Chemical and Process Engineering
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Huntsman LDPE Project
Low Density Polyethylene										Client: Simon-Carves
2005/6	Senior Process Engineer
Project Background
SembCorp Simon-Carves Limited were initially awarded a contract by Huntsman Petrochemicals UK Ltd (now Sabic), for the front end engineering design (FEED) of the LDPE process using ExxonMobil’s technology, They were subsequently awarded a further contract for the detailed design, supply and construction of this plant at their Wilton complex in the North East of England. This is the world’s largest single stream LDPE plant, with a total capacity of 400,000 tonnes per year.
Summary of Process			Click here for a simplified diagram of the process.
The process (from ExxonMobil) is based on a high pressure tubular reactor. Ethylene is supplied to the plant and compressed using primary and secondary compressors to a pressure of around 3000 bar.
Courtesy Simon-Carves				The compressed Ethylene gas is preheated and passed through a number of tubular reactor zones (total length nearly 3km) into which organic peroxides, dissolved in a straight chain saturated hydrocarbon, are injected to initiate the reaction. An unsaturated hydrocarbon can be added to the ethylene as a modifier to control the melt index. An organic aldehyde can also be used as a modifier for the production of medium density polyethylene. The reactors are double pipe units with pressurised hot water flowing in the annuli. The polymerisation reaction is highly exothermal and the energy removed from the reaction is converted in to steam at various pressures for use within the LDPE plant and elsewhere on complex. Excess unused ethylene is separated after the reactors and recycled to the reactor feed system. The polymer melt is mixed with master-batch additives in an extruder to yield the final product. Some ethylene is carried forward through the extruder and the pellets are degassed with air during intermediate storage prior to being sent to final product storage.
Specific Responsibilities
Assessment of utility requirements at FEED stage. Pressurised hot water system for reactor temperature control. Boiler feed water preparation Distribution and return of utilities. Battery limits co-ordination.
Pressurised Water System
The purpose of pressurised hot water system is initially to bring the reactor metalwork up to running temperature and then to remove the immense quantity of heat generated by the reaction (c 50MW). The system also allows controlled de-fouling of the reactors. The high operating temperature of the process requires the water to be pressurised to around 30 barg to avoid vaporisation occurring especially at the hot reactor walls. This required the specification of pumps, heat exchangers, control valves etc serving 6 reactor reaction zones and associated cooling zones, recycle gas cooling and as well as extrusion temperature control. A 50 m^3 expansion drum also formed part of this system.									Courtesy Simon-Carves
Utility Systems
The detailed design of the utility systems covered steam, condensate, cooling tower water distribution, raw and process water, plant and instrument air, high and low pressure nitrogen, boiler feed water and silo wash water systems. For use within the plant, imported steam (up to 65 barg) from the Wilton power station is let down to high pressure (32 barg) using enthalpy control. The pressure is then subsequently let down to medium (10 barg) and low (3.5 barg) pressures. Condensate from these systems is recovered at different pressures to minimise the amount of flash steam formed in the condensate pipe work. The plant also has steam and condensate switching station to allow the use of either MP or LP steam for jacketing and tracing. For operations that produce excess steam, a direct contact condenser was specified, with the resulting condensate being used as cooling tower make-up water. The boiler feed water system required the specification of de-aerator and boiler feed water pumps. After de-aeration of recovered condensate together with demineralised water imported from the power station, it is chemically treated for use in process waste-heat boilers, steam de-super-heaters and for make-up to closed circuit water systems. Liaison with the plant Owner was required in relation to battery limit and water treatment requirements.
Documents Produced Battery limits schedules. Process descriptions. Utility demands. P&IDs Control system narratives. Process calculations and data sheets for centrifugal and reciprocating pumps, shell and tube heat exchangers, pressure vessels. Specialist Software "InTools" for creating process specifications for instrumentation. "PDMS Reality Review" for 3D plant layout visualisation. "TASC5" for shell and tube heat exchanger design.							Instrument data sheets for pressure, temperature, flow and level transmitters. Specifications for steam pressure reducing and de-super-heating stations. Calculations in support of instrument and pressure regulator specifications. Calculations for pressure drop, line sizing and control valve size checks. Process duty steam trap specifications.