Technology

CNPC (China National Petroleum Corporation) and its subsidiary PetroChina have independently developed a complete set of technology integrating catalyst design, process engineering, and equipment manufacturing for producing high-quality α-olefin comonomers, specifically 1-hexene and 1-butene.The development of this technology encompasses the entire production chain from catalyst synthesis to final product purification. 

Catalyst System Architecture and Chemistry

The heart of CNPC's flexible comonomer technology lies in its four-component catalyst system:

1. A chromium-based main catalyst serves as the active metal center for ethylene coordination and subsequent carbon-carbon bond formation. 
2. A phosphorus-nitrogen containing ligand coordinates to the chromium center and creates a specific steric and electronic environment around the metal. The ligand design incorporates both phosphorus and nitrogen donor atoms, which work synergistically to control the size of the coordination sphere and influence the mechanism by which ethylene molecules approach and react at the active site, enabling the catalyst to achieve high selectivity for either dimerization to 1-butene or trimerization to 1-hexene depending on the specific ligand structure and reaction conditions employed.
3. A methylaluminoxane (MAO) functions as a catalyst promoter or activator. The ratio of MAO to chromium is carefully controlled to optimize catalyst activity while minimizing aluminum incorporation into the product stream.
4. A halogenated accelerator acts as an electron donor to fine-tune the electronic properties of the catalytic system. 

The catalyst system operates under temperate conditions, typically between 90 and 100 degrees Celsius, which reduces energy requirements for heating and cooling, simplifies materials of construction requirements for the reactor system, minimizes side reactions that can lead to undesired polymer formation or catalyst deactivation, reduces catalyst consumption per tonne of product, and minimizes the burden on downstream separation and purification systems, alltogether reducing operating costs.

Reactor Design and Operating Conditions for Ethylene Trimerization

The trimerization reaction takes place in a specially designed tank reactor that incorporates advanced features to optimize gas-liquid mass transfer and ensure uniform catalyst distribution throughout the reaction volume, addressing the need to efficiently transfer ethylene gas into the liquid phase by incorporating a combination mixing paddle design coupled with a forced circulation system that creates intensive mixing and high interfacial area between the gas and liquid phases.

The reactor operates at elevated pressure, typically in the range of 100 to 110 bar, which serves multiple purposes in the process design:

• The high pressure increases the solubility of ethylene in the reaction solvent to maintain high reaction rates
• It also helps to keep reaction products in the liquid phase, facilitating continuous removal of product from the reactor while maintaining steady-state operation
• The combination of moderate temperature and elevated pressure creates an operating regime that balances reaction kinetics, catalyst stability, and practical engineering considerations

Reaction steps:

1. Ethylene feedstock is continuously introduced into the reactor, where it rapidly dissolves into the liquid phase
2. The four-component catalyst is continuously injected into the reactor at carefully controlled rates, with the individual components either pre-mixed or introduced separately depending on the specific implementation
3. The fast reaction kinetics enabled by the highly active catalyst system and efficient mass transfer prevent the accumulation of reactive intermediates that could undergo further reactions to form polymeric materials.

1- Hexene Recovery and Purification Process 

Following the trimerization reaction, the reactor effluent enters a multi-stage separation and purification train designed to remove catalyst residues, recover unreacted materials, and produce polymer-grade 1-hexene meeting stringent purity specifications:

1. The first stage of this train is a waste liquid water washing section where the organic reaction mixture contacts water to extract soluble catalyst residues and polar by-products, critical for removing chromium species and aluminum compounds
2. After water washing, the organic phase proceeds to a vacuum pumping and solvent recovery system, where pressure is reduced substantially below atmospheric pressure, which causes volatile components including unreacted ethylene, light hydrocarbon by-products, and process solvent to vaporize and separate from the heavier 1-hexene product.
3. The vapors are collected and condensed, with ethylene being compressed and recycled back to the reactor feed while the recovered solvent is purified and returned to the process. 
4. The trimerization product mixture then enters a rough separation stage where it is separated based on boiling point differences, removing the bulk of light and heavy impurities in a single distillation operation
5. The partially purified stream from the rough separation stage then enters the a rectification second stage, which consists of a three-distillation tower arrangement:
   1. The first tower removes light ends including butenes, butanes, and other C₄ and lighter components that were not completely separated in the rough distillation. These light components are either recycled to the reactor if they contain unreacted ethylene or sent to fuel gas depending on their composition.
   2. The second tower performs the critical separation of 1-hexene from heavier oligomerization products including octenes, decenes, and higher molecular weight materials. This tower operates with a high reflux ratio and a large number of theoretical stages to achieve the sharp separation necessary for producing high-purity 1-hexene overhead while recovering valuable heavy products as bottoms.
   3. The third tower in the rectification sequence serves as a final polishing step, removing trace impurities and ensuring that the 1-hexene product meets all specifications for use as a polyethylene comonomer (purity spec > 99.5 %, Chlorine < 1 mg/kg, metal impurities < detection limits).

Flexible Operation Capability and Product Selectivity

CNPC's comonomer technology can flexibly produce either 1-butene or 1-hexene via respectively selective dimerization or trimerization of ethylene, which is achieved through careful control of catalyst composition and reaction conditions, and specific seperation conditions: 

• Catalyst design: These two reaction pathways share common catalytic intermediates but diverge at a critical point in the catalytic cycle where the metallacycle intermediate can either undergo reductive elimination to release product or can insert another ethylene molecule to continue growing the carbon chain. Both use ethylene oligomerization over chromium‑based P–N ligand catalysts activated with MAO.
• Reaction conditions: Operating conditions are adjusted such as reaction temperature, pressure, ethylene concentration, and the ratios of various catalyst components. This flexibility is achieved through what the patent describes as a binary catalytic system. Both are run in liquid‑phase, high‑pressure stirred reactors (tank reactors) with intensive gas–liquid mixing and similar temperature/pressure ranges (roughly 90–100 °C, 10+ MPa class, exact numbers vary by implementation).
• Separation and purtification: Both processes require downstream water‑washing, solvent recovery and multi‑tower distillation to remove catalyst residues, recover solvent and separate the target α‑olefin from lighter/heavier by‑products. Exact tower duties, tray numbers, and cut points would not be identical in a plant configured to make 1‑butene as the primary product.