Korotov N.A
IV year student of the Institute of Metallurgy
and Chemical Technology S.B. Leonova of Irkutsk State Technical University
CALCINATION OF PETROLEUM COKE
The task of deepening refining - one of the
most important in the Russian oil refining. Most realistic way to achieve this
goal - the construction of new and reconstruction of existing plants, coking of
petroleum residues.
Calcination process provides a valuable carbon
product - coke.
The resulting crude coking coke are
chemically stable and inert material comprises 88-95% carbon, 3.4% hydrogen,
2.1% nitrogen, 0,58-6% sulfur and 7.1% oxygen.
Petroleum coke is a strategic product that must
constantly be supplied to the smelter. Skilled use of coke in the ferrous and
nonferrous metallurgy, as well as in the production of graphite products
require coke calcination step [1].
The world market in different regions of the
world requires an increasing amount of calcined coke, primarily for the
aluminum industry.
In Russia, more than 80% of calcined coke need
in the aluminum industry. According to the company RUSAL, coke deficit will
grow and by 2017 was about 1 milliontons.
Calcination is crucial, because solves the
following tasks: reducing moisture and devolatilization; ordered structure of
coke, increasing the true density; decrease in electrical resistance;
significant increase in commodity cost of coke by improving its quality.
Calcining of petroleum coke may be carried out
in a rotary tube furnace (Fig. 1). The rotary kiln is a steel pipe with a
diameter of 3.0 - 4.0 m and a length of 45-60 meters, inside a refractory
lined, is inclined at a small angle to the horizontal. The rotation of the
furnace and the slope provides a sliding movement of the rolls and stirring
coke layer thickness of 45-50 cm. The crude coke is loaded at the upper end and
moves along the furnace in a zone of high temperature calcined and discharged
at the lower end. Removal and partial combustion of volatiles gives energy for
calcination. To support combustion furnace counter to the flow of coke is fed
fuel and air. Calciner performs functions: conveyor - moves from 1050 t / h of
coke; countercurrent heat exchanger - heats Loy coke from 20 to 1300˚S;
High Temperature Reactor - produces cracking and combustion of volatiles, the
ordering structure of coke [1].
Furnace chamber is divided into three temperature
zones, corresponding to the stages of calcification:
- Zone 1. Warming and drying of coke: to
400˚S;
- Zone 2: Remove and volatile; 400 - 800 /
1000˚S;
- Zone 3. Seal and growth of crystallites: 1000
-1200 / 1300˚S.
Crude petroleum coke contains 6 - 8% moisture,
which is removed in Zone 1. Coke mixed in the furnace as lipkoplastichnoy mass
with a large angle of repose, a sliding - roll of held high on the wall of the
furnace.
The length of the zone depends on the moisture
content.
In the zone 2 are removed, and thermal cracking
of the volatile substances. Cox is heated to ~ 800 / 1000˚C,
"swell" from the output of volatile and may fluidize, leading to a
longitudinal "Landslide".
Coke calcination is completed in the
calcination zone 3 by the energy of combustion of the fuel. At temperatures
above 800˚S enhanced decomposition of hydrocarbons into hydrogen and
pyrolytic carbon. The latter has the ability to be deposited in the pores of
coke, which helps to seal the coke. Coke layer is heated to 1250 ± 50? C.
Residual hydrogen is reduced to <0.1%. The two-dimensional graphite
structure with loss of lateral connections become more mobile and begin to be
ordered in three-dimensional crystallites. The formation of crystalline
structures is found on electron grams in the form of point reflections. It is
believed that in the initial state raw cokes have a spatial structure of a
polymer consisting of ordered in a two-dimensional plane of the condensed
aromatic rings. The rings are linked in the polymer side chain carbon
constituting disordered part. The excess free energy causes the spontaneous
transition of petroleum coke in a more stable state of the two- and
three-dimensional ordering. Calcination accelerates ordering. Degradation
products of side chains are removed as volatiles. Two-dimensional plane
samouporyadochivayutsya in bundles of parallel layers and form microparticles -
crystallites [2].
Effect of calcination on the properties of
coke.Calciner control parameters are: the final calcination temperature, affects
the value of the actual density, resistivity, crystallite size and reactivity;
calcination time, determined by the length of the furnace, speed, angle and
load: effect on the basic properties of coke; the quantity of air flow: effect
on the yield of coke, fuel consumption, emissions; mode burners specifies
calcination temperature affects fuel consumption, coke yield, the amount of
emissions.
How to increase the productivity of the plant
calcination? Mixing to the extent possible with conventional raw coke
alternative varieties (lower quality) to reduce raw material costs.Increasing
the productivity of individual kilns.Improving the efficiency of the cooling of
coke at high load.Construction of parallel lines calcination. Sieving /
crushing and mixing of different grades of calcined petroleum coke. Designing
control systems environment for the production of calcined nipple using raw
coke in blends with coke sulfur content up to 6 wt%. Improving sampling systems
for controlling the operations of mixing green and calcined coke.Organization
of effective dust control and management of the Straits on the installation as
a whole, including calcined coke dust control oil [3].
Most operating systems RF calcination need
major reconstruction with the aim of increasing the yield of coke, reduce
costs, increase energy efficiency, automation of the process. Increase
turnaround equipped with modern means of control and management, sanitation and
environmental conditions.
List of references
1.
SunyaevZ.I. Production, upgrading and use of petroleum coke - M .: Chemistry,
1973 - 296s.
2. Gimaev
R.I. Refining and petrochemical synthesis. - Ufa: UOU, 1974. - Vol. 16.
3.
Glagoleva O.F. Petroleum coke // World of petroleum products. - ¹3. - 2009. -
39-41s.