i CHEMICAL ENQlNEERlNG REVIEWS I
UNIT OPERATIONS REVIEW
1I
Absorption and Humidification
THERE
was modest activity in the development of packings and general testing during the past year. Most efforts were directed toward obtaining additional data for certain types of Stedman packing. In the field of capacity data and processes a few interesting contributions appeared; in particular, additional data on the hot-carbonate carbon dioxide-removal process were given. I n the study of fundamentals several articles treated absorption theories, penetration, and surface renewal. In one, the correlation of diffusion coefficients was considered, and new data in wetted-wall columns were given. Column Studies
I n order to correlate pressure drop for two-phase flow through towers, carrying Stedman packing, Morton and King (27) modified the Carman equation for flow through porous media. The proposed correlation involves a term that pertains to voidage of the packing. Liquid rate and flow characteristics of the discontinuous (liquid) phase undoubtedly affect voidage. Regardless of reflux rate, a constant thickness of liquid film around the apertures was assumed.
This in effect postulates that voidage remained constant for the reflux rates that were operative. A comparison of the proposed correlation with experimental data shows satisfactory agreement for low irrigation rates. Deviations become more severe at elevated liquid rates. This was attributed to the advent of loading of the packing. The experiments also supported the now generally accepted belief that loading and flooding are phenomena that develop gradually and not suddenly a t a definite flow point, as has been postulated earlier. Morton, Cerigo, and King (20) also reported holdup and flooding data for reduced pressure operation with Stedman packing 6 inches in diameter. For triangular pyramid-type Stedman packing holdup below the loading point was expressed by H = 0.095R0.6S where H i s column holdup in gallons per cubic foot of tower and R is the liquid irrigation rate in gallons per hour per square foot of tower cross section. T h e limiting vapor velocity a t the flooding point, in feet per second, was given by
MAX LEVA, a consulting chemical engineer, has worked in fluidization research and in the development of tower packings, gas absorption, and gas drying installations. A native of Ludwigshafen, Germany, Leva holds a B.S. from the University of Cincinnati and M.S. from Carnegie Institute of Technology. He is a registered professional engineer and member of the ACS and AIChE.
CHIN-YUNG WEN is teaching chemical engineering at West Virginia University. He holds q B.S. degree from the National Taiwan University and a M.S. and Ph.D. from West Virginia University. Wen has done research on gas absorption, fluidization, and solid-gas flow. He is a member of Sigma Xi and Phi Lambda Upsilon and several technical societies.
where G and L are liquid and gas rates, and pa and pL are gas and liquid densities. A new industrial distillation and absorption packing, Spraypak, investigated by McWilliams and others (79), consists of a single layer of expanded metal. Graphical correlations were given for pressure drop, liquid holdup, and entrainment. Spraypak was claimed to have a lower pressure drop than bubble cap plates, handling the same throughput, and H E T P values were also reported lower. Liquid-liquid holdup was investigated by Wicks and Beckmann (27). Toluene was the dispersed and water the continuous phase. Rings were used as packing and sizes ranged from inch through all intermediate sizes to 1 inch. Column diameters were 3, 4, and 6 inches. T h e data were correlated empirically by a n equation based on dimensional analysis, Phenomenonwise three different types of holdup were observed. The dispersed-phase drops that rise freely to the interface were described as “free holdup.” “Operational holdup” includes free holdup, in addition to droplets that are freed from the interstices by pulsations. Under “total holdup” they defined the entire amount of dispersed phase in the effective packed volume at any time. T h e possible effect of packing arrangement in liquid-liquid flooding was examined statistically by Johnson and Beeckmans (73). Their studies disclosed that the phenomenon is definitely influenced by arrangement of pieces. The results were presented in form of a n equation. Results of wetted area studies in a column 5 inches in diameter were reported by Hikita and Kataoka (70). Packings investigated were rings 15, 25, and 35 mm. in diameter. Liquids used were aqueous solutions of methanol and glycerol. As found earlier by others. in the operating range below the loading zone gas rate had no effect on wetted area, but extent of wetted area was liquidrate dependent. The data were correlated by
5 at
=
0.0464 L’I3
(g)
m
where m = -1.42d-0.” a, = wetted area VOL. 49, NO. 3, PART II
MARCH 1957
457
The hot-carbonate process for removal of existing processes, during regeneration.
= total packing area L = liquid rate: kg. per sq. meter per hour = surface tension, dynes per cni. 0 = packing diameter. cm. G!
