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Regeneration of Carbon Dioxide Saturated Monoethanolamine-Glycol Aqueous Solutions at Atmospheric Pressure in a Packed Bubble Reactor Owen J. Curnow,*,† Susan P. Krumdieck,*,‡ and Elizabeth M. Jenkins‡,§ Departments of Chemistry and Mechanical Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand, and ASCO Carbon Dioxide Limited, 7 Canterbury Street, PO Box 16-134, Christchurch, New Zealand
A method has been investigated for regeneration of CO2-saturated monoethanolamine (MEA) aqueous solution at low temperature and atmospheric pressure, with a rapid regeneration rate and low evaporation solution loss. The solution of 50 wt % MEA, 35 wt % water, and 15 wt % glycol was saturated with CO2 through addition of dry ice, and the regeneration rate was measured as a function of temperature and flow rate of an inert gas through the solution in a packed bubble reactor. An infrared CO2 detector was used to measure the CO2 concentration in the flushing gas to determine total percent regeneration over a period of time at a given temperature. The bubbling flow through the reactor was found to greatly enhance the regeneration rate and total amount of regeneration. It is possible with the packed bubble reactor to regenerate MEA solutions to 40% CO2 at temperatures below 90 °C. 1. Introduction Amines such as monoethanolamine (MEA) have been used for many years in industrial processes to absorb CO2 from combustion gas streams and to “sweeten” natural gas.1 In these applications, the CO2 concentration in the gas stream can be in the range of 15-50%, and the standard packed column or trickle plate column with compressed gas is used. MEA has also been employed to maintain a functional atmosphere on board submarines, where the air CO2 partial pressure is far lower than in industrial applications.2,3 A comfortable working environment should not exceed 1.5% CO2, so it is not critical that the scrubber achieve complete removal of CO2 in the process. In industrial processes, very high temperatures (>100 °C) are used to regenerate MEA solutions in packedbed columns.1 The gas flow is usually high-pressure steam, which provides the heat of reaction and carries the CO2 out of the reactor. The regeneration process is usually done at a temperature in excess of the boiling temperature.1 The chemical kinetics of regeneration increases with temperature, but the CO2 is released into solution. Thus, for industrial trickle column technology, the regeneration rate can be limited by any of the physical processes involved, including the solubility of CO2 in the solution, the diffusion and/or convection transport through the liquid flow to the gas interface, desorption from the liquid-gas interface, and diffusion into the gas stream. Standard chemical engineering design methods are employed to tailor the column size, flow rates, and temperatures to the process require* To whom correspondence should be addressed. O.J.C.: tel, 64-3-364-2819; fax, 64-3-364-2110; e-mail, owen.curnow@ canterbury.ac.nz. S.P.K.: tel, 64-3-364-2987; fax, 64-3-3642063; e-mail,
[email protected]. † Department of Chemistry, University of Canterbury. ‡ Department of Mechanical Engineering, University of Canterbury. § ASCO Carbon Dioxide Limited.
