Separation of liquid mixtures by capillary distillation

Desulznatron,8 1 ( 199 1) 129- 160 Elsevler Science Publishers B V., Amsterdam

Separation of Liquid Mixtures by Capillary Distillation G. C. YEH, B V YEH, B J. RATIGAN, S. J. CORRENTI, M. S. YEH, D. W. PITAKOWSKI, W. FLEMING, D. B. RITZ and J. A. LARIVIERE Department of Chemical Engrneenng, Vdlanova, PA 19085

Villanova Unrverslty,

SUMMARY Captllary dtsttllation ut111zcsthe elfects of the solid-ltqutd interfacial molecular forces, 1 c. dispersion mteractions and polar interactions, to alter the vapor-ltqutd equthbna of hquid mtxtures to be separated by distillation using capillary plates -fracttonating plates havmg capillary-type passages (1, la, lb, lc and Id). Several azeotropic systems have been investigated and their separations have been demonstrated to various industries in the U.S.A. interested in liquid separation thus showing the practicality and usefulness of this radically new separation technology (16-18). Numerous bmary solutions have been investigated for their separation by capillary distillatton resultmg m unusually high separation and thermal eflictenctes with very small pressure drops (2-S, 9-15, 19) In thts paper, only some typical separatton expenmcnts and thctr results arc reported to show the very unique nature of captllary disttllatton The systems reported herein include acetic actdwater mtxturcs and azeotroptc mixtures of ethyl alcohol-water, of I-propyl alcohol-water, of acetone-methyl alcohol, and of acetonitrile-ethylene dichloride. Very htgh Murphree plate cflicicncics exceeding 100% (often several hundreds or thousands percent) were obtained m most cases due to the fact that the concentratton of the vapor leaving the capillary plate, yn, is higher than that whtch would have been obtained if the vapor leaving the plate had been in cquihbrtum wnh the hqutd leavmg the plate, yi. In many instances, Murphree plate effictenctes(%) obtained were negative numbers due to the fact that the conccntratton of the vapor lrom the plate below, yn+l, IS higher than yi indtcatmg that the vapor-hqutd equthbrta have been altered by the sohd-hqutd intcrfactal molecular forces. The elfects of the vapor rate and liquid height upon Murphree plate effictenctcs and pressure drop per plate, and the thermal cfftctencres of capillary plate reported earlier (1, 20) are recapitulated to illustrate the very umque operational characteristtcs of captllary dtsttllatton The separation characteristics, espectally mcchamsms of capillary dtstillatton , is not completely understood at this time. 129

130

INTRODUCTION Captllary dtsttllatton utth/,cs the cffccts of the sohd-liquid mtcrfactal molecular forces, 1 c dtspcrston mtcractions and polar interacttons, to alter the vapor-liquid cquthbna ofthc liquid mtxturcs to be separated by dtstillation using capillary plates -fracttonatmg plates having capillary-type passages. The capillary plates also promote the suction nuclcatc condensation of the vapor m contact, the suction captllary flow of the condcnsatc formed and the suction nucleate boiling of the liquid on them under nonnal operational condtttons thus resulting m htgh separation and thermal cffictcnctcs with small prcssurc drop (l-5, 20) As early as 1959, G C Ych (2) successfully dtsttllcd several liqutd mixtures mcludmg accttc acid-water system, ethyl alcohol-water system, methyl alcohol-watct system and mtnc acid-water system usmg smtcrcd porous stamlcss steel plates (port SIX range 0 S-12 7 mtcrons dnd porosny 41%) and obtamcd Murphrcc plate cfftctcnctes grcatcr than one-hundred percent for nearly all the cxpcriments Durmg these early studies, Ych found that the smtercd porous stainless steel plates employed had altcrcd the rclattvc volattlntes of the liquid components m the hqutd mtxtures and greatly shafted the azeotropic composmon n-tthe ethanol-water system Later, Ych (3) was able to produce products whose compositions hc on opposite sides of the normal aLcotroptc composition for the ethanol-water system Other hqutds and a/,eotropcs mcludmg furfural-water system, benzene-ethyl alcohol system and bcnzcnc-chloroform system have also been dtsttllcd successfully by Yeh (4) usmg smtcrcd porous glass or metal plates Smcc thcsc early studtcs made by Ych, numerous binary soluttons have been separated by captllary dtsttllatton rcsultmg m unusually htgh separation and thermal cl fictcnctes wtth very small prcssurc drops (12- l&20). The practtcaltty of scparatton of scvcral a/.eotroptc systems by capillary dtsttllatton has been dcmonstratcd to vanous mdustncs m the U S A interested m liquid separations (lo- 18) The effects of vapor rate and ltqutd hctght upon Murphree plate efftciencies and pressure drop per plate, and the thermal effictencies of captllary plates were reported earher (1,20). Vapor rates have shown negligible effects on Murphree plate efftctcnctcs and prcssurc drop across captllary plates Ltqutd hctghts have been found to have a small cffcct on Murphrcc plate effictcnctes The thermal cl fictenctcs for the captllaty plates used have been found to bc nearly 100% m most ol the cxpcnmcntal runs studtcd rcgardlcss of the opcratmg condmons used It rcquncs only a very small tcmpcrature dtffcrcnttal across each captllary plate to opcratc a cdptllary dtcttllatton column successfully The cffccts of the soled-ltqutd mtcrfactal molecular forces upon the vaporhqutd cqutltbrta of hqutd mtxturcs m small captllancs, for which normal vaporhqutd cquthbna had been dctcrmmed and pubhshcd, wcrc studied for etghty-four binary soluttons mcludmg hydrocarbon solvents, halogcnated hydrocarbons, orgamc sulfur compounds, monohydnc alcohols, phenols, ketones, ammcs, esters and water (16, 10, 22-27) Most systems studied by the authors show an mcreasc m relattve volattitty when placed m small captllartes although some systems show a dccrcasc or even a rcvcrsal m rclattvc volanhty The thcorcttcally

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predicted vapor-liquid equihbna agreed very well wtth the expenmental values obtamed for monohydnc alcohol-water systems (20, 21, 23, 25-27) For those systems whose vapor-ltqutd equilibrta arc reversed when placed in small captllartes, separation by ‘Rcverscd Distillation’ using capillary plates may be feastblc. The feastbtlity of a few systems was established cxpcnmentally in 1983 (16). Separation mechamsms of capillary disttllatton for both zeotropic and azeotroptc mlxturcs were studied and a model for an ideal stage in capillary dtstillation was proposed (16, 20). Mcchamsms and rates of stmultancous heat and mass transfer across a capillary plate wet-c analyzed in terms of capillary vaporization, capillary condensate flow, and captllary condcnsatton. A complex rclattonship was developed to descnbe the mtcracnon bctwecn captllary vaponzation and capillary condensatron (6) Several columns havmg various mstdc dtamctcrs rdngmg between 2 to 6 inches and porous mclalltc plates of dtffcrent designs have been tested successfully for their opcrattonal charactcristtcs (6, 7) A pilot plant utilizmg smtered porous glass plates has been but It and Its testmg 1s m progress (18, 21). The mam ObJccl of thts paper is to report some of the highhghts of the research results obtained m the rcccnt studtcs made on the separation charactcnsttcs of capillary dtstillatton

