DEPARTMENT OF PETROLEUM ENGINEERING AND APPLIED GEOPHYSICS TPG4140 NA NATURAL TURAL GAS
Fractionation of Natural Gas Liquids to produce LPG
Submitted To
Prof. Jon Steinar Gudmundsson
Submitted By
Ahmad, Rafiq Malla, Majed A. Osama El-Majzoub Det Hladky ,Maros Shadman Far, Amir Usman, Muhammad
Trondheim Nov 24, 2011
Table of Contents Abstract.......................................................................................................................................................... .......................................................................................................................................................... i 1.
Introduction ............................................................. ................................................................................................................................. ......................................................................... ..... 1 1.1
Properties ........................................................... ............................................................................................................................. ....................................................................... ..... 1
1.2
Uses of LPG ........................................................ .......................................................................................................................... ....................................................................... ..... 3
1.3 2.
Natural Gas Liquids Processing ............................................................ ............................................................................................................. ................................................. 5 2.1
LPG Recovery Processes............................................................... ................................................................................................................ ................................................. 5
2.1.1
Recontacting-compression ............................................................ ................................................................................................... ....................................... 6
2.1.2
Refrigeration ............................................................. ......................................................................................................................... ............................................................ 6
2.1.3
Lean oil absorption.................................................... absorption................................................................................................................ ............................................................ 6
2.1.4
Adsorption................................................................. Adsorption............................................................................................................................. ............................................................ 6
2.2
LPG Manufacturing ........................................................... ....................................................................................................................... ............................................................ 6
2.2.1
Acid gas removal .................................................................. ................................................................................................................... ................................................. 7
2.2.2
Extraction Unit .......................................................... ...................................................................................................................... ............................................................ 7
2.2.3
Fractionation Unit ................................................................ ................................................................................................................. ................................................. 7
2.2.3.1
Deethanizer Section ......................................................... .......................................................................................................... ................................................. 8
2.2.3.2
Depropanizer Section ................................................................. ........................................................................................................ ....................................... 8
2.2.3.3
Debutanizer Section ......................................................... .......................................................................................................... ................................................. 9
2.2.4
3.
The future for LPG ................................................................ ................................................................................................................. ................................................. 3
Product Treatment Plant ............................................................... ...................................................................................................... ....................................... 9
2.3
Feed Specifications for NGL Fractionation ........................................................ .................................................................................. .......................... 10
2.4
Product Specifications.................................................................. ................................................................................................................. ............................................... 10
Simulation .......................................................... ............................................................................................................................ ................................................................................. ............... 12 3.1 3.2
Feed conditioning ............................................................. ....................................................................................................................... .......................................................... 12 Fractionation columns ............................................................. ............................................................................................................ ............................................... 13 3.2.1
Deethanizer .................................................................. ................................................................................................................. ............................................... 13
3.2.2
Debutanizer.............................................................................................. Debutanizer............................ ................................................................................. ............... 14
3.2.3
Depropanizer ........................................................... .......................................................................................................... ............................................... 15
3.2.4
4.
Butane splitter ............................................................... .................................................................................................... ..................................... 16
LPG Transport .............................................................. .................................................................................................................................. ....................................................................... ... 17 4.1 4.2
Continuous flow of LPG – Pipe system ............................................................. ....................................................................................... .......................... 17 Discrete (bulk) means of LPG transport ........................................................ .................................................................................. .......................... 18
Table of Contents Abstract.......................................................................................................................................................... .......................................................................................................................................................... i 1.
Introduction ............................................................. ................................................................................................................................. ......................................................................... ..... 1 1.1
Properties ........................................................... ............................................................................................................................. ....................................................................... ..... 1
1.2
Uses of LPG ........................................................ .......................................................................................................................... ....................................................................... ..... 3
1.3 2.
Natural Gas Liquids Processing ............................................................ ............................................................................................................. ................................................. 5 2.1
LPG Recovery Processes............................................................... ................................................................................................................ ................................................. 5
2.1.1
Recontacting-compression ............................................................ ................................................................................................... ....................................... 6
2.1.2
Refrigeration ............................................................. ......................................................................................................................... ............................................................ 6
2.1.3
Lean oil absorption.................................................... absorption................................................................................................................ ............................................................ 6
2.1.4
Adsorption................................................................. Adsorption............................................................................................................................. ............................................................ 6
2.2
LPG Manufacturing ........................................................... ....................................................................................................................... ............................................................ 6
2.2.1
Acid gas removal .................................................................. ................................................................................................................... ................................................. 7
2.2.2
Extraction Unit .......................................................... ...................................................................................................................... ............................................................ 7
2.2.3
Fractionation Unit ................................................................ ................................................................................................................. ................................................. 7
2.2.3.1
Deethanizer Section ......................................................... .......................................................................................................... ................................................. 8
2.2.3.2
Depropanizer Section ................................................................. ........................................................................................................ ....................................... 8
2.2.3.3
Debutanizer Section ......................................................... .......................................................................................................... ................................................. 9
2.2.4
3.
