Natural gas can contain appreciable amounts ofinert nitrogen. For nitrogen content between 8 - 50mole% it is often economical to remove this nitro-

gen to increase the heating value of the gas in order tomeet pipeline contractual calorific value specification andto reduce gas compression requirements.

A 275 MMSCFD facility processes natural gas fromthe Bhit field, Pakistan, removing carbon dioxide, nitro-gen and hydrocarbon condensate to upgrade the gasquality for export into the national transmission system.The plant is operated by Eni Pakistan, with joint venturepartners being: Kirthar Pakistan B.V., Oil and GasDevelopment Corporation and Premier Kufpec Pakistan.

The heart of the plant is a cryogenic nitrogen rejectionunit (NRU), which was designed, supplied and commis-sioned by Costain Oil, Gas & Process Ltd, Manchester,U.K. Costain nitrogen rejection units have been installedin Europe, North Africa and Asia processing up to 300MMSCFD of natural gas and feed contents of 8 - 55%nitrogen.

The Bhit plant makes use of proven cryogenic processtechnology. The plant is highly efficient, has low powerconsumption and has a simple machinery configuration,which optimised the overall capital cost. The plant wassuccessfully commissioned in 2003 and provides reliableand efficient operation at up to 110% of design capacity.The Bhit NRU is the first plant of this type in Pakistanand at the present time is the largest in Asia.

Bhit field developmentThe Bhit gas discovery is part of the Kirtharexploration concession in Sindh Province,Pakistan. The natural gas contains approxi-mately 20% nitrogen and is therefore of a rel-atively low calorific value.

The processing facility (Figure 1)receives sulfur free saturated gas from theBhit field wells, removes condensate andwater in a common reception unit, thenprocesses the gas in two trains. Theprocesses include pretreatment, nitrogenremoval and export gas compression beforeexport into the Indus Right Bank pipeline (Sui toKarachi), part of the national transmission sys-tem. The pretreatment sections ensure the gas issuitable for cryogenic nitrogen removal through thefollowing major unit operations:

A boost fromN2rejection

Rupert Millward and Brian Dreaves, Costain Oil, Gas & Process Ltd, UK, present a casestudy on anitrogen rejection unit installed at the Bhit field inPakistan for the production of high quality natural gas.

Reprinted from HYDROCARBON ENGINEERING JULY 2004

� Carbon dioxide removal to 50 ppmv using DEA.� Complete water removal on regenerable molecular

sieve.

Each train was designed to deliver 135 MMSCFD ofexport gas from a feed with nitrogen content between 16 -20% (approximately 845 Btu/ft3 gross calorific value). TheNRU processes approximately 100 MMSCFD of feed gaswith the rest bypassing the cryogenic unit direct to exportcompression. The overall gas is upgraded to a maximum of7.5 mole% nitrogen (950 Btu/ft3).

Project lifecycle commitmentCostain’s commitment began in the project’s infancy with aconceptual design study. The study examined the availablegas analysis, gave a techno/economic evaluation of theavailable processing routes and developed an outline flow-sheet and cost for the gas treatment plant with particularfocus on compression power and the cryogenic heart of thetreatment plant, the NRUs.

The next phase of the project was to develop the projectdefinition and commence the procurement of the long leaditems. A basic engineering package was produced, whichoptimised the export compressor and NRU together for max-imum energy efficiency, reviewed the available compressoroptions and gave recommendations for the optimum com-pressor and prime mover configuration. The NRU flow-sheets and layouts were developed to allow definition of theoverall facility. Early procurement activities for the long leaditems were started such that following sanction, the detailedengineering and supply phase of the project could be exe-cuted in minimum time.

The project was then executed on a 12 month, fast trackcontract to design and supply the NRU. Four NRU coldboxes, 500 t of steelwork, 5000 m of pipework, 1500 valves,instruments and accessories were delivered on time and tobudget. Aquater, a Snamprogetti Group company, was

responsible for the construction activities, whileSnamprogetti Engineering BV was responsible for thedesign and engineering.

