Novel Spray Tower for CO2 Capture Using Uniform Spray of

Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 3065−3075

pubs.acs.org/IECR

Novel Spray Tower for CO2 Capture Using Uniform Spray of Monosized Absorbent Droplets Minki Cho,† Sookab Lee,† Munkyoung Choi,‡ and Jin W. Lee*,† †

Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang, 790-784, South Korea InGineers Inc., Jinae-dong, Gimhae-si, Gyeongnam 316-11, South Korea

‡

ABSTRACT: Conventional spray towers emit absorbent droplets that are nonuniform in size and spatial distribution; as a result, capture efficiency is degraded. This paper presents the design and evaluation of a novel spray tower for capturing CO2 with much increased capture efficiency. The tower uses a multinozzle plate, and can spray sorbent drops of almost the same size vertically and quite uniformly across the flow cross section. Uniform size and spatial distribution of droplets greatly increased the capture efficiency η, although much bigger drops were used, achieving η ∼ 95%, which has rarely been reported to date for CO2 capture by a spray tower. Experimental results were compared with the predictions by numerical simulations in excellent agreement. The new spray tower has a number of strong points that are favorable for scaling up of diameter and length.

1. INTRODUCTION The importance of CO2 capture is increasing, due to desires to mitigate global warming.1 Of a variety of techniques for postcombustion CO2 capture, chemical absorption using an aqueous solution of amine or ammonia (NH3) is considered to be closest to practical use, because this method has advantages, including high capture efficiency η and high throughput,2−5 simple mechanism and structure, low regeneration temperature, and easy utilization of waste heat.6 Chemical absorption using liquid absorbent is usually realized in either a packed tower or a spray tower: the bubble column is not considered to be practical due to the excessive amount of absorbent that it needs.7 A spray tower is the simplest type of chemical absorption reactor, in which sorbent solution is sprayed into a rising gas stream that contains CO2; small droplets of various sizes form, and fall in the direction opposite to the gas flow. Droplet size affects their falling velocities and the amount of CO2 that they absorb. The concentrations of CO2 and sorbent molecules that determine the absorption mass flux vary from position to position and also among droplets of different sizes. A packed tower is another type of absorption reactor; it is filled with a large number of small packing elements; sorbent solution supplied at the top flows down over the surface of the packing elements to form a liquid film on their surfaces. It shares the same characteristics as the spray tower, except that sorbent solution forms a liquid film © 2018 American Chemical Society

instead of droplets, and that the absorption characteristics differ among films of different thickness. Any practical CO2 capture technique must simultaneously satisfy two conflicting goals of high capture efficiency and low cost. All current CO2 capture technologies under study and development have put higher priority on capture efficiency than on cost, and have used excessive amount of absorbent or excessively long towers to get capture efficiency η > 90%. The use of an excessive amount of absorbent results in high energy cost, and an excessively long tower results in high construction cost, both of which are critical barriers to commercialization of CO2 capture. Energy cost for CO2 capture consists of two components, one for the fluid flow and the other for heating the sorbent solution to regenerate it; both costs are proportional to the liquid (sorbent) flow rate Ql. Construction cost is determined by the tower size and the type and amount of packing material. In general, the efficiency of mass transfer between two flow streams is determined by the total mass transfer coefficient hA, where h is the effective mass transfer coefficient per unit area and A is the total surface area of absorbent liquid contained in Received: Revised: Accepted: Published: 3065

December 22, 2017 February 7, 2018 February 13, 2018 February 13, 2018 DOI: 10.1021/acs.iecr.7b05309 Ind. Eng. Chem. Res. 2018, 57, 3065−3075

