U.K. Nitrogen-removal Plant Starts Up

U.K. nitrogen-removal plant starts up

Michael J. Healy, Adrian J. Finn, Les Halford

Costain Oil Gas & Process Ltd.

Manchester, U.K.

The gas-treatment plant at Connah's Quay, North Wales, U.K., is dominated

by a view of the cold boxes, ground flare, and compressor house.

Connah's Quay Gas Treatment Plant

The £40 million natural-gas-treatment plant at Connah's Quay, North Wales,

U.K., and owned and operated by the U.K. utility PowerGen, started up in

November 1997. It processes up to 200 MMscfd of natural gas, reducing

sulfur compounds and rejecting sufficient nitrogen to make the sales gas

suitable for export to the U.K. National Transmission System (NTS).

This plant is part of projects for processing and commercializing natural gas

from the Liverpool Bay development. It was designed, installed, and

commissioned by Costain Oil, Gas & Process Ltd., Manchester.

Nitrogen is removed by cryogenic separation that employs the Joule-

Thomson principle in a series of integrated distillation columns. This process

was found to be optimal compared with less-sophisticated schemes and is

sufficiently tolerant to CO2 to avoid an upstream CO2-removal step.

Costain undertook the project on a turnkey basis.

Four fields

The Liverpool Bay development (Fig. 1) consists of four offshore fields in the

Irish Sea off the North Wales coast. Although oil is exported directly from the

Douglas platform, natural gas is brought ashore at the Point of Ayr terminal

through a 20-in., 33-km subsea pipeline.

The Point of Ayr gas terminal has a design capacity of 300 MMscfd. The inlet

facilities remove methanol (used for hydrate inhibition), water, and

condensate. The dry gas is then sweetened with an amine-based solvent.

Hydrogen sulfide content is reduced to 3.3 ppm, mercaptans to 35 ppm.

The hydrocarbon dew point of the gas is reduced by a mechanical

refrigeration unit to typical pipeline quality, and the gas is then exported

through a 27-km underground pipeline at 30 bar along the North Wales Coast

to Connah's Quay (Fig. 2).

Connah's Quay is the location of a new 1,430-megawatt, combined-cycle gas

turbine power station. The fuel-gas requirement of 230 MMscfd leaves a

surplus of 70 MMscfd in the pipeline from the Point of Ayr terminal. The gas

required for power generation is routed through an aboveground installation

directly to the power station.

The balance is passed to the gas-treatment plant where it is purified and

compressed before being exported through a 30-in., 3-km pipeline under the

Dee Estuary to Burton Point where it connects into the NTS.

The gas-treatment plant normally expects to process 70 MMscfd but has been

designed to treat up to 200 MMscfd to cater for additional gas if the power

station is operated at reduced capacity.

Process design

While the natural gas is of adequate quality for power generation, it requires

treatment for the NTS. The Wobbe Index is less than permitted because of

the high nitrogen content (up to 11%), and the level of sulfur compounds must

be substantially reduced (Table 1).

A cryogenic process reduces the nitrogen content to less than 5% (Fig. 3).

This process consists of a double distillation-column system with an upstream

preseparation column to give low overall power consumption, a simple

configuration of process compressors, and tolerance to carbon dioxide.

Such triple-column systems have been used previously in the U.S. and

Western Europe for natural gas of similar nitrogen content.1 2 A rigorous

evaluation of alternative process flowsheets concluded that this configuration

was optimum for this particular application.

The heart of the process, cryogenics, requires deep gas drying. This is

achieved by molecular-sieve adsorbers which are sized to remove not only

water but also the sulfur compounds. Mercury is removed from the natural gas

before the cryogenic section to protect the brazed aluminum heat exchangers.

The treated gas from the cryogenic section is compressed to NTS pressure

and exported from the plant.

18-month turnkey

Costain's work included conceptual design and process optimization, basic

and detailed engineering, procurement, construction, and commissioning.

Along with the process plant, the contract included all necessary

infrastructure: firewater systems, control building, power distribution, flare,

sulfur-compounds incinerator, distributed control system (DCS), and process-

safeguarding systems.

The project was undertaken on a fixed-price basis with PowerGen monitoring

progress and performance against agreed "milestones" to ensure that

contract intent was met throughout.

The critical path for the 18-month, fast-track schedule was specification and

procurement of the sales-gas compressors and the design and fabrication of

the cryogenic cold boxes.

The selected compressors were multistage centrifugal types. From both an

economic and environmental standpoint, electric drives were selected.

Costain's previous experience with such machines on hydrocarbon service

and process development undertaken before contract award enabled

procurement of these machines within the first 6 weeks of the contract

program.

