Hydrate Process for Land Transport of Natural Gas in India

Gas Transport as Hydrate

Mohamed Iqbal Pallipurath
TKM College of Engineering, Kollam, Kerala, India

Abstract

A feasibility study is conducted on the new process of transportation of Natural Gas using Gas Hydrate as applicable to land transport in India. India’s vast resources of stranded gas fields can be economically brought to the consumer via this technology. Current methodologies are discussed and an appropriate form is presented in the context of domestic road and rail networks. Formation, storage, transport and recovery issues are addressed. Economic viability is investigated. Formation and Transport are redesigned to suit domestic conditions of labour and road/rail system.

1.1 Introduction

Natural gas usage continues to grow in response to the need to cope with global environmental issues while securing a expedient and stable source of energy. Currently, the long distance ocean transport of natural gas to consumer markets is achieved by an integrated Liquefied Natural Gas (hereinafter called LNG) system. However, this system needs huge capital investments for LNG production facilities, LNG carriers and so on, which are economic only with very large-scale gas fields.
A ubiquitous question in the oil industry is what to do with the associated gas in fields which have no gas pipeline. The oil industry has apparently come to a point where new field developments will not be undertaken unless the associated gas problem is solved. Several such associated gas situations can be identified in India. The term “stranded gas” has been used for situations where the field is remote or where the field is located in deep water. The expression “marginal gas” has been used for situations where the field is too small to justify a gas pipeline for long-term production. These associated gas situations apply also to non-associated gas.
The great amount of stranded gas India has cannot be economically transported via current technology. Accordingly, the development of a simpler, lower cost natural gas transportation system is urgently required to meet the growing domestic and global demand for natural gas.
Gas hydrates are clathrates where the guest gas molecules are occluded in a lattice of host water molecules. With all cavities of Type I structure occupied by methane molecules, the volume ratio of gas (at standard temperature and pressure) to water can be as high as 185. Theoretical investigations have been carried out during the past five decades to do this in gas hydrates. Even though the investigations proved the concept of storing natural gas in hydrates technically feasible, applications stayed in the laboratory stage because of complexities of the process, low hydrate formation rates and high overheads.

2.1 Storing and transporting associated gas: State of the Art

Storage and transportation of natural gas in hydrate form have been investigated recently, primarily in Japan, Norway and England.
Japan seeks commercialisation of a natural gas hydrate process to contend with liquefied natural gas (LNG) in transportation.
Pilot plants capable of generating 600 kg to 1,000 kg of gas hydrates per day are under construction or testing in these countries.

The Japanese process and Norwegian dry process share the following characteristics:

• a hydrate slurry is formed in a high pressure, continuous stirred tank reactor;
• a series of treatments pack the low energy-density slurry into high-energy density dry hydrate; and
• natural gas hydrates are stored and transported at atmospheric pressure and temperature of 1°F or lower.

British Gas Group (BG) keeps the gas hydrates in slurry state throughout the formation, storage and transportation process.
Because of the hydrate slurry’s lower energy density than LNG, multiple tankers would be necessary to compete with a single LNG tanker.
The Japanese, British and Norwegian processes are designed primarily to transport natural gas to compete with LNG.

3.1 Formation: Experimental Equipment

The laboratory equipment is designed to study the viability of a non-stirred system to occlude natural gas in gas hydrates with a minimum of labor. Surfactant aided formation method is used since the addition of Sodium Dodecyl Sulphate can increase the reaction rate 700 times. A simple hydrate formation/ storage/ decomposition tank without any moving parts is favored to reduce maintenance, labor and capital costs. Customarily, it could take days with distilled water to initiate the nucleation of hydrates and achieve appreciable growth in a non-stirred system. Even after hydrate nucleation is initiated in a dormant system, a thin solid film forms across the gas/liquid interface that separates the gas and liquid phase, thus drastically slowing hydrate formation. Also, when gas hydrate crystals mature, as much as 80% to 90% of interstitial water of the crystals may remain unreacted. A 2300-ml, SS304 stainless steel, laboratory hydrate test cell (10 cm. diameter, 30 cm high) with a removable top flange sealed with a Teflon gasket, is designed to conform to ASME Pressure vessel code. The laboratory cell has the following basic capabilities:

• Heat or cool pressurized cell contents upon demand;
• Continuously monitor pressure and temperatures in cell;
• Collect data continuously and store on computer;
• View contents and video record when desired; and
• Sustain constant pressure while measuring inlet gas flow rate.

