A Mass-Transfer Model of Nitric Oxide Removal In a Rotating Drum

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Kinetics, Catalysis, and Reaction Engineering

A Mass-Transfer Model of Nitric Oxide Removal In a Rotating Drum Biofilter Coupled with FeII(EDTA) Absorption Jun chen, Jiali Wu, Jun Wang, Shihan Zhang, and Jian-Meng Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00966 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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Industrial & Engineering Chemistry Research

A Mass-Transfer Model of Nitric Oxide Removal In a Rotating Drum Biofilter Coupled with FeII(EDTA) Absorption Jun Chena*, Jiali Wub, Jun Wangb, Shihan Zhangb, Jianmeng Chena*

a

Engineering Research Center of the Ministry of Education for Bioconversion and

Biopurification, Zhejiang University of Technology, Hangzhou, 310032, P.R. China b

College of Environment, Zhejiang University of Technology, Hangzhou, 310032, P.R. China *

Corresponding Author: Jun Chen; Jianmeng Chen

E-mail: [email protected]; [email protected]

Phone: +86-571-88320386;

Fax: +86-571-88320881;

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: The biological nitric oxide (NO) reduction rate in a rotating drum biofilter (RDB) mainly depends on the mass transfer between the gas and liquid phase because of the poor solubility of NO. This work, aimed to facilitate the gas–liquid (GL) mass transfer of NO in RDB, the FeII(EDTA) was used to capture NO and porous drum was employed to increase the eddy current effect of the system. The denitrification rate of GL-RDB was stabilized at about 95% during the long-term operation. A dynamic model of GL-RDB was established to optimize the denitrification performance of this innovative RDB. Experimental results revealed that the developed model well described NO removal in GL-RDB. Model analysis also showed that the rotational speed and inlet NO concentration were the critical parameters in determining of the RBD performance. This work may provide fundamental theory for the further study of GL-RDB.

KEYWORDS: Mass transfer, Rotating drum biofilter, Dynamic model, Nitric oxide


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1 INDUCTION The nitrogen oxides (NOx) are mainly produced by industrial production and vehicle exhaust.1, 2 NOx and its secondary pollutants induced by NOx such as photochemical smog, acid rain, ozone hole, and PM2.5, are harmful to the human health.2-4 A number of processes for the removal of nitrogen oxides (NOx) emitted from stationary combustion facilities have been developed, such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR).5, 6 However, SCR requires expensive catalysts, SNCR requires high temperatures, (i.e., increased process costs). In addition to the dry denitrification above, there are also wet denitrification such as oxidation denitrification7 and complexation reduction denitration.8,

9

And all of these processes require the

continuous addition of oxidizing agents or reducing agents to maintain the denitrification efficiency. Therefore, biological NOx removal techniques may be promising alternatives to the conventional techniques. Traditional bioreactors require a relatively long residence time due to the slow mass transfer of NO from the gas to the liquid phase. To overcome these problems, an innovative bioreactor called a rotating drum biofilter (RDB) was widely developed.10, 11 Initially, RDB were used to deal with volatile organic pollutants (VOCs). In view of the excellent performance of RDB in gas–liquid mass transfer, the drawback of poor aqueous solubility of NO can be overcome.12, 13 However, the excess shear rates brought about by high rotating speeds may damage the cells in biological fluids;14 conversely, vortex mixing is an efficient laminar gas–liquid mixing mode that can reduce



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such shear damage to microorganisms. Pedley et al.15 observed the eddy current effect when fluid flow through a movable wall. Furthermore, due to the special layer package structure of RDB, biomass is unevenly distributed along the depths,11 thereby causing unstable performance of a biofilter. The innovative biofilter gas–liquid rotating drum biofilters (GL-RDB) combined with eddy current wave reduce microorganism damage by shear, and stabilize the removal efficiency of the bioreactors. Meanwhile, FeII(EDTA) is an efficient and economical chelating agent for NO absorption that has been used in many biological-denitrification reactors to enhance the mass transfer efficiency and proven to play an important role in the biological denitrification process.16-18 The rate constant of the chelation reaction is 2.4 × 108 M-1s-1, and the stability constant is 2.1 × 106 M-1 at 298 K,19 so this reaction is considered as a quick response in the corresponding modeling process. The main reactions involved in the chemical absorption–biological reduction can be expressed as Fe (EDTA) + NO → Fe (EDTA) − NO

(1)

4Fe (EDTA) + O + 4H → 4Fe (EDTA) + 2H O

(2)

12Fe (EDTA) + 6N + 6H O + 6CO

(3)

