Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Effecting a change in a polymerization process in response...
Reexamination Certificate
2001-08-08
2002-12-24
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Effecting a change in a polymerization process in response...
C526S059000
Reexamination Certificate
active
06498219
ABSTRACT:
The present invention relates to a method of on-line monitoring and control of monomer conversion in emulsion polymerization in a reactor, in particular in semicontinuous and continuous emulsion polymerization processes on an industrial scale.
Safety aspects play a prominent role in production processes in chemical industry. Chemical production processes are therefore usually monitored continually in order to avoid possible dangerous situations which could lead to explosions or to release of chemicals.
Many chemical reactions, for example emulsion polymerization, proceed exothermically and are therefore associated with the liberation of heat. If, in such a reaction system, less heat is removed than is generated by reaction of the starting materials the resulting temperature rise in the system can lead to a self-accelerating reaction. This is referred to as a “runaway” reaction. In a closed reactor system, a temperature rise is also associated with an increase in the internal pressure in the reactor.
A reactor for exothermic chemical reactions therefore has, in addition to cooling devices for efficient removal of heat, specific safety devices for release of pressure, for example safety valves or special “catch tank” systems which make it possible for the contents of the reactor to be quickly emptied into a safety tank. As a basic safety requirement, the process should always be carried out in such a way that the safety devices are not actuated even under unfavorable conditions, i.e. in the case of a spontaneous, adiabatic runaway reaction of the mixture present in the reactor. To realize this basic principle, reaction monitoring aided by the process control system is usually provided. The essential task of this reaction monitoring is to ensure the safety of the process and to limit the process risk at every point in time during the reaction in the running process.
Up to now, reaction monitoring has usually been based on fixed apparatus-dependent and formulation-independent limit values for feed amounts and/or rates for the starting materials and on monitoring of temperature differences.
These fixed limit values necessitate very large safety margins; optimization of the process conditions in terms of economics is only possible within narrow limits in the case of such concepts.
However, to optimize the space-time yield while maintaining plant safety, it is necessary to replace these rigid limit values by more flexible limit values based on up-to-date measurements while the reaction is running.
In emulsion polymerization, the starting materials (essentially monomers, emulsifiers, water, initiators and stabilizers) are introduced according to a predetermined addition strategy into the reactor where the emulsified monomer droplets are converted into polymer particles with liberation of heat.
Continuous reaction monitoring of emulsion polymerization therefore consists essentially of two elements:
monitoring of a threatened runaway reaction by actuating an alarm if a particular maximum internal temperature in the reactor is exceeded; and
monitoring/actuation of an alarm in the case of monomer accumulation.
An accumulation of monomers in the reactor is, firstly, associated with the risk of the reaction ceasing. However, an accumulation of monomers at the same time also represents an incalculable safety risk should an adiabatic runaway reaction of the reaction mixture occur. Reliable reaction monitoring therefore requires that the reaction enthalpy present in the reactor as a result of accumulative monomers but not yet liberated be known exactly at every point in time.
Various methods of monitoring monomer accumulation are already known.
In the “de Haas” reaction monitoring method, the setting of the regulating valves for steam and cooling water supply to the temperature-control bath of the reactor is monitored. This variant has the advantage that it can be implemented relatively simply. It employs instrumentation which is already present for controlling the reaction. However, for this same reason, the method cannot be used as a safety device of requirement class 5 (DIN 19250 or SIL III as per IEC 61508). In addition, certain effects such as reactor fouling or a deterioration in heat removal if the viscosity of the reaction mixture rises cannot be taken into account. The increase in the internal pressure in the reactor which occurs in the case of a runaway reaction is also not taken into account. Furthermore, this method of reaction monitoring reaches its limitations in the case of reactions which are provided with a relatively complex regulation strategy.
A further known method of monitoring monomer accumulation is to monitor the minimum initially charged amount of inerts (for instance deionized water) and the maximum flow for the monomer feed. However, this monitoring method allows only a relatively restricted flexibility with regard to the formulations and the operating procedure for the reactor. In itself it is not sufficient for monitoring the start of the reaction or a cessation of the reaction and must therefore be combined with organizational measures and, if appropriate, the “de Haas” reaction monitoring method. This method also does not explicitly take into account pressures which may possibly occur. This method is unfavorable from an economic point of view since, owing to the rigid limit values for amounts, relatively large safety margins have to be allowed.
A further known method is to monitor the temperature difference between the internal reactor temperature and the reactor bath temperature after reaching a “worst case” amount. The “worst case” amount is the maximum amount of monomers which can be permitted to run into the reactor without occurrence of a polymerization reaction while still leading to conditions within the safety margins in the case of a runaway reaction. The “worst case” amount can be determined on the basis of measured flows with the aid of a model. The calculation is then carried out by means of a simplified heat balance which takes into account only the introduced heat flows. However, this method, too, does not explicitly take into account pressures which possibly occur in the case of a runaway reaction. The monitoring of a rigid limit value for the temperature difference between internal reactor temperature and reactor bath temperature does not take into account the influences of reactor fouling and the viscosity. In addition, this method has only restricted usability in the case of reactors having extended cooling opportunities such as external heat exchangers or reflux condensers.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved method of on-line monitoring and control of monomer conversion in emulsion polymerization, which method makes possible more economical process conditions combined with unaltered, high plant safety and, in particular, is also usable for reactors having extended cooling opportunities and in processes having complex regulation strategies.
We have found that this object is achieved by the method described in claim
1
. The method of the invention comprises
a) selecting an initialization time t
0
=0 and assigning a particular original heat content Q
0
to the reactor for this point in time,
b) as from the initialization time, continuously determining the heat Q
IN
, introduced into the reactor, the reaction enthalpy Q
RE
introduced and the heat Q
OUT
removed from the reactor,
c) calculating the heat which has not been removed Q
AD
according to the following balance
Q
AD
(
t
)=
Q
0
+Q
IN
(
t
)+
Q
RE
(
t
)−
Q
OUT
(
t
),
d) calculating the maximum internal temperature T
AD
which occurs in the case of a spontaneous adiabatic reaction from the heat which has not been removed Q
AD
(t) and the instantaneous internal temperature T(t) of the reactor and,
e) if the calculated maximum internal temperature T
AD
exceeds the instantaneous internal temperature T(t) of the reactor by a particular margin, implementing measures which prevent a further rise in the heat which
Birk Joachim
Hauff Thomas
Klostermann Rainer
Kröner Hubertus
BASF - Aktiengesellschaft
Teskin Fred
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