Page 80 - Fluid Catalytic Cracking Handbook

58 Fluid Catalytic Cracking Handbook

UOP Test Method 313 is commonly employed to determine the
basic nitrogen content of FCC feed. The feed sample is first mixed
50/50 with acetic acid. The mixture is then titrated with perchloric acid.
ASTM Method D-3228 (or chemiluminescent nitrogen detector) is
often employed to measure the total nitrogen and involves converting
all the nitrogen in the feed to ammonia and then titrating it with
standard sulfuric acid. Another method (ASTM D-4629) is used to
measure the total nitrogen through oxidative combustion and chemilu-
minescent detection.

Sulfur

FCC feedstocks contain sulfur in the form of organic-sulfur com-
pounds such as mercaptan, sulfide, and thiophenes. Frequently, as the
residue content of crade oil increases, so does the sulfur content (Table
2-5). Total sulfur in FCC feed is determined by the wavelength
dispersive x-ray fluorescence spectrometry method (ASTM D-2622).
The results are expressed as elemental sulfur.
Although desulfurization is not the goal of cat cracking operations,
approximately 50% of sulfur in the feed is converted to H 2S. In
addition, the remaining sulfur compounds in the FCC products are
lighter and can be desulfurized by low-pressure hydrodesulfuriza-
tion processing.
In the FCC, H 2S is formed principally by the catalytic decomposi-
tion of non-thiophenic (non-ring) sulfur compounds. Table 2-3 shows
the effects of feedstock sulfur compounds on H 2S production.
As with H 2S, the distribution of sulfur among the other FCC pro-
ducts depends on several factors, which include feed, catalyst type,
conversion, and operating conditions. Feed type and residence time are
the most significant variables. Sulfur distribution in FCC products of
several feedstocks is shown in Table 2-4. Figure 2-9 illustrates the
sulfur distribution as a function of the unit conversion.
For nonhydrotreated feeds at 78 vol% conversion, about 50 wt% of
the sulfur in the feed is converted to hydrogen sulfide (H 2S). The
remaining 50% of the sulfur is distributed approximately as follows:
» 6 wt% in gasoline
« 23 wt% in light cycle oil
* 15 wt% in decanted oil
• 6 wt% in coke