A Superheated Nitrogen Cryostat of 32 cm diameter surrounds the test cell. Alternately steam can be passed through the jacket to heat the setup. The Nitrogen Cryostat can maintain ± 0.1 K of the set point to as low as 253 K. Insulation encloses the test cell and exterior jacket. A transducer and a set of 12 PT100 probes monitor pressure and temperatures. The Methane cylinder regulator maintains a desired pressure in the test cell as gas occludes into hydrates; the regulator can maintain a pressure within ± 10 kPa. Flow rate of gas into the test cell during hydrate formation is monitored with a gas flow meter that has a capability of 0 sccm to 5,000 sccm, at an accuracy within 1% of full scale and a repeatability of within 0.25% of flow rate. A data acquisition system records the outputs from gas flow meter, PT100s and pressure transducer on a computer. The data is permanently incorporated into a database using in-house developed software.
Sodium dodecyl sulfate (SDS or NaDS) (CH3(CH2)11OSO3Na) (FW 288.38), also known as sodium lauryl sulfate (SLS), is an ionic surfactant that is used in the experimental setup to reduce the hydrate formation time. Literature attributes 700 times increase in formation rate by addition of SDS. The molecule has a tail of 12 carbon atoms, attached to a sulfate group, giving the molecule the amphiphilic properties required of a detergent.

It is prepared by sulphation of 1-dodecanol (lauryl alcohol, CH3(CH2)10CH2OH) followed by neutralisation with sodium carbonate. It is used in both industrially produced and home-made cosmetics.
Like all detergent surfactants (including soaps), it removes oils from the skin, and can cause skin irritation. It is also irritating to the eyes. The critical micelle concentration in pure water is 0.008 M, and the aggregation number at this concentration is around 50.
A micelle (also micella, plural micellae) is an aggregate of the surfactant molecules dispersed in a liquid colloid. The process of forming micelles is known as micellization. Micelles are often globular in shape, but other shapes are possible, including ellipsoids, cylinders, bilayers, and vesicles. The shape of a micelle is controlled largely by the molecular geometry of its surfactant molecules, but micelle shape also depends on the conditions (such as temperature or pH, and the type and concentration of any added salt).
Individual surfactant molecules that are in the colloid but are not part of a micelle are called "monomers." In water, the hydrophilic "heads" of surfactant molecules are always in contact with water, regardless of whether the surfactants exist as monomers or as part of a micelle. However, the hydrophobic "tails" of surfactant molecules have less contact with water when they are part of a micelle. In a micelle, the hydrophobic tails of several surfactant molecules assemble into an oil-like core that has less contact with water. In contrast, surfactant monomers are surrounded by water molecules that create a "cage" of molecules connected by hydrogen bonds. This water cage is similar to a clathrate and has an ice-like crystal structure. In a non polar solvent, the hydrophilic groups form the core of the micelle, and the hydrophobic groups remain on the surface of the micelle (so-called reverse micelle).
Micelles only form when the concentration of surfactant is greater than the critical micelle concentration (CMC), and the temperature of the system is greater than the critical micelle temperature, or Krafft temperature. The formation of micelles can be understood using thermodynamics: micelles can form spontaneously because of a balance between entropy and enthalpy. In water, the hydrophobic effect is the driving force for micelle formation, despite the fact that assembling surfactant molecules together reduces their entropy. Broadly speaking, above the CMC, the entropic penalty of assembling the surfactant molecules is less than the entropic penalty of the caging water molecules. Also important are enthalpic considerations, such as the electrostatic interactions that occur between charged (or ionic) surfactants.
Micelles composed of ionic surfactants are surrounded by a "cloud" of tightly-bound ions. Because these ions have a charge opposite or counter to the charge of the ionic surfactant, they are called counterions. Although the bound counterions partially neutralize a charged micelle (by up to 90%), the effects of micelle charge may be important relatively far from the micelle, and ionic micelles can influence many properties of the mixture, including its electrical conductivity. However, adding salt to a colloid containing micelles can decrease the strength of electrostatic interactions and lead to larger ionic micelles.
An analytical balance is used to weigh surfactant. Powdered sodium dodecyl sulphate (SDS) is used in the tests; the 98%+ pure SDS (with no alcohols in the residuals) has a molecular weight of 288.4 g/mol. Double-distilled water is used in the surfactant solutions.
Methane of 99.99% purity is used initially. Later tests will also be conducted with natural gas.
3.2 Formation: Procedure, Laboratory Feasibility Study
The cell is filled with surfactant-water solution to displace all gases. The hydrocarbon gas is then injected to displace water to a predetermined water level. The system is cooled to 275 K to 278 K under a pressure too low for hydrates to form. Pressure is then raised to the operating pressure during a 2 to 3 minute span by admitting gas into the cell; measurement of gas mass admitted is made with a flow meter. Hydrate formation is tracked through monitored temperatures, pressures and mass flows continuously displayed and recorded on the computer. During the experimental run, cell interior is observed with an endoscope monitor, and the video is recorded on computer with voice over commentary.

