Reaction Modeling

ARTICLE pubs.acs.org/IECR

Experimental Measurements and Mass Transfer/Reaction Modeling for an Industrial NOx Absorption Process Kyle G. Loutet,*,† Andres Mahecha-Botero,†,‡ Tony Boyd,† Steven Buchi,† Doug Reid,§ and Clive M.H. Brereton† †

NORAM Engineering and Constructors Limited, 200 Granville Street, Suite 1800, Vancouver, British Columbia, Canada V6C 1S4 Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia, Canada V6T 1Z3 § Huntsman Polyurethanes (U.K.) Limited, P.O. Box 99, Redcar, United Kingdom TS10 4YA ‡

ABSTRACT: An industrial reactive separation process for NOx absorption into water and nitric acid in a world-scale mononitrobenzene (MNB) plant in Redcar, U.K., was investigated. Gas- and liquid-phase concentrations were measured in situ, while operating conditions such as temperature, pressure, and absorbent flow were varied. Furthermore, a comprehensive mass transfer/reaction model was developed in the process simulator Aspen Plus to simulate the absorption process, with the field process data used for validation. The model accurately predicted NOx removal from the gas (i.e., typically within 3.5% of the plant data). Other key findings are that the model successfully predicted changes in performance when key parameters such as temperature and pressure were varied. Furthermore, addition of a bleaching section was investigated, resulting in a significant improvement in NOx removal. Currently, the completed model is used to simulate existing and proposed NOx absorption systems and to develop new and innovative designs for NOx capture.

1. INTRODUCTION With numerous kinetic and equilibrium gas- and liquid-phase reactions and mass and heat transfer phenomena, absorption of NOx gases (i.e., NO, NO2, N2O3, N2O4, HNO2, HNO3) is one of the more complex absorption operations in existence.1 Industrial NOx absorption is utilized in the manufacture of nitric acid and in the treatment of waste gases. With the industrialized world increasingly conscious of its environmental footprint and with governments adopting ever-stricter environmental regulations, designing NOx columns for the purpose of pollution abatement is of particular interest. Lowering atmospheric NOx emissions is an important regulatory goal, as illustrated by the European Union’s First Daughter Directive2 which seeks to cut the annual mean NO2 concentration to 40 μg/m3. When NOx gases encounter atmospheric water vapor they can combine to form nitric acid, a contributor to acid rain.3 In Shanghai, China, rainwater pH is observed to decrease in the cold season, when NOx emissions are highest due to increased burning of coal and cold start-up of commuter vehicles.4 Deposited acid rain can in turn cause environmental damage in the form of acidified freshwater and habitat loss for freshwater species and soil acidification and the subsequent adverse effect on plant life.5 Additionally, acid rain can deposit on human-made structures and cause structural damage.6 The release of NOx to the atmosphere can also have a negative effect from an aesthetic standpoint. When NO is released to the atmosphere it reacts with oxygen, ozone, and radicals to form NO2,7 the brown, odorous, and toxic compound that is largely responsible for smog in urban areas.8 However, for many industrial (i.e., noncombustion) NOx removal processes, increasingly tight water regulations are also influencing plant r 2011 American Chemical Society

designs, as the conventional alkali scrubber results in high levels of nitrates and nitrites in effluents. Reliable and accurate modeling of NOx absorption operations therefore represents a useful tool in improving designs and ultimately meeting and exceeding environmental targets for both air and water. 1.1. Industrial Nitration Process. In NORAM Engineering’s patented nitration process (U.S. Patent 5,313,0099), an aromatic hydrocarbon, benzene, is continuously nitrated, using nitric acid as a feedstock, resulting in the production of mononitrobenzene (MNB). In downstream plants, the product MNB is normally converted to aniline, which is used in the production of methylene diphenyl diisocyanate (MDI) and ultimately used as a feedstock in the production of polyurethane plastic. Byproduct NOx gases produced in the nitration step are separated from the liquid products and sent to a NOx absorption column where they are treated counter-currently under pressure with water. The nitric acid produced in the NOx column is collected and recycled to the nitration step, displacing fresh nitric acid feed, lowering the NOx in the plant vent, and reducing the nitrate/nitrite levels in the plant effluent (U.S. Patent 5,963,87810). In the NOx column, the gases and air are fed to the bottom of the column and contacted with water. A portion of the liquid outlet is typically cooled and recirculated to the top and/or to an intermediate point in the column. The column can contain packed or trayed sections or a combination of both. Figure 1 shows a simplified version of the NOx absorption Received: February 26, 2010 Accepted: December 20, 2010 Revised: August 20, 2010 Published: January 13, 2011 2192

dx.doi.org/10.1021/ie100436p | Ind. Eng. Chem. Res. 2011, 50, 2192–2203

Industrial & Engineering Chemistry Research process used in collecting process data for validation of the model. Other variations on the presented configuration have been used industrially, including elimination of the effluent stream. 1.2. Modeling of NOx Absorption. A number of mathematical models which incorporate reaction/mass transfer kinetics have been proposed for the NOx system in the past. Models such as those of Suchak et al.,11 Ramanand and Rao,12 and Pradhan and Joshi13 use the lumped parameter to predict the overall √ rate of absorption of select species. The lumped parameter, H kD, often referred to as the “absorption factor”, is derived from penetration or two-film theory and accounts for diffusion through the gas phase followed by first-order reaction in the liquid film while also taking into account phase equilibrium.14 A typical mass transfer expression using the lumped parameter is shown below, using

Figure 1. Example of NORAM’s NOx recovery process utilized in the Wilton MNB plant in Redcar, U.K.

