UNIVERSITY OF APPLIED SCIENCES EMDEN-LEER

Department of Chemical Engineering

Semester Project:

―C3MR Natural Gas Liquefaction Process Simulation‖

Student Name: Elisavet Michailidi

Supervisor: Prof. S. Steinegeweg

Emden, February 2014

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TABLE OF CONTENTS

A. THEORETICAL PART

1. Introduction ..................................................................... 5

2. Brief History of Liquefied Natural Gas ............................ 6

3. Lng Production-Industrial Processes ............................... 7

3.1. Cascade Processes .................................................................. 8

3.1.1. Linde Process ..................................................................... 8

3.2. Mixed Refrigerant Processes ................................................... 9

3.2.1. SMR (PRICO) Process ....................................................... 9

3.2.2. DMR Process ....................................................................10

3.2.3. C3MR Process ...................................................................10

3.2.4. APX Process .....................................................................11

4. Gas Purification Processes ............................................ 12

4.1. Acid Gas Removal .................................................................13

4.2. Dehydration ...........................................................................14

4.3. Mercury Removal ..................................................................15

4.4. Nitrogen Rejection .................................................................15

B.MODEL

1. Introduction .................................................................. 16

2. Aspen Hysys Simulator .................................................. 16

3. Thermodynamic Model Selection ................................... 16

3.1. Peng-Robinson Equation of State .............................. 17

4. Feed .............................................................................. 18

5. Process Description ...................................................... 18

5.1. Purification Section ....................................................... 18

5.2. Compressor Train Section ............................................. 20

5.2.1. Mixed Refrigerant Composition ................................ 20

5.3. Propane Pre-Cooling Section ......................................... 20

5.4. Liquefaction Section ...................................................... 21

6. Energy Analysis ............................................................ 24

7. Conclusions................................................................... 26

Citations .............................................................................. 27

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A. THEORETICAL PART

1. INTRODUCTION

Natural gas is a mixture of gaseous hydrocarbons with varying

quantities of nonhydrocarbons, which are normally considered

impurities. Natural gas is composed mainly of methane. In addition,

it usually contains minor quantities of heavier hydrocarbons and

varying amounts of gaseous nonhydrocarbons such as nitrogen,

carbon dioxide, and hydrogen sulfide.

Natural gas is set to become one of the most important primary

energy sources for the 21st century. As the cleanest fossil fuel, it

possesses many advantages such as giving off a great deal of heating

energy when it burns and emits lower levels of potentially harmful

byproducts and thus, it is expected to be one of the most promising

energy resources in the coming decades (Natural Gas Supply

Association, 2010).

Natural gas is transported either in pipelines or in liquefied

natural gas carriers after exploration and treatment. The liquefied

natural gas is produced by the liquefaction process of natural gas,

which refrigerates natural gas sources from ambient temperature to

around −162 oC in atmospheric pressure.

The principal reason for liquefying natural gas is a 600-fold

reduction in volume that occurs with the vapor-to-liquid phase

change. In its naturally occurring vapor state, natural gas is a bulky

energy source, which is difficult to handle. Storage of the vapor

requires huge underground caverns or large telescoping storage

tanks. Transporting natural gas from production sources to points of

consumption necessitates large pipeline networks and a considerably

higher transportation cost (Fig. 1.) Thus, only overland or somewhat

shorter undersea routes can be considered. Finally, natural gas at

atmospheric pressure is too bulky to be considered as a fuel for

transportation purposes. Liquefaction of natural gas serves to

overcome these obstacles, and permits transport over larger

distances and more diverse application of liquefied natural gas

(LNG) as an energy source (Foss, 2007).

Fig. 1 Natural gas transportation cost

Source: American Institute of Gas Technology

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2. BRIEF HISTORY OF LIQUEFIED NATURAL

GAS

Natural gas liquefaction dates back to the 19th century when

British chemist and physicist Michael Faraday experimented with

liquefying different types of gases, including natural gas. German

engineer Karl Von Linde built the first practical compressor

refrigeration machine in Munich in 1873. The first LNG plant was

built in West Virginia in 1912 and began operation in 1917. The first

commercial liquefaction plant was built in Cleveland, Ohio, in 1941.