The future for LPG ................................................................ ................................................................................................................. ................................................. 3
Product Treatment Plant ............................................................... ...................................................................................................... ....................................... 9
2.3
Feed Specifications for NGL Fractionation ........................................................ .................................................................................. .......................... 10
2.4
Product Specifications.................................................................. ................................................................................................................. ............................................... 10
Simulation .......................................................... ............................................................................................................................ ................................................................................. ............... 12 3.1 3.2
Feed conditioning ............................................................. ....................................................................................................................... .......................................................... 12 Fractionation columns ............................................................. ............................................................................................................ ............................................... 13 3.2.1
Deethanizer .................................................................. ................................................................................................................. ............................................... 13
3.2.2
Debutanizer.............................................................................................. Debutanizer............................ ................................................................................. ............... 14
3.2.3
Depropanizer ........................................................... .......................................................................................................... ............................................... 15
3.2.4
4.
Butane splitter ............................................................... .................................................................................................... ..................................... 16
LPG Transport .............................................................. .................................................................................................................................. ....................................................................... ... 17 4.1 4.2
Continuous flow of LPG – Pipe system ............................................................. ....................................................................................... .......................... 17 Discrete (bulk) means of LPG transport ........................................................ .................................................................................. .......................... 18
4.2.1
Vessel tanker transport .............................................................. ................................................................................................... ..................................... 19
4.2.2
Rail, truck, car transport ........................................................ ............................................................................................. ..................................... 20
4.3
Economic analysis .................................................................. ....................................................................................................... ..................................... 20
Conclusions and Discussions ........................................................................ ....................................................................................................................... ............................................... 23 References .................................................................. .................................................................................................................................... ................................................................................ .............. 24 Appendix ......................................................... ........................................................................................................................... ............................................................................................ .......................... 26
List of Tables Table 1: Typical Properties of LPG ................................................................................................................ 2 Table 2:Fractionator types for LPG Production (Abdel-Aal, H. K.et al 2003) ................................................ 9 Table 3:Different Contaminants in LPG (Abdel-Aal, H. K.et al 2003) ............................................................ 9 Table 4: Specifications of feed for NGL fractionation unit (Manley ,D.B.) .................................................. 10 Table 5:Product specifications for LPG (Manley ,D.B) ................................................................................ 11 Table A1:Simulation reults .........................................................................................................................27
List of Figures Figure 1:Blockdiagram for LPG Manufacturing (Parkash ,Surinder ,2009) .................................................. 7 Figure 2: Typical Fractionator train for NGL (Parkash ,Surinder 2009) ........................................................ 8 Figure3: Feed conditioning alt.1 ................................................................................................................. 12 Figure 4: Feed conditioning alt.2 ................................................................................................................ 13 Figure 5:Feed conditioning alt.3 ................................................................................................................. 13 Figure 6: Deethanizer column ..................................................................................................................... 14 Figure 7: Debutanizer column ..................................................................................................................... 15 Figure 8: Depropanizer column................................................................................................................... 15 Figure 9: Butane splitter ............................................................................................................................. 16 Figure A1: Process Flow Sheet (Simulation sheet from Hysys) ................................................................... 26 Figure A2. Distribution Chain (World LP Gas Association 2009)................................................................. 28 Figure A3: Mid-America Pipeline (Willbros Group, Inc. 2011) .................................................................... 29 Figure A4: Jamnanagar New Delhi ............................................................................................................. 29 Figure A5: SST,SSM ..................................................................................................................................... 30 Figure A6: amerigas Canister ...................................................................................................................... 30 Figure A7: World LPG gas consumption (Fundamentals of the World Gas Industry, 2008) ....................... 31 FigureA 8: World autogas consumption (Fundamentals of the World Gas Industry, 2008) ...................... 31
Abstract
Raw natural gas contains valuable heavier hydrocarbons such as ethane, propane, butane and fraction of higher hydrocarbons. These associated hydrocarbons, known as natural gas liquids (NGL), must be recovered from the gas in order to control the dew point of natural gas stream and to earn revenue by selling these components as products for different industries. Natural gas liquids are fractionated to produce LPG. The purpose of this report is to see the method to fractionate different NGL’s to produce
LPG. Different processes for LPG production and recovery from natural gas are discussed. Further, process has been described for LPG production from NGL by fractionation. Simulation for fractionation columns has been done in order to investigate the material and energy balance. An overview of LPG transportation through canisters and pipeline, which is a new thing, has been highlighted in the report. Economic analysis and future market for LPG has been highlighted to see whether this product can be an alternative to high fuel consumption and demands or not.
Fractionation of NGL to produce LPG 1. Introduction Natural gas is one of the world’s favorite and promising fuels. Transportation of gas is not
something easy therefore converting this gas into liquid simplifies and eases the transportation process. Liquefied Petroleum Gas, which is a super pressurized gas stored in a liquid form in tanks or canisters, is a known type of Natural gas that we are going to look very close into. LPG is a flammable mixture of hydrocarbon gases used as a fossil fuel closely linked to oil, almost two third of the LPG that is used is extracted directly from the Earth in the same way as Natural gas. The rest is manufactured indirectly from petroleum drilled from the Earth in wells. (Crude oil) LPG is considered to be a mixture of two flammable nontoxic gases known as propane (C 3H8) and butane (C 4H10). Propylene and butylenes are present in small concentrations too. Mainly the LPG gas is of no odor which makes it hard for people to detect the leakage if it happens, so a small amount of a pungent gas such as ethanethiol are added to help people smell potentially dangerous gas leaks.