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N2O4 as an example.11 RN2 O4 ¼ aHN2 O4 ððkDÞN2 O4 Þ0:5 ðpiN2 O4 - pbN2 O4 Þ

ð1Þ

Recently, however, numerous attempts have been made at developing models which eliminate the use of the lumped parameter to give better accuracy. Patwardhan and Joshi15 developed a model consisting of coupled second-order differential equations based on two-film theory which considers the diffusion and film reaction of each NOx component in each phase; the model is ultimately solved using the Shooting Method.16 Hupen and Kenig17 in turn developed a rate-based model based on two-film theory and used Aspen Custom Modeler18 as a solving tool. A review of gas absorption operations in general is contained in a book by Treybal.19 In the present study, the Aspen Plus18 simulation environment is selected as the computational tool in order to take advantage of some key features: 1 Built-in physical and chemical properties, 2 Ability to customize the physical property database, specifically through addition of user-defined Henry’s coefficients, 3 Built-in and robust counter-current and iterative solving capability, 4 Ability to account for mass transfer rates in addition to equilibrium considerations, 5 Straightforward user interface. A RadFrac block operating with the RateSep add-on is used to simulate the reactive absorption system.18 In this model, information about the type of packing or trays, the height and diameter of the column, and the location of inlet and outlet streams is specified. Also, this is currently the only model in Aspen Plus in which rate-based mass and heat transfer calculations are possible. RateSep refers to the rate-based add-on to RadFrac. Although past studies have attempted to characterize industrial and pilot-scale NOx columns, reliable, industrial-scale process data from which mathematical models can be confidently verified are lacking in the literature. The experimenters therefore gathered operating data for a NOx column in an existing industrial plant at a variety of process conditions, and the data

Figure 2. Wilton MNB plant in Redcar, U.K. 2193

dx.doi.org/10.1021/ie100436p |Ind. Eng. Chem. Res. 2011, 50, 2192–2203

Industrial & Engineering Chemistry Research

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Table 1. Measured and Derived Data Points for the Wilton MNB Plant in Redcar, U.K. (used for validation) sump temp.

pressure

inlet NOx flow

inlet air flow

recirculation rate

demineralized water

outlet nitric acid

trial

(°C)

(kPa)

(kmol/h)

(kmol/h)

(kg/h)

flow to top (m3/h)

concentration (% wt)

1

25.5

371

1.49

15.5

43 969

1.45

7.7

2

25.4

371

1.45

15.1

43 981

4.53

7.8

3

25.1

371

1.45

12.9

44 000

4.95

7.8

4

27.1

411

1.53

13.4

44 384

4.98

10.5

5

39.9

371

1.61

13.9

44 345

5.05

10.5

6

56.6

371

1.66

13.2

44 312

5.02

10.2

7 8

28.6 29.1

371 371

1.41 1.23

12.7 13.3

44 371 44 505

5.03 5.02

9.5 10.9

9

29.0

371

1.20

13.1

44 521

1.60

11.8

10

29.5

336

1.27

13.4

44 555

5.03

12.6

11

28.7

371

1.51

14.2

44 431

4.99

12.0

12

27.9

271

1.56

13.4

43 555

5.01

11.6

13

28.6

371

0.84

12.7

43 111

4.97

11.6

14

28.6

270

0.90

12.7

43 111

4.97

12.6

15 16

28.6 28.6

270 270

0.94 0.97

12.4 12.7

43 111 43 111

4.97 4.97

13.2 13.8

17

28.6

270

1.01

12.9

43 111

4.97

14.4

were used to verify the Aspen Plus model. The plant, shown in Figure 2, is located in Redcar, U.K., and has been in operation since its commissioning in 1997. It is currently the world’s largest operating MNB production plant. The NOx absorption column is the shorter tower on the left.

2. EXPERIMENTAL STUDIES In the industrial plant, MNB is produced from benzene and nitric acid using NORAM’s patented nitration process. When benzene is reacted with nitric acid to produce crude MNB, a number of byproducts are generated. The overall reaction is represented below. C 6 H6 benzene

þ HNO3 f C6 H5 NO2 þ H2 O þ byproducts nitric acid

MNB

water

Among the byproducts is nitric oxide (NO), the primary NOx species. Downstream of the nitration step, NO gas is separated from the crude MNB, mixed with air, compressed, and sent to the NOx absorption column. The NOx column is fed demineralized water at the top which flows down the column, absorbs NOx, and exits as a nitric acid solution. In this particular plant the column consists of two packed beds in series, with a significant volume of weak nitric acid recirculated back to the lower packed section, as shown in Figure 1. A small flow of demineralized water is also fed to the bottom bed. Product nitric acid from the bottom of the column is recycled back to the nitration reactor, which slightly reduces the fresh nitric acid consumption of the plant. Both beds are packed with KochGlitsch IMTP-25 (25 mm) stainless steel packing, and both have inner diameters of 1.2 m. The upper bed is 2.9 m in length, while the lower bed is 6.1 m in length. 2.1. Experimental Design. Careful selection of operating conditions in the NOx column is crucial for maximizing NOx capture. Generally speaking, low temperature and high pressure are favorable, primarily because NO oxidation to NO2 is kinetically favored by low temperature and is third order in pressure. In addition to temperature and pressure, other parameters to consider in the design of the column include configuration

