An empirical model of absorption of nitric oxide with ammoniacal cobalt (II) solutions in a Spray Tower

Chemical Engineering Research and Design 1 4 8 ( 2 0 1 9 ) 240–250

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An empirical model of absorption of nitric oxide with ammoniacal cobalt (II) solutions in a Spray Tower Haiming Wang a , Qinghai Li b,c , Changfu You b,c , Zhongchao Tan a,c,∗ a

Department of Mechanical and Mechatronics Engineering, University of Waterloo, Ontario, N2L 3G1, Canada Department of Energy and Power Engineering, Tsinghua University, Beijing, 100084, China c Tsinghua University – University of Waterloo Joint Research Centre for Micro/Nano Energy & Environment Technology, Tsinghua University, Beijing, 100084, China b

a r t i c l e

i n f o

a b s t r a c t

Article history:

An empirical model was developed for the prediction of the absorption efficiency of nitric

Received 3 September 2018

oxide (NO) into ammoniacal cobalt (II) solution in terms of gas flow rate, liquid flow rate,

Received in revised form 4 March

and temperature. The model parameters were determined experimentally using a spray-

2019

ing tower with initial absorbent solution pH of 10.0–10.2 for temperatures in the range of

Accepted 9 June 2019

281.15–323.15 K. The reactions between Co2+ complexes and NO were found to be in the

Available online 17 June 2019

fast pseudo-first-order reaction regime. The correlation between the overall mass transfer coefficient and the gas and liquid flow rates was determined for NO absorption. The liq-

Keywords:

uid flow rate (0.225 to 0.730 L/min) had negligible effect on the NO absorption efficiency for

NO absorption model

the conditions with excessive Co2+ complexes in the solution due to the fast reaction rates.

Ammoniacal cobalt

Temperature showed detrimental effects on the NO absorption for conditions in this study.

Wet scrubbing

© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Mass transfer coefficient

1.

Introduction

a great potential in simultaneous desulfurization and denitrification (Ding et al., 2014; Fang et al., 2011; Hutson et al., 2008; Liu et al., 2018a,

Effective control of nitrogen oxides (NOx ) emissions from stationary sources is important to public health and the environment. NOx is an air pollutant that contributes to acid rain, smog, secondary aerosol par-

2016; Long et al., 2005; Luo et al., 2018; Zhao et al., 2014, 2016). Spraying tower is usually used as an effective liquid absorption reactor for its

ticles, and tropospheric ozone (Kim et al., 2006; Oluwoye et al., 2017; Wang et al., 2017). Among all the NOx emission control technologies, selective catalytic reduction (SCR) is currently the most widely used

2018b). Therefore, it is also chosen as the reactor in this study. The desulfurization efficiencies in most simultaneous desulfurization and

low pressure drop and large interfacial contact area (Liu et al., 2017,

one (Ma et al., 2016). However, catalyst fouling caused by SO2 , mineral

denitrification processes can approach 100%, but it is much lower for denitrification. The main reason is that nitric oxide (NO) comprises of

dust and water vapor leads to frequent replacement of the catalysts. Novel catalysts have been developed to improve the catalyst tolerance (Chen et al., 2011; Gao et al., 2018), but they are also challenged by the

about 95% of NOx in a typical flue gas, and it cannot be absorbed efficiently in aqueous solutions due to the low solubility of NO. A novel absorbent is needed for effective NOx removal.

narrow temperature window matching the catalyst activity. For example, it has been reported that the NOx removal efficiency dropped when

There are several approaches to the improvement of the NO absorption, including oxidation of insoluble NO to soluble NO2 and formation

the load of boilers was reduced (Jiang et al., 2009; Meng et al., 2015). Liquid absorption offers an alternative to, yet cost-effective option

a complex with NO. Several absorbents have been developed for these purposes; they include H2 O2 , FeSO4 , NaClO2 , KMnO4 , Fe(II)-EDTA, and

for the NOx emission control for its simplicity and tolerance to the

ammoniacal cobalt(II) (Chien and Chu, 2000; Ding et al., 2014; Fang et al.,

flue gas compounds (Adewuyi and Sakyi, 2013; Liu et al., 2018a; Yu and Tan, 2014). More importantly, many researchers believe that it has

2011, 2013; Long et al., 2004; Wei et al., 2009; Zhang et al., 2016). For the

∗

Corresponding author at: Department of Mechanical and Mechatronics Engineering, University of Waterloo, Ontario, N2L 3G1, Canada. E-mail address: [email protected] (Z. Tan). https://doi.org/10.1016/j.cherd.2019.06.012 0263-8762/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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Chemical Engineering Research and Design 1 4 8 ( 2 0 1 9 ) 240–250

