- Email: [email protected]
Vacuum stripping of butane from water in a packed column
VACUUh3
STRiPFiNG
OF
BUTANE
FROM
WATER
IN
A PACKED
CC3LlJMN
When the freezing or hydrating of brine in a desalting process fur producing fresh water is achieved by means of an extraneous refrigerating agent. whether it is an almost immiscible gas. such as butane. or a gas capable of forming hydrates, such as propane. CH,ClF or other halogenaced hydrocarbons, the outgoing products of brine and fresh water contain various amounts of the dissolved refrigerant_ In the secondary refrigerant freezing process, the butane content in the product streams priur to dcpsing can vary between 80-140 ppm, depending upon the operating conditions. The hydrating agents arc much more soluble, and the range is roughly from 0.1 to 3 % by weight. The purpose of this study was to obtain data on the vacuum stripping of butane from water. and to analp and interpret these data by means of current models of mass transfer, thus providing a sound basis for design, Butane was stripped from water down to concentrations as low as 0.6 ppm and the mass transfer rate was found to be liquid-diffusion controlled. The coefficients did not disagree with the predictive correlations of others, but a straightforward comparison was not possible. No particutar design problems are foreseen in the stripping of butane down to very fow concentrations, although additional data to predict heights of liquid transfer units at conditions of very high liquid to gas fiow ratios would be useful. SYMBOLS
a
-
%I” c
-
spe&i~ packing surface per unit packed volume (for 4 mm Berl saddles, this was found by extrapolation (18) to be 14.3 cmz/cm3) effective gas liquid exchange area per unit packed volume, cmz/cm3 the concentration of butane in the bulk liquid ppm or g moles/liter
* Present address: 62 rue &q¬
54 Nancy. France, Lk5alinoriofl, 9 (1971) X51-366
J.-E. BAJOLLE,
352
P. A. RICE AND
A. J. BARDUHP;
c-6 - the concentration
of butane in the liquid at the gas-liquid interface, g moles/liter the equiiibrium concentration of butane in the liquid phase at the column temperature and pressure, ppm the concentration of butane in the liquid phase at the&h sampling point the nominal diameter of the packing. 0.4 cm for the Berl saddles used here
c*
-
=, dP
-
4
-
the diameter of a sphere with the same surface area as one piece of
4.
-
the diffusivity
HE
-
IfO‘ k(T)
-
k,
-
P
-
= 0.52 cm for 4 mm Berl saddles (see text) of butane in the liquid phase. cm2/sec the height of an individual liquid transfer unit, cm the height of an overa liquid transfer unit. cm packing
Henry’s law constant for dilute solutions of butane in water, Torr
the gas phase mass transfer coefficient, (g mole/(cm*)(Torr)(sec) the liquid phase m&s transfer coefficient for butane in water, cm/set kL A-0, - the overall liquid phase mass transfer coefficient for butane in water. cm/set JG?,., - ko,, for the first or top section of the packed column k otz - koL for the second section of the packed column - the liquid volumetric flow rate. cm3fmin IF. NhiLB - the specific mass transfer rate of butane from the bulk &quid to the gasIiquid interface, g moles&cm2 )(min) ‘VoL - the number of overall liquid transfer units Nil, - the Reynolds number for the liquid phase defined as d&v J%c - the Schmidt number for the liquid phase defined as r/D, Ns, - the Stanton number defined as koLaG,dJu v *SIX - the Stanton number for the first or top section of the packed column n;,,, - the Stanton number for the second section of the packed column the partial pressure of butane in the gas phase, Torr PB the partial pressure of butane at the gas-liquid interface, Ton PB, the partial pressure of water in the gas phase, Torr PW P - the total pressure in the packed column, Torr T - the temperature within the packed column. “K t4 - the supeficial velocity of the liquid in the packed column, cm/set or cm/min UC - the superficial velocity of gas in the packed column. cm/xc z - the length measured from the top of the packed column, cm - the length of a section of packed column = 30 cm &I v - the kinematic viscosity, cm2,&c liquid density, g/cm3
Dcsalitwtion, 9 (1971) 351-366
VAC3JU.M STRiPi’fNG
OF BUTANE
FROM WATER
353
lNTRODUCTlOS
30th the secondary refrigerant freezing process and the hydrate process for desalting saline waters yield two product streams which. if not degassed, would contain excessive amounts of volatile refrigerant. In freezing, this means 80 to 140 ppm of butane and in hydrating it means 0.1 to 3 wt.% CH2CIF (31). CF,C& (12). or other halogenated hydrocarbon. These gases must be recovered and recycled down to about 1 to 10 ppm to JCCOWJ their economic value and/or to prevent explosion hazards, and probably to the 0.1 ppm range to meet public health standards for drinking water. When the dissolved gas is to be recovered after removal. it is importznt that the method be uncomplicated since we are producing a product (water) worth roughly U.S. SO.10 to 0.20 per ton. which is an order of magnitude less than the cheapest of mined chemicals, e.g. limestone or salt anti two or three orders of magnitude less than the cheapest manufactured chemicals. e.g. sulfuric acid or soda ash. Removal by air or flue g:::rsstripping thus seems out of the question since the subsequent recovery is difficult. Simple flashing of the product streams at reduced pressures is usefui, and probably necessary, but not sufficient since the approach to equilibrium in a single stage flash is not very close because of the very short residence times in spray chitmbers. Starting with a water solution saturated with butane at 1 atm, containing say 100 ppm butane, and flashing at 0.1 atm would remove the bulk of the dissolved butane and allow the recovery of the removed gas by compression of the vapors to the condensing pressure. Depending on plant size, the cost of the refrigerant, and other economic factors the ffashing could be done in more than one stage to save compression costs. Also, if the refrigerant were inexpensive it is possible that the partially stripped products could then be further stripped with non-condensible gas down to 0.1 ppm with no provisions for recovery of the liberated refrigerant. If the pressure of the final stage flash were equal to the vapor pressure of water, the equilibrium solubiiity of the refrigerant would be zero and there would be 70 tower limits on the refrigerant concentrations in the efihtent water or brine except those set by the mass transfer rates. Since, however, any reasonableapproach to equilibrium requires more time than normally available in a spray, the use of a packed column for further treatment of the flashed liquid seems necessary when goals of fractional ppm are set for the product. This is true also at higher pressures. The process taking place in such a packed column is not quite the same as what is normally called stripping. We refer to it here as stripping since it is what the designer will call it, and it is a process to provide time for an approach to equilibrium in which a gas evolves from a liquid. The flow is countercurrent also but it differs from conventional stripping in that the gas flow is very small compared to the liquid and is not an independent variable but is zero at the bottom of the
Desalination.
9 (1971)
351-366
J.-E.
354 column.
BAJOLLE.
P. A. RICE
AND
A. 3. BARDUHN
This does not complicate
the design as long as the liquid film controls, from the rate equations. Very few results have been reported so far on the actuat efficiency of a process designed to remove butane from a water solution. An estimation of the height of a transfer unit in a packed column based upon CO, absorption can be found in (2). The butane content is said to have been reduced to less than 0.2 ppm in a packed column at 35 Torr (3). but no specific apparatus is known to have been designed and set up to investigate in detail this particular mass transfer
since the gas flow rate is eliminated
OpeTdiUll.
