CO2 Capture and Crystallization of Ammonia Bicarbonate in a Lab-Scale Scrubber

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Article CO2 Capture and Crystallization of Ammonia Bicarbonate in a Lab-Scale Scrubber

Pao Chi Chen * and Shun Chao Yu

Department of Chemical and Materials Engineering, Lunghwa University of Science and Technology, Taoyuan 33306, Taiwan; [email protected] * Correspondence: [email protected]

Received: 21 November 2017; Accepted: 12 January 2018; Published: 16 January 2018

Abstract: A lab-scale bubble-column scrubber is used to capture CO2 gas and produce ammonia bicarbonate (ABC) using aqueous ammonia as an absorbent under a constant pH and temperature. ◦ The CO2 concentration is adjusted by mixing N2 and CO2 in the range of 15–60 vol % at 55 C. The process variables are the pH of the solution, temperature, gas-flow rate and the concentration of gas. The effects of the process variables on the removal efficiency (E), absorption rate (RA) and overall mass-transfer coefficient (KGa) were explored. A multiple-tube mass balance model was used to −4 −3 determine RA and KGa, in which RA and KGa were in the range of 2.14 × 10 –1.09 × 10 mol/(s·L) and 0.0136–0.5669 1/s, respectively. Results found that, RA showed an obvious increase with the increase in pH, inlet gas concentration and gas temperature, while KGa decreased with an increase in inlet gas concentration. Using linear regression, an empirical expression for KGa/E was obtained. On the other hand, ammonia bicarbonate crystals could be produced at a pH of 9.5 when the gas concentration was higher than 30% and γ (=Fg/FA, the gas-liquid molar flow rate ratio) ≥ 1.5.

Keywords: ammonia bicarbonate; carbon dioxide; mass-transfer coefﬁcient; capture; bubble-column scrubber

1. Introduction The total global carbon dioxide emissions are increasing by the day, which causes the temperature to rise rapidly, thus, creating severe impacts on the global ecology. Therefore, the issues of CO2 have attracted great attention with the most attention on the CO2 emission of coal fired power plants and steel plants [1]. There are two main kinds of CO2 capturing methods currently under study, namely, pre-combustion and post-combustion [2]. The technology of the pre-combustion method is to primary treat fossil fuels with steam and air or oxygen, in order to separate the fossil fuel into carbon dioxide and hydrogen and then, to store the carbon dioxide and use the hydrogen for combustion meaning no carbon dioxide would be produced. In the post-combustion method, the emitted CO2 is about 15% which is mainly captured by chemical absorption. The absorbents used for chemical absorption include the alkaline solutions of all levels, such as amine, ammonia, sodium hydroxide and hot potassium carbonate [3–6]. Among numerous absorption technologies and absorbents, the monoethanolamine (MEA) process is the most popular and already operates in more than a thousand units which is comparable with 700 plants for potassium carbonate solution [6]. However, the MEA process still has some disadvantages including low absorption efficiency, low CO2 loading, high regeneration energy requirements and the container corrosion issue [3,7]. In the case of using MEA as the absorbent, the absorbed carbon dioxide is stored in the liquid phase in a liquid state, which might be released again due to the influence of the pH value and the huge volume. Therefore, in some studies, MEA is changed to aqueous ammonia as the absorbent, which is used to absorb the carbon dioxide as reported in the literature [1,3,8].

Crystals 2018, 8, 39; doi:10.3390/cryst8010039 www.mdpi.com/journal/crystals Crystals 2018, 8, 39 2 of 13 aqueous ammonia as the absorbent, which is used to absorb the carbon dioxide as reported in the literature [1,3,8]. The advantages of using aqueous ammonia as the absorbent are high absorption load, low Crystals 2018, 8, 39 2 of 13 regeneration energy requirement, low corrosiveness, lower cost and higher removal efficiency [9–11]. Another advantage is that an aqueous ammonia solution can be used to simultaneously The advantages of using aqueous ammonia as the absorbent are high absorption load, low remove CO2, SO2 and NOx [12]. Moreover, the other reasons for using aqueous ammonia as an regeneration energy requirement, low corrosiveness, lower cost and higher removal efﬁciency [9–11]. absorbent solution are due to its high CO2 loading capacity, low regeneration energy at a relatively Another advantage is that an aqueous ammonia solution can be used to simultaneously remove CO , low temperature (~8 °C) and reuse of ammonia and stripped water vapor for the distillation2 of SO2 and NOx [12]. Moreover, the other reasons for using aqueous ammonia as an absorbent solution aqueous carbonated ammonia [1,4,13]. The CO2 absorption process using aqueous ammonia are due to its high CO2 loading capacity, low regeneration energy at a relatively low temperature solution(~8 ◦C) depends and reuse on of the ammonia temperature, and stripped ion concentration water vapor for and the pH distillation value of of the aqueous solution carbonated where more carbonammonia dioxide [1, 4is,13 absorbed]. The CO at2 absorptiona higher pH process value using and aqueousmore ammonia ammonia is solution created depends [3,14]. Moreover, on the a highertemperature, aqueous ionammoni concentrationa concentration and pH value or lower of the solution CO2 conc whereentration more carbon would dioxide cause is the absorbed absorption efficiencyat a higher to pH decline, value andwhich more would ammonia affect is created the absorption [3,14]. Moreover, of carbon a higher dioxide. aqueous The ammonia carbonate dissociationconcentration degree or lowerin the CO aqueous2 concentration phase is would shown cause in Figure the absorption 1 [15,16 efﬁciency]. Within to a decline,pH range which of pH = 7–9,would as shown affect by the α absorption1, the solution of carbon mainly dioxide. contains The carbonate bicarbonate; dissociation when degree the pH in thevalue aqueous is above phase 10, as α shownis shown by α2 in, theFigure solution1[ 15,16 mainly]. Within contains a pH range carbonate; of pH = and 7–9, as when shown the by pH1, value the solution is less mainly than 6, the contains bicarbonate; when the pH value is above 10, as shown by α2, the solution mainly contains solution mainly contains carbonic acid (H2CO3*). carbonate; and when the pH value is less than 6, the solution mainly contains carbonic acid (H2CO3*).

Figure 1. Inﬂuence of pH values on carbonate dissociation, re-plotted in here [15,16]. Figure 1. Influence of pH values on carbonate dissociation, re-plotted in here [15,16].

