28 • LNG journal • The World’s Leading LNG publication

PROCESSING

UK’s Gasconsult ZR-LNG liquefaction technologyaims to make impact with new mid-scale solutionBill Howe, Chief Executive, and Geoff Skinner and Tony Maunder, directors, of Gasconsult Ltd.

Most LNG production takes place in

large-scale plants using the Air Products

(APCI) C3MR® and Conoco Phillips

cascade technologies. These plants have

single Train LNG outputs of up to 7.8

million tonnes/year and all have been

built onshore.

Recently interest has developed in

so-called mid-scale LNG production of

up to 2 million tonnes/year, particularly

for exploitation of smaller gas fields

with reserves of around 1 trillion

cubic feet.

Although it is possible to scale down

the large-scale plants for these smaller

applications, the end result is not capital

efficient, and less complex solutions are

being sought.

FieldsMany small fields are offshore, and can

only practically be developed with

floating LNG production units (FLNG).

For offshore installations, operators

are looking for FLNG production

technologies that are more robust and

less complex than existing land-based

installations.

Safety considerations offshore also

favour minimum inventory of

hydrocarbons in the liquefaction unit,

especially of liquid hydrocarbons.

The ZR-LNG™ process has been

developed by Gasconsult Ltd. to meet

these criteria. It is an extremely simple

scheme comprising only 2 compressor

packages and 7 major equipment items.

It has 20 percent lower capital and

operating cost than other mid-scale

liquefaction technologies.

Further, in avoiding the use of liquid

hydrocarbon refrigerants, ZR-LNG™

addresses the safety concerns of offshore

operators.

Studies by Gasconsult indicate that

much of the energy saving of the ZR-

LNG™ process results from the use of

methane-rich gas in the refrigeration

cycle in place of nitrogen.

Factors advantaging the methane-

cycle over the nitrogen cycle are

elaborated below.

Relative capital expenditure and

operational expenditure versus single

mixed refrigerant (SMR) and the dual

nitrogen expander process are provided

in the graphic.

Notwithstanding the focus of this

paper on the nitrogen cycle comparison,

ZR-LNGTM Capex and Opex is also

advantageous compared with SMR

designs for mid-scale projects.

Mid-Scale practiceIntensive efforts have been made to

adapt SMR and dual mixed refrigerant

(DMR) schemes for FLNG and other mid-

scale applications.

However, a marked preference is

emerging for reduction and if possible

elimination of production/import of liquid

hydrocarbon refrigerants offshore.

All LNG production involves large-

scale storage of hydrocarbon in the form

of the LNG product, but higher molecular

weight hydrocarbon refrigerants

particularly propane are seen as more

hazardous, due to a tendency for any

leakage to accumulate in enclosed spaces,

such as between decks.

Due to these safety concerns, there is a

current trend to expander-based

processes using nitrogen for FLNG

applications.

There is also a preference for processes

not requiring distillation columns, which

can be vulnerable to ship motion.

Many nitrogen cycle variants have

been proposed, and the first medium-

scale FLNG application has now been

announced for Petronas in Malaysia, a

1.2 million tonnes/year unit using the

APCI AP-NTM process.

IssuesSignificant issues with nitrogen cycles

compared with mixed refrigerant

processes are higher power requirement

(kWh/tonne of LNG product) and larger

line sizes.

This discourages the use of nitrogen

cycle schemes, particularly for higher

plant capacities. While power demand

depends on local factors, including

feedstock composition and pressure,

ambient air/seawater temperature and

type of cooling (air or water), large base

load plants typically require 300

kWh/tonne LNG of shaft power for the

process compressor drives in the

liquefaction unit. Simpler SMR schemes

proposed for smaller capacity plants

typically fall in the range 360-400

kWh/tonne.

The comparative drive power for a

2-expander nitrogen cycle is 420

kWh/tonne LNG, 40 percent higher than

large refrigerant based plants.

It can be argued that energy efficiency

is unimportant as LNG plants are

usually built in sparsely populated areas

where gas is cheap.

Figure 2: The potential impact on ZR-LNG energy efficiency for shortfalls inexpander, compressor and separator efficiencies

Figure 1: A simplified schematic of one variant of the ZR-LNG™ process fora 1 million tonne per annum plant using the Gasconsult method

Figure 3: Relative Capex and Opex data for the ZR-LNG process based onGasconsult studies

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LNG journal • April 2013 • 29

PROCESSING

However, even with low cost feedstock,

there are compelling arguments for

improving energy efficiency:

� The resulting lower power

consumption reduces

the size of the

compression package

and the capex of the

plant; which in turn

� Reduces the weight

of the plant, an

important offshore cost

consideration; and

� The lower power

requirement reduces

associated CO2

emissions

TechnologyThe need to reduce the

power demand for an

expander-based process

while preserving the

operational simplicity of the

nitrogen cycle has led to the

development of the ZR-

LNG™ process.