The purpose
of this study
was to obtain
butane from water, and to analyze basis for design.
data on the vacuum
and interpret
stripping
these d;ita to provide
of
a sound
EXPEBthiEMTAL
Appamrus The
apparatus
was designed
IO study
the cKccts of pressure
and flow rare
or? the rate of butane removal from an aqueous solution flowing down a packed column. As long as the column pressure is above the vapor pressure of water, the heat effects are negligible and the column remains nearly isothermal, This was the case dealt with throughout this study. liquid !rops
1
rotameter butane
+
Fig. 1. Apparatus.
needle
valve
T
stopcock
Q
three-uoy
stopcock
-M
lo monometer
-S
to sampling
ctrcuit
VACUUM
STRlPPING
OF BUTANE
F%OM WATER
355
Fig. 1 is a sketch of the apparatus consisting mainly of a feed tank and stripping column. The feed tank is an J 8” high and f 5” diameter cytindcr, constructed of stainfess steef and connected to a butane cylinder equipped with a pressure regulator. Another inlet is used 10 fill the tank. Inside the tank, an jmmersion pump continuously agitates the solution to reduce saturation time. A three-section glass packed-column was adapted for use as a stripping co‘fumn. The packing in each. of the 30 cm iong, 5 cm diameter, cylindrical sections, is 4 mm ceramic Berf saddles. Each section is sealed to the uther by a ?~~6~ g&s joint. The bottom is a 50 Iiter spherical fiask, the top of which is joined to a g&s tube connected to a small pump used for removing the liquid. The top glass joint
of the column has two tubes for incoming piqued and outgoing where a thermometer is inserted to indicate the vapor temperature.
vapor, a joict and a pressure
WP* At the bottom of each of the two last sections, a glass tube with a stopcock is placed under the packing and atlows samples to be collected. The first section is divided into three packed stages of equal height, each section with a sampling tube and a thermometer at its base. The sampling tubes, each fitted with a stopcock, and the thermomrters, arc diarnetrjcalf~ opposed and are mounted in such a way that their extremities are wet by the same liquid. Both are under a small cone-shaped screen which holds the packing. There is a hole in the center of the screen, so that the flow is sufficient for good sampling, but not large enough to perturb significantly the flow pattern in the column. fn addition. there is a sampling tube and thermometer al the entry point of the feed into the column, The column pressure is regulated by a needte-vatve in the outfet gas line which goes to the vacuum pump.. The liquid feed rate is controlled by valves and by the rotational speed a centrifugal pump, which is itself controlled by a variable resistance. A rotameter mounted between the feed tank and the column is used to measure the liquid flow rates. The sampling flask consist of a sm&J g&s b&o equipped with two stopcocks so that it can be evacuated to the column pressure before filling. Procedrcte
The sotution of butane is prepared first. The column is then ffushed with an atmosphere of butane to remove the air from xhe system. The vacuum pump is turned on and the pressure in the column adjusted to the desired operating value. The pump can either be continuousIy run while the vacuum is controlled by the needle valve. or be turned on intermittently to make up for small increases in pressure due to stripping_ After the feed is admitted, steady-state in the column is reached in about 30 minutes. Liquid samples are then taken from the top of the column and at each sampling pint. Care is taken to hold the sample bulb at a Desalination,9 (1971) 351-366
356
J.-E.
BAJOLLE,
P. A. RlCE AND A. J. BARDUHN
pressure only slightly less than that in the column to eliminate change in sample composition. The silmples are analyzed for butane using a Beckman Model 915 Total Carbon Analyzer. To complete the run the vapor temperature and the liquid temperature at several points in the column are recorded. No significant temperature change along the height of the column is observed.
A literature survey was conducted on n-butane solubility in order to obtain the liquid-vapor equilibrium curves at different pressure and derive thermodynamic data. After Winkler’s earlier estimations (4). Morrison and Billet (5) were the tirst to give reliable data. Their measurements involved a manometric but precise
method.
They
investigated
over
the range
! atmosohere. Further studies showed
good
agreement
with
(6-13). Morrison
Kreshcck
- Schneider
0
Wetioufct
- Mottk - Stolter - Coffin
X
Morrison Rcomcr
+
Clouisn - PolQlosc
at a total
pressure
of
- Scherogo
- Billet - Soge
- Locey
Epuo¶ion
I
to 75°C
using more sophisticated equipment, and Billet’s results, and showed that
A
0
10°C
5 RECIPROCAL
of
line is:
&
3:4
TEMPERATURE
= IOCWT
316
-
I-K I-’
Fig. 2. Equilibrium conamtration of butane in water (total P =
1 am).
~~SU/iMtiOil.
9 (1971)
351-366
VACUUM
STRIPPING
OF BUTANE
FROM WATER
357
Henry’s inw related the solubiiity to the total pressure quite satisfactorily over the range O-1 atmosphere. These data have been fitted to a curue of In C* BS tOWj3”, C* being expressed in ppm and T in degrees Kdvin. With a srandsrd deviation of less than 2.5% in C* between 10°C and 35”C, the analytical relationship obtained at t atmosphere was (see Fig. 2): In c* = t8.2914 -
11.5149
fooo
( )
“““““““““” + 2.15249
Henry’s law constant k(T) can be calculated atmosphere by means of the relation:
moo 2
( ) ---Y-
from the equilibrium
curve at one
Tn the mnge of interest flWC to 4U’C), X-(T) can be represented by a straight line, The equation of this line is: k(T)
very well
= 1.1333 lob T- 3.0273 108 Tort-/mole fraction = 0.3513 T - 93.84 Torrjppm with T = “K
For a given total pressure, the equilibrium solubility also varies considerably with the temperature and this effect is amplified further toward the low pressures, because the water vapor becomes dominant in the gz~ phase as tbe total pressure becomes near the vapor pressure of water. For inst;rncc, in going from 25% to 20°C there is a 25y; soiubility increase a t 200 Torr total pressure while this percektage grows to 160% at 30 Turr total pressure. Because the precision of temperature measurement is about the same for all pressures, the lower the pressure, the lower the accuracy of the estimation of the salubility.
Twenty-three runs were made covering a range of 6 diRerent Row rates, from 40 mlimin to 280 mt/min, and five different column pressures. The pressures chosen were 30,45,85, 105 and 130 Torr, Temperatures were always near ambient which was 25 rt 2 YJ. At 25°C the vapor pressure of water is 23.76 Torr and k(T) is 10.9 Torr/ppm. Note that at the column pressures used, the feed from the tank was afways supersaturated with butane and some gashing took place after the rotameter and especially in the column above the packing. The vapor thus evolved left through the vapor line and did nat affect the stripping action in the cr>lumn. The feed samples CI at the top of the column measured the butane concentration in the true liquid feed to the top of the packing. fn 21 out of the 23 runs even the true eofumn feed had an equilibrium partial pressure of butane greater than the tota! pressure and thus fuE_rher spontaneous flashing (or boiling) was possible in the packing. With the large liquid area exposed Desdinatim~ 9 (1971) 351-366
J.-E.
