Polymerization experiments were conducted in an existing laboratory setup (Figure 4.1). [22, 23, 41] The setup can be divided into four main parts: The raw material supply including the propylene purification unit, the polymerization reactor, the control units and the data acquisition system.
4.1.1 Raw material supply and purification
The used Ziegler-Natta catalysts are sensitive to feed stream impurities such as water, oxygen and other polar compounds. In order to achieve high activities and good reproducibility, the raw materials were purified before entering the reactor. In particular, the monomer was purified by a multi-stage purification system.
Propylene (Air Liquide) was supplied in gas bottles with a purity of 99.5 mol%. A diaphragm pump (ORLITA MhS 18/5, ProMinent) guaranteed the monomer conveyance through the five stage purification system. In the first column, molecular sieve with a pore size of 4 Å was used to remove traces of water. In the second stage, an oxidized copper catalyst (PuriStar®
R3-12, BASF) assured the removal of sulfur, arsine, H2S and COS compounds. In the third column, a 50:50 mixture of Selexsorb® CDL/COS (BASF) was used to adsob a wide range of polar compounds. The fourth column contained PuriStar® R3-17 (BASF) in order to remove CO. The last stage consisted of two columns filled with Dynocel 680 which has a high capacity for polar organic and acid gas compounds. For improved removal of impurities, the liquid propylene was recycled several times through the purification system at a pressure of 40 bar.
Hydrogen (Linde) of purity 5.0 was supplied in gas bottles and applied as chain transfer agent. It was used as received without further purification.
In-house nitrogen (liquid storage tank, Air Liquide) of quality 5.0 was used for inertization of the reactor system. Further purification was performed by passing the gas over an Oxisorb®
cartridge (Linde) in order to remove oxygen traces.
Figure 4.1: Experimental setup for the gas phase polymerization of propylene.
Table 4.1: Gas supply and quality.
Gas Supplier Purity Propylene Air Liquide 2.5
Hydrogen Linde 5.0
Nitrogen Air Liquide 5.0
4.1.2 Lab-scale polymerization reactor
The gas phase polymerization experiments were performed in a 5 L horizontal stirred tank reactor under industrial relevant conditions (Figure 4.2). The reactor is designed for pressures up to 40 bar and temperatures up to 120 °C. The stainless steel reactor is cooled and heated via the jacket using a commercial thermostat (Proline RP 855, Lauda) with a maximum heating power of 3.5 kW and a maximum cooling power of 1.6 kW. A thermal oil (Therminol® ADX 10, Fragol) was used as thermostating liquid. Since propylene is fed to the reactor in the liquid state, cooling is additionally caused by monomer evaporation. However, this only amounts up to a fraction of about 20 % as the specific heat of evaporation is 18.4 kJ/mol [115] and the specific reaction heat is 102 kJ/mol [116]. The temperature is measured in the center of the reactor with a Pt100. The pressure is measured with a digital pressure sensor (IUT-10, WIKA) within a range of 0 to 40 bar and a relative error of 0.15 %.
The anchor-type stirrer is rotated via a magnetic coupling (bmd 300, Büchi) driven by a three phase variable gear motor (Büchi). Stirring speeds up to 800 rpm can be realized.
The gases and the catalyst system are inserted via the top of the reactor. Propylene is fed via a liquid phase mass flow controller (Flomega 5882, Brooks) and hydrogen via a gas phase mass flow controller (MF50S, Brooks). The catalyst system is injected directly with liquid propylene from the purification unit without passing the mass flow controller. The fast injection fluid flow is caused by the pressure difference of liquid propylene at 40 bar and the reactor pressure.
A slide vane rotary vacuum pump (P 6 Z, Ilmvac) and the in-house nitrogen are connected to the reactor for inertization.
Figure 4.2: Horizontal gas phase polymerization reactor.
4.1.3 Measurement of reaction kinetics and control units
The polymerization reactor is operated in semi-batch mode at isobaric and isothermal conditions. During the reaction, gaseous propylene is converted to solid polypropylene. The density of the solid is much higher than the density of the gas. The consequent pressure drop is compensated by constantly feeding monomer to the reactor. At constant pressure and temperature, the mass flow of propylene into the reactor is equal to the gross propylene consumption by reaction which is proportional to the polymerization rate or activity of the catalyst. Thus the reaction rate is accessible during the polymerization experiment.
In order to maintain isobaric conditions, monomer is fed to the reactor in a closed pressure control loop. The pressure value is transmitted from the WIKA pressure sensor to a PID controller (i16, Omega i-Series, Newport) via a voltage signal. Depending on the difference between the measured pressure value and the set point, the controller forwards a voltage signal to a control box (Brooks Instrument) which regulates the propylene flow via the mass flow controller.
Isothermal conditions are achieved by a cascade closed loop control which is available via the Lauda thermostat. The cascaded control consists of an outer and an inner PID controller.
The outer PID controller compares the reactor temperature with the manually entered temperature set point and adjusts the inner thermostat bath temperature set point in order to maintain the target reactor temperature. The inner PID controller manipulates the heating power in order to achieve the set thermostat bath temperature.
4.1.4 Data acquisition
The commercial software DASYLab 9 (National Instruments) was used for data acquisition. A snapshot of the interface during a typical polymerization experiment is shown in Figure 4.3.
During an experiment, pressure, temperature and mass flows are collected, processed and displayed. The data of each experiment is recorded and stored in an ASCII-file. For safety reasons, the process inputs, i.e. the set temperature and pressure, could also be set manually directly at the corresponding controllers.
Figure 4.3: User interface of DASYLab 9 during a polymerization experiment.