Concentration and plate efficiency in distillation columns

Chemical

EngiaeeringScience, 1964,

Vol

8 pp

Concentration

Pergsmon Press Ltd,

and plate efficiency in distillation W.

Laboratory

168 to lflf3

for Pbyslcs

R. VAN WIJK

and Meteorology,

and H

A

Agricultural

Recezved 23Td March

C

columns

THIJSSEN

C’mverslty

FVagenmgen,

Netherlands

1954

Summary-The Influence of hqmd coneentratlon and reflux ratlo on plate efficiency has been studled for the system n-heptane-methylcyclohexane usmg a sieve plate column ulth 8 trays A deflmte decrease m plate efficiency 1s found if the concentration of one component IS of the order of 0 1 mole per cent but the decrease m plate efficiency is less than reported for the mixture ethanol-water The system n-heptane-methylcyclohexane appears to follow Raoult’s law better than would follow from the exlstmg eqmhbrmm data R&urn&--En utdlsant une colonne adlabatlque Q hult plateaux avec le $ystemc heptane-normalmethylcyclohexane, les auteurs ont dtudl& l’mfluence de la cornposItIon sur I’efflcacltC des plateaux L’efflcacltB dlmmue lorsque le tltre molalre d’un des deux composants s’abawe au-devsous de 0 I Cette dlmmutlon reste pourtant mf&eure ii celle qul a 8td slgnalde pour les systCmes eau-ethanol ou eau-propanol Le systiime heptane normal-methyl cyclohexane se conforme mleux i la 101 de RaoJt que l’on ne seralt ament i le penser d’apr& les don&es d’Cquhbre pubhkes

of concentration might therefore partly be due to other factors Further a small error m the locatlon of the equlhbrmm curye produces

INTRODUCTION

The questlon has often been raised as to whether plate efficiency m chstdlatlon columns ~&pen& upon the concentration the mixture

of

the

components

m

maccuracles

m

the

calculations

of

the

plate

a sharp increase

eficlencles, which become extremely large m the nelghbourhood of the azeotroplc composltlon

from a value below 5 per cent to approximately 85 per cent m plate efficiency with ethyl-alcoholwater mixtures m a bubblecap column, If the

Separations of Isotopes [4, 5, 61 show that m a wetted wall column the H E T P remams finite at extremely low concentrations but no definite

alcohol

values

KEYES

and

BYMAN

[l]

concentration

found

increased

from

0 to

10

m Murphree plate efficiency near the azeotrope of lsopropanol-water has been reported by LANGDON and KEYES [2] (also m a bubblecap column vanatlon column benzene.

)

CHAKVEKILIAN

of

the

H.E.T

found

[a]

P

m

a

bearmg

for the system carbontetrachlordThe number of deal plates varied from

tension,

the

volumetric

and the time of contact different composltlons.

hqud

reported

High

numbers

components,

for example,

of the order

of 1 p p m or less occur and a decrease of plate efficiency for low concentrations would be very important for practical apphcations It IS for

wall

this reason mvestlgated

4 at 10 mole per cent Ccl, to 2 at 80 mole per cent In these mvestlgations, however, the slope of the eqmhbrmm curve the hqud vlscoslty, the surface

been

In the chstdlatlon of flavours from fruit Juice extremely low concentrations of the flavour

hnear

a wetted

have

of plates have been reported for the concentration of flavours from apple lulce although the components mvolved are very volatile [7]

mole per cent. They found a decrease m efficiency near the azestroplc composltlon. A rapd decrease

that the present authors have the influence of concentration on

plate efficiency in a specially constructed column m which it has been possible to determme the

velocity

efficiency of the mdlvdual plates and to study the other factors which might mfluence plate

varied markedly at the The reported influence 153

W R

WIJK and H A C

VAX

Concentr&.tlonand plate elbclency m dlstdlatlon columns

THIJSSEN

efficiency separately The system chosen for the experiments was n heptane-methylcyclohexane, various reflux ratios and concentrations have been used The propertles of this system which 1s very suitable for the present Table I Table