358
BAJOLLE,
P. A. RlCE AND A. 3. BABLXIHN
in the packing and the thin films of water present this seems unlikely, however, and no visuat evidence of bubbles forming in the liquid was noted, although this might be difficult fo detect. Nucleation of a butane vapor bubble seems improbable also when it is considered that only I to 4 molecules per million of fettd are butane. ‘If. in fact, such boiling did take place it would probably increase the expected mass transfer rates both by decrerising diffusion path lengths (and thus increasing kL) and by increasing the gas-liquid interfacial area (u& but the process of diffusion stitt would have to occur between the bulk liquid and the gas-fiquid interface. pro$les Table I gives rhe con~nt~tion prufiles along the column for theexperimental flow rates and pressures in terms of n-butane ppm. The concentration at the bottom 0,. the columr, C,, was usually as close to the ~ni~ibrjnrn salue, C*, at given operating conditions as we couid measure, and thus it was not possibte to cafcufate driving forces or mass transfer coefficients for the bottom sectiohof the packing, Cuttcetttraticn
40
30 45 85
1.8 6.2 II.7
0.6 ZO 6.7
0.6 20 5.7
0.6 2.0 5.7
30 45 85 105
4.3 6.9 11.7 7.5 26.2 23.1
3.0 3.2 6.9 7.5 19.7 17.7
1.8 2.0 5.7 7.5 11.0 J6.4
0.6 2.0 5.7 7.5 9.8 16.4
85 IO5 130
5.4 37.0 13.7 17-O 19.6
1.2 9.7 9.4 11.0 127
a5 4.8 7.0 7.5 9.8
0.6 2.0 5.7
45 85 t30
11.7 13.6 15.8
7.4
2.0
iii
42 5.3 9.6
210
30 45 85 130
10.2 8.0 24.7 29.3
7.2 5.6 17.7 TO.7
3.7 2.0 11.7 15.0
O-6 2.0 5.7 9.8
2?0
45 83
54.0 23.0
12.0 37.8
8.5 12.0
4.6 8.0
7r3
130 zoo f3f
!70
30 45
;i
;:i
Desalination~ 9 (1971) 351-356
VACUUM
STRIPPING
OF BUTANE
FROM WATER
359
ANALYSlS The experimental concentration profiles were used to calculate overall liquid mass transfer coeficients. lf u designates the superficial velocity of the liquid, overall liquid mass transfer and a,,, interfacial gas-liquid k3t* a mass in the phase, between heights 2 Z + of the yields:
u
‘-
dC dZ
=
-
k,,o,,(C
- c*)
For a given set of experimental conditions, assuming difute solutions and no temperature change, u and C* are constant. This equation can, therefore, he integrated over a given section of the column to yield:
c a-1 and C, are the concentrations of the iiquid before and after section rr, rcspectivcly. ‘fhc left hand side of these equations is the number of overall liquid transfer units, while the right hand side is the length of the section divided by the height of an overalf fiquid transfer unit. The data were correlated in the form of a Stanton number for mass transfer defined as: .
From each experimental concentration profile. only two values of the Stanton number were calculated, one for each of the two top sections; since the ~o~~~tratjo~ change in the last section is so smati. Because the concentration changes are larger, the Stanton numbers determined for the first section are more precise than those determined for rhe second. The precision of the Stanton number is also better at the higher flow rates. The results shown in Table fl are represented by the curves A& and Nst, vs N,, in Fig. 3. Because there was not much difference in the Stanton numbers observed at different totaf cotumn pressures, an average vafue was computed for each flow rate, These curves can be fitted with second degree polynomials with 3 good approximation to yield the following analytic expressions: N St* =3.42x lo-%‘,2 - 5.95 x lo-%&2*93X &r* = 1.94 x iW-4NR~ - 3.91 X t0-3N,,-i-2.53 In the above expressions,
lo-* x to-”
the ATRtis for the liquid and is defined as udJv_ DtWillil#tion,9 (1971) 351-366
360
J.-E BAJOLLE,
TABLE
P. A. RICE AND A. J. BARDUHN
II
6.
6.
23.9
30 45 85 130 ZOO
1.5 4. 5. 3.3 s.
4.
18.5
4.08 4.1
4.1
18.8
3.55
45 85 105 130
1.95 215 2.7 3.3
2.7
13.2
275 2.85
2.8
13.7
I70
4.47
45 85
1.85 22
2.
9.2
2.45
2.45
11.9
210
5.53
30 45 85 130
I .45 1.65 1.58 1.79
1.65
6.7
2.
2.05
9.6
280
7.37
4s 85
I.20 1.45
1.35
4.0
1.5 1.9
1.70
7.1
40
1.05
85
70
1.84
135
From the values of Nsl, the product of the overall mass transfer coeflkient and the gas liquid interracial area, ko,_~~,, can be calculated. Because several mass transfer theories (14) predict a linear dependence of the liquid mass transfer coeflkient on (NRC)*for low mass transfer rates, k,,,u,, and kOL,uGt were plotted against (NR.)* in Fig. 4. These curves show two regions. The coefficients initially increase linearly with the square root of NRC and then abruptly decrease after Reynolds numbeti of 3.5 and 4.5. The equation of &a,, w (Nn,)*, for IV,, Z 4, was found to be: koLaGt =
1.4 1 x 10-3(N,,)f(sec-‘)
It is useful to compare the observed overall mass transfer coefficients with those predicted from generalized correlations. The mass transkr rate can be written in terms of the overall and individual mass transfer coefficients as Nu, = koL(C-C*)
= kL(C-Ci)=
k&pB,-pB)s
If the overall liquid mass transfer coefficient does not change with the total pressure, Desalirmion.
9 (1971) 351-366
VACUUM
STRIPPING
OF BUTANE
361
FROM WATER
1e-
0
12-
8 -
IO8 642-
Fig. 3. Variation of the Stanton number with Reynolds number the resistance
to mass transfer in the vapor phase is negligible and the overall mass transfer coc&cient becomes equal to the liquid mass transfer coefficient. There does appear to be an increase in (C, - Ct)/(C, - C*) with pressure in Tabie II but it is small and irregular. This implies some gas phase resistance but that the liquid phase resistance is controlling and k,, z k,. After obtaining an extensive series of data on mass transfer in packed columns, Shulman (f5) and his co-workers proposed the following correlation:
For the experimental flow rates, the values of k, can thus be predicted from Shulman’s correlation, if the packing and fluid properties are known. D, was recently measured and the value I.04 x IO- ’ cm*/sec at 25 “C was found (16). The value of d, was found to be 0.52 cm by extrapolating data (17) on larger Beri saddles down to a nominal size of4 mm. and the values of k, From the values of kOLuCL obtained experimentally, from Shulman’s correlations, values of acL, the gas-liquid interfacial area, can be calculated and compared with vaIucs obtained from other workers. The calculated values of u,, for the experimental fiow rates are given in Table III, Desdinafion, 9 (1971) 351-366
J.-E
362
0
BNOLLE,
First
AND
A. J. BARDUHN
Section
A Second
Fig. 4. Variation
P. A. RICE
Section
of the overall liquid mass transfer coefficient with
number.
together with the percentage of effectively utifized total packing area, defined by: o/Outilized area = 100
OGL - a
For 4 mm packing, u was found by extrapolating Berl saddles packing to be 14.3 cm’lcm’. TABLE
III
CoMpurAnoh’OF
1.41
2.48
4.77 6.01 . 7.43 9.90
data (17) observed for larger
ocr.
40 70 135 170 210 280
k&mJsec) x IO-’
korrra (see-l) x 10-3
aa_(cnStc~
o/n utilized stfea
1.51 1.94 2.61 2.89 3.18 3.62
1.41 1.91 2.63 2.31 2.07 x.45
0.093 0.098 0.101 0.080 0.065 0.046
0.65 0.69 0.71
0.56 O-45 0.32
De.WzirUfion. 9 (1971) 351-366
VACUUhf
STRG’PING
OF BUTANE
FROM
363
WATER
An examination of this table shows that ucL tends to decrease as the flow rate increases beyond a certain N,,. This result is similar to the results of Shulman (IGf and Fe&nger (13). As the Iiquid ffow increases, the numerous crevices at the points of contact in a smal packing become increasingly gooded, thus exposing a relatively smalt surface area. The percentage of total packing surface effectively utilized is accordingly very low. in our wse less than 1 7:. For a given flow rate this percentage has been observed by Shulman to decrease as the nominal size of the packing decreases, although the packing specific surface increases. For II = 4.77 cmjmin, this percentage has been computed as a function of the nornina size of the packing, using Fethngeis data. The percent utihzed arca is afmost indepe&dent of the gas velocity until near the loading point, although the gas velocity used is approximately equal to the avewgc gas velocity in the top section of the column. The results are shown in Fig. 5. and the point obtained in this study does not disagree with those of Feltinger as reported by Shutman (15). Our resutts are thus not inconsistent with those obtained under more usual stripping cunditions, although the test of consistency is quite rough because of the extrapolation of packing parameters involved and the lack of other data for small sized Berl saddles. The results do suggest, however, that large scale butane stripping systems can be designed using either the correlations of Shutmnn to determine the hquid mass transfer coefticient and the values of eRectivety utilized packing area., or the data of others on values of k,uo, for various packings when stripping sligh,ly soluble gases.
