Thesis on
STUDIES ON PURIFICATION OF NATURAL GAS
USING CRYOGENIC TECHNOLOGY
Submitted by
BISWAJIT DEBNATH
Class Roll no: 001310302012
Examination Roll No: M4CHE1508
Registration No: 124707 of 2013-14
Session: 2013-2015
Master of Chemical Engineering
Project Supervisor
Prof. (Dr.) Kajari Kargupta
This project report is submitted towards the completion of
Master of Engineering degree in Chemical Engineering
DEPARTMENT OF CHEMICAL ENGINEERING
JADAVPUR UNIVERSITY
KOLKATA 700 032
Faculty of Engineering & Technology Department of Chemical Engineering
Jadavpur University Kolkata 700032
DECLARATION OF ORIGINALITY
I, Sri Biswajit Debnath, declare that this thesis is my own work and has not been submitted in any form for another degree at any university or other institute of tertiary education. Information derived from the published and unpublished work of others has been acknowledged in the text and a list of references is given in this thesis. I also declare that I have pursued the Master of Chemical Engineering course in accordance with the requirements of the universitys regulation, Research practice and ethical policies have been complied with appropriately Name: Biswajit Debnath
Exam Roll No: M4CHE1508
Class Roll Number: 00131302012
Thesis Title: Studies on Purification of Natural Gas Using Cryogenic
Technology.
Signed: ___________________ Date: ___________
Faculty of Engineering & Technology Department of Chemical Engineering
Jadavpur University Kolkata 700032
C E R T I F I C A T E
This is to certify that Mr. Biswajit Debnath, a final year student of
Master of Chemical Engineering Examination student of Chemical
Engineering department, Jadavpur University (Class Roll No:
001310302012; Examination Roll No: M4CHE1508; Registration No:
124707 of 2013-14), has completed the thesis work titled Studies on
Purification of Natural Gas Using Cryogenic Technology under the
supervision of Prof. (Dr.) Kajari Kargupta during his Masters
Curriculum. This work has not been reported earlier anywhere and can be
approved for submission in partial fulfillment of the course work.
________________________ Prof. (Dr.) Kajari Kargupta Thesis Supervisor Department of Chemical Engineering Jadavpur University
Prof. (Dr.) Chandan Ghua Head of the department Department of Chemical Engineering Jadavpur University
Dean Faculty of Engineering and Technology Jadavpur University
Faculty of Engineering & Technology Department of Chemical Engineering
Jadavpur University Kolkata 700032
CERTIFICATE OF APPROVAL
The foregoing thesis, entitled as Studies on Purification of Natural Gas
Using Cryogenic Technology is hereby approved by the committee of
final examination for evaluation of thesis as a creditable study of an
engineering subject carried out and presented by Mr. Biswajit Debnath
(Class Roll No: 001310302012; Examination Roll No: M4CHE1508;
Registration No: 124707 of 2013-14) in a manner satisfactory to warrant
its acceptance as a perquisite to the degree of Master of Automobile
Engineering. It is understood that by this approval, the undersigned do not
necessarily endorse or approve any statement made, opinion expressed or
conclusion drawn therein, but approve the thesis only for the purpose for
which it is submitted.
Committee of final examination for evaluation of thesis
To my family, my friends and
my inspirations (Late Suptish C. Nandy,
Late Jim Morrison and Late B.B. King)
i
C O N T E N T S
List of Figures v
List of Tables viii
Nomenclature ix
Acknowledgement 1
ABSTRACT 3
CHAPTER 1: INTRODUCTION 4-23
1.0 Energy Demand
1.1 Natural Gas
1.2 Existing Carbon Capture and Storage Technologies
1.2.1 Chemical Looping
1.2.2 Absorption
1.2.3 Adsorption
1.2.4 Membrane Technology
1.2.4 Cryogenic Technology
1.3 Research Methodology
CHAPTER 2: LITERATURE REVIEW 24-39
2.0 Historical Background
2.1 Detailed Literature Review
2.1.1 Thermodynamics and Solidification
2.1.2 Conventional Cryogenic Technology
2.1.3 Non-conventional Cryogenic Technology
2.1.4 Hybrid Technology
2.2 Research Gap
2.3 Objectives
CHAPTER 3: MODEL DESCRIPTION 40 52
3.0 Problem Statement
3.1 Nucleation Theory
3.2 Description of packed bed setup
3.3 Experimental Procedure
ii
3.4 Transport Phenomena Model for CO2 Separation on Single Packing
3.4.1 Mass-transfer and kinetics of nucleation
3.4.2 Energy-Balance Equation
3.4.3 Thermodynamic, mass and heat transfer correlations used for simulation
3.5 Transport phenomena model for CO2 capture using cryogenically cooled packed
bed with multiple Packing
3.5.1 Modeling of Desublimation of Carbon dioxide inside the
Capture/Deposition Zone of the Packed Column
3.6 Solution Technique
CHAPTER 4: RESULTS & DISCUSSION 53 85
4.0 Desublimation kinetics of Carbon dioxide on a single packing from gas stream
4.1 Results of simulation with pure carbon dioxide feed gas stream
4.1.1 Variation of frost layer thickness () with time at different inlet gas flowrate
4.1.2 Rate of frost layer deposition with time at different inlet gas flowrate
4.1.3 Rate of change of frost layer thickness with time at different inlet gas
flowrate
4.1.4 Variation of Interface Temperature with time at different inlet gas flowrate
4.2 Results of simulation with carbon dioxide and methane mixture as feed gas stream
4.2.1 Variation of frost layer thickness () with time at different bulk pressure
4.2.2 Rate of change of frost layer deposition with time at different inlet gas
flowrate
4.2.3 Variation of Particle Temperature with time at different bulk Pressure
4.2.4 Variation of Interface Temperature with time at different inlet gas flowrate
4.3 Effect variation of CO2 percentage on Frost Layer Thickness
4.4 Effect of CO2 composition on Particle Temperature
4.5 Dynamics of CO2 capture inside a cryogenically cooled packed bed with pure CO2 as feed
4.5.1 Variation of frost layer thickness with time at different position along the
bed for co current flow
4.5.2 Variation of frost layer thickness with time at different position along the
bed for counter current flow
iii
4.5.3 Contour plot of frost layer thickness with time and axial position for co
current flow
4.5.4 Contour plot of frost layer thickness with time and axial position for counter
current flow
4.5.5 Surface plot of frost layer thickness with time and axial position for counter
current flow
4.5.6 Growth of frost layer thickness with time and axial position for counter
current flow
4.5.7 Variation of outlet mass flowrate thickness with time and axial position for
counter current flow
4.5.8 Surface Plot of frost layer thickness with time and axial position for counter
current flow
4.5.9 Percentage separation of carbon dioxide of with axial position at different
time for counter current flow
4.5.10 Contour plot of percentage separation of carbon dioxide with time and
axial position for counter current flow
4.5.11 Contour plot of frost layer deposition on only packing surface
(heterogeneous nucleation) with time and axial position for counter current flow
4.5.11 Surface plot of frost layer deposition on only packing surface
(heterogeneous nucleation) with time and axial position for counter current flow
4.5.12 Contour plot of frost layer deposition on only bed wall (heterogeneous
nucleation) with time and axial position for counter current flow
4.5.13 Surface plot of frost layer deposition on only bed wall (heterogeneous
nucleation) with time and axial position for counter current flow
4.5.14 Effect of homogeneous nucleation on saturation time during the capture
cycle
4.5.15 Effect of inlet gas feed flowrate on bed saturation time during the capture
cycle
4.6 Dynamics of CO2 capture inside a cryogenically cooled packed bed with 80%
CO2 as feed
4.6.1 Variation of frost layer thickness with time at different position along the
bed for counter current flow
iv
4.6.2 Variation of normalized mass flow with axial distance at different time
for 80% CO2 composition in feed gas mixture and countercurrent flow
4.7 Validation of simulation results with experimental results
CHAPTER 5: CONCLUSION 86 87
5.0 Conclusion
5.1 Future Scope
References 98 101
Appendix I 102 104
Appendix II 105 106
v
List of figures
Fig no Description
Fig. 1 Demand of Natural Gas by Region projected to 2035.
Fig. 2 Reserves to Production (R/P) ratios of natural gas by regions
Fig.3 Natural Gas Consumption by sector, 2013
Fig. 4 Natural Gas Consumption by country, 2013
Fig. 5 Distribution of high CO2 gas fields by country.
Fig. 6 General Carbon Capture and Storage process.
Fig. 7 Post, Pre and Oxyfuel combustion processes.
Fig. 8 Process flow diagram of a typical amine-solvent (MDEA)-based chemical
absorptionsystem for the separation of CO2 and other acid gases from natural gas.
Fig. 9 Pressure Temperature Phase Diagram for CO2.
