EFFECTS OF GUEST GAS ON PELLETIZING PERFORMANCE OF NATURAL GAS HYDRATE (NGH) PELLETS

Tetsuro Murayama1 , Wataru Iwabuchi, Masato Ito, Masahiro TakahashiChiba Technology Center1, Research & Development Headquarters

Mitsui Engineering &Shipbuilding CO., LTD., 1, Yawatakaigan-dori, Ichihara-City, Chiba, JAPAN

ABSTRACTMitsui Engineering & Shipbuilding has been successfully producing natural gas hydrate artificially and converting it into pellet form for easy handling and transportation. Such NGH pellets are stabilized at minus 20 degrees Celsius under atmospheric pressure, and it has been a continuing challenge to produce NGH pellets with higher quality and lower dissociation rate, in order to improve the economics of transporting and storing the pellets. For this reason, a newly-

designed molding test apparatus has been developed; and noting the relationship between molding pressure and hydrate concentration ratio, which is an indicator of the molding solid quality, tests with various compression pressures were carried out to investigate characteristics of hydrate compression and pelletizing, as well as the relationship between guest gas and pelletizing performance. Hydrate slurry composed of hydrate particles and water is formed in a reactor up to 10 wt% of the hydrate ratio. Then, the slurry is supplied to the test apparatus and filtered by a

cylindrical filter for dewatering, so that the hydrate ratio becomes 30 wt%. The concentrated hydrate is mechanically compressed by a piston, and pelletized to produce pellets with a high quality ratio. The typical cylindrical diameter and length of the hydrate solid are 100 mm and 50mm, respectively. The high quality NGH pellets were formed by the piston under a pressure of 15MPaG; and a NGH ratio of 90 wt% was successfully achieved. In the filtration process, the filtration resistance was obtained from the rate of slurry feed and the differential pressure of the

concentrated hydrate. In the compression process, a correlation among volume changes of slurry, compression molding pressure and molding speed were observed. We concluded that the filtration resistance of NGH is larger than that of methane hydrate, and that there is a correlation between the value of molding pressure and hydrate concentration.

Keywords: gas hydrates, NGH, pellet

Corresponding author: Phone: +81 (0)436 41 1930 Fax +81 (0)436 43 1785 E-mail: murayama@ mes.co.jp

INTRODUCTIONMitsui Engineering & Shipbuilding is

developing a natural gas transport chain using hydrate as a transport medium. Natural gas hydrate (NGH) is formed by gas-liquid contact in normal

temperature, the water under the high pressure,and has a grain size of 20-30 μm. The formed hydrate is in the form of slurry consisting of water and NGH particles. In order to use NGH as a gas transport medium, it is important to increase the amount of NGH per unit mass, or the NGH ratio.

A higher NGH ratio allows, for example, for a

reduction in the number of transport ships, leading to a reduction in the total cost of the chain.

The dissociation rate of NGH can be minimized, due to the self-preservation effect, when cooled to -20ºC, which is convenient for storage under

atmospheric pressure. The NGH ratio of the slurry in the reactor is about 10 wt%. Cooling NGH

under this condition results in a reduced amount of gas per unit mass and lower energy density, which is an economic disadvantage. The NGH ratio needs to be increased to increase energy density.

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011),Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

NGH powder is obtained by dewatering the slurry. However, the powder has low bulk densityand requires a large volume of space for transport. It is therefore necessary to increase the NGH ratio and reduce the volume of the powder for transportpurposes.

In order to resolve these issues, we have developed a machine (hereinafter the ‘Hydrate Pellet Machine (HPM)’) to pelletize NGH directly from the NGH slurry. This paper describes the operating process of the machine, and reports on the performance of the pellets obtained, as well as

the data on the filtration and compression of hydrate obtained in the development of the machine.

OPERATION PROCESS AND MERIT

The HPM is a machine designed to pelletize hydrate using a piston-cylinder mechanism. Table 1 shows the specifications of the HPM. Figure 1shows an overall view of the HPM.

Table 1 Specifications of the HPM

Name Attribute Specifications

HPM

Max. compression pressurePellet sizeProduction capacity

25 MPa100 dia. × 50 l.10 kg/h_NGH

Figure 1 Overall view of the HPM

The cylinder and compression piston have a 100 mm circular cross section. There is a shut-off piston at the outlet end surface of the cylinder. The pellet solidified inside the cylinder is ejected by

opening and closing the piston. As shown in Figure 2, part of the cylinder wall consists of a cylindrical filtering screen.

Figure 2 Cylindrical filtering screen

Figure 3 illustrates the operation of the HPM.

A) Constant rate filtration

B) Constant pressure filtration

C) Compression

D) Ejection& refrigerate

Hydrateslurry

Filtrate

Figure 3 Operation of the HPM

A) The NGH ratio of the NGH slurry formed in the agitation-type reactor is adjusted to 10 w%.The slurry is pumped into the HPM by a displacement pump. There is a poppet valve at the

end of the compression piston. The NGH slurry is fed into the cylinder when the valve is opened.The slurry pumped into the cylinder is filtered by the cylindrical filtering screen to separate NGH particles. The filtrate is returned to the reactor and circulated for reuse. The NGH particles are

deposited over time on the inner surface of the cylindrical screen.

