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.