Nomenclature Gas–liquid interfacial area, m2 /m3 Temperature dependent constant in Eqs. (37) and (39), unless otherwise specified Absorbent in liquid B CL NO concentration in liquid, kmol/m3 D Diffusivity in water, cm2 /s Enhancement factor E E∞ Enhancement factor for an instantaneous reaction h Spraying column height, m Henry’s law constant, kPa·m3 /kmol H Ha Hatta number Overall mass transfer coefficient, KG kmol/(m2 ·h·kPa) kG Mass transfer coefficient in gas film, kmol/(m2 ·h·kPa) Mass transfer coefficient in liquid film, m/s kL m, n, p, q Reaction orders with respect to reactant, unless otherwise specified M Mole weight, g/mol The adsorption rate of NO in a solution, N kmol/m3 ·h Pressure, kPa P Q Volumetric flow rate, L/min  Mole flow rate, kmol/h Q Universal gas constant (kPa·m3 )/(kmol·K) R S The cross sectional area of the column, m2 t Time, s Temperature, K T V Volume of the reactor, m3 Vb,NO Molar volume of NO at its normal boiling temperature y Mole fraction of NO in bulk gas Mole fraction of NO at equilibrium with the NO y* concentration in bulk liquid Mole ratio of NO to inert gas in bulk gas Y z Stoichiometric coefficient a A, A1

oxidation of NO to NO2 , in addition to the high costs associated with the process by strong oxidants (Fang et al., 2011; Liu et al., 2018a), unwanted side reactions between NO2 and H2 O Eq. (1) would release NO again.

3NO2 + H2 O → 2HNO3 +NO

(1)

Yu et al. (2012) compared the performances of H2 O2 , Fe(II)-EDTA, and ammoniacal cobalt(II) solutions in terms of NO absorption. They found that ammoniacal cobalt(II) was a superior absorbent. Ammoniacal cobalt(II) could absorb NO effectively and form nitrosyl complexes and, in the presence of O2 , oxidize it into nitrate. Moreover, the main byproducts are ammonium nitrite/nitrate and ammonium sulfite/sulfate if both SO2 and NOx exist in the system. These byproducts can be further processed for the production of fertilizers (Resnik et al., 2004). A Co(II) ion can bind with up to six ammonia molecules in an ammonia containing cobaltous solution (Yu and Tan, 2014). As shown in Table S1 in Supporting Information (SI), six Co(II) complexes can be formed in the ammoniacal cobalt(II) solution, i.e. [CoAm ]2+ (A = NH3 , m = 1–6). Among these Co(II) complexes, only [Co(NH3 )5 ]2+ (penta-amminecobalt (II)) and [Co(NH3 )6 ]2+ (hexa-amminecobalt (II)) can react effectively with NO via reactions in Eqs. (2) and (3). (Yu and Tan, 2013)

2[Co(NH3 )5 (H2 O)]

2[Co(NH3 )6 ]

2+

2+

+ 2NO ↔ [Co(NH3 )5 (NO)2 Co(NH3 )5 ]

+ 2NO ↔ [Co(NH3 )5 (NO)2 Co(NH3 )5 ]

4+

4+

+ 2H2 O

+ 2NH3

(2)

(3)

The composition in a cobalt (II) system depends on the pH, temperature, and ammonium concentration of the solution. [Co(NH3 )5 ]2+ and [Co(NH3 )6 ]2+ dominate the Co(II) complexes when pH is greater than 9.5 (T = 303.15 K and [NH4 + ] = 2 mol/L) (Yu and Tan, 2013, 2014). Kinetic studies are also necessary to better understanding of the absorption process. Mao et al. (2008, 2009) reported the equilibrium constant for the reaction between NO and hexa-amminecobalt (II) ions without the analysis of the cobalt (II) ammonia system. The reaction rate constant and equilibrium constant were reported later by Yu and Tan (2013, 2014) by considering the parallel reactions between [Co(NH3 )5 ]2+ /[Co(NH3 )6 ]2+ and NO. The overall mass transfer coefficient (KG a) of NO from gas phase into liquid phase is important to the engineering design of reactors such as an absorption column (Fu et al., 2012). However, the information of KG a for the absorption of NO into

Greek Letters ˚ Association factor for water  Viscosity of water, Pa·s  Residence time (s) NO absorption efficiency ␩

ammoniacal cobalt (II) solution is still missing in literature. It is also difficult to use existing kinetic parameters for the prediction of the NO

Subscripts (2), (3) Reactions (2) and (3) B Absorbent in liquid Gas G H2 O Water L Liquid i Gas–liquid interfacial Inert gas I Inlet in m, n, p, q Reaction orders with respect to reactant Nitric oxide NO Outlet out Total bulk gas T

developed for the absorption of NO into an ammoniacal cobalt(II) solu-

removal efficiency. A practical model is needed for the prediction of NO removal efficiency in terms of typical operating parameters such as gas flow rate, liquid flow rate, and temperature. This model is expected to be useful to system optimization and operation. In the present study, therefore, a practical empirical model was

Superscripts m, n, p, q Reaction orders with respect to reactant (2), (3) Reactions (2) and (3)

tion. The correlation between the overall NO mass transfer coefficient and the gas and liquid flow rates was obtained experimentally. Then an empirical equation was developed for NO absorption efficiency in terms of gas flow rate (QG ), liquid flow rate (QL ) and temperature (T). The key parameters in the model were determined using the data obtained under different QG , QL , and T. Furthermore, the influences of SO2 and CO2 in flue gas on the NO absorption were discussed for future studies.

2.

Analytical

Consider an element with the height of dh in the cross-flow packed column shown in Fig. 1. Gases enters the column from the bottom and liquid falls downward from the top. The liquid and inert gas flow rates are denoted as QL and QI ’, respectively. C and Y stand for the NO concentration in liquid and the mole ratio of NO to the inert gas, respectively.