Fig. 10 Gibbs Free energy difference for homogeneous and heterogeneous nucleation
Fig. 11 The Packed Bed Experimental Setup
Fig. 12 Schematic diagram of a packed bed and a single packing with frost layer
Fig. 13 Schematic Diagram explaining the Algorithm
Fig. 14 Effect of inlet gas flow rate on carbon dioxide frost layer thickness ()
Fig. 15 Effect of inlet gas flow rate on rate of deposition of carbon dioxide frost layer
Fig. 16 Effect of inlet gas flow rate on rate of change of carbon dioxide frost layer
Fig. 17 Effect of inlet gas flow rate on Interfacial Temperature (Ti)
Fig. 18 Effect of bulk pressure on frost layer thickness with time
Fig. 19 Effect of inlet gas flow rate on rate of deposition of frost layer thickness
Fig. 20 Effect of bulk pressure on Particle Temperature (Tp)
Fig. 21 Effect of inlet gas flow rate on Interfacial Temperature (Ti)
Fig. 22 Effect of variation of carbon dioxide in feed gas stream on frost layer thickness
Fig. 23 Effect of variation of carbon dioxide in feed gas stream on Particle Temperature
(Tp)
Fig. 24 Growth of CO2 frost with time at different axial position of the bed for
Co current flow at 5 lpm inlet gas flowrate and Temperature Profile 3
Fig. 25 Growth of CO2 frost with time at different axial position of the bed for
Counter current flow at 5 lpm inlet gas flowrate and Temperature Profile
Fig. 26 Contour plot of CO2 frost vs. time vs. Axial position for
vi
Co current flow at 5 lpm inlet gas flowrate and Temperature Profile 3
Fig. 27 Contour plot of CO2 frost vs. time vs. Axial position for Counter current flow at 5
lpm inlet gas flowrate and Temperature Profile 1
Fig. 28 Contour plot of CO2 frost vs. time vs. Axial position for Counter current flow at
10 lpm inlet gas flowrate and Temperature Profile 3
Fig. 29 Surface plot of CO2 frost vs. time vs. Axial position for Counter current flow at
5 lpm inlet gas flowrate and Temperature Profile 3
Fig. 30 Growth of CO2 frost with time at different axial position of the for Counter
current flow at 10 lpm inlet gas flowrate and Temperature Profile 3
Fig. 31 Variation of outlet Mass flowrate with time at different axial position of the for
Counter current flow at 10 lpm inlet gas flowrate and Temperature Profile 3
Fig. 32 Surface plot of outlet mass flowrate vs. time vs. Axial position for Counter
current flow at 10 lpm inlet gas flowrate and Temperature Profile 2
Fig. 33 Percentage separation of CO2 frost vs. Axial position at different time for
Counter current flow at 5 lpm feed flowrate and Temperature Profile 3
Fig. 34 Contour plot of percentage separation of CO2 vs. time vs. Axial position for
Counter current flow at 5 lpm inlet gas flowrate and Temperature Profile 3
Fig. 35 Contour plot of deposition of CO2 frost on packing vs. time vs. Axial position
for Counter current flow at 5 lpm inlet gas flowrate and Temperature Profile 3
Fig. 36 Surface plot of deposition of CO2 frost on packing vs. time vs. Axial position for
Counter current flow at 5 lpm inlet gas flowrate and Temperature Profile 3
Fig. 37 Contour plot of deposition of CO2 frost on bed wall vs. time vs. Axial position
for Counter current flow at 5 lpm inlet gas flowrate and Temperature Profile 3
Fig. 38 Surface plot of deposition of CO2 frost on bed wall vs. time vs. Axial position
for Counter current flow at 5 lpm inlet gas flowrate and Temperature Profile 3
Fig. 39 Effect of Homogeneous Nucleation on the saturation time for the capture cycle
for Counter current flow at 5 lpm inlet gas flowrate and Temperature Profile 3
Fig. 40 Effect of inlet gas feed flowrate on the saturation time for the capture cycle for
Counter current flow at 5 lpm inlet gas flowrate and Temperature Profile 3
Fig. 41 Growth of CO2 frost with time at different axial position of the bed for Counter
current flow at 5 lpm inlet feed gas flowrate and Temperature Profile 3
Fig. 42 Variation of normalized mass flow with axial distance at different time for
Counter current flow at 5 lpm inlet gas flowrate and Temperature Profile 3
vii
Fig. 43 Comparison of simulation and experimental results of variation of percentage
CO2 with time at different bed temperature profiles for Counter current flow at 5
lpm gas flowrate
viii
List of tables
Table no. Description
1 Typical Composition of natural Gas
2 U.S. pipeline composition specifications for natural gas delivery
3 PVTX experimental data for carbon dioxide mixtures
ix
NOMENCLATURE Symbol Property
mass of CO2 frost deposited on the packing
mass transfer co-efficient in the CO2 stream side
bulk pressure in the flue gas
saturation pressure of CO2 at the interface
t time
h convective heat transfer co-efficient
latent heat of desublimation for CO2
T temperature within the frost layer
radial distance from the surface of the packing
density of the glass packing
specific heat of the packing material
thermal conductivity of the CO2 frost
frost layer thickness
Sh Sherwood number
Nu Nusselts number
Re Reynolds number
Pr Prandtls number
Sc Schmidts number
Density of CO2 frost
Total number of packing particle in a single layer
Mass of CO2 flowing into the column
Mass of CO2 flowing out of the column
Q Volumetric flow rate of CO2 in
CO2 Density of CO2 in gas phase
x
Mass of N2 flowing out of the column
Density of nitrogen gas in the column
Mass fraction of CO2 in exit stream of single layer
mg Mass flow rate of the CO2 stream
Cg Specific heat capacity of CO2 gas
Tg Temperature of the flowing CO2 stream
Ap Cross-sectional area of the packed column
n Packing density
1
Acknowledgement
It would be a great pleasure for me to take the opportunity to humbly express my gratitude for
the innumerable gestures of help, cooperation and encouragement which I have received from
my teachers, friends and all of my well wishers during this course. First of all, I would like to
express my immense gratitude to the Chemical Engineering Department of Jadavpur University
for assigning me the project entitled Studies on Purification of Natural Gas using Cryogenic
Technology.
I am deeply indebted to my Project Supervisor Prof. (Dr.) Kajari Kargupta, for
providing me with an opportunity to work on this interesting field which has a great impact on
making a clean environment upon successful exploitation. Her insight and expertise in the field
of modeling and simulation have enriched me more than anything else. Also I would like to take
this opportunity to acknowledge that her guidance not only helped to complete my project but
also showed me new light in my life. She has been always there even when I pretended to
understand everything and anything but I didnt. Her commitment, dedication and undisputed
love towards me have helped me to grow both as a human being as well as in the field of
chemical engineering.
I am very grateful to Prof. (Dr.) Chandan Guha, Head of the Department, Chemical
Engineering Department and all other faculty members for their help and cooperation. I want to
thank all the teachers and staff of the Chemical Engineering Department.
I would like to extend my thanks to our lab mates who have supported me. My sincere
appreciation also extends to all my colleagues and others who have provided assistance at
various occasions. I would also like to thank Ms. Shubhanwita Saha, Mrs. Punam
Mukhopadhyay, Mr. Rahul Baidya, Mr. Rayan Kundu, Ms. Upasana Das and Ms. Aryama
Raychaudhuri for their help in peer reviewing the thesis and help with the English. I would also
like to extend my gratitude towards Ms. Ditipriya Hazra, Ms. Sanghamitra Das, Mr. Riju De and
Mr. Shambojit Roy for their help in downloading research papers which I were unable to access.
2
I would also like to acknowledge Ms. Eapsita Pahari, Ms. Suchismita Paul, Mrs.
Moumita Sardar, Ms. Shimanti Chandra and my specially good friend MON for their help and
mental support when I was struggling to figure out things regarding this project.
And last but not least I would like thank Lokenath Baba, Jim Morrison (JimDa), Lucile
II, Mr. Bapi Debnath my father, Mrs. Rekha Debnath my mother, Mr. Swarup Mondal my meso,
Ms. Sayantani Mondal my sister and my maternal grandparents for their love, blessings, faith in
me and all other kinds of support during the period of my Masters Degree.
Biswajit Debnath
P.G. Student
Department of Chemical Engineering
Jadavpur University
3
ABSTRACT
The growing concern of low carbon foot print and energy demand it is now necessary to invade
the impure gas wells and proper technology for methane enrichment. In this study cryogenic
separation of carbon dioxide has been carried out in the solid vapour zone, from pure carbon
dioxide and carbon dioxide gas mixtures. A two step model has been considered the mass
transfer from bulk gas to the packing and the nucleation of carbon dioxide frost on the packing.
These two resistances are considered to be in parallel. A single packing was considered for
transport phenomena modeling and there after it was embedded into a multiple packing model.
