How control valves can enhance polymerization processing
Demand for top-quality polymers is rising throughout the world with Asia currently in the lead. A brisk growth in polyethylene and polypropylene over the next 10 years is expected mostly in Asia, Africa, the Middle East, Eastern Europe and Central-South America. Though markets are demanding higher quality and cleaner polymers, polyolefin producers want to cut downtime to avoid production losses that represent significant costs.
Playing a part in efficient polymerization
Modern polyolefin production processes are extremely efficient at producing large quantities of polymer with less capital investment. However, since polymerizations are highly exothermic reactions, efficient heat removal remains a key concern in polyolefin reactors.
Valves have a critical role in ensuring that the polymerization process is stable, flexible and controlled. Proper valve performance improves the accuracy of throughput and stable reactor temperature control. Reliability is also key for efficient cooling and avoiding unnecessary valve and piping costs.
The choice of valve type depends on the actual process in question and specific conditions within the application. Many applications within polyolefin flow control require custom-made, license-specific or engineered globe, gate, plug or butterfly valves for severe conditions, such as product discharge, flashing, cavitation and catalyst feed.
A modular approach makes valves both standard and custom-made. When the body, actuation and positioning are based on industry standards, the packing structure, seat type, trim coating or diagnostic capabilities can be custom-made to suit the application. This makes the valves reliable, economical and easy to service.
Rotary valves to avoid flammability
Rotary eccentric plug valves should be considered as a choice in applications that involve fine polymer powder, suspended solids or erosive catalysts. Alkyl aluminum is an example of a commonly used flow media for polymerization, which may accumulate to valve packing and can also be a safety risk at the plant. If the metal alkyl is exposed to the air, it may ignite.
A rotary valve is the perfect solution for such cases as it does not have a rising stem. This eliminates the problem of the alkyl aluminum sticking to the stem when exposed to air, which can cause leakage. Additionally, the valve’s scraping seat design clears debris every time the valve cycles, further contributing to safe operations.
Picture: Metso’s rotary stem valves.
Eccentric plug valves resist flashing
Polyolefin reactors produce a tremendous amount of heat, since polymerizations are highly exothermic reactions. Therefore, efficient heat removal is a key concern. Regardless of the reactor’s cooling method, it is essential to control the desired reaction rates and associated fouling. Flashing is a phenomenon associated with certain types of reactors with water cooling, resulting in severe erosion and leakage of carbon steel valves with metal back seals and downstream piping. So the valves controlling the flows need to resist flashing.
The valves should be installed as close to the cooling tank as possible, allowing high velocity flows to mix in the tank space instead of hitting the piping walls. Proper sizing is important for an optimized opening angle. Moreover, an eccentric plug valve with a flow to close or in a reverse-flow direction, balanced plug design and stainless steel body is highly recommended to avoid any flashing damage in the valve body.
Picture: Neles® Finetrol™ eccentric plug valve.
Ball valves withstand high-cycle polymerization
Fluidized-bed type, gas-phase polymer processes are very economical for producing commodity-grade polymers, making them popular with polymer producers. These reactors require a product discharge system to remove polymer from the reactor and flash off the monomer. The valves need to withstand a high-cycling rate of 400,000 up to million cycles per year and extremely fast cycle times of less than 3 seconds. Additionally, the valves must cope with abrasive media, whereas the valve seats and bearings must resist the growth of polymer in cavities and grooves.
Ball valves equipped with high-cycle piston actuators and polymer-proof metal seats have been the choice for many polymer producers in plants with a product discharge system. This is due to the polymer-proof metal seat that prevents polymer from penetrating behind the seat and seizing the valve. Its anti-abrasive feature makes it a perfect selection for hard polymer and more abrasive catalysts, which are nowadays widely used to improve the end-product polymer quality.
The simple principle of a valve seating, where the seat has continuous contact with the ball, allows reliable, long-lasting tightness and cycle life in demanding applications.
Picture: Neles® modular ball valve construction.
Globe valves with clean flow media
A rising stem globe valve is an economical and easy way of controlling flows with a variety of trims available. A rising stem globe valve is a viable choice for polyolefin processing with relatively clean flow media, like in in a gaseous monomer feed or recycle loop that use a cyclone outlet reactor to eliminate fines from the recycle stream.
Picture on the right: Neles® globe valve.
Boosting the process
To satisfy the growing global demand for top-notch polymers, polymer producers need to ensure that all units in the polymerization process are maximally efficient and reliable.
Choosing the right and reliable control valves can help reduce safety risks related to flammable substances, improve overall plant productivity and uptime, as well as contribute to savings in hardware and labor costs created by the repair and replacement of failed systems caused by defective valves. Using properly designed control valves that meet the tough requirements of the polymerization process is a proven way for producers to boost plant performance.
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