Fellinger Doto Reported in Shulmon Q These Data
l
u = 4-f? cm/min us = 0.8 cm/min
( L = $35 cmbmin
)
t 5 NDMfNAt
StZE
icmf
OF CERAMIC
BERL
SADDLE
Fig. 5, Pacent of suurfacr:area used for mass transfer t*s nominaI size of 3erl saddie.
364
J.-E.
BAIOLLE,
P. A.
RICE
AND
A.
J. BARDUHN
The designer will normally have at hand the unstripped product rates from the desalting plant along with their actual butane content and desired final ppm of butane. Flashing of these streams at some reduced pressure will remove much of this dissolved butane. but the extent to which this occurs was not studied in the work reported here. A study of spray flashing is being conducted and will he
reported later. Presuming that the designer can predict the composition effluent
from
a flashing
operation
of the
he will then let this be the feed to a stripping
column. To design the column he then needs to estimate (3) an allowable liquid rate in lbs/hr ft”, and
for any given packing,
(b) the height of a transfer unit expected. A packed column is certainly indicated since at the low pressures used the pressure drop in the gas phase must be very smallUnfortunately, the conditions under which this type of column
operates
arc out of the range of the data reported in the literature on flooding and the extent to which mass transfer rates are applicable is uncertain. It seems necessary that additional experimental work be done under conditions more near to those expected. The unusual conditions for this operation are: (a) the very low gas to liquid ratios encountered, and (b) the extremely low concentrations of the solute required for the final product. We believe that the study reported here has shown that very low concentrations in the fractional ppm range present no new design problems and that butane can be reduced from say 1 to 0.1 ppm (or probably even 0. I to 0.01 ppm) in the same packed column that would reduce it from 100 to 10 ppm, and that the number of liquid transfer units required to make any of these reductions would be In 10 or 2.3 when the pressure is set at the vapor pressure of water, that is, nothing unexpected is likely to occur at very low concentrations. The low gas to liquid ratio presents new problems. For example, with a feed containing 29 ppm butane the mass ratio of gas to liquid at the top of the column (where it is a maximum) is about 1:50,000 or a little more, depending
on how much water vapor leaves with the butane. None of the existing correlations on flooding can be extrapolated to this condition with confidence. It would be desirable to use high liquid rates to reduce the required column diameters, of course, but it is not clear whether flooding is an important factor at all. It is possible that the adverse effect of high liquid rates on the liquid transfer unit height would determine an optimum liquid rate well before any hydrodynamic restrictions occurred_ Values of I?,_ appear to rise sharply for liquid rates above 25,ooO lbs/hr ft2 for I/2 in. to l-If2 in. ring packing.
Perry (f9) has reviewed much of the design data for flooding and liquid Dcsabation*
9 (1971)
351-366
VACUUM
5TRIPPiKG
OF BUTAhT. FROM WASR
365
transfer unit heights in packed towers. The data there (compiled by Sherwood and Holloway (20)) on values of NL for desorption of O,, HZ, and CO, should be applicable to butane desorption if corrections for Schmidt numbers are made. The main question in using these data is the effect of gas rate. This is said to be unimportant at low liquid rates, but others (Perry (19)) say that at high liquid rates, the gas rate has an adverse effect when it is very low due to excessive backmixing in the gas phase, This may be unimportant when the equilibrium solubility is near 0, but the cause of the adverse effect is only speculative and the effect may exist in butane stripping cotumns not using an Independent strippingagent. To be safe it is recommended that before a firm design is accepted, some tests be made to determine HL for liquid rates of 10,000 to 30,000 Ibs/hr ft* in a packed tower in which butane (or oxygen) is removed under vacuum with no independent stripping %crs* Since the equations of Sherwood and Holloway show HL to increase as about the Ct.25 power of the liquid rate- the total tower vofume will decrease rapidfy at higher liquid loadings and values of at least 10,000 lbsihr ft’ and possibly
much higher are likely to be economical.
From the experimental data, the variation of the mass transfer coefficient (k,,u,,) with the Reynolds number was obtained for stripping butane from water in a packed column down to concentrations as low as 0.6 ppm. For the 4 mm Bcrl saddle packing used, this coefficient was found first to increase linearly with the square root of the Reynolds number until a maximum is reached abruptly at a Reynolds number of about 4, and then to decrease slowly. The mass transfer was experimentally shown to beliquid-diffusion controlled. The interfacial areas calculated from the experimental mass transfer data and Shulman’s correlation for the liquid phase mass transfer coefficients were not inconsistent with measurements of interfaciaf area on other packings. The results obtained are expected to be valid as long as the thermal effects are not significant, that is, as long as the column is operated at a total pressure not below the vapor pressure of water.
This research was supported of the Interior.
by the Office of Sahne Water, US. Department
Desdination, 9 (1971) 351-366
366
J.-E. BAJOttE,
P. A. RKX AND A. 3. BARDUHN
4. L. W. WISX~X. Chem. Rev., 34 (1901) 1408. 5. 1. J. M muu.sos AL~DF. 8m.m. f. Chem. Sue., (1952) 3819. 6, W_ F. CU~N AND M. F_ Powx_m~, J_ Am. Chpm. SOE., 74 (19S2) 4817. 7. N. N. Rurmn, B_ H. SAGE.*xE)W. N. L~EY, hai. Eng. CAem,, 44 ($952) 609, 8. C. I). McAwm, Name, 2#.I (1953) 109;t. 9. A. Wunstr, J. Fh_vs_Uwm. 67 (1963) 2079. 10: D. B. WETXN~~R, S. K. Maw, L. STOLLERANF) R. L. Corn. .I. Am. C/tern. six, 96 (1%) 508. 1I. C. D. MCAUBJWE,Am. Cftem, Sot.. Dis_ Pefml. Chem., Preprints. 9 (3,(1964) 275. 12 G. C_ KRESH~~C&H. S~ZH~‘E~IXR ma3 H. A. SCHERAGE, J. P&m. Ctwm~, 69 (1%5) 3132. 13. E. J. FAR’KAS, A&. Ckm., 37 (9) $1%5) 1173. 14. R. B. BIRD, W. E. Smvmr ml) E. N. LIGKT~OO~,Thzs.qxw~Phcwrncmz, John Wiiq, ww York, NY.. 1960.p. 539. 15. H. L. SHUWN et al., A.f.Ch.&f.. 1 (1955) 247, 253.295; 5 (1959) 290; 6 (1960) 175,463; 3 (1963) 479.
