Engineering Chemistry S Datta

396 ENGINEERING CHEMISTRY
Similar to cracking, reforming is also thermal and catalytic. Thermal reforming has
been almost completely replaced by catalytic reforming.
The catalysts used are molybdena, chromia supported on alumina, platinum on silica or
alumina. The latter is more active, but gets poisoned easily by the impurities like S, As, N etc.
Reforming is a once through process. Fluid bed and moving bed processes are run at
higher temperature and lower pressures and fixed bed processes are run at higher pressures
(temperature 450°C–520°C, pressure 5–45 kg/cm 2 ) (Fig. 18.9).
Low boiling
(to avoid explosive
reaction with H ) 2
H2
compressed
and recycled
Compressor
(25 atm)
Diethanolamine
(DEA)
Gas
H2S
scrubber
Fractionator
Reformed
or
stabilized
gasoline
Gasoline
feed stock
Fractionator
Residue
Heater
at 500°C
Fraction
b.p.
100–170°C
Reactor
containing
catalytic
bed
(Pt–Al O ) 2 2
Reactor
(Pt–
AlO)
2 3
Reactor
(Pt–
AlO)
2 3 Condenser
To
solvent
DEA
recover
Heater
at 500°C
Heater
at 500°C
Fig. 18.9 Fixed-bed catalytic reforming process.
Refining of Gasoline
The straight-run gasoline obtained from the fractionation of crude petroleum contains
undesirable constituents like unsaturated straight chain hydrocarbons and sulfur compounds.
The former leads to gum and sludge formation and the latter leads to corrosion of engines and
pollution of atmosphere. These are removed by refining processes.
(i) Objectionable odour forming mercaptans and H 2
S make the oil sour. Oil sample
treated with sodium plumbite and a little sulfur converts the sulfur compounds into
disulfides (Doctor’s process). The black PbS precipitates and the other disulfides are
extracted with a suitable solvent, rendering the foul smelling stock to sweet smelling
or odourless stock and the process is called sweetening process.
2RSH + Na 2
PbO 2
⎯⎯→ Pb(RS) 2
+ 2NaOH
Pb(RS) 2
+ S ⎯⎯→ PbS + RSSR
disulfide
(ii) Gasoline is percolated through “Fuller’s earth” which absorbs the olefins and coloring
matters present in it.
(iii) The storing quality of the refined gasoline is improved by adding different inhibitors
of oxidations to it.
(iv) Refined gasoline is blended with suitable fractions of catalytically cracked gasoline
to impart good combustion qualities.