Modeling of a Packed Bubble Column for Methyl Nitrite Regeneration

Article pubs.acs.org/IECR

Modeling of a Packed Bubble Column for Methyl Nitrite Regeneration Based on Reaction Kinetics and Mass Transfer Zhenhua Li, Weihan Wang, Jing Lv, and Xinbin Ma* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ABSTRACT: The process of dimethyl oxalate synthesis via CO gaseous phase coupling reaction involves two reactions: CO coupling reaction to produce dimethyl oxalate and the regeneration of methyl nitrite. These two reactions are mutually cycled, and matching reaction rates of both reactions are required. For this purpose, both reactions need to be simulated to provide guidance for scaling up the process. In this paper, the kinetics of methyl nitrite synthesis from NO, NO2, and methanol at atmospheric pressure was obtained by using a stirred reactor. Based on this kinetics equation and mass transfer, a mathematical model for methyl nitrite regeneration reaction from O2, NO, and methanol in a packed bubble column reactor was proposed. Using this model, the effects of reaction temperature, N2 volume fraction, NO/O2 molar ratio, and superficial gas velocity on the methyl nitrite yield were predicted and compared with experimental results. It was found that the model predictions agreed well with experimental data.

1. INTRODUCTION Ethylene glycol (EG) is an important organic compound used in a large number of industrial processes, such as energy, chemical, automotive, and textile.1 A variety of processes have been developed for the synthesis of EG.2,3 Among these synthetic processes, the hydrogenation of dimethyl oxalate (DMO) to EG is considered to be a promising one with potential benefits both environmental and economic.4,5 The synthesis of DMO is the key step for EG production, which includes a CO coupling reaction (eq 1a) and the methyl nitrite (MN) regeneration reaction (eq 1b) with the overall reaction as indicated by eq 1c.6 2CO + 2CH3ONO → (COOCH3)2 + 2NO

(1a)

2CH3OH + 0.5O2 + 2NO → 2CH3ONO + H 2O

(1b)

requiring a molar ratio of NO and NO2 larger than 1:1. The concentration of NO2 will affect the concentration of N2O3 and then the MN regeneration rate. Chen11 has investigated this reaction under varied NO/O2 molar ratios. It was found that the reaction rate was mainly controlled by mass transfer in the liquid film. However, the kinetics was obtained in a bubble column reactor without eliminating the effect of mass transfer. A model reactor for kinetics study was set up by eliminating the effect of mass transfer, and the kinetics of MN synthesis from NO, NO2, and methanol was further studied in this work. Based on NO oxidation kinetics from the literature, MN regeneration kinetics, and mass transfer, a mathematical model depicting MN regeneration reaction from O2, NO, and methanol in a packed bubble column reactor was established. Experiments were carried out in a semicontinuous packed bubble column reactor to investigate the effects of the operation parameters on the MN regeneration reaction. The results were compared with the model predictions.

2CO + 0.5O2 + 2CH3OH → (COOCH3)2 + H 2O (1c)

The overall reaction indicates that the atom economy of DMO synthesis is 86.8% atom economy with only H2O as a byproduct. To maximize the real atom economy of the process, the CO coupling reaction and MN regeneration need to be optimized and occur at the same reaction rate. Studies on the catalysis and kinetics of the CO coupling reaction have been reported; the kinetics is much related to the catalyst used.7−9 For MN regeneration reaction, a few works have been reported but no mathematical model for MN regeneration in a bubble reactor was found.10−14 Liu10 has investigated the reaction kinetics of MN synthesis from N2O3 and methanol at temperatures of 278−298 K with N2O3 being replaced by a mixture of NO and NO2 (molar ratio 1:1). It was concluded that MN synthesis was a fast pseudofirst-order reaction and the reaction rate was affected only by the partial pressure of NO in gaseous phase. However, this kinetics has its limitations because NO is usually in excess in MN regeneration in order to make O2 convert completely, © 2013 American Chemical Society

2. PRINCIPLES OF MN REGENERATION The MN regeneration reaction undergoes a multistep process. In the presence of O2, NO undergoes irreversible oxidation in gaseous phase to form NO2, which is expressed by eq 2a. The kinetics of NO oxidation to NO2 has been reported,15,16 and the NO oxidation rate is second order with respect to NO and first order with respect to O2. Two reversible reactions occurred in the gaseous phase to form N2O3 and N2O4, as shown by eqs 2b and 2c.17,18 Since the forward and backward reaction rates of eqs 2b and 2c are very high, the concentrations of NO, NO2, N2O3, and N2O4 can be considered always to be in equilibrium.19,20 Nitrogen oxides will react with methanol in Received: Revised: Accepted: Published: 2814

October 29, 2012 January 13, 2013 January 25, 2013 January 25, 2013 dx.doi.org/10.1021/ie302966g | Ind. Eng. Chem. Res. 2013, 52, 2814−2823

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic diagram of the experimental apparatus: (1) NO cylinder; (2) O2 cylinder; (3) N2 cylinder; (4) mass flowmeter; (5) tubular reactor; (6) stirred cell; (7) magnetic stirrers; (8) driving motor; (9) condenser; (10) soap film meter; (11) gas chromatograph; (12) tail gas absorber; (13) absolute methanol; (14) plunger pump; (15) liquid reservoir; (16) peristaltic pump; (17) water bath.

