Li, Cheng

Li, Junming

Wang, Buxuan

Xie, Wenxin

Zhongyi, Beijing

English

Purdue E-Pubs

Purdue UniversityPurdue e-PubsInternational Refrigeration and Air ConditioningConference School of Mechanical Engineering2008Theoretical Investigation on CondensingCharacteristics of Air and Oil Vapor MixturesCheng LiTsinghua UniversityJunming LiTsinghua UniversityBuxuan WangTsinghua UniversityWenxin XieShengye Co.Beijing ZhongyiShengye Co.Follow this and additional works at: http://docs.lib.purdue.edu/iraccThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.htmlLi, Cheng; Li, Junming; Wang, Buxuan; Xie, Wenxin; and Zhongyi, Beijing, "Theoretical Investigation on Condensing Characteristicsof Air and Oil Vapor Mixtures" (2008). International Refrigeration and Air Conditioning Conference. Paper 907.http://docs.lib.purdue.edu/iracc/907 2212, Page 1 International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008 THEORETICAL INVESTIGATION ON CONDENSING CHARACTERISTICS OF AIR AND OIL VAPOR MIXTURES Cheng Li?   Jun-Ming Li?*   Bu-Xuan Wang?   Wen-Xin Xie2?Key Laboratory for Thermal Science and Power Engineering of Ministry of Education  Department of Thermal Engineering, Tsinghua University Beijing 100084, China Beijing Zhongyi Shengye Co Ltd, Beijing 150078, China *Tel: +8610-62771001, Fax: +8610-62770209   Email: lijm@tsinghua.edu.cnABSTRACT The condensation process of the volatilized oil-vapor is calculated for certain given constitutes of air and oil-vapor mixtures by using finite difference method. The results indicated that there are three distinct temperature regions in the whole cooling process; each corresponds with special refrigeration processes. The temperature region from 308.15 K to 276.15 K is the prepared cooling for removing the water vapor by condensation which will effects the later condensing of the oil-vapor, the temperature region from 276.15 K to 238.15 K is the condensation process of high carbon organic substances, and the temperature region from 238.15 K to198.15 K is the condensation region for relative low carbon organic substances. According to the calculation results, some key problems were discussed in the condensing recovery apparatus and system design.  Key Words: oil-vapor, condensation recovery, convective heat transfer INTRODUCTION Refrigeration condensation of volatilized oil-vapor is one method of oil vapor recovery (Zhao et al. 2001; Li et al.2006; Tang et al. 2007). Such a method includes a complex condensation process with mixtures of several kinds of condensable vapors and mixtures of dry air and water vapor. The volatilized oil-vapors are harmful to the environment (Wang, 2007; Chen, 2007). Besides, they badly affect qualities of oil and gasoline and induce danger of fire and blast. The condensable oil vapors are main light alkyls and alkenes. The number of carbon atoms in such a molecule is among 4~6. In air and oil-vapor mixtures, factors of light hydrocarbons with different partial pressure and physical characteristics, such as saturated pressure, saturated temperature and triple points, are destined to complex condensing processes. Condensation of water vapor(Kang and Kim, 1994), which plays a main role in the primary cooling process before temperature decreases to water freezing point, should be non a neglectable factor, because of its relative humidity of about 60%. And pre-removing of water vapor is a necessary step for the refrigeration used for oil vapor recovery. Condensation of light hydrocarbons is a typical multi-mixtures process with the feasibility of Marangoni condensation for small fractions of kinds of hydrocarbon vapors and their different boiling points. And the immiscible solution, because of the existence of water liquid, leads to the possibility of dropwise condensation. Complex heat transfer and enhancement mechanism have been discussed (Ma et al, 2008), while few investigations aimed at the air and oil-vapor mixtures condensation (Peng, 2000). The purpose of this paper is to discuss condensation process of the volatilized oil-vapor, in view of theoretical investigation. CALCULATION METHOD The petroleum or oil constitutes of the complicated organic hydrocarbons. Every hydrocarbon holds different fraction, which is attributed to exploitation location in the world and the exploiting stage. And the volatized oil  2212, Page 2 International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008 components could be different relative to the local temperature, measurement to sun radiation and value of octane for gasoline. One case shows in table 1. Table 1.volume fraction of a certain air and oil vapor mixtures component 4Cn ? 