I

- Phyncal

are collected

e1pec gravity 30°F/800F

II

Summary

Viscosity tabs)

ep

of plate

dzmenszons 38

Inslde diameter of column, cm

m

Pldte spacmg, em

12 0

Holes/plate

97 0 075

Hole diameter, cm Drdhng pattern

propertzes of the system ~~ohmane

0/0n-heptane

purpose

Table

Iktdnce

n-heptane-

Slope of the eqml curve

Eqmbteral

triangular 03

between hole centres, cm

Weir height, cm

10

Inside diameter of downpipe, cm

10

via a pilot valve and a siphon-flowmeter reboller The flowrate of coohng water 0 774

0 50

0 731

loo

0 888

0 30

condenser

1 076

controlled

with

the

aid

of

a

rotameter, and the current m the electric heating plate of the reboller stablhzed to wlthm 0.5% The system dlstllled was prepared from pure

1000 0 21

was

to the m the

0 931

grade n-heptane of the Phllhps Petroleum Company and pure grade methylcyclohexane

EQIJIPMEWT

Drug Houses

The mdex

of the Brltlsh

of refractlon

remained

A column was used composed of nine sections, four of which had a glass wall, the other five were of metal and were provided with a device for

unchanged after percolation over dry slhca gel. A contmuous record of the vapour temperature of all sections was obtamed by means of a

measurmg

Brown

hquld flow rate and for samplmg

Each

of

the

samples

of f

0 02 mole ‘$&n-heptane The

perature of which could be regulated by means of a set of heatmg cods For the measurements of the hqmd rates on the trays a new type of which combmes

refractometer

were

the refractive index has been developed

by Prof. VAN HEEL and coworkers With this apparatus concentration

by I.5 cm of glass blanket, the glass sectlons Every sectlon was surrounded by a double wall by a Jacket constructed partly of glass, the tem-

was developed

recorder

composltlons

determined by means of The refractometer used,

and reflux divider were mounted directly over the top section The measuring sections were Insulated

siphon-flowmeter

1%point

The

sectlon except the highest one contamed a sieve plate Llqmd flow-rate, temperature, composltlon and heat transfer through the wall could be The condenser measured m a measurmg sectlon

m Delft [8]. differences

could be indicated.

was

directly

calibrated

by test samples composed of the pure components by welghmg The refractive mdlces themselves Samples of 0 3 to 0 4 cc were not calculated

; with this quantity no interference with the operation of the column was observed were drawn

The

hqmd

remammg

m

the

sample

hne

was

small dlmenslons (7 cm) and a measuring range of 0 01-l cc/see with an accuracy of about 3% Sample taps specially deslgned for mmlmum sample holdup were provided on all measurmg sectlons and on two of the glass ones AdditIonal

blown back mto the column Just before samphng After steady-state operation was mdlcated by the

details

Jacket temperatures mmutes

regardmg

the

tray

and

column

constancy of temperature and of the composltlon of the samples at the top-plate a complete set of measurements was made every two hours The

design

are given m Table II A complete description of the column ~111 be presented m a subsequent paper. Overhead vapour IS wholly condensed For runs under partial reflux a part flowed by gravity

were

adJusted

every

10

PROCEDURE

Runs were made both at total reflux and at partial reflux with reflux ratios ranging from 15J

W

R VAN WIJK and H A C! THIJJSSEN Concentration and plate efficiency in distillation columns

to

L/1/=05

The sets of runs at

L/V=08

total reflux covered almost the entlre composltlon range of the bmary system n-heptane-methylcyclohexane The dlffuslonal drlvmg force * Y n+1 - y, ranged from 0 085 mole y0 at 1 or at 99 mole ok n-heptane to 1 8 mole y. at 50

LEWIS

mole

1s essentially

The Murphree plate efficiency which depends upon the intensity and type of mlxmg of the liquor on and of the vapour below the plate as well as on the term m V/L was related by W. K.

o/o n-heptane

The superficial vapour velocity for the total reflux runs at 50 mole oh n-heptane was varied m regular increments from 10 to 27 cm/set The lower ’ dlstlllatlon

carefully established four glass windows

hmlt

or weep-pomt,

was

Smce

for

a bubble

local efficiency the plate-and

the trays did not yet start leaking. The upper hmlt was determined by the dlmenslons of the

The

the runs at partial

overall

defined

For the

amounted,

reflux from

ranging

formation

rather

That for velocltles 238

to

500 cm/see

than tube flow occurs,

m the

results

m molal

composltlon

successive

top section were determined.

