Scholarly article on topic 'Absorption of carbon dioxide into aqueous potassium carbonate promoted by boric acid'

Absorption of carbon dioxide into aqueous potassium carbonate promoted by boric acid Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — Ujjal K. Ghosh, Sandra E. Kentish, Geoff W. Stevens

Abstract The absorption of carbon dioxide with alkanolamine or potassium carbonate solvents has gained widespre ad acceptance for the removal of CO2 from natural gas and H 2. However, alkanolamine solutions are prone to oxidative degradation at high temperature. The main advantages of potassium carbonate solution for CO2 removal are the high chemical solubility of CO2 in the carbonate/ bicarbonate system, low solvent costs and the low energy requirement for solvent regeneration. The major challenge concerning absorption of CO2 into aqueous solutions of potassium carbonate is a relatively slow rate of reaction in the liquid phase causing low mass transfer rates. It is often advantageous to add a promoter to increase the absorption rate. While piperazine is often used for this purpose, there are some environmental health concerns with this approach. In this work we consider boric acid as an alternative promoter. Absorption of CO2 in a wetted -wall column by aqueous potassium carbonate solution with and without various concentrations of boric acid as promoter was measured under conditions in which the reaction of CO2 was of pseudo-first order. The equilibrium partial pressure and the rate of absorption of CO2 were measured in 30.0 wt% potassium carbonate and 1.0 to 5.0 wt% of boric acid at 50 to 80 ∘C. The overall pseudo -first-order reaction rate constants were determined fro m the kinetic measurements. The addition of small amounts of boric acid to potassium carbonate resulted in a significant enhancement of CO2 absorption rates.

Academic research paper on topic "Absorption of carbon dioxide into aqueous potassium carbonate promoted by boric acid"

Ама11вЫ» опкн et тмяйбмяЛгба.сап

SdenceDIrect

Energy Procedía

ELSEVIER

Energy Procedía 1 (2009) 1075-1081

www.elsevier.com/locate/procedia

GHGT-9

Absorption of carbon dioxide into aqueous potassium carbonate

promoted by boric acid

Ujjal K Ghosh*, Sandra E Kentish and Geoff W Stevens

The absorption of carbon dioxide with alkanolamine or potassium carbonate solvents has gained widespread acceptance for the removal of C02 from natural gas and H2. However, alkanolamine solutions are prone to oxidative degradation at high temperature. The main advantages of potassium carbonate solution for C02 removal are the high chemical solubility of CO 2 in the carbonate/ bicarbonate system, low solvent costs and the low energy requirement for solvent regeneration. The major challenge concerning absorption of C02 into aqueous solutions of potassium carbonate is a relatively slow rate of reaction in the liquid phase causing low mass transfer rates. It is often advantageous to add a promoter to increase the absorption rate. While piperazine is often used for this purpose, there are some environmental health concerns with this approach. In this work we consider boric acid as an alternative promoter.

Absorption of C02 in a wetted -wall column by aqueous potassium carbonate solution with and without various concentrations of boric acid as promoter was measured under conditions in which the reaction of CO 2 was of pseudo-first order. The equilibrium partial pressure and the rate of absorption of C02 were measured in 30.0 wt% potassium carbonate and 1.0 to 5.0 wt% of boric acid at 50 to 80 °C. The overall pseudo -first-order reaction rate constants were determined fro m the kinetic measurements. The addition of small amounts of boric acid to ootassium carbonate resulted in a sisnificant enhancement of CO absorotion rates. © 2009 Elsevier Ltd. All rights reserved.

Keywords: GO 2 absorption; Potassium carbonate; Promoter; Boric acid

1. Introduction

Carbon dioxide (C02) is a very important greenhouse gas that is emitted during the burning of fossil fuels. The most likely options for separation and capture of C02 from the flue gases of large combustion plants include chemical absorption, physical and chemical adsorption and gas -separation membranes.