4Ci ? 4Cc ? ?? 4Cn ?? 4Ci 5Cn ? 5Ci ? 6Cn ? hydrocarbons fractionvol (%) 2.420 2.894 3.724 2.944 3.24 0.434 6.156 6.884 28.696 The oil vapor is actually a mixture consisted of kinds hydrocarbons, the reason only considering certain components is that the revelation from oil refineries and gasoline indicates that only parts of the hydrocarbons can volatilize easily. Date measured from (Peng, 2000) declares that 34.3vol% hydrocarbons components exist in air and oil-vapor mixtures in LuoYang Petroleum and Chemical Plant. And the main components are 4C ? 5C  and 6C . Three Parameters Method steam pressure equation (Hatsopoulous and Keenan, 1891) is adopted and verified by the date from software NIST as following )(Tf)(Tfpln r(1)r(0)r ???                                                                                  (1) ?here                        6rrr)0( 169347T.028862lnT.1T09648.692714.5f ????                                 6rrr)1( 43577T.0472lnT.13T6875.152518.15f ????   )43577.04721ln.1315.6875-(15.2518)1697347.028862ln.109648.692714.5lnp( 6-161c ??????? ???????? ?The ?  values and eccentricity factor ? for calculated hydrocarbons showed in table 2. Table2.values of ?  parameter and eccentricity factor component  ? ? component ? ?4Cn ? 0.6413 0.1966 ?? 4Ci 0.6320 0.1898 4Ci ? 0.6405 0.1811 5Cn ? 0.6584 0.2492 4Cc ? 0.6210 0.1833 5Ci ? 0.6538 0.2260 ?? 4Cn 0.6400 0.2429 6Cn ? 0.6737 0.3008 Software Nist employed to validate the Three Parameters Method is plotted in Figure 1. The calculated and compared component is butane. Results indicate that departure of the calculated date by Three Parameter Method is no more than 2% from the Nist one, so that the method can be acceptable. 140 160 180 200 220 240 260 280 300 320050000100000150000200000250000300000 Three ParameterNIST datumsaturated pressure(Pa)temperature(K)Figure 1: Saturated temperature and pressure relation of butane steam  2212, Page 3 International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008 The gas and oil-vapor mixtures listed in table 1 are considered as ideal gas mixtures. So every component in the gas phase satisfies the ideal gas state equation iii RTnVp ?                                                                                               (2) which can be supported by data from Table 4, with the calculating temperature 293.15 K. Compressibility factors of components in Table 2 are more close to 1, which indicates that the gas and oil-vapor mixtures can be considered as ideal gas mixtures. Especially, volume fractions of components listed in Table 1 is less than 8%, which means every hydrocarbon shares no more than 8% of the total pressure. Therefore, the ideal gas mixture assumption is reasonable.   Table 2 compressibility factors (Z) of hydrocarbons in 20 degrees centigrade and 1 atm. component methane ethane propane butane pentane hexane Z 0.9981 0.9920 0.9834 0.9682 0.9450 0.9190 The condensation of one component did not effected by others for ideal gas mixtures, so that when analyzing a certain component hydrocarbon condensation, the others could be treated as a certain ideal gas, the state equations of the analyzed component in two different points would be as 1111 RTnVp ?2222 RTnVp ?When temperature drop is tiny enough in the cooling process so that there is a little difference between 2T  and 1T ,considering 2T  equals 1T , the error can be accepted. So the condensation quantity can be easily expressed as follows with the overall pressure p constant: )(2221121 ppppppRTVnnn ????????                                                                      (3) Where, n?  is the condensation quantity of the analyzed component with its saturated partial pressure. And the assumption above can clearly be true if the temperature drop is small enough. Therefore, in any temperature drop from the above three intersected temperature intervals, a certain component condensation can be calculated accordingly, with the saturated temperature judged by the Three Parameters Method state equation described above. CALCULATION RESULTS AND DISCUSSION According to equation (3), the condensation process is plotted in Figure 2 for the case showed in Table 2 with calculating temperature step 0.5  in every step and the condensation of water vapor ? is not included. It is seen from Figure 2 that only 6Cn ? condenses in the first intervals with temperature drop from 276.15 K to 238.15 K. Thereasons are 6Cn?  has a relative