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Gas Transport as Hydrate
Mohamed Iqbal Pallipurath
Cryogenic Engineering Centre, IIT Kharagpur, W. Bengal, 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. Figures 1 to 4 show schematics of the
different processes.
Figure 1. Japanese Hydrate production process (MES, 2002)
Figure 2 BG slurry hydrate production process (Fitzgerald, 2001).
Figure 3.Norwegian dry hydrate process (Gudmundsson and Hveding, 1995)
Figure 4. Norwegian crude/hydrate slurry process (Gudmundsson and Mork, 2001)
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.
Figure 5. Sodium dodecyl sulfate
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. Figure 5 from literature [2] illustrates hydrates
typically formed in a chilled test cell during a 5-day to 10-day period in which a
hydrocarbon gas pressurizes a distilled water phase. A slow, random growth of
hydrate crystals is evident.
Figure 6. Typical hydrates formed in a chilled test cell[2].
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 (Figure 7).
Figure 7. This three-stage design incorporates attributes enhancing gas hydrate
storage economics that minimize labor.[2]
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.
Literature [2] provides us with some data plots for a similar setup. In Figure 8, plots
of feed gas flow rates and vessel pressures vs. time are given when gas is admitted
manually.
Figure 8. The above chart results are based on manually admitted gas.[2]
The downward spikes in the figure represent rates of batches of gas manually input;
superposed are the corresponding pressure spikes to about 550+ psig in the vessel.
Immediately after each batch gas input, hydrates form and drop the pressure,
signifying the formation and collection of hydrates on the fins.[2]
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.
Figure 9 Hydrate Loading Location
4.2 Transportation: Pelletisation
Figure 10 Pelletiser (MES)[3]
Figure 11 Hydrate Pellets (2 cm diameter) MES [3]
Figure 10 shows the continuous pelletiser, and Figure 11 shows 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 Maximum 23 m
Truck - Pony Trailer Combination Maximum 23 m
Truck - Full Trailer Combination Maximum 23 m
Box Length Limit
Maximum 20.0 m
Truck - Pony Trailer Combination Maximum 20.0 m
Truck - Full Trailer Combination
Trailer Length Limits
Semi-trailer Maximum 16.2 m
Full Trailer Maximum 12.5 m
Pony Trailer 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 Min 6.25 m/Max 12.5 m
Full Trailer Minimum 6.25 m
Pony Trailer Minimum 6.25 m
Effective Rear Overhang
Straight Truck Maximum 4.0 m
Semi-trailer Maximum 35% of wheelbase
Full Trailer Maximum 35% of wheelbase
Pony Trailer 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 Minimum 3.0 m
Single Axle to Tandem axle Minimum 3.0 m
Single Axle to Tridem Axle Minimum 5.0 m
Tandem Axle to Tandem Axle Minimum 5.0 m
Tandem Axle to Tridem Axle 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 N/A 8000 kg1
Tractor N/A 5500 kg1
Tandem Steering2 Straight Truck 1.2 m to 1.85 m 16 000 kg
Single (other than Single Tires N/A 6000 kg
steering axle) Dual Tires N/A 9100 kg
Tandem Straight Truck, less than 1.2 m 9100 kg
(including tandem Tractor, Trailer 1.2 m to 1.85 m 18 000 kg
equivalent axle) and Semi-trailer greater than 1.85 m 9100 kg
Tridem Semi-trailer less than 2.4 m 18 000 kg
(including tridem 2.4 m to less than 3.0 m 21 000 kg
equivalent axle) 3.0 m to less than 3.6 m 24 000 kg
3.6 m to 3.7 m 26 000 kg
greater than 3.7 m 18 000 kg
Triaxle Semi-trailer less than 2.4 m 18 000 kg
2.4 m to less than 3.0 m 18 000 kg
3.0 m to less than 3.6 m 18 000 kg
3.6 m to 4.9 m 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 Maximum 1000 kg greater or less than the weight of an
Equalization adjacent axle in the same axle group
Tire Loading
- per mm of tire width Maximum 10 kg/mm