(packing type, packed height, column diameter, etc.), recirculation rate, and demineralized water flow (i.e., the concentration of weak nitric acid being circulated). Through the development of a reliable simulation tool, the manipulation of all of these parameters can be tested and the NOx absorption process can be optimized. The validity of the simulation tool is tested by comparison with actual plant data, the selection and collection of which is explained in the following sections. Throughout the program, the MNB production rate in the plant was held relatively constant. Normal operating conditions in the NOx column are a pressure of 371 kPa and a temperature of 25 °C. Due to a number of factors in the plant such as safety, the constant MNB production rate, process control, and environmental regulations, the manipulation of all column parameters was limited while for some parameters it was simply not possible. For example, recirculation rate was more or less constant across the data points, while temperature, pressure, NOx flow, air flow, and demineralized water flow were not varied greatly. Fortunately, it was possible to experimentally obtain a few data points of elevated temperature and elevated and reduced pressure, which serve to improve confidence in the model’s versatility and its ability to correctly predict trends in the plant operation. Data, some measured and some derived, at a variety of conditions in the NOx column are summarized in Table 1. The temperature shown is the sump temperature, which is the temperature right at the bottom of the column. 2.2. Experimental Materials and Methods. Distributed control system (DCS) controllers continuously measured the operating temperature and pressure in the NOx column, air intake rate, flow of liquid nitric acid from the column, demineralized water flow to the top and bottom beds, recirculation rate to the bottom bed, and nitric acid concentration in the liquid exiting the bottom of the column. Readings from the DCS controllers were provided to the NORAM experimenters by plant operators. To ensure that DCS readings were representative of steady-state conditions, numerous readings were taken over a time period and averaged. Readings were recorded every 5 min with an average of approximately 45 min spent on each trial operating point. 2194



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Table 2. Steady-State Material and Energy Balances for Each Stage18 description mole balance for bulk liquid mole balance for bulk vapor mole balance

equation FjL xFij

þ Lj - 1 xi, j - 1 þ NijL þ rijL - Lj xij ¼ 0

FjV yFij þ Vj þ 1 yi, j þ 1 - NijV þ rijV - Vj yij ¼ 0 fL

NijI þ rij - NijL ¼ 0

for liquid film mole balance

NijV þ rijf V - NijI ¼ 0

for vapor film energy balance

FjL HjFL þ Lj - 1 HjL- 1 þ QjL þ qLj - Lj HjL ¼ 0

for bulk liquid energy balance

FjV HjFV þ Vj þ 1 HjVþ 1 þ QjV - qVj - Vj HjV ¼ 0

for bulk vapor energy balance

qIj - qLj ¼ 0

for liquid film energy balance

qVj - qIj ¼ 0

for vapor film phase equilibrium

yIij - Kij xIij ¼ 0

at interface summation equation

n X

for bulk liquid summation equation for bulk vapor summation equation for liquid film summation equation for vapor film

i¼1 n X i¼1 n X i¼1 n X i¼1

Figure 3. Representation of Aspen rate-based model of stage.

xij - 1 ¼ 0 yij - 1 ¼ 0 xIij - 1 ¼ 0 yIij - 1 ¼ 0

A Lancom Series 3 flue gas analyzer continuously recorded concentrations of a number of gaseous species (O 2, NO, NO2, CO, CO2) in the plant vent downstream of the NOx recovery column. The analyzer was provided by an independent testing company, and readings from the analyzer were gathered manually. Accuracies of the gas analysis readings are within 1% for O2, 0.5% for CO2, and 2% for all others. Liquid samples were analyzed in house at the Wilton plant laboratory, with HNO 3 concentration determined by titration and HNO2 concentration determined by liquid chromatography.

3. MODEL DEVELOPMENT The RadFrac model in Aspen Plus operating with the RateSep add-on is used to simulate the NOx absorption process. This particular model is selected for its counter-current solving capability, ability to account for reactions in both phases, ability to incorporate electrolyte systems, and with the RateSep add-on, its rate-based heat and mass transfer calculation abilities. 3.1. Details on Mathematical Model. The Radfrac block with the RateSep add-on is a rate-based unit operation model consisting of equations for material and energy balances, mass transfer rates, heat transfer rates, phase equilibria, and summation equations, and these are illustrated in Table 2.18 The Radfrac block must first be solved in equilibrium mode, and the solution in equilibrium mode forms the initial guess once the block is switched to rate-based mode. The model solves the system stagewise, with a stage representing a section of packing or a

Figure 4. Model variation with stages specified for a packed bed.

tray. As explained in the Aspen Plus Users Guide,18 the equations in Table 2 are solved for each stage. In order to calculate bulk properties upon which mass/energy flux, reaction rate, holdup, and pressure drop calculations are based, a mixed flow model is utilized in Aspen Plus. Liquid and vapor phase bulk properties (flow, composition, temperature, pressure) are therefore calculated based on the outlet conditions for each phase leaving a given stage. A representation of a ratebased stage in Aspen Plus is shown in Figure 3.18 In Aspen Plus rate-based distillation, chemical reactions are modeled as occurring in both the bulk and the film. Reaction rates for equilibrium reactions are adjusted by Aspen until the equilibrium condition is established at that point in the film. Note that the thickness of the film is also calculated by the model as the average mass transfer coefficient divided by the average diffusivity.18 In terms of the actual mass transfer coefficients, the equations that are used in the various sets of correlations are presented in Appendix A. Subscript i refers to an individual component while subscript j refers to the stage number. Newton’s method is employed to solve the system of algebraic equations simultaneously. Available for the calculation of mass transfer coefficients and interfacial area in random packing are correlations 2195



