Text: Sari Aronen
Photos: Metso & Shutterstock
The propane dehydrogenation process, known as PDH, is used to supply polymer grade propylene from propane to meet the growing demands of the propylene market, independent of a steam cracker or fluid catalytic cracking (FCC) unit. It provides a dedicated, reliable source of propylene to give more control over propylene feedstock costs.
Propylene is a basic building block of the chemical industry and can be used to produce a variety of products including plastics and solvents. Currently, the demand for propylene in Asia is growing rapidly, mainly due to increasing demand for its downstream product polypropylene. China’s propylene consumption accounts for more than 15% of worldwide demand and is growing at approximately 5-6% per year. Meanwhile, PDH technology has been constantly improved, lowering the investment and operation costs. Due to these reasons, the use of PDH technology is constantly growing in China. Nowadays Chinese propane dehydrogenation plants tend to be on a very large scale; one plant, for example, is producing in excess of 600 000 tpy of propylene.
The feedstock represents a large portion of the total production cost of PDH. The PDH economics are largely dependent on the price differential between propane and propylene. Thus, PDH also has a very bright future in areas rich in propane resources, such as the Middle East or North America. In these areas, PDH technology could be used to convert propane derived from shale gas to propylene. The propane dehydrogenation process operates at a high temperature (approximately 540 ˚C) to convert propane into propylene in the presence of a catalyst. The process consists of a reactor section, product recovery section and catalyst regeneration section.
A careful selection of valves for the propane dehydrogenation process will increase the efficiency, productivity and safety of the process.
Sari Aronen, Application Manager, Metso
Continuous catalyst regeneration
Over a period of time, due to the high operating temperature the catalyst becomes coated with coke, a natural byproduct of the process, and therefore requires regeneration. Catalyst activity is maintained by continuous catalyst regeneration (CCR) or by shutting down reactors one-by-one and regenerating the reactor using the regeneration air. The CCR is where the catalyst is continuously withdrawn from the reactor, then regenerated and fed back to the reactor bed. The CCR is not only used in the PDH process, but also commonly used in continuous catalytic reforming with the same function. A series of lock hoppers, typically four complete lock hopper arrangements, is used to move catalyst from the reactor to the regenerator and eventually back into the reactor.
A constant non-declining yield is important in PDH economics. This is achieved by the CCR section, which ensures the reactors are continuously supplied with freshly regenerated catalyst and that product yields are maintained at fresh catalyst levels. Critical valves in catalyst handling lock hoppers, venting, catalyst withdrawal and addition play an important role in successful catalyst regenerating process performance. Poorly performing valves in the process must be serviced because they will have a direct impact on efficiency. Valves should be specifically designed to meet the process requirements, such as UOP specification 671. For these critical valves in the CCR section, care must be taken in material selection and seat construction in order to avoid any wear or particles entering the seat cavities and adhering to sealing surfaces.
Metal seated valves, such as our ball valves, have been widely used for critical catalyst handling applications. Hard coatings should be applied to raise the surface hardness of the ball to provide long component life in this highly abrasive service. Special seat construction is recommended in this critical application as catalyst fines behind the seat can cause the required torque to increase enough to exceed the maximum output capability of the actuator. This design has proved its long lasting tightness over years of frequent cycling and catalyst handling.
In the CCR section, the catalyst addition system is the point in the process where new catalyst is added to replace the quantity of catalyst that is withdrawn and discarded from the system after it can no longer be regenerated. The new catalyst flows by gravity into the system through a catalyst addition hopper at ambient temperature. Soft seated ball valves with catalyst friendly design are typically used as a solution in this case. A safety interlock system is a typical requirement to prevent both the valve above and below the lock hopper from opening at the same time.
Reactor and product recovery
In the reactor section of propane dehydrogenation, heaters are used to maintain the desired reaction temperature by continuously supplying heat, because the reaction is endothermic. Conventionally, heater pass control valves have been rising stem globe valve designs. However, during the heater cycle, some coke and sticky oil may start to accumulate in the rising stem gland packing. Leakage and sticking reduces the accuracy of throughput control, affecting heater performance and process efficiency while posing environmental and safety risks. Unscheduled maintenance can be costly and risky, and can financially impact plant productivity. The gland design of rotary control valves is inherently reliable and will not suffer the leakage problems typically associated with conventional globe designs. This is because a rotary stem does not move process media into gland packing in the same way as a rising stem.
