oil and chemical process industries, particularly when such processes as steam distillation, heterogeneous azeotropic distillation and solvent drying are used.
Chemical
ELSEVIER
Engineering
and Processing.
33
( 1994)
429-436
Local rates of heat and mass transfer during condensation non-eutectic vapours of binary immiscible liquids Osman Depurtment
of Chemicul
Engineering,
TOBITAK
Received
Tutkun*
Murmara
21 February
of
Research
Centre,
1994; accepted
P.O.
4 August
Box 21, 41470 Gebze-Kocaeli,
Turkq
1994
Abstract Heat and mass transfer data are reported for the condensation of vapours of immiscible liquids. Condensation occurred on the outside of 0.0254 m diameter horizontal copper tube. Mixtures of organic vapours in steam have been studied, such mixtures being rich in steam and removed from the eutectic composition. The condensate flow pattern has been determined experimentally. In systems where the density ratio of the organic to water liquids is less than one, a standing-drop pattern is observed; when the ratio is greater than one, channeling flow is the main feature. For systems removed from the eutectic composition, diffusional resistance through the vapour phase adjacent to the vapour-liquid interface has a major effect on the transfer rates. Ke~~ordsr
Heat
transfer;
Mass
transfer:
Local
rates;
Condensation;
1. Introduction
In recent years a fair amount of attention has been given to the condensation of vapours of immiscible liquids. This is a process which occurs frequently in the oil and chemical process industries, particularly when such processes as steam distillation, heterogeneous azeotropic distillation and solvent drying are used. Kirkbride [l] made the first approach to the investigation of this problem. Subsequently, many more experimenters have published data on the subject and most have presented equations to fit their own data [ 1~ 11, 141. These studies have been mainly restricted to eutectic mixtures, where the vapour phase does not present a resistance to heat and mass transfer. However, the discrepancies between the condensate film heat-transfer coefficients reported, even with the same fluids and tube surface, are so great that most of the correlations proposed are limited only to the experimental conditions used, and hence cannot be used successfully for design purposes. The most accurate
* Present address: TR-54040 Adapazari. 0255-2701/94/$7.00
Faculty
of
Engineering,
Sakarya
University,
Turkey. (0 1994 ~ Elsevier
SSDIO255-2701(94)00519-2
Science
S.A. All rights
reserved
Non-eutectic
vapours;
correlations
are
and
[3], the
Turner
those
Binary
immiscible
of Bernhardt predictions
liquids
et al. [2] and of both
being
Akers
to within
when compared with all of the data. The fact that the condensate formed during this condensation process consists of two liquid phases results in condensate patterns markedly different to the laminar films occurring during the condensation of most pure components. This pattern must have a marked effect upon the conductive resistance offered by the condensate. Hazelton and Baker [4] described six ideal patterns of condensate flow. However only three were observed in their study, namely film drop, standing drop and channelling. In a condensation process, however, a combination of all three may occur. The most informative visual study of the condensation process of immiscible liquids was made by Bernhardt et al. [2]. Visual information was obtained by means of high-speed motion pictures taken through a microscope using the vertical end of a 25.4 mm diameter copper cylinder. These workers removed much of the previous guesswork by using both probe and dyeing techniques to show that the standing drops were of water while the film consisted of the organic phase. Heat-transfer rates obtained with the film drop and the +25%
430
0. Turkun / Chemical Engineering mnd Processing 33 (1994) 429-436
standing drop are comparable, whereas channelling flow results in a higher heat transfer [3-51. Hence, the conductive resistance presented by the condensate layer depends on the condensate flow. The factor that is subject to most disagreement is the relationship between the heat-transfer coefficient and the temperature difference. The elIect of the temperature difference across the condensate film on the heat-transfer coefficient is not well understood. Some workers claim no dependence [4,6,7], whereas other data indicate a decrease in the film coefficient as AT, increases [2,8-lo] whilst other data show an increase in the coefficient as AT, increases [5,9.11]. The condensation of a binary vapour mixture differs in several respects from that of a pure vapour. The essential difference between the condensation of a pure componcnt and a binary mixture is the resistance to heat and mass transfer in the vapour phase in the latter case. In general, for a binary mixture, this implies that the liquid composition at the interface is different from the vapour composition at the interface, with both being different from the mean compositions of the respective phases at that point in the process. The non-uniformity in the composition of each phase means that there are diffusional resistances to the transfer of mass in each phase. In the present work, emphasis is made on the condensation of vapours of immiscible liquids when the composition of the vapour mixture is removed from the eutectic conditions. Using such conditions, the rates of heat and mass transfer, and the vapour and condensate compositions, have been measured experimentally at six
I’
different locations in the condenser when the vapour and coolant flows were concurrent at atmospheric pressure. The condensate flow patterns were also determined for all the systems studied.