\:'erted area \vas virtually independent of packed height. Effective \vetted surface area i n Raschig ring coliimns \vas also the subjrct of a revieiv by St'hitt (25),1vho summarized past developments, based on direct nirasiirements and absorption behavior. ,\ nomograph permitting solution of problems that pertain to gas-liquid packed toiver operation \vas presented by Jacobs (72). The nomograph is based on a flooding correlation prrsented earlier? and the data extend to Kaschis rings and Intalox saddles. A brief study in distillation. described ti>. Kirschbauin (77). einpliasiztd the importance of ~ v a l l effect in packed Raschig ring coluinns. One-inch Kaschig rings \vere examined in t\vo 400mm.-diameter columns. One column had a smooth inside \vall: whereas the other column carried transverse ribs a t close pitch. I n the latter the ring a r rangement adjacent to the \\.all was thoroughly irregular. Height effect studirs disclosed a better performance in this t \ p of colurnn, \\-hich \vas atiribured to tlie absence or rniniiriizcd intensit)- of \\.all efrect.
approach. 'I-'hr valmr press~ir~: o f siiicoii tetrafluoride o \ ~ rfluosilicic acid \\'as found negligible ; hence, in gas al)sorption there was no back pressure fruni the
liquid phase. The chief rrsistancr !\.as fo~indto be in tlie gas film, as evidrnccd by the virtually nonexistent intrrcc1)t ivhen 1 'Iif;a \\'as plotted against Q.3, N e i v absorption data for tlie s).steiii sulfur dioxide--watcr ~ v c r robtained 11) Parkison ('22). The I-inch Raschi? rinq to\ver \vas 8 inches i n diameter, packrd t i l ) to 2 fecr. Concrntrarions of stilfiur diand 5 10 l o r ; , oxide ivere 0.5 to 1.5:; ~Temperaturcs~ v c r e70' F. :I f'c\\, r u n s ivere also niadc a t 5(lc and 90' F. Tcniperaturc dependence \vas i n agrernieiit with t h a t alread!. established b!. SVIiitiic!and L-ivian. 'The capacity data ~veredvpendent on both liquid and gas raws. .Ibsorption of carbon dioxide in T ~ L I \vas invrstiqarcd b!, Fiijita a n d Ha!-nka\va ( ( j ) in a packed to\vt:r. iis \vel1 as under conditions of so-called "rod-like irrigation." T h e toiver bringing abour this condition is iiierel!. dii rmpty shcll. rlirough ivhich rod-like strrarns of \\-;iter pass do\vnivard from coplier tubes. There is no splashing. For some s1)ccific opvrating conditions the rodlikc to\\'er apprarcd superior to thc. Kaschiq-rinq and Berl saddlc-i)acicd to~ver. C a p i c ir!. data ~ v v r ccorreliitrd for tlie packrd tu\\-rr by I),,
Capacity Datu \Vhyness (26) reported 0 1 1 ~ h absorpc tion of silicon tetrafluoride by ivatcr. .I s!-stem of descending ivater drops as \vel1 as a \\-etted-wall colurnn \vas used. For absorption by drops. the concentration of fluosilicic acid in the effluent increascd \vith partial pressure of silicon tetrafluoride in the gas until the latter had reached a value of about 80 mm. of mercury. A further partial pressure increase had no effect on eRuent concentration. Microscopic investigation indicated that formation of a silica film near the surface of the drops might have hindered further absorption. This should be kept i n mind in the specification of height of a
spray absorber. )Vetted-wall absorption data Icere satisfactorily correlated by a t\vo-filni
458
CO, offers
\\-here I),,
=
O,025(4L'qL?{
\\-vr.rr. and h[uller ((5). T h r i r column \vas 100 mni. in diameter, packed a b o u ~2000 m i n . hiqh \vith Kaschig rinqs 10 inrn. i n dianietrr. I n vsscncc, t h e y rneasurcd t h c rate of aninionia roncrntration increase i n tlir dcsccndirig liquid. F r m i a consideration of material balancc: iind tlic. definition of K,;a in ternis of qas i'ati's ;inti terininal coiiceiitra~iOiis.they arrivcd at an cxpr(mion 11 hich rclatrd concentration incrcmc of ammonia in the liquid directly t o parlicd hci5ht. T h u s : \\a[cl'
INDUSTRIAL AND ENGINEERING CHEMISTRY
= =
\vherc ,r is the conccniration of atrirrioriia thc liquid. c ' is iin intryration u i n stant. cs is rlic conce~itriiti~n in and ii is the coltirrin height. .Is was tci ~c.sp(:cwd, ii ~ i l o o i f ii i's. log x produced Ibr tlie niqjor portiun through the colunin a lincar rrlation, but tlierc secrried t o Le a deviation from linearity in the 1)asc o f the coluinr~. .I'liis could have I)cc:n caused ti!. nialdis tri bu tion. Cooling roiver pcrforniance \vas dcscribrd b y Itiazurni (77). Heat and inass transfrr data \vcrc obtaiiicd f o i , construction \vitli rcd\vood and Xlasonitc slats. 'T'lic c:ffei'c.t of ronstruction \vas indicated and ii gt.ncralizing equation was cstcd. (;as absorption calculations ciutiined by T,llis ( f ) , I'or two(:otnpc~nen~; and inu1ticornt)oiieni systems the graphical nirthod of evalualion of the nuniher of equilibrium stagrs \vas givrn ivitli emphasis o n ii varying ratio of liquid to gas rate as pertains to high conccntr;ilions of solute in the inlct strcams. Packing cal)acit)- data for '/ 4-inch Raschig ring-s in 21 small distillation-estrac.tivc distillatirin unit. operatiny \ \ i r h nirtIi\ 1c!.cloliesane-loiitciic xvith and !vithuul furfural, \yere givm by C h r n r r , E l l i s 2 and (;ranville (7). I n borh cases ()})tiinuin operation \vas found near t l i r loacliii
