Theoretical Investigation on Condensing Characteristics of Air and Oil Vapor Mixtures

Transcription

1 Pudue Univesity Pudue e-pubs Intenational Refigeation and Ai Conditioning Confeence School of Mechanical Engineeing 2008 Theoetical Investigation on Condensing Chaacteistics of Ai and Oil Vapo Mixtues Cheng Li Tsinghua Univesity Junming Li Tsinghua Univesity Buxuan Wang Tsinghua Univesity Wenxin Xie Shengye Co. Beijing Zhongyi Shengye Co. Follow this and additional woks at: Li, Cheng; Li, Junming; Wang, Buxuan; Xie, Wenxin; and Zhongyi, Beijing, "Theoetical Investigation on Condensing Chaacteistics of Ai and Oil Vapo Mixtues" (2008). Intenational Refigeation and Ai Conditioning Confeence. Pape This document has been made available though Pudue e-pubs, a sevice of the Pudue Univesity Libaies. Please contact epubs@pudue.edu fo additional infomation. Complete poceedings may be acquied in pint and on CD-ROM diectly fom the Ray W. Heick Laboatoies at Heick/Events/odelit.html

2 2212, Page 1 THEORETICAL INVESTIGATION ON CONDENSING CHARACTERISTICS OF AIR AND OIL VAPOR MIXTURES Cheng Li Jun-Ming Li * Bu-Xuan Wang Wen-Xin Xie 2 Key Laboatoy fo Themal Science and Powe Engineeing of Ministy of Education Depatment of Themal Engineeing, Tsinghua Univesity Beijing , China Beijing Zhongyi Shengye Co Ltd, Beijing , China *Tel: , Fax: lijm@tsinghua.edu.cn ABSTRACT The condensation pocess of the volatilized oil-vapo is calculated fo cetain given constitutes of ai and oil-vapo mixtues by using finite diffeence method. The esults indicated that thee ae thee distinct tempeatue egions in the whole cooling pocess; each coesponds with special efigeation pocesses. The tempeatue egion fom K to K is the pepaed cooling fo emoving the wate vapo by condensation which will effects the late condensing of the oil-vapo, the tempeatue egion fom K to K is the condensation pocess of high cabon oganic substances, and the tempeatue egion fom K to K is the condensation egion fo elative low cabon oganic substances. Accoding to the calculation esults, some key poblems wee discussed in the condensing ecovey appaatus and system design. Key Wods: oil-vapo, condensation ecovey, convective heat tansfe INTRODUCTION Refigeation condensation of volatilized oil-vapo is one method of oil vapo ecovey (Zhao et al. 2001; Li et al. 200; Tang et al. 2007). Such a method includes a complex condensation pocess with mixtues of seveal kinds of condensable vapos and mixtues of dy ai and wate vapo. The volatilized oil-vapos ae hamful to the envionment (Wang, 2007; Chen, 2007). Besides, they badly affect qualities of oil and gasoline and induce dange of fie and blast. The condensable oil vapos ae main light alkyls and alkenes. The numbe of cabon atoms in such a molecule is among 4~. In ai and oil-vapo mixtues, factos of light hydocabons with diffeent patial pessue and physical chaacteistics, such as satuated pessue, satuated tempeatue and tiple points, ae destined to complex condensing pocesses. Condensation of wate vapo(kang and Kim, 1994), which plays a main ole in the pimay cooling pocess befoe tempeatue deceases to wate feezing point, should be non a neglectable facto, because of its elative humidity of about 0%. And pe-emoving of wate vapo is a necessay step fo the efigeation used fo oil vapo ecovey. Condensation of light hydocabons is a typical multi-mixtues pocess with the feasibility of Maangoni condensation fo small factions of kinds of hydocabon vapos and thei diffeent boiling points. And the immiscible solution, because of the existence of wate liquid, leads to the possibility of dopwise condensation. Complex heat tansfe and enhancement mechanism have been discussed (Ma et al, 2008), while few investigations aimed at the ai and oil-vapo mixtues condensation (Peng, 2000). The pupose of this pape is to discuss condensation pocess of the volatilized oil-vapo, in view of theoetical investigation. CALCULATION METHOD The petoleum o oil constitutes of the complicated oganic hydocabons. Evey hydocabon holds diffeent faction, which is attibuted to exploitation location in the wold and the exploiting stage. And the volatized oil Intenational Refigeation and Ai Conditioning Confeence at Pudue, July 14-17, 2008