Many solvents have been applied to gas treating, but the most effective solvents are generally considered to be aqueous alkanolamines or hot potassium carbonate solvents. Primary and secondary amines react with C02toform amine carbamates. Aqueous primary amines, such as monoethanolamine, have been shown to have fast reaction rates [1]. The commonly used secondary amines, such as diethanolamine, have been shown to have somewhat slower reaction rates than primary amines. Previous investigators have explored the solubility and reaction rate of

Cooperative Research Centre for Greenhouse Gas Technologies (C02CRC), Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia

Abstract

Corresponding author. Tel: +61-3-83448863; Fax: +61-3-83444153 E-mail address: ughosh@unimelb.edu.au

doi:10.1016/j.egypro.2009.01.142

C02 in aqueous potassium carbonate [2-4]. Alkanolamines have an advantage over the hot potassium carbonate process in that the absorption rate of C02 by amines is fast; however, the heat of absorption is also high. In contrast, absorption into potassium carbonate has a heat of absorption similar to physical solvents, but is limited by slow absorption rates. The current state-of-the-art technology for C02 removal from flue gases is generally considered to be a 30 wt% (7 M) aqueous solution of monoethanolamine (MEA). MEA has a high capacity for C02 and high rate of absorption, but its performance is limited by several factors, including a high heat of absorption, oxidative degradation at high temperature and corrosion issues. In particular, aqueous potassium carbonate solutions are commonly used in hot carbonate processes for bulk C02 removal because of their low cost, large capacity, ease of handling, and relative ease of regeneration.

It has long been recognized that the rate of C02 absorption in carbonate solutions can be increased through the use of various additives, including amines and hydration catalysts. C02 absorption in amine-potash mixtures was studied by Ellis [5], who compared his results with theoretical predictions. Danckwerts and McNeil [6] described the effects of a hydration catalyst, sodium arsenite, on C02 absorption in mixed amine-potash solutions. In both investigations, the solutions contained relatively high concentrations of amine (> 1M) and, from a practical point of view, such solutions could not be used conveniently in conventional carbonate processes. Roberts and Danckwerts [7] worked on the absorption of CO 2 in a solution containing potassium carbonate and potassium arsenite. In the absence of arsenite the C02 reacts predominantly with the hydroxyl ions present, which are in low concentration (e.g. 10"4 g /litre.) in carbonate-bicarbonat e mixtures so that the reaction is relatively slow, although faster than the reaction with water. The arsenite ions catalyse the latter, and thus accelerate the reaction of CO 2 and hence the processes of absorption and desorption. The increase in rate is proportional to the concentration of arsenic added. The addition of 1M arsenic (about 10 per cent by weight As203) to an equimolal C03 /HC03" mixture increases the rate of reaction of C02 from about 1.8 to about 140 sec"1 at 25°C. Sharma and Danckwerts [8] tested the absorption of carbon dioxide into carbonate buffer with arsenite, formaldehyde and hypochlorite as catalyst. They had found that anions which have a negatively charged oxygen atom and at least one hydroxyl group attached to the same central atom, and in which the negative charge is not delocalized by resonance effects, form a single group which obeys the Bronsted relationship.

Sharma and Danckwerts [9] also studied the effect of catalytic promoters for the absorption of C02 in potassium carbonate solution. They selected different anions of telluric acid, germanic acid, arsenious acid, silicic acid, chloral hydrate, chloral alcoholate, hydrates of butyl chloral, glyoxal, formaldehyde, acetaldehyde, diacetal as catalyst. They had also tested trichloroethanol, triflouroethanol, hydrogen peroxide, some sugars, and sulphurous, selenous, tellurous, phosphoric and phenyl arsenic acids and phenol.

Cullinane and Rochelle [10] worked on absorption of carbon dioxide with aqueous potassium carbonate promoted by piperazine. They used 20 -30 wt% of K2C03 and 0.6 M piperazine in the temperature range 40-80°C. The addition of 0.6 m piperazine to a 20 wt% potassium carbonate system decreases the C02 equilibrium partial pressure by approximately 85% at intermediat e CO 2 loading. The addition of 0.6 m piperazine to 20 wt% potassium carbonate increases the rate of C02 absorption by an order of magnitude at 60oC. The rate of C02 absorption in the promoted solution compares favourably to that of 5 .0 M MEA.