1.1
Properties
LPG is as twice as heavy as air and half as heavy as water and it is colorless and odorless. LPG can be compressed at a ratio of 1:250 which enables it to be marked in portable containers in liquid form as mentioned above. LPG also produces less air pollutants and carbon dioxide than most other fuels; it helps to reduce the emissions of the typical house
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Fractionation of NGL to produce LPG by almost 1.5 tones of CO 2 a year. LPG reduced black carbon emissions as well which are the second biggest contributor of global warming and causes serious health hazards. LPG has a high heating (the amount of heat released during combustion of a specified amount of it) of 12,467 kcal/m³ which is much higher than the average heat value of most Natural gas (9350 kcal/m 3). Also LPG has a very high Wobbe index (an indicator of the interchangeability of fuel gases: WI
GrossHeatValue
SpecificGravity
) of 73.5-87.5 MJ/Sm3 which is a
high combustion energy output. LPG can be used as an alternative fuel to natural gas (methane) in residential, commercial and industrial applications, as an alternative to gasoline for automotive fuel purposes, and as a feedstock in petrochemical applications. Both propane and butane are gaseous hydrocarbons at normal temperatures (15 degrees Celsius) and atmospheric pressure. However, they can be stored and distributed in liquid from at temperatures of under -42 degrees and -2 degrees Celsius for propane and butane respectively. The Fig below shows the typical properties of LPG. Table 1: Typical Properties of LPG
Property
Propane
Butane
Liquid Density
0.50-0.51
0.57-0.58
Conversion(Ltr per ton)
1968
1732
Gas Density/air
1.40-1.55
1.90-2.10
Boiling Point (C)
-45
-2
Latent Heat of Vaporization
358 KJ/Kg
372 KJ/Kg
Specific Heat(as liquid)
0.60 Btu/deg
0.57 Btu/deg
Sulfur Content
0-0.02%
0-0.02%
Calorific Value
2,500 Btu/ft3
3,270 Btu/ft3
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Fractionation of NGL to produce LPG 1.2
Uses of LPG
LPG is used as fuel, especially for vehicles such as cars and motorcycles, also as an aerosol propellant and refrigerant to avoid damage to the ozone. It is an advantage to use LPG as a fuel for vehicles because it burns cleaner than petrol and diesel. Another use is as a refrigerant. Propane gas and butane gas are used to make hydrocarbon refrigerants. Hydrocarbons are known to be more energy efficient and cheaper than other chemicals, which is why it is suitable to be used as refrigerants. Another popular use is as a cooking fuel. LPG is very popular, especially among countries like India and other Asian countries. LPG is used as a cooking fuel for households and even businesses such as restaurants. As for propane, it is more popularly being used as fuel for barbeques and portable stoves. This is because propane has a low boiling point, so it will vaporize once it is released from the container. Butane, on the other hand, is famously bottled as fuel for lighters and deodorants. When propane and butane combine together, they become LPG. LPG can be used as a back-up or secondary fuel in generating the energy for the household. For example, in order to heat water in winter, LPG is used alongside a solar panel to provide enough energy for this purpose.
1.3
The future for LPG
LP Gas has played a valuable role in meeting the world’s energy needs. In the future, LPG
has the opportunity to enhance this role by also helping to combat climate change. By releasing fewer harmful pollutants when used as a domestic and automotive fuel source LPG is not only a cleaner alternative but also a healthy one. It seems that the portable nature of bottled LPG, combined with its clean burning characteristics, presents an immediate winning solution to rapidly expand the availability of modern energy to those that have been without it. LPG can claim to be ahead of its time, for its clean-burning, low-carbon advantage is available at once, so that even using today’s technology, most industries can exceed Kyoto
greenhouse gas reduction targets by switching to LPG. LPG produces lower greenhouse gas
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Fractionation of NGL to produce LPG emissions compared to conventional energy supplies in every application it is used, from stationary applications such as water heating, space heating, cooking and industrial boilers to transportation applications.
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Fractionation of NGL to produce LPG 2. Natural Gas Liquids Processing Raw natural gas contains valuable heavier hydrocarbons when extracted from the well head. The heavier hydrocarbons which are associated with the raw natural gas are ethane, propane, butane and natural gasoline (condensate from). These associated hydrocarbons are called natural gas liquids. These NGL components must be recovered to control the dew point of natural gas stream and also to earn revenue by selling out the separated components. Following are the different processes used to separate impurities:
Oil and condensate removal
Water Removal
Separation of natural gas liquids
Sulfur and carbon dioxide removal
Our aim/objective of this report is to study the fractionation of natural gas liquids to produce LPG, (Abdel-Aal, H. K et.al 2003). We will discuss first different LPG manufacturing processes.
2.1
LPG Recovery Processes
Natural gas mainly contains methane and smaller amounts of ethane, propane, butane and heavier hydrocarbons along with varying amount of water vapors, carbon dioxide, sulfur compounds and other non-hydrocarbons. Ethane, propane, butane and propane are known as associated gases. The removal of these gases from raw natural gas is necessary to meet the desired consumer specifications of natural gas and to extract valuable products such as LPG from natural gas. Various techniques are used to recover LPG from natural gas/oil. 1. Recontacting-compression 2. Refrigeration 3. Absorption 4. Adsorption 5. A combination of above
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Fractionation of NGL to produce LPG 2.1.1 Recontacting-compression This process is normally used for the recovery of LPG from crude oil fractionator. This technique is hardly used in gas industry. The top product from a crude oil fractionator consists of lighter fractions namely methane, ethane, propane and butane. This top product stream is compressed, combined with top liquid product, cooled and fed to the separator. The liquid phase from separator is passed through Deethanizer and the vapor phase containing some LPG fractions is used as fuel gas. Liquid product of Deethanizer is LPG. The recovery of LPG by this technique is 75% (Elvers , Barbara 2008). 2.1.2 Refrigeration This technique is more common for recovery of LPG from gas streams. The principle behind this technique is to refrigerate the gas stream and LPG fractions are obtained. Recovered fractions are fractionated to get the LPG components. The technique is employed in three different processes:
Low temperature separation
Expander Plants
Combined Processes
2.1.3 Lean oil absorption This method employs the hydrocarbon oil to recover lighter fractions. This process is used in refineries and also in gas processing plants. LPG recovery by this process is 98%. 2.1.4 Adsorption Adsorbents are used in this process so that gas molecules are bonded to the surface. Normally silica gel, activated carbon and alumina are used as adsorbent. The LPG recovery by this process is significantly lower than other two processes.
