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Propane dehydrogenation: Part 3 – reactor and product recovery

This time, let’s take a look into the reactor and product recovery section of propane dehydrogenation (PDH) process and valves.

Controlling the flows

Simplified PDH reactor heater.

Simplified PDH reactor heater.

In the reactor section, heaters are used to maintain the desired reaction temperature by supplying heat continuously since the reaction is endothermic. Conventionally, heater coil control valves (1.)  have been rising-stem globe-valve designs. During the heater cycle, some coke and hydrocarbons may start to accumulate in the rising-stem gland packing, especially when processing hydrocarbons at high temperatures. Leakage and sticking reduces the accuracy of throughput control, affecting heater performance and process efficiency while posing environmental and safety risks. The gland design of rotary control valves, like Metso’s intelligent control valves, is inherently reliable and will not suffer the leakage problems typically associated with conventional globe designs. This is due to the rotary stem not tending to move process media into the gland packing such as rising stem. This is important aspect when processing light hydrocarbons and hydgrogen in particular. For the heater fuel gas control (3.), rotary control valves bring wide rangeability and high turndown ratio that makes it possible to use one single valve for start-up and normal operations instead of two globe valve configuration. The same applies for hydrogen recycle and control valves, where a proper hydrogen-to-hydrocarbon ratio calls for accuracy, rangeability and high performance.

What’s the function of a dryer in PDH?

Metal seated ball valve for dryer applications from Metso Automation.

Metal seated ball valve for dryer applications from Metso Automation.

In PDH process, dryers are used to remove trace amounts of water to prevent freezing in the subsequent  cold product separation and to remove hydrogen sulphide. A typical dryer consists of two or more columns packed with molecular sieves. As the wet stream is processed in one column, the other is regenerating. The dryer switching valves play an important role in directing the inlet/outlet stream of gas between the dryer columns, thus switching the columns from an adsorption phase into the regeneration phase in a preset sequence. The valves have to withstand these fluctuations in temperature (from ambient to around 250°C) 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, therefore 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, like Metso’s ball and triple eccentric disc valves, have been widely used for these kinds of demanding switching applications. Special hard coatings, such as carbides, are commonly used in this type of application.

Double seated disc valve (left) from Metso Automation compared to a gate valve (right).

Double seated disc valve (left) from Metso Automation compared to a gate valve (right).

As an example of application based valve selection is a 600 000 tpy PDH plant in China where high performance triple eccentric disc valves with a double seat design were selected as dryer valve over double gate valve. The valves with a double seat design can manage medium with large temperature differences between two sides of the valve and keep bi-directional tightness, which provides a compact solution for dryer valves instead of large size double gate valves. Compared with double gate valves, high performance triple eccentric disc valves have many advantages such as less weight and cost saving. The rotary stem does not tend to move process media into the gland packing like a rising stem, and it is able to tolerate the piping forces.

Remember safety…

Looking for valves to PDH process heater burner shutoff and ESD? Do you need safe gas depressurizing for your selective hydrogenation reactor? Careful selection of valves for the propane dehydrogenation process provides ways to increase process efficiency, productivity and safety. Single-source responsibility for all components manufactured, fully tested and configured will ensure the proper performance of valves from reactor section to product recovery and catalyst regeneration throughout the whole life cycle. 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.

Related posts:

Safe offshore production – part 2: converting data into valuable information
Safe offshore production – part 1: challenges and role of automation

 


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Sari Aronen

Oil & Gas Application Manager, Metso Flow Control

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