Simulations were carried out to study the spacio- temporal evolution of frost layer thickness both
in single packing and multiple packing. The effects of bulk pressure and composition of carbon
dioxide on frost layer thickness, rate of change of frost layer thickness, rate of frost deposition,
interfacial temperature and partial temperature were simulated. The results showed that higher
carbon dioxide concentration results in higher frost layer thickness. Simulations were also
carried out for the total bed. Contour and 3D surface plots of frost layer thickness and mass flow
rate reveals that effective separation depends on inlet pressure flow rate, temperature profile and
flow configuration. They also affect the frost deposition. It was also found that counter current
flow configuration with respect to liquid nitrogen ensures better separation than co-current flow
configuration. The significance of the study is in design and optimization of cryogenic separation
of carbon dioxide from flue gas and natural gas.
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5
CHAPTER
1 INTRODUCTION
6
1.0 Energy Demand The appetite for oil, natural gas, and other energy sources is growing dramatically, with
worldwide energy consumption projected to increase by more than 40 percent by 2035. The
growing demand is fueled by a population that is predicted to increase 25 percent in the next 20
years, with most of that growth in countries with emerging economies, such as China and India.
This phase of very high energy consumption growth is driven by the industrialization and
electrification of non-OECD economies, notably China. The 2002-2012 decade recorded the
largest ever growth of energy consumption in volume terms over any ten year period. There is a
clear long-run shift in energy growth from the OECD to the non-OECD. Virtually all (95%) of
the projected growth is in the non-OECD, with energy consumption growing at 2.3% p.a. OECD
energy consumption, by contrast, grows at just 0.2% p.a. over the whole period and is expected
to fall from 2030 onwards. By sector, industry will always remain the dominant source of growth
for primary energy consumption, both directly and indirectly (in the form of electricity). Industry
accounts for more than half of the growth of energy consumption. Although it is forecasted that
the growth in renewable (6.4% p.a.) is going to be the fastest amongst all the fuels but in the final
decade (considering a projection till 2035) gas is the largest single contributor to growth that
being the fastest growing among fossil fuels (1.9% p.a.) and the only one to grow more rapidly
than total energy. Rising energy demand from economic output and improved standards of living
will likely put added pressure on energy supplies. For example, in China alone, demand is
expected to increase by 75 percent by 2035. Simultaneously worldwide consumption of
petroleum and other liquid fuels raised from 87 MMbbl/d in 2010 to 98 MMbbl/d in 2020 and
projected to rise to 119 MMbbl/d in 2040. The growth in other liquid supplies is attributed to by-
products of natural gas production (in the case of NGPL) and government policies aimed at
increasing the use of alternative liquid fuels in the transportation sector. Other liquid supplies
account for between 14% and 17% of total liquid fuel supplies throughout the projection period
of 2040. Energy demand will grow, especially in the non-OECD (Organization for Economic
Co-operation and Development) countries, which accounts for much of the uncertainty about
future demand growth.
Global demand for natural gas is projected to grow by 1.9% p.a., reaching 497 Bcf/d by
2035, with non-OECD growth (2.7% p.a.) outpacing the OECD (1% p.a.). Global gas supply is
expected to grow to 172 Bcf/d by 2035. Shale gas is the fastest growing source of supply (6.5%
7
Fig. 1: Demand of Natural Gas by Region projected to 2035
(B.P. Energy Outlook 2035)
p.a.), providing nearly half of the growth in global gas. On the demand side, shale gas gives US
natural gas a competitive advantage relative to other fuels. This is already visible in the power
sector, where gas is likely to continue to grow (0.5% p.a.) at the expense of coal, despite the
rapid expansion of renewables. Next, gas is expected to gain market share in the industrial sector,
from 39% in 2012 to 42% by 2035. And finally, gas will start to penetrate the transport sector.
Gas is the fastest growing fuel (18% p.a.) in a sector where overall demand is falling (-0.9%
p.a.). By 2035 gas will account for 8% of US transport sector fuels, almost matching biofuels.
1.1 Natural Gas Natural gas is used primarily as a fuel and as a raw material in manufacturing. It is used in
home furnaces, water heaters, and cooking stoves. As an industrial fuel, it is used in brick,
cement, and ceramic-tile kilns; in glass making; for generating steam in water boilers; and as a
clean heat source for sterilizing instruments and processing foods. As a raw material in
petrochemical manufacturing, natural gas is used to produce hydrogen, sulfur, carbon black, and
ammonia. Ethylene, an important petrochemical, is also produced from natural gas.
8
The discovery of natural gas dates from ancient times in the Middle East. Thousands of
years ago, it was noticed that natural gas seeps ignited by lightning created burning springs. In
Persia, Greece, or India, people built temples around these eternal flames for their religious
practices. However, the energy value of natural gas was not recognized until approximately 900
BC in China, and the Chinese drilled the first known natural gas well in 211 BC.
Natural gas exists in nature under pressure in rock reservoirs in the Earths crust, either in
conjunction with and dissolved in heavier hydrocarbons and water or by itself. It is produced
from the reservoir similarly to or in conjunction with crude oil. Natural gas has been formed by
the degradation of organic matter accumulated in the past millions of years. The principal
constituent of natural gas is methane. Other constituents are paraffinic hydrocarbons such as
ethane, propane, and the butanes. Many natural gases contain nitrogen as well as carbon dioxide
and hydrogen sulfide. Trace quantities of argon, hydrogen, and helium may also be present. The
composition of natural gas can vary widely. Table 1-1 outlines the typical makeup of natural gas
before it is refined.
Table 1: Typical Composition of natural Gas (Wikipedia)
Name Formula Volume (%)
Methane CH4 70-90%
Ethane, Propane & Butane C2H6, C3H8, C4H10 0-20%
Carbon Dioxide CO2 0-8%
Oxygen O2 0-0.2%
Nitrogen N2 0-5%
Hydrogen sulphide H2S 0-5%
Rare gases A, He, Ne, Xe trace
Natural gas can also contain a small proportion of C5+ hydrocarbons. When separated, this
fraction is a light gasoline. Some aromatics such as benzene, toluene, and xylenes can also be
present, raising safety issues due to their toxicity. Natural gas can contain other contaminants
too. Acid contaminants, such as mercaptans (R-SH), carbonyl sulfide (COS), Carbon dioxide
(CO2) and carbon disulfide (CS2) might be present in small quantities. Mercury can also be
present either as a metal in vapour phase or as an organo-metallic compound in liquid fractions.
Concentration levels are generally very small, but even at very small concentration levels,
9
mercury can be detrimental due its toxicity and its corrosive properties (reaction with aluminium
alloys).
According to ExxonMobil Energy Outlook Report 2015, global demand for natural gas is
projected to rise by 65 percent from 2010 to 2040, the largest volume growth of any energy
source. The extensive utilization of NG has led to decreased NG reserves to production ratio over
regions of the world, among which Middle East has the highest ratio as shown in Figure 2.
Fig. 2: Reserves to Production (R/P) ratios of natural gas by regions
Natural gas, also called the prince of hydrocarbons as it has many applications. The
proportion of the natural gas consumed for energy production in major fields including
industrial, commercial, residential, transportation and in generating electricity for the year 2009
is shown on Fig. 3.
10
Fig.3: Natural Gas Consumption by sector, 2013
Natural gas consumption is the highest in United States and Russia, followed by North America
and Middle East. Figure 4 shows the natural gas domestic consumption worldwide in 2013.
Fig.4: Natural Gas Consumption by country, 2013 (Enerdata.net)
11
Natural gas consists primarily of methane (70-90% of the total component) and other light and
heavier hydrocarbons. The impurities present in natural gas need to be removed to meet the
pipeline quality standard (NaturalGas.org 2010). The allowable amounts of common impurities
in U.S. for the delivery of the natural gas to the pipe line are given below.
As one of the major contaminates in natural gas feeds, carbon dioxide must optimally be
removed as it reduces the energy content of the gas and affect the selling price of the natural gas.
Moreover, it becomes acidic and corrosive in the presence of water that has a potential to
damage the pipeline and the equipment system. In addition, when the issue of transportation of
the natural gas to a very far distance is a concern, the use of pipelines will be too expensive so
that Liquefied Natural Gas (LNG), Gas to Liquid (GTL) and chemicals are considered to be an
alternative option. In LNG processing plant, while cooling the natural gas to a very low
temperature, the CO2 can be frozen and block pipeline systems and cause transportation
drawback. Hence, the presence of CO2 in natural gas remains one of the challenging gas
separation problems in process engineering for CO2/CH4 systems.
Table 2. U.S. pipeline composition specifications for natural gas delivery
(Al-Juaied 2004; Baker 2004)
Components U.S. Pipeline Specification
Hydrocarbons (C3+) 950 1050 Btu/scf dew point -20OC
CO2 < 2 mol%
H2S < 4 ppm
H2O < 0.1 gm/m3 (
12
Fig. 5: Distribution of high CO2 gas fields by country (Maqsood et al., 2014)
The chaos around the world with crude oil and due to increased demand on natural gas has
stimulated the researchers to develop, design and modify cryogenic technology for Natural Gas
separation. Presence of carbon dioxide (CO2) and other sour gases in varying quantities in
different natural gas resources has endorsed several innovative sequestrating technologies to be
invented to mitigate the anthropogenic CO2 emission as well as maintaining greenhouse gas level
in the atmosphere. And research is still going on.
