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 at Metso Flow Control Finland. Stay tuned also for his next post, where he discusses the reactor discharge system with complex piping and valves controlling the discharge flows.
Valves enhance fluidized-bed polymerization: Part 1 – Reactor cycle gas system
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.
Challenges to overcome
The feed control valves require good control and rangeability so that the properties of the resulting polymer can be properly adjusted. Perhaps somewhat surprisingly, certain types of catalysts have been shown to have lower activity if an excess amount of hydrogen is present, making accurate control for the hydrogen feed even more process critical.
The result of improper seat selection
For the catalyst feed valves, it is crucial that the valves are able to withstand the abrasive nature of some modern catalysts. In case pyrophoric catalysts are being used, the valve needs to have a tight shaft seal to prevent catalyst from escaping via the shaft, resulting in combustion due to exposure to air.
Due to the exothermic nature of the process, sufficient cooling is necessary to ensure smooth operation. Since the process parameters may be adjusted to produce a different type of polymer, the flow of cooling water also needs to be adjustable to cope with the changes. Depending on the source of the water, it may also have impurities.
Emergency safety vent valves are required to conform to modern safety standards. It is imperative that these valves work even after longer periods of non-operation to allow gas venting if over-pressurization were to occur.
Valves up to the task
For optimal feed control, segment valves offer excellent accuracy and wide rangeability. If accurate control is even more critical, as is the case for the feed of hydrogen, globe valves offer the best possible control accuracy.
Only metal-seated valves are sufficiently tough to withstand the abrasiveness of newer catalyst types. Ball valves offer a robust stem to ball connection increasing toughness further and a live loaded gland packing ensures that pyrophoric catalysts will not come in contact with air.
The cooling water control valve is a similar application to that of the feed valves, so a segment valve is the valve of choice. In addition, isolation valves are needed that must provide tight shut-off and long life. Triple eccentric disc valves offer the best of both worlds, with minimal wear keeping the valve tight and maintenance costs low
Metal-seated ball valve with an intelligent safety solenoid
Finally, for the emergency safety vents reliable operation and tightness is critical. Metal-seated ball valves with a spring-operated cylinder actuator and an intelligent safety solenoid with diagnostic features ensure that the valve is operable in case of emergency.
Related post: Part 2 – Product discharge system