The LNG was stored in tanks at atmospheric pressure. The

liquefaction of natural gas raised the possibility of its transportation

to distant destinations. In January 1959, the world's first LNG

tanker, The Methane Pioneer, a converted World War ll liberty

freighter containing five, 7,000 barrel equivalent aluminum

prismatic tanks with balsa wood supports and insulation of plywood

and urethane, carried an LNG cargo from Lake Charles, Louisiana to

Canvey Island, United Kingdom. This event demonstrated that large

quantities of liquefied natural gas could be transported safely across

the ocean. Following the successful performance of The Methane

Pioneer, the British Gas Council proceeded with plans to implement

a commercial project to import LNG from Venezuela to Canvey

Island. However, before the commercial agreements could be

finalized, large quantities of natural gas were discovered in Libya

and in the gigantic Hassi R' Mel field in Algeria, which are only half

the distance to England as Venezuela. With the start-up of the 260

million cubic feet per day (MMcfd) Arzew GL4Z or Camel plant in

1964, the United Kingdom became the world's first LNG importer

and Algeria the first LNG exporter. Algeria has since become a

major world supplier of natural gas as LNG.

After the concept was shown to work in the United Kingdom,

additional liquefaction plants and import terminals were constructed

in both the Atlantic and Pacific regions. Four marine terminals were

built in the United States between 1971 and 1980. They are in Lake

Charles (operated by CMS Energy), Everett, Massachusetts

(operated by SUEZ through their Distrigas subsidiary), Elba Island,

Georgia (operated by El Paso Energy), and Cove Point, Maryland

(operated by Dominion Energy). After reaching a peak receipt

volume of 253 BCF (billion cubic feet) in 1979, which represented

1.3 percent of U.S. gas demand, LNG imports declined because a

gas surplus developed in North America and price disputes occurred

with Algeria, the sole LNG provider to the U.S. at that time. The

Elba Island and Cove Point receiving terminals were subsequently

mothballed in 1980 and the Lake Charles and the Everett terminals

suffered from very low utilization.

The first exports of LNG from the U.S. to Asia occurred in 1969

when Alaskan LNG was sent to Japan. Alaskan LNG is derived

from natural gas that is produced by ConocoPhillips and Marathon

from fields in Cook Inlet in the southern portion of the state of

Alaska, liquefied at the Kenai Peninsula LNG plant (one of the

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oldest, continuously operated LNG plants in the world) and shipped

to Japan. In 1999, the first Atlantic Basin LNG liquefaction plant in

the western hemisphere came on production in Trinidad. This event,

coupled with an increase in demand for natural gas in the U.S.

particularly for power generation and an increase in U.S. natural gas

prices, resulted in a renewed interest in the U.S. market for LNG. As

a result, the two mothballed LNG receiving terminals have been

reactivated. Elba Island was reactivated in 2001. In October 2002,

the Federal Energy Regulatory Commission (FERC) gave approval

to Dominion Resources for its plans to re-open Cove Point LNG

facility in 2003; first shipments to the reactivated terminal were

received in fall 2006. In April 2005 the world's first offshore, ship-

based regasification facility was set in operation in the Gulf of

Mexico by Excelerate Energy. Additionally, a number of approved,

planned, and proposed projects are under development.

3. LNG PRODUCTION-INDUSTRIAL PROCESSES

LNG (Liquefied Natural Gas), is natural gas in its liquid form.

Liquefaction of the natural gas is achieved by cooling it down to

approximately -162oC in atmospheric pressure. In these conditions,

it transits from the gaseous to the liquid state, thus reducing its

volume by 600 times. Another way to succeed the phase transition is

increasing the gas pressure up to the gas critical pressure point.

However, because of the high critical pressure of methane, LNG

must be produced commercially by refrigeration.

