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Файл:Chemistry, technology and properties of synthetic rubber. Tutorial
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rubbers production data in the USSR and Russia in the period from
1988 to 2004 inclusive.
Table 4.
The production of some types of SR in the USSR and Russia
The USSR
Russia
Type of rubber 1988 1998 2001 2004
Isoprene 986.8 221.5 351.6 406.1
Butadiene 368.2 84.6 95.8 129.6
Styrene-butadiene
545 143 238.1 307.1
(α -methylstyrene)
Butyl rubber 56.4 84.6 95.8 129.6
Other 203.2 85 168.2 141.2
Total 2159.6 618.7 919.5 1113.6
Half of the produced domestic rubber is exported. At the
same time, the share of isoprene rubber exports is 28%, butadiene 40%, BSR - 50%, while the export of butyl rubber accounts for 95%
of its production volume.
In 2005, four companies attained to more than 80% of the
total production volume of rubber and latex in our country. For
example, OAO "Nizhnekamskneftekhim" produces 27% of the total
SR, LLC "Togliattikauchuk" - 23%, "Voronezhsintezkauchuk" 19%, JSC "Kauchuk" (Sterlitamak) - 15%. The share of OJSC
"Omsky Kautchuk" accounts for 8% of rubber produced in Russia,
JSC "Efremov plant of SR" - 5%, JSC "Krasnoyarsk plant SR" - 3%.
The structure of tires production in Russia has significantly changed
and come up to the world structure in recent years. The share of
passenger car tires in the production volume considerably increased
and in 2004 was 63%. For example, in Western Europe and the
United States this rate is from 72 to 90%.
A change in the structure of tire production has caused
changings in tire requirements and their production processes.
Among the most priority characteristics for tire consumers were grip
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on different road carpets (asphalt, wet asphalt, snow, ice), rolling
loss, tires load-bearing and geometric inhomogeneity and materials
consumption. In this regard, the trend of production of solution
polymerized butadiene-styrene rubber has appeared. Solution
styrene-butadiene rubber ensures higher wet grip and lower rolling
losses with the same wear resistance, as compared with the emulsion
rubber. Significant increase in tire grip, the vehicle stability and
roadability are also achieved with the introduction of 8.5 wt. fraction
of polyisoprene in a rubber compound with a predominant content of
3,4-units.
The general trend in world production of SR is the growth of
the role of thermoplastic elastomers (TPE), the main advantages of
which are nearly complete wasteless recycling, reuse and elimination
of vulcanization stage.
Over the last decade owing to the opening of new generation
of metallocene catalysts the manufacturing of previously unknown
stereoblock propylene rubbers and copolymers of ethylene with
higher α-olefins became possible.
World demand for elastomers (NR and SR) in the next 30
years could double from 18 million tons in 2005 to 36 million tons in
2035, while maintaining the typical ratio SR: NR = 60: 40 for the
present.
1.4 Main Stages of Polymerization Processes
In the vast majority of cases, in the industrial production of synthetic
rubber there are used the polymerization processes, which, in spite of
the variety, can be represented by a single flow scheme: basic
components preparation of polymerization process of extraction and
treatment of the polymer. A few processes in which the polymer is
formed by the polycondensation are also described by the same
sequence of operations (except that the second stage is the
polycondensation).
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Technical implementation of all stages of the process must
ensure the maximum yield of product per reactor volume unit at the
least cost to raw materials, capital investment, energy, etc. Besides
this approach which is common to the whole chemical engineering
for obtaining rubbers and other polymers, the set of properties of the
product, defined by chemical structure and the polymer structure, is
crucial. Therefore, in the technology of rubbers synthesis kinetic
peculiarities of processes that affect the productivity of the
equipment cannot be considered out of the reached molecular weight
characteristics and microstructure of the polymer. The solution of
these two related problems is implementation and the right selection
of the conditions applied at each stage of the processes.
The first stage of the process – the preparation of the source
components – typically involves the preparation of solutions (rarely
emulsions or suspensions) of certain concentrations and their dosing
into the polymerization apparatus. The process of purification of
monomers, solvents and other materials of the polymerization system
is as a rule a separate technological cycle and is not included in the
preparation of components stage. Processes of dosing, dilution,
filtration, heating (or cooling), etc. occurring at this stage usually do
not have any distinguishing features and can be equipped with a
standard chemical equipment.
