IAPG 2008

Simulation of Amine Plants: Fundamental Models and Limitations

2das Jornadas Técnicas Sobre Acondicionamiento del Gas Natural

30 de Septiembre al 3 de Octubre de 2008

El Calafate, Argentina

Jenny Seagraves

INEOS Oxide

GAS/SPEC Technology Group

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Topics of Presentation

General history and overview of fundamental models

refer to paper and references in papers for more details

Case Studies

Important considerations or ideas for designing or optimizing an amine plant

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History and Fundamentals of Amine Simulation Models

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Improved simulation model are developed as solvent technologies evolve and amine plant become more

complex….

MEA SpecialtyTEA DEA DGA Amine

1930 1940 1950 1960 1970 1980 & Beyond….

MDEA DIPA

Simple Models(Hand Calculations)

Complex ComputerModels

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Simulation of MDEA and newer specialty solvents...

MDEA-based and specialty solvents more difficult to simulate

contain MDEA and sometimes blends of chemicals that yield specific treating characteristics

have components with different reaction kinetics

MDEA solvent have different temperature profile than MEA or DEA.

Simplified computer calculations are dangerously misleading for MDEA and specialty amine designs

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Improved Simulation is Needed asAmine Plant Designs Evolve...

While 20 trays absorber & regenerator designs are still most common ….

We now are designing amine plants with

multiple feeds and side draws

Complex multi-staged flash to reduce energy

New mass transfer devices to get more capacity

» new packing material or trays

» or a combination of the two.

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Example of Amine Plant with Multi-feeds and Flash

Syngas

Lean Amine

Rich Amine

Semi-lean

CO2

T = 130 F (50 C)

Reboiler

Absorber

Regenerator

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Definitions

Vapor Liquid Equilibrium (VLE)

Defines the solution chemistry / chemical species present

model determines the maximum limit of H2S and CO2 absorbed

Reaction Rates

Defines how quickly H2S and CO2 are absorbed

H2S react instantaneously with amines and CO2 react at various rates depending on type of amine.

Mass Transfer Rate

Define the surface area and how quickly the surface area is refreshed for H2S and CO2 absorption

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Vapor Liquid Equilibriumionization of water2 H2O H3O+ + OH- (eq. 1)

dissociation of hydrogen sulfideH2O + H2S H3O+ + HS- (eq. 2)

dissociation of bisulfideH2O + HS- H3O+ + S2- (eq. 3)

dissociation of carbon dioxide2 H2O + CO2 H3O+ + HCO3- (eq. 4)

dissociation of bicarbonateH2O + HCO3- H3O+ + CO32- (eq. 5)

dissociation of protonated alkanolamineH2O + RR’R’’NH+ H3O+ + R’R’R’’N (eq. 6)

carbamate reversion to bicarbonate RR’NCOO- + H2O RR’NH + HCO3- (eq. 7)

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Vapor Liquid Equilibrium

The equations governing chemical equilibria for equations 1 to 7 may be written as:

K = i (xi i )i (eq. 8)

where, K is the equilibrium constantxi is the mole fraction of species

ii is the activity coefficient of

species i i is the stoichiometric coefficient

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Chemical Kinetics and Mass Transfer

Ni = Ei k°i,L a (yi interface - yi Bulk) (eq 8)

NI = transfer rateEi = enhancement factor (accounts for chemical reaction)k°i,L = Mass transfer coefficient a = interfacial areayi

interface= acid gas conc. at interface (from Henry’s law)yi

bulk = acid gas conc. in bulk (from VLE)

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Evolution of Amine Simulation

Pre 1980s - Equilibrium Stage Approach was only method

Uses simplified estimates

Estimate chemical species in solution

Uses tray efficiencies lump reaction and mass transfer rates

Adequate for simulation of MEA and DEA

Not accurate for MDEA, specialty solvents, and complex amine mixtures

Still used in many commercial simulators today

After 1980s - Mass Transfer Rate Based Approach

More rigorous

Calculate exact chemical species present in solution

Calculate reaction and mass transfer rates

Accurate for MEA, DEA, MDEA, and Specialty amine solvents

Can be extended to systems with heat stable salts and other components if data is available

Used in only a few simulators

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History of Mass Transfer Rate Based Simulation Approach

Idea to combine mass transfer with chemical reactions in amine simulation came about as a result of works by Astarita, Weiland, Katti, and others.

