(S.P.Prokopchuk,
12, Kosmonavtov avenue, flat 30, Vinnytsa-21,
Ukraine, 21021)
Abstract:
it
gives a short description of influence of the catalysts,
polar solvents, reaction duration, temperature, pressure,
hydrocarbons concentration in the reaction mixture, oxygen
concentration in the oxidating gas, addition of water into
the oxidation zone, removal of the reaction products out of
the oxidation zone, step-by-step temperature change in the
oxidation zone, step-by-step introduction of the oxygen
containing gas into the oxidation zone on yield and
selectivity of end
products
formation at hydrocarbons liquid phase oxidation.
Keywords:
oxidation, modification, hydrocarbons, selectivity, yield,
catalysts.
Introduction
Hydrocarbons
liquid phase oxidation refers to one of the most available
methods of oxygen containing substances obtaining. However,
this method has not been widely adopted in industry. This
can be explained by formation of a great number of
by-products when end products are formed at liquid phase
oxidation since the oxidation process goes by branch radical
mechanism and it is difficult to regulate it. That is why
selectivity of end products in many processes of
hydrocarbons oxidation does not exceed 30- 40 %. Development
of science and industry enables complex approach to
hydrocarbons liquid phase oxidation process regulation using
both constructional formations of oxidation reactors and
influence of different technological parameters in order to
increase end products selectivity. In view of availability
and cheapness of this process increase of end products
selectivity over 50 % can make this method competitive with
respect to other methods of obtaining of oxygen containing
products.
There are
different ways of increase of selectivity of end products
formation at hydrocarbons liquid phase oxidation. They are:
the use of catalytic systems, effect of technological
parameters change and details of construction of oxidation
reactors.
Discussion
Interaction of
catalytic systems on modification of hydrocarbons
liquidphase oxidation
The widespread
catalytic system relates with the use of metals compounds of
variable valence. As numerous investigations show the
compounds of metals Co, Mn, Cr, Mo etc. are used at
hydrocarbons liquid phase catalytic oxidation
(1)
.
The role of
these catalysts consists in their interaction with primary
products of oxidation – hydro peroxides and peroxiradicals
as well as with secondary products of oxidation- peroacids,
alkyoxyradicals, acyloxyradicals. Interaction of catalysts
with primary products of hydrocarbons oxidation, with
hydroxyl peroxides in particular, exerts considerable
influence on the end products yield. Each of the catalysts
reacts differently with hydroperoxides, thereby partially or
completely changes the ways of conversion of hydrocarbons
liquid phase oxidation intermediate products and influences
their yield.
Cobalt catalyst
interacting with hydroperoxide passes from two-valence state
to three-valence state and promotes hydrocarbon molecule
cleavage with formation of aldehydes, alkyloxy – and
acyloxyradicals which are basically converted to carbon
acids.
Manganese
catalyst (Mn - catalyst ) interacting with hydroperoxide
passes from two-valence state to unstable three-valence
state (2)
. Then
this manganese ion absorbs oxygen molecule and passes to six
or seven – valence state forming intermediate complex
compounds with oxyhydrocarbon radical which is cleavaged to
aldehydes and acyl radicals.
Molybdenum
catalyst (Mo – catalyst) partially stabilizes hydro-
peroxide owing to formation of intermediate complex compound
(3,4)
. This
compound decomposes to oxyradicals which are converted to
ketones or to alcohols without hydrocarbon molecule
cleavage. Thus, cobalt catalyst promotes hydrocarbon
molecule cleavage, manganese catalyst promotes deeper
hydrocarbon oxidation, molybdenum catalyst promotes
formation of oxygen containing compounds with hydrocarbon
molecule keeping. That is why molybdenum catalyst is used
for epoxide reactions of nonsaturated hydrocarbons by
hydroperoxides. Cobalt and manganese catalyst is used to
obtain carbon acids at hydrocarbons oxidation.
To increase
control properties of catalysts it is possible to use
complex catalysts which are composed of two, three or four
compounds of different metals. The uses of such catalysts
enable to increase their control properties. In particular,
utilization of manganese catalysts during light hydrocarbons
fractions oxidation increases considerably the yield of
formic acid and decreases the yield of propionic acid. The
use of two – complex manganese
– chromium (Mn – Cr – catalyst) catalyst at great yield of
formic acid increases the yield of propionic acid, thereby
increases the total selectivity of formation of lower carbon
acids (5)
. On the
whole, complex
catalysts
activity differs slightly from simple catalysts, since each
of catalysts shows its catalytic properties basically during
interaction with hydro peroxides.