ments: for example, maximum CO2 transport for dry ice production and maximum CO2 removal for natural gas sweetening. New applications of amine to scrub CO2 to below 10 ppm include the air side of alkaline fuel cells (AFC’s)4 and the reformed fuel side of both AFC’s and proton exchange membrane (PEM)5 fuel cells. In these new energy supply applications, the amine absorptiondesorption cycle represents a balance of plant subsystem; thus, parasitic energy usage is a critical issue. Any viable CO2 scrubber for fuel cells should have minimal energy and maintenance requirements. An additional consideration is the footprint, or space and complexity, added to the total system by the scrubber component. The present study of low-temperature regeneration is part of a larger program to develop a lowenergy, low-cost, low-maintenance, compact scrubber technology that can continuously remove CO2 to meet fuel cell specifications.6 This paper reports low-temperature regeneration of amine by dramatically increasing the active surface area and liquid turbulence of a hot amine solution through the use of a packed bubble reactor. In a solution with high amine concentration, and thus high CO2 solubility, the regeneration rate of a fully saturated amine solution is limited by the desorption rate of the evolved CO2 out of the solution at the liquid-gas surface interface, as shown in Figure 1.7-9 The effective surface area of a gas-liquid interface is increased by using a bubble column.10,11 However, the gas hold-up in a bubble column can be very limited, because bubbles coalesce together as they travel upward, and the liquid can become eruptive as the upward buoyancy force of a large bubble overcomes the weight of the fluid above. The gas flow that can be injected through a bubble column is even more limited in high-viscosity liquids.12 Our group has proposed the use of a packed bubble reactor as a means to achieving a highly turbulent bubble-gas mixture with high gas throughput to reduce liquid side diffusion and transport resistance and to reduce gas
10.1021/ie049428n CCC: $30.25 © 2005 American Chemical Society Published on Web 01/22/2005
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Figure 1. CO2 regeneration process from a liquid volume of saturated MEA solution, with a desorption limiting surface area As, to a volume of gas.
Figure 2. Schematic diagram of the packed bubble reactor and a photograph of the actual packing demonstrating how the packing maintains bubble size, roughly equal to half the packing radius, and generates mixed-phase turbulent flow conditions without causing excess liquid splash.
side diffusion resistance.6 Figure 2 shows a schematic diagram of the packed bubble reactor. Packing with a high surface area to volume ratio was used. Gas flow rates up to 10 times that of unpacked bubble columns are possible with this type of packing, which is usually used for mixing processes. 2. Experimental Section The experimental setup for regeneration is shown in Figure 3. The gas supply was preheated by passage through a copper coil immersed in a 70 °C oil bath. This was found to heat the gas to approximately 30 °C and was sufficient to prevent temperature drops in the reaction solution. Prior to immersion of the packed bubble reactor in the oil bath, the oil bath was heated to approximately 20 °C above the desired temperature. Immersion of the reactor in the oil bath would then cool the oil bath to the desired operating temperature, after which the oil bath was maintained at a constant temperature. For runs made without bubbling an inert gas through the solution, inert gas was passed over the solution. The reactor was packed with 5 mm stainless steel Pall rings. Outgasses from the reactor were cooled with a
Figure 3. Illustration of the apparatus for the regeneration of CO2-saturated MEA-glycol aqueous solutions. The inert flush gas (N2) is preheated in an oil bath and injected into the packed bubble reactor in a controlled-temperature oil bath (TB). Before it is passed through the IR CO2 detector, the gas stream passes through a cold water condenser and a salt ice bath.
water condenser to recover vapors and were then passed through a -20 °C salt/ice bath to remove any residual vapors before going to the detector. The volume of CO2 released was detected using a Draeger Polytron Transmitter (IR-CO2). CO2 volumes were calculated by measurement of the CO2 percentage in the outgas, taking into account gas temperature, the inert gas flow rate, and the CO2 contribution to the flow rate. Total gas flow rate was measured with a floating-sphere flow meter placed in the line prior to the 70 °C preheating oil bath. MEA and ethylene glycol were obtained from Union Carbide and APS Chemicals, respectively, and dry ice was obtained from either BOC Gases Limited or ASCO Carbon Dioxide Limited. The gas supply for regeneration was dinitrogen from an in-house supply. The glass packed bubble reactor was built by the Chemistry Department of the University of Canterbury. Oil bath temperatures and the reactor solution temperature were monitored and/or controlled by OMRON or Heidolph EKT 3001 temperature controllers. The probe in the reactor solution had to be regularly protected with a silicone sealer. CO2-saturated solutions were prepared by addition of dry ice to 500 mL of a 50 wt % MEA, 35 wt % glycol, and 15 wt % water solution. The solution was warmed to 40 °C and additional CO2 added to ensure saturation of the solution. The increase in weight of the solution was used to determine that the initial CO2 content was around 0.5 mol of CO2 per mole of MEA. 13C-NMR spectroscopy confirmed that the CO2 was present as the carbamate salt [HOCH2CH2NH3][HOCH2CH2NHCO2].13,14 3. Results Experiments were conducted to investigate the effect of inert gas flow rate and temperature on the regeneration rate and total regeneration with time for fully CO2 saturated solutions. Results were examined by calculating the % regeneration from the measured CO2 flow rate in the flushing gas and the known initial CO2 content of the solution:
% regeneration(t) ) cumulative moles of CO2 regenerated at time t initial moles of CO2 in saturated solution
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Figure 4. Behavior of regeneration at various temperatures of the oil heating bath (TB) without gas injection.
