Format And Type Fonts CCHHEEMMIICCAALL EENNGGIINNEEEERRIINNGG TTRRAANNSSAACCTTIIOONNSS VOL. 45, 2015 A publication of The Italian Association of Chemical Engineering www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Sharifah Rafidah Wan Alwi, Jun Yow Yong, Xia Liu Copyright © 2015, AIDIC Servizi S.r.l., ISBN 978-88-95608-36-5; ISSN 2283-9216 DOI: 10.3303/CET1545032 Please cite this article as: Kansha Y., Mizuno H., Kotani Y., Ishizuka M., Fu Q., Tsutsumi A., 2015, Numerical investigation of energy saving potential for self-heat recuperation, Chemical Engineering Transactions, 45, 187-192 DOI:10.3303/CET1545032 187 Numerical Investigation of Energy Saving Potential for Self-Heat Recuperation Yasuki Kansha a , Hiroyuki Mizuno a , Yui Kotani a , Masanori Ishizuka a , Chunfeng Song b , Qian Fu a , Atsushi Tsutsumi*, a a Collaborative Research Center for Energy Engineering, Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku Tokyo 153-8505, Japan b School of Environmental Science and Engineering,Tianjin University, 92 Wei Jin Road, Nankai District, Tianjin, 300072 P.R. China, a-tsu2mi@iis.u-tokyo.ac.jp Recently, energy saving technology has attracted increased interest in many countries for suppressing global warming and reducing the use of fossil fuels. Self-heat recuperation technology has recently been developed for energy saving of chemical processes. It has the characteristics whereby total process heat can be recirculated within the process, leading to a marked reduction in energy consumption. The authors have developed a simple calculation technique for the minimum energy required for thermal processes that was derived numerically from the view point of irreversibility and exergy loss for heat transfer. In addition, the authors reported that the actual energy required for a self-heat recuperative thermal process is almost the same as the value derived from this calculation technique. In this paper, the authors examined the minimum energy required for thermal processes with/without heat recovery or based on self-heat recuperation using the cold process stream properties and also evaluated the energy saving potential of these processes using process simulation. Since the results obtained from the calculation method can be used as target values of heat recovery technology, this investigation supports process intensification and is promising for industries to examine the energy saving potential when designing a thermal process. 1. Introduction The reduction of CO2 emission has become a major target in efforts to suppress global warming. The combustion of fossil fuel for heating produces a large amount of CO2. For this reason, energy saving technology has attracted increased interest in many countries. So far, to reduce the energy consumption, heat recovery technology – Pinch Analysis, which exchanges heat between the hot and cold streams in a process, has been applied to thermal processes from 1980s (Linnhoff and Hindmarsh, 1983). Although this technology is a powerful technology for energy saving of the chemical process, an additional heat source may be required depending on the minimum temperature difference between hot and cold streams for heat exchange (Linnhoff, 1993). This additional heat is often produced by fuel combustion, leading to CO2 production. Wechsung et al. (2010) developed a heat integration technology with pressure changes at sub- ambient temperature and successfully reduced the energy consumption of LNG plant. To expand this technology, Fu and Gundersen (2013) developed a vapour recompression process for the cryogenic air separation. Recently, the authors have developed a self-heat recuperation technology based on exergy destruction minimization to reduce the energy consumption of thermal processes from different aspects (Kansha et al. 2009). Applying the self-heat recuperation technology to thermal processes, not only the latent heat but also the sensible heat of the process stream, i.e. all process heat, can be circulated into the processes without any heat addition. As a result, the energy consumption and exergy destruction of a process can be greatly reduced in the steady state. In fact, this technology has been applied to several processes as case 188 studies and shows large energy saving potentials in these processes such as azeotropic distillation (Kansha et al., 2010) and hydro-desulfurization reaction