QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:

B.CATALYST AND ADDITIVES:

Q-32:

What conditions or contaminants will deactivate ZSM-5 additive? What is the half-life of ZSM-5 in clean feed operation? Will contaminants such as vanadium, sodium or other metals adversely affect the propylene selectivity of ZSM-5? What is the best way to monitor the effectiveness of the ZSM-5?

A-32:

 The deactivation mechanism for FCC catalyst is primarily related to unit cell size reduction and, eventually, collapse or sintering of the zeolite crystal. The mechanism of ZSM-5 deactivation is quite different.

The deactivation mechanism is simply the dealumination of the ZSM-5 crystal, and activity is lost through the loss of active aluminum sites. The crystal structure does not collapse. The activity retention and half-life of the ZSM-5 additive in the circulating inventory is strongly affected by hydrothermal conditions within the regenerator with temperature being the dominant variable.
ZSM-5 additive activity is less affected by contaminant metals than is FCC catalyst due to the fact that heavy feed molecules containing contaminant metals, such as vanadium, are less likely to crack on ZSM-5. ZSM-5 will therefore maintain its activity longer than  the FCC catalyst.

It is worth pointing out that a unit experiencing high equilibrium vanadium levels will likely experience a loss in conversion, which will reduce LPG yields. This loss may give the appearance of a ZSM-5 effect. The propylene selectivity will likely remain unaffected.

The activity retention difference between ZSM-5 and FCC catalyst will increase as the equilibrium metals level increases. Intercat has evaluated ZSM-5 additive half-lives for several units and found a typical half-life of about 18 days, with a minimum of 2 days and a maximum of 36 days.

ZSM-5 additive activity in an operating unit is strongly affected by the catalyst replacement rate. Units having a very high replacement rate present a higher average ZSM-5 activity than units with very low changeout rate. A paper presented at the 2000 American Chemical Society conference investigated the subject of LPG selectivity differences in detail. This study reviewed additives having different ZSM-5 crystal content, different levels of additive additions in the FCC, additives from different manufacturers, additives with different silica-to-alumina ratios, and additives steamed at different severities.

The results of the study   demonstrate that if one additive were more selective than another, the propylene yield would fall on a different line, which did not occur. All additives tested at all concentrations fell on the same line. We also found that the propylene yield increases faster than butylene yield and that higher delta LPG yield leads to higher propylene yields
.
Additive zeolite content, type, method of manufacture, and steaming severity have no effect on the selectivity of the final LPG product. The ratio of propylene to butylene in the final product depends only on how much LPG is made. The conclusion is that ZSM-5 additive selectivity is determined by the zeolite structure alone. Therefore, measuring activity differences is more important than looking for selectivity differences with a standard ZSM-5 additive. (Please note that these results apply only to standard ZSM-5 technology.)

While propylene selectivities are determined by the ZSM-5 crystal structure, the activity and stability of the various additives are determined by the crystal stabilization technology employed plus the interaction of the crystal with the matrix. An additive containing properly stabilized ZSM-5 crystal, combined with a strong matrix, will result in excellent activity retention with superior propylene yields when compared to other technologies.

Intercat and other suppliers have invested significantly in development of ZSM-5 additive technology, which is reflected in our broad product portfolio. Intercat possesses an extensive range of ZSM-5 additives maximizing propylene, butylene, and octanes. Additionally, Intercat produces ZSM-5 additives that minimize LPG increase for wet gas compressor-limited operations.

There are several selectivity-based ratios that can be used in monitoring ZSM-5 performance. These include: propane olefinicity, propylene yield vs. LPG, propylene vs. conversion, propylene vs. butylene, and propylene vs. gasoline. The most important of these ratios are the propane olefinicity and the propylene-to-LPG.