Optimized Heat Exchanger Network design of GTL (Gas-To-Liquid) process

Transcription

1 October 2014, Volume 5, No.5 International Journal of Chemical and Environmental Engineering Optimized Heat Exchanger Network design of GTL (Gas-To-Liquid) process Sangsun Lee a ; Dongju Moon b ; Sungwon Hwang a* a Department of Chemistry & Chemical engineering, Inha University, Korea b Clean Energy Center, Korea Institute of Science and Technology (KIST), Korea * Corresponding author : sungwon.hwang@inha.ac.kr Abstract: GTL (Gas-to-Liquid) technology converts natural gas into liquid fuel products, and it has received much attention by researchers especially under the circumstance of continuous conventional oil price increase. The GTL technology typically includes reforming, Fischer Tropsch (F-T), and upgrading processes that generally require high pressure and temperature operating conditions. Therefore, optimal heat exchanger network of GTL is essential to maximize waste heat recovery, minimizing the consumption of heating fuel and CO2 release from fired heaters. In this work, heat exchanger networks (HEN) of GTL process are analyzed and optimized based on economic evaluation such as capital and operating costs. First, GTL process was developed by using commercial process design software such as Aspen HYSYS. Then, the mass and energy balance of the heat transfer equipment is transferred to Aspen energy analyzer to evaluate various cases of heat exchanger networks. The results showed that the methodology enabled us to achieve optimal HEN with higher energy efficiency of the GTL process. Keywords: GTL; Gas-To-Liquid; Process; Heat exchanger network; Optimization; Heat efficiency. 1. Introduction Fossil fuels have been used as a main source of energy for human being since the 1 st world war. Therefore, the refinery and petrochemical industry has shown much development for the last few decades. Nowadays, environmental issues such as climate change and global warming became major threat to human life, and much discussion about world energy usage has been made. For example, engineers and scientists in industry make enormous efforts to increase the energy efficiencies of the processes. The GTL (Gas to Liquid) process that converts natural gas or light hydrocarbons such as methane, ethane and propane to longer chained hydrocarbons such as gasoline, diesel and fuel oil has recently received much attention by scientists. According to World Bank report, over 150 billion cubic metres ( cu ft) of natural gas are flared or vented annually, and this amount is equivalent to 25% of the United States' gas consumption or 30% of the European Union's annual gas consumption [1]. Therefore, the GTL process is expected to be one of key technologies that reduce waste of energy sources. The GTL process is mainly composed of three individual processes such as reforming, Fischer-Tropsch (FT), and upgrading units. One of disadvantages of using this technology is that the processes consume high amounts of energy for reaction that converts light hydrocarbons to long-chained hydrocarbons and separation of the products. Therefore, the process of GTL technology should be fully analyzed in terms of energy efficiency. For example, it is necessary to examine if the heat exchanger network is optimally designed to recover waste heat that is generated from the process. Furthermore, it is quite important to optimize the integration of steam and power generation system to maximize energy efficiency of the process. Figure 1. Composite curve Figure 1. Composite curve

