Seeding reactor

A micro-mixed, seeded reactor will produce a broad polymer distribution with a high molecular weight tail and polydispersity index that approaches 2 at large degrees of polymerization. 

Figure 23. Production of yellow iron oxide by the precipitation (A) and Penniman (B) processes a) Tank b) Pigment reactor c) Seed reactor d) Pigment reactor with scrap basket e) Filter f) Dryer g) Mill...

Specific turbidity histories are also plotted vs. dimensionless time for a continuous emulsion polymerization run the samples were withdrawn from the second reactor of a continuous train where the first reactor is a small seeding reactor. Part A of Figure 3 shows the particle size behaviour during start up all monomer, water, initiator and soap feedrates were kept constant until the process reached a steady state. In part B, the soap concentration in the seed reactor was increased a decrease in the particle size was expected and it is clearly shown from the specific turbidity measurements. 

Fig. 33 Effect of monomer concentration Mpi fed to the first tubular seeding reactor with back mixing on the number of polymer particles produced (Sq (NaLS)=6.25 g/dm -water, Iq (KPS)=1.25 g/dm -water, Mpi (St)=variable 50 °C. Experimental data empty circles, particle number observed at t=40 min in a batch reactor filled circles, steady-state particle number observed in the first tubular seeding reactor operated with mean residence time r=40 min)...

A comparison between the single CSTR and the same reactor with a seeding CSTR reactor is shown in Figures 2 and 3- With the seeding reactor present, all property oscillations are eliminated, while maintaining essentially the same productivity and quality (conversion and particle size distribution). 

Figure 1. Comparison of reactor configurations single CSTR (Base Case) versus same size CSTR with small seeding reactor (New Reactor Configuration).

These concepts have been exploited in the design of the reactor system and control scheme shown in Figure 5.14. In this configuration, all of the initiator, most of the surfactant and part of the water and monomer are fed to the PFR. The remainder of the surfactant, monomer and water bypass the PFR, and are fed directly to the first CSTR. The initiator flow rate to the PFR is used to control monomer conversion. The bypass of water around the PFR and directly into the CSTR is used to control particle nucleation, and, hence, particle size. The variation of the aqueous flow split produces fluctuations in the surfactant concentration in the seeding reactor, resulting in changes in the particle number and thereby allowing for particle size control. It should be... 

The process flow diagram is shown in Figure 1.17. The plant comprises four 2 m (seed 1) and four 20 m (seed 2) seed reactors and six identical 120 m main fermenters (nominal volumes). All reactors share similar geometry. Table 1.9 notes some key mass balance information. The fed-batch fermentation results in a final liquid harvest volume of 85 m. ... 

Figure 7 Continuous emulsion polymerization (a) conversion and number of particles vs. residence time for a single CSTR (b) reactor system with a small seeding reactor (c) conversion and number of particles vs. time with a seeding reactor.

Particle nucleation and particle growth are important steps in emulsion polymerization because they affect the overall polymerization rate and polymer properties. Thus, initiator concentration and surfactant type/concentra-tion have significant effects on the polymerization kinetics. In a batch emulsion process, particle nucleation and growth steps can be separated to some extent by employing a multistage reaction process. In a continuous process, both particle nucleation and growth steps occur simultaneously unless a seed reactor is provided to separate these two effects. In general, latex particle size distributions obtained by batch and continuous processes are quite different. 

In the most common production method, the semibatch process, about 10% of the preemulsified monomer is added to the deionised water in the reactor. A shot of initiator is added to the reactor to create the seed. Some manufacturers use master batches of seed to avoid variation in this step. Having set the number of particles in the pot, the remaining monomer and, in some cases, additional initiator are added over time. Typical feed times ate 1—4 h. Lengthening the feeds tempers heat generation and provides for uniform comonomer sequence distributions (67). Sometimes skewed monomer feeds are used to offset differences in monomer reactivity ratios. In some cases a second monomer charge is made to produce core—shell latices. At the end of the process pH adjustments are often made. The product is then pumped to a prefilter tank, filtered, and pumped to a post-filter tank where additional processing can occur. When the feed rate of monomer during semibatch production is very low, the reactor is said to be monomer starved. Under these... 

The batch process is similar to the semibatch process except that most or all of the ingredients are added at the beginning of the reaction. Heat generation during a pure batch process makes reactor temperature control difficult, especially for high soHds latices. Seed, usually at 5—10% soHds, is routinely made via a batch process to produce a uniform particle-size distribution. Most kinetic studies and models are based on batch processes (69). 