More carbon dioxide is absorbed by aqueous ammonia at a higher pH value; in addition, more More carbon dioxide is absorbed by aqueous ammonia at a higher pH value; in addition, more ammonia is created at a higher pH value. Therefore, the best pH value will facilitate the absorption ammoniaprocess, is as created shown at in a Figure higher2. EquationspH value. (1)–(3) Therefore, are the the formulas best pH for value ammonia will dissociation facilitate the in water.absorption process,As ammonia as shown will in be Figure separated 2. Equations due to the (1) effect–(3) of are the the pH formulas value and for a higher ammonia pH value dissociation can help in the water. As ammoniaformation ofwill NH3(aq), be separated NH3 can due be separated to the effect out of of the the solution pH value [16]. and As shown a higher in Figure pH value2, it is can known help the formationthat, when of NH pH3 =(aq), 9.5 theNH mole3 can fraction be separated of NH3 isout about of th 0.8,e solution which could [16]. be A as veryshown good in operatingFigure 2, it is knowncondition that, inwhen terms pH of ammonium = 9.5 the molebicarbonate fraction generation. of NH3 is about 0.8, which could be a very good operating condition in terms of ammonium+ bicarbonate− generation. NH4 + OH = NH3(aq) + H2O (1)   NH 4  OH  NH3 (aq)  H2O (1) NH3(aq) = NH3(q) (2) + − H2O = H + OH (3) (2) NH3(aq)  NH3(q)

  (3) H2O  H  OH

Crystals 2018, 8, 39 3 of 13 Crystals 2018, 8, 39 3 of 13

1.00

0.80

0.60

0.40 3 NH (mole fraction)NH 0.20

0.00 4 6 8 10 12 14 pH

FigureFigure 2. 2. DissociationDissociation of of ammonium ammonium hydroxide hydroxide varies with pH, re-plotted in here [[16].16].

Many scholars have studied the absorption of CO2 using an aqueous ammonia solution and Many scholars have studied the absorption of CO2 using an aqueous ammonia solution and have haveadopted adopted different different scrubbers scrubbers for their for their studies, studies, such su asch the as stirredthe stirred cell cell [4,17 [4,17],], sieve sieve plate plate column column [9], [9],falling falling film [film18], absorption[18], absorption column column [19] and [19] spray and tower spray [20 ]tower scrubbers. [20] Inscrubbers. literature, In the literature, suitability the of suitabilityan absorber of is an determined absorber is by determined the level of by the the mass level transfer of the coefficient. mass transfer However, coefficient. we can However, also use thewe can also use the absorption factor (φ) to make the comparison, which is the moles of CO2 that are absorption factor (ϕ) to make the comparison, which is the moles of CO2 that are able to be absorbed ableper moleto be perabsorbed unit volume per mole and per is defined,unit volume as follows: and is defined, as follows:

Fg E ϕ = FgE ϕ = (4)(4) FAAVB where FgFg (mol/s)(mol/s) representsrepresents the the molar molar flow flow rate rate ofthe of feedthe feed gas, Egas,is the E is absorption the absorption efficiency, efficiency,FA (mol/s) FA (mol/s)is the molar is the flow molar rate flow of therate solvent of the solvent and VB and(L)is V theB (L)is volume the volume of the absorber. of the absorber. Regarding the bubblebubble column,column, thethe absorptionabsorption factor factor is is 5 5 to to 10 10 times times that that of of the the packed packed column column of ofhigh-efficiency high-efficiency structured structured packing packing material, material, which which indicates indicates that underthat under the same the same conditions, conditions, in order in orderto obtain to obtain the same the absorption same absorption factor, the factor, volume the ofvolume the packed of the column packed of column high-efficiency of high-efficiency structured structuredpacking material packing should material be 5should to 10 timesbe 5 to that 10of times the bubblethat of column;the bubble and column; while the and volume while ofthe a volumeconventional of a packedconventional column, packed needs column, to be 130needs times to be that 130 of times the bubblethat of the column bubble to column have the to samehave theabsorption same absorption factor as the factor bubble as columnthe bubble [21 ].column The above [21]. results The above show thatresults the show bubble that column the bubble shows columnsuperior shows performance superior and performance many scholars and use many the scho bubblelars column use the as bubble the absorber column and as the object absorber of study and to objectthis end of [ 5study,22–24 ].to Tothis summarize, end [5,22–24]. using aqueousTo summ ammoniaarize, using as the aqueous absorbent ammonia requires highas the load absorbent and low requiresenergy for high regeneration. load and low In addition,energy for according regeneration to the. In absorption addition, according factor comparison, to the absorption the absorption factor comparison,factor of the bubblethe absorption column factor is higher of the than bubble other colu typesmn of is absorbers. higher than other types of absorbers. From literatureliterature survey, survey, capturing capturing CO 2COusing2 using aqueous aqueous ammonia ammonia has two has possible: two onepossible: is absorption one is absorptionwithout precipitation without precipitation and the other and is the absorption other is absorption with precipitation, with precipitation, depending depending on the pH on of the pHsolution. of the Insolution. addition, In precipitationaddition, precipitation of ABC in of the ABC bubble-column in the bubble-column scrubber is scrubber feasible is as feasible compared as comparedpacked column packed and column sieve tray. and However, sieve tray. the However, research regardingthe research to theregarding capture to of the CO 2captureusing aqueous of CO2 usingammonia aqueous in a bubble-columnammonia in a bubble-column scrubber is scare. scrubber This indicates is scare. thatThis no indicates technique that data, no technique including data,precipitation including rate precipitation of ABC, absorption rate of ABC, rate andabsorpti overallon rate mass-transfer and overall coefficient, mass-transfer can becoefficient, found in can the beliterature. found in Due the toliterature. this, a study Due of to capturing this, a study CO 2ofusing capturing aqueous CO ammonia2 using aqueous solution ammonia in a bubble-column solution in ascrubber bubble-column is required, scrubber including is requir absorptioned, including without absorption precipitation withou andt absorptionprecipitation with and precipitation. absorption withTherefore, precipitation. the purposes Therefore, of this the research purposes are of to this understand research theare phenomenato understand in the absorptionphenomena of in CO the2 absorptionin the bubble-column of CO2 in scrubber;the bubble-column to find the scrubbe effect ofr; process to find variables the effect on of the process absorption variables rate, overallon the absorptionmass-transfer rate, coefficient, overall mass-transfer precipitation coefficient, of ABC; and precipitation to search for of the ABC; better and process to search in thefor capturethe better of processCO2 using in the aqueous capture ammonia of CO2 using as absorbent. aqueous ammonia as absorbent.

2. Absorption Model and Solution Chemistry In a bubble-column scrubber, a gas mixture containing carbon dioxide (A) and nitrogen (B) flows into a bubble column from the bottom and continues through the distributor, thus, coming

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2. Absorption Model and Solution Chemistry Crystals 2018, 8, 39 4 of 13 In a bubble-column scrubber, a gas mixture containing carbon dioxide (A) and nitrogen (B) flows into a bubble column from the bottom and continues through the distributor, thus, coming into into continuous contact with the aqueous ammonia solution flowing into the column from the top. continuous contact with the aqueous ammonia solution flowing into the column from the top. The two The two streams come into contact within the column’s countercurrent simultaneously. The streams come into contact within the column’s countercurrent simultaneously. The diffusing of CO2 diffusing of CO2 gas inside a gas bubble in the solution can be described by a Two-film model, as gas inside a gas bubble in the solution can be described by a Two-film model, as shown in Figure3. shown in Figure 3. The CO2 gas at pressure P diffuses from the left side to the gas-liquid interface The CO gas at pressure P diffuses from the left side to the gas-liquid interface and enters into the and enters2 into the liquid side, where it is absorbed by the aqueous ammonia solution. At the liquid side, where it is absorbed by the aqueous ammonia solution. At the interface, the relationship interface, the relationship between the partial pressure of the CO2 and the dissolution of the CO2 in between the partial pressure of the CO2 and the dissolution of the CO2 in the liquid can be expressed the liquid can be expressed by Harry’s Law (PA = HCA). According to this model, the absorption rate by Harry’s Law (P = HC ). According to this model, the absorption rate and overall mass transfer and overall mass transferA A coefficient can be determined using measurable quantities [21]. coefficient can be determined using measurable quantities [21].