2 Figure 2. Relationship between T min and total cost For the optimization of heat exchanger networks, two different approaches were suggested. Hernan`n Rodera [2] proposed a mathematical modeling, while Alan P. Rossiter [3] graphical approach, which was so called, pinch technology. Linnhoff and Flower developed pinch analysis, of which methodology minimize energy consumption of the process by estimating thermodynamically feasible energy consumption and achieve the target by optimizing the heat recovery system such as heat exchanger networks [4, 5]. In this research, the pinch analysis was applied to the GTL process to minimize energy consumption of the technology. For this, Aspen HYSYS R was used for the modeling of the process, and Aspen Energy Analyzer TM was used to analyze the energy efficiency of the existing heat exchanger networks in the process. 1.1 Defining energy target Energy target defines the maximum amounts of heat duties to be transferred from hot process streams to cold process streams achieving minimum amount of energy consumption, required by the utilities. Figure 1 shows a composite curve that is developed based on temperature and enthalpy data, obtained from hot and cold streams in the process. This composite curve indicates energy target and discrepancy of energy usage between the target and the current status of the existing unit. 1.2 Fundamentals of pinch analysis One of the first steps to do with a composite curve is to examine if T min is properly adjusted. T min is the temperature difference of hot and cold streams at the closest approach between these two streams. In general, as T min becomes bigger, the process consumes more utilities, while it might require smaller capital cost for heat exchangers due to higher temperature driving force. On the other hand, as T min becomes it requires higher capital cost for the heat exchangers, while overall operating cost becomes smaller. Therefore, appropriate Tmin should be selected based on the economic analysis and practical constraints in terms of operation and design. This trade-off between operating and capital costs is illustrated in figure 2. Once T min is selected, the composite curve of the existing process should be analyzed based on the following rules. LPG Natural Gas Reforming Syngas Fischer- Tropsch Hydro Upgrading Gasoline Diesel Base oil Figure 3. Concept of GTL process Figure 4. GTL process simulation model Figure 5. Heat Exchanger Network Simulation model 308

3 There should not be any heat tr ansfer across the pinch. No heating utility is used below the pinch. No cooling utility is used above the pinch 2. GTL process As briefly described in Introduction, the GTL process includes three processes: reforming, Fischer-Tropsch (FT) and upgrading as shown in figure 3. First, the reforming process converts natural gas into syngas. Secondly, F-T process converts syngas into longchained hydrocarbons. Thirdly, the upgrading process converts unnecessary heavy hydrocarbons to lighter liquid transport fuels such as gasoline and diesel. Following reactions (1) - (3) represents three main reaction mechanisms of reforming process, while reaction (4) shows the reaction mechanism of F-T process. Because most reactions in GTL process are exothermic at high pressure, the generated heat from the exothermic reactions is available for either preheating of feed stream or steam generation. Furthermore, because most reactions occur at high temperature in the processes, the high enthalpy from the reactants and products after the reactor is also available. As a result, developing an optimum heat exchanger network is crucial to achieve high energy efficiency of the GTL process. (1) CH 4 + 3/2O 2 CO + 2H 2 O (2) CH 4 + H 2 O CO + 3H 2 H 298 = 520KJ/mol H 298 = -206KJ/mol exchanger units, total heat exchanger surface area, and expected operating cost of both initial GTL model and final GTL model after pinch analysis. As shown in Figure 4 and 5, initial GTL model shows poor heat transfer efficiency compared with the final model after pinch analysis. 2.2 Minimum utility usage Fig 6 and 7 shows the selected optimal T min of 19 after analyzing trade-off of total capital and operating costs based initial GTL model and complete composite curves based on the selected T min. The composite curves show a pinch point and utility consumption targets with minimum cooling and heating usage,. 2.3 Optimize cases Although minimum cooling and heating usage are calculated by pinch analysis, applying them to real process requires careful consideration. For example, if excess amount of heat is identified, it is necessary to consider various options to recover the heat (e.g. steam generation, heat tranfer between process streams, etc.). Therefore, economic analysis of each case study should be carried out. In this study, the following options were considered to maximize energy recovery of GTL process. Maximize heat recovery with economizer - In initial model, there was little heat exchange between the process steams, so that heat transfer depends too much on the usage of heating and cooling utilities. (3) CO + H 2 O CO 2 + H 2 H 298 = 41KJ/mol (4) (2n+1)H2 + nco CnH(2n+2) + nh2o H 298 = -167KJ/mol 2.1 Basic data of initial GTL model Figure 6. Trade-offs between total cost and T In this study, the GTL process includes 14 individual process streams, 5 hot utilities, and 8 cold utilities. Table 1 summarizes heating and cooling duties, number of heat 309 Figure 7. Composite curve of GTL process Power recovery system with steam turbine - GTL process consumes high amounts of electric power. For the reason, excess heat is used to produce steam, and the electric power is then produced through steam turbine. Methane recycle system with fired heater - Because reforming process operates with high temperature, pre-heater duty is also very significant. There have been a number of studies over methane combustion system, which reduces the duty of pre-heater. In GTL process, the byproducts contain significant