The cold-box designs required careful study early in the basic engineering of

the project to ensure constructability was adequately addressed. The size and

configuration of the heat exchangers, columns, separators, and pipe work

required detailed evaluation to ensure the most cost-effective overall solution.

Transport and lifting limitations were considered in detail because of the

restrictions of the site layout and limited access constraints.

To maintain project schedule, hazard and operability studies were completed

early with a "design freeze" implemented by the fourth month of the schedule.

This was key in ensuring the project schedule could be met at a controlled

cost.

To afford maximum time for civil works and to minimize site-construction work,

it was agreed that the plant should consist of several preassembled modules:

12 modules whose total weight exceeded 1,000 metric tons. The largest

modules were the two cold boxes, key in ensuring satisfactory operation of

the cryogenic process.

These were also critical items to the schedule. With overall shipping

dimensions exceeding 49 m x 5.5 m x 4.5 m, transport and lifting logistics

required detailed planning.

Use of a multidisciplined three-dimensional electronic model on plant-design

management system (PDMS) reduced design detailing man-hours and

enabled model reviews to be conducted rapidly, thus ensuring good layout,

tight bulk-materials control, and error-free construction drawings (Fig. 4).

These were all factors in ensuring effective "single pass" design and

construction in the least amount of time. Field and module rework was kept to

an absolute minimum and represented less than 2% of costs with no program

slippage.

The use of PDMS is especially valuable for cold-box design. It simplifies

hydraulic analysis, enables layout to be generated and quickly assessed, and

often results in reduced box size and cost (Fig. 5).

A reliability, operability, and maintainability (ROM) analysis of the complete

plant was performed during the detailed-engineering phase with an in-house

program linked to a database.

This step helped to target those plant areas with low availability so that

solutions could be found to meet overall plant availability criteria. Overall plant

availability is designed to exceed 98%.

Cold-box modules

The two cold boxes, shop-fabricated near the construction site, comprise

complete package units in which aluminum plate-fin heat exchangers,

distillation columns, separators, piping, and instrument-sensing elements are

housed. The housing is a carbon steel frame, clad with plate (Fig. 6 ).

The equipment inside the cold boxes is a combination of aluminum and

stainless steel. Exchangers are of aluminum alloys; vessels and distillation

columns are of stainless steel. Cold equipment such as plate-fin exchangers

are supported on stainless steel beams that are insulated from the carbon-

steel frame via heat-resistant supports.

Similarly, heat-resistant supports are used between the carbon-steel frame

and vessels mounted from the cold-box floor. The cold boxes stand on

elevated piers, ensuring an air gap between the base of the box and the

ground.

Piping between equipment is mostly of stainless steel with some aluminum.

All piping connections are welded to prevent leakage from the process into

the cold-box housing. Changes in pipe material from stainless steel to

aluminum are made with proprietary design bimetallic transition pieces

especially developed for the cryogenic industry.

All lines passing through the cladding are of stainless steel or similar material

and terminate with external flanges. Cold lines that penetrate the cladding

have specially designed and fabricated thermal shunts to protect carbon-steel

plate from brittle fracture.

Insulation of the cold equipment is achieved by filling the internal void of the

cold box with free flowing nonflammable expanded perlite after erection at

site. To ensure a safe atmosphere in the cold-box housing, the internal void is

continuously purged with dry nitrogen.

The cold-box housing is provided with a purge/pressure-control vacuum break

system and protected by an emergency relief manway in case of process

leakage.

Gas drying

Fig. 7 shows the gas drying and sulfur-removal section.

Molecular sieve is in three internally insulated vessels in a two-duty/one-

regenerating arrangement. Water is removed to less than 1 ppm to prevent

ice forming in the downstream cryogenic process. Hydrogen sulfide is

removed to less than 1 ppm and total sulfur to less than 15 ppm to achieve

sales-gas specification.

The molecular sieves are regenerated with hot waste nitrogen from the

cryogenic section of the plant to desorb water and sulfur compounds that are

then destroyed in a thermal oxidizer. The resulting sulfur dioxide is dispersed

to atmosphere through a 45-m stack.

The design of this system was fully evaluated against the Best-Available-

Technology-Not-Entailing-Excessive-Cost requirements of the U.K.

Environmental Agency.

If, for any reason, one of the molecular-sieve vessels is unavailable, the

control sequence enables plant operation to continue with two beds to ensure

high availability.

