Basell Spherizone Technology

Maurizio Dorini and Gabriele Mei

17.1

Introduction

The family of polymers, and in particular polyolefins, is important in the modern world because of the very high number of applications in all fields. Among plastic materials, polypropylene is one of the most important, having undergone rapid growth since its discovery in 1954 by the Nobel Prize winner G. Natta of Politecnico di Milano. Today, polypropylene demand is about 40 million ton per year with a market share, among all thermoplastic materials, of about 26%, which is second only to polyethylene (39%).

Competition, in the market of polypropylene licensing technologies, has been a driving force to improve the available processes, with the aim of reducing the investment and variable costs obtained by a simplification of the process, reduction of raw materials and utilities consumptions, improving also the environmental impact with lower gas emission and liquid effluents. The evolution of catalysts and technology has allowed the polypropylene properties to be expanded, to fulfill the market demand and to widen its application.

The predecessor company of BASELL (Montecatini Edison, Himont, Montell, Basell) has always been strongly committed to the research and development of new catalysts for polyolefin, and specifically polypropylene, production and in the continuous improvement of the production processes.

17.2

Technology Evolution

The evolution of the processes for polypropylene production is strictly connected to the improvement of the catalyst system.

By 1950 Ziegler had worked on the growth of alkyl chains by insertion of ethylene into the Al—C bond oftrialkyl-aluminium, mainly focused in the field of polyethylene. Natta, from the beginning, attempted propylene polymerization, succeeding in

March 1954. Natta's research group fractionated the obtained polymer and found that 40% of it was a hard, high-melting, insoluble fraction.

Natta's discovery soon found industrial application with the first plant, set up by Montecatini in Ferrara, starting production in 1956.

Table 17.1 gives a brief summary of PP catalyst evolution.

In the 1960s, polypropylene processes, operated batchwise, employed firstgeneration low yield catalysts (<1000kg-PP per kg catalyst) in mechanically stirred reactors filled with an inert heavy hydrocarbon diluent used to keep the crystalline polymer suspended; propylene was dissolved in the diluent, allowing to operate the reactor at low pressure. Polymer produced with these catalysts had unacceptably high Ti residual metals and contained 10% atactic polypropylene, which required separation. Removal of catalyst residues involved treatment of the polymer with alcohol, multiple organic and/or water washings, polymer/diluent separation via centrifugation or filtration, multistage drying and elaborate diluent/amorphous separation systems. These processes were costly and difficult to operate, and also required extensive water treatment facilities, and catalyst residue disposal systems. The environmental impact was quite high.

As the demand of polypropylene increased, it was necessary to increase the plant capacity and to use continuous processes, this was possible thanks to second-generation catalysts with increased yield (6000-15 000 kg-PP per kg catalyst) and isotacticity, but not yet to an extent that allowed simplification of the production process. Several different processes were developed: slurry, solution, bulk, gas phase.

In the slurry processes the batch reactors were replaced with continuous stirred vessels operated in series, which ran full or under level control. The operating pressure depended on the selected solvent, the most common being hexane, but also heptane, kerosene and butane were used.

Table 17.1 Performance of different catalyst generations.

Generation

Catalyst

Yield (kgPP

I.I.

Morphology

Process

composition

per g-Cat)

(wt%)

control

requirements

1st

TiClj/AlClj + DEAC

1

90-94

Not possible

Deashing + Atactic removal

2nd

TiCl3 + DEAC

10-15

94-97

Possible

Deashing

3rd

TiCl4/ester/ MgCl2 + AlRs/ ester

15-30

90-95

Possible

Atactic removal

4th

TiCl4/diester/ MgCl2 + TEA/ silane (HY/HS)

30-60

95-99

Possible

5th

TiCl4/diether/ MgCl2 + TEA

70-120

95-99

Possible

TiCl4/

40-70

90-99

Possible

succinate/

MgCl2 + TEA

6th

Zirconocene + MAO

90-99

Possible

Bulk processes (liquid monomer) were operated at higher pressure and have the advantages of a higher reaction rate because of the high monomer concentration, and the absence of diluent.

