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This chemical engineering ebook discusses findings gained through work with the NOx abatement system at Radford Facility and Army Ammunition Plant (RFAAP). Removal of harmful substances from flue-gas emissions has garnered increased priority in the chemical industry in preceding decades, as governmental restrictions on these substances become more stringent and as national awareness concerning environmental quality and resource utilization continues to grow. These reasons make the study of NOx abatement an important and challenging endeavor. This work concerns itself specifically with reduction of NOx in flue-gas emissions from stationary sources. First we present an overview of current technology and approaches to controlling NOx for stationary sources. Next, we focus in on one particular approach to control of NOx within the context of a case study of the technology used at the Radford Facility and Army Ammunition Plant. RFAAP employs a scrubber/absorber tower followed in series by a selective catalytic reduction (SCR) reaction vessel in their NOx abatement system. We use as the method of study computer simulations within ASPEN Plus, a process simulation software package for chemical plants. We develop three different models with which to characterize NOx abatement at RFAAP, a conversion model, an equilibrium model and a kinetic model. The conversion-reaction model approximates the absorption and SCR reactions with constant percentage extent-of-reaction values. Though useful for initial investigation and mass balance information, we find the conversion model's insensitivity to process changes to be unacceptable for in-depth study of the case of NOx absorption and SCR. The equilibrium-reaction model works on the assumption that all the reactions reach chemical equilibrium. For the conditions studied here, we find the equilibrium model accurately simulates NOx absorption but fails in the case of SCR. Therefore, we introduce a kinetic-reaction model to handle the SCR. The SCR reactions prove to be highly rate-dependant and the kinetic approach performs well. The final evolution of the ASPEN Plus simulation uses an equilibrium model for the absorption operation and a kinetic model for the SCR. We explore retrofit options using this combined model and propose process improvements. We end this work with observations of the entire project in the form of conclusions and recommendations for improving the operation of the NOx abatement system through process-parameter optimization and equipment-retrofit schemes.

By leading the reader through the process by which we arrived at a successful and highly informative computer model for NOx absorption and SCR, we hope to educate the reader on the subtleties of NOx abatement by absorption and SCR. We attempt to break down the numerous complex processes to present a less daunting prospect to the engineer challenged with the application of current NOx removal technology. In addition, we introduce the reader to the power and usefulness of computer modeling in instances of such complexity. The model teaches us about the details of the process and helps us develop concrete information for its optimization. Ideally, the reader could use a similar approach in tackling related operations and not confine the usefulness of this thesis to NOx absorption and SCR.

The audiences that we think would benefit from exposure to this thesis are the following: · Environmental engineers with a NOx problem; · Process engineers interested in optimization tools; · Design engineers exploring flue-gas treatment options; · Combustion engineer desiring to learn about SCR; · Chemists and mathematicians intrigued by the complexities of NOx absorption chemistry.

Content:

CHAPTER 1:
Introduction

1.1 Why Nitrogen Oxides?
1.2 Why Computer Modeling?
1.3 An Overview of This Thesis
1.4 Nomenclature

Chapter 2:
Literature Review

2.1 Nitrogen Oxides in the Environment
2.1.1 Hazards Associated with Nitrogen Oxide Compounds in the Environment
2.1.2 Sources of NOx Gases Released to the Environment

2.2 Various NOx Control Techniques for Flue-Gas Emissions from Stationary Sources

2.2.1 Wet Processes
2.2.1.1 Wet Scrubbing
2.2.1.2 Extended Absorption

2.2.2 Dry Processes

2.2.2.1 Non-Selective Catalytic Reduction (NSCR)
2.2.2.2 Selective Catalytic Reduction (SCR)
2.2.2.3 Selective Non-Catalytic Reduction (SNCR)

2.3 Catalyst Varieties for Selective Catalytic Reduction (SCR)

2.3.1 Catalyst Blends
2.3.2 Catalyst Configurations
2.3.3 Effect of Water on the SCR Reactions

2.4 NOx Gas Absorption into Aqueous Media in Conjunction with SCR

2.4.1 Equipment
2.4.1.1 Scrubber/Absorber
2.4.1.2 Demister
2.4.1.3 Process Heating Equipment
2.4.1.4 Catalyst Vessel
2.4.1.5 Stack
2.4.1.6 NOx Analyzers and Process Control Equipment

2.4.2 Species Involved
2.4.3 Reactions in the System
2.4.4 Flue-Gas Treatment Versus Nitric Acid Production
2.4.5 Current Research
2.4.6 Modifications and Retrofit Options for Existing NOx Absorption Plants

2.4.6.1 Cooling
2.4.6.2 High-Pressure Operation
2.4.6.3 Addition of H2O2
2.4.6.4 Oxidation of Nitric Oxide

2.5 Computer Modeling Techniques

2.5.1 Special Challenges of NOx Absorption
2.5.2 ASPEN Plus
2.5.3 Fundamental Approach

2.6 Summary
2.7 Nomenclature
2.8 References

Chapter 3
Introduction to the NOx Abatement Process at RFAAP and Computer Simulation of the NOx Abatement Process as a Conversion-Reaction Model

3.1 Introduction
3.1.1 An Overview
3.1.2 Introduction to NOx Removal at RFAAP
3.1.3 Purpose of NOx Abatement at RFAAP
3.1.4 Process Description

3.2 Detailed Explanation of Process Equipment

3.2.1 Scrubber/Absorber
3.2.2 Demister
3.2.3 Heat Exchangers and Process Heaters
3.2.4 Catalyst Vessel
3.2.5 NOx Abatement System Input and Output Flow Rates