4 amounts of light hydrocarbons, and a portion of the byproduct is consumed as a fuel for a pre-heater. 3.4 Maximize heat recovery Table 2 summarizes heat exchange data between hot and cold streams in the GTL process. In case of H2 hot stream, it goes through fired heater as a preheating step and the temperature increases up to 900 before the reactor. Then, the product stream after the reactor is cooled down to the temperature of 30. Furthermore, H4 hot stream is cooled down from to 30. Both of these two streams have critical impacts on overall energy efficiency of the GTL process because they consume high amounts of heat duties from the heating and cooling utilities. Table 1. Stream data of initial model Initial model Pinch model Heating [kj/h] 3.38e e+08 Cooling [kj/h] 3.69e e+09 Number of units Total Area [m 2 ] 8.32e e+05 Operating cost [$/s] Capital cost [$] 7.01e e+08 Total cost [$/s] Table 2. Heat transfer data of process stream Stream name (hot) Inlet T [ ] Outlet T [ ] Enthalpy [kj/h] Stream name (cold) Inlet T Outlet T Enthalpy [kj/h] H e+07 C e+08 H e+09 C e+08 H e+08 C e+08 H e+09 C e+09 H e+07 C e+08 H C e+08 C e06 Therefore, it is necessary to split these two streams or to use the economizer at different temperatures to eliminate any inefficiency. Fig 7 compares the simulation results of initial and the final GTL models. Table 3 also compares heat transfer area and cost of heat exchanger before and after building up heat exchanger networks. 2.5 Power generation by HP steam turbine Alternative way to increase the energy efficiency of the GTL process is to use a steam boiler to produce HP steam and produce electric power through steam turbine. Figure 9 shows the simulation results after integration of steam system with electric power system that uses steam turbine. According to the results of the simulation, it is found that about 651.8ton/h of HP steam and about 61.11ton/h of MP steam were generated from each boiler. Waste heat of steam was mainly used to produce HP steam. Consequently, these steams generate about 230.9MW of electric power with steam turbine. 2.6 Methane recycle for Heating The simulation results show that high amount of methane are accumulated in a recycle system of the GTL process. Pure methane has heat value of about 1,000 Btu per cubic feet, so the methane-carbon dioxide mixture is expected to have a heat value of about 600 Btu per cubic feet [10]. In this study, there are rich light hydro-carbon total amount of produced methane reached up to 42.37ton/h. A portion of methane can be used as a fuel for fired heater, of which total heat duty is equivalent to about 5.54e+8kJ/h. Summation with heat of other waste hydrocarbon is almost 3.7e+6kJ/h. It can also reduce heating cost. 2.7 Methane recycle for Heating The simulation results show that high amount of methane are accumulated in a recycle system of the GTL process. Pure methane has heat value of about 1,000 Btu per cubic feet, so the methane-carbon dioxide mixture is expected to have a heat value of about 600 Btu per cubic feet [10]. In this study, there are rich light hydro-carbon total amount of produced methane reached up to 42.37ton/h. A 310