In early 1980s, GAS/SPEC funded a series of research projects to developed the first amine simulator that combined

rigorous vapor-liquid-equilibrium (VLE) modeling

with mass transfer and chemical reactions calculations

Mass Transfer Rate-based simulation has been used and refined over the last 20+ years by the GAS/SPEC group

Available in certain simulators such as

GAS/SPEC APS Simulator (proprietary simulation program)

Commercially available ProTreat Simulator (Optimized Gas Treating Inc.)

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What is mass transfer rate-based?

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Example of GAS/SPEC APS Simulation

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Most Basic Amine Simulation Models

PredictedPlant Performance

Simulation

Material Balance

Tray Efficiency

Phase Equilibrium

Properties

Use tray efficiencies to account for

•mass transfer•reaction rates

Efficiencies are empirically derived

Ignore tower internals•use equivalent stages to represent a given number of trays or packing height

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Mass Transfer Rate-based Simulations

PredictedPlant Performance

SimulationMaterial Balance

Reaction Kinetics

Phase Equilibrium

Properties

Mass Transfer(Tower internals)

More detailed approach

Avoid the use of efficiencies

Considers differences in reaction rates of H2S and CO2

Consider Mass Transfer rate of absorption in different tower internals (trays, packing, etc.)

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Advantages of MT Rate-based Models

Makes more rigorous and accurate prediction inside column

temperature profile

reaction or absorption zone

identify trouble area in the column

» equilibrium limits

» areas of corrosion concerns due to high temperatures

1

3

5

7

9

11

13

15

17

19

21

100 110 120 130 140 150

Temperature (F)

Tra

y N

um

ber

ProTreat

Actual

Example of Actual vs Predicted

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Equilibrium Stage Approach

No one-to-one correspondence of theoretical stage with position in column

3 trays per stage ? Or 4 trays per stage?…etc.

Difficult to locate exact temperature and composition of feeds and side draws

Top Tray

Stage 3

Tray location?Stage 2

Tray location?Temp?Composition?

Stage 1

Feed

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M.T. Rate-based Approach

Know temperature and composition on every actual tray

Can accurately locate optimum points for feeds and side draws

Top Tray

Tray is known

Tray is knownTemp is known

Feed

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Case Studies

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Case Study 1

High pressure coal bed methane gas

requires CO2 removal only

plant have ability to treat a portion of the natural gas and blend to meet 3 mol% CO2 spec

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Case 1 - Flow Diagram

AMINECOOLER

REFLUX CONDENSER

REFLUX ACCUMULATOR

LEAN /RICHCROSS-EXCHANGER

FEED

FILTER TRAIN

TREATED GAS

REBOILER

ABSORBERREGEN

RICH AMINE

LEAN AMINE

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Benchmark Performance Tests

Test 1 Test 2 Test 3Raw Gas Flow (Nm3/h) 235500 232100 200900 Temperature (oC) 40 40 40 Pressure (kPa) 6881 6881 6881 CO2 (mol%) 4.29 4.29 4.21

Lean Solvent Flow (m3/h) 227 186 227 Temp (oC) 40 43 39 Wt% MDEA 48 48 48

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Performance Compared to Simulation

Test 1 Test 2 Test 3Solvent Rate (m3/h) 227 186 227Gas Rate (Nm3/h) 235500 232100 200900

Treated Gas Measured CO2 (mol%) 1.54 1.98 1.20 Predicted CO2 (mol%) 1.57 1.95 1.20Lean Amine Actual mol/mol 0.008 0.008 0.007 Predicted mol/mol 0.0075 0.0059 0.0046Rich Amine Predicted mol/mol 0.310 0.403 0.294

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Performance Compared to Simulation

Test 1 Test 2 Test 3Solvent Rate (m3/h) 227 186 227Gas Rate (Nm3/h) 235500 232100 200900

Treated Gas Measured CO2 (mol%) 1.54 1.98 1.20 Predicted CO2 (mol%) 1.57 1.95 1.20Lean Amine Actual mol/mol 0.008 0.008 0.007 Predicted mol/mol 0.0075 0.0059 0.0046Rich Amine Predicted mol/mol 0.310 0.403 0.294

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Actual versus Simulation Predicted Temperature

1

3

5

7

9

11

13

15

17

19

21

38 49 60 71

Temperature (°C)

Tra

y N

um

ber

1

3

5

7

9

11

13

15

17

19

21

38 49 60 71 82

Temperature (°C)

Tra

y N

um

ber

1

3

5

7

9

11

13

15

17

19

21

38 49 60

Temperature (°C)

Tra

y N

um

ber

Test 1 - Absorber Test 2 - Absorber Test 3 - Absorber

Actual temperature measurements

Simulated Temperatures

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Significance of Temperature Profile

Concern with Temperature Profile because

higher and broader profile have corrosion implications

outlet gas temperature increase load on downstream dehydration equipment

high temperature may limit capacity or cause plant to go off spec - difficult to absorb CO2

» near equilibrium loading

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Tower Temperature Profiles

GAS/SPEC technical service engineers use these temperature scans of towers to troubleshoot amine plant. This is a method to monitor performance

Poor liquid distribution

Broad temperature profile throughout

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Options for More Capacity

Customer wants more capacity out of the plant

However CO2 level in inlet gas is rising!