Despite of the fact that several literatures is available on removal of sulfur containing
gases and higher hydrocarbons, now the focus must be shifted towards the removal of carbon
dioxide efficiently form gas stream. With growing economy and population driving the demand
of energy associated with lean and green slogans have made it imperative for the researchers to
innovate and materialize new technologies to deal with the natural gas reserves containing CO2
up to 80%. Natural gas is mostly considered as a "clean" fuel as compared to other fossil fuels,
the natural gas found in reservoirs deposit is not necessarily "clean" and free of impurities. One
third of the proven natural gas reserves are estimated to be sour. Malaysia alone constitutes more
than 13 Tscf of undeveloped natural gas because of the high CO2 concentration. In some gas
fields, the concentration of CO2 exceeds 70 percent (Darman & Harum, 2006). Therefore
separation of carbon dioxide is necessary to maintain the selling price of the natural gas with
13
quality because CO2 reduces the calorific value of the gas making it economically unfeasible.
The high content of CO2 in natural gas enhances the formation of carbonic acids and dry ice
causing corrosion and clogging of delivery pipelines. Hence, the removal of CO2 from the
natural gas is important for maintaining the quality of the product to satisfy the customer. Also,
purification of raw natural gas is necessary so as to not facilitate pipeline corrosion and to satisfy
the pipeline standards for different NG companies. Therefore, the impurities must be removed to
meet the pipe-line quality standard specifications as a consumer fuel, enhance the calorific value
of the natural gas, avoid pipelines and equipment corrosion and further overcome related process
bottle necks.
1.2 Existing Carbon Capture and Storage Technologies Raw natural gas collected from the wells is often impure. Purification is carried out at different
stages to meet different pipeline specification, as standardized internationally & nationally. CO2
being one of the most notorious acid gases purification is important as to meet standards &
commercialization.
Fig. 6: General Carbon Capture and Storage process
14
Different existing technologies are there which are widely used for CO2 separation from natural
gas stream as well as flue gas streams which evolves in different processes in industries. Existing
technologies includes absorption, adsorption, cryogenic technology, membrane technology,
chemical looping etc. These technologies have been developed over the years in order to meet
environmental regulations, pipeline specifications and optimize the operational costing. Figure 6
represents a general Carbon Capture and Storage process.
Fig. 7: Post, Pre and Oxyfuel combustion processes
Oxyfuel Combustion is the process by which nearly pure oxygen fires power plants instead of
air, producing a flue gas stream comprising water and carbon dioxide. The water is condensed
which leaves the pure CO2 stream. Generally, there exists as Air Separation Unit (ASU) which is
built on the front end of Oxyfuel combustion power plants, which employs cryogenic technology
to distill out Oxygen from air. This process is very difficult & energy intensive. Since, firing is
carried out with pure Oxygen, it causes high flame temperature. As a result the boilers are
required to be rebuilt & circulation of flue gas is required to control heat flux & flame
15
temperature. An ASU and Oxyfuel combustion decrease plant electrical output by approximately
30% (Nielson 2012).
In post combustion carbon capture process, the CO2 is captured from the flue gas after fuel
combustion [Fig. 7]. While this process can be retrofitted to any existing process plant, dilute
CO2 concentration present on the flue gas at low pressure makes it energy intensive to capture as
a large volume of gases are required to be handled.
In the following section, different CCS technologies have been discussed in details.
1.2.1 Chemical Looping Chemical looping is a process in which oxygen, typically from air, oxidizes a metal oxide
particle in an oxidizing reactor, releasing heat and is then transported to a reducing reactor where
it mixes with fuel & converts to a less oxidized state. In this case, generally two fluidized beds
are interconnected that allows the metal oxide particles to circulate between both reactors. Here
the metal oxide acts as an oxygen carrier, no other elements of air comes into contact with fuel
and doesnt mix up with CO2 produced from fuel oxidation. As a result, the flue gas stream
contains only water & CO2. The disadvantage is that metal oxides are expensive & deactivate
with time. Actually, the cycling temperatures of the metal oxides represent a significant amount
of entropy generation with the loss in Gibbs free energy associated with it (Nielson 2012).
1.2.2 Absorption Absorption process is one of the most well known, established, industrially applicable state of
the art technologies in natural gas purification process where a component of a gaseous phase is
contacted with a liquid in which it is preferentially soluble. Most of the conventional CO2
removal processes from natural gas are based on either chemical or physical or simultaneous
physical-chemical absorption process. Absorption is usually carried out in a countercurrent tower
(column), through which liquid descends and gas ascends. The reverse process (which is also
termed as stripping process, desorption) is employed when it is required to remove the absorbed
gases from the solvent for the purpose of recovery of the gas or the solvent or both. The
efficiency of any sorbent to absorb CO2 or H2S is expressed by its loading capacity. The two
major cost intensive factors associated with the absorption process include: i) the solvent
circulation rate and loading capacity required to achieve the degree of sweetening and ii) the
16
high energy consumption regarding the regeneration of the solvent (Biruh Shimekit and Hilmi
Mukhtar 2012, Kidnay and Parrish, 2006, Mullick 2014).
Absorption process can be classified into two categories- a) physical absorption process
and b) chemical absorption process. In physical absorption process implemented for CO2
removal the major controlling parameters are temperature, pressure of the feed gas stream and
partial pressure of the acid gas components present in the feed gas stream. A condition of low
feed temperature and high partial pressure (10 bars or more) is favorable for commercialization
of the process physical solvents have weak affinity towards the acid gas components. Hence, it is
necessary to employ high solvent circulation rate and limited loading capacity. In this case, less
energy is required for the regeneration step compared to traditional amine based processes as it is
carried out in low pressure. However, if the carbon dioxide is to be utilized for Enhanced Oil
Recovery (EOR), the cost of compressing the gas increases energy requirement. This method can
be implemented where the feed stream is rich in carbon dioxide and the product purity doesnt
matter a lot (Yeo et al, 2012). Chemical absorption process, takes place as exothermic reaction
between the chemical sorbent and the target acid gas component (CO2) at low temperature.
During the process, strong chemical bonds are formed between the target component and the
functional group of the chemical sorbent. The percentage separation of acid gas component is
dependent on the loading capacity of the sorbent predetermined by the available active sites (Yeo
et al., 2012, Christopher et. al., 2008). In the chemical process industry two types of chemical
solvents are used 1) Aqueous amine solution and 2) Carbonate solution. Selective absorption of
acid gases by exothermic chemical reaction with amine groups is the key concept of this process
and it is widely applied in the natural gas industries. The common amine based solvents used for
the absorption process are monoethanolamine (MEA), diethanolamine (DEA) triethanolamine
(TEA), diisopropanolamine (DIPA), diglycolamine (DGA) and methyldiethanolamine (MDEA)
that reacts with the acid gas (CO2 and H2S) to form a complex or bond. The basicity is provided
by the amine function, and it provides reactivity to remove the acid gases. The hydroxyl groups
serve to increase the solubility of amine in water. This effect reduces the vapor pressure of the
amines so that less is lost out the top of the absorber or stripper (Glasscock 1990, Biruh
Shimekit and Hilmi Mukhtar 2012). The Sour gases are brought in contact with the amine
solution in a countercurrent flow through an absorption column allowing the sorbent to strip out
CO2 or H2S selectively from the NG. The sweetened gas comes out from the top of the column
17
where as the sorbent loaded with sour gas components exits from the bottom which is further
directed to another column for regeneration. The stripping or desorption is carried out at a higher
temperature (about 373 K 473 K) giving out a high concentrated pure CO2 stream after
dehydration. The regenerated solvent is cooled down and recycled to the top of the column.
Absoprtion is well known for being the easiest and most common method for acid gas removal
from natural gas; but high solvent regeneration cost, equipment corrosion by acid compounds,
low loading capacity, amine degradation by SOx, NOx, HCl, HF, and O2 in flue gas, high solvent
circulation rate etc are some of the barrier towards the wide application in real field. (Liu et. al.,
2009; Olajire, 2010).
Fig. 8: Process flow diagram of a typical amine-solvent (MDEA)-based chemical absorption system
for the separation of CO2 and other acid gases from natural gas (Hubbard, 2010; Kohl and Nielsen,
1997)
CO2 removal from sour gas stream at a high pressure and temperature by using hot alkali
carbonate solutions like potassium carbonate (K2CO3) or sodium carbonate (NaCO3) was first
implemented in 1950s (Kohl and Nielsen, 1997). The process was first commercialized by US
Bureau of Mines as Benfield Process in 1954. Since the rate of absorption of CO2 by carbonate
solution increases with rise in temperature, the process is carried out at a high temperature 383 K
389 K. In cryogenic process units, the stringent allowance for the presence of very low CO2
level endorsed the modification of the process design. The new Hi-Pure design combines the
18
amine solution and carbonate solution process to enhance the rate of CO2 absorption (Rufford et
al., 2012).