4th Edition 1963,pp. 18-25 to 1837. 20. T. K_ Sw’swwt‘, AND F. A. f...HOLLOH’AY,?‘ims. Am. Inst. Chtm. figrs.,
36 ~1940) 39
Desnlinnrion.9 (1971) 351-366
STRiPFiNG
OF
BUTANE
FROM
WATER
IN
A PACKED
CC3LlJMN
When the freezing or hydrating of brine in a desalting process fur producing fresh water is achieved by means of an extraneous refrigerating agent. whether it is an almost immiscible gas. such as butane. or a gas capable of forming hydrates, such as propane. CH,ClF or other halogenaced hydrocarbons, the outgoing products of brine and fresh water contain various amounts of the dissolved refrigerant_ In the secondary refrigerant freezing process, the butane content in the product streams priur to dcpsing can vary between 80-140 ppm, depending upon the operating conditions. The hydrating agents arc much more soluble, and the range is roughly from 0.1 to 3 % by weight. The purpose of this study was to obtain data on the vacuum stripping of butane from water. and to analp and interpret these data by means of current models of mass transfer, thus providing a sound basis for design, Butane was stripped from water down to concentrations as low as 0.6 ppm and the mass transfer rate was found to be liquid-diffusion controlled. The coefficients did not disagree with the predictive correlations of others, but a straightforward comparison was not possible. No particutar design problems are foreseen in the stripping of butane down to very fow concentrations, although additional data to predict heights of liquid transfer units at conditions of very high liquid to gas fiow ratios would be useful. SYMBOLS
a
-
%I” c
-
spe&i~ packing surface per unit packed volume (for 4 mm Berl saddles, this was found by extrapolation (18) to be 14.3 cmz/cm3) effective gas liquid exchange area per unit packed volume, cmz/cm3 the concentration of butane in the bulk liquid ppm or g moles/liter
* Present address: 62 rue &q¬
54 Nancy. France, Lk5alinoriofl, 9 (1971) X51-366
J.-E. BAJOLLE,
352
P. A. RICE AND
A. J. BARDUHP;
c-6 - the concentration
of butane in the liquid at the gas-liquid interface, g moles/liter the equiiibrium concentration of butane in the liquid phase at the column temperature and pressure, ppm the concentration of butane in the liquid phase at the&h sampling point the nominal diameter of the packing. 0.4 cm for the Berl saddles used here
c*
-
=, dP
-
4
-
the diameter of a sphere with the same surface area as one piece of
4.
-
the diffusivity
HE
-
IfO‘ k(T)
-
k,
-
P
-
= 0.52 cm for 4 mm Berl saddles (see text) of butane in the liquid phase. cm2/sec the height of an individual liquid transfer unit, cm the height of an overa liquid transfer unit. cm packing
Henry’s law constant for dilute solutions of butane in water, Torr
the gas phase mass transfer coefficient, (g mole/(cm*)(Torr)(sec) the liquid phase m&s transfer coefficient for butane in water, cm/set kL A-0, - the overall liquid phase mass transfer coefficient for butane in water. cm/set JG?,., - ko,, for the first or top section of the packed column k otz - koL for the second section of the packed column - the liquid volumetric flow rate. cm3fmin IF. NhiLB - the specific mass transfer rate of butane from the bulk &quid to the gasIiquid interface, g moles&cm2 )(min) ‘VoL - the number of overall liquid transfer units Nil, - the Reynolds number for the liquid phase defined as d&v J%c - the Schmidt number for the liquid phase defined as r/D, Ns, - the Stanton number defined as koLaG,dJu v *SIX - the Stanton number for the first or top section of the packed column n;,,, - the Stanton number for the second section of the packed column the partial pressure of butane in the gas phase, Torr PB the partial pressure of butane at the gas-liquid interface, Ton PB, the partial pressure of water in the gas phase, Torr PW P - the total pressure in the packed column, Torr T - the temperature within the packed column. “K t4 - the supeficial velocity of the liquid in the packed column, cm/set or cm/min UC - the superficial velocity of gas in the packed column. cm/xc z - the length measured from the top of the packed column, cm - the length of a section of packed column = 30 cm &I v - the kinematic viscosity, cm2,&c liquid density, g/cm3
Dcsalitwtion, 9 (1971) 351-366
VAC3JU.M STRiPi’fNG
OF BUTANE
FROM WATER
353
lNTRODUCTlOS
30th the secondary refrigerant freezing process and the hydrate process for desalting saline waters yield two product streams which. if not degassed, would contain excessive amounts of volatile refrigerant. In freezing, this means 80 to 140 ppm of butane and in hydrating it means 0.1 to 3 wt.% CH2CIF (31). CF,C& (12). or other halogenated hydrocarbon. These gases must be recovered and recycled down to about 1 to 10 ppm to JCCOWJ their economic value and/or to prevent explosion hazards, and probably to the 0.1 ppm range to meet public health standards for drinking water. When the dissolved gas is to be recovered after removal. it is importznt that the method be uncomplicated since we are producing a product (water) worth roughly U.S. SO.10 to 0.20 per ton. which is an order of magnitude less than the cheapest of mined chemicals, e.g. limestone or salt anti two or three orders of magnitude less than the cheapest manufactured chemicals. e.g. sulfuric acid or soda ash. Removal by air or flue g:::rsstripping thus seems out of the question since the subsequent recovery is difficult. Simple flashing of the product streams at reduced pressures is usefui, and probably necessary, but not sufficient since the approach to equilibrium in a single stage flash is not very close because of the very short residence times in spray chitmbers. Starting with a water solution saturated with butane at 1 atm, containing say 100 ppm butane, and flashing at 0.1 atm would remove the bulk of the dissolved butane and allow the recovery of the removed gas by compression of the vapors to the condensing pressure. Depending on plant size, the cost of the refrigerant, and other economic factors the ffashing could be done in more than one stage to save compression costs. Also, if the refrigerant were inexpensive it is possible that the partially stripped products could then be further stripped with non-condensible gas down to 0.1 ppm with no provisions for recovery of the liberated refrigerant. If the pressure of the final stage flash were equal to the vapor pressure of water, the equilibrium solubiiity of the refrigerant would be zero and there would be 70 tower limits on the refrigerant concentrations in the efihtent water or brine except those set by the mass transfer rates. Since, however, any reasonableapproach to equilibrium requires more time than normally available in a spray, the use of a packed column for further treatment of the flashed liquid seems necessary when goals of fractional ppm are set for the product. This is true also at higher pressures. The process taking place in such a packed column is not quite the same as what is normally called stripping. We refer to it here as stripping since it is what the designer will call it, and it is a process to provide time for an approach to equilibrium in which a gas evolves from a liquid. The flow is countercurrent also but it differs from conventional stripping in that the gas flow is very small compared to the liquid and is not an independent variable but is zero at the bottom of the
Desalination.
9 (1971)
351-366
J.-E.
354 column.
BAJOLLE.
P. A. RICE
AND
A. 3. BARDUHN
This does not complicate
the design as long as the liquid film controls, from the rate equations. Very few results have been reported so far on the actuat efficiency of a process designed to remove butane from a water solution. An estimation of the height of a transfer unit in a packed column based upon CO, absorption can be found in (2). The butane content is said to have been reduced to less than 0.2 ppm in a packed column at 35 Torr (3). but no specific apparatus is known to have been designed and set up to investigate in detail this particular mass transfer
since the gas flow rate is eliminated
OpeTdiUll.
