In this post we continue discussing the polyolefin processing by looking at the product discharge system, piping and valves controlling the flows.
One of the distinct advantages associated with gas-phase processes is the relative ease of separating the reacted polymer from the unreacted monomer. However, calling the process simple is quite an understatement as the product discharge system is often more complex than the reactor cycle gas system itself.
This complexity is directly reflected in the demands for the valves as well. Proper valve performance improves the accuracy of throughput control and favorably affects overall plant performance.
Discharge of the product
In a reactor, gaseous monomer, comonomers and proprietary catalysts combine to produce a dry, polymer resin. This resin – along with hydrocarbon vapor, unspent catalyst, and possibly polymer sheets and chunks – flows from the reactor to the product discharge system (PDS) where unreacted gas is separated from the product and returned to the reactor.
The PDS consists of one or more product chambers, as well as an assortment of valves and piping for product conveyance, isolation, and gas venting and recycling. Downstream from the PDS, the product is purged with nitrogen to remove any remaining absorbed hydrocarbons. Finally, the granular polymer is pelletized with application-appropriate additives.
Demanding proper valve performance
Valves play a central role in the product discharge system, and proper valve performance is critical in the discharge line(s) including the product chamber(s).
The valve should be able to withstand a high-cycling rate, which could be over 200 000 or even 400 000 cycles per year. In most applications, fast cycle times of less than 2 to 3 seconds are required. Therefore, high-cycle design and construction of both valves and actuators are essential.
High-pressure differences over the valve, combined with pipe vibration and blending forces, require that valves are highly rigid and have long-lasting tightness. Valves must be able to withstand the abrasive nature of the media, and valve seats and bearings must resist the growth of polymer in cavities and grooves.
Meeting the demands
Metal-seated ball valves equipped with high-cycle piston actuators, such as rugged trunnion-mounted ball valves, have been the choice for many polymer producers for critical applications in the gas-phase product discharge system. This choice has been made due to the polymer-proof metal seat that prevents valve seizure and the anti-abrasive feature that withstands the hard polymers and abrasive catalysts found in polymer production plants today.
Valves which only handle purge gas and have minimal to no contact with polymer resin or catalyst can alternatively utilize soft-seated ball valves, since the potential for wear to occur due to contact with abrasive materials is low.
The use of rotary valve technology provides a way to keep the volatile hydrocarbon emissions low during the whole cycle life. This is based on a certified, emission-proof stem sealing with live-loaded packing that is designed to keep its tightness, even in very high-cycle applications.
An intelligent, integrated PDS valve controller with unique embedded diagnostic features further enhances valve performance. It allows users to perform predictive maintenance and guarantees the availability of high-cycling PDS valves.
The right combination
Using properly designed valves, actuators and controllers that meet the tough requirements of gas-phase polymerization is a proven way for producers to boost plant performance. Proper feed control valves allow production of different grade polymers using the same valves and piping, reducing investment costs. Maintenance and downtime is also reduced as polymer build-up in valves is inhibited and intelligent valve controllers allow potential problems to be corrected beforehand.
Text by Tommi Räsänen. For additional information, please contact lindsay.coutinho@valmet.com
This blog post has been up-dated in July 2020, due to company name change to Neles.
Related post: Part 1 – Reactor cycle gas system