Due to the flow resistance of the filtrate passing through this particle bed, the pressure difference between the inside and outside of the screen increases. The slurry feed stops upon detection

that the pressure difference has reached a certain value.

B, C) In order to condense the NGH particle bed, the compression piston is advanced to apply mechanical force. As the piston is advanced, the NGH particle bed is condensed and an NGH pelletis formed in front of the piston. As the cylinder volume decreases, the pelletization of the particle

bed progresses and the reaction force to the piston increases.

Since the travel speed of the piston is kept constant by the hydraulic system, the compression pressure can be gradually increased. When the maximum compression pressure is reached (the

compression pressure can be set to any value up to 25 MPa), constant pressure control is maintained.

There are low- and high-pressure hydraulic pumps, which automatically switch over depending on the compacting pressure and the travel speed.

D) The pellet formed at a given pressure opens the shut-off cylinder and is ejected by the compression cylinder into the pellet cooling tank shown at the bottom-left of Figure 4. The pellet cooling tank and the shut-off cylinder are filled with hexane. The pellet is ejected into the hexane.

M

Reactor

Slurrypump

keep pellet under refrigeration

Mix

edga

s

Wat

er

Balance tank

Hexane

Mixed gas

Pellet tank

Oil hydrauliccylinder

Oil hydrauliccylinder

10wt%

filtrate

coverpi ston

circulation of therefrigerant liquid

ball valve

Figure 4 HPM flow diagram

The pressure of the pellet cooling tank increases by an amount equivalent to the volume of the pellet ejected. In order to offset this pressure increase, the pressure is communicated to the gas phase of the reactor via the pressure balance tank

installed above the HPM. The NGH pellet, which has higher density than the hexane, settles in the hexane and piles up at the bottom of the cooling tank.

A small amount of leak water ejected with the pellet is separated by a stainless steel mesh at the bottom the tank. The bottom of the tank is kept cooled to -20ºC and the ejected pellet is

cooled quickly. The HPM outlet temperature is

kept above the freezing point by providing an appropriate distance between the HPM and the cooling tank.

The operating hydraulic pressure at the inlet of the hydraulic cylinder of the compression piston was measured by a pressure transducer. The pellet

compression pressure used was obtained by subtracting the pressure (5.4 MPaG) of the high-pressure vessel from the measured cylinderpressure multiplied by the ratio of the cross-sectional area of the hydraulic cylinder and the cross-sectional area of the hydrate piston.

The piston position was measured (with a magnetostrictive sensor) as the displacement of the hydraulic cylinder rod. During operation, the compression pressure, the piston position and the screen pressure difference were continuously recorded. The flow rate and density of the slurry

on the feed side and the filtrate side were measured by a Coriolis mass flowmeter. The HPMuses a system to perform filtration and subsequent compression in the same container. This system is used for the following reasons:

An expensive high-pressure vessel can be eliminated by performing both processes in the same container.

There is no need for transferring the slurry between equipment, and therefore no need for transfer equipment, requiring only a small

installation area. All processes on the NGH particle bed are

performed in the liquid, making it possible to minimize gas bubbles entrained in the pellet during the condensation of NGH.

The temperature of the machine can be easily

controlled by circulating the liquid, eliminating the need for complicated jacket cooling.

TES T RES ULT

From a mechanical perspective, as a method of compressing hydrate, the process offers an ideal means to rapidly produce pellets with a high NGH ratio at low compacting pressure.

Pellet general view

Figures 5 and 6 show the pellet produced by the HPM. The pellet has a smooth surface and a transparent gray color. It is a 100 × 50 mmcylindrical pellet.

Filtration: Mean filtration ratio resistance

In the filtering process, the 10 wt% slurry from the reactor is condensed to 30 wt% using the discharge pressure of the slurry pump.

Figure 5 Pellet (side view)

Figure 6 Pellet (front view)

For the continuous operation of the HPM, the amount of NGH fed to the cylinder needs to be kept constant. The NGH particle bed filtered from the slurry grows over time inside the cylindrical

filtering screen (the inner diameter side). Flow resistance is generated when the filtrate passes through the particle bed. The amount of NGH to the cylinder is kept constant by monitoring the

pressure difference between the inside and outside (the outer diameter side) of the screen.

Generally, theoretical guidelines for the design and operation of a filter can be explained by Ruth’s filtration theory [6], except for special cases.