The purpose
of this study
was to obtain
butane from water, and to analyze basis for design.
data on the vacuum
and interpret
stripping
these d;ita to provide
of
a sound
EXPEBthiEMTAL
Appamrus The
apparatus
was designed
IO study
the cKccts of pressure
and flow rare
or? the rate of butane removal from an aqueous solution flowing down a packed column. As long as the column pressure is above the vapor pressure of water, the heat effects are negligible and the column remains nearly isothermal, This was the case dealt with throughout this study. liquid !rops
1
rotameter butane
+
Fig. 1. Apparatus.
needle
valve
T
stopcock
Q
three-uoy
stopcock
-M
lo monometer
-S
to sampling
ctrcuit
VACUUM
STRlPPING
OF BUTANE
F%OM WATER
355
Fig. 1 is a sketch of the apparatus consisting mainly of a feed tank and stripping column. The feed tank is an J 8” high and f 5” diameter cytindcr, constructed of stainfess steef and connected to a butane cylinder equipped with a pressure regulator. Another inlet is used 10 fill the tank. Inside the tank, an jmmersion pump continuously agitates the solution to reduce saturation time. A three-section glass packed-column was adapted for use as a stripping co‘fumn. The packing in each. of the 30 cm iong, 5 cm diameter, cylindrical sections, is 4 mm ceramic Berf saddles. Each section is sealed to the uther by a ?~~6~ g&s joint. The bottom is a 50 Iiter spherical fiask, the top of which is joined to a g&s tube connected to a small pump used for removing the liquid. The top glass joint
of the column has two tubes for incoming piqued and outgoing where a thermometer is inserted to indicate the vapor temperature.
vapor, a joict and a pressure
WP* At the bottom of each of the two last sections, a glass tube with a stopcock is placed under the packing and atlows samples to be collected. The first section is divided into three packed stages of equal height, each section with a sampling tube and a thermometer at its base. The sampling tubes, each fitted with a stopcock, and the thermomrters, arc diarnetrjcalf~ opposed and are mounted in such a way that their extremities are wet by the same liquid. Both are under a small cone-shaped screen which holds the packing. There is a hole in the center of the screen, so that the flow is sufficient for good sampling, but not large enough to perturb significantly the flow pattern in the column. fn addition. there is a sampling tube and thermometer al the entry point of the feed into the column, The column pressure is regulated by a needte-vatve in the outfet gas line which goes to the vacuum pump.. The liquid feed rate is controlled by valves and by the rotational speed a centrifugal pump, which is itself controlled by a variable resistance. A rotameter mounted between the feed tank and the column is used to measure the liquid flow rates. The sampling flask consist of a sm&J g&s b&o equipped with two stopcocks so that it can be evacuated to the column pressure before filling. Procedrcte
The sotution of butane is prepared first. The column is then ffushed with an atmosphere of butane to remove the air from xhe system. The vacuum pump is turned on and the pressure in the column adjusted to the desired operating value. The pump can either be continuousIy run while the vacuum is controlled by the needle valve. or be turned on intermittently to make up for small increases in pressure due to stripping_ After the feed is admitted, steady-state in the column is reached in about 30 minutes. Liquid samples are then taken from the top of the column and at each sampling pint. Care is taken to hold the sample bulb at a Desalination,9 (1971) 351-366
356
J.-E.
BAJOLLE,
P. A. RlCE AND A. J. BARDUHN
pressure only slightly less than that in the column to eliminate change in sample composition. The silmples are analyzed for butane using a Beckman Model 915 Total Carbon Analyzer. To complete the run the vapor temperature and the liquid temperature at several points in the column are recorded. No significant temperature change along the height of the column is observed.
A literature survey was conducted on n-butane solubility in order to obtain the liquid-vapor equilibrium curves at different pressure and derive thermodynamic data. After Winkler’s earlier estimations (4). Morrison and Billet (5) were the tirst to give reliable data. Their measurements involved a manometric but precise
method.
They
investigated
over
the range
! atmosohere. Further studies showed
good
agreement
with
(6-13). Morrison
Kreshcck
- Schneider
0
Wetioufct
- Mottk - Stolter - Coffin
X
Morrison Rcomcr
+
Clouisn - PolQlosc
at a total
pressure
of
- Scherogo
- Billet - Soge
- Locey
Epuo¶ion
I
to 75°C
using more sophisticated equipment, and Billet’s results, and showed that
A
0
10°C
5 RECIPROCAL
of
line is:
&
3:4
TEMPERATURE
= IOCWT
316
-
I-K I-’
Fig. 2. Equilibrium conamtration of butane in water (total P =
1 am).
~~SU/iMtiOil.
9 (1971)
351-366
VACUUM
STRIPPING
OF BUTANE
FROM WATER
357
Henry’s inw related the solubiiity to the total pressure quite satisfactorily over the range O-1 atmosphere. These data have been fitted to a curue of In C* BS tOWj3”, C* being expressed in ppm and T in degrees Kdvin. With a srandsrd deviation of less than 2.5% in C* between 10°C and 35”C, the analytical relationship obtained at t atmosphere was (see Fig. 2): In c* = t8.2914 -
11.5149
fooo
( )
“““““““““” + 2.15249
Henry’s law constant k(T) can be calculated atmosphere by means of the relation:
moo 2
( ) ---Y-
from the equilibrium
curve at one
Tn the mnge of interest flWC to 4U’C), X-(T) can be represented by a straight line, The equation of this line is: k(T)
very well
= 1.1333 lob T- 3.0273 108 Tort-/mole fraction = 0.3513 T - 93.84 Torrjppm with T = “K
For a given total pressure, the equilibrium solubility also varies considerably with the temperature and this effect is amplified further toward the low pressures, because the water vapor becomes dominant in the gz~ phase as tbe total pressure becomes near the vapor pressure of water. For inst;rncc, in going from 25% to 20°C there is a 25y; soiubility increase a t 200 Torr total pressure while this percektage grows to 160% at 30 Turr total pressure. Because the precision of temperature measurement is about the same for all pressures, the lower the pressure, the lower the accuracy of the estimation of the salubility.
Twenty-three runs were made covering a range of 6 diRerent Row rates, from 40 mlimin to 280 mt/min, and five different column pressures. The pressures chosen were 30,45,85, 105 and 130 Torr, Temperatures were always near ambient which was 25 rt 2 YJ. At 25°C the vapor pressure of water is 23.76 Torr and k(T) is 10.9 Torr/ppm. Note that at the column pressures used, the feed from the tank was afways supersaturated with butane and some gashing took place after the rotameter and especially in the column above the packing. The vapor thus evolved left through the vapor line and did nat affect the stripping action in the cr>lumn. The feed samples CI at the top of the column measured the butane concentration in the true liquid feed to the top of the packing. fn 21 out of the 23 runs even the true eofumn feed had an equilibrium partial pressure of butane greater than the tota! pressure and thus fuE_rher spontaneous flashing (or boiling) was possible in the packing. With the large liquid area exposed Desdinatim~ 9 (1971) 351-366
J.-E.