on a gas basis

Yn-1

1s

(1)

5/n* - y,_,

n-heptane-methylcyclohexane of

the

vapour

In order to avoid

properties

entermg

and

are calculated

this, Murphree from

the

pomt

number

of

are

hardly

varying)

this

procedure

may be substituted for a determmatlon of The number of overall mdlvldual plate efficlencles gas transfer umts 1s defined as follows --

s G-1

N O@-

of the very small difference between

real

tration range (slope of the eqmhbrmm curve, dlffuslonal vapour drlvmg force and physical

19 to 27 three

_~!!

=

trays

In these runs only 7 samples of each the lowest At the end of each and top section were taken run the hqmd flowrates of tray l-3-5-7 and the

the

overall gas phase transfer units of the entire column As for the runs at total reflux the mtegratlon 1s carried out over a very narrow concen-

hquld samples of the trays 1-3-5-6-7-8 overhead vapour were taken, and three samples of tray 6 For the runs 12, 13 to 27 It was unnecessary to analyse

each tray on account

bystem

efficiencies

soon as steady state operation was attamed After this a sample was taken every 15 minutes or four and the rapour and 19

the

can be

traces of volatile lmpurltles or water a small amount of dlstdlate was withdrawn slowly as

12, 13 and

I D

equation

Yn -

=

composltlons

bubble

shown from hydraulic conslderatlons In order to eliminate the influence of possible

In each run except,

It

leaving a plate are nearly equal especially at the low- and high composltlon range Under such condltlons the subtraction of two numbers of nearly the same value 1s likely to qlve questionable

to 238 cm/set for the mmlmum rate to 642 cm/set for the maximum rate and to 405 cm/set for the holes

17tm

efficiency

-

t,amed on 16 5-19 3 cm/set

runs at partial

plate

pomt

Eo,

of the range of stable operation. The dlstlllatlon rate of the other total reflux runs was also mamm the holes

which

factors.

appeared to he Just between pomt efficiency [lo] and taking

by the followmg

reflux were made at a smgle superficial vapour velocity of 17 cm/set, which IS m the centre

velocltles

such

mto account that the caps lmpete the mixing, It may be expected that the plate- and pomt efficlencles m this study are almost mdentlcal.

siphon-flowmeters. Entramment appeared to be still negligible To avold the effect of a varying

The vapour

of

1s a valid representation of plate performance. The real values of pomt efficlencles he m a range between those of the plate and pomt efficlencles

by observing through the the lowest rate at which

vapour rate on plate efficiency,

point efficiency

mdependent

must, however, be remembered that neither the plate efficiency nor the local or pomt efficiency

hmlt was determined by the lowest rate that permltted stable tray

operation , this lower

[9] to the Murphree

In

the interval

Y,

yn_l 1

varies from c__ Y*n 155

(2)

Y -?J*

Yl

to y, to

?&-1

the 1

1

integral ___ Y --Y* Its average

Y*n+l - Ynn’

W R

VAN M’IJK and H A C THIJSSEN Table III

Run

Super&al

ratlo

vapour

Overpressure

velocity

cm/set I

in the reboder

Liquor composltlon, of tray number

(mole

cm of water

Molal vapour Drffusronal

total reflux ”

rum

1

3

4

10 _____

10

10

1.0

10 ---

10

10

16 8 -~

10 1

26 6

13 5

13 8 ~--_

24 2

16 5

10 0 ~-

10 0

16 0

10 0

10 0 -~__

130

110

47 50 53 54 56 57 -___

2 2 2 6 25 55

46 8 49 9 53 1 54 7 562 57 7

49 0

97 97 97 97 98 98

25 52 74 95 08 12

95 07

95 01

48 51 54 55 57 58

59 20 _____

59 1

59 6

96 77 __~-

12

--

._

110

[ 140

13 -___

18

y0 n-heptane)

composrtion

vapour

(Y7&+1-

and plate efficrency m drstrllatron columns

. 1 3 5 6 7 8

Per

Summary tIf the

number

Molal hquor-vapour

Concentration

top tray,

%

drrvmg force mole

1 80 -__

Murphree plate efficrency cent calculated accordmg to eq 5 plate

52 60 73 81 90 96

97 29 97 59 97 82 97 92 9797 98 08

99 02

98 15 ____

98 22

0 15

0 15

98 98 98 98 98 98

__

~~ 96 64

36 36 36 85 32 82

66 27

%

YtJ

Accuracy of Murphree (per cent)