Figure 3. A two-ﬁlm model showing the absorption of CO2 in an aqueous ammonia solution. Figure 3. A two-film model showing the absorption of CO2 in an aqueous ammonia solution.

The solution’s chemistry for the NH3-CO2-H2O system is presented in Table1[ 25]. Ionization of The solution’s chemistry for the NH3-CO2-H2O system is presented in Table 1 [25]. Ionization of each component after the absorption of the CO2 into the aqueous ammonia solution can be expressed each component after the absorption of the CO2 into the aqueous ammonia solution can be expressed by six equations, as shown in Table1. Equations (6) and (7) show the carbonation of CO 2 dissolved in an by six equations, as shown in Table 1. Equations (6) and (7) show the carbonation of CO2 dissolved in aqueous solution. Equations (8)–(10) show that NH3 as a weak base will cause the chemical absorption an aqueous solution. Equations (8)–(10) show that NH3 as a weak base will cause the chemical of the weak acid CO2. In the absorbed solution, the various species include CO2 and bicarbonate, absorption of the weak acid CO2. In the absorbed solution, the various species include CO2 and carbonate,bicarbonate, carbamate carbonate, anions carbamate and ammonium anions and cation. ammonium In addition, cation. Table In addition,1 and Equation Table 1 (10)and includeEquation the formation of ammonia bicarbonate (ABC) and NH4HCO3(s), as this solid can precipitate in the (10) include the formation of ammonia bicarbonate (ABC) and NH4HCO3(s), as this solid can scrubberprecipitate depending in the scrubber on the operatingdepending conditions. on the operat However,ing conditions. the concentrations However, the of chemicalconcentrations species of can vary when the solution pH, CO2 concentration, temperature and ammonia concentration are chemical species can vary when the solution pH, CO2 concentration, temperature and ammonia changed.concentration Thus, howare changed. to control Thus, the process how to variables control becomesthe process significant variables in the becomes ammonia significant capture study.in the Theammonia absorption capture rate study. and overall The absorption mass transfer rate coefficientand overall can mass be determinedtransfer coefficient when CO can2 isbe captured determined by an aqueous ammonia solution [5]. Therefore, once the inlet and outlet concentrations of CO2 can be when CO2 is captured by an aqueous ammonia solution [5]. Therefore, once the inlet and outlet measured at a steady state; the absorption rate, Equation (11) and overall mass transfer coefficient, concentrations of CO2 can be measured at a steady state; the absorption rate, Equation (11) and Equationoverall mass (13), transfer can be evaluated,coefficient, as Eq shownuation in(13), Table can2 .be Alternatively, evaluated, as the shownKGa incan Table be estimated 2. Alternatively, using Equation (14) when the outlet CO2 concentration is near zero. In Equation (14), (CA − HCL) is the the KGa can be estimated using Equation (14) when the outlet CO2 concentration is nearav zero. In arithmetic mean between− the inlet and outlet. Equation (14), (CA HCL )av is the arithmetic mean between the inlet and outlet.

Table 1. Solution chemistry for NH3-CO2-H2O system [25]. ↔ + + − H 2O H OH (5) + ↔ + + − CO2 H 2O H HCO 3 (6) − ↔ + + 2− HCO3 H CO3 (7) + ↔ + + − NH 3 H 2O NH 4 OH (8) + − ↔ − + NH 3 HCO3 NH 2COO H 2O (9) + + − ↔ NH 4 HCO3 NH 4 HCO3 (s) (10)

Crystals 2018, 8, 39 5 of 13

Table 1. Solution chemistry for NH3-CO2-H2O system [25].

+ − H2O ↔ H + OH (5) + − CO2 + H2O ↔ H + HCO3 (6) − + 2− HCO3 ↔ H + CO3 (7) + − NH3 + H2O ↔ NH4 + OH (8) − − NH3 + HCO3 ↔ NH2COO + H2O (9) + − NH4 + HCO3 ↔ NH4 HCO3(s) (10)

Table 2. Absorption rate and overall mass-transfer coefﬁcient used in this work.

h 1−y y i R = FA1 1 − A1 A2 (11) A VL yA1 1−yA2

PA1 1 FA1 = Qg × RT × 60 (12) Q K a = g × ln CA1 (13) G VL CA2 RA KGa = (14) (CA −HCL)av MT Rp = τ (15) τ = VL (16) QL

3. Experimental Procedure The experimental device used is shown in Figure4. The inside diameter of the column is 5 cm and the gas distributor is a perforated plate designed with four holes per square centimeter and each hole is 1 mm in diameter. Initially, a known amount of aqueous ammonia solution (14%) is introduced into the column, into which a pH electrode is inserted. The solution is adjusted to the desired pH value by adding a known concentration of aqueous solution (28%). The experiment starts when the mixed N2–CO2 gas, in the range of 15–60% CO2 gas, begins bubbling through the column. The gas flow rates are in the range of 3 to 5 L/min. A gas heater and circulating cooling water tank are used to control the gas and solution temperatures. The mixed gas is delivered to the heater, which maintains the inlet gas temperature at 55 ◦C. The operating pH value is in the range of 9.5–11.5. The temperature in the bubble column is maintained at 25–60 ◦C by a water cooling system (Deng Yng, D-620, New Taipei, Taiwan). The operating time is 5–7 h, depending on the operating conditions. During the operation, the pH of the solution is decreased due to absorption in the scrubber. In order to hold the pH value of the solution at a desired value during operation, an aqueous ammonia solution is introduced into the column through the action of a pH controller (Suntex, PC-310, New Taipei, Taiwan). The overflow solution automatically flows out to the reservoir. The volume of added solution is recorded every ten minutes during the operation. A digital gauge pressure meter measures the inlet gas pressure. A CO2 gas meter (Guardian Plus, D600, Hartford, USA) is used to measure the concentration of the CO2 gas at the outlet of the bubble column (y2). When the concentration of CO2 gas changes to a steady state, the solutions are withdrawn using a syringe to measure the total dissolved CO2 in the solution. At the end of the operation, the solution is poured into a vessel and the volume of the solution (VL) is measured. In addition, the slurry solution is separated using a filter. The crystal size distribution is measured by a particle size analyzer (Galai, CIS-50, Or Akiva, Israel). The crystals obtained are characterized by X-ray (Rigaku, D/MAX-2200/PC, Tokyo, Japan), FESEM (Jeol, JSM-6500F, Tokyo, Japan) and EA (Thermo, Flash EA1112, Cambridge, UK ), respectively. A total of twenty runs are carried out in this work. The operating conditions and physical properties of ABC are, as shown in Table3. Crystals 2018, 8, 39 6 of 13 Crystals 2018, 8, 39 6 of 13

FigureFigure 4. Capture 4. Capture of CO of2 using CO2 usinga lab-scale a lab-scale bubble-column bubble-column scrubber. scrubber. 1. CO2 gas 1. COtank;2 gas 2. N tank;2 gas 2.tank; N2 gas 3. Gas-flowtank; 3. meter; Gas-ﬂow 4. Gas-flow meter; 4.meter; Gas-ﬂow 5. Bubble-column meter; 5. Bubble-column scrubber; 6. pH scrubber; control; 6. 7. pH Digital control; Pressure 7. Digital Gauge;Pressure 8. Tubing Gauge; pump; 8. Tubing 9. Solvent; pump; 10. 9. CO Solvent;2 meter; 10. 11. CO Cooling2 meter; machine; 11. Cooling 12. machine;Reservoir; 12. Digital Reservoir; thermometer.Digital thermometer.