process (Matsuda et al., 2010b). Moreover, the authors have developed a simple calculation technique for the minimum energy required for thermal processes that was derived numerically from the point of view of irreversibility (Kansha et al., 2013a). The authors reported that the energy required for a self-heat recuperative thermal process is almost the same as the derived value. In this paper, we examined the minimum energy required for thermal processes with/without heat recovery or based on self-heat recuperation using the process stream properties. Furthermore, we showed that the results obtained from our calculation method can be fixed as target values of heat recovery technology to evaluate the energy saving potential of these processes using process simulation. 2. Energy required for thermal processes A process stream is heated in a thermal process to satisfy the condition of the following process, as shown in Figure 1(a). Tin and Tout are the input and output temperatures of the process stream to the heater. F is the flow rate of the process stream. By using heat capacity (CP(T)) of the process stream, the received heat amount (Q) of the process stream is expressed by the following equation Eq(1):  o u t in )( P T T dTTFCQ (1) This equation can be simply rewritten when CP is constant during temperature change; )( inoutP TTFCQ  (2) This heat amount (Q) is the required heat amount to increase temperature from Tin to Tout. Figure 1(b) shows the temperature–heat diagram which shows this relationship. Tin Tout F Q Heat amount Te m p e ra tu re Tin Tout (a) (b) Heater Figure 1: Conventional thermal process: a) flow diagram; b) temperature–heat diagram Figure 2(a) shows the flow diagram of feed–effluent heat exchange system as an example of conventional heat recovery. We assumed that the following process after this system does not have any enthalpy change for clear explanation. Hence, hot stream is perfectly the same as the cold stream. In this case, heat exchange duty in the heat exchanger is maximized. The received heat amount (Q) of the process stream to increase temperature from Tin to Tout can be represented by Eq(1). However, all of the heat cannot be exchanged with effluent stream due to the second law of thermodynamics. The heat amount supplied by the additional heater (q) is expressed by the following equation; 189  o u t 1 mid , )( P T T dTTFCq (3) This amount is same as the cooling amount at cooler in Figure 2(a) due to the energy conservation law. According to the above mentioned assumptions, ΔTht which means the temperature difference between hot and cold (process stream) streams equals Tout – Tmid, 1 and Tmid, 2 – Tin. This means that feed (cold) and effluent (hot) curve in the temperature–heat diagram as shown in Figure 2(b) are in parallel with ΔT difference. Using ΔTht, Eq(3) can be simply rewritten as; htP TFCq  (4) In addition, ΔTht is defined by the heat exchanger properties. Tin Tmid, 1 F Q Heat amount Te m p e ra tu re Tin Tout (a) (b) Tmid, 2 Tout Tout Tin Heater Cooler Heat exchanger Tmid, 1 Tmid, 2 Q q Figure 2: Conventional thermal process with heat recovery: a) flow diagram; b) temperature–heat diagram Kansha et al. (2013a) reported the required energy for thermal process based on self-heat recuperation as shown in Figure 3 is almost same as the exergy destruction for heat transfer. This exergy destruction (EXdest) for heat transfer can be represented by; )/()/( 0Tatht 0 Tatht dest TTTT dSTF dEX    (5) where S is entropy of the process stream, T0 is the standard temperature and ΔTht at T is the temperature difference between process (hot and cold) streams at process stream temperature T (Tin < T < Tout). Kansha et al. (2013a) also reported that temperature difference between process (hot and cold) streams during heat exchange in the thermal process based on self-heat recuperation using compression does not change significantly. This means that heat capacity is not significantly affected by compression. If we assume that the heat exchanger type of this thermal process is the same as the thermal process with feed- effluent heat exchanger, ΔTht at T can be set to ΔTht. Hence, entropy and heat capacity has the following relationship; dT T TC dS )( P (6) 190 The energy required (W) for the thermal process based on self-heat recuperation can be derived by the following equation when process stream temperature is close to T0 and ΔT is much smaller than T0.           