3.3 Formation: Follow Up

Results from the successful laboratory feasibility study will be used to design a scaled-up proof-of-concept process. Literature [2] provides these consequences of using surfactant solutions:

• gas hydrate formation rates in the non-stirred system will be increased by 2.5 orders of magnitude;
• hydrate particles will be self-packed as they form in the formation vessel; and
• interstitial water of the hydrate mass will react to near completion.

Gas-hydrate formation rates—If a gas hydrate storage process is to be practical for industrial applications, then natural gas must be occluded in gas hydrates at a rapid rate. This property coupled with the economic requirement of a non-stirred system, creates a particularly difficult problem because a pressurized and chilled quiescent water/natural-gas system develops a thin hydrate film at the water-gas interface that acts as a barrier to mass transport.

R.E. Rogers and Y. Zhong [1] found that by adding about 284 ppm of SDS, the rate of formation could be increased by a factor greater than about 700.
Physical properties, such as surface tension, of water-surfactant solutions change abruptly at the critical micellar concentration (CMC) where surfactant molecules organize and orient their hydrophilic heads and hydrophobic tails. However, the concentration of 284 ppm SDS used effectively in the experiments in literature [2] is well below the CMC measured to be about 2,700 ppm at ambient conditions. Literature [2] reports that by repeating pressure, temperature and surface area in the test cell while varying SDS concentration that hydrate induction time decreased rapidly with SDS concentration until a threshold concentration is reached at hydrate-forming conditions, whereupon no further decrease occurred with added surfactant. This threshold concentration at hydrate conditions is found in literature [2] to be about 242 ppm. It is thought that increased solubility of hydrocarbon gases in the water at hydrate-forming conditions may increase SDS solubility at these low temperatures and enhance micelle formation to the lower 242 ppm.
3.4 Formation: Self-packing of gas hydrates
Ordinarily, unreacted interstitial water adsorbed on hydrate particles can occupy as much as 80% to 90% of the total volume of the hydrate mass – an important consideration when economics dictates that volume of storage be minimized. However, when water of the SDS solution goes into the hydrate molecular structure, surfactant is excluded into interstitial water where it promotes hydrate formation of that interstitial water. Hydrates are promoted in the interstitial water because surfactant solution concentrated in the interstices continues to help solubilize natural gas, and the surface areas of the surrounding hydrate particles provide large interfacial areas for further reaction. The literature [2] reports that hydrates formed from surfactant solutions accumulate as a porous mass of orderly packed small particles through which natural gas can permeate and contact unreacted interstitial water.

3.5 Formation: Scaled-up process design

The performance of the laboratory process in literature [2] indicated a scaled-up process could be designed to incorporate notable process attributes enhancing economics of gas hydrate storage of natural gas. These attributes suggest a simple process that minimizes labor

A proof-of-concept hydrate gas storage process is designed to form, store and decompose 140 scm of natural gas in gas hydrates. In this proof-of-concept process, hydrates form with no stirring at 1.6°C and 3.7 MPa from a water solution containing surfactant above its critical threshold concentration at hydrate-forming conditions. As hydrates form, the mass accumulates on cold, solid surfaces placed at the liquid-gas interface. These metal surfaces serve to transfer heat in formation and decomposition steps, but they also adsorb and collect hydrates during formation. The process is designed so hydrates attach to the solid interfaces and, as the water level drops, the solid hydrate particles grow radially from those surfaces until the vessel is filled.