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Table 3. Gas-Phase Reaction Set reaction

stoichiometry

R1

2NO þ O2 f 2NO2

R2

2NO2 T N2 O4

R3

NO þ NO2 T N2 O3

R4

N2 O3 þ H2 O T 2HNO2

R5

N2 O4 þ H2 O T HNO3 þ HNO2

developed by Onda et al.,20 Bravo and Fair,21 and Billet and Schultes.22 All three sets of correlations were tested, and the Bravo and Fair21 correlations were selected because model predictions most closely matched plant data when these correlations were used, as presented in section 4. For rate-based heat transfer calculations, Aspen Plus uses the Chilton and Coburn method. The equations used in each correlation are shown in Appendix A. The primary assumptions made in the model are listed below. 1 Kinetic and potential energy changes are negligible. 2 The NOx column operates in steady state under steady-flow conditions. 3 The NOx system consists of the components N2, O2, H2O, NO, NO2, N2O3, N2O4, HNO3, HNO2, H3Oþ, OH-, NO3-, and NO2-. 4 Adiabatic operation. 5 Twenty stages specified for each packed section (determined from testing as discussed below). 6 Mass transfer parameters calculated using Bravo and Fair21 correlation. 7 Chilton and Colburn method used to calculate heat transfer rates. Assumption 5 requires further explanation. When specifying a rate-based packed bed in Aspen Plus, the user must still specify the number of stages for that bed. Increasing the number of stages reduces the integration step size and theoretically improves accuracy. The upper bed in the industrial plant contains Koch-Glitsch IMTP-25 (25 mm) packing with a packed height of 2.9 m. Figure 4 shows how the outlet NOx concentration from the column decreases while increasing the number of stages specified for the upper bed for one data set. Clearly, the curve levels off as the number of segments increases. With 20 segments, a NOx concentration of 381 ppmv is achieved compared to 301 ppmv with 40 segments. The measured value for this trial is 242 ppmv. Although using 40 segments yields better accuracy, it also greatly destabilizes the model. Above 20 segments, convergence without mass and energy balance errors is extremely difficult and for most trials convergence is simply not possible. In an attempt to balance accuracy and model stability, 20 segments are chosen for the upper and lower beds. 3.2. Reaction System and Kinetics. Numerous experimental studies of the NOx absorption system have been performed in

forward and reverse rate expressions r1f ¼

k1 ½NO2 ½O2 2

r2f ¼ k2 ½NO2 2 ; r2r ¼

k2 ½N2 O4 K2

r3f ¼ k3 ½NO½NO2 ; r3r ¼

r4f ¼ k4 ½N2 O3 ½H2 O; r4r ¼

r5f ¼ k5 ½N2 O4 ½H2 O; r5r ¼

k3 ½N2 O3 K3 k4 ½HNO2 2 K4

k5 ½HNO3 ½HNO2 K5

Table 4. Liquid-Phase Reaction Set reaction

stoichiometry

rate expression

R6

2NO2 þ H2 O f HNO2 þ HNO3

r6f ¼ k6 ½NO2 2

R7

N2 O3 þ H2 O f 2HNO2

r7f ¼ k7 ½N2 O3

R8

N2 O4 þ H2 O f HNO3 þ HNO2

r8f ¼ k8 ½N2 O4

R9

3HNO2 f HNO3 þ H2 O þ 2NO

r9f ¼ k9

½HNO2 4 p2NO

the past. The kinetics of NO oxidation to NO2 in the gas phase were first reported by Bodenstein.23 Further studies of this reaction and others in the gas phase have been completed and reviewed by Joshi et al.1 Reactions in the liquid phase are also important in the NOx absorption system and have been studied in depth. Equilibrium considerations in the liquid phase have been compiled by Schwartz and White.24 The temperaturedependent kinetics of one particularly important liquid-phase reaction, that is, decomposition of nitrous acid to nitric acid, are reported by Wendel and Pigford.25 This reaction also forms NO, which is subsequently desorbed to the gas phase, bringing the NOx absorption process full circle. The reaction system in the NOx system is complex, with over 40 reactions in total.1 In the present model and in all mathematical models found in the literature, the reactions system is simplified. The gas-phase reaction system used by Patwardhan and Joshi15 is adopted for the present model and shown in Table 3. The liquid-phase reaction system employed by Hupen and Kenig17 is adopted for the present model and shown in Table 4. Additionally, three electrolyte reactions are considered in the liquid phase and are shown in Table 5. The defaults contained in Aspen Plus for these three reactions are accepted. The most important reaction in NOx absorption is R1.1 NO is essentially insoluble in water and nitric acid; thus, without NO oxidation, negligible absorption takes place. It is an unusual reaction in that it is kinetically favored by low temperature while nearly all reactions are kinetically favored by high temperature. Rate expressions for the five gas-phase reactions are shown in Table 3, similar to those given in Patwardhan and Joshi.15 Kinetic rate expressions for the liquid phase are shown in Table 4, based 2196



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on the liquid-phase reaction set given in Hupen and Kenig.17 For R9, the expression given by Wendel and Pigford25 is used. 3.2.1. Kinetic Parameters. With the exception of electrolyte reactions, all reactions are modeled kinetically in Aspen Plus. This is because plug flow reactor models, which are used in the model to simulate vapor spaces and sections of pipe, can only handle kinetically defined reactions. Two of the gas-phase reactions, R2 and R3, are commonly modeled in the literature as equilibrium reactions.15 In Aspen Plus, kinetic reactions can be specified as reversible or irreversible. For reversible reactions, the forward rate constant is inputted directly by defining the reaction activation energy. The reverse rate constant, however, is defined indirectly by inputting the temperature-dependent equilibrium constant for the reaction. In order to model them kinetically, they are given so-called “near-infinite” kinetics, allowing the species to reach equilibrium values instantly. As shown in Table 6, which uses R2 as an example, by giving the reactions very fast kinetics, they can Table 5. Electrolyte Reaction Set reaction

stoichiometry

R10

2H2 O T H3 O þ þ OH -

R11

HNO3 þ H2 O T H3 O þ þ NO3 -

R12

HNO2 þ H2 O T H3 O þ þ NO2 -

Table 6. Model Verification for Near-Infinite Kinetics kinetic

theoretical

constant

equilibrium constant

equilibrium constant predicted by the model

[m3 kmol-1 s-1]

