Lecture 6: Condenser and Reboiler Design

Transcription

1 Lecure 6: Conenser an Reboiler Design Pressure rop calculaion i. Tube sie pressure rop In case of ube sie conensaion: For conensaion in he ube sie by aking one-half of he convenional pressure rop relaion can be use. 2 1 fg L n p P 12, psi (1.14) S i Where, f = fricion facor G =mass velociy [lb. h -1.f -2 ] L =Tube lengh [f] n =Number of ube passes p i =Tube ID [f] S =Specific graviy of he ube sie flui =Viscosiy correcion facor = 1, = viscosiy of he ube sie flui; w = viscosiy of waer) ( w ii. Shell sie pressure rop In case of shell sie conensaion: Similarly for conensaion in he shell sie: 2 1 fgs DS nb 1 Ps 12, psi (1.15) DS e s s Subscrip s inicaes shell sie flui. n b = number of baffles D e = Equivalen iameer for he shell [f] Join iniiaive of IITs an IISc Fune by MHRD Page 33 of 41

2 Temperaure NPTEL Chemical Engineering Chemical Engineering Design - II Calculae all flui propery a film emperauret f. No reurn loss calculaion is require for he conensing flui. In case of non-conensing flui (single phase flow), use he convenional pressure rop relaion De-superheaing an sub-cooling De-superheaing is ifferen from conensaion of a saurae vapor. The sensible hea shoul be remove firs o e-superhea he vapor o obain he saurae vapor. Similarly, he saurae liqui is o be furher coole own (sub-coole) by exracing sensible hea below he boiling poin. The emperaure profile is shown in Figure 1.9 for he conensaion of superheae vapor o obain he sub-coole liqui from he same exchanger. The mean emperaure ifference an hea ransfer coefficien shoul be calculae iniviually for each secion if he egree of superhea/ sub-cool is large. The weighe mean emperaure ifference an overall ransfer co-efficien can be use o esign he conensers if hea loa ue o sensible hea ransfer in each uni abou 25% of laen hea ransfer. Oherwise, i is convenien o esign separae e-superheaer an subcooling exchangers. The calculaions for eail suy can be foun ou in reference [3] (page ). T superhea T sa T sub-cool Desuper -heaing Conensaion Subcooling LMTD esuperh LMTD conensaio LMTD subcooli Hea ransfer Figure 1.9. Conensaion wih e-superheaing an sub-cooling [2]. Join iniiaive of IITs an IISc Fune by MHRD Page 34 of 41

3 Pracice problem: Design a horizonal conenser for he conensaion of 45,000 lb/h of almos pure normal propyl alcohol available a 15 psig. A his pressure, he boiling poin of n-propyl alcohol is 244 F. Waer available in he emperaure range of 95 o 120 F can be as he coolan. The maximum pressure rop of 2 psi an 10 psi is permissible for he vapor phase an waer respecively Reboilers Classificaion of reboilers There are hree major ypes of reboilers: i. Thermosyphon naural circulaion reboiler: The boiling occurs insie he ubes in verical hermosyphon reboiler an insie shell in horizonal hermosyphon reboiler (Figure 1.10). In verical hermosyphon reboiler, he liqui circulaion occurs ue o ensiy ifference beween vapor-liqui mixure (wo phase) in he exchanger from he reboiler an he liqui hrough he owncomer o he reboiler. Avanages: mos economical because no pump is require. Limiaions: no suiable for heavily viscous flui; high consrucion cos for he insallaion of he column base a suiable elevaion o ge hermosyphon effec; no suiable for low emperaure ifference processes ue o boiling poin elevaion impose by saic hea. Figure Thermosyphon reboiler [5]. (a) Horizonal hermosyphon reboiler. (b) Verical hermosyphon reboiler Join iniiaive of IITs an IISc Fune by MHRD Page 35 of 41