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Chemical Engineering Research and Design 1 4 8 ( 2 0 1 9 ) 240–250

Fig. 1 – Material balance and the mass transfer process in the packed column. Assume uniform distribution of gas and liquid phases within the cross section of the column. Mass balance with respect to NO is described as 

QL dC = −QI dY

NO into ammoniacal cobalt solution. Ha is a parameter that quantifies the effects of chemical reactions on mass transfer (Danckwerts, 1970). The absorption can be assumed pseudofirst-order (Hartono et al., 2009) if

(4) 3 < Ha << E∞

The variation of concentration, dC, due to the absorption of NO can be determined from the adsorption rate of NO into the solution, NNO , as follows. QL dC = NNO Sdh

where E∞ is the enhancement factor. For an instantaneous reaction (van Swaaij and Versteeg, 1992),



(5) E∞ =

Combination of Eqs. (4) and (5) leads to 

NNO Sdh = −QI dY

(6)

The mole ratio of NO to inert gas, Y, in bulk phase can be deduced from the mole fraction of NO in total bulk gas, y: Y=

y 1−y

[NO∗] =



NNO Sdh = −QG dy

(8)

According to the double-film theory (Lewis and Whitman, 1924), illustrated on the right side of Fig. 1, the NO absorption rate, NNO , is NNO = KG aPNO = KG aPT (y − y∗)



DB DNO

(12)

PNO,i H

(13)

Since it is impractical to determine PNO,i at the interface, the partial pressure of NO in the bulk gas, PNO , can be used for the estimation of E∞ . Henry’s law constant for NO in water is temperature dependent and it can be quantified using Eq. (14) (Sander, 2015).



H = 5.263 × 104 · exp −1600

1 T

−

1 298.2

 (14)

The diffusivity of NO in water, DNO , in Eq. (12), can be estimated by the Wilke–Chang equation (Wilke and Chang, 1955):

(9)

where the equilibrium fraction of NO, y*, can be estimated by Henry’s law: PT y∗ = HCL

DNO [B] + DB z [NO∗]

where the stoichiometric coefficient for the overall reaction is z = 2. The diffusivity of B (Co2+ in this study) in water, DB , is 6.3 × 10−6 cm2 ·s−1 for cobalt salts in water (Patil et al., 1993); the solubility of NO in water, [NO*], is also governed by Henry’s law, that is,

(7)

In this study, y is much less than one because the concentration of NO used was 0.1 vol%. Therefore, the variation of the gas flow rate caused by the absorption of NO is negligible and the inert gas flow rate QI ’ in Eq. (6) is close to the total gas flow QG ’. Then Eq. (6) can then be rewritten as

(11)

(10)

The NO concentration in liquid phase, CL , is negligible if the reaction between the absorbents and NO is in the fast pseudo-first-order reaction regime. Hatta number (Ha) can be used to verify whether this is true for the absorption of

DNO = 7.4 × 10−12 (˚MH2 O )0.5

T 0.6 H2 O Vb,NO

(15)

where the association factor for water ˚ is 2.26 (Theodore and Ricci, 2011); the molar volume of NO, Vb,NO , at its normal boiling temperature is 23.6 cm3 /mol (Wilke and Chang, 1955). Then the value of E∞ in Eq. (12) can be determined and it is 1.74 × 104 at the temperature of 303.15 K. The Hatta number is given as follows for the reactions between a gaseous species A (NO in this study) and a liquid

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Chemical Engineering Research and Design 1 4 8 ( 2 0 1 9 ) 240–250

reactant B with the reaction orders of m and n for A and B, respectively (Danckwerts, 1970). 1 Ha = kL



2 m−1 kmn DNO Bn cNO,i m+1

(16)

(y*) in Eq. (10) can be assumed zero. As a result, the adsorption rate of NO, Eq. (9), is simplified as NNO = KG aPT y

(22)

Combination of Eqs. (8) and (22) gives In this study, however, both [Co(NH3 )5 ]2+ and [Co(NH3 )6 ]2+ present in the solution competing for NO. In another word, Reactions (2) and (3) take place simultaneously. Onda et al. (1970) discussed two irreversible parallel reactions, namely, reaction between A and B(2) with m-th order for A and n-th order for B(2) . Reaction between A and B(3) with p-th order for A and q-th order for B(3) . B(2) and B(3) represent the [Co(NH3 )5 ]2+ in Eq. (2) and [Co(NH3 )6 ]2+ in Eq. (3) in this study. The modified Hatta number is then



  q p−m  m + 1 kpq B(3) cNO,i Ha = M 1+ · p+1

M=

(17)

kmn Bn(2)

m−1 kmn DNO Bn(2) cNO,i 2 · m+1 k2L

 DNO

 2 m+1





(2)

DNO

(3) + kpq B(3)

2 (3) q p−1 k B c p + 1 pq (3) NO,i

 (19)

PT Sh

yin yout

 (24)

Then the overall mass transfer coefficient can be determined with experimental data for the inlet and outlet mole ratios of NO, and the gas flow rate.  The molar flow rate of gas, QG , in Eq. (23) can be determined using Eq. (25) with known volumetric flow rate, QG . n V

(25)

where n is the total amount of gas inside the reactor. Combining Eqs. (23) and (25) and the ideal gas law, we have KG aPNO Sdh = −

Q G PT dy RT

(26)

Note that the time for gas to penetrate through the element (dh) in Fig. 1 is dt =

dh QG /S

(27)

Then Eq. (26) can be rewritten as

(20)

1 dPNO RT dt

(28)

Eq. (28) can be rearranged for integration.