Figure 5. Rate of CO2 evolution with time in a 140 °C oil bath with no gas passing through the solution. The boiling point was reached after approximately 30 min in this run.
In some experiments, the MEA solution temperature changes with time, and in many cases we wish to compare % regeneration over time at different temperatures. A process temperature, T*, is defined as the solution temperature measured throughout the experiment, TR(t), normalized with the solution boiling point, TBP. T* was calculated using absolute temperature. To analyze the results, we have plotted the nondimensional process parameter (% regeneration)/T* versus processing time, t.
T*(t) )
TR(t) TBP
3.1. Regeneration Rate with No Gas Injection. To interpret the effects of passing an inert gas through the solution during regeneration, studies were first done to look at the behavior of the solution with the gas passing over, rather than through, the solution. Figure 4 shows the % regeneration behavior of CO2-saturated solutions as they are heated with an oil bath at various temperatures with no gas injection into the solution. The runs are labeled according to the temperature of the heated oil bath, TB. The 100 °C bath is barely able to evolve CO2. The baths at 110-130 °C start to evolve a burst of CO2 when the temperature of the solution reaches approximately 90 °C and reaches a maximum rate of CO2 evolution at 95-98 °C before the rate of CO2 evolution drops down. The total amount of regeneration seen in this burst of CO2 evolution represents 7-10% total regeneration. The “burst”, as shown in more detail in Figure 5, was observed to occur with the onset of nucleation boiling at about TBP ) 98 °C, where the resultant vapor bubbles act to flush some CO2 into the passing gas stream. With the oil bath heated to 140 °C, CO2 evolution also begins at about 90 °C and there is a similar burst of CO2 evolution at 95-98 °C; however, the subsequent higher temperatures reached in the solution lead to boiling of the solution at 105 °C and CO2 evolution that rapidly increases as the vapors carry the CO2 out of the solution. The rate of CO2 evolution fluctuates significantly in response to the boiling process (Figure 5). 3.2. Effect of Gas Injection on Initial Regeneration Stage. We investigated the effect of preheating the solution prior to gas injection. The result is shown in Figure 6, in which the solution temperature is also shown. Very little CO2 is evolved during the period with
Figure 6. Comparison of 110 °C regeneration rate (A) for inert gas flow (6.8 L/min) during preheat and (B) with no bubble gas flow during the first 20 min. The nondimensional scale for both experiments is the CO2 removed compared to the initial saturated content, divided by the MEA solution temperature normalized by the boiling temperature.