Some solvents
are also of interest since they can be used as catalytic
systems. Liquid substances the molecules of which have
strongly polarized chemical bonds or active centers which
consist of a polarized atom refer to them. Fluorcontaining
compounds also refer to such solvents. They can be the
carries of oxygen molecule or ligands in complex compounds
formed at the expense of hydroperoxides or peroxiradicals,
since fluor is the most electronegative element.
Such catalytic
systems can be used as catalysts in some processes of
hydrocarbons liquid phase oxidation, in particular, in the
process of obtaining of propylene oxide by direct oxidation
of propylene.
Interaction of
basic technological parameters on modification of
hydrocarbons liquid phase oxidation.
The process of
hydrocarbons oxidation can be also controlled by changing
the basic technological parameters: reaction duration,
temperature, pressure,
hydrocarbons
concentration in the reaction mixture and oxygen
concentration in the oxidizing gas
(6)
. It is
possible to influence the process of hydrocarbons liquid
phase oxidation itself by changing the reaction duration
only at the stage of initiation of the oxidation process
during the induction period, since in 15-30 minutes after
oxidation starting during induction period the rate of
radical oxidation process increases considerably and only in
1-2 hours it begins to decrease gradually. Such decrease of
reaction rate can apparently be explained by formation of
radicals hydroxide in the process of oxidation which partly
recombine the radicals formed in the process of hydrocarbone
molecule cleavage. By changing the reaction duration it is
possible to increase selectivity of formation of the
reaction primary and secondary products. At the reaction
duration 0.5-2 hours selectivity of formation of primary
products increases: hydroperoxides, ketones and alcohols of
the corresponding hydrocarbons.
At the reaction
duration more than 2 hours the yield of main products of
oxidation and selectivity of formation of oxidation
secondary products increase: carbon acids, alcohols, ketones
and aldehydes formed at the expense of hydrocarbons
oxidizing distruction. Practically, hydrocarbons oxidation
reaction duration is determined by experiment depending on
the process type, periodic or continuous, and oxidation
conditions (temperature).
In the process
of hydrocarbons liquid phase oxidation it is possible to
control partly the yield of the reaction products by
controlling the temperature change. At low temperature the
induction period increases, oxidation rate decreases and the
yield of primary products increases.
At high
temperature the reaction rate increases and the yield of the
oxidation secondary products which are formed at thermal
decomposition of the hydrocarbon which is being oxidized
increases. There is optimum temperature of oxidation
reaction at which maximum yield of end products is achieved.
By changing the
pressure in the process of hydrocarbons liquid phase
oxidation it is possible to change insignificantly
hydrocarbon concentration in the reaction mixture and the
reaction rate. Practically, the pressure is chosen
experimentally depending on the hydrocarbon to be oxidized
and reaction temperature so as to keep hydrocarbon in liquid
state during oxidation.
Hydrocarbon
concentration in the reaction mixture influences, the
process of the hydrocarbon liquid phase oxidation in the
following way.
Hydrocarbons
oxidation in inert solvents at their concentration below 25
% leads to stopping of radical oxidation process since it
becomes difficult to transfer radical to another molecule of
hydrocarbon.
When
hydrocarbon concentration in reaction mixture is 30-40 % the
oxidation process takes place with less degree of branching
and with greater selectivity of final products formation
owing to oxidation cell effect.
When
hydrocarbon concentration is great in the reaction mixture
or during pure hydrocarbons oxidation the branching process
of hydrocarbons liquid phase oxidation with end products
selectivity decrease prevails. Practically, the hydrocarbon
concentration in the reaction mixture has its optimum value
at which the greatest yield and the greatest selectivity of
end products formation is achieved. By changing the oxygen
concentration in oxidizing gas it is also possible to
influence insignificantly the process of hydrocarbons liquid
phase oxidation. When the oxygen concentration is up to 12
vol. % the yield of reaction primary and secondary products
increases insignificantly, their molecules contain small
quantity of oxygen (aldehydes, low-molecular alcohols and
ketones). Under these conditions the reaction rate and
hydrocarbon fractional conversion decrease. Increase of
oxygen concentration up to 21 % increases the reaction rate
and increases insignificantly the yield of reaction products
containg maximum quantity of oxygen in the molecule (carbon
acids, carbon dioxide). Further increase of oxygen
concentration to 36 % at insignificant increase of hydrogens
fractional conversion increases the yield of the carbon
dioxide and decreases selectivity of end products formation.