5 portion of methane can be used as a fuel for fired heater, of which total heat duty is equivalent to about 5.54e+8kJ/h. Summation with heat of other waste hydrocarbon is almost 3.7e+6kJ/h. It can also reduce heating cost. Initial model Developed model Heating [kj/h] 3.38e e+09 Cooling [kj/h] 3.69e e+09 Number of units Total Area [m 2 ] 8.32e e+05 Operating cost [$/s] Capital cost [$] 7.01e e+07 Total cost [$/s] Figure 8. Initial heat exchanger network model Figure 9. Upgrade heat exchanger network model 2.8 Optimization of each option Optimization of each option Previously heat exchanger network optimization has been considered separately from steam generation system or methane recycles system. Therefore, in this case these individual systems were combined together, and the results showed that about 61.11ton/h of MP steam could be produced further. - Integration of steam system with electric power system - Optimization of a portion of methane recycle as a fuel for a fired heater - Integration of the above three cases Table 4 compares the values of minimum utility targets, operation cost, capital cost and total cost, etc. 2.9 Compare to pinch approach Four options were considered in this case as follows. - Heat exchanger network optimization Table 3. Compare of Stream data Conclusion This study aimed to maximize the energy efficiency of the process by optimizing the heat exchanger networks and

6 the electric power generation system. For this, the following tasks were completed. Develop the existing GTL process model with Aspen HYSYS R Analyze the existing process with Aspen Energy Analyzer TM Compare the energy consumption of the unit against the target Develop various case studies and optimize the heat exchanger networks Choose the best heat exchanger networks after economic analysis review Table 4. Compare each case to pinch approach Utility usage Pinch Heat recovery Power recovery Optimized Value Percent Value percent Value percent Heating [kj/h] 8.049e e e e Cooling [kj/h] 1.113e e e e Total Area [m 2 ] 4.652e e e e Heating [$/s] Cooling [$/s] Cost Operating [$/s] Capital [$] 1.01e e e e Total [$/s] As a result, optimization of heat exchanger networks resulted in overall energy efficiency increase of the process by consuming less amounts of utilities such as fuel oil / gas, cooling water, electric power, etc., and by maximizing power generation. By attaching heat exchanger network, the heat efficiency increased by almost 90% of final optimized model. However, it should be noted that some practical constraints such as safety, operating issues were not taken into account during this study. ACKNOWLEDGMENT This work was supported by a Special Education Program for Offshore Plant by the Ministry of Trade, Industry and Energy Affairs (MOTIE). This work was supported by Korea Institute of Science and Technology and funded by Ministry of Trade, Industry and Energy, Korea. [6] J. jaehak, L. inbeom, and J. geunsu., Optimal synthesis for retrofitting heat exchanger networks. Journal of the Korean Institute of Chemical Engineers, (1991). 29: [7] Energy Efficiency and Renewable Energy Team., How to calculate the true cost of steam, Industrial Technologies Program Energy Efficiency and renewable energy, (2003). DC [8] Xu Hao. Martina Elissa Djatmiko. Yuanyuan Xu. Yining Wang. Jie Chang. and Yongwang Li. Simulation Analysis of a Gas-to- Liquid Process Using Aspen Plus, Chem. Eng. Technol. (2008), 31, No. 2, [9] Buping Bao. Mahmoud M. El-Halwagi. and nimir O. Elbashir. Simulation, integration, and economic analysis of gas-to-liquid processes, Fuel Processing Technology 91, (2010), [10] Hae-Pyeong Lee. In-Young Lee. and Kyong-Ok Yoo. The system separation method for the optimal target of heat exchanger network synthesis with multiple pinches. Korean J. of chem. Eng. 12(5), (1995), [11] Bodo Linnhoff. Helen Dunford. and Robin Smith. Heat integration of distillation columns into overall processes. Chemical Engineering Science, (1983), Vol. 38, No. 8, REFERENCES [1] World Bank, GGFR Partners Unlock Value of Wasted Gas", World Bank 14 December Retrieved 17 March [2] Herman`n Rodera, Multipurpose Heat-Exchanger Networks for Heat Integration Across Plants. School of Chemical Engineering and Material Science, (2001). 40: [3] Alan P. Rossiter, Improve Energy Efficiency via Heat Integration. Rossiter&Associates. American institute of Chemical Engineers. (2010). [4] Linnhoff, B. and Flower, J. R.: AIChE J., 15, 10(1978) [5] Tjoe, T. N. and Linnhoff, B.: Chem. Eng., April 28, 47(1986) 312