517 54 5 55 6 57 0 58 4

1 80

86

85 ~-

1 80

75

0 085

74

67

0 267

77 ~___

78

--

0 276

1 77

72

84 5

efficiency 15

15

15

1

45

16

17

27

16

10

1 1 1 is 0 if the - ii*, - Yn-1 +y* n+1 -Y n varlatlon 1s consldered as linear m this small

graphically A plot of equation (5) IS given m Fig. 1, where E,, 1s plotted against N,, with the term C as parameter Obviously the absolute

Interval.

value

value

integral

N OGT

Inserting

this

average

value

m

the

of NOGT 1s not

represent&e

for

the

in eq. 2 gives

= 0 5

1

kY,-Yn-1) 1

substltutlon

N mT=“”

(

y

of equation

1 (

(EOG,

+

(3)

R*-Yn-l+Fn+d

(1)

m (3)

EOG~+~

X

gives

y”=,,?nj’

?I1

(4)

If it 1s assumed that E,, 1s independent of the term (y*, - y,*) and the slope of the eqmhbrmm curve and slope of the operating line are both constant

over

the

range

of composltlons

sidered, the relation between now be wntten as follows -

E,,

con-

and NOGT may

-

No,, EOG

m

which

C =

=

F+r$ n

05.n(l+C) can

40,

Frg 1 Relatronshlp between EoG, overall gas plate efficiency, and NOG,, number of overall gas-phase transfer umts

be

determmed

of the

entire

column,

parameter

n 156

w&h

accordmg

C = to eq

yn -

yn-1

Y n+1 -Y, (5).

as

W R. VANWIJK and H. A C THIJSSEN Table 111I

Summary

-

Run number

I 19

_____~

Molal hquor-vapour ratio --

Supcrflclal vapour velocity cm/set

_ Overpressure m the reboller cm of water

~-10 _____ 16 8 IO 0 ---

Liquor composition, (mole % n-heptane) of twy number 1 8 5 6 7 8 Molal vapour composition top tray,

of

I

total

-

20

24 --10

25 -__ 10

16 5

19 3

193

100 ---

11 5

11 5

97 77

9689

98 53

98 00

23

10

10

--

16 5

__ 16 5

16 5

11 0 ---

11

-10 0

0

199

382

448

5 11

728

2 96 --__

586

692

7 94

1108

018

0 34

cent Murphree plate efficiency calculated accordmg to eq 5

?O 5

76 5

770

Accuracy of Murphree plate efficiency (per cent)

-_

0 45

041

800

10

0 12%I

78 5 ---

10

75 4

modified

vahd for the runs at total reflux m this study, smce (y*, - y,+) as well as m are constant The vapour concentrations on mdlvldual

per cent

eq

(5).

97 OG

9565

0246 __-

79 4

817 -__

832

17

10

3

10

27

036

-

Murphree efficiency since C decreases for increasing values of V/L, therefore L\‘ooT decreases at constant values of E,, Equation 5 1s rigorously

from

9318

0169 --

-_

__

-

can be calculated

10

10 -__ 187

-_-

0 62

.- --

--

trays

26 ---

-_

%

DiffusIonal vapour drivmg force mole % (Yn+l - Yn) Per

Concentration and plate efficiency m distillation columns

Gibbs-Duhem

[12] and appeared m the concentration n-heptane

equation

to

of

REDLICH

be highly mconslstent range from 80-100 mole

On our request

the eqmh-

brmm data were redetermined by the “ Central Institute of Physical and Chemical Constants, Netherlands ” The new data fitted Raoult’s law to wlthm the accuracy of the experlmental error (0 1 mole o/o n-heptane), which 1s m accordance