[m3 kmol-1]

[m3 kmol-1]

0

1.0 10

122.9

5.1

-95.8%

1.0 101

122.9

63.5

-48.3%

1.0 102

122.9

122.0

-0.7%

1.0 103

122.9

122.1

-0.7%

1.0 109

122.9

122.1

-0.7%

% error

essentially be forced to equilibrium, that is, as the kinetic factor is increased, the reaction gets closer and closer to achieving equilibrium in a small time period. In the model, R2 and R3 are given a kinetic factor of 109 m3 kmol-1 s-1 to ensure equilibrium is immediately reached. All kinetic parameters and their references are given in Table 7. Reactions R2-R5 are modeled as reversible reactions, and thus, incorporation of an equilibrium parameter for each of these reactions is required.15 These are shown in Table 8. Note that by default, Aspen Plus treats electrolyte reactions as equilibrium reactions. 3.3. Species Mass Transfer. Mass transfer kinetics of the NOx system have been previously characterized. Miller14 used data from industrial-scale nitric acid production columns to derive a temperature- and acid-strength-dependent relationship √ for the so-called lumped parameter, H kD, for the primary mass transfer species, N2O4. Denbigh and Prince,30 Wendel and and Hoftizer and Kwanten32 reported Pigford,25 Kramers et al.,31√ temperature-dependent H kD values for N2O4 absorption into water, while Werner et al.33 report values for varying nitric acid 27 Sherwood et al.,34 and Newman strength. Studies by Corriveau, √ 35 and Carta report H kD for the component, N2O3 in water and other solutions. With the RateSep add-on, Aspen Plus calculates binary mass transfer coefficients in both phases using built-in correlations. Interfacial area, heat transfer coefficients, and liquid hold-up are also calculated through the use of correlations. The correlations that are implemented depend on whether the column contains trays or packing and what types of trays/packing are being used. 3.4. Phase Equilibria. In order to ensure correct phase equilibrium calculations, the components NO, NO2, N2O3, N2O4, N2, O2, and HNO2 are defined as Henry’s components. For N2 and O2, Henry’s law parameters contained in the Aspen Plus databank are used, while for the other components, Henry’s law parameters are manually defined based on literature information. A summary of the inputted Henry’s law constants and their sources are summarized in Table 9. Temperature-dependent relationships use temperature units of Kelvin. Water is the solvent in all cases.

Table 7. Kinetic Reaction Parameters parameter

value

units

k1

(10((652.1/T)-0.7356))((RT)/101 325)2

ref

m6 kmol-2 s-1

9

Joshi et al.1

-1 -1

3

k2

10

k3

109

m3 kmol-1 s-1

this work, section 3.1

k4 k5

41 000 250

m3 kmol-1 s-1 m3 kmol-1 s-1

Patwardhan and Joshi15 Patwardhan and Joshi15

k6

104.67209

m3 kmol-1 s-1

Lee and Schwartz26

k7

10

4.23044

10

((-4139/T)þ16.3415)

10

((-6200/T)þ20.1979)

k8 k9

m kmol

this work, section 3.1

s

-1

Corriveau27

s

-1

Wendel and Pigford25

s

2

-3 -1

9

atm m kmol

s

Wendel and Pigford25

Table 8. Equilibrium Reaction Parameters parameter K2

value ((2993/T)-9.223)

(10

units

)((RT)/101 325)

(10 )((RT)/101 325) 10((-20.83/T)-0.5012)

K5

10((-965.5/T)-1.481)

-1

Bronsted28

3

-1

Beattie and Bell29 Patwardhan and Joshi15

m kmol

((2072/T)7.234)

K3 K4

remarks and/or refs

3

m kmol

Patwardhan and Joshi15 2197



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Table 9. Henry’s Law Constants component NO NO2 N2O3 N2O4 HNO2

Henry’s law constant

units

1.84 10-5e(1500((1/T)-1/298))

kmol m-3 kPa-1

Sander36

-3

-1

Sander36

-1

Schwartz and White24

-1

Schwartz and White24

-1

Sander36

-4 (2500((1/T)-1/298))

1.18 10 e

kmol m

-3

-3

5.92 10

kmol m

-2

-3

1.38 10

kmol m

-1 (8700((1/T)-1/298))

-3

4.80 10 e

kmol m

remarks and/or refs

kPa kPa kPa kPa

In past models such as that of Patwardhan and Joshi,15 HNO3 is also defined as a Henry’s component. The vapor-liquid equilibria for HNO3 in water based on the Aspen Plus defaults is found to be consistent with data in Perry and Green;37 so, HNO3 is excluded from the Henry’s components list. The Henry’s law constants provided in Table 9 apply to the form of Henry’s law shown below. ½i ¼ H Pi