Stainless steel 304L comprises the pressure vessel, and its shell is 0.9 m. inside diameter and 0.94 m. outside diameter. The working length of the pressure vessel, which will be used in the vertical position, is 1.8 m. The top ellipsoidal dome is Teflon-coated on the inside to prevent hydrate buildup from blocking exit ports.
The jacket surrounding the pressure vessel is made of 3 mm. thick 304L stainless steel. Baffles direct the flow of circulating water-glycol solution through the jacket. The gap between jacket and pressure vessel is 3 cm.
Thirty finned heat exchanger tubes, which are symmetrical, extend into the pressure vessel -15 tubes for entering fluid and 15 for exiting fluid. The 30 tubes are brought into three concentric doughnut-shaped ring headers;12 outlet tubes exit the ring headers and extend through the top dome of the pressure vessel. Hydrates also build symmetrically upon the heat-exchanger tubes and fins. At the end of the process, hydrates from adjacent heat-exchanger tubes/fins should touch but leave flow paths to the exit ports at the top of the vessel.
The heat exchanger tubes are designed to withstand a maximum external pressure of 5 MPa; the minimum internal pressure is 0.3 MPa for the circulating glycol solution. The design temperature is -6°C to 45°C to accommodate heating or cooling in forming or decomposing hydrates. The fins increase hydrate formation rate in two ways. Formation rate is directly proportional to the interfacial surface area and is dependent on heat transfer rate, a parameter dependent on surface area.
The pressure vessel and internal heat exchanger will be fabricated to American Society of Mechanical Engineers (ASME) standards as given by the 2001 edition of ASME Boiler and Pressure Vessel Code, Section VIII, Division 1.
A chiller capable of circulating glycol-water solution at the required flow rate and temperature would be of 12-ton refrigeration capacity. Glycol-water solution will be circulated from the chiller through the heat exchanger/ adsorber inside the formation tank; the solution will flow in parallel through the formation tank’s exterior jacket.
A surge tank for decomposition gases is provided. A deionized water supply and boiler to burn off-gases are provided in the design.
After having purged the vessel of air, the procedure is to fill it about two-thirds full with water/surfactant solution, cool the system and establish 3.7 MPa with natural gas. Thereafter, a constant-pressure regulator admits makeup gas to maintain 3.7 MPa as hydrates form, self-pack on the heat exchanger fins and drop the water level. No further labor is needed until the vessel is full of gas hydrates that contain 140 scm gas.

3.6 Formation: Conclusion

Formidable problems (forming hydrates rapidly, collecting and packing hydrates, and
reacting interstitial water) to make natural gas storage in gas hydrates an economically
viable process are overcome by forming the hydrates from a surfactant solution. In the
feasibility study, a non-stirred laboratory test cell could be filled with hydrates in less
than 3 hours with a capacity of 156 vol/vol. The important attributes of the laboratory
process are incorporated in the design for a proof-of concept scale-up. Simplicity and
minimum labor requirements are stressed in the design. The process is designed to
store 140 scm of natural gas in gas hydrates to be formed from surfactant solutions at
3.7 MPa and 1.6°C. A finned-tube heat exchanger accommodates latent-heat transfer
during hydrate formation and decomposition, but the exchanger also serves to collect
by adsorption and symmetrically pack hydrate particles as they form. The final design
of proof-of-concept facility is based on experimental results of the laboratory
feasibility study.

4.1 Transportation: Overview

Transportation is either by road or by rail. The process starts at the location where the hydrate is formed as described above. This location is where the gas hydrate is loaded.

4.2 Transportation: Pelletisation

NGH pellets of Ø20mm made from powdered NGH. Compression molding produces pellets with superior strength, which is important during prolonged storage and loading/unloading. This process also produces pellets with better sphericity, which is important for better fluidity. Greater pellet homogeneity is also achieved in this pelletising process. NGH can be pelletised under 2-3 MPa compression resulting in round pellets. Pellet static collapse tests [3] also confirmed that pellets can bear about 0.21 MPa, which is more than the collapse pressure calculated by the static compression load from pellets’ own weight and by the vibration/acceleration of the land transport vehicle.

4.3 Transportation: Storage Prior to loading

After the pelletisation the pellets are stored in a cooled (-15oC, 1 atm) cylindrical storage tank. Refrigeration is by means of glycol water circulated in a jacket surrounding the storage tank. It is estimated that a 12 tonne refrigeration plant would suffice. The level of stored pellets should not exceed 5 m to prevent crushing. Pellet transfer to the storage tank is by means of a conveyor belt from the pelletisation plant.

4.4 Transportation: Loading

The pellets are loaded onto the trailer/ rail wagon by conveyor belt connected to an open trap door on the side of the storage tank. The pellets are fed to the conveyor belt by gravity, the trap door controlling the rate of feed. storage tank is built such that the bottom of the tank is above the level of the top of the truck/ wagon, the conveyor belt being horizontal and moving at 0.5 m/s.

4.5 Transportation: Transportation

Transportation may be either by road using refrigerated trailer trucks, or by rail using special refrigerated rail wagons such as are already in use by Indian railways to transport perishable goods like marine produce or vegetable/fruits. The temperature to be maintained inside the truck/wagon is between -15oC and -10oC.

