Reactor drop size distribution

The area between phases A is the surface area of the drops. It will clearly be a strong function of the stirring characteristics (we assume that stirring is always fast enough to mix both phases). The presence of surfactants, drop size distributions, stirrer design, and circulation patterns. Interfacial area is frequently an unknown in emulsion reactors, but the above formulation should be applicable. Another complication in emulsion reactors is the fact that mass transfer coefficients depend strongly on drop size and stirring rate. The relevant parameter in an emulsion reactor is A km wilh neither factor known very well. 

In this section, several cases where there is a spread in drop size distribution will be calculated first for an ideal piston flow reactor in which all liquid parts have the same residence time distribution, and, finally, also the case of a CSTR in which there is a spread in drop size will be calculated, but only for the case of zero-order drop conversion. 

Fig. 13. Conversion f versus dimensionless reaction time 8 for piston flow reactor with drop size distribution zero-order drop conversion.

Continuous Stirred Tank Reactor with Nonuniform Drop Size Distribution... 

In a continuous stirred tank reactor in which the dispersed phase is segregated, a spread in the drop size distribution is present, and there is mass transfer limitation, the spread in the concentration distribution will... 

The effect of coalescence and break-up of droplets on the yield of chemical reactions was studied by Villermaux (33). Micromixing effects may occur even in batch reactors if there is a drop size distribution and mass-transfer control. Although practical rules for the design and scale-up of liquid-liquid reactors are available as Oldshue showed in the case of alkylation (152), many problems remain unsolved (.5) mass transfer effects, high hold-up fractions (> 20 %), large density differences, high viscosities, influence of surfactants. 

Liquid-liquid reactions, like their gas-liquid counterparts, are affected by the physical and chemical characteristics of the system and also by the mechanical features of the equipment. Dispersion phenomena such as the coalescence and breakage of droplets, drop size distribution, and phase inversion will also affect both conversion and selectivity (see, e.g., Sprow, 1967 Laddha and Degaleesan, 1976 Delichatsios and Probstein, 1976 Tavlarides and Stamatoudis, 1981 Backes et al., 1990 Hernandez et al., 1993 Kumar et al., 1993 Brooks and Richmond, 1994). Analysis of dispersion in reactors is a vast field in itself and will not be considered in this book, except as a feature of the equipment. 

The volume fraction of drops in commercial suspension polymerization reactors is usually high and drop coalescence cannot be ignored. In liquid-liquid dispersions the drop size distributions (DSDs) depend on the breakage and on coalescence processes. 

When a water-miscible polymer is to be made via a suspension process, the continuous phase is a water-immiscible fluid, often a hydrocarbon. In such circumstances the adjective inverse is often used to identify the process [118]. The drop phase is often an aqueous monomer solution which contains a water-soluble initiator. Inverse processes that produce very small polymer particles are sometimes referred to as inverse emulsion polymerization but that is often a misnomer because the polymerization mechanism is not always analogous to conventional emulsion polymerization. A more accurate expression is either inverse microsuspension or inverse dispersion polymerization. Here, as with conventional suspension polymerization, the polymerization reaction occurs inside the monomer-containing drops. The drop stabilizers are initially dispersed in the continuous (nonaqueous phase). If particulate solids are used for drop stabilization, the surfaces of the small particles must be rendered hydrophobic. Inverse dispersion polymerization is used to make water-soluble polymers and copolymers from monomers such as acrylic acid, acylamide, and methacrylic acid. These polymers are used in water treatment and as thickening agents for textile applications. Beads of polysaccharides can also be made in inverse suspensions but, in those cases, the polymers are usually preformed before the suspension is created. Physical changes, rather than polymerization reactions, occur in the drops. Conventional stirred reactors are usually used for inverse suspension polymerization and the drop size distribution can be fairly wide. However, Ni et al. [119] found that good control of DSD and PSD could be achieved in the inverse-phase suspension polymerization of acrylamide by using an oscillatory baffled reactor. 

Population Balance Approach. The use of mass and energy balances alone to model polymer reactors is inadequate to describe many cases of interest. Examples are suspension and emulsion polymerizations where drop size or particle distribution may be of interest. In such cases, an accounting for the change in number of droplets or particles of a given size range is often required. This is an example of a population balance. 

Deionized water (720 g), sodium lauryl sulfate (4.3 g), dioctanoyl peroxide (40 g), and acetone (133 g) were emulsified using an ultrasonic probe for 10 minutes. The step 1 polystyrene seed (48.0 g seed, 578 g latex) was added to the emulsion together with lauryl sulfate (0.8 g) and acetone (29.6 g). The mixture was transferred to a flask and left to agitate at approximately 25°C for 48 hours. Acetone was then removed and the solution added to a 5-liter double-walled glass reactor. The temperature was increased to 40°C while styrene (336 g) and divinyl benzene (0.88 g) were added drop-wise over approximately 60 minutes. After 4 hours the mixture was treated with deionized water (1200 g), potassium iodide (1.28 g), and polyvinyl pyrrolidone (18.48 g) with the temperature increased to 70°C. The polymerization continued for 6 hours at 70°C and 1 hour at 90°C. Styrene-based oligomer particles with a diameter of 1.7 pm and with a narrow size distribution were obtained. 

Batch suspension reactors are, theoretically, the kinetic equivalent of water-cooled mass reactors. The major new problems are stabilization of the viscous polymer drops, prediction of particle size distribution, etc. Particle size distribution was found to be determined early in the polymerization by Hopff et al. (28, 29,40). Church and Shinnar (12) applied turbulence theory to explain the stabilization of suspension polymers by the combined action of protective colloids and turbulent flow forces. Suspension polymerization in a CSTR without coalescence is a prime example of the segregated CSTR treated by Tadmor and Biesenberger (51) and is discussed below. In a series of papers, Goldsmith and Amundson (23) and Luss and Amundson (39) studied the unique control and stability problems which arise from the existence of the two-phase reaction system. 

Valentas and Amundson (V3) studied the performance of continuous flow dispersed phase reactors as affected by droplet breakage processes and size distribution of the droplets. Various reaction cases with and without mass transfer were studied for both completely mixed or completely segregated dispersed phase. Droplet size distribution is shown to have a considerable effect on the efliciency of a segregated reaction system. They indicated that polydispersed drop populations require a larger reactor volume to obtain the same conversion as a monodispersed system for zero-order (or mass-transfer-controlled) reactions in higher conversion regions. As the dispersed phase becomes completely mixed, the distribution of droplet sizes becomes less important. These interactions are un-... 

The conditions used for pellet forming can have a major influence on several important catalyst properties, including pore size distribution, pellet strength, and abrasion resistance. Both the size and shape of catalyst pellets affect the pressure drop across a packed bed reactor and also, as indicated earlier, affect the Thiele modulus and thus the effectiveness factor. Recently, monolith catalysts have begun to be used in circumstances where low-pressure drop and/or... 

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