TableTable 3. Operating 3. Operating conditions conditions and physic and physicalal properties properties of ABC of ABCcrystals. crystals.

OperatingOperating Condition Condition Gas-flow rate (L/min) 3–5 Gas-ﬂow rate (L/min) 3–5 Concentration of CO2 15–60% Concentration of CO2 15–60% pH pH9.5 9.5 OperatingOperating time (h) time (h) 5–7 5–7 ◦ Gas inlet temperatureGas inlet temperature (°C) ( C) 55 55 Working temperature in the scrubber (◦C) 25–60 Working temperature in the scrubber (°C) 25–60 Physical Property of ABC Physical Property of ABC 3 Density (kg/mDensity3) (kg/m ) 1.581.58 Molecular weight 79.06 Molecular weight ◦ 79.06 Solubility (g/L-H2O) (20 ( C)) 220 SolubilityDecomposition (g/L-H2O) (20 temperature (°C)) (◦C) 35–60220 Decomposition temperature (°C) 35–60 4. Results and Discussion 4. Results and Discussion The results listed in Table4 include the RA, KGa and E. It is found that, the removal The results listed in Table 4 include the RA, KGa and E. It is found that, the removal efficiency E −4 efﬁciency E obtained here is in the range of 10.9–100%; the RA was in the range of 3.2122 × 10 – obtained here is− 4in the range of 10.9–100%; the RA was in the range of 3.2122 × 10−4–10.999 × 10−4 10.999 × 10 mol/(s·L) and the KGa is in the range of 0.0136–0.3302 L/s. In addition, the ABC crystals mol/(s·L)are formed and the at aKG pHa is of in 9.5, the such range as of Nos. 0.0136–0.3302 3, 4, 6, 7, 8, 10,L/s. 11 In and addition, 12. For the other ABC pH crystals values, noare ABC formed crystals at a pHcould of be9.5, obtained. such as Nos. 3, 4, 6, 7, 8, 10, 11 and 12. For other pH values, no ABC crystals could be obtained. Table 4. Operating conditions and experimental data obtained in this work. Table 4. Operating conditions and experimental data obtained in this work. Q Q R × 104 K a No. pH g y (%) y (%) T (◦C) L γ (-) A G E (%) (L/min) 1 2 (mL/min) · KGa(1/s) No. pH Qg (L/min) y1 (%) y2 (%) T (°C) QL (mL/min) γ (-) RA × 104 (mol/(s(mol/(s·L))L)) E (%) (1/s) 1 9.5 3 15 7.95 25 3.66 0.58 3.9178 0.0794 47.0 1 9.5 3 15 7.95 25 3.66 0.58 3.9178 0.0794 47.0 2 9.5 3 30 17.7 25 3.30 1.33 5.1831 0.0447 41.3 2 39.5 9.5 3 3 30 50 17.7 38.8 25 25 3.30 4.90 1.33 1.50 5.1831 5.7170 0.0447 0.0194 41.3 22.4 3 49.5 9.5 3 3 50 60 38.8 53.5 25 25 4.90 4.96 1.50 1.75 5.7170 7.1506 0.0194 0.0143 22.4 10.9 4 59.5 9.5 3 4 60 15 53.5 8.7 25 25 4.96 2.22 1.75 1.33 7.1506 3.5858 0.0143 0.0692 10.9 42.1 5 69.5 9.5 4 4 15 30 8.7 22 25 25 2.22 3.51 1.33 1.71 3.5858 5.0876 0.0692 0.0327 42.1 26.7 6 79.5 9.5 4 4 30 50 22 39.3 25 25 3.51 4.73 1.71 2.05 5.0876 7.0207 0.0327 0.0136 26.7 21.4 7 89.5 9.5 4 4 50 60 39.3 51.7 25 25 4.73 6.63 2.05 1.77 7.0207 9.3765 0.0136 0.0199 21.4 13.8 8 99.5 9.5 4 5 60 15 51.7 7.56 25 25 6.63 3.30 1.77 1.11 9.3765 5.4895 0.0199 0.1142 13.8 49.6 9 109.5 9.5 5 5 15 30 7.56 22.9 25 25 3.30 0.60 1.11 12.24 5.4895 7.0572 0.1142 0.0506 49.6 23.6 10 9.5 5 30 22.9 25 0.60 12.24 7.0572 0.0506 23.6 11 9.5 5 50 39.6 25 7.28 1.69 9.0338 0.0299 20.8

Crystals 2018, 8, 39 7 of 13

Table 4. Cont.

Q Q R × 104 K a CrystalsNo. 2018, pH 8, 39 g y (%) y (%) T (◦C) L γ (-) A G E (%)7 of 13 (L/min) 1 2 (mL/min) (mol/(s·L)) (1/s) 1211 9.5 9.5 5 5 60 50 50.6 39.6 25 25 7.10 7.28 2.07 1.69 10.999 9.0338 0.0299 0.0241 20.815.8 1312 10.0 9.5 3 5 15 60 10.0 50.6 25 25 2.96 7.10 0.75 2.07 3.2122 10.999 0.0241 0.0608 15.833.3 1413 10.510.0 3 3 15 15 5.3 10.0 25 25 8.89 2.96 0.24 0.75 4.1095 3.2122 0.0608 0.1020 33.364.6 14 10.5 3 15 5.3 25 8.89 0.24 4.1095 0.1020 64.6 15 11.0 3 15 3.7 25 9.73 0.22 4.8295 0.1573 75.3 15 11.0 3 15 3.7 25 9.73 0.22 4.8295 0.1573 75.3 16 11.5 3 15 0 25 37.63 0.058 6.3943 0.2083 100 16 11.5 3 15 0 25 37.63 0.058 6.3943 0.2083 100 1717 10.010.0 4 4 15 15 5.5 5.5 25 25 7.08 7.08 0.41 0.41 7.4126 7.4126 0.1808 0.1808 63.3 1818 10.010.0 4 4 15 15 0 0 40 40 4.14 4.14 0.67 0.67 5.9145 5.9145 0.1963 0.1963 96.7 1919 10.010.0 4 4 15 15 0 0 50 50 8.71 8.71 0.31 0.31 6.7421 6.7421 0.2381 0.2381 100 2020 10.010.0 4 4 15 15 0 0 60 60 46.13 46.13 0.053 0.053 9.0425 9.0425 0.3302 0.3302 100

4.1.4.1. Effect Effect of of Process Process Variables Variables on on E E

Figure 5a shows the effect of the CO2 inlet concentrations at various gas-flow rates on the Figure5a shows the effect of the CO 2 inlet concentrations at various gas-flow rates on the removal removal efficiency of CO2. It is found that E decreases with the increase in CO2 gas concentration, efficiency of CO2. It is found that E decreases with the increase in CO2 gas concentration, while the whileeffect ofthe the effect gas of flow the rate gas was flow not rate obvious. was not On obvious. the other On hand, the other the effects hand, of the the effects pH level of the and pH gas level inlet andtemperature gas inlet oftemperatureE are significant, of E are asshown significant, in Figure as shown5b,c. The in resultsFigure show5b,c. thatThe resultsE increases show with that the E increasesincrease inwith the the pH levelincrease and in gas the flow pH ratelevel at and the gasgas inlet.flow Therate analysisat the gas of inlet. the gas-liquid The analysis molar of flow the gas-liquidrate ratio, γmolar, as listed flow in rate Table ratio,4, finds γ, as that listed the in removal Table efficiency4, finds that is higherthe removal than 60% efficiency when γisis higher lower thanthan 60% 0.5, indicatingwhen γ is thelower influence than 0.5, of indicatingγ on E. the influence of γ on E.