(5:1−6:1) and different reaction temperatures (298−318 K). NO and O2 were premixed in a tubular reactor at 298 K and atmospheric pressure with residence time over 10 s, and no oxygen was detected at the exit of the tubular reactor. Fresh methanol was introduced into the reactor by a plunger pump at a certain flow rate, and the liquid was pumped off the reactor by a peristaltic pump at the same flow rate to keep the height of the liquid phase at 20 mm. A magnetic stirrer was used to make the liquid concentration uniform. The reaction temperature was controlled by circulating the water at a certain temperature with an accuracy of ±1 K. After the desired temperature was attained, N2 was introduced into the reactor to replace air, and then the mixture of NO and NO2 was introduced into the reactor. The gaseous phase was kept uniform by a mechanical stirrer. The outlet gas mixture was passed through a cold trap at 273 K to make the water and methanol condense. The noncondensable gas was analyzed using an online gas chromatograph. 3.2.2. Packed Bubble Column Reactor. The experiment setup is schematically shown in Figure 2. The bubble reactor consisted of three parts. The upper condenser section was 200

the liquid phase with N2O3 being transformed to MN and water while N2O4 is transformed to MN and nitric acid, as indicated by eqs 2d and 2e. 2NO + O2 → 2NO2

(2a)

NO + NO2 ⇌ N2O3

(2b)

2NO2 ⇌ N2O4

(2c)

2CH3OH + N2O3 → 2CH3ONO + H 2O

(2d)

CH3OH + N2O4 → CH3ONO + HNO3

(2e)

Usually total divalent nitrogen oxides (NO*) and total tetravalent nitrogen oxides (NO2*) are defined to describe the mixture of nitrogen oxides. The NO* concentration is the sum of concentrations of NO and N2O3, while the NO2* concentration is the sum of concentrations of NO2, N2O3, and double the concentration of N2O4.21 Due to the thermodynamic limitation, the concentrations of N2O3 and N2O4 are too low to be detected. Thus, it is reasonable to consider that the concentrations of NO* and NO2* are approximately equal to the concentrations of NO and NO2, respectively. Therefore, eqs 2b−2e can be summarized as the following equations: 2CH3OH + NO + NO2 → 2CH3ONO + H 2O

(3a)

CH3OH + 2NO2 → CH3ONO + HNO3

(3b)

3. EXPERIMENTAL SECTION 3.1. Materials. The purity of the methanol used (A.R., Jiangfeng Chemical Reagent Co., Ltd., China) was 99.9 wt %. The N2 and O2 used (Liufang Gas Technology Co., Ltd., Tianjin, China) were both of 99.99% purity. The NO used (Date Gas Co., Ltd., Dalian, China) was of 99.5% purity. 3.2. Experimental Setup and Procedure. 3.2.1. Model Reactor for Kinetics Measurement: Stirred Reactor. The experiment setup is schematically shown in Figure 1. The stirred reactor was 80 mm in height and 89 mm in inner diameter. Methanol (124 mL) was added to the reactor, and the height of the liquid phase was 20 mm. A series of experiments were conducted at different NO/O2 molar ratios

Figure 2. Schematic diagram of the experimental apparatus: (1) mixer; (2) packed bubble reactor; (3) cold trap; (4) gas sample; (5) flowmeter; (6) metering pump; (7) methanol; (8) condenser; (9) water bath. 2815

dx.doi.org/10.1021/ie302966g | Ind. Eng. Chem. Res. 2013, 52, 2814−2823


Article

mm in length and 76 mm in inner diameter. It was maintained at 268 K to make methanol and water condense. The low reaction section was 200 mm in length and 25 mm in inner diameter, which was packed with 95 mL of Raschig ring porcelain. The reaction temperature was controlled by circulating water from a thermostatic water bath. The middle transition section was 100 mm in length and 25 mm in inner diameter for gas−liquid separation. In each experiment, 40 mL of methanol was charged into the reactor. N2 was introduced to replace the air inside the reactor. Then, N2, NO, and O2 were introduced to the reactor at a certain flow rate, and the reaction was conducted at atmospheric pressure. The uncondensed gas was analyzed online by a SP2100 gas chromatograph with a TCD detector. An oxydipropionylnitrile packed column, a Porapak-QS packed column, and a 5A molecular sieve packed column were used. Hydrogen was used as the carrier gas with a flow rate of 25 mL/min. The temperatures of injection, column, and detector were 353, 353, and 373 K, respectively. An external standard method was used to obtain the concentration of component i in outlet gas, xi. 3.3. Calculation of MN Regeneration Rate and MN Yield. The flow rate of outlet gas (G) was measured by a soap film flowmeter. The generating rate of MN was obtained by eq 4. RMN =

GxMN 60a(RT )

x NO = x NOx

* 0 + x NO ) − ( R + RNO2 ) NO 2 * *

(8a)

0 G 0x NO − RNO2 2

* 0 0 G 0(x NO + x NO ) − ( R + RNO2 ) NO (8b) 2 * * Since NO/O2 molar ratios in all experiments were no less than the stoichiometry ratio of 4:1, O2 was selected as the key component for calculating O2 conversion, NO2 yield, and MN yield as follows:

XO2 =

G 0xO02 − GxO2 G 0xO02

·100% (9a)

YNO2 =

1 Gx NO2 ·100% 2 G 0xO02

(9b)

YMN =

1 GxMN ·100% 4 G 0xO02

(9c)

4. KINETICS MODEL FOR MN REGENERATION FROM NO, NO2, AND METHANOL 4.1. Mass Transfer Resistance. The experiments of MN synthesis at different gaseous or liquid stirring speeds were carried out to evaluate the effect of external mass transfer resistance. The gaseous stirring speed was varied from 50 to 300 rpm while keeping the other operating parameters constant. Figure 3 shows that the MN generating rate increases