METHOD AND APPARATUS Captllary dtsttllauon uses a plate-type apparatus Plates conststmg at least m part of a porous structure havmg captllary-type passages cxtcndlng between openmgs on each stdc of the plarc arc cmploycd m the place of the conventional perforated plate, or SICVCplate. The capillary-type passages arc adapted to bc wetted and Itllcd by the hquid condensate resulting from the condensatton of a vapor mtxturc. Conscqucntly, menisci are formed m the capillary-type passages and the gcncratton of capillary prcssurcs cause the condensed ltqutd to bc lodged wtthin and transmitted through the capillary passages while preventmg the passage of vapor. A vapor mixture produced by botlmg or evaporatmg the hqutd mixture is brought mto contact wtth one stde of the plate, where the vapor mtxturc IS substantially condensed m and near the adJaCCnt openmgs of such passages, and the condensate lormcd IS transmitted through the passages to the other stdc ol the plate where It IS botled or evaporated m and adjacent to the opcnmgs ol such passages The capillary pressure differential requncd to cause the condcnsatc to flow 1s produced by the boiling or cvaporatmg of the condcnsatc, and the heat rcquircd for such boiling or evaporatmg IS obtamcd by transfer of the latent heat of condensation and of the scnstble heat of the vapor mixture through the plate The thickness of the plate 1s typically a Icw tlmcs the dmrnetcr of the captllary-type passages. In pracncc, more than one capillary distillanon stage are used in the apparatus The vapor mixlurc havmg left the vaporizatron stde of the first plate is

132 condensed on the adjacent side, condensation side of a second plate, with the resultant condensate again being transmitted through the second plate. Thus, a series of successive condensations and the subsequent vaporization are carried out, thereby employing a series of capillary-type plates in a manner that appears similar to the conventional rectification in a conventional plate column, but which is totally different in mechanism and effect. The principle and method of capillary distillation can be better understood by referring to Fig. 1 which is a simplified model for an ideal stage in capillary distillation. Using total reflux for illustration, the method of capillary distillation of a liquid mixture can be readily seen. If the feed solution having a composition x. is boiled, the equilibrium vapor mixture generated will have a composition YE. The vapor mixture, when condensed on the condensation side of the first capillary plate, will have the identical composition xl. If the resultant condensate is transmitted through the capillary plate and boiled on the vaporization side of the plate under the influence of the capillary forces discussed above, the equilibrium vapor mixture formed ~111have a composition yz, as shown on the Y, - X, curve for vapor-liquid equilibrium in capillaries. The newly formed vapor mixture will be contacted by the liquid on top pf the same plate and exchanges mass as it nses through the hquid. If one assumes that the vapor mixture leaving the liquid is in equilibrium with the liquid, or that the liquid has the composition xcl, the resultant equilibrium composition of the vapor mixture leaving the liquid will have a composition ys as shown on Y - X curve for normal vapor-liquid equilibria From the above explanation, it IS clear that one theroretical stage in capillary distillation constitutes one capillary theoretical stage (from pomt A to point B) and one normal theoretical stage (from point B to point C). In the case where the Y, - X, curve and the Y- X curve are identical, a single stage m captllary distillation ~111~111be equal to two normal theorettcal stages n-t the conventtonal dtsttllatton. By rcpcating the procedure described above for constructmg one theorettcal stage in captllary distillation, the number of theoretical capillary dlsnllatton stages required for a given separation with a given reflux ratio may be readily dctermmed. The actual number of stages needed would be dependent on the separation efficiency of each plate, and is always larger than the number of thcorettcal plates. From the above discussion on one theorettcal stage in capillary dtsttllation, tt is obvtous that the degree of separation by one capillary dtstillatton stage would bc much greater than that of conventtonal distillation stage and the Murphrce plate cflictency dclincd lor conventtonal plates would have no real meaning for captllary dtsttllanon plates It would be more meanmglul to detcrmme the number of equtvalcnt thcorencal stages per capillary plate than to find the Murphree plate cftictency of the plate smce the Murphree plate efliciency in a capillary plate ISdcternuncd only by the tcrmmal tlutd compositions and does not represent the dcgrce or cxtcnt of separation the plate actually obtained For capillary dtsullatton, the Murphrcc cfficicncy greater than 1 0 simply means that the concentratton of the vapor leavmg the plate is higher than the normal

133

Y-X y;

__-_____--__-__

y;z_____t

0

A5

_____

x0

.

__

Xl

Xd

1

&z

x2

x3

1.0

Liquid Compontion, X or x Flgure 1. Model for One Theoretical Stage m Capillary Distillation of a Zeotropic Mixture

x0

Xl

XA

x2

xdx3

xc2

1.0

Llqmd Composition, X or x Figure 2 Model for One Theoretical Stage m Capillary Dlstlllatlon of an Azeotropic Mixture

134

-

-

a

To atmosphere

Coolmg water out

Coolmg water m

ermometer

Figure 3. Experimental Capillary Dlstillauon Apparatus

135

equilibrtum concentratton The Murphrce efficiency less than 1.O would mean that the concentration of the vapor is lesser than the normal equrlibrium concentratton Thcrcfore, the maximum experimental values of Murphree efficiency may be consrdcrcd an indicatton of the departure of the Y, - X, curve from the Y - X curve of the system. Because of its unique mechanisms, capillary distillation can be used for separatmg azeouopes whose Y, - X, curves differ from the normal Y - X curves. This is done by dcsrgnmg and carrying out the captllary distillation operation m such a way that the capillary vaponzatton step on one plate takes place at the nght point on the Y, - X, curve so that the liquid having a concentration below the normal azeotroptc composttron IS evaporated to yteld a vapor having a concentratton above the normal azcotropic compositton, as shown m Fig 2 Referring to Rg 2, if a liquid mixture having a composition xo is boiled the equrlibrium vapor mixture generated ~111have a composition y:. The vapor mixture, when condensed on the condensation side of the first caprllary plate will have the rdentrcal composition xl Thts condensate is transmitted to the vaponzauon srde of the plate and boded to produce the equrhbrium vapor mixture having a compostnon y:t by the capillary vaporizatron discussed above

The

concentrations y: and xl arc below the normal azeotroptc composition yf, and XA The concentrations y$ and xc1 are above the azeotroprc composition ye and XA. Thus, products of two composnions, y: and y:t, on the opposite sides of the azeotropic compositions, ye or XA, are produced. Upon the normal vaponzatlon from the liquid on top of the plate, the equihbrium vapor mtxture produced will have a compostnon yz before contacting with the next plate at which the vapor mixture 1s condensed to have the same composition x2 thus repeatmg the same separatton cycle for each plate as described in reference to Fig 1 For a mmimum boning azeotrope shown m Fig 2, the separation cycle in each capillary plate will form a loop as shown and net separation is obtained from one plate to another if the Y, - X, curve 1s located above the Y - X curve, as shown m Fig. 2 (see Table 4 for separatton data). In some azeotropes, the Y, - X, curves have also an azeotroptc composition, but differ from their normal azeotroptc compositions. For these hqutd mixtures, a capillary dtsttllation plate IS employed to separate their normal azeotropic mtxtures and a conventional sieve plate may be used in separating their capillary azeotropic mixtures. Thus, both capillary drstillation plates and conventtonal sieve plates may be employed in the same distrllatton apparatus to separate an azeotrope having an azeotroprc composition on both the Y - X curve and the Y, - X, curve As may be obvious from Ftg 1 and Fig 2, m the modeling of one theoretical stage for capillary disttllation the following ideal conditrons are assumed:

136 a) the equlhbnum vaporization of the feed solution at xo; bJ the total condensation of the equilibrium vapory: formed m a) on the condensation side (viz. the lower side) of the capillary plate, forming xl; c) the equilibrium vaponzation of the condensate at xl formed in b) at the vaporization side (vis. the upper side) of the capillary plate after being transported to the top of the plate. Here, it is assumed that the condensate was not mixed with the liquid on the top of the plate before it vapon7as

to form

YE,.

the total condcnsatlon of the cqulhbnum vapor at yrl lormcd m c) mto the liquid on the top of pIa& formmg xcl; e> the equihbnum vaponzation for the condcnsalc at xc1 formed in d)

d)

forming vapor y;, and linally 0

the total condensation of the equilibrium vapor at yl on the condensation side of Lhcplate above lo repeal the cycle The idcal conditions assumed for each of the above steps are unrealistic,

especially the formalion of xcl, y; and y2 smce the mixmg lakes place between the condcnsalc from caplllanes and the liquid on top of the plate, and between the vapor from caplllanes and the vapor from the liquid on top ol Ihe plate Therefore, a real scparatlon stage in capillary dislillalion should be less than a theoretical scparatlon slagc which is Ihc sum ofonc idcal stage based on Y - X curve and one Ideal stage based on Y, - X, curve

SEPARATION EXPERIMENTS AND RESULTS Several expenmenlal and pilot cquipmcni have been constructed and tested. They vary only m si/,e and capacity, but not funclional features. Each has a boiler, a glass column conncctmg to the lop of the still and conwsimg of a number of sections of glass bcadcd pressure pipe, a rcflux sphtler, and a condenser. The column was dcslgncd m scctlons for easy conslructlon and assembly. Each section can take a capillary play and has provislons for measuring temperature and sampling both vapor and liquid The capillary plate is fitted between the gaskcl lmcr and the beaded end of the glass pipe section These column sections arc flanged together by special coupling. Electrical heating is used for boihng the liquid mixture in the still and heating the column, which is thermally insulated. As many as ten capillary plates have been installed in the column for testing in Lhcpast although the apparatus is designed to take as many plates as desired For each run, the vapor rate, the rcflux ratio, the llquld height, and the cooling waicr raic arc controlled

137

Ag 3 IS a schcmauc dlaglam of a typical capillary distillation apparatus including the live major components the ~1111 1, the column sections containing the capillary plates 2, the rcllux spllttcr 3, the condcnscr 4, and the top scctlon 5 The elcctnc hcatcrs for the still and the column, the sampling ports for both liquid and vapor provldcd for each plate, the thcrmocouplcs used in monitonng the temperature of the fhnds and of the plates arc not shown in Fig 3. The rcflux ratio IS controlled by the rcflux splitlcr 3, which is acluatcd by a solenoid that is controlled by an on-off type Llmcr(not shown). The timer recc~ves a signal from the vapor tcmpcrature sensor 6, and conlrols the solenoid through the time setting desired. Many different types of capillary plates have been construclcd and tested. The structure of these plates mcludcs single metal mesh scrccs, multllayer metal mesh screens, smtcrcd porous metal plates and photoctched metal screens Smcc the thermal conductlvlty of the plate IS very important, usually conductors were used as plate matcrrals They mcludc copper, brass, stamless steel, alummum, etc. The si/.es of capillary plates lcslcd wcrc I Inch, 2 inch, 3 inch, and 5 Inch In diameter, and Ihc dlamctcrs of capillary passages m these plates vary between a few microns and a few hundred microns. Other than the sintcrcd porous metal plates used, all of the plalcs used have a uniform SW capillary passages. Both the sintcrcd porous mcial plaies and the muhilaycr metal mesh screen plates have interconnected captllary passages while the single metal mesh screen plates and the pholoctchcd metal screen plates have unconncclcd capillary passages. The thickness of the plates used varies bctwccn a few thousandths of an Inch and to one sixteenth of an inch All of the plates were dcslgned and machined to lit to the column dcslgn A hole was bored through each plate to allow a downcomer to fit properly The dlmcnslons of the downcomer Hcrc detcrmmcd cxperimcntally to allow smooth operation under the spcclflcd cxpcnmcmal condllions includmg vapor rate, liquid rate, liquid height, etc To allow for quick adJUStmCntof hquld height above a plalc, a machine thread was cut m the top of the downcomer, and the hclght of the downcomcr can bc adjusted by movmg the position of the two machmcd nuts In Ihc opposltc stdcs of Ihc plate. The four capillary plates used are hstcd in Table 1

Table 1. A.

Spccificatlon of Ihc Capillary PINGSUsed Smtcred Porous Stainless Steel Plate-1 Manufacturer. Paul Trimly Micro Corp., Cortland, NY, USA Thlckncss. l/16 inch (0 159 cm) Port SK Dlstnbution: See Fig. 9 Porosity* 44% Dlamcter: 2.625 mchcs (6.668 cm)

138

B.

Sintered Porous Stainless Steel Plate-2 Manufacturer: Brunswick Technetics, DcLand, FL, USA Product Number: FM 1104 SS Percent Density: 40% by volume Thickness: 0.062 inches (0 1575 cm) Area Denstty: 1 042 lbs./ft2 Median Pore Size* 13.5 microns Surface Arca: 3900 in.2/lb. (55,471 cm2.kg)

C.

Sintered Porous Stainless Steel Plate-3 Manufacturer Brunswick Tcchnetics, DcLand, FL, USA Product Number. FM 134 Thickness. 0.035 inches (0.0889 cm) Pcrccnt Density: 53% by volume Pore Size Range: 2.5-280 microns Surface Ama. Approx 3200 in.2/lb.

D.

Photo-etch Stainless Steel Screen Manufacturer: Buckbee-Mears Co., ST. Paul, MN, USA Thickness: 0.005 inch (0.0127 cm) Hole size: 0.006 inch (0 0152 cm) % open area 22 Diameter: 4.0 mchcs (10.165 cm)

A hypoderrmc needle and syringe were used for takmg both hqutd and vapor samples from the separate samplmg ports, which are equipped with a capillary tube leading to the desired sampling point. The pressure differential across each capillary plate was measured on a monometer. The pressure above the top plate was maintained at atmospheric pressure. Liquid mixtures of various organic and inorganic compounds, including several azeotropes, have been separated in accordance with the following procedure: (1) The height of the liquid pool on each capillary plate was established by adjusting the elevation of the inlet port of the downcomer leading from the plate. (2) All the sample probes were positioned at the desired sampling points to be ready for sampling. (3) The distilling flask, or still, was filled with the liquid mixture to be separated. (4) The column heater was adjusted so that the temperature on the outer wall of the column was identical to the temperature of the reflux.