I ~
x
liquid phase diffiisivit) liquid mass velocit!. ii = effective contact area per unit tc11vrr volurne p = drnsiry and \.iscc,sil\. o I liquid Z = toivci- heiTlii Henson, Field, and Ha)-nr.s ( I ) dcscribed further their process for absoi.t)ing carbon dioxide from inert atmospheres? by using hot carbonate solutions. T h e studies were made in pilot installations Lvhere the towers were 6 and 8 inches in diameter and u p LO 30 feet i n height. One of the advantages of the new process over older existing processes is considerable steam saving during regeneration. Comparisons ivith the monoethanolamine process are given. Further data for the system ammoi~iaI,
considerable steam saving, over
ing point. Kirschbauni and others ( I , $ ) have continued their earlier distillatioii studies, \\-herein [ h e effect of pressurr o n rectification \\.asconsidered. Fundamentals
.I rrvic\c of the film rlieory and pcnctration theory \vas given b y 1)arick-
ABSORPTION AND HUMIDIFICATION werts ( 3 ) . The effects of chemical reaction based on Higbie’s and Danckwerts’ model of surface renewal were discussed. Neither the film model nor the simple penetration models seem to be valid for correlation of observed absorption rates. The invalidity of the simple penetration theory is attributed to the uncertainty of the surface-age distribution function, greater rates of surface renewal caused by relatively fast flow of liquid in some regions, and slower rates of renewal caused by relatively stagnant liquid in other locations of the packings. For this reason over-all effects are not readily calculated, unless the distribution of k L values throughout the packing is known. T h e only case where the penetration theory might be applicable is in prediction of the effect of chemical absorption in industrial equipment, and only then when small scale data are available from small scale equipment that operates essentially similarly from a hydrodynamic point of view. “Surface rejuvenation” was preferred to “surface renewal” because a complete renewal between two smooth surfaces of packing does not occur. Based essentially on the Higbie-Danckwerts film model Hanratty (9) derived a rate equation and a concentration profile equation for mass transfer between a turbulent fluid and a solid surface. The probability function +(e,) has been assumed to have the form
where A and 7 are constants, 0, is total contact time between a fluid and the wall, and n = I, 2, 3. , . . T h e transfer equation was solved for the assumed surface-age distribution function. The measured concentration profile lies between +(&) = Ae-@clr and +(&) = & = constant. Althpugh these results d o not necessarily prove that the discontinuous film model is a description of actuality, the agreement between predicted velocity profile by the model and that of measured values suggests the possibility of a closer approximation of the mass transfer mechanism, if proper age-probability functions are selected. I n order to show that the Danckwerts surface renewal concept may be applicable to the mass transfer of solid-liquid systems, Johnson and Huang (74)studied the rates of dissolution of several organic solids from smoothed flat surfaces into turbulent liquids in a n agitated vessel. Their experimental correlation indicated an exponent of 0.5 for the Schmidt number and thus agreed with k L c q / D in Danckwerts’ rate equation, based on surface renewal. The authors concluded that this provides new and significant evidence for the applicability of the Danckwerts theory of surface renewal,
