mass transfer and structure of bubbly flows in a system of co2 disposal into the ocean by a gas-lift...
TRANSCRIPT
Chemical Engineering Science 54 (1999) 4945}4951
Mass transfer and structure of bubbly #ows in a system of CO2
disposal into the ocean by a gas-lift column
Takayuki Saito!,*, Takeo Kajishima", Katsumi Tsuchiya#, Sanai Kosugi$!National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki 305-8569, Japan
"Faculty of Engineering, Osaka University, Japan#Faculty of Engineering, Tokushima University, Japan
$Sumitomo Metal Industries Ltd., Japan
Abstract
A new method for ocean sequestration of low-purity CO2
gas emitted from "red power plant is developed. This is a gas-lift pumpsystem, named progressive gas lift advanced dissolution (P-GLAD) system, to dissolve only CO
2gas of combustion gas in seawater at
shallow waters and to transport CO2-rich seawater to great depths. The system is an inverse-J pipeline set at the ocean at a depth
between 200 and 3000 m and a releasing system of indissoluble gas. To improve the e$ciency of the P-GLAD, one should elucidatebubbly #ows accompanying gas phase dissolution formed in the gas-lift column of the system. In the present paper, "rst, the authorsdiscuss mass transfer in bubbly #ows of pure CO
2gas and "ltrated tap water along the pipe axis in laboratory-scale P-GLAD
of 25 mm in diameter and 7.69 m in height. Second, mass transfer in bubbly #ows of mixed gas (95% volume of CO2
and 5% volumeof pure air) and "ltrated tap water in the same setup is discussed. The mass transfer coe$cient of CO
2in the later system has the
values of 3.1]10~4}8.5]10~5. It is shown that the mass transfer coe$cient is a function of the distance from the gas injection.Finally, the performance of the system is elucidated on the basis of the experimental and numerical investigations. The laboratory-scale P-GLAD dissolved over 98.5% of CO
2injected in the liquid phase. ( 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Mass transfer; Bubbly #ow; Gas-lift pump; Carbon dioxide; Ocean disposal
1. Introduction
The CO2
concentration in the atmosphere reached355 ppm in 1992 (IPCC, 1995). The global warmingmainly due to the increase in the atmospheric CO
2concentration is a serious matter. Therefore, counter-measures that can economically and e!ectively treat hugeamounts of CO
2(23 GtC/yr) emitted by human activities
should be devised. Ocean sequestration of CO2is a hope-
ful option to mitigate the global warming.An e$cient method of CO
2sequestration at sea must
isolate huge amounts of CO2
from the atmosphere forlong term at low-cost and low-energy consumption. Todeal with this problem, the gas lift advanced dissolution(GLAD) system was invented for ocean sequestration ofpure CO
2gas (Saito & Kajishima, 1997; Saito, Kajishima
& Nagaosa, 1996; Saito, Kajishima, Nagaosa &Kitamura, 1997; Kajishima, Saito, Nagaosa & Hatano,
*Corresponding author. Tel.: 00-81-298-58-8522; fax: 00-81-298-58-8508.
E-mail address: [email protected] (T. Saito)
1995; Kajishima, Saito, Nagaosa & Kosugi, 1997a, b).Furthermore, the GLAD system for low-purity CO
2gas
(hereafter P-GLAD; Progressive-GLAD) as shown inFig. 1 is an advanced system to improve the total e$-ciency of the ocean sequestration of CO
2(Saito,
Kajishima & Tsuchiya, 1998).In the present paper, mass transfer coe$cients in
bubbly #ows of pure CO2
and those of mixed gas of CO2
and air are measured and calculated by visualization ofthe #ow "elds and digital image processing. We considernumerical models of bubbly #ows in a gas-lift columnwith gas-phase dissolution. First, the mass transfer coe$-cients will be shown as a function of the distance from thegas injection point. We will discuss the mass transfercoe$cient and dissolution rate of CO
2gas in the direc-
tion of the pipe axis, i.e. the pro"le of the mass transfercoe$cient in a gas-lift column. Moreover, the di!erencebetween k
Lof a pure CO
2system and that of a low-purity
CO2system is discussed. Third, the pro"le of gas dissolu-
tion rate in the gas-lift column will be discussed.Finally, the computational results are compared with theexperimental ones.