4 ii. iii. Force circulaion reboiler: The liqui is fe by means of a pump. Force circulaion reboilers wih verical or horizonal ubes boiling may be esigne. Force circulaion reboilers are similar o verical hermosiphon reboilers, excep he pump is use for he circulaion of he liqui an he ho liqui flows insie column. To calculae he hea ransfer coefficien i is generally assume ha, hea is ransferre only by force convecion. The usual meho of shell an ube exchanger esign can be use. Avanage: suiable for viscous an highly fouling fluis. Disavanage: high pumping an mainenance cos; pump is require o circulae he boiling liqui hrough he ubes an back ino he column. Kele reboiler: The ube bunle is immerge in a pool of liqui a he base of he column in an oversize shell (Figure 1.11). Kele reboiler is also calle a submerge bunle reboiler. The heigh of he ube bunle is usually 40-60% of he shell ID. The submergence of he ube bunle is assure by an overflow weir a heigh of ypically 5-15 cm from he upper surface of opmos ubes. Avanage: suiable for vacuum operaion an high vaporizaion rae up o abou 80% of he fee. Limiaions: low hea ransfer rae han oher ypes as here is no liqui circulaion (low velociy); no appropriae for fouling fluis; kele reboiler is no suiable for hea sensiive maerials as i has higher resience ime. The bunle iameer D b, can be obaine from he empirical equaion ([2] page ): D 1/ n1 n b o K1 where, D b= bunle iameer [mm], n = number of ubes, iameer [mm]. The values of he consans K1 an n1 are in Table 1.5. (1.16) o = ube ousie Join iniiaive of IITs an IISc Fune by MHRD Page 36 of 41

5 Table 1.5. Consans use o calculae he ube bunle iameer. Pich ype Consans Number of ube passes ( n ) Triangular K ( PT 1.25o) n Square ( P 1.25 ) T o 1 K n Design of kele reboiler Figure Kele ype reboiler [1]. The Kern meho for esigning of Kele reboiler for isohermal boiling is summarize below. I is assume ha he egree of sub-cooling an super-heaing of he col flui is negligible i.e. vaporizaion of close boiling compouns wih negligible super-heaing of vapors forme. i. Make energy balance an eermine he hea uy. ii. Calculae of flui propery a he caloric emperaure (or a arihmeic mean emperaure) as alreay shown. iii. Follow he same guieline an esign requiremens for shell conaining he vaporing liqui. iv. Calculaion of hea ransfer co-efficien Join iniiaive of IITs an IISc Fune by MHRD Page 37 of 41

6 Calculaion of iniviual hea ransfer co-efficien ho flui: The calculaion of hea ransfer co-efficien of he ho flui can be performe similarly as in case of esign of shell an ube hea exchanger for single phase. Calculaion of iniviual hea ransfer coefficien of he boiling liqui: The Kern meho is iscusse here o calculae he iniviual hea ransfer co-efficien of he boiling liqui by rial an error proceure. Kern [2] recommens ha he maximum allowable vaporizing film coefficiens: Bu/h.f 2 F for naural or force circulaion vaporizing organics Bu/h.f 2 F for naural or force circulaion vaporizing aqueous soluion of low concenraion. The maximum allowable hea flux: Bu/(h)f 2 ) for force circulaion reboilers an Bu/(h)f 2 ) for naural circulaion reboilers vaporizing organics Bu/(h)f 2 ) for boh force or naural circulaion reboilers vaporizing aqueous soluion. Assume ha h (assm) = 300 Bu/h.f 2 F for organics or 1000 Bu/h.f 2 F for waer. Wih his assume value, calculae he ube wall emperaure ( T w ): T w T Where, h( avg ) h hio ( Thc Th ( avg )) ( h h ) io i io hi ( i io o ube ID an o ube OD) (1.17) T h( avg ) = Average emperaure of he ho flui T hc =Caloric emperaure of he ho flui Now, re-eermine h cal (laen hea ransfer) from he Figure 1.12 corresponing o ( T ). ( is he col flui boiling emperaure). w Coninue he calculaion ill, h cal h ( assm). If he calculae hcal is greaer han he maximum hea ransfer co-efficien of 300 Bu/h.f 2 F for organics an 1000 Bu/h.f 2 F for waer, ake h cal = 300 Bu/h.f 2 F for organics an h cal =1000 Bu/h.f 2 F for waer. Calculae he overall hea ransfer-coefficien ( U ) incluing he ir facors. Join iniiaive of IITs an IISc Fune by MHRD Page 38 of 41

