Transportation of Natural Gas Using Liquid Carriers at Ambient Temperature

Ben Thompson

Purpose of this Work

• In this work we evaluate the use of an existing storage method in transportation of natural gas using ships across the ocean.

• We consider the method to store natural gas in liquid hydrocarbon mixtures at moderate pressures and ambient temperature (OU/CBME patent).

• For this we will consider well-known storage architectures (Tube bundles & Coselle units)

• We will compare with the Liquefied method (LNG) and the High Pressure compressed gas method (CNG)

Overview• Existing technology

• Previous work

• Present analysis

• Conclusions and recommendations

Existing Technology

• Existing Patent: “High-Energy Density Storage of Natural Gas at Moderate Temperatures” (Supergas™)

• Natural gas dissolved in pure liquid propane.▫ Initial evaluation was at low temperatures and

moderate to high pressures.

▫ Maximum loading : 70 mol % methane at lower temperatures.

Existing Technology

•Coselle units

•Composite piping

http://www.ngvrus.ru/images/15_41.jpg

http://i234.photobucket.com/albums/ee274/biopact3/biopact_coselle_CNG.jpg

http://www.lnf.infn.it/esperimenti/dear/hbp-700.jpg

Compressed Natural Gas (CNG)• Natural gas is highly

compressed to pressures around 3000 psia.

• Ambient temperature.

• Cost effective when shipping distance is between 200 and 2500 miles.

http://www.marinelog.com/IMAGESMMIV/cng2.jpg

http://dsp-psd.pwgsc.gc.ca/Collection/C89-4-70-1998E.pdf

Compressed Natural Gas Carrier

Coselle units Tube bundles

Liquefied Natural Gas• Ambient pressure.

• Temperature: -161 ºC

• Cost effective with shipping distances greater than 2500 miles.

http://www.ferc.gov/images/photogallery/lng_sksummit.jpghttp://ahmadberlian.blogsome.com/images/LNG_tanker.jpg

Density comparison • Densities:- CNG – 128 kg/m3

- LNG – 410 kg/m3

- Supergas ™ – 329 kg/m3

- This mixture has less methane (about 1/3 of the total mass) than CNG or LNG contributing to the density.

• Moles of Methane per cubic meter- Supergas ™ : 4800 at 80 degrees Fahrenheit and 1500 psi

- LNG: 25000 - CNG: 6400 at 60 degrees Fahrenheit and 3000 psi

• LNG has the highest energy content and Supergas ™ at this temperature has the lowest.

Capacities• 145,000 ton capacity

tanker traveling at 18 knots.

• Every ship has same capacity so costs increase almost linearly to reach new distances.

• 14, 275 tons of natural gas could be stored on this tanker.

• Propane and equipment account for rest of weight.

0 2000 4000 6000 8000 10000 120000

5

10

15

20

25

30

35

Ships Required vs Shipping Distance

1 Million tpa

2 Million tpa

3.5 Million tpa

Shipping Distance (miles)

Sh

ips

Req

uir

ed (

# o

f sh

ips)

Initial Conclusions• Use of carbon fiber reinforced piping to ship

natural gas in hydrocarbon carrier.

• 70 mol % methane mixture.

• 30 °F and 1500 psia.

Issues we investigated• Variations in Density predictions and the

possible error in profitability.

• ASME codes.

• Use of other solvents.

• Loading and unloading costs.

Density Prediction Methods

• Soave-Redlich-Kwong (SRK)

• Peng-Robinson (PR)

• BWRS

• 50/50 molar mixture of propane and methane.

• The above methods were modified using their respective liquid density calculation methods. API method was the default, but it was not used.

Comparison of prediction methodsPressure vs Liquid Density

17.6

19.6

21.6

23.6

25.6

1200 1700 2200 2700

Pressure (psi)

Liqu

id D

ensi

ty (l

b/ft

3 )

SRK1

SRK3

SRK4

PR1

BWRS

2.0 % variation between SRK and PR equations of state.

SRK

PR

Loading Station Specifics

• Equipment shown to the right.