In the second stage polymerization reaction proceeds, and the
high molecular weight compound – rubber is formed. Conditions
typical for chemical engineering are relatively rare in the process,
and typical equipment is suitable only in certain manifactures.
The feature of the polymerization step is quite significant
heat. When joining of the molecule of unsaturated monomer to the
growing chain one double bond breaks and two single bonds form, so
the theoretical value of reaction heat is easy to calculate. If we take
the average values of bond energies E
Q
= 2 E
R
C-C
- E
≈ 95 kJ/mol. (1.3)
C=C
C=C
and E
, we will obtain:
C-C
Actually, this value is slightly smaller, and the greater the size
or number of substituents in the monomer molecule, the lower the
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heat of polymerization. Obviously, this is due to the fact that part of
the energy is spent on overcoming the steric hindrances while
growing of the chain.
Polymerization with ring opening is also an exothermic
reaction, meanwhile, the more strained the ring, the higher the heat.
For example, ethylene oxide (a very strained three-membered ring) is
polymerized with the thermal effect of 104.6 kJ/mol (2374 kJ/kg),
whereas for tetrahydrofuran (low strained five-membered ring) QR =
21 kJ/mol (292 kJ/kg).
Other features of polymerization stage are a significant
viscosity of the reaction medium, the possibility of the polymer
sticking to the walls of the equipment, etc., which makes the
occurring processes of mass and heat transfer complicated, and
requires special types of equipment.
Microkinetics that studies chemical reactions, complicated by
processes of mass and heat transfer, underlies the theory of
calculating the polymerization apparatus. Since the temperature of
the process greatly and variously affects the speed of the various
reactions occurring during the polymer formation, the vessel design
should ensure strict set temperature and the working capacity of
equipment in a wide range of modes.
During the conversion of monomer into the rubber, there are
manifold and often multi-step processes of mass and heat transfer,
such as diffusion of the monomer to the active centers,
rearrangement of resulting macromolecules, the local heat release (by
means of the viscous friction energy dissipation when stirring) and
its removal from the system, etc. Therefore, analyzing the
polymerization processes and their modeling it is important to
combine theoretical and practical generalization of the both transport
processes at the molecular level and the processes of convective
mass and heat exchange for the reactor (or a group of reactors) as a
whole.
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1.5 Methods of Polymerization
The polymerization processes of synthetic rubber are carried out in
different conditions, according to the method of initiation, the nature
of the monomer, the desired microstructure of the polymer, etc.
The processes of bulk polymerization of the monomer seem
to be the most simple, as they do not require any solvents, thinners
and others additional components. Meanwhile, the monomer could
be liquid or gaseous.
The process of liquid-phase polymerization could occur in
two ways. 1) The polymer and the polymerization initiator are
soluble in the monomer, the process begins and continues in the
solution the viscosity of which greatly increases as the monomer
exhausts. Therefore, in conventional reactors only the initial stage of
the process can be conducted with intensive stirring, when the
reaction mass remains sufficiently mobile. To complete the reaction
the solution of the polymer in the monomer must be moved into
small molds where the process takes place without stirring up to high
conversion degrees. 2) The polymer is not soluble in the monomer;
the polymerization process begins in a homogeneous system, and
then continues in the particles of polymer, swollen in the monomer.
In this case the chain termination reactions are complicated, and the
polymer has high molecular weight values.
Another way of polymerization in the bulk of the monomer is
gas-phase process in which the monomer is used in the form of gas.
The formation of the polymer begins and develops on the surface of
the catalyst, resulting in a two-phase system throughout the process.
At that rate, the conditions of removal of polymerization heat are
substantially improving (with the help of remote heat exchangers,
where the circulating monomer is cooled). The obtaining the
butadiene rubber (SRB) on the metallic sodium in such a way is
associated with serious problems, which are its periodicity,
complexity of the catalyst deactivation and rubber perfection
procedures, a low level of process mechanization and automation.
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Hereupon, the gas-phase polymerization in this way is out of dated
and finds limited use.