139

(5) A clean syringe was used to take each liquid sample of 1.0 mtllihter, after 0.5 mtllilitcr of the hqutd had been wtthdrawn and dtscarded from each samplmg port; and a larger clean syringe was used to take each vapor sample of 10 milhlitcrs from a separate sampling port. (6) Both the liqutd and vapor samples were taken to ascertain steady-state conditions in each run(7) Each sample was analyzed immediately after withdrawal by both refractometry and chromatography. (8) Temperatures and pressures were read for every sample taken and sampling for each run was repeated at least five times in order to confirm the results (9) The vapor rate was determined by mcasurmg the rate of the condensation m the condenser. The vapor rate was vaned by varying the heat mput using a vanable transformer The Murphree plate cffictenctes Ev for separation based on vapor composttions were calculated using the standard relationship: E, = Yn - Yn+l * Yn - Yn+l

Ul

In a manner analogous to the expression for the Murphree plate efficiency, the following expresston for efficiency of heat transfer for the plate tn

PI can be readily shown to be related exponentially to the overall coefficient of heat transfer of the plate, the heat capacities of the vapors, the heat capacities of the liquids, the latent heats of condensation of all the components, and the contact time between the vapor and the liquid on the plate n. It has been found experimentally that T v n IS practically equal to TL.n. for every run. For this reason, an alternate and useful definition for the efficiency of heat transfer has been employed in an attempt to find any possible effects that some important operatronal conditions may have upon the efficiency of heat transfer across the capillary plates tested. For this purpose, the following efficiency defined in terms of the temperature of the hquid entering the plate n, Tf_n., and that of the liquid leaving the same plate, n.n+ 1 was employed. g

2LJTL n+l

[31

140

A.

Separation of Mixtures of Ethvlene Dichloride and Acetonitrile

The experimental results of capillary distillation runs carried out with a binary liquid mixture consisting of ethylene dichloride and acetonitrile, in a concentration range from 19 to 36 mole percent acetonitrile, are set forth in Fig. 5,6,7, and 8 and are discussed hereinafter. These results include runs where the level of the liquid above the capillary plates was maintained at 0,0.25,0.50, 1.O and 2.0 inches for various runs. The capillary plates used in this experiment was the sintered porous stainless steel plate-l listed in Table 1. It has a diameter of 2 inches and only two plates were used. Both the normal Y - X curve and the experimentally determined Y, - X, curve at 1 atm are shown in Fig. 4, which indicates that the azeotropic composition appears to have shifted when the system 1.0 0.8 0.6 0.4

$

s

0.2 0.0

0.2 0.4 0.6 0.8 1.0 Llqurd Composruon, X or x Figure 4. Acetonitrile-Ethylene Dichloride System: Y-X Relation at 1 atm 0.0

Vapor Rate,

(

mdhm31: 1 cm2sec 1

Figure 5. Murphree Plate Efficiency vs. Vapor Rate

0

1.00

A

200

141

is placed m small captllanes. As may be obvtous from Fig. 4, this system is considered to be very dtfficu’n to separate by ordmary disttllatton as it has small relative volattlmes and an azeotmpic composition at 60.2 mole % acetronitrile in a bulk solunon at one atmosphere. For the above reasons, it was chosen for studying the separatton characteristics of capillary disttllation.

Ld

^ ox ti 3

Vapor Rate Through Plate 15

A 00s

10

0

015

A

0 22

0.5

.

032

e

iii jj

( milllmo,:~ cm%ec

1

a

Llquld Hclght on Plate, inches

Figure 6 Murphree Plate Effictcncy vs Ltquid Height In Fig. 5, plate efficiency IS shown as a function of the vapor rates for the various liquid levels In Fig 6, the same data are prescntcd to show the plate efficiency as a functron of hqurd height on the plate for various vapor rates As will be noted, the observed plate cflictenctes generally were much higher than those typically achieved with conventional perforated or sieve plates although only a single plate was used In addition, the plate efficiencies for the system tested were found generally to increase with the height of liquid on the plate, but only up to a liquid level of about one inch For liquid levels over on mch in height, the plate efftcrency decreased with the lower vapor rates. This may account for the apparent opumum liquid height suggested by the data presented m Fig 6 It is very important to note that the plate efficienctes observed when no liquid pool was maintained on the captllary plate (I.e. at 0 inch liquid height) ranged between about 40 and 70 pcrccent This obsctved fact 1s clear evidence that the vapor mixture actually condensed and lodged in the porous structure of the captllary plate with the consequent capillary vaporizatton from the capillary plate. With a conventtonal sieve plate under the same circumstances (i.e. in the absence of any liquid pool on the plate) the plate efficiency would have been zero since no enrichment of the vapor passing such a plate would be expected in the absence of a hqurd pool on its top As will be noted in Fig 5, the observed plate efficiencies appear to be unaffected by varying the vapor rate at liqurd levels of one inch and lower The superficial vapor rates for the capillary plate employed were lower than those

142

commonly expcnenccd wtth convcnuonal SICVC or bubble-cap plates However, the allowable maxrmum vapor rate for a captllary plate can bc mcrcascd crther by increasmg the porostty and the thermal conducttvity of the plate, or by reducing the thickness of the plate In Fig 7, the prcssurc drop across the capillary plate IS shown as a function of vapor rate for the various liquid heights As might bc expected, the pressure drop was found to Increase with the height of liquid on the capillary plate but to bc rclattvcly mdcpcndcnt of the vapor ralc

4

22

Lqud Hc~ghton I’hc

F

(mc tic\)

3-W

::E= 0 0

I 1

l

0 00

0

0 2s

0

0 50

0

100

A

2 (x)

2

mdlimo :. Vapor Rate, ( cm*w_ 1

Frgurc 7 Plate Prcssurc Drop vs Vapor Rate In pnncrplc, thcrc should bc lmle prcssurc drop across a caprllary plate whtch IScomplctcly wcttcd by the condcnscd hqutd unless there IS heat loss lo the surroundmgs. Smcc the prcssurc drop data rcportcd m Fig. 7 arc for a caprllary plate whrch was completely wcttablc, the obscrvcd pressure drop must be primarily due to the stattc prcssurc of liquid above the plate that the vapor must withstand. The remammg mcrcmcnt of the pressure drop may be attnbuted only to heat loss and the possrblc blowmg of vapor through larger ports m the plate Since a caprllary prcssurc ot one inch water corresponds to a pore radtus of 220 microns, an opcratmg prcssurc drop of two mchcs water at a liqutd lcvcl of one inch, as shown m Frg 7, would mdrcatc that some vapor blowing through the plate could have been occunng through pores larger than 220 mrcrons m radms, assuming ncghgrblc heat loss to the surroundmgs From the pore-srzc dtstrtbutton curve of Frg 8, It can bc seen that 40% by volume of the pores m the capillary plate cmploycd m the cxpcnmcntal runs wcrc larger than 220 microns m

143

Fl&wrc8 Pore SK Dlstnbutlon of Plate radius Accordmgly, 11may bc concluded that In the capillary plate cmploycd, both the capillary dlsnllatlon mcchanlsm and Ihc conventlonal Steve plate dlstlllatlon mechamsm contnbutcd to the obscrvcd cfficlcnclcs Needless to say, a capillary plate can bc rcadlly constructed to have a pore-size distribution of any dcsircd profile As noted above with rcfcrcncc to Fig. 5, 6 and 7, the observed plate cfficicnclcs wcrc much hlghcr, frcqucntly m excess of 100 percent and as high as 175 percent, than those commonly cxpcncnccd with conventional plates This is due to the mcchamsm through which capillary plates work as already dlscussed above Smcc Murphrcc plate cfflclcncy has a very llmltcd sigmflcance m capillary dlstlllatlon, the true cxtcnt of \ep,lratlon by each plate I\ better rcprescntcd by the numbcl of cqulvalcnt theorctlcal stages of the plate NC. which can bc dctcrmmed rcadlly by the usual stalrcase constructlon bctwccn the mltlal and final vapor composltlons across the plate using the normal Y - X curve As cxamplcs, the number of cqulvalcnt thcorctlcal stages for some typical runs arc shown with thclr Murphrcc cl ficlcnclcs m Table 2 for comparison w1t.hEn. Table 2