Another kind of technology called the hybrid absorption process technology has been
developed where the effects of physical and chemical absorption processes have been combined
in a single unit operation by using mixed solvents. Sulfinol-D licensed by Shell Global Solutions,
Sulfinol-M, Amisol process licensed by Lurgi are some of the well known hybrid absorption
processes. (Rajani, 2004, Kohl and Nielsen, 1997).
1.2.3 Adsorption The process of adsorption can be described as the adhesion or retention of selective components
of feed gas stream brought into contact to the surface of certain solid adsorbent as the result of
the force of field at the surface. The reverse process is known as desorption in which the
adsorbed foreign molecules are released. High pressure and low temperature is favorable for
adsorption whereas low pressure and high temperature is suitable for regeneration or desorption.
In natural gas industries, removal of water, sulphur, mercury and heavy hydrocarbons are carried
out based on adsorption. Although adsorption process is rarely applied for bulk separation of
CO2 from CH4, there are kinetics-based adsorption processes that have been implemented in
USA for the recovery of methane from landfill gas. These gases mainly comprises of methane
(50-65%), carbon dioxide (35-50%), a trace amount of nitrogen and sulfur compounds. In this
process, carbon molecular sieve is used as the adsorbent. In use of this process, it can be possible
to recover more than 90% methane with 87-89% purity (Yang 1997, Tagliabue et al., 2009).
Depending on the nature and strength of the surface forces, adsorptive gas separation process can
be divided into two types a) physical adsorption and b) chemisorption. In physical adsorption
the gas molecules are adsorbed in the surface pores, there is no chemical reaction associated with
it. In chemisorption there is a formation of a chemical bond between the sorbate and the solid
surface (covalent interaction of CO2 and the surface of the adsorbent) which facilitates larger
adsorption capacity. These kinds of interactions are strong, highly specific, and often not easily
reversible. Chemisorption is sometimes employed for removal of trace concentrations of
contaminants. However, the difficulty of regeneration makes such systems unviable for most
process applications. The forces of physical adsorption are weaker (a combination of Van der
Waals forces and electrostatic forces) than the forces of chemisorptions which means the heats of
19
physical adsorption are lower. Since there is no covalent bond formation the adsorbent is more
easily regenerated. Physical adsorption at a surface is so fast, and the kinetics of physical
adsorption is usually controlled by mass or heat transfer rather than by the intrinsic rate of the
surface process (Meyers 2001).Based on regeneration methods, adsorption process is most
commonly divided into temperature swing adsorption (TSA), pressure swing adsorption (PSA)
and displacement desorption. In TSA, desorption is achieved by increasing the temperature of
the adsorption bed, either by applying heat to the bed or by purging with a hot purge gas.
Thermal swing adsorption is very reliable to remove minor component. The main limitation is
the adsorption cycle time that is required to cool down the bed. Moreover, high energy
requirements and large heat loss are on the cards. (Mersmann et al. 2011). PSA is a well known
technology for the removal of CO2 from gaseous streams containing methane. In PSA,
regeneration is carried out by lowering the operating partial pressure to desorb the adsorbate
(Kerry 2007). This can be obtained either by depressurization or by evacuation or by
implementing both. It is more suitable for bulk separation. Another type of desorption is
Displacement desorption. It is similar to purge gas stripping as the temperature and pressure are
maintained constant, but instead of an inert purge, an adsorbable species is used to displace the
adsorbed component from the bed. It is generally used when desorption by pressure swing or
thermal swing fails to be practical. Fluidised and moving bed operations (Seader and Henley,
2006), and fixed- bed electrothermal-swing adsorption (ESA) (An et al., 2011; Grande and
Rodrigues, 2008) are some of the less commonly applied adsorption techniques. The most
important factors for sustainable application of this technology are a) high selectivity and good
adsorption capacity of the target component, b) high surface area available for adsorption, c) fast
adsorption/desorption rate, d) physical and chemical stability, e) high regenerability and f) cost
of the adsorbent. Most commonly used commercially available adsorbents are - Activated
carbon, zeolites, molecular sieves etc. However, some novel adsorbents has been developed for
higher CO2 adsorption capacity Metal-organic frameworks (MOFs), zeolitic imidazolate
frameworks (ZIFs), surface functionalised silicas and porous carbons. Despite of novel
innovations and improved application specific researches this technology possesses some
disadvantages a) Very large, thick walled and heavy weighted adsorption tower which needs
high maintenance cost, b) low CO2 selectivity and adsorptivity of the available adsorbents, c)
high cost and low efficiency of CO2 adsorption in natural gas industries and d) production of
20
large amount of waste water and sludge (Hao et al., 2011, Biruh Shimekit and Hilmi
Mukhtar 2012, Mullick 2014 ).
1.2.4 Membrane Technology A membrane is a thin barrier placed between two phases or mediums, which allow one or more
constituents to selectively pass from one medium to the other in the presence of an appropriate
driving force while retaining the rest (Binay K Dutta, 2009). The separation of gas mixtures
with membranes has emerged from being a laboratory curiosity to becoming a rapidly growing,
commercially viable alternative to traditional methods of gas separation within the last two
decades. Membrane gas separation has become one of the most significant new unit operations to
emerge in the chemical industry in the last 25 years (Prasad et al. 1994). The important criteria
for selecting membrane materials for gas separation are based on the following key factors (a)
intrinsic membrane permselectivity (b) ability of the membrane material to resist swelling
induced plasticization (chemical resistance, which is quite rare but mostly fulfilled by inorganic
membranes) and (c) ability to process the membrane material into a useful asymmetric
morphology with good mechanical strength under adverse thermal and feed mixture conditions.
The polymer membrane material should have good interaction and sorption capacity preferably
with one of the components of the mixture for an effective separation (Biruh Shimekit and
Hilmi Mukhtar 2012).The first large scale industrial application of gas separation membrane
came to the market in the year 1980. It was launched by Permea used for hydrogen separation
and it was called PRISM. Since that inception the business of membranes has grown into a
$150 million/year and it will continue to grow in the coming future. There has been a lot of
research on this topic for last three decades but not even 10 type of polymer is available for
commercial gas separation. Most importantly, these polymers were not specifically designed for
gas separation hence improvement in such area is important that can provide higher selectivity in
gas separation. Apart from this, membranes must be thermally and chemically stable, resistive to
ageing, plasticization (for polymeric membranes), cost effective and must be easy to scale up.
Membrane technologies evolved from the discovery of Acid gas removal technique from natural
gas. The removal of CO2 from natural gas is the only large scale membrane based process in
practice today. There are more than 200 plants have been installed and most of them are installed
by Kvaerner-GMS, Universal Oil Products and Cynara (Baker et al. 2004). Membranes used
21
for gas separation are mainly classified into three categories based on the structure and materials:
polymeric membranes, inorganic membranes and mixed matrix membranes. During the last two
decades dozens of new polymers have been described in the literature, which have been
developed for gas separation. The largest group among these are probably polyimides. Cellulose
acetate Ethyl cellulose, Polycarbonate, brominated, Polydimethylsiloxane, Polymide (Matrimid),
Polymethylpentene, Polyphenyleneoxide, Polysulfone are the other polymers which are of
practical importance for gas separation. Cellulose acetate, polysulfone and polyimides are by far
the most important polymers for gas separation membranes (Nunes and Peinemann 2001).
Based on the operating temperature whether below or above the polymer glass transition
temperature, polymeric membranes can further be classified as rubbery polymers and glassy
polymers. In spite of, simple flow configuration and low cost, these polymeric membranes
cannot compete the conventional amine solvent absorption process due to low permeability, less
selectivity, degradation over time, non resistive against corrosive and high temperature
environment, unable to handle large volume of gas stream, low thermal and chemical stability.
Another major problem with polymeric membranes is the probability of plasticization of the
membrane after a limit of pressure of CO2 (Tin et. al., 2004, Larikov et. al., 2011, Rufford et
al. 2012). The current market for inorganic membranes for gas separation is extremely small. It
is not believed that the market share of inorganic membranes will increase significantly in the
near future. The main obstacle is their high price and some principle difficulties during
reproducible large-scale production. On the other hand fascinating research results have been
published in the recent past such as, unmatched selectivity for carbon dioxide/methane
separation with ceramic membranes (Tsai et al. 2000). There are different types of inorganic
membranes like ceramic membrane, nanoporous carbon membrane, Perovskite-type oxide
membranes etc. Dense inorganic membranes are prepared by spreading a thin metal layer of
palladium, nickel, silver, zirconia etc. and since all of these are expensive metals, sophisticated
handling is required. Also, the high capital cost, unstable mechanical properties, low
permeability reduce the applicability extensively. Ceramic, carbon and zeolite membranes are
commonly used for CO2/CH4 separation. A new development in the field of inorganic
membranes is zeolite based membranes. Silicalite-1 MFI membrane, Y-type zeolite membrane,
SAPO-34 zeolite membrane, KY-type zeolite membrane, DDR type zeolite membrane, A-type
zeolite membrane and T-type zeolite membrane (Yeo et al, 2012) are various types of zeolite
22
membranes which can be applied for CO2 separation from CH4 based on competitive adsorption.
