Reactors for the low-temperature polymerization
On a large scale, synthetic butyl rubbers for adhesives, coating and sealing materials as well as for the tire industry are polymerized at low temperatures. The solution polymerization in hexane or precipitation polymerization in dichloromethane is catalyzed by aluminum chloride. To achieve a high molecular weight, the highly exothermic reaction must be controlled at low temperatures ranging from -90 ° C to -100 ° C.
These low-temperature reactors are designed as draft tube reactors with tube bundles in the outer ring. The process medium is guided inside the tubes. The draft tube allows defined reaction conditions by a narrow residence time distribution in these continuous processes. Due to the immediate conversion of the reactants in continuous operation, the accumulation of unreacted monomers and thus the risk of a runaway reaction is avoided with an appropriate monitoring. The requirements for such reactors far exceed, what is usually necessary in draft tubes as in continuous crystallizers:
- High flow velocities in the cooling tubes of approx. 4 m / s to prevent deposits of precipitating polymer
- Flow-directing internals to get an uniform distribution of the diverted flow into the tubes to ensure consistent and sufficient flow velocity in all tubes and to avoid dead zones, especially at the bottom
- Rapid mixing of the feed streams to achieve the rubber’s quality parameters
- Minimal flow losses, since the heat loss by friction must be removed by the cooling unit.
- Safe shaft sealing of the bottom entry agitator without dead zones which could foster uncontrolled precipitation.
The agitator is designed as an axial pump which is adapted to the geometry of the vessel and internals, thus contributing significantly to cost-effectiveness. On the one hand, the high efficiency is determined by the guide vanes. These are adapted to the outflow angle of the impeller blades to achieve an almost swirl-free axial flow with minimal pressure loss. On the other hand, the blade geometry of the impellers with an approximate airfoil profile contributes to a separation-free flow around the blades and thus to the efficiency.
Double-acting mechanical seals with their supply systems are used to seal the shaft introduced from below into the reactor. These seals are proven and suitable for harsh operating conditions. A permanent, monitored flushing of the seal with the process fluid hexane or dichloromethane prevents the polymerization in the gaps of the sealing space. Special attention must be paid to the operating conditions at low temperatures. The mechanical seal must be heated to avoid sticking of the barrier fluid in the sealing surfaces and that the elastomers such as the O-rings become brittle.
The example of the low-temperature reactor shows that only the careful adjustment of all components with regard to mixing, heat removal, hydrodynamics and operational safety leads to a reliable, energy and cost-efficient production of synthetic rubbers.