FOSSIL FUEL CLEANING PROCESSES 415
contents of petroleum stocks are mainly in the form of thio-
phenes and thiophanes and these can be removed only by
catalytic decomposition in the presence of hydrogen. The
Union Oil Company has developed a cobalt molybdate
desulfurization catalyst capable of handling the full range of
petroleum stocks encountered in refi ning operations. Even
the more refractory sulfur compounds associated with these
stocks are removed. This catalyst exhibited excellent abra-
sion resistance and heat stability, retaining its activity and
strength after calcination in air at temperature as high as
1470 F.^8 Cobalt molybdate may be considered a chemical
union of cobalt oxide and molybdic oxide, CoO · MoO 3. The
high activity of this compound is due to an actual chemical
combination of these oxides with a resultant alteration of the
spacing of the various atoms in the crystal lattice.^8 Catalyst
life is two to fi ve years. Catalyst poisons consisted of carbon,
sulfur nitrogen and polymers. Regeneration is accomplished
at 700 to 1200F using air with steam or fl ue gases.
The fundamental reactions in desulfurization are as
follows:
General ReactionC n H m Sp x H 2 → C n Hm 2 x (^) 2 p p H 2 S
Desulfurization of ethyl mercaptan
C 2 H 5 SH H 2 → C 2 H 6 H 2 S ∆ H 19.56 kg cal/mole
Desulfurization of diethyl sulfi de
(C 2 H 5 ) 2 S 2H 2 → 2C 2 H 6 H 2 S ∆ H 36.54 kg cal/mole
Desulfurization of thiophene
C 4 H 4 S 4H 2 →C 4 H 10 H 2 S ∆ H 73.26 kg cal/mole
Desulfurization of amylene
C 5 H 10 H 2 →C 5 H 12 ∆ H −33.48 kg cal/mole.
The change in heat content for all these reactions is negative,
indicating that they take place with evolution of heat. The
sulfur content in Middle East Gas Oil, a typical feed, is 1.25%
by weight. The pilot plant data shows that the heat effect is not
serious and whole process can be treated as isothermal.
The chemical reaction process on the catalyst is postu-
lated to proceed on the surface of the catalyst by interaction
of the sulfur-bearing molecules and hydrogen atoms formed
through activated absorption of hydrogen molecules.^9 Oil
molecules are more strongly absorbed than hydrogen mol-
ecules, and therefore may preferentially cover part of the
surface, leaving less surface available for dissociation of
hydrogen molecules. In the presence of diluent, namely, N 2 ,
it can also compete for free sites on the surface, and accord-
ingly may cause a reduction in the concentration of hydro-
gen on the surface, thus giving the lower rate constant when
working with H 2 −N 2 mixture.
Conversion of the sulfur compounds to hydrogen sulfi de
and saturated hydrocarbons occurs by cleavage of the sulfur
to carbon bonds; essentially no C—C bonds are broken.
Residuum Desulfurization
The H-Oil-process ( Cities Service ) In order to meet the need
for an effi cient method of desulfurizing residual oils with-
out the complexities encountered in the myriad of existing
fi xed bed catalytic systems, Cities Service developed what is
known as the H-Oil system.
Although fi xed bed catalytic reactors had been extensively
used for desulfurizing distillate oils, desulfurization of residual
oil in a fi xed bed reactor presented several diffi culties:
- the high temperature rise through the bed tended
to cause hot spots and coking, - the presence of solids in the feed and the forma-
tion of tar-like coke deposits on the catalyst tended
to cause a gradual build-up of pressure drop over
they catalyst bed and - because of the relatively rapid deactivation of the
catalysts, system shut down for catalyst replacement
occurred often, on the order of six times yearly.
To overcome these problems an ebbulated bed reactor
was designed. Figure 3 is a simplifi ed drawing of reactor
workings.
The feed oil is mixed with the recycle and makeup
hydrogen gas and enters the bottom of the reactor. It passes
up through the distributor plate which distributes the oil and
gas evenly across the reactor.
The reaction zone consists of a liquid phase with gas
bubbling through and with the catalyst particles suspended
in the liquid, and in random motion. It is a back-mixed, iso-
thermal reactor, with a temperature gradient between any
two points in the reactor no greater than 5F.
Due to the catalyst suspension in liquid phase, cata-
lyst particles do not tend to adhere to one another, causing
blockage of fl ow. Any solids present in the feed pass directly
through the reactor. Reactor pressure drop is constant.
One of the more important aspects of the ebbulated
bed reactor system is that periodic shutdowns for catalyst
replacement is not necessary. Daily catalyst replacement
results in a steady state activity.
Table 2 shows examples of H-Oil desulfurization perfor-
mance with atmospheric and vacuum residuals. In addition,
investment and operating cost data are shown to illustrate
the important effect of feed stock characteristics on overall
economics.
Cases 1–3 describe processing of three atmospheric resid-
ual feeds. The Kuwait Residuum treated in case 1 is a high
sulfur oil containing relatively low metals content (60 PPM).
Therefore, the rate of catalyst deactivation is low and operat-
ing conditions are set to minimize hydrocracking and maxi-
mize desulfurization. In fact, only 2–3% naphtha and 9–10%
middle distillate are produced. The actual chemical hydrogen
consumption is fairly close to the estimated needed to remove
the sulfur. For many atmospheric residuals which are not too
high in metals, this case is typical to give maximum production
of low sulfur fuel oil at minimum conversion and hydrogen
consumption.
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