The design process made extensive use of a multi-disci-plined three dimensional electronic model on plant-designmanagement system (PDMS) for plant layout, materials def-inition and control and development of fully detailed con-struction packages. This allowed construction of the plantwith a minimum of site technical queries and without directsupervision by Costain. The 500 t of ancillary platforms,which were trial erected in the supplier’s works before dis-patch, were erected without a single site technical query.

Once the plant reached mechanical completion, Costainengineers directed the pre-commissioning effort on the NRUand from the availability of first gas, process engineers pro-vided 24 hour supervision of the NRU startup and operationthrough to successful completion of the plant performancetests.

Costain’s commitment to the project continues to thisday with input to development and debottlenecking schemesto further increase output, which already exceeds specifica-tion by more than 10%.

Nitrogen rejection overviewFor small gas flows of below about 50 MMSCFD, pressureswing adsorption (PSA), semi-permeable membranes ornon-cryogenic absorption can be considered as options fornitrogen removal. However, for larger gas volumes, wherethere is a need to separate nitrogen/methane to an appro-priate purity and recovery, the above processes require rel-atively high power consumption and capital cost. The onlyeconomical process technology for large flowrates is cryo-genic distillation. The power consumption required by acryogenic unit to effect separation is not excessive in com-parison to the power consumption that would be requiredjust to deliver gas to the pipeline system.

The process of designing for nitrogen rejection must con-sider the most cost effective overall process including forfeed compression, pretreatment, nitrogen removal and prod-uct gas compression. There are a range of process designsfor the cryogenic removal of nitrogen from natural gas, whichlead to alternative power requirements and machinery con-figurations. An integrated approach to overall facility designis required, as the most cost effective NRU does not neces-sarily lead to the most cost effective overall scheme whenfeed and product compression are taken into account. Thecost of machinery is a key factor, which requires the processand machinery to be optimised together.

The pre-treatment facilities required for nitrogen rejectionare essentially similar to those required for conventional gasprocessing, which can include removal of carbon dioxide,sulfur, water and hydrocarbon dewpoint control. Deeperremoval may be required to avoid freezing at the coldertemperatures present in the NRU. The removal of heavyhydrocarbons (aromatics in particular) is conventional prac-tice on low temperature gas plants and many systems havebeen installed using lean oil absorption, adsorption and par-tial condensation. Removal is important to ensure that freez-ing does not occur and to improve process efficiency.

Cryogenic nitrogen rejectionBecause of the dominant impact of product compression ontotal plant cost, the cryogenic process cycle must be highlyefficient. For nitrogen rejection units these issues are wellunderstood, which makes the choice of process cycle rela-tively straightforward.

The key parameter for process cycle selection is nitro-gen content. Feed pressure, flowrate and contaminant levels

Figure 2. Nitrogen removal unit and gas compression.

Figure 1.Bhit processing steps.

Reprinted from HYDROCARBON ENGINEERING JULY 2004

are also of importance but it is the nitrogen content whichessentially dictates the cryogenic cycle.

Process selection and optimisation for nitrogen rejectionis an exercise in balancing the cryogenic process efficiency,flowsheet complexity and cost against the cost of compres-sion. The machinery configuration needs to be carefullyaddressed to minimise power consumption.

The capital cost and power consumption of the selectedNRU process is influenced by the feed flowrate to the cryo-genic process. It is conventional practice for a portion of feedgas to bypass the cryogenic process to minimise both fac-tors. This means that the nitrogen level in the NRU producthydrocarbon stream must be reduced below the overallsales gas specification in order that the blended total exportgas is on specification.

The rejected nitrogen stream contains a small quantity ofhydrocarbon (predominantly methane). The hydrocarboncontent of the nitrogen vent stream is dictated by environ-mental and economic criteria and is typically set between 0.5 - 2 mole %. The economic optimum methane level in thevent stream is derived from a relatively straightforward evaluation of revenue loss against capital and operatingcost. Depending on local environmental regulations thisstream is vented, re-injected or incinerated.