4.5.1 Highway Transport

National Highways in India is the class of roads maintained by the Central Government and is the main long-distance roadways. The NH's constitute about 58,000 km, i.e. around 2% of the total road network in India, but carries nearly 40 % of the total road traffic. The varied climatic, demographic and traffic situation prevents these highways from having a uniform character. These may be six laned in some parts, to even non-metalled stretches in remote places. Many NH’s are still undergoing up-gradation or even construction. There are long NH's to connect the metros together, as well as short shoots off the highway to give connectivity to the nearby ports or harbours. The longest NH is the NH7 which goes all the way from Varanasi in Uttar Pradesh to Kanyakumari at the southern most point of the Indian mainland, in Tamil Nadu covering a distance of 2369 km, and passing through the metros like Jabalpur, Nagpur, Hyderabad and Bangalore. The shortest NH is the NH47A, which is a 6 km stretch to the Ernakulam - Kochi Port.

Very few of India's highways are concretised, the most notable being the Mumbai-Pune Expressway.

India has launched a massive highway up gradation called the Golden Quadrilateral Project which connects the four metros by four lane highways. Work is scheduled to be completed in December 2006.
Until then transporting hydrate on Indian roads needs special design of the carrier’s suspension on the flat bed 8 wheeler.
Part 1 - Vehicle Weight and Dimension Limits

A.

General Dimensional Limits

Dimension

Limit

Overall Height Limit

Maximum 3.5 m

Overall Width Limit

Maximum 2.6 m1,2

Overall Length Limits
Tractor Semi-trailer
Truck - Pony Trailer Combination
Truck - Full Trailer Combination

Maximum 23 m
Maximum 23 m
Maximum 23 m

Box Length Limit

Truck - Pony Trailer Combination
Truck - Full Trailer Combination

Maximum 20.0 m
Maximum 20.0 m

Trailer Length Limits
Semi-trailer
Full Trailer
Pony Trailer

Maximum 16.2 m
Maximum 12.5 m
Maximum 12.5 m

1

An outside rear-vision mirror may extend up to 300 mm on each side of a vehicle or combination of vehicles.

2

Auxiliary equipment or devices not designed or used to carry cargo may extend up to 100 mm on each side of a vehicle or combination of vehicles.

B.

Dimensional Controls - Wheelbases, Interaxle Spacings, Overhangs, Setback and Track Width

Dimension

Limit

Tractor Wheelbase

Maximum 6.2 m

Trailer Wheelbase
Semi-trailer
Full Trailer
Pony Trailer

Min 6.25 m/Max 12.5 m
Minimum 6.25 m
Minimum 6.25 m

Effective Rear Overhang
Straight Truck
Semi-trailer
Full Trailer
Pony Trailer

Maximum 4.0 m
Maximum 35% of wheelbase
Maximum 35% of wheelbase
Maximum 4.0 m

Rear Overhang1

Maximum 2.0 m

Front Overhang2

Maximum 1.0 m

Kingpin Setback (Semi-trailer)

Maximum 2.0 m radius

Track Width
Semi-trailer, Full Trailer and Pony Trailer

Minimum 2.5 m

Minimum Interaxle Spacing Requirements
Single Axle to Single Axle
Single Axle to Tandem axle
Single Axle to Tridem Axle
Tandem Axle to Tandem Axle
Tandem Axle to Tridem Axle

Minimum 3.0 m
Minimum 3.0 m
Minimum 5.0 m
Minimum 5.0 m
Minimum 5.5 m

1

Cargo may overhang the rear, if the overall length and effective rear overhang limits are respected. Red warning flags are required on the rear of the cargo when the rear overhang exceeds 1.0 m.

2

Cargo may overhang the front, if the overall length limit for the vehicle or vehicle combination is not exceeded, and in the case of a semi-trailer, the cargo does not extend beyond a 2.0 m radius about the kingpin.

C.

Axle Weight Limits

Axle Type

Application

Spread Range

Weight Limit

Steering

Straight Truck
Tractor

N/A
N/A

8000 kg1
5500 kg1

Tandem Steering2

Straight Truck

1.2 m to 1.85 m

16 000 kg

Single (other than
steering axle)

Single Tires
Dual Tires

N/A
N/A

6000 kg
9100 kg

Tandem
(including tandem
equivalent axle)

Straight Truck,
Tractor, Trailer
and Semi-trailer

less than 1.2 m
1.2 m to 1.85 m
greater than 1.85 m

9100 kg
18 000 kg
9100 kg

Tridem
(including tridem
equivalent axle)

Semi-trailer

less than 2.4 m
2.4 m to less than 3.0 m
3.0 m to less than 3.6 m
3.6 m to 3.7 m
greater than 3.7 m

18 000 kg
21 000 kg
24 000 kg
26 000 kg
18 000 kg

Triaxle

Semi-trailer

less than 2.4 m
2.4 m to less than 3.0 m
3.0 m to less than 3.6 m
3.6 m to 4.9 m

18 000 kg
18 000 kg
18 000 kg
18 000 kg

1

Steering axle loads can be as high as 9100 kg for a vehicle or combination of vehicles provided the load carrying capacity of the axles, tires, and all other components is not exceeded, and the tire loading does not exceed 10 kg/mm of tire width; however, no increase in the specified maximum gross vehicle weight limit for the configuration will be permitted with higher steering axle loads.

