Economical structural design of natural gas processing plants

Case study of a butane treatment unit illustrates effective structural design procedures and highlights construction challenges encountered in new and expansion projects.

Osama Bedair

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Article Summary

Natural gas processing separates hydrocarbons and fluids from pure natural gas to produce dry gas suitable for pipeline transportation. Raw natural gas is first collected from oil wells then processed at collection points using separator vessels to remove fluids and other impurities. Gas processing plants convert natural gas to other products such as gasoline, ethane, butane, and propane. In some cases, hydrocarbon materials such as ethane, propane, and butane must be extracted from natural gas prior to pipeline transportation. Fractionation converts raw material into refined products transported using pipelines to various processing units for further processing.

Liquefied petroleum gas (LPG) refers to propane or butane (either separate or in a mix), maintained in a liquid state under specific pressure / temperature within a vessel. LPG is a valuable energy source and is widely used as a chemical feedstock for the petrochemical and agriculture industries. Butane is a hydrocarbon gas (C4H10) that is colourless, odourless, flammable, and can be easily liquefied. Butane is used as fuel for portable stoves/barbecues, a propellant in aerosols, refrigerants, and a feedstock to manufacture ethylene and butadiene, a key ingredient of synthetic rubber. Butane extractions occur in a closed-loop extraction system. These units are closed, devoid of atmosphere, and recover the gas to its original vessel.

Design cycles of natural gas processing plants are complicated and require close interaction between process, mechanical, and structural engineering disciplines. The engineering design is normally staged into several gates that require the owner’s approval for funding. Delays in construction projects due to engineering ambiguities may result in substantial losses in the form of interest on construction loans, management/staff time, and an increase in commodities prices as a result of the continuous inflation of material prices.

Significant work has dealt with chemical and mechanical design improvements compared to structural aspects. Patience and Bockrath1 presented a butane oxidation technique used in circulating a fluidised bed reactor to produce anhydride from n-butane Wu, et al2 presented a study to enhance catalytic performance for butane oxidation. Sáez, et al3 performed experimental tests employing a diesel oil burner to study the combustion process of liquid butane. A dual pumping and injection system was designed to operate with pressures varying from 0.8 to 2.0 MPa. They also performed a feasibility study to modify the combustion technology of diesel oil burners to use liquid butane as an alternative fuel. Yang, et al4 presented a process model that utilises n-butane compounds to extract organic pollutants from contaminated water. Removal efficiencies for hydrophobic pollutants were greater than 90%. Removal of residual butane in treated effluent was achieved by depressurisation, air stripping, and elevating operating temperature. Mokhatab and Poe5 introduced real-time optimisation models in the gas processing industry. The concept enables operating facilities to respond efficiently and effectively to changing feed rates and composition, equipment condition, and dynamic processing economics. Liu, et al6 presented modelling and optimisation tools for the petroleum refinery process. Bulasara, et al7 presented a study to revamp heat exchangers in process plants to evaluate various commercial aspects. Lulianelli and Drioli8 presented a review of recent developments of gas separation technology used in the petrochemical industry and refineries. The review also highlighted the importance of membrane reactors for fuel processing and membrane-based pretreatments and integrated membrane gas separation systems. Other aspects dealing with process design or butane process plants are presented by Gallagher9 and Meyers10. Bedair11-15 addressed various design aspects of structural members used in heavy industry.

Not much information is available on the structural design improvements of natural gas processing plants. Most effort is directed towards process or chemical design aspects. Investigators and plants owners have given barely any attention to improving structural design aspects of hydrocarbon facilities.  Minimal literature has addressed structural engineering requirements of natural gas processing plants. Furthermore, most of the design provisions available in the North American codes of practice16-22 deal with residential structures.