2. Experimental
The apparatus used in this investigation is shown in Fig. 1. It consisted essentially of a boiler (A), an experimental condenser (B), a stripper column (C), a main storage vessel (D) and a coolant tank (E). The vapour mixtures generated flowed through a pipe to the experimental condenser. The condenser was constructed of two concentric horizontal tubes, cooling water flowing through the inner tube and vapour mixture flowing through the annulus. The outer surface of the larger tube was lagged. The vapour condensed on the outside of the surface of the inner tube. Excess vapour was allowed to pass to a vent condenser (L). The shell of the condenser was fitted with an internal trough which collected the condensate as it ran off the tube. This trough was divided into six separate compartments so that any variation in the condensate flow rate and composition could be studied along the tube length. Three observation ports were also provided at the centre of the shell so that the nature of condensate flow pattern could be investigated. The condensate running from the trough (M), the shell (J) and the vent condenser (L) were fed into a single glass receiving vessel (N).
I
c 0 E F
Fig.
1. Flow diagram of the experimental
details
apparatus
StrIpper column Main sl’omge vessel Coolant tank I-sion heaters
employed.
K L M N P R s T
Coolant pump vent condenser Trough Receiving vessel Coaloscer Rotameters Valves Vent
431
The larger tube into which the inner tube was placed consisted of a stainless-steel shell of 0.0762 m inside diameter and 0.9144m in length. As stated above, condensation took place on the outside of the inner tube which was a horizontal oxidized copper tube of 0.0254 m in diameter and 0.9144 m in length. Mixtures of organic vapours in steam were studied. These mixtures, which consisted of toluene, n-heptane, cyclohexane and trichloroethylene with water, were rich in steam. Before taking any real data, the inner tube was well oxidized during trial runs employed in testing of the rig when both water vapour and toluenelwater systems were used over a period of time. At the end of this period the tarnished colour of the tube surface indicated that it was completely oxidized. Great care was also exercised to remove inert gases from the system. To this end, sufficient excess vapour was allowed to enter the vent condenser and a vent line (T) was provided in the system for this purpose. The rates of heat and mass transfer, the vapour and condensate compositions, and the coolant and bulk vapour temperatures were measured at six equally spaced locations in the condenser under atmospheric pressure when concurrent flows of vapour and coolant water were employed. The temperatures of the wall, bulk coolant and vapour were determined using calibrated chromel/constantan thermocouples. All thermocouples were calibrated at six different temperatures, i.e at the melting point of ice, the boiling points of distilled water and highly pure chemicals, and ambient temperature. Each could be fitted to a second-order equation. Thermocouple readings were recorded using a multichannel data logger with ice/water as the reference cold junction. In order to determine the condensate flow patterns of the runs, the condensation process was photographed through an observation window. 3. Results and discussion A liquid -vapour eutectic for two immiscible liquids is characterized by a point on the temperature/composition phase diagram for a binary mixture (see Fig. 2). Since there are three phases in equilibrium and two components, there is only one degree of freedom, i.e. the system is determined if either the temperature or the pressure is specified. Point E in Fig. 2 is called the eutectic point and its temperature and composition are referred to as the eutectic temperature T, and the . eutectic composttton JJ,~. For many systems, particularly organic/water systems, mutual solubility is negligible and thus the activity coefficients may be taken as unity. The eutectic point can then be estimated from the vapour pressure data for both components at a given total pressure P,, assuming ideal gas behaviour [9,12]. Pf + Pq = P f
(1)
COMPOSITION Fig. 2. Equilibrium temperature/composition pletely immiscible binary system.
