Ind. Eng. Chem. Res. 2008, 47, 145-153
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SEPARATIONS Performance of Spray Column for CO2 Capture Application Jeffery Kuntz and Adisorn Aroonwilas* Faculty of Engineering, UniVersity of Regina, Regina, Saskatchewan, Canada S4S 0A2
The mass-transfer performance of a spray column was tested experimentally for the removal of carbon dioxide (CO2) from a gas stream by using an aqueous solution of monoethanolamine (MEA). The experiments were performed in absorption columns over ranges of process conditions. The performance of the spray was evaluated and presented in terms of the overall mass-transfer coefficient (KGae). It was found that the performance of the spray column varies significantly with the tested process parameters, including gas flow rate, liquid flow rate, CO2 partial pressure, MEA concentration, CO2 loading, and size of spray nozzle. The results were further analyzed for the fundamental information on effective area (ae) and gas-phase mass-transfer coefficient (kG). The performance of the spray column was also compared to that of a packed column. The comparison demonstrates a great potential of using the spray column in the CO2 capture application. 1. Introduction Capture and storage of carbon dioxide (CO2) produced by fossil fuel combustion for power generation is one of the important strategies for reducing or sustaining atmospheric levels of greenhouse gas (GHG) emissions. Today, there are a variety of technologies capable of capturing CO2 from industrial flue gas streams, including gas absorption, cryogenic separation, membrane separation, and adsorption. Gas absorption into an aqueous alkanolamine solution is the most well-established of the technologies available for CO2 capture.1 This technology has been proven for more than a half century to work successfully in the oil and gas as well as chemical industries. However, it is widely recognized that the cost of CO2 capture by gas absorption is still prohibitively high for the environmental application. The reduction of CO2 capture cost can be attained through development of cost-effective and energy-efficient absorption processes. This could be achieved by improving absorption performance of alkanolamines and also by design of gas-liquid contactors used in the process. The commonly used alkanolamines for CO2 absorption are monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), methyldiethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP). An appropriate selection of these alkanolamines and their mixing formulations to be used in the process could result in a reduction in the size of process equipment and/or a reduced energy consumption for the process, providing a reduced cost of CO2 capture. In the same token, use of proper gas-liquid contactors could result in a significant reduction in capital and maintenance costs of the process. The improvement of CO2 absorption performance by alternative contactor design is the primary focus of this work. For a number of years, the capture of CO2 from gas streams has been well-documented and many techniques have been studied. The absorption of CO2 using packed and tray columns * To whom correspondence should be addressed. Tel.: (306) 3372469. Fax: (306) 585-4855. E-mail:
[email protected].
has produced a large amount of published data with many types of solvents and column internals being tested.2-9 On the contrary, the spray column has been studied by few, and to date, there has been little data published in the area of CO2 capture.10 The main use of the spray column has been for the removal of sulfur dioxide (SO2) from flue gas streams in a flue gas desulfurization (FGD) unit. Specific areas of study are the design of the column, the orientation of the nozzles, and the solvents used.11-13 There have been a few attempts at CO2 removal utilizing a spray column, and the focus was on the absorption using sodium hydroxide (NaOH).14-16 The specific case of the CO2 absorption in a spray column using an alkanoalmine such as MEA to date has no data published. This provides no masstransfer information for comparison of solvents as well as various types of nozzles and their performance. Therefore, there is a need for further study in this area to examine the feasibility of spray column. This work reports the mass-transfer performance of a spray column for CO2 capture using aqueous solutions of alkanolamine MEA. The performance of the spray column was evaluated experimentally under various conditions to reveal effects of process parameters, including CO2 partial pressure in the gas phase, gas flow rate, liquid flow rate, concentration of MEA, CO2 loading of absorption solution, and size of spray nozzle. A set of absorption experiments using a packed column was also conducted to determine the performance of the conventional column for comparison purposes. In addition, fundamental information on effective mass-transfer area (ae) and gas-phase mass-transfer coefficient (kG) analyzed from the experimental results is also reported. 2. Absorption Experiments 2.1. Experimental Apparatus. The apparatus used in the absorption experiments is shown in Figure 1. The experiments were conducted in two separate columns, i.e., spray column and packed column. The spray column was constructed of acrylic plastic that was 0.55 m high and had an inside diameter (i.d.) of 0.10 m. The spray column was operated with one of three