(2) 0.0099 mol/L, respectively. The reaction rate constants, kmn (3) 6 7 −1 and kpq , at 303.15 K were 7.57 × 10 and 1.12 × 10 L·mol ·s−1

respectively (Yu and Tan, 2014). According to the Higbie penetration theory (Higbie, 1935), the liquid phase mass transfer coefficient is

kL = 2

ln

KG aPNO = −



The relative abundance of [Co(NH3 )5 ]2+ and [Co(NH3 )6 ]2+ in the ammoniacal cobalt solution depends on the pH and temperature of the solution. For the original pH of 10 and the temperature of 303.15 K, the penta- and hexa-amminecobalt (II) accounted for 54.44% and 24.77%, respectively, of the total cobalt (II) concentration in the ammoniacal cobalt system. Details can be found in our earlier studies (Yu and Tan, 2013, 2014). Therefore, B(2) and B(3) in Eq. (20) equal to 0.0218 and







QG

KG a =



m−1 + kmn Bn(2) cNO,i

(2) kmn B(2)

The overall mass transfer coefficient for NO absorption in ammoniacal cobalt solution can be determined by integrating Eq. (23), and it is described as

QG = Q G

By regression analysis and graphical method, m, n, p, and q can be determined and they are equal to one for the absorption of NO in the ammoniacal cobalt solution (Yu and Tan, 2014). Then Eq. (19) can be simplified as 1 Ha = kL

(23)

(18)

In order to use the Eqs. (17) and (18) above, the concentration of cobalt (II) in the bulk liquid has to be maintained in excess of the concentration of NO at the gas–liquid interface. In this case, Reactions (2) and (3) are deemed to proceed forward only (Yu and Tan, 2014). Combing Eqs. (17) and (18) one can get the Hatta number in Eq. (19). 1 Ha = kL



KG aPT ySdh = −QG dy

DNO t

(21)

Finally, the calculated Hatta number is 4.13 × 103 , which is much greater than 3 but less than E∞ (1.74 × 104 ). Therefore, the absorption of NO into an ammoniacal cobalt (II) solution is believed to take place in the fast pseudo-first-order regime. Accordingly, the equilibrium fraction of NO in the liquid phase



Pout

Pin

dPNO =− PNO





RTKG adt

(29)

0

where  is the residence time of the gas phase in the reactor and =

V QG

(30)

The absorption efficiency can be described in terms of the NO partial pressure as =1−

PNO,out PNO,in

(31)

Finally, the NO absorption efficiency, , can be calculated using Eq. (32).



 = 1 − exp −KG aRTV

1 QG

 (32)

Eq. (32) indicates that the absorption efficiency depends on gas flow rate, temperature, the configuration of the reactor

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Chemical Engineering Research and Design 1 4 8 ( 2 0 1 9 ) 240–250

Table 1 – Test conditions. Name

Parameter

Value

NO concentration Oxygen concentration Gas temperature Liquid temperature Gas flow rate Liquid flow rate

NO (ppmv) O2 (vol%) TG (◦ C) TL (◦ C) QG (L/min) QL (L/min)

1000 5 45 8–50 1–3 0.225–0.73

(i.e. volume of the column) and the mass transfer coefficient, KG a. However, it is inconvenient to using Eq. (32) for the prediction of NO adsorption efficiency in practice primarily because mass transfer coefficient (KG a) is a dependent variable. The mass transfer coefficient changes with temperature, gas and liquid flow rates, and the inlet and outlet NO concentrations. Therefore, the correlation between the overall mass transfer coefficient and the operational conditions need to be determined first for the prediction of the total NO absorption efficiency using Eq. (32).

3.

Experimental

The schematic diagram of the experimental setup is shown in Fig. 2. The adsorption of nitric oxide took place in a spraying column made of 316 stainless steel. The column is 500 mm long with an inner diameter of 100 mm. About 247 g of plastic spheres with the diameter of 25 mm (Water Purification Company, Gongyi, China) were packed inside the column. Details can be found in Fig. S1 in Supplementary Information (SI). Simulated flue gas was used for testing. It was a mixture of N2 (Grade 4.8), NO (2.5% balanced in N2 ) and air (grade zero). All gases were purchased from Praxair Inc., Canada. The gas flow rates were regulated by mass flow controllers (Cole-Parmer, Canada) with an accuracy of ±0.3%. All gases passed through an in-line gas mixer (Model 3/8-40-3-6-2, Koflo Inc., Illinois, USA) for mixing. A three-way valve downstream the mixer was used for bypass when needed. The simulated flow gas was heated by an in-line heater before entering the spraying column. The temperature of the in-line heater was maintained at 45 ± 0.5 ◦ C by a circulating thermostatic water bath. The temperature of the simulated flue gas was monitored by a digital thermometer (Model 4045, Control Company). The pre-heated flue gas then entered the spray tower from the bottom and exited from the top. The test gas flow rate (QG ) varied from 1 to 3 L/min for different experimental tests. The oxygen concentration in flue gas was regulated at 5 vol% for all tests. The operational conditions are listed in Table 1. The cobalt (II)-ammonia solution was prepared following the procedure that was reported in previous studies (Yu et al., 2012), and it is briefly summarized as follows. All chemicals used herein for solution preparation were purchased from Sigma-Aldrich Co. LLC. Certain amount of cobalt(II) nitrate hexahydrate (Co(NO3 )2 ·6H2 O, ACS reagent, ≥98%) and ammonium chloride (NH4 Cl, ACS reagent) were weighed by an analytical balance with a resolution of 0.01 g (Model RK-1142193, Denver Instrument Inc., USA). They were then mixed with aqueous ammonia (NH3 ·H2 O; ACS reagent) and deionized ˜ wt%. water in a beaker until its concentration reached 2830 NH4 Cl/NH3 ·H2 O buffer solution was prepared by adding NH4 Cl into the solution. The buffer solution was used to ensure ready supply of NH3 ligand and avoid the formation of hydroxide. The cobalt(II)-ammonia solution was prepared in a 500 mL beaker placed on a stirring heating plate (Model SP142025Q,