Article

Industrial & Engineering Chemistry Research

spray tower, at the same high gas-to-liquid flow ratio of 20 (mol/mol). This inferior η of spray towers compared to packed towers can be attributed to several factors, including the following: potential loss of droplets to the reactor wall by adhesion (“wall loss”); high nonuniformities in spatial droplet distribution, in drop size, and in gas velocity; and drop coalescence by collision between drops. The first three factors originate from the characteristics of the spray nozzle: a common spray nozzle generates a conical spray of droplets with a wide size variation, and the difference in inertia among droplets of different sizes results in nonuniform spatial distribution of droplets across the flow cross section. Small droplets may flow backward along the high-velocity gas flow, and large droplets may reach the wall and be lost. Collision between drops due to nonzero relative velocity increases the drop size and size dispersion, resulting in a decrease in surface area. As droplet size increases, the amount of absorbent mass and the time required for internal diffusion and chemical reaction increase, but falling speed increases; combined, these factors reduce the degree of absorbent use and thereby degrade η of a spray tower. When the number density of sorbent drops exceeds a limit, the mass transfer rate may even decrease due to the much reduced gas concentration in the region of high droplet population.13 In a packed-bed reactor, an absorbing surface forms on the liquid film that flows over the surface of the packing material. The falling speed of the liquid is made very low (i.e., a few centimeters per second even in large reactors) by the friction from the solid packing material, and this slowness is the determining factor for the high η of a packed tower, as is expected from eq 1. Because a typical packed bed is composed of packing elements of rather uniform size, the liquid film thickness averaged over a few element units is nearly uniform and the spatial distribution of the films is uniform throughout the reactor; these uniformities enable efficient capture of CO2. Although current packed towers absorb CO2 more efficiently than spray towers do, options for further improvement in η are limited: film thickness cannot be controlled easily because it is predominantly determined by the liquid flow rate, and absorption or sorbent saturation proceeds only from the gas interface to the solid interface; both of these attributes are disadvantages of a packed-bed reactor. On the contrary, although current spray towers have lower η values than packed towers, optimization of injection of droplets (i.e., drop-size and spatial distributions) can provide much room for η improvement of spray towers.11 In a spray tower, absorption or sorbent saturation proceeds in three principal directions from the gas interface toward the droplet center, and the surface area−volume ratio is 6 times larger than that for a liquid film of the same thickness as the droplet diameter d. Even for d values 2 or 3 times the film thickness, the surface area per total liquid volume is 3 or 2 times larger, respectively, compared to the planar liquid film. This is a very strong point of a spray tower relative to a packed tower. The best absorption performance is expected for uniform distribution of monosized droplets in uniform gas flow. However, droplets generated by typical spray nozzles have quite a large size variation with a geometric standard deviation σ > 2.0. For polydisperse droplets, the carried mass increases but “capture capacity” or captured CO2 per droplet mass decreases as droplet mass increases, and the average capture capacity decreases as the size distribution becomes wide. Furthermore, for conical spray nozzles, some large droplets can

the reactor. A is proportional to the total amount of absorbent liquid contained in the reactor, i.e., the product Ql and liquid falling time tl = L/Vl, where L is tower length and Vl is liquid velocity. The determining parameter for the capture efficiency, with the specific surface area per liquid volume implicit, can be expressed as hA ∼ hQ ltl ∼ hQ l(L /Vl )

(1)

Equation 1 implies that hA or η can be enhanced by changing h, Ql, L, Vl, or some combination of them. However, increasing Ql increases energy cost, increasing L increases construction cost, and decreasing Vl requires use of additional friction elements such as the packing material and thereby increases construction and maintenance costs. Therefore, the best strategy to increase hA is to increase h, but research with this goal is scarce. Use of a vortex flow in the gas stream to increase h has not yielded promising results.8−10 If liquid drops move in a swirling gas flow, the swirl can increase the mass transfer in the gas phase and also the internal circulation in liquid drops. When alkaline solutions are used to capture CO2, mass transfer on the gas side is sufficient,11 and mass transfer on the liquid side is the limiting factor. However, the effect of vortex motion in the gas flow has been studied in association with mass transfer in the gas side, and the effect on mass transfer through the increased internal circulation has not been studied. When the speed of mass transfer in the gas side is fast enough, h can be increased only by increasing mass transfer on the liquid side. Surface-absorbed CO2 molecules must diffuse or disperse into the liquid to meet sorbent molecules; this process takes time and increases as drop size or film thickness increases, and the required absorbent flow increases with increase in the number of interior absorbent molecules that exit the reactor unused. Recent numerical simulations verified that the internal circulating flow inside a sorbent liquid drop increased the internal dispersion of surface-absorbed CO2, and that the change of overall h for CO2 capture during the absorption process was almost universal when plotted versus the degree of CO2 saturation.12 Liquid-side dispersion is increased by this internal circulation, but no other practical method than using a swirling gas flow exists that can be used to control or strengthen the internal circulation and the resulting h for a single liquid drop. Because h in eq 1 represents the overall coefficient averaged over all drops or films in the reactor, average h can be increased even if h for each drop or film is not changed at all: when h is dependent on drop size or film thickness, average h changes with the size distribution. This is simple reasoning with a solid theoretical basis, but the approach has never been practically tried before. This work is the first attempt to increase average h substantially by adjusting the drop-size distribution. Theoretically, the spray tower has advantages that allow it to achieve higher η values than the packed tower: in a spray tower, the total surface area between gas mixture and absorbent liquid can be greatly increased, and CO2 dispersion or diffusion in the absorbent liquid phase can proceed in three principal directions, whereas in a packed tower, the liquid film is rather thick, and CO2 diffusion occurs in just one direction. However, in practice, η vs absorbent flow rate per CO2 flow Ql/Qg is usually better for the packed tower than for the spray tower, at both lab scale and pilot scale: typical capture efficiency of labscale systems is 95% for the packed tower and 85% for the 3066