The dry sulfur-free feed gas from the molecular sieves containing 8-11%

nitrogen passes to the cryogenic nitrogen-removal system (Fig. 8) which

produces a natural-gas stream containing less than 1.5 mol % nitrogen and a

nitrogen-rich off-gas containing less than 1.5 mol % methane.

The depth of nitrogen rejection allows approximately 30% of the feed to

bypass the cryogenic process and be sent directly to sales-gas compression,

the NTS specification allowing up to 5% nitrogen.

The cryogenic process is based on two main separations. In the

preseparation column, the feed gas is separated into a methane stream and a

nitrogen-enriched stream suitable to be passed to the downstream double

column system.

The preseparation column allows more than 50% of the methane to be

separated and recovered at higher operating temperatures (-100? C.) than the

downstream double-column system. This reduces the work of separation,

hence minimizing sales-gas compression power,3 and it also increases

tolerance to the carbon dioxide in the feed gas.1 4 5

Costain Oil, Gas & Process6 also applied this concept for the British Gas

North Morecambe terminal farther north on the English coast.7 The

preseparation step upgrades the nitrogen level and reduces the feed rate to

the double-column system, which further improves the efficiency of the overall

cryogenic process.

The associated reduction in column size and exchanger size for the double-

column system reduced cold-box size and cost.

In the low-temperature, double-column system (Fig. 9), the nitrogen-enriched

gas is separated (lowest temperature -185? C.) into the reject-nitrogen stream

containing less than 1.5% methane and a low-pressure methane stream. This

stream is pumped to 10 bar and passed to the sales-gas compressors.

A common methane reboiler/nitrogen-reflux condenser links the two columns

operating at 25 bar and 1.5 bar, respectively. This double-column system

resembles that employed extensively in oxygen production and avoids the

need for complex heat pumps and excessive machinery.8 9

The Control of Industrial Major Accident Hazard regulations apply to the site,

and the plant has operated in accordance with the Integrated Pollution Control

requirements of the 1990 Environmental Protection Act.

This requirement dominated much of the thinking behind the process and

engineering design.

Helium removal

The natural-gas feed to the gas-treatment plant contains up to 600 ppm

helium. Although insufficient for economical recovery, the helium must be

removed to prevent it blanketing the reboiler/condenser in the double-column

system.

A refluxing heat exchanger10 takes a vapor stream from the lower column

condenser, using refrigeration from the rich liquid stream to condense out

nitrogen and leaving a helium-rich purge stream.

The refluxing heat exchanger has the equivalent of several equilibrium stages

and provides a 30% rich helium stream, which is passed into the reject-

nitrogen stream.

The reflux-exchanger action resembles the upper part of a distillation column

but has two distinct advantages over a column:

1. The temperature difference between the feed stream and the stream

providing refrigeration is small, thus giving high efficiency.

2. The exchanger has in effect a large number of partial-condensation

stages, so that temperature and composition differences between

vapor and liquid are small and separation is effected near to

equilibrium conditions.

Capacity control

The turndown requirements for the unit were to be able to run at any capacity

between 30 MMscfd and 200 MMscfd and to be able to move between plant

capacities at a ramp rate of 2 MMscfd/min.

Facilitating this requirement were the following design features:

Compression. The main compression system consisted of four

machines, each having a capacity of 25%-that is, 50 MMscfd-with each

motor rated at 4.1 megawatts. These compressors can deliver between

55 and 75 barg, depending on NTS pipeline pressure.

As the plant throughput increases, the suction pressure to the machine rises

and the control system would automatically start another compressor. As the

plant throughput decreases, the sales-gas flow would decrease and the

control system would automatically stop one of the compressors.

Molecular sieves. At minimum feed-gas nitrogen content-that is, 8%-all

of the reject nitrogen produced by the cold box is required for mol-sieve

regeneration.

To ensure there was always sufficient reject nitrogen for regeneration, the

molecular sieves ran on a variable on stream time, depending on the feed gas

flow rate. The on stream time was considerably extended during turndown

conditions based on flow totalizers, which automatically ensured complete

regeneration. This approach also reduced overall regeneration-energy needs.

Process-control system. Flow rate to the gas-treatment plant is

controlled; any change in the required flow is input to the DCS.

By application of "feed-forward control," the DCS then increases the feed flow

at the maximum ramp rate allowed and adjusts the set points of all cold-box

controllers to the values required for the new feed flow rate.

The main reason for this approach is the relatively rapid ramp rate and the

need to maintain process temperatures and liquid levels. Some controllers

use temperature set points cascaded onto flow controllers.

As temperature controllers are normally slow acting, feed-forward control

ensured that the temperature controller did not retard the ramp rate.