In those

cases m which these concentrations were measured. excellent agreement was found with the experl-

with

The Murphree point efficiencies mental values. of the runs at partial reflux were calculated both

the experiments

of CHVTZEK, HAAYE and

Sr~a [lS] who could not detect any deviation from ldeahty ln the hqurd state above 40°C In rlew

from equation (5) and the mdlvldual tray composltlons Vapour composltlons m the partial reflux runs could be directly calculated from hqmd samples and flowrates The discrepancy

of this lt was considered that Raoult’s law could be applied with confidence The vapour pressures of n-heptane and -methylcyclohexane were obtamed from Circular No C 461 of the

of the molal hqmd flowrates between the trays 1, 3, 5, 7, and the topsectlon amounted at most to 3% which mduces a maxlmum error of 1 5% In all m the posltlon of the operating line

Natlonal

Bueau

of Standards

RESULTS

AND Drsc~-ssn~

runs the column could be kept adlabatlc

to wlthm

0 5%. The

I’otal rejlux Table 111 summarize\ the value of pIate efficiency aIong with the composltlons

data

of

vapour

BROMILEY and

liquid

eqmhbrmm

QUIGGLE [ll]

were tested

of by a

the

trays,

orerpressure

157

superficial

m the reboder.

vapour

velocity

and

Liquid composltlons

W. R VAN WIJK and H A C THIJSSEN

Concentration and plate ef%lency m dlstdlatlon columns

In order to examme whether this drop m efficiency near the extremes of the concentration range 1s not caused by a small devlatlon of the system from ldeahty, the followmg testing method was introduced

80

Superfiflal

vapour

veloaty

cm/set

-

Fig 2 Effect of vapour velocity on plate efficiency at total reflux Ys,+l - Y, = 1 80 mole per cent

m the table are an average of three analyses The effect of column throughout upon the local Murphree efficiency IS shown to be small, cf Fig. 2 which 1s m general agreement with the literature on this sublect Murphree efficlencles at a constant superlficlal vapour velocity of 16 5-19 Sft/sec are plotted m Fig 3 as a function of molal hqmd composltlon It 1s apparent that the efficiency varies markedly with composltlon m the ranges of low and of high concentrations. This variation can not be attributed to a lack of preclslon m the calculation of the Murphree efficiency (See the hst of accuracies m Table III). 90

Mole % n heptane

Fig

4

-

Assummg that Raoult’s law holds m the nelghbourhood of 50 mole o/0the values of,Murphree efficlencles m this range were calculated From these a new eqmhbrmm curve was calculated, assuming that the efficiency remams constant m the entire concentration range The relative

‘I

r~~~l,~,l,,,I~,~I,,II

6o0

20

LO

* 60

80

100

I, _OOq

Mole % n heptane 8.

m /qu/d

Expenmental relatwe volatdlttes If effclencles are independent of the liquid composition A rl ideal 0 D!experimental

reflux

Superficial vapour 19 8 cm/sue.

10

60

80

100

In bgu/d Mole % n heptane In lquld

Effects of hqmd concentration on plate efficiency

total

I, 20

velocity

16 5-

158

Fig

5

Thermodynamic testmg graph eqmbbnum curve

of

the

-

new

and plate efficmncym dlstdlatloncolumns W R. VANWIJKand H A C TEIJSSEN Concentration

volatlhtles of this new curve are plotted along wrth the ideal volatlhties agamst the hqmd composltlon m Fig 4 REDLICH’S modified Gibbs-Duhem equation was now applied to test the consistency of this calculated eqmhbrmm In Fig 5 the logarithm of the ratio of the curve activity coefficients IS plotted against the liquid composition.

The values of log ?! are obtained Ya from the followmg equation log ?! = log Gc- log 2 2 YZ 1

log fi dx = 0

As the curve does not conform to s

0

3’2

HATCHER and SAGE [14] m an mvestlgation of the influence of natural and forced convection upon the transfer of n-butane to and from hydrocarbon liquids found for natural convectron conditions a fairly regular decrease of the masstransfer coefficient for the hqmd film as the concentration of the n-butane m the hqmd phase approached that at the interphase, while this effect appeared to be neghglble with forced convection, which is m accordance with Fig. 6. In the runs wrth partial Pa&al rejhx reflux the distillate was returned to the reborler m order to obtain a stationary state In Fig. 7 the analysis of the mdivldual hqmd samples of partial reflux run 17 are plotted against their In the same graph correspondmg tray numbers

