Chemical Technology August 2015

Chemical Technology • August 2015

issue in cold climate operation.

Figure 6 shows one of the configurations using propane

as an intermediate fluid in LNG vaporisers. The configura-

tion uses two heat exchangers operating in series. The first

exchanger uses the heat of condensation of propane to

vaporise the LNG at the cryogenic temperature range, and

the second heat exchanger uses the sensible heat of sea-

water at the higher temperature range. Since direct contact

with seawater occurs at higher temperatures, freezing of

seawater is avoided. The second exchanger is also used to

vaporise the intermediate fluid propane that is recirculated

to the first exchanger.

In addition to LNG heating, this configuration can also be

used to generate power. In this configuration, high pressure

propane or butane expanders can be expanded in a gas

expander operating in a closed loop Rankine cycle.

The vaporised LNG can also be used to generate power

using a natural gas expander in an open cycle configura-

tion. In most instances, when natural gas is supplied to a

power plant, the gas pressure is lower than the LNG sendout

pressure. Typically, LNG sendout pressure is at 100 barg

or higher, and is required to be letdown to 30 barg at the

power plant inlet. The potential work from the letdown

operation can be recovered using a natural gas expander

for power generation. Sufficient power can be produced to

operate the LNG regasification facility. Power generation

using LNG as the cold heat sink is attractive since power

can be generated without the use of fuel gas or emissions.

References

• Mak, J, Patel, D “LNG Vaporiser Selection Based on Site

Conditions”, paper presented at the LNG 17 Conference,

Houston, Texas, USA (May 30-31, 2013).

• Mokhatab, S, Mak, J, Wood, D, Valalppil, J, “Handbook

of Liquefied Natural Gas”, Elsevier Publishing, October

2013.

Figure 3: Typical Ambient Air Vaporiser

Figure 4: Improved Defrosting on Ambient Air Vaporiser

Figure 5: Glycol-water Intermediate Fluid Vaporiser integration

with different heat sources

Figure 6: IFV LNG Vaporisers in Rankine Cycle

DESIGN AND MATERIALS OF CONSTRUCTION