Several obstacles such as expensive material, difficulty in producing the thin porous structure,
insufficient mechanical strength necessitates further research and investigation for successful
deployment in industries. Due to the different drawbacks of the existing membrane processes, it
is highly desirable to provide an alternative cost effective membrane which combines
homogeneously interpenetrating polymeric matrices for ease of processibility and inorganic
particle for high permeability and selectivity well above the upper-bound limit (Shekhawat,
Luebke et al. 2003, Biruh Shimekit and Hilmi Mukhtar 2012).). The combinations of the
superior gas selectivities of molecular sieves with the processibility of polymeric membranes
have attracted many researchers. The hybrid membranes consisting of inorganic molecular sieves
and polymers are often referred to as mixed matrix membranes (Mahajan and Koros, 2000,
Nunes and Peinemann 2001). It is still in developmental stage. The major challenges for
successful exploitation of membrane technology are a) high permeability and b) high selectivity.
Natural gas obtained from geological sources is at a high pressure. Hence high driving force is
there for permeation. But flue gas is generally at a low pressure. Therefore, high compression is
needed for permeation through membrane which makes the process overall process expensive
(Mullick 2014).
1.2.5 Cryogenic Technology Cryogenics is the science that studies the production and behavior of materials at very low
temperatures spanning the range between 100 K (-173oC) and absolute zero (0 K or -273oC). The
word cryogenics has been derived from two Greek words cryos which means icy cold and
genes means born i.e. "the production of freezing cold"; however, the term is used today as
a synonym for the low-temperature state. And with the chaos around the world with crude oil the
demand of natural gas have increased. This has stimulated the researchers and they have shown
the anarchy to develop, design and modify cryogenic technology for Natural Gas separation. Of
course, a novel method which can simultaneously capture, remove and transport carbon dioxide
holds enormous promises for real life industrial applications. Cryogenic technology is
advantageous over other existing amine absorption or PSA/TSA based processes A) No
chemicals and solvents are required by the process, hence no recurring consumable costs; B) No
process makeup water supply and further treatment are required; C) No process heating systems
23
are required; D) No solvent regeneration equipments are required; E) Water is removed
immediately downstream of the inlet separator so corrosion potential is scant; F) carbon dioxide
is available at higher pressure and can be used for Enhanced Oil Recovery or sequestration
purposes; G) Natural Gas Liquids (NGL) obtained as a by-product which has good market
potential; H) There is no chances for foaming and I) No special winterization requirements for
cold climates. Moreover, if process integration is employed and the required cold duty is
obtained at relatively low costs from a liquefied natural gas (LNG) re-gasification terminal,
cryogenic carbon capture becomes extremely attractive and economically feasible. Despite of so
many advantages, this technology still could not replace the conventional processes described
before. The main reason for this is the high cooling cost for the process. At atmospheric pressure,
CO2 directly de-sublimes from gas phase to solid phase below the saturation temperature. But for
operation in the gas-liquid zone, high compression of the feed gas is necessary which increases
the capital cost. Another problem is the solid formation, often columns are plugged by solid CO2
clusters and ice that comes from the water vapor freezing in the CO2 feed mixture. That is why,
the researchers are currently motivated to develop and establish the desublimation process
industrially (Mullick 2014).
The cryogenic technology can be classified into two major sections a) Conventional Cryogenic
Technologies and b) Non-conventional Cryogenic Technologies. In the next chapter, these
technologies have been discussed in details.
24
1.3 Research Methodology
The work was carried out as follows
Firstly, a detailed literature review was carried out. More than 80 sources of literature including
journal papers, conference proceedings, thesis works, books, articles and websites have been
reviewed. The references cited in each relevant literature were examined to find out additional
sources of information.
Secondly, the research gap was identified from the detailed literature review and the research
questions were formulated.
Then, according to our research question, the work was divided into two sections a)
Experimentation and b) Transport phenomena modeling and Simulation. The work being a joint
collaboration between Universiti of Tecknologi PETRONAS and Jadavpur University, the
experimental part was carried out at Universiti of Tecknologi PETRONAS and the transport
phenomena modeling and simulation were carried by us at Jadavpur University.
Experiments were conducted in a setup at different initial bed temperature profiles both in
counter current and co-current configurations. Feed composition, flowrate and initial temperature
were varied and experiments were conducted.
Then, a two step transport phenomena model was developed considering a single packing to
study the spacio temporal evolution and kinetics of the frost layer on a single packing, which
was further used to compute the frost layer of the total packed bed.
Thereafter, the model equations were solved using the GEARs algorithm (ode15s library
function available in MATLAB) in MATLAB considering a pseudo homogeneous state. The
initial bed temperature profiles obtained from the experiments has been used for the simulation.
The results obtained were analyzed to study the spacio temporal evolution of frost layer and to
predict the frost layer kinetics.
25
CHAPTER 2
LITERATURE REVIEW
26
2.0 Historical Background It is believed that the first ever successful liquefaction of any cryogenic gas was carried out not
earlier than 1877, by a French mining engineer Cailletet who produced a mist of liquid oxygen
droplets. He succeeded in pre-cooling a container filled with oxygen at 300 atm and then
expanding the gas by suddenly opening the valve of the container. Around the same time, a
Swiss physicist Pictet was also independently successful in liquefying oxygen by cascade
cooling.
A breakthrough was made in London in 1892 when James Dewar developed the vacuum
jacketed double-walled containers with silvered inner walls. This invention facilitated successful
liquefaction of hydrogen and helium in large quantities by 1898. In the mean time, Linde was
granted a patent on air liquefaction in Germany in 1885 and became the pioneer in industrial
scale production of cryogenic liquids.
In 1902, a French engineer Claude established L Air Liquid to develop and produce his
air liquefaction system in which a large fraction of cooling was obtained by using an expansion
engine. Five years later, in 1907, Linde installed the first air liquefaction plant in the USA.
During the period between the two World Wars, a number of developments took place in the
field of cryogenics. After the World War II, in 1947, Collins, a mechanical engineer, developed
an efficient cryostat for liquefaction of helium at MIT, USA. This could be used for safe and
sustained maintenance of temperature for experimental studies between ambient and 2 K. The
impact of this development was so remarkable in boosting the confidence level of researches
engaged in cryogenic applications that anything earlier than this era was jocularly referred to as
BC or Before Collins. The newer developments are continually taking place even today
(Mukhopadhyay 2010).
2.1 Detailed Literature Review A huge number of literatures are available on carbon dioxide capture and recovery technologies
based on cryogenic separation. Extensive study showed that these technologies can be classified
into three categories - (A) Conventional cryogenic technology, (B) Non-conventional cryogenic
technology and (C) Hybrid technology. Other than the technological aspects, the internal
thermodynamics and the science associated with the governing phenomena have also been
27
considered in this review of reported studies. The following sections have been developed to
give a brief idea on the subject considered in this thesis.
2.1.1 Thermodynamics and Solidification The research on Natural gas purification using cryogenic technology is totally based on
the science of Thermodynamics. The Cryogenic Technology finds its origin in the Second Law
of Thermodynamics. And then there is Equation of State that describes the properties of fluid
mixtures and provides the mathematical relationships between the state functions associated with
the process. Concern toward understanding of the mechanisms of these technologies and the
proper design of the equipment necessitates a deep look into the thermodynamic analysis of CO2
- natural gas phase equilibrium.
Fig.9: Pressure Temperature Phase Diagram for CO2
The above figure shows the Pressure Temperature phase diagram for pure Carbon
Dioxide. It clearly shows that at higher pressure within the cryogenic limits, the CO2 becomes
solid. The general or conventional cryogenic distillation columns operate at a higher pressure and
often the solidification of the CO2 becomes a problem for the Heat Exchangers associated. That
is why the non-conventional ones are operated at normal atmospheric or lower pressure and to
28
eliminate the problem of solid CO2 packed beds are employed. In the latter, the zone of operation
is the Solid vapour region of the phase diagram.
Thermodynamic data for the methane-carbon dioxide binary system is available at three
primary sources in literature. These data sets also include the solid-liquid-vapour region of the
binary mixture. The first set of data was reported by Donnely et. al. (1954). This data includes
three phase data points from -78.6 to 57.78 0C. Critical conditions for CH4-CO2 system were
also presented. Different data points for SLE were also measured. Vapour liquid equilibrium for
different pressure ranges was also determined. Pikaar (1959) provided the second set of data. A
constant volume cell was used to measure the 1, 3, 5, 10 and 20 % CO2 frost lines along with the
dew and bubble point lines of theses mixtures in the region of three phase locus. Davis et. al.