0009-2509/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved.PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 2 1 6 - X
Fig. 1. Concept and principle of the P-GLAD system. The main part ofthe P-GLAD system is an inverse-J pipeline (1) and (2), and an indissol-uble gas releasing system (3). The low-purity CO
2gas (a) is injected into
the dissolution pipe (1) at a depth of 200}400 m. An upward current isgenerated in the pipe by a gas-lift e!ect. CO
2included in the bubbles
dissolves into seawater while the bubbles rise in the pipe. Ambientseawater (b) is #owing at the bottom of the pipe. Indissoluble gas ofN
2and O
2should be released at the top of the pipe by an indissoluble
gas releasing system (3). On the other hand, the drainpipe (2) is used asa pass to transport and to release the CO
2-rich seawater (c) at great
depths of 1000}3000 m. The pumping e!ect is strong enough to trans-port CO
2-rich seawater over long distance. The density of CO
2-rich
seawater is larger than that of the ambient seawater. An additionaldownward current is generated in the drainpipe. Thus, the bubbledissolution and the transportation of CO
2-rich seawater to great depths
are enhanced in the P-GLAD system by the pressure energy of thecompressed gas. A P-GLAD system (Diameter of dissolution pipe"500 mm, ¸
1"400 m, ¸
2"380 m, ¸
3"300 m, ¸
4"100 m, ¸
5"
1100 m, Gas composition: 95% CO2
and 5% N2
in volume, kL"
2]10~4 m/s) shows the following ability by numerical simulation; super-"cial velocity of liquid phase"1.27 m/s at gas injection rate"1 kg/s).
2. Experimental setup
Fig. 2 shows a schematic diagram of the setup used inthe present investigation. The dissolution pipe (1) anddrainpipe (2) are made of acrylic and are transparent,25 mm in diameter and 7.69 m in height. These pipescorrespond with the P-GLAD system. The dissolutionpipe is connected to two pressure-vessels (4) made ofstainless steel and are 106.3 mm in diameter and 8.19 min height. The drainpipe is placed inside the vessel. Onecan observe the #ows in the drainpipe through small
Fig. 2. Experimental setup.
windows (6). An indissoluble gas releasing device (3)releases indissoluble gas of N
2and O
2. Pure CO
2gas
(99.9% purity) and pure air (CO(1 ppm, CO2(1 ppm
and CH4(1 ppm) were supplied by gas cylinders (8) and
(9), respectively, and are well mixed in a gas mixingaccumulator (10). The mixed gas is injected into thedissolution pipe through a gas-injector (5) at 840 mm (i.e.34 pipe diameters) from the bottom. The gas-injector hasan annular structure. The inner pipe is equipped with 108small capillaries of 0.78 mm in diameter. This injectorcan form almost uniform bubbles for any #ow rate of themixed gas within the range of the present study (Saitoet al., 1997).
Tap water is supplied at the top of each pressure vesselafter "ltration by a 1 lm "lter. We carry out the experi-ments under over#owing condition. The supplied waterand the gas}liquid mixtures have a temperature between287 and 290 K. The experiments are carried out underatmospheric pressure.
We measure the super"cial velocity of liquid phaseJL
by an electromagnetic #owmeter (A), the temperatureand static pressure in the dissolution pipe by thermocouples (B) and pressure transducers (C), respectively.Mass #ow controllers (E) and (F) measure each gas #owrate Q
CO2and Q
AIR. We employ two sets of high-speed
video systems of 500 frames/s and stroboscopes of 5 ls of#ushing rate (C) to visualize the bubbly #ows in thedissolution pipe. The video systems are mounted ona camera lifter (7).