(4)

LC HNO3 60a

(5)

Since the experiment in the packed bubble reactor was semicontinuously operated, it was hard to get the instant formation rate of nitric acid. The average formation rate of nitric acid was obtained by plotting the amount of nitric acid in liquid phase versus different reaction times, and the slope of the curve is the nitric acid formation rate. Based on the stoichiometry of two reactions for MN regeneration, as shown in eqs 3a and 3b, the main reaction rate Rr and the side reaction rate Rs were obtained by the following equations: Rr =

G

0 (x NO

x NO2 = x NOx

In the MN regeneration kinetics test, the concentration of nitric acid in outlet liquid was analyzed by an acid−base titration method. The sample was neutralized by titrating NaOH solution (0.05 mol/L), using phenolphthalein as an indicator. The generated rate of nitric acid was obtained by eq 5. RHNO3 =

0 G 0x NO − RNO

0

1 (RMN − RHNO3) 2

R s = RHNO3

Figure 3. Effect of gas stirring speed on MN generating rate. T = 303 K; G = 500 mL/min; N2 volume fraction = 80%; NO/O2 molar ratio = 5:1; L = 2 mL/min; liquid stirring speed = 150 rpm.

(6a)

with the gaseous stirring speed increasing from 50 to 150 rpm. However, relatively little change is observed at the speed between 150 and 300 rpm. The effect of liquid stirring speed was further investigated, as shown in Figure 4. Obviously, the MN generating rate is stable within the liquid stirring speed varied from 100 to 250 rpm. These results imply that the mass transfer resistances in both gaseous phase and liquid phase are not the rate-controlling steps. Thus, the following kinetics experiments in this work were conducted at 200 rpm gaseous stirring speed and 150 rpm liquid stirring speed. 4.2. Effect of Liquid Flow Rate. In the MN synthesis reaction, the generated H2O and HNO3 are accumulated in the liquid phase and decrease the concentration of methanol in the liquid, which would impact the physical and chemical

(6b)

Since the peaks of NOx were overlapped mutually in the gas chromatogram, the analytical concentration should be the sum of the concentrations of NOx. According to mass balance, the reaction rates of NO* and NO2* were obtained by RNO = R ra (7a) * RNO2 = R ra + 2R sa (7b) * Because the concentrations of NO* and NO2* approximately equal the concentrations of NO and NO2, respectively, the molar fractions of NO and NO2 in outlet gas were expressed as 2816



Article

Figure 4. Effect of liquid stirring speed on MN generating rate. T = 303 K; G = 500 mL/min; N2 volume fraction = 80%; NO/O2 molar ratio = 5:1; L = 2 mL/min; gas stirring speed = 200 rpm.

Figure 6. Steady-state concentration profiles for diffusion and reaction in a liquid film.

properties of the liquid phase. It is expected that the liquid flow rate may influence the liquid phase composition and then the MN formation rate. A series of experiments were carried out with the liquid flow rate varied from 0.5 to 3 mL/min. As shown in Figure 5, MN and nitric acid generating rates are not influenced by the outlet liquid flow rate. The following experiments in this work were conducted at 2 mL/min liquid flow rate.

In this experiment, we found that the nitric acid formation rate is so much lower than the MN formation rate that reaction 3b could be ignored. Using a two-film model, the material balances of NO and NO2 in a microunit volume in the liquid film are expressed as follows: ⎞⎤ ⎛ dC ⎛ dC B ⎞ ⎡ d2C B ⎜ − DB ⎟ − ⎢ − D B⎜ B + d Z ⎟⎥ − R r d Z = 0 2 ⎝ dZ ⎠ ⎢⎣ dZ ⎠⎥⎦ ⎝ dZ (11a)

⎛ dC ⎞⎤ ⎛ dC C ⎞ ⎡ d2CC ⎜ −DC ⎟ − ⎢ −DC⎜ C + ⎟⎥ − R r d Z = 0 d Z ⎝ dZ ⎠ ⎣⎢ d Z2 ⎝ dZ ⎠⎥⎦ (11b)

Equations 11a and 11b can be simplified as − DB −DC

d2C B d Z2 d2CC d Z2

= R r = k rC EC BCC

(12a)

= R r = k rC EC BCC

(12b)

Figure 5. Effect of liquid flow rate on MN and nitric acid generating rate. T = 303 K; G = 500 mL/min; N2 volume fraction = 80%; NO/O2 molar ratio = 5:1; gas stirring speed = 200 rpm; liquid stirring speed = 150 rpm.

NO2 in this reaction is selected as the key component in MN regeneration because it is insufficient compared with NO. The following expression is obtained from eqs 12a and 12b: D C B = C B* − C (CC* − CC) DB (13)

4.3. Kinetics Equation. Several kinetics equations for MN regeneration have been reported, and all of them describe MN regeneration rate as a quick gas−liquid reaction in the liquid film with a power-law expression.10−12 In our study, the kinetics model was proposed to express the MN formation rate with the concentration of reactants as follows:

Substituting eq 13 into eq 12b and assuming η = dCC/dZ, we obtained:

R r = k rC EC BCC

η

kC dη = r E C BCC dC C DC

(14)

On the basis of the concentration profiles in Figure 6, the boundary conditions for eq 14 are as follows:

(10)

where kr is the rate constant of the regeneration reaction and Ci is the molar concentration of component i in the liquid film. In this work, subscripts “A”, “B”, “C”, “D”, and “E” denote O2, NO, NO2, MN, and methanol, respectively. According to the two-film model, the profiles of components in the liquid film are shown in Figure 6. Since methanol concentration has little change within the liquid film, CE can be considered a constant.