3 2212, Page 2 components could be diffeent elative to the local tempeatue, measuement to sun adiation and value of octane fo gasoline. One case shows in table 1. Table 1.volume faction of a cetain ai and oil vapo mixtues component n C4 i C c C 4 4 nc 4 i C 4 n C5 i C5 n C hydocabons faction vol (%) The oil vapo is actually a mixtue consisted of kinds hydocabons, the eason only consideing cetain components is that the evelation fom oil efineies and gasoline indicates that only pats of the hydocabons can volatilize easily. Date measued fom (Peng, 2000) declaes that 34.3vol% hydocabons components exist in ai and oil-vapo mixtues in LuoYang Petoleum and Chemical Plant. And the main components ae C 4 C 5 andc. Thee Paametes Method steam pessue equation (Hatsopoulous and Keenan, 1891) is adopted and veified by the date fom softwae NIST as following (0) (1) ln p f (T ) f (T ) (1) hee (0) f T lnT T f (1) T lnT T 1-1 ( lnpc ln ) ( ln ) The values and eccenticity facto fo calculated hydocabons showed in table 2. Table2.values of paamete and eccenticity facto component component n C i C i C n C c C i C n C n C Softwae Nist employed to validate the Thee Paametes Method is plotted in Figue 1. The calculated and compaed component is butane. Results indicate that depatue of the calculated date by Thee Paamete Method is no moe than 2% fom the Nist one, so that the method can be acceptable satuated pessue(pa) Thee Paamete NIST datum tempeatue(k) Figue 1: Satuated tempeatue and pessue elation of butane steam Intenational Refigeation and Ai Conditioning Confeence at Pudue, July 14-17, 2008