Therefore, it is important to highlight requirements for the structural design of natural gas processing plants. This article presents a butane treatment unit (BTU) case study to illustrate effective structural design procedures and highlight construction challenges encountered in new and expansion projects. The layout of the BTU is presented to describe the facilities used in gas processing. Then, a brief overview of the geometric characteristics of the facility is provided to identify loading mechanisms. Structural design criteria developed for each facility’s transportation, installation, and operation are also discussed. Simplified numerical modelling strategies are described to simulate load transfer and soil/structure interactions. The article also provides recommendations and guidelines for engineers to use in practice.

Butane treatment unit description
The BTU is part of the naphtha hydrotreating facility and consists of:
- Horizontal (HPR) and vertical (VPR) pipe racks
- Pumps and compressor house (PCH)
- Butane reactor (BR)
- Steel modules (SM-01), (SM-02)
- Substation building (SB)
- Control building (CB)
- Heat exchangers, horizontal and vertical vessels
- Underground services, roads, and parking

Note that the layout of the BTU may vary depending on the process design. The layout in Figure 1 was designed to boost refinery production of synthetic light crude oil to 232 mbbl/d. The orange arrow shows the direction of north (N). Two main pipe racks support the pipelines transporting process material from/or to the BTU. These are denoted as horizontal (HPR) and vertical (VPR) pipe racks in Figure 1. HPR supports pipelines and cable trays running in the east-west direction and consists of seven modules. VPR runs from south-north and consists of 13 steel modules.

The butane reactor (BR) is located in the north-west (NW) quadrant of the BTU and is supported by octagonal concrete foundations. The pumps and compressor house (PCH) is located in the southwest quadrant of the BTU. This facility consists of steel framing that partitions each piece of equipment and concrete pad base supports. Mechanical equipment is shown in orange. Steel module SM-01 is placed at the lower half of the BTU and supports heat exchangers, pipelines, and cable trays. Some heat exchangers are stacked to optimise the floor space. Butane steel module SM-02 is located in the northwest quadrant of BTU and supports heat exchangers, pipelines, and cable trays. Columns of steel modules (SM01) and (SM-02) are supported on pile foundations.

The substation building (SB) is shown in yellow and located in the southeast quadrant. This building consists of two elevated floors and hosts critical electrical equipment. The building size and layout are determined by the required interior equipment spacing. The control building (CB) houses critical instrumentation controls and is occupied almost continuously. Plant design criteria were used to determine the spacing between control buildings and process units where explosion potential is high.

The soil profile consists of four main soil layers. The first layer is sandy with pockets of clay, with a thickness of 3.0m. The second layers consists of silty clay with traces of coarse and an average thickness of 5m. The third layer is made up of silty sand mixed with very dense to hard cemented siltstone. This layer is very stiff and considered the bearing layer for pile foundations. Groundwater level was detected at about 3.0-4.0m depth.

BTU structural design criteria
Steel modules were designed for erection, operation, testing, and transportation load conditions. The dead load consists of the total self-weight of structural steel, equipment, permanent fixtures, fireproofing, insulation, fixed partitions, piping, and electrical material. The weight of cable trays, safety ladders, cages, and junction boxes was also taken into account. The grating dead load was approximated using linear load distribution along the runs. The dead loads of pipes with small diameters were converted into an equivalent uniform distribution. The pipe hydro-test load was not applied concurrently with wind or earthquake loads. Equipment dead loads included equipment weight, insulation, fireproofing, permanent fixtures, and attachments. Fireproofing material density was 25 KN/m3. Cable tray weight was approximated using linear load distribution along the supporting beam spans. Concrete paving weight was approximated as 25 KN/m3.

Live loads included temporary/maintenance loads and moveable partitions. Floorplates and grating loads were designed for a live load of 4.8 KPa. Static loads and impact forces at start-up or normal operation were obtained from the mechanical datasheet. Equipment load was extracted from vendor drawings. Wind, snow, and earthquake loads were calculated using NBC (2010). The notional load was added to the sway effects for all load combinations. The translational load effect produced by notional lateral loads at each level was approximated using 0.005 times factored gravity loads contributed by that storey.

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