed integer nonlinear program (MINLP) based on a superstructure approach. The results of a practical case study indicate that significant energy saving can be achieved via the novel retrofit strategy. Keywords: steam system, integration, MINLP, retrofit 1. Introduction Steam systems are an important part of most processing sites, where energy consumption influences the costs enormously. Better energy management of utility system can lead to remarkable cost savings. Such cost savings can be accomplished by more effective energy regulation of the system or energy integration with neighboring sites in industrial parks. Much work has been published on the design and optimization of steam systems. Nishio et al. (1980) and Chou and Shih (1987) introduced the methodology based on thermodynamic principles to design the steam distribution network (SDN). Papoulias and Grossmann (1983) presented the superstructure-based mathematical formulation, considering the total annual cost (TAC) as the objective to find the best network design. It should be noted that studies mentioned above addressed the design of SDN assuming all units operate at full load or at fixed efficiency to satisfying demands and conditions. Thence, Aguilar et al. (2007) and Chen and Lin (2010) proposed a robust computational tool to address design and operational problems for flexible utility plants, considering structural and operational parameters as variables to be optimized simultaneously. In an Eco-Industrial Park (EIP), the basic idea is that businesses cooperate with each other in an attempt to reduce waste and pollution, efficiently share re-sources (such as materials, water, energy, and natural resources), and increase economic gains and improve environmental quality to help achieve sustainable development (Mattila et al., 2010). In this paper, the work is to develop an approach taking into account energy integration among companies. The proposed steam system model has ability to tackle the retrofit design in industry.

2 Steam Power Plant Retrofit in an Oil Refinery Plant Problem Statement The problem studied in this article focuses on an existing steam power plant in a refinery. The corresponding layout and configuration of the site is presented in Figure 1. The plant is consisted of three boilers, four levels of steam headers and four steam turbines, which produces steam and electricity to satisfying heating and power demands of process plants. In this case, steam import is available from vicinal steam plants, where a large amount of waste heat is recovered from neighbor companies and converted into steam. This paper will seek the better way to improve and enhance the site efficiency. In order to fulfill this work, the superstructure of steam system is presented in Figure 2, which consists of possible combinations of different type units. The MINLP model will be developed to find the optimal network and to determine the best configurations. The objective is to find an efficient steam system which has both the potential to improve the economic efficiency of the plant and to reduce the negative environmental impact. Figure 1 Steam distribution network for the existing plant (base case). Figure 2 Superstructure for steam distribution network.

3 892 C. L. Chen et al. 3. Model Formulation Figure 2 presents a typical SDN, which is the combination of types of units and all possible flow connections. According to the network, steam is generated in steam boilers, and collected and distributed through steam headers. When steam is expanded in steam turbines, the units produce power and steam at high, medium, or low pressures, which can satisfy requirements of inside processing plants. Moreover, to provide the chance of energy integration with company-partnering, steam and power that are imported or exported are allowable Boiler bfw f f bd f b B (1) b bi bi ii ii f H q f h f H b B (2) bfw deaer bd sat,l b b bi bi bi i ii ii Steam turbine w = w t TS (3) i, i' I ii' ii't jj tj z z i, i' I, i i', t T (4) t t ii't z z t TS (5) jj tj z w t z t TS, j J (6) t tj tj tj z f t z i, i' I, i i', t T (7) t ii't ii't ii't z w t z i, i' I, i i', t T (8) t ii't ii't ii't 3.2. Steam header = f f f f F f ld ps imp,s bi i'it i'i i i i bb i' I tt i' I i' i i' i f f F f f f i I pd vent exp,s ii't ii' i i i i i' I tt i' I i' > i i' i f h f h f h f H F H f H ld deaer ps ps imp,s imp,s bi bi i'it i'it i'i i' i i i i i bb i' I tt i' I i' i i' i = ( pd vent exp,s ) ii't ii' i i i i i i' I tt i' I i' > i i' i f f F f f f h i I (9) (10) 3.3. Power balance dem,s w w W j J (11) tts tj mj j mm

4 Steam Power Plant Retrofit in an Oil Refinery Plant 893 w w w W w imp,e dem,e mj exp,e ii't m i, i' I tte mm jj ii' 3.4. MINLP model w w cw C f C q t C f u bu tt bb uu min xω C f C f C w C w i i i i ii imp,s imp,s exp,s exp,s imp,e imp,e exp,e exp,e bd bfw ld vent c exp,s imp,s w f, f, f, f, f, f, f, f, f, f, f, f, f, bi bi b bu ii ' t ii ' i i i i i exp,e imp,e h, h, h, q, q, w, w, w, w, w, bi ii ' t i b t ii ' t mj tj x exp,e imp,e z, z, z, z, z, z, z, z bi b ii ' t m tj t b B, i I, j J, m M, t T, u U t hrs (12) (13) Table 1: Site conditions Table 2: Steam and power demands Total working hours 8,000 h/y Steam demands (101 kg/cm 2 ) 13.0 t/h Fuel oil LHV 42,800 kj/kg Steam demands (20.6 kg/cm 2 ) t/h Electric price 3.8 NT$/kWh Steam demands (4.5 kg/cm 2 ) t/h Steam price (import) 1,080 NT$/t Power demands 81.3 MW Fuel price 24,730 NT$/t Shaft power demand MW Shaft power demand MW Shaft power demand MW 4. Case Study An industrial case study is presented to illustrate how the site performance can be improved. Figure 1 shows the existing layout and current operating conditions. Tables 1 and 2 show the site conditions and steam and power demands. It is found that the existing steam plant must supply steam and power for satisfying heating and driving devices. Note that 35.0 ton/hr steam is imported from one neighboring company in early stage. The properties of the import steam is 18 bar and 215 and is lower than that of the system required (20.6 bar, 260 ). Therefore, the import steam is supplied directly to the process for heating without integrating it into the steam plant Case 1 Retrofit without energy integration of import steam In the first case, the strategy which includes retrofit of steam systems is studied but without integration of import steam into system. The optimum solution for the retrofit problem is shown in Figure 3. The result suggests that Boiler B1 increases its steam production to ton/hr in order to satisfy steam requirements. An additional turbine ST5 for generating electricity is installed, which is located between 101 and 20.6 bar steam headers. In addition, it is found that steam turbine ST1 is modified to produce 20.6 and 4.5 bar steam instead of the original 20.6 bar and condensing stream. These turbines produce steam to satisfying 20.6 bar steam requirement. Consequently, boilers B3 and B4 are suggested to turn off and the flow rate of let-down station ( bar)