4 2212, Page 3 The gas and oil-vapo mixtues listed in table 1 ae consideed as ideal gas mixtues. So evey component in the gas phase satisfies the ideal gas state equation p iv n irt (2) i which can be suppoted by data fom Table 4, with the calculating tempeatue K. Compessibility factos of components in Table 2 ae moe close to 1, which indicates that the gas and oil-vapo mixtues can be consideed as ideal gas mixtues. Especially, volume factions of components listed in Table 1 is less than 8%, which means evey hydocabon shaes no moe than 8% of the total pessue. Theefoe, the ideal gas mixtue assumption is easonable. Table 2 compessibility factos (Z) of hydocabons in 20 degees centigade and 1 atm. component methane ethane popane butane pentane hexane Z The condensation of one component did not effected by othes fo ideal gas mixtues, so that when analyzing a cetain component hydocabon condensation, the othes could be teated as a cetain ideal gas, the state equations of the analyzed component in two diffeent points would be as p 1V1 n 1RT1 p 2V2 n 2RT2 When tempeatue dop is tiny enough in the cooling pocess so that thee is a little diffeence between T 2 andt 1, consideing T equals 2 T 1, the eo can be accepted. So the condensation quantity can be easily expessed as follows with the oveall pessue p constant: V p p2 n n1 n2 ( p1 p2 ) (3) RT1 p p2 Whee, n is the condensation quantity of the analyzed component with its satuated patial pessue. And the assumption above can clealy be tue if the tempeatue dop is small enough. Theefoe, in any tempeatue dop fom the above thee intesected tempeatue intevals, a cetain component condensation can be calculated accodingly, with the satuated tempeatue judged by the Thee Paametes Method state equation descibed above. CALCULATION RESULTS AND DISCUSSION Accoding to equation (3), the condensation pocess is plotted in Figue 2 fo the case showed in Table 2 with calculating tempeatue step 0.5 in evey step and the condensation of wate vapo is not included. It is seen fom Figue 2 that only n C condenses in the fist intevals with tempeatue dop fom K to K. The easons ae n C has a elative high boiling point and the high faction in the mixtue. In fact, moe than 80% wate vapo condensed when the ai and oil-vapo mixtues each K. So in an oil-vapos ecovey system, the fist inteval is impotant. Because the wate condensate could solidified to additive on the heat exchange sufaces, it will affect the pocess of heat tansfe. Theefoe, the pe-cooling is impotant, which is the so called fist egion. When the tempeatue deceases fom K to K, nealy 80% n C has changed to liquid and i C5 condenses a lot also. But othe components just begin condensation. In this tempeatue dop egion, the emnant wate vapo condenses almost and become fost, which bings lots toubles to the actual heat exchanges and the ability to be povided with the fost emove measuement should be consideed futhe. All the hydocabon components happen to condense in the thid egion, i.e. tempeatue dop fom K to K. Thee exists a lage phase-change heat load in the thid egion, which is geat consumption to the efigeating load as cost will be moe if the same heat load stands at much lowe tempeatue point than the envionment one. In this inteval, moe than 80% hydocabon components can be effectively eclaimed. And almost 99% components ae eclaimed, if the lowest cooling tempeatue eaches minus 90. In the whole condensation pocess, the condensation ate could be quite diffeent. The tempeatue inteval between 233 K~190 K shaes a high condensation ate fo hydocabons C and 4 C 5. Evey hydocabon condenses faste at its condensation beginning. Oil-vapos ecovey efficiency is 82%, when setting the lowest tempeatue as and 9% can obtain, when the lowest tempeatue is K. Intenational Refigeation and Ai Conditioning Confeence at Pudue, July 14-17, 2008

5 2212, Page 4 Figue 2(a): Relations of cooling tempeatue and component condensation faction Figue 2(b): Relations of cooling tempeatue and component condensation faction Accoding to the calculation esults, the impotant factos should be consideed fo the design of an oil vapo ecovey efigeation system as following. The fist, pe-cooling should be adapted to gas and oil-vapo mixtues so as to minimize the enegy consumption. The second, the thid egion efigeating machine occupies the position with chaacteistics of elative low tempeatue (lowe than the teminal tempeatue of the teated gas mixtue) and elative big efigeating loads, which is contibuted to the main hydocabon phase-change couse. If the oveall efigeating machines ae selfgovened thee cycles, the thid intevals powe consumption of compesso will be amazed big, due to the elative high envionment tempeatue compaed to that of the efigeant tempeatue in the enty to the compesso. Such a big potential diffeence fo heat tansfe is not poposed. And huge powe consumption in compesso is at the expense of potential diffeence. To solve the poblem, the cascaded efigeation cycle is advised in the second and the thid pats, which means efigeation load of the second cycle espectively seve the second class efigeation load and the thid class condense. Table 4 below gives contast between the unattached cycle and the cascaded efigeation one. It is clea that the cascaded efigeation cycle can obviously impove the oveall efigeation C fo ou poblem. Table 4 Cop compaison between unattached and ovelap coupled efigeating system case unattached cascaded efigeation one and expot ai pe-cooling C And the thid, the tube and shell heat exchange is pevalently advised (e.g. Song, 2007; Tan, 2008). Fo the finned tube heat exchange, to avoid the elative bad heat tansfe capacity of gas-gas heat exchange, seconday efigeant can be a consideable scheme in the heat load tanspot between gas and oil-vapo mixtues and efigeant. Theefoe, the gas velocity could not be big enough, which futhe wosens the heat tansfe coefficient of gas mixtues. So heat tansfe enhancement and optimization could be vey impotant. CONCLUSIONS Intenational Refigeation and Ai Conditioning Confeence at Pudue, July 14-17, 2008