The newly developed gas-phase polymerization processes in a
fluidized bed of powdered catalyst overcame the majority of
mentioned shortcomings, but these processes have not become
widespread so far.
In recent decades, the polymerization in solution has become
the main method of producing synthetic rubber in our country. It
should be noted that when manufacturing the SR all the processes of
solution polymerization are ionic. In most commercial systems there
are solvents in which the source monomer (or mixture of monomers)
and the resulting polymer are highly soluble. The reaction mass
preserves the homogeneity during the whole process and as the
monomer converses to polymer its viscosity increases significantly.
The resulting product (the polymerizate) is a solution of rubber and
unpolymerized monomer, in addition, it contains the remains of a
catalyst, so the rubber separation is related to the deactivation of the
catalyst and distillation of monomer and solvent. The features of the
process are the high viscosity of the reaction medium, making it
difficult to eliminate the polymerization heat, and the possible
sticking of polymer to the reactor walls.
The version of the solution polymerization is less commonly
used, when the polymer is not soluble in the solvent, and
polymerizate is a suspension of the swollen polymer. At the same
polymer concentration the viscosity of the dispersion is always lower
than the viscosity of the solution, which facilitates heat elimination
and allows higher concentrations of monomer in the source solution.
This reduces energy consumption during the subsequent separation
of the polymer and solvent recovery.
The emulsion polymerization is widespread in the global SR
industry and is always radical. During the polymerization, the source
aqueous emulsion of the monomers turns into a colloidal dispersion
of polymer (latex) with particle sizes of 30 to 300 nm. The low
viscosity of the reaction medium makes it easy to take away the
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polymerization heat, but the unpolymerized monomers distillation
and rubber separation from latex is rather complicated and energyintensive processes. However, these costs are much lower than when
separating rubber from the solution and solvent recovering.
Therefore, generally, emulsion rubbers have a lower cost than
solvent rubbers of the same type. The resulting rubber is usually
contaminated with the remains of an emulsifier and inorganic salts;
besides, radical polymerization mechanism does not allow to obtain
stereoregular rubbers.
Large scale solution and emulsion polymerization are usually
continuous processes in the cascade of reactors operating in the mode
close to the ideal mixing, whereas the work of the whole cascade
close to the mode of plugflow reactors. The higher the reactor
volume the lower cost per unit of product, and the higher the level of
automation, and the closer to the optimum process conditions, the
higher the single aggregate economic capacity. But with the growth
the reactor volume the problems of mixing and especially reaction
heat removal become more complicated, and now it is commonly
used the polymerizers of 16-20 m3.
1.6 Heat Exchange in Polymerization
One of the most intensive methods of heat removal from the reaction
volume is the evaporation of part of the monomer or solvent with
their condensation in a separate condenser and return to the
apparatus. However, the evaporation out of highly viscous mediums
is accompanied by considerable foaming, which makes it hard to
implement this interesting method in industrial scale. If to avoid
foaming (eg, as receiving the ethylene-propylene rubber in liquid
propylene), or to suppress it by antifoamers, the heat removal by
evaporation will be very promising.
When the convective heat removal in a continuous process of
polymerization the heat balance for each unit of the cascade can be
written as:
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QP + QN = QM + QF, (1.4)
where: QP - the heat of polymerization; QN - the viscous flow energy
dissipation; QM - the heat consumed in heating the incoming products
to the temperature in the reactor; QF - the heat revealed through the
heat exchange surface.
Qp = Gp × [M] × ∆X × rp, (1.5)
QM = Gp × cp × (tp - t
QF = F × α × (tr - t
(1.6)
in),
(1.7)
w),
where: Gp, cp - the amount and heat capacity of the reaction mass
supplied to the apparatus; [M] - monomer concentration; ∆X - the
increment of the degree of conversion (conversion) of monomer in
the apparatus; rr - the monomer polymerization heat; α - the
coefficient of heat transfer from the reaction mass to the wall; F - the
heat exchange surface; tp, t
- the temperature in the reactor, of the
in, tw
incoming reaction mass and wall of the apparatus, relatively.