Eickmeyer [11-12] reported an improved, catalyzed process for the removal of C02 from gas mixtures using a solution containing 15-40% by weight of potassium carbonate in which the absorption efficiency is enhanced by the addition of sodium or potassium vanadate and sodium or potassium borate. This author found that use of the above described catalysts permits significant reductions in solution circulation rates (up to 45%), which leads to equipment and utility economies. However, he did not elaborate on the effect of borate on enhancement of C02 absorption rate into potassium carbonate solution. Eickmeyer [13] also reported the removal of C02 andH2S from gaseous mixtures using potassium carbonate and potassium borate. Amine borates were also used as catalysts. The use of some inorganic additives, such as arsenious anhydride (As203), and selenious and tellurous acid, as solution activator are mentioned.

Field [14] described the use of an aqueous scrubbing solution for the separation of C02 from gas mixtures containing potassium carbonate in presence of a sodium or potassium borate and a sodium or potassium salt of a vanadium oxy acid. The solution is resistant to oxidative degradation in the presence of oxygen-containing gases.

No rate data of CO2 absorption was reported. Imada et al. [15] reported the absorption of CO2 in lithium silicate promoted by potassium carbonate and sodium carbonate. The mole ratio of the absorption promoter to s odium carbonate/ potassium carbon ate was used in a range from 0.15 to 0.3. The amount of promoter used was in the range of 2 to 4% of lithium silicate used.

From the above discussion it is evident that different compounds, some alkanolamines as well as some inorganic compounds have been used to enhance the rate of C02 absorption into aqu eous potassium carbonate solution. However, many promoters are carcinogenic in nature and some others are not stable at stripper conditions. In this regard boric acid is environmentally friendly and can be used as a promoter. In this study we report the us efulness of boric acid as a promoter to enhance the rate of C02 absorption into aqueous potassium carbonate solution.

2. Theory

The gas film mass transfer coefficient, kg, in a wetted wall column was determined by Bishnoi [16] using S02 absorption into 0.1 M NaOH. The results of these experiments are correlated with a form for kg proposed by Hobler [17].

Sh = 1.075 fReSc-) ®

The Reynolds number is defined as u p d

Re = (2)

where u is the linear velocity of the gas, p is the density, and ^ is the viscosity. Also, d is the hydraulic diameter of the annulus and h is the height of the wetted wall column (varying). The Schmidt number is

& =(3) P Dœ 2

DC02 is the diffusion coefficient of C02. The gas film transfer coefficient can be found from the following definition of the Sherwood number.

Sh =-L_ (4)

where T is the temperature and R is the gas constant.

The flux of C02 can be characterized by the overall gas phase mass transfer coefficient.

Nco2 = Ka (Pco2>i - P'co2) (5)

The mass transfer coefficient, KG, is calculated as the slope of the flux versus the log mean pressure, P lm. The P lm is defined as a log mean average of bulk gas partial pressures of C02 across the wetted wall column and is assumed representative of PC02 bp Pco 2J„ - Pco lPut (6)

= In(Pco„„/Pco^ )

The equilibrium partial pressure, P C02, can be foundin Kohl and Riesenfeld [18].

An expression forthe liquid film resistance, which includes the kinetics of C02 absorption, can be written as

Nco2 = k'g (Pco2,, - P'co2) (7)

where kg' is a normalized flux, a mass transfer coefficient for the partial pressure driving force across the liquid film.

The normalized flux is calcul ated from the following expression.

= f— - -f (8)

' I K„ k I

For C02 absorption by amines or carbonate, chemical reaction effects typically dominate the physical mass transfer inside the liquid film; therefore, kg'can be related to the rate of C02 absorption as well as the kinetics o f the absorption process.

3. Experimental

A wetted wall column (WWC) shown in Figure 1 was used for C02 absorption rate experiments in this work. The wetted wall column consists of a stainless steel tube with a 1.26 cm outer diameter. The length of the tube can be adjusted as it was extended from the liquid feed line into the column housing. The total contact area is calcul ated as the longitudinal area of the tube and the area of the top of the column (considered a hemisphere due to the shape of liquidfilm). The column is housed inside a glass cylinder with an inside diameter of 2.54 cm to provide the gasliquid contact chamber. The gas phase hydraulic diameter (outer diameter minus inner diameter) of the enclosure is 1.26 cm, giving a cross-sectional area for gas flow of 3.8cm 2. The chamber is enclosed inside a second glass cylinder (12.5 cm OD) that serves as an insulating bath with circulating heat transfer fluid.