3.3 Introduction to ASPEN Plus Simulation of NOx Abatement

3.3.1 Purpose for ASPEN Plus Simluation of NOx Abatement
3.3.2 Motivation for a Conversion-Reaction Model
3.3.3 ASPEN Simulation Procedure
3.3.4 The Conversion-Reaction Model

3.4 Discussion of the Conversion Model Results

3.4.1 The Preliminary ASPEN Model
3.4.1.1 Scrubber/Absorber
3.4.1.2 Demister
3.4.1.3 Heat Exchangers and Process Heaters
3.4.1.4 Catalyst Vessel

3.4.2 Absorption-Column Performance
3.4.3 Sensitivity Analysis

3.4.3.1 Fume-Feed Temperature
3.4.3.2 Top-Stage Pressure
3.4.3.3 Water Flow Rate

3.5 Conclusions

3.5.1 Conclusions Regarding Process Variables
3.5.2 Problems with the First Equilibrium Model

3.6 Recommendations
3.8 Nomenclature
3.9 References

Chapter 4:
Computer Simulation of the NOx Abatement Process as an Equilibrium-Reaction Model

4.1 Introduction
4.1.1 An Overview
4.1.2 Motivation for an Equilibrium-Absorption Model
4.1.3 Complexities of NOx Absorption
4.1.4 Simplification of Reaction Mechanism
4.1.5 Assumptions Made for the Equilibrium Model
4.1.5.1 Assumption I: Neglect Reaction (4.1)
4.1.5.2 Assumption II: Treat Reaction (4.2) as Being in Instantaneous
Equilibrium
4.1.5.3 Assumption III: Combine Reactions (4.4) and 4.(5)
4.1.5.4 Assumption IV: Neglect the N2O3 Pathway
4.1.5.5 Assumption V: Eliminate HNO2
4.1.5.6 Assumption VI: Assume Vapor-Phase Acid Concentrations Are
Negligible
4.1.5.7 Assumption VII: Neglect Reaction

4.1.6 Equilibrium Model of the SCR Unit

4.2 Discussion of the First Equilibrium Model

4.2.1 Results of the First Equilibrium Model
4.2.2 Sensitivity Analyses for the First Equilibrium Model
4.2.2.1 Column-Tray Number
4.2.2.2 Column Cooling
4.2.2.3 Fume-Feed Temperature
4.2.2.4 Filtered-Water Feed Temperature
4.2.2.5 Cooling-Jacket Duty
4.2.2.6 Column Pressure
4.2.2.7 NO Feed Rate to the Scrubber/Absorber
4.2.2.8 NO2 Feed Rate to the Scrubber/Absorber
4.2.2.9 Filtered-Water Flow Rate to Top of Column
4.2.2.10 NO Feed Rate to the Catalyst Vessel
4.2.2.11 Ammonia Feed Rate to the Catalyst Vessel
4.2.2.12 Steam Feed Rate to the Catalyst Vessel

4.2.3 Advantages of the First Equilibrium Model
4.2.4 Disadvantages of the First Equilibrium Model

4.3 The Second Equilibrium Model

4.3.1 Motivation for a Second Equilibrium Model
4.3.2 Vaporization Efficiencies
4.3.3 Results of the Second Equilibrium Model
4.3.4 Sensitivity Analyses
4.3.4.1 Column Cooling: Fume-Feed Temperature
4.3.4.2 Top-Stage Pressure
4.3.4.3 NO2 Feed Flow Rate

4.3.5 Advantages of the Second Equilibrium Model
4.3.6 Disadvantages of the Second Equilibrium Model

4.4 Conclusions

4.4.1 Conclusions Regarding Process Parameters
4.4.2 Itemized Conclusions

4.5 Recommendations
4.6 Nomenclature
4.7 References

Chapter 5:
Computer Simulation of the NOx Abatement Process as a Kinetic-Reaction Model

5.1 Introduction
5.2 The Kinetic-Reaction Model
5.2.1 Motivation for a Kinetic-Reaction Absorption Model
5.2.2 Mechanism of Kinetic Reactions
5.2.3 Characteristics of the Kinetic Model

5.3 Discussion of the Kinetic Model

5.3.1 Results of the Kinetic Model
5.3.2 Sensitivity Analysis
5.3.2.1 NO Flow Rate to Catalyst Vessel
5.3.2.2 Ammonia Flow Rate to Catalyst Vessel
5.3.2.3 Steam Flow Rate to Catalyst Vessel
5.3.2.4 Oxygen Percentage in Feed to Catalyst Vessel
5.3.2.5 Pressure of Catalyst Vessel Feed

5.4 Conclusions
5.5 Recommendations
5.6 Nomenclature
5.7 References

Chapter 6:
Retrofit and Economics for NOx Absorption with Selective Catalytic Reduction (SCR)

6.1 Introduction
6.1.1 An Overview
6.1.2 Retrofit Considerations Particular to RFAAP

6.2 Simple Retrofit Options

6.2.1 Side Water Feeds and Draws
6.2.2 Cooling the Fume Stream
6.2.3 Cooling the Bottom-Acid Recycle

6.3 Complex Retrofit Design Options

6.3.1 Heat Recovery
6.3.2 Acid Distillation
6.3.3 Cooling Trays or a Cooling Jacket
6.3.4 Alternative Approach to Cooling the Fume Stream
6.3.5 Ozone
6.3.6 Hydrogen Peroxide

6.4 Retrofit Design Economics
6.5 Conclusions
6.6 Recommendations
6.7 Nomenclature
6.8 References

Chapter 7
Conclusions, Recommendations, and Observations

7.1 Modeling and ASPEN Plus Computer Simulation
7.1.1 Modeling NOx Absorption
7.1.2 Modeling SCR

7.2 Process Improvements

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