Traces of moisture can degrade your Polypropylene quality and poison your catalyst

Polypropylene is one of the most versatile and important thermoplastics. Compared to Polyethylene, it is even stronger, stiffer and harder. Supplemented with additives, many properties like ultraviolet and chemical resistance, thermal stability and flame resistance can be enhanced and used for bottles, toys, casings, furniture, fabrics, and many more. Polypropylene can be produced by three different processes that includes a Ziegler-Natta catalyst. The suspension (“Slurry”) process, the bulk polymerization process and the gas phase process with solid catalyst.

The density, as well as thermal behaviours (crystallization, melting and degradation) of the final product, can be controlled and improved by involving a comonomer like 1-Hexene in the polymerization process.

Moisture must be rigorously excluded because even traces of water may result in the deactivation of the catalyst. Even if the solubility of water in 1-Hexene is very small (Figure 1), dehydration is mandatory.

Figure 1: Solubility of Moisture in 1-Hexene
Figure 1: Solubility of Moisture in 1-Hexene

Depending on the plant size, the dehydration unit for the 1-Hexene comonomere is equipped of several molecular sieve vessels. The example (Figure 2) shows a two bed unit for dehydration below 5 ppmw.

Once the 1-Hexene passed the first vessel, the moisture is analyzer to detect moisture breakthrough in case that the molecular sieve is exhausted or damaged. After the second molecular sieve, the moisture is again measured in the outlet stream of the dehydration unit. Molecular sieves require a periodical (several days) regeneration with hot gas.

Figure 2: Schematic of a simplified molecular sieve dehydration unit
Figure 2: Schematic of a simplified molecular sieve dehydration unit

Typical Process Conditions:

Composition:            pure 1-Hexene

Temperature:            10 to 40 °C
(50 to 104 °F)

Pressure:                    300 to 600 kPag
                                    43 to 87 psig

Moisture:                    greater than 25 ppmw (feed)
Less than 5 ppmw (outlet)

A reliable trace moisture analyzer is essential for safe and efficient operation:

  • Early detection of moisture break-through
  • Assurance that the moisture at the outlet is below 5 ppmw to protect that catalyst
  • Define the regeneration intervals for process optimization and energy saving

The BARTEC BENKE Hygrophil F is a multi-channel trace moisture analyzer with an extremely robust but accurate moisture sensor. The standard calibration down to a moisture content of 0.05 ppmw is usually validated by an independent and accredited laboratory. Inline or at-line installations are possible with sensor retraction armature or sample conditioning systems.

They HYGROPHIL F is designed for the needs of the oil, gas and chemical industry

The core element of the moisture sensor is an optical thin film element made of silicon dioxide and zirconium dioxide. The microporous stack of alternating high and low refracting optical layers is forming a Fabry-Pérot Interfermoter with a distinctive and reliably detectable reflection minimum in the spectral range around 820 nm.

Selective to water, molecules can diffuse into the porous Fabry-Pérot element, which then performs a shift of the reflection minimum in proportion to the actual water vapour pressure. The spectral shift is detected and evaluated with a compact high-resolution polychromator, which is located in the evaluation unit together with the light-emitting diode. The interconnecting fiber optic cable can be up to 800 m long because of the detection of an optical minimum instead of an intensity change.

The evaluation unit processes and calculates the present dew point temperature as well as other units such as water vapour pressure, parts per million, mg/m3 and furthermore. Several analog outputs, MODBUS, PROFIBUS RTU and via TCP/IP as well as relay contacts are equipped as standard for interconnection to the control system.

Figure 3: Schematic of the Measurement Principle
Figure 3: Schematic of the Measurement Principle

Thousands of installations worldwide prove the advantage of this measurement principle:

  • Robust sensor construction for outstanding long-term stability
  • Sensor validation at process conditions by an accredited and independent laboratory on request
  • Easy sensor cleaning and almost no maintenance required
  • In-line installation with sensor retraction armature possible (fast response and emission-free)
  • Flexible on-line solutions with customized sample conditioning systems
  • Certified for safe operation in hazardous area (ATEX, IECEx, CSA, TR CU)


Learn more about the BARTEC HYGROPHIL F 5673 moisture analyzer on our website or call and speak to one of our technical representatives at 1-800-912-9262.