LNG vaporizers using various refrigerants as intermediate fluid: Comparison of the required heat transfer area
Introduction
The liquefication of gas for large quantity transportation and thereafter re-gasification of cryogenic fluid at terminals has been widely utilized in the supply of liquefied natural gas (LNG), oxygen, nitrogen and carbon dioxide, etc. Due to the global energy crisis and environmental deterioration, the LNG industry is booming worldwide. There has been intensive research interest in the process and equipment development concerning the feed gas conditioning (Wen et al., 2011, Wen et al., 2012, Yang et al., 2014), natural gas liquid recovery (He and Ju, 2014, Vatani et al., 2013, Wang et al., 2013) and liquefication (Alabdulkarem et al., 2011, Castillo and Dorao, 2013, Hatcher et al., 2012) in the LNG production stage, as well as the vaporization (Gavelli, 2010, Kataoka et al., 1973, Koichi et al., 1992) and cold energy recovery (Liu and Guo, 2011, Lu and W, 2009, Shi and Che, 2009) in the LNG re-gasification stage. As for the LNG re-gasification process, various vaporizers have been developed, including the open-rack vaporizer, submerged combustion vaporizer, ambient air vaporizer and intermediate fluid vaporizer (IFV), etc.
An IFV is typically a compact shell-and-tube heat exchanger with three parts, namely an evaporator to vaporize the intermediate fluid (IF) by a heat source, a condenser to release the latent heat of IF to LNG, and a thermolator to heat the natural gas (NG) to the specified temperature before use. The evaporator and condenser of an IFV are often arranged in one single, large shell, while the thermolator is installed either on the said shell or independent of it (Iwasaki and Asada, 2002, Iwasaki et al., 2000, Yamamoto et al., 2002). The heat source fluid and LNG flow in the tube sides of the evaporator and condenser, respectively. The boiling and condensation of IF take place in the shell side, transferring the heat from source fluid indirectly to LNG.
The conventional heat source fluid in an IFV is seawater. While warm water or an aqueous solution of glycol can be alternative choice in the place where seawater cannot be used from the standpoint of environmental protection, or in the case where a cold energy recovery system is combined (Iwasaki and Asada, 2002). As for the IF, some major requirements include moderate saturation temperature, large latent heat and low viscosity (Iwasaki and Asada, 2002, Liu et al., 2013, Pu et al., 2014, Yamamoto et al., 2002).
The advantages of the IFV over the other types of LNG vaporizers include (Dendy and Nanda, 2008, Liu et al., 2013, Patel et al., 2013, Pu et al., 2014): (a) better adaptability than the open-rack vaporizer, i.e. no icing problem and low requirement for seawater quality; (b) better energy efficiency than the submerged combustion vaporizer, i.e. no additional consumption of NG for combustion and low carbon emission; and (c) better robustness than the ambient air vaporizer, i.e. no frosting problem and less sensitivity to ambient condition. Besides, the IFV is superior for the floating storage and re-gasification unit and the cold energy recovery system due to its compact volume and indirect heat transfer. As a result, investigations on this new kind of vaporizer have been growing fast recently (Bai et al., 2013, Chen and Chen, 2010, Iwasaki and Asada, 2002, Iwasaki et al., 2000, Liu et al., 2013, Pu et al., 2014).
Due to the highly integrated configuration, the design of an IFV is more complex than conventional shell-and-tube heat exchangers. By applying energy balance and determining the heat transfer rate with heat transfer coefficient (HTC) correlation, Bai et al. (2013) proposed a method for calculating the heat transfer area of an IFV. The distributed parameter model (DPM) was utilized by dividing each section into elements, in order to account for the effects caused by the large temperature difference (Pacio and Dorao, 2011). The Cooper correlation (Cooper, 1984) and the modified Nusselt formula (Jung et al., 2004a) were adopted respectively for the boiling and condensation HTC predictions of propane. Using the similar thermal model, Pu et al. (2014) published their recent work on analysing the performance of an IFV under different process variables, including the temperature and mass flow rate of inlet seawater, and the pressure and mass flow rate of inlet LNG.
The selection of IF and its saturation condition can influence significantly the HTC and thereafter the required heat transfer area (HTA) of an IFV system. However, the IFs in the reported applications are predominantly limited to propane (Bai et al., 2013, Iwasaki and Asada, 2002, Liu et al., 2013, Pu et al., 2014). Unfortunately, the influences of the IF and its saturation condition on the IFV performance have been so far rarely reported in the open literature.
Considering the major requirements for IFs, it is proposed that some hydrocarbons (e.g., propane, butane or mixed refrigerants) may be used in an IFV (Patel et al., 2013, Sohn et al., 2012). As a matter of fact, various hydrocarbons have recently been accepted or proposed for use in refrigeration, due to their advantages of low cost, availability and environmental friendliness. These include propylene, propane, isobutane, butane, and dimethylether (DME), etc (Devotta et al., 1997, Fine, 1997, IEA's Heat Pump Center, 2002, Jung et al., 1999, Kruse, 1996, Li, 2011).
In this paper, numerical calculations have been conducted to compare the required heat transfer areas of the IFVs utilizing the candidate refrigerants of propylene, propane, isobutane, butane and DME, and operating under the saturation temperatures from 263.15 to 268.15 K. The limiting heat transfer steps have been identified for the evaporator and condenser when utilizing different refrigerants, in order to provide some guidance for potential process intensification. The results reported here are of help for assessing the applicability of a refrigerant and its saturation condition in an IFV at the design phase.
Section snippets
Physical model
The undergoing heat transfer process in a typical IFV is schematically shown in Fig. 1. The inlet mass flow rate, pressure and temperature of seawater to the thermolator were defined as msw, Psw, and Tsw1. The outlet temperatures of seawater from the thermolator and evaporator were denoted as Tsw2 and Tsw3, respectively. On the other hand, the inlet mass flow rate, pressure and temperature of LNG to the condenser were expressed as mng, Png, and Tng1, respectively. The outlet temperature of LNG
Solution procedure
The solution procedure is schematically shown in Fig. 2. After inputting the design specifications such as mng, Png, Tng1, Tng3 and Tsw1, and the selected parameters including IF, Tsat, Psw, Tsw2 and Tsw3, the unknown process parameters of msw and Tng2 were determined by the energy balance of the whole IFV system and that of the thermolator. Once the inlet and outlet temperatures of seawater and LNG/NG in each section were obtained, the three heat exchangers (i.e. the thermolator, evaporator
Results and discussion
For the LNG re-gasification task listed in Table 1, the calculated mass flow rate of seawater msw, LNG outlet temperature from the condenser Tng2 and required HTA of the thermolator Ath are 2986.25 kg s−1, 216.37 K and 671.52 m2, respectively. Since these results are not influenced by the IF type and its saturation condition, they will not be further discussed.
Conclusions
Numerical calculations have been conducted to compare the required HTAs of the IFVs utilizing the candidate refrigerants of propylene, propane, isobutane, butane and DME, and operating under their corresponding saturation temperatures from 263.15 to 268.15 K. The results show that the HTA of the evaporator increases while that of the condenser decreases with the saturation temperature. The limiting heat transfer steps are identified as the external boiling of DME, isobutane and butane in the
Acknowledgement
The authors gratefully acknowledge the financial support from National Natural Science Foundation of China under the grant number of 21406046.
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