A number of processes have been commercialized for LNG

production (Martin, Pigourier, & Fischer, 2009). Commercial

processes can be distinguished into two categories:

Cascade Processes

Mixed Refrigerant Processes

The major liquefaction processes, used in the industry, are the

following:

AP-X Process

Cascade Process developed by Conoco Phillips

DMR Process developed by Shell

LNG Processes

Cascade Processes

Mixed Refrigerant Processes

With pre-cooling

Without pre-cooling

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SMR Process (also refered as PRICO)

Linde Process

C3MR Process (also refered as APCI) developed by Air

Products

3.1. CASCADE PROCESSES

Each refrigerant is established as a separate closed-loop

refrigerator that supplies refrigeration at discrete temperature levels.

Typically, propane, ethylene, and methane are used to provide a

wide, balanced range of refrigeration. After compression three

temperature levels for each of the three refrigerants form a nine

stage cascade. Each of these temperature levels corresponds to a

preset pressure letdown for evaporating the refrigerant in heat

exchange with the natural gas feed and a separate refrigerant stream

that requires cooling. Heat is removed from the natural gas at

successively lower temperatures; i.e., the refrigerant is boiled at

successively lower pressure. Heat is rejected to ambient air or water

via the warmest refrigerant, generally propane, and compressor

aftercoolers. The methane refrigerant loop is open in that it is

combined with the natural gas feed and after the final pressure

letdown the liquid methane forms part of the LNG product. The

wide range of boiling points for the refrigerant components also

means that some of the heavier components are compressedto higher

pressures than actually requiredfor their condensation to ensure

condensation of the lighter, lower boiling components such

asnitrogen and methane. Such a recompression penalty cannot be

avoided without a certain degree of separation of the refrigerant

components such as occurs in precooled mixed refrigerant processes

(Hammer, Lubcke, & Kettner, 2012).

Fig. 2 Cascade Process Simplified Flow Diagram

3.1.1. LINDE PROCESS

This process is a three cycle process, like the cascade process,

but with mixed refrigerant on all cycles (Fig.3.). Compared to the

cascade, the efficiency is better, as mixed refrigerants allows to have

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a closer approach. However, the power is not the same on all three

cycles, unlike the new cascade. Plate-fin exchangers are used on the

first cycle, and spiral wound exchangers on the two colder cycles.

Fig. 3 LINDE Process Simplified Process Diagram

3.2. MIXED REFRIGERANT PROCESSES

The mixed refrigerant cycle (MRC) uses a single mixed

refrigerant instead of multiple pure refrigerants as the cascade cycle.

The mixed refrigerant normally consists of nitrogen, ethane,

propane, butane and pentane. Such a mixture evaporates over a

temperature trajectory instead of at a constant evaporating point and

this has large benefits for the total process. The refrigeration effect

will be distributed over a range of temperatures and accordingly the

overall temperature difference between the natural gas and mixed

refrigerant is small. Small driving temperature differences give

operation nearer to reversibility; leading to a higher thermodynamic

efficiency. Simultaneously, the power requirement will be lower and

the entire machinery smaller (Finn, 2009 ). Some of MRC

technologies are; Single mixture process (SMR), Mixed refrigerant

with propane pre-cooling (C3/MR), Dual mixed refrigerant process

(DMR), Mixed and AP-X. Most existing natural gas liquefaction

plants operate on the mixed refrigerant processes, with the propane

pre-cooled mixed refrigerant process being the most widely used.

Mixed Refrigerant processes can be divided into (a) Mixed

Refrigerant Processes without pre-cooling and (b) Mixed

Refrigerant Processes with precooling.