6 2212, Page 5 Gas and oil-vapo mixtues condensing with the pupose of ecovey oil-vapo is a vey complex pocess. This pape pays main attention to the condensation ecovey of kinds of hydocabon components and thei elations with tempeatue points. Evey component in the gas mixtue, including the condensable gas and the non-condensable, is a esistance to othe components diffusion. Based on the calculation esults, some conclusions could be obtained as followings. Condensation ate in diffeent tempeatue point could be quite diffeent, with the elative low tempeatue egion existing big condensation ate fo evey condensable component and a lage phase-change heat load. Dehumidification pocess fom K to K is impotant and to decease the fost effects to next futhe efigeating pocess. And The elative high boiling point and the high factions (such as C in pesent wok) condensate in the fist egion. The tempeatue egion fom K to K is the condensation pocess of high cabon oganic substances, and the tempeatue egion fom K to K is the condensation egion fo elative low cabon oganic substances. The tempeatue inteval between 233 K~190 K shaes a high condensation velocity fo hydocabons C 4 and C 5. And cascaded efigeation one is a ational stategy to impove oveall Cop, because of the elative lage heat load. NOMENCLATURE P contast pessue, P P Pc ( ) Subscipts P total pessue (Pa) i a vapo component T contast tempeatue, T T Tc ( ) 1 condition befoe condensation Z compessibility facto ( ) 2 condition afte condensation R mola gas constant 1 ( J mol K ) c citical point eccenticity facto ( ) b odinay boiling point dimensionless tempeatue, T b Tc ( ) n mola ate (mol s -1 ) n mola condensation ate (mol s -1 ) COP coefficient of pefomance, (W/W) REFERENCES Tang, Y, W., Huang, Y, CH., Yang, J., 2007, Cuent Situation, Tend and Stategies of Intenational Picing of China's Oil Impot, Resouces cience, vol. 29(1) Li J, F., Chen, Y, L., Li, B, L., Li, Y, Z., Li, X, Z., 200, Developing Status of Oil-gas Recovey Technology and its Pospect in China, Envionmental Potection of Oil &Gas Field. Vol. 1 no.1 Zhao, G, G., Zhao G, Y., 2001, Discussion on Oil Gas Recovey in Oil Stoage Systems, Petoleum Refiney Engineeing, vol. 31 no. 8 Chen, G., 2007, Tend of China Cud Oil Impot, China Custom, no. 0:p.41 Wang, C, J., 2007, Ameican Oil-Gas Recovey and Envionmental Potection, Commecial Economy Studies, vol. 10 no. 02 Peng, G, Q., 2000, A Discussion on Oil/Gas Condensation Recovey, Petoleum Poducts Application Reseach.vol.18 o.4 Kang, H, C., Kim. M. H., 1994, Effect of Non-Condensable Gas and Wavy Inteface on the Condensation Heat Tansfe in a nealy Hoizontal Plate, Nuclea Engineeing and Design. vol Ma, X, H., Zhou, X, D., Lan, Z., Zhang, Y., 2008, Condensation Heat Tansfe Enhancement in the Pesence of Non- Condensable Gas Using the Intefacial Effect of Dopwise Condensation, Int. J. Heat and Mass Tansfe., no. 51:p Yoshio, U., Wang, S, X., 2004 Chaacteistic cuves and the pomotion effect of ethanol on steam condensation heat tansfe, Int. J. Heat and Mass Tansfe., no.47:p Hatsopoulous, G, N., Keenan, J, H., 1981, Pinciple of Geneal Themodynamics, Kiege Publishing Cop., New Yok R.E, p ACKNOWLEDGEMENT The eseach was suppoted by Beijing Natual Science Foundation (No ) and the National High Technology Reseach and Development Pogam of China (83 Pogam, No. 200AA05Z207) Intenational Refigeation and Ai Conditioning Confeence at Pudue, July 14-17, 2008

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