(1962) measured the three phase locus for the binary mixture from the triple point of carbon
dioxide to -175.61oC. In addition to that they also measured the composition of vapour and liquid
phase along the solid-liquid-vapour locus of methane carbon dioxide system. Eggemen et. al.
(2005) found that unreliable CO2 freezing temperature predictions are being made by several of
the commercial process simulators typically used by gas processors. In general, they found the
existing experimental data were adequate and that thermodynamic models, both equation of state
and activity coefficient based, can be used to make accurate predictions of CO2 freezing
temperatures. However, previous works was not adequately addressed how to properly apply
these models within a process simulation. According to them it was the improper formulation of
the CO2 freezing calculations was the cause of the unreliable predictions made by the
commercial process simulators. They showed how to properly formulate the thermodynamic
calculations to be used for prediction of CO2 solids formation. Procedures for heat exchangers,
expanders and columns have been discussed. Common pitfalls (convergence to spurious roots,
convergence to physically meaningful but useless solutions, non-convergence of numerical
algorithms, improper formulation of temperature safety margins, etc.) can be avoided by using
these procedures.
Many other phase equilibrium experimental data for CO2 is available in literature. A summary of
this is given below.
29
Table 3: PVTX experimental data for carbon dioxide mixtures (Maqsood et al. 2014)
Source Year Type Mixture Temperature range
(oC)
Pressure range
(bar)
Donnelly and Katz
(1954)
1954 TPxy CO2-CH4 -106 to 29 20-74
Kaminishi et al.
(1968)
1968 TPxy CO2/CO, CO2/Ar, CO2/CH4,
CO2/CO/H2
- 50 to 10 24-200
Neumann and Walch
(1968)
1968 TPxy CO2-CH4 -65, -228 to -53.15 440-690
Arai et al. (1971) 1971 PVTX CO2-CH4, CO2-N2 -20 to 15 50-150
Sarashina et al. (1971) 1971 PVTX CO2-CH4-N2 -40 to 0 60-100
Davalos et al. (1976) 1976 TPxy CO2-CH4 -43 to -23 9-85
Hwang et al. (1976) 1976 TPxy CO2-CH4 -120 to -54 20-65
Mraw et al. (1978) 1978 TPxy CO2-CH4 -184 ton -65 5-63
Somait and Kidnay
(1978)
1978 TPxy CO2-CH4, CO2-N2 -3 30-120
Al-Sahhaf et al.
(1983)
1983 TPxy CO2-CH4, CO2-N2, CO2-CH4-
N2
-54 to -23 5.8-160.15
Magee and Ely (1988) 1988 TPxy CO2-CH4 -48 to 127 20-350
Ely et al. (1989) 1989 TPxy CO2-CH4, CO2-N2 -23 to 57 23-320
Trappehl and Knapp
(1987)
1989 TPxy CO2-CH4, CO2-N2 -53 20-120
Al-Sahhaf (1990) 1990 TPxy CO2-CH4-N2 -43 to -23 62.1-100.34
Xu et al. (1992) 1992 TPxy CO2-CH4, CO2-N2, CO2-CH4-
N2
15 and 20 51.1-91.1
Seitz et al. (1996) 2002 PVTX CO2-CH4-N2 50-250 199-999
2.1.2 Conventional Cryogenic Technology Conventional Cryogenic Technology is a very old process where the operation is carried out at a
very low temperature. Conventional Cryogenic Technology includes simple Cryogenic
distillation and extractive distillation. Cryogenic distillation is carried out at an extremely low
temperature and high pressure to separate CO2 and other components based on their different
boiling point. This method directly produces liquid CO2 or CO2 vapor at a high pressure which
reduces extra costs of compression for storage purpose. This technology is not economical and
30
energetically viable for dilute gas streams. One of the major operating problems for this method
is solid formation and choking of column in the older top section of the distillation columns in
both low and high pressure ranges. Solid formation during separation of CO2 has been reported
in literature (Katz & Donnely , 1954). Gas Processors Association (RR-10, 1974) published
liquid phase composition of CO2- CO4 binary gas mixture at S-L-V locus. Maqsood et al. (2014)
predicted the solid formation in the distillation column inside the convergence look utilizing
GPA the data they found that increasing the column pressure might be help full to avoid solid
formation. However methane loss will increase and the purity of methane will be questionable.
Henson et al. (2001) derived a low order wave model for cryogenic nitrogen purification
column and compared the MATLAB simulated results with a first principles model developed
within the commercial dynamic simulator HYSYS. Plant (Hyprotech). The authors used the non
linear wave modelling concept to model the cryogenic nitrogen separation column and
performed rigorous modelling of the combined reboiler/condenser assembly and verified the
model using the dynamic simulator. The model is capable of producing acceptable prediction of
composition responses for various types of disturbances. However, the constant wave pattern
assumption used in the wave model development invariably leads to some degree of modelling
error. They proposed on-line model adaptation as a possible approach to overcome the constant
wave shape assumption. Panopoulos et. al. (2013) suggested a cryogenic method for recovery
of H2 and CH4 from a rich-CO2 stream in a pre-combustion carbon capture system. They
modeled their process using Aspen Plus based on differences in thermodynamic properties and
evaluated the effects of it on the efficiency of the system. They also studied the effect of
operating parameters of the (Purification & Compression Unit) PCU integration on the
performance of the system. Maqsood et. al. (2014) synthesized efficient cryogenic distillation
sequence for purification of natural gas having medium and high concentration of carbon dioxide
contained. Calculations for conventional and hybrid distillation column sequences were
performed using the heuristic and evolutionary strategies. Three different sequences direct,
indirect and mixed were chosen for different feed compositions selected from the literature. It
was found that direct sequence is the best options for separation of CO2 from natural gas with
different feed composition in respect of minimum vapour flow, marginal vapour flow and energy
requirements. They also found hybrid cryogenic network requires considerably lower energy
31
and showed significant reduction in capital cost of the columns compared to the conventional
cryogenic network.
Maqsood et. al. (2014) presented a techno- economic evaluation of cryogenics network for
separation of carbon dioxide from natural gas with different feed compositions. Equipment
sizing and cost estimation has been carried out for the cryogenic networks using the co relations
provided in the literature.
Maqsood et. al. (2015) conducted a study with different configurations of cryogenic distillation
networks to remove carbon dioxide from natural gas feed with high heavy hydro carbon
contained they investigated three different case studies along with conventional cryogenic
network. They found that higher operating pressure leads to reduction of energy requirements but
operational problems in distillation columns can arise due to thermodynamic behavior of
methane - carbon dioxide system.
Extractive Distillation
In order to prevent solid formation inside the distillation column extractive distillation is
a suitable option. Holmes et al. (1982) patented and extractive distillation process, introducing
heavier hydro carbon (C2-C5 alkanes) or other non polar liquids which are miscible with methane
at the column conditions in the condenser section of the distillation column. This help to avoid
solidification of carbon-dioxide. Holmes et al. (1983) executed pilot studies on cryogenic acid
gas / hydrocarbon separation process and validated the work. Valencia et al. (1985) patented a
method for separating carbon dioxide from methane using Helium as an additive which
facilitated the separation process and prevented solid carbon dioxide formation. However the
separation of He from CH4 at higher pressure is quite problematic. Atkinson et al. (1988)
introduced a dual pressure distillation process intended for the removal of high concentration
carbon dioxide from methane. It comprises of two distillation columns operating indifferent
pressure to avoid carbon dioxide solidification. ZareNezhad et al.(2009) reported an extractive
distillation technique for producing CO2 enriched injection gas for enhance oil recovery (EOR)
fields. No external solvent is required in this case. In this technique a part of natural gas liquid
stream comprising of iso-butane and heavier components were added to the top of a CO2 stripper
followed by ethane and propane stripping of ethane and heavier components stream. Due to this
the azeotropic mixture of carbon dioxide and ethane breaks down which increases the tray
efficiency. Berstad et al. (2012) reported presented a low-temperature process for CO2 removal
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from natural gas before liquefaction. They simulated a three distillation column network by using
C5 as an additive in ASPEN Hysys with PengRobinson equation of State for the separation of
CO2 from natural gas. Berstad et al. (2013) presented a review on low temperature carbon
dioxide technologies and discussed their potential. This gives us detailed idea about the
applicability, energy efficiency of the technologies discussed with the base line technology.
2.1.3 Non-conventional Cryogenic Technology
There is always an alternative way to everything as they say that the grass is always greener on
the other side. Researchers found a few alternative ways to discover the real potential of the
cryogenic technologies. These alternative things are referred to as the Non-Conventional
Cryogenic Technologies. These non-conventional cryogenic technologies focuses on separation
of carbon dioxide by utilizing one of the major disadvantages of the conventional cryogenic
processes into its working principle. Actually the solidification of CO2 in the solid vapour zone
is utilized to desublimate carbon dioxide from the gas stream.