358
BAJOLLE,
P. A. RlCE AND A. 3. BABLXIHN
in the packing and the thin films of water present this seems unlikely, however, and no visuat evidence of bubbles forming in the liquid was noted, although this might be difficult fo detect. Nucleation of a butane vapor bubble seems improbable also when it is considered that only I to 4 molecules per million of fettd are butane. ‘If. in fact, such boiling did take place it would probably increase the expected mass transfer rates both by decrerising diffusion path lengths (and thus increasing kL) and by increasing the gas-liquid interfacial area (u& but the process of diffusion stitt would have to occur between the bulk liquid and the gas-fiquid interface. pro$les Table I gives rhe con~nt~tion prufiles along the column for theexperimental flow rates and pressures in terms of n-butane ppm. The concentration at the bottom 0,. the columr, C,, was usually as close to the ~ni~ibrjnrn salue, C*, at given operating conditions as we couid measure, and thus it was not possibte to cafcufate driving forces or mass transfer coefficients for the bottom sectiohof the packing, Cuttcetttraticn
40
30 45 85
1.8 6.2 II.7
0.6 ZO 6.7
0.6 20 5.7
0.6 2.0 5.7
30 45 85 105
4.3 6.9 11.7 7.5 26.2 23.1
3.0 3.2 6.9 7.5 19.7 17.7
1.8 2.0 5.7 7.5 11.0 J6.4
0.6 2.0 5.7 7.5 9.8 16.4
85 IO5 130
5.4 37.0 13.7 17-O 19.6
1.2 9.7 9.4 11.0 127
a5 4.8 7.0 7.5 9.8
0.6 2.0 5.7
45 85 t30
11.7 13.6 15.8
7.4
2.0
iii
42 5.3 9.6
210
30 45 85 130
10.2 8.0 24.7 29.3
7.2 5.6 17.7 TO.7
3.7 2.0 11.7 15.0
O-6 2.0 5.7 9.8
2?0
45 83
54.0 23.0
12.0 37.8
8.5 12.0
4.6 8.0
7r3
130 zoo f3f
!70
30 45
;i
;:i
Desalination~ 9 (1971) 351-356
VACUUM
STRIPPING
OF BUTANE
FROM WATER
359
ANALYSlS The experimental concentration profiles were used to calculate overall liquid mass transfer coeficients. lf u designates the superficial velocity of the liquid, overall liquid mass transfer and a,,, interfacial gas-liquid k3t* a mass in the phase, between heights 2 Z + of the yields:
u
‘-
dC dZ
=
-
k,,o,,(C
- c*)
For a given set of experimental conditions, assuming difute solutions and no temperature change, u and C* are constant. This equation can, therefore, he integrated over a given section of the column to yield:
c a-1 and C, are the concentrations of the iiquid before and after section rr, rcspectivcly. ‘fhc left hand side of these equations is the number of overall liquid transfer units, while the right hand side is the length of the section divided by the height of an overalf fiquid transfer unit. The data were correlated in the form of a Stanton number for mass transfer defined as: .
From each experimental concentration profile. only two values of the Stanton number were calculated, one for each of the two top sections; since the ~o~~~tratjo~ change in the last section is so smati. Because the concentration changes are larger, the Stanton numbers determined for the first section are more precise than those determined for rhe second. The precision of the Stanton number is also better at the higher flow rates. The results shown in Table fl are represented by the curves A& and Nst, vs N,, in Fig. 3. Because there was not much difference in the Stanton numbers observed at different totaf cotumn pressures, an average vafue was computed for each flow rate, These curves can be fitted with second degree polynomials with 3 good approximation to yield the following analytic expressions: N St* =3.42x lo-%‘,2 - 5.95 x lo-%&2*93X &r* = 1.94 x iW-4NR~ - 3.91 X t0-3N,,-i-2.53 In the above expressions,
lo-* x to-”
the ATRtis for the liquid and is defined as udJv_ DtWillil#tion,9 (1971) 351-366
360
J.-E BAJOLLE,
TABLE
P. A. RICE AND A. J. BARDUHN
II
6.
6.
23.9
30 45 85 130 ZOO
1.5 4. 5. 3.3 s.
4.
18.5
4.08 4.1
4.1
18.8
3.55
45 85 105 130
1.95 215 2.7 3.3
2.7
13.2
275 2.85
2.8
13.7
I70
4.47
45 85
1.85 22
2.
9.2
2.45
2.45
11.9
210
5.53
30 45 85 130
I .45 1.65 1.58 1.79
1.65
6.7
2.
2.05
9.6
280
7.37
4s 85
I.20 1.45
1.35
4.0
1.5 1.9
1.70
7.1
40
1.05
85
70
1.84
135
From the values of Nsl, the product of the overall mass transfer coeflkient and the gas liquid interracial area, ko,_~~,, can be calculated. Because several mass transfer theories (14) predict a linear dependence of the liquid mass transfer coeflkient on (NRC)*for low mass transfer rates, k,,,u,, and kOL,uGt were plotted against (NR.)* in Fig. 4. These curves show two regions. The coefficients initially increase linearly with the square root of NRC and then abruptly decrease after Reynolds numbeti of 3.5 and 4.5. The equation of &a,, w (Nn,)*, for IV,, Z 4, was found to be: koLaGt =
1.4 1 x 10-3(N,,)f(sec-‘)
It is useful to compare the observed overall mass transfer coefficients with those predicted from generalized correlations. The mass transkr rate can be written in terms of the overall and individual mass transfer coefficients as Nu, = koL(C-C*)
= kL(C-Ci)=
k&pB,-pB)s
If the overall liquid mass transfer coefficient does not change with the total pressure, Desalirmion.
9 (1971) 351-366
VACUUM
STRIPPING
OF BUTANE
361
FROM WATER
1e-
0
12-
8 -
IO8 642-
Fig. 3. Variation of the Stanton number with Reynolds number the resistance
to mass transfer in the vapor phase is negligible and the overall mass transfer coc&cient becomes equal to the liquid mass transfer coefficient. There does appear to be an increase in (C, - Ct)/(C, - C*) with pressure in Tabie II but it is small and irregular. This implies some gas phase resistance but that the liquid phase resistance is controlling and k,, z k,. After obtaining an extensive series of data on mass transfer in packed columns, Shulman (f5) and his co-workers proposed the following correlation:
For the experimental flow rates, the values of k, can thus be predicted from Shulman’s correlation, if the packing and fluid properties are known. D, was recently measured and the value I.04 x IO- ’ cm*/sec at 25 “C was found (16). The value of d, was found to be 0.52 cm by extrapolating data (17) on larger Beri saddles down to a nominal size of4 mm. and the values of k, From the values of kOLuCL obtained experimentally, from Shulman’s correlations, values of acL, the gas-liquid interfacial area, can be calculated and compared with vaIucs obtained from other workers. The calculated values of u,, for the experimental fiow rates are given in Table III, Desdinafion, 9 (1971) 351-366
J.-E
362
0
BNOLLE,
First
AND
A. J. BARDUHN
Section
A Second
Fig. 4. Variation
P. A. RICE
Section
of the overall liquid mass transfer coefficient with
number.
together with the percentage of effectively utifized total packing area, defined by: o/Outilized area = 100
OGL - a
For 4 mm packing, u was found by extrapolating Berl saddles packing to be 14.3 cm’lcm’. TABLE
III
CoMpurAnoh’OF
1.41
2.48
4.77 6.01 . 7.43 9.90
data (17) observed for larger
ocr.
40 70 135 170 210 280
k&mJsec) x IO-’
korrra (see-l) x 10-3
aa_(cnStc~
o/n utilized stfea
1.51 1.94 2.61 2.89 3.18 3.62
1.41 1.91 2.63 2.31 2.07 x.45
0.093 0.098 0.101 0.080 0.065 0.046
0.65 0.69 0.71
0.56 O-45 0.32
De.WzirUfion. 9 (1971) 351-366
VACUUhf
STRG’PING
OF BUTANE
FROM
363
WATER
An examination of this table shows that ucL tends to decrease as the flow rate increases beyond a certain N,,. This result is similar to the results of Shulman (IGf and Fe&nger (13). As the Iiquid ffow increases, the numerous crevices at the points of contact in a smal packing become increasingly gooded, thus exposing a relatively smalt surface area. The percentage of total packing surface effectively utilized is accordingly very low. in our wse less than 1 7:. For a given flow rate this percentage has been observed by Shulman to decrease as the nominal size of the packing decreases, although the packing specific surface increases. For II = 4.77 cmjmin, this percentage has been computed as a function of the nornina size of the packing, using Fethngeis data. The percent utihzed arca is afmost indepe&dent of the gas velocity until near the loading point, although the gas velocity used is approximately equal to the avewgc gas velocity in the top section of the column. The results are shown in Fig. 5. and the point obtained in this study does not disagree with those of Feltinger as reported by Shutman (15). Our resutts are thus not inconsistent with those obtained under more usual stripping cunditions, although the test of consistency is quite rough because of the extrapolation of packing parameters involved and the lack of other data for small sized Berl saddles. The results do suggest, however, that large scale butane stripping systems can be designed using either the correlations of Shutmnn to determine the hquid mass transfer coefticient and the values of eRectivety utilized packing area., or the data of others on values of k,uo, for various packings when stripping sligh,ly soluble gases.