DOI: 10.1021/acs.iecr.7b05309 Ind. Eng. Chem. Res. 2018, 57, 3065−3075

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Figure 1. Schematic of the experimental setup.

the bottom through a perforated distributor plate to generate a uniform gas velocity profile, and aqueous NH3 was injected from the top of the tower through the nozzle plate to form a countercurrent flow arrangement between the gas flow and liquid droplets. The nozzle plate was a thin sheet of plastic or metal with numerous microholes fabricated in a lattice pattern, and was connected to the absorbent liquid reservoir. Aqueous NH3 was supplied to the reservoir by a mass flow controller, and the flow rate of absorbent liquid and the size of sprayed droplets were controlled by adjusting the pressure backing the liquid reservoir. The nozzle plate covered most of the flow cross section of the tower, and microholes in the nozzle were arranged in a pattern so that the sprayed droplets were nearly monodisperse in size and the spatial distribution of the droplets was uniform in a particular pattern throughout the reactor space. The concentration of CO2 ([CO2]) in the gas mixture was controlled using two mass flow controllers, one for N2 and the other for CO2, and the gas mixture was fed through a mixing chamber. The gas was introduced into the tower through a distribution panel installed at the bottom to ensure a uniform gas velocity (Figure 1). A demister panel was installed on the ceiling of the tower to capture small droplets from the exiting gas flow. An infrared CO2 analyzer (IR600, Hitech Instruments) monitored [CO2] in the gas mixture at both the gas inlet and the gas outlet; the measurement error was ±2%. Experiments were repeated three times for each set of operating conditions to quantify the repeatability of the results. Dimensions of the spray tower and operating conditions (Table 1) are in the ranges reported in other reports.15−19 To facilitate a direct comparison of η with other reports, [NH3] was fixed at 8 wt % and [CO2] at 15 vol %, similar to the conditions used in other reports.15−19 The temperature of the system was fixed at 25 °C. Gas flow rate of 36 L/min corresponds to a velocity of 0.057 m/s, and the gas flow rates were equivalent to a Reynolds number 180 ≤ Re ≤ 490. Although NH3 solution was used in this study, the effect of the new design on η relative to conventional spray towers is expected to remain similar, at least qualitatively, irrespective of the type of absorbent solution. η was calculated using [CO2] measured at the inlet (xg,i) and the outlet (xg,o) of the gas flow as

be lost to the reactor wall due to their high inertia, and spray droplets are distributed nonuniformly across the reactor cross section. All these factors contribute to the deterioration of η, and the size polydispersity is the most dominant factor. The aim of this work was to develop a novel highly efficient spray tower reactor with much higher CO2 η than typical spray towers at the same operating conditions. The distribution of gas flow, size of the sorbent droplets, and spatial distribution of spray droplets were made nearly uniform using a multinozzle plate with numerous micronozzle holes arranged in an optimal pattern. Wall loss was nearly eliminated by injecting sorbent droplets vertically downward from the nozzle plate, and because of the uniform droplet size, fly back of small droplets did not occur: droplets small enough to be carried out by the gas flow were not generated. The idea of uniform spray tower was first studied by Hixson and Scott for physical absorption of ammonia with only a small number of big water drops.14 In this study the technique was far improved with finer droplets in huge number of sprays, and that in the presence of slow chemical reaction. CO2 capture was improved greatly, and a very high η close to 95% was obtained, which is the highest reported for CO2 by a small spray tower. This is the first attempt to improve the overall mass transfer coefficient itself in wet chemical absorption, which is the basic parameter for the capture efficiency, and that does not impose a cost burden. The spray tower developed in this study is a novel structure that has not been studied before; its efficiency is much better than any of the existing spray towers and is competitive with that of a packed tower. The configuration of the new tower is appropriate for easy scale-up with further efficiency increase expected due to the increase in residence time as reactor scale increases. Compared to a packed tower, substantial reduction in the construction and maintenance costs is guaranteed, because packing materials are not used.

