Vulcan Series Chloride Removal Technology VGP CRT 2000 and ...

4.0 REMOVAL OF CHLORIDES FROM CATALYTIC REFORMER PRODUCT STREAMS
4.1 Introduction
Traditionally chlorides have been removed from catalytic reformer products streams
using fixed beds of adsorbents. For many years alumina has been used as the
adsorbent. Alumina offered a reasonable capacity for chlorides and was sold at low
commodity prices.
In recent years alumina users have become more aware of its limitations and its
popularity has decreased. The main reasons why alumina limitations have become more
apparent in recent years:
- Analytical techniques have become better and are able to detect slip of chlorides from the
alumina beds.
- Recognition that, alumina beds promote the synthesis of organic chlorides which slip
from the bed.
- Higher severity operations (such as CCR and cyclic style units) are more common and
the problems with alumina are particularly noticeable on these units
- Increasing regulations on disposal of spent catalysts has caused problems for users of
alumina.
4.2 Alumina as a chloride guard
To appreciate the limitations of alumina it is first necessary to understand how alumina
removes chlorides. The alumina surface is covered with active sites, which are either
negatively charged hydroxyl groups or positively charged aluminum sites. These polar
sites are capable of attracting polar molecules, and the more polar the molecule the
greater the attraction.
Hydrogen chloride is quite polar (dipole moment of 1.08 Debyes) and will be adsorbed to
the surface of the alumina. For the more active sites on the alumina there may even be
some disassociation of the hydrogen chloride with the negatively charged chloride
aligning with the aluminum sites and the positively charged hydrogen ion being released
from the surface. However, most of the hydrogen chloride is not disassociated.
Thus most of the hydrogen chloride is removed by physical adsorption on the surface
(rather than by chemisorption where there is actually a rearrangement of the electron
structure between the surface and adsorbing molecule-taking place). The physisorbed
intermolecular forces are weak and sensitive to changes in the operating environment.
The actual concentration of hydrogen chloride at the surface of the alumina is determined
by equilibrium with the hydrogen chloride partial pressure in the gas. This equilibrium
constant is sensitive to changes in:
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