Diseño de la Base de Datos

A stirred tank crystallizer (STC) is generally designed as a round bottomed vessel where heating and cooling is often applied through a jacket (15). Mixing is achieved through the addition of a stirrer unit, frequently an over-head stirrer, which is attached to an impeller shaft to mix the fluids within the vessel; a schematic is shown in Figure 2.10. The main challenges in a traditional batch STC include obtaining good mixing and heat transfer (9), as the circulation rate must be high enough to maintain total suspension of any crystals present (3) but not too high to cause vortices and lead to excessive crystal breakage, as mentioned in Section 2.1.5.2. Mixing can be promoted by the addition of static baffles in STC (9). However, non-uniformity has been shown to occur even when turbulences are achieved in the STC; and get worse when the vessel size increases, giving rise to large mixing gradients during industrial operation (55). The use of continuous STCs is often used in industry is generally through either several tanks in series or a MSMPR system. MSMPR is mixed-suspension, mixed-product removal system where after a hold time, the products are removed and the reactants inputted into the vessel at the same rate.

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Overhead Motor

Stirrer Guide

Stirred Tank Vessel

Stainless Steel Baffle Inserts

Figure 2.10 – Schematic of a typical stirred tank crystallizer

Poor heat transfer within the vessel is another consequence of this non-uniformity and this means that the outer edges of the reaction liquid (those nearest the jacket) are generally at a different temperature to the centre of the vessel (56); transferring the heat produced from the jacket into the centre is not easy with any real speed (45). A lag time can consequently occur elongating the crystallization time and disrupting the crystallization, e.g. nucleation may occur around the outer edges of the vessel before the centre (57), which depending on vessel size could lead to unwanted seeding of the supersaturated solution (28).

It has be shown that when a higher stirrer speed is used in the STC, the number of impurity inclusions present in the product crystals are higher than in crystals produced using lower stirrer speeds (46, 58). As the stirrer speed is increased within STC, the energy input for crystal-crystal, crystal-impeller and crystal-wall collisions is high, which then raises the nucleation rate (58) through collision breeding. This would lower the MSZW; the potential effect of this is likely to be unwanted nucleation of a system when temperature variations exceed the MSZW boundary.

Within the STC the controlling dimensionless number is the Reynolds number, equation 2.5, which is related to the impeller. This number influences the fluid friction and heat and mass transfer within the system; with a higher value indicating improvement in these parameters as the system enters the turbulent mixing regime.

34 𝑅𝑒 = ^{𝜌𝑁𝑠𝐷𝑠2}

𝜇_𝑠 (2.5)

Where ρ = fluid density [kg m^-3], Ns = agitator speed (RPS or s^-1), Ds = impeller diameter [m] and µ = the fluid viscosity [Ns m^-2].

In order to compare the performance of different vessels, mixing conditions for both vessels were selected so that the power density or dissipation rate is approximately equal. The power density of a crystallizer is defined as the “time averaged power dissipation divided by the system volume” (8). For the STC determinations of this have been well documented (59), and can be calculated using equation 2.6.

P

V = ^POρNS3DS5

V_st [W m⁻³] (2.6)

Where P/V = the power density or mixing intensity [W m^-3] and VST = the volume of fluid within the stirred tank [m³]. PO = the power number of the impeller (60), which varies with the impeller type and is obtained from literature. It should be noted that the other parameters are the same as those used in equation 2.5.

The STC has been utilized for crystallization for many years and consequently there is a large body of research in connection with the nucleation, growth and purity effects in the STC.

Within un-seeded systems, nucleation has been shown to occur sporadically (31). Due to this, large batch-to-batch variations can occur (61) leading to a serious problem (62) especially in industry. The rate of production of these primary nuclei also has an effect on the secondary nucleation and consequently the growth rate of the system. Due to the differences between vessel and impeller diameter, mixing gradients can occur within the vessel when the region close to the impeller is ‘better’ mixed than the outer edges, any nuclei located here are likely to undergo more crystal breakage than those in the poorly mixed regions (3). This could lead to a higher rate of secondary nucleation at the centre producing smaller crystals throughout the process. Another consequence of the mixing gradient is the production of agglomerates in the poorly mixed outer edges. Here, single crystals are likely to ‘stick’ together during the process as they are in close contact and are not likely to be moved apart (51), resulting in a large size distribution. This large size distribution is generally unwanted (63) and often has an effect on the final purity, generally lowering the value obtained. This is probably due to a combination of effects, with the smallest crystals having a rough surface to which impure mother liquor often sticks (3, 25) and the largest agglomerates trapping mother liquor between the single crystals (25).

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The addition of seeds is a widely used methodology for minimising primary nucleation (20). The addition of seeds lowers the supersaturation required to nucleate (20) and therefore reduces the effect of the cooling rate and supersaturation on the system, adding a degree of control to the crystallization process.