Working Fluid Selection for Organic Rankine Cycle ... - ACS Publications

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Working Fluid Selection for Organic Rankine Cycle (ORC) Considering the Characteristics of Waste Heat Sources Haoshui Yu,† Xiao Feng,*,‡ and Yufei Wang† †

State Key Laboratory of Heavy Oil Processing, New Energy Institute, China University of Petroleum, Beijing 102249, China School of Chemical Engineering & Technology, Xi’an Jiaotong University, Xi’an 710049, China

‡

ABSTRACT: Organic Rankine cycle (ORC) is a promising way for low temperature waste heat utilization. The performance of an ORC system is strongly related to the working fluid. Therefore, working fluid selection is a primary task in customizing an ORC to a specific background process for waste heat recovery. In refineries, there are huge amounts of waste heat, in which some waste heat streams simultaneously release latent heat and sensible heat, and the target temperature is constrained in most cases. This paper focuses on the working fluid selection of the ORC recovering different types of waste heat in refineries. In this paper, the waste heat sources in refineries are classified into three types (i.e., sensible heat source, combined heat source, and latent heat source) according to their characteristics. The impacts of waste heat sources characteristics (i.e., waste heat type, inlet and target temperatures of waste heat, the ratio of latent to sensible heat) on the working fluid selection are investigated. The results show that the characteristics of waste heat exert great influence on working fluid selection. Different types of waste heat source present different thermodynamic criteria for optimum working fluids. The inlet temperature (Tin) and target temperature (Ttar) of waste heat, and the ratio of latent to sensible heat (R) of waste heat source have influence on the selection of working fluid. However, the degree of influence depends on the waste heat types. Thermodynamic criteria on working fluid selection for each type of waste heat sources are drawn in this paper.

1. INTRODUCTION Organic Rankine cycle (ORC) is a promising technology that can upgrade low-temperature heat into electricity.1 In refineries, there are huge amounts of low temperature waste heat sources, which are usually cooled by the cooling utility or discharged to the environment directly.2 Utilization of theses waste heat can not only improve the energetic efficiency, but also reduce greenhouse gas emissions. The working fluid of an ORC determines thermal efficiency, safety, stability, environmental impact, and economic profitability of the system.3 In recent years, working fluid selection for ORC has drawn significant attention. Different performance evaluation criteria lead to different optimum working fluids. Therefore, a reasonable evaluation criterion is the key issue for working fluid selection. The performance evaluation criteria based on the first law of thermodynamics include thermal efficiency, heat recovery efficiency, and expansion ratio.4 Hung et al.5 used thermal efficiency as the criterion for working fluid selection. The results showed that thermal efficiency is closely related to the latent heat of working fluid at low pressure. Lai et al.6 conducted the working fluid selection research for hightemperature ORC based on thermal efficiency. Yu et al.7 proposed a new method for simultanous selection of working fluid and operating conditions for an ORC recovering waste heat with maximum net power output as the working fluid selection criterion. Tchanche et al.8 found that working fluids with high boiling point are competitive in thermal efficiency. Liu et al.9 used total heat-recovery efficiency and heat availability instead of thermal efficiency as the evaluation criteria to optimize the working fluid and operating conditions. The results demonstrated that total heat-recovery efficiency and heat availability are more accurate than the thermal efficiency to © XXXX American Chemical Society

reflect the performance of the ORC in recovering low temperature waste heat. The performance evaluation criteria based on the second law of thermodynamics include exergy efficiency, exergy destruction rate, exergy recovery efficiency, etc.4 Heberle et al.10 found that working fluids with low critical temperatures should be favored in parallel circuits based on exergy analysis. In another paper, exergy destruction rates of each component of an ORC were quantified.11 The evaporator presented the greatest energy destructions among all the components. The performance evaluation criteria considering the economics include net power output per heat exchanger area, minimum annual cost, annualized net profit, unit cost net profit, and so on.4 Hettiarachchi et al.12 chose the ratio of heat transfer surface to generated power as an optimization criterion for the selection of the working fluid. Quoilin et al.13 investigated an ORC system with total heat recovery efficiency, overall system efficiency, and sepcific investment cost being the evaluation criteria. Toffolo et al.14 proposed a method that considered multicriteria for working fluid selection. Guo et al.15 have also done similar research on working fluid selection for an ORC driven by low temperature geothermal sources. However, all the above works concentrated on the working fluid selection for an ORC driven by exhasut gas, solar energy, and geothermal resources. To the best of our knowledge, the research on working fluid selection for an ORC driven by refinery waste heat is limited. Only Jung et al.16 and Song et Received: June 23, 2015 Revised: December 19, 2015 Accepted: January 15, 2016

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DOI: 10.1021/acs.iecr.5b02277 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research al.17 investigated the ORC system applied to refineries. Jung et al.16 investigated the working fluids selection of an ORC to utilize a liquid stream of kerosene. Song et al.17 investigated the ORC system recovering multistrand waste heat sources. However, the waste heat sources are all sensible heat sources. Both papers do not consider the influence of waste heat characteristics on working fluid selection. The waste heat sources in refineries are much more complicated primarily due to the fact that the temperature range of waste heat is wide, and in some cases, latent heat and sensible heat would be released simultaneously. Additionally, in most cases, the target temperature of the waste heat stream is constrained to meet the specific requirements. As a result, the effect of characteristics of waste heat source on working fluid selection is unnegligible. Therefore, this paper conducts working fluid selection considering the characteristics of the waste heat source. The innovation of this paper is to classify the waste heat sources in refineries into three types and investigate the impact of waste heat source characteristics on working fluid selection.