Number of Equ~valcntThcorctlcal Stages Ne

Run Number 1 2 9

yn 0 326 0 320 0 326

19

0 308

20 23 24 27

0 307 0 292 0291 0.287

Y; 0 321 0 312 0 307 0 282 0 286 0 274 0 274 0 270

Yn-1 0 276 0 3 10 0 267 0 244 0 242 0 238 0 233 0 231

En 1 12 1 20 1 ox

Ne 1 26 1 26

1 66

1 61 2 64 2 35 2.45 2 40

1 45 151 1 39 1 44

1 so

144

As may be seen from Table 2, the number of equivalent theoretical stage for all runs are greater than their Murphree cfficicncles In Run No. 19, the En value indicates that the dtstancc between the Y, - X, curve and 45O diagonal for acetonitnlc-cthylenc dichloride mixture is at least 66% greater than that between the Y - X curve and 45O diagonal m the composition range studled. The experimentally determined Y, - X, curve is shown m Fig. 4 for comparislon with the normal Y - X curve. The departure of Y, - X, curve from the normal Y - X curve may be due mainly to the polar interactions caused by the polar molecules of acetonitnle and partly to the large dlfkrence m molar volume bctwcen the two components. From Table 2, it may be seen that the number of cqulvalent theoretical stages is unusually large for a run having a high Murphree efficiency although the two values are not related directly m any obvious ways. The efficiencies of heat transfer across capillary plates were determmed usmg Eq 3, and correlated with the temperature differential between the bollmg llquld in the still Tb and the condensate leaving the condcnscr Tc when only a single plate was employed As may be seen with refcrencc to Fig 9, the thermal effclency &I IS in general close to 100 percent for nearly every run. It shows little dependence on the temperature differential, Tb-Tc, which was at most only 6OC. Because of the extremely high efficiency of heat transfer for nearly every run observed, it * was not possible to find experimentally an &I effect of any operatlonal conditions upon the efficiency .

*ia

G 5 b 12 zi !I g

1*02 -1 1.01 100

0.99 0.98 097 0.96 095

0

.**‘o’

‘0‘J. yb*p;$y . ‘0. .

l

;.p..lq’., . . .. . . .

’ ’

I

I

I

I

I

1

2

3 Tb-Tc

4

5

Figure 9. Effect of Tb - Tc on Thermal Efficiency

6

145

Seoaranon of Ethanol-Water Mtxturcs

B

Both the Y - X curve and the Y, - X, curves, at 1 atm for this system, are shown in Fig 10 The thcorcttcally prcdtctcd Y, - X, curve agrees very well with the expcrimcntal data obtnmcd for Yc vs. Xc, which have been published earlier (21, 23, 24, and 27) The two vapor-liquid equilibria curves at ethanol concentrattons above approxtmatcly 20 molt % differ considerably, and no azeotropic compositton appears to exist when the system IS placed in small capillaries.

UC-W

g

0.8

E 06 & 04

/#!fzcc 0

Y-X

8 0.2

$ 0.0

Normal CapdIary Expcrlmental

0.0 0 2 0.4 0 6 0 8 1 0

LJQUID COMl’OSITK>N, X or x

Figure 10 Ethanol-Water System. Y-X Relation at SOOC

In order to study the separation charactensttc of capillary distillation for azeotropic mixtures having a minimum boiling point, the experiments were concentrated in separating mtxturcs having concentration several mole % below and above the azeotropic composttion of89.4 mole % at one atmosphere. The smtered porous stamless steel plate-2 of Table 1 was used for separation experiments The results of three separation runs are given in Table 3 The plate effictencics above 100% means that the concentratton yn of the vapor leaving the plate is htghcr than that of the normal equthbnum vapor y:; and the negative plate efftcienctes were obtatncd when the concentratton of the vapor leaving the plate below yn+l is htgher than y;‘. The time required to reach a steady-state for these separation runs involving an azeotropic composition was considerably longer than the elapsed time of each run because of the separatton mechamsms of the capillary distillation as described m reference to Fig 2

146

Table 3. Run:

Plate #

Scparauon of Ethanol-Waler Mlxturcs l-ET Reflux: Total Liquid Hctght. 1.5 Inches Elapsed Time 2 hours X

Y

Y*

Ev

82.10 83 80 85 20 86 20

260.1 205 7 132 1 126.8

Bottom

1.5 mchcs Elapsed Ttme 2 hours

Plate # Bottom 1 2 3 4 Run:

(mo;(c %) 83.770 84.309 85 193 85 992 86 491

Y (mole %) 84.629 85.364 86 027 86.54 1 87 053

* (mie %)

Ev (mole %)

85.19 86.00 86 30 86 50

131.0 1043 1883 -1248 8

Y* (mole %)

Ev (mole %)

87 50 87 55 87 90 88 10

1606 -241 8 376 3 -1095 2

3-ET Reflux. Total Ltqutd Hetght 1 5 mchcs Elapsed Time 3 hours

Plate #

X

Bottom 1 2 3 4

(mole %) 86 751 87.123 87.455 87 821 88 061

Y (mole %) 87 287 87 629 87 820 88 121 88 351

In order to test the practtcality of captllary dtsttllatton n-t separatmg a mimmum bothng pomt azeotrope compostuon, ethanol-water system was chosen

147

since both the theoretically predlctcd Y, - X, curve and the experimentally determined Y, - X, data agree well for this system The photo-ctchcd stainless steel screen of Table 1 was chosen as the capillary plate to be used because the plate has a definable geometric structure and thcrcforc Ihc cxpcnment can bc easily repeated m future if ncccssary, although Lhchole slzc IS too large and the thickness IS too small to bc very clfcct~vc as a capillary plate Only two stage (viz. two capillary plalcs) wcrc employed For each run, a feed solullon 01 Ihe deslrcd concenlratlon was synthesized using 200 proof ethanol and dlstlllcd water Each run used total reflux and a hquld height of 1 5 inches Both hquld and vapor samples were taken and analyzed for each run lo pcrm~l Lhc dctermmallon of the vapor and liquid concentrations for each plate. The sampling was repeated al least five times dunng each of the three steady-stale runs and their average concentration values aTereponcd in Table 4. As may be noted from the table, Run No 6A was conducted in the concentration range below Lhe azcotroplc conccnlration As expected, both the liquid and vapor conccntrallons of clhanol incrcasc as the vapor nscs across each capillary plate bcforc rcachmg die ayeolropic composllion Run No 4 was made with the feed conccntralion of 88 55 ethanol At a steady state, the liquid concentration CLJ+] m the sl11lpot IS 88 l%, on ~hc bollom plate CL n IS 91.8% and that on the top plate CL n_ 1 IS 92 6%, indicating that the aLeotropc was broken by the bottom plate mlo two products one having a composlllon below the azeotroplc point and the other having a composition above the azeotropic point

Table 4.

