A good mix of positive material properties - moisture and chemical resistance, best tensile strength and high flexibility at the same time - ensure a continuously increasing demand. LDPE (low-density polyethylene) is now an integral part of everyday life and guarantees, through solid packaging, the safe transport of food all the way into the fridge in your kitchen. The thermal polymerisation and conversion of gaseous ethylene into polyethylene in granular form takes place under pressures of up to 3,200 bar, temperatures of up to 300 °C and the addition of catalysts.
The production process takes place in 5 steps:
In addition to the tube reactor process visualised here, the autoclave reactor process is used as an alternative, with both processes using WIKA measurement technology solutions in a variety of forms.
In order to achieve a consistently high throughput with constant quality, the operators are dependent on maintaining several critical process parameters - pressure, temperature, flow rate, level - in the respective plant sections.
In gaseous form, fresh ethylene is mixed with recycled ethylene and the gas is compressed to 250 - 300 bar while adding initiators. The process pressure is then increased in several steps to 1,800 … 3,200 bar using a hypercompressor. The required final pressure is determined by the respective reactor concept. The heat generated during compression is removed in a systematic manner using high-pressure intercoolers.
To ensure that the desired polymerisation is started and proceeds in a controlled manner, the media temperature and media pressure at each stage must be precisely matched. In particular, fast-responding temperature sensors and pressure sensors help users to monitor compressors and coolers safely and efficiently. The special design of the sensors means they can withstand pressures of up to 6,000 bar.
Highly vibration-resistant temperature sensors monitor the correct bearing temperature on the compressor, which is subjected to heavy stress throughout.
After the desired media temperature in the preheater has been set precisely, polymerisation takes place in several steps in the high-pressure tubular reactor, which can be up to 4,000 metres long, at pressures between 1,800 ... 3,200 bar and temperatures between 180 ... 320 °C.
By selectively adding several catalysts (co-monomers) in several successive tubular sections at different temperature levels, long-chain (and at the same time widely branched) polyethylene polymers are produced from individual monomers. The desired quality of the end product can be specifically adjusted by precisely dosing the composition and quantity. Pressure monitoring of the closely controlled high-pressure injection pump plays a critical role.
Similarly, media pressure and media temperature at the inlet and outlet of the tubular reactor are monitored with particularly fast responding sensors. They are available in pressure ranges of 2,500 bar, 4,000 bar, 6,000 bar up to 15,000 bar.
Manufacturing and testing processes adapted for high pressures, such as vacuum brazing, X-ray and high-pressure testing, are part of the established processes at WIKA. In the autoclave reactor, on the other hand, the parameters are measured directly in the reactor at several points.
Following polymerisation, the LDPE media stream is decompressed to around 300 bar and cooled down. The high-pressure separator then separates the desired polymer melt from the unreacted ethylene gas components. Approximately 60 % is not converted in a single cycle and is returned to the compressor as recycling gas.
In the low-pressure separator, further separation of cooling oil and wax components takes place. In order to monitor the separation continuously and correctly and also to prevent any possible deposit of the medium within dead spaces, the flush-mounted process transmitter solution, consisting of a tubular diaphragm seal and smart pressure transmitters, with pressure rating up to 600 bar, is used at this point in many LDPE plants.
In an extruder, the polymer melt is processed into small solid granular pellets. During the extrusion process, the remaining ethylene gas components are separated and various additives, such as colouring agents, UV stabilisers, antistatic or fire-retardant additives, are added in order to optimally adapt the plastic for its subsequent uses.
Bayonet-sprung temperature sensors, with constant contact pressure, are used to monitor the heating temperature. In the subsequent process step, the moisture of the pellets is reduced to minimum residual moisture by means of hot air drying systems or dehumidification systems, depending on the desired residual moisture content.
The LDPE pellets are stored in high silos until they are transported as bulk material or bagged in the desired containers. In order to prevent the accumulation of a possible explosive gas mixture inside, nitrogen is usually injected at several points as a purging gas to remove the outgassing components of the LDPE pellets in a controlled manner.
The constant purge flow is monitored precisely, either using a meter run or a compact measuring orifice with a differential pressure transmitter mounted, and thus contributes to permanent safety in the system.
Consultation, design, implementation – all from one source
Precise calibration instruments are the starting point for resolving your test requirements. However, they only form one part of a high-performance calibration system. From our extensive product range, we can design a complete and individual solution for you which contains all the relevant components: with adaptability for test items, pressure and vacuum supply, components for pressure control and fine adjustment, through to voltage supply and multimeters for the calibration of electrical test items.
Our particular strength lies in the project planning, development and the building of complete, individual, application-specific systems – from simple manual work stations through to fully automated test systems in production lines.
Calibration technology and calibration services
Are you looking for suitable calibration equipment for your applications? Get an overview of our wide range of calibrators. Use our calibration service for pressure, temperature, force, flow and electrical measurands. We calibrate your references and test equipment independently of manufacturers in our accredited calibration laboratories, or directly on your premises.
WIKA will support you with additional services from our trained experts. Service technicians will make sure that your measuring instruments, in case of breakdowns and repairs, are fully functional again in a short space of time. From pressure controllers and diaphragm seal systems to calibration baths - all from one source. We install your measuring units and provide support for the commissioning of the instrumentation. Through our local experts, we can be reached worldwide, are quickly available and tuned to individual circumstances. Try it for yourself.