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CHAPTER-1 INTRODUCTION

1.1 Carbon Capture

As the climate change fear has achieved a great importance, various methods have been

developed to mitigate CO2 emission. For the last 5 decades there has been growing concern as

the average global temperature is increasing at a moderate rate The main cause behind in CO2 is

due to various activities adopted by human beings which directly or indirectly contribute to the

rise in CO2 concentration. The main source for the increase in CO2 concentration in the

atmosphere is electricity generation sector which mainly comprises of Natural gas and Coal fired

power plants. So there is a urgent need of deploying CO2 mitigation technologies on this sector

which will provide a cleaner environment in future.

1.2 CO2 is a greenhouse gas

CO2 is essential to life on Earth. Greenhouse gases, including CO2, prevent some of the sun's

heat from escaping back into space, keeping the Earth warm enough for plants and animals to

survive.

Common, naturally-occurring greenhouse gases in the atmosphere that can trap some of this heat

include water vapour, CO2, methane and nitrous oxide.

CO2 is a vital part of the food chain for most living creatures. It is also used to put the fizz in

soft drinks, beer and champagne.

1.3 Excess of CO2

CO2 naturally moves into and out of the atmosphere. For example, plants take up and use CO2 to

produce energy, and animals breathe out CO2 made from using energy. The greatly increased

amount of CO2 in the atmosphere resulting from human invention and industrialization, however,

is causing the Earths temperature to rise rapidly.

When fossil fuels are burnt in a power plant to make electricity, large amounts of CO2 are

released into the atmosphere. CO2 is released from the ground into the atmosphere during natural

gas production.[5]

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CHAPTER-2 CARBON CAPTURE AND SEQUESTRATION

There are three basic types of CO2 capture: pre-combustion, post-combustion and oxyfuel

combustion.

2.1 Pre-combustion capture

Pre-combustion capture processes convert fuel into a gaseous mixture of hydrogen and CO2. The

hydrogen is separated and can be burnt without producing any CO2. The CO2 can then be

compressed for transport and storage. The fuel conversion steps required for pre-combustion are

more complex than the processes involved in post-combustion, making the technology more

difficult to apply to existing power plants.

Fig 1.1 Mechanism of precombustion capture

2.2 Post-combustion capture

Post-combustion processes separate CO2 from combustion exhaust gases. CO2 can be captured

using liquid solvent or other separation methods. In an absorption-based approach, once

absorbed by the solvent, the CO2 is released by heating to form a high purity CO2 stream.

Fig 1.2 Mechanism of postcombustion capture

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2.3 Oxyfuel combustion

Oxyfuel combustion processes use oxygen rather than air for combustion of fuel. This produces

exhaust gas that is mainly water vapor and CO2 that can be easily separated to produce a high

purity CO2 stream.[1]

Fig 1.3 Mechanism of oxyfuel combustion

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CHAPTER-3 AMINE PROCESSING

3.1 AMINE PROCESS

In most of the industries absorption with the help of chemical solvents, which is also known as

chemical absorption is the commercially most widely used process to remove acid gas(mainly

CO2 and H2S) from various gas streams. Currently preferred chemical solvents for acid gas

removal by chemical absorption are amine based absorbents. Alkanolamines, which are the

combinations of alcohols and ammonia, are the mostly preferred solvents for removing acid gas.

In addition to natural gas processing, chemical absorption of acid gases by alkanolamines has

been utilized in a various industries like petroleum refining, CO2 capture from combustion and

flue gases, removal of CO2 from synthesis gas in ammonia or hydrogen plants.

3.2 DETAILED DESCRIPTION OF THE PROCESS

The process used for capturing CO2 using MDEA can be divided into 3 different sections:

1. Cooling of Flue gas and its compression

2. Absorption of CO2 and solvent regeneration

3. Compression of CO2

A detailed flowsheet of the process is given below:

Fig 2.1 Flowsheet for CO2 capture by MDEA solvent

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3.3 COOLING OF FLUE GAS AND COMPRESSION

The absorber for the CO2-H2O-MDEA system should operate at temperature of around 40C and

therefore, the gases temperature at the inlet of the absorber should lie in the temperature ranging

from 40-50C. Usually, the flue gases temperature at the exhaust in industries ranges from 110-

120C and hence, the flue gases must be cooled before feeding it to the absorber. Sometimes wet

flue gas desulfurization scrubber is used for cooling the flue gases. If the flue gases have not

been through a scrubber then cooling is done by other means. Direct contact cooling tower

(DCC) is used for cooling the flue gases by feeding it the tower.DCC may be tray tower or

packed tower where counter-current flow of flue gases.

The flue gas coming out of the DCC needs to be compressed and therefore, it is sent to a blower.

Because of the upward movement of the flue gas in the absorber (tray column), the pressure of

the flue gas needs to be raised before feeding to the absorber. Along with the pressure,

temperature is also increased. Flue gas needs to be scrubbed prior to chemical absorption with

MDEA to remove NOx, SOx and other impurities, which react irreversibly with MDEA to form

heat stable salts which cannot be reclaimed. The recommended concentration for NO2 should be

less than 20 ppm. Similarly the recommended concentration for SOx should be less than 10

ppmv for MDEA solvent. A wet electrostatic precipitator or a mist eliminator must be employed

in the flue gas desulfurization unit so as to remove SO3, which can form sulfuric acid aerosol in

scrubbers which can cause corrosion.

3.4 SOLVENT REGENRATION

The absorber used is a tray column, where vapor and liquid leaving the stage are in equilibrium.