diagram
for a com-
Three possible modes of condensation of vapours of immiscible liquids can be envisaged depending on the composition of the bulk vapour mixture relative to the eutectic composition (points M or M’ and P in Fig. 2). The details are discussed elsewhere [13]. In Fig. 3, the experimentally measured condensate film heat-transfer coefficients along the tube length for the representative runs have bene plotted together with the film coefficients predicted from the shared-surface model of Bernhardt et al. [2] and the film-drop model of Akers and Turner [3], given below: h, = h, 0, + h,o, h,
= 0.725
[
3
(2)
o.25 r1
(3)
where p and i. are averaged by weight fraction and k by volume fraction. The quantity AT, = TE - T, is the temperature difference within the condensate film. It is apparent from Fig, 3 that the coefficients predicted from Eqs. (2) and (3) are in error by as much as 150% when compared to the experimental data. Although some data agree to within +20%, the majority predicted from Eq. (2) are in error by 50% to 150%. The former group of data relate mainly to the organicrich condensate while the latter belong to the water-rich ones. There is also a third group of data which lie below the _t 20% agreement line which agree reasonably well with the channelling model recommended by Akers and Turner [3], i.e. h, = 0.8
w, 1, h, + w,l,h, w, i., + ~~1,
1
(4)
The condensate film coefficients increase along the tube length. This is unexpected. The vapour mixtures condensed were water-rich and hence the water content in the condensate decreased along the tube length. It might have been expected that the resistance presented by the condensate layer would have increased, but this
0. Tutkun / Chemical Engineering and Processing 33 (1994) 429-436
432
x r UXQ-
x
Y
o Pkers-Turner (film-drop)
x
&kers-lxmrlchannelling)
q B.arnhardt et al
3oal-
xExperimentol
lollbluene/Water
I
2ooo
1 oAk-T.kerNil’m-drop1 lAkSrs-Wtiomdlilgl et d rExperimental
q Ber&udt
_
.
.
f
.
L
3cnol 0
1
010
020
01)
Rxition
OLO
from
a50
060
Inlet Iml
070
0.80
1 09c
Fig. 3. Comparison of the experimental condensate heat-transfer coefficients with those predicted from Eqs. (2)-(4) along the condenser length.
was not the case. One possible explanation contradiction could lie in the condensate flow The standing drop pattern was observed with water, n-heptanelwater and cyclohexane/water
while channelling flow occurred with the trichloroethylene/water system, as shown in Fig. 4. Droplet coverage of water on the tube surface could be varied along the tube length by changing the condensate composition. This was, in fact, observed by Bernhardt et al. [2], i.e the higher the water content of the condensate the higher the droplet coverage of the surface. Polley and Calus [5] showed that if a dense coverage of standing drops was present, the coefficients decreased and led to a greater temperature difference within the condensate film. In addition, the organic film also hinders the formation of standing drops, thereby reducing the drop population and causing the heat-transfer coefficient to increase. Further confirmation that the condensate pattern changes with condensate composition has been obtained in the present work. For this purpose, the condensate