C B = C B*,

Z = 0,

Z = δR ,

C B = C BL ,

CC = CC*

CC = 0,

η=0

Because η < 0 η=− 2817

D k rC E ⎡ 2 DC ⎤ CC ⎥ CC ⎢C B* − C CC* + DC ⎣ DB 3 DB ⎦

(15)



Article

The NO2 mass transfer rate through the gas−liquid interface is NC|Z = 0 = −DC(η)Z = 0 =

Table 3. Rate Constants of MN Regeneration at Different Temperatures

⎡ 1 DC *⎤ * DCk rC E⎢C B* − CC ⎥ CC 3 DB ⎦ ⎣ (16)

As the NO2 mass transfer and reaction achieve steady state, there is R r = NC|Z = 0 =

⎡ 1 DC *⎤ * DCk rC E⎢C B* − CC ⎥ CC 3 DB ⎦ ⎣

(17)

The enhancement factor, β, is defined as β=

Rr = kLCCC*

DCk rC E ⎡ 1 DC *⎤ C* − CC ⎥ 2 ⎢ B 3 DB ⎦ (kLC) ⎣

(18)

The mass transfer coefficient kLC was determined by Shi et al.22 using the method of CO2 absorption into aqueous solutions of sodium hydroxide. When the resistance in gas film is neglected,11 the concentrations of NO and NO2 in gas−liquid interface can be obtained by Henry’s law: p C B* = B HB (19a) CC* =

T/K

PNO/ kPa

PNO2/ kPa

Rr × 104/ (mmol/cm2·s)

kr × 10−9/ (cm6/mmol2·s)

298 298 298 303 303 303 308 308 308 413 413 413 418 418 418

3.70 5.56 8.28 3.73 5.61 8.30 3.76 5.62 8.34 3.78 5.63 8.35 3.80 5.65 8.36

0.40 0.34 0.29 0.37 0.29 0.27 0.34 0.28 0.23 0.32 0.27 0.22 0.30 0.25 0.21

4.08 4.21 4.41 4.11 4.24 4.42 4.16 4.26 4.44 4.18 4.28 4.46 4.22 4.30 4.47

2.52 2.46 2.52 2.97 3.29 2.92 3.55 3.64 3.84 3.91 3.96 4.26 4.52 4.68 4.62

pC HC

(19b)

In this work, the Henry’s constants of NO and NO2 in methanol at different temperatures are listed in Table 1, Table 1. Henry’s Constants of NO and NO2 in Methanol Solvent at Different Temperatures T/K

HNO × 10−3/(kPa·cm3/mmol)

HNO2 × 10−3/(kPa·cm3/mmol)

298 303 308 313 318

0.9497 0.9997 1.0509 1.1031 1.1564

3.1710 3.4035 3.6443 3.8934 4.1509

Figure 7. Arrhenius plot of the MN regeneration reaction. Rr =

5. REACTOR MODEL 5.1. Determination of the Byproduct Accompanying MN Regeneration. As mentioned above, the formation rate of nitric acid was obtained by plotting molar quantities of nitric acid in the liquid phase versus reaction time after the reaction reached steady state. One fitting result is shown in Figure 8, where the slope is considered to be the formation rate of nitric acid. To assess the side reaction accompanying MN regeneration, a series of MN regeneration experiments at different NO/O2 molar ratios were carried out. As shown in Figure 9, the rate ratios of the main reaction (eq 3a) to the side reaction (eq 3b) are all larger than 50:1 at NO/O2 molar ratios ranging from 4:1 to 8:1. Since the MN yield is about 85%, the yield of nitric acid should be less than 2%. Therefore, the side reaction between NO2 and methanol to nitric acid could be ignored when the reactor model was set up to predict MN regeneration. Because O2 conversion is almost 100%, it can be deduced that the generated NO2 is not consumed completely. The main byproduct during the MN regeneration reaction should be the intermediate product, NO2.

Table 2. Diffusion Constants of NO and NO2 Estimated by Wilke and Chang’s Equation at Different Temperatures DNO × 105/(cm2/s)

DNO2 × 105/(cm2/s)

298 303 308 313 318

4.6098 4.9590 5.3635 5.7843 6.2215

5.5270 5.9458 6.4308 6.9353 7.4595

⎛ 23725 ⎞⎛ pB 1 DC pC ⎞ ⎟⎜ − DC(3.70 × 107) exp⎜ − ⎟C E ⎝ RT ⎠⎝ HB 3 DB HC ⎠

(20)

estimated by a molecular thermodynamic model.23 The diffusion coefficients of NO and NO2 were calculated according to the Wilke−Chang equation,24 and the estimated results are listed in Table 2. The regeneration kinetics experiments were performed at different temperatures from 298 to 318 K. The rate constants at various temperatures were calculated by eq 17. The results are listed in Table 3. Figure 7 shows the logarithm of the reaction rate constant kr as a function of reciprocal reaction temperature. The kinetics equation can be represented as