2.2
LPG Manufacturing
LPG is produced by fractionation of natural gas liquids and from crude oil by distillation, catalytic cracking, delayed cokers and hydrocrackers. LPG manufacturing process starts with acid gas removal and extraction unit, then fractionation unit and ends with the product treatment plant. The simple process is described in the following block diagram. (Parkash ,Surinder ,2009)
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Fractionation of NGL to produce LPG
Raw Natural Gas
Acid Gas Removal
Extraction Unit
Product
Finished Product
from gas well
Fractionation Unit
Treatment Unit LPG Fig1:Blockdiagram for LPG Manufacturing 1 (Parkash ,Surinder ,2009)
2.2.1 Acid gas removal Raw gas from the well head is received in knock out drums to separate gas and liquid phases. The oil field gases contain corrosive acid gases like CO 2 and H2S.Removal of these gases is necessary to further process the gas for LPG production or more products. These acid gases are removed by amine treatment or Benfield processes .After this, acid gases free natural gas is sent to extraction unit. 2.2.2 Extraction Unit The feed of extraction unit is the combination of associated gases and condensate. The product streams are divided into three steps .One having the liquid stream rich in propane, butane, and gasoline is sent to the fractionation tower for LPG production and other two streams to the product gas unit for further processing. 2.2.3 Fractionation Unit Liquid stream consisting of ethane, propane, butane and pentane is treated in the fractionator trains to separate them and sold as LPG. Complete process flow sheet is shown in figure 2. Fractionation tower consists of three columns: Deethanizer, Depropanizer and Debutanizer. The whole process description is as follows.
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Fractionation of NGL to produce LPG
Figure 2: Typical Fractionator train for NGL (Parkash ,Surinder 2009)
2.2.3.1
Deethanizer Section
Raw gas containing associated gases is fed from the top of the Deethanizer. Deethanizer operated at approximately 390lb/in 2. We separated out ethane from this column. The overhead product is ethane in the form of vapors, which is partially condensed in the o
condenser by using propane at 20 F and collected in the reflux drum. Condensed product is recycled to the Deethanizer tower and non-condensed vapors (mainly ethane) are sent to the fuel gas system. Temperature inside tower is maintained by supplying heat from reboiler. The bottom product from Deethanizer enters into the next columns, depropanizer. 2.2.3.2
Depropanizer Section
The pressure of Deethanizer bottom product is reduced to 290 lb/in 2 and then entered into the depropanizer. The overhead product of this column is propane rich and is condensed in the condenser by using cooling water. The condensed product is collected in the reflux drum. Some amount of this is refluxed back to the column. Heat is supplied through direct fired heater.
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Fractionation of NGL to produce LPG 2.2.3.3 Debutanizer Section 2 Depropanizer bottom product is expanded to a pressure of 110 lb/in and fed to the top of the tower. Propane is separated as top product and condensed further in condenser by using cooling water. Bottom products are the heavier hydrocarbons. Fractionators of different types are commonly used in gas plant. Commonly used fractionators for LPG are listed in the table below. Table 2:Fractionator types for LPG Production (Abdel-Aal, H. K.et al 2003)
Type of fractionator
Feed
Top product
Bottom product
Demethanizer
C1/C2
Methane
Ethane
Deethanizer
LPG
Ethane
Propane plus
Depropanizer
Deethanizer bottoms
Debutanizer
Depropanizer bottoms Butanes
Deisobutanizer
Debutanizer top
Propane
Butanes plus Natural gasoline(pentanes plus)
Isobutane
Normal butane
2.2.4 Product Treatment Plant Propane and butane products separated from the fractionation plant contain some impurities as residual water, H 2S, Carbon disulfide and sulfur compounds. These impurities should be removed in order to meet the desired product specifications. The contaminants and their reasons for removal have been listed in the table below. Table 3:Different Contaminants in LPG (Abdel-Aal, H. K.et al 2003)
Contaminants
Reasons for Removal
Hydrogen sulfide
Safety and Environmental
Carbon dioxide
Corrosion control
Carbonyl sulfide
Product specification
Carbon disulfide
Prevention of freeze out
Mercaptans
Organic sulfides
Prevention of catalyst
Nitrogen
poisoning in downstream facilities
Water
at low temperatures
Numerous processes are available to remove contaminants but two of them are the most important and commonly used.
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Fractionation of NGL to produce LPG a. Absorptive Purification b. Adsorptive Purification
2.3
Feed Specifications for NGL Fractionation
Feed for NGL (Natural gas liquid) fractionation plants comes from upstream processing plants, which receives feed directly from gas reservoirs. Feed composition is different from different reservoirs. Feed composition is important for design considerations. The feed for NGL fractionation trains contain methane, ethane, propane, butane and heavier ones. The feed composition for NGL fractionation column is shown in the table below. Table 4: Specifications of feed for NGL fractionation unit (Manley ,D.B.)