12Fe (EDTA) − NO + C H O !""""""""""#

Surprisingly, FeII(EDTA) and FeIII(EDTA) may be directly involved in the nitric oxide reductase (NOR) reduction during denitrification, based on the available midpoint potential between NO reductases and FeII(EDTA)/FeIII(EDTA) system.20 However, according to some studies on biofilm-electrode reactors, FeII(EDTA) is directly involved


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in the microbial reduction of chelate FeII(EDTA)-NO as a dominant electron donor.21 In the present work, the holes were made on the surface of a drum to increase the formation of eddy currents in the gas phase and improve the gas–liquid mass-transfer efficiency. Simultaneously, FeII(EDTA) was also added into the liquid phase to enhance the mass transfer in the biofilter. Subsequently, a mathematical model was developed to describe the NO removal profile in the GL-RDB. This model was verified by experimental data, and analyses of the effects of inlet NO concentration, empty-bed-residence time (EBRT), and rotating speed on NO removal efficiency were conducted to obtain the optimum operation conditions. 2 Materials and methods 2.1 Rotating drum biofilters An innovative GL-RDB was developed and evaluated in this investigation. A schematic of this set-up is presented in Figure 1. The RDB consisted of a gas distributor, reactor, nutrient solution reflux device, and gas-analysis unit. The GL-RDB retained its closed organic glass chamber with a stainless-steel drum frame and three layers of spongy synthetic media as reported in our previous work.16 However, in the present work, porous drum walls were used to enhance the turbulence in the gas phase. The simulated gas enters the RDB from the upper pipeline device and then passes through both sides of the rotating porous drum and drum walls, resulting in numerous eddy currents formed in the gas phase. Due to the strong vortex effect in gas phase, NO rapidly diffuses to the liquid film on the surface of the biofilm, finally is reduced to



nitrogen gas through denitrification in the biofilm. [Figure 1 about here] 2.2 Chemical absorption–biological reduction system A concentrated active sludge was collected from a secondary sedimentation tank in the Hangzhou QiGe Wastewater Treatment Plant, China. The supernatant was used to inoculate the GL-RDB after settlement. The supernatant was then added to the GL-RDB, and activated-sludge acclimation was carried out using NaNO3 as sole nitrogen source and glucose as the sole carbon source. After the removal efficiency reached 95%, the nitrogen source in the GL-RDB was gradually replaced by nitrosyl compounds (FeII(EDTA)-NO), i.e., FeII(EDTA) was added to chelate the NO in the GL-RDB. After the microorganisms were successful acclimated, the formation process of biofilm on the carrier started. The nutrient solution had the following compositions (per liter): glucose (10000 mg), KH2PO4 (500 mg), K2HPO4 (500 mg), MgSO4.7H2O (100 mg), CaCl2 (50 mg), CuSO4 (1 mg), FeSO4 (1 mg), MnSO4 (5 mg), Na2MoO4 (1 mg), and ZnCl2 (1 mg). 2.3 Analytical methods The NO concentrations in the gas inlet and outlet were measured with a NO-NO2-NOx analyzer (0–100 mg m−3, Thermo Electron Co., USA). The synthetic gases of NO and N2 was controlled with mass-flow controllers (MFC) (Beijing Sevenstar Qualiflow Electronic Equipment Manufacturing Co., Ltd. model D07-12A). The concentrations of nitrate nitrogen and nitrite ions were measured by ion chromatography (DX-500,


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DionexCorporation). FeII(EDTA)-NO concentration was detected by its absorbance at 420 nm using a UV/visible spectrophotometer (UNICO (Shanghai) Instruments Co., Ltd.). pH was measured using a pH meter (Huaguang Wireless Electric Co., Hangzhou, China). 3 Model development Holes in the GL-RDB were drilled at both ends of drum in contrast to those in multilayer RDB,22 in which convective diffusion and eddy diffusion simultaneously occur. The following assumptions were used to describe the phenomena that occurred in the GL-RDB: 1) The treatment of NO by the GL-RDB was considered as a steady-state process, in particular, the effect of oxygen on this can be seen as constant. 2) The GL-RDB rotated at constant speed, and the experience of any point at the same radius of packing radial was uniform. Therefore, the liquid phase can be regarded as stationary, and the terms of molecular diffusion and convective diffusion in the liquid equilibrium can be neglected. 3) Biofilm was a steady-state membrane with uniform thickness, only existed on the surface of the filler, and no absorption occurred in the empty layer. 4) No microorganisms existed in the gas and solid phases. 5) NO concentration was always uniform in the gas phase, and the evenly distributed gas streams moved along the radial and axial directions. 6) At the gas–liquid interface, the absorption of NO by the liquid phase followed Henry's law.



