Advanced Process Design of Nitric Acid Plants

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Dec 4, 1998 - Nitric Acid Conference. Valley Lodge .... Ammonia oxidation is sensitive to temperature, pressure, reactor space ... Bubble Point. Dew Point ...

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Advanced Process Design of Nitric Acid Plants Ralph Grob and Paul Mathias Aspen Technology, Inc. 4 December 1998 Presented at: Nitric Acid Conference Valley Lodge, Magaliesburg 3-4 December, 1998 © 1998 AspenTech All Rights Reserved.

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AspenTech’s Plantelligence Solution TM

Design Enable the True Potential to be attained

Determine the True Potential to be achieved

Model Operate

Manage

Evaluate performance against True Potential

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The Opportunity : Enormous Economic Returns

The Opportunity

Industry Average Plantelligence™

Best Practices

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Core Competencies

ERP

Design Operate

Models

Manage

Business Process Expertise IT Integration Manufacturing and Integrated Supply Chain Expertise Deep Process Knowledge Intelligent Field

OCS Plantelligence™

Electronics , Computer Engineering, Network Communication © 1998 AspenTech All Rights Reserved.

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Solids and Electrolytes Plus“ • Industry-specific layered product. - Specialized property and equipment models - Molecular and stream attributes (e.g., PSD) - Standard, proven process simulations

• Expertise in targeted industries. - We know, understand your technology - Technology transfer, consulting and support

• Long-term commitment to customer success. Plantelligence™

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TARGETED INDUSTRIES

• Inorganic chemicals • Specialty chemicals • Mining and metals

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Solids and Electrolytes Plus (SEP™) Process Simulator COMP1

Phase Equilibria • Vapor-liquid-solid • VaporVapor-liquidliquid-liquidliquid-solid

R1OUT

Reactor

Databanks • Ions • Dilute electrolytes • Solids

Physical Properties • Cp, H, G, S • U • K

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Stream Structure • PSD moments • PSD • Ions

VAP-A

SEP1

GAS1

POWDER1

• SteadySteady-state simulation • AspenPlus“ • Dynamic Simulation • AspenDynamics“

Unit Operations • crystallizer • ion exchange • dryers • electrolytic cells

Process Models • Fertilizers • Caustics • Metals • Amines • etc. Thermodynamic Models • Zemaitis • Chen • Pitzer • EOS © 1998 AspenTech All Rights Reserved.

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Core Capabilities of SEP Technology • •



Comprehensive strength of Aspen Engineering suite Focus on engineering science of systems containing solids and electrolytes Expertise in process engineering of target industries - inorganic chemicals, metals and mining

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S E P Models for the Fertilizer Industry

NH3

Nitric Acid Ammonium Nitrate

CO2

Urea

Phosphate Rock Sulfur

Potash Ore Plantelligence™

Sulfuric Potash Acid

Potash

Diammonium Phosphate Phosphoric Acid

Box Code Red - Available Yellow - Under development White - Planned © 1998 AspenTech All Rights Reserved.

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Benefits of Process Modeling • Gain a deeper understanding of the process • Investigate process enhancements, safety • Improve control • Improve environmental compliance • Reduce energy costs • Utilize wide range of raw-material blends Plantelligence™

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Benefits of Nitric Acid Modeling • MARKET FORCES • 80% into fertilizer industry - demand is cyclical • OPERATIONAL CHALLENGES • Emissions limitations are critical when demand is high • Yield is critical when demand is low • Ammonia oxidation is sensitive to temperature, pressure, reactor space velocity - catalyst losses must be minimized • Absorber must be optimized for off design and normal conditions, and within NOx emissions limits • CAPITAL PROJECTS • Debottleneck absorber • Add air compression, new columns • Evaluate piping changes to reduce side reactions, improve yields Plantelligence• Add advanced or multi-variable control ™

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Challenges of Nitric Acid Modeling • Multiple reactions occur in most equipment, including pipes, gas coolers, heat exchangers and condensers • Performance is flow rate and pressure sensitive • Special physical properties are needed for nitric acid VLE and heat of mixing calculations • Many recycle streams for heat and power integration • Absorption tower has rate-limited and equilibrium vapor and liquid reactions, rate-limited mass transfer, and cooling coils on most trays Plantelligence™