4946 T. Saito et al. / Chemical Engineering Science 54 (1999) 4945}4951
3. Visualization of 6ow 5eld and digital image processing
3.1. Visualization of yow xeld
Fig. 3 shows the location of the high-speed videocamera, the strobe, the dissolution pipe and an acrylicrectangular water jacket with square cross section. Thewater-jacket covers the visualized section of the dissolu-tion pipe, and removes parallax and distortion of theimage. The high-speed video cameras and remote con-trolled zoom lens are mounted on a precision opticalstage that has 63 of freedom. We take shadow images ofbubbly #ows in the dissolution pipe from two directionsintersecting perpendicularly each other by two sets of thehigh-speed video cameras and the strobes. The followingprocedure describes the visualization of the #ow "eld.(1) Attaching four acrylic scaling plates on each side ofthe water jacket, we take high-speed video images of thescale and the dissolution pipe "lled with water (Fig. 4(a)).(2) Detaching the scaling plates, we take high-speed videoimages of the dissolution pipe "lled with water (Fig. 4(b)).These are base images for image processing. (3) We takehigh-speed video images of bubbly #ows (Fig. 4(c)). It wascon"rmed that the total distortion of an image taken bythis visualizing system and procedure is smaller than0.5% ("2.5 pixel).
3.2. Digital image processing
The high-speed video systems record the images onS-VHS video tapes. A computer captures the original
Fig. 3. Visualizing system of #ow "eld. The optical axis of the high-speed video camera, the zoom lens and the strobes intersects perpen-dicularly to the vertical axis of the dissolution pipe. Light intensityirradiated on the observed section was adjusted almost uniform.
images and stores them as ti!-format "les through animage digitizer. At this stage, we repeat digitizing of theoriginal image 10 times, and averaged them. The aver-aged images are processed according to the followingprocedure. First, we process the averaged image byemploying a median "lter and remove random noise.Second, we calculate the absolute di!erence of the inten-sity level of each pixel from the base image (Fig. 5(a)) andthe "ltered image of the bubbly #ow (Fig. 5(b)). Then weobtain an image of only bubbles (Fig. 5(c)). Third, a bi-nary image of bubbles is obtained (Fig. 5(d)). Finally,from the binary image we obtain location of the center ofgravity (x, y, z), long diameter D
1, and short diameter D
2,
velocity ux, u
y, u
zof each bubble, and the number of
bubbles n in a section.
4. Numerical modeling and computational method
A numerical method for gas}liquid two-phase #owdeveloped by the authors is employed (Kajishima& Saito, 1996). In this section the outline of the computa-tion is described. As the temperature is assumed to beconstant, the energy equation is not needed. Thus, thebasic equations of conservation of mass and momentumare as follows:
LoLt
#
L(omum)
Lz"Q
G, (1)
L(omum)
Lt#
L(aoGu2G)
Lz#
L[(1!a)oLu2L]
Lz
"!
Lp
Lz!og!
4qw
DP
, (2)
where om"ao
G#(1!a)o
Land o
mum"ao
GuG#
(1!a)oLuL.
The void fraction is evaluated from the mass conserva-tion of the gas phase
L(aoG)
Lt#
L(aoGuG)
Lz"Q
G!C
G, (3)
CG"5.24o
GkL(a2/D2
PD
z)1@3, (4)
where Dzis the length of a computational cell.
In addition, we apply the drift #ux model to determinethe velocity of each phase. The gas-phase velocity isestimated as
uG"B
0J#;
G, (5)
where J"JG#J
L, J
G"au
Gand J
L"(1!a)u
L. The
liquid-phase velocity is given by uL"(J!au
G)/(1!a).
For bubbly #ow, B0
and;G
(Zuber & Findlay, 1965) are
B0"1.2!0.2Jo
G/o
L,
;G"1.41[p
Lg(o
L!o
G)/o2
L]1@4. (6)
T. Saito et al. / Chemical Engineering Science 54 (1999) 4945}4951 4947
Fig. 4. Samples of high-speed video images in visualizing procedure. The vertical axis, the horizontal axis and scales are printed on the acrylic scalingplates. The plate is the same size as the sides of the water jacket. The vertical axes of the plates, parallel to each other, and the vertical axis of the pipeare all in one plane. The horizontal axes and the optical axis of the high-speed video camera are all in one plane.