2. METHODS FOR EXPERIMENTATION AND ANALYSIS 2.1. Experimental Setup and Conditions. The system (Figure 1) used in the experiments was a simple cylindrical tube with a multinozzle plate installed at the top. The tower was made of transparent polycarbonate to allow visual observation of the spray. A mixture of N2 and CO2 gases was supplied from 3067


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2.2. Characteristics of the Droplet Spray Used in This Study. η of a spray tower is sensitively dependent on the sizes of the spray droplets. Decrease in droplet size is correlated with decrease in the time required for internal diffusion of the surface-absorbed gas molecules, and with increase in the residence time of the droplet because its falling velocity decreases. Therefore, a decrease in droplet size can increase the capture rate, but also increase the risk that the droplet will fly backward in high-velocity gas flow. In small lab-scale spray towers, gas velocity is a few centimeters per second, and typical drop size (Sauter mean diameter, SMD) is 30−50 μm15,17 or ∼100 μm.20−22 In a spray generated by a common spray nozzle with SMD = 50 μm and σ = 2.0, very small droplets of d < SMD/2 (=25 μm) contain 0.013% of the mass, and very big drops of d > 2 SMD (=100 μm) contain 87% of the mass. The high mass fraction contained in big drops makes the average capture much lower than that estimated using average drop size (SMD). The wide variation of drop size causes multiple factors that reduce the overall η: these include nonuniform spatial distribution of drops, loss of big drops to the reactor wall, and loss of small

Table 1. Characteristic Conditions of the Experimental Systems category geometry

operating conditions

η [%] =

parameter tower diam (m) tower height (m) gas flow rate (L/min) gas concn (vol %) liquid flow rate (L/min) liquid concn (wt %) temp (°C)

this study

Lim19

0.115

0.1

0.1, 0.2, 0.3

1.0, 1.5

0.5

1.0

13−36

10−28

25−150

15

5−15

6.6−37.5

0.2−0.3

0.13−0.26

0.2−0.65

8

2−26

5−20

25

20−55

15

Q g,ixg,i − Q g,oxg,o Q g,ixg,i

Qing15

× 100 (2)

where Qg,i is the gas flow rate at the inlet and Qg,o is the gas flow rate at the outlet.

Figure 2. (a) Photographs of falling 300-μm droplets generated from a single nozzle and (b) nondimensionalized variation of drop size and geometric standard deviation with liquid flow rate. Ruler scales of 1 mm are seen at the left end of (a). 3068


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Industrial & Engineering Chemistry Research droplets into the exiting gas flow. Thus, in this study, the spray tower and nozzle perforation pattern were designed to generate absorbent droplets with almost uniform size, and to spray them vertically in a uniform spatial distribution. This configuration minimizes liquid loss due to fly back and wall loss, so the average η becomes close to that for the droplet of the mean size. To generate droplets of controlled uniform size at regular periods, a special nozzle plate was developed; the front plates was perforated by a large number of contoured nozzle holes, and liquid flow was controlled by the back pressure. The nozzle holes generated droplets of almost uniform size, and the drop size was nearly insensitive to variation of pressure or liquid flow rate. Drop size was measured in two ways: (i) falling drops were pictured with a high-speed camera and the size was measured from the photographed images; (ii) drops were collected on a glass plate and the volume was measured from the images. Drop size parameters, mean and geometric standard deviation, measured with two different methods, were in good agreement. To facilitate future application to large-scale (high-gas-flow) systems, rather big droplets of 300-μm diameter were used in this study. The terminal velocity of a 300-μm droplet is ∼1.1 m/s,23 so 300-μm droplets can be used in towers with gas velocity of 0.2−0.5 m/s with drop-to-gas velocity ratio of 2−5. The nozzle plate generated droplets with SMD ∼ 310 μm and narrow σ ∼ 1.2, and both the drop size and size distribution were almost independent of the liquid flow rate (Figure 2). Insensitivity of the size and size distribution of the generated droplets from a nozzle plate is very advantageous in that the liquid flow rate can be varied without affecting changes in capture characteristics, as occurs in conventional spray nozzles. Figure 3 shows the sprays and droplets ejected from a nozzle plate: droplets were nearly uniform in size with almost the same size deviation of σ ∼ 1.2 as that from a single nozzle. Droplets were ejected vertically downward, and the loss of droplets to the tube wall due to conical ejection with typical spray nozzles was almost negligible. This is another strong point of the nozzle plate, in that the spray tower can be made longer without worrying much about η deterioration through wall loss, which is a common feature of conventional spray towers. To check the effect of the residence time or total surface area, tower lengths of 1.0 and 1.5 m were tested. In both cases the nozzle plate generated droplets of the same size at the same velocity. 2.3. Numerical Simulation for a Model Spray Tower. In order to check the validity of the basic concept of the novel spray tower and also to compare the experimental results with theoretical predictions, the capture performance of a model spray tower was numerically simulated. The system for analysis was a vertical channel of constant cross section with a uniform gas flow. Gas mixture containing CO2 was introduced at a uniform velocity from the bottom, and droplets were injected downward uniformly from the top. To facilitate the analysis, some simplifying assumptions were introduced. Gas velocity was uniform throughout the reactor space, and droplets fell at constant terminal speeds relative to the gas flow: terminal velocity was calculated from the equation given by Beard, which is valid for droplets with internal circulating flow and is valid for 0.03 ≤ Re ≤ 280.24 Polydisperse drop-size distribution was represented by the log-normal distribution which is widely used for spray droplets.23 Drop size was assumed unchanged by absorption, and the drop-size