The solution process was complex and expensive. The product range was somewhat restricted because a special, high-temperature catalyst was required.

The gas-phase process was a simple one but, even with the second-generation catalyst, atactic PP and catalyst residuals were left in the final polymer; consequently, the product quality suffered the presence of atactic polymer and catalyst residuals (stiffness, color, resistance to oxidation).

The first major improvement in the manufacturing process came in the 1970s with the discovery of the milled, active MgCl2 support for PE, the extension to PP with the use of electron donors, and of the combination of internal and external electron donors to promote the iso-index without relinquishing catalyst yield; this brought about third-generation, high yield catalysts (15 000-30 000 kg-PP per kg catalyst), eliminating the need for catalyst residue removal (Ti level below 5 ppm), but the atactic content was still unacceptably high.

The consequences on the manufacturing process were the elimination of catalyst deactivation and removal of a section, with a positive impact on installation and variable costs and also on the environment by removing all the alcohol and water treatment that generated effluents from the plant.

The process still used solvent and the solvent recovery system from the atactic polymer was still in place.

Figure 17.1 shows a block diagram ofthe notable process simplification on moving from second- (low yield slurry process) to third-generation catalysts (high yield slurry process).

In the 1980s, fourth-generation high yield, high selectivity (HY/HS) catalysts (30 000kg-PP per kg-catalyst, isotactic index 95-99%) provided a real breakthrough in process simplification, eliminating the need for catalyst and atactic removal.

The polymer flake size and shape is an enlarged copy in size and shape of the catalyst particle. The average diameter of the polymer particles depends on the average diameter of the catalyst and on the extent of polymerization.

The catalyst can have a granular or spherical form, it can be tailored to a very high isotactic index, very high extent, quite broad or narrow molecular weight distribution, and so on. The ratio between the average diameter ofthe flake and the diameter ofthe catalyst is called the replication factor.

An additional characteristic ofthe catalyst was a longer activity that, together with its very high porosity (allowing encapsulation of a large amount of ethylene propylene rubbers inside the particle), opened the way to a large expansion ofthe PP Impact Copolymer range. Development ofthe spherical support with controlled morphology was another factor that affected technology simplification, allowing the polymer to be handled through the process using standard control valves, avoiding the handling of a large amount of fines, without fouling or clogging.

The high bulk density and the large average diameter allowed high gas velocities in a fluidized gas-phase reactor with limited entrainment, boosting the possibility of

Basell Montedison Slurry Technology
PELLETIZED PRODUCT

LOW YIELD SLURRY PROCESS

Figure 17.1 Comparison of low and high yield Montedison slurry processes.

Spheripol Reactor
PELLETIZED PRODUCT

HIGH YIELD SLURRY PROCESS

Hydrogen

Propylene

Ethylene

Pelletised product

Figure 17.2 Block diagram of the Spheripol process.

Pelletised product

Figure 17.2 Block diagram of the Spheripol process.

heat removal by external gas cooling, and so keeping the fluidized gas reactor small. This further simplified the process and improved product quality.

Refinement of the bulk polymerization reactor and of the gas-phase reactor led to the development of the Spheripol process in 1982 (Figure 17.2).

The Spheripol process was a significant step change in the polypropylene process, allowing a broadening of product capabilities and a considerable saving in terms of investment costs as well as operating costs.

The Spheripol process has been very successful in the PP technology market -since 1982, when the first plant was started up in Brindisi, Italy, over 100 plants have come into operation or are under construction worldwide.

The Spheripol process has been recognized as an environmentally friendly process, being listed in the Polymers BREF, European Commission - IPPC: Integrated Pollution Prevention and Control Best Available Techniques in the production of Polymers dated October 2006.

17.3

Spherizone Technology

Despite the success of the Spheripol process, Basell continue research aimed at improving product characteristics, widening the range of grades and developing the next generation of PP technology, the Spherizone process.

Catalyst components

Polymerisation

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