Crystallisation/Precipitation

Crystallisation processes are used in a wide range of industries including chemical, petrochemical and pharmaceutical industries. The chemical industry uses crystallisation as a means to produce bulk chemicals and also as a separation technology for purification.

Crystallisation is a very important and widely used process for delivering “product” in its final form. For example a large % of pharmaceuticals are purified by crystallisation. Another example is sugar which is crystallised for sale.

Crystals are formed in a super saturated solution and build into the final form.

The point of nucleation - where the first crystal molecules come out of solution is critical. 

Very often the correct - or incorrect - crystal form (polymorph) is determined at nucleation. Often a “seed” is required to start the crystallisation, but once started may proceed rapidly. The size and shape of the crystals are important to the product.

Controlling the final shape and size is difficult, particularly as it is often difficult to measure on line and the crystallisation process is often non linear in behaviour.

In addition, the scaling up of crystallisation is challenging as reaction rates are affected by macro meso and micro flow rates as volumes increase beyond lab scale.

Photos: Jacketed glass-lined reactor with 4 measurement planes to observe crystallisation and other reactions , and, glass based probe for use in similar vessels.

This shows the results from two batch Barium Sulphate crystallisation experiments with poor mixing (top) and good mixing (bottom). The 4 circular images on the right hand side are conductivity tomograms from the 4 circular measurement planes on the vessel. 

Electrical process tomography scans the conductivity or dielectric at 100's of points in a process vessel. 

When two ions in solution come together to form a crystal there is a very large change in electrical properties.  By mapping conductivity changes in a vessel process tomography can provide many tools to investigate crystallisation:

• mapping high conductivity regions can help determine supersaturation regions; their size, dynamics, concentration variations etc
• measuring conductivity throughout a vessel can provide important kinetic data on crystallisation
• due to its high sensitivity to electrical changes, resistance tomography can provide information on the onset of crystallisation
• acoustic measurements delivers a rich data source on the particle size distribution of crystals

The p2+ is the most appropriate instrument to monitor crystallisation in aqueous systems.

Work has been carried out with 4-plane, 8-plane and tomography probes, all producing useful data for monitoring crystallisation.

Furthermore, process tomography can be used to help characterise separation and purification of the crystals.  Examples where the technology has been deployed include liquid-liquid separation and filtration.

Ultrasound Spectrocopy can also provide information with regards to quality of crystallisation and particle size distribution.

Key benefits include:

• early identification of on-set of crystallisation

• determination of homogeneity / heterogeneity of reaction conditions

• visualisation of process dynamics during crystallisation at different process scales

• control of process conditions to ensure optimal crystal size distribution

• control of crystallisation process

• improved crystallisation process

Above: Different stages of crystals formation and chart plotting FBRM and ERT measurements during crystallisation (Ricard et al., 2005)

Register to access our "Application of process tomography as a tool for monitoring and scale-up of crystallisation" and other "crystallisation" case studies available from the Download section on the right.

Publications:

T.L. Rodgers, D.R. Stephenson, M. Cooke, T.A. York, R. Mann (2009) Tomographic imaging during semibatch reactive precipitation of barium sulphate in a stirred vessel, Chemical Engineering Research and Design, Vol. 87, No. 4, pp 615-626

Bolton, GT, Bennett, M, Wang, M, Qiu, C, Wright, M, Stanley, SJ and Rhodes, D (2007) Development of an electrical tomographic system for operation in a remote, acidic and radioactive environment, Chemical Engineering Journal, Vol. 130, Issues 2-3, pp 165-169

Stanley SJ, Tomographic imaging during reactive precipitation: mixing with chemical reaction, Chemical Engineering Science, 2006, 61 (23), pp 7850-7863

Kagoshima, M. and Mann, R. (2005) Interactions of Precipitation and Fluid Mixing with Model Validation by Electrical Tomography, Chemical Engineering Research and Design, Vol. 83, No. 7, pp 806-810

Stanley, SJ, Mann, R and Primrose, KM (2005) Interrogation of a precipitation reaction by electrical resistance tomography, AIChE Journal, Vol. 51, No. 2, 607-614

Ricard, F, Brechtelsbauer, C, Xu, XY, Lawrence, CJ (2005) Monitoring of multiphase pharmaceutical processes using electrical resistance tomography, Chemical Engineering Research and Design, 83, A7, pp 794-805

For more information about this paper, please contact ITS.

In the press:

• Nuclear Engineering -Waste Management - July 2008 - Crystal Clear
• Pharmamanufacturing.com - 2009 - Scaling up and controlling crystallization
• The Chemical Engineer Feb 2009 - Seeing is believing

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