Liquid
Feed
Ethane
Propane
Iso-Butane
N-Butane
Methane,C1
0.5
1.36
Ethane,C2
37.0
95.14
7.32
Propane,C3
26.0
3.50
90.18
Isobutane,iC4
7.2
96.0
4.50
N-Butane,C4
14.8
2.0
95.0
Gasoline
volume%
Butanes,
2.0
2.50
Iso-pentane,iC5
5.0
Pentanes
3.5
3.0 33.13 0.50
23.52
N-pentane,NC5 N-hexane,NC6
4.0
26.90
N-heptane,NC7
2.0
13.45
2.4
Product Specifications
The product specification for LPG plant is shown in the following table. This data has been provided by the US Gas Processors Association. The products specifications must be met to sale the qualitative gas. Vapor pressure and temperature are the most important parameters which should be controlled during operation.
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Fractionation of NGL to produce LPG Table 5:Product specifications for LPG (Manley ,D.B) Product
Commercial
Commercial
Commercial propane – Propane
characteristics
propane
butane
butane mix
Composition
mainly
propane
and propene
mainly
butane
and butene
mainly
mixes
HD-
5a
of not less than
propane – propene and
90 % propane;
butane – butene
not more than 5 % propene
Vapor (max.)
pressure at
208
70
208
208
of – 37
36
36
– 37
100°F,
psig b Temperature
volatile residue: °F b Butane and heavier,
2.5
2.5
vol% Pentane
2.0
and
2.0
heavier, vol%
0.05
0.05
Oil stain observation
pass c
pass c
Volatile
15
Residual
matter,
,
mL
15
15
10
sulfur,grains/100 cu ft Moisture content Free water content
pass d
pass d none
none
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Fractionation of NGL to produce LPG 3. Simulation The modelling of LPG extraction from fractionation of NGL is done using Aspen Hysys as simulation tool. The process is divided into 5 sections. These sections are feed conditioning, Deethanizer, depropanizer, debutanizer and butane splitter. Each section is described as under.
3.1
Feed conditioning
The feed streams are NGL and their compositions are given in appendix. The first feed stream is coming from separation unit from well stream and the second feed from dehydration unit. The temperature and pressure of the feeds are given to be 25°C and 30 bar but they have different flow rates. These feeds are to be processed in order to extract LPG products that are propane, isobutane and n-butane. The products were selected based on demand in LPG market. Before the Deethanizer column the feeds are to be conditioned. There are three alternatives for conditioning. First alternative is to mix both feeds before Deethanizer column and then expand the mixed feed. A separator is used to remove the lighter hydrocarbons that are methane and ethane. An illustration of this alternative is shown in figure (3)
Figure3: Feed conditioning alt.1
The second alternative is to expansion and separation of methane in each stream. The bottom stream from the separators in each stream are mixed in mixer and then sent to deethanizer as shown in figure (4). 12 | P a g e
Fractionation of NGL to produce LPG
Figure 4: Feed conditioning alt.2
The third alternative is to mix both feeds, expand the mixed feed and let it into the Deethanizer column as shown in figure (5). In this project the third alternative was considered to be the best choice.
Figure 5:Feed conditioning alt.3
3.2
Fractionation columns
3.2.1 Deethanizer In this project no refrigeration is used in order to minimize the cost. The first column is deethanizer where no condenser is used and the top product gases (methane and ethane) are withdrawn from top if gas phase. The feed is fed to the column from top. See Figure (6).
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Fractionation of NGL to produce LPG The condition inside the column is set such as 26 bar pressure and number of trays to be 15 trays. The specification in the deethanizer is selected in order to converge the column. Since there is no condenser, only one specification would be enough to converge the column. In this column the bottom product rate was used since both methane and ethane are light and would be difficult to stay in the bottom stream. Other specifications such as component recovery and component fraction were used but it was difficult to converge the column. The number of trays in the column is very important, since the more the number of stages the higher the column will be and the more expensive expensive the cost will be but in the same time the purer the product will be. The optimum number of stages in this project is 15 stages.
Figure 6: Deethanizer column
3.2.2 Debutanizer Here the debutanizer was used before depropanizer for economic reason so that the next separation will be easier and the depropanizer will be smaller. In debutanizer the butane and lighter hydrocarbons are withdrawn as top products and condensed in condenser while heavier hydrocarbons are withdrawn from bottom as bottom products. A debutanizer model is shown in figure (7). In modelling the debutanizer two specifications are required in order to converge the column since both condenser and reboiler are present. In this case both distillate rate and component recovery of both propane and butane in top are selected. In modelling the
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Fractionation of NGL to produce LPG debutanizer, specification has an influence of the purity of the top products as it is expected over 99.9% of propane and 966% of butane recovered in the top product. The number of stages were 15 and top and bottom pressure were 16 and 17 bar respectively.
Figure 7: Debutanizer column
3.2.3 Depropanizer Depropanizer separates propane from butane with a similar modelling to debutanizer. Propane is withdrawn from top as top product after condensing and butane as bottom product. Figure (8) shows depropanizer column. Since there are only butane and pr opane in the feed, the modelling is easier. Both component recovery and component ratio are selected as specification. With thses specifications over 99.8% of propane was recovered in the top product. The condition inside the column is as follow: 15 stages, top and bottom pressure 9 and 10bar respectively.
