Добавил:
ivanov666
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Chemistry, technology and properties of synthetic rubber. Tutorial
.pdf
exchange surface. To intensify the process of heat release there is a
need to circulatory motion of the reaction mixture with the required
flow of built-in tubular heat exchange elements. It should be taken
into account that for the cross flow of tubes the heat transfer
coefficient is 4-5 times higher than for the longitudinal flow.
1.7 Rubber Separation
In a typical rubber synthesis processes the reaction mass coming out
of the last polymerizer of the cascade is either a solution
(polymerizate) or latex, and in both cases except for the polymer and
the thinner contains unpolymerized monomer, components of the
catalytic complex (or initiating system), etc. Therefore, when
separating the rubber it is required to deactivate the catalyst
(initiator), to wash the polymer from its remains, to recover the
monomer, to separate the polymer from the solvent or an aqueous
dispersion medium and to add the antioxidants simultaneously.
In ionic polymerization catalyst deactivation process is
usually carried out by adding compounds into polymerizate that
destroy the active catalytic complexes meanwhile the radical
processes are interrupted by adding corresponding inhibitors
(stoppers) in the system.
There are several methods of separating the rubber from the
solutions, the most important are:
• Precipitation of the polymer by adding to the solution liquids
insolubilizing the rubber, followed by the processes of filtration,
drying of rubber and separation of solvents and monomers mixture;
• Evaporation of polymerizate to complete removal of all
volatiles from the polymer (mainly solvent and monomer);
• Dispersion of polymerizate in hot water with the
simultaneous feed of polymerizate and steam to the crumb former
(water degassing); meanwhile, there is a removal of volatile products
in the vapor phase and the polymer is separated as crumbs dispersed
41
in water, which are then separated and dried. The method of water
degassing is essential in obtaining solution synthetic rubber.
In emulsion polymerization, first unpolymerized monomers are
distilled from the reaction mixture (degassing of latex), and only then
rubber is separated by coagulation of latex.
Physicochemically, the polymerizate water degassing and
latex degassing have much in common, since in both cases there are
complex heat-mass exchange processes in multicomponent and
multiphase polymer - hydrocarbon - water - vapor systems. However,
a different state of the source and final products sets different
technological challenges and requires the use of irregular equipment.
The final operations on rubber separation from the aqueous
dispersion for solution and emulsion polymerization are almost
identical.
2 EXAMPLES OF TECHNOLOGICAL PROCESSES OF
PRODUCTION OF SYNTHETIC RUBBER
2.1 Obtaining Solution Stereoregular Isoprene Rubber Using
Anionic Coordination Catalysts
Currently, the industries use modified Ziegler-Natta catalysts
containing three (or more) components. Modifiers are electron donor
compounds of the Lewis bases type (amines, ethers, thioethers,
alcohols, phenols) that are added in certain ratio to one component or
to the catalytic complex. The use of the modified Ziegler-Natta
catalysts based on AlR3 and TiCl4 in the polymerization of isoprene
has the following advantages: increased rate of polymerization,
reduced content of the gel fraction in the polyisoprene, there is the
possibility of polymerization to higher conversions without affecting
the optimal properties of rubber, polymerization process becomes
less sensitive to trace and overstatement of the temperature, the
presence of electron donors has no effect on the microstructure of
polyisoprene.
42
When anionic coordination polymerization, the incorporation
9
H2CCHCHCHCH
+
C
l
9
of the following molecule of the monomer into the polymer chain is
preceded by its coordination with the components of the growing
chain end (active center).
First organometallic compounds of transition metals were
used by K. Ziegler and G. Natta as catalysts for olefins in the early
1950s. Ziegler-Natta catalysts are mostly heterogeneous, however,
homogeneous catalysts are also known. In some cases, it depends on
the order of mixing the reagents. For example, in case of insertion of
TiI2Cl2 into solution containing triisobutylaluminum (TIBA) and
butadiene the catalyst will be homogenous. Provided that TiI2Cl2 is
entered into the TIBA solution without butadiene, the catalyst is
heterogeneous.
The interaction of transition metal halides such as TiCl4 and
organoaluminum compounds such as triisobutylaluminum Al(iC4H9)3 underlies the synthesis of Ziegler-Natta catalysts:
TiCl4(i-C4H9)3Al
H3C
CH
CH
CH
+
-700C
Cl
Cl
Ti
CH
2
i-C4H
Cl
Cl
3
i-C4H
9
Al
i-C4H
9
-0,5 i-C
4H8
-0,5 i-C4H
0
-55
C
Cl
C
H
3
10
CH
CH
CH
CH
Ti
2
i-C4H
Cl
Al i-C4H
Cl
9
43
In the Ziegler-Natta catalysts formation process there are two
Reco
m
bination
main reactions. They are the alkylation of transition metal ion and
the formation of an octahedral complex with a vacant (coordinating)
orbital. The result of the alkylation is the formation of carbanion transition metal ion bonding which is the place of insertion of
monomer molecules and the anion growth of chain. Coordination of
diene monomers by the transition metal ion stipulates the formation
of stereoregular polymer.