ð2Þ

3.5. Properties Calculation. The Electrolyte Non-Random Two Liquid (NRTL) property method is selected in Aspen Plus. This is an activity coefficient-based model that can handle electrolyte components.18 Since the NOx system contains nitric acid and nitrous acid, an electrolyte-capable property method is the natural choice. For the most part, physical properties contained in the Aspen Plus databank are used, while select data are inputted in certain cases. One such example is the component nitrous acid, HNO2. Although the component is contained in the databank, there is very little physical property data associated with it, so little that flash calculations on streams containing HNO2 are not possible. Since little data is available about HNO2 in the literature, the component is simply recreated in Aspen Plus. This is done by inputting the component’s molecular structure and allowing the simulator to approximate its physical properties based on that input. 3.6. Model Solution. Model convergence proves to be one of the most challenging parts of the process. Once the model is set up, it is first run with the RadFrac blocks operating in equilibrium mode and no reactions. Then, while still in equilibrium mode, reactions are incorporated throughout the column and the model is rerun. Once that converges, the RadFrac blocks are switched to rate-based mode and are again rerun and converged. If recirculation is desired, this is incorporated and the model is resimulated. For each flowsheet iteration, up to 200 iterations are required to solve the RadFrac block and up to 500 flowsheet iterations may be required to solve the recycle loops; therefore, up to 100 000 total iterations may be required to solve the block. If there are multiple RadFrac blocks operating counter-currently, this number increases drastically.

4. RESULTS AND DISCUSSION The upper and lower beds for the Wilton NOx column are studied, with gas-phase concentrations exiting the lower bed determined by way of material balance. The upper bed is studied first, since it is simpler than the bottom bed in that there is no recirculation and no cooling. For both the upper and the lower beds, all three sets of mass transfer correlations are tested in order to determine which gives the best results. Note that the results are presented in percentage NOx removal as opposed to absolute outlet NOx values for each trial. This has been done to protect the confidentiality of the plant owner.

Figure 5. Model validation for upper packed bed.

4.1. Upper Bed Performance. The NOx concentration entering the top bed is typically on the order of 11 000 ppmv. For each correlation, the model consistently underpredicts the removal of NOx species, with the Bravo and Fair21 random packing correlation yielding the best results and Billet and Schultes22 the worst. In terms of the outlet NOx concentration, the Bravo and Fair21 correlation is 81.8% higher than the measured value on average. In terms of percentage removal of NOx species, however, the Bravo and Fair21 correlation is within 2.8% of the measured removal. The average percent removal of NOx species in the upper bed was experimentally measured to be 97.2%. Measured NOx removal and predictions from each of the three available correlations are shown in Figure 5. The model can also be used to predict column profiles, including temperature, pressure, flow, and composition. Shown in Figure 6 is the predicted NOx content in the vapor phase across the upper packed bed using trial 2 as an example. Also shown are the inlet and outlet NOx concentrations in the vapor phase. From a design perspective, the predicted column profiles can be used to assess column performance by identifying dead zones, that is, regions in the column where NOx concentration is unchanged. 4.2. Lower Bed Performance. The same procedure applied to the upper bed was applied to the lower bed. The lower bed, however, is much more complicated than the upper bed in that it includes a large volume of nitric acid in circulation and cooling. Furthermore, inlet NOx concentrations are an order of magnitude higher in the bottom bed than in upper bed with an average inlet concentration of 9% by volume. Model predictions match up closely with the data in terms of both vent gas NOx concentration and percent removal of NOx species. The correlations slightly overestimated NOx removal for most of the trials with Onda et al.20 offering marginally better results than Bravo and Fair.21 Convergence was not achievable using the Billet and Schultes22 correlation. In terms of outlet NOx 2198



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Figure 6. NOx profile for upper packed bed trial 2.

Figure 8. NOx profile for lower packed bed trial 2.

Figure 7. Model validation for lower packed bed.

Figure 9. Pressure effects for the upper packed bed.

concentration, the Onda et al.20 correlation gives results within 25.7%, while in terms of percent removal of NOx species, the correlation is within 3.5%. Results are shown in Figure 7. All other subsequent simulations use the Bravo and Fair21 random packing correlations in order to ensure top and bottom beds are simulated with the same correlations. Figure 8 shows the predicted NOx profile in the vapor phase for the lower packed bed, again using trial 2 as an example. Unlike the upper bed where the NOx concentration trends downward smoothly, the lower bed shows a leveling off of the NOx content about one-third of the way up the column. Identifying such phenomena demonstrates the power of the simulation exercise. In this case, it appears that the lower bed could be decreased in height without sacrificing significant NOx removal, resulting in a more efficient and ultimately cheaper design. It is also interesting to note that the NOx concentration increases slightly right at the top of the packed section. This is likely due to the flashing of dissolved NOx species that are being fed to the top of the section via the weak nitric acid recirculation stream. Prevention of this phenomenon is discussed in section 4.5. 4.3. Influence of Key Operating Parameters. In order to determine the effects of different operating parameters, certain specific parameters are manipulated across the set of trials. Again, due to plant limitations with regard to safety, production, and process control, manipulation of parameters was constrained.

4.3.1. Effect of Pressure. The third-order dependence on pressure in the kinetically controlled gas-phase oxidation of NO to NO2 makes operating pressure a crucial tuning parameter in maximizing column performance. Generally speaking, increasing pressure will improve the performance of a NOx column. The majority of trials are operated at a pressure of 371 kPa. Trials 4, 10, and 12, however, are operated with pressures of 411, 336, and 271 kPa, respectively. Therefore, the study of the effect of pressure used trials 2, 4, 10, and 12 because they offer different pressures with all other parameters held relatively constant. The results presented in Figures 9 and 10 illustrate the model’s ability to correctly predict changes in column performance as a result of changes in pressure. Agreement is especially good for the upper packed bed. 4.3.2. Effect of Temperature. Temperature is another important parameter in the NOx system. The gas-phase oxidation of NO to NO2 is kinetically favored by low temperature; therefore, keeping the NOx column at low temperature is desirable from that standpoint. On the other hand, the other kinetic reactions and species mass transfer are favored by elevated temperature, so that is another consideration. The majority of the trials operate in the temperature range 25-29 °C. Trials 5 and 6, however, offer elevated temperatures of 40 and 57 °C, respectively. Trials 5, 6, and 7 are thus good points for observing the effect of temperature, since pressure is identical across the three trials and inlet NOx flows, demineralized water flows, and 2199



Figure 10. Pressure effects for the lower packed bed.