Figure 1. Variation of thermal efficiencies with evaporation temperature for all the preselected working fluids with condensation temperature of 45 °C.

specified evaporation and condensation temperatures, the higher thermal efficiency will be obtained provided the critical temperature is higher. The results are consistent with those obtained by Mago et al.25 and Aljundi.26 It can also be concluded that the differences of thermal efficiencies for all the working fluids are tiny at low evaporation temperatures, and the differences increase with the increase of evaporation temperature. As far as the thermal efficiency is concerned, it is definite that the working fluid with higer critical temperature shows higher thermal efficiency. However, thermal efficiency is not the sole factor affecting the power output. Since net power output is equal to the product of thermal efficiency and the amount of waste heat recovered (Qrecovered), thermal efficiency and Qrecovered should be simultaneously considered while screening working fluid. This suggests that there are two ways to improve the net power output of an ORC system: improving the heat input and enhancing the heat-work conversion efficiency. Qrecovered reflects the ability of working fluid to recover waste heat. The abilities of different working fluids to recover waste heat need to be taken into consideration during working fluid selection. The abilities to recover waste heat of different working fluids vary with the waste heat conditions. Therefore, it is necessary to investigate the effect of waste heat characteristics on the working fluid selection. The characteristics of waste heat include heat capacity flow rate of waste heat, inlet and target temperatures of waste heat, and the ratio of latent heat to sensible heat. According to these characteristics, waste heat sources are classified into three types as shown in Figure 2 (i.e., sensible heat source, combined heat source, and latent heat source). A sensible heat source releases heat with obvious temperature change. The temperature− enthalpy (T−H) curve is an oblique line, and the ratio of latent to sensible heat (R) is equal to 0. Flue gas is an example of this type of waste heat source. The combined heat source undergoes a nearly isothermal phase change process and a temperature-changing process. The T−H curve is a combination of an oblique line and nearly a horizontal line, and R is greater than 0. The waste heat released by a distillation column overhead total condenser is an example of this type of waste heat source. The latent heat source undergoes only a nearly isothermal phase change process. The T−H curve is nearly a horizontal line, and R is infinite. The waste heat released by a distillation column overhead partial condenser is an example of

2. WORKING FLUIDS PRESELECTION AND WASTE HEAT SOURCES CLASSIFICATION Since environmental regulation is becoming more and more stringent, the environmental effects must be taken into consideration during working fluid selection.18 CFCs (chlorofluorocarbons) have been banned from production and application for high ODP (ozone depletion potential).19 The preselected working fluids are chlorine free in this paper. The working fluids used in ORC system are basically categorized in three groups based on the slope of saturation vapor curves in T−S (temperature−entropy) diagram,20 which are dry, isentropic, and wet working fluids. Since wet working fluids tend to form droplets at the outlet of a turbine,21 which may cause the corrosion of turbine blades, wet working fluids are excluded in this paper. Eight preselected working fluids considered in this work are listed in Table 1. The thermodynamic properties are obtained in Aspen Plus.22 Table 1. Preselected Working Fluids and Their Properties working fluids

chemical formula

Tc (°C)

Pc (bar)

ODP

type

R227EA R236FA R600A R236EA R600 R245FA R601A R601

C3HF7 C3H2F6-D1 C4H10−2 C3H2F6 C4H10−1 C3H3F5-D1 C5H12−2 C5H12−1

101.68 124.92 134.65 139.23 151.97 154.05 187.25 196.55

29.12 32.19 36.40 34.12 37.96 36.40 33.80 33.70

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

dry dry dry dry dry isentropic dry dry

In general, the heat in the condenser is removed by cooling water.23 Therefore, the condensation temperature is restricted to the ambient temperature. In this paper, the condensation temperature is set as 45 °C. At specified evaporation and condensation temperatures, the thermal efficiency is a constant for each working fluid. With the condensation temperature fixed at 45 °C, the thermal efficiency of all the preselected working fluid at various evaporation temperatures is plotted in Figure 1. The evaporation temperature should not be higher than the critical temperature for subcritical cycle to avoid the problems caused by a supercritical cycle.24 It is clear that the higher the critical temperature, the wider temperature range a working fluid can evaporate. As shown in Figure 1, with the B


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investigate the effect of waste heat inlet temperature on working fluid selection, the target temperature will be unrestricted. In our previous work,7 we proposed a new method for simultaneous selection of working fluid and operating conditions for an ORC recovering waste heat sources whose target temperatures are unconstrained. The results demonstrated that under various waste heat inlet temperatures, the working fluids, whose critical temperatures are slightly lower (about 25−35 °C) than the inlet temperature, are selected as the optimum working fluids. In this case, critical temperature of the working fluid is the vital parameter for working fluid selection. Since the power output equals to the product of waste heat recovered and thermal efficiency, both waste heat recovered and thermal efficiency have influence on the power output. The working fluids, whose temperature is slightly lower than waste heat inlet temperature, can recover all or most of the available waste heat and show acceptable thermal efficiency, and so gives the maximum power output. The working fluid, whose critical temperature is far lower than Tin, has poor thermal efficiency, even though they can recover the waste heat totally. The working fluids, with critical temperatures higher than Tin, present high thermal efficiency, but Qrecovered is much smaller compared with that of lower critical temperature working fluids. In this case, the critical temperature of working fluid is the primary factor that should be taken into consideration in working fluid selection. The cases of constrained target temperatures will be considered in the next section. 3.2. Effect of Waste Heat Target Temperature on Working Fluid Selection for Sensible Heat Sources. Waste heat inlet temperature determines the choice of optimum working fluid when the target temperature of waste heat is unconstrained. The variation of waste heat inlet temperature leads to the variation of optimum working fluid. In this part, we will examine the effect of target temperature on the working fluid selection. With the inlet temperature of waste heat fixed at 150 °C, when the target temperatures of the waste heat are unconstrained, 100 and 120 °C respectively, the results are as follows. 3.2.1. When Waste Heat Inlet Temperature Is 150 °C and the Target Temperature Is Unrestricted, the Maximum Available Waste Heat Is 100 × (150 − 55) = 9500 kW. Figure 3 shows the variation of net power output and Qrecovered with evaporation temperature. It is apparent that R236FA gives