When preparing the catalyst a low-activity diene monomer piperylene - is introduced in the reaction metal complex system. It is
required to occupy the coordination site by the compound similar to
diene monomers and to replace it easily on this molecule after the
feeding of the monomers (beginning of polymerization).
Using transition metal ions of variable oxidation level Ti(IV) or V(V) for the preparation of the catalytic complex there are
redox reactions accompanied by reduction of metal ion: Ti(IV) →
Ti(III) → Ti(II); V(V) → V(IV) → V(III) → V(II). When metal ion
transits in lower oxidation degree the catalyst loses its catalytic
activity. This process is called the aging of catalyst. Especially fast
aging of the catalytic complex goes with the vanadium catalysts. To
extend the catalyst activity rating time the reactivators are input in
catalytic reaction system. The scheme shows an oxidation
mechanism of the ion V(II) (reactivation of the catalyst) under
hexachlorocyclopentadiene.
e
-
+
+
+
V(II)
Cl
V(III)
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
ClCl
Cl
+
V(II)
Cl
Рекомбинация
According to the currently accepted mechanism the chain
growth is preceded by coordination of the monomer. Regardless of
the type of transition metal and the nature of the ligands before the
introduction stage of the monomer on the C-Mt-bonding in the metal
44
complex catalyst the π-allyl complex is formed. The formation of
C
l
C
l
C
H
C
H
1,4-cis-polydiene is preceded by the anti-configuration of the π-allyl
complex. According to the Cossey - Allemand mechanism the
growing polymer chain and the coordinating orbital interchange in
each act of growth:
CH
CH
CH
CH
Cl
Ti
2
H2C
2
CH CH
CH
Cl
Al i-C4H
9
H2C CH CH CH
Cl
2
2
CH
CH
CH
Cl
Ti
2
CH
2
H2C
CH
CH
CH
2
Cl
CH
Cl
Al i-C4H
9
H2C CH CH CH
CH
2
2
2
Chain termination is mainly related to the spontaneous
reaction of hydride transfer in which from the end of the growing
chain the complex comes off with the formation of a transition-metal
hydride, which due to the interaction with the molecule of the
monomer re-generates the π-allyl group. It is also possible the
transfer of hydride-ion per molecule of the monomer at the moment
of its coordination. Received hydride transition metal compound
interacting with the molecule of the monomer gives an active catalyst
45
complex over, and both reactions can be regarded together as a chain
C
H
3
2
C
H
transfer to monomer.
CH
CH
CH
CH
2
2
+ HMtXn
+ C4H
6
HC
CH
CH
CH
2
MtXn
–
H
2
HC
CH
CH
MtXn
Due to the fact that the diene fragment is formed at the end of
the residual polymer chain the reaction of chain transfer to polymer
becomes possible. The result is branch points in polymer.
In process of polymerization with Ziegler-Natta catalysts the transfer
chain is also possible by means of the trialkylaluminum, provided
that it is taken in excess when preparing the catalyst complex.
~~CH2-MtXn + AlR3 → ~~CH2-AlR2 + R-MtXn.
In anionic coordination polymerization the chain can be
transferred through hydrogen, whose introduction into the reaction
mixture can control the molecular weight of polydiene:
CH
CH
CH
3
HMtXn+
2
HC
CH
CH
CH
2
MtXn
+ H
2
46
Technical implementation of isoprene rubber production with
various catalytic systems has no vital differences. Thus, the process
of obtaining isoprene rubber SKI-3 in isopentane solution consists of
the following operations: drying of the solvent; the preparation of the
catalyst complex; the polymerization of isoprene; deactivation of the
catalyst; washing and stabilization of polymerizate; the rubber
separation from the solvent; separation of rubber from the pulp;
rubber drying and packaging. Auxiliary operations are preparing a
stabilizer suspension and stopper solution; preparation of
ingredients of anti-agglomerator; azeotropic drying of return
solvent; distillation of C4 fraction and the rectification of isopentaneisoprene fraction.
2.2 Drying of Solvent
Fig.1 The scheme of solvent drying and regeneration of alumogel:
1, 5, 11 - pumps; 2, 12 - coolers; 31 - 33 - dryers, 4, 10 - tanks; 6 -
separator; 7,8 - condenser; 9 - precipitating tank; 13 - gas blower; 14
- pipe still.
I - solvent from the storage; II - liquid propane; III - steam; IV -
water; V - water for stripping organic compounds; VI - solvent to the
store; VII - nitrogen; VIII - solvent to the alkaline cleaning; IX -
solvent to polymerization; X - chilled water.