Option 1 - Continue to treat with MDEA

Treat to just below 3% CO2 specification

Option 2 - Upgrade to a Specialty Solvent

Treat CO2 to low levels of < 1000 ppm

then blend with untreated gas to meet 3% CO2 specification

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Max Capacity with MDEA

0

100000

200000

300000

400000

500000

600000

3.5 4 4.5 5 5.5 6 6.5

Inlet CO2, mol%

Gas

Flo

w, N

m3/

h

Treated

Bypassed

Combined

Pipeline Max

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Max Capacity with Specialty Solvent

0

100000

200000

300000

400000

500000

600000

700000

800000

900000

3.5 4 4.5 5 5.5 6 6.5

Inlet CO2, mol%

Ga

s F

low

, N

m3

/h

Treated

Bypassed

CombinedPipeline Max

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Results after Conversion

MDEA CS-2010Flow to Absorber (Nm3/h) 235500 232100Inlet CO2, mol% 4.29 4.5Outlet CO2, mol% 1.54 < 0.1Amine Flow, Nm3/h 227 202

Max Total Gas Capacity (Nm3/h) 446400 502200

Currently limited by capacity of downstream pipeline

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Conclusions - Case 1

Demonstrates use of simulation tool to

accurately predict temperature and CO2 in the column.

identify opportunities for optimization of existing plant

make decision on how to best utilize assets for present and future treating conditions

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Case Study 2

Offshore natural gas application

H2S and CO2 removal

Simulations used to

design original plant

modify plant to adapt to changing process conditions

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Case 2 - Flow Diagram

AMINECOOLER

REFLUX CONDENSER

REFLUX ACCUMULATOR

LEAN /RICHCROSS-EXCHANGER

FEED

FILTER TRAIN

TREATED GAS

REBOILER

ABSORBERREGEN

RICH AMINE

LEAN AMINE

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Original Design Treating Conditions

Inlet Gas Flow (Nm3/h) 502200Inlet Gas Pressure (kPa) 7419Inlet Gas Temp (°C) 49

Gas Composition: CO2 (mol%) 3.25 H2S (mol%) 1.35

Treated Gas Specification: CO2 (mol%) < 1 H2S (ppmv) < 4

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Key Design Decisions

Prior to INEOS involvement, customer decided on

30 tray absorber (3.35 meters diameter with 10 cm weir height)

design based on generic MDEA

plant was already designed with “Equilibrium Stage”-based simulator

Use of 30 trays is unusual in an offshore application due to weight consideration

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Simulation - Design Rate

Gas Flow (Nm3/h) 502200Feed Tray from Top 30MDEA Conc. (wt% ) 50%Circulation Rate (m3/h) 545

Treated Gas CO2 (mol%) 0.92 H2S (ppmv) < 1 ppm

Lean Loadings / Rich Loadings H2S (mol/mol) 0.0002 / 0.13 CO2 (mol/mol) 0.005 / 0.23

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Variations operating conditions were also simulated...

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Simulations for Changing Condition

Limited heat source at certain times

57% of design duty available

Plant will operate at reduced rate

Increased CO2 pickup at reduced rate

How to operate plant to minimize CO2 pickup

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Alternatives for Operating at Reduced Rates

502200 Nm3/hReboiler Duty = X30 traysCO2 Out = 0.92 mol%

30 trays279000 Nm3/h340 m3/h of 50wt% MDEAReboiler Duty = 0.57 XCO2 Out = 0.59 mol%

19 trays279000 Nm3/h340 m3/h of 50 wt% MDEAReboiler Duty = 0.57XCO2 Out = 0.99 mol%

Scenario 1

Scenario 2

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Outcome of Simulations

Feed points added to trays 30, 24, 19 to allow for flexibility under changing conditions

Tray 30

Tray 24

Tray 19

Feed

ABSORBER

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Prior to Startup

Plant needed lower CO2 level

Minimize corrosion in downstream pipeline

Old spec 1% CO2 ; New spec 1000 ppmv CO2

In order to maximize CO2 removal, customer has 2 options

Option 1 - Continue with MDEA

» Higher amine circulation rate, L/V

» Use all 30 trays

Option 2 - Specialty amine solvent

» Treat with less trays and less circulation

Customer decide to proceed startup with MDEA and then upgrade to a specialty solvent.