- per tire (except steering Maximum 3000 kg
axles)
Part 2 - Vehicle Weights and Dimensions Limits by Configuration
Category 1: Tractor Semi-trailer
Section 1 - Dimension Limits
Figure 12 Tractor Semi-Trailer
DIMENSION LIMIT
Overall Length Maximum 23 m1
Overall Width Maximum 2.6 m
Overall Height Maximum 4.15 m
Tractor
Wheelbase Maximum 6.2 m
Tandem axle spread Minimum 1.2 m/Maximum 1.85 m
Semi-trailer
Length Maximum 16.2 m
Wheelbase Minimum 6.25 m/Maximum 12.5 m
Kingpin setback Maximum 2.0 m radius
Effective rear overhang Maximum 35% of wheelbase
Tandem axle spread Minimum 1.2 m/Maximum > 1.85 m
Tridem axle spread Minimum 2.4 m/Maximum 3.7 m
Triaxle axle spread Minimum 2.4 m/Maximum 4.8 m
Track width Minimum 2.5 m/Maximum 2.6 m
Interaxle Spacings
Single Axle to Single, Tandem or Tridem Minimum 3.0 m
Axle Minimum 5.0 m
Tandem Axle to Tandem Axle Minimum 5.5 m
Tandem Axle to Tridem Axle
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
Figure 13 Tractor Semi-Trailer
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 Maximum 18 000 kg
Axle spread > 1.85 m Maximum 9100 kg
Tridem Axle (including tridem equivalent axle)
Axle spread 2.4 m to less than 3.0 m Maximum 21 000 kg
Axle spread 3.0 m to less than 3.6 m Maximum 24 000 kg
Axle spread 3.6 m to 3.7 m Maximum 26 000 kg
Axle spread greater than 3.7 m Maximum 18 000 kg
Triaxle Axle
Axle spread 2.4 m to less than 3.0 m Maximum 18 000 kg
Axle spread 3.0 m to less than 3.6 m Maximum 18 000 kg
Axle spread 3.6 m to 4.9 m 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 Maximum 32 600 kg
Four axles - with semi-trailer tandem spread > 1.85 m Maximum 23 700 kg
Five axles - with tandem spreads 1.2 m to 1.85 m Maximum 41 500 kg
Five axles - with semi-trailer tandem spread > 1.85 m Maximum 32 600 kg
Six axles - with tridem spread 2.4 m to < 3.0 m Maximum 41 500 kg
Six axles - with tridem spread 3.0 m to < 3.6 m Maximum 41 500 kg
Six axles - with tridem spread 3.6 m to 3.7 m Maximum 41 500 kg
Six axles - with tridem spread > 3.7 m Maximum 41 500 kg
Six axles - with triaxle spread 2.4 m to < 3.0 m Maximum 41 500 kg
Six axles - with triaxle spread 3.0 m to < 3.6 m Maximum 41 500 kg
Six axles - with triaxle spread 3.6 m to 4.9 m 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= 4 / 3( r 3 ) hydrate = 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 /( d 2 / 4 )
= 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
2gGt0
Calculate the Shock velocity V =2(9.81)(20)(0.027)/3.14
= 3.38 m/s
2 f n V
Then Fragilty = Output G = 10 = Gout =2(3.14)(fn)(3.38)/9.81
g
So fn = 4.62 Hz
V
Dynamic Deflection d d = 3.38/2(3.14)(4.62)
2 f n
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 pellets (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
Figure 14 Sandwich Type Vibration Isolating Mount
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
A commercial sample available which matches our design has dimensions as shown
in figure 14
4.5.2 Rail Transport
Figure 15. Standard dimensions of rolling stock. Indian Railways.
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= 4 / 3( r 3 ) hydrate
= 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 /( d 2 / 4 )
= 3119.1 Pa
= 0.0031 MPa << 0.21 MPa
So there is no possibility of crushing of pellets
4.6 Transportation: Unloading
Figure16 Unloading and regasification stage.
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].
Figure 17. The Capacity-Distance Diagram [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.
An overlap region shown in Figure 17, reflects the wide range of conditions that
affect the gas technology selected for a particular application.
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
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[8] Department of road transport and highways http://morth.nic.in/index.htm
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