7 Hea ransfer co-efficien (h), Bu/(hr)(f 2 )( o F) NPTEL Chemical Engineering Chemical Engineering Design - II Maximum for waer Maximum for organics 100 Laen hea ransfer Sensible hea ransfer by force convecion (T w -), o F Temperaure ifference beween ube wall an boiling liqui Figure Naural circulaion boiling an sensible hea ransfer [3]. v. Decie ype of exchanger i.e. fixe ube shee or U- shell (use U-ube reboiler for large emperaure ifference), ube size (iameer, lengh, ube pich), layou, effecive ube lengh. A ube pich of beween 1.5 o 2 imes he ubes OD shoul be use o avoi vapor blankeing. Q A vi. Calculae exchanger area ( A ) an number of ubes ( n ). U ( LMTD ) L o The number of ubes shoul be calculae base on he effecive ube lengh for U-ube reboilers. The effecive ube lengh is less han physical ube lengh ue o U-ben. vii. Calculae he hea flux= Q A [Bu/(h.f 2 )]. This value shoul be less han he maximum hea flux of Bu/(h)f 2 ) for force circulaion reboilers vaporizing organics an Bu/(h)f 2 ) for boh force or naural circulaion Join iniiaive of IITs an IISc Fune by MHRD Page 39 of 41

8 viii. reboilers vaporizing aqueous soluion. Oherwise, go o sep # v, repea he calculaion unil wihin he allowable limis. Check for allowable vapor velociy ( u v ) ([3] page 749): The maximum vapor velociy u v (m/s) a he liqui surface shoul be less han ha given by he expression below o avoi oo much enrainmen. u v 1/ 2 l v 0.2 v (1.18) where, = liqui ensiy an, = vapor ensiy l l If his crierion is no saisfie, go o sep # v an revise he calculaion. ix. Pressure rop calculaion Tube sie pressure rop (ho flui): The pressure rop calculaion of he ho flui can be carrie ou as alreay presene. Shell sie pressure rop (vaporizing liqui): There will be negligible hyrosaic hea for he flow of liqui from he column o reboilers (low circulaion velociy) if he liqui level above he ube bunle is no oo high. Therefore, shell sie pressure rop may be consiere negligible. x. Calculae over surface an over esign xi. Go for mechanical esign Design problem: Gasoline (65 API graviy) flow rae of 60,000 lb/h wih a small boiling range a 400 F is o be vaporize o form 37,050 lb/h vapor a an operaing pressure of 200 psig. Use gas oil (30 API graviy) in he emperaure range from 600 o 500 F a 120 psig operaing pressure as he heaing meium. A ube sie pressure rop of 10 psi is allowable. Design a suiable Kele reboiler o serve he purpose. Join iniiaive of IITs an IISc Fune by MHRD Page 40 of 41

9 References [1]. Inian Sanar (IS: ): Specificaion for Shell an Tube Type Hea Exchangers, BIS 2007, New Delhi. [2]. R. K. Sinno, Coulson & Richarson s Chemical Engineering: Chemical Engineering Design (volume 6), Buerworh-Heinemann, 3 r e [3]. D. Q. Kern, Process Hea Transfer, McGraw-Hill Book Company, In. e [4] Dua B.K. Hea Transfer-Principles an Applicaions, PHI Pv. L., New Delhi, 1 s e [5] James R. Couper; W. Roy Penney, James R. Fair, Sanley M. Walas, Chemical Process Equipmen: selecion an esign, Elsevier Inc., 2 n e Join iniiaive of IITs an IISc Fune by MHRD Page 41 of 41