Scparallon of Mmlmum Boiling Point Azeotropic Composltlons

Run No 4 Vapor ralc = 0.2364 mIs/scc Feed concentration = 88.50 molt % clhanol Stillpot boiling point = 78 0 f l°C Trial Number 1 2 3 4 5

xn+ 1 (mole %) 88.1 88.1 88.1 88.2 88 1

xn Yn (mole %) (molt %) 89.4 91.8 89.4 91.8 90.0 91 8 90.0 91 6 90 2 91 8

xn-1 (mole %) 92.6 92 6 91 8 91 8 91.8

Yn-1 (mole %) 90.0 90.0 90.2 89.8 90 8

148

Run No. 5 Vapor rate = 0 1773 ml&cc Feed concentratton = 91.2 mole % ethanol Stillpot boiling point = 77.5 k l°C

Trial Number 1 2 3 4

xn+l (mole %) 90 9 90 9 90 8 90 9

xn Yn (mole %) (mole %) 94 7 90 9 94.9 91 7 94 2 91 8 95 7 91 8

xn- 1 (mole %) 94 9 94 9 95 7 95.7

Yn-1 (mole %) 91 8 92 9 92 8 92 2

xn-1 (mole %) 86 8 86 0 86 0 86 8 86 0

Yn-1 (mole %) 88 6 88 7 89 4 89.4 88.7

Run No. GA Vapor rate = 0 1970 mls/sec Feed concentration = 80.8 mole % ethanol Stillpot boilmg point = 78.0 + l°C

Tnal Number 1 2 3 4 5

xn+l (mole %) 804 80 4 80 5 804 80.4

xn Yn (mole %) (mole %) 84 0 87 6 84.0 84.9 84.0 87 6 84.0 87 6 84 0 87.6

At the top plate, both the liquid and vapor concentrations arc mcreascd. It is important to note that when the liquid concentration is above the azeotmpic point, the vapor concentration is less than the liquid concentration as expected from the normal vapor-liquid equihbrmm curve The data of Run No 4 clearly indicate that the azeotrope was separated by capillary distillation Run No. 5 was made usmg the feed concentration of 91 2% ethanol, which IS above the azcotropic point of 89.4%. Both the hquid and vapor conccntrattons were mcreascd shghtly by the two plates used Agam, the vapor concentration IS lower than the hquid concentration on each plate as may bc expected horn the normal vapor-liquid equihbnum curve As discussed earlier in reference to Fig. 2, m the separation of an azeotropic mixture by capillary distillation both the normal and the capillary vapor-liquid equihbria control the separation effect, and the vapor concentration can increase and then decrease at each stage by gomg through a separation cycle This separation cycle is repeated at each stage as a vapor m condensed at the lower side of a capillary plate and a new vapor IS formed from the upper side of

149 the plate This scparanon mechsmsm 01 captllary dtsttllatton rcqutrcs a longer time for each stage and the cnttrc column to reach a true steady state Smcc the system approaches n true steady state very slowly, rt 1s qmte possible that the above three runs have ncvcr rcachcd the true steady state. Photoctchcd plates with smaller holes and larger thtckness would have given bcttcr separatton result and arc thcreforc rccommcnded strongly Smce Y, vs X, curve shows no azcotroptc point, thus system can bc separated completely by capillary dtsttllatton wtth addittonal stages Separatton of 1-Propanol-Waler Mixtures

C.

Both the normal Y - X curve and the captllary Y, - X, curve at 1 atm are shown m Ftg 11 which indtcatcd that no a/.cotroptc compostnon 1s formed when the system was chosen to test the scparauon charactcnsttcs of captllary dtsttllatton for liquid mtxtures havmg an s/cotroprc compostnon at a lower conccntratlon (43 6 % I-Propanol) The \mtcrcd porous stamlcss rtccl plate-3 of Table 1 was employed for the scparatton cxpcnmcntc

1.0

08 06 /Z!Z?!IYc-x, 0.4 02

A 0 0

Normal Capillary Expcnmcnd

Y-X

00

0 0 0.2 0 4 0 6 0 8 I 0 LIQUID COMI’OSITION, X or x

Figure 11 1-Propanol-Water System. Y-X Relauon at 40°C

More than forty separation runs at I-propanol concentrations ranging between 1 0 mole % and 60 molt % have been made Two typical runs starting with very dtlute soluttons and cndmg up with solunons having concentratrons exceeding the azeotroptc composmon (viz. 43.6 mole %) arc reported m Table 5 The elapsed time for Run I-PW was 16 hours and that for Run 2-PW was 20 hours, although both runs had*ncver reached the steady state. Steady-state would require several days to reach smcc the rate of separation of an azeotrope is extremely slow as described above in rcfemnce to Fig. 2 Very high Murphrce plate efficiencies Ev excccdmg 100% or negattvc plate cffictcncies were again obtained.

150 Scparauon of I-Propanol-Water Mlxturcs

Table 5 Run:

I-PW Rcflux- Total Ltquid Hctght 1 5 mchcs Elapsed Tlmc* 16 hours Vapor Rate 0 4 ml/we

Plate #

X

Bottom 1 2 3 4

(mole %) 5 198 37 555 43 146 43 410 44 161

Y (mole %) 37 839 43.159 43 850 44 155 44 454

Y* (molt %) 41 43 43 43

Ev (mole %)

10 13 30 80

163 1 -2383 0 -55.5 -84 2

Y* (molt %)

Ev (mole %)

Run 2-PW Reflux: Total Liquid Height 1 5 inches Elapsed Trme 20 hours Plate # Bottom 1 2 3 4

D

X

(molt %) 1 886 25 228 41 258 43 566 44.409

Y (molt %) 21 548 41 895 44 086 44 197 44 287

39 OS 42 10 43 29 43.90

1163 1068 8 -139 4 -30.3

Senaratton of Acetone-Methanol Mixtures

Both the normal Y - X and the theoretically predicted Y, - X, curve are shown n-rFig 12 which show that the Y, - X, curve dcpans greatly from the Y - X curve at 1 atm and no azeotroptc composttion IS formed when the system IS placed in small capillaries. Therefore, this system should be an ideal system to demonstrate the beneficial effect of capillary distillanon Although several runs were made m the entire concentration range, the results of a typical separation run starting with 11.16 mole % acetone are shown m Table 6. With only six capillary plates (the sintered porous stainless steel plate-3 of Table l), the acetone

151

00

02

0.4

06

LIQUID COMI’OSITION.

08

10

X or x

Frgure 12 Acctonc-Methanol System Y-X Rclauon at 4S°C concentration was increased from 11 16% to 66 77% The same separation would require at least 11 thcorctical plates in the ordmary dtstillatron The Murphree plate effrcrencrcs E v obuuned were all above 100% At the concentratrons above 70 molt % acctonc, the scparatron would be slower smcc there is an aLcotroprc composrtron on the normal Y - X curve at 79.6 mole % acetone Separation if Acetone-Methanol Mixtures

Table 6. Run:

2-AM Reflux: Total Liqurd Herght: 1 5 inches Elapsed Trme* 3 hours Vapor Rate: 0.85 ml/see

Plate #

X

(mole %) Bottom 1 2 3 4 5 6 Condensate

11 168 21095 34.522 44.342 46 390 58 811 64 200 66.778

Y (mole %)

Y* (mole %)

Ev (mole %)

22.649 36.788 53.009 54.950 60.160 65.160 68 279

35.5 1 47.50 53.65 55.80 63 20 66 12

109.4 151.4 302.8 691 2 182.5 342 9

152

E.