composition was studied experimentally over the following ranges: toluene/water, 1.5% to 75% toluene; n-heptanelwater, 4.5% to 86.6% n-heptane; cyclohexane/water, 49% to 85% cyclohexane; and trichloroethylene/water, 30% to 81% trichloroethylene, all percentages being expressed by volume. The measured condensate film heat-transfer coefficients were compared with data quoted in the literature, the majority of which corresponded to the eutectic composition. However, since all the experimental data were not eutectic only those relatively close to the eutectic composition were used for comparative purposes. Figure 5 shows such a comparison between the data obtained in this work with those quoted elsewhere. It is apparent from Fig. 5(a) that the experimental data obtained for the toluene/water system over the range 2000-6000 W me2 K-’ lie between the data of Sykes and Marchello [9] and those of Deakin [ 141. It is interesting to note that the condensing surfaces employed by three other sets of workers [6,9,14] were also oxidized copper. Thus, it seems that the condensate pattern obtained on apparently similar tubes can be different. These differences in the coefficients may be attributed to the fact that the properties of the tube surface vary with time. Polley and Calus [5] initially reported a channelling pattern with the n-heptanelwater system. However, this pattern later transformed to a standing-drop type using the same tube with same system. and hence gave a lower heat-transfer coefficient. Similar behaviour has been observed in the present work. Thus, in the very early trial runs with the toluenelwater system a channelling-type condensate flow was obtained. However, in later experiments with the same system under identical conditions, the flow pattern shifted towards the standing drop as seen from Fig. 4(a), in agreement with the flow pattern described by Baker and Mueller [6]. This may be attributed to a change in the surface properties of the tube. However, a mixture of channelling and standing drop types was
for this pattern. toluene/ systems,
0. Tutkun 1 Chemical Engineering and Processing 33 (1994) 429-436
8 6
I
I
10
I
433
I Ii)11
I
lalbluene/VJater
E
oBdw-WellerD=33.4mm) sB&a-kao
0
0 -25.4mml
lStecanel+StandcrtlD=laOm~~
B
4
% 3 Y AZ
n
SykS (0=34.9mml oDeakin UJ=25.4mm) xExper~mental lD=25.Lmm)
1
0
q D
. :..
-__.
&
x0 Ox .a
“’
--__
l
$ “sr”
“,o
‘Lr
Nuselt fxe&ti~~-------, for
w%
0
.,vD
4
1
_--
pure toluene
--__ 1
I I 1 10 _ I I 6 .( bl n-Heptane/Water
I
,
If,,,
,
I
I
I
lllll
I
I
x*
oBdcer-f-kmUer(D=33.4mml x Experknental ID=25.4mml
6
xx
I-
s
f
Ix
-4
$
%
d z
--b_
I
I
1
I I
10 I * Tel cyKlohe~/Water
6
oDich-Ponter
xExfm-immtd
0 0
0
Nusseit pr&t;;---for pve n-heptane
I
I I,,,,
I
I
lllll
--__
0
0
--__ 0
---
1
I
I I
I
Y
1
0 0
2
-
_
lD=266mnl U?=25.4mnt
(b) z4-
Nusselt predktio;---‘--,_ for pure cycbhexane
-+--_-_
B
08 k
---__
----___ Nusselt &&on -----_ fa pure trlchlomethylene I
I
2
Illl~~~
4 Flm
6 8 temperature
-----_
I 10 20 diiference IK 1
I P
Fig. 5. Comparison of condensate film heat-transfer coefficients tained in the present work with those quoted in the literature.
(4 Fig. 4. Observed condensate flow patterns: (a) toluene/water; (b) n-heptane/water; (c) cyclohexane/water; and (d) trichloroethylene/ water.