(a) pH = 9.5 (b) T = 25 °C (c) pH = 10.0

Figure 5. Effects of process parameters on the removal efficiency of CO2: (a) effect of y1 on E at pH = Figure 5. Effects of process parameters on the removal efﬁciency of CO2:(a) effect of y1 on E at pH = 9.5; 9.5; (b) effect of pH on E at T = 25 °C; (c) effect of T on E at pH = 10.0. (b) effect of pH on E at T = 25 ◦C; (c) effect of T on E at pH = 10.0.

4.2. Effect of Process Variables on the RA and KGa 4.2. Effect of Process Variables on the RA and KGa A plot of RA versus y1 at various gas-flow rate absorption rates is shown in Figure 6a. The A plot of RA versus y1 at various gas-flow rate absorption rates is shown in Figure6a. The results results show that RA increases with an increase in y1, while RA is higher at higher gas flow rates. In show that RA increases with an increase in y1, while RA is higher at higher gas flow rates. In addition, addition, the results find that the RA increases with the pH and gas input concentrations, as shown the results find that the RA increases with the pH and gas input concentrations, as shown in Figure6b,c. in Figure 6b,c. Alternatively, a wetted-wall column for CO2 absorption test is reported in literatures Alternatively, a wetted-wall column for CO absorption test is reported in literatures by different 2 −4 −3 by different scholars, who obtain RA in the range of− 42.15 × 10 –4.48−3 × 10 mol/(L·s) [13] and 2.05− 4× scholars, who obtain RA in the range of 2.15 × 10 –4.48 × 10 mol/(L·s) [13] and 2.05 × 10 – 10−4–5.53 × 10−3 mol/(L·s) [26], respectively. Some of these values are higher than that reported in 5.53 × 10−3 mol/(L·s) [26], respectively. Some of these values are higher than that reported in here, −4 −3 −3 −3 here, 3.21 −×4 10 –1.10 × −103 mol/(L·s). In addition, the higher RA (2.07 × 10 −–7.243 × 10 mol/(L·s))−3 can 3.21 × 10 –1.10 × 10 mol/(L·s). In addition, the higher RA (2.07 × 10 –7.24 × 10 mol/(L·s)) be obtained when aqueous hydrazine is used as a solvent for CO2 capture [27]. can be obtained when aqueous hydrazine is used as a solvent for CO2 capture [27]. The effects of y1, Qg, pH and T on the overall mass transfer are also investigated. Figure 7a The effects of y1, Qg, pH and T on the overall mass transfer are also investigated. Figure7a shows that the effect of y1 on KGa at various Qg is obvious. It also shows that KGa decreases with an shows that the effect of y1 on KGa at various Qg is obvious. It also shows that KGa decreases with increase in y1 indicating a higher mass resistance at higher gas concentrations. In addition, KGa an increase in y1 indicating a higher mass resistance at higher gas concentrations. In addition, KGa increases with the increase in Qg, which indicates that increasing the gas-liquid contact surface area increases with the increase in Q , which indicates that increasing the gas-liquid contact surface area and turbulence mixing results ing a more effective contact. The effects of pH levels and gas inlet and turbulence mixing results in a more effective contact. The effects of pH levels and gas inlet temperatures on KGa are expressed in Figure 7b,c. The effects of the pH level and temperature on KGa are obvious, indicating the significance of pH and temperature. However, the KGa, as obtained here, is in the range of 0.0136–0.3302 1/s, which is comparable with other solvents, such as MEA solvent which is in the range of 0.0342–0.881 1/s [28] and NaOH/BaCl2/H2O, which is in the ranges of 0.0651–0.3396 1/s [5] and 0.015–0.14 1/s for a stirred-tank scrubber using an aqueous ammonia solution as the scrubbing solution [4]. On the other hand, from data reported in literatures

Crystals 2018, 8, 39 8 of 13

temperatures on KGa are expressed in Figure7b,c. The effects of the pH level and temperature on KGa are obvious, indicating the signiﬁcance of pH and temperature. However, the KGa, as obtained here, is in the range of 0.0136–0.3302 1/s, which is comparable with other solvents, such as MEA CrystalsCrystals 2018 2018, ,8 8, ,39 39 88 of of 13 13 solvent which is in the range of 0.0342–0.881 1/s [28] and NaOH/BaCl2/H2O, which is in the ranges of 0.0651–0.3396 1/s [5] and 0.015–0.14 1/s for a stirred-tank scrubber using an aqueous ammonia [8,13,26,27],[8,13,26,27], thethe KKGGaa cancan bebe recalculatedrecalculated forfor comparison.comparison. TheThe recalculatedrecalculated valuesvalues ofof wetted-wallwetted-wall solution as the scrubbing solution [4]. On the other hand, from data reported in literatures [8,13,26,27], columncolumn teststests areare 0.03469–0.26580.03469–0.2658 1/s1/s [13][13] andand 0.02551–0.43360.02551–0.4336 1/s1/s [26],[26], respectively;respectively; whilewhile thethe the K a can be recalculated for comparison. The recalculated values of wetted-wall column tests are recalculatedrecalculatedG valuesvalues forfor packedpacked columncolumn areare inin thethe rarangenge ofof 0.486–2.430.486–2.43 1/s1/s [8][8] whenwhen aa specificspecific areaarea ofof 0.03469–0.2658 1/s2 [133 ] and 0.02551–0.4336 1/s [26], respectively; while the recalculated values for packings,packings, 500500 mm2/m/m3, , isis available.available. Moreover,Moreover, thethe KKGGaa isis 2.212.21 1/s1/s whenwhen usingusing aqueousaqueous hydrazinehydrazine packed column are in the range of 0.486–2.43 1/s [8] when a speciﬁc area of packings, 500 m2/m3, solventsolvent [27]. [27]. is available. Moreover, the KGa is 2.21 1/s when using aqueous hydrazine solvent [27].