C02 Analyzer

Saturator

Figure 1. Schematic of Wetted Wall Column experiment

The liquid solution is contained in a reservoir with a total volume of 3 L. The fluid is pushed from the reservoir to the wetted-wall column by a gear pump through a coil submerged in a heated circulator. The fluid flows up through the middle of the column, overflows at the top, and is evenly distributed on the outer surface of the column. The liquid continuously flows from the bottom of the column to the solution reservoir. A liquid rotameter indicates the volumetric flowrate of the liquid. Gas enters near the base of the column, counter-currently contacting the fluid as it flows up to the gas outlet.

Nitrogen and C02 are contained in cylinders and the flowrate is governed by Aalborg mass flow controllers. The metered gases are mixed and saturated with water at the operating temperature of the column in a sealed vessel immersed in a heat bath. After saturation, the gas is introduced to the bottom of the column and flows counter-currently past the liquid film. The gas exits the top of the column and is routed to a condenser before venting. This allows a dry sample to be taken for C02 analysis using a Varian Gas Chromatograph. The wetted wall column height used for all experiments was 14 cm. Gas flow rate was maintained at 5-6 litres per minute, so that the gas phase resistance is very high.

Heating Media.

ID 2.54 cm

Heating Media

Figure 2. Sketch of Wetted Wall Column

4. Results and discussion

In this work, addition of boric acid into aqueous potassium carbonate solution increases the rate of carbon dioxide absorption. The effect of boric acid addition into aqueous potassium carbonate to rate o f CC^ absorption is shown in

Figure 3. It can be seen from Figure 3 that the normalized flux has been increased from 3 x 10_11 to 5.35 x 10"11

gmole cm " s- Pa" as the boric acid concentration increases from 0 to 3 wt% in 30 wt% potassium carbonate solution at60°C.

5.50E-011

£ 5.00E-011

o 4.50E-011

O 4.00E-011

G T3 ü

S 3.50E-011

X 3.00E-011

Figure 3. Variation of carbon dioxide absorption rate with concentration of boric acid in aqueous solution of

potassium carbonate and boric acid

Concentration of K CO :30wt%

Gas flowrate: 5.8 Lmin ' Liquid flow rate: 58 mL min 1 Temperature: 60 C

0.0 0.5 1.0 1.5 2.0 2.5

Concentration of H , BO,, wt%

C02 reacts with aqueous solution of potassium carb onate solution as follows:

CO 2 + H p o H+ + HCO-

co ; + H+ o HCO -co1 + OH - o HCO-

co : + ho o hco - + oh -

(10) (11) (12)

CS PL,

3.60E-011

3.20E-011

2.80E-011

2.40E-011

2.00E-011

1.60E-011

1.20E-011

Concentration ofK 2CO 3: 30 wt%

Gas flow rate: 5.8 L min 1 Liquid flow rate: 58 mL min 1 Concentration ofH 3B03: 1.0 wt%

Temperature, C

Figure 4. Variation of carbon dioxide absorption rate with temperature in aqueous solution of potassium carbonate

and boric acid

Both reactions (9) and (11) are slow and rate determining, the former being much slower than the later [19]. The increase in rate of C02 absorption by addition of boric acid may be due to its catalytic effect on reaction (11). The effect of temperature on rate of C02 absorption is shown in Figure 4. It can be seen from Figure 4 that the normalized flux has been increased from 1.45 x 10 ~n to 3.33 x 10~n gmole cm"2s-1Pa"1 as the temperature increases from 40°C to 60°C in 30 wt% potassium carbonate solution with 1.0 wt% boric acid concentration. In this temperature range, a reduction in gas solubility is countered by an increase in sorption kinetics. Table 1 shows a comparative rate data in different solutions. The rate in boric acid promoted carbonate solution is not comparable with that in amine solution. However, the experimental benefits and robustness of this system offer significant advantages over the amine based systems.