3.2.1. SMR (PRICO) PROCESS

The PRICO SMR process is the simplest of the four processes

studied. The process was first used in 1981 at the Skikda LNG plant

in Algeria. Three liquefaction trains using the process have been

built and operated over the last 23 years. Fig. 2 shows a simplified

flow sheet, which consists of a single LNG heat exchanger, a

separate feed/product and refrigerant system, a compressor with an

associated after-cooler, suction scrubbers a separator and pump. The

SMR process has the lowest equipment count compared to the other

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processes. In the SMR process the feed gas enters the LNG

exchanger at feed conditions and is cooled against the cold

refrigerant stream to the required LNG storage conditions of less

than -155 oC. The cold low-pressure refrigerant stream also acts to

condense the high-pressure refrigerant stream prior to the pressure

let down stage that provides the necessary heat exchanger cold side

temperature differential.

Fig. 4 PRICO Process Simplified Flow Sheet

3.2.2. DMR PROCESS

This process (Fig. 5.) is a dual mixed refrigerant process, with

different power on the two cycles, and with two spiral-wound

exchangers. Having mixed refrigerant on the first cycle allows to

have a smaller condenser, and also to remove the propane

compressor bottleneck: For propane compressors, the compressor

size, thus the capacity of the unit is limited by the mach number at

the tip of the blades.

Fig. 5 Shell DMR Process Simplified Flow Sheet

3.2.3. C3MR PROCESS

In the early 1970s, a third generation of processes, precooled

mixed-refrigerant processes, developed. The most widely used

process employs two separate refrigeration systems, a propane

cascade refrigerant loop in series with a mixed refrigerant cycle that

incorporates propane, ethane, methane, and nitrogen as components.

A basic schematic of the C3MR process is shown in Figure 6.

Natural gas is pre-cooled to about -35oC by propane. After

precooling, it passes up through a tube circuit in the main cryogenic

heat exchanger where it is liquefied and sub-cooled to between -

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150oC to -162

oC by mixed refrigerant (MR) flowing down on the

shell-side. To pre-cool the natural gas, propane is compressed to a

high enough pressure such that it can be condensed by ambient air

or cooling water. Liquid propane is then let down in pressure in a

series of stages, further reducing its temperature and allowing it to

provide refrigeration to the natural gas.

The propane is also used to pre-cool the mixed refrigerant which has

been compressed after exiting the bottom of the MCHE. After pre-

cooling, the partially condensed mixed refrigerant is separated in a

high pressure separator. The vapor and liquid streams pass through

separate tube circuits in the MCHE where they are further cooled,

liquefied, and sub-cooled. The two sub-cooled streams are let down

in pressure, further reducing their temperatures. As the mixed

refrigerant vaporizes and flows downward on the shell side of the

MCHE, it provides refrigeration for liquefying and sub-cooling the

natural gas. The vaporized mixed refrigerant is then recompressed.

The use of a single component pre-cooling fluid with a staged

pressure let-down provides for an efficient, easy to control pre-

cooling step. The use of a mixed refrigerant for liquefaction and

sub-cooling in a single exchanger permits boiling of the refrigerant

over a temperature range, leading to high efficiency when it is most

crucial. In this way, the C3MR cycle minimizes the number of

equipment items and control loops while maintaining the highest

efficiency on the market. These advantages lead to minimal plant

complexity, easier operation, and high availability (M. J. Roberts,

2004).

Fig. 6 C3/MR Process Simplified Flow Diagram

3.2.4. APX PROCESS

In this process (Fig.7), a cycle similar to the precooled mixed-

refrigerant process is used to precool and liquefy the LNG.

However, the liquefied LNG is subcooled in a nitrogen refrigeration

closed-loop process cycle. Nitrogen gas is compressed, cooled to

near ambient conditions with cooling water or ambient air, and then

further cooled to cryogenic conditions by expansion to lower

pressure. The gaseous nitrogen is then used to subcool LNG, after

which it is returned to be recompressed, completing the refrigeration

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cycle. By employing the nitrogen refrigeration cycle to subcool the

LNG, the mixed-refrigerant only has to cool the LNG to about -115 oC. This allows for a larger production capacity without a substantial

increase in equipment size.