1616 Q =3 L/min Qg g =3 L/min Q =4L/min Qg g =4L/min Q =5L/min 1212 Qg g =5L/min

88 -4 -4 A A R (10 (10 mol/L-s) R R (10 (10 mol/L-s) R 44

00 00 20406080 20406080

y1y (%) (%) 1

((aa)) pH pH = = 9.5 9.5 ( (bb)) T T = = 25 25 °C °C ( (cc)) pH pH = = 10.0 10.0

FigureFigure 6.6. EffectsEffects ofof processprocess variablesvariables onon thethe absorptionabsorption rate:rate: ((aa)) effecteffect ofof yy11 onon RRAA atat pHpH == 9.5;9.5; Figure 6. Effects of process variables on the absorption rate: (a) effect of y1 on RA at pH = 9.5; (b) effect ((bb)) effect effect of of pH pH on on R RAA ◦at at T T = = 25 25 °C; °C; ( (cc)) effect effect of of T T on on R RAA at at pH pH = = 10.0. 10.0. of pH on RA at T = 25 C; (c) effect of T on RA at pH = 10.0.

0.160.16 Q =3L/min Qg g =3L/min Q = 4L/min Qg g = 4L/min Q =5L/min 0.120.12 Qg g =5L/min -1 -1 0.080.08 G G K a(s K a(s ) K a(s K a(s )

0.040.04

0.000.00 00 20406080 20406080 y (%) 1y (%) 1 ((aa)) pH pH = = 9.5 9.5 ( (bb)) T T = = 25 25 °C °C ((cc)) pH pH = = 10.0 10.0

FigureFigure 7.7. EffectsEffects ofof processprocess variablesvariables onon KKGGaa showingshowing thethe significantsignificant parameters:parameters: ((aa)) effecteffect ofof yy11 onon Figure 7. Effects of process variables on KGa showing the signiﬁcant parameters: (a) effect of y1 on KGa KKGGaa at at pH pH = = 9.5; 9.5; ( (bb)) effect effect of of pH pH on on K KGGaa at at T T = =◦ 25 25 °C; °C; ( (cc)) effect effect of of T T on on K KGGaa at at pH pH = = 10.0. 10.0. at pH = 9.5; (b) effect of pH on KGa at T = 25 C; (c) effect of T on KGa at pH = 10.0.

4.3.4.3. Characterization Characterization of of ABC ABC Crystals Crystals 4.3. Characterization of ABC Crystals WhileWhile thethe formationformation ofof ABCABC isis foundfound here,here, itit isis notnot foundfound underunder allall conditions,conditions, suchsuch asas Nos.Nos. 3,3, While the formation of ABC is found here, it is not found under all conditions, such as Nos. 3, 4, 6, 4,4, 6,6, 7,7, 8,8, 10,10, 1111 andand 1212 atat aa pHpH levellevel ofof 9.5.9.5. FigureFigure 88 forfor No.No. 1212 showsshows thethe crystalcrystal sizesize distributiondistribution 7, 8, 10, 11 and 12 at a pH level of 9.5. Figure8 for No. 12 shows the crystal size distribution measured measuredmeasured usingusing aa CIS-1CIS-1 particleparticle sizesize analyzer.analyzer. TheThe majormajor distributiondistribution isis inin thethe sizesize rangerange ofof 30–10030–100 using a CIS-1 particle size analyzer. The major distribution is in the size range of 30–100 µm. On the μμm.m. OnOn thethe otherother hand,hand, whenwhen thethe crystalscrystals areare examinexamineded byby SEM,SEM, asas alsoalso shownshown inin FigureFigure 8,8, smallsmall other hand, when the crystals are examined by SEM, as also shown in Figure8, small irregular crystals irregularirregular crystals crystals are are observed observed indicating indicating the the fracture fracture property property of of ABC ABC crystals. crystals. are observed indicating the fracture property of ABC crystals.

Crystals 2018, 8, 39 9 of 13 CrystalsCrystals 20182018, 8,, 839, 39 9 9of of 13 13

Figure 8. A plot of wt % vs. L showing the crystal size distribution of ABC (No. 12). FigureFigure 8. 8. AA plot plot of of wt wt % % vs. vs. L Lshowing showing the the crystal crystal size size distribution distribution of of ABC ABC (No. (No. 12). 12). Figure 9 shows the XRD spectra for the three runs, as compared with the standard ABC Figure 9 shows the XRD spectra for the three runs, as compared with the standard ABC crystalsFigure bought9 shows at the the market. XRD spectraThe major for thepeaks three for runs, a standard as compared sample with are the29.7 standard (131), 16.6 ABC (020), crystals 21.9 crystals bought at the market. The major peaks for a standard sample are 29.7 (131), 16.6 (020), 21.9 (012)bought and at 24.5 the market. (200), which The major are peaksthe major for a standardpeaks for sample ABC arecrystals, 29.7(131), as reported 16.6 (020), by 21.9 Meng (012) [29]. and (012) and 24.5 (200), which are the major peaks for ABC crystals, as reported by Meng [29]. However,24.5 (200), the which peaks are theof the major three peaks samples for ABC exhibit crystals, the assame reported peaks by as Meng the [standard29]. However, sample, the thus, peaks However, the peaks of the three samples exhibit the same peaks as the standard sample, thus, confirmingof the three the samples formation exhibit of theABC same crystals peaks in as this the work. standard Alternatively, sample, thus, EA confirming analyses according the formation to N, of confirming the formation of ABC crystals in this work. Alternatively, EA analyses according to N, H,ABC O and crystals C for in ABC this work. crystals Alternatively, are shown EAin Figure analyses 10 according. Analysisto finds N, H, that O andall elements C for ABC are crystals close areto H, O and C for ABC crystals are shown in Figure 10. Analysis finds that all elements are close to thatshown of a in standard Figure 10 sample,. Analysis indicating finds that that all the elements precipitates are close obtained to that ofhere a standard are confirmed sample, to indicating be ABC that of a standard sample, indicating that the precipitates obtained here are confirmed to be ABC crystals.that the precipitates obtained here are confirmed to be ABC crystals. crystals.

Standard sample Standard sample

No.4 No.4

No.8 No.8 No.12 No.12

FigureFigure 9. 9. XRDXRD spectra spectra for for ABC ABC crystals. crystals. Figure 9. XRD spectra for ABC crystals.

Crystals 2018, 8, 39 10 of 13 CrystalsCrystals 2018 2018, 8, 839, 39 1010 of of13 13

FigureFigureFigure 10. 10. 10. EAEA EA analysis analysis analysis for for for ABC ABC ABC crystals crystals crystals co compared comparedmpared with with with a a standarda standard standard sample. sample. sample.

Finally, the precipitation rate for ABC crystals could be evaluated using Equations (15) and Finally,Finally, the the precipitation precipitation rate rate for for ABC ABC crystals crystals could could be evaluated be evaluated using using Equations Equations (15) and(15) (16) and (16)(16) when when the the slurry slurry density density (M (MT) Tand) and mean mean residence residence time time (T ()T are) are obtained. obtained. Figure Figure 11 11 shows shows that that R pR p when the slurry density (MT) and mean residence time (T) are obtained. Figure 11 shows that Rp is is isincreased increased with with an an increase increase in in y1 y at1 at various various rates rates of of Q gQ. gThe. The results results show show that, that, the the gas gas concentration concentration increased with an increase in y1 at various rates of Qg. The results show that, the gas concentration hashas significant significant effect effect on on the the R pR valuep value as as compared compared with with Q gQ. gAlternatively,. Alternatively, analysis analysis of of γ γfinds finds that that the the has signiﬁcant effect on the Rp value as compared with Qg. Alternatively, analysis of γ ﬁnds that the precipitates of ABC can be obtained when γ is higher than 1.5; however, no precipitates are precipitatesprecipitates of of ABC ABC can can be obtainedbe obtained when whenγ is higher γ is higher than 1.5; than however, 1.5; however, no precipitates no precipitates are observed are observed when γ is lower than 1.5, regardless of the pH level. whenobservedγ is lower when than γ is lower 1.5, regardless than 1.5, ofregardless the pH level. of the pH level.

