Page 206 - Synthetic Fuels Handbook

192 CHAPTER SIX

exothermic nature of the hydroretorting reactions, less energy input is required per barrel
of product obtained. Furthermore, there is practically no upper or lower limit on the grade
of oil shale that can be treated.
Hydrocracking is a cracking process in which higher-molecular-weight hydrocarbons
pyrolyze to lower-molecular-weight paraffins and olefins in the presence of hydrogen. The
hydrogen saturates the olefins formed during the cracking process. Hydrocracking is used
to process low-value stocks with high heavy metal content. It is also suitable for highly
aromatic feeds that cannot be processed easily by conventional catalytic cracking. Shale
oils are not highly aromatic, whereas coal liquids are very highly aromatic.
Middle-distillate (often called mid-distillate) hydrocracking is carried out with a noble
metal catalyst. The average reactor temperature is 480°C, and the average pressure is
around 130 to 140 atm. The most common form of hydrocracking is carried out as a two-
stage operation. The first stage is to remove nitrogen compounds and heavy aromatics
from the raw crude, whereas the second stage is to carry out selective hydrocracking reac-
tions on the cleaner oil from the first stage. Both stages are processed catalytically. Once
the hydrocracking stages are over, the products go to a distillation section that consists of
a hydrogen sulfide stripper and a recycle splitter. Commercial hydrocracking processes
include Gulf HDS, H-Oil, IFP Hydrocracking, Isocracking, LC-Fining, Microcat-RC (also
known as M-Coke), Mild Hydrocracking, Mild Resid Hydrocracking (MRH), Residfining,
Unicracking, and Veba Combi-Cracking (VCC).
Arsenic removed from the oil by hydrotreating remains on the catalyst, generating a
material that is a carcinogen, an acute poison, and a chronic poison. The catalyst must be
removed and replaced when its capacity to hold arsenic is reached. Unocal found that its
disposal options were limited.

6.6 THE FUTURE

Oil shale still has a future and remains a viable option for the production of liquid fuels.
Many of the companies involved in earlier oil shale projects still hold their oil shale technol-
ogy and resource assets. The body of knowledge and understanding established by these
past efforts provides the foundation for ongoing advances in shale oil production, min-
ing, retorting, and processing technology and supports the growing worldwide interest and
activity in oil shale development. In fact, in many cases, the technologies developed to pro-
duce and process kerogen oil from shale have not been abandoned, but rather mothballed
for adaptation and application at a future date when market demand would increase and
major capital investments for oil shale projects could be justified.
The fundamental problem with all oil shale technologies is the need to provide large
amounts of heat energy to decompose the kerogen to liquid and gas products. More than
1 t of shale must be heated to temperatures in the range 454 to 537ºC (850º to 1000ºF) for
each barrel of oil generated, and the heat supplied must be of relatively high quality to reach
retorting temperature. Once the reaction is complete, recovering sensible heat from the
hot rock is very desirable for optimum process economics. This leads to three areas where
new technology could improve the economics of oil recovery: (a) recovering heat from the
spent shale, (b) disposal of spent shale, especially if the shale is discharged at temperatures
where the char can catch fire in the air, and (c) concurrent generation of large volumes of
carbon dioxide.
The heat recovery from hot solids is generally not efficient, unless it is in the area of
fluidized bed technology. However, to apply fluidized bed technology to oil shale would
require grinding the shale to sizes less than about 1 mm, an energy intensive task that would
result in an expensive disposal problem. However, such fine particles might be used in a