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After Startup

After startup, the plant experienced foaming

Plant had difficulty treating at high capacity

Not making the 1% CO2 spec with MDEA

Problem was caused by

Hydrocarbon coming into the plant

High amine flow and high tray count required by MDEA seem to worsen foaming problem

» operate with only 19 trays

» over-circulate to keep the CO2 level down

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Conversion to Specialty Solvent

After operating with MDEA for 5 months, customer converted to GAS/SPEC* CS-2000 solvent

Running conversion.

Now plant treating at full capacity of 450 MMSCFD

Meeting < 1000 ppmv CO2 spec

Only the bottom 19 trays were needed

Reduction in foaming tendency

» better separation / filtration

» higher loading decrease HC solubility

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Conclusions - Case 2

Ideally want to design a plant with fewer trays and higher rich loadings

to reduce capital cost

to minimize hydrocarbon absorption

Simulation used to determined alternative feed points to improve plant flexibility

Simulations helped adapt plant to new treating requirements with a specialty solvent

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Case Study 3

Natural gas plant

plant faced with rising CO2 composition

Originally 7.8 mol%

CO2 is now over 10%

Plant operation was unstable because high outlet CO2 caused coldbox to freeze

Goal is to increase capacity and stabilize plant operations

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Operating Conditions versus Simulated

Flow (Nm3/h) 34600 Temperature (°C) 11 Pressure (kPa) 4440 Inlet CO2 (mol%) 10.2 Actual CO2 Out (ppm) 10 Predicted CO2 Out (ppm) 10

Lean Solvent Flow (m3/h) 82 Temperature (°C) 48 Wt% GAS/SPEC CS-2020 50

Rich Solvent Temperature (°C) 79 to 81 Predicted Temp (°C) 81

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Operating Conditions versus Simulated

Flow (Nm3/h) 34600 Temperature (°C) 11 Pressure (kPa) 4440 Inlet CO2 (mol%) 10.2 Actual CO2 Out (ppm) 10 Predicted CO2 Out (ppm) 10

Lean Solvent Flow (m3/h) 82 Temperature (°C) 48 Wt% GAS/SPEC CS-2020 50

Rich Solvent Temperature (°C) 79 to 81 Predicted Temp (°C) 81

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Effect of Rate on CO2 Concentration

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15

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21

23

1 10 100 1000 10000 100000

CO2 in Vapor, ppmv

Tra

y #

(Top

dow

n)

36800 Nm3/h

35700 Nm3/h

34600 Nm3/h

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Effect of Rate on CO2 Loadings

1

3

5

7

9

11

13

15

17

19

21

23

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

Loading, mol/mol

Tra

y #

(Top

dow

n)

36800 Nm3/h

35700 Nm3/h

34600 Nm3/h

Little CO2 absorption

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Effect of Rate on Column Temperature

1

3

5

7

9

11

13

15

17

19

21

23

120 130 140 150 160 170 180 190 200 210

Temperature, °F

Tra

y #

(Top

dow

n)

36800 Nm3/h

35700 Nm3/h

34600 Nm3/h

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Outcome - Case 3

Plant personnel confirmed maximum rate of 34600 Nm3/h

Client considering upgrading pumps and exchangers in order to increase/maintain capacity as inlet CO2 rises

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Conclusions - Case 3

MT Rate based simulation gave insight on effect of gas rate on treat and temperature profile

Allows plant to make informed decisions for future

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Conclusions

Discussed the advantages of Mass Transfer Rate Based Simulation over other simulation methods

Case studies have shown

accuracy of column temperature/composition prediction

effect of mass transfer (tray count) on performance

how to use simulator to design/modify in changing conditions

the importance in considering temperature effects

IAPG 2008

Acknowledgement

Ulises Cruz - INEOS

Andy Sargent - INEOS

Ralph Weiland - Optimized Gas Treating, Inc.

* GAS/SPEC and CS-2000 are trademarks of INEOS Oxide

TM ProTreat is a trademark of Optimized Gas Treating, Inc.

IAPG 2008

QUESTIONS?