Figure 3. Photographs of the spray from a nozzle plate generating 300μm droplets uniformly across the flow cross section: (top) whole view; (bottom left) showing a few sprays falling straight down; (bottom right) magnified view with separate droplets identified.

distribution was uniform all over the reactor space. Droplet loss to the wall was neglected. Aqueous ammonia was used as the absorbent, and the gas phase was a mixture of N2 and CO2. The gas concentration was uniform across any cross section of the reactor, and the average CO2 concentration on the liquid-side interface was assumed to be proportional to the free stream gas concentration Cg,s ∼ HCg; this assumption is valid because CO2 diffusion in the gas phase is much faster than the chemical absorption inside the droplet.25 Physical and chemical properties were constant, H was expressed as a function of temperature,26 and the reaction rate constant k was borrowed from refs 26 and 27. Heating by mass absorption was neglected. Numerical analysis for a counterflow reactor in steady state requires a formula for the mass transfer coefficient, expressed only in terms of local (Eulerian) conditions of flow and concentration, not time. A proper empirical formula has been developed for CO2 mass flux into an aqueous ammonia droplet:11 3069


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Figure 4. Degree of saturation ϕ for droplets of various sizes in a lab-scale spray tower with dCMD = 180 μm and σ = 2.0: (top) change with position along the reactor and (bottom) at the exit.

mg″ = k

⎡ 2.15 × 10−6 Cg,sC l ⎢ ⎢ C l,i 0.5 ⎣

capture by a d(i) droplet multiplied by the number of d(i) droplets, n(i).

0.48

Cl ⎞ ⎥ 1.40 × 10 ⎛ ⎜ ⎟⎟ 1 + − ⎜ C l,i ⎠ ⎥⎦ d 0.3 ⎝ −7

dmg (i , j) = mg″(i , j) A(i)

2.5⎤

(4)

imax

dmg (j) =

(3)

∑ dmg (i , j) n(i) i=1

(5)

Then the gas flow rate, gas concentration, and liquid concentration were changed reflecting the amount of gas absorbed. Mass balance equations for each cell yielded a set of simultaneous equations for the CO2 concentration in the gas mixture and the droplets and equations for the sorbent concentration in droplets of different sizes; the equations were solved iteratively using MATLAB. The size distribution of the droplets generated from a spray nozzle was assumed log-normal with geometric mean or count median diameter dCMD and geometric standard deviation σ.23 If size distribution is represented by a finite number of discrete sizes (d(i); i = 1 ∼ imax) and size intervals (Δd(i); i = 1 ∼ imax),

It quantifies the absorption mass flux into an absorbent droplet as a function of local and instantaneous gas concentration (Cg) and absorbent concentration (Cl) together with droplet size (d) and chemical reaction speed (k). Cl,i is the initial absorbent concentration entering the absorber. The reactor space was divided into a sufficient number of unit cells along the reactor length, and droplets of different sizes were treated separately. For each cell j, the amount of CO2 absorbed by a single droplet of size d(i) and surface area A(i) = πd(i)2, dmg(i,j), was calculated as eq 4 using eq 3, and total capture in cell j, dmg(j), was obtained by summing up the 3070


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3.2. Fundamental Features of the Experimental ηCurve of the New Spray Tower. First, the gas flow was varied with the liquid flow fixed in the 1-m tower, and η was plotted versus the molar flow rate ratio (Nl/Ng) of NH3 to CO2 in the range 5−20. Measured η values were very consistent over repeated measurements, with maximum variance of ±2% for most cases, and increased smoothly with the liquid-to-gas flow rate ratio over the whole flow range; it was also well-fitted by an exponential curve (Figure 5). The efficiency of heat/mass

size frequency or probability f(i) can be approximated as eq 6, and the number n(i) of d(i) droplets generated per second can be calculated from the total liquid flow rate Ql as eq 7, where q(i) denotes the volume of a droplet of size d(i). f (i ) =

