A new process is born
First application of a new process for producing linear alpha-olefins

Linear alpha-olefins (LAOs) have many applications in the chemical industry. They have a broad range of hydrocarbons of different lengths, based on a single molecule: ethy-lene. Linde, with the Saudi-Arabian company Sabic, has developed a technologically simple and particu-larly economical process for producing LAOs, which is now realized in a commercial plant.

Plastics or plasticizers, polishing agents, detergents or candles: the linear alpha-olefin (LAO) group is the basis for an entire series of products. From the chemical viewpoint, they are linear hydrocarbons having a double bond between the first and second carbon atoms. Thus the double bond is in the alpha position, giving the linear alpha-olefins their name. LAOs are produced primarily through oligomerization of ethylene. Combining two or more ethylene molecules gives hydrocarbon chains of various lengths, always having an even number of carbon atoms:
C4 = 1-butene, C6 = 1-hexene, C8 = 1-octene, C10 = 1-decene, etc.

With the a-Sablin technology, it is possible to produce LAOs particularly economically with a simple reactor design and a new type of catalyst system. a-Sablin was developed by Linde Engineering in cooperation with the Saudi-Arabian company Sabic (Saudi Arabian Basic Industries Corporation). The first world-scale commercial plant is now being built for one of the Sabic subsidiaries.
LAOs are used to produce various products, depending on their chain lengths. Short-chain LAOs (C4 to C8) are used primarily as comonomers in producing polyethylene. Medium range LAOs (C8 to C12) are raw materials for producing synthetic lubricants. LAOs in the range of C12 to C18 are used to produce detergents, and the long-chain LAOs (C18+) can be used directly as lubricants and drilling fluids.
Use of LAOs as comonomers for producing polyethylene has the greatest market share, 50%. Insertion of the LAO molecules into the long molecular chain of polyethylene can intentionally change the physical properties of the polymer, so that polymers with very different product characteristics can be produced. Up to 20% of 1-butene, 1-hexene or 1-octene are used in modern polymerization processes.
As the plastics market has been growing for years, the comonomers are the driving force for the continually growing need for LAOs. The predicted growth rate, 5 to 10%, is above that for all but a very few other chemical intermediates.
As none of the long-established processes are available for licensing, Linde started to develop its own technology to produce LAO in 1993. In the Institute for Chemical Physics (ICP) in Chernogolovka, Russia, Linde found a cooperation partner which already had many years of experience in ethylene oligomerization. After development of the fundamentals of the process, the project was continued after 1997 with Sabic. A pilot plant was built and the technology was further developed to be ready for marketing. Today, Sabic and Linde hold all the rights to the technology, which now has the name a-Sablin.
The cooperation with Sabic had the following objectives:
Optimization of the catalyst system and of the operating conditions
Reactor modeling and reactor design
Construction and operation of a pilot plant
Development of models and tools to scale up the technology.
The development of the a-Sablin technology was concluded in 2001 with the successful operation of the pilot plant and release of the technology for licensing. In 2002, Linde began planning and construction of a world-scale a-Sablin plant for Jubail United Petrochemical Company (Untited), a subsidiary of Sabic. This first commercial plant will be completed in 2006. The value of the contract for the facility to be constructed in Al-Jubail, Saudi Arabia is approx. $200 million.
Basic Chemistry
The addition reaction of olefins to form dimers, trimers, etc., is called oligomerization. If ethylene is used, the products are linear alpha olefins. The a-Sablin process uses a two-component catalyst system: a patented zirconium compound and a commercially available aluminum alkyl as the co-catalyst (Figure 2). The reaction conditions can be kept moderate at 20 to 30 bar and 60 to 100 °C. Chain growth and chain termination occur in the same reactor, so this is a single-step system. Chain growth and termination are determined by the molar ratio of aluminum (Al) to Zirconium (Zr). Thus, it is possible to get the desired product distribution by simply adjusting the amounts of catalyst and co-catalyst. With a high Al/Zr ratio, for instance, one can produce a product mixture that contains 80% C4 to C8 LAOs.
The active catalyst complex is characterized by high activity and selectivity. Up to 20 tons of LAO products can be produced with one kilogram of the zirconium compound.
In contrast to the a-Sablin technology, the established processes operate at substantially higher pressures (up to 200 bar) and higher temperatures (up to 300 °C). Some of them are based on the Ziegler reaction, in which triethylaluminum, for instance, is used as the starting material for the catalyst system. With the other processes, other reaction steps such as isomerization and metathesis are sometimes applied, in addition to the oligomerization reaction, to get the desired product distribution. Like the a-Sablin technology, all the processes operate exclusively in the liquid phase with a solvent, or use the products formed as solvents.
The a-Sablin pilot plant
After intensive laboratory tests, Sabic and Linde started work on the engineering of the pilot plant in 1998. The plant modules were assembled at Linde Engineering and then shipped to Sabic Research & Technology at Riyadh. There, they were incorporated into the existing pilot plant infrastructure. The plant was successfully put into operation in the spring of 2000.
The pilot plant is designed for a maximum reactor throughput of 15 kg/h (including the solvent). That amounts to a LAO capacity of about 50 tons/year. The reactor geometry resembles a representative segment of a commercial reactor, so that it is suitable for testing scale-up methods and tools. Aside from the complete reaction section, the pilot plant has a separation section to fractionate the reaction product into typical product cuts, as well as all recycles (e. g., ethylene, solvent). The ratio of the Zr catalyst to the Al co-catalyst can be varied over a wide range in the reaction section.
Operation of the pilot plant provided ample operating experience on an industrial scale. That involved the following points in particular:
