(48a) Operation of an Industrial Smb Unit for P-Xylene Separation Accounting for Adsorbent Ageing Problems
Among the first and relevant applications of the Simulated Moving Bed (SMB) concept, as patented by Brougthon and Gerhold (1961), was the separation of p-xylene from C8 isomers mixtures at the petrochemical aromatic sites. With Parex® process from UOP (Broughton et al., 1970), and the later introduction of the Eluxyl® process from IFP (Ash et al., 1994), it is possible that around 60 % of the total world p-xylene production is now performed by the application of this kind of adsorptive technology combined with an isomerisation reactor (Minceva and Rodrigues 2007). Stressing out the all importance that p xylene represents nowadays as one of the biggest world commodities, used as a raw material in the manufacture of polyethylene terephthalate (PET) in production of polyester fibbers, molded plastics, films, and blown beverage bottles, it can be perceived that although p-xylene separation has been one of the first is still quite prominent in the SMB separation field. 50 years of experience on SMB application to p-xylene separation, given it the rank of ?Old? application, nevertheless, and despite all the work published or patented innovations related with this technology, just a few refer to the adsorbent ageing problems among this type of units (Minceva and Rodrigues, 2002). The information considering this issue remains restricted to the plant operators and technology owners and mainly particle size re-distribution and capacity decline are mentioned as a result of adsorbent ageing leading to process performance decrease. It is from the common wisdom the use of guard beds, as in the gas processing industry, avoiding impurities to enter in the separation process, or more elaborated methods such as online adsorbent removal (Pilliod et al., 2006a,b) and innovative fluid distribution apparatus (Frey et al., 2006), to mitigate fines production. Nevertheless, ageing problems such as ?parasite? reactions with the solid or liquid phase, and other attrition effects, leading to fine and channelling continue to appear as time goes by during these units operation. By treating adsorbent ageing as a disturbance it is possible to compensate it by applying SMB online controllers as pointed out by Engell or Morari research groups (Natarajan and Lee, 2000; Abel et al., 2004, or in Engell, 2007). However, the implementation of model predictive control methods should also be based on robust ageing models as compensative measures. Additionally, when the problem is an all complete new unit design the study and analysis of adsorbent ageing factors can have a significant weight on the financial management choices involved on these projects. The possibility of extended life to one of the most predominant economical factors on these units design, the adsorbent cost, can have considerable influence on all unit complete design, if not it the unit viability. Additionally, concepts as sustainable development (Brundtland et al., 1987), the problematic of resources depletion and eco-efficient techniques have to be taken into account in the 21st century innovation path. Extending the sieve life is also a way of driving this kind of separation technology throughout the sustainable lane. A general study on SMB adsorbent ageing has recently been presented by Rodrigues in last AIChE meeting (Sá Gomes et al., 2006), or the more detailed study submitted to publication (Sá Gomes et al., 2007), but still modelling-based research is required to define the ageing problem consequences and indicate strategies that can be used to mitigate capacity loss or other deactivation effects on this particular industrial separation, as involving the use on the so-called non-conventional operating modes as Varicol®, as pointed out in Sá Gomes et al., 2007 for ageing in chiral separations case. Therefore, is objective of this work the presentation of a complete analysis to the adsorbent ageing effect on the performance parameters of an industrial SMB unit for p-xylene separation as well as the development of some strategies to overcome this problem.
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