high boiling point and the high fraction in the mixture. In fact, more than 80% water vapor condensed when the air and oil-vapor mixtures reach 276.15 K. So in an oil-vapors recovery system, the first interval is important. Because the water condensate could solidified to additive on the heat exchanger surfaces, it will affect the process of heat transfer. Therefore, the pre-cooling is important, which is the so called first region. When the temperature decreases from 276.15 K to 238.15 K, nearly 80% 6Cn ? has changed to liquid and 5Ci?condenses a lot also. But other components just begin condensation. In this temperature drop region, the remnant water vapor condenses almost and become frost, which brings lots troubles to the actual heat exchangers and the ability to be provided with the frost remove measurement should be considered further. All the hydrocarbon components happen to condense in the third region, i.e. temperature drop from 238.15 K to 198.15 K. There exists a large phase-change heat load in the third region, which is great consumption to the refrigerating load as cost will be more if the same heat load stands at much lower temperature point than the environment one. In this interval, more than 80% hydrocarbon components can be effectively reclaimed. And almost 99% components are reclaimed, if the lowest cooling temperature reaches minus 90 ?.In the whole condensation process, the condensation rate could be quite different. The temperature interval between 233 K~190 K shares a high condensation rate for hydrocarbons 4C and 5C . Every hydrocarbon condenses faster at its condensation beginning. Oil-vapors recovery efficiency is 82%, when setting the lowest temperature as 198.15. and 96% can obtain, when the lowest temperature is 183.15K.  2212, Page 4 International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008 Figure 2(a):  Relations of cooling temperature and component condensation fractionFigure 2(b):  Relations of cooling temperature and component condensation fraction According to the calculation results, the important factors should be considered for the design of an oil vapor recovery refrigeration system as following. The first, pre-cooling should be adapted to gas and oil-vapor mixtures so as to minimize the energy consumption.  The second, the third region refrigerating machine occupies the position with characteristics of relative low temperature (lower than the terminal temperature of the treated gas mixture) and relative big refrigerating loads, which is contributed to the main hydrocarbon phase-change course. If the overall refrigerating machines are self-governed three cycles, the third intervals power consumption of compressor will be amazed big, due to the relative high environment temperature compared to that of the refrigerant temperature in the entry to the compressor. Such a big potential difference for heat transfer is not proposed. And huge power consumption in compressor is at the expense of potential difference. To solve the problem, the cascaded refrigeration cycle is advised in the second and the third parts, which means refrigeration load of the second cycle respectively serve the second class refrigeration load and the third class condenser. Table 4 below gives contrast between the unattached cycle and the cascaded refrigeration one. It is clear that the cascaded refrigeration cycle can obviously improve the overall refrigeration C??for our problem. Table 4 Cop comparison between unattached and overlap coupled refrigerating system case unattached cascaded refrigeration one and export air pre-cooling C?? 0.604 1.56 And the third, the tube and shell heat exchanger is prevalently advised (e.g. Song, 2007; Tan, 2008). For the finned tube heat exchanger, to avoid the relative bad heat transfer capacity of gas-gas heat exchanger, secondary refrigerant can be a considerable scheme in the heat load transport between gas and oil-vapor mixtures and refrigerant. Therefore, the gas velocity could not be big enough, which further worsens the heat transfer coefficient of gas mixtures. So heat transfer enhancement and optimization could be very important. CONCLUSIONS  2212, Page 5 International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008 Gas and oil-vapor mixtures condensing with the purpose of recovery oil-vapor is a very complex process. This paper pays main attention to the condensation recovery of kinds of hydrocarbon components and their relations with temperature points. Every component in the gas mixture, including the condensable