10.1021/ie061702l CCC: $40.75 © 2008 American Chemical Society Published on Web 12/11/2007
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Ind. Eng. Chem. Res., Vol. 47, No. 1, 2008
Figure 1. Schematic diagram of CO2 absorption apparatus. Table 1. Specifications of Spray Nozzles and Packing Used in the Present Study P-20 spray nozzle orifice diameter (mm) spray angle (deg) liquid flow rate at 1 bar (L/min) liquid flow rate at 5 bar (L/min)
0.508 90 0.153 0.341
P-28 spray nozzle orifice diameter (mm) spray angle (deg) liquid flow rate at 1 bar (L/min) liquid flow rate at 5 bar (L/min)
0.711 90 0.296 0.662
P-40 spray nozzle orifice diameter (mm) spray angle (deg) liquid flow rate at 1 bar (L/min) liquid flow rate at 5 bar (L/min)
1.02 90 0.638 0.1.43
Mellapak 500Y packing element height (m) specific area (m2/m3) void fraction corrugation angle (deg)
0.20 500 0.91 45
316 stainless steel nozzles manufactured by BETE Industrial Spray Nozzles (models P-20, P-28, and P-40). The specifications of the nozzles used can be found in Table 1. The nozzle position within the column can be adjusted vertically to prevent the spray from hitting the column wall, thus eliminating the wall effect. The nozzles were operated through a range of liquid flow rates and pressures to assess the capacity and effectiveness of each nozzle utilized. Similar to the spray column, the packed column was also constructed of acrylic plastic with an i.d. of 0.10 m but had a height of 0.80 m. The packed column was fitted with Mellapak 500Y structured packing provided by Sulzer Brothers Limited, Winterhur, Switzerland. The Mellapak 500Y was chosen because of its great mass-transfer performance for the absorption of CO2.17 There were two sections of the packing used that provided a height of 0.40 m. The packing was installed with each layer rotated by 90° with respect to the previous one.18 Table 1 also provides the specifications of the Mellapak 500Y packing. The CO2 concentrations of gas streams entering and leaving the columns were needed for evaluating the overall mass-transfer coefficient (KGae). For this reason, there was a
sampling point before the gas entered the column and at the point where the gas exited the column. The CO2 concentrations were measured using an infrared gas analyzer (model 302WP, Nova Analytical Systems Inc.). The reading range of the analyzer was 0.0-20.0% of CO2 by volume with the accuracy of (2% of the full-scale reading. Auxiliary equipment such as a liquid pump, liquid tanks, and gas flow meters were also used in this work. A stainless steel gear pump purchased from Cole-Parmer was used to deliver the liquid absorption solution to the top of the spray and packed columns. The pump was driven by “NEMA 4X” digital dispensing drive that kept speed control to (0.3%. Two liquid tanks (20 L capacity) made of high-density polyethylene served as reservoirs for supplying and receiving the absorption solution. Three calibrated gas flow meters were used to regulate the flow of the air and CO2. They were correlated variable-area meters, which were calibrated using a Humonics-Optiflow soap flow meter. The maximum flow rates were 32 L/min and 220 L/min for air and 20 L/min for CO2. 2.2. Experimental Procedures. The experiments began by preparing the feed liquid solution at the desired MEA concentration and CO2 loading for each experimental run. Air from a central supply line and CO2 from a cylinder were introduced and regulated by gas flow meters to the desired flow rates. The two gas streams were then mixed to the desired gas-phase CO2 concentration and introduced into the bottom of the column through a single gas line. The gas mixture once in the column passed through a dispersion outlet to disperse uniformly across the column. At the same time, the prepared liquid solution from the feed reservoir was pumped to the top of the column, where it entered the column through either a spray nozzle (for the spray column) or the liquid distributor (for the packed column). This brought the gas and liquid into contact countercurrently, and the CO2 in the gas phase was absorbed. The gas then carried out through the top of the column while the CO2 rich solution was collected out of the bottom of the column in the liquid receiver. To obtain reliable experimental results, the absorption process was operated until steady-state conditions were reached. The steady-state condition was indicated when the CO2 concentration of the exit gas held constant. The time for the spray column to reach steady state was ∼7-10 min, while the time for the packed column was ∼15 min. It was at these times that the CO2 concentration of the exit gas was measured and a liquid sample was collected out of the bottom of the column. The liquid sample was analyzed for CO2 content (or CO2 loading) and used to verify the CO2 absorption rates that were calculated from the gas-phase data. The total concentration of MEA was determined by titration with standard 1.0 N hydrochloric acid (HCl) solutions using methyl orange as the indicator. Then, the determination of CO2 content in the liquid was determined by acidifying a precisely measured quantity of the sample by adding excess HCl solution. The CO2 gas was released, and it was captured by a precision gas buret ((1.0 mL). The amount of released CO2 was then used to calculate the CO2 loading of the liquid solution. 