The softened seawater is fed with dry or slaked lime (dolime) to a reactor. After precipitation in the reactor, a flocculating agent is added and the slurry is pumped to a thickener where the precipitate settles. The spent seawater overflows the thickener and is returned to the sea. A portion of the thickener underflow is recirculated to the reactor to seed crystal growth and improve settling and filtering characteristics of the precipitate. The remainder of the thickener underflow is pumped to a countercurrent washing system. In this system the slurry is washed with freshwater to remove the soluble salts. The washed slurry is vacuum-filtered to produce a filter cake that contains about 50% Mg(OH)2. Typical dimensions for equipment used in the seawater process may be found in the Hterature (75). 

The culture is incubated at a temperature of 28°C in the reactor for 60 hours with mechanicai agitation and constant aeration. The resulting broth is seeded into 600 liters of a sterile culture medium contained in a metal fermenting vat 1,800 liters in capacity and prepared according to the following formulation ... 

The rate of polymerization with styrene-type monomers is directly proportional to the number of particles formed. In batch reactors most of the particles are nucleated early in the reaction and the number formed depends on the emulsifier available to stabilize these small particles. In a CSTR operating at steady-state the rate of nucleation of new particles depends on the concentration of free emulsifier, i.e. the emulsifier not adsorbed on other surfaces. Since the average particle size in a CSTR is larger than the average size at the end of the batch nucleation period, fewer particles are formed in a CSTR than if the same recipe were used in a batch reactor. Since rate is proportional to the number of particles for styrene-type monomers, the rate per unit volume in a CSTR will be less than the interval-two rate in a batch reactor. In fact, the maximum CSTR rate will be about 60 to 70 percent the batch rate for such monomers. Monomers for which the rate is not as strongly dependent on the number of particles will display less of a difference between batch and continuous reactors. Also, continuous reactors with a particle seed in the feed may be capable of higher rates. 

One of the most promising ways of dealing with conversion oscillations is the use of a small-particle latex seed in a feed stream so that particle nucleation does not occur in the CSTRs. Berens (3) used a seed produced in another reactor to achieve stable operation of a continuous PVC reactor. Gonzalez used a continuous tubular pre-reactor to generate the seed for a CSTR producing PMMA latex. 

In this work, the characteristic "living" polymer phenomenon was utilized by preparing a seed polymer in a batch reactor. The seed polymer and styrene were then fed to a constant flow stirred tank reactor. This procedure allowed use of the lumped parameter rate expression given by Equations (5) through (8) to describe the polymerization reaction, and eliminated complications involved in describing simultaneous initiation and propagation reactions. 

Three polymer seeds were prepared in a batch reactor. The reactor with styrene and benzene was cooled to 0 C in an ice bath, initiator was injected into the reactor and reaction began with a gradual increase in temperature. Table II presents the initial conditions used in preparing the seed polymer and the molecular weights of the seed polymer. The molecular weight distribution of the pol3nner seeds are shown in Figure 5. 

Experimental Procedure. For the initial start-up of the continuous tirred tank reactor, the mixing speed and bath temperature were adjusted with the reactor full of solvent. The polymer seed and monomer feed rates were then adjusted simultaneously. 

Evaluation of Mixing Models. The micro-mixed reactor will produce polymer disttibutions with increasing amounts of high molecular weight tail as the degree of polymerization of the polymer product increases over that of the original seed polymer. 

This trend is illustrated by the curves for the micro-mixed reactor in Figures 8 through 14. Also characteristic of the seeded, micro-mixed reactor is the convergence of the polydispersity index to 2 for a high degree of polymerization. This trend is illustrated to some extent in Table VI which presents the calculated degrees of polymerizations. 

Largest polymer chain length in polymer distribution Smallest polymer chain length in seed distribution and reactor effluent Initiation rate constant Propagation rate constant... 

Fraction passing through reaction zone in by-pass reactor o Superscript denotes feed stream of seed polymer... 

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. 

Nomura and Fujita (12.), Figures 8 and 9, styrene/MMA copolymerization in a batch reactor at 50 °C using seed particles. 

Data of Nomura and Funita (12). The predictive capabilities of EPM for copolymerizations are shown in Figures 8-9. Nomura has published a very extensive set of seeded experimental data for the system styrene-MMA. Figures 8 and 9 summarize the EPM calculations for two of these runs which were carried out in a batch reactor at 50 °C at an initiator concentration of 1.25 g dm 3 water. The concentration of the seeded particles was 6x10 dm 3 and the total mass of monomer was 200 g dm 3. The ratio of the mass of MMA to the total monomer was 0.5 and 0.1 in Figures 8 and 9 respectively. The agreement between the measured and predicted values of the total monomer conversion, the copolymer composition, and the concentration of the two monomers in the latex particles is excellent. The transition from Interval II to Interval III is predicted satisfactorily. In accordance with the experimental observations, EPM predicted no new particle formation under the conditions of this run. 

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