The filtration rate q is given by equation (1).

w

Pjq

A

Q N

f

(1)

Q [m3/s]: filtrate volume flow rateAf [m2]: filtration areaq [m/s]: filtration rate

P [Pa]: added pressure difference (inlet/outlet pressure difference)jN [-]: effective filtration area factor (a variable to correct the reduction of filtration area due to the

growth of the particle bed) [7]

μ [Pa･s]: filtrate viscosity

w [kg/m2]: amount of NGH per unit area of particle bedα [m/kg]: mean filtration ratio resistance

Figure 7 shows measurements of the mean filtration ratio resistance using (a) the HPM and

(b) a batch filter prior to the design of the HPM.The mean filtration ratio resistance α [m/kg]

depends on the added pressure difference P [Pa]. As the pressure increases, the cake is compressed and the flow resistance of the filtrate increases. The mean filtration ratio resistance α in NGH is given by the following empirical equation:

55.09100.4 P (2)

As the filtration progresses, the particle bed grows and the filtration resistance increases, resulting in a gradual decrease in filtration rate (constant pressure filtration). The filtration rate needs to be kept constant to minimize the filtration time. In the HPM, a displacement pump is used for

this purpose.With the growth of the filtration particle bed,

the filtration resistance increases and the filtration pressure gradually increases. The displacement pump can keep the flow rate constant, independent of this pressure. As a result, the filtration rate remains constant (constant rate filtration) during

the better part of the filtration process, allowing

the filtration process to be completed in a short period of time.

1109

2109

3109

4109

5109

0.2 0.25 0.3 0.35 0.4 0.5 0.6 0.8 1

Mea

n s

pec

ific

filtr

atio

n r

esi

stan

ce [

m/k

g]

Differential pressure P[MPa]

HPMelement test

Approx.

Figure 7 Relationship between the pressure

difference and the mean filtration ratio resistance

Relationship between the compression pressure and the hydrate ratio

Compression pressure is applied by the

compression piston to further condense the cake, which has been condensed by filtration to an NGH ratio of 30 wt%. Figure 8 shows the relationship between the compression pressure and the hydrate ratio, obtained by the HPM.

0

20

40

60

80

100

0 5 10 15 20 25 30

NG

H r

atio

[ w

% ]

Piston under pressure [ MPa ]

LNG CH4

Figure 8 Relationship between the compression

pressure and the NGH ratio

For both methane and NGH, the NGH ratio increases with increasing compression pressure,making it possible to produce pellets with a high purity of more than 90%. The optimal compression pressure differs between methane and

NGH. This is probably due to the difference in hydrate grain size.

The NGH ratio tends to decrease at 9 MPa or higher for methane, and at 17 MPa or higher for NGH. This is probably due to the involvement of phenomena resulting from the compression

process, such as the friction between the particles, the plastic deformation of the particles, and the heat generated by the compression of the gassurrounded by the NGH particles.

This mechanism needs to be explained and quantified in future studies.

Resolution properties of the pellet

Figure 9 shows the dissociation rate of the HPM-produced pellet under atmospheric pressure.The pellet shows a high dissociation rate initially.

Subsequently, the amount of dissociation decreases due to the self-preservation effect. An NGH ratio of more than 70 wt% is maintained, even after the passage of two weeks.

0

20

40

60

80

100

0 100 200 300 400 500

NG

H r

atio

[ w %

]

Elapsed time [ hour ]

LNG-1LNG-2

Figure 9 Dissociation rate of the NGH pellet

CONCLUSION

We have developed a hydrate pellet machine(HPM) using a piston-cylinder mechanism to

condense and pelletize hydrate. The NGH ratio and dissociation rate of the pellet produced by the HPM were more than 90% and less than 0.1%, respectively. Data obtained usin g the HPM will be used to increase its processing capacity for commercial applications, reduce its size, improve

its performance by reducing its power consumption, and reduce its operating costs.

REFERENCES

[1] Kanda H., Uchida K., Nakamura K., SuzukiT .(2005). Economics and energy requirements on natural gas ocean transportation in form of

natural gas hydrate (NGH) pellets, Proceedings of 5th International Conference on Gas Hydrate, Trondheim, Norway.[2] Iwasaki T., Katoh Y., Nagamori S., Takahashi S., Oya N.(2005). Continuous Natural Gas Hydrate Pellet Production (NGHP) by Process

Development Unit (PDU), Proceedings of 5thInternational Conference on Gas Hydrate, Trondheim, Norway.[3] Takahashi M., Iwasaki T., Katoh Y., Uchida K.(2005), Experimental Research on Mixed Hydrate Pellet Production and Dissociation,

Proceedings of 5th International Conference on Gas Hydrate, T rondheim, Norway.[4] Watanabe S., Takahashi S., Mizubayashi H.,Murata S., Murakami H.(2008), A Demonstration Project of NGH Land Transportation System ,Manuscript for 6th International Conference on

Gas Hydrate, Vancouver, Canada[5] Takahashi M., Moriya H., Katoh Y., Iwasaki T .(2008), Development of Natural Gas Hydrate (NGH) Pellet Production System by Bench Scale Unit for Transportation and Storage of NGH Pellet, Manuscript for 6th International

Conference on Gas Hydrate, Vancouver, Canada.[6] Ruth,B.F. Ind.End.Chem. 1935 : 27 :708.[7] Shirato,M.,T.Murase and K.Kobayashi Filter Sep.1968.:5:219.