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Sep 10, 2012
Adsorption and molecular sieve technologies have been widely used throughout various processing industries for decades to separate hydrocarbons or dry gases. Automation, including switching valve selection has an important role to play in the successful and sustainable molecular sieve adsorption process, often critical for the whole process plant availability.
Neles has a long history of flow control solutions to adsorptive processes, such as cracked gas dryers and LNG dryers. Valve selection depends on the actual application and specific requirements. There are, however, a lot of similar challenges to be found in molecular sieve valve applications, and this aspect puts automation companies, such as Neles, in an interesting position as a link between different industries with profound application knowledge and experience.
What is molecular sieving?
Typical molecular sieve consists of two or more columns packed with molecular sieves, typically zeolite based adsorbents. The special characteristics include reversible adsorption of various gas or liquid compounds and a network of cavities and narrow pores offering a very high internal surface area. As the wet or sour stream is processed in one column, the other is regenerating. Depending on the volumes being treated, a multi adsorber system can be implemented. The molecular sieve adsorbs the impurities in a downflow direction. When the molecular sieve bed approaches its maximum saturation, the inlet stream is switched to the second column and regeneration is started in the first column. Molecular sieves can be regenerated by heating the adsorbent with hot gas, steam or by changing the partial pressure (pressure swing). Regeneration gas flows counter currently to the process gas at temperatures of 200 – 320 ˚C. Countercurrent regeneration is used to prevent the aging of the adsorbent and therefore enhances the number of cycles. Cooling with dry gas follows regeneration.
Ball or butterfly?
The switching valves in molecular sieving play an important role in switching the columns from an adsorption phase into the regeneration and cooling phase in a preset sequence. The valves have to withstand fluctuations in temperature and frequent cycling while keeping the tightness in both flow directions over the years of operation.
Care must be taken in material selection and seat construction in order to avoid any wear with particles coming from mol sieve beds entering the seat cavities and adhering to sealing surfaces. Fugitive emission control is very important in hydrocarbon processing to avoid hazards to the environment and to health.
Neles Trunnion mounted ball valve.
Metal seated valves, such as Neles’ ball and butterfly valves, have been widely used for these kinds of demanding switching applications. Trunnion mounted ball valves are selected for their reliable operation and excellent response with high pressure differentials. This design has proved its long lasting tightness over years of frequent switching with molecular sieve dust present and constant temperature changes. It seems to be a trend nowadays, that butterfly valves are considered in certain cases, where the sizes are large and pressures at moderate levels (10 – 20 bar).
Triple eccentric disc designs provide an interesting option for switching molecular sieves. It is important, however to fully understand the process conditions for correct valve selection to ensure continuous drying or separation of fluids for application in question.
Transparency to valve’s performance
It is important for the owner/operator to be able to see how the valve is performing in this critical application, especially with regards to predicting and planning the maintenance activities. Intelligent digital controllers with diagnostic capabilities combined with asset management solutions, such as Neles SwitchGuard™ and FieldCare, take the start up, operation and maintenance planning for molecular sieve switching applications to an advanced level. Most importantly, during the course of the process it is possible to see what is going on in the automated switching valves in the molecular sieve. This is often critical for the whole plant’s availability, for example in LNG liquefaction or refinery isomerisation.
Reliable molecular sieve switching valves will support the use and development of cleaner and more economical processes and fuels, such as isomerisation, LNG and bioethanol, where molecular sieve adsorption plays an important role in total process availability and sustainable fuel production.