In this patented process,

nitrogen is replaced as the

refrigerant by the feed

natural gas itself.

With this development,

the net liquefaction unit

drive power in a temperate

zone application is reduced

to 310-350 kWh/tonne LNG;

depending on the feedstock

composition/pressure.

With its simplicity, low

energy consumption and

low capital cost ZR-LNG™

is suitable for both onshore

and offshore application up

to a capacity of 2 million

tonnes/year LNG.

The ZR-LNG™ process

can operate on a full range

of hydrocarbon gases,

including very lean feeds

containing insufficient C2+

for production of a

hydrocarbon refrigerant.

DescriptionA simplified schematic of

one variant of the ZR-

LNG™ process for a

1 million tonne per annum

plant is provided in Figure

1 on the previous page.

After conventional pre-

treatment and drying, the

feed gas is cooled in the first

part of heat exchange block HX1 to

condense natural gas liquids (NGL),

which are removed in separator SP1.

The vapour overhead from SP1 then

mixes with part of the recycle gas stream,

which has also been precooled in the first

part of HX1.

This combined stream is further

cooled in the second part of HX1 and is

then divided. One portion passes to

partially liquefying expander EXP2,

which discharges into separator SP2.

ConocoPhillips is committed to protecting the environment that we all share. Employed in LNG facilities around the world for over four decades, the

ConocoPhillips Optimized Cascade® Process continues to set new standards in the design and operation of efficient and cost-effective LNG facilities. The technology has strong environmental advantages, including:

• Advanced aeroderivative gas turbines.

• Integrated waste heat recovery.

• Gas and liquid expanders.

• Inlet air cooling of gas turbines.

• Integrated NGL extraction.

• Minimized plot space requirements through equipment modularization.

• Minimized flaring.

Our environmental advantages give you the confidence you need.

All the reasons. All the confidence.

To discover additional reasons why you should choose the ConocoPhillips Optimized Cascade® Process, please contact us at:

Web site: LNGlicensing.ConocoPhillips.com

e-mail: LNGprocess@ConocoPhillips.com

phone: 01-713-235-2127

© ConocoPhillips Company. 2010. All rights reserved. Optimized Cascade is a trademark of ConocoPhillips Company.

p18-32:LNG 3 12/04/2013 15:04 Page 13

30 • LNG journal • The World’s Leading LNG publication

PROCESSING

A second portion is condensed in

the third part of heat exchanger HX1.

This condensate and the liquid phase

from SP2 are both let down into the

LNG product tank. The balance of the

recycle gas is passed through a first

expander EXP1. The outlet gas from

EXP1 is combined with the vapour

phase from SP2 leaving the third part

of HX1.

The mixture, having cooled incoming

feed and recycle gas, then passes to

compressor CP1 at near ambient

temperature, forming the recycle gas.

Energy recovered in EXP1 and EXP2

reduces the net power requirement for

the recycle gas compressor

CP1 by over 35 percent.

A portion of the low

pressure flash gas plus boil-

off gas from LNG storage

may be used as fuel for

example in a gas engine or

gas turbine to generate the

balance of the power

required for the process.

The remaining low

pressure flash gas is further

compressed in CP2 to rejoin

the main flow of recycle gas.

A major advantage of

ZR-LNG™ stems from its

simplicity, a typical plant

comprising only two

compressor packages plus

seven major equipment

items.

As the process has

no external cryogenic

refrigerant cycle with

associated top-up system,

several equipment items

are eliminated, together

with the associated piping,

valves, electrics plus civil

and structural works. This

results in a significant

reduction in complexity and

capital cost, whilst also

securing a thermal

efficiency higher than

nitrogen expander and

SMR cycles.

ProvenequipmentAll equipment included in

the ZR-LNG™ process

scheme is proven in

operation. In developing

ZR-LNG™ performance

data, process efficiency and

operating experience were

confirmed with potential

suppliers of all critical

equipment.

In common with other

LNG technologies, the

compressors in ZR-LNG™

may use the full range of

drivers including electric

motors, gas engines, gas

and steam turbines.

A feature of the process

is the availability of

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LNG journal • April 2013 • 31

PROCESSING

flash/boil-off gas at low pressure. With

the recent development of 4-stroke gas

engines with unit outputs up to 20 MWe,

their use to burn the flash/boil-off gas is

an interesting option,

particularly for smaller

plants and FLNG.

Deployment of multiple

engines to generate electric

power may improve plant

availability compared

to traditional direct

mechanical turbine drives.