Figure 2. Classification of waste heat sources.

this type of waste heat source. For latent heat source, all the working fluids can recover the waste heat totally. There is no difference in the ability to recover waste heat for all the preselected working fluids. Therefore, working fluid selection for latent heat source reduces to screening the working fluid presenting highest thermal efficiency. Therefore, in this paper, we mainly focus on the working fluid selection for the first two types of waste heat sources. In the process of working fluid selection, the condensation temperature and the minimum approach temperature are set as 45 and 10 °C, respectively. Therefore, the minimum attainable temperature of a waste heat stream is 55 °C. The maximum available waste heat is the heat released from the inlet temperature to 55 °C. The isentropic turbine efficiency and the pump efficiency are both assumed as 0.8. The heat capacity flow rate of waste heat source is taken as 100 kW/°C for the sensible waste heat. The results of all the preselected working fluids under different waste heat conditions and evaporation temperatures are simulated in Aspen Plus.

3. WORKING FLUID SELECTION FOR SENSIBLE HEAT SOURCES For a sensible heat source, inlet and target temperatures are most important characteristics of the waste heat source. The effect of inlet and target temperatures on the working fluid selection should be investigated. 3.1. Effect of Waste Heat Inlet Temperature on Working Fluid Selection for Sensible Heat Sources. To

Figure 3. Variation of net power output and waste heat recovered with evaporation temperature (Tin = 150 °C; Ttar, unconstrained). C


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Figure 4. Variation of net power output and waste heat recovered with evaporation temperature (Tin = 150 °C; Ttar = 100 °C).

fluids. R236FA whose critical temperature is lower than 150 °C (Tin) is selected as the optimum working fluids due to its acceptable thermal efficiency and advantageous ability to recover waste heat. 3.2.2. When the Inlet Temperature of Waste Heat Is 150 °C and the Target Temperature Is 100 °C, the Maximum Available Waste Heat Is 100 × (150 − 100) = 5000 kW. Figure 4 shows the variation of net power output and Qrecovered with evaporation temperature. It is apparent that R600A gives the maximum power output. The variation of power output with evaporation temperature for all the working fluids presented two trends. Trend (i): For R227EA and R236FA whose critical temperatures are much lower than the waste heat inlet temperature (150 °C), the power output increases with evaporation temperature. It should be noted that R227EA could recover the waste heat totally under all the evaporation temperatures. Although R227EA can recover the waste heat totally, it cannot evaporate at high temperatures due to the limitation of its critical temperature. Then the thermal efficiency for R227EA is not satisfactory in this case. R236FA can evaporate at 120 °C, and Qrecovered is close to the maximum available heat. Therefore, the power output of R236FA is larger than that of R227EA. Trend (ii): For other working fluids with higher critical temperatures, the net power outputs increase with evaporation temperature to the maximum, after which the power outputs decrease. However, all the preselected working fluids can recover the waste heat totally at low evaporation temperatures, which means that the differences of all the working fluids in the ability to recover waste heat become narrower compared with the first case, in which the target temperature is unconstrained. It is interesting that the optimum evaporation temperature at which the power output reaches maximum is the highest evaporation temperature at which the waste heat can be recovered totally. The reason is that once the evaporation temperature is higher than the maximum temperature at which the waste heat recovered totally, the pinch point between working fluid and waste heat source will limit the amount of waste heat recovered. The amount of waste heat recovered decreases sharply with the increase of waste heat recovered. Among these working fluids, the optimum evaporation temperature of R600A is much higher than the others, which means under the circumstance that each working fluid recovers the waste heat totally, R600A shows the highest thermal

the maximum power output. The variation of power output with evaporation temperature for all the working fluids presented two trends. Trend (i): For R227EA and R236FA whose critical temperatures are much lower than waste heat inlet temperature of 150 °C (i.e., the difference between critical temperature and 150 °C is more than 20 °C), the power output increases with evaporation temperature. It should be noted that R227EA could recover the waste heat totally under all the evaporation temperatures. Therefore, the graph of the variation of waste heat recovered with evaporation temperature is a horizontal line. Although R227EA can recover the waste heat totally, it cannot evaporate at high temperatures due to the limitation of critical temperature. Then the thermal efficiency for R227EA is not satisfactory in this case. Although R236FA cannot recover waste heat totally, it can recover most of the available heat and evaporate at high temperatures. Higher evaporation temperature leads to higher thermal efficiency. R236FA shows an inflection point in Figure 3 of waste heat recovered around 118 °C. The reason is that the latent heat decreases faster in the near critical region higher than 118 °C. The power output of R236FA is larger than that of R227EA. Trend (ii): For other working fluids (R600A and R236EA) whose critical temperatures are close to or higher than 150 °C (Tin), the net power outputs increase first, and after they reach the maximum, they decrease with the evaporation temperature. Qrecovered decreases with the increase of evaporation temperature all the way. Thermal efficiencies increase with the increase of evaporation temperature constantly. Qrecovered and thermal efficiency present two opposite change tendencies with the increase of evaporation temperature. Therefore, there exists a trade-off between Qrecovered and thermal efficiency. Thermal efficiency and Qrecovered determine the optimum evaporation temperature jointly. Since Qrecovered for working fluids whose critical temperature is higher than waste heat inlet temperature is much smaller than that of working fluids whose critical temperature is lower than waste heat inlet temperature, working fluids with critical temperatures higher than 150 °C present lower net power output. Among working fluids whose critical temperature is lower than 150 °C (waste heat inlet temperature), R227EA can recover the waste heat totally, but the thermal efficiency is too low compared with that of R236FA. R236FA presents satisfactory Qrecovered and moderate thermal efficiency. The power output of R236FA is the largest among all the working D


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Figure 5. Variation of net power output and waste heat recovered with evaporation temperature (Tin = 150 °C; Ttar = 120 °C).