Fig. 7 APX Process Simplified Flow Diagram

4. GAS PURIFICATION PROCESSES

Natural gas consists primarily of methane. However, small

quantities of CO2, H2S, H2O, heavier than CH4 hydrocarbons and N2

can also be found, depending on the natural gas source. Such

compounds are considered as impurities. Thus, 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 and

avoid pipelines and equipment corrosion.

The technologies that are widely used to treat the natural gas

include absorption processes, adsorption processes, cryogenic

condensation and membranes. The technologies and their

improvement have been developed over the years to treat certain

types of gas with the aim of optimizing capital cost and operating

cost, meet gas specifications and environmental purposes.

The type and desing of purification processes is determined

taking into consideration factors such as nature and amount of

contaminants in the feed gas, the amount of every contaminants

present in feed gas and the targeted removal capacity, amount of

hydrocarbon in the gas, pipeline specification, capital and operating

cost, amount of gas to be processed, desired selectivity, conditions at

which the feed gas is available for processing are the major factors

that should also be considered (Shimekit & Mukhtar, 2006).

Especially the liquefaction process, requires the reduction of

certain contaminants, such as H2O and CO2 to sufficiently low levels

to prevent not only their corrosive effects but also the formation of

solids, known as hydrates, which form in low temperature

conditions. (Foss, 2007) As a result, LNG is typically made up

almost only from methane as shown in Fig. 8.

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Fig. 8 LNG Typical Composition

A typical purification process for LNG plant includes:

Acid Gas Removal

Dehydration

Mercury Removal

Nitrogen Rejection

4.1. ACID GAS REMOVAL

Removal of the sour gas components is one of the most common

aims of natural gas treating. The quality demands on purified gas

and the anticipated type of processing or further use of the separated

components are quite varied. As a result, a choice of many different

processesis available (Kohl & Riesenfeld, 1975).

The wet absorption (solvent based) acid gas removal still

remains clearly the most cost effective for base load LNG

applications. Developments in cryogenic and membrane CO2

removal have yet to threaten the position of the solvent based

processes when deep removal of CO2 for LNG production is

required (Klinkenbijl, M.L.Dillon, & Heyman, 1999).

Three basic types of liquid absorption processes are available:

Physical absorption processes, which use a solvent that

physically absorbs CO2, H2S and organic sulphur

components. Examples are the Purisol and Selexol processes.

Physical solvents can be applied advantageously when the

partial pressure of the contaminants are high, the treated gas

specification is moderate and large gas volumes have to be

purified. Physical solvents also absorb significant quantities

of hydrocarbons, which obviously is a disadvantage.

Chemical absorption processes, which chemically absorb

the H2S, and CO2. Organic sulphur components do not

chemically react with the solvent. Common examples are

amine processes, using aqueous solutions of alkanol amines

such as MEA, DEA, MDEA and DIPA. Chemical solvents

are specifically suitable when contaminants at relatively low

95%

5%

LNG Typical Composition

Methane Others

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partial pressure have to be removed to very low

concentrations. Chemical solvents will not remove

mercaptans down to low levels due to the low solubility of

these components. An advantage however is that there is

minimum co-absorption of hydrocarbons. Due to the

chemical reaction between the solvent and CO2 and H2S, the

regeneration energy requirements are normally higher than

for a physical solvent.

Mixed solvents, are a mixture of a chemical and a physical

solvent. The most widely known process is the Shell Sulfinol

Process, which applies a mixture of sulfolane, water and

DIPA or or MDEA.

Adsorption processes for the removal of hydrogen sulfide and

carbon dioxide are used in natural gas treatment when only small

amounts of acidic components must be removed. Activated charcoal

and zeolitic molecular sieves are used as adsorbents. Molecular

sieves are used widely for the purification of natural gas when it is

used as feedstock for a cryogenic plant for production of liquefied

gas. In these plants, even low levels of carbon dioxide cause

problems because the gas freezes in the low-temperature unit and

can lead to blockage.