Figure 8: Depropanizer column
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Fractionation of NGL to produce LPG 3.2.4 Butane splitter The last column is butane splitter in which butane is separated in iso-butane as top product and nbutane as bottom product. In butane splitter distillate rate and component recovery are selected as specification. The separation of iso and n-butane is more difficult and therefore the number of stages is higher than in depropanizer. With this specifications and manipulating the pressure in the column an iso-butane recovery of 96.6% was achieved. A model of butane splitter is shown in figure (9).
Figure 9: Butane splitter
Note: All simulation results have been pasted in table A1 in appendix.
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Fractionation of NGL to produce LPG 4.
LPG Transport
Demand for LPG is growing constantly. It is used in all energy requiring areas, particularly residential and commercial sectors of developed or developing countries. It is expected, that with population growth bonded with energy demand, the use of clean liquid and gaseous fuels will continue to increase. At the same time, the historic levels of oil prices are pushing the transportation demands of LPG. New transportation projects are expected to come, alongside with many already in planning stage. With increasing access to LPG and many new market possibilities, complex and innovative solutions for transport problems will play important role in those projects. The LPG distribution chain can be seen in fig 2A in appendix. Due to the fact, that LPG in normal conditions (1 bar, 20°C) has gaseous form, unequal distribution in area, seasonal consumption and static, highly localized production, many problems had to be overcome and wide network of transport systems was developed. To fulfill certain pressure and market requirements, two main types of LPG distribution was introduced:
Continuous flow of gas characterized by all types of pipe technologies, providing cheap, constant and simple access to LPG at the expense of high preliminary investments, localized storage terminals and additional extended network of delivery.
“Discrete” (bulk) means of transport characterized by moving certain amount of LPG in
pressurized canisters carried by cars, trains, ships. Each of those two branches contains special types of transport and is described by different safety measures and precautions. Due to their complexity it’s necessary to describe that in
few paragraphs bellow.
4.1
Continuous flow of LPG – Pipe system
LPG flow in certain time of its migration to consumers through pipelines. Although the initial construction expanses ale high, the result if are build and correctly maintained is significant. This way of LPG transport is most economic and safe, and moreover, has some other advantages such as :
Better availability of LPG in hardly accessible areas
Significant LPG transport reduction costs 17 | P a g e
Fractionation of NGL to produce LPG
Environmental impact in terms of reducing exhaust emissions and energy consumptions
LPG pipeline system is not as wide as NG and oil pipeline system, however, it still follows all “LP gas production – consumer” steps. Also, it is not evenly spread over the globe and
differs by continents and final usage. Most widespread network is in Europe, where LPG is mainly used in residential areas for water heating. The pipes are mostly made from carbon steel and are 1” to 6” wide. Different situation occurs on USA, where LPG is used for distributed power generation. As a result, pipe network is not so wide, but the existing pipes are larger, and generate higher gas flow to Gas plants. One example is double pipeline from New Mexico to Minnesota and Wisconsin – Mid-America Pipeline. First system devoted only for LPG. It is 3540 km ANSI900 system of 4”to 10” pipeline, was constructed in 1960, and includes 6 delivery, 2 operating
terminals, 14 pumping stations and underground storage. The pipeline map is shown in fig. A3 in appendix . Similar example with different goal was established in India, where 33.6 million Indian households in 2001 were using LPG for cooking. Primary cooking usage, lack of developed infrastructure, and vast unsupplied areas had lead to building 1900 km LPG mostly 12” to 16” pipeline network. 1300 km branch connects Jamnagar on the west with New Delhi area
on the north. And 600 km of network is connecting Vizag on the east coast with midlands Secenderabad. The LPG transmission system has a capacity 3.8 MMTPA LPG. The Jamnagar – New Delhi branch can be seen on figure A4 in appendix.
4.2
Discrete (bulk) means of LPG transport
The discrete transport system is completely opposite than Continuous system. Where pipe system was rigid in delivery, the discrete system is flexible, in the terms of place, amount and time. The same works for price, initial investments are lover in comparison, but overall expenses are higher in order. As a result, other than pipeline systems are used whenever the pipes would be economically unjustifiable. Detailed view to each type can be read below.
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Fractionation of NGL to produce LPG 4.2.1 Vessel tanker transport Provide the vast majority of LPG discrete transportation, and major amount of transported LPG is transported by sea. There is a fleet of more than 1000 tankers around the globe. Those tankers are not single-function vessels dedicated only for LPG. By the nature of LPG, every ship capable of carrying pressurized, semi-pressurized or refrigerated LPG can work as a LPG tanker. Due to the fact that it is safer to transport huge amounts under low temperature, rather than high pressure, there are three types of LPG vessels, divided by size, derived from system of keeping the gas in liquid form.
Pressurized tankers (18 bar, ambient temperatures) used in short to medium haul trades. Carrying capacity differs from 3 000 – 10 000 m3.
Semi-pressurized tankers (5-8 bar, -15 ± 5°C) for medium haul trades, with capacity 10 000 – 30 000 m3.