no gas being passed through the solution. Upon bubbling, there is rapid CO2 evolution, much faster than is observed in the boiling process (see Figure 5), accompanied by a 5 °C drop in the solution temperature due to the endothermic process of evaporation. 3.3. Effect of Injection Gas Flow Rate. To investigate the effect of gas flow rate on the rate of regeneration, gas was passed through saturated solutions at a variety of flow rates. Thermal equilibrium takes some time to reach, during which most of the regeneration has occurred. It is clear from the results illustrated in Figure 7 that, at flow rates of less than 6 L/min, the flow rate has a significant impact on the rate of regeneration. This may be influenced by three factors: one is the gas side, where low air flow will lower the diffusion rate, increasing the mass transport resistance. Another factor is the lower gas-liquid surface area present with lower gas flows. The final factor is on the liquid side, involving the turbulence induced by greater air flow rate. We observed that the three higher gas flow experiments appeared more like a froth, while the lower gas flow experiments looked like bubbles travelling up through a liquid. Above 6 L/min, the rate of regeneration is invariant with the flow rate, implying that the inert gas is
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of regeneration of an aqueous amine solution carried out by boiling 10 mL of a 2.5 M MEA solution at 100 °C and found that 50% of the regeneration was complete within 10 min.14 The rates of regeneration are much faster with bubbling than without bubbling. In comparison of Figure 8 to Figure 4, even the 100 °C bath with bubbling gives a faster rate of regeneration than the 140 °C oil bath without gas injection, in which the solution has reached the boiling point. The asymptotic behavior of the regeneration rate is expected, as each solution will eventually reach the equilibrium CO2 solution concentration, which is a function of temperature for both the CO2 solution solubility and the CO2-MEA reaction. Figure 7. Rate of regeneration at various inert gas flow rates with a nominal bath temperature of 110 °C.
Figure 8. Behavior of regeneration at various temperatures of the oil heating bath (TB) with gas injection (6.8 L/min).
carrying away the evolved CO2 as fast as it is generated and higher flow rates provide no increase in the rate of regeneration. It should be noted that increasing the flow rate affects neither the bubble residence time in the solution nor the size of the bubbles. Therefore, a doubling of the flow rate corresponds to a doubling of the number of bubbles and, hence, a doubling of the effective surface area between the solution and the inert gas. Increased gas flow rate also represents a reduced gas bulk CO2 concentration. Thus, the upper limit in CO2 regeneration rate with increased gas flow rate indicates that a different step in the regeneration process becomes the limiting factor. 3.4. Effect of Temperature on Packed Bubble Regeneration. Evaluation of the effect of temperature on the rate of regeneration with a gas passing through the solution was performed with a fixed inert gas flow rate of 6.8 L/min. The results, illustrated in Figure 8, show the expected trend that the rate of regeneration increases with temperature. What we found most surprising was that regeneration was at least 50% complete while the temperature was still increasing and had not even reached 90 °C. Unlike the cases without bubbling, the behavior at the different temperatures was essentially the same, but with different rates of CO2 evolution and degree of regeneration. Almost half of the regeneration of the 500 mL solution was complete within 50 min in each case. Hook has reported the rate
4. Conclusions Industrial applications of packed-column systems using the temperature-swing absorption-desorption capabilities of amines for CO2 processing cannot be scaled down to small-size, low-temperature, low-pressure, low-energy scrubbing of air. A new method for scrubbing air using a packed bubble column absorber and regenerator has been proposed. The new method is under development for alkaline fuel cells and other CO2 management applications.15 These applications will require low-temperature regeneration of the amine solution at atmospheric pressure. This work reports the findings that a high molar concentration monoethanolamine solution can be fully regenerated to the solubility and reaction limit at temperatures below 100 °C at atmospheric pressure. The chemical regeneration rate is