4.2. DEHYDRATION

The sweet gas leaving the acid gas removal step is still saturated

with water. Especially when the gas will be used as a feed for LNG

production, it is essential that water should be completely removed

in order to prevent hydrates formation.

Cryogenic Dehydration. The wet gas is cooled until the

components to be removed precipitate by condensationor formation

of hydrates

Dehydration by Absorption Processes. Standardized

dehydration plants using glycol absorption are employed most

widely. In the absorber, glycol and gas are brought in contact

counter currently. Triethylene glycol (TEG) is used in preference to

other glycols (mono- and diethylene glycols) because of its high

absorption capacity for water vapor, its low vapor pressure (small

losses from evaporation), and its high thermal stability

Adsorptive Dehydration. In adsorptive dehydration the gas is

brought in contact with molecular sieves, silica gel, or SORBEAD

(i.e., Na2O- containing SiO2 ). Dew points < -70 oC are attainable

with adsorption plants. This is particularly necessary for cryogenic

plants and liquid natural gas (LNG) plants, where traces of water

and carbon dioxide can lead to blockage by ice formation.

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4.3. MERCURY REMOVAL

Mercury removal is normally done with a fixed bed adsorption

step. Commonly used adsorbents are sulphur impregnated carbons,

in which the mercury reacts with sulphur to form the stable mercuric

sulphide. A standard molecular sieve will also absorb Hg but

regeneration is impossible. An alternative approach is the silver-

impregnated molecular sieve (UOP HgSIV). In principle this

molecular sieve can be regenerated, however the release of mercury

from the molecular sieve bed would require dedicated material

selections in the regeneration gas treating section.

4.4. NITROGEN REJECTION

Although not common, nitrogen is sometimes removed and

rejected using one of the three processes (Gas Processes,

Hydrocarbon Processing, 2002):

Cryogenic process (Nitrogen Rejection Unit), using low

temperature distillation.

Absorption process, using lean oil or a special solvent as the

absorbent.

Adsorption process, using activated carbon or molecular

sieves as the adsorbent.

Membrane Processes (under development)

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B. MODEL

1. INTRODUCTION

Propane precooling mixed component refrigerant process

(C3/MR) represents 80% of the commercial used processes. The

process has proven to be efficient, flexible, reliable, and cost

competitive (M. J. Roberts, 2004).

For these reasons the a C3MR process, using synthetic natural

gas (SNG) from the methanisation process, was selected to be

simulated. Simulation of the process has been conducted using

Aspen Hysys® version V.8. process simulation software. The PR

equation of state is used for thermodynamic properties calculations

both for the natural gas and the refrigerants.

2. ASPEN HYSYS SIMULATOR

Chemical process modeling is a computer modeling technique

used in chemical engineering process design. It typically involves

using purpose-built software to define a system of interconnected

components, which are then solved so that the steady-state or

dynamic behavior of the system can be predicted. The system

components and connections are represented as a Process Flow

diagram. Computer-aided process design programs, often referred to

as process simulators, flow sheet simulators, or flow sheeting

packages, are widely used in process design.

Aspen HYSYS by Aspen Technology is one of the major

process simulators that are widely used in chemical and

thermodynamic process industries today. Aspen HYSYS is the

industry leading simulation software for oil & gas, refining, and

engineering processes. With an extensive array of unit operations,

specialized work environments, and a robust solver, modeling in

Aspen HYSYS V8 enables the user to (Sittler & Ajikutira, 2013):

Improve equipment design and performance

Monitor safety and operational issues in the plant

Optimize processing capacity and operating conditions

Identify energy savings opportunities and reduce GHG

emissions

Perform economic evaluation to realize savings in the

process design

3. THERMODYNAMIC MODEL SELECTION

An equation of state (EOS) is a functional relationship between

state variables — usually a complete set of such variables. Most

EOS are written to express functional relationships between P, T and

V. For simulation of a LNG production process, a reliable equation

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of state (EoS) is needed for thermodynamic data predictions.

literature.