Separatton of Water-Acetic Actd Mtxturcs

Thts is another sysicm whtch IS known to bc very difftcult to scparatc by dtstillatton Both the normal Y - X curve and the thcorcttcally prcdtctcd Y, - X, curve at 1 atm arc shown m Fig 13 whtch mdlcatcs that the Y, - X, curve departs from the Y - X curve only sltghtly as both water and acettc acid cxhtbn strong polar mlcracttons with surlaccs m small captllartcs The smtcrcd porous stainless steel plate-2 of Table 1 wa\ u\cd a\ the captllary plate m the scparatton

Ftgurc 13 Water-Accttc Acid System Y-X Rclauon

expertmcnt Only two runs wcrc catrtcd out* the results arc shown in Table 7. Because of the cxcessivc corroston of the captllary plates by acetic actd at high concentrations, the runs wcrc brtcf and stopped a few hours after starting Those plates near the top of the column, via plates #5, and #6, have ncvcr reached a steady state although plate cffictcnctcs excccdtng 100%~or ncgattve plate cfficiencics wcrc obtalncd for other plates A disttllatmn column uttllLmg smtemd porous glass plates have been butlt and is bcmg tcstcd for scparatton of corrosive liquid mtxturcs such as wiWr-iUctlc actd system

153

Scparatlon of Waler-Acctlc Acid Mlxturcs

Table 7 Run

I-WAc Rcllux Total Lqwd Hcqht. 1.S mchcs Elapsed Tlmc* 2 5 hours Vapor Rate 0 133 ml/see

Plate #

X

(molt

Bottom 1 2 3 4 5 6 Condensate

94,)

71 023 73 929 7.5 032 79 908 81 647 X4 824 92 016 92 776

Y

(molt %) 68 57 1 8 1.999 83 824 x5 836 87 540 88 338 93 916

y*

EV

(molt %)

(molt %,)

Xl 81 84 86 88 94

107 9 -2645 0 531 5 1664 106 4 75.8

02 93 20 83 29 38

2-WAc Run Reflux Total Liquid Hclght 1 5 mchcs Elapsed Tune 2 5 hours Vapor Rate 0 143 ml/a Plate # Bottom 1 2 3 4 5 6 Condcnsatc

(mol(c %) 83 778 87 703 87 2x5 90 760 91 so2 92 024 97 714 94 999

(mok o/o) 86 892 91 098 91 520 93 507 oj & 1 94 382 9s 173

Y* (molt o/o) 91.20 92 13 92 58 93 72 95 45 9x 17

Ev (molt %I) 97 6 1317 8 1875 269 5 22 0 20 9

154

Seoaration of Methanol-Water

F.

Thts system was studied because its theoretically predicted Y, - X, curve lies below the normal Y - X curve at 1 atm with an azeotropic point, indicating that the relative volatility of methanol to water dccrcascs when the system is placed in small captllarics The system is known to be easily separable by the ordinary dlsttllation although its separation may bc very diffcrcnt by capillary distillation, as may be expected from the Y, - X, curve prcdictcd (set Ftg 14)

00

02 Llquld

04

06

Composltwn,

OX

IO

X or x

Figure 14. Methanol-Water System: Y-X Relation at 760 mm Hg The smtered porous stainless steel plate-3 of Table 1 was employed as the capillary plate for the expcnmcnt. The results of four typtcal runs are given m Table 8. From Run No. 7-MW, the separation in the concentration range between 1 mole % and 20 mole % methanol, where an azcotropic composition is formed m capillaries, appears to be quite different from what would be expected in the ordinary dtsttllatton according to the normal Y - X curve In this run, It takes 6 stages of capillary disttllation to separate what can be separated in one theoretical stage by the ordmary distillation as may be expected form the normal Y - X curve In Run No 6-MW, from 44 mole % to 64 mole % methanol, It takes 6 stages of capillary dlsttllatton to separate what can be separated n-tone theoretical stage by the ordmary distillation, as may bc expected from the normal Y - X curve. It 1s mterestmg to pomt out that in Run No 4-MW and Run No 5 MW, only one capillary plate is nccdcd to completely separate methanol-water mixtures having concentrattons of ethanol higher than 70 mole %. The same separation by the ordinary dtstillatton would rcquirc more than 8 theorettcal stages. The results of the separation of methanol-water mixtures illustrate very well the unique charactenstics of captllary dtsttllatton. They indicate clearly that the separation of capillary dtsttllatton IS affected by both the normal Y - X curve and the captllary Y, - X, curve

155

Table 8.

Scparatton of Methanol-Water Mtxlurcs 4MW Reflux. Total Feed 62.90 mole % methanol Elapsed Time 24 hours

Run:

Plate # Bottom 1 2 3 4 5 6

X

(molt %) 65 30 70 60 71 12 72 OX 74.31 76.11 78 76

Run: 5-MW Reflux. Total, Feed Plate # Bottom 1

2 3 4 5 6

Y (molt %) 83.94 100 00 100 00 100 00 100 00

100.00 100.00

74 73 molt % mcthitnol, Elapsed Ttmc

(molt %) 71 96 74 72 76.00 76.92 77 70 79 28 80.93

Bottom 1

2 3 4 5 6

X

(molt %) 44 54 50 73 53.28 57.81 58.04 59 00 64 01

Ev (molt %) 430 56

8 hours

*

X

Ev

(molYc%) 100 00 100 00 100.00 100 00 100 00 100 00 100 00

Run: 6-MW Reflux. Total, Feed: 49 39 molt % methanol, Plate #

Y* (molt %) 85 13 87.67 88.18 88 50 89 68 90 05 91.33

Y (molt %) 77.70 86.83 83 15 86.85 86 78 8841 89 69

(moYlc%) 88 46 89 76 90.03 90.44 90 82 91.58 92.38

(molt %)

Elapsed Ttme. 5 hours )‘* (molt %) 75.00 78.50 79 79 82.07 82 18 82 67 85 01

Ev (mole %) 1145.00 -342 59 -39.66 -37 65

156

Run: 7-MW Rcflux: Total; Feed 6 23 molt o/omethanol, Elapsed Ttmc 2 hours Plate #

X

Bottom 1 2 3 4 5 6

(mole %) 1.89 3.01 3 21 4 31 5 31 6 69 20.01

Y (mole %I) 14.48 1s 74 23.56 27 08 36 43 37 23 59 14

Yr (molt %) 1 11 1675 17.76 23 30 28 34 3s 30 57 90

Ev (mole 8) 55.51 387 13 -1353 85 742 06 -70 80 106 00

DESIGN AND OPERATIONAL CONSIDERATIONS The following considerattons are important in designing and operating a capillary distillation apparatus in order for the apparatus to produce the unique effect of capillary distillauon discussed above. A.

Pressure

The pressure dtffercntral of the rising vapor between two plates APv m the vapor phase must be larger than the sum of the static pressure of the liquid on top of the plate AhL and the pressure drop of the condensate transmittmg through the capillary passages APLVc APvlAhL+APLc Since the magnitude of APLc. is very small and far less than the capillary pressure APc which is the driving force for the flow of the condensate, m general APv must be kept larger than AhL. APv>AhL

[51

When the ve condition is not met, “weeping” of the liquid may occur the pressure differential of the nsing vapor between two plates APv is lfabo too large and exceeds the capllla;y pressure AP,, the rising vapor will then blow through the capillary passages producing no effects of captllary distillation as may be obvious. Therefore, the pressure differential of the nsing vapor between two plates must be kept smaller than the capillary pressure.