Production and qualification of representative LAO products for specific applications.
Confirmation of catalyst and reactor performance with respect to activity, selectivity, and productivity; determination of the optimal reaction conditions and recycles.
Demonstration of catalyst deactivation and catalyst removal.
Confirmation of the material concept for the reactor section and the separation section.
Tool for trouble-shooting of commercial LAO plants.
The LAO reactor concept
Oligomerization of ethylene to linear alpha-olefins occurs by homogeneous catalysis in a bubble column reactor, with the solvent and the catalyst components, which are also liquid, fed into its 2-phase layer (Figure 3). Ethylene is introduced via a gas distribution system to the bottom section. The long-chain alpha-olefins and the solvent are removed from the reactor by means of a side drawoff from the two-phase layer. The short-chain alpha-olefins and excess ethylene leave the reactor through the reactor overhead, corresponding to the thermo-dynamic phase equilibrium. Part of the alpha-olefins formed and part of the solvent are condensed in an integral condenser and serve as internal reflux. The ethylene feed to the LAO reactor provides the raw material for the oligomerization reaction, and provides intense mixing of the 2-phase layer. As the oligomerization is a strongly exothermic reaction, the ethylene also acts as a cooling agent to remove the heat of reaction from the LAO reactor.
The function of the ethylene as a coolant is one of the characteristic and unique features of the a-Sablin technology. With this function, it is possible to avoid using externally cooled heat exchanger tubes connected to the reaction system, which would be prone to fouling and plugging by even traces of polymerization.
In the a-Sablin LAO reactor, on the other hand, the heat of reaction is removed by the circulation of ethylene through the reactor by heating the ethylene. A smaller portion of the heat of reaction is also removed from the 2-phase layer by evaporative cooling and by reflux of cold solvent to the reactor. Cooling systems needing regular cleaning are not required.
Another important function of the ethylene circulation is that of effective and homogeneous mixing of the reaction mass in the 2-phase layer. Assurance of a homogeneous reaction mass is particularly important to avoid local hot spots that would degrade the product qualities.
Reactor modeling
A reactor model was prepared as part of the development work for the a-Sablin technology. It is being used as a versatile tool for technology optimization and reactor design. With this reactor model it is possible to predict product distributions, to determine the optimal operating conditions for the desired distribution, and to establish the reactor geometry.
The basis of the reactor model is a kinetic model of the fundamental reactions. The basic equations for the reactor kinetics were determined in cooperation with well-known technical universities. Aside from the mass and energy balances and the fluid dynamic relations, a closed mathematical model of the bubble column reactor, including the reflux condenser, was established. Further studies on the following issues were done to prove the validity of the model computations:
Physical and chemical properties of the reaction partners
Thermodynamic properties of the components
Hydrodynamic characteristics.
Extensive tests in laboratory plants and in the pilot plant, as well as experience from comparable technologies provided the bases for a reliable reactor design.
Separation of the LAOs
The heavy (long-chain) alpha-olefins, with the solvent and the dissolved catalyst are removed from the 2-phase layer of the LAO reactor in liquid form in a side-drawoff. In the next step, the catalyst components still contained in this stream are deactivated and separated from the alpha-olefins (Figure 8). Then the LAO fraction after catalyst removal, together with the fraction of light (short-chain) alpha-olefins is sent to C2/C4 separation. Part of the overhead product from this column is returned to the LAO reactor in the ethylene cycle. To prevent accumulation of inert components in the ethylene loop, a small volume is removed from the loop as the “C2 purge”. The bottoms from the C2/C4 separation are sent to further fractionations, in which they are separated into the desired end products. The solvent is also recovered in the separation section and returned to the LAO reactor.
Only conventional distillation technology is used in the entire product separation process. Other expensive purification steps for the products are not required, since the catalyst produces directly high-purity alpha-olefins.
Most of the established LAO technologies are characterized by catalyst systems with relatively low activities and by industrial plants that allow only suboptimal reaction conditions. Technologies based on the Ziegler chain growth reaction, with aluminum alkyls, require very high ethylene pressures (more than 200 bar) and high temperatures (more than 300 °C). The reaction pressure can be reduced if nickel-phosphane / boro-hydride systems are used, but then the molecular weight distribution range of the LAO products becomes broader. Furthermore, the established LAO technologies are generally not available for licensing. Those points were the initial basis for development of the a-Sablin technology with the objective of being able o produce LAO in a competitive process independently of the established manufacturers and technologies.
Well-proven plant design
For the a-Sablin technology the following references are available:
Laboratory plants at Linde and Sabic: Different types of laboratory plants (batch plants as well as continuous plants) at Linde and at Sabic have been operated for many thousand hours.
LAO pilot plant: The LAO pilot plant at the Sabic R&D facilities in Riyadh was operated for several years to optimize and demonstrate the technology, as well as to produce representative product fractions in technical quantities, which were then used to qualify them in polyethylene plants.
Linde olefin plants: The reference list of Linde olefin plants (ethylene plants, steam crackers) built by Linde in recent decades includes more than 30 world-scale plants. All those plants include a fractionation section for the separation of components similar to the separation section of a LAO plant. As a consequence, the full expertise and experience from design and operation of all these olefins plants is utilized for the design and operation of the separation section of a LAO plant.