In the petroleum industry, two cubic EoS are generally used: the

SRK and the PR EoS (Ahmed, 1997):

they are simple and capable of fast calculations

they apply in both liquid and vapour phases

they are applicable over wide ranges of pressures and

temperatures

they estimate accurate densities

However, a slightly better performance around critical

conditions makes the PR EOS more preferred to gas/condensate

systems (Firoozabadi, 1989)

3.1. PENG-ROBINSON EQUATION OF STATE

The Peng-Robinson EOS has become the most popular equation

of state for natural gas systems in the petroleum industry. During the

decade of the 1970’s, D. Peng was a PhD student of Prof. D.B.

Robinson at the University of University of Alberta (Edmonton,

Canada).

Peng and Robinson introduced the following modified vdW

EOS:

Where:

The generalized expression for the temperature-dependant

parameter is given by:

Where:

With:

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4. FEED

In the current process, synthetic natural gas (SNG) from the

methanisation process is used as a feed to the unit. The composition

of the feed is shown in Table 1.

Mole Fractions

CH4 0.5960

N2 0.01

H2O 0.3510

H2 0.0260

CO2 0.0170

Table 1 SNG Feed Composition

5. PROCESS DESCRIPTION

The model, simulated in this project is based on the C3MR process.

As shown at the figure below, the process can be divided in four

―sections‖,

Fig. 9 Process Description Overview

5.1. PURIFICATION SECTION

At first step, SNG, with the composition shown in Table 1,

enters the purification where the bulk of water is separated with a

• Water Separation

• CO2 Removal

• Dehydration

• Nitrogen Rejection

Purification Section

• Natural Gas pre-cooling

• Mixed Refrigerant cooling

Propane precooling

Section

• Increasing the pressure of the refrigerant mixture

Compressor Train Section

• Cooling the gas down to its liquefaction temperature

Liquefaction Section

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phase separator. After this step, the gas appears the following

composition (Table 2):

Mole Fractions

CH4 0.9162

N2 0.0154

H2O 0.0024

H2 0.0400

CO2 0.0261

Table 2 SNG Composition after H2O Separation

Subsequently, CO2, H2O traces and nitrogen are removed by

absorption in MEA, adsorption in molecular sieves and membrane

separation respectively. Finally, the purified natural gas with the

composition shown on Table 3, enters the propane pre-cooling

section at temperature of 20 oC and 10 bar pressure.

% composition

CH4 95,82

H2 4,18

Table 3 SNG Composition after Purification

Fig. 10 Purification Section

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5.2. COMPRESSOR TRAIN SECTION

In this section, a train of two compressors is used to increase in

two steps the pressure of the refrigerant mixture up to 17 bars.

Fig. 11 Compressor Train Section

5.2.1. MIXED REFRIGERANT COMPOSITION

A mixture of hydrocarbons and N2 is used to achieve the

desired refrigerant characteristics in liquefaction. The refrigerant

mixture evaporates over a temperature range similar to that of the

process cooling demand. The desired result is that the the

temperature – enthalpy warming curve of the mixed refrigerant

closely tracks the natural gas cooling curve in the main heat

exchangers This reduces the energy demand of the process. The

mole fraction of refrigerants' composition were obtained either by

trial and error method based on cooling curves appeared in the main

heat exchangers.

The mole fraction of refrigerants' composition were obtained

either by trial and error method based on cooling curves appeared in

the main heat exchangers

The composition of the refrigerant mixture selected for the

current process is shown below (Table 4).

Mole Fraction

Methane 0.32

Ethane 0.33

Butane 0.1

Nitrogen 0.25

Table 4 Mixed Refrigerant Composition

5.3. PROPANE PRE-COOLING SECTION

In this section, the SNG from the purification section enters a

heat exchanger where it is cooled, by propane, to -35 oC. A simple

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refrigerant cycle is used. Furthermore, propane is also used to cool

down the refrigerant mixture to -12 oC. Then, the cool refrigerant

mixture enters a separator where it is separated to gas and liquid

phase. Finally, the two streams along with the pre-cooled SNG enter

the liquefaction section.