From the absorbers bottom, flue gas is fed whereas lean amine solvent is fed from the top. The

loading of the lean amine stream which is entering to the absorber from the top is between 0.3-

0.35 and rich amine stream leaving the absorber has a loading close to 0.8. In a MDEA system,

the loading is defined on a mole basis as given by

The amine stream which is stripped off CO2 is referred to as Lean amine off i.e. the amine stream

entering from the absorbers top. If the amine stream has CO2 loaded in it, then it is known as

rich amine i.e. the stream leaving from the absorbers bottom. The lean amine stream is entering

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inside the absorber through the second stage from the top and make-up water is entering at the

top stage.

The rich amine leaving at the absorbers bottom is sent to the heat exchanger, which is also

known as cross-heat exchanger, via a pump. In the cross heat-exchanger, the heat is exchanged

between the rich amine stream from the absorber and the lean amine stream from the desorber.

3.5 SOLVENT RECLAIMATION

A MDEA reclaimer is used necessarily when the flue gas is coming out from coal fired power

plants to treat the stable salts that are formed because of SOx and NOx. These salts should not

accumulate on the amine stream as it is not desirable since it reduces the solvents capacity for

absorbing CO2. The removal of the purge stream of MDEA solvent is done and is then sent to the

reclaimer where strong alkali like NaOH and heat are added because of which heat stable salts

can be dissociated contributing to the recovery of the solvent

3.6 CO2 COMPRESSION

The CO2 gas coming out from the desorbers top needs to be dried and should be compressed

before sending it for storage. Drying is one of the important steps as the presence of even small

amount moisture in the stream can corrode the pipelines, which are used for transporting CO2.

Typically, a reciprocating compressor with 4 stages is used for cooling. The compressor is

employed for compressing the CO2 to a pressure of 90 atm, after which the liquid CO2 can be

pumped through pump with the discharge pressure of 130 atm.[6]

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There are various thermodynamics equations like Wilson, NRTL and UNIQUAC etc. the

generated data from experiment and calculated data should have minimum error which is

achieved by data regression. Data regression follows the least square method.

Problem statement-Analysis of CO2 absorption using 50% MDEA solvent at 70C in

Aspen Plus[4]

Steps for regression using VLE data for ethanol-ethyl acetate system.

Temperature p(CO2)

343 95.7 0.2367

343 117.7 0.279

343 177.2 0.3582

343 220.9 0.4029

343 273.6 0.4718

343 306.7 0.4834

343 379.1 0.5259

343 430.1 0.5489

343 486.7 0.5858

343 488.3 0.5894

343 581.4 0.6058

343 688.1 0.6609

343 776.9 0.6786

343 813.4 0.6898

=

+1) =

=1 Mole fraction of MDEA=

=0.1308

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CHAPTER-4 STEPS OF REGRESSION

1. Start Aspen Plus and create a new run.[2]

2. In the Home Tab of the Ribbon, in the Run Mode group, click Regression.

3. Enter the components on the Components-Specification-Selection sheet

4. Select the property method. Use the Methods-Specification-Global sheet to choose

property method (WILSON).

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5. Enter experimental data

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Use the Data- Mixture form to enter the VLE data

6. Specify the regression case.

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7. Specify additional regression cases.

8. Run the regression.

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9. Examine the result on the Regression-Results- Parameter form.

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10. On the Home tab of the ribbon, in the Plot group, open the plot gallery and click Residual

to plot the residual of pressure for case VLE.[3]

Merged graph by E-NRTL Model & WILSON Model

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CHAPTER-5 CONCLUSIONS

To carry out simulation and modeling for the CO2 capture process with MDEA, the E-NRTL and

Wilson model was used to correlate vapor-liquid equilibrium properties of CO2-H2O-MDEA

system with the experimental data available for the CO2-H2O-MDEA system. The E-NRTL

model is validated to predict vapor-liquid equilibrium (VLE) and partial pressure of CO2 of the

MDEA-H2O-CO2 system with temperature ranging 70C, concentration of MDEA up to 50wt %,

and loadings of CO2 close to 0.5. The model provides a representation for thermodynamic

property for the CO2- H2O-MDEA system over a wider range of conditions and gives more-

reliable predictions than those from Wilson works.The relation of CO2 capture on the lean

loading, desorber temperature and pressure was sufficient to ensure the completion of the

simulation and designing of the Carbon Capture and Storage work.

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References

1. Post-combustion CO2 Capture with Chemical Absorption: A State-of-the-art Review(M.

Wanga* A. Lawala, P. Stephensonb, J. Siddersb, C.Ramshawa and H. Yeunga process

Systems Engineering Group, School of Engineering, Cranfield University, UK. bRWE

npower, UK.)

2. Aspen plus help

3. Aspen user manual

4. Solubility of Carbon Dioxide in 30 mass % Monoethanolamine and 50 mass %

Methyldiethanolamine Solutions (Sholeh Mamun, Roger Nilsen, and Hallvard

F.Svendsen*)Department of Chemical Engineering, Norwegian University of Science

and Technology,N-7491 Trondheim, Norway

5. Carbon Dioxide Capture by Chemical Absorption:A Solvent Comparison Study by

Anusha Kothandaraman B. Chem. Eng. Institute of Chemical Technology, University of

Mumbai, 2005 M.S. Chemical Engineering Practice Massachusetts Institute of

Technology, 2006

6. Aspen Simulation of CO2 absorption system with various Amine Solution Seok Kim,

Hyung-Taek Kim Dept. of Energy Studies, Ajou University Wonchon-dong San-5,

Paldal-gu