Fig. 5. Procedure of digital image processing.
Wall friction qw
is modeled as
qw"jo
LJ2L/8(1!a)1.75. (7)
The "nite di!erence method has been appliedto discretize the basic equations. The formal accuracy
is fourth order in space and "rst order in time.The implicit time marching scheme for density andpressure is employed to deal with the high compress-ibility of the gas phase (Kajishima & Saito,1997).
4948 T. Saito et al. / Chemical Engineering Science 54 (1999) 4945}4951
5. Results and discussion
5.1. Experimental results on mass transfer coezcient
The mass transfer coe$cient kL
is obtained from
dCB
dtA
P¸"k
LA
T(C!C
=), (8)
where t is the time, AP
the pipe cross-sectional area, and¸ the length of the measured section. For calculatingkL
one needs CB, A
Tand (C
S!C
=). In this investigation,
these are functions of z. Considering that the shapeof the bubbles is an oblate ellipsoidal described by(x2#y2)/[D
1(z)]2#z2/[D
2(z)]2"1, we can calculate
the bubble volume <MB(z) and interfacial AM
B(z). The gas
#ow rate at z, QG(z) is expressed by
QG(z)"n(u6
B/¸)<M
B(z), (9)
where ¸ is the length of the measured section. An electro-magnetic #owmeter and pressure transducers, respec-tively, measure the water #ow rate, Q
Land pressure
inside the pipe, p(z). Hence, we obtain CB(z) by
CB(z)"C(p(z)#4p
L/DM
B(z))
QG(z)
QL#Q
G(z)DNbR¹, (10)
z is replaced by t via
t"z2NPz
0
u(z) dz"z/u6B. (11)
Hence, Eq. (8) is transformed into
CB(tu6
B)"C(p(tu6
B)#4p
L/DM
B(z))
QG(tu6
B)
QL#Q
G(tu6
B)DNbR¹,
b"1. (12)
Assuming no oxygen and no nitrogen absorption, themolar concentration of CO
2gas is given by
CCO2
(tu6B)"C
B(tu6
B)!C
AIR. (13)
C=
is given by
C="C
0#C
CO2(z
0)!C
CO2(z), (14)
where C0
is the molar concentration of CO2
in ambientwater. The total interfacial area of the bubbles is given by
AT"
AMB
(tu6B)
<MB
(tu6B)
QG(tu6
B)
QL#Q
G(tu6
B). (15)
Fig. 6 shows the mass transfer coe$cient kL
in bubbly#ows of pure CO
2gas and tap water as a function of z.
Fig. 7 shows the same for bubbly #ows of mixed gas (95%volume of CO
2and 5% volume of pure air) and tap
water. These "gures show the mass transfer pro"le as
Fig. 6. Pro"les of mass transfer coe$cient in the #ows of pure CO2
andtap water.
Fig. 7. Pro"les of mass transfer coe$cient in the #ows of 95% CO2and
tap water.
a function of z in the dissolution pipe of the laboratory-scale P-GLAD system. In both bubbly #ows, k
Lde-
creases with increase in z. The mass transfer coe$cient inthe pure CO
2bubbly #ows is larger than that in low-
purity CO2
bubbly #ows. Considering the diameter ofthe bubble, the present values of k
Lin the pure CO
2bubbly #ows are similar to those obtained by the pre-vious investigation (Motarjemi & Jameson, 1978).
5.2. Dissolution ratio of CO2 gas
The amount of CO2
gas dissolved is important whenconsidering the performance of the P-GLAD system. Wede"ne the dissolution ratio of CO
2gas, R
D(z) by
RD"[C
B(z
0)!C
B(z)]/C
B(z
0). (16)
Fig. 8 shows the pro"le of RD(z) in the dissolution pipe
in the case of pure CO2, and Fig. 9 for 95% CO
2gas.