3. Results and Discussion More than 400 absorption runs were conducted for both spray and packed columns over ranges of operating and design conditions as listed in Table 2. The experimental results have been presented in terms of the volumetric overall mass-transfer coefficient (KGae) and are reported as functions of the process
Ind. Eng. Chem. Res., Vol. 47, No. 1, 2008 147 Table 2. Test Conditions of Spray and Packed Columns parameter
Table 3. Variation in Mass Flux of CO2 Across Gas-Liquid Interface at Different Conditionsa
conditions
absorption solvent gas phase gas flow rate feed CO2 partial pressure liquid phase liquid flow rate MEA concentration CO2 loading temperature
monoethanolamine (MEA)
conditions
up to 764 m3/m2‚h 5, 10, 15 kPa up to 10.3 m3/m2‚h 3, 5, 7 kmol/m3 0, 0.15, 0.25, 0.35, 0.45 mol/mol 25 °C
effect of CO2 partial pressure (PCO2) PCO2 ) 15 kPa (reference) PCO2 ) 5 kPa effect of gas flow rate (G) G ) 15 m3/m2‚h (reference) G ) 76 m3/m2‚h G ) 152 m3/m2‚h G ) 382 m3/m2‚h G ) 573 m3/m2‚h G ) 764 m3/m2‚h effect of CO2 loading (RCO2) RCO2 ) 0.00 mol/mol (reference) RCO2 ) 0.15 mol/mol RCO2 ) 0.25 mol/mol RCO2 ) 0.35 mol/mol RCO2 ) 0.45 mol/mol
relative mass flux (NCO2/NCO2,reference) 1.00 1.13 1.00 1.18 1.37 1.63 1.72 1.75 1.00 0.85 0.73 0.57 0.34
a The mass flux values were calculated from a reaction-diffusion model using equations presented by Wellek et al.:19 DA ∂2CA/∂x2 ) ∂CA/∂t + k2CACB and DB ∂2CB/∂x2 ) ∂CB/∂t + zk2CACB.
Figure 2. Effect of CO2 partial pressure on KGae (nozzle ) P-20, liquid flow rate ) 1.53 m3/m2‚h, and gas flow rate ) 382-764 m3/m2‚h).
parameters that were tested. The KGae was determined by using the following equation,6
KGae )
(
GI
/ P(yCO2,G - yCO ) 2
)( ) dYCO2,G dZ
(1)
where GI is inert gas flow rate in kmol/m2‚h, P is total pressure / on the system in kPa, Z is column height in m, yCO2,G and yCO 2 are the mole fraction of CO2 in the gas stream and the equilibrium mole fraction of CO2, respectively, and YCO2,G is the mole ratio of CO2 in the gas stream. The followings are the highlights of the results. 3.1. Mass-Transfer Coefficient of Spray Column. 3.1.1. Effect of CO2 Partial Pressure. In the gas absorption application, the gas-phase CO2 concentration varies considerably as the gas stream travels through the absorption column. The variation in the CO2 concentration defines the change in CO2 partial pressure prevailing along the height of the column. It was found in this study that the CO2 partial pressure has a great impact on the overall mass-transfer performance of the spray column, i.e., the KGae decreased as the partial pressure increased from 5 to 15 kPa. The effect of CO2 partial pressure can be illustrated clearly in Figure 2, where the KGae is plotted directly against the partial pressure. The reduction in KGae value holds true at different CO2 loadings. By considering mass flux of CO2 absorption (NCO2), an increase in CO2 partial pressure leads to an increasing amount of CO2 transferred into the liquid phase as shown in Table 3 (simulation results of reaction-diffusion model). However, the increasing mass flux occurs in a lower extent compared to the change in partial pressure (because of a greater degree of reactive depletion at the gas-liquid interface), causing the mass-transfer coefficient representing the specific mass-transfer rate per unit driving force to reduce as partial pressure increases. This follows the overall mass-transfer equation (eq 1) that the KGae decreases as the mass-transfer
/ driving force P(yCO2,G - yCO ) in the gas phase increases. It can 2 also be observed from Figure 2 that the effect of partial pressure becomes less, especially at the CO2 loading of >0.25 mol/mol and the partial pressure of >10 kPa. The restricted diffusion and amount of reactive MEA in the liquid phase is speculated to be the cause of this behavior. Mass-transfer phenomena under the high CO2 loading condition is mainly controlled by the CO2 reaction in the liquid, thus resulting in only a small change in the amount of CO2 absorbed as the partial pressure increases. 3.1.2. Effect of Gas Flow Rate. The gas flow rate affects the overall mass-transfer coefficient, but only to a certain point. As the gas flow rate increases, so does the KGae coefficient. This behavior illustrates the gas-phase controlled mass transfer, taking place especially within the low range of the gas flow rate (
