Polymerization continuous

Equations (84)-(89) can be solved to provide a description of the dynamic behavior of a continuous well-stirred system. Analytical solutions may be derived for the steady-state case by setting all derivative terms to zero. Assuming no inflow of radicals, Eq. (92) results for Aq. 

For most cases, the radical lifetime is much shorter than the average residence time. Thus outflow of radicals can be neglected, resulting in the familiar expression, Eq. (93), for Aq. 

Substitution of these expressions into the monomer balance leads to a nonlinear relationship between conversion and 0 [Eq. (95)]. 

Assuming no inflow of polymer into the reactor and the long-chain hypothesis, the steady-state values for DPn and DPw in the absence of LCB = 0) are given by Eq. (83). LCB and reaction with terminal double bonds broaden the MWD significantly [81]. 

For monofunctional initiator, this expression becomes much simpler 

The main distinction of polymerization in continuous stirred tank reactors (CSTRs) from batch/plug-fiow polymerization is the distribution of reactor residence times. In a single CSTR, the distribution of molecules in residence time (i.e., the probability to be in the reactor during time period t is given by [37, 38] 

Obviously, the increase in the number of reactors in a series approaches the process to the batch one because residence time distribution becomes narrower thus, in the kth reactor [37] 

A series of r identical continuous stirred tank reactors is considered 

Monomer, solvent, initiator, and other components, if appropriate, are fed into the first reactor at a constant volumetric rate. The concentrations of monomer, solvent, and initiator in the input stream are Mo, S, and Iq, respectively. 

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The polymerization of monomers to form hydrocarbon resins is typically carried out by either the direct addition of catalyst to a hydrocarbon fraction or by the addition of feed to a solvent—catalyst slurry or solution. Most commercial manufacturers use a continuous polymerization process as opposed to a batch process. Reactor temperatures are typically in the range of 0—120°C. 

As the polymer molecular weight increases, so does the melt viscosity, and the power to the stirrer drive is monitored so that an end point can be determined for each batch. When the desired melt viscosity is reached, the molten polymer is discharged through a bottom valve, often under positive pressure of the blanketing gas, and extmded as a ribbon or as thick strands which are water-quenched and chopped continuously by a set of mechanical knives. Large amounts of PET are also made by continuous polymerization processes. PBT is made both by batch and continuous polymerization processes (79—81). 

For continuing polymerization to occur, the ion pair must display reasonable stabiUty. Strongly nucleophilic anions, such as C/ , are not suitable, because the ion pair is unstable with respect to THE and the alkyl haUde. A counterion of relatively low nucleophilicity is required to achieve a controlled and continuing polymerization. Examples of anions of suitably low nucleophilicity are complex ions such as SbE , AsF , PF , SbCf, BE 4, or other anions that can reversibly coUapse to a covalent ester species CF SO, FSO, and CIO . In order to achieve reproducible and predictable results in the cationic polymerization of THE, it is necessary to use pure, dry reagents and dry conditions. High vacuum techniques are required for theoretical studies. Careful work in an inert atmosphere, such as dry nitrogen, is satisfactory for many purposes, including commercial synthesis. 

Continuous polymerization systems offer the possibiUty of several advantages including better heat transfer and cooling capacity, reduction in downtime, more uniform products, and less raw material handling (59,60). In some continuous emulsion homopolymerization processes, materials are added continuously to a first ketde and partially polymerized, then passed into a second reactor where, with additional initiator, the reaction is concluded. Continuous emulsion copolymerizations of vinyl acetate with ethylene have been described (61—64). Recirculating loop reactors which have high heat-transfer rates have found use for the manufacture of latexes for paint appHcations (59). 

Continuous Polymerization. A typical continuous flow diagram for the vinyl acetate polymerisation is shown in Figure 12. The vinyl acetate is fed to the first reactor vessel, in which the mixture is purged with an inert gas such as nitrogen. Alternatively, the feed may be purged before being introduced to the reactor (209). A methanol solution containing the free-radical initiator is combined with the above stream and passed directiy and continuously into the first reactor from which a stream of the polymerisation mixture is continuously withdrawn and passed to subsequent reactors. More initiator can be added to these reactors to further increase the conversion. 

Continuous polymerization in a staged series of reactors is a variation of this process (82). In one example, a mixture of chloroprene, 2,3-dichloro-l,3-butadiene, dodecyl mercaptan, and phenothiazine (15 ppm) is fed to the first of a cascade of 7 reactors together with a water solution containing disproportionated potassium abietate, potassium hydroxide, and formamidine sulfinic acid catalyst. Residence time in each reactor is 25 min at 45°C for a total conversion of 66%. Potassium ion is used in place of sodium to minimize coagulum formation. In other examples, it was judged best to feed catalyst to each reactor in the cascade (83). 

FIG. 23-23 Batch and continuous polymerizations, (a) Polyethylene in a tiihiilar flow reactor, up to 2 km long hy 6,4 cm ID, (h) Batch process for polystyrene, (c) Batch-continuous process for polystyrene, (d) Suspension (head) process for polyvinylchloride, (e) Emulsion process for polyvinylchloride, (Ray and Laurence, in Lapidus and Amundson, eds, Chemical Reactor Theory Review, Frentice-Hall, 1977. )... 