Schah et al. (2011) conducted a feasibility study of Carbon capture by desublimation. They
modeled the process using ASPEN Plus featuring finned-plate heat exchangers. The process
cooled the incoming flue gas with a condensing heat exchanger and there after another heat
exchanger desublimated the remaining water vapour in the flue gas. Finally a third heat
exchanger desublimated the Carbon dioxide. In this model, both water and carbon dioxide froze
and desublimated directly on the heat exchanger surface there by requiring periodical
regeneration of the heat exchanger. An economic analysis compared to a common MEA
absorption showed that this desublimation process has superior capture performance that requires
comparatively less energy penalty. A continuous process that avoids losses due to regeneration
employee parallel heat exchanger trains makes it really attractive. Clodic et al. (Clodic et al.
2005; Clodic and Younes 2002; Clodic and Younes 2003; Perrotin and Clodic 2005; Clodic
and Younes 2006; Clodic and Younes 2006) have patented a desublimating carbon capture
system quite similar to the one that Schah model. Schahs design employs a series of heat
exchangers that operates at successively decreasing temperatures. The first one condenses water,
the latter remaining water vapour and the last one desublimates carbon dioxide. Clodic uses a
flat-plate heat exchanger for the desublimating stage instated of the finned heat exchanger as
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used by Schah. The cold and clean flue gas exits the last exchanges going through a regenerating
heat exchanger to cool the incoming flue gas. After the CO2 & the H2O loading in the heat
exchanger reaches a maximum, the flue gas is diverted into a parallel system. Then the heat
exchanger enters the regenerations mode. During this period valves isolate the frozen CO2 as
they warm up. This melts the CO2 and pressurizes the system. The water and CO2 flow out of the
heat exchanger as liquids under pressure. The heat exchanger pressure drops back to that of the
flue gas, the heat exchangers cool back to cryogenic temperatures; and there by resuming the
process. Certain challenges exist in the work of Clodic. They are - (1) The sublimating CO2
creates an insulating layer between the heat exchanger and the flue gas decreasing the heat
transfer and increasing the pressure drop; (2) the system is inherently a semi-batch or batch
process; (3), the amount of CO2 that is captured is small compared to the mass of the heat
exchangers on which it collects, and cycling the large mass of heat exchanger material from the
capture temperature of around 140 K to the CO2 melting temperature of about 220 K generates
large amounts of entropy and decreases the process efficiency and (4) pressurizing the heat
exchangers at commercial scale will require a valve that sustains 8-70 bar pressure in a duct that
is nominally 30 feet in diameter, which presents a significant practical (rather than fundamental)
problem.
Cryogenically Cooled Packed Bed
Tuinier et. al., (2010, 2011) has developed a novel process concept for carbon capture and
storage based on dynamic packed beds with a moving interface between water and carbon
dioxide. The process concept is based on the periodic operation of cryogenically cooled packed
beds. The process cycle consists of three consecutive steps cooling, capture and recovery steps
respectively. A front of desublimating- sublimating CO2 is formed and it moves based in the
temperature profile inside the bed. Nitrogen initially cools the packed bed. Once flue gas enters
the packed bed, water condenses and freezes, then CO2 desublimates onto the packing material.
As the bed reaches maximum H2O and CO2 loading, the flue gas is diverted to a parallel system
and pure CO2 flows into the bed above the desublimation temperature but below the freezing
temperature of H2O. The solid CO2 in the bed sublimates and leaves with the CO2 stream. Warm
nitrogen then evaporates the water, followed by a recycle steam of cold clean flue gas to cool the packing material. After the capture cycle, the bed is regenerated for further use. The process was
34
reported for atmospheric pressure separation of low CO2 flue gases using dynamic beds. This
process can be made continuous if three columns are built in parallel. The temporal evolution of
axial temperature, concentration, and mass deposition profiles occurring in the beds can be well
described by a validated one-dimensional pseudo-homogeneous axially dispersed plug flow
model. Aliredza (2013) reported the experimental and simulation work on recovery of CO2
using cryogenic packed bed. Figure 9 shows the schematic representation of cryogenic packed
bed. Abul Hassan et al. (2013) developed an experimental setup for cryogenic separation of
CO2 from natural gas with high CO2 content. In this study, the CO2 concentration was used up to
70%. The separation principle of CO2 from natural gas was based on desublimation principle in
countercurrent cryogenic packed bed. They also simulated the cryogenic packed considering a 1
dimensional pseudo homogeneous model (Abul Hassan et al. 2013). Due very steep gradients
of temperature and concentration the solution is tough to get. Finite difference, Forward-Time
Central Space (FTCS) scheme is used for simultaneous numerical solution of the model. The
scheme was found to be efficient as the results compared with the experimental ones showed
promising potential for industrialization. Multiple cryogenic packed beds have been used for
simultaneous dehydration as well as CO2 separation by Karen (Karen Hui, 2013) and Abul
Hassan et al. (Abul hassan et al., 2014). Aditi Mullick (2014) have modeled and simulated the
separation of CO2 onto cryogenically cooled packing using reduced order transport phenomena
models. The transport mechanism and dynamics of cryogenic CO2 capture have been addressed
using a model based on single packing. The nucleation kinetics and growth rate of deposition of
CO2 frost on a single cooled packing is studied using a two-step model, which takes into account
the diffusion from supersaturated gas phase to the gas solid interface and relatively slower
crystallization kinetics and nucleation on a heterogeneous surface (packing) in series. Recently,
Abul Hassan et al. published a paper where they have explored the minimization of energy
consumption for a counter current switched packed bed intended to separate CO2 and other
components of natural gas (Abul hassan et al., 2014). They conducted experiments with a
switched packed bed setup by changing different operating parameters and compared the results
with other co-current or jacket cooled constant temperature configurations. They also
investigated the effects of the important process parameters initial temperature profiles of the
cryogenic bed, feed composition, and feed flow rate on energy requirement, bed saturation, bed
pressure and cycling times. The energy consumption of countercurrent switched packed bed was
35
compared with the conventional cryogenic distillation process and it saves 662 kJ energy per kg
CO2; for a constant inlet feed composition. The effect of feed composition on the energy
requirement revealed that countercurrent switched cryogenic packed beds have potential for
substantial energy savings during purification of natural gas with high CO2 content.
Stirling Coolers
Song et al. has developed a cryogenic carbon capture technology similar to Clodic, but has
managed to make it a truly continuous process (Song, Kitamura et al. 2012; Song, Kitamura et
al. 2012; Song, Kitamura et al. 2012; Song, Kitamura et al. 2013). Song uses a similar three
heat exchanger design but uses Stirling coolers (SC) instead of plate heat exchangers. A Stirling
cooler generates an acoustic pulse that creates a refrigeration effect inside a pulse tube cold
finger (Hu, Dai et al. 2010). Stirling coolers have high efficiency, high reliability, and small
footprint and volume. The first SC pre-cools and dehydrates the flue gas. The condensed water
leaves as a separate stream, while the cool flue gas continues to a second SC. The second SC
desublimates the CO2 as a solid on the surface of the cold finger, while the clean flue gas
exhausts. A mechanical scraping rod is used to keep the surface of that heat exchanger clean,
while solid CO2 falls into a storage chamber where at third SC provides cooling to keep the CO2
in a solid state. Song et al., (2013) evaluated the properties of this free piston Stirling cooler
system and briefly compared its performance with other cryogenic methods. They found that this
approach is better than LN and LNG in terms of energy consumption but high pre-chill time,
vibration and less deposition area are disadvantages for this system. They suggested integrating
this method with amine methods is an effective approach. They are looking forward to make this
a vibration proof system. Song et al. (2013) experimentally tested the performance of the Free
Piston Stirling Cooler (FPSC) system for CO2 capture. The effect of flowrate of the gas stream
and temperature of FPSC was investigated in detail. They found that the system can capture 95%
CO2 from simulated flue gas and consume 0.55MJ of electrical energy per kg of carbon dioxide
recovered which is the least compared to other dominant technologies. Song et al., (2014)
presented a process simulation and energy analysis of cryogenic CO2 capture process based on
Stirling coolers. They simulated the overall energy flow Stirling coolers based cryogenic CO2 capture process. Theoretical analysis of the energy consumption for each of the unit in the
capture system was also under taken. They also compared energy consumption of this process
36
with other established technologies and claimed there process to be least energy consuming (1.6
MJ/KG CO2). Furthermore; they extended their work and investigated the influence of capture
conditions on the performance on the systems; based on three levels and variables and in central
composite design. They optimized the system with the objective of maximum CO2 recovery,
CO2 productivity and minimum energy consumption. They compared there result with
experimental data. They found that under the optimal condition 95.20% CO2 can be removed.
Song et al., (2015) investigated the Co-efficient Of Performance (COP) of the Fitted Piston
based Stirling coolers (FPSC). The key parameters were also investigated in order to improve the
COP. It was found mat
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