Fig. 12 Natural Gas C3 Pre-Cooling Section

Fig. 13 Refrigerant Mixture C3 Pre-Cooling Section

Although refrigerant cooling can take place in several cooling

stages, only one stage is shown in the model for simplicity.

5.4. LIQUEFACTION SECTION

In this section, three fin plate cryogenic heat exchangers are used

to cool down the SNG to -165 oC which is the liquefaction

temperature.

The pre-cooled natural gas, along with the two streams of

refrigerant mixture, enters the first heat exchanger (LNG-101) where

it is cooled down to -90 oC. Then, it enters the LNG-102 heat

exchanger where it is further cooled down to -140 oC.

Subsequently, gas liquefaction is achieved in the third heat

exchanger (LNG-103) which cools the SNG down to -165 oC.

Finally, the stream, coming out from LNG-103 is expanded,

through a valve, from 10 bar to 1 bar, which is the pressure of LNG

storage. The expanded stream enters a flashing drum, where it is

separated to gas and liquid phases. The liquid phase, which is the

final LNG product is leaded to storage tanks, while the gaseous

phase is leaded to the flare.

The flow diagram of this section is shown on figure 14.

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Fig. 14 SNG Liquefaction Section

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As mentioned above, the desired result is that the the

temperature – enthalpy warming curve of the mixed refrigerant

closely tracks the natural gas cooling curve in the main heat

exchangers This reduces the energy demand of the process. The hot-

cold composite curves for the three heat exchangers are shown

below.

Fig. 15 T-Q LNG-101

Fig. 16 T-Q LNG-102

Fig. 17 T-Q LNG-103

24

6. ENERGY ANALYSIS

It is known that the main energy demand for liquefaction

processes is the energy consumed at the compressors. In the current

process four compressors exist:

K-100 and K-101, both used at MR Compressor train

section

K-102 used for compressing propane at MR propane

cooling cycle and

K-103 used at propane pre-cooling section of SNG

The power consumption for all the compressors is shown below:

Energy Demand for Compression

Compressor

K-100 334,1 kW

K-101 301,3 kW

K-102 22,55 kW

K-103 275,6 kW

Total Energy

Demand

933,55 kW

Table 5 Energy Demand for Compression

Molar Flow 62,2 kgmole/h

Mass Flow 961,4 kg/h

HHV Mass

(High Heating Value of

Stream “Natural Gas to

LNG Section” )

55,61 MJ/kg

LHV Mass

(Low Heating Value of

Stream “Natural Gas to

LNG Section”)

50,42 MJ/kg

Table 6 Properties of SNG Entering the Liquefaction Section

25

Fig. 18 C3MR Process Flow Sheet

26

7. DYNAMIC MODELING

As a final step, an effort to simulate the current process in

Dynamic Mode was made.

A simplified model of the C3MR process was built with Aspen

Hysys in Dynamic Mode (Fig. 19).

Fig. 19 Simplified C3MR Process- Dynamic Simulation

Simple shell-and-tube heat exchangers are used instead of fin

plate cryogenic heat exchangers, in order to solve the case with the

―Dynamic Assistant‖ tool, provided by Hysys.

It is vital that the LNG product has a certain temperature

( about -162 oC) at atmospheric pressure and the mixed refrigerant

enters the system at a constant pressure. Thus, three IC controllers

(PIC 100, PIC 101 and TIC 100) are used to control the pressure of

the reftrigerant mixture, the pressure of the LNG Product and the

temperature of the LNG Product respectively.

27

8. CONCLUSIONS

As expected to happen, by closing of two curves in cooling

curve diagrams, we could reduce total duty and increase the

efficiency of LNG production.

The mole fraction of refrigerants' composition were obtained

either by trial and error method based on cooling curves

appeared in the main heat exchangers. It can be realized that

while the curves for the LNG-101 and LNG-102 heat

exchangers are good, the curves for LNG-103 needs further

optimization.

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