much higher temperature of the microreactor compared to the temperature of the
surrounding diluent. This upper stable point must be avoided because otherwise it
can lead to a situation where the polymer grains are swollen with the hydrocarbon
diluent or even melt and then stick together throughout the whole reactor, with the
consequence that the reactor can no longer be controlled and must be shut down. In
the worst case, the whole reactor content must be removed and the reactor cleaned.
It is now known how to design the catalyst particle, how to react the catalyst with
the aluminumorganic cocatalyst, and how to run the polymerization process to
avoid overheating of the microreactor, even for super-active catalysts [32, 33].
The mesoscale level comprises all processes inside the slurry phase. At this level,
there are no process-relevant chemical reactions. Here run the transport processes
for ethene, hydrogen to control average molecular mass, and e.g., 1-butene or other
1-olefines to control density. Further, the heat transfer from the polymerizing
polymer grains to the diluent and then to the cooler system is located here. It
is important to establish a stable stationary state for both concentrations of the
reacting components and temperature, with stable concentration and temperature
gradients in the polymerization reactor from top to bottom and vice versa. It is
further essential that the bulk mixing time is far below average residence time
for each vessel. This is achieved using modern state-of-the-art stirrer systems with
blades or impellers or turbines at different heights [34–36]. On the basis of this
know-how, it is possible to construct polymerization vessels with increasing size,
up to 200 m
3 . The relevant criteria will be discussed in Sect. 3.2.
The macroscale level comprises the polymerization reactor as a whole.
Based on a detailed understanding of all processes on the microscale and mesoscale
levels, it was possible to develop a process model indicating which intensive
and extensive process variables must be controlled within which ranges to hold
the relevant product data in the required ranges [37–39]. It was a great surprise
that only a very limited number of variables have to be controlled and it was
further surprising to see a nearly perfect agreement between calculated and
measured process data. It is now possible to run large plants with throughputs up
to 50 tons/h safely, without uncontrolled shutdowns, with less pollution, with
outstanding product qualities, and with high reproducibility.
Polyethylene products are no longer only commodity grades but are now used for
technical applications like pipes for drinking water, waste water, industrial piping,
and gas transport systems; containers for industrial packaging, especially for
the transport of dangerous goods; automotive fuel tanks; extremely tough films;
and many more applications [32, 33, 40–50]. Using the cascade technology with
two or even three reactors in series (advanced cascade process), and introducing
the catalyst only into the first reactor, this technology reaches an outstanding
flexibility in product development because no catalyst change is necessary for
the development of new grades. Only the process parameters of the different
reactors must be modified. This is an outstanding advantage and opens a lot of
opportunities for further product development.
62
L.L. Bo ¨hm
surrounding diluent. This upper stable point must be avoided because otherwise it
can lead to a situation where the polymer grains are swollen with the hydrocarbon
diluent or even melt and then stick together throughout the whole reactor, with the
consequence that the reactor can no longer be controlled and must be shut down. In
the worst case, the whole reactor content must be removed and the reactor cleaned.
It is now known how to design the catalyst particle, how to react the catalyst with
the aluminumorganic cocatalyst, and how to run the polymerization process to
avoid overheating of the microreactor, even for super-active catalysts [32, 33].
The mesoscale level comprises all processes inside the slurry phase. At this level,
there are no process-relevant chemical reactions. Here run the transport processes
for ethene, hydrogen to control average molecular mass, and e.g., 1-butene or other
1-olefines to control density. Further, the heat transfer from the polymerizing
polymer grains to the diluent and then to the cooler system is located here. It
is important to establish a stable stationary state for both concentrations of the
reacting components and temperature, with stable concentration and temperature
gradients in the polymerization reactor from top to bottom and vice versa. It is
further essential that the bulk mixing time is far below average residence time
for each vessel. This is achieved using modern state-of-the-art stirrer systems with
blades or impellers or turbines at different heights [34–36]. On the basis of this
know-how, it is possible to construct polymerization vessels with increasing size,
up to 200 m
3 . The relevant criteria will be discussed in Sect. 3.2.
The macroscale level comprises the polymerization reactor as a whole.
Based on a detailed understanding of all processes on the microscale and mesoscale
levels, it was possible to develop a process model indicating which intensive
and extensive process variables must be controlled within which ranges to hold
the relevant product data in the required ranges [37–39]. It was a great surprise
that only a very limited number of variables have to be controlled and it was
further surprising to see a nearly perfect agreement between calculated and
measured process data. It is now possible to run large plants with throughputs up
to 50 tons/h safely, without uncontrolled shutdowns, with less pollution, with
outstanding product qualities, and with high reproducibility.
Polyethylene products are no longer only commodity grades but are now used for
technical applications like pipes for drinking water, waste water, industrial piping,
and gas transport systems; containers for industrial packaging, especially for
the transport of dangerous goods; automotive fuel tanks; extremely tough films;
and many more applications [32, 33, 40–50]. Using the cascade technology with
two or even three reactors in series (advanced cascade process), and introducing
the catalyst only into the first reactor, this technology reaches an outstanding
flexibility in product development because no catalyst change is necessary for
the development of new grades. Only the process parameters of the different
reactors must be modified. This is an outstanding advantage and opens a lot of
opportunities for further product development.
62
L.L. Bo ¨hm