Fully refrigerated vessels (ambient pressure, -43°C for pure propane) are used for long haul trades, with high LPG demand, e.g. Japan. Those carrying capacity varies from 30 3
000 to 100 000 m . There is also considerable fleet of very small vessels used mainly for coastal, short sea and inland-river trades. Because the LPG vapors are highly flammable, without scent and unrecognizable by naked eye, some safety precautions was introduced to avoid leakage or any disaster. The containers for LPG have usually strong walls, and are shielded from outside area by innerwall layer of inert gas, mostly nitrogen. Also, because of possibility to carry other gases, before loading are cleaned by CNG vapors blow. Vessel transport itself can be considered as simple and safe in comparison to on-loading and offloading. There are two main systems of transfer nowadays, static shore terminal (SST), and single point mooring system (SPM). SPM consist of onshore LPG storage facility and LPG offshore buoy, connected by flexible pipelines. Thanks to the pipeline connection, SPM can be considered as middle-step between pipe and vessel transport. Floating buoy allows connection of all types of tankers, 360° movement grants tanker possibility to moor from all sides depending on bathymetry 19 | P a g e
Fractionation of NGL to produce LPG and environmental condition at given time. In comparison with SST, SPM grants much quicker transfer, independent of weather and lower maintenance costs. Both SST and SPM are illustrated in Fig.A5, SST-SPM. After vessels offload the cargo, other types of distribution come to play. 4.2.2 Rail, truck, car transport Rail with truck transport fulfills the role of ship on the land. Following receipt at terminals from ships, LPG is transported to localized storage stations for further distribution by smaller cars, or to the consumers directly, mostly Industry. For instance in Italy is LPG exclusively transported by trains, where no pipeline network was developed. Because of the highest number of single transports and high concentration of population in comparison to pipe or ship, certain precautions were taken. The LPG is mostly kept liquefied by pressure, therefore canisters of all types must be robust, able to keep high internal pressure and sustain significant damage without leakage. Next, canisters never can be fulfilled more than 40% of water equivalent capacity. This is necessary because of high differences of external temperature changes. So the gas inside the canister is allowed to evaporate, without significant increase on pressure. As well, canisters must be equipped with overpressure valves and mechanical safety cover around valve. In terms of visual recognition, international regulations demands white or red color, and exact signs on visible places, showing the content of canister. Example of unified canister can be seen on fig.A6 in appendix.
4.3
Economic analysis
Supplies of LPG are continually rising, closely followed by prices. And, the estimations show, that it will have positive future. The demand for LPG on market over years is stable, that in combination with rising supply would for commonly thinking mind signalize, easing of prices. However, the situations on markets looks different. For illustration, the price of propane rose during last few years from 500 dollars/tone to 900 d/t in late 2007 and 1500 d/t in 2011. This increase is however inevitable. It is caused by many factors, one of the biggest, is close link to price of crude oil and constantly weakening dollar. Quick rise of prices choke the demand for LPG few times in 20 | P a g e
Fractionation of NGL to produce LPG the past. Recovery came hand to hand with rising prices mainly thanks to constant petrochemical demand and never-ending rising of petrol prices.
Main sources of LPG
Refinery production – increasing
Crude oil associated gases processing – moderate increase
Non-associated natural gas processing – significant increase due to new started up discoveries and large volumes originated from Qatar, Iran, UAE and Nigeria.
In the past was estimated, that petrochemical industry will not be able to consume these increasing amounts, resulting in LPG price moderation. Unfortunately, as actual prices show, it didn’t happen. Future estimations in this direction are unwise.
Demand
Petrochemical industry in western Europe and Middle East - rising
Middle East, Asia and Africa domestic sector – rising
Transport sector (autogas) in Europe and Asia – Pacific – rising
North America - falling around 2,3m t/year
The increase in demand is highly fractionated, by timescale, geographically or by sector. See Fig.A7 in appendix. In domestic sector, the increase is stable thanks to easy access to LPG cylinders, growing infrastructure, substitution of other types of fuel by LPG etc. Also, domestic market in Asia shows positive numbers. Population and income are rising quicker than grid based energy sources, resulting in higher demand for flexible LPG. Similarity with natural gas is next key reason for rising LPG demand. For industry, possibility to relative easy switch from natural gas to LPG means high valued advantage, mainly in long term NG prices predictions. LPG is used also as a backup plan for NG using industry, and not surprisingly, for large-scale capital investors. Autogas sector was rising in last years probably in highest pace from all sectors. Over year growth in consumption is app. 6%. Two thirds of this consumption is located in only 7 21 | P a g e
Fractionation of NGL to produce LPG countries. This trend is not caused only by usual gasoline and diesel rising prices, but by cars manufacturers. Their new introduced cars running on autogas, are another key for demand. This boom is illustrated on South Korean example. Where, almost 80% of autogas consuming cars, was introduced by domestic manufactures. Similar picture occur in Turkey, where government – car manufacturers cooperation led to 16% growth in LPG consumption. See Fig.A8 in appendix. Despite the continuously rising prices, the clean burning LPG remains a growing source of energy. And future estimations, although was wrong sometimes, confirms its rising role in energy-demanding world.
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Fractionation of NGL to produce LPG
Conclusions and Discussions
LPG can be an alternative fuel for vehicles as it burns cleaner and greenhouse gas emissions can be controlled. So, it can help us to combat climate change.
LPG can be recovered from natural gas liquids by different methods. Refrigeration method is more common of all because recovery from this process is 98%.
There are three alternatives to condition the feed stream for fractionation unit. It has been found to be the best choice to mix the two feed streams from the wells having different compositions and feeding it to deethanizer first.
Cost for fractionation operation can be reduced by adapting refrigeration of ….