Batch polymerization is still used. However, most new processes use continuous polymerization and direct spinning. 

The chemical changes that can occur during processing and effect product performances include (1) continued polymerization and cross-linking, which increases viscosity (2) depolymerization or damaging of molecules, which reduces viscosity and (3)... 

PA-6,6 is made from the relatively expensive materials hexamethylene diamine and adipic acid. An alternative synthesis of PA-6,6 from adiponitrile and hexamethylene diamine utilizing water is under investigation.16 PA-6 can be synthesized in a continuous process at atmospheric pressure, but reaction times are very long as the ring-opening initiation step is particularly slow. The reaction time can be shortened considerably by carrying out prepolymerization in the presence of excess water at pressure however, this makes the continuous polymerization process more complex. Copolymers with amide units of uniform length (diamides) are relatively new the diamide units are able to crystallize easily and have a thermally stable crystalline structure. 

Continuous polymerization processes for PA-6,6 have been reported for over 30 years.5,6,28 Prepolymerization in tubular (Fig. 3.21) or baffled reactors is particularly well suited to continuous polymerization. The polymerization of prepolymers to high-molecular-weight materials in a continuous process is more difficult to control as small differences is molecular weights result in large differences in viscosities. Viscosity differences result in different hold-up times in die reactor and thus nonhomogeneous products. 

Continuous polymerization processes, 167 Convergent method, 8 COOH-terminated poly(ethylene adipate), 95-96... 

Continuous Polymerizations As previously mentioned, fifteen continuous polymerizations in the tubular reactor were performed at different flow rates (i.e. (Nj g) ) with twelve runs using identical formulations and three runs having different emulsifier and initiator concentrations. A summary of the experimental runs is presented in Table IV and the styrene conversion vs reaction time data are presented graphically in Figures 7 to 9. It is important to note that the measurements of pressure and temperature profiles, flow rate and the latex properties indicated that steady state operation was reached after a period corresponding to twice the residence time in the tubular reactor. This agrees with Ghosh s results ). 

The yield that can be attained by a semibatch process is generally higher because the semibatch run starts from scratch, with maximum values of both variables Cg (o) = Cg and k] (o) = k . However, the yield from a continuous run in which t equals the batch time is governed by the product of Cg (t) and kj (t), so > and k (t) = k °. Because neither of these conditions is likely to be fulfilled completely, a continuous polymerization in a backmix reactor will probably always fail to attain the Y attainable by a semibatch reactor at the same t. However, several backmix reactors in series will approach the behavior of a plug flow continuous reactor, which is equivalent to a semibatch reactor. 

Continuous Simulation, C0NGAS. There are no published data available on propylene continuous polymerization suitable to check the accuracy of the C0NGAS model. However, there is an equation for yield vs. time published by Wisseroth (3 ) for a completely backmixed continuous reactor ... 

Effects of starvation feeding in semicontinuous or continuous polymerization systems. 

Reactor Design. The continuous polymerization reactions in this investigation were performed in a 50 ml pyrex glass reactor. The mixing mechanism utilized two mixing impellers and a Chemco magnet-drive mechanism. 

Experimental Apparatus. The experimental apparatus used in the continuous polymerization reactions of this investigation was constructed and used by Ahmad (27) for earlier studies of isoprene... 

Figure 6. Experimental apparatus for continuous polymerization of styrene ((D) Needle valve (X) t)dlve ( ) pressure regulator check valve (— ) ther-...

There are many variations on this theme. Fed-batch and continuous emulsion polymerizations are common. Continuous polymerization in a CSTR is dynamically unstable when free emulsifier is present. Oscillations with periods of several hours will result, but these can be avoided by feeding the CSTR with seed particles made in a batch or tubular reactor. 

A continuous polymerization train consisting of two stirred tanks in series is used to copolymerize styrene, rx = 0.41, and acrylonitrile, vy = 0.04. The flow rate to the first reactor is 3000 kg/h and a conversion of 40% is expected. Makeup styrene is fed to the second reactor and a conversion of 30% (based on the 3000 kg/h initial feed) is expected there. What should be the feed composition and how much styrene should be fed to the second reactor if a copolymer containing 58 wt% styrene is desired ... 

As an example of the use of MIXCO.TRIAD, an analysis of comonomer triad distribution of several ethylene-propylene copolymer samples will be delineated. The theoretical triad Intensities corresponding to the 2-state B/B and 3-state B/B/B mixture models are given In Table VI. Abls, et al (19) had earlier published the HMR triad data on ethylene-propylene samples made through continuous polymerization with heterogeneous titanium catalysts. The data can be readily fitted to the two-state B/B model. The results for samples 2 and 5 are shown In Table VII. The mean deviation (R) between the observed and the calculated Intensities Is less than 1% absolute, and certainly less than the experimental error In the HMR Intensity determination. 

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