Trend (ii): For other working fluids whose critical temperatures are higher, the net power outputs increase first and then decrease after they reach the maximum. These working fluids can recover the waste heat totally except for evaporating at high temperatures close to the waste heat inlet temperature. For each working fluid, the optimum evaporation temperature is still the highest evaporation temperature at which the waste heat can be recovered totally. This is similar to section 3.2.2. The maximum power outputs of R600 (356 kW), R245FA (357 kW), R601A (356 kW), and R601 (359 kW) are almost the same. Among all the optimum evaporation temperatures, R601 evaporating at 120 °C presents the highest thermal efficiency compared with other working fluids evaporating at their respective optimum evaporation temperatures. Therefore, R601 is selected as the optimum working fluid under these waste heat conditions. However, R600, R245FA, and R601A can also be the optimum working fluid as the maximum power outputs of these working fluids are very close to that of R601. To sum up, when the waste heat inlet temperature is fixed at 150 °C and the waste heat target temperatures are unconstrained, 100 and 120 °C, respectively, the corresponding optimum working fluids are R236FA, R600A, and R601. With the increase of the target temperature, the critical temperatures of the optimum working fluids increase. The reason is that with the increase of the target temperature, the differences of all the working fluids in the ability to recover waste heat are lessened. Then thermal efficiency becomes the dominated factor needed to be considered while screening working fluid. When the target temperature is higher than 100 °C, the optimum evaporation temperature for each working fluid is the highest temperature at which the waste heat can be recovered totally. The net power output and Qrecovered decrease sharply beyond the optimum evaporation temperature. This indicates that for a single working fluid, Qrecovered exerts much more influence on the net power output than does thermal efficiency. Therefore, the optimum working fluid tends to be the one with higher critical temperature with the increase of the waste heat target temperature. When the target temperature is higher than 120 °C, the optimum working fluids is always R601. 3.3. Summary of Working Fluid Selection for Sensible Heat Sources. Sensible heat sources are common waste heat source in refineries. For this type of waste heat source, inlet and target temperatures exert great influence on the working fluid

efficiency. Therefore, the power output of R600A is the maximum. Since R227EA cannot evaporate at 120 °C and R236FA cannot recover waste heat totally at 120 °C, R600A is selected as the optimum working fluid under this waste heat conditions. Compared with section 3.2.1, this case indicates that working fluids with higher critical temperature tends to become the optimum working fluid with the increase of target temperature. Although critical temperatures of R227EA, R236FA, R600A, and R236FA are lower than 150 °C, the critical temperatures of R600A and R236EA are more close to 150 °C. Therefore, the amount of waste heat recovered shows different variations with evaporation temperature as shown in Figures 3 and 4. In Figure 3, the waste heat recovered by R227EA is a horizontal line, which demonstrates that the amount of waste heat recovered remains the same under all evaporation temperatures. Therefore, the power output increases with evaporation temperature. The amount of waste heat recovered by R236FA decreases first and then increases slightly with evaporation temperature. For R600A and R236EA, the amount of waste heat recovered decreases with evaporation temperature. As the thermal efficiency increases with evaporation temperature, the power output for R600A and R236FA increases first and then decreases with evaporation temperature. Therefore, the reason why R600A and R236EA present different trends from R227EA and R236FA is that the critical temperatures of R600A and R236EA are close to the inlet temperature of waste heat, and the amounts of waste heat recovered show different trends. 3.2.3. When the Inlet Temperature of Waste Heat Is 150 °C and the Target Temperature Is 120 °C, the Maximum Available Waste Heat Is 100 × (150 − 120) = 3000 kW. Figure 5 shows the variation of the net power output and Qrecovered with evaporation temperature. It is apparent that R601 gives the maximum power output. The variation of the power output with evaporation temperature for all the working fluids presented two trends. Trend (i): For R227EA, R236FA, and R600A whose critical temperatures are lower compared with other working fluids, the power output increases with the increase of evaporation temperature all the way. These three working fluids could recover the waste heat totally under all the evaporation temperatures. Although these three working fluids can recover the waste heat totally, they cannot evaporate at high temperatures due to the limitation of the critical temperature. Their net power outputs are not satisfactory. E


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Industrial & Engineering Chemistry Research selection. On the basis of the above cases, the following conclusions can be obtained. When the target temperature of waste heat is unconstrained, the inlet temperature of the waste heat determines the optimum working fluid. The working fluid whose critical temperature is about 25−35 °C lower than the waste heat inlet temperature has satisfactory waste heat recovering ability and acceptable thermal efficiency. This kind of working fluid gives the maximum net power output under this circumstance. When the target temperature is constrained, the higher the target temperature, the higher the critical temperature of the optimum working fluid. With the increase of waste heat target temperature, the waste heat curve is close to a horizontal line. Under this circumstance, the ability to recover waste heat for all the working fluids converges with the increase of waste heat target temperature. The optimum working fluid tends to be the one with higher critical temperature but depends on the difference of waste heat inlet and target temperature. When the target temperature of the waste heat is higher than 120 °C, the optimum working fluid is R601.