in out htPhtP ln o u t in T T TFC T dT TFCW T T (7) This energy required (W) is supplied to the system by work as the net energy required for compression. To compare the values (Q, q, W) from Eqs(2), (4) and (7), we can simply estimate the energy required amount for the thermal processes with/without heat recovery or based on self-heat recuperation using process stream properties. F Tin F Q Heat amount Te m p e ra tu re Tin Tout (a) (b) Tmid, 4 Tout Tout Tin Cooler Heat exchanger Tmid, 3 Tmid, 4 Tmid, 3 ΔTht at T Figure 3: Thermal process based on self-heat recuperation: a) flow diagram; b) temperature–heat diagram 3. Simulations 3.1 Thermal process for gas stream Kansha et al. (2009) reported comparisons between the energy required for self-heat recuperative processes and the conventional counterparts. According to the above-mentioned paper, the process simulation was conducted using PRO/II Ver. 8.1. As a real fluid, butane was used for the gas stream. In the calculations for all cases, the streams were heated from 300 K to a set temperature Tin, and the flow rate of the stream, F, was 100 kmol/h (= 5,812 kg/h). The Soave–Redlich–Kwong equation of state was used considering the real gas stream. The minimum temperature difference for heat exchange was assumed to be 10 K. The pressure ratio in the compressor was set to maintain a constant temperature increase of 10 K owing to compression. The efficiency of the heat exchanger was 100 % (i.e. no heat loss), and the adiabatic efficiencies of the compressor and expander were 100 %. CP of butane at standard condition was 1.72 kJ kg -1 K -1 . A comparison between the energy required (Q, q, W) for the thermal processes calculated from Eqs(2), (4) and (7) as shown in Table 1, and the energy required for the thermal processes calculated by the simulation as shown in Table 2 was conducted. It can be seen from Tables 1 and 2 that the energy required for the thermal processes by the above- mentioned simple calculations are almost consistent with the simulation results. In simple calculations, increase in Q was proportional to increase in Tout and q was kept at a constant value of 27.7 kW because CP was assumed to be constant for simple calculations, and hot and cold streams were the same stream. It can be estimated that the difference of values mainly came from temperature dependence of heat capacity (CP). This effect can be understood by the fact that the increase of q depends on Tout. At the same 191 time, it can be observed that the difference of W between calculation and simulation becomes larger with the increase in temperature. This is because the denominator of Eq(5) becomes larger at higher temperatures. Table 1: Energy required for the thermal processes (Calculation) Tout [K] Q [kW] q [kW] W [kW] 350 138.6 27.7 4.3 400 277.2 27.7 8.0 450 415.8 27.7 11.2 Table 2: Energy required for the thermal processes (Simulation), Kansha et al. (2009) Tout [K] Q [kW] q [kW] W [kW] 350 147.7 31.0 4.4 400 313.7 34.6 8.6 450 497.0 38.0 12.5 3.2 Methanol production In a methanol production process (LPMEOH™ Demonstration Unit, Heydom et al., 2003) as a case study, we compared the energy required (Q, q, W) for the thermal processes calculated from Eq(2), Eq(4) and Eq(7) with the energy required for the thermal processes calculated by the simulation using PRO/II Ver. 9.0 (Invensys, SimSci) (Kansha et al., 2013b). The feed stream (CO, H2, etc.) was provided from reformer, this stream was mixed with recycle stream and converted to methanol as shown in Figure 4 (Kansha et al., 2014). In this simulation, the Soave–Redlich–Kwong equation was selected for the thermodynamics data and 100 % adiabatic efficiency was assumed for the compressors. The reactor was assumed to be an isothermal reactor. In addition, the minimum temperature difference for the heat exchangers was fixed at 10 K for all the heat exchangers, and the heat and pressure losses from the system were assumed to be negligible. To ensure the effect of self-heat recuperation and heat recovery, the flow diagram was modified and the feed and reactor conditions were fixed at 313.7 K and 5.07 MPa, and 478.7 K and 5.07 MPa. Flow rate to the reactor was 43,700 kg/h and CP of this stream was 2.22 kJ kg -1 K -1 . By using stream number in Figure 4, stream 1 was feed stream to the heater at 313.7 K and stream 