Bhit NRU process selectionThe Bhit reservoir pressure declines with time and at somepoint feed compression will be needed upstream of the NRU.It is clearly desirable to delay investment in upstream com-pression facilities but not if operation of the NRU at low feedpressure has a major effect on process efficiency, becausethis will result in a large power requirement and high capitalcost for the export gas compression system. An evaluationwas performed as part of the conceptual study to identify theoptimum NRU feed pressure and feed compression strategy.The NRU feed pressure selected gave a good compromisebetween initial capital cost of the NRU and deferment of feedcompression.

The Bhit process flowsheet was selected following rigor-ous techno/economic screening of alternative schemes fornitrogen/methane separation taking into account the impacton pre-treatment and compression. The selected schemewas the triple column process, which had the advantage of arelatively simple machinery configuration and good energyefficiency, which made it the optimum scheme on cost andtechnical evaluation criteria.

The triple column process takes advantage of the effi-ciency and simplicity of the classical air separation doublecolumn process, which is appropriate for NRUs for high nitro-gen content gases, while being able to handle the lower con-centrations of nitrogen in the Bhit feed gas. The major advan-tage of the double column process is that for a relatively highnitrogen feed content there is no need for a complex heatpump cycle1, 2 which is required for the older single columnprocesses. Instead, machinery is limited to methane pumps,export and feed compression (where required). The triplecolumn process uses the simple expediency of removingsufficient methane from the feed gas in a ‘preseparation’ col-umn to produce a ‘high nitrogen content gas’ for subsequentprocessing in the double column without adding additionalmachinery.

The concept of a preseparation column was first appliedby Costain for the British Gas North Morecambe terminal onthe North West coast of England3, 4 and subsequently for theutility company PowerGen, Connahs Quay facility in NorthWales, U.K5. The preseparation column both upgrades thenitrogen level and reduces the feed rate to the downstream

nitrogen rejection system, which improvesthe overall energy efficiency.

Although the triple scheme was consid-ered the best flowsheet, implementationwas made difficult due to transport con-straints for the remote site. However, minormodifications to the triple column schemeproved suitably attractive compared to thecompeting processes. The modificationsenabled sufficient reflux to be provided tothe nitrogen rejection column while meetingthe required methane levels in the nitrogenvent gas and achieving good energy effi-ciency. The advantages of the triple columnscheme were maintained in that it allowed asimple configuration of process compressors and good toler-ance to carbon dioxide.

Bhit NRU process designThe dry feed gas from the molecular sieves, containing 16 - 20% nitrogen, passes to the cryogenic nitrogen removalsystem, which produces a natural gas stream containingminimal nitrogen, a nitrogen rich offgas containing lowmethane content and a hydrocarbon condensate stream.The depth of nitrogen rejection allows approximately 30% ofthe feed to bypass the cryogenic process and be sentdirectly to sales gas compression.

The feed gas includes some heavy hydrocarbon con-tent (aromatics in particular) which require removal inorder to meet the pipeline hydrocarbon dewpoint specifi-cation and to avoid freezing in the colder sections of thecold box. A partial condensation step was integrated in theNRU and the heavier components are removed and sentto the fuel system or storage.

The cryogenic process (Figure 2) is based on two mainseparations. In the pre-separation column, the feed gas isseparated at relatively warm temperatures into a stream ofmethane of the required quality for export and a nitrogenenriched stream suitable to be passed onto the final nitrogenrejection section. The preseparation column allows a largeportion of the methane to be separated and recovered athigher operating temperatures (-100 ˚C) than the down-stream separation system. This reduces the work of separa-tion and hence minimises sales gas compression power6.

In the low temperature, final nitrogen rejection step

Figure 4. Model of a typical NRUplate fin heat exchanger cold box.

Figure 3. Nitrogen rejection column.

(Figure 3), the nitrogen enriched gas is separated into thereject-nitrogen stream, which is vented to atmosphere and alow pressure liquid methane stream. The pressure of themethane stream is raised by a vertical multistage cryogenicpump and passed to the sales gas compressors.

Compressor selectionA key element in successful NRU design is the optimisationof the NRU product stream conditions to allow economiccompressor selection. In the basic engineering phaseCostain’s engineers worked closely with Eni Pakistan Ltdand compressor suppliers to establish the optimum fit andcompressor configuration. The task was to select compres-sors which could achieve the process requirement for theNRU whilst being sufficiently flexible in design and operationto achieve the variable discharge pressure requirements andminimise the load on the available power generation systemat startup.