Figure 11. Temperature effects for the upper packed bed.

recirculation rates vary only slightly. The results in Figure 11 show increasing NOx capture with increasing temperature, which suggests that the important factors at play in the upper bed are mass transfer and perhaps reactions other than NO oxidation. The results in Figure 12 show the trend of decreasing column performance with increasing temperature, which suggests that NO oxidation is the important factor at play in the lower section. For both the upper and the lower packed sections, the trends are well predicted by the model. The fact that the manipulation of an important column parameter such as temperature can be advantageous from one standpoint and disadvantageous from another standpoint illustrates the complexity of the NOx absorption system. 4.4. Discussion of Discrepancy Between Model and Experimental Results. The Aspen Plus model uses experimentally established, empirical relations to predict mass/heat transfer and holdup. As shown in the preceding sections, discrepancies exist between the experimental data and the model for the three sets of mass transfer correlations that are used in the current study. Some possible reasons for these discrepancies are presented herein. The Onda et al.20 correlations were developed experimentally based on distillation systems consisting of gases, water, and organic solvents. More specifically, the correlations were developed based on data from benzene-toluene, methanol-water, and ethanolwater mixtures. Furthermore, the correlations were developed using different packing types, namely, 15 mm Raschig rings and 0.5 in. Berl saddles, constructed of glass, ceramic, and PVC. The use

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Figure 12. Temperature effects for the lower packed bed.

of organic liquids and different packing types in that experimental study are likely contributing factors to the discrepancy between experimental and model predictions in the current study. The Bravo and Fair21 correlations were developed based in large part on the previous work20 described above, and hence, some of the same problems arise. The systems considered were distillation systems of organic solvents and organic solvents with water. Also, absorption systems consisting of ammonia/water/ air and oxygen/water/air were studied. Packing types studied included Berl saddles, Raschig rings, and Pall rings varying in size from 0.5 to 3 in. and made of either ceramic or steel. Given the range of packing sizes and the study of systems of gas absorption into water, it is not surprising that Bravo and Fair21 yields better results than Onda et al.20 Differences in packing type still exist, and this could contribute to discrepancies between model and experimental model predictions in the present study. The Billet and Schultes22 correlations produce the least accurate results in the present study. The correlations are based on experimental data from over 50 different test systems and more than 70 packing types, including structured and arranged.38 Furthermore, rather than relying on an empirical calculation approach like the other correlations, the calculations are based on a fluid dynamics approach.38 Since it appears to be the most robust of the three available sets of correlations, the poor results are unexpected. It must be noted, however, that for the upper packed section, Billet and Schultes22 predicts only marginally worse results than the other correlations. In addition, the performance of Billet and Schultes22 for the lower bed cannot be adequately assessed since convergence was not possible. Further work is thus require to improve convergence and ultimately assess the performance of the Billet and Schultes22 correlations in rate-based NOx absorption in Aspen Plus. 4.5. Model Implementation To Optimize Process Performance. The Aspen Plus model can be used to rate existing designs, to design new NOx recovery columns, and, perhaps most interestingly, to experiment with completely new design configurations in order to improve performance and meet new environmental regulations. This ability to test innovative new designs represents NORAM’s key driver for development of the model. An illustrative example is the implementation of a socalled bleaching section. The idea here is that after the liquid exits the bottom of the NOx column, it is contacted counter-currently with a NOx-free gas (i.e., air) in order to remove dissolved NOx species and HNO2 in the weak nitric acid, which may be 2200



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to the upper bed could be reduced. Other areas where the model can be used for process optimization include the height and diameter of the packed columns, the flow of recirculated acid, temperature, and pressure among others.

Figure 13. NOx absorption system with incorporation of a bleaching section.

5. CONCLUSIONS A rate-based Aspen Plus model for counter-current NOx absorption into water and nitric acid solutions was developed using industrial-scale plant (i.e., the Wilton MNB plant in Redcar, U.K.) data for validation. Several correlations are tested, and the Bravo and Fair21 random packing correlations prove to be the most dependable for the system of study. Further development is required to extend the model’s application over a wider range of conditions. Specifically, the present model is not validated at nitric acid strengths greater than 15% by weight. The model successfully predicts the removal of NOx species, with an average discrepancy between experimental and predicted results of 2.8% for the upper packed bed and 3.5% for the lower packed bed. The model also successfully predicts trends in column performance for changing temperature and pressure. In addition to providing a tool for rating existing NOx absorption systems, the model provides NORAM with the ability to develop designs for new and existing chemical plants. This includes the design of innovative new configurations such as the addition of a bleaching section presented in section 4.5. ’ APPENDIX A: MASS TRANSFER, INTERFACIAL AREA, AND HEAT TRANSFER CORRELATIONS Please note that all correlations are presented as given in Aspen Plus 2006.5 Users Guide.18 Onda et al.20 Correlations !0:333 L 0:4 μ g L 0 0:667 - 0:5 ScL, i, k ðap dp Þ ðA.1Þ ki, k ¼ 0:0051ðRe L Þ FLt

