Page 216 - Fluid Catalytic Cracking Handbook

184 Fluid Catalytic Cracking Handbook

* Direct blending, where the C 4's are blended into the refinery's
gasoline pool to regulate vapor pressure and to enhance the octane
number. However, new gasoline regulations require reduction of
the vapor pressure, thus displacing a large volume of C 4's for
alternative uses.
* Alkylation, where the olefins are reacted with isobutane to make
a very desirable gasoline blending stock. A Iky late is an attractive
blending component because it has no aromatics or sulfur, low vapor
pressure, low end point, and high research and motor octane ratings.
* MTBE, where isobutylene is reacted with methanol to produce an
oxygenate gasoline additive called methyl tertiary butyl ether
(MTBE). MTBE is added to gasoline to meet the minimum oxygen
requirement for "reformulated" gasoline.
The LPG yield and its olefinicity can be increased by:
* Changing to a catalyst, which minimizes "hydrogen transfer"
reactions
» Increasing the conversion
* Decreasing residence time, particularly the amount of time product
vapors spend in the reactor housing before entering the main column
» Adding ZSM-5 catalyst additive

An FCC catalyst containing zeolite with a low hydrogen transfer
rate reduces resaturation of the olefins in the riser. As stated in Chapter
4, primary cracking products in the riser are highly olefinic. Most of
these olefins are in the gasoline boiling range; the rest appear in the
LPG and LCO boiling range.
The LPG olefins do not crack further, but they can become saturated
by hydrogen transfer. The gasoline and LCO range olefins can be
cracked again to form gasoline range olefins and LPG olefins. The
olefins in the gasoline and LCO range can also cyclize to form
cycloparaffins. The cycloparaffins can react through H 2 transfer with
olefins in the LPG and gasoline to produce aromatics and paraffins.
Therefore, a catalyst that inhibits hydrogen transfer reactions will
increase olefinicity of the LPG,
The conversion increase is accomplished by manipulating the fol-
lowing operating conditions:
* Increasing the reactor temperature. Increasing the reactor tem-
perature beyond the peak gasoline yield results in overcracking