Figure 6. T−H diagram of waste heat source and working fluids evaporating at different temperatures (R = 0.25; Tin = 120 °C; Ttar, unconstrained).

4. WORKING FLUID SELECTION FOR COMBINED HEAT SOURCES Combined heat sources contain both sensible heat and latent heat. The ratio of latent to sensible heat (R) definitely affects the choice of the optimum working fluid. R is the primary factor that needs to be investigated for combined heat sources. When R is small (about less than 0.1), the waste heat source is similar to a sensible heat source. The working fluid selection thermodynamic criterion for sensible source can be applied to this case. When R is large (about greater than 5), the T−H curve of a waste heat source approximates a horizontal line. The working fluid selection is similar to the working fluid selection for latent heat source. When R is moderate, the effect of R on the working fluid selection is investigated as follows. 4.1. Effect of the Ratio of Latent to Sensible Heat on Working Fluid Selection for Combined Heat Sources. To investigate the effect of R on the working fluid selection, the waste heat inlet temperature and target temperature are fixed. When the waste heat target temperature is unconstrained and inlet temperature is 120 °C, working fluid selection with R to be 0.25, 0.5, 1, and 2, respectively, is shown as follows. 4.1.1. When the Inlet Temperature Is 120 °C and the Target Temperature Is Unconstrained, the Maximum Available Sensible Heat Is (120 − 55) × 100 = 6500 kW. When R is 0.25, the latent heat is 6500 × 0.25 = 1625 kW. The maximum available waste heat is 6500 + 1625 = 8125 kW. The red line is the waste heat curve shown in Figure 6. Figure 7 shows the variation of the net power output and Qrecovered with evaporation temperature. It is apparent that R227EA gives the maximum power output. R227EA presents better ability to recover waste heat than other working fluids. It can recover the waste heat totally (8125 kW) at evaporation temperature higher than 95 °C. Qrecovered values of other working fluids are much less than that of R227EA. The T−H diagrams of R227EA evaporating at 100 °C, and R601 evaporating at 100 and 110 °C, are illustrated in Figure 6. When R227EA and R601 both evaporate at 100 °C, R227EA can recover the waste heat totally (8125 kW), and R601 can recover only 3870 kW. When R601 evaporates at 100 and 110 °C, respectively, R601 can recover 3870 kW and 2586 kW correspondingly. It can be concluded that both working fluid and evaporating temperature exert great influence on Qrecovered,

which determines the net power output. Although the thermal efficiency of R227EA is the lowest among all the working fluids, Qrecovered of R227EA is much higher than the other working fluids. The effect of Qrecovered on the net power output is more remarkable than that of thermal efficiency under this circumstance. Therefore, R227EA is selected as the optimum working fluid. 4.1.2. When the Inlet Temperature Is 120 °C and the Target Temperature Is Unconstrained, the Maximum Available Sensible Heat Is (120−55) × 100 = 6500 kW. When R is 0.5, the latent heat is 6500 × 0.5 = 3250 kW. The maximum available waste heat is 6500 + 3250 = 9750 kW. The T−H diagram of the waste heat is the red line shown in Figure 8. Figure 9 shows the variation of the net power output and Qrecovered with evaporation temperature. It is apparent that R236FA gives the maximum power output. Figure 8 illustrates the T−H diagram of R227EA evaporating at 80 and 100 °C, respectively, and R236FA evaporating at 110 °C is also shown in Figure 8. R227EA presents the best ability to recover waste heat, which can recover the waste heat totally (9750 kW) under all the evaporation temperatures. R236FA can recover 8030 kW waste heat evaporating at 110 °C. Although R227EA presents distinct advantage in the ability to recover waste heat, it cannot evaporate at temperatures higher than its critical temperature. Therefore, R227EA cannot attain satisfactory thermal efficiency. However, R236FA can evaporate at 110 °C, which is the maximum attainable evaporation temperature with the inlet temperature of waste heat being 120 °C. While evaporating at 110 °C, R236FA can recover 8030 kW, which is 82.35% of the maximum available waste heat. Thermal efficiency and Qrecovered of R236FA are both satisfactory. Therefore, the power output of R236FA is greater than that of R227EA. Compared with other working fluids with higher critical temperatures, R236FA can recover much more waste heat. The T−H diagrams of other working fluids with higher critical temperature are omitted for the sake of clarity. R236FA is selected as the optimum working fluid under this waste heat condition. 4.1.3. When the Inlet Temperature Is 120 °C and the Target Temperature Is Unconstrained, the Maximum F


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Figure 7. Variation of net power output and waste heat recovered with evaporation temperature (R = 0.25; Tin = 120 °C; Ttar, unconstrained).

Figure 10. T−H diagram of waste heat source and working fluids evaporating at different temperatures (R = 1; Tin = 120 °C; Ttar, unconstrained).


Figure 11 shows the variation of the net power output and Qrecovered with evaporation temperature. It is apparent that R600A gives the maximum power output. The variations of the net power output increase with the increase of evaporation temperature for all the working fluids. R227EA, R236FA, and

Available Sensible Heat Is (120−55) × 100 = 6500 kW. When R is 1, the latent heat is 6500 × 1 = 6500 kW. The maximum available waste heat is 6500 + 6500 = 13000 kW. The T−H diagram of the waste heat is the red line shown in Figure 10.

Figure 9. Variation of net power output and waste heat recovered with evaporation temperature (R = 0.5; Tin = 120 °C; Ttar, unconstrained). G


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Figure 11. Variation of net power output and waste heat recovered with evaporation temperature (R = 1; Tin = 120 °C; Ttar, unconstrained).