Thermo Scientific, Canada). After stirring for 10 min, 100 ml more of deionized water was added to the solution and resumed stirring for 5 more minutes. The final solution was used as absorbent. 2 L of fresh absorbent was used for each test. The pH of the solution was monitored by a digital pH meter with an accuracy of ±0.01 (Model pH 700, Oakton Instruments, USA). The initial pH value was in the range of 10.–0.1<– –>. The concentrations of Co2+ , NH4 + , and ammonia in the solution were 0.04, 0.4, and 1.84 mol/L, respectively. In a typical test, the absorbent solution was first transferred to the circulating thermostatic bath that was set at the desired temperature. The solution was delivered to a spray nozzle (Model 1/8G-3001.4, IHS Engineering 360) in the tower by a peristaltic pump (Model 77200-62, Cole Parmer). The flow rate, varying from 0.225 to 0.73 L/min, was monitored by a liquid flowmeter (Model TMR1-010099, Cole Parmer). The liquid moved downward through the packed bed, and it was circulated back into the thermostatic bath. The temperatures of the solution were monitored at the inlet and outlet of the tower reactor. Average temperature was used as the operation temperature of the reactor. Gas concentrations were measured using a Fourier transform infrared (FTIR) gas analyzer (MultiGas 2030, MKS Instruments Inc.). The data at the inlet was collected via the by-pass route every 20 s after the readings became stable. The NO concentration at the outlet was recorded continuously by the same FTIR. Slipping ammonia was absorbed by H2 SO4 with a concentration 95–98 wt% upstream of the FTIR. The corresponding NO absorption efficiency is calculated from the inlet and outlet NO concentrations using Eq. (33). =

yin − yout yin

(33)

Fig. 3 shows the typical NO absorption efficiencies vs. operation time with different liquid flow rates. The removal efficiency decreased slowly over time. With a liquid flow rate of 0.225 L/min, the efficiency decreased by 2.4% over 30 min from 87.6% to 85.5%. Therefore, the NO absorption efficiency was assumed to be stable within 30 min. The average concentration of NO at the outlet of the column was used for the calculation of KGa and used for the validation of efficiency prediction in the following discussion. The error bars in the following figures are for the maximum and minimum of outlet NO concentrations in the tests.

4.

Results and discussion

4.1. Determination of the overall mass transfer coefficient Fig. 3 shows the overall mass transfer coefficients and the NO absorption efficiencies measured with different liquid flow rates. The overall mass transfer coefficient increased slightly from 0.042 to 0.047 kmol m−3 h−1 kPa−1 when the liquid loading increased from 0.225 to 0.73 L/min. In general, a greater liquid loading improves the gas–liquid interface area and increases the mass transfer coefficient. In this study, however, the slight increase in KG a did not lead to an obvious increase in the NO removal efficiency. As seen in Fig. 3(b), the average NO removal efficiency increased from 86.2% to 89.2% when the liquid flow rate increased from 0.225 to 0.73 L/min. This result indicates that there was sufficient Co2+ complexes in the spraying tower for NO absorption even at the liquid loading of

Chemical Engineering Research and Design 1 4 8 ( 2 0 1 9 ) 240–250

245

Fig. 2 – Schematic diagram of the experimental setup.

Fig. 4 – Effects of the gas flow rate on NO absorption efficiency and the overall mass transfer coefficient (QL = 0.73 L/min, T = 303.15 K). transfer between the gas and liquid due to the increased gas velocity at the gas–solid interface. KG a increased from 0.041 to 0.047 kmol·m−3 h-1 ·kPa-1 as the gas flow rate increased from 2 to 3 L/min. According to the double film theory, the overall mass transfer coefficient, KG a, can be expressed in terms of the mass transfer resistances in gas and liquid films: 1 1 H = + KG a kG a EkL a

Fig. 3 – The overall mass transfer coefficient KG a and variation of NO removal efficiency with liquid flow rate. (a) KG a; (b) NO removal efficiency. (QG = 3 L/min, T = 303.15 K). 0.225 L/min. Therefore, the rate controlling step was the mass transfer in the gas and water films rather than the chemical reactions. Fig. 4 shows the effects of gas flow rate on NO removal efficiency and the overall mass transfer coefficient. A high gas flow rate corresponding to a short gas residence time resulted in a decrease of NO removal efficiency from 94.5% to 89.2%. Nevertheless, increasing the gas flow rate enhanced the mass

(34)