and that their type of apparatus can be used for further confirmation of the theory, especially for the case where mass transfer is accompanied by a chemical reaction. A graphical correlation of binary gas diffusion coefficients was developed by Fair and Lerner ( 5 ) ,based on the Hirschfelder-Bird-Spotz diffusion equation and the theorem of corresponding states. Despite the fact that the critical diffusion coefficient and “reduced” diffusion coefficient which they defined d o not possess great physical significance, the authors were able to show the “barrier gas ratio” (the ratio of the critical diffusion coefficient for various gases through a single barrier gas and the critical diffusion coefficient for these gases through air) to be independent of the properties of the diffusing gas. This enables rapid calculation. T h e accuracy of the method a t high pressure (near the critical value) may not be too good. Cairns and Roper (2) continued their studies of wetted-wall columns. From their adiabatic dehumidification data they concluded that the prediction of Colburn and Drew applies. Correlation of the increased heat transfer rates requires, however, consideration of P,,/P and a (where PB, is the log-mean partial pressure of the nondiffusing component in the gas film, P is total pressure, and a is the ratio of the sensible heat carried by the diffusing vapors and the heat transferred in absence of mass transfer) and not a alone. Mass transfer studies in a wetted-wall tower carrying a falling film of water and carbon dioxide were made by Kamei and others (75). The experimental values of height of transfer unit per unit test section were smaller than the theoretical values derived by Pigford (23) based on true molecular diffusion in a perfect laminar liquid layer. T h e deviation is attributed to ripples on the surface of the laminar film. For carbon dioxide absorption without gas flow
where = height of a transfer unit, feet HL 2 = height of wetted-wall column ReL = Reynolds number, pertaining to falling liquid film PL, PL = viscosity, density of liquid D L = molecular diffusivity Results of another mass transfer study in a wetted-wall tower were reported by Schwarz and Hoelscher (24), who investigated concentration profiles of water vapor a t various elevations. T h e descending films were essentially free of ripples and the air stream had developed to complete turbulence. Many of the discrepancies in the literature, concerning
mass transfer data, were believed due to lack of consideration of entrance effects. T h e mass transfer rates observed did not reach constancy until the downstream distance had exceeded six column diameters. Results of a n adiabatic humidification study in a perforated plate tower were reported by Kamei, Takamatsu, and Nakazaki (76). Another study of Yoshida and Hyodo (28) involved the vaporization of organic solvents from their wet-bulb surface into water-wet air. Essentially saturation relationships were presented for the cases where nonhygroscopic and hygroscopic-type solvents were involved.
literature Cited Benson, H. E., Field, J. H., Haynes, W. P., Chem. Eng. Progr. 52, 433,
10 (1956). Cairns, R. C . , Roper, G. H., Chem. Eng. Sci. 4, 221-8 (1955). Danckwerts. P. V., A.Z.Ch.E. Journal 1.456-63‘11955): Ellis, S. R. ‘M., Petroleum ReJner 35, NO. 2,127-31 (1956). Fair, J. R., Lerner, B. J., A.Z.Ch.E. Journal 2, 1, 13 (1956). Fujita, S., Hayakawa, T., Chem. Eng. (Japun) 20, 113-117 (1956). Garner. F. H.. Ellis. S.R. M.. Granville,’W. H.‘. J . Inst. Petroleum 42, 148-54 (1956). Guyer, A., Guyer, A., Jr., Muller, F., Helv. Chim. Acta 38, 1545-53 (1955); 50, No. 4. Hanratty, T. J., A.Z.Ch.E. Journal 2, 359-62 (1956). Hikita, H., Kataoka, T., Chem. Eng. ( J u p a n ) 20, 528-33 (1956). Inazumi. H.. Zbid.. 19. 579-85 (1955). Jacobs, ’J. K., Petroieum Refiner 35, NO. 6, 187-8 (1956). Johnson, A. I., Beeckmans, J. M. L., Can. J . Technol. 33, 434-44 (1955). Johnson, A. I., Huang, C. J., A . I . Ch.E. Journal 2,412-19 (1956). Kamei. S.. Oishi. J.. Iiiima.. H.., Itoi,‘M.,’Kamada, 6f., ehem. Eng. ( J a p a n ) 20, 65-70 (1956). Kamei, S., Takamatsu, T., Nakazaki, S., Zbid., 20, 23-7 (1956). Kirschbaum, E., Chem. Zng. Tech. 28, 639 (1956). Kirschbaum, E., Busch, W., Billet, R., Zbid., 28,475-80 (1956). McWilliams, J. A , , Pratt, H. R. C., Dell, F. R., Jones, D. A., Trans. Znst. Chem. Engrs. (London) 34, 1743- 11956). ,Morton, Frank, Cerigo, D. G., King, P. J.,Zbid., 34, 146 (1956). Morton, Frank, Kine, P. J., Zbid., 34,155 (1956). Parkison, R. V., Tappi 39, 522-7 (Julv 1956). Pigford, R. L.’, Ph.D. thesis, University of Illinois, 1941. Schwarz, W. H., Hoelscher, H. E., A.Z.Ch.E. Journal 2,101-6 (1956). Whitt, F. R., Brit. Chem. Eng. 1956, 439. Whyness, A. L., Trans. Znst. Chem. Engrs. (London) 34, 117-26 (1956). Wicks, C. E., Beckmann, R. B., A.I.CI1.E. Journal 1, No. 4, 426-33 (1955). Yoshida, T., Hyodo, T Chem. Eng. (Jupnn) 20, 528-40 (19k6).
VOL. 49, NO. 3, PART II
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MARCH 1957
459