R600A can recover the waste heat totally under all the evaporation temperatures. Since R600A shows the highest thermal efficiency, R600A gives the maximum power output among these three working fluids. R236EA can recover the waste heat completely at all the evaporation temperatures except for 110 °C. The power output of R236EA is almost the same as that of R600A. For other working fluids, Qrecovered values are much less than that of R600A, and so the net power output is not satisfactory. The T−H diagrams of R227EA evaporating at 100 °C, R600A evaporating at 110 °C, and R601 evaporating at 110 °C are shown in Figure 10. Although R227EA can recover the waste heat totally, R227EA cannot evaporate at 110 °C due to the limitation of critical temperature. The thermal efficiency of R227EA evaporating at 100 °C is less than that of R600A evaporating at 110 °C. Although the thermal efficiency of R601 evaporating at 110 °C is higher than that of R600A evaporating at 110 °C, Qrecovered is only 10345 kW, which is much less than that of R600A. R600A recovers the waste heat totally at 110 °C, and the corresponding thermal efficiency is satisfactory. Therefore, R600A is selected as the optimum working fluid under this waste heat condition. 4.1.4. When the Inlet Temperature Is 120 °C and the Target Temperature Is Unconstrained, the Maximum Available Sensible Heat Is (120−55) × 100 = 6500 kW. When R is 2, the latent heat is 6500 × 3 = 13 000 kW. The maximum available waste heat is 6500 + 13000 = 19 500 kW. The T−H diagram of the waste heat is the red line shown in Figure 12. Figure 13 shows the variation of the net power output and Qrecovered with evaporation temperature. It is apparent that R601 gives the maximum power output. The power output increases with evaporation temperature for all the working fluids. All the working fluids can recover the waste heat totally under all the evaporation temperatures. There is no difference in the ability to recover waste heat for all the working fluids under this waste heat condition. Since R601 shows the maximum thermal efficiency among all the working fluids, the net power output of R601 is the maximum. With R being 0.25, 0.5, 1, and 2, respectively, the optimum working fluids are R227EA, R236FA, R600A, and R601 correspondingly. The critical temperature of the optimum working fluid increases with the increase of R. When R is small, there exist obvious differences in Qrecovered among all the working fluids. That means all the working fluids show obvious

Figure 12. T−H diagram of waste heat source and working fluids evaporating at different temperatures (R = 2; Tin = 120 °C; Ttar, unconstrained).

differences in their ability to recover waste heat. However, when R increases, the difference of Qrecovered for all the working fluids reduces. Take the case in section 4.1.4 for example, when R is 2, all the working fluids can recover the waste heat totally. There is no difference in Qrecovered for all the working fluids. If R is greater than 2 for the above case, it is definitely that the optimum working fluid is still R601. Therefore, there exists a specific ratio, beyond which all the working fluid can recover the waste heat totally. When R is less than 2 in the above case, not all the working fluids can recover the waste heat totally. There exists a difference in the ability to recover waste heat. Therefore, R is a primary factor that affects the working fluid selection for combined heat source. If R is greater than a specific value, there is no difference in Qrecovered for all the working fluids. Thus, only the thermal efficiency affects net power output. We can select the one with maximum critical temperature as optimum working fluid. If R is within a certain range, the thermal efficiency and Qrecovered should be taken into consideration simultaneously. Under this circumstance, the inlet temperature and target temperature of a waste heat source may have influence on the working fluid selection. 4.2. Effect of Inlet Temperature of Waste Heat on Working Fluid Selection for Combined Heat Sources. In this part, we will explore the effect of the waste heat inlet temperature on the working fluid selection. To investigate the effect of the inlet temperature on the working fluid selection, R and the target temperature should be fixed. When the target H


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Figure 13. Variation of net power output and waste heat recovered with evaporation temperature (R = 2; Tin = 120 °C; Ttar, unconstrained).

temperature of the waste heat is unconstrained and R is set as 0.5, working fluid selection results under the inlet temperature 120, 150, and 190 °C are as follows. 4.2.1. When the Inlet Temperature Is 120 °C and the Target Temperature Is Unconstrained, the Maximum Available Sensible Heat Is (120 − 55) × 100 = 6500 kW. Since R is fixed at 0.5, the latent heat is 6500 × 0.5 = 3250 kW. This is just the case in section 4.1.2. The optimum working fluid is R236FA. 4.2.2. When the Inlet Temperature Is 150 °C and the Target Temperature Is Unconstrained, the Maximum Available Sensible Heat Is (150 − 55) × 100 = 9500 kW. Since R is fixed at 0.5, the latent heat is 9500 × 0.5 = 4750 kW. The maximum available waste heat is 9500 + 4750 = 14 250 kW. The T−H diagram of waste heat is the red line as shown in Figure 14.