The gas-phase mass transfer coefficient, kG , and the liquidphase mass transfer coefficient, kL , in the double stirring reactor for NO absorption were positively correlated to the stirring speed (Sada et al., 1978). The mass transfer coefficients increased with the stirring speed due to the enhanced turbulence at the gas–liquid interface. Similarly, the high gas velocity in the spray tower improved the mass transfer in the gas film. However, the improvement of overall mass transfer was shadowed by the negative effects caused by the shortened residence time and lowered the NO remove efficiency. Fig. 5 shows the effects of the gas residence time on the NO removal efficiency and overall mass transfer coefficient at the constant gas to liquid flow rate ratio (QG /QL = 4.1). As discussed

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Chemical Engineering Research and Design 1 4 8 ( 2 0 1 9 ) 240–250

Fig. 5 – Effects of residence time on NO removal efficiency and overall mass transfer coefficient under the same gas–liquid flow rate ratio of 4.1. (a. QG = 1 L/min, QL = 0.243 L/min,  = 236 s; b. QG = 2 L/min, QL = 0.486 L/min,  = 118 s; c. QG = 3 L/min, QL = 0.73 L/min,  = 79 s; T = 303.15 K). Table 2 – Regression and the t-test results for Eq. (36). Parameter

value

Standard error

t-value

P-value

lnA1 x1 x2

−3.34 0.08 0.26

0.0297 0.0196 0.0226

112.7 3.890 11.28

3.71E-8 0.0177 3.52E-4

earlier, the increase in gas and liquid flow rate enhanced the mass transfer across the gas-phase interface and increased the overall mass transfer coefficient, KG a. However, the NO removal efficiency dropped from 98.9% to 89.2% due to the shortened residence time. The variation of QG had a greater influence than QL on the NO removal efficiency. By comparing the data in Fig. 5 (b) and Fig. 4, which are for the same gas flow rate of 2 L/min, one can see little difference in NO removal efficiency, which was only 0.7% when the liquid flow rate dropped from 0.73 L/min (in Fig. 4) to 0.486 L/min (in Fig. 5b).It further confirms the minor impact of liquid flow rate on the absorption process for the conditions in this study. The aforementioned kinetic and experimental data show that the overall mass transfer coefficient is closely related to the gas and liquid flow rates in the spraying tower. They affect the individual mass transfer of NO in gas and liquid films. Therefore, the over mass transfer coefficient can be described in terms of gas and liquid flow rates. Similar to the relation for absorption of CO2 into ammonia (Zeng et al., 2011) and H2 S into sodium hypochlorite (Turpin et al., 2008) using spraying tower, the correlation of the overall mass transfer coefficient can be described using Eq. (35). KG a = A1 QLx1 QGx2

(35)

By applying ln on both sides, Eq. (35) can be changed into a liner relationship as ln (KG a) = ln A1 + x1 ln QL + x2 ln QG

(36)

The three parameters, A1 , x1 and x2 , can be determined by linear regression of experimental data, which are listed in Table 2.

Fig. 6 – Comparison of the overall mass transfer coefficients from experimental data and the proposed correlation. To determine the significance of each parameter obtained for Eq. (36), a t-test was performed with the null hypothesis of H0 : lnA1 = 0, x1 = 0, and x2 = 0 at a significance level of 0.05. Table 2 summarizes the regression parameters and the corresponding t-test results. All the P values are less than 0.05, which means the null hypotheses are rejected. It can be concluded that the regression parameters, lnA1 , x1 , and x2 , are significant. Then the following correlation between the overall mass transfer coefficient and gas and liquid flow rates was determined as KG a = 0.0353QL0.08 QG0.26 (R2 = 0.96)

(37)

Fig. 6 shows the comparison between the values of KG a determined experimentally and calculated using Eq. (37). The differences are less than 5%. This indicates the correlation can appropriately describe the effects of the gas and liquid flow rates on the mass transfer of NO into ammoniacal cobalt solutions. As shown in Eq. (37), the power for QG is greater than that of QL . It further confirms that gas flow rate has a greater influence on NO absorption. One should notice that A1 is not dependent on the gas or liquid flow rate but the temperature. This important relationship is elaborated in the next section.

4.2.

Prediction of NO absorption efficiency

As mentioned above, the overall mass transfer coefficient depends on the operation conditions, including temperature, gas flow rate, and liquid flow rate. It is of practical importance to describe the NO removal efficiency in terms of these operational conditions. This relationship can be obtained by substituting Eq. (37) into Eq. (32) followed by a simple manipulation of the equations. Finally, the NO absorption efficiency can be calculated using Eq. (38).

 = 1 − exp

−A ×

QL0.08 QG0.74

 (38)

where A is a parameter related to the operation temperature and coupled with A1 in Eq. (35). It can be determined with one set of experimental data obtained at different temperatures. Fig. 7 shows the variation of A for the operation temperatures

Chemical Engineering Research and Design 1 4 8 ( 2 0 1 9 ) 240–250

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transfer coefficient decreased from 0.053 kmol·m−3 h−1 ·kPa−1 at 281.15 K to 0.024 kmol·m−3 h−1 ·kPa−1 at 323.15 K. There are several reasons behind this detrimental effect of the temperature on the NO absorption. Firstly, the absorption of NO into the solution at a high temperature is hindered by the low solubility of NO. The NO concentration at the gas–liquid interface, CNO,i , can be described by Henry’s law as CNO,i =

Fig. 7 – Non-linear fitting of A to the NO removal efficiency under different temperatures.

