catalyst. Introducing ethene, the polymerization process starts to generate a high
molecular mass polyethylene. Under the given reaction conditions (temperature
below 100
C), the polyethylene is insoluble in the hydrocarbon diluent and
precipitates as a powder to form a slurry (slurry polymerization process). This
process was performed in a 2 L glass autoclave [1, 2]. For industrial use, it was
of essential importance that this process was protected by a patent [3, 4]. Within
a very short time, a technical plant came on stream and in 1955 the first products
were offered on the market [5–9]. This new process was regarded as so important
that a small plant was presented at the Brussels World Exhibition in 1958 to
demonstrate this technology and the product [10]. In his Nobel Prize lecture in
1963, Ziegler could show how fast his invention had been transferred into technical
processes worldwide [2].
Today, this process is operated in principle in the same way as described by
Ziegler et al. [1–4]. However, over the six decades since the discovery enormous
progress has been achieved, and a detailed understanding of all relevant processes
on all scales of this technology is available. The process is now run using superactive catalysts on the basis of a MgCl 2 particle loaded with TiCl 4 [11–21]. These
catalysts are at least two orders of magnitude more active than the catalysts used
by Ziegler and colleagues. The activity under comparable reaction conditions
reaches more than 100 kg polymer per gram titanium (the amount of polymer is
usually related to the active component of the catalyst, in this case to titanium)
resulting in a catalyst residue of less than 10 ppm Ti, usually in the range of around
1 ppm Ti or even less. From a technical point of view, this is an outstanding
advantage because all catalyst residues can remain in the polymer without any
treatment to save product quality.
The whole process can be separated into three clear distinguishable levels
(micro-, macro-, and mesoscale) as first proposed by Ray [22]. The most important
processes are running on the microscale level, meaning inside and at the surface of
the growing polymer particle. Here all relevant chemical reactions take place.
The polymerization reaction is well understood on the basis of the Cossee–Arlman
model [23–25]. All other relevant chemical processes are known based on a
reaction model published in 1978 [26]. This reaction model was used to develop
an advanced process control strategy, as described later. On the microscale level,
the most important process is the transfer of a catalyst particle to a polymer grain.
This process is called the particle-forming process [27]. Because all reactions
take place inside this particle, it has to be regarded as a small reactor, called
a microreactor. Different models have been published for this particle-forming
process, but the simplest model, the polymeric flow model [27, 28], was found to
describe this process quite well. The polymerization reaction is highly exothermic.
Therefore, it is very important at least for the technical process to avoid overheating
of these microreactors. Wicke et al. has shown that there are two stable points with
respect to the temperature and concentration profiles inside and at the surface of
such microreactors [29–31]. There is a stable point controlled by kinetics, with
small temperature gradients inside and at the surface of the polymerizing particle,
and a diffusion-controlled stable point with strong temperature gradients and a
The Slurry Polymerization Process with Super-Active Ziegler-Type Catalyst. . .
61
molecular mass polyethylene. Under the given reaction conditions (temperature
below 100
C), the polyethylene is insoluble in the hydrocarbon diluent and
precipitates as a powder to form a slurry (slurry polymerization process). This
process was performed in a 2 L glass autoclave [1, 2]. For industrial use, it was
of essential importance that this process was protected by a patent [3, 4]. Within
a very short time, a technical plant came on stream and in 1955 the first products
were offered on the market [5–9]. This new process was regarded as so important
that a small plant was presented at the Brussels World Exhibition in 1958 to
demonstrate this technology and the product [10]. In his Nobel Prize lecture in
1963, Ziegler could show how fast his invention had been transferred into technical
processes worldwide [2].
Today, this process is operated in principle in the same way as described by
Ziegler et al. [1–4]. However, over the six decades since the discovery enormous
progress has been achieved, and a detailed understanding of all relevant processes
on all scales of this technology is available. The process is now run using superactive catalysts on the basis of a MgCl 2 particle loaded with TiCl 4 [11–21]. These
catalysts are at least two orders of magnitude more active than the catalysts used
by Ziegler and colleagues. The activity under comparable reaction conditions
reaches more than 100 kg polymer per gram titanium (the amount of polymer is
usually related to the active component of the catalyst, in this case to titanium)
resulting in a catalyst residue of less than 10 ppm Ti, usually in the range of around
1 ppm Ti or even less. From a technical point of view, this is an outstanding
advantage because all catalyst residues can remain in the polymer without any
treatment to save product quality.
The whole process can be separated into three clear distinguishable levels
(micro-, macro-, and mesoscale) as first proposed by Ray [22]. The most important
processes are running on the microscale level, meaning inside and at the surface of
the growing polymer particle. Here all relevant chemical reactions take place.
The polymerization reaction is well understood on the basis of the Cossee–Arlman
model [23–25]. All other relevant chemical processes are known based on a
reaction model published in 1978 [26]. This reaction model was used to develop
an advanced process control strategy, as described later. On the microscale level,
the most important process is the transfer of a catalyst particle to a polymer grain.
This process is called the particle-forming process [27]. Because all reactions
take place inside this particle, it has to be regarded as a small reactor, called
a microreactor. Different models have been published for this particle-forming
process, but the simplest model, the polymeric flow model [27, 28], was found to
describe this process quite well. The polymerization reaction is highly exothermic.
Therefore, it is very important at least for the technical process to avoid overheating
of these microreactors. Wicke et al. has shown that there are two stable points with
respect to the temperature and concentration profiles inside and at the surface of
such microreactors [29–31]. There is a stable point controlled by kinetics, with
small temperature gradients inside and at the surface of the polymerizing particle,
and a diffusion-controlled stable point with strong temperature gradients and a
The Slurry Polymerization Process with Super-Active Ziegler-Type Catalyst. . .
61