2⎞ ⎛ [ln(d(i)/d 1 CMD)] ⎟ exp⎜ − 2π d(i) ln σ 2[ln σ ]2 ⎝ ⎠

n(i) = ntotf (i) Δd(i) =

(6)

Q [f (i) Δd(i)]

l imax ∑i = 1 [q(i)

f (i) Δd(i)]

(7)

Even when polydisperse droplets of a known size distribution at generation are evenly supplied across the reactor cross section, the size distribution and droplet number density in the reactor space are different from those at generation, because droplets fall at velocities that are related to their sizes. Therefore, the number of d(i) droplets in the reactor space is determined by modifying the generation rate by a factor that is inversely proportional to the falling velocity Vf(i) = VT(i) − Vg, in such a way that the generation rate from the nozzle should be equal to the number density multiplied by the falling velocity for each droplet size. At the start of simulation, reactor height L, gas velocity Vg, liquid flow rate Ql, and droplet size distribution were specified. Once the sorbent flow rate and droplet size distribution are given, the gas velocity and number density of droplets at each spatial grid point can be determined in advance from eqs 6 and 7, and small droplets for which the terminal velocity is lower than the gas velocity were removed from the analysis domain.

3. RESULTS AND DISCUSSION 3.1. Possibility of Enhancing Capture Capacity with Controlled Droplet Size Distribution. In capturing CO2 with liquid sorbent, the most important factor for the capture performance is the droplet size because all parameters associated with capture performance are closely related to the droplet diameter: the time required for full droplet saturation is roughly proportional to td ∼ d2/D, the falling velocity is to Vf ∼ VT ∼ d1 − d2, and the effective diffusion coefficient in the presence of internal circulating flow varies as D ∼ d1.5.11 Also, the gas velocity is limited to the terminal falling velocity of the droplets, VT > Vg. The next important factor is the sorbent flow rate, usually represented by the ratio Ql/Qg to the gas flow rate. When spray droplets are polydisperse in size, droplets of different sizes become partially saturated to different degrees after passing through the same distance. The degree of saturation of droplets at the exit of a reactor is determined by the ratio tr/td of falling or residence time to diffusion time; this ratio decreases as droplet size increases, roughly as d−3.5 ≤ tr/td ≤ d−2.5 (d−2 ≤ (tr ∼ VT−1) ≤ d −1; td ∼ d1.5). This relationship implies that by the time a certain droplet is almost saturated, another droplet with twice the diameter is saturated only to ∼20% or requires a tower of almost 5 times as long to attain the same degree of saturation. Since big drops contain more mass ∼ d3 but much lower saturation (or capture per mass), the presence of big drops reduces the average degree of saturation, resulting in a lower η with a given sorbent flow or a higher sorbent flow required to achieve a given η. A sample case of dCMD = 180 μm and σ = 2.0 was analyzed, and the results shown in Figure 4 are in agreement with the reasoning given above.

Figure 5. Capture efficiency of the 1-m tower at various liquid-to-gas flow rate molar ratios together with fitted exponential curves: (top) gas flow varied with liquid flow fixed and (bottom) either liquid flow or gas flow varied with the other flow fixed.

transfer in a counterflow exchanger can be expressed as a function of total mass transfer coefficient and mass flux ratio, which approaches an exponential function when the mass ratio is very big or small as34 η = 1 − exp[−ahA(Q l /Q g)] = 1 − exp[−a′hA(Nl /Ng)] mg /ml ≪ 1

(8)

where a and a′ are proportionality constants that depend on the material properties and inlet concentrations of the gas and liquid, h is the effective or average mass transfer coefficient, and A is the total surface area of the droplets for mass transfer. The mass flow rate ratio (mg/ml) in this study was about 0.1, at 3071


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Industrial & Engineering Chemistry Research which the η-curve is well approximated by an exponential function. Second, liquid flow was varied with the gas flow fixed. The curve of η versus the mole ratio was almost the same as when the gas flow was varied with the liquid flow fixed (Figure 5). This finding is consistent with the expectation based on the correlation for the mass transfer coefficient of Choi et al.:11 the mass transfer coefficient was proportional to the local gas concentration and local liquid concentration. When the change rate of any variable is proportional to itself, overall change varies as an exponential function of time or flow path. Therefore, the η-curve obtained with varying gas flow at the same liquid flow is equivalent to the η-curve obtained with varying liquid flow at the same gas flow, once the η-curve is plotted versus the ratio Nl/Ng of the mole flow rate of absorbent to CO2. The η-curves obtained using towers of lengths 1.0 and 1.5 m are shown, and also compared with numerical predictions in Figure 6. First of all, it has to be noted that the 1.5-m tower

terminal velocity, and passed through the tower at a higher velocity than the terminal velocity. Therefore, the number of droplets or total surface area for gas absorption was smaller than can be estimated based on the terminal velocity condition. 3.3. Comparison with Other Spray Towers. Experimental results with the 1-m tower of this study were compared with results from two lab-scale spray towers of refs 15 and 19 (Figure 7). At the lowest flow-rate ratio of 5, the η values of the