FigureFigureFigure 11.11. 11. A A plotplot plot ofof of RRp pR vs.p vs. y1 yatat1 atvarious various various values values values of of of QQ gQg showingshowingg showing the the the influence inﬂuence influence of of of yy1 1 yand1and and QQ gQ.g .g.

4.4.4.4.4.4. Strategy Strategy Strategy of of ofCO CO CO22 Capture2Capture Capture Using Using Using Aqueous Aqueous Aqueous Ammonia Ammonia Ammonia Solution Solution Solution

TheTheThe K GKaaG aobtained obtained herehere herecan can can be be usedbe used used to to calculate to calculate calculate the the scrubber the scrubber scrubber size size [16 size]. [16]. Generally [16]. Generally Generally speaking, speaking, speaking, a larger a a largerKlargerGa means K GKaG ameans that means a smallerthat that a asmaller scrubber smaller scrubber sizescrubber is required, size size is isrequired, while required, a higher while whileE ashows ahigher higher an E effective Eshows shows reductionan an effective effective in reductionCOreduction2 emissions. in in CO CO2 Thus,emissions.2 emissions. obtaining Thus, Thus, a obtaining larger obtainingKG a andlargera largerE isK GK theaG anda and major E Eis is focusthe the major major in process focus focus in design. in process process Figure design. design. 12 FigureshowsFigure 12 that 12 shows KshowsGa increased that that K GKaG increaseda with increased an increase with with an an in increase Eincrease, showing in in E , aE showing, linear showing relationship. a alinear linear relationship. relationship. In order to In clarify, In order order the to to clarify,targetclarify, ofthe Kthe Gtargeta targetand ofE of areK GKa setG anda and at E 0.2 Eare L/sare set set and at at 0.2 80%, 0.2 L/s L/s respectively. and and 80%, 80%, respectively. Therefore,respectively. the Therefore, Therefore, ﬁgure can the the be figure dividedfigure can can into be be dividedfourdivided areas, into into i.e., four four A, areas, B,areas, C i.e., and i.e., A, D, A, B, in B, C which Cand and D, CD, in shows in which which a C higher Cshows showsK aG highera andhigherE K. GK OnaG anda theand E contrary,.E On. On the the contrary, B contrary, shows B a B showslowershows KaG loweraa lowerand KE G,Ka especiallyG anda and E ,E especially, especially in the case in in ofthe the the case case formation of of the the formation offormation ABC. Excluding of of ABC. ABC. Excluding theExcluding formation the the formation of formation ABC data, of of ABCKABCGa/ Edata, data,can K only GKaG/Ea be/ Ecan correlatedcan only only be be withcorrelated correlated the pH with ofwith the the the solution pH pH of of the and the solutionγ solution, as parameters and and γ ,γ as, Tas ,parameters theparameters concentration T ,T the, the concentrationandconcentration ﬂow-rate and are and includedflow-rate flow-rate in are γare. included The included regression in in γ .γ The. resultsThe regression regression are shown, results results as are follows: are shown, shown, as as follows: follows:

KKKG aaa −0.−2060.206 GG ===00.8826.08826.8826γ −γ0.206exp(expexp((−0−0.1681.01681.1681pHpHpH) ) ) (17)(17)(17) EEE

Crystals 20182018, ,88, ,39 39 11 of 13 Crystals 2018, 8, 39 11 of 13 The regression error in Equation (17) is 12.68%. This equation can be used to predict KGa when E, ThepH and regression γ are given. errorerror inIninEquation thisEquation way, (17) the(17) is sizeis 12.68%. 12.68%. of the This scrubbThis equation equationer can canbe can estimated. be be used used to predictInto addition,predictKG aKwhen Gina whenorderE, pH toE, obtainpHand andγ arethe γ given.areABC given. at In a thishigher In way,this E way, thevalue, size the a of sizetwo-scrubber the of scrubber the scrubb in can serieser be can estimated. is be required. estimated. In Figure addition, In addition, 13 inshows order in a toorderpossible obtain to E obtainprocessthe ABC the for at ABC CO a higher2 atcapture a highervalue, using E avalue, an two-scrubber aqueous a two-scrubber ammonia in series insolution is series required. isin required.a bubble Figure 13column.Figure shows 13 a shows possible a possible process processfor CO2 forcapture CO2 capture using an using aqueous an aqueous ammonia ammonia solution solution in a bubble in a column.bubble column. 0.50 0.50 Without formation of ABC WithWithout formation formation of ABC of ABC 0.40 With formation of ABC 0.40

0.30 -1 0.30 -1

G A C

K a(s ) a(s K 0.20 G A C

K a(s ) a(s K 0.20 B D B D 0.10 0.10

0.00 0 20406080100 0.00 0 20406080100E(%) E(%) Figure 12. A plot of KGa vs. E showing the performance of the scrubbing process. Figure 12. A plot of KGa vs. E showing the performance of the scrubbing process. Figure 12. A plot of KGa vs. E showing the performance of the scrubbing process.

Figure 13.13. AA combination combination of of the the crystallization crystallization and and non-crystallization non-crystallization of ABC of inABC CO 2incapture CO2 capture process Figureprocessusing an 13.using aqueous A ancombination aqueous ammonia ammonia solution.of the crystallization solution. and non-crystallization of ABC in CO2 capture process using an aqueous ammonia solution. 5. Conclusions Conclusions 5. Conclusions A bubble-column scrubber wa wass successfully used used to explore the capture of CO2 and the precipitationA bubble-column ofof ABC ABC crystals. crystals.scrubber ABC ABCwa crystalss successfullycrystals can becan generated usedbe generatedto atexplore a pH at levelthe a ofcapturepH 9.5 level in theof of concentrationCO 9.52 and in the concentrationprecipitation ofrange ABC of 30–60%crystals. andABC γ ≥crystals 1.5. The can removal be generated efficiency atcan a bepH controlled level of to9.5 a desiredin the concentration range of 30–60% and γ ≥ 1.5. The removal efficiency can be controlled to a desired

Crystals 2018, 8, 39 12 of 13 range of 30–60% and γ ≥ 1.5. The removal efficiency can be controlled to a desired level when the pH of the solution and inlet gas temperature were effectively adjusted. The absorption rates and overall mass-transfer coefficient obtained were comparable with other solvents, resulting in positive development for the capture of CO2 using an aqueous ammonia solution. Data showed that KGa/E can be correlated with pH and γ to obtain an empirical equation for KGa/E. Finally, reducing the emission of CO2 from flue gas using an aqueous ammonia solution is found to be flexible both with and without precipitation depending on the requirement of CO2 capture technology.

Acknowledgments: The authors acknowledge the financial support of the MOST in Taiwan ROC (MOST-106-2221-E-262-015). Author Contributions: Pao Chi Chen conceived and designed the experiments and wrote the paper, while Shun Chao Yu performed the experiments and analyzed the data. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Han, K.; Ahn, C.K.; Lee, M.S. Performance of an ammonia-based CO2 capture pilot facility in iron and steel industry. Int. J. Greenh. Gas Control 2014, 27, 239–246. [CrossRef] 2. Leung, D.Y.C.; Caramanna, C.; Maroto-Valer, M.M. An overview of current status of carbon dioxide capture and storage technologies. Renew. Sustain. Energy Rev. 2014, 39, 426–443. [CrossRef]

3. Yeh, A.C.; Bai, H. Comparison of ammonia and monoethanolamine solvents to reduce CO2 greenhouse gas emissions. Sci. Total Environ. 1999, 228, 121–133. [CrossRef] 4. Chen, P.C.; Lin, L.C. Capture of carbon dioxide using aqueous ammonia in a lab-scale stirred-tank scrubber. Adv. Mater. Res. 2014, 955–959, 1927–1934. [CrossRef]

5. Chen, P.C.; Shi, W.; Du, R.; Chen, V. Scrubbing of CO2 greenhouse gases, accompanied by precipitation in a continuous bubble-column scrubber. Ind. Eng. Chem. Res. 2008, 47, 6336–6343. [CrossRef]

6. Borhani, T.N.G.; Azarpour, A.; Akbari, V.; Wan Alwi, S.R.; Manan, Z.A. CO2 capture with potassium carbonate solutions: A state-of-the-art review. Int. J. Greenh. Gas Control 2015, 41, 142–162. [CrossRef]

7. Bai, H.; Yeh, A.C. Removal of CO2 greenhouse gas by ammonia scrubbing. Ind. Eng. Chem. Res. 1997, 36, 2490–2493. [CrossRef]

8. Dave, N.; Do, T.; Puxty, G.; Rowland, R.; Feron, P.H.M.; Attalla, M.I. CO2 capture by aqueous amines and aqueous ammonia—A Comparison. Energy Procedia 2009, 1, 949–954. [CrossRef]

9. Diao, Y.F.; Zheng, X.Y.; He, B.S.; Chen, C.H.; Xu, X.C. Experimental study on capuring CO2 greenhouse gas by ammonia scrubbing. Energy Convers. Manag. 2004, 45, 2283–2296. [CrossRef] 10. Huang, H.; Chang, S.G. Method to regenerate ammonia for the capture of carbon dioxide. Energy Fuels 2002, 16, 904–910. [CrossRef]

11. Darde, V.; Thomsen, K.; van Well, W.J.M.; Stenby, E.H. Chilled ammonia process for CO2 capture. Energy Procedia 2009, 1, 1035–1042. [CrossRef]

12. Resnik, K.P.; Yeh, J.T.; Pennline, H.W. Aqua ammonia process for simultaneous removal of CO2, SO2 and NOx. Int. J. Environ. Technonl. Manag. 2004, 4, 89–104. [CrossRef] 13. Liu, J.; Wang, S.; Zhao, B.; Tong, H.; Chen, C. Absorption of carbon dioxide in aqueous ammonia. Energy Procedia 2009, 1, 933–940. [CrossRef]

14. Yeh, J.T.; Resnik, K.P.; Rygle, K.; Pennline, H.W. Semi-batch absorption and regeneration studies for CO2 capture by aqueous ammonia. Fuel Process. Technol. 2005, 86, 1533–1546. [CrossRef] 15. Chen, P.C.; Liu, S.M.; Jang, C.J.; Hwang, R.C.; Yang, Y.L.; Lee, J.S.; Jang, J.S. Interpretation of gas-liquid reactive crystallization data using a size-independent agglomeration model. J. Cryst. Growth 2003, 257, 333–343. [CrossRef] 16. Morel, F.M.M. Principles of Aqueous Chemistry; John Wiley & Sons: New York, NY, USA, 1983; Chapter 4; pp. 127–177. ISBN 0-471-08683-5.

17. Shen, J.F.; Yang, Y.M.; Ma, J.R. Promotion mechanism for CO2 absorption into partially carbonated ammonia solutions. J. Chem. Eng. Jpn. 1999, 32, 378–381. [CrossRef] 18. Xiao, J.; Li, C.C.; Li, M.H. Kinetics of absorption of carbon dioxide into aqueous solutions of 2-amino-2-methyl-1-propanol + monoethanolamin. Chem. Eng. Sci. 2000, 55, 161–175. [CrossRef] Crystals 2018, 8, 39 13 of 13

19. Meng, L.; Burris, S.; Bui, H.; Pan, W.P. Development of an analytical method for distinguishing ammonia

bicarbonate form the products of an aqueous ammonia CO2 scrubber. Anal. Chem. 2005, 77, 5947–5952. [CrossRef][PubMed]

20. Ma, S.; Zang, B.; Song, H.; Chen, G.; Yang, J. Research on mass transfer of CO2 absorption using ammonia solution in spray tower. Int. J. Heat Mass Transf. 2013, 67, 696–703. [CrossRef] 21. Chen, P.C. Absorption of Carbon Dioxide in a Bubble Column Scrubber. Greenhouse Gases; Liu, G., Ed.; InTech: Rijeka, Croatia, 2012; Chapter 5; pp. 95–116. 22. Petrov, P.; Ewert, G.; Rohm, H.J. Chemisorptive removal of carbon dioxide from process streams using a reactive bubble column with simultaneous production of usable materials. Chem. Eng. Technol. 2006, 29, 1084–1089. [CrossRef] 23. Sánchez, O.; Michaud, S.; Escudié, R.; Delgenès, J.-P.; Bernet, N. Liquid mixing and gas–liquid mass transfer in a three-phase inverse turbulent bed reactor. Chem. Eng. J. 2005, 114, 1–7. [CrossRef] 24. Zhao, B.; Su, Y.; Peng, Y.C. Effect of reactor geometry on aqueous ammonia-based carbon dioxide capture in bubble column reactors. Int. J. Greenh. Gas Control 2013, 17, 481–487. [CrossRef]

25. Mathias, P.M.; Reddy, S.; Connell, J.P.O. Quantitative evaluation of the aqueous-ammonia process for CO2 capture using fundamental data and thermodynamic analysis. Energy Procedia 2009, 1, 1227–1234. [CrossRef]

26. Puxty, G.; Rowland, R.; Attalla, M. Comparison of the rate of CO2 absorption into ammonia and manoethanolamine. Chem. Eng. Sci. 2010, 65, 915–922. [CrossRef] 27. Lee, K.H.; Lee, B.; Lee, J.H.; You, J.K.; Park, K.T.; Baek, I.H.; Hur, N.H. Aqueous hydrazine as a promising candidate for capyuring carbon dioxide. Int. J. Greenh. Gas Control 2014, 29, 256–262. [CrossRef]

28. Chen, P.C.; Luo, Y.X.; Cai, P.W. CO2 Capture using monoethanolamine in a bubble-column scrubber. Chem. Eng. Technol. 2015, 38, 274–282. [CrossRef] 29. Meng, L. Development of an Analytical Method for Distinguishing Ammonium Bicarbonate from the

Products of an Aqueous Ammonia CO2 Scrubber and the Characterization of Ammonium Bicarbonate. Master’s Thesis, Western Kentucky University, Bowling Green, KY, USA, December 2004.

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