Fellinger Doto Reported in Shulmon Q These Data
l
u = 4-f? cm/min us = 0.8 cm/min
( L = $35 cmbmin
)
t 5 NDMfNAt
StZE
icmf
OF CERAMIC
BERL
SADDLE
Fig. 5, Pacent of suurfacr:area used for mass transfer t*s nominaI size of 3erl saddie.
364
J.-E.
BAIOLLE,
P. A.
RICE
AND
A.
J. BARDUHN
The designer will normally have at hand the unstripped product rates from the desalting plant along with their actual butane content and desired final ppm of butane. Flashing of these streams at some reduced pressure will remove much of this dissolved butane. but the extent to which this occurs was not studied in the work reported here. A study of spray flashing is being conducted and will he
reported later. Presuming that the designer can predict the composition effluent
from
a flashing
operation
of the
he will then let this be the feed to a stripping
column. To design the column he then needs to estimate (3) an allowable liquid rate in lbs/hr ft”, and
for any given packing,
(b) the height of a transfer unit expected. A packed column is certainly indicated since at the low pressures used the pressure drop in the gas phase must be very smallUnfortunately, the conditions under which this type of column
operates
arc out of the range of the data reported in the literature on flooding and the extent to which mass transfer rates are applicable is uncertain. It seems necessary that additional experimental work be done under conditions more near to those expected. The unusual conditions for this operation are: (a) the very low gas to liquid ratios encountered, and (b) the extremely low concentrations of the solute required for the final product. We believe that the study reported here has shown that very low concentrations in the fractional ppm range present no new design problems and that butane can be reduced from say 1 to 0.1 ppm (or probably even 0. I to 0.01 ppm) in the same packed column that would reduce it from 100 to 10 ppm, and that the number of liquid transfer units required to make any of these reductions would be In 10 or 2.3 when the pressure is set at the vapor pressure of water, that is, nothing unexpected is likely to occur at very low concentrations. The low gas to liquid ratio presents new problems. For example, with a feed containing 29 ppm butane the mass ratio of gas to liquid at the top of the column (where it is a maximum) is about 1:50,000 or a little more, depending
on how much water vapor leaves with the butane. None of the existing correlations on flooding can be extrapolated to this condition with confidence. It would be desirable to use high liquid rates to reduce the required column diameters, of course, but it is not clear whether flooding is an important factor at all. It is possible that the adverse effect of high liquid rates on the liquid transfer unit height would determine an optimum liquid rate well before any hydrodynamic restrictions occurred_ Values of I?,_ appear to rise sharply for liquid rates above 25,ooO lbs/hr ft2 for I/2 in. to l-If2 in. ring packing.
Perry (f9) has reviewed much of the design data for flooding and liquid Dcsabation*
9 (1971)
351-366
VACUUM
5TRIPPiKG
OF BUTAhT. FROM WASR
365
transfer unit heights in packed towers. The data there (compiled by Sherwood and Holloway (20)) on values of NL for desorption of O,, HZ, and CO, should be applicable to butane desorption if corrections for Schmidt numbers are made. The main question in using these data is the effect of gas rate. This is said to be unimportant at low liquid rates, but others (Perry (19)) say that at high liquid rates, the gas rate has an adverse effect when it is very low due to excessive backmixing in the gas phase, This may be unimportant when the equilibrium solubility is near 0, but the cause of the adverse effect is only speculative and the effect may exist in butane stripping cotumns not using an Independent strippingagent. To be safe it is recommended that before a firm design is accepted, some tests be made to determine HL for liquid rates of 10,000 to 30,000 Ibs/hr ft* in a packed tower in which butane (or oxygen) is removed under vacuum with no independent stripping %crs* Since the equations of Sherwood and Holloway show HL to increase as about the Ct.25 power of the liquid rate- the total tower vofume will decrease rapidfy at higher liquid loadings and values of at least 10,000 lbsihr ft’ and possibly
much higher are likely to be economical.
From the experimental data, the variation of the mass transfer coefficient (k,,u,,) with the Reynolds number was obtained for stripping butane from water in a packed column down to concentrations as low as 0.6 ppm. For the 4 mm Bcrl saddle packing used, this coefficient was found first to increase linearly with the square root of the Reynolds number until a maximum is reached abruptly at a Reynolds number of about 4, and then to decrease slowly. The mass transfer was experimentally shown to beliquid-diffusion controlled. The interfacial areas calculated from the experimental mass transfer data and Shulman’s correlation for the liquid phase mass transfer coefficients were not inconsistent with measurements of interfaciaf area on other packings. The results obtained are expected to be valid as long as the thermal effects are not significant, that is, as long as the column is operated at a total pressure not below the vapor pressure of water.
This research was supported of the Interior.
by the Office of Sahne Water, US. Department
Desdination, 9 (1971) 351-366
366
J.-E. BAJOttE,
P. A. RKX AND A. 3. BARDUHN
4. L. W. WISX~X. Chem. Rev., 34 (1901) 1408. 5. 1. J. M muu.sos AL~DF. 8m.m. f. Chem. Sue., (1952) 3819. 6, W_ F. CU~N AND M. F_ Powx_m~, J_ Am. Chpm. SOE., 74 (19S2) 4817. 7. N. N. Rurmn, B_ H. SAGE.*xE)W. N. L~EY, hai. Eng. CAem,, 44 ($952) 609, 8. C. I). McAwm, Name, 2#.I (1953) 109;t. 9. A. Wunstr, J. Fh_vs_Uwm. 67 (1963) 2079. 10: D. B. WETXN~~R, S. K. Maw, L. STOLLERANF) R. L. Corn. .I. Am. C/tern. six, 96 (1%) 508. 1I. C. D. MCAUBJWE,Am. Cftem, Sot.. Dis_ Pefml. Chem., Preprints. 9 (3,(1964) 275. 12 G. C_ KRESH~~C&H. S~ZH~‘E~IXR ma3 H. A. SCHERAGE, J. P&m. Ctwm~, 69 (1%5) 3132. 13. E. J. FAR’KAS, A&. Ckm., 37 (9) $1%5) 1173. 14. R. B. BIRD, W. E. Smvmr ml) E. N. LIGKT~OO~,Thzs.qxw~Phcwrncmz, John Wiiq, ww York, NY.. 1960.p. 539. 15. H. L. SHUWN et al., A.f.Ch.&f.. 1 (1955) 247, 253.295; 5 (1959) 290; 6 (1960) 175,463; 3 (1963) 479.
4th Edition 1963,pp. 18-25 to 1837. 20. T. K_ Sw’swwt‘, AND F. A. f...HOLLOH’AY,?‘ims. Am. Inst. Chtm. figrs.,
36 ~1940) 39
Desnlinnrion.9 (1971) 351-366