Proceedings of the 6th International Conference on Process Systems Engineering (PSE ASIA)

25 - 27 June 2013, Kuala Lumpur.

Liquefaction of Carbon Dioxide with Ammonia

Absorption Chiller System and its Energy

Reduction

Seeyub Yanga, Yeong Su Jeonga, Chiseob Leeb, Seong Pill Chob and Chonghun Han a

a Seoul National University, Department of Chemical and Biological Engineering,

311dong 415ho Gwanak-ro 1 Gwanak-gu, Seoul,151-744 Republic of Korea b KEPCO E&C, 2354 Younggudae-ro, Giheung-gu, Yongin-si, Gyeonggi-do, 446-713,

Republic of Korea

Abstract

Carbon Capture and Sequestration (CCS) processes are researched throughout the

world and considered as a good bridge technology toward low carbon future.

Absorption chiller is an energy efficient option for liquefaction of carbon dioxide since

it uses heat energy as a source for refrigeration not electrical energy. It is especially

efficient when waste heat is abundant like power plants. The carbon dioxide compressor

duty to 30 bar is 59.8kWh/tonCO2. Ternary system of ammonia/water/sodium

hydroxide shows better performance than conventional binary absorption chiller system;

e.g. low reboiler temperature and duty.

Keywords: CCS; Carbon Dioxide, Absorption Chiller, Sodium Hydroxide

1. Introduction

Carbon Capture and Sequestration (CCS) processes are researched throughout the

world and considered as a good bridge technology toward low carbon future. After

carbon dioxide is captured from flue gas of the pulverized coal plant, it needs to be

compressed to supercritical phase or cooled to liquid phase for the transportation to the

storage site. This preparation process for the transportation is known to reduce 3-4%

energy efficiency of the power plant. (Gottlicher, 2004) For the amine-based capture

process, liquefaction process using Joule-Thompson cycle consumes about 100kWh/ton

CO2. (Aspelund, 2007)

Absorption chiller is a viable option for liquefaction since it uses heat energy as a

source for refrigeration not electrical energy. Typical power plants have abundant waste

heat so that absorption chiller can be powerful. Some LNG-based plants use absorption

chiller technology to improve cycle efficiency. (Mortazavi, 2010) And some papers

showed that absorption chiller can have the least energy requirement among the

refrigeration technology when proper condition is achieved. (Alabdulkarem, 2012) In

this paper, carbon dioxide liquefaction using absorption chiller will be presented.

578 S. Yang et al

2. Modeling bases

2.1. Ammonia-water absorption chiller system

Ammonia-water system is one of the common refrigerant-absorbent combinations in

the absorption chiller system. Figure 1 shows basic principle for the refrigeration system.

Rich solution of refrigerant and solvent is pumped to the desirable high pressure and

then it is separated in the generator. Distillated refrigerant is condensed to the liquid

phase and expanded to be a cold temperature and evaporated in the evaporator. Bottom

stream of the generator, which is often called lean solution, is mixed with outlet stream

of the evaporator to be an aqueous phase.

Main operation energy is provided in the generator to separate ammonia and lean

solution. Practical generator temperature of ammonia/water system is about 440K.

(Darwish, 2008)

Figure 1. Dühring plot for Absorption Chiller Liquefaction System

2.2. Ammonia-water-sodium hydroxide absorption chiller system

Some additives are known to affect separation of ammonia-water. LiBr, LiCl, and

LiNO3 strengthen ammonia-water attraction to make absorption easier, while base

hydroxide like LiOH and KOH weakens their attraction to make boiling temperature

lower than binary solution. (Reiner, 1991) In this paper, solution of ammonia-water-

sodium hydroxide is used with following salting-out effect.

NH3 + H2O ↔ NH4+ + OH- (1)

NaOH → Na+ + OH- (2)

This ternary system is simulated with electrolyte NRTL (non-random two-liquid

mixture) model. Electrolyte binary parameters are obtained from Aspen Plus Property

data set SYSOP15, which has no default value for electrolyte-water pair. SYSOP15

reproduces experimental pressure data better than GMELCC regression from Steiu et al,

especially better ammonia and sodium hydroxide rich ternary solution. (Steiu, 2009)

Figure 2 shows the result. Experimental data is from Salavera et al. (Salavera, 2005)

10wt% and 20wt% ammonia is free from sodium hydroxide and 35wt% and 40wt%

ammonia has 4wt% sodium hydroxide.

Liquefaction of Carbon Dioxide with Ammonia Absorption Chiller System and its

Energy Reduction 579

Figure 2. Thermodynamic property experimental data and simulation estimation

2.3. Process flow diagram

Commercial process simulator Aspen Plus 7.3 is used to simulate this system. Figure 3

shows basic design for the ammonia-water-sodium hydroxide absorption chiller carbon

dioxide system. The capture process is based on the Boryeng power plant MEA

(monoethanolamine) based capture pilot. The composition data is from author’s

previous work. (Seeyub, 2012)

Figure 3. Process Flow Diagram for NH3-H2O-NaOH Absorption Chiller

0

200

400

600

800

1000

1200

283.15 293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15

P

r

e

s

s

u

r

e(

k

P

a)

Temperature(K)

10wt%

20wt%

35wt%

40wt%

10wt%sim

20wt%sim

35wt%sim

40wt%sim

580 S. Yang et al

For the stream name, A stands for ammonia, W stands for weak solution and R stands

for rich solution. Reverse osmosis membrane separation efficiency is assumed 99%.

(Steiu, 2008) Some modelling parameters are shown in Table 1.

Table 1. Modelling parameters

Parameters Value

Absorber pressure 3 bar

Generator pressure 16 bar

NaOH concentration 5wt%

Rich ammonia mole concentration 0.416

Lean ammonia mole concentration 0.273

3. Results and Discussion

3.1. Simulation result

The carbon dioxide compressor duty to 30 bar is 59.8kWh/tonCO2, when efficiency is

assumed 80%. Pressure-enthalpy diagram is shown in Figure 4. Pressure axis value is

log-scale. So electricity demand for the liquefaction is lowered by 1/3 when compared

to the research results from other carbon dioxide conditioning process.

Figure 4. Pressure-enthalpy diagram for carbon dioxide liquefaction

1

10

100

-4.2E+08 -4.1E+08 -4E+08 -3.9E+08 -3.8E+08

Pre

ssure

(bar)

Molar Enthalpy (J/kmol)

Liquefaction of Carbon Dioxide with Ammonia Absorption Chiller System and its

Energy Reduction 581

3.2. Conclusion

Absorption chiller can remarkably reduce energy requirement for liquefaction of

carbon dioxide as it uses waste heat. Ternary system of introducing sodium hydroxide

to ammonia/water system is necessary for reduction of boiling temperature.

Acknowledgements

This research was supported by the second phase of the Brain Korea 21 Program in

2013, Institute of Chemical Processes in Seoul National University, Strategic

Technology Development and Energy Efficiency & Resources Development of the

Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded

by the Ministry of Knowledge Economy (2010201020006D-12-2-100) and grant from

the LNG Plant R&D Center funded by the Ministry of Land, Transportation and

Maritime Affairs (MLTM) of the Korean government.

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