Figure 7. Efficiency curves of new uniform spray tower (symbols) and conventional spray towers (solid curves).

conventional spray towers were almost the same as that of the novel spray tower, but thereafter increased much more slowly than the novel spray tower with increased absorbent flow, reaching only η ∼ 80% at Ql/Qg = 18, which was much lower than η ∼ 92% of the novel spray tower. The η-curves for conventional spray towers could not be fitted to a smooth exponential function over the whole range. Usually, the curves were steep at low η or low Ql, and then flattened as η or Ql increased. In order to clarify the reason for the change in the curve shape, the experimental results of Lim et al.19 were compared with numerical results obtained with L = 1 m, dCMD = 100 μm, and three different values of σ = 2.0, 2.4, and 2.6 (Figure 8). Experimental results showed a reasonable agreement with the numerical results for σ = 2.4 at low absorbent flows of Ql/Qg < 10, and with those for σ = 2.6 at high absorbent flows of Ql/Qg > 18. It is thought that this trend

Figure 6. Comparison between experimental results and numerical simulations for two different tower lengths of 1.0 and 1.5 m: dCMD = 300 μm, σ = 1.2, and k = 10 m3/(mol·s).

showed a very high η = 94.7% at mole ratio 20; to the best of our knowledge this is the highest η reported for a spray tower with CO2 and NH3. These experiments prove that a spray tower that emits a uniform spray of monodisperse droplets behaves very well in a developed quasi-steady manner, and that its η is far better than those of existing spray towers. The capture efficiency of 94.7% at Nl/Ng = 20 is the best CO2capture efficiency that has ever been reported with a spray tower that uses NH3. Furthermore, as is noticed in Figure 6, the experimental results are in excellent agreement with numerical results obtained with dCMD = 300 μm and σ = 1.2 for both L = 1.0 and 1.5 m, and the good agreement can be attributed to the uniform spatial distribution of droplets and uniform drop size. The best-fit value for the reaction rate constant was k = 10 m3/ (mol·s), which is in the low range of the values reported in the literature.27−33 This is thought to result from the unsteady effect due to the use of big drops in a short tower, as is described below. The inertial characteristic time τ required for velocity equilibration between a 300-μm drop and air flow is ∼0.25 s,23 but the residence times of droplets in the 1.0- and 1.5-m systems used in this study were 0.46 and 0.86 s, respectively. Velocity equilibration to 99% requires ∼3τ, so droplets ejected from the nozzle plate at a high velocity did not reach their

Figure 8. Comparison between experimental results (solid curve) and numerical simulations (dotted curves) for a conventional spray tower for L = 1 m, d = 100 μm, and varied σ = 2.0, 2.4, and 2.6. Experimental results of this study are also shown as solid symbols. 3072


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However, for the new spray tower with the nozzle plate, the tower length can be increased without much increase in wall loss, and the basic nature of capture performance is expected to remain almost the same. An increased L should result in a reduced liquid flow required for any particular η. In large-scale towers gas flow becomes turbulent, and turbulence may accelerate drop coagulation, resulting in performance degradation: a separate study is needed to complete practical design technology. Furthermore, the same characteristics of the new spray tower can be used in a regenerator with almost a similar increase in stripping efficiency: as droplet size increases, the required regeneration temperature increases, so it can be reduced appreciably when a monodisperse spray is used; as a result, the heating (energy) cost or the reactor size of regeneration can be reduced. Although this conclusion applies to the simplest condition of uniform gas velocity and uniform spatial distribution of droplets, the general trend may also be applicable to other conditions of gas or sorbent distribution and to other types of sorbents.