47
The purified isoprene fraction from the intermediate storage (see
Figure 1) is piped to the cooler 2, where it is cooled by propane
evaporating at 0°C; separation of vapor-liquid mixture is realized
through the separator. Isopentane-isoprene fraction cooled to 10°C is
fed for drying to the dryer 3, filled by alumogel. The dried solvent is
sent to the polymerization. Dryers operate in periodic mode: one
dryer is busy, the second is on the regeneration and the third is idle.
The solvent from dryer which is switched over to regeneration is
moved to container 4, from which is pumped periodically by the
pump 5 to alkaline cleaning. Before the regeneration the dryer gets
steamed thoroughly with superheated steam with a temperature of
400-450°C. Steamed hydrocarbons and the water vapor come from
the dryer 3 to the separator 6 and then to the condenser 7, cooled by
industrial water. Uncondensed hydrocarbon vapors come into the
condenser 8 cooled by chilled water. Hydrocarbons condensate with
the water drain into the precipitating tank 9, where the upper organic
layer goes by gravity into the tank 10, and then is fed by the pump 11
to the storage. The bottom layer from the precipitating tank 9 is
directed to the stripping of hydrocarbons.
After steaming of dryer with superheated steam the alumogel
is regenerated with hot nitrogen. Compressed nitrogen under the
pressure of 0.25 MPa is supplied to the system of nitrogen circulation
for recharge in the admission line of gas blower 13, from there into
the furnace 14, where it is heating to 400°C, and then to the dryers.
Nitrogen is heated in the furnace 14 through the heat released during
the combustion of fuel gas which is supplied from the grid. The hot
nitrogen from the dryer 3 goes through the separator 6, is cooled in
condensers 7 and 8 and in cooler 12, then it is fed into the admission
line of gas blower 13, fueled by fresh nitrogen and heated in furnace
14. The circulation of nitrogen by gas blower 13 through the furnace
14 and dryer 3 is performed within 6-7 h, then dryers 3 are cooled to
10°C by circulating cold nitrogen. At the end of the cooling dryers 3
are switched over to receiving the isopentane-isoprene fraction.
48
2.3 Preparation of Catalyst Complex
Preparation of the catalyst complex (Figure 2) is carried out in an
atmosphere of nitrogen, purified from the impurities of oxygen and
water. Titanium tetrachloride is fed from a warehouse to the
measuring tank 1. At the presence of impurities titanium
tetrachloride re evaporates in stripping column 2 filled with packing
copper shavings. The purified product is condensed in a water
condenser 3, cooled in the brine cooler 4 and goes to the measuring
tank 5.
Fig.2 Diagram for catalyst complex producing when obtaining SKI-3
1, 5, 6, 7, 8, 11 — measuring tank; 2 — stripping column; 3 —
condenser; 4 — cooler; 9 — reactor; 10 — pump.
I — nitrogen; II — titanium tetrachloride; III — steam; IV —
triisobutylaluminum; V — toluene; VI — modifier; VII — liquid
propane; VIII — catalyst complex to polymerization; IX — brine; X
— slurry.
Catalyst complex is prepared in the reactor 9, equipped with a
stirrer and a jacket, which is supplied with the coolant. The reactor 9
49
is purged with nitrogen, and then it is loaded with the appropriate
amount of toluene from the measuring tank 7 and
triisobutylaluminum from the measuring tank 6, the stirrer switchs on
and the necessary amount of titanium tetrachloride and the modifier
is supplied from the measuring tank 5 and 8, respectively. The
loading order of components of the catalyst complex can be varied.
The circulation of the insoluble in toluene catalyst complex is carried
out by pump 10 to prevent its sedimentation when forming by
mixing. The heat released during the interaction of the components is
removed by coolant fed into the reactor jacket 9. The ready-to-use
catalyst complex is pumped into the measuring tank 11, in which it is
stored when cooling and is dosed to the polymerization.
Preparation temperature and the ratio of components in the
modified Ziegler-Natta catalysts have a significant impact on their
activity: the complexes prepared at low temperatures (below -70°C)
are more active than complexes prepared at high temperatures. The
rate of polymerization of isoprene on the modified catalyst is higher
than the two-component one, which allows reducing the dosage of
catalyst and increasing the stability of polyisoprene by reducing the
content of the transition metals. There are used anisole, n-chloranil,
and diphenyl oxide as modifiers. To have stable activity of
heterogeneous catalyst and lack of agglomeration of its particles
(only in that case the polymerization process achieves its
reproducibility) conditions should provide the necessary heat and
mass transfer, eliminating the course of secondary reactions which
result in products that reduce the activity and stereospecificity of the
catalyst complex and degrade the properties of the polymer.
Control of preparation of the modified catalyst complex is
carried out regarding composition of its heterogeneous or soluble
part, electrochemical or magnetic parameters. There has been
developed automated control methods and corresponding equipment
that will provide the production of the catalytic complex of high
activity, uniform composition and given ratio of components.
50
Соседние файлы в предмете [НЕСОРТИРОВАННОЕ]