Table 1: C02 absorption rates in different solutions at 60oC

Solutions condition k's (Normalized flux) gmole cm"/s"1Pa"1

1.8 MK,CO, (P *ro2 = 500 Pa) 1 1.1 x 10"11

1.8MK2C03+0.6MPZ(P*co2 = 500Pa) 1 1.2xl0"lu

7 M MEA (P *rn?=500 Pa) 1 1.6xl0"lu

2.7 M K2C03 + 0.48 M H3B03 (P *C02 = 99Pa) 2 5.35xl0"u

1 Ref [10]

2 This work

5. Conclusions

This paper shows that the addition of 3.0% boric acid to potassium carbonate system can increase the kinetics of uptake of C02 by a factor of 2. Temperature is also shown to play an important role. The rate is still below that obtained by the amine based solvents, and further research is required to optimize this system. The experimental benefits and robustness of this system offer significant advantages over the amine based systems.

6. References

[1] Astarita, G. Carbon dioxide absorption in aqueous ethanolamine solutions, Chemical Engineering Science, 1961, 16, 202-207.

[2] Benson, H. E.; Field, J. H.; Jimeson, R. M. C02 absorption employing hot potassium carbonate solutions, Chemical Engineering Progress, 1954, 50(7), 356-364.

[3] Benson, H. E.; Field, J. H.; Haynes, W. P. Improved process for C02 absorption uses hot carbonate solutions, Chemical Engineering Progress, 1956, 52 (10), 433 -438.

[4] Tosh, S.; Field, J. H.; Benson, H.E., Haynes, W.P. Equilibrium study of the system potassium carbonate, potassium bicarbonate, carbon dioxide, and water, United States Bureau of Mines, 5484, 1959.

[5] Ellis, J. E. The correlation of absorption rates of C02 by alkaline and amine solutions in packed columns, Transactions of the Institution of Chemical Engineers, I960, 38, 216-224J.

[6] Danckwerts, P. V.; McNeil, K. M. The effects of catalysis on rates of absorption of C02 into aqueous amine-potash solutions, Chemical Engineering Science, 1967, 22, 925-930.

[7] Roberts D.; Danckwerts, P. V. Kinetics of C02 absorption in alkaline solutions-I: Transient absorption rates and catalysis by arsenite, Chemical Engineering Science, 1962, 17, 961-969.

[8] Sharma, M. M.; Danckwerts, P. V. Catalysis by Broensted bases of the reaction between C02 and water, Transactions of the Faraday Society, 1963 , 59, 386-95.

[9] Sharma, M. M.; Danckwerts, P. V. Fast reactions of C02 in alkaline solutions - (a) carbonate buffers with arsenite, formaldehyde, and hypochlorite as catalysts, (b) aqueous monoisopropanol amine (1 -amino-2-propanol) solutions, Chemical Engineering Science, 1963, 18, 729-735.

[10] Cullinane, J. T.; Rochelle, G. T. Carbon dioxide absorption with aqueous potassium carbonate promoted bypiperazine, Chemical Engineering Science, 2004, 59(17), 3619 -3630.

[11] Eickmeyer, A. G. For bulk removal of acid gases, costs favor the hot-carbonate process, Chemical Engineer, 1958,65(17) 113-116.

[12] Eickmeyer, A.G. Removal of C02 from gas mixtures, US Patent No. 4430312, 1984.

[13] Eickmeyer, A.G. Method and compositions for removing acid gases from gaseous mixtures, US Patent No. 4271132, 1981.

[14] Field, J. H. Separation of C02 from gas mixtures, US Patent No. 3907969, 1975.

[15] Imada, T.; Kato M.; Essaki, K. Carbon dioxide absorbent and carbon dioxide separation apparatus, US Patent Application No. 20070072769, 2007.

[16] Bishnoi, S. and Rochelle, G. T. Absorption of Carbon Dioxide into Aqueous Piperazine: Reaction Kinetics, Mass Transfer and Solubility, Chemical Engineering Science, 2000, 5 5(22), 5531-5543.

[17] Hobler, T. Mass Transfer and Absorbers, 1966, Oxford, Pergamon Press.

[18] Kohl, A. L. and Riesenfeld, F. C. Gas Purification, 198 5, Houston, GulfPublishmg.

[19] Sanyal, D.; Vasistha, N. and Saraf, D. N. Modeling of carbon dioxide absorber using hot carbonate process, Industrial Engineering and Chemistry Research, 1988, 27, 2149-2156.