Another important application in heaters is fuel gas control; accurate, reliable control valves play a significant role in reducing operating costs. Proper combustion maximizes heat transfer, which minimises fuel gas consumption and related costs. Variations in fuel gas composition and different operating conditions at start up, normal operation and shut down mean that such valves must control various loads, which requires wide rangeability from the valve. This is typically solved by using a split range configuration with globe valves. Another method is to use rotary control valves with wide rangeability as high as 150: 1, such as Neles V-port segment valves. This way, the wide rangeability allows accurate control with both small flows and large valve openings with a single valve solution.
In the reactor section of propane dehydrogenation, dryers are used to remove trace amounts of water formed from the catalyst regeneration, and to remove hydrogen sulfide. A typical dryer consists of two or more columns packed with molecular sieves. As the wet stream is processed in one column, the other column is regenerating. The dryer switching valves play an important role in directing the inlet/outlet stream of gas between the dryer columns, hence switching the columns from an adsorption phase into the regeneration phase in a preset sequence. High temperature gas or hydrogen (approximately 250 ˚C) is used to regenerate the adsorption bed. The valves have to withstand these fluctuations in temperature together with high pressure, all while keeping the tightness in both flow directions over years of operation.
The molecular sieve dryer beds tend to release dust during the regeneration cycle. Care must be taken in material selection and seat construction in order to avoid any wear or particles entering the seat cavities and adhering to sealing surfaces. Different stroking profiles are often required for opening and closing to minimise the bed dust release and pressure shocks. Metal seated valves, such as Metso’s ball and butterfly valves, have been widely used for these kinds of demanding switching applications. For the most demanding applications, trunnion mounted ball valves are selected for their reliable operation and excellent response with high pressure differentials.
Trunnion mounted designs give lower friction and operating torque. Seat construction ensures durable tightness in both directions, even in extreme conditions. This design has proved its long lasting tightness over years of frequent switching with molecular sieve dust present and constant temperature changes. Special hard coatings, such as carbides, are commonly used in this type of application. For smaller sizes and lower pressure differentials, seat supported ball valves have been used for floating ball designs that ensure long lasting tightness with metal seats and low shut off pressures. Triple eccentric disc valves provide an interesting option in large size applications for dryer valves, where pressures remain at a moderate level. The triple offset metal seat design is well suited to this frequent cycling at high temperature and to abrasive applications because it can withstand long periods of operation without losing bidirectional tightness. Long lasting tightness is ensured by mechanically induced disc and seat contact, which does not rely on differential pressure and a rugged one-piece seat design.
High performance triple eccentric disc valves with double seat design can manage medium with large temperature difference between two sides of the valve and to keep bidirectional tightness. This provides a single valve solution for dryer valves instead of double gate valves in a large size. Compared with double gate valves, high performance triple eccentric disc valves have many advantages, including less weight and cost saving. Similarly to a rising stem, the rotary stem does not tend to move process media into gland packing, and is able to tolerate the piping forces.
After careful selection and sizing of the valve, actuator and instrumentation, seeing how the valve is performing in critical applications, such as with lock hopper valves or dryer valves, is possible. This helps to predict and plan the maintenance activities. Digital control valve positioners provide digital communications, but intelligent valve controllers have embedded valve diagnostics and online monitoring capabilities to predict valve failures and maintenance needs. They also provide additional safety for staff and the process.
An intelligent valve condition monitoring system enables a systematic approach. Valves that need maintenance are identified and the provision of spare parts, appropriate tools and service work can be preplanned to reduce problems and risks in the process plant. Failures or malfunctions are detected before problems occur. The valve’s diagnostic history, current status, performance and future performance can all be seen. Intelligent digital controllers take the start up, operation and maintenance planning for propane dehydrogenation applications to a new level. Most importantly, it is possible to see, during the course of the process, what is going on at the process critical valves, such as the lock hopper valves in CCR section or switching valves in dryers.
Careful selection of valves for the propane dehydrogenation process will increase the efficiency, productivity and safety of the process. An intelligent valve controller provides the means for simple and reliable instrumentation with transparency to valves’ performance while the process is running. Intelligent and reliable valves will support the use and development of propylene production through propane dehydrogenation that provides the means for broader sources of feed.
Previously published in Results flow control 1/2015 and in Hydrocarbon Engineering magazine, December 2012 issue, as 'In a reliable role'.