Understanding how molecular sieving works
A typical molecular sieve consists of two or more columns packed with molecular sieves, such as zeolite-based adsorbents. Special characteristics of these sieves include reversible adsorption of various gas or liquid compounds and a network of cavities and narrow pores offering an extremely high internal surface area.
As wet or sour steam is processed in one column, the other is regenerating. The molecular sieve adsorbs impurities in a down-flow direction. When the molecular sieve bed reaches maximum saturation, the inlet stream switches to the second column and regeneration starts in the first column.
Molecular sieves can be regenerated by heating the adsorbent with hot gas, steam or by changing the partial pressure. Regeneration gas flows countercurrent to the process gas to prevent the adsorbent from aging, thereby increasing the number of cycles. Cooling with dry gas follows regeneration.
Choosing the right switching valve
Valves used in molecular sieving play a crucial role in switching the columns from an adsorption phase to regeneration and cooling in a preset sequence. They must withstand wide temperature fluctuations and frequent cycling while maintaining tightness in both flow directions for many years.
Material selection and seat construction are key, as they reduce the number of particles entering the seat cavities from the molecular sieve beds and thereby minimize wear from particles adhering to the sealing surfaces. Fugitive emission control is essential in hydrocarbon processing to avoid environmental and health hazards.
Metal-seated ball and butterfly valves have been widely used in such demanding switching applications.
Trunnion-mounted ball valves provide reliable operation and excellent response to high pressure differentials. This design offers proven long-lasting tightness over years of frequent switching despite molecular sieve dust and constant temperature changes. Butterfly valves are optimal for large sizes and moderate pressure levels (10–20 bar). Triple-eccentric disc designs provide a viable option for switching molecular sieves.
It is vital, however, to understand process conditions in order to select the right valve that will ensure continuous drying or the separation of fluids.
Transparency provides view on performance
For critical applications, such as LNG liquefaction or refinery isomerization, the plant must be able to assess the valve’s performance, especially when predicting or planning maintenance activities.
Intelligent digital controllers, such as Neles® SwitchGuard™ with diagnostic capabilities combined with asset management solutions, take start-up operations and maintenance planning for switching applications to a higher level by providing a transparent view of how the automated switching valves are performing in the molecular sieve. These tools further secure an entire plant’s availability.
With the race to cleaner fuels that comply with the coming regulations, molecular sieve adsorption will take on an even bigger role in helping plants reach total process availability and sustainable fuel production. The selection of reliable molecular sieve switching valves will also grow in importance as they support the use and development of cleaner and more economical processes and fuels, including isomerization, LNG and bioethanol.
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Oil & Gas Application Manager, Metso Flow Control