Figure 14. Effect of adding a bleaching section below the bottom section of the column on NOx leaving the bottom bed.

kVi, k ¼

important in the subsequent storage and use of the recovered weak nitric acid. Also, undesirable flashing of dissolved NOx species upon injection of recirculated weak nitric acid into the upper column section can be avoided, thereby reducing NOx in the tail gas from the column. This bleaching section, which in effect is a NOx stripper, could be added as a separate column in an existing plant or incorporated into a new NOx column (as illustrated in Figure 13). The system in Figure 13 is essentially the same as the one depicted in Figure 1 except for the additional section located below the lower bed. The effect of the addition of bleaching trays is illustrated in Figure 14 for the first six trials at the Wilton plant. On average, the outlet NOx concentration from the bottom bed is predicted to drop by 3404 ppmv. This implies that NOx emissions in the outlet gas could be reduced. As environmental regulations on air emissions continue to tighten with respect to the discharge of NOx, improvements such as these could be the difference between 200 and 100 ppmv in the outlet gas. It could also mean that by improving the performance of the lower bed, either the upper bed could be made smaller or the demineralized water flow

8 < 2:0Re0:7 Sc0:333 ap DV ðap dp Þ - 2 for dp < 0:015 m V i, k V , i, k

-2 0:333 V : 5:23Re0:7 for dp > 0:015 m V ScV , i, k ap Di, k ðap dp Þ

ðA.2Þ aI ¼ aw At hp

ðA.3Þ

"

!# 0:75 σc 0:1 - 0:05 0:2 ReL FrL WeL aw ¼ ap 1 - exp - 1:45 σ Bravo and Fair21 Correlations 0:4 kLi, k ¼ 0:0051ðRe0 L Þ0:667 ScL-, i0:5 , k ðap dp Þ

kVi, k ¼

μL g FLt

!0:333

ðA.4Þ ðA.5Þ

8 < 2:0Re0:7 Sc0:333 ap DV ðap dp Þ - 2 for dp < 0:015 m V i, k V , i, k

-2 0:333 V : 5:23Re0:7 for dp > 0:015 m V ScV , i, k ap Di, k ðap dp Þ

ðA.6Þ 2201


Industrial & Engineering Chemistry Research aI ¼ ae A t h p

ðA.7Þ

ae ¼ 19:78ap ðCaL ReV Þ0:392

σ 0:5 h0:4 p

Billet and Schultes22 Correlations !0:167 sffiffiffiffiffiffiffiffi !0:333 L DLi, k us gF L t ki, k ¼ CL μL dk ap kVi, k

¼ CV

1 pffiffiffi ε - ht

rffiffiffiffiffi ap V 0:75 0:333 D Re ScV , i, k dh i, k V

aI ¼ ae A t h p L

uL dh F 1:5 ae ¼ ap pffiffiffiffiffiffiffiffiffi s L t μ ap dh

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ðA.8Þ

ðA.9Þ

ðA.10Þ ðA.11Þ

10:75 ! - 0:2 0 ! - 0:45 L L 2 L 2 ðu Þ d F ðu Þ h s s tA @ σ gdh ðA.12Þ

Chilton and Coburn39 Correlations h ¼ L

L k F L CLP

V

hV ¼ k F V CVP

!2=3

λL F L CLP D

ðA.13Þ

L

!2=3

λV F V CVP D

V

ðA.14Þ

’ AUTHOR INFORMATION Corresponding Author

*Tel.: 604-681-2030. E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Huntsman Polyurethanes (UK) Ltd. and their Wilton operating staff for allowing access to their facility and supplying the crucial plant data required for model validation. ’ NOTATION a = specific surface area, m-1 A = cross-sectional area, m2 C = mass transfer coefficient parameter for Billet and Schultes22 correlation Ca = capillary number (μu/σ) CP = specific molar heat capacity, J kmol-1 K-1 d = nominal size, m D = diffusivity, m2 s-1 F = feed molar flow rate, kmol s-1 Fr = Froude number (u2/gd) g = gravitational constant, m s-2 h = heat transfer coefficient, W m-2 K-1 h = height of section, m H = Henry’s law constant, kmol m-3 kPa-1 k = binary mass transfer coefficient, m s-1 m s-1 hk = average mass transfer coefficient, -1 L = liquid molar flow rate, kmol s N = mass transfer rate, kmol s-1 P = pressure, kPa

Q = heat input to a stage, W q = heat transfer rate, W r = reaction rate, kmol s-1 R = specific rate of mass transfer, kmol m-3 R = universal gas constant, m3 kPa kmol-1 K-1 Re = Reynolds number (Fdu/μ) Re0 = Reynolds number based on wetted or effective surface area (Fdu/μ) Sc = Schmidt number (μ/Fd) T = temperature, K u = velocity, m s-1 V = vapor molar flow rate, kmol s-1 We = Weber number (Fu2d/σ) x = liquid mole fraction y = vapor mole fraction [ ] = liquid phase molar concentration, kmol m-3 Greek Letters

ε = void fraction in packing λ = thermal conductivity, W m-1 K-1 μ = viscosity, Pa s F = density, kg m-3 = molar density, kmol m-3 σ = surface tension, N m-1 Subscripts

c = critical e = effective packing f = forward i = component j = stage k = component L = liquid p = packing r = reverse s = superficial t = total column V = vapor Superscripts

b = bulk F = feed f = film I = interface L = liquid V = vapor Abbreviations

DCS = distributed control system MDI = methylene diphenyl diisocyanate MNB = mononitrobenzene NRTL = nonrandom two liquid

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