Polymerisation Residence time

Performing what is known as post-condensation. Most step polymerisations are exothermic and, consequently, the equilibrium constant K decreases with increasing temperature. Hence, one way to increase the molecular mass would be to decrease the polymerisation temperature, but kinetics prohibits using a too low temperature as it will lead to an excessively long residence time in the reactor and/or too high viscosities. Thus, in order to reach very high molecular... 

The development of Nd-based catalysts has permitted the industrial realisation of a polymerisation process for cis- 1,4-polybutadiene in which temperature varies in the range 20-50 °C. In such a process, the polymerisation proceeds relatively fast the average residence time of the monomer varies in the range 0.5-4 h to achieve 90% butadiene conversion. 

The above gas-phase reaction is a polymerisation process which causes deviation from linear kinetics. Long residence-time performance is essential in large industrial reactions for forming hard coatings, where the residence time of the feed precursor is measured in minutes, whereas typical residence times for small tubular reactions is on the order of 1 s or less. 

It is not only the heat exchanger that has to be carefully monitored conditions upstream of the heat exchanger may promote fouling of the heat transfer surface. For instance long residence times in upstream equipment during "warm up" procedures may generate particulate matter, e.g. from polymerisation reactions, that subsequently deposit in a downstream heat exchanger. 

There are two common types of continuous reactors continuous stirred tank reactors (CSTRs) (53), and plug flow reactors (PFRs). CSTRs are simply large tanks that are ideally well-mixed (such that the emulsion composition is uniform throughout the entire reactor volume) in which the polymerisation takes place. CSTRs are operated at a constant overall conversion. CSTRs are often used in series or trains to build up conversion incrementally. Styrene-butadiene rubber has been produced in this manner. Not all latex particles spend the same amount of time polymerising in a CSTR. Some particles exit sooner than others, producing a distribution of particle residence times, diameters and compositions. 

Details are given of a non-steady-state operation for controlling latex particle size distribution by using a continuous emulsion polymerisation of vinyl acetate. The experiment was conducted in a continuously stirred tank reactor under conditions below the critical micelle concentration of the emulsifier. The mean residence time was switched alternately between two values in the nonsteady-state operation to induce oscillations in monomer conversion in time. The effect of the switching operation on particle size distribution is discussed. 13 refs. 

Reaction engineering of polymerisation processes is a promising field where new advances are expected in the next few years. Early work by Denbigh (1) shows that when chain propagation time is well below the residence time, as is the case of polyadditions, continuous stirred-tank reactors (CSTR) insure a product with a narrower spread in statistical properties. For the case of continuous emulsion... 

Rotation has been proposed by several organisations to enhance polymerisation reactions. An early reference was made by Ramshaw (1993) to a US patent taken out in 1964 by DuPont Company which highlighted the benefits of polymerising in thin films at up to 400°C with a residence time of seconds. Not all subseqnent inventions have jumped in at the deep end in producing rotating reactors. 

While it should be self evident that a rational reactor design demands knowledge of both the fluid dynamic environment and the detailed process kinetics, the latter are rarely available. In many instances this leads to the severe limitation of many important reactions due to an inadequate fluid dynamic intensity. Some of these are known to be fast, e.g. liquid phase nitrations, while others (incorrectly) are assumed to be slow, e.g. most polymerisations. In these circumstances the pragmatic approach is to use a high intensity reactor for each system and then to assess the impact upon the space-time productivity. Obviously, an intrinsically slow system is resistant to further acceleration and this will rapidly become evident. One significant qualification of this contention involves the very short residence time in the SDR compared with its conventional counterparts. In certain reactions the process temperature is restricted to one that avoids product breakdown in the time... 

In the SDR it was shown that the average inCTease in conversion in one disc pass for the 40% prepolymer feed was of the order of 10 15% and about 16% for the 60% prepolymer feed. With residence times of the order of 1-2 seconds, the polymerisation rate in the SDR is estimated to be between 12 000% per hour to 64000% per hour, as opposed to 10 50% per hour in conventional reactors operating at more extreme conditions. Molecular weights and polydispersity indices were almost unchanged in comparison to the feed. 

However the department is known for its particle technology group (with its information science service), its work on polymerisation reactions and residence time distribution theories, all originating with research groups. 

Prior to polymerisation, the Hiillips catalyst must be activated at hi tenperature to stabilise the chromium as a surface chromate, the precursor to the active site. This is achieved by treatment with dry air in a fluidised bed using tenperatures of 500 to 950 C and residence times of vp to 12 hours. UJ)on exposure to ethylene in the reactor the Cr (VI) sites are progressively reduced to the active sites vhich catalyse the polymerisation. 

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