T/K

pC HC

2818



Article

tration of methanol was 99.9 wt %. Even though the generated H2O would enrich in the liquid phase, the concentration of methanol in the reactor was still much larger than 40 wt %. Therefore, the impact of the variation of methanol concentration during the reaction in the semicontinuous process can be ignored. 5.3. Reactor Model. The model for the MN regeneration reaction in a packed bubble column reactor at steady-state operation is proposed by assuming the following: 1. The gaseous phase moves from the bottom of the column in a plug flow and follows ideal gas behavior. 2. The concentration of methanol in the liquid film is regarded as a constant. The partial pressure of methanol is regarded as its saturated vapor pressure. 3. MN generated from the regeneration reaction diffuses into the gas phase quickly. However, MN can dissolve in methanol. The amount of dissolved MN in methanol is considered to be the saturated value at a certain temperature. 4. There are no concentration and temperature gradients in the radial direction. 5. The gas and liquid holdups are uniform throughout the column. Therefore, the value of the effective interfacial area and mass transform coefficient are also uniform throughout the column. The MN regeneration reaction contains two-step reactions given in eqs 2a and 3a. On the basis of the two-film theory, the mass balances for O2, NO, NO2, and MN can be expressed as

Figure 8. Formation rate of nitric acid. T = 303 K; uG = 2.43 cm/s; N2 volume fraction = 60%; NO/O2 molar ratio = 6:1; methanol volume = 40 mL; initial methanol concentration = 99.9 wt %.

Figure 9. Effect of NO/O2 molar ratio on MN yield and Rr/Rs ratio. T = 303 K; uG = 2.43 cm/s; N2 volume fraction = 60%; NO/O2 molar ratio = 4:1; methanol volume = 40 mL; initial methanol concentration = 99.9 wt %.

5.2. Effect of Methanol Concentration. The effect of the initial methanol concentration was investigated over the range 20−99.9 wt %, and the result is shown in Figure 10. It is obvious that the methanol concentration shows a negligible impact on the MN yield at methanol concentrations higher than 40 wt %. Only when the methanol concentration is lower than 40 wt %, the MN yield has a sharp drop with the decrease of methanol concentration. In this work, the initial concen-

uG0

dCAG = −R oεG dh

(21a)

uG0

dC BG = −2R oεG − R raL dh

(21b)

uG0

dCCG = 2R oεG − R raL dh

(21c)

uG0

dC DG = 2R raL − M dh

(21d)

with NO oxidation rate Ro and regeneration reaction rate Rr described by R o = koCAG(C BG)2 R r = kLβCC* = kLCC*

(22a)

⎞ DCk rC E ⎛ D * − 1 C CC*⎟ C ⎜ B 3 DB ⎠ (kLC)2 ⎝

(22b)

where G0 is the gas inlet flow rate, and M in eq 21d is the dissolved rate of MN in the liquid phase. The reaction rate constant of NO oxidation is expressed as15 ⎛ 7733 ⎞ ⎟ ko = 1.2 × 103 exp⎜ ⎝ RT ⎠

(23)

In this work, the partial pressure of methanol in gas is considered to be the saturated steam pressure. According to Henry’s law and the ideal gas equation, the concentrations of NO and NO2 in the gas−liquid interface were obtained by the following equations: Figure 10. Effect of methanol concentration on MN yield. T = 308 K; uG = 2.43 cm/s; N2 volume fraction = 80%; NO/O2 molar ratio = 5:1; methanol volume = 40 mL.

C B* = 2819

RTC BG HB

(24a)


Industrial & Engineering Chemistry Research RTCCG HC

CC* =

Article

(24b)

Considering the volume changes in the gas phase, the concentrations of O2, NO, NO2, and MN in gaseous phase are given by CAG =

C BG =

CCG =

C DG =

0 CAG (1 − XA )

1 + σAxA0XA

(25a)

0 C BG (1 − XB)

1 + σAxA0XA

(25b)

0 2CAG YC

1 + σAxA0XA

(25c)

0 4CAG YD

1 + σAxA0XA

(25d)

The gas holdup (εG) and average bubble diameter (dvs) are defined as25,26 −1/8 ⎛ μL ⎞1/4 ⎛ σLg ⎞ ⎜ ⎟ εG = 1.2⎜ ⎟ ⎜ uG 3/4 ⎟ ⎝ σL ⎠ ⎝ ρL ⎠

⎛ gD 2ρ ⎞−0.5⎛ gD 3 ⎞−0.12 ⎛ u d vs T L ⎟⎟ ⎜ T2 ⎟ ⎜⎜ G = 26⎜⎜ σ DT ν ⎝ gDT ⎝ ⎠ ⎝ L ⎠ L

(26)

⎞−0.12 ⎟⎟ ⎠

Figure 11. Effect of temperature on MN regeneration (uG = 2.43 cm/ s; N2 volume fraction = 80%; NO/O2 molar ratio = 5:1; methanol volume = 40 mL). (a) Calculated NO2 and MN yields as functions of the liquid height. (b) Experimental and calculated results of total MN yield at different temperatures.