D.

Other Weight Related Limits

Axle Groups - Load Equalization

Maximum 1000 kg greater or less than the weight of an adjacent axle in the same axle group

Tire Loading
- per mm of tire width
- per tire (except steering axles)

Maximum 10 kg/mm
Maximum 3000 kg

Part 2 - Vehicle Weights and Dimensions Limits by Configuration
Category 1: Tractor Semi-trailer
Section 1 - Dimension Limits

DIMENSION

LIMIT

Overall Length

Maximum 23 m1

Overall Width

Maximum 2.6 m

Overall Height

Maximum 4.15 m

Tractor
Wheelbase
Tandem axle spread

Maximum 6.2 m
Minimum 1.2 m/Maximum 1.85 m

Semi-trailer
Length
Wheelbase
Kingpin setback
Effective rear overhang
Tandem axle spread
Tridem axle spread
Triaxle axle spread
Track width

Maximum 16.2 m
Minimum 6.25 m/Maximum 12.5 m
Maximum 2.0 m radius
Maximum 35% of wheelbase
Minimum 1.2 m/Maximum > 1.85 m
Minimum 2.4 m/Maximum 3.7 m
Minimum 2.4 m/Maximum 4.8 m
Minimum 2.5 m/Maximum 2.6 m

Interaxle Spacings
Single Axle to Single, Tandem or Tridem Axle
Tandem Axle to Tandem Axle
Tandem Axle to Tridem Axle

Minimum 3.0 m
Minimum 5.0 m
Minimum 5.5 m

1

A tractor semi-trailer while being used to transport poles, pipe or material that cannot be dismembered shall have a maximum overall length limit of 25 m.

Category 1: Tractor Semi-trailer
Section 2 - Weight Limits
WEIGHT

LIMIT

Axle Weight Limits:

Steering Axle

Maximum 5500 kg1

Single Axle (dual tires)

Maximum 9100 kg

Tandem Axle (including tandem equivalent axle)
Axle spread 1.2 m to 1.85 m
Axle spread > 1.85 m

Maximum 18 000 kg
Maximum 9100 kg

Tridem Axle (including tridem equivalent axle)
Axle spread 2.4 m to less than 3.0 m
Axle spread 3.0 m to less than 3.6 m
Axle spread 3.6 m to 3.7 m
Axle spread greater than 3.7 m

Maximum 21 000 kg
Maximum 24 000 kg
Maximum 26 000 kg
Maximum 18 000 kg

Triaxle Axle
Axle spread 2.4 m to less than 3.0 m
Axle spread 3.0 m to less than 3.6 m
Axle spread 3.6 m to 4.9 m

Maximum 18 000 kg
Maximum 18 000 kg
Maximum 18 000 kg

Gross Vehicle Weight Limits:

Highways

Three axles

Maximum 23 700 kg

Four axles- with tandem spread 1.2 m to 1.85 m
Four axles - with semi-trailer tandem spread > 1.85 m

Maximum 32 600 kg
Maximum 23 700 kg

Five axles - with tandem spreads 1.2 m to 1.85 m
Five axles - with semi-trailer tandem spread > 1.85 m

Maximum 41 500 kg
Maximum 32 600 kg

Six axles - with tridem spread 2.4 m to < 3.0 m
Six axles - with tridem spread 3.0 m to < 3.6 m
Six axles - with tridem spread 3.6 m to 3.7 m
Six axles - with tridem spread > 3.7 m

Maximum 41 500 kg
Maximum 41 500 kg
Maximum 41 500 kg
Maximum 41 500 kg

Six axles - with triaxle spread 2.4 m to < 3.0 m
Six axles - with triaxle spread 3.0 m to < 3.6 m
Six axles - with triaxle spread 3.6 m to 4.9 m

Maximum 41 500 kg
Maximum 41 500 kg
Maximum 41 500 kg

1

The maximum steering axle weight can be as high as 9100 kg for a vehicle or combination of vehicles if the load carrying capacity of the axle, tires, and all other components is not exceeded, and the tire loading does not exceed 10 kg/mm of width; however, the maximum gross vehicle weight limit will be based on a steering axle weight of 5500 kg.