It must be concluded

that the variation of the Murphree efficiency with concentration can not be attributed to either maccuracles or wrong equihbrmm curve and that an influence of the dlffusronal driving force upon efficiency exists. A similar effect for the system ethanol-water (KEYES and BYMAN, [l]) 1s represented m Fig. 6 where EoG 1s plotted against liquid composltron with vapour rate as parameter The influence of the dlffuslonal driving force appears to decrease at increasing vapour rate Owing to the hmited accuracy of sample analysis and varying physical properties with concentration no further conclusions wdl be drawn

I

471

t

I9

I

3

I

I

I

I

5

I

I

II,,,

I

7 Tray number

CL -

for a typlcalpartialrefluxrun Fig. 7 Tray composltlons . By hquldanalysis CalculatedcurveI for EOG = constant - CalculatedcurveII for Em = f (Y*,+, - Y,)

curve (I) represents the hqmd cornpositrons calculated accordmg to the BAKER-STOCKHARDTprocedure [16] so constant plate efficlencles over all

Mole %

ethanol m liquid

-

Fig 8 Effect of superficud vapour velocity and hqmd composition on plate efficiency for the system ethanol-

water

[I]

the trays and a curve (II) representmg hqmd compositions giving plate efficiencies which are a same function of the diffusional vapour dnvmg force as found from the total reflux runs at the same vapour rate From a separate study it appeared that the samples of tray 3 and 5 mrght have contamed some condensed vapour (too high It is therefore concentration of n-heptane) concluded that curve II must be preferred. A 159

W R ‘C’ANWIJK and H A C THIJSSEN

Conoentratlon

downward trend of efficiency with decreasmg reflux ratlo 1s m&c&d m Fig 8 It has been stated that the plate eficlency still increases at mcreasmg values of L/V above unity

m the bottom

section

(15)

In that

and plate efficiency m dlsbllatlon

columns

It 1s therefore concluded that the plate efficiency 1s dependent on the term mL/V, according to the relation between the overall-and mdlvldual number

case

of transfer

units $-

= L OG

well as on the dlffuslonal

drlvmg

+ m*

Ng

as

NL

force

ACKNOWLEDGMENT

The authors

are mdepted to Mr DE HAAS VAN the Netherlands Orgamsatlon DORS~EI~ of A T.A -T N 0 for loaning the van Heel precision refractometer NOTATIOX = overall pomt efhnency, based on gas concentrations AV, = number of gas-film transfer umts NL = number of hquld-film transfer units. No,+= overall number of transfer umts, based on gas EoG

/, IL,, ,I, ,, ,/* 08

09

Reflux

ratlo

concentrations L = molar ilpa flow (g mol/sec) V = molar vapour flow (g mol/sec) rm = slope of equdlbrnun curve dy*/& n = number of plates. P = 7 aFour * ressure of pure component OT= relative volatlllty y = actlvlty coeff y = mole per cent in vapour c = mole per cent in ilqula y* = eqmhbrmm value of y correspondmg

10

L/V

-

8 Influence of rcflux i-&lo on plate elhclency Superfimal vapour velocity 1’7 cm/set Average concentration of n-heptane m hqmd 50 mole per cent fig

tfre-relatlonshlp

between

Ay* and E,,

cannot

be

explamed only by the effect of a smmltaneous decrease or increase of the dlffuslonal dnvmg force.

REFERENCES

PI

KhYhb, D B and BYMAN, L

PI

LANGDOX,

W

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iM and KLYES, D B

111 Ing

Expt

Sta

Bull

1941 328

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White,

J

W , Food Research

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Lswrs,

W

K

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IlO1

SHILLING, G D , BEyha, G H and WATSON, C C

WI

BROMTLEY, E

WI

REDLICH, 0

P31 P41 WI P-Y

C and QUIGGLE, D

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, Ind Eng Chem 1948 40 341 CRUTZEN, J L , HAA~E, R and SIEG, L , Z fur Naturforsch 1950 5a Go0 HATCHER, J B and SAGE, B H , Ind Eng Chem 194133 443 WI1 LIAMS, G C , STmGha, E K , and NICHOLS, J H , Chem Eng Prog 1950 46 7. BAKER, T and STOCKHARDT, J S , Ind Eng. Chem 1939 22 376

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