RD(z) rapidly increases for z(3 m, and most of the CO
2gas injected dissolves into the water phase in the sectionbetween the gas injection and z"3 m. It takes a max-imum value of 0.986, 0.990 and 0.985 in pure CO
2system
for each gas injection rate, respectively. In the case of95% CO
2the maximum values are 0.979, 0.955 and
0.964. The maximum values in the later system is a littlebit smaller than those in the former. However, P-GLADshows satisfactory performance of CO
2gas dissolution.
T. Saito et al. / Chemical Engineering Science 54 (1999) 4945}4951 4949
Fig. 8. Pro"le of gas dissolution ratio in the #ows of pure CO2
and tapwater.
Fig. 9. Pro"le of gas dissolution ratio in the #ows of 95% CO2
and tapwater.
Fig. 10. Comparison of numerical and experimental results on pro"lesof void fraction.
5.3. Numerical results
We compare computational results on pro"les of thevoid fraction in the dissolution pipe with experimentalones in order to con"rm our numerical modeling andcomputational scheme. Here, k
L"2]10~4 m/s is ad-
opted in Eq. (4). Fig. 10 shows a comparison for bubbly#ows of pure CO
2and tap water. The computational
results show satisfactory agreement with the experi-
mental ones. Hence, our modeling and numerical schemeare considered to be reasonable. Near the gas injection,however, the di!erence between computational and ex-perimental results is large. This is caused by the modelingof the gas dissolution rate, i.e. Eq. (4). The mass transfercoe$cient, k
Lis given as a constant value in the numer-
ical modeling of gas dissolution rate. The coe$cientshould be modeled as a function of D
B, void fraction and
so on. We need further investigation to improve themodel.
6. Conclusion
We discussed bubbly #ows in a newly developed sys-tem for CO
2disposal into the ocean. The system utilizes
a gas-lift pump to dissolve CO2
gas into seawater and totransport the CO
2solution to the deep ocean. High-
speed video cameras and digital image processing wereused to examine bubbly #ows in a gas-lift column ofa laboratory-scale system. We discussed pro"les of themass transfer coe$cient, the gas dissolution ratio and thevoid fraction in the gas-lift column, experimentally andnumerically; that is, we elucidated one-dimensional (pipeaxial direction) structure of mass transfer in the gas-liftcolumn. The performance of dissolving of pure CO
2gas
as well as low-purity CO2gas is very high, and 95.0}99%
of the CO2
gas injected is dissolved in water phase. Toimprove the performance of the system for low-purityCO
2gas, we need further investigation.
Notation
A interfacial area, m2
B0
coe$cient of drift #ux model, dimensionlessC
Bmole of gas phase, mol/m3
CS
concentration of gas phase in surroundingliquid, mol/m3
C=
background concentration, mol/m3
D diameter, mg gravitational acceleration, m/s2J super"cial velocity, m/skL
mass transfer coe$cient, m/s¸ length of visualized section, mn number of bubbles, dimensionlessp pressure, PaQ #ow rate or injection rate, m3/sR gas constant, J/(molK)R
Dgas dissolution ratio, dimensionless
t time, s¹ temperature, Ku velocity, m/s; drift velocity, m/s< volume, m3
x span wise direction, m
4950 T. Saito et al. / Chemical Engineering Science 54 (1999) 4945}4951
y horizontal direction, mz main stream direction, m
Greek letters
a void fraction, dimensionlessb compressibility factor, dimensionlessC dissolution rate, m3/sj friction factor, dimensionlesso density, kg/m3
p surface tension, N/mqw
wall friction, Pa
Subscripts
AIR Pure airB bubbleCO
2carbon dioxide
G gas phase¸ liquid phasem gas}liquid mixtureP pipeS surrounding liquid of bubble¹ totalR background0 ambient or gas injection1 long2 short- average
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