Refrigeration is maintained inside the insulated trucks similar to the units currently in use for trailer trucks transporting marine goods. The power for the refrigerant compressor comes from the truck engine through a generator coupled to the engine which in turn powers the compressor motor.

4.5.1.a Design Check for Crushing of Pellets

Diameter of pellet = 0.02 m
Number of tiers of pellets possible in a vehicle on Indian roads. = H/0.02
Where H is the height permissible for the carrier
Now the maximum permissible height of vehicle on Indian roads=3.5 m
So, the maximum height of carrier would be around 2.5 m
So number of tiers = 2.5/0.02=125
So maximum load on one pellet = (125-1)* weight of one pellet
Now weight of one pellet= = 0.0041 N
So load on one pellet = 124(0.0041) = 0.51 N
Pressure on one pellet = 0.51 / projected area of one pellet
= 0.51 /( )
= 1620.3 Pa
= 0.0016 MPa << 0.21 MPa
So there is no possibility of crushing of pellets

4.5.1.b Design of Vibration Isolators

Shipping container type vibration isolation is opted for. Shipping Container Mounts (also called sandwich mounts) consist of two metal plates with an elastomer bonded between them. The composition and configuration of the elastomer determines the static and dynamic properties of the part. Sandwich mounts have excellent capacity for energy control, and they exhibit linear shear load deflection characteristics through a significant deflection range.

Let us design for a maximum shock of 20 g incurred as a half sine shock pulse delivered in t millisecond.

If the average pothole on Indian road is of 0.3 m major dimension, a vehicle traveling at 40 kmph will take 0.3(3600)/40(1000) seconds to cross it.
So t = 0.027 s

Let the fragility of hydrate be taken as 10g
Calculate the Shock velocity =2(9.81)(20)(0.027)/3.14
= 3.38 m/s
Then Fragilty = Output G = 10 = =2(3.14)(fn)(3.38)/9.81
So fn = 4.62 Hz
Dynamic Deflection = 3.38/2(3.14)(4.62)
dd = 0.117 m

so minimum thickness of sandwich mount = 0.117/1.5 =0.078 m = 7.8 cm
The maximum static load is the weight of hydrate + weight of refrigerated container
= LBH(packing ratio)+ Wc
= (23)(2.6)(2.5)(980)(0.7) + 50000 = 152,557 N
If we use 6 sandwich mounts, then static load on a mount is 152557/6
= 25426 N
Choose a sandwich mount with static load capacity of 26000 N and Thickness 8 cm


A = 228.6 mm B = 228.6 mm C = 190.5 mm D = 190.5 mm E = 12.9 mm
F = 101.6 mm G = 165.1 mm Flange Thickness = 4.8 mm

4.5.2 Rail Transport

Refrigeration for the insulated railway wagon is maintained much as the current refrigerated wagons use, i.e. from a combination of a/c generator run off the shaft of rolling stock, and the battery pack below the wagon. The refrigerant compressor runs off the generator when in motion and off the battery pack when stationary. The generator is belt driven by the wheel shaft, and has a typical capacity of 40 kW. Desired temperature range is -15oC to -10oC at atmospheric pressure. Maximum level to which pellets are loaded is 4 m.

4.5.2.a Design Check for Crushing of Pellets

Diameter of pellet = 0.02 m
Number of tiers of pellets possible in a vehicle on Indian rail. = H/0.02
Where H is the height permissible for the carrier
Now the maximum permissible height of rolling stock on Indian rail=4.8 m (see figure 15)
So number of tiers = 4.8/0.02= 240
So maximum load on one pellet = (240-1)* weight of one pellet
Now weight of one pellet=
= 0.0041 N
So load on one pellet = 239(0.0041) = 0.98 N
Pressure on one pellet = 0.98 / projected area of one pellet
= 0.98 /( )
= 3119.1 Pa
= 0.0031 MPa << 0.21 MPa
So there is no possibility of crushing of pellets

4.6 Transportation: Unloading

4.7 Transportation: Storage after Unloading

After the unloading the pellets are stored in a cooled (-15oC, 1 atm) cylindrical storage tank. Refrigeration is by means of glycol water circulated in a jacket surrounding the storage tank. It is estimated that a 12 tonne refrigeration plant would suffice. The level of stored pellets should not exceed 5 m to prevent crushing. Pellet transfer to the storage tank is by means of a combination of screw lift and conveyor belt from the carrier.