5 894 C. L. Chen et al. can be reduced to 58.1 ton/hr, which results in better energy utilization. The optimum operating cost is NT $ million/year, which corresponds to a 13.8 % reduction compared to the base case. Steam turbines ST1 and ST5 provide a new route for operating so that the site performance is improved. Figure 3 Optimal SDN for the retrofit without energy integration of import steam Case 2 Retrofit with energy integration of import steam As introduced previously, an initial design considers that the flow rate of import steam is constant and the steam is supplied to the process directly for heating. In this section, steam import integrated into the steam system will be discussed. An ejector is a simplified type of vacuum pump or compressor, where high pressure operating fluid can bring low pressure fluid to produce a higher pressure fluid. Therefore, steam-jet ejector is applied in order to integrate different level steams. Steam import is available from neighbouring plants, where the properties are 18 bar and 215 and the maximum steam supply is 120 ton/hr. It is desired to produce steam whose properties are not lower than 20 bar and 260, because it is feasible and convenient to use this level steam not only for heating but also for driving devices of process plants. To integrate import steam by the ejector, the entrainment ratio is determined, which is defined by the flow rate ratio of lower pressure import steam over higher pressure steam. In this case, the entrainment ratio is estimated according the design procedures illustrated in Perry's Handbook (1999), and its value is Then, it can be applied to ensure that properties of discharge steam from ejectors are higher than the requirements. The optimal flow sheet which considers retrofit and integration design is shown in Figure 4. Steam-jet ejector is installed and applied to integrate lower grade steam into the site ton/hr steam (101 bar) is treated as operating fluid to carry 92.5 ton/hr import steam (18 bar) to produce the medium pressure steam (20.6 bar). Consequently, boiler B1 decreases steam production to 150 ton/hr. Steam turbine ST1 is retrofitted to produce 20.6 and 4.5 bar steams. One additional new turbine ST5 is installed and is located between 101 and 20.6 bar steam headers, which is applied to generate electricity and produce 20.6 bar steam. Therefore, boilers B3 and B4 are turned off since steam requirement can be from ejector and steam turbines. It is also noted that flow rates of all let-down stations are reduced to 0 ton/hr, where the energy of let-down streams can be utilized by ejector and steam turbines. The overall operating cost of the steam plant has reduced by 15.1 % (NT $ million/year) compared to the base case. In summary,

6 Steam Power Plant Retrofit in an Oil Refinery Plant 895 the result recommends increasing steam import from neighbouring plants and reducing very high pressure steam production from the boiler. It also shows that energy integration in industrial parks can bring profit and break limitation to improve site performance. Figure 4 Optimal SDN for the retrofit with energy integration of import steam. 5. Conclusion A systematic methodology based on the superstructure has been proposed for improving the performance of the existing steam system in a petroleum refinery. In this work, two elements containing steam and electricity are regarded as energy sources to be integrated, which are available from the vicinal companies' energy systems. By the retrofit, significant reduction has achieved in operating cost by 13.8 %. On the other hand, to promote the cooperation of neighboring companies in EIPs, energy share was accomplished via the increase of steam import, where the corresponding cost saving is 15.1 %. The usability of import steam was overcome by utilizing the steam-jet ejector to produce the desired properties of steam. It is mentioned that the proposed model can be easily extended to various situations if different problem are met. References C.C. Chou, Y.S. Shih, 1987, A thermodynamic approach to the design and synthesis of plant utility systems, Ind. Eng. Chem. Res. 26, C.L. Chen, C.Y. Lin, 2010, A flexible structural and operational design of steam systems, Chemical Engineering Transactions, 21, M. Nishio, J. Itoh, K. Shiroko, T. Umeda, 1980, A thermodynamic approach to steam-power system design, Ind. Eng. Chem. Proc. Des. Dev. 19, O. Aguilar, S.J. Perry, J.K. Kim, R. Smith, 2007, Design and optimization of flexible utility systems subject to variable conditions, Part 2: Methodology and Applications, Chem. Eng. Res. Des. 85(A8), Perry's Chemical Engineer's Handbook Seventh Edition, 1999, McGraw-Hill Companies. S.A. Papoulias, I.E. Grossmann, 1983, A structural optimization approach in process synthesis. I: Utility System, Comput. Chem. Eng. 7(6), T.J. Mattila, S. Pakarinen, L. Sokka, 2010, Quantifying the total environmental impacts of an industrial symbiosis - a comparison of process-, hybrid and input-output life cycle assessment. Environ. Sci. Technol. 44,