Kerry Clunie, Pfaudler-Balfour, explains how a combination of axially pumping and radially pumping impellers is a suitable solution for crystallisation applications.

Many pharmaceutical and chemical manufacturers have processes which involve crystallisation (the process of formation of solid crystals precipitating from a solution). More specifically, crystallisation is seen in the manufacture of API’s (Active Pharmaceutical Ingredients) in the pharmaceutical industry where glass-lined vessels are heavily used as batch reactors in the crystallisation process.

The crystallisation process:

Solution – Supersaturated Liquid added to vessel.

Distribute – Small “SEED” Crystals.

Suspend – Solids fully suspended.

Cool     – Vessel contents cooled to induce crystallisation.

Drain – Solution drained out of vessel.

Filter – Solids recovered from Mother Liquor.

The draining of the solution from the vessel is often carried out in batches, due to the filter or centrifuge having a smaller capacity than the discharging vessel – a process that can lead to several problems. 

Crystals that are in suspension must stay in suspension at all times, which can prove difficult at low levels within the vessel.  If suspension is poor, this creates a potential for solids to settle on the bottom dish of the vessel, which can lead to:

•Batch to batch contamination if solids are left over

•Bottom dish glass abrasion due to solids swirling on the surface of the bottom dish.

Historically, conical bottom vessels were used as they are useful for low level mixing. However, these types of vessel have proven to be inferior to standard torispherical bottom vessels for solid suspension duties.

In order to solve the problem we need to first define and then prioritise the process issues:

1. Solid Suspension.

2. Low Level Solid Suspension.

3. Heat Transfer from Vessel Walls.

The general solution for solid suspension processes is to supply an Axially pumping impeller. These draw the fluid from directly above the impeller and then pump it down to the bottom dish of the tank and up around the vessel’s walls, thus promoting top-to-bottom mixing. This flow pattern is effective for keeping solids in suspension and is also efficient in heat transfer applications with the fluid flowing up the vessel walls. However, Axial impellers are not suitable for low level applications. This is because there must be space below the blades for the impeller to pump to. In these cases, Radially pumping impellers are required as the fluid is pumped out towards the vessel walls with no space required below.

So, a solution can be offered that combines both impellers without compromising any of the priorities. Use both types of impeller!

Using a Radially pumping impeller close to the bottom and an Axially pumping impeller further up the shaft creates both a top-to-bottom flow pattern and the low level mixing that would be required.

This solution has been validated and is being used increasingly by pharmaceutical companies and engineering houses.

In order to fully validate the equipment, Pfaudler consulted ANSYS, a  provider of engineering simulation software and consulting services. Taking a typical Full Scale Production reactor, Pfaudler and  ANSYS worked together to model and validate the crystallisation process at 100% volume (4000 L) and 25% volume (1000 L).

ANSYS was supplied with:

•AutoCAD files of the vessel, both impellers, agitator shaft and the baffle.

•Conditions of the process – Impeller Speed, Solids Vol%, Density, etc..

A computational model (mesh) was built and then the 3D studio was used to model the particle motion and interaction with the liquid. ANSYS produced 3-Dimensional pictures and animations showing solids distribution, velocities and fluid strain.

Model Details:

•Sliding mesh model used to rotate the impellers, time step of 0.01sec used.

?•Eulerian granular model used to model the secondary solid phase.

?•Initial steady state multiple reference frame model used as starting condition for the sliding mesh.

?•10 Seconds of real time transient simulation run, to obtain stable solid distribution.

?•Liquid surface assumed to be quiescent in both cases and represented by a frictionless flat boundary.

Conclusions: For the Full Vessel (4000 L), ANSYS has proved that:

1. The dual flight agitator fully suspends all solids (Volume Fraction 4.64%) with only 0.04% variation across the vessel.

2. There is good top-to-bottom mixing across the bottom dish and vessel walls maximising heat transfer.

Both of these points are essential for controlled crystal production.

For the 25% Full Vessel (1000 L), ANSYS has proved that:

1. With one Radially Pumping Impeller, all solids (Volume Fraction 20.45%) are fully suspended with only 0.38% variation.

This highlights that no solids are settled on the bottom dish.


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