40
ob-
reported by Sykes and Marchello 191, whereas channelling flow was observed by Deakin [ 141. It should be emphasized that the condensate patterns observed, and depicted in Fig. 4, feature only the central portion of the tube, whereas the experimental data reported in Fig. 5(a) generally correspond to the later portions of the
434
0. Tutkun / Chemical Engineering and Processing 33 (1994) 429-436
tube. If the pattern does change as a result of changes in the condensate composition and flow rate, then it would be expected that the standing-drop type would shift towards the channelling type. This could then explain the higher coefficients obtained in this work relative to those obtained by others. For the n-heptane/water system, experimental data for 60% w/w to 75% w/w n-heptane in the condensate have been compared with the only other available data of Baker and Mueller [6]. Although these workers stated that heptane isomers were used, this would not explain the large discrepancy existing between the two data sets as is apparent from Fig. 5(b). The data of Baker and Mueller [6] correspond to a standing-drop type flow pattern and if channelling flow occurred in the present work due to changes in the condensate composition and flow rate as discussed above, then this may explain the discrepancy between the two data sets for this system. Figure 5(c) shows that another major discrepancy occurs between the experimental data for the cyclohexane/water system with 63% w/w to 75% w/w cyclohexane in the condensate and the cyclohexane/water data of Ponter and Diah [8], the eutectic composition for this system being 91.5% w/w cyclohexane. As acknowledged by Ponter and Diah [8], their data for all the systems studied are consistently lower than those of most workers; however, they did not report the condensate pattern. If it is assumed that this was of the standing-drop type, then the discrepancy might be explained in a similar manner to that discussed for the n-heptane/water system. Finally, the experimental data for the trichloroethylene/water system with 68% w/w to 84% w/w trichloroethylene in the condensate are compared with the data of Baker and Mueller [6], Baker and Tsao [ 71 and Deakin [ 141 in Fig. 5(d), the eutectic composition in this case being approximately 94%. As can be seen from Fig. 5(d), the experimental data lie within the range of data obtained by these workers in this case. In general, the experimental data compare reasonably well with the data reported in the literature, as shown in Fig. 5. The conductive resistance exhibited by the condensate layer depends upon the condensate flow pattern, as discussed by Tutkun [ 151. From Fig. 5 it is also evident that the effect of the film temperature difference on the film coefficient is not certain. The data obtained in the present study do not follow a, Nusselt-type trend, although the condensate film coefficients decrease as AT, increases. However, the dependence of AT, on h may be masked by the condensate composition which seems to affect the condensate flow. Apart from the surface characteristics of the tube and condensate composition, the density ratio of the organic phase to the water phase is also a likely parameter affecting the Row mechanism. In the toluene/water, n-heptane/water and
cyclohexane/water systems, since the organic phase is the lighter phase, a standing drop pattern was obtained. However, the flow mechanism was of the channelling type in the trichloroethylene/water system where the organic phase is the heavier one. The significance of density ratio effects together with surface forces has also been recognized by Ponter and Diah [8] for condensation on two different surfaces. It should be noted that all the inlet vapour compositions were non-eutectic and water-rich, ranging (in mol% organic) from 2.8% to 33% for toluene/water, 11% to 43% for n-heptane/water, 30% to 43% for cyclohexane/water and 20% to 37% for the trichloroethylene/water system. Since both components were condensing, the interfacial conditions were eutectic and may be predicted from Eq. (1). When a vapour mixture of non-eutectic composition M enters a condenser, the composition of the mixture will move towards point P as shown in Fig. 2, i.e. the composition of organic y,, in the vapour phase will increase. The condensate composition of organic also increases and approaches the bulk vapour composition along the tube length, as shown in Fig. 6. Since the eutectic composition y,, is fixed, when both vapours are condensing the composition difference between the bulk vapour and the interface, ylb - y,,, decreases, and hence the diffusional resistance in the vapour phase boundary decreases along the tube length as is apparent from Fig. 6. The measured condensation rates of organic and water along the condenser length are shown in Fig. 7. In most runs, the condensation rates of organic and water increased. This is expected since the diffusional resistance presented by the vapour phase will continuously decrease until the composition reaches the eutectic. These results further indicate that diffusional resistance is significant during the condensation of noneutectic vapours of immiscible liquids.
036
I
A.‘, ,/
,,,_-
d
/’
Fig. 6. Axial distribution tion along the condenser
of bulk vapour length.
and condensate
composi-
0. Tutkun 1 Chemical Engineering
I,
j.00
090
and Processing
435
33 (1994) 429-436
the eutectic point. As a result, heat and mass transfer rates increase in the same direction. In the experiments reported here, there was an increase in the rate of condensation of the organic phase of up to 600%. (2) The inlet vapour compositions, as mole fractions of organic, are all non-eutectic and water-rich as follows: toluene/water, 0.028 I y,,, 2 0.329; n-heptane/ water, 0.110 5 ylb s 0.426; cyclohexane/water, 0.298 2 ylb s 0.428; and trichloroethylene/water, 0.200