Figure 15 shows the variation of the net power output and Qrecovered with the evaporation temperature. It is apparent that R600 gives the maximum net power output. The T−H diagrams of R600A evaporating at 130 °C, R600 evaporating at 140 °C, and R601A evaporating at 140 °C are shown in Figure 14. Although R600A can recover the waste heat totally at 130 °C, it cannot evaporate at 140 °C due to the limitation of the critical temperature. The thermal efficiency of R600A evaporating at 130 °C is less than that of R600 evaporating at 140 °C. Although the thermal efficiency of R601A evaporating at 140 °C is higher than that of R600 evaporating at 140 °C, Qrecovered is only 10 349 kW, which is much less than that of R600. R600 evaporating at 140 °C can recover 14 126 kW waste heat, accounting for 99.1% of the maximum available waste heat, and the corresponding thermal efficiency is acceptable. Therefore, R600 gives the maximum power output. 4.2.3. When the Inlet Temperature Is 190 °C and the Target Temperature Is Unconstrained, the Maximum Sensible Heat Is (190−55) × 100 = 13500 kW. Since R is fixed at 0.5, the latent heat is 13 500 × 0.5 = 6750 kW. The maximum available waste heat is 13 500 + 6750 = 20 250 kW. The T−H diagram of the waste heat is the red line shown in Figure 16. Figure 17 shows the variation of the net power output and Qrecovered with evaporation temperature. It is apparent that R601 gives the maximum power output. The net power output increases with evaporation temperature for all the working fluids. All the working fluids can recover the waste heat totally (20 250 kW). There is no difference in the waste heat recovering ability under this waste heat conditions. Now the working fluid selection is narrowed to selecting the working fluid with maximum thermal efficiency. The T−H diagram of R227EA evaporating at 100 °C, R245FA evaporating at 150 °C, and R601 evaporating at 180 °C is drawn in Figure 16. R227EA and R245FA cannot evaporate at high temperatures due to the limitation of the critical temperatures. It is clear that all the working fluids can recover the waste heat totally under all attainable evaporation temperatures. Under this circumstance, the critical temperature determines the thermal efficiency, which is the sole factor influencing the net power output. Then we should select the working fluid that can reach the maximum thermal efficiency. Since R601 can evaporate at the maximum attainable evaporation temperature (180 °C), it shows the maximum thermal efficiency. R601 is the optimum working fluid under this waste heat condition.

Figure 14. T−H diagram of waste heat source and working fluids evaporating at different temperatures (R = 0.5; Tin = 150 °C; Ttar, unconstrained). I


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Figure 15. Variation of net power output and waste heat recovered with evaporation temperature (R = 0.5; Tin = 150 °C; Ttar, unconstrained).

The above results are similar to the case in section 4.1.4. In the case of 4.1.4, the waste heat inlet temperature is 120 °C and R is 2. All the working fluids can recover the waste heat totally, and there is no difference in waste heat recovering ability in that case. Compared with the above case, it can be concluded that waste heat inlet temperature has influence on the minimum ratio of latent to sensible heat, beyond which all the working fluids can recover the waste heat totally, and the working fluid selection reduces to selecting the one with highest critical temperature. The minimum ratios of latent to sensible heat under various inlet temperatures are obtained with the above method with many cases as shown in Table 2. It is clear that the higher the inlet temperature of the waste heat, the smaller the minimum R. When the target temperature is unconstrained and R is 0.5, with the waste heat inlet temperatures being 120, 150, and 190 °C, respectively, the optimum working fluids are R236FA, R600, and R601 correspondingly. The critical temperatures of optimum working fluids increase with the increase of waste heat inlet temperature. It can be seen that the critical temperature of the optimum working fluid may be higher than the waste heat inlet temperature. With a specified waste heat inlet temperature, there exists a minimum ratio, beyond which all the working fluids can recover the waste heat totally, and the working fluid selection reduces to select the one with the maximum critical temperature. When the target temperature


Figure 17. Variation of net power output and waste heat recovered with evaporation temperature (R = 0.5; Tin = 190 °C; Ttar, unconstrained). J


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Industrial & Engineering Chemistry Research Table 2. Minimum R under Different Waste Heat Inlet Temperatures (Ttar, Unconstrained) Tin (°C)

100

110

120

130

140

150

160

170

180

190

minimum R

2.3

2

1.7

1.4

1

0.8

0.7

0.6

0.5

0.4

140 °C. The net power output of R600 evaporating at 140 °C is much larger than that at 130 °C. R601 can evaporate at 140 °C, and the thermal efficiency is higher than that of R600, but Qrecovered of R601 is only 7215 kW, which is much less than that of R600. Therefore, R600A shows advantages in both thermal efficiency and Qrecovered, and the net power output of R600A is the maximum. 4.3.3. When the Inlet Temperature Is 150 °C and the Target Temperature Is 110 °C, the Maximum Sensible Heat Is (150 − 110) × 100 = 4000 kW. Since R is fixed at 0.5, the latent heat is 4000 × 0.5 = 2000 kW. The maximum available waste heat can be 4000 + 2000 = 6000 kW. The T−H diagram of the waste heat is the red line shown in Figure 20. Figure 21 shows the variation of the net power output and Qrecovered with evaporation temperature. It is apparent that R600 gives the maximum power output. The variation of power output with evaporation temperature is similar to that in section 4.3.2. The optimum working fluid is still R600. To sum up, when Tin and R are fixed, the optimum working fluids are R600 under various target temperatures. With the variation of waste heat target temperature, the ability to recover waste heat does not vary dramatically between all the working fluids. Therefore, the optimum working fluid is R600 under all the target temperatures. Therefore, compared with Tin and R, the target temperature of the waste heat has less influence on the working fluid selection. 4.4. Summary of Working Fluid Selection for Combined Waste Heat Sources. For combined waste heat sources, R is the most important parameter affecting working fluid selection. At specified inlet and target temperatures, there exist a minimum R, beyond which all the working fluids can recover the waste heat totally, and the working fluid with the maximum critical temperature is designated as the optimum working fluid. When R is less than the minimum ratio, thermal efficiency and Qrecovered should be taken into consideration simultaneously in working fluid selection. The minimum R is associated with the inlet temperature of the waste heat. The minimum R under various inlet temperatures is calculated in this paper. With fixed R, inlet temperature of waste heat has great effect on the working fluid selection, and the target temperature has little effect on the working fluid selection.