Debutanizer has been used before depropanizer in order to become it more economical and to make separation easy at next stage.
The separation of iso and n-butane is more difficult and more number of stages are required in order to do the separation which in turn increases the overall cost.
LPG transportation cost can be reduced by employing the pipeline technology. Although these projects are underway but it would be proved as a safe and economic.
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Fractionation of NGL to produce LPG References
[1] Abdel-Aal, H. K. , Aggour ,Mohamed, Fahim, M. Al. (2003) “ Petroleum and Gas Field Processing”, Marcel Dekker INC, New York Basel, pp- 317-329 [2] Asian development bank “PROJECT COMPLETION REPORT ON THE LPG PIPELINE PROJECT (Loan 1591- IND) IN INDIA “;(2003)
[3] Bahnassi ,Essam , Khouri,Abdul Rahman , Alderton,Peter , Fleshman ,James “Achieving product specifications for ethane through to pentane plus from NGL fractionation plants”,
(AIChE Fall Conference, Foster Wheeler); 2005
[4] Elvers ,Barbara (2008) “ Handbook of fuels: energy sources for transportation”, Wiley VCH-Verlag GmbH & Co, pp-142-149 [5] Fundamentals of the World Gas Industry, 2008, http://91.121.21.64/page_attachments/0000/0330/Petroleum_Economist.pdf [6] Gail India Limited, 2011, http://gail.nic.in/gailnewsite/businesses/lpgpipeline.html
[7] Manley.D.B. “Thermodynamically efficient distillation: NGL fractionation”; Department of chemical engineering, university of missouri – rolla, Rolla, Missouri
[8] Naturalgas.org 2011, http://naturalgas.org/naturalgas/transport.asp [9] Parkash ,Surinder (2009) “Petroleum Fuels Manufacturing Handbook: including Specialty Products and Sustainable Manufacturing Techniques ” McGraw -Hill Professional Publishing,
pp-4-10 [10] René Raaijmakers, “Offshore terminals for the transportation of Liquefied Petroleum Gas”, Bluewater Offshore Production Systems (USA), Inc., http://www.bluewater-
offshore.com/downloads/PT26_LPG%20SPM_bluewater.pdf
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Fractionation of NGL to produce LPG [11] Stellman ,J.M. (1998) “Encyclopaedia of Occupational Health and Safety”, International Labor Office, Geneva [12] Stopford ,Martin (2009) “Maritime economics”, 3rd edition, Rutledge [13] Total 2011, http://www.totalgaz.com/Gaz/International.nsf/VS_OPM/8AFAE12EA35EC563C125718100509959? OpenDocument
[14] Willbros Group, Inc. 2011, “LPG pipeline system”; http://willbros.com/About/Areas-ofOperation/United-States-398.html
[15] World LP Gas Association 2009, http://www.worldlpgas.com/what-is-lp-gas/interactivemaps/
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Fractionation of NGL to produce LPG
Appendix Figure A1: Process Flow Sheet (Simulation sheet from Hysys)
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Fractionation of NGL to produce LPG Table A1:Simulation Results (Material Streams)
Vapor
Feed1
Feed2
Feed
Feed in
C1.C2
C3+
C3+ in
C5+
0.0000
0.0000
0.0000
0.0000
0.9985
0.0000
0.3064
0.0000
Fraction Temp.
C
25.0
25.0
24.87
25.06
37.54
246.7
235.2
251.7
Pressur
KPa
3000
3000
3000
2600
1800
2600
1700
1700
Molar
Kgm
225.9
122.2
348.1
348.1
62.09
286.0
286.0
242.0
Flow
ol/hr
Mass
Kg/h
2.500e+
8000
3.300e+
3.300e+
1487.0
3.151e+
3.151e+
2.916e+
Flow
r
004
004
004
004
004
004
Liquid
M /h
Volume
r
e
3
36.00
13.38
49.39
49.39
4.119
45.27
45.27
41.02
-
-
-
-
-
-
-
-
5.499e+
1.986e+
7.485e+
7.485e+
5.203e+
5.016e+
5.016e+
4.524e+
007
007
007
007
007
007
007
007
flow Heat
kJ/hr
Flow
Vapor
C3.C4
C3.C4 in
C4
C3
C4 in
iC4
nC4
0.0000
0.2062
0.0000
0.0000
0.2392
0.0000
0.0000
Fraction Temperature
C
76.67
56.50
74.71
22.82
46.17
5.095
50.93
Pressure
KPa
1600
1000
1000
900
500
190
500
Molar Flow
Kg
44
44
29.19
14.81
29.19
10.07
19.12
2356
2356
1702
653.1
1702
583.2
1119
4.243
4.243
2.954
1.289
2.954
1.038
1.916
kJ/
-
-
-
-
-
-
-
hr
5.499e+00
1.986e+0
7.485e+0
7.485e+0
5.203e+0
5.016e+0
5.016e+0
7
07
07
07
07
07
07
mol /hr Mass Flow
Kg/ hr
Liquid flow
M3 /hr
Heat Flow
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Fractionation of NGL to produce LPG Figure A2. Distribution Chain (World LP Gas Association 2009)
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Fractionation of NGL to produce LPG Figure A3: Mid-America Pipeline (Willbros Group, Inc. 2011)
Figure A4: Jamnanagar New Delhi
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Fractionation of NGL to produce LPG Figure A5: SST,SSM (René Raaijmakers)
Figure A6: amerigas Canister
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