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NITRIC ACID - PROPERTIES • Pure water - cooling water and steam • Modified RKS for polar nonelectrolyte mixtures • Electrolyte NRTL

Fine-tuned parameters high, known accuracy Plantelligence™

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Electrolyte NRTL and Chemistry High Accuracy for thermodynamic properties: • Vapor-liquid equilibrium • Enthalpy • Density

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Partial Pressures of HNO3 and H2O Over Aqueous Nitric Acid 1

Partial Pressure (bar)

100°C 0.1 60°C 0.01

0.001

0.0001 0 Plantelligence™

0.2

0.4 0.6 x (HNO3)

0.8

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Txy Diagram for Aqueous Nitric Acid at 1 Atmosphere 400

Temperature (K)

390 Dew Point

380 Bubble Point

370

360

350 0 Plantelligence™

0.2

0.4

0.6

x, y (HNO3)

0.8

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NITRIC ACID PLANT SECTIONS • Gas-mixing • Ammonia oxidation • Nitric oxide oxidation • Absorption • Tail-gas treatment

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Nitric Acid Flowsheet Sections • Front Section Air Compressor Liquid Ammonia

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Oxidation of NH3 to NO Vaporizers

NO Gas

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Nitric Acid Flowsheet Sections • Middle Section NOx Gas NO Gas

Oxidation of NO to NO2

Dimerization of NO2 to N2O4

Condensation to form aqueous HNO3 Nitric Acid Condensate

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Nitric Acid Flowsheet Sections • Towers Section

Water NOx Gas Nitric Acid Condensate

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Absorption of N2O4 into water to form HNO3

Turbine Expansion Degas Product Stream

Exhaust

Nitric Acid Product

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Countercurrent NOx Gas Cooler NOx Gas Coolant Out

NOx Gas Coolant In

• Problem • Two streams exchange heat with countercurrent flow • One stream has two reactions • Solution • Model with RPLUG and specify a countercurrent coolant • Use a design-spec to determine the exit temperature of the coolant Plantelligence™

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Absorption Tower

• Several vapor and liquid phase reactions • Rate-limited liquid-vapor mass transfer • UA-limited heat transfer

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Rigorous Tower Model Reactions and MassTransfer Species Vapor Phase

Interface

Liquid Phase

mo NO(L) N O (G) mo N O (L) HNO (G) mo HNO (L) H O (G) mo H O (L) NO(G)

o 2NO mo N O

2NO + O2 2NO2

2

2

4

2

2

3

2

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4

4

N2O4 + H2O

mo HNO

3

+ HNO2

3

2

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NITRIC ACID ABSORBER • Rate-based mass transfer, chemical reaction • Bulk vapor and liquid on each tray perfectly mixed • Optimum set of 4 gas-phase reaction • Mass-transfer through gas and liquid films • Estimated heat transfer to cooling coils • Negligible heat loss to surroundings

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NITRIC ACID ABSORBER Four gas-phase reactions: 2NO + O2 o 2NO2 2NO2 mo N2O4 N2O4 + H2O o HNO3 + HNO2 3HNO2 o H2O + HNO3 + 2NO Plantelligence™

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Features of AspenTech’s Nitric Acid Plant Model • Simultaneous rate-limited and equilibrium reactions are modeled in pipes, heat exchangers, and condensers • Property models were developed using Aspen Plus electrolyte capability • Compressors modeled using performance curves • Pressure drop calculated for each piece of equipment based on volumetric flowrate Plantelligence™

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Features of AspenTech’s Nitric Acid Plant Model • Model is segmented for simulation of part of plant or entire plant • Rigorous and efficient absorber model for accurate absorber and plant simulations • Special summary report of nitric acid concentration and production rates, cooling water usage, power requirements, and flue gas composition

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USES OF NITRIC ACID PLANT MODEL • Verification of plant design • Day-to-day analysis of equipment performance • Plant optimization - Optimize cost function subject to equipment performance and operating constraints • Debottlenecking studies - is tower or compressor limiting?

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