fixed at 55 °C, the minimum ratios are obtained under various waste heat inlet temperatures as shown in Table 2. 4.3. Effect of Target Temperature of Waste Heat on Working Fluid Selection for Combined Heat Sources. To investigate the effect of the target temperature of waste heat on working fluid selection, R and the inlet temperature will be fixed. The inlet temperature of the waste heat is set as 150 °C and R is set as 0.5, which is less than the minimum ratio under waste heat inlet temperature 150 °C. Working fluid selection results with the target temperatures of 55, 80, and 110 °C are as follows. 4.3.1. When the Inlet Temperature Is 150 °C and the Target Temperature Is 55 °C, the Maximum Available Sensible Heat Is (150 − 55) × 100 = 9500 kW. Since R is fixed at 0.5, the latent heat is 9500 × 0.5 = 4750 kW. The maximum available waste heat is 9500 + 4750 = 14 250 kW. This is just the case in section 4.2.2. The optimum working fluid is R600. 4.3.2. When the Inlet Temperature Is 150 °C and the Target Temperature Is 80 °C, the Maximum Available Sensible Heat Is (150 − 80) × 100 = 7000 kW. Since R is fixed at 0.5, the latent heat is 7000 × 0.5 = 3500 kW. The maximum available waste heat is 7000 + 3500 = 10 500 kW. The T−H diagram of the waste heat is the red line shown in Figure 18.

5. WORKING FLUID SELECTION FOR LATENT HEAT SOURCES For latent heat source, the heat transfer behavior between the working fluid and waste heat is not a problem. The waste heat can be recovered totally by any working fluid. There is no difference in the ability to recover waste heat for all the working fluids. Therefore, the working fluid selection is narrowed to selecting the working fluid with a maximum thermal efficiency, which means selecting the working fluid with the highest critical temperature. Among all the preselected working fluids, R601 is the optimum working fluid for a latent heat source.

Figure 18. T−H diagram of waste heat source and working fluids evaporating at different temperatures (R = 0.5; Tin = 150 °C; Ttar = 80 °C).

Figure 19 shows the variation of the net power output and Qrecovered with evaporation temperature. It is apparent that R600 gives the maximum power output. The T−H diagrams of R600 evaporating at 130 and 140 °C, and R601 evaporating at 140 °C are drawn in Figure 18. R600 can evaporate at 140 °C, that is, the maximum attainable evaporating temperature with Qrecovered being 10 409 kW, which is very close to the maximum available waste heat. Although R600 can recover waste heat totally at 130 °C, the thermal efficiency is much less than that at

6. CONCLUSION Since the waste heat sources in refineries are more complex than the conventional low temperature heat source, it is necessary to consider the characteristics of a waste heat source K


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Figure 19. Variation of net power output and waste heat recovered with evaporation temperature (R = 0.5; Tin = 150 °C; Ttar = 80 °C).

(1) For a sensible heat source, the following conclusions are drawn. (a) When the waste heat target temperature is unconstrained, the working fluid whose critical temperature is lower than the waste heat inlet temperature 25− 35 °C is the optimum working fluid. (b) When the target temperature is constrained, the higher the target temperature, the higher the critical temperature of the optimum working fluid. When the target temperature of the waste heat source is higher than 120 °C, R601 is selected as the optimum working fluid. When the target temperature is less than 120 °C, the critical temperature of the optimum working fluid may be above or below the waste heat inlet temperature, which depends on the difference between the inlet and target temperatures. (2) For a combined waste heat source, the following conclusions are drawn. (a) R is the most important parameter affecting the working fluid selection for combined waste heat sources. When R is small (about less than 0.1), the working fluid selection is similar to that for sensible heat sources. When R is large (about greater than 5), the working fluid selection is similar to that for latent heat sources. (b) When inlet and target temperatures are fixed and R is within a certain range, the optimum working fluid tends to be the one with higher critical temperature with the increase of R. (c) When the target temperature is unconstrained, the minimum R,

Figure 20. T−H diagram of waste heat source and working fluids evaporating at different temperatures (R = 0.5; Tin = 150 °C; Ttar = 110 °C).

while selecting a working fluid. In this paper, working fluid selection is studied for waste heat sources classified into three categories, that is, sensible, combined, and latent heat sources.

Figure 21. Variation of net power output and waste heat recovered with evaporation temperature (R = 0.5; Tin = 150 °C; Ttar = 110 °C). L


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Industrial & Engineering Chemistry Research beyond which all the working fluids can recover the waste heat totally, is obtained under various waste heat inlet temperatures. When the ratio of latent to sensible heat is greater than the minimum R, working fluid selection reduces to select the one with the maximum thermal efficiency. R601 is selected as the optimum working fluid in this case. (d) When the target temperature and R are fixed, the inlet temperature exerts great influence on the working fluid selection. The higher the waste heat inlet temperature, the higher the critical temperature of the optimum working fluid. (e) When the inlet temperature and R are fixed, the target temperature of waste heat has little effect on working fluid selection. (3) For a latent heat source, there is no difference in the ability to recover waste heat for all the working fluids, that is, all the working fluids can recover the waste heat completely. Therefore, the working fluid selection is narrowed to select the working fluid with the maximum thermal efficiency. Among the preselected working fluids in this paper, R601 presents the highest thermal efficiency.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China under Grant No. 21576286 and SINOPEC under Grant No. 313109 is gratefully acknowledged.

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NOMENCLATURE H = enthalpy ORC = organic Rankine cycle ODP = ozone depletion potential Pc = critical pressure Qrecovered = the amount of waste heat recovered R = the ratio of latent to sensible heat S = entropy Tc = critical temperature T = temperature Tin = waste heat inlet temperature Ttar = waste heat target temperature

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REFERENCES

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