(27)

Therefore, the effective gas−liquid interfacial area (aL) is given by aL =

6εG d vs

the suitable temperature range for the MN regeneration reaction. 6.2. Effect of N2 Volume Fraction. N2 is used as an inert gas in the regeneration system for security consideration. The effect of the initial N2 volume fraction on the MN yield was investigated over the range 50−80%. As observed in Figure 12a, the NO2 yield increases first along with the reactor height until reaching a maximum value and then decreases. Increasing the initial N2 volume fraction reduces the MN yield but increases the NO2 yield at the exit of reactor. This is due to the positive effect of high reactant concentration on both NO oxidation in gaseous phase and the interphase mass transfer. Figure 12b shows that the calculated result coincides well with the experimental value. On the basis of this result and the security consideration, the suitable N2 volume fraction was 65−80%. 6.3. Effect of NO/O2 Molar Ratio. Because O2 is required to be completely consumed in the regeneration reaction, the NO/O2 molar ratio must be higher than the stoichiometric ratio 4:1. The effect of the NO/O2 molar ratio was investigated over the range 4:1 to 8:1 by varying the NO/O2 molar ratio at a constant volume fraction of N2. As observed in Figure 13a, the MN yield increased while the NO2 yield decreased with an increase of the NO/O2 molar ratio. The calculated result is consistent with the experimental result, as shown in Figure 13b. Hence, the suitable NO/O2 molar ratio range for MN regeneration reaction is from 5:1 to 6:1. 6.4. Effect of Superficial Gas Velocity. The effect of superficial gas velocity on the MN yield was investigated from 2.43 to 4.85 cm/s, and the results are shown in Figure 14a. It is obvious that the MN yield decreases while the NO2 yield increases with an increase of superficial gas velocity. On one

(28)

The mass transfer coefficient of the liquid film is calculated according to the following equation:27 3/4 1/2 0.15DA ⎛ νL ⎞ ⎛ d vsuGρL ⎞ ⎟⎟ kL = ⎜ ⎟ ⎜⎜ d vs ⎝ DA ⎠ ⎝ μL ⎠

(29)

6. MODEL PREDICTION AND COMPARISON WITH EXPERIMENTAL RESULTS Using the above reactor model, the effects of reaction temperature, N2 volume fraction, NO/O2 molar ratio, and superficial gas velocity (uG) on the MN yield were predicted and compared with the experimental results. 6.1. Effect of Temperature. The effect of temperature on the MN yield was investigated over the range 298−323 K, and the results are shown in Figure 11. With the increase of temperature, the NO oxidation reaction rate decreases,28 while both the mass transfer rate and MN regeneration rate increase. Figure 11a shows that both NO2 and MN yields varied little with temperature. The experimental and calculated results for the MN yield as a function of reaction temperature are shown in Figure 11b. Both indicate that the MN yield changes little when the temperature is varied from 298 to 323 K. Increasing the temperature decreases the solubility of MN in methanol,29 which is favorable to the MN yield. However, at high temperature, more energy consumption is needed for condensing methanol vapor. Hence, 303−313 K would be 2820



Article

Figure 12. Effect of N2 volume fraction on MN regeneration (T = 308 K; uG = 2.43 cm/s; NO/O2 molar ratio = 5:1; methanol volume = 40 mL). (a) Calculated NO2 and MN yields as functions of the liquid height. (b) Experimental and calculated results of total MN yield at different N2 volume fractions.

Figure 13. Effect of NO/O2 molar ratio on MN regeneration (T = 308 K; uG = 2.43 cm/s; N2 volume fraction = 80%; methanol volume = 40 mL). (a) Calculated NO2 and MN yields as functions of the liquid height. (b) Experimental and calculated results of total MN yield at different NO/O2 molar ratios.

6.5. Comparison of Experimental and Calculated Data. Figure 15 provides the comparison of calculated MN yield with experimental MN yield. Compared with the experimental results, the calculated MN yields show a negative deviation. Two reasons are provided to explain this deviation: (1) The small amount of MN generated from NO2 with methanol is ignored in this model, which makes the calculated results a little lower than the experimental values. (2) The dissolved MN in methanol may be not saturated in an actual process. As observed, the relative errors between the experimental and calculated yields are all below 10%. This suggests that the reactor model we proposed fits well with the experimental results and is feasible for simulation in a scale-up MN regeneration process.

hand, when the superficial gas velocity is higher, NO oxidation goes faster and the instantaneous NO/NO2 molar ratio in the gaseous phase is lower. On the other hand, an increase in the superficial gas velocity reduces the residence time of gases. These have a negative effect on the MN yield. As observed in Figure 14b, both experimental and calculated results show that a higher MN yield could be achieved at a lower gas flow rate. The suitable inlet gas flow rate is 2.43−3.03 cm/s. Figure 14b indicates that the calculated result coincides well with the experimental value at low gas velocity but it deviates from the experimental value at high gas velocity. Here we provide the following reasons: (1) The introduction of a packing prevents a rapid escape of bubbles in the column because the packing provides the bubbles a tortuous path, resulting in longer traveling distance for bubbles in the column and then an increase in gas holdup. This effect of packing structure is more significant at higher superficial gas velocity.30−32 However, in this work, the effect of packing structure on gas holdup was not considered. This may be one reason for the difference of calculated and experimental methyl nitrite yields. (2) The amount of dissolved MN in methanol is considered to be a constant as its saturated value in this model, which was affected only by temperature. Actually, the amount of dissolved MN in methanol increased slightly with time until its saturated value in methanol, which was also influenced by gas velocity.33 With the increase of the superficial gas velocity, the quantity of dissolved MN in liquid should be a little decreased. This may be another reason for the difference of calculated and experimental MN yields.