Therefore, the efficiency of heat dissipation is influenced by
technological factors (temperature, rate of polymerization, monomer
concentration, etc.) and structural characteristics of the equipment (F,
α, QN). .Knowing these parameters of the process, it is possible to
define the role of each member of the equation and draw some
conclusions.
Quantity of heat released in the device due to the
polymerization reaction depends primarily on the values of ∆X and
[M], since the amount of fed products is usually specified by the
volume of the reactors of the cascade, and the value of rp for the
system is constant. From the standpoint of simplifying the degassing
and reducing costs of the monomer regeneration it is advisable to
carry out the processes to the highest possible conversion, limited by
the quality of the resulting polymer or the kinetics of the
polymerization process. As a rule, the increment of conversion ∆X
decreases from the first apparatus to the last in the cascade of
polymerizers mainly due to lowering of monomer concentrations.
For the more uniform distribution of heat load over the reactors of
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cascade it is reasonable sometimes to carry out the fractional feed of
the monomer.
To create highly productive polymerization processes it is
necessary to ensure the highest possible output concentration of
polymer in the reaction mass. However, during the solution
polymerization, the viscosity of the system substantially increases
with the growth of concentration, which is especially characteristic
for solutions of flexible chain polymers, namely rubbers. Maximum
allowable viscosity is achieved at concentrations of polymer in
solution 11-13%. In consideration of the high conversion of
monomers, the initial concentration of monomer [M] in the solution
fed to the polymerization does not exceed 15% wt. In emulsions, the
concentration of monomers in the initial reaction mass is much
higher (30-50% wt.).
When heat removing through the heat exchange surface the
temperature difference (tr-tw) is very important which is often limited
by the terms of the polymerization process, the ability of the polymer
deposition on cooled surfaces, or economic considerations.
Depending on the polymerization temperature tp the different
refrigerants are used:
Temperature of
Refrigerant
polymerization, °C
50 ÷ 80 industrial water
10 ÷ 50 chilled water, brine
0 ÷ 10 brine, ammonia, propane
-80 ÷ -100 Ethylene
The polymerization heat removal by means of heating
supplied cooled reaction mass is important for the fast processes of
polymerization. In principle, with a sufficiently large difference (tptin) the process can be transferred to the autothermal mode, i.e. the
entire heat released in the apparatus is spent on heating the incoming
products, and the cooling through the heat exchange surface is
unnecessary. At relatively low temperatures of polymerization, the
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difference (tp-tin) cannot be large, and this mode is possible only at
very low values of [M] or ∆X.
In the synthetic rubber industry, both heat removal methods
are used in combination, moreover the largest value of (tp-tin) is
characteristic for the first machine of the cascade, where the
increment of the monomer conversion is the largest and, therefore,
the heat release is also the highest in the cascade.
It is important to consider the viscous flow heat QN, which
depends on the properties of the reaction mixture, and mixing
intensity. With the increase in the rotary speed n of mixer the number
of heat released in the apparatus increases, and at the same time (but
much less) the heat transfer through the cooling surface enhances:
QN ≈ A × na; QF ≈ B × nb, (1.8)
where A and B - coefficients of proportionality, a ≈ 2 ÷ 3; b ≈ 0,35 ÷
0,65.
In this regard, the work of polimerizer is not reasonable under
any conditions. Figure 1.1 schematically shows the change in the
ratio of the heat balance equation summands for different rates of
stirring (for two values of Qp). It is clear that the process will proceed
without raising the temperature, provided that:
∆Q = (QF + QM) - (Qp + QN) ≥ 0. (1.9)
Autothermal mode in these systems cannot be realized as QM
<Qp and heat removal through the heat exchange surface is required.
At higher values of Qp (curve 2) this apparatus cannot be used in any
mixing rates, as ∆Q <0. At smaller values of Qp (curve 1) reactor
provides the heat removal in the range of mixing rates n1 ÷ n2, when
∆Q ≥ 0, and it is possible to find the optimal rotation speed of the
mixer no, where ∆Q reaches its maximum value.
For economic reasons it is advisable to operate at mixer
rotation speeds less than no. This is due to the fact that reducing the
rotary speed of mixer the heat transfer efficiency reduces less than
the relative energy costs for mixing.
For the large volume reactors the technology problem of
polymerization heat removal is usually solved by increasing the heat
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