&
WI

157

If a capillary plate havmg extrcmcly small capillary passages IS used, the captllary prcssurc APc is very large. Under the ctrcumstancc, APv maybe large enough to force the liquid mstdc the downcomcr to flow upward through the plate above. This would crcatc a flood-hkc condttton and thcrcforc must be prevented by employing a modcratc vapor rate. In summary, the pressure of the vapor between two plates APv should be kept larger than the static pressure of liqutd on the upper plate AhL but smaller than the captllary pressun: APc as: APc>APv>Ah~

B

[71

Vaoor Rate Ltmttattons

If a capillary plate dots not have enough captllary passages to allow fast transmission of condcnscd vapor or if the captllary passages arc too long and too small so that the rcsrstance to the condcnsatc flow IS too great, the condensed vapor can not be transmtttcd through the plate at the rate the vapor is condensed. Under the above condittons, the consumption of the liquid on top of the plate due to botlmg would be grcatcr than the supply of the liqutd through capillary flow of the condensate, and consequently, the liquid on top of the plate will deplete. This situation 1s caused by the limitation m captllary flow and may be prevented by employing a plate havmg a suitable capillary structure or by regulating the vapor rate to suit the existing plate

C.

Flooding

As tn conventional sieve plates, “floodtng” or “prtmmg” like phenomenon may occur inside a captllary dtsttllatton apparatus having a small plate-spacing or treating a liqutd mixture containing a surface active compound when boiling of hqutd on top of the plate is too rapid. In a conventional distrllation apparatus, flooding may occur due to an excessive pressure drop actoss each plate which rcsultcd IS a very large pressure diffcrcntial existing in the apparatus. However, flooding of this kind ~111occur less likely in a capillary distillation apparatus, m whtch the prcssurc drop across each plate is not only very small but also affected by the capillary pressure D

Ltautd Entramment

Unlike conventional steve plates, entrainment of the liquid by vapor flow may not occur between two plates in capillary distillation. Since the vapor formed on each plate condenses at the lower side of the plate above and does not bubble through the ltqutd on the plate, the liquid entrainment in a capillary distillation apparatus is not ltkcly to occur..

158

E.

Heat Transfer Rcautrcmcnts

As already discussed above, the efficiency of heat transfer in a captllary distillatton apparatus is cxtrcmcly htgh, and it rcqutrcs only a small temperature differential across the plate to operate smoothly Thcrcforc, the liqutd on top of the plate may boll under subcoolcd condtuons under the mfluencc of the captllary pressure The thermal rcststanccs and tcmpcrature dtffcrcnttal across each plate are directly rclatcd to mass transfer and other operational vartablcs, and the analysis of thcsc matters 1squnc mvolved as rcportcd earhcr (6)

CONCLUSIONS AND RECOMMENDATIONS The separation charactcrtsttcs of capillary dtsttllatton IS very umque Although the cffictcncy of scp‘u-anon IS affcctcd by both the normal Y - X relationship and the captllary Y, - X, relationship, thctr effects and the mtcracttons arc not fully understood at thts tnnc The model of one theorcttcal stage for capillary disttllatton ptopo\cd m this paper IS based on scvcral ovcrstmphficd assumpuons whtch ,trc not met in the real sttuatton It IS safe to say that while captllary disulltton can scparatc most hquid mtxturcs wtth cffictcnctcs htghcr than that of the ordmary disttllation, tt scparatcj poorly those liquid mixtures whose Y, - X, curve hcs below the Y - X curve such as the mcthanolwater system Very few hquid mtxturcs appear to have this tcndcncy. The prcssurc drop per plate m a captllary dtstillation apparatus IS very small, and unhke the conventional plate-type apparatus, the pressure drops across all of the captllary plates m a captllary dtstillation apparatus arc al fectcd by the capillary prcssurc across the mcntsci m cap~llancs which oppose the stattc pressure of the hquid It IS highly rccommcndcd that further \tudics bc c,trncd out on captllary dtstillation, c~pcctally on Its application to the acparatton of various types of azcotropcs, cxpcrimcntal dctcrmmation of v,ipor-ltquid cquilibrta of ltquid mtxtures m small captllancs and scparauon mcchamsms NOTATION

Ev Ah

NC

ap T ; Y

= Murphrce plate cl ficicncy m Eq 1 = static pressure, dyncs/cm2 = number of equivalent thcorctical stages = pressure drop or dtffcrcnnal, dyncs/cm2 = tcmpcraturc, OK or OC = ltquid conccntranon, molt fractton or molt pcrccnt = cqutlrbnum liquid conccntrauon, molt fraction = vapor conccntriiuon, molt fraction or molt pcrccnt = cqutltbnum vapor concentration, molt fraction = thermal cfficicncy m Eq 2 through 3

159

SUBSCRIPTS c L V

n n+l

= capillary conditions. = lqu~d-5latc condmons = vapor-slate conditions = nth plate condition5 = n+l th pl,itc, 01 wll conditions

REFERENCES 1 la lb Ic Id. 2 3 4 5. 6 7 8 9. 10. 11 12 13 14.

G C Ych, Scparat~on of Lqnd Mlxturcs, U S. Patcnl No. 4,118,285 (Ott 3, 1978) G C Ych, Scpnratlon ol Lquld Mixtures, Canadian Pdtcnt No 1,056,760 (June 19, 1979) G C Ych, Scparatlon of Lquld Mlxturcs, U K Patent No 1548,299 July 11, 1979) G C Ych, Scpa~wonc DI M~\clc DI L~qu~th, Italian Patcn~ No 1,060,214 (Aug 10, 1982) G C Yeh, Proccdc ct appikriulp)ur la scpamon dc melanges hquldes, French Patent No 2,309,262 (Dee 11, 1982) G C. Ych, Unpubhshcd work, Auburn Umverslty, Auburn, AL (1959) G C Ych, Unpubllshcd work, VIllanova Umvcrslty, Vlllanova, PA (1966) G C. Ych, Unpubhshcd work, Villanova Umvcrsity, Villanova, PA (1967) B J Riltlgan, Ciiplllary DlstIllilLlon,Master Thesis (Chcm Eng ), Villanova Umvcrslty, Vlllanova, PA (1974) J V Scolc~c, Caplll,lry DIS~III;NIOII, Mask1 Thcs~s (Chcm Eng), VlllimOva Umvcrslly, Vlll:m~~a, PA (1978) J G Wllbcrg, Cap~llaty Dlstllli~tlon, Scmot Thcsls (Chcm Eng ), VIllanova Unlvcrslty, Vlllanova, PA (1978) S J Corrcnu, Cilplll:lry Dlstlllatlon, Scmor Thcs~s (Chcm Eng) VIllanova Umvcrslly, Vlllimova, PA (1979) M C Cashlon, C:iplllilry Dlstlll,ttlon, Scmor Thcsls (Chcm. Eng ) V&mova Umvcrslly, V~llanova, PA (1980) S A Popp, Capillary Distlllat~on, Scmor Thcsls (Chcm Eng ) Vdlanova University, Villanova, PA (1981) D M Richmond, C;ipllliiry Dlstlllation, Scmor Thcsls (Chcm Eng.) V~llanova Umvcr\ity, V~llnnova, PA (1982) D W PIti~tkOWShl, Ciiplll;iry Dlstllliitlon, Scmor Thcsls (Chcm Eng ) Villanova Umvcrslty, V~lla~wva. PA (1983) W Flcmmg, Capllliiry D~ct~lhl~on, Scmor Thccls (Chcm Eng ) Vdlanova Umvcrslty, Vill,mova, PA (1985) D B RIII, Caplll;\ry Dlstlllatlon, Scnlor Thcsls (Chcm Eng ) V~llanova Unwcrsitp, Vlllnnovn, PA (1986)

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