2 was feed stream to the reactor at 478.7 K. Because of the isothermal reactor, stream 3 was the effluent stream from reactor at 478.7 K and stream 4 was output stream from cooler at 313.7 K. Heater duty (Q) of the thermal process without heat recovery was calculated to be 4.4 MW. At the same time, the energy required for the thermal process with heat recovery (q) and that based on self-heat recuperation (W) were numerically calculated to be 0.27 MW and 0.11 MW. According to the simulation results, these three values (Q, q, W) were 4.5, 0.48, and 0.10 MW, as summarized in Table 3. From this comparison, it can be seen that the energy required (Q, q, W) for the thermal processes calculated from Eq(2), Eq(4) and Eq(7) followed the tendency of the simulation results. However, the energy required for the thermal process with heat recovery (q) calculated from Eq(4) was not fitted well to the simulation values. The reason for this difference is that feed stream (1→2) did not receive enough heat from effluent stream (3→4) due to a pinch point in the heat exchanger. The effluent stream contains much methanol, in which its sensible and latent heats has to be exchanged with sensible heat of feed stream. Thus, the composite curves of feed and effluent stream in the heat exchanger are not in parallel. In fact, the temperature of the exited feed stream from the heat exchanger was 18 K lower than the reaction temperature (478.7 K). Table 3: Comparison of Energy required for the thermal processes in the methanol production Q [MW] q [MW] W [MW] Calculation 4.4 0.27 0.11 Simulation 4.5 0.48 0.10 192 Separator Reactor Heater Cooler Recycle Crude Methanol Feed Target 1 2 3 4 Figure 4: Simple flow diagram for methanol production 4. Conclusions In this paper, we proposed a simple calculation method to obtain the energy required for the thermal processes with/without heat recovery or based on self-heat recuperation using the process stream properties. The results obtained from this simple calculation method were well-fitted to the simulation results. Therefore, this method can estimate the energy saving potential of heat recovery and self-heat recuperation without simulation and the obtained results can be used as target values for energy saving. This investigation supports process intensification and is promising for industries to examine the energy saving potential when designing a thermal process. References Fu C., Gundersen T., 2013, Recuperative vapor recompression heat pumps in cryogenic air separation processes, Energy, 59, 708–718. Heydorn E.C., Diamond B.W., Lilly R.D., 2003, Commercial-scale demonstration of the liquid phase methanol (LPMEOH TM ) process, Final Report (Volume 2: Project Performance and Economics) DE- FC22-92PC90543, Department of Energy National Energy Technology Laboratory, Washington DC, USA. Kansha Y., Tsuru N., Sato K., Fushimi C., Tsutsumi A., 2009, Self-heat recuperation technology for energy saving in chemical processes, Ind. Eng. Chem. Res., 48, 7682–7686. Kansha Y., Tsuru N., Fushimi C., Tsutsumi A., 2010, New design methodology based on self-heat recuperation for production by azeotropic distillation, Energy Fuels, 24, 6099–6102. Kansha Y., Aziz M., Kishimoto A., Tsutsumi A., 2013a, Evaluation of a self-heat recuperative thermal process based on thermodynamic irreversibility and exergy, J. Chem. Eng. Japan, 46, 87–91. Kansha Y., Ishizuka M., Tsutsumi A., 2013b, Development of innovative methanol synthesis process based on self-heat recuperation, Chemical Engineering Transactions, 35, 37–42. Kansha Y., Ishizuka M., Song C., Tsutsumi A., 2014, An innovative methanol synthesis process based on self-heat recuperation, App. Therm. Eng., 70, 1189–1194. Linnhoff, B., Hindmarsh, E., 1983, The pinch design method for heat exchanger networks, Chem. Eng. Sci., 38, 745–763. Linnhoff, B., 1993, Pinch analysis-a state-of-the-art overview, Chem. Eng. Res. Des., 71, 503–522. Matsuda K., Kawazuishi K., Hirochi Y., Sato R., Kansha Y., Fushimi C., Shikatani Y., Kunikiyo H., Tsutsumi A., 2010b, Advanced energy saving in the reaction section of hydro-desulfurization process with self-heat recuoperation technology, App. Therm. Eng., 30, 2300–2306. Wechsung, A., Aspelund, A., Gundersen, T., Barton P.I., 2010, Synthesis of Heat Exchanger Networks at Subambient Conditions with Compression and Expansion of Process Streams, AIChE J., 57, 2090– 2108.