Construction strategy

The bulk of the equipment arrived at site as preassembled modules. This

ensured much of the fabrication work was performed in controlled workshop

conditions, thus enhancing quality control.

This approach also minimized schedule risk (associated with working in a

coastal environment) and maximized the time available at site for completion

of civil and building work before onset of mechanical hook-up.

Preassembled units included:

Methane preseparation column. This column was trayed, insulated,

fitted with ladders, platforms, external pipe work, instrumentation, and

lighting, and transported to site. With this approach, any high-level

work was minimized, and the column in its fully dressed state was

complete within days of delivery to site

Preassembled pipe racks. These racks were fully fabricated, fire

proofed, loaded with pipe work, painted, insulated, and tested before

shipment to site. Laser alignment, with particular attention to site

surveying of the preformed concrete-support piers, ensured hook-up of

these units without need for any rework on site and minimum amount

of scaffolding.

Adsorber and suction-drum control and switching valves. These valves

were connected complete with all necessary pipe work,

instrumentation, insulation, platforms, ladders, and lighting.

Filters and filtration equipment. This equipment includes associated

piping, platforms, ladders, instrumentation, and lighting.

Such preassembly dramatically reduced the time for mechanical and electrical

field construction compared to industry norms. Commencement of module

installation to mechanical completion required less than 10 weeks.

Because of the overall sizes of the heating medium heater, ground flare, and

thermal oxidizer, these items were site assembled. They were contracted as

turnkey packages from the proprietary equipment suppliers.

The control of site construction work was especially important because the

plant location, adjacent the River Dee, is a "site of special scientific interest";

strict environmental conditions controlled all stages of the project including

minimization of emissions and site and plant appearance.

Approximately 100,000 man-hr of engineering work and project management

were expended on the project in addition to almost 500,000 man-hr in

construction and module assembly. The site work force peaked at

approximately 300.

The plant was built in full compliance with U.K. Health and Safety Executive

Construction and Design Management regulations and has undergone

considerable monitoring to ensure these new regulations are adhered to.

In scope, construction work was managed under the U.K. National Agreement

for the Engineering Construction Industry, the site being nominated as part of

the overall power-plant construction project.

References

1. Streich, M., "N2 Removal from Natural Gas," Hydrocarbon Processing,

April 1970, p. 86.

2. "Ethane and helium recovery," Hydrocarbon Processing, April 1984, p.

66.

3. O'Brien, J.V., and Maloney, J.J., "Continuous improvement in nitrogen

rejection unit design," Hydrocarbon Engineering, September 1997, p.

68.

4. Rathmann, W., and Grimm, P., "Separation of NGL, Nitrogen and

Helium from Natural Gases and Associated Gases," Linde Reports on

Science and Technology, p. 14, Vol. 34, 1982.

5. Boocock, R., and Trautmann, S., "Nitrogen Rejection Units in Natural

Gas Processing," Gas Processors Association European Chapter

Meeting, London, Sept. 26, 1991.

6. Finn, A.J., and Kennett, A.J., "Separation of nitrogen from methane-

containing gas streams," U.K. Patent No. 2208699.

7. Mayer, M., and Crowe, T., "The North Morecambe Onshore Terminal,"

Gas Processors Association European Chapter Meeting, London, Sept.

28, 1995.

8. Limb, D.I., "Poland's Natural Gas Will Fuel Major Helium Buildup,"

Chemical Engineering, December 1974.

9. Duckett, M., and Ruhemann, M., "Cryogenic Gas Separation," The

Chemical Engineer, December 1985, p. 14.

10.Finn, A.J., "Enhance Gas Processing with Reflux Heat Exchangers,"

Chemical Engineering, May 1994, p. 142.

The Authors

Mike Healy is process engineering manager at Costain Oil, Gas & Process

Ltd., Manchester, U.K. Previously, he worked for M.W. Kellogg Ltd., London.

Healy holds a BS in chemical engineering from Nottingham University and is a

fellow of the Institution of Chemical Engineers and a chartered engineer in the

U.K.

Adrian Finn is chief process engineer with responsibility for process

development at Costain Oil, Gas & Process. He holds a BS Tech in chemical

engineering and fuel technology from Sheffield University and an MS from

Leeds University. He is a fellow of the Institution of Chemical Engineers and is

a chartered engineer in the U.K. and a member of the Gas Processors

Association.

Les Halford is operations director at Costain. He has a B.Tech in chemical

engineering from Bradford University and an MS in construction management

from Cranfield Business School. He is a member of the Institution of Chemical

Engineers and is a chartered engineer in the U.K.