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7) The physical properties of NO in nutrient solution were the same as those in water, and the element N did not accumulate in liquid. All NO species changed to N2 by denitrification reaction under anaerobic conditions. 3.1 Gas mass balance The mass transfer of NO in gas characteristic cell including convection diffusion, molecular diffusion, and gas-to-liquid phase transfer. Due to the holes at both sides of drum, the mass-transfer direction of NO was changed, thereby resulting in eddy diffusion. The reaction equation that described the mass accumulation rate in the characteristic cell was as follows: %&'∗ %)

= +,

% - &'∗ % -

− .,

%&'∗ %

− /,0

12

+ 3

% - &'∗ %4-

(4)

where 3 is the eddy diffusion coefficient. When 5 = 67 , 8, = 87 , 9 = 97 . In the weak unsteady flow state, the mass-transfer rate can be approximated as follows: 3

= :(8,∗ − 80 )

(5)

× ?6@ .A< × +,

(6)

% - &'∗ %4-

According the Lamourelle,23, 24 : can be express as :=

7.7< =

Vortex flow is typically unsteady, so increasing the intake air on both sides of the GL-RDB would have resulted in weak unsteady vortex flow. In addition, the oscillatory Reynolds number, 6@ plays an important impact role in the generation and characteristics of eddy current waves, as defined by the ratio of inertial force to viscous force. Therefore25,


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6@ =

BC D

(7)

where E is the tube flow rate, ℎ is the channel diameter, and G is the kinematic viscosity. 3.2 Liquid mass balance In the liquid mass-transfer balance, apart from convection diffusion, the processes of molecular diffusion and gas-to-liquid phase transfer occur, thereby augmenting the transmission of the liquid and biological phases without eddy diffusion. %&H %)

= +0

When 5 = 0, CI = C0 .

% - &H % -

− .0

%&H %

+ /,0

12

− /0I

1-

(8)

The innovative GL-RDB introduces FeII(EDTA) to enhance the mass-transfer performance of the reactor, and the enhancement factor L was used to represent the increase in mass-transfer rate. Many studies have shown that the chelation between FeII(EDTA) and NO is irreversible,26, 27 because the equilibrium constant of the chelation reaction or FeII(EDTA)-NO stability constants are sufficiently large to ignore the inverse reaction. According to the estimation of inverse reaction enhancement factor in a reaction,28 L can be written as

L = 1 +

MNOPP (QRST) ×&NOPP (QRST) MH ×&U

H

(9)

where the +4V PP(WMXY) was 5.5 × 10-6 cm2/s estimated from the research.29 Although FeII(EDTA) participates in reactions (1) to (3) in the system, only reaction (2) induces FeII(EDTA) to loses its the chelating ability, i.e., FeII(EDTA) is converted to FeIII(EDTA). According to assumption 1) that the concentration of FeII(EDTA) was constant during the



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steady operation of GL-RDB, the concentration of FeII(EDTA) can be measured and calculated by subtracting FeIII(EDTA) at steady state. 3.3 Biological mass balance The mass change of NO in biological phase was mainly affected by two aspects, i.e., the transfer of NO from the liquid to the biological and the biological metabolism. Consequently, the biological mass balance can be shown as %&Z %)

= +I

% - &Z

When \ = 0, 8I = 80 ; \ = ], −+I ^

%[ -

%&Z %[

−6

_`

[ab

(10) = −+I ^

%&c %[

_`

[ab

3.4 Solid phase mass balance For solid-phase microelements, the mass balance can be shown as

When \ = ], /Id = +I ^

%&c

%&g %[

%)

_`

= /Id e1

2 e1- e1f

(11)

[ab

3.5 Model solutions Due to the turbulence of eddy currents, the effects of molecular diffusion and convective diffusion can be neglected; thus combined with assumption 5), the mass-transfer equation can be simplified as

5 = 67 , 8, = 87 5 = 0, CI = C0

.,

%&'∗ %

.0

= − 1 /,0 + :(8,∗ − 80 )

(12)

= − 1 /,0 + 1 /0I

(13)

2

%&H %

2

-


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+I

\ = 0, 8I = 80

% - &Z %[ -

=6

(14)

Combining eq. (13) with assumption 2) yields /,0 = /0I

(15)

According to eq. (14) and Fick's first law, eq. (16) can be expressed as /,0 = /0I = ] × 6

(16)

Based on the low solubility of NO, and the Monod equation, eq. (17) can be written as 6 = hi × jI ×

kg

&H

kg

(17)

To simplify the calculation, the complex items can be replaced by l = hi × jI ×

, so 6 can be expressed as

6 = l × 80

(18)

In the gas–liquid two-phase mass transfer, the liquid mass-transfer coefficient is

Considering that

qnH n'

m0 =

nH

opH p'

(19)

= r

A Mass-Transfer Model of Nitric Oxide Removal In a Rotating Drum

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