7. CONCLUSIONS The kinetics of MN synthesis from NO, NO2, and methanol was obtained using a model reactor suitable for gas−liquid reaction. MN regeneration from O2, NO, and methanol in a packed bubble column reactor was further studied in the consideration of NO 2 as the main byproduct in the regeneration process. Based on the kinetics equation and plug flow assumption, a mathematical model depicting MN regeneration reaction in the packed bubble reactor was proposed to predict the MN yield at varied reaction conditions. As a result, the calculated MN yield by the model coincided well with the experimental data, which proved that the model was effective for guiding the scaling up of the MN regeneration process. Both the calculated and experimental data showed that the gaseous phase composition and superficial gas velocity had 2821



■

Article

ACKNOWLEDGMENTS

Financial support from the National Key Project for the 11th Five Year Plan (Grant 2006BAE02B00) and the Program of Introducing Talents of Discipline to Universities (B06006) is greatly appreciated.

■

Figure 14. Effect of superficial gas velocity on MN regeneration (T = 308 K; N2 volume fraction = 80%; NO/O2 molar ratio = 5:1; methanol volume = 40 mL). (a) Calculated NO2 and MN yields as functions of the liquid height. (b) Experimental and calculated results of total MN yield at different superficial gas velocities.

Figure 15. Comparison of calculated and experimental MN yields.

great influences on the MN yield. The suitable operation conditions were determined to be as follows: reaction temperature of 303−313 K, NO/O2 molar ratio of 5:1−6:1, N2 content of 65−80% volume fraction, and superficial gas velocity of 2.43−3.03 cm/s.

■

NOMENCLATURE a = cross-sectional area of the stirred reactor, cm2 aL = effective gas−liquid interfacial area of unit volume, cm−1 Ci = molar concentration of component i in liquid flim, mmol/mL CiG = molar concentration of component i in gaseous phase, mmol/mL C*i = molar concentration of component i in gas−liquid interface, mmol/mL CiL = molar concentration of component i in bulk liquid, mmol/mL Di = diffusion coefficient of component i in liquid, cm2/s dvs = bubble diameter, cm G = gas flow rate, mL/min G0 = initial gas flow rate, mL/min Hi = Henry’s constant of component i, kPa·cm3/mmol h = height of liquid, cm kL = mass transfer coefficient in packed bubble column reactor, cm/s kLC = mass transfer coefficient of NO2 in stirred reactor, cm/ s ko = reaction rate constant of reaction 2a, mL2/(mmol2·s) kr = reaction rate constant of reaction 3a, mL2/(mmol2·s) L = liquid flow rate, mL/min M = dissolved rate of MN in liquid phase, mmol/(mL·s) Ni = mass transfer rate of component i through gas−liquid interface, mmol/(cm2·s) pi = partial pressure of component i, kPa pi* = partial pressure of component i in gas−liquid interface, kPa R = gas constant, 8.314 J/(mol·K) RMN = generating rate of MN, mmol/(cm2·s) RHNO3 = generating rate of nitric acid, mmol/(cm2·s) RNO* = consumption rate of the total divalent nitrogen oxides, mmol/s RNO2* = consumption rate of the total tetravalent nitrogen oxides, mmol/s Ro = reaction rate of reaction 2a, mmol/(mL·s) Rr = reaction rate of reaction 3a, mmol/(cm2·s) Rs = reaction rate of reaction 3b, mmol/(cm2·s) T = temperature, K uG = superficial gas velocity, cm/s u0G = initial gas velocity, cm/s Xi = conversion of component i xi = molar fraction of component i in gaseous phase x0i = initial molar fraction of component i in gaseous phase Yi = yield of component i yi = molar fraction of component i in liquid phase Z = coordinate

Greek Symbols

AUTHOR INFORMATION

β = enhancement factor δL = thickness of liquid film, cm δR = film thickness for reaction zone, cm εG = gas holdup σA = expansion factor to O2

Corresponding Author

*E-mail: [email protected]. Fax: +86-22-87401818. Notes

The authors declare no competing financial interest. 2822



Article

Subscripts

(24) Wilke, C. R.; Chang, P. Correlation of Diffusion Coefficients in Dilute Solutions. AIChE J. 1955, 1, 264. (25) Froment, G. F.; Bischoff, K. B.; De Wilde, J. Chemical Reactor Analysis and Design; Wiley: New York, 1990. (26) Akita, K.; Yoshida, F. Bubble Size, Interfacial Area, and LiquidPhase Mass Transfer Coefficient in Bubble Columns. Ind. Eng. Chem. Res. 1974, 13, 84. (27) Schuegerl, K.; Luecke, J.; Lehmann, J.; Wagner, F. Application of Tower Bioreactors in Cell Mass Production. Adv. Biochem. Eng. 1978, 8, 63. (28) De Paiva, J. L.; Kachan, G. C. Modeling and Simulation of a Packed Column for NOx Absorption with Hydrogen Peroxide. Ind. Eng. Chem. Res. 1998, 37, 609. (29) Liu, N. Solubility Determination of Ethyl/Methyl Nitrite and Study on Mass Transfer Thereof. Thesis, Tianjin University, Tianjin, China, 2009. (30) Therning, P.; Rasmuson, A. Liquid Dispersion and Gas Holdup in Packed Bubble Columns at Atmospheric Pressure. Chem. Eng. J. 2001, 81, 69. (31) Shah, M.; Kiss, A. A.; Zondervan, E.; Van Der Schaaf, J.; De Haan, A. B. Gas holdup, axial dispersion, and mass transfer studies in bubble columns. Ind. Eng. Chem. Res. 2012, 51, 14268. (32) Moustiri, S.; Hebrard, G.; Roustan, M. Effect of a New High Porosity Packing on Hydrodynamics of Bubble Columns. Chem. Eng. Process. 2002, 44, 419. (33) Li, R. Simulation Study on Carbon Monoxide CouplingRegeneration Cycling Process to Produce Diethyl Oxalate. Thesis, Tianjin University, Tianjin, China, 2009.