• Mixer consists of 10 foot long, 24 inch ID stainless steel pipe.

• Upwards of 80,000 hp compressor power requirement.

Compression Compression

Heat Exchanger (Cooling)

Heat Exchanger (Cooling)

Mixer

Methane Dissolution issues

• Additives to the mixture could increase the amount of methane dissolved within the propane.

• Both propane and methane are nonpolar, so any substance increasing nonpolar attractions will help this.

• Introduction of a fraction of some polar substance might cause this, but an agitator would be required for a continuous phase.

Unloading Station Specifics

• Equipment: - Heat exchanger

- Flash drum - Expander - Distillation column (no condenser due to

large energy requirements to condense methane)

• Specifics:- 15 theoretical trays.

- Pre-cooling to 45 ◦F - Pressure drop in flash drum to 1000 psi - Column operating pressure of 500 psi - Column operating temperature of 20 ◦F - Only 98.9% of propane is recovered and it

should be higher.

Projected Income for Various Shipment Methods and various Thermodynamic prediction methods

01

23

45

67

8

Pro

jec

ted

In

co

me

, $

MM

Coselle units

Stainless steelpipes

Compositereinforced steelpipes

SRKPR

BWRS

Economic Comparison

Thermodynamic Method effect:1.8% variation for Coselle units, 2.0% variation for composite pipes., 0.6% variation in stainless steel pipes.

Net Profit

0

500

1000

1500

2000

2500

Equations of State

Net

Pro

fit

(MM

$)

Coselle Units

Stainless Steel Pipes

Composite Piping

SRKPR BW

RS

Economic Comparison

Coselle units and composite piping are on the scale of billions of net profit per year.

Economic Comparison

Supergas ($/ton) LNG ($/ton)0

10

20

30

40

50

60

70

80

90

Comparison of Operating Costs

1 million tpa

2 million tpa

3.5 million tpa

Op

erat

ing

Co

st (

$/to

n)

• Operating costs of LNG decrease with increased production.

• This assumes constant operation.

• Supergas method assumes only operation on days of loading/unloading.

Economic Comparison

0 1000 2000 3000 4000 5000 6000 7000 8000

-250

-200

-150

-100

-50

0

Net Profit vs Shipping Distance

$10/ton

$20/ton

$30/ton

$40/ton

$50/ton

$60/ton

$70/ton

$80/ton

$90/ton

$100/tonShipping Distance

Net

Pro

fit

($M

M)

• Shipping costs are the deterrent in the profitability of this method.

• Higher capacity tankers or lower cost tankers could limit this cost and make method profitable.

• Only high prices could make this method profitable.

Economic Analysis

•LNG requires fewer ships to complete the specified capacity.

•Shipping costs rely on charter rate of $65,000.

0 2000 4000 6000 8000 10000 120000

50

100

150

200

250

Shipping Cost Comparison: LNG vs Supergas

Supergas 100mmtpa

Supergas 200mmtpa

Supergas 350 mmtpa

LNG 100 mmtpa

LNG 200 mmtpa

LNG 350 mmtpa

Shipping Distance, miles

Sh

ipp

ing

Co

st,

$MM

ASME Codes

•Carbon fiber composite piping

•Required for low weight to be competitive

•No codes exist •Legal and safety

issues were not analyzed

Other Solvents

• Other solvents explored:

▫ Heavier hydrocarbons in pure form and in mixtures.

▫ Acetone: 3000 psia for equivalent molar mixture

▫ Cyclohexane and other hydrocarbons.

• These solvents do not have any higher capacity for carrying methane at moderate pressures.

Conclusions • Transportation using Supergas™ and carbon

reinforced pipes is not more profitable than LNG at any distance.

• Required shipping costs to meet the capacity supplied by LNG keep this method from being economical.

• Possible errors in the thermodynamic prediction methods for density only affect the profitability by 2.0% maximum

• Research on additives to enhance methane diffusion into solvent might be advisable.

• At high gas costs, around $100/ton, it may be profitable for short distances. But, LNG would gain the same benefits.