gas and the non-condensable, is a resistance to other components diffusion. Based on the calculation results, some conclusions could be obtained as followings. ? Condensation rate in different temperature point could be quite different, with the relative low temperature region existing big condensation rate for every condensable component and a large phase-change heat load. ? Dehumidification process from 308.15 K to 276.15 K is important and to decrease the frost effects to next further refrigerating process. And The relative high boiling point and the high fractions (such as 6C in present work) condensate in the first region. ? The temperature region from 276.15 K to 238.15 K is the condensation process of high carbon organic substances, and the temperature region from 238.15 K to198.15 K is the condensation region for relative low carbon organic substances. ? The temperature interval between 233 K~190 K shares a high condensation velocity for hydrocarbons 4Cand 5C . And cascaded refrigeration one is a rational strategy to improve overall Cop, because of the relative large heat load. NOMENCLATURE                                                    rP              contrast  pressure, cr PPP ?                           (–)                                              SubscriptsP               total pressure                                                   (Pa)                                 i       a vapor component rT              contrast temperature, cr TTT ?                     (–)                                   1      condition before condensation Z               compressibility factor                                     (–)                                   2      condition after condensation R              molar gas constant                                           ( 1KmolJ ?? )                  c      critical point ?              eccentricity factor                                            (–)                                   b      ordinary boiling point ?               dimensionless temperature, cb TT??             (–)                                    n               molar rate                                                         (mol s-1)n?            molar condensation rate                                   (mol s-1)COP         coefficient of performance,                              (W/W) REFERENCES Tang, Y, W., Huang, Y, CH., Yang, J., 2007, Current Situation, Trend and Strategies of International Pricing of China's Oil Import, Resources ?cience, vol. 29(1) Li J, F., Chen, Y, L., Li, B, L., Li, Y, Z., Li, X, Z., 2006, Developing Status of Oil-gas Recovery Technology and its Prospect in China, Environmental Protection of Oil &Gas Field. Vol. 16 no.1 Zhao, G, G., Zhao G, Y., 2001, Discussion on Oil Gas Recovery in Oil Storage Systems, Petroleum Refinery Engineering,vol. 31 no. 8 Chen, G., 2007, Trend of China Crud Oil Import, China Custom, no. 06:p.41 Wang, C, J., 2007, American Oil-Gas Recovery and Environmental Protection, Commercial Economy Studies, vol. 10 no. 026Peng, G, Q., 2000, A Discussion on Oil/Gas Condensation Recovery, Petroleum Products Application Research.vol.18 ?o.4  Kang, H, C., Kim. M. H., 1994, Effect of Non-Condensable Gas and Wavy Interface on the Condensation Heat Transfer in a nearly Horizontal Plate, Nuclear Engineering and Design.  vol. 149. Ma, X, H., Zhou, X, D., Lan, Z., Zhang, Y., 2008, Condensation Heat Transfer Enhancement in the Presence of Non-Condensable Gas Using the Interfacial Effect of Dropwise Condensation, Int. J. Heat and Mass Transfer., no. 51:p. 1728-1737. Yoshio, U., Wang, S, X., 2004 Characteristic curves and the promotion effect of ethanol on steam condensation heat transfer, Int. J. Heat and Mass Transfer., no.47:p.4507-4516. Hatsopoulous, G, N., Keenan, J, H., 1981, Principle of General Thermodynamics, Krieger Publishing Cop., New York R.E, p. 3-28. ACKNOWLEDGEMENT                                                                                                                                                                                       The research was supported by Beijing Natural Science Foundation (No. 3071001) and the National High Technology Research and Development Program of China (863 Program, No. 2006AA05Z207) 

Theoretical Investigation on Condensing Characteristics of Air and Oil Vapor Mixtures

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Theoretical Investigation on Condensing Characteristics of Air and Oil Vapor Mixtures

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