cal = calculated from the model exp = experimental result

■

REFERENCES

(1) Yue, H.; Zhao, Y.; Ma, X.; Gong, J. Ethylene Glycol: Properties, Synthesis, and Applications. Chem. Soc. Rev. 2012, 41, 4218. (2) Xu, G. H.; Li, Y. C.; Li, Z. H.; Wang, H. J. Kinetics of the Hydrogenation of Diethyl Oxalate to Ethylene Glycol. Ind. Eng. Chem. Res. 1995, 34, 2371. (3) Kawabe, K. Development of Highly Selective Process for MonoEthylene Glycol Production from Ethylene Oxide via Ethylene Carbonate Using Phosphonium Salt Catalyst. Catal. Surv. Asia 2010, 14, 111. (4) Yin, A.; Guo, X.; Dai, W. L.; Li, H.; Fan, K. Highly Active and Selective Copper-containing HMS Catalyst in the Hydrogenation of Dimethyl Oxalate to Ethylene Glycol. Appl. Catal., A 2008, 349, 91. (5) Xu, G.; Ma, X.; He, F.; Chen, H. F. Characteristics of Catalyst for Carbon Monoxide Coupling Reaction. Ind. Eng. Chem. Res. 1995, 34, 2379. (6) Yamamoto, Y. Vapor Phase Carbonylation Reactions Using Methyl Nitrite Over Pd Catalysts. Catal. Surv. Asia 2010, 14, 103. (7) Gao, X.; Zhao, Y.; Wang, S.; Yin, Y.; Wang, B.; Ma, X. A Pd-Fe/ α-Al2O3/Cordierite Monolithic Catalyst for CO Coupling to Oxalate. Chem. Eng. Sci. 2011, 66, 3513. (8) Ji, Y.; Liu, G.; Li, W.; Xiao, W. The Mechanism of CO Coupling Reaction to Form Dimethyl Oxalate over Pd/α-Al2O3. J. Mol. Catal. A: Chem. 2009, 314, 63. (9) Uchiumi, S. I.; Ataka, K.; Matsuzaki, T. Oxidative Reactions by a Palladium-Alkyl Nitrite System. J. Organomet. Chem. 1999, 576, 279. (10) Liu, G.; Ji, Y.; Li, W. Kinetic Study on Methyl Nitrite Synthesis from Methanol and Dinitrogen Trioxide. Chem. Eng. J. 2010, 157, 483. (11) Chen, J.; Xu, G.; Li, Z.; Chen, H. Kinetics of Regeneration Reaction for CO Coupling. J. Chem. Ind. Eng. 1993, 44, 66 (in Chinese). (12) Wang, H.; Li, G. Kinetic Study on the Synthesis of Ethyl Nitrite by the Reaction of C2H5OH, O2, and NO in a Trickle Bed Reactor. Chem. Eng. J. 2010, 163, 422. (13) Nishihira, K., Tanaka, S., Yoshida, S. Process for Producing Alkyl Nitrite. U.S. Patent 6,191,302, 2001. (14) Sugise, R., Tanaka, S., Ii, H., Mii, K. Process for Producing Alkyl Nitrite. U.S. Patent 7,714,160, 2010. (15) Joshi, J. B.; Mahajani, V. V.; Juvekar, V. A. Absorption of NOx Gases. Chem. Eng. Commun. 1985, 33, 1. (16) Atkinson, R.; Baulch, D.; Cox, R.; Crowley, J.; Hampson, R.; Hynes, R.; Jenkin, M.; Rossi, M.; Troe, J. Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry: Volume IGas Phase Reactions of Gas Phase Reactions of Ox, HOx, NOx and SOx Species. Atmos. Chem. Phys. 2004, 4, 1461. (17) Jethani, K. R.; Suchak, N. J.; Joshi, J. B. Modeling and Simulation of a Spray Column for NOx Absorption. Comput. Chem. Eng. 1992, 16, 11. (18) Hofytizer, P. J.; Kwanten, F. J. G. Absorption of Nitrous Gases; CRC Press: Cleveland, OH, 1972. (19) Pradhan, M. P.; Joshi, J. B. Absorption of NOx Gases in Plate Column: Selective Manufacture of Sodium Nitrite. Chem. Eng. Sci. 2000, 55, 1269. (20) Pradhan, M. P.; Joshi, J. B. Absorption of NOx Gases in Aqueous NaOH Solutions: Selectivity and Optimization. AIChE J. 1999, 45, 38. (21) Andrew, S. P. S.; Hanson, D. D1. The Dynamics of Nitrous Gas Absorption. Chem. Eng. Sci. 1961, 14, 105. (22) Shi, Y.; Littlejohn, D.; Chang, S. G. Kinetics of NO Absorption in Aqueous Iron(II) Bis(2,3-dimercapto-1-propanesulfonate) Solutions Using a Stirred Reactor. Ind. Eng. Chem. Res. 1996, 35, 1668. (23) Xu, Y.; Hu, Y.; Liu, G. Molecular Thermodynamics of Gas Solubility (II) Henry’s Constants of Gases in Polar Solvents. J. Chem. Ind. Eng. 1987, 38, 137 (in Chinese). 2823


Modeling of a Packed Bubble Column for Methyl Nitrite Regeneration

Recommend Documents