Page 74 - Fluid Catalytic Cracking Handbook

52 Fluid Catalytic Cracking Handbook

are such that the Furol viscosity of oil is numerically one-tenth (-^-)
of the Universal viscosity at the same temperature.

Conradson, Rams bottom, Microcarbon,
and Heptane Insolubles

One area of cat cracking not fully understood is the proper determi-
nation of carbon residue of the feed and how it affects the unit's coke
make. Carbon residue is defined as the carbonaceous residue formed
after thermal destruction of a sample. Cat crackers are generally
limited in coke burn capacity, therefore, the inclusion of residue in
the feed produces more coke and forces a reduction in FCC throughput.
Conventional gas oil feeds generally have a carbon residue less than
0.5 wt%; for feeds containing resid, the number can be as high as 15 wt%.
Four popular tests are presently used to measure carbon residue or
concarbon of FCC feedstocks:

» Conradson
« Ramsbottom
• Micro-method
* Heptane insolubles
The object is to indicate the relative coke forming tendency of
feedstocks. Each test has advantages and disadvantages, but none of
them provide a rigorous definition of carbon residue or asphaltenes.
The Conradson test (ASTM D-189) measures carbon residue by
evaporative and destructive distillation. The sample is placed in a
preweighed sample dish. The sample is heated, using a gas burner,
until vapor ceases to burn and no blue smoke is observed. After
cooling, the sample dish is reweighed to calculate the percent carbon
residue. The test, though popular, is not a good measure of the coke-
forming tendency of FCC feed because it indicates thermal, rather than
catalytic, coke. In addition, the test is labor intensive and is usually
not reproducible, and the procedure tends to be subjective.
The Ramsbottom test (ASTM D-524) is also used to measure carbon
residue. The test calls for introducing 4 grams of sample into a pre-
weighed glass bulb, then inserting the bulb in a heated bath for 20
minutes. The bath temperature is maintained at 1,027°F (553°C). After
20 minutes, the sample bulb is cooled and reweighed. Compared with
the Conradson test, Ramsbottom is more precise and reproducible,