Valves have a critical role in ensuring that the fluidized-bed type of gas-phase polymerization is stable, flexible and controlled. Any valve leakage poses both an environmental and safety issue due to risk of fire, toxicity, and the volatility of gaseous hydrocarbons.
Our this week’s post about gas phase polymerization reactors and valves is presented by Tommi Räsänen, who has been studying these processes during his internship this summer here. Stay tuned also for his next post, where he discusses the reactor discharge system with complex piping and valves controlling the discharge flows.
Having only recently delved into the industrial polymerization processes, I was surprised to find out how many different processes there are for producing polyolefins. As each of these offer their own distinct advantages, there is no single process that could be considered a standard among the industry even though some processes are more used than others.
The processes are often categorized according to the phase of the raw material feed. Gas-phase processes are an attractive option due to being economical and energy-efficient. Many gas-phase processes utilize one or more fluidized-bed reactors with possible other reactor types in the same process.
Valves have a critical role in ensuring that the fluidized-bed type of gas-phase polymerization is stable, flexible and controlled. Any valve leakage poses both an environmental and safety issue due to risk of fire, toxicity, and the volatility of gaseous hydrocarbons.
The reactor gas cycle
Gas-phase monomer and comonomer are fed into the process together with hydrogen and enter a fluidized-bed reactor. Proprietary catalysts, which initiate the polymerization reaction, and possibly cocatalysts, enhancing catalyst activity, are mixed with nitrogen and fed into the reactor. The gas flow rate into the reactor is adjusted so that the formed polymer particles are fluidized, allowing them to be removed via a product discharge system.
Polymerization is an exothermic process, meaning that heat is released in the reaction. Therefore an efficient heat removal system is crucial. To combat the generated heat, per pass conversion is deliberately kept low to allow large amounts of gas to be recycled. This gas exits through the top of the reactor and is compressed to enhance heat transfer, before entering a heat exchanger where it is cooled. After this, the cooled gas re-enters the reactor.