Interaction of
details construction of reactors on the modification of
hydrocarbons
liquid phase oxidation.
The yield and
selectivity of end products formation at hydrocarbons liquid
phase oxidation can be changed at the expense of water
adding into the oxidation zone, by removing the reaction
products from oxidation zone, by stage change of the
reaction temperature and by stage introduction of
oxygencontaining gas into the oxidation zone in the way of
changing of construction of oxidation reactors. Addition of
water (to 10 mass. %) into reaction mixture decreases
insignificantly oxidation reaction rate and increases
selectivity of forming of some reaction products, in
particular, lower carbon acids. Further increase of water in
reaction
mixture
decreases significantly hydrocarbons oxidation rate and when
water is added over 50 mass % the oxidation reaction
practically stops.
While addition
of water solutions of carbon acids up to 20-30 mass %
increases selectivity of lower carbon axids formation at
insignificant decrease of oxidation rate and hydrocarbons
fractional conversion. The role of water additions into the
reaction mixture is appearantly explained by water
absorbtion of active radicals, by transfer of some products
of reaction into water layer. Since most of the products
oxidize themselves in the reaction zone one of the ways to
increase their yield and selectivity is removing of these
products out of the oxidation zone. For that it is
10
necessary to
carry out the oxidation process in the reactor of “Arlift”
type in which the oxidant is recirculated by means of
temperature difference in the reaction zone and in cooling
system, followed by with a rawal of the oxidate lower water
layer from the oxidation zone. Carrying out of such process
also enables to increase the yield of primary products of
the oxidation reaction.
Temperature
change by stages enables to increase or to decrease the
reaction rate of hydrocarbons thermal cleavage under
different oxidation conditions, thus enabling to influence
the yield and selectivity of end products. Step-by-step
introduction of the oxygen containing gas into oxidation
zone enables to introduce oxygen at different temperatures
and influence oxidation cell effect simultaneously when
hydrocarbons are oxidized more deeply, thereby to change
partly the yield and selectivity of end products.
Conclusions
In some
processes of hydrocarbons liquid phase oxidation it is not
sufficient to use one of the above mentioned factors to
influence the yield and selectivity of end products, this
makes these methods less competitive with respect to other
methods . Whereas combination of different path –ways of
modification of hydrocarbons liquid phase oxidation enables
to influence
sufficiently the yield and selectivity of end products
formation, thus increasing competitiveness of these methods.
Reference
1. Prokopchuk
S.P., Abadjev S.S., Shevchuk V.U. Catalytic oxidation
n-pentane in
the liquid phase. Ukr. Chem. Journ. Kyiv, 1983. Vol. 45, 5, p.p.505-508.
2. Prokopchuk
S.P., Abadjev S.S., Shevchuk V.U. The way of formation
of n-pentane
liquid phase oxidation. Ukr. Chem. Journ. Kyiv, 1985, Vol.
51, 1,
p.p. 89-93.
3. Prokopchuk
S.P., Shevchuk V.U., Ivakht B.V. Catalytic conjugate
oxidation
n-pentane and propylene. Ukr. Chem. Journ. Kyiv, 1988, Vol.
54, 5,
p.p. 840-843.
4. Prokopchuk
S.P., Abadjev S.S., Shevchuk V.U. Joint receiving of the
propylene oxyde
and lower aliphatic carbolic acids. Chem. Ind.,
Moscow, 1988,
11,
p.p.666-668.
5. Prokopchuk
S.P., Morozova L.P. The complex catalysts influence on
the n-pentane
oxydation. Reports of Vinnitsia National Medical
University,
Vinnitsia, 2005.Vol. 9, 1, p.p.
52-54.
6. Prokopchuk
S.P., Abadjev S.S., Shevchuk V.U., Makedonskyi O.O. The
choice optimum
conditions of the process of receiving carbolic acids.
Chem. Ind.,
Moscow, 1987,
7,
p.p. 399-401.
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