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Abstract(s)
The aim of this work is to contribute for the development of adsorption based
separation processes with considerable potential for commercial application on the
refining industry, namely, in the separation of high research octane number (HRON)
paraffins from light naphtha fractions. The development of an adsorption process
requires first a detailed knowledge of equilibria and kinetics of adsorption and their
impact on the dynamic response of an adsorption column. Accordingly, we start
collecting single and mixture adsorption equilibrium isotherms of C6 isomers, n-hexane
(nHEX), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB), and 2,2-
dimethylbutane (22DMB), from breakthrough experiments in zeolite beta. This
adsorbent was selected because its pore system posses interesting characteristics for
the separation of HRON dibranched C6 from their low research octane number (LRON)
monobranched isomers. It was found that the sorption hierarchy in zeolite beta was
most favourable towards the linear isomer and least favourable towards the dibranched
ones. Zeolite beta demonstrated significant selectivity to discriminate
between mono and dibranched C6 isomers, especially at low coverage. Based on an
analysis of sorption events at the molecular level, a Tri-Site Langmuir model (TSL) was
developed to interpret the equilibrium data with good accuracy.
Sorption kinetics studied by zero-length chromatography technique allowed us to
find the nature of controlling diffusion mechanism; for nHEX and 3MP macropore
diffusion is controlling. For 23DMB and 22DMB, the system is governed apparently by
both macropore and micropore diffusion.
The dynamics of equimolar C5/C6 paraffin fractions in a fixed bed of zeolite beta was
studied. Breakthrough experiments demonstrate that the sorption hierarchy is
temperature-dependent. At 583 K, an enriched HRON fraction of 22DMB, iso-pentane
(iPEN) and 23DMB can be selectively separated from the isomerate feed. For the case
of feed mixtures with the typical composition of the hydroisomerization reactor
product, the enriched fraction contains LRON n-pentane (nPEN) which decreases the
octane quality of the product obtained. However, the use of a layered bed with zeolite
5A and zeolite beta can displace the nPEN from the enriched fraction, resulting in a
maximum octane number of about 92.5 points. Aspen Adsim was used to simulate the
dynamic behaviour of the C5/C6 fraction in a non-isothermal and non-adiabatic bed
giving a good description of the set of experimental data. An optimal design of a
mono/dibranched separation process can be achieved by properly tuning the
operating temperature and the zeolite 5A/zeolite beta ratio on a layered fixed bed. The performance of a layered pressure swing adsorption (PSA) process for the
separation of HRON paraffins from a C5/C6 light naphtha fraction is simulated using a
detailed, adiabatic single column PSA model. A zeolite 5A layer is used for selective
adsorption of LRON n-paraffins while a zeolite beta layer is used to reduce the
concentration of the LRON 3MP in the HRON fraction. The effects of various independent
process variables (zeolite 5A-to-zeolite beta ratio, purge-to-feed ratio, cycle time,
depressurization mode and operating temperature) on the process performance
(product RON, HRON molecules recovery, HRON purity, and process productivity) are
evaluated. It is demonstrated that an optimal zeolite 5A-to-zeolite beta ratio can
improve the product average RON of up to 1.0 point comparatively to existing
processes using zeolite 5A only. Moreover, process simulations demonstrated that an
increase of 20 K in the operating temperature results in octane gain of 0.2 RON.
The study and development of membrane technologies was also included in this
work as an alternative to PSA processes. The preparation of supported zeolite beta
membranes was successfully achieved by exploring several combinations of seeding
techniques and synthesis methods. The surface of the membranes was completely
covered by well intergrown crystals. The quality of the membranes was tested by
means of pervaporation of ethanol/1,3,5-triisopropylbenzene mixtures together with
permporometry experiments. The performance in the vapour separation of quaternary
equimolar mixtures of C6 isomers showed that permeate flux decreases as the
branching degree increases following the order: nHEX>>3MP>23DMB>22DMB. In the
retentate, the fractions of 3MP and nHEX decrease while the concentration of
dibranched isomers is increased compared to the feed composition. The RON of the
quaternary mixture was enhanced up to 5 points with the best synthesized membrane.
The potential application of the novel metal-organic frameworks (MOFs) as an
alternative to zeolites was also addressed. A screening study for mixtures of C6 isomers
was performed in three different MOFs.The first is a rigid zirconium terephthalate UiO-
66, which possesses two types of cages of diameter 12 Å and 9 Å; the second is a
chromium trimesate MIL-100(Cr), which possesses a rigid structure with giant cages
accessible through 5-9 Å microporous windows; and the third is the flexible Zn2
(BDC)2(H2O)2·(DMF) (MOF-2), in which the pore system contains 1-D large channels.
Multicomponent equimolar experiments show that UiO-66 exhibits inverse shape
selectivity for C6 isomers, being the retention governed by the rotational freedom of
the molecules in the small cages. In the MIL-100(Cr), the sorption hierarchy is similar to
the one found in zeolite beta. Finally, MOF-2 exhibits extraordinary n/iso selectivity, by
making use of an unusual guest-dependent dynamic behaviour to exclusively take up
nHEX, while hindering the access of branched C6 isomers to the pore system.
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Citation
Bárcia, Patrick da Silva (2010). Separation of light naphtha for the octane upgrading of gasoline: adsorption and membrane technologies and new adsorbents. Porto: FEUP. Tese de Doutoramento em Chemical and Biological Engineering