5 Regasification

Regasification is by means of water at 20oC or higher if available in the ambient is sprayed into the regasifier which contains pellets. In Indian climes this process is particularly cheap whereas in European countries, the water has to be heated before injection [10]. The hydrate melts in the tanks and the natural gas is ducted in a large-diameter duct from the gasifier to compressors. This process can actually be done in the truck/wagon itself to avoid constructing a separate gasifier chamber. Of the 4 metros in India, 3 have ambient conditions above 20oC most of the year round thereby making this a very cost effective process as compared to the European processes.

6 Discussion

CAPACITY-DISTANCE DIAGRAM
Parameters influencing the feasibility of natural gas developments include the size of the resource, the distance to the market, the size of the market and the technology used. Of the 150 TCM (trillion cubic metres) world reserves of natural gas, 38% are in the Former Soviet Union, 35% in the Middle East, 9% in OECD-countries and 18% the rest of the world. Of the natural gas fields world-wide still to be developed, about 80% are less than 7 BCM in size, and about one half of the fields are considered to contain stranded gas [7]. Assuming a project life of about 20 years, a 7 BCM field size will sustain a delivery of 0.35 BCM per year. The technology used needs to be appropriate for the size of the resource. Pipelines are universally used to transport natural gas from field resource to market. Economy-of-scale effects influence what transport capacity and distance a particular pipeline will be feasible. For short distances and large capacity, natural gas pipelines are more feasible [7].

The diagram illustrates what stranded gas technologies are likely to be appropriated with respect to distance and capacity. LNG is generally considered appropriate for large-volumes for long-distances; GTL (gas to liquid) is generally considered appropriate for medium-to-low volumes for long-distances [7]. Offshore pipelines in India less than 1000 km in length are generally considered appropriate for large-volumes, for example above 1 BCM. CNG, GTW and NGH technologies are considered appropriate for medium-to-low volumes and medium-to-short distances.

Conclusion

NGH technology in 2002 was about 12% lower in cost (CAPEX) than LNG technology [7]. With pipelines in danger of sabotage in today’s volatile political climate, NGH technology looks particularly bright. With indigenous NGH technology, and low cost of transportation, our dependence on piped NG from foreign countries can be decreased. When NGH technology is widely adopted and matures, its costs are expected to decrease.
Regasification of NGH in particular is especially suited to the Indian clime and can reduce costs by as much as 75%.

References

[1] Rogers, R.E. and Zhong, Y., Surfactant Process for Promoting Gas Hydrate Formation and Application of the Same, U.S. Patent No. 6,389,820 (2002).
[2] R.E. Rogers, Yu Zhong, R. Arunkumar, J.A. Etheridge, L.E. Pearson, J. McCown
and K. Hogancamp. Gas Hydrate Storage Process for Natural Gas.
[3] J. S. Gudmundsson and F. Hveding. TRANSPORT OF NATURAL GAS AS FROZEN HYDRATE. Proceedings, 5th International Offshore and Polar Engineering Conference The Hague, The Netherlands, June 11-16,1995
[4] J.S. Gudmundsson, V. Andersson and O.I. Levik and M. Parlaktuna. Hydrate Concept for Capturing Associated Gas. 1998 SPE European Petroleum Conference
The Hague, The Netherlands, 20-22 October 1998
[5] Toru Iwasaki, Yuuichi Katoh, Takashi Arai, Kiyoshi Horiguchi and Kazuyoshi Matsu. DEVELOPMENT OF A HYDRATE-BASED NATURAL GAS TRANSPORTATION SYSTEM. http://nippon.zaidan.info/seikabutsu/2002/00223/contents/040.htm
[6] Hideyuki Shirota, Hikida Kenjiro, Yasuharu Nakajima, Susumu Ota, Tatsuya Takaoki, Toru Iwasaki, Kazunari Ohgaki. USE OF HYDRATE FOR NATURAL GAS TRANSPORTATION. http://nippon.zaidan.info/seikabutsu/2002/00223/contents/042.htm
[7] Jón S. Gudmundsson, Oscar F. Graff. HYDRATE NON-PIPELINE TECHNOLOGY FOR TRANSPORT OF NATURAL GAS
[8] Department of road transport and highways http://morth.nic.in/
[9] Indian Railways Rolling Stock - I http://irfca.org/faq/faq-stock.html, http://irfca.org/docs/stock-dimensions-1971.html
[10] J.S. Gudmundsson and A. Børrehaug. FROZEN HYDRATE FOR TRANSPORT OF NATURAL GAS. 2nd International Conference on Natural Gas Hydrate, June 2-6, 1996, Toulouse, France. http://www.ipt.ntnu.no/~ngh/library/paper3.html