Page 159 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: GTY/GWT P2: GLM Final
Encyclopedia of Physical Science and Technology EN006K-933 June 29, 2001 12:14

Fuel Chemistry 269

CO + 2H 2 −→ CH 3 OH (Methanol Synthesis) widely in Germany during the Second World War, due to
poor economics it was discontinued after the war. This
2CH 3 OH −→ CH 3 OCH 3 + H 2 O
process is used in South Africa (Sasol) for reasons other
(Methanol-to-Gasoline)
than economics.
nCO + 2nH 2 −→ ( CH 2 )n + H 2 O
(Fischer–Tropsch Synthesis) 4. Developments in Liquefaction Processes
nCO + (2n + 1)H 2 −→ ( CH 2 ) n+1 + H 2 O. Lower operating temperatures are desirable in direct liq-
uefaction processes since higher temperatures tend to pro-
Direct liquefaction of coal can be achieved with and with-
mote cracking of molecules producing more gaseous and
out catalysts using pressures of 250 to 700 atmospheres
solid products at the expense of liquids. Similarly, lower
◦
and temperatures ranging between 425 and 480 C. In the
pressures are desirable from an ease and cost-of-operation
indirect liquefaction process, coal is gasified to produce
point of view. Recent research efforts in the area of direct
synthesis gas and cleaned to remove impurity gases and
liquefaction have concentrated on reducing the operating
solids. The processes used to clean the gases depend on
pressure, improving the separation process by using a hy-
the impurities.
drogen donor solvent (Consol Synthetic Fuels Process),
The principal variables that affect the yield and distri-
operation without catalysts (Solvent Refined Coal (SRC)),
bution of products in direct liquefaction are the solvent
and by using a solvent without catalysts (Exxon Donor
properties such as stability and hydrogen transfer capa-
Solvent) but using external catalytic rehydrogenation of
bility, coal rank and maceral composition, reaction condi-
the solvent. Catalytic effects in liquefaction are due to the
tions, and the presence or absence of catalytic effects. Bi-
inherent mineral matter in the coal and to added catalysts.
tuminous coals are the most suitable feed-stock for direct
Recent research efforts have focused on the area of multi-
liquefaction as they produce the highest yields of desir-
stage liquefaction to minimize hydrogen consumption and
able liquids, although most coals (except anthracites) can
maximize overall process yields.
be converted into liquid products. Medium rank coals are
Later versions of Sasol plant (Sasol 2 and 3 units which
themostreactive(reactfast)underliquefactionconditions.
also use indirect liquefaction process) have used only syn-
Among the various petrographic components, the sum of
thol reactors to increase the yield of gasoline and have
the vitrinite and liptinite maceral contents correlates well
reacted excess methane with steam to produce more CO
with the total yield of liquid products.
and H 2 .
Recent developments include producing liquid fuels
3. Liquefaction Processes
from synthesis gas through an intermediate step of con-
The Bergius process was the first commercially available verting the synthesis gas into methanol at relatively low
liquefaction process. It was developed during the First operating pressures (750–1500 psi) and temperatures
◦
World War and involves dissolving coal in a recycled sol- (205–300 C). Methanol is then converted into a range of
vent oil and reacting with hydrogen under high pressures liquid hydrocarbons. The use of zeolite catalysts (as de-
ranging from 200 to 700 atmospheres. An iron oxide cat- veloped by Mobil) has enabled the direct production of
alyst is also employed. The temperatures in the reactor gasoline from methanol with high efficiency.
were in the range of 425–480 C. Light and heavy liquid
◦
fractions are separated from the ash to produce gasoline X. LIQUID FUELS
and recycle oil, respectively. In general, 1 ton of coal pro-
duces about 150–170 liters of gasoline, 190 liters of diesel Liquid fuels are obtained by refining naturally occurring
fuel, and 130 liters of fuel oil. Separation of ash and heavy crude oil. Like coal, crude oil from different places can
liquids, and erosion due to cyclic pressurization, posed dif- differ in composition because of the precursor materi-
ficulties which caused the process to be taken out of use als and the conditions for transformation organic matter
after the war. to crude oil. Crude oil is a complex naturally occurring
In the first generation, in a commercially operated, in- liquid containing mostly hydrocarbons and some com-
direct liquefaction process called Fischer–Tropsch syn- pounds containing N, S, and O atoms. Crude oil consists of
thesis, coal is gasified first using the high-pressure Lurgi paraffins (straight-chain and branched-chain compounds),
gasifier and the synthesis gas is reacted over an iron-based naphthenes (cyclo paraffins), and aromatics (benzene and
catalyst either in a fixed-bed Arge reactor or a fluidized- its derivatives).
bedSyntholreactor.Dependingonthereactionconditions, The average composition of crude oils from various
the products obtained consist of a wide range of hydro- parts of the world does not vary significantly. However,
carbons. Although this process was developed and used because of the variations in viscosity, density, sulfur, and