occurs because with conventional spray nozzles the drop-size distribution widens as pressure increases, so the mass fraction of big droplets increases, and the incremental gain in η decreases, compared to the gain that would occur when the drop-size distribution does not change. In conventional spray towers, spatial distribution of droplets is not uniform, and the spatial nonuniformity makes an additional contribution to the degradation of capture performance. Also the increased dropsize variation at high liquid injection can increase the loss of small droplets, the efficient absorbers, due to coagulation or scrubbing by big drops. In that sense, it is safe to accept that the effect of spatial nonuniformity is reflected or contained in the adverse effect represented by σ, the nonuniformity of drop size. This behavior is common to all spray towers that use conical sprays.13 For this reason, the newly designed tower that produces a uniform spray showed a much better η than the conventional spray towers. The total mass transfer coefficient estimated from eq 3 at high Ql was almost double that of the conventional spray tower. Considering that the size of the droplets used in this study was about 300 μm and the SMD of the droplets used in the conventional spray tower of Lim19 was 100 μm,13 the higher efficiency of the uniform spray tower than of the conventional spray tower is even more surprising. The η increase can be attributed to the uniformity of the size and spatial distribution of the drops, and also to reduction in wall loss and fly back. η increases as the length of the flow path increases, but a small conventional spray tower cannot be made longer than a certain limit because wall loss and coagulation become too severe. Conventional spray towers cannot be used for large-scale absorption, because gas velocity is limited due to the very high fly back of small droplets, the efficient absorbers. In contrast, the length of the new spray tower can be increased without much difficulty, because wall loss is very small due to the vertical injection pattern. Also gas velocity can be increased without much concern, because falling velocity of the absorbing droplets can be optimized easily by choosing a proper drop size, and fly back can be made negligible due to the narrow size distribution of the generated droplets. 3.4. Further Improvement and Practical Implementation. This work was just a test of the feasibility of the novel spray tower concept using uniform spray of monodisperse droplets for capturing CO2 with high enough efficiency. To facilitate stable generation of monodisperse droplets in a passive mode and also future application to large-scale systems, rather large drops (300 μm) were used in short reactors (1.0, 1.5 m). Even in the 1.5-m tower, the residence time of the droplets was only 0.86 s, which was much shorter than the 3−4 s for the droplets in typical spray towers. Despite that short contact time, η attained the record high value of ∼95%. To further increase η or to maintain the same η at smaller liquid flow, total surface area or contact time must be increased, but a large-scale system requires a high gas velocity at the same time. In conventional spray towers, increased gas velocity may blow small droplets backward and thereby lose them (fly back); because small droplets contribute to the most efficient gas absorption, this loss can reduce η. Even in a packed bed, increased gas velocity thickens the liquid film and thereby decreases mass transfer. The simplest way of increasing the surface area or contact time is to use a long tower. For conventional spray towers, coagulation among droplets of different size is very severe, and capture performance gets degraded very rapidly with tower length, which limits the use of spray towers to short lengths.

4. CONCLUSION The paper describes a new type of spray tower that eliminates or reduces the shortcomings of conventional spray towers. The tower generates spray droplets of nearly uniform size by using a nozzle plate with many nozzle holes. These droplets are injected vertically to form a nearly uniform spray across the tower cross section. Because large droplets (∼300 μm) are injected vertically, droplet loss by adhesion to the wall and by fly back was almost entirely eliminated. The CO2 η was much higher than that of the typical spray tower, and the effective total mass transfer coefficient almost twice as high. η reached ∼95%, which is the best-ever η reported to date for CO2 capture with NH3 solution in a spray tower. Experimental results were in excellent agreement with numerical predictions. The basic conditions of this study are appropriate for application to larger systems than used here; increase in tower height can be expected to increase η further.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +82-54-279-2170. Fax: +82-54-279-3199. ORCID

Minki Cho: 0000-0001-6812-9896 Jin W. Lee: 0000-0002-3533-6757 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Korea Research Foundation (KRF) grants funded by the Korean government (MOE) (No. 2013R-1A1A-2057752).

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NOMENCLATURE a = proportionality constant used in eq A = surface area (m2) C = mole concentration (mol/m3) d = droplet diameter (μm) D = diameter of tower (m) f = probability density or frequency h = mass transfer coefficient (kg/m2/s)

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H = Henry constant k = reaction rate constant (m3/(mol·s)) L = length or height of tower (m) mg″̋ = absorption mass flux into a droplet (kg/(m2·s)) N = mole flow rate (mol/s) n = number of droplets P = pressure (Pa) q = droplet volume (m3) Q = volume flow rate (m3/s) Re = Reynolds number based on drop diameter and terminal velocity SMD = Sauter mean diameter (μm) t = time (s) T = temperature (K) V = velocity (m/s) x = volume fraction (m3/m3) y = mass fraction (kg/kg) Greek Symbols

η = capture efficiency (%) ϕ = degree of saturation σ = geometric standard deviation τ = characteristic time (s) Subscripts

c = cross section d = diffusion CMD = count median diameter f = falling g = CO2 gas i = inlet condition, initial l = liquid, sorbent max = maximum o = outlet condition ref = reference s = surface T = terminal tot = total

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Novel Spray Tower for CO2 Capture Using Uniform Spray of

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