sufficiently fast at temperatures above 80 °C but is limited by diffusion through the liquid to a liquid-gas interface where desorption can occur. Injecting an inert gas flow into a heated packed bubble column reactor produces vigorous mixing, smaller aggregate diffusion path, and greatly increased liquid-gas interface area. The gas-injected packed bubble regenerator was shown to accomplish full regeneration (50% of saturation loading) at temperatures significantly below those used in industrial processes. The regeneration rate in a vessel of fully saturated MEA solution at 100 °C was measured to be nearly negligible with no gas injection. When inert gas was bubbled through the solution, the solution was regenerated rapidly, more rapidly in fact than by boiling the solution at 140 °C. The influence of the injected gas flow rate and the regeneration temperature were studied in separate experiments. Higher regeneration rates and more complete total regeneration were achieved with higher gas flow rates. A higher gas flow rate produces greater turbulence, lower aggregate gas bulk CO2 concentration, and increased liquid-gas interface area, each of which are contributing factors to the regeneration rate. Increased injection gas flow rate was shown to correlate with increasing regeneration rate up to a point. In these experiments, once a fully turbulent mixture was achieved, at a gas flow rate of 6 L/min through 500 mL of heated MEA solution, then further increase of gas flow rate did not result in increased regeneration rate. The extent of full regeneration achieved in the packed bubble reactor with high gas flow rate was observed to depend on the temperature in a predictable manner according to the regeneration chemistry. More complete regeneration could be accomplished at higher temper-
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ature. The primary finding of the research was that regeneration with the packed bubble column at 100 °C was far faster than in a stirred beaker with no injected gas at 140 °C. Acknowledgment This research was supported by the New Zealand Foundation for Research Science and Technology and by ASCO Carbon Dioxide Limited. Literature Cited (1) Kohl, A. L.; Riesenfeld, F. C. In Gas Purification, 4th ed.; Gulf Publishing: Houston, TX, 1985. (2) Gustafson, P. R. In The Present Status of Chemical research in Atmosphere Purification and Control on Nuclear-Powered Submarines; Miller, R. R., Piatt, V. R., Eds.; NRL Report 5630; Naval Research Laboratory: Washington, DC, 1961; Chapter 7. (3) Ravner, H.; Blachly, C. H. In The Present Status of Chemical research in Atmosphere Purification and Control on NuclearPowered Submarines; Piatt, V. R., White, J. C., Eds.; NRL Report 5814; Naval Research Laboratory: Washington, DC, 1962; Chapter 5. (4) Larminie, J.; Dicks, A. Fuel Cells Explained; Wiley: 2000. (5) Larminie, J.; Dicks A. In Fuel Cell Systems Explained, 2nd ed.; Wiley: Sussex, U.K., 2003; pp 137-138. (6) Krumdieck, S.; Wallace, J.; Green, M.; Marsh, K.; Clelland, J.; Green, M.; Brown, M. (Canterprise Limited) In Apparatus for continuous carbon dioxide absorption. New Zealand Provisional Patent Application No. 514666, 2001.
(7) Sartori, G.; Savage, D. W. Sterically Hindered Amines for Carbon Dioxide Removal from Gases. Ind. Eng. Chem. Fundam. 1983, 22, 239. (8) Maddox, R. N.; Mains, G. J.; Rahman, M. A. Reactions of Carbon Dioxide and Hydrogen Sulfide with Some Alkanolamines. Ind. Eng. Chem. Res. 1987, 26, 27. (9) Crooks, J. E.; Donnellan, J. P. Kinetics of the Reaction Between Carbon Dioxide and Tertiary Amines. J. Org. Chem. 1990, 55, 1372. (10) Calderbank, P. H. In Gas Absorption from Bubbles. The Chemical Engineer, CE209-CE233, October, 1967. (11) Mashelkar, R. A., Bubble Columns, Br. Chem. Eng. 1970, 15, 1297. (12) Kohl, A. L.; Nielsen R. B. Gas Purification, 5th ed.; Gulf Publishing: Houston, TX, 1997. (13) Suda, T.; Iwaki, T.; Mimura, T. Facile Determination of Dissolved Species in CO2-Amine-H2O System by NMR Spectroscopy. Chem. Lett. 1996, 9, 777. (14) Hook, R. J. An Investigation of Some Sterically Hindered Amines as Potential Carbon Dioxide Scrubbing Compounds. Ind. Eng. Chem. Res. 1997, 36, 1779. (15) Krumdieck, S.; Wallace, J.; Green, M.; Marsh, K.; Clelland, J.; Green, M.; Brown, M. (Canterprise Limited) Apparatus for continuous carbon dioxide absorption, International Application Published Under The Patent Cooperation Treaty (PCT), WO 03/ 031028, April 17, 2003.
Received for review June 30, 2004 Revised manuscript received October 12, 2004 Accepted November 19, 2004 IE049428N