Like all processes, ZR-

LNG™ is dependent for

reliable operation on key

equipment performing as

designed. Studies have been

carried out to evaluate the

vulnerability of ZR-LNG™

to non-performance of

critical equipment.

ImpactFigure 2: Indicates the

potential impact on

ZR-LNGTM energy

efficiency for shortfalls in

expander, compressor and

separator efficiencies.

These indicate a robust

tolerance to equipment

underperformance against

performance guarantees

provided by well qualified

suppliers.

Gasconsult has

examined the differences

between methane and

nitrogen as gaseous

refrigerants. Part of the

energy saving of the ZR-

LNG™ process over the

nitrogen cycle is accounted

for at a high level by the

higher molar specific heat

of methane (around 22

percent) and lower

compression power per mol

(5 percent).

The overall explanation

is however more complex

as methane and

nitrogen display differing

characteristics relative to

an ideal gas. Other factors

thus contribute to the

low specific power of

ZR-LNG™ including the

cycle conditions (pressure/

temperature), the

arrangements for

extraction of flash gas as

fuel for compressor drivers and use of a

liquefying expander in the ZR-LNG™

patented configuration.

The overall benefit of the ZR-LNG™

cycle relative to nitrogen was further

investigated by performing comparative

HYSYS simulations.

Several nitrogen cycle variants were

evaluated using the same machine

efficiencies, loop pressure drops, heat

exchanger temperature approaches and

heat in-leakage as the methane-rich

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p18-32:LNG 3 12/04/2013 15:05 Page 15

32 • LNG journal • The World’s Leading LNG publication

PROCESSING

gas expansion in the ZR-LNG™

configuration.

These studies indicate up to 30

percent reduced suction volume and 20

percent less aggregate installed machine

kW for the ZR-LNG™ process.

The net power requirement for the

nitrogen systems was typically 70-90

kWh/tonne higher than for ZR-LNG™.

This analysis is consistent with

published data for dual nitrogen

expander schemes and provides further

verification of the superiority of the

ZR-LNG™ scheme.

ConclusionThe ZR-LNGTM process is positioned

as a simpler lower capital and operating

cost process than both nitrogen expander

cycles and SMR schemes. Its energy

efficiency approaches that of complex

large-scale base load LNG plants.

Relative Capex and Opex data based

on Gasconsult studies are shown in

Figure 3 on page 28.

Relative to SMR schemes, ZR-LNG™

has a lower equipment count, permits

more rapid start-up and has reduced

flaring duty.

Compared with nitrogen expander

schemes, ZR-LNG™ benefits from the

higher specific heat and lower

compression power of methane relative

to nitrogen, and process configuration

benefits; resulting in lower power

requirements and lower capital cost.

Lower recycle gas flow rates in turn

reduce recycle gas line sizes, a significant

issue with nitrogen cycles.

ZR-LNG™ has particular advantages

for FLNG applications including low

weight, low liquid hydrocarbon

inventory and no storage of cryogenic

refrigerants. �

Bill HoweChief Executive of Gasconsult LtdOn graduation (B.Sc. Hons Chemical Engineering) Billworked for two years on major UK construction projectsand subsequently accumulated over 25 years ofinternational experience providing management,engineering and construction services to majorcorporations and government entities. He spent over 20years with the Foster Wheeler Group a leading processplants contractor holding the positions of Director ofSales at Foster Wheeler Energy Limited UK andManaging Director of Foster Wheeler‘s South Africanaffiliate. He has interacted with and delivered majorprojects for clients in the power generation, oil refining,synthetic fuels and environmental sectors. Hisgeographical experience covers Europe, Africa, theMiddle East, South East Asia, China and the US.

Geoff Skinner Director of Gasconsult LtdHe graduated from Oxford with a BSc and an MA inchemistry. From 1958 to 1965 he worked forHumphreys & Glasgow (now part of JacobsEngineering), while taking a postgraduate course inchemical engineering at Battersea Polytechnic. Hejoined Foster Wheeler in 1965. From 1981 to 1986 hewas Technical Director of Foster Wheeler SynfuelsCorporation in Livingston, New Jersey. On his return tothe UK, he worked on a variety hydrogen and syngasrelated technologies. Since 1998, Geoff has acted as aconsultant to several multinational and has registereda number of patents including LNG liquefactionprocesses.

Tony Maunder Director of Gasconsult LtdHe has degrees in Mechanical Sciences and ChemicalEngineering from Cambridge University. After workingwith ICI General Chemicals, he spent 16 years in theengineering contracting industry, finally with FosterWheeler Energy. From 1980 till 1993 he worked for BPResearch and BP Engineering on evaluation of researchprojects, natural gas conversion to liquids, synthesisgas and fuels, including service in Venezuela. From1993 he worked as an independent consultant until theformation of Gasconsult in 2000.

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