The selected configuration split the compression dutyover two independent multistage API 617 machines withintercooling and electric motor drives. A 2 x 50% capacityarrangement was selected to give the optimum balance ofreliability, flexibility, operability and installed cost.

Cold box modules The cold box is the key unit of a cryogenic process. Wheretransport restrictions allow, they are shop fabricated modulesin which the plate-fin exchangers, distillation columns, sepa-rators, control valves and temperature instruments arehoused (Figure 4). The housing is a carbon steel frame, cladwith carbon steel plate.

The equipment inside the cold box is constructed of alu-minium or stainless steel. The plate-fin heat exchangersare of aluminium alloys whereas the columns and separa-tors are of stainless steel. Cold equipment is supported onstainless steel beams, which are insulated from the carbonsteel frame via heat resistant supports.

All piping connections are welded to ensure no processleakage can occur within the cold box. Changes in pipematerial from stainless steel to aluminium are made utilis-ing proprietary fusion-welded transition joints developedspecifically for the cryogenic industry.

Insulation of the cold box equipment is achieved by fill-ing the internal void with free flowing expanded perlite aftererection at site.

For the Bhit project, the logistics of transporting the coldbox modules to the job site presented a severe challengeto the design and construction teams. After a detailedreview, the largest size that could be safely transported wasspecified at 20 m x 5 m x 5 m.

NRU cold boxes utilising the triple column process aretypically 30+ m tall, and so an engineering review of all-available options was carried out. Consideration was givento building the cold boxes in situ, as is the practice for majorair separation plants. However the extreme location,scarcity of skilled resources for the critical fabrication of alu-minium components and the requirement for absolutecleanliness made this unattractive. The option selected wasto adjust the process specification and layout of the coldboxes in order to achieve the given space envelope.

A design was adopted that featured two cold boxeslinked by a number of cross platforms. All control valvesand major process lines were designed external to the coldbox to maximise the space available for process equipmentwithin the cold box envelope. A highly integrated designwas selected for the main feed exchanger, which utilisedthe supplier’s maximum furnace capacity. The exchangerhas some 22 process connections and weighs 35 t.

Plant startupCostain’s experienced commissioning and process engi-neers played a vital role in supporting the Eni Pakistan pro-duction team in the successful plant start up. As the plantneared mechanical completion, a commissioning engineerwas on site to guide the construction contractors throughthe requirements for drying and cleaning a cryogenic plant.From the availability of first gas, process engineers were onhand to supervise the Eni Pakistan production team duringstart up to bring the NRU up to required throughput. Theengineers remained onsite to safeguard production, to pro-vide trouble shooting assistance and for the training of theEni Pakistan production team until performance testing ofthe entire facility was completed.

References1. LIMB, D.I., ‘Poland’s Natural Gas Will Fuel Major Helium Buildup’,

Chemical Engineering, 9 December 1974, pp 80.2. DUCKETT, M., RUHEMANN, M., ‘Cryogenic Gas Separation’, The

Chemical Engineer, December 1985, pp 14.3. FINN, A.J., KENNETT, A.J., ‘Separation of nitrogen from methane-

containing gas streams’, U.K. Patent No. 2208699.4. MAYER, M., CROWE, T., ‘The North Morecambe Onshore Terminal’,

Gas Processors Association European Chapter Meeting, London,September 28, 1995.

5. HEALY, M.J., FINN, A.J., HALFORD, L., ‘U.K. nitrogen removal plantstarts up’, Oil & Gas Journal, 1 February 1999, pp 36.

6. O’BRIEN, J.V., MALONEY, J.J., ‘Continuous improvement in nitrogenrejection unit design’, Hydrocarbon Engineering, September 1997,

pp 68.____________________________________________�

Reprinted from HYDROCARBON ENGINEERING JULY 2004

Figure 6. The highly integrated NRU, with the preseparation column in the foreground next tothe cold boxes and cryogenic pumps.

Figure 5. Gas is processed in two identical trains.The NRU cold boxes dominate the process areas.