PNO H

(41)

Elevating system temperature would increase the Henry’s law constant in Eq. (41). Consequently, it reduces the NO concentration at the interface, which means less NO can be transferred into the liquid film. This can be confirmed by the experimental data in Fig. 9 (b). In addition, the solubility of oxygen in the liquid was lower at a higher temperature. Dissolved oxygen is necessary for the reaction between hexamminecobalt(II) and NO via the following chemical reactions (Long et al., 2004). [(NH3 )5 Co − O − O − Co(NH3 )5 ] Cl4 +[Co(NH3 )5 NO] Cl2 +2NH4 Cl → 2 [Co(NH3 )6 ] Cl3 + [Co(NH3 )5 NO2 ] Cl2 + H2 O (42)

2 [Co(NH3 )5 NO2 ] Cl2 + H2 O + 4NH3 → NH4 NO2 + NH4 NO3 +2 [Co(NH3 )6 ] Cl2

Fig. 8 – Comparison between the efficiencies calculated using Eq. (40) and those determined experimentally. in the range of 281.15–323.15 K. Then the relationship between A and T was obtained by non-linear regression. A = 6.2 − 2.6 × 10−8 exp

 T  17.2



R2 = 0.99

(39)

Finally, Eq. (38) can be written as Eq. (40) for the data within this work.  = 1 − exp



−8

2.6 × 10

exp

 T  17.2



− 6.2 ×

QL0.08

K(2) = 1.90 × 107 exp

 3598.5  T

 1476.4  T

(44)

(45)

(40)

Fig. 8 shows the comparison between the calculated efficiencies and experimental data for different temperatures, and gas and liquid flow rates. The errors are much less than ±5%, indicating the validity of the empirical equation for the system in this work.

4.2.1.

[(NH3 )5 Co–O–O–Co(NH3 )5 ]2+ is a diamagnetic salt of a binuclear cobalt complex ion, and a strong oxidant (Ochiai, 1973; Schaefer, 1968). It reacts with [Co(NH3 )5 (NO)2 Co(NH3 )5 ]4+ to form soluble nitrate and nitrite. Therefore, the reduced solubility of O2 at a high temperature slows down the oxidation of NO and the formation of [Co(NH3 )6 ]2+ . Furthermore, the equilibrium of the reversible reaction between Co (II) complexes and NO (Eqs. 2 and 3) is broken when temperature changes. The equilibrium constants of Eqs. (2) and (3) were reported in our earlier study (Yu and Tan, 2013), and they are described using Eqs. (44) and (45), respectively.

K(3) = 3.56 × 1011 exp



QG0.74

(43)

Effects of temperature on NO removal efficiency

Fig. 9 shows the calculated NO removal efficiencies for temperatures in the range of 281.15–323.15 K. The gas flow rate was maintained at 3 L/min for all tests, and the constant liquid flow rate, 0.73 L/min. Results in Fig. 9 show that the NO removal efficiency dropped from 92.0% to 67.4% when the temperature increased from 281.15 K to 323.15 K. The overall mass

Both equations show that the equilibrium constants decrease with the increase of temperature. They also indicate that the chemical equilibrium tends to move toward the left side of Reactions (2) and (3) at an elevated temperature, which decreases the absorption ability of the cobalt based solvent. Cobalt complex [Co(NH3 )6 ]3+ is more stable than [Co(NH3 )6 ]2+ in an ammoniacal cobalt solution (Yu et al., 2012), yet the [Co(NH3 )6 ]3+ is not as effective as [Co(NH3 )6 ]2+ in binding with the dissolved NO. At a high temperature, the [Co(NH3 )6 ]2+ tend to be converted into [Co(NH3 )6 ]3+ . Consequently the absorption efficiency is reduced. In industrial applications, a low temperature is preferred for the NO absorption into an ammoniacal cobalt (II) solution. As seen in Fig. 9 (a), the difference is only 2.0% between the NO removal efficiencies measured at 283.15 K and 298.15 K. In

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Chemical Engineering Research and Design 1 4 8 ( 2 0 1 9 ) 240–250

Fig. 9 – Effects of temperature on the NO absorption performance. a: NO removal efficiency; b: overall mass transfer coefficient. pseudo-first-order regime, where the dissolved NO is absorbed instantaneously by the cobalt (II) species. As the reaction is not the rate controlling step for NO absorption, further increase in QL would not cause much difference in the NO absorption efficiency. However, the absorption efficiency would decrease dramatically at a much lower liquid flow rate as indicated by the calculation results. Take a gas flow rate of 2 L/min for example (the red-dash-line in Fig. 10), the NO absorption efficiency would decrease from 94% to about 81% when the liquid flow rate drops from 0.4 to 0.01 L/min. At a low liquid flow rate, the mass transfer resistance increased for NO absorption as indicated by the reduced overall mass transfer coefficient (Eq. 37). More NO could penetrate through the column without contacting with the liquid surface.

4.2.3. Fig. 10 – Effects of the gas and liquid flow rates on NO absorption efficiency (calculations for the temperature of 303.15 K). practice, excessive reduction in flue gas temperature requires a great energy consumption but with much less gain in NO removal efficiency. It is therefore recommended the operation temperature to be around room temperature.

4.2.2. Effects of gas and liquid flow rates on NO removal efficiency Fig. 10 shows the effects of gas and liquid flow rates on the NO absorption efficiency. As indicated in the empirical equation (Eq. 40), liquid flow rate has a power of 0.08, which is about 9 times less than that of gas flow rate (0.74). Therefore, QG has a much greater impact on the NO removal efficiency than QL does for the system in this study. For each gas flow rate, the absorption efficiency becomes insensitive to the variation of the liquid flow rate after it reaches certain point. This can be demonstrated by the calculated results in Fig. 10. In a typical test, 2 L of the ammoniacal cobalt (II) solution was used without replacement or regeneration. The total amount of absorbent in the solution is 0.08 mol, which is much more than needed for the NO absorbed within the operation time. With a gas flow rate of 3 L/min, which is the largest gas flow rate (largest NO dosage) tested in this study, only about 5% of the Co2+ complexes were consumed during the operation time. Therefore, there was sufficient absorbent in the solution for NO absorption, even though the solution was tested without replacement or regeneration. In addition, the reaction between the absorbent and NO is in the fast

Limitations of the work

It should be noted that the absorption of NO in a spraying tower is a complex process, which depends on many factors besides the three parameters discussed in the present study. The pH of the cobalt (II) solution could change the cobalt (II) complex composition. At pH10, the two reactive species for NO absorption, [Co(NH3 )5 ]2+ and [Co(NH3 )6 ]2+ , account for 54.44% and 24.77% of the total cobalt (II), respectively. At pH9, the [Co(NH3 )5 ]2+ and [Co(NH3 )6 ]2+ composition decrease to only 7.16% and 0.26%, respectively. Thus the decrease of the pH would negatively affect the NO removal. In practice, the absorbent solution needs to be kept at a level above pH 10 by make-up solution to ensure effective NO absorption. The influence of the solution pH was not included in the current model. For future study, it would be reasonable to enclose the effects of pH by replacing the liquid flow rate QL in Eq. (40) with the molar flow rates of the reactive species which depend on the pH and the amount of cobalt ions. Another important factor influencing NO absorption efficiency is the composition of the flue gas. In real industrial processes, multiple pollutants such as SO2 and CO2 coexist in the flue gas. They can react with NH3 in the ammoniacal cobalt solution and change the pH of the solution. Yet the simulated flue gas used in the present study was NO balanced in 5 vol% O2 and N2 . We conducted the simultaneous absorption of NO, SO2 , and CO2 in a bubble column using the same ammoniacal cobalt solution. The SO2 removal efficiency maintained above 95%, while the NO absorption efficiency decreased gradually from 91% to 65.9% and 41.1% with the addition of 1920 ppmv and 2810 ppmv SO2 , respectively. Details can be found in Fig. S2 (a). Sulfites (SO3 2− ) formed by the absorption of SO2 consumes the diamagnetic complex, [(NH3 )5 Co–O–O–Co(NH3 )5 ]2+ ,

Chemical Engineering Research and Design 1 4 8 ( 2 0 1 9 ) 240–250

and convert [Co(NH3 )6 ]2+ into [Co(NH3 )6 ]3+ (Mao et al., 2008). [Co(NH3 )6 ]3+ is less active than [Co(NH3 )6 ]2+ in the absorption of NO. By adding Na2 SO3 into the ammoniacal cobalt (II) system, the concentration of Co2+ was found to decrease gradually (Fig. S2 (b)), indicating that Co2+ in solution can be consumed by the sulfites. The concentration of CO2 (10–20%) is much higher than those of SO2 and NO in the flue gas, and the adverse effect is even more obvious than SO2 (Fig. S3). With 9.94% of CO2 , the NO removal efficiency decreased from 92.8% to 66.7% in 20 min. Carbon dioxide mainly react with the excessive ammonia in the solution. It may also lower the solution pH, which further affects the cobalt (II) complex composition as discussed earlier. The pH of the solution decreased from 10.15 to 9.01 in 30 min. It can be seen from above that a much more comprehensive model is needed in the future work. In general, since the absorption of CO2 affects the NO absorption mainly through the changing of pH, the effects of CO2 can be integrated into the molar flow rates of each reactive species in the model, which is similar to the effect of solution pH. As for SO2 , the influence could possibly be expressed in the constant A in Eq. (38).

5.

Conclusions

The absorption of NO by ammoniacal cobalt (II) solution was studied in a spraying reactor. The overall mass transfer coefficient of the absorption process was obtained at different operation conditions including gas flow rate, liquid flow rate, and temperature. It was found that the liquid flow rate in the range of 0.225 to 0.730 L/min had little influence on the NO absorption efficiency, primarily due to the fast reaction rate. The reactions between the Co2+ and NO were found to be in the fast pseudo-first-order regime. The increase of temperature adversely affected the absorption of NO. Finally, an empirical model was proposed based on the kinetic analysis and experimental data. It can predict the NO removal efficiency with an error of ±5%.

Acknowledgement The authors would like to acknowledge the financial support from the International S&T Cooperation Program of China (Grant No. 2015DFG61910) and the Ontario-China Research & Innovation Program (OCRIF-